UC Irvine

In preparations of synaptic terminals (synaptosomes) isolated from rat brain, the activity of phospholipase A2 (PLA2), a phospholipid hydrolase that serves a central function in signal transduction, was inhibited in a Ca(2+)-dependent manner by incubation with 60 mM K+ or with the Ca(2+)-selective ionophore ionomycin. Reversal by alkaline phosphatase treatment suggested that this inhibitory effect resulted from phosphorylation of a synaptosomal protein substrate. When lysed synaptosomes were incubated with Ca2+/calmodulin (CaM), purified Ca2+/CAM-dependent protein kinase II (Ca2+/CaM-dependent PK II) and ATP, PLA2 activity in lysates was nearly abolished within 10 min. This effect was accompanied by a marked decrease in the Vmax of the enzyme and little or no change in the Km. Furthermore, Ca2+/CaM with ATP but without exogenous Ca2+/CaM-dependent PK II partially inhibited PLA2 activity, and this effect was prevented by treating the lysates with a selective peptide inhibitor of Ca2+/CaM-dependent PK II. In contrast, incubation of intact synaptosomes with 4 beta-phorbol 12-myristate 13-acetate or of lysed synaptosomes with purified protein kinase C had little or no effect on PLA2 activity. The results strongly suggest that the Ca(2+)-dependent inhibition of PLA2 activity observed in intact nerve endings was produced by activation of the multifunctional Ca2+/CaM-dependent PK II. A membrane-permeable adenylyl cyclase activator, forskolin, enhanced PLA2 activity in intact synaptosomes, and cAMP-dependent protein kinase potentiated PLA2 activity in lysed synaptosomes. Furthermore, another broad-spectrum protein kinase present in synaptic terminals, casein kinase II, also potentiated PLA2 activity in lysed synaptosomes. The effects of both protein kinases were associated with a decrease in Km and no change in Vmax. The results suggest that PLA2 activity in synaptic terminals is subject to bidirectional control by distinct signal transduction pathways. Moreover, mutually antagonistic effects of the Ca2+/CaM-dependent PK II and PLA2 pathways provide a possible molecular mechanism for bidirectional modulation of neurotransmitter release.