Renal adaptation to phosphate deprivation in the Hyp mouse with X-linked hypophosphatemia

A novel Mixture Model Method for identification of differentially expressed genes from DNA microarray data

Background

The main goal in analyzing microarray data is to determine the genes that are differentially expressed across two types of tissue samples or samples obtained under two experimental conditions. Mixture model method (MMM hereafter) is a nonparametric statistical method often used for microarray processing applications, but is known to over-fit the data if the number of replicates is small. In addition, the results of the MMM may not be repeatable when dealing with a small number of replicates. In this paper, we propose a new version of MMM to ensure the repeatability of the results in different runs, and reduce the sensitivity of the results on the parameters.

Results

The proposed technique is applied to the two different data sets: Leukaemia data set and a data set that examines the effects of low phosphate diet on regular and Hyp mice. In each study, the proposed algorithm successfully selects genes closely related to the disease state that are verified by biological information.

Conclusion

The results indicate 100% repeatability in all runs, and exhibit very little sensitivity on the choice of parameters. In addition, the evaluation of the applied method on the Leukaemia data set shows 12% improvement compared to the MMM in detecting the biologically-identified 50 expressed genes by Thomas et al. The results witness to the successful performance of the proposed algorithm in quantitative pathogenesis of diseases and comparative evaluation of treatment methods.

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.

Vitamin D metabolism and phosphate transport in developing kidney: effect of diet and mutation

In order to obtain a better understanding of the molecular mechanisms involved in phosphate reabsorption and vitamin D hormone production by mammalian kidney, we have devoted our efforts to the study of a mutant mouse model (Hyp). Studies from our laboratory have demonstrated that Na(+)-dependent phosphate transport is significantly reduced in renal brush border membrane vesicles derived from Hyp mice and that the regulation of the renal mitochondrial enzymes which metabolize 25-hydroxyvitamin D3 (25-OH-D3) is impaired in the mutant strain. The demonstration of abnormal phosphate transport and 25-OH-D3 metabolism in proximal tubule cells derived from Hyp kidney after 6-8 days in culture indicates that the mutant renal phenotype is independent of circulating factors and, therefore, intrinsic to the kidney. However, the precise relationship between these two proximal tubular abnormalities is poorly understood. Because the Hyp mutation segregates as a Mendelian trait, it is very likely that one mutant gene is responsible for the biochemical and clinical phenotype. Several hypotheses are put forth to explain the nature of the primary mutation in the Hyp mouse.

Calcitonin stimulation of renal 25-hydroxyvitamin D-1 alpha-hydroxylase activity in hypophosphatemic mice. Evidence that the regulation of calcitriol production is not universally abnormal in X-linked hypophosphatemia

Hypophosphatemia (Hyp) mice have defective regulation of 25(OH)D-1 alpha-hydroxylase activity in response to hypophosphatemia, hypocalcemia, and parathyroid hormone (PTH) administration. However, recent observations support the existence of anatomically distinct, independently regulated renal 1 alpha-hydroxylase systems in mammalian proximal convoluted and straight tubules. To more completely define the extent of the 1 alpha-hydroxylase regulatory defect in Hyp-mice, we compared enzyme maximum velocity in normal and mutants after infusion of calcitonin. Upon stimulation, renal 1 alpha-hydroxylase activity increased to similar levels in normal and Hyp-mouse renal homogenates. Moreover, time-course and dose-dependence studies revealed similar patterns of response in the animal models. Subsequently, we examined whether PTH and calcitonin stimulatory effects on enzyme activity are mediated through different mechanisms. In both animal models administration of PTH and calcitonin increased enzyme activity to levels greater than those obtained after maximal stimulation by either hormone alone, consistent with additive effects. These observations indicate that a calcitonin-sensitive component of 1 alpha-hydroxylase is not compromised in the X-linked hypophosphatemic syndrome.

Effect of the X-linked Hyp mutation on N-ethylmaleimide labelling of proteins in renal brush border membrane

The X-linked dominant mutation, hypophosphataemia (gene symbol, Hyp) is expressed in the laboratory mouse as deficient phosphate transport at the renal brush border membrane (BBM) of proximal nephron. In an attempt to identify proteins which mediate phosphate transport, we treated renal BBM vesicles prepared from mutant male (Hyp/Y) and normal male (+/Y) littermates, with radiolabelled N-ethylmaleimide (NEM), in the presence or absence of arsenate of arsenate which is a competitive inhibitor of phosphate transport. Polyacrylamide gel electrophoresis revealed labelling of membrane proteins in the 40-45 kDa range; addition of arsenate during NEM treatment inhibited labelling. These findings indicate a 40-45 kDa protein as a component of the renal BBM phosphate transport system(s). We found no difference between protein labelling of the renal BBM from Hyp/Y and +/Y mice.

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.

