Renal Phosphate Transport Research Articles

The international X-linked hypophosphataemia (XLH) registry (NCT03193476): rationale for and description of an international, observational study

BackgroundX-linked hypophosphataemia (XLH) is a rare, hereditary, progressive and lifelong phosphate wasting disorder characterised by pathological elevations in fibroblast growth factor (FGF) 23 concentration and activity; XLH has an incidence of approximately 1 in 20–25,000 individuals. Excess FGF23 activity leads to increased phosphate excretion in the kidneys – mediated by downregulation of renal tubular phosphate transporters – and reduced phosphate absorption in the intestines – due to impaired vitamin D activation. This results in impaired bone growth and mineralisation, short and disproportionate stature, leg bowing, musculoskeletal pain, spontaneous dental abscesses, rickets, and osteomalacia. The spectrum of manifestations differs between paediatric and adult patients. Those involved in the treatment of this condition face many challenges, including a lack of robust natural history and demographic data. This multicentre, international, rare-disease patient registry (XLH Registry) was established to address the paucity of data in XLH and to help inform future clinical practice.ResultsThe XLH Registry collects standard diagnostic and monitoring practice data, including (where applicable) diagnosis and disease progression history, treatment regimens and family history; the protocol does not mandate any interventions or clinical assessments. The XLH Registry aims to recruit 1200 paediatric and adult patients with XLH over 10 years, and several data analyses and peer-reviewed publications are expected to be generated throughout this period. A post-authorisation safety study for Bburosumab, for which the registry Sponsor is the marketing authorisation holder, will be nested as a sub-study within the XLH Registry via a subsequent protocol amendment.ConclusionThe data collected within this rare-disease patient registry will be utilised to synthesise real-world evidence to inform the management of XLH, to improve the quality of life and standard of care of patients living with this rare debilitating disease.

Androgen action on renal calcium and phosphate handling: Effects of bisphosphonate treatment and low calcium diet

Renal calcium and phosphate handling is an important contributor to mineral homeostasis and bone health and the androgen receptor (AR) is highly expressed in the kidney. We investigated the short term effects of androgen deprivation on renal calcium and phosphate reabsorption, independent of their effects on bone. Two weeks following orchidectomy (ORX) of adult mice, bone loss occurred along with hypercalciuria, which was similarly prevented by testosterone and dihydrotestosterone supplementation. Treatment with bisphosphonates prior to ORX also inhibited hypercalciuria, indicating that the calcium flux originated from the bone. Renal calcium and phosphate transporter expression was increased post-ORX, independent of bisphosphonates. Furthermore, androgen deprivation appeared to stimulate local synthesis of 1,25(OH)2D3. When bisphosphonate-treated mice were fed a low calcium diet, bone resorption was no longer blocked and secondary hyperparathyroidism developed, which was more pronounced in ORX mice than sham-operated mice. In conclusion, this study shows that androgen deprivation increased renal calcium and phosphate transporter expression, independent of bone, and underlines the importance of adequate intestinal calcium supply in circumstances of androgen deprivation and bisphosphonate treatment.

Hypophosphatemic Rickets with Hypercalciuria: A Novel Homozygous Mutation in SLC34A3 and Literature Review

Hypophosphatemic rickets with hypercalciuria (HHRH) is a rare, recessively-inherited form of rickets caused by homozygous or compound heterozygous mutations in the SLC34A3 gene that encodes the renal tubular phosphate transporter protein NaPi2c. The bone phenotype varies from severe rickets to no disease. Accurate diagnosis is important as the treatment differs from other forms of rickets. The patient was a 12-year-old boy from the Indian subcontinent with florid hypophosphatemic rickets. A targeted gene panel to search for mutations in genes associated with inherited forms of rickets was performed. We also completed a literature search of published cases of HHRH. The targeted gene panel demonstrated a novel homozygous SLC34A3 mutation: c.1339 G>A (p.Ala447Thr). His parents were heterozygous for the mutation. In our literature review we found that people with homozygous SLC34A3 mutations were more likely to have rickets than those with compound heterozygous mutations (85% versus 45%, p<0.002) and that serum phosphate z scores were lower in those with rickets than those without (-3.3 with a standard deviation of 1.5 versus -2.1 with a standard deviation of 1.5, p<0.005). The bone phenotype of HHRH is related to the nature of the mutation and serum phosphate levels. Targeted gene panels can aid in the accurate diagnosis of inherited forms of rickets, and facilitate correct treatment.

