Distribution and role of calcium-independent phospholipase A2 in the brain

Abstract

One class of phospholipase A2 (PLA2) that does not require calcium for its activity is the cytosolic calcium-independent PLA2 (iPLA2). In brain tissues, the basal expression and activity of iPLA2 is higher than either cytosolic calcium-dependent PLA2 (cPLA2) or secretory calcium-dependent PLA2 (sPLA2) (Molloy et al. 1998; Farooqui et al. 1999), and its protein expression decreases during aging (Aid and Bosetti, 2007). iPLA2 is not only responsible for regulation of membrane phospholipid homeostasis (?housekeeping?) in cells (Balsinde et al. 1995), but also plays important roles in intracellular signal transduction. The present study aims to elucidate the role and distribution of iPLA2 in the brain. iPLA2 immunoreactivity was observed in structures derived from the telencephalon, whereas structures derived from the diencephalon were lightly labeled. The midbrain, vestibular, trigeminal and inferior olivary nuclei, and the cerebellar cortex were densely labeled. Immunoreactivity was observed on the nuclear envelope of neurons, dendrites and axon terminals using electron microscopy. Drug-induced tremulous oral movements have been linked to human Parkinsonian tremor (Salamore et al. 1998). Inhibitors of iPLA2 could induce vacuous chewing movements (VCM) in rats. Striatal injections of iPLA2 inhibitor, bromoenol lactone (BEL), resulted in significantly increased VCM in Wistar rats from 2 to 5 days after injection. Significantly increased VCM was also observed after intrathalamic or intracortical injections of BEL. In contrast, no significant effect was observed after BEL injection into the cerebellum. The effects of BEL were replicable using another PLA2 inhibitor, methyl arachidonyl fluorophosphonate (MAFP). These findings suggest that increased VCM after MAFP injection was because of inhibition of iPLA2. The observations with BEL and MAFP point to a role for inhibition of PLA2 enzymatic activity in VCM. Prepulse inhibition (PPI) of the acoustic startle reflex has been widely used as a model of sensory information processing and sensorimotor gating (Graham, 1975; Kumari and Sharma, 2002). Our results demonstrate that systemic administration of the non-specific PLA2 inhibitor, quinacrine, resulted in significantly decreased PPI of the auditory startle reflex at 76, 80, and 84 decibel (dB), compared to saline injected controls. Rats that received intrastriatal injection of antisense oligonucleotide to iPLA2 also showed significant reduction in PPI at prepulse intensities of 76 and 84 dB compared to scrambled sense injected controls. iPLA2 inhibition apparently has the same effect as increased dopamine receptor stimulation, in terms of its effects on PPI. These findings appear consistent with the previous findings that iPLA2 inhibition causes VCM. We have shown that mitochondrial permeability transition pore (MPTP) is essential in meditating the effect of BEL on exocytosis in PC12 cells. Blocking of MPTP with the inhibitors bongkrekic acid (BKA) and cyclosporine A (CsA) resulted in reduced exocytosis after intracellular addition of BEL. p-trifluromethoxy carbonyl cyanide phenyl hydrazone (FCCP) which depolarizes mitochondria transition potential and deplete mitochondrial calcium did not have any effects on exocytosis after addition of BEL. The above results are consistent with previous studies that overexpression of iPLA2 has a protective effect on the mitochondria. It is possible that inhibition of iPLA2 causes damage to the mitochondria and release of calcium resulting in exocytosis. The results highlight the importance of normal ?housekeeping? phospholipase A2 in maintaining the normal function of neurons.