Neuropilin-1 promotes VEGFR-2 trafficking through Rab11 vesicles thereby specifying signal output. Blood. 2011;118:816-826. [PMID: 21586748 DOI: 10.1182/blood-2011-01-328773]

A proteomics approach to isolating neuropilin-dependent α5 integrin trafficking pathways: neuropilin 1 and 2 co-traffic α5 integrin through endosomal p120RasGAP to promote polarised fibronectin fibrillogenesis in endothelial cells

Integrin trafficking to and from membrane adhesions is a crucial mechanism that dictates many aspects of a cell's behaviour, including motility, polarisation, and invasion. In endothelial cells (ECs), the intracellular traffic of α5 integrin is regulated by both neuropilin 1 (NRP1) and neuropilin 2 (NRP2), yet the redundancies in function between these co-receptors remain unclear. Moreover, the endocytic complexes that participate in NRP-directed traffic remain poorly annotated. Here we identify an important role for the GTPase-activating protein p120RasGAP in ECs, promoting the recycling of α5 integrin from early endosomes. Mechanistically, p120RasGAP enables transit of endocytosed α5 integrin-NRP1-NRP2 complexes to Rab11+ recycling endosomes, promoting cell polarisation and fibronectin (FN) fibrillogenesis. Silencing of both NRP receptors, or p120RasGAP, resulted in the accumulation of α5 integrin in early endosomes, a loss of α5 integrin from surface adhesions, and attenuated EC polarisation. Endothelial-specific deletion of both NRP1 and NRP2 in the postnatal retina recapitulated our in vitro findings, severely impairing FN fibrillogenesis and polarised sprouting. Our data assign an essential role for p120RasGAP during integrin traffic in ECs and support a hypothesis that NRP receptors co-traffic internalised cargoes. Importantly, we utilise comparative proteomics analyses to isolate a comprehensive map of NRP1-dependent and NRP2-dependent α5 integrin interactions in ECs.

Multifunctional nanocoatings with synergistic controlled release of zinc ions and cytokines for precise modulation of vascular intimal reconstruction

Vascular stent implantation remains the major therapeutic method for cardiovascular diseases currently. We here introduced crucial biological functional biological function factors (SDF-1α, VEGF) and vital metal ions (Zn2+) into the stent surface to explore their synergistic effect in the microenvironment. The combination of the different factors is known to effectively regulate cellular inflammatory response and selectively regulate cell biological behavior. Meanwhile, in the implemented method, VEGF and Zn2+ were loaded into heparin and poly-l-lysine (Hep-PLL) nanoparticles, ensuring a controlled release of functional molecules with a multi-factor synergistic effect and excellent biological functions in vitro and in vivo. Notably, after 150 days of implantation of the modified stent in rabbits, a thin and smooth new intima was obtained. This study offers a new idea for constructing a modified surface microenvironment and promoting tissue repair.

Endothelial Neuropilin-1: a multifaced signal transducer with an emerging role in inflammation and atherosclerosis beyond angiogenesis

Neuropilin-1 (NRP1) is a transmembrane glycoprotein expressed by several cell types including, neurons, endothelial cells (ECs), smooth muscle cells, cardiomyocytes and immune cells comprising macrophages, dendritic cells and T cell subsets. Since NRP1 discovery in 1987 as an adhesion molecule in the frog nervous system, more than 2300 publications on PubMed investigated the function of NRP1 in physiological and pathological contexts. NRP1 has been characterised as a coreceptor for class 3 semaphorins and several members of the vascular endothelial growth factor (VEGF) family. Because the VEGF family is the main regulator of blood and lymphatic vessel growth in addition to promoting neurogenesis, neuronal patterning, neuroprotection and glial growth, the role of NRP1 in these biological processes has been extensively investigated. It is now established that NRP1 promotes the physiological growth of new vessels from pre-existing ones in the process of angiogenesis. Furthermore, several studies have shown that NRP1 mediates signalling pathways regulating pathological vascular growth in ocular neovascular diseases and tumour development. Less defined are the roles of NRP1 in maintaining the function of the quiescent established vasculature in an adult organism. This review will focus on the opposite roles of NRP1 in regulating transforming growth factor β signalling pathways in different cell types, and on the emerging role of endothelial NRP1 as an atheroprotective, anti-inflammatory factor involved in the response of ECs to shear stress.

Trafficking dynamics of VEGFR1, VEGFR2, and NRP1 in human endothelial cells

The vascular endothelial growth factor (VEGF) family of cytokines are key drivers of blood vessel growth and remodeling. These ligands act via multiple VEGF receptors (VEGFR) and co-receptors such as Neuropilin (NRP) expressed on endothelial cells. These membrane-associated receptors are not solely expressed on the cell surface, they move between the surface and intracellular locations, where they can function differently. The location of the receptor alters its ability to 'see' (access and bind to) its ligands, which regulates receptor activation; location also alters receptor exposure to subcellularly localized phosphatases, which regulates its deactivation. Thus, receptors in different subcellular locations initiate different signaling, both in terms of quantity and quality. Similarly, the local levels of co-expression of other receptors alters competition for ligands. Subcellular localization is controlled by intracellular trafficking processes, which thus control VEGFR activity; therefore, to understand VEGFR activity, we must understand receptor trafficking. Here, for the first time, we simultaneously quantify the trafficking of VEGFR1, VEGFR2, and NRP1 on the same cells-specifically human umbilical vein endothelial cells (HUVECs). We build a computational model describing the expression, interaction, and trafficking of these receptors, and use it to simulate cell culture experiments. We use new quantitative experimental data to parameterize the model, which then provides mechanistic insight into the trafficking and localization of this receptor network. We show that VEGFR2 and NRP1 trafficking is not the same on HUVECs as on non-human ECs; and we show that VEGFR1 trafficking is not the same as VEGFR2 trafficking, but rather is faster in both internalization and recycling. As a consequence, the VEGF receptors are not evenly distributed between the cell surface and intracellular locations, with a very low percentage of VEGFR1 being on the cell surface, and high levels of NRP1 on the cell surface. Our findings have implications both for the sensing of extracellular ligands and for the composition of signaling complexes at the cell surface versus inside the cell.

Biomechanical forces and force-triggered drug delivery in tumor neovascularization

Tumor angiogenesis is one of the typical hallmarks of tumor occurrence and development, and tumor neovascularization also exhibits distinct characteristics from normal blood vessels. As the number of cells and matrix inside the tumor increases, the biomechanical force is enhanced, specifically manifested as solid stress, fluid stress, stiffness, and topology. This mechanical microenvironment also provides shelter for tumors and intensifies angiogenesis, providing oxygen and nutritional support for tumor progression. During tumor development, the biomechanical microenvironment also emerges, which in turn feeds back to regulate the tumor progression, including tumor angiogenesis, and biochemical and biomechanical signals can regulate tumor angiogenesis. Blood vessels possess inherent sensitivity to mechanical stimuli, but compared to the extensive research on biochemical signal regulation, the study of the regulation of tumor neovascularization by biomechanical signals remains relatively scarce. Biomechanical forces can affect the phenotypic characteristics and mechanical signaling pathways of tumor blood vessels, directly regulating angiogenesis. Meanwhile, they can indirectly regulate tumor angiogenesis by causing an imbalance in angiogenesis signals and affecting stromal cell function. Understanding the regulatory mechanism of biomechanical forces in tumor angiogenesis is beneficial for better identifying and even taming the mechanical forces involved in angiogenesis, providing new therapeutic targets for tumor vascular normalization. Therefore, we summarized the composition of biomechanical forces and their direct or indirect regulation of tumor neovascularization. In addition, this review discussed the use of biomechanical forces in combination with anti-angiogenic therapies for the treatment of tumors, and biomechanical forces triggered delivery systems.

Dual targeting of DR5 and VEGFR2 molecular pathways by multivalent fusion protein significantly suppresses tumor growth and angiogenesis

Destroying tumor vasculature is a relevant therapeutic strategy due to its involvement in tumor progression. However, adaptive resistance to approved antiangiogenic drugs targeting VEGF/VEGFR pathway requires the recruitment of additional targets. In this aspect, targeting TRAIL pathway is promising as it is an important component of the immune system involved in tumor immunosurveillance. For dual targeting of malignant cells and tumor vascular microenvironment, we designed a multivalent fusion protein SRH-DR5-B-iRGD with antiangiogenic VEGFR2-specific peptide SRH at the N-terminus and a tumor-targeting and -penetrating peptide iRGD at the C-terminus of receptor-selective TRAIL variant DR5-B. SRH-DR5-B-iRGD obtained high affinity for DR5, VEGFR2 and αvβ3 integrin in nanomolar range. Fusion of DR5-B with effector peptides accelerated DR5 receptor internalization rate upon ligand binding. Antitumor efficacy was evaluated in vitro in human tumor cell lines and primary patient-derived glioblastoma neurospheres, and in vivo in xenograft mouse model of human glioblastoma. Multivalent binding of SRH-DR5-B-iRGD fusion efficiently stimulated DR5-mediated tumor cell death via caspase-dependent mechanism, suppressed xenograft tumor growth by >80 %, doubled the lifespan of xenograft animals, and inhibited tumor vascularization. Therefore, targeting DR5 and VEGFR2 molecular pathways with SRH-DR5-B-iRGD protein may provide a novel therapeutic approach for treatment of solid tumors.

