Clinical consequences of mutations in sodium phosphate cotransporters

Inflammatory, Metabolic, and Coagulation Effects on Medial Arterial Calcification in Patients with Peripheral Arterial Disease

Calcium deposits in the vessel wall in the form of hydroxyapatite can accumulate in the intimal layer, as in atherosclerotic plaque, but also in the medial layer, as in medial arterial calcification (MAC) or medial Möenckeberg sclerosis. Once considered a passive, degenerative process, MAC has recently been shown to be an active process with a complex but tightly regulated pathophysiology. Atherosclerosis and MAC represent distinct clinical entities that correlate in different ways with conventional cardiovascular risk factors. As both entities coexist in the vast majority of patients, it is difficult to estimate the relative contribution of specific risk factors to their development. MAC is strongly associated with age, diabetes mellitus, and chronic kidney disease. Given the complexity of MAC pathophysiology, it is expected that a variety of different factors and signaling pathways may be involved in the development and progression of the disease. In this article, we focus on metabolic factors, primarily hyperphosphatemia and hyperglycemia, and a wide range of possible mechanisms by which they might contribute to the development and progression of MAC. In addition, we provide insight into possible mechanisms by which inflammatory and coagulation factors are involved in vascular calcification processes. A better understanding of the complexity of MAC and the mechanisms involved in its development is essential for the development of potential preventive and therapeutic strategies.

Phosphatonins: From Discovery to Therapeutics

OBJECTIVE

Phosphate is crucial for cell signaling, energy metabolism, nucleotide synthesis, and bone mineralization. The gut-bone-parathyroid-kidney axis is influenced by parathyroid hormone, 1,25-dihydroxyvitamin D, and phosphatonins, especially fibroblast growth factor 23 (FGF23). These hormones facilitate maintenance of phosphate homeostasis. This review summarizes current knowledge regarding the phosphate homeostasis, phosphatonin pathophysiology, and clinical implications of FGF23-related hypophosphatemic disorders, with specific focus on burosumab treatment.

METHOD

A focused literature search of PubMed was conducted.

RESULTS

Phosphatonins including FGF23, secreted frizzled-related protein 4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 7 play a pathogenic role in several hypophosphatemic disorders. Excess FGF23 inhibits sodium-dependent phosphate cotransporters (NaPi-2a and NaPi-2c), resulting in hyperphosphaturia and hypophosphatemia. Additionally, FGF23 suppresses 1,25-dihydroxyvitamin D synthesis in the proximal renal tubule, and thus, it indirectly inhibits intestinal phosphate absorption. Disorders of FGF23-related hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia/McCune-Albright syndrome, and tumor-induced osteomalacia (TIO). Complications of conventional therapy with oral phosphate and vitamin D analogs comprise gastrointestinal distress, hypercalcemia, nephrocalcinosis, and secondary/tertiary hyperparathyroidism. In both children and adults with XLH and TIO, the anti-FGF23 antibody burosumab exhibits a favorable safety profile and is associated with healing of rickets in affected children and improvement of osteomalacia in both children and adults.

CONCLUSION

The treatment paradigm for XLH and TIO is changing based on data from recent clinical trials. Research suggest that burosumab is effective and safe for pediatric and adult patients with XLH or TIO.

Tmem174, a regulator of phosphate transporter prevents hyperphosphatemia

Renal type II sodium-dependent inorganic phosphate (Pi) transporters NaPi2a and NaPi2c cooperate with other organs to strictly regulate the plasma Pi concentration. A high Pi load induces expression and secretion of the phosphaturic hormones parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) that enhance urinary Pi excretion and prevent the onset of hyperphosphatemia. How FGF23 secretion from bone is increased by a high Pi load and the setpoint of the plasma Pi concentration, however, are unclear. Here, we investigated the role of Transmembrane protein 174 (Tmem174) and observed evidence for gene co-expression networks in NaPi2a and NaPi2c function. Tmem174 is localized in the renal proximal tubules and interacts with NaPi2a, but not NaPi2c. In Tmem174-knockout (KO) mice, the serum FGF23 concentration was markedly increased but increased Pi excretion and hypophosphatemia were not observed. In addition, Tmem174-KO mice exhibit reduced NaPi2a responsiveness to FGF23 and PTH administration. Furthermore, a dietary Pi load causes marked hyperphosphatemia and abnormal NaPi2a regulation in Tmem174-KO mice. Thus, Tmem174 is thought to be associated with FGF23 induction in bones and the regulation of NaPi2a to prevent an increase in the plasma Pi concentration due to a high Pi load and kidney injury.

The human pathogenic 91del7 mutation in SLC34A1 has no effect in mineral homeostasis in mice

Kidneys are key regulators of phosphate homeostasis. Biallelic mutations of the renal Na⁺/phosphate cotransporter SLC34A1/NaPi-IIa cause idiopathic infantile hypercalcemia, whereas monoallelic mutations were frequently noted in adults with kidney stones. Genome-wide-association studies identified SLC34A1 as a risk locus for chronic kidney disease. Pathogenic mutations in SLC34A1 are present in 4% of the general population. Here, we characterize a mouse model carrying the 91del7 in-frame deletion, a frequent mutation whose significance remains unclear. Under normal dietary conditions, 12 weeks old heterozygous and homozygous males have similar plasma and urinary levels of phosphate as their wild type (WT) littermates, and comparable concentrations of parathyroid hormone, fibroblast growth factor 23 (FGF-23) and 1,25(OH)₂ vitamin D₃. Renal phosphate transport, and expression of NaPi-IIa and NaPi-IIc cotransporters, was indistinguishable in the three genotypes. Challenging mice with low dietary phosphate did not result in differences between genotypes with regard to urinary and plasma phosphate. Urinary and plasma phosphate, plasma FGF-23 and expression of cotransporters were similar in all genotypes after weaning. Urinary phosphate and bone mineral density were also comparable in 300 days old WT and mutant mice. In conclusion, mice carrying the 91del7 truncation do not show signs of impaired phosphate homeostasis.

