Distinct signalling pathways regulate sprouting angiogenesis from the dorsal aorta and the axial vein

Endothelial cell transitions in zebrafish vascular development

In recent decades, developmental biologists have come to view vascular development as a series of progressive transitions. Mesoderm differentiates into endothelial cells; arteries, veins and lymphatic endothelial cells are specified from early endothelial cells; and vascular networks diversify and invade developing tissues and organs. Our understanding of this elaborate developmental process has benefitted from detailed studies using the zebrafish as a model system. Here, we review a number of key developmental transitions that occur in zebrafish during the formation of the blood and lymphatic vessel networks.

ACVR1/ALK2-p21 signaling axis modulates proliferation of the venous endothelium in the retinal vasculature

The proliferation of the endothelium is a highly coordinated process to ensure the emergence, expansion, and homeostasis of the vasculature. While Bone Morphogenetic Protein (BMP) signaling fine-tunes the behaviors of endothelium in health and disease, how BMP signaling influences the proliferation of endothelium and therefore, modulates angiogenesis remains largely unknown. Here, we evaluated the role of Activin A Type I Receptor (ACVR1/ALK2), a key BMP receptor in the endothelium, in modulating the proliferation of endothelial cells. We show that ACVR1/ALK2 is a key modulator for the proliferation of endothelium in the retinal vessels. Loss of endothelial ALK2 leads to a significant reduction in endothelial proliferation and results in fewer branches/endothelial cells in the retinal vessels. Interestingly, venous endothelium appears to be more susceptible to ALK2 deletion. Mechanistically, ACVR1/ALK2 inhibits the expression of CDKN1A/p21, a critical negative regulator of cell cycle progression, in a SMAD1/5-dependent manner, thereby enabling the venous endothelium to undergo active proliferation by suppressing CDKN1A/p21. Taken together, our findings show that BMP signaling mediated by ACVR1/ALK2 provides a critical yet previously underappreciated input to modulate the proliferation of venous endothelium, thereby fine-tuning the context of angiogenesis in health and disease.

Bifenox induces hepatotoxicity and vascular toxicity in zebrafish embryos via ROS production and alterations in signaling pathways

Existing evidence shows that currently used pesticides pose toxicological risks to exposed wildlife. Chemically, bifenox belongs to diphenyl ethers, a well-known group of herbicides. Its mechanism of action primarily involves inducing lipid peroxidation and blocking protoporphyrinogen oxidases. Toxicity of diphenyl ether herbicides has been elucidated in animal cells; however, in vivo toxicological evaluations of bifenox are required to determine its unexpected effects. This study aimed to determine the negative effects of bifenox, and its effects on higher eukaryotes. We found that early stages of zebrafish embryo exposed to bifenox demonstrated increased mortality and physiological defects, based on the LC50 value. Bifenox severely inhibited blood vessel growth by reducing key elements of complex connectivity; fluorescently tagged transgenic lines (fli1a:EGFP) showed morphological changes. Additionally, transgenic lines that selectively identified hepatocytes (fabp10a:DsRed) showed reduced fluorescence, indicating that bifenox may inhibit liver development. To evaluate the level of oxidative stress, we used 2',7'-dichlorofluorescein diacetate (DCFH-DA) probes in zebrafish embryos to identify the underlying mechanisms causing developmental damage. Our findings demonstrate that exposure to bifenox causes abnormalities in the hepatic and cardiovascular systems during zebrafish embryogenesis. Therefore, this study provides new information for the evaluation of toxicological risks of bifenox in vertebrates.

The cytoskeleton adaptor protein Sorbs1 controls the development of lymphatic and venous vessels in zebrafish

BACKGROUND

Lymphangiogenesis, the formation of lymphatic vessels, is tightly linked to the development of the venous vasculature, both at the cellular and molecular levels. Here, we identify a novel role for Sorbs1, the founding member of the SoHo family of cytoskeleton adaptor proteins, in vascular and lymphatic development in the zebrafish.

RESULTS

We show that Sorbs1 is required for secondary sprouting and emergence of several vascular structures specifically derived from the axial vein. Most notably, formation of the precursor parachordal lymphatic structures is affected in sorbs1 mutant embryos, severely impacting the establishment of the trunk lymphatic vessel network. Interestingly, we show that Sorbs1 interacts with the BMP pathway and could function outside of Vegfc signaling. Mechanistically, Sorbs1 controls FAK/Src signaling and subsequently impacts on the cytoskeleton processes regulated by Rac1 and RhoA GTPases. Inactivation of Sorbs1 altered cell-extracellular matrix (ECM) contacts rearrangement and cytoskeleton dynamics, leading to specific defects in endothelial cell migratory and adhesive properties.

CONCLUSIONS

Overall, using in vitro and in vivo assays, we identify Sorbs1 as an important regulator of venous and lymphatic angiogenesis independently of the Vegfc signaling axis. These results provide a better understanding of the complexity found within context-specific vascular and lymphatic development.

A defined clathrin-mediated trafficking pathway regulates sFLT1/VEGFR1 secretion from endothelial cells

FLT1/VEGFR1 negatively regulates VEGF-A signaling and is required for proper vessel morphogenesis during vascular development and vessel homeostasis. Although a soluble isoform, sFLT1, is often mis-regulated in disease and aging, how sFLT1 is trafficked and secreted from endothelial cells is not well understood. Here we define requirements for constitutive sFLT1 trafficking and secretion in endothelial cells from the Golgi to the plasma membrane, and we show that sFLT1 secretion requires clathrin at or near the Golgi. Perturbations that affect sFLT1 trafficking blunted endothelial cell secretion and promoted intracellular mis-localization in cells and zebrafish embryos. siRNA-mediated depletion of specific trafficking components revealed requirements for RAB27A, VAMP3, and STX3 for post-Golgi vesicle trafficking and sFLT1 secretion, while STX6, ARF1, and AP1 were required at the Golgi. Live-imaging of temporally controlled sFLT1 release from the endoplasmic reticulum showed clathrin-dependent sFLT1 trafficking at the Golgi into secretory vesicles that then trafficked to the plasma membrane. Depletion of STX6 altered vessel sprouting in 3D, suggesting that endothelial cell sFLT1 secretion influences proper vessel sprouting. Thus, specific trafficking components provide a secretory path from the Golgi to the plasma membrane for sFLT1 in endothelial cells that utilizes a specialized clathrin-dependent intermediate, suggesting novel therapeutic targets.

