The interaction of endoglin with beta-arrestin2 regulates transforming growth factor-beta-mediated ERK activation and migration in endothelial cells

Proteomic analysis reveals key differences in pro-stromal corneal tissue between highly myopic males and females

Background and purpose

Nowadays, myopia has become a highly prevalent disease globally, especially in East Asia. Epidemiological studies have found that there may be sex differences in the occurrence and progression of myopia, with females having a higher incidence of myopia and higher risk of myopia progression. The purpose of this study was to explore the sex differences in myopic cornea using corneal stroma removed by small incision lenticule extraction (SMILE) surgery.

Methods

The corneal stroma of females with high myopia (FH) and males with high myopia (MH) were subjected to proteomic assays. Proteomic-related data were statistically analyzed using software such as MaxQuan, KAAS, Proteome Discovery, etc. The total number of proteins in the cornea and the proteins specifically expressed in the two groups were counted, and the differentially expressed proteins in the two groups were identified by expression fold change >2 and p-value <0.05, and volcano plots were constructed, and functional enrichment analysis, subcellular organelle analysis, and molecular interaction were implemented.

Results

Ten samples from each group were analyzed. Twenty-seven proteins were down-regulated and 27 proteins were up-regulated in the FH group, of which 23 proteins were up-regulated in the range of 2-10-fold and 4 proteins were up-regulated in the range of >10-fold. Comparative proteomic analysis of the cornea of male and female patients with high myopia revealed that the expression of corneal extracellular matrix and collagen I, III, V, and VIII-associated proteins were increased in the cornea of female patients, and the transforming growth factor-β (TGF-β)/Smad pathway was an important pathway obtained by functional analysis.

Conclusion

Comparative proteomic analysis of cornea from male and female patients with high myopia revealed increased expression of proteins related to extracellular matrix and collagen I, III, V, and VIII in female patients, and the TGF-β/Smad pathway was an important pathway obtained from the functional analysis, suggesting that extracellular matrix remodeling and collagen fiber synthesis may be more active in the cornea of female patients.

Single-cell and bulk RNA-seq unveils the immune infiltration landscape associated with cuproptosis in cerebral cavernous malformations

BACKGROUND

Cerebral cavernous malformations (CCMs) are vascular abnormalities associated with deregulated angiogenesis. Their pathogenesis and optimal treatment remain unclear. This study aims to investigate the molecular signatures of cuproptosis, a newly identified type of cell death, associated with CCMs development.

METHODS

Bulk RNA sequencing (RNA-seq) from 15 CCM and 6 control samples were performed with consensus clustering and clustered to two subtypes based on expression levels of cuproptosis-related genes (CRGs). Differentially expressed genes and immune infiltration between subtypes were then identified. Machine learning algorithms including the least absolute shrinkage and selection operator and random forest were employed to screen for hub genes for CCMs associated with cuproptosis. Furthermore, Pathway enrichment and correlation analysis were used to explore the functions of hub genes and their association with immune phenotypes in CCMs. An external dataset was then employed for validation. Finally, employing the Cellchat algorithm on a single-cell RNA-seq dataset, we explored potential mechanisms underlying the participation of these hub genes in cell-cell communication in CCMs.

RESULTS

Our study revealed two distinct CCM subtypes with differential pattern of CRG expression and immune infiltration. Three hub genes (BTBD10, PFDN4, and CEMIP) were identified and validated, which may significantly associate with CCM pathogenesis. These genes were found to be significantly upregulated in CCM endothelial cells (ECs) and were validated through immunofluorescence and western blot analysis. Single-cell RNA-seq analysis revealed the cellular co-expression patterns of these hub genes, particularly highlighting the high expression of BTBD10 and PFDN4 in ECs. Additionally, a significant co-localization was also observed between BTBD10 and the pivotal cuproptosis gene FDX1 in Mki67+ tip cells, indicating the crucial role of cuproptosis for angiogenesis in CCMs. The study also explored the cell-cell communication between subcluster of ECs expressing these hub genes and immune cells, particularly M2 macrophages, suggesting a role for these interactions in CCM pathogenesis.

CONCLUSION

This study identifies molecular signatures linking cuproptosis to CCMs pathogenesis. Three hub genes-PFDN4, CEMIP, and BTBD10-may influence disease progression by modulating immunity. Further research is needed to understand their precise disease mechanisms and evaluate their potential as biomarkers or therapeutic targets for CCMs.

NRP1 interacts with endoglin and VEGFR2 to modulate VEGF signaling and endothelial cell sprouting

Endothelial cells express neuropilin 1 (NRP1), endoglin (ENG) and vascular endothelial growth factor receptor 2 (VEGFR2), which regulate VEGF-A-mediated vascular development and angiogenesis. However, the link between complex formation among these receptors with VEGF-A-induced signaling and biology is yet unclear. Here, we quantify surface receptor interactions by IgG-mediated immobilization of one receptor, and fluorescence recovery after photobleaching (FRAP) measurements of the mobility of another coexpressed receptor. We observe stable ENG/NRP1, ENG/VEGFR2, and NRP1/VEGFR2 complexes, which are enhanced by VEGF-A. ENG augments NRP1/VEGFR2 interactions, suggesting formation of tripartite complexes bridged by ENG. Effects on signaling are measured in murine embryonic endothelial cells expressing (MEEC+/+) or lacking (MEEC-/-) ENG, along with NRP1 and/or ENG overexpression or knockdown. We find that optimal VEGF-A-mediated phosphorylation of VEGFR2 and Erk1/2 requires ENG and NRP1. ENG or NRP1 increase VEGF-A-induced sprouting, becoming optimal in cells expressing all three receptors, and both processes are inhibited by a MEK1/2 inhibitor. We propose a model where the maximal potency of VEGF-A involves a tripartite complex where ENG bridges VEGFR2 and NRP1, providing an attractive therapeutic target for modulation of VEGF-A signaling and biological responses.

A novel tumor immunotherapy-related signature for risk stratification, prognosis prediction, and immune status in hepatocellular carcinoma

Immunotherapy as a strategy to deal with cancer is increasingly being used clinically, especially in hepatocellular carcinoma (HCC). We aim to create an immunotherapy-related signature that can play a role in predicting HCC patients' survival and therapeutic outcomes. Immunotherapy-related genes were discovered first. Clinical information and gene expression data were extracted from GSE140901. By a series of bioinformatics methods to analyze, overlapping genes were used to build an immunotherapy-related signature that could contribute to predict both the prognosis of people with hepatocellular carcinoma and responder to immune checkpoint blockade therapy of them in TCGA database. Differences of the two groups in immune cell subpopulations were then compared. Furthermore, A nomogram was constructed, based on the immunotherapy-related signature and clinicopathological features, and proved to be highly predictive. Finally, immunohistochemistry assays were performed in HCC tissue and normal tissue adjacent tumors to verify the differences of the four genes expression. As a result of this study, a prognostic protein profile associated with immunotherapy had been created, which could be applied to predict patients' response to immunotherapy and may provide a new perspective as clinicians focus on non-apoptotic treatment for patients with HCC.

Endoglin, a Novel Biomarker and Therapeutical Target to Prevent Malignant Peripheral Nerve Sheath Tumor Growth and Metastasis

PURPOSE

Malignant peripheral nerve sheath tumors (MPNST) are highly aggressive soft-tissue sarcomas that lack effective treatments, underscoring the urgent need to uncover novel mediators of MPNST pathogenesis that may serve as potential therapeutic targets. Tumor angiogenesis is considered a critical event in MPNST transformation and progression. Here, we have investigated whether endoglin (ENG), a TGFβ coreceptor with a crucial role in angiogenesis, could be a novel therapeutic target in MPNSTs.

EXPERIMENTAL DESIGN

ENG expression was evaluated in human peripheral nerve sheath tumor tissues and plasma samples. Effects of tumor cell-specific ENG expression on gene expression, signaling pathway activation and in vivo MPNST growth and metastasis, were investigated. The efficacy of ENG targeting in monotherapy or in combination with MEK inhibition was analyzed in xenograft models.

RESULTS

ENG expression was found to be upregulated in both human MPNST tumor tissues and plasma-circulating small extracellular vesicles. We demonstrated that ENG modulates Smad1/5 and MAPK/ERK pathway activation and pro-angiogenic and pro-metastatic gene expression in MPNST cells and plays an active role in tumor growth and metastasis in vivo. Targeting with ENG-neutralizing antibodies (TRC105/M1043) decreased MPNST growth and metastasis in xenograft models by reducing tumor cell proliferation and angiogenesis. Moreover, combination of anti-ENG therapy with MEK inhibition effectively reduced tumor cell growth and angiogenesis.

