High-affinity binding sites for 125I-labelled pancreatic secretory phospholipase A2 in rat brain

The role of secretory phospholipase A₂ in the central nervous system and neurological diseases

Secretory phospholipase A2 (sPLA2s) are small secreted proteins (14-18 kDa) and require submillimolar levels of Ca(2+) for liberating arachidonic acid from cell membrane lipids. In addition to the enzymatic function, sPLA2 can exert various biological responses by binding to specific receptors. Physiologically, sPLA2s play important roles on the neurotransmission in the central nervous system and the neuritogenesis in the peripheral nervous system. Pathologically, sPLA2s are involved in the neurodegenerative diseases (e.g., Alzheimer's disease) and cerebrovascular diseases (e.g., stoke). The common pathology (e.g., neuronal apoptosis) of Alzheimer's disease and stroke coexists in the mixed dementia, suggesting common pathogenic mechanisms of the two neurological diseases. Among mammalian sPLA2s, sPLA2-IB and sPLA2-IIA induce neuronal apoptosis in rat cortical neurons. The excess influx of calcium into neurons via L-type voltage-dependent Ca(2+) channels mediates the two sPLA2-induced apoptosis. The elevated concentration of intracellular calcium activates PKC, MAPK and cytosolic PLA2. Moreover, it is linked with the production of reactive oxygen species and apoptosis through activation of the superoxide producing enzyme NADPH oxidase. NADPH oxidase is involved in the neurotoxicity of amyloid β peptide, which impairs synaptic plasticity long before its deposition in the form of amyloid plaques of Alzheimer's disease. In turn, reactive oxygen species from NADPH oxidase can stimulate ERK1/2 phosphorylation and activation of cPLA2 and result in a release of arachidonic acid. sPLA2 is up-regulated in both Alzheimer's disease and cerebrovascular disease, suggesting the involvement of sPLA2 in the common pathogenic mechanisms of the two diseases. Thus, our review presents evidences for pathophysiological roles of sPLA2 in the central nervous system and neurological diseases.

L-type voltage-dependent calcium channel is involved in the snake venom group IA secretory phospholipase A2-induced neuronal apoptosis

Snake venom group IA secretory phospholipase A2 (sPLA2-IA) is known as a neurotoxin. Snake venom sPLA2s are neurotoxic in vivo and in vitro, causing synergistic neurotoxicity to cortical cultures when applied with toxic concentrations of glutamate. However, it has not yet been cleared sufficiently how sPLA2-IA exerts neurotoxicity. Here, we found sPLA2-IA induced neuronal cell death in a concentration-dependent manner. This death was a delayed response requiring a latent time for 6h. sPLA2-IA-induced neuronal cell death was accompanied with apoptotic blebbing, condensed chromatin, and fragmented DNA, exhibiting apoptotic features. NMDA receptor blockers suppressed the neurotoxicity of sPLA2-IA, but an AMPA receptor blocker did not. Interestingly, L-type voltage-dependent Ca(2+) channel (L-VDCC) blocker significantly protected neurons from the sPLA2-IA-induced apoptosis. On the other hand, neither N-VDCC blockers nor P/Q-VDCC blocker did. In conclusion, we demonstrated that sPLA2-IA induced neuronal cell death via apoptosis. Furthermore, the present study suggests that not only NMDA receptor but also L-VDCC contributed to the neurotoxicity of snake venom sPLA2-IA.

Genetic association between the phospholipase A2 gene and unipolar affective disorder: a multicentre case-control study

The co-segregation in one pedigree of bipolar affective disorder with Darier's disease whose gene is on chromosome 12q23-q24.1, and findings from linkage and association studies with the neighbouring gene of phospholipase A2 (PLA2) indicate that PLA2 may be considered as a candidate gene for affective disorders. All relevant genetic association studies, however, were conducted on bipolar patients. In the present study, the possible association between the PLA2 gene and unipolar affective disorder was examined on 321 unipolar patients and 604 controls (all personally interviewed), recruited from six countries (Belgium, Bulgaria, Croatia, Germany, Greece, and Italy) participating in the European Collaborative Project on Affective Disorders. After controlling for population group and gender, one of the eight alleles of the investigated marker (allele 7) was found to be more frequent among unipolar patients with more than three major depressive episodes than among controls (P<0.01); genotypic association was also observed, under the dominant model of genetic transmission (P<0.02). In addition, presence of allele 7 was correlated with a higher frequency of depressive episodes (P<0.02). These findings suggest that structural variations at the PLA2 gene or the chromosomal region around it may confer susceptibility for unipolar affective disorder.

