Posttranscriptional regulation of the proximal tubule NaPi-II transporter in response to PTH and dietary P(i)

(Pro)renin Receptor Regulates Phosphate Homeostasis in Rats via Releasing Fibroblast Growth Factor-23

Phosphate (Pi) is one of the basic necessities required for sustenance of life and its metabolism largely relies on excretory function of the kidney, a process chiefly under the endocrine control of bone-derived fibroblast growth factor 23 (FGF23). However, knowledge gap exists in understanding the regulatory loop responsible for eliciting phophaturic response to Pi treatment. Here, we reported a novel role of (pro)renin receptor (PRR) in mediating phosphaturic response to Pi treatment via upregulation of FGF23 production. Male Sprague-Dawley rats were pretreated for 5 days via osmotic pump-driven infusion of a PRR antagonist PRO20 or vehicle, and then treated with high Pi (HP) solution as drinking fluid for the last 24 h. PRO20 reduced HP-induced Pi excretion by 42%, accompanied by blunted upregulation of circulating FGF23 and parathyroid hormone (PTH) and downregulation of renal Na/Pi-IIa expression. In cultured osteoblast cells, exposure to HP induced a 1.56-fold increase in FGF23 expression, which was blunted by PRO20 or siRNA against PRR. Together, these results suggest that activation of PRR promotes phosphaturic response through stimulation of FGF23 production and subsequent downregulation of renal Na/Pi-IIa expression.

The Causes of Hypo- and Hyperphosphatemia in Humans

Phosphate homeostasis involves several major organs that are the skeleton, the intestine, the kidney, and parathyroid glands. Major regulators of phosphate homeostasis are parathormone, fibroblast growth factor 23, 1,25-dihydroxyvitamin D, which respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption, through the modulation of expression of transcellular transporters at the intestinal and/or renal tubular level. Any acquired or genetic dysfunction in these major organs or regulators may induce hypo- or hyperphosphatemia. The causes of hypo- and hyperphosphatemia are numerous. This review develops the main causes of acquired and genetic hypo- and hyperphosphatemia.

Significant Species Differences in Intestinal Phosphate Absorption between Dogs, Rats, and Monkeys

A treatment for hyperphosphatemia would be expected to reduce mortality rates for CKD and dialysis patients. Although rodent studies have suggested sodium-dependent phosphate transporter type IIb (NaPi-IIb) as a potential target for hyperphosphatemia, NaPi-IIb selective inhibitors failed to achieve efficacy in human clinical trials. In this study, we analyzed phosphate metabolism in rats, dogs, and monkeys to confirm the species differences. Factors related to phosphate metabolism were measured and intestinal phosphate absorption rate was calculated from fecal excretion in each species. Phosphate uptake by intestinal brush border membrane vesicles (BBMV) and the mRNA expression of NaPi-IIb, PiT-1, and PiT-2 were analyzed. In addition, alkaline phosphatase (ALP) activity was evaluated. The intestinal phosphate absorption rate, including phosphate uptake by BBMV and NaPi-IIb expression, was the highest in dogs. Notably, urinary phosphate excretion was the lowest in monkeys, and their intestinal phosphate absorption rate was by far the lowest. Dogs and rats showed positive correlations between V_max/K_m of phosphate uptake in BBMV and NaPi-IIb expression. Although phosphate uptake was observed in the BBMV of monkeys, NaPi-IIb expression was not detected and ALP activity was low. This study revealed significant species differences in intestinal phosphate absorption. NaPi-IIb contributes to intestinal phosphate uptake in rats and dogs. However, in monkeys, phosphate is poorly absorbed due to the slight degradation of organic phosphate in the intestine.

Mechanisms and regulation of epithelial phosphate transport in ruminants: approaches in comparative physiology

Ruminants have a unique utilization of phosphate (P_i) based on the so-called endogenous P_i recycling to guarantee adequate P_i supply for ruminal microbial growth and for buffering short-chain fatty acids. Large amounts of P_i enter the gastrointestinal tract by salivary secretion. The high saliva P_i concentrations are generated by active secretion of P_i from blood into primary saliva via basolateral sodium (Na⁺)-dependent P_i transporter type II. The following subsequent intestinal absorption of P_i is mainly carried out in the jejunum by the apical located secondary active Na⁺-dependent P_i transporters NaPi IIb (SLC34A2) and PiT1 (SLC20A1). A reduction in dietary P_i intake stimulates the intestinal P_i absorption by increasing the expression of NaPi IIb despite unchanged plasma 1,25-dihydroxyvitamin D₃ concentrations, which modulate P_i homeostasis in monogastric species. Reabsorption of glomerular filtrated plasma P_i is mainly mediated by the P_i transporters NaPi IIa (SLC34A1) and NaPi IIc (SLC34A3) in proximal tubule apical cells. The expression of NaPi IIa and the corresponding renal Na⁺-dependent P_i capacity were modulated by high dietary phosphorus (P) intake in a parathyroid-dependent manner. In response to reduced dietary P_i intake, the expression of NaPi IIa was not adapted indicating that renal P_i reabsorption in ruminants runs at a high level allowing no further increase when P intake is diminished. In bones and in the mammary glands, Na⁺-dependent P_i transporters are able to contribute to maintaining P_i homeostasis. Overall, the regulation of P_i transporter activity and expression by hormonal modulators confirms substantial differences between ruminant and non-ruminant species.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Renal localization and regulation by dietary phosphate of the MCT14 orphan transporter