CONCLUSIONS

Our data unveil a tumor-promoting function of ENG in MPNSTs and support the use of this protein as a novel biomarker and a promising therapeutic target for this disease.

Endothelial β-arrestins regulate mechanotransduction by the type II bone morphogenetic protein receptor in primary cilia

Modulation of endothelial cell behavior and phenotype by hemodynamic forces involves many signaling components, including cell surface receptors, intracellular signaling intermediaries, transcription factors, and epigenetic elements. Many of the signaling mechanisms that underlie mechanotransduction by endothelial cells are inadequately defined. Here we sought to better understand how β-arrestins, intracellular proteins that regulate agonist-mediated desensitization and integration of signaling by transmembrane receptors, may be involved in the endothelial cell response to shear stress. We performed both in vitro studies with primary endothelial cells subjected to β-arrestin knockdown, and in vivo studies using mice with endothelial specific deletion of β-arrestin 1 and β-arrestin 2. We found that β-arrestins are localized to primary cilia in endothelial cells, which are present in subpopulations of endothelial cells in relatively low shear states. Recruitment of β-arrestins to cilia involved its interaction with IFT81, a component of the flagellar transport protein complex in the cilia. β-arrestin knockdown led to marked reduction in shear stress response, including induction of NOS3 expression. Within the cilia, β-arrestins were found to associate with the type II bone morphogenetic protein receptor (BMPR-II), whose disruption similarly led to an impaired endothelial shear response. β-arrestins also regulated Smad transcription factor phosphorylation by BMPR-II. Mice with endothelial specific deletion of β-arrestin 1 and β-arrestin 2 were found to have impaired retinal angiogenesis. In conclusion, we have identified a novel role for endothelial β-arrestins as key transducers of ciliary mechanotransduction that play a central role in shear signaling by BMPR-II and contribute to vascular development.

TGF-β superfamily co-receptors in cancer

Transforming growth factor-β (TGF-β) superfamily signaling via their cognate receptors is frequently modified by TGF-β superfamily co-receptors. Signaling through SMAD-mediated pathways may be enhanced or depressed depending on the specific co-receptor and cell context. This dynamic effect on signaling is further modified by the release of many of the co-receptors from the membrane to generate soluble forms that are often antagonistic to the membrane-bound receptors. The co-receptors discussed here include TβRIII (betaglycan), endoglin, BAMBI, CD109, SCUBE proteins, neuropilins, Cripto-1, MuSK, and RGMs. Dysregulation of these co-receptors can lead to altered TGF-β superfamily signaling that contributes to the pathophysiology of many cancers through regulation of growth, metastatic potential, and the tumor microenvironment. Here we describe the role of several TGF-β superfamily co-receptors on TGF-β superfamily signaling and the impact on cellular and physiological functions with a particular focus on cancer, including a discussion on recent pharmacological advances and potential clinical applications targeting these co-receptors.

Simple oligonucleotide-based multiplexing of single-cell chromatin accessibility

Microdroplet single-cell ATAC-seq is widely used to measure chromatin accessibility, however, highly scalable and simple sample multiplexing procedures are not available. Here, we present a transposome-assisted single nucleus barcoding approach for ATAC-seq (SNuBar-ATAC) that utilizes a single oligonucleotide adaptor for multiplexing samples during the existing tagmentation step and does not require a pre-labeling procedure. The accuracy and scalability of SNuBar-ATAC was evaluated using cell line mixture experiments. We applied SNuBar-ATAC to investigate treatment-induced chromatin accessibility dynamics by multiplexing 28 mice with lung tumors that received different combinations of chemo, radiation, and targeted immunotherapy. We also applied SNuBar-ATAC to study spatial epigenetic heterogeneity by multiplexing 32 regions from a human breast tissue. Additionally, we show that SNuBar can multiplex single cell ATAC and RNA multiomic assays in cell lines and human breast tissue samples. Our data show that SNuBar is a highly accurate, easy-to-use, and scalable system for multiplexing scATAC-seq and scATAC and RNA co-assay experiments.

Review on the current treatment status of vein of Galen malformations and future directions in research and treatment

INTRODUCTION

Vein of Galen malformations (VOGMs) represent a rare pathologic entity with often catastrophic natural history. The advances in endovascular treatment in recent years have allowed for a paradigm shift in the treatment and outcome of these high-flow shunts, even though their pathogenetic mechanisms and evolution remain in part obscure.

AREAS COVERED

The overall management of VOGMs requires a tailored case-to-case approach, starting with in utero detection and reserving endovascular treatment for indicated cases. Lately, the advances in translational research with whole-genome sequencing and the coupling with cellular-level hemodynamics attempt to shed more light in the pathogenesis and evolution of these lesions. At the same time the advances in endovascular techniques allow for more safety and tailored technical strategy planning. Furthermore, the advances in MRI techniques allow a better understanding of their vascular anatomy. In view of these recent advances and by performing a PUBMED literature review of the last 15 years, we attempt a review of the evolutions in the imaging, management, endovascular treatment and understanding of underlying mechanisms for VOGMs.

EXPERT OPINION

The progress in the fields detailed in this review appears very promising in better understanding VOGMs and expanding the available therapeutic arsenal.

A bioinformatic analysis of the inhibin-betaglycan-endoglin/CD105 network reveals prognostic value in multiple solid tumors

Inhibins and activins are dimeric ligands belonging to the TGFβ superfamily with emergent roles in cancer. Inhibins contain an α-subunit (INHA) and a β-subunit (either INHBA or INHBB), while activins are mainly homodimers of either βA (INHBA) or βB (INHBB) subunits. Inhibins are biomarkers in a subset of cancers and utilize the coreceptors betaglycan (TGFBR3) and endoglin (ENG) for physiological or pathological outcomes. Given the array of prior reports on inhibin, activin and the coreceptors in cancer, this study aims to provide a comprehensive analysis, assessing their functional prognostic potential in cancer using a bioinformatics approach. We identify cancer cell lines and cancer types most dependent and impacted, which included p53 mutated breast and ovarian cancers and lung adenocarcinomas. Moreover, INHA itself was dependent on TGFBR3 and ENG/CD105 in multiple cancer types. INHA, INHBA, TGFBR3, and ENG also predicted patients' response to anthracycline and taxane therapy in luminal A breast cancers. We also obtained a gene signature model that could accurately classify 96.7% of the cases based on outcomes. Lastly, we cross-compared gene correlations revealing INHA dependency to TGFBR3 or ENG influencing different pathways themselves. These results suggest that inhibins are particularly important in a subset of cancers depending on the coreceptor TGFBR3 and ENG and are of substantial prognostic value, thereby warranting further investigation.

The Role of Endoglin in Hepatocellular Carcinoma

Endoglin (CD105) is a type-1 integral transmembrane glycoprotein and coreceptor for transforming growth factor-β (TGF-β) ligands. The endoglin/TGF-β signaling pathway regulates hemostasis, cell proliferation/migration, extracellular matrix (ECM) synthesis and angiogenesis. Angiogenesis contributes to early progression, invasion, postoperative recurrence, and metastasis in hepatocellular carcinoma (HCC), one of the most widespread malignancies globally. Endoglin is overexpressed in newly formed HCC microvessels. It increases microvessel density in cirrhotic and regenerative HCC nodules. In addition, circulating endoglin is present in HCC patients, suggesting potential for use as a diagnostic or prognostic factor. HCC angiogenesis is dynamic and endoglin expression varies by stage. TRC105 (carotuximab) is an antibody against endoglin, and three of its clinical trials were related to liver diseases. A partial response was achieved when combining TRC105 with sorafenib. Although antiangiogenic therapy still carries some risks, combination therapy with endoglin inhibitors or other targeted therapies holds promise.

Endoglin in the Spotlight to Treat Cancer

A spotlight has been shone on endoglin in recent years due to that fact of its potential to serve as both a reliable disease biomarker and a therapeutic target. Indeed, endoglin has now been assigned many roles in both physiological and pathological processes. From a molecular point of view, endoglin mainly acts as a co-receptor in the canonical TGFβ pathway, but also it may be shed and released from the membrane, giving rise to the soluble form, which also plays important roles in cell signaling. In cancer, in particular, endoglin may contribute to either an oncogenic or a non-oncogenic phenotype depending on the cell context. The fact that endoglin is expressed by neoplastic and non-neoplastic cells within the tumor microenvironment suggests new possibilities for targeted therapies. Here, we aimed to review and discuss the many roles played by endoglin in different tumor types, as well as the strong evidence provided by pre-clinical and clinical studies that supports the therapeutic targeting of endoglin as a novel clinical strategy.

