β-Catenin-dependent transcription is central to Bmp-mediated formation of venous vessels

A β-catenin chromobody-based probe highlights endothelial maturation during vascular morphogenesis in vivo

Visualization of protein dynamics is a crucial step in understanding cellular processes. Chromobodies, fluorescently labelled single-domain antibodies, have emerged as versatile probes for live cell imaging of endogenous proteins. However, how these chromobodies behave in vivo and how accurately they monitor tissue changes remain poorly explored. Here, we generated an endothelial-specific β-catenin chromobody-derived probe and analyzed its expression pattern during cardiovascular development in zebrafish. Using high-resolution confocal imaging, we show that the chromobody signal correlates with the localization of β-catenin in the nucleus and at cell-cell junctions, and thereby can be used to assess endothelial maturation. Loss of Cadherin 5 strongly affects the localization of the chromobody at the cell membrane, confirming the cadherin-based adherens junction role of β-catenin. Furthermore, using a genetic model to block blood flow, we observed that cell junctions are compromised in most endothelial cells but not in the endocardium, highlighting the heterogeneous response of the endothelium to the lack of blood flow. Overall, our data further expand the use of chromobodies for in vivo applications and illustrate their potential to monitor tissue morphogenesis at high resolution.

Aggf1 Specifies Hemangioblasts at the Top of Regulatory Hierarchy via Npas4l and mTOR-S6K-Emp2-ERK Signaling

BACKGROUND

Hemangioblasts are mesoderm-derived multipotent stem cells for differentiation of all hematopoietic and endothelial cells in the circulation system. However, the underlying molecular mechanism is poorly understood.

METHODS

CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (type II CRISPR RNA-guided endonuclease) editing was used to develop aggf1-/- and emp2-/- knockout zebra fish. Whole-mount in situ hybridization and transgenic Tg(gata1-EGFP [enhanced green fluorescent protein]), Tg(mpx-EGFP), Tg(rag2-DsRed [discosoma sp. red fluorescent protein]), Tg(cd41-EGFP), Tg(kdrl-EGFP), and Tg(aggf1-/-;kdrl-EGFP) zebra fish were used to examine specification of hemangioblasts and hematopoietic stem and progenitor cells (HSPCs), hematopoiesis, and vascular development. Quantitative real-time polymerase chain reaction and Western blot analyses were used for expression analysis of genes and proteins.

RESULTS

Knockout of aggf1 impaired specification of hemangioblasts and HSPCs, hematopoiesis, and vascular development in zebra fish. Expression of npas4l/cloche-the presumed earliest marker for hemangioblast specification-was significantly reduced in aggf1-/- embryos and increased by overexpression of aggf1 in embryos. Overexpression of npas4l rescued the impaired specification of hemangioblasts and HSPCs and development of hematopoiesis and intersegmental vessels in aggf1-/- embryos, placing aggf1 upstream of npas4l in hemangioblast specification. To identify the underlying molecular mechanism, we identified emp2 as a key aggf1 downstream gene. Similar to aggf1, emp2 knockout impaired the specification of hemangioblasts and HSPCs, hematopoiesis, and angiogenesis by increasing the phosphorylation of ERK1/2 (extracellular signal-regulated protein kinase 1/2). Mechanistic studies showed that aggf1 knockdown and knockout significantly decreased the phosphorylated levels of mTOR (mammalian target of rapamycin) and p70 S6K (ribosomal protein S6 kinase), resulting in reduced protein synthesis of Emp2 (epithelial membrane protein 2), whereas mTOR activator MHY1485 (4,6-dimorpholino-N-(4-nitrophenyl)-1,3,5-triazin-2-amine) rescued the impaired specification of hemangioblasts and HSPCs and development of hematopoiesis and intersegmental vessels and reduced Emp2 expression induced by aggf1 knockdown.

CONCLUSIONS

These results indicate that aggf1 acts at the top of npas4l and becomes the earliest marker during specification of hemangioblasts. Our data identify a novel signaling axis of Aggf1 (angiogenic factor with G-patch and FHA domain 1)-mTOR-S6K-ERK1/2 for specification of hemangioblasts and HSPCs, primitive and definitive hematopoiesis, and vascular development. Our findings provide important insights into specification of hemangioblasts and HSPCs essential for the development of the circulation system.

Local angiogenic interplay of Vegfc/d and Vegfa controls brain region-specific emergence of fenestrated capillaries

Fenestrated and blood-brain barrier (BBB)-forming endothelial cells constitute major brain capillaries, and this vascular heterogeneity is crucial for region-specific neural function and brain homeostasis. How these capillary types emerge in a brain region-specific manner and subsequently establish intra-brain vascular heterogeneity remains unclear. Here, we performed a comparative analysis of vascularization across the zebrafish choroid plexuses (CPs), circumventricular organs (CVOs), and retinal choroid, and show common angiogenic mechanisms critical for fenestrated brain capillary formation. We found that zebrafish deficient for Gpr124, Reck, or Wnt7aa exhibit severely impaired BBB angiogenesis without any apparent defect in fenestrated capillary formation in the CPs, CVOs, and retinal choroid. Conversely, genetic loss of various Vegf combinations caused significant disruptions in Wnt7/Gpr124/Reck signaling-independent vascularization of these organs. The phenotypic variation and specificity revealed heterogeneous endothelial requirements for Vegfs-dependent angiogenesis during CP and CVO vascularization, identifying unexpected interplay of Vegfc/d and Vegfa in this process. Mechanistically, expression analysis and paracrine activity-deficient vegfc mutant characterization suggest that endothelial cells and non-neuronal specialized cell types present in the CPs and CVOs are major sources of Vegfs responsible for regionally restricted angiogenic interplay. Thus, brain region-specific presentations and interplay of Vegfc/d and Vegfa control emergence of fenestrated capillaries, providing insight into the mechanisms driving intra-brain vascular heterogeneity and fenestrated vessel formation in other organs.

Single-cell transcriptomic analysis of vascular endothelial cells in zebrafish embryos

Vascular endothelial cells exhibit substantial phenotypic and transcriptional heterogeneity which is established during early embryogenesis. However, the molecular mechanisms involved in establishing endothelial cell diversity are still not well understood. Zebrafish has emerged as an advantageous model to study vascular development. Despite its importance, the single-cell transcriptomic profile of vascular endothelial cells during zebrafish development is still missing. To address this, we applied single-cell RNA-sequencing (scRNA-seq) of vascular endothelial cells isolated from zebrafish embryos at the 24 hpf stage. Six distinct clusters or subclusters related to vascular endothelial cells were identified which include arterial, two venous, cranial, endocardial and endothelial progenitor cell subtypes. Furthermore, we validated our findings by characterizing novel markers for arterial, venous, and endocardial cells. We experimentally confirmed the presence of two transcriptionally different venous cell subtypes, demonstrating heterogeneity among venous endothelial cells at this early developmental stage. This dataset will be a valuable resource for future functional characterization of vascular endothelial cells and interrogation of molecular mechanisms involved in the establishment of their heterogeneity and cell-fate decisions.

