Control of Mast Cell Regulated Exocytosis by Munc13 Proteins

Abstract

Mast cells (MCs) are involved in pathogen defense and inflammatory reactions. Upon stimulation, they release substances stored in their granules via regulated exocytosis. In other cell types, Munc13 proteins play essential roles in regulated exocytosis. We found that MCs express Munc13-2 and -4, and we studied their roles using global and conditional knockout (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 histological evidence of defective MC degranulation, and not with changes in MC development, distribution, numbers or morphology. In vitro assays revealed that the defective MC response in the absence of Munc13-4 was limited to regulated exocytosis, leaving other MC secretory effector responses intact. Single cell capacitance measurements in MCs from mouse mutants with different expression levels of Munc13-4 in their MCs showed that as levels of Munc13-4 decrease, the rate of exocytosis declines first, and the total amount of exocytosis follows. A requirement for Munc13-2 in MC exocytosis was revealed only in the absence of Munc13-4. Electrophysiology and electron microscopy studies showed that the number of multigranular compound events (granule-to-granule homotypic fusion) was severely reduced in the absence of Munc13-4. We conclude that while 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 IgE-mediated anaphylactic response.