Endoglin Protein Interactome Profiling Identifies TRIM21 and Galectin-3 as New Binding Partners

Abstract

Endoglin is a 180-kDa glycoprotein receptor primarily expressed by the vascular endothelium and involved in cardiovascular disease and cancer. Heterozygous mutations in the endoglin gene (ENG) cause hereditary hemorrhagic telangiectasia type 1, a vascular disease that presents with nasal and gastrointestinal bleeding, skin and mucosa telangiectases, and arteriovenous malformations in internal organs. A circulating form of endoglin (alias soluble endoglin, sEng), proteolytically released from the membrane-bound protein, has been observed in several inflammation-related pathological conditions and appears to contribute to endothelial dysfunction and cancer development through unknown mechanisms. Membrane-bound endoglin is an auxiliary component of the TGF-β receptor complex and the extracellular region of endoglin has been shown to interact with types I and II TGF-β receptors, as well as with BMP9 and BMP10 ligands, both members of the TGF-β family. To search for novel protein interactors, we screened a microarray containing over 9000 unique human proteins using recombinant sEng as bait. We find that sEng binds with high affinity, at least, to 22 new proteins. Among these, we validated the interaction of endoglin with galectin-3, a secreted member of the lectin family with capacity to bind membrane glycoproteins, and with tripartite motif-containing protein 21 (TRIM21), an E3 ubiquitin-protein ligase. Using human endothelial cells and Chinese hamster ovary cells, we showed that endoglin co-immunoprecipitates and co-localizes with galectin-3 or TRIM21. These results open new research avenues on endoglin function and regulation.

Potential Role of Circulating Endoglin in Hypertension via the Upregulated Expression of BMP4

“…In this line, endoglin deficiency in HHT1 endothelial cells, or overexpression in human cells, can regulate the expression of a wide range of target genes at the transcript level [65][66][67]. Also, a number of potential novel interactors of sEng located in the nucleus have been recently identified [68]. Furthermore, sEng bound to BMP9 is able to intracellularly signal via membrane-bound endoglin in endothelial cells, rather than being an inhibitory ligand trap [24].…”

Endoglin: Beyond the Endothelium

“…Sol-eng was originally described to inhibit angiogenesis by acting as a ligand trap for the endoglin ligand TGF-β [31] or, as has been more commonly reported, BMP-9 [32,33]. Interestingly, recent data indicate that in addition to being an inhibitory ligand trap, increased circulating monomeric sol-eng might stimulate BMP-9 signaling via binding to endothelial endoglin.…”

Potential Second-Hits in Hereditary Hemorrhagic Telangiectasia

“…This is in agreement with the fragile mucocutaneous telangiectases that easily break, leading to the frequent nose and gastrointestinal bleedings present in HHT patients. An active search for novel endoglin-specific interactors has allowed the identification of multiple proteins, suggesting the involvement of endoglin in ALK1-independent pathways; nonetheless, the functional characterization and relevance of the novel interactors to the HHT1 field remains to be established [39,43]. Overall, it can be postulated that the phenotypic differences between HHT1 and HHT2 could arise, at least in part, from a subthreshold endoglin participation in these ALK1-independent signaling pathways.…”