Sequential Link of Kiss-And-Run Mechanism and Classical Exocytosis at Hippocampal Synapses

Abstract

The mechanisms behind highly dynamic variation of synaptic vesicle release have been shown to be linked to different vesicle pools, variable modes of mobilization, calcium levels, protein phosphorylation and a variety of endocytosis mechanisms. This study presents a model that may explain seemingly contradicting exocytosis mechanism like ‘kiss-and-run’ vs full vesicle fusion. It was found that the onset of FM1-43 release of from individual synapses can be delayed by up to a few seconds after the start of stimulation. This phenomenon is linked to modified opening kinetics of a fusion pore that connects the vesicle lumen to the extracellular space. Some synapses were completely inactive during the first few seconds after start of the stimulation, despite immediate rise of calcium. A low concentration of staurosporine, which has been used to induce ‘kiss-and-run’ exocytosis, increased the proportion of delayed synapses. Vesicle fusion was never delayed after start of stimulation, when it was monitored with pH-sensitive synaptopHlourin, indicating an instantaneous formation of a fusion pore that allowed rapid equilibration of the vesicle lumenal pH but prevented release of FM1-43. The delay of FM release could be explained by the transient formation of a fusion pore (“kiss-and-run”) that expanded to full vesicle fusion after a few seconds. Our observations point to a regulated exocytosis mechanism in hippocampal synapses that combines ‘kiss-and-run’- and full vesicle fusion exocytosis in a sequential process.