Identification of Novel Vascular Genes Downstream of Islet2 and Nr2f1b Transcription Factors

The genetic regulation of vascular development is not elucidated completely. We previously characterized the transcription factors Islet2 (Isl2) and Nr2f1b as being critical for vascular growth. In this study, we further performed combinatorial microarrays to identify genes that are potentially regulated by these factors. We verified the changed expression of several targets in isl2/nr2f1b morphants. Those genes expressed in vessels during embryogenesis suggested their functions in vascular development. We selectively assayed a potential target follistatin a (fsta). Follistatin is known to inhibit BMP, and BMP signaling has been shown to be important for angiogenesis. However, the fsta’s role in vascular development has not been well studied. Here, we showed the vascular defects in ISV growth and CVP patterning while overexpressing fsta in the embryo, which mimics the phenotype of isl2/nr2f1b morphants. The vascular abnormalities are likely caused by defects in migration and proliferation. We further observed the altered expression of vessel markers consistent with the vascular defects in (fli:fsta) embryos. We showed that the knockdown of fsta can rescue the vascular defects in (fli:fsta) fish, suggesting the functional specificity of fsta. Moreover, the decreased expression of fsta rescues abnormal vessel growth in isl2 and nr2f1b morphants, indicating that fsta functions downstream of isl2/nr2f1b. Lastly, we showed that Isl2/Nr2f1b control vascular development, via Fsta–BMP signaling in part. Collectively, our microarray data identify many interesting genes regulated by isl2/nr2f1b, which likely function in the vasculature. Our research provides useful information on the genetic control of vascular development.

Identification and characterization of a special type of subnuclear structure: AGGF1-coated paraspeckles

AGGF1 is an angiogenic factor with G-Patch and FHA domains 1 described by our group. Gain-of-function mutations in AGGF1 cause Klippel-Trenaunay syndrome, whereas somatic loss-of-function mutations cause cancer. Paraspeckles are small membraneless subnuclear structures with a diameter of 0.5-1 μm, and composed of lncRNA NEAT1 as the scaffold and three core RNA-binding proteins NONO, PSPC1, and PSF. Here, we show that AGGF1 is a key regulatory and structural component of paraspeckles that induces paraspeckle formation, forms an outside rim of paraspeckles, wraps around the NONO/PSF/PSPC1/NEAT1 core, and regulates the size and number of paraspeckles. AGGF1-paraspeckles are larger (>1 μm) than conventional paraspeckles. RNA-FISH in combination with immunostaining shows that AGGF1, NONO, and NEAT1_2 co-localize in 20.58% of NEAT1_2-positive paraspeckles. Mechanistically, AGGF1 interacts with NONO, PSF, and HNRNPK, and upregulates NEAT1_2, a longer, 23 kb NEAT1 transcript with a key role in regulation of paraspeckle size and number. RNA-immunoprecipitation shows that AGGF1 interacts with NEAT1, which may be another possible mechanism underlying the formation of AGGF1-paraspeckles. NEAT1_2 knockdown reduces the number and size of AGGF1-paraspeckles. Functionally, AGGF1 regulates alternative RNA splicing as it decreases the exon skipping/inclusion ratio in a CD44 model. AGGF1 is also localized in some nuclear foci without NEAT1 or NONO, suggesting that AGGF1 is an important liquid-liquid phase separation (LLPS) driver for other types of AGGF1-positive nuclear condensates (referred to as AGGF1-bodies). Our results identify a special type of AGGF1-coated paraspeckles and provide important insights into the formation, structure, and function of paraspeckles.

Receptor and Molecular Mechanism of AGGF1 Signaling in Endothelial Cell Functions and Angiogenesis

Objective

Angiogenic factor AGGF1 (angiogenic factor with G-patch and FHA [Forkhead-associated] domain 1) promotes angiogenesis as potently as VEGFA (vascular endothelial growth factor A) and regulates endothelial cell (EC) proliferation, migration, specification of multipotent hemangioblasts and venous ECs, hematopoiesis, and vascular development and causes vascular disease Klippel-Trenaunay syndrome when mutated. However, the receptor for AGGF1 and the underlying molecular mechanisms remain to be defined.

Approach and Results

Using functional blocking studies with neutralizing antibodies, we identified [alpha]5[beta]1 as the receptor for AGGF1 on ECs. AGGF1 interacts with [alpha]5[beta]1 and activates FAK (focal adhesion kinase), Src (proto-oncogene tyrosine-protein kinase), and AKT (protein kinase B). Functional analysis of 12 serial N-terminal deletions and 13 C-terminal deletions by every 50 amino acids mapped the angiogenic domain of AGGF1 to a domain between amino acids 604-613 (FQRDDAPAS). The angiogenic domain is required for EC adhesion and migration, capillary tube formation, and AKT activation. The deletion of the angiogenic domain eliminated the effects of AGGF1 on therapeutic angiogenesis and increased blood flow in a mouse model for peripheral artery disease. A 40-mer or 15-mer peptide containing the angiogenic domain blocks AGGF1 function, however, a 15-mer peptide containing a single amino acid mutation from -RDD- to -RGD- (a classical RGD integrin-binding motif) failed to block AGGF1 function.

Conclusions

We have identified integrin [alpha]5[beta]1 as an EC receptor for AGGF1 and a novel AGGF1-mediated signaling pathway of [alpha]5[beta]1-FAK-Src-AKT for angiogenesis. Our results identify an FQRDDAPAS angiogenic domain of AGGF1 crucial for its interaction with [alpha]5[beta]1 and signaling.

Molecular and Cellular Mechanisms of Vascular Development in Zebrafish

The establishment of a functional cardiovascular system is crucial for the development of all vertebrates. Defects in the development of the cardiovascular system lead to cardiovascular diseases, which are among the top 10 causes of death worldwide. However, we are just beginning to understand which signaling pathways guide blood vessel growth in different tissues and organs. The advantages of the model organism zebrafish (Danio rerio) helped to identify novel cellular and molecular mechanisms of vascular growth. In this review we will discuss the current knowledge of vasculogenesis and angiogenesis in the zebrafish embryo. In particular, we describe the molecular mechanisms that contribute to the formation of blood vessels in different vascular beds within the embryo.

Zebrafish Vascular Mural Cell Biology: Recent Advances, Development, and Functions

Recruitment of mural cells to the vascular wall is essential for forming the vasculature as well as maintaining proper vascular functions. In recent years, zebrafish genetic tools for mural cell biology have improved substantially. Fluorescently labeled zebrafish mural cell reporter lines enable us to study, with higher spatiotemporal resolution than ever, the processes of mural cell development from their progenitors. Furthermore, recent phenotypic analysis of platelet-derived growth factor beta mutant zebrafish revealed well-conserved organotypic mural cell development and functions in vertebrates with the unique features of zebrafish. However, comprehensive reviews of zebrafish mural cells are lacking. Therefore, herein, we highlight recent advances in zebrafish mural cell tools. We also summarize the fundamental features of zebrafish mural cell development, especially at early stages, and functions.

