STUDIES ON REGULATING MECHANISMS OF INTRACELLULAR MOVEMENT OF THE INSULIN GRANULES

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Prior to exocytosis, the secretory granules are synthesized, carried in the cytoplasm, and distributed in the vicinity of the plasma membrane. In the pancreatic beta cell, these regulatory steps upstream exocytosis have been mainly discussed on the basis of the measurement of insulin released into the extracellular space, and very few papers have been published on the direct measurement and quantification of these pre-exocytotic steps. Our study supported by this grant, demonstrated that the preexocytotic steps are crucial for the regulation of insulin release and, we demonstrated some controlling mechanisms characteristic for insulin release ; 1) intracellular movement of the insulin granules do not have any fixed direction as reported for the synaptic granules in neuronal cells, 2 ) myosin II (very recently reported to be myosin IIA by others) is a motor protein of the insulin granules, 3) intracellular movement of the insulin granule is activated by mobilized Ca2+ from internal stores (not Ca2+ influx) and cAMP, 4) the activation is via phosphorylation of myosin light chain by myosin light chain kinase, and via that of unknown substrate by protein kinase A.Furthermore, we found TPA-responsive protein kinase C is responsible for the granule docking to the plasma membrane. Our study demonstrated that second messengers and resultant protein phosphorylation are dividing works in the insulin secretory cascade. This contributes to investigation of phasic release of insulin by glucose, which is impaired in type 2 diabetes mellitus.