Analysis of glucose homeostasis and beta cell mass in a mouse model of type 2 diabetes deficient in the pro-apoptotic protein BIM
AffiliationMedicine (St Vincent's)
Document TypePhD thesis
Access StatusOpen Access
© 2017 Dr Jingjing Ge
Loss of beta cell mass is a feature of type 2 diabetes and evidence suggests that apoptosis contributes to this loss. Previously in our lab, we identified that the pro-apoptotic BH3-only proteins BIM and PUMA are required for glucose-induced apoptosis of islet cells in vitro. High concentrations of glucose induce ER and oxidative stress that activate the downstream intrinsic apoptosis of beta cells through a BIM and PUMA dependent pathway. Islets isolated from human pancreas donors had increased expression of BIM in type 2 diabetic patients compared to non-diabetic pancreas donors. This suggests that BIM may play an important role in the loss of beta cell mass in humans as well. In this project, my aim was to investigate the role of the pro-apoptotic molecule BIM in loss of beta cell mass in vivo in type 2 diabetes. I tested the hypothesis that inhibiting BIM in type 2 diabetes will enhance islet survival and improve glucose homeostasis. Chapter 2 describes the role of BIM in the type 2 diabetic Leprdb/db model by studying metabolism. Global deficiency of BIM significantly protected Leprdb/db mice by improving glucose homeostasis. Loss of BIM decreased non-fasting and fasting glucose, and improved insulin sensitivity and glucose tolerance in Leprdb/db mice. Previous data from our lab suggest that whole body BIM deletion results in increased insulin sensitivity in muscle, liver and adipose tissue. This is due to a role for BIM in mitochondrial function, and a preference for BIM-deficient cells to utilize lipids in metabolism. Therefore, to examine the effects of BIM deficiency on beta cell death, we generated mice with beta cell specific deficiency of BIM. In contrast to global knockouts, BIM deficiency only in the beta cells of Leprdb/db mice did not improve glucose homeostasis. These findings suggest that the improved metabolic phenotype of global BIM deficiency results from enhanced insulin sensitivity and not prevention of beta cell apoptosis. In chapter 3, the role of BIM in beta cell survival in type 2 diabetes was studied in Leprdb/db mice with global BIM deficiency or beta cell specific BIM deficiency. We observed a striking increase in islet size in BIM-deficient Leprdb/db mice by histology, while the beta-cell size was similar between Leprdb/dbBIM-/- and Leprdb/dbBIM+/+ mice. This suggests that the increase in islet size was due to an increase in the number of beta cells. Quantification of islet volume with optical projection tomography revealed a significant increase in islet volume in BIM-deficient mice compared with wild-type Leprdb/db mice. We detected a reduced number of TUNEL positive islets cells in BIM-deficient compared with wild type Leprdb/db islets. We also observed about 2-fold increase in proliferating beta cells in Leprdb/dbBIM-/- mice compared to that in Leprdb/dbBIM+/+ mice. However, there was no change in islet size in the Leprdb/db mice with beta cell specific deletion of BIM. These results indicate that BIM does not play a direct role in beta cell apoptosis in type 2 diabetic conditions, but its global inhibition generates a compensatory model by increasing beta cell mass, and this is likely due to the improved insulin sensitivity as a result of BIM inhibition. Last, to test the hypothesis that BIM inhibition contributes to improvement of glucose homeostasis and beta cell expansion through regulating insulin sensitivity, we generated a model with severe insulin resistance using the insulin antagonist S961. Impaired glucose homeostasis and beta cell expansion was comparable in BIM-deficient and wild-type controls after S961 treatment. This indicates that BIM inhibition could not contribute to glucose homeostasis or beta cell expansion in the total absence of insulin sensitivity. This finding supports the idea that BIM’s function in regulating diabetes is related to insulin sensitivity. Overall, my results in this thesis suggest that inhibition of BIM in beta cells only cannot prevent type 2 diabetes, however, global inhibition of BIM contributes to improvement of glucose homeostasis and beta cell expansion in type 2 diabetes through regulating insulin sensitivity.
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