Endothelial cell self-fusion during vascular pruning

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.

TMEM215 Prevents Endothelial Cell Apoptosis in Vessel Regression by Blunting BIK-Regulated ER-to-Mitochondrial Ca Influx

BACKGROUND

In developmental and pathological tissues, nascent vessel networks generated by angiogenesis require further pruning/regression to delete nonfunctional endothelial cells (ECs) by apoptosis and migration. Mechanisms underlying EC apoptosis during vessel pruning remain elusive. TMEM215 (transmembrane protein 215) is an endoplasmic reticulum-located, 2-pass transmembrane protein. We have previously demonstrated that TMEM215 knockdown in ECs leads to cell death, but its physiological function and mechanism are unclear.

METHODS

We characterized the role and mechanism of TMEM215 in EC apoptosis using human umbilical vein endothelial cells by identifying its interacting proteins with immunoprecipitation-mass spectrometry. The physiological function of TMEM215 in ECs was assessed by establishing a conditional knockout mouse strain. The role of TMEM215 in pathological angiogenesis was evaluated by tumor and choroidal neovascularization models. We also tried to evaluate its translational value by delivering a Tmem215 small interfering RNA (siRNA) using nanoparticles in vivo.

RESULTS

TMEM215 knockdown in ECs induced apoptotic cell death. We identified the chaperone BiP as a binding partner of TMEM215, and TMEM215 forms a complex with and facilitates the interaction of BiP (binding immunoglobin protein) with the BH (BCL-2 [B-cell lymphoma 2] homology) 3-only proapoptotic protein BIK (BCL-2 interacting killer). TMEM215 knockdown triggered apoptosis in a BIK-dependent way and was abrogated by BCL-2. Notably, TMEM215 knockdown increased the number and diminished the distance of mitochondria-associated endoplasmic reticulum membranes and increased mitochondrial calcium influx. Inhibiting mitochondrial calcium influx by blocking the IP3R (inositol 1,4,5-trisphosphate receptor) or MCU (mitochondrial calcium uniporter) abrogated TMEM215 knockdown-induced apoptosis. TMEM215 expression in ECs was induced by physiological laminar shear stress via EZH2 downregulation. In EC-specific Tmem215 knockout mice, induced Tmem215 depletion impaired the regression of retinal vasculature characterized by reduced vessel density, increased empty basement membrane sleeves, and increased EC apoptosis. Moreover, EC-specific Tmem215 ablation inhibited tumor growth with disrupted vasculature. However, Tmem215 ablation in adult mice attenuated lung metastasis, consistent with reduced Vcam1 expression. Administration of nanoparticles carrying Tmem215 siRNA also inhibited tumor growth and choroidal neovascularization injury.

CONCLUSIONS

TMEM215, which is induced by blood flow-derived shear stress via downregulating EZH2, protects ECs from BIK-triggered mitochondrial apoptosis mediated by calcium influx through mitochondria-associated ER membranes during vessel pruning, thus providing a novel target for antiangiogenic therapy.

Up-to-date molecular medicine strategies for management of ocular surface neovascularization

Ocular surface neovascularization and its resulting pathological changes significantly alter corneal refraction and obstruct the light path to the retina, and hence is a major cause of vision loss. Various factors such as infection, irritation, trauma, dry eye, and ocular surface surgery trigger neovascularization via angiogenesis and lymphangiogenesis dependent on VEGF-related and alternative mechanisms. Recent advances in antiangiogenic drugs, nanotechnology, gene therapy, surgical equipment and techniques, animal models, and drug delivery strategies have provided a range of novel therapeutic options for the treatment of ocular surface neovascularization. In this review article, we comprehensively discuss the etiology and mechanisms of corneal neovascularization and other types of ocular surface neovascularization, as well as emerging animal models and drug delivery strategies that facilitate its management.

Mathematical models of developmental vascular remodelling: A review

Over the past 40 years, there has been a strong focus on the development of mathematical models of angiogenesis, while developmental remodelling has received little such attention from the mathematical community. Sprouting angiogenesis can be seen as a very crude way of laying out a primitive vessel network (the raw material), while remodelling (understood as pruning of redundant vessels, diameter control, and the establishment of vessel identity and hierarchy) is the key to turning that primitive network into a functional network. This multiscale problem is of prime importance in the development of a functional vasculature. In addition, defective remodelling (either during developmental remodelling or due to a reactivation of the remodelling programme caused by an injury) is associated with a significant number of diseases. In this review, we discuss existing mathematical models of developmental remodelling and explore the important contributions that these models have made to the field of vascular development. These mathematical models are effectively used to investigate and predict vascular development and are able to reproduce experimentally observable results. Moreover, these models provide a useful means of hypothesis generation and can explain the underlying mechanisms driving the observed structural and functional network development. However, developmental vascular remodelling is still a relatively new area in mathematical biology, and many biological questions remain unanswered. In this review, we present the existing modelling paradigms and define the key challenges for the field.

Mechanisms of skin vascular maturation and maintenance captured by longitudinal imaging of live mice

A functional network of blood vessels is essential for organ growth and homeostasis, yet how the vasculature matures and maintains homeostasis remains elusive in live mice. By longitudinally tracking the same neonatal endothelial cells (ECs) over days to weeks, we found that capillary plexus expansion is driven by vessel regression to optimize network perfusion. Neonatal ECs rearrange positions to evenly distribute throughout the developing plexus and become positionally stable in adulthood. Upon local ablation, adult ECs survive through a plasmalemmal self-repair response, while neonatal ECs are predisposed to die. Furthermore, adult ECs reactivate migration to assist vessel repair. Global ablation reveals coordinated maintenance of the adult vascular architecture that allows for eventual network recovery. Lastly, neonatal remodeling and adult maintenance of the skin vascular plexus are orchestrated by temporally restricted, neonatal VEGFR2 signaling. Our work sheds light on fundamental mechanisms that underlie both vascular maturation and adult homeostasis in vivo.