Transluminal Pillars-Their Origin and Role in the Remodelling of the Zebrafish Caudal Vein Plexus

Intussusceptive pillars, regarded as a hallmark of intussusceptive angiogenesis, have been described in developing vasculature of many organs and organisms. The aim of this study was to resolve the question about pillar formation and their further maturation employing zebrafish caudal vein plexus (CVP). The CVP development was monitored by in vivo confocal microscopy in high spatio-temporal resolution using the transgenic zebrafish model Fli1a:eGPF//Gata1:dsRed. We tracked back the formation of pillars (diameter ≤ 4 µm) and intercapillary meshes (diameter > 4 µm) and analysed their morphology and behaviour. Transluminal pillars in the CVP arose via a combination of sprouting, lumen expansion, and/or the creation of intraluminal folds, and those mechanisms were not associated directly with blood flow. The follow-up of pillars indicated that one-third of them disappeared between 28 and 48 h post fertilisation (hpf), and of the remaining ones, only 1/17 changed their cross-section area by >50%. The majority of the bigger meshes (39/62) increased their cross-section area by >50%. Plexus simplification and the establishment of hierarchy were dominated by the dynamics of intercapillary meshes, which formed mainly via sprouting angiogenesis. These meshes were observed to grow, reshape, and merge with each other. Our observations suggested an alternative view on intussusceptive angiogenesis in the CVP.

An H2 S-BMP6 Dual-Loading System with Regulating Yap/Taz and Jun Pathway for Synergistic Critical Limb Ischemia Salvaging Therapy

Critical limb ischemia, the final course of peripheral artery disease, is characterized by an insufficient supply of blood flow and excessive oxidative stress. H2 S molecular therapy possesses huge potential for accelerating revascularization and scavenging intracellular reactive oxygen species (ROS). Moreover, it is found that BMP6 is the most significantly up-expressed secreted protein-related gene in HUVECs treated with GYY4137, a H2 S donor, based on the transcriptome analysis. Herein, a UIO-66-NH2 @GYY4137@BMP6 co-delivery nanoplatform to strengthen the therapeutic effects of limb ischemia is developed. The established UIO-66-NH2 @GYY4137@BMP6 nanoplatform exerts its proangiogenic and anti-oxidation functions by regulating key pathways. The underlying molecular mechanisms of UIO-66-NH2 @GYY4137@BMP6 dual-loading system lie in the upregulation of phosphorylated YAP/TAZ and Jun to promote HUVECs proliferation and downregulation of phosphorylated p53/p21 to scavenge excessive ROS. Meanwhile, laser-doppler perfusion imaging (LDPI), injury severity evaluation, and histological analysis confirm the excellent therapeutic effects of UIO-66-NH2 @GYY4137@BMP6 in vivo. This work may shed light on the treatment of critical limb ischemia by regulating YAP, Jun, and p53 signaling pathways based on gas-protein synergistic therapy.

Development of a Highly In Vivo Efficacious Dual Antitumor and Antiangiogenic Organoiridium Complex as a Potential Anti-Lung Cancer Agent

While the phenomenal clinical success of blockbuster platinum (Pt) drugs is highly encouraging, the inherent and acquired resistance and dose-limiting side effects severely limit their clinical application. To find a better alternative with translational potential, we synthesized a library of six organo-IrIII half-sandwich [(η5-CpX)Ir(N∧N)Cl]+-type complexes. In vitro screening identified two lead candidates [(η5-CpXPh)Ir(Ph2Phen)Cl]+ (5, CpXPh = tetramethyl-phenyl-cyclopentadienyl and Ph2Phen = 4,7-diphenyl-1,10-phenanthroline) and [(η5-CpXBiPh)Ir(Ph2Phen)Cl]+ (6, CpXBiPh = tetramethyl-biphenyl-cyclopentadienyl) with nanomolar IC50 values. Both 5 and 6 efficiently overcame Pt resistance and presented excellent cancer cell selectivity in vitro. Potent antiangiogenic properties of 6 were demonstrated in the zebrafish model. Satisfyingly, 6 and its nanoliposome Lipo-6 presented considerably higher in vivo antitumor efficacy as compared to cisplatin, as well as earlier reported IrIII half-sandwich complexes in mice bearing the A549 non-small lung cancer xenograft. In particular, complex 6 is the first example of this class that exerted dual in vivo antiangiogenic and antitumor properties.

Functional characterization of stap2b in zebrafish vascular development

The genetic control and signaling pathways of vascular development are not comprehensively understood. Transcription factors Islet2 (Isl2) and nr2f1b are critical for vascular growth in zebrafish, and further transcriptome analysis has revealed potential targets regulated by isl2/nr2f1b. In this study, we focused on the potential activation gene signal-transducing adaptor protein 2b (stap2b) and revealed a novel role of stap2b in vascular development. stap2b mRNA was expressed in developing vessels, suggesting stap2b plays a role in vascularization. Knocking down stap2b expression by morpholino injection or Crispr-Cas9-generated stap2b mutants caused vascular defects, suggesting a role played by stap2b in controlling the patterning of intersegmental vessels (ISVs) and the caudal vein plexus (CVP). The vessel abnormalities associated with stap2b deficiency were found to be due to dysregulated cell migration and proliferation. The decreased expression of vascular-specific markers in stap2b morphants was consistent with the vascular defects observed. In contrast, overexpression of stap2b enhanced the growth of ISVs and reversed the vessel defects in stap2b morphants. These data suggest that stap2b is necessary and sufficient to promote vascular development. Finally, we examined the interaction between stap2b and multiple signaling. We showed that stap2b regulated ISV growth through the JAK-STAT pathway. Moreover, we found that stap2b was regulated by Notch signaling to control ISV growth, and stap2b interacted with bone morphogenetic protein signaling to contribute to CVP formation. Altogether, we demonstrated that stap2b acts downstream of the isl2/nr2f1b pathway to play a pivotal role in vascular development via interaction with multiple signaling pathways.

