Characterization of receptors for osteogenic protein-1/bone morphogenetic protein-7 (OP-1/BMP-7) in rat kidneys

Background. Osteogenic protein-1/bone morphogenetic protein-7 (OP-1/BMP-7), a member of the transforming growth factor-beta superfamily, has been show n to prevent kidney damage from ischemia/reperfusion injury in rats. The mo lecular events involved in OP-l action on kidney are not yet understood. Methods. In this study, we evaluated the biodistribution of I-125-labeled O P-l in rat kidneys. Adult rats received a single intravenous injection of 2 50 mug I-125-labeled OP-l per kg body wt, a dose that was effective in prot ecting kidneys from ischemic injury. Tissue localization, in situ hybridiza tion, and immunostaining with a specific receptor antibody were performed t o identify OP-l cellular targets. Also, isolated plasma membranes from kidn ey cortex and medulla regions were analyzed to identify and characterize re ceptor structural components that recognize OP-1. Results. At 10 and 180 minutes following injection, the relative uptake of I-125-labeled OP-l was consistently higher in kidney cortex than in medulla region. Upon autoradiography, kidney tissue sections revealed that OP-l bo und to the convoluted tubule epithelium, glomeruli, and collecting ducts. M oreover, in situ hybridization and immunostaining methods have shown locali zation of mRNA transcripts and the protein for BMP receptor type TI in the cortex and medulla in similar areas as I-125-labeled OP-1. Bulk membranes ( enriched with plasma membranes) isolated from the cortex and medulla region s of kidney each bound specifically to I-125-OP-1, and the binding of I-125 -labeled OP-l was inhibited by unlabeled OP-l in a dose-dependent manner. H owever, platelet-derived growth factor, transforming growth factor-p, insul in-like growth factor, fibroblast growth factors, and other members of BMP family such as BMP-2 and cartilage-derived morphogenetic protein-1/growth a nd differentiation factor-5 (CDMP-1/GDF-5) failed to inhibit the binding of I-125-labeled OP-l to receptors, suggesting a high degree of specificity w ith which OP-l bound to kidney receptors. Scatchard analysis of quantitativ e binding data indicated that the OP-l receptors of kidney contained a sing le class of high-affinity binding sites for OP-l with an association consta nt (K-a) of 2.26 X 10(9) mol/L-1 and a binding capacity of 1.01 pmol of OP- l per mg membrane protein. When analyzed by a ligand blot technique, plasma membranes isolated from kidney cortex and medulla each showed the presence of a prominent specific band with a relative molecular mass (M-r) of 100 k D. Further analysis by Western blotting indicated that an antibody raised a gainst BMP type II receptor effectively recognized the 100 kD OP-l binding component of kidney plasma membranes. Conclusions. We demonstrated, to our knowledge for the first time, the pres ence of membrane-bound, specific, high affinity OP-l receptors in rat kidne y tissues, which are likely to mediate OP-l actions in the kidney. The majo r OP-1-binding component of the kidney appears to be a long form of BMP typ e II receptor with a M-r of 100 kD. In vivo and in vitro evidence suggests that the cellular targets for OP-l are convoluted tubule epithelium, glomer uli, and collecting ducts. OP-l does not share receptor binding properties with other growth factors, including BMP-2 and CDMP-1, suggesting that its mode of action in kidney appears to be specific.