The islet of Langerhans is the functional unit responsible for glucose-stimulated insulin secretion (GSIS), and thus plays a key role in blood glucose homeostasis. The importance of the islet is demonstrated by the proven ability of islet transplants to reverse Type I diabetes pathologies in human patients. Further, it has long been known that an islet yields -10 fold greater insulin response to glucose than an equivalent mass of isolated p-cells. The long-term goal of this project is to understand the multicellular mechanisms of islet function, and their role in the regulation of blood glucose under normal and pathological conditions. In many ways, the islet appears to function as a syncytium, which exhibits synchronous behavior of membrane action potentials, Ca2+ oscillations, and pulsatile insulin secretion across all p-cells in the islet. Despite the importance of the islet, we still have only a rudimentary understanding of the basic multicellular mechanisms that lead to these synchronous behaviors and to enhanced GSIS. The islet is also made up of different cell types, and very little is known about the interplay between the different cells. We hypothesize that gap junction coupling between fl cells in the islet leads to the synchronous behaviors, and this in turn, accounts for a majority of the increased insulin response from intact islets over isolated/?ce//s, although paracrine signaling between fi-cells and between the different cell types also plays a modulatory role. The validity and limits of this hypothesiswill be tested via three specific aims: 1) Determine the relative conductance strength of gap junction coupling between p-cells in terms of the KATP channel conductance;2) Determine the role of intra- and intercellular metabolism in the glucose-stimulated Ca2+ oscillations in p-cells within intact islets; and 3) Determine the metabolic, membrane potential, and Ca2+ dynamic changes of the a-cell in response to glucose, and the influences of p-cell activity on these responses. Using our unique quantitative optical imaging methods and novel microfluidic devices, the dynamics of these molecular mechanisms can be followed quantitatively in living cells within intact islets. These investigations will also utilize several available transgenic and tissue-specific knock-out mouse models with demonstrated phenotypes, as well as traditional biochemical and molecular biological approaches. Lay Summary: We propose to study the function of pancreatic islets, which contain the insulin-secreting p- cells. Two health-related motivations underlie the proposed work. First, a better understanding of pancreatic islet function is necessary to increase the success rate of islet transplants, which are a promising strategy to cure Type I diabetes. Second, understanding the cell-to-cell communication pathways may lead to new drugs that enhance insulin secretion for the treatment of Type II diabetes.
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