Angiopoietin 1 causes vessel enlargement, without angiogenic sprouting, during a critical developmental period

Mesenchymal glioma stem cells trigger vasectasia-distinct neovascularization process stimulated by extracellular vesicles carrying EGFR

Targeting neovascularization in glioblastoma (GBM) is hampered by poor understanding of the underlying mechanisms and unclear linkages to tumour molecular landscapes. Here we report that different molecular subtypes of human glioma stem cells (GSC) trigger distinct endothelial responses involving either angiogenic or circumferential vascular growth (vasectasia). The latter process is selectively triggered by mesenchymal (but not proneural) GSCs and is mediated by a subset of extracellular vesicles (EVs) able to transfer EGFR/EGFRvIII transcript to endothelial cells. Inhibition of the expression and phosphorylation of EGFR in endothelial cells, either pharmacologically (Dacomitinib) or genetically (gene editing), abolishes their EV responses in vitro and disrupts vasectasia in vivo. Therapeutic inhibition of EGFR markedly extends anticancer effects of VEGF blockade in mice, coupled with abrogation of vasectasia and prolonged survival. Thus, vasectasia driven by intercellular transfer of oncogenic EGFR may represent a new therapeutic target in a subset of GBMs.

The Angiopoietin Signaling Pathway Is Involved in Inflammatory Processes in Hospitalized COVID-19 Patients

The viral agent SARS-CoV-2 clearly affects several organ systems, including the cardiovascular system. Angiopoietins are involved in vascular integrity and angiogenesis. Angiopoietin-1 (Ang1) promotes vessel stabilization, while angiopoietin-2 (Ang2), which is usually expressed at low levels, is significantly elevated in inflammatory and angiogenic conditions. Interleukin-6 (IL-6) is known to induce defective angiogenesis via the activation of the Ang2 pathway. Vasculitis and vasculopathy are some of the defining features of moderate to severe COVID-19-associated systemic disease. We investigated the serum levels of angiopoietins, as well as interleukin-6 levels and anti-SARS-CoV2 IgG titers, in hospitalized COVID-19 patients across disease severity and healthy controls. Ang2 levels were elevated in COVID-19 patients across all severity compared to healthy controls, while Ang1 levels were decreased. The patients with adverse outcomes (death and/or prolonged hospitalization) had relatively lower and stable Ang1 levels but continuously elevated Ang2 levels, while those who had no adverse outcomes had increasing levels of both Ang1 and Ang2, followed by a decrease in both. These results suggest that the dynamic levels of Ang1 and Ang2 during the clinical course may predict adverse outcomes in COVID-19 patients. Ang1 seems to play an important role in controlling Ang2-related inflammatory mechanisms in COVID-19 patients. IL-6 and anti-SARS-CoV2 spike protein IgG levels were significantly elevated in patients with severe disease. Our findings represent an informative pilot assessment into the role of the angiopoietin signaling pathway in the inflammatory response in COVID-19.

Intracameral Injection of AAV-DJ.COMP-ANG1 Reduces the IOP of Mice by Reshaping the Trabecular Outflow Pathway

Purpose

The angiopoietin-1 (ANG1)-TIE signaling pathway orchestrates the development and maintenance of the Schlemm's canal (SC). In this study, we investigated the impact of adeno-associated virus (AAV)-mediated gene therapy with cartilage oligomeric matrix protein-ANG1 (COMP-ANG1) on trabecular outflow pathway.

Methods

Different serotypes of AAVs were compared for transduction specificity and efficiency in the anterior segment. The selected AAVs encoding COMP-ANG1 or ZsGreen1 (control) were delivered into the anterior chambers of wild-type C57BL/6J mice. The IOP and ocular surface were monitored regularly. Ocular perfusion was performed to measure the outflow facility and label flow patterns of the trabecular drainage pathway. Structural features of SC as well as limbal, retinal, and skin vessels were visualized by immunostaining. Ultrastructural changes in the SC and trabecular meshwork were observed under transmission electron microscopy.

Results

AAV-DJ could effectively infect the anterior segment. Intracameral injection of AAV-DJ.COMP-ANG1 lowered IOP in wild-type C57BL/6J mice. No signs of inflammation or angiogenesis were noticed. Four weeks after AAV injection, the conventional outflow facility and effective filtration area were increased significantly (P = 0.005 and P = 0.04, respectively). Consistently, the area of the SC was enlarged (P < 0.001) with increased density of giant vacuoles in the inner wall (P = 0.006). In addition, the SC endothelia lay on a more discontinuous basement membrane (P = 0.046) and a more porous juxtacanalicular tissue (P = 0.005) in the COMP-ANG1 group.

Conclusions

Intracamerally injected AAV-DJ.COMP-ANG1 offers a significant IOP-lowering effect by remodeling the trabecular outflow pathway of mouse eyes.

Extracellular Vesicles from Human Umbilical Cord-Derived MSCs Affect Vessel Formation In Vitro and Promote VEGFR2-Mediated Cell Survival

Mesenchymal stromal cell (MSC)-derived extracellular vesicles (EVs) have emerged as novel tools in regenerative medicine. Angiogenesis modulation is widely studied for the treatment of ischaemic diseases, wound healing, and tissue regeneration. Here, we have shown that EVs from human umbilical cord-derived MSCs can affect VEGFR2 signalling, a master regulator of angiogenesis homeostasis, via altering the phosphorylation of AKT. This translates into an inhibition of apoptosis, promoting exclusively cell survival, but not proliferation, in human microvascular endothelial cells. Interestingly, when comparing EVs from normoxic cells to those obtained from hypoxia (1% O₂) preconditioned cells, hypoxia-derived EVs appear to have a slightly enhanced effect. Furthermore, when studied in a longer term endothelial-fibroblast co-culture angiogenesis model in vitro, both EV populations demonstrated a positive effect on vessel formation, evidenced by increased vessel networks with tubes of significantly larger diameters. Our data reveals that EVs selectively target components of the angiogenic pathway, promoting VEGFR2-mediated cell survival via enhancement of AKT activation. Our data show that EVs are able to enhance specific components of the VEGF signalling pathway and may have therapeutic potential to support endothelial cell survival.

Ioxynil and diethylstilbestrol increase the risks of cardiovascular and thyroid dysfunction in zebrafish

Endocrine disruption results from exposure to chemicals that alter the function of the endocrine system in animals. Chronic 60 days of exposure to a low dose (0.1 μM) of ioxynil (IOX) or diethylstilbestrol (DES) via food was used to determine the effects of these chemicals on the physiology of the heart and thyroid follicles in juvenile zebrafish. Immunofluorescence analysis and subsequent 3D morphometric analysis of the zebrafish heart revealed that chronic exposure to IOX induced ventricle deformation and significant volume increase (p < 0.001). DES exposure caused a change in ventricle morphology, but volume was unaffected. Alongside, it was found that DES exposure upregulated endothelial related genes (angptl1b, mhc1lia, mybpc2a, ptgir, notch1b and vwf) involved in vascular homeostasis. Both IOX and DES exposure caused a change in thyroid follicle morphology. Notably, in IOX exposed juveniles, thyroid follicle hypertrophy was observed; and in DES-exposed fish, an enlarged thyroid field was present. In summary, chronic exposure of juvenile zebrafish to IOX and DES affected the heart and the thyroid. Given that both chemicals are able to change the morphology of the thyroid it indicates that they behave as endocrine disruptive chemicals (EDCs). Heart function dynamically changes thyroid morphology, and function and hence it is likely that the observed cardiac effects of IOX and DES are the source of altered thyroid status in these fish.

