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.

Bmper, which is orthologous to Drosophila melanogaster crossveinless 2, is a secreted factor that regulates Bmp activity in a tissue- and stage-dependent manner. Both pro- and anti-Bmp activities have been postulated for Bmper, although the molecular mechanisms through which Bmper affects Bmp signaling are unclear. In this paper, we demonstrate that as molar concentrations of Bmper exceed Bmp4, Bmper dynamically switches from an activator to an inhibitor of Bmp4 signaling. Inhibition of Bmp4 through a novel endocytic trap-and-sink mechanism leads to the efficient degradation of Bmper and Bmp4 by the lysosome. Bmper-mediated internalization of Bmp4 reduces the duration and magnitude of Bmp4-dependent Smad signaling. We also determined that Noggin and Gremlin, but not Chordin, trigger endocytosis of Bmps. This endocytic transport pathway expands the extracellular roles of selective Bmp modulators to include intracellular regulation. This dosage-dependent molecular switch resolves discordances among studies that examine how Bmper regulates Bmp activity and has broad implications for Bmp signal regulation by secreted mediators.

Abstract-Bone morphogenetic proteins (BMPs) are involved in embryonic and adult blood vessel formation in health and disease. BMPER (BMP endothelial cell precursor-derived regulator) is a differentially expressed protein in embryonic endothelial precursor cells. In earlier work, we found that BMPER interacts with BMPs and when overexpressed antagonizes their function in embryonic axis formation. In contrast, in a BMPER-deficient zebrafish model, BMPER behaves as a BMP agonist. Furthermore, lack of BMPER induces a vascular phenotype in zebrafish that is driven by disarray of the intersomitic vasculature. Here, we investigate the impact of BMPER on endothelial cell function and signaling and elucidate its role in BMP-4 function in gain-and loss-of-function models. As shown by Western blotting and immunocytochemistry, BMPER is an extracellular matrix protein expressed by endothelial cells in skin, heart, and lung. We show that BMPER is a downstream target of FoxO3a and consistently exerts activating effects on endothelial cell sprouting and migration in vitro and in vivo. Accordingly, when BMPER is depleted from endothelial cells, sprouting is impaired. In terms of BMPER related intracellular signaling, we show that BMPER is permissive and necessary for Smad 1/5 phosphorylation and induces Erk1/2 activation. Most interestingly, BMPER is necessary for BMP-4 to exert its activating role in endothelial function and to induce Smad 1/5 activation. Vice versa, BMP-4 is necessary for BMPER activity. Taken together, BMPER is a dose-dependent endothelial cell activator that plays a unique and pivotal role in fine-tuning BMP activity in angiogenesis. Key Words: BMPER Ⅲ bone morphogenetic proteins Ⅲ vascular biology Ⅲ endothelial cell function Ⅲ signaling A ngiogenesis is a basic biological event that is involved in embryonic development but also in adult physiological and pathological conditions, such as inflammation, tumor growth, atherosclerosis, or response to ischemia. This process depends on the orchestrated function of intra-and extracellular proteins, many of which are conserved from embryonic development through adulthood. 1 Bone morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)-␤ superfamily. Originally, they have been identified by their ability to induce ectopic bone formation and have been extensively studied during embryonic development, in which they control axis formation and organogenesis. Today, more than 20 BMP-related proteins and a number of BMP modulating proteins have been identified. 2 A growing body of evidence suggests that they serve as important regulators in vascular development and disease. 3 BMPs are extracellular proteins that signal through cell surface complexes of type I and type II serine/threonine kinase receptors. On activation, the receptors mediate intracellular signaling mainly through the Smad 1/5 transcription factors. BMP signaling is regulated at several levels: activity of R-Smads (1/5) is modulated by facilitating (eg, Smad 4) or inhibitory (eg,...

