Analysis of Dll4 regulation reveals a combinatorial role for Sox and Notch in arterial development

The molecular and cellular hematopoietic stem cell specification niche

Hematopoietic stem cells (HSCs) are a population of tissue-specific stem cells that reside in the bone marrow of adult mammals, where they self-renew and continuously regenerate the adult hematopoietic lineages over the life of the individual. Prominence as a stem cell model and clinical usefulness have driven interest in understanding the physiologic processes that lead to the specification of HSCs during embryonic development. High-efficiency directed differentiation of HSCs by the instruction of defined progenitor cells using sequentially defined instructive molecules and conditions remains impossible, indicating that comprehensive knowledge of the complete set of precursor intermediate identities and required inductive inputs remains incompletely understood. Recently, interest in the molecular and cellular microenvironment where HSCs are specified from endothelial precursors-the "specification niche"-has increased. Here we review recent progress in understanding these niche spaces across vertebrate phyla, as well as how a better characterization of the origin and molecular phenotypes of the niche cell populations has helped inform and complicate previous understanding of signaling required for HSC emergence and maturation.

Eph-ephrin signaling couples endothelial cell sorting and arterial specification

Cell segregation allows the compartmentalization of cells with similar fates during morphogenesis, which can be enhanced by cell fate plasticity in response to local molecular and biomechanical cues. Endothelial tip cells in the growing retina, which lead vessel sprouts, give rise to arterial endothelial cells and thereby mediate arterial growth. Here, we have combined cell type-specific and inducible mouse genetics, flow experiments in vitro, single-cell RNA sequencing and biochemistry to show that the balance between ephrin-B2 and its receptor EphB4 is critical for arterial specification, cell sorting and arteriovenous patterning. At the molecular level, elevated ephrin-B2 function after loss of EphB4 enhances signaling responses by the Notch pathway, VEGF and the transcription factor Dach1, which is influenced by endothelial shear stress. Our findings reveal how Eph-ephrin interactions integrate cell segregation and arteriovenous specification in the vasculature, which has potential relevance for human vascular malformations caused by EPHB4 mutations.

Molecular and Spatial Signatures of Mouse Embryonic Endothelial Cells at Single-Cell Resolution

BACKGROUND

Mature endothelial cells (ECs) are heterogeneous, with subtypes defined by tissue origin and position within the vascular bed (ie, artery, capillary, vein, and lymphatic). How this heterogeneity is established during the development of the vascular system, especially arteriovenous specification of ECs, remains incompletely characterized.

METHODS

We used droplet-based single-cell RNA sequencing and multiplexed error-robust fluorescence in situ hybridization to define EC and EC progenitor subtypes from E9.5, E12.5, and E15.5 mouse embryos. We used trajectory inference to analyze the specification of arterial ECs (aECs) and venous ECs (vECs) from EC progenitors. Network analysis identified candidate transcriptional regulators of arteriovenous differentiation, which we tested by CRISPR (clustered regularly interspaced short palindromic repeats) loss of function in human-induced pluripotent stem cells undergoing directed differentiation to aECs or vECs (human-induced pluripotent stem cell-aECs or human-induced pluripotent stem cell-vECs).

RESULTS

From the single-cell transcriptomes of 7682 E9.5 to E15.5 ECs, we identified 19 EC subtypes, including Etv2+Bnip3+ EC progenitors. Spatial transcriptomic analysis of 15 448 ECs provided orthogonal validation of these EC subtypes and established their spatial distribution. Most embryonic ECs were grouped by their vascular-bed types, while ECs from the brain, heart, liver, and lung were grouped by their tissue origins. Arterial (Eln, Dkk2, Vegfc, and Egfl8), venous (Fam174b and Clec14a), and capillary (Kcne3) marker genes were identified. Compared with aECs, embryonic vECs and capillary ECs shared fewer markers than their adult counterparts. Early capillary ECs with venous characteristics functioned as a branch point for differentiation of aEC and vEC lineages.

CONCLUSIONS

Our results provide a spatiotemporal map of embryonic EC heterogeneity at single-cell resolution and demonstrate that the diversity of ECs in the embryo arises from both tissue origin and vascular-bed position. Developing aECs and vECs share common venous-featured capillary precursors and are regulated by distinct transcriptional regulatory networks.

Transcription factors regulating vasculogenesis and angiogenesis

Transcription factors (TFs) play a crucial role in regulating the dynamic and precise patterns of gene expression required for the initial specification of endothelial cells (ECs), and during endothelial growth and differentiation. While sharing many core features, ECs can be highly heterogeneous. Differential gene expression between ECs is essential to pattern the hierarchical vascular network into arteries, veins and capillaries, to drive angiogenic growth of new vessels, and to direct specialization in response to local signals. Unlike many other cell types, ECs have no single master regulator, instead relying on differing combinations of a necessarily limited repertoire of TFs to achieve tight spatial and temporal activation and repression of gene expression. Here, we will discuss the cohort of TFs known to be involved in directing gene expression during different stages of mammalian vasculogenesis and angiogenesis, with a primary focus on development.

Engineered hematopoietic and immune cells derived from human pluripotent stem cells

For the past decade, significant advances have been achieved in human hematopoietic stem cell (HSC) transplantation for treating various blood diseases and cancers. However, challenges remain with the quality control, amount, and cost of HSCs and HSC-derived immune cells. The advent of human pluripotent stem cells (hPSCs) may transform HSC transplantation and cancer immunotherapy by providing a cost-effective and scalable cell source for fundamental studies and translational applications. In this review, we discuss the current developments in the field of stem cell engineering for hematopoietic stem and progenitor cell (HSPC) differentiation and further differentiation of HSPCs into functional immune cells. The key advances in stem cell engineering include the generation of HSPCs from hPSCs, genetic modification of hPSCs, and hPSC-derived HSPCs for improved function, further differentiation of HPSCs into functional immune cells, and applications of cell culture platforms for hematopoietic cell manufacturing. Current challenges impeding the translation of hPSC-HSPCs and immune cells as well as further directions to address these challenges are also discussed.

Sox7-positive endothelial progenitors establish coronary arteries and govern ventricular compaction

The cardiac endothelium influences ventricular chamber development by coordinating trabeculation and compaction. However, the endothelial-specific molecular mechanisms mediating this coordination are not fully understood. Here, we identify the Sox7 transcription factor as a critical cue instructing cardiac endothelium identity during ventricular chamber development. Endothelial-specific loss of Sox7 function in mice results in cardiac ventricular defects similar to non-compaction cardiomyopathy, with a change in the proportions of trabecular and compact cardiomyocytes in the mutant hearts. This phenotype is paralleled by abnormal coronary artery formation. Loss of Sox7 function disrupts the transcriptional regulation of the Notch pathway and connexins 37 and 40, which govern coronary arterial specification. Upon Sox7 endothelial-specific deletion, single-nuclei transcriptomics analysis identifies the depletion of a subset of Sox9/Gpc3-positive endocardial progenitor cells and an increase in erythro-myeloid cell lineages. Fate mapping analysis reveals that a subset of Sox7-null endothelial cells transdifferentiate into hematopoietic but not cardiomyocyte lineages. Our findings determine that Sox7 maintains cardiac endothelial cell identity, which is crucial to the cellular cross-talk that drives ventricular compaction and coronary artery development.

