In a phenotype-driven, genetic screen to identify novel genes that affect ?-cell function, we found a mutation in Sec61a1 in a family of mice with diabetes. Sec61a1 encodes the protein conducting subunit of the endoplasmic reticulum (ER) translocation channel, which is responsible for protein import into the ER. Diabetes in Sec61a1 mutant mice is correlated with ?-cell apoptosis and massively distended ER. The distended ER is consistent with ER stress, an accumulation of unfolded proteins within the ER. The cell mounts a response to ER stress, the unfolded protein response, which includes up regulation of protein folding chaperones (e.g. Bip), and, in the case that ER stress cannot be relieved, up regulation of the proapoptotic transcription factor Chop. Bip and Chop are both up regulated in diabetic Sec61a1 mutant animals. Thus, our hypothesis is that Sec61a1 mutation leads to ER stress, ?-cell apoptosis, and diabetes. The experiments in this proposal are designed to test this hypothesis, to understand the mechanism by which Sec61a1 mutation leads to ER stress, and to ask if ER stress plays a role in the ?-cell dysfunction that is part of type 2 diabetes.
The development of type 2 diabetes (T2D) is a two-hit process involving insulin resistance in peripheral tissues and a failure of pancreatic ?-cells to compensate for insulin resistance by increased insulin secretion. Prospective studies in humans have established the causative role of both insulin resistance and ?-cell dysfunction in T2D. What is the nature of the ?-cell failure that accompanies T2D? Autopsy studies have shown that T2D is associated with a relative decrease in ?-cell mass due to apoptosis, a finding that has been supported in mouse models. There is evidence in the literature to support any of several mechanisms of ?-cell loss, including oxidative stress, glucotoxicity, lipotoxicity, and cytokine-induced cell death. In these studies we will examine the role of endoplasmic reticulum (ER) stress and ER stress-induced apoptosis in ?- cell dysfunction. Evidence for a role of ER stress in ?-cell death comes from human genetic syndromes marked by neonatal diabetes, and various mouse models including a novel mouse model of diabetes from our own lab. Via a forward genetic approach in the mouse, we have identified a family of diabetic mice with a mutation in the ER translocation channel protein Sec61a1. Preliminary data indicate that the mice become diabetic because they lose ?-cells beginning at about four weeks of age. This loss correlates with an increase in ER stress. The mechanism of ER stress likely relates to a defect in protein processing or degradation due to the Sec6a1 mutation. The studies described herein seek to exploit this and other mouse models of diabetes to answer some fundamental questions about the nature of ?-cell dysfunction in T2D. How does this mutation in Sec61a1 lead to ER stress, ?-cell apoptosis, and diabetes and does ER stress play a role in the ?- cell dysfunction normally seen in T2D? It is hoped that an increased understanding of ?-cell dysfunction will lead to more effective drugs against T2D.
