Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

ENG

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101593

Synonyms

Historical Background

Endoglin (OMIM 131195), also known as cluster of differentiation CD105, was originally identified in endothelial cells by immunofluorescence using a monoclonal antibody (mAb 44G4) raised against a human pre-B leukemia cell line (Gougos and Letarte 1988). Later it was shown that endoglin is also expressed outside the endothelium in pro-fibrogenic and immune cells (Meurer et al. 2014; Ojeda-Fernandez et al. 2016). It is an integral membrane-bound disulfide-linked 180 kDa homodimeric receptor that acts as an auxiliary receptor for ligands of the transforming growth factor-β (TGF-β) superfamily. Endoglin interacts with the TGF-β signaling receptors and influences Smad-dependent and Smad-independent effects. The endoglin gene produces two splice variants (i.e., S- and L-endoglin) which cause a different response outcome due to the sequence variation of their cytoplasmic tails. Regulated ectodomain shedding by matrix metalloprotease-14 (MMP-14) converts endoglin into a soluble form (i.e., sol-Eng). In humans, several hundred mutations of the endoglin gene are known which lead to a haploinsufficiency and give rise to an autosomal dominant bleeding disorder termed hereditary hemorrhagic telangiectasia type 1 (HHT1). HHT1 is characterized by localized angiodysplasia, arteriovenous malformations (AVMs), and various vascular lesions, mainly on the face, lips, hands, and gastrointestinal mucosa. In experimental endoglin knockout mice models it was shown that homozygous endoglin loss is embryonically lethal due to cardiovascular abnormalities. Therefore, it was initially proposed that endoglin function is mainly associated with the cardiovascular system and vascular remodeling. However, it was also demonstrated that endoglin is a crucial mediator during liver fibrogenesis that enhances TGF-β-driven Smad1/5 phosphorylation and α-smooth muscle actin production during hepatic insult (Meurer et al. 2011).

Chromosomal Localization, Structure, and ENG Gene Variants

The human ENG gene was mapped to the long arm of human chromosome 9 (9q34→qter) by Southern blot analysis of DNA isolated from human-hamster somatic cell hybrids and by fluorescent in situ hybridization coupled with 4’,6-Diamidin-2-phenylindol (DAPI) banding on human chromosomes (Fernández-Ruiz et al. 1993). The detailed chromosomal assignment was subsequently predicted from the fact that the mouse homolog is located on chromosome 2 directly in the close proximity of the adenylate kinase-1 gene that is syntenic to human chromosome subband 9q34.11 (Fig. 1) (Qureshi et al. 1995).
ENG, Fig. 1

Chromosomal localization of human, mouse, and ratENGgenes. The human ENG gene is located on the long arm of chromosome 9, subregion 9q34.11 (upper panel). The murine homolog maps to the proximal region of chromosome 2 in close proximity to marker D2mit33 (middle panel), and the rat homolog was assigned to the short arm of chromosome 3 (lower panel). The three ideograms were created with the Genome Decoration Page (www.ncbi.nlm.nih.gov/genome/tools/gdp)

The human ENG gene contains 15 exons numbered 1 to 14, where exon 9 is split into 9a and 9b (McAllister et al. 1994). Beside the full length endoglin (FL-Eng), a splice variant has been identified, i.e., short-endoglin (S-Eng), that is characterized by the retention of intron 14 in the mature mRNA causing a shortened C-terminal tail with a different amino acid sequence (Bellon et al. 1993). The expression of S-Eng is increased in senescent endothelial cells in which the splicing factor-2 (ASF/SF2) interferes with the process of splicing (Blanco and Bernabeu, 2011). In contrast to human and mouse, splicing of endoglin in rat results in a longer S-Eng protein product with a peptide insertion in the regular C-terminus. Moreover, the usage of alternative polyadenylation sites generates two mRNAs with different sizes of yet unknown function (Meurer et al. 2011).

