Functional ephrin-B2 expression for promotive interaction between arterial and venous vessels in postnatal neovascularization

Multifaceted Role of Notch Signaling in Vascular Health and Diseases

Notch signaling is evolutionarily conserved from Drosophila to mammals and it functions as an essential modulator of vascular growth and development by directing endothelial cell specification, proliferation, migration, arteriovenous differentiation, inflammation, and apoptosis. The interplay between Notch and other signaling pathways plays a homeostatic role by modulating multiple vascular functions, including permeability regulation, angiogenesis, and vascular remodeling. This review explores current knowledge on Notch signaling in vascular development, homeostasis, and disease. It also discusses recent developments in understanding how endothelial Notch signaling affects vascular inflammation via cytokines or aberrant shear stress in endothelial cells while addressing the reciprocal relationship between Notch signaling and inflammation.

EphrinB2-mediated CDK5/ISL1 pathway enhances cardiac lymphangiogenesis and alleviates ischemic injury by resolving post-MI inflammation

EphrinB2 (erythropoietin-producing hepatoma interactor B2) is a key Eph/ephrin family member, promoting angiogenesis, vasculogenesis, and lymphangiogenesis during embryonic development. However, the role of EphrinB2 in cardiac lymphangiogenesis following myocardial infarction (MI) and the potential molecular mechanism remains to be demonstrated. This study revealed that EphrinB2 prevented ischemic heart post-MI from remodeling and dysfunction by activating the cardiac lymphangiogenesis signaling pathway. Deletion of EphrinB2 impaired cardiac lymphangiogenesis and aggravated adverse cardiac remodeling and ventricular dysfunction post-MI. At the same time, overexpression of EphrinB2 stimulated cardiac lymphangiogenesis which facilitated cardiac infiltrating macrophage drainage and reduced inflammation in the ischemic heart. The beneficial effects of EphrinB2 on improving clearance of inflammatory response and cardiac function were abolished in Lyve1 knockout mice. Mechanistically, EphrinB2 accelerated cell cycling and lymphatic endothelial cell proliferation and migration by activating CDK5 and CDK5-dependent ISL1 nuclear translocation. EphrinB2 enhanced the transcriptional activity of ISL1 at the VEGFR3 (FLT4) promoter, and VEGFR3 inhibitor MAZ51 significantly diminished the EphrinB2-mediated lymphangiogenesis and deteriorated the ischemic cardiac function. We uncovered a novel mechanism of EphrinB2-driven cardiac lymphangiogenesis in improving myocardial remodeling and function after MI.

Recent advances of the Ephrin and Eph family in cardiovascular development and pathologies

Erythropoietin-producing hepatoma (Eph) receptors, comprising the largest family of receptor tyrosine kinases (RTKs), exert profound influence on diverse biological processes and pathological conditions such as cancer. Interacting with their corresponding ligands, erythropoietin-producing hepatoma receptor interacting proteins (Ephrins), Eph receptors regulate crucial events like embryonic development, tissue boundary formation, and tumor cell survival. In addition to their well-established roles in embryonic development and cancers, emerging evidence highlights the pivotal contribution of the Ephrin/Eph family to cardiovascular physiology and pathology. Studies have elucidated their involvement in cardiovascular development, atherosclerosis, postnatal angiogenesis, and, more recently, cardiac fibrosis and calcification, suggesting a promising avenue for therapeutic interventions in cardiovascular diseases. There remains a need for a comprehensive synthesis of their collective impact in the cardiovascular context. By exploring the intricate interactions between Eph receptors, ephrins, and cardiovascular system, this review aims to provide a holistic understanding of their roles and therapeutic potential in cardiovascular health and diseases.

Notch Signaling-Modified Mesenchymal Stem Cell Patch Improves Left Ventricular Function via Arteriogenesis Induction in a Rat Myocardial Infarction Model

For ischemic cardiomyopathy (ICM) with limited therapeutic options, the induction of arteriogenesis has the potential to improve cardiac function through major restoration of blood flow. We hypothesized that transplantation of a Notch signaling-modified mesenchymal stem cell (SB623 cell) patch would induce angiogenesis and arteriogenesis in ischemic lesions, leading to improvement of left ventricular (LV) function in a rat ICM model. Two weeks after the induction of ischemia, SB623 cell patch transplantation into ICM rats (SB group, n = 10) or a sham operation (no-treatment group, n = 10) was performed. The LV ejection fraction was significantly improved at 6 weeks after SB623 cell patch transplantation (P < 0.001). Histological findings revealed that the number of von Willebrand factor (vWF)-positive capillary vessels (P < 0.01) and alpha smooth muscle actin (αSMA)- and vWF-positive arterioles with a diameter greater than 20 µm (P = 0.002) was significantly increased in the SB group, suggesting the induction of angiogenesis and arteriogenesis. Moreover, rat cardiomyocytes treated with SB623 cell patch transplantation showed upregulation of ephrin-B2 (P = 0.03) and EphB4 (P = 0.01) gene expression, indicating arteriogenesis induction. In conclusion, SB623 cell patch transplantation improved LV function by inducing angiogenesis and arteriogenesis in a rat ICM model.

Vascular Injury in the Zebrafish Tail Modulates Blood Flow and Peak Wall Shear Stress to Restore Embryonic Circular Network

Mechano-responsive signaling pathways enable blood vessels within a connected network to structurally adapt to partition of blood flow between organ systems. Wall shear stress (WSS) modulates endothelial cell proliferation and arteriovenous specification. Here, we study vascular regeneration in a zebrafish model by using tail amputation to disrupt the embryonic circulatory loop (ECL) at 3 days post fertilization (dpf). We observed a local increase in blood flow and peak WSS in the Segmental Artery (SeA) immediately adjacent to the amputation site. By manipulating blood flow and WSS via changes in blood viscosity and myocardial contractility, we show that the angiogenic Notch-ephrinb2 cascade is hemodynamically activated in the SeA to guide arteriogenesis and network reconnection. Taken together, ECL amputation induces changes in microvascular topology to partition blood flow and increase WSS-mediated Notch-ephrinb2 pathway, promoting new vascular arterial loop formation and restoring microcirculation.