Fibroblast growth factor 23 and phosphate homeostasis.

The current review highlights recent advances in the area of renal tubular phosphate transport and its regulation by fibroblast growth factor 23 (FGF23), a potent regulator of phosphate homeostasis. Recent studies demonstrate that FGF23 binds to both membrane and soluble form of α-klotho to activate FGF receptor signaling pathways. Parathyroid hormone and FGF23 equivalently decrease sodium-dependent phosphate cotransport but the effect is not additive, suggesting a shared but not synergistic mechanism of action. Crosstalk occurs downstream of parathyroid hormone-receptor and FGF23-receptor signaling and converge at the level of the scaffolding protein, sodium-hydrogen exchanger regulatory factor-1. A novel mechanism for phosphate efflux through the basolateral membrane of renal proximal tubular epithelia via an atypical G-protein coupled receptor, Xenotropic and polytropic retrovirus receptor 1 (XPR1), was recently identified. Conditional deletion of Xpr1 gene in renal proximal tubules in mice leads to hypophosphatemic rickets and Fanconi syndrome establishing an important role for XPR1 in phosphate homeostasis. A novel anti-FGF23 antibody, burosumab, was recently approved to treat X-linked hypophosphatemia, a human disorder of FGF23 excess. Significant advances in understanding the cellular and molecular aspects of renal tubular phosphate transport and its regulation by FGF23 has led to the discovery of novel therapeutics to treat human disorders of phosphate homeostasis.

Evaluating pathogenicity of SLC34A3-Ser192Leu, a frequent European missense variant in disorders of renal phosphate wasting

Loss-of-function mutations of SLC34A3 represent an established cause of a distinct renal phosphate wasting disorder termed hereditary hypophosphatemic rickets with hypercalciuria (HHRH). SLC34A3 encodes the renal phosphate transporter NaPi2c expressed at the apical brush border of proximal renal tubules. Substitution of p.Ser192Leu is one of the most frequent genetic changes among HHRH patients in Europe, but has never been systematically evaluated, clinically or on a cellular level. Identification of a 32-year-old female with a homozgyous c.575C>T, p.Ser192Leu substitution enabled a more comprehensive assessment of the impact of this missense variant. Clinically, the patient showed renal phosphate wasting and nephrocalcinosis without any bone abnormalities. Heterozygous carriers of deleterious SLC34A3 variants were previously described to harbor an increased risk of kidney stone formation and renal calcification. We hence examined the frequency of p.Ser192Leu variants in our adult kidney stone cohort and compared the results to clinical findings of previously published cases of both mono- and biallelic p.Ser192Leu changes. On a cellular level, p.Ser192Leu-mutated transporters localize to the plasma membrane in different cellular systems, but lead to significantly reduced transport activity of inorganic phosphate upon overexpression in Xenopus oocytes. Despite the reduced function in ectopic cellular systems, the clinical consequences of p.Ser192Leu may appear relatively mild, at least in our index patient, and can potentially be missed in clinical practice.

Genetic Variants Associated with Circulating Fibroblast Growth Factor 23.

