Absence of vitamin D receptor in mature osteoclasts results in altered osteoclastic activity and bone loss

Mature osteoclasts express the vitamin D receptor (VDR) and are able to synthesise and respond to 1,25(OH)2D3 via CYP27B1 enzyme activity. Whether vitamin D signalling within osteoclasts is necessary for the regulation of osteoclastic bone resorption in an in vivo setting is unclear. To determine the requirement for the VDR- and CYP27B1-mediated activity in mature osteoclasts, conditional deletion mouse models were created whereby either Vdr or Cyp27b1 gene was inactivated by breeding either Vdrfl/fl or Cyp27b1fl/fl mice with Cathepsin K-Cre transgenic mice (CstkCre) to generate CtskCre/Vdr−/− and CtskCre/Cyp27b1−/− mice respectively. To account for potential CtskCre-meaited off-target deletion of Vdr, Dmp1Cre were also used determine the effect of Vdr deletion in osteocytes. Furthermore, CtskCre/Vdr−/− mice were ovariectomised (OVX) to assess the role of VDR in osteoclasts under bone-loss conditions and bone marrow precursor cells were cultured under osteoclastogenic conditions to assess osteoclast formation. Six-week-old CtskCre/Vdr−/− female mice demonstrated a 15% decrease in femoral BV/TV (p < 0.05). In contrast, BV/TV remained unchanged in CtskCre/Cyp27b1−/− mice as well as in Dmp1Cre/VDR−/− mice. When CtskCre/Vdr−/− mice were subjected to OVX, the bone loss that occurred in CtskCre/Vdr−/− was predominantly due to a diminished volume of thinner trabeculae when compared to control levels. These changes in bone volume in CtskCre/Vdr−/− mice occurred without an observable histological change in osteoclast numbers or size. However, while cultured bone marrow-derived osteoclasts from CtskCre/Vdr−/− mice were marginally increased when compared to VDRfl/fl mice, elevated expression of genes such as Cathepsin K, Nfatc1 and VATPase was observed. Collectively, these data indicate that the absence of VDR in mature osteoclasts causes exacerbated bone loss in young mice and during OVX which is associated with enhanced osteoclastic activity and without increased osteoclastogenesis.