A src family tyrosine kinase inhibits neurotransmitter release from neuronal cells

Tyrosine kinases are expressed in many tissues, particularly in the central nervous system, and regulate various cellular functions. We report here that a src family tyrosine kinase-specific inhibitor, PP2, enhances neurotransmitter release from PC12 cells and primary cultured neurons. PP2 enhances only Ca(2+)-dependent release; it does not affect basal release. These effects result from an enhancement of vesicular exocytosis and not from the reuptake or refilling of neurotransmitters because Ca(2+)-dependent secretion of an exogenously expressed reporter protein, the human growth hormone (hGH), is also enhanced by PP2. Overexpression of constitutive active v-src, but not of a kinase-inactive mutant, suppressed Ca(2+)-dependent release. In PP2-treated cells, Pyk2, paxillin, and some other proteins showed a decrease in tyrosine phosphorylation, and the enhancement of tyrosine phosphorylation of these proteins in response to Ca(2+) influx was also reduced. Electron and fluorescence microscopy showed that PP2 treatment induced morphological change and decreased phalloidin reactivity at the filopodium-like structures on the processes of PC12 cells. Interestingly, inhibition of actin polymerization with cytochalasin D and latrunculin A enhanced Ca(2+)-dependent, but not basal, release. It is possible that a src family tyrosine kinase, through the regulation of actin dynamics, has an inhibitory function to regulate neurotransmitter release.