Normal molecular size of the Na(+)-phosphate cotransporter and normal Na(+)-dependent binding of phosphonoformic acid in renal brush border membranes of X-linked Hyp mice

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

The functional unit of the renal type IIa Na+/Pi cotransporter is a monomer

The composition of the functional unit of the rat renal type IIa Na(+)/P(i) cotransporter (NaPi-IIa) was investigated by using two approaches based on the differential sensitivities of the wild type (WT) and mutant S460C proteins to 2-aminoethylmethanethiosulfonate hydrobromide (MTSEA), a charged cysteine modifier. Transport activity of S460C is completely blocked after incubation in MTSEA, whereas that of the WT remains unaffected. First, Xenopus laevis oocytes were coinjected with cRNAs coding for the WT and S460C in different proportions, and the transport inhibition after MTSEA incubation was assayed by electrophysiology. The relationship between MTSEA inhibition and proportion of cRNA was consistent with that for a functional monomer. Second, concatameric proteins were constructed that either comprised two WT proteins (WT-WT), two S460C mutants (S460C-S460C), or one of each (WT-S460C). Western blots of oocytes injected with fusion protein cRNA showed bands at approximately 200 kDa, whereas a main band at approximately 90 kDa was obtained for the WT cRNA alone. The kinetic properties of concatamers were the same as for the single proteins. Transport activity of the WT-WT concatamer was unaffected by MTSEA incubation, fully inhibited for S460C-S460C, but 50% inhibited for WT-S460C. This behavior was also consistent with NaPi-IIa being a functional monomer.

Involvement of disulphide bonds in the renal sodium/phosphate co-transporter NaPi-2

The rat renal brush border membrane sodium/phosphate co-transporter NaPi-2 was analysed in Western blots with polyclonal antibodies raised against its N-terminal and C-terminal segments. Under reducing conditions, proteins of 45-49 and 70-90 kDa (p45 and p70) were detected with N-terminal antibodies, and proteins of 40 and 70-90 kDa (p40 and p70) were detected with C-terminal antibodies. p40 and p45 apparently result from a post-translational cleavage of NaPi-2 but remain linked through one or more disulphide bonds. Glycosidase digestion showed that both polypeptides are glycosylated; the cleavage site could thus be located between Asn-298 and Asn-328, which have been shown to constitute the only two N-glycosylated residues in NaPi-2. In the absence of reducing agents, both N-terminal and C-terminal antibodies detected p70 and a protein of 180 kDa (p180), suggesting the presence of p70 dimers. Much higher concentrations of beta-mercaptoethanol were required to produce a given effect in intact membrane vesicles than in solubilized proteins, indicating that the affected disulphide bonds are not exposed at the surface of the co-transporter. Phosphate transport activity decreased with increasing concentrations of reducing agents [beta-mercaptoethanol, dithiothreitol and tris-(2-carboxyethyl)phosphine] and was linearly correlated with the amount of p180 detected. The target sizes estimated from the radiation-induced loss of intensity of p40, p70 and p180 were all approx. 190 kDa, suggesting that NaPi-2 exists as an oligomeric protein in which the subunits are sufficiently close to one another to allow substantial energy transfer between the monomers. When protein samples were pretreated with beta-mercaptoethanol [2.5% and 5% (v/v) to optimize the detection of p40 and p70] before irradiation, target sizes estimated from the radiation-induced loss of intensity of p40 and p70 were 74 and 92 kDa respectively, showing the presence of disulphide bridges in the molecular structure of NaPi-2.

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.

Factors affecting the stability of the renal sodium/phosphate symporter during its solubilization and reconstitution

Phosphate is reabsorbed across the brush-border membrane of the proximal tubule by a specific sodium-dependent symporter. Like the other brush-border membrane transport proteins of the kidney, the phosphate carrier remains to be isolated in a functional state. To establish a set of parameters that allow to preserve its biological activity, the phosphate carrier was solubilized under systematically varied conditions and reconstituted into proteoliposomes. Successful reconstitution was achieved only when the extraction buffer contained lipids extracted from the renal brush-border membrane. Glycerol, an osmolyte which reduces the water activity of the solution, was also required. It could however be replaced by 150 mM sodium or potassium phosphate. Below this concentration and in the presence of glycerol, the ionic strength of the solution had little effect on the stability of the transporter, but sodium phosphate could not be replaced by sodium chloride. Phosphate transport in reconstituted vesicles depended on the concentration of detergent and pH of the extraction buffer. Finally, transport activity was increased when solubilization was carried out in the presence of a reducing agent, dithiothreitol. These results should be helpful during the purification and further characterization of the renal phosphate symporter.