Background— The adaptive growth of blood vessels is an important protective mechanism in cardiovascular disease. However, the underlying regulatory mechanisms of this process are only partly understood. Recently, small endogenous RNAs (microRNAs [miRNAs]) were found to play an important role in embryonic and postnatal vascular development. Here, we used miRNA transcriptome analysis after induction of hind-limb ischemia in mice to screen for miRNAs involved in adaptive blood vessel growth following arterial occlusion. Methods and Results— Using miRNA arrays, we explored the miRNA expression profile during adaptive neovascularization. We describe specific changes in miRNA expression patterns and show that miRNA-100 is significantly downregulated after induction of hind-limb ischemia in mice. Our data demonstrate that miR-100 modulates proliferation, tube formation, and sprouting activity of endothelial cells and migration of vascular smooth muscle cells and functions as an endogenous repressor of the serine/threonine protein kinase mammalian target of rapamycin (mTOR). Whereas miR-100 inhibition increased mTOR levels in endothelial cells, overexpression of miR-100 reduced mTOR expression and consequently attenuated cellular proliferation. Supporting this notion, overexpression of an mTOR construct lacking the miRNA binding site rescued the inhibitory effect of miR-100 on cell proliferation. Accordingly, miR-100 inhibition by specific antagomirs in vivo stimulated angiogenesis and resulted in functional improvement of perfusion after femoral artery occlusion in mice. In contrast, treatment with the mTOR inhibitor rapamycin had the opposite effect. Conclusions— Our data demonstrate that miR-100 has an antiangiogenic function and represses mTOR signaling in endothelial and vascular smooth muscle cells. Inhibition of miR-100 could be a novel approach for the modulation of blood vessel growth and other mTOR-dependent processes.

We present the isolation and functional analysis of a transformer2 homologue Mdtra2 in the housefly Musca domestica. Compromising the activity of this gene by injecting dsRNA into embryos causes complete sex reversal of genotypically female individuals into fertile males, revealing an essential function of Mdtra2 in female development of the housefly. Mdtra2 is required for female-specific splicing of Musca doublesex (Mddsx) which structurally and functionally corresponds to Drosophila dsx, the bottom-most regulator in the sex-determining pathway. Since Mdtra2 is expressed in males and females, we propose that Mdtra2 serves as an essential co-factor of F, the key sex-determining switch upstream of Mddsx. We also provide evidence that Mdtra2 acts upstream as a positive regulator of F supporting genetic data which suggest that F relies on an autocatalytic activity to select and maintain the female path of development. We further show that repression of male courtship behavior by F requires Mdtra2. This function of F and Mdtra2 appears not to be mediated by Mddsx, suggesting that bifurcation of the pathway at this level is a conserved feature in the genetic architecture of Musca and Drosophila.

Endothelial cell migration is an important step during angiogenesis, and its dysregulation contributes to aberrant neovascularization. The bone morphogenetic proteins (BMPs) are potent stimulators of cell migration and angiogenesis. Using microarray analyses, we find that myosin-X (Myo10) is a BMP target gene. In endothelial cells, BMP6-induced Myo10 localizes in filopodia, and BMP-dependent filopodial assembly decreases when Myo10 expression is reduced. Likewise, cellular alignment and directional migration induced by BMP6 are Myo10 dependent. Surprisingly, we find that Myo10 and BMP6 receptor ALK6 colocalize in a BMP6-dependent fashion. ALK6 translocates into filopodia after BMP6 stimulation, and both ALK6 and Myo10 possess intrafilopodial motility. Additionally, Myo10 is required for BMP6-dependent Smad activation, indicating that in addition to its function in filopodial assembly, Myo10 also participates in a requisite amplification loop for BMP signaling. Our data indicate that Myo10 is required to guide endothelial migration toward BMP6 gradients via the regulation of filopodial function and amplification of BMP signals.

Summary. Histidine-rich glycoprotein (HRG) is a serum protein belonging to the cystatin superfamily. HRG may play a regulatory role in hemostasis and innate immunity. However, this role is uncertain because of a lack of rigorous testing in an animal model. We generated mice lacking the translation start point of exon 1 of the Hrg gene, effectively resulting in a null mutation (Hrg -/-). The mice were viable and fertile but had no HRG in their blood. Antithrombin activity in the plasma of Hrg -/-mice was higher than in the plasma of heterozygous Hrg +/-or wild-type Hrg +/+ mice. The prothrombin time was shorter in Hrg -/-mice than in Hrg +/-and Hrg +/+ mice. Bleeding time after tail tip amputation in Hrg -/-mice was shorter than in Hrg +/+ mice. The spontaneous fibrinolytic activity in clotted blood of Hrg -/-mice was higher than in Hrg +/+ mice. These findings suggest that HRG plays a role as both an anticoagulant and an antifibrinolytic modifier, and may regulate platelet function in vivo.

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