Viral vascular endothelial growth factors vary extensively in amino acid sequence, receptor-binding specificities, and the ability to induce vascular permeability yet are uniformly active mitogens

Viruses exploit growth factor mechanisms to achieve augmented pathogenicity and promote tumorigenesis

Cellular homeostasis is regulated by growth factors (GFs) which orchestrate various cellular processes including proliferation, survival, differentiation, motility, inflammation and angiogenesis. Dysregulation of GFs in microbial infections and malignancies have been reported previously. Viral pathogens exemplify the exploitation of host cell GFs and their signalling pathways contributing to viral entry, virulence, and evasion of anti-viral immune responses. Viruses can also perturb cellular metabolism and the cell cycle by manipulation of GF signaling. In some cases, this disturbance may promote oncogenesis. Viral pathogens can encode viral GF homologues and induce the endogenous biosynthesis of GFs and their corresponding receptors or manipulate their activity to infect the host cells. Close investigation of how viral strategies exploit and regulate GFs, a will shed light on how to improve anti-viral therapy and cancer treatment. In this review, we discuss and provide insights on how various viral pathogens exploit different GFs to promote viral survival and oncogenic transformation, and how this knowledge can be leveraged toward the design of more efficient therapeutics or novel drug delivery systems in the treatment of both viral infections and malignancies.

Orf Virus IL-10 and VEGF-E Act Synergistically to Enhance Healing of Cutaneous Wounds in Mice

Orf virus (OV) is a zoonotic parapoxvirus that causes highly proliferative skin lesions which resolve with minimal inflammation and scarring. OV encodes two immunomodulators, vascular endothelial growth factor (VEGF)-E and interleukin-10 (ovIL-10), which individually modulate skin repair and inflammation. This study examined the effects of the VEGF-E and ovIL-10 combination on healing processes in a murine wound model. Treatments with viral proteins, individually and in combination, were compared to a mammalian VEGF-A and IL-10 combination. Wound biopsies were harvested to measure re-epithelialisation and scarring (histology), inflammation, fibrosis and angiogenesis (immunofluorescence), and gene expression (quantitative polymerase chain reaction). VEGF-E and ovIL-10 showed additive effects on wound closure and re-epithelialisation, and suppressed M1 macrophage and myofibroblast infiltration, while allowing M2 macrophage recruitment. The viral combination also increased endothelial cell density and pericyte coverage, and improved collagen deposition while reducing the scar area. The mammalian combination showed equivalent effects on wound closure, re-epithelialisation and fibrosis, but did not promote blood vessel stabilisation or collagen remodeling. The combination treatments also differentially altered the expression of transforming growth factor beta isoforms, Tgfβ1 and Tgfβ3. These findings show that the OV proteins synergistically enhance skin repair, and act in a complimentary fashion to improve scar quality.

Deriving Immune Modulating Drugs from Viruses-A New Class of Biologics

Viruses are widely used as a platform for the production of therapeutics. Vaccines containing live, dead and components of viruses, gene therapy vectors and oncolytic viruses are key examples of clinically-approved therapeutic uses for viruses. Despite this, the use of virus-derived proteins as natural sources for immune modulators remains in the early stages of development. Viruses have evolved complex, highly effective approaches for immune evasion. Originally developed for protection against host immune responses, viral immune-modulating proteins are extraordinarily potent, often functioning at picomolar concentrations. These complex viral intracellular parasites have "performed the R&D", developing highly effective immune evasive strategies over millions of years. These proteins provide a new and natural source for immune-modulating therapeutics, similar in many ways to penicillin being developed from mold or streptokinase from bacteria. Virus-derived serine proteinase inhibitors (serpins), chemokine modulating proteins, complement control, inflammasome inhibition, growth factors (e.g., viral vascular endothelial growth factor) and cytokine mimics (e.g., viral interleukin 10) and/or inhibitors (e.g., tumor necrosis factor) have now been identified that target central immunological response pathways. We review here current development of virus-derived immune-modulating biologics with efficacy demonstrated in pre-clinical or clinical studies, focusing on pox and herpesviruses-derived immune-modulating therapeutics.

Circulation of orf viruses containing the NZ7-like vascular endothelial growth factor (VEGF-E) gene type in India

Orf, a poxviral skin infection of small ruminants is caused by orf virus (ORFV) of the genus Parapoxvirus of the Poxviridae family. Vascular endothelial growth factor (VEGF) is an important virulence factor that is responsible for proliferative lesions in parapoxviral infections. VEGF gene shows high intra- and inter-species variability. Two variants of VEGF have been described globally in ORFV, viz. NZ2- and NZ7-like. In the present study, ORFV isolates of different geographic regions of India were analysed on the basis of the VEGF gene. Indian ORFV isolates showed 95.7-100 % nucleotide (nt) and 78.4-99.3 % amino acid (aa) identity with each other, except ORFV-Assam/LK/14 and ORFV-Meghalaya/03 which shared 85.1-88.35 % and 79.1-81.8 % identity, at nt and aa levels, respectively with other Indian ORFV isolates. All Indian ORFVs under the study demonstrated 83.5-99.1 % nt and 80.5-97.9 % aa identity with NZ7-like VEGF as compared to 41.2-44.8 % nt and 30.7-38.4 % aa identity with NZ2-like VEGF on comparison with global ORFV strains. Phylogenetic analysis based on the VEGF gene showed two clusters of ORFV in which the Indian ORFVs clustered with NZ7-like VEGF from global ORFV strains, mostly from China. Despite the considerable variation, VEGF protein from Indian ORFV strains showed conserved VEGF homology domain with eight cysteine residues. Homology modeling of Indian ORFV strains predicted the presence of extended Loop 3 similar to NZ7-like VEGF. Therefore, the present study showed the circulation of ORFV strains with comparatively less variable NZ7-like VEGF in India which implicates its importance in the epidemiology of ORFV infections in the country.

Poxvirus-Induced Vascular Angiogenesis Mimicking Pyogenic Granuloma

The orf virus, a member of poxvirus family, is a zoonotic parapoxvirus endemic in many countries, mostly seen among sheep, goats, oxen, and may be transmitted to humans. Orf virus infections may induce ulceration, papulonodular, pustular, or ecthyma lesions in the skin. Rarely, orf virus provokes extensive vasculoendothelial proliferation by encoding an apparent homolog of the mammalian vascular endothelial growth factor family of molecules. The vascular endothelial growth factor-like viral gene product is expressed early during infection and could be responsible for the induction of endothelial proliferation. Here, a 6-year-old male patient with poxvirus-induced widespread vascular angiogenesis is presented, which developed ten days after a thermal burn.

Exploitation of receptor tyrosine kinases by viral-encoded growth factors

Receptor tyrosine kinases (RTKs) are essential components of cell communication pathways utilized from the embryonic to adult stages of life. These transmembrane receptors bind polypeptide ligands, such as growth factors, inducing signalling cascades that control cellular processes such as proliferation, survival, differentiation, motility and inflammation. Many viruses have acquired homologs of growth factors encoded by the hosts that they infect. Production of growth factors during infection allows viruses to exploit RTKs for entry and replication in cells, as well as for host and environmental dissemination. This review describes the genetic diversity amongst virus-derived growth factors and the mechanisms by which RTK exploitation enhances virus survival, then highlights how viral ligands can be used to further understanding of RTK signalling and function during embryogenesis, homeostasis and disease scenarios.

