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

Phage-Displayed Mimotopes of SARS-CoV-2 Spike Protein Targeted to Authentic and Alternative Cellular Receptors

The evolution of the SARS-CoV-2 virus during the COVID-19 pandemic was accompanied by the emergence of new heavily mutated viral variants with increased infectivity and/or resistance to detection by the human immune system. To respond to the urgent need for advanced methods and materials to empower a better understanding of the mechanisms of virus’s adaptation to human host cells and to the immuno-resistant human population, we suggested using recombinant filamentous bacteriophages, displaying on their surface foreign peptides termed “mimotopes”, which mimic the structure of viral receptor-binding sites on the viral spike protein and can serve as molecular probes in the evaluation of molecular mechanisms of virus infectivity. In opposition to spike-binding antibodies that are commonly used in studying the interaction of the ACE2 receptor with SARS-CoV-2 variants in vitro, phage spike mimotopes targeted to other cellular receptors would allow discovery of their role in viral infection in vivo using cell culture, tissue, organs, or the whole organism. Phage mimotopes of the SARS-CoV-2 Spike S1 protein have been developed using a combination of phage display and molecular mimicry concepts, termed here “phage mimicry”, supported by bioinformatics methods. The key elements of the phage mimicry concept include: (1) preparation of a collection of p8-type (landscape) phages, which interact with authentic active receptors of live human cells, presumably mimicking the binding interactions of human coronaviruses such as SARS-CoV-2 and its variants; (2) discovery of closely related amino acid clusters with similar 3D structural motifs on the surface of natural ligands (FGF1 and NRP1), of the model receptor of interest FGFR and the S1 spike protein; and (3) an ELISA analysis of the interaction between candidate phage mimotopes with FGFR3 (a potential alternative receptor) in comparison with ACE2 (the authentic receptor).

A Structural Overview of Vascular Endothelial Growth Factors Pharmacological Ligands: From Macromolecules to Designed Peptidomimetics

The vascular endothelial growth factor (VEGF) family of cytokines plays a key role in vasculogenesis, angiogenesis, and lymphangiogenesis. VEGF-A is the main member of this family, alongside placental growth factor (PlGF), VEGF-B/C/D in mammals, and VEGF-E/F in other organisms. To study the activities of these growth factors under physiological and pathological conditions, resulting in therapeutic applications in cancer and age-related macular degeneration, blocking ligands have been developed. These have mostly been large biomolecules like antibodies. Ligands with high affinities, at least in the nanomolar range, and accurate structural data from X-ray crystallography and NMR spectroscopy have been described. They constitute the main focus of this overview, which evidences similarities and differences in their binding modes. For VEGF-A ligands, and to a limited extent also for PlGF, a transition is now observed towards developing smaller ligands like nanobodies and peptides. These include unnatural amino acids and chemical modifications for designed and improved properties, such as serum stability and greater affinity. However, this review also highlights the scarcity of such small molecular entities and the striking lack of small organic molecule ligands. It also shows the gap between the rather large array of ligands targeting VEGF-A and the general absence of ligands binding other VEGF members, besides some antibodies. Future developments in these directions are expected in the upcoming years, and the study of these growth factors and their promising therapeutic applications will be welcomed.

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.

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.

Mechanisms of immunomodulation by mammalian and viral decoy receptors: insights from structures

Immune responses are regulated by effector cytokines and chemokines that signal through cell surface receptors. Mammalian decoy receptors - which are typically soluble or inactive versions of cell surface receptors or soluble protein modules termed binding proteins - modulate and antagonize signalling by canonical effector-receptor complexes. Viruses have developed a diverse array of molecular decoys to evade host immune responses; these include viral homologues of host cytokines, chemokines and chemokine receptors; variants of host receptors with new functions; and novel decoy receptors that do not have host counterparts. Over the past decade, the number of known mammalian and viral decoy receptors has increased considerably, yet a comprehensive curation of the corresponding structure-mechanism relationships has not been carried out. In this Review, we provide a comprehensive resource on this topic with a view to better understanding the roles and evolutionary relationships of mammalian and viral decoy receptors, and the opportunities for leveraging their therapeutic potential.

