Localization of RANKL (Receptor activator of NF kappa B ligand) mRNA and protein in skeletal and extraskeletal tissues

RANKL (receptor activator of NF kappa B ligand) is a membrane-associated os teoblastic molecule, and along with macrophage-colony-stimulating factor, i s crucial for osteoclast formation. RANKL is known to be strongly expressed in osteoblasts and lymphoid tissues. We have sought to determine the skele tal and extraskeletal sites of production of RANKL mRNA and protein using t he techniques of in situ hybridization and immunohistochemistry. Expression of RANKL mRNA and protein were determined in the developmental progression of endochondral bone formation in mouse, intramembranous bone formation in a rabbit model (mRNA only), in human giant cell tumors of bone, and at ext raskeletal sites in the mouse. RANKL mRNA was expressed in prehypertrophic and hypertrophic chondrocytes at day E15 embryonic mouse long bone, and its expression was maintained at these sites throughout development. In newbor n and adult mice, high levels of RANKL mRNA were expressed in mesenchymal c ells of the periosteum and in mature osteoblasts, while megakaryocytes with in the marrow microenvironment expressed RANKL mRNA from 1 week of age, Imm unohistochemical analysis revealed a similar localization pattern of RANKL protein at the sites described. In the intramembranous bone formation model , RANKL mRNA was expressed in mesenchymal cells and in actively synthesizin g osteoblasts, but not in flattened lining osteoblasts or late osteocytes, Expression of RANKL mRNA and protein in osteoclasts was variable with those within resorption lacunae showing the strongest signal/staining. Likewise, expression varied in osteoclasts from giant cell tumor of bone with a mino rity of tartrate-resistant acid phosphatase-positive multinucleated cells h aving no detectable RANKL mRNA or protein. In extraskeletal tissues, RANKL mRNA and protein were detected in the brain, heart, kidney, skeletal muscle , and skin throughout mouse development, suggesting the possibility of seve ral other functions of the molecule. RANKL was also developmentally regulat ed, as evidenced by its expression in the intestine, liver, and lung at E15 and newborn mouse but not in the adult. (C) 1999 by Elsevier Science Inc. All rights reserved.