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Sorting through the extensive and confusing roles of sortilin in metabolic disease

Open AccessPublished:June 17, 2022DOI:https://doi.org/10.1016/j.jlr.2022.100243

      Abstract

      Sortilin is a post-Golgi trafficking receptor homologous to the yeast vacuolar protein sorting receptor 10 (VPS10). The VPS10 motif on sortilin is a 10-bladed β-propeller structure capable of binding more than 50 proteins, covering a wide range of biological functions including lipid and lipoprotein metabolism, neuronal growth and death, inflammation, and lysosomal degradation. Sortilin has a complex cellular trafficking itinerary, where it functions as a receptor in the trans-Golgi network, endosomes, secretory vesicles, multivesicular bodies, and at the cell surface. In addition, sortilin is associated with hypercholesterolemia, Alzheimer’s disease, prion diseases, Parkinson’s disease, and inflammation syndromes. The 1p13.3 locus containing SORT1, the gene encoding sortilin, carries the strongest association with LDL-C of all loci in human genome-wide association studies. However, the mechanism by which sortilin influences LDL-C is unclear. Here, we review the role sortilin plays in cardiovascular and metabolic diseases and describe in detail the large and often contradictory literature on the role of sortilin in the regulation of LDL-C levels.

      Supplementary key words

      Abbreviations:

      AAV (adeno-associated virus), ADAM (a disintegrin and metalloprotease), AP (adaptor protein), apoB-100 (apolipoprotein B-100), ATF3 (cyclic adenosine monophosphate transcription factor 3), CAD (coronary artery disease), CD (chow diet), C/EBPα (CCAAT-enhancer-binding protein alpha), CELSR2 (cadherin EGF LAG seven-pass G-type receptor 2), CES1 (carboxylesterase 1), CI (cation-independent), CRE (C-rich element), DLK1 (delta-like noncanonical Notch ligand 1), ER (endoplasmic reticulum), EV (extracellular vesicle), FCR (fractional catabolic rate), GGA (Golgi-localized, γ-adaptin ear-containing ADP-ribosylation factor-binding protein), GLUT4 (glucose transporter 4), GSV (GLUT4 storage vesicle), GWAS (genome-wide association study), HF/HC (high-fat/high-cholesterol diet), HFD (high-fat diet), IL-6 (interleukin 6), LDLR (LDL receptor), MPR (mannose 6-phosphate receptor), mTORC1 (mammalian target of rapamycin complex 1), NTR1 (neurotensin receptor 1), PCBP (poly-rC-binding protein), PI3K (phosphoinositide-3-kinase), PKC (protein kinase C), p75NTR (p75 neurotrophin receptor), proBDNF (pro-brain derived neurotrophic factor), proNGF (pro-nerve growth factor), PSRC1 (proline- and serine-rich coiled-coil 1), PVC (prevacuolar endosome compartment), RAP (receptor-associated protein), SMC (smooth muscle cell), TC (total cholesterol), TG (triglyceride), Tg (transgenic), TGN (trans-Golgi network), UTR (untranslated region), VPS10 (vacuolar protein sorting 10), VSMC (vascular smooth muscle cell), WAT (white adipose tissue), WD (Western diet), WT (wild-type)
      Sortilin (SORT1) was first purified and cloned by affinity chromatography of membrane protein extracts from human brain using receptor-associated protein (RAP) as bait (
      • Petersen C.M.
      • Nielsen M.S.
      • Nykjaer A.
      • Jacobsen L.
      • Tommerup N.
      • Rasmussen H.H.
      • et al.
      Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.
      ). RAP is an endoplasmic reticulum (ER)/Golgi-localized molecular chaperone involved in the folding and processing of members of the LDL receptor (LDLR) family. By binding to these receptors, RAP prevents premature binding of ligands (
      • Bu G.
      • Geuze H.J.
      • Strous G.J.
      • Schwartz A.L.
      39 kDa receptor-associated protein is an ER resident protein and molecular chaperone for LDL receptor-related protein.
      ,
      • Sarti M.
      • Farquhar M.G.
      • Orlando R.A.
      The receptor-associated protein (RAP) interacts with several resident proteins of the endoplasmic reticulum including a glycoprotein related to actin.
      ,
      • Willnow T.E.
      • Armstrong S.A.
      • Hammer R.E.
      • Herz J.
      Functional expression of low density lipoprotein receptor-related protein is controlled by receptor-associated protein in vivo.
      ,
      • Willnow T.E.
      • Rohlmann A.
      • Horton J.
      • Otani H.
      • Braun J.R.
      • Hammer R.E.
      • et al.
      RAP, a specialized chaperone, prevents ligand-induced ER retention and degradation of LDL receptor-related endocytic receptors.
      ). Sortilin was the first receptor not seemingly related to the LDLR family that was found to bind to RAP. Sortilin instead is homologous with yeast vacuolar protein sorting 10 (VPS10) and the cation-dependent and cation-independent mannose 6-phosphate receptors (CD-MPR and CI-MPR), which traffic newly synthesized lysosomal enzymes toward the lysosome. Indeed, soon after its discovery, sortilin was shown to transport several resident lysosomal enzymes to the lysosome (
      • Lefrancois S.
      • Zeng J.
      • Hassan A.J.
      • Canuel M.
      • Morales C.R.
      The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin.
      ,
      • Ni X.
      • Morales C.R.
      The lysosomal trafficking of acid sphingomyelinase is mediated by sortilin and mannose 6-phosphate receptor.
      ,
      • Canuel M.
      • Korkidakis A.
      • Konnyu K.
      • Morales C.R.
      Sortilin mediates the lysosomal targeting of cathepsins D and H.
      ) as well as traffic other proteins for lysosomal degradation (
      • Strong A.
      • Ding Q.R.
      • Edmondson A.C.
      • Millar J.S.
      • Sachs K.V.
      • Li X.Y.
      • et al.
      Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.
      ,
      • Evans S.F.
      • Irmady K.
      • Ostrow K.
      • Kim T.
      • Nykjaer A.
      • Saftig P.
      • et al.
      Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation.
      ,
      • Nielsen M.S.
      • Jacobsen C.
      • Olivecrona G.
      • Gliemann J.
      • Petersen C.M.
      Sortilin/neurotensin receptor-3 binds and mediates degradation of lipoprotein lipase.
      ,
      • Lv Y.
      • Yang J.
      • Gao A.
      • Sun S.
      • Zheng X.
      • Chen X.
      • et al.
      Sortilin promotes macrophage cholesterol accumulation and aortic atherosclerosis through lysosomal degradation of ATP-binding cassette transporter A1 protein.
      ,
      • Li J.
      • Wang Y.
      • Matye D.J.
      • Chavan H.
      • Krishnamurthy P.
      • Li F.
      • et al.
      Sortilin 1 modulates hepatic cholesterol lipotoxicity in mice via functional interaction with liver carboxylesterase 1.
      ).
      The initial observation that sortilin binds to RAP suggested that it may be involved in lipoprotein trafficking with the cell. Evidence for such a role came from human genetics. Four genome-wide association studies (GWASs), all published in the same year, found several noncoding SNPs in linkage disequilibrium located at an intergenic region on chromosome 1p13.3 that are strongly associated with circulating LDL-C levels (
      • Kathiresan S.
      • Melander O.
      • Guiducci C.
      • Surti A.
      • Burtt N.P.
      • Rieder M.J.
      • et al.
      Six new loci associated with blood low-density lipoprotein cholesterol, high-density lipoprotein cholesterol or triglycerides in humans.
      ,
      • Willer C.J.
      • Sanna S.
      • Jackson A.U.
      • Scuteri A.
      • Bonnycastle L.L.
      • Clarke R.
      • et al.
      Newly identified loci that influence lipid concentrations and risk of coronary artery disease.
      ,
      • Sandhu M.S.
      • Waterworth D.M.
      • Debenham S.L.
      • Wheeler E.
      • Papadakis K.
      • Zhao J.H.
      • et al.
      LDL-cholesterol concentrations: a genome-wide association study.
      ,
      • Wallace C.
      • Newhouse S.J.
      • Braund P.
      • Zhang F.
      • Tobin M.
      • Falchi M.
      • et al.
      Genome-wide association study identifies genes for biomarkers of cardiovascular disease: serum urate and dyslipidemia.
      ,
      • Samani N.J.
      • Erdmann J.
      • Hall A.S.
      • Hengstenberg C.
      • Mangino M.
      • Mayer B.
      • et al.
      Genomewide association analysis of coronary artery disease.
      ). Three genes are located at this locus: cadherin EGF LAG seven-pass G-type receptor 2 (CELSR2), proline- and serine-rich coiled-coil 1 (PSRC1), and SORT1. Follow-up analysis of the key SNP rs646776 revealed that it impacts the mRNA expression of all three genes in human liver, with the largest regulatory effect on SORT1 mRNA (
      • Kathiresan S.
      • Melander O.
      • Guiducci C.
      • Surti A.
      • Burtt N.P.
      • Rieder M.J.
      • et al.
      Six new loci associated with blood low-density lipoprotein cholesterol, high-density lipoprotein cholesterol or triglycerides in humans.
      ).
      The GWAS findings led to attempts by many groups to reveal the molecular mechanism behind the association of hepatic SORT1 expression with LDL-C. Studies in cell lines and mouse model systems have led to contradictory results on the role of sortilin in cholesterol metabolism, the most notable regarding the directionality of the effect of sortilin on apolipoprotein B-100 (apoB-100) trafficking and VLDL secretion in hepatocytes. Studies by Musunuru et al. (
      • Musunuru K.
      • Strong A.
      • Frank-Kamenetsky M.
      • Lee N.E.
      • Ahfeldt T.
      • Sachs K.V.
      • et al.
      From noncoding variant to phenotype via SORT1 at the 1p13 cholesterol locus.
      ) and Kjolby et al. (
      • Kjolby M.
      • Andersen O.M.
      • Breiderhoff T.
      • Fjorback A.W.
      • Pedersen K.M.
      • Madsen P.
      • et al.
      Sort1, encoded by the cardiovascular risk locus 1p13.3, is a regulator of hepatic lipoprotein export.
      ) found that sortilin regulates VLDL secretion from hepatocytes, thereby affecting LDL-C levels, as VLDL is the precursor of LDL. However, the data published by Musunuru et al. (
      • Musunuru K.
      • Strong A.
      • Frank-Kamenetsky M.
      • Lee N.E.
      • Ahfeldt T.
      • Sachs K.V.
      • et al.
      From noncoding variant to phenotype via SORT1 at the 1p13 cholesterol locus.
      ) showed sortilin to be a negative regulator of VLDL secretion by trafficking the apoB-100-containing lipoprotein toward the lysosome for degradation, whereas Kjolby et al. (
      • Kjolby M.
      • Andersen O.M.
      • Breiderhoff T.
      • Fjorback A.W.
      • Pedersen K.M.
      • Madsen P.
      • et al.
      Sort1, encoded by the cardiovascular risk locus 1p13.3, is a regulator of hepatic lipoprotein export.
      ) showed sortilin to be a positive regulator of VLDL secretion by trafficking it toward the plasma membrane. These two articles were the foundation for a multitude of studies from several groups, but the reason for the discrepant results is still unknown.
      In addition to its role in CVD, a large body of work has established sortilin as a regulator of neuronal development and maintenance and in the pathogenesis of neurological and mood disorders, including Alzheimer’s disease, frontotemporal lobar degeneration, Parkinson’s disease, depression, and anxiety (please see refs. 
      • Malik A.R.
      • Willnow T.E.
      VPS10P domain receptors: sorting out brain health and disease.
      ,
      • Al-Yozbaki M.
      • Acha-Sagredo A.
      • George A.
      • Liloglou T.
      • Wilson C.M.
      Balancing neurotrophin pathway and sortilin function: its role in human disease.
      ,
      • Glerup S.
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      • Vaegter C.B.
      Sortilins in neurotrophic factor signaling.
      ,
      • Carlo A.S.
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      Sorting receptor sortilin-a culprit in cardiovascular and neurological diseases.
      for excellent reviews on this topic).
      This review is divided into two major sections. The first revisits fundamental aspects of sortilin’s structure and function, including the tissue distribution and regulation of its expression, the cellular pathways by which it traffics, and its known ligands. The second half reviews and discusses the role that sortilin plays in cardiovascular and metabolic disease, including its involvement in lipoprotein and cholesterol metabolism, and its potential as a drug target. The primary goal of this review is to pull together the more well-recognized (and controversial) ways in which sortilin influences cholesterol metabolism with ones that may have been overshadowed, to better understand the complexity of sortilin’s function.

