Pre- and Post-Synaptic Roles of Synaptotagmin-7 in Exocytosis

Abstract

Synaptotagmin-7 (Syt-7) serves as a calcium-sensor for regulated exocytosis in the central and peripheral nervous systems. Its high sensitivity allows tunable secretory responses to a range of stimuli that result in graded increases in intracellular calcium. Despite the importance of Syt-7, questions remain as to its localization and specific functions in nervous tissue. Here, these issues were examined at a key effector arm of the sympathetic nervous system - the adrenal medulla - with mice lacking endogenous Syt-7 (Syt-7 KO). First, in terms of localization, we find that in the axons and terminals of neurons innervating medullary chromaffin cells, as well as in chromaffin cells themselves, Syt-7 is punctate in appearance consistent with its sorting to secretory organelles. Functionally, in neurons, we find that Syt-7 is required for a form of short-term synaptic enhancement termed facilitation. Post-synaptically, in adrenal chromaffin cells, secretion is strongly compromised in the absence of Syt-7. Moreover, Syt-7 KO chromaffin cells release their cargos with faster kinetics than do WT cells. To further clarify the role of Syt-7 in exocytosis, purified dense core vesicles expressing only Syt-7, or the other major chromaffin cell isoform, Syt-1, were triggered to fuse on synthetic planar supported bilayers bearing t-SNAREs. These in vitro reconstitution studies demonstrated that Syt-7 confers substantially greater calcium sensitivity to vesicle fusion than Syt-1 and slows the rate at which cargos are released, recapitulating the key cellular observations. Our work shows that by virtue of its sorting and biochemistry, Syt-7 serves important functional roles as a regulator of exocytosis at pre- and post-synaptic sites within the sympatho-adrenal system.