Synaptic Specializations Mediated by Synaptotagmin Isoforms

Abstract

The timing of neurotransmitter release in response to presynaptic firing varies widely across synapses in the brain. The release of synaptic vesicles can be synchronous and occur within milliseconds of an action potential, or asynchronous and persist for tens of milliseconds. Release can also undergo short-term facilitation in which it is enhanced by two closely timed action potentials. The molecular properties of synapses that determine the relative timing of synaptic vesicle release have remained poorly understood. We have found that Synaptotagmin 7 (Syt7) plays a major role in multiple aspects of synaptic transmission. Syt7 mediates short-term facilitation at almost all synapses where it is present, including seven different synapses tested here. Loss of Syt7 also eliminated facilitation that was activated by trains of stimuli at Purkinje cell and vestibular synapses, revealing an interplay between short-term facilitation and depression that maintains constant synaptic strength. Changes in the initial probability of release were not detectable in Syt7 KOs at three different synapses, and removal of Syt7 did not reduce the rate of recovery from depression in cerebellar Purkinje cell synapses. Facilitation could be rescued in Syt7 KOs by presynaptic expression of Syt7, but not Ca2+-insensitive Syt7. At cerebellar parallel fibers we found that Syt7 can mediate both facilitation and asynchronous release, but there are additional mechanisms of facilitation and asynchronous release in the absence of Syt7. In contrast to facilitation, the presence or absence of Syt7 does not determine whether asynchronous release occurs. Syt7 was most strongly expressed at Purkinje cell synapses in the cerebellar nuclei, even though no asynchronous release could be detected. Inhibitory synapses in the inferior olive are exclusively asynchronous, and we found that release synchrony is instead determined by the absence of Synaptotagmin 1 and 2. Syt7 plays a role in shaping the kinetics of asynchronous release in the inferior olive, but does not control the magnitude of asynchronous release. Our findings suggest that Syt7 could be a Ca sensor for facilitation that is prominent at many synapses, and that it may also play a role in asynchronous release in combination with other additional mechanisms.