Characterization of the role of the Synaptotagmin family as calcium sensors in facilitation and asynchronous neurotransmitter release

Ca²⁺ influx into presynaptic nerve terminals activates synaptic vesicle exocytosis by triggering fast synchronous fusion and a slower asynchronous release pathway. In addition, a brief rise in Ca²⁺ after consecutive action potentials has been correlated with a form of short-term synaptic plasticity with enhanced vesicle fusion termed facilitation. Although the synaptic vesicle protein Synaptotagmin 1 (Syt1) has been implicated as the Ca²⁺ sensor for synchronous fusion, the molecular identity of the Ca²⁺ sensors that mediate facilitation and asynchronous release is unknown. To test whether the synchronous Ca²⁺ sensor, Syt1, or the asynchronous Ca²⁺ sensor is involved in facilitation, we analyzed whether genetic elimination of Syt1 in Drosophila results in a concomitant impairment in facilitation. Our results indicate that Syt1 acts as a redundant Ca²⁺ sensor for facilitation, with the asynchronous Ca²⁺ sensor contributing significantly to this form of short-term plasticity. We next examined whether other members of the Drosophila Syt family functioned in Ca²⁺-dependent asynchronous release or facilitation in vivo. Genetic elimination of other panneuronally expressed Syt proteins did not alter these forms of exocytosis, indicating a non-Syt Ca²⁺ sensor functions for both facilitation and asynchronous release. In light of these findings, the presence of two presynaptic Ca²⁺ sensors can be placed in a biological context, a Syt1-based Ca²⁺ sensor devoted primarily to baseline synaptic transmission and a second non-Syt Ca²⁺ sensor for short-term synaptic plasticity and asynchronous release.