HTATIP2 regulates arteriogenic activity in monocytes from patients with limb ischemia

Use of autologous cells isolated from elderly patients with multiple comorbidities may account for the modest efficacy of cell therapy in patients with chronic limb threatening ischemia (CLTI). We aimed to determine whether proarteriogenic monocyte/macrophages (Mo/MΦs) from patients with CLTI were functionally impaired and to demonstrate the mechanisms related to any impairment. Proarteriogenic Mo/MΦs isolated from patients with CLTI were found to have an impaired capacity to promote neovascularization in vitro and in vivo compared with those isolated from healthy controls. This was associated with increased expression of human HIV-1 TAT interactive protein-2 (HTATIP2), a transcription factor known to suppress angiogenesis/arteriogenesis. Silencing HTATIP2 restored the functional capacity of CLTI Mo/MΦs, which was associated with increased expression of arteriogenic regulators Neuropilin-1 and Angiopoietin-1, and their ability to enhance angiogenic (endothelial tubule formation) and arteriogenic (smooth muscle proliferation) processes in vitro. In support of the translational relevance of our findings, silencing HTATIP2 in proarteriogenic Mo/MΦs isolated from patients with CLTI rescued their capacity to enhance limb perfusion in the ischemic hindlimb by effecting greater angiogenesis and arteriogenesis. Ex vivo modulation of HTATIP2 may offer a strategy for rescuing the functional impairment of pro-angio/arteriogenic Mo/MΦs prior to autologous delivery and increase the likelihood of clinical efficacy.

Neuropilin-1 is essential for vascular endothelial growth factor A-mediated increase of sensory neuron activity and development of pain-like behaviors

ABSTRACT

Neuropilin-1 (NRP-1) is a transmembrane glycoprotein that binds numerous ligands including vascular endothelial growth factor A (VEGFA). Binding of this ligand to NRP-1 and the co-receptor, the tyrosine kinase receptor VEGFR2, elicits nociceptor sensitization resulting in pain through the enhancement of the activity of voltage-gated sodium and calcium channels. We previously reported that blocking the interaction between VEGFA and NRP-1 with the Spike protein of SARS-CoV-2 attenuates VEGFA-induced dorsal root ganglion (DRG) neuronal excitability and alleviates neuropathic pain, pointing to the VEGFA/NRP-1 signaling as a novel therapeutic target of pain. Here, we investigated whether peripheral sensory neurons and spinal cord hyperexcitability and pain behaviors were affected by the loss of NRP-1. Nrp-1 is expressed in both peptidergic and nonpeptidergic sensory neurons. A CRIPSR/Cas9 strategy targeting the second exon of nrp-1 gene was used to knockdown NRP-1. Neuropilin-1 editing in DRG neurons reduced VEGFA-mediated increases in CaV2.2 currents and sodium currents through NaV1.7. Neuropilin-1 editing had no impact on voltage-gated potassium channels. Following in vivo editing of NRP-1, lumbar dorsal horn slices showed a decrease in the frequency of VEGFA-mediated increases in spontaneous excitatory postsynaptic currents. Finally, intrathecal injection of a lentivirus packaged with an NRP-1 guide RNA and Cas9 enzyme prevented spinal nerve injury-induced mechanical allodynia and thermal hyperalgesia in both male and female rats. Collectively, our findings highlight a key role of NRP-1 in modulating pain pathways in the sensory nervous system.

Biology and therapeutic targeting of vascular endothelial growth factor A

The formation of new blood vessels, called angiogenesis, is an essential pathophysiological process in which several families of regulators have been implicated. Among these, vascular endothelial growth factor A (VEGFA; also known as VEGF) and its two tyrosine kinase receptors, VEGFR1 and VEGFR2, represent a key signalling pathway mediating physiological angiogenesis and are also major therapeutic targets. VEGFA is a member of the gene family that includes VEGFB, VEGFC, VEGFD and placental growth factor (PLGF). Three decades after its initial isolation and cloning, VEGFA is arguably the most extensively investigated signalling system in angiogenesis. Although many mediators of angiogenesis have been identified, including members of the FGF family, angiopoietins, TGFβ and sphingosine 1-phosphate, all current FDA-approved anti-angiogenic drugs target the VEGF pathway. Anti-VEGF agents are widely used in oncology and, in combination with chemotherapy or immunotherapy, are now the standard of care in multiple malignancies. Anti-VEGF drugs have also revolutionized the treatment of neovascular eye disorders such as age-related macular degeneration and ischaemic retinal disorders. In this Review, we emphasize the molecular, structural and cellular basis of VEGFA action as well as recent findings illustrating unexpected interactions with other pathways and provocative reports on the role of VEGFA in regenerative medicine. We also discuss clinical and translational aspects of VEGFA. Given the crucial role that VEGFA plays in regulating angiogenesis in health and disease, this molecule is largely the focus of this Review.

Vascular endothelial growth factor isoforms differentially protect neurons against neurotoxic events associated with Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, the chronic and progressive deterioration of memory and cognitive abilities. AD can be pathologically characterised by neuritic plaques and neurofibrillary tangles, formed by the aberrant aggregation of β-amyloid and tau proteins, respectively. We tested the hypothesis that VEGF isoforms VEGF-A₁₆₅a and VEGF-A₁₆₅b, produced by differential splice site selection in exon 8, could differentially protect neurons from neurotoxicities induced by β-amyloid and tau proteins, and that controlling expression of splicing factor kinase activity could have protective effects on AD-related neurotoxicity in vitro. Using oxidative stress, β-amyloid, and tau hyperphosphorylation models, we investigated the effect of VEGF-A splicing isoforms, previously established to be neurotrophic agents, as well as small molecule kinase inhibitors, which selectively inhibit SRPK1, the major regulator of VEGF splicing. While both VEGF-A₁₆₅a and VEGF-A₁₆₅b isoforms were protective against AD-related neurotoxicity, measured by increased metabolic activity and neurite outgrowth, VEGF-A₁₆₅a was able to enhance neurite outgrowth but VEGF-A₁₆₅b did not. In contrast, VEGF-A₁₆₅b was more effective than VEGF-A₁₆₅a in preventing neurite "dieback" in a tau hyperphosphorylation model. SRPK1 inhibition was found to significantly protect against neurite "dieback" through shifting AS of VEGFA towards the VEGF-A₁₆₅b isoform. These results indicate that controlling the activities of the two different isoforms could have therapeutic potential in Alzheimer's disease, but their effect may depend on the predominant mechanism of the neurotoxicity-tau or β-amyloid.

KIF13B mediates VEGFR2 recycling to modulate vascular permeability

Excessive vascular endothelial growth factor-A (VEGF-A) signaling induces vascular leakage and angiogenesis in diseases. VEGFR2 trafficking to the cell surface, mediated by kinesin-3 family protein KIF13B, is essential to respond to VEGF-A when inducing angiogenesis. However, the precise mechanism of how KIF13B regulates VEGF-induced signaling and its effects on endothelial permeability is largely unknown. Here we show that KIF13B-mediated recycling of internalized VEGFR2 through Rab11-positive recycling vesicle regulates endothelial permeability. Phosphorylated VEGFR2 at the cell-cell junction was internalized and associated with KIF13B in Rab5-positive early endosomes. KIF13B mediated VEGFR2 recycling through Rab11-positive recycling vesicle. Inhibition of the function of KIF13B attenuated phosphorylation of VEGFR2 at Y951, SRC at Y416, and VE-cadherin at Y685, which are necessary for endothelial permeability. Failure of VEGFR2 trafficking to the cell surface induced accumulation and degradation of VEGFR2 in lysosomes. Furthermore, in the animal model of the blinding eye disease wet age-related macular degeneration (AMD), inhibition of KIF13B-mediated VEGFR2 trafficking also mitigated vascular leakage. Thus, the present results identify the fundamental role of VEGFR2 recycling to the cell surface in mediating vascular permeability, which suggests a promising strategy for mitigating vascular leakage associated with inflammatory diseases.