      Structure and function of sortilin

      Sortilin (encoded by the SORT1 gene) is a ∼100 kDa type I transmembrane protein and member of the mammalian VPS10 family of post-Golgi trafficking receptors (Fig. 1A) (
      • Hermey G.
      The Vps10p-domain receptor family.
      ,
      • Willnow T.E.
      • Petersen C.M.
      • Nykjaer A.
      VPS10P-domain receptors - regulators of neuronal viability and function.
      ). The defining feature of this protein family is the presence of a ∼700 amino acid luminal/extracellular VPS10 domain, which folds into three structural domains: a large N-terminal 10-bladed β-propeller structure and two small C-terminal cysteine-rich domains (together designated the “ten cysteine consensus” or 10CC module) (
      • Quistgaard E.M.
      • Madsen P.
      • Groftehauge M.K.
      • Nissen P.
      • Petersen C.M.
      • Thirup S.S.
      Ligands bind to Sortilin in the tunnel of a ten-bladed beta-propeller domain.
      ) (Fig. 1B). Following the luminal domain, each receptor has a transmembrane domain followed by a short cytoplasmic/intracellular tail of 40–60 amino acids.
      Figure thumbnail gr1
      Fig. 1Sortilin is a member of the VPS10 family. A: Sortilin is a member of the mammalian VPS10 family of receptors along with SorLA, SorCS1, SorCS2, and SorCS3, which have a large luminal/extracellular segment containing a VPS10 domain, a transmembrane domain, and a short cytoplasmic/intracellular tail. Diagram adapted from Malik and Willnow (
      • Malik A.R.
      • Willnow T.E.
      VPS10P domain receptors: sorting out brain health and disease.
      ). B: The VPS10 domain folds into a 10-bladed β-propeller and cysteine-rich 10CC module, as determined by Quistgaard et al. (
      • Quistgaard E.M.
      • Madsen P.
      • Groftehauge M.K.
      • Nissen P.
      • Petersen C.M.
      • Thirup S.S.
      Ligands bind to Sortilin in the tunnel of a ten-bladed beta-propeller domain.
      ). Protein Data Bank ID: 3F6K.

      Tissue distribution

      Sortilin is expressed in a variety of tissues and cell types. In adult humans, it is highly expressed in tissues like the brain, spinal cord, heart, and skeletal muscle, and lowly expressed in the liver, kidney, pancreas, spleen, and small intestine (
      • Petersen C.M.
      • Nielsen M.S.
      • Nykjaer A.
      • Jacobsen L.
      • Tommerup N.
      • Rasmussen H.H.
      • et al.
      Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.
      ). In the adult human brain, it is predominantly expressed in neurons with regional and neuronal cell-type variability (
      • Xu S.Y.
      • Zhang Q.L.
      • Zhang Q.
      • Wan L.
      • Jiang J.
      • Tu T.
      • et al.
      Regional and cellular mapping of sortilin immunoreactivity in adult human brain.
      ). In adult C57BL/6 (B6) mice, sortilin is highly expressed in the hypothalamus, brain, and white adipose tissue (WAT) and lowly expressed in liver and skeletal muscle. In several tissues in mice, including lung, kidney, and pancreas, it is highly expressed during development and then downregulated in adulthood (
      • Boggild S.
      • Molgaard S.
      • Glerup S.
      • Nyengaard J.R.
      Spatiotemporal patterns of sortilin and SorCS2 localization during organ development.
      ). There is a high differential expression in the central nervous system during embryonal development in mice (
      • Hermans-Borgmeyer I.
      • Hermey G.
      • Nykjaer A.
      • Schaller C.
      Expression of the 100-kDa neurotensin receptor sortilin during mouse embryonal development.
      ,
      • Boggild S.
      • Molgaard S.
      • Glerup S.
      • Nyengaard J.R.
      Highly segregated localization of the functionally related vps10p receptors sortilin and SorCS2 during neurodevelopment.
      ). Sortilin is also expressed in immune cells (
      • Patel K.M.
      • Strong A.
      • Tohyama J.
      • Jin X.
      • Morales C.R.
      • Billheimer J.
      • et al.
      Macrophage sortilin promotes LDL uptake, foam cell formation, and atherosclerosis.
      ,
      • Mortensen M.B.
      • Kjolby M.
      • Gunnersen S.
      • Larsen J.V.
      • Palmfeldt J.
      • Falk E.
      • et al.
      Targeting sortilin in immune cells reduces proinflammatory cytokines and atherosclerosis.
      ,
      • Herda S.
      • Raczkowski F.
      • Mittrücker H.-W.
      • Willimsky G.
      • Gerlach K.
      • Kühl A.A.
      • et al.
      The sorting receptor sortilin exhibits a dual function in exocytic trafficking of interferon-γ and granzyme A in T cells.
      ,
      • Fauchais A.L.
      • Lalloue F.
      • Lise M.C.
      • Boumediene A.
      • Preud'homme J.L.
      • Vidal E.
      • et al.
      Role of endogenous brain-derived neurotrophic factor and sortilin in B cell survival.
      ).

      Regulation of sortilin expression

      Sortilin (SORT1) expression is tightly regulated at the transcriptional, post-transcriptional, and post-translational levels by many DNA and RNA binding proteins and signaling pathways in a cell- and tissue-specific manner.

      Transcriptional regulation

      At the DNA level, sortilin expression is regulated in a tissue-specific manner by the transcription factors CCAAT-enhancer-binding protein alpha (C/EBPα), cyclic adenosine monophosphate transcription factor 3 (ATF3), peroxisome proliferator-activated receptor gamma, and signal transducer and activator of transcription 1, and by DNA methylation. Human GWAS have identified SNPs near the SORT1 gene, located in a noncoding region between the two neighboring genes CELSR2 and PSRC1, which affect the expression of SORT1, CELSR2, and PSRC1 in a tissue-specific manner (
      • Kathiresan S.
      • Melander O.
      • Guiducci C.
      • Surti A.
      • Burtt N.P.
      • Rieder M.J.
      • et al.
      Six new loci associated with blood low-density lipoprotein cholesterol, high-density lipoprotein cholesterol or triglycerides in humans.
      ,
      • Musunuru K.
      • Strong A.
      • Frank-Kamenetsky M.
      • Lee N.E.
      • Ahfeldt T.
      • Sachs K.V.
      • et al.
      From noncoding variant to phenotype via SORT1 at the 1p13 cholesterol locus.
      ,
      • Schadt E.E.
      • Molony C.
      • Chudin E.
      • Hao K.
      • Yang X.
      • Lum P.Y.
      • et al.
      Mapping the genetic architecture of gene expression in human liver.
      ,
      • Linsel-Nitschke P.
      • Heeren J.
      • Aherrahrou Z.
      • Bruse P.
      • Gieger C.
      • Illig T.
      • et al.
      Genetic variation at chromosome 1p13.3 affects sortilin mRNA expression, cellular LDL-uptake and serum LDL levels which translates to the risk of coronary artery disease.
      ). The association of these SNPs with the expression of multiple genes suggests that variation at this locus may have a regional effect on gene expression. Musunuru et al. (
      • Musunuru K.
      • Strong A.
      • Frank-Kamenetsky M.
      • Lee N.E.
      • Ahfeldt T.
      • Sachs K.V.
      • et al.
      From noncoding variant to phenotype via SORT1 at the 1p13 cholesterol locus.
      ) discovered that the minor allele of rs12740374 increases the expression of SORT1 by creating a binding site for the C/EBP transcription factors in the liver. Furthermore, forced expression of C/EBPα specifically induced SORT1 expression in hepatocytes but not embryonic cells or adipocytes.
      Obesity in humans is associated with downregulation of sortilin at the mRNA and protein levels in subcutaneous WAT (
      • Kaddai V.
      • Jager J.
      • Gonzalez T.
      • Najem-Lendom R.
      • Bonnafous S.
      • Tran A.
      • et al.
      Involvement of TNF-α in abnormal adipocyte and muscle sortilin expression in obese mice and humans.
      ) and liver (
      • Bi L.
      • Chiang J.Y.
      • Ding W.X.
      • Dunn W.
      • Roberts B.
      • Li T.
      Saturated fatty acids activate ERK signaling to downregulate hepatic sortilin 1 in obese and diabetic mice.
      ). Similarly, sortilin mRNA and protein expression is downregulated in the liver, gonadal WAT, and skeletal muscle in response to high-fat diet-induced obesity and genetic obesity (ob/ob) in B6 mice (
      • Kaddai V.
      • Jager J.
      • Gonzalez T.
      • Najem-Lendom R.
      • Bonnafous S.
      • Tran A.
      • et al.
      Involvement of TNF-α in abnormal adipocyte and muscle sortilin expression in obese mice and humans.
      ,
      • Ai D.
      • Baez J.M.
      • Jiang H.
      • Conlon D.M.
      • Hernandez-Ono A.
      • Frank-Kamenetsky M.
      • et al.
      Activation of ER stress and mTORC1 suppresses hepatic sortilin-1 levels in obese mice.
      ), making them a good model system for studying the regulation of sortilin expression in obesity. Overnutrition results in hyperactivation of mammalian target of rapamycin complex 1 (mTORC1) and activation of the ER stress response. Ai et al. (
      • Ai D.
      • Baez J.M.
      • Jiang H.
      • Conlon D.M.
      • Hernandez-Ono A.
      • Frank-Kamenetsky M.
      • et al.
      Activation of ER stress and mTORC1 suppresses hepatic sortilin-1 levels in obese mice.
      ) demonstrated that ATF3, which is rapidly induced by ER stress downstream of phospho-eukaryotic initiation factor 2a, binds to a site in the proximal Sort1 promoter and acts as a transcriptional repressor in liver and adipose tissue. Obesity induces inflammation and a proinflammatory environment, which activates Toll-like receptors and subsequent nuclear factor kappa-light-chain-enhancer of activated B cells activation and ATF3 transcription. Multiple cytokines that are key inflammatory mediators regulate the expression of Sort1 mRNA. TNFα controls Sort1 mRNA expression in adipocytes and skeletal muscle partly through a peroxisome proliferator-activated receptor gamma-dependent mechanism (
      • Kaddai V.
      • Jager J.
      • Gonzalez T.
      • Najem-Lendom R.
      • Bonnafous S.
      • Tran A.
      • et al.
      Involvement of TNF-α in abnormal adipocyte and muscle sortilin expression in obese mice and humans.
      ). IFN-γ controls hepatic Sort1 levels through the signal transducer and activator of transcription 1 transcription factor, which is activated and bound to the Sort1 gene upon IFN-γ treatment, reducing the expression of Sort1 (
      • Pirault J.
      • Polyzos K.A.
      • Petri M.H.
      • Ketelhuth D.F.J.
      • Back M.
      • Hansson G.K.
      The inflammatory cytokine interferon-gamma inhibits sortilin-1 expression in hepatocytes via the JAK/STAT pathway.
      ). In addition, in a mouse model that is deficient in regulatory T cells, hepatic Sort1 mRNA expression is significantly reduced, likely through the coincident dramatic increase in hepatic ATF3 in these mice (
      • Klingenberg R.
      • Gerdes N.
      • Badeau R.M.
      • Gistera A.
      • Strodthoff D.
      • Ketelhuth D.F.
      • et al.
      Depletion of FOXP3+ regulatory T cells promotes hypercholesterolemia and atherosclerosis.
      ).