Combination Treatment of TRPV4 Agonist with Cisplatin Promotes Vessel Normalization in an Animal Model of Oral Squamous Cell Carcinoma

Background and Objectives: Oral squamous cell carcinoma (OSCC) is the sixth most common malignancy in the world. Transient receptor potential vanilloid 4 (TRPV4) channel has been shown to be involved in angiogenesis in multiple types of tumors. However, not much is known about TRPV4′s involvement in OSCC. Thus, in this study, we investigate the effect of administering a TRPV4 agonist on angiogenesis in OSCC. Materials and Methods: Thirty-six Sprague Dawley (SD) rats were used in this study. 4-nitroquinoline 1-oxide (4NQO) was used to induce OSCC. Cisplatin (an anticancer drug), and GSK1016790A (an agonist for TRPV4) was used in this study. Immunohistochemistry was employed to examine the TRPV4 expression. An RT2 Profiler PCR Array was performed for gene expression analysis of TRPV4, vascular growth factors that correspond directly with angiogenesis, such as angiopoietin (Ang-1 and Ang-2), and tyrosine kinase (Tie-1 and Tie-2) receptors. Tumor vessel maturity was assessed by microvessel density and microvessel-pericyte-coverage index. Results: RT2 profiler PCR array showed significant elevated levels of Ang-1 (2.1-fold change; p < 0.05) and Tie-2 (4.5-fold change; p < 0.05) in OSCC following the administration of a combination of GSK1016790A and cisplatin. Additionally, the combination treatment significantly reduced the microvessel density (p < 0.01) and significantly increased the percentage of microvessels covered with pericytes (p < 0.01) in OSCC. Furthermore, tumor size was significantly reduced (p < 0.05) in rats that received cisplatin alone. The combination treatment also greatly reduced the tumor size; however, the data were not statistically significant. Conclusions: The findings suggest that combining a TRPV4 agonist with cisplatin for treatment of OSCC promote vessels normalization via modulation of Ang-1/Tie-2 pathway.

Gastric Cancer-Derived Extracellular Vesicles (EVs) Promote Angiogenesis via Angiopoietin-2

Simple Summary

Angiogenesis is the formation of new blood vessels, which is essential for gastric cancer growth and metastasis. Angiopoietin-2 is a key driver of tumor angiogenesis and has recently emerged as a promising target for antiangiogenic therapy. Extracellular vesicles play an important role in tumor progression including angiogenesis. We explored the crosstalk between gastric cancer and endothelial cells mediated by vesicles, with a specific focus on angiopoietin-2. We show that primary gastric cancer and omental metastasis tissues express angiopoietin-2. We isolated gastric cancer vesicles and demonstrated that they induce the proliferation, migration, invasion, and tube formation of endothelial cells. Characterization of the angiogenic profile of these vesicles revealed high levels of proangiogenic proteins including angiopoietin-2. Using angiopoietin-2 knockdown, we demonstrate that angiopoietin-2 mediates the proangiogenic effects of the gastric cancer vesicles. Our findings suggest a new mechanism via which gastric cancer cells induce angiogenesis. Such a mechanism may be used as a target for cancer therapy.

Abstract

Angiogenesis is an important control point of gastric cancer (GC) progression and metastasis. Angiopoietin-2 (ANG2) is a key driver of tumor angiogenesis and metastasis, and it has been identified in primary GC tissues. Extracellular vesicles (EVs) play an important role in mediating intercellular communication through the transfer of proteins between cells. However, the expression of ANG2 in GC-EVs has never been reported. Here, we characterized the EV-mediated crosstalk between GC and endothelial cells (ECs), with particular focus on the role of ANG2. We first demonstrate that ANG2 is expressed in GC primary and metastatic tissues. We then isolated EVs from two different GC cell lines and showed that these EVs enhance EC proliferation, migration, invasion, and tube formation in vitro and in vivo. Using an angiogenesis protein array, we showed that GC-EVs contain high levels of proangiogenic proteins, including ANG2. Lastly, using Lenti viral ANG2-shRNA, we demonstrated that the proangiogenic effects of the GC-EVs were mediated by ANG2 through the activation of the PI3K/Akt signal transduction pathway. Our data suggest a new mechanism via which GC cells induce angiogenesis. This knowledge may be utilized to develop new therapies in gastric cancer.

COVID-19 and dys-regulation of pulmonary endothelium: implications for vascular remodeling

Coronavirus disease-2019 (COVID-19), the disease caused by severe acute respiratory syndrome-coronavirus-2, has claimed more than 4.4 million lives worldwide (as of 20 August 2021). Severe cases of the disease often result in respiratory distress due to cytokine storm, and mechanical ventilation is required. Although, the lungs are the primary organs affected by the disease, more evidence on damage to the heart, kidney, and liver is emerging. A common link in these connections is the cardiovascular network. Inner lining of the blood vessels, called endothelium, is formed by a single layer of endothelial cells. Several clinical manifestations involving the endothelium have been reported, such as its activation via immunomodulation, endotheliitis, thrombosis, vasoconstriction, and distinct intussusceptive angiogenesis (IA), a unique and rapid process of blood-vessel formation by splitting a vessel into two lumens. In fact, the virus directly infects the endothelium via TMPRSS2 spike glycoprotein priming to facilitate ACE-2-mediated viral entry. Recent studies have indicated a significant increase in remodeling of the pulmonary vascular bed via intussusception in patients with COVID-19. However, the lack of circulatory biomarkers for IA limits its detection in COVID-19 pathogenesis. In this review, we describe the implications of angiogenesis in COVID-19, unique features of the pulmonary vascular bed and its remodeling, and a rapid and non-invasive assessment of IA to overcome the technical limitations in patients with COVID-19.

The Amino-Terminal Oligomerization Domain of Angiopoietin-2 Affects Vascular Remodeling, Mammary Gland Tumor Growth, and Lung Metastasis in Mice

Angiopoietin-2 (ANGPT2) is a context-dependent TIE2 agonistic or antagonistic ligand that induces diverse responses in cancer. Blocking ANGPT2 provides a promising strategy for inhibiting tumor growth and metastasis, yet variable effects of targeting ANGPT2 have complicated drug development. ANGPT2₄₄₃ is a naturally occurring, lower oligomeric protein isoform whose expression is increased in cancer. Here, we use a knock-in mouse line (mice expressing Angpt2₄₄₃), a genetic model for breast cancer and metastasis (MMTV-PyMT), a syngeneic melanoma lung colonization model (B16F10), and orthotopic injection of E0771 breast cancer cells to show that alternative forms increase the diversity of Angpt2 function. In a mouse retina model of angiogenesis, expression of Angpt2₄₄₃ caused impaired venous development, suggesting enhanced function as a competitive antagonist for Tie2. In mammary gland tumor models, Angpt2₄₄₃ differentially affected primary tumor growth and vascularization; these varying effects were associated with Angpt2 protein localization in the endothelium or in the stromal extracellular matrix as well as the frequency of Tie2-positive tumor blood vessels. In the presence of metastatic cells, Angpt2₄₄₃ promoted destabilization of pulmonary vasculature and lung metastasis. In vitro, ANGPT2₄₄₃ was susceptible to proteolytical cleavage, resulting in a monomeric ligand (ANGPT2_DAP) that inhibited ANGPT1- or ANGPT4-induced TIE2 activation but did not bind to alternative ANGPT2 receptor α5β1 integrin. Collectively, these data reveal novel roles for the ANGPT2 N-terminal domain in blood vessel remodeling, tumor growth, metastasis, integrin binding, and proteolytic regulation. SIGNIFICANCE: This study identifies the role of the N-terminal oligomerization domain of angiopoietin-2 in vascular remodeling and lung metastasis and provides new insights into mechanisms underlying the versatile functions of angiopoietin-2 in cancer.See related commentary by Kamiyama and Augustin, p. 35.

