Synaptotagmin 7 differentially regulates synaptic vesicle trafficking at tonic and phasic synapses

Synaptic vesicle (SV) release probability can differ across neuronal subtypes, but the molecular mechanisms establishing such differences remain incompletely understood. In the current study we demonstrate that Synaptotagmin 7 (SYT7), a Ca²⁺-sensor regulating SV trafficking, is enriched at Drosophila tonic Ib synapses compared with phasic Is synapses. Loss of SYT7 preferentially enhanced evoked release at Ib terminals, whereas overexpression strongly suppressed fusion at both neuronal subtypes. High-frequency stimulation partially overcomes this inhibition, producing robust facilitation. Ultrastructural analysis revealed that SYT7 overexpression preserved morphologically docked SVs while promoting accumulation of enlarged presynaptic cisternae and redistribution of SV-associated proteins. Conversely, SYT7 loss reduced activity-induced presynaptic cisternae, suggesting an important role in regulating the bulk endocytosis pathway. Together, these findings identify differential SYT7 abundance as a contributor to tonic and phasic synaptic diversity and reveal a role for SYT7 in coordinating SV release and presynaptic membrane cycling.