      Post-transcriptional regulation

      Sortilin expression is regulated by a variety of mechanisms at the RNA level. A network of RNA-binding proteins, including TAR-DNA binding protein 43 (TDP-43), heterogeneous nuclear ribonucleoprotein L (hnRNP L), polypyrimidine tract-binding protein (PTB), and hnRNP A1/A2, is involved in the proper splicing of Sort1 mRNA (
      • Prudencio M.
      • Jansen-West K.R.
      • Lee W.C.
      • Gendron T.F.
      • Zhang Y.-J.
      • Xu Y.-F.
      • et al.
      Misregulation of human sortilin splicing leads to the generation of a nonfunctional progranulin receptor.
      ,
      • Polymenidou M.
      • Lagier-Tourenne C.
      • Hutt K.R.
      • Huelga S.C.
      • Moran J.
      • Liang T.Y.
      • et al.
      Long pre-mRNA depletion and RNA missplicing contribute to neuronal vulnerability from loss of TDP-43.
      ,
      • Tann J.Y.
      • Wong L.W.
      • Sajikumar S.
      • Ibanez C.F.
      Abnormal TDP-43 function impairs activity-dependent BDNF secretion, synaptic plasticity, and cognitive behavior through altered Sortilin splicing.
      ,
      • Mohagheghi F.
      • Prudencio M.
      • Stuani C.
      • Cook C.
      • Jansen-West K.
      • Dickson D.W.
      • et al.
      TDP-43 functions within a network of hnRNP proteins to inhibit the production of a truncated human SORT1 receptor.
      ). Poly-rC-binding proteins 1 and 2 (PCBP1 and PCBP2) stabilize Sort1 mRNA by recognizing the C-rich element (CRE) in the 3′ untranslated region (UTR) (
      • Yabe-Wada T.
      • Philpott C.C.
      • Onai N.
      PCBP2 post-transcriptionally regulates sortilin expression by binding to a C-rich element in its 3' UTR.
      ,
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      ). The nucleotide-binding ability of PCBP1 and PCBP2 is impaired by zinc ions, and alterations in intracellular zinc affect Sort1 expression. In differentiated PC12 cells, C2C12 myotubes, and rat skeletal muscles, Sort1 expression is positively regulated by glucose through a post-transcriptional mechanism involving 5′ adenosine monophosphate-activated protein kinase and mTORC1, possibly through enhancement of protein translation (
      • Kawashima K.I.
      • Ishiuchi Y.
      • Konnai M.
      • Komatsu S.
      • Sato H.
      • Kawaguchi H.
      • et al.
      Glucose deprivation regulates the progranulin-sortilin axis in PC12 cells.
      ,
      • Ariga M.
      • Yoneyama Y.
      • Fukushima T.
      • Ishiuchi Y.
      • Ishii T.
      • Sato H.
      • et al.
      Glucose deprivation attenuates sortilin levels in skeletal muscle cells.
      ). In addition, the microRNAs miR-182 and miR378a-3p have been shown to bind to the 3′ UTR of Sort1 mRNA, decreasing Sort1 mRNA levels and sortilin protein (
      • Zhang Z.
      • Jiang W.
      • Yang H.
      • Lin Q.
      • Qin X.
      The miR-182/SORT1 axis regulates vascular smooth muscle cell calcification in vitro and in vivo.
      ,
      • Zhang T.
      • Shi H.
      • Liu N.
      • Tian J.
      • Zhao X.
      • Steer C.J.
      • et al.
      Activation of microRNA-378a-3p biogenesis promotes hepatic secretion of VLDL and hyperlipidemia by modulating ApoB100-Sortilin1 axis.
      ).

      Post-translational regulation

      At the protein level, sortilin expression is regulated by palmitoylation, ubiquitination, and phosphorylation of its cytoplasmic tail. Palmitoylation of cysteine 783 in the tail of sortilin stabilizes sortilin protein (
      • McCormick P.J.
      • Dumaresq-Doiron K.
      • Pluviose A.S.
      • Pichette V.
      • Tosato G.
      • Lefrancois S.
      Palmitoylation controls recycling in lysosomal sorting and trafficking.
      ). Nonpalmitoylated sortilin is ubiquitinated by the “neural precursor cell expressed developmentally downregulated 4” E3 ubiquitin protein ligase (NEDD4) and internalized into the lysosomal compartment via the endosomal sorting complexes required for transport pathway for degradation (
      • Dumaresq-Doiron K.
      • Jules F.
      • Lefrancois S.
      Sortilin turnover is mediated by ubiquitination.
      ). Sortilin is post-translationally downregulated in the liver and gonadal WAT in obesity (
      • Kaddai V.
      • Jager J.
      • Gonzalez T.
      • Najem-Lendom R.
      • Bonnafous S.
      • Tran A.
      • et al.
      Involvement of TNF-α in abnormal adipocyte and muscle sortilin expression in obese mice and humans.
      ,
      • Bi L.
      • Chiang J.Y.
      • Ding W.X.
      • Dunn W.
      • Roberts B.
      • Li T.
      Saturated fatty acids activate ERK signaling to downregulate hepatic sortilin 1 in obese and diabetic mice.
      ,
      • Ai D.
      • Baez J.M.
      • Jiang H.
      • Conlon D.M.
      • Hernandez-Ono A.
      • Frank-Kamenetsky M.
      • et al.
      Activation of ER stress and mTORC1 suppresses hepatic sortilin-1 levels in obese mice.
      ,
      • Chen C.
      • Li J.
      • Matye D.J.
      • Wang Y.
      • Li T.
      Hepatocyte sortilin 1 knockout and treatment with a sortilin 1 inhibitor reduced plasma cholesterol in Western diet-fed mice.
      ,
      • Li J.
      • Bi L.
      • Hulke M.
      • Li T.
      Fish oil and fenofibrate prevented phosphorylation-dependent hepatic sortilin 1 degradation in Western diet-fed mice.
      ,
      • Li J.
      • Chen C.
      • Li Y.
      • Matye D.J.
      • Wang Y.
      • Ding W.X.
      • et al.
      Inhibition of insulin/PI3K/AKT signaling decreases adipose Sortilin 1 in mice and 3T3-L1 adipocytes.
      ,
      • Li J.
      • Matye D.J.
      • Li T.
      Insulin resistance induces posttranslational hepatic sortilin 1 degradation in mice.
      ,
      • Li J.
      • Matye D.J.
      • Wang Y.
      • Li T.
      Sortilin 1 knockout alters basal adipose glucose metabolism but not diet-induced obesity in mice.
      ). Saturated fatty acids downregulate hepatic sortilin protein through activation of ERK, which phosphorylates serine 793 in the cytoplasmic tail of sortilin. This phosphorylation event is followed by ubiquitination of lysine 818 and lysosomal degradation (
      • Bi L.
      • Chiang J.Y.
      • Ding W.X.
      • Dunn W.
      • Roberts B.
      • Li T.
      Saturated fatty acids activate ERK signaling to downregulate hepatic sortilin 1 in obese and diabetic mice.
      ,
      • Li J.
      • Bi L.
      • Hulke M.
      • Li T.
      Fish oil and fenofibrate prevented phosphorylation-dependent hepatic sortilin 1 degradation in Western diet-fed mice.
      ). Oxidized LDL activates ERK signaling to downregulate sortilin expression in liver sinusoidal endothelial cells (
      • Zhang Q.
      • Lin W.
      • Tian L.
      • Di B.
      • Yu J.
      • Niu X.
      • et al.
      Oxidized low-density lipoprotein activates extracellular signal-regulated kinase signaling to downregulate sortilin expression in liver sinusoidal endothelial cells.
      ). In C2C12 myotubes, saturated fatty acids induce downregulation of sortilin via mechanisms involving protein kinase C (PKC) (
      • Tsuchiya Y.
      • Hatakeyama H.
      • Emoto N.
      • Wagatsuma F.
      • Matsushita S.
      • Kanzaki M.
      Palmitate-induced down-regulation of sortilin and impaired GLUT4 trafficking in C2C12 myotubes.
      ).
      Sortilin protein is a target of insulin signaling through the insulin/phosphoinositide-3-kinase/protein kinase B (insulin/PI3K/AKT) signaling cascade, whereby insulin increases sortilin protein expression. In hepatocytes, casein kinase II is activated by insulin signaling and phosphorylates serine 825 in the cytoplasmic tail of sortilin, inducing sortilin expression. Inhibition of PI3K signaling or prevention of sortilin phosphorylation induces the lysosomal degradation of sortilin (
      • Li J.
      • Matye D.J.
      • Li T.
      Insulin resistance induces posttranslational hepatic sortilin 1 degradation in mice.
      ). Hepatic sortilin is also a target of leptin signaling, potentially through the action of leptin to stimulate insulin receptor substrate-mediated PI3K activity (
      • Jun J.Y.
      • Ma Z.
      • Pyla R.
      • Segar L.
      Leptin treatment inhibits the progression of atherosclerosis by attenuating hypercholesterolemia in type 1 diabetic Ins2(+/Akita):apoE(-/-) mice.
      ). Interestingly, the insulin/PI3K/AKT signaling cascade also regulates sortilin protein in adipocytes through an unknown mechanism but not through phosphorylation of serine 825 (
      • Li J.
      • Chen C.
      • Li Y.
      • Matye D.J.
      • Wang Y.
      • Ding W.X.
      • et al.
      Inhibition of insulin/PI3K/AKT signaling decreases adipose Sortilin 1 in mice and 3T3-L1 adipocytes.
      ).
      There is great interest in the significance of the downregulation of sortilin in WAT and liver in obesity. The role of insulin and inflammatory cytokine signaling in regulating liver, adipose, and skeletal muscle sortilin stability suggests that inflammation and impaired insulin signaling (insulin resistance) contribute to reduced sortilin protein in these tissues in obesity.

