Tip Cell Selection Research Articles

SVEGFR1 Is Enriched in Hepatic Vein Blood-Evidence for a Provisional Hepatic Factor Candidate?

Background: Pulmonary arteriovenous malformations (PAVMs) are common sequelae of palliated univentricular congenital heart disease, yet their pathogenesis remain poorly defined. In this preliminary study, we used paired patient blood samples to identify potential hepatic factor candidates enriched in hepatic vein blood.Methods: Paired venous blood samples were collected from the hepatic vein (HV) and superior vena cava (SVC) from children 0 to 10 years with univentricular and biventricular congenital heart disease (n = 40). We used three independent protein analyses to identify proteomic differences between HV and SVC blood. Subsequently, we investigated the relevance of our quantified protein differences with human lung microvascular endothelial assays.Results: Two independent protein arrays (semi-quantitative immunoblot and quantitative array) identified that soluble vascular endothelial growth factor receptor 1 (sVEGFR1) is significantly elevated in HV serum compared to SVC serum. Using ELISA, we confirmed the previous findings that sVEGFR1 is enriched in HV serum (n = 24, p < 0.0001). Finally, we studied the quantified HV and SVC serum levels of sVEGFR1 in vitro. HV levels of sVEGFR1 decreased tip cell selection (p = 0.0482) and tube formation (fewer tubes [p = 0.0246], shorter tube length [p = 0.0300]) in vitro compared to SVC levels of sVEGFR1.Conclusions: Based on a small heterogenous cohort, sVEGFR1 is elevated in HV serum compared to paired SVC samples, and the mean sVEGFR1 concentrations in these two systemic veins cause pulmonary endothelial phenotypic differences in vitro. Further research is needed to determine whether sVEGFR1 has a direct role in pulmonary microvascular remodeling and PAVMs in patients with palliated univentricular congenital heart disease.

In vitro Three-Dimensional Sprouting Assay of Angiogenesis using Mouse Embryonic Stem Cells for Vascular Disease Modeling and Drug Testing.

Recent advances in induced pluripotent stem cells (iPSC) and gene editing technologies enable the development of novel human cell-based disease models for phenotypic drug discovery (PDD) programs. Although these novel devices could predict the safety and efficacy of investigational drugs in humans more accurately, their development to the clinic still strongly rely on mammalian data, notably the use of mouse disease models. In parallel to human organoid or organ-on-chip disease models, the development of relevant in vitro mouse models is therefore an unmet need for evaluating direct drug efficacy and safety comparisons between species and in vivo and in vitro conditions. Here, a vascular sprouting assay that utilizes mouse embryonic stem cells differentiated into embryoid bodies (EBs) is described. Vascularized EBs cultured onto 3D-collagen gel develop new blood vessels that expand, a process called sprouting angiogenesis. This model recapitulates key features of in vivo sprouting angiogenesis-formation of blood vessels from a pre-existing vascular network-including endothelial tip cell selection, endothelial cell migration and proliferation, cell guidance, tube formation, and mural cell recruitment. It is amenable to screening for drugs and genes modulating angiogenesis and shows similarities with recently described three-dimensional (3D) vascular assays based on human iPSC technologies.

Active perception during angiogenesis: filopodia speed up Notch selection of tip cells in silico and in vivo.

How do cells make efficient collective decisions during tissue morphogenesis? Humans and other organisms use feedback between movement and sensing known as ‘sensorimotor coordination’ or ‘active perception’ to inform behaviour, but active perception has not before been investigated at a cellular level within organs. Here we provide the first proof of concept in silico/in vivo study demonstrating that filopodia (actin-rich, dynamic, finger-like cell membrane protrusions) play an unexpected role in speeding up collective endothelial decisions during the time-constrained process of ‘tip cell’ selection during blood vessel formation (angiogenesis). We first validate simulation predictions in vivo with live imaging of zebrafish intersegmental vessel growth. Further simulation studies then indicate the effect is due to the coupled positive feedback between movement and sensing on filopodia conferring a bistable switch-like property to Notch lateral inhibition, ensuring tip selection is a rapid and robust process. We then employ measures from computational neuroscience to assess whether filopodia function as a primitive (basal) form of active perception and find evidence in support. By viewing cell behaviour through the ‘basal cognitive lens' we acquire a fresh perspective on the tip cell selection process, revealing a hidden, yet vital time-keeping role for filopodia. Finally, we discuss a myriad of new and exciting research directions stemming from our conceptual approach to interpreting cell behaviour.This article is part of the theme issue ‘Basal cognition: multicellularity, neurons and the cognitive lens’.

