Distinct and redundant functions of Esama and VE-cadherin during vascular morphogenesis

The sprouting angiogenesis and vascular dysfunction triggered by bisphenol S and tetrabromobisphenol S through disrupting vascular endothelial-cadherin in zebrafish

Exogenous chemical toxicants may be important inducers of pathological angiogenesis diseases. However, few studies have investigated the associations between pathological angiogenesis diseases and chemical toxicant exposures, and the specific mechanism by which chemical toxicants induce sprouting angiogenesis is unclear. In this study, zebrafish were exposed to bisphenol S (BPS, 1-100 μg/L) and tetrabromobisphenol S (TBBPS, 0.1 and 10 μg/L) from the embryonic stage to the larval stage to investigate how pollutants interfere with angiogenesis and the function of ectopic sprouting vessels. The results showed that BPS and TBBPS promoted ectopic sprouting angiogenesis in different types of vascular plexuses, including the posterior cardinal vein (PCV) and superficial choroidal vessels (SOVs), at different developmental time points. Proteomic analyses of eGFP-positive endothelial cells (ECs) isolated from Tg(flk1: eGFP) zebrafish revealed that both BPS and TBBPS induced ectopic angiogenesis by acting on vascular endothelial-cadherin (VE-cadherin) and activating downstream proangiogenic signaling. In ectopic sprouting vessels induced by BPS and TBBPS, increased endothelial permeability resulted in white blood cell recruitment. Human oxidized lipids also tended to deposit in these ectopic vessels following BPS and TBBPS exposure. These findings suggest that chemical toxicant-induced ectopic angiogenesis is an important cause of vascular dysfunction and related diseases.

The Triad of Blood-Brain Barrier Integrity: Endothelial Cells, Astrocytes, and Pericytes in Perinatal Stroke Pathophysiology

Pediatric stroke, a significant cause of long-term neurological deficits in children, often arises from disruptions within neurovascular unit (NVU) components. The NVU, a dynamic ensemble of astrocytes, endothelial cells, pericytes, and microglia, is vital for maintaining cerebral homeostasis and regulating vascular brain development. Its structural integrity, particularly at the blood-brain barrier (BBB), depends on intercellular junctions and the basement membrane, which together restrict paracellular transport and shield the brain from systemic insults. Dysfunction in this intricate system is increasingly linked to pediatric stroke and related cerebrovascular conditions. Mutations disrupting endothelial cell adhesion or pericyte-endothelial interactions can compromise BBB stability, leading to pathological outcomes such as intraventricular hemorrhage in the germinal matrix, a hallmark of vascular brain immaturity. Additionally, inflammation, ferroptosis, necroptosis, and autophagy are key cellular processes influencing brain damage and repair. Excessive activation of these mechanisms can exacerbate NVU injury, whereas targeted therapeutic modulation offers potential pathways to mitigate damage and support recovery. This review explores the cellular and molecular mechanisms underlying NVU dysfunction, BBB disruption, and subsequent brain injury in pediatric stroke. Understanding the interplay between genetic mutations, environmental stressors, and NVU dynamics provides new insights into stroke pathogenesis. The susceptibility of the germinal matrix to vascular rupture further emphasizes the critical role of NVU integrity in early brain development. Targeting inflammatory pathways and cell death mechanisms presents promising strategies to preserve NVU function and improve outcomes for affected neonates.

Combined forces of hydrostatic pressure and actin polymerization drive endothelial tip cell migration and sprouting angiogenesis

Cell migration is a key process in the shaping and formation of tissues. During sprouting angiogenesis, endothelial tip cells invade avascular tissues by generating actomyosin-dependent forces that drive cell migration and vascular expansion. Surprisingly, endothelial cells (ECs) can still invade if actin polymerization is inhibited. In this study, we show that endothelial tip cells employ an alternative mechanism of cell migration that is dependent on Aquaporin (Aqp)-mediated water inflow and increase in hydrostatic pressure. In the zebrafish, ECs express aqp1a.1 and aqp8a.1 in newly formed vascular sprouts in a VEGFR2-dependent manner. Aqp1a.1 and Aqp8a.1 loss-of-function studies show an impairment in intersegmental vessels formation because of a decreased capacity of tip cells to increase their cytoplasmic volume and generate membrane protrusions, leading to delayed tip cell emergence from the dorsal aorta and slower migration. Further inhibition of actin polymerization resulted in a greater decrease in sprouting angiogenesis, indicating that ECs employ two mechanisms for robust cell migration in vivo. Our study thus highlights an important role of hydrostatic pressure in tissue morphogenesis.

