Differentiation Of Insulin-producing Beta-cells Research Articles

Characterization and regulation of the hb9/mnx1 beta-cell progenitor specific enhancer in zebrafish

Differentiation of insulin producing beta-cells is a genetically well defined process that involves functions of various conserved transcription factors. Still, the transcriptional mechanisms underlying specification and determination of beta-cell fate are poorly defined. Here we provide the description of a beta-cell progenitor specific enhancer as a model to study initial steps of beta-cell differentiation. We show that evolutionary non-conserved upstream sequences of the zebrafish hb9 gene are required and sufficient for regulating expression in beta-cells prior to the onset of insulin expression. This enhancer contains binding sites for paired-box transcription factors and two E-boxes that in EMSA studies show interaction with Pax6b and NeuroD, respectively. We show that Pax6b is a potent activator of endodermal hb9 expression and that this activation depends on the beta-cell enhancer. Using genetic approaches we show that pax6b is crucial for maintenance but not induction of pancreatic hb9 transcription. As loss of Pax6b or Hb9 independently results in the loss of insulin expression, the data reveal a novel cross-talk between the two essential regulators of early beta-cell differentiation. While we find that the known pancreatic E-box binding proteins NeuroD and Ngn3 are not required for hb9 expression we also show that removal of both E-boxes selectively eliminates pancreatic specific reporter expression. The data provide evidence for an Ngn3 independent pathway of beta-cell specification that requires function of currently not specified E-box binding factors.

Immunotherapy of Type 1 Diabetes: Where Are We and Where Should We Be Going?

Type 1 diabetes (T1D) is a chronic autoimmune disorder characterized by destruction of insulin-producing pancreatic beta cells. Many broad-based immunosuppressive and antigen-specific immunoregulatory therapies have been and are currently being evaluated for their utility in the prevention and treatment of T1D. Looking forward, this review discusses the potential therapeutic use of antigen-specific tolerance strategies, including tolerance induced by "tolerogenic" antigen-presenting cells pulsed with diabetogenic antigens and transfer of induced or expanded regulatory T cells, which have demonstrated efficacy in nonobese diabetic (NOD) mice. Depending on the time of therapeutic intervention in the T1D disease process, antigen-specific immunoregulatory strategies may be employed as monotherapies, or in combination with short-term tolerance-promoting immunoregulatory drugs and/or drugs promoting differentiation of insulin-producing beta cells from endogenous progenitors.

Pdx1 Level Defines Pancreatic Gene Expression Pattern and Cell Lineage Differentiation

The absence of Pdx1 and the expression of brain-4 distinguish alpha-cells from other pancreatic endocrine cell lineages. To define the transcription factor responsible for pancreatic cell differentiation, we employed the reverse tetracycline-dependent transactivator system in INS-I cell-derived subclones INSralphabeta and INSrbeta to achieve tightly controlled and conditional expression of wild type Pdx1 or its dominant-negative mutant, as well as brain-4. INSralphabeta cells express not only insulin but also glucagon and brain-4, while INSrbeta cells express only insulin. Overexpression of Pdx1 eliminated glucagon mRNA and protein in INSralphabeta cells and promoted the expression of beta-cell-specific genes in INSrbeta cells. Induction of dominant-negative Pdx1 in INSralphabeta cells resulted in differentiation of insulin-producing beta-cells into glucagon-containing alpha-cells without altering brain4 expression. Loss of Pdx1 function alone in INSrbeta cells, which do not express endogenous brain-4 and glucagon, was also sufficient to abolish the expression of genes restricted to beta-cells and to cause alpha-cell differentiation. In contrast, induction of brain-4 in INSrbeta cells initiated detectable expression of glucagon but did not affect beta-cell-specific gene expression. In conclusion, Pdx1 confers the expression of pancreatic beta-cell-specific genes, such as genes encoding insulin, islet amyloid polypeptide, Glut2, and Nkx6.1. Pdx1 defines pancreatic cell lineage differentiation. Loss of Pdx1 function rather than expression of brain4 is a prerequisite for alpha-cell differentiation.

The Pax4 gene is essential for differentiation of insulin-producing beta cells in the mammalian pancreas.

The mammalian pancreas contains two distinct cell populations: endocrine cells which secrete hormones into the bloodstream, and exocrine cells, which secrete enzymes into the digestive tract. The four endocrine cell types found in the adult pancreas-(alpha, beta, delta and PP-synthesize glucagon, insulin, somatostatin and pancreatic polypeptide, respectively. All of these endocrine cells arise from common multipotent precursors, which coexpress several hormones when they start to differentiate. Expression of some homeobox genes in the early developing pancreas has been reported. The Pax4 gene is expressed in the early pancreas, but is later restricted to beta cells. Inactivation of Pax4 by homologous recombination results in the absence of mature insulin- and somatostatin-producing cells (beta and delta, respectively) in the pancreas of Pax4 homozygous mutant mice, but glucagon-producing alpha cells are present in considerably higher numbers. We propose that the early expression of Pax4 in a subset of endocrine progenitors is essential for the differentiation of the beta and delta cell lineages. A default pathway would explain the elevated number of alpha cells in the absence of Pax4.