Dermatopathology of Orf Virus (Malaysian Isolates) in Mice Experimentally Inoculated at Different Sites with and without Dexamethasone Administration

Orf is a clinical manifestation of parapoxvirus infection often fatal in goats and sheep especially when they are under stress or influenced by unfavorable environment. This study investigated the pathogenicity of two Orf virus isolates (ORFV UPM1/14 and UPM2/14) and host response in mouse model by using different inoculation sites with/without prior exposure to dexamethasone. Treatments with dexamethasone served as an immunosuppressant that may mimic stress situation in affected animals. Groups of five mice were given intradermal injection of 0.2 mL of tissue culture infective dose 50 (TCID₅₀) of UPM1/14 (Group 1) and UPM2/14 (Group 2) at the dorsum (Group 1A; Group 2A), ear pinna (Group 1B; Group 2B), and labial commissure (Group 1C; Group 2C). An inoculum 0.2 mL of UPM1/14 was administered to animals treated with dexamethasone (n=5; 5 mg/kg/day intraperitoneally) and nondexamethasone (n=5) groups at the dorsum, ear pinna, and labial commissure. No significant difference (p>0.05) was observed in the mean lesion scores among the groups of different inoculation sites or between dexamethasone-treated and nontreated groups. However, there was a significant difference (p<0.05) in the mean stratum thickness of affected skin following inoculation with UPM2/14 isolate at the ear pinna and labial commissure. Histopathology examination revealed keratosis, acanthosis, and ballooning degeneration in the skin of affected mice. Orf virus DNA was detected in the skin samples by targeting F1L and B2L virus-specific genes in polymerase chain reaction (PCR) assay. Intradermal inoculation with UPM1/14 or UPM2/14 isolate produced a mild skin lesion in mice, and there was no significant difference in orf disease manifestation despite variation of inoculation sites. Similarly, short-term dexamethasone administration gave no adverse effects on pathogenicity of orf virus isolates.

VEGF Receptor-2 Activation Mediated by VEGF-E Limits Scar Tissue Formation Following Cutaneous Injury

Objective: Vascular endothelial growth factor (VEGF) family members are critical regulators of tissue repair and depending on their distinct pattern of receptor specificity can also promote inflammation and scarring. This study utilized a receptor-selective VEGF to examine the role of VEGF receptor (VEGFR)-2 in scar tissue (ST) formation. Approach: Cutaneous skin wounds were created in mice using a 4 mm biopsy punch and then treated until closure with purified VEGF-E derived from orf virus stain NZ-2. Tissue samples were harvested to measure gene expression using quantitative PCR and to observe ST formation through histological examination and changes in cell populations by immunofluorescence. Results: VEGFR-2-activation with VEGF-E increased expression of anti-inflammatory cytokine interleukin (IL)-10 and reduced macrophage infiltration and myofibroblast differentiation in wounded skin compared with controls. VEGF-E treatment also increased microvascular density and improved pericyte coverage of blood vessels in the healing wounds. The ST that formed following treatment with VEGF-E was reduced in size and showed improved collagen structure. Innovation: The role of VEGFR-2 activation in wound epithelialization and angiogenesis is well established; but its contribution to ST formation is unclear. This study tests the effect of a selective VEGFR-2 activation on ST formation following cutaneous wounding in a murine model. Conclusion: VEGFR-2 stimulation can enhance the quality of skin repair, at least, in part, through the induction of IL-10 expression and dampening of wound inflammation and fibrosis. Therapies that selectively activate VEGFR-2 may therefore be beneficial to treat impaired healing or to prevent excess scarring.

Treatment of limb wounds of horses with orf virus IL-10 and VEGF-E accelerates resolution of exuberant granulation tissue, but does not prevent its development

Bandaging of limb wounds in horses leads to formation of exuberant granulation tissue (EGT) that retards healing due to protracted inflammation, aberrant vascularisation and delayed epithelialisation. EGT is not observed if wounds are left undressed or when wounds are on the body. A previous study showed that short-term administration of proteins derived from orf virus dampened inflammation and promoted epithelialisation of open wounds in horses. Here, we investigated the impact of orf virus interleukin-10 and vascular endothelial growth factor-E on the development and resolution of EGT. Excisional wounds were created on the forelimb of four horses, and bandages were maintained until full healing to induce EGT formation. Matching body wounds were created to ensure EGT was limited to the limb, and to differentiate the effects of the viral proteins on normal healing and on EGT formation. Viral proteins or the hydrogel vehicle control were administered topically to site-matched wounds at day 1, with repeat administration at day 8. Wound healing and EGT formation were monitored macroscopically. Wound margin samples were harvested at 2, 7 and 14 days, and at full healing, with histology used to observe epithelialisation, immunofluorescence used to detect inflammatory cells, angiogenesis and cell death, and qPCR to measure expression of genes regulating inflammation and angiogenesis. Limb wounds developed EGT, and exhibited slower healing than body wounds. Viral protein treatment did not accelerate healing at either location nor limit EGT formation in limb wounds. Treatment of limb wounds did however increase epithelialisation and angiogenesis, without dampening inflammatory cell infiltration or gene expression. The healed wounds also had less occlusion and death of blood vessels and fewer epidermal rete ridges following viral protein treatment. These findings indicate that the viral protein treatment does not suppress wound inflammation or EGT formation, but does promote vascular and epidermal repair and EGT resolution.

Emerging Roles for VEGF-D in Human Disease

Blood vessels and lymphatic vessels are located in many tissues and organs throughout the body, and play important roles in a wide variety of prevalent diseases in humans. Vascular endothelial growth factor-D (VEGF-D) is a secreted protein that can promote the remodeling of blood vessels and lymphatics in development and disease. Recent fundamental and translational studies have provided insight into the molecular mechanisms by which VEGF-D exerts its effects in human disease. Hence this protein is now of interest as a therapeutic and/or diagnostic target, or as a potential therapeutic agent, in a diversity of indications in cardiovascular medicine, cancer and the devastating pulmonary condition lymphangioleiomyomatosis. This has led to clinical trial programs to assess the effect of targeting VEGF-D signaling pathways, or delivering VEGF-D, in angina, cancer and ocular indications. This review summarizes our understanding of VEGF-D signaling in human disease, which is largely based on animal disease models and clinicopathological studies, and provides information about the outcomes of recent clinical trials testing agonists or antagonists of VEGF-D signaling.

The fibrinolysis inhibitor α2-antiplasmin restricts lymphatic remodelling and metastasis in a mouse model of cancer

Remodelling of lymphatic vessels in tumours facilitates metastasis to lymph nodes. The growth factors VEGF-C and VEGF-D are well known inducers of lymphatic remodelling and metastasis in cancer. They are initially produced as full-length proteins requiring proteolytic processing in order to bind VEGF receptors with high affinity and thereby promote lymphatic remodelling. The fibrinolytic protease plasmin promotes processing of VEGF-C and VEGF-D in vitro, but its role in processing them in cancer was unknown. Here we explore plasmin's role in proteolytically activating VEGF-D in vivo, and promoting lymphatic remodelling and metastasis in cancer, by co-expressing the plasmin inhibitor α₂-antiplasmin with VEGF-D in a mouse tumour model. We show that α₂-antiplasmin restricts activation of VEGF-D, enlargement of intra-tumoural lymphatics and occurrence of lymph node metastasis. Our findings indicate that the fibrinolytic system influences lymphatic remodelling in tumours which is consistent with previous clinicopathological observations correlating fibrinolytic components with cancer metastasis.

