Abnormal adenosine 3'.5'-monophosphate stimulation of renal 1,25-dihydroxyvitamin D production in hyp mice: evidence that 25-hydroxyvitamin D-1 alpha-hydroxylase dysfunction results from aberrant intracellular function

Disordered regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase gene expression by phosphorus in X-linked hypophosphatemic (hyp) mice

X-linked hypophosphatemic (Hyp) mice exhibit hypophosphatemia, impaired renal phosphate reabsorption, defective skeletal mineralization, and disordered regulation of vitamin D metabolism: In Hyp mice, restriction of dietary phosphorus induces a decrease in serum concentration of 1,25-dihydroxyvitamin D and renal activity of 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase), and induces an increase in renal activity of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase). In contrast, in wild-type mice, phosphorus restriction stimulates renal 1alpha-hydroxylase gene expression and suppresses that of 24-hydroxylase. To determine the molecular basis for the disordered regulation of vitamin D metabolism in Hyp mice, we determined renal mitochondrial 1alpha-hydroxylase activity and the renal abundance of p450c1alpha and p450c24 mRNA in wild-type and Hyp mice fed either control, low-, or high-phosphorus diets for 5 d. In wild-type mice, phosphorus restriction increased 1alpha-hydroxylase activity and p450c1alpha mRNA expression by 6-fold and 3-fold, respectively, whereas in the Hyp strain the same diet induced changes of similar magnitude but opposite in direction. Phosphorus supplementation was without effect in wild-type mice, whereas in Hyp mice the same diet induced 3-fold and 2-fold increases, respectively, in enzyme activity and p450c1alpha mRNA abundance. In wild-type mice, both renal 1alpha-hydroxylase activity and p450c1alpha mRNA abundance varied inversely and significantly with serum phosphorus concentrations, whereas in Hyp mice the relationship between both renal parameters and serum phosphorus concentration was direct. In Hyp mice, phosphorus restriction induced a significant increase in renal p450c24 mRNA abundance, in contrast to the lack of effect observed in wild-type mice. The present findings demonstrate that regulation of both the p450c1alpha and p45024 genes by phosphorus is disordered in Hyp mice at the level of renal 1alpha-hydroxylase activity and renal p450c1alpha and p450c24 mRNA expression.

Abnormal regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase activity in X-linked hypophosphatemia: a translational or post-translational defect

The hyp mouse exhibits abnormal metabolic/hormonal regulation of renal 25(OH)D-1alpha-hydroxylase activity. Whether this results from aberrant transcriptional regulation of the 1alpha-hydroxylase gene, CYP27B1, remains unknown. To investigate this possibility, we compared phosphate and parathyroid hormone effects on renal proximal convoluted tubule and thyrocalcitonin effects on proximal straight tubule enzyme activity and mRNA expression in normal and hyp mice. We assayed 25(OH)D-1alpha-hydroxylase activity by measuring 1,25(OH)2D production and mRNA by ribonuclease protection. Phosphate-depleted mice exhibited a 3-fold increment of 25(OH)D-1alpha-hydroxylase activity compared with normals, whereas hyp mice displayed no enhanced enzyme function. Phosphate-depleted mice concurrently displayed a 2-fold increase in mRNA transcripts; in contrast, despite failure to alter enzyme activity, hyp mice exhibited a similar increment in mRNA transcripts. Parathyroid hormone stimulation of normal mice increased 25(OH)D-1alpha-hydroxylase activity 10-fold, while eliciting only a 2-fold increment in hyp mouse enzyme function. This disparity occurred despite increments of mRNA transcripts to comparable levels (22.2 +/- 3.5- vs. 19.9 +/- 1.8-fold). The dissociation between phosphate- and parathyroid hormone-mediated transcriptional activity and protein function was not universal. Thus, thyrocalcitonin stimulation of normal and hyp mice resulted in comparable enhancement of mRNA transcripts and enzyme activity. These observations indicate that abnormal regulation of vitamin D metabolism in hyp mice occurs in the proximal convoluted tubule and results, not from aberrant transcriptional regulation, but from a defect in translational or post-translational activity.