Structural determinants of NH3 and NH4+ transport by mouse Rhbg, a renal Rh glycoprotein

Renal Rhbg is localized to the basolateral membrane of intercalated cells and is involved in NH3/NH4+ transport. The structure of Rhbg is not yet resolved; however, a high-resolution crystal structure of AmtB, a bacterial homolog of Rh, has been determined. We aligned the sequence of Rhbg to that of AmtB and identified important sites of Rhbg that may affect transport. Our analysis positioned three conserved amino acids, histidine 183 (H183), histidine 342 (H342), and tryptophan 230 (W230), within the hydrophobic pore where they presumably serve to control NH3 transport. A fourth residue, phenylalanine 128 (F128) was positioned at the upper vestibule, presumably contributing to recruitment of NH4+ We generated three mutations each of H183, H342, W230, and F128 and expressed them in frog oocytes. Immunolabeling showed that W230 and F128 mutants were localized to the cell membrane, whereas H183 and H342 staining was diffuse and mostly intracellular. To determine function, we compared measurements of NH3/NH4+ and methyl amine (MA)/methyl ammonium (MA+)-induced currents, intracellular pH, and surface pH (pHs) among oocytes expressing the mutants, Rhbg, or injected with H2O. In H183 and W230 mutants, NH4+-induced current and intracellular acidification were inhibited compared with that of Rhbg, and MA-induced intracellular alkalinization was completely absent. Expression of H183A or W230A mutants inhibited NH3/NH4+- and MA/MA+-induced decrease in pHs to the level observed in H2O-injected oocytes. Mutations of F128 did not significantly affect transport of NH3 or NH4+ These data demonstrated that mutating H183 or W230 caused loss of function but not F128. H183 and H342 may affect membrane expression of the transporter.

Against the odds? De novo structure determination of a pilin with two cysteine residues by sulfur SAD

Exploiting the anomalous signal of the intrinsic S atoms to phase a protein structure is advantageous, as ideally only a single well diffracting native crystal is required. However, sulfur is a weak anomalous scatterer at the typical wavelengths used for X-ray diffraction experiments, and therefore sulfur SAD data sets need to be recorded with a high multiplicity. In this study, the structure of a small pilin protein was determined by sulfur SAD despite several obstacles such as a low anomalous signal (a theoretical Bijvoet ratio of 0.9% at a wavelength of 1.8 Å), radiation damage-induced reduction of the cysteines and a multiplicity of only 5.5. The anomalous signal was improved by merging three data sets from different volumes of a single crystal, yielding a multiplicity of 17.5, and a sodium ion was added to the substructure of anomalous scatterers. In general, all data sets were balanced around the threshold values for a successful phasing strategy. In addition, a collection of statistics on structures from the PDB that were solved by sulfur SAD are presented and compared with the data. Looking at the quality indicator R(anom)/R(p.i.m.), an inconsistency in the documentation of the anomalous R factor is noted and reported.

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.

Thermodynamic and structural description of allosterically regulated VEGFR-2 dimerization

VEGFs activate 3 receptor tyrosine kinases, VEGFR-1, VEGFR-2, and VEGFR-3, promoting angiogenic and lymphangiogenic signaling. The extracellular receptor domain (ECD) consists of 7 Ig-homology domains; domains 2 and 3 (D23) represent the ligand-binding domain, whereas the function of D4-7 is unclear. Ligand binding promotes receptor dimerization and instigates transmembrane signaling and receptor kinase activation. In the present study, isothermal titration calorimetry showed that the Gibbs free energy of VEGF-A, VEGF-C, or VEGF-E binding to D23 or the full-length ECD of VEGFR-2 is dominated by favorable entropic contribution with enthalpic penalty. The free energy of VEGF binding to the ECD is 1.0-1.7 kcal/mol less favorable than for binding to D23. A model of the VEGF-E/VEGFR-2 ECD complex derived from small-angle scattering data provided evidence for homotypic interactions in D4-7. We also solved the crystal structures of complexes between VEGF-A or VEGF-E with D23, which revealed comparable binding surfaces and similar interactions between the ligands and the receptor, but showed variation in D23 twist angles. The energetically unfavorable homotypic interactions in D4-7 may be required for re-orientation of receptor monomers, and this mechanism might prevent ligand-independent activation of VEGFR-2 to evade the deleterious consequences for blood and lymph vessel homeostasis arising from inappropriate receptor activation.

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.

