Artery/vein specification is governed by opposing phosphatidylinositol-3 kinase and MAP kinase/ERK signaling

Eph-ephrin signaling couples endothelial cell sorting and arterial specification

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

Building human artery and vein endothelial cells from pluripotent stem cells, and enduring mysteries surrounding arteriovenous development

Owing to their manifold roles in health and disease, there have been intense efforts to synthetically generate blood vessels in vitro from human pluripotent stem cells (hPSCs). However, there are multiple types of blood vessel, including arteries and veins, which are molecularly and functionally different. How can we specifically generate either arterial or venous endothelial cells (ECs) from hPSCs in vitro? Here, we summarize how arterial or venous ECs arise during embryonic development. VEGF and NOTCH arbitrate the bifurcation of arterial vs. venous ECs in vivo. While manipulating these two signaling pathways biases hPSC differentiation towards arterial and venous identities, efficiently generating these two subtypes of ECs has remained challenging until recently. Numerous questions remain to be fully addressed. What is the complete identity, timing and combination of extracellular signals that specify arterial vs. venous identities? How do these extracellular signals intersect with fluid flow to modulate arteriovenous fate? What is a unified definition for endothelial progenitors or angioblasts, and when do arterial vs. venous potentials segregate? How can we regulate hPSC-derived arterial and venous ECs in vitro, and generate organ-specific ECs? In turn, answers to these questions could avail the production of arterial and venous ECs from hPSCs, accelerating vascular research, tissue engineering, and regenerative medicine.

HTATIP2 regulates arteriogenic activity in monocytes from patients with limb ischemia

Use of autologous cells isolated from elderly patients with multiple comorbidities may account for the modest efficacy of cell therapy in patients with chronic limb threatening ischemia (CLTI). We aimed to determine whether proarteriogenic monocyte/macrophages (Mo/MΦs) from patients with CLTI were functionally impaired and to demonstrate the mechanisms related to any impairment. Proarteriogenic Mo/MΦs isolated from patients with CLTI were found to have an impaired capacity to promote neovascularization in vitro and in vivo compared with those isolated from healthy controls. This was associated with increased expression of human HIV-1 TAT interactive protein-2 (HTATIP2), a transcription factor known to suppress angiogenesis/arteriogenesis. Silencing HTATIP2 restored the functional capacity of CLTI Mo/MΦs, which was associated with increased expression of arteriogenic regulators Neuropilin-1 and Angiopoietin-1, and their ability to enhance angiogenic (endothelial tubule formation) and arteriogenic (smooth muscle proliferation) processes in vitro. In support of the translational relevance of our findings, silencing HTATIP2 in proarteriogenic Mo/MΦs isolated from patients with CLTI rescued their capacity to enhance limb perfusion in the ischemic hindlimb by effecting greater angiogenesis and arteriogenesis. Ex vivo modulation of HTATIP2 may offer a strategy for rescuing the functional impairment of pro-angio/arteriogenic Mo/MΦs prior to autologous delivery and increase the likelihood of clinical efficacy.

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

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

rasa1-related arteriovenous malformation is driven by aberrant venous signalling

Arteriovenous malformations (AVMs) develop where abnormal endothelial signalling allows direct connections between arteries and veins. Mutations in RASA1, a Ras GTPase activating protein, lead to AVMs in humans and, as we show, in zebrafish rasa1 mutants. rasa1 mutants develop cavernous AVMs that subsume part of the dorsal aorta and multiple veins in the caudal venous plexus (CVP) - a venous vascular bed. The AVMs progressively enlarge and fill with slow-flowing blood. We show that the AVM results in both higher minimum and maximum flow velocities, resulting in increased pulsatility in the aorta and decreased pulsatility in the vein. These hemodynamic changes correlate with reduced expression of the flow-responsive transcription factor klf2a. Remodelling of the CVP is impaired with an excess of intraluminal pillars, which is a sign of incomplete intussusceptive angiogenesis. Mechanistically, we show that the AVM arises from ectopic activation of MEK/ERK in the vein of rasa1 mutants, and that cell size is also increased in the vein. Blocking MEK/ERK signalling prevents AVM initiation in mutants. Alterations in venous MEK/ERK therefore drive the initiation of rasa1 AVMs.

Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.

Hypoxic Postconditioning Promotes Angiogenesis After Ischemic Stroke

Although hypoxic postconditioning (HPC) has a protective effect on ischemic stroke, its effect on angiogenesis after ischemic stroke is still unclear. This study was designed to investigate the effects of HPC on angiogenesis after ischemic stroke and to preliminarily study the mechanism involved. Oxygen-glucose deprivation (OGD)-intervened bEnd.3 (mouse brain-derived Endothelial cell. 3) was used to simulate cerebral ischemia. Cell counting kit-8 (CCK-8), Cell BrdU proliferation, wound healing, Transwell and tube formation assays were used to evaluate the effect of HPC on the cell viability, proliferation, migration (horizontal and vertical migration), morphogenesis and tube formation of bEnd.3. A middle cerebral artery occlusion (MCAO) model was made in C57 mice to simulate focal cerebral ischemia. Rod rotation test, corner test, modified neurological severity score (mNSS), and balance beam walking test were used to evaluate the effect of HPC on the neurological impairment of mice. Immunofluorescence staining was used to evaluate the effect of HPC on angiogenesis in mice. The angiogenesis-related proteins were evaluated and quantified using western blot. Results showed that HPC significantly promoted proliferation, migration and tube formation of bEnd.3. HPC significantly reversed the neurological deficit of MCAO mice. Moreover, HPC significantly promoted angiogenesis in the peri-infarct area, and angiogenesis was found to be positively correlated with the improvement of neurological impairment. The HPC mice showed higher PLCλ and ALK5 than did MCAO. We conclude that HPC improves the neurological deficit caused by focal cerebral ischemia by promoting angiogenesis. Furthermore, the effect of HPC on improving angiogenesis may be related to PLCλ and ALK5.

Genetics of brain arteriovenous malformations and cerebral cavernous malformations

Cerebrovascular malformations comprise abnormal development of cerebral vasculature. They can result in hemorrhagic stroke due to rupture of lesions as well as seizures and neurological defects. The most common forms of cerebrovascular malformations are brain arteriovenous malformations (bAVMs) and cerebral cavernous malformations (CCMs). They occur in both sporadic and inherited forms. Rapidly evolving molecular genetic methodologies have helped to identify causative or associated genes involved in genesis of bAVMs and CCMs. In this review, we highlight the current knowledge regarding the genetic basis of these malformations.