Effects of shear stress on differentiation of stem cells into endothelial cells

Stem cell transplantation is an appealing potential therapy for vascular diseases and an indispensable key step in vascular tissue engineering. Substantial effort has been made to differentiate stem cells toward vascular cell phenotypes, including endothelial cells (ECs) and smooth muscle cells. The microenvironment of vascular cells not only contains biochemical factors that influence differentiation but also exerts hemodynamic forces, such as shear stress and cyclic strain. More recently, studies have shown that shear stress can influence the differentiation of stem cells toward ECs. A deep understanding of the responses and underlying mechanisms involved in this process is essential for clinical translation. This review highlights current data supporting the role of shear stress in stem cell differentiation into ECs. Potential mechanisms and signaling cascades for transducing shear stress into a biological signal are proposed. Further study of stem cell responses to shear stress will be necessary to apply stem cells for pharmacological applications and cardiovascular implants in the realm of regenerative medicine.

The Role of Eph Receptors and Ephrins in Corneal Physiology and Diseases

The cornea, while appearing to be simple tissue, is actually an extremely complex structure. In order for it to retain its biomechanical and optical properties, perfect organization of its cells is essential. Proper regeneration is especially important after injuries and in the course of various diseases. Eph receptors and ephrin are mainly responsible for the proper organization of tissues as well as cell migration and communication. In this review, we present the current state of knowledge on the role of Eph and ephrins in corneal physiology and diseases, in particular, we focused on the functions of the epithelium and endothelium. Since the role of Eph and ephrins in the angiogenesis process has been well established, we also analyzed their influence on conditions with corneal neovascularization.

Selective Endothelial Hyperactivation of Oncogenic KRAS Induces Brain Arteriovenous Malformations in Mice

OBJECTIVE

Brain arteriovenous malformations (bAVMs) are a leading cause of hemorrhagic stroke and neurological deficits in children and young adults, however, no pharmacological intervention is available to treat these patients. Although more than 95% of bAVMs are sporadic without family history, the pathogenesis of sporadic bAVMs is largely unknown, which may account for the lack of therapeutic options. KRAS mutations are frequently observed in cancer, and a recent unprecedented finding of these mutations in human sporadic bAVMs offers a new direction in the bAVM research. Using a novel adeno-associated virus targeting brain endothelium (AAV-BR1), the current study tested if endothelial KRAS^G12V mutation induces sporadic bAVMs in mice.

METHODS

Five-week-old mice were systemically injected with either AAV-BR1-GFP or -KRAS^G12V . At 8 weeks after the AAV injection, bAVM formation and characteristics were addressed by histological and molecular analyses. The effect of MEK/ERK inhibition on KRAS^G12V -induced bAVMs was determined by treatment of trametinib, a US Food and Drug Administration (FDA)-approved MEK/ERK inhibitor.

RESULTS

The viral-mediated KRAS^G12V overexpression induced bAVMs, which were composed of a tangled nidus mirroring the distinctive morphology of human bAVMs. The bAVMs were accompanied by focal angiogenesis, intracerebral hemorrhages, altered vascular constituents, neuroinflammation, and impaired sensory/cognitive/motor functions. Finally, we confirmed that bAVM growth was inhibited by trametinib treatment.

INTERPRETATION

Our innovative approach using AAV-BR1 confirms that KRAS mutations promote bAVM development via the MEK/ERK pathway, and provides a novel preclinical mouse model of bAVMs which will be useful to develop a therapeutic strategy for patients with bAVM. ANN NEUROL 2021;89:926-941.

Sonic hedgehog is expressed in human brain arteriovenous malformations and induces arteriovenous malformations in vivo

Abnormalities in arterial versus venous endothelial cell identity and dysregulation of angiogenesis are deemed important in the pathophysiology of brain arteriovenous malformations (AVMs). The Sonic hedgehog (Shh) pathway is crucial for both angiogenesis and arterial versus venous differentiation of endothelial cells, through its dual role on the vascular endothelial growth factor/Notch signaling and the nuclear orphan receptor COUP-TFII. In this study, we show that Shh, Gli1 (the main transcription factor of the Shh pathway), and COUP-TFII (a target of the non-canonical Shh pathway) are aberrantly expressed in human brain AVMs. We also show that implantation of pellets containing Shh in the cornea of Efnb2/LacZ mice induces growth of distinct arteries and veins, interconnected by complex sets of arteriovenous shunts, without an interposed capillary bed, as seen in AVMs. We also demonstrate that injection in the rat brain of a plasmid containing the human Shh gene induces the growth of tangles of tortuous and dilated vessels, in part positive and in part negative for the arterial marker αSMA, with direct connections between αSMA-positive and -negative vessels. In summary, we show that the Shh pathway is active in human brain AVMs and that Shh-induced angiogenesis has characteristics reminiscent of those seen in AVMs in humans.

Notch signaling-modified mesenchymal stem cells improve tissue perfusion by induction of arteriogenesis in a rat hindlimb ischemia model

Notch signaling-modified human mesenchymal stem cell, SB623 cell, is a promising cell therapy product for ischemic stroke. With the aim to expand indications for their use for critical limb-threatening ischemia (CLTI), we hypothesized that SB623 cells improved tissue perfusion by inducing angiogenesis or arteriogenesis in a hindlimb ischemia model rat. In Sprague–Dawley rats, hindlimb ischemia was generated by femoral artery removal, then seven days after ischemic induction 1 × 10⁵ SB623 cells or PBS was injected into the ischemic adductor muscle. As compared with the PBS group, tissue perfusion was significantly increased in the SB623 group. While capillary density did not vary between the groups, αSMA- and vWF-positive arterioles with a diameter  > 15 μm were significantly increased in the SB623 group. Whole transcriptome analysis of endothelial cells co-cultured with SB623 cells showed upregulation of the Notch signaling pathway as well as several other pathways potentially leading to arteriogenesis. Furthermore, rat muscle treated with SB623 cells showed a trend for higher ephrin-B2 and significantly higher EphB4 expression, which are known as arteriogenic markers. In the hindlimb ischemia model, SB623 cells improved tissue perfusion by inducing arteriogenesis, suggesting a promising cell source for treatment of CLTI.

