Exocytosis is at the base of fundamental physiological processes such as neurotransmission and hormone release and elucidating its molecular mechanisms is a major step towards deciphering the immense complexity of brain function. It is well established that synaptotagmins (syts) are essential components of the release machinery that couple the entry of Ca2+ to the fusion of vesicles and subsequent release of their contents. However, many aspects of syt function, and of the interplay between different syt isoforms, in triggering the release process are as yet unknown. In the present project I propose to investigate the roles of different syt isoforms on exocytosis in chromaffin cells and hippocampal neurons. Since both syt-1 and syt-7 cooperatively participate in exocytosis I will first study the modulation of fusion pore properties by these isoforms. This will be done by studying fusion pore formation through single-spike amperometry in chromaffin cells obtained from syt-1, syt-7 and syt-1/syt-7 knockout mice. I will also virally re-express wild type syts and chimeras between both proteins to identify the domains involved in slow and fast secretion. Additionally, I expect to clarify whether vesicles containing syt-1 and syt-7 are recruited during separate phases of release (fast vs. slow) using constructs that express these isoforms tagged with a pH-sensitive YFP. Finally, I will seek to identify a possible function of syt-7 in hippocampal neurons by first studying its subcellular distribution in vesicles vs plasma membrane. Additionally, I will study whether syt-7 plays a differential role in release between GABAergic and glutamatergic neurons. My proposed multidisciplinary approach offers great potential to tackle these questions and is expected to yield original results that may help in deciphering the precise role of different syt isoforms in exocytosis. This will contribute, on the longer term, to our understanding of neuronal communication.
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