Abstract
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.
ENDOGLIN INVOLVEMENT IN HEREDITARY HEMORRHAGIC TELANGIECTASIA
Hereditary hemorrhagic telangiectasia (HHT) or Rendu-Osler-Weber syndrome is a vascular hereditary autosomal dominant disease associated with epistaxis, telangiectasias, gastrointestinal hemorrhages, and arteriovenous malformations in lung, liver, and brain (; ). The first gene identified as being involved in HHT was ENDOGLIN (ENG) that maps to chromosome 9 (; ), representing between 39 and 59% of the total HHT population, a frequency that depends on the geographic location. Several 100 different pathogenic mutations have been described in ENG giving rise to the HHT1 subtype (). Haploinsufficiency is accepted as the underlying mechanism of HHT1 pathogenicity, but the fine cellular and molecular machinery involved remains to be elucidated.
ENDOGLIN STRUCTURE, EXPRESSION, AND FUNCTION
Human endoglin is a type I integral membrane protein with a large extracellular (EC) domain (561 amino acids), a single hydrophobic transmembrane (TM) domain, and a short cytosolic tail (Figure 1A). Endoglin is a glycosylated protein expressed as a 180-kDa disulfide-linked homodimer in endothelial cells (). The predominant expression of endoglin in endothelial cells suggests that these are the target cells in HHT, where endoglin haploinsufficiency reveals its pathogenicity. Structurally, endoglin belongs to the zona pellucida (ZP) family of proteins that share a juxtamembrane ZP domain of 260 amino acid residues in their EC region (). This ZP domain encodes an RGD tripeptide that is a prototypic member of a family of motifs involved in integrin-based interactions with EC matrix and certain cell surface proteins (Takada et al., 2007). Recently, it has been demonstrated that endothelial endoglin interacts with leukocyte integrin α5β1 via its RGD motif, suggesting a regulatory role for endoglin in transendothelial leukocyte trafficking (). The RGD motif is present in endoglin from primates, while functionally related RGD-like motifs are found in endoglin from rodents and other mammals, stressing the physiological importance and high conservation of this motif (; ). Next to the ZP domain is the orphan domain, which is located in the NH2-terminus of the protein (). The orphan domain is involved in the binding to members of the TGF-β superfamily (activin, TGF-β, and BMP families) in accordance with the role of endoglin as an auxiliary TGF-β receptor (). Thus, endoglin modulates the endothelial response to TGF-β-related ligands, including cell proliferation, apoptosis, vascular remodeling, angiogenesis, wound healing, or fibrosis (). Of note, most of ENG mutations associated with HHT1 map to the EC domain, suggesting that the biological function of this domain is critical for the disease (). The cytosolic domain of endoglin is phosphorylated at Ser/Thr/Tyr residues and it can be targeted by serine/threonine () and tyrosine () kinases. It has been shown that the endoglin phosphorylation status can influence its subcellular localization () and degradation (). The cytoplasmic domain of endoglin also regulates actin cytoskeletal organization, microtubule-based transport machinery, and cell migration (; ), through its specific binding to different cytosolic proteins ().
FIGURE 1
ENDOGLIN IN INTEGRIN-MEDIATED SIGNALING AND CELLULAR FUNCTION
Integrins are ubiquitous cell surface receptors involved in cell–cell and cell–EC matrix interactions (Takada et al., 2007) that play a relevant role in vascular biology (). The functional role of endothelial endoglin as a counter-receptor for leukocytes’ integrins has been recently reported (). Interestingly, inflammatory leukocyte recruitment, which involves integrin-mediated cell–cell interactions, is critical for proper vascular remodeling. Several animal models of inflammation and vascular repair have shown that endoglin haploinsufficiency leads to an abnormal leukocyte infiltration and function that depends on the target organ or the triggering stimulus (van Laake et al., 2006; ; ; ; ; ). Thus, an inflammation model of dextran sodium sulfate (DSS)-induced chronic colitis revealed more leukocyte infiltration in the gut and a more severe colitis phenotype in endoglin heterozygous (Eng+/-) mice relative to control animals (; ; ). After myocardial infarction, a greater deterioration in cardiac function was observed in Eng+/- compared to control mice, although host inflammatory leukocyte numbers in the infarct zone were similar; however, defects in vessel formation and heart function in Eng+/- mice were rescued by injection of leukocytes from healthy human donors, but not by leukocytes from HHT1 patients (van Laake et al., 2006). Moreover, upon permanent distal middle cerebral artery occlusion, Eng+/- mice showed larger infarct/atrophic volumes associated with fewer infiltrating macrophages, suggesting that endoglin deficiency impairs brain injury recovery by impairing macrophage homing, delaying inflammation resolution, and reducing angiogenesis (). Furthermore, decreased inflammation-induced leukocyte trafficking to peritoneum and lungs was found in Eng+/- mice treated with the inflammatory agents carrageenan or lipopolysaccharide (LPS), respectively (). The impaired leukocyte trafficking was observed for neutrophils, lymphocytes, monocytes, and basophiles. The nature of this migratory process was assessed using in vitro cell adhesion and transmigration studies. Leukocytes treated with the chemokine CXCL12, an integrin activator, strongly adhered to endoglin-coated plates and to endoglin-expressing endothelial cells. Similarly, leukocyte transmigration through adherent cell monolayers was dependent on the presence of endoglin. Both, endoglin-dependent cellular adhesion and transmigration processes involved the leukocyte integrin α5β1 via the endoglin RGD motif (). Based on these results, a hypothetical model is illustrated in Figure 1B.
