Beta1 integrin establishes endothelial cell polarity and arteriolar lumen formation via a Par3-dependent mechanism

Controlled activation of morphogenesis to generate a functional human microvasculature in a synthetic matrix

Understanding the role of the extracellular matrix (ECM) in vascular morphogenesis has been possible using natural ECMs as in vitro models to study the underlying molecular mechanisms. However, little is known about vascular morphogenesis in synthetic matrices where properties can be tuned toward both the basic understanding of tubulogenesis in modular environments and as a clinically relevant alternative to natural materials for regenerative medicine. We investigated synthetic, tunable hyaluronic acid (HA) hydrogels and determined both the adhesion and degradation parameters that enable human endothelial colony-forming cells (ECFCs) to form efficient vascular networks. Entrapped ECFCs underwent tubulogenesis dependent on the cellular interactions with the HA hydrogel during each stage of vascular morphogenesis. Vacuole and lumen formed through integrins α(5)β(1) and α(V)β(3), while branching and sprouting were enabled by HA hydrogel degradation. Vascular networks formed within HA hydrogels containing ECFCs anastomosed with the host's circulation and supported blood flow in the hydrogel after transplantation. Collectively, we show that the signaling pathways of vascular morphogenesis of ECFCs can be precisely regulated in a synthetic matrix, resulting in a functional microvasculature useful for the study of 3-dimensional vascular biology and toward a range of vascular disorders and approaches in tissue regeneration.

Blood vessel tubulogenesis requires Rasip1 regulation of GTPase signaling

Cardiovascular function depends on patent blood vessel formation by endothelial cells (ECs). However, the mechanisms underlying vascular "tubulogenesis" are only beginning to be unraveled. We show that endothelial tubulogenesis requires the Ras interacting protein 1, Rasip1, and its binding partner, the RhoGAP Arhgap29. Mice lacking Rasip1 fail to form patent lumens in all blood vessels, including the early endocardial tube. Rasipl null angioblasts fail to properly localize the polarity determinant Par3 and display defective cell polarity, resulting in mislocalized junctional complexes and loss of adhesion to extracellular matrix (ECM). Similarly, depletion of either Rasip1 or Arhgap29 in cultured ECs blocks in vitro lumen formation, fundamentally alters the cytoskeleton, and reduces integrin-dependent adhesion to ECM. These defects result from increased RhoA/ROCK/myosin II activity and blockade of Cdc42 and Rac1 signaling. This study identifies Rasip1 as a unique, endothelial-specific regulator of Rho GTPase signaling, which is essential for blood vessel morphogenesis.

LUMENating blood vessels

The acquisition of a lumen is an essential step in vascular morphogenesis. In this issue of Developmental Cell, Xu et al. (2011) show that the small GTPase Rasip is a critical regulator of cytoskeleton dynamics and cell adhesion, which together drive the emergence of vascular lumens.

Combinatorial and distinct roles of α₅ and α₄ integrins in stress erythropoiesis in mice

To delineate the role of specific members of β₁ integrins in stress erythropoiesis in the adult, we compared the response to phenylhydrazine stress in 3 genetically deficient models. The survival of β₁-conditionally deficient mice after phenylhydrazine is severely compromised because of their inability to mount a successful life saving splenic erythroid response, a phenotype reproduced in β₁(Δ/Δ) reconstituted animals. The response of bone marrow to phenylhydrazine-induced stress was, unlike that of spleen, appropriate in terms of progenitor cell expansion and mobilization to peripheral blood although late differentiation defects qualitatively similar to those in spleen were present in bone marrow. In contrast to β₁-deficient mice, α₄(Δ/Δ) mice showed only a kinetic delay in recovery and similar to β₁(Δ/Δ), terminal maturation defects in both bone marrow and spleen, which were not present in VCAM-1(Δ/Δ) mice. Convergence of information from these comparative studies lends new insight to the distinct in vivo roles of α₄ and α₅ integrins in erythroid stress, suggesting that the presence of mainly α₅β₁ integrin in all hematopoietic progenitor cells interacting with splenic microenvironmental ligands/cells is instrumental for their survival and accumulation during hemolytic stress, whereas presence of α₄ or of both α₅ and α₄, is important for completion of terminal maturation steps.