The role of blood flow in vessel remodeling and its regulatory mechanism during developmental angiogenesis

Vessel remodeling is essential for a functional and mature vascular network. According to the difference in endothelial cell (EC) behavior, we classified vessel remodeling into vessel pruning, vessel regression and vessel fusion. Vessel remodeling has been proven in various organs and species, such as the brain vasculature, subintestinal veins (SIVs), and caudal vein (CV) in zebrafish and yolk sac vessels, retina, and hyaloid vessels in mice. ECs and periendothelial cells (such as pericytes and astrocytes) contribute to vessel remodeling. EC junction remodeling and actin cytoskeleton dynamic rearrangement are indispensable for vessel pruning. More importantly, blood flow has a vital role in vessel remodeling. In recent studies, several mechanosensors, such as integrins, platelet endothelial cell adhesion molecule-1 (PECAM-1)/vascular endothelial cell (VE-cadherin)/vascular endothelial growth factor receptor 2 (VEGFR2) complex, and notch1, have been shown to contribute to mechanotransduction and vessel remodeling. In this review, we highlight the current knowledge of vessel remodeling in mouse and zebrafish models. We further underline the contribution of cellular behavior and periendothelial cells to vessel remodeling. Finally, we discuss the mechanosensory complex in ECs and the molecular mechanisms responsible for vessel remodeling.

Angiogenic Modification of Microfibrous Polycaprolactone by pCMV-VEGF165 Plasmid Promotes Local Vascular Growth after Implantation in Rats

Insufficient vascular growth in the area of artificial-material implantation contributes to ischemia, fibrosis, the development of bacterial infections, and tissue necrosis around the graft. The purpose of this study was to evaluate angiogenesis after implantation of polycaprolactone microfiber scaffolds modified by a pCMV-VEGF165-plasmid in rats. Influence of vascularization on scaffold degradation was also examined. We investigated flat microfibrous scaffolds obtained by electrospinning polycaprolactone with incorporation of the pCMV-VEGF-165 plasmid into the microfibers at concentrations of 0.005 ng of plasmid per 1 mg of polycaprolactone (0.005 ng/mg) (LCGroup) and 0.05 ng/mg (HCGroup). The samples were subcutaneously implanted in the interscapular area of rats. On days 7, 16, 33, 46, and 64, the scaffolds were removed, and a histological study with a morphometric evaluation of the density and diameter of the vessels and microfiber diameter was performed. The number of vessels was increased in all groups, as well as the resorption of the scaffold. On day 33, the vascular density in the HCGroup was 42% higher compared to the control group (p = 0.0344). The dose-dependent effect of the pCMV-VEGF165-plasmid was confirmed by enhanced angiogenesis in the HCGroup compared to the LCGroup on day 33 (p-value = 0.0259). We did not find a statistically significant correlation between scaffold degradation rate and vessel growth (the Pearson correlation coefficient was ρ = 0.20, p-value = 0.6134). Functionalization of polycaprolactone by incorporation of the pCMV-VEGF165 plasmid provided improved vascularization within 33 days after implantation, however, vessel growth did not seem to correlate with scaffold degradation rate.

Endothelial CYP2J2 overexpression restores the BRB via METTL3-mediated ANXA1 upregulation

Blood-retinal barrier (BRB) breakdown is responsible for multiple ocular diseases, such as diabetic retinopathy, age-related macular degeneration, and retinal vascular occlusive diseases. Increased vascular permeability contributes to vasogenic edema and tissue damage, with consequent adverse effects on vision. Herein, we found that endothelial CYP2J2 overexpression maintained BRB integrity after ischemia-reperfusion injury and consequently protected against retinal ganglion cell loss. Oxidative stress repressed endothelial ANXA1 expression in vivo and in vitro. CYP2J2 upregulated methyltransferase-like 3 (METTL3) expression and hence promoted ANXA1 translation via ANXA1 m⁶ A modification in endothelium under oxidative stress. CYP2J2 maintained the distribution of endothelial tight junctions and adherens junctions in an ANXA1-dependent manner. Endothelial ANXA1 plays an indispensable role in vascular homeostasis and stabilization during development. Endothelial ANXA1 deletion disrupted retinal vascular perfusion as well as BRB integrity. CYP2J2 metabolites restored BRB integrity in the presence of ANXA1. Our findings identified the CYP2J2-METTL3-ANXA1 pathway as a potential therapeutic target for relieving BRB impairments.

Single-cell RNA sequencing profiling of mouse endothelial cells in response to pulmonary arterial hypertension

AIMS

Endothelial cell (EC) dysfunction drives the initiation and pathogenesis of pulmonary arterial hypertension (PAH). We aimed to characterize EC dynamics in PAH at single-cell resolution.

METHODS AND RESULTS

We carried out single-cell RNA sequencing (scRNA-seq) of lung ECs isolated from an EC lineage-tracing mouse model in Control and SU5416/hypoxia-induced PAH conditions. EC populations corresponding to distinct lung vessel types, including two discrete capillary populations, were identified in both Control and PAH mice. Differential gene expression analysis revealed global PAH-induced EC changes that were confirmed by bulk RNA-seq. This included upregulation of the major histocompatibility complex class II pathway, supporting a role for ECs in the inflammatory response in PAH. We also identified a PAH response specific to the second capillary EC population including upregulation of genes involved in cell death, cell motility, and angiogenesis. Interestingly, four genes with genetic variants associated with PAH were dysregulated in mouse ECs in PAH. To compare relevance across PAH models and species, we performed a detailed analysis of EC heterogeneity and response to PAH in rats and humans through whole-lung PAH scRNA-seq datasets, revealing that 51% of up-regulated mouse genes were also up-regulated in rat or human PAH. We identified promising new candidates to target endothelial dysfunction including CD74, the knockdown of which regulates EC proliferation and barrier integrity in vitro. Finally, with an in silico cell ordering approach, we identified zonation-dependent changes across the arteriovenous axis in mouse PAH and showed upregulation of the Serine/threonine-protein kinase Sgk1 at the junction between the macro- and microvasculature.

CONCLUSION

This study uncovers PAH-induced EC transcriptomic changes at a high resolution, revealing novel targets for potential therapeutic candidate development.

Vascularized pancreas-on-a-chip device produced using a printable simulated extracellular matrix

The extracellular matrix (ECM) influences cellular behavior, function, and fate. The ECM surrounding Langerhans islets has not been investigated in detail to explain its role in the development and maturation of pancreatic β-cells. Herein, a complex combination of the simulated ECM (sECM) has been examined with a comprehensive analysis of cell response and a variety of controls. The most promising results were obtained from group containing fibrin, collagen type I, Matrigel®, hyaluronic acid, methylcellulose, and two compounds of functionalized, ionically crosslinking bacterial cellulose (sECMbc). Even though the cell viability was not significantly impacted, the performance of group of sECMbc showed 2 to 4x higher sprouting number and length, 2 to 4x higher insulin secretion in static conditions, and 2 to 10x higher gene expression of VEGF-A, Endothelin-1, and NOS3 than the control group of fibrin matrix (sECMf). Each material was tested in a hydrogel-based, perfusable, pancreas-on-a-chip device and the best group - sECMbc has been tested with the drug Sunitinib to show the extended possibilities of the device for both diabetes-like screening as well as PDAC chemotherapeutics screening for potential personal medicine approach. It proved its functionality in 7 days dynamic culture and is suitable as a physiological tissue model. Moreover, the device with the pancreatic-like spheroids was 3D bioprintable and perfusable.

