1α-Hydroxylase gene ablation and P_isupplementation inhibit renal calcification in mice homozygous for the disruptedNpt2agene

Abstract

-Arnaud. 1␣-Hydroxylase gene ablation and P i supplementation inhibit renal calcification in mice homozygous for the disrupted Na-P i cotransporter gene Npt2a. Am J Physiol Renal Physiol 286: F675-F681, 2004. First published December 2, 2003 10.1152/ ajprenal.00362.2003.-Disruption of the major renal Na-phosphate (P i) cotransporter gene Npt2a in mice leads to a substantial decrease in renal brush-border membrane Na-P i cotransport, hypophosphatemia, and appropriate adaptive increases in renal 25-hydroxyvitamin D 3-1␣-hydroxylase (1␣OHase) activity and the serum concentration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D]. The latter is associated with increased intestinal Ca absorption, hypercalcemia, hypercalciuria, and renal calcification in Npt2 Ϫ/Ϫ mice. To determine the contribution of elevated serum 1,25(OH) 2D levels to the development of hypercalciuria and nephrocalcinosis in Npt2 Ϫ/Ϫ mice, we examined the effects of 1␣OHase gene ablation and long-term Pi supplementation on urinary Ca excretion and renal calcification by microcomputed tomography. We show that the urinary Ca/creatinine ratio is significantly decreased in Npt2 Ϫ/Ϫ /1␣OHase Ϫ/Ϫ mice compared with Npt2 Ϫ/Ϫ mice. In addition, renal calcification, determined by estimating the calcified volume to total renal volume (CV/TV), is reduced by ϳ80% in Npt2 Ϫ/Ϫ /1␣OHase Ϫ/Ϫ mice compared with that in Npt2 Ϫ/Ϫ mice. In Npt2 Ϫ/Ϫ mice derived from dams fed a 1% Pi diet and maintained on the same diet, we observed a significant decrease in urinary Ca/creatinine that was also associated with ϳ80% reduction in CV/TV when compared with counterparts fed a 0.6% diet. Taken together, the present data demonstrate that both 1␣OHase gene ablation and P i supplementation inhibit renal calcification in Npt2 Ϫ/Ϫ mice and that 1,25(OH)2D is essential for the development of hypercalciuria and nephrocalcinosis in the mutant strain. phosphate wasting; hypophosphatemia; hypercalciuria; nephrocalcinosis; 1,25-dihydroxyvitamin D THE TYPE IIA Na-phosphate (P i ) cotransporter Npt2a is the most abundant Na-P i cotransporter in mouse kidney (23) and is expressed in the brush-border membrane (BBM) of proximal tubular cells (6) where the bulk of filtered P i is reabsorbed. Studies in our laboratory demonstrated that mice homozygous for the disrupted Npt2a gene (Npt2) exhibit an increase in urinary P i excretion, an ϳ80% loss in BBM Na-P i cotransport, and significant hypophosphatemia (2). In addition, Npt2 Ϫ/Ϫ mice fail to respond to P i deprivation with an adaptive increase in BBM Na-P i cotransport (11) and to parathyroid hormone (PTH) with a decrease in transport (27). These findings underscore the significant role of Npt2a in renal P i reabsorption and its regulation by dietary P i and PTH.Hypophosphatemia, induced by feeding a low-P i diet, is an important stimulus for increased renal synthesis of 1,25-dihydroxyvitamin D 3 [1,25(OH) 2 D] by the cytochrome P-450 25-hydroxyvitamin D 3 -1␣-hydroxylase (CYP27B1, hereafter referred to as 1␣OHase) (9). We showed that hypophosphatemi...

Intestinal and renal adaptation to a low-Pi diet of type II NaPi cotransporters in vitamin D receptor- and 1αOHase-deficient mice

“…Decreased urinary excretion of P i was not paralleled by detectable changes in the abundance of the NaP i -IIa protein as one would assume to occur because of increased levels of PTH reported for VDR-and 1␣OHase-deficient mice (7,23). In contrast to our findings, slight decreases in NaP i -IIa abundance have been reported in VDRdeficient mice (23), 1,25(OH) 2 D-deficient rats (25), and 1␣OHase-deficient weanling mice (26). Because the regulation of NaP i -IIa by PTH may be impaired by the lack of 1,25(OH) 2 D (10), kidney slices of adapted mice were incubated with PTH.…”

“…After 5 days on a high-P i diet, serum P i in the VDR Ϫ/Ϫ animals did not differ from that of the wild types, whereas the 1␣OHase Ϫ/Ϫ animals showed moderate hypophosphatemia, which is in agreement with earlier reports (7,26). In contrast to data published by others (23), the serum P i concentration in VDR-deficient mice fed a high-P i diet was not different from that of the wild-type mice.…”

“…Under such conditions, additional downregulation of NaP i -IIa by PTH may be too small to be detected. Of note, reported reductions in NaP i -IIa content (23,25,26) were observed in animals fed a normal P i diet containing 0.6 -0.8% P i .…”

Autosomal-Recessive Mutations in SLC34A1 Encoding Sodium-Phosphate Cotransporter 2A Cause Idiopathic Infantile Hypercalcemia

“…Similar observations were made by Tenenhouse and colleagues who describe a normalization of calcium metabolism in Slc34a1 knockout mice by either additional ablation of 1a-hydroxylase (CYP27B1) or by high phosphate supplementation. 19 Importantly, these data are in line with the clinical observation made in patient P9.1 who remained hypercalcemic after rehydration and cessation of vitamin D supplements (Figure 3), but showed a rapid improvement of his clinical condition and a complete reversal of biochemical abnormalities, including an increase in FGF23, after phosphate supplementation.…”

Correction of the mineralization defect in hyp mice treated with protease inhibitors CA074 and pepstatin

“…Also, these mice have a predisposition to develop massive renal stones [13]. Secondary ablation of the 1-alpha-hydroxylase gene to produce a double null-mutant corrects the renal stones defect [79]. Remarkably, in patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH), the systemic abnormalities mirror the NPT2 null-mutant mice except for the bone phenotype [80,81].…”