Structural and Functional Insights into Endoglin Ligand Recognition and Binding

Endoglin, a type I membrane glycoprotein expressed as a disulfide-linked homodimer on human vascular endothelial cells, is a component of the transforming growth factor (TGF)-β receptor complex and is implicated in a dominant vascular dysplasia known as hereditary hemorrhagic telangiectasia as well as in preeclampsia. It interacts with the type I TGF-β signaling receptor activin receptor-like kinase (ALK)1 and modulates cellular responses to Bone Morphogenetic Protein (BMP)-9 and BMP-10. Structurally, besides carrying a zona pellucida (ZP) domain, endoglin contains at its N-terminal extracellular region a domain of unknown function and without homology to any other known protein, therefore called the orphan domain (OD). In this study, we have determined the recognition and binding ability of full length ALK1, endoglin and constructs encompassing the OD to BMP-9 using combined methods, consisting of surface plasmon resonance and cellular assays. ALK1 and endoglin ectodomains bind, independently of their glycosylation state and without cooperativity, to different sites of BMP-9. The OD comprising residues 22 to 337 was identified among the present constructs as the minimal active endoglin domain needed for partner recognition. These studies also pinpointed to Cys350 as being responsible for the dimerization of endoglin. In contrast to the complete endoglin ectodomain, the OD is a monomer and its small angle X-ray scattering characterization revealed a compact conformation in solution into which a de novo model was fitted

Mice lacking endoglin in macrophages show an impaired immune response

24 p.-9 fig.-1 tab. Ojeda Fernández, Luisa et al.Endoglin is an auxiliary receptor for members of the TGF-β superfamily and plays an important role in the homeostasis of the vessel wall. Mutations in endoglin gene (ENG) or in the closely related TGF-β receptor type I ACVRL1/ALK1 are responsible for a rare dominant vascular dysplasia, the Hereditary Hemorrhagic Telangiectasia (HHT), or Rendu-OslerWeber syndrome. Endoglin is also expressed in human macrophages, but its role in macrophage function remains unknown. In this work, we show that endoglin expression is triggered during the monocyte-macrophage differentiation process, both in vitro and during the in vivo differentiation of blood monocytes recruited to foci of inflammation in wild-type C57BL/6 mice. To analyze the role of endoglin in macrophages in vivo, an endoglin myeloid lineage specific knock-out mouse line (Engfl/flLysMCre) was generated. These mice show a predisposition to develop spontaneous infections by opportunistic bacteria. Engfl/flLysMCre mice also display increased survival following LPS-induced peritonitis, suggesting a delayed immune response. Phagocytic activity is impaired in peritoneal macrophages, altering one of the main functions of macrophages which contributes to the initiation of the immune response. We also observed altered expression of TGF-β1 target genes in endoglin deficient peritoneal macrophages. Overall, the altered immune activity of endoglin deficient macrophages could help to explain the higher rate of infectious diseases seen in HHT1 patients.This work was funded by: Ministerio de Economía y Competitividad of Spain (SAF2011-23475 to LMB; SAF2013-43421-R and SAF2010- 19222 to CB.Peer reviewe

Endoglin in liver fibrosis

Liver fibrosis occurs in most types of chronic liver diseases and is characterized by excessive accumulation of extracellular matrix proteins, leading to disruption of tissue function and eventually organ failure. Transforming growth factor (TGF)-β represents an important pro-fibrogenic factor and aberrant TGF-β action has been implicated in many disease processes of the liver. Endoglin is a TGF-β co-receptor expressed mainly in endothelial cells that has been shown to differentially regulates TGF-β signal transduction by inhibiting ALK5-Smad2/3 signalling and augmenting ALK1-Smad1/5 signalling. Recent reports demonstrating upregulation of endoglin expression in pro-fibrogenic cell types such as scleroderma fibroblasts and hepatic stellate cells have led to studies exploring the potential involvement of this TGF-β co-receptor in organ fibrosis. A recent article by Meurer and colleagues now shows that endoglin expression is increased in transdifferentiating hepatic stellate cells in vitro and in two different models (carbon tetrachloride intoxication and bile duct ligation) of liver fibrosis in vivo. Moreover, they show that endoglin overexpression in hepatic stellate cells is associated with enhanced TGF-β-driven Smad1/5 phosphorylation and α-smooth muscle actin production without altering Smad2/3 signaling. These findings suggest that endoglin may play an important role in hepatic fibrosis by altering the balance of TGF-β signaling via the ALK1-Smad1/5 and ALK-Smad2/3 pathways and raise the possibility that targeting endoglin expression in transdifferentiating hepatic stellate cells may represent a novel therapeutic strategy for the treatment of liver fibrosis