DNMT3A inhibits E2F1-induced arterial marker expression and impairs angiogenesis in human umbilical artery endothelial cells

Arterial marker genes EphrinB2 and HEY2 are essential for cardiovascular development and postnatal neovascularization. Our previous study confirmed that E2F1 could activate the transcription of EphrinB2 and HEY2 in human mesenchymal stem cells; however, the detailed mechanism has not been resolved yet. In this study, we focused on the interaction between E2F1 and DNMT3A, a de novo DNA methyltransferase, on regulating the expression of EphrinB2 and HEY2, and explored the potential mechanisms. Gain- and loss-of-function experiments implicated the positive effect of E2F1 on the expression of EphrinB2 and HEY2 and tube formation in human umbilical artery endothelial cells. Accumulation of DNMT3A decreased the levels of EphrinB2 and HEY2, and impaired tube formation induced by E2F1, while inhibiting DNMT3A by RNA interference augmented their expression and angiogenesis in E2F1-trasfected cells. We then asked whether the low expressions of EphrinB2 and HEY2 induced by DNMT3A are related to the methylation status of their promoters. Surprisingly, the methylation status of the CpG islands in the promoter region was not significantly affected by overexpression of exogenous DNMT3A. Furthermore, the interaction between E2F1 and DNMT3A was confirmed by co-immunoprecipitation. DNMT3A could inhibit the transcription of EphrinB2 and HEY2 promoters by affecting the binding of E2F1 to its recognition sequences as revealed by luciferase reporter assay and chromatin immunoprecipitation. These results identified a novel mechanism underlying the cooperation of DNMT3A with E2F1 on regulating target gene expression, and revealed their roles in the angiogenic process.

Insight into the Role of Angiopoietins in Ageing-Associated Diseases

Angiopoietin (Ang) and its receptor, TIE signaling, contribute to the development and maturation of embryonic vasculature as well as vascular remodeling and permeability in adult tissues. Targeting both this signaling pathway and the major pathway with vascular endothelial growth factor (VEGF) is expected to permit clinical applications, especially in antiangiogenic therapies against tumors. Several drugs targeting the Ang-TIE signaling pathway in cancer patients are under clinical development. Similar to how cancer increases with age, unsuitable angiogenesis or endothelial dysfunction is often seen in other ageing-associated diseases (AADs) such as atherosclerosis, Alzheimer's disease, type 2 diabetes, chronic kidney disease and cardiovascular diseases. Thus, the Ang-TIE pathway is a possible molecular target for AAD therapy. In this review, we focus on the potential role of the Ang-TIE signaling pathway in AADs, especially non-cancer-related AADs. We also suggest translational insights and future clinical applications of this pathway in those AADs.

The critical role of the interplays of EphrinB2/EphB4 and VEGF in the induction of angiogenesis

The significant role of VEGF (vascular endothelial growth factor) as an angiogenesis inducer is well recognized. Besides VEGF, EphrinB2/EphB4 also plays essential roles in vascular development and postnatal angiogenesis. Compared with classical proangiogenic factors, not only does EphrinB2/EphB4 promote sprouting of new vessels, it is also involved in the vessel maturation. Given their involvement in many physiologic and pathological conditions, EphB4 and EphrinB2 are increasingly recognized as attractive therapeutic targets for angiogenesis-related diseases through modulating their expression and function. Previous works mainly focused on the individual role of VEGF and EphrinB2/EphB4 in angiogenesis, respectively, but the correlation between EphrinB2/EphB4 and VEGF in angiogenesis has not been fully disclosed. Here, we summarize the structure and bidirectional signaling of EphrinB2/EphB4, provide an overview on the relationship between EphrinB2/EphB4 signaling and VEGF pathway in angiogenesis and highlight the associated potential usefulness in anti-angiogenetic therapy.

EFNB2 facilitates cell proliferation, migration, and invasion in pancreatic ductal adenocarcinoma via the p53/p21 pathway and EMT

Ephrin-2 (EFNB2) is expressed at abnormally high levels in some neoplasms, such as squamous cell carcinoma of the head and neck and colorectal cancer. Its overexpression is associated with the malignant progression of tumors. However, the expression of EFNB2 in pancreatic ductal adenocarcinoma (PDAC) has not been thoroughly studied. EFNB2 expression was evaluated by quantitative real-time PCR, immunohistochemistry, and western blotting. Furthermore, the association between its expression levels and the clinicopathological features of PDAC patients was explored. To determine the underlying mechanisms of EFNB2, we transfected PDAC cells with small interfering RNA and performed in vitro and in vivo experiments. EFNB2 expression levels were significantly increased in cancer tissues and were associated with PDAC clinical stage and Ki67 expression. The down-regulation of EFNB2 inhibited cell proliferation by up-regulating p53/p21-mediated G0/G1 phase blockade. Knockdown of EFNB2 decreased the migration and invasion of PDAC cells by blocking epithelial-mesenchymal transition. These results suggested that EFNB2 may participate in the development of PDAC by promoting cell proliferation, migration, and invasion. Thus, EFNB2 is a potential target for the diagnosis and treatment of PDAC.

EphrinB2 activation enhances angiogenesis, reduces amyloid-β deposits and secondary damage in thalamus at the early stage after cortical infarction in hypertensive rats

Cerebral infarction causes secondary neurodegeneration and angiogenesis in thalamus, which impacts functional recovery after stroke. Here, we hypothesize that activation of ephrinB2 could stimulate angiogenesis and restore the secondary neurodegeneration in thalamus after cerebral infarction. Focal cerebral infarction was induced by middle cerebral artery occlusion (MCAO). Secondary damage, angiogenesis, amyloid-β (Aβ) deposits, levels of ephrinB2 and receptor for advanced glycation end product (RAGE) in the ipsilateral thalamus were determined by immunofluorescence and immunoblot. The contribution of ephrinB2 to angiogenesis was determined by siRNA-mediated knockdown of ephrinB2 and pharmacological activation of ephrinB2. The results showed that formation of new vessels and ephrinB2 expression was markedly increased in the ipsilateral thalamus at seven days after MCAO. EphrinB2 knockdown markedly suppressed angiogenesis coinciding with increased Aβ accumulation, neuronal loss and gliosis in the ipsilateral thalamus. In contrast, clustered EphB2-Fc significantly enhanced angiogenesis, alleviated Aβ accumulation and the secondary thalamic damage, which was accompanied by accelerated function recovery. Additionally, activation of ephrinB2 significantly reduced RAGE levels in the ipsilateral thalamus. Our findings suggest that activation of ephrinB2 promotes angiogenesis, ameliorates Aβ accumulation and the secondary thalamic damage after cerebral infarction. Additionally, RAGE might be involved in Aβ clearance by activating ephrinB2 in the thalamus.

