Multiple forms of "kiss-and-run" exocytosis revealed by evanescent wave microscopy.
ABSTRACT Exocytotic release of neuropeptides and hormones is generally believed to involve the complete merger of the secretory vesicle with the plasma membrane. However, recent data have suggested that "kiss-and-run" mechanisms may also play a role. Here, we have examined the dynamics of exocytosis in pancreatic MIN6 beta cells by imaging lumen- (neuropeptide Y/pH-insensitive yellow fluorescent protein; NPY.Venus) or vesicle membrane-targeted fluorescent probes (synaptobrevin-2/enhanced green fluorescent protein; synapto.pHluorin, or phosphatase on the granule of insulinoma-enhanced green fluorescent protein, phogrin.EGFP) by evanescent wave microscopy. Unexpectedly, NPY.Venus release events occurred much less frequently (13%-40% maximal rate) than those of synapto.pHluorin, even though the latter molecule, but not phogrin.EGFP, usually diffused away from the site of fusion. Thus, the majority of exocytosis occurs in these cells by kiss-and-run events that involve either the release of small molecules only, small molecules and selected membrane proteins, or all soluble cargoes ("pure," "mixed," and "full" kiss-and-run, respectively). Changes in the activity of synaptotagmin IV, achieved here by overexpression of the wild-type protein, may allow different stimuli to alter the ratio of these events, and thus the release of selected vesicle cargoes.
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ABSTRACT: Pancreatic β cell dysfunction is pathognomonic of type 2 diabetes mellitus (T2DM) and is driven by environmental and genetic factors. β cell responses to glucose and to incretins such as glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are altered in the disease state. While rodent β cells act as a coordinated syncytium to drive insulin release, this property is unexplored in human islets. In situ imaging approaches were therefore used to monitor in real time the islet dynamics underlying hormone release. We found that GLP-1 and GIP recruit a highly coordinated subnetwork of β cells that are targeted by lipotoxicity to suppress insulin secretion. Donor BMI was negatively correlated with subpopulation responses to GLP-1, suggesting that this action of incretin contributes to functional β cell mass in vivo. Conversely, exposure of mice to a high-fat diet unveiled a role for incretin in maintaining coordinated islet activity, supporting the existence of species-specific strategies to maintain normoglycemia. These findings demonstrate that β cell connectedness is an inherent property of human islets that is likely to influence incretin-potentiated insulin secretion and may be perturbed by diabetogenic insults to disrupt glucose homeostasis in humans.The Journal of clinical investigation 09/2013; · 15.39 Impact Factor
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ABSTRACT: Glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells is critical to the maintenance of blood glucose homeostasis in animals. Both decrease in pancreatic beta-cell mass and defects in beta-cell function contribute to the onset of diabetes, although the underlying mechanisms remain largely unknown. Molecular imaging techniques can help beta-cell study in a number of ways. High-resolution fluorescence imaging techniques provide novel insights into the fundamental mechanisms underlying GSIS in isolated beta-cells or in situ in pancreatic islets, and dynamic changes of beta-cell mass and function can be noninvasively monitored in vivo by imaging techniques such as positron emission tomography and single-photon emission computed tomography. All these techniques will contribute to the better understanding of the progression of diabetes and the search for the optimized therapeutic measures that reverse deficits in beta-cell mass and function.Journal of Molecular Medicine 05/2013; · 4.77 Impact Factor
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ABSTRACT: The higher organization of β-cells into spheroid structures termed islets of Langerhans is critical for the proper regulation of insulin secretion. Thus, rodent β-cells form a functional syncytium that integrates and propagates information encoded by secretagogues, producing a "gain-of-function" in hormone release through the generation of coordinated cell-cell activity. By contrast, human islets possess divergent topology, and this may have repercussions for the cell-cell communication pathways that mediate the population dynamics underlying the intraislet regulation of insulin secretion. This is pertinent for type 2 diabetes mellitus pathogenesis, and its study in rodent models, because environmental and genetic factors may converge on these processes in a species-specific manner to precipitate the defective insulin secretion associated with glucose intolerance. The aim of the present minireview is therefore to discuss the structural and functional underpinnings that influence insulin secretion from human islets, and the possibility that dyscoordination between individual β-cells may play an important role in some forms of type 2 diabetes mellitus.Molecular Endocrinology 11/2013; · 4.75 Impact Factor