Munc13-1 couples DAG and Ca ²⁺ signaling to dynamic vesicle priming, synaptic short-term plasticity, and posttetanic potentiation

Synaptic strength and plasticity are fine-tuned by neuromodulation and use-dependent second-messenger signaling. Presynaptic diacylglycerol (DAG), Ca²⁺, and Ca²⁺-calmodulin signaling converge on the essential synaptic vesicle (SV) priming protein Munc13-1 via its regulatory C₁, C₂B, and CaM-binding domains. Using brainstem-specific heterozygous mice expressing a DAG-binding-deficient Munc13-1 variant (Munc13-1^H567K), we compared synaptic transmission in situ at glutamatergic calyx of Held synapses carrying either a single Munc13-1^H567K or a single Munc13-1^wt allele. Munc13-1^H567K/- synapses show enhanced initial strength but impaired steady-state release and slower recovery from depression. These deficits result from an increased initial abundance of fully primed SVs and a loss of activity-dependent acceleration of SV priming. Posttetanic potentiation (PTP) is strongly reduced in Munc13-1^H567K/- synapses and either increased or attenuated by C₂B mutations that enhance or weaken Ca²⁺-phospholipid binding. Our data identify Munc13-1 as a target of presynaptic TrkB-phospholipase C-γ signaling and demonstrate that C₁ and C₂B domain-dependent regulation of Munc13-1 determines synaptic strength and shapes short-term plasticity and PTP.