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

Anti-angiogenic Potential of Trans-chalcone in an In Vivo Chick Chorioallantoic Membrane Model: An ATP Antagonist to VEGFR with Predicted Blood-brain Barrier Permeability

BACKGROUND

Glioblastoma multiforme (GBM) is characterized by massive tumorinduced angiogenesis aiding tumorigenesis. Vascular endothelial growth factor A (VEGF-A) via VEGF receptor 2 (VEGFR-2) constitutes majorly to drive this process. Putting a halt to tumordriven angiogenesis is a major clinical challenge, and the blood-brain barrier (BBB) is the prime bottleneck in GBM treatment. Several phytochemicals show promising antiangiogenic activity across different models, but their ability to cross BBB remains unexplored.

METHODS

We screened over 99 phytochemicals having anti-angiogenic properties reported in the literature and evaluated them for their BBB permeability, molecular interaction with VEGFR-2 domains, ECD2-3 (extracellular domains 2-3) and TKD (tyrosine kinase domain) at VEGF-A and ATP binding site, cell membrane permeability, and hepatotoxicity using in silico tools. Furthermore, the anti-angiogenic activity of predicted lead Trans-Chalcone (TC) was evaluated in the chick chorioallantoic membrane.

RESULTS

Out of 99 phytochemicals, 35 showed an efficient ability to cross BBB with a probability score of > 0.8. Docking studies revealed 30 phytochemicals crossing benchmark binding affinity < -6.4 kcal/mol of TKD with the native ligand ATP alone. Out of 30 phytochemicals, 12 showed moderate to low hepatotoxicity, and 5 showed a violation of Lipinski's rule of five. Our in silico analysis predicted TC as a BBB permeable anti-angiogenic compound for use in GBM therapy. TC reduced vascularization in the CAM model, which was associated with the downregulation of VEGFR-2 transcript expression.

CONCLUSION

The present study showed TC to possess anti-angiogenic potential via the inhibition of VEGFR-2. In addition, the study predicted TC to cross BBB as well as a safe alternative for GBM therapy, which needs further investigation.

Dual gene-activated dermal scaffolds regulate angiogenesis and wound healing by mediating the coexpression of VEGF and angiopoietin-1

The vascularization of dermal substitutes is a key challenge in efforts to heal deep skin defects. In this study, dual gene-activated dermal scaffolds (DGADSs-1) were fabricated by loading nanocomposite particles of polyethylenimine (PEI)/multiple plasmid DNAs (pDNAs) encoding vascular endothelial growth factor and angiopoietin-1 at a ratio of 1:1. In a similar manner, DGADSs-2 were loaded with a chimeric plasmid encoding both VEGF and Ang-1. In vitro studies showed that both types of DGADSs released PEI/pDNA nanoparticles in a sustained manner; they demonstrated effective transfection ability, leading to upregulated expression of VEGF and Ang-1. Furthermore, both types of DGADSs promoted fibroblast proliferation and blood vessel formation, although DGADSs-1 showed a more obvious promotion effect. A rat full-thickness skin defect model showed that split-thickness skin transplanted using a one-step method could achieve full survival at the 12th day after surgery in both DGADSs-1 and DGADSs-2 groups, and the vascularization time of dermal substitutes was significantly shortened. Compared with the other three groups of scaffolds, the DGADSs-1 group had significantly greater cell infiltration, collagen deposition, neovascularization, and vascular maturation, all of which promoted wound healing. Thus, compared with single-gene-activated dermal scaffolds, DGADSs show greater potential for enhancing angiogenesis. DGADSs with different loading modes also exhibited differences in terms of angiogenesis; the effect of loading two genes (DGADSs-1) was better than the effect of loading a chimeric gene (DGADSs-2). In summary, DGADSs, which continuously upregulate VEGF and Ang-1 expression, offer a new functional tissue-engineered dermal substitute with the ability to activate vascularization.

Expansion and collapse of VEGF diversity in major clades of the animal kingdom

Together with the platelet-derived growth factors (PDGFs), the vascular endothelial growth factors (VEGFs) form the PDGF/VEGF subgroup among cystine knot growth factors. The evolutionary relationships within this subgroup have not been examined thoroughly to date. Here, we comprehensively analyze the PDGF/VEGF growth factors throughout all animal phyla and propose a phylogenetic tree. Vertebrate whole-genome duplications play a role in expanding PDGF/VEGF diversity, but several limited duplications are necessary to account for the temporal pattern of emergence. The phylogenetically oldest PDGF/VEGF-like growth factor likely featured a C-terminus with a BR3P signature, a hallmark of the modern-day lymphangiogenic growth factors VEGF-C and VEGF-D. Some younger VEGF genes, such as VEGFB and PGF, appeared completely absent in important vertebrate clades such as birds and amphibia, respectively. In contrast, individual PDGF/VEGF gene duplications frequently occurred in fish on top of the known fish-specific whole-genome duplications. The lack of precise counterparts for human genes poses limitations but also offers opportunities for research using organisms that diverge considerably from humans. Sources for the graphical abstract: 326 MYA and older [1]; 72-240 MYA [2]; 235-65 MYA [3].

Meta-analysis of single-cell and single-nucleus transcriptomics reveals kidney cell type consensus signatures

While the amount of studies involving single-cell or single-nucleus RNA-sequencing technologies grows exponentially within the biomedical research area, the kidney field requires reference transcriptomic signatures to allocate each cluster its matching cell type. The present meta-analysis of 39 previously published datasets, from 7 independent studies, involving healthy human adult kidney samples, offers a set of 24 distinct consensus kidney cell type signatures. The use of these signatures may help to assure the reliability of cell type identification in future studies involving single-cell and single-nucleus transcriptomics while improving the reproducibility in cell type allocation.

Structural insights into regulation of CCN protein activities and functions

CCN proteins play important functions during development, in repair mechanisms following tissue injury, as well as in pathophysiologic mechanisms of metastasis of cancer. CCNs are secreted proteins that have a multimodular structure and are categorized as matricellular proteins. Although the prevailing view is that CCN proteins regulate biologic processes by interacting with a wide array of other proteins in the microenvironment of the extracellular matrix, the molecular mechanisms of action of CCN proteins are still poorly understood. Not dissuading the current view, however, the recent appreciation that these proteins are signaling proteins in their own right and may even be considered preproproteins controlled by endopeptidases to release a C-terminal bioactive peptide has opened new avenues of research. Also, the recent resolution of the crystal structure of two of the domains of CCN3 have provided new knowledge with implications for the entire CCN family. These resolved structures in combination with structural predictions based upon the AlphaFold artificial intelligence tool provide means to shed new light on CCN functions in context of the notable literature in the field. CCN proteins have emerged as important therapeutic targets in several disease conditions, and clinical trials are currently ongoing. Thus, a review that critically discusses structure - function relationship of CCN proteins, in particular as it relates to interactions with other proteins in the extracellular milieu and on the cell surface, as well as to cell signaling activities of these proteins, is very timely. Suggested mechanism for activation and inhibition of signaling by the CCN protein family (graphics generated with BioRender.com ).

Elucidating activation and deactivation dynamics of VEGFR-2 transmembrane domain with coarse-grained molecular dynamics simulations

The vascular endothelial growth factor receptor 2 (VEGFR-2) is a member of receptor tyrosine kinases (RTKs) and is a dimeric membrane protein that functions as a primary regulator of angiogenesis. As is usual with RTKs, spatial alignment of its transmembrane domain (TMD) is essential toward VEGFR-2 activation. Experimentally, the helix rotations within TMD around their own helical axes are known to participate importantly toward the activation process in VEGFR-2, but the detailed dynamics of the interconversion between the active and inactive TMD forms have not been clearly elucidated at the molecular level. Here, we attempt to elucidate the process by using coarse grained (CG) molecular dynamics (MD) simulations. We observe that inactive dimeric TMD in separation is structurally stable over tens of microseconds, suggesting that TMD itself is passive and does not allow spontaneous signaling of VEGFR-2. By starting from the active conformation, we reveal the mechanism of TMD inactivation through analyzing the CG MD trajectories. We observe that interconversions between a left-handed overlay and a right-handed one are essential for the process of going from an active TMD structure to the inactive form. In addition, our simulations find that the helices can rotate properly when the overlaying structure of the helices interconverts and when the crossing angle of the two helices changes by larger than ~40 degrees. As the activation right after the ligand attachment on VEGFR-2 will take place in the reverse manner of this inactivation process, these structural aspects will also appear importantly for the activation process. The rather large change in helix configuration for activation also explains why VEGFR-2 rarely self-activate and how the activating ligand structurally drive the whole VEGFR-2. This mechanism of TMD activation / inactivation within VEGFR-2 may help in further understanding the overall activation processes of other RTKs.