Fibroblast growth factor 23 (FGF23), a bone-derived hormone that regulates phosphorus and vitamin D metabolism, contributes to the pathogenesis of mineral and bone disorders in CKD and is an emerging cardiovascular risk factor. Central elements of FGF23 regulation remain incompletely understood; genetic variation may help explain interindividual differences. We performed a meta-analysis of genome-wide association studies of circulating FGF23 concentrations among 16,624 participants of European ancestry from seven cohort studies, excluding participants with eGFR<30 ml/min per 1.73 m2 to focus on FGF23 under normal conditions. We evaluated the association of single-nucleotide polymorphisms (SNPs) with natural log-transformed FGF23 concentration, adjusted for age, sex, study site, and principal components of ancestry. A second model additionally adjusted for BMI and eGFR. We discovered 154 SNPs from five independent regions associated with FGF23 concentration. The SNP with the strongest association, rs17216707 (P=3.0×10-24), lies upstream of CYP24A1, which encodes the primary catabolic enzyme for 1,25-dihydroxyvitamin D and 25-hydroxyvitamin D. Each additional copy of the T allele at this locus is associated with 5% higher FGF23 concentration. Another locus strongly associated with variations in FGF23 concentration is rs11741640, within RGS14 and upstream of SLC34A1 (a gene involved in renal phosphate transport). Additional adjustment for BMI and eGFR did not materially alter the magnitude of these associations. Another top locus (within ABO, the ABO blood group transferase gene) was no longer statistically significant at the genome-wide level. Common genetic variants located near genes involved in vitamin D metabolism and renal phosphate transport are associated with differences in circulating FGF23 concentrations.

AMP-activated kinase is a regulator of fibroblast growth factor 23 production

Fibroblast growth factor 23 (FGF23) is a proteohormone regulating renal phosphate transport and vitamin D metabolism as well as inducing left heart hypertrophy. FGF23-deficient mice suffer from severe tissue calcification, accelerated aging and a myriad of aging-associated diseases. Bone cells produce FGF23 upon store-operated calcium ion entry (SOCE) through the calcium selective ion channel Orai1. AMP-activated kinase (AMPK) is a powerful energy sensor helping cells survive states of energy deficiency, and AMPK down-regulates Orai1. Here we investigated the role of AMPK in FGF23 production. Fgf23 gene transcription was analyzed by qRT-PCR and SOCE by fluorescence optics in UMR106 osteoblast-like cells while the serum FGF23 concentration and phosphate metabolism were assessed in AMPKα1-knockout and wild-type mice. The AMPK activator, 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) down-regulated, whereas the AMPK inhibitor, dorsomorphin dihydrochloride (compound C) and AMPK gene silencing induced Fgf23 transcription. AICAR decreased membrane abundance of Orai1 and SOCE. SOCE inhibitors lowered Fgf23 gene expression induced by AMPK inhibition. AMPKα1-knockout mice had a higher serum FGF23 concentration compared to wild-type mice. Thus, AMPK participates in the regulation of FGF23 production invitro and invivo. The inhibitory effect of AMPK on FGF23 production is at least in part mediated by Orai1-involving SOCE.

Genetic Variants Associated with Circulating Parathyroid Hormone.

Parathyroid hormone (PTH) is a primary calcium regulatory hormone. Elevated serum PTH concentrations in primary and secondary hyperparathyroidism have been associated with bone disease, hypertension, and in some studies, cardiovascular mortality. Genetic causes of variation in circulating PTH concentrations are incompletely understood. We performed a genome-wide association study of serum PTH concentrations among 29,155 participants of European ancestry from 13 cohort studies (n=22,653 and n=6502 in discovery and replication analyses, respectively). We evaluated the association of single nucleotide polymorphisms (SNPs) with natural log-transformed PTH concentration adjusted for age, sex, season, study site, and principal components of ancestry. We discovered associations of SNPs from five independent regions with serum PTH concentration, including the strongest association with rs6127099 upstream of CYP24A1 (P=4.2 × 10-53), a gene that encodes the primary catabolic enzyme for 1,25-dihydroxyvitamin D and 25-dihydroxyvitamin D. Each additional copy of the minor allele at this SNP associated with 7% higher serum PTH concentration. The other SNPs associated with serum PTH concentration included rs4074995 within RGS14 (P=6.6 × 10-17), rs219779 adjacent to CLDN14 (P=3.5 × 10-16), rs4443100 near RTDR1 (P=8.7 × 10-9), and rs73186030 near CASR (P=4.8 × 10-8). Of these five SNPs, rs6127099, rs4074995, and rs219779 replicated. Thus, common genetic variants located near genes involved in vitamin D metabolism and calcium and renal phosphate transport associated with differences in circulating PTH concentrations. Future studies could identify the causal variants at these loci, and the clinical and functional relevance of these variants should be pursued.