Differential Receptor Binding and Regulatory Mechanisms for the Lymphangiogenic Growth Factors Vascular Endothelial Growth Factor (VEGF)-C and -D

VEGF-C and VEGF-D are secreted glycoproteins that induce angiogenesis and lymphangiogenesis in cancer, thereby promoting tumor growth and spread. They exhibit structural homology and activate VEGFR-2 and VEGFR-3, receptors on endothelial cells that signal for growth of blood vessels and lymphatics. VEGF-C and VEGF-D were thought to exhibit similar bioactivities, yet recent studies indicated distinct signaling mechanisms (e.g. tumor-derived VEGF-C promoted expression of the prostaglandin biosynthetic enzyme COX-2 in lymphatics, a response thought to facilitate metastasis via the lymphatic vasculature, whereas VEGF-D did not). Here we explore the basis of the distinct bioactivities of VEGF-D using a neutralizing antibody, peptide mapping, and mutagenesis to demonstrate that the N-terminal α-helix of mature VEGF-D (Phe⁹³–Arg¹⁰⁸) is critical for binding VEGFR-2 and VEGFR-3. Importantly, the N-terminal part of this α-helix, from Phe⁹³ to Thr⁹⁸, is required for binding VEGFR-3 but not VEGFR-2. Surprisingly, the corresponding part of the α-helix in mature VEGF-C did not influence binding to either VEGFR-2 or VEGFR-3, indicating distinct determinants of receptor binding by these growth factors. A variant of mature VEGF-D harboring a mutation in the N-terminal α-helix, D103A, exhibited enhanced potency for activating VEGFR-3, was able to promote increased COX-2 mRNA levels in lymphatic endothelial cells, and had enhanced capacity to induce lymphatic sprouting in vivo. This mutant may be useful for developing protein-based therapeutics to drive lymphangiogenesis in clinical settings, such as lymphedema. Our studies shed light on the VEGF-D structure/function relationship and provide a basis for understanding functional differences compared with VEGF-C.

Short-term treatment of equine wounds with orf virus IL-10 and VEGF-E dampens inflammation and promotes repair processes without accelerating closure

Healing is delayed in limb wounds relative to body wounds of horses, partly because of sustained inflammation and inefficient angiogenesis. In laboratory animals, proteins derived from orf virus modulate these processes and enhance healing. We aimed to compare immune cell trafficking and the inflammatory, vascular, and epidermal responses in body and limb wounds of horses and then to investigate the impact of orf virus interleukin-10 and vascular endothelial growth factor-E on these processes. Standardized excisional wounds were created on the body and forelimb of horses and their progression monitored macroscopically until healed. Tissue samples were harvested to measure the expression of genes regulating inflammation and repair (quantitative polymerase chain reaction) and to observe epithelialization (histology), innate immune cell infiltration, and angiogenesis (immunofluorescence). Delayed healing of limb wounds was characterized by intensified and extended pro-inflammatory signaling and exacerbated innate immune response, concomitant with the absence of anti-inflammatory eIL-10. Blood vessels were initially more permeable and then matured belatedly, concomitant with retarded production of angiogenic factors. Epithelial coverage was achieved belatedly in limb wounds. Viral proteins were administered to wounds of one body and one limb site/horse at days 1-3, while wounds at matching sites served as controls. Treatment dampened pro-inflammatory gene expression and the innate immune response in all wounds. It also improved angiogenic gene expression, but primarily in body wounds, where it altered blood vessel density and myofibroblast persistence. Moreover, the viral proteins increased epithelialization of all wounds. The short-term viral protein therapy did not, however, improve the healing rate of wounds in either location, likely due to suboptimal dosing. In conclusion, we have further detailed the processes contributing to protracted healing in limb wounds of horses and shown that short-term administration of viral proteins exerts several promising though transient effects that, if optimized, may positively influence healing.

VEGF receptor-2-specific signaling mediated by VEGF-E induces hemangioma-like lesions in normal and in malignant tissue

Viral VEGF-E (ovVEGF-E), a homolog of VEGF-A, was discovered in the genome of Orf virus. Together with VEGF-A, B, C, D, placental growth factor (PlGF) and snake venom VEGF (svVEGF), ovVEGF-E is a member of the VEGF family of potent angiogenesis factors with a bioactivity similar to

VEGF-A

it induces proliferation, migration and sprouting of cultured vascular endothelial cells and proliferative lesions in the skin of sheep, goat and man that are characterized by massive capillary proliferation and dilation. These biological functions are mediated exclusively via its interaction with VEGF receptor-2 (VEGFR-2). Here, we have generated transgenic mice specifically expressing ovVEGF-E in β-cells of the endocrine pancreas (Rip1VEGF-E; RVE). RVE mice show an increase in number and size of the islets of Langerhans and a distorted organization of insulin and glucagon-expressing cells. Islet endothelial cells of RVE mice hyper-proliferate and form increased numbers of functional blood vessels. In addition, the formation of disorganized lymphatic vessels and increased immune cell infiltration is observed. Upon crossing RVE single-transgenic mice with Rip1Tag2 (RT2) transgenic mice, a well-studied model of pancreatic β-cell carcinogenesis, double-transgenic mice (RT2;RVE) display hyper-proliferation of endothelial cells resulting in the formation of hemangioma-like lesions. In addition, RT2;RVE mice exhibit activated lymphangiogenesis at the tumor periphery and increased neutrophil and macrophage tumor infiltration and micro-metastasis to lymph nodes and lungs. These phenotypes markedly differ from the phenotypes observed with the transgenic expression of the other VEGF family members in β-cells of normal mice and of RT2 mice.

VEGF-D promotes pulmonary oedema in hyperoxic acute lung injury

Leakage of fluid from blood vessels, leading to oedema, is a key feature of many diseases including hyperoxic acute lung injury (HALI), which can occur when patients are ventilated with high concentrations of oxygen (hyperoxia). The molecular mechanisms driving vascular leak and oedema in HALI are poorly understood. VEGF‐D is a protein that promotes blood vessel leak and oedema when overexpressed in tissues, but the role of endogenous VEGF‐D in pathological oedema was unknown. To address these issues, we exposed Vegfd‐deficient mice to hyperoxia. The resulting pulmonary oedema in Vegfd‐deficient mice was substantially reduced compared to wild‐type, as was the protein content of bronchoalveolar lavage fluid, consistent with reduced vascular leak. Vegf‐d and its receptor Vegfr‐3 were more highly expressed in lungs of hyperoxic, versus normoxic, wild‐type mice, indicating that components of the Vegf‐d signalling pathway are up‐regulated in hyperoxia. Importantly, VEGF‐D and its receptors were co‐localized on blood vessels in clinical samples of human lungs exposed to hyperoxia; hence, VEGF‐D may act directly on blood vessels to promote fluid leak. Our studies show that Vegf‐d promotes oedema in response to hyperoxia in mice and support the hypothesis that VEGF‐D signalling promotes vascular leak in human HALI. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

A Simple Bioassay for the Evaluation of Vascular Endothelial Growth Factors

The analysis of receptor tyrosine kinases and their interacting ligands involved in vascular biology is often challenging due to the constitutive expression of families of related receptors, a broad range of related ligands and the difficulty of dealing with primary cultures of specialized endothelial cells. Here we describe a bioassay for the detection of ligands to the vascular endothelial growth factor receptor-2 (VEGFR-2), a key transducer of signals that promote angiogenesis and lymphangiogenesis. A cDNA encoding a fusion of the extracellular (ligand-binding) region of VEGFR-2 with the transmembrane and cytoplasmic regions of the erythropoietin receptor (EpoR) is expressed in the factor-dependent cell line Ba/F3. This cell line grows in the presence of interleukin-3 (IL-3) and withdrawal of this factor results in death of the cells within 24 hr. Expression of the VEGFR-2/EpoR receptor fusion provides an alternative mechanism to promote survival and potentially proliferation of stably transfected Ba/F3 cells in the presence of a ligand capable of binding and cross-linking the extracellular portion of the fusion protein (i.e., one that can cross-link the VEGFR-2 extracellular region). The assay can be performed in two ways: a semi-quantitative approach in which small volumes of ligand and cells permit a rapid result in 24 hr, and a quantitative approach involving surrogate markers of a viable cell number. The assay is relatively easy to perform, is highly responsive to known VEGFR-2 ligands and can accommodate extracellular inhibitors of VEGFR-2 signaling such as monoclonal antibodies to the receptor or ligands, and soluble ligand traps.

