Title: Understanding mechanisms of beta cell susceptibility to type 1 diabetes
Description: Type 1 diabetes mellitus (T1D) is an autoimmune disease characterized by the inflammation of the insulin-producing pancreatic beta cells, eventually leading to beta cell loss and the inability to maintain glucose homeostasis. Understanding the mechanisms of beta cell-intrinsic factors that influence the maintenance of cellular defenses and contribute to cell death when deregulated will be crucial in efforts to treat or prevent beta cell loss in individuals who are prone to autoimmunity. Through my thesis work, I have investigated beta cell-specific etiologies of T1D through both a candidate-based approach using beta cell specific deletion of a susceptibility gene and an unbiased global exploration of beta cell factors that regulate the predisposition to insulitic injury. Protein tyrosine phosphatase N2 (PTPN2) is a T1D candidate gene that has been shown to be critical for modulating inflammation by regulating T cell activation. PTPN2 is also highly expressed in human and murine beta cells and it has been shown to be critical for beta cell function in vivo and inhibit inflammatory stimuli-mediated beta cell apoptosis in vitro, suggesting that PTPN2 mediated defense against inflammation is two pronged negative regulation of inflammatory immune cells and elevation of a beta cell intrinsic defense.

To examine whether PTPN2 regulates beta cell loss upon cytotoxic stimuli by bolstering beta cell defense mechanisms in vivo, I deleted PTPN2 in the beta cells (Ptpn2 beta-KO) and subjected the mice to the diabetogenic agent streptozotocin (STZ). Animals deficient in beta cell PTPN2 are more susceptible to STZ induced diabetes and have poor survival due to hyperglycemia. While investigating the mechanism of PTPN2-mediated beta cell defense, I have discovered that PTPN2 interacts with pyruvate kinase M2 (PKM2), a key metabolic enzyme that normally resides in the cytosol. In response to STZ, PKM2 translocates to the nuclei of diabetic beta cells, and the lack of PTPN2 results in the hyper-accumulation of nuclear PKM2, suggesting that PTPN2 mediates nuclear export of PKM2 in stressed beta cells. In the nucleus, PKM2 mediates the transcriptional activation of key proapototic genes, which is attenuated when I modulate nuclear PKM2 ex vivo, in effect reconstituting the function of PTPN2. Together, deregulation of PTPN2 mediated nuclear export of PKM2 leading to excessive transcriptional activation of proapoptotic genes may be the mechanism for exacerbated diabetes in the Ptpn2 beta KO mice.

To identify novel candidates that function in the beta cells to influence beta cell susceptibility to insulitic injury, I established RNA transcriptome and CpG dinucleotide methylome profiles of islets isolated from insulitis susceptible NOD and insulitis resistant NOR mice, prior to the onset of insulitis. Integrating these profiles with the genes nested in the human diabetic loci from the genome wide association studies, I identified several novel candidate genes that may be involved in T1D pathogenesis in a beta cell specific manner. Moreover, I also examined non CpG methylation, which appears to influence gene expression independently of CpG methylation. Collectively, my studies have expanded the understanding of beta cell-specific factors that regulate cellular defense to insulitis and may have expanded the therapeutic possibilities by implicating PKM2, inhibition of which is the focus of many cancer therapy research.