Synaptotagmin Has an Essential Function in Synaptic Vesicle Positioning for Synchronous Release in Addition to Its Role as a Calcium Sensor

A multitude of synaptic proteins interact at the active zones of nerve terminals to achieve the high temporal precision of neurotransmitter release in synchrony with action potentials. Though synaptotagmin has been recognized as the Ca(2+) sensor for synchronous release, it may have additional roles of action. We address this question at the calyx of Held, a giant presynaptic terminal, that allows biophysical dissection of multiple roles of molecules in synaptic transmission. Using high-level expression recombinant adenoviruses, in conjunction with a stereotactic surgery in postnatal day 1 rats, we overcame the previous inability to moleculary perturb the calyx by overexpression of a mutated synaptotagmin. We report that this mutation leaves intrinsic Ca(2+) sensitivity of vesicles intact while it destabilizes the readily releasable pool of vesicles and loosens the tight coupling between Ca(2+) influx and release, most likely by interfering with the correct positioning of vesicles with respect to Ca(2+) channels.