Identification of a Munc13-sensitive step in chromaffin cell large dense-core vesicle exocytosis

Mammals have adrenal glands, which secrete the stress hormone adrenaline as well as other hormones into the bloodstream. These molecules are produced in chromaffin cells, where they are packaged into compartments called large dense-core vesicles (LDCVs). To release the hormones into the bloodstream, the vesicles bind to and fuse with the membrane that surrounds the cell. This process – which is called exocytosis – is triggered by increases in the level of calcium ions inside the cells. Exocytosis also enables nerve cells to release chemical signals at junctions (known as synapses) with other nerve cells. These signals are packaged within another type of vesicle called 'synaptic' vesicles, which also release their contents by fusing with the cell membrane. However, it is not clear whether the two types of vesicle carry out exocytosis in the same way. Exocytosis requires that the vesicles physically attach to the membrane and undergo a process termed 'priming', which enables them to fuse quickly with the membrane in response to an increase in calcium ion levels. In synaptic vesicles, both of these processes – physical membrane attachment and priming – appear to occur in a single step that requires a family of proteins called the Munc13 proteins. Here, Man et al. investigate whether the Munc13 proteins are also essential for LDCV exocytosis in the chromaffin cells of mice. The experiments reveal that in contrast to synaptic vesicles, the initial binding of LDCVs to membranes does not require Munc13 proteins. However, the loss of one member of the family called Munc13-2 dramatically reduces the fusion of LDCVs with the membrane of chromaffin cells. Further experiments reveal that different Munc13 proteins differ in their ability to drive the exocytosis of LDCVs. Man et al. use a mathematical model of LDCV exocytosis, which reveals that Munc13 plays an important role in the first part of the priming step. Together, these findings show that synaptic vesicles and LDCVs use different mechanisms to bind to membranes, but are primed for fusion in a similar way.