Endothelial cell mechanics and dynamics in angiogenesis

The efficient distribution of oxygen and metabolites is critical for embryonic development and growth as well as tissue homeostasis. This is achieved by endothelial cells forming and maintaining a closed, circulatory network of tubular blood vessels. Endothelial cells are highly plastic cells with the capability to generate diverse dynamic responses at different stages of vessel development in order to build vessel networks of tissue-specific patterns and morphologies. In this review, we discuss new conceptual advances gained from in vitro and in vivo models of angiogenesis on the control of endothelial cell dynamics. We highlight the complex interplay between mechanical cues, actin cytoskeleton and endothelial behaviors, and the emerging importance of hydrostatic pressure in complementing actin-dependent mechanisms to regulate endothelial cell mechanics and angiogenesis. Understanding these processes provides insights into vascular repair and regeneration mechanisms.

Initiation of lumen formation from junctions via differential actomyosin contractility regulated by dynamic recruitment of Rasip1

De novo lumen formation necessitates the precise segregation of junctional proteins from apical surfaces, yet the underlying mechanisms remain unclear. Using a zebrafish model, we develop a series of molecular reporters, photo-convertible and optogenetic tools to study the establishment of apical domains. Our study identifies Rasip1 as one of the earliest apical proteins recruited, which suppresses actomyosin contractility at junctional patches by inhibiting NMII, thereby allowing for the sustained outward flow of junctional complexes. Following the establishment of apical compartments, Rasip1 shuttles between junctions and the apical compartments in response to local high tension. Rasip1 confines Cdh5 to junctions by suppressing apical contractility. Conversely, the recruitment of Rasip1 to junctions is regulated by Heg1 and Krit1 to modulate contractility along junctions. Overall, de novo lumen formation and maintenance depend on the precise control of contractility within apical compartments and junctions, orchestrated by the dynamic recruitment of Rasip1.

Neurons enhance blood-brain barrier function via upregulating claudin-5 and VE-cadherin expression due to glial cell line-derived neurotrophic factor secretion

Blood-brain barrier (BBB) prevents neurotoxins from entering central nervous system. We aimed to establish and characterize an in vitro triple co-culture BBB model consisting of brain endothelial cells hCMEC/D3, astrocytoma U251 cells, and neuroblastoma SH-SY5Y cells. Co-culture of SH-SY5Y and U251 cells markedly enhanced claudin-5 and VE-cadherin expression in hCMEC/D3 cells, accompanied by increased transendothelial electrical resistance and decreased permeability. Conditioned medium (CM) from SH-SY5Y cells (S-CM), U251 cells (U-CM), and co-culture of SH-SY5Y and U251 cells (US-CM) also promoted claudin-5 and VE-cadherin expression. Glial cell line-derived neurotrophic factor (GDNF) levels in S-CM and US-CM were significantly higher than CMs from hCMEC/D3 and U-CM. Both GDNF and US-CM upregulated claudin-5 and VE-cadherin expression, which were attenuated by anti-GDNF antibody and GDNF signaling inhibitors. GDNF increased claudin-5 expression via the PI3K/AKT/FOXO1 and MAPK/ERK pathways. Meanwhile, GDNF promoted VE-cadherin expression by activating PI3K/AKT/ETS1 and MAPK/ERK/ETS1 signaling. The roles of GDNF in BBB integrity were validated using brain-specific Gdnf silencing mice. The developed triple co-culture BBB model was successfully applied to predict BBB permeability. In conclusion, neurons enhance BBB integrity by upregulating claudin-5 and VE-cadherin expression through GDNF secretion and established triple co-culture BBB model may be used to predict drugs' BBB permeability.

Nuclear SUN1 stabilizes endothelial cell junctions via microtubules to regulate blood vessel formation

