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1 Present address of J. Xu: Division of Experimental Medicine, Harvard Institute of Medicine and Beth Israel Deaconess Medical Center, Boston, MA 02115.
Phospholipase A2 (PLA2) belongs to a family of enzymes that catalyze the cleavage of fatty acids from the sn-2 position of phospholipids. There are more than 19 different isoforms of PLA2 in the mammalian system, but recent studies have focused on three major groups, namely, the group IV cytosolic PLA2, the group II secretory PLA2 (sPLA2), and the group VI Ca2+-independent PLA2. These PLA2s are involved in a complex network of signaling pathways that link receptor agonists, oxidative agents, and proinflammatory cytokines to the release of arachidonic acid (AA) and the synthesis of eicosanoids. PLA2s acting on membrane phospholipids have been implicated in intracellular membrane trafficking, differentiation, proliferation, and apoptotic processes. All major groups of PLA2 are present in the central nervous system (CNS). Therefore, this review is focused on PLA2 and AA release in neural cells, especially in astrocytes and neurons. In addition, because many neurodegenerative diseases are associated with increased oxidative and inflammatory responses, an attempt was made to include studies on PLA2 in cerebral ischemia, Alzheimer's disease, and neuronal injury due to excitotoxic agents.
Information from these studies has provided clear evidence for the important role of PLA2 in regulating physiological and pathological functions in the CNS.
) recognized the important role of arachidonic acid (AA) in the central nervous system (CNS) in the '70s when he observed the rapid and transient release of this fatty acid in the brain due to seizure and cerebral ischemia. The “Bazan effect” has since stimulated over 30 years of investigations attempting to unravel mechanisms regulating AA release from membrane phospholipids in the CNS.
Phospholipids in CNS membranes are enriched in polyunsaturated fatty acids (PUFAs) (
). Under normal conditions, free fatty acids (FFAs) released by PLA2 are rapidly taken up by membrane phospholipids through an energy-dependent process involving CoA and ATP (
). To date, limited information is available on the structure and functions of acyltransferases. However, recent advances in molecular biological techniques have aided in the identification of many genes encoding different groups of PLA2 and have provided new information on the properties and functions of these molecules.
PLA2 (EC3.1.1.4.) belongs to a family of enzymes that catalyze the cleavage of fatty acids from the sn-2 position of phospholipids. These enzymes are not only important for maintenance of cell membrane phospholipids; they also play a key role in regulating the release of AA, a precursor for synthesis of eicosanoids. In the mammalian system, more than 19 different isoforms of PLA2 have been identified, and different PLA2s have been shown to participate in physiological events related to cell injury, inflammation, and apoptosis (
). Recent studies have focused on three major groups of PLA2: the group IV calcium-dependent cytosolic PLA2 (cPLA2), the group II secretory PLA2 (sPLA2), and the group VI Ca2+-independent PLA2 (iPLA2) (
). The present review is devoted to PLA2 in neural cells in the CNS, especially the signaling pathways regulating different PLA2s in neurons and astrocytes (see Fig. 1). Because PLA2s have been implicated in the pathology of a number of neurodegenerative diseases, an attempt was also made to include recent studies describing the different groups of PLA2s in cerebral ischemia, Alzheimer's disease (AD), and neuronal injury due to excitotoxic agents. To facilitate updated information, a website linking data concerning PLA2s in different neural cells and their involvement in neurodegenerative diseases has been created: http://www.pla2.com. The authors plan to update the information in this site periodically.
