MOLECULAR CHARACTERIZATION AND IN-SITU LOCALIZATION OF MURINE ENDOGLIN REVEAL THAT IT IS A TRANSFORMING GROWTH-FACTOR-BETA BINDING-PROTEIN OF ENDOTHELIAL AND STROMAL CELLS

Endoglin is an integral membrane glycoprotein predominantly expressed on human endothelial cells and recently shown to bind transforming gro wth factor-beta 1 (TGF beta 1) with high affinity. We now report the c loning and sequencing of a full-length murine endoglin complementary D NA of 2902 base pairs which hybridizes specifically with a single mess enger RNA (mRNA) species. The polypeptide of 653 amino acids has an ov erall identity of 72% with human and porcine endoglin. The transmembra ne and cytoplasmic domains of all three proteins differ by two to four amino acids and are 70% identical to the corresponding regions of the TGF beta binding protein, betaglycan. Relative levels of murine endog lin mRNA were estimated by polymerase chain reaction and found to be h igh in ovary and uterus, intermediate in heart and muscle, and low in placenta and spleen. In situ hybridization and immunofluorescence conf irmed that murine endoglin, like its human counterpart, is present in blood vessels and capillaries in all tissues examined. In addition, th e stromal cells in the connective tissue of intestine, stomach, heart, muscle, uterus, ovary, and testis were strongly and specifically reac tive with complementary RNA probes and with a polyclonal antibody to e ndoglin; epithelial cell layers were distinctly unreactive. This distr ibution is. similar to that of extracellular TGF beta 1, particularly in heart and uterus, and suggests that endoglin on stromal fibroblast- like cells might be regulating access of TGF beta 1 to the signaling r eceptor complex. NCTC-2071 fibroblasts in culture were shown to expres s high levels of endoglin mRNA by polymerase chain reaction. After che mical cross-linking with [I-125]TGF beta 1 and immuneprecipitation wit h the polyclonal antihuman endoglin serum, a radiolabeled band of mel wt 180,000 corresponding to dimeric endoglin was observed under nonred ucing conditions, whereas a single band of mol wt 90,000 was seen unde r reducing conditions. Thus murine fibroblast endoglin is capable of b inding TGF beta 1. Future studies should establish the specialized rol e of endoglin in the TGF beta receptor complex of endothelial and stro mal cells.