Receptors that bind to PEDF and their therapeutic roles in retinal diseases

Retinal neovascular, neurodegenerative, and inflammatory diseases represented by diabetic retinopathy are the main types of blinding eye disorders that continually cause the increased burden worldwide. Pigment epithelium-derived factor (PEDF) is an endogenous factor with multiple effects including neurotrophic activity, anti-angiogenesis, anti-tumorigenesis, and anti-inflammatory activity. PEDF activity depends on the interaction with the proteins on the cell surface. At present, seven independent receptors, including adipose triglyceride lipase, laminin receptor, lipoprotein receptor-related protein, plexin domain-containing 1, plexin domain-containing 2, F1-ATP synthase, and vascular endothelial growth factor receptor 2, have been demonstrated and confirmed to be high affinity receptors for PEDF. Understanding the interactions between PEDF and PEDF receptors, their roles in normal cellular metabolism and the response the initiate in disease will be accommodating for elucidating the ways in which inflammation, angiogenesis, and neurodegeneration exacerbate disease pathology. In this review, we firstly introduce PEDF receptors comprehensively, focusing particularly on their expression pattern, ligands, related diseases, and signal transduction pathways, respectively. We also discuss the interactive ways of PEDF and receptors to expand the prospective understanding of PEDF receptors in the diagnosis and treatment of retinal diseases.

Evolution and diversification of Mountain voles (Rodentia: Cricetidae)

The systematics of the Cricetid genus Neodon have long been fraught with uncertainty due to sampling issues and a lack of comprehensive datasets. To gain better insights into the phylogeny and evolution of Neodon, we systematically sampled Neodon across the Hengduan and Himalayan Mountains, which cover most of its range in China. Analyses of skulls, teeth, and bacular structures revealed 15 distinct patterns corresponding to 15 species of Neodon. In addition to morphological analyses, we generated a high-quality reference genome for the mountain vole and generated whole-genome sequencing data for 47 samples. Phylogenomic analyses supported the recognition of six new species, revealing a long-term underestimation of Neodon diversity. We further identified positively selected genes potentially related to high-elevation adaptation. Together, our results illuminate how climate change caused the plateau to become the centre of Neodon origin and diversification and how mountain voles have adapted to the hypoxic high-altitude plateau environment.

Bioactive VEGF-C from E. coli

Vascular endothelial growth factor-C (VEGF-C) stimulates lymphatic vessel growth in transgenic models, via viral gene delivery, and as a recombinant protein. Expressing eukaryotic proteins like VEGF-C in bacterial cells has limitations, as these cells lack specific posttranslational modifications and provisions for disulfide bond formation. However, given the cost and time savings associated with bacterial expression systems, there is considerable value in expressing VEGF-C using bacterial cells. We identified two approaches that result in biologically active Escherichia coli-derived VEGF-C. Expectedly, VEGF-C expressed from a truncated cDNA became bioactive after in vitro folding from inclusion bodies. Given that VEGF-C is one of the cysteine-richest growth factors in humans, it was unclear whether known methods to facilitate correct cysteine bond formation allow for the direct expression of bioactive VEGF-C in the cytoplasm. By fusing VEGF-C to maltose-binding protein and expressing these fusions in the redox-modified cytoplasm of the Origami (DE3) strain, we could recover biological activity for deletion mutants lacking the propeptides of VEGF-C. This is the first report of a bioactive VEGF growth factor obtained from E. coli cells circumventing in-vitro folding.

Preparation, In Vitro Affinity, and In Vivo Biodistribution of Receptor-Specific 68Ga-Labeled Peptides Targeting Vascular Endothelial Growth Factor Receptors

As angiogenesis plays a key role in tumor growth and metastasis, the angiogenic process has attracted scientific interest as a target for diagnostic and therapeutic agents. Factors influencing angiogenesis include the vascular endothelial growth factor (VEGF) family and the two associated receptor types (VEGFR-1 and VEGFR-2). VEGFR-1/-2 detection and quantification in cancer lesions are essential for tumor process management. As a result of the advantageous pharmacokinetics and image contrast, peptides radiolabeled with PET emitters have become interesting tools for the visualization of VEGFR-1/-2-positive tumors. In this study, we prepared ⁶⁸Ga-labeled peptides containing 15 (peptide 1) and 23 (peptide 2) amino acids as new PET tracers for tumor angiogenic process imaging.

METHODS

The peptides were conjugated with NODAGA-tris(t-Bu ester) and subsequently radiolabeled with [⁶⁸Ga]Ga-chloride. The prepared [⁶⁸Ga]Ga-NODAGA-peptide 1 and [⁶⁸Ga]Ga-NODAGA-peptide 2 were tested for radiochemical purity and saline/plasma stability. Consequently, the binding affinity toward VEGFRs was assessed in vitro on human glioblastoma and kidney carcinoma cells. The found peptide receptor affinity was compared with the calculated values in the PROtein binDIng enerGY prediction (PRODIGY) server. Finally, the biodistribution study was performed on BALB/c female mice to reveal the basic pharmacokinetic behavior of radiopeptides.

RESULTS

The in vitro affinity testing of [⁶⁸Ga]Ga-NODAGA-peptides 1 and 2 showed retained receptor binding as characterized by equilibrium dissociation constant (K_D) values in the range of 0.5-1.2 μM and inhibitory concentration 50% (IC₅₀) values in the range of 3.0-5.6 μM. Better binding properties of peptide 2 to VEGFR-1/-2 were found in the PRODIGY server. The biodistribution study on mice showed remarkable accumulation of both peptides in the kidneys and urinary bladder with a short half-life after intravenous application. The in vitro plasma stability of [⁶⁸Ga]Ga-NODAGA-peptide 2 was superior to that of [⁶⁸Ga]Ga-NODAGA-peptide 1.

CONCLUSIONS

The obtained results demonstrated a high radiolabeling yield with no need for purification and preserved binding potency of ⁶⁸Ga-labeled peptides 1 and 2 toward VEGFRs in cancer cells. The peptide-receptor protein interaction assessed in protein-peptide docking determined the strongest interaction of peptide 2 with domain 2 of VEGFR-2 in addition to a more acceptable plasma stability (t_1/2 = 120 min) than that for peptide 1. We found both radiolabeled peptides very potent in their receptor binding, which makes them suitable imaging agents. The rapid transition of the radiopeptides into the urinary tract indicates suitable pharmacokinetic characteristics.

Cholenic acid derivative UniPR1331 impairs tumor angiogenesis via blockade of VEGF/VEGFR2 in addition to Eph/ephrin

Angiogenesis, the formation of new blood vessels from preexisting ones, is crucial for tumor growth and metastatization, and is considered a promising therapeutic target. Unfortunately, drugs directed against a specific proangiogenic growth factor or receptor turned out to be of limited benefit for oncology patients, likely due to the high biochemical redundancy of the neovascularization process. In this scenario, multitarget compounds that are able to simultaneously tackle different proangiogenic pathways are eagerly awaited. UniPR1331 is a 3β-hydroxy-Δ⁵-cholenic acid derivative, which is already known to inhibit Eph-ephrin interaction. Here, we employed an analysis pipeline consisting of molecular modeling and simulation, surface plasmon resonance spectrometry, biochemical assays, and endothelial cell models to demonstrate that UniPR1331 directly interacts with the vascular endothelial growth factor receptor 2 (VEGFR2) too. The binding of UniPR1331 to VEGFR2 prevents its interaction with the natural ligand vascular endothelial growth factor and subsequent autophosphorylation, signal transduction, and in vitro proangiogenic activation of endothelial cells. In vivo, UniPR1331 inhibits tumor cell-driven angiogenesis in zebrafish. Taken together, these data shed light on the pleiotropic pharmacological effect of UniPR1331, and point to Δ⁵-cholenic acid as a promising molecular scaffold for the development of multitarget antiangiogenic compounds.

