Brothers and Sisters: Molecular Insights Into Arterial-Venous Heterogeneity

The molecular differences between arteries and veins are genetically predetermined and are evident even before the first embryonic heart beat. Although ephrinB2 and EphB4 are expressed in cells that will ultimately differentiate into arteries and veins respectively, many other genes have been shown to play a significant role in cell fate determination. The expression patterns of ephrinB2 and EphB4 are restricted to arterial-venous boundaries, and Eph/ephrin signaling provides repulsive cues at arterial-venous boundaries that are thought to prevent intermixing of arterial- and venous-fated cells. However, the maintenance of arterial-venous fate is susceptible to some degree of plasticity. Thus, in response to signals from the ambient microenvironment and shear stress, there is flow-mediated intercalation of the arteries and veins that ultimately leads to the formation of a functional, closed-loop circulation. In addition, cells in the blood vessels of each organ undergo epigenetic, morphologic and functional adaptive changes that are specific to the proximate function of their cognate organ(s). These adaptive changes result in an inter-organ and intra-organ vessel heterogeneity that manifest clinically in a disparate response of different organs to identical risk factors and injury in the same animal. In this review, we will focus on the molecular and physiologic factors influencing arterial-venous heterogeneity between and within different organ(s). We will explore arterial-venous differences in selected organs as well as their respective endothelial cell architectural organization that results in their inter- and intra-organ heterogeneity

p68RacGAP Is a Novel GTPase-activating Protein That Interacts with Vascular Endothelial Zinc Finger-1 and Modulates Endothelial Cell Capillary Formation

The endothelium is required for maintenance of vascular integrity and homeostasis during vascular development and in adulthood. However, little is known about the coordinated interplay between transcription factors and signaling molecules that regulate endothelial cell-dependent transcriptional events. Vascular endothelial zinc finger-1 (Vezf1) is a zinc finger-containing transcription factor that is specifically expressed within the endothelium during vascular development. We have previously shown that Vezf1 potently activates transcription of the endothelin-1 promoter. We now report the identification of p68RacGAP, a novel Vezf1-interacting 68-kDa RhoGAP domain-containing protein. p68RacGAP mRNA is highly expressed in vascular endothelial cells by Northern blot analysis, and immunohistochemical staining of adult mouse tissues identified p68RacGAP in endothelial cells, vascular smooth muscle cells, and epithelial cells in vivo. Rac1 and Vezf1 both bind avidly to p68RacGAP, suggesting that p68RacGAP is not only a GTPase-activating protein for Rac1 but that p68RacGAP may also be part of the protein complex that binds to and modulates Vezf1 transcriptional activity. Functionally p68RacGAP specifically activates the GTPase activity of Rac1 in vivo but not Cdc42 or RhoA. In addition, p68RacGAP potently inhibits Vezf1/DB1-mediated transcriptional activation of the human endothelin-1 promoter and modulates endothelial cell capillary tube formation. Taken together, these data suggest that p68RacGAP is a multifunctional regulatory protein that has a Rac1-specific GTPase-activating activity, regulates transcriptional activity of the endothelin-1 promoter, and is involved in the signal transduction pathway that regulates endothelial cell capillary tube formation during angiogenesis

BMPER, a Novel Endothelial Cell Precursor-Derived Protein, Antagonizes Bone Morphogenetic Protein Signaling and Endothelial Cell Differentiation

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