The Interplay Between Brain Vascular Calcification and Microglia

Vascular calcifications are characterized by the ectopic deposition of calcium and phosphate in the vascular lumen or wall. They are a common finding in computed tomography scans or during autopsy and are often directly related to a pathological condition. While the pathogenesis and functional consequences of vascular calcifications have been intensively studied in some peripheral organs, vascular calcification, and its pathogenesis in the central nervous system is poorly characterized and understood. Here, we review the occurrence of vessel calcifications in the brain in the context of aging and various brain diseases. We discuss the pathomechanism of brain vascular calcification in primary familial brain calcification as an example of brain vessel calcification. A particular focus is the response of microglia to the vessel calcification in the brain and their role in the clearance of calcifications.

PDGF-BB is involved in phosphate regulation via the phosphate transporters in human neuroblastoma SH-SY5Y cells

Inorganic phosphate (Pi) is the second most abundant inorganic ion in the body. Since abnormalities in Pi metabolism are risk factors for various diseases, serum Pi levels are strictly controlled. Type-III sodium-dependent Pi transporters, PiT-1 (encoded by SLC20A1) and PiT-2 (encoded by SLC20A2), are distributed throughout the tissues of the body, including the central nervous system, and are known to be responsible for extracellular to intracellular Pi transport. Platelet-derived growth factor (PDGF) is a major growth factor of mesenchymal cells. PDGF-BB, a homodimer of PDGF-B, regulates intracellular Pi by increasing PiT-1 expression in vascular smooth muscle cells. However, the effects of PDGF-BB on Pi transporters in neurons have yet to be reported. Here, we investigated the effect of PDGF-BB on Pi transporters in human neuroblastoma SH-SY5Y cells. PDGF-BB did not induce SLC20A1 mRNA expression, but it increased the intracellular uptake of Pi via PiT-1 in SH-SY5Y cells. Among the signaling pathways associated with PDGF-BB, AKT signaling was shown to be involved in the increase in Pi transport. In addition, the PDGF-BB-induced increase in Pi mediated neuroprotective effects in SLC20A2-suppressed cells, in an in vitro model of the pathological condition found in idiopathic basal ganglia calcification. Moreover, the increase in Pi uptake was found to occur through promotion of intracellular PiT-1 translocation to the plasma membrane. Overall, these results indicate that PDGF-BB exerts neuroprotective effects via Pi transport, and they demonstrate the potential utility of PDGF-BB against abnormal Pi metabolism in neurons.

Hyperphosphatemia increases inflammation to exacerbate anemia and skeletal muscle wasting independently of FGF23-FGFR4 signaling

Elevations in plasma phosphate concentrations (hyperphosphatemia) occur in chronic kidney disease (CKD), in certain genetic disorders, and following the intake of a phosphate-rich diet. Whether hyperphosphatemia and/or associated changes in metabolic regulators, including elevations of fibroblast growth factor 23 (FGF23) directly contribute to specific complications of CKD is uncertain. Here, we report that similar to patients with CKD, mice with adenine-induced CKD develop inflammation, anemia, and skeletal muscle wasting. These complications are also observed in mice fed high phosphate diet even without CKD. Ablation of pathologic FGF23-FGFR4 signaling did not protect mice on an increased phosphate diet or mice with adenine-induced CKD from these sequelae. However, low phosphate diet ameliorated anemia and skeletal muscle wasting in a genetic mouse model of CKD. Our mechanistic in vitro studies indicate that phosphate elevations induce inflammatory signaling and increase hepcidin expression in hepatocytes, a potential causative link between hyperphosphatemia, anemia, and skeletal muscle dysfunction. Our study suggests that high phosphate intake, as caused by the consumption of processed food, may have harmful effects irrespective of pre-existing kidney injury, supporting not only the clinical utility of treating hyperphosphatemia in CKD patients but also arguing for limiting phosphate intake in healthy individuals.

Characteristics and therapeutic potential of sodium-dependent phosphate cotransporters in relation to idiopathic basal ganglia calcification

Type-III sodium-dependent phosphate transporters 1 and 2 (PiT 1 and PiT 2, respectively) are proteins encoded by SLC20A1 and SLC20A2, respectively. The ubiquitous distribution of SLC20A1 and SLC20A2 mRNAs in mammalian tissues supports the housekeeping maintenance and homeostasis of intracellular inorganic phosphate (Pi), which is absorbed from interstitial fluid for normal cellular functions. SLC20A2 variants have been found in patients with idiopathic basal ganglia calcification (IBGC), also known as Fahr's disease or primary familial brain calcification (PFBC). Thus, disrupted Pi homeostasis is considered one of the major factors in the pathogenic mechanism of IBGC. In this paper, among the causative genes of IBGC, we focused specifically on PiT2, and its potential for a therapeutic target of IBGC.

Slc20a1b is essential for hematopoietic stem/progenitor cell expansion in zebrafish

Hematopoietic stem and progenitor cells (HSPCs) are able to self-renew and can give rise to all blood lineages throughout their lifetime, yet the mechanisms regulating HSPC development have yet to be discovered. In this study, we characterized a hematopoiesis defective zebrafish mutant line named smu07, which was obtained from our previous forward genetic screening, and found the HSPC expansion deficiency in the mutant. Positional cloning identified that slc20a1b, which encodes a sodium phosphate cotransporter, contributed to the smu07 blood phenotype. Further analysis demonstrated that mutation of slc20a1b affects HSPC expansion through cell cycle arrest at G2/M phases in a cell-autonomous manner. Our study shows that slc20a1b is a vital regulator for HSPC proliferation in zebrafish early hematopoiesis and provides valuable insights into HSPC development.

