Blocking VEGFR-3 suppresses angiogenic sprouting and vascular network formation

Endothelial Ca 2+ oscillations reflect VEGFR signaling-regulated angiogenic capacity in vivo

Sprouting angiogenesis is a well-coordinated process controlled by multiple extracellular inputs, including vascular endothelial growth factor (VEGF). However, little is known about when and how individual endothelial cell (EC) responds to angiogenic inputs in vivo. Here, we visualized endothelial Ca²⁺ dynamics in zebrafish and found that intracellular Ca²⁺ oscillations occurred in ECs exhibiting angiogenic behavior. Ca²⁺ oscillations depended upon VEGF receptor-2 (Vegfr2) and Vegfr3 in ECs budding from the dorsal aorta (DA) and posterior cardinal vein, respectively. Thus, visualizing Ca²⁺ oscillations allowed us to monitor EC responses to angiogenic cues. Vegfr-dependent Ca²⁺ oscillations occurred in migrating tip cells as well as stalk cells budding from the DA. We investigated how Dll4/Notch signaling regulates endothelial Ca²⁺ oscillations and found that it was required for the selection of single stalk cell as well as tip cell. Thus, we captured spatio-temporal Ca²⁺ dynamics during sprouting angiogenesis, as a result of cellular responses to angiogenic inputs.

DOI:http://dx.doi.org/10.7554/eLife.08817.001

Throughout life, new blood vessels grow out like branches from existing vessels in a process called “sprouting angiogenesis”. This involves some of the endothelial cells that line the inner surface of the blood vessel migrating outwards, creating a vessel sprout made up of tip cells and stalk cells.

Sprouting is controlled by two opposing signaling systems. One pathway is triggered by a molecule called vascular endothelial growth factor (VEGF). This molecule binds to receptor proteins to activate a range of signaling processes that stimulate endothelial cells to become tip cells, and so encourage the formation of new sprouts. However, it was not known exactly when or how the endothelial cells respond to these signals.

By contrast, the Notch signaling pathway inhibits sprouting angiogenesis. The two signaling pathways interact with each other: VEGF signaling in tip cells activates Notch signaling in neighboring cells, which then prevents VEGF signaling in these cells. This feedback mechanism helps a new sprout to form by suppressing tip-like activity in the cells surrounding a new tip cell, forcing these cells to become stalk cells.

Activating VEGF receptors also causes brief increases, or oscillations, in the level of calcium ions inside the endothelial cells. Now, Yokota, Nakajima et al. have investigated VEGF activity by genetically engineering zebrafish embryos so that fluorescent proteins inside their endothelial cells emit more light when calcium ion levels inside the cell increase. As zebrafish embryos are transparent, this change in fluorescence can be seen in the living animal. Imaging the embryos revealed that calcium ion oscillations occur in both tip and stalk cells in response to VEGF signaling as they bud from vessels. Notch signaling can also regulate the calcium ion oscillations; this controls whether an individual cell becomes a tip or a stalk cell, and restricts the number of stalk cells in the sprout.

The flow of blood through the vessels is also thought to influence calcium ion oscillations in endothelial cells. Future studies could therefore use the imaging technique developed by Yokota, Nakajima et al. to investigate how blood flow influences the development of new blood vessels.

DOI:http://dx.doi.org/10.7554/eLife.08817.002

Lymphatic Vascular Regeneration: The Next Step in Tissue Engineering

The lymphatic system plays a crucial role in interstitial fluid drainage, lipid absorption, and immunological defense. Lymphatic dysfunction results in lymphedema, fluid accumulation, and swelling of soft tissues, as well as a potentially impaired immune response. Lymphedema significantly reduces quality of life of patients on a physical, mental, social, and economic basis. Current therapeutic approaches in treatment of lymphatic disease are limited. Over the last decades, great progress has been made in the development of therapeutic strategies to enhance vascular regeneration. These solutions to treat vascular disease may also be applicable in the treatment of lymphatic diseases. Comparison of the organogenic process and biological organization of the vascular and lymphatic systems and studies in the regulatory mechanisms involved in lymphangiogenesis and angiogenesis show many common features. In this study, we address the similarities between both transport systems, and focus in depth on the biology of lymphatic development. Based on the current advances in vascular regeneration, we propose different strategies for lymphatic tissue engineering that may be used for treatment of primary and secondary lymphedema.

Isolation and characterization of circulating lymphatic endothelial colony forming cells

RATIONALE

The identification of circulating endothelial progenitor cells has led to speculation regarding their origin as well as their contribution to neovascular development. Two distinct types of endothelium make up the blood and lymphatic vessel system. However, it has yet to be determined whether there are distinct lymphatic-specific circulating endothelial progenitor cells.

OBJECTIVE

This study aims to isolate and characterize the cellular properties and global gene expression of lymphatic-specific endothelial progenitor cells.

METHODS AND RESULTS

We isolated circulating endothelial colony forming cells (ECFCs) from whole peripheral blood. These cells are endothelial in nature, as defined by their expression of endothelial markers and their ability to undergo capillary morphogenesis in three-dimensional culture. A subset of isolated colonies express markers of lymphatic endothelium, including VEGFR-3 and Prox-1, with low levels of VEGFR-1, a blood endothelial marker, while the bulk of the isolated cells express high VEGFR-1 levels with low VEGFR-3 and Prox-1 expression. The different isolates have differential responses to VEGF-C, a lymphatic endothelial specific cytokine, strongly suggesting that there are lymphatic specific and blood specific ECFCs. Global analysis of gene expression revealed key differences in the regulation of pathways involved in cellular differentiation between blood and lymphatic-specific ECFCs.

CONCLUSION

These data indicate that there are two distinguishable circulating ECFC types, blood and lymphatic, which are likely to have discrete functions during neovascularization.

Involvement of phosphoinositide 3-kinase class IA (PI3K 110α) and NADPH oxidase 1 (NOX1) in regulation of vascular differentiation induced by vascular endothelial growth factor (VEGF) in mouse embryonic stem cells

The impact of reactive oxygen species and phosphoinositide 3-kinase (PI3K) in differentiating embryonic stem (ES) cells is largely unknown. Here, we show that the silencing of the PI3K catalytic subunit p110α and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 (NOX1) by short hairpin RNA or pharmacological inhibition of NOX and ras-related C3 botulinum toxin substrate 1 (Rac1) abolishes superoxide production by vascular endothelial growth factor (VEGF) in mouse ES cells and in ES-cell-derived fetal liver kinase-1(+) (Flk-1(+)) vascular progenitor cells, whereas the mitochondrial complex I inhibitor rotenone does not have an effect. Silencing p110α or inhibiting Rac1 arrests vasculogenesis at initial stages in embryoid bodies, even under VEGF treatment, as indicated by platelet endothelial cell adhesion molecule-1 (PECAM-1)-positive areas and branching points. In the absence of p110α, tube-like structure formation on matrigel and cell migration of Flk-1(+) cells in scratch migration assays are totally impaired. Silencing NOX1 causes a reduction in PECAM-1-positive areas, branching points, cell migration and tube length upon VEGF treatment, despite the expression of vascular differentiation markers. Interestingly, silencing p110α but not NOX1 inhibits the activation of Rac1, Ras homologue gene family member A (RhoA) and Akt leading to the abrogation of VEGF-induced lamellipodia structure formation. Thus, our data demonstrate that the PI3K p110α-Akt/Rac1 and NOX1 signalling pathways play a pivotal role in VEGF-induced vascular differentiation and cell migration. Rac1, RhoA and Akt phosphorylation occur downstream of PI3K and upstream of NOX1 underscoring a role of PI3K p110α in the regulation of cell polarity and migration.