Renal expression of the sodium/phosphate cotransporter gene, Npt2, is not required for regulation of renal 1 alpha-hydroxylase by phosphate

Several reports have suggested that the regulation of renal 1,25-dihydroxyvitamin D [1,25-(OH)(2)D] synthesis by extracellular phosphate (Pi) is dependent on normal transepithelial Pi transport by the renal tubule. Mice homozygous for the disrupted Na/Pi cotransporter gene Npt2 (Npt2(-/-)) exhibit renal Pi wasting, an approximately 85% decrease in renal brush border membrane Na/Pi cotransport, hypophosphatemia, and an increase in serum 1,25-(OH)(2)D concentration. We undertook 1) to determine the mechanism for the increased circulating levels of 1,25-(OH)(2)D in Npt2(-/-) mice and 2) to establish whether renal 1alpha-hydroxylase was appropriately regulated by dietary Pi in the absence of Npt2 gene expression. On a control diet, the 2.5-fold increase in the serum 1,25-(OH)(2)D concentration in Npt2(-/-) mice, relative to that in Npt2(+/+) littermates, is associated with a corresponding increase in renal mitochondrial 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity and messenger RNA (mRNA) abundance. A low Pi diet elicits an increase in serum 1,25-(OH)(2)D concentration, renal 1alpha-hydroxylase activity, and mRNA abundance in Npt2(+/+) and Npt2(-/-) mice to similar levels in both mouse strains. A high Pi diet has no effect on serum 1,25-(OH)(2)D concentration, renal 1 alpha-hydroxylase activity, or mRNA abundance in Npt2(+/+) mice, but normalizes these parameters in Npt2(-/-) mice. In addition, renal 24-hydroxylase mRNA abundance is significantly reduced in Npt2(-/-) mice compared with that in Npt2(+/+) mice under all dietary conditions. In summary, we demonstrate that 1) increased renal synthesis of 1,25-(OH)(2)D is responsible for the increased serum 1,25-(OH)(2)D concentration in Npt2(-/-) mice; and 2) renal 1alpha-hydroxylase gene expression is appropriately regulated by dietary manipulation of serum Pi in both Npt2(+/+) and Npt2(-/-) mice. Thus, intact renal Na/Pi cotransport is not required for the regulation of renal 1alpha-hydroxylase by Pi.

Growth hormone normalizes renal 1,25-dihydroxyvitamin D3-24-hydroxylase gene expression but not Na+-phosphate cotransporter (Npt2) mRNA in phosphate-deprived Hyp mice

The murine X-linked Hyp mutation is characterized by decreased renal expression of type II Na+-phosphate (Pi) cotransporter (Npt2) mRNA and an abnormal vitamin D response to Pi deprivation. The latter is manifest by an aberrant fall in serum 1,25-dihydroxyvitamin D3 (1,25(OH)2D) levels that is associated with an increase in renal 1,25(OH)2D-24-hydroxylase (24-hydroxylase), the first enzyme in the C-24 oxidation pathway. Because growth hormone (GH) enhances renal Na+-Pi cotransport and permits the adaptive 1,25(OH)2D response in Pi-deprived hypophysectomized rats, we examined the effects of GH on vitamin D metabolism and renal Npt2 mRNA abundance in Hyp mice fed control and low Pi diets. GH significantly decreased renal 24-hydroxylase activity (0.202+/-0.020 to 0.098+/-0.008 pmol/mg of protein/minute, p < 0.05) and mRNA abundance, relative to beta-actin mRNA (299+/-13 to 78+/-14, p < 0.05), in Hyp mice fed the low Pi diet but had no effect on either parameter in mutants fed the control diet. Moreover, after GH treatment, renal 24-hydroxylase gene expression was no longer elevated in Pi-deprived Hyp mice relative to mutants fed control diet. In contrast, GH did not correct the serum concentration of 1,25(OH)2D in Pi-deprived Hyp mice. We also demonstrate that GH did not normalize renal Npt2 mRNA expression, relative to beta-actin mRNA, in Hyp mice fed either control or low Pi diets. The present data demonstrate that normalization of renal 24-hydroxylase gene expression in Pi-deprived Hyp mice by GH is not sufficient to correct the serum concentration of 1,25(OH)2D and is not associated with an alteration in renal Npt2 mRNA expression.