Endothelial cells line all blood vessels, where they coordinate blood vessel formation and the blood-tissue barrier via regulation of cell-cell junctions. The nucleus also regulates endothelial cell behaviors, but it is unclear how the nucleus contributes to endothelial cell activities at the cell periphery. Here, we show that the nuclear-localized linker of the nucleoskeleton and cytoskeleton (LINC) complex protein SUN1 regulates vascular sprouting and endothelial cell-cell junction morphology and function. Loss of murine endothelial Sun1 impaired blood vessel formation and destabilized junctions, angiogenic sprouts formed but retracted in SUN1-depleted sprouts, and zebrafish vessels lacking Sun1b had aberrant junctions and defective cell-cell connections. At the cellular level, SUN1 stabilized endothelial cell-cell junctions, promoted junction function, and regulated contractility. Mechanistically, SUN1 depletion altered cell behaviors via the cytoskeleton without changing transcriptional profiles. Reduced peripheral microtubule density, fewer junction contacts, and increased catastrophes accompanied SUN1 loss, and microtubule depolymerization phenocopied effects on junctions. Depletion of GEF-H1, a microtubule-regulated Rho activator, or the LINC complex protein nesprin-1 rescued defective junctions of SUN1-depleted endothelial cells. Thus, endothelial SUN1 regulates peripheral cell-cell junctions from the nucleus via LINC complex-based microtubule interactions that affect peripheral microtubule dynamics and Rho-regulated contractility, and this long-range regulation is important for proper blood vessel sprouting and junction integrity.

Involvement of transmembrane protein 184a during angiogenesis in zebrafish embryos

Angiogenesis, the outgrowth of new blood vessels from existing vasculature, is critical during development, tissue formation, and wound healing. In response to vascular endothelial growth factors (VEGFs), endothelial cells are activated to proliferate and move towards the signal, extending the vessel. These events are directed by VEGF-VEGF receptor (Vegfr2) signal transduction, which in turn is modulated by heparan sulfate proteoglycans (HSPGs). HSPGs are glycoproteins covalently attached to HS glycosaminoglycan chains. Transmembrane protein 184a (Tmem184a) has been recently identified as a heparin receptor, which is believed to bind heparan sulfate chains in vivo. Therefore, Tmem184a has the potential to fine-tune interactions between VEGF and HS, modulating Vegfr2-dependent angiogenesis. The function of Tmem184a has been investigated in the regenerating zebrafish caudal fin, but its role has yet to be evaluated during developmental angiogenesis. Here we provide insights into how Tmem184a contributes to the proper formation of the vasculature in zebrafish embryos. First, we find that knockdown of Tmem184a causes a reduction in the number of intact intersegmental vessels (ISVs) in the zebrafish embryo. This phenotype mimics that of vegfr2b knockout mutants, which have previously been shown to exhibit severe defects in ISV development. We then test the importance of HS interactions by removing the binding domain within the Tmem184a protein, which has a negative effect on angiogenesis. Tmem184a is found to act synergistically with Vegfr2b, indicating that the two gene products function in a common pathway to modulate angiogenesis. Moreover, we find that knockdown of Tmem184a leads to an increase in endothelial cell proliferation but a decrease in the amount of VE-cadherin present. Together, these findings suggest that Tmem184a is necessary for ISVs to organize into mature, complete vessels.

Mechanical Aspects of Angiogenesis

Simple Summary

The formation of new blood vessels from already existing ones is a process of high clinical relevance, since it is of great importance for both physiological and pathological processes. In regard to tumors, the process is crucial, since it ensures the supply with nutrients and the growth of the tumor. The influence of mechanical factors on this biological process is an emerging field. Until now, the shear force of the blood flow has been considered the main mechanical parameter during angiogenesis. This review article provides an overview of further mechanical cues, with particular focus on the surrounding extracellular matrix impacting the cell behavior and, thus, regulating angiogenesis. This underlines the enormous importance of the mechanical properties of the extracellular matrix on cell biological processes and shows how changing the mechanics of the extracellular matrix could be used as a possible therapeutic approach in cancer therapy.

Abstract

Angiogenesis is of high clinical relevance as it plays a crucial role in physiological (e.g., tissue regeneration) and pathological processes (e.g., tumor growth). Besides chemical signals, such as VEGF, the relationship between cells and the extracellular matrix (ECM) can influence endothelial cell behavior during angiogenesis. Previously, in terms of the connection between angiogenesis and mechanical factors, researchers have focused on shear forces due to blood flow. However, it is becoming increasingly important to include the direct influence of the ECM on biological processes, such as angiogenesis. In this context, we focus on the stiffness of the surrounding ECM and the adhesion of cells to the ECM. Furthermore, we highlight the mechanical cues during the main stages of angiogenesis: cell migration, tip and stalk cells, and vessel stabilization. It becomes clear that the different stages of angiogenesis require various chemical and mechanical cues to be modulated by/modulate the stiffness of the ECM. Thus, changes of the ECM during tumor growth represent additional potential dysregulations of angiogenesis in addition to erroneous biochemical signals. This awareness could be the basis of therapeutic approaches to counteract specific processes in tumor angiogenesis.