Transcriptional regulators of arterial and venous identity in the developing mammalian embryo

The complex and hierarchical vascular network of arteries, veins, and capillaries features considerable endothelial heterogeneity, yet the regulatory pathways directing arteriovenous specification, differentiation, and identity are still not fully understood. Recent advances in analysis of endothelial-specific gene-regulatory elements, single-cell RNA sequencing, and cell lineage tracing have both emphasized the importance of transcriptional regulation in this process and shed considerable light on the mechanism and regulation of specification within the endothelium. In this review, we discuss recent advances in our understanding of how endothelial cells acquire arterial and venous identity and the role different transcription factors play in this process.

Sox17 protects human brain microvascular endothelial cells from AngII-induced injury by regulating autophagy and apoptosis

Intracranial aneurysm (IA), is a localized dilation of the intracranial arteries, the rupture of which is catastrophic. Hypertension is major IA risk factor that mediates endothelial cell damage. Sox17 is highly expressed in intracranial vascular endothelial cells, and GWAS studies indicate that its genetic alteration is one of the major genetic risk factors for IA. Vascular endothelial cell injury plays a vital role in the pathogenesis of IA. The genetic ablation of Sox17 plus hypertension induced by AngII can lead to an increased incidence of intracranial aneurysms had tested in the previous animal experiments. In order to study the underlying molecular mechanisms, we established stable Sox17-overexpressing and knockdown cell lines in human brain microvascular endothelial cells (HBMECs) first. Then flow cytometry, western blotting, and immunofluorescence were employed. We found that the knockdown of Sox17 could worsen the apoptosis and autophagy of HBMECs caused by AngII, while overexpression of Sox17 had the opposite effect. Transmission electron microscopy displayed increased autophagosomes after the knockdown of Sox17 in HBMECs. The RNA-sequencing analysis shown that dysregulation of the Sox17 gene was closely associated with the autophagy-related pathways. Our study suggests that Sox17 could protect HBMECs from AngII-induced injury by regulating autophagy and apoptosis.

DLL4 promotes partial endothelial-to-mesenchymal transition at atherosclerosis-prone regions of arteries

Flowing blood regulates vascular development, homeostasis and disease by generating wall shear stress which has major effects on endothelial cell (EC) physiology. Low oscillatory shear stress (LOSS) induces a form of cell plasticity called endothelial-to-mesenchymal transition (EndMT). This process has divergent effects; in embryos LOSS-induced EndMT drives the development of atrioventricular valves, whereas in adult arteries it is associated with inflammation and atherosclerosis. The Notch ligand DLL4 is essential for LOSS-dependent valve development; here we investigated whether DLL4 is required for responses to LOSS in adult arteries. Analysis of cultured human coronary artery EC revealed that DLL4 regulates the transcriptome to induce markers of EndMT and inflammation under LOSS conditions. Consistently, genetic deletion of Dll4 from murine EC reduced SNAIL (EndMT marker) and VCAM-1 (inflammation marker) at a LOSS region of the murine aorta. We hypothesized that endothelial Dll4 is pro-atherogenic but this analysis was confounded because endothelial Dll4 negatively regulated plasma cholesterol levels in hyperlipidemic mice. We conclude that endothelial DLL4 is required for LOSS-induction of EndMT and inflammation regulators at atheroprone regions of arteries, and is also a regulator of plasma cholesterol.

The tissue-specific transcriptional landscape underlines the involvement of endothelial cells in health and disease

Endothelial cells (ECs) that line vascular and lymphatic vessels are being increasingly recognized as important to organ function in health and disease. ECs participate not only in the trafficking of gases, metabolites, and cells between the bloodstream and tissues but also in the angiocrine-based induction of heterogeneous parenchymal cells, which are unique to their specific tissue functions. The molecular mechanisms regulating EC heterogeneity between and within different tissues are modeled during embryogenesis and become fully established in adults. Any changes in adult tissue homeostasis induced by aging, stress conditions, and various noxae may reshape EC heterogeneity and induce specific transcriptional features that condition a functional phenotype. Heterogeneity is sustained via specific genetic programs organized through the combinatory effects of a discrete number of transcription factors (TFs) that, at the single tissue-level, constitute dynamic networks that are post-transcriptionally and epigenetically regulated. This review is focused on outlining the TF-based networks involved in EC specialization and physiological and pathological stressors thought to modify their architecture.

PI(4,5)P2-dependent regulation of endothelial tip cell specification contributes to angiogenesis

Dynamic positioning of endothelial tip and stalk cells, via the interplay between VEGFR2 and NOTCH signaling, is essential for angiogenesis. VEGFR2 activates PI3K, which phosphorylates PI(4,5)P₂ to PI(3,4,5)P₃, activating AKT; however, PI3K/AKT does not direct tip cell specification. We report that PI(4,5)P₂ hydrolysis by the phosphoinositide-5-phosphatase, INPP5K, contributes to angiogenesis. INPP5K ablation disrupted tip cell specification and impaired embryonic angiogenesis associated with enhanced DLL4/NOTCH signaling. INPP5K degraded a pool of PI(4,5)P₂ generated by PIP5K1C phosphorylation of PI(4)P in endothelial cells. INPP5K ablation increased PI(4,5)P₂, thereby releasing β-catenin from the plasma membrane, and concurrently increased PI(3,4,5)P₃-dependent AKT activation, conditions that licensed DLL4/NOTCH transcription. Suppression of PI(4,5)P₂ in INPP5K-siRNA cells by PIP5K1C-siRNA, restored β-catenin membrane localization and normalized AKT signaling. Pharmacological NOTCH or AKT inhibition in vivo or genetic β-catenin attenuation rescued angiogenesis defects in INPP5K-null mice. Therefore, PI(4,5)P₂ is critical for β-catenin/DLL4/NOTCH signaling, which governs tip cell specification during angiogenesis.

The blood vasculature instructs lymphatic patterning in a SOX7-dependent manner

Despite a growing catalog of secreted factors critical for lymphatic network assembly, little is known about the mechanisms that modulate the expression level of these molecular cues in blood vascular endothelial cells (BECs). Here, we show that a BEC-specific transcription factor, SOX7, plays a crucial role in a non-cell-autonomous manner by modulating the transcription of angiocrine signals to pattern lymphatic vessels. While SOX7 is not expressed in lymphatic endothelial cells (LECs), the conditional loss of SOX7 function in mouse embryos causes a dysmorphic dermal lymphatic phenotype. We identify novel distant regulatory regions in mice and humans that contribute to directly repressing the transcription of a major lymphangiogenic growth factor (Vegfc) in a SOX7-dependent manner. Further, we show that SOX7 directly binds HEY1, a canonical repressor of the Notch pathway, suggesting that transcriptional repression may also be modulated by the recruitment of this protein partner at Vegfc genomic regulatory regions. Our work unveils a role for SOX7 in modulating downstream signaling events crucial for lymphatic patterning, at least in part via the transcriptional repression of VEGFC levels in the blood vascular endothelium.