YULINK regulates vascular formation in zebrafish and HUVECs

BACKGROUND

The distinct arterial and venous cell fates are dictated by a combination of various genetic factors which form diverse types of blood vessels such as arteries, veins, and capillaries. We report here that YULINK protein is involved in vasculogenesis, especially venous formation.

METHODS

In this manuscript, we employed gene knockdown, yeast two-hybrid, FLIM-FRET, immunoprecipitation, and various imaging technologies to investigate the role of YULINK gene in zebrafish and human umbilical vein endothelial cells (HUVECs).

RESULTS

Knockdown of YULINK during the arterial-venous developmental stage of zebrafish embryos led to the defective venous formation and abnormal vascular plexus formation. Knockdown of YULINK in HUVECs impaired their ability to undergo cell migration and differentiation into a capillary-like tube formation. In addition, the phosphorylated EPHB4 was decreased in YULINK knockdown HUVECs. Yeast two-hybrid, FLIM-FRET, immunoprecipitation, as well as imaging technologies showed that YULINK colocalized with endosome related proteins (EPS15, RAB33B or TICAM2) and markers (Clathrin and RHOB). VEGF-induced VEGFR2 internalization was also compromised in YULINK knockdown HUVECs, demonstrating to the involvement of YULINK.

CONCLUSION

This study suggests that YULINK regulates vasculogenesis, possibly through endocytosis in zebrafish and HUVECs.

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.

Generating human artery and vein cells from pluripotent stem cells highlights the arterial tropism of Nipah and Hendra viruses

Stem cell research endeavors to generate specific subtypes of classically defined "cell types." Here, we generate >90% pure human artery or vein endothelial cells from pluripotent stem cells within 3-4 days. We specified artery cells by inhibiting vein-specifying signals and vice versa. These cells modeled viral infection of human vasculature by Nipah and Hendra viruses, which are extraordinarily deadly (∼57%-59% fatality rate) and require biosafety-level-4 containment. Generating pure populations of artery and vein cells highlighted that Nipah and Hendra viruses preferentially infected arteries; arteries expressed higher levels of their viral-entry receptor. Virally infected artery cells fused into syncytia containing up to 23 nuclei, which rapidly died. Despite infecting arteries and occupying ∼6%-17% of their transcriptome, Nipah and Hendra largely eluded innate immune detection, minimally eliciting interferon signaling. We thus efficiently generate artery and vein cells, introduce stem-cell-based toolkits for biosafety-level-4 virology, and explore the arterial tropism and cellular effects of Nipah and Hendra viruses.

Repurposing drugs to treat cardiovascular disease in the era of precision medicine

Drug repurposing is the use of a given therapeutic agent for indications other than that for which it was originally designed or intended. The concept is appealing because of potentially lower development costs and shorter timelines than are needed to produce a new drug. To date, drug repurposing for cardiovascular indications has been opportunistic and driven by knowledge of disease mechanisms or serendipitous observation rather than by systematic endeavours to match an existing drug to a new indication. Innovations in two areas of personalized medicine — computational approaches to associate drug effects with disease signatures and predictive model systems to screen drugs for disease-modifying activities — support efforts that together create an efficient pipeline to systematically repurpose drugs to treat cardiovascular disease. Furthermore, new experimental strategies that guide the medicinal chemistry re-engineering of drugs could improve repurposing efforts by tailoring a medicine to its new indication. In this Review, we summarize the historical approach to repurposing and discuss the technological advances that have created a new landscape of opportunities.

Drugs can be repurposed for new therapeutic indications. In this Review, Mercola and colleagues summarize the latest techniques for systematic drug repurposing and re-engineering, which could increase the pace, efficiency and cost-effectiveness of drug discovery for the treatment of cardiovascular disease.

Contemporary technologies are expected to make drug repurposing large-scale, systematic and deliberate rather than opportunistic.

New experimental and computational tools harness patient genomics for drug repurposing.

Discovery of repurposed drugs on the basis of patient genomics has implications for precision prescribing of medicines to treat individual patients.

The treatment of rare, monogenic diseases, which often provide too little return on investment to incentivize conventional drug discovery, might benefit because the molecular aetiologies of these diseases are well suited to the discovery of drug repurposing candidates.

In the spotlight: the role of TGFβ signalling in haematopoietic stem and progenitor cell emergence

Haematopoietic stem and progenitor cells (HSPCs) sustain haematopoiesis by generating precise numbers of mature blood cells throughout the lifetime of an individual. In vertebrates, HSPCs arise during embryonic development from a specialised endothelial cell population, the haemogenic endothelium (HE). Signalling by the Transforming Growth Factor β (TGFβ) pathway is key to regulate haematopoiesis in the adult bone marrow, but evidence for a role in the formation of HSPCs has only recently started to emerge. In this review, we examine recent work in various model systems that demonstrate a key role for TGFβ signalling in HSPC emergence from the HE. The current evidence underpins two seemingly contradictory views of TGFβ function: as a negative regulator of HSPCs by limiting haematopoietic output from HE, and as a positive regulator, by programming the HE towards the haematopoietic fate. Understanding how to modulate the requirement for TGFβ signalling in HSC emergence may have critical implications for the generation of these cells in vitro for therapeutic use.

Mapping the developing human cardiac endothelium at single-cell resolution identifies MECOM as a regulator of arteriovenous gene expression

AIMS

Coronary vasculature formation is a critical event during cardiac development, essential for heart function throughout perinatal and adult life. However, current understanding of coronary vascular development has largely been derived from transgenic mouse models. The aim of this study was to characterize the transcriptome of the human foetal cardiac endothelium using single-cell RNA sequencing (scRNA-seq) to provide critical new insights into the cellular heterogeneity and transcriptional dynamics that underpin endothelial specification within the vasculature of the developing heart.

METHODS AND RESULTS

We acquired scRNA-seq data of over 10 000 foetal cardiac endothelial cells (ECs), revealing divergent EC subtypes including endocardial, capillary, venous, arterial, and lymphatic populations. Gene regulatory network analyses predicted roles for SMAD1 and MECOM in determining the identity of capillary and arterial populations, respectively. Trajectory inference analysis suggested an endocardial contribution to the coronary vasculature and subsequent arterialization of capillary endothelium accompanied by increasing MECOM expression. Comparative analysis of equivalent data from murine cardiac development demonstrated that transcriptional signatures defining endothelial subpopulations are largely conserved between human and mouse. Comprehensive characterization of the transcriptional response to MECOM knockdown in human embryonic stem cell-derived EC (hESC-EC) demonstrated an increase in the expression of non-arterial markers, including those enriched in venous EC.