      Cellular trafficking itinerary of sortilin

      Sortilin is a post-Golgi trafficking receptor

      Sortilin was the first VPS10 domain-containing mammalian protein to be discovered. The domain was first identified in Saccharomyces cerevisiae in the sorting receptor protein known as VPS10. Primarily localized in the late Golgi compartment, VPS10 interacts with soluble vacuolar hydrolases, including carboxypeptidase Y (CPY) and proteinase A (PrA), and traffics them to a prevacuolar endosome compartment (PVC) (
      • Marcusson E.G.
      • Horazdovsky B.F.
      • Cereghino J.L.
      • Gharakhanian E.
      • Emr S.D.
      The sorting receptor for yeast vacuolar carboxypeptidase Y is encoded by the VPS10 gene.
      ,
      • Cooper A.A.
      • Stevens T.H.
      Vps10p cycles between the late-Golgi and prevacuolar compartments in its function as the sorting receptor for multiple yeast vacuolar hydrolases.
      ,
      • Westphal V.
      • Marcusson E.G.
      • Winther J.R.
      • Emr S.D.
      • van den Hazel H.B.
      Multiple pathways for vacuolar sorting of yeast proteinase A.
      ). At the PVC, VPS10 releases its ligand and recycles back to the Golgi apparatus for additional rounds of sorting. The hydrolase continues to the vacuole. VPS10 was recognized as being analogous to the CD-MPR and the CI-MPR in mammalian cells. Newly synthesized lysosomal enzymes acquire a mannose 6-phosphate moiety as they pass through the cis-Golgi. MPRs then bind these enzymes in the trans-Golgi network (TGN) and traffic them to an endosomal compartment. The lysosomal enzymes dissociate from the MPRs in the endosome, where the enzymes continue to the lysosome and the MPRs recycle back to the TGN. The majority of the MPRs traffic between the TGN and endosomes, but some traffic to the cell surface to internalize extracellular lysosomal enzymes (
      • Kornfeld S.
      Structure and function of the mannose 6-phosphate/insulinlike growth factor II receptors.
      ,
      • Kornfeld S.
      • Mellman I.
      The biogenesis of lysosomes.
      ). The similarity of sortilin to VPS10 and the MPRs prompted initial studies that investigated the involvement of sortilin in targeting lysosomal enzymes to the lysosome in mammalian cells. As predicted, sortilin was found to traffic between the TGN and endosomes (
      • Petersen C.M.
      • Nielsen M.S.
      • Nykjaer A.
      • Jacobsen L.
      • Tommerup N.
      • Rasmussen H.H.
      • et al.
      Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.
      ,
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ,
      • Takatsu H.
      • Katoh Y.
      • Shiba Y.
      • Nakayama K.
      Golgi-localizing, gamma-adaptin ear homology domain, ADP-ribosylation factor-binding (GGA) proteins interact with acidic dileucine sequences within the cytoplasmic domains of sorting receptors through their Vps27p/Hrs/STAM (VHS) domains.
      ,
      • Seaman M.N.
      Cargo-selective endosomal sorting for retrieval to the Golgi requires retromer.
      ,
      • Canuel M.
      • Lefrancois S.
      • Zeng J.
      • Morales C.R.
      AP-1 and retromer play opposite roles in the trafficking of sortilin between the Golgi apparatus and the lysosomes.
      ,
      • Mari M.
      • Bujny M.V.
      • Zeuschner D.
      • Geerts W.J.
      • Griffith J.
      • Petersen C.M.
      • et al.
      SNX1 defines an early endosomal recycling exit for sortilin and mannose 6-phosphate receptors.
      ,
      • Kim E.
      • Lee Y.
      • Lee H.J.
      • Kim J.S.
      • Song B.S.
      • Huh J.W.
      • et al.
      Implication of mouse Vps26b-Vps29-Vps35 retromer complex in sortilin trafficking.
      ), mediating the lysosomal targeting of prosaposin (PSAP) (
      • Lefrancois S.
      • Zeng J.
      • Hassan A.J.
      • Canuel M.
      • Morales C.R.
      The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin.
      ,
      • Zeng J.
      • Hassan A.J.
      • Morales C.R.
      Study of the mouse sortilin gene: effects of its transient silencing by RNA interference in TM4 sertoli cells.
      ,
      • Hassan A.J.
      • Zeng J.
      • Ni X.
      • Morales C.R.
      The trafficking of prosaposin (SGP-1) and GM2AP to the lysosomes of TM4 Sertoli cells is mediated by sortilin and monomeric adaptor proteins.
      ,
      • Lefrancois S.
      • Canuel M.
      • Zeng J.
      • Morales C.R.
      Inactivation of sortilin (a novel lysosomal sorting receptor) by dominant negative competition and RNA interference.
      ,
      • Canuel M.
      • Bhattacharyya N.
      • Balbis A.
      • Yuan L.
      • Morales C.R.
      Sortilin and prosaposin localize to detergent-resistant membrane microdomains.
      ,
      • Zeng J.
      • Racicott J.
      • Morales C.R.
      The inactivation of the sortilin gene leads to a partial disruption of prosaposin trafficking to the lysosomes.
      ,
      • Yuan L.
      • Morales C.R.
      A stretch of 17 amino acids in the prosaposin C terminus is critical for its binding to sortilin and targeting to lysosomes.
      ,
      • Wahe A.
      • Kasmapour B.
      • Schmaderer C.
      • Liebl D.
      • Sandhoff K.
      • Nykjaer A.
      • et al.
      Golgi-to-phagosome transport of acid sphingomyelinase and prosaposin is mediated by sortilin.
      ), GM2 ganglioside activator protein (GM2AP) (
      • Lefrancois S.
      • Zeng J.
      • Hassan A.J.
      • Canuel M.
      • Morales C.R.
      The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin.
      ,
      • Hassan A.J.
      • Zeng J.
      • Ni X.
      • Morales C.R.
      The trafficking of prosaposin (SGP-1) and GM2AP to the lysosomes of TM4 Sertoli cells is mediated by sortilin and monomeric adaptor proteins.
      ,
      • Lefrancois S.
      • Canuel M.
      • Zeng J.
      • Morales C.R.
      Inactivation of sortilin (a novel lysosomal sorting receptor) by dominant negative competition and RNA interference.
      ), acid sphingomyelinase (ASM) (
      • Ni X.
      • Morales C.R.
      The lysosomal trafficking of acid sphingomyelinase is mediated by sortilin and mannose 6-phosphate receptor.
      ,
      • Wahe A.
      • Kasmapour B.
      • Schmaderer C.
      • Liebl D.
      • Sandhoff K.
      • Nykjaer A.
      • et al.
      Golgi-to-phagosome transport of acid sphingomyelinase and prosaposin is mediated by sortilin.
      ,
      • Jin S.
      • Yi F.
      • Zhang F.
      • Poklis J.L.
      • Li P.L.
      Lysosomal targeting and trafficking of acid sphingomyelinase to lipid raft platforms in coronary endothelial cells.
      ,
      • Bao J.X.
      • Jin S.
      • Zhang F.
      • Wang Z.C.
      • Li N.
      • Li P.L.
      Activation of membrane NADPH oxidase associated with lysosome-targeted acid sphingomyelinase in coronary endothelial cells.
      ), and cathepsins D and H (
      • Canuel M.
      • Korkidakis A.
      • Konnyu K.
      • Morales C.R.
      Sortilin mediates the lysosomal targeting of cathepsins D and H.
      ).
      Subsequent studies continue to elucidate a much more complex trafficking itinerary of sortilin. In addition to shuttling between the TGN and endosomes, sortilin can also traffic through the constitutive secretory pathway (
      • Herda S.
      • Raczkowski F.
      • Mittrücker H.-W.
      • Willimsky G.
      • Gerlach K.
      • Kühl A.A.
      • et al.
      The sorting receptor sortilin exhibits a dual function in exocytic trafficking of interferon-γ and granzyme A in T cells.
      ), sort into the regulated secretory pathway in specialized cell types (
      • Shi J.
      • Kandror K.V.
      Sortilin is essential and sufficient for the formation of Glut4 storage vesicles in 3T3-L1 adipocytes.
      ,
      • Chen Z.Y.
      • Ieraci A.
      • Teng H.
      • Dall H.
      • Meng C.X.
      • Herrera D.G.
      • et al.
      Sortilin controls intracellular sorting of brain-derived neurotrophic factor to the regulated secretory pathway.
      ,
      • Morris N.J.
      • Ross S.A.
      • Lane W.S.
      • Moestrup S.K.
      • Petersen C.M.
      • Keller S.R.
      • et al.
      Sortilin is the major 110-kDa protein in GLUT4 vesicles from adipocytes.
      ), function as an endocytosis receptor at the cell surface (
      • Nielsen M.S.
      • Jacobsen C.
      • Olivecrona G.
      • Gliemann J.
      • Petersen C.M.
      Sortilin/neurotensin receptor-3 binds and mediates degradation of lipoprotein lipase.
      ,
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ,
      • Morinville A.
      • Martin S.
      • Lavallee M.
      • Vincent J.P.
      • Beaudet A.
      • Mazella J.
      Internalization and trafficking of neurotensin via NTS3 receptors in HT29 cells.
      ,
      • Nilsson S.K.
      • Christensen S.
      • Raarup M.K.
      • Ryan R.O.
      • Nielsen M.S.
      • Olivecrona G.
      Endocytosis of apolipoprotein A-V by members of the low density lipoprotein receptor and the VPS10p domain receptor families.
      ,
      • Prabakaran T.
      • Nielsen R.
      • Satchell S.C.
      • Mathieson P.W.
      • Feldt-Rasmussen U.
      • Sorensen S.S.
      • et al.
      Mannose 6-phosphate receptor and sortilin mediated endocytosis of alpha-galactosidase A in kidney endothelial cells.
      ), and aid in exosome formation and release (
      • Wilson C.M.
      • Naves T.
      • Vincent F.
      • Melloni B.
      • Bonnaud F.
      • Lalloue F.
      • et al.
      Sortilin mediates the release and transfer of exosomes in concert with two tyrosine kinase receptors.
      ,
      • Goettsch C.
      • Hutcheson J.D.
      • Aikawa M.
      • Iwata H.
      • Pham T.
      • Nykjaer A.
      • et al.
      Sortilin mediates vascular calcification via its recruitment into extracellular vesicles.
      ,
      • Pinet S.
      • Bessette B.
      • Vedrenne N.
      • Lacroix A.
      • Richard L.
      • Jauberteau M.O.
      • et al.
      TrkB-containing exosomes promote the transfer of glioblastoma aggressiveness to YKL-40-inactivated glioblastoma cells.
      ,
      • Itoh S.
      • Mizuno K.
      • Aikawa M.
      • Aikawa E.
      Dimerization of sortilin regulates its trafficking to extracellular vesicles.
      ) (Fig. 2A).
      Figure thumbnail gr2
      Fig. 2Cellular trafficking of sortilin. A: 1) Sortilin is converted from its proform to its mature form in the TGN by furin cleavage of its propeptide. Mature sortilin can exit the TGN through three different routes, depending on the cell type, 2) through anterograde sorting to endosomes, which requires binding of APs GGA and/or AP-1 to its cytoplasmic tail, 3) through the constitutive secretory pathway, or 4) through the regulated secretory pathway in specialized cell types. Sortilin does not localize to mature secretory granules, and it is unclear how it exits immature secretory granules, indicated by dotted arrows. When sortilin reaches the cell surface, it can be 5) shed from the surface, or 6) endocytosed, which requires AP-2 binding. The majority of sortilin at the plasma membrane is rapidly endocytosed. 7) Sortilin is returned from endosomes to the TGN by retrograde sorting, which requires interaction with the retromer complex and PACS-1. 8) Sortilin is also involved in sorting cargo to multivesicular bodies and can itself be secreted from the cell in exosomes. Highlighted box indicates a process that occur in specialized cell types. Diagram adapted from Malik and Willnow et al. (
      • Malik A.R.
      • Willnow T.E.
      VPS10P domain receptors: sorting out brain health and disease.
      ). B: Important sorting motifs located in the cytoplasmic tails of sortilin, CI-MPR, CD-MPR, and yeast VPS10 include a tyrosine-based motif (YXXΦ) and an acidic cluster dileucine motif (DXXLL) that overlaps with an acidic cluster motif ([DE]XXXL[LI]), which are important for AP binding. Thickness of the underline indicates relative potency of the motif for the transport and AP binding indicated. MVB, multivesicular body; PACS-1, phosphofurin acidic cluster sorting protein 1.