Transcription factor induction of vascular blood stem cell niches in vivo

The hematopoietic niche is a supportive microenvironment composed of distinct cell types, including specialized vascular endothelial cells that directly interact with hematopoietic stem and progenitor cells (HSPCs). The molecular factors that specify niche endothelial cells and orchestrate HSPC homeostasis remain largely unknown. Using multi-dimensional gene expression and chromatin accessibility analyses in zebrafish, we define a conserved gene expression signature and cis-regulatory landscape that are unique to sinusoidal endothelial cells in the HSPC niche. Using enhancer mutagenesis and transcription factor overexpression, we elucidate a transcriptional code that involves members of the Ets, Sox, and nuclear hormone receptor families and is sufficient to induce ectopic niche endothelial cells that associate with mesenchymal stromal cells and support the recruitment, maintenance, and division of HSPCs in vivo. These studies set forth an approach for generating synthetic HSPC niches, in vitro or in vivo, and for effective therapies to modulate the endogenous niche.

Environmentally relevant concentrations of fipronil selectively disrupt venous vessel development in zebrafish embryos/larvae

The pesticide fipronil is widely dispersed in aquatic environments and frequently detected in the general population. Although the adverse effects on embryonic growth by fipronil exposure have been extensively documented, the early responses for its developmental toxicity are largely unknown. In the present study, we explored the sensitive targets of fipronil, focusing on vascular injury using zebrafish embryos/larvae and cultured human endothelial cells. Exposure to 5-500 μg/L fipronil at the early stage impeded the growth of sub-intestinal venous plexus (SIVP), caudal vein plexus (CVP), and common cardinal veins (CCV). The damages on venous vessels occurred at exposure to the environmentally relevant concentration as low as 5 μg/L fipronil, whereas no significant change was observed in general toxicity indexes. In contrast, vascular development of the dorsal aorta (DA) or intersegmental artery (ISA) was not affected. In addition, the mRNA levels of vascular markers and vessel type-specific function genes exhibited significant decreases in venous genes, including nr2f2, ephb4a, and flt4, but no appreciable change in arterial genes. Likewise, the more pronounced changes in cell death and cytoskeleton disruption were shown in human umbilical vein endothelial cells as compared with human aortic endothelial cells. Furthermore, molecular docking supported a stronger affinity of fipronil and its metabolites to the proteins correlated with venous development, such as BMPR2 and SMARCA4. These results reveal the heterogeneity in developing vasculature responsive to fipronil's exposure. The preferential impacts on the veins confer higher sensitivity, allowing them to be appropriate targets for monitoring fipronil's developmental toxicity.

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.

Temporally and regionally distinct morphogenetic processes govern zebrafish caudal fin blood vessel network expansion

Blood vessels form elaborate networks that depend on tissue-specific signalling pathways and anatomical structures to guide their growth. However, it is not clear which morphogenetic principles organize the stepwise assembly of the vasculature. We therefore performed a longitudinal analysis of zebrafish caudal fin vascular assembly, revealing the existence of temporally and spatially distinct morphogenetic processes. Initially, vein-derived endothelial cells (ECs) generated arteries in a reiterative process requiring vascular endothelial growth factor (Vegf), Notch and cxcr4a signalling. Subsequently, veins produced veins in more proximal fin regions, transforming pre-existing artery-vein loops into a three-vessel pattern consisting of an artery and two veins. A distinct set of vascular plexuses formed at the base of the fin. They differed in their diameter, flow magnitude and marker gene expression. At later stages, intussusceptive angiogenesis occurred from veins in distal fin regions. In proximal fin regions, we observed new vein sprouts crossing the inter-ray tissue through sprouting angiogenesis. Together, our results reveal a surprising diversity among the mechanisms generating the mature fin vasculature and suggest that these might be driven by separate local cues.

A defined clathrin-mediated trafficking pathway regulates sFLT1/VEGFR1 secretion from endothelial cells

FLT1/VEGFR1 negatively regulates VEGF-A signaling and is required for proper vessel morphogenesis during vascular development and vessel homeostasis. Although a soluble isoform, sFLT1, is often mis-regulated in disease and aging, how sFLT1 is trafficked and secreted from endothelial cells is not well understood. Here we define requirements for constitutive sFLT1 trafficking and secretion in endothelial cells from the Golgi to the plasma membrane, and we show that sFLT1 secretion requires clathrin at or near the Golgi. Perturbations that affect sFLT1 trafficking blunted endothelial cell secretion and promoted intracellular mis-localization in cells and zebrafish embryos. siRNA-mediated depletion of specific trafficking components revealed requirements for RAB27A, VAMP3, and STX3 for post-Golgi vesicle trafficking and sFLT1 secretion, while STX6, ARF1, and AP1 were required at the Golgi. Depletion of STX6 altered vessel sprouting in a 3D angiogenesis model, indicating that endothelial cell sFLT1 secretion is important for proper vessel sprouting. Thus, specific trafficking components provide a secretory path from the Golgi to the plasma membrane for sFLT1 in endothelial cells that utilizes a specialized clathrin-dependent intermediate, suggesting novel therapeutic targets.