Endothelial cell talin1 is essential for embryonic angiogenesis

Using Tln1(fl/fl);CreER mice, we show that tamoxifen-induced inactivation of the talin1 gene throughout the embryo produces an angiogenesis phenotype that is restricted to newly forming blood vessels. The phenotype has a rapid onset in early embryos, resulting in vessel defects by 48 h and death of the embryo within 72 h. Very similar vascular defects were obtained using a Tie2-Cre endothelial cell-specific Tln1 knockout, a phenotype that was rescued by expression of a Tln1 mini-gene in endothelial cells. We show that endothelial cells, unlike most other cell types, do not express talin2, which can compensate for loss of talin1, and demonstrate for the first time that endothelial cells in vivo lacking talin1 are unable to undergo the cell spreading and flattening required to form vessels.

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

Jagged1 in the portal vein mesenchyme regulates intrahepatic bile duct development: insights into Alagille syndrome

Mutations in the human Notch ligand jagged 1 (JAG1) result in a multi-system disorder called Alagille syndrome (AGS). AGS is chiefly characterized by a paucity of intrahepatic bile ducts (IHBD), but also includes cardiac, ocular, skeletal, craniofacial and renal defects. The disease penetration and severity of the affected organs can vary significantly and the molecular basis for this broad spectrum of pathology is unclear. Here, we report that Jag1 inactivation in the portal vein mesenchyme (PVM), but not in the endothelium of mice, leads to the hepatic defects associated with AGS. Loss of Jag1 expression in SM22α-positive cells of the PVM leads to defective bile duct development beyond the initial formation of the ductal plate. Cytokeratin 19-positive cells are detected surrounding the portal vein, yet they are unable to form biliary tubes, revealing an instructive role of the vasculature in liver development. These findings uncover the cellular basis for the defining feature of AGS, identify mesenchymal Jag1-dependent and -independent stages of duct development, and provide mechanistic information for the role of Jag1 in IHBD formation.

An anteroposterior wave of vascular inhibitor downregulation signals aortae fusion along the embryonic midline axis

Paracrine signals, both positive and negative, regulate the positioning and remodeling of embryonic blood vessels. In the embryos of mammals and birds, the first major remodeling event is the fusion of bilateral dorsal aortae at the midline to form the dorsal aorta. Although the original bilaterality of the dorsal aortae occurs as the result of inhibitory factors (antagonists of BMP signaling) secreted from the midline by the notochord, it is unknown how fusion is later signaled. Here, we report that dorsal aortae fusion is tightly regulated by a change in signaling by the notochord along the anteroposterior axis. During aortae fusion, the notochord ceases to exert its negative influence on vessel formation. This is achieved by a transcriptional downregulation of negative regulators while positive regulators are maintained at pre-fusion levels. In particular, Chordin, the most abundant BMP antagonist expressed in the notochord prior to fusion, undergoes a dramatic downregulation in an anterior to posterior wave. With inhibitory signals diminished and sustained expression of the positive factors SHH and VEGF at the midline, fusion of the dorsal aortae is signaled. These results demonstrate a novel mechanism by which major modifications of the vascular pattern can occur through modulation of vascular inhibitors without changes in the levels of positive vascular regulators.