CONCLUSIONS

scRNA-seq of the human foetal cardiac endothelium identified distinct EC populations. A predicted endocardial contribution to the developing coronary vasculature was identified, as well as subsequent arterial specification of capillary EC. Loss of MECOM in hESC-EC increased expression of non-arterial markers, suggesting a role in maintaining arterial EC identity.

Notch Signaling in HSC Emergence: When, Why and How

The hematopoietic stem cell (HSC) sustains blood homeostasis throughout life in vertebrates. During embryonic development, HSCs emerge from the aorta-gonads and mesonephros (AGM) region along with hematopoietic progenitors within hematopoietic clusters which are found in the dorsal aorta, the main arterial vessel. Notch signaling, which is essential for arterial specification of the aorta, is also crucial in hematopoietic development and HSC activity. In this review, we will present and discuss the evidence that we have for Notch activity in hematopoietic cell fate specification and the crosstalk with the endothelial and arterial lineage. The core hematopoietic program is conserved across vertebrates and here we review studies conducted using different models of vertebrate hematopoiesis, including zebrafish, mouse and in vitro differentiated Embryonic stem cells. To fulfill the goal of engineering HSCs in vitro, we need to understand the molecular processes that modulate Notch signaling during HSC emergence in a temporal and spatial context. Here, we review relevant contributions from different model systems that are required to specify precursors of HSC and HSC activity through Notch interactions at different stages of development.

Single-cell transcriptome of early hematopoiesis guides arterial endothelial-enhanced functional T cell generation from human PSCs

Hematopoietic differentiation of human pluripotent stem cells (hPSCs) requires orchestration of dynamic cell and gene regulatory networks but often generates blood cells that lack natural function. Here, we performed extensive single-cell transcriptomic analyses to map fate choices and gene expression patterns during hematopoietic differentiation of hPSCs and showed that oxidative metabolism was dysregulated during in vitro directed differentiation. Applying hypoxic conditions at the stage of endothelial-to-hematopoietic transition in vitro effectively promoted the development of arterial specification programs that governed the generation of hematopoietic progenitor cells (HPCs) with functional T cell potential. Following engineered expression of the anti-CD19 chimeric antigen receptor, the T cells generated from arterial endothelium-primed HPCs inhibited tumor growth both in vitro and in vivo. Collectively, our study provides benchmark datasets as a resource to further understand the origins of human hematopoiesis and represents an advance in guiding in vitro generation of functional T cells for clinical applications.

Developmental Perspectives on Arterial Fate Specification

Blood vessel acquisition of arterial or venous fate is an adaptive phenomenon in response to increasing blood circulation during vascular morphogenesis. The past two decades of effort in this field led to development of a widely accepted paradigm of molecular regulators centering on VEGF and Notch signaling. More recent findings focused on shear stress-induced cell cycle arrest as a prerequisite for arterial specification substantially modify this traditional understanding. This review aims to summarize key molecular mechanisms that work in concert to drive the acquisition of arterial fate in two distinct developmental settings of vascular morphogenesis: de novo vasculogenesis of the dorsal aorta and postnatal retinal angiogenesis. We will also discuss the questions and conceptual controversies that potentially point to novel directions of investigation and possible clinical relevance.

Rgs4 is a regulator of mTOR activity required for motoneuron axon outgrowth and neuronal development in zebrafish

The Regulator of G protein signaling 4 (Rgs4) is a member of the RGS proteins superfamily that modulates the activity of G-protein coupled receptors. It is mainly expressed in the nervous system and is linked to several neuronal signaling pathways; however, its role in neural development in vivo remains inconclusive. Here, we generated and characterized a rgs4 loss of function model (MZrgs4) in zebrafish. MZrgs4 embryos showed motility defects and presented reduced head and eye sizes, reflecting defective motoneurons axon outgrowth and a significant decrease in the number of neurons in the central and peripheral nervous system. Forcing the expression of Rgs4 specifically within motoneurons rescued their early defective outgrowth in MZrgs4 embryos, indicating an autonomous role for Rgs4 in motoneurons. We also analyzed the role of Akt, Erk and mechanistic target of rapamycin (mTOR) signaling cascades and showed a requirement for these pathways in motoneurons axon outgrowth and neuronal development. Drawing on pharmacological and rescue experiments in MZrgs4, we provide evidence that Rgs4 facilitates signaling mediated by Akt, Erk and mTOR in order to drive axon outgrowth in motoneurons and regulate neuronal numbers.

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.

Somatic Mosaicism in the Pathogenesis of de novo Cerebral Arteriovenous Malformations: A Paradigm Shift Implicating the RAS-MAPK Signaling Cascade

Cerebral arteriovenous malformations (AVMs) are leading causes of lesional hemorrhagic stroke in both the pediatric and young adult population, with sporadic AVMs accounting for the majority of cases. Recent evidence has identified somatic mosaicism in key proximal components of the RAS-MAPK signaling cascade within endothelial cells collected from human sporadic cerebral AVMs, with early preclinical models supporting a potential causal role for these mutations in the pathogenesis of these malformations. Germline mutations that predispose to deregulation of the RAS-MAPK signaling axis have also been identified in hereditary vascular malformation syndromes, highlighting the key role of this signaling axis in global AVM development. Herein, we review the most recent genomic and preclinical evidence implicating somatic mosaicism in the RAS-MAPK signaling pathway in the pathogenesis of sporadic cerebral AVMs. Also, we review evidence for RAS-MAPK dysregulation in hereditary vascular malformation syndromes and present a hypothesis suggesting that this pathway is central for the development of both sporadic and syndrome-associated AVMs. Finally, we examine the clinical implications of these recent discoveries and highlight potential therapeutic targets within this signaling pathway.

Endothelial sprouting, proliferation, or senescence: tipping the balance from physiology to pathology

Therapeutic modulation of vascular cell proliferation and migration is essential for the effective inhibition of angiogenesis in cancer or its induction in cardiovascular disease. The general view is that an increase in vascular growth factor levels or mitogenic stimulation is beneficial for angiogenesis, since it leads to an increase in both endothelial proliferation and sprouting. However, several recent studies showed that an increase in mitogenic stimuli can also lead to the arrest of angiogenesis. This is due to the existence of intrinsic signaling feedback loops and cell cycle checkpoints that work in synchrony to maintain a balance between endothelial proliferation and sprouting. This balance is tightly and effectively regulated during tissue growth and is often deregulated or impaired in disease. Most therapeutic strategies used so far to promote vascular growth simply increase mitogenic stimuli, without taking into account its deleterious effects on this balance and on vascular cells. Here, we review the main findings on the mechanisms controlling physiological vascular sprouting, proliferation, and senescence and how those mechanisms are often deregulated in acquired or congenital cardiovascular disease leading to a diverse range of pathologies. We also discuss alternative approaches to increase the effectiveness of pro-angiogenic therapies in cardiovascular regenerative medicine.

