CA SENSING FOR EXOCYTOSIS

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DESCRIPTION: (Applicant's Abstract) The secretion of neurotransmitter and hormones from neurons and neuroendocrine cells is a highly regulated process. It is now widely accepted that a rise in intracellular [Ca2+] rapidly triggers secretion from excitable cells. However, it has recently become clear that Ca2+ also slowly modulates ("primes") release, in part through activation of protein kinase C (PKC), which, in turn, accelerates the rate that secretory vesicles become ready to be released. Therefore it is likely that there are multiple fast (triggering) and slow (modulating) Ca2+ sensors for exocytosis. A long-range goal of the investigator is to understand how Ca2+ triggers exocytosis from excitable cells and how exocytosis is regulated by Ca2+ and other second messengers. The goal of this project is to characterize fast and slow Ca2+ sensing for exocytosis in individual cells using optical and electrophysiological techniques which allow both fine control of [Ca2+] and high-time-resolution measurements of exocytosis. The 3 aims are: Aim I. To determine how the protein SNAP-25 is involved in Ca2+ priming and triggering steps. The effect of mutations of SNAP-25 on exocytosis will be measured to test the hypothesis that the C-terminus of the protein participates in both Ca2+-priming and triggering steps. Aim II. To determine how fast Ca2+ can prime exocytosis. Experiments will elevate [Ca2+]i in 2 steps to sequentially prime and trigger secretion to test the hypothesis that Ca2+ priming occurs in less than 1 second. Aim III. To quantify the ionic selectivity of the Ca2+ trigger for exocytosis. Multivalent cations such as Sr2+, Ba2+, and Pb2+ can act as "Ca2+ surrogates" in triggering exocytosis and other Ca2+-activated cellular responses. The ability of Ca2+ surrogates to rapidly trigger exocytosis will be measured to provide clues about the approximate size, flexibility and accessibility of the Ca2+-binding cavity of the triggering Ca2+ sensor. Achieving these aims will provide new insights into the mechanisms whereby secretion is regulated. Such basic knowledge is essential to understand complex processes such as short-term memory formation in the brain, the modulation of insulin secretion by glucagon in the endocrine pancreas, and the neurotoxicity of Pb2+ in the central nervous system.