Since it has been realized that endoglin mutations are causative for HHT1 (McAllister et al. 1994), a wealth of different ENG mutations and variants causing altered expression or formation of aberrant protein products have been identified, and the number is still growing. Actually, the ENG database that is hosted by the Department of Pathology at the University of Utah (http://arup.utah.edu/database/ENG/ENG_welcome.php) contains 500 entries of genetic variants (retrieved on December 1, 2016) that have different genotype-phenotype correlations with regard to clinical significance (benign, suspected benign, uncertain, suspected pathogenic, pathogenic). Nevertheless, mutations are not spread randomly in the genomic sequence, and mutations are preferentially found in the orphan domain and in the N-terminal part of the zona pellucida (ZP) domain in which three highly conserved cysteines (Cys363, Cys382, and Cys412) are exceptionally prone to mutations (Llorca et al. 2007).

Endoglin: A Type I Transmembrane Receptor

Endoglin is a type I transmembrane glycoprotein receptor that forms a homodimer of two subunits, each with an apparent molecular weight of 95 kDa that is stabilized by multiple disulfide bonds (Gougos and Letarte, 1988). Each monomer contains a signal peptide of 25 amino acids (aa), a large extracellular domain (ECD, 561 aa), a single transmembranal portion (25 aa), and a short intracellular region (47 aa). In human endoglin, the ECD majorly consists of an orphan domain (Glu26-Ile359) that has no homology to other protein domain fold motifs and a zona pellucida (ZP) domain (Gln360-Gly586) that can be further subdivided into a ZP-N (Gln360-Ser457) and a ZP-C (Pro458-Gly586) subdomain. The ZP domain is a general protein polymerization module of ∼260 aa, which is found at the C-terminus of many secreted eukaryotic glycoproteins and contains eight strictly conserved cysteines (Fig. 2) that are involved in disulfide bridge formation (Jovine et al. 2005). Although the functions of ZP domains vary tremendously, they are hydrophobic, often strongly glycosylated, involved in shedding to generate a soluble form, responsible for protein-protein interactions, and finally highly expressed in the corresponding tissues in which they occur (Jovine et al. 2005). Detailed deletion and substitution studies have shown that Cys582 in human FL-Eng is involved in intermolecular disulfide binding and that the six cysteines between Cys330 and Cys412 are necessary to mediate receptor dimerization (Guerrero-Esteo et al. 2002).
ENG, Fig. 2

Multiple protein sequence alignment of endoglin from different species. The primary sequences of bovine (BT, AAI14688), pig (SS, NP_999196), rat (RN, AAS67893), mouse (MM, NP_031958), human (HS, NP_001108225), and rabbit (OC, XP_008249251) endoglin proteins that are deposited in the GenBank (http://www.ncbi.nlm.nih.gov) were aligned using the Clustal Omega tool (www.ebi.ac.uk/Tools/msa/clustalo). In the alignment, conserved aa are marked by asterisk (*), positions that carry aa with strongly similar properties by a colon (:), and positions with weakly similar properties by a period (.), respectively. The Arginine-Glycin-Aspartic (RGD) acid sequence motif in human endoglin (aa 399-aa 401) is underlined. Please note that the highest degree of homology is found at the C-terminal regions encompassing the cytosolic part of endoglin, while endoglin from the different species share only approximately 60 –80% sequence identity in their extracellular domains. The different structural features of endoglin are marked in light blue (secretory signal peptide, 1–25 aa), grey (orphan domain, aa 26-aa 359), light green (ZP domain, aa 360-aa 586), orange (transmembrane region, aa 587- aa 613), and red (cytoplasmic domain, aa 614-aa 658). The 14 highly conserved cysteine residues and an additional cysteine at aa position 582 in human endoglin that is involved in dimerization are marked in blue font. Potential proteolytic cleavage sites and a confirmed MMP-14 recognition site at position 586 and 587 relative to human endoglin are marked in red font. Verified N-linked glycosylation sites within human endoglin are marked in green font

Among TGF-β-family receptors, endoglin and betaglycan constitute the TGF-β type III receptor family. Both receptors share a high degree of similarity, especially in their intracellular domain that is also the most conserved region between endoglin from different species (cf. Fig. 2), implying that this region has important functions (Jovine et al. 2005). Especially because both receptors lack enzymatic activities in their short C-terminal domains, they participate in TGF-β signaling (see below) via protein-protein interactions which are either direct or modulated by endoglin phosphorylation (see below). Compared to betaglycan, the signaling specificity of endoglin is majorly determined by its ECD (Letamendia et al. 1998).