Molecular genetic analysis of orf virus: a poxvirus that has adapted to skin

Orf virus is the type species of the Parapoxvirus genus of the family Poxviridae. It induces acute pustular skin lesions in sheep and goats and is transmissible to humans. The genome is G+C rich, 138 kbp and encodes 132 genes. It shares many essential genes with vaccinia virus that are required for survival but encodes a number of unique factors that allow it to replicate in the highly specific immune environment of skin. Phylogenetic analysis suggests that both viral interleukin-10 and vascular endothelial growth factor genes have been "captured" from their host during the evolution of the parapoxviruses. Genes such as a chemokine binding protein and a protein that binds granulocyte-macrophage colony-stimulating factor and interleukin-2 appear to have evolved from a common poxvirus ancestral gene while three parapoxvirus nuclear factor (NF)-κB signalling pathway inhibitors have no homology to other known NF-κB inhibitors. A homologue of an anaphase-promoting complex subunit that is believed to manipulate the cell cycle and enhance viral DNA synthesis appears to be a specific adaptation for viral-replication in keratinocytes. The review focuses on the unique genes of orf virus, discusses their evolutionary origins and their role in allowing viral-replication in the skin epidermis.

Comparative genomic sequence analysis of Chinese orf virus strain NA1/11 with other parapoxviruses

Orf virus (ORFV) is a typical member of the genus Parapoxvirus. The parapoxvirus genome consists of highly variable terminal regions and relatively conserved central regions with a high G + C content. In our previous study, a novel ORFV strain, NA1/11, was isolated from northeastern China. To fully characterize this strain, we sequenced the entire genome of NA1/11 and conducted a comparative analysis using multiple parapoxviruses. The genomic sequence of NA1/11 was found to consist of 137,080 nucleotides with a G + C content of 63.6 %, but it did not contain the terminal hairpin sequence. Alignment of ORFs from NA1/11 with NZ2, IA82 and SA00 revealed several highly variable ORFs, while the most evident ones are ORFs 001, 103, 109-110, 116 and 132. An odd phenomenon in the region of ORFs 118-120 is that the non-coding fragments are almost as long as the coding fragments. By comparative analysis of inverted terminal repeats, we identified one repeat motif and a long conserved fragment. By comparing the ITRs of SA00 with those of three other ORFVs, more clues were obtained about the correlation between ITR sequence and host adaption. Comparison of the NA1/11 genome with the sequences of other strains of ORFV revealed highly variable regions, thus providing new insights into the genetic diversity of ORFV.

Lymphovascular and neural regulation of metastasis: shared tumour signalling pathways and novel therapeutic approaches

The progression of cancer is supported by a wide variety of non-neoplastic cell types which make up the tumour stroma, including immune cells, endothelial cells, cancer-associated fibroblasts and nerve fibres. These host cells contribute molecular signals that enhance primary tumour growth and provide physical avenues for metastatic dissemination. This article provides an overview of the role of blood vessels, lymphatic vessels and nerve fibres in the tumour microenvironment and highlights the interconnected molecular signalling pathways that control their development and activation in cancer. Further, this article highlights the known pharmacological agents which target these pathways and discusses the potential therapeutic uses of drugs that target angiogenesis, lymphangiogenesis and stress-response pathways in the different stages of cancer care.

The propeptides of VEGF-D determine heparin binding, receptor heterodimerization, and effects on tumor biology

VEGF-D is an angiogenic and lymphangiogenic glycoprotein that can be proteolytically processed generating various forms differing in subunit composition due to the presence or absence of N- and C-terminal propeptides. These propeptides flank the central VEGF homology domain, that contains the binding sites for VEGF receptors (VEGFRs), but their biological functions were unclear. Characterization of propeptide function will be important to clarify which forms of VEGF-D are biologically active and therefore clinically relevant. Here we use VEGF-D mutants deficient in either propeptide, and in the capacity to process the remaining propeptide, to monitor the functions of these domains. We report for the first time that VEGF-D binds heparin, and that the C-terminal propeptide significantly enhances this interaction (removal of this propeptide from full-length VEGF-D completely prevents heparin binding). We also show that removal of either the N- or C-terminal propeptide is required for VEGF-D to drive formation of VEGFR-2/VEGFR-3 heterodimers which have recently been shown to positively regulate angiogenic sprouting. The mature form of VEGF-D, lacking both propeptides, can also promote formation of these receptor heterodimers. In a mouse tumor model, removal of only the C-terminal propeptide from full-length VEGF-D was sufficient to enhance angiogenesis and tumor growth. In contrast, removal of both propeptides is required for high rates of lymph node metastasis. The findings reported here show that the propeptides profoundly influence molecular interactions of VEGF-D with VEGF receptors, co-receptors, and heparin, and its effects on tumor biology.

VEGF-A is necessary and sufficient for retinal neuroprotection in models of experimental glaucoma

Vascular endothelial growth factor A (VEGF-A) is a validated therapeutic target in several angiogenic- and vascular permeability–related pathological conditions, including certain cancers and potentially blinding diseases, such as age-related macular degeneration and diabetic retinopathy. We and others have shown that VEGF-A also plays an important role in neuronal development and neuroprotection, including in the neural retina. Antagonism of VEGF-A function might therefore present a risk to neuronal survival as a significant adverse effect. Herein, we demonstrate that VEGF-A acts directly on retinal ganglion cells (RGCs) to promote survival. VEGF receptor-2 signaling via the phosphoinositide-3-kinase/Akt pathway was required for the survival response in isolated RGCs. These results were confirmed in animal models of staurosporine-induced RGC death and experimental hypertensive glaucoma. Importantly, we observed that VEGF-A blockade significantly exacerbated neuronal cell death in the hypertensive glaucoma model. Our findings highlight the need to better define the risks associated with use of VEGF-A antagonists in the ocular setting.

Vascular endothelial growth factor-D: signaling mechanisms, biology, and clinical relevance

Vascular endothelial growth factor-D (VEGF-D) is a secreted glycoprotein that promotes growth of blood vessels (angiogenesis) and lymphatic vessels (lymphangiogenesis), and can induce remodeling of large lymphatics. VEGF-D enhances solid tumor growth and metastatic spread in animal models of cancer, and in some human cancers VEGF-D correlates with metastatic spread, poor patient outcome, and, potentially, with resistance to anti-angiogenic drugs. Hence, VEGF-D signaling is a potential target for novel anti-cancer therapeutics designed to enhance anti-angiogenic approaches and to restrict metastasis. In the cardiovascular system, delivery of VEGF-D in animal models enhanced angiogenesis and tissue perfusion, findings which have led to a range of clinical trials testing this protein for therapeutic angiogenesis in cardiovascular diseases. Despite these experimental and clinical developments, our knowledge of the signaling mechanisms driven by VEGF-D is still evolving--here we explore the biology of VEGF-D, its signaling mechanisms, and the clinical relevance of this growth factor.

The vascular endothelial growth factor (VEGF)-E encoded by orf virus regulates keratinocyte proliferation and migration and promotes epidermal regeneration

Vascular endothelial growth factor (VEGF)-A, a key regulator of cutaneous blood vessel formation, appears to have an additional role during wound healing, enhancing re-epithelialization. Orf virus, a zoonotic parapoxvirus, induces proliferative skin lesions that initiate in wounds and are characterized by extensive blood vessel formation, epidermal hyperplasia and rete ridge formation. The vascular changes beneath the lesion are largely due to viral-expressed VEGF-E. This study investigated using mouse skin models whether VEGF-E can induce epidermal changes such as that seen in the viral lesion. Injection of VEGF-E into normal skin increased the number of endothelial cells and blood vessels within the dermis and increased epidermal thickening and keratinocyte number. Injection of VEGF-E into wounded skin, which more closely mimics orf virus lesions, increased neo-epidermal thickness and area, promoted rete ridge formation, and enhanced wound re-epithelialization. Quantitative RT-PCR analysis showed that VEGF-E did not induce expression of epidermal-specific growth factors within the wound, but did increase matrix metalloproteinase (MMP)-2 and MMP-9 expression. In cell-based assays, VEGF-E induced keratinocyte migration and proliferation, responses that were inhibited by a neutralizing antibody against VEGF receptor (VEGFR)-2. These findings demonstrate that VEGF-E, both directly and indirectly, regulates keratinocyte function, thereby promoting epidermal regeneration.