MCT14 is an orphan transporter belonging to the SLC16 transporter family mediating the transport of monocarboxylates, aromatic amino acids, creatine, and thyroid hormones. The expression, tissue localization, regulation, and function of MCT14 are unknown. In mouse MCT14 mRNA abundance is highest in kidney. Using a newly developed and validated antibody, MCT14 was localized to the luminal membrane of the thick ascending limb of the loop of Henle colocalizing in the same cells with uromodulin and NKCC2. MCT14 mRNA and protein was found to be highly regulated by dietary phosphate intake in mice being increased by high dietary phosphate intake at both mRNA and protein level. In order to identify the transport substrate(s), we expressed MCT14 in Xenopus laevis oocytes where MCT14 was integrated into the plasma membrane. However, no transport was discovered for the classic substrates of the SLC16 family nor for phosphate. In summary, MCT14 is an orphan transporter regulated by phosphate and highly enriched in kidney localizing to the luminal membrane of one specific nephron segment.

Hypophosphatemic rickets: lessons from disrupted FGF23 control of phosphorus homeostasis

Fibroblast growth factor-23 (FGF23) regulates phosphate reabsorption in the kidney and therefore plays an essential role in phosphate balance in humans. There is a host of defects that ultimately lead to excess FGF23 levels and thereby cause renal phosphate wasting and hypophosphatemic rickets. We describe the genetic, pathophysiologic, and clinical aspects of this group of disorders with a focus on X-linked hypophosphatemia (XLH), the best characterized of these abnormalities. We also discuss autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR) and tumor-induced osteomalacia (TIO) in addition to other rarer FGF23-mediated conditions. We contrast the FGF23-mediated disorders with FGF23-independent hypophosphatemia, specifically hypophosphatemic rickets with hypercalciuria (HHRH). Errant diagnosis of hypophosphatemic disorders is common. This review aims to enhance the recognition and appropriate diagnosis of hypophosphatemia and to guide appropriate treatment.

Exome sequencing identifies a novel homozygous mutation in the phosphate transporter SLC34A1 in hypophosphatemia and nephrocalcinosis

CONTEXT

Two Argentinean siblings (a boy and a girl) from a nonconsanguineous family presented with hypercalcemia, hypercalciuria, hypophosphatemia, low parathyroid hormone (PTH), and nephrocalcinosis.

OBJECTIVE

The goal of this study was to identify genetic causes of the clinical findings in the two siblings.

DESIGN

Whole exome sequencing was performed to identify disease-causing mutations in the youngest sibling, and a candidate variant was screened in other family members by Sanger sequencing. In vitro experiments were conducted to determine the effects of the mutation that was identified.

PATIENTS AND OTHER PARTICIPANTS

Affected siblings (2 y.o. female and 10 y.o male) and their parents were included in the study. Informed consent was obtained for genetic studies.

RESULTS

A novel homozygous mutation in the gene encoding the renal sodium-dependent phosphate transporter SLC34A1 was identified in both siblings (c.1484G>A, p.Arg495His). In vitro studies showed that the p.Arg495His mutation resulted in decreased phosphate uptake when compared to wild-type SLC34A1.

CONCLUSIONS

The homozygous G>A transition that results in the substitution of histidine for arginine at position 495 of the renal sodium-dependent phosphate transporter, SLC34A1, is involved in disease pathogenesis in these patients. Our report of the second family with two mutated SLC34A1 alleles expands the known phenotype of this rare condition.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

Regulation of phosphate transport by fibroblast growth factor 23 (FGF23): implications for disorders of phosphate metabolism

There are a number of hypophosphatemic disorders due to renal phosphate wasting that cannot be explained by elevated levels of parathyroid hormone. The circulating factors responsible for the phosphaturia have been designated as phosphatonins. Studies of patients with tumor-induced osteomalacia and other genetic diseases of phosphate metabolism have resulted in the identification of a number of hormones that regulate phosphate homeostasis, including matrix extracellular phosphoglycoprotein (MEPE), secreted frizzled-related protein 4 (sFRP-4), dentin matrix protein 1 (DMP1), fibroblast growth factor 7 (FGF7), fibroblast growth factor 23 (FGF23), and Klotho. Our understanding of the actions of these hypophosphatemic peptides has been enhanced by studies in mice either overexpressing or not expressing these hormones. This review focuses on FGF23 since its regulation is disordered in diseases that affect children, such as X-linked hypophosphatemia, autosomal dominant and recessive hypophosphatemic rickets as well as chronic kidney disease. Recent studies have shown that FGF23 is unique among the FGFs in its requirement for Klotho for receptor activation. Here, we also discuss new potentially clinically important data pointing to the receptor(s) that mediate the binding and action of FGF23 and Klotho.