Fig. 1A scheme representing different PLA2 pathways in astrocytes and neurons.
cPLA2 belongs to the group IV PLA2s. Although three isoforms, i.e., cPLA2α, -β, and -γ, have been identified, the 85 kDa cPLA2α has been studied most extensively. This protein is comprised of a C2 domain and multiple phosphorylation sites, including two consensus sites (S505 and S727) for phosphorylation by mitogen-activated protein kinases (MAPKs) (
). Translocation of cPLA2 has also been shown to participate in intracellular membrane trafficking processes, such as those governing the Golgi and endocytic pathways (
). In macrophages, as well as in other cell systems, agents including G protein-coupled receptor agonists, calcium ionophores, phorbol esters, and zymogens can activate cPLA2, resulting in AA release (
). Through its linkage to receptor-mediated signaling pathways, cPLA2 is an important PLA2 for rapid AA release in cells and for modulating a number of intracellular processes.
sPLA2
The sPLA2 family consists of multiple groups (I, II, III, V, X, and XII) of enzymes characterized by a conserved Ca2+ binding loop and a conserved histidine residue in the catalytic domain (
). The group II sPLA2s, including IIA, IIC, IID, IIE, and IIF isoforms, are low-molecular-weight proteins (∼14 kDa) with secretory sequences. Genes for many of the group II sPLA2 isoforms are clustered in chromosome 1 (
). Of the group II sPLA2s, the IIA enzyme has been studied extensively because of its involvement in inflammatory processes in the peripheral systems (
). In the CNS, group IIA sPLA2 mRNA is expressed in cultured astrocytes and can be induced in response to proinflammatory cytokines [tumor necrosis factor alpha (TNFα), interleukin-1β (IL-1β), and interferon gamma (IFNγ)] (
Stimulation of group II phospholipase A2 mRNA expression and release in an immortalized astrocyte cell line (DITNC) by LPS, TNFα, and IL-1β. Interactive effects.
The iPLA2 family is comprised of group VIA and VIB. Group VIA enzyme has at least five splice variants, all with ankyrin repeats, whereas group VIB iPLA2 lacks ankyrin repeats but consists of a signal motif for peroxisome localization (
). Fractionation of bovine brain cytosol by column chromatography resulted in two fractions, a 110 kDa iPLA2 fraction, which prefers hydrolysis of diacyl-glycero-3-phosphoethanolamine, and a 39 kDa iPLA2 fraction, which selectively acts on 1-alkenyl-2-acyl-glycero-3-phosphoethanolamine (ethanolamine plasmalogen, PEpl) (
). Although iPLA2s are generally regarded as housekeeping enzymes for the maintenance of membrane phospholipids, recent studies have revealed novel functional roles for this group of enzymes, i.e., regulation of vascular smooth muscle contraction (
Distinct roles of two intracellular phospholipase A2s in fatty acid release in the cell death pathway: proteolytic fragment of type IVA cytosolic phospholipase A2α inhibits stimulus-induced arachidonate release, whereas that of group VI Ca2+-independent phospholipase A2 augments spontaneous fatty acid release.
) indicated that >70% of PLA2 activity in normal rat brain could be attributed to iPLA2.
PLA2 IN ASTROCYTES
Astrocytes are the major cell type in the CNS and play multiple functional roles in providing nutrient support to neurons, modulating Ca2+ homeostasis, and regulating neurotransmission, as well as mediating host defense functions. Astrocytes have been shown to contain all major groups of PLA2 (
). Therefore, these cells have been used to study the roles of different groups of PLA2 in normal physiological and pathological functions.
Response to receptor agonists
Although many G protein-coupled receptors are expressed in astrocytes, there is considerable interest in the P2Y nucleotide receptors in these cells. One reason for this interest is that in the brain, ATP is stored at high concentrations in synaptic vesicles and is coreleased with neurotransmitters during neuronal excitation (
). Therefore, P2Y receptors in astrocytes may constitute an important mechanism for mediating communication between neurons and glial cells. Activation of P2Y receptors by extracellular nucleotides such as ATP/UTP has been shown to cause an increase in intracellular Ca2+ concentrations ([Ca2+]i) as well as activation of a number of signaling pathways (
). In astrocytes, activation of P2Y receptors is implicated in reactive gliosis, a pathological condition associated with a number of neurodegenerative diseases (
Mitogenic signaling by ATP/P2Y purinergic receptors in astrocytes: involvement of a calcium-independent protein kinase C, extracellular signal-regulated protein kinase pathway distinct from the phosphatidylinositol-specific phospholipase C/calcium pathway.