Functional antibody characterization via direct structural analysis and information-driven protein-protein docking

Detailed description of the mechanism of action of the therapeutic antibodies is essential for the functional characterization and future optimization of potential clinical agents. We recently developed KD035, a fully human antibody targeting vascular endothelial growth factor receptor 2 (VEGFR2). KD035 blocked VEGF‐A, and VEGF‐C‐mediated VEGFR2 activation, as demonstrated by the in vitro binding and competition assays and functional cellular assays. Here, we report a computational model of the complex between the variable fragment of KD035 (KD035(Fv)) and the domains 2 and 3 of the extracellular portion of VEGFR2 (VEGFR2(D2‐3)). Our modeling was guided by a priori experimental information including the X‐ray structures of KD035 and related antibodies, binding assays, target domain mapping and comparison of KD035 affinity for VEGFR2 from different species. The accuracy of the model was assessed by molecular dynamics simulations, and subsequently validated by mutagenesis and binding analysis. Importantly, the steps followed during the generation of this model can set a precedent for future in silico efforts aimed at the accurate description of the antibody–antigen and more broadly protein–protein complexes.

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.

A Novel Human Long Noncoding RNA SCDAL Promotes Angiogenesis through SNF5-Mediated GDF6 Expression

Angiogenesis is essential for vascular development. The roles of regulatory long noncoding RNAs (lncRNAs) in mediating angiogenesis remain under-explored. Human embryonic stem cell-derived mesenchymal stem cells (hES-MSCs) are shown to exert more potent cardioprotective effects against cardiac ischemia than human bone marrow-derived MSCs (hBM-MSCs), associated with enhanced neovascularization. The purpose of this study is to search for angiogenic lncRNAs enriched in hES-MSCs, and investigate their roles and mechanisms. AC103746.1 is one of the most highly expressed intergenic lncRNAs detected in hES-MSCs versus hBM-MSCs, and named as SCDAL (stem cell-derived angiogenic lncRNA). SCDAL knockdown significantly reduce the angiogenic potential and reparative effects of hES-MSCs in the infarcted hearts, while overexpression of SCDAL in either hES-MSCs or hBM-MSCs exhibits augmented angiogenesis and cardiac function recovery. Mechanistically, SCDAL induces growth differentiation factor 6 (GDF6) expression via direct interaction with SNF5 at GDF6 promoter. Secreted GDF6 promotes endothelial angiogenesis via non-canonical vascular endothelial growth factor receptor 2 activation. Furthermore, SCDAL-GDF6 is expressed in human endothelial cells, and directly enhances endothelial angiogenesis in vitro and in vivo. Thus, these findings uncover a previously unknown lncRNA-dependent regulatory circuit for angiogenesis. Targeted intervention of the SCDAL-GDF6 pathway has potential as a therapy for ischemic heart diseases.

Lymphatic Malformations: Genetics, Mechanisms and Therapeutic Strategies

Lymphatic vessels maintain tissue fluid homeostasis by returning to blood circulation interstitial fluid that has extravasated from the blood capillaries. They provide a trafficking route for cells of the immune system, thus critically contributing to immune surveillance. Developmental or functional defects in the lymphatic vessels, their obstruction or damage, lead to accumulation of fluid in tissues, resulting in lymphedema. Here we discuss developmental lymphatic anomalies called lymphatic malformations and complex lymphatic anomalies that manifest as localized or multifocal lesions of the lymphatic vasculature, respectively. They are rare diseases that are caused mostly by somatic mutations and can present with variable symptoms based upon the size and location of the lesions composed of fluid-filled cisterns or channels. Substantial progress has been made recently in understanding the molecular basis of their pathogenesis through the identification of their genetic causes, combined with the elucidation of the underlying mechanisms in animal disease models and patient-derived lymphatic endothelial cells. Most of the solitary somatic mutations that cause lymphatic malformations and complex lymphatic anomalies occur in genes that encode components of oncogenic growth factor signal transduction pathways. This has led to successful repurposing of some targeted cancer therapeutics to the treatment of lymphatic malformations and complex lymphatic anomalies. Apart from the mutations that act as lymphatic endothelial cell-autonomous drivers of these anomalies, current evidence points to superimposed paracrine mechanisms that critically contribute to disease pathogenesis and thus provide additional targets for therapeutic intervention. Here, we review these advances and discuss new treatment strategies that are based on the recently identified molecular pathways.

Use of NanoBiT and NanoBRET to monitor fluorescent VEGF-A binding kinetics to VEGFR2/NRP1 heteromeric complexes in living cells

BACKGROUND AND PURPOSE

VEGF-A is a key mediator of angiogenesis, primarily signalling via VEGF receptor 2 (VEGFR2). Endothelial cells also express the co-receptor neuropilin-1 (NRP1) that potentiates VEGF-A/VEGFR2 signalling. VEGFR2 and NRP1 had distinct real-time ligand binding kinetics when monitored using BRET. We previously characterised fluorescent VEGF-A isoforms tagged at a single site with tetramethylrhodamine (TMR). Here, we explored differences between VEGF-A isoforms in living cells that co-expressed both receptors.

EXPERIMENTAL APPROACH

Receptor localisation was monitored in HEK293T cells expressing both VEGFR2 and NRP1 using membrane-impermeant HaloTag and SnapTag technologies. To isolate ligand binding pharmacology at a defined VEGFR2/NRP1 complex, we developed an assay using NanoBiT complementation technology whereby heteromerisation is required for luminescence emissions. Binding affinities and kinetics of VEGFR2-selective VEGF165 b-TMR and non-selective VEGF165 a-TMR were monitored using BRET from this defined complex.

KEY RESULTS

Cell surface VEGFR2 and NRP1 were co-localised and formed a constitutive heteromeric complex. Despite being selective for VEGFR2, VEGF165 b-TMR had a distinct kinetic ligand binding profile at the complex that largely remained elevated in cells over 90 min. VEGF165 a-TMR bound to the VEGFR2/NRP1 complex with kinetics comparable to those of VEGFR2 alone. Using a binding-dead mutant of NRP1 did not affect the binding kinetics or affinity of VEGF165 a-TMR.

CONCLUSION AND IMPLICATIONS

This NanoBiT approach enabled real-time ligand binding to be quantified in living cells at 37°C from a specified complex between a receptor TK and its co-receptor for the first time.

Therapeutic Efficacy of a Novel Acetylated Tetrapeptide in Animal Models of Age-Related Macular Degeneration

It has been shown previously that a novel tetrapeptide, Arg-Leu-Tyr-Glu (RLYE), derived from human plasminogen inhibits vascular endothelial growth factor (VEGF)-induced angiogenesis, suppresses choroidal neovascularization in mice by an inhibition of VEGF receptor-2 (VEGFR-2) specific signaling pathway. In this study, we report that a modified tetrapeptide (Ac-RLYE) showed improved anti-choroidal neovascularization (CNV) efficacy in a number of animal models of neovascular age-related macular degeneration (AMD) which include rat, rabbit, and minipig. The preventive and therapeutic in vivo efficacy of Ac-RLYE via following intravitreal administration was determined to be either similar or superior to that of ranibizumab and aflibercept. Assessment of the intraocular pharmacokinetic and toxicokinetic properties of Ac-RLYE in rabbits demonstrated that it rapidly reached the retina with minimal systemic exposure after a single intravitreal dose, and it did not accumulate in plasma during repetitive dosing (bi-weekly for 14 weeks). Our results suggested that Ac-RLYE has a great potential for an alternative therapeutics for neovascular (wet) AMD. Since the amino acids in human VEGFR-2 targeted by Ac-RLYE are conserved among the animals employed in this study, the therapeutic efficacies of Ac-RLYE evaluated in those animals are predicted to be observed in human patients suffering from retinal degenerative diseases.

Proteolytic Cleavages in the VEGF Family: Generating Diversity among Angiogenic VEGFs, Essential for the Activation of Lymphangiogenic VEGFs

Simple Summary

Vascular endothelial growth factors (VEGFs) regulate the growth of blood and lymphatic vessels. Some of them induce the growth of blood vessels, and others the growth of lymphatic vessels. Blocking VEGF-A is used today to treat several types of cancer (“antiangiogenic therapy”). However, in other diseases, we would like to increase the activity of VEGFs. For example, VEGF-A could generate new blood vessels to protect from heart disease, and VEGF-C could generate new lymphatics to counteract lymphedema. Clinical trials are testing the latter concept at the moment. Because VEGF-C and VEGF-D are produced as inactive precursors, we propose that novel drugs could also target the enzymatic activation of VEGF-C and VEGF-D. However, because of the delicate balance between too much and too little vascular growth, a detailed understanding of the activation of the VEGFs is needed before such concepts can be converted into safe and efficacious therapies.