Refueling the Ischemic CNS: Guidance Molecules for Vascular Repair

Stroke patients have only limited therapeutic options and often remain with considerable disabilities. To promote neurological recovery, angiogenesis in the ischemic peri-infarct region has been recognized as an encouraging therapeutic target. Despite advances in mechanistic understanding of vascular growth and repair, effective and safe angiogenic treatments are currently missing. Besides the most intensively studied angiogenic growth factors, recent research has indicated that the process of vascular sprouting and migration also requires the participation of guidance molecules, many of which were initially identified as regulators of axonal growth. Here, we review the inhibitory and growth-promoting effects of guidance molecules on the vascular system and discuss their potential as novel angiogenic targets for neurovascular diseases.

Essential roles of EphrinB2 in mammalian heart: from development to diseases

EphrinB2, a membrane-tethered ligand preferentially binding to its receptor EphB4, is ubiquitously expressed in all mammals. Through the particular bidirectional signaling, EphrinB2 plays a critical role during the development of cardiovascular system, postnatal angiogenesis physiologically and pathologically, and cardiac remodeling after injuries as an emerging role. This review highlights the pivotal involvement of EphrinB2 in heart, from developmental cardiogenesis to pathological cardiac remodeling process. Further potential translational therapies will be discussed in targeting EphrinB2 signaling, to better understand the prevention and treatment of cardiovascular diseases.

Multivalent biomaterial platform to control the distinct arterial venous differentiation of pluripotent stem cells

Vascular endothelial cells (ECs) differentiated from pluripotent stem cells have enormous potential to be used in a variety of therapeutic areas such as tissue engineering of vascular grafts and re-vascularization of ischemic tissues. To date, various protocols have been developed to differentiate stem cells toward vascular ECs. However, current methods are still not sufficient to drive the distinct arterial venous differentiation. Therefore, developing refined method of arterial-venous differentiation is critically needed to address this gap. Here, we developed a biomaterial platform to mimic multivalent ephrin-B2/EphB4 signaling and investigated its role in the early arterial and venous specification of pluripotent stem cells. Our results show immobilized ephrinB2 or EphB4 on hydrogel substrates have a distinct effect on arterial venous differentiation by regulating several arterial venous markers. When in combination with Wnt pathway agonist or BMP4 signaling, the ephrin-B2/EphB4 biomaterial platform can create diverging EC progenitor populations, demonstrating differential gene expression pattern across a wide range of arterial and venous markers, as well as phenotypic markers such as anti-thrombotic, pro-atherogenic and osteogenic genes, that are consistent with the in vivo expression patterns of arterial and venous ECs. Importantly, this distinct EC progenitor population cannot be achieved by current methods of applying soluble factors or hemodynamic stimuli alone, illustrating that fine-tuning of developmental signals using the biomaterial platform offers a new approach to better control the arterial venous differentiation of stem cells.

Shear stress promotes differentiation of stem cells from human exfoliated deciduous teeth into endothelial cells via the downstream pathway of VEGF-Notch signaling

Effects of shear stress on endotheliaxl differentiation of stem cells from human exfoliated deciduous teeth (SHEDs) were investigated. SHEDs were treated with shear stress, then reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was performed to analyse the mRNA expression of arterial markers and western blot analysis was performed to analyse protein expression of angiogenic markers. Additionally, in vitro matrigel angiogenesis assay was performed to evaluate vascular-like structure formation. The secreted protein expression levels of the vascular endothelial growth factor (VEGF) of SHEDs after shear stress was also quantified using corresponding ELISA kits. Untreated SHEDs seeded on Matrigel cannot form vessel-like structures at any time points, whereas groups treated with shear stress formed a few vessel-like structures at 4, 8 and 12 h. When SHEDs were treated with EphrinB2-siRNA for 24, the capability of vessel-like structure formation was suppressed. After being treated with shear stress, the expression of VEGF, VEGFR2, DLL4, Notch1, EphrinB2, Hey1 and Hey2 (arterial markers) gene expression was significantly upregulated, moreover, the protein levels of VEGFR2, EphrinB2, CD31, Notch1, DLL4, Hey1, and Hey2 were also significantly up-regulated. Both the mRNA and protein expression levels of EphB4 (venous marker) were downregulated. The average VEGF protein concentration in supernatants secreted by shear stress treated SHEDs groups increased significantly. In conclusion, shear stress was able to induce arterial endothelial differentiation of stem cells from human exfoliated deciduous teeth, and VEGF-DLL4/Notch‑EphrinB2 signaling was involved in this process.

EphrinB2/EphB4 pathway in postnatal angiogenesis: a potential therapeutic target for ischemic cardiovascular disease

Ischemic cardiovascular disease remains one of the leading causes of morbidity and mortality in the world. Proangiogenic therapy appears to be a promising and feasible strategy for the patients with ischemic cardiovascular disease, but the results of preclinical and clinical trials are limited due to the complicated mechanisms of angiogenesis. Facilitating the formation of functional vessels is important in rescuing the ischemic cardiomyocytes. EphrinB2/EphB4, a novel pathway in angiogenesis, plays a critical role in both microvascular growth and neovascular maturation. Hence, investigating the mechanisms of EphrinB2/EphB4 pathway in angiogenesis may contribute to the development of novel therapeutics for ischemic cardiovascular disease. Previous reviews mainly focused on the role of EphrinB2/EphB4 pathway in embryo vascular development, but their role in postnatal angiogenesis in ischemic heart disease has not been fully illustrated. Here, we summarized the current knowledge of EphrinB2/EphB4 in angiogenesis and their interaction with other angiogenic pathways in ischemic cardiovascular disease.

Membrane-mediated regulation of vascular identity

Vascular diseases span diverse pathology, but frequently arise from aberrant signaling attributed to specific membrane-associated molecules, particularly the Eph-ephrin family. Originally recognized as markers of embryonic vessel identity, Eph receptors and their membrane-associated ligands, ephrins, are now known to have a range of vital functions in vascular physiology. Interactions of Ephs with ephrins at cell-to-cell interfaces promote a variety of cellular responses such as repulsion, adhesion, attraction, and migration, and frequently occur during organ development, including vessel formation. Elaborate coordination of Eph- and ephrin-related signaling among different cell populations is required for proper formation of the embryonic vessel network. There is growing evidence supporting the idea that Eph and ephrin proteins also have postnatal interactions with a number of other membrane-associated signal transduction pathways, coordinating translation of environmental signals into cells. This article provides an overview of membrane-bound signaling mechanisms that define vascular identity in both the embryo and the adult, focusing on Eph- and ephrin-related signaling. We also discuss the role and clinical significance of this signaling system in normal organ development, neoplasms, and vascular pathologies.