High-Throughput Imaging of Blood Flow Reveals Developmental Changes in Distribution Patterns of Hemodynamic Quantities in Developing Zebrafish

Mechanical forces from blood flow and pressure (hemodynamic forces) contribute to the formation and shaping of the blood vascular network during embryonic development. Previous studies have demonstrated that hemodynamic forces regulate signaling and gene expression in endothelial cells that line the inner surface of vascular tubes, thereby modifying their cellular state and behavior. Given its important role in vascular development, we still know very little about the quantitative aspects of hemodynamics that endothelial cells experience due to the difficulty in measuring forces in vivo. In this study, we sought to determine the magnitude of wall shear stress (WSS) exerted on ECs by blood flow in different vessel types and how it evolves during development. Utilizing the zebrafish as a vertebrate model system, we have established a semi-automated high-throughput fluorescent imaging system to capture the flow of red blood cells in an entire zebrafish between 2- and 6-day post-fertilization (dpf). This system is capable of imaging up to 50 zebrafish at a time. A semi-automated analysis method was developed to calculate WSS in zebrafish trunk vessels. This was achieved by measuring red blood cell flow using particle tracking velocimetry analysis, generating a custom-made script to measure lumen diameter, and measuring local tube hematocrit levels to calculate the effective blood viscosity at each developmental stage. With this methodology, we were able to determine WSS magnitude in different vessels at different stages of embryonic and larvae growth and identified developmental changes in WSS, with absolute levels of peak WSS in all vessel types falling to levels below 0.3 Pa at 6 dpf. Additionally, we discovered that zebrafish display an anterior-to-posterior trend in WSS at each developmental stage.

Shear stress switches the association of endothelial enhancers from ETV/ETS to KLF transcription factor binding sites

Endothelial cells (ECs) lining blood vessels are exposed to mechanical forces, such as shear stress. These forces control many aspects of EC biology, including vascular tone, cell migration and proliferation. Despite a good understanding of the genes responding to shear stress, our insight into the transcriptional regulation of these genes is much more limited. Here, we set out to study alterations in the chromatin landscape of human umbilical vein endothelial cells (HUVEC) exposed to laminar shear stress. To do so, we performed ChIP-Seq for H3K27 acetylation, indicative of active enhancer elements and ATAC-Seq to mark regions of open chromatin in addition to RNA-Seq on HUVEC exposed to 6 h of laminar shear stress. Our results show a correlation of gained and lost enhancers with up and downregulated genes, respectively. DNA motif analysis revealed an over-representation of KLF transcription factor (TF) binding sites in gained enhancers, while lost enhancers contained more ETV/ETS motifs. We validated a subset of flow responsive enhancers using luciferase-based reporter constructs and CRISPR-Cas9 mediated genome editing. Lastly, we characterized the shear stress response in ECs of zebrafish embryos using RNA-Seq. Our results lay the groundwork for the exploration of shear stress responsive elements in controlling EC biology.

Zebrafish as a tractable model of human cardiovascular disease

Mammalian models including non-human primates, pigs and rodents have been used extensively to study the mechanisms of cardiovascular disease. However, there is an increasing desire for alternative model systems that provide excellent scientific value while replacing or reducing the use of mammals. Here, we review the use of zebrafish, Danio rerio, to study cardiovascular development and disease. The anatomy and physiology of zebrafish and mammalian cardiovascular systems are compared, and we describe the use of zebrafish models in studying the mechanisms of cardiac (e.g. congenital heart defects, cardiomyopathy, conduction disorders and regeneration) and vascular (endothelial dysfunction and atherosclerosis, lipid metabolism, vascular ageing, neurovascular physiology and stroke) pathologies. We also review the use of zebrafish for studying pharmacological responses to cardiovascular drugs and describe several features of zebrafish that make them a compelling model for in vivo screening of compounds for the treatment cardiovascular disease. LINKED ARTICLES: This article is part of a themed issue on Preclinical Models for Cardiovascular disease research (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.5/issuetoc.

Analyzing Vessel Regression and Endothelial Apoptosis as a Component of Angiogenic Vessel Remodeling

Angiogenic vessel remodeling is a critical step in establishing a hierarchical vessel network. Vessel networks rapidly expand through angiogenesis in response to pro-angiogenic factors. This leads to an initially dense vessel network that requires selective regression of vessel branches to establish a hierarchical conduit for blood flow, a process known as pruning. This involves migration of endothelial cells from low-flow vessels to adjacent high-flow vessels and generally occurs independently of cell death. Vessels may also regress in response to other stimuli, including reduced metabolic demand, redundancy, and pathological stimuli. In these contexts, widespread vessel regression typically occurs and involves loss of endothelial cells by apoptotic cell death. Thus, vessel remodeling occurs via both apoptosis independent and dependent vessel regression. In this chapter, we outline a semi-automated method for quantifying vessel regression using the neonatal model of angiogenesis. We further provide instruction on analyzing endothelial apoptosis in this model.

A cell atlas of microbe-responsive processes in the zebrafish intestine

Gut microbial products direct growth, differentiation, and development in animal hosts. However, we lack system-wide understanding of cell-specific responses to the microbiome. We profiled cell transcriptomes from the intestine, and associated tissue, of zebrafish larvae raised in the presence or absence of a microbiome. We uncovered extensive cellular heterogeneity in the conventional zebrafish intestinal epithelium, including previously undescribed cell types with known mammalian homologs. By comparing conventional to germ-free profiles, we mapped microbial impacts on transcriptional activity in each cell population. We revealed intricate degrees of cellular specificity in host responses to the microbiome that included regulatory effects on patterning and on metabolic and immune activity. For example, we showed that the absence of microbes hindered pro-angiogenic signals in the developing vasculature, causing impaired intestinal vascularization. Our work provides a high-resolution atlas of intestinal cellular composition in the developing fish gut and details the effects of the microbiome on each cell type.