Three-Dimensional Structure of Endoglin

By use of single-particle electron microscopy and atomic modeling, a first three-dimensional structure of the ∼140 kDa ECM of human endoglin was proposed (Llorca et al. 2007). These pioneering studies revealed that endoglin is structured into three distinct domains (Orphan domain, ZP-N, and ZP-C) with a defined structural arrangement (Fig. 3). Although the resolution of this model is rather low (25 Å), this model is a very suitable framework to understand how endoglin interacts with the TGF-β receptor system. The position and the proposed structure of the ZP domain would be compatible with the affinity of endoglin for the TGF-β receptor type I (TβRI, ALK5) and II (TβRII) that is a prerequisite for the clustering and signaling of the TGF-β receptor complex. In addition, the existence of a potential highly conserved protease cleavage site at position Arg437-Lys438-Lys439 (RKK) that is located within the linker between ZP-C and ZP-N is in agreement with the proposed bipartite structure of the ZP that usually contains protease-sensitive sites (Llorca et al. 2007). However, the biochemical elucidation using cleavage site mutants showed that the natural cleavage site is located at the end of the ZP-C domain at position Gly586-Leu587 and that endoglin shedding depends on direct interaction with MMP-14 (Hawinkels et al. 2010). Nevertheless, the information provided by the proposed model is also an important tool to understand the molecular impact of missense mutations that affect the disulfide bridges or other critical amino acids located at sensitive structural regions necessary to guarantee proper processing or three-dimensional fold. Thereby it turned out that mutations in the orphan domain tend to be retained in the endoplasmic reticulum whereas mutations in the ZP or the intracellular domain reach the plasma membrane (Ali et al. 2011).
ENG, Fig. 3

Proposed three-dimensional fold of the extracellular domain of the human endoglin monomer. Endoglin is a transmembrane type I receptor consisting out of an N-terminal signal peptide (aa 1- aa 25) that is cleaved upon synthesis, an orphan domain (aa 26- aa 359), a ZP domain (aa 360- aa 589), a single transmembranal portion (aa 590- aa 613), and a short intracellular region (aa 614- aa 658). The structure was predicted with Jmol, version 14.2.15 (http://www.jmol.org) using the structure coordinates that were generated by single-particle electron microscopy reconstruction (Llorca et al. 2007) and kindly provided by Dr. Carmelo Bernabeu (Centro de Investigaciones Biológicas, CSIC, Madrid, Spain)

Nevertheless, respective domains have important functional implications for the interaction with the signaling receptors. Recent experiments using the fluorescence recovery after photobleaching technique have underpinned the view that endoglin forms stable dimers that function as a scaffold for binding TβRII, ALK5, and ALK1 and that ALK5 bind to endoglin with differential dependence on TβRII playing a crucial role in recruiting ALK5 to the receptor complex (Pomeraniec et al. 2015).

Endoglin itself is not able to activate downstream signaling partners through phosphorylation. However, it modulates protein interaction and signaling complex assembly/composition via direct physical binding to signaling partners (Fig. 4). The different endoglin subdomains on one hand are able to interact with the TGF-β signaling receptors. Binding to the ECD of FL-endoglin is independent of the signaling receptor activity. Whereas physical interaction of the intracellular domain of endoglin with TβRII and the type I receptors (ALK5 and ALK1) is dependent on the activation state of the binding partners. In contrast, endoglin interacts with ALK5 and ALK1 only if the kinase domain is inactive, whereas TβRII remains associated in its active and inactive forms (Guerrero-Esteo et al. 2002). Furthermore, endoglin regulates eNOS activity by binding to eNOS, and the regulatory phosphatase PP2a (Xu et al. 2014) regulates the composition of focal adhesions and cytoskeletal organization by interaction with Zyxin/ZRP-1 (Sanz-Rodriguez et al. 2004). Moreover trafficking of endoglin is regulated by association with β-arrestin-2 and GIPC (Lee and Blobe, 2007; Lee et al. 2008).
ENG, Fig. 4