1,25-Dihydroxyvitamin D Alone Improves Skeletal Growth, Microarchitecture, and Strength in a Murine Model of XLH, Despite Enhanced FGF23 Expression.

X-linked hypophosphatemia (XLH) is characterized by impaired renal tubular reabsorption of phosphate owing to increased circulating FGF23 levels, resulting in rickets in growing children and impaired bone mineralization. Increased FGF23 decreases renal brush border membrane sodium-dependent phosphate transporter IIa (Npt2a) causing renal phosphate wasting, impairs 1-α hydroxylation of 25-hydroxyvitamin D, and induces the vitamin D 24-hydroxylase, leading to inappropriately low circulating levels of 1,25-dihydroxyvitamin D (1,25D). The goal of therapy is prevention of rickets and improvement of growth in children by phosphate and 1,25D supplementation. However, this therapy is often complicated by hypercalcemia and nephrocalcinosis and does not always prevent hyperparathyroidism. To determine if 1,25D or blocking FGF23 action can improve the skeletal phenotype without phosphate supplementation, mice with XLH (Hyp) were treated with daily 1,25D repletion, FGF23 antibodies (FGF23Ab), or biweekly high-dose 1,25D from d2 to d75 without supplemental phosphate. All treatments maintained normocalcemia, increased serum phosphate, and normalized parathyroid hormone levels. They also prevented the loss of Npt2a, α-Klotho, and pERK1/2 immunoreactivity observed in the kidneys of untreated Hyp mice. Daily treatment with 1,25D decreased urine phosphate losses despite a marked increase in bone FGF23 mRNA and in circulating FGF23 levels. Daily 1,25D was more effective than other treatments in normalizing the growth plate and metaphyseal organization. In addition to being the only therapy that normalized lumbar vertebral height and body weight, daily 1,25D therapy normalized bone geometry and was more effective than FGF23Ab in improving trabecular bone structure. Daily 1,25D and FGF23Ab improved cortical microarchitecture and whole-bone biomechanical properties more so than biweekly 1,25D. Thus, monotherapy with 1,25D improves growth, skeletal microarchitecture, and bone strength in the absence of phosphate supplementation despite enhancing FGF23 expression, demonstrating that 1,25D has direct beneficial effects on the skeleton in XLH, independent of its role in phosphate homeostasis. © 2016 American Society for Bone and Mineral Research.

Deregulated Renal Calcium and Phosphate Transport during Experimental Kidney Failure.

Impaired mineral homeostasis and inflammation are hallmarks of chronic kidney disease (CKD), yet the underlying mechanisms of electrolyte regulation during CKD are still unclear. Here, we applied two different murine models, partial nephrectomy and adenine-enriched dietary intervention, to induce kidney failure and to investigate the subsequent impact on systemic and local renal factors involved in Ca2+ and Pi regulation. Our results demonstrated that both experimental models induce features of CKD, as reflected by uremia, and elevated renal neutrophil gelatinase-associated lipocalin (NGAL) expression. In our model kidney failure was associated with polyuria, hypercalcemia and elevated urinary Ca2+ excretion. In accordance, CKD augmented systemic PTH and affected the FGF23-αklotho-vitamin-D axis by elevating circulatory FGF23 levels and reducing renal αklotho expression. Interestingly, renal FGF23 expression was also induced by inflammatory stimuli directly. Renal expression of Cyp27b1, but not Cyp24a1, and blood levels of 1,25-dihydroxy vitamin D3 were significantly elevated in both models. Furthermore, kidney failure was characterized by enhanced renal expression of the transient receptor potential cation channel subfamily V member 5 (TRPV5), calbindin-D28k, and sodium-dependent Pi transporter type 2b (NaPi2b), whereas the renal expression of sodium-dependent Pi transporter type 2a (NaPi2a) and type 3 (PIT2) were reduced. Together, our data indicates two different models of experimental kidney failure comparably associate with disturbed FGF23-αklotho-vitamin-D signalling and a deregulated electrolyte homeostasis. Moreover, this study identifies local tubular, possibly inflammation- or PTH- and/or FGF23-associated, adaptive mechanisms, impacting on Ca2+/Pi homeostasis, hence enabling new opportunities to target electrolyte disturbances that emerge as a consequence of CKD development.

