Deconstructing complexin function in activating and clamping Ca2+-triggered exocytosis by comparing knockout and knockdown phenotypes

Complexin, a presynaptic protein that avidly binds to assembled SNARE complexes, is widely acknowledged to activate Ca2+-triggered exocytosis. In addition, studies of invertebrate complexin mutants and of mouse neurons with a double knockdown (DKD) of complexin-1 and -2 suggested that complexin maintains the readily releasable pool (RRP) of vesicles and clamps spontaneous exocytosis. In contrast, studies of mouse neurons with a double knockout (DKO) of complexin-1 and -2, largely carried out in hippocampal autapses, did not detect changes in the RRP size or in spontaneous exocytosis. To clarify complexin function, we here directly compared in two different preparations, cultured cortical and olfactory bulb neurons, the phenotypes of complexin DKD and DKO neurons. We find that complexin-deficient DKD and DKO neurons invariably exhibit a similar to 50% decrease in vesicle priming. Moreover, the DKD consistently increased spontaneous exocytosis, but the DKO did so in cortical but not olfactory bulb neurons. Furthermore, the complexin DKD but not the complexin DKO caused a compensatory increase in complexin-3 and -4 mRNA levels; overexpression of complexin-3 but not complexin-1 increased spontaneous exocytosis. Complexin-3 but not complexin-1 contains a C-terminal lipid anchor attaching it to the plasma membrane; addition of a similar lipid anchor to complexin-1 converted complexin-1 from a clamp into an activator of spontaneous exocytosis. Viewed together, our data suggest that complexin generally functions in priming and Ca2+ triggering of exocytosis, and additionally contributes to the control of spontaneous exocytosis dependent on the developmental history of a neuron and on the subcellular localization of the complexin.