Group III secreted phospholipase A2 causes apoptosis in rat primary cortical neuronal cultures

Secreted phospholipase A(2) (sPLA(2)) has been shown to cause both necrotic and apoptotic neuronal injury in the central nervous system. The acute effects of group III sPLA(2) (sPLA(2)-III) include an ability to potentiate glutamate-induced necrosis and calcium influx in primary rat neuronal cultures. Thus, a brief exposure to glutamate and sPLA(2)-III results in an immediate potentiated calcium influx and a potentiated necrosis the following day after treatment, compared to addition of either agonist alone. In contrast to these acute effects, sPLA(2) from both groups IB and IIA have been shown to induce neuronal cell death via apoptosis. Here, we offer the first evidence that group III sPLA(2) potently causes extensive neuronal apoptosis in rat cortical cultures, as evidenced by DNA fragmentation. Distinct from this effect, even high concentrations of glutamate did not cause neuronal damage under the same conditions, and there was no evidence of sPLA(2) damage to cortical astrocytes.

Porcine pancreatic group IB secretory phospholipase A2 potentiates Ca2+ influx through L-type voltage-sensitive Ca2+ channels

Secretory phospholipase A(2) (sPLA(2)) exhibits neurotoxicity in the central nervous system. There are high-affinity binding sites of the porcine pancreatic group IB sPLA(2) (sPLA(2)-IB) in the brain. sPLA(2)-IB causes neuronal cell death via apoptosis in the rat cerebral cortex. Although apoptosis is triggered by an influx of Ca(2+) into neurons, it has not yet been ascertained whether the Ca(2+) influx is associated with the neurotoxicity of sPLA(2)-IB. We thus examined the possible involvement of Ca(2+) in the neurotoxicity of sPLA(2)-IB in the primary culture of rat cortical neurons. sPLA(2)-IB induced neuronal cell death in a concentration- and time-dependent manner. This death was accompanied by condensed chromatin and fragmented DNA, exhibiting apoptotic features. Before apoptosis, sPLA(2)-IB markedly enhanced the influx of Ca(2+) into neurons. A calcium chelator suppressed neurons from sPLA(2)-IB-induced neuronal cell death in a concentration-dependent manner. An L-type voltage-sensitive Ca(2+) channel (L-VSCC) blocker significantly protected the sPLA(2)-IB-potentiated influx of Ca(2+). On the other hand, blockers of N-VSCC and P/Q-VSCC did not. An L-VSCC blocker protected neurons from sPLA(2)-IB-induced neuronal cell death. In addition, the L-VSCC blocker ameliorated the apoptotic features of sPLA(2)-IB-treated neurons. Neither an N-VSCC blocker nor P/Q-VSCC blockers affected the neurotoxicity of the enzyme. In conclusion, these findings demonstrate that the influx of Ca(2+) into neurons play an important role in the neurotoxicity of sPLA(2)-IB. Furthermore, the present study suggests that L-VSCC contribute to the sPLA(2)-IB-potentiated influx of Ca(2+) into neurons.

Group IB secretory phospholipase A(2)induces cell death in the cultured cortical neurons: a possible involvement of its binding sites

In primary cultures of rat cortical neurons, group IB secretory phospholipase A(2) (sPLA(2)-IB) induced cell death. In rat cortical membranes, there were high affinity binding sites of [125I]sPLA(2)-IB. The high-affinity binding sites were decreased by sPLA(2)-IB and anti-sPLA(2) receptor immunoglobulin G (anti-sPLA(2)R IgG). Furthermore, anti-sPLA(2)R IgG caused neuronal cell death in a concentration-dependent manner. The present study suggests that sPLA(2)-IB induces neuronal cell death via its high-affinity binding sites.

