Role of intracellular phosphate in the regulation of renal phosphate transport during development

Role of transporters in regulating mammalian intracellular inorganic phosphate

This review summarizes the current understanding of the role of plasma membrane transporters in regulating intracellular inorganic phosphate ([Pi]In) in mammals. Pi influx is mediated by SLC34 and SLC20 Na+-Pi cotransporters. In non-epithelial cells other than erythrocytes, Pi influx via SLC20 transporters PiT1 and/or PiT2 is balanced by efflux through XPR1 (xenotropic and polytropic retrovirus receptor 1). Two new pathways for mammalian Pi transport regulation have been described recently: 1) in the presence of adequate Pi, cells continuously internalize and degrade PiT1. Pi starvation causes recycling of PiT1 from early endosomes to the plasma membrane and thereby increases the capacity for Pi influx; and 2) binding of inositol pyrophosphate InsP8 to the SPX domain of XPR1 increases Pi efflux. InsP8 is degraded by a phosphatase that is strongly inhibited by Pi. Therefore, an increase in [Pi]In decreases InsP8 degradation, increases InsP8 binding to SPX, and increases Pi efflux, completing a feedback loop for [Pi]In homeostasis. Published data on [Pi]In by magnetic resonance spectroscopy indicate that the steady state [Pi]In of skeletal muscle, heart, and brain is normally in the range of 1–5 mM, but it is not yet known whether PiT1 recycling or XPR1 activation by InsP8 contributes to Pi homeostasis in these organs. Data on [Pi]In in cultured cells are variable and suggest that some cells can regulate [Pi] better than others, following a change in [Pi]Ex. More measurements of [Pi]In, influx, and efflux are needed to determine how closely, and how rapidly, mammalian [Pi]In is regulated during either hyper- or hypophosphatemia.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Animal models with pathological mineralization phenotypes

Extracellular matrix mineralization is important for mechanical stability of the skeleton and for calcium and phosphate storage. Professional mineral-disposing cell types are hypertrophic chondrocytes, odontoblasts, ameloblasts and osteoblasts. Since ectopic mineralization causes tissue dysfunction mineralization inhibitors and promoting factors have to be kept in close balance. The most prominent inhibitors are fetuin-A, matrix-Gla-protein (MGP), SIGBLING proteins and pyrophosphate. In spite of their ubiquitous presence, their loss entails a specific rather than a stereotypic pattern of ectopic mineralization. Typical sites of pathological mineral accumulation are connective tissues, articular cartilage, and vessels. Associated common human pathologies are degenerative joint disorders and arteriosclerosis. This article gives a summary on what we have learned from different mouse models with pathologic mineralization phenotypes about the role of these inhibitors and the regulation of mineralization promoting factors.

Basolateral phosphate transport in renal proximal-tubule-like OK cells

It is generally assumed that phosphate (Pi) effluxes from proximal tubule cells by passive diffusion across the basolateral (BL) membrane. We explored the mechanism of BL Pi efflux in proximal tubule-like OK cells grown on permeable filters and then loaded with 32P. BL efflux of 32P was significantly stimulated (P < 0.05) by exposing the BL side of the monolayer to 12.5 mM Pi, to 10 mM citrate, or by acid-loading the cells, and was inhibited by exposure to 0.05 mM Pi or 25 mM HCO3; by contrast, BL exposure to high (8.4) pH, 40 mM K+, 140 mM Na gluconate (replacing NaCl), 10 mM lactate, 10 mM succinate, or 10 mM glutamate did not affect BL 32P efflux. These data are consistent with BL Pi efflux from proximal tubule-like cells occurring, in part, via an electro-neutral sodium-sensitive anion transporter capable of exchanging two moles of intracellular acidic H2PO4- for each mole of extracellular basic HPO4= or for citrate.

