FGF23 acts directly on renal proximal tubules to induce phosphaturia through activation of the ERK1/2-SGK1 signaling pathway

Fibroblast growth factor-23 (FGF23) is a bone-derived endocrine regulator of phosphate homeostasis which inhibits renal tubular phosphate reabsorption. Binding of circulating FGF23 to FGF receptors in the cell membrane requires the concurrent presence of the co-receptor αKlotho. It is still controversial whether αKlotho is expressed in the kidney proximal tubule, the principal site of phosphate reabsorption. Hence, it has remained an enigma as to how FGF23 downregulates renal phosphate reabsorption. Here, we show that renal proximal tubular cells do express the co-receptor αKlotho together with cognate FGF receptors, and that FGF23 directly downregulates membrane expression of the sodium-phosphate cotransporter NaPi-2a by serine phosphorylation of the scaffolding protein Na(+)/H(+) exchange regulatory cofactor (NHERF)-1 through ERK1/2 and serum/glucocorticoid-regulated kinase-1 signaling.

Akt2/PKBbeta-sensitive regulation of renal phosphate transport

AIM

The protein kinase B (PKB)/Akt is known to stimulate the cellular uptake of glucose and amino acids. The kinase is expressed in proximal renal tubules. The present study explored the influence of Akt/PKB on renal tubular phosphate transport.

METHODS

The renal phosphate transporter NaPi-IIa was expressed in Xenopus oocytes with or without PKB/Akt and Na(+) phosphate cotransport determined using dual electrode voltage clamp. Renal phosphate excretion was determined in Akt2/PKBbeta knockout mice (akt2(-/-)) and corresponding wild-type mice (akt2(+/+)). Transporter protein abundance was determined using Western blotting and phosphate transport by (32)P uptake into brush border membrane vesicles.

RESULTS

The phosphate-induced current in NaPi-IIa-expressing Xenopus oocytes was significantly increased by the coexpression of Akt/PKB. Phosphate excretion [micromol per 24 h per g BW] was higher by 91% in akt2(-/-) than in akt2(+/+) mice. The phosphaturia of akt2(-/-) mice occurred despite normal transport activity and expression of the renal phosphate transporters NaPi-IIa, NaPi-IIc and Pit2 in the brush border membrane, a significantly decreased plasma PTH concentration (by 46%) and a significantly enhanced plasma 1,25-dihydroxyvitamin D(3) concentration (by 46%). Moreover, fractional renal Ca(2+) excretion was significantly enhanced (by 53%) and bone density significantly reduced (by 11%) in akt2(-/-) mice.

CONCLUSIONS

Akt2/PKBbeta plays a role in the acute regulation of renal phosphate transport and thus contributes to the maintenance of phosphate balance and adequate mineralization of bone.

Delayed transport of tissue-nonspecific alkaline phosphatase with missense mutations causing hypophosphatasia

Hypophosphatasia is a rare genetic disease characterized by diminished bone and tooth mineralization due to deficient activity of tissue-nonspecific alkaline phosphatase (TNSALP). The disease is clinically heterogeneous due to different mutations in the TNSALP gene. In order to determine whether mutated TNSALP proteins may be sequestered, degraded, or subjected to delay in their transport to the cell membrane, we built a plasmid expressing a YFP-TNSALP fluorescent fusion protein allowing the observation of cellular localization in live cells by fluorescence confocal microscopy at different time points after transfection. We studied five mutants (c. 571G>A, c. 653T>C, c. 746G>T, c. 1363G>A and c. 1468A>T) exhibiting various levels of in vitro residual enzymatic activity. While the wild-type protein reached the membrane within the first 24h after transfection, the mutants reached the membrane with delays of 24, 48 or 72 h. For all of the tested mutations, accumulation of the mutated proteins, mainly in the Golgi apparatus, was observed. We concluded that reduced ALP activity of these TNSALP mutants results from structural disturbances and delay in membrane anchoring, and not from compromised catalytic activity.

