BACKGROUND: Interactions between the receptor activator of the NF-kappaB ligand (RANKL) and its receptor, RANK, are important in the terminal differentiation and activation of osteoclasts. In the current investigation, we examine the feasibility of using genetically modified mesenchymal stem cells (MSCs), C3H10T1/2 cells as a platform for the sustained systemic delivery of therapeutic proteins into the circulation in an osteoporosis model, and investigate retroviral-mediated gene therapy of RANK-Fc as a means of ameliorating ovariectomy (OVX)-induced bone resorption. METHODS: C3H10T1/2 cells were transduced with a MSCV-based retroviral vector containing cDNA of a fusion protein combining the extracellular domain of murine RANK with the human immunoglobulin constant domain (MSCV-RANK-Fc-eGFP). Young adult female mice were subjected to OVX or sham surgery, followed by treatment with transduced cells or PBS 4 weeks later. The expression of RANK-Fc by these cells was assessed, both in vitro and in vivo. Total bone mineral density (BMD) was measured and GFP expression was examined. RESULTS: Transduced cells produced biologically active RANK-Fc in vitro and in vivo. Mice that were subjected to OVX followed by treatment with cells transduced with MSCV-RANK-Fc-eGFP 4 weeks later contained no significant but higher total BMD than either the control vector or PBS-treated mice after 8 weeks. Higher GFP expression was attained in the liver, spleen, and intra-abdominal fat of mice treated with MSCV-RANK-Fc-eGFP. CONCLUSION: The data collectively indicate that C3H10T1/2 cells are effectively transduced with a MSCV-based retrovirus, and are capable of secreting biologically active RANK-Fc in vitro and in vivo. Moreover, gene therapy facilitating the sustained delivery of RANK-Fc may be an effective method to reverse OVX-induced osteoporosis.