YULINK regulates vascular formation in zebrafish and HUVECs

BACKGROUND

The distinct arterial and venous cell fates are dictated by a combination of various genetic factors which form diverse types of blood vessels such as arteries, veins, and capillaries. We report here that YULINK protein is involved in vasculogenesis, especially venous formation.

METHODS

In this manuscript, we employed gene knockdown, yeast two-hybrid, FLIM-FRET, immunoprecipitation, and various imaging technologies to investigate the role of YULINK gene in zebrafish and human umbilical vein endothelial cells (HUVECs).

RESULTS

Knockdown of YULINK during the arterial-venous developmental stage of zebrafish embryos led to the defective venous formation and abnormal vascular plexus formation. Knockdown of YULINK in HUVECs impaired their ability to undergo cell migration and differentiation into a capillary-like tube formation. In addition, the phosphorylated EPHB4 was decreased in YULINK knockdown HUVECs. Yeast two-hybrid, FLIM-FRET, immunoprecipitation, as well as imaging technologies showed that YULINK colocalized with endosome related proteins (EPS15, RAB33B or TICAM2) and markers (Clathrin and RHOB). VEGF-induced VEGFR2 internalization was also compromised in YULINK knockdown HUVECs, demonstrating to the involvement of YULINK.

CONCLUSION

This study suggests that YULINK regulates vasculogenesis, possibly through endocytosis in zebrafish and HUVECs.

Exploring RAB11A Pathway to Hinder Chronic Myeloid Leukemia-Induced Angiogenesis In Vivo

Neoangiogenesis is generally correlated with poor prognosis, due to the promotion of cancer cell growth, invasion and metastasis. The progression of chronic myeloid leukemia (CML) is frequently associated with an increased vascular density in bone marrow. From a molecular point of view, the small GTP-binding protein Rab11a, involved in the endosomal slow recycling pathway, has been shown to play a crucial role for the neoangiogenic process at the bone marrow of CML patients, by controlling the secretion of exosomes by CML cells, and by regulating the recycling of vascular endothelial factor receptors. The angiogenic potential of exosomes secreted by the CML cell line K562 has been previously observed using the chorioallantoic membrane (CAM) model. Herein, gold nanoparticles (AuNPs) were functionalized with an anti-RAB11A oligonucleotide (AuNP@RAB11A) to downregulate RAB11A mRNA in K562 cell line which showed a 40% silencing of the mRNA after 6 h and 14% silencing of the protein after 12 h. Then, using the in vivo CAM model, these exosomes secreted by AuNP@RAB11A incubated K562 did not present the angiogenic potential of those secreted from untreated K562 cells. These results demonstrate the relevance of Rab11 for the neoangiogenesis mediated by tumor exosomes, whose deleterious effect may be counteracted via targeted silencing of these crucial genes; thus, decreasing the number of pro-tumoral exosomes at the tumor microenvironment.

Endothelial VEGFR Coreceptors Neuropilin-1 and Neuropilin-2 Are Essential for Tumor Angiogenesis

Neuropilin (NRP) expression is highly correlated with poor outcome in multiple cancer subtypes. As known coreceptors for VEGFRs, core drivers of angiogenesis, past investigations have alluded to their functional roles in facilitating tumorigenesis by promoting invasive vessel growth. Despite this, it remains unclear as to whether NRP1 and NRP2 act in a synergistic manner to enhance pathologic angiogenesis. Here we demonstrate, using NRP1 ECKO , NRP2 ECKO , and NRP1/NRP2 ECKO mouse models, that maximum inhibition of primary tumor development and angiogenesis is achieved when both endothelial NRP1 and NRP2 are targeted simultaneously. Metastasis and secondary site angiogenesis were also significantly inhibited in NRP1/NRP2 ECKO animals. Mechanistic studies revealed that codepleting NRP1 and NRP2 in mouse-microvascular endothelial cells stimulates rapid shuttling of VEGFR-2 to Rab7+ endosomes for proteosomal degradation. Our results highlight the importance of targeting both NRP1 and NRP2 to modulate tumor angiogenesis.

Significance

The findings presented in this study demonstrate that tumor angiogenesis and growth can be arrested completely by cotargeting endothelial NRP1 and NRP2. We provide new insight into the mechanisms of action regulating NRP-dependent tumor angiogenesis and signpost a novel approach to halt tumor progression.

Matrix stiffness in regulation of tumor angiogenesis

Angiogenesis is one of the most common malignant features of solid tumors such as liver cancer, pancreatic cancer, and gastrointestinal tumors, which is the basis of tumor growth, invasion, and metastasis. It is also an important target of anti-tumor therapy. Tumor angiogenesis is usually triggered by biochemical, hypoxic, and biomechanical factors in the microenvironment. The regulation of biochemical signals and hypoxic microenvironment in tumor angiogenesis have been widely documented, but the role of biomechanical signals in tumor angiogenesis has gradually begun to be uncovered in recent years. The vasculature system is naturally sensitive to mechanical stimuli. Recent studies have highlighted the important regulatory effects of biomechanical stimuli, such as matrix stiffness, fluid shear stress, and vascular lumen pressure, on the phenotype and functions of tumor blood vessels. In this paper, we summarize the new progress and internal mechanisms of matrix stiffness-mediated effects on tumor angiogenesis.

VEGFR endocytosis: Implications for angiogenesis

The binding of vascular endothelial growth factor (VEGF) superfamily to VEGF receptor tyrosine kinases (VEGFRs) and co-receptors regulates vasculogenesis, angiogenesis and lymphangiogenesis. A recurring theme is that dysfunction in VEGF signaling promotes pathological angiogenesis, an important feature of cancer and pro-inflammatory disease states. Endocytosis of basal (resting) or activated VEGFRs facilitates signal attenuation and endothelial quiescence. However, increasing evidence suggest that activated VEGFRs can continue to signal from intracellular compartments such as endosomes. In this chapter, we focus on the evolving link between VEGFR endocytosis, signaling and turnover and the implications for angiogenesis. There is much interest in how such understanding of VEGFR dynamics can be harnessed therapeutically for a wide range of human disease states.

Neuropilin-1 deficiency in vascular smooth muscle cells is associated with hereditary hemorrhagic telangiectasia arteriovenous malformations

Patients with hereditary hemorrhagic telangiectasia (HHT) have arteriovenous malformations (AVMs) with genetic mutations involving the activin-A receptor like type 1 (ACVRL1 or ALK1) and endoglin (ENG). Recent studies have shown that Neuropilin-1 (NRP-1) inhibits ALK1. We investigated the expression of NRP-1 in livers of patients with HHT and found that there was a significant reduction in NRP-1 in perivascular smooth muscle cells (SMCs). We used Nrp1^SM22KO mice (Nrp1 was ablated in SMCs) and found hemorrhage, increased immune cell infiltration with a decrease in SMCs, and pericyte lining in lungs and liver in adult mice. Histologic examination revealed lung arteriovenous fistulas (AVFs) with enlarged liver vessels. Evaluation of the retina vessels at P5 from Nrp1^SM22KO mice demonstrated dilated capillaries with a reduction of pericytes. In inflow artery of surgical AVFs from the Nrp1^SM22KO versus WT mice, there was a significant decrease in Tgfb1, Eng, and Alk1 expression and phosphorylated SMAD1/5/8 (pSMAD1/5/8), with an increase in apoptosis. TGF-β1–stimulated aortic SMCs from Nrp1^SM22KO versus WT mice have decreased pSMAD1/5/8 and increased apoptosis. Coimmunoprecipitation experiments revealed that NRP-1 interacts with ALK1 and ENG in SMCs. In summary, NRP-1 deletion in SMCs leads to reduced ALK1, ENG, and pSMAD1/5/8 signaling and reduced cell death associated with AVM formation.

A widespread viral entry mechanism: The C-end Rule motif-neuropilin receptor interaction

Many phylogenetically distant animal viruses, including the new coronavirus severe acute respiratory syndrome coronavirus 2, have surface proteins with polybasic sites that are cleaved by host furin and furin-like proteases. Other than priming certain viral surface proteins for fusion, cleavage generates a carboxy-terminal RXXR sequence. This C-end Rule (CendR) motif is known to bind to neuropilin (NRP) receptors on the cell surface. NRPs are ubiquitously expressed, pleiotropic cell surface receptors with important roles in growth factor signaling, vascular biology, and neurobiology, as well as immune homeostasis and activation. The CendR–NRP receptor interaction promotes endocytic internalization and tissue spreading of different cargo, including viral particles. We propose that the interaction between viral surface proteins and NRPs plays an underappreciated and prevalent role in the transmission and pathogenesis of diverse viruses and represents a promising broad-spectrum antiviral target.

