Formation of the Regulated Secretory Pathway: a View from the Cytosol

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Abstract The ability to release signaling molecules, such as peptide hormones, neuropeptides, and many growth factors, in response to an appropriate extracellular stimulus, is central to physiology, behavior, and development. The secretory vesicles mediating this regulated secretion are called large dense core vesicles (LDCVs). They bud from the trans-Golgi network (TGN) where their soluble cargo aggregates to form a dense core, but the cellular mechanisms, and in particular, the cytosolic machinery that produces these secretory vesicles are still not well understood. Recent published studies in C. Elegans have identified HID-1 as a cytosolic factor involved in neuropeptide sorting, but its function remains unknown. HID-1 is a peripheral membrane protein associated with the TGN and its expression is restricted to neuroendocrine cells, suggesting that it might contribute to LDCV biogenesis. Our preliminary loss-of-function experiments with rat neuroendocrine HID-1 knockout (KO) cells generated by genome-editing have confirmed that HID-1 is indeed required for mammalian neuroendocrine secretion. We will now 1) determine how HID-1 contributes to LDCV formation using a combination of biochemical and imaging assays, 2) establish how HID-1 targets specifically to the TGN, and 3) identify the mechanism regulating the reversible binding of HID-1 to the TGN and assess its significance for LDCV formation. These studies will provide invaluable information about the cell biology and biochemistry of a novel peripheral membrane protein involved in membrane trafficking with important implications for our understanding of neuroendocrine secretion.