GTP-Binding Protein 1-Like (GTPBP1l) Regulates Vascular Patterning during Zebrafish Development

Genetic regulation of vascular patterning is not fully understood. Here, we report a novel gene, gtpbp1l (GTP-binding protein 1-like), that regulates vascular development in zebrafish. Amino acid sequence comparison and a phylogenetic study showed that gtpbp1l is conserved in vertebrates. Gtpbp1l mRNA is expressed in the vasculature during embryogenesis. Knockdown of gtpbp1l by morpholino impairs the patterning of the intersegmental vessel (ISV) and caudal vein plexus (CVP), indicating the role of gtpbp1l in vasculature. Further apoptosis assays and transgenic fish tests suggested that vascular defects in gtpbp1l morphants are not due to cell death but are likely caused by the impairment of migration and proliferation. Moreover, the altered expression of vessel markers is consistent with the vascular defects in gtpbp1l morphants. Finally, we revealed that gtpbp1l is regulated by VEGF/notch and BMP signaling. Collectively, these findings showed that gtpbp1l plays a critical role in vascular patterning during zebrafish development.

Environmental and intrinsic modulations of venous differentiation

Endothelial cells in veins differ in morphology, function and gene expression from those in arteries and lymphatics. Understanding how venous and arterial identities are induced during development is required to understand how arterio-venous malformations occur, and to improve the outcome of vein grafts in surgery by promoting arterialization of veins. To identify factors that promote venous endothelial cell fate in vivo, we isolated veins from quail embryos, at different developmental stages, that were grafted into the coelom of chick embryos. Endothelial cells migrated out from the grafted vein and their colonization of host veins and/or arteries was quantified. We show that venous fate is promoted by sympathetic vessel innervation at embryonic day 11. Removal of sympathetic innervation decreased vein colonization, while norepinephrine enhanced venous colonization. BMP treatment or inhibition of ERK enhanced venous fate, revealing environmental neurotransmitter and BMP signaling and intrinsic ERK inhibition as actors in venous fate acquisition. We also identify the BMP antagonist Noggin as a potent mediator of venous arterialization.

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.

Pro-Angiogenetic Effects of Purified Extracts from Helix aspersa during Zebrafish Development

Helix aspersa is a species of land snail belonging to the Helicidae family, widespread in the Mediterranean and continental area up to Northern Europe. In some areas it is appreciated as a food, but is mostly considered a parasite of gardens and cultivated fields. The mucus of Helix aspersa has found multiple applications in the cosmetic and health fields. In the present study, we investigated for the first time the angiogenetic properties of purified extracts from Helix aspersa using a transgenic zebrafish line Tg (kdrl:EGFP). The angiogenesis induced by purified snail extracts was demonstrated by their capability to increase the three well-established parameters of angiogenesis: generation of intersegmental vessels, modeling of caudal venous plexus, and formation of sub-intestinal venous plexus. The effects appeared to be mediated by the vascular endothelial growth factor (VEGF) pathway, being prevented by pretreatment of embryos with the selective VEGF receptor antagonist SU5416, and supported by the increased VEGF mRNA levels found in snail-extract-treated embryos. Insufficient vascular supply is underlined by low VEGF signaling, primarily because of its indispensable role in preventing capillary loss. Our findings might have a pharmacological impact by counteracting VEGF hypofunction and promoting angiogenesis to maintain adequate microvascular and vascular density in normal and suffering tissues and organs.

Nitrobenzoate-Derived Compound X8 Impairs Vascular Development in Zebrafish

Proper growth and patterning of blood vessels are critical for embryogenesis. Chemicals or environmental hormones may interfere with vascular growth and cause developmental defects. Nitrobenzoate-based compounds have been demonstrated to have a wide range of biological and pharmacological functions, leading to the development of numerous 4-nitrobenzoate derivatives for clinical application. In this study, we tested a novel nitrobenzoate-derived compound, X8, and investigated its effects on vascular development using zebrafish as a model organism. We first determined the survival rate of embryos after the addition of exogenous X8 (0.5, 1, 3, 5, and 10 μM) to the fish medium and determined a sublethal dose of 3 μM for use in further assays. We used transgenic fish to examine the effects of X8 treatment on vascular development. At 25–32 h postfertilization (hpf), X8 treatment impaired the growth of intersegmental vessels (ISVs) and caudal vein plexuses (CVPs). Moreover, X8-treated embryos exhibited pericardial edema and circulatory defects at 60–72 hpf, suggesting the effects of X8 in vasculature. Apoptosis tests showed that the vascular defects were likely caused by the inhibition of proliferation and migration. To investigate the molecular impacts underlying the defects in the vasculature of X8-treated fish, the expression levels of vascular markers, including ephrinb2, mrc1, and stabilin, were assessed, and the decreased expression of those genes was detected, indicating that X8 inhibited the expression of vascular genes. Finally, we showed that X8 treatment disrupted exogenous GS4012-induced angiogenesis in Tg(flk:egfp) zebrafish embryos. In addition, vascular defects were enhanced during cotreatment with X8 and the VEGFR2 inhibitor SU5416, suggesting that X8 treatment causes vascular defects mediated by disruption of VEGF/VEGFR2 signaling. Collectively, our findings indicate that X8 could be developed as a novel antiangiogenic agent.