The P-type ATPase transporter ATP7A promotes angiogenesis by limiting autophagic degradation of VEGFR2

VEGFR2 (KDR/Flk1) signaling in endothelial cells (ECs) plays a central role in angiogenesis. The P-type ATPase transporter ATP7A regulates copper homeostasis, and its role in VEGFR2 signaling and angiogenesis is entirely unknown. Here, we describe the unexpected crosstalk between the Copper transporter ATP7A, autophagy, and VEGFR2 degradation. The functional significance of this Copper transporter was demonstrated by the finding that inducible EC-specific ATP7A deficient mice or ATP7A-dysfunctional ATP7Amut mice showed impaired post-ischemic neovascularization. In ECs, loss of ATP7A inhibited VEGF-induced VEGFR2 signaling and angiogenic responses, in part by promoting ligand-induced VEGFR2 protein degradation. Mechanistically, VEGF stimulated ATP7A translocation from the trans-Golgi network to the plasma membrane where it bound to VEGFR2, which prevented autophagy-mediated lysosomal VEGFR2 degradation by inhibiting autophagic cargo/adapter p62/SQSTM1 binding to ubiquitinated VEGFR2. Enhanced autophagy flux due to ATP7A dysfunction in vivo was confirmed by autophagy reporter CAG-ATP7Amut -RFP-EGFP-LC3 transgenic mice. In summary, our study uncovers a novel function of ATP7A to limit autophagy-mediated degradation of VEGFR2, thereby promoting VEGFR2 signaling and angiogenesis, which restores perfusion recovery and neovascularization. Thus, endothelial ATP7A is identified as a potential therapeutic target for treatment of ischemic cardiovascular diseases.

The endosomal RIN2/Rab5C machinery prevents VEGFR2 degradation to control gene expression and tip cell identity during angiogenesis

Sprouting angiogenesis is key to many pathophysiological conditions, and is strongly regulated by vascular endothelial growth factor (VEGF) signaling through VEGF receptor 2 (VEGFR2). Here we report that the early endosomal GTPase Rab5C and its activator RIN2 prevent lysosomal routing and degradation of VEGF-bound, internalized VEGFR2 in human endothelial cells. Stabilization of endosomal VEGFR2 levels by RIN2/Rab5C is crucial for VEGF signaling through the ERK and PI3-K pathways, the expression of immediate VEGF target genes, as well as specification of angiogenic 'tip' and 'stalk' cell phenotypes and cell sprouting. Using overexpression of Rab mutants, knockdown and CRISPR/Cas9-mediated gene editing, and live-cell imaging in zebrafish, we further show that endosomal stabilization of VEGFR2 levels is required for developmental angiogenesis in vivo. In contrast, the premature degradation of internalized VEGFR2 disrupts VEGF signaling, gene expression, and tip cell formation and migration. Thus, an endosomal feedforward mechanism maintains receptor signaling by preventing lysosomal degradation, which is directly linked to the induction of target genes and cell fate in collectively migrating cells during morphogenesis.

Neuropilin 1 Regulation of Vascular Permeability Signaling

The vascular endothelium acts as a selective barrier to regulate macromolecule exchange between the blood and tissues. However, the integrity of the endothelium barrier is compromised in an array of pathological settings, including ischemic disease and cancer, which are the leading causes of death worldwide. The resulting vascular hyperpermeability to plasma molecules as well as leukocytes then leads to tissue damaging edema formation and inflammation. The vascular endothelial growth factor A (VEGFA) is a potent permeability factor, and therefore a desirable target for impeding vascular hyperpermeability. However, VEGFA also promotes angiogenesis, the growth of new blood vessels, which is required for reperfusion of ischemic tissues. Moreover, edema increases interstitial pressure in poorly perfused tumors, thereby affecting the delivery of therapeutics, which could be counteracted by stimulating the growth of new functional blood vessels. Thus, targets must be identified to accurately modulate the barrier function of blood vessels without affecting angiogenesis, as well as to develop more effective pro- or anti-angiogenic therapies. Recent studies have shown that the VEGFA co-receptor neuropilin 1 (NRP1) could be playing a fundamental role in steering VEGFA-induced responses of vascular endothelial cells towards angiogenesis or vascular permeability. Moreover, NRP1 is involved in mediating permeability signals induced by ligands other than VEGFA. This review therefore focuses on current knowledge on the role of NRP1 in the regulation of vascular permeability signaling in the endothelium to provide an up-to-date landscape of the current knowledge in this field.

VEGF and VEGFR2 bind to similar pH-sensitive sites on fibronectin, exposed by heparin-mediated conformational changes

Physical interactions between vascular endothelial growth factor (VEGF), a central player in blood endothelial cell biology, and fibronectin, a major fibrillar protein of the extracellular matrix, are important determinants of angiogenic activity in health and disease. Conditions signaling the need for new blood vessel growth, such as hypoxia and low extracellular pH, increase VEGF–fibronectin interactions. These interactions can be further fine-tuned through changes in the availability of the VEGF-binding sites on fibronectin, regulated by conformational changes induced by heparin and heparan sulfate chains within the extracellular matrix. These interactions may alter VEGF bioavailability, generate gradients, or alter the way VEGF is recognized by and activates its cell-surface receptors. Here, using equilibrium and kinetic studies, we discovered that fibronectin can also interact with the extracellular domain of the VEGF receptor 2 (VEGFR2). The VEGFR2-binding sites on fibronectin show great similarity to the VEGF-binding sites, as they were also exposed upon heparin-induced conformational changes in fibronectin, and the interaction was enhanced at acidic pH. Kinetic parameters and affinities for VEGF and VEGFR2 binding to fibronectin were determined by surface plasmon resonance measurements, revealing two populations of fibronectin-binding sites for each molecule. Our data also suggest that a VEGF/VEGFR2/fibronectin triple complex may be formed by VEGF or VEGFR2 first binding to fibronectin and subsequently recruiting the third binding partner. The formation of such a complex may lead to the activation of distinct angiogenic signaling pathways, offering new possibilities for clinical applications that target angiogenesis.

Tubulin carboxypeptidase activity of vasohibin-1 inhibits angiogenesis by interfering with endocytosis and trafficking of pro-angiogenic factor receptors

Receptor endocytosis is crucial for integrating extracellular stimuli of pro-angiogenic factors, including vascular endothelial growth factor (VEGF), into the cell via signal transduction. VEGF not only triggers various angiogenic events including endothelial cell (EC) migration, but also induces the expression of negative regulators of angiogenesis, including vasohibin-1 (VASH1). While we have previously reported that VASH1 inhibits angiogenesis in vitro and in vivo, its mode of action on EC behavior remains elusive. Recently VASH1 was shown to have tubulin carboxypeptidase (TCP) activity, mediating the post-translational modification of microtubules (MTs) by detyrosination of α-tubulin within cells. However, the role of VASH1 TCP activity in angiogenesis has not yet been clarified. Here, we showed that VASH1 detyrosinated α-tubulin in ECs and suppressed in vitro and in vivo angiogenesis. In cultured ECs, VASH1 impaired endocytosis and trafficking of VEGF receptor 2 (VEGFR2), which resulted in the decreased signal transduction and EC migration. These effects of VASH1 could be restored by tubulin tyrosine ligase (TTL) in ECs, suggesting that detyrosination of α-tubulin negatively regulates angiogenesis. Furthermore, we found that detyrosinated tubulin-rich MTs were not adequate as trafficking rails for VEGFR2 endocytosis. Consistent with these results, inhibition of TCP activity of VASH1 led to the inhibition of VASH1-mediated suppression of VEGF-induced signals, EC migration, and in vivo angiogenesis. Our results indicate a novel mechanism of VASH1-mediated inhibition of pro-angiogenic factor receptor trafficking via modification of MTs.