Spatiotemporal single-cell RNA sequencing of developing chicken hearts identifies interplay between cellular differentiation and morphogenesis

Single-cell RNA sequencing is a powerful tool to study developmental biology but does not preserve spatial information about tissue morphology and cellular interactions. Here, we combine single-cell and spatial transcriptomics with algorithms for data integration to study the development of the chicken heart from the early to late four-chambered heart stage. We create a census of the diverse cellular lineages in developing hearts, their spatial organization, and their interactions during development. Spatial mapping of differentiation transitions in cardiac lineages defines transcriptional differences between epithelial and mesenchymal cells within the epicardial lineage. Using spatially resolved expression analysis, we identify anatomically restricted expression programs, including expression of genes implicated in congenital heart disease. Last, we discover a persistent enrichment of the small, secreted peptide, thymosin beta-4, throughout coronary vascular development. Overall, our study identifies an intricate interplay between cellular differentiation and morphogenesis.

ALK1 regulates the internalization of endoglin and the type III TGF-β receptor

Complex formation and endocytosis of transforming growth factor-β (TGF-β) receptors play important roles in signaling. However, their interdependence remained unexplored. Here, we demonstrate that ALK1, a TGF-β type I receptor prevalent in endothelial cells, forms stable complexes at the cell surface with endoglin and with type III TGF-β receptors (TβRIII). We show that ALK1 undergoes clathrin-mediated endocytosis (CME) faster than ALK5, type II TGF-β receptor (TβRII), endoglin, or TβRIII. These complexes regulate the endocytosis of the TGF-β receptors, with a major effect mediated by ALK1. Thus, ALK1 enhances the endocytosis of TβRIII and endoglin, while ALK5 and TβRII mildly enhance endoglin, but not TβRIII, internalization. Conversely, the slowly endocytosed endoglin has no effect on the endocytosis of either ALK1, ALK5, or TβRII, while TβRIII has a differential effect, slowing the internalization of ALK5 and TβRII, but not ALK1. Such effects may be relevant to signaling, as BMP9-mediated Smad1/5/8 phosphorylation is inhibited by CME blockade in endothelial cells. We propose a model that links TGF-β receptor oligomerization and endocytosis, based on which endocytosis signals are exposed/functional in specific receptor complexes. This has broad implications for signaling, implying that complex formation among various receptors regulates their surface levels and signaling intensities.

Beta-Arrestins and Receptor Signaling in the Vascular Endothelium

The vascular endothelium is the innermost layer of blood vessels and is a key regulator of vascular tone. Endothelial function is controlled by receptor signaling through G protein-coupled receptors, receptor tyrosine kinases and receptor serine-threonine kinases. The β-arrestins, multifunctional adapter proteins, have the potential to regulate all of these receptor families, although it is unclear as to whether they serve to integrate signaling across all of these different axes. Notably, the β-arrestins have been shown to regulate signaling by a number of receptors important in endothelial function, such as chemokine receptors and receptors for vasoactive substances such as angiotensin II, endothelin-1 and prostaglandins. β-arrestin-mediated signaling pathways have been shown to play central roles in pathways that control vasodilation, cell proliferation, migration, and immune function. At this time, the physiological impact of this signaling has not been studied in detail, but a deeper understanding of it could lead to the development of novel therapies for the treatment of vascular disease.

Endoglin: An 'Accessory' Receptor Regulating Blood Cell Development and Inflammation

Transforming growth factor-β1 (TGF-β1) is a pleiotropic factor sensed by most cells. It regulates a broad spectrum of cellular responses including hematopoiesis. In order to process TGF-β1-responses in time and space in an appropriate manner, there is a tight regulation of its signaling at diverse steps. The downstream signaling is mediated by type I and type II receptors and modulated by the ‘accessory’ receptor Endoglin also termed cluster of differentiation 105 (CD105). Endoglin was initially identified on pre-B leukemia cells but has received most attention due to its high expression on activated endothelial cells. In turn, Endoglin has been figured out as the causative factor for diseases associated with vascular dysfunction like hereditary hemorrhagic telangiectasia-1 (HHT-1), pre-eclampsia, and intrauterine growth restriction (IUPR). Because HHT patients often show signs of inflammation at vascular lesions, and loss of Endoglin in the myeloid lineage leads to spontaneous inflammation, it is speculated that Endoglin impacts inflammatory processes. In line, Endoglin is expressed on progenitor/precursor cells during hematopoiesis as well as on mature, differentiated cells of the innate and adaptive immune system. However, so far only pro-monocytes and macrophages have been in the focus of research, although Endoglin has been identified in many other immune system cell subsets. These findings imply a functional role of Endoglin in the maturation and function of immune cells. Aside the functional relevance of Endoglin in endothelial cells, CD105 is differentially expressed during hematopoiesis, arguing for a role of this receptor in the development of individual cell lineages. In addition, Endoglin expression is present on mature immune cells of the innate (i.e., macrophages and mast cells) and the adaptive (i.e., T-cells) immune system, further suggesting Endoglin as a factor that shapes immune responses. In this review, we summarize current knowledge on Endoglin expression and function in hematopoietic precursors and mature hematopoietic cells of different lineages.

Arrb2 promotes endothelial progenitor cell-mediated postischemic neovascularization

Background and aim: Modulating biological functions of endothelial progenitor cells (EPCs) is essential for therapeutic angiogenesis in ischemic vascular diseases. This study aimed to explore the role and molecular mechanisms of β-arrestin 2 (Arrb2) in EPCs biology and angiogenic therapy. Methods: The influence of Arrb2 on postischemic neovascularization was evaluated in Arrb2-deficient mice. The proliferation, apoptosis, and various functions of EPCs were analyzed in vitro by manipulating the expression of Arrb2. Finally, the in vivo effect of Arrb2 on EPC-mediated neovascularization was investigated in a mouse model of hind-limb ischemia (HLI). Results: Arrb2-deficient mice exhibited impaired blood flow recovery based on laser Doppler measurements and reduced capillary density in the adductor muscle after unilateral HLI. Arrb2-deficient mice also showed restricted intraplug angiogenesis in subcutaneously implanted Matrigel plugs. In vitro, lentivirus-mediated Arrb2 overexpression promoted EPC proliferation, migration, adhesion, and tube formation, whereas Arrb2 knockdown had opposite effects. In addition, the overexpression of Arrb2 in EPCs protected them from hypoxia-induced apoptosis and improved intraplug angiogenesis ex vivo. Mechanistically, Arrb2 interacted with and activated extracellular signal-regulated kinase (ERK)1/2 and protein kinase B (Akt) signaling pathways. Finally, the transplantation of EPCs overexpressing Arrb2 resulted in a significantly higher blood flow restoration in ischemic hind limb and higher capillary density during histological analysis compared with control or Arrb2-knockdown EPC-treated nude mice. Conclusions: The data indicated that Arrb2 augmented EPC-mediated neovascularization through the activation of ERK and Akt signaling pathways. This novel biological function of Arrb2 might provide a potential therapeutic option to promote EPCs in the treatment of ischemic vascular diseases.

Resveratrol Promotes Tumor Microvessel Growth via Endoglin and Extracellular Signal-Regulated Kinase Signaling Pathway and Enhances the Anticancer Efficacy of Gemcitabine against Lung Cancer

The synergistic anticancer effect of gemcitabine (GEM) and resveratrol (RSVL) has been noted in certain cancer types. However, whether the same phenomenon would occur in lung cancer is unclear. Here, we uncovered the molecular mechanism by which RSVL enhances the anticancer effect of GEM against lung cancer cells both in vitro and in vivo. We established human lung adenocarcinoma HCC827 xenografts in nude mice and treated them with GEM and RSVL to detect their synergistic effect in vivo. Tumor tissue sections from nude mice were subjected to hematoxylin and eosin staining for blood vessel morphological observation, and immunohistochemistry was conducted to detect CD31-positive staining blood vessels. We also established the HCC827-human umbilical vein endothelial cell (HUVEC) co-culture model to observe the tubule network formation. Human angiogenesis antibody array was used to screen the angiogenesis-related proteins in RSVL-treated HCC827. RSVL suppressed the expression of endoglin (ENG) and increased tumor microvessel growth and blood perfusion into tumor. Co-treatment of RSVL and GEM led to more tumor growth suppression than treatment of GEM alone. Mechanistically, using the HCC827-HUVEC co-culture model, we showed that RSVL-suppressed ENG expression was accompanied with augmented levels of phosphorylated extracellular signal-regulated kinase (ERK) 1/2 and increased tubule network formation, which may explain why RSVL promoted tumor microvessel growth in vivo. RSVL promoted tumor microvessel growth via ENG and ERK and enhanced the anticancer efficacy of GEM. Our results suggest that intake of RSVL may be beneficial during lung cancer chemotherapy.