Strategies to Enhance the Efficiency of Endothelial Progenitor Cell Therapy by Ephrin B2 Pretreatment and Coadministration with Smooth Muscle Progenitor Cells on Vascular Function during the Wound-Healing Process in Irradiated or Nonirradiated Condition

Endothelial progenitor cell (EPC) transplantation has beneficial effects for therapeutic neovascularization. We therefore assessed the effect of a therapeutic strategy based on EPC administation in the healing of radiation-induced damage. To improve cell therapy for clinical use, we used pretreatment with ephrin B2-Fc (Eph-B2-Fc) and/or coadministration with smooth muscle progenitor cells. At day 3, EPCs promoted dermal wound healing in both nonirradiated and irradiated mice by 1.2- and 1.15-fold, respectively, compared with animals injected with phosphate-buffered saline. In addition, EPCs also improved skin-blood perfusion and capillary density in both irradiated and nonirradiated mice compared with PBS-injected animals. We also demonstrated that activation with Eph-B2-Fc increased wound closure by 1.6-fold compared with unstimulated EPCs in nonirradiated mice. Interestingly, the beneficial effect of Eph-B2-Fc was abolished in irradiated animals. In addition, we found that Eph-B2-Fc stimulation did not improve EPC-induced vascular permeability or adhesiveness compared to unstimulated EPCs. We hypothesized that this effect was due to high oxidative stress during irradiation, leading to inhibition of EPCs' beneficial effect on vascular function. In this line, we demonstated that, in irradiated conditions, N-acetyl-l-cysteine treatment restored the beneficial effect of EPC stimulation with Eph-B2-Fc in the wound healing process. In conclusion, stimulation by Eph-B2-Fc improved the beneficial effect of EPCs in physiological conditions and irradiated conditions only in association with antioxidant treatment. Additionally, cotherapy was beneficial in pathological conditions.

Ephrin B2 and EphB4 selectively mark arterial and venous vessels in cerebral arteriovenous malformation

OBJECTIVES

Ephrin type B receptor 4 (EphB4, Eph receptor) selectively binds ephrin B2 (Eph ligand). EphB4/ephrin B2 is involved in embryonic vessel development, vascular remodelling and pathological vessel formation in adults (including tumour angiogenesis). Binding of vascular endothelial growth factor (VEGF)-A to the endothelial-specific receptor VEGF receptor-2 is the main extracellular signal triggering angiogenic response. Little is known about the role of EphB4/ephrin B2 during angiogenesis and arteriovenous plasticity in cerebral arteriovenous malformation (cAVM). This study investigated EphB4 and ephrin B2 expression in cAVM.

METHODS

Haemorrhagic (H-AVM) and nonhaemorrhagic (NH-AVM) specimens of AVM nidus, obtained after microsurgical cAVM resection, and normal superficial temporal artery (STA) specimens, were analysed retrospectively. VEGF-A, EphB4 and ephrin B2 expression were studied by immunohistochemistry and immunoblotting.

RESULTS

In cAVM (10 H-AVM; 10 NH-AVM), VEGF-A was immunocytochemically localized to endothelial cells; strong endothelial cell staining was found for EphB4 in veins and ephrin B2 in arteries. Normal STA (n = 10) did not express EphB4 or ephrin B2. EphB4 and ephrin B2 expression was greater in H-AVM than in NH-AVM.

CONCLUSIONS

Endothelial cells are more active in H-AVM than NH-AVM. EphB4 and ephrin B2 play important roles in neovascularization and arteriovenous differentiation/plasticity. These data provide new insights into the aetiology of cAVM and lay a foundation for further study. The notch pathway induced by VEGF-A may be a key signalling pathway in this process.

Vascular endothelial growth factor-A inhibits EphB4 and stimulates delta-like ligand 4 expression in adult endothelial cells

BACKGROUND

During vein graft adaptation to the arterial circulation, vascular endothelial growth factor (VEGF) A expression transiently increases before becoming downregulated; however, the role of VEGF-A in venous remodeling is not clear. In addition, although VEGF-A stimulates angiogenesis and determines arterial identity in nascent arterial endothelial cells (EC), the role of VEGF-A in regulating identity in adult venous EC is also not clear.

MATERIALS AND METHODS

EC, wild type (EphB4+/+) or heterozygous knockout (EphB4+/-), were stimulated with VEGF-A (0-100 ng/mL) and examined with quantitative polymerase chain reaction and western blotting.

RESULTS

VEGF-A (100 ng/mL) inhibited expression of EphB4 and stimulated expression of delta-like ligand 4 (dll4) but did not stimulate either notch or EphrinB2 expression in adult venous EC. Pretreatment with VEGF receptor 2-neutralizing antibody abolished VEGF-stimulated downregulation of EphB4 but not the upregulation of dll4. Pretreatment with PD98059 or wortmannin showed that VEGF-A downregulation of EphB4 and upregulation of dll4 are mitogen-activated protein kinase kinase and extracellular signal-regulated kinase dependent but phosphatidylinositol 3 kinase-Akt independent. Compared with VEGF-induced EphB4 downregulation and dll4 upregulation in control EC, reduced EphB4 signaling in EphB4+/- EC showed even further downregulation of EphB4 and upregulation of dll4.

CONCLUSIONS

Despite the genetic programming of arterial and venous EC fate, VEGF-A can repress venous identity in adult venous EC without induction of arterial identity. These changes in adult EC in vitro recapitulate the changes in identity described during vein graft adaptation to the arterial environment in vivo.

A microarray analysis of angiogenesis modulation effect of Xuefu Zhuyu Decoction on endothelial cells

OBJECTIVE

To study the angiogenesis modulation mechanism of Xuefu Zhuyu Decoction () on the endothelial cell line ECV304.

METHODS

ECV304 cells were treated with 2.5% Xuefu Zhuyu Decoction-containing serum (XFZYD-CS) for 24 h, 48 h or 72 h. Thiazolyl blue tetrazolium bromide (MTT), fluorescence activating cell sorter (FACS), migration, adhesion and in vitro tube formation assays were conducted to confirm an angiogenesis effect of XFZYD at 3 time points. An analysis of angiogenesis regulator profiles was performed at 3 times with real-time polymerase chain reaction (RT-PCR) superarray.