Axon guidance receptors: Endocytosis, trafficking and downstream signaling from endosomes

During the development of the nervous system, axons extend through complex environments. Growth cones at the axon tip allow axons to find and innervate their appropriate targets and form functional synapses. Axon pathfinding requires axons to respond to guidance signals and these cues need to be detected by specialized receptors followed by intracellular signal integration and translation. Several downstream signaling pathways have been identified for axon guidance receptors and it has become evident that these pathways are often initiated from intracellular vesicles called endosomes. Endosomes allow receptors to traffic intracellularly, re-locating receptors from one cellular region to another. The localization of axon guidance receptors to endosomal compartments is crucial for their function, signaling output and expression levels. For example, active receptors within endosomes can recruit downstream proteins to the endosomal membrane and facilitate signaling. Also, endosomal trafficking can re-locate receptors back to the plasma membrane to allow re-activation or mediate downregulation of receptor signaling via degradation. Accumulating evidence suggests that axon guidance receptors do not follow a pre-set default trafficking route but may change their localization within endosomes. This re-routing appears to be spatially and temporally regulated, either by expression of adaptor proteins or co-receptors. These findings shed light on how signaling in axon guidance is regulated and diversified - a mechanism which explains how a limited set of guidance cues can help to establish billions of neuronal connections. In this review, we summarize and discuss our current knowledge of axon guidance receptor trafficking and provide directions for future research.

Epsins in vascular development, function and disease

Epsins are a family of adaptor proteins involved in clathrin-dependent endocytosis. In the vasculature, epsins 1 and 2 are functionally redundant members of this family that are expressed in the endothelial cells of blood vessels and the lymphatic system throughout development and adulthood. These proteins contain a number of peptide motifs that allow them to interact with lipid moieties and a variety of proteins. These interactions facilitate the regulation of a wide range of cell signaling pathways. In this review, we focus on the involvement of epsins 1 and 2 in controlling vascular endothelial growth factor receptor signaling in angiogenesis and lymphangiogenesis. We also discuss the therapeutic implications of understanding the molecular mechanisms of epsin-mediated regulation in diseases such as atherosclerosis and diabetes.

Elements of the Endomucin Extracellular Domain Essential for VEGF-Induced VEGFR2 Activity

Endomucin (EMCN) is the type I transmembrane glycoprotein, mucin-like component of the endothelial cell glycocalyx. We have previously shown that EMCN is necessary for vascular endothelial growth factor (VEGF)-induced VEGF receptor 2 (VEGFR2) internalization and downstream signaling. To explore the structural components of EMCN that are necessary for its function and the molecular mechanism of EMCN in VEGF-induced endothelial functions, we generated a series of mouse EMCN truncation mutants and examined their ability to rescue VEGF-induced endothelial functions in human primary endothelial cells (EC) in which endogenous EMCN had been knocked down using siRNA. Expression of the mouse full-length EMCN (FL EMCN) and the extracellular domain truncation mutants ∆21-81 EMCN and ∆21-121 EMCN, but not the shortest mutant ∆21-161 EMCN, successfully rescued the VEGF-induced EC migration, tube formation, and proliferation. ∆21-161 EMCN failed to interact with VEGFR2 and did not facilitate VEGFR2 internalization. Deletion of COSMC (C1GalT1C1) revealed that the abundant mucin-type O-glycans were not required for its VEGFR2-related functions. Mutation of the two N-glycosylation sites on ∆21-121 EMCN abolished its interaction with VEGFR2 and its function in VEGFR2 internalization. These results reveal ∆21-121 EMCN as the minimal extracellular domain sufficient for VEGFR2-mediated endothelial function and demonstrate an important role for N-glycosylation in VEGFR2 interaction, internalization, and angiogenic activity.

Raftlin is recruited by neuropilin-1 to the activated VEGFR2 complex to control proangiogenic signaling

Background

VEGFR2 (vascular endothelial growth factor receptor 2) is the major pro-angiogenic receptor in endothelial cells. Compared to other members of the receptor tyrosine kinase family, we know relatively few VEGFR2 signaling partners. Our objective was to use mass spectrometry-based proteomics to identify novel binding partners of activated VEGFR2.

Methods

We created an endothelial cell line stably expressing GFP-tagged VEGFR2 and isolated activated receptor complexes. Analysis by mass spectrometry identified raftlin as a novel binding partner of VEGFR2.

Results

We found that raftlin is recruited to the activated VEGFR2 complex via the co-receptor Nrp1 (neuropilin-1). We show that raftlin regulates the surface levels of Nrp1 in endothelial cells, controlling the availability of Nrp1 for VEGFR2 interaction. Raftlin stabilizes active VEGFR2 at the cell surface by inhibiting endocytosis of the activated receptor. Raftlin also promotes recycling of internalized VEGFR2 to the cell surface. Raftlin alters the signaling outcomes of VEGFR2 activation, inhibiting the activation of p38 and FAK (focal adhesion kinases) specifically. Both pathways are linked to cell migration in endothelial cells, and raftlin inhibits endothelial cell migration in response to VEGF.

Conclusion

Nrp1 is an important co-receptor for VEGFR2; however, its functions are still only partially understood. We show that raftlin works with Nrp1 in endothelial cells to control intracellular trafficking of the activated VEGFR2. This modulates the response to VEGF and controls endothelial cell migration.

Electronic supplementary material

The online version of this article (10.1007/s10456-020-09715-z) contains supplementary material, which is available to authorized users.

Heparin/poly-l-lysine nanoplatform with growth factor delivery for surface modification of cardiovascular stents: The influence of vascular endothelial growth factor loading

The rapid re-endothelialization of the vascular stent surface is desirable for preventing thrombosis or reducing restenosis. Many biological factors that promote the biological behavior of endothelial cells have been used for the surface modification of stents. Vascular endothelial growth factor (VEGF), which plays an important role in angiogenesis, induces strong vascular growth. In this study, we investigated different VEGF concentrations (50 to 500 ng/ml) to determine the optimum concentration for biocompatibility. First, VEGF-loaded heparin/poly-l-lysine (Hep-PLL) nanoparticles were created by electrostatic interactions. Then, the VEGF-loaded nanoparticles were immobilized on dopamine-coated 316 L stainless steel (SS) surfaces. The physical and chemical properties of the modified surface were characterized and the biocompatibility was evaluated in vitro. The results indicated that the VEGF-loaded nanoparticles were immobilized successfully on the 316LSS surface, as evidenced by the results of Alcian Blue staining and water contact angle (WCA) measurements. The low platelet adhesion and activation indicated that the modified surfaces had good blood compatibility. The modified surfaces showed a good inhibitory effect on smooth muscle cells, indicating that they inhibited tissue hyperplasia. In addition, the modified surfaces significantly promoted endothelial cell adhesion, proliferation, migration, and biological activity, especially VEGF concentration was 350 ng/ml (NPV350). The optical VEGF concentration of the surface modified Hep-PLL nanoparticles was 350 ng/ml. The proposed method shows promise for potential applications for cardiovascular devices.

MicroRNA-148a/b-3p regulates angiogenesis by targeting neuropilin-1 in endothelial cells

MicroRNAs (miRs) are crucial regulators of vascular endothelial cell (EC) functions, including migration, proliferation, and survival. However, the role of most miRs in ECs remains unknown. Using RNA sequencing analysis, we found that miR-148a/b-3p expression was significantly downregulated during the differentiation of umbilical cord blood mononuclear cells into outgrowing ECs and that decreased miR-148a/b-3p levels were closely related to EC behavior. Overexpression of miR-148a/b-3p in ECs significantly reduced migration, filamentous actin remodeling, and angiogenic sprouting. Intriguingly, the effects of decreased miR-148a/b-3p levels were augmented by treatment with vascular endothelial growth factor (VEGF). Importantly, we found that miR-148a/b-3p directly regulated neuropilin-1 (NRP1) expression by binding to its 3′-untranslated region. In addition, because NRP1 is the coreceptor for VEGF receptor 2 (VEGFR2), overexpression of miR-148a/b-3p inhibited VEGF-induced activation of VEGFR2 and inhibited its downstream pathways, as indicated by changes to phosphorylated focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), and p38 mitogen-activated protein kinase. Collectively, our results demonstrate that miR-148a/b-3p is a direct transcriptional regulator of NRP1 that mediates antiangiogenic pathways. These data suggest that miR-148a/b-3p is a therapeutic candidate for overcoming EC dysfunction and angiogenic disorders, including ischemia, retinopathy, and tumor vascularization.

A small regulatory RNA molecule helps prevent the development of new blood vessels, a finding that could have implications for the treatment of vascular disease and cancer. Young-Guen Kwon from Yonsei University in Seoul, South Korea, and colleagues cataloged all the microRNAs expressed during the differentiation of umbilical cord blood stem cells into precursors of the cells that line the inside of blood vessels. One microRNA in particular stood out for its association with cellular differentiation. The authors showed that this microRNA, called miR-148a/b-3p, directly binds part of the gene transcript encoding neuropilin-1, thereby blocking the production of this receptor for growth factors involved in new blood vessel formation. Modulating the activity of miR-148a/b-3p could have therapeutic value for disorders marked by aberrant blood vessel growth.