STIMULATION OF RAT TYPE IIA SODIUM DEPENDENT PHOSPHATE COTRANSPORTER (NAPI-2) BY SERUM AND GLUCOCORTICOID-INDUCED KINASE SGK1 AND NA+/H+ EXCHANGE REGULATING FACTOR NHERF2

Inorganic phosphate (Pi) is reabsorbed in the renal proximal convoluted tubules mainly via the electrogenic sodium dependent phosphate cotransporter NaPi Type IIa (NaPi-IIa). The isoforms of NaPi-IIa have been cloned from different species such as rat (NaPi-2). Serum and glucocorticoid-induced kinase 1 (SGK1) and rat sodium dependent phosphate cotransporter (NaPi-2) are highly expressed in the brush border membrane (BBM) of proximal tubule cells. The significance of the kinase in regulation of sodium dependent phosphate cotransporter (NaPi-2) has, however, remained elusive. On the other hand, the carboxyl-terminal tail of NaPi-2 contains information for apical expression, and interacts by means of its three terminal amino acids with several PSD95/DglA/ZO-1-like domains (PDZ)-containing proteins such as Na+/H+ exchanger 3 regulatory factors NHERF1 or NHERF2. Both, NHERF1 and NHERF2 modulate the targeting and trafficking of several proteins into the plasma membrane. Trafficking of the Na+/H+ exchanger NHE3 is controlled by NHE regulating factor NHERF2 and serum and glucocorticoid-inducible kinase SGK1. To test for a possible involvement in NaPi-2 regulation, cRNA encoding NaPi-2 was injected into Xenopuslaevis oocytes with or without additional injection of cRNA encoding SGK1 and/or NHERF2. Using two-electrode voltage-clamp, the transport activity was quantified as the substrate-induced current. Exposure to 1 mM phosphate induced an inward current (IP) in NaPi-2 expressing oocytes but not in water injected oocytes. Coexpression of SGK1 in NaPi-2 expressing oocytes significantly stimulated the phosphate-induced inward current. Moreover, coexpression of NHERF2 also significantly stimulated the phosphate-induced inward current in NaPi-2 expressing oocytes. The effect of SGK1 on NaPi-2 is mimicked by additional coexpression of NHERF2. The observations suggest that SGK1 and NHERF2 regulate NaPi-2 activity and are thus likely to participate in the stimulatory effect of some hormones, such as growth hormone and insulin, on renal phosphate transport. The present results thus disclose novel signaling mechanisms regulating NaPi-2 activity and renal phosphate transport, which may be important for regulation of phosphate homeostasis.

Role of Na+/H+ exchanger regulatory factor 1 in forward trafficking of the type IIa Na+-Pi cotransporter.