Serum hyaluronidase aberrations in metabolic and morphogenetic disorders

Hyaluronidases are endo-glycosidases that degrade both hyaluronan (hyaluronic acid) (HA) and chondroitin sulfates. Deficiency of hyaluronidase activity has been predicted to result in a phenotype similar to that observed in mucopolysaccharidosis (MPS). In the present study, we surveyed a variety of patients with phenotypes similar to those observed in MPS, but without significant mucopolysacchariduria to determine if some are based on aberrations in serum hyaluronidase (Hyal-1) activity. The study included patients with well-characterized dysmorphic disorders occurring on genetic basis, as well as those of unkown etiology. The purpose of the study was to establish how wide spread were abnormalities in levels of circulating Hyal-1 activity. A simple and sensitive semi-quantitative zymographic procedure was used for the determination of activity. Levels of both beta-N-acetylglucosaminidase and beta-glucuronidase whose activities contribute to the total breakdown of hyaluronan (HA) were also measured, as well as the concentration of circulating HA. Among 48 patients with bone or connective tissue abnormalities, low levels of Hyal-1 activity were found in six patients compared to levels in 100 healthy donors (2.0-3.2 units/microL vs 6(+/- 1 SE) units/microL). These six patients exhibited a wide spectrum of clinical abnormalities, in particular shortened extremities: they included three patients with unknown causes of clinical symptoms, one patient with Sanfilippo disease, one of the seven patients with achondroplasia, and one with hypophosphotemic rickets. Normal levels of serum Hyal-1 activities were found in patients with Morquio disease, GM1 gangliosidosis, I cell-disease, 6 of the 7 patients with achondroplasia, Marfan's-syndrome and Ehlers-Danlos syndrome. No patient totally lacked serum Hyal-1 activity. Serum HA concentration was elevated in patients with Sanfilippo A and I-cell disease. Determination of serum and leukocyte Hyal-1 and serum HA may be useful to evaluate patients with metabolic and morphogenetic disorders.

Evidence for abnormal translational regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase activity in the hyp-mouse

Hyp-mice exhibit abnormal regulation of 25-hydroxyvitamin D [25(OH)D]-1alpha-hydroxylase activity. Previous observations suggest such aberrant modulation is posttranscriptional. To investigate this possibility further, we examined whether hyp-mice manifest abnormal translation of 25(OH)D-1alpha-hydroxylase mRNA. We compared phosphate, parathyroid, and calcitonin effects on renal 25(OH)D-1alpha-hydroxylase protein as well as mRNA and enzyme activity in normal and hyp-mice. We assayed protein by Western blots, mRNA by real-time RT-PCR, and enzyme activity by measuring 1,25-dihydroxyvitamin D production. Although phosphate-depleted mice exhibited enhanced enzyme function, with significantly increased mRNA and protein expression, hyp-mice comparably increased mRNA but failed to augment enzyme activity, concordant with an inability to increase protein expression. PTH stimulation increased mRNA and protein expression as well as enzyme activity in normal mice but in hyp-mice, despite effecting mRNA enhancement, did not increment enzyme function or protein. The inability of hypophosphatemia and PTH to increase 25(OH)D-1alpha-hydroxylase activity and protein expression in hyp-mice was not universal because calcitonin stimulation was normal, suggesting proximal convoluted tubule localization of the defect. These data, in accord with absent undue enhancement of protein expression in hyp-mice treated with protease inhibitors, establish that abberrant regulation of vitamin D metabolism results from abnormal translational activity.

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.

Structure and function of disease-causing missense mutations in the PHEX gene

The PHEX gene that is mutated in patients with X-linked hypophosphatemia (XLH) encodes a protein homologous to the M13 family of zinc metallopeptidases. The present study was undertaken to assess the impact of nine PHEX missense mutations on cellular trafficking, endopeptidase activity, and protein conformation. Secreted forms of wild-type and mutant PHEX proteins were generated by PCR mutagenesis; these included C85R, D237G, Y317F, G579R, G579V, S711R, A720T, and F731Y identified in XLH patients, and E581V, which in neutral endopeptidase 24.11 abolishes catalytic activity but not plasma membrane localization. The wild-type and D237G, Y317F, E581V, and F731Y proteins were terminally glycosylated and secreted into the medium, whereas the C85R, G579R, G579V, S711R, and A720T proteins were trapped inside the transfected cells. Growing the cells at 26 C permitted the secretion of G579V, S711R, and A720T proteins, although the yield of rescued G579V was insufficient for further analysis. Endopeptidase activity of secreted and rescued PHEX proteins, assessed using a novel internally quenched fluorogenic peptide substrate, revealed that E581V and S711R are completely inactive; D237G and Y317F exhibit 50-60% of wild-type activity; and A720T and F731Y retain full catalytic activity. Conformational analysis by limited proteolysis demonstrated that F731Y is more sensitive to trypsin and D237G is more resistant to endoproteinase Glu-c than the wild-type protein. Thus, defects in protein trafficking, endopeptidase activity, and protein conformation account for loss of PHEX function in XLH patients harboring these missense mutations.

Up-regulation of liver glucose-6-phosphatase in x-linked hypophosphatemic mice

We previously showed that a phosphate-deficient diet resulting in hypophosphatemia upregulated the catalytic subunit p36 of rat liver glucose-6-phosphatase, which is responsible for hepatic glucose production. A possible association between phosphate and glucose homeostasis was now further evaluated in the Hyp mouse, a murine homologue of human X-linked hypophosphatemia. We found that in the Hyp mouse as in the dietary Pi deficiency model, serum insulin was reduced while glycemia was increased, and that liver glucose-6-phosphatase activity was enhanced as a consequence of increased mRNA and protein levels of p36. In contrast, the Hyp model had decreased mRNA and protein levels of the putative glucose-6-phosphate translocase p46 and liver cyclic AMP was not increased as in the phosphate-deficient diet rats. It is concluded that in genetic as in dietary hypophosphatemia, elevated glucose-6-phosphatase activity could be partially responsible for the impaired glucose metabolism albeit through distinct mechanisms.