Role of ClC-5 in renal endocytosis is unique among ClC exchangers and does not require PY-motif-dependent ubiquitylation

Inactivation of the mainly endosomal 2Cl(-)/H(+)-exchanger ClC-5 severely impairs endocytosis in renal proximal tubules and underlies the human kidney stone disorder Dent's disease. In heterologous expression systems, interaction of the E3 ubiquitin ligases WWP2 and Nedd4-2 with a "PY-motif" in the cytoplasmic C terminus of ClC-5 stimulates its internalization from the plasma membrane and may influence receptor-mediated endocytosis. We asked whether this interaction is relevant in vivo and generated mice in which the PY-motif was destroyed by a point mutation. Unlike ClC-5 knock-out mice, these knock-in mice displayed neither low molecular weight proteinuria nor hyperphosphaturia, and both receptor-mediated and fluid-phase endocytosis were normal. The abundances and localizations of the endocytic receptor megalin and of the Na(+)-coupled phosphate transporter NaPi-2a (Npt2) were not changed, either. To explore whether the discrepancy in results from heterologous expression studies might be due to heteromerization of ClC-5 with ClC-3 or ClC-4 in vivo, we studied knock-in mice additionally deleted for those related transporters. Disruption of neither ClC-3 nor ClC-4 led to proteinuria or impaired proximal tubular endocytosis by itself, nor in combination with the PY-mutant of ClC-5. Endocytosis of cells lacking ClC-5 was not impaired further when ClC-3 or ClC-4 was additionally deleted. We conclude that ClC-5 is unique among CLC proteins in being crucial for proximal tubular endocytosis and that PY-motif-dependent ubiquitylation of ClC-5 is dispensable for this role.

Sex differences in adaptive downregulation of pre-macula densa sodium transporters with ANG II infusion in mice

An increase in blood pressure (BP) due to angiotensin II (ANG II) infusion or other means is associated with adaptive pressure natriuresis due to reduced sodium reabsorption primarily in proximal tubule (PT) and thick ascending limb (TAL). We tested the hypothesis that male and female mice would show differential response to ANG II infusion with regard to the regulation of the protein abundance of sodium transporters in the PT and TAL and that these responses would be modulated by aging. Young (approximately 3 mo) and old (approximately 21 mo) male and female mice were infused with ANG II at 800 ng x kg body wt(-1) x min(-1) by osmotic minipump for 7 days or received a sham operation. ANG II increased mean arterial pressure (MAP), measured by radiotelemetry, significantly more in male mice of both ages (increased approximately 30-40 mmHg), compared with females (increased approximately 15-25 mmHg). On day 1, MAP was also significantly increased in old mice, relative to young (P = 0.01). ANG II infusion was associated with a significant decline in plasma testosterone (to <30% of control male) in male mice and rise in young female mice (to 478% of control female). No sex differences were found in the upregulation of the sodium hydrogen exchanger abundance on Western blots observed with ANG II infusion or the downregulation of the sodium phosphate cotransporter; however, aging did impact on some of these changes. Male mice (especially young) also had significantly reduced levels of the TAL bumetanide-sensitive Na-K-2Cl cotransporter (to 60% of male control), while young females showed an increase (to 126% of female control) with ANG II infusion. These sex differences do not support impaired pressure natriuresis in male mice, but might reflect a greater need and attempt to mount an appropriately BP-metered natriuretic response by additional downregulation of TAL sodium reabsorption.

Phosphate: known and potential roles during development and regeneration of teeth and supporting structures

Inorganic phosphate (P(i)) is abundant in cells and tissues as an important component of nucleic acids and phospholipids, a source of high-energy bonds in nucleoside triphosphates, a substrate for kinases and phosphatases, and a regulator of intracellular signaling. The majority of the body's P(i) exists in the mineralized matrix of bones and teeth. Systemic P(i) metabolism is regulated by a cast of hormones, phosphatonins, and other factors via the bone-kidney-intestine axis. Mineralization in bones and teeth is in turn affected by homeostasis of P(i) and inorganic pyrophosphate (PPi), with further regulation of the P(i)/PP(i) ratio by cellular enzymes and transporters. Much has been learned by analyzing the molecular basis for changes in mineralized tissue development in mutant and knock-out mice with altered P(i) metabolism. This review focuses on factors regulating systemic and local P(i) homeostasis and their known and putative effects on the hard tissues of the oral cavity. By understanding the role of P(i) metabolism in the development and maintenance of the oral mineralized tissues, it will be possible to develop improved regenerative approaches.