Formation of cardiovascular tubes in invertebrates and vertebrates

The cardiovascular system developed early in evolution and is pivotal for the transport of oxygen, nutrients, and waste products within the organism. It is composed of hollow tubular structures and has a high level of complexity in vertebrates. This complexity is, at least in part, due to the endothelial cell lining of vertebrate blood vessels. However, vascular lumen formation by endothelial cells is still controversially discussed. For example, it has been suggested that the lumen mainly forms via coalescence of large intracellular vacuoles generated by pinocytosis. Alternatively, it was proposed that the vascular lumen initiates extracellularly between adjacent apical endothelial cell surfaces. Here we discuss invertebrate and vertebrate cardiovascular lumen formation and highlight the possible modes of blood vessel formation. Finally, we point to the importance of a better understanding of vascular lumen formation for treating human pathologies, including cancer and coronary heart disease.

Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis

Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.

When Cre-mediated recombination in mice does not result in protein loss

Cre/loxP recombination enables cellular specificity and, in the case of inducible systems, temporal control of genomic deletions. Here we used a SM22α tamoxifen-inducible Cre line to inactivate β1 integrin in adult smooth muscle. Interestingly, analysis of two distinct β1 loxP transgenic mice revealed vastly different outcomes after β1 integrin deletion. Lethality occurred 4 weeks postinduction in one Cre/loxP line, while no apparent phenotype was seen in the other line. Genetic analysis revealed appropriate DNA excision in both cases; however, differences were found in the degree of protein loss with absolutely no change in protein levels in the model that lacked a phenotype. Seeking to understand protein persistence despite appropriate recombination, we first validated the flox allele using a constitutive Cre line and demonstrated its ability to mediate effective protein inactivation. We then examined the possibility of heterozygous cell selection, protein turnover, and deletion efficiency with no success for explaining the phenotype. Finally, we documented the presence of the Cre-recombination episomal product, which persisted in tissue samples with no protein loss. The product was only noted in cells with low proliferative capacity. These findings highlight the potential for protein expression from the products of Cre-recombinase excised genes, particularly when deletion occurs in low turnover populations.

Vascular remodeling of the vitelline artery initiates extravascular emergence of hematopoietic clusters

The vitelline artery is a temporary structure that undergoes extensive remodeling during midgestation to eventually become the superior mesenteric artery (also called the cranial mesenteric artery, in the mouse). Here we show that, during this remodeling process, large clusters of hematopoietic progenitors emerge via extravascular budding and form structures that resemble previously described mesenteric blood islands. We demonstrate through fate mapping of vascular endothelium that these mesenteric blood islands are derived from the endothelium of the vitelline artery. We further show that the vitelline arterial endothelium and subsequent blood island structures originate from a lateral plate mesodermal population. Lineage tracing of the lateral plate mesoderm demonstrates contribution to all hemogenic vascular beds in the embryo, and eventually, all hematopoietic cells in the adult. The intraembryonic hematopoietic cell clusters contain viable, proliferative cells that exhibit hematopoietic stem cell markers and are able to further differentiate into myeloid and erythroid lineages. Vitelline artery-derived hematopoietic progenitor clusters appear between embryonic day 10 and embryonic day 10.75 in the caudal half of the midgut mesentery, but by embryonic day 11.0 are sporadically found on the cranial side of the midgut, thus suggesting possible extravascular migration aided by midgut rotation.

Resolving cell-cell junctions: lumen formation in blood vessels

Formation of a patent vascular lumen is essential for the transport of oxygen, nutrients and waste products to and from tissues. No matter whether the blood vessel arises from vasculogenesis or angiogenesis, endothelial cells (EC) first have to form a cord, which subsequently lumenizes, in order to generate a functional vessel. During these processes, cellular junctions rearrange between adjacent ECs and are involved in EC polarization as a prerequisite for lumen formation. Here we review the role of EC junctions in vascular lumen formation within different vascular beds.

