Soluble Endoglin Specifically Binds Bone Morphogenetic Proteins 9 and 10 via Its Orphan Domain, Inhibits Blood Vessel Formation, and Suppresses Tumor Growth

Abstract

Endoglin (CD105), a transmembrane protein of the transforming growth factor ␤ superfamily, plays a crucial role in angiogenesis. Mutations in endoglin result in the vascular defect known as hereditary hemorrhagic telangiectasia (HHT1). The soluble form of endoglin was suggested to contribute to the pathogenesis of preeclampsia. To obtain further insight into its function, we cloned, expressed, purified, and characterized the extracellular domain (ECD) of mouse and human endoglin fused to an immunoglobulin Fc domain. We found that mouse and human endoglin ECD-Fc bound directly, specifically, and with high affinity to bone morphogenetic proteins 9 and 10 (BMP9 and BMP10) in surface plasmon resonance (Biacore) and cell-based assays. We performed a function mapping analysis of the different domains of endoglin by examining their contributions to the selectivity and biological activity of the protein. Endoglin (CD105) is a homodimeric glycosylated cell-surface protein of 180 kDa previously identified as a co-receptor belonging to the TGF␤ superfamily (1). Several lines of evidence support an important role of endoglin in cardiovascular development and vascular remodeling (2). Loss-of-function mutations in endoglin are implicated in the vascular disorder hereditary hemorrhagic telangiectasia type 1 (HHT1), 3 which is a bleeding disorder characterized by arteriovenous malformations in the brain, lungs, and liver and is attributed to haploinsufficiency (3-5). Homozygous endoglin knock-out mice die during early gestation due to the lack of development of normal mature blood vessels (6). Adult endoglin heterozygous mice (7-8) or mice with a conditional mutation in the endoglin gene (9) exhibited similar angiogenic abnormalities and were used as animal models for HHT1. Mutations in the endoglin gene found in numerous HHT1 patients are localized most exclusively in the extracellular domain (4). The specific role of endoglin in the vascular dysplasia observed in HHT patients is not known, but it is likely to be related to the role of TGF␤ family signaling in angiogenesis (2, 10). Interestingly, another form of HHT, known as HHT2, which is also characterized by the presence of telangiectases as well as arteriovenous malformations in brain, lungs and liver, results from the loss of TGF␤ type I receptor ALK1 (11), which suggests an interrelatedness between endoglin and ALK1 and possibly involvement of the same ligand(s) in the mechanism of action of both molecules.A soluble form of endoglin has been observed in the serum of patients with different types of solid malignancies (12) and of pregnant women suffering from preeclampsia, a disease leading to vascular permeability (13), hypertension, and placental abruption (14). This soluble form, which reportedly results from partial shedding of the membrane-bound form of endoglin by the matrix metalloproteinase 14 (MT1-MMP) (15), a phenomenon also observed for the other type III receptor betaglycan (16), has been proposed to act as a scavenger or trap for circulating TGF␤ family liga...

Safety, Pharmacokinetics, Pharmacodynamics, and Antitumor Activity of Dalantercept, an Activin Receptor–like Kinase-1 Ligand Trap, in Patients with Advanced Cancer

“…Also, in clinical development as an antitumor agent is a chimeric antibody (TRACON) capable of neutralizing endoglin (CD105; ref. 31), a proangiogenic protein in the TGFb superfamily that binds BMP9/BMP10 (22,45,46), with loss-of-function mutations resulting in another type of vascular dysplasia, HHT-1 (15,47). In patients with advanced, refractory tumors (31), anti-endoglin antibody was associated with infusion reactions and immunogenicity distinct from effects described for dalantercept in the present study, whereas other adverse effects such as anemia and telangiectasias were observed in both studies.…”

Anti-human Activin Receptor-like Kinase 1 (ALK1) Antibody Attenuates Bone Morphogenetic Protein 9 (BMP9)-induced ALK1 Signaling and Interferes with Endothelial Cell Sprouting

“…Because ALK1 is known to bind the co-receptor endoglin upon ligand binding (15,16), we subsequently examined the effect of anti-hALK1 antibody on ALK1-endoglin complex formation. In HEK293T cells, human ALK1-HA and human endoglin-FLAG were co-transfected (Fig.…”

“…ALK1 and endoglin have been shown to engage in a complex, although whether this is ligand-dependent or -independent is debated (15,16).…”

Structure of bone morphogenetic protein 9 procomplex

“…Although a member of the BMP subfamily and possessing chondrogenic and osteogenic activity, BMP9 is expressed in liver and is required for properly organized blood and lymphatic vascular development (11,12). Mutations in the prodomain of BMP9, in its receptor Alk1, and in its coreceptor endoglin cause phenotypically overlapping hereditary hemorrhagic telangiectasias (13)(14)(15).…”