Structural analysis of vascular endothelial growth factor receptor-2/ligand complexes by small-angle X-ray solution scattering

Receptor tyrosine kinases play essential roles in tissue development and homeostasis, and aberrant signaling by these molecules is the basis of many diseases. Understanding the activation mechanism of these receptors is thus of high clinical relevance. We investigated vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs), which regulate blood and lymph vessel formation. We analyzed the structural changes in the extracellular receptor domain that were induced by ligand binding and that represent the initial step in transmembrane signaling, culminating in the activation of the intracellular receptor kinase domain. High-resolution structural information for the ligand binding domain became available recently, but the flexibility of the extracellular domain and inhomogeneous glycosylation of VEGFRs have prevented the production of highly diffracting crystals of the entire extracellular domain so far. Therefore, we chose to further investigate VEGFR structure by small-angle X-ray scattering in solution (SAXS). SAXS data were combined with independent distance restraint determination obtained by mass spectrometric analysis of chemically cross-linked ligand/receptor complexes. With these data, we constructed a structural model of the entire extracellular receptor domain in the unbound form and in complex with VEGF.

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.

Autotaxin signaling via lysophosphatidic acid receptors contributes to vascular endothelial growth factor-induced endothelial cell migration

Important roles for vascular endothelial growth factor (VEGF) and autotaxin (ATX) have been established for embryonic vasculogenesis and cancer progression. We examined whether these two angiogenic factors cooperate in regulation of endothelial cell migratory responses. VEGF stimulated expression of ATX and LPA1, a receptor for the ATX enzymatic product lysophosphatidic acid (LPA), in human umbilical vein endothelial cells. Knockdown of ATX expression significantly decreased mRNA levels for the receptors LPA1, LPA2, S1P1, S1P2, S1P3, and VEGFR2 and abolished cell migration to lysophosphatidylcholine, LPA, recombinant ATX, and VEGF. Migration to sphingosylphosphorylcholine and sphinogosine-1-phosphate was also reduced in ATX knockdown cells, whereas migration to serum remained unchanged. Furthermore, ATX knockdown decreased Akt2 mRNA levels, whereas LPA treatment strongly stimulated Akt2 expression. We propose that VEGF stimulates LPA production by inducing ATX expression. VEGF also increases LPA1 signaling, which in turn increases Akt2 expression. Akt2 is strongly associated with cancer progression, cellular migration, and promotion of epithelial-mesenchymal transition. These data show a role for ATX in maintaining expression of receptors required for VEGF and lysophospholipids to accelerate angiogenesis. Because VEGF and ATX are upregulated in many cancers, the regulatory mechanism proposed in these studies could apply to cancer-related angiogenesis and cancer progression. These data further suggest that ATX could be a prognostic factor or a target for therapeutic intervention in several cancers.

Unique signal transduction of the VEGF family members VEGF-A and VEGF-E

Both VEGF (vascular endothelial growth factor)-A and Orf-virus-encoded VEGF-E bind and activate VEGFR (VEGF receptor)-2; however, only VEGF-A binds VEGFR-1. To understand the biological differences between VEGF-A and VEGF-E in vivo, we established transgenic mouse models. K14 (keratin-14)-promoter-driven VEGF-E transgenic mice showed a significant increase in mature blood vessels. However, K14–VEGF-A transgenic mice exhibited severe inflammation and oedema with increased angiogenesis, as well as lymphangiogenesis and lymph vessel dilatation. K14–VEGF-A transgenic mice deficient in VEGFR-1 signalling (K14–VEGF-A-tg/VEGFR-1 TK−/− mice) showed decreases in oedema and inflammation with less recruitment of macrophage-lineage cells, suggesting an involvement of VEGFR-1 in these adverse effects. VEGFE might be more useful than VEGFA for pro-angiogenic therapy.

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.