Abstract

Specific proteolytic cleavages turn on, modify, or turn off the activity of vascular endothelial growth factors (VEGFs). Proteolysis is most prominent among the lymph­angiogenic VEGF-C and VEGF-D, which are synthesized as precursors that need to undergo enzymatic removal of their C- and N-terminal propeptides before they can activate their receptors. At least five different proteases mediate the activating cleavage of VEGF-C: plasmin, ADAMTS3, prostate-specific antigen, cathepsin D, and thrombin. All of these proteases except for ADAMTS3 can also activate VEGF-D. Processing by different proteases results in distinct forms of the “mature” growth factors, which differ in affinity and receptor activation potential. The “default” VEGF-C-activating enzyme ADAMTS3 does not activate VEGF-D, and therefore, VEGF-C and VEGF-D do function in different contexts. VEGF-C itself is also regulated in different contexts by distinct proteases. During embryonic development, ADAMTS3 activates VEGF-C. The other activating proteases are likely important for non-developmental lymphangiogenesis during, e.g., tissue regeneration, inflammation, immune response, and pathological tumor-associated lymphangiogenesis. The better we understand these events at the molecular level, the greater our chances of developing successful therapies targeting VEGF-C and VEGF-D for diseases involving the lymphatics such as lymphedema or cancer.

Plump endothelial cells integrated into pre-existing venules contribute to the formation of 'mother' and 'daughter' vessels in pyogenic granuloma: possible role of galectin-1, -3 and -8

INTRODUCTION

Pyogenic granuloma (PG) is a reactive inflammatory vascular lesion of the skin and mucous membranes, characterised by the presence of enlarged venules and seamed and seamless capillaries with plump endothelial cells (EC), and numerous macrophages. EC activation upregulates the synthesis of galectins and induces their translocation to the EC surface promoting angiogenesis and lymphangiogenesis, particularly galectin-1 (Gal-1), Gal-3 and Gal-8. However, the presence and distribution of Gal-1, -3 and -8, as well as their implications in the pathogenesis of PG, has not been considered.

MATERIALS AND METHODS

Eight biopsies from patients diagnosed with PG were selected. The presence of PECAM-1/CD31, IL-1β, VEGF-C, VEGFR-2, VEGFR-3, integrin β1, CD44, fibronectin and Gal-1, -3 and -8 was assessed by immunofluorescence staining using confocal laser scanning microscopy.

RESULTS AND DISCUSSION

Immunostaining revealed that these molecules were present in the enlarged venules with plump ECs, in some macrophages and other immune cells. We propose that macrophages release VEGF-A and VEGF-C inducing VEGFR-2/VEGFR-3 expression and activation, leading macrophages to transdifferentiate into plump ECs that might integrate into pre-existing venules, contributing to the formation of enlarged venules with transluminal bridges and capillaries. EC activation, induced by certain cytokines, has been shown to stimulate galectin expression and changes in the cellular localisation through association and activation of specific EC surface glycoproteins. Therefore, it is plausible that Gal-1, -3 and -8, acting in a concerted manner, could be mediating the transdifferentiation of macrophages into plump ECs and facilitating their migration and incorporation into the new vessels.

LAY SUMMARY

In this study, immunostaining of pyogenic granuloma (PG) tissue sections showed immunoreactivity for PECAM-1/CD31, IL-1β, VEGF-C, VEGFR-2 and VEGFR-3, and galectin-1, -3 and -8 in enlarged venules with plump endothelial cells (EC), as well as in some macrophages and other immune cells. Interestingly, enlarged and thin-walled transient vessels lined by PECAM-1/CD31 and VEGFR-2 immunopositive ECs that form from pre-existing normal venules in response to VEGF-A (called 'mother' vessels [MV]) and that undergo intraluminal bridging evolving into various types of capillaries (called 'daughter' vessels [DV]) have been observed in benign and malignant tumours, in physiological and pathological angiogenesis as well as in vascular malformations, suggesting an important role for VEGF-A and VEGFR-2 in such a process. However, it is not only the mechanisms by which the MVs evolve in different types of DVs that remains to be elucidated, but also whether the cells that form intraluminal bridges proceed from locally activated ECs or whether they are derived from bone marrow precursors or from resident macrophages.Given that the formation of homodimers by Gal-1 and Gal-8 and pentamers by Gal-3 to generate gal-glycan lattices at the cell surface and in the extracellular space has been shown, it is possible that in PG tissue Gal-1, -3 and -8, through their binding partners, form a supramolecular structure at the surface of ECs and plump ECs, macrophages and in the extracellular space that might be mediating the transdifferentiation of macrophages into plump ECs and facilitating the migration and incorporation of these cells into the pre-existing venules, thus contributing to the formation of MVs and DVs.

Structural Basis for Vascular Endothelial Growth Factor Receptor Activation and Implications for Disease Therapy

Vascular endothelial growth factors (VEGFs) bind to membrane receptors on a wide variety of cells to regulate diverse biological responses. The VEGF-A family member promotes vasculogenesis and angiogenesis, processes which are essential for vascular development and physiology. As angiogenesis can be subverted in many disease states, including tumour development and progression, there is much interest in understanding the mechanistic basis for how VEGF-A regulates cell and tissue function. VEGF-A binds with high affinity to two VEGF receptor tyrosine kinases (VEGFR1, VEGFR2) and with lower affinity to co-receptors called neuropilin-1 and neuropilin-2 (NRP1, NRP2). Here, we use a structural viewpoint to summarise our current knowledge of VEGF-VEGFR activation and signal transduction. As targeting VEGF-VEGFR activation holds much therapeutic promise, we examine the structural basis for anti-angiogenic therapy using small-molecule compounds such as tyrosine kinase inhibitors that block VEGFR activation and downstream signalling. This review provides a rational basis towards reconciling VEGF and VEGFR structure and function in developing new therapeutics for a diverse range of ailments.

Molecular Bases of VEGFR-2-Mediated Physiological Function and Pathological Role

The vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) play crucial roles in vasculogenesis and angiogenesis. Angiogenesis is an important mechanism in many physiological and pathological processes, and is involved in endothelial cell proliferation, migration, and survival, then leads to further tubulogenesis, and finally promotes formation of vessels. This series of signaling cascade pathways are precisely mediated by VEGF/VEGFR-2 system. The VEGF binding to the IgD2 and IgD3 of VEGFR-2 induces the dimerization of the receptor, subsequently the activation and trans-autophosphorylation of the tyrosine kinase, and then the initiation of the intracellular signaling cascades. Finally the VEGF-activated VEGFR-2 stimulates and mediates variety of signaling transduction, biological responses, and pathological processes in angiogenesis. Several crucial phosphorylated sites Tyr801, Try951, Try1175, and Try1214 in the VEGFR-2 intracellular domains mediate several key signaling processes including PLCγ-PKC, TSAd-Src-PI3K-Akt, SHB-FAK-paxillin, SHB-PI3K-Akt, and NCK-p38-MAPKAPK2/3 pathways. Based on the molecular structure and signaling pathways of VEGFR-2, the strategy of the VEGFR-2-targeted therapy should be considered to employ in the treatment of the VEGF/VEGFR-2-associated diseases by blocking the VEGF/VEGFR-2 signaling pathway, inhibiting VEGF and VEGFR-2 gene expression, blocking the binding of VEGF and VEGFR-2, and preventing the proliferation, migration, and survival of vascular endothelial cells expressing VEGFR-2.

Evolutionary Differences in the Vegf/Vegfr Code Reveal Organotypic Roles for the Endothelial Cell Receptor Kdr in Developmental Lymphangiogenesis

Lymphatic vascular development establishes embryonic and adult tissue fluid balance and is integral in disease. In diverse vertebrate organs, lymphatic vessels display organotypic function and develop in an organ-specific manner. In all settings, developmental lymphangiogenesis is considered driven by vascular endothelial growth factor (VEGF) receptor-3 (VEGFR3), whereas a role for VEGFR2 remains to be fully explored. Here, we define the zebrafish Vegf/Vegfr code in receptor binding studies. We find that while Vegfd directs craniofacial lymphangiogenesis, it binds Kdr (a VEGFR2 homolog) but surprisingly, unlike in mammals, does not bind Flt4 (VEGFR3). Epistatic analyses and characterization of a kdr mutant confirm receptor-binding analyses, demonstrating that Kdr is indispensible for rostral craniofacial lymphangiogenesis, but not caudal trunk lymphangiogenesis, in which Flt4 is central. We further demonstrate an unexpected yet essential role for Kdr in inducing lymphatic endothelial cell fate. This work reveals evolutionary divergence in the Vegf/Vegfr code that uncovers spatially restricted mechanisms of developmental lymphangiogenesis.