      Sorting motifs and adaptor proteins

      Like VPS10 and the MPRs, the cytoplasmic tail of sortilin harbors sorting motifs required for the binding of adaptor proteins (APs), Golgi-localized, γ-adaptin ear-containing ADP-ribosylation factor-binding proteins (GGAs), retromer, and other proteins that regulate the trafficking of sortilin (Fig. 2B). Transport of sortilin from the TGN to endosomes is regulated by the binding of GGA1, GGA2, and AP-1 to an acidic cluster dileucine motif (DXXLL, where X is any amino acid) and overlapping acidic cluster motif ([DE]XXXL[LI]) at the far C-terminus of the sortilin tail (
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ,
      • Takatsu H.
      • Katoh Y.
      • Shiba Y.
      • Nakayama K.
      Golgi-localizing, gamma-adaptin ear homology domain, ADP-ribosylation factor-binding (GGA) proteins interact with acidic dileucine sequences within the cytoplasmic domains of sorting receptors through their Vps27p/Hrs/STAM (VHS) domains.
      ,
      • Canuel M.
      • Lefrancois S.
      • Zeng J.
      • Morales C.R.
      AP-1 and retromer play opposite roles in the trafficking of sortilin between the Golgi apparatus and the lysosomes.
      ,
      • Jacobsen L.
      • Madsen P.
      • Nielsen M.S.
      • Geraerts W.P.
      • Gliemann J.
      • Smit A.B.
      • et al.
      The sorLA cytoplasmic domain interacts with GGA1 and -2 and defines minimum requirements for GGA binding.
      ,
      • Cramer J.F.
      • Gustafsen C.
      • Behrens M.A.
      • Oliveira C.L.
      • Pedersen J.S.
      • Madsen P.
      • et al.
      GGA autoinhibition revisited.
      ,
      • Baltes J.
      • Larsen J.V.
      • Radhakrishnan K.
      • Geumann C.
      • Kratzke M.
      • Petersen C.M.
      • et al.
      sigma1B adaptin regulates adipogenesis by mediating the sorting of sortilin in adipose tissue.
      ). The dileucine is the most critical part of the motif (
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ), and it is essential that it be positioned at the C-terminus, as adding extra amino acids to the C-terminus of sortilin (such as a tag) has been shown to inhibit GGA binding (
      • Cramer J.F.
      • Gustafsen C.
      • Behrens M.A.
      • Oliveira C.L.
      • Pedersen J.S.
      • Madsen P.
      • et al.
      GGA autoinhibition revisited.
      ). AP-1 binding to a tyrosine-based motif in the cytoplasmic tail of sortilin (YXXΦ, where X is any amino acid and Φ is a bulky hydrophobic residue) may also be involved in the TGN-to-endosome transport. At the endosome, the tyrosine-based motif is a potent signal for retromer binding and required for its efficient retrieval back to the TGN for further rounds of sorting (
      • Seaman M.N.
      Cargo-selective endosomal sorting for retrieval to the Golgi requires retromer.
      ,
      • Canuel M.
      • Lefrancois S.
      • Zeng J.
      • Morales C.R.
      AP-1 and retromer play opposite roles in the trafficking of sortilin between the Golgi apparatus and the lysosomes.
      ,
      • Mari M.
      • Bujny M.V.
      • Zeuschner D.
      • Geerts W.J.
      • Griffith J.
      • Petersen C.M.
      • et al.
      SNX1 defines an early endosomal recycling exit for sortilin and mannose 6-phosphate receptors.
      ,
      • Kim E.
      • Lee Y.
      • Lee H.J.
      • Kim J.S.
      • Song B.S.
      • Huh J.W.
      • et al.
      Implication of mouse Vps26b-Vps29-Vps35 retromer complex in sortilin trafficking.
      ,
      • Harbour M.E.
      • Breusegem S.Y.
      • Antrobus R.
      • Freeman C.
      • Reid E.
      • Seaman M.N.
      The cargo-selective retromer complex is a recruiting hub for protein complexes that regulate endosomal tubule dynamics.
      ). There is evidence that a nearby hydrophobic motif, FLV in sortilin and WLM in CI-MPR, may be part of a bipartite retromer binding site (
      • Suzuki S.W.
      • Chuang Y.S.
      • Li M.
      • Seaman M.N.J.
      • Emr S.D.
      A bipartite sorting signal ensures specificity of retromer complex in membrane protein recycling.
      ,
      • Seaman M.N.
      Identification of a novel conserved sorting motif required for retromer-mediated endosome-to-TGN retrieval.
      ). This site has similarity to the FYVF site in VPS10 that is required for its retrieval from the PVC in yeast (
      • Cereghino J.L.
      • Marcusson E.G.
      • Emr S.D.
      The cytoplasmic tail domain of the vacuolar protein sorting receptor Vps10p and a subset of VPS gene products regulate receptor stability, function, and localization.
      ). Ceroid-lipofuscinosis neuronal protein 5 (CLN5) has been shown to be required for retromer binding to sortilin at early endosomes (
      • Mamo A.
      • Jules F.
      • Dumaresq-Doiron K.
      • Costantino S.
      • Lefrancois S.
      The role of ceroid lipofuscinosis neuronal protein 5 (CLN5) in endosomal sorting.
      ), and AP-5, a relatively uncharacterized AP, can interact with sortilin and function as a backup to the retromer in retrieving sortilin from the endosome (
      • Hirst J.
      • Itzhak D.N.
      • Antrobus R.
      • Borner G.H.H.
      • Robinson M.S.
      Role of the AP-5 adaptor protein complex in late endosome-to-Golgi retrieval.
      ). In addition, Rab7b is important for the formation of transport carriers that move sortilin between the TGN and endosomes (
      • Progida C.
      • Nielsen M.S.
      • Koster G.
      • Bucci C.
      • Bakke O.
      Dynamics of Rab7b-dependent transport of sorting receptors.
      ). Calnuc regulates the activity of Rab7 in this process and is also involved in the recruitment of retromer to endosomes (
      • Larkin H.
      • Costantino S.
      • Seaman M.N.
      • Lavoie C.
      Calnuc Function in Endosomal Sorting of Lysosomal Receptors.
      ).
      Post-translational modifications of the sorting motifs regulate AP binding, and therefore the trafficking and localization of sortilin. Palmitoylation of cysteine 783 in the cytoplasmic tail of sortilin (nine amino acids N-terminal to the YXXΦ motif) by the aspartic acid-histidine-histidine-cysteine-containing palmitoyltransferase 15 (DHHC-15) is required for efficient retromer binding and retrieval of sortilin from endosomes (
      • McCormick P.J.
      • Dumaresq-Doiron K.
      • Pluviose A.S.
      • Pichette V.
      • Tosato G.
      • Lefrancois S.
      Palmitoylation controls recycling in lysosomal sorting and trafficking.
      ). Palmitoylation is not required for AP-1 binding, suggesting that this modification is required for exit of sortilin from the endosomes but not from the TGN. Mutation of the palmitoylation site results in the trapping of sortilin in endosomes. Unable to recycle, it is subject to ubiquitination and degradation.
      The acidic cluster motif in the cytoplasmic tail of sortilin contains a serine residue (serine 825) that can be phosphorylated by casein kinase II (
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ). Investigation of whether this acidic cluster or the phosphorylation status of its serine residue affects binding of APs and trafficking of sortilin has generated complicated results (
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ,
      • Takatsu H.
      • Katoh Y.
      • Shiba Y.
      • Nakayama K.
      Golgi-localizing, gamma-adaptin ear homology domain, ADP-ribosylation factor-binding (GGA) proteins interact with acidic dileucine sequences within the cytoplasmic domains of sorting receptors through their Vps27p/Hrs/STAM (VHS) domains.
      ). There is speculation that the hydrophilic nature of the serine residue, but not its phosphorylation status, is important, as has been shown for the sorting of the CI-MPR (
      • Chen H.J.
      • Yuan J.
      • Lobel P.
      Systematic mutational analysis of the cation-independent mannose 6-phosphate/insulin-like growth factor II receptor cytoplasmic domain. An acidic cluster containing a key aspartate is important for function in lysosomal enzyme sorting.
      ), or that its phosphorylation status is important for binding of GGA2 but not GGA1 (
      • Jacobsen L.
      • Madsen P.
      • Nielsen M.S.
      • Geraerts W.P.
      • Gliemann J.
      • Smit A.B.
      • et al.
      The sorLA cytoplasmic domain interacts with GGA1 and -2 and defines minimum requirements for GGA binding.
      ). Phosphofurin acidic cluster sorting protein 1 (PACS-1) binds phosphorylated acidic clusters, mediating retrograde Golgi-endosome transport (
      • Molloy S.S.
      • Anderson E.D.
      • Jean F.
      • Thomas G.
      Bi-cycling the furin pathway: from TGN localization to pathogen activation and embryogenesis.
      ) and may play a role in the retrieval of sortilin (
      • Takatsu H.
      • Katoh Y.
      • Shiba Y.
      • Nakayama K.
      Golgi-localizing, gamma-adaptin ear homology domain, ADP-ribosylation factor-binding (GGA) proteins interact with acidic dileucine sequences within the cytoplasmic domains of sorting receptors through their Vps27p/Hrs/STAM (VHS) domains.
      ,
      • Scott G.K.
      • Fei H.
      • Thomas L.
      • Medigeshi G.R.
      • Thomas G.
      A PACS-1, GGA3 and CK2 complex regulates CI-MPR trafficking.
      ). The YXXΦ motif (YSVL) also contains a serine residue, one that can be phosphorylated by Rac-p21-activated kinases 1–3 (
      • Pallesen L.T.
      • Gustafsen C.
      • Cramer J.F.
      • Petersen S.V.
      • Thirup S.S.
      • Madsen P.
      • et al.
      PAK kinases target sortilin and modulate its sorting.
      ). The phosphorylation of this serine residue alters the affinity for AP-1 binding and changes the intracellular localization of sortilin, supporting prior evidence that the YXXΦ motif is involved in TGN-to-endosome transport through AP-1 binding, in addition to being a potent internalization signal through AP-2 binding at the plasma membrane.
      The molecules involved in directing sortilin into the secretory pathways are not as well elucidated. Entry into the regulated secretory pathway requires interaction with still unidentified APs (
      • Tooze S.A.
      Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells.
      ). Huntingtin-associated protein 1 (HAP1) may be involved (
      • Yang M.
      • Lim Y.
      • Li X.
      • Zhong J.H.
      • Zhou X.F.
      Precursor of brain-derived neurotrophic factor (proBDNF) forms a complex with Huntingtin-associated protein-1 (HAP1) and sortilin that modulates proBDNF trafficking, degradation, and processing.
      ). Although it is not an AP itself, it may aid in AP recruitment to the cytoplasmic tail of sortilin. Proteins do not require specific interaction with APs to exit the TGN into the constitutive secretory pathway. It is unknown how sortilin’s entry into this pathway is regulated; however, the “sorting for entry” model of Golgi sorting indicates that proteins enter the constitutive secretory pathway by default if not directly or indirectly bound by APs for regulated secretory pathway or endosome targeting (
      • Tooze S.A.
      Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells.
      ). Therefore, sortilin’s entry into the constitutive secretory pathway may be indirectly regulated by post-translational modifications of its cytoplasmic tail that affect binding of APs and entry into these other pathways.
      Sortilin molecules that reach the cell surface can have up to three different fates, depending on the cell type. The majority of sortilin receptors at the plasma membrane are rapidly endocytosed. Others become a substrate for a disintegrin and metalloprotease 10 (ADAM10) (
      • Evans S.F.
      • Irmady K.
      • Ostrow K.
      • Kim T.
      • Nykjaer A.
      • Saftig P.
      • et al.
      Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation.
      ,
      • Navarro V.
      • Vincent J.P.
      • Mazella J.
      Shedding of the luminal domain of the neurotensin receptor-3/sortilin in the HT29 cell line.
      ,
      • Ogawa K.
      • Ueno T.
      • Iwasaki T.
      • Kujiraoka T.
      • Ishihara M.
      • Kunimoto S.
      • et al.
      Soluble sortilin is released by activated platelets and its circulating levels are associated with cardiovascular risk factors.
      ), which cleaves the luminal domain from the transmembrane and cytosolic domains, shedding it from the cell in a soluble form. ADAM17/TNFα-converting enzyme may also be involved in cleaving sortilin (
      • Hermey G.
      • Sjogaard S.S.
      • Petersen C.M.
      • Nykjaer A.
      • Gliemann J.
      Tumour necrosis factor alpha-converting enzyme mediates ectodomain shedding of Vps10p-domain receptor family members.
      ), but this is controversial and may be cell type specific (
      • Evans S.F.
      • Irmady K.
      • Ostrow K.
      • Kim T.
      • Nykjaer A.
      • Saftig P.
      • et al.
      Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation.
      ). After the luminal domain is cleaved, the C-terminal fragment left behind in the cell membrane can become a substrate for γ-secretase, potentially aiding in the degradation of the fragment (
      • Nyborg A.C.
      • Ladd T.B.
      • Zwizinski C.W.
      • Lah J.J.
      • Golde T.E.
      Sortilin, SorCS1b, and SorLA Vps10p sorting receptors, are novel gamma-secretase substrates.
      ). The majority of the cleavage by ADAM10 occurs at the cell surface; however, soluble sortilin has also been detected intracellularly from cleavage by ADAM10 in the secretory pathway, leading to its constitutive secretion from the cell (
      • Evans S.F.
      • Irmady K.
      • Ostrow K.
      • Kim T.
      • Nykjaer A.
      • Saftig P.
      • et al.
      Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation.
      ). In certain cell types such as neurons, sortilin that reaches the plasma membrane can hetero-oligomerize with the p75 neurotrophin receptor (p75NTR), allowing it to bind pro-nerve growth factor (proNGF) and transmit a signal for apoptosis (see refs.
      • Malik A.R.
      • Willnow T.E.
      VPS10P domain receptors: sorting out brain health and disease.
      ,
      • Al-Yozbaki M.
      • Acha-Sagredo A.
      • George A.
      • Liloglou T.
      • Wilson C.M.
      Balancing neurotrophin pathway and sortilin function: its role in human disease.
      ,
      • Glerup S.
      • Nykjaer A.
      • Vaegter C.B.
      Sortilins in neurotrophic factor signaling.
      ,
      • Carlo A.S.
      • Nykjaer A.
      • Willnow T.E.
      Sorting receptor sortilin-a culprit in cardiovascular and neurological diseases.
      for reviews).
      In the steady state, sortilin is predominantly localized to the TGN and endosomes, with a small amount localized (∼10%) at the cell surface (
      • Petersen C.M.
      • Nielsen M.S.
      • Nykjaer A.
      • Jacobsen L.
      • Tommerup N.
      • Rasmussen H.H.
      • et al.
      Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.
      ,
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ,
      • Takatsu H.
      • Katoh Y.
      • Shiba Y.
      • Nakayama K.
      Golgi-localizing, gamma-adaptin ear homology domain, ADP-ribosylation factor-binding (GGA) proteins interact with acidic dileucine sequences within the cytoplasmic domains of sorting receptors through their Vps27p/Hrs/STAM (VHS) domains.
      ,
      • Mari M.
      • Bujny M.V.
      • Zeuschner D.
      • Geerts W.J.
      • Griffith J.
      • Petersen C.M.
      • et al.
      SNX1 defines an early endosomal recycling exit for sortilin and mannose 6-phosphate receptors.
      ,
      • Morinville A.
      • Martin S.
      • Lavallee M.
      • Vincent J.P.
      • Beaudet A.
      • Mazella J.
      Internalization and trafficking of neurotensin via NTS3 receptors in HT29 cells.
      ). At the cell surface, the tyrosine-based motif is a potent signal for internalization by AP-2 (
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ). The acidic cluster dileucine, to which AP-2 can bind, also plays a role in sortilin internalization, but to a much lesser extent (
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ). Mutation of the tyrosine-based motif results in the accumulation of sortilin at the plasma membrane (
      • Strong A.
      • Ding Q.R.
      • Edmondson A.C.
      • Millar J.S.
      • Sachs K.V.
      • Li X.Y.
      • et al.
      Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.
      ,
      • Nielsen M.S.
      • Madsen P.
      • Christensen E.I.
      • Nykjaer A.
      • Gliemann J.
      • Kasper D.
      • et al.
      The sortilin cytoplasmic tail conveys Golgi-endosome transport and binds the VHS domain of the GGA2 sorting protein.
      ), indicating that the steady-state localization of sortilin can be deceiving and that a large number of the receptors reach the cell surface but are rapidly internalized.