Analysis of genetic information from the antlers of Rangifer tarandus (reindeer) at the rapid growth stage

Reindeer is the only deer species in which both males and females regularly grow antlers, providing an excellent model for studying the rapid growth and annual regeneration of antlers. The study of genetic information from reindeer is the basis for revealing the unique mechanism of antler growth. In the present study, we obtained 18.86 GB of clean reads, which were assembled to obtain 94,575 unigenes (average length: 704.69). Among these reads, 30,980 sequences were identified by searching a database of known proteins and then annotated with Gene Ontology (GO) terms, Clusters of Orthologous Groups (COG) classifications and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. All 7,480 simple sequence repeats (SSRs) were detected. A total of 84,435 and 82,226 high-quality single-nucleotide polymorphisms (SNPs) were identified in male and female reindeer, respectively. We identified 31 genes that were highly expressed in reindeer antlers. These genes regulate cell activities that are closely associated with the process of rapid tissue growth. Our results provide a basis for studying reindeer antlers and for further studying the molecular genetics, population genetics, and functional genomics of reindeer.

Continuous endoglin (CD105) overexpression disrupts angiogenesis and facilitates tumor cell metastasis

Endoglin (CD105) is an auxiliary receptor for members of the TFG-β superfamily. Whereas it has been demonstrated that the deficiency of endoglin leads to minor and defective angiogenesis, little is known about the effect of its increased expression, characteristic of several types of cancer. Angiogenesis is essential for tumor growth, so high levels of proangiogenic molecules, such as endoglin, are supposed to be related to greater tumor growth leading to a poor cancer prognosis. However, we demonstrate here that endoglin overexpression do not stimulate sprouting or vascularization in several in vitro and in vivo models. Instead, steady endoglin overexpression keep endothelial cells in an active phenotype that results in an impairment of the correct stabilization of the endothelium and the recruitment of mural cells. In a context of continuous enhanced angiogenesis, such as in tumors, endoglin overexpression gives rise to altered vessels with an incomplete mural coverage that permit the extravasation of blood. Moreover, these alterations allow the intravasation of tumor cells, the subsequent development of metastases and, thus, a worse cancer prognosis.

Endoglin Trafficking/Exosomal Targeting in Liver Cells Depends on N-Glycosylation

Injury of the liver involves a wound healing partial reaction governed by hepatic stellate cells and portal fibroblasts. Individual members of the transforming growth factor-β (TGF-β) superfamily including TGF-β itself and bone morphogenetic proteins (BMP) exert diverse and partially opposing effects on pro-fibrogenic responses. Signaling by these ligands is mediated through binding to membrane integral receptors type I/type II. Binding and the outcome of signaling is critically modulated by Endoglin (Eng), a type III co-receptor. In order to learn more about trafficking of Eng in liver cells, we investigated the membranal subdomain localization of full-length (FL)-Eng. We could show that FL-Eng is enriched in Caveolin-1-containing sucrose gradient fractions. Since lipid rafts contribute to the pool of exosomes, we could consequently demonstrate for the first time that exosomes isolated from cultured primary hepatic stellate cells and its derivatives contain Eng. Moreover, via adenoviral overexpression, we demonstrate that all liver cells have the capacity to direct Eng to exosomes, irrespectively whether they express endogenous Eng or not. Finally, we demonstrate that block of N-glycosylation does not interfere with dimerization of the receptor, but abrogates the secretion of soluble Eng (sol-Eng) and prevents exosomal targeting of FL-Eng.

Endoglin as an Adhesion Molecule in Mature and Progenitor Endothelial Cells: A Function Beyond TGF-β

Endoglin (ENG) is a transmembrane glycoprotein expressed on endothelial cells that functions as a co-receptor for several ligands of the transforming growth factor beta (TGF-β) family. ENG is also a recognized marker of angiogenesis and mutations in the endoglin gene are responsible for Hereditary Hemorrhagic Telangiectasia (HHT) type 1, a vascular disease characterized by defective angiogenesis, arteriovenous malformations, telangiectasia, and epistaxis. In addition to its involvement in the TGF-β family signaling pathways, several lines of evidence suggest that the extracellular domain of ENG has a role in integrin-mediated cell adhesion via its RGD motif. Indeed, we have described a role for endothelial ENG in leukocyte trafficking and extravasation via its binding to leukocyte integrins. We have also found that ENG is involved in vasculogenic properties of endothelial progenitor cells known as endothelial colony forming cells (ECFCs). Moreover, the binding of endothelial ENG to platelet integrins regulate the resistance to shear during platelet-endothelium interactions under inflammatory conditions. Because of the need for more effective treatments in HHT and the involvement of ENG in angiogenesis, current studies are aimed at identifying novel biological functions of ENG which could serve as a therapeutic target. This review focuses on the interaction between ENG and integrins with the aim to better understand the role of this protein in blood vessel formation driven by progenitor and mature endothelial cells.

Role of Endoglin (CD105) in the Progression of Hepatocellular Carcinoma and Anti-Angiogenic Therapy

The liver is perfused by both arterial and venous blood, with a resulting abnormal microenvironment selecting for more-aggressive malignancies. Hepatocellular carcinoma (HCC) is the most frequent primary liver cancer, the sixth most common cancer globally, and the third leading cause of cancer-related mortality worldwide. HCC is characterized by its hypervascularization. Improving the efficiency of anti-angiogenic treatment and mitigation of anti-angiogenic drug resistance are the top priorities in the development of non-surgical HCC therapies. Endoglin (CD105), a transmembrane glycoprotein, is one of the transforming growth factor β (TGF-β) co-receptors. Involvement of that protein in angiogenesis of solid tumours is well documented. Endoglin is a marker of activated endothelial cells (ECs), and is preferentially expressed in the angiogenic endothelium of solid tumours, including HCC. HCC is associated with changes in CD105-positive ECs within and around the tumour. The large spectrum of endoglin effects in the liver is cell-type- and HCC- stage-specific. High expression of endoglin in non-tumour tissue suggests that this microenvironment might play an especially important role in the progression of HCC. Evaluation of tissue expression, as well as serum concentrations of this glycoprotein in HCC, tends to confirm its role as an important biomarker in HCC diagnosis and prognosis. The role of endoglin in liver fibrosis and HCC progression also makes it an attractive therapeutic target. Despite these facts, the exact molecular mechanisms of endoglin functioning in hepatocarcinogenesis are still poorly understood. This review summarizes the current data concerning the role and signalling pathways of endoglin in hepatocellular carcinoma development and progression, and provides an overview of the strategies available for a specific targeting of CD105 in anti-angiogenic therapy in HCC.

Human genetics and molecular mechanisms of vein of Galen malformation

Vein of Galen malformations (VOGMs) are rare developmental cerebrovascular lesions characterized by fistulas between the choroidal circulation and the median prosencephalic vein. Although the treatment of VOGMs has greatly benefited from advances in endovascular therapy, including technical innovation in interventional neuroradiology, many patients are recalcitrant to procedural intervention or lack accessibility to specialized care centers, highlighting the need for improved screening, diagnostics, and therapeutics. A fundamental obstacle to identifying novel targets is the limited understanding of VOGM molecular pathophysiology, including its human genetics, and the lack of an adequate VOGM animal model. Herein, the known human mutations associated with VOGMs are reviewed to provide a framework for future gene discovery. Gene mutations have been identified in 2 Mendelian syndromes of which VOGM is an infrequent but associated phenotype: capillary malformation-arteriovenous malformation syndrome ( RASA1) and hereditary hemorrhagic telangiectasia ( ENG and ACVRL1). However, these mutations probably represent only a small fraction of all VOGM cases. Traditional genetic approaches have been limited in their ability to identify additional causative genes for VOGM because kindreds are rare, limited in patient number, and/or seem to have sporadic inheritance patterns, attributable in part to incomplete penetrance and phenotypic variability. The authors hypothesize that the apparent sporadic occurrence of VOGM may frequently be attributable to de novo mutation or incomplete penetrance of rare transmitted variants. Collaboration among treating physicians, patients' families, and investigators using next-generation sequencing could lead to the discovery of novel genes for VOGM. This could improve the understanding of normal vascular biology, elucidate the pathogenesis of VOGM and possibly other more common arteriovenous malformation subtypes, and pave the way for advances in the diagnosis and treatment of patients with VOGM.