Sox17 mediates adult arterial endothelial cell adaptation to hemodynamics

Sox17 is a critical regulator of arterial identity during early embryonic vascular development. However, its role in adult endothelial cells (ECs) are not fully understood. Sox17 is highly expressed in arterial ECs but not in venous ECs throughout embryonic development to adulthood suggesting that it may play a functional role in adult arteries. Here, we investigated Sox17 mediated phenotypical changes in adult ECs. To precisely control the temporal expression level of Sox17, we designed a tetracycline-inducible lentiviral gene expression system to express Sox17 selectively in cultured venous ECs. We confirmed that Sox17-induced ECs exhibit a gene profile favoring arterial and tip cell identity. Furthermore, in comparison to control ECs, Sox17-activated ECs under shear leads to greater expression of arterial markers and suppression of venous identity. These data suggest that Sox17 enables greater hemodynamic adaptability of ECs in response to fluid shear stress. Here, we also demonstrate key morphogenic behaviors of Sox17-mediated ECs. In both vasculogenic and angiogenic 3D fibrin gel studies, Sox17-mediated ECs prefer to form cohesive vessels with one another while interfering the vessel formation of the control ECs. Sox17-mediated ECs elicit hyper-sprouting behavior in the presence of pericytes but not fibroblasts, suggesting Sox17 mediated sprouting frequency is dependent on supporting cell type. Using a microfluidic chip, we also show that Sox17-mediated ECs maintain thinner diameter vessels that do not widen under interstitial flow like the control ECs. Taken together, these data showed that Sox17 mediated EC gene expression and phenotypical changes are highly modulated in the context of biomechanical stimuli, suggesting Sox17 plays a role in regulating the arterial ECs adaptability under arterial hemodynamics as well as tip cells behavior during angiogenesis and vasculogenesis. The results from this study may be valuable in improving vein graft adaptation to arterial hemodynamics and bioengineering microvasculature for tissue engineering applications.

Analysis of Vascular Morphogenesis in Zebrafish

Analysis of cardiovascular development in zebrafish embryos has become a major driver of vascular research in recent years. Imaging-based analyses have allowed the discovery or verification of morphologically distinct processes and mechanisms of, e.g., endothelial cell migration, angiogenic sprouting, tip or stalk cell behavior, and vessel anastomosis. In this chapter, we describe the techniques and tools used for confocal imaging of zebrafish endothelial development in combination with general experimental approaches for molecular dissection of involved signaling pathways.

De Novo Generation of Human Hematopoietic Stem Cells from Pluripotent Stem Cells for Cellular Therapy

The ability to manufacture human hematopoietic stem cells (HSCs) in the laboratory holds enormous promise for cellular therapy of human blood diseases. Several differentiation protocols have been developed to facilitate the emergence of HSCs from human pluripotent stem cells (PSCs). Most approaches employ a stepwise addition of cytokines and morphogens to recapitulate the natural developmental process. However, these protocols globally lack clinical relevance and uniformly induce PSCs to produce hematopoietic progenitors with embryonic features and limited engraftment and differentiation capabilities. This review examines how key intrinsic cues and extrinsic environmental inputs have been integrated within human PSC differentiation protocols to enhance the emergence of definitive hematopoiesis and how advances in genomics set the stage for imminent breakthroughs in this field.

JAG1-NOTCH4 mechanosensing drives atherosclerosis

Endothelial cell (EC) sensing of disturbed blood flow triggers atherosclerosis, a disease of arteries that causes heart attack and stroke, through poorly defined mechanisms. The Notch pathway plays a central role in blood vessel growth and homeostasis, but its potential role in sensing of disturbed flow has not been previously studied. Here, we show using porcine and murine arteries and cultured human coronary artery EC that disturbed flow activates the JAG1-NOTCH4 signaling pathway. Light-sheet imaging revealed enrichment of JAG1 and NOTCH4 in EC of atherosclerotic plaques, and EC-specific genetic deletion of Jag1 (Jag1ECKO) demonstrated that Jag1 promotes atherosclerosis at sites of disturbed flow. Mechanistically, single-cell RNA sequencing in Jag1ECKO mice demonstrated that Jag1 suppresses subsets of ECs that proliferate and migrate. We conclude that JAG1-NOTCH4 sensing of disturbed flow enhances atherosclerosis susceptibility by regulating EC heterogeneity and that therapeutic targeting of this pathway may treat atherosclerosis.

A SOX17-PDGFB signaling axis regulates aortic root development

Developmental etiologies causing complex congenital aortic root abnormalities are unknown. Here we show that deletion of Sox17 in aortic root endothelium in mice causes underdeveloped aortic root leading to a bicuspid aortic valve due to the absence of non-coronary leaflet and mispositioned left coronary ostium. The respective defects are associated with reduced proliferation of non-coronary leaflet mesenchyme and aortic root smooth muscle derived from the second heart field cardiomyocytes. Mechanistically, SOX17 occupies a Pdgfb transcriptional enhancer to promote its transcription and Sox17 deletion inhibits the endothelial Pdgfb transcription and PDGFB growth signaling to the non-coronary leaflet mesenchyme. Restoration of PDGFB in aortic root endothelium rescues the non-coronary leaflet and left coronary ostium defects in Sox17 nulls. These data support a SOX17-PDGFB axis underlying aortic root development that is critical for aortic valve and coronary ostium patterning, thereby informing a potential shared disease mechanism for concurrent anomalous aortic valve and coronary arteries.

Prediction and Experimental Validation of a New Salinity-Responsive Cis-Regulatory Element (CRE) in a Tilapia Cell Line

Transcriptional regulation is a major mechanism by which organisms integrate gene x environment interactions. It can be achieved by coordinated interplay between cis-regulatory elements (CREs) and transcription factors (TFs). Euryhaline tilapia (Oreochromis mossambicus) tolerate a wide range of salinity and thus are an appropriate model to examine transcriptional regulatory mechanisms during salinity stress in fish. Quantitative proteomics in combination with the transcription inhibitor actinomycin D revealed 19 proteins that are transcriptionally upregulated by hyperosmolality in tilapia brain (OmB) cells. We searched the extended proximal promoter up to intron1 of each corresponding gene for common motifs using motif discovery tools. The top-ranked motif identified (STREME1) represents a binding site for the Forkhead box TF L1 (FoxL1). STREME1 function during hyperosmolality was experimentally validated by choosing two of the 19 genes, chloride intracellular channel 2 (clic2) and uridine phosphorylase 1 (upp1), that are enriched in STREME1 in their extended promoters. Transcriptional induction of these genes during hyperosmolality requires STREME1, as evidenced by motif mutagenesis. We conclude that STREME1 represents a new functional CRE that contributes to gene x environment interactions during salinity stress in tilapia. Moreover, our results indicate that FoxL1 family TFs are contribute to hyperosmotic induction of genes in euryhaline fish.

Epigenetic Regulation of Endothelial Cell Lineages During Zebrafish Development-New Insights From Technical Advances

Epigenetic regulation is integral in orchestrating the spatiotemporal regulation of gene expression which underlies tissue development. The emergence of new tools to assess genome-wide epigenetic modifications has enabled significant advances in the field of vascular biology in zebrafish. Zebrafish represents a powerful model to investigate the activity of cis-regulatory elements in vivo by combining technologies such as ATAC-seq, ChIP-seq and CUT&Tag with the generation of transgenic lines and live imaging to validate the activity of these regulatory elements. Recently, this approach led to the identification and characterization of key enhancers of important vascular genes, such as gata2a, notch1b and dll4. In this review we will discuss how the latest technologies in epigenetics are being used in the zebrafish to determine chromatin states and assess the function of the cis-regulatory sequences that shape the zebrafish vascular network.