Effects of antioxidant MnTBAP on angiogenesis in newborn mice with hyperoxic lung injury

BACKGROUND

Oxygen toxicity mediated by reactive oxygen species (ROS) plays an essential role in the development of bronchopulmonary dysplasia in premature infants. By reducing oxidative stress, antioxidants protect the immature lung. We studied the effects of MnTBAP, a catalytic antioxidant on angiogenesis and alveolar growth following neonatal hyperoxia.

METHODS

Newborn mouse litters randomized to room air (RA) or >95% O2 for 72 hours from day 4 (D4) to D7 to receive either MnTBAP (10 mg/kg/d) or saline intraperitoneally (every 24 h for three doses). Lungs harvested for angiogenic gene expression, protein expression, and histopathology post-hyperoxia exposure. Radial alveolar count (RAC), mean linear intercept (MLI) and vessel density assessed by histopathology.

RESULTS

Angiogenic gene expression was significantly lower in the hyperoxia group compared to the RA group. The protein expression for VEGF and its receptor, VEGFR1, was significantly lower following treatment with MnTBAP compared to hyperoxia alone. Expression of VEGFR2, Angiopoietin-1 and TIE2, were substantially higher in the RA groups compared to hyperoxia groups with or without MnTBAP. Hyperoxia groups demonstrated alveolar simplification. MnTBAP reduced vessel density and failed to improve alveolar growth following hyperoxia.

CONCLUSIONS

MnTBAP, a catalytic antioxidant, does not offer protection from hyperoxia-induced alveolar impairment. The lack of angiogenic upregulation by MnTBAP may contribute to alveolar simplification in newborn mice.

Gastrointestinal vascular permeability changes following spinal cord injury

BACKGROUND

Gastrointestinal (GI) dysfunction is observed clinically after spinal cord injury (SCI) and contributes to the diminished long-term quality of life. Our study examined the acute and chronic GI vascular changes that occur following SCI. We demonstrated that the GI vascular tract in SCI mice becomes compromised during the acute phase of injury and persists into the chronic phase of injury.

METHODS

Gastrointestinal vasculature permeability was measured using dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) at 48 hours, and 2 and 4 weeks following contusion spinal cord injury. Angiopoietin-1, a vascular stabilizing protein, was administered intravenously following injury. Intestinal contractile activity assessments were performed following the last imaging session.

KEY RESULTS

Our results indicated that a single administration of Ang-1 reduced vascular permeability at 48 hours but the effect was only transient. However, when the treatment paradigm was changed from a single administration to multiple administrations of Ang-1 following contusion injury, our DCE MRI data indicated a significant decrease in GI vascular permeability 4 weeks after injury compared with vehicle control treated animals. This improved GI vascular permeability was associated with improved sustained intestinal contractile activity. We also demonstrated that Ang-1 reduced the expression of sICAM-1 in the ileum compared with the saline-treated group.

CONCLUSIONS AND INFERENCES

We show that the GI vasculature is compromised in the acute and chronic phase of injury following spinal contusion. Our results also indicate that multiple administrations of Ang-1 can attenuate GI vascular permeability, possibly reduce inflammation, and improve sustained agonist-induced contraction compared with saline treatment.

Angiopoietin-1 Knockout Mice as a Genetic Model of Open-Angle Glaucoma

Purpose

A leading cause of blindness worldwide, glaucoma is often caused by elevated intraocular pressure (IOP) due to impaired aqueous humor outflow from the anterior chamber through Schlemm's canal (SC) and the trabecular meshwork. Despite the large clinical burden, glaucoma research and drug development are hindered by a limited selection of preclinical models that accurately recapitulate human disease. Here, we propose that Angpt1 conditional knockout mice may provide one such model. Angiopoietin/TEK (ANGPT/TEK) signaling is crucial for SC formation and integrity in mice and humans, and mice lacking TEK or its ligand ANGPT1 develop a hypomorphic SC insufficient for normal aqueous humor outflow.

Methods

We used a comprehensive histology and physiology approach to characterize the glaucoma phenotype of Angpt1 inducible knockout mice, especially focusing on retina morphology and function.

Results

Angpt1 deletion resulted in persistent ocular hypertension beginning in the first month after birth and leading to decreased visual acuity with age due to glaucomatous neuropathy. In the neural retina, we identified marked and specific loss of the retinal ganglion cells, whereas other retinal neurons exhibited largely normal morphology and patterning. Electroretinogram recordings demonstrated reduced scotopic threshold response, further indicating loss of retinal ganglion cell function.

Conclusions

These findings highlight the potential of Angpt1 conditional knockout mice as a valuable new glaucoma model.

Translational Relevance

Currently, few reliable, rapid-onset genetic glaucoma models are available, and Angpt1 knockout mice will provide an additional tool for studies of IOP-induced neural damage, mechanisms of disease progression, and novel treatment strategies.

Growth factor signaling pathways in vascular development and disease

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

Differential roles of p38 MAPK and ERK1/2 in angiopoietin-2-mediated rat pulmonary microvascular endothelial cell apoptosis induced by lipopolysaccharide

Angiopoietin-2 (Ang-2) is a Tie-2 ligand that destabilizes vascular structures, enhances vascular permeability and induces vascular regression and endothelial cell apoptosis. Although there is evidence for the involvement of the Ang/Tie2 axis in acute lung injury (ALI), the underlying mechanisms involved in Ang-2-induced cell apoptosis are not well understood. In this study, whether Ang-2 contributes to microvascular endothelial cell injury and mediates lipopolysaccharide (LPS)-induced endothelial cell apoptosis and its associated signaling pathways was investigated. Exposure of rat pulmonary microvascular endothelial cells (RPMVECs) to LPS, Ang-2 and related inhibitors was performed to measure the expression levels of Ang-2, the activation of mitogen-activated protein kinases (MAPKs), the phosphorylation of extracellular signal-regulated kinase (ERK)1/2, and expression of the apoptosis-related proteins Bax and Bcl-2 using western blotting, reverse transcription-quantitative polymerase chain reaction, flow cytometry and fluorescence microscopy. The expression of Ang-2 in the RPMVECs was increased by LPS independent of time. The phosphorylation of p38 MAPK and ERK1/2 was significantly upregulated and the activation of apoptosis-related proteins Bax and Bcl was mediated by Ang-2. In addition, inhibition of the p38 pathway by SB203580 attenuated the Ang-2-mediated cell apoptosis, but inhibition of the ERK1/2 pathway by PD98059 exerted an anti-apoptotic effect against Ang-2. In conclusion, LPS-induced apoptosis is partly mediated via stimulation of p38 and ERK1/2 signaling pathways, where Ang-2 acts an inflammation-related factor to participate in the course of cell apoptosis in RPMVECs.