DLL4 promotes continuous adult intestinal lacteal regeneration and dietary fat transport

The small intestine is a dynamic and complex organ that is characterized by constant epithelium turnover and crosstalk among various cell types and the microbiota. Lymphatic capillaries of the small intestine, called lacteals, play key roles in dietary fat absorption and the gut immune response; however, little is known about the molecular regulation of lacteal function. Here, we performed a high-resolution analysis of the small intestinal stroma and determined that lacteals reside in a permanent regenerative, proliferative state that is distinct from embryonic lymphangiogenesis or quiescent lymphatic vessels observed in other tissues. We further demonstrated that this continuous regeneration process is mediated by Notch signaling and that the expression of the Notch ligand delta-like 4 (DLL4) in lacteals requires activation of VEGFR3 and VEGFR2. Moreover, genetic inactivation of Dll4 in lymphatic endothelial cells led to lacteal regression and impaired dietary fat uptake. We propose that such a slow lymphatic regeneration mode is necessary to match a unique need of intestinal lymphatic vessels for both continuous maintenance, due to the constant exposure to dietary fat and mechanical strain, and efficient uptake of fat and immune cells. Our work reveals how lymphatic vessel responses are shaped by tissue specialization and uncover a role for continuous DLL4 signaling in the function of adult lymphatic vasculature.

Vesicular trafficking mechanisms in endothelial cells as modulators of the tumor vasculature and targets of antiangiogenic therapies

A common feature of solid tumors is their ability to incite the formation of new blood and lymph vessels trough the processes of angiogenesis and lymphangiogenesis, respectively, to support tumor growth and favor metastatic dissemination. As a result of the lack of feedback regulatory control mechanisms or due to the exacerbated presence of pro-angiogenic signals within the tumor microenvironment, the tumor endothelium receives continuous signals to sprout and develop, generating vessels that are structurally and functionally abnormal. An emerging mechanism playing a central role in shaping the tumor vasculature is the endothelial-vesicular network that regulates trafficking/export and degradation of key signaling proteins and membrane receptors, including the vascular endothelial growth-factor receptor-2/3 and members of the Notch pathway. Here we will discuss recent evidence highlighting how vesicular trafficking mechanisms in endothelial cells contribute to pathological angiogenesis/lymphangiogenesis and can provide novel and exploitable targets in antiangiogenic therapies.

Anti-metastatic action of FAK inhibitor OXA-11 in combination with VEGFR-2 signaling blockade in pancreatic neuroendocrine tumors

The present study sought to determine the anti-tumor effects of OXA-11, a potent, novel small-molecule amino pyrimidine inhibitor (1.2 pM biochemical IC(50)) of focal adhesion kinase (FAK). In studies of cancer cell lines, OXA-11 inhibited FAK phosphorylation at phospho-tyrosine 397 with a mechanistic IC(50) of 1 nM in TOV21G tumor cells, which translated into functional suppression of proliferation in 3-dimensional culture with an EC(50) of 9 nM. Studies of OXA-11 activity in TOV21G tumor-cell xenografts in mice revealed a pharmacodynamic EC(50) of 1.8 nM, indicative of mechanistic inhibition of pFAK [Y397] in these tumors. OXA-11 inhibited TOV21G tumor growth in a dose-dependent manner and also potentiated effects of cisplatin on tumor cell proliferation and apoptosis in vitro and on tumor growth in mice. Studies of pancreatic neuroendocrine tumors in RIP-Tag2 transgenic mice revealed OXA-11 suppression of pFAK [Y397] and pFAK [Y861] in tumors and liver. OXA-11 given daily from age 14 to 17 weeks reduced tumor vascularity, invasion, and when given together with the anti-VEGFR-2 antibody DC101 reduced the incidence, abundance, and size of liver metastases. Liver micrometastases were found in 100 % of mice treated with vehicle, 84 % of mice treated with OXA-11, and 79 % of mice treated with DC101 (19-24 mice per group). In contrast, liver micrometastases were found in only 52 % of 21 mice treated with OXA-11 plus DC101, and those present were significantly smaller and less numerous. Together, these findings indicate that OXA-11 is a potent and selective inhibitor of FAK phosphorylation in vitro and in vivo. OXA-11 slows tumor growth, potentiates the anti-tumor actions of cisplatin and--when combined with VEGFR-2 blockade--reduces metastasis of pancreatic neuroendocrine tumors in RIP-Tag2 mice.

Cyclic AMP Response Element Binding Protein Mediates Pathological Retinal Neovascularization via Modulating DLL4-NOTCH1 Signaling

Retinal neovascularization is the most common cause of moderate to severe vision loss in all age groups. Despite the use of anti-VEGFA therapies, this complication continues to cause blindness, suggesting a role for additional molecules in retinal neovascularization. Besides VEGFA and VEGFB, hypoxia induced VEGFC expression robustly. Based on this finding, we tested the role of VEGFC in pathological retinal angiogenesis. VEGFC induced proliferation, migration, sprouting and tube formation of human retinal microvascular endothelial cells (HRMVECs) and these responses require CREB-mediated DLL4 expression and NOTCH1 activation. Furthermore, down regulation of VEGFC levels substantially reduced tip cell formation and retinal neovascularization in vivo. In addition, we observed that CREB via modulating the DLL4-NOTCH1 signaling mediates VEGFC-induced tip cell formation and retinal neovascularization. In regard to upstream mechanism, we found that down regulation of p38β levels inhibited hypoxia-induced CREB-DLL4-NOTCH1 activation, tip cell formation, sprouting and retinal neovascularization. Based on these findings, it may be suggested that VEGFC besides its role in the regulation of lymphangiogenesis also plays a role in pathological retinal angiogenesis and this effect depends on p38β and CREB-mediated activation of DLL4-NOTCH1 signaling.

Dll4 Blockade in Stromal Cells Mediates Antitumor Effects in Preclinical Models of Ovarian Cancer

The Notch ligand delta-like 4 (Dll4) has been identified as a promising target in tumor angiogenesis in preclinical studies, and Dll4 inhibitors have recently entered clinical trials for solid tumors, including ovarian cancers. In this study, we report the development of REGN421 (enoticumab), a fully human IgG1 monoclonal antibody that binds human Dll4 with sub-nanomolar affinity and inhibits Notch signaling. Administering REGN421 to immunodeficient mice engineered to express human Dll4 inhibited the growth of several human tumor xenografts in association with the formation of nonfunctional tumor blood vessels. In ovarian tumor xenograft models, Dll4 was expressed specifically by the tumor endothelium, and Dll4 blockade by human-specific or mouse-specific Dll4 antibodies exerted potent antitumor activity, which relied entirely on targeting Dll4 expressed by tumor stromal cells but not by the tumor cells themselves. However, Dll4 blockade reduced Notch signaling in both blood vessels and tumor cells surrounding the blood vessels, suggesting that endothelial-expressed Dll4 might induce Notch signaling in adjacent ovarian tumor cells. The antitumor effects of targeting Dll4 were augmented significantly by simultaneous inhibition of VEGF signaling, whereas this combined blockade reversed normal organ vascular changes induced by Dll4 blockade alone. Overall, our findings deepen the rationale for antibody-based strategies to target Dll4 in ovarian cancers, especially in combination with VEGF blockade.

