Insulin granule dynamics in pancreatic beta cells

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The mechanism of impaired insulin secretion in type 2 diabetes (TTDM) is poorly understood. In health, approximately 75 percent of secreted insulin is released in discrete pulses with a periodicity of approximately 6 minutes, and modulation of the magnitude of these pulses serves to regulate the insulin secretion rate. In patients with TTDM, the rate of insulin secretion is impaired by a selective reduction of the pulse mass (amount of insulin released during a pulse), not pulse frequency. In addition, in TTDM first phase insulin secretion in response to a glucose bolus is impaired. These deficits are present even though there appears to be abundant stored insulin in the islets of patients with TTDM. The incretin hormone glucagon-like peptide-1 restores pulsatility and first phase secretion. Taken together these observations suggest that the number of insulin granules available for rapid discharge in a discrete insulin pulse or first phase secretion is deficient in TTDM. Our overall hypothesis for the present studies is that the mechanisms of impaired insulin secretion in TTDM is a decrease in the readily releasable pool of insulin granules. Three specific aims test this overall hypothesis: Aim 1: Test the hypothesis that impaired insulin secretion in TTDM is due to a reduction in the size of the readily releasable pool of insulin granules. Aim 2: Test the hypothesis that the mechanism leading to this deficit is insufficient docking of granules from the reserve pool. Aim 3: Test the hypothesis that the readily releasable pool and insulin secretion can be restored by agents that enhance granules docking and/or inhibit undocking. We will use the method of total internal reflection fluorescence microscopy (TIRFM), a method that enables the visualization of individual granules near the plasma membrane within living secretory cells. We are well positioned to address these hypotheses with the following resources: (1) A fully established apparatus for TIRFM. (2) Access to a number of rodent models of TTDM, including GK and ZDF rats and a transgenic rat model in which human IAPP is expressed. (3) Access to human islets. (4) The support of the USC Diabetes Research Center.