Chicken receptor activator of nuclear factor-kappaB ligand induces formation of chicken osteoclasts from bone marrow cells and also directly activates mature osteoclasts.

Receptor activator of nuclear factor-kappaB ligand (RANKL), which functions as a major determinant of osteoclast differentiation and activation, is a type II transmembrane protein and is expressed in osteoblasts-stromal cells. The aim of this study was to clarify the role of chicken RANKL (chRANKL) in chicken osteoclast differentiation and to determine its effect on mature chicken osteoclasts. In the present study, chRANKL protein was first cloned and expressed in Escherichia coli. We then treated chicken bone marrow cells with chRANKL protein and found that it induced the formation of chicken osteoclast-like multinucleated cells in a dose-dependent manner in the presence of human macrophage colony-stimulating factor. Moreover, the addition of chicken osteoprotegerin could block the effect of chRANKL with regard to osteoclast-like multi-nucleated cell formation and bone resorption. Using primary cultures of chicken osteoclasts on bone slices, we also found that bone resorption pits per cell increased with chRANKL concentration in a dose-dependent manner. The chRANKL-treated hens exhibited increased blood Ca(++) levels within 2 h after injection, showing that chRANKL also activates osteoclasts in vivo. These results clearly indicate that the expressed protein is functional and may also be a critical factor for chicken osteoclastogenesis and bone resorption.