[Show abstract][Hide abstract] ABSTRACT: Isl-1 is essential for the survival and ensuing differentiation of pancreatic endocrine progenitors. Isl-1 remains expressed in all adult pancreatic endocrine lineages; however, its specific function in the postnatal pancreas is unclear. Here we determine whether Isl-1 plays a distinct role in the postnatal β-cell by performing physiological and morphometric analyses of a tamoxifen-inducible, β-cell-specific Isl-1 loss of function mouse: Isl-1(L/L); Pdx1-CreER(Tm). Ablating Isl-1 in postnatal β-cells reduced glucose tolerance without significantly reducing β-cell mass or increasing β-cell apoptosis. Rather, islets from Isl-1(L/L); Pdx1-CreER(Tm) mice showed impaired insulin secretion. To identify direct targets of Isl-1, we integrated high-throughput gene expression and Isl-1 chromatin occupancy using islets from Isl-1(L/L); Pdx1-CreER(Tm) mice and βTC3 insulinoma cells, respectively. Ablating Isl-1 significantly affected the β-cell transcriptome, including known targets Insulin and MafA as well as novel targets Pdx1 and Slc2a2. Using ChIP-Seq and luciferase reporter assays we found that Isl-1 directly occupies functional regulatory elements of Pdx1 and Slc2a2. Thus, Isl-1 is essential for postnatal β-cell function, directly regulates Pdx1 and Slc2a2, and has a mature β-cell cistrome distinct from that of pancreatic endocrine progenitors.
[Show abstract][Hide abstract] ABSTRACT: All pancreatic endocrine cell types arise from a common endocrine precursor cell population, yet the molecular mechanisms that establish and maintain the unique gene expression programs of each endocrine cell lineage have remained largely elusive. Such knowledge would improve our ability to correctly program or reprogram cells to adopt specific endocrine fates. Here, we show that the transcription factor Nkx6.1 is both necessary and sufficient to specify insulin-producing beta cells. Heritable expression of Nkx6.1 in endocrine precursors of mice is sufficient to respecify non-beta endocrine precursors towards the beta cell lineage, while endocrine precursor- or beta cell-specific inactivation of Nkx6.1 converts beta cells to alternative endocrine lineages. Remaining insulin(+) cells in conditional Nkx6.1 mutants fail to express the beta cell transcription factors Pdx1 and MafA and ectopically express genes found in non-beta endocrine cells. By showing that Nkx6.1 binds to and represses the alpha cell determinant Arx, we identify Arx as a direct target of Nkx6.1. Moreover, we demonstrate that Nkx6.1 and the Arx activator Isl1 regulate Arx transcription antagonistically, thus establishing competition between Isl1 and Nkx6.1 as a critical mechanism for determining alpha versus beta cell identity. Our findings establish Nkx6.1 as a beta cell programming factor and demonstrate that repression of alternative lineage programs is a fundamental principle by which beta cells are specified and maintained. Given the lack of Nkx6.1 expression and aberrant activation of non-beta endocrine hormones in human embryonic stem cell (hESC)-derived insulin(+) cells, our study has significant implications for developing cell replacement therapies.
[Show abstract][Hide abstract] ABSTRACT: A decrease in the expression of Islet-1 (Isl-1), an islet transcription factor, has been reported in several physiological settings of reduced β-cell function. Here, we investigate whether an increased level of Isl-1 in islet cells can enhance β-cell function and/or mass. We demonstrate that transgenic mice with Isl-1 overexpression display improved glucose tolerance and enhanced insulin secretion without significant changes in β cell mass. From our microarray study, we identify approximately 135 differentially expressed genes in the islets of Isl-1 overexpressing mice that have been implicated to function in numerous biological processes including protein trafficking, metabolism and differentiation. Using real-time PCR we have confirmed upregulation of Caps2, Sec14l4, Slc2a10, P2rx7, Afamin, and Neurogenin 3 that may in part mediate the observed improved insulin secretion in Isl-1 overexpressing mice. These findings show for the first time that Isl-1 is a key factor in regulating adult β cell function in vivo, and suggest that Isl-1 elevation could be beneficial to improve glucose homeostasis.
[Show abstract][Hide abstract] ABSTRACT: Aristaless related homeodomain protein (Arx) specifies the formation of the pancreatic islet α-cell during development. This cell type produces glucagon, a major counteracting hormone to insulin in regulating glucose homeostasis in adults. However, little is known about the factors that regulate Arx transcription in the pancreas. In this study, we showed that the number of Arx(+) cells was significantly reduced in the pancreata of embryos deficient for the Islet-1 (Isl-1) transcription factor, which was also supported by the reduction in Arx mRNA levels. Chromatin immunoprecipitation analysis localized Isl-1 activator binding sites within two highly conserved noncoding regulatory regions (Re) in the Arx locus, termed Re1 (+5.6 to +6.1 kb) and Re2 (+23.6 to +24 kb). Using cell line-based transfection assays, we demonstrated that a Re1- and Re2-driven reporter was selectively activated in islet α-cells, a process mediated by Isl-1 in overexpression, knockdown, and site-directed mutation experiments. Moreover, Arx mRNA levels were up-regulated in islet α-cells upon Isl-1 overexpression in vivo. Isl-1 represents the first known activator of Arx transcription in α-cells, here established to be acting through the conserved Re1 and Re2 control domains.
[Show abstract][Hide abstract] ABSTRACT: The major role of glucagon is to promote hepatic gluconeogenesis and glycogenolysis to raise blood glucose levels during hypoglycemic conditions. Several animal models have been established to examine the in vivo function of glucagon in the liver through attenuation of glucagon via glucagon receptor knockout animals and pharmacological interventions. To investigate the consequences of glucagon loss to hepatic glucose production and glucose homeostasis, we derived mice with a pancreas specific ablation of the alpha-cell transcription factor, Arx, resulting in a complete loss of the glucagon-producing pancreatic alpha-cell. Using this model, we found that glucagon is not required for the general health of mice but is essential for total hepatic glucose production. Our data clarifies the importance of glucagon during the regulation of fasting and postprandial glucose homeostasis.