The mammalian pancreas is a mixed exocrine and endocrine gland that, in most species, arises from ventral and dorsal buds which subsequently merge to form the pancreas. In both mouse and rat the first histological sign of morphogenesis of the dorsal pancreas is a dorsal evagination of the duodenum at the level of the liver at around the 22-25-somite stage, and shortly thereafter a ventral evagination appears as a derivative of the liver diverticulum. Low levels of insulin gene transcripts are already present and restricted to the dorsal foregut endoderm at 20 somites, suggesting that pancreas- or insulin gene-specific transcriptional factors are present in this region before the onset of morphogenesis. Insulin-promoter-factor 1 (IPF1) is a homeodomain protein which, in the adult mouse pancreas, is selectively expressed in the beta-cells and binds to and transactivates the insulin promoter. In mouse embryos, IPF1 expression is restricted to the developing pancreatic anlagen and is initiated when the foregut endoderm is committed to a pancreatic fate. We now show that mice homozygous for a targeted mutation in the Ipf1 gene selectively lack a pancreas. The mutant pups survive fetal development but die within a few days after birth. The gastrointestinal part and all other internal organs were normal in appearance. No pancreatic tissue and no ectopic expression of insulin or pancreatic amylase could be detected in mutant embryos and neonates. These findings show that IPF1 is needed for the formation of the pancreas and suggest that it acts to determine the fate of common pancreatic precursor cells and/or to regulate their propagation.

The pancreas contains both exocrine and endocrine cells, but the molecular mechanisms controlling the differentiation of these cell types are largely unknown. Despite their endodermal origin, pancreatic endocrine cells share several molecular characteristics with neurons, and, like neurons in the central nervous system, differentiating endocrine cells in the pancreas appear in a scattered fashion within a field of progenitor cells. This indicates that they may be generated by lateral specification through Notch signalling. Here, to test this idea, we analysed pancreas development in mice genetically altered at several steps in the Notch signalling pathway. Mice deficient for Delta-like gene 1 (Dll1) or the intracellular mediator RBP-Jkappa showed accelerated differentiation of pancreatic endocrine cells. A similar phenotype was observed in mice over-expressing neurogenin 3 (ngn 3) or the intracellular form of Notch3 (a repressor of Notch signalling). These data provide evidence that ngn3 acts as proendocrine gene and that Notch signalling is critical for the decision between the endocrine and progenitor/exocrine fates in the developing pancreas.

To study the late ␤-cell-specific function of the homeodomain protein IPF1/PDX1 we have generated mice in which the Ipf1/Pdx1 gene has been disrupted specifically in ␤ cells. These mice develop diabetes with age, and we show that IPF1/PDX1 is required for maintaining the ␤ cell identity by positively regulating insulin and islet amyloid polypeptide expression and by repressing glucagon expression. We also provide evidence that IPF1/PDX1 regulates the expression of Glut2 in a dosage-dependent manner suggesting that lowered IPF1/ PDX1 activity may contribute to the development of type II diabetes by causing impaired expression of both Glut2 and insulin.

The mammalian pancreas is a specialized derivative of the primitive gut endoderm and controls many homeostatic functions through the activity of its component exocrine acinar and endocrine islet cells. The LIM homeodomain protein ISL1 is expressed in all classes of islet cells in the adult and its expression in the embryo is initiated soon after the islet cells have left the cell cycle. ISL1 is also expressed in mesenchymal cells that surround the dorsal but not ventral evagination of the gut endoderm, which together comprise the pancreatic anlagen. To define the role of ISL1 in the development of the pancreas, we have now analysed acinar and islet cell differentiation in mice deficient in ISL1 function. Dorsal pancreatic mesenchyme does not form in ISL1-mutant embryos and there is an associated failure of exocrine cell differentiation in the dorsal but not the ventral pancreas. There is also a complete loss of differentiated islet cells. Exocrine, but not endocrine, cell differentiation in the dorsal pancreas can be rescued in vitro by provision of mesenchyme derived from wild-type embryos. These results indicate that ISL1, by virtue of its requirement for the formation of dorsal mesenchyme, is necessary for the development of the dorsal exocrine pancreas, and also that ISL1 function in pancreatic endodermal cells is required for the generation of all endocrine islet cells.

OBJECTIVE-The G-protein-coupled receptor Gpr40 is expressed in ␤-cells where it contributes to free fatty acid (FFA) enhancement of glucose-stimulated insulin secretion (1-4). However, other sites of Gpr40 expression, including the intestine, have been suggested. The transcription factor IPF1/PDX1 was recently shown to bind to an enhancer element within the 5Ј-flanking region of Gpr40 (5), implying that IPF1/PDX1 might regulate Gpr40 expression. Here, we addressed whether 1) Gpr40 is expressed in the intestine and 2) Ipf1/Pdx1 function is required for Gpr40 expression. RESEARCH DESIGN AND METHODS-In the present study, Gpr40 expression was monitored by X-gal staining using Gpr40 reporter mice and by in situ hybridization. Ipf1/Pdx1-null and ␤-cell specific mutants were used to investigate whether Ipf1/ Pdx1 controls Gpr40 expression. Plasma insulin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucose levels in response to acute oral fat diet were determined in Gpr40 mutant and control mice.RESULTS-Here, we show that Gpr40 is expressed in endocrine cells of the gastrointestinal tract, including cells expressing the incretin hormones GLP-1 and GIP, and that Gpr40 mediates FFA-stimulated incretin secretion. We also show that Ipf1/Pdx1 is required for expression of Gpr40 in ␤-cells and endocrine cells of the anterior gastrointestinal tract.CONCLUSIONS-Together, our data provide evidence that Gpr40 modulates FFA-stimulated insulin secretion from ␤-cells not only directly but also indirectly via regulation of incretin secretion. Moreover, our data suggest a conserved role for Ipf1/Pdx1 and Gpr40 in FFA-mediated secretion of hormones that regulate glucose and overall energy homeostasis. Diabetes