Ifi30 Is Required for Sprouting Angiogenesis During Caudal Vein Plexus Formation in Zebrafish

Interferon-gamma-inducible protein 30 (IFI30) is a critical enzyme that mainly exists in immune cells and functions in reducing protein disulfide bonds in endocytosis-mediated protein degradation. Regardless of this, it is also found to be expressed in vascular system. However, the functions of IFI30 in vascular development remains unknown. Vascular network formation is a tightly controlled process coordinating a series of cell behaviors, including endothelial cell (EC) sprouting, proliferation, and anastomosis. In this work, we analyzed the function of zebrafish Ifi30, orthologous to the human IFI30, in vascular development during embryogenesis. The ifi30 gene was found to be highly expressed in the caudal vein plexus (CVP) region of zebrafish embryos. Morpholino-mediated Ifi30 knockdown in zebrafish resulted in incomplete CVP formation with reduced loop numbers, area, and width. Further analyses implied that Ifi30 deficiency impaired cell behaviors of both ECs and macrophages, including cell proliferation and migration. Here, we demonstrate a novel role of IFI30, which was originally identified as a lysosomal thiol reductase involved in immune responses, in CVP development during embryogenesis. Our results suggest that Ifi30 is required for sprouting angiogenesis during CVP formation, which may offer an insight into the function of human IFI30 in angiogenesis under physiological or pathological conditions.

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.

Interaction of transcription factors Islet2 and Nr2f1b to control vascular patterning during zebrafish development

Many regulators controlling arterial identity are well described; however, transcription factors that promote vein identity and vascular patterning have remained largely unknown. We previously identified the transcription factors Islet2 (Isl2) and Nr2f1b required for specification of the vein and tip cell identity mediated by notch pathway in zebrafish. However, the interaction between Isl2 and Nr2f1b is not known. In this study, we report that Nr2f2 plays minor roles on vein and intersegmental vessels (ISV) growth and dissect the genetic interactions among the three transcription factors Isl2, Nr2f1b, and Nr2f2 using a combinatorial knockdown strategy. The double knockdown of isl2/nr2f1b, isl2/nr2f2, and nr2f1b/nr2f2 showed the enhanced defects in vasculature including less completed ISV, reduced veins, and ISV cells. We further tested the genetic relationship among these three transcription factors. We found isl2 can regulate the expression of nr2f1b and nr2f2, suggesting a model where Isl2 functions upstream of Nr2f1b and Nr2f2. We hypothsized that Isl2 and Nr2f1b can function together through cis-regulatory binding motifs. In-vitro luciferase assay results, we showed that Isl2 and Nr2f1b can cooperatively enhance gene expression. Moreover, co-immunoprecipitation results indicated that Isl2 and Nr2f1b interact physically. Together, we showed that the interaction of the Nr2f1b and Nr2f2 transcription factors in combination with the Islet2 play coordinated roles in the vascular development of zebrafish.

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

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

An ATX-LPA6-Gα13-ROCK axis shapes and maintains caudal vein plexus in zebrafish

Lysophosphatidic acid (LPA) is a potential regulator of vascular formation derived from blood. In this study, we utilized zebrafish as a model organism to monitor the blood vessel formation in detail. Zebrafish mutant of ATX, an LPA-producing enzyme, had a defect in the caudal vein plexus (CVP). Pharmacological inhibition of ATX resulted in a fusion of the delicate vessels in the CVP to form large sac-like vessels. Mutant embryos of LPA₆ receptor and downstream Gα₁₃ showed the same phenotype. Administration of OMPT, a stable LPA-analog, induced rapid CVP constriction, which was attenuated significantly in the LPA₆ mutant. We also found that blood flow-induced CVP formation was dependent on ATX. The present study demonstrated that the ATX-LPA₆ axis acts cooperatively with blood flow and contributes to the formation and maintenance of the CVP by generating contractive force in endothelial cells.

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Blocking an ATX-LPA₆-Gα₁₃-ROCK axis causes malformation of the caudal vein plexus

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The axis also contributes to maintaining the fine structure of the caudal vein plexus

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Activation of LPA₆ induces vasoconstriction

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Caudal vein plexus formation evoked by blood flow is dependent on an ATX-LPA₆ axis

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.

Developmental Angiogenesis Requires the Mitochondrial Phenylalanyl-tRNA Synthetase

Background: Mitochondrial aminoacyl-tRNA synthetases (mtARSs) catalyze the binding of specific amino acids to their cognate tRNAs and play an essential role in the synthesis of proteins encoded by mitochondrial DNA. Defects in mtARSs have been linked to human diseases, but their tissue-specific pathophysiology remains elusive. Here we examined the role of mitochondrial phenylalanyl-tRNA synthetase (FARS2) in developmental angiogenesis and its potential contribution to the pathogenesis of cardiovascular disease.

Methods: Morpholinos were injected into fertilized zebrafish ova to establish an in vivo fars2 knock-down model. A visualization of the vasculature was achieved by using Tg (fli1: EGFP)^y1 transgenic zebrafish. In addition, small interference RNAs (siRNAs) were transferred into human umbilical vein endothelial cells (HUVECs) to establish an in vitro FARS2 knock-down model. Cell motility, proliferation, and tubulogenesis were determined using scratch-wound CCK8, transwell-based migration, and tube formation assays. In addition, mitochondria- and non-mitochondria-related respiration were evaluated using a Seahorse XF24 analyzer and flow cytometry assays. Analyses of the expression levels of transcripts and proteins were performed using qRT-PCR and western blotting, respectively.

Results: The knock-down of fars2 hampered the embryonic development in zebrafish and delayed the formation of the vasculature in Tg (fli1: EGFP)^y1 transgenic zebrafish. In addition, the siRNA-mediated knock-down of FARS2 impaired angiogenesis in HUVECs as indicated by decreased cell motility and tube formation capacity. The knock-down of FARS2 also produced variable decreases in mitochondrial- and non-mitochondrial respiration in HUVECs and disrupted the regulatory pathways of angiogenesis in both HUVECs and zebrafish.

Conclusion: Our current work offers novel insights into angiogenesis defects and cardiovascular diseases induced by FARS2 deficiency.