) further demonstrated the role of the extracellular signal-regulated protein kinase (ERK) and protein kinase C (PKC) pathways for enhancing cPLA2 phosphorylation and stimulating AA release in murine astrocytes. ATP, acting on the P2Y2 receptors in astrocytes, could also mediate the release of docosahexaenoic acid (DHA) (
Docosahexaenoic acid and arachidonic acid release in rat brain astrocytes is mediated by two separate isoforms of phospholipase A2 and is differently regulated by cyclic AMP and Ca2+.
Docosahexaenoic acid and arachidonic acid release in rat brain astrocytes is mediated by two separate isoforms of phospholipase A2 and is differently regulated by cyclic AMP and Ca2+.
Role of protein kinase C alpha and mitogen-activated protein kinases in endothelin-1-stimulation of cytosolic phospholipase A2 in iris sphincter smooth muscle.
Endothelin-1-induced arachidonic acid release by cytosolic phospholipase A2 activation in rat vascular smooth muscle via extracellular signal-regulated kinases pathway.
Role of protein kinase C alpha and mitogen-activated protein kinases in endothelin-1-stimulation of cytosolic phospholipase A2 in iris sphincter smooth muscle.
). Many G protein-coupled receptors are linked to phospholipase C and the release of inositol trisphosphates and diacylglycerols, which are second messengers for Ca2+ mobilization and activation of PKC, respectively. Increases in both [Ca2+]i and PKC are important factors in the translocation and phosphorylation of cPLA2. However, studies in NIH3T3 cells stably expressed with the serotonin 5HT2A receptor indicated that instead of the phospholipase C pathway, 5HT-stimulated PLA2 and AA release involved in both the Gi/o-associated G-mediated ERK1/2 and the G12/13-coupled, Rho-mediated p38 MAP kinase pathways (
). These results illustrate the complexity of different intracellular signaling pathways in the regulation of cPLA2.
Response to oxidative agents
Reactive oxygen species (ROS) are produced in biological systems through both enzymatic and nonenzymatic mechanisms. Excessive generation of ROS in the CNS has been implicated in neuronal damage resulting from cerebral ischemia and in AD. Oxidant compounds such as H2O2 have been shown to cause perturbation of cell membrane integrity and alteration of mitochondrial function, resulting in an increase in [Ca2+]i (
). In fact, H2O2 is a naturally occurring oxidant produced by a number of intracellular reactions, and excessive production of this compound is associated with signaling pathways (
Mediation by arachidonic acid metabolites of the H2O2-induced stimulation of mitogen-activated protein kinases (extracellular-signal-regulated kinase and c-Jun NH2-terminal kinase).
). On the other hand, a study in mesangial cells that were transfected with cPLA2 and/or sPLA2 demonstrated the involvement of cPLA2 and sPLA2 in H2O2-induced AA release (
Cross-talk between cytosolic phospholipase A2alpha (cPLA2alpha) and secretory phospholipase A2 (sPLA2) in hydrogen peroxide-induced arachidonic acid release in murine mesangial cells. SPLA2 regulates cPLA2alpha activity that is responsible for arachidonic acid release.
