Secretion and membrane fusion are fundamental cellular processes regulating ER-Golgi transport in protein maturation, plasma membrane recycling, cell division, sexual reproduction, acid secretion, and the release of enzymes, hormones and neurotransmitters, to name just a few. It is therefore no surprise that defects in secretion and membrane fusion give rise to diseases like diabetes, Alzheimer’s, Parkinson’s, acute gastroduodenal diseases, gastroesophageal reflux disease, intestinal infections due to inhibition of gastric acid secretion, biliary diseases resulting from malfunction of secretion from hepatocytes, polycystic ovarian disease as a result of altered gonadotropin secretion, and Gitelman disease associated with growth hormone deficiency and disturbances in vasopressin secretion, are only a few examples. Understanding cellular secretion and membrane fusion will therefore help not only to advance our understanding of these vital cellular and physiological processes, but in the development of drugs to help ameliorate secretory defects, provide insight into our understanding of cellular entry and exit of viruses and other pathogens, and in the development of smart drug delivery systems. Thus the role of secretion and membrane fusion in health and disease is profound. Studies in the last ten years reveal the molecular mechanism of secretion and membrane fusion in cells, which will be discussed in this article. Membrane-bound secretory vesicles containing their cargo, dock and fuse at specialized plasma membrane structures called porosomes. Secretory vesicles swell, resulting in a buildup of intravesicular pressure, which helps in expulsion of contents from within vesicles. The discovery of the porosome, its isolation, structure and dynamics, its biochemical composition and functional reconstitution, will be provided. Following docking of secretory vesicles at the base of porosomes, the molecular mechanism of fusion of the vesicle membrane at the porosome membrane, will be outlined. Finally, studies revealing the molecular mechanism of secretory vesicle swelling resulting in expulsion of vesicular contents, will be discussed. With these findings a new understanding of cell secretion has emerged, and confirmed by a number of laboratories.
Earlier electrophysiological studies on mast cells suggested the existence of fusion pores at the cell plasma membrane (PM), which became continuous with the secretory vesicle membrane after stimulation of secretion . Atomic force microscopy (AFM) has confirmed the existence of the fusion pore and its structure and dynamics in both exocrine [2,3] and neuroendocrine cells [4,5] at near nm resolution and in real time. Fusion pores in NG108-15 nerve cells have also been reported . Isolated live pancreatic acinar cells in physiological buffer, when imaged with the AFM [2,3], reveal at the apical PM a group of circular ‘pits’ measuring 0.4-1.2 μm in diameter which contain smaller ‘depressions’ (Fig. 1). Each depression averages between 100 and 150 nm in diameter, and typically 3-4 depressions are located within a pit. The basolateral membrane of acinar cells is however, devoid of either pits or depressions. High-resolution AFM images of depressions in live cells further reveal a cone-shaped morphology (Fig. 1). The depth of each depression cone measures 15-30 nm. Similarly, growth hormone (GH) secreting cells of the pituitary gland and chromaffin cells possess pits and depression structures at their PM [4,5], suggesting their universal presence in secretory cells. Exposure of pancreatic acinar cells to a secretagogue (mastoparan) results in a time-dependent increase (20-35%) in depression diameter, followed by a return to resting size on completion of secretion [2, 3] (Fig. 2). No demonstrable change in pit size is detected following stimulation of secretion . Enlargement of depression diameter and an increase in its relative depth after exposure to secretagoguescorrelated with increased secretion. Conversely, exposure of pancreatic acinar cells to cytochalasin B, a fungal toxin that inhibits actin polymerization, results in a 15-20% decrease in depression size and a consequent 50-60% loss in secretion . Results from these studies suggest that depressions are the fusion pores in pancreatic acinar cells. Furthermore, these studies demonstrate the involvement of actin in regulation of both the structure and function of depressions. Analogous to pancreatic acinar cells, examination of resting GH secreting cells of the pituitary  and chromaffin cells of the adrenal medulla  also reveal the presence of pits and depressions at the cell PM (Fig. 3). Depressions in resting GH cells measure 154 4.5 nm (mean ± SE) in diameter. Exposure of GH cells to a secretagogue results in a 40% increase in depression diameter (215 ± 4.6 nm; p Fig. 4). These studies confirm depressions to be the fusion pores or porosomes in pancreatic acinar cells where membrane-bound secretory vesicles dock and fuse to release vesicular contents. Similarly, in somatotrophs of the pituitary, gold-tagged growth hormone-specific antibody is found to selectively localize at depressions following stimulation of secretion , again identifying depressions in GH cells as fusion pores or porosomes. Furthermore, recent studies in the laboratory using both AFM and EM, reveal the presence of porosomes in neurons, β-cells of the endocrine pancreas, and in mast cells (Fig. 5 and 6, unpublished).