Receptor tyrosine kinase targeting in glioblastoma: performance, limitations and future approaches

From all central nervous system tumors, gliomas are the most common. Nowadays, researchers are looking for more efficient treatments for these tumors, as well as ways for early diagnosis. Receptor tyrosine kinases (RTKs) are major targets for oncology and the development of small-molecule RTK inhibitors has been proven successful in cancer treatment. Mutations or aberrant activation of the RTKs and their intracellular signaling pathways are linked to several malignant diseases, including glioblastoma. The progress in the understanding of malignant glioma evolution has led to RTK targeted therapies with high capacity to improve the therapeutic response while reducing toxicity. In this review, we present the most important RTKs (i.e. EGFR, IGFR, PDGFR and VEGFR) currently used for developing cancer therapeutics together with the potential of RTK-related drugs in glioblastoma treatment. Also, we focus on some therapeutic agents that are currently at different stages of research or even in clinical phases and proved to be suitable as re-purposing candidates for glioblastoma treatment.

Creation of different bioluminescence resonance energy transfer based biosensors with high affinity to VEGF

In age-related macular degeneration (AMD) or diabetic retinopathy (DR), hypoxia and inflammatory processes lead to an upregulation of the vascular endothelial growth factor (VEGF) expression and thereby to pathological neovascularisation with incorrectly formed vessels prone to damage, thus increasing the vascular permeability and the risk of bleeding and oedema in the retina. State of the art treatment is the repeated intraocular injection of anti-VEGF molecules. For developing improved individualized treatment approaches, a minimally invasive, repeatable method for in vivo quantification of VEGF in the eye is necessary. Therefore, we designed single molecule eBRET2 VEGF biosensors by directly fusing a Renilla luciferase mutant (Rluc8) N-terminal and a green fluorescent protein (GFP2) C-terminal to a VEGF binding domain. In total, 10 different VEGF biosensors (Re01- Re10) were generated based on either single domains or full length of VEGF receptor 1 or 2 extracellular regions as VEGF binding domains. Full length expression of the biosensors in HEK293-T cells was verified via Western Blot employing an anti-Rluc8-IgG. Expression of alternative splice variants was eliminated through the deletion of the donor splice site by introduction of a silent point mutation. In all ten biosensors the energy transfer from the Rluc8 to the GFP2 occurs and generates a measurable eBRET2 ratio. Four biosensors show a relevant change of the BRET ratio (ΔBR) after VEGF binding. Furthermore, each biosensor shows a unique detection range for VEGF quantification and especially Re06 and Re07 have a high sensitivity in the range of in vivo VEGF concentrations in the eye, previously measured by invasive methods. In conclusion, we generated several eBRET2 biosensors that are suitable for VEGF quantification in vitro and could identify two eBRET2 biosensors, which may be suitable for non-invasive in vivo VEGF quantification with an implantable device.

Dual Action of Sulfated Hyaluronan on Angiogenic Processes in Relation to Vascular Endothelial Growth Factor-A

Pathological healing characterized by abnormal angiogenesis presents a serious burden to patients’ quality of life requiring innovative treatment strategies. Glycosaminoglycans (GAG) are important regulators of angiogenic processes. This experimental and computational study revealed how sulfated GAG derivatives (sGAG) influence the interplay of vascular endothelial growth factor (VEGF)₁₆₅ and its heparin-binding domain (HBD) with the signaling receptor VEGFR-2 up to atomic detail. There was profound evidence for a HBD-GAG-HBD stacking configuration. Here, the sGAG act as a “molecular glue” leading to recognition modes in which sGAG interact with two VEGF₁₆₅-HBDs. A 3D angiogenesis model demonstrated the dual regulatory role of high-sulfated derivatives on the biological activity of endothelial cells. While GAG alone promote sprouting, they downregulate VEGF₁₆₅-mediated signaling and, thereby, elicit VEGF₁₆₅-independent and -dependent effects. These findings provide novel insights into the modulatory potential of sGAG derivatives on angiogenic processes and point towards their prospective application in treating abnormal angiogenesis.

Genomic and transcriptomic comparison of post-radiation versus sporadic sarcomas

Post-radiation sarcomas are rare secondary cancers arising from radiation therapies. To date, few genetic specificities have been described for such malignancies and the oncogenesis of sarcomas with complex genetics (both sporadic and post-radiation) remains largely misunderstood. We performed genomic and transcriptomic analyses on 77 post-radiation sarcomas using DNA-array and RNA sequencing. Consequently, we were able to investigate changes in copy number variations, transcriptome profiling, fusion gene expression, and mutational landscapes. We compare these data to a reference cohort of 93 sporadic sarcomas. At genomic level, similar chromosomal complexity was observed both in post-radiation and sporadic sarcomas with complex genetics. We found more frequent CDKN2A and CDKN2B (coding for p14/p16 and p15 proteins, respectively; at 9p21.3) losses in post-radiation (71%) than in sporadic tumors (39%; P = 6.92e-3). Among all detected fusion genes and punctual variations, few specificities were observed between these groups and such alterations are not able to drive a strong and specific oncogenesis. Recurrent MYC amplifications (96%) and KDR variants (8%) were detected in post-radiation angiosarcomas, in agreement with the literature. Transcriptomic analysis of such angiosarcomas revealed two distinct groups harboring different genomic imbalances (in particular gains of 17q24.2-17qter) with different clinical courses according to patient's vital status. Differential gene expression analysis permitted to focus on the immune response as a potential actor to tumor aggressiveness. Histochemistry validated a lower inflammation and lower immune infiltrate at tumor periphery for highly aggressive angiosarcomas. Our results provide new genomic and transcriptomic information about post-radiation sarcomas. The techniques we used (RNA-seq and DNA-arrays) did not highlight major differences in sarcomas with complex genetics depending on the radiation context, revealing similar patterns of transcriptomic profiles and chromosomal copy number variations. Additional characterizations, particularly whole genome sequencing, could measure changes in DNA following radiation therapy in such malignancies and may precise their oncogenesis.

N-GlyDE: a two-stage N-linked glycosylation site prediction incorporating gapped dipeptides and pattern-based encoding

N-linked glycosylation is one of the predominant post-translational modifications involved in a number of biological functions. Since experimental characterization of glycosites is challenging, glycosite prediction is crucial. Several predictors have been made available and report high performance. Most of them evaluate their performance at every asparagine in protein sequences, not confined to asparagine in the N-X-S/T sequon. In this paper, we present N-GlyDE, a two-stage prediction tool trained on rigorously-constructed non-redundant datasets to predict N-linked glycosites in the human proteome. The first stage uses a protein similarity voting algorithm trained  on both glycoproteins and non-glycoproteins to predict a score for a protein to improve glycosite prediction. The second stage uses a support vector machine to predict N-linked glycosites by utilizing features of gapped dipeptides, pattern-based predicted surface accessibility, and predicted secondary structure. N-GlyDE's final predictions are derived from a weight adjustment of the second-stage prediction results based on the first-stage prediction score. Evaluated on N-X-S/T sequons of an independent dataset comprised of 53 glycoproteins and 33 non-glycoproteins, N-GlyDE achieves an accuracy and MCC of 0.740 and 0.499, respectively, outperforming the compared tools. The N-GlyDE web server is available at http://bioapp.iis.sinica.edu.tw/N-GlyDE/ .

Investigation of Site-Specific Differences in Glycan Microheterogeneity by N-Glycopeptide Mapping of VEGFR-IgG Fusion Protein

A biosimilar fusion protein VEGFR-IgG consisting of vascular endothelial growth factor receptors 1 and 2 (VEGFR-1, VEGFR-2) and the Fc portion of human IgG1 was prepared for this study. The prepared fusion protein was expected to possess a total of five N-linked glycosylation sites: two sites in the VEGFR-1 region, two sites in the VEGFR-2 region, and one site in the human IgG Fc region. For site-specific glycan analysis, the fusion protein was hydrolyzed with trypsin, and the resulting tryptic digests were analyzed by liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI MS/MS). The expected N-linked glycosylation sites were successfully identified and site-specific glycopeptide mapping was completed by Integrated GlycoProteome Analyzer (I-GPA) for the resulting raw tandem mass data. Finally, it was clearly confirmed that N-linked glycans for each glycosylation site showed significantly different patterns in microheterogeneity, which may indicate certain functions for each glycosylation site in the protein. Based on the mapping results, the unique features in glycan microheterogeneity for the five glycosylation sites of VEGFR-IgG fusion protein were compared site-specifically and further discussed to understand the functional meaning of each glycosylation pattern.