Astrocytes can readily respond to proinflammatory agents and lipopolysaccharides (LPS), causing the induction of a number of genes through activation of the nuclear factor κB pathway. In primary rat astrocytes, cytokines (TNFα, IL-1β, and IFNγ) stimulated the increase in prostaglandin E2 (PGE2) production, which was preceded by an increase in cyclooxygenase-2 (COX-2) and sPLA2 mRNA but not COX-1 and cPLA2 mRNA (
) further indicated the role of sPLA2 in cytokine-induced production of PGE2. Because C57Bl/6 mice lack the group IIA sPLA2 gene due to a frame shift mutation (
). On the other hand, a study with lung fibroblasts isolated from cPLA2-deficient mice also showed less PGE2 production as compared with fibroblasts from the wild-type mice, suggesting an important role for cPLA2 in the PGE2 pathway (
) showed that exposure of rat astrocytes to proinflammatory cytokines (TNFα, IL-1β, and IFNγ) for 16 h followed with cPLA2 agonists (ATP and PMA) for 30 min resulted in 70-fold higher production of PGE2 as compared with astrocytes stimulated with ATP and PMA without prior cytokine exposure. Results from the above studies suggest that in pathological conditions associated with an increase in inflammation, a sustained increase in proinflammatory cytokines in the brain may enhance the response of G protein-coupled receptors to produce higher levels of eicosanoids. In a recent study in our laboratory, exposure of murine astrocytes to 1L-1β for 12 h resulted in an increase in COX-2 and cPLA2 immunoreactivity (G. Y. Sun et al., unpublished observations). Confocal microscopic examination indicated that COX-2 and cPLA2 are localized in the perinuclear area (Fig. 2). In a recent report by Pardue, Rapoport, and Bosetti (
Fig. 2Confocal microscopy showing localization of cytosolic PLA2 (cPLA2) and cyclooxygenase-2 (COX-2) in the perinuclear area of primary murine astrocytes following treatment with interleukin-1β (IL-1β) (10 ng/ml) for 12 h. Astrocytes were plated on coverslips coated with poly-d-lysine and grown to 50% confluence. Cells were treated with IL-1β and were washed with Balch Buffer II (BBII) containing 25 mM HEPES, 75 mM potassium acetate, and 5 mM EGTA. Immediately after treatment, cells were fixed with 2% paraformaldehyde in phosphate-buffered saline (pH 7.2) at 25°C for 30 min. Cells were permeabilized and blocked by incubation for 30 min at 25°C with 0.1% (w/v) saponin and 10% (v/v) normal donkey serum diluted in BBII. Cells were incubated with primary antibodies (1:50 rabbit polyclonal anti-cPLA2 IgG or 1:100 goat polyclonal anti-rat COX-2 IgG) diluted in BBII. Incubation with secondary antibodies (1:200 Cy3-conjugated donkey anti-rabbit IgG or 1:200 Cy5-conjugated donkey anti-goat IgG) was carried out at 25°C for 4 h. After three washes with BBII, the coverslips were rinsed with distilled water and mounted on glass slides with Mowilol. The stained cells were viewed with a confocal microscope (Bio-Rad Lasershop 2000).
In addition to IL-1β, other cytokines, such as TNFα, can also activate COX-2 and stimulate signaling pathways leading to cPLA2 phosphorylation and AA release (
Signaling mechanisms involved in the activation of arachidonic acid metabolism in human astrocytoma cells by tumor necrosis factor alpha: phosphorylation of cytosolic phospholipase A2 and transactivation of cyclooxygenase-2.
). As shown in the study with human astrocytoma cells (1321N1), TNFα-stimulated phosphorylation of cPLA2 involved the c-Jun and p38 MAP kinase pathways but not the ERK pathway (
Signaling mechanisms involved in the activation of arachidonic acid metabolism in human astrocytoma cells by tumor necrosis factor alpha: phosphorylation of cytosolic phospholipase A2 and transactivation of cyclooxygenase-2.
). In addition to group IIA sPLA2, group V sPLA2 was also present in astrocytes, and TNFα stimulated both types of sPLA2, albeit through different time courses and different pathways (
). These studies demonstrate that different cytokines can activate different isoforms of sPLA2 in astrocytes.
