Abstract
Calcium release from intracellular stores influences synaptic response timing and magnitude. Despite the critical role of inositol trisphosphate (IP3)- and ryanodine receptor (RyR)-dependent calcium release in regulating synaptic strength, the upstream signaling mechanisms that govern IP3 receptor or RyR activity remain elusive. Here, we provide evidence that the ArfGAP-containing protein Asap modulates NMJ morphogenesis and synaptic calcium homeostasis by activating IP3-mediated calcium release from the endoplasmic reticulum (ER) via the phospholipase C-beta (PLCĪ²) signaling pathway. Using CRISPR/Cas9-engineered Asap mutants and genetically encoded calcium sensors, we demonstrate that loss of Asap leads to elevated resting synaptic calcium, resulting in increased evoked amplitude, elevated spontaneous miniature frequency, and reduced synaptic failures under low extracellular calcium conditions. Additional pharmacological and genetic manipulations of calcium regulatory pathways further support the role of increased resting intracellular calcium in driving enhanced neurotransmission in Asap-deficient synapses. Consistent with the role of Asap's ArfGAP domain in NMJ morphogenesis and intracellular calcium regulation, expressing a GDP-locked form of Arf6 (Arf6DN) or knocking down Arf6 in Asap mutants not only rescues Asap-associated synaptic defects but also normalizes synaptic calcium levels. Furthermore, epistatic analysis revealed that attenuation of IP3-signaling components in animals constitutively expressing Arf6CA normalized the NMJ morphological defects and synaptic functions. Together, these findings provide novel insights into the role of Asap-Arf6-PLCĪ² signaling in IP3-regulated calcium dynamics, sustaining both structural and functional synaptic plasticity.
