Renal Na(+)-phosphate cotransport in X-linked Hyp mice responds appropriately to Na+ gradient, membrane potential, and pH

Functional characterization of the human intestinal NaPi-IIb cotransporter in hamster fibroblasts and Xenopus oocytes

The recently cloned NaPi-IIb cotransporter is an apical membrane protein that is involved in the absorption of phosphate in the intestine. To expedite functional and structural studies, the human intestinal NaPi-IIb cotransporter was stably expressed in hamster fibroblast (PS120) cells. The hNaPi-IIb cDNA stably transfected cells exhibited a 1.8-fold higher sodium-dependent phosphate uptake than vector DNA transfected cells, and had a K(m) for Pi of approximately 106 microM and a K(m) for Na(+) of approximately 34 mM. The hNaPi-IIb cotransporter was also expressed in Xenopus oocytes and it exhibited a K(m) for Pi of approximately 113 microM and a K(m) for Na(+) of approximately 65 mM. The hNaPi-IIb cotransporter expressed in both PS120 cells and oocytes was inhibited by high external pH. Furthermore, the phosphate uptake mediated by the hNaPi-IIb cotransporter was inhibited by 5 mM phosphonoformic acid (PFA), 1 mM arsenate and 100 nM phorbol myristate acetate (PMA). These results demonstrate that the human intestinal NaPi-IIb cotransporter is functional when expressed in hamster fibroblasts, and that this model system may be useful in the future to identify NaPi-IIb cotransporter-specific inhibitors.

Renal Na(+)-phosphate cotransporter gene expression in X-linked Hyp and Gy mice

The X-linked Hyp and Gy mutations are murine homologues of X-linked hypophosphatemia (XLH), a dominant disorder of phosphate (Pi) homeostasis characterized by growth retardation, rickets, hypophosphatemia and decreased renal tubular maximum for Pi reabsorption relative to glomerular filtration rate (Tmp/GFR). In Hyp and Gy mice, the decrease in Tmp/GFR is associated with a reduction in renal brush-border membrane (BBM) Na(+)-Pi cotransport that can be ascribed to a decrease in renal-specific, Na(+)-Pi cotransporter (NPT2) mRNA and protein abundance. Although renal NPT2 gene expression is reduced in Hyp and Gy mice, the NPT2 gene does not map to the X chromosome. These findings exclude NPT2 as a candidate gene for murine and human X-linked hypophosphatemias and suggest that genes at the Hyp, Gy and XLH (HYP) loci are involved in regulation of NPT2 gene expression. Both Hyp and Gy mice respond to low Pi diet with an increase in BBM Na(+)-Pi cotransport, NPT2 mRNA and protein. The increase in NPT2 protein in Pi-depleted mice far exceeds the increase in NPT2 mRNA, suggesting that translational or post-translational mechanisms are involved in the adaptive process. NPT2 protein is localized to the apical surface of the proximal tubule, where immunostaining in both normal and Hyp mice is increased in response to low Pi diet. Pi-deprived Hyp and Gy mice fail to show an increase in Tmp/GFR, indicating that adaptation at the BBM is not sufficient for the overall increase in Tmp/GFR in response to low Pi diet.

Renal expression of Na+-phosphate cotransporter mRNA and protein: effect of the Gy mutation and low phosphate diet