Estrogen downregulates the proximal tubule type IIa sodium phosphate cotransporter causing phosphate wasting and hypophosphatemia

Estrogen treatment causes significant hypophosphatemia in patients. To determine the mechanisms responsible for this effect, we injected ovariectomized rats with either 17beta-estradiol or vehicle for three days. Significant renal phosphate wasting and hypophosphatemia occurred in estrogen-treated rats despite a decrease in their food intake. The mRNA and protein levels of the renal proximal tubule sodium phosphate cotransporter (NaPi-IIa) were significantly decreased in estradiol-treated ad-libitum or pair-fed groups. Estrogen did not affect NaPi-III or NaPi-IIc expression. In ovariectomized and parathyroidectomized rats, 17beta-estradiol caused a significant decrease in NaPi-IIa mRNA and protein expression compared to vehicle. Estrogen receptor alpha isoform blocker significantly blunted the anorexic effect of 17beta-estradiol but did not affect the downregulation of NaPi-IIa. Our studies show that renal phosphate wasting and hypophosphatemia induced by estrogen are secondary to downregulation of NaPi-IIa in the proximal tubule. These effects are independent of food intake or parathyroid hormone levels and likely not mediated through the activation of estrogen receptor alpha subtype.

Hormonal regulation of phosphate homeostasis in goats during transition to rumination

Regulatory processes in phosphorus (P) homeostasis in small ruminants are quite different compared to monogastric animals. Adaptive responses of modulating hormones [parathyroid hormone (PTH) and calcitriol] to feeding variable amounts of P are lacking. Therefore, the aim of this study was to examine the influence of high dietary P intake (control diet: 4 g kg(-1) dry matter; high-P diet: 8 g kg(-1) dry matter) on the expression levels of PTH receptor (PTHR), vitamin D receptor (VDR) and Na+-dependent Pi transporters (NaPi II) in kidney and jejunum of goats starting rumination. After 3 months of feeding, plasma phosphate (Pi) and PTH concentrations were increased in the high-P diet group, whereas calcium and calcitriol were not changed. The intestinal Na+-dependent Pi transport capacity was not influenced by a high-P diet and the expression of jejunal VDR, PTHR and NaPi IIb was not modified. Interestingly, renal Na+-dependent Pi transport capacity was significantly reduced and concomitantly the expression of PTHR and NaPi IIa was decreased. In conclusion, the adaptive response of renal Pi reabsorption in goats, which were in transition from non-ruminant to ruminant stage was comparable to that of monogastric animals. In contrast, the modulation of the intestinal Pi absorption was like in adult ruminants.

Unchanged expression of the sodium-dependent phosphate cotransporter NaPi-IIa despite diurnal changes in renal phosphate excretion

Renal phosphate excretion is subjected to circadian rhythmicity. The bulk of filtered inorganic phosphate (P(i)) is reabsorbed by the sodium-dependent phosphate cotransporter NaPi-IIa. The regulation of proximal tubular phosphate reabsorptive capacity is largely attributed to the altered abundance of NaPi-IIa residing in the brush border membrane (BBM) of proximal tubular cells. Therefore, we examined if the diurnal rise in renal phosphate excretion is accompanied by a corresponding change in NaPi-IIa expression. Renal phosphate excretion, creatinine clearance, and serum phosphate were determined at consecutive time points in rats, starting from 8 a.m. until 5 p.m. During this period, renal phosphate excretion (fractional P(i) excretion) increased more than eightfold until 5 p.m. compared to the morning values at 8 a.m. In addition, serum phosphate and creatinine clearance as well as the calculated tubular phosphate threshold increased. Neither immunoblot analysis of BBMs nor immunohistochemical staining for NaPi-IIa yielded evidence for a lower abundance of NaPi-IIa in kidneys collected in the afternoon compared to those in the morning. However, kidneys sampled in the afternoon showed a small decrease (14%) in (32)P uptakes into BBM vesicles (BBMVs). Thus, the diurnal rise in renal phosphate excretion was associated with a mild reduction in the sodium-dependent phosphate transport rate in proximal tubular BBMs. There was no apparent downregulation of NaPi-IIa abundance and only a small reduction in Na(+)-dependent Pi-transport activity. Thus, the diurnal changes in urinary phosphate excretion appear to be mainly related to changes in serum phosphate and tubular threshold but not to NaPi-IIa expression.