Additional lines of evidence support a tight link between integrins and endoglin functions: (i) on overexpression of endoglin, fibroblasts tend to form clusters that involve α5β1 integrin and are inhibited by coating the culture plates with an RGD-containing fragment of fibronectin (); (ii) analysis of integrin-rich sites of focal adhesion, isolated with RGD-labeled microspheres, shows that endoglin regulates cell migration and focal adhesion composition via interaction through its cytoplasmic domain with the protein zyxin present in focal adhesion sites (); (iii) binding of human pathogenic bacteria to the carcinoembryonic antigen (CEA) in epithelial cells, triggers de novo expression of endoglin, which, in turn changes focal adhesion composition, activating β1 integrins and inducing a dramatic increase in the ECM-binding capacity of the cells (); (iv) endoglin mediates fibronectin/α5β1 integrin and TGF-β pathway crosstalk in endothelial cells via the internalization of the cell surface α5β1 integrin/endoglin complex (Tian et al., 2012); and (v) protein and gene expression analysis have shown that ectopic endoglin represses the synthesis of several members of the integrin family and modulates integrin-related biological functions, including cell adhesion and transmigration (; ). Moreover, endoglin is able to antagonize the integrins’ downstream MAPK pathway, by inhibiting ERK signaling and altering the subcellular distribution of activated ERK (; ). Taken together, these data strongly suggest a close physical and functional association between endoglin and integrins and prompt the question of whether the adhesion role of endothelial endoglin, as a counter-receptor for leukocytes’ integrins, is involved in the HHT pathogenesis.
IS THE ROLE OF ENDOGLIN IN CELL–CELL ADHESION INVOLVED IN HHT PATHOGENESIS?
A common feature in HHT patients is the presence of vascular lesions (telangiectasias and arteriovenous malformations) that lead to a loss of the intervening capillary network that connects the arteriole with the venule (). However, it remains unclear why the vascular lesions appear only at distinct sites within certain organs, rather than being present throughout the body and in all organs/tissues. To explain this paradox, the need for a trigger such as inflammation, vascular injury, angiogenic stimuli, or ischemia has been postulated (; ; ). In HHT1, this trigger would synergize with endoglin haploinsufficiency to generate the vascular lesion (; ). Interestingly, those stimuli are usually associated with the upregulated expression of endoglin in endothelial cells and an inflammatory cell infiltrate (Torsney et al., 2002; ), suggesting the need of both, endoglin function and leukocyte infiltration, in the vascular repair/remodeling process. Indeed, it has been reported that endoglin plays a crucial role in leukocyte-mediated vascular repair (van Laake et al., 2006). Of note, HHT skin telangiectasias show a perivascular mononuclear cell infiltrate, including lymphocytes and monocytes/macrophages (). This is in agreement with the leukocyte infiltration occurring during angiogenesis and vascular remodeling (). Therefore, it can be hypothesized that the function of endothelial endoglin as an adhesion counter-receptor for leukocyte’s integrins is involved in HHT1 pathogenesis (Figure 2). According to this hypothetical model, in healthy subjects the capillary network subjected to an inflammatory stimulus is infiltrated with leukocytes that contribute to the vascular repair/remodeling. By contrast, in HHT1 patients endoglin haploinsufficiency impairs leukocyte infiltration, leading to a defective vascular repair/remodeling. As a consequence, the capillary network would disappear and only a preferential vessel remains that eventually becomes the arterio-venous shunt.
FIGURE 2
CONCLUSION AND PERSPECTIVES
A novel role for endothelial endoglin as a cellular adhesion receptor has been reported. Here, we postulate a model attempting to relate the cell adhesion function of endoglin to the pathobiology of HHT. In the HHT1 setting, endoglin protein levels may not reach the minimum threshold to achieve the optimal function in leukocyte adhesion mediated by integrins. This endoglin haploinsufficiency alters leukocyte transmigration and may synergize with a trigger such as inflammation to generate the vascular lesion. The function of endoglin as an adhesion counter-receptor for integrins may help to understand the pathogenicity in HHT and may be a potential therapeutic target in HHT. Modulators of endoglin/integrin cell adhesion are putative drugs to be tested. In addition to the endothelial cell–leukocyte adhesion, it would be of interest to investigate whether the adhesion of endothelial cells with mural cells also involves the endoglin/integrin interaction.
Statements
Acknowledgments
This study was supported by grants from the Ministerio de Economia y Competitividad of Spain (SAF2010-19222 and SAF2013-43421-R to Carmelo Bernabeu; SAF2010-1588, SAF2013-45784-R, and RD12/0021/0032-REDINREN to José M. Lopez-Novoa). The Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER) and REDINREN are initiatives of the Instituto de Salud Carlos III (ISCIII) of Spain supported by FEDER funds.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
HHT, endoglin, integrins, cell adhesion, endothelial cells, inflammation, RGD
Citation
Rossi E, Lopez-Novoa JM and Bernabeu C (2015) Endoglin involvement in integrin-mediated cell adhesion as a putative pathogenic mechanism in hereditary hemorrhagic telangiectasia type 1 (HHT1). Front. Genet. 5:457. doi: 10.3389/fgene.2014.00457
Received
15 October 2014
Accepted
12 December 2014
Published
07 January 2015
Volume
5 - 2014
Edited by
Michelle Letarte, University of Toronto and Hospital for Sick Children, Canada
Reviewed by
Christina Stuelten, National Cancer Institute, USA; Theodora Katsila, University of Patras, Greece
Copyright
© 2015 Rossi, Lopez-Novoa and Bernabeu.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Carmelo Bernabeu, Centro de Investigaciones Biológicas – Consejo Superior de Investigaciones Científicas and Centro de Investigación Biomédica en Red de Enfermedades Raras, Calle Ramiro de Maeztu 9, Madrid 28040, Spain e-mail: bernabeu.c@cib.csic.es
This article was submitted to Genetic Disorders, a section of the journal Frontiers in Genetics.
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