The blood flow-klf6a-tagln2 axis drives vessel pruning in zebrafish by regulating endothelial cell rearrangement and actin cytoskeleton dynamics

Recent studies have focused on capillary pruning in various organs and species. However, the way in which large-diameter vessels are pruned remains unclear. Here we show that pruning of the zebrafish caudal vein (CV) from ventral capillaries of the CV plexus in different transgenic embryos is driven by endothelial cell (EC) rearrangement, which involves EC nucleus migration, junction remodeling, and actin cytoskeleton remodeling. Further observation reveals a growing difference in blood flow velocity between the two vessels in CV pruning in zebrafish embryos. With this model, we identify the critical role of Kruppel-like factor 6a (klf6a) in CV pruning. Disruption of klf6a functioning impairs CV pruning in zebrafish. klf6a is required for EC nucleus migration, junction remodeling, and actin cytoskeleton dynamics in zebrafish embryos. Moreover, actin-related protein transgelin 2 (tagln2) is a direct downstream target of klf6a in CV pruning in zebrafish embryos. Together these results demonstrate that the klf6a-tagln2 axis regulates CV pruning by promoting EC rearrangement.

Vascular remodeling is critical for vascular physiology and pathology. The primitive vascular plexus formed by angiogenesis, subsequently undergoes extensive vascular remodeling to establish a functionally and hierarchically branched network of blood vessels. Vascular remodeling mainly consists of vessel pruning and fusion. Endothelial cell rearrangement plays an essential role in vessel pruning, which involves endothelial cell migration and polarity. Dysfunction of flow-induced vascular remodeling will cause arteriovenous malformation and impair reperfusion of ischemia stroke. In this study, we show that the large-diameter vessel of the caudal vein is pruned from ventral capillaries of the caudal vein plexus in zebrafish embryos. With this model, we observe a growing difference in blood flow velocity between two branches in vessel pruning. We identify that the klf6a-tagln2 axis regulates CV pruning by promoting endothelial cell rearrangement and junction remodeling. Our results suggest that the caudal vein formation is an ideal model for screening the potential genes involved in vascular remodeling-related disease.

The Role of BMP Signaling in Endothelial Heterogeneity

Bone morphogenetic proteins (BMPs), which compose the largest group of the transforming growth factor-β (TGF-ß) superfamily, have been implied to play a crucial role in diverse physiological processes. The most intriguing feature of BMP signaling is that it elicits heterogeneous responses from cells with equivalent identity, thus permitting highly context-dependent signaling outcomes. In endothelial cells (ECs), which are increasingly perceived as a highly heterogeneous population of cells with respect to their morphology, function, as well as molecular characteristics, BMP signaling has shown to elicit diverse and often opposite effects, illustrating the innate complexity of signaling responses. In this review, we provide a concise yet comprehensive overview of how outcomes of BMP signaling are modulated in a context-dependent manner with an emphasis on the underlying molecular mechanisms and summarize how these regulations of the BMP signaling promote endothelial heterogeneity.

The BMP Pathway in Blood Vessel and Lymphatic Vessel Biology

Bone morphogenetic proteins (BMPs) were originally identified as the active components in bone extracts that can induce ectopic bone formation. In recent decades, their key role has broadly expanded beyond bone physiology and pathology. Nowadays, the BMP pathway is considered an important player in vascular signaling. Indeed, mutations in genes encoding different components of the BMP pathway cause various severe vascular diseases. Their signaling contributes to the morphological, functional and molecular heterogeneity among endothelial cells in different vessel types such as arteries, veins, lymphatic vessels and capillaries within different organs. The BMP pathway is a remarkably fine-tuned pathway. As a result, its signaling output in the vessel wall critically depends on the cellular context, which includes flow hemodynamics, interplay with other vascular signaling cascades and the interaction of endothelial cells with peri-endothelial cells and the surrounding matrix. In this review, the emerging role of BMP signaling in lymphatic vessel biology will be highlighted within the framework of BMP signaling in the circulatory vasculature.

Zebrafish Vascular Development: General and Tissue-Specific Regulation

Circulation is required for the delivery of oxygen and nutrition to tissues and organs, as well as waste collection. Therefore, the heart and vessels develop first during embryogenesis. The circulatory system consists of the heart, blood vessels, and blood cells, which originate from the mesoderm. The gene expression pattern required for blood vessel development is predetermined by the hierarchical and sequential regulation of genes for the differentiation of mesodermal cells. Herein, we review how blood vessels form distinctly in different tissues or organs of zebrafish and how vessel formation is universally or tissue-specifically regulated by signal transduction pathways and blood flow. In addition, the unsolved issues of mutual contacts and interplay of circulatory organs during embryogenesis are discussed.

FHA domain of AGGF1 is essential for its nucleocytoplasmic transport and angiogenesis

Angiogenic factor with G-patch and FHA domains 1 (AGGF1) exhibits a dynamic distribution from the nucleus to the cytoplasm in endothelial cells during angiogenesis, but the biological significance and underlying mechanism of this nucleocytoplasmic transport remains unknown. Here, we demonstrate that the dynamic distribution is essential for AGGF1 to execute its angiogenic function. To search the structural bases for this nucleocytoplasmic transport, we characterized three potential nuclear localization regions, one potential nuclear export region, forkhead-associated (FHA), and G-patch domains to determine their effects on nucleocytoplasmic transport and angiogenesis, and we show that AGGF1 remains intact during the dynamic subcellular distribution and the region from 260 to 288 amino acids acts as a signal for its nuclear localization. The distribution of AGGF1 in cytoplasm needs both FHA domain and 14-3-3α/β. Binding of AGGF1 via FHA domain to 14-3-3α/β is required to complete the transport. Thus, we for the first time established structural bases for the nucleocytoplasmic transport of AGGF1 and revealed that the FHA domain of AGGF1 is essential for its nucleocytoplasmic transport and angiogenesis.

Optogenetic investigation of BMP target gene expression diversity

Signaling molecules activate distinct patterns of gene expression to coordinate embryogenesis, but how spatiotemporal expression diversity is generated is an open question. In zebrafish, a BMP signaling gradient patterns the dorsal-ventral axis. We systematically identified target genes responding to BMP and found that they have diverse spatiotemporal expression patterns. Transcriptional responses to optogenetically delivered high- and low-amplitude BMP signaling pulses indicate that spatiotemporal expression is not fully defined by different BMP signaling activation thresholds. Additionally, we observed negligible correlations between spatiotemporal expression and transcription kinetics for the majority of analyzed genes in response to BMP signaling pulses. In contrast, spatial differences between BMP target genes largely collapsed when FGF and Nodal signaling were inhibited. Our results suggest that, similar to other patterning systems, combinatorial signaling is likely to be a major driver of spatial diversity in BMP-dependent gene expression in zebrafish.