PLA2 IN MICROGLIAL CELLS
In addition to astrocytes, little is known about PLA2 in other types of glial cells, such as the microglial cells and the oligodendroglial cells. This is due in part to difficulties in isolating sufficient quantities of these cells for biochemical analysis. Microglial cells are immune-active cells and exhibit many properties similar to those of macrophages and astrocytes (
). Therefore, there is substantial interest in the role of PLA2 in the inflammatory responses in these cells. In N9 microglial cells, PLA2 inhibitors could inhibit LPS-induced TNFα release, suggesting an involvement of PLA2 in the cytokine pathway (
). Although the murine-derived BV-2 microglial cells lack the group IIA sPLA2, they contain high levels of cPLA2 (G. Y. Sun, unpublished observations). In BV-2 cells, AA release stimulated by IFNγ and PMA was PKC and ERK dependent, suggesting the involvement of cPLA2 in mediating the AA release in these cells (G. Y. Sun, unpublished observations). In human microglial cells, LPS was capable of inducing COX-2 mRNA expression and PGE2 production (
), can stimulate AA release in neurons. Furthermore, oxidant compounds such as H2O2 could further enhance AA release stimulated by a number of neurotransmitter agonists (
). Although there is evidence for the involvement of Ca2+ and cPLA2 in the AA release from neurons, the signaling pathways leading to cPLA2 activation remain unclear.
Studies in vivo have demonstrated cPLA2 mRNA expression in hippocampal neurons (
In vivo activation of N-methyl-D-aspartate receptors in the rat hippocampus increases prostaglandin E2 extracellular levels and triggers lipid peroxidation through cyclooxygenase-mediated mechanisms.
Interleukin-1 beta-induced substance P release from rat cultured primary afferent neurons driven by two phospholipase A2 enzymes: secretory type IIA and cytosolic type IV.
) indicated that cultured rat neurons expressed both cPLA2 and sPLA2 and that both PLA2s were involved in the cytokine-induced release of substance P from these neurons. Using specific antibodies, the study by Matsuzawa et al. (
) detected group IIA sPLA2 in brain synaptosomes. Furthermore, release of group IIA sPLA2 from synaptosomes was observed upon depolarization with high concentrations of potassium (
Synergy by secretory phospholipase A2 and glutamate on inducing cell death and sustained arachidonic acid metabolic changes in primary cortical neuronal cultures.
) have provided evidence for the presence of a neuronal receptor for group IIA sPLA2. Low concentrations of sPLA2-OS2 (from Taipan snake venom) enhanced glutamate excitotoxicity, leading to neuron death (
Secreted phospholipase A2-induced neurotoxicity and epileptic seizures after intracerebral administration: an unexplained heterogeneity as emphasized with paradoxin and crotoxin.
). Some of these effects were attributed to the ability of sPLA2 to increase Ca2+ influx through stimulation of an L-type voltage-sensitive Ca2+ channel (
Most tumor cells contain elevated levels of PLA2, and increased production of eicosanoids has been implicated in cell growth. Using specific PLA2 inhibitors, van Rossum et al. (
) demonstrated the involvement of cPLA2 activity in cell cycle progression, especially from G1 to S phase in neuroblastoma (N2A) cells. In human neuroblastoma LA-N-1 cells, an isoform of iPLA2 was shown to specifically utilize phosphatidylethanolamine (PE) and PE-plasmalogen as substrates (
). Differentiation of these cells with retinoic acid was marked by an increase in iPLA2 activity in the nuclei, suggesting a role for this PLA2 in regulating nuclear membrane functions (
Interleukin-1beta induced cyclooxygenase 2 expression and prostaglandin E2 secretion by human neuroblastoma cells: implications for Alzheimer's disease.
). Therefore, studies with neuroblastoma cells have revealed novel functions of iPLA2.