GM-CSF ameliorates microvascular barrier integrity via pericyte-derived Ang-1 in wound healing

Skin wound healing involves complex coordinated interactions of cells, tissues, and mediators. Maintaining microvascular barrier integrity is one of the key events for endothelial homeostasis during wound healing. Vasodilation is observed after vasoconstriction, which causes blood vessels to become porous, facilitates leukocyte infiltration and aids angiogenesis at the wound-area, postinjury. Eventually, vessel integrity has to be reestablished for vascular maturation. Numerous studies have found that granulocyte macrophage colony-stimulating factor (GM-CSF) accelerates wound healing by inducing recruitment of repair cells into the injury area and releases of cytokines. However, whether GM-CSF is involving in the maintaining of microvascular barrier integrity and the underlying mechanism remain still unclear. Aim of this study was to investigate the effects of GM-CSF on modulation of microvascular permeability in wound healing and underlying mechanisms. Wound closure and microvascular leakage was investigated using a full-thickness skin wound mouse model after GM-CSF intervention. The endothelial permeability was measured by Evans blue assay in vivo and in vitro endothelium/pericyte co-culture system using a FITC-Dextran permeability assay. To identify the source of angiopoietin-1 (Ang-1), double staining is used in vivo and ELISA and qPCR are used in vitro. To determine the specific effect of Ang-1 on GM-CSF maintaining microvascular stabilization, Ang-1 siRNA was applied to inhibit Ang-1 production in vivo and in vitro. Wound closure was significantly accelerated and microvascular leakage was ameliorated after GM-CSF treatment in mouse wound sites. GM-CSF decreased endothelial permeability through tightening endothelial junctions and increased Ang-1 protein level that was derived by perictye. Furthermore, applications of siRNAAng-1 inhibited GM-CSF mediated protection of microvascular barrier integrity both in vivo and in vitro. Our data indicate that GM-CSF ameliorates microvascular barrier integrity via pericyte-derived Ang-1 during wound healing.

Tissue Engineering the Vascular Tree

A major hurdle in the field of tissue engineering and regenerative medicine remains the design and construction of larger (> 1 cm3) in vitro tissues for biological studies and transplantation. While there has been success in creating three-dimensional (3D) capillary networks, relatively large arteries (diameter >3-5 mm), and more recently small arteries (diameter 500 μm-1 mm), there has been no success in the creation of a living dynamic blood vessel network comprising of arterioles (diameter 40-300 μm), capillaries, and venules. Such a network would provide the foundation to supply nutrients and oxygen to all surrounding cells for larger tissues and organs that require a hierarchical vascular supply. In this study, we describe the different technologies and methods that have been employed in an effort to create individual vessels and networks of vessels to support engineered tissues for in vivo and in vitro applications. A special focus is placed on the generation of blood vessels with average dimensions that span from microns (capillaries) to a millimeter (large arterioles). We also identify major challenges while exploring new opportunities to create model systems of the entire vascular tree, including arterioles and venules.

Angiopoietin-1 is required for Schlemm's canal development in mice and humans

Primary congenital glaucoma (PCG) is a leading cause of blindness in children worldwide and is caused by developmental defects in 2 aqueous humor outflow structures, Schlemm's canal (SC) and the trabecular meshwork. We previously identified loss-of-function mutations in the angiopoietin (ANGPT) receptor TEK in families with PCG and showed that ANGPT/TEK signaling is essential for SC development. Here, we describe roles for the major ANGPT ligands in the development of the aqueous outflow pathway. We determined that ANGPT1 is essential for SC development, and that Angpt1-knockout mice form a severely hypomorphic canal with elevated intraocular pressure. By contrast, ANGPT2 was dispensable, although mice deficient in both Angpt1 and Angpt2 completely lacked SC, indicating that ANGPT2 compensates for the loss of ANGPT1. In addition, we identified 3 human subjects with rare ANGPT1 variants within an international cohort of 284 PCG patients. Loss of function in 2 of the 3 patient alleles was observed by functional analysis of ANGPT1 variants in a combined in silico, in vitro, and in vivo approach, supporting a causative role for ANGPT1 in disease. By linking ANGPT1 with PCG, these results highlight the importance of ANGPT/TEK signaling in glaucoma pathogenesis and identify a candidate target for therapeutic development.

Angiopoietin-Tie signalling in the cardiovascular and lymphatic systems

Endothelial cells that form the inner layer of blood and lymphatic vessels are important regulators of vascular functions and centrally involved in the pathogenesis of vascular diseases. In addition to the vascular endothelial growth factor (VEGF) receptor pathway, the angiopoietin (Ang)-Tie system is a second endothelial cell specific ligand-receptor signalling system necessary for embryonic cardiovascular and lymphatic development. The Ang-Tie system also regulates postnatal angiogenesis, vessel remodelling, vascular permeability and inflammation to maintain vascular homoeostasis in adult physiology. This system is implicated in numerous diseases where the vasculature has an important contribution, such as cancer, sepsis, diabetes, atherosclerosis and ocular diseases. Furthermore, mutations in the TIE2 signalling pathway cause defects in vascular morphogenesis, resulting in venous malformations and primary congenital glaucoma. Here, we review recent advances in the understanding of the Ang-Tie signalling system, including cross-talk with the vascular endothelial protein tyrosine phosphatase (VE-PTP) and the integrin cell adhesion receptors, focusing on the Ang-Tie system in vascular development and pathogenesis of vascular diseases.

Therapeutic targeting of the angiopoietin-TIE pathway

The endothelial angiopoietin (ANG)-TIE growth factor receptor pathway regulates vascular permeability and pathological vascular remodelling during inflammation, tumour angiogenesis and metastasis. Drugs that target the ANG-TIE pathway are in clinical development for oncological and ophthalmological applications. The aim is to complement current vascular endothelial growth factor (VEGF)-based anti-angiogenic therapies in cancer, wet age-related macular degeneration and macular oedema. The unique function of the ANG-TIE pathway in vascular stabilization also renders this pathway an attractive target in sepsis, organ transplantation, atherosclerosis and vascular complications of diabetes. This Review covers key aspects of the function of the ANG-TIE pathway in vascular disease and describes the recent development of novel therapeutics that target this pathway.

Activated protein C binds directly to Tie2: possible beneficial effects on endothelial barrier function

Activated protein C (APC) is a natural anticoagulant with strong anti-inflammatory, anti-apoptotic, and barrier stabilizing properties. These cytoprotective properties of APC are thought to be exerted through its pathway involving the binding of APC to endothelial protein C receptor and cleavage of protease-activated receptors. In this study, we found that APC enhanced endothelial barrier integrity via a novel pathway, by binding directly to and activating Tie2, a transmembrane endothelial tyrosine kinase receptor. Binding assays demonstrated that APC competed with the only known ligands of Tie2, the angiopoietins (Angs). APC bound directly to Tie2 (Kd ~3 nM), with markedly stronger binding affinity than Ang2. After binding, APC rapidly activated Tie2 to enhance endothelial barrier function as shown by Evan's blue dye transfer across confluent cell monolayers and in vivo studies. Blocking Tie2 restricted endothelial barrier integrity. This study highlights a novel mechanism by which APC binds directly to Tie2 to enhance endothelial barrier integrity, which helps to explain APC's protective effects in vascular leakage-related pathologies.

Phase Ib Safety, Two-Dose Study of MultiGeneAngio in Patients with Chronic Critical Limb Ischemia

Critical limb ischemia (CLI) is the most severe presentation of peripheral arterial disease. We developed cell-based therapy entailing intra-arterial injection of autologous venous endothelial cells (ECs) modified to express angiopoietin 1, combined with autologous venous smooth muscle cells (SMCs) modified to express vascular endothelial growth factor. This combination promoted arteriogenesis in animal models and was safe in patients with limiting claudication. In an open-label, phase Ib study, we assessed the safety and efficacy of this therapy in CLI patients who failed or were unsuitable for surgery or intravascular intervention. Of 23 patients enrolled, 18 with rest pain or non-healing ulcers (Rutherford categories 4 and 5) were treated according to protocol, and 5 with significant tissue loss (Rutherford 6) were treated under compassionate treatment. Patients were assigned randomly to receive 1 × 10⁷ or 5 × 10⁷ (EC-to-SMC ratio, 1:1) of the cell combination. One-year amputation-free survival rate was 72% (13/18) for Rutherford 4 and 5 patients; all 5 patients with Rutherford 6 underwent amputation. Of the 12 with unhealing ulcers at dosing, 6 had complete healing and 2 others had >66% reduction in ulcer size. Outcomes did not differ between the dose groups. No severe adverse events were observed related to the therapy.

