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

Abstract

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.

Keywords: ECFCs; EPC; HHT1; TGF-β; endoglin; endothelial progenitors; integrins.

Figure 1

Hypothetical model of Endoglin and its role as TGF-β co-receptor and adhesion molecule in different physiological contexts. (A) Canonical pathway that implicates Eng as a co-receptor of TGF-β in ECs. Bone morphogenetic protein 9 (BMP9), and other members of the TGF-β family, bind to an EC receptor complex composed by the type I (R-I) receptor named ALK1 and the type II (R-II) receptor, both exhibiting serine/threonine kinase activity, as well as the auxiliary receptor endoglin. Upon ligand binding, the R-II transphosphorylates ALK1, which subsequently propagates the signal by phosphorylating the receptor-regulated Smad (R-Smad) family of proteins Smad1/5/8. Phosphorylated R-Smads form heteromeric complexes with Smad4, translocating into the nucleus to regulate the transcriptional activity of different target genes. BMP9, Endoglin, ALK1 and Smad4 proteins are encoded by GDF2, ENG, ACVRL1 and MADH4 genes, whose pathogenic mutations give rise to HHT5, HHT1, HHT2, and JPHT, respectively (30). (B) Endothelial endoglin as an adhesion molecule involving its RGD sequence when binding to cell surface integrins from leukocytes, VMCs or platelets. (C) Leukocyte transmigration through the vessel endothelium. Under inflammatory conditions, different soluble factors are released leading to leukocyte adhesion and transmigration through ECs. This process is mediated, at least, by the interaction between leukocyte integrin α5β1 and endothelial endoglin involving the specific recognition of the RGD motif in Eng (22). (D) Adhesion between endothelial cells and vascular mural cells (VMCs). This intercellular association involves, at least, the interaction between integrin α5β1 in VMCs and endothelial endoglin via the specific recognition of the RGD motif in Eng (23). (E) Platelet-dependent hemostasis. Endothelial endoglin via its RGD binds to platelet integrins αIIbβ3 and α5β1, contributing to stabilize platelets adhesion to endothelium (24).

Figure 2

Endoglin in ECFCs. (A) Role of endoglin matrigel vascularization in vivo. Matrigel plugs were mixed with either ECFCs treated with control siRNA plus mesenchymal stem cells (MSCs) (left) or with ECFCs treated with endoglin specific siRNA plus MSCs (right). The Matrigel mixture was injected into nude mice and the number of vessels was analyzed after 1 week. Control plugs display a marked vascularization with functional vessels (presence of erythrocytes). Plugs with endoglin-silenced ECFCs display less vascularization than controls, suggesting an important role for this protein in cell adhesion (23). (B) MSCs combined with ECFCs, accelerate muscle recovery in a mouse model of hind limb ischemia, through an endoglin-dependent mechanism. After femoral ligature and retro-orbital injection of ECFC+MSC Doppler analysis shows a revascularization induced by co-injection of ECFCs plus MSCs (left). This synergistic effect is abolished when endoglin is previously silenced in ECFCs (right) (61). (C) Mechanisms involved in the synergy between ECFCs and MSCs. (i) Mutual paracrine effect between ECFCs and MSCs involving growth factors like VEGF and PDGFBB (76, 77). (ii) ECFCs-induced differentiation of MSCs into perivascular cells via Notch-Jagged1 (72) (iii) Adhesion between ECFCs and MSCs involving endoglin (61).