PLA2 IN NEURODEGENERATIVE DISEASES
Information regarding the roles of different types of PLA2 in neurodegenerative diseases is sketchy, primarily because of the complex cellular network and the presence of different cell types in the brain. In this review, an attempt was made to cover studies on PLA2 in cerebral ischemia, AD, and neurodegeneration due to excitotoxic compounds. For a better coverage of PLA2 in other neurological and psychiatric disorders, including alcoholism, epilepsy, schizophrenia, and affective disorders, readers are encouraged to visit our web site (http://www.pla2.com).
Cerebral ischemia
Cessation of blood flow in cerebral ischemia (stroke) is known to trigger a number of physiological and biochemical changes, including rapid energy depletion, release of excitatory amino acid transmitters, neuronal membrane depolarization, and influx of Ca2+. Many of these changes are associated with an increase in oxidative stress, resulting in the production of ROS, which in turn, are important factors underlying delayed neuron cell death (
). In the rat focal cerebral ischemia model induced by the occlusion of the middle cerebral artery (MCA), a biphasic increase in FFAs was observed, one during the ischemic period and another at ∼16 h after reperfusion (
). In the early phase of ischemia, FFA accumulation was attributed to activation of Ca2+-dependent cPLA2 as well as to an inhibition of the energy-dependent reacylation process (
). The second phase of FFA increase was attributed to upregulation of the group IIA sPLA2 in reactive astrocytes in the penumbral area (G. Y. Sun, unpublished observations). The increase in sPLA2, together with that of other lipid mediators in reactive astrocytes, is in agreement with the increased inflammatory response observed during this period of cerebral ischemia (
). In another form of MCA occlusion induced by a photochemical mechanism, an increase in group IIA sPLA2 activity was associated with ischemia-induced neuronal apoptosis (
In the forebrain model of global cerebral ischemia, delayed neuron death was found in the hippocampal CA1 area 2 to 7 days after ischemia-reperfusion. A study by Lauritzen, Heurteaux, and Lazdunski (
) indicated a biphasic upregulation of group IIA sPLA2 mRNA in rat brain after transient global ischemia. In another study, analysis of fatty acids using different phospholipase inhibitors provided evidence that the FFA release resulting from global ischemia-reperfusion was mainly due to the activation of the Ca2+-dependent cPLA2 (
). In immature rats following hypoxic ischemia, neuron death in the CA1 hippocampal area was accompanied by an increase in COX-2 and cPLA2 immunoreactivity (
). Thus, neuronal damage due to ischemic injury may involve COX-2, cPLA2, and sPLA2, depending on the cell type, time course, and type of ischemic insult.
AD
Increased deposition of amyloid plaques infiltrated by reactive astrocytes and microglial cells is a major pathological landmark of AD. Aggregated forms of amyloid β (Aβ) peptides, particularly Aβ1-42, have been shown to elicit cytotoxic effects resulting in neuron cell death (
Beta-amyloid stimulation of inducible nitric-oxide synthase in astrocytes is interleukin-1beta- and tumor necrosis factor-alpha (TNF alpha)-dependent, and involves a TNF alpha receptor-associated factor and NFkappaB-inducing kinase-dependent signaling mechanism.
Plasmalogen deficiency in early Alzheimer's disease subjects and in animal models: molecular characterization using electrospray ionization mass spectrometry.
), a decrease in plasmenyl phospholipids was detected in the white matter of AD brain during the early stage of the disease, when only mild cognitive impairments were apparent. Plasmalogens are synthesized in peroxisomes. However, it is not clear whether the decrease in these phospholipids in the AD brain is associated with a peroxisomal disorder. Because PEpls in brain are highly enriched in DHA (
), a deficiency in these phospholipids may also lead to a decrease in DHA, which in turn, may have important implications in brain function, including learning ability (
There is accumulating evidence for the involvement of specific PLA2s in AD brain pathology. In two separate studies, a decrease in PLA2 activity was found in the parietal and temporal cortex (
). On the other hand, immunohistochemical studies showed an increase in cPLA2 immunoreactivity associated with the glial fibrillary acidic protein-positive astrocytes in the AD brain (
). In a recent gene array study, profiling of 12,633 genes in the hippocampal CA1 area of AD patients indicated an increase in cPLA2 and COX-2 expression, as well as upregulation of a number of apoptotic and proinflammatory genes (
Gene expression profiling of 12633 genes in Alzheimer hippocampal CA1: transcription and neurotrophic factor down-regulation and up-regulation of apoptotic and pro-inflammatory signaling.