Molecular size of the functional complex and protein subunits of the renal phosphate symporter

The oligomeric structure of the rabbit renal brush-border membrane sodium/phosphate cotransporter was examined with the radiation inactivation and fragmentation technique. The size of its functional complex (its "radiation inactivation size") was estimated from the rate of decay of its sodium-dependent transport activity as a function of the radiation dose. A radiation inactivation size of 223 +/- 42 kDa was obtained. The polypeptide constituting the monomeric unit of the Na1+/Pi symporter was detected by immunoblotting with polyclonal anti-peptide antibodies directed against the 14 amino acid C-terminal portion of the symporter molecule. Its apparent molecular size estimated by comparison with standards following SDS-polyacrylamide gel electrophoresis was 64,000. This value is in good agreement with its known molecular mass of 51,797 Da calculated from the amino acid sequence deducted from the nucleotide sequence of its gene since this protein is probably glycosylated. The loss of labeling intensity of the polypeptide of M(r) = 64,000 was also measured as a function of radiation dose. The molecular size calculated from these data (its "target size") was 165 +/- 20 kDa. The target size estimated for the rat phosphate cotransporter was 184 +/- 46 kDa, and its previously reported radiation inactivation size was 234 +/- 14 kDa. These results strongly suggest that the renal Na1+/Pi cotransporter exists as an oligomeric protein, probably a homotetramer. The fact that the values obtained for the target size are about 3/4 those obtained for the radiation inactivation size of these cotransport proteins indicates that their subunits are closely associated since most of their subunits appear to be fragmented by a single ionizing radiation hit.

Immunodetection and characterization of proteins implicated in renal sodium/phosphate cotransport

Polyclonal antibodies raised against the 14-amino acid C-terminal portion of the rabbit renal brush-border membrane Na+/Pi cotransporter, as deduced from the nucleotide sequence of the cloned NaPi-1 gene, were used for Western blot analysis of renal brush-border membrane proteins from rat, rabbit and beef. Proteins of 65 kDa from the rat, 64 kDa from the rabbit, and 38, 66, 77, 92, 110, 176 and 222 kDa from the beef were specifically labelled. The affinity of the antibodies was much greater, however, for the proteins of the rat and rabbit than for those of the beef. The rat 65-kDa antigen was readily detected in brush-border membranes isolated from kidney cortex, but was absent from the basolateral membrane and the cytosolic and microsomal fractions of this tissue, in agreement with the subcellular localization of the Na+/Pi cotransporter. This antigen was however several-fold more abundant in the juxtamedullary portion of the cortex than in the outer portion. Despite a strong stimulation in phosphate transport, a low-phosphate diet had little influence on the amount of antigen detected. An additional peptide-displaceable band corresponding to a protein of 250 kDa appeared when beta-mercaptoethanol was omitted during electrophoresis, in agreement with the possibility that disulfide bonds may be involved in the regulation of renal phosphate transport activity.

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.

Regulation of the phosphate (Pi) concentration in UMR 106 osteoblast-like cells: effect of Pi, Na+ and K+

Osteoblast-like cells possess Na-dependent transporters which accumulate orthophosphate (Pi) from the extracellular medium. This may be important in bone formation. Here we describe parallel measurements of Pi uptake and cellular [Pi] in such cells from the rat (UMR 106-01 and UMR 106-06) and human (OB), and in non-osteoblastic human fibroblasts (Detroit 532 (DET)). In UMR 106-01, cellular [Pi] was weakly dependent on extracellular [Pi] and higher than expected from passive transport alone. [32Pi]-uptake was inhibited by Na deprivation, but paradoxically increased on K deprivation. With Na, 87 per cent of cellular 32P was found in organic phosphorus pools after only 5 min. Na deprivation also decreased cellular [Pi], in both UMR 106-01 and DET, but the decrease was smaller than that in [32Pi]-uptake. Ouabain decreased [32Pi]-uptake and cellular [Pi] in DET, but not in UMR 106-01. Regulation of cellular [Pi] is therefore at least partly dependent on Na/Pi co-transport, but this does not seem to be an exclusive property of osteoblasts.