Fluid flow as a driver of embryonic morphogenesis

Fluid flow is a powerful morphogenic force during embryonic development. The physical forces created by flowing fluids can either create morphogen gradients or be translated by mechanosensitive cells into biological changes in gene expression. In this Primer, we describe how fluid flow is created in different systems and highlight the important mechanosensitive signalling pathways involved for sensing and transducing flow during embryogenesis. Specifically, we describe how fluid flow helps establish left-right asymmetry in the early embryo and discuss the role of flow of blood, lymph and cerebrospinal fluid in sculpting the embryonic cardiovascular and nervous system.

Ets1 functions partially redundantly with Etv2 to promote embryonic vasculogenesis and angiogenesis in zebrafish

ETS transcription factors play an important role in the specification and differentiation of endothelial cells during vascular development. Despite previous studies, the role of the founding member of the ETS family, Ets1, in vascular development in vivo is only partially understood. Here, we generated a zebrafish ets1 mutant by TALEN genome editing and tested functional redundancy between Ets1 and a related ETS factor Etv2/Etsrp/ER71. While zebrafish ets1^-/- mutants have a normal functional vascular system, etv2^-/-;ets1^-/embryos had more severe angiogenic defects and lower expression levels of kdr and kdrl, the two zebrafish homologs of the mammalian Vascular Endothelial Growth Factor Receptor 2 VEGFR2/Flk1, than etv2^-/-embryos. Expression of constitutively active Mitogen-Activated Protein Kinase1 (MAP2K1) within endothelial cells partially rescued this angiogenic defect. Interestingly, ets1^-/- embryos displayed extensive apoptosis within the trunk vasculature despite exhibiting normal vascular patterning. Loss of Ets1 combined with a partial knockdown of Etv2 function resulted in a decrease in endothelial cell numbers in the axial vasculature, which argues for a role of Ets1 in promoting vasculogenesis. We also demonstrate that although both Ets1 and Etv2 can induce ectopic vascular marker expression in zebrafish embryos, Ets1 activity is dependent on MAPK-mediated phosphorylation of its Thr30 and Ser33 residues, while Etv2 activity is not. Together, our results identify a novel function of Ets1 in regulating endothelial cell survival during vasculogenesis in vivo. Based on these findings, we propose a revised model of how Ets1 and Etv2 play unique and partially redundant roles to promote vascular development.

Somatic Gain of KRAS Function in the Endothelium Is Sufficient to Cause Vascular Malformations That Require MEK but Not PI3K Signaling

Supplemental Digital Content is available in the text.

Rationale:

We previously identified somatic activating mutations in the KRAS (Kirsten rat sarcoma viral oncogene homologue) gene in the endothelium of the majority of human sporadic brain arteriovenous malformations; a disorder characterized by direct connections between arteries and veins. However, whether this genetic abnormality alone is sufficient for lesion formation, as well as how active KRAS signaling contributes to arteriovenous malformations, remains unknown.

Objective:

To establish the first in vivo models of somatic KRAS gain of function in the endothelium in both mice and zebrafish to directly observe the phenotypic consequences of constitutive KRAS activity at a cellular level in vivo, and to test potential therapeutic interventions for arteriovenous malformations.

Methods and Results:

Using both postnatal and adult mice, as well as embryonic zebrafish, we demonstrate that endothelial-specific gain of function mutations in Kras (G12D or G12V) are sufficient to induce brain arteriovenous malformations. Active KRAS signaling leads to altered endothelial cell morphogenesis and increased cell size, ectopic sprouting, expanded vessel lumen diameter, and direct connections between arteries and veins. Furthermore, we show that these lesions are not associated with altered endothelial growth dynamics or a lack of proper arteriovenous identity but instead seem to feature exuberant angiogenic signaling. Finally, we demonstrate that KRAS-dependent arteriovenous malformations in zebrafish are refractory to inhibition of the downstream effector PI3K but instead require active MEK (mitogen-activated protein kinase kinase 1) signaling.

Conclusions:

We demonstrate that active KRAS expression in the endothelium is sufficient for brain arteriovenous malformations, even in the setting of uninjured adult vasculature. Furthermore, the finding that KRAS-dependent lesions are reversible in zebrafish suggests that MEK inhibition may represent a promising therapeutic treatment for arteriovenous malformation patients.

Anti-angiogenic effects of VEGF stimulation on endothelium deficient in phosphoinositide recycling

Anti-angiogenic therapies have generated significant interest for their potential to combat tumor growth. However, tumor overproduction of pro-angiogenic ligands can overcome these therapies, hampering success of this approach. To circumvent this problem, we target the resynthesis of phosphoinositides consumed during intracellular transduction of pro-angiogenic signals in endothelial cells (EC), thus harnessing the tumor’s own production of excess stimulatory ligands to deplete adjacent ECs of the capacity to respond to these signals. Using zebrafish and human endothelial cells in vitro, we show ECs deficient in CDP-diacylglycerol synthase 2 are uniquely sensitive to increased vascular endothelial growth factor (VEGF) stimulation due to a reduced capacity to re-synthesize phosphoinositides, including phosphatidylinositol-(4,5)-bisphosphate (PIP2), resulting in VEGF-exacerbated defects in angiogenesis and angiogenic signaling. Using murine tumor allograft models, we show that systemic or EC specific suppression of phosphoinositide recycling results in reduced tumor growth and tumor angiogenesis. Our results suggest inhibition of phosphoinositide recycling provides a useful anti-angiogenic approach.

Tumors can overproduce pro-angiogenic ligands overcoming currently approved anti-angiogenic therapies and hindering their success. Here, the authors show that targeting phosphoinositide recycling during tumor angiogenesis harnesses the tumor’s own production of angiogenic ligands to deplete adjacent endothelial cells of the capacity to respond to these signals.

5F peptide promotes endothelial differentiation of bone marrow stem cells through activation of ERK1/2 signaling

Synthetic apolipoprotein A-I (apoA-I) mimetic peptide 5F exhibits anti-atherosclerotic ability with largely unknown mechanism(s). Bone marrow (BM)-derived endothelial progenitor cells (EPCs) play a critical role in vascular integrity and function. The objective of the present study was to evaluate the effect of 5F on endothelial differentiation of BM stem cells and related mechanisms. Murine BM multipotent adult progenitor cells (MAPCs) were induced to differentiate into endothelial cells in vitro with or without 5F. The expression of endothelial markers vWF, Flk-1 and CD31 was significantly increased in the cells treated with 5F with enhanced in vitro vascular tube formation and LDL uptake without significant changes on proliferation and stem cell maker Oct-4 expression. Phosphorylated ERK1/2, not Akt, was significantly increased in 5F-treated cells. Treatment of MAPCs with PD98059 or small interfering RNA against ERK2 substantially attenuated ERK1/2 phosphorylation, and effectively prevented 5F-induced enhancement of endothelial differentiation of MAPCs. In vivo studies revealed that 5F increased EPCs number in the BM in mice after acute hindlimb ischemia that was effectively prevented with PD98059 treatment. These data supported the conclusion that 5F promoted endothelial differentiation of MAPCs through activation of ERK1/2 signaling.