Angiogenic factor AGGF1 acts as a tumor suppressor by modulating p53 post-transcriptional modifications and stability via MDM2

Angiogenesis factors are widely known to promote tumor growth by increasing tumor angiogenesis in the tumor microenvironment, however, little is known whether their intracellular function is involved in tumorigenesis. Here we show that AGGF1 acts as a tumor suppressor by regulating p53 when acting inside tumor cells. AGGF1 antagonizes MDM2 function to inhibit p53 ubiquitination, increases the acetylation, phosphorylation, stability and expression levels of p53, activates transcription of p53 target genes, and regulates cell proliferation, cell cycle, and apoptosis. AGGF1 also interacts with p53 through the FHA domain. Somatic AGGF1 variants in the FHA domain in human tumors, including p.Q467H, p.Y469 N, and p.N483T, inhibit AGGF1 activity on tumor suppression. These results identify a key role for AGGF1 in an AGGF1-MDM2-p53 signaling axis with important functions in tumor suppression, and uncover a novel trans-tumor-suppression mechanism dependent on p53. This study has potential implications in diagnosis and therapies of cancer.

[Live Imaging of Angiogenesis during Wound Healing]

Repair of injured tissues requires angiogenesis, the growth of new blood vessels from pre-existing ones. Cutaneous wound healing is a complex and dynamic process by which skin tissue repairs itself after injury; however, how endothelial cells and pericytes form new blood vessels during cutaneous wound angiogenesis remains unclear. We recently developed a fluorescence-based live imaging system to analyze cutaneous wound angiogenesis in adult zebrafish. Employing this system, we found that endothelial cells and pericytes remain in a quiescent state in normal skin tissue, whereas cutaneous injury immediately activates both types of cells to induce angiogenesis. At 2 days post-injury (dpi), the injured vessels elongated, and some uninjured vessels became tortuous and began to sprout new branches. Then, vessel sprouting, elongation, bifurcation, and anastomosis progressively occurred to form the tortuous and disorganized vascular networks observed at 6 dpi. Thereafter, blood vessel tortuosity gradually decreased through the regression of excessive vessels, thereby leading to the formation of well-organized vessel networks at 42 dpi. Pericytes are thought to detach from the vessel wall to promote endothelial cell sprouting upon the induction of angiogenesis. However, not only endothelial cells but also pericytes proliferated to form pericyte-covered tortuous blood vessels in response to cutaneous injury, revealing an unexpected role of pericytes in cutaneous wound angiogenesis. Therefore, this live-imaging system for adult zebrafish is anticipated to make a valuable contribution to research advancements in understanding the angiogenesis that occurs during tissue repair.

Angiogenic Factor with G Patch and FHA Domains 1 (AGGF1) Acts as Diagnostic Biomarker and Adverse Prognostic Factor of Hepatocellular Carcinoma (HCC): Evidence from Bioinformatic Analysis

Background

Angiogenic factor with G patch and FHA domains 1 (AGGF1) is a novel identified initiator of angiogenesis through promoting the proliferation of endothelial cells. The continuous angiogenesis plays a key role in the growth, invasion, and metastasis of hepatocellular carcinoma (HCC), while the diagnostic and prognostic roles of AGGF1 for HCC need to be further studied.

Material/Methods

The mRNA sequencing datasets and clinical features of HCC patients were extracted from The Cancer Genome Atlas database. The relationship between clinical features and AGGF1 expression was analyzed by Wilcoxon test. Further validation explorations were carried out using online database Oncomine. The diagnostic receiver operating characteristic curves of AGGF1 and alpha-fetoprotein were compared to examine the diagnostic efficacy of AGGF1. Survival analysis and Gene Set Enrichment Analysis were performed to explore the prediction value and potential mechanism of AGGF1 dysregulation in HCC.

Results

Comprehensive overexpression of AGGF1 was observed in HCC, correlating with poor overall survival. Upregulated level of AGGF1 was statistically associated with poor differentiated histological grade, advanced cancer stage and T classification. AGGF1 was a more effective diagnostic marker than alpha-fetoprotein in HCC. Several important pathways related to HCC including pathway in cancer and P53 signaling pathway were differentially enriched in the high AGGF1 expression phenotype.

Conclusions

AGGF1 was a potential diagnostic and prognostic marker for poor clinical outcomes in HCC patients. Moreover, vital pathways regulated by AGGF1 in HCC may include regulation of autophagy, Wnt signaling pathway, pathway in cancer, cell cycle, and P53 signaling pathway.

Role of ADTRP (Androgen-Dependent Tissue Factor Pathway Inhibitor Regulating Protein) in Vascular Development and Function

Background

The physiological function of ADTRP (androgen‐dependent tissue factor pathway inhibitor regulating protein) is unknown. We previously identified ADTRP as coregulating with and supporting the anticoagulant activity of tissue factor pathway inhibitor in endothelial cells in vitro. Here, we studied the role of ADTRP in vivo, specifically related to vascular development, stability, and function.

Methods and Results

Genetic inhibition of Adtrp produced vascular malformations in the low‐pressure vasculature of zebrafish embryos and newborn mice: dilation/tortuosity, perivascular inflammation, extravascular proteolysis, increased permeability, and microhemorrhages, which produced partially penetrant lethality. Vascular leakiness correlated with decreased endothelial cell junction components VE‐cadherin and claudin‐5. Changes in hemostasis in young adults comprised modest decrease of tissue factor pathway inhibitor antigen and activity and increased tail bleeding time and volume. Cell‐based reporter assays revealed that ADTRP negatively regulates canonical Wnt signaling, affecting membrane events downstream of low‐density lipoprotein receptor‐related protein 6 (LRP6) and upstream of glycogen synthase kinase 3 beta. ADTRP deficiency increased aberrant/ectopic Wnt/β‐catenin signaling in vivo in newborn mice and zebrafish embryos, and upregulated matrix metallopeptidase (MMP)‐9 in endothelial cells and mast cells (MCs). Vascular lesions in newborn Adtrp^−/− pups displayed accumulation of MCs, decreased extracellular matrix content, and deficient perivascular cell coverage. Wnt‐pathway inhibition reversed the increased mmp9 in zebrafish embryos, demonstrating that mmp9 expression induced by Adtrp deficiency was downstream of canonical Wnt signaling.

Conclusions

Our studies demonstrate that ADTRP plays a major role in vascular development and function, most likely through expression in endothelial cells and/or perivascular cells of Wnt‐regulated genes that control vascular stability and integrity.

