Munc13 proteins control regulated exocytosis in mast cells

Mast cells (MCs) are involved in host defenses against pathogens and inflammation. Stimulated MCs release substances stored in their granules via regulated exocytosis. In other cell types, Munc13 (mammalian homolog of Caenorhabditis elegans uncoordinated gene 13) proteins play essential roles in regulated exocytosis. Here, we found that MCs express Munc13-2 and -4, and we studied their roles using global and conditional knock-out (KO) mice. In a model of systemic anaphylaxis, we found no difference between WT and Munc13-2 KO mice, but global and MC-specific Munc13-4 KO mice developed less hypothermia. This protection correlated with lower plasma histamine levels and with histological evidence of defective MC degranulation but not with changes in MC development, distribution, numbers, or morphology. In vitro assays revealed that the defective response in Munc13-4-deficient MCs was limited to regulated exocytosis, leaving other MC secretory effector responses intact. Single cell capacitance measurements in MCs from mouse mutants differing in Munc13-4 expression levels in their MCs revealed that as levels of Munc13-4 decrease, the rate of exocytosis declines first, and then the total amount of exocytosis decreases. A requirement for Munc13-2 in MC exocytosis was revealed only in the absence of Munc13-4. Electrophysiology and EM studies uncovered that the number of multigranular compound events (i.e. granule-to-granule homotypic fusion) was severely reduced in the absence of Munc13-4. We conclude that although Munc13-2 plays a minor role, Munc13-4 is essential for regulated exocytosis in MCs, and that this MC effector response is required for a full anaphylactic response.