Notch signaling and taxis mechanisms regulate early stage angiogenesis: A mathematical and computational model

During angiogenesis, new blood vessels sprout and grow from existing ones. This process plays a crucial role in organ development and repair, in wound healing and in numerous pathological processes such as cancer progression or diabetes. Here, we present a mathematical model of early stage angiogenesis that permits exploration of the relative importance of mechanical, chemical and cellular cues. Endothelial cells proliferate and move over an extracellular matrix by following external gradients of Vessel Endothelial Growth Factor, adhesion and stiffness, which are incorporated to a Cellular Potts model with a finite element description of elasticity. The dynamics of Notch signaling involving Delta-4 and Jagged-1 ligands determines tip cell selection and vessel branching. Through their production rates, competing Jagged-Notch and Delta-Notch dynamics determine the influence of lateral inhibition and lateral induction on the selection of cellular phenotypes, branching of blood vessels, anastomosis (fusion of blood vessels) and angiogenesis velocity. Anastomosis may be favored or impeded depending on the mechanical configuration of strain vectors in the ECM near tip cells. Numerical simulations demonstrate that increasing Jagged production results in pathological vasculatures with thinner and more abundant vessels, which can be compensated by augmenting the production of Delta ligands.

Retinal myeloid cells regulate tip cell selection and vascular branching morphogenesis via Notch ligand Delta-like 1

During angiogenesis, single endothelial cells (EC) specialize into tip cells that guide vessel sprouting towards growth factor gradients and instruct the adjacent vessel stalk. The balance between tip and stalk cells is regulated by endothelial Notch signalling through the expression of Notch ligand Delta-like 4 (Dll4) in tip cells, which suppresses a tip cell fate in adjacent stalk cells. Here we show, using genetic reporter and conditional deletion strategies, that myeloid cells regulate tip cell numbers and Dll4 expression via the Notch ligand Dll1 during vascular development in the retina. Dll1 is selectively expressed by a subpopulation of retinal myeloid cells, which progressively localizes to the sprouting vascular network. Conditional, myeloid-specific deletion of Dll1 impairs endothelial Dll4 tip-stalk gradient resulting in an increase of endothelial tip cells and EC filopodia, accompanied by an increase in vascular density and branching. In vitro, co-culture of human EC with monocyte-derived macrophages induced Dll1 upregulation in macrophages and Dll4 upregulation and an endothelial tip cell signature in EC. Furthermore, culturing human EC on recombinant DLL1 induced endothelial Dll4 expression and a tip cell program, indicating that changes are Dll1-dependent. Thus, myeloid cells regulate tip cell fate and angiogenesis through expression of Notch ligand Dll1.

Activation of Notch signaling by soluble Dll4 decreases vascular permeability via a cAMP/PKA-dependent pathway.

The Notch ligand delta-like ligand 4 (Dll4), upregulated by VEGF, is a key regulator of vessel morphogenesis and function, controlling tip and stalk cell selection during sprouting angiogenesis. Inhibition of Dll4 results in hypersprouting, nonfunctional, poorly perfused vessels, suggesting a role for Dll4 in the formation of mature, reactive, functional vessels, with low permeability and able to restrict fluid and solute exchange. We tested the hypothesis that Dll4 controls transvascular fluid exchange. A recombinant protein expressing only the extracellular portion of Dll4 [soluble Dll4 (sDll4)] induced Notch signaling in endothelial cells (ECs), resulting in increased expression of vascular-endothelial cadherin, but not the tight junctional protein zonula occludens 1, at intercellular junctions. sDll4 decreased the permeability of FITC-labeled albumin across EC monolayers, and this effect was abrogated by coculture with the γ-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester. One of the known molecular effectors responsible for strengthening EC-EC contacts is PKA, so we tested the effect of modulation of PKA on the sDll4-mediated reduction of permeability. Inhibition of PKA reversed the sDll4-mediated reduction in permeability and reduced expression of the Notch target gene Hey1. Knockdown of PKA reduced sDLL4-mediated vascular-endothelial cadherin junctional expression. sDll4 also caused a significant decrease in the hydraulic conductivity of rat mesenteric microvessels in vivo. This reduction was abolished upon coperfusion with the PKA inhibitor H89 dihydrochloride. These results indicate that Dll4 signaling through Notch activation acts through a cAMP/PKA pathway upon intercellular adherens junctions, but not tight junctions, to regulate endothelial barrier function. NEW & NOTEWORTHY Notch signaling reduces vascular permeability through stimulation of cAMP-dependent protein kinase A.

