The vitamin D receptor (VDR) is thought to mediate the biological effects of 1,25-dihydroxyvitamin D. We have generated VDR knockout mice that develop abnormalities in mineral ion homeostasis, accompanied by osteomalacia, rickets and hyperparathyroidism by 4 weeks of age. Analogous to humans with VDR mutations, these mice have hair perinatally, but develop alopecia. Although VDR null mice with normal mineral ion levels have histologically, histomorphometrically and biomechanically normal long bones, there remains considerable controversy as to whether the VDR and its ligand play a critical role in the skeleton. To address this hypothesis, we propose to isolate primary calvarial osteoblasts from the VDR null mice and examine their ability to form mineralized bone nodules. We will also address whether VDR ablation influences the program of osteoblast differentiation in this system. Because there is increasing evidence that the two major hormones involved in mineral ion homeostasis, PTH and 1,25-dihydroxyvitamin D, have different effects on osteoblasts that give rise to endochondral and intramembranous bone, similar studies will be performed in stromal cells isolated from the long bones of the VDR null mice and control littermates. The effects of VDR status and mineral ion homeostasis on bone morphogenesis will also be examined, using ectopic bone formation assays. Hair reconstitution assays have demonstrated that the VDR null keratinocyte is responsible for the hair cycle defect that causes alopecia. Preliminary studies in VDR null mice that express the human VDR under the keratin 14 promoter support these findings. These mice will be further characterized, as will signaling pathways involved in hair morphogenesis, to determine if a defect in these pathways in the VDR null mice is responsible for the defect in the hair cycle. Keratinocyte stem cells, thought to provide a source of cells for cyclic regeneration of the lower part of the hair follicle, will also be quantitatively and qualitatively assessed.
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