RESULTS

At 48 h, XFZYD-CS induced ECV304 significantly improved cell viability, number in S phase, migration, adhesion and tube formation. At 24 h and 72 h, only cell migration was elevated. Microarray results showed that the expression of 27 angiogenesis-related genes was changed.

CONCLUSION

XFZYD-CS treatment induced angiogenesis on ECV304 cells with significant cellcular changes occurring at 48 h and genetic changes as early as 24 h.

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.

Vessel arterial-venous plasticity in adult neovascularization

OBJECTIVE

Proper arterial and venous specification is a hallmark of functional vascular networks. While arterial-venous identity is genetically pre-determined during embryo development, it is unknown whether an analogous pre-specification occurs in adult neovascularization. Our goal is to determine whether vessel arterial-venous specification in adult neovascularization is pre-determined by the identity of the originating vessels.

METHODS AND RESULTS

We assessed identity specification during neovascularization by implanting isolated microvessels of arterial identity from both mice and rats and assessing the identity outcomes of the resulting, newly formed vasculature. These microvessels of arterial identity spontaneously formed a stereotypical, perfused microcirculation comprised of the full complement of microvessel types intrinsic to a mature microvasculature. Changes in microvessel identity occurred during sprouting angiogenesis, with neovessels displaying an ambiguous arterial-venous phenotype associated with reduced EphrinB2 phosphorylation.

CONCLUSIONS

Our findings indicate that microvessel arterial-venous identity in adult neovascularization is not necessarily pre-determined and that adult microvessels display a considerable level of phenotypic plasticity during neovascularization. In addition, we show that vessels of arterial identity also hold the potential to undergo sprouting angiogenesis.

Pitavastatin-induced angiogenesis and arteriogenesis is mediated by Notch1 in a murine hindlimb ischemia model without induction of VEGF

Notch signaling is reported to regulate angiogenesis, interacting with vascular endothelial growth factor (VEGF) signaling. HMG CoA reductase inhibitors (statins) also alter Notch signaling in vascular cells, but the mechanism and involvement of Notch and VEGF signaling in statin-mediated angiogenesis remain unclear. Here, we examined how statins activate the endothelial Notch1, and promote angiogenesis and arteriogenesis. We examined blood flow recovery after hindlimb ischemia in wild-type (WT) and Notch1 mutant mice treated with or without pitavastatin (3 mg/kg/day, p.o.). Although VEGF induction was not altered in ischemic limbs, pitavastatin promoted blood flow recovery in ischemic limbs in control mice but not in Notch1 mutant mice. Furthermore, pitavastatin induced endothelial ephrinB2 downstream of Notch1 and increased the density of both capillaries and arterioles in the ischemic limbs of WT but not of Notch1 mutant mice. Pitavastatin (100 nmol/l) rapidly activated γ-secretase and Notch1 in human umbilical vein endothelial cells without VEGF induction, which was suppressed by pharmacological inhibition and knockdown of Akt. Pitavastatin also augmented endothelial proliferation and tube formation on Matrigel, which were suppressed by either γ-secretase inhibition or knockdown of Notch1. Pitavastatin-induced microvascular sprouting was also impaired in Notch1 mutant aortic explants. Taken together, pitavastatin activates Notch1 through Akt-dependent stimulation of γ-secretase in endothelial cells, and thereby increases vasculogenesis without VEGF induction.

The stent-eluting drugs sirolimus and paclitaxel suppress healing of the endothelium by induction of autophagy

Clinical studies have indicated that the stent-eluting drugs sirolimus and paclitaxel impact restenosis; however, it is still elusive how these drugs affect the vascular endothelium at the molecular and cellular levels. The purpose of this study was to determine whether sirolimus and paclitaxel induce molecular and cellular alterations in the vascular endothelium. Endothelial regrowth was assessed in human aortic endothelial cells and rat aortic endothelium. Molecular and cellular alterations were analyzed in human aortic endothelial cells by Western blot analysis, transmission electron microscopy, and immunofluorescence staining. Green fluorescent protein-LC3 mice were used to analyze autophagic endothelium. Here, we show that sirolimus and paclitaxel differentially induce self-digesting autophagy in vascular endothelial cells with changes in expression of LC3B, p53, and Bcl-2, considerably suppressing re-endothelialization and revascularization. These results suggest that phenotypic alteration in the endothelium by sirolimus or paclitaxel might affect the rates of late stent thrombosis, myocardial infarction, and mortality.

Arterial-venous specification during development

The major arteries and veins of the vertebrate circulatory system are formed early in embryonic development, before the onset of circulation, following de novo aggregation of "angioblast" progenitors in a process called vasculogenesis. Initial embryonic determination of artery or vein identity is regulated by variety of genetic factors that work in concert to specify endothelial cell fate, giving rise to 2 distinct components of the circulatory loop possessing unique structural characteristics. Work in multiple in vivo animal model systems has led to a detailed examination of the interacting partners that determine arterial and venous specification. We discuss the hierarchical arrangement of many signaling molecules, including Hedgehog (Hh), vascular endothelial growth factor (VEGF), Notch, and chicken ovalbumin upstream-transcription factor II (COUP-TFII) that promote or inhibit divergent pathways of endothelial cell fate. Elucidation of the functional role of these genetic determinants of blood vessel specification together with the epigenetic factors involved in subsequent modification of arterial-venous identity will allow for potential new therapeutic targets for vascular disorders.

Ephrin-B2 expression critically influences Nipah virus infection independent of its cytoplasmic tail

Background

Cell entry and cell-to-cell spread of the highly pathogenic Nipah virus (NiV) requires binding of the NiV G protein to cellular ephrin receptors and subsequent NiV F-mediated fusion. Since expression levels of the main NiV entry receptor ephrin-B2 (EB2) are highly regulated in vivo to fulfill the physiological functions in axon guidance and angiogenesis, the goal of this study was to determine if changes in the EB2 expression influence NiV infection.