      Ligands and binding sites

      Sortilin is a multiligand receptor, trafficking and binding a number of soluble and membrane proteins of varying size that have diverse and often unrelated functions. Over 50 different proteins have been identified to bind sortilin and/or have altered trafficking or signaling upon manipulation of sortilin expression or function (Table 1). Neurotensin was the first ligand identified and is the only one that has been co-crystalized with sortilin, revealing its binding in a small pocket inside the tunnel of the 10-bladed β-propeller of sortilin’s VPS10 domain (
      • Quistgaard E.M.
      • Madsen P.
      • Groftehauge M.K.
      • Nissen P.
      • Petersen C.M.
      • Thirup S.S.
      Ligands bind to Sortilin in the tunnel of a ten-bladed beta-propeller domain.
      ). Competitive binding measurements have demonstrated that other ligands, including proNGF, pro-brain derived neurotrophic factor (proBDNF), and progranulin (PRGN) likely bind in a distinct but overlapping region with that of neurotensin (
      • Quistgaard E.M.
      • Madsen P.
      • Groftehauge M.K.
      • Nissen P.
      • Petersen C.M.
      • Thirup S.S.
      Ligands bind to Sortilin in the tunnel of a ten-bladed beta-propeller domain.
      ,
      • Trabjerg E.
      • Abu-Asad N.
      • Wan Z.
      • Kartberg F.
      • Christensen S.
      • Rand K.D.
      Investigating the conformational response of the sortilin receptor upon binding endogenous peptide- and protein ligands by HDX-MS.
      ,
      • Serup Andersen O.
      • Boisguerin P.
      • Glerup S.
      • Skeldal S.
      • Volkmer R.
      • Willnow T.E.
      • et al.
      Identification of a linear epitope in sortilin that partakes in pro-neurotrophin binding.
      ), revealing that at least part of the binding site of proneurotrophins is located outside the tunnel of the β-propeller (
      • Serup Andersen O.
      • Boisguerin P.
      • Glerup S.
      • Skeldal S.
      • Volkmer R.
      • Willnow T.E.
      • et al.
      Identification of a linear epitope in sortilin that partakes in pro-neurotrophin binding.
      ).
      Table 1Sortilin ligands
      PathwayLigandReferences
      Lipid relatedApolipoprotein A-V (apoA-V)(
      • Nilsson S.K.
      • Christensen S.
      • Raarup M.K.
      • Ryan R.O.
      • Nielsen M.S.
      • Olivecrona G.
      Endocytosis of apolipoprotein A-V by members of the low density lipoprotein receptor and the VPS10p domain receptor families.
      )
      Apolipoprotein B-100 (apoB-100)(
      • Strong A.
      • Ding Q.R.
      • Edmondson A.C.
      • Millar J.S.
      • Sachs K.V.
      • Li X.Y.
      • et al.
      Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.
      ,
      • Kjolby M.
      • Andersen O.M.
      • Breiderhoff T.
      • Fjorback A.W.
      • Pedersen K.M.
      • Madsen P.
      • et al.
      Sort1, encoded by the cardiovascular risk locus 1p13.3, is a regulator of hepatic lipoprotein export.
      ,
      • Chamberlain J.M.
      • O'Dell C.
      • Sparks C.E.
      • Sparks J.D.
      Insulin suppression of apolipoprotein B in McArdle RH7777 cells involves increased sortilin 1 interaction and lysosomal targeting.
      ,
      • Sparks R.P.
      • Guida W.C.
      • Sowden M.P.
      • Jenkins J.L.
      • Starr M.L.
      • Fratti R.A.
      • et al.
      Sortilin facilitates VLDL-B100 secretion by insulin sensitive McArdle RH7777 cells.
      ,
      • Amengual J.
      • Guo L.
      • Strong A.
      • Madrigal-Matute J.
      • Wang H.
      • Kaushik S.
      • et al.
      Autophagy is required for sortilin-mediated degradation of apolipoprotein B100.
      )
      Apolipoprotein E (apoE)(
      • Carlo A.S.
      • Gustafsen C.
      • Mastrobuoni G.
      • Nielsen M.S.
      • Burgert T.
      • Hartl D.
      • et al.
      The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-beta peptide in the brain.
      )
      Apolipoprotein J/clusterin (apoJ)(
      • Wang Y.
      • Qin X.
      • Paudel H.K.
      Amyloid beta peptide promotes lysosomal degradation of clusterin via sortilin in hippocampal primary neurons.
      )
      ATP-binding cassette subfamily A member 1 (ABCA1)(
      • Lv Y.
      • Yang J.
      • Gao A.
      • Sun S.
      • Zheng X.
      • Chen X.
      • et al.
      Sortilin promotes macrophage cholesterol accumulation and aortic atherosclerosis through lysosomal degradation of ATP-binding cassette transporter A1 protein.
      )
      Delta like non-canonical Notch ligand 1 (DLK1)(
      • Baltes J.
      • Larsen J.V.
      • Radhakrishnan K.
      • Geumann C.
      • Kratzke M.
      • Petersen C.M.
      • et al.
      sigma1B adaptin regulates adipogenesis by mediating the sorting of sortilin in adipose tissue.
      )
      Lipoprotein lipase (LPL)(
      • Nielsen M.S.
      • Jacobsen C.
      • Olivecrona G.
      • Gliemann J.
      • Petersen C.M.
      Sortilin/neurotensin receptor-3 binds and mediates degradation of lipoprotein lipase.
      )
      Carboxylesterase 1 (CES1)(
      • Li J.
      • Wang Y.
      • Matye D.J.
      • Chavan H.
      • Krishnamurthy P.
      • Li F.
      • et al.
      Sortilin 1 modulates hepatic cholesterol lipotoxicity in mice via functional interaction with liver carboxylesterase 1.
      )
      Proprotein convertase subtilisin/kexin type 9 (PCSK9)(
      • Gustafsen C.
      • Kjolby M.
      • Nyegaard M.
      • Mattheisen M.
      • Lundhede J.
      • Buttenschon H.
      • et al.
      The hypercholesterolemia-risk gene SORT1 facilitates PCSK9 secretion.
      )
      Neurotrophin relatedp75 neurotrophin receptor (p75NTR)(
      • Teng H.K.
      • Teng K.K.
      • Lee R.
      • Wright S.
      • Tevar S.
      • Almeida R.D.
      • et al.
      ProBDNF induces neuronal apoptosis via activation of a receptor complex of p75NTR and sortilin.
      ,
      • Nykjaer A.
      • Lee R.
      • Teng K.K.
      • Jansen P.
      • Madsen P.
      • Nielsen M.S.
      • et al.
      Sortilin is essential for proNGF-induced neuronal cell death.
      ,
      • Skeldal S.
      • Sykes A.M.
      • Glerup S.
      • Matusica D.
      • Palstra N.
      • Autio H.
      • et al.
      Mapping of the interaction site between sortilin and the p75 neurotrophin receptor reveals a regulatory role for the sortilin intracellular domain in p75 neurotrophin receptor shedding and apoptosis.
      )
      pro-brain-derived neurotrophic factor (proBDNF)(
      • Evans S.F.
      • Irmady K.
      • Ostrow K.
      • Kim T.
      • Nykjaer A.
      • Saftig P.
      • et al.
      Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation.
      ,
      • Fauchais A.L.
      • Lalloue F.
      • Lise M.C.
      • Boumediene A.
      • Preud'homme J.L.
      • Vidal E.
      • et al.
      Role of endogenous brain-derived neurotrophic factor and sortilin in B cell survival.
      ,
      • Tann J.Y.
      • Wong L.W.
      • Sajikumar S.
      • Ibanez C.F.
      Abnormal TDP-43 function impairs activity-dependent BDNF secretion, synaptic plasticity, and cognitive behavior through altered Sortilin splicing.
      ,
      • Chen Z.Y.
      • Ieraci A.
      • Teng H.
      • Dall H.
      • Meng C.X.
      • Herrera D.G.
      • et al.
      Sortilin controls intracellular sorting of brain-derived neurotrophic factor to the regulated secretory pathway.
      ,
      • Yang M.
      • Lim Y.
      • Li X.
      • Zhong J.H.
      • Zhou X.F.
      Precursor of brain-derived neurotrophic factor (proBDNF) forms a complex with Huntingtin-associated protein-1 (HAP1) and sortilin that modulates proBDNF trafficking, degradation, and processing.
      ,
      • Teng H.K.
      • Teng K.K.
      • Lee R.
      • Wright S.
      • Tevar S.
      • Almeida R.D.
      • et al.
      ProBDNF induces neuronal apoptosis via activation of a receptor complex of p75NTR and sortilin.
      ,
      • Richner M.
      • Pallesen L.T.
      • Ulrichsen M.
      • Poulsen E.T.
      • Holm T.H.
      • Login H.
      • et al.
      Sortilin gates neurotensin and BDNF signaling to control peripheral neuropathic pain.
      ,
      • Zamani M.
      • Eslami M.
      • Nezafat N.
      • Hosseini S.V.
      • Ghasemi Y.
      Evaluating the effect of BDNF Val66Met polymorphism on complex formation with HAP1 and Sortilin1 via structural modeling.
      )
      pro-nerve growth factor (proNGF)(
      • Nykjaer A.
      • Lee R.
      • Teng K.K.
      • Jansen P.
      • Madsen P.
      • Nielsen M.S.
      • et al.
      Sortilin is essential for proNGF-induced neuronal cell death.
      ,
      • Feng D.
      • Kim T.
      • Ozkan E.
      • Light M.
      • Torkin R.
      • Teng K.K.
      • et al.
      Molecular and structural insight into proNGF engagement of p75NTR and sortilin.
      ,
      • Rogers M.L.
      • Bailey S.
      • Matusica D.
      • Nicholson I.
      • Muyderman H.
      • Pagadala P.C.
      • et al.
      ProNGF mediates death of Natural Killer cells through activation of the p75NTR-sortilin complex.
      ,
      • Nakamura K.
      • Namekata K.
      • Harada C.
      • Harada T.
      Intracellular sortilin expression pattern regulates proNGF-induced naturally occurring cell death during development.
      ,
      • Clewes O.
      • Fahey M.S.
      • Tyler S.J.
      • Watson J.J.
      • Seok H.
      • Catania C.
      • et al.
      Human ProNGF: biological effects and binding profiles at TrkA, P75NTR and sortilin.
      )
      Proneurotrophin-3 (proNT-3)(
      • Tauris J.
      • Gustafsen C.
      • Christensen E.I.
      • Jansen P.
      • Nykjaer A.
      • Nyengaard J.R.
      • et al.
      Proneurotrophin-3 may induce Sortilin-dependent death in inner ear neurons.
      )
      Tropomyosin receptor kinase A (TRKA)(
      • Vaegter C.B.
      • Jansen P.
      • Fjorback A.W.
      • Glerup S.
      • Skeldal S.
      • Kjolby M.
      • et al.
      Sortilin associates with Trk receptors to enhance anterograde transport and neurotrophin signaling.
      )
      Tropomyosin receptor kinase B (TRKB)(
      • Vaegter C.B.
      • Jansen P.
      • Fjorback A.W.
      • Glerup S.
      • Skeldal S.
      • Kjolby M.
      • et al.
      Sortilin associates with Trk receptors to enhance anterograde transport and neurotrophin signaling.
      )
      Tropomyosin receptor kinase C (TRKC)(
      • Vaegter C.B.
      • Jansen P.
      • Fjorback A.W.
      • Glerup S.
      • Skeldal S.
      • Kjolby M.
      • et al.
      Sortilin associates with Trk receptors to enhance anterograde transport and neurotrophin signaling.
      )
      Neurotensin relatedNeurotensin(
      • Quistgaard E.M.
      • Madsen P.
      • Groftehauge M.K.
      • Nissen P.
      • Petersen C.M.
      • Thirup S.S.
      Ligands bind to Sortilin in the tunnel of a ten-bladed beta-propeller domain.
      ,
      • Morinville A.
      • Martin S.
      • Lavallee M.
      • Vincent J.P.
      • Beaudet A.
      • Mazella J.
      Internalization and trafficking of neurotensin via NTS3 receptors in HT29 cells.
      ,
      • Mazella J.
      • Zsurger N.
      • Navarro V.
      • Chabry J.
      • Kaghad M.
      • Caput D.
      • et al.
      The 100-kDa neurotensin receptor is gp95/sortilin, a non-G-protein-coupled receptor.
      ,
      • Quistgaard E.M.
      • Groftehauge M.K.
      • Madsen P.
      • Pallesen L.T.
      • Christensen B.
      • Sorensen E.S.
      • et al.
      Revisiting the structure of the Vps10 domain of human sortilin and its interaction with neurotensin.
      )
      Neurotensin receptor 1 (NTR1)(
      • Ouyang Q.
      • Zhou J.
      • Yang W.
      • Cui H.
      • Xu M.
      • Yi L.
      Oncogenic role of neurotensin and neurotensin receptors in various cancers.
      ,
      • Martin S.
      • Navarro V.
      • Vincent J.P.
      • Mazella J.
      Neurotensin receptor-1 and-3 complex modulates the cellular signaling of neurotensin in the HT29 cell line.
      )
      Neurotensin receptor 2 (NTR2)(
      • Beraud-Dufour S.
      • Coppola T.
      • Massa F.
      • Mazella J.
      Neurotensin receptor-2 and-3 are crucial for the anti-apoptotic effect of neurotensin on pancreatic beta-TC3 cells.
      )
      Amyloid precursor protein (APP) relatedAmyloid-precursor-like protein 2 (APLP2)(
      • Butkinaree C.
      • Canuel M.
      • Essalmani R.
      • Poirier S.
      • Benjannet S.
      • Asselin M.C.
      • et al.
      Amyloid precursor-like protein 2 and sortilin do not regulate the PCSK9 convertase-mediated low density lipoprotein receptor degradation but interact with each other.
      )
      Amyloid precursor protein (APP)(
      • Gustafsen C.
      • Glerup S.
      • Pallesen L.T.
      • Olsen D.
      • Andersen O.M.
      • Nykjaer A.
      • et al.