Activation of Notch signalling by soluble Dll4 decreases permeability via a cAMP/PKA‐dependent pathway

The Notch ligand Delta like ligand 4 (Dll4), which is upregulated by VEGF during angiogenesis, is a key regulator of vessel morphogenesis, maturation and function. It has been widely described as a regulator of tip and stalk cell selection during sprouting angiogenesis. Inhibition of Dll4 results in hyper‐sprouting, non‐functional and poorly perfused vessels, which suggests a role for Dll4 in the formation of mature, reactive, functional vessels, that have low permeability and are able to restrict fluid and solute exchange across vessel walls. We therefore tested the hypothesis that Dll4 plays a role in controlling transvascular fluid exchange. Using a recombinant protein expressing only the extracellular portion of Dll4 (soluble Dll4: sDll4), we were able to induce Notch signalling in endothelial cells (EC), resulting in an increased expression of VE‐Cadherin at intercellular junctions. sDll4 decreased permeability of FITC labelled albumin across EC monolayers and this effect was abrogated by co‐culture with DAPT. One of the known molecular effectors responsible for strengthening EC‐EC contacts is the cyclic AMP dependent protein kinase PKA, so we tested the effect of modulation of PKA on sDll4‐mediated reduction of permeability. Inhibition of PKA reversed the sDll4‐mediated reduction in permeability and reduced Hey‐1 expression. We showed that sDll4 caused a significant decrease in the hydraulic conductivity of rat mesenteric microvessels in vivo. This reduction was abolished upon co‐perfusion with the PKA inhibitor H89 dihydrochloride. These results indicate that Dll4 signalling, through Notch activation, acts upon intercellular adherens junctions through a cAMP/PKA pathway, to regulate endothelial barrier function.Support or Funding InformationSupported the British Heart FoundationThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Defective endothelial cell migration in the absence of Cdc42 leads to capillary-venous malformations.

Formation and homeostasis of the vascular system requires several coordinated cellular functions, but their precise interplay during development and their relative importance for vascular pathologies remain poorly understood. Here, we investigated the endothelial functions regulated by Cdc42 and their in vivo relevance during angiogenic sprouting and vascular morphogenesis in the postnatal mouse retina. We found that Cdc42 is required for endothelial tip cell selection, directed cell migration and filopodia formation, but dispensable for cell proliferation or apoptosis. Although the loss of Cdc42 seems generally compatible with apical-basal polarization and lumen formation in retinal blood vessels, it leads to defective endothelial axial polarization and to the formation of severe vascular malformations in capillaries and veins. Tracking of Cdc42-depleted endothelial cells in mosaic retinas suggests that these capillary-venous malformations arise as a consequence of defective cell migration, when endothelial cells that proliferate at normal rates are unable to re-distribute within the vascular network.

Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays

Activation of vascular endothelial cells (ECs) by growth factors initiates a cascade of events during angiogenesis in vivo consisting of EC tip cell selection, sprout formation, EC stalk cell proliferation, and ultimately vascular stabilization by support cells. Although EC functional assays can recapitulate one or more aspects of angiogenesis in vitro, they are often limited by undefined substrates and lack of dependence on key angiogenic signaling axes. Here, we designed and characterized a chemically-defined model of endothelial sprouting behavior in vitro using human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs). We rapidly encapsulated iPSC-ECs at high density in poly(ethylene glycol) (PEG) hydrogel spheres using thiol-ene chemistry and subsequently encapsulated cell-dense hydrogel spheres in a cell-free hydrogel layer. The hydrogel sprouting array supported pro-angiogenic phenotype of iPSC-ECs and supported growth factor-dependent proliferation and sprouting behavior. iPSC-ECs in the sprouting model responded appropriately to several reference pharmacological angiogenesis inhibitors of vascular endothelial growth factor, NF-κB, matrix metalloproteinase-2/9, protein kinase activity, and β-tubulin, which confirms their functional role in endothelial sprouting. A blinded screen of 38 putative vascular disrupting compounds from the US Environmental Protection Agency’s ToxCast library identified six compounds that inhibited iPSC-EC sprouting and five compounds that were overtly cytotoxic to iPSC-ECs at a single concentration. The chemically-defined iPSC-EC sprouting model (iSM) is thus amenable to enhanced-throughput screening of small molecular libraries for effects on angiogenic sprouting and iPSC-EC toxicity assessment. Statement of SignificanceAngiogenesis assays that are commonly used for drug screening and toxicity assessment applications typically utilize natural substrates like MatrigelTM that are difficult to spatially pattern, costly, ill-defined, and may exhibit lot-to-lot variability. Herein, we describe a novel angiogenic sprouting assay using chemically-defined, bioinert poly(ethylene glycol) hydrogels functionalized with biomimetic peptides to promote cell attachment and degradation in a reproducible format that may mitigate the need for natural substrates. The quantitative assay of angiogenic sprouting here enables precise control over the initial conditions and can be formulated into arrays for screening. The sprouting assay here was dependent on key angiogenic signaling axes in a screen of angiogenesis inhibitors and a blinded screen of putative vascular disrupting compounds from the US-EPA.

Temporal modulation of collective cell behavior controls vascular network topology.

Vascular network density determines the amount of oxygen and nutrients delivered to host tissues, but how the vast diversity of densities is generated is unknown. Reiterations of endothelial-tip-cell selection, sprout extension and anastomosis are the basis for vascular network generation, a process governed by the VEGF/Notch feedback loop. Here, we find that temporal regulation of this feedback loop, a previously unexplored dimension, is the key mechanism to determine vascular density. Iterating between computational modeling and in vivo live imaging, we demonstrate that the rate of tip-cell selection determines the length of linear sprout extension at the expense of branching, dictating network density. We provide the first example of a host tissue-derived signal (Semaphorin3E-Plexin-D1) that accelerates tip cell selection rate, yielding a dense network. We propose that temporal regulation of this critical, iterative aspect of network formation could be a general mechanism, and additional temporal regulators may exist to sculpt vascular topology.

Computational Models of Sprouting Angiogenesis and Cell Migration: Towards Multiscale Mechanochemical Models of Angiogenesis

Angiogenesis, the formation of new bloods vessels from the existing vasculature, is a process that is essential during development and regeneration of tissues, and that plays a major role in diseases like cancer. Computational models have been designed to obtain a better under- standing of the mechanisms behind angiogenesis. In this paper we review computational models of sprouting angiogenesis. These models can be subdivided into three categories: models that mainly focus on tip cell migration, models that make a distinction between the role of tip cells and stalk cells, and models that consider cell shape dynamics. Many models combine discrete modeling of individual cells with continuous modeling of the extracellular matrix (ECM) and diffusing solutes, in this way resulting in a hybrid model. We discuss their merits in unraveling the role of certain factors for vascular network formation, such as the role of (chemotactic, haptotactic, contact) guid- ance cues in the dynamics and morphology of vascular network formation, and the role of cell-cell interactions that govern tip cell selection and phenotypic changes in general. At the same time, we identify a need for the inclusion of cell mechanical principles in models of angiogenesis, in partic- ular for the description of cell migration, cell-matrix and cell-cell interaction, as the generation of cellular forces is key to cell migration. To further underline this we review models of single cell migration that incorporate such principles, which could be the starting point for formulating novel models of angiogenesis that respect the fundamental laws of classical mechanics at the cell level. As the generation of cellular forces is strongly mediated by pro-angiogenic signals, such models must couple cell mechanical principles to molecular signaling into multiscale mechanochemical models of angiogenesis. Finally, a tight coupling between models and experiments will be re- quired to facilitate model improvements and the generation of novel insights on the regulation of angiogenesis.

Quantitative assessment of angiogenesis, perfused blood vessels and endothelial tip cells in the postnatal mouse brain.

