Notch signaling regulates remodeling and vessel diameter in the extraembryonic yolk sac.

BACKGROUND: The signaling cascades that direct the morphological differentiation of the vascular system during early embryogenesis are not well defined. Several signaling pathways, including Notch and VEGF signaling, are critical for the formation of the vasculature in the mouse. To further understand the role of Notch signaling during endothelial differentiation and the genes regulated by this pathway, both loss-of-function and gain-of-function approaches were analyzed in vivo. RESULTS: Conditional transgenic models were used to expand and ablate Notch signaling in the early embryonic endothelium. Embryos with activated Notch1 signaling in the vasculature displayed a variety of defects, and died soon after E10.5. Most notably, the extraembryonic vasculature of the yolk sac displayed remodeling differentiation defects, with greatly enlarged lumens. These phenotypes were distinct from endothelial loss-of-function of RBPJ, a transcriptional regulator of Notch activity. Gene expression analysis of RNA isolated from the yolk sac endothelia of transgenic embryos indicated aberrant expression in a variety of genes in these models. In particular, a variety of secreted factors, including VEGF and TGF-ß family members, displayed coordinate expression defects in the loss-of-function and gain-of-function models. CONCLUSIONS: Morphological analyses of the in vivo models confirm and expand the understanding of Notch signaling in directing endothelial development, specifically in the regulation of vessel diameter in the intra- and extraembryonic vasculature. Expression analysis of these in vivo models suggests that the vascular differentiation defects may be due to the regulation of key genes through the Notch-RBPJ signaling axis. A number of these genes regulated by Notch signaling encode secreted factors, suggesting that Notch signaling may mediate remodeling and vessel diameter in the extraembryonic yolk sac via autocrine and paracrine cell communication. We propose a role for Notch signaling in elaborating the microenvironment of the nascent arteriole, suggesting novel regulatory connections between Notch signaling and other signaling pathways during endothelial differentiation.

Early mammalian erythropoiesis requires the Dot1L methyltransferase.

Histone methylation is an important regulator of gene expression; its coordinated activity is critical in complex developmental processes such as hematopoiesis. Disruptor of telomere silencing 1-like (DOT1L) is a unique histone methyltransferase that specifically methylates histone H3 at lysine 79. We analyzed Dot1L-mutant mice to determine influence of this enzyme on embryonic hematopoiesis. Mutant mice developed more slowly than wild-type embryos and died between embryonic days 10.5 and 13.5, displaying a striking anemia, especially apparent in small vessels of the yolk sac. Further, a severe, selective defect in erythroid, but not myeloid, differentiation was observed. Erythroid progenitors failed to develop normally, showing retarded progression through the cell cycle, accumulation during G0/G1 stage, and marked increase in apoptosis in response to erythroid growth factors. GATA2, a factor essential for early erythropoiesis, was significantly reduced in Dot1L-deficient cells, whereas expression of PU.1, a transcription factor that inhibits erythropoiesis and promotes myelopoiesis, was increased. These data suggest a model whereby DOT1L-dependent lysine 79 of histone H3 methylation serves as a critical regulator of a differentiation switch during early hematopoiesis, regulating steady-state levels of GATA2 and PU.1 transcription, thus controlling numbers of circulating erythroid and myeloid cells.