      Sortilin and SorLA display distinct roles in processing and trafficking of amyloid precursor protein.
      ,
      • Yang M.
      • Virassamy B.
      • Vijayaraj S.L.
      • Lim Y.
      • Saadipour K.
      • Wang Y.J.
      • et al.
      The intracellular domain of sortilin interacts with amyloid precursor protein and regulates its lysosomal and lipid raft trafficking.
      )
      β-site amyloid precursor protein cleaving enzyme 1 (BACE1)(
      • Finan G.M.
      • Okada H.
      • Kim T.W.
      BACE1 retrograde trafficking is uniquely regulated by the cytoplasmic domain of sortilin.
      )
      Cytokine relatedCardiolipin-like cytokine/cytokine-like factor-1 (CLC/CLF-1)(
      • Larsen J.V.
      • Hansen M.
      • Moller B.
      • Madsen P.
      • Scheller J.
      • Nielsen M.
      • et al.
      Sortilin facilitates signaling of ciliary neurotrophic factor and related helical type 1 cytokines targeting the gp130/leukemia inhibitory factor receptor beta heterodimer.
      )
      Ciliary neurotrophic factor (CNTF)(
      • Larsen J.V.
      • Hansen M.
      • Moller B.
      • Madsen P.
      • Scheller J.
      • Nielsen M.
      • et al.
      Sortilin facilitates signaling of ciliary neurotrophic factor and related helical type 1 cytokines targeting the gp130/leukemia inhibitory factor receptor beta heterodimer.
      )
      Glycoprotein 130/leukemia inhibitory factor receptor β (gp130/LIFRβ)(
      • Larsen J.V.
      • Hansen M.
      • Moller B.
      • Madsen P.
      • Scheller J.
      • Nielsen M.
      • et al.
      Sortilin facilitates signaling of ciliary neurotrophic factor and related helical type 1 cytokines targeting the gp130/leukemia inhibitory factor receptor beta heterodimer.
      )
      Interferon-α (IFN-α)(
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      )
      Interferon-γ (IFN-γ)(
      • Mortensen M.B.
      • Kjolby M.
      • Gunnersen S.
      • Larsen J.V.
      • Palmfeldt J.
      • Falk E.
      • et al.
      Targeting sortilin in immune cells reduces proinflammatory cytokines and atherosclerosis.
      ,
      • Herda S.
      • Raczkowski F.
      • Mittrücker H.-W.
      • Willimsky G.
      • Gerlach K.
      • Kühl A.A.
      • et al.
      The sorting receptor sortilin exhibits a dual function in exocytic trafficking of interferon-γ and granzyme A in T cells.
      ,
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      )
      Interleukin-6 (IL-6)(
      • Mortensen M.B.
      • Kjolby M.
      • Gunnersen S.
      • Larsen J.V.
      • Palmfeldt J.
      • Falk E.
      • et al.
      Targeting sortilin in immune cells reduces proinflammatory cytokines and atherosclerosis.
      ,
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      )
      Interleukin-10 (IL-10)(
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      )
      Interleukin-12 (IL-12)(
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      )
      Interleukin-17A (IL-17A)(
      • Yabe-Wada T.
      • Matsuba S.
      • Takeda K.
      • Sato T.
      • Suyama M.
      • Ohkawa Y.
      • et al.
      TLR signals posttranscriptionally regulate the cytokine trafficking mediator sortilin.
      )
      Neuropoietin(
      • Larsen J.V.
      • Hansen M.
      • Moller B.
      • Madsen P.
      • Scheller J.
      • Nielsen M.
      • et al.
      Sortilin facilitates signaling of ciliary neurotrophic factor and related helical type 1 cytokines targeting the gp130/leukemia inhibitory factor receptor beta heterodimer.
      )
      Lysosomal proteinsAcid sphingomyelinase (ASM)(
      • Ni X.
      • Morales C.R.
      The lysosomal trafficking of acid sphingomyelinase is mediated by sortilin and mannose 6-phosphate receptor.
      ,
      • Jin S.
      • Yi F.
      • Zhang F.
      • Poklis J.L.
      • Li P.L.
      Lysosomal targeting and trafficking of acid sphingomyelinase to lipid raft platforms in coronary endothelial cells.
      )
      Cathepsin D(
      • Canuel M.
      • Korkidakis A.
      • Konnyu K.
      • Morales C.R.
      Sortilin mediates the lysosomal targeting of cathepsins D and H.
      )
      Cathepsin H(
      • Canuel M.
      • Korkidakis A.
      • Konnyu K.
      • Morales C.R.
      Sortilin mediates the lysosomal targeting of cathepsins D and H.
      )
      Prosaposin (PSAP)(
      • Lefrancois S.
      • Zeng J.
      • Hassan A.J.
      • Canuel M.
      • Morales C.R.
      The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin.
      ,
      • Hassan A.J.
      • Zeng J.
      • Ni X.
      • Morales C.R.
      The trafficking of prosaposin (SGP-1) and GM2AP to the lysosomes of TM4 Sertoli cells is mediated by sortilin and monomeric adaptor proteins.
      ,
      • Zeng J.
      • Racicott J.
      • Morales C.R.
      The inactivation of the sortilin gene leads to a partial disruption of prosaposin trafficking to the lysosomes.
      ,
      • Yuan L.
      • Morales C.R.
      A stretch of 17 amino acids in the prosaposin C terminus is critical for its binding to sortilin and targeting to lysosomes.
      ,
      • Wahe A.
      • Kasmapour B.
      • Schmaderer C.
      • Liebl D.
      • Sandhoff K.
      • Nykjaer A.
      • et al.
      Golgi-to-phagosome transport of acid sphingomyelinase and prosaposin is mediated by sortilin.
      )
      OtherActivin(
      • Kwon S.
      • Christian J.L.
      Sortilin associates with transforming growth factor-beta family proteins to enhance lysosome-mediated degradation.
      )
      Adiponectin(
      • Karki S.
      • Chakrabarti P.
      • Huang G.
      • Wang H.
      • Farmer S.R.
      • Kandror K.V.
      The multi-level action of fatty acids on adiponectin production by fat cells.
      )
      α-galactosidase A (α-Gal A)(
      • Prabakaran T.
      • Nielsen R.
      • Satchell S.C.
      • Mathieson P.W.
      • Feldt-Rasmussen U.
      • Sorensen S.S.
      • et al.
      Mannose 6-phosphate receptor and sortilin mediated endocytosis of alpha-galactosidase A in kidney endothelial cells.
      )
      α-synuclein(
      • Lv Y.C.
      • Gao A.B.
      • Yang J.
      • Zhong L.Y.
      • Jia B.
      • Ouyang S.H.
      • et al.
      Long-term adenosine A1 receptor activation-induced sortilin expression promotes alpha-synuclein upregulation in dopaminergic neurons.
      )
      Bone morphogenic protein 4 (BMP4)(
      • Kwon S.
      • Christian J.L.
      Sortilin associates with transforming growth factor-beta family proteins to enhance lysosome-mediated degradation.
      )
      Epidermal growth factor receptor (EGFR)(
      • Li Q.
      • Ma W.
      • Li T.
      Sortilin as a new membrane inhibitor of EGFR trafficking for overcoming resistance to EGFR inhibitors in non-small cell lung cancer.
      ,
      • Al-Akhrass H.
      • Naves T.
      • Vincent F.
      • Magnaudeix A.
      • Durand K.
      • Bertin F.
      • et al.
      Sortilin limits EGFR signaling by promoting its internalization in lung cancer.
      )
      Gelsolin(
      • Chen C.C.
      • Chiou S.H.
      • Yang C.L.
      • Chow K.C.
      • Lin T.Y.
      • Chang H.W.
      • et al.
      Secreted gelsolin desensitizes and induces apoptosis of infiltrated lymphocytes in prostate cancer.
      )
      Glucose transporter 4 (GLUT4) storage vesicles(
      • Shi J.
      • Kandror K.V.
      Sortilin is essential and sufficient for the formation of Glut4 storage vesicles in 3T3-L1 adipocytes.
      ,
      • Morris N.J.
      • Ross S.A.
      • Lane W.S.
      • Moestrup S.K.
      • Petersen C.M.
      • Keller S.R.
      • et al.
      Sortilin is the major 110-kDa protein in GLUT4 vesicles from adipocytes.
      ,
      • Lin B.Z.
      • Pilch P.F.
      • Kandror K.V.
      Sortilin is a major protein component of Glut4-containing vesicles.
      ,
      • Shi J.
      • Kandror K.V.
      The luminal Vps10p domain of sortilin plays the predominant role in targeting to insulin-responsive Glut4-containing vesicles.
      ,
      • Huang G.
      • Buckler-Pena D.
      • Nauta T.
      • Singh M.
      • Asmar A.
      • Shi J.
      • et al.
      Insulin responsiveness of glucose transporter 4 in 3T3-L1 cells depends on the presence of sortilin.
      ,
      • Pan X.
      • Zaarur N.
      • Singh M.
      • Morin P.
      • Kandror K.V.
      Sortilin and retromer mediate retrograde transport of Glut4 in 3T3-L1 adipocytes.
      ,
      • Hashiramoto M.
      • James D.E.
      Characterization of insulin-responsive GLUT4 storage vesicles isolated from 3T3-L1 adipocytes.
      ,
      • Ariga M.
      • Nedachi T.
      • Katagiri H.
      • Kanzaki M.
      Functional role of sortilin in myogenesis and development of insulin-responsive glucose transport system in C2C12 myocytes.
      )
      GM2 ganglioside activator pseudogene (GM2AP)(
      • Lefrancois S.
      • Zeng J.
      • Hassan A.J.
      • Canuel M.
      • Morales C.R.
      The lysosomal trafficking of sphingolipid activator proteins (SAPs) is mediated by sortilin.
      ,
      • Hassan A.J.
      • Zeng J.
      • Ni X.
      • Morales C.R.
      The trafficking of prosaposin (SGP-1) and GM2AP to the lysosomes of TM4 Sertoli cells is mediated by sortilin and monomeric adaptor proteins.
      )
      Golgi phosphoprotein 4 (GPP130)(
      • Venkat S.
      • Linstedt A.D.
      Manganese-induced trafficking and turnover of GPP130 is mediated by sortilin.
      )
      Na+/Cl cotransporter (NCC)(
      • Zhou B.
      • Zhuang J.
      • Gu D.
      • Wang H.
      • Cebotaru L.
      • Guggino W.B.
      • et al.
      WNK4 enhances the degradation of NCC through a sortilin-mediated lysosomal pathway.
      )
      Phosphatidylinositol (3,4,5)-triphosphate (PIP3)(
      • Sparks R.P.
      • Jenkins J.L.
      • Miner G.E.
      • Wang Y.
      • Guida W.C.
      • Sparks C.E.
      • et al.
      Phosphatidylinositol (3,4,5)-trisphosphate binds to sortilin and competes with neurotensin: implications for very low density lipoprotein binding.
      )
      Prion (PrPC and PrPSc)(
      • Uchiyama K.
      • Tomita M.
      • Yano M.
      • Chida J.
      • Hara H.
      • Das N.R.
      • et al.
      Prions amplify through degradation of the VPS10P sorting receptor sortilin.
      )
      Progranulin (PGRN)(
      • Prudencio M.
      • Jansen-West K.R.
      • Lee W.C.
      • Gendron T.F.
      • Zhang Y.-J.
      • Xu Y.-F.
      • et al.
      Misregulation of human sortilin splicing leads to the generation of a nonfunctional progranulin receptor.
      ,
      • Hu F.
      • Padukkavidana T.
      • Vægter C.B.
      • Brady O.A.
      • Zheng Y.
      • Mackenzie I.R.
      • et al.
      Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin.
      ,
      • Zheng Y.
      • Brady O.A.
      • Meng P.S.
      • Mao Y.
      • Hu F.
      C-terminus of progranulin interacts with the beta-propeller region of sortilin to regulate progranulin trafficking.
      )
      Prorenin receptor (PRR)(
      • Lu X.
      • Meima M.E.
      • Nelson J.K.
      • Sorrentino V.
      • Loregger A.
      • Scheij S.
      • et al.
      Identification of the (Pro)renin receptor as a novel regulator of low-density lipoprotein metabolism.
      )
      Receptor-associated protein (RAP)(
      • Petersen C.M.
      • Nielsen M.S.
      • Nykjaer A.
      • Jacobsen L.
      • Tommerup N.
      • Rasmussen H.H.
      • et al.
      Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.
      )
      Sonic hedgehog (SHH)(
      • Campbell C.
      • Beug S.
      • Nickerson P.E.
      • Peng J.
      • Mazerolle C.
      • Bassett E.A.
      • et al.
      Sortilin regulates sorting and secretion of Sonic hedgehog.
      )
      Thyroglobulin(
      • Botta R.
      • Lisi S.
      • Pinchera A.
      • Giorgi F.
      • Marcocci C.
      • Taddei A.R.
      • et al.
      Sortilin is a putative postendocytic receptor of thyroglobulin.
      ,
      • Botta R.
      • Lisi S.
      • Rotondo Dottore G.
      • Vitti P.
      • Marino M.
      Binding of thyroglobulin (Tg) to the low-density lipoprotein receptor-associated protein (RAP) during the biosynthetic pathway prevents premature Tg interactions with sortilin.
      )
      TWIK-related potassium channel 1 (TREK-1)(
      • Mazella J.
      • Petrault O.
      • Lucas G.
      • Deval E.
      • Beraud-Dufour S.
      • Gandin C.
      • et al.
      Spadin, a sortilin-derived peptide, targeting rodent TREK-1 channels: a new concept in the antidepressant drug design.
      ,
      • Moreno S.
      • Devader C.M.
      • Pietri M.
      • Borsotto M.
      • Heurteaux C.
      • Mazella J.
      Altered Trek-1 Function in Sortilin Deficient Mice Results in Decreased Depressive-Like Behavior.
      )
      A comprehensive list of known ligands or receptor binding partners of sortilin.

