We report the detailed developmental expression profiles of three endothelial specific receptor tyrosine kinases (RTKs) flk-1, tek, tie, as well as vascular endothelial growth factor (VEGF), the flk-1 ligand. We also examined the expression of the other VEGF receptor, flt-1, during placental development. flk-1, tek, and tie transcripts were detected sequentially at one-half day intervals starting at E7.0, suggesting that each of these RTKs play a unique role during vascularization of the mouse embryo. All three RTKs were expressed in the extraembryonic and embryonic mesoderm in regions that eventually give rise to the vasculature. Except for the expression of tek and flk-1 in the mesoderm of the amnion, the expression of these RTKs from E8.5 onwards was virtually indistinguishable. An abundant amount of flt-1 transcripts was found in the spongiotrophoblast cells of the developing placenta from E8.0 onwards. This cellular compartment is located between the maternal and labyrinthine layers of the placenta, which both express VEGF. VEGF transcripts were detected as early as E7.0 in the endoderm juxtaposed to the flk-1 positive mesoderm, and later in development VEGF expression displayed an expression profile both contiguous with that of flk-1, and also in tissues found some distance from the flk-1-expressing endothelium. These results suggest a possible dual role for VEGF which includes a chemotactic and/or a cellular maintenance role for VEGF during vascularization of the mouse embryo. o 1995 Wiley-Liss, Inc.

SummaryIncreasing evidence suggests that loss of β cell characteristics may cause insulin secretory deficiency in diabetes, but the underlying mechanisms remain unclear. Here, we show that Rfx6, whose mutation leads to neonatal diabetes in humans, is essential to maintain key features of functionally mature β cells in mice. Rfx6 loss in adult β cells leads to glucose intolerance, impaired β cell glucose sensing, and defective insulin secretion. This is associated with reduced expression of core components of the insulin secretion pathway, including glucokinase, the Abcc8/SUR1 subunit of KATP channels and voltage-gated Ca2+ channels, which are direct targets of Rfx6. Moreover, Rfx6 contributes to the silencing of the vast majority of “disallowed” genes, a group usually specifically repressed in adult β cells, and thus to the maintenance of β cell maturity. These findings raise the possibility that changes in Rfx6 expression or activity may contribute to β cell failure in humans.