Isoform-Specific Roles of ERK1 and ERK2 in Arteriogenesis

Despite the clinical importance of arteriogenesis, this biological process is poorly understood. ERK1 and ERK2 are key components of a major intracellular signaling pathway activated by vascular endothelial growth (VEGF) and FGF2, growth factors critical to arteriogenesis. To investigate the specific role of each ERK isoform in arteriogenesis, we used mice with a global Erk1 knockout as well as Erk1 and Erk2 floxed mice to delete Erk1 or Erk2 in endothelial cells, macrophages, and smooth muscle cells. We found that ERK1 controls macrophage infiltration following an ischemic event. Loss of ERK1 in endothelial cells and macrophages induced an excessive macrophage infiltration leading to an increased but poorly functional arteriogenesis. Loss of ERK2 in endothelial cells leads to a decreased arteriogenesis due to decreased endothelial cell proliferation and a reduced eNOS expression. These findings show for the first time that isoform-specific roles of ERK1 and ERK2 in the control of arteriogenesis.

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

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

Heterogeneity of Vascular Endothelial Cells, De Novo Arteriogenesis and Therapeutic Implications in Pancreatic Neuroendocrine Tumors

Arteriogenesis supplies oxygen and nutrients in the tumor microenvironment (TME), which may play an important role in tumor growth and metastasis. Pancreatic neuroendocrine tumors (pNETs) are the second most common pancreatic malignancy and are frequently metastatic on presentation. Nearly a third of pNETs secrete bioactive substances causing debilitating symptoms. Current treatment options for metastatic pNETs are limited. Importantly, these tumors are highly vascularized and heterogeneous neoplasms, in which the heterogeneity of vascular endothelial cells (ECs) and de novo arteriogenesis may be critical for their progression. Current anti-angiogenetic targeted treatments have not shown substantial clinical benefits, and they are poorly tolerated. This review article describes EC heterogeneity and heterogeneous tumor-associated ECs (TAECs) in the TME and emphasizes the concept of de novo arteriogenesis in the TME. The authors also emphasize the challenges of current antiangiogenic therapy in pNETs and discuss the potential of tumor arteriogenesis as a novel therapeutic target. Finally, the authors prospect the clinical potential of targeting the FoxO1-CD36-Notch pathway that is associated with both pNET progression and arteriogenesis and provide insights into the clinical implications of targeting plasticity of cancer stem cells (CSCs) and vascular niche, particularly the arteriolar niche within the TME in pNETs, which will also provide insights into other types of cancer, including breast cancer, lung cancer, and malignant melanoma.

Evolutionarily conserved regulation of sleep by epidermal growth factor receptor signaling

The genetic bases for most human sleep disorders and for variation in human sleep quantity and quality are largely unknown. Using the zebrafish, a diurnal vertebrate, to investigate the genetic regulation of sleep, we found that epidermal growth factor receptor (EGFR) signaling is necessary and sufficient for normal sleep levels and is required for the normal homeostatic response to sleep deprivation. We observed that EGFR signaling promotes sleep via mitogen-activated protein kinase/extracellular signal-regulated kinase and RFamide neuropeptide signaling and that it regulates RFamide neuropeptide expression and neuronal activity. Consistent with these findings, analysis of a large cohort of human genetic data from participants of European ancestry revealed that common variants in genes within the EGFR signaling pathway are associated with variation in human sleep quantity and quality. These results indicate that EGFR signaling and its downstream pathways play a central and ancient role in regulating sleep and provide new therapeutic targets for sleep disorders.

Streamlining drug discovery assays for cardiovascular disease using zebrafish

Introduction: In the last decade, our armamentarium of cardiovascular drug therapy has expanded significantly. Using innovative functional genomics strategies such as genome editing by CRISPR/Cas9 as well as high-throughput assays to identify bioactive small chemical compounds has significantly facilitated elaboration of the underlying pathomechanism in various cardiovascular diseases. However, despite scientific progress approvals for cardiovascular drugs has stagnated significantly compared to other fields of drug discovery and therapy during the past years.Areas covered: In this review, the authors discuss the aspects and pitfalls during the early phase of cardiovascular drug discovery and describe the advantages of zebrafish as an in vivo organism to model human cardiovascular diseases (CVD) as well as an in vivo platform for high-throughput chemical compound screening. They also highlight the emerging, promising techniques of automated read-out systems during high-throughput screening (HTS) for the evaluation of important cardiac functional parameters in zebrafish with the potential to streamline CVD drug discovery.Expert opinion: The successful identification of novel drugs to treat CVD is a major challenge in modern biomedical and clinical research. In this context, the definition of the etiologic fundamentals of human cardiovascular diseases is the prerequisite for an efficient and straightforward drug discovery.

Molecular regulation of arteriovenous endothelial cell specification

The systemic circulation depends upon a highly organized, hierarchal blood vascular network that requires the successful specification of arterial and venous endothelial cells during development. This process is driven by a cascade of signaling events (including Hedgehog, vascular endothelial growth factor (VEGF), Notch, connexin (Cx), transforming growth factor-beta (TGF- β), and COUP transcription factor 2 (COUP-TFII)) to influence endothelial cell cycle status and expression of arterial or venous genes and is further regulated by hemodynamic flow. Failure of endothelial cells to properly undergo arteriovenous specification may contribute to vascular malformation and dysfunction, such as in hereditary hemorrhagic telangiectasia (HHT) and capillary malformation-arteriovenous malformation (CM-AVM) where abnormal vessel structures, such as large shunts lacking clear arteriovenous identity and function, form and compromise peripheral blood flow. This review provides an overview of recent findings in the field of arteriovenous specification and highlights key regulators of this process.

