Exocytosis in insulin secreting cells - role of SNARE-proteins
Abstract: Type 2 diabetes is marked by deterioration in pancreatic β-cell function. SNARE-proteins are crucial for the fusion of insulin granules with the plasma membrane, a prerequisite for insulin secretion. The aim of this thesis has been to investigate the exocytotic process in β-cells with a specific focus on the function and significance of the two SNARE-proteins SNAP-25 and syntaxin 1. SNAP-25 and syntaxin 1 was discovered to be situated in clusters along the plasma membrane. Immunoneutralization of syntaxin 1 and SNAP-25 resulted in a strongly reduced exocytotic response of primed granules in close association with the voltage-dependent calcium channels. Cholesterol is an essential component of the plasma membrane. Desorption of cholesterol from the plasma membrane in β-cells were accompanied with an overall reduction in the response of β-cells, from insulin secretion to exocytosis. We believe this is due to disturbance of a basic mechanism. Indeed, we found that SNAP-25 migrated from the plasma membrane out to the cytosol. The stimulating effect of cAMP on insulin secretion is implemented through different pathways. One pathway is through cAMP-GEFII and its downstream affector RIM2. We show that SNAP-25 binds to both cAMP-GEFII and RIM2, and that this binding mediates the effects of cAMP on exocytosis. The blind-drunk mouse carries a mutation in SNAP-25 that results in increased binding affinities within the SNARE-complex and the consequence in pancreatic β-cells are impaired vesicle recycling and granule exocytosis. These results together demonstrate the significance of a functional exocytotic machinery for β-cells to respond satisfying to an elevation in blood glucose.
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