Endosomes and Autophagy: Regulators of Pulmonary Endothelial Cell Homeostasis in Health and Disease

Significance: Alterations in oxidant/antioxidant balance injure pulmonary endothelial cells and are important in the pathogenesis of lung diseases, such as Acute Respiratory Distress Syndrome (ARDS), ischemia/reperfusion injury, pulmonary arterial hypertension (PAH), and emphysema. Recent Advances: The endosomal and autophagic pathways regulate cell homeostasis. Both pathways support recycling or degradation of macromolecules or organelles, targeted to endosomes or lysosomes, respectively. Thus, both processes promote cell survival. However, with environmental stress or injury, imbalance in endosomal and autophagic pathways may enhance macromolecular or organelle degradation, diminish biosynthetic processes, and cause cell death. Critical Issues: While the role of autophagy in cellular homeostasis in pulmonary disease has been investigated, the role of the endosome in the lung vasculature is less known. Furthermore, autophagy can either decrease or exacerbate endothelial injury, depending upon inciting insult and disease process. Future Directions: Diseases affecting the pulmonary endothelium, such as emphysema, ARDS, and PAH, are linked to altered endosomal or autophagic processing, leading to enhanced degradation of macromolecules and potential cell death. Efforts to target this imbalance have yielded limited success as treatments for lung injuries, which may be due to the complexity of both processes. It is possible that endosomal trafficking proteins, such as Rab GTPases and late endosomal/lysosomal adaptor, MAPK and MTOR activator 1, may be novel therapeutic targets. While endocytosis or autophagy have been linked to improved function of the pulmonary endothelium in vitro and in vivo, further studies are needed to identify targets for modulating cellular homeostasis in the lung.

Regulation of VEGFR2 trafficking and signaling by Rab GTPase-activating proteins

Vascular endothelial growth factor receptor-2 (VEGFR2) and its ligands (VEGFs) are crucial players in vasculogenesis and angiogenesis. General blocking of this signaling system with antibodies or small molecule inhibitors is an established strategy to treat cancer and age-related macular degeneration. Nevertheless, the activated receptor can signal to discrete downstream signaling pathways and the equilibrium between these pathways is modulated by coreceptors and distinct isoforms of VEGF. Here we investigated the influence of Rab GTPase activating proteins (RabGAPs) on VEGFR2 signaling, tube formation, and migration of endothelial cells. We demonstrate that members of the TBC1D10 subfamily of RabGAPs have opposite effects. Whereas TBC1D10A leads to increased Erk1/2 signaling, TBC1D10B lowered Erk1/2 and p38 signaling and reduced tube formation in vitro. TBC1D10A is a RabGAP acting on RAB13 that was shown before to play a role in angiogenesis and we could indeed show colocalization of these two proteins with VEGFR2 in activated cells. In addition, we observed that cells expressing TBC1D10B show lower expression of VEGFR2 and NRP1 on filopodia of activated cells. Taken together, our systematic analysis of influence of RabGAPs on VEGFR2 signaling identifies the TBC1D10 subfamily members as modulators of angiogenesis.

Neuropilin-2 contributes to tumor progression in preclinical models of small intestinal neuroendocrine tumors

The identification of novel regulators of tumor progression is a key challenge to gain knowledge on the biology of small intestinal neuroendocrine tumors (SI-NETs). We recently identified the loss of the axon guidance protein semaphorin 3F as a protumoral event in SI-NETs. Interestingly the expression of its receptor neuropilin-2 (NRP-2) was still maintained. This study aimed at deciphering the potential role of NRP-2 as a contributor to SI-NET progression. The role of NRP-2 in SI-NET progression was addressed using an approach integrating human tissue and serum samples, cell lines and in vivo models. Data obtained from human SI-NET tissues showed that membranous NRP-2 expression is present in a majority of tumors, and is correlated with invasion, metastatic abilities, and neovascularization. In addition, NRP-2 soluble isoform was found elevated in serum samples from metastatic patients. In preclinical mouse models of NET progression, NRP-2 silencing led to a sustained antitumor effect, partly driven by the downregulation of VEGFR2. In contrast, its ectopic expression conferred a gain of aggressiveness, driven by the activation of various oncogenic signaling pathways. Lastly, NRP-2 inhibition led to a decrease of tumor cell viability, and sensitized to therapeutic agents. Overall, our results point out NRP-2 as a potential therapeutic target for SI-NETs, and will foster the development of innovative strategies targeting this receptor. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Comparison of the ligand-binding properties of fluorescent VEGF-A isoforms to VEGF receptor 2 in living cells and membrane preparations using NanoBRET

Background and Purpose

Vascular endothelial growth factor A (VEGF‐A) is a key mediator of angiogenesis. A striking feature of the binding of a fluorescent analogue of VEGF₁₆₅a to nanoluciferase‐tagged VEGF receptor 2 (VEGFR2) in living cells is that the BRET signal is not sustained and declines over time. This may be secondary to receptor internalisation. Here, we have compared the binding of three fluorescent VEGF‐A isoforms to VEGFR2 in cells and isolated membrane preparations.

Experimental Approach

Ligand‐binding kinetics were monitored in both intact HEK293T cells and membranes (expressing nanoluciferase‐tagged VEGFR2) using BRET between tagged receptor and fluorescent analogues of VEGF₁₆₅a, VEGF₁₆₅b, and VEGF₁₂₁a. VEGFR2 endocytosis in intact cells expressing VEGFR2 was monitored by following the appearance of fluorescent ligand‐associated receptors in intracellular endosomes using automated quantitative imaging.

Key Results

Quantitative analysis of the effect of fluorescent VEGF‐A isoforms on VEGFR2 endocytosis in cells demonstrated that they produce a rapid and potent translocation of ligand‐bound VEGFR2 into intracellular endosomes. NanoBRET can be used to monitor the kinetics of the binding of fluorescent VEGF‐A isoforms to VEGFR2. In isolated membrane preparations, ligand‐binding association curves were maintained for the duration of the 90‐min experiment. Measurement of the k_off at pH 6.0 in membrane preparations indicated shorter ligand residence times than those obtained at pH 7.4.

Conclusions and Implications

These studies suggest that rapid VEGF‐A isoform‐induced receptor endocytosis shortens agonist residence times on the receptor (1/k_off) as VEGFR2 moves from the plasma membrane to the intracellular endosomes.

Neuropilin-1 aggravates liver cirrhosis by promoting angiogenesis via VEGFR2-dependent PI3K/Akt pathway in hepatic sinusoidal endothelial cells

Background

We have revealed that neuropilin-1 (NRP-1) promoted hepatic stellate cell activation and liver fibrosis through its profibrogenic signalling pathways. However, the role of NRP-1 in angiogenesis in hepatic sinusoidal endothelial cells (HSECs) during liver cirrhosis remains unclear.

Methods

The correlation between NRP-1 expression and angiogenesis was evaluated in both human and murine cirrhotic liver tissues by immunohistochemical staining, quantitative real-time PCR, and western blotting. In addition, the role and mechanism of NRP-1 in regulating VEGFR2-dependent angiogenesis was identified in endothelial cells (ECs) in vitro. Moreover, liver histocultures were used to test the therapeutic effect of NRP-1 blocking in liver fibrosis.

Findings

Higher expression of NRP-1 in HSECs was detected, which was positively correlated with angiogenesis in liver cirrhosis. In vitro, NRP-1 knockdown suppressed the expression and activation of VEGFR2, accompanied by reduced ability of the vascular tube formation and the migration of ECs. Conversely, NRP-1 overexpression upregulated VEGFR2, promoted tube formation, and the migration of ECs. Mechanistically, NRP-1 modulated the expression of VEGFR2 by regulating FAK and its kinase activity. Furthermore, NRP-1 promoted VEGFR2-dependent angiogenesis via the PI3K/Akt pathway in HSECs. Blocking NRP-1 function reduced intrahepatic angiogenesis and fibrosis-associated factors in the in vitro liver histocultures.

Interpretation

NRP-1 promotes angiogenesis by upregulating the expression and activation of VEGFR2 through the PI3K/Akt signalling pathway in liver cirrhosis. This study highlights the possibility of therapeutically targeting NRP-1 for the treatment of cirrhosis.

Fund

National Natural Science Foundation of China (No. 81570551; 81770607; 81600469; 81401868), Key Research project of Shandong Province (No. 2016GSF201008; 2017GSF218053), Natural Science Foundation of Shandong Province (No. ZR2017MH102), National Science and Technology Major Project of China (No. 2018ZX10302206-001-006).