Endothelial Heterogeneity in Development and Wound Healing

The vasculature is comprised of endothelial cells that are heterogeneous in nature. From tissue resident progenitors to mature differentiated endothelial cells, the diversity of these populations allows for the formation, maintenance, and regeneration of the vascular system in development and disease, particularly during situations of wound healing. Additionally, the de-differentiation and plasticity of different endothelial cells, especially their capacity to undergo endothelial to mesenchymal transition, has also garnered significant interest due to its implication in disease progression, with emphasis on scarring and fibrosis. In this review, we will pinpoint the seminal discoveries defining the phenotype and mechanisms of endothelial heterogeneity in development and disease, with a specific focus only on wound healing.

Endothelial Semaphorin 3fb regulates Vegf pathway-mediated angiogenic sprouting

Vessel growth integrates diverse extrinsic signals with intrinsic signaling cascades to coordinate cell migration and sprouting morphogenesis. The pro-angiogenic effects of Vascular Endothelial Growth Factor (VEGF) are carefully controlled during sprouting to generate an efficiently patterned vascular network. We identify crosstalk between VEGF signaling and that of the secreted ligand Semaphorin 3fb (Sema3fb), one of two zebrafish paralogs of mammalian Sema3F. The sema3fb gene is expressed by endothelial cells in actively sprouting vessels. Loss of sema3fb results in abnormally wide and stunted intersegmental vessel artery sprouts. Although the sprouts initiate at the correct developmental time, they have a reduced migration speed. These sprouts have persistent filopodia and abnormally spaced nuclei suggesting dysregulated control of actin assembly. sema3fb mutants show simultaneously higher expression of pro-angiogenic (VEGF receptor 2 (vegfr2) and delta-like 4 (dll4)) and anti-angiogenic (soluble VEGF receptor 1 (svegfr1)/ soluble Fms Related Receptor Tyrosine Kinase 1 (sflt1)) pathway components. We show increased phospho-ERK staining in migrating angioblasts, consistent with enhanced Vegf activity. Reducing Vegfr2 kinase activity in sema3fb mutants rescues angiogenic sprouting. Our data suggest that Sema3fb plays a critical role in promoting endothelial sprouting through modulating the VEGF signaling pathway, acting as an autocrine cue that modulates intrinsic growth factor signaling.

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 tolerance to hypoxia is defined by a time-sensitive response of the gene regulatory network in sea urchin embryos

Deoxygenation, the reduction of oxygen level in the oceans induced by global warming and anthropogenic disturbances, is a major threat to marine life. This change in oxygen level could be especially harmful to marine embryos that use endogenous hypoxia and redox gradients as morphogens during normal development. Here, we show that the tolerance to hypoxic conditions changes between different developmental stages of the sea urchin embryo, possibly due to the structure of the gene regulatory networks (GRNs). We demonstrate that during normal development, the bone morphogenetic protein (BMP) pathway restricts the activity of the vascular endothelial growth factor (VEGF) pathway to two lateral domains and this restriction controls proper skeletal patterning. Hypoxia applied during early development strongly perturbs the activity of Nodal and BMP pathways that affect the VEGF pathway, dorsal-ventral (DV) and skeletogenic patterning. These pathways are largely unaffected by hypoxia applied after DV-axis formation. We propose that the use of redox and hypoxia as morphogens makes the sea urchin embryo highly sensitive to environmental hypoxia during early development, but the GRN structure provides higher tolerance to hypoxia at later stages.

Summary: The use of hypoxia and redox gradients as morphogens makes sea urchin early development sensitive to environmental hypoxia. This sensitivity decreases later, possibly due to the gene regulatory network structure.

Vascular endothelial cell specification in health and disease

There are two vascular networks in mammals that coordinately function as the main supply and drainage systems of the body. The blood vasculature carries oxygen, nutrients, circulating cells, and soluble factors to and from every tissue. The lymphatic vasculature maintains interstitial fluid homeostasis, transports hematopoietic cells for immune surveillance, and absorbs fat from the gastrointestinal tract. These vascular systems consist of highly organized networks of specialized vessels including arteries, veins, capillaries, and lymphatic vessels that exhibit different structures and cellular composition enabling distinct functions. All vessels are composed of an inner layer of endothelial cells that are in direct contact with the circulating fluid; therefore, they are the first responders to circulating factors. However, endothelial cells are not homogenous; rather, they are a heterogenous population of specialized cells perfectly designed for the physiological demands of the vessel they constitute. This review provides an overview of the current knowledge of the specification of arterial, venous, capillary, and lymphatic endothelial cell identities during vascular development. We also discuss how the dysregulation of these processes can lead to vascular malformations, and therapeutic approaches that have been developed for their treatment.

Endothelial struts enable the generation of large lumenized blood vessels de novo

De novo blood vessel formation occurs through coalescence of endothelial cells (ECs) into a cord-like structure, followed by lumenization either through cell-^1-3 or cord-hollowing^4-7. Vessels generated in this manner are restricted in diameter to one or two ECs, and these models fail to explain how vasculogenesis can form large-diameter vessels. Here, we describe a model for large vessel formation that does not require a cord-like structure or a hollowing step. In this model, ECs coalesce into a network of struts in the future lumen of the vessel, a process dependent upon bone morphogenetic protein signalling. The vessel wall forms around this network and consists initially of only a few patches of ECs. To withstand external forces and to maintain the shape of the vessel, strut formation traps erythrocytes into compartments to form a rigid structure. Struts gradually prune and ECs from struts migrate into and become part of the vessel wall. Experimental severing of struts resulted in vessel collapse, disturbed blood flow and remodelling defects, demonstrating that struts enable the patency of large vessels during their formation.

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.