Control of dynamic cell behaviors during angiogenesis and anastomosis by Rasip1

Organ morphogenesis is driven by a wealth of tightly orchestrated cellular behaviors, which ensure proper organ assembly and function. Many of these cell activities involve cell-cell interactions and remodeling of the F-actin cytoskeleton. Here, we analyze the requirement for Rasip1 (Ras-interacting protein 1), an endothelial-specific regulator of junctional dynamics, during blood vessel formation. Phenotype analysis of rasip1 mutants in zebrafish embryos reveals distinct functions of Rasip1 during sprouting angiogenesis, anastomosis and lumen formation. During angiogenic sprouting, loss of Rasip1 causes cell pairing defects due to a destabilization of tricellular junctions, indicating that stable tricellular junctions are essential to maintain multicellular organization within the sprout. During anastomosis, Rasip1 is required to establish a stable apical membrane compartment; rasip1 mutants display ectopic, reticulated junctions and the apical compartment is frequently collapsed. Loss of Ccm1 and Heg1 function mimics the junctional defects of rasip1 mutants. Furthermore, downregulation of ccm1 and heg1 leads to a delocalization of Rasip1 at cell junctions, indicating that junctional tethering of Rasip1 is required for its function in junction formation and stabilization during sprouting angiogenesis.

Summary:In vivo analysis of rasip1 mutants reveals multiple roles for Rasip1 during angiogenic sprouting, anastomosis and lumen formation, including stabilization of tricellular junctions to permit coordinated cell rearrangements and multicellular tube formation.

Prmt5 promotes vascular morphogenesis independently of its methyltransferase activity

During development, the vertebrate vasculature undergoes major growth and remodeling. While the transcriptional cascade underlying blood vessel formation starts to be better characterized, little is known concerning the role and mode of action of epigenetic enzymes during this process. Here, we explored the role of the Protein Arginine Methyl Transferase Prmt5 in blood vessel formation as well as hematopoiesis using zebrafish as a model system. Through the combination of different prmt5 loss-of-function approaches we highlighted a key role of Prmt5 in both processes. Notably, we showed that Prmt5 promotes vascular morphogenesis through the transcriptional control of ETS transcription factors and adhesion proteins in endothelial cells. Interestingly, using a catalytic dead mutant of Prmt5 and a specific drug inhibitor, we found that while Prmt5 methyltransferase activity was required for blood cell formation, it was dispensable for vessel formation. Analyses of chromatin architecture impact on reporter genes expression and chromatin immunoprecipitation experiments led us to propose that Prmt5 regulates transcription by acting as a scaffold protein that facilitates chromatin looping to promote vascular morphogenesis.

Blood vessel formation is an essential developmental process required for the survival of all vertebrates. The vascular anatomy and the mechanisms involved in vessel formation are highly conserved among vertebrates. Hence, we used zebrafish as a model, to decipher the role and the mode of action of Prmt5, an enzyme known to regulate gene expression, in vascular morphogenesis and in blood cell formation in vivo. Using different approaches, we highlighted a key role of Prmt5 during both processes. However, we found that while blood cell formation required Prmt5 enzymatic activity, vascular morphogenesis was independent on its activity. Prmt5 has been proposed as a therapeutic target in many diseases, including cancer. Yet, we show here that Prmt5 acts at least in part independently of its methyltransferase activity to regulate vascular morphogenesis. By shedding light on a mechanism of action of Prmt5 that will be insensitive to enzymatic inhibitors, our data calls forth the design of alternative drugs. In addition, this non-canonical function of Prmt5 may have a more pervasive role than previously thought in physiological conditions, i.e. during development, but also in pathological situations such as in tumor angiogenesis and certainly deserves more attention in the future.

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.

Endothelial Cell Dynamics in Vascular Development: Insights From Live-Imaging in Zebrafish

The formation of the vertebrate vasculature involves the acquisition of endothelial cell identities, sprouting, migration, remodeling and maturation of functional vessel networks. To understand the cellular and molecular processes that drive vascular development, live-imaging of dynamic cellular events in the zebrafish embryo have proven highly informative. This review focusses on recent advances, new tools and new insights from imaging studies in vascular cell biology using zebrafish as a model system.