Modulation of potassium channels in the hearts of transgenic and mutant mice with altered polyamine biosynthesis

Inward rectification of cardiac I(K1)channels was modulated by genetic manipulation of the naturally occurring polyamines. Ornithine decarboxylase (ODC) was overexpressed in mouse heart under control of the cardiac alpha -myosin heavy chain promoter (alpha MHC). In ODC transgenic hearts, putrescine and cadaverine levels were highly elevated ( identical with 35-fold for putrescine), spermidine was increased 3.6-fold, but spermine was essentially unchanged. I(K1)density was reduced by identical with 38%, although the voltage-dependence of rectification was essentially unchanged. Interestingly, the fast component of transient outward (I(to,f)) current was increased, but the total outward current amplitude was unchanged. I(K1)and I(to)currents were also studied in myocytes from mutant Gyro (Gy) mice in which the spermine synthase gene is disrupted, leading to a complete loss of spermine. I(K1)current densities were not altered in Gy myocytes, but the steepness of rectification was reduced indicating a role for spermine in controlling rectification. Intracellular dialysis of myocytes with putrescine, spermidine and spermine caused reduction, no change and increase of the steepness of rectification, respectively. Taken together with kinetic analysis of I(K1)activation these results are consistent with spermine being a major rectifying factor at potentials positive to E(K), spermidine dominating at potentials around and negative to E(K), and putrescine playing no significant role in rectification in the mouse heart.

Molecular cloning of a novel human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, GalNAc-T8, and analysis as a candidate autosomal dominant hypophosphatemic rickets (ADHR) gene

The UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) family of enzymes initiates mucin-like O-glycosylation of specific proteins. Using exon-prediction analysis on genomic sequence from human chromosome 12p13.3, we identified novel exons that shared significant homology with the ppGaNTases. cDNA library screening and RT-PCR produced the complete coding sequence of a novel human ppGaNTase family member, designated GalNAc-T8. The open reading frame (ORF) of GalNAc-T8 codes for a 637 amino acid, type-II membrane protein that is 45-60% identical to the other mammalian ppGaNTases. GalNAc-T8 shares high homology within the functional regions of the known ppGaNTases; however, the enzyme possesses a novel residue substitution within a characteristic motif of the catalytic domain. Northern analysis of multiple human tissue mRNAs demonstrated that the 5.0 and 2.1kb GalNAc-T8 transcripts are widely expressed. The metabolic disorder autosomal dominant hypophosphatemic rickets (ADHR) was previously mapped to the region of chromosome 12p13.3 in which GalNAc-T8 resides. Using a positional-candidate strategy for identifying the ADHR gene, GalNAc-T8 was subjected to mutational analysis in DNA from ADHR individuals. We detected multiple polymorphisms in the human GalNAc-T8 ORF, but did not find ADHR mutations. In summary, these studies identified the human GalNAc-T8 gene, as well as multiple genomic polymorphisms that will be useful for further understanding the structure-function relations of the ppGaNTases.

Comparison of total alkaline phosphatase and three assays for bone-specific alkaline phosphatase in childhood and adolescence

We compared serum levels of total alkaline phosphatase (TAP) and bone-specific alkaline phosphatase (BAP) as determined by three different assays (lectin affinity electrophoresis, immunoradiometric assay, enzyme-linked immunosorbent assay) in subjects aged 5-20 years suffering from X-linked hypophosphatemic rickets (n = 14), chronic renal failure (n = 10) and chronic cholestatic liver disease (n = 16). Results were compared to controls of the same age and were expressed as standard deviation scores (SDS). TAP correlated significantly with BAP (r > 0.9 for each assay; p < 0.001) in controls. In children with cholestatic diseases, TAP (median SDS + 2.0) was elevated, but BAP, as measured by the electrophoretic assay, was within the reference range for most patients (median SDS: -0.4; p = 0.003 for the difference between the median SDS of TAP and BAP). In contrast, results for BAP as determined by the two immunoassays were not significantly different from TAP in any of the three patient groups (p > 0.05 in each group for both assays). In this study, the two immunoassays did not have a detectable advantage over lectin affinity electrophoresis in the determination of BAP.

A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium

X-linked hypophosphatemic rickets (HYP) is a dominant disorder characterised by impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules. By positional cloning, we have isolated a candidate gene from the HYP region in Xp22.1. This gene exhibits homology to a family of endopeptidase genes, members of which are involved in the degradation or activation of a variety of peptide hormones. This gene (which we have called PEX) is composed of multiple exons which span at least five cosmids. Intragenic non-overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients, which suggest that the PEX gene is involved in the HYP disorder.

