RIM and MUNC13 membrane-binding domains are essential for neuropeptide secretion.

Abstract

Neurons release neurotransmitters from synaptic vesicles (SVs) and neuropeptides from dense-core vesicles (DCVs). The presynaptic proteins RIM and MUNC13 play key roles in both pathways. It remains unclear how DCVs are targeted to release sites and whether RIM and MUNC13 are involved in this process. Here, we show that three membrane-binding domains in RIM and MUNC13 regulate DCV exocytosis differently from SV exocytosis. Using neuropeptide secretion assays with single-vesicle resolution and peptidomics analysis of endogenous neuropeptide release in MUNC13/RIM null neurons, we demonstrate that MUNC13 is essential for DCV exocytosis. The RIM N terminus prevents MUNC13 degradation via the proteasome, and inhibiting proteasomal degradation partially rescues DCV exocytosis in RIM's absence. Unlike SV exocytosis, the PIP2-binding RIM C2B domain and MUNC13 C1-C2B polybasic face are redundant for DCV exocytosis, while the lipid-binding MUNC13 C2C domain is crucial. These results show that RIM and MUNC13 synergistically regulate DCV exocytosis through membrane interactions and reveal new mechanistic differences between SV and DCV exocytosis.