Synapse location during growth depends on glia location.

Synaptic contacts are largely established during embryogenesis and are then maintained during growth. To identify molecules involved in this process, we conducted a forward genetic screen in C. elegans and identified cima-1. In cima-1 mutants, synaptic contacts are correctly established during embryogenesis, but ectopic synapses emerge during postdevelopmental growth. cima-1 encodes a solute carrier in the SLC17 family of transporters that includes sialin, a protein that when mutated in humans results in neurological disorders. cima-1 does not function in neurons but rather functions in the nearby epidermal cells to correctly position glia during postlarval growth. Our findings indicate that CIMA-1 antagonizes the FGF receptor (FGFR), and does so most likely by inhibiting FGFR's role in epidermal-glia adhesion rather than signaling. Our data suggest that epidermal-glia crosstalk, in this case mediated by a transporter and the FGF receptor, is vital to preserve embryonically derived circuit architecture during postdevelopmental growth.

Glia keep synapse distribution under wraps.

The wiring of the nervous system requires that axons navigate to the correct targets and maintain their correct positions during developmental growth. In this issue, Shao et al. (2013) now reveal a crucial new role for glia in preserving correct synaptic connectivity during developmental growth.

SLC17A3 rs9379800 and Ischemic Stroke Susceptibility at the Northern Region of Malaysia.

OBJECTIVES: The relationships of Paired Like Homeodomain 2 (PITX2), Ninjurin 2 (NINJ2), TWIST-Related Protein 1 (TWIST1), Ras Interacting Protein 1 (Rasip1), Solute Carrier Family 17 Member 3 (SLC17A3), Methylmalonyl Co-A Mutase (MUT) and Fer3 Like BHLH Transcription Factor (FERD3L) polymorphisms and gene expression with ischemic stroke have yet to be determined in Malaysia. Hence, this study aimed to explore the associations of single nucleotide polymorphisms (SNPs) and gene expression with ischemic stroke risk among population who resided at the Northern region of Malaysia. MATERIALS AND METHODS: Study subjects including 216 ischemic stroke patients and 203 healthy controls were recruited upon obtaining ethical clearance. SNP genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism assays. Gene expression levels were quantified by real-time polymerase chain reaction assays. Statistical and genetic analyses were conducted with SPSS version 22.2, PLINK version 1.07 and multifactor dimensionality reduction software. RESULTS: Study subjects with G allele, CG or GG genotypes of SLC17A3 rs9379800 demonstrated increased risk of ischemic stroke with the odds ratios ranging from 1.76-fold to 3.14-fold (p<0.05). When stratified study subjects according to the ethnicity, SLC17A3 rs9379800 G allele and CG genotype contributed to 2.14- and 2.96-fold of ischemic stroke risk among Malay population significantly, in the multivariate analysis (p<0.05). However, no significant associations were observed for PITX2, NINJ2, TWIST1, Rasip1, and MUT polymorphisms with ischemic stroke risk in the multivariate analysis for the pooled cases and controls as well as when stratified them according to the ethnicity. Lower mRNA expression levels of Rasip1, SLC17A3, MUT and FERD3L were observed among cases (p<0.05). After FDR adjustment, the mRNA level of SLC17A3 remained significantly associated with ischemic stroke among Malay population (q=0.034). CONCLUSION: In conclusion, this study suggests that SLC17A3 rs9379800 polymorphism and its gene expression contribute to significant ischemic stroke risk among Malaysian population, particularly the Malay who resided at the Northern Region of the country. Our findings can provide useful information for the future diagnosis, management and treatment of ischemic stroke patients.

Olsalazine Sodium Increases Renal Urate Excretion by Modulating Urate Transporters in Hyperuricemic Animals.

