The Role of Microtubule Sliding in Regulation of Insulin Secretion

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Abstract Diabetes mellitus is a major metabolic disorder currently affecting 5–10% of the population in the western societies. In type-2 diabetes, which accounts for 90% of all diabetes, insulin is not released into the bloodstream in sufficient amounts. Insulin secretion is a function of pancreatic beta cells. Beta cells have to secrete restricted doses of insulin, in order to reduce blood sugar to normal levels but do not completely deplete it; this requires tight coordination between intracellular insulin storage and secretion. Our data indicate that this coordination is regulated by cytoskeletal polymers microtubules (MTs), which are known to serve as intracellular highways; molecular motors such as kinesin-1, move along MTs to transport and park membrane organelles and insulin granules at specific cellular locations. Our lab has shown that the dense MTs in pancreatic beta cells restrain insulin granules in “cages”, restricting insulin granule availability for glucose- stimulated insulin secretion. Previous studies have shown that targeted inactivation of kinesin-1 leads to hyperglycemia and the authors concluded that this is due to loss of trafficking of insulin by kinesin-1. Whether the MT network is remodeled and configured by this kinesin as well is not known and therefore may be part of the underlying hyperglycemic phenotype observed in the targeted inactivation of conventional kinesin in the pancreas. One possibility for resolution of these cages is through the active transport of MTs by a kinesin motor. Interestingly, I have found that glucose stimulation promotes the novel MT sliding process in beta cells. I hypothesize that MT sliding resolves “caged” insulin granules and/or impacts MT network configuration in pancreatic beta cells for their proper function. My specific aims are (1) to determine glucose-dependent pathways and mechanisms that facilitate MT sliding and (2) to elucidate the role of MT sliding in MT- dependent regulation of insulin release. I proposed to utilize high-end approaches for high- and super- resolution biological imaging, computational modeling, and various glucose secretion assays to achieve these aims. The discovery of MTs serving as regulatory components in beta cells is a new and exciting topic in the field.