Regulation of synaptic vesicle accumulation and axon terminal remodeling during synapse formation by distinct Ca2+ signaling

The synaptic vesicle accumulation and subsequent morphological remodeling of axon terminals are characteristic features of presynaptic differentiation of zebrafish olfactory sensory neurons. The synaptic vesicle accumulation and axon terminal remodeling are regulated by protein kinase A and calcineurin signaling, respectively. To investigate upstream signals of presynaptic differentiation, we focused on Ca<SU2+</SU signaling as Ca<SU2+</SU/calmodulin is required for the activation of both calcineurin and some adenylyl cyclases. We here showed that application of Ca<SU2+</SU/calmodulin inhibitor or olfactory sensory neuron-specific expression of calmodulin inhibitory peptide suppressed both synaptic vesicle accumulation and axon terminal remodeling. Thus, the trigger of presynaptic differentiation could be Ca<SU2+</SU release from intracellular stores or Ca<SU2+</SU influx. Application of a phospholipase C inhibitor or olfactory sensory neuron-specific expression of inositol 1,4,5-trisphosphate (IP3) 5-phosphatase suppressed synaptic vesicle accumulation, but not morphological remodeling. In contrast, application of a voltage-gated Ca<SU2+</SU channel blocker or expression of Kir2.1 inward rectifying potassium channel prevented the morphological remodeling. We also provided evidence that IP3 signaling acted upstream of protein kinase A signaling. Our results suggest that IP3-mediated Ca<SU2+</SU/calmodulin signaling stimulates synaptic vesicle accumulation and subsequent neuronal activity-dependent Ca<SU2+</SU/calmodulin signaling induces the morphological remodeling of axon terminals.