Chloride channel diseases resulting from impaired transepithelial transport or vesicular function

The transport of anions across cellular membranes is crucial for various functions, including the control of electrical excitability of muscle and nerve, transport of salt and water across epithelia, and the regulation of cell volume or the acidification and ionic homeostasis of intracellular organelles. Given this broad range of functions, it is perhaps not surprising that mutations in Cl- channels lead to a large spectrum of diseases. These diverse pathologies include the muscle disorder myotonia, cystic fibrosis, renal salt loss in Bartter syndrome, kidney stones, deafness, and the bone disease osteopetrosis. This review will focus on diseases related to transepithelial transport and on disorders involving vesicular Cl- channels.

Role of membrane microdomains in PTH-mediated down-regulation of NaPi-IIa in opossum kidney cells

BACKGROUND

Parathyroid hormone (PTH) rapidly down-regulates type IIa sodium-dependent phosphate transporter (NaPi-IIa) via an endocytic pathway. Since the relationship between PTH signaling and NaPi-IIa endocytosis has not been explored, we investigated the role of membrane microdomains in this process.

METHODS

We examined the submembrane localization of NaPi-IIa in opossum kidney (OK-N2) cells that stably expressed human NaPi-IIa, and searched for a PTH-induced specific phosphorylating substrate on their membrane microdomains by immunoblotting with specific antibody against phospho substrates of protein kinases.

RESULTS

We found that NaPi-IIa was primarily localized in low-density membrane (LDM) domains of the plasma membrane; PTH reduced the levels of immunoreactive NaPi-IIa in these domains. Furthermore, PTH activated both protein kinase A (PKA) and protein kinase Calpha (PKCa) and increased the phosphorylation of 250 kD and 80 kD substrates; this latter substrate was identified as ezrin, which a member of the ezrin-radixin-moesin (ERM) protein family. In response to PTH, ezrin was phosphorylated by both PKA and PKC. Dominant negative ezrin blocked the reduction in NaPi-IIa expression in the LDM domains that was induced by PTH.

CONCLUSION

These data suggest that NaPi-IIa and PTH-induced phosphorylated proteins that include ezrin are compartmentalized in LDM microdomains. This compartmentalization may play an important role in the down-regulation of NaPi-IIa via endocytosis.

What have we learnt about the regulation of phosphate metabolism?

PURPOSE OF REVIEW

The search for hormones which specifically regulate phosphate metabolism has fuelled recent tantalizing studies. These studies have been motivated by diseases involving renal phosphate wasting, including tumor-induced osteomalacia, X-linked hypophosphatemic rickets, and autosomal dominant hypophosphatemia. This review focuses on likely candidate 'phosphatonins' and their possible physiological significance.

RECENT FINDINGS

Candidate phosphatonins include fibroblast growth factor 23, matrix extracellular phosphoglycoprotein, stanniocalcin, and Frizzled-related protein 4. Fibroblast growth factor 23 has emerged as the prime candidate explaining pathophysiology of these diseases. FGF-23 is expressed in most tumors in tumor-induced osteomalacia. Serum fibroblast growth factor 23 is increased in most patients with X-linked hypophosphatemic rickets and tumor-induced osteomalacia. Injection of recombinant fibroblast growth factor 23 induces phosphaturia, hypophosphatemia, and suppression of 1,25-dihydroxyvitamin D in animals. Many unanswered questions remain, including the relationship between PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) mutations and elevated fibroblast growth factor 23. It is also not clear whether these candidate phosphatonins play a role in phosphate or vitamin D metabolism in healthy humans, or that this role is endocrine. The most compelling evidence derives from the fibroblast growth factor 23-knockout mouse which shows hyperphosphatemia and increased serum 1,25-dihydroxyvitamin D. A physiologically relevant phosphatonin should explain renal adaptation to variable dietary phosphate intake. The tissue source and determinants of serum fibroblast growth factor 23 are unknown.

SUMMARY

Pathophysiological and animal studies serve as a logical foundation on which to base further questions of human physiology. The definition of what is or is not a phosphatonin may need to be refined. There is a need to return to 'old-fashioned' human physiology studies to place recent findings in perspective.

Evidence for a bone-kidney axis regulating phosphate homeostasis

A novel circulation phosphaturic hormone is postulated to regulate systemic phosphate homeostasis. Two new studies reveal that the phosphaturic factor FGF-23 is increased in hypophosphatemic subjects with McCune-Albright syndrome and that secreted frizzled-related protein-4 (sFRP-4), a factor produced by tumors derived from subjects with tumor-induced osteomalacia, also has phosphaturic activity. It remains to be established whether FGF-23 and sFRP-4 represent two distinct phosphatonins or are somehow integrated in a novel phosphate-regulating bone-kidney axis.