Sodium-phosphate transport in the kidney and intestine of the hypophosphatemic mouse

X-linked hypophosphatemic vitamin D-resistant rickets is the most common inherited form of vitamin D-resistant rickets in man. The current studies were designed to characterize the defect in the sodium (Na+)-phosphate transporter in the (Hyp) mouse model. The slope of initial rate of phosphate uptake was significantly decreased in the kidney but not in intestinal brush border membranes of the (Hyp) mice compared with genetically matched controls. Phosphate uptake by the basolateral membranes of the intestine and kidney was similar in the (Hyp) and control mice. Kinetic analysis of phosphate uptake by renal brush border membranes showed a Vmax of 0.32 +/- 0.06 and 1.6 +/- 0.1 nmol/mg protein per 15 s (P < 0.01) and Km of 0.07 +/- 0.06 and 0.39 +/- 0.05 mM in (Hyp) and control mice respectively (P < 0.05). Vmax and Kmax of jejunal uptake of phosphate were similar in (Hyp) and control mice. To confirm these findings, we expressed the Na(+)-phosphate transporter in Xenopus laevis oocytes. Na(+)-dependent phosphate uptake in the oocytes was expressed 6 days after renal and intestinal poly(A)+ RNA injection, however, uptake values were significantly lower in oocytes injected with renal poly(A)+ RNA from the (Hyp) mice compared with controls (P < 0.01). No differences were noted in phosphate uptake by oocytes injected with poly(A)+ RNA from the jejunum of the (Hyp) or control mice. These studies suggest that the defect in the (Hyp) mice is localized to the kidney and is secondary to diminished activity and/or function of the Na(+)-phosphate transporter.

25-Hydroxyvitamin D-24-hydroxylase in phytohemagglutinin-stimulated lymphocytes: intermediate bioresponse to 1,25-dihydroxyvitamin D3 of cells from parents of patients with vitamin D-dependent rickets type II

A method for assay of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) activity in phytohemagglutinin (PHA)-stimulated lymphocytes was applied to determine whether vitamin D-dependent rickets type II (VDDR II) is hereditary. In normal lymphocytes incubated with PHA for 3 days, maximal and half-maximal responses of 24-hydroxylase were observed after exposure to 10(-8) mol/L and (1.3 +/- 0.4) x 10(-9) mol/L 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], respectively. These responses were similar to those of cultured skin fibroblasts. In contrast, after exposure to 10(-8), 10(-7), and 10(-6) mol/L 1,25-(OH)2D3, no 24-hydroxylase activity was detected in cells from patients with VDDR II, and intermediate activity was observed in cells from their parents. These findings indicated the presence of an intracellular receptor-effector system for 1,25-(OH)2D3 in peripheral lymphocytes. Heterozygotes of VDDR II could be identified, and autosomal recessive inheritance of the disease was demonstrated. Detection of heterozygotes of this disease was not possible by assay of inhibition of thymidine incorporation, another marker of the function of 1,25-(OH)2D3 in PHA-stimulated lymphocytes. Therefore, assay of 24-hydroxylase induction reflected the receptor status more closely than assay of inhibition of DNA biosynthesis. The assay of 24-hydroxylase activity in PHA-stimulated lymphocytes described here will be useful for diagnosis of VDDR II and study of families of patients with this disease.

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

Previously we have established that abnormal regulation of renal 25-hydroxyvitamin D (25OHD)-1 alpha-hydroxylase in Hyp mice involves the PTH-adenylate cyclase component of enzyme activation. However, it remains unknown if the muted effects of PTH result from 1) abnormal second messenger production or 2) an intracellular defect limiting enzyme activation. To distinguish between these possibilities, we compared cAMP stimulation of renal 25OHD-1 alpha-hydroxylase in normal, phosphate-depleted normal, and Hyp mice. Administration of N6-monobutyryl cAMP iv (200 mg/kg/day) increased enzyme activity in normal (4.1 +/- 1.7 vs. 40.7 +/- 7.0 fmol/mg kidney.min) and phosphate-depleted mice (13.3 +/- 1.8 vs. 78.2 +/- 10.4) to a level significantly greater than that achieved in Hyp mice (7.4 +/- 1.1 vs. 22.7 +/- 3.6). Moreover, similar to our observations after PTH stimulation, the apparent abnormal cAMP effect did not result from an altered time course of enzyme activation or a rightward shift in the dose response. Collectively, these data indicate that abnormal regulation of 1,25-dihydroxyvitamin D production in Hyp mice results from aberrant intracellular regulation of 25OHD-1 alpha-hydroxylase, a defect probably related to deranged phosphate transport in the renal tubule.

