The hallmark of type 1 diabetes is the specific destruction of islet a-cells. Although an autoimmune mechanism has been suggested to underlie the loss of a-cells, the events that initiate and promote the death of beta-cells are still not fully understood. Therefore, a knowledge of the factors and mechanisms that regulate the apoptotic events and contribute to a-cell destruction are crucial to design experiments to prevent or reverse type 1 diabetes in a therapeutically efficient manner. This proposal seeks to explore the hypothesis that physiological apoptosis is one of the significant events that initiates the cascade of events that lead to beta-cell death. To test this hypothesis we propose to use a model of beta-cell IGF deficiency for our studies - the beta-cell-specific insulin receptor and IGF-1 receptor double knockout mouse, (betaDKO), that exhibits enhanced beta-cell apoptosis during the third week after birth that coincides with the weaning period. We propose to define the death signaling pathways that are mobilized before the onset of beta-cell apoptosis in the betaDKO mice. Furthermore, we plan to dissect the links between insulin/IGF-I signaling and pro-and antiapoptotic pathways involving the Bcl-2 family members using freshly isolated islets and clonal beta-cells derived from the double knockouts.
The Specific Aims of the study are to: 1) Evaluate the alterations in components of the Bcl-2 pathway in primary islets and clonal a-cell lines derived from a model of beta-cell IGF deficiency (betaDKO mouse) and investigate the susceptibility of the mutant beta-cells to apoptotic stimuli in vivo and in vitro; 2) Test the hypothesis that beta-cell over-expression of Bcl-xL in the aDKO mice will protect the mutant beta-cells from apoptosis; and 3) Dissect the pathways and molecular mechanisms that link insulin/IGF-I signaling with pro-and anti-apoptotic pathways with special reference to the Bcl-2 family members using primary islets and clonal beta-cell lines derived from betaDKO mice. We believe this application is extremely timely and innovative for this RFA for several reasons. First, the engagement of Dr. Korsmeyer, a leader in programmed cell death, will bring his extraordinary expertise and reagents, critical for the field of type 1 diabetes and will satisfy one of the major requirements of this RFA. Second, the availability of a model of IGF deficiency in islet beta-cells - the betaDKO mouse - provides us with a unique tool to evaluate the consequences of lack of survival signals on beta-cell death during the weaning period. Finally, the exciting finding from Dr. Korsmeyer and colleagues that the BH3-only member, BAD, forms a complex with glucokinase in mitochondria and is involved in the co-ordinate similar link operates in islets. This is especially significant in the light of findings from Dr. Kulkarni's lab of a potential role for insulin and/or IGF-I as anti-apoptotic factors that can also modulate the expression of glucokinase in beta-cells and the increasing significance of mitochondria in the pathogenesis of type 1 and type 2 diabetes.
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