The X-linked Gy mutation is closely linked, but not allelic, to Hyp and is characterized by rickets, hypophosphatemia, decreased renal tubular maximum for phosphate (Pi) reabsorption (TmP) and a specific reduction in renal brush-border membrane (BBM) Na+-Pi cotransport. Gy mice, like their normal littermates, respond to a low-Pi diet with an increase in BBM Na+-Pi cotransport, but fail to show an adaptive increase in Tmp. Using an antibody raised against the NH2 terminal peptide of the rat renal-specific Na+-Pi cotransporter (NaPi-2) and a NaPi-2 cDNA probe, we examined the effect of the Gy mutation and low-Pi diet (0.03% Pi) on NaPi-2 protein and mRNA abundance. The reduction in BBM Na+-Pi cotransport in Gy mice (51 +/- 5% of normal, P < 0.05) was associated with a decrease in NaPi-2 protein (46 +/- 12% of normal, P < 0.05) and mRNA abundance (76 +/- 5%, P < 0.05). The low-Pi diet elicited a two- to three-fold increase in Na+-Pi cotransport in both normal and Gy mice that was accompanied by a large increase in NaPi-2 protein (10.2-fold in normal and 16.9-fold in Gy mice) and a modest increase in NaPi-2 mRNA (1.3-fold in both mouse strains, P < 0.05). The present data demonstrate that (1) the renal defect in BBM Pi transport in Gy mice can be ascribed to a deficit in NaPi-2 protein and mRNA abundance, (2) both normal and Gy mice respond to low Pi with an adaptive increase in NaPi-2 protein that exceeds the increase in Na+-Pi cotransport activity and NaPi-2 mRNA, (3) the adaptive increase in NaPi-2 protein and mRNA are not sufficient for the overall increase in TmP following Pi restriction.

Comparison of 22-oxacalcitriol and 1,25(OH)2D3 on bone metabolism in young X-linked hypophosphatemic male mice

Using a mouse model (Hyp) of human hypophosphatemic vitamin D-resistant rickets [X-linked hypophosphatemia (XLH)], we compared the effects of 22-oxa-1,25-dihydroxyvitamin D3 (OCT) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on restoring defects in mineral and skeletal metabolism. Hyp/Y mice received OCT or 1,25(OH)2D3 at doses of 0.05-0.25 micron.kg-1.day-1 for 4 wk. OCT normalized serum calcium levels, whereas 1,25(OH)2D3 produced hypercalcemia in Hyp/Y. OCT and 1,25(OH)2D3 also normalized serum phosphate levels and increased urinary calcium levels. Additionally, OCT and 1,25(OH)2D3 reduced elevated urinary pyridinoline levels and suppressed urinary adenosine 3',5'-cyclic monophosphate levels to normal. Bone ash content was low in Hyp/Y, and OCT was more effective than 1,25(OH)2D3 in reversing this defect. Histomorphometric analysis of bone turnover, mineralization rate, and osteoid content demonstrated comparable responses with OCT and 1,25(OH)2D3, although the highest dose of 1,25(OH)2D3 resulted in increased osteoid content and delayed mineralization. OCT appears to be more effective and definitely less toxic than 1,25(OH)2D3 in reversing skeletal lesions in Hyp/Y mice and may prove to be the drug of choice in the treatment of childhood XLH.

Phosphate transport in immortalized cell cultures from the renal proximal tubule of normal and Hyp mice: evidence that the HYP gene locus product is an extrarenal factor

Whether renal phosphate wasting in X-linked hypophosphatemia (XLH) results from an intrinsic renal or humoral defect remains controversial. In studies of the murine homolog of XLH, harboring the Simian Virus 40 (SV40) large T antigen, we obviated the influence of renal cell heterogeneity and impressed memory by comparing Na(+)-phosphate cotransport in immortalized cells from the S1 segment of the proximal tubule. Cells from SV40 transgenic normal and Hyp mice exhibit characteristics of differentiated proximal tubule cells including gluconeogenesis and alkaline phosphatase activity. Surprisingly, however, we found two distinct populations of cells from the S1 proximal tubule of both normal and Hyp mice. In one, PTH treatment increases cAMP accumulation, while in the other both PTH and thyrocalcitonin enhance cAMP production. Kinetic parameters for Na(+)-phosphate cotransport were similar in both subpopulations of cells from normal (Km, 0.29 +/- 0.03 vs. 0.39 +/- 0.04 mM; Vmax, 4.6 +/- 0.6 vs. 5.2 +/- 0.4 nmol/mg/5 minutes) and Hyp mice (0.33 +/- 0.02 vs. 0.26 +/- 0.04; 6.0 +/- 0.7, 4.8 +/- 0.6). More importantly, phosphate transport in S1 cells of either subpopulation from Hyp mice is no different than that of normals. These data indicate that renal proximal tubule cells from Hyp mice have intrinsically normal phosphate transport and support the hypothesis that a humoral abnormality underlies renal phosphate wasting in XLH.