Results

Surprisingly, transfection of increasing EB2 plasmid concentrations reduced cell-to-cell fusion both in cells expressing the NiV glycoproteins and in cells infected with NiV. This effect was attributed to the downregulation of the NiV glycoproteins from the cell surface. In addition to the influence on cell-to-cell fusion, increased EB2 expression significantly reduced the total amount of NiV-infected cells, thus interfered with virus entry. To determine if the negative effect of elevated EB2 expression on virus entry is a result of an increased EB2 signaling, receptor function of a tail-truncated and therefore signaling-defective ΔcEB2 was tested. Interestingly, ΔcEB2 fully functioned as NiV entry and fusion receptor, and overexpression also interfered with virus replication.

Conclusion

Our findings clearly show that EB2 signaling does not account for the striking negative impact of elevated receptor expression on NiV infection, but rather that the ratio between the NiV envelope glycoproteins and surface receptors critically influence cell-to-cell fusion and virus entry.

Possible novel targets for therapeutic angiogenesis

An increasing number of studies about the molecular basis of angiogenesis are rapidly disclosing novel signal pathways involved in the blood vessel formation process. This review will focus on bone morphogenic proteins, Hedgehog, Notch, ephrins, neuropilins, neurotrophins and netrins. These recently discovered angiogenesis mediators are involved in vascular development during embryogenesis and, interestingly, they are shared between the nervous and vascular systems. They represent new potential targets in the vasculature and suggest novel therapeutic opportunities.

Involvement of endothelial ephrin-B2 in adhesion and transmigration of EphB-receptor-expressing monocytes

The vascular endothelium is a crucial interface that controls the recruitment of circulating leukocytes. Based on the luminal expression of the ephrin-B2 ligand by endothelial cells (ECs) and the expression of EphB receptors (EphBRs) by circulating monocytes, we hypothesized that EphBR-ephrinB interactions are involved in monocyte adhesion. Adhesion experiments with monocytic cells were performed on ECs that overexpressed either full-length ephrin-B2 or cytoplasmically truncated ephrin-B2 (DeltaC-ephrin-B2). Atomic force microscopy confirmed similar adhesive strengths of EphBR-expressing J774 cells to ECs that either overexpressed full-length ephrin-B2 or truncated DeltaC-ephrin-B2 (1-minute interaction). Yet, adhesion experiments under static or flow conditions for 30 minutes demonstrated the preferential adhesion of monocytic cells to ECs that overexpressed full-length ephrin-B2 but not to DeltaC-ephrin-B2 or to ECs that had been mock transduced. Adhesion was blocked by ephrin-B2-specific and EphBR-specific antibodies. Correspondingly, adhesion of EphB4-receptor-overexpressing monocytes to ephrin-B2-positive ECs was further augmented. Trafficking experiments of cell-surface molecules revealed that, prior to internalization, the resulting EphB4-receptor-ephrin-B2 complex translocated from the luminal surface to inter-endothelial junctions. Lastly, full-length ephrin-B2 in ECs was also involved in monocyte transmigration. Collectively, our study identifies a role of EphBR-ephrinB interactions as a new step in the cascade of events leading to monocyte adhesion and transmigration through the vascular endothelium.

The role of Ephs, Ephrins, and growth factors in Kaposi sarcoma and implications of EphrinB2 blockade

Kaposi sarcoma (KS) is associated with human herpesvirus (HHV)-8 and is dependent on the induction of vascular endothelial growth factors (VEGFs). VEGF regulates genes that provide arterial or venous identity to endothelial cells, such as the induction of EphrinB2, which phenotypically defines arterial endothelial cells and pericytes, and represses EphB4, which defines venous endothelial cells. We conducted a comprehensive analysis of the Eph receptor tyrosine kinases to determine which members are expressed and therefore contribute to KS pathogenesis. We demonstrated limited Eph/Ephrin expression; notably, the only ligand highly expressed is EphrinB2. We next studied the biologic effects of blocking EphrinB2 using the extracellular domain of EphB4 fused with human serum albumin (sEphB4-HSA). sEphB4-HSA inhibited migration and invasion of the KS cells in vitro in response to various growth factors. Finally, we determined the biologic effects of combining sEphB4-HSA and an antibody to VEGF. sEphB4-HSA was more active than the VEGF antibody, and combination of the 2 had at least additive activity. sEphB4-HSA reduced blood vessel density, pericyte recruitment, vessel perfusion, and increased hypoxia, with an associated increase in VEGF and DLL4 expression. The combination of sEphB4-HSA and VEGF antibody is a rational treatment combination for further investigation.

Interleukin-3 promotes expansion of hemopoietic-derived CD45+ angiogenic cells and their arterial commitment via STAT5 activation

Interleukin-3 (IL-3) released by infiltrating inflammatory cells in different pathologic settings contributes to organ and tumor angiogenesis. Here we demonstrate that IL-3 expands a subset of CD45+ circulating angiogenic cells clonally derived from the hemopoietic progenitors. Moreover, CD45+ cells exposed to IL-3 acquire arterial specification and contribute to the formation of vessels in vivo. Depletion of signal transducer and activator of transcription 5 (STAT5) provides evidence that IL-3–mediated cell expansion and arterial morphogenesis rely on STAT5 activation. In addition, by means of Tie2-transgenic mice, we demonstrate that STAT5 also regulates IL-3–induced expansion and arterial specification of bonemarrow–derived CD45+ cells. Thus, our data provide the first evidence that, in inflammatory microenvironments containing IL-3, angiogenic cells derived from hemopoietic precursors can act as adult vasculogenic cells. Moreover, the characterization of the signaling pathway regulating these events provides the rationale for therapeutically targeting STAT5 in these pathologic settings.

Environmental enrichment counteracts Alzheimer's neurovascular dysfunction in TgCRND8 mice

We and others have recently demonstrated that cognitive and physical stimulation in form of environmental enrichment reduces cerebral beta-amyloid (Abeta) deposition in transgenic mouse models of Alzheimer's disease. This effect was independent from amyloid precursor protein (APP) expression or processing and rather a consequence of enhanced clearance of Abeta. However, the detailed mechanisms remain unclear. In the present study, we show that environmental enrichment in TgCRND8 mice (carrying human APP(Swedish+Indiana)) affect the neurovascular unit by increased angiogenesis and differential regulation of Abeta receptor/transporter molecules, namely up-regulation of LRP1, ApoE and A2M as well as down-regulation of RAGE so that brain to blood Abeta clearance is facilitated. These results suggest a hitherto unknown effect of environmental enrichment counteracting the vascular dysfunction in Alzheimer diseased brain.