Abnormal parathyroid hormone stimulation of 25-hydroxyvitamin D-1 alpha-hydroxylase activity in the hypophosphatemic mouse. Evidence for a generalized defect of vitamin D metabolism

Abnormal regulation of vitamin D metabolism is a feature of X-linked hypophosphatemic rickets in man and of the murine homologue of the disease in the hypophosphatemic (Hyp)-mouse. We previously reported that mutant mice have abnormally low renal 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity for the prevailing degree of hypophosphatemia. To further characterize this defect, we examined whether Hyp-mouse renal 1 alpha-hydroxylase activity responds normally to other stimulatory and inhibitory controls of enzyme function. We studied stimulation by parathyroid hormone (PTH) using: (a) a calcium-deficient (0.02% Ca) diet to raise endogenous PTH; or (b) 24-h continuous infusion of 0.25 IU/h bovine PTH via osmotic minipump. In both cases enzyme activity of identically treated normal mice increased to greater levels than those attained by Hyp-mice. The relative inability of PTH to stimulate 1 alpha-hydroxylase activity is not a function of the hypophosphatemia in the Hyp-mouse since PTH-infused, phosphate-depleted normal mice sustained a level of enzyme activity greater than that of normal and Hyp-mice. In further studies we investigated inhibition of enzyme activity by using: (a) a calcium-loaded (1.2% Ca) diet to suppress endogenous PTH; or (b) 24-h continuous infusion of 0.2 ng/h 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). The 1 alpha-hydroxylase activity of normal and Hyp-mice was significantly reduced to similar absolute levels following maintenance on the calcium-loaded diet. Further, infusion of 1,25(OH)2D3 caused a comparable reduction of 1 alpha-hydroxylase activity in normal, Hyp-, and phosphate-depleted normal mice. These observations indicate that the inhibitory control of 1 alpha-hydroxylase by reduced levels of PTH or increased 1,25(OH)2D3 concentrations is intact in the mutants. However, the inability of PTH and hypophosphatemia to stimulate enzyme activity in a manner analogous to that in normal and phosphate-depleted mice indicates that a generalized defect of 1 alpha-hydroxylase regulation is manifest in Hyp-mice.

Protein kinase activity and protein kinase inhibitor in mouse kidney: effect of the X-linked Hyp mutation and vitamin D status

cAMP-dependent protein kinase, Ca+2- and phospholipid-dependent protein kinase, and protein kinase inhibitor activity were examined in renal homogenates and 20,000 X g supernatant fractions of normal and Hyp mice. In both genotypes, 70% of total renal cAMP-dependent protein kinase activity was recovered in the soluble fraction in which the activity ratio (without cAMP to with cAMP) of the enzyme was 0.35. The requirement for cAMP was not different for protein kinase of normal and mutant littermates, with an apparent Km for cAMP of 0.05 microM in both genotypes. Furthermore, vitamin D and calcium deficiencies did not significantly affect cAMP-dependent protein kinase activity in normal and Hyp mouse kidney. The concentration of the heat-stable protein kinase inhibitor protein in the 20,000 X g supernatant fraction was identical in normal and Hyp kidney. Whereas protein kinase inhibitor levels were increased 1.8-fold by vitamin D and calcium deficiencies in normal mice (P less than 0.001), no such increase was detectable in Hyp mice. Ca+2- and phospholipid-dependent-protein kinase (protein kinase C) activity in the 20,000 X g supernatant fraction comprised 50% of the total activity of kidney homogenates of both normal and mutant mice. The initial rate of protein kinase C was increased 1.5-fold in kidney supernatants of Hyp mice (P less than 0.001). In contrast, protein kinase C was not significantly different from normal in supernatant fractions of heart, spleen, and liver prepared from Hyp mice. The present demonstration of abnormally high renal protein kinase C activity in Hyp mice may serve to explain the relationship between the previously reported renal defects in brush border membrane phosphate transport and vitamin D metabolism in the mutant strain and elucidate the nature of the primary defect in the Hyp mouse.

Vitamin D-dependent rickets type I in pigs

We have bred a strain of pigs with an inherited condition of hypocalcaemic rickets, transmitted by an autosomal-recessive mechanism. Homozygous (affected) piglets grew at half the rate of their heterozygous (clinically normal) littermates, and developed profound hypocalcaemia with severe secondary hyperparathyroidism and hypophosphataemia by 8 weeks of age. In the hypocalcaemic piglets, plasma 1,25-dihydroxyvitamin D levels were low or undetectable, and 24,25-dihydroxyvitamin D3 levels were also reduced despite 25-hydroxyvitamin D3 levels being 2-fold higher. There was no detectable 25-hydroxyvitamin D3-1- or -24-hydroxylase enzyme activity in renal homogenates prepared from affected animals. Plasma and intestinal calcium-binding protein levels were reduced in the hypocalcaemic piglets. Sucrose density gradient analysis of intestinal cytosol, prepared in high-salt buffer, revealed the presence of a similar amount of a specific less than 4.2S 1,25-dihydroxyvitamin D3 binder in both groups of piglets. Administration of pharmacological doses of vitamin D3 to affected animals reversed the hypocalcaemia. We conclude that this strain of pigs has vitamin D-dependent rickets type I.

