No abstract

The development of endothelial cell precursors is essential for vasculogenesis. We screened for differentially expressed transcripts in endothelial cell precursors in developing mouse embryoid bodies. We cloned a complete cDNA encoding a protein that contains an amino-terminal signal peptide, five cysteine-rich domains, a von Willebrand D domain, and a trypsin inhibitor domain. We termed this protein BMPER (bone morphogenetic protein [BMP]-binding endothelial cell precursor-derived regulator). BMPER is specifically expressed in flk-1-positive cells and parallels the time course of flk-1 induction in these cells. In situ hybridization in mouse embryos demonstrates dorsal midline staining and staining of the aorto-gonadal-mesonephric region, which is known to host vascular precursor cells. BMPER is a secreted protein that directly interacts with BMP2, BMP4, and BMP6 and antagonizes BMP4-dependent Smad5 activation. In Xenopus embryos, ventral injection of BMPER mRNA results in axis duplication and downregulation of the expression of Xvent-1 (downstream target of Smad signaling). In an embryoid body differentiation assay, BMP4-dependent differentiation of endothelial cells in embryoid bodies is also antagonized by BMPER. Taken together, our data indicate that BMPER is a novel BMP-binding protein that is expressed by endothelial cell precursors, has BMP-antagonizing activity, and may play a role in endothelial cell differentiation by modulating local BMP activity.

SummaryTbx5 and Tbx20 act synergistically to control vertebrate heart morphogenesis

XB/U-cadherin is a maternal Xenopus cadherin which mediates interblastomere adhesion in early embryogenesis. In order to explore its role in gastrulation, we expressed a cytoplasmic deletion mutant of XB/U-cadherin (XB delta c38) under the control of the CMV promoter in Xenopus embryos. This truncated XB-cadherin fails to form complexes with catenins and does not mediate cell-cell aggregation as shown by transfection of mouse Ltk- cells. Injections of the deletion for XB/U-cadherin into the dorsal-marginal region of four cell stage embryos resulted in a dominant negative expression of the cadherin mutant after MBT. Two different phenotypes were observed in a dose dependent manner: high doses (125-250 pg DNA) led to severe distortions of the gastrulation movement. Involution of the mesoderm was impaired, posterior mesoderm migrated laterally around the blastopore and formed two bands of axial tissue. Low doses (up to 50 pg DNA) resulted in embryos of a posteriorized phenotype with disorganized neural structures. Both phenotypes could be rescued by coinjection of cDNA constructs containing wild-type XB/U-cadherin. Injections of constructs encoding a XB/U-cadherin protein truncated both in its extracellular and cytoplasmic domains yielded normal phenotypes. These results suggest that a proper function of XB/U-cadherin is essential for mesoderm movements during gastrulation.