Mechanism of renal phosphate retention during growth

We have previously demonstrated that the retention of phosphate required for growth is due to a a high Vmax of the Na(+)-Pi cotransport system located in the brush border membrane of the proximal tubule. Because of this and other similarities between adaptation of the kidney to a high Pi demand (growth) and that to low Pi supply, we measured the levels of NaPi-2 mRNA and cDNA present in kidney cortex of 3- and > 12-week-old rats. Like in Pi depletion, Western blots revealed that a 80 to 85 kDa protein recognized by a polyclonal antibody directed against the N-terminal region of the NaPi-2 protein was 2.3-fold more abundant in renal microvilli of the young than of adult animals. However, unlike in Pi depletion, Northern blot analysis failed to reveal a significant difference between mRNA levels at the two ages. Furthermore, suppression of NaPi-2 mRNA activity by annealing with antisense oligomers, or removal of the NaPi-2 transcripts by subtractive hybridization did not affect the rate of Na(+)-Pi cotransport induced in oocytes by polyA RNA of rapidly growing animals, while abolishing the ability of the renal cortical polyA RNA of adult rats to encode for Na(+)-Pi cotransport. RT-PCR of subtracted polyA RNA using primers specific for a region conserved in NaPi type II (Pi modulated) cotransporters yielded a product that was 98% homologous with that region, despite the absence of NaPi-2 cDNA. The results of these experiments demonstrate that the polyA RNA from kidneys of young animals contains unique mRNA transcripts able to encode for a NaPi protein homologous to, but distinct from NaPi-2.

Parathyroid hormone gene expression in hypophosphatemic rats

Phosphate is central to bone metabolism and we have therefore studied whether parathyroid hormone (PTH) is regulated by dietary phosphate in vivo. Weanling rats were fed diets with different phosphate contents for 3 wk: low phosphate (0.02%), normal calcium (0.6%), normal phosphate (0.3%), and calcium (0.6%); high phosphate (1.2%), high calcium (1.2%). The low phosphate diet led to hypophosphatemia, hypercalcemia, and increased serum 1,25(OH)2D3 together with decreased PTH mRNA levels (25 +/- 8% of controls, P < 0.01) and serum immunoreactive PTH (4.7 +/- 0.8: 22.1 +/- 3.7 pg/ml; low phosphate: control, P < 0.05). A high phosphate diet led to increased PTH mRNA levels. In situ hybridization showed that hypophosphatemia decreased PTH mRNA in all the parathyroid cells. To separate the effect of low phosphate from changes in calcium and vitamin D rats were fed diets to maintain them as vitamin D-deficient and normocalcemic despite the hypophosphatemia. Hypophosphatemic, normocalemic rats with normal serum 1,25(OH)2D3 levels still had decreased PTH mRNAs. Nuclear transcript run-ons showed that the effect of low phosphate was posttranscriptional. Calcium and 1,25(OH)2D3 regulate the parathyroid and we now show that dietary phosphate also regulates the parathyroid by a mechanism which remains to be defined.

NMR-visible intracellular P(i) and phosphoesters during regulation of Na(+)-P(i) cotransport in opossum kidney cells

There is an inverse relationship between intracellular concentration of P(i) ([P(i)]i) in the kidney and maximum velocity (Vmax) of Na(+)-P(i) cotransport in brush-border membrane vesicles both in P(i)-deprived and growing animals. However, at any given [P(i)]i, the Vmax is substantially higher in growing than in P(i)-deprived animals. This suggests that growth and P(i) depletion act on P(i) transport via different mechanisms. We tested this hypothesis by measuring the nuclear magnetic resonance-visible phosphate and the Vmax of Na(+)-P(i) cotransport in proximal tubule-like cells [opossum kidney (OK) cells] cultured in vitro. OK cells incubated in 1 mM extracellular P(i) had a [P(i)]i of 1.1 +/- 0.2 mM and a P(i) uptake of 1.47 +/- 0.06 nmol/mg in 5 min. Exposure of OK cells to P(i)-free medium decreased [P(i)]i by 80 +/- 7% (P < 0.01) and stimulated P(i) transport by 34 +/- 7% (P < 0.05). Exposure of OK cells to 10(-8) M insulin-like growth factor I (IGF-I) increased P(i) transport by 25 +/- 8% (P < 0.05) but did not affect [P(i)]i. The stimulation of Vmax produced by IGF-I was additive to that due to P(i) restriction. In addition, P(i) deprivation decreased the phosphomonoesters by 0.66 +/- 0.04-fold (P < 0.05) and increased the phosphodiesters by 2.5 +/- 0.5-fold (P < 0.01). Treatment with IGF-I increased both the phosphomonoesters (1.2 +/- 0.1-fold) and the phosphodiesters (4.1 +/- 0.6-fold). These results support the assumption that low P(i) supply and IGF-I stimulate Na(+)-P(i) cotransport by independent mechanisms.