During development and in various diseases of the CNS, new blood vessel formation starts with endothelial tip cell selection and vascular sprout migration, followed by the establishment of functional, perfused blood vessels. Here we describe a method that allows the assessment of these distinct angiogenic steps together with antibody-based protein detection in the postnatal mouse brain. Intravascular and perivascular markers such as Evans blue (EB), isolectin B4 (IB4) or laminin (LN) are used alongside simultaneous immunofluorescence on the same sections. By using confocal laser-scanning microscopy and stereological methods for analysis, detailed quantification of the 3D postnatal brain vasculature for perfused and nonperfused vessels (e.g., vascular volume fraction, vessel length and number, number of branch points and perfusion status of the newly formed vessels) and characterization of sprouting activity (e.g., endothelial tip cell density, filopodia number) can be obtained. The entire protocol, from mouse perfusion to vessel analysis, takes ∼10 d.

Roles of VEGF-A signalling in development, regeneration, and tumours.

Angiogenesis, the formation of new networks of blood vessels, has essential roles in embryonic development, organ homeostasis and disease progression. Several signalling molecules, such as vascular endothelial growth factors (VEGFs), fibroblast growth factors (FGFs), transforming growth factor (TGF)-β and angiopoietin-1 and 2, are known to be key regulators of blood vessel development and network patterning. Among these, the roles of VEGF-A and its receptors in vessel morphogenesis are understood best. VEGF-A signalling plays a crucial role in embryonic development through the regulation of angiogenesis. VEGF-A regulates most of the endothelial response, such as the proliferation and migration of endothelial cells (ECs), vascular permeability and the selection of tip and stalk cells. VEGF-A signalling also regulates organ homeostasis in adults. If an organ is exposed to severe injury, VEGF-A induces the release of paracrine factors from ECs, which increase the rate of regeneration of the organ. VEGF-A signalling also has an important role in the progression of angiogenesis-related diseases, especially cancer. Consequently, many agents that block VEGF-A have been developed and reported as useful tools for the inhibition of the growth and metastatic spread of tumours. Here, we summarize recent reports of the multiple functions of VEGF-A signalling during development, organ regeneration and tumour progression.

The 10 Most Read Articles Published in Circulation Research in 2013

The 10 Most Read Articles Published in <i>Circulation Research</i> in 2013

Synaptojanin-2 Binding Protein Stabilizes the Notch Ligands DLL1 and DLL4 and Inhibits Sprouting Angiogenesis

The formation of novel blood vessels is initiated by vascular endothelial growth factor. Subsequently, DLL4-Notch signaling controls the selection of tip cells, which guide new sprouts, and trailing stalk cells. Notch signaling in stalk cells is induced by DLL4 on the tip cells. Moreover, DLL4 and DLL1 are expressed in the stalk cell plexus to maintain Notch signaling. Notch loss-of-function causes formation of a hyperdense vascular network with disturbed blood flow. This study was aimed at identifying novel modifiers of Notch signaling that interact with the intracellular domains of DLL1 and DLL4. Synaptojanin-2 binding protein (SYNJ2BP, also known as ARIP2) interacted with the PDZ binding motif of DLL1 and DLL4, but not with the Notch ligand Jagged-1. SYNJ2BP was preferentially expressed in stalk cells, enhanced DLL1 and DLL4 protein stability, and promoted Notch signaling in endothelial cells. SYNJ2BP induced expression of the Notch target genes HEY1, lunatic fringe (LFNG), and ephrin-B2, reduced phosphorylation of ERK1/2, and decreased expression of the angiogenic factor vascular endothelial growth factor (VEGF)-C. It inhibited the expression of genes enriched in tip cells, such as angiopoietin-2, ESM1, and Apelin, and impaired tip cell formation. SYNJ2BP inhibited endothelial cell migration, proliferation, and VEGF-induced angiogenesis. This could be rescued by blockade of Notch signaling or application of angiopoietin-2. SYNJ2BP-silenced human endothelial cells formed a functional vascular network in immunocompromised mice with significantly increased vascular density. These data identify SYNJ2BP as a novel inhibitor of tip cell formation, executing its functions predominately by promoting Delta-Notch signaling.