Functional aspects of the endoglin C-terminus. Depicted are the C-termini of human, mouse, and rat endoglin. The variant part of the splice forms is indicated in green. The primarily phosphorylated residues are shown in red. The PDZ domain is colored in dark blue bold font. Protein binding sites are marked with grey boxes. (upper part) Phosphorylation of the endoglin C-terminus. The numbering of the residues is done according to the human sequence. Putatively phosphorylated residues are highlighted above the sequence. Phosphorylated sites are shown in red circles below the effector kinase. Kinases are boxed in green and corresponding references are indicated above the sequence. (lower part) Protein binding to the endoglin C-terminus. Interacting proteins are boxed in green below the sequence. The functional relevance of interacting partners and references are indicated

Endoglin Phosphorylation and TGF-β Signaling

Phosphorylation of endoglin occurs primarily on serine residues and to a lesser extent on threonine residues, whereas tyrosine involvement is questionable (Lastres et al. 1994). This posttranslational modification is performed principally by TGF-β receptors (Koleva et al. 2006). FL-Eng is phosphorylated at Ser646/Ser649 by ALK5 (Ray et al. 2010), Thr640/Thr654 by ALK1 (Koleva et al. 2006), and Ser634/Ser635 by TβRII, respectively (Koleva et al. 2006). Functionally, FL-Eng itself inhibits autophosphorylation of TβRII but enhances phosphorylation of ALK5 by TβRII leading to a stronger Smad2 transcriptional activity (Guerrero-Esteo et al. 2002). Moreover, ALK1 phosphorylation and binding of endoglin was observed only in the presence of TGF-β1, and this phosphorylation leads to loss of FL-Eng from focal adhesions (Koleva et al. 2006). This functional association of endoglin and ALK1 potentiates TGF-β signaling via the ALK1 branch and significantly modulates proliferative and adhesive properties of endothelial cells (Blanco et al. 2005). On the other side, the transient overexpression of endoglin inhibits TGF-β/ALK5 signaling and enhances BMP-7/Smad1/Smad5 pathway (Scherner et al. 2007). In addition, phosphorylation of threonine Thr650 in the C-terminus of endoglin regulates the interaction with the scaffolding protein β-arrestin-2 (Lee and Blobe 2007). Moreover, the PDZ domain found at the C-terminal end of endoglin is not only a binding motive for GIPC (Lee et al. 2008) but also regulates the phosphorylation of serine residues in the C-terminus by the type I TGF-β receptors (Koleva et al. 2006). Recently it was shown that tyrosine phosphorylation of the membrane proximal intracellular 612Tyr-Ile-Tyr614-motive regulates endoglin trafficking. Direct phosphorylation of this motive by src leads to internalization of endoglin – and the interacting receptors ALK1/TβRII – and lysosomal degradation (Pan et al. 2014).

Endoglin in Health and Disease

Gene mutations that affect human endoglin function are inherited as autosomal dominant disorders and may cause AVMs in different organs, including brain, lung, and liver. These are highly characteristic in hereditary hemorrhagic telangiectasia (HHT, Osler-Weber-Rendu syndrome), and the formation of AVMs occurs in both small and large blood vessels. This leads to epistaxis, gastrointestinal bleeding, and microcytic anemia due to iron deficiency, along with characteristic mucocutaneous telangiectasia. AVMs are found in pulmonary, hepatic, and cerebral vascular tissue.