Pax9 is essential for granulopoiesis but dispensable for erythropoiesis in zebrafish

Paired Box (Pax) gene family, a group of transcription regulators have been implicated in diverse physiological processes. However, their role during hematopoiesis which generate a plethora of blood cells remains largely unknown. Using a previously reported single cell transcriptomics data, we analyzed the expression of individual Pax family members in hematopoietic cells in zebrafish. We have identified that Pax9, which is an essential regulator for odontogenesis and palatogenesis, is selectively localized within a single cluster of the hematopoietic lineage. To further analyze the function of Pax9 in hematopoiesis, we generated two independent pax9 knock-out mutants using the CRISPR-Cas9 technique. We found that Pax9 appears to be an essential regulator for granulopoiesis but dispensable for erythropoiesis during development, as lack of pax9 selectively decreased the number of neutrophils with a concomitant decrease in the expression level of neutrophil markers. In addition, embryos, where pax9 was functionally disrupted by injecting morpholinos, failed to increase the number of neutrophils in response to pathogenic bacteria, suggesting that Pax9 is not only essential for developmental granulopoiesis but also emergency granulopoiesis. Due to the inability to initiate emergency granulopoiesis, innate immune responses were severely compromised in pax9 morpholino-mediated embryos, increasing their susceptibility and mortality. Taken together, our data indicate that Pax9 is essential for granulopoiesis and promotes innate immunity in zebrafish larvae.

Slc39a5-mediated zinc homeostasis plays an essential role in venous angiogenesis in zebrafish

Angiogenesis is a precise process mediated by a variety of signals and the environmental niche. Although the essential trace element zinc and its homeostasis are essential for maintaining proper cellular functions, whether zinc plays a role in angiogenesis is currently unknown. Using zebrafish embryos as a model system, we found that zinc treatment significantly increased the expression of the slc39a5 gene, which encodes the zinc transporter Slc39a5. Moreover, knocking down slc39a5 expression using either a morpholino or CRISPR/Cas9-mediated gene editing led to cardiac ischaemia and an accumulation of red blood cells in the caudal vein plexus (CVP), as well as delayed venous sprouting and fewer vascular loops in the CVP region during early development. Further analysis revealed significantly reduced proliferation and delayed cell migration in the caudal vein of slc39a5 morphants. At the mechanistic level, we found increased levels of systemic zinc in slc39a5-deficient embryos, and chelating zinc restored CVP development. In addition, we found that zinc overload in wild-type embryos leads to impaired CVP formation. Taken together, these results indicate that Slc39a5 plays a critical role in endothelial sprouting and migration in venous angiogenesis by regulating zinc homeostasis.

P130Cas/bcar1 mediates zebrafish caudal vein plexus angiogenesis

P130CAS/BCAR1 belongs to the CAS family of adaptor proteins, with important regulatory roles in cell migration, cell cycle control, and apoptosis. Previously, we and others showed that P130CAS mediates VEGF-A and PDGF signalling in vitro, but its cardiovascular function in vivo remains relatively unexplored. We characterise here a novel deletion model of P130CAS in zebrafish. Using in vivo microscopy and transgenic vascular reporters, we observed that while bcar1−/− zebrafish showed no arterial angiogenic or heart defects during development, they strikingly failed to form the caudal vein plexus (CVP). Endothelial cells (ECs) within the CVP of bcar1−/− embryos produced fewer filopodial structures and did not detach efficiently from neighbouring cells, resulting in a significant reduction in ventral extension and overall CVP area. Mechanistically, we show that P130Cas mediates Bmp2b-induced ectopic angiogenic sprouting of ECs in the developing embryo and provide pharmacological evidence for a role of Src family kinases in CVP development.

Apelin signaling drives vascular endothelial cells toward a pro-angiogenic state

To form new blood vessels (angiogenesis), endothelial cells (ECs) must be activated and acquire highly migratory and proliferative phenotypes. However, the molecular mechanisms that govern these processes are incompletely understood. Here, we show that Apelin signaling functions to drive ECs into such an angiogenic state. Zebrafish lacking Apelin signaling exhibit defects in endothelial tip cell morphology and sprouting. Using transplantation experiments, we find that in mosaic vessels, wild-type ECs leave the dorsal aorta (DA) and form new vessels while neighboring ECs defective in Apelin signaling remain in the DA. Mechanistically, Apelin signaling enhances glycolytic activity in ECs at least in part by increasing levels of the growth-promoting transcription factor c-Myc. Moreover, APELIN expression is regulated by Notch signaling in human ECs, and its function is required for the hypersprouting phenotype in Delta-like 4 (Dll4) knockdown zebrafish embryos. These data provide new insights into fundamental principles of blood vessel formation and Apelin signaling, enabling a better understanding of vascular growth in health and disease.

In vivo dynamic analysis of BMP-2-induced ectopic bone formation

Bone morphogenetic protein (BMP)-2 plays a central role in bone-tissue engineering because of its potent bone-induction ability. However, the process of BMP-induced bone formation in vivo remains poorly elucidated. Here, we aimed to establish a method for intravital imaging of the entire process of BMP-2-induced ectopic bone formation. Using multicolor intravital imaging in transgenic mice, we visualized the spatiotemporal process of bone induction, including appearance and motility of osteoblasts and osteoclasts, angiogenesis, collagen-fiber formation, and bone-mineral deposition. Furthermore, we investigated how PTH1-34 affects BMP-2-induced bone formation, which revealed that PTH1-34 administration accelerated differentiation and increased the motility of osteoblasts, whereas it decreased morphological changes in osteoclasts. This is the first report on visualization of the entire process of BMP-2-induced bone formation using intravital imaging techniques, which, we believe, will contribute to our understanding of ectopic bone formation and provide new parameters for evaluating bone-forming activity.