Indications of lymphatic endothelial differentiation and endothelial progenitor cell activation in the pathology of proliferative diabetic retinopathy

PURPOSE

Proliferative diabetic retinopathy (PDR) is characterized by ischaemia- and inflammation-induced neovascularization, but the pathological vascular differentiation in PDR remains poorly characterized. Here, endothelial progenitor and growth properties, as well as potential lymphatic differentiation, were investigated in the neovascular membrane specimens from vitrectomized patients with PDR.

METHODS

The expression of pan-endothelial CD31 (PECAM-1), ETS-related gene (ERG), α-smooth muscle actin (α-SMA), and stem/progenitor cell marker CD117 (c-kit) and cell proliferation marker Ki67 was investigated along with the markers of lymphatic endothelial differentiation (vascular endothelial growth factor receptor (VEGFR)-3; prospero-related homeobox gene-1 (Prox-1), lymphatic vessel endothelial receptor [LYVE)-1 and podoplanin (PDPN)] by immunohistochemistry. Lymphocyte antigen CD45 and pan-macrophage marker CD68 were likewise investigated.

RESULTS

All specimens displayed CD31, ERG and α-SMA immunoreactivity in irregular blood vessels. Unexpectedly, VEGFR3 and Prox-1 lymphatic marker positive vessels were also detected in several tissues. Prox-1 was co-expressed with CD117 in lumen-lining endothelial cells and adjacent cells, representing putative endothelial stem/progenitor cells and pro-angiogenic perivascular cells. Immunoreactivity of CD45 and CD68 was detectable in all investigated diabetic neovessel specimens. PDPN immunoreactivity was also detected in irregular lumen-forming structures, but these cells lacked CD31 and ERG that mark blood and lymphatic endothelium.

CONCLUSIONS

Although the inner part of human eye is physiologically devoid of lymphatic vessels, lymphatic differentiation associated with endothelial stem/progenitor cell activation may be involved in the pathogenesis of human PDR. Further studies are warranted to elucidate whether targeting lymphatic factors could be beneficial in the treatment of patients with the sight-threatening forms of DR.

Fenofibrate inhibits the expression of VEGFC and VEGFR-3 in retinal pigmental epithelial cells exposed to hypoxia

The aim of the present study was to examine the mechanisms through which fenofibrate inhibits the ability of human retinal pigment epithelial cells (RPE cells) exposed to hypoxia to stimulate the proliferation and migration of human umbilical vein endothelial cells (HUVECs). For this purpose, RPE cells and HUVECs were divided into the following groups: RPE-normoxia, RPE + fenofibrate, RPE-hypoxia, RPE hypoxia + fenofibrate; HUVECs normal culture and HUVECs + RPE-hypoxia culture supernatant. RPE cell hypoxia was induced by cobalt(II) chloride (CoCl₂). A superoxide anion probe was used to measure the production of superoxide anion, which is indicative of hypoxic conditions. Cell proliferation was assessed by MTT assay, and the expression of vascular endothelial growth factor C (VEGFC) and vascular endothelial growth factor receptor-3 (VEGFR-3) in the RPE cell culture supernatant was measured by enzyme-linked immunosorbent assay (ELISA). The migration ability of the HUVECs was determined by scratch-wound assay, and the angiogenic ability of the HUVECs was examined by measuring cell lumen formation. The mRNA and protein expression levels of VEGFC and VEGFR-3 in the RPE cells were measured by RT-qPCR and western blot analysis, respectively. Our results revealed that fenofibrate inhibited the increase in the expression and release of VEGFC and VEGFR-3 into the RPE cell culture supernatant induced by exposure to hypoxia. The culture of HUVECs in medium supernatant of RPE cells epxosed to hypoxia enhanced the viability and migration ability of the HUVECs and promoted lumen formation; these effects were inhibited by fenofibrate. In conclusion, our data demonstrated that the exposure of RPE cells to hypoxia induced the expression and release of VEGFC and VEGFR-3 into the cell culture supernatant. The culture of HUVECs in conditioned medium from RPE cells exposed to hypoxia increased VEGFC and VEGFR-3 expression, and promoted the proliferation and migration of the HUVECs, as well as capillary tube formation, suggesting that RPE cells play an important role in the formation of choroidal neovascularization resulting from hypoxia. Fenofibrate inhibited the upregulation of VEGFC and VEGFR-3 in the RPE cells exposed to hypoxia, and thus reduced the ability of HUVECs to form new blood vessels.

A FAK scaffold inhibitor disrupts FAK and VEGFR-3 signaling and blocks melanoma growth by targeting both tumor and endothelial cells

Melanoma has the highest mortality rate of all skin cancers and a major cause of treatment failure is drug resistance. Tumors heterogeneity requires novel therapeutic strategies and new drugs targeting multiple pathways. One of the new approaches is targeting the scaffolding function of tumor related proteins such as focal adhesion kinase (FAK). FAK is overexpressed in most solid tumors and is involved in multiple protein-protein interactions critical for tumor cell survival, tumor neovascularization, progression and metastasis. In this study, we investigated the anticancer activity of the FAK scaffold inhibitor C4, targeted to the FAK-VEGFR-3 complex, against melanomas. We compared C4 inhibitory effects in BRAF mutant vs BRAF wild type melanomas. C4 effectively caused melanoma tumor regression in vivo, when administered alone and sensitized tumors to chemotherapy. The most dramatic effect of C4 was related to reduction of vasculature of both BRAF wild type and V600E mutant xenograft tumors. The in vivo effects of C4 were assessed in xenograft models using non-invasive multimodality imaging in conjunction with histologic and molecular biology methods. C4 inhibited cell viability, adhesion and motility of melanoma and endothelial cells, specifically blocked phosphorylation of VEGFR-3 and FAK and disrupted their complexes. Specificity of in vivo effects for C4 were confirmed by a decrease in tumor FAK and VEGFR-3 phosphorylation, reduction of vasculogenesis and reduced blood flow. Our collective observations provide evidence that a small molecule inhibitor targeted to the FAK protein-protein interaction site successfully inhibits melanoma growth through dual targeting of tumor and endothelial cells and is effective against both BRAF wild type and mutant melanomas.

Regulation of signaling interactions and receptor endocytosis in growing blood vessels

Blood vessels and the lymphatic vasculature are extensive tubular networks formed by endothelial cells that have several indispensable functions in the developing and adult organism. During growth and tissue regeneration but also in many pathological settings, these vascular networks expand, which is critically controlled by the receptor EphB4 and the ligand ephrin-B2. An increasing body of evidence links Eph/ephrin molecules to the function of other receptor tyrosine kinases and cell surface receptors. In the endothelium, ephrin-B2 is required for clathrin-dependent internalization and full signaling activity of VEGFR2, the main receptor for vascular endothelial growth factor. In vascular smooth muscle cells, ephrin-B2 antagonizes clathrin-dependent endocytosis of PDGFRβ and controls the balanced activation of different signal transduction processes after stimulation with platelet-derived growth factor. This review summarizes the important roles of Eph/ephrin molecules in vascular morphogenesis and explains the function of ephrin-B2 as a molecular hub for receptor endocytosis in the vasculature.

Regulation of vascular endothelial growth factor receptor function in angiogenesis by numb and numb-like

OBJECTIVE

Vascular endothelial growth factor (VEGF) signaling is a major regulator of physiological and pathological angiogenesis. VEGF receptor activity is strongly controlled by endocytosis, which can terminate or enhance signal transduction in the angiogenic endothelium, but the exact molecular regulation of these processes remains incompletely understood. We have therefore examined the function of Numb family clathrin-associated sorting proteins in angiogenesis.