FOXO1 represses sprouty 2 and sprouty 4 expression to promote arterial specification and vascular remodeling in the mouse yolk sac

Establishing a functional circulatory system is required for post-implantation development during murine embryogenesis. Previous studies in loss-of-function mouse models showed that FOXO1, a Forkhead family transcription factor, is required for yolk sac (YS) vascular remodeling and survival beyond embryonic day (E) 11. Here, we demonstrate that at E8.25, loss of Foxo1 in Tie2-cre expressing cells resulted in increased sprouty 2 (Spry2) and Spry4 expression, reduced arterial gene expression and reduced Kdr (also known as Vegfr2 and Flk1) transcripts without affecting overall endothelial cell identity, survival or proliferation. Using a Dll4-BAC-nlacZ reporter line, we found that one of the earliest expressed arterial genes, delta like 4, is significantly reduced in Foxo1 mutant YS without being substantially affected in the embryo proper. We show that FOXO1 binds directly to previously identified Spry2 gene regulatory elements (GREs) and newly identified, evolutionarily conserved Spry4 GREs to repress their expression. Furthermore, overexpression of Spry4 in transient transgenic embryos largely recapitulates the reduced expression of arterial genes seen in conditional Foxo1 mutants. Together, these data reveal a novel role for FOXO1 as a key transcriptional repressor regulating both pre-flow arterial specification and subsequent vessel remodeling within the murine YS.

Finding and Verifying Enhancers for Endothelial-Expressed Genes

Identification and analysis of enhancers for endothelial-expressed genes can provide crucial information regarding their upstream transcriptional regulators. However, enhancer identification can be challenging, particularly for people with limited access or experience of bioinformatics, and transgenic analysis of enhancer activity patterns can be prohibitively expensive. Here we describe how to use publicly available datasets displayed on the UCSC Genome Browser to identify putative endothelial enhancers for mammalian genes. Furthermore, we detail how to utilize mosaic Tol2-mediated transgenesis in zebrafish to verify whether a putative enhancer is capable of directing endothelial-specific patterns of gene expression.

A transcription factor is the target of propranolol treatment in infantile hemangioma

Propranolol is a nonselective β-adrenergic receptor (AR) blocker that has been the first-line therapy for problematic infantile hemangioma (IH), the most frequent childhood vascular tumor. Although IHs are benign and eventually regress spontaneously, at least 15% of patients require treatment. Despite the extensive use of propranolol for IH treatment, its mode of action remains unclear. In this issue of the JCI, Seebauer et al. investigated the cellular and molecular consequences of propranolol treatment on IH vascular tumor formation in a murine model of IH. The efficacy of propranolol was independent of its β-AR blocker activity and was attributable to the direct targeting of the transcription factor SOX18, which, in turn, reduced hemangioma blood vessel formation. We believe these results will guide clinical translation for the use of more efficient and safer therapies for IH and possibly for other vascular anomalies in which SOX18 plays a role.

Analysis of Placental Arteriovenous Formation Reveals New Insights Into Embryos With Congenital Heart Defects

The placental vasculature provides the developing embryo with a circulation to deliver nutrients and dispose of waste products. However, in the mouse, the vascular components of the chorio-allantoic placenta have been largely unexplored due to a lack of well-validated molecular markers. This is required to study how these blood vessels form in development and how they are impacted by embryonic or maternal defects. Here, we employed marker analysis to characterize the arterial/arteriole and venous/venule endothelial cells (ECs) during normal mouse placental development. We reveal that placental ECs are potentially unique compared with their embryonic counterparts. We assessed embryonic markers of arterial ECs, venous ECs, and their capillary counterparts-arteriole and venule ECs. Major findings were that the arterial tree exclusively expressed Dll4, and venous vascular tree could be distinguished from the arterial tree by Endomucin (EMCN) expression levels. The relationship between the placenta and developing heart is particularly interesting. These two organs form at the same stages of embryogenesis and are well known to affect each other's growth trajectories. However, although there are many mouse models of heart defects, these are not routinely assessed for placental defects. Using these new placental vascular markers, we reveal that mouse embryos from one model of heart defects, caused by maternal iron deficiency, also have defects in the formation of the placental arterial, but not the venous, vascular tree. Defects to the embryonic cardiovascular system can therefore have a significant impact on blood flow delivery and expansion of the placental arterial tree.

A dominant-negative SOX18 mutant disrupts multiple regulatory layers essential to transcription factor activity

Few genetically dominant mutations involved in human disease have been fully explained at the molecular level. In cases where the mutant gene encodes a transcription factor, the dominant-negative mode of action of the mutant protein is particularly poorly understood. Here, we studied the genome-wide mechanism underlying a dominant-negative form of the SOX18 transcription factor (SOX18^RaOp) responsible for both the classical mouse mutant Ragged Opossum and the human genetic disorder Hypotrichosis-lymphedema-telangiectasia-renal defect syndrome. Combining three single-molecule imaging assays in living cells together with genomics and proteomics analysis, we found that SOX18^RaOp disrupts the system through an accumulation of molecular interferences which impair several functional properties of the wild-type SOX18 protein, including its target gene selection process. The dominant-negative effect is further amplified by poisoning the interactome of its wild-type counterpart, which perturbs regulatory nodes such as SOX7 and MEF2C. Our findings explain in unprecedented detail the multi-layered process that underpins the molecular aetiology of dominant-negative transcription factor function.

Endothelial ontogeny and the establishment of vascular heterogeneity

The establishment of distinct cellular identities was pivotal during the evolution of Metazoa, enabling the emergence of an array of specialized tissues with different functions. In most animals including vertebrates, cell specialization occurs in response to a combination of intrinsic (e.g., cellular ontogeny) and extrinsic (e.g., local environment) factors that drive the acquisition of unique characteristics at the single-cell level. The first functional organ system to form in vertebrates is the cardiovascular system, which is lined by a network of endothelial cells whose organ-specific characteristics have long been recognized. Recent genetic analyses at the single-cell level have revealed that heterogeneity exists not only at the organ level but also between neighboring endothelial cells. Thus, how endothelial heterogeneity is established has become a key question in vascular biology. Drawing upon evidence from multiple organ systems, here we will discuss the role that lineage history may play in establishing endothelial heterogeneity.

Cerebrovascular development: mechanisms and experimental approaches

The cerebral vasculature plays a central role in human health and disease and possesses several unique anatomic, functional and molecular characteristics. Despite their importance, the mechanisms that determine cerebrovascular development are less well studied than other vascular territories. This is in part due to limitations of existing models and techniques for visualisation and manipulation of the cerebral vasculature. In this review we summarise the experimental approaches used to study the cerebral vessels and the mechanisms that contribute to their development.