Hyperuricemia is mainly the result of relative underexcretion of urate. Urate is mainly eliminated by kidney and several important transporters expressed on the membrane of renal tubular cells involved in urate excretion. Olsalazine sodium was screened from 3167 authorized small compounds/drugs, targeting xanthine oxidoreductase. In previous study, we reported that olsalazine sodium significantly reduced the serum urate levels, and the anti-hyperuricemic activity linked with inhibiting urate formation by reducing the activity of xanthine oxidoreductase. The current research aimed to assess olsalazine sodium renal urate excretion and likely molecular mechanism. The results showed that administration of olsalazine sodium 5.0 mg/kg decreased the levels of serum urate in hyperuricemic rats, and noticeably improved the fractional excretion of urate and urate clearance, exhibiting an uricosuric action. Moreover, olsalazine sodium (2.5, 5.0, 10.0 mg/kg) reduced the level of blood urea nitrogen in rats. Further study showed that olsalazine sodium reduced the mRNA expression of urate reabsorptive transporter glucose transporter 9 (GLUT9), increased the mRNA expression of urate secretory transporters, organic anion transporter 1 (OAT1), OAT3 and type 1 sodium-dependent phosphate transporter (NPT1) as well as the protein expression of OAT3 in the kidney in hyperuricemic mice. In conclusion, olsalazine sodium exhibited a promotion of urate excretion in kidney by increasing the expression of OAT3.

Identification of six novel susceptibility loci for dyslipidemia using longitudinal exome-wide association studies in a Japanese population.

Recent genome-wide association studies have identified various dyslipidemia-related genetic variants. However, most studies were conducted in a cross-sectional manner. We thus performed longitudinal exome-wide association studies of dyslipidemia in a Japanese population. We used ~244,000 genetic variants and clinical data of 6022 Japanese individuals who had undergone annual health checkups for several years. After quality control, the association of dyslipidemia-related phenotypes with 24,691 single nucleotide polymorphisms (SNPs) was tested using the generalized estimating equation model. In total, 82 SNPs were significantly (P < 2.03 × 10-6) associated with dyslipidemia phenotypes. Of these SNPs, four (rs74416240 of TCHP, rs925368 of GIT2, rs7969300 of ATXN2, and rs12231744 of NAA25) and two (rs34902660 of SLC17A3 and rs1042127 of CDSN) were identified as novel genetic determinants of hypo-HDL- and hyper-LDL-cholesterolemia, respectively. A replication study using the cross-sectional data of 8310 Japanese individuals showed the association of the six identified SNPs with dyslipidemia-related traits.

The Time-Feature of Uric Acid Excretion in Hyperuricemia Mice Induced by Potassium Oxonate and Adenine.

Hyperuricemia is an important risk factor of chronic kidney disease, metabolic syndrome and cardiovascular disease. We aimed to assess the time-feature relationship of hyperuricemia mouse model on uric acid excretion and renal function. A hyperuricemia mouse model was established by potassium oxonate (PO) and adenine for 21 days. Ultra Performance Liquid Chromatography was used to determine plasma uric acid level. Hematoxylin-eosin staining was applied to observe kidney pathological changes, and Western blot was used to detect renal urate transporters' expression. In hyperuricemia mice, plasma uric acid level increased significantly from the 3rd day, and tended to be stable from the 7th day, and the clearance rate of uric acid decreased greatly from the 3rd day. Further study found that the renal organ of hyperuricemia mice showed slight damage from the 3rd day, and significantly deteriorated renal function from the 10th day. In addition, the expression levels of GLUT9 and URAT1 were upregulated from the 3rd day, while ABCG2 and OAT1 were downregulated from the 3rd day, and NPT1 were downregulated from the 7th day in hyperuricemia mice kidney. This paper presents a method suitable for experimental hyperuricemia mouse model, and shows the time-feature of each index in a hyperuricemia mice model.

Intestinal secretion of indoxyl sulfate as a possible compensatory excretion pathway in chronic kidney disease.

Indoxyl sulfate (IS) is a protein-bound uremic toxin that progressively accumulates in plasma during chronic kidney disease (CKD), and its accumulation is associated with the progression of CKD. This study examined the intestinal secretion of IS using in situ single-pass intestinal perfusion in a rat model of renal insufficiency, MRP2- and BCRP-overexpressing Sf9 membrane vesicles, and Caco-2 cell monolayers. An in situ single-pass perfusion study in CKD model rats demonstrated that a small amount of IS is secreted into intestinal lumen after iv administration of IS, and the clearance increased AUC-dependently. An excess amount of IS (3 mm) partially inhibited the MRP2- and BCRP-mediated uptake of specific fluorescent substrates, CDCF and Lucifer yellow, respectively, into the membrane vesicles, although IS was not taken up at a physiological concentration, 10 µm. In the Caco-2 cell monolayers, the IS transport was higher in the absorptive direction than in the secretory direction (p < 0.05). p-Aminohippuric acid (PAH) strongly inhibited IS transport in both directions (absorptive, p = 0.142; secretory, p < 0.01). Given that the blood IS levels are much higher than those in the intestinal lumen, it is possible that this unknown PAH-sensitive system contributes to the intestinal IS secretion. Although in situ inhibition study is needed to confirm that this unknown transporter mediates the in vivo intestinal secretion of IS, we speculate that this unknown active efflux system works as a compensatory excretion pathway for excess organic anions such as IS especially in end-stage renal disease.