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.

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.

Chemerin regulates normal angiogenesis and hypoxia-driven neovascularization

Chemerin is a multifunctional protein initially characterized in our laboratory as a chemoattractant factor for leukocyte populations. Its main functional receptor is CMKLR1. We identified previously chemerin as an anti-tumoral factor inhibiting the vascularization of tumor grafts. We show here that overexpression of bioactive chemerin in mice results in a reduction of the density of the retinal vascular network during its development and in adults. Chemerin did not affect vascular sprouting during the post-natal development of the network, but rather promoted endothelial cell apoptosis and vessel pruning. This phenotype was reversed to normal in CMKLR1-deficient mice, demonstrating the role of this receptor. Chemerin inhibited also neoangiogenesis in a model of pathological proliferative retinopathy, and in response to hind-limb ischemia. Mechanistically, PTEN and FOXO1 antagonists could almost completely restore the density of the retinal vasculature, suggesting the involvement of the PI3-kinase/AKT pathway in the chemerin-induced vessel regression process.

Methods to quantify endothelial cell front-rear polarity in vivo and in vitro

PURPOSE OF REVIEW

Endothelial cell (EC) front-rear (axial) polarization in response to chemokines and shear stress is fundamental for angiogenesis. This review provides an overview of the in vitro and in vivo methods that are currently available to quantify EC axial polarity.

RECENT FINDINGS

Innovative methodologies and new animal models have been developed to evaluate EC axial polarity. Micropatterning, wound healing and microfluidic assays allow interrogation of signalling mechanisms in vitro. Mouse and zebrafish transgenic lines, in combination with advances in imaging techniques and computational tools, enable interrogation of physiological functions of EC axial polarity in vascular biology during development and in pathology in vivo.

SUMMARY

We present a literature-based review of the methods available to study EC polarity. Further refinement of quantitative methods to analyse EC axial polarity using deep learning-based computational tools will generate new understanding on the aetiology of vascular malformations.

From remodeling to quiescence: The transformation of the vascular network

The vascular system is essential for embryogenesis, healing, and homeostasis. Dysfunction or deregulated blood vessel function contributes to multiple diseases, including diabetic retinopathy, cancer, hypertension, or vascular malformations. A balance between the formation of new blood vessels, vascular remodeling, and vessel quiescence is fundamental for tissue growth and function. Whilst the major mechanisms contributing to the formation of new blood vessels have been well explored in recent years, vascular remodeling and quiescence remain poorly understood. In this review, we highlight the cellular and molecular mechanisms responsible for vessel remodeling and quiescence during angiogenesis. We further underline how impaired remodeling and/or destabilization of vessel networks can contribute to vascular pathologies. Finally, we speculate how addressing the molecular mechanisms of vascular remodeling and stabilization could help to treat vascular-related disorders.

The effect of absent blood flow on the zebrafish cerebral and trunk vasculature

The role of blood flow in vascular development is complex and context-dependent. In this study, we quantify the effect of the lack of blood flow on embryonic vascular development on two vascular beds, namely the cerebral and trunk vasculature in zebrafish. We perform this by analysing vascular topology, endothelial cell (EC) number, EC distribution, apoptosis, and inflammatory response in animals with normal blood flow or absent blood flow. We find that absent blood flow reduced vascular area and EC number significantly in both examined vascular beds, but the effect is more severe in the cerebral vasculature, and severity increases over time. Absent blood flow leads to an increase in non-EC-specific apoptosis without increasing tissue inflammation, as quantified by cerebral immune cell numbers and nitric oxide. Similarly, while stereotypic vascular patterning in the trunk is maintained, intra-cerebral vessels show altered patterning, which is likely to be due to vessels failing to initiate effective fusion and anastomosis rather than sprouting or path-seeking. In conclusion, blood flow is essential for cellular survival in both the trunk and cerebral vasculature, but particularly intra-cerebral vessels are affected by the lack of blood flow, suggesting that responses to blood flow differ between these two vascular beds.

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

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

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

Endothelial cell mechanics and blood flow forces in vascular morphogenesis

The vertebrate cardiovascular system is made up by a hierarchically structured network of highly specialised blood vessels. This network emerges during early embryogenesis and evolves in size and complexity concomitant with embryonic growth and organ formation. Underlying this plasticity are actin-driven endothelial cell behaviours, which allow endothelial cells to change their shape and move within the vascular network. In this review, we discuss the cellular and molecular mechanisms involved in vascular network formation and how these intrinsic mechanisms are influenced by haemodynamic forces provided by pressurized blood flow. While most of this review focusses on in vivo evidence from zebrafish embryos, we also mention complementary findings obtained in other experimental systems.

Diverse cellular origins of adult blood vascular endothelial cells

During embryonic stages, vascular endothelial cells (ECs) originate from the mesoderm, at specific extraembryonic and embryonic regions, through a process called vasculogenesis. In the adult, EC renewal/replacement mostly depend on local resident ECs or endothelial progenitor cells (EPCs). Nevertheless, contribution from circulating ECs/EPCs was also reported. In addition, cells lacking from EC/EPC markers with in vitro extended plasticity were shown to originate endothelial-like cells (ELCs). Most of these cells consist of mesenchymal stromal progenitors, which would eventually get mobilized from the bone marrow after injury. Based on that, current knowledge on different mouse and human bone marrow stromal cell (BM-SC) subpopulations, able to contribute with mesenchymal stromal/stem cells (MSCs), is herein reviewed. Such analyses underline an unexpected heterogeneity among sinusoidal LepR⁺ stromal/CAR cells. For instance, in a recent report a subgroup of LepR⁺ stromal/CAR progenitors, which express GLAST and is traced in Wnt1^Cre;R26R^Tom mice, was found to contribute with ELCs in vivo. These GLAST ​⁺ ​Wnt1⁺ BM-SCs were shown to get mobilized to the peripheral blood and to contribute with liver regeneration. Other sources of ELCs, such as adipose, neural and dental pulp tissues, were also published. Finally, mechanisms likely involved in the enhanced cellular plasticity properties of bone marrow/adipose tissue stromal cells, able to originate ELCs, are assessed. In the future, strategies to analyze the in vivo expression profile of stromal cells, with MSC properties, in combination with screening of active genomic regions at the single cell-level, during early postnatal development and/or after injury, will likely help understanding properties of these ELC sources.