SOX17 Loss-of-Function Mutation Underlying Familial Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) refers to a rare, progressive disorder that is characterized by occlusive pulmonary vascular remodeling, resulting in increased pulmonary arterial pressure, right-sided heart failure, and eventual death. Emerging evidence from genetic investigations of pediatric-onset PAH highlights the strong genetic basis underpinning PAH, and deleterious variants in multiple genes have been found to cause PAH. Nevertheless, PAH is of substantial genetic heterogeneity, and the genetic defects underlying PAH in the overwhelming majority of cases remain elusive. In this investigation, a consanguineous family suffering from PAH transmitted as an autosomal-dominant trait was identified. Through whole-exome sequencing and bioinformatic analyses as well as Sanger sequencing analyses of the PAH family, a novel heterozygous SOX17 mutation, NM_022454.4: c.379C>T; p. (Gln127*), was found to co-segregate with the disease in the family, with complete penetrance. The nonsense mutation was neither observed in 612 unrelated healthy volunteers nor retrieved in the population genetic databases encompassing the Genome Aggregation Database, the Exome Aggregation Consortium database, and the Single Nucleotide Polymorphism database. Biological analyses using a dual-luciferase reporter assay system revealed that the Gln127*-mutant SOX17 protein lost the ability to transcriptionally activate its target gene NOTCH1. Moreover, the Gln127*-mutant SOX17 protein exhibited no inhibitory effect on the function of CTNNB1-encode β-catenin, which is a key player in vascular morphogenesis. This research firstly links SOX17 loss-of-function mutation to familial PAH, which provides novel insight into the molecular pathogenesis of PAH, suggesting potential implications for genetic and prognostic risk evaluation as well as personalized prophylaxis of the family members affected with PAH.

The complexity of tumour angiogenesis based on recently described molecules

Tumour angiogenesis is a crucial factor associated with tumour growth, progression, and metastasis. The whole process is the result of an interaction between a wide range of different molecules, influencing each other. Herein we summarize novel discoveries related to the less known angiogenic molecules such as galectins, pentraxin-3, Ral-interacting protein of 76 kDa (RLIP76), long non-coding RNAs (lncRNAs), B7-H3, and delta-like ligand-4 (DLL-4) and their role in the process of tumour angiogenesis. These molecules influence the most important molecular pathways involved in the formation of blood vessels in cancer, including the vascular endothelial growth factor (VEGF)-vascular endothelial growth factor receptor interaction (VEGFR), HIF1-a activation, or PI3K/Akt/mTOR and JAK-STAT signalling pathways. Increased expression of galectins, RLIP76, and B7H3 has been proven in several malignancies. Pentraxin-3, which appears to inhibit tumour angiogenesis, shows reduced expression in tumour tissues. Anti-angiogenic treatment based mainly on VEGF inhibition has proved to be of limited effectiveness, leading to the development of drug resistance. The newly discovered molecules are of great interest as a potential source of new anti-cancer therapies. Their role as targets for new drugs and as prognostic markers in neoplasms is discussed in this review.

SOX7 suppresses endothelial-to-mesenchymal transitions by enhancing VE-cadherin expression during outflow tract development

The endothelial-to-mesenchymal transition (EndMT) is a critical process that occurs during the development of the outflow tract (OFT). Malformations of the OFT can lead to the occurrence of conotruncal defect (CTD). SOX7 duplication has been reported in patients with congenital CTD, but its specific role in OFT development remains poorly understood. To decipher this, histological analysis showed that SRY-related HMG-box 7 (SOX7) was regionally expressed in the endocardial endothelial cells and in the mesenchymal cells of the OFT, where EndMT occurs. Experiments, using in vitro collagen gel culture system, revealed that SOX7 was a negative regulator of EndMT that inhibited endocardial cell (EC) migration and resulted in decreased number of mesenchymal cells. Forced expression of SOX7 in endothelial cells blocked further migration and improved the expression of the adhesion protein vascular endothelial (VE)-cadherin (VE-cadherin). Moreover, a VE-cadherin knockdown could partly reverse the SOX7-mediated repression of cell migration. Luciferase and electrophoretic mobility shift assay (EMSA) demonstrated that SOX7 up-regulated VE-cadherin by directly binding to the gene's promoter in endothelial cells. The coding exons and splicing regions of the SOX7 gene were also scanned in the 536 sporadic CTD patients and in 300 unaffected controls, which revealed four heterozygous SOX7 mutations. Luciferase assays revealed that two SOX7 variants weakened the transactivation of the VE-cadherin promoter. In conclusion, SOX7 inhibited EndMT during OFT development by directly up-regulating the endothelial-specific adhesion molecule VE-cadherin. SOX7 mutations can lead to impaired EndMT by regulating VE-cadherin, which may give rise to the molecular mechanisms associated with SOX7 in CTD pathogenesis.

ETS factors are required but not sufficient for specific patterns of enhancer activity in different endothelial subtypes

Correct vascular differentiation requires distinct patterns of gene expression in different subtypes of endothelial cells. Members of the ETS transcription factor family are essential for the transcriptional activation of arterial and angiogenesis-specific gene regulatory elements, leading to the hypothesis that they play lineage-defining roles in arterial and angiogenic differentiation directly downstream of VEGFA signalling. However, an alternative explanation is that ETS binding at enhancers and promoters is a general requirement for activation of many endothelial genes regardless of expression pattern, with subtype-specificity provided by additional factors. Here we use analysis of Ephb4 and Coup-TFII (Nr2f2) vein-specific enhancers to demonstrate that ETS factors are equally essential for vein, arterial and angiogenic-specific enhancer activity patterns. Further, we show that ETS factor binding at these vein-specific enhancers is enriched by VEGFA signalling, similar to that seen at arterial and angiogenic enhancers. However, while arterial and angiogenic enhancers can be activated by VEGFA in vivo, the Ephb4 and Coup-TFII venous enhancers are not, suggesting that the specificity of VEGFA-induced arterial and angiogenic enhancer activity occurs via non-ETS transcription factors. These results support a model in which ETS factors are not the primary regulators of specific patterns of gene expression in different endothelial subtypes.

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Vein-specific enhancers can contain essential ETS motifs.

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VEGFA induced an increase in ETS binding at vein, arterial and angiogenic enhancers.

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VEGFA stimulation cannot induce vein-specific enhancer activity.

Importance of endothelial Hey1 expression for thoracic great vessel development and its distal enhancer for Notch-dependent endothelial transcription

Thoracic great vessels such as the aorta and subclavian arteries are formed through dynamic remodeling of embryonic pharyngeal arch arteries (PAAs). Previous work has shown that loss of a basic helix-loop-helix transcription factor Hey1 in mice causes abnormal fourth PAA development and lethal great vessel anomalies resembling congenital malformations in humans. However, how Hey1 mediates vascular formation remains unclear. In this study, we revealed that Hey1 in vascular endothelial cells, but not in smooth muscle cells, played essential roles for PAA development and great vessel morphogenesis in mouse embryos. Tek-Cre-mediated Hey1 deletion in endothelial cells affected endothelial tube formation and smooth muscle differentiation in embryonic fourth PAAs and resulted in interruption of the aortic arch and other great vessel malformations. Cell specificity and signal responsiveness of Hey1 expression were controlled through multiple cis-regulatory regions. We found two distal genomic regions that had enhancer activity in endothelial cells and in the pharyngeal epithelium and somites, respectively. The novel endothelial enhancer was conserved across species and was specific to large-caliber arteries. Its transcriptional activity was regulated by Notch signaling in vitro and in vivo, but not by ALK1 signaling and other transcription factors implicated in endothelial cell specificity. The distal endothelial enhancer was not essential for basal Hey1 expression in mouse embryos but may likely serve for Notch-dependent transcriptional control in endothelial cells together with the proximal regulatory region. These findings help in understanding the significance and regulation of endothelial Hey1 as a mediator of multiple signaling pathways in embryonic vascular formation.