APPROACH AND RESULTS

We show that Numb proteins are expressed by endothelial cells during retinal angiogenesis in mice. Inducible inactivation of the Numb/Numbl genes in the postnatal endothelium led to impaired vessel growth, reduced endothelial proliferation and sprouting, and decreased VEGF receptor activation. Biochemistry and cell biology experiments established that Numb can interact with VEGFR2 and VEGFR3 and controls VEGF receptor activation in response to ligand stimulation. Experiments in cultured endothelial cells showed that Numb proteins counteract VEGF receptor degradation and promote VEGFR2 recycling back to the plasma membrane.

CONCLUSIONS

Numb proteins control VEGF receptor endocytosis, signaling, and recycling in endothelial cells, which promotes the angiogenic growth of blood vessels.

Vascularisation of the central nervous system

The developing central nervous system (CNS) is vascularised through the angiogenic invasion of blood vessels from a perineural vascular plexus, followed by continued sprouting and remodelling until a hierarchical vascular network is formed. Remarkably, vascularisation occurs without perturbing the intricate architecture of the neurogenic niches or the emerging neural networks. We discuss the mouse hindbrain, forebrain and retina as widely used models to study developmental angiogenesis in the mammalian CNS and provide an overview of key cellular and molecular mechanisms regulating the vascularisation of these organs.

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CNS vascularisation is initiated during embryonic development.

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CNS vascularisation is studied in the mouse forebrain, hindbrain and retina models.

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Neuroglial cells interact with endothelial cells to promote angiogenesis.

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Neuroglial cells produce growth factors and matrix cues to pattern vessels.

Overview of Methods for Overcoming Hindrance to Drug Delivery to Tumors, with Special Attention to Tumor Interstitial Fluid

Every drug used to treat cancer (chemotherapeutics, immunological, monoclonal antibodies, nanoparticles, radionuclides) must reach the targeted cells through the tumor environment at adequate concentrations, in order to exert their cell-killing effects. For any of these agents to reach the goal cells, they must overcome a number of impediments created by the tumor microenvironment (TME), beginning with tumor interstitial fluid pressure (TIFP), and a multifactorial increase in composition of the extracellular matrix (ECM). A primary modifier of TME is hypoxia, which increases the production of growth factors, such as vascular endothelial growth factor and platelet-derived growth factor. These growth factors released by both tumor cells and bone marrow recruited myeloid cells form abnormal vasculature characterized by vessels that are tortuous and more permeable. Increased leakiness combined with increased inflammatory byproducts accumulates fluid within the tumor mass (tumor interstitial fluid), ultimately creating an increased pressure (TIFP). Fibroblasts are also up-regulated by the TME, and deposit fibers that further augment the density of the ECM, thus, further worsening the TIFP. Increased TIFP with the ECM are the major obstacles to adequate drug delivery. By decreasing TIFP and ECM density, we can expect an associated rise in drug concentration within the tumor itself. In this overview, we will describe all the methods (drugs, nutraceuticals, and physical methods of treatment) able to lower TIFP and to modify ECM used for increasing drug concentration within the tumor tissue.

A Case of Abnormal Lymphatic-Like Differentiation and Endothelial Progenitor Cell Activation in Neovascularization Associated with Hemi-Retinal Vein Occlusion

Purpose

Pathological vascular differentiation in retinal vein occlusion (RVO)-related neovessel formation remains poorly characterized. The role of intraocular lymphatic-like differentiation or endothelial progenitor cell activity has not been studied in this disease.

Methods

Vitrectomy was performed in an eye with hemi-RVO; the neovessel membrane located at the optic nerve head was removed and subjected to immunohistochemistry. Characterization of the neovascular tissue was performed using hematoxylin and eosin, α-smooth muscle actin, and the pan-endothelial cell (EC) adhesion molecule CD31. The expression of lymphatic EC markers was studied by lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1), podoplanin (PDPN), and prospero-related homeobox protein 1 (Prox-1). Potential vascular stem/progenitor cells were identified by active cellular proliferation (Ki67) and expression of the stem cell marker CD117.

Results

The specimen contained blood vessels lined by ECs and surrounded by pericytes. Immunoreactivity for LYVE-1 and Prox-1 was detected, with Prox-1 being more widely expressed in the active Ki67-positive lumen-lining cells. PDPN expression was instead found in the cells residing in the extravascular tissue. Expression of the stem cell markers CD117 and Ki67 suggested vascular endothelial progenitor cell activity.

Conclusions

Intraocular lymphatic-like differentiation coupled with progenitor cell activation may be involved in the pathology of neovessel formation in ischemia-induced human hemi-RVO.

Lymphangiogenesis and Inflammation-Looking for the "Missing Pieces" of the Puzzle

Several papers about lymphangiogenesis and inflammation focused on the detailed and complicated descriptions of the molecular pathways accompanying both non-tumor and tumor inflammatory-induced lymphatic vessel development. Many authors are tempted to present inflammatory-induced lymphangiogenesis in pathologic conditions neglecting the role of inflammatory cells during embryonic lymphatic vessel development. Some of the inflammatory cells are largely characterized in inflammatory-induced lymphangiogenesis, while others as mast cells, eosinophils, or plasma cells are less studied. No phenotypic characterization of inflammation-activated lymphatic endothelial cell is available in this moment. Another paradox is related to the existence of few papers regarding lymphangiogenesis inside lymphoid organs and for their related pathology. There are still several "missing pieces of such a big puzzle" of lymphangiogenesis and inflammation, with a direct impact on the ineffectiveness of the anti-inflammatory therapy as lymphangiogenesis inhibitors. The present paper will focus on the controversial issues of lymphangiogenesis and inflammation.

Notch functions in developmental and tumour angiogenesis by diverse mechanisms

The Notch signalling pathway is a key regulator of developmental and tumour angiogenesis. Inhibition of Delta-like 4 (Dll4)-mediated Notch signalling results in hyper-sprouting, demonstrating that Notch regulates tip-stalk cell identity in developing tissues and tumours. Paradoxically, Dll4 blockade leads to reduced tumour growth because the newly growing vessels are poorly perfused. To explore the potential for targeting Notch, we developed Notch inhibitors, termed the Notch1 decoys. A Notch1 decoy variant containing all 36 epidermal growth factor (EGF)-like repeats of the extracellular domain of rat Notch1 has been shown to inhibit both Dll and Jagged class Notch ligands. Thus this Notch1 decoy functions differently than Dll4-specific blockade, although it has the potential to inhibit Dll4 activity. Expression of the Notch1 decoy in mice disrupted tumour angiogenesis and inhibited tumour growth. To understand the mechanism by which Notch blockade acts, it is important to note that Notch can function in multiple cell types that make up the vasculature, including endothelial cells and perivascular cells. We investigated Notch function in retinal microglia and determined how myeloid-expressed Notch can influence macrophages and angiogenesis. We found that myeloid-specific loss of Notch1 reduced microglia recruitment and led to improper microglia localization during retinal angiogenesis. Thus either pharmacological inhibition of Notch signalling or genetic deficiencies of Notch function in microglia leads to abnormal angiogenesis.