GWAS of clinically defined gout and subtypes identifies multiple susceptibility loci that include urate transporter genes.

OBJECTIVE: A genome-wide association study (GWAS) of gout and its subtypes was performed to identify novel gout loci, including those that are subtype-specific. METHODS: Putative causal association signals from a GWAS of 945 clinically defined gout cases and 1213 controls from Japanese males were replicated with 1396 cases and 1268 controls using a custom chip of 1961 single nucleotide polymorphisms (SNPs). We also first conducted GWASs of gout subtypes. Replication with Caucasian and New Zealand Polynesian samples was done to further validate the loci identified in this study. RESULTS: In addition to the five loci we reported previously, further susceptibility loci were identified at a genome-wide significance level (p<5.0×10-8): urate transporter genes (SLC22A12 and SLC17A1) and HIST1H2BF-HIST1H4E for all gout cases, and NIPAL1 and FAM35A for the renal underexcretion gout subtype. While NIPAL1 encodes a magnesium transporter, functional analysis did not detect urate transport via NIPAL1, suggesting an indirect association with urate handling. Localisation analysis in the human kidney revealed expression of NIPAL1 and FAM35A mainly in the distal tubules, which suggests the involvement of the distal nephron in urate handling in humans. Clinically ascertained male patients with gout and controls of Caucasian and Polynesian ancestries were also genotyped, and FAM35A was associated with gout in all cases. A meta-analysis of the three populations revealed FAM35A to be associated with gout at a genome-wide level of significance (p meta =3.58×10-8). CONCLUSIONS: Our findings including novel gout risk loci provide further understanding of the molecular pathogenesis of gout and lead to a novel concept for the therapeutic target of gout/hyperuricaemia.

Genome-wide association study of serum iron phenotypes in premenopausal women of European descent.

A genome-wide association study was performed on 1130 premenopausal women to detect common variants associated with three serum iron-related phenotypes. Total iron binding capacity was strongly associated (p=10(-14)) with variants in and near the TF gene (transferrin), the serum iron transporting protein, and with variants in HFE (p=4×10(-7)), which encodes the human hemochromatosis gene. Association was also detected between percent iron saturation (p=10(-8)) and variants in the chromosome 6 region containing both HFE and SLC17A2, which encodes a phosphate transport protein. No significant associations were detected with serum iron, but variants in HFE were suggestive (p=10(-6)). Our results corroborate prior studies in older subjects and demonstrate that the association of these genetic variants with iron phenotypes can be detected in premenopausal women.

Wide expression of type I Na+-phosphate cotransporter 3 (NPT3/SLC17A2), a membrane potential-driven organic anion transporter.

Membrane potential (Δψ)-driven and Cl(-)-dependent organic anion transport is a primary function of the solute carrier family 17 (SLC17) transporter family. Although the transport substrates and physiological relevance of the major members are well understood, SLC17A2 protein known to be Na(+)-phosphate cotransporter 3 (NPT3) is far less well characterized. In the present study, we investigated the transport properties and expression patterns of mouse SLC17A2 protein (mNPT3). Proteoliposomes containing the purified mNPT3 protein took up radiolabeled p-aminohippuric acid (PAH) in a Δψ- and Cl(-)-dependent manner. The mNPT3-mediated PAH uptake was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDs) and Evans blue, common inhibitors of SLC17 family members. The PAH uptake was also inhibited by various anionic compounds, such as hydrophilic nonsteroidal anti-inflammatory drugs (NSAIDs) and urate. Consistent with these observations, the proteoliposome took up radiolabeled urate in a Δψ- and Cl(-)-dependent manner. Immunohistochemistry with specific antibodies against mNPT3 combined with RT-PCR revealed that mNPT3 is present in various tissues, including the hepatic bile duct, luminal membranes of the renal urinary tubules, maternal side of syncytiotrophoblast in the placenta, apical membrane of follicle cells in the thyroid, bronchiole epithelial cells in the lungs, and astrocytes around blood vessels in the cerebrum. These results suggested that mNPT3 is a polyspecific organic anion transporter that is involved in circulation of urate throughout the body.