TGF-β family co-receptor function and signaling

Transforming growth factor-β (TGF-β) family members, which include TGF-βs, activins and bone morphogenetic proteins, are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues. These secreted protein ligands signal via single-transmembrane Type I and Type II serine/threonine kinase receptors and intracellular SMAD transcription factors. Deregulation in signaling has been implicated in a broad array of diseases, and implicate the need for intricate fine tuning in cellular signaling responses. One important emerging mechanism by which TGF-β family receptor signaling intensity, duration, specificity and diversity are regulated and/or mediated is through cell surface co-receptors. Here, we provide an overview of the co-receptors that have been identified for TGF-β family members. While some appear to be specific to TGF-β family members, others are shared with other pathways and provide possible ways for signal integration. This review focuses on novel functions of TGF-β family co-receptors, which continue to be discovered.

TGF-β1 in Vascular Wall Pathology: Unraveling Chronic Venous Insufficiency Pathophysiology

Chronic venous insufficiency and varicose veins occur commonly in affluent countries and are a socioeconomic burden. However, there remains a relative lack of knowledge about venous pathophysiology. Various theories have been suggested, yet the molecular sequence of events is poorly understood. Transforming growth factor-beta one (TGF-β1) is a highly complex polypeptide with multifunctional properties that has an active role during embryonic development, in adult organ physiology and in the pathophysiology of major diseases, including cancer and various autoimmune, fibrotic and cardiovascular diseases. Therefore, an emphasis on understanding its signaling pathways (and possible disruptions) will be an essential requirement for a better comprehension and management of specific diseases. This review aims at shedding more light on venous pathophysiology by describing the TGF-β1 structure, function, activation and signaling, and providing an overview of how this growth factor and disturbances in its signaling pathway may contribute to specific pathological processes concerning the vessel wall which, in turn, may have a role in chronic venous insufficiency.

Endoglin Mediates Vascular Maturation by Promoting Vascular Smooth Muscle Cell Migration and Spreading

Supplemental Digital Content is available in the text.

Objective—

Endoglin, a transforming growth factor-β superfamily coreceptor, is predominantly expressed in endothelial cells and has essential roles in vascular development. However, whether endoglin is also expressed in vascular smooth muscle cells (VSMCs), especially in vivo, remains controversial. Furthermore, the roles of endoglin in VSMC biology remain largely unknown. Our objective was to examine the expression and determine the function of endoglin in VSMCs during angiogenesis.

Approach and Results—

Here, we determine that endoglin is robustly expressed in VSMCs. Using CRISPR/CAS9 knockout and short hairpin RNA knockdown in the VSMC/endothelial coculture model system, we determine that endoglin in VSMCs, but not in endothelial cells, promotes VSMCs recruitment by the endothelial cells both in vitro and in vivo. Using an unbiased bioinformatics analysis of RNA sequencing data and further study, we determine that, mechanistically, endoglin mediates VSMC recruitment by promoting VSMC migration and spreading on endothelial cells via increasing integrin/FAK pathway signaling, whereas endoglin has minimal effects on VSMC adhesion to endothelial cells. In addition, we further determine that loss of endoglin in VSMCs inhibits VSMC recruitment in vivo.

Conclusions—

These studies demonstrate that endoglin has an important role in VSMC recruitment and blood vessel maturation during angiogenesis and also provide novel insights into how discordant endoglin function in endothelial and VSMCs may regulate vascular maturation and angiogenesis.

The role of endoglin in post-ischemic revascularization

Following arterial occlusion, blood vessels respond by forming a new network of functional capillaries (angiogenesis), by reorganizing preexisting capillaries through the recruitment of smooth muscle cells to generate new arteries (arteriogenesis) and by growing and remodeling preexisting collateral arterioles into physiologically relevant arteries (collateral development). All these processes result in the recovery of organ perfusion. The importance of endoglin in post-occlusion reperfusion is sustained by several observations: (1) endoglin expression is increased in vessels showing active angiogenesis/remodeling; (2) genetic endoglin haploinsufficiency in humans causes deficient angiogenesis; and (3) the reduction of endoglin expression by gene disruption or the administration of endoglin-neutralizing antibodies reduces angiogenesis and revascularization. However, the precise role of endoglin in the several processes associated with revascularization has not been completely elucidated and, in some cases, the function ascribed to endoglin by different authors is controversial. The purpose of this review is to organize in a critical way the information available for the role of endoglin in several phenomena (angiogenesis, arteriogenesis and collateral development) associated with post-ischemic revascularization.

The CXCL10/CXCR3 axis promotes cardiac microvascular endothelial cell migration via the p38/FAK pathway in a proliferation-independent manner

CXCL10 is a chemokine with potent chemotactic activity for immune and non-immune cells expressing its receptor CXCR3. Previous studies have demonstrated that CXCL10 is involved in myocardial infarction. However, the role of CXCL10 in cardiac microvascular endothelial cell (CMEC) regulation and related mechanisms remains unclear. In this study, we investigated the effects of CXCL10 on the CMEC migration and explored its potential molecular mechanism by wound healing, cell proliferation and viability analysis. Furthermore, migration-related signaling pathways, including FAK, Erk, p38 and Smad, were examined by Western blotting. We found that CXCL10 significantly promotes CMEC migration under normal conditions and during hypoxia/ischemia. However, no significant differences in CMEC proliferation and viability were observed with or without CXCL10 treatment. CXCL10-mediated CMEC migration was greatly blocked by treatment with an anti-CXCR3 antibody. Although CXCL10 treatment promoted phosphorylation and activation of the FAK, Erk, and p38 pathways during hypoxia/ischemia, CXCL10-mediated CMEC migration was significantly blocked by p38 and FAK inhibitors, but not by an Erk inhibitor. Furthermore, CXCL10-mediated FAK activation was suppressed by the p38 inhibitor. These findings indicated that the CXCL10/CXCR3 pathway promotes the migration of CMECs under normal conditions and during hypoxia/ischemia in a proliferation-independent manner, at least in part, through regulation of the p38/FAK pathways.

Tendon progenitor cells in injured tendons have strong chondrogenic potential: the CD105-negative subpopulation induces chondrogenic degeneration

To study the cellular mechanism of the tendon repair process, we used a mouse Achilles tendon injury model to focus on the cells recruited to the injured site. The cells isolated from injured tendon 1 week after the surgery and uninjured tendons contained the connective tissue progenitor populations as determined by colony-forming capacity, cell surface markers, and multipotency. When the injured tendon-derived progenitor cells (inTPCs) were transplanted into injured Achilles tendons, they were not only integrated in the regenerating area expressing tenogenic phenotype but also trans-differentiated into chondrogenic cells in the degenerative lesion that underwent ectopic endochondral ossification. Surprisingly, the micromass culture of the inTPCs rapidly underwent chondrogenic differentiation even in the absence of exogenous bone morphogenetic proteins or TGFβs. The cells isolated from human ruptured tendon tissues also showed connective tissue progenitor properties and exhibited stronger chondrogenic ability than bone marrow stromal cells. The mouse inTPCs contained two subpopulations one positive and one negative for CD105, a coreceptor of the TGFβ superfamily. The CD105-negative cells showed superior chondrogenic potential in vitro and induced larger chondroid degenerative lesions in mice as compared to the CD105-positive cells. These findings indicate that tendon progenitor cells are recruited to the injured site of tendons and have a strong chondrogenic potential and that the CD105-negative population of these cells would be the cause for chondroid degeneration in injured tendons. The newly identified cells recruited to the injured tendon may provide novel targets to develop therapeutic strategies to facilitate tendon repair.

VEGF, Notch and TGFβ/BMPs in regulation of sprouting angiogenesis and vascular patterning

The blood vasculature is constantly adapting to meet the demand from tissue. In so doing, branches may form, reorganize or regress. These complex processes employ integration of multiple signalling cascades, some of them being restricted to endothelial and mural cells and, hence, suitable for targeting of the vasculature. Both genetic and drug targeting experiments have demonstrated the requirement for the vascular endothelial growth factor (VEGF) system, the Delta-like-Notch system and the transforming growth factor β (TGFβ)/bone morphogenetic protein (BMP) cascades in vascular development. Although several of these signalling cascades in part converge into common downstream components, they differ in temporal and spatial regulation and expression. For example, the pro-angiogenic VEGFA is secreted by cells in need of oxygen, presented to the basal side of the endothelium, whereas BMP9 and BMP10 are supplied via the bloodstream in constant interaction with the apical side to suppress angiogenesis. Delta-like 4 (DLL4), on the other hand, is provided as an endothelial membrane bound ligand. In the present article, we discuss recent data on the integration of these pathways in the process of sprouting angiogenesis and vascular patterning and malformation.