Altered osteoblast gluconeogenesis in X-linked hypophosphatemic mice is associated with a depressed intracellular pH

We have studied gluconeogenesis and intracellular pH levels in normal (+/Y) and X-linked hypophosphatemic (Hyp/Y) mice. Compared with +/Y littermates, Hyp/Y mouse osteoblasts showed a higher rate of glucose production from fructose (10-fold), glutamine, and malate, but no significant difference when alpha-ketoglutarate was used as substrate. The activities of the pentose cycle enzymes, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase, were not different in the two osteoblast preparations. Examination of intracellular pH (pHi) using the double excitation of the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) revealed a significantly lower pHi in Hyp/Y mouse osteoblasts compared with +/Y mouse osteoblasts (7.01 +/- 0.03 n = 10 versus 7.15 +/- 0.04 n = 8, respectively; P < 0.05). These results show for the first time that osteoblasts are capable of glucose production and that glucose production is altered in the Hyp/Y mouse osteoblast. As altered gluconeogenesis has been associated with reduced intracellular pH in other systems, a similar mechanism may be operative in the Hyp/Y mouse osteoblast. The observed defects may be intrinsic to the Hyp phenotype as the alterations in intracellular pH and gluconeogenesis persisted in vitro, or they may represent impressed memory from the in vivo state and the presumed circulating factor that influences phosphate transport.

Phosphate transport in osteoblasts from normal and X-linked hypophosphatemic mice

Human hypophophatemic vitamin D-resistant rickets (X-linked hypophosphatemia-XLH) is characterized by hypophosphatemia, a decreased tubular reabsorption of phosphate (P(i)) and defective skeleton mineralization. Utilizing a mouse model (Hyp) of XLH, which demonstrates biological abnormalities and skeletal defects of XLH, we analyzed sodium-dependent phosphate transport in isolated osteoblasts derived from the calvaria of normophosphatemic and hypophosphatemic mice. Initial rates of phosphate uptake by normal and Hyp osteoblasts showed similar slopes. Osteoblasts from both normal and Hyp mice exhibited saturable, sodium-dependent phosphate transport with apparent Vmax and Km values not significantly different (normal mice, Vmax = 24.30 +/- 3.45 nmol/mg prot. 10 min, Km = 349.49 +/- 95.20 mumol/liter; Hyp mice, Vmax = 23.03 +/- 3.41 nmol/mg prot. 10 min, Km = 453.64 +/- 106.93 mumol/liter, n = 24). No differences were found in the ability of normal and Hyp osteoblasts to respond to P(i) transport after 5 hours of P(i) deprivation. Both cell types exhibited a similar increase in cAMP in response to PTH. The accumulated results demonstrate that P(i) uptake and transport in normal and Hyp mouse osteoblasts is a sodium-dependent saturable process. As osteoblast P(i) uptake and transport is apparently normal in the Hyp mouse model of XLH, the "osteoblastic failure" described for the Hyp mouse should be attributed to other mechanism(s).

Renal Na(+)-phosphate cotransport in murine X-linked hypophosphatemic rickets. Molecular characterization