The diagnosis of HHT is often based on these clinical features, and genetic testing is not always needed for diagnosis. Typical features associated with HHT are summarized in consensus guidelines for HHT screening that are known as the “Curaçao criteria” and include information about epistaxis, family history, telangiectasias, and visceral lesions (Shovlin et al. 2000). These features arise in high frequency in HHT patients, usually by the age of 40 (Fig. 5), and HHT diagnosis is classified as definite if three of the “Curaçao criteria” are present.
ENG, Fig. 5

Estimated prevalence of vascular malformations and clinical features in HHT. The autosomal dominant genetic disorder HHT provokes changes in angiogenesis affecting the proper functionality of endothelium, smooth muscle cells, and pericytes. As a consequence, large (arteriovenous) and small vascular (telangiectasias) malformations arise. Large arteriovenous malformations (AVMs) in HHT1 most often are found in the liver, lung, and brain. Small vascular malformations and cosmetically displeasing skin lesions are found on the lips, tongue, hands, nose, and ears and are common in HHT. The frequency of each clinical feature is depicted. References for more details are indicated

Nevertheless, in cases of suspected patients without a clear HHT diagnosis and without mutations in the known HHT genes, additional biomarkers for the diagnosis of the disease are needed. Promising candidates are next-generation sequencing (NGS) and potential biomarkers detected in plasma and serum. These include proteins involved in vascular biology as well as regulatory microRNAs and long noncoding RNAs (reviewed by Botella et al. 2015). A new approach relies on IR-spectroscopy of peripheral blood plasma to analyze the “metabolic change pattern” which turned out to be significantly different between HHT patients and individuals from the control group (Lux et al. 2013).

Telangiectasias in the nose, causing characteristic nosebleeds, and on the skin of the hands, tongue, lips, and ears are the most typical clinical symptoms associated with HHT (Fig. 6). More critical is the formation of AVMs that can be life-threatening. Pulmonary AVMs that occur in 40% of people with HHT may predict the formation of small blood clots that cause stroke. Cerebral AVMs that are found in 5–20% of HHT patients might be highly variable in regard to size, structure, and location. Liver AVMs that are found in up to 75% of all patients may occasionally cause heart failure, especially when an HHT patient forms clots and large shunts forcing the heart to pump high volumes of extra blood through the low-resistance pathway of these hepatic AVMs. Nowadays, a large variety of medical imaging techniques have been developed with the ability to visualize the occurrence or severity of AVMs in internal organs of HHT patients (Olitsky 2010) providing the basis to estimate the necessity of surgical treatments (Fig. 7). To prevent the frequency of fatal clinical events such as stroke, high-output heart failure, pulmonary hypertension, and hemorrhage, the embolization of visceral AVMs is a valuable course of treatment.
ENG, Fig. 6

Representative telangiectasias found in HHT1 patients. (a) Multiple telangiectasias in the tongue and in the area of the upper lip. (b) Telangiectasias of hands are preferably found in the fingertips. (c) Blood crusts in the area of the external nose in a patient resulting from recent nosebleeds. (d) Endoscopic view through the right nasal cavity as assessed by argon plasma coagulation endoscopic procedure. Typically, multiple telangiectasias are found on the inferior turbinate, the septum, the nasal floor, and the middle turbinate. These are covered by a thin, respiratory epithelium that is very vulnerable and can easily become the cause of nosebleeds. (e) Telangiectasias in the outer ear region. (f) Contact endoscopy of the buccal mucosa showing telangiectasias appearing as a dilated vascular section between afferent and efferent vessels. All images are case courtesy of Dr. Urban Geisthoff (Department of Otorhinolaryngology, Essen University Hospital, Essen, Germany)