Regulation of TGF-β receptor hetero-oligomerization and signaling by endoglin

Endoglin is a modulator of TGF-β signaling in endothelial cells. We show that it forms stable homodimers serving as a scaffold for binding TβRII, ALK5, and ALK1. ALK1 and ALK5 bind endoglin differentially, with TβRII recruiting ALK5. Signaling data indicate a role for this receptor complex in balancing TGF-β signaling between Smad1/5/8 and Smad2/3.

Complex formation among transforming growth factor-β (TGF-β) receptors and its modulation by coreceptors represent an important level of regulation for TGF-β signaling. Oligomerization of ALK5 and the type II TGF-β receptor (TβRII) has been thoroughly investigated, both in vitro and in intact cells. However, such studies, especially in live cells, are missing for the endothelial cell coreceptor endoglin and for the ALK1 type I receptor, which enables endothelial cells to respond to TGF-β by activation of both Smad2/3 and Smad1/5/8. Here we combined immunoglobulin G–mediated immobilization of one cell-surface receptor with lateral mobility studies of a coexpressed receptor by fluorescence recovery after photobleaching (FRAP) to demonstrate that endoglin forms stable homodimers that function as a scaffold for binding TβRII, ALK5, and ALK1. ALK1 and ALK5 bind to endoglin with differential dependence on TβRII, which plays a major role in recruiting ALK5 to the complex. Signaling data indicate a role for the quaternary receptor complex in regulating the balance between TGF-β signaling to Smad1/5/8 and to Smad2/3.

Endoglin Regulation of Smad2 Function Mediates Beclin1 Expression and Endothelial Autophagy

Autophagy is the targeted degradation of proteins and organelles critical for homeostasis and cell survival. Transforming growth factor β (TGF-β) differentially regulates autophagy in a context-specific manner, although the precise intracellular mechanisms remain less clear. Importantly, how TGF-β controls autophagic responses in endothelial cells (EC) during angiogenesis is unknown. Here we identified endoglin, an EC-specific TGF-β co-receptor essential for angiogenesis, as a key determinant of autophagy. Among the two opposing TGF-β Smad pathways in the EC system (Smad1/5/8 and Smad2/3), we found Smad2 as the major transcriptional regulator of autophagy that targets beclin1 (BECN1) gene expression. Smad2, but not Smad3, acts as a repressor upstream of the BECN1 promoter region. Overall, endoglin promotes autophagy by impeding Smad2 transcriptional repressor activity. Notably, increased beclin1 levels upon Smad2 knockdown directly correlated with enhanced autophagy during angiogenesis. Taken together, these results establish endoglin as a critical mediator of autophagy and demonstrate a new transcriptional mechanism by which Smad2 inhibits angiogenesis.

Endoglin involvement in integrin-mediated cell adhesion as a putative pathogenic mechanism in hereditary hemorrhagic telangiectasia type 1 (HHT1)

Mutations in the endoglin gene (ENG) are responsible for ∼50% of all cases with hereditary hemorrhagic telangiectasia (HHT). Because of the absence of effective treatments for HHT symptoms, studies aimed at identifying novel biological functions of endoglin which could serve as therapeutic targets of the disease are needed. Endoglin is an endothelial membrane protein, whose most studied function has been its role as an auxiliary receptor in the TGF-β receptor complex. However, several lines of evidence suggest the involvement of endoglin in TGF-β-independent functions. Endoglin displays, within its zona pellucida domain, an RGD motif, which is a prototypic sequence involved in integrin-based interactions with other proteins. Indeed, we have recently described a novel role for endothelial endoglin in leukocyte trafficking and extravasation via its interaction with leukocyte integrins. In addition, functional, as well as protein and gene expression analysis have shown that ectopic endoglin represses the synthesis of several members of the integrin family and modulates integrin-mediated cell adhesions. This review focuses on the tight link between endoglin and integrins and how the role of endothelial endoglin in integrin-dependent cell adhesion processes can provide a better understanding of the pathogenic mechanisms leading to vascular lesions in endoglin haploinsufficient HHT1 patients.

Lung Cancer Biomarkers

Lung cancer is the most frequently occurring cancer in the world and continually leads in mortality among cancers. The overall 5-year survival rate for lung cancer has risen only 4% (from 12% to 16%) over the past 4 decades, and late diagnosis is a major obstacle in improving lung cancer prognosis. Survival of patients undergoing lung resection is greater than 80%, suggesting that early detection and diagnosis of cancers before they become inoperable and lethal will greatly improve mortality. Lung cancer biomarkers can be used for screening, detection, diagnosis, prognosis, prediction, stratification, therapy response monitoring, and so on. This review focuses on noninvasive diagnostic and prognostic biomarkers. For that purpose, our discussion in this review will focus on biological fluid-based biomarkers. The body fluids include blood (serum or plasma), sputum, saliva, BAL, pleural effusion, and VOC. Since it is rich in different cellular and molecular elements and is one of the most convenient and routine clinical procedures, serum or plasma is the main source for the development and validation of many noninvasive biomarkers. In terms of molecular aspects, the most widely validated ones are proteins, some of which are used in the clinical sector, though in limited accessory purposes. We will also discuss the lung cancer (protein) biomarkers in clinical trials and currently in the validation phase with hundreds of samples. After proteins, we will discuss microRNAs, methylated DNA, and circulating tumor cells, which are being vigorously developed and validated as potential lung cancer biomarkers. The main aim of this review is to provide researchers and clinicians with an understanding of the potential noninvasive lung cancer biomarkers in biological fluids that have recently been discovered.

Smad-independent pathway involved in transforming growth factor β1-induced Nox4 expression and proliferation of endothelial cells

NADPH oxidase-derived reactive oxygen species are important for various cellular functions, including proliferation. Endothelial cells predominantly express the Nox4 isoform of NADPH oxidase, but it is not entirely clear how it is regulated. In this study, we investigated the signalling pathways involved in transforming growth factor-β1 (TGF-β1)-induced Nox4 expression and the proliferation of human microvascular endothelial cells (HMECs). TGF-β1 stimulated Nox4 messenger RNA and protein expression in HMECs. TGF-β1-induced Nox4 also increased hydrogen peroxide production, which was inhibited by diphenyleneiodonium and EUK134. The acute treatment of HMECs with TGF-β1 enhanced the phosphorylation of Smad2 and extracellular signal-regulated kinase (ERK) 1/2, without affecting p38MAPK, Akt, or Jun N-terminal kinase 1/2 (JNK1/2) pathways. Further, inhibition of Smad2 signalling using an inhibitor of activin receptor-linked kinase 5 SB431542 reduced TGF-β1-induced Nox4 expression, while inhibition of ERK1/2 with the inhibitor of mitogen-activated protein kinase kinase 1/2 U0126 decreased both basal and TGF-β1-induced Nox4 expression. Inhibition of ERK1/2 phosphorylation with U0126 did not affect Smad2 phosphorylation. Finally, TGF-β1 enhanced endothelial cell proliferation, which was reduced by U0126 but not by SB431542. These findings suggest that the non-canonical pathway ERK1/2 regulates Nox4 expression and may be involved in TGF-β1-induced proliferation of endothelial cells, which is vital during angiogenesis and vascular development.

Investigation of endoglin wild-type and missense mutant protein heterodimerisation using fluorescence microscopy based IF, BiFC and FRET analyses

The homodimeric transmembrane receptor endoglin (CD105) plays an important role in angiogenesis. This is highlighted by mutations in its gene, causing the vascular disorder HHT1. The main role of endoglin function has been assigned to the modulation of transforming growth factor β and bone morphogenetic protein signalling in endothelial cells. Nevertheless, other functions of endoglin have been revealed to be involved in different cellular functions and in other cell types than endothelial cells. Compared to the exploration of its natural function, little experimental data have been gathered about the mode of action of endoglin HHT mutations at the cellular level, especially missense mutations, and to what degree these might interfere with normal endoglin function. In this paper, we have used fluorescence-based microscopic techniques, such as bimolecular fluorescence complementation (BiFC), immunofluorescence staining with the endoglin specific monoclonal antibody SN6, and protein interaction studies by Förster Resonance Energy Transfer (FRET) to investigate the formation and cellular localisation of possible homo- and heterodimers composed of endoglin wild-type and endoglin missense mutant proteins. The results show that all of the investigated missense mutants dimerise with themselves, as well as with wild-type endoglin, and localise, depending on the position of the affected amino acid, either in the rough endoplasmic reticulum (rER) or in the plasma membrane of the cells. We show that the rER retained mutants reduce the amount of endogenous wild-type endoglin on the plasma membrane through interception in the rER when transiently or stably expressed in HMEC-1 endothelial cells. As a result of this, endoglin modulated TGF-β1 signal transduction is also abrogated, which is not due to TGF-β receptor ER trafficking interference. Protein interaction analyses by FRET show that rER located endoglin missense mutants do not perturb protein processing of other membrane receptors, such as TβRII, ALK5 or ALK1.