Polymorphisms in GCKR, SLC17A1 and SLC22A12 were associated with phenotype gout in Han Chinese males: a case-control study.

BACKGROUND: Gout is a common arthritic disease resulting from elevated serum uric acid (SUA) level. A large meta-analysis including 28,141 individuals identified nine single nucleotide polymorphisms (SNPs) associated with altered SUA level in a Caucasian population. However, raised SUA level alone is not sufficient for the development of gout arthritis and most of these SNPs have not been studied in a Han Chinese population. Here, we performed a case-control association analysis to investigate the relationship between these SUA correlated SNPs and gout arthritis in Han Chinese. METHODS: A total of 622 ascertained gout p9atients and 917 healthy controls were genotyped. Genome-wide significant SNPs, rs12129861, rs780094, rs734553, rs742132, rs1183201, rs12356193, rs17300741 and rs505802 in the previous SUA study, were selected for our analysis. RESULTS: No deviation from the Hardy-Weinberg equilibrium was observed either in the case or control cohorts (corrected p > 0.05). Three SNPs, rs780094 (located in GCKR, corrected p = 1.78E(-4), OR = 0.723), rs1183201 (located in SLC17A1, corrected p = 1.39E(-7), OR = 0.572) and rs505802 (located in SLC22A12, corrected p = 0.007, OR = 0.747), were significantly associated with gout on allelic level independent of potential cofounding traits. While the remaining SNPs were not replicated. We also found significant associations of uric acid concentrations with these three SNPs (rs780094 in GCKR, corrected p = 3.94E(-5); rs1183201 in SLC17A1, corrected p = 0.005; rs505802 in SLC22A12, corrected p = 0.003) and of triglycerides with rs780094 (located in GCKR, corrected p = 2.96E(-4)). Unfortunately, SNP-SNP interactions for these three significant SNPs were not detected (rs780094 vs rs1183201, p = 0.402; rs780094 vs rs505802, p = 0.434; rs1183201 vs rs505802, p = 0.143). CONCLUSIONS: Three SUA correlated SNPs in Caucasian population, rs780094 in GCKR, rs1183201 in SLC17A1 and rs505802 in SLC22A12 were confirmed to be associated with gout arthritis and uric acid concentrations in Han Chinese males. Considering genetic differences among populations and complicated pathogenesis of gout arthritis, more validating tests in independent populations and relevant functional experiments are suggested in future.

[Polymorphisms of SLC17A1 gene and their interaction with alcohol drinking among Uygur patients with hyperuricemia].

OBJECTIVE: To investigate the correlation between polymorphisms of uric acid transporter related gene SLC17A1 and hyperuricemia (HUA) among ethnic Uygur patients from Xinjiang. METHODS: A case-control study was carried out, which enrolled 1036 patients with hyperuricemia and 1031 healthy controls. Two single nucleotide polymorphisms (SNPs) of the SLC17A1 gene were determined with Sequenom MassARRAY. Crossover analysis was used to assess the effect of interaction between above SNPs and alcohol drinking on uric acid level. RESULTS: Genotypic and allelic frequencies of the SLC17A1 gene at the two loci in the two groups were compared. The CT genotype of the rs9467596 locus and TC genotype of the rs2096386 locus showed a higher risk for hyperuricemia (OR=1.334, 95%CI:1.082-1.644; OR=1.242, 95%CI:1.015-1.519, respectively). Crossover analysis also revealed that the SLC17A1 rs2096386 polymorphism has a positive interaction with alcohol drinking in a multiplication model (ORint=1.21, P<0.05, OR>1). CONCLUSION: SNP rs9467596 and rs2096386 of the SLC17A1 gene may have a correlation between hyperuricemia and alcohol drinking among Uygur patients.

Exploiting Leishmania tarentolae cell-free extracts for the synthesis of human solute carriers.

Cell-free protein production offers a versatile alternative to complement in vivo expression systems. However, usage of prokaryotic