Characterization of cis-acting element in renal NaPi-2 cotransporter mRNA that determines mRNA stability

Hypophosphatemia leads to an increase in Na(+)-P(i) cotransporter (NaPi-2) mRNA levels. This increase is posttranscriptional and correlates with a more stable transcript mediated by the terminal 698 nt of the NaPi-2 mRNA. A 71-nt binding element was identified with renal proteins from rats fed control and low-P(i) (-P(i)) diet. The binding of -P(i) renal proteins to this transcript was increased compared with control proteins. The functionality of the cis element was demonstrated by an in vitro degradation assay. -P(i) renal proteins stabilized transcripts that included the cis element compared with control renal extracts. The full-length NaPi-2 transcript, but not control transcripts, was stabilized by -P(i) extracts. Insertion of the binding element into green fluorescent protein (GFP) as a reporter gene decreased chimeric GFP mRNA levels in transfection experiments. Our results suggest that the protein-binding region of the NaPi-2 mRNA functions as a cis-acting instability element. In hypophosphatemia there is increased binding to the cis-acting element and subsequent stabilization of NaPi-2 mRNA.

Characterization of a type IIb sodium-phosphate cotransporter from zebrafish (Danio rerio) kidney

Zebrafish (Danio rerio) express two isoforms of the type IIb Na-dependent P(i) cotransporter (NaPi). Type NaPi-IIb1 has previously been cloned and characterized. Here, we report the cloning of the NaPi-IIb2 transcript from zebrafish kidney, its localization, and its functional characterization. RT-PCR with renal RNA and degenerate NaPi-IIb-specific primers resulted in a specific fragment. 3'-Rapid amplification of cDNA ends yielded a product that contained typical NaPi-IIb characteristics such as a cysteine-rich COOH terminus and a PDZ (PSD95- Dlg-zona occludens-1) binding motif. Several approaches were unsuccessful at cloning the 5' end of the transcript; products lacked an in-frame start codon. The missing information was obtained from an EST (GenBank accession number ). The combined clone displayed a high degree of homology with published type IIb cotransporter sequences. Specific antibodies were raised against a COOH-terminal epitope of both NaPi-IIb1 and NaPi-IIb2 isoforms. Immunohistochemical mapping revealed apical expression of both isoforms in zebrafish renal and intestinal epithelia, as well as in bile ducts. The novel clone was expressed in oocytes, and function was assayed by the two-electrode voltage-clamp technique. The function of the new NaPi-IIb2 clone was found to be significantly different from NaPi-IIb1 despite strong structural similarities. NaPi-IIb2 was found to be strongly voltage sensitive, with higher affinities for both sodium and phosphate than NaPi-IIb1. Also, NaPi-IIb2 was significantly less sensitive to external pH than NaPi-IIb1. The strong structural similarity but divergent function makes these zebrafish transporters ideal models for the molecular mapping of functionally important regions in the type II NaPi-cotransporter family.

Acute and chronic effect of dietary phosphorus restriction on protein expression in young rat renal proximal tubules

Renal proximal tubules play a vital role in phosphorus (P) homeostasis. It is well known that dietary P restriction up-regulates the activities of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1-OHase), an enzyme that is involved in activation of vitamin D and thereby maintaining P balance. However, the mechanism involved in such regulation is not known. In the present study, we aim to identify proteins that might be involved in the renal adaptation to dietary P restriction using a proteomic approach. Renal proximal tubules were harvested from young rats fed either normal P diet or low P diet (LPD) for 1 to 7 days. Western blotting analysis of 1-OHase and signaling proteins in insulin-like growth factor I axis indicated an increase in expression of these proteins upon dietary P restriction. Using two-dimensional electrophoresis, we found that LPD reduced the total number of protein species expressed in renal proximal tubules. Differentially expressed proteins were analyzed and located using the software Melanie III, and their identities were found using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Our results showed that beta-actin, gamma-actin, major urinary protein, phosphatidylinositol transfer protein beta isoform, and G1/S-specific cyclin D3 are up-regulated and nonspecific lipid transfer protein is down-regulated by LPD.

Glucocorticoid regulation and glycosylation of mouse intestinal type IIb Na-P(i) cotransporter during ontogeny

We sought to characterize expression of an apically expressed intestinal Na-P(i) cotransporter (Na-P(i)-IIb) during mouse ontogeny and to assess the effects of methylprednisolone (MP) treatment. In control mice, Na-P(i) uptake by intestinal brush-border membrane vesicles was highest at 14 days of age, lower at 21 days, and further reduced at 8 wk and 8-9 mo of age. Na-P(i)-IIb mRNA and immunoreactive protein levels in 14-day-old animals were markedly higher than in older groups. MP treatment significantly decreased Na-P(i) uptake and Na-P(i)-IIb mRNA and protein expression in 14-day-old mice. Additionally, the size of the protein was smaller in 14-day-old mice. Deglycosylation of protein from 14-day-old and 8-wk-old animals with peptide N-glycosidase reduced the molecular weight to the predicted size. We conclude that intestinal Na-P(i) uptake and Na-P(i)-IIb expression are highest at 14 days and decrease with age. Furthermore, MP treatment reduced intestinal Na-P(i) uptake approximately threefold in 14-day-old mice and this reduction correlates with reduced Na-P(i)-IIb mRNA and protein expression. We also demonstrate that Na-P(i)-IIb is an N-linked glycoprotein and that glycosylation is age dependent.