Antiangiogenic molecules from marine actinomycetes and the importance of using zebrafish model in cancer research

Blood vessel sprouting from pre-existing vessels or angiogenesis plays a significant role in tumour progression. Development of novel biomolecules from marine natural sources has a promising role in drug discovery specifically in the area of antiangiogenic chemotherapeutics. Symbiotic actinomycetes from marine origin proved to be potent and valuable sources of antiangiogenic compounds. Zebrafish represent a well-established model for small molecular screening and employed to study tumour angiogenesis over the last decade. Use of zebrafish has increased in the laboratory due to its various advantages like rapid embryo development, optically transparent embryos, large clutch size of embryos and most importantly high genetic conservation comparable to humans. Zebrafish also shares similar physiopathology of tumour angiogenesis with humans and with these advantages, zebrafish has become a popular model in the past decade to study on angiogenesis related disorders like diabetic retinopathy and cancer. This review focuses on the importance of antiangiogenic compounds from marine actinomycetes and utility of zebrafish in cancer angiogenesis research.

Location, Location, Location: How Vascular Specialization Influences Hematopoietic Fates During Development

During embryonic development, sequential waves of hematopoiesis give rise to blood-forming cells with diverse lineage potentials and self-renewal properties. This process must accomplish two important yet divergent goals: the rapid generation of differentiated blood cells to meet the needs of the developing embryo and the production of a reservoir of hematopoietic stem cells to provide for life-long hematopoiesis in the adult. Vascular beds in distinct anatomical sites of extraembryonic tissues and the embryo proper provide the necessary conditions to support these divergent objectives, suggesting a critical role for specialized vascular niche cells in regulating disparate blood cell fates during development. In this review, we will examine the current understanding of how organ- and stage-specific vascular niche specialization contributes to the development of the hematopoietic system.

Coronary vessel formation in development and disease: mechanisms and insights for therapy

The formation of new blood vessels after myocardial infarction (MI) is essential for the survival of existing and regenerated cardiac tissue. However, the extent of endogenous revascularization after MI is insufficient, and MI can often result in ventricular remodelling, progression to heart failure and premature death. The neutral results of numerous clinical trials that have evaluated the efficacy of angiogenic therapy to revascularize the infarcted heart reflect our poor understanding of the processes required to form a functional coronary vasculature. In this Review, we describe the latest advances in our understanding of the processes involved in coronary vessel formation, with mechanistic insights taken from developmental studies. Coronary vessels originate from multiple cellular sources during development and form through a number of distinct and carefully orchestrated processes. The ectopic reactivation of developmental programmes has been proposed as a new paradigm for regenerative medicine, therefore, a complete understanding of these processes is crucial. Furthermore, knowledge of how these processes differ between the embryonic and adult heart, and how they might be more closely recapitulated after injury are critical for our understanding of regenerative biology, and might facilitate the identification of tractable molecular targets to therapeutically promote neovascularization and regeneration of the infarcted heart.

Coronary Revascularization During Heart Regeneration Is Regulated by Epicardial and Endocardial Cues and Forms a Scaffold for Cardiomyocyte Repopulation

Defective coronary network function and insufficient blood supply are both cause and consequence of myocardial infarction. Efficient revascularization after infarction is essential to support tissue repair and function. Zebrafish hearts exhibit a remarkable ability to regenerate, and coronary revascularization initiates within hours of injury, but how this process is regulated remains unknown. Here, we show that revascularization requires a coordinated multi-tissue response culminating with the formation of a complex vascular network available as a scaffold for cardiomyocyte repopulation. During a process we term "coronary-endocardial anchoring," new coronaries respond by sprouting (1) superficially within the regenerating epicardium and (2) intra-ventricularly toward the activated endocardium. Mechanistically, superficial revascularization is guided by epicardial Cxcl12-Cxcr4 signaling and intra-ventricular sprouting by endocardial Vegfa signaling. Our findings indicate that the injury-activated epicardium and endocardium support cardiomyocyte replenishment initially through the guidance of coronary sprouting. Simulating this process in the injured mammalian heart should help its healing.

Nfatc1 Promotes Interstitial Cell Formation During Cardiac Valve Development in Zebrafish

RATIONALE

The transcription factor NFATC1 (nuclear factor of activated T-cell 1) has been implicated in cardiac valve formation in humans and mice, but we know little about the underlying mechanisms. To gain mechanistic understanding of cardiac valve formation at single-cell resolution and insights into the role of NFATC1 in this process, we used the zebrafish model as it offers unique attributes for live imaging and facile genetics.

OBJECTIVE

To understand the role of Nfatc1 in cardiac valve formation.

METHODS AND RESULTS

Using the zebrafish atrioventricular valve, we focus on the valve interstitial cells (VICs), which confer biomechanical strength to the cardiac valve leaflets. We find that initially atrioventricular endocardial cells migrate collectively into the cardiac jelly to form a bilayered structure; subsequently, the cells that led this migration invade the ECM (extracellular matrix) between the 2 endocardial cell monolayers, undergo endothelial-to-mesenchymal transition as marked by loss of intercellular adhesion, and differentiate into VICs. These cells proliferate and are joined by a few neural crest-derived cells. VIC expansion and a switch from a promigratory to an elastic ECM drive valve leaflet elongation. Functional analysis of Nfatc1 reveals its requirement during VIC development. Zebrafish nfatc1 mutants form significantly fewer VICs due to reduced proliferation and impaired recruitment of endocardial and neural crest cells during the early stages of VIC development. With high-speed microscopy and echocardiography, we show that reduced VIC formation correlates with valvular dysfunction and severe retrograde blood flow that persist into adulthood. Analysis of downstream effectors reveals that Nfatc1 promotes the expression of twist1b-a well-known regulator of epithelial-to-mesenchymal transition.

CONCLUSIONS

Our study sheds light on the function of Nfatc1 in zebrafish cardiac valve development and reveals its role in VIC formation. It also further establishes the zebrafish as a powerful model to carry out longitudinal studies of valve formation and function.

Dll4 Suppresses Transcytosis for Arterial Blood-Retinal Barrier Homeostasis

RATIONALE

Central nervous system has low vascular permeability by organizing tight junction (TJ) and limiting endothelial transcytosis. While TJ has long been considered to be responsible for vascular barrier in central nervous system, suppressed transcytosis in endothelial cells is now emerging as a complementary mechanism. Whether transcytosis regulation is independent of TJ and its dysregulation dominantly causes diseases associated with edema remain elusive. Dll4 signaling is important for various vascular contexts, but its role in the maintenance of vascular barrier in central nervous system remains unknown.

OBJECTIVE

To find a TJ-independent regulatory mechanism selective for transcytosis and identify its dysregulation as a cause of pathological leakage.

METHODS AND RESULTS

We studied transcytosis in the adult mouse retina with low vascular permeability and employed a hypertension-induced retinal edema model for its pathological implication. Both antibody-based and genetic inactivation of Dll4 or Notch1 induce hyperpermeability by increasing transcytosis without junctional destabilization in arterial endothelial cells, leading to nonhemorrhagic leakage predominantly in the superficial retinal layer. Endothelial Sox17 deletion represses Dll4 in retinal arteries, phenocopying Dll4 blocking-driven vascular leakage. Ang II (angiotensin II)-induced hypertension represses arterial Sox17 and Dll4, followed by transcytosis-driven retinal edema, which is rescued by a gain of Notch activity. Transcriptomic profiling of retinal endothelial cells suggests that Dll4 blocking activates SREBP1 (sterol regulatory element-binding protein 1)-mediated lipogenic transcription and enriches gene sets favorable for caveolae formation. Profiling also predicts the activation of VEGF (vascular endothelial growth factor) signaling by Dll4 blockade. Inhibition of SREBP1 or VEGF-VEGFR2 (VEGF receptor 2) signaling attenuates both Dll4 blockade-driven and hypertension-induced retinal leakage.