). These findings are in agreement with the increased oxidative and inflammatory responses and presence of reactive astrocytes associated with AD pathology (
Beta-amyloid stimulation of inducible nitric-oxide synthase in astrocytes is interleukin-1beta- and tumor necrosis factor-alpha (TNF alpha)-dependent, and involves a TNF alpha receptor-associated factor and NFkappaB-inducing kinase-dependent signaling mechanism.
Phospholipases as mediators of amyloid beta peptide neurotoxicity: an early event contributing to neurodegeneration characteristic of Alzheimer's disease.
Discussion of the role of the extracellular signal-regulated kinase-phospholipase A2 pathway in production of reactive oxygen species in Alzheimer's disease.
Excitotoxic compounds such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-OH-dopamine have been shown to cause neurodegeneration resulting in Parkinson-like symptoms. Quinacrine, a nonselective PLA2 inhibitor, significantly reduced MPTP-induced dopamine loss (
). Mice deficient in cPLA2 were shown to exhibit more resistance to MPTP neurotoxicity than wild-type mice, further supporting a role of cPLA2 in mediating MPTP neurotoxicity (
Kainic acid (KA) is a subclass of glutamate receptor agonists, and systemic and/or local administration of this compound can result in seizures and neurodegeneration. Upregulation of cPLA2 expression was found in hippocampal neurons after injury induced by injection of KA into the brain (
). Electron microscopic examination indicated two phases of upregulation of cPLA2 in the hippocampus following KA injection. The first phase was attributed to an increase in cPLA2 in the neurons, and the second phase (after 1 week) was attributed to an adaptive response associated with gliosis (
). These studies demonstrate a role for cPLA2 in hippocampal neuronal injury resulting from excitotoxic compounds.
SUMMARY
A review of recent studies clearly demonstrates the important role of PLA2 in mediating normal and pathological functions in the CNS. However, studies of PLA2 in the CNS are complicated by the presence of different types of cells and the complex signaling pathways generated by different agonists (Fig. 1). Studies with astrocytes in culture reveal a link between cPLA2 and the G protein-coupled receptors and sPLA2 and the transcriptional pathways induced by proinflammatory cytokines. These studies have provided new information on mechanisms for regulating different groups of PLA2 in neural cells in the CNS.
These studies also reveal several areas requiring further studies: 1) Because iPLA2 comprises a large portion of PLA2 activity in the CNS, future studies should be directed toward a better understanding of the structure and function of different isoforms of iPLA2 in the brain. 2) Because AA release is associated with neuronal excitation, further studies are needed to clearly identify the types of PLA2 and the signaling pathways regulating AA release in neurons. 3) Studies with astrocytes may provide more information regarding the physiological roles of cPLA2 in intracellular trafficking as well as in other intracellular functions. 4) Because microglial cells have been implicated in the pathology of many neurodegenerative diseases, more studies are needed to demonstrate the role of PLA2 in the inflammatory response of these cells. With the advancement of molecular biological techniques, and as specific antibodies targeted to different PLA2s become more readily available, it can be projected that more studies will focus on defining the roles of different PLA2s in neurodegenerative diseases. It is anticipated that the new information will be important for the development of novel therapeutic strategies to combat damage resulting from upregulation of PLA2 in the affected neurodegenerative disorders.
Acknowledgments
Thanks are due to Dr. MaryKay Orgill for help in reading the manuscript. This research was supported in part by National Institutes of Health Grant DHHS P01 AG-18357.