Endothelial Snail Regulates Capillary Branching Morphogenesis via Vascular Endothelial Growth Factor Receptor 3 Expression

Vascular branching morphogenesis is activated and maintained by several signaling pathways. Among them, vascular endothelial growth factor receptor 2 (VEGFR2) signaling is largely presented in arteries, and VEGFR3 signaling is in veins and capillaries. Recent reports have documented that Snail, a well-known epithelial-to-mesenchymal transition protein, is expressed in endothelial cells, where it regulates sprouting angiogenesis and embryonic vascular development. Here, we identified Snail as a regulator of VEGFR3 expression during capillary branching morphogenesis. Snail was dramatically upregulated in sprouting vessels in the developing retinal vasculature, including the leading-edged vessels and vertical sprouting vessels for capillary extension toward the deep retina. Results from in vitro functional studies demonstrate that Snail expression colocalized with VEGFR3 and upregulated VEGFR3 mRNA by directly binding to the VEGFR3 promoter via cooperating with early growth response protein-1. Snail knockdown in postnatal mice attenuated the formation of the deep capillary plexus, not only by impairing vertical sprouting vessels but also by downregulating VEGFR3 expression. Collectively, these data suggest that the Snail-VEGFR3 axis controls capillary extension, especially in vessels expressing VEGFR2 at low levels.

A Novel Missense Mutation in FLT4 Causes Autosomal Recessive Hereditary Lymphedema

BACKGROUND

Primary lymphedema covers around 10% of all lymphedema cases. Most cases segregate as an autosomal dominant trait and rarely manifest autosomal recessive inheritance. Our research aimed to map and ultimately to hunt the mutation that causes hereditary lymphedema in an extended consanguineous Muslim family consisting of several affected individuals.

METHODS AND RESULTS

We attempted molecular diagnosis by applying homozygosity mapping and whole genome linkage analysis. A candidate locus of 2.3 Mb located on chromosome 5q35.3 was identified, yielding an overall LOD score of 3.18. This locus has been previously linked to congenital lymphedema, namely by the FLT4 gene. Mutations in FLT4 that were previously described in Muslim-Israeli families were discarded as culprit using sequence analysis. Sanger sequencing the gene revealed a novel missense variant in exon 28 (NM_182925.4: c.3704C>G; p.Ser1235Cys). This variant has perfect segregation within the extended family and was not previously reported in either common or pathogenic variants databases.

CONCLUSIONS

Our mutation is the first reported pathogenic variant located outside the tyrosine kinase domains of the VEGFR3 receptor, and the second to portray autosomal recessive inheritance. The homozygous substitution of serine by cysteine at position 1235 affects protein tyrosine kinase activity, possibly through a null effect mechanism rather than a negative dominant effect. Our variant is associated with a mild phenotype, possibly reflecting some residual receptor activity, most probably attributed to the variant's location beyond the TK domains.

VEGF-trap aflibercept significantly improves long-term graft survival in high-risk corneal transplantation

BACKGROUND

Graft failure because of immune rejection remains a significant problem in organ transplantation, and lymphatic and blood vessels are important components of the afferent and efferent arms of the host alloimmune response, respectively. We compare the effect of antihemangiogenic and antilymphangiogenic therapies on alloimmunity and graft survival in a murine model of high-risk corneal transplantation.

METHODS

Orthotopic corneal transplantation was performed in hemevascularized and lymph-vascularized high-risk host beds, and graft recipients received subconjunctival vascular endothelial growth factor (VEGF)-trap, anti-VEGF-C, sVEGFR-3, or no treatment, beginning at the time of surgery. Fourteen days after transplantation, graft hemeangiogenesis and lymphangiogenesis were evaluated by immunohistochemistry. The frequencies of Th1 cells in regional lymphoid tissue and graft-infiltrating immune cells were evaluated by flow cytometry. Long-term allograft survival was compared using Kaplan-Meier curves.

RESULTS

VEGF-trap significantly decreased graft hemangiogenesis as compared to the control group and was most effective in reducing the frequency of graft-infiltrating immune cells. Anti-VEGF-C and sVEGFR3 significantly decreased graft lymphangiogenesis and lymphoid Th1 cell frequencies as compared to control. VEGF-trap (72%), anti-VEGF-C (25%), and sVEGFR-3 (11%) all significantly improved in the 8-week graft survival compared to control (0%), although VEGF-trap was significantly more effective than both anti-VEGF-C (P < 0.05) and sVEGFR-3 (P < 0.05).

CONCLUSION

In a clinically relevant model of high-risk corneal transplantation in which blood and lymphatic vessels are present and treatment begins at the time of transplantation, VEGF-trap is significantly more effective in improving long-term graft survival as compared to anti-VEGF-C and sVEGFR-3, but all approaches improve survival when compared to untreated control.

Role of hypoxia and vascular endothelial growth factors in lymphangiogenesis

Hypoxia is a major condition for the induction of angiogenesis during tumor development but its role in lymphangiogenesis remains unclear. Blood and lymphatic vasculatures are stimulated by growth factors from the vascular endothelial family: the VEGFs. In this review, we investigate the role of hypoxia in the molecular regulation of synthesis of lymphangiogenic growth factors VEGF-A, VEGF-C, and VEGF-D. Gene expression can be regulated at transcriptional and translational levels by hypoxia. Despite strong regulation of DNA transcription induced by hypoxia-inducible factors (HIFs), the majority of cellular stresses such as hypoxia lead to inhibition of cap-dependent translation of the mRNA, resulting in downregulation of protein synthesis. Here, we describe how translation initiation of VEGF mRNAs is induced by hypoxia through an internal ribosome entry site (IRES)-dependent mechanism. Considering the implication of the lymphatic vasculature in metastatic dissemination, it seems crucial to understand the hypoxia-induced molecular regulation of lymphangiogenic growth factors to obtain new insights for cancer therapy.

Vascular endothelial growth factor receptor 3 signaling contributes to angioobliterative pulmonary hypertension

The mechanisms involved in the development of severe angioobliterative pulmonary arterial hypertension (PAH) are multicellular and complex. Many of the features of human severe PAH, including angioobliteration, lung perivascular inflammation, and right heart failure, are reproduced in the Sugen 5416/chronic hypoxia (SuHx) rat model. Here we address, at first glance, the confusing and paradoxical aspect of the model, namely, that treatment of rats with the antiangiogenic vascular endothelial growth factor (VEGF) receptor 1 and 2 kinase inhibitor, Sugen 5416, when combined with chronic hypoxia, causes angioproliferative pulmonary vascular disease. We postulated that signaling through the unblocked VEGF receptor VEGFR3 (or flt4) could account for some of the pulmonary arteriolar lumen-occluding cell growth. We also considered that Sugen 5416-induced VEGFR1 and VEGFR2 blockade could alter the expression pattern of VEGF isoform proteins. Indeed, in the lungs of SuHx rats we found increased expression of the ligand proteins VEGF-C and VEGF-D as well as enhanced expression of the VEGFR3 protein. In contrast, in the failing right ventricle of SuHx rats there was a profound decrease in the expression of VEGF-B and VEGF-D in addition to the previously described reduction in VEGF-A expression. MAZ51, an inhibitor of VEGFR3 phosphorylation and VEGFR3 signaling, largely prevented the development of angioobliteration in the SuHx model; however, obliterated vessels did not reopen when animals with established PAH were treated with the VEGFR3 inhibitor. Part of the mechanism of vasoobliteration in the SuHx model occurs via VEGFR3. VEGFR1/VEGFR2 inhibition can be initially antiangiogenic by inducing lung vessel endothelial cell apoptosis; however, it can be subsequently angiogenic via VEGF-C and VEGF-D signaling through VEGFR3.