Understanding the Journey of Human Hematopoietic Stem Cell Development

Hematopoietic stem cells (HSCs) surface during embryogenesis leading to the genesis of the hematopoietic system, which is vital for immune function, homeostasis balance, and inflammatory responses in the human body. Hematopoiesis is the process of blood cell formation, which initiates from hematopoietic stem/progenitor cells (HSPCs) and is responsible for the generation of all adult blood cells. With their self-renewing and pluripotent properties, human pluripotent stem cells (hPSCs) provide an unprecedented opportunity to create in vitro models of differentiation that will revolutionize our understanding of human development, especially of the human blood system. The utilization of hPSCs provides newfound approaches for studying the origins of human blood cell diseases and generating progenitor populations for cell-based treatments. Current shortages in our knowledge of adult HSCs and the molecular mechanisms that control hematopoietic development in physiological and pathological conditions can be resolved with better understanding of the regulatory networks involved in hematopoiesis, their impact on gene expression, and further enhance our ability to develop novel strategies of clinical importance. In this review, we delve into the recent advances in the understanding of the various cellular and molecular pathways that lead to blood development from hPSCs and examine the current knowledge of human hematopoietic development. We also review how in vitro differentiation of hPSCs can undergo hematopoietic transition and specification, including major subtypes, and consider techniques and protocols that facilitate the generation of hematopoietic stem cells.

Sec14l3 potentiates VEGFR2 signaling to regulate zebrafish vasculogenesis

Vascular endothelial growth factor (VEGF) regulates vasculogenesis by using its tyrosine kinase receptors. However, little is known about whether Sec14-like phosphatidylinositol transfer proteins (PTP) are involved in this process. Here, we show that zebrafish sec14l3, one of the family members, specifically participates in artery and vein formation via regulating angioblasts and subsequent venous progenitors’ migration during vasculogenesis. Vascular defects caused by sec14l3 depletion are partially rescued by restoration of VEGFR2 signaling at the receptor or downstream effector level. Biochemical analyses show that Sec14l3/SEC14L2 physically bind to VEGFR2 and prevent it from dephosphorylation specifically at the Y¹¹⁷⁵ site by peri-membrane tyrosine phosphatase PTP1B, therefore potentiating VEGFR2 signaling activation. Meanwhile, Sec14l3 and SEC14L2 interact with RAB5A/4A and facilitate the formation of their GTP-bound states, which might be critical for VEGFR2 endocytic trafficking. Thus, we conclude that Sec14l3 controls vasculogenesis in zebrafish via the regulation of VEGFR2 activation.

The growth factor VEGF is known to regulate vasculogenesis but the downstream pathways activated are unclear. Here, the authors report that Sec14l3, a member of the PITP (phosphatidyl inositol transfer proteins) family regulates the formation of zebrafish vasculature by promoting VEGFR2 endocytic trafficking.

Developing zebrafish disease models for in vivo small molecule screens

The zebrafish is a model organism that allows in vivo studies to be performed at a scale usually restricted to in vitro studies. As such, the zebrafish is well suited to in vivo screens, in which thousands of small molecules are tested for their ability to modify disease phenotypes in zebrafish disease models. Numerous approaches have been developed for modeling human diseases in zebrafish, including mutagenesis, transgenesis, pharmacological approaches, wounding, and exposure to infectious or cancerous agents. We review the various strategies for modeling human diseases in zebrafish and discuss important considerations when developing zebrafish models for use in in vivo small molecule screens.

Effect of voluntary exercise on intracellular signalling pathways of angiogenesis in the sciatic nerve of type 1 diabetic castrated male rats

Objectives

Impaired angiogenesis in sciatic nerve is a major complication of diabetic neuropathy. Protein kinase B (AKT) and extracellular signal regulated kinase (ERK) signaling pathways play critical roles during capillary-like network formation in the angiogenesis process.

Methods

Twenty-four adult male Wistar rats (weight: 250–300 g) were used in the research. The rats were randomly divided into four groups (n = 6): (1) diabetic (Dia), (2) diabetic + castration (Dia-Cas), (3) diabetic + exercise (Dia-E), and (4) diabetic + castration + exercise (Dia-Cas-E). Type 1 diabetes (T1D) was induced with streptozotocin (50 mg/kg). After 6 weeks, sciatic nerve was separated and used for histological evaluation and determination of phosphorylated AKT (P-AKT) and phosphorylated ERK (P-ERK) levels by ELISA method.

Results

Glucose levels decreased in the Dia-E group compared to the Dia-Cas group (p < 0.01). In addition, our finding shows that exercise in the Dia-Cas group diminished blood glucose levels compared to the Dia-Cas group but this effect of exercise was not significant. Voluntary exercise in the diabetic castrated group decreased P-AKT protein and increased P-ERK 1/2 protein levels in the sciatic tissue compared to the diabetes group significantly (p < 0.05). Histopathological findings showed that Dia-Cas group with 6-week exercise training significantly raised the number of microvascular density in the sciatic tissue of diabetic rats compared to the diabetic group (p < 0.05).

Conclusions

Voluntary exercise in diabetic rats increases angiogenesis in the sciatic nerve. The possible mechanism is the increase of P-ERK 1/2 but not P-AKT levels in the sciatic nerve of T1D rats.

Screening drugs for myocardial disease in vivo with zebrafish: an expert update

INTRODUCTION

Our understanding of the complexity of cardiovascular disease pathophysiology remains very incomplete and has hampered cardiovascular drug development over recent decades. The prevalence of cardiovascular diseases and their increasing global burden call for novel strategies to address disease biology and drug discovery. Areas covered: This review describes the recent history of cardiovascular drug discovery using in vivo phenotype-based screening in zebrafish. The rationale for the use of this model is highlighted and the initial efforts in the fields of disease modeling and high-throughput screening are illustrated. Finally, the advantages and limitations of in vivo zebrafish screening are discussed, highlighting newer approaches, such as genome editing technologies, to accelerate our understanding of disease biology and the development of precise disease models. Expert opinion: Full understanding and faithful modeling of specific cardiovascular disease is a rate-limiting step for cardiovascular drug discovery. The resurgence of in vivo phenotype screening together with the advancement of systems biology approaches allows for the identification of lead compounds which show efficacy on integrative disease biology in the absence of validated targets. This strategy bypasses current gaps in knowledge of disease biology and paves the way for successful drug discovery and downstream molecular target identification.

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.

