Synaptotagmin-1 and-7 are functionally overlapping Ca2+ sensors for exocytosis in adrenal chromaffin cells

Synaptotagmin-1, the canonical isoform of the synaptotagmin family, is a Ca2+ sensor for fast synchronous neurotransmitter release in forebrain neurons and chromaffin cells. Even though deletion of synaptotagmin-1 abolishes fast exocytosis in chromaffin cells, it reduces overall secretion by only 20% because of the persistence of slow exocytosis. Therefore, another Ca2+ sensor dominates release in these cells. Synaptotagmin-7 has a higher Ca2+ affinity and slower binding kinetics than synaptotagmin-1, matching the proposed properties for the second, slower Ca2+ sensor. Here, we examined Ca2+-triggered exocytosis in chromaffin cells from KO mice lacking synaptotagmin-7, and from knockin mice containing normal levels of a mutant synaptotagmin-7 whose C2B domain does not bind Ca2+. In both types of mutant chromaffin cells, Ca2+-triggered exocytosis was decreased dramatically. Moreover, in chromaffin cells lacking both synaptotagmin-1 and -7, only a very slow release component, accounting for approximate to 30% of WT exocytosis, persisted. These data establish synaptotagmin-7 as a major Ca2+ sensor for exocytosis in chromaffin cells, which, together with synaptotagmin-1, mediates almost all of the Ca2+ triggering of exocytosis in these cells, a surprising result, considering the lack of a role of synaptotagmin-7 in synaptic vesicle exocytosis.