Endothelial-Specific EphA4 Negatively Regulates Native Pial Collateral Formation and Re-Perfusion following Hindlimb Ischemia

Leptomeningeal anastomoses play a critical role in regulating vascular re-perfusion following obstruction, however, the mechanisms regulating their development remains under investingation. Our current findings indicate that EphA4 receptor is a novel negative regulator of collaterogenesis. We demonstrate that EphA4 is highly expressed on pial arteriole collaterals at post-natal day (P) 1 and 7, then significantly reduced by P21. Endothelial cell (EC)-specific loss of EphA4, EphA4f/f/Tie2::Cre (KO), resulted in an increase in the density but not diameter of pial collaterals compared to WT mice. ECs isolated from KO mice displayed a 3-fold increase in proliferation, enhanced migration, tube formation and elevated levels of phospho(p)-Akt compared to WT ECs. Attenuating p-Akt, using LY294002, reduced the proliferative and migration effects in the KO ECs. RNAseq analysis also revealed altered expression patterns for genes that regulate cell proliferation, vascular development, extracellular matrix and immune-mediate responses, namely MCP-1, MMP2 and angiopoietin-1. Lastly, we show that induction of hindlimb ischemia resulted in accelerated re-perfusion, collateral remodeling and reduced tissue necrosis in the absence of EC-specific EphA4 compared to WT mice. These findings demonstrate a novel role for EphA4 in the early development of the pial collateral network and suggests a role in regulating vascular remodeling after obstruction.

A phase I trial of ANG1/2-Tie2 inhibitor trebaninib (AMG386) and temsirolimus in advanced solid tumors (PJC008/NCI♯9041)

Background There is crosstalk between the ANG-Tie2 and the PI3K/Akt/mTOR pathways. Combined ANG1/2 and mTOR blockade may have additive anti-cancer activity. The combination of trebananib, an inhibitor of ANG1/2-Tie2 interaction, with temsirolimus was evaluated in patients with advanced solid tumors to determine tolerability, maximum tolerated dose (MTD), and preliminary antitumor activity. Methods Patients were enrolled using 3 + 3 design, and were given intravenous trebananib and temsirolimus on Day 1, 8, 15 and 22 of a 28-day cycle. Dose limiting toxicities (DLTs) were evaluated during cycle 1. Peripheral blood was collected for evaluation of Tie2-expressing monocytes (TEMs) and thymidine phosphorylase (TP). Sparse pharmacokinetic (PK) sampling for trebananib drug levels was performed on Day 1 and 8 of cycle 2. Results Twenty-one patients were enrolled, 6 at dose level (DL) 1, 7 at DL −1, and 8 at DL −2. No effect of temsirolimus on trebananib PK was observed. The most common treatment-related adverse events (AEs) were: fatigue (81 %), edema (62 %), anorexia (57 %), nausea (52 %), rash (43 %) and mucositis (43 %). The most common grade ≥ 3 AEs included lymphopenia (28 %) and fatigue (28 %). The MTD was exceeded at DL-2. Of 18 response evaluable patients, 1 partial response was observed (ER+/HER2−/PIK3CA mutant breast cancer) and 4 patients had prolonged SD ≥ 24 weeks. No correlation with clinical benefit was observed with change in number TEMs or TP expression in TEMs with treatment. Conclusions The MTD was exceeded at trebananib 10 mg/kg weekly and temsirolimus 20 mg weekly, with frequent overlapping toxicities including fatigue, edema, and anorexia.

Treatment of newborn mice with inhibitors of vascular endothelial growth factor receptor tyrosine kinase induces abnormal retinal vascular patterning

We have previously reported that treatment of newborn mice with KRN633, a vascular endothelial growth factor (VEGF) receptor tyrosine kinase inhibitor, delayed retinal vascularization leading to abnormal retinal vascular growth and patterns. To determine whether similar abnormalities are observed in newborn mice treated with other VEGF receptor tyrosine kinase inhibitors, we administered axitinib to mice on the day of birth and on the following day. When compared with control pups, a significant delay in retinal vascularization was observed in pups treated with axitinib (5 mg/kg). Axitinib-treated pups had a very dense capillary network on postnatal day (P) 6 and fewer central arteries and veins on P8 and P12. Central veins, but not arteries, were significantly enlarged on P8. These abnormalities were similar to those observed in KRN633-treated pups and probably represent a common phenotype induced by short-term treatment with VEGF receptor inhibitors in newborn mice. Therefore, mice treated postnatally with VEGFR inhibitors could serve as an animal model for studying the mechanisms of retinal vascular formation and patterning.

PTK7+ Mononuclear Cells Express VEGFR2 and Contribute to Vascular Stabilization by Upregulating Angiopoietin-1

OBJECTIVE

In angiogenesis, circulating mononuclear cells are recruited to vascular lesions; however, the underlying mechanisms are poorly understood.

APPROACH AND RESULTS

Here, we characterize the functional role of protein tyrosine kinase 7 (PTK7)-expressing CD11b(+) mononuclear cells in vitro and in vivo using a mouse model of angiogenesis. Although the frequencies of PTK7(+)CD11b(+) cells in the bone marrow remained similar after vascular endothelial growth factor-A-induced neovascularization, we observed an 11-fold increase in the cornea. Importantly, vascular endothelial growth factor-A-induced chemotaxis of PTK7(+) cells was mediated by vascular endothelial growth factor receptor 2. In a coculture with endothelial cells, PTK7(+)CD11b(+) cells stabilized the vascular network for 2 weeks by expressing high levels of angiopoietin-1. The enhanced vascular stability was abolished by knockdown of angiopoietin-1 in PTK7(+)CD11b(+) cells and could be restored by angiopoietin-1 treatment.

CONCLUSIONS

We conclude that PTK7 expression in perivascular mononuclear cells induces vascular endothelial growth factor receptor 2 and angiopoietin-1 expression and thus contributes to vascular stabilization in angiogenesis.

R3 receptor tyrosine phosphatases: conserved regulators of receptor tyrosine kinase signaling and tubular organ development

R3 receptor tyrosine phosphatases (RPTPs) are characterized by extracellular domains composed solely of long chains of fibronectin type III repeats, and by the presence of a single phosphatase domain. There are five proteins in mammals with this structure, two in Drosophila and one in Caenorhabditis elegans. R3 RPTPs are selective regulators of receptor tyrosine kinase (RTK) signaling, and a number of different RTKs have been shown to be direct targets for their phosphatase activities. Genetic studies in both invertebrate model systems and in mammals have shown that R3 RPTPs are essential for tubular organ development. They also have important functions during nervous system development. R3 RPTPs are likely to be tumor suppressors in a number of types of cancer.