Mechanisms Ensuring Endothelial Junction Integrity Beyond VE-Cadherin

Endothelial junctions provide blood and lymph vessel integrity and are essential for the formation of a vascular system. They control the extravasation of solutes, leukocytes and metastatic cells from blood vessels and the uptake of fluid and leukocytes into the lymphatic vascular system. A multitude of adhesion molecules mediate and control the integrity and permeability of endothelial junctions. VE-cadherin is arguably the most important adhesion molecule for the formation of vascular structures, and the stability of their junctions. Interestingly, despite this prominence, its elimination from junctions in the adult organism has different consequences in the vasculature of different organs, both for blood and lymph vessels. In addition, even in tissues where the lack of VE-cadherin leads to strong plasma leaks from venules, the physical integrity of endothelial junctions is preserved. Obviously, other adhesion molecules can compensate for a loss of VE-cadherin and this review will discuss which other adhesive mechanisms contribute to the stability and regulation of endothelial junctions and cooperate with VE-cadherin in intact vessels. In addition to adhesion molecules, endothelial receptors will be discussed, which stimulate signaling processes that provide junction stability by modulating the actomyosin system, which reinforces tension of circumferential actin and dampens pulling forces of radial stress fibers. Finally, we will highlight most recent reports about the formation and control of the specialized button-like junctions of initial lymphatics, which represent the entry sites for fluid and cells into the lymphatic vascular system.

Imaging-based vascular-related biomarkers for early detection of acetaminophen-induced liver injury

Acetaminophen (APAP) is the foremost cause of drug-induced liver injury in the Western world. Most studies of APAP hepatotoxicity have focused on the hepatocellular injury, but current hepatocyte-related biomarkers have delayed presentation time and a lack of sensitivity. APAP overdose can induce hepatic microvascular congestion, which importantly precedes the injury of hepatocytes. However, the underlying molecular mechanisms remain unclear. It is imperative to discover and validate sensitive and specific translational biomarkers of APAP-induced liver injury. Methods: In this study, we assessed APAP toxicity in sinusoidal endothelial cells and hepatocytes in mice treated with overdose APAP at different time points. The underlying mechanisms of APAP overdose induced sinusoidal endothelial cell injury were investigated by RT2 Profiler PCR arrays. The impact of APAP overdose on endothelial cell function was assessed by pseudovessel formation of endothelial cells in 2D Matrigel and in vivo hepatic vascular integrity using multiphoton microscopy. Finally, the effects of APAP overdose on oxygen levels in the liver and hepatic microcirculation were evaluated by contrast enhanced ultrasonography. Potential imaging-based vascular-related markers for early detection of APAP induced liver injury were assessed. Results: Our study confirmed that hepatic endothelial cells are an early and direct target for APAP hepatotoxicity. ICAM1-related cellular adhesion pathways played a prominent role in APAP-induced endothelial cell injury, which was further validated in primary human sinusoidal endothelial cells and human livers after APAP overdose. APAP overdose impacted pseudovessel formation of endothelial cells and in vivo hepatic vascular integrity. Use of ultrasound to detect APAP-induced liver injury demonstrated that mean transit time, an imaging-based vascular-related biomarker, was more sensitive and precise for early detection of APAP hepatotoxicity and monitoring the treatment response in comparison with a conventional blood-based biomarker. Conclusion: Imaging-based vascular-related biomarkers can identify early and mild liver injury induced by APAP overdose. With further development, such biomarkers may improve the assessment of liver injury and the efficacy of clinical decision-making, which can be extended to other microvascular dysfunction of deep organs.

VE-Cadherin and Anastomosis of Blood Vessels Formed by Dental Stem Cells

It is known that dental pulp stem cells (DPSCs) can be induced to differentiate into vasculogenic endothelial (VE) cells. However, the process that results in sprouting and anastomosis of DPSC-derived vessels remains unclear. Here, we performed studies to understand the mechanisms underpinning the anastomosis of the host vasculature with blood vessels generated by DPSCs (a model for mesenchymal stem cells). VE-cadherin-silenced primary human DPSCs seeded in tooth slice/scaffolds and transplanted into the subcutaneous space of immunodeficient mice generated fewer functional blood vessels (i.e., anastomosed with the host vasculature) than control DPSCs transduced with scrambled sequences. Both VE-cadherin-silenced and mitogen-activated protein kinase kinase 1 (MEK1)-silenced cells showed a decrease in the number of capillary sprouts in vitro. Interestingly, DPSC stably transduced with a VE-cadherin reporter demonstrated that vascular endothelial growth factor (VEGF) induces VE-cadherin expression in sprouting DPSCs undergoing anastomosis, but not in quiescent DPSCs. To begin to understand the mechanisms regulating VE-cadherin, we stably silenced MEK1 and observed that VEGF was no longer able to induce VE-cadherin expression and capillary sprout formation. Notably ERG, a transcriptional factor downstream from MEK/ERK, binds to the promoter region of VE-cadherin (chip assay) and is induced by VEGF in DPSCs. Collectively, these data defined a signaling pathway triggered by VEGF that results in phosphorylation of MEK1/ERK and activation of ERG leading to expression of VE-cadherin, which is required for anastomosis of DPSC-derived blood vessels. In conclusion, these results unveiled a signaling pathway that enables the generation of functional blood vessels upon vasculogenic differentiation of DPSCs.