Endothelial alpha5 and alphav integrins cooperate in remodeling of the vasculature during development

Integrin cell adhesion receptors and fibronectin, one of their extracellular matrix ligands, have been demonstrated to be important for angiogenesis using functional perturbation studies and complete knockout mouse models. Here, we report on the roles of the alpha5 and alphav integrins, which are the major endothelial fibronectin receptors, in developmental angiogenesis. We generated an integrin alpha5-floxed mouse line and ablated alpha5 integrin in endothelial cells. Unexpectedly, endothelial-specific knockout of integrin alpha5 has no obvious effect on developmental angiogenesis. We provide evidence for genetic interaction between mutations in integrin alpha5 and alphav and for overlapping functions and compensation between these integrins and perhaps others. Nonetheless, in embryos lacking both alpha5 and alphav integrins in their endothelial cells, initial vasculogenesis and angiogenesis proceed normally, at least up to E11.5, including the formation of apparently normal embryonic vasculature and development of the branchial arches. However, in the absence of endothelial alpha5 and alphav integrins, but not of either alone, there are extensive defects in remodeling of the great vessels and heart resulting in death at ~E14.5. We also found that fibronectin assembly is somewhat affected in integrin alpha5 knockout endothelial cells and markedly reduced in integrin alpha5/alphav double-knockout endothelial cell lines. Therefore, neither alpha5 nor alphav integrins are required in endothelial cells for initial vasculogenesis and angiogenesis, although they are required for remodeling of the heart and great vessels. These integrins on other cells, and/or other integrins on endothelial cells, might contribute to fibronectin assembly and vascular development.

EGFL7 meets miRNA-126: an angiogenesis alliance

Blood vessels form de novo through the tightly regulated programs of vasculogenesis and angiogenesis. Both processes are distinct but one of the steps they share is the formation of a central lumen, when groups of cells organized as vascular cords undergo complex changes to achieve a tube-like morphology. Recently, a protein termed epidermal growth factor-like domain 7 (EGFL7) was described as a novel endothelial cell-derived factor involved in the regulation of the spatial arrangement of cells during vascular tube assembly. With its impact on tubulogenesis and vessel shape EGFL7 joined the large family of molecules governing blood vessel formation. Only recently, the molecular mechanisms underlying EGFL7's effects have been started to be elucidated and shaping of the extracellular matrix (ECM) as well as Notch signaling might very well play a role in mediating its biological effects. Further, findings in knock-out animal models suggest miR-126, a miRNA located within the egfl7 gene, has a major role in vessel development by promoting VEGF signaling, angiogenesis and vascular integrity. This review summarizes our current knowledge on EGFL7 and miR-126 and we will discuss the implications of both bioactive molecules for the formation of blood vessels.

Endothelial lumen signaling complexes control 3D matrix-specific tubulogenesis through interdependent Cdc42- and MT1-MMP-mediated events

Here, we define an endothelial cell (EC) lumen signaling complex involving Cdc42, Par6b, Par3, junction adhesion molecule (Jam)-B and Jam-C, membrane type 1-matrix metalloproteinase (MT1-MMP), and integrin alpha(2)beta(1), which coassociate to control human EC tubulogenesis in 3D collagen matrices. Blockade of both Jam-B and Jam-C using antibodies, siRNA, or dominant-negative mutants completely interferes with lumen and tube formation resulting from a lack of Cdc42 activation, inhibition of Cdc42-GTP-dependent signal transduction, and blockade of MT1-MMP-dependent proteolysis. This process requires interdependent Cdc42 and MT1-MMP signaling, which involves Par3 binding to the Jam-B and Jam-C cytoplasmic tails, an interaction that is necessary to physically couple the components of the lumen signaling complex. MT1-MMP proteolytic activity is necessary for Cdc42 activation during EC tube formation in 3D collagen matrices but not on 2D collagen surfaces, whereas Cdc42 activation is necessary for MT1-MMP to create vascular guidance tunnels and tube networks in 3D matrices through proteolytic events. This work reveals a novel interdependent role for Cdc42-dependent signaling and MT1-MMP-dependent proteolysis, a process that occurs selectively in 3D collagen matrices and that requires EC lumen signaling complexes, to control human EC tubulogenesis during vascular morphogenesis.