VEGF and Notch in Tip and Stalk Cell Selection

Sprouting angiogenesis is a dynamic process in which endothelial cells collectively migrate, shape new lumenized tubes, make new connections, and remodel the nascent network into a hierarchically branched and functionally perfused vascular bed. Endothelial cells in the nascent sprout adopt two distinct cellular phenotypes--known as tip and stalk cells--with specialized functions and gene expression patterns. VEGF and Notch signaling engage in an intricate cross talk to balance tip and stalk cell formation and to regulate directed tip cell migration and stalk cell proliferation. In this article, we summarize the current knowledge and implications of the tip/stalk cell concepts and the quantitative and dynamic integration of VEGF and Notch signaling in tip and stalk cell selection.

MOSAIC: a multiscale model of osteogenesis and sprouting angiogenesis with lateral inhibition of endothelial cells.

The healing of a fracture depends largely on the development of a new blood vessel network (angiogenesis) in the callus. During angiogenesis tip cells lead the developing sprout in response to extracellular signals, amongst which vascular endothelial growth factor (VEGF) is critical. In order to ensure a correct development of the vasculature, the balance between stalk and tip cell phenotypes must be tightly controlled, which is primarily achieved by the Dll4-Notch1 signaling pathway. This study presents a novel multiscale model of osteogenesis and sprouting angiogenesis, incorporating lateral inhibition of endothelial cells (further denoted MOSAIC model) through Dll4-Notch1 signaling, and applies it to fracture healing. The MOSAIC model correctly predicted the bone regeneration process and recapitulated many experimentally observed aspects of tip cell selection: the salt and pepper pattern seen for cell fates, an increased tip cell density due to the loss of Dll4 and an excessive number of tip cells in high VEGF environments. When VEGF concentration was even further increased, the MOSAIC model predicted the absence of a vascular network and fracture healing, thereby leading to a non-union, which is a direct consequence of the mutual inhibition of neighboring cells through Dll4-Notch1 signaling. This result was not retrieved for a more phenomenological model that only considers extracellular signals for tip cell migration, which illustrates the importance of implementing the actual signaling pathway rather than phenomenological rules. Finally, the MOSAIC model demonstrated the importance of a proper criterion for tip cell selection and the need for experimental data to further explore this. In conclusion, this study demonstrates that the MOSAIC model creates enhanced capabilities for investigating the influence of molecular mechanisms on angiogenesis and its relation to bone formation in a more mechanistic way and across different time and spatial scales.

Born to Die

Blood vessel network formation is a pivotal process of physiologic development as well as pathological conditions such as tumor growth, diabetic retinopathy and cardiovascular diseases1,2. During neovascularization, vasculogenesis is followed by the well described process of sprouting angiogenesis, including tip cell induction and selection, sprout elongation and stalk formation, leading to the formation of a dense primary vascular network1,2. To match vessel perfusion with the local metabolic demand of the tissue, initial sprouting angiogenesis is followed by vessel pruning and maturation to form a hierarchically defined vascular network of large arteries and veins branching into smaller capillaries1,2. Although the phenomenon of vessel regression has already been described by Rouget in 18373 and further characterized by Ashton in 19614, the mechanisms of vessel pruning and the signaling pathways controlling this process are still largely undefined. (SELECT FULL TEXT TO CONTINUE)

Stalk Cell Phenotype Depends on Integration of Notch and Smad1/5 Signaling Cascades

Gradients of vascular endothelial growth factor (VEGF) induce single endothelial cells to become leading tip cells of emerging angiogenic sprouts. Tip cells then suppress tip-cell features in adjacent stalk cells via Dll4/Notch-mediated lateral inhibition. We report here that Smad1/Smad5-mediated BMP signaling synergizes with Notch signaling during selection of tip and stalk cells. Endothelium-specific inactivation of Smad1/Smad5 in mouse embryos results in impaired Dll4/Notch signaling and increased numbers of tip-cell-like cells at the expense of stalk cells. Smad1/5 downregulation in cultured endothelial cells reduced the expression of several target genes of Notch and of other stalk-cell-enriched transcripts (Hes1, Hey1, Jagged1, VEGFR1, and Id1-3). Moreover, Id proteins act as competence factors for stalk cells and form complexes with Hes1, which augment Hes1 levels in the endothelium. Our findings provide in vivo evidence for a regulatory loop between BMP/TGFβ-Smad1/5 and Notch signaling that orchestrates tip- versus stalk-cell selection and vessel plasticity.