The Zebrafish Cardiac Endothelial Cell-Roles in Development and Regeneration

In zebrafish, the spatiotemporal development of the vascular system is well described due to its stereotypical nature. However, the cellular and molecular mechanisms orchestrating post-embryonic vascular development, the maintenance of vascular homeostasis, or how coronary vessels integrate into the growing heart are less well studied. In the context of cardiac regeneration, the central cellular mechanism by which the heart regenerates a fully functional myocardium relies on the proliferation of pre-existing cardiomyocytes; the epicardium and the endocardium are also known to play key roles in the regenerative process. Remarkably, revascularisation of the injured tissue occurs within a few hours after cardiac damage, thus generating a vascular network acting as a scaffold for the regenerating myocardium. The activation of the endocardium leads to the secretion of cytokines, further supporting the proliferation of the cardiomyocytes. Although epicardium, endocardium, and myocardium interact with each other to orchestrate heart development and regeneration, in this review, we focus on recent advances in the understanding of the development of the endocardium and the coronary vasculature in zebrafish as well as their pivotal roles in the heart regeneration process.

2,3,7,8-Tetrachlorodibenzo-p-dioxin exposure disrupts development of the visceral and ocular vasculature

The aryl hydrocarbon receptor (AHR) has endogenous functions in mammalian vascular development and is necessary for mediating the toxic effects of a number of environmental contaminants. Studies in mice have demonstrated that AHR is necessary for the formation of the renal, retinal, and hepatic vasculature. In fish, exposure to the prototypic AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces expression of the AHR biomarker cyp1a throughout the developing vasculature and produces vascular malformations in the head and heart. However, it is not known whether the vascular structures that are sensitive to loss of AHR function are also disrupted by aberrant AHR activation. Here, we report that TCDD-exposure in zebrafish disrupts development of 1) the subintestinal venous plexus (SIVP), which vascularizes the developing liver, kidney, gut, and pancreas, and 2) the superficial annular vessel (SAV), an essential component of the retinal vasculature. Furthermore, we determined that TCDD exposure increased the expression of bmp4, a key molecular mediator of SIVP morphogenesis. We hypothesize that the observed SIVP phenotypes contribute to one of the hallmarks of TCDD exposure in fish - the failure of the yolk sac to absorb. Together, our data describe novel TCDD-induced vascular phenotypes and provide molecular insight into critical factors producing the observed vascular malformations.

Inhibitory Effects of Euphorbia ebracteolata Hayata Extract ECB on Melanoma-Induced Hyperplasia of Blood Vessels in Zebrafish Embryos

Melanoma is a serious malignant form of skin cancer. Euphorbiaceae compound B (ECB, 2,4-dihydroxy-6-methoxy-3-methylacetophenone) is an acetophenone compound that is isolated from Euphorbia ebracteolata Hayata (EEH), an herbaceous perennial, and has antitumor activity. Here, we transplanted human melanoma cells into zebrafish embryos to establish a zebrafish/melanoma model. We showed that this model can be used to evaluate the therapeutic effect of EEH and ECB and discussed its potential mechanism of action. The results showed that ECB was an active ingredient of EEH in inhibiting melanoma-induced hyperplasia of blood vessels in zebrafish embryos, similar to the angiogenic inhibitor vatalanib. ECB inhibited the number and length of subintestinal veins (p < 0.05), as well as the distribution of melanoma in zebrafish embryos (p < 0.05). More importantly, unlike vatalanib, ECB only inhibited melanoma-induced abnormal and excessive growth of blood vessels in xenografts. In addition, ECB inhibited the mRNA expression of vegfr2 and vegfr3 in zebrafish. Both vegfr2 and vegfr3 are essential genes that regulate blood vessel formation and upregulate the expression of p53 and casp3a genes in zebrafish. Together, the above-mentioned results indicate that ECB has a potential antimelanoma effect in vivo, which may be mediated by inhibiting vascular endothelial growth factor receptors.

Contribution of cell death signaling to blood vessel formation

The formation of new blood vessels is driven by proliferation of endothelial cells (ECs), elongation of maturing vessel sprouts and ultimately vessel remodeling to create a hierarchically structured vascular system. Vessel regression is an essential process to remove redundant vessel branches in order to adapt the final vessel density to the demands of the surrounding tissue. How exactly vessel regression occurs and whether and to which extent cell death contributes to this process has been in the focus of several studies within the last decade. On top, recent findings challenge our simplistic view of the cell death signaling machinery as a sole executer of cellular demise, as emerging evidences suggest that some of the classic cell death regulators even promote blood vessel formation. This review summarizes our current knowledge on the role of the cell death signaling machinery with a focus on the apoptosis and necroptosis signaling pathways during blood vessel formation in development and pathology.

Building the complex architectures of vascular networks: Where to branch, where to connect and where to remodel?

The cardiovascular system is the first organ to become functional during vertebrate embryogenesis and is responsible for the distribution of oxygen and nutrients to all cells of the body. The cardiovascular system constitutes a circulatory loop in which blood flows from the heart through arteries into the microvasculature and back through veins to the heart. The vasculature is characterized by the heterogeneity of blood vessels with respect to size, cellular architecture and function, including both larger vessels that are found at defined positions within the body and smaller vessels or vascular beds that are organized in a less stereotyped manner. Recent studies have shed light on how the vascular tree is formed and how the interconnection of all branches is elaborated and maintained. In contrast to many other branched organs such as the lung or the kidney, vessel connection (also called anastomosis) is a key process underlying the formation of vascular networks; each outgrowing angiogenic sprout must anastomose in order to allow blood flow in the newly formed vessel segment. It turns out that during this "sprouting and anastomosis" process, too many vessels are generated, and that blood flow is subsequently optimized through the removal (pruning) of low flow segments. Here, we reflect on the cellular and molecular mechanisms involved in forming the complex architecture of the vasculature through sprouting, anastomosis and pruning, and raise some questions that remain to be addressed in future studies.