Endothelial endoglin is involved in inflammation: role in leukocyte adhesion and transmigration

Human endoglin is an RGD-containing transmembrane glycoprotein identified in vascular endothelial cells. Although endoglin is essential for angiogenesis and its expression is up-regulated in inflammation and at sites of leukocyte extravasation, its role in leukocyte trafficking is unknown. This function was tested in endoglin heterozygous mice (Eng+/-) and their wild-type siblings Eng+/+ treated with carrageenan or LPS as inflammatory agents. Both stimuli showed that inflammation-induced leukocyte transendothelial migration to peritoneum or lungs was significantly lower in Eng+/- than in Eng+/+ mice. Leukocyte transmigration through cell monolayers of endoglin transfectants was clearly enhanced in the presence of endoglin. Coating transwells with the RGD-containing extracellular domain of endoglin, enhanced leukocyte transmigration, and this increased motility was inhibited by soluble endoglin. Leukocytes stimulated with CXCL12, a chemokine involved in inflammation, strongly adhered to endoglincoated plates and to endoglin-expressing endothelial cells. This endoglin-dependent adhesion was abolished by soluble endoglin, RGD peptides, the anti-integrin α5β1 inhibitory antibody LIA1/2 and the chemokine receptor inhibitor AMD3100. These results demonstrate for the first time that endothelial endoglin interacts with leukocyte integrin α5β1 via its RGD motif, and this adhesion process is stimulated by the inflammatory chemokine CXCL12, suggesting a regulatory role for endoglin in transendothelial leukocyte trafficking. © 2013 by The American Society of Hematology.Ministerio de Economia y Competitividad of Spain (SAF2010-61 827, SAF2010-15 881); Centro de Investigacion Biomedica en Red de Enfermedades Raras (CIBERER); Red de Investigacion Cooperativa en Enfermedades Renales (REDINDEN)Peer Reviewe

Endoglin expression in breast tumor cells suppresses invasion and metastasis and correlates with improved clinical outcome

Tumor growth factor-beta (TGF-beta) signaling in cancer has been implicated in growth suppression of early lesions and enhancing tumor cell invasion and metastasis. However, the cellular mechanisms that determine this signaling output in individual tumors are still largely unknown. In endothelial cells, TGF-beta signaling is modulated by the TGF-beta co-receptor endoglin (CD105). Here we demonstrate that endoglin is expressed in a subset of invasive breast cancers and cell lines and is subject to epigenetic silencing by gene methylation. Endoglin downregulation in non-tumorigenic MCF10A breast cells leads to the formation of abnormal acini in 3D culture, but does not promote cell migration or transformation. In contrast, in the presence of activated ErbB2, endoglin downregulation in MCF10A cells leads to enhanced invasion into a 3D matrix. Consistent with these data, ectopic expression of endoglin in MDA-MB-231 cells blocks TGF-beta-enhanced cell motility and invasion and reduces lung colonization in an in vivo metastasis model. Unlike endothelial cells, endoglin does not modulate Smad-mediated TGF-beta signaling in breast cells but attenuates the cytoskeletal remodeling to impair cell migration and invasion. Importantly, in a large cohort of invasive breast cancers, lack of endoglin expression in the tumor cell compartment correlates with ENG gene methylation and poor clinical outcome. Oncogene (2011) 30, 1046-1058; doi:10.1038/onc.2010.488; published online 1 November 201