Activity of renal 25-hydroxyvitamin D3-1 alpha-hydroxylase in a case of X-linked hypophosphataemic rickets

In 1974, a 2-year-old boy was diagnosed as having X-linked hypophosphataemic rickets (XLH) because of severe rickets and hypophosphataemia. The vitamin D metabolite concentrations, blood and urine chemistry and renal 25-hydroxyvitamin D3 (25OHD3)-1 alpha-hydroxylase were measured in 1982 (about 2 weeks after withdrawal of medication). 1 alpha-hydroxylase was 392 pg/mg tissue/20 min in the patient, which was high compared with aged-matched controls (69.7 +/- 28.5 pg/mg tissue/20 min, mean +/- SD, n = 7). Our present studies showed that the 1 alpha-hydroxylase activity in the patient with XLH was elevated. Therefore, the normal or low 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) concentrations in XLH patients could be due to accelerated catabolism of 1,25-(OH)2D3 or abnormally regulated 25OHD3-1 alpha-hydroxylase in response to hypophosphataemia, although significantly elevated above that in normal controls.

Investigation of the mechanism for abnormal renal 25-hydroxyvitamin D3-1-hydroxylase activity in the X-linked Hyp mouse

Renal mitochondria from mutant hypophosphatemic male mice (Hyp/Y) fed a vitamin D-deficient, low calcium diet synthesize significantly less 1,25-dihydroxyvitamin D3 than mitochondria from normal male (+/Y) littermates on the same diet. Kinetic studies reveal that maximum velocity (Vmax) for 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) is lower in Hyp/Y relative to +/Y mice (0.21 +/- 0.02 vs. 1.06 +/- 0.12 pmol/mg protein X min) whereas the apparent Michaelis-Menten constant (Km) for the reaction is not different in both genotypes (0.55 +/- 0.05 vs. 0.50 +/- 0.08 microM). The presence of an inhibitor for 1-hydroxylase activity in renal mitochondria of Hyp/Y mice was ruled out by estimating enzyme activity in mixtures of renal mitochondria from +/Y and Hyp/Y mice. Phosphate in the incubation medium stimulated 1-hydroxylase activity in +/Y mitochondria. In Hyp/Y mice, the stimulation achieved was smaller in magnitude and the added phosphate did not restore mutant 1-hydroxylase activity to normal. The vitamin D-deficient, low calcium diet led to a significant and comparable increase in serum PTH and urinary excretion of cAMP in +/Y and Hyp/Y, suggesting that the mutant strain had an appropriate PTH response to the diet-induced fall in serum calcium. Furthermore, the fractional excretion index of phosphate which is significantly greater in Hyp/Y than +/Y mice fed the control diet increased 3-fold in both genotypes fed the vitamin D-deficient, low calcium diet. These results suggest that the abnormal renal 1-hydroxylase response in Hyp mice is not the result of generalized renal resistance to PTH in the mutant strain and suggest that the defect in Hyp/Y mice may reside at a regulatory step subsequent to cAMP production.

Abnormal regulation of 25-hydroxyvitamin D3-1 alpha-hydroxylase activity by calcium and calcitonin in renal cortex from hypophosphatemic (Hyp) mice

25-Hydroxyvitamin D3-1 alpha-hydroxylase activity was assayed in primary serum-free monolayer tissue culture of renal cortical cells from hypophosphatemic (Hyp) mice and normal litter mates. Morphological and growth characteristics of cells from the two genotypes were indistinguishable. Basal enzyme activity was not significantly different in either type of cell over a wide range of substrate concentration. The enzyme from both genotypes was stimulated by PTH and suppressed by increased phosphate concentration in the culture medium. Whereas 1 alpha-hydroxylase activity in cells from normal mice was increased in low calcium medium and suppressed in high calcium medium, the enzyme in cells from Hyp mice was not altered by similar changes in the medium calcium concentration. Salmon calcitonin caused a significant increase in 1 alpha-hydroxylase in cells from normal mice, but did not stimulate enzyme activity in cells from Hyp mice. These studies indicate that control of 1 alpha-hydroxylase activity is abnormal in renal cortical cells from Hyp mice. Impaired control of this enzyme could result in the inappropriately low circulating concentrations of 1,25-dihydroxyvitamin D3 that have been observed in humans with hypophosphatemic rickets and in the relatively low activity of 1 alpha-hydroxylase in renal cortical homogenates of Hyp mice compared to that in normal mice on a low phosphate diet.