Na+/H+ exchanger regulatory factor (NHERF1) plays a critical role in the renal transport of phosphate by binding to Na+-Pi cotransporter (NpT2a) in the proximal tubule. While the association between NpT2a and NHERF1 in the apical membrane is known, the role of NHERF1 to regulate the trafficking of NpT2a has not been studied. To address this question, we performed cell fractionation by sucrose gradient centrifugation in opossum kidney (OK) cells placed in low-Pi medium to stimulate forward trafficking of NpT2a. Immunoblot analysis demonstrated expression of NpT2a and NHERF1 in the endoplasmic reticulum (ER)/Golgi. Coimmunoprecipitation demonstrated a NpT2a-NHERF1 interaction in the ER/Golgi. Low-Pi medium for 4 and 8 h triggered a decrease in NHERF1 in the plasma membrane with a corresponding increase in the ER/Golgi. Time-lapse total internal reflection fluorescence imaging of OK cells placed in low-Pi medium, paired with particle tracking and mean square displacement analysis, indicated active directed movement of NHERF1 at early and late time points, whereas NpT2a showed active movement only at later times. Silence of NHERF1 in OK cells expressing green fluorescent protein (GFP)-NpT2a resulted in an intracellular accumulation of GFP-NpT2a. Transfection with GFP-labeled COOH-terminal (TRL) PDZ-binding motif deleted or wild-type NpT2a in OK cells followed by cell fractionation and immunoprecipitation confirmed that the interaction between NpT2a and NHERF1 was dependent on the TRL motif of NpT2a. We conclude that appropriate trafficking of NpT2a to the plasma membrane is dependent on the initial association between NpT2a and NHERF1 through the COOH-terminal TRL motif of NpT2a in the ER/Golgi and requires redistribution of NHERF1 to the ER/Golgi.

The couple fibroblast growth factor 23 (FGF23)/Klotho

The fibrobalst growth factor 23 (FGF23) is a protein secreted in the plasma by bone cells. FGF23 controls phosphate and calcitriol plasma concentration. Its main physiological targets are the kidney and the parathyroid gland. Its production is stimulated by phosphate intake and plasma calcitriol levels. FGF23 binds a receptor at the cell surface that is composed by the association of a FGF receptor with the protein Klotho. Klotho expression is restricted to few organs including the kidney and the parathyroid gland. Klotho, which is expressed at the cell surface, can also be released in the plasma after an enzymatic cleavage. The role of the circulating form of Klotho is unknown. In addition to its role as a co-receptor Klotho may control the stability of the renal sodium phosphate transporter NPT2a and the calcium channel TRPV5 via enzymatic properties. FGF23 or Klotho gene disruption in mice leads to similar phenotypes except for the levels of FGF23, undetectable or increased respectively. FGF23 plasma concentration rises from the early stage of renal insufficiency to prevent plasma phosphate concentration from increasing. In parallel Klotho production seems to diminish. These modifications trigger the secondary hyperparathyroidism observed in renal insufficiency. The increase in FGF23 concentration and the decrease in Klotho levels are associated in animals and in human as well to an augmentation of mortality especially from cardiovascular causes and to heart modification: hypertrophy or dysfunction.

Klotho/fgf23‐Independent and ERα Mediated Downregulation of NaPi‐IIa and NaPi‐IIc by Estrogen in the Mouse Kidney Proximal Tubule

In these studies, we examined the effect of estrogen on renal inorganic phosphate (Pi) transport and the roles of its receptor isoforms (ERα and ERβ) and Klotho/FGF23 pathway in this effect in mice. Mice with null mutations (KO) of ERα or ERβ or Klotho and their wild‐type (WT) were placed in metabolic cages and injected daily with 17β‐estradiol or vehicle for 3 days. Food intake, urine volume and Pi excretion were measured daily. The results indicate that estrogen‐treated WT mice exhibited a significant increase in renal Pi wasting despite a reduction in food intake. Molecular studies showed a significant downregulation of NaPi‐IIa and NaPi‐IIc proteins with no change in their mRNA expression levels in the kidney. Estrogen‐induced reduction in food intake, phosphaturia and downregulation of NaPi‐IIa and NaPi‐IIc proteins were observed in ERβ KO but not in ERα KO mice. Further, Klotho KO mice exhibited a significant downregulation of NaPi‐IIa and hypophosphatemia in response to estrogen. In a cell line stably co‐expressing both ERα and ERβ, 24h of estrogen downregulated NaPi‐IIa protein, when cells are transiently transfected with a plasmid containing ORF‐3'UTR but not 5'UTR‐ORF of the mouse NaPi‐IIa transcript. In conclusion, estrogen causes phosphaturia in mice through the downregulation NaPi‐IIa and NaPi‐IIc protein abundance in the kidney. This effect is independent of Klotho/FGF23 pathway and is mediated through the activation of ERα. 3'UTR region of mouse NaPi‐IIa mRNA likely plays an important role in the inhibition of its translation by estrogen.