Tying the knot: the cystine signature and molecular-recognition processes of the vascular endothelial growth factor family of angiogenic cytokines

The cystine-knot motif, made up of three intertwined disulfide bridges, is a unique feature of several toxins, cyclotides and growth factors, and occurs in a variety of species, including fungi, insects, molluscs and mammals. Growth factor molecules containing the cystine-knot motif serve as ligands for a diverse range of receptors and play an important role in extracellular signalling. This superfamily of polypeptides comprises several homodimeric and heterodimeric molecules that are central characters in both health and disease. Amongst these molecules are a group of proteins that belong to the vascular endothelial growth factor (VEGF) subfamily. The members of this family are known angiogenic factors that regulate processes leading to blood vessel formation in physiological and pathological conditions. The focus of the present review is on the structural characteristics of proteins that belong to the VEGF family and on signal-transduction pathways that become initiated via the VEGF receptors.

Proteolytic processing of vascular endothelial growth factor-D is essential for its capacity to promote the growth and spread of cancer

VEGF-D is a mitogen for endothelial cells that promotes tumor growth and metastatic spread in animal models, and expression of which correlates with lymph node metastasis in some human cancers. It is secreted from the cell as a full-length form with propeptides flanking a central region containing binding sites for VEGFR-2 and VEGFR-3, receptors that signal for angiogenesis and lymphangiogenesis. The propeptides can be cleaved from VEGF-D, enhancing affinity for VEGFR-2 and VEGFR-3 in vitro; however, the importance of this processing in cancer is unclear. To explore the necessity of processing for the effects of VEGF-D in cancer, we use a mutant full-length form that cannot be processed, and show that, in contrast to full-length VEGF-D that is processed, this mutant does not promote tumor growth and lymph node metastasis in a mouse tumor model. Processing of VEGF-D is required for tumor angiogenesis, lymphangiogenesis, and recruitment of tumor-associated macrophages. These observations may be explained by the requirement of processing for VEGF-D to bind neuropilin receptors and activate VEGFR-2. Our results indicate that proteolytic processing is necessary for VEGF-D to promote the growth and spread of cancer, and suggest that enzymes catalyzing this processing could be targets for antimetastatic therapeutics.

The VD1 neutralizing antibody to vascular endothelial growth factor-D: binding epitope and relationship to receptor binding

Vascular endothelial growth factor-D (VEGF-D) is a secreted protein that promotes tumor growth and metastatic spread in animal models of cancer. Expression of VEGF-D in prevalent human cancers was reported to correlate with lymph node metastasis and patient outcome-hence, this protein is a potential target for novel anticancer therapeutics designed to restrict tumor growth and spread. Here, we define the binding site in VEGF-D of a neutralizing antibody, designated VD1, which blocks the interaction of VEGF-D with its cell surface receptors vascular endothelial growth factor receptor (VEGFR)-2 and VEGFR-3 and is being used for the development of therapeutic antibodies. We show by peptide-based mapping and site-directed mutagenesis that the VD1 binding site includes the five residues (147)NEESL(151) and that immunization with a synthetic peptide containing this motif generates antibodies that neutralize VEGF-D. The tertiary structure of VEGF-D indicates that the (147)NEESL(151) epitope is located in the L2 loop of the growth factor, which is important for receptor binding. Mutation of any of these five residues influences receptor binding; for example, mutations to E148, which abolished binding to VD1, impaired the interaction with VEGFR-2 but enhanced binding to VEGFR-3. This structure/function study indicates that the VD1 binding epitope is part of the receptor binding site of VEGF-D, identifies a region of VEGF-D critical for binding of receptors and explains why VD1 does not bind other members of the VEGF family of growth factors.

Effect of peptide bioregulator on healing of excision wounds in old animals

Aging is associated with reduction of protein synthesis in cells, which leads to deceleration of proliferative processes in various tissues. Recovery of damaged skin sites was stimulated with a peptide bioregulator chondrolux. This agent is based on an extract from calf cartilage and bone tissue. Its effect on healing of excision wounds was studied in old rabbits. Morphological analysis of the wound surface during various periods of healing was carried out by electron microscopy. The results indicate that chondrolux application to the wound surface stimulated and optimized the reparative process. Active development of granulation tissue was observed as early as on day 14 after wound infliction (vs. days 21-28 in control animals). Acceleration of wound healing was paralleled by an increase in functional activities of fibroblast organelles.

Structural determinants of growth factor binding and specificity by VEGF receptor 2

Vascular endothelial growth factors (VEGFs) regulate blood and lymph vessel formation through activation of three receptor tyrosine kinases, VEGFR-1, -2, and -3. The extracellular domain of VEGF receptors consists of seven immunoglobulin homology domains, which, upon ligand binding, promote receptor dimerization. Dimerization initiates transmembrane signaling, which activates the intracellular tyrosine kinase domain of the receptor. VEGF-C stimulates lymphangiogenesis and contributes to pathological angiogenesis via VEGFR-3. However, proteolytically processed VEGF-C also stimulates VEGFR-2, the predominant transducer of signals required for physiological and pathological angiogenesis. Here we present the crystal structure of VEGF-C bound to the VEGFR-2 high-affinity-binding site, which consists of immunoglobulin homology domains D2 and D3. This structure reveals a symmetrical 22 complex, in which left-handed twisted receptor domains wrap around the 2-fold axis of VEGF-C. In the VEGFs, receptor specificity is determined by an N-terminal alpha helix and three peptide loops. Our structure shows that two of these loops in VEGF-C bind to VEGFR-2 subdomains D2 and D3, while one interacts primarily with D3. Additionally, the N-terminal helix of VEGF-C interacts with D2, and the groove separating the two VEGF-C monomers binds to the D2/D3 linker. VEGF-C, unlike VEGF-A, does not bind VEGFR-1. We therefore created VEGFR-1/VEGFR-2 chimeric proteins to further study receptor specificity. This biochemical analysis, together with our structural data, defined VEGFR-2 residues critical for the binding of VEGF-A and VEGF-C. Our results provide significant insights into the structural features that determine the high affinity and specificity of VEGF/VEGFR interactions.

The genome of pseudocowpoxvirus: comparison of a reindeer isolate and a reference strain

Parapoxviruses (PPV), of the family Poxviridae, cause a pustular cutaneous disease in sheep and goats (orf virus, ORFV) and cattle (pseudocowpoxvirus, PCPV and bovine papular stomatitis virus, BPSV). Here, we present the first genomic sequence of a reference strain of PCPV (VR634) along with the genomic sequence of a PPV (F00.120R) isolated in Finland from reindeer (Rangifer tarandus tarandus). The F00.120R and VR634 genomes are 135 and 145 kb in length and contain 131 and 134 putative genes, respectively, with their genome organization being similar to that of other PPVs. The predicted proteins of F00.120R and VR634 have an average amino acid sequence identity of over 95%, whereas they share only 88 and 73% amino acid identity with the ORFV and BPSV proteomes, respectively. The most notable differences were found near the genome termini. F00.120R lacks six and VR634 lacks three genes seen near the right terminus of other PPVs. Four genes at the left end of F00.120R and one in the middle of both genomes appear to be fragmented paralogues of other genes within the genome. VR634 has larger than expected inverted terminal repeats possibly as a result of genomic rearrangements. The high G+C content (64%) of these two viruses along with amino acid sequence comparisons and whole genome phylogenetic analyses confirm the classification of PCPV as a separate species within the genus Parapoxvirus and verify that the virus responsible for an outbreak of contagious stomatitis in reindeer over the winter of 1999-2000 can be classified as PCPV.