Novel Dent disease 1 cellular models reveal biological processes underlying ClC-5 loss-of-function

Dent disease 1 (DD1) is a rare X-linked renal proximal tubulopathy characterized by low molecular weight proteinuria and variable degree of hypercalciuria, nephrocalcinosis and/or nephrolithiasis, progressing to chronic kidney disease. Although mutations in the electrogenic Cl-/H+ antiporter ClC-5, which impair endocytic uptake in proximal tubule cells, cause the disease, there is poor genotype-phenotype correlation and their contribution to proximal tubule dysfunction remains unclear. To further discover the mechanisms linking ClC-5 loss-of-function to proximal tubule dysfunction, we have generated novel DD1 cellular models depleted of ClC-5 and carrying ClC-5 mutants p.(Val523del), p.(Glu527Asp) and p.(Ile524Lys) using the human proximal tubule-derived RPTEC/TERT1 cell line. Our DD1 cellular models exhibit impaired albumin endocytosis, increased substrate adhesion and decreased collective migration, correlating with a less differentiated epithelial phenotype. Despite sharing functional features, these DD1 cell models exhibit different gene expression profiles, being p.(Val523del) ClC-5 the mutation showing the largest differences. Gene set enrichment analysis pointed to kidney development, anion homeostasis, organic acid transport, extracellular matrix organization and cell-migration biological processes as the most likely involved in DD1 pathophysiology. In conclusion, our results revealed the pathways linking ClC-5 mutations with tubular dysfunction and, importantly, provide new cellular models to further study DD1 pathophysiology.

Identification and Characterization of Osmoregulation Related MicroRNAs in Gills of Hybrid Tilapia Under Three Types of Osmotic Stress

Researchers have increasingly suggested that microRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate gene expression and protein translation in organs and respond to abiotic and biotic stressors. To understand the function of miRNAs in osmotic stress regulation of the gills of hybrid tilapia (Oreochromis mossambicus ♀ × Oreochromis urolepis hornorum ♂), high-throughput Illumina deep sequencing technology was used to investigate the expression profiles of miRNAs under salinity stress (S, 25‰), alkalinity stress (A, 4‰) and salinity-alkalinity stress (SA, S: 15‰, A: 4‰) challenges. The results showed that 31, 41, and 27 upregulated and 33, 42, and 40 downregulated miRNAs (P < 0.05) were identified in the salt stress, alkali stress, and saline-alkali stress group, respectively, which were compared with those in the control group (C). Fourteen significantly differently expressed miRNAs were selected randomly and then validated by a quantitative polymerase chain reaction. On the basis of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis, genes related to osmoregulation and biosynthesis were enriched in the three types of osmotic stress. In addition, three miRNAs and three predicted target genes were chosen to conduct a quantitative polymerase chain reaction in the hybrid tilapia and its parents during 96-h osmotic stress. Differential expression patterns of miRNAs provided the basis for research data to further investigate the miRNA-modulating networks in osmoregulation of teleost.

High phosphate impairs arterial endothelial function through AMPK-related pathways in mouse resistance arteries

AIMS

In patients with renal disease, high serum phosphate shows a relationship with cardiovascular risk. We speculate that high phosphate (HP) impairs arterial vasodilation via the endothelium and explore potential underlying mechanisms.

METHODS

Isolated vessel relaxation, endothelial function, glomerular filtration rate (GFR), oxidative stress status and protein expression were assessed in HP diet mice. Mitochondrial function and protein expression were assessed in HP-treated human umbilical vein endothelial cells (HUVECs).

RESULTS

High phosphate (1.3%) diet for 12 weeks impaired endothelium-dependent relaxation in mesenteric arteries, kidney interlobar arteries and afferent arterioles; reduced GFR and the blood pressure responses to acute administration of acetylcholine. The PPARα/LKB1/AMPK/eNOS pathway was attenuated in the endothelium of mesenteric arteries from HP diet mice. The observed vasodilatory impairment of mesenteric arteries was ameliorated by PPARα agonist WY-14643. The phosphate transporter PiT-1 knockdown prevented HP-mediated suppression of eNOS activity by impeding phosphorus influx in HUVECs. Endothelium cytoplasmic and mitochondrial reactive oxygen species (ROS) were increased in HP diet mice. Moreover HP decreased the expression of mitochondrial-related antioxidant genes. Finally, mitochondrial membrane potential and PGC-1α expression were reduced by HP treatment in HUVECs, which was partly restored by AMPKα agonist.

CONCLUSIONS

HP impairs endothelial function by reducing NO bioavailability via decreasing eNOS activity and increasing mitochondrial ROS, in which the AMPK-related signalling pathways may play a key role.

Specific populations of urinary extracellular vesicles and proteins differentiate type 1 primary hyperoxaluria patients without and with nephrocalcinosis or kidney stones

BACKGROUND

Primary hyperoxaluria type 1 (PH1) is associated with nephrocalcinosis (NC) and calcium oxalate (CaOx) kidney stones (KS). Populations of urinary extracellular vesicles (EVs) can reflect kidney pathology. The aim of this study was to determine whether urinary EVs carrying specific biomarkers and proteins differ among PH1 patients with NC, KS or with neither disease process.

METHODS

Mayo Clinic Rare Kidney Stone Consortium bio-banked cell-free urine from male and female PH1 patients without (n = 10) and with NC (n = 6) or KS (n = 9) and an eGFR > 40 mL/min/1.73 m2 were studied. Urinary EVs were quantified by digital flow cytometer and results expressed as EVs/ mg creatinine. Expressions of urinary proteins were measured by customized antibody array and results expressed as relative intensity. Data were analyzed by ANCOVA adjusting for sex, and biomarkers differences were considered statistically significant among groups at a false discovery rate threshold of Q < 0.20.

RESULTS

Total EVs and EVs from different types of glomerular and renal tubular cells (11/13 markers) were significantly (Q < 0.20) altered among PH1 patients without NC and KS, patients with NC or patients with KS alone. Three cellular adhesion/inflammatory (ICAM-1, MCP-1, and tissue factor) markers carrying EVs were statistically (Q < 0.20) different between PH1 patients groups. Three renal injury (β2-microglobulin, laminin α5, and NGAL) marker-positive urinary EVs out of 5 marker assayed were statistically (Q < 0.20) different among PH1 patients without and with NC or KS. The number of immune/inflammatory cell-derived (8 different cell markers positive) EVs were statistically (Q < 0.20) different between PH1 patients groups. EV generation markers (ANO4 and HIP1) and renal calcium/phosphate regulation or calcifying matrixvesicles markers (klotho, PiT1/2) were also statistically (Q < 0.20) different between PH1 patients groups. Only 13 (CD14, CD40, CFVII, CRP, E-cadherin, EGFR, endoglin, fetuin A, MCP-1, neprilysin, OPN, OPGN, and PDGFRβ) out of 40 proteins were significantly (Q < 0.20) different between PH1 patients without and with NC or KS.