Src-mediated post-translational regulation of endoglin stability and function is critical for angiogenesis

Endoglin is a transforming growth factor β (TGF-β) co-receptor essential for angiogenesis and tumor vascularization. Endoglin modulates the crucial balance between pro- and anti-angiogenic signaling by activin receptor-like kinase (ALK) 1, 5, and TGF-β type II (TβRII) receptors. Despite its established role in physiology and disease, the mechanism of endoglin down-regulation remains unknown. Here we report that the conserved juxtamembrane cytoplasmic tyrosine motif ((612)YIY(614)) is a critical determinant of angiogenesis. Src directly phosphorylates this motif to induce endoglin internalization and degradation via the lysosome. We identified epidermal growth factor (EGF) and vascular endothelial growth factor (VEGF) as Src-activators that induce endoglin turnover following (612)YIY(614) phosphorylation. Interestingly, Src phosphorylation of endoglin-(612)YIY(614) was also an important process for receptor down-regulation by TRACON105 (TRC105), an endoglin-targeting antibody currently in clinical trials. The regulation of (612)YIY(614) phosphorylation was critical for angiogenesis, as both the phosphomimetic and unphosphorylatable mutants impaired endothelial functions including proliferation, migration, and capillary tube formation. Collectively, these findings establish Src and pro-angiogenic mitogens as critical mediators of endoglin stability and function.

Effects of the combination of TRC105 and bevacizumab on endothelial cell biology

Endoglin, or CD105, is a cell membrane glycoprotein that is overexpressed on proliferating endothelial cells (EC), including those found in malignancies and choroidal neovascularization. Endoglin mediates the transition from quiescent endothelium, characterized by the relatively dominant state of Smad 2/3 phosphorylation, to active angiogenesis by preferentially phosphorylating Smad 1/5/8. The monoclonal antibody TRC105 binds endoglin with high avidity and is currently being tested in phase 1b and phase 2 clinical trials. In this report, we evaluated the effects of TRC105 on primary human umbilical vascular endothelial cells (HUVEC) as a single agent and in combination with bevacizumab. As single agents, both TRC105 and bevacizumab efficiently blocked HUVEC tube formation, and the combination of both agents achieved even greater levels of inhibition. We further assessed the effects of each drug on various aspects of HUVEC function. While bevacizumab was observed to inhibit HUVEC viability in nutrient-limited medium, TRC105 had little effect on HUVEC viability, either alone or in combination with bevacizumab. Additionally, both drugs inhibited HUVEC migration and induced apoptosis. At the molecular level, TRC105 treatment of HUVEC lead to decreased Smad 1/5/8 phosphorylation in response to BMP-9, a primary ligand for endoglin. Together, these results indicate that TRC105 acts as an effective anti-angiogenic agent alone and in combination with bevacizumab.

Endoglin in liver fibrogenesis: Bridging basic science and clinical practice

Endoglin, also known as cluster of differentiation CD105, was originally identified 25 years ago as a novel marker of endothelial cells. Later it was shown that endoglin is also expressed in pro-fibrogenic cells including mesangial cells, cardiac and scleroderma fibroblasts, and hepatic stellate cells. It is an integral membrane-bound disulfide-linked 180 kDa homodimeric receptor that acts as a transforming growth factor-β (TGF-β) auxiliary co-receptor. In humans, several hundreds of mutations of the endoglin gene are known that give rise to an autosomal dominant bleeding disorder that is characterized by localized angiodysplasia and arteriovenous malformation. This disease is termed hereditary hemorrhagic telangiectasia type I and induces various vascular lesions, mainly on the face, lips, hands and gastrointestinal mucosa. Two variants of endoglin (i.e., S- and L-endoglin) are formed by alternative splicing that distinguishes from each other in the length of their cytoplasmic tails. Moreover, a soluble form of endoglin, i.e., sol-Eng, is shedded by the matrix metalloprotease-14 that cleaves within the extracellular juxtamembrane region. Endoglin interacts with the TGF-β signaling receptors and influences Smad-dependent and -independent effects. Recent work has demonstrated that endoglin is a crucial mediator during liver fibrogenesis that critically controls the activity of the different Smad branches. In the present review, we summarize the present knowledge of endoglin expression and function, its involvement in fibrogenic Smad signaling, current models to investigate endoglin function, and the diagnostic value of endoglin in liver disease.

Antibody-directed coupling of endoglin and MMP-14 is a key mechanism for endoglin shedding and deregulation of TGF-β signaling

Endoglin is a transforming growth factor β (TGF-β) coreceptor that serves as a prognostic, diagnostic and therapeutic vascular target in human cancer. A number of endoglin ectodomain-targeting antibodies (Abs) can effectively suppress both normal and tumor-associated angiogenesis, but their molecular actions remain poorly characterized. Here we define a key mechanism for TRACON105 (TRC105), a humanized monoclonal Ab in clinical trials for treatment of advanced or metastatic tumors. TRC105, along with several other endoglin Abs tested, enhance endoglin shedding through direct coupling of endoglin and the membrane-type 1 matrix metalloproteinase (MMP)-14 at the cell surface to release the antiangiogenic factor, soluble endoglin (sEng). In addition to this coupling process, endoglin shedding is further amplified by increased MMP-14 expression that requires TRC105 concentration-dependent c-Jun N-terminal kinase (JNK) activation. There were also notable counterbalancing effects on canonical Smad signaling in which TRC105 abrogated both the steady-state and TGF-β-induced Smad1/5/8 activation while augmenting Smad2/3 activation. Interestingly, TRC105-induced sEng and aberrant Smad signaling resulted in an excessive migratory response through enhanced stress fiber formation and disruption of endothelial cell–cell junctions. Collectively, our study defines endoglin shedding and deregulated TGF-β signaling during migration as major mechanisms by which TRC105 inhibits angiogenesis.

Overexpression of endoglin modulates TGF-β1-signalling pathways in a novel immortalized mouse hepatic stellate cell line

Hepatic stellate cells (HSCs) play a major role in the pathogenesis of liver fibrosis. Working on primary HSCs requires difficult isolation procedures; therefore we have generated and here characterize a mouse hepatic stellate cell line expressing GFP under control of the collagen 1(I) promoter/enhancer. These cells are responsive to pro-fibrogenic stimuIi, such as PDGF or TGF-β1, and are able to activate intracellular signalling pathways including Smads and MAP kinases. Nevertheless, due to the basal level of activation, TGF-β1 did not significantly induce GFP expression contrasting the TGF-β1 regulated endogenous collagen I expression. We could demonstrate that the accessory TGF-β-receptor endoglin, which is endogenously expressed at very low levels, has a differential effect on signalling of these cells when transiently overexpressed. In the presence of endoglin activation of Smad1/5/8 was drastically enhanced. Moreover, the phosphorylation of ERK1/2 was increased, and the expression of vimentin, α-smooth muscle actin and connective tissue growth factor was upregulated. Endoglin induced a slight increase in expression of the inhibitor of differentiation-2 while the amount of endogenous collagen type I was reduced. Therefore, this profibrogenic cell line with hepatic stellate cell origin is not only a promising novel experimental tool, which can be used in vivo for cell tracing experiments. Furthermore it allows investigating the impact of various regulatory proteins (e.g. endoglin) on profibrogenic signal transduction, differentiation and hepatic stellate cell biology.