Isoxanthohumol modulates angiogenesis and inflammation via vascular endothelial growth factor receptor, tumor necrosis factor alpha and nuclear factor kappa B pathways

Angiogenesis and inflammation are becoming distinguished players in the pathogenesis of many heterogeneous diseases, such as diabetes, cardiovascular disease, and cancer. Therefore, it is crucial to study new compounds that are able to modulate these events. Isoxanthohumol (IXN) is a polyphenol with antioxidant, anti-inflammatory, and antiangiogenic properties. The aim of this study was to evaluate the effects of IXN on blood vessel proliferation and maturation and describe underlying molecular mechanisms in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). Angiogenic profile of IXN was analyzed by retinal angiogenesis at different time points. IXN modulation of angiogenic and inflammatory signaling pathways was evaluated by Western blotting on EC and VSMC cultures. IXN inhibited by 20% sprouting angiogenesis and decreased vascular coverage by mural cells up to 39%. IXN of 10 µM also decreased inflammatory signals, namely tumor necrosis factor alpha (TNF-α) (26 and 40%) and factor nuclear kappa B (24 and 42%) in human aortic smooth muscle cells (HASMCs) and human umbilical vein endothelial cells (HUVECs). Angiogenic regulators, including vascular endothelial growth factor receptor 2 (HUVEC, 55%), angiopoietins 1 (HUVEC, 39%; HASMC, 35%), angiopoietin 2 (HUVEC, 38%), and Tie2 (HUVEC, 56%) were also inhibited by 10 µM of IXN treatments. Akt activation was reduced by 47% in HUVEC-treated cells and Erk activation was also reduced by 52 and 69% upon IXN treatment of HUVEC and HASMC. IXN seems to regulate in vivo vascular proliferation and stabilization and the EC-VSMC-inflammatory crosstalk, leaving this molecule as an interesting nutritional player for angiogenesis and inflammation-related diseases.

Microvascular repair: post-angiogenesis vascular dynamics

Vascular compromise and the accompanying perfusion deficits cause or complicate a large array of disease conditions and treatment failures. This has prompted the exploration of therapeutic strategies to repair or regenerate vasculatures, thereby establishing more competent microcirculatory beds. Growing evidence indicates that an increase in vessel numbers within a tissue does not necessarily promote an increase in tissue perfusion. Effective regeneration of a microcirculation entails the integration of new stable microvessel segments into the network via neovascularization. Beginning with angiogenesis, neovascularization entails an integrated series of vascular activities leading to the formation of a new mature microcirculation, and includes vascular guidance and inosculation, vessel maturation, pruning, AV specification, network patterning, structural adaptation, intussusception, and microvascular stabilization. While the generation of new vessel segments is necessary to expand a network, without the concomitant neovessel remodeling and adaptation processes intrinsic to microvascular network formation, these additional vessel segments give rise to a dysfunctional microcirculation. While many of the mechanisms regulating angiogenesis have been detailed, a thorough understanding of the mechanisms driving post-angiogenesis activities specific to neovascularization has yet to be fully realized, but is necessary to develop effective therapeutic strategies for repairing compromised microcirculations as a means to treat disease.

Sequential, timely and controlled expression of hVEGF165 and Ang-1 effectively improves functional angiogenesis and cardiac function in vivo

We aimed to control the gene expression of vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang-1) in the ischemic heart to explore the feasibility of sequential, timely and controlled multigene expression as a means of improving therapeutic angiogenesis in vivo. Adult rabbit myocardial infarction models were surgically established (n=120). Hypoxia-inducible factor-1α-hypoxic response element (HIF1α-HRE) and Tet (tetracycline)-On advanced gene control systems were reconstructed for controlled expression of the human VEGF165 (hVEGF165) and Ang-1 genes, respectively. Recombinant adeno-associated viruses (rAAV)-9HRE-hVEGF165 and rAAV-TRE-Tight-Ang-1 were delivered into the ischemic myocardium for 12 weeks. Reverse transcription-polymerase chain reaction, western blotting and immunofluorescence staining were used to detect gene and protein expression. Vessel functionality, vascular permeability and animal cardiac function were also evaluated. Under the control of the HIF1α-HRE and Tet-On gene control systems, the expression of the exogenous hVEGF165 and Ang-1 genes was consistent in the ischemia control. In the sequential group, we found that the number of functional vessels with a larger diameter and more vascular branches was increased, and vascular permeability was significantly reduced. In addition, animal heart function was significantly improved compared with the non-sequential and hVEGF165- or Ang-1-only groups (P<0.05, P<0.05, respectively). Sequential, timely and controlled expression of the hVEGF165 and Ang-1 genes in vivo is a new therapeutic angiogenesis strategy that can effectively promote functional vessel regeneration and can improve cardiac function in ischemic heart disease.

New insights into the developmental mechanisms of coronary vessels and epicardium

During heart development, the epicardium, which originates from the proepicardial organ (PE), is a source of coronary vessels. The PE develops from the posterior visceral mesoderm of the pericardial coelom after stimulation with a combination of weak bone morphogenetic protein and strong fibroblast growth factor (FGF) signaling. PE-derived cells migrate across the heart surface to form the epicardial sheet, which subsequently seeds multipotent subepicardial mesenchymal cells via epithelial-mesenchymal transition, which is regulated by several signaling pathways including retinoic acid, FGF, sonic hedgehog, Wnt, transforming growth factor-β, and platelet-derived growth factor. Subepicardial endothelial progenitors eventually generate the coronary vascular plexus, which acquires an arterial or venous phenotype, connects with the sinus venosus and aortic sinuses, and then matures through the recruitment of vascular smooth muscle cells under the regulation of complex growth factor signaling pathways. These developmental programs might be activated in the adult heart after injury and play a role in the regeneration/repair of the myocardium.

Enhanced Angpt1/Tie2 signaling affects the differentiation and long-term repopulation ability of hematopoietic stem cells

Angiopoietin-1 (Angpt1) signaling via the Tie2 receptor regulates vascular and hematopoietic systems. To investigate the role of Angpt1-Tie2 signaling in hematopoiesis, we prepared conditionally inducible transgenic (Tg) mice expressing a genetically engineered Angpt1, cartridge oligomeric matrix protein (COMP)-Angpt1. The effects of COMP-Angpt1 overexpression in osteoblasts on hematopoiesis were then investigated by crossing COMP-Angpt1 Tg mice with Col1a1-Cre Tg mice. Interestingly, peripheral blood analyses showed that 4 week (wk)-old (but not 8 wk-old) Col1a1-Cre+/COMP-Angpt1+ mice had a lower percentage of circulating B cells and a higher percentage of myeloid cells than Col1a1-Cre-/COMP-Angpt1+ (control) mice. Although there were no significant differences in the immunophenotypic hematopoietic stem and progenitor cell (HSPC) populations between Col1a1-Cre+/COMP-Angpt1+ and control mice, lineage(-)Sca-1(+)c-Kit(+) (LSK) cells isolated from 8 wk-old Col1a1-Cre+/COMP-Angpt1+ mice showed better long-term bone marrow reconstitution ability. These data indicate that Angpt1-Tie2 signaling affects the differentiation capacity of hematopoietic lineages during development and increases the stem cell activity of HSCs.

The complex role of angiopoietin-2 in the angiopoietin-tie signaling pathway

The angiopoietin-Tie signaling system is a vascular-specific receptor tyrosine kinase pathway that is essential for normal vascular development. Although the basic functioning of the pathway is understood, many uncertainties remain about the role of certain members of the pathway, particularly angiopoietin-2 (Ang2), in pathological vascular remodeling and angiogenesis. We summarize the components of the angiopoietin-Tie pathway and then focus on studies that highlight the role of Ang2 in disease settings, including cancer and inflammation. The expression of Ang2 is elevated in many cancers and types of inflammation, which prompted the development of specific reagents to block its interaction with the Tie2 receptor. The application of these reagents in preclinical models of inflammation and cancer has begun to elucidate the role of Ang2 in vascular remodeling and disease pathogenesis and has led to emerging clinical tests of Ang2 inhibitors.