The nptA gene of Vibrio cholerae encodes a functional sodium-dependent phosphate cotransporter homologous to the type II cotransporters of eukaryotes

The nptA gene of Vibrio cholerae has significant protein sequence homology with type II sodium-dependent phosphate (P(i)) cotransporters found in animals but not previously identified in prokaryotes. The phylogeny of known type II cotransporter sequences indicates that nptA may be either an ancestral gene or a gene acquired from a higher eukaryotic source. The gene was cloned into an expression vector under the control of an inducible promoter and expressed in Escherichia coli. The results demonstrate that nptA encodes a functional protein with activity similar to that of the animal enzyme, catalyzing high-affinity, sodium-dependent P(i) uptake with comparable affinities for both sodium and phosphate ions. Furthermore, the activity of NptA is influenced by pH, again in a manner similar to that of the NaPi-2a subtype of the animal enzyme, although it lacks the corresponding REK motif thought to be responsible for this phenomenon. P(i) uptake activity, a component of which appeared to be sodium dependent, was increased in V. cholerae by phosphate starvation. However, it appears from the use of a reporter gene expressed from the nptA promoter that none of this activity is attributable to the induction of expression from nptA. It is thus proposed that the physiological function of NptA protein may be the rapid uptake of P(i) in preparation for rapid growth in nutrient-rich environments and that it may therefore play a role in establishing infection.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Murine and human type I Na-phosphate cotransporter genes: structure and promoter activity

Na-phosphate (P(i)) cotransporters in the apical membrane of renal proximal tubular cells play a major role in the maintenance of P(i) homeostasis. Although two such cotransporters, Npt1 and Npt2, have been identified, little is known about the function and regulation of Npt1. We cloned and characterized the murine (Npt1) and human (NPT1) genes, isolated the 5'-flanking region of Npt1, and analyzed its promoter activity. Npt1 is approximately 29 kb with 12 exons, whereas NPT1 is approximately 49 kb with one additional exon. The Npt1 promoter has a TATA-like box but no CAAT box, and the transcription start site was identified by primer extension and 5'-rapid amplification of cDNA ends. Transfection of opossum kidney cells with Npt1 promoter-reporter gene constructs demonstrated significant activity in a 570-bp fragment that was completely inhibited by cotransfection with the transcription factor, hepatocyte nuclear factor (HNF)-3 beta. Deletion of 200 bp from the 3'-end of the 570-bp fragment abrogated its promoter activity. In addition, promoter activity of a 4.5-kb fragment, but not the 570-bp fragment, was stimulated fourfold by cotransfection with HNF-1 alpha. Other well-characterized cis-acting elements were identified in the Npt1 promoter. We suggest that Npt1 expression is transcriptionally regulated and provide a basis for the investigation of Npt1 function by targeted mutagenesis.

Absence of aquaporin-4 water channels from kidneys of the desert rodent Dipodomys merriami merriami

Recently, we found that aquaporin-4 (AQP4) is expressed in the S3 segment of renal proximal tubules of mice but not in rat proximal tubules. Because mice have relatively larger papillae than rats, it was proposed that the renal distribution of AQP4 in various species could be related to their maximum urinary concentrating ability. Therefore, kidneys and other tissues of Merriam's desert kangaroo rat, Dipodomys merriami merriami, which produce extremely concentrated urine (up to 5,000 mosmol/kgH(2)O), were examined for AQP4 expression and localization. Contrary to our expectation, AQP4 immunostaining was undetectable in any region of the kidney, and the absence of AQP4 protein was confirmed by Western blotting. By freeze fracture electron microscopy, orthogonal arrays of intramembraneous particles (OAPs) were not detectable in plasma membranes of principal cells and proximal tubules. However, AQP4 protein was readily detectable in gastric parietal and brain astroglial cells. Northern blotting failed to detect AQP4 mRNA in kangaroo rat kidneys, whereas both in situ hybridization and RT-PCR experiments did reveal AQP4 mRNA in collecting ducts and proximal tubules of the S3 segment. These results suggest that renal expression of AQP4 in the kangaroo rat kidney is regulated at the transcriptional or translational level, and the absence of AQP4 may be critical for the extreme urinary concentration that occurs in this species.