Three-Dimensional Architecture of Glomerular Endothelial Cells Revealed by FIB-SEM Tomography

Focused-ion beam-scanning electron microscopic (FIB-SEM) tomography enables easier acquisition of a series of ultrastructural, sectional images directly from resin-embedded biological samples. In this study, to clarify the three-dimensional (3D) architecture of glomerular endothelial cells (GEnCs) in adult rats, we manually extracted GEnCs from serial FIB-SEM images and reconstructed them on an Amira reconstruction software. The luminal and basal surface structures were clearly visualized in the reconstructed GEnCs, although only the luminal surface structures could be observed by conventional SEM. The luminal surface visualized via the reconstructed GEnCs was quite similar to that observed through conventional SEM, indicating that 3D reconstruction could be performed with high accuracy. Thus, we successfully described the 3D architecture of normal GEnCs in adult rats more clearly and precisely than ever before. The GEnCs were found to consist of three major subcellular compartments, namely, the cell body, cytoplasmic ridges, and sieve plates, in addition to two associated subcellular compartments, namely, the globular protrusions and reticular porous structures. Furthermore, most individual GEnCs made up a “seamless” tubular shape, and some of them formed an autocellular junction to make up a tubular shape. FIB-SEM tomography with reconstruction is a powerful approach to better understand the 3D architecture of GEnCs. Moreover, the morphological information revealed in this study will be valuable for the 3D pathologic evaluation of GEnCs in animal and human glomerular diseases and the structural analysis of developmental processes in the glomerular capillary system.

Corneal angiogenic privilege and its failure

The cornea actively maintains its own avascular status to preserve its ultimate optical function. This corneal avascular state is also defined as "corneal angiogenic privilege", which results from a critical and sensitive balance between anti-angiogenic and pro-angiogenic mechanisms. In our review, we aim to explore the complex equilibrium among multiple mediators which prevents neovascularization in the resting cornea, as well as to unveil the evolutive process which leads to corneal angiogenesis in response to different injuries.

Differential Thresholds of Proteasome Activation Reveal Two Separable Mechanisms of Sensory Organ Polarization in C. elegans

Cephalization is a major innovation of animal evolution and implies a synchronization of nervous system, mouth, and foregut polarization to align alimentary tract and sensomotoric system for effective foraging. However, the underlying integration of morphogenetic programs is poorly understood. Here, we show that invagination of neuroectoderm through de novo polarization and apical constriction creates the mouth opening in the Caenorhabditis elegans embryo. Simultaneously, all 18 juxta-oral sensory organ dendritic tips become symmetrically positioned around the mouth: While the two bilaterally symmetric amphid sensilla endings are towed to the mouth opening, labial and cephalic sensilla become positioned independently. Dendrite towing is enabled by the pre-polarized sensory amphid pores intercalating into the leading edge of the anteriorly migrating epidermal sheet, while apical constriction-mediated cell–cell re-arrangements mediate positioning of all other sensory organs. These two processes can be separated by gradual inactivation of the 26S proteasome activator, RPN-6.1. Moreover, RPN-6.1 also shows a dose-dependent requirement for maintenance of coordinated apical polarization of other organs with apical lumen, the pharynx, and the intestine. Thus, our data unveil integration of morphogenetic programs during the coordination of alimentary tract and sensory organ formation and suggest that this process requires tight control of ubiquitin-dependent protein degradation.

Blocking endothelial apoptosis revascularizes the retina in a model of ischemic retinopathy

Aberrant, neovascular retinal blood vessel growth is a vision-threatening complication in ischemic retinal diseases. It is driven by retinal hypoxia frequently caused by capillary nonperfusion and endothelial cell (EC) loss. We investigated the role of EC apoptosis in this process using a mouse model of ischemic retinopathy, in which vessel closure and EC apoptosis cause capillary regression and retinal ischemia followed by neovascularization. Protecting ECs from apoptosis in this model did not prevent capillary closure or retinal ischemia. Nonetheless, it prevented the clearance of ECs from closed capillaries, delaying vessel regression and allowing ECs to persist in clusters throughout the ischemic zone. In response to hypoxia, these preserved ECs underwent a vessel sprouting response and rapidly reassembled into a functional vascular network. This alleviated retinal hypoxia, preventing subsequent pathogenic neovascularization. Vessel reassembly was not limited by VEGFA neutralization, suggesting it was not dependent on the excess VEGFA produced by the ischemic retina. Neutralization of ANG2 did not prevent vessel reassembly, but did impair subsequent angiogenic expansion of the reassembled vessels. Blockade of EC apoptosis may promote ischemic tissue revascularization by preserving ECs within ischemic tissue that retain the capacity to reassemble a functional network and rapidly restore blood supply.

Marcksl1 modulates endothelial cell mechanoresponse to haemodynamic forces to control blood vessel shape and size

The formation of vascular tubes is driven by extensive changes in endothelial cell (EC) shape. Here, we have identified a role of the actin-binding protein, Marcksl1, in modulating the mechanical properties of EC cortex to regulate cell shape and vessel structure during angiogenesis. Increasing and depleting Marcksl1 expression level in vivo results in an increase and decrease, respectively, in EC size and the diameter of microvessels. Furthermore, endothelial overexpression of Marcksl1 induces ectopic blebbing on both apical and basal membranes, during and after lumen formation, that is suppressed by reduced blood flow. High resolution imaging reveals that Marcksl1 promotes the formation of linear actin bundles and decreases actin density at the EC cortex. Our findings demonstrate that a balanced network of linear and branched actin at the EC cortex is essential in conferring cortical integrity to resist the deforming forces of blood flow to regulate vessel structure.

During lumen formation in blood vessels, endothelial cells become exposed to hemodynamic forces that induce membrane blebbing and changes in cell shape. Here, the authors show endothelial cells develop an actin-based protective mechanism in the cell cortex that prevents excessive blebbing to control cell shape and vessel diameter.

ELAVL1 Role in Cell Fusion and Tunneling Membrane Nanotube Formations with Implication to Treat Glioma Heterogeneity

Homotypic and heterotypic cell fusions via permanent membrane fusions and temporal tunneling nanotube formations in the glioma microenvironment were recently documented in vitro and in vivo and mediate glioma survival, plasticity, and recurrence. Chronic inflammation, a hypoxic environment, aberrant mitochondrial function, and ER stress due to unfolded protein accumulation upregulate cell fusion events, which leads to tumor heterogeneity and represents an adaptive mechanism to promote tumor cell survival and plasticity in cytotoxic, nutrient-deprived, mechanically stressed, and inflammatory microenvironments. Cell fusion is a multistep process, which consists of the activation of the cellular stress response, autophagy formation, rearrangement of cytoskeletal architecture in the areas of cell-to-cell contacts, and the expression of proinflammatory cytokines and fusogenic proteins. The mRNA-binding protein of ELAV-family HuR is a critical node, which orchestrates the stress response, autophagy formation, cytoskeletal architecture, and the expression of proinflammatory cytokines and fusogenic proteins. HuR is overexpressed in gliomas and is associated with poor prognosis and treatment resistance. Our review provides a link between the HuR role in the regulation of cell fusion and tunneling nanotube formations in the glioma microenvironment and the potential suppression of these processes by different classes of HuR inhibitors.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