Coexpression of angiopoietin-1 with VEGF increases the structural integrity of the blood-brain barrier and reduces atrophy volume

Vascular endothelial growth factor (VEGF)-induced neovasculature is immature and leaky. We tested if coexpression of angiopoietin-1 (ANG1) with VEGF improves blood-brain barrier (BBB) integrity and VEGF neuroprotective and neurorestorative effects using a permanent distal middle cerebral artery occlusion (pMCAO) model. Adult CD-1 mice were injected with 2 × 10(9) virus genomes of adeno-associated viral vectors expressing VEGF (AAV-VEGF) or ANG1 (AAV-ANG1) individually or together in a 1:1 ratio into the ischemic penumbra 1 hour after pMCAO. AAV-LacZ was used as vector control. Samples were collected 3 weeks later. Compared with AAV-LacZ, coinjection of AAV-VEGF and AAV-ANG1 reduced atrophy volume (46%, P=0.004); injection of AAV-VEGF or AAV-ANG1 individually reduced atrophy volume slightly (36%, P=0.08 and 33%, P=0.09, respectively). Overexpression of VEGF reduced tight junction protein expression and increased Evans blue extravasation. Compared with VEGF expression alone, coexpression of ANG1 with VEGF resulted in upregulation of tight junction protein expression and reduction of Evans blue leakage (AAV-ANG1/AAV-VEGF: 1.4 ± 0.3 versus AAV-VEGF: 2.8 ± 0.7, P=0.001). Coinjection of AAV-VEGF and AAV-ANG1 induced a similar degree of angiogenesis as injection of AAV-VEGF alone (P=0.85). Thus, coexpression of ANG1 with VEGF improved BBB integrity and resulted in better neuroprotection compared with VEGF expression alone.

Pericytes: developmental, physiological, and pathological perspectives, problems, and promises

Pericytes, the mural cells of blood microvessels, have recently come into focus as regulators of vascular morphogenesis and function during development, cardiovascular homeostasis, and disease. Pericytes are implicated in the development of diabetic retinopathy and tissue fibrosis, and they are potential stromal targets for cancer therapy. Some pericytes are probably mesenchymal stem or progenitor cells, which give rise to adipocytes, cartilage, bone, and muscle. However, there is still confusion about the identity, ontogeny, and progeny of pericytes. Here, we review the history of these investigations, indicate emerging concepts, and point out problems and promise in the field of pericyte biology.

Thyroid hormone promotes remodeling of coronary resistance vessels

Low thyroid hormone (TH) function has been linked to impaired coronary blood flow, reduced density of small arterioles, and heart failure. Nonetheless, little is known about the mechanisms by which THs regulate coronary microvascular remodeling. The current study examined the initial cellular events associated with coronary remodeling induced by triiodothyronine (T3) in hypothyroid rats. Rats with established hypothyroidism, eight weeks after surgical thyroidectomy (TX), were treated with T3 for 36 or 72 hours. The early effects of T3 treatment on coronary microvasculature were examined morphometrically. Gene expression changes in the heart were assessed by quantitative PCR Array. Hypothyroidism resulted in arteriolar atrophy in the left ventricle. T3 treatment rapidly induced small arteriolar muscularization and, within 72 hours, restored arteriolar density to control levels. Total length of the capillary network was not affected by TX or T3 treatment. T3 treatment resulted in the coordinate regulation of Angiopoietin 1 and 2 expression. The response of Angiopoietins was consistent with vessel enlargement. In addition to the well known effects of THs on vasoreactivity, these results suggest that THs may affect function of small resistance arteries by phenotypic remodeling of vascular smooth muscle cells (VSMC).

Pericyte requirement for anti-leak action of angiopoietin-1 and vascular remodeling in sustained inflammation

Blood vessel leakiness is an early, transient event in acute inflammation but can also persist as vessels undergo remodeling in sustained inflammation. Angiopoietin/Tie2 signaling can reduce the leakiness through changes in endothelial cells. The role of pericytes in this action has been unknown. We used the selective PDGF-B-blocking oligonucleotide aptamer AX102 to determine whether disruption of pericyte-endothelial crosstalk alters vascular leakiness or remodeling in the airways of mice under four different conditions: i) baseline, ii) acute inflammation induced by bradykinin, iii) sustained inflammation after 7-day infection by the respiratory pathogen Mycoplasma pulmonis, or iv) leakage after bradykinin challenge in the presence of vascular stabilization by the angiopoietin-1 (Ang1) mimic COMP-Ang1 for 7 days. AX102 reduced pericyte coverage but did not alter the leakage of microspheres from tracheal blood vessels at baseline or after bradykinin; however, AX102 exaggerated leakage at 7 days after M. pulmonis infection and increased vascular remodeling and disease severity at 14 days. AX102 also abolished the antileakage effect of COMP-Ang1 at 7 days. Together, these findings show that pericyte contributions to endothelial stability have greater dependence on PDGF-B during the development of sustained inflammation, when pericyte dynamics accompany vascular remodeling, than under baseline conditions or in acute inflammation. The findings also show that the antileakage action of Ang1 requires PDGF-dependent actions of pericytes in maintaining endothelial stability.

Targeting microvasculature for neuroprotection after SCI

Spinal cord injury (SCI) is characterized by secondary degeneration, which leads to tissue loss at the epicenter and subsequent functional deficits. This review provides insight into the pathophysiology of microvascular dysfunction and endothelial cell loss, which are among the earliest responses during the first postinjury day. The enigmatic role of the angiogenic response in the penumbra around the lost tissue, which occurs during the first 2 weeks, is also discussed. The importance of stabilizing and rescuing the injured vasculature is now well-recognized, and several pharmacological and genetic treatments have emerged in the past few years. We conclude with suggestions for future experimental research, including development of vascular-selective treatments and exploitation of genetic models. In summary, vascular dysfunction following SCI is an important contributor to neurological deficits, as proposed long ago. However, there now appears to be new and potentially powerful opportunities for treating acute SCI by targeting the vascular responses.

Context-dependent role of angiopoietin-1 inhibition in the suppression of angiogenesis and tumor growth: implications for AMG 386, an angiopoietin-1/2-neutralizing peptibody

AMG 386 is an investigational first-in-class peptide-Fc fusion protein (peptibody) that inhibits angiogenesis by preventing the interaction of angiopoietin-1 (Ang1) and Ang2 with their receptor, Tie2. Although the therapeutic value of blocking Ang2 has been shown in several models of tumorigenesis and angiogenesis, the potential benefit of Ang1 antagonism is less clear. To investigate the consequences of Ang1 neutralization, we have developed potent and selective peptibodies that inhibit the interaction between Ang1 and its receptor, Tie2. Although selective Ang1 antagonism has no independent effect in models of angiogenesis-associated diseases (cancer and diabetic retinopathy), it induces ovarian atrophy in normal juvenile rats and inhibits ovarian follicular angiogenesis in a hormone-induced ovulation model. Surprisingly, the activity of Ang1 inhibitors seems to be unmasked in some disease models when combined with Ang2 inhibitors, even in the context of concurrent vascular endothelial growth factor inhibition. Dual inhibition of Ang1 and Ang2 using AMG 386 or a combination of Ang1- and Ang2-selective peptibodies cooperatively suppresses tumor xenograft growth and ovarian follicular angiogenesis; however, Ang1 inhibition fails to augment the suppressive effect of Ang2 inhibition on tumor endothelial cell proliferation, corneal angiogenesis, and oxygen-induced retinal angiogenesis. In no case was Ang1 inhibition shown to (a) confer superior activity to Ang2 inhibition or dual Ang1/2 inhibition or (b) antagonize the efficacy of Ang2 inhibition. These results imply that Ang1 plays a context-dependent role in promoting postnatal angiogenesis and that dual Ang1/2 inhibition is superior to selective Ang2 inhibition for suppression of angiogenesis in some postnatal settings.

Tumour vasculature and angiogenic profile of paediatric pilocytic astrocytoma; is it much different from glioblastoma?