X-linked hypophosphatemia. A phenotype in search of a cause

XLH is an important disease, it is the subject of several classic articles in the medical sciences (Scriver et al., 1991), and it has been an important stimulus to study renal hypophosphatemias and how they are involved in rickets and osteomalacia (Scriver, 1974; Scriver and Tenenhouse, 1991). Renal transport is the major determinant of phosphate homeostasis in mammals and it is unlikely that this important biochemical parameter would have been left by evolution to a single renal transport system. Together physiologists and geneticists found that the mammalian kidney has several gene products dedicated to phosphate transport. That has implications for biochemists in search of a membrane protein to clone and explain XLH, for example. Let us suppose the transporter affected in XLH is cloned. Will it be the product of the XLH (or Hyp or Gy) locus? One will not know until the transporter gene is mapped. There is no question of the X-chromosome locus product being protein kinase C for example, since it maps to autosomes. But where does one start in the search for the X-chromosome locus? With the elusive putative diffusible factor or with the transporter, or perhaps with an enzyme in vitamin D hormone metabolism? Which goes to say that it is necessary to know the phenotype to arrive at the right locus. Or is it? Sufficient physical mapping of region Xp22.31-p21.3 will eventually lead to positional cloning of the Hyp gene. What will it be?(ABSTRACT TRUNCATED AT 250 WORDS)

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

To investigate the mechanism for the 50% decrease in Vmax of the high-affinity phosphate transport system in the renal brush-border membrane of X-linked Hyp mice, we compared the effects of external Na+ concentration, membrane potential, pH, phosphonoformic acid (PFA), and arsenate on Na(+)-Pi cotransport in brush-border membrane vesicles prepared from normal mice and Hyp littermates. The affinity of the Na(+)-Pi cotransport system for Na+ (apparent Km = 60 +/- 7 and 64 +/- 2 mM for normal and Hyp mice, respectively) and the Na(+)-Pi stoichiometry estimated from Hill plots (2.5 +/- 0.2 and 2.9 +/- 0.6 for normal and Hyp mice, respectively) were similar in brush-border membranes of both strains. Inside-negative membrane potential, generated by anions of different permeabilities, stimulated Na(+)-Pi cotransport and inside-positive membrane potential generated by valinomycin, and a K+ gradient (outside greater than inside) inhibited Na(+)-Pi cotransport to the same extent in brush-border membranes derived from normal mice and Hyp littermates. The pH dependence of Na(+)-Pi cotransport was similar in brush-border membrane vesicles of normal and Hyp mice. The ratio of Na(+)-Pi cotransport measured at pH 7.5 relative to that at pH 6.5 was 2.9 +/- 0.6 in normal mice and 2.9 +/- 0.7 in Hyp mice. PFA was a competitive inhibitor of Na(+)-Pi cotransport in brush-border membranes of both normal and Hyp mice. However, the apparent Ki for PFA was significantly lower in Hyp mice (0.31 +/- 0.01 and 0.19 +/- 0.02 mM in normal and Hyp mice, respectively, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular size of the renal sodium/phosphate symporter in native and reconstituted systems

The size of the renal sodium/phosphate symporter was estimated with the radiation inactivation technique in isolated bovine brush border membrane vesicles and after reconstitution in proteoliposomes. The functional unit of the native phosphate carrier had a radiation inactivation size of 172 +/- 17 kDa. Identical values were obtained for the reconstituted carrier whether it was irradiated before or after the formation of the proteoliposomes (161 +/- 9 and 159 +/- 11 kDa, respectively). The sodium-independent uptake of phosphate was not affected significantly by radiation doses up to 10 Mrad. This activity is therefore not due to the reconstitution of a large phosphate-binding protein such as alkaline phosphatase. Furthermore, bromotetramisole, a specific inhibitor of phosphate binding to this enzyme, had no significant effect on the uptake of phosphate by the proteoliposomes.

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.