CONCLUSIONS

In the retina, Sox17-Dll4-SREBP1 signaling axis controls transcytosis independently of TJ in superficial arteries among heterogeneous regulations for the whole vessels. Uncontrolled transcytosis via dysregulated Dll4 underlies pathological leakage in hypertensive retina and could be a therapeutic target for treating hypertension-associated retinal edema.

Activation of the Arterial Program Drives Development of Definitive Hemogenic Endothelium with Lymphoid Potential

Understanding the pathways guiding the development of definitive hematopoiesis with lymphoid potential is essential for advancing human pluripotent stem cell (hPSC) technologies for the treatment of blood diseases and immunotherapies. In the embryo, lymphoid progenitors and hematopoietic stem cells (HSCs) arise from hemogenic endothelium (HE) lining arteries but not veins. Here, we show that activation of the arterial program through ETS1 overexpression or by modulating MAPK/ERK signaling pathways at the mesodermal stage of development dramatically enhanced the formation of arterial-type HE expressing DLL4 and CXCR4. Blood cells generated from arterial HE were more than 100-fold enriched in T cell precursor frequency and possessed the capacity to produce B lymphocytes and red blood cells expressing high levels of BCL11a and β-globin. Together, these findings provide an innovative strategy to aid in the generation of definitive lymphomyeloid progenitors and lymphoid cells from hPSCs for immunotherapy through enhancing arterial programming of HE.

Late developing cardiac lymphatic vasculature supports adult zebrafish heart function and regeneration

The cardiac lymphatic vascular system and its potentially critical functions in heart patients have been largely underappreciated, in part due to a lack of experimentally accessible systems. We here demonstrate that cardiac lymphatic vessels develop in young adult zebrafish, using coronary arteries to guide their expansion down the ventricle. Mechanistically, we show that in cxcr4a mutants with defective coronary artery development, cardiac lymphatic vessels fail to expand onto the ventricle. In regenerating adult zebrafish hearts the lymphatic vasculature undergoes extensive lymphangiogenesis in response to a cryoinjury. A significant defect in reducing the scar size after cryoinjury is observed in zebrafish with impaired Vegfc/Vegfr3 signaling that fail to develop intact cardiac lymphatic vessels. These results suggest that the cardiac lymphatic system can influence the regenerative potential of the myocardium.

Molecular mechanistic insights: The emerging role of SOXF transcription factors in tumorigenesis and development

Over the last decade, the development and progress of next-generation sequencers incorporated with classical biochemical analyses have drastically produced novel insights into transcription factors, including Sry-like high-mobility group box (SOX) factors. In addition to their primary functions in binding to and activating specific downstream genes, transcription factors also participate in the dedifferentiation or direct reprogramming of somatic cells to undifferentiated cells or specific lineage cells. Since the discovery of SOX factors, members of the SOXF (SOX7, SOX17, and SOX18) family have been identified to play broad roles, especially with regard to cardiovascular development. More recently, SOXF factors have been recognized as crucial players in determining the cell fate and in the regulation of cancer cells. Here, we provide an overview of research on the mechanism by which SOXF factors regulate development and cancer, and discuss their potential as new targets for cancer drugs while offering insight into novel mechanistic transcriptional regulation during cell lineage commitment.

Novel role of sex-determining region Y-box 7 (SOX7) in tumor biology and cardiovascular developmental biology

The sex-determining region Y-box 7 (Sox7) is an important member of the SOX F family, which is characterized by a high-mobility-group DNA-binding domain. Previous studies have demonstrated the role of SOX7 in cardiovascular development. SOX7 expression could be detected in normal adult tissues. Furthermore, the expression levels of SOX7 were different in different tumors. Most studies showed the downregulation of SOX7 in tumors, while some studies reported its upregulation in tumors. In this review, we first summarized the upstream regulators (including transcription factors, microRNAs (miRNAs), long noncoding RNAs (lncRNAs) and some exogenous regulators) and downstream molecules (including factors in the Wnt/β-catenin signaling pathway and some other signaling pathways) of SOX7. Then, the roles of SOX7 in multiple tumors were presented. Finally, the significance of divergent SOX7 expression during cardiovascular development was briefly discussed. The information compiled in this study characterized SOX7 during tumorigenesis and cardiovascular development, which should facilitate the design of future research and promote SOX7 as a therapeutic target.

Endothelial Cell Development and Its Application to Regenerative Medicine

The formation and remodeling of a functional circulatory system is critical for sustaining prenatal and postnatal life. During embryogenesis, newly differentiated endothelial cells require further specification to create the unique features of distinct vessel subtypes needed to support tissue morphogenesis. In this review, we explore signaling pathways and transcriptional regulators that modulate endothelial cell differentiation and specification, as well as applications of these processes to stem cell biology and regenerative medicine. We also summarize recent technical advances, including the growing utilization of single-cell sequencing to study vascular heterogeneity and development.

Cerebrovascular endothelial cells form transient Notch-dependent cystic structures in zebrafish

We identify a novel endothelial membrane behaviour in transgenic zebrafish. Cerebral blood vessels extrude large transient spherical structures that persist for an average of 23 min before regressing into the parent vessel. We term these structures "kugeln", after the German for sphere. Kugeln are only observed arising from the cerebral vessels and are present as late as 28 days post fertilization. Kugeln do not communicate with the vessel lumen and can form in the absence of blood flow. They contain little or no cytoplasm, but the majority are highly positive for nitric oxide reactivity. Kugeln do not interact with brain lymphatic endothelial cells (BLECs) and can form in their absence, nor do they perform a scavenging role or interact with macrophages. Inhibition of actin polymerization, Myosin II, or Notch signalling reduces kugel formation, while inhibition of VEGF or Wnt dysregulation (either inhibition or activation) increases kugel formation. Kugeln represent a novel Notch-dependent NO-containing endothelial organelle restricted to the cerebral vessels, of currently unknown function.

Lysophosphatidic acid-induced YAP/TAZ activation promotes developmental angiogenesis by repressing Notch ligand Dll4

Lysophosphatidic acid (LPA) is a potent lipid mediator with various biological functions mediated through six G protein-coupled receptors (GPCRs), LPA1-6. Previous studies have demonstrated that LPA-Gα12/Gα13 signaling plays an important role in embryonic vascular development. However, the responsible LPA receptors and underlying mechanisms are poorly understood. Here, we show a critical role of LPA4 and LPA6 in developmental angiogenesis. In mice, Lpa4;Lpa6 double knockout (DKO) embryos were lethal due to global vascular deficiencies, and endothelial cell (EC)-specific Lpa4;Lpa6 DKO retinas had impaired sprouting angiogenesis. Mechanistically, LPA activated the transcriptional regulators YAP and TAZ through LPA4/LPA6-mediated Gα12/Gα13-Rho-ROCK signaling in ECs. YAP/TAZ knockdown increased β-catenin- and Notch intracellular domain (NICD)-mediated endothelial expression of the Notch ligand delta-like 4 (DLL4). Fibrin gel sprouting assay revealed that LPA4/LPA6, Gα12/Gα13, or YAP/TAZ knockdown consistently blocked EC sprouting, which was rescued by a Notch inhibitor. Of note, the inhibition of Notch signaling also ameliorated impaired retinal angiogenesis in EC-specific Lpa4;Lpa6 DKO mice. Overall, these results suggest that the Gα12/Gα13-coupled receptors LPA4 and LPA6 synergistically regulate endothelial Dll4 expression through YAP/TAZ activation. This could in part account for the mechanism of YAP/TAZ-mediated developmental angiogenesis. Our findings provide a novel insight into the biology of GPCR-activated YAP/TAZ.