Mechanism of rapid phosphate (Pi) transport adaptation to a single low Pi meal in rat renal brush border membrane

Previous studies have shown that the adaptive response of tubular inorganic phosphate (Pi) transport to Pi deprivation is detectable in the whole kidney 24 h after switching rats from a high (HPD) to a low (LPD) Pi diet. In the present work we report on a more rapid adaptive response of the sodium (Na)-dependent Pi transport system located in the luminal membrane of the proximal tubule and its relation with changes in phosphatemia an parathyroid hormone status. Rats were fed HPD and trained to eat their daily ration within 1 h. After two weeks of equilibration half of the animals received a single LPD ration. 1, 2 and 4 h after the end of food consumption the animals were either sacrificed for renal cortical brush border membrane vesicle (BBMV) isolation or used for determining plasma Pi concentration, urinary excretion of Pi and cAMP. The results indicate that 2 and 4 h after the end of feeding, the Na-dependent Pi transport in BBMV was stimulated by 70 and 140% respectively in intact rats exposed for the first time to LPD. This response was preceded by a significant fall in plasma Pi concentration (HPD: 2.46 +/- 0.03, LPD: 2.04 +/- 0.05 mM), in the urinary excretion of Pi (HPD: 899.0 +/- 68.1; LPD: 6.5 +/- 3.3 mumol/ml GFR) and cAMP (HPD: 76.9 +/- 7.4, LPD: 48.2 +/- 1.4 pmol/ml GF). This last result suggested a rapid inhibition of PTH after one single LPD feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal handling of phosphate in the salivary glands of X-linked hypophosphataemic mice

In X-linked hypophosphataemia (Hyp gene) there is reduced renal tubule re-absorption of phosphate and an osteomalacic bone disorder. To determine whether altered phosphate transport also occurs in the salivary gland, saliva was analysed from normal and Hyp mice at 10 weeks of age. The effect of plasma inorganic phosphate on the composition of saliva was controlled by placing both genotypes on either a control diet or a low-P diet for three days. Inorganic phosphate, total phosphate, calcium, sodium, potassium, chloride, amylase, sialic acid, protein, osmotic pressure, percentage water, percentage inorganic solids and sample volume were measured. The Hyp mice fed the control diet showed a significant decrease in salivary-inorganic phosphate, total phosphate, osmotic pressure and sialic acid. These four variables were similarly lower in the normal mice fed the low-phosphate diet. The changes seem to stem from the low plasma inorganic phosphate and not from expression of the Hyp gene in the salivary gland. The total evidence from various organ systems suggests that the altered phosphate transport in X-linked hypophosphataemia may be restricted to the kidney and is not a generalized phosphate-transport deficiency.

Normal milk composition in lactating X-linked hypophosphatemic mice despite continued hypophosphatemia

Patients with X-linked hypophosphatemia and mice bearing the Hyp gene have reduced renal tubular reabsorption of phosphate and an osteomalacic bone disease. To test if altered phosphate transport also exists in the mammary glands, milk was analyzed from normal (n = 9) and heterozygous Hyp (n = 8) mice 14 days after giving birth. Inorganic phosphate, total phosphate, calcium, magnesium, sodium, and potassium were measured; percent cream, fat, water, and nonfat organic solids were measured; and protein was measured. No significant differences (NSD) were found except for greater sodium in Hyp milk. There was also NSD in litter weight. The lactating Hyp had a lower body weight and remained hypophosphatemic relative to lactating normals, but both groups had higher plasma phosphate than nonlactating controls of the same genotype. The data suggest that Hyp mice can accumulate a normal amount of phosphate in their milk despite the plasma phosphate being two-thirds of normal. These data, with other recent reports of different organ systems, suggest that the altered phosphate transport activity may be restricted to the kidney.

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.

Increased urinary excretion of cyclic nucleotides in X-linked hypophosphatemic (Hyp) mice

Hyp mice, a model for human X-linked hypophosphatemia, had elevated urinary cyclic AMP, cyclic GMP, and magnesium excretion compared to normal mice. The data suggest a renal origin of the urinary cyclic nucleotides. No significant differences in plasma cyclic AMP and cyclic GMP were observed between genotypes.

Autosomal hypophosphataemic bone disease responds to 1,25-(OH)2D3

We diagnosed non X-linked hypophosphataemic bone disease in a 38-month-old girl. Findings included: genu varum, shortened stature, fasting hypophosphataemia (2.3-2.5 mg/100 ml; 0.74-0.81 mmol/l), diminished theoretical renal threshold for phosphate (TmP/GFR), and osteomalacia without rickets. One patient (the father) had fasting hypophosphataemia (2.3-2.7 mg/100 ml; 0.74-0.87 mmol/l) and low TmP/GFR without osteomalacia or shortened stature. Treatment of the girl with 1,25-(OH)2D3 (1 microgram a day) raised the level of serum phosphorus, improved tubular reabsorption of phosphate, and healed the bone deformity; this combination of responses is not present in X-linked hypophosphataemia. There was no correction of hypophosphataemia or TmP/GFR with 1,25-(OH)2D3 treatment (1-3 micrograms a day) in the father.

Alkaline phosphatase activity does not mediate phosphate transport in the renal-cortical brush-border membrane

We studied (1) the effect of primary modulators of phosphate transport, namely the hypophosphataemic mouse mutant (Hyp) and low-phosphorus diet, on alkaline phosphatase activity in mouse renal-cortex brush-border membrane vesicles and (2) the effect of several primary inhibitors of alkaline phosphatase on phosphate transport. Brush-border membrane vesicles from Hyp-mouse kidney had 50% loss of Na+-dependent phosphate transport, but only 18% decrease in alkaline phosphatase activity. The low-phosphorus diet effectively stimulated Na+/phosphate co-transport in brush-border membrane vesicles (+ 118%), but increased alkaline phosphatase activity only slightly (+13%). Levamisole (0.1 mM) and EDTA (1.0 mM) inhibited brush-border membrane-vesicle alkaline phosphatase activity of 82% and 93% respectively, but had no significant effect on Na+/phosphate co-transport. We conclude that alkaline phosphatase does not play a direct role in phosphate transport across the brush-border membrane of mouse kidney.