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 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)

Crosstransplantation of kidneys in normal and Hyp mice. Evidence that the Hyp mouse phenotype is unrelated to an intrinsic renal defect

Although deranged phosphate transport is the fundamental abnormality in X-linked hypophosphatemic (XLH) rickets, it remains unknown if this defect is the consequence of an intrinsic kidney abnormality or aberrant production of a humoral factor. To discriminate between these possibilities, we examined phosphate homeostasis in normal and Hyp mice, subjected to renal crosstransplantation. We initially evaluated the effects of uninephrectomy on the indices of phosphate metabolism that identify the mutant biochemical phenotype. No differences were found in the serum phosphorus concentration, fractional excretion of phosphate (FEP), or tubular reabsorption of phosphate per milliliter of glomerular filtrate (TRP) in uninephrectomized normal and Hyp mice, compared with sham-operated controls. Subsequently, single kidneys from normal or Hyp mice were transplanted into normal and Hyp mouse recipients. Normal mice transplanted with normal kidneys and Hyp mice engrafted with mutant kidneys exhibited serum phosphorus, FEP, and TRP no different from those of uninephrectomized normal and Hyp mice, respectively. However, engraftment of normal kidneys in Hyp mice and mutant kidneys in normal mice affected neither serum phosphorus (4.69 +/- 0.31 and 8.25 +/- 0.52 mg/dl, respectively) nor FEP and TRP of the recipients. These data indicate that the Hyp mouse phenotype is neither corrected nor transferred by renal transplantation. Further, they suggest that the phosphate transport defect in Hyp mice, and likely X-linked hypophosphatemia, is the result of a humoral factor, and is not an intrinsic renal abnormality.

Dissociation of phosphaturia and 25(OH)D-1 alpha-hydroxylase trophism using a novel analogue of parathyroid hormone

Certain parathyroid hormone (PTH) analogues have been shown to selectively impair some but not all physiological actions of PTH. In this study, transaminated rat (r) PTH [TA-rPTH-(1-34)], a PTH analogue that differs from the rPTH-(1-34) fragment in that the NH2-terminal alanine is converted to pyruvate, was infused into mice to determine its properties in vivo and specifically to determine whether stimulation of 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity was more dependent on concomitant renal handling of phosphate or on generation of adenosine 3',5'-cyclic monophosphate (cAMP). High-performance liquid chromatography-purified TA-rPTH-(1-34) was infused into C57BL mice at 10 or 30 pmol/h for 24 h. At 30 pmol/h, TA-rPTH-(1-34) was comparable with rPTH-(1-34) in its hypophosphatemic and phosphaturic effects but was less potent than rPTH-(1-34) in raising serum calcium. TA-rPTH-(1-34) was markedly less effective in stimulating renal 1 alpha-hydroxylase than rPTH-(1-34). Stimulation of urinary cAMP excretion occurred after infusion with TA-rPTH-(1-34), but this effect was significantly less than that seen with rPTH-(1-34). These findings indicate that PTH-induced hypophosphatemia and phosphaturia can be uncoupled from PTH stimulation of 1 alpha-hydroxylase. Furthermore, cAMP-related signal transduction appears to be more significant in regulation of 1 alpha-hydroxylase than mechanisms that mediate PTH-sensitive phosphate transport, independent of cAMP.

Defective bone formation by Hyp mouse bone cells transplanted into normal mice: evidence in favor of an intrinsic osteoblast defect

The hypophosphatemic (Hyp) mouse is an animal model for human hypophosphatemic vitamin D-resistant rickets. We have reported that bone cells isolated from Hyp mice born to homozygous mutant females produce abnormal bone when transplanted into normal mice. To test whether an environmentally acquired defect of the mutant cells contributed to the impaired bone formation observed in transplants, periostea and osteoblasts from normal and Hyp littermates were transplanted intramuscularly into normal animals. To test more specifically for an hypophosphatemia-induced cell alteration before transplantation, bone cells isolated from phosphate-depleted normal mice were transplanted into normal animals. The bone nodules formed in 2 week transplants were characterized by measuring their osteoid thickness and volume. Impaired bone formation was evidenced in Hyp transplants compared to normal littermate transplants by increased osteoid thickness and volume. In contrast to cells from mutant mice, cells isolated from normal mice with comparable hypophosphatemia produced normal bone. These results indicate that the inability of Hyp osteoblasts to produce normal bone when placed in a normal environment is not the consequence of prior exposure to an altered environmental but likely of an intrinsic cellular abnormality. These observations add further support to the concept that the osteoblast is an important target for the Hyp mutation.

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.