Structure-function analysis of VEGF receptor activation and the role of coreceptors in angiogenic signaling

Vascular endothelial growth factors (VEGFs) constitute a family of six polypeptides, VEGF-A, -B, -C, -D, -E and PlGF, that regulate blood and lymphatic vessel development. VEGFs specifically bind to three type V receptor tyrosine kinases (RTKs), VEGFR-1, -2 and -3, and to coreceptors such as neuropilins and heparan sulfate proteoglycans (HSPG). VEGFRs are activated upon ligand-induced dimerization mediated by the extracellular domain (ECD). A study using receptor constructs carrying artificial dimerization-promoting transmembrane domains (TMDs) showed that receptor dimerization is necessary, but not sufficient, for receptor activation and demonstrates that distinct orientation of receptor monomers is required to instigate transmembrane signaling. Angiogenic signaling by VEGF receptors also depends on cooperation with specific coreceptors such as neuropilins and HSPG. A number of VEGF isoforms differ in binding to coreceptors, and ligand-specific signal output is apparently the result of the specific coreceptor complex assembled by a particular VEGF isoform. Here we discuss the structural features of VEGF family ligands and their receptors in relation to their distinct signal output and angiogenic potential.

Proteomic characterization of HIV-modulated membrane receptors, kinases and signaling proteins involved in novel angiogenic pathways

Background

Kaposi's sarcoma (KS), hemangioma, and other angioproliferative diseases are highly prevalent in HIV-infected individuals. While KS is etiologically linked to the human herpesvirus-8 (HHV8) infection, HIV-patients without HHV-8 and those infected with unrelated viruses also develop angiopathies. Further, HIV-Tat can activate protein-tyrosine-kinase (PTK-activity) of the vascular endothelial growth factor receptor involved in stimulating angiogenic processes. However, Tat by itself or HHV8-genes alone cannot induce angiogenesis in vivo unless specific proteins/enzymes are produced synchronously by different cell-types. We therefore tested a hypothesis that chronic HIV-replication in non-endothelial cells may produce novel factors that provoke angiogenic pathways.

Methods

Genome-wide proteins from HIV-infected and uninfected T-lymphocytes were tested by subtractive proteomics analyses at various stages of virus and cell growth in vitro over a period of two years. Several thousand differentially regulated proteins were identified by mass spectrometry (MS) and >200 proteins were confirmed in multiple gels. Each protein was scrutinized extensively by protein-interaction-pathways, bioinformatics, and statistical analyses.

Results

By functional categorization, 31 proteins were identified to be associated with various signaling events involved in angiogenesis. 88% proteins were located in the plasma membrane or extracellular matrix and >90% were found to be essential for regeneration, neovascularization and angiogenic processes during embryonic development.

Conclusion

Chronic HIV-infection of T-cells produces membrane receptor-PTKs, serine-threonine kinases, growth factors, adhesion molecules and many diffusible signaling proteins that have not been previously reported in HIV-infected cells. Each protein has been associated with endothelial cell-growth, morphogenesis, sprouting, microvessel-formation and other biological processes involved in angiogenesis (p = 10^-4 to 10^-12). Bioinformatics analyses suggest that overproduction of PTKs and other kinases in HIV-infected cells has suppressed VEGF/VEGFR-PTK expression and promoted VEGFR-independent pathways. This unique mechanism is similar to that observed in neovascularization and angiogenesis during embryogenesis. Validation of clinically relevant proteins by gene-silencing and translational studies in vivo would identify specific targets that can be used for early diagnosis of angiogenic disorders and future development of inhibitors of angiopathies. This is the first comprehensive study to demonstrate that HIV-infection alone, without any co-infection or treatment, can induce numerous "embryonic" proteins and kinases capable of generating novel VEGF-independent angiogenic pathways.

Snake venom Vascular Endothelial Growth Factors (VEGF-Fs) exclusively vary their structures and functions among species

Vascular endothelial growth factor (VEGF-A) and its family proteins are crucial regulators of blood vessel formation and vascular permeability. Snake venom has recently been shown to be an exogenous source of unique VEGF (known as VEGF-F), and now, two types of VEGF-F with distinct biochemical properties have been reported. Here, we show that VEGF-Fs (venom-type VEGFs) are highly variable in structure and function among species, in contrast to endogenous tissue-type VEGFs (VEGF-As) of snakes. Although the structures of tissue-type VEGFs are highly conserved among venomous snake species and even among all vertebrates, including humans, those of venom-type VEGFs are extensively variegated, especially in the regions around receptor-binding loops and C-terminal putative coreceptor-binding regions, indicating that highly frequent variations are located around functionally key regions of the proteins. Genetic analyses suggest that venom-type VEGF gene may have developed from a tissue-type gene and that the unique sequence of its C-terminal region was generated by an alteration in the translation frame in the corresponding exons. We further verified that a novel venom-type VEGF from Bitis arietans displays unique properties distinct from already known VEGFs. Our results may provide evidence of a novel mechanism causing the generation of multiple snake toxins and also of a new model of molecular evolution.

Orf virus VEGF-E NZ2 promotes paracellular NRP-1/VEGFR-2 coreceptor assembly via the peptide RPPR

Vascular endothelial growth factors (VEGFs) interact with the receptor tyrosine kinases (RTKs) VEGFR-1, -2, and -3; neuropilins (NRPs); and heparan sulfate (HS) proteoglycans. VEGF RTKs signal to downstream targets upon ligand-induced tyrosine phosphorylation, while NRPs and HS act as coreceptors that lack enzymatic activity yet modulate signal output by VEGF RTKs. VEGFs exist in various isoforms with distinct receptor specificity and biological activity. Here, a series of mammalian VEGF-A splice variants and orf virus VEGF-Es, as well as chimeric and mutant VEGF variants, were characterized to determine the motifs required for binding to NRP-1 in the absence (VEGF-E) or presence (VEGF-A(165)) of an HS-binding sequence. We identified the carboxyterminal peptides RPPR and DKPRR as the NRP-1 binding motifs of VEGF-E and VEGF-A, respectively. RPPR had significantly higher affinity for NRP-1 than DKPRR. VEGFs containing an RPPR motif promoted HS-independent coreceptor complex assembly between VEGFR-2 and NRP-1, independent of whether these receptors were expressed on the same or separate cells grown in cocultures. Functional studies showed that stable coreceptor assembly by VEGF correlated with its ability to promote vessel formation in an embryoid body angiogenesis assay.

Infectious spleen and kidney necrosis virus ORF48R functions as a new viral vascular endothelial growth factor

Infectious spleen and kidney necrosis virus (ISKNV) causes a pandemic and serious disease in fish. Infection by ISKNV causes epidermal lesions, in which petechial hemorrhages and abdominal edema are prominent features. ISKNV ORF48R contains a domain similar to that of the platelet-derived growth factor and vascular endothelial growth factor (VEGF) families of proteins. ISKNV ORF48R showed higher similarity to the VEGFs encoded by Megalocytivirus and Parapoxvirus than to those encoded in fish and mammals. We used zebrafish as a model and constructed a recombinant plasmid containing the DNA sequence of ISKNV ORF48R to study ISKNV infection. The plasmid was microinjected into zebrafish embryos at the one-cell stage. Overexpression of the ISKNV ORF48R gene results in pericardial edema and dilation at the tail region of zebrafish embryos, suggesting that ISKNV ORF48R induces vascular permeability. ISKNV ORF48R is also able to stimulate a striking expression of flk1 in the zebrafish dorsal aorta and the axial vein. Furthermore, ISKNV ORF48R, while cooperating with zebrafish VEGF(121), can stimulate more striking expression of flk1 than can either ISKNV ORF48R or zebrafish VEGF(121) alone. However, decreased expression of FLK-1 by gene knockdown results in the disappearance of pericardial edema and dilation at the tail region of zebrafish embryos induced by overexpression of ISKNV ORF48R in the early stages of embryonic development.