CONCLUSIONS

These results imply activation of distinct renal tubular and interstitial cell populations and processes associated with KS and NC, and suggest specific populations of urinary EVs and proteins are potential biomarkers to assess the pathogenic mechanisms between KS versus NC among PH1 patients.

Urinary phosphate-containing nanoparticle contributes to inflammation and kidney injury in a salt-sensitive hypertension rat model

Although disturbed phosphate metabolism frequently accompanies chronic kidney disease (CKD), its causal role in CKD progression remains unclear. It is also not fully understood how excess salt induces organ damage. We here show that urinary phosphate-containing nanoparticles promote kidney injury in salt-sensitive hypertension. In Dahl salt-sensitive rats, salt loading resulted in a significant increase in urinary phosphate excretion without altering serum phosphate levels. An intestinal phosphate binder sucroferric oxyhydroxide attenuated renal inflammation and proteinuria in this model, along with the suppression of phosphaturia. Using cultured proximal tubule cells, we confirmed direct pathogenic roles of phosphate-containing nanoparticles in renal tubules. Finally, transcriptome analysis revealed a potential role of complement C1q in renal inflammation associated with altered phosphate metabolism. These data demonstrate that increased phosphate excretion promotes renal inflammation in salt-sensitive hypertension and suggest a role of disturbed phosphate metabolism in the pathophysiology of hypertensive kidney disease and high salt-induced kidney injury.

Expression of Phosphatonin-Related Genes in Sheep, Dog and Horse Kidneys Using Quantitative Reverse Transcriptase PCR

Simple Summary

Traditionally, it has been thought that control of body phosphorus was secondary to the tighter control of calcium. However, over the last 20 years, an extensive system for control of body phosphorus by proteins called phosphatonins has been shown to exist. Most research on phosphatonins has been done in rat or mouse models. This paper looks at whether important proteins and phosphorus channels in the phosphatonin pathways are present in the kidneys of dogs, horses and sheep. The results showed that all of the components of the phosphatonin system are present in these species, but that there are species differences in which protein or channel is most common, and in the relationships between the proteins and channels. This research is important because the phosphatonin system is involved in the progression of chronic kidney disease in humans and animals, and differences in the systems between animal species may affect treatment of chronic kidney disease.

Abstract

The aim of this preliminary study was to determine the relative expression of phosphatonin pathway-related genes in normal dog, sheep and horse kidneys and to explore the relationships between the different genes. Kidneys were collected post-mortem from 10 sheep, 10 horses and 8 dogs. RNA was extracted, followed by reverse transcriptase quantitative polymerase chain reaction for fibroblast growth factor receptor 1 IIIc (FGFR1IIIC), sodium-phosphate co-transporter (NPT) 1 (SLC17A1), NPT2a (SLC34A1), NPT2c (SLC34A3), parathyroid hormone 1 receptor (PTH1R), klotho (KL), vitamin D receptor (VDR), 1a-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24A1). NPT2a was highly expressed in the dog kidneys, compared with those of the horses and sheep. NPT1 had greatest expression in horses and sheep, although the three different NPTs all had relatively similar expression in sheep. There was little variability in FGFR1IIIc expression, particularly in the dogs and horses. FGFR1IIIc expression was negatively correlated with NPT genes (except NPT2a in sheep), while NPT genes were all positively correlated with each other. Unexpectedly, klotho was positively correlated with NPT genes in all three species. These results provide the basis for further research into this important regulatory system. In particular, species differences in phosphatonin gene expression should be considered when considering the pathogenesis of chronic kidney disease.

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.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Role of sodium-dependent Pi transporter/Npt2c on Pi homeostasis in klotho knockout mice different properties between juvenile and adult stages

SLC34A3/NPT2c/NaPi-2c/Npt2c is a growth-related NaPi cotransporter that mediates the uptake of renal sodium-dependent phosphate (Pi). Mutation of human NPT2c causes hereditary hypophosphatemic rickets with hypercalciuria. Mice with Npt2c knockout, however, exhibit normal Pi metabolism. To investigate the role of Npt2c in Pi homeostasis, we generated α-klotho-/- /Npt2c-/- (KL2cDKO) mice and analyzed Pi homeostasis. α-Klotho-/- (KLKO) mice exhibit hyperphosphatemia and markedly increased kidney Npt2c protein levels. Genetic disruption of Npt2c extended the lifespan of KLKO mice similar to that of α-Klotho-/- /Npt2a-/- mice. Adult KL2cDKO mice had hyperphosphatemia, but analysis of Pi metabolism revealed significantly decreased intestinal and renal Pi (re)absorption compared with KLKO mice. The 1,25-dihydroxy vitamin D3 concentration was not reduced in KL2cDKO mice compared with that in KLKO mice. The KL2cDKO mice had less severe soft tissue and vascular calcification compared with KLKO mice. Juvenile KL2cDKO mice had significantly reduced plasma Pi levels, but Pi metabolism was not changed. In Npt2cKO mice, plasma Pi levels began to decrease around the age of 15 days and significant hypophosphatemia developed within 21 days. The findings of the present study suggest that Npt2c contributes to regulating plasma Pi levels in the juvenile stage and affects Pi retention in the soft and vascular tissues in KLKO mice.

