Inactivation of the 25-Hydroxyvitamin D 1α-Hydroxylase and Vitamin D Receptor Demonstrates Independent and Interdependent Effects of Calcium and Vitamin D on Skeletal and Mineral Homeostasis

Abstract

We employed a genetic approach to determine whether deficiency of 1,25-dihydroxyvitamin D (1,25(OH) 2 D) and deficiency of the vitamin D receptor (VDR) produce the same alterations in skeletal and calcium homeostasis and whether calcium can subserve the skeletal functions of 1,25(OH) 2 D and the VDR. Mice with targeted deletion of the 25-hydroxyvitamin D 1␣-hydroxylase (1␣(OH)ase ؊/؊ ) gene, the VDR gene, and both genes were exposed to 1) a high calcium intake, which maintained fertility but left mice hypocalcemic; 2) this intake plus three times weekly injections of 1,25(OH) 2 D 3 , which normalized calcium in the 1␣(OH)ase ؊/؊ mice only; or 3) a "rescue" diet, which normalized calcium in all mutants. These regimens induced different phenotypic changes, thereby disclosing selective modulation by calcium and the vitamin D system. Parathyroid gland size and the development of the cartilaginous growth plate were each regulated by calcium and by 1,25(OH) 2 D 3 but independent of the VDR. Parathyroid hormone secretion and mineralization of bone reflected ambient calcium levels rather than the 1,25(OH) 2 D/VDR system. In contrast, increased calcium absorption and optimal osteoblastogenesis and osteoclastogenesis were modulated by the 1,25(OH) 2 D/VDR system. These studies indicate that the calcium ion and the 1,25(OH) 2 D/VDR system exert discrete effects on skeletal and calcium homeostasis, which may occur coordinately or independently.Vitamin D plays a major role in modulating calcium and skeletal homeostasis and exerts a significant influence on the growth and differentiation of a variety of tissues (1-3). Vitamin D is absorbed from the diet and generated in skin by exposure to ultraviolet light. The secosteroid is transported in blood bound to vitamin D-binding protein (4)

The calcium-sensing receptor and 25-hydroxyvitamin D–1α-hydroxylase interact to modulate skeletal growth and bone turnover

“…On the normal diet, the 1a(OH)ase À/À mice remained hypocalcemic, with severe secondary hyperparathyroidism, consistent with our previous findings. (15) After deletion of the full-length CaSR in the double mutants, serum calcium remained very low in the absence of active vitamin D, both parathyroid gland size and circulating PTH concentrations were even greater than in the 1a(OH)ase À/À mice, and hypophosphatemia was present. On the rescue diet, mean serum calcium levels were normalized in 1a(OH)ase À/À mice, as described previously, (15) but rose above normal in the double mutants, likely because of impaired renal calcium excretion owing to the absence of the renal action of CaSR in the setting of increased gastrointestinal absorption of calcium because of the rescue diet.…”

“…Mutant mice and control littermates were maintained in a virus-and parasite-free barrier facility and exposed to a 12-/12-hour light/dark cycle. At approximately 21 days of age, mice were weaned and maintained on drinking water containing 1.5% calcium gluconate and either a normal diet of autoclaved chow containing 1% calcium, 0.85% phosphorus, 0% lactose, and 2.2 U/g of vitamin D (Ralston Purina Co., St. Louis, MO, USA) or a ''rescue'' diet (15) of gamma-irradiated chow containing 20% lactose, 2% calcium, 1.25% phosphorus, and 2.2 U/g of vitamin D (TD96348, Harlan Teklad, Madison, WI, USA). No significant differences in any parameter determined were observed in wild-type mice on a normal or a rescue diet.…”

“…No significant differences in any parameter determined were observed in wild-type mice on a normal or a rescue diet. (15) Consequently, control data are shown for the wild-type mice on the normal diet. Animals were euthanized at about 3 months of age.…”

“…(13,14) We wished to test the hypothesis that CaSR has an important role to play in skeletal function in older growing animals and therefore crossed the Casr À/À animals with 1a(OH)ase À/À mice to ascertain the postnatal consequences of the absence of the CaSR on skeletal homeostasis and to examine the interaction of [Ca 2þ ] e and 1,25(OH) 2 D in the double mutants on modulating skeletal function. We exposed the different genetic models to two different environments: a normal diet, on which 1a(OH)ase À/À mice are known to be hypocalcemic and hypophosphatemic, and a high-calcium, high-phosphorus ''rescue'' diet (15) containing 20% lactose, which is known to normalize serum calcium and phosphate in the absence of either active vitamin D or the vitamin D receptor.…”

Vitamin D, a modulator of musculoskeletal health in chronic kidney disease

“…It permits more calcium to enter the cell, which is translocated into the circulation, ensuring the availability of sufficient calcium and phosphate for adequate mineralization of the newly formed bone matrix to avoid rickets/osteomalacia. 1,25(OH) 2 D induces skeletal anabolism and couples the activity of osteoblasts and osteoclasts through the regulation of several genes including osteopontin, osteocalcin, and the Wnt receptor LRP5 30, 31. Indeed, vitamin D stimulates the expression of LRP5, which, together with sclerostin, Dkk1, and frizzled, constitutes the Wnt pathway, a critical process for skeletal mineralization that is tissue specific.…”