Functional significance of vascular endothelial growth factor receptors on gastrointestinal cancer cells

Vascular endothelial growth factor (VEGF) has been shown to be the major mediator of physiologic and pathologic angiogenesis. VEGF was initially thought to be an endothelial cell specific ligand, but recently, VEGF has been shown to mediate tumor cell function via activation of receptors on tumor cells themselves. Here, we review the expression patterns and binding profiles of the VEGF receptors and their ligands on gastrointestinal tumor cells. Furthermore, we describe the current knowledge in regards to the function of these receptors on tumor cells. Elucidating the function of VEGF receptors on tumor cells should help us to better understand the potential mechanisms of action of anti-VEGF therapies.

The C-terminus of viral vascular endothelial growth factor-E partially blocks binding to VEGF receptor-1

Vascular endothelial growth factor (VEGF) family members play important roles in embryonic development and angiogenesis during wound healing and in pathological conditions such as tumor formation. Parapoxviruses express a new member of the VEGF family which is a functional mitogen that specifically activates VEGF receptor (VEGFR)-2 but not VEGFR-1. In this study, we show that deletion from the viral VEGF of a unique C-terminal region increases both VEGFR-1 binding and VEGFR-1-mediated monocyte migration. Enzymatic removal of O-linked glycosylation from the C-terminus also increased VEGFR-1 binding and migration of THP-1 monocytes indicating that both the C-terminal residues and O-linked sugars contribute to blocking viral VEGF binding to VEGFR-1. The data suggest that conservation of the C-terminal residues throughout the viral VEGF subfamily may represent a means of reducing the immunostimulatory activities associated with VEGFR-1 activation while maintaining the ability to induce angiogenesis via VEGFR-2.

Major amino acid sequence variants of viral vascular endothelial growth factor are functionally equivalent during Orf virus infection of sheep skin

Orf virus infection causes a contagious pustular dermatitis characterized by extensive vascular changes that have been linked to a virally encoded vascular endothelial growth factor (VEGF). The VEGF genes of different strains of orf virus can vary extensively in amino acid sequence. Functional analyses of two major variant VEGF proteins derived from orf virus strains, NZ2 and NZ7, have revealed quantitative differences in biological activities and receptor binding specificities suggesting that these viral VEGFs could have different roles in the pathology of orf virus infection. In this study, we show that both orf virus strains express equivalent levels of the viral VEGF variants and during infection of sheep skin induce comparable levels of vascularization, edema, epidermal rete ridge and scab formation. Recombinants of orf virus NZ2 and NZ7 strains in which the variant VEGF genes were disrupted showed markedly reduced vascular changes and evidence of partially attenuated viral growth. These results demonstrate that despite substantial differences in sequence and biological activity in vitro, these virally expressed virulence factors are functionally equivalent in their natural host, contributing equally to orf virus pathology.

Bovine papular stomatitis virus encodes a functionally distinct VEGF that binds both VEGFR-1 and VEGFR-2

Bovine papular stomatitis virus (BPSV), a member of the genus Parapoxvirus, causes proliferative dermatitis in cattle and humans. Other species of the genus cause similar lesions, the nature of which has been attributed, at least in part, to a viral-encoded vascular endothelial growth factor (VEGF) that induces vascularization and dermal oedema through VEGF receptor-2 (VEGFR-2). The results of this study showed that BPSV strain V660 encodes a novel VEGF and that the predicted BPSV protein showed only 33-52% amino acid identity to VEGFs encoded by the other species of the genus. BPSV VEGF showed higher identity to mammalian VEGF-A (51%) than the other parapoxviral VEGFs (31-46%). Assays of the purified BPSV VEGF (BPSVV660VEGF) demonstrated that it was also functionally more similar to VEGF-A, as it showed significant binding to VEGFR-1 and induced monocyte migration. Like VEGF-A and the other viral VEGFs, BPSVV660VEGF bound VEGFR-2 with high affinity. Sequence analysis and structural modelling of BPSVV660VEGF revealed specific residues, outside the known receptor-binding face, that are predicted either to influence VEGF structure or to mediate binding directly to the VEGFRs. These results indicate that BPSVV660VEGF is a biologically active member of the VEGF family and that, via its interaction with VEGFR-2, it is likely to contribute to the proliferative and highly vascularized nature of BPSV lesions. This is also the first example of a viral VEGF acting via VEGFR-1 and influencing haematopoietic cell function. These data suggest that BPSVV660VEGF is an evolutionary and functional intermediate between VEGF-A and the other parapoxviral VEGFs.

Augmentation of vascular permeability of VEGF is enhanced by KDR-binding proteins

VEGF165 is a key regulator of angiogenesis and a potent vascular permeability factor. Using snake venom proteins as tools, we demonstrate the enhanced vascular leakage of VEGF by KDR-binding proteins. The snake venom-derived KDR-specific VEGF, vammin, potently enhanced vascular leakage compared with other known permeability-enhancing factors including VEGF165, while KDR-bp from snake venom, a KDR antagonist of endothelial cell growth was a very weak permeability enhancer. Unexpectedly when co-administrated, KDR-bp synergistically enhanced either vammin or VEGF165-stimulated vascular leakage, despite its antagonistic effect on cell growth. This augmenting effect was specifically observed in the combined administration of KDR-bp with either VEGF165 or vammin, but not other combination of known permeability-enhancing factors. We further demonstrated a similar increased vascular leakage by the combined administration of VEGF165 and TIMP-3, the only known endogenous antagonist of KDR. Our findings implicate TIMP-3 as a critical player in the vascular leakage-enhancing effect of VEGF165 in vivo.

Genus Parapoxvirus

Highly contagious pustular skin infections of sheep, goats and cattle that were unwittingly transmitted to humans from close contact with infected animals, have been the scourge of shepherds, herdsmen and dairy farmers for centuries. In more recent times we recognise that these proliferative pustular lesions are likely to be caused by a group of zoonotic viruses that are classified as parapoxviruses. In addition to infecting the above ungulates, parapoxviruses have more recently been isolated from seals, camels, red deer and reindeer and most have been shown to infect man. The parapoxviruses have one of the smallest genomes of the poxvirus family (140 kb) yet share over 70% of their genes with the most virulent members. Like other poxviruses, the central core of the genomes encode factors for virus transcription and replication, and structural proteins, whereas the terminal regions encode accessory factors that give the parapoxvirus group many of its unique features. Several genes of parapoxviruses are unique to this genus and encode factors that target inflammation, the innate immune responses and the development of acquired immunity. These factors include a homologue of mammalian interleukin (IL)-10, a chemokine binding protein and a granulocyte-macrophage colony stimulating factor /IL-2 binding protein. The ability of this group to reinfect their hosts, even though a cell-mediated memory response is induced during primary infection, may be related to their epitheliotropic niche and the immunomodulators they produce. In this highly localised environment, the secreted immunomodulators only interfere with the local immune response and thus do not compromise the host’s immune system. The discovery of a vascular endothelial growth factor-like gene may explain the highly vascular nature of parapoxvirus lesions. There are many genes of parapoxviruses which do not encode polypeptides with significant matches with protein sequences in public databases, separating this genus from most other mammalian poxviruses. These genes appear to be involved in inhibiting apoptosis, manipulating cell cycle progression and degradation of cellular proteins that may be involved in the stress response, thus allowing the virus to subvert intracellular antiviral mechanisms and enhance the availability of cellular molecules required for replication. Parapoxviruses in common with Molluscum contagiosum virus lack a number of genes that are highly conserved in other poxviruses, including factors for nucleotide metabolism, serine protease inhibitors and kelch-like proteins. It is apparent that parapoxviruses have evolved a unique repertoire of genes that have allowed adaptation to the highly specialised environment of the epidermis.

