Regulation of spontaneous neurotransmission and homeostatic synaptic plasticity by synaptotagmin-1 disease variants at the SNARE primary interface

<i>De novo</i> mutations in synaptotagmin-1 (syt1) cause a rare neurodevelopmental disorder, manifesting in global developmental delay, ophthalmic abnormalities, infantile hypotonia, facial dysmorphisms, absent speech, EEG abnormalities, and hyperkinetic movements, ranging from moderate to severe. Here, we evaluate eleven patient-relevant mutations spanning the Ca²⁺ binding domains of syt1-C2A and -C2B impact neurotransmission. We found that the mutation causing the most severe impact on neurotransmission, p.N341S, triggers aberrant spontaneous neurotransmission and occludes homeostatic synaptic plasticity signaling pathways. Our results suggest that potential phosphorylation of this newly introduced Ser residue underlies the functional change. A serine missense mutation creates a novel phosphorylation site as a broad spectrum protein kinase inhibitor rescues spontaneous neurotransmission. We identify key residues, localized to the primary interface between syt1 and SNAP-25, responsible for this shift in syt1 function in synaptic vesicle release. Substituting neutral amino acids at residue 341 alters the interaction of the Ser mutation, with double mutations in the surrounding amino acids in the primary interface rescuing synaptic function. These results provide a framework for how a syt1 point mutation introduces a substrate for phosphorylation and disrupts intermolecular interactions at the primary interface with SNAP-25 altering spontaneous neurotransmission and homeostatic plasticity.