Synapse and Active Zone Assembly in the Absence of Presynaptic Ca2+ Channels and Ca2+ Entry

Presynaptic CaV2 channels are essential for Ca2+-triggered exocytosis. In addition, there are two competing models for their roles in synapse structure. First, Ca2+ channels or Ca2+ entry may control synapse assembly. Second, active zone proteins may scaffold CaV2s to presynaptic release sites, and synapse structure is CaV2 independent. Here, we ablated all three CaV2s 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 Ca2+ entry did not disrupt active zone assembly. Although CaV2 knockout impaired the localization of β subunits, α2δ-1 localized normally. Rescue with CaV2 restored exocytosis, and CaV2 active zone targeting depended on the intracellular C-terminus. We conclude that synapse assembly is independent of CaV2s or Ca2+ entry through them. Instead, active zone proteins recruit and anchor CaV2s via CaV2 C-termini. [Display omitted] •CaV2s mediate evoked synaptic vesicle release and cannot be replaced by CaV1 or CaV3•Active zone nano-assemblies and docking persist after removing CaV2s or Ca2+ entry•α2δ-1 localization within nerve terminals is broad and does not require CaV2s•CaV2 C-termini and their binding to active zone proteins mediate CaV2 targeting CaV2 channels provide Ca2+ for triggering neurotransmitter release at central synapses. Held et al. remove all CaV2s and Ca2+ entry and find that synaptic protein nano-assemblies persist and synapse ultrastructure is unimpaired despite abolishing synaptic transmission. Rescue experiments establish that CaV2 C-termini are important for channel anchoring at active zones.