In This Issue

HomeCirculation ResearchVol. 110, No. 4In This Issue Free AccessIn BriefPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessIn BriefPDF/EPUBIn This Issue Originally published17 Feb 2012https://doi.org/10.1161/RES.0b013e31824d9f4bCirculation Research. 2012;110:523Dll1 Controls Vascular Branching (p 530)Notch ligand Dll1 is an extravascular cue that tells vessels where to branch, report Napp et al.Download figureDownload PowerPointDuring angiogenesis, the branching of new vessels begins when specific cells in the vessel wall become tip cells. The expression of the Notch ligand, Dll4, defines tip cells and prevents neighboring cells from adopting the same fate. Dll4 is induced by the external growth factor VEGF (vascular endothelial growth factor), which then prompts sprouting of the vessel led by the tip cell. But VEGF does not work alone. Napp et al now show that Notch ligand, Dll1, is also essential for tip cell selection. The team studied mice that had a heterozygous deletion of Dll1 and found that in the newborn retina, a site of abundant angiogenesis, there were fewer tip cells and fewer vessel branches. Expression analysis revealed that Dll1 was never found in the developing vessel's endothelial cells themselves but rather was immediately adjacent to the vessels at the sites of tip cell formation. In vitro studies showed that recombinant Dll1 could induce human endothelial cells to upregulate tip cell–specific genes, including Dll4. Understanding the molecular mechanisms of angiogenesis has far-reaching implications in areas such as development, wound healing, and tumor formation.Left-Right Asymmetry in Vascular Cells (p 551)Given a choice of direction, vascular mesenchymal cells turn right and reveal an inherent asymmetry, say Chen et al.Download figureDownload PowerPointAlthough animals might appear to be symmetrical on the outside, their innards reveal a striking left-right asymmetry in both the positioning of organs and the structure of the organs themselves. In the early embryo, the beating of cilia on the surface of certain cells sets up left-right asymmetry by washing molecules in one direction over neighboring cells. This model implies that cells themselves have an inherent left-right asymmetry, but the random orientation of cells in culture makes testing that theory rather difficult. Chen et al noticed that vascular mesenchymal cells (VMCs) form distinct patterns when grown on plates striped with adherent and nonadherent surfaces. Closer inspection revealed that as proliferating VMCs filled the adherent stripes and were forced to venture toward nonadherent stripes, they instinctively turned right, eventually leading to the observed patterns. This preference was dependent on intracellular stress fiber formation and was not unique to VMCs; 1 other cell type, ST2, showed a left turn preference, whereas 2 other cell types showed no preference. These findings could be important not only in studies of development but also in tissue engineering, say the team.Bmp and Myocardial Differentiation (p 578)de Pater et al have determined when and where BMP is needed during heart development.Download figureDownload PowerPointDespite its name, bone morphogenetic protein (BMP) is required for differentiation of a variety of tissues, including the myocardium. In vitro studies of cardiomyocyte differentiation, however, have suggested seemingly contradictory roles for BMP. Pater et al have now set things straight, in the zebrafish at least. They showed that although BMP signaling is present in cardiac progenitor cells in the early fish embryo and is required for induction of cardiomyocytes, it is later detrimental to heart development and is downregulated. The team showed that inhibition of BMP signaling between 10 and 24 hours after fertilization reduced the numbers of atrial and ventricular cardiomyocytes, but inhibition of BMP after 24 hours had no effect. Once cardiomyocyte differentiation was under way, BMP activity in the embryo was normally inhibited by a signaling factor called Smad6. This reduction in BMP activity was essential for proper morphogenesis of the heart, because reactivation led to ectopic expression of BMP-target genes and impaired chamber formation. Understanding precisely when and where BMP is required should help researchers improve their in vitro methods for cardiomyocyte differentiation of embryonic or induced pluripotent stem cells.Written by Ruth Williams Previous Back to top Next FiguresReferencesRelatedDetails February 17, 2012Vol 110, Issue 4 Advertisement Article InformationMetrics © 2012 American Heart Association, Inc.https://doi.org/10.1161/RES.0b013e31824d9f4b Originally publishedFebruary 17, 2012 PDF download Advertisement