Glucose homeostasis is achieved by the coordinated efforts of insulin secretion from pancreatic islets and insulin action-glucose uptake by skeletal muscle and adipose tissues. Both processes are controlled by SNARE-mediated exocytosis events, and when these events go awry, Type 2 diabetes ensues. The t-SNARE protein, Syntaxin 4, and its regulatory binding partner, Munc18c, are now recognized as essential lcomponents of the exocytic mechanisms for both processes, although the detailed molecular mechanisms remain unresolved. The long term goal here is to understand the molecular, cellular and physiological basis for the dysregulation of glucose homeostasis leading to the onset of Type 2 diabetes. Towards this goal, published and preliminary data described herein reveal the beginnings of a detailed 'switch mechanism'conserved in beta cells and adipocytes involving the stimulus-induced phosphorylation of Munc18c and its subsequent preference for binding to a new partner, Doc2b. The objective of this application is to delineate the molecular mechanisms by which Munc18 and Doc2b proteins facilitate regulated exocytosis in insulinsecreting and insulin-responsive cell types to control glucose homeostasis. The central hypothesis is that Munc18c functions as the key molecular switch to transiently activate Syntaxin 4 and to promote vesicle delivery events in a stimulus-induced fashion. Supporting this are new findings which show: a) Requirement for Munc18c in a new pre-docking step of exocytosis termed 'syntaxin activation';b) Candidate Munc18c modifying factors to catalyze its tyrosine-phosphorylation present in adipocytes and in beta cells;c) New Munc18c binding factors involved in cytoskeletal remodeling and granule delivery/positioning. This hypothesis will be tested in three specific aims: 1) Determine the requirement for Munc18c-Doc2b interaction in regulating GLUT4 translocation and whole body glucose homeostasis;2) Delineate the role of Munc18-Doc2b protein complexes in biphasic insulin secretion;3) Elucidate the mechanisms by which Munc18c functions in Syntaxin 4-activation and in granule delivery/mobilization processes to facilitate insulin exocytosis. Studies will be accomplished using Doc2b and Munc18 knockout mice for whole body analyses of homeostasis and tissues there from for tissue-specific effects upon biphasic insulin release (islet perifusion) and skeletal muscle glucose uptake (GLUT4 translocation). MIN6 beta cells and 3T3L1 adipocytes will be utilized for molecular/biochemical dissection of spatial and temporal changes in proteinprotein interactions coupled with visualization of dynamic changes in granule/vesicle exocytosis events using microscopy and biochemical subcellular fractionation analyses. Gaining knowledge of how Munc18c regulates these exocytic events will mark progress towards the long-term goal of modulating, perhaps simultaneously, both insulin secretion and insulin-stimulated glucose uptake in order to improve glucose homeostasis in the patient.
Type 2 diabetes has been coined a 'two-hit'disease;one 'hit'is dysfunction of glucose clearance by the skeletal muscle and adipose tissues, and another 'hit'is dysfunction of insulin secretion by the pancreatic islet beta cells. Aberrant abundance and function of the Munc18c protein and its binding partners have been reported in diabetic human muscle and islets and therefore have the potential to mediate cross-talk and underlie both 'hits'. Our research using novel rodent models supports an important and required role for Munc18c, and now it is imperative that the detailed molecular mechanisms involving Munc18c in these processes in skeletal muscle, adipose and islets be elucidated. Conservation of Munc18c-based mechanisms carries great potential for development of novel therapeutic strategies that could simultaneously tackle both 'hits'of this disease to improve the livelihood of people with diabetes.
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