VEGF and FGF signaling during head regeneration in hydra

BACKGROUND

Vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) signaling pathways play important roles in the formation of the blood vascular system and nervous system across animal phyla. We have earlier reported VEGF and FGF from Hydra vulgaris Ind-Pune, a cnidarian with a defined body axis, an organized nervous system and a remarkable ability of regeneration. We have now identified three more components of VEGF and FGF signaling pathways from hydra. These include FGF-1, FGF receptor 1 (FGFR-1) and VEGF receptor 2 (VEGFR-2) with a view to deciphering their possible roles in regeneration.

METHODS

In silico analysis of proteins was performed using Clustal omega, Swiss model, MEGA 7.0, etc. Gene expression was studied by whole mount in situ hybridization. VEGF and FGF signaling was inhibited using specific pharmacological inhibitors and their effects on head regeneration were studied.

RESULTS

Expression patterns of the genes indicate a possible interaction between FGF-1 and FGFR-1 and also VEGF and VEGFR-2. Upon treatment of decapitated hydra with pharmacological inhibitor of FGFR-1 or VEGFR-2 for 48 h, head regeneration was delayed in treated as compared to untreated, control regenerates. When we studied the expression of head specific genes HyBra1 and HyKs1 and tentacle specific gene HyAlx in control and treated regenerates using whole mount in situ hybridization, expression of all the three genes was found to be adversely affected in treated regenerates.

CONCLUSIONS

The results suggest that VEGF and FGF signaling play important roles in regeneration of hypostome and tentacles in hydra.

N-Glycosylation regulates ligand-dependent activation and signaling of vascular endothelial growth factor receptor 2 (VEGFR2)

The tumor microenvironment and proinflammatory signals significantly alter glycosylation of cell-surface proteins on endothelial cells. By altering the N-glycosylation machinery in the endoplasmic reticulum and Golgi, proinflammatory cytokines promote the modification of endothelial glycoproteins such as vascular endothelial growth factor receptor 2 (VEGFR2) with sialic acid-capped N-glycans. VEGFR2 is a highly N-glycosylated receptor tyrosine kinase involved in pro-angiogenic signaling in physiological and pathological contexts, including cancer. Here, using glycoside hydrolase and kinase assays and immunoprecipitation and MS-based analyses, we demonstrate that N-linked glycans at the Asn-247 site in VEGFR2 hinder VEGF ligand-mediated receptor activation and signaling in endothelial cells. We provide evidence that cell surface-associated VEGFR2 displays sialylated N-glycans at Asn-247 and, in contrast, that the nearby sites Asn-145 and Asn-160 contain lower levels of sialylated N-glycans and higher levels of high-mannose N-glycans, respectively. Furthermore, we report that VEGFR2 Asn-247-linked glycans capped with sialic acid oppose ligand-mediated VEGFR2 activation, whereas the uncapped asialo-glycans favor activation of this receptor. We propose that N-glycosylation, specifically the capping of N-glycans at Asn-247 by sialic acid, tunes ligand-dependent activation and signaling of VEGFR2 in endothelial cells.

KLK3/PSA and cathepsin D activate VEGF-C and VEGF-D

Vascular endothelial growth factor-C (VEGF-C) acts primarily on endothelial cells, but also on non-vascular targets, for example in the CNS and immune system. Here we describe a novel, unique VEGF-C form in the human reproductive system produced via cleavage by kallikrein-related peptidase 3 (KLK3), aka prostate-specific antigen (PSA). KLK3 activated VEGF-C specifically and efficiently through cleavage at a novel N-terminal site. We detected VEGF-C in seminal plasma, and sperm liquefaction occurred concurrently with VEGF-C activation, which was enhanced by collagen and calcium binding EGF domains 1 (CCBE1). After plasmin and ADAMTS3, KLK3 is the third protease shown to activate VEGF-C. Since differently activated VEGF-Cs are characterized by successively shorter N-terminal helices, we created an even shorter hypothetical form, which showed preferential binding to VEGFR-3. Using mass spectrometric analysis of the isolated VEGF-C-cleaving activity from human saliva, we identified cathepsin D as a protease that can activate VEGF-C as well as VEGF-D.

The lymphatic system is composed of networks of vessels that drain fluids from the body’s tissues and filter it back into the blood. Growing these vessels depends on a factor known as VEGF-C, which is released in an inactive form and must be cut by enzymes before it can work. One enzyme that is known to activate the VEGF-C signal when the early embryo is developing is ADAMTS3. If this signal fails to switch on this can result in a condition known as lymphedema – whereby problems in the lymphatic system cause tissues to swell due to insufficient drainage. However, it is unknown whether the VEGF-C signal can be activated by enzymes other than ADAMTS3.

To investigate this Jha, Rauniyar et al. tested a specific family of proteins commonly found in the human prostate, which have previously been predicted to act on VEGF-C. This revealed that the lymphatic vessel growth factor can also be activated by an enzyme found in seminal fluid called prostate specific antigen, or PSA for short. To see if enzymes in other bodily fluids could switch on VEGF-C, different components of human saliva were separated and tested to see which could cut inactive VEGF-C. This showed that VEGF-C could be converted to an active form by another enzyme called cathepsin D.

Unexpectedly, Jha, Rauniyar et al. found that VEGF-C was also present in semen. For conception to occur PSA must liquify the semen following ejaculation. It was discovered that PSA activates VEGF-C just as the semen starts to liquify, suggesting that the lymphatic vessel growth factor might also play an important role in reproduction. In addition to VEGF-C, both PSA and cathepsin D were found to activate another growth factor called VEGF-D, which has an unknown role in the human body.

VEGF-C helps the spread of tumors, and blocking the two enzymes that activate this growth factor may be a new therapeutic approach for cancer. However, more work is needed to validate which types of tumor, if any, use these enzymes to activate VEGF-C. In addition, understanding the relationship between PSA and VEGF-C could help improve our knowledge of human reproduction.

The RTK Interactome: Overview and Perspective on RTK Heterointeractions

Receptor tyrosine kinases (RTKs) play important roles in cell growth, motility, differentiation, and survival. These single-pass membrane proteins are grouped into subfamilies based on the similarity of their extracellular domains. They are generally thought to be activated by ligand binding, which promotes homodimerization and then autophosphorylation in trans. However, RTK interactions are more complicated, as RTKs can interact in the absence of ligand and heterodimerize within and across subfamilies. Here, we review the known cross-subfamily RTK heterointeractions and their possible biological implications, as well as the methodologies which have been used to study them. Moreover, we demonstrate how thermodynamic models can be used to study RTKs and to explain many of the complicated biological effects which have been described in the literature. Finally, we discuss the concept of the RTK interactome: a putative, extensive network of interactions between the RTKs. This RTK interactome can produce unique signaling outputs; can amplify, inhibit, and modify signaling; and can allow for signaling backups. The existence of the RTK interactome could provide an explanation for the irreproducibility of experimental data from different studies and for the failure of some RTK inhibitors to produce the desired therapeutic effects. We argue that a deeper knowledge of RTK interactome thermodynamics can lead to a better understanding of fundamental RTK signaling processes in health and disease. We further argue that there is a need for quantitative, thermodynamic studies that probe the strengths of the interactions between RTKs and their ligands and between different RTKs.

Human Recombinant VEGFR2D4 Biochemical Characterization to Investigate Novel Anti-VEGFR2D4 Antibodies for Allosteric Targeting of VEGFR2

VEGF-A/VEGFR2 complex is the major signaling pathway involved in angiogenesis and the inhibition of this axis retards tumor growth and inflammatory disorders progression, reducing vessel sprouting. Signaling by VEGFR2 requires receptor dimerization and a well-defined orientation of monomers in the active dimer. The extracellular portion of receptor is composed of seven Ig-like domains, of which D2-3 are the ligand binding domains, while D4 and D7, establishing homotypic contacts, allosterically regulate receptor activity. The allosteric targeting of VEGFR2 represents a promising alternative to study neovascular disorders overcoming drawbacks related to competition with VEGF. In this work, we expressed in bacterial host domain 4 of VEGFR2 (VEGFR2D4). After protein refolding, we characterized the purified domain and administered it in mice for monoclonal antibodies production. One of them, mAbD4, was tested in ELISA assays, showing a nanomolar affinity for VEGFR2D4. Finally, the methodology here described could contribute to the development of antibodies which can allosterically bind VEGFR2 and therefore to be used for imaging purposes or to modulate receptor signaling.