Regulatory pathways governing murine coronary vessel formation are dysregulated in the injured adult heart

The survival of ischaemic cardiomyocytes after myocardial infarction (MI) depends on the formation of new blood vessels. However, endogenous neovascularization is inefficient and the regulatory pathways directing coronary vessel growth are not well understood. Here we describe three independent regulatory pathways active in coronary vessels during development through analysis of the expression patterns of differentially regulated endothelial enhancers in the heart. The angiogenic VEGFA-MEF2 regulatory pathway is predominantly active in endocardial-derived vessels, whilst SOXF/RBPJ and BMP-SMAD pathways are seen in sinus venosus-derived arterial and venous coronaries, respectively. Although all developmental pathways contribute to post-MI vessel growth in the neonate, none are active during neovascularization after MI in adult hearts. This was particularly notable for the angiogenic VEGFA-MEF2 pathway, otherwise active in adult hearts and during neoangiogenesis in other adult settings. Our results therefore demonstrate a fundamental divergence between the regulation of coronary vessel growth in healthy and ischemic adult hearts.

How coronary vessels develop and respond to injury is not fully understood. Here, the authors use murine enhancer:reporter models to identify three transcriptional pathways active in different parts of coronary vasculature. These also contribute to neovascularization in the injured neonatal, but not adult, heart.

Veins and Arteries Build Hierarchical Branching Patterns Differently: Bottom-Up versus Top-Down

A tree-like hierarchical branching structure is present in many biological systems, such as the kidney, lung, mammary gland, and blood vessels. Most of these organs form through branching morphogenesis, where outward growth results in smaller and smaller branches. However, the blood vasculature is unique in that it exists as two trees (arterial and venous) connected at their tips. Obtaining this organization might therefore require unique developmental mechanisms. As reviewed here, recent data indicate that arterial trees often form in reverse order. Accordingly, initial arterial endothelial cell differentiation occurs outside of arterial vessels. These pre-artery cells then build trees by following a migratory path from smaller into larger arteries, a process guided by the forces imparted by blood flow. Thus, in comparison to other branched organs, arteries can obtain their structure through inward growth and coalescence. Here, new information on the underlying mechanisms is discussed, and how defects can lead to pathologies, such as hypoplastic arteries and arteriovenous malformations.

Venous identity requires BMP signalling through ALK3

Venous endothelial cells are molecularly and functionally distinct from their arterial counterparts. Although veins are often considered the default endothelial state, genetic manipulations can modulate both acquisition and loss of venous fate, suggesting that venous identity is the result of active transcriptional regulation. However, little is known about this process. Here we show that BMP signalling controls venous identity via the ALK3/BMPR1A receptor and SMAD1/SMAD5. Perturbations to TGF-β and BMP signalling in mice and zebrafish result in aberrant vein formation and loss of expression of the venous-specific gene Ephb4, with no effect on arterial identity. Analysis of a venous endothelium-specific enhancer for Ephb4 shows enriched binding of SMAD1/5 and a requirement for SMAD binding motifs. Further, our results demonstrate that BMP/SMAD-mediated Ephb4 expression requires the venous-enriched BMP type I receptor ALK3/BMPR1A. Together, our analysis demonstrates a requirement for BMP signalling in the establishment of Ephb4 expression and the venous vasculature.

The establishment of functional vasculatures requires the specification of newly formed vessels into veins and arteries. Here, Neal et al. use a combination of genetic approaches in mice and zebrafish to show that BMP signalling, via ALK3 and SMAD1/5, is required for venous specification during blood vessel development.

Rescue of hematopoietic stem/progenitor cells formation in plcg1 zebrafish mutant

Hematopoietic stem/progenitor cells (HSPC) in zebrafish emerge from the aortic hemogenic endothelium (HE) and migrate towards the caudal hematopoietic tissue (CHT), where they expand and differentiate during definitive hematopoiesis. Phospholipase C gamma 1 (Plcγ1) has been implicated for hematopoiesis in vivo and in vitro and is also required to drive arterial and HSPC formation. Genetic mutation in plcg1-/- (y10 allele) completely disrupts the aortic blood flow, specification of arterial fate, and HSPC formation in zebrafish embryos. We previously demonstrated that ginger treatment promoted definitive hematopoiesis via Bmp signaling. In this paper, we focus on HSPC development in plcg1-/- mutants and show that ginger/10-gingerol (10-G) can rescue the expression of arterial and HSPC markers in the HE and CHT in plcg1-/- mutant embryos. We demonstrate that ginger can induce scl/runx1 expression, and that rescued HE fate is dependent on Bmp and Notch. Bmp and Notch are known to regulate nitric oxide (NO) production and NO can induce hematopoietic stem cell fate. We show that ginger produces a robust up-regulation of NO. Taken together, we suggest in this paper that Bmp, Notch and NO are potential players that mediate the effect of ginger/10-G for rescuing the genetic defects in blood vessel specification and HSPC formation in plcg1-/- mutants. Understanding the molecular mechanisms of HSPC development in vivo is critical for understanding HSPC expansion, which will have a positive impact in regenerative medicine.

Kruppel-like factor 4 regulates developmental angiogenesis through disruption of the RBP-J-NICD-MAML complex in intron 3 of Dll4

Angiogenesis is a multistep process that requires highly regulated endothelial cell (EC) behavior. The transcription factor Krüppel-like factor 4 (KLF4) is a critical regulator of several basic EC functions; we have recently shown that KLF4 disturbs pathological (tumor) angiogenesis by mediating the expression of members of VEGF and Notch signaling pathways. Notch signaling is central to orchestration of sprouting angiogenesis but little is known about the upstream regulation of Notch itself. To determine the role of KLF4 in normal (developmental) angiogenesis, we used a mouse retinal angiogenesis model. We found that endothelial-specific overexpression of KLF4 in transgenic mice (EC-K4 Tg) leads to increased vessel density, branching and number of tip cell filopodia as assessed on postnatal day 6 (P6). The hypertrophic vasculature seen with sustained KLF4 overexpression is not stable and undergoes prominent remodeling during P7-P12 resulting in a normal appearing retinal vasculature in adult EC-K4 Tg mice. We find that KLF4 inhibits Delta-like 4 (DLL4) expression in the angiogenic front during retinal vascular development. Furthermore, in an oxygen-induced retinopathy model, overexpression of KLF4 results in decreased vaso-obliteration and neovascular tuft formation that is similar to genetic or pharmacologic DLL4 inhibition. Mechanistically, we show that KLF4 disables the activity of the essential Notch transcriptional activator RBP-J by interfering with binding of co-activators NICD and MAML at intron 3 of the Notch ligand DLL4. In summary, our experimental results demonstrate a regulatory role of KLF4 in developmental angiogenesis through regulation of DLL4 transcription.