AIMS

Pilocytic astrocytomas are the most frequent brain tumours in children. Because of their high vascularity, this study aimed to obtain insights into potential angiogenic related therapeutic targets in these tumours by characterization of the vasculature and the angiogenic profile. In this study 59 paediatric pilocytic astrocytomas were compared with 62 adult glioblastomas, as a prototype of tumour angiogenesis.

METHODS

Microvessel density, vessel maturity in terms of basement membrane and pericyte coverage, and turnover of both endothelial and tumour cells, and vascular endothelial growth factor (VEGF) expression were evaluated in tumour tissue, immunohistochemically stained with, respectively, CD34, collagen IV, smooth muscle actin, Ki67/CD34, caspase-3/CD34 and VEGF(-A-D). As an indicator for vessel stability the angiopoietin (ANGPT)-1/ANGPT-2 balance was calculated using Real Time RT-PCR.

RESULTS

Pilocytic astrocytoma and glioblastoma showed similar fractions of vessels covered with basement membrane and pericytes. Overlapping ANGPT-1/ANGPT-2 balance and VEGF-A expression were found. Pilocytic astrocytoma had fewer but wider vessels compared with glioblastoma. Turnover of endothelial and tumour cells were relatively lower in pilocytic astrocytoma. Within pilocytic astrocytoma, higher ANGPT-1/ANGPT-2 balance was correlated with fewer apoptotic endothelial cells. Lower numbers of vessels were correlated with higher VEGF-A expression.

CONCLUSIONS

Despite the fact that pilocytic astrocytoma showed a different vessel architecture compared with glioblastoma, a critical overlap in vessel immaturity/instability and the angiogenic profile was seen between both tumours. These findings suggest encouraging possibilities for targeting angiogenesis (for instance with anti-VEGF) as a therapeutic strategy in pilocytic astrocytoma.

Loss of cerebral cavernous malformation 3 (Ccm3) in neuroglia leads to CCM and vascular pathology

Communication between neural cells and the vasculature is integral to the proper development and later function of the central nervous system. A mechanistic understanding of the interactions between components of the neurovascular unit has implications for various disorders, including cerebral cavernous malformations (CCMs) in which focal vascular lesions form throughout the central nervous system. Loss of function mutations in three genes with proven endothelial cell autonomous roles, CCM1/krev1 interaction trapped gene 1, CCM2, and CCM3/programmed cell death 10, cause familial CCM. By using neural specific conditional mouse mutants, we show that Ccm3 has both neural cell autonomous and nonautonomous functions. Gfap- or Emx1-Cre-mediated Ccm3 neural deletion leads to increased proliferation, increased survival, and activation of astrocytes through cell autonomous mechanisms involving activated Akt signaling. In addition, loss of neural CCM3 results in a vascular phenotype characterized by diffusely dilated and simplified cerebral vasculature along with formation of multiple vascular lesions that closely resemble human cavernomas through cell nonautonomous mechanisms. RNA sequencing of the vascular lesions shows abundant expression of molecules involved in cytoskeletal remodeling, including protein kinase A and Rho-GTPase signaling. Our findings implicate neural cells in the pathogenesis of CCMs, showing the importance of this pathway in neural/vascular interactions within the neurovascular unit.

Rapid remodeling of airway vascular architecture at birth

Recent advances have documented the development of lung vasculature before and after birth, but less is known of the growth and maturation of airway vasculature. We sought to determine whether airway vasculature changes during the perinatal period and when the typical adult pattern develops. On embryonic day 16.5 mouse tracheas had a primitive vascular plexus unlike the adult airway vasculature, but instead resembling the yolk sac vasculature. Soon after birth (P0), the primitive vascular plexus underwent abrupt and extensive remodeling. Blood vessels overlying tracheal cartilage rings regressed from P1 to P3 but regrew from P4 to P7 to form the hierarchical, segmented, ladder-like adult pattern. Hypoxia and HIF-1α were present in tracheal epithelium over vessels that survived but not where they regressed. These findings reveal the plasticity of airway vasculature after birth and show that these vessels can be used to elucidate factors that promote postnatal vascular remodeling and maturation.

Selective activation of sphingosine 1-phosphate receptors 1 and 3 promotes local microvascular network growth

Proper spatial and temporal regulation of microvascular remodeling is critical to the formation of functional vascular networks, spanning the various arterial, venous, capillary, and collateral vessel systems. Recently, our group has demonstrated that sustained release of sphingosine 1-phosphate (S1P) from biodegradable polymers promotes microvascular network growth and arteriolar expansion. In this study, we employed S1P receptor-specific compounds to activate and antagonize different combinations of S1P receptors to elucidate those receptors most critical for promotion of pharmacologically induced microvascular network growth. We show that S1P(1) and S1P(3) receptors act synergistically to enhance functional network formation via increased functional length density, arteriolar diameter expansion, and increased vascular branching in the dorsal skinfold window chamber model. FTY720, a potent activator of S1P(1) and S1P(3), promoted a 107% and 153% increase in length density 3 and 7 days after implantation, respectively. It also increased arteriolar diameters by 60% and 85% 3 and 7 days after implantation. FTY720-stimulated branching in venules significantly more than unloaded poly(D, L-lactic-co-glycolic acid). When implanted on the mouse spinotrapezius muscle, FTY720 stimulated an arteriogenic response characterized by increased tortuosity and collateralization of branching microvascular networks. Our results demonstrate the effectiveness of S1P(1) and S1P(3) receptor-selective agonists (such as FTY720) in promoting microvascular growth for tissue engineering applications.

Angiopoietin-2-driven vascular remodeling in airway inflammation

Vascular remodeling is a feature of chronic inflammation during which capillaries transform into venules that expand the region of the vasculature in which leakage and leukocyte emigration both occur. Recently, we found that angiopoietin/Tie2 receptor signaling drives the transformation of capillaries into venules at an early stage of the sustained inflammatory response in the airways of mice infected with Mycoplasma pulmonis. However, the precise contributions of both angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) are not clear. In this study, we sought to determine the contribution of Ang2 to this vascular remodeling. Ang2 mRNA expression levels increased and phosphorylated Tie2 immunoreactivity in mucosal blood vessels decreased, indicative of diminished receptor signaling after infection. Selective inhibition of Ang2 throughout the infection by administration of either of two distinct function-blocking antibodies reduced the suppression of Tie2 phosphorylation and decreased the remodeling of mucosal capillaries into venules, the amount of leukocyte influx, and disease severity. These findings are consistent with Ang2 acting as an antagonist of Tie2 receptors and the reduction of Tie2 phosphorylation in endothelial cells rendering the vasculature more responsive to cytokines that promote both vascular remodeling and the consequences of inflammation after M. pulmonis infection. By blocking such changes, Ang2 inhibitors may prove beneficial in the treatment of sustained inflammation in which vascular remodeling, leakage, and leukocyte influx contribute to its pathophysiology.

Formation of cardiovascular tubes in invertebrates and vertebrates

The cardiovascular system developed early in evolution and is pivotal for the transport of oxygen, nutrients, and waste products within the organism. It is composed of hollow tubular structures and has a high level of complexity in vertebrates. This complexity is, at least in part, due to the endothelial cell lining of vertebrate blood vessels. However, vascular lumen formation by endothelial cells is still controversially discussed. For example, it has been suggested that the lumen mainly forms via coalescence of large intracellular vacuoles generated by pinocytosis. Alternatively, it was proposed that the vascular lumen initiates extracellularly between adjacent apical endothelial cell surfaces. Here we discuss invertebrate and vertebrate cardiovascular lumen formation and highlight the possible modes of blood vessel formation. Finally, we point to the importance of a better understanding of vascular lumen formation for treating human pathologies, including cancer and coronary heart disease.