During pregnancy, the energy requirements of the fetus impose changes in maternal metabolism. Increasing insulin resistance in the mother maintains nutrient flow to the growing fetus, while prolactin and placental lactogen counterbalance this resistance and prevent maternal hyperglycemia by driving expansion of the maternal population of insulin-producing β-cells1–3. However, the exact mechanisms by which the lactogenic hormones drive β-cell expansion remain uncertain. Here we show that serotonin acts downstream of lactogen signaling to drive β-cell proliferation. Serotonin synthetic enzyme Tph1 and serotonin production increased sharply in β-cells during pregnancy or after treatment with lactogens in vitro. Inhibition of serotonin synthesis by dietary tryptophan restriction or Tph inhibition blocked β-cell expansion and induced glucose intolerance in pregnant mice without affecting insulin sensitivity. Expression of the Gαq-linked serotonin receptor Htr2b in maternal islets increased during pregnancy and normalized just prior to parturition, while expression of the Gαi-linked receptor Htr1d increased at the end of pregnancy and postpartum. Blocking Htr2b signaling in pregnant mice also blocked β-cell expansion and caused glucose intolerance. These studies reveal an integrated signaling pathway linking β-cell mass to anticipated insulin need during pregnancy. Modulators of this pathway, including medications and diet, may affect the risk of gestational diabetes4.

Insulin from the β-cells of the pancreatic islets of Langerhans controls energy homeostasis in vertebrates, and its deficiency causes diabetes mellitus. During embryonic development, the transcription factor Neurogenin3 initiates the differentiation of the β-cells and other islet cell types from pancreatic endoderm, but the genetic program that subsequently completes this differentiation remains incompletely understood. Here we show that the transcription factor Rfx6 directs islet cell differentiation downstream of Neurogenin3. Mice lacking Rfx6 failed to generate any of the normal islet cell types except for pancreatic-polypeptide-producing cells. In human infants with a similar autosomal recessive syndrome of neonatal diabetes, genetic mapping and subsequent sequencing identified mutations in the human RFX6 gene. These studies demonstrate a unique position for Rfx6 in the hierarchy of factors that coordinate pancreatic islet development in both mice and humans. Rfx6 could prove useful in efforts to generate β-cells for patients with diabetes.

OBJECTIVE-The generation of distinct cell types during the development of the pancreas depends on sequential changes in gene expression. We tested the hypothesis that microRNAs (miRNAs), which limit gene expression through posttranscriptional silencing, modulate the gene expression cascades involved in pancreas development.RESEARCH DESIGN AND METHODS-miRNAs were cloned and sequenced from developing pancreata, and expression of a subset of these genes was tested using locked nucleic acid in situ analyses. To assess the overall contribution of miRNAs to pancreatic development, Dicer1, an enzyme required for miRNA processing, was conditionally deleted from the developing pancreas.RESULTS-Sequencing of small RNAs identified over 125 miRNAs, including 18 novel sequences, with distinct expression domains within the developing pancreas. To test the developmental contribution of these miRNAs, we conditionally deleted the miRNA processing enzyme Dicer1 early in pancreas development. Dicer-null animals displayed gross defects in all pancreatic lineages, although the endocrine cells, and especially the insulin-producing ␤-cells, were most dramatically reduced. The endocrine defect was associated with an increase in the notchsignaling target Hes1 and a reduction in the formation of endocrine cell progenitors expressing the Hes1 target gene neurogenin3. CONCLUSIONS-The expression of a unique profile of miRNAs is required during pancreas development and is necessary for ␤-cell formation. Diabetes 56:2938-2945, 2007 P ancreatic organogenesis begins at embryonic day (e)9.5 in the mouse embryo with the budding of the dorsal anlagen from the prospective gut endoderm (1). Pancreatic duodenal homeobox-1 (Pdx-1) appears in the same area 1 day earlier and is expressed in all pancreas progenitors (2,3). As the pancreatic program continues, the expression of the basic helixloop-helix factor neurogenin3 initiates the differentiation of the endocrine cells. Neurogenin3 expression peaks coincidently with the "secondary transition," a time of rapid endocrine cell generation that occurs between e13.5 and e14.5 (4,5). The cell-specific factors Onecut1, Tcf1, Tcf2, and Sox9, as well as the Notch signaling pathway, regulate Neurog3 expression during development (5-9).Many tissues of the developing organism express microRNAs (miRNAs), which are small (ϳ20 nt) RNAs that mediate posttranscriptional gene silencing by interacting with the RNA-induced silencing complex and binding to the 3Ј untranslated region of their cognate messenger RNA targets (10,11). Here, we hypothesize that miRNAs are expressed in the developing pancreas, that they are necessary for the normal development of pancreatic ␤-cells, and that they may be involved in regulating genes important for normal pancreas morphogenesis. Ultimately, an intimate understanding of how miRNAs govern pancreas development may be necessary for stem cell-based therapies for all forms of diabetes. RESEARCH DESIGN AND METHODSMice were housed on a 12-h light/dark cycle in a controlled climate accordin...