Partial reduced Pi transport function of PiT-2 might not be sufficient to induce brain calcification of idiopathic basal ganglia calcification

Idiopathic basal ganglia calcification (IBGC) is a rare intractable disease characterized by abnormal mineral deposits, including mostly calcium in the basal ganglia, thalamus, and cerebellum. SLC20A2 is encoding the phosphate transporter PiT-2 and was identified in 2012 as the causative gene of familial IBGC. In this study, we investigated functionally two novel SLC20A2 variants (c.680C > T, c.1487G > A) and two SLC20A2 variants (c.82G > A, c.358G > C) previously reported from patients with IBGC. We evaluated the function of variant PiT-2 using stable cell lines. While inorganic phosphate (Pi) transport activity was abolished in the cells with c.82G > A, c.358G > C, and c.1487G > A variants, activity was maintained at 27.8% of the reference level in cells with the c.680C > T variant. Surprisingly, the c.680C > T variant had been discovered by chance in healthy members of an IBGC family, suggesting that partial preservation of Pi transport activity may avoid the onset of IBGC. In addition, we confirmed that PiT-2 variants could be translocated into the cell membrane to the same extent as PiT-2 wild type. In conclusion, we investigated the PiT-2 dysfunction of four SLC20A2 variants and suggested that a partial reduced Pi transport function of PiT-2 might not be sufficient to induce brain calcification of IBGC.

Pericytes in Primary Familial Brain Calcification

Pericytes are perivascular cells along capillaries that are critical for the development of a functional vascular bed in the central nervous system and other organs. Pericyte functions in the adult brain are less well understood. Pericytes have been suggested to mediate functional hyperemia at the capillary level, regulate the blood-brain barrier and to give rise to scar tissue after spinal cord injury. Furthermore, pericyte loss has been suggested to precede cognitive decline in mouse models of Alzheimer's disease. Despite this observation, there is no convincing causality between pericyte loss and the pathogenesis of Alzheimer's disease. However, recent loss-of-function mutations in PDGFB and PDGFRB genes have implicated pericytes as the principle cell type affected in primary familiar brain calcification (PFBC), a neuropsychiatric disorder with dominant inheritance. Here we review the role of the PDGFB/PDGFRB signaling pathway in pericyte development and briefly discuss homeostatic functions of pericytes in the brain. We provide an overview of recent studies with mouse models of PFBC and discuss suggested pathogenic mechanisms for PFBC with special reference to pericytes.

Expression and function of Slc34 sodium-phosphate co-transporters in skeleton and teeth

Under normal physiological condition, the biomineralization process is limited to skeletal tissues and teeth and occurs throughout the individual's life. Biomineralization is an actively regulated process involving the progressive mineralization of the extracellular matrix secreted by osteoblasts in bone or odontoblasts and ameloblasts in tooth. Although the detailed molecular mechanisms underlying the formation of calcium-phosphate apatite crystals are still debated, it is suggested that calcium and phosphate may need to be transported across the membrane of the mineralizing cell, suggesting a pivotal role of phosphate transporters in bone and tooth mineralization. In this context, this short review describes the current knowledge on the role of Slc34 Na⁺-phosphate transporters in skeletal and tooth mineralization.

NAD metabolism and the SLC34 family: evidence for a liver-kidney axis regulating inorganic phosphate

The solute carrier 34 (SLC34) family of membrane transporters is a major contributor to Pi homeostasis. Many factors are involved in regulating the SLC34 family. The roles of the bone mineral metabolism factors parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) in Pi homeostasis are well studied. Intracellular Pi is thought to be involved in energy metabolism, such as ATP production. Under certain conditions of altered energy metabolism, plasma Pi concentrations are affected by the regulation of a Pi shift into cells or release from the tissues. We recently investigated the mechanism of hepatectomy-related hypophosphatemia, which is thought to involve an unknown phosphaturic factor. Hepatectomy-related hypophosphatemia is due to impaired nicotinamide adenine dinucleotide (NAD) metabolism through its effects on the SLC34 family in the liver-kidney axis. The oxidized form of NAD, NAD⁺, is an essential cofactor in various cellular biochemical reactions. Levels of NAD⁺ and its reduced form NADH vary with the availability of dietary energy and nutrients. Nicotinamide phosphoribosyltransferase (Nampt) generates a key NAD⁺ intermediate, nicotinamide mononucleotide, from nicotinamide and 5-phosphoribosyl 1-pyrophosphate. The liver, an important organ of NAD metabolism, is thought to release metabolic products such as nicotinamide and may control NAD metabolism in other organs. Moreover, NAD is an important regulator of the circadian rhythm. Liver-specific Nampt-deficient mice and heterozygous Nampt mice have abnormal daily plasma Pi concentration oscillations. These data indicate that NAD metabolism in the intestine, liver, and kidney is closely related to Pi metabolism through the SLC34 family. Here, we review the relationship between the SLC34 family and NAD metabolism based on our recent studies.

Primary familial brain calcification with a novel SLC20A2 mutation: Analysis of PiT-2 expression and localization

Primary familial brain calcification (PFBC) is an autosomal dominant rare disorder characterized by bilateral and symmetric brain calcifications, and neuropsychiatric manifestations. Four genes have been linked to PFBC: SLC20A2, PDGFRB, PDGFB, and XPR1. In this study, we report molecular and clinical data of a PFBC patient carrying a novel SLC20A2 mutation and we investigate the impact of the mutation on PiT-2 expression and function. Sanger sequencing of SLC20A2, PDGFRB, PDGFB, XPR1 led to the identification of a novel duplication of twelve nucleotides (c.1876_1887dup/ p.Trp626_Thr629dup) in SLC20A2 gene. SLC20A2 encodes for a cell membrane transporter (PiT-2) involved in maintenance of inorganic phosphate homeostasis. We performed an analysis of expression and functionality of PiT-2 protein in patient primary cultured fibroblasts. In patient fibroblasts, the mutation does not affect PiT-2 expression but alter sub-cellular localization. The Pi-uptake assay revealed a less Pi depletion in patient than in control fibroblasts, suggesting that SLC20A2 duplication may impair Pi internalization. This is the first study reporting sub-cellular expression analysis of mutant PiT-2 in primary cultured fibroblasts from a PFBC patient, showing that p.Trp626_Thr629dup in SLC20A2 alters PiT-2 sub-cellular localization and reduces Pi-uptake, leading to onset of PFBC in our patient.