Interference With ESAM (Endothelial Cell-Selective Adhesion Molecule) Plus Vascular Endothelial-Cadherin Causes Immediate Lethality and Lung-Specific Blood Coagulation

Objective:

Vascular endothelial (VE)-cadherin is of dominant importance for the formation and stability of endothelial junctions, yet induced gene inactivation enhances vascular permeability in the lung but does not cause junction rupture. This study aims at identifying the junctional adhesion molecule, which is responsible for preventing endothelial junction rupture in the pulmonary vasculature in the absence of VE-cadherin.

Approach and Results:

We have compared the relevance of ESAM (endothelial cell-selective adhesion molecule), JAM (junctional adhesion molecule)-A, PECAM (platelet endothelial cell adhesion molecule)-1, and VE-cadherin for vascular barrier integrity in various mouse tissues. Gene inactivation of ESAM enhanced vascular permeability in the lung but not in the heart, skin, and brain. In contrast, deletion of JAM-A or PECAM-1 did not affect barrier integrity in any of these organs. Blocking VE-cadherin with antibodies caused lethality in ESAM −/− mice within 30 minutes but had no such effect in JAM-A −/− , PECAM-1 −/− or wild-type mice. Likewise, induced gene inactivation of VE-cadherin caused rapid lethality only in the absence of ESAM. Ultrastructural analysis revealed that only combined interference with VE-cadherin and ESAM disrupted endothelial junctions and caused massive blood coagulation in the lung. Mechanistically, we could exclude a role of platelet ESAM in coagulation, changes in the expression of other junctional proteins or a contribution of cytoplasmic signaling domains of ESAM.

Conclusions:

Despite well-documented roles of JAM-A and PECAM-1 for the regulation of endothelial junctions, only for ESAM, we detected an essential role for endothelial barrier integrity in a tissue-specific way. In addition, we found that it is ESAM which prevents endothelial junction rupture in the lung when VE-cadherin is absent.

Blood Flow Suppresses Vascular Anomalies in a Zebrafish Model of Cerebral Cavernous Malformations

Rationale:

Pathological biomechanical signaling induces vascular anomalies including cerebral cavernous malformations (CCM), which are caused by a clonal loss of CCM1/KRIT1 (Krev interaction trapped protein 1), CCM2/MGC4607, or CCM3/PDCD10. Why patients typically experience lesions only in lowly perfused venous capillaries of the cerebrovasculature is completely unknown.

Objective:

In contrast, animal models with a complete loss of CCM proteins lack a functional heart and blood flow and exhibit vascular anomalies within major blood vessels as well. This finding raises the possibility that hemodynamics may play a role in the context of this vascular pathology.

Methods and Results:

Here, we used a genetic approach to restore cardiac function and blood flow in a zebrafish model of CCM1. We find that blood flow prevents cardiovascular anomalies including a hyperplastic expansion within a large Ccm1-deficient vascular bed, the lateral dorsal aorta.

Conclusions:

This study identifies blood flow as an important physiological factor that is protective in the cause of this devastating vascular pathology.

Recent insights into vascular development from studies in zebrafish

PURPOSE OF REVIEW

Zebrafish has provided a powerful platform to study vascular biology over the past 25 years, owing to their distinct advantages for imaging and genetic manipulation. In this review, we summarize recent progress in vascular biology with particular emphasis on vascular development in zebrafish.

RECENT FINDINGS

The advent of transcription activator-like effector nuclease and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 genome-editing technologies has dramatically facilitated reverse genetic approaches in zebrafish, as in other models. Here, we highlight recent studies on vascular development in zebrafish which mainly employed forward or reverse genetics combined with high-resolution imaging. These studies have advanced our understanding of diverse areas in vascular biology, including transcriptional regulation of endothelial cell differentiation, endothelial cell signaling during angiogenesis and lymphangiogenesis, vascular bed-specific developmental mechanisms, and perivascular cell recruitment.

SUMMARY

The unique attributes of the zebrafish model have allowed critical cellular and molecular insights into fundamental mechanisms of vascular development. Knowledge acquired through recent zebrafish work further advances our understanding of basic mechanisms underlying vascular morphogenesis, maintenance, and homeostasis. Ultimately, insights provided by the zebrafish model will help to understand the genetic, cellular, and molecular underpinnings of human vascular malformations and diseases.