Sox7 controls arterial specification in conjunction with hey2 and efnb2 function

SoxF family members have been linked to arterio-venous specification events and human pathological conditions, but in contrast to Sox17 and Sox18, a detailed in vivo analysis of a Sox7 mutant model is still lacking. In this study we generated zebrafish sox7 mutants to understand the role of Sox7 during vascular development. By in vivo imaging of transgenic zebrafish lines we show that sox7 mutants display a short circulatory loop around the heart as a result of aberrant connections between the lateral dorsal aorta (LDA) and either the venous primary head sinus (PHS) or the common cardinal vein (CCV). In situ hybridization and live observations in flt4:mCitrine transgenic embryos revealed increased expression levels of flt4 in arterial endothelial cells at the exact location of the aberrant vascular connections in sox7 mutants. An identical circulatory short loop could also be observed in newly generated mutants for hey2 and efnb2. By genetically modulating levels of sox7, hey2 and efnb2 we demonstrate a genetic interaction of sox7 with hey2 and efnb2. The specific spatially confined effect of loss of Sox7 function can be rescued by overexpressing the Notch intracellular domain (NICD) in arterial cells of sox7 mutants, placing Sox7 upstream of Notch in this aspect of arterial development. Hence, sox7 levels are crucial in arterial specification in conjunction with hey2 and efnb2 function, with mutants in all three genes displaying shunt formation and an arterial block.

Simultaneous in vivo imaging of blood and lymphatic vessel growth in Prox1-GFP/Flk1::myr-mCherry mice

The ability to visually observe angiogenesis and lymphangiogenesis simultaneously and repeatedly in living animals would greatly enhance our understanding of the inter-dependence of these processes. To generate a mouse model that allows such visualization via in vivo fluorescence imaging, we crossed Prox1-GFP mice with Flk1::myr-mCherry mice to generate Prox1-GFP/Flk1::myr-mCherry mice, in which lymphatic vessels emit green fluorescence and blood vessels emit red fluorescence. Corneal neovascularization was induced in these mice using three injury models: implantation of a vascular endothelial growth factor (VEGF) pellet, implantation of a basic fibroblast growth factor (bFGF) pellet, and alkali burn injury. Vessel growth was observed in vivo by stereomicroscopy on days 0, 3, 7 and 10 after pellet implantation or alkali injury as well as in flat-mounted corneas via confocal microscopy after the final in vivo imaging time point. We observed blood and lymphatic vessel growth in all three models, with the most significant growth occurring from days 0-7. Upon VEGF stimulation, the growth kinetics of blood and lymphatic vessels were similar. Blood vessels exhibited similar growth patterns in VEGF- and bFGF-stimulated corneas. Alkali burn injury induced robust angiogenesis and lymphangiogenesis. The intrinsic fluorescence of blood and lymphatic endothelial cells in Prox1-GFP/Flk1::myr-mCherry mice permitted simultaneous in vivo imaging of angiogenesis and lymphangiogenesis. This allowed us to differentiate the processes as well as observe their inter-dependence, and will be valuable in development of therapies targeting angiogenesis and/or lymphangiogenesis.

Intramembrane binding of VE-cadherin to VEGFR2 and VEGFR3 assembles the endothelial mechanosensory complex

VE-cadherin plays a critical role in endothelial shear stress mechanotransduction by interacting with VEGFRs through their transmembrane domains.

Endothelial responses to fluid shear stress are essential for vascular development and physiology, and determine the formation of atherosclerotic plaques at regions of disturbed flow. Previous work identified VE-cadherin as an essential component, along with PECAM-1 and VEGFR2, of a complex that mediates flow signaling. However, VE-cadherin’s precise role is poorly understood. We now show that the transmembrane domain of VE-cadherin mediates an essential adapter function by binding directly to the transmembrane domain of VEGFR2, as well as VEGFR3, which we now identify as another component of the junctional mechanosensory complex. VEGFR2 and VEGFR3 signal redundantly downstream of VE-cadherin. Furthermore, VEGFR3 expression is observed in the aortic endothelium, where it contributes to flow responses in vivo. In summary, this study identifies a novel adapter function for VE-cadherin mediated by transmembrane domain association with VEGFRs.

Vascular endothelial growth factor receptor 3 controls neural stem cell activation in mice and humans

Neural stem cells (NSCs) continuously produce new neurons within the adult mammalian hippocampus. NSCs are typically quiescent but activated to self-renew or differentiate into neural progenitor cells. The molecular mechanisms of NSC activation remain poorly understood. Here, we show that adult hippocampal NSCs express vascular endothelial growth factor receptor (VEGFR) 3 and its ligand VEGF-C, which activates quiescent NSCs to enter the cell cycle and generate progenitor cells. Hippocampal NSC activation and neurogenesis are impaired by conditional deletion of Vegfr3 in NSCs. Functionally, this is associated with compromised NSC activation in response to VEGF-C and physical activity. In NSCs derived from human embryonic stem cells (hESCs), VEGF-C/VEGFR3 mediates intracellular activation of AKT and ERK pathways that control cell fate and proliferation. These findings identify VEGF-C/VEGFR3 signaling as a specific regulator of NSC activation and neurogenesis in mammals.

Pericytes as targets in hereditary hemorrhagic telangiectasia

Defective paracrine Transforming Growth Factor-β (TGF-β) signaling between endothelial cells and the neighboring mural cells have been thought to lead to the development of vascular lesions that are characteristic of Hereditary Hemorrhagic Telangiectasia (HHT). This review highlights recent progress in our understanding of TGF-β signaling in mural cell recruitment and vessel stabilization and how perturbed TGF-β signaling might contribute to defective endothelial-mural cell interaction affecting vessel functionalities. Our recent findings have provided exciting insights into the role of thalidomide, a drug that reduces both the frequency and the duration of epistaxis in individuals with HHT by targeting mural cells. These advances provide opportunities for the development of new therapies for vascular malformations.

Association between the polymorphisms of the vascular endothelial growth factor gene and metabolic syndrome

Vascular endothelial growth factor (VEGF) is a major angiogenic factor. Increased levels of VEGF have been reported in patients with metabolic syndrome (MetS). The role of VEGF polymorphisms in MetS susceptibility, however, has not been reported previously. Thus, the present study was performed to analyze the associations between the VEGF -634G>C and 936C>T polymorphisms and the patients with MetS. A total of 320 patients with MetS (mean age, 49.86±11.76 years) and 320 healthy subjects (mean age, 50.94±8.43 years) were enrolled in the study. The VEGF -634G>C polymorphism in the 5'-untranslated region (UTR) and 936C>T polymorphism in 3'-UTR were analyzed by polymerase chain reaction-restriction fragment length polymorphism. The VEGF -634G>C polymorphism significantly affected MetS susceptibility. The CC genotype of the -634G>C polymorphism was significantly associated with an increased risk of MetS [adjusted odds ratio (AOR)=3.973; 95% confidence interval (CI), 2.321-6.799; P<0.0001]. AORs of the dominant (GG vs. GC+CC) and recessive models (GG+GC vs. CC) between the cases and controls were 2.569 (95% CI, 1.657-3.983; P<0.0001) and 2.163 (95% CI, 1.475-3.171; P=0.0001), respectively. Haplotypes of -634G>C and 936C>T were also associated with MetS susceptibility. When the haplotype data were stratified by gender, the association remained only in males. The -634G>C polymorphism was also associated with the subgroups of MetS risk components by the stratification analysis. The 936C>T polymorphism was, however, not associated with the MetS susceptibility. The present study demonstrates that the VEGF -634G>C polymorphism and haplotypes may be a genetic determinant for the MetS susceptibility. To the best of our knowledge, this is the first study on the significant association of the VEGF polymorphisms in MetS patients. To confirm the effects of the VEGF polymorphisms on MetS, further functional and population studies are required.