Recent advances in pediatric metabolic bone disease: the consequences of altered phosphate homeostasis in renal insufficiency and hypophosphatemic vitamin D-resistant rickets

Over the past decade our understanding of the pathogenesis of altered mineral homeostasis in chronic renal failure (CRF) and X-linked hypophosphatemic vitamin D-resistant rickets (XLH) has increased, and has provided a rational approach for the use of the 1 alpha-hydroxylated analogues of vitamin D in their therapy. Recent evidence suggests that intracellular phosphate (Pi) retention in CRF plays a major role in decreasing serum 1,25-dihydroxyvitamin D (1,25(OH)2D) levels, which are responsible for the progressive rise in serum parathyroid hormone (PTH) concentrations through the direct action of 1,25(OH)2D on the parathyroid gland. 1,25(OH)2D levels affect the number of intracellular 1,25(OH)2D receptors, preproPTH mRNA levels and the set point for calcium suppression of PTH release. Further in experimental CRF, the maintenance of normal 1,25(OH)2D levels prevents parathyroid gland hyperplasia. These studies indicate that depressed renal 1 alpha-hydroxylase activity due to Pi retention is a major factor in directly increasing PTH secretion, which in turn contributes significantly to the severity of renal osteodystrophy. Thus the aim of therapy in early CRF should be to maintain normal levels of 1,25(OH)2D which can be achieved by either dietary Pi restriction and oral Pi binders or by administering small doses of 1 alpha-hydroxylated metabolites. The long term consequences of these two different therapeutic regimens still need to be assessed. In XLH, evidence is rapidly accumulating that alterations in 1 alpha-hydroxylase activity secondary to impaired Pi handling by the proximal renal tubule, results in decreased serum 1,25(OH)2D levels, which might be responsible for a number of the associated abnormalities documented in both treated and untreated XLH patients. These abnormalities include decreased calcium and Pi absorption by the intestine and low normal serum calcium values. In vitamin D- and Pi-treated patients 1,25(OH)2D levels are further depressed, with a resultant increase in PTH values, and the development of tertiary hyperparathyroidism in a small number of patients. The use of 1 alpha-hydroxylated analogues rather than vitamin D together with Pi supplements decreases the severity of hyperparathyroidism, improves Pi absorption from the intestine and markedly ameliorates the degree of osteomalacia. Whether long-term therapy with these analogues will prevent the development of tertiary hyperparathyroidism in patients with XLH is unclear.

Normal regulation of calcitriol production in Gy mice. Evidence for biochemical heterogeneity in the X-linked hypophosphatemic diseases

Phenotypic heterogeneity in X-linked hypophosphatemic rickets (XLH) is ascribed to variable penetrance of the genetic abnormality. However, studies of hypophosphatemic (Hyp) and gyrorotary (Gy) mice indicate that mutations at different loci along the X chromosome may underlie the genetically transmitted hypophosphatemic disorders. Thus, genetic heterogeneity may be a determinant of the phenotypic variability in XLH. To determine if such variance includes biochemical diversity, we examined whether Gy mice, similar to Hyp mice, exhibit abnormal regulation of renal 25-hydroxyvitamin D (25[OH]D)-1 alpha-hydroxylase. Serum phosphorus in Gy (4.7 +/- 0.3 mg/dl) and phosphate (P)-depleted mice (4.9 +/- 0.4) was significantly less than normal (8.4 +/- 0.5). Consistent with P depletion, the Gy mice exhibited enhanced renal 25(OH)D-1 alpha-hydroxylase activity (9.3 +/- 0.6 fmol/mg kidney per min), similar to that of P-depleted normals (9.1 +/- 1.5), but significantly greater than that of controls (3.1 +/- 0.3). Such normal enzyme responsiveness was confirmed upon PTH stimulation (1 IU/h s.c.), which revealed that Gy mice increased renal 1-hydroxylase (59 +/- 7.7) similarly to normals (65 +/- 7.7) and P-depleted animals (58.4 +/- 7.8). Calcitonin administration also enhanced enzyme function comparably in the animal models. Evidence confirming normally responsive calcitriol production in untreated Gy mice included increased serum 1,25-dihydroxyvitamin D levels, gastrointestinal calcium absorption, and urinary calcium. The normally regulated vitamin D metabolism in Gy mice indicates that biochemically diverse disease may result from mutations in the gene family regulating renal P transport and underlying X-linked hypophosphatemia. We suspect such heterogeneity is due to altered P transport at variable segments of the proximal convoluted tubule.