GDNF enhances human blood-nerve barrier function in vitro via MAPK signaling pathways

The human blood-nerve barrier (BNB) formed by endoneurial microvascular endothelial cells, serves to maintain the internal microenvironment in peripheral nerves required for normal axonal signal transduction to and from the central nervous system. The mechanisms of human BNB formation in health and disease are not fully elucidated. Prior work established a sufficient role for glial-derived neurotrophic factor (GDNF) in enhancing human BNB biophysical properties following serum withdrawal in vitro via RET-tyrosine kinase-dependent cytoskeletal remodeling. The objective of the study was to ascertain the downstream signaling pathway involved in this process and more comprehensively determine the molecular changes that may occur at human BNB intercellular junctions under the influence of GDNF. Proteomic studies suggested expression of several mitogen-activated protein kinases (MAPKs) in confluent GDNF-treated endoneurial endothelial cells following serum withdrawal. Using electric cell-substrate impedance sensing to continuously measure transendothelial electrical resistance and static transwell solute permeability assays with fluoresceinated small and large molecules to evaluate BNB biophysical function, we determined MAPK signaling was essential for GDNF-mediated BNB TEER increase following serum withdrawal downstream of RET-tyrosine kinase signaling that persisted for up to 48 hours in vitro. This increase was associated with reduced solute permeability to fluoresceinated sodium and high molecular weight dextran. Specific GDNF-mediated alterations were detected in cytoskeletal and intercellular junctional complex molecular transcripts and proteins relative to basal conditions without exogenous GDNF. This work provides novel insights into the molecular determinants and mechanisms responsible for specialized restrictive human BNB formation in health and disease.

Wnt/β-catenin signaling regulates VE-cadherin-mediated anastomosis of brain capillaries by counteracting S1pr1 signaling

Canonical Wnt signaling is crucial for vascularization of the central nervous system and blood-brain barrier (BBB) formation. BBB formation and modulation are not only important for development, but also relevant for vascular and neurodegenerative diseases. However, there is little understanding of how Wnt signaling contributes to brain angiogenesis and BBB formation. Here we show, using high resolution in vivo imaging and temporal and spatial manipulation of Wnt signaling, different requirements for Wnt signaling during brain angiogenesis and BBB formation. In the absence of Wnt signaling, premature Sphingosine-1-phosphate receptor (S1pr) signaling reduces VE-cadherin and Esama at cell-cell junctions. We suggest that Wnt signaling suppresses S1pr signaling during angiogenesis to enable the dynamic junction formation during anastomosis, whereas later S1pr signaling regulates BBB maturation and VE-cadherin stabilization. Our data provides a link between brain angiogenesis and BBB formation and identifies Wnt signaling as coordinator of the timing and as regulator of anastomosis.

Epithelial-mesenchymal transition in haematopoietic stem cell development and homeostasis

Epithelial-mesenchymal transition (EMT) is a morphogenetic process of cells that adopt an epithelial organization in their developmental ontogeny or homeostatic maintenance. Abnormalities in EMT regulation result in many malignant tumours in the human body. Tumours associated with the haematopoietic system, however, are traditionally not considered to involve EMT and haematopoietic stem cells (HSCs) are generally not associated with epithelial characteristics. In this review, we discuss the ontogeny and homeostasis of adult HSCs in the context of EMT intermediate states. We provide evidence that cell polarity regulation is critical for both HSC formation from embryonic dorsal aorta and HSC self-renewal and differentiation in adult bone marrow. HSC polarity is controlled by the same set of surface and transcriptional regulators as those described in canonical EMT processes. With an emphasis on partial EMT, we propose that the concept of EMT can be similarly applied in the study of HSC generation, maintenance and pathogenesis.

Junction-based lamellipodia drive endothelial cell rearrangements in vivo via a VE-cadherin-F-actin based oscillatory cell-cell interaction

Angiogenesis and vascular remodeling are driven by extensive endothelial cell movements. Here, we present in vivo evidence that endothelial cell movements are associated with oscillating lamellipodia-like structures, which emerge from cell junctions in the direction of cell movements. High-resolution time-lapse imaging of these junction-based lamellipodia (JBL) shows dynamic and distinct deployment of junctional proteins, such as F-actin, VE-cadherin and ZO1, during JBL oscillations. Upon initiation, F-actin and VE-cadherin are broadly distributed within JBL, whereas ZO1 remains at cell junctions. Subsequently, a new junction is formed at the front of the JBL, which then merges with the proximal junction. Rac1 inhibition interferes with JBL oscillations and disrupts cell elongation-similar to a truncation in ve-cadherin preventing VE-cad/F-actin interaction. Taken together, our observations suggest an oscillating ratchet-like mechanism, which is used by endothelial cells to move over each other and thus provides the physical means for cell rearrangements.