      Regulation of ligand binding and trafficking

      Sortilin is synthesized as a proprotein and converted to its mature form in the late Golgi by furin cleavage of its propeptide. The propeptide binds inside the tunnel of the β-propeller with high affinity and inhibits binding of some of sortilin’s ligands in the early secretory pathway, including neurotensin (
      • Quistgaard E.M.
      • Madsen P.
      • Groftehauge M.K.
      • Nissen P.
      • Petersen C.M.
      • Thirup S.S.
      Ligands bind to Sortilin in the tunnel of a ten-bladed beta-propeller domain.
      ,
      • Serup Andersen O.
      • Boisguerin P.
      • Glerup S.
      • Skeldal S.
      • Volkmer R.
      • Willnow T.E.
      • et al.
      Identification of a linear epitope in sortilin that partakes in pro-neurotrophin binding.
      ,
      • Munck Petersen C.
      • Nielsen M.S.
      • Jacobsen C.
      • Tauris J.
      • Jacobsen L.
      • Gliemann J.
      • et al.
      Propeptide cleavage conditions sortilin/neurotensin receptor-3 for ligand binding.
      ) and RAP (
      • Munck Petersen C.
      • Nielsen M.S.
      • Jacobsen C.
      • Tauris J.
      • Jacobsen L.
      • Gliemann J.
      • et al.
      Propeptide cleavage conditions sortilin/neurotensin receptor-3 for ligand binding.
      ). Interestingly, binding of the propeptide to sortilin does not block the binding of all ligands, including proNGF and proBDNF (
      • Serup Andersen O.
      • Boisguerin P.
      • Glerup S.
      • Skeldal S.
      • Volkmer R.
      • Willnow T.E.
      • et al.
      Identification of a linear epitope in sortilin that partakes in pro-neurotrophin binding.
      ), supporting the view that sortilin has multiple binding sites for ligands. Recently, a small molecule that specifically binds to “binding site 2” (the site where neurotensin does not bind) was shown to augment binding of neurotensin to sortilin binding site 1, suggesting that site 2 is an allosteric regulator of site 1 binding (
      • Sparks R.P.
      • Arango A.S.
      • Jenkins J.L.
      • Guida W.C.
      • Tajkhorshid E.
      • Sparks C.E.
      • et al.
      An Allosteric Binding Site on Sortilin Regulates the Trafficking of VLDL, PCSK9, and LDLR in Hepatocytes.
      ).
      Since sortilin can bind to multiple ligands and traffic them through several possible pathways in the same cell, various questions emerge: Upon binding of a particular ligand, what determines which of the multiple trafficking pathways are pursued? What is the link between specific ligand binding and recruitment of the appropriate APs to transport a ligand to its correct destination? The answers to these questions are largely unknown. However, recent work by Trabjerg et al. (
      • Trabjerg E.
      • Abu-Asad N.
      • Wan Z.
      • Kartberg F.
      • Christensen S.
      • Rand K.D.
      Investigating the conformational response of the sortilin receptor upon binding endogenous peptide- and protein ligands by HDX-MS.
      ) using hydrogen/deuterium exchange mass spectrometry found that different ligands exhibit distinct conformational impacts on sortilin. These specific ligand binding-induced conformational changes extend into the membrane-proximal domain of sortilin, and potentially across the membrane, possibly affecting AP binding. This hints at a mechanism by which sortilin mediates diverse ligand-dependent trafficking. Another possibility is that ligands destined for different pathways localize to different regions of the Golgi, prior to their interaction with sorting receptors. Recently, the Bonifacino group provided direct evidence for this additional level of protein sorting in the Golgi, where there is early segregation of different sets of proteins that are destined for different pathways, well before their export in transport carriers (
      • Chen Y.
      • Gershlick D.C.
      • Park S.Y.
      • Bonifacino J.S.
      Segregation in the Golgi complex precedes export of endolysosomal proteins in distinct transport carriers.
      ). This creates regions of the Golgi that generate carriers destined for the constitutive secretory pathway that are distinct from regions that generate carriers destined for the endolysosomal system, for example. Therefore, it is possible that sortilin localized to the section of the Golgi that buds transport carriers destined for the endolysosomal system only has access to proteins that have been presorted for this pathway. Generation of these carriers and their targeting to the endolysosomal pathway would require AP binding to receptors. Similarly, sortilin localized to the section of the Golgi that generates transport carriers destined for the constitutive secretory pathway would only have access to proteins that have been presorted for this pathway. However, these carriers would be generated and targeted independently of APs.
      Interestingly, in some cases, sortilin has been shown to traffic the same ligand to different pathways depending on the cellular context. For example, under normal conditions, sortilin targets proBDNF to the regulated secretory pathway in neurons. However, under conditions where the cell has excess proBDNF, sortilin targets this excess to the endolysosomal system for degradation (
      • Evans S.F.
      • Irmady K.
      • Ostrow K.
      • Kim T.
      • Nykjaer A.
      • Saftig P.
      • et al.
      Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation.
      ,
      • Chen Z.Y.
      • Ieraci A.
      • Teng H.
      • Dall H.
      • Meng C.X.
      • Herrera D.G.
      • et al.
      Sortilin controls intracellular sorting of brain-derived neurotrophic factor to the regulated secretory pathway.
      ). In hepatocytes, sortilin may traffic apoB-100 toward the secretory pathway for secretion or toward the lysosome for degradation, depending on the metabolic context (
      • Chamberlain J.M.
      • O'Dell C.
      • Sparks C.E.
      • Sparks J.D.
      Insulin suppression of apolipoprotein B in McArdle RH7777 cells involves increased sortilin 1 interaction and lysosomal targeting.
      ,
      • Sparks R.P.
      • Guida W.C.
      • Sowden M.P.
      • Jenkins J.L.
      • Starr M.L.
      • Fratti R.A.
      • et al.
      Sortilin facilitates VLDL-B100 secretion by insulin sensitive McArdle RH7777 cells.
      ,
      • Conlon D.M.
      • Schneider C.V.
      • Ko Y.A.
      • Rodrigues A.
      • Guo K.
      • Hand N.J.
      • et al.
      Sortilin restricts secretion of apolipoprotein B-100 by hepatocytes under stressed but not basal conditions.
      ). The mechanism underlying these switches is unknown.
      Ligand binding is also regulated by dimerization of sortilin at low pH. During sortilin’s transport between the TGN, cell surface, endosomes, and other vesicular compartments, it is exposed to dramatic fluctuations in pH. Ligands tend to show high affinity for sortilin at neutral pH but have a reduced or a complete loss of affinity at acidic pH (
      • Petersen C.M.
      • Nielsen M.S.
      • Nykjaer A.
      • Jacobsen L.
      • Tommerup N.
      • Rasmussen H.H.
      • et al.
      Molecular identification of a novel candidate sorting receptor purified from human brain by receptor-associated protein affinity chromatography.
      ,
      • Gustafsen C.
      • Kjolby M.
      • Nyegaard M.
      • Mattheisen M.
      • Lundhede J.
      • Buttenschon H.
      • et al.
      The hypercholesterolemia-risk gene SORT1 facilitates PCSK9 secretion.
      ,
      • Gustafsen C.
      • Glerup S.
      • Pallesen L.T.
      • Olsen D.
      • Andersen O.M.
      • Nykjaer A.
      • et al.
      Sortilin and SorLA display distinct roles in processing and trafficking of amyloid precursor protein.
      ,
      • Munck Petersen C.
      • Nielsen M.S.
      • Jacobsen C.
      • Tauris J.
      • Jacobsen L.
      • Gliemann J.
      • et al.
      Propeptide cleavage conditions sortilin/neurotensin receptor-3 for ligand binding.
      ,
      • Conticello S.G.
      • Kowalsman N.D.
      • Jacobsen C.
      • Yudkovsky G.
      • Sato K.
      • Elazar Z.
      • et al.
      The prodomain of a secreted hydrophobic mini-protein facilitates its export from the endoplasmic reticulum by hitchhiking on sorting receptors.
      ), consistent with release of ligands in secretory granules or late endosomes. Recent reports by several groups have revealed that low pH triggers sortilin to undergo a conformational change and dimerize, causing the collapse of the binding site in the tunnel of the β-propeller and release of the ligand (
      • Itoh S.
      • Mizuno K.
      • Aikawa M.
      • Aikawa E.
      Dimerization of sortilin regulates its trafficking to extracellular vesicles.
      ,
      • Yabe-Wada T.
      • Matsuba S.
      • Unno M.
      • Onai N.
      Crystal structure of the ligand-free form of the Vps10 ectodomain of dimerized Sortilin at acidic pH.
      ,
      • Leloup N.
      • Lossl P.
      • Meijer D.H.
      • Brennich M.
      • Heck A.J.R.
      • Thies-Weesie D.M.E.
      • et al.
      Low pH-induced conformational change and dimerization of sortilin triggers endocytosed ligand release.
      ,
      • Januliene D.
      • Andersen J.L.
      • Nielsen J.A.
      • Quistgaard E.M.
      • Hansen M.
      • Strandbygaard D.
      • et al.
      Acidic environment induces dimerization and ligand binding site collapse in the Vps10p domain of sortilin.
      ) (Fig. 3). Sortilin is predominantly a monomer at neutral pH and predominantly a dimer at acidic pH. It dimerizes through the top face of its β-propeller, opposite the 10CC module. Hydrophobic loops that protrude from the blades of the β-propeller at the dimer interface are important for dimer formation. In addition, disruption and formation of Coulombic repulsions between charged residues (
      • Leloup N.
      • Lossl P.
      • Meijer D.H.
      • Brennich M.
      • Heck A.J.R.
      • Thies-Weesie D.M.E.
      • et al.
      Low pH-induced conformational change and dimerization of sortilin triggers endocytosed ligand release.
      ), salt bridges (
      • Leloup N.
      • Lossl P.
      • Meijer D.H.
      • Brennich M.
      • Heck A.J.R.
      • Thies-Weesie D.M.E.
      • et al.
      Low pH-induced conformational change and dimerization of sortilin triggers endocytosed ligand release.
      ), and disulfide bonds (
      • Itoh S.
      • Mizuno K.
      • Aikawa M.
      • Aikawa E.
      Dimerization of sortilin regulates its trafficking to extracellular vesicles.
      ) are important for the conformational changes and monomer-dimer shift that occurs upon pH change. Only structures of the sortilin luminal domain were determined, but the structure of the soluble sortilin dimer reveals that the C termini of the luminal domains are in close proximity to each other, indicating that the 2-fold axis that describes the dimer is oriented perpendicular to the cell surface (
      • Leloup N.
      • Lossl P.
      • Meijer D.H.
      • Brennich M.
      • Heck A.J.R.
      • Thies-Weesie D.M.E.
      • et al.
      Low pH-induced conformational change and dimerization of sortilin triggers endocytosed ligand release.
      ). The ligand binding site located in the tunnel of the β-propeller undergoes a conformational change in the monomer-dimer transition that triggers release of ligand from sortilin (
      • Leloup N.
      • Lossl P.
      • Meijer D.H.
      • Brennich M.
      • Heck A.J.R.
      • Thies-Weesie D.M.E.
      • et al.
      Low pH-induced conformational change and dimerization of sortilin triggers endocytosed ligand release.
      ). Januliene et al. (
      • Januliene D.
      • Andersen J.L.
      • Nielsen J.A.
      • Quistgaard E.M.
      • Hansen M.
      • Strandbygaard D.
      • et al.
      Acidic environment induces dimerization and ligand binding site collapse in the Vps10p domain of sortilin.