Parapoxvirus of red deer in New Zealand encodes a variant of viral vascular endothelial growth factor

Parapoxvirus of red deer in New Zealand (PVNZ), a species of the Parapoxvirus genus, causes scabby lesions on the skin and the velvet of red deer. The three other species of the genus have each been shown to encode homologs of vascular endothelial growth factor (VEGF). We report here that PVNZ strain RD86 also encodes a VEGF and that the predicted PVNZ protein shows only 37-54% amino acid identity to VEGFs encoded by the other species of the genus. Despite this extensive sequence divergence, assays of purified PVNZ VEGF (PVNZ(RD86)VEGF) demonstrated that it shares the unique VEGF receptor (VEGFR) binding profile of the other parapoxvirus VEGFs, in that it binds VEGFR-2 and induces VEGFR-2-mediated proliferation of Ba/F3-derived cells, but does not bind VEGFR-1 or VEGFR-3. In contrast to some other viral VEGFs, it does not bind neuropilin-1. Our results indicate that PVNZ(RD86)VEGF is a biologically active member of the VEGF family and is likely to contribute to the proliferative and highly vascularized nature of PVNZ lesions. Our data also reveal that all members of the genus encode a VEGF and that an extraordinary degree of inter-species sequence variation is a general feature of the parapoxvirus VEGFs.

Molecular and functional diversity of vascular endothelial growth factors

Members of the vascular endothelial growth factor (VEGF) family are crucial regulators of neovascularization and are classified as cystine knot growth factors that specifically bind cellular receptor tyrosine kinases VEGFR-1, VEGFR-2, and VEGFR-3 with high but variable affinity and selectivity. The VEGF family has recently been expanded and currently comprises seven members: VEGF-A, VEGF-B, placenta growth factor (PlGF), VEGF-C, VEGF-D, viral VEGF (also known as VEGF-E), and snake venom VEGF (also known as VEGF-F). Although all members are structurally homologous, there is molecular diversity among the subtypes, and several isoforms, such as VEGF-A, VEGF-B, and PlGF, are generated by alternative exon splicing. These splicing isoforms exhibit differing properties, particularly in binding to co-receptor neuropilins and heparin. VEGF family proteins play multiple physiological roles, such as angiogenesis and lymphangiogenesis, while exogenous members (viral and snake venom VEGFs) display activities that are unique in physiology and function. This review will highlight the molecular and functional diversity of VEGF family proteins.

Localization of heparin- and neuropilin-1-recognition sites of viral VEGFs

VEGF-A165 plays a central role in neovascularization. The biological activities of VEGF-A165 are largely mediated through KDR. VEGF-A165 also binds to cellular coreceptors, neuropilin-1 (NP-1), and heparin, via its C-terminal domain, resulting in functional modulation. Parapoxvirus-encoded VEGFs (PV-VEGFs), which recognize KDR, possess basic amino acid clusters in their C-terminal regions. Some PV-VEGFs may interact with NP-1; however, the NP-1- and heparin-binding properties have not been fully characterized. Here, we demonstrate that the heparin- and NP-1-binding region of PV-VEGFs is located in its C-terminal tail. Furthermore, the two arginine residues adjacent to the C-terminus greatly contribute to both interactions.

Crystal Structure of the Orf Virus NZ2 Variant of Vascular Endothelial Growth Factor-E

Mammalian vascular endothelial growth factors constitute a family of polypeptides, vascular endothelial growth factor (VEGF)-A, -B, -C, -D and placenta growth factor (PlGF), that regulate blood and lymphatic vessel development. VEGFs bind to three types of receptor tyrosine kinases, VEGF receptors 1, 2, and 3, that are predominantly expressed on endothelial and some hematopoietic cells. Pox viruses of the Orf family encode highly related proteins called VEGF-E that show only 25-35% amino acid identity with VEGF-A but bind with comparable affinity to VEGFR-2. The crystal structure of VEGF-E NZ2 described here reveals high similarity to the known structural homologs VEGF-A, PlGF, and the snake venoms Vammin and VR-1, which are all homodimers and contain the characteristic cysteine knot motif. Distinct conformational differences are observed in loop L1 and particularly in L3, which contains a highly flexible GS-rich motif that differs from all other structural homologs. Based on our structure, we created chimeric proteins by exchanging selected segments in L1 and L3 with the corresponding sequences from PlGF. Single loop mutants did not bind to either receptor, whereas a VEGF-E mutant in which both L1 and L3 were replaced gained affinity for VEGFR-1, illustrating the possibility to engineer receptor-specific chimeric VEGF molecules. In addition, changing arginine 46 to isoleucine in L1 significantly increased the affinity of VEGF-E for both VEGF receptors.

Chimeric VEGF-E(NZ7)/PlGF promotes angiogenesis via VEGFR-2 without significant enhancement of vascular permeability and inflammation

OBJECTIVE

Vascular endothelial growth factor (VEGF) plays critical roles in the regulation of angiogenesis and lymphangiogenesis. However, tissue edema, hemorrhage, and inflammation occur when VEGF-A is used for angiogenic therapy. To design a novel angiogenic factor without severe side effects, we examined the biological function of chimeric VEGF-E(NZ7)/placental growth factor (PlGF), which is composed of Orf-Virus(NZ7)-derived VEGF-E(NZ7) and human PlGF1, in a transgenic (Tg) mouse model.

METHODS AND RESULTS

A strong angiogenic response was observed in both VEGF-E(NZ7)/PlGF and VEGF-A165 Tg mice. Notably, the vascular leakage of VEGF-E(NZ7)/PlGF-induced blood vessels was 4-fold lower than that of VEGF-A165-induced blood vessels. Furthermore, the monocyte/macrophage recruitment in the skin of VEGF-E(NZ7)/PlGF Tg mice was approximately 8-fold decreased compared with that of VEGF-A165 Tg mice. In addition, the lymphatic vessels in VEGF-E(NZ7)/PlGF Tg mice were structurally normal, whereas they were markedly dilated in VEGF-A165 Tg mice, possibly because of the high vascular leakage. Receptor binding assay demonstrated that VEGF-E(NZ7)/PlGF was the ligand only activating VEGF receptor (VEGFR)-2.

CONCLUSIONS

These results indicated that neither the hyperpermeability in response to simultaneous stimulation of VEGFR-1 and VEGFR-2 nor VEGFR-1-mediated severe inflammation was associated with VEGF-E(NZ7)/PlGF-induced angiogenesis. The unique receptor binding property may shed light on VEGF-E(NZ7)/PlGF as a novel candidate for therapeutic angiogenesis.

The role of VEGF receptors in angiogenesis; complex partnerships

Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development and homeostasis but also have profound effects on neural cells. VEGFs are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and upon stimulation with growth factors such as transforming growth factors, interleukins or platelet-derived growth factor. VEGFs bind to three variants of type III receptor tyrosine kinases, VEGF receptor 1, 2 and 3. Each VEGF isoform binds to a particular subset of these receptors giving rise to the formation of receptor homo- and heterodimers that activate discrete signaling pathways. Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparan sulfate, integrins or cadherins. Here we summarize the knowledge accumulated since the discovery of these proteins more than 20 years ago with the emphasis on the signaling pathways activated by VEGF receptors in endothelial cells during cell migration, growth and differentiation.