A Series of Tubes: The C. elegans Excretory Canal Cell as a Model for Tubule Development

Formation and regulation of properly sized epithelial tubes is essential for multicellular life. The excretory canal cell of C. elegans provides a powerful model for investigating the integration of the cytoskeleton, intracellular transport, and organismal physiology to regulate the developmental processes of tube extension, lumen formation, and lumen diameter regulation in a narrow single cell. Multiple studies have provided new understanding of actin and intermediate filament cytoskeletal elements, vesicle transport, and the role of vacuolar ATPase in determining tube size. Most of the genes discovered have clear homologues in humans, with implications for understanding these processes in mammalian tissues such as Schwann cells, renal tubules, and brain vasculature. The results of several new genetic screens are described that provide a host of new targets for future studies in this informative structure.

Fluid Shear Stress Sensing by the Endothelial Layer

Blood flow produces mechanical frictional forces, parallel to the blood flow exerted on the endothelial wall of the vessel, the so-called wall shear stress (WSS). WSS sensing is associated with several vascular pathologies, but it is first a physiological phenomenon. Endothelial cell sensitivity to WSS is involved in several developmental and physiological vascular processes such as angiogenesis and vascular morphogenesis, vascular remodeling, and vascular tone. Local conditions of blood flow determine the characteristics of WSS, i.e., intensity, direction, pulsatility, sensed by the endothelial cells that, through their effect of the vascular network, impact WSS. All these processes generate a local-global retroactive loop that determines the ability of the vascular system to ensure the perfusion of the tissues. In order to account for the physiological role of WSS, the so-called shear stress set point theory has been proposed, according to which WSS sensing acts locally on vessel remodeling so that WSS is maintained close to a set point value, with local and distant effects of vascular blood flow. The aim of this article is (1) to review the existing literature on WSS sensing involvement on the behavior of endothelial cells and its short-term (vasoreactivity) and long-term (vascular morphogenesis and remodeling) effects on vascular functioning in physiological condition; (2) to present the various hypotheses about WSS sensors and analyze the conceptual background of these representations, in particular the concept of tensional prestress or biotensegrity; and (3) to analyze the relevance, explanatory value, and limitations of the WSS set point theory, that should be viewed as dynamical, and not algorithmic, processes, acting in a self-organized way. We conclude that this dynamic set point theory and the biotensegrity concept provide a relevant explanatory framework to analyze the physiological mechanisms of WSS sensing and their possible shift toward pathological situations.

Endothelial cells on the move: dynamics in vascular morphogenesis and disease

The vascular system is a hierarchically organized network of blood vessels that play crucial roles in embryogenesis, homeostasis and disease. Blood vessels are built by endothelial cells – the cells lining the interior of blood vessels – through a process named vascular morphogenesis. Endothelial cells react to different biomechanical signals in their environment by adjusting their behavior to: (1) invade, proliferate and fuse to form new vessels (angiogenesis); (2) remodel, regress and establish a hierarchy in the network (patterning); and (3) maintain network stability (quiescence). Each step involves the coordination of endothelial cell differentiation, proliferation, polarity, migration, rearrangements and shape changes to ensure network integrity and an efficient barrier between blood and tissues. In this review, we highlighted the relevance and the mechanisms involving endothelial cell migration during different steps of vascular morphogenesis. We further present evidence on how impaired endothelial cell dynamics can contribute to pathology.

Extracranial arteriovenous malformations: from bedside to bench

Arteriovenous malformation (AVM) is defined as a fast-flow vascular anomaly that shunts blood from arteries directly to veins. This short circuit of blood flow contributes to progressive expansion of draining veins, resulting in ischaemia, tissue deformation and in some severe cases, congestive heart failure. Various medical interventions have been employed to treat AVM, however, management of which remains a huge challenge because of its high recurrence rate and lethal complications. Thus, understanding the underlying mechanisms of AVM development and progression will help direct discovery and a potential cure. Here, we summarize current findings in the field of extracranial AVMs with the aim to provide insight into their aetiology and molecular influences, in the hope to pave the way for future treatment.

Blood Flow Forces in Shaping the Vascular System: A Focus on Endothelial Cell Behavior

The endothelium is the cell monolayer that lines the interior of the blood vessels separating the vessel lumen where blood circulates, from the surrounding tissues. During embryonic development, endothelial cells (ECs) must ensure that a tight barrier function is maintained whilst dynamically adapting to the growing vascular tree that is being formed and remodeled. Blood circulation generates mechanical forces, such as shear stress and circumferential stretch that are directly acting on the endothelium. ECs actively respond to flow-derived mechanical cues by becoming polarized, migrating and changing neighbors, undergoing shape changes, proliferating or even leaving the tissue and changing identity. It is now accepted that coordinated changes at the single cell level drive fundamental processes governing vascular network morphogenesis such as angiogenic sprouting, network pruning, lumen formation, regulation of vessel caliber and stability or cell fate transitions. Here we summarize the cell biology and mechanics of ECs in response to flow-derived forces, discuss the latest advances made at the single cell level with particular emphasis on in vivo studies and highlight potential implications for vascular pathologies.

Endothelial responses to shear stress in atherosclerosis: a novel role for developmental genes

Flowing blood generates a frictional force called shear stress that has major effects on vascular function. Branches and bends of arteries are exposed to complex blood flow patterns that exert low or low oscillatory shear stress, a mechanical environment that promotes vascular dysfunction and atherosclerosis. Conversely, physiologically high shear stress is protective. Endothelial cells are critical sensors of shear stress but the mechanisms by which they decode complex shear stress environments to regulate physiological and pathophysiological responses remain incompletely understood. Several laboratories have advanced this field by integrating specialized shear-stress models with systems biology approaches, including transcriptome, methylome and proteome profiling and functional screening platforms, for unbiased identification of novel mechanosensitive signalling pathways in arteries. In this Review, we describe these studies, which reveal that shear stress regulates diverse processes and demonstrate that multiple pathways classically known to be involved in embryonic development, such as BMP-TGFβ, WNT, Notch, HIF1α, TWIST1 and HOX family genes, are regulated by shear stress in arteries in adults. We propose that mechanical activation of these pathways evolved to orchestrate vascular development but also drives atherosclerosis in low shear stress regions of adult arteries.

