Role of Munc13 Isoforms in Regulating Large Dense Core Vesicle Exocytosis in Chromaffin Cells

Abstract

The process of vesicular exocytosis is a fast, tightly regulated and Ca2+-triggered event mediated by SNARE complex formation. Although it is known that factors like Munc18, Munc13 and CAPS are important in setting up the SNARE core fusion machinery in a process called priming, the underlying molecular mechanism is not well understood. Munc13 is of critical importance in priming synaptic vesicles (SVs) for release, and SV exocytosis in hippocampal synapses is completely shut down in the absence of Munc13-1 and Munc13-2, whereas in the case of synaptic dense core vesicles (DCVs) the synaptic preference of DCV release is lost. CAPS was originally identified as a factor which reconstitutes secretion in permeabilised neuroendocrine cells, and has since been recognised as important in regulated release of DCVs in C. elegans and large dense core vesicles (LDCVs) in neuroendocrine chromaffin cells, as well as in SV exocytosis in neurons. Although overexpression studies had also implicated Munc13s in LDCV release in chromaffin cells, in this cell type no LDCV release deficit had ever been demonstrated in their absence, and CAPSs proteins had been suggested to be the main regulators of LDCV exocytosis. To gain a more complete picture of potential differences in the regulation of SV and LDCV exocytosis, we investigated the role of different Munc13 isoforms in chromaffin cell LDCV exocytosis. This study is the first to report a deficit in chromaffin cell LDCV exocytosis in the absence of Munc13 isoforms. The ubiquitous (ub)Munc13-2 is the dominant isoform in murine chromaffin cells, and its deletion results in reductions of 60% of the fast burst component, of 52% of the slow burst component and of 72% of the sustained component, which is a more drastic reduction of release than in chromaffin cells of CAPS-deficient mice. Munc13-1 expression is low in perinatal adrenal glands, and its deletion alone did not result in significant changes in exocytosis, however its function in LDCV release became apparent in the absence of Munc13-2. Munc13-1 appears to mostly contribute to the slow burst and sustained component of release. By contrast, deletion of Baiap3, another Munc13 isoform with relatively high expression in adrenal medulla, did not lead to changes in LDCV exocytosis and its overexpression could not rescue the release deficit of Munc13-1/2-deficient cells. The remaining Munc13 isoforms, bMunc13-2, Munc13-3 and Munc13-4 are not expressed in perinatal adrenal glands and do not contribute to LDCV exocytosis in this cell type. Taken together, our findings show that ubMunc13-2 and Munc13-1 regulate LDCV exocytosis in chromaffin cells. Thus, at least in mammals, both Munc13s and CAPS proteins are critical in the regulation of both SV and LDCV exocytosis in neurons as well as in neuroendocrine cells.