Vdr expression in osteoclast precursors is not critical in bone homeostasis

The long-recognized role of the vitamin D endocrine system is to maintain stable serum calcium concentrations, which are ensured by a complex interplay between parathyroid gland, kidney, intestine, and bone. However, although VDR is expressed in osteoclastogenic cells, the contribution of VDR-mediated signaling to osteoclast differentiation and activity remains undefined. We therefore deleted Vdr expression efficiently and specifically in myeloid cells by use of M lysozyme-driven Cre expression, which targets granulocytes, monocytes, macrophages and osteoclasts (Vdr^myel- mice). Bone and calcium homeostasis were investigated under basal conditions and in conditions of increased bone remodeling, by feeding Vdr^myel- and Vdr^myel+ (wildtype) mice either a normal (1%) or a low (0.02%) calcium diet from weaning onwards. Vdr^myel- mice developed normally and were normocalcemic at the age of 8 weeks, both at the normal and the low calcium diet. No differences in trabecular or cortical bone mass were observed between Vdr^myel- mice and their wildtype littermates. Dietary calcium restriction resulted in a comparable reduction of trabecular bone mass (40%) and cortical thickness (48%) in Vdr^myel- and Vdr^myel+ mice, pointing to a massive transfer of calcium from the bone to the serum. In agreement with these results, osteoclastic differentiation of hematopoietic cells of Vdr^myel- mice, either induced by M-CSF and RANKL, or cocultured with osteoblasts, occurred as efficiently as osteoclastogenesis from Vdr^myel+ mice. In conclusion, our data do not support a role for osteoclastic Vdr signaling in the control of bone homeostasis.