Comparison of EphA receptor tyrosine kinases and ephrinA ligand expression to EphB-ephrinB in vascularized corneas

PURPOSE

Eph cell surface receptors and their ligands, ephrins, are involved in neuronal patterning and neovascularization. Our purpose is to compare and characterize the expression of ephrinA ligands and EphA receptors to ephrinB ligands and EphB receptors in excised mouse corneal tissue, in corneal epithelial and keratocyte cell lines, and during corneal angiogenesis.

METHODS

Mouse corneal epithelial cells and keratocytes were immortalized using SV40T antigen viral infection of primary cultures. The immortalized epithelial cells and keratocytes were cloned and characterized using antibodies to keratin, vimentin, integrin alpha5beta1, and alpha-smooth muscle actin. Basic fibroblast growth factor pellets were implanted to induce corneal neovascularization. The eyes of wild-type, ephrinB2(tlacZ/+), and EphB4(tlacZ/+) heterozygous mice were harvested and sectioned 7 days after pellet implantation. Confocal immunohistochemistry was performed to compare the expression of the Eph/ephrinA family (EphA1-8, ephrinA1-5) and Eph/ephrinB family (EphB1-4, EphB6 ephrinB1-3).

RESULTS

EphA1, EphA3, ephrinA1, ephrinA2, EphB1, EphB4, ephrinB1, and ephrinB2 were detected in wild-type mouse corneal epithelial cells and keratocytes. EphA2 was immunolocalized only in epithelial cells. Also, EphA3, ephrinA1, EphB1, EphB4, and ephrinB1 were immunolocalized to the corneal epithelium and stroma. In the vascularized corneas, ephrinB1 was immunolocalized mainly to the keratocytes around the vessels, and ephrinB2, EphB1, and EphB4 were colocalized mainly with CD31 to the vascular endothelial cells.

CONCLUSIONS

The characterization of ephrin ligand and Eph receptor expression during cornea angiogensis in this study suggests that the Eph/ephrin family of receptor tyrosine kinases and their ligands may play a role in the regulation of corneal angiogenesis.

Venous identity is lost but arterial identity is not gained during vein graft adaptation

OBJECTIVES

Ephrin ligands and Eph receptors are signaling molecules that are differentially expressed on arteries and veins during development. We examined whether Eph-B4, a venous marker, and Ephrin-B2, an arterial marker, are regulated during vein graft adaptation in humans and aged rats.

METHODS AND RESULTS

Eph-B4 transcripts and immunodetectable protein are downregulated in endothelial and smooth muscle cells of patent vein grafts in both humans and in aged rats, whereas Ephrin-B2 transcripts and protein are not strongly induced. Other markers of arterial identity, including dll4 and notch-4, are also not induced during vein graft adaptation in aged rats. Because VEGF-A is upstream of the Ephrin-Eph pathway, and expression of VEGF-A is induced only at early time points after exposure of the vein to the arterial environment, we inhibited VEGF-A in vein grafts using an siRNA-based approach. Vein grafts treated with siRNA directed against VEGF-A demonstrated a thicker intima-media containing alpha-actin, consistent with arterialization, but did not contain Eph-B4 or Ephrin-B2.

CONCLUSIONS

Venous identity is preserved in the veins of aged animals, but is lost during adaptation to the arterial circulation; arterial markers are not induced. Markers of vessel identity are plastic in adults and their selective regulation may mediate vein graft adaptation to the arterial environment in aged animals and humans.

PSGL-1-mediated activation of EphB4 increases the proangiogenic potential of endothelial progenitor cells

Endothelial progenitor cell (EPC) transplantation has beneficial effects for therapeutic neovascularization; however, only a small proportion of injected cells home to the lesion and incorporate into the neocapillaries. Consequently, this type of cell therapy requires substantial improvement to be of clinical value. Erythropoietin-producing human hepatocellular carcinoma (Eph) receptors and their ephrin ligands are key regulators of vascular development. We postulated that activation of the EphB4/ephrin-B2 system may enhance EPC proangiogenic potential. In this report, we demonstrate in a nude mouse model of hind limb ischemia that EphB4 activation with an ephrin-B2-Fc chimeric protein increases the angiogenic potential of human EPCs. This effect was abolished by EphB4 siRNA, confirming that it is mediated by EphB4. EphB4 activation enhanced P selectin glycoprotein ligand-1 (PSGL-1) expression and EPC adhesion. Inhibition of PSGL-1 by siRNA reversed the proangiogenic and adhesive effects of EphB4 activation. Moreover, neutralizing antibodies to E selectin and P selectin blocked ephrin-B2-Fc-stimulated EPC adhesion properties. Thus, activation of EphB4 enhances EPC proangiogenic capacity through induction of PSGL-1 expression and adhesion to E selectin and P selectin. Therefore, activation of EphB4 is an innovative and potentially valuable therapeutic strategy for improving the recruitment of EPCs to sites of neovascularization and thereby the efficiency of cell-based proangiogenic therapy.

Notch ligand Delta-like 1 is essential for postnatal arteriogenesis

Growth of functional arteries is essential for the restoration of blood flow to ischemic organs. Notch signaling regulates arterial differentiation upstream of ephrin-B2 during embryonic development, but its role during postnatal arteriogenesis is unknown. Here, we identify the Notch ligand Delta-like 1 (Dll1) as an essential regulator of postnatal arteriogenesis. Dll1 expression was specifically detected in arterial endothelial cells, but not in venous endothelial cells or capillaries. During ischemia-induced arteriogenesis endothelial Dll1 expression was strongly induced, Notch signaling activated and ephrin-B2 upregulated, whereas perivascular cells expressed proangiogenic vascular endothelial growth factor, and the ephrin-B2 activator EphB4. In heterozygous Dll1 mutant mice endothelial Notch activation and ephrin-B2 induction after hindlimb ischemia were absent, arterial collateral growth was abrogated and recovery of blood flow was severely impaired, but perivascular vascular endothelial growth factor and EphB4 expression was unaltered. In vitro, angiogenic growth factors synergistically activated Notch signaling by induction of Dll1, which was necessary and sufficient to regulate ephrin-B2 expression and to induce ephrin-B2 and EphB4-dependent branching morphogenesis in human arterial EC. Thus, Dll1-mediated Notch activation regulates ephrin-B2 expression and postnatal arteriogenesis.