Vascular remodeling is governed by a VEGFR3-dependent fluid shear stress set point

Vascular remodeling under conditions of growth or exercise, or during recovery from arterial restriction or blockage is essential for health, but mechanisms are poorly understood. It has been proposed that endothelial cells have a preferred level of fluid shear stress, or ‘set point’, that determines remodeling. We show that human umbilical vein endothelial cells respond optimally within a range of fluid shear stress that approximate physiological shear. Lymphatic endothelial cells, which experience much lower flow in vivo, show similar effects but at lower value of shear stress. VEGFR3 levels, a component of a junctional mechanosensory complex, mediate these differences. Experiments in mice and zebrafish demonstrate that changing levels of VEGFR3/Flt4 modulates aortic lumen diameter consistent with flow-dependent remodeling. These data provide direct evidence for a fluid shear stress set point, identify a mechanism for varying the set point, and demonstrate its relevance to vessel remodeling in vivo.

DOI:http://dx.doi.org/10.7554/eLife.04645.001

Blood and lymphatic vessels remodel their shape, diameter and connections during development, and throughout life in response to growth, exercise and disease. This process is called vascular remodeling.

The endothelial cells that line the inside of blood and lymphatic vessels are constantly exposed to the frictional force from flowing blood, termed fluid shear stress. Changes in shear stress are sensed by the endothelial cells, which trigger vascular remodeling to return the stress to the original level. It has been proposed that remodeling is governed by a preferred level of fluid shear stress, or set point, against which deviations in the shear stress are compared. Thus, changing the fluid flow through a blood vessel increases or decreases shear stress, which results in the vessel remodeling to restore the original level of shear stress. Like all remodeling, this process involves inflammation to recruit white blood cells, which assist with the process.

Baeyens et al. investigated whether such a shear stress set point exists and what its biological basis might be using cultured endothelial cells from human umbilical veins. These cells remained stable and in a resting state when a particular level of shear stress was applied to them; above or below this shear stress level, the cells produced an inflammatory response like that seen during vascular remodeling. This suggests that these cells do indeed have a set point for shear stress. The same response occurred in human lymphatic endothelial cells, although in these cells the shear stress set point was much lower, correlating with the low flow in lymphatic vessels.

Baeyens et al. then discovered that the shear stress set point is related to the level of a protein called VEGFR3 in the cells, which was recently found to participate in shear stress sensing. Endothelial cells from lymphatic vessels normally produce much greater quantities of VEGFR3 than those from blood vessels. Reducing the amount of VEGFR3 in lymphatic endothelial cells increased the set point shear stress, while increasing the levels in blood vessel cells decreased the set point. This suggests that the levels of this protein account for the difference in the response of these two cell types. Baeyens et al. then tested this pathway by reducing the levels of VEGFR3 in zebrafish embryos and in adult mice. In both animals, this caused arteries to narrow, showing that VEGFR3 levels also control sensitivity to shear stress—and hence vascular remodeling—inside living creatures.

Understanding in detail how vascular remodeling is regulated could help improve treatments for a wide range of cardiovascular conditions. To do so, further work will be needed to develop methods to control the sensitivity of endothelial cells to shear stress and to identify other proteins that might specifically control the narrowing or the expansion of vessels in human patients.

DOI:http://dx.doi.org/10.7554/eLife.04645.002

New pharmaceutical treatment of gastric MALT lymphoma: anti-angiogenesis treatment using VEGF receptor antibodies and celecoxib

In addition to eradication of Helicobacter pylori, chemotherapy with anticancer agents, and radiation therapy, the treatment with molecular target drugs including rituximab, a CD20 antagonist, is one of the promising new regimens. The mucosa-associated lymphoid tissue (MALT) lymphoma is histologically characterized by rich distribution of the microvascular network consisting of the immature capillaries, lymphatics and venules, and this microvascular network could be the target of the new pharmacotherapy in addition to the direct action on the accumulated B lymphocytes. We have established the animal model of the gastric MALT lymphoma by the Helicobacter heilmannii (H. heilmannii) peroral infection of C57BL/6 mice. The disease induced by this model is very similar to the human counterpart, because of the lymphoepithelial lesion characteristic of the human MALT lymphoma as well as the rich vascularization and localization of vascular endothelial growth factor (VEGF) and its receptors, Flt-1, Flk-1 and Flt-4. By administering VEGF receptor antibodies or celecoxib, one of the cyclooxygenase 2 inhibitors, we were able to induce a significant decrease in the size of the tumor and the apoptotic changes of the endothelial cells of the microvascular network. These antiangiogenic strategies were suggested to be candidates for the new pharmacological treatment of gastric MALT lymphoma, when other treatments are not effective.

Chronic inflammation, lymphangiogenesis, and effect of an anti-VEGFR therapy in a mouse model and in human patients with aspiration pneumonia

Chronic inflammation induces lymphangiogenesis and blood vessel remodelling. Since aged pneumonia patients often have repeated episodes of aspiration pneumonia, the pathogenesis may involve chronic inflammation. For lymphangiogenesis, VEGFR-3 and its ligand VEGF-C are key factors. No previous studies have examined chronic inflammation or vascular changes in aspiration pneumonia or its mouse models. In lung inflammation, little is known about the effect of blocking VEGFR-3 on lung lymphangiogenesis and, moreover, its effect on the disease condition. This study aimed to establish a mouse model of aspiration pneumonia, examine the presence of chronic inflammation and vascular changes in the model and in patients, and evaluate the effect of inhibiting VEGFR-3 on the lymphangiogenesis and disease condition in this model. To induce aspiration pneumonia, we repeated inoculation of pepsin at low pH and LPS into mice for 21-28 days, durations in which bronchioalveolar lavage and plasma leakage in the lung suggested the presence of exaggerated inflammation. Conventional and immunohistochemical analysis of tracheal whole mounts suggested the presence of chronic inflammation, lymphangiogenesis, and blood vessel remodelling in the model. Quantitative RT-PCR of the trachea and lung suggested the involvement of lymphangiogenic factor VEGF-C, VEGFR-3, and pro-inflammatory cytokines. In the lung, the aspiration model showed the presence of chronic inflammation and exaggerated lymphangiogenesis. Treatment with the VEGFR inhibitor axitinib or the VEGFR-3 specific inhibitor SAR131675 impaired lymphangiogenesis in the lung and improved oxygen saturation in the aspiration model. Since the lung is the main site of aspiration pneumonia, the changes were intensive in the lung and mild in the trachea. Human lung samples also showed the presence of chronic inflammation and exaggerated lymphangiogenesis, suggesting the relevance of the model to the disease. These results suggest lymphatics in the lung as a new target of analysis and therapy in aspiration pneumonia.

VEGFR3 does not sustain retinal angiogenesis without VEGFR2

Angiogenesis, the formation of new blood vessels, is regulated by vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs). VEGFR2 is abundant in the tip cells of angiogenic sprouts, where VEGF/VEGFR2 functions upstream of the delta-like ligand 4 (DLL4)/Notch signal transduction pathway. VEGFR3 is expressed in all endothelia and is indispensable for angiogenesis during early embryonic development. In adults, VEGFR3 is expressed in angiogenic blood vessels and some fenestrated endothelia. VEGFR3 is abundant in endothelial tip cells, where it activates Notch signaling, facilitating the conversion of tip cells to stalk cells during the stabilization of vascular branches. Subsequently, Notch activation suppresses VEGFR3 expression in a negative feedback loop. Here we used conditional deletions and a Notch pathway inhibitor to investigate the cross-talk between VEGFR2, VEGFR3, and Notch in vivo. We show that postnatal angiogenesis requires VEGFR2 signaling also in the absence of Notch or VEGFR3, and that even small amounts of VEGFR2 are able to sustain angiogenesis to some extent. We found that VEGFR2 is required independently of VEGFR3 for endothelial DLL4 up-regulation and angiogenic sprouting, and for VEGFR3 functions in angiogenesis. In contrast, VEGFR2 deletion had no effect, whereas VEGFR3 was essential for postnatal lymphangiogenesis, and even for lymphatic vessel maintenance in adult skin. Knowledge of these interactions and the signaling functions of VEGFRs in blood vessels and lymphatic vessels is essential for the therapeutic manipulation of the vascular system, especially when considering multitargeted antiangiogenic treatments.