Rational Design of Antiangiogenic Helical Oligopeptides Targeting the Vascular Endothelial Growth Factor Receptors

Tumor angiogenesis, essential for cancer development, is regulated mainly by vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs), which are overexpressed in cancer cells. Therefore, the VEGF/VEGFR interaction represents a promising pharmaceutical target to fight cancer progression. The VEGF surface interacting with VEGFRs comprises a short α-helix. In this work, helical oligopeptides mimicking the VEGF-C helix were rationally designed based on structural analyses and computational studies. The helical conformation was stabilized by optimizing intramolecular interactions and by introducing helix-inducing C^α,α-disubstituted amino acids. The conformational features of the synthetic peptides were characterized by circular dichroism and nuclear magnetic resonance, and their receptor binding properties and antiangiogenic activity were determined. The best hits exhibited antiangiogenic activity in vitro at nanomolar concentrations and were resistant to proteolytic degradation.

Theoretical Insights into the Retinal Dynamics of Vascular Endothelial Growth Factor in Patients Treated with Ranibizumab, Based on an Ocular Pharmacokinetic/Pharmacodynamic Model

Neovascular age-related macular degeneration (wet AMD) results from the pathological angiogenesis of choroidal capillaries, which leak fluid within or below the macular region of the retina. The current standard of care for treating wet AMD utilizes intravitreal injections of anti-VEGF antibodies or antibody fragments to suppress ocular vascular endothelial growth factor (VEGF) levels. While VEGF suppression has been demonstrated in wet AMD patients by serial measurements of free-VEGF concentrations in aqueous humor samples, it is presumed that anti-VEGF molecules also permeate across the inner limiting membrane (ILM) of the retina as well as the retinal pigmented epithelium (RPE) and suppress VEGF levels in the retina and/or choroidal regions. The latter effects are inferred from serial optical coherence tomography (OCT) measurements of fluid in the retinal and sub-retinal spaces. In order to gain theoretical insights to the dynamics of retinal levels of free-VEGF following intravitreal injection of anti-VEGF molecules, we have extended our previous two-compartment pharmacokinetic/pharmacodynamic (PK/PD) model of ranibizumab-VEGF suppression in vitreous and aqueous humors to a three-compartment model that includes the retinal compartment. In the new model, reference values for the macromolecular permeability coefficients between retina and vitreous ( p_ILM) and between retina and choroid ( p_RPE) were estimated from PK data obtained in rabbit. With these values, the three-compartment model was used to re-analyze the aqueous humor levels of free-VEGF obtained in wet AMD patients treated with ranibizumab and to compare them to the simulated retinal levels of free-VEGF, including the observed variability in PK and PD. We have also used the model to explore the impact of varying p_ILM and p_RPE to assess the case in which an anti-VEGF molecule is impermeable to the ILM and to assess the potential effects of AMD pathology on the RPE barrier. Our simulations show that, for the reference values of p_ILM and p_RPE, the simulated duration of VEGF suppression in the retina is approximately 50% shorter than the observed duration of VEGF suppression in the aqueous humor, a finding that may explain the short duration of suppressed disease activity in the "high anti-VEGF demand" patients reported by Fauser and Muether ( Br. J. Ophthalmol. 2016, 100, 1494-1498 ). At 10-fold lower values of p_RPE, the durations of VEGF suppression in the retina and aqueous humor are comparable. Lastly we have used the model to explore the impact of dose and binding parameters on the duration and depth of VEGF suppression in the aqueous and retinal compartments. Our simulations with the three-compartment PK/PD model provide new insights into inter-patient variability in response to anti-VEGF therapy and offer a mechanistic framework for developing treatment regimens and molecules that may prolong the duration of retinal VEGF suppression.

Molecular Pharmacology of VEGF-A Isoforms: Binding and Signalling at VEGFR2

Vascular endothelial growth factor-A (VEGF-A) is a key mediator of angiogenesis, signalling via the class IV tyrosine kinase receptor family of VEGF Receptors (VEGFRs). Although VEGF-A ligands bind to both VEGFR1 and VEGFR2, they primarily signal via VEGFR2 leading to endothelial cell proliferation, survival, migration and vascular permeability. Distinct VEGF-A isoforms result from alternative splicing of the Vegfa gene at exon 8, resulting in VEGFxxxa or VEGFxxxb isoforms. Alternative splicing events at exons 5⁻7, in addition to recently identified posttranslational read-through events, produce VEGF-A isoforms that differ in their bioavailability and interaction with the co-receptor Neuropilin-1. This review explores the molecular pharmacology of VEGF-A isoforms at VEGFR2 in respect to ligand binding and downstream signalling. To understand how VEGF-A isoforms have distinct signalling despite similar affinities for VEGFR2, this review re-evaluates the typical classification of these isoforms relative to the prototypical, “pro-angiogenic” VEGF165a. We also examine the molecular mechanisms underpinning the regulation of VEGF-A isoform signalling and the importance of interactions with other membrane and extracellular matrix proteins. As approved therapeutics targeting the VEGF-A/VEGFR signalling axis largely lack long-term efficacy, understanding these isoform-specific mechanisms could aid future drug discovery efforts targeting VEGF receptor pharmacology.

Characterization of a drug-targetable allosteric site regulating vascular endothelial growth factor signaling

Vascular endothelial growth factors (VEGFs) regulate blood and lymph vessel development upon activation of three receptor tyrosine kinases (VEGFRs). The extracellular domain of VEGFRs consists of seven Ig-homology domains, of which D2-3 form the ligand-binding site, while the membrane proximal domains D4-7 are involved in homotypic interactions in ligand-bound receptor dimers. Based on low-resolution structures, we identified allosteric sites in D4-5 and D7 of vascular endothelial growth factor receptor 2 (VEGFR-2) accomplishing regulatory functions. Allosteric inhibition of VEGFR-2 signaling represents an attractive option for the treatment of neovascular diseases. We showed earlier that DARPin^® binders to domains D4 or D7 are potent VEGFR-2 inhibitors. Here we investigated in detail the allosteric inhibition mechanism of the domain D4 binding inhibitor D4b. The 2.38 Å crystal structure of D4b in complex with VEGFR-2 D4-5, the first high-resolution structure of this VEGFR-2 segment, indicates steric hindrance by D4b as the mechanism of inhibition of receptor activation. At the cellular level, D4b triggered quantitative internalization of VEGFR-2 in the absence of ligand and thus clearance of VEGFR-2 from the surface of endothelial cells. The allosteric VEGFR-2 inhibition was sufficiently strong to efficiently inhibit the growth of human endothelial cells at suboptimal dose in a mouse xenograft model in vivo, underlining the therapeutic potential of the approach.

Population exposure-safety analysis of cediranib for Phase I and II studies in patients with cancer

AIMS

A multistudy analysis of cediranib, a potent, selective inhibitor of all three vascular endothelial growth factor receptors (VEGFR-1, -2 and -3), was conducted to establish population exposure-safety models for the relationship of cediranib exposure to the safety endpoints, diastolic and systolic blood pressure (DBP and SBP) and diarrhoea in cancer patients. These models were applied to predict safety outcomes for different cediranib dose regimens.

METHODS

Models for hypertension and diarrhoea were constructed based on data from 10 Phase I and three Phase II studies comprising 631 cancer patients following cediranib once-daily oral dosing. Daily DBP and SBP were simultaneously characterized using indirect response models for predicted cediranib concentration-time courses, while daily diarrhoea events were modelled as ordered categorical variables with a proportional odds model with a Markov element for predicted average cediranib concentrations.

RESULTS

For 20 mg cediranib once-daily oral administration, the mean increase in DBP and SBP was predicted to be 7 (95% CI 3-13) and 8 mmHg (95% CI 3-16), respectively, while the probability of mild diarrhoea, but not the severity, was predicted to increase over time. Severe diarrhoea was predicted to be resolved rapidly upon discontinuation of cediranib treatment.

CONCLUSIONS

Maximum blood pressure increase was observed within the first few days of cediranib treatment, consistent with the pharmacokinetic profile of cediranib reaching steady state in about 5 days. The probability of diarrhoea increased with cediranib concentration but was far more dependent on the status of diarrhoea predicted on the previous day.

