Bone morphogenetic protein and blood vessels: New insights into endothelial cell junction regulation

Purpose of reviewBMP signaling is an important regulator of vascular development and homeostasis, and perturbations of BMP pathway components are linked to vascular disease. However, until recently BMP's broad requirements in many developmental programs delayed cause-and-effect and mechanistic studies of its vascular role in vivo. This review covers recent findings that illuminate the role of BMP signaling in endothelial cells of blood vessels, and highlights effects of BMP signaling on endothelial cell junctions and vascular barrier function.Recent findingsBMP signaling in endothelial cells of blood vessels is context-dependent, and can either be pro-angiogenic and promote vascular sprouting, or antiangiogenic and promote vascular homeostasis. I discuss how distinct BMP signaling inputs impact blood vessel formation and function, with emphasis on new studies that investigate how BMP signaling affects endothelial cell junctions and vascular permeability.SummaryBMP signaling is important but complex in endothelial cells of blood vessels, with multiple distinct inputs leading to opposing cellular behaviors and phenotypic outputs in ways that are poorly understood. Endothelial cell-cell junctions are a target of BMP signaling, and junction stability can be tuned in either direction by BMP inputs. Several human diseases have perturbed junctions linked to BMP signaling changes

Endoglin moves and shapes endothelial cells

Vascular malformations result from improper blood vessel responses to molecular and mechanical signals. Two studies now show that endothelial cell migration and cell shape changes are perturbed in mutants lacking the TGFβ/BMP co-receptor endoglin, leading to arteriovenous shunts. Endoglin coordinates endothelial cell responses to ligand-receptor signalling and flow-mediated mechanical cues

New plugs for CCM bleeds

CCMs are central nervous system vascular lesions that affect ~1 in 200 humans. They are characterized by abnormal blood flow and hemorrhage that can lead to neurologic impairment, seizures, and stroke. Most CCM research has focused on the underlying genetic mutations and the cellular mechanisms leading to endothelial cell dysfunction, which results in the dilated and tortuous vessels that characterize the lesions. It has been assumed that hemorrhage results from defective endothelial cell junctions (ie, a leaky bag) and/or rupture as a result of fragile vessels and disrupted blood flow. An intriguing new study by Lopez-Ramirez et al shifts focus to the coagulant properties of endothelial cells and shows that CCM lesions have increased anticoagulant proteins that contribute to the bleeding phenotype and may provide novel therapeutic targets

Feeding cancer's sweet tooth: Specialized tumour vasculature shuttles glucose in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal neoplasm characterized by a 'fortress' of thick collagen fibres, abundant myofibroblasts, and paradoxically reduced vascularization compared to normal pancreas. Despite these features, PDAC shows no reduction in the uptake of glucose that fuels tumour cell survival. In new work published in The Journal of Pathology, Saiyin and colleagues have identified a novel adaptation of PDAC tumour endothelium; namely, 'hairy-like' basal microvilli that increase the total vascular surface area and correlate with regions of highest glucose uptake. Since basal microvilli are not present on normal pancreatic blood vessels, their presence may add diagnostic value and blocking their function is a potential new treatment strategy for PDAC. This novel finding of basal microvilli on PDAC endothelium is a striking example of how phenotypic plasticity in tumour blood vessels contributes to tumour growth and progression, independent of conventional modes of angiogenesis

The versatility and paradox of BMP signaling in endothelial cell behaviors and blood vessel function

Blood vessels expand via sprouting angiogenesis, and this process involves numerous endothelial cell behaviors, such as collective migration, proliferation, cell–cell junction rearrangements, and anastomosis and lumen formation. Subsequently, blood vessels remodel to form a hierarchical network that circulates blood and delivers oxygen and nutrients to tissue. During this time, endothelial cells become quiescent and form a barrier between blood and tissues that regulates transport of liquids and solutes. Bone morphogenetic protein (BMP) signaling regulates both proangiogenic and homeostatic endothelial cell behaviors as blood vessels form and mature. Almost 30 years ago, human pedigrees linked BMP signaling to diseases associated with blood vessel hemorrhage and shunts, and recent work greatly expanded our knowledge of the players and the effects of vascular BMP signaling. Despite these gains, there remain paradoxes and questions, especially with respect to how and where the different and opposing BMP signaling outputs are regulated. This review examines endothelial cell BMP signaling in vitro and in vivo and discusses the paradox of BMP signals that both destabilize and stabilize endothelial cell behaviors

Developmental SMAD6 loss leads to blood vessel hemorrhage and disrupted endothelial cell junctions