Endoglin haploinsufficiency promotes fibroblast accumulation during wound healing through Akt activation

Accurate regulation of dermal fibroblast function plays a crucial role in wound healing. Many fibrotic diseases are characterized by a failure to conclude normal tissue repair and the persistence of fibroblasts inside lesions. In the present study we demonstrate that endoglin haploinsufficiency promotes fibroblast accumulation during wound healing. Moreover, scars from endoglin-heterozygous (Eng^+/−) mice show persisting fibroblasts 12 days after wounding, which could lead to a fibrotic scar. Endoglin haploinsufficiency results in increased proliferation and migration of primary cultured murine dermal fibroblasts (MDFs). Moreover, Eng^+/− MDF have diminished responses to apoptotic signals compared with control cells. Altogether, these modifications could explain the augmented presence of fibroblasts in Eng^+/− mice wounds. We demonstrate that endoglin expression regulates Akt phosphorylation and that PI3K inhibition abolishes the differences in proliferation between endoglin haploinsufficient and control cells. Finally, persistent fibroblasts in Eng^+/− mice wound co-localize with a greater degree of Akt phosphorylation. Thus, endoglin haploinsufficiency seems to promote fibroblast accumulation during wound healing through the activation of the PI3K/Akt pathway. These studies open new non-Smad signaling pathway for endoglin regulating fibroblast cell function during wound healing, as new therapeutic opportunities for the treatment of fibrotic wounds.

Endoglin for tumor imaging and targeted cancer therapy

INTRODUCTION

Although cancer treatment has evolved substantially in the past decades, cancer-related mortality rates are still increasing. Therapies targeting tumor angiogenesis, crucial for the growth of solid tumors, mainly target vascular endothelial growth factor (VEGF) and have been clinically applied during the last decade. However, these therapies have not met high expectations, which were based on therapeutic efficacy in animal models. This can partly be explained by the upregulation of alternative angiogenic pathways. Therefore, additional therapies targeting other pro-angiogenic pathways are needed.

AREAS COVERED

The transforming growth factor (TGF)-β signaling pathway plays an important role in (tumor) angiogenesis. Therefore, components of this pathway are interesting candidates for anti-angiogenic therapy. Endoglin, a co-receptor for various TGF-β family members, is specifically overexpressed in tumor vessels and endoglin expression is associated with metastasis and patient survival. Therefore, endoglin might be a good candidate for anti-angiogenic therapy. In this review, we discuss the potential of using endoglin to target the tumor vasculature for imaging and therapeutic purposes.

EXPERT OPINION

Considering the promising results from various in vitro studies, in vivo animal models and the first clinical trial targeting endoglin, we are convinced that endoglin is a valuable tool for the diagnosis, visualization and ultimately treatment of solid cancers.

Endoglin mediates fibronectin/α5β1 integrin and TGF-β pathway crosstalk in endothelial cells

Both the transforming growth factor β (TGF-β) and integrin signalling pathways have well-established roles in angiogenesis. However, how these pathways integrate to regulate angiogenesis is unknown. Here, we show that the extracellular matrix component, fibronectin, and its cellular receptor, α5β1 integrin, specifically increase TGF-β1- and BMP-9-induced Smad1/5/8 phosphorylation via the TGF-β superfamily receptors endoglin and activin-like kinase-1 (ALK1). Fibronectin and α5β1 integrin increase Smad1/5/8 signalling by promoting endoglin/ALK1 cell surface complex formation. In a reciprocal manner, TGF-β1 activates α5β1 integrin and downstream signalling to focal adhesion kinase (FAK) in an endoglin-dependent manner. α5β1 integrin and endoglin form a complex on the cell surface and co-internalize, with their internalization regulating α5β1 integrin activation and signalling. Functionally, endoglin-mediated fibronectin/α5β1 integrin and TGF-β pathway crosstalk alter the responses of endothelial cells to TGF-β1, switching TGF-β1 from a promoter to a suppressor of migration, inhibiting TGF-β1-mediated apoptosis to promote capillary stability, and partially mediating developmental angiogenesis in vivo. These studies provide a novel mechanism for the regulation of TGF-β superfamily signalling and endothelial function through crosstalk with integrin signalling pathways.

Endoglin inhibits ERK-induced c-Myc and cyclin D1 expression to impede endothelial cell proliferation

Endoglin is an endothelial-specific transforming growth factor beta (TGF-β) co-receptor essential for angiogenesis and vascular remodeling. Endoglin regulates a wide range of cellular processes, including cell adhesion, migration, and proliferation, through TGF-β signaling to canonical Smad and Smad-independent pathways. Despite its overall pro-angiogenic role in the vasculature, the underlying mechanism of endoglin action is poorly characterized. We previously identified β-arrestin2 as a binding partner that causes endoglin internalization from the plasma membrane and inhibits ERK signaling towards endothelial migration. In the present study, we examined the mechanistic role of endoglin and β-arrestin2 in endothelial cell proliferation. We show that endoglin impedes cell growth through sustained inhibition of ERK-induced c-Myc and cyclin D1 expression in a TGF-β-independent manner. The down-regulation of c-Myc and cyclin D1, along with growth-inhibition, are reversed when the endoglin/β-arrestin2 interaction is disrupted. Given that TGF-β-induced Smad signaling potently represses c-Myc in most cell types, our findings here show a novel mechanism by which endoglin augments growth-inhibition by targeting ERK and key downstream mitogenic substrates.

BAMBI regulates angiogenesis and endothelial homeostasis through modulation of alternative TGFβ signaling

BACKGROUND

BAMBI is a type I TGFβ receptor antagonist, whose in vivo function remains unclear, as BAMBI(-/-) mice lack an obvious phenotype.

METHODOLOGY/PRINCIPAL FINDINGS

Identifying BAMBI's functions requires identification of cell-specific expression of BAMBI. By immunohistology we found BAMBI expression restricted to endothelial cells and by electron microscopy BAMBI(-/-) mice showed prominent and swollen endothelial cells in myocardial and glomerular capillaries. In endothelial cells over-expression of BAMBI reduced, whereas knock-down enhanced capillary growth and migration in response to TGFβ. In vivo angiogenesis was enhanced in matrigel implants and in glomerular hypertrophy after unilateral nephrectomy in BAMBI(-/-) compared to BAMBI(+/+) mice consistent with an endothelial phenotype for BAMBI(-/-) mice. BAMBI's mechanism of action in endothelial cells was examined by canonical and alternative TGFβ signaling in HUVEC with over-expression or knock-down of BAMBI. BAMBI knockdown enhanced basal and TGFβ stimulated SMAD1/5 and ERK1/2 phosphorylation, while over-expression prevented both.

CONCLUSIONS/SIGNIFICANCE

Thus we provide a first description of a vascular phenotype for BAMBI(-/-) mice, and provide in vitro and in vivo evidence that BAMBI contributes to endothelial and vascular homeostasis. Further, we demonstrate that in endothelial cells BAMBI interferes with alternative TGFβ signaling, most likely through the ALK 1 receptor, which may explain the phenotype observed in BAMBI(-/-) mice. This newly described role for BAMBI in regulating endothelial function has potential implications for understanding and treating vascular disease and tumor neo-angiogenesis.

Endoglin regulates PI3-kinase/Akt trafficking and signaling to alter endothelial capillary stability during angiogenesis

Endoglin (CD105) is an endothelial-specific transforming growth factor β (TGF-β) coreceptor essential for angiogenesis and vascular homeostasis. Although endoglin dysfunction contributes to numerous vascular conditions, the mechanism of endoglin action remains poorly understood. Here we report a novel mechanism in which endoglin and Gα-interacting protein C-terminus-interacting protein (GIPC)-mediated trafficking of phosphatidylinositol 3-kinase (PI3K) regulates endothelial signaling and function. We demonstrate that endoglin interacts with the PI3K subunits p110α and p85 via GIPC to recruit and activate PI3K and Akt at the cell membrane. Opposing ligand-induced effects are observed in which TGF-β1 attenuates, whereas bone morphogenetic protein-9 enhances, endoglin/GIPC-mediated membrane scaffolding of PI3K and Akt to alter endothelial capillary tube stability in vitro. Moreover, we employ the first transgenic zebrafish model for endoglin to demonstrate that GIPC is a critical component of endoglin function during developmental angiogenesis in vivo. These studies define a novel non-Smad function for endoglin and GIPC in regulating endothelial cell function during angiogenesis.

Oligomeric interactions of TGF-β and BMP receptors

Transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) cytokines participate in a multiplicity of ways in the regulation of numerous physiological and pathological processes. Their wide-ranging biological functions are controlled by several mechanisms, including regulation of transcription, complex formation among the signaling receptors (oligomerization) and with co-receptors, binding of the receptors to scaffolding proteins or their targeting to specific membrane domains. Here, we address the generation of TGF-β and BMP receptor homo- and hetero-oligomers and its roles as a mechanism capable of fast regulation of signaling by these crucial cytokines. We examine the available biochemical, biophysical and structural evidence for the ternary structure of these complexes, and the possible roles of homomeric and heteromeric receptor oligomers in signaling.