Tipping off endothelial tubes: nitric oxide drives tip cells

Angiogenesis, the formation of new blood vessels from pre-existing vessels, is a complex process that warrants cell migration, proliferation, tip cell formation, ring formation, and finally tube formation. Angiogenesis is initiated by a single leader endothelial cell called "tip cell," followed by vessel elongation by "stalk cells." Tip cells are characterized by their long filopodial extensions and expression of vascular endothelial growth factor receptor-2 and endocan. Although nitric oxide (NO) is an important modulator of angiogenesis, its role in angiogenic sprouting and specifically in tip cell formation is poorly understood. The present study tested the role of endothelial nitric oxide synthase (eNOS)/NO/cyclic GMP (cGMP) signaling in tip cell formation. In primary endothelial cell culture, about 40% of the tip cells showed characteristic sub-cellular localization of eNOS toward the anterior progressive end of the tip cells, and eNOS became phosphorylated at serine 1177. Loss of eNOS suppressed tip cell formation. Live cell NO imaging demonstrated approximately 35% more NO in tip cells compared with stalk cells. Tip cells showed increased level of cGMP relative to stalk cells. Further, the dissection of NO downstream signaling using pharmacological inhibitors and inducers indicates that NO uses the sGC/cGMP pathway in tip cells to lead angiogenesis. Taken together, the present study confirms that eNOS/NO/cGMP signaling defines the direction of tip cell migration and thereby initiates new blood vessel formation.

Hand2 is an essential regulator for two Notch-dependent functions within the embryonic endocardium

The basic-helix-loop-helix (bHLH) transcription factor Hand2 plays critical roles during cardiac morphogenesis via expression and function within myocardial, neural crest, and epicardial cell populations. Here, we show that Hand2 plays two essential Notch-dependent roles within the endocardium. Endocardial ablation of Hand2 results in failure to develop a patent tricuspid valve, intraventricular septum defects, and hypotrabeculated ventricles, which collectively resemble the human congenital defect tricuspid atresia. We show endocardial Hand2 to be an integral downstream component of a Notch endocardium-to-myocardium signaling pathway and a direct transcriptional regulator of Neuregulin1. Additionally, Hand2 participates in endocardium-to-endocardium-based cell signaling, with Hand2 mutant hearts displaying an increased density of coronary lumens. Molecular analyses further reveal dysregulation of several crucial components of Vegf signaling, including VegfA, VegfR2, Nrp1, and VegfR3. Thus, Hand2 functions as a crucial downstream transcriptional effector of endocardial Notch signaling during both cardiogenesis and coronary vasculogenesis.

Quantitative assessment of angiogenesis, perfused blood vessels and endothelial tip cells in the postnatal mouse brain

During development and in various diseases of the CNS, new blood vessel formation starts with endothelial tip cell selection and vascular sprout migration, followed by the establishment of functional, perfused blood vessels. Here we describe a method that allows the assessment of these distinct angiogenic steps together with antibody-based protein detection in the postnatal mouse brain. Intravascular and perivascular markers such as Evans blue (EB), isolectin B4 (IB4) or laminin (LN) are used alongside simultaneous immunofluorescence on the same sections. By using confocal laser-scanning microscopy and stereological methods for analysis, detailed quantification of the 3D postnatal brain vasculature for perfused and nonperfused vessels (e.g., vascular volume fraction, vessel length and number, number of branch points and perfusion status of the newly formed vessels) and characterization of sprouting activity (e.g., endothelial tip cell density, filopodia number) can be obtained. The entire protocol, from mouse perfusion to vessel analysis, takes ∼10 d.

Multiplexed protein profiling after aneurysmal subarachnoid hemorrhage: characterization of differential expression patterns in cerebral vasospasm

Cerebral vasospasm is a major contributor to delayed morbidity following aneurysmal subarachnoid hemorrhage. We sought to evaluate differential plasma protein levels across time in patients with aneurysmal subarachnoid hemorrhage to identify potential biomarkers and to better understand the pathogenesis of cerebral vasospasm. Nine female patients with aneurysmal subarachnoid hemorrhage underwent serial analysis of 239 different serum protein levels using quantitative, multiplexed immunoassays (DiscoveryMAP 250+ v2.0, Myriad RBM, Austin, TX, USA) on post-hemorrhage days 0 and 5. A repeated measures analysis of variance determined that mean protein concentration decreased significantly in patients who developed vasospasm versus those who did not for alpha-2-macroglobulin (F [1.00,7.00]=16.33, p=0.005), angiogenin (F [1.00,7.00]=7.65, p=0.028), apolipoprotein A-IV (F [1.00,7.00]=6.308, p=0.040), granulocyte colony-stimulating factor (F [1.00,7.00]=9.08, p=0.020), macrophage-stimulating protein (F [1.00,7.00]=24.21, p=0.002), tetranectin (F [1.00,7.00]=5.46, p<0.039), vascular endothelial growth factor receptor 3 (F [1.00,7.00]=6.94, p=0.034), and significantly increased for vitronectin (F [1.00,7.00]=5.79, p=0.047). These biomarkers may be of value in detecting cerebral vasospasm, possibly aiding in the identification of patients at high-risk prior to neurological deterioration.

The Prox1-Vegfr3 feedback loop maintains the identity and the number of lymphatic endothelial cell progenitors

The lack of Prox1 activity results in the complete absence of lymphatic endothelial cells (LECs). Here, Srinivasan et al. identified Vegfr3, the cognate receptor of the lymphangiogenic growth factor Vegfc, as a dosage-dependent, direct in vivo target of Prox1. Vegfr3 regulates Prox1 by establishing a feedback loop necessary to maintain the identity of LEC progenitors, and Vegfc-mediated activation of Vegfr3 signaling is necessary to maintain Prox1 expression in LEC progenitors.

The mammalian lymphatic vasculature is important for returning fluids from the extracellular tissue milieu back to the blood circulation. We showed previously that Prox1 dosage is important for the development of the mammalian lymphatic vasculature. The lack of Prox1 activity results in the complete absence of lymphatic endothelial cells (LECs). In Prox1 heterozygous embryos, the number of LECs is reduced because of a decrease in the progenitor pool in the cardinal vein. This reduction is caused by some progenitor cells being unable to maintain Prox1 expression. In this study, we identified Vegfr3, the cognate receptor of the lymphangiogenic growth factor Vegfc, as a dosage-dependent, direct in vivo target of Prox1. Using various mouse models, we also determined that Vegfr3 regulates Prox1 by establishing a feedback loop necessary to maintain the identity of LEC progenitors and that Vegfc-mediated activation of Vegfr3 signaling is necessary to maintain Prox1 expression in LEC progenitors. We propose that this feedback loop is the main sensing mechanism controlling the number of LEC progenitors and, as a consequence, the number of budding LECs that will form the embryonic lymphatic vasculature.