ENG, Fig. 7

Arteriovenous malformations found in HHT1 patients. In patients suffering from HHT1, AVMs that typically occur in the lungs, liver, and brain can be identified by different medical imaging techniques. Exemplarily, a computed tomography (CT) scan of the brain (A), a X-ray of the frontal lung (B), contrast-enhanced CT scans of the coronal (C) and axial (D) arterial phase of the lungs, and a digital subtraction angiography (DSA) of the liver (E) are depicted. The different imaging techniques show multiple AVMs (marked by white arrows) in the internal organs. (A) Case courtesy of Dr. Jamie McDonald (Department of Radiology, HHT Center Department of Pathology, University of Utah, Salt Lake City, USA). (B–E) Case courtesy of Prof. Frank Gaillard, Dr. Andrew Dixon, and Dr. Nasir Siddiqui (from http://radiopaedia.org) with rIDs 5621, 10,510, and 12,906)

Furthermore, symptomatic treatment with angiogenesis inhibitors or antihormonal agents that only affects the symptoms and not the cause are widely applied (Meurer et al. 2014). In some patients, the use of thalidomide, lenalidomide, and humanized anti-VEGF monoclonal antibody (bevacizumab) that possess antiangiogenic activities are suitable to reduce the incidence of nasal and gastrointestinal bleedings. Likewise, the β-receptor blocker propanolol used in prophylaxis of esophageal variceal bleeding in patients with liver cirrhosis is beneficial when locally administered in the nose mucosa to control epitaxis. Other studies showed that the estrogen receptor antagonist tamoxifen and the selective estrogen receptor modulator raloxifene can both reduce episodes of epistaxis and transfusion requirements in patients suffering from nasal vascular malformations. However, these therapies are limited by severe side effects and the need to administer respective drugs for long periods.

Endoglin in Liver Fibrosis and Hepatocellular Carcinoma

During liver fibrosis and cirrhosis, the excessive accumulation of extracellular matrix (ECM) proteins promotes hepatic scarring and eventually leads to organ failure. In all these processes, TGF-β is the most effective fibrogenic cytokine that induces fibrosis through multiple mechanisms, including direct activation of hepatic stellate cells (HSC), stimulation of ECM production, as well as prompting the synthesis of tissue inhibitors of matrix metalloproteinases (TIMPs) that prevent ECM degradation.

Endoglin inhibits the ALK5-Smad2/3 signaling branch and promotes ALK1-Smad1/5 signaling thereby promoting pro-fibrogenic activities of TGF-β (Meurer et al. 2011). In addition, endoglin expression is increased in activated HSC in vitro and in murine models of liver injury in vivo (Meurer et al. 2005). Since HSC are the major source for ECM production in liver fibrosis, the increase of endoglin is a critical factor that significantly contributes to the development of hepatic fibrosis. This could be underscored in myofibroblast-like HSC cell lines in which transient overexpression of endoglin leads to an increased expression of the activation marker α-SMA and the matricellular protein connective tissue growth factor (CTGF) (Meurer et al. 2011; Meurer et al. 2013). In addition, membrane-bound endoglin is expressed in a subset of newly formed microvessels in hepatocellular carcinoma (HCC) and has therefore been suggested as a complementary biomarker that may be used as a parameter to distinguish benign from malignant liver nodules (Segatelli et al. 2014), and to estimate angiogenesis in HCC (Yao et al. 2007). In addition, it was shown that the detection of soluble endoglin in the serum might be used as a complementary biomarker to assess the development of HCC in cirrhotic patients (Yagmur et al. 2007).

Experimental Models in Endoglin Research

As outlined above, endoglin deficiency in humans has a strong phenotype and is responsible for many diseases, such as HHT, preeclampsia liver fibrosis, and cancer. To experimentally address the biological functions in the pathogenesis of these diseases, several murine endoglin knockout models were developed (Fig. 8). Mice that are homozygous deficient for endoglin are embryonically lethal and die because of defects in vascular development. To overcome the problems of embryonic lethality, Allinson and colleagues generated a mouse in which the endoglin gene is flanked by loxP sites at exons 5 and 6 (Allinson et al. 2007). These mice show a normal phenotype and were taken in several independent studies to disrupt endoglin expression in different cell types including vascular smooth muscle cells, endothelial cells, LysM positive macrophages, and neural crest- and somite-derived Pax3-positive vascular precursor cells by use of the Cre-loxP genetic recombination system (Tual-Chalot et al. 2015).
ENG, Fig. 8