The BMP pathway regulates developmental processes including angiogenesis, yet its signaling outputs are complex and context-dependent. Recently, we showed that SMAD6, an intracellular BMP inhibitor expressed in endothelial cells, decreases vessel sprouting and branching both in vitro and in zebrafish. Genetic deletion of SMAD6 in mice results in poorly characterized cardiovascular defects and lethality. Here, we analyzed the effects of SMAD6 loss on vascular function during murine development. SMAD6 was expressed in a subset of blood vessels throughout development, primarily in arteries, while expression outside of the vasculature was largely confined to developing cardiac valves with no obvious embryonic phenotype. Mice deficient in SMAD6 died during late gestation and early stages of postnatal development, and this lethality was associated with vessel hemorrhage. Mice that survived past birth had increased branching and sprouting of developing postnatal retinal vessels and disorganized tight and adherens junctions. In vitro, knockdown of SMAD6 led to abnormal endothelial cell adherens junctions and increased VE-cadherin endocytosis, indicative of activated endothelium. Thus, SMAD6 is essential for proper blood vessel function during murine development, where it appears to stabilize endothelial junctions to prevent hemorrhage and aberrant angiogenesis

Excess centrosomes induce p53-dependent senescence without DNA damage in endothelial cells

Tumor blood vessels support tumor growth and progression. Centrosomes are microtubule organization centers in cells, and often up to 30% of tumor endothelial cells (ECs) acquire excess (>2) centrosomes. Although excess centrosomes can lead to aneuploidy and chromosome instability in tumor cells, how untransformed ECs respond to excess centrosomes is poorly understood. We found that the frequency of primary human ECs with excess centrosomes was quickly reduced in a p53-dependent manner. Excess centrosomes in ECs were associated with p53 phosphorylation at Ser33, increased p21 levels, and decreased cell proliferation and expression of senescence markers, but independent of DNA damage and apoptosis. Aspects of the senescence-associated phenotype were also observed in mouse ECs that were isolated from tumors with excess centrosomes. Primary ECs with excess centrosomes in vascular sprouts also had elevated Ser33 p53 phosphorylation and expressed senescence markers. Our work demonstrates that nontransformed ECs respond differently to excess centrosomes than do most tumor cells-they undergo senescence in vascular sprouts and vessels, which suggests that pathologic outcomes of centrosome overduplication depend on the transformation status of cells

Structure and evolution of the human IKBA gene

IκBα belongs to a gene family whose members are characterized by their 6-7 Ankyrin repeats, which allow them to interact with members of the Rel family of transcription factors. We have sequenced a human IκBα genomic clone to determine its gene structure. The human IκBα gene (IKBA) has six exons and five introns that span approximately 3.5 kb. This genomic organization is similar to that of other members of the Ankyrin gene family. The humanIKBAgene shares similar intron/exon boundaries with the humanBCL3andNFKB2genes, which is consistent with their conserved Ankyrin repeats. To examine further the evolutionary relationship between human IκBα and other members of its gene family, we performed a phylogenetic analysis. Although the resulting phylogenetic tree does not identify a common ancestor of the IκBα gene family, it indicates that this family diverges into two groups based on structure and function

Expression and inducibility of vascular adhesion receptors in development

ICAM-1 and VCAM are cell adhesion receptors expressed on vascular endothelial cells, and their expression is up-regulated as part of the inflammatory response. Tumor-derived Py-4-1 cells were a source of murine endothelial cells, and they showed increased lymphocyte binding when incubated with TNFα. Py-4-1 cells stimulated with TNFα or LPS also had elevated levels of ICAM-1 and VCAM RNAs, with maximum levels at 2 h. Developmental expression of adhesion receptors was assayed in murine yolk sacs and in cystic embryoid bodies (CEBs) that differentiate in vitro from murine embryonic stem cells. ICAM-1 and VCAM RNAs were expressed in unstimulated yolk sacs and CEBs. Expression was inducible in CEBs by exposure to LPS at all developmental stages, beginning at day 6. The time course of RNA induction in CEBs was similar to that of the Py-4-1 endothelial cell line. ICAM-1 protein expression was localized to vascular and hematopoietic blood islands in the CEBs. These results show that embryonic cells respond to inflammatory mediators by up-regulating expression of vascular adhesion receptors, and they imply an early ontogeny for the endothelial cell signal transduction pathway necessary for leukocyte recruitment

Blood Vessel Patterning on Retinal Astrocytes Requires Endothelial Flt-1 (VEGFR-1)

Feedback mechanisms are critical components of many pro-angiogenic signaling pathways that keep vessel growth within a functional range. The Vascular Endothelial Growth Factor-A (VEGF-A) pathway utilizes the decoy VEGF-A receptor Flt-1 to provide negative feedback regulation of VEGF-A signaling. In this study, we investigated how the genetic loss of flt-1 differentially affects the branching complexity of vascular networks in tissues despite similar effects on endothelial sprouting. We selectively ablated flt-1 in the post-natal retina and found that maximum induction of flt-1 loss resulted in alterations in endothelial sprouting and filopodial extension, ultimately yielding hyper-branched networks in the absence of changes in retinal astrocyte architecture. The mosaic deletion of flt-1 revealed that sprouting endothelial cells flanked by flt-1-/- regions of vasculature more extensively associated with underlying astrocytes and exhibited aberrant sprouting, independent of the tip cell genotype. Overall, our data support a model in which tissue patterning features, such as retinal astrocytes, integrate with flt-1-regulated angiogenic molecular and cellular mechanisms to yield optimal vessel patterning for a given tissue