Coactivator-independent vitamin D receptor signaling impairs intestinal calcium transport in mice, leading to severe rickets, which 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 1a,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?AF2) to study coactivator-independent VDR signaling. We confirm that ligand-induced transcriptional activation was disabled because the mutant VDR?AF2 protein was unable to interact with coactivators. Systemic Vdr?AF2 mice developed short, undermineralized bones with enlarged 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?AF2 mice. Our findings in osteoblast- and osteoclast-specific Vdr?AF2 mice did not recapitulate this bone phenotype indicating coactivator-independent VDR effects are more important in other organs. On the other hand, RNA-seq analysis of duodenum and kidney revealed a repression of VDR  target genes in systemic Vdr?AF2 mice, which was not observed in Vdr-/- mice. These genes could provide new insights in the compensaory (re)absorption of minerals that are crucial for bone homeostasis. In summary, coactivator-independent VDR effects contribute to mineral and bone homeostasis. Overall design: To investigate the differential effects of co-activator independent VDR signaling versus absent VDR signaling on mineral homeostasis, we performed RNA seq analysis in duodenum and kidney samples of Vdr-/- and VdrAF2 mice. Herefor, we used 8-week-old female mice to obtain 6 Vdr wildtype samples, 4 Vdr-/- samples and 4 Vdr AF2 samples of two different tissues, duodenum and kidney, which makes 28 samples in total. Mice were maintained on a diet high in calcium, phosphate and lactose (Inotiv, TD.96348) to circumvent effects of hypocalcemia and hypophosphatema, at least in the Vdr-/- mice.