Paton, C. D.; Hoppa, M. B.

In neurons, the endoplasmic reticulum (ER) forms an extensive network that establishes membrane contact sites (MCSs) with various organelles including the plasma membrane (PM). While MCSs are known to regulate lipid exchange and Ca2+ signaling, their specific roles in synaptic transmission remain poorly understood. Here, we demonstrate that the ER resident proteins VAPA and VAPB are essential for organizing presynaptic Ca2+ exchange and mobilizing synaptic vesicles. We show that the loss of VAP impairs Ca2+ loading into both the ER and mitochondria during electrical activity. This regulation occurs primarily through VAP interactions with voltage-gated potassium channels (Kv2) at the PM. Our data suggest that the Kv2-VAP complex organizes presynaptic Ca2+ signaling outside of the active zone. Without this scaffold, synaptic vesicles become trapped in the reserve pool and fail to participate in exocytosis. These findings reveal a novel role for Kv2-VAP MCSs in coordinating organelle Ca2+ signaling and the synaptic vesicle cycle.