Neuropilins Controlling Cancer Therapy Responsiveness

Neuropilins (NRPs) are cell surface glycoproteins, acting as co-receptors for secreted Semaphorins (SEMAs) and for members of the vascular endothelial growth factor (VEGF) family; they have been initially implicated in axon guidance and angiogenesis regulation, and more recently in cancer progression. In addition, NRPs have been shown to control many other fundamental signaling pathways, especially mediated by tyrosine kinase receptors (RTKs) of growth factors, such as HGF (hepatocyte growth factor), PDGF (platelet derived growth factor) and EGF (epidermal growth factor). This enables NRPs to control a range of pivotal mechanisms in the cancer context, from tumor cell proliferation and metastatic dissemination, to tumor angiogenesis and immune escape. Moreover, cancer treatment failures due to resistance to innovative oncogene-targeted drugs is typically associated with the activity of alternative RTK-dependent pathways; and neuropilins’ capacity to control oncogenic signaling cascades supports the hypothesis that they could elicit such mechanisms in cancer cells, in order to escape cytotoxic stress and therapeutic attacks. Intriguingly, several studies have recently assayed the impact of NRPs inhibition in combination with diverse anti-cancer drugs. In this minireview, we will discuss the state-of-art about the relevance of NRPs as potential predictive biomarkers of drug response, and the rationale to target these proteins in combination with other anticancer therapies.

Sec14l3 potentiates VEGFR2 signaling to regulate zebrafish vasculogenesis

Vascular endothelial growth factor (VEGF) regulates vasculogenesis by using its tyrosine kinase receptors. However, little is known about whether Sec14-like phosphatidylinositol transfer proteins (PTP) are involved in this process. Here, we show that zebrafish sec14l3, one of the family members, specifically participates in artery and vein formation via regulating angioblasts and subsequent venous progenitors’ migration during vasculogenesis. Vascular defects caused by sec14l3 depletion are partially rescued by restoration of VEGFR2 signaling at the receptor or downstream effector level. Biochemical analyses show that Sec14l3/SEC14L2 physically bind to VEGFR2 and prevent it from dephosphorylation specifically at the Y¹¹⁷⁵ site by peri-membrane tyrosine phosphatase PTP1B, therefore potentiating VEGFR2 signaling activation. Meanwhile, Sec14l3 and SEC14L2 interact with RAB5A/4A and facilitate the formation of their GTP-bound states, which might be critical for VEGFR2 endocytic trafficking. Thus, we conclude that Sec14l3 controls vasculogenesis in zebrafish via the regulation of VEGFR2 activation.

The growth factor VEGF is known to regulate vasculogenesis but the downstream pathways activated are unclear. Here, the authors report that Sec14l3, a member of the PITP (phosphatidyl inositol transfer proteins) family regulates the formation of zebrafish vasculature by promoting VEGFR2 endocytic trafficking.

NRP-1 interacts with GIPC1 and SYX to activate p38 MAPK signaling and cancer stem cell survival

Epidermal cancer stem cells (ECS cells) comprise a limited population of cells that form aggressive, rapidly growing, and highly vascularized tumors. VEGF-A/NRP-1 signaling is a key driver of the ECS cell phenotype and aggressive tumor formation. However, relatively less is known regarding the downstream events following VEGF-A/NRP-1 interaction. In the present study, we show that VEGF-A/NRP-1, GIPC1, and Syx interact to increase RhoA-dependent p38 MAPK activity to enhance ECS cell spheroid formation, invasion, migration, and angiogenic potential. Inhibition or knockdown of NRP-1, GIPC1 or Syx attenuates RhoA and p38 activity to reduce the ECS cell phenotype, and NRP-1 knockout, or pharmacologic inhibition of VEGF-A/NRP-1 interaction or RhoA activity, reduces p38 MAPK activity and tumor growth. Moreover, expression of wild-type or constitutively-active RhoA, or p38, in NRP1-knockout cells, restores p38 activity and the ECS cell phenotype. These findings suggest that NRP-1 forms a complex with GIPC1 and Syx to activate RhoA/ROCK-dependent p38 activity to enhance the ECS cell phenotype and tumor formation.

The DCBLD receptor family: emerging signaling roles in development, homeostasis and disease

The discoidin, CUB, and LCCL domain-containing (DCBLD) receptor family are composed of the type-I transmembrane proteins DCBLD1 and DCBLD2 (also ESDN and CLCP1). These proteins are highly conserved across vertebrates and possess similar domain structure to that of neuropilins, which act as critical co-receptors in developmental processes. Although DCBLD1 remains largely uncharacterized, the functional and mechanistic roles of DCBLD2 are emerging. This review provides a comprehensive discussion of this presumed receptor family, ranging from structural and signaling aspects to their associations with cancer, physiology, and development.

Context-dependent regulation of receptor tyrosine kinases: Insights from systems biology approaches

Receptor tyrosine kinases (RTKs) are cell membrane proteins that provide cells with the ability to sense proteins in their environments. Many RTKs are essential to development and organ growth. Derangement of RTKs-by mutation or by overexpression-is central to several developmental and adult disorders including cancer, short stature, and vascular pathologies. The mechanism of action of RTKs is complex and is regulated by contextual components, including the existence of multiple competing ligands and receptors in many families, the intracellular location of the RTK, the dynamic and cell-specific coexpression of other RTKs, and the commonality of downstream signaling pathways. This means that both the state of the cell and the microenvironment outside the cell play a role, which makes sense given the pivotal location of RTKs as the nexus linking the extracellular milieu to intracellular signaling and modification of cell behavior. In this review, we describe these different contextual components through the lens of systems biology, in which both computational modeling and experimental "omics" approaches have been used to better understand RTK networks. The complexity of these networks is such that using these systems biology approaches is necessary to get a handle on the mechanisms of pathology and the design of therapeutics targeting RTKs. In particular, we describe in detail three concrete examples (involving ErbB3, VEGFR2, and AXL) that illustrate how systems approaches can reveal key mechanistic and therapeutic insights. This article is categorized under: Biological Mechanisms > Cell Signaling Models of Systems Properties and Processes > Mechanistic Models Translational, Genomic, and Systems Medicine > Therapeutic Methods.

TFEB controls vascular development by regulating the proliferation of endothelial cells

Transcription factor TFEB is thought to control cellular functions—including in the vascular bed—primarily via regulation of lysosomal biogenesis and autophagic flux. Here, we report that TFEB also orchestrates a non‐canonical program that controls the cell cycle/VEGFR2 pathway in the developing vasculature. In endothelial cells, TFEB depletion halts proliferation at the G1‐S transition by inhibiting the CDK4/Rb pathway. TFEB‐deficient cells attempt to compensate for this limitation by increasing VEGFR2 levels at the plasma membrane via microRNA‐mediated mechanisms and controlled membrane trafficking. TFEB stimulates expression of the miR‐15a/16‐1 cluster, which limits VEGFR2 transcript stability and negatively modulates expression of MYO1C, a regulator of VEGFR2 trafficking to the cell surface. Altered levels of miR‐15a/16‐1 and MYO1C in TFEB‐depleted cells cause increased expression of plasma membrane VEGFR2, but in a manner associated with low signaling strength. An endothelium‐specific Tfeb‐knockout mouse model displays defects in fetal and newborn mouse vasculature caused by reduced endothelial proliferation and by anomalous function of the VEGFR2 pathway. These previously unrecognized functions of TFEB expand its role beyond regulation of the autophagic pathway in the vascular system.

S1P1 receptor phosphorylation, internalization, and interaction with Rab proteins: effects of sphingosine 1-phosphate, FTY720-P, phorbol esters, and paroxetine

Sphingosine 1-phosphate (S1P) and FTY720-phosphate (FTYp) increased intracellular calcium in cells expressing S1P₁ mCherry-tagged receptors; the synthetic agonist was considerably less potent. Activation of protein kinase C by phorbol myristate acetate (PMA) blocked these effects. The three agents induced receptor phosphorylation and internalization, with the action of FTYp being more intense. S1P₁ receptor–Rab protein (GFP-tagged) interaction was studied using FRET. The three agents were able to induce S1P₁ receptor–Rab5 interaction, although with different time courses. S1P₁ receptor–Rab9 interaction was mainly increased by the phorbol ester, whereas S1P₁ receptor–Rab7 interaction was only increased by FTYp and after a 30-min incubation. These actions were not observed using dominant negative (GDP-bound) Rab protein mutants. The data suggested that the three agents induce interaction with early endosomes, but that the natural agonist induced rapid receptor recycling, whereas activation of protein kinase C favored interaction with late endosome and slow recycling and FTYp triggered receptor interaction with vesicles associated with proteasomal/lysosomal degradation. The ability of bisindolylmaleimide I and paroxetine to block some of these actions suggested the activation of protein kinase C was associated mainly with the action of PMA, whereas G protein-coupled receptor kinase (GRK) 2 (GRK2) was involved in the action of the three agents.