Regulation of hormone-sensitive renal phosphate transport.

Phosphate is essential for growth and maintenance of the skeleton and for generating high-energy phosphate compounds. Evolutionary adaptation to high dietary phosphorous in humans and other terrestrial vertebrates involves regulated mechanisms assuring the efficient renal elimination of excess phosphate. These mechanisms prominently include PTH, FGF23, and Vitamin D, which directly and indirectly regulate phosphate transport. Disordered phosphate homeostasis is associated with pathologies ranging from kidney stones to kidney failure. Chronic kidney disease results in hyperphosphatemia, an elevated calcium×phosphate product with considerable morbidity and mortality, mostly associated with adverse cardiovascular events. This chapter highlights recent findings and insights regarding the hormonal regulation of renal phosphate transport along with imbalances of phosphate balance due to acquired or inherited diseases states.

The Role of Sodium-Dependent Phosphate Transporter in Phosphate Homeostasis.

Inorganic phosphate (Pi) is an essential compound for several biologic functions. Pi levels outside the normal range, however, contribute to several pathological processes. Hypophosphatemia leads to bone abnormalities, such as rickets/osteomalacia. Hyperphosphatemia contributes to vascular calcification in patients with chronic kidney disease and hemodialysis patients and is independently associated with cardiac mortality.Pi homeostasis is regulated by the coordinated function of renal and intestinal sodium-dependent phosphate (NaPi) transporters with dietary Pi, parathyroid hormone, 1,25-dihydroxyvitamin D3, and fibroblast growth factor 23. The type II NaPi transporter/SLC34 family, with three members identified to date, is mainly responsible for Pi homeostasis in the body. SLC34A1 and SCL34A3 are predominantly expressed in the kidney, whereas SLC34A2 is expressed in the small intestine. The role of each SLC34 in the body was recently established by studies of gene-targeted mice. Mutation of SLC34A1 causes Fanconi syndrome and mutation of SLC34A3 causes autosomal recessive hereditary hypophosphatemic rickets with hypercalciuria. SLC34A2 is thought to be a major intestinal NaPi transporter and mutation of SLC34A2 causes pulmonary alveolar microlithiasis. A detailed understanding of Pi regulation in the body is important toward maintaining health.

New Developments in CKD-MBD. Molecular mechanisms of intestinal and renal phosphate transport

Intestinal inorganic phosphate (Pi) absorption and renal Pi reabsorption are regulated by parathyroid hormone, fibroblast growth factor 23, or 1,25-dihydroxyvitamin D, which modulate the expression of Pi transporters. Identification, functional characterization and analysis of knockout mice of Pi transporters contributed to clarify the role of each transporter in Pi homeostasis. Furthermore, the molecular mechanisms will lead to the development of new pharmaceutical research approaches.

Renal phosphate transporters

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

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

Two Argentinean siblings (a boy and a girl) from a nonconsanguineous family presented with hypercalcemia, hypercalciuria, hypophosphatemia, low parathyroid hormone (PTH), and nephrocalcinosis. The goal of this study was to identify genetic causes of the clinical findings in the two siblings. 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. 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. 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. 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.

Enigmatic Cassandra: renal FGF23 formation in polycystic kidney disease

Fibroblast growth factor 23 (FGF23) counteracts phosphate excess and tissue calcification. Phosphate intake, Ca(2+), parathyroid hormone, and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) stimulate FGF23 release from bone. FGF23 inhibits renal 1,25(OH)2D3 formation and phosphate reabsorption. Spichtig and colleagues demonstrate that FGF23 is generated in rodent polycystic kidneys, leading to an increase in plasma FGF23 concentration before reduction in kidney function. FGF23 fails to appreciably downregulate renal phosphate transporter and 1α-25OH-vitamin D hydroxylase activities. Unraveling underlying mechanisms may open diagnostic and therapeutic opportunities.