Neuroprotective effect of 5-aminolevulinic acid against low inorganic phosphate in neuroblastoma SH-SY5Y cells

PiT-1 (encoded by SLC20A1) and PiT-2 (encoded by SLC20A2) are type-III sodium-dependent phosphate cotransporters (NaPiTs). Recently, SLC20A2 mutations have been found in patients with idiopathic basal ganglia calcification (IBGC), and were predicted to bring about an inability to transport Pi from the extracellular environment. Here we investigated the effect of low Pi loading on the human neuroblastoma SH-SY5Y and the human glioblastoma A172 cell lines. The results show a different sensitivity to low Pi loading and differential regulation of type-III NaPiTs in these cells. We also examined whether 5-aminolevulinic acid (5-ALA) inhibited low Pi loading-induced neurotoxicity in SH-SY5Y cells. Concomitant application of 5-ALA with low Pi loading markedly attenuated low Pi-induced cell death and mitochondrial dysfunction via the induction of HO-1 by p38 MAPK. The findings provide us with novel viewpoints to understand the pathophysiology of IBGC, and give a new insight into the clinical prevention and treatment of IBGC.

Dietary Phosphorus Intake and the Kidney

Although phosphorus is an essential nutrient required for multiple physiological functions, recent research raises concerns that high phosphorus intake could have detrimental effects on health. Phosphorus is abundant in the food supply of developed countries, occurring naturally in protein-rich foods and as an additive in processed foods. High phosphorus intake can cause vascular and renal calcification, renal tubular injury, and premature death in multiple animal models. Small studies in human suggest that high phosphorus intake may result in positive phosphorus balance and correlate with renal calcification and albuminuria. Although serum phosphorus is strongly associated with cardiovascular disease, progression of kidney disease, and death, limited data exist linking high phosphorus intake directly to adverse clinical outcomes. Further prospective studies are needed to determine whether phosphorus intake is a modifiable risk factor for kidney disease.

Effect of Osteocyte-Ablation on Inorganic Phosphate Metabolism: Analysis of Bone-Kidney-Gut Axis

In response to kidney damage, osteocytes increase the production of several hormones critically involved in mineral metabolism. Recent studies suggest that osteocyte function is altered very early in the course of chronic kidney disease. In the present study, to clarify the role of osteocytes and the canalicular network in mineral homeostasis, we performed four experiments. In Experiment 1, we investigated renal and intestinal Pi handling in osteocyte-less (OCL) model mice [transgenic mice with the dentin matrix protein-1 promoter-driven diphtheria toxin (DT)-receptor that were injected with DT]. In Experiment 2, we administered granulocyte colony-stimulating factor to mice to disrupt the osteocyte canalicular network. In Experiment 3, we investigated the role of osteocytes in dietary Pi signaling. In Experiment 4, we analyzed gene expression level fluctuations in the intestine and liver by comparing mice fed a high Pi diet and OCL mice. Together, the findings of these experiments indicate that osteocyte ablation caused rapid renal Pi excretion (P < 0.01) before the plasma fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) levels increased. At the same time, we observed a rapid suppression of renal Klotho (P < 0.01), type II sodium phosphate transporters Npt2a (P < 0.01) and Npt2c (P < 0.05), and an increase in intestinal Npt2b (P < 0.01) protein. In OCL mice, Pi excretion in feces was markedly reduced (P < 0.01). Together, these effects of osteocyte ablation are predicted to markedly increase intestinal Pi absorption (P < 0.01), thus suggesting that increased intestinal Pi absorption stimulates renal Pi excretion in OCL mice. In addition, the ablation of osteocytes and feeding of a high Pi diet affected FGF15/bile acid metabolism and controlled Npt2b expression. In conclusion, OCL mice exhibited increased renal Pi excretion due to enhanced intestinal Pi absorption. We discuss the role of FGF23-Klotho on renal and intestinal Pi metabolism in OCL mice.

Control of phosphate balance by the kidney and intestine

The prevention and correction of hyperphosphatemia are major goals of the treatment of chronic kidney disease (CKD)-bone mineral disorders, and thus, Pi balance requires special attention. Pi balance is maintained by intestinal absorption, renal excretion, and bone accretion. The kidney is mainly responsible for the plasma Pi concentration. In CKD, reduced glomerular filtration rate leads to various Pi metabolism abnormalities, and Pi absorption in the small intestine also has an important role in Pi metabolism. Disturbances in Pi metabolism are mediated by a series of complex changes in regulatory hormones originating from the skeleton, intestine, parathyroid gland, and kidney. In this review, we describe the regulation of type II sodium-dependent Pi co-transporters by the kidney and intestine, including the regulation of Pi transport, circadian rhythm, and the vicious circle between salivary Pi secretion and intestinal Pi absorption in animals with and without CKD.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Regulation of renal phosphate handling: inter-organ communication in health and disease

In this review, we focus on the interconnection of inorganic phosphate (Pi) homeostasis in the network of the bone-kidney, parathyroid-kidney, intestine-kidney, and liver-kidney axes. Such a network of organ communication is important for body Pi homeostasis. Normalization of serum Pi levels is a clinical target in patients with chronic kidney disease (CKD). Particularly, disorders of the fibroblast growth factor 23/klotho system are observed in early CKD. Identification of phosphaturic factors from the intestine and liver may enhance our understanding of body Pi homeostasis and Pi metabolism disturbances in CKD patients.