Monomeric gremlin is a novel vascular endothelial growth factor receptor-2 antagonist

Angiogenesis plays a key role in various physiological and pathological conditions, including inflammation and tumor growth. The bone morphogenetic protein (BMP) antagonist gremlin has been identified as a novel pro-angiogenic factor. Gremlin promotes neovascular responses via a BMP-independent activation of the vascular endothelial growth factor (VEGF) receptor-2 (VEGFR2). BMP antagonists may act as covalent or non-covalent homodimers or in a monomeric form, while VEGFRs ligands are usually dimeric. However, the oligomeric state of gremlin and its role in modulating the biological activity of the protein remain to be elucidated.

Here we show that gremlin is expressed in vitro and in vivo both as a monomer and as a covalently linked homodimer. Mutagenesis of amino acid residue Cys141 prevents gremlin dimerization leading to the formation of gremlin^C141A monomers. Gremlin^C141A monomer retains a BMP antagonist activity similar to the wild-type dimer, but is devoid of a significant angiogenic capacity. Notably, we found that gremlin^C141A mutant engages VEGFR2 in a non-productive manner, thus acting as receptor antagonist. Accordingly, both gremlin^C141A and wild-type monomers inhibit angiogenesis driven by dimeric gremlin or VEGF-A₁₆₅. Moreover, by acting as a VEGFR2 antagonist, gremlin^C141A inhibits the angiogenic and tumorigenic potential of murine breast and prostate cancer cells in vivo.

In conclusion, our data show that gremlin exists in multiple forms endowed with specific bioactivities and provide new insights into the molecular bases of gremlin dimerization. Furthermore, we propose gremlin monomer as a new inhibitor of VEGFR2 signalling during tumor growth.

Arg-Leu-Tyr-Glu tetrapeptide inhibits tumor progression by suppressing angiogenesis and vascular permeability via VEGF receptor-2 antagonism

The tetrapeptide Arg-Leu-Tyr-Glu (RLYE) is known to inhibit vascular endothelial growth factor-A (VEGF-A)-induced angiogenesis in vitro. Herein, we examined its underlying mechanism and antitumor activity associated with vascular remodeling. RLYE inhibited VEGF-A-induced angiogenesis in a mouse model and suppressed VEGF-A-induced angiogenic signal cascades in human endothelial cells. However, RLYE showed no inhibitory effect on VEGF-A-induced proliferation and migration of multiple myeloma cells expressing VEGF receptor (VEGFR)-1, but not VEGFR-2. In addition, RLYE showed no inhibitory effect on angiogenic activities induced by VEGF-B, basic fibroblast growth factor, epithermal growth factor, sphingosine-1-phosphate, and placental growth factor. RLYE bound specifically to VEGFR-2 at the VEGF-A binding site, thereby blocking VEGF-A-VEGFR-2 binding and VEGF-A-induced VEGFR-2 internalization. The RLYE peptide inhibited tumor growth and metastasis via suppression of tumor angiogenesis in tumor-bearing mice. Moreover, RLYE showed a synergistic effect of the cytotoxic agent irinotecan on tumor cell apoptosis and tumor progression via tumor vessel normalization due to stabilization of VE-cadherin-mediated adherens junction, improvement of pericyte coverage, and inhibition of vascular leakage in tumors. Our results suggest that RLYE can be used as an antiangiogenic and tumor blood vessel remodeling agent for inhibition of tumor growth and metastasis by antagonizing VEGFR-2, with the synergistic anti-cancer effect via enhancement of drug delivery and therapeutic efficacy.

VEGF-B Levels in the Vitreous of Diabetic and Non-Diabetic Patients with Ocular Diseases and Its Correlation with Structural Parameters

Vascular endothelial growth factor B (VEGF-B) is one of the enigmatic members of the VEGF family. The knowledge gap about VEGF-B expression and how its levels are altered in diabetic eyes were the focus of this investigation that was addressed by comparing and correlating vitreous VEGF-B between diabetic and non-diabetic patients. VEGF-B levels were measured by enzyme-linked immunosorbent assay in vitreous samples (n = 33) from diabetic (n = 25) and non-diabetic (n = 8) patients. Results were compared between groups. Optical coherence tomography from diabetic patients was evaluated for central retinal thickness (CRT) and macular volume (MV). Mean vitreous VEGF-B concentration was higher in diabetic (18.82 ± 1.44 pg/mL) vs. non-diabetic patients (17.90 ± 0.32 pg/mL) (p = 0.006), and in proliferative diabetic retinopathy (PDR) (19.03 ± 1.52 pg/mL) vs. non-PDR (NPDR) patients (18.18 ±0.96 pg/mL) (p = 0.025). In diabetic retinopathy (DR) patients, correlation between VEGF-B and CRT (μm) was positive and moderate: r_s = 0.441 (p ≤ 0.05) and the correlation between VEGF-B and MV (mm³) was positive and robust: r_s = 0.716 (p ≤ 0.01). VEGF-B levels are overexpressed in vitreous of diabetic patients, and the levels are higher in developed stages of DR. Correlation results show that CRT and MV increase with increased levels of VEGF-B. Targeting VEGF-B inhibition may have therapeutic beneficial implications.

Contribution of vascular endothelial growth factor receptor-2 sialylation to the process of angiogenesis

Vascular endothelial growth factor receptor-2 (VEGFR2) is the main pro-angiogenic receptor expressed by endothelial cells (ECs). Using surface plasmon resonance, immunoprecipitation, enzymatic digestion, immunofluorescence and cross-linking experiments with specific sugar-binding lectins, we demonstrated that VEGFR2 bears both α,1-fucose and α(2,6)-linked sialic acid (NeuAc). However, only the latter is required for VEGF binding to VEGFR2 and consequent VEGF-dependent VEGFR2 activation and motogenic response in ECs. Notably, downregulation of β-galactoside α(2,6)-sialyltransferase expression by short hairpin RNA transduction inhibits VEGFR2 α(2,6) sialylation that is paralleled by an increase of β-galactoside α(2,3)-sialyltransferase expression. This results in an ex-novo α(2,3)-NeuAc sialylation of the receptor that functionally replaces the lacking α(2,6)-NeuAc, thus allowing VEGF/VEGFR2 interaction. In keeping with the role of VEGFR2 sialylation in angiogenesis, the α(2,6)-NeuAc-binding lectin Sambucus nigra (SNA) prevents VEGF-dependent VEGFR2 autophosphorylation and EC motility, proliferation and motogenesis. In addition, SNA exerts a VEGF-antagonist activity in tridimensional angiogenesis models in vitro and in the chick-embryo chorioallantoic membrane neovascularization assay and mouse matrigel plug assay in vivo. In conclusion, VEGFR2-associated NeuAc plays an important role in modulating VEGF/VEGFR2 interaction, EC pro-angiogenic activation and neovessel formation. VEGFR2 sialylation may represent a target for the treatment of angiogenesis-dependent diseases.

The optimum marker for the detection of lymphatic vessels

Podoplanin, lymphatic vessel endothelial hyaluronic acid receptor-1, prospero-related homeobox-1 and vascular endothelial growth factor receptor 3 have been demonstrated to have crucial roles in the development of the lymphatic system and lymphangiogenesis process by combining with their corresponding receptors. Thus, the four markers have been widely used in labelling lymphatic vessels for the detection of lymphangiogenesis and lymphatic vessel invasion. Numerous authors have aimed to identify the roles of these four markers in the lymphatic system and the mechanisms have been partly clarified at the molecular level. The aim of the present review was to comprehensively clarify the characteristics and latent action modes of the four markers in order to determine which is the best one for the detection of lymphangiogenesis and lymphatic vessel invasion.

Structure and dynamics of the insulin receptor: implications for receptor activation and drug discovery

Recently, major progress has been made in uncovering the mechanisms of how insulin engages its receptor and modulates downstream signal transduction. Here, we present in detail the current structural knowledge surrounding the individual components of the complex, binding sites, and dynamics during the activation process. A novel kinase triggering mechanism, the 'bow-arrow model', is proposed based on current knowledge and computational simulations of this system, in which insulin, after its initial interaction with binding site 1, engages with site 2 between the fibronectin type III (FnIII)-1 and -2 domains, which changes the conformation of FnIII-3 and eventually translates into structural changes across the membrane. This model provides a new perspective on the process of insulin binding to its receptor and, thus, could lead to future novel drug discovery efforts.