Coactivator-independent vitamin D receptor signaling causes severe rickets in mice, that is not prevented by a diet high in calcium, phosphate, and lactose

The vitamin D receptor (VDR) plays a critical role in the regulation of mineral and bone homeostasis. Upon binding of 1 alpha,25-dihydroxyvitamin D3 to the VDR, the activation function 2 (AF2) domain repositions and recruits coactivators for the assembly of the transcriptional machinery required for gene transcription. In contrast to coactivator-induced transcriptional activation, the functional effects of coactivator-independent VDR signaling remain unclear. In humans, mutations in the AF2 domain are associated with hereditary vitamin D-resistant rickets, a genetic disorder characterized by impaired bone mineralization and growth. In the present study, we used mice with a systemic or conditional deletion of the VDR-AF2 domain (Vdr Delta AF2) to study coactivator-independent VDR signaling. We confirm that ligand-induced transcriptional activation was disabled because the mutant VDR Delta AF2 protein was unable to interact with coactivators. Systemic Vdr Delta AF2 mice developed short, undermineralized bones with dysmorphic growth plates, a bone phenotype that was more pronounced than that of systemic Vdr knockout (Vdr-/-) mice. Interestingly, a rescue diet that is high in calcium, phosphate, and lactose, normalized this phenotype in Vdr-/-, but not in Vdr Delta AF2 mice. However, osteoblast- and osteoclast-specific Vdr Delta AF2 mice did not recapitulate this bone phenotype indicating coactivator-independent VDR effects are more important in other organs. In addition, RNA-sequencing analysis of duodenum and kidney revealed a decreased expression of VDR target genes in systemic Vdr Delta AF2 mice, which was not observed in Vdr-/- mice. These genes could provide new insights in the compensatory (re)absorption of minerals that are crucial for bone homeostasis. In summary, coactivator-independent VDR effects contribute to mineral and bone homeostasis.