PDGF, pericytes and the pathogenesis of idiopathic basal ganglia calcification (IBGC)

Platelet-derived growth factors (PDGFs) are important mitogens for various types of mesenchymal cells, and as such, they exert critical functions during organogenesis in mammalian embryonic and early postnatal development. Increased or ectopic PDGF activity may also cause or contribute to diseases such as cancer and tissue fibrosis. Until recently, no loss-of-function (LOF) mutations in PDGF or PDGF receptor genes were reported as causally linked to a human disease. This changed in 2013 when reports appeared on presumed LOF mutations in the genes encoding PDGF-B and its receptor PDGF receptor-beta (PDGF-Rβ) in familial idiopathic basal ganglia calcification (IBGC), a brain disease characterized by anatomically localized calcifications in or near the blood microvessels. Here, we review PDGF-B and PDGF-Rβ biology with special reference to their functions in brain-blood vessel development, pericyte recruitment and the regulation of the blood-brain barrier. We also discuss various scenarios for IBGC pathogenesis suggested by observations in patients and genetically engineered animal models of the disease.

Vascular Calcification and Stone Disease: A New Look towards the Mechanism

Calcium phosphate (CaP) crystals are formed in pathological calcification as well as during stone formation. Although there are several theories as to how these crystals can develop through the combined interactions of biochemical and biophysical factors, the exact mechanism of such mineralization is largely unknown. Based on the published scientific literature, we found that common factors can link the initial stages of stone formation and calcification in anatomically distal tissues and organs. For example, changes to the spatiotemporal conditions of the fluid flow in tubular structures may provide initial condition(s) for CaP crystal generation needed for stone formation. Additionally, recent evidence has provided a meaningful association between the active participation of proteins and transcription factors found in the bone forming (ossification) mechanism that are also involved in the early stages of kidney stone formation and arterial calcification. Our review will focus on three topics of discussion (physiological influences-calcium and phosphate concentration-and similarities to ossification, or bone formation) that may elucidate some commonality in the mechanisms of stone formation and calcification, and pave the way towards opening new avenues for further research.

Renal phosphate transporters

PURPOSE OF REVIEW

Phosphate homeostasis is tightly controlled by the coordinated activity of bone, kidney, intestine, and parathyroid gland. The renal phosphate transporters have emerged as key regulators of both total body phosphate homeostasis and serum phosphate concentration. This review focuses on the latest updates in phosphate transport and transporters with an emphasis on renal phosphate transporters.

RECENT FINDINGS

Structure function analysis of type II sodium phosphate cotransporters has revealed motifs with significant similarity to those seen in other sodium-coupled solute transporters, identifying key amino acid residues important for solute binding and transport. Previously unidentified regulators of these transporters have been found, although their physiologic significance and interaction with more traditional regulators have not been established. Type II and type III sodium phosphate cotransporters play critical roles in bone, choroid plexus, and vascular physiology and pathophysiology.

SUMMARY

Increasing knowledge of structure function relationships for sodium phosphate cotransporters, as well as greater appreciation for the complexity of their regulation and role in renal and nonrenal tissue, brings the promise of newer, more specific treatments for disorders of phosphate homeostasis.

VIDEO ABSTRACT

http://links.lww.com/CONH/A10.

Wakame (Undaria pinnatifida ) modulates hyperphosphatemia in a rat model of chronic renal failure

In chronic renal failure, inorganic phosphate (Pi) retention speeds up the progression to end-stage renal disease. The current therapy for hyperphosphatemia in patients with chronic renal failure consists of dietary Pi restriction combined with administration of Pi binders, but each therapy has practical problems. Thus, the discovery of foods or nutrients that inhibit Pi absorption may be useful for the treatment of hyperphosphatemia. In the present study, we investigated whether wakame (Undaria pinnatifida) is a useful food for the prevention of hyperphosphatemia in a rat model of renal failure. Feeding a diet containing 5% wakame significantly decreased plasma and urinary Pi levels and increased the amount of fecal Pi. In addition, wakame significantly reduced plasma blood urea nitrogen and plasma Pi levels in 5/6 nephrectomized rats fed a high-Pi diet. Biochemical analyses showed that the reduction of intestinal Pi absorption is the main reason for the decrease in plasma Pi levels in rats fed a diet containing wakame. In addition, feeding alginic acid and fucoidan, major components of wakame fiber, was effective in reducing plasma Pi levels in normal rats. Finally, we concluded that wakame may be a useful food for the prevention of hyperphosphatemia in rodents.

Regulation of serum phosphate

The regulation of serum phosphate, an acknowledged risk factor for chronic kidney disease and cardiovascular mortality, is poorly understood. The discovery of fibroblast growth factor 23 (FGF23) as a key regulator of renal phosphate handling and activation of vitamin D has revolutionized our comprehension of phosphate homeostasis. Through as yet undetermined mechanisms, circulating and dietary phosphate appear to have a direct effect on FGF23 release by bone cells that, in turn, causes renal phosphate excretion and decreases intestinal phosphate absorption through a decrease in vitamin D production. Thus, the two major phosphaturic hormones, PTH and FGF23, have opposing effects on vitamin D production, placing vitamin D at the nexus of phosphate homeostasis. While our understanding of phosphate homeostasis has advanced, the factors determining regulation of serum phosphate level remain enigmatic. Diet, time of day, season, gender, age and genetics have all been identified as significant contributors to serum phosphate level. The effects of these factors on serum phosphate have major implications for what is understood as 'normal' and for studies of phosphate homeostasis and metabolism. Moreover, other hormonal mediators such as dopamine, insulin-like growth factor, and angiotensin II also affect renal handling of phosphate. How the major hormone effects on phosphate handling are regulated and how the effect of these other factors are integrated to yield the measurable serum phosphate are only now beginning to be studied.

Homeostasis, the milieu intérieur, and the wisdom of the nephron

The concept of homeostasis has been inextricably linked to the function of the kidneys for more than a century when it was recognized that the kidneys had the ability to maintain the "internal milieu" and allow organisms the "physiologic freedom" to move into varying environments and take in varying diets and fluids. Early ingenious, albeit rudimentary, experiments unlocked a wealth of secrets on the mechanisms involved in the formation of urine and renal handling of the gamut of electrolytes, as well as that of water, acid, and protein. Recent scientific advances have confirmed these prescient postulates such that the modern clinician is the beneficiary of a rich understanding of the nephron and the kidney's critical role in homeostasis down to the molecular level. This review summarizes those early achievements and provides a framework and introduction for the new CJASN series on renal physiology.