Novel vascular endothelial growth factor D variants with increased biological activity

Members of the vascular endothelial growth factor (VEGF) family play a pivotal role in angiogenesis and lymphangiogenesis. They are potential therapeutics to induce blood vessel formation in myocardium and skeletal muscle, when normal blood flow is compromised. Most members of the VEGF/platelet derived growth factor protein superfamily exist as covalently bound antiparallel dimers. However, the mature form of VEGF-D (VEGF-D(DeltaNDeltaC)) is predominantly a non-covalent dimer even though the cysteine residues (Cys-44 and Cys-53) forming the intersubunit disulfide bridges in the other members of the VEGF family are also conserved in VEGF-D. Moreover, VEGF-D bears an additional cysteine residue (Cys-25) at the subunit interface. Guided by our model of VEGF-D(DeltaNDeltaC), the cysteines at the subunit interface were mutated to study the effect of these residues on the structural and functional properties of VEGF-D(DeltaNDeltaC). The conserved cysteines Cys-44 and Cys-53 were found to be essential for the function of VEGF-D(DeltaNDeltaC). More importantly, the substitution of the Cys-25 at the dimer interface by various amino acids improved the activity of the recombinant VEGF-D(DeltaNDeltaC) and increased the dimer to monomer ratio. Specifically, substitutions to hydrophobic amino acids Ile, Leu, and Val, equivalent to those found in other VEGFs, most favorably affected the activity of the recombinant VEGF-D(DeltaNDeltaC). The increased activity of these mutants was mainly due to stabilization of the protein. This study enables us to better understand the structural determinants controlling the biological activity of VEGF-D. The novel variants of VEGF-D(DeltaNDeltaC) described here are potential agents for therapeutic applications, where induction of vascular formation is required.

Neuropilin-1 in regulation of VEGF-induced activation of p38MAPK and endothelial cell organization

Vascular endothelial growth factor (VEGF)-A regulates vascular development and angiogenesis. VEGF isoforms differ in ability to bind coreceptors heparan sulfate (HS) and neuropilin-1 (NRP1). We used VEGF-A165 (which binds HS and NRP1), VEGF-A121 (binds neither HS nor NRP1), and parapoxvirus VEGF-E-NZ2 (binds NRP1 but not HS) to investigate the role of NRP1 in organization of endothelial cells into vascular structures. All 3 ligands induced similar level of VEGFR-2 tyrosine phosphorylation in the presence of NRP1. In contrast, sprouting angiogenesis in differentiating embryonic stem cells (embryoid bodies), formation of branching pericyte-embedded vessels in subcutaneous matrigel plugs, and sprouting of intersegmental vessels in developing zebrafish were induced by VEGF-A165 and VEGF-E-NZ2 but not by VEGF-A121. Analyses of recombinant factors with NRP1-binding gain- and loss-of-function properties supported the conclusion that NRP1 is critical for VEGF-induced sprouting and branching of endothelial cells. Signal transduction antibody arrays implicated NRP1 in VEGF-induced activation of p38MAPK. Inclusion of the p38MAPK inhibitor SB203580 in VEGF-A165-containing matrigel plugs led to attenuated angiogenesis and poor association with pericytes. Our data strongly indicate that the ability of VEGF ligands to bind NRP1 influences p38MAPK activation, and formation of functional, pericyte-associated vessels.

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.

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.

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.

Structure of a VEGF–VEGF receptor complex determined by electron microscopy

Receptor tyrosine kinases are activated upon ligand-induced dimerization. Here we show that the monomeric extracellular domain of vascular endothelial growth factor (VEGF) receptor-2 (VEGFR-2) has a flexible structure. Binding of VEGF to membrane-distal immunoglobulin-like domains causes receptor dimerization and promotes further interaction between receptor monomers through the membrane-proximal immunoglobulin-like domain 7. By this mechanism, ligand-induced dimerization of VEGFR-2 can be communicated across the membrane, activating the intracellular tyrosine kinase domains.

Dynamical behavior of the vascular endothelial growth factor: Biological implications

The vascular endothelial growth factor (VEGF) seems to be the most important regulator of physiological and pathological angiogenesis, being, for this reason, a favorite target for therapies against angiogenesis-related diseases. VEGF is a homodimer in which the monomers are formed by beta-strands interconnected on the poles by three loops. A recent work showed that an intimate relationship between loops-1 and -3 is required for high affinity binding to the receptors (Kiba et al., J Biol Chem 2003;278:13453-13461). In this work, we report the results of a 10-ns molecular dynamics simulation of VEGF. We analyzed the dynamical behavior of the protein (using a dynamical cross-correlation map) and found that it is governed by a high degree of correlation between the motions of the loops. We also performed a principal component analysis and found an overall motion in which the opposite poles are projected against each other, just like the movement of the wings of a butterfly. From the biological point of view, it is likely that this motion would facilitate receptor binding since VEGF must enter a restricted cavity formed by the two subunits of the receptor.