The X-linked Hyp mouse is characterized by a specific defect in proximal tubular phosphate (Pi) reabsorption that is associated with a decrease in Vmax of the high affinity Na(+)-Pi cotransport system in the renal brush border membrane. To understand the mechanism for Vmax reduction, we examined the effect of the Hyp mutation on renal expression of Na(+)-Pi cotransporter mRNA and protein. Northern hybridization of renal RNA with a rat, renal-specific Na(+)-Pi cotransporter cDNA probe (NaPi-2) (Magagnin et al. 1993. Proc. Natl. Acad. Sci. USA. 90:5979-5983.) demonstrated a reduction in a 2.6-kb transcript in kidneys of Hyp mice relative to normal littermates (NaPi-2/beta-actin mRNA = 57 +/- 6% of normal in Hyp mice, n = 6, P < 0.01). Na(+)-Pi cotransport, but not Na(+)-sulfate cotransport, was approximately 50% lower in Xenopus oocytes injected with renal mRNA extracted from Hyp mice when compared with that from normal mice. Hybrid depletion experiments documented that the mRNA-dependent expression of Na(+)-Pi cotransport in oocytes was related to NaPi-2. Western analysis demonstrated that NaPi-2 protein is also significantly reduced in brush border membranes of Hyp mice when compared to normals. The present data demonstrate that the specific reduction in renal Na(+)-Pi cotransport in brush border membranes of Hyp mice can be ascribed to a proportionate decrease in the abundance of Na(+)-Pi cotransporter mRNA and protein.

Renal adaptation to phosphate deprivation: lessons from the X-linked Hyp mouse

The X-linked Hyp mutation, a murine homologue of X-linked hypophosphatemia in humans, is characterized by renal defects in phosphate reabsorption and vitamin D metabolism. In addition, the renal adaptive response to phosphate deprivation in mutant Hyp mice differs from that of normal littermates. While Hyp mice fed a low phosphate diet retain the capacity to exhibit a significant increase in renal brush-border membrane sodium-phosphate cotransport in vitro, the mutants fail to show an adaptive increase in maximal tubular reabsorption of phosphate per volume of glomerular filtrate (TmP/GFR) in vivo. Moreover, unlike their normal counterparts, Hyp mice respond to phosphate restriction with a fall in the serum concentration of 1,25-dihydroxyvitamin D [1,25(OH)2D] that can be ascribed to increased renal 1,25(OH)2D catabolism. The dissociation between the adaptive brush-border membrane phosphate transport response and the TmP/GFR and vitamin D responses observed in Hyp mice is also apparent in X-linked Gy mice and hypophysectomized rats. Based on these findings and the notion that transport across the brush-border membrane reflects proximal tubular function, we suggest that the adaptive TmP/GFR response requires the participation of 1,25(OH)2D or a related metabolite and that a more distal segment of the nephron is the likely target for the 1,25(OH)2D-dependent increase in overall tubular phosphate conservation.

X-linked hypophosphataemia: a homologous phenotype in humans and mice with unusual organ-specific gene dosage

XLH (X-linked hypophosphataemia, gene symbol HYP, McKusick 307800, 307810) and its murine counterparts (Hyp and Gy) map to a conserved segment on the X-chromosome (Xp 22.31-p.21.3, human; distal X, mouse). Gene dosage has received relatively little attention in the long history of research on this disease, which began over 50 years ago. Bone and teeth are sites of the principal disease manifestations in XLH (rickets, osteomalacia, interglobular dentin). Newer measures of quantitative XLH phenotypes reveal gene dose effects in bone and teeth with heterozygous values distributed between those in mutant hemizygotes and normal homozygotes. On the other hand, serum phosphate concentrations (which are low in the mutant phenotype and thereby contribute to bone and tooth phenotypes) do not show gene dosage. In Hyp mice serum values in mutant hemizygotes, mutant homozygotes and heterozygotes are similar. Phosphate homeostasis reflects its renal conservation. Renal absorption of phosphate on a high-affinity, Na+ ion-gradient coupled system in renal brush border membrane is impaired and gene dosage is absent at this level; the mutant phenotype is fully dominant. Synthesis and degradation of 1,25(OH)2D are also abnormal in XLH (and Hyp), but gene dosage in these parameters has not yet been measured. An (unidentified) inhibitory trans-acting product of the X-linked locus, affecting phosphate transport and vitamin D metabolism, acting perhaps through cytosolic protein kinase C, could explain the renal phenotype. But why would it have a normal gene dose effect in bone and teeth? Since the locus may have duplicated (to form Hyp and Gy), and shows evidence of variable expression in different organs (inner ear, bone/teeth, kidney), it may have been recruited during evolution to multiple functions.