Effect of Disulfide Bond Incorporation on the Structure and Activity of Endostatin Peptide

The structure and function of a 27-a.a. fragment of the N-terminal sequence of human endostatin (ES-Zn) were compared to those of the mutant peptide (ES-SSZn) obtained by adding Cys-Pro-Ala to the endostatin N-terminus and substituting Asn16 for Cys ensuring formation of a disulfide bond. Structural comparison of ES-Zn and ES-SSZn by far-UV circular dichroism (CD), intrinsic fluorescence, and molecular dynamics simulation methods revealed significant structural perturbations in ES-SSZn, such as elimination of the β-sheet conformer, modification of the N-terminal loop structure, and reorganization of dynamic properties of the entire peptide backbone. ES-SSZn was approximately 2 and 3 times less efficient than ES-Zn and the full-length human endostatin, respectively, in the induction of caspase-3-dependent apoptosis in human umbilical vein endothelial cells (HUVECs) in vitro (p < 0.05). In contrast, treatment of metastatic 4T1 breast tumors in mice with ES-Zn and ES-SSZn (5 mg/kg body weight daily) for 14 days resulted in similar regression of tumor size, comparable downregulation of angiogenesis (CD31 and CD34) and cell proliferation (Ki67), and therefore, the same extent of apoptosis induction (TUNEL, p53, and Bcl-2) for both peptides (as compared to the untreated controls). Western blot analysis of HUVEC and 4T1 tumor lysates revealed the same levels of suppression of key signaling mediators Akt and ERK1/2 by ES-Zn and ES-SSZn. Contrary to the earlier studies, our results showed that the function of the 1-27 endostatin fragment is independent of its overall structure. Stabilization of the N-terminal loop structure by the disulfide bond incorporation causes relief from structural deviations.

Arterial identity of hemogenic endothelium: a key to unlock definitive hematopoietic commitment in human pluripotent stem cell cultures

Human pluripotent stem cells (hPSCs) have been suggested as a potential source for the de novo production of blood cells for transfusion, immunotherapies, and transplantation. However, even with advanced hematopoietic differentiation methods, the primitive and myeloid-restricted waves of hematopoiesis dominate in hPSC differentiation cultures, whereas cell surface markers to distinguish these waves of hematopoiesis from lympho-myeloid hematopoiesis remain unknown. In the embryo, hematopoietic stem cells (HSCs) arise from hemogenic endothelium (HE) lining arteries, but not veins. This observation led to a long-standing hypothesis that arterial specification is an essential prerequisite to initiate the HSC program. It has also been established that lymphoid potential in the yolk sac and extraembryonic vasculature is mostly confined to arteries, whereas myeloid-restricted hematopoiesis is not specific to arterial vessels. Here, we review how the link between arterialization and the subsequent definitive multilineage hematopoietic program can be exploited to identify HE enriched in lymphoid progenitors and aid in in vitro approaches to enhance the production of lymphoid cells and potentially HSCs from hPSCs. We also discuss alternative models of hematopoietic specification at arterial sites and recent advances in our understanding of hematopoietic development and the production of engraftable hematopoietic cells from hPSCs.

FoxO1 transcriptional activities in VEGF expression and beyond: a key regulator in functional angiogenesis?

FoxO1 has emerged as an important regulator of angiogenesis. Recent work published in this Journal shows that FoxO1 regulates VEGF expression in keratinocytes and is required for angiogenesis in wound healing. Since FoxO1 also regulates CD36 transcription, and endothelial cell differentiation and vascular maturation, this transcription factor may be essential for the formation of functional vascular networks via coupling the regulation of CD36 in vascular endothelial cells under physiological and pathological conditions. Although many outstanding questions remain to be answered, the mechanisms by which FoxO1 regulates VEGF in keratinocytes provide insight into the development of functional angiogenesis and further our understanding of vascular biology. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Arterial Venous Differentiation for Vascular Bioengineering

The development processes of arteries and veins are fundamentally different, leading to distinct differences in anatomy, structure, and function as well as molecular profiles. Understanding the complex interaction between genetic and epigenetic pathways, as well as extracellular and biomechanical signals that orchestrate arterial venous differentiation, is not only critical for the understanding of vascular diseases of arteries and veins but also valuable for vascular tissue engineering strategies. Recent research has suggested that certain transcriptional factors not only control arterial venous differentiation during development but also play a critical role in adult vessel function and disease processes. This review summarizes the signaling pathways and critical transcription factors that are important for arterial versus venous specification. We focus on those signals that have a direct relation to the structure and function of arteries and veins, and have implications for vascular disease processes and tissue engineering applications.

VEGF/PKD-1 signaling mediates arteriogenic gene expression and angiogenic responses in reversible human microvascular endothelial cells with extended lifespan

Microvascular ECs (MVECs) are an ideal model in angiogenesis research. The aim of this study was to determine vascular endothelial growth factor (VEGF)/protein kinase D1 (PKD-1) signaling in expression of arteriogenic genes in human MVECs. To achieve this aim, we transduced specific SV40 large T antigen and telomerase into primary human dermal MVECs (HMVEC-D) to establish reversible HMVECs with extended lifespan (HMVECi-D). HMVECi-D was then exposed to VEGF/VEGF-inducer GS4012 or transduced with constitutively active protein kinase PKD-1 (PKD-CA). Quantitative RT-PCR was performed to detect arteriogenic gene expression. Furthermore, the angiogenic capacity in response to VEGF pathway was evaluated by Matrigel tube-formation and proliferation assays. We observed that VEGF/PKD-1 signaling axis significantly stimulated the expression of arteriogenic genes and promoted EC proliferation, along with downregulation of CD36 expression. Intriguingly, overexpression of PKD-CA also resulted in formation of tip cell morphology, accompanied by increased mRNA of delta-like ligand 4 (DLL4). In conclusion, we have successfully established and characterized HMVECi-D, and showed that VEGF/PKD-1 signaling axis increases angiogenic and arteriogenic gene expression. These studies suggest that the axis may regulate arteriolar differentiation through changing MVEC gene expression.

FGF mediated MAPK and PI3K/Akt Signals make distinct contributions to pluripotency and the establishment of Neural Crest

Early vertebrate embryos possess cells with the potential to generate all embryonic cell types. While this pluripotency is progressively lost as cells become lineage restricted, Neural Crest cells retain broad developmental potential. Here, we provide novel insights into signals essential for both pluripotency and neural crest formation in Xenopus. We show that FGF signaling controls a subset of genes expressed by pluripotent blastula cells, and find a striking switch in the signaling cascades activated by FGF signaling as cells lose pluripotency and commence lineage restriction. Pluripotent cells display and require Map Kinase signaling, whereas PI3 Kinase/Akt signals increase as developmental potential is restricted, and are required for transit to certain lineage restricted states. Importantly, retaining a high Map Kinase/low Akt signaling profile is essential for establishing Neural Crest stem cells. These findings shed important light on the signal-mediated control of pluripotency and the molecular mechanisms governing genesis of Neural Crest.