Dual Inhibition of BMP and WNT Signals Promotes Pancreatic Differentiation from Human Pluripotent Stem Cells

Pathological or functional loss of pancreatic beta cells is the cause of diabetes. Understanding how signaling pathways regulate pancreatic lineage and searching for combinations of signal modulators to promote pancreatic differentiation will definitely facilitate the robust generation of functional beta cells for curing hyperglycemia. In this study, we first tested the effect of several potent BMP inhibitors on pancreatic differentiation using human embryonic stem cells. Next, we examined the endodermal lineage bias upon potent BMP inhibitor treatment and further checked the crosstalk between signal pathways governing endodermal lineage determination. Furthermore, we improved current pancreatic differentiation system based on the signaling pathway study. Finally, we used human-induced pluripotent stem cells to validate our finding. We found BMP inhibitors indeed not only blocked hepatic lineage but also impeded intestinal lineage from human definitive endoderm unexpectedly. Signaling pathway analysis indicated potent BMP inhibitor resulted in the decrease of WNT signal activity and inhibition of WNT could contribute to the improved pancreatic differentiation. Herein, we combined the dual inhibition of BMP and WNT signaling and greatly enhanced human pancreatic progenitor differentiation as well as beta cell generation from both embryonic stem cells and induced pluripotent stem cells. Conclusively, our present work identified the crosstalk between the BMP and WNT signal pathways during human endoderm patterning and pancreas specification, and provided an improved in vitro pancreatic directed differentiation protocol from human pluripotent stem cells.

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.

High levels of serum sclerostin and DKK1 in a case of Klippel-Trénaunay syndrome

Klippel-Trénaunay syndrome (KTS) is described as a complex syndrome characterized by various combinations of capillary, venous, and lymphatic malformations associated with bone and soft tissue hypertrophy. We report a case of a 67-year-old postmenopausal Caucasian women with KTS that shows elevated levels of sclerostin and Dickkopf-related protein 1 (DKK1). Dual-energy X-ray absorptiometry (DXA) BMD T-scores at lumbar spine and femur were normal. Serum calcium and phosphorus levels were consistently normal, 25-hydroxyvitamin D (25OHD) < 30 ng/mL, and normal parathyroid hormone (PTH). Turnover markers (serum osteocalcin [OCN], and carboxy-terminal cross-linking telopeptide of type 1 collagen [CTx]) were in the reference limits. It is interesting to note that the serum levels of sclerostin and DKK-1 were significantly higher in our patient with KTS than in a healthy volunteer (control), without impact on bone mineral density and bone formation markers. In fact, in our patient, the BMD at lumbar spine and femur was normal, and osteocalcin was not suppressed. Based on what is known, we would have expected to find low levels of the inhibitors of the Wnt system, perhaps we can explain the data as a response to the compensation for β-catenin hyper-transformation.

Angiotensin II increases angiogenesis by NF-κB-mediated transcriptional activation of angiogenic factor AGGF1

Angiogenic factor with G-patch and FHA domains 1 (AGGF1) is involved in vascular development, angiogenesis, specification of hemangioblasts, and differentiation of veins. When mutated, however, it causes Klippel-Trenaunay syndrome, a vascular disorder. In this study, we show that angiotensin II (AngII)-the major effector of the renin-angiotensin system and one of the most important regulators of the cardiovascular system-induces the expression of AGGF1 through NF-κB, and that AGGF1 plays a key role in AngII-induced angiogenesis. AngII significantly up-regulated the levels of AGGF1 mRNA and protein in HUVECs at concentrations of 10-40 μg/ml but not >60 μg/ml. AngII type 1 receptor (AT1R) inhibitor losartan inhibited AngII-induced up-regulation of AGGF1, whereas AT2R inhibitor PD123319 further increased AngII-induced up-regulation of AGGF1. Up-regulation of AGGF1 by AngII was blocked by NF-κB inhibitors, and p65 binds directly to a binding site at the promoter/regulatory region of AGGF1 and transcriptionally activates AGGF1 expression. AngII-induced endothelial tube formation was blocked by small interfering RNAs (siRNAs) for RELA (RELA proto-oncogene, NF-κB subunit)/p65 or AGGF1, and the effect of RELA siRNA was rescued by AGGF1. AngII-induced angiogenesis from aortic rings was severely impaired in Aggf1^+/- mice, and the effect was restored by AGGF1. These data suggest that AngII acts as a critical regulator of AGGF1 expression through NF-κB, and that AGGF1 plays a key role in AngII-induced angiogenesis.-Si, W., Xie, W., Deng, W., Xiao, Y., Karnik, S. S., Xu, C., Chen, Q., Wang, Q. K. Angiotensin II increases angiogenesis by NF-κB-mediated transcriptional activation of angiogenic factor AGGF1.

Transgenic Expression of A Venous Malformation Related Mutation, TIE2-R849W, Significantly Induces Multiple Malformations of Zebrafish

A TIE2 mutation causing arginine-to-tryptophan substitution at residue 849 (TIE2-R849W) is commonly identified in heredofamilial venous malformation. However, there is no in vivo model to confirm the pathogenic role of TIE2-R849W. Humanized TIE2-R849W plasmid was constructed via PCR-mediated site-directed mutagenesis. After transcription and micro-injection, TIE2-R849W significantly induces multiple malformations in zebrafish: caudal vein plexus (CVP) defect, eye abnormalities, forebrain formation perturbations, and mandibular malformation. Histologically, these phenotypes accompany aphakia, confused retina plexiform layer, abnormal mandibular cartilage, ectopic myelencephalon proliferation and aberrant location of neurogliocytes. According to qRT-PCR, except for high expression of egfl7, the other CVP-related genes cd146, nr2f1a, and s1pr1 are not significantly different from control. TIE2-R849W also induced upregulation of the wnt signaling pathway. Gene array in vitro shows that under the effect of TIE2-R849W, consistent with high expression of pik3 and foxo1, high levels of egfl7, wnt9a, lrp5 and dkk1 were partly confirmed. This in vivo model directly identifies the venous-related pathogenic role of TIE2-R849W. Under up-regulation of TIE2-R849W, egfl7 could be considered a potential reason for venous defects. Moreover, the wnt pathway may perform an important role as a key trigger for head multi-malformations.

How to Plumb a Pisces: Understanding Vascular Development and Disease Using Zebrafish Embryos

Our vasculature plays diverse and critical roles in homeostasis and disease. In recent decades, the use of zebrafish has driven our understanding of vascular development into new areas, identifying new genes and mechanisms controlling vessel formation and allowing unprecedented observation of the cellular and molecular events that shape the developing vasculature. Here, we highlight key mechanisms controlling formation of the zebrafish vasculature and investigate how knowledge from this highly tractable model system has informed our understanding of vascular disease in humans.