Knockdown of the sodium-dependent phosphate co-transporter 2b (NPT2b) suppresses lung tumorigenesis

The sodium-dependent phosphate co-transporter 2b (NPT2b) plays an important role in maintaining phosphate homeostasis. In previous studies, we have shown that high dietary inorganic phosphate (Pi) consumption in mice stimulated lung tumorigenesis and increased NPT2b expression. NPT2b has also been found to be highly expressed in human lung cancer tissues. The association of high expression of NPT2b in the lung with poor prognosis in oncogenic lung diseases prompted us to test whether knockdown of NPT2b may regulate lung cancer growth. To address this issue, aerosols that contained small interfering RNA (siRNA) directed against NPT2b (siNPT2b) were delivered into the lungs of K-ras (LA1) mice, which constitute a murine model reflecting human lung cancer. Our results clearly showed that repeated aerosol delivery of siNPT2b successfully suppressed lung cancer growth and decreased cancer cell proliferation and angiogenesis, while facilitating apoptosis. These results strongly suggest that NPT2b plays a role lung tumorigenesis and represents a novel target for lung cancer therapy.

Localization of type-III sodium-dependent phosphate transporter 2 in the mouse brain

Type-III sodium-dependent phosphate transporters 1 and 2 (PiT-1 and PiT-2, respectively) are proteins encoded by SLC20A1 and SLC20A2, respectively. The ubiquitous distribution of PiT-1 and PiT-2 mRNAs in mammalian tissues is in agreement with the housekeeping maintenance of homeostasis of intracellular inorganic phosphate (Pi), which is absorbed from interstitial fluid for normal cellular functions. Recently, mutations of SLC20A2 have been found in patients with idiopathic basal ganglia calcification (IBGC), also known as Fahr's disease. However, the localization of PiT-2 in the brain has not been clarified yet. Therefore, the aim of this study is to clarify the distribution of PiT-2 expression in the mouse brain. Our biochemical and immunohistochemical analyses using a polyclonal antibody (Ab) and a monoclonal Ab showed that PiT-2 was ubiquitously expressed throughout the brain. In terms of the cellular type, PiT-2 was predominantly detected in neurons; it colocalized with β-tubulin III in the cerebral cortex and with calbindin D-28K in Purkinje cells. Additionally, PiT-2 immunopositivity was detected in the microtubule-associated protein 2-positive neuronal dendrites in the cerebral cortex. However, colocalization with PiT-2 immunopositivity was not observed in the synaptophysin-positive nerve terminals. PiT-2 was also expressed in astrocytes and vascular endothelial cells. Our results indicate that PiT-2 plays an important role in the maintenance of cellular Pi homeostasis in neurons, astrocytes, and endothelial cells. This finding is a milestone in the study of the function of PiT-2 in the normal mouse brain and particularly in the brains of patients with Fahr's disease.

Parathyroid hormone (PTH) decreases sodium-phosphate cotransporter type IIa (NpT2a) mRNA stability

The acute inhibitory effects of parathyroid hormone (PTH) on proximal tubule Na+-K+-ATPase (Na-K) and sodium-dependent phosphate (NaPi) transport have been extensively studied, while little is known about the chronic effects of PTH. Patients with primary hyperparathyroidism, a condition characterized by chronic elevations in PTH, exhibit persistent hypophosphatemia but not significant evidence of salt wasting. We postulate that chronic PTH stimulation results in differential desensitization of PTH responses. To address this hypothesis, we compared the effects of chronic PTH stimulation on Na-Pi cotransporter (Npt2a) expression and Na-K activity and expression in Sprague Dawley rats, transgenic mice featuring parathyroid-specific cyclin D1 overexpression (PTH-D1), and proximal tubule cell culture models. We demonstrated a progressive decrease in brush-border membrane (BBM) expression of Npt2a from rats treated with PTH for 6 h or 4 days, while Na-K expression and activity in the basolateral membranes (BLM) exhibited an initial decrease followed by recovery to control levels by 4 days. Npt2a protein expression in PTH-D1 mice was decreased relative to control animals, whereas levels of Na-K, NHERF-1, and PTH receptor remained unchanged. In PTH-D1 mice, NpT2a mRNA expression was reduced by 50% relative to control mice. In opossum kidney proximal tubule cells, PTH decreased Npt2a mRNA levels. Both actinomycin D and cycloheximide treatment prevented the PTH-mediated decrease in Npt2a mRNA, suggesting that the PTH response requires transcription and translation. These findings suggest that responses to chronic PTH exposure are selectively regulated at a posttranscriptional level. The persistence of the phosphaturic response to PTH occurs through posttranscriptional mechanisms.

Functional expression, purification and reconstitution of the recombinant phosphate transporter Pho89 of Saccharomyces cerevisiae

The Saccharomyces cerevisiae high-affinity phosphate transporter Pho89 is a member of the inorganic phosphate (Pi) transporter (PiT) family, and shares significant homology with the type III Na(+)/Pi symporters, hPit1 and hPit2. Currently, detailed biochemical and biophysical analyses of Pho89 to better understand its transport mechanisms are limited, owing to the lack of purified Pho89 in an active form. In the present study, we expressed functional Pho89 in the cell membrane of Pichia pastoris, solubilized it in Triton X-100 and foscholine-12, and purified it by immobilized nickel affinity chromatography combined with size exclusion chromatography. The protein eluted as an oligomer on the gel filtration column, and SDS/PAGE followed by western blotting analysis revealed that the protein appeared as bands of approximately 63, 140 and 520 kDa, corresponding to the monomeric, dimeric and oligomeric masses of the protein, respectively. Proteoliposomes containing purified and reconstituted Pho89 showed Na(+)-dependent Pi transport activity driven by an artificially imposed electrochemical Na(+) gradient. This implies that Pho89 operates as a symporter. Moreover, its activity is sensitive to the Na(+) ionophore monensin. To our knowledge, this study represents the first report on the functional reconstitution of a Pi-coupled PiT family member.