Functional role of ATP binding to synapsin I in synaptic vesicle trafficking and release dynamics

Synapsins (Syns) are synaptic vesicle (SV)-associated proteins involved in the regulation of synaptic transmission and plasticity, which display a highly conserved ATP binding site in the central C-domain, whose functional role is unknown. Using molecular dynamics simulations, we demonstrated that ATP binding to SynI is mediated by a conformational transition of a flexible loop that opens to make the binding site accessible; such transition, prevented in the K269Q mutant, is not significantly affected in the absence of Ca2+ or by the E373K mutation that abolishes Ca2+ -binding. Indeed, the ATP binding to SynI also occurred under Ca2+ -free conditions and increased its association with purified rat SVs regardless of the presence of Ca2+ and promoted SynI oligomerization. However, although under Ca2+ -free conditions, SynI dimerization and SV clustering were enhanced, Ca2+ favored the formation of tetramers at the expense of dimers and did not affect SV clustering, indicating a role of Ca2+ -dependent dimer/tetramer transitions in the regulation of ATP-dependent SV clustering. To elucidate the role of ATP/SynI binding in synaptic physiology, mouse SynI knock-out hippocampal neurons were transduced with either wild-type or K269Q mutant SynI and inhibitory transmission was studied by patch-clamp and electron microscopy. K269Q-SynI expressing inhibitory synapses showed increased synaptic strength due to an increase in the release probability, an increased vulnerability to synaptic depression and a dysregulation of SV trafficking, when compared with wild-type SynI-expressing terminals. The results suggest that the ATP-SynI binding plays predocking and postdocking roles in the modulation of SV clustering and plasticity of inhibitory synapses.