Endoglin is an accessory protein of multiple signalling receptor complexes of the TGF-ß superfamily, implications on the development of hereditary hemorrhagic telangiectasia type 1 (HHT1)

grantor: University of Toronto'Endoglin' (CD105) is the gene mutated in the autosomal dominant vascular disorder HHT1 and codes for a major integral membrane glycoprotein expressed on all vasculature. In endothelial cells, activated monocytes, and leukemic cells, endoglin associates with TGF-ß1, -ß3, and the heteromeric TGF-ß signalling receptor complex, consisting of type I and II ser/thr kinases. This thesis investigates the biochemical function of endoglin by determining the molecular mechanism underlying HHT1, and by studying the nature of endoglin interaction with TGF-ß receptors. Endoglin expression in members of confirmed HHT1 families was analyzed, and compared to normal. Both endothelial cells and activated monocytes were found to express about half levels of fully processed cell surface endoglin which represents expression from the normal allele. A search for mutant proteins reveals that proteins if detectable, are transiently expressed intracellularly, and not at the cell surface where endoglin functions. This thesis demonstrates haploinsufficiency is the predominant mechanism underlying the HHT1 phenotype, thus a critical level of endoglin is required in vasculature, and possibly other cells where it is expressed. Endoglin modulates some TGF-ß1 responses in certain cell types. However, I show HHT1 HUVEC do not exhibit altered proliferative responses to TGF-ß1. 'ALK1', the target gene for HHT2, codes for an orphan type I receptor that functions in BMP signalling pathways. Endoglin and ALK1 are likely involved in a common pathway suggesting endoglin could interact with other members of the TGF-ß superfamily. I tested this hypothesis and show endoglin interacts with TGF-ß1, -ß3, activin-A, BMP-2 and BMP-7, but cannot bind ligands without coexpression of the respective ligand binding kinase receptors. Endoglin specifically interacts with multiple heteromeric signalling complexes for these ligands, but does not disrupt their formation. My results suggest that endoglin does not act as a true receptor, and I define endoglin here as an accessory protein for the TGF-ß superfamily that could modulate the signalling capacity of the receptor complex. Endoglin function in endothelial, hematopoietic and other cell types, is thus highly dependent on the repertoire of TGF-ß family receptors the cells express.Ph.D