Abnormal renal mitochondrial 25-hydroxyvitamin D3-1-hydroxylase activity in the vitamin D and calcium deficient X-linked Hyp mouse

Despite severe hypophosphatemia, plasma levels of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) are not significantly elevated in the Hyp mouse, a murine homologue of X-linked hypophosphatemia in man. To examine the effect of the Hyp mutation on vitamin D hormone biosynthesis, the metabolism of 25-hydroxyvitamin D3 (25-OH-D3) by isolated renal mitochondria was studied. The ability of a vitamin D deficient, low calcium diet to stimulate renal mitochondrial 25-hydroxyvitamin D3-1-hydroxylase activity (1-OHase) in normal mice (n = 22) and Hyp littermates (n = 20) was examined. Both genotypes responded to the diet with an increase in 1-OHase activity and a decrease in 25-hydroxyvitamin D3-24-hydroxylase activity (24-OHase). The increase in 1-OHase activity, however, was significantly lower in Hyp mice (8-fold) than in normal littermates (13-fold, P less than 0.001). In spite of depressed 1-OHase in the mutant strain, enzyme activity was significantly correlated with serum calcium concentration in both normal and Hyp mice. The present results provide direct evidence for an abnormal 1-OHase response in renal mitochondria of Hyp mouse.

Abnormal regulation of renal 25-hydroxyvitamin D-1 alpha-hydroxylase activity in the X-linked hypophosphatemic mouse

Abnormal vitamin D metabolism has been suspected in patients with X-linked hypophosphatemic rickets (XLH) and X-linked hypophosphatemic mice (Hyp-mice), the murine homologue of the human disease. We compared 25(OH)D-1 alpha-hydroxylase activity in the Hyp-mouse kidney to that in normal and phosphate-depleted mouse kidney. Weanling normal and Hyp-mice were fed a 0.6% phosphorus diet; phosphate-depleted mice received a 0.02% phosphorus diet. At 8-10 wk of age the serum phosphorus values in Hyp (3.35 +/- 0.12 mg/dl) and phosphate-depleted mice (3.83 +/- 0.56) were not significantly different. Despite the similar magnitude of phosphate depletion, however, the maximum levels of 25(OH)D-1 alpha-hydroxylase activity were disparate: phosphate-depleted mouse kidney had profoundly increased activity compared to normal (17.04 +/- .104 vs. 4.96 +/- 0.23 fmol 1,25(OH)2D3 produced/mg kidney per min) while Hyp-mouse kidney had a fourfold lesser increment (8.18 +/- 0.62). These data indicate that phosphate depletion is a potent stimulus of 25(OH)D-1 alpha-hydroxylase activity in the (C57BL6J) mouse. Moreover, the results show that abnormal regulation of 25(OH)D-1 alpha-hydroxylase activity is manifest in the Hyp-mouse.

Abnormal 24-hydroxylation of 25-hydroxyvitamin D in the X-linked hypophosphatemic mouse

The effect of extracellular phosphate on the control of 25-hydroxyvitamin D3 24-hydroxylase was studied in normal mice and littermates with X-linked hypophosphatemic rickets (Hyp). 24-Hydroxylase activity and plasma concentrations of 24,25-dihydroxyvitamin D3 were significantly higher in Hyp mice than in normal mice when both groups were fed a normal diet containing 1.22% calcium (Ca) and 0.8% phosphorus (Pi). The differential in 24-hydroxylase activity was exaggerated when serum phosphate was reduced in normal mice by means of a low Pi diet or increased in Hyp mice by means of a high Pi diet. Differences in 24-hydroxylase activity between the two groups of mice were also demonstrated in the presence of varying Pi concentrations in vitro. Thus, in both Hyp and normal mice, 24-hydroxylase activity is influenced in a qualitatively similar manner by serum Pi. Plasma concentrations of 1,25-dihydroxyvitamin D3 were the same in normal and Hyp mice. The data are consistent with the hypothesis that control the renal metabolism of 25-hydroxyvitamin D3 in Hyp mice is reset such tht 24-hydroxylase activity is inappropriate high for the prevailing serum phosphate over a wide range of concentrations.

Parathyroid hormone effects on serum 1,25-dihydroxyvitamin D levels in patients with X-linked hypophosphatemic rickets: evidence for abnormal 25-hydroxyvitamin D-1-hydroxylase activity

