title: Complexin II plays a positive role in Ca2+-triggered exocytosis by facilitating vesicle priming.
abstract: SNARE-mediated exocytosis is a multistage process central to synaptic transmission and hormone release. Complexins (CPXs) are small proteins that bind very rapidly and with a high affinity to the SNARE core complex, where they have been proposed recently to inhibit exocytosis by clamping the complex and inhibiting membrane fusion. However, several other studies also suggest that CPXs are positive regulators of neurotransmitter release. Thus, whether CPXs are positive or negative regulators of exocytosis is not known, much less the stage in the vesicle life cycle at which they function. Here, we systematically dissect the vesicle stages leading up to exocytosis using a knockout-rescue strategy in a mammalian model system. We show that adrenal chromaffin cells from CPX II knockout mice exhibit markedly diminished releasable vesicle pools (comprising the readily and slowly releasable pools), while showing no change in the kinetics of fusion pore dilation or morphological vesicle docking. Overexpression of WT CPX II-but not of SNARE-binding-deficient mutants-restores the size of the the releasable pools in knockout cells, and in WT cells it markedly enlarges them. Our results show that CPXs regulate the size of the primed vesicle pools and have a positive role in Ca(2+)-triggered exocytosis.
authors.affiliation: Department of Physiology and Biophysics, Keck School of Medicine, Zilkha Neurogenetic Institute, University of Southern California, Los Angeles, CA 90089, USA.
authors.forename: Haijiang
authors.initials: H
authors.lastname: Cai
authors.forename: Kerstin
authors.initials: K
authors.lastname: Reim
authors.forename: Frederique
authors.initials: F
authors.lastname: Varoqueaux
authors.forename: Sompol
authors.initials: S
authors.lastname: Tapechum
authors.forename: Kerstin
authors.initials: K
authors.lastname: Hill
authors.forename: Jakob B
authors.initials: JB
authors.lastname: Sørensen
authors.forename: Nils
authors.initials: N
authors.lastname: Brose
authors.forename: Robert H
authors.initials: RH
authors.lastname: Chow
mesh_terms: D033942:Adaptor Proteins, Vesicular Transport; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002395:Catecholamines / Q000378:metabolism; D019439:Chromaffin Cells / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D005089:Exocytosis / Q000502:physiology*; D005260:Female; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D008817:Mice, Mutant Strains; D008854:Microscopy, Electron; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D050600:SNARE Proteins / Q000378:metabolism; D022142:Secretory Vesicles / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D033942:Adaptor Proteins, Vesicular Transport; D002395:Catecholamines; D009419:Nerve Tissue Proteins; D050600:SNARE Proteins; C095993:complexin II; D002118:Calcium