Evolution of the Na-P(i) cotransport systems

Membrane transport systems for P(i) transport are key elements in maintaining homeostasis of P(i) in organisms as diverse as bacteria and human. Two Na-P(i) cotransporter families with well-described functional properties in vertebrates, namely NaPi-II and NaPi-III, show conserved structural features with prokaryotic origin. A clear vertical relationship can be established among the mammalian protein family NaPi-III, a homologous system in C. elegans, the yeast system Pho89, and the bacterial P(i) transporter Pit. An alternative lineage connects the mammalian NaPi-II-related transporters with homologous proteins from Caenorhabditis elegans and Vibrio cholerae. The present review focuses on the molecular evolution of the NaPi-II protein family. Preliminary results indicate that the NaPi-II homologue cloned from V. cholerae is indeed a functional P(i) transporter when expressed in Xenopus oocytes. The closely related NaPi-II isoforms NaPi-IIa and NaPi-IIb are responsible for regulated epithelial Na-dependent P(i) transport in all vertebrates. Most species express two different NaPi-II proteins with the exception of the flounder and Xenopus laevis, which rely on only a single isoform. Using an RT-PCR-based approach with degenerate primers, we were able to identify NaPi-II-related mRNAs in a variety of vertebrates from different families. We hypothesize that the original NaPi-IIb-related gene was duplicated early in vertebrate development. The appearance of NaPi-IIa correlates with the development of the mammalian nephron.

A dibasic motif involved in parathyroid hormone-induced down-regulation of the type IIa NaPi cotransporter

Type II NaPi cotransporters are expressed in the apical membrane of P(i)-(re)absorbing epithelia: the type IIa in renal proximal tubule and the type IIb in small intestine. Parathyroid hormone (PTH) leads to a retrieval from the apical membrane of the type IIa NaPi cotransporter. The type IIa cotransporter is also expressed in opossum kidney (OK) cells, and its expression is under the control of PTH. In the present study, we identified the molecular "domains" involved in the PTH-induced retrieval of the type IIa NaPi cotransporter. Wild-type mouse type IIa (mIIa) and type IIb (mIIb) as well as several mIIa-mIIb chimeras and site-directed mutants were fused to the enhanced green fluorescent protein and transfected into OK cells. We found that mIIa but not mIIb was internalized and degraded after incubation with 1-34 (or 3-34) PTH. Using chimeras, we found that the N and C termini were not required in this effect, whereas a "domain" located between residues 216 and 658 seemed to be necessary. This region contains two putative intracellular loops with highly conserved sequences between mIIa and mIIb; in the last intracellular loop, two charged amino acids of type IIa (K(503)R(504)) are replaced by uncharged residues in type IIb (N(520)I(521)). We generated two mutants in which these residues were interchanged: mIIaNI and mIIbKR. Similarly to mIIa, the mIIbKR mutant was endocytosed in response to 1-34 PTH; in contrast, mIIaNI behaved as mIIb and was not internalized. In conclusion, a dibasic amino acid motif (K(503)R(504)) located in the last intracellular loop of the type IIa NaPi cotransporter is essential for its PTH-induced retrieval.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Altered expression of renal AQPs and Na(+) transporters in rats with lithium-induced NDI

Lithium (Li) treatment is often associated with nephrogenic diabetes insipidus (NDI). The changes in whole kidney expression of aquaporin-1 (AQP1), -2, and -3 as well as Na-K-ATPase, type 3 Na/H exchanger (NHE3), type 2 Na-Pi cotransporter (NaPi-2), type 1 bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1), and thiazide-sensitive Na-Cl cotransporter (TSC) were examined in rats treated with Li orally for 4 wk: protocol 1, high doses of Li (high Na(+) intake), and protocol 2, low doses of Li (identical food and normal Na(+) intake in Li-treated and control rats). Both protocols resulted in severe polyuria. Semiquantitative immunoblotting revealed that whole kidney abundance of AQP2 was dramatically reduced to 6% (protocol 1) and 27% (protocol 2) of control levels. In contrast, the abundance of AQP1 was not decreased. Immunoelectron microscopy confirmed the dramatic downregulation of AQP2 and AQP3, whereas AQP4 labeling was not reduced. Li-treated rats had a marked increase in urinary Na(+) excretion in both protocols. However, the expression of several major Na(+) transporters in the proximal tubule, loop of Henle, and distal convoluted tubule was unchanged in protocol 2, whereas in protocol 1 significantly increased NHE3 and BSC-1 expression or reduced NaPi-2 expression was associated with chronic Li treatment. In conclusion, severe downregulation of AQP2 and AQP3 appears to be important for the development of Li-induced polyuria. In contrast, the increased or unchanged expression of NHE3, BSC-1, Na-K-ATPase, and TSC indicates that these Na(+) transporters do not participate in the development of Li-induced polyuria.