Glutamate signalling and secretory phospholipase A2 modulate the release of arachidonic acid from neuronal membranes

The lipid mediators generated by phospholipases A(2) (PLA(2)), free arachidonic acid (AA), eicosanoids, and platelet-activating factor, modulate neuronal activity; when overproduced, some of them become potent neurotoxins. We have shown, using primary cortical neuron cultures, that glutamate and secretory PLA(2) (sPLA(2)) from bee venom (bv sPLA(2)) and Taipan snake venom (OS2) elicit synergy in inducing neuronal cell death. Low concentrations of sPLA(2) are selective ligands of cell-surface sPLA(2) receptors. We investigated which neuronal arachidonoyl phospholipids are targeted by glutamate-activated cytosolic calcium-dependent PLA(2) (cPLA(2)) and by sPLA(2). Treatment of (3)H-AA-labeled cortical neurons with mildly toxic concentrations of sPLA(2) (25 ng/ml, 1.78 nM) for 45 min resulted in a two- to threefold higher loss of (3)H-AA from phosphatidylcholine (PC) than from phosphatidylethanolamine (PE) and in minor changes in other phospholipids. A similar profile, although of greater magnitude, was observed 20 hr posttreatment. Glutamate (80 microM) induced much less mobilization of (3)H-AA than did sPLA(2) and resulted in a threefold greater degradation of (3)H-AA PE than of (3)H-AA PC by 20 hr posttreatment. Combining sPLA(2) and glutamate resulted in a greater degradation of PC and PE, and the N-methyl-D-aspartate receptor antagonist MK-801 only blocked glutamate effects. Thus, activation of the arachidonate cascade induced by glutamate and sPLA(2) under experimental conditions that lead to neuronal cell death involves the hydrolysis of different (perhaps partially overlapping) cellular phospholipid pools.

Group IB secretory phospholipase A2 induces neuronal cell death via apoptosis

Group IB secretory phospholipase A2 (sPLA2-IB) mediates cell proliferation, cell migration, hormone release and eicosanoid production via its receptor in peripheral tissues. In the CNS, high-affinity binding sites of sPLA2-IB have been documented. However, it remains obscure whether sPLA2-IB causes biologic or pathologic response in the CNS. To this end, we examined effects of sPLA2-IB on neuronal survival in primary cultures of rat cortical neurons. sPLA2-IB induced neuronal cell death in a concentration-dependent manner. This death was a delayed response requiring a latent time for 6 h; sPLA2-IB-induced neuronal cell death was accompanied with apoptotic blebbing, condensed chromatin, and fragmented DNA, exhibiting apoptotic features. Before cell death, sPLA2-IB liberated arachidonic acid (AA) and generated prostaglandin D2 (PGD2) from neurons. PGD2 and its metabolite, Delta12-PGJ2, exhibited neurotoxicity. Inhibitors of sPLA2 and cyclooxygenase-2 (COX-2) significantly suppressed not only AA release, but also PGD2 generation. These inhibitors significantly prevented neurons from sPLA2-IB-induced neuronal cell death. In conclusion, we demonstrate a novel biological response, apoptosis, of sPLA2-IB in the CNS. Furthermore, the present study suggests that PGD2 metabolites, especially Delta12-PGJ2, might mediate sPLA2-IB-induced apoptosis.

The regulation of AMPA receptor-binding sites

A wide variety of mechanisms have been identified that can regulate the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor complex. Modulation has been shown to occur at the nucleic acid level via RNA editing and alternative splicing. At the posttranslational level, processes such as phosphorylation, glycosylation, chemical modification of reactive groups on the receptor proteins, interaction with a putative receptor-associated modulatory protein, and changes in the lipid environment have been reported to regulate receptor binding and function. In this review, we discuss general aspects of the cell biology, pharmacology, and function of AMPA receptors. In particular, we focus on some factors shown to modulate agonist binding and discuss possible molecular mechanisms underlying the regulation observed.