Hold Me, but Not Too Tight-Endothelial Cell-Cell Junctions in Angiogenesis

Endothelial cell-cell junctions must perform seemingly incompatible tasks during vascular development-providing stable connections that prevent leakage, while allowing dynamic cellular rearrangements during sprouting, anastomosis, lumen formation, and functional remodeling of the vascular network. This review aims to highlight recent insights into the molecular mechanisms governing endothelial cell-cell adhesion in the context of vascular development.

Spatiotemporal endothelial cell - pericyte association in tumors as shown by high resolution 4D intravital imaging

Endothelial cells and pericytes are integral cellular components of the vasculature with distinct interactive functionalities. To study dynamic interactions between these two cells we created two transgenic animal lines. A truncated eNOS (endothelial nitric oxide synthase) construct was used as a GFP tag for endothelial cell evaluation and an inducible Cre-lox recombination, under control of the Pdgfrb (platelet derived growth factor receptor beta) promoter, was created for pericyte assessment. Also, eNOStag-GFP animals were crossed with the already established Cspg4-DsRed mice expressing DsRed fluorescent protein in pericytes. For intravital imaging we used tumors implanted in the dorsal skinfold of these transgenic animals. This setup allowed us to study time and space dependent complexities, such as distribution, morphology, motility, and association between both vascular cell types in all angiogenetic stages, without the need for additional labeling. Moreover, as fluorescence was still clearly detectable after fixation, it is possible to perform comparative histology following intravital evaluation. These transgenic mouse lines form an excellent model to capture collective and individual cellular and subcellular endothelial cell – pericyte dynamics and will help answer key questions on the cellular and molecular relationship between these two cells.

Angiotensin II inhibits the protein expression of ZO‑1 in vascular endothelial cells by downregulating VE‑cadherin

Angiotensin II (Ang II) is reported to be involved in the development of various cardiovascular diseases by disrupting microvessel permeability, however, the underlying mechanism remains to be elucidated. The present study aimed to investigate the mechanism by which Ang II disrupts microvascular permeability. Rat endothelial cells were subjected to primary culture and identification. Cells in passages 4‑7 were then used for the following experiments. The cells were divided into control, Ang II, and Ang II + valsartan groups, and reverse transcription‑quantitative polymerase chain reaction and western blot analyses were perform to evaluate the expression of zonula occludens‑1 (ZO‑1) and vascular endothelial (VE)‑cadherin in the cells. The distribution of ZO‑1 protein was also detected using immunofluorescence assays. It was found that, compared with the control group, lower expression levels of ZO‑1 and VE‑cadherin were present in the Ang II group (P<0.01). ZO‑1 was also irregularly distributed at the periphery of the cells. In addition, the overexpression of VE‑cadherin reversed the effect of Ang II on the expression and distribution of ZO‑1 in endothelial cells. Together, these results suggested that Ang II inhibited the protein expression of ZO‑1 in vascular endothelial cells by downregulating VE‑cadherin, thus destroying the tight junctions between endothelial cells, which may also be the mechanism by which Ang II is involved in the development of cardiovascular diseases.

Live imaging molecular changes in junctional tension upon VE-cadherin in zebrafish

Forces play diverse roles in vascular development, homeostasis and disease. VE-cadherin at endothelial cell-cell junctions links the contractile acto-myosin cytoskeletons of adjacent cells, serving as a tension-transducer. To explore tensile changes across VE-cadherin in live zebrafish, we tailored an optical biosensor approach, originally established in vitro. We validate localization and function of a VE-cadherin tension sensor (TS) in vivo. Changes in tension across VE-cadherin observed using ratio-metric or lifetime FRET measurements reflect acto-myosin contractility within endothelial cells. Furthermore, we apply the TS to reveal biologically relevant changes in VE-cadherin tension that occur as the dorsal aorta matures and upon genetic and chemical perturbations during embryonic development.

Mechanical forces play a crucial role during morphogenesis, but how these are sensed and transduced in vivo is not fully understood. Here the authors apply a FRET tension sensor to live zebrafish and study changes in VE-cadherin tension at endothelial cell-cell junctions during arterial maturation.

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

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