Generation of novel genetically modified rats to reveal the molecular mechanisms of vitamin D actions

Recent studies have suggested that vitamin D activities involve vitamin D receptor (VDR)-dependent and VDR-independent effects of 1α,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) and 25-hydroxyvitamin D(3) (25(OH)D(3)) and ligand-independent effects of the VDR. Here, we describe a novel in vivo system using genetically modified rats deficient in the Cyp27b1 or Vdr genes. Type II rickets model rats with a mutant Vdr (R270L), which recognizes 1,25(OH)(2)D(3) with an affinity equivalent to that for 25(OH)D(3), were also generated. Although Cyp27b1-knockout (KO), Vdr-KO, and Vdr (R270L) rats each showed rickets symptoms, including abnormal bone formation, they were significantly different from each other. Administration of 25(OH)D(3) reversed rickets symptoms in Cyp27b1-KO and Vdr (R270L) rats. Interestingly, 1,25(OH)(2)D(3) was synthesized in Cyp27b1-KO rats, probably by Cyp27a1. In contrast, the effects of 25(OH)D(3) on Vdr (R270L) rats strongly suggested a direct action of 25(OH)D(3) via VDR-genomic pathways. These results convincingly suggest the usefulness of our in vivo system.