Trafficking of Adhesion and Growth Factor Receptors and Their Effector Kinases

Cell adhesion to macromolecules in the microenvironment is essential for the development and maintenance of tissues, and its dysregulation can lead to a range of disease states, including inflammation, fibrosis, and cancer. The biomechanical and biochemical mechanisms that mediate cell adhesion rely on signaling by a range of effector proteins, including kinases and associated scaffolding proteins. The intracellular trafficking of these must be tightly controlled in space and time to enable effective cell adhesion and microenvironmental sensing and to integrate cell adhesion with, and compartmentalize it from, other cellular processes, such as gene transcription, protein degradation, and cell division. Delivery of adhesion receptors and signaling proteins from the plasma membrane to unanticipated subcellular locales is revealing novel biological functions. Here, we review the expected and unexpected trafficking, and sites of activity, of adhesion and growth factor receptors and intracellular kinase partners as we begin to appreciate the complexity and diversity of their spatial regulation.

Gain of affinity for VEGF165 binding within the VEGFR2/NRP1 cellular complex detected by an HTRF-based binding assay

Neuroplin 1 (NRP1), a transmembrane protein interacting with Vascular Endothelial Growth Factor VEGF-A₁₆₅ (called here VEGF165) and the tyrosine kinase Receptor 2 (VEGFR2) promote angiogenesis and vascular homeostasis. In a pathophysiological context, several studies suggested that VEGFR2 and NRP1 mediate tumor development and progression. Given the involvement of the VEGF165 network in promoting tumor angiogenesis, NRP1, VEGFR2 and VEGF165 have been identified as targets for anti-angiogenic therapy. No binding assay exists to monitor specifically the binding of VEGF165 to the VEGFR2/NRP1 complex in intact cells. We established a binding assay based on the homogenous time-resolved fluorescence (HTRF®) technology. This unique binding assay enables to assess the interaction of VEGF165 with VEGFR2 or NRP1 within the VEGFR2/NRP1 complex. Ligand binding saturation experiments revealed that VEGF165 binds the VEGFR2/NRP1 complex at the cell surface with a ten to twenty-fold higher affinity compared to SNAP-VEGFR2 or SNAP-NRP1 receptors alone not engaged in the heteromeric complex. The assay allows characterizing the impact of NRP1 ligands on VEGF165 to the complex. It shows high specificity, reproducibility and robustness, making it compatible with high throughput screening (HTS) applications for identifying new VEGF165 antagonists selective for NRP1 or the VEGFR2/NRP1 complex.

Real-Time Ligand Binding of Fluorescent VEGF-A Isoforms that Discriminate between VEGFR2 and NRP1 in Living Cells

Fluorescent VEGF-A isoforms have been evaluated for their ability to discriminate between VEGFR2 and NRP1 in real-time ligand binding studies in live cells using BRET. To enable this, we synthesized single-site (N-terminal cysteine) labeled versions of VEGF₁₆₅a, VEGF₁₆₅b, and VEGF₁₂₁a. These were used in combination with N-terminal NanoLuc-tagged VEGFR2 or NRP1 to evaluate the selectivity of VEGF isoforms for these two membrane proteins. All fluorescent VEGF-A isoforms displayed high affinity for VEGFR2. Only VEGF₁₆₅a-TMR bound to NanoLuc-NRP1 with a similar high affinity (4.4 nM). Competition NRP1 binding experiments yielded a rank order of potency of VEGF₁₆₅a > VEGF₁₈₉a > VEGF₁₄₅a. VEGF₁₆₅b, VEGF-Ax, VEGF₁₂₁a, and VEGF₁₁₁a were unable to bind to NRP1. There were marked differences in the kinetic binding profiles of VEGF₁₆₅a-TMR for NRP1 and VEGFR2. These data emphasize the importance of the kinetic aspects of ligand binding to VEGFR2 and its co-receptors in the dynamics of VEGF signaling.

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VEGF₁₆₅a, VEGF₁₂₁a, and VEGF₁₆₅b were single-site labeled with tetramethylrhodamine

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NanoBRET quantified that VEGF-A isoforms have similar binding properties at VEGFR2

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NRP1 expressed in live cells does not bind VEGF₁₆₅b, VEGF₁₂₁a, VEGF-Ax, or VEGF₁₁₁a

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VEGFR2 and NRP1 have markedly distinct kinetic profiles binding VEGF₁₆₅a-TMR

Physiological Role of Vascular Endothelial Growth Factors as Homeostatic Regulators

The vascular endothelial growth factor (VEGF) family of proteins are key regulators of physiological systems. Originally linked with endothelial function, they have since become understood to be principal regulators of multiple tissues, both through their actions on vascular cells, but also through direct actions on other tissue types, including epithelial cells, neurons, and the immune system. The complexity of the five members of the gene family in terms of their different splice isoforms, differential translation, and specific localizations have enabled tissues to use these potent signaling molecules to control how they function to maintain their environment. This homeostatic function of VEGFs has been less intensely studied than their involvement in disease processes, development, and reproduction, but they still play a substantial and significant role in healthy control of blood volume and pressure, interstitial volume and drainage, renal and lung function, immunity, and signal processing in the peripheral and central nervous system. The widespread expression of VEGFs in healthy adult tissues, and the disturbances seen when VEGF signaling is inhibited support this view of the proteins as endogenous regulators of normal physiological function. This review summarizes the evidence and recent breakthroughs in understanding of the physiology that is regulated by VEGF, with emphasis on the role they play in maintaining homeostasis. © 2017 American Physiological Society. Compr Physiol 8:955-979, 2018.

Long Non-Coding RNA Malat1 Regulates Angiogenesis in Hindlimb Ischemia

Angiogenesis is a complex process that depends on the delicate regulation of gene expression. Dysregulation of transcription during angiogenesis often leads to various human diseases. Emerging evidence has recently begun to show that long non-coding RNAs (lncRNAs) may mediate angiogenesis in both physiological and pathological conditions; concurrently, underlying molecular mechanisms are largely unexplored. Previously, our lab identified metastasis associates lung adenocarcinoma transcript 1 (Malat1) as an oxygen-glucose deprivation (OGD)-responsive endothelial lncRNA. Here we reported that genetic deficiency of Malat1 leads to reduced blood vessel formation and local blood flow perfusion in mouse hind limbs at one to four weeks after hindlimb ischemia. Malat1 and vascular endothelial growth factor receptor 2 (VEGFR2) levels were found to be increased in both cultured mouse primary skeletal muscle microvascular endothelial cells (SMMECs) after 16 h OGD followed by 24 h reperfusion and in mouse gastrocnemius muscle that underwent hindlimb ischemia followed by 28 days of reperfusion. Moreover, Malat1 silencing by locked nucleic acid (LNA)-GapmeRs significantly reduced tube formation, cell migration, and cell proliferation in SMMEC cultures. Mechanistically, RNA subcellular isolation and RNA-immunoprecipitation experiments demonstrate that Malat1 directly targets VEGFR2 to facilitate angiogenesis. The results suggest that Malat1 regulates cell-autonomous angiogenesis through direct regulation of VEGFR2.

Computer Simulation of TSP1 Inhibition of VEGF-Akt-eNOS: An Angiogenesis Triple Threat

The matricellular protein thrombospondin-1 (TSP1) is a potent inhibitor of angiogenesis. Specifically, TSP1 has been experimentally shown to inhibit signaling downstream of vascular endothelial growth factor (VEGF). The molecular mechanism of this inhibition is not entirely clear. We developed a detailed computational model of VEGF signaling to Akt-endothelial nitric oxide synthase (eNOS) to investigate the quantitative molecular mechanism of TSP1 inhibition. The model demonstrated that TSP1 acceleration of VEGFR2 degradation is sufficient to explain the inhibition of VEGFR2 and eNOS phosphorylation. However, Akt inhibition requires TSP1-induced phosphatase recruitment to VEGFR2. The model was then utilized to test various strategies for the rescue of VEGF signaling to Akt and eNOS. Inhibiting TSP1 was predicted to be not as effective as CD47 depletion in rescuing signaling to Akt. The model further predicts that combination strategy involving depletion of CD47 and inhibition of TSP1 binding to CD47 is necessary for effective recovery of signaling to eNOS. In all, computational modeling offers insight to molecular mechanisms involving TSP1 interaction with VEGF signaling and provides strategies for rescuing angiogenesis by targeting TSP1-CD47 axis.