Cardioprotective Effect of Crocin Combined with Voluntary Exercise in Rat: Role of Mir-126 and Mir-210 in Heart Angiogenesis

Background

Crocin is reported to have a wide range of biological activities such as cardiovascular protection. Recent epidemiologic studies have shown that exercise reduces cardiovascular morbidity and mortality in the general population.

Objective

The aim of this study was to evaluate the effect of crocin and voluntary exercise on miR-126 and miR-210 expression levels and angiogenesis in the heart tissue.

Methods

Animals were divided into 4 groups: control, exercise, crocin, and exercise-crocin. Animals received oral administration of crocin (50 mg/kg) or performed voluntary exercise alone or together for 8 weeks. Akt, ERK1/2 protein levels, miR-126 and miR-210 expression were measured in the heart tissue. Immunohistochemical method was used to detect CD31 in the heart tissue.

Results

Akt and ERK1/2 levels of the heart tissue were higher in crocin treated group and voluntary exercise trained group after 8 weeks. Combination of crocin and exercise also significantly enhanced Akt and ERK1/2 levels in the heart tissue. MiR-126, miR-210 expression and CD31 in the heart increased in both crocin and voluntary exercise groups compared with control group. In addition, combination of exercise and crocin amplified their effect on miR-126 and miR-210 expression, and angiogenesis.

Conclusion

Crocin and voluntary exercise improve heart angiogenesis possibly through enhancement of miR-126 and miR-210 expression. Voluntary exercise and diet supplementation with crocin could have beneficial effects in prevention of cardiovascular disease.

Endoglin prevents vascular malformation by regulating flow-induced cell migration and specification through VEGFR2 signalling

Loss-of-function (LOF) mutations in the endothelial cell (EC) enriched gene endoglin (ENG) causes the human disease hereditary haemorrhagic telangiectasia-1, characterized by vascular malformations promoted by vascular endothelial growth factor A (VEGFA). How ENG deficiency alters EC behaviour to trigger these anomalies is not understood. Mosaic ENG deletion in the postnatal mouse rendered Eng LOF ECs insensitive to flow-mediated venous to arterial migration. Eng LOF ECs retained within arterioles acquired venous characteristics and secondary ENG-independent proliferation resulting in arterio-venous malformation (AVM). Analysis following simultaneous Eng LOF and overexpression (OE) revealed that ENG OE ECs dominate tip cell positions and home preferentially to arteries. ENG knock-down altered VEGFA-mediated VEGFR2 kinetics and promoted AKT signalling. Blockage of PI3K/AKT partly normalised flow-directed migration of ENG LOF ECs in vitro and reduced the severity of AVM in vivo. This demonstrates the requirement of ENG in flow-mediated migration and modulation of VEGFR2 signalling in vascular patterning.

Vegf signaling promotes vascular endothelial differentiation by modulating etv2 expression

Vasculogenesis involves the differentiation of vascular endothelial progenitors de novo from undifferentiated mesoderm, their migration and coalescence to form the major embryonic vessels and the acquisition of arterial or venous identity. Vascular Endothelial Growth Factor (Vegf) signaling plays multiple roles during vascular development. However, its function during embryonic vasculogenesis has been controversial. Previous studies have implicated Vegf signaling in either regulating arteriovenous specification or overall vascular endothelial differentiation. To clarify the role of Vegf in embryonic vasculogenesis and identify its downstream targets, we used chemical inhibitors of Vegf receptor (Vegfr) signaling in zebrafish embryos as well as zebrafish genetic mutants. A high level of chemical inhibition of Vegfr signaling resulted in the reduction of overall vascular endothelial marker gene expression, including downregulation of both arterial and venous markers, ultimately leading to the apoptosis of vascular endothelial cells. In contrast, a low level of Vegfr inhibition specifically blocked arterial specification while the expression of venous markers appeared largely unaffected or increased. Inhibition of Vegfr signaling prior to the initiation of vasculogenesis reduced overall vascular endothelial differentiation, while inhibition of Vegfr signaling starting at mid-somitogenesis stages largely inhibited arterial specification. Conversely, Vegf overexpression resulted in the expansion of both arterial and pan-endothelial markers, while the expression of several venous-specific markers was downregulated. We further show that Vegf signaling affects overall endothelial differentiation by modulating the expression of the ETS transcription factor etv2/ etsrp. etv2 expression was downregulated in Vegfr- inhibited embryos, and expanded in Vegfaa-overexpressing embryos. Furthermore, vascular-specific overexpression of etv2 in Vegfr-inhibited embryos rescued defects in vascular endothelial differentiation. Similarly, vegfaa genetic mutants displayed a combination of the two phenotypes observed with chemical Vegfr inhibition: the expression of arterial and pan-endothelial markers including etv2 was downregulated while the expression of most venous markers was either expanded or unchanged. Based on these results we propose a revised model which explains the different phenotypes observed upon inhibition of Vegf signaling: low levels of Vegf signaling promote overall vascular endothelial differentiation and cell survival by upregulating etv2 expression, while high levels of Vegf signaling promote arterial and inhibit venous specification.

Highly Efficient Differentiation of Endothelial Cells from Pluripotent Stem Cells Requires the MAPK and the PI3K Pathways

Pluripotent stem cells are a promising source of endothelial cells (ECs) for the treatment of vascular diseases. We have developed a robust protocol to differentiate human induced pluripotent stem cells (hiPSCs) and embryonic stem cells (hESCs) into ECs with high purities (94%-97% CD31⁺ and 78%-83% VE-cadherin⁺ ) in 8 days without cell sorting. Passaging of these cells yielded a nearly pure population of ECs (99% of CD31⁺ and 96.8% VE-cadherin⁺ ). These ECs also expressed other endothelial markers vWF, Tie2, NOS3, and exhibited functions of ECs such as uptake of Dil-acetylated low-density lipoprotein and formation of tubes in vitro or vessels in vivo on matrigel. We found that FGF2, VEGF, and BMP4 synergistically induced early vascular progenitors (VPs) from hiPSC-derived mesodermal cells. The MAPK and PI3K pathways are crucial not only for the initial commitment to vascular lineages but also for the differentiation of vascular progenitors to ECs, most likely through regulation of the ETS family transcription factors, ERG and FLI1. We revealed novel roles of the p38 and JNK MAPK pathways on EC differentiation. Furthermore, inhibition of the ERK pathway markedly promoted the differentiation of smooth muscle cells. Finally, we demonstrate that pluripotent stem cell-derived ECs are capable of forming patent blood vessels that were connected to the host vasculature in the ischemic limbs of immune deficient mice. Thus, we demonstrate that ECs can be efficiently derived from hiPSCs and hESCs, and have great potential for vascular therapy as well as for mechanistic studies of EC differentiation. Stem Cells 2017;35:909-919.