Experimental models in HHT research. Three different endoglin knockout models are available in which different parts of the endoglin gene (ΔEx9-Ex11, ΔEx1-Ex2, or Δ609bp promoter/Ex1) are deleted. In all of these models, the homozygous inactivation causes embryonic lethality at 10–11.5 days post coitum because of defects in vascular development. Mice in which Ex5 and Ex6 of the Eng gene were floxed with lox P sites show a normal phenotype. Using the Cre-lox recombination technology, these mice were taken to specifically disrupt the endoglin gene in vascular smooth muscle cells, endothelial cells, LysM positive macrophages, and neural crest- and somite-derived Pax3-positive vascular precursor cells. In addition, this mouse strain was used to establish a model in which endoglin can be disrupted in all cell types upon tamoxifen treatment. Heterozygous mice carrying one copy of the Δ609bp promoter/Ex1 allele were used to isolate TGF-β modifier genes and to characterize endoglin function in embryonic endothelial cell permeability. Other suitable experimental tools in endoglin research are the targeted suppression of endoglin by siRNA and the overexpression of endoglin by retroviral transduction. References for more details are given

Primary cultures of endothelial cells that are generated from heterozygous mice carrying only one functional Eng allele are another experimental tool that is suitable to investigate biological functions of endoglin in vascular pathology. Similarly, other studies have shown that the application of siRNA targeting endoglin expression in human and murine endothelial cells is suitable to reduce the levels of endoglin mRNA and protein and to develop a good antiangiogenic therapeutic potential in vivo.

There is also the possibility to transiently or stably overexpress the different splice variants (i.e., S- and L-Eng) in vitro and in vivo (Lastres et al. 1994). In a pioneering study this approach was used to demonstrate that S-Eng has antiangiogenic properties in cancer development (Pérez-Gómez et al. 2005).

Summary

Endoglin, also known as cluster of differentiation CD105, was originally identified in 1988 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 an auxiliary TGF-β receptor. In humans, a large variety of mutations and variants of the endoglin gene are identified that give rise to an autosomal dominant bleeding disorder that is characterized by localized angiodysplasia and AVMs. This disease, termed hereditary hemorrhagic telangiectasia type I, induces various vascular lesions, mainly on the face, lips, hands, and gastrointestinal mucosa. Mice lacking functional Eng gene show embryonic lethality at day 10–11.5 post coitum due to major defects in angiogenesis and heart development. For experimental endoglin research, mice that are heterozygous for mutations or conditional knockout mice that allow cell-specific inactivation of the Eng gene at different times during development have become attractive tools in endoglin research. 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 (sol-Eng) is formed by shedding mediated by MMP-14 that cleaves within the extracellular juxtamembrane region, which is competent to modulate signaling and bears potential as a complementary biomarker in some pathological settings. Endoglin interacts with the TGF-β signaling receptors and influences Smad-dependent and Smad-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.

See Also

Notes

Acknowledgments

RW is supported by grants from the German Research Foundation (SFB/TRR57, P13/Q3) and a grant from the Interdisciplinary Centre for Clinical Research within the Faculty of Medicine at the RWTH Aachen University (IZKF Aachen, Project E7-6). The authors are grateful to Sabine Weiskirchen for preparing line drawings, Dr. J. MacDonald (Department of Radiology, Hereditary Hemorrhagic Telangiectasia Center, University of Utah, Salt Lake City, UT, USA) and Dr. U. Geisthoff (Department of Otorhinolaryngology, Essen University Hospital, Essen, Germany) for providing photographs. In addition, the authors would like to thank the colleagues Prof. Frank Gaillard, Dr. Andrew Dixon, and Dr. Nasir Siddiqui that deposited radiology images in the Radiopaedia.org resource and allowed us to use them in our work.

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute of Molecular Pathobiochemistry, Experimental Gene Therapy and Clinical ChemistryRWTH University Hospital AachenAachenGermany