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 1 alpha,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.