Synaptotagmin IV overexpression inhibits depolarization-induced exocytosis in PC12 cells

Depolarization-induced vesicle exocytosis is a complex mechanism involving a number of proteins. In this process, synaptotagmins work as members of the Ca(2+)-sensing system that triggers the fusion of the synaptic vesicle with the plasma membrane. Synaptotagmin IV (SytIV), an immediate-early gene induced by depolarization in PC12 pheochromocytoma cells and in the hippocampus, has been suggested to work as a negative regulator of neurotransmitter release. Unlike other synaptotagmins, SytIV has an evolutionarily conserved substitution of an aspartate to a serine in the Ca(2+) coordination site of its C2A domain, preventing SytIV from binding anionic lipids in a Ca(2+)-dependent fashion. We used the secretion of human growth hormone (hGH) as a reporter system with which to examine the effects of overexpressing SytIV and other depolarization-induced immediate-early genes (the protein kinases KID-1, SIK, and PIM-1 and the transcription factors rTLE3 and Nurr1) on depolarization-induced vesicle exocytosis in PC12 cells. SytIV overexpression resulted in decreased depolarization-induced hGH release. However, conversion of the unique serine in SytIV to an aspartate eliminated this inhibitory activity. In addition, rTLE3 overexpression produced only a modest increase in spontaneous vesicle exocytosis, whereas KID-1, SIK, PIM-1, and Nurr1 overexpression had no effect on depolarization-induced exocytosis.