Regulating Distinct Cell Lineages in the Pancreatic Islet

Type I and type II diabetes mellitus are associated with a loss of functioning insulin-producing β cells in the pancreas. Understanding the mechanism of normal islet and β cell development will be an important step in developing possible treatments for the disease. Nkx2.2 is essential for proper β cell differentiation. Nkx2.2 mice show a complete absence of insulin-producing β cells, a 90% reduction of glucagon-producing α cells, and an increase in ghrelin-producing cells. Nkx2.2 contains three conserved domains: the tinman domain (TN), homeodomain (HD), and NK2-specific domain (SD). The SD domain is highly conserved among Nk2 family members and across species. However, its function remains largely unknown. In order to further understand the molecular interactions involving Nkx2.2 in the developing mouse pancreas, we have generated a mouse line containing mutations in the NK2-SD domain. We show that SD mutant mice have a decrease in β cell numbers as well as a decrease in the β cell markers, NeuroD, Nkx6.1, Ins1 and Ins2. However, there is no change in α cell numbers or the α cell markers, Glucagon and Irx2. Unlike the persistent upregulation of Ghrelin in the Nkx2.2 mice, Nkx2.2SD/SD mice display a transient increase in Ghrelin expression, which normalizes by birth. Additionally, polyhormonal cells are seen as early as E12.5 and persist postnatally. Postnatally, the mice show morphological changes in islet size and the proximity of their islets to the ducts. Moreover, they show a continuing loss of β cells and the persistence of polyhormonal cells resulting in severe hyperglycemia. Mechanistically, Nkx2.2 has been shown to interact in a protein complex involving several methylation factors. We show that the SD domain is necessary for the interaction of Nkx2.2 and Dnmt1, the maintenance methyltransferase. We further show that there is a loss of methylation in the α cell gene Arx in sorted β cells of the Nkx2.2 SD/SD mice as well as global hypomethylation in the Nkx2.2 SD/SD mice. These data suggest that Nkx2.2 is responsible for proper methylation patters of islet specific genes in the developing pancreas, which is important for β cell development and the formation of normal islet cell identities