Real-time 3D visualization of cellular rearrangements during cardiac valve formation

During cardiac valve development, the single-layered endocardial sheet at the atrioventricular canal (AVC) is remodeled into multilayered immature valve leaflets. Most of our knowledge about this process comes from examining fixed samples that do not allow a real-time appreciation of the intricacies of valve formation. Here, we exploit non-invasive in vivo imaging techniques to identify the dynamic cell behaviors that lead to the formation of the immature valve leaflets. We find that in zebrafish, the valve leaflets consist of two sets of endocardial cells at the luminal and abluminal side, which we refer to as luminal cells (LCs) and abluminal cells (ALCs), respectively. By analyzing cellular rearrangements during valve formation, we observed that the LCs and ALCs originate from the atrium and ventricle, respectively. Furthermore, we utilized Wnt/β-catenin and Notch signaling reporter lines to distinguish between the LCs and ALCs, and also found that cardiac contractility and/or blood flow is necessary for the endocardial expression of these signaling reporters. Thus, our 3D analyses of cardiac valve formation in zebrafish provide fundamental insights into the cellular rearrangements underlying this process.

Coral-Derived Natural Marine Compound GB9 Impairs Vascular Development in Zebrafish

Blood vessels in vertebrates are established and genetically controlled in an evolutionarily-conserved manner during embryogenesis. Disruption of vascular growth by chemical compounds or environmental hormones may cause developmental defects. This study analyzed the vascular impacts of marine compound GB9 in zebrafish. GB9 was isolated from the marine soft coral Capnella imbricata and had shown anti-neuroinflammatory and anti-nociceptive activities. However, the role of GB9 on vascular development has not been reported. We first tested the survival rate of embryos under exogenous 5, 7.5, 10, and 15 μM GB9 added to the medium and determined a sub-lethal dosage of 10 μM GB9 for further assay. Using transgenic Tg(fli:eGFP) fish to examine vascular development, we found that GB9 treatment impaired intersegmental vessel (ISV) growth and caudal vein plexus (CVP) patterning at 25 hours post-fertilization (hpf) and 30 hpf. GB9 exposure caused pericardial edema and impaired circulation at 48–52 hpf, which are common secondary effects of vascular defects and suggest the effects of GB9 on vascular development. Apoptic cell death analysis showed that vascular defects were not caused by cell death, but were likely due to the inhibition of migration and/or proliferation by examining ISV cell numbers. To test the molecular mechanisms of vascular defects in GB9-treated embryos, we examined the expression of vascular markers and found the decreased expression of vascular specific markers ephrinb2, flk, mrc1, and stabilin. In addition, we examined whether GB9 treatment impairs vascular growth due to an imbalance of redox homeostasis. We found an enhanced effect of vascular defects during GB9 and H₂O₂ co-treatment. Moreover, exogenous N-acetyl-cysteine (NAC) treatment rescued the vascular defects in GB9 treated embryos. Our results showed that GB9 exposure causes vascular defects likely mediated by the imbalance of redox homeostasis.

Pushing Yap into the Nucleus with Shear Force

Endothelial cells line blood vessels and experience shear stress from blood flow. In this issue of Developmental Cell, Nakajima and colleagues (2017) show that in zebrafish Yap responds to blood flow by translocating into the nucleus, where it drives a genetic program to maintain vascular stability.

Targeting AGGF1 (angiogenic factor with G patch and FHA domains 1) for Blocking Neointimal Formation After Vascular Injury

Background

Despite recent improvements in angioplasty and placement of drug‐eluting stents in treatment of atherosclerosis, restenosis and in‐stent thrombosis impede treatment efficacy and cause numerous deaths. Research efforts are needed to identify new molecular targets for blocking restenosis. We aim to establish angiogenic factor AGGF1 (angiogenic factor with G patch and FHA domains 1) as a novel target for blocking neointimal formation and restenosis after vascular injury.

Methods and Results

AGGF1 shows strong expression in carotid arteries; however, its expression is markedly decreased in arteries after vascular injury. AGGF1^+/− mice show increased neointimal formation accompanied with increased proliferation of vascular smooth muscle cells (VSMCs) in carotid arteries after vascular injury. Importantly, AGGF1 protein therapy blocks neointimal formation after vascular injury by inhibiting the proliferation and promoting phenotypic switching of VSMCs to the contractile phenotype in mice in vivo. In vitro, AGGF1 significantly inhibits VSMCs proliferation and decreases the cell numbers at the S phase. AGGF1 also blocks platelet‐derived growth factor‐BB–induced proliferation, migration of VSMCs, increases expression of cyclin D, and decreases expression of p21 and p27. AGGF1 inhibits phenotypic switching of VSMCs to the synthetic phenotype by countering the inhibitory effect of platelet‐derived growth factor‐BB on SRF expression and the formation of the myocardin/SRF/CArG‐box complex involved in activation of VSMCs markers. Finally, we show that AGGF1 inhibits platelet‐derived growth factor‐BB–induced phosphorylation of MEK1/2, ERK1/2, and Elk phosphorylation involved in the phenotypic switching of VSMCs, and that overexpression of Elk abolishes the effect of AGGF1.

Conclusions

AGGF1 protein therapy is effective in blocking neointimal formation after vascular injury by regulating a novel AGGF1‐MEK1/2‐ERK1/2‐Elk‐myocardin‐SRF/p27 signaling pathway.

Fine-tune regulation of carboxypeptidase N1 controls vascular patterning during zebrafish development

Vascular development is regulated by complicated signals and molecules in vertebrates. In this study, we characterized a novel function of carboxypeptidase N1 (Cpn1) in the vasculature. We show that cpn1 mRNA is expressed in developing vessels. The knockdown of cpn1 by morpholino injection impairs the growth of intersegmental vessels (ISV) and caudal vein plexus (CVP), suggesting the role of cpn1 in vascular development. We showed that vascular defects are not caused by cell death but are due to the impairment of migration and proliferation. Consistent with vascular growth defects, loss of cpn1 affects the expression of the vascular markers flt4, mrc1, flk, stabilin, and ephrinb2. Furthermore, the overexpression of cpn1 impaired the growth of ISV and CVP, but the remodeling expression of vascular markers was different from the knockdown of cpn1, indicating the differential regulation mechanisms in cpn1-overexpressing embryos. We examine the interaction between cpn1 and multiple signals and observed that cpn1 is regulated by Notch/VEGF signals for ISV growth and likely regulates BMP signals for CVP patterning. In conclusion, we demonstrate that cpn1 has a critical role in the vascular development of zebrafish. We also reveal a fine-tune regulation of cpn1 that controls vascular patterning mediated by multiple signals.

Yap/Taz transcriptional activity is essential for vascular regression via Ctgf expression and actin polymerization

Vascular regression is essential to remove redundant vessels during the formation of an efficient vascular network that can transport oxygen and nutrient to every corner of the body. However, no mechanism is known to explain how major blood vessels regress during development. Here we use the dorsal part of the caudal vein plexus (dCVP) in Zebrafish to investigate the mechanism of regression and discover a new role of Yap/Taz in vascular regression. During regression, Yap/Taz is activated by blood circulation in the endothelial cells. This leads to induction of Ctgf and actin polymerization. Interference with Yap/Taz activation decreased Ctgf production, which decreased actin polymerization and vascular regression. These results implicate a novel role of Yap/Taz in vascular regression.