Patients with X-linked hypophosphatemic rickets (XLH) have normal or marginally low serum 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels despite manifesting hypophosphatemia and phosphate depletion, which increase 1,25-(OH)2D production in many animal species. These data are consistent with the possibility that regulation of vitamin D metabolism is abnormal in XLH. However, controversy concerning the role of phosphate in the regulation of 25-hydroxyvitamin D-1-hydroxylase activity in man has raised doubt about this proposed defect. The presence of a defect in vitamin D metabolism could be established if hormonal or metabolic factors, other than hypophosphatemia, were unable to stimulate 25-hydroxyvitamin D-1-hydroxylase activity normally in patients with XLH. Thus, we compared the effects of parathyroid hormone infusion on serum 1,25-(OH)2D levels in patients with XLH and normals. In response to iv infusion of parathyroid extract (200 U at 0915 and 1700 h), the serum 1,25-(OH)2D concentration increased 218% above base line (from 34.0 +/- 3.0 to 108.8 +/- 2.5 pg/ml) in normals and only 68% (from 30.6 +/- 3.0 to 48.8 +/- 5.5 pg/ml) in patients with XLH. The disparate response occurred in spite of an equivalent increase in urinary cAMP excretion in the normals (from 3.00 +/- 0.14 to 8.70 +/- 0.25 mumol/g creatinine . 24 h) and XLH patients (from 3.10 +/- 0.39 to 8.30 +/- 1.0 mumol/g creatinine . 24 h) as well as equivalent decreases in the renal tubular maximum for the reabsorption of phosphate per liter glomerular filtrate (1.2 +/- 0.1 and 0.9 +/- 0.2 mg/dl, respectively). These observations support the possibility that regulation of vitamin D metabolism is abnormal in XLH.

Early biochemical findings in familial hypophosphataemic, hyperphosphaturic rickets and response to treatment

Regular biochemical measurements were made in 4 babies, each of whom had one parent with familial hypophosphataemic, hyperphosphaturic rickets. Hypophosphataemia developed by 2 months and levels of alkaline phosphatase had increased by 3 months in all four. Decreased tubular reabsorption of phosphate and x-ray changes of rickets did not develop until 6 months in 3 of the babies. In the fourth these abnormalities developed at 9 days and 3 months. The babies were treated with oral phosphate and small doses of 1-alpha-hydroxy-cholecalciferol. The rickets healed readily in 3 babies and their linear growth kis within the normal range. Healing took much longer in the remaining child and his linear growth is below the 3rd centile. Hypercalcaemia has not been a problem of treatment.

Hormone-sensitive adenylate cyclase along the nephron of genetically hypophosphatemic mice

The response of the adenylate cyclase (AC) activity to PTH and calcitonin was measured along the nephron of normal (N) and mutant hypophosphatemic (Hyp) mice of the C 57 BL/6J strain, using in vitro single tubule AC microassay. In each experiment, a Hyp mouse was paired to a N mouse from the same litter. In the presence of PTH (10 U/ml), AC activities (femtomoles cAMP per millimeter of tubule per 30-min incubation) were reduced in the proximal convoluted tubule of Hyp mice as compared to N mice in all experiments (448 +/- (SEM) 46 vs. 831 +/- 79, N = 4, P less than 0.01). Some decrease in AC response to PTH also was noted in the cortical portion of the thick ascending limb of the loop of Henle (476 +/- 70 in Hyp mice vs. 719 +/- 83 in N mice, N = 4, P = NS). The Hyp and N AC responses to PTH were similar in the "bright" and "granular" portions of the distal convoluted tubule (1524 +/- 177 in Hyp mice and 1538 +/- 228 in N mice, N = 4). The other segments tested were not responsive to PTH (except the pars recta of the proximal tubule). In the presence of salmon calcitonin (10 ng/ml), a striking 5- to 12-fold increase in AC activity of the "bright" and "granular" portions of the distal convoluted tubule was observed in each Hyp mouse as compared to its paired N control (2434 +/- 618 vs. 399 +/- 56, N = 6, P less than 0.01). The AC response to calcitonin was also increased, though to a lesser extnet (Hyp/N = 1.8) in the "light" portion of the distal tubule (590 +/- 60 in Hyp and 352 +/- 36 in N mice, P less than 0.01). Other segments of the mouse nephron were also observed to contain calcitonin-sensitive AC, but the responses were of limited magnitude only and were not statistically different in Hyp and N mice. Dose-response curves showed that the decrease of the response to PTH in the proximal tubule as well as the increase of the response to calcitonin in the distal tubule were present in Hyp mice for the whole range of hormone concentrations tested. In both structures, the apparent Km for the cyclase activation by the hormone was similar in the Hyp and its paired N mouse.

Serum 25-hydroxyvitamin D in infantile rickets

In small children with nutritional vitamin D deficiency, the serum concentration of 25-hydroxyvitamin D (25-OH-D), the major circulating metabolite of vitamin D, was correlated with the stage of clinical disease. It was low (16 to 20 ng/ml) but within the normal range in the earliest (hypocalcemic) stage of the deficiency syndrome and decreased (less than 15 ng/ml) in the more advanced stages. In patients with familial hypophosphatemia (X-linked dominant), mean serum 25-OH-D concentration was the same as in age-matched normal controls. Evidence is presented that endogenous parathyroid hormone may have a role in the depletion of serum 25-OH0D in deficiency states.