Endothelial responses to shear stress in atherosclerosis: a novel role for developmental genes

Flowing blood generates a frictional force called shear stress that has major effects on vascular function. Branches and bends of arteries are exposed to complex blood flow patterns that exert low or low oscillatory shear stress, a mechanical environment that promotes vascular dysfunction and atherosclerosis. Conversely, physiologically high shear stress is protective. Endothelial cells are critical sensors of shear stress but the mechanisms by which they decode complex shear stress environments to regulate physiological and pathophysiological responses remain incompletely understood. Several laboratories have advanced this field by integrating specialized shear-stress models with systems biology approaches, including transcriptome, methylome and proteome profiling and functional screening platforms, for unbiased identification of novel mechanosensitive signalling pathways in arteries. In this Review, we describe these studies, which reveal that shear stress regulates diverse processes and demonstrate that multiple pathways classically known to be involved in embryonic development, such as BMP-TGFβ, WNT, Notch, HIF1α, TWIST1 and HOX family genes, are regulated by shear stress in arteries in adults. We propose that mechanical activation of these pathways evolved to orchestrate vascular development but also drives atherosclerosis in low shear stress regions of adult arteries.

Tumor angiogenesis: causes, consequences, challenges and opportunities

Tumor vascularization occurs through several distinct biological processes, which not only vary between tumor type and anatomic location, but also occur simultaneously within the same cancer tissue. These processes are orchestrated by a range of secreted factors and signaling pathways and can involve participation of non-endothelial cells, such as progenitors or cancer stem cells. Anti-angiogenic therapies using either antibodies or tyrosine kinase inhibitors have been approved to treat several types of cancer. However, the benefit of treatment has so far been modest, some patients not responding at all and others acquiring resistance. It is becoming increasingly clear that blocking tumors from accessing the circulation is not an easy task to accomplish. Tumor vessel functionality and gene expression often differ vastly when comparing different cancer subtypes, and vessel phenotype can be markedly heterogeneous within a single tumor. Here, we summarize the current understanding of cellular and molecular mechanisms involved in tumor angiogenesis and discuss challenges and opportunities associated with vascular targeting.

Artery-vein specification in the zebrafish trunk is pre-patterned by heterogeneous Notch activity and balanced by flow-mediated fine-tuning

How developing vascular networks acquire the right balance of arteries, veins and lymphatic vessels to efficiently supply and drain tissues is poorly understood. In zebrafish embryos, the robust and regular 50:50 global balance of intersegmental veins and arteries that form along the trunk prompts the intriguing question of how does the organism keep ‘count’? Previous studies have suggested that the ultimate fate of an intersegmental vessel (ISV) is determined by the identity of the approaching secondary sprout emerging from the posterior cardinal vein. Here, we show that the formation of a balanced trunk vasculature involves an early heterogeneity in endothelial cell behaviour and Notch signalling activity in the seemingly identical primary ISVs that is independent of secondary sprouting and flow. We show that Notch signalling mediates the local patterning of ISVs, and an adaptive flow-mediated mechanism subsequently fine-tunes the global balance of arteries and veins along the trunk. We propose that this dual mechanism provides the adaptability required to establish a balanced network of arteries, veins and lymphatic vessels.

Highlighted Article: A stepwise dual mechanism involving Notch signalling and flow provides the adaptability required to establish a balanced network of arteries and veins in the zebrafish trunk.

Endothelial Cells Filopodia in the Anastomosis of Central Nervous System Capillaries

In this article we explore filopodia of endothelial cells (ECs) in the developing central nervous system (CNS) using the Golgi method and transmission electron microscopy. Filopodia of ECs play a crucial role in the anastomosis of growing capillaries of the CNS. The leading ECs filopodia from approaching capillaries interconnect forming complex conglomerates that precede the anastomotic event. The contacting filopodia form narrow spaces between them filled with proteinaceous basal lamina material. The original narrow spaces coalesce into larger ones leading to the formation of a single one that will interconnect (anastomose) the two approaching capillaries. The four leading ECs (two for each approaching capillary) become the wall of the newly formed post-anastomotic CNS capillaries. These new CNS capillaries are very small with narrow and irregular lumina that might permit the passage of fluid but not yet of blood cells. Eventually, their lumen enlarges and permits the passage of blood cells.

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.

How cells fuse

Brukman et al. review cell–cell fusion mechanisms, focusing on the identity of the fusogens that mediate these processes and the regulation of their activities.

Cell–cell fusion remains the least understood type of membrane fusion process. However, the last few years have brought about major advances in understanding fusion between gametes, myoblasts, macrophages, trophoblasts, epithelial, cancer, and other cells in normal development and in diseases. While different cell fusion processes appear to proceed via similar membrane rearrangements, proteins that have been identified as necessary and sufficient for cell fusion (fusogens) use diverse mechanisms. Some fusions are controlled by a single fusogen; other fusions depend on several proteins that either work together throughout the fusion pathway or drive distinct stages. Furthermore, some fusions require fusogens to be present on both fusing membranes, and in other fusions, fusogens have to be on only one of the membranes. Remarkably, some of the proteins that fuse cells also sculpt single cells, repair neurons, promote scission of endocytic vesicles, and seal phagosomes. In this review, we discuss the properties and diversity of the known proteins mediating cell–cell fusion and highlight their different working mechanisms in various contexts.

Multi-sample SPIM image acquisition, processing and analysis of vascular growth in zebrafish

To quantitatively understand biological processes that occur over many hours or days, it is desirable to image multiple samples simultaneously, and automatically process and analyse the resulting datasets. Here, we present a complete multi-sample preparation, imaging, processing and analysis workflow to determine the development of the vascular volume in zebrafish. Up to five live embryos were mounted and imaged simultaneously over several days using selective plane illumination microscopy (SPIM). The resulting large imagery dataset of several terabytes was processed in an automated manner on a high-performance computer cluster and segmented using a novel segmentation approach that uses images of red blood cells as training data. This analysis yielded a precise quantification of growth characteristics of the whole vascular network, head vasculature and tail vasculature over development. Our multi-sample platform demonstrates effective upgrades to conventional single-sample imaging platforms and paves the way for diverse quantitative long-term imaging studies.

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.