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 Ca(2+) 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 Ca(2+) sensor dominates release in these cells. Synaptotagmin-7 has a higher Ca(2+) affinity and slower binding kinetics than synaptotagmin-1, matching the proposed properties for the second, slower Ca(2+) sensor. Here, we examined Ca(2+)-triggered exocytosis in chromaffin cells from KO mice lacking synaptotagmin-7, and from knockin mice containing normal levels of a mutant synaptotagmin-7 whose C(2)B domain does not bind Ca(2+). In both types of mutant chromaffin cells, Ca(2+)-triggered exocytosis was decreased dramatically. Moreover, in chromaffin cells lacking both synaptotagmin-1 and -7, only a very slow release component, accounting for approximately 30% of WT exocytosis, persisted. These data establish synaptotagmin-7 as a major Ca(2+) sensor for exocytosis in chromaffin cells, which, together with synaptotagmin-1, mediates almost all of the Ca(2+) triggering of exocytosis in these cells, a surprising result, considering the lack of a role of synaptotagmin-7 in synaptic vesicle exocytosis.
Schonn
Jean-Sébastien
JS
Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany.
Maximov
Anton
A
Lao
Ye
Y
Südhof
Thomas C
TC
Sørensen
Jakob B
JB
D000311:Adrenal Glands / Q000166:cytology* / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D019439:Chromaffin Cells / Q000166:cytology / Q000378:metabolism*; D005089:Exocytosis; D017353:Gene Deletion; D007700:Kinetics; D008564:Membrane Potentials; D051379:Mice; D018345:Mice, Knockout; D017434:Protein Structure, Tertiary; D050863:Synaptotagmin I / Q000378:metabolism*; D050857:Synaptotagmins / Q000737:chemistry / Q000378:metabolism*
D050863:Synaptotagmin I; D050857:Synaptotagmins; D002118:Calcium