Differences in Expression of Genes Involved in Bone Development and Morphogenesis in the Walls of Internal Thoracic Artery and Saphenous Vein Conduits May Provide Markers Useful for Evaluation Graft Patency

Coronary artery bypass grafting (CABG) is one of the most efficient procedures for patients with advanced coronary artery disease. From all the blood vessels with the potential to be used in this procedure, the internal thoracic artery (ITA) and the saphenous vein (SV) are the most commonly applied as aortocoronary conduits. Nevertheless, in order to evaluate the graft patency and efficiency effectively, basic knowledge should be constantly expanding at the molecular level as well, as the understanding of predictive factors is still limited. In this study, we have employed the expressive microarray approach, validated with Real-Time Quantitative Polymerase Chain Reaction (RT-qPCR), to analyze the transcriptome of both venous and arterial grafts. Searching for potential molecular factors, we analyzed differentially expressed gene ontologies involved in bone development and morphogenesis, for the possibility of discovery of new markers for the evaluation of ITA and SV segment quality. Among three ontological groups of interest—“endochondral bone morphogenesis”, “ossification”, and “skeletal system development”—we found six genes common to all of them. BMP6, SHOX2, COL13A1, CSGALNACT1, RUNX2, and STC1 showed differential expression patterns in both analyzed vessels. STC1 and COL13A1 were upregulated in ITA samples, whereas others were upregulated in SV. With regard to the Runx2 protein function in osteogenic phenotype regulation, the RUNX2 gene seems to be of paramount importance in assessing the potential of ITA, SV, and other vessels used in the CABG procedure. Overall, the presented study provided valuable insight into the molecular background of conduit characterization, and thus indicated genes that may be the target of subsequent studies, also at the protein level. Moreover, it has been suggested that RUNX2 may be recognized as a molecular marker of osteogenic changes in human blood vessels.

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.

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.

Ivabradine promotes angiogenesis and reduces cardiac hypertrophy in mice with myocardial infarction

OBJECTIVE

We investigated the underlying mechanism of ivabradine (IVA) in promoting angiogenesis and reducing cardiac hypertrophy in mice with myocardial infarction (MI).

METHODS

Nineteen mice were randomly assigned into three groups as follows: sham group (10 ml/kg/day phosphate buffer saline (PBS), n=6), model group (MI and 10 ml/kg/day PBS, n=6) and IVA group (MI and 10 mg/kg/day IVA, n=7). All groups received an intragastric gavage for four weeks. Heart and body mass were measured. Cardiac function and heart rate were assessed by echocardiography and electrocardiography, respectively. The collagen deposition, area of cardiomyocytes, and number of capillaries were evaluated using Masson's staining, anti-wheat germ agglutinin (WGA) staining, and platelet endothelial cell adhesion molecule-1 (CD31) staining, respectively. The protein kinase B (Akt)- endothelial nitric oxide synthase (eNOS) signaling and p-38 mitogen-activated protein kinase (MAPK) family in myocardium were determined by western blot.

RESULTS

IVA treatment greatly improved cardiac dysfunction and suppressed cardiac hypertrophy at 4 weeks after MI (p<0.05). Heart rate and fibrotic area of IVA group declined notably compared to those of the model group (p<0.05). IVA administration substantially reduced cardiomyocyte size and increased capillary formation (p<0.05). Besides, IVA medication can enhance Akt-eNOS signaling and inhibit p38 MAPK phosphorylation in the heart of mice with MI (p<0.05).

CONCLUSION

IVA can perform two functions, the promotion of angiogenesis and the reduction of cardiac hypertrophy, both of which were closely associated with Akt-eNOS signaling activation and p38 MAPK inhibition.

Growth factor signaling pathways in vascular development and disease

Angiogenic blood vessel growth is essential to ensure organs receive adequate blood supply to support normal organ function and homeostasis. Angiogenesis involves a complex series of cellular events through which new vessels grow out from existing vasculature. Growth factor signaling, layered over a range of other signaling inputs, orchestrates this process. The response of endothelial cells (ECs) to growth factor signals must be carefully controlled through feedback mechanisms to prevent excessive vessel growth, remodeling or destabilization. In this article, we summarize recent findings describing how ECs respond to growth factor signals during blood vessel development and homeostasis and how perturbation of these responses can lead to disease.

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.

Vascular Development

The vascular system forms as a branching network of endothelial cells that acquire identity as arterial, venous, hemogenic, or lymphatic. Endothelial specification depends on gene targets transcribed by Ets domain-containing factors, including Ets variant gene 2 (Etv2), together with the activity of chromatin-remodeling complexes containing Brahma-related gene-1 (Brg1). Once specified and assembled into vessels, mechanisms regulating lumen diameter and axial growth ensure that the structure of the branching vascular network matches the need for perfusion of target tissues. In addition, blood vessels provide important morphogenic cues that guide or direct the development of organs forming around them. As the embryo grows and lumen diameters increase, smooth muscle cells wrap around the nascent vessel walls to provide mechanical strength and vasomotor control of the circulation. Increasing mechanical stretch and wall strain promote smooth muscle cell differentiation via coupling of actin cytoskeletal remodeling to myocardin and serum response factor-dependent transcription. Remodeling of artery walls by developmental signaling pathways reappears in postnatal blood vessels during physiological and pathological adaptation to vessel wall injury, inflammation, or chronic hypoxia. Recent reports providing insights into major steps in vascular development are reviewed here with a particular emphasis on studies that have been recently published in Arteriosclerosis, Thrombosis, and Vascular Biology.

Blood flow-induced Notch activation and endothelial migration enable vascular remodeling in zebrafish embryos

Arteries and veins are formed independently by different types of endothelial cells (ECs). In vascular remodeling, arteries and veins become connected and some arteries become veins. It is unclear how ECs in transforming vessels change their type and how fates of individual vessels are determined. In embryonic zebrafish trunk, vascular remodeling transforms arterial intersegmental vessels (ISVs) into a functional network of arteries and veins. Here we find that, once an ISV is connected to venous circulation, venous blood flow promotes upstream migration of ECs that results in displacement of arterial ECs by venous ECs, completing the transformation of this ISV into a vein without trans-differentiation of ECs. Arterial blood flow initiated in two neighboring ISVs prevents their transformation into veins by activating Notch signaling in ECs. Together, different responses of ECs to arterial and venous blood flow lead to formation of a balanced network with equal numbers of arteries and veins.