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

Osteogenic protein-1/bone morphogenetic protein-7 (OP-1/BMP-7), a member of the transforming growth factor-beta superfamily, has been shown to prevent kidney damage from ischemia/reperfusion injury in rats. The molecular events involved in OP-1 action on kidney are not yet understood.

In this study, we evaluated the biodistribution of (125)I-labeled OP-1 in rat kidneys. Adult rats received a single intravenous injection of 250 microg (125)I-labeled OP-1 per kg body wt, a dose that was effective in protecting kidneys from ischemic injury. Tissue localization, in situ hybridization, and immunostaining with a specific receptor antibody were performed to identify OP-1 cellular targets. Also, isolated plasma membranes from kidney cortex and medulla regions were analyzed to identify and characterize receptor structural components that recognize OP-1.

At 10 and 180 minutes following injection, the relative uptake of (125)I-labeled OP-1 was consistently higher in kidney cortex than in medulla region. Upon autoradiography, kidney tissue sections revealed that OP-1 bound to the convoluted tubule epithelium, glomeruli, and collecting ducts. Moreover, in situ hybridization and immunostaining methods have shown localization of mRNA transcripts and the protein for BMP receptor type II in the cortex and medulla in similar areas as (125)I-labeled OP-1. Bulk membranes (enriched with plasma membranes) isolated from the cortex and medulla regions of kidney each bound specifically to (125)I-OP-1, and the binding of (125)I-labeled OP-1 was inhibited by unlabeled OP-1 in a dose-dependent manner. However, platelet-derived growth factor, transforming growth factor-beta, insulin-like growth factor, fibroblast growth factors, and other members of BMP family such as BMP-2 and cartilage-derived morphogenetic protein-1/growth and differentiation factor-5 (CDMP-1/GDF-5) failed to inhibit the binding of (125)I-labeled OP-1 to receptors, suggesting a high degree of specificity with which OP-1 bound to kidney receptors. Scatchard analysis of quantitative binding data indicated that the OP-1 receptors of kidney contained a single class of high-affinity binding sites for OP-1 with an association constant (Ka) of 2.26 x 109 mol/L-1 and a binding capacity of 1.01 pmol of OP-1 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 (Mr) of 100 kD. Further analysis by Western blotting indicated that an antibody raised against BMP type II receptor effectively recognized the 100 kD OP-1 binding component of kidney plasma membranes.

We demonstrated, to our knowledge for the first time, the presence of membrane-bound, specific, high-affinity OP-1 receptors in rat kidney tissues, which are likely to mediate OP-1 actions in the kidney. The major OP-1-binding component of the kidney appears to be a long form of BMP type II receptor with a Mr of 100 kD. In vivo and in vitro evidence suggests that the cellular targets for OP-1 are convoluted tubule epithelium, glomeruli, and collecting ducts. OP-1 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.