Synapse and Active Zone Assembly in the Absence of Presynaptic Ca Channels and Ca Entry.

Presynaptic Ca2 channels are essential for Ca-triggered exocytosis. In addition, there are two competing models for their roles in synapse structure. First, Ca channels or Ca entry may control synapse assembly. Second, active zone proteins may scaffold Ca2s to presynaptic release sites, and synapse structure is Ca2 independent. Here, we ablated all three Ca2s using conditional knockout in cultured hippocampal neurons or at the calyx of Held, which abolished evoked exocytosis. Compellingly, synapse and active zone structure, vesicle docking, and transsynaptic nano-organization were unimpaired. Similarly, long-term blockade of action potentials and Ca entry did not disrupt active zone assembly. Although Ca2 knockout impaired the localization of β subunits, α2δ-1 localized normally. Rescue with Ca2 restored exocytosis, and Ca2 active zone targeting depended on the intracellular C-terminus. We conclude that synapse assembly is independent of Ca2s or Ca entry through them. Instead, active zone proteins recruit and anchor Ca2s via Ca2 C-termini.