REFERENCES
Bazan N.G.
Effects of ischemia and electroconvulsive shock on free fatty acid pool in the brain.
Stimulation of group II phospholipase A2 mRNA expression and release in an immortalized astrocyte cell line (DITNC) by LPS, TNFα, and IL-1β. Interactive effects.
Distinct roles of two intracellular phospholipase A2s in fatty acid release in the cell death pathway: proteolytic fragment of type IVA cytosolic phospholipase A2α inhibits stimulus-induced arachidonate release, whereas that of group VI Ca2+-independent phospholipase A2 augments spontaneous fatty acid release.
Mitogenic signaling by ATP/P2Y purinergic receptors in astrocytes: involvement of a calcium-independent protein kinase C, extracellular signal-regulated protein kinase pathway distinct from the phosphatidylinositol-specific phospholipase C/calcium pathway.
Docosahexaenoic acid and arachidonic acid release in rat brain astrocytes is mediated by two separate isoforms of phospholipase A2 and is differently regulated by cyclic AMP and Ca2+.
Role of protein kinase C alpha and mitogen-activated protein kinases in endothelin-1-stimulation of cytosolic phospholipase A2 in iris sphincter smooth muscle.
Endothelin-1-induced arachidonic acid release by cytosolic phospholipase A2 activation in rat vascular smooth muscle via extracellular signal-regulated kinases pathway.
Mediation by arachidonic acid metabolites of the H2O2-induced stimulation of mitogen-activated protein kinases (extracellular-signal-regulated kinase and c-Jun NH2-terminal kinase).
Cross-talk between cytosolic phospholipase A2alpha (cPLA2alpha) and secretory phospholipase A2 (sPLA2) in hydrogen peroxide-induced arachidonic acid release in murine mesangial cells. SPLA2 regulates cPLA2alpha activity that is responsible for arachidonic acid release.
Signaling mechanisms involved in the activation of arachidonic acid metabolism in human astrocytoma cells by tumor necrosis factor alpha: phosphorylation of cytosolic phospholipase A2 and transactivation of cyclooxygenase-2.
In vivo activation of N-methyl-D-aspartate receptors in the rat hippocampus increases prostaglandin E2 extracellular levels and triggers lipid peroxidation through cyclooxygenase-mediated mechanisms.
Interleukin-1 beta-induced substance P release from rat cultured primary afferent neurons driven by two phospholipase A2 enzymes: secretory type IIA and cytosolic type IV.
Synergy by secretory phospholipase A2 and glutamate on inducing cell death and sustained arachidonic acid metabolic changes in primary cortical neuronal cultures.
Secreted phospholipase A2-induced neurotoxicity and epileptic seizures after intracerebral administration: an unexplained heterogeneity as emphasized with paradoxin and crotoxin.
Interleukin-1beta induced cyclooxygenase 2 expression and prostaglandin E2 secretion by human neuroblastoma cells: implications for Alzheimer's disease.
Beta-amyloid stimulation of inducible nitric-oxide synthase in astrocytes is interleukin-1beta- and tumor necrosis factor-alpha (TNF alpha)-dependent, and involves a TNF alpha receptor-associated factor and NFkappaB-inducing kinase-dependent signaling mechanism.
Plasmalogen deficiency in early Alzheimer's disease subjects and in animal models: molecular characterization using electrospray ionization mass spectrometry.
Gene expression profiling of 12633 genes in Alzheimer hippocampal CA1: transcription and neurotrophic factor down-regulation and up-regulation of apoptotic and pro-inflammatory signaling.
Phospholipases as mediators of amyloid beta peptide neurotoxicity: an early event contributing to neurodegeneration characteristic of Alzheimer's disease.
Discussion of the role of the extracellular signal-regulated kinase-phospholipase A2 pathway in production of reactive oxygen species in Alzheimer's disease.
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