The winged helix transcription factor Foxc1a is essential for somitogenesis in zebrafish

Previous studies identified zebrafish foxc1a and foxc1b as homologs of the mouse forkhead gene, Foxc1. Both genes are transcribed in the unsegmented presomitic mesoderm (PSM), newly formed somites, adaxial cells, and head mesoderm. Here, we show that inhibiting synthesis of Foxc1a (but not Foxc1b) protein with two different morpholino antisense oligonucleotides blocks formation of morphological somites, segment boundaries, and segmented expression of genes normally transcribed in anterior and posterior somites and expression of paraxis implicated in somite epithelialization. Patterning of the anterior PSM is also affected, as judged by the absence of mesp-b, ephrinB2, and ephA4 expression, and the down-regulation of notch5 and notch6. In contrast, the expression of other genes, including mesp-a and papc, in the anterior of somite primordia, and the oscillating expression of deltaC and deltaD in the PSM appear normal. Nevertheless, this expression is apparently insufficient for the maturation of the presumptive somites to proceed to the stage when boundary formation occurs or for the maintenance of anterior/posterior patterning. Mouse embryos that are compound null mutants for Foxc1 and the closely related Foxc2 have no morphological somites and show abnormal expression of Notch signaling pathway genes in the anterior PSM. Therefore, zebrafish foxc1a plays an essential and conserved role in somite formation, regulating both the expression of paraxis and the A/P patterning of somite primordia.     

Pancreatic Islet Vasculature Adapts to Insulin Resistance Through Dilation and Not Angiogenesis

Pancreatic islets adapt to insulin resistance through a complex set of changes, including β-cell hyperplasia and hypertrophy. To determine if islet vascularization changes in response to insulin resistance, we investigated three independent models of insulin resistance: ob/ob, GLUT4^+/−, and mice with high-fat diet–induced obesity. Intravital blood vessel labeling and immunocytochemistry revealed a vascular plasticity in which islet vessel area was significantly increased, but intraislet vessel density was decreased as the result of insulin resistance. These vascular changes were independent of islet size and were only observed within the β-cell core but not in the islet periphery. Intraislet endothelial cell fenestration, proliferation, and islet angiogenic factor/receptor expression were unchanged in insulin-resistant compared with control mice, indicating that islet capillary expansion is mediated by dilation of preexisting vessels and not by angiogenesis. We propose that the islet capillary dilation is modulated by endothelial nitric oxide synthase via complementary signals derived from β-cells, parasympathetic nerves, and increased islet blood flow. These compensatory changes in islet vascularization may influence whether β-cells can adequately respond to insulin resistance and prevent the development of diabetes.Pancreatic islets are highly vascularized, and this feature is critical for β-cells to rapidly sense the blood glucose and secrete insulin into the systemic circulation (1,2). Islet vascularization begins early in pancreas development and is maintained in adulthood as a consequence of islet cell production of angiogenic factors such as vascular endothelial growth factor-A (VEGF-A) and angiopoietin-1 (Ang-1) (3–6). These factors recruit endothelial cells (ECs), stimulate blood vessel growth and maturation, and in the case of VEGF-A, promote formation of EC fenestrations (5,6). In addition, ECs adjacent to pancreatic epithelium reciprocally influence islet cell differentiation and development (7,8).β-Cells have a remarkable ability to respond to changes in an organism’s metabolic state, such as changes in the blood glucose or increased insulin requirements. For example, when insulin resistance develops, β-cells of the pancreatic islet can dramatically increase insulin production and secretion with an increase of β-cell mass, thus maintaining normoglycemia (9,10). In this way, mouse models with marked insulin resistance and humans with obesity-related insulin resistance are hyperinsulinemic but not hyperglycemic. The mechanisms underlying this β-cell adaptation to insulin resistance and their subsequent failure in some individuals who develop type 2 diabetes are incompletely understood.Because of the highly vascularized state of pancreatic islets and the marked changes in β-cell size and number in the setting of insulin resistance, we hypothesized that the islet vasculature must adapt to these changes in β-cell mass and insulin requirements. We envisioned that a hyperplastic islet, like a growing tumor mass, would increase production of angiogenic factors to increase its vascular supply with expanding β-cell mass (11). To test this hypothesis, we examined islet vascularization in three mouse models of insulin resistance and found, unexpectedly, that islet vessel density was decreased, not increased, and that the intraislet vasculature became markedly dilated whereas vessels in the exocrine tissue were unchanged. The dilation of intraislet capillaries was independent of islet size, suggesting the vascular adaptation may primarily support increased β-cell insulin secretory demand rather than β-cell mass expansion. Moreover, these vascular changes were accompanied by an increase in islet parasympathetic innervation. Our results indicate that the metabolic state influences islet angioarchitecture and innervation, suggesting that islet neurovascular remodeling may influence whether β-cells can adequately respond to insulin resistance and maintain normoglycemia.