Ftr82 Is Critical for Vascular Patterning during Zebrafish Development

Cellular components and signaling pathways are required for the proper growth of blood vessels. Here, we report for the first time that a teleost-specific gene ftr82 (finTRIM family, member 82) plays a critical role in vasculature during zebrafish development. To date, there has been no description of tripartite motif proteins (TRIM) in vascular development, and the role of ftr82 is unknown. In this study, we found that ftr82 mRNA is expressed during the development of vessels, and loss of ftr82 by morpholino (MO) knockdown impairs the growth of intersegmental vessels (ISV) and caudal vein plexus (CVP), suggesting that ftr82 plays a critical role in promoting ISV and CVP growth. We showed the specificity of ftr82 MO by analyzing ftr82 expression products and expressing ftr82 mRNA to rescue ftr82 morphants. We further showed that the knockdown of ftr82 reduced ISV cell numbers, suggesting that the growth impairment of vessels is likely due to a decrease of cell proliferation and migration, but not cell death. In addition, loss of ftr82 affects the expression of vascular markers, which is consistent with the defect of vascular growth. Finally, we showed that ftr82 likely interacts with vascular endothelial growth factor (VEGF) and Notch signaling. Together, we identify teleost-specific ftr82 as a vascular gene that plays an important role for vascular development in zebrafish.

Osteocrin, a peptide secreted from the heart and other tissues, contributes to cranial osteogenesis and chondrogenesis in zebrafish

The heart is an endocrine organ, as cardiomyocytes (CMs) secrete natriuretic peptide (NP) hormones. Since the discovery of NPs, no other peptide hormones that affect remote organs have been identified from the heart. We identified osteocrin (Ostn) as an osteogenesis/chondrogenesis regulatory hormone secreted from CMs in zebrafish. ostn mutant larvae exhibit impaired membranous and chondral bone formation. The impaired bones were recovered by CM-specific overexpression of OSTN. We analyzed the parasphenoid (ps) as a representative of membranous bones. In the shortened ps of ostn morphants, nuclear Yap1/Wwtr1-dependent transcription was increased, suggesting that Ostn might induce the nuclear export of Yap1/Wwtr1 in osteoblasts. Although OSTN is proposed to bind to NPR3 (clearance receptor for NPs) to enhance the binding of NPs to NPR1 or NPR2, OSTN enhanced C-type NP (CNP)-dependent nuclear export of YAP1/WWTR1 of cultured mouse osteoblasts stimulated with saturable CNP. OSTN might therefore activate unidentified receptors that augment protein kinase G signaling mediated by a CNP-NPR2 signaling axis. These data demonstrate that Ostn secreted from the heart contributes to bone formation as an endocrine hormone.

ERK activation in endothelial cells is a novel marker during neovasculogenesis

Vasculogenesis is essential during early development to construct networks transporting oxygen, blood and nutrients. Tip and stalk cells are specialized endothelial cells involved in novel vessel formation because of their behavior such as sprouting as a leading cell and following tip cell. However, the spatiotemporal details determining the emergence of these cells are unknown. Here, we first show that the ERK activity in endothelial cells represents the precursor of tip and stalk cells for vasculogenesis in zebrafish. We identified that tip and stalk cells for intersegmental vessel (ISV) formation were already specialized in the dorsal aorta (DA) before sprouting. Furthermore, similar specialization was observed in tip cells during parachordal vessel (PAV) formation in lymphangiogenesis. We also identified that the ERK activity was required for specialized cells to emerge from existing blood vessels. Our data show that the ERK activity is a novel marker for determining the emergence of cells in both angiogenesis and lymphangiogenesis.

Building the drains: the lymphatic vasculature in health and disease

The lymphatic vasculature is comprised of a network of endothelial vessels found in close proximity to but separated from the blood vasculature. An essential tissue component of all vertebrates, lymphatics are responsible for the maintenance of fluid homeostasis, dissemination of immune cells, and lipid reabsorption under healthy conditions. When lymphatic vessels are impaired due to invasive surgery, genetic disorders, or parasitic infections, severe fluid build-up accumulates in the affected tissues causing a condition known as lymphedema. Malignant tumors can also directly activate lymphangiogenesis and use these vessels to promote the spread of metastatic cells. Although their first description goes back to the times of Hippocrates, with subsequent anatomical characterization at the beginning of the 20th-century, the lack of identifying molecular markers and tools to visualize these translucent vessels meant that investigation of lymphatic vessels fell well behind research of blood vessels. However, after years under the shadow of the blood vasculature, recent advances in imaging technologies and new genetic and molecular tools have accelerated the pace of research on lymphatic vessel development. These new tools have facilitated both work in classical mammalian models and the emergence of new powerful vertebrate models like zebrafish, quickly driving the field of lymphatic development back into the spotlight. In this review, we summarize the highlights of recent research on the development and function of the lymphatic vascular network in health and disease. WIREs Dev Biol 2016, 5:689-710. doi: 10.1002/wdev.246 For further resources related to this article, please visit the WIREs website.

Angiogenic Factor AGGF1 Activates Autophagy with an Essential Role in Therapeutic Angiogenesis for Heart Disease

AGGF1 is an angiogenic factor with therapeutic potential to treat coronary artery disease (CAD) and myocardial infarction (MI). However, the underlying mechanism for AGGF1-mediated therapeutic angiogenesis is unknown. Here, we show for the first time that AGGF1 activates autophagy, a housekeeping catabolic cellular process, in endothelial cells (ECs), HL1, H9C2, and vascular smooth muscle cells. Studies with Atg5 small interfering RNA (siRNA) and the autophagy inhibitors bafilomycin A1 (Baf) and chloroquine demonstrate that autophagy is required for AGGF1-mediated EC proliferation, migration, capillary tube formation, and aortic ring-based angiogenesis. Aggf1^+/- knockout (KO) mice show reduced autophagy, which was associated with inhibition of angiogenesis, larger infarct areas, and contractile dysfunction after MI. Protein therapy with AGGF1 leads to robust recovery of myocardial function and contraction with increased survival, increased ejection fraction, reduction of infarct areas, and inhibition of cardiac apoptosis and fibrosis by promoting therapeutic angiogenesis in mice with MI. Inhibition of autophagy in mice by bafilomycin A1 or in Becn1^+/- and Atg5 KO mice eliminates AGGF1-mediated angiogenesis and therapeutic actions, indicating that autophagy acts upstream of and is essential for angiogenesis. Mechanistically, AGGF1 initiates autophagy by activating JNK, which leads to activation of Vps34 lipid kinase and the assembly of Becn1-Vps34-Atg14 complex involved in the initiation of autophagy. Our data demonstrate that (1) autophagy is essential for effective therapeutic angiogenesis to treat CAD and MI; (2) AGGF1 is critical to induction of autophagy; and (3) AGGF1 is a novel agent for treatment of CAD and MI. Our data suggest that maintaining or increasing autophagy is a highly innovative strategy to robustly boost the efficacy of therapeutic angiogenesis.

Treatment with the angiogenic factor AGGF1 dramatically improves survival and cardiac function in mouse models for coronary artery disease and myocardial infarction by activating autophagy and angiogenesis.

Coronary artery disease is the number one killer disease worldwide. Recently, therapeutic angiogenesis has been proposed as an attractive new strategy for treating this and other ischemic diseases. This study establishes the angiogenic factor AGGF1 as a novel target and agent that can successfully treat coronary artery disease and acute myocardial infarction and dramatically improve survival and cardiac function in mouse models. We present the unexpected finding that AGGF1 has these effects via activating autophagy, and that autophagy is essential for therapeutic angiogenesis in animals. We find that AGGF1 is a novel master regulator of autophagy not only in endothelial cells but also in all other cell types examined in the study. Mechanistically, AGGF1 activates autophagy by activating JNK, which leads to activation of the Vps34 lipid kinase and assembly of the Becn1-Vps34-Atg14 complex involved in the initiation of autophagy. The study thus provides a link connecting the therapeutic angiogenesis and autophagy pathways in heart disease.

Development of the lymphatic system: new questions and paradigms

The lymphatic system is a blind-ended network of vessels that plays important roles in mediating tissue fluid homeostasis, intestinal lipid absorption and the immune response. A profound understanding of the development of lymphatic vessels, as well as of the molecular cues governing their formation and morphogenesis, might prove essential for our ability to treat lymphatic-related diseases. The embryonic origins of lymphatic vessels have been debated for over a century, with a model claiming a venous origin for the lymphatic endothelium being predominant. However, recent studies have provided new insights into the origins of lymphatic vessels. Here, we review the molecular mechanisms controlling lymphatic specification and sprouting, and we discuss exciting findings that shed new light on previously uncharacterized sources of lymphatic endothelial cells.

Yap/Taz transcriptional activity in endothelial cells promotes intramembranous ossification via the BMP pathway

Osteogenesis is categorized into two groups based on developmental histology, intramembranous and endochondral ossification. The role of blood vessels during endochondral ossification is well known, while their role in intramembranous ossification, especially the intertissue pathway, is poorly understood. Here, we demonstrate endothelial Yap/Taz is a novel regulator of intramembranous ossification in zebrafish. Appropriate blood flow is required for Yap/Taz transcriptional activation in endothelial cells and intramembranous ossification. Additionally, Yap/Taz transcriptional activity in endothelial cells specifically promotes intramembranous ossification. BMP expression by Yap/Taz transactivation in endothelial cells is also identified as a bridging factor between blood vessels and intramembranous ossification. Furthermore, the expression of Runx2 in pre-osteoblast cells is a downstream target of Yap/Taz transcriptional activity in endothelial cells. Our results provide novel insight into the relationship between blood flow and ossification by demonstrating intertissue regulation.

BMP signaling in vascular biology and dysfunction

The vascular system is critical for developmental growth, tissue homeostasis and repair but also for tumor development. Bone morphogenetic protein (BMP) signaling has recently emerged as a fundamental pathway of the endothelium by regulating cardiovascular and lymphatic development and by being causative for several vascular dysfunctions. Two vascular disorders have been directly linked to impaired BMP signaling: pulmonary arterial hypertension and hereditary hemorrhagic telangiectasia. Endothelial BMP signaling critically depends on the cellular context, which includes among others vascular heterogeneity, exposure to flow, and the intertwining with other signaling cascades (Notch, WNT, Hippo and hypoxia). The purpose of this review is to highlight the most recent findings illustrating the clear need for reconsidering the role of BMPs in vascular biology.

Tip cell-specific requirement for an atypical Gpr124- and Reck-dependent Wnt/β-catenin pathway during brain angiogenesis

Despite the critical role of endothelial Wnt/β-catenin signaling during central nervous system (CNS) vascularization, how endothelial cells sense and respond to specific Wnt ligands and what aspects of the multistep process of intra-cerebral blood vessel morphogenesis are controlled by these angiogenic signals remain poorly understood. We addressed these questions at single-cell resolution in zebrafish embryos. We identify the GPI-anchored MMP inhibitor Reck and the adhesion GPCR Gpr124 as integral components of a Wnt7a/Wnt7b-specific signaling complex required for brain angiogenesis and dorsal root ganglia neurogenesis. We further show that this atypical Wnt/β-catenin signaling pathway selectively controls endothelial tip cell function and hence, that mosaic restoration of single wild-type tip cells in Wnt/β-catenin-deficient perineural vessels is sufficient to initiate the formation of CNS vessels. Our results identify molecular determinants of ligand specificity of Wnt/β-catenin signaling and provide evidence for organ-specific control of vascular invasion through tight modulation of tip cell function.

DOI:http://dx.doi.org/10.7554/eLife.06489.001

Organs develop alongside the network of blood vessels that supply them with oxygen and nutrients. One way that new blood vessels grow is by sprouting out of the side of an existing vessel, via a process called angiogenesis. This process relies on signals that are received by the endothelial cells that line the inner wall of blood vessels, and that direct the cells to form a new ‘sprout’, consisting of tip and stalk cells.

In the developing brain, the Wnt/β-catenin signaling pathway helps direct the formation of blood vessels. In this pathway, a member of a protein family called Wnt signals to specific proteins on the surface of the cells lining the blood vessels. Much effort has gone into uncovering the identity of these proteins, with many studies looking at blood vessel development in the brain of mouse embryos.

In this study, Vanhollebeke et al. turned to zebrafish embryos to uncover new regulators of angiogenesis and define their roles during the multi-step process of blood vessel development in the brain. A variety of experimental techniques were used to alter and study the activity of different Wnt signaling pathway components. These experiments revealed that the Wnt7a and Wnt7b proteins signal to an endothelial cell membrane protein complex containing the proteins Gpr124 and Reck.

Vanhollebeke et al. then created ‘mosaic’ zebrafish embryos, which contained two genetically distinct types of cells—cells that were missing one of the components of Wnt/β-catenin signaling pathway, and wild-type cells. Visualizing the growth of the vessels showed that all the new blood vessels that sprouted had normal tip cells. However, the cells in the stalk of the sprout could be either normal or missing a signaling protein.

These findings demonstrate that Wnt/β-catenin signaling controls the pattern of blood vessel development in the brain by acting specifically on the invasive behaviors of the tip cells of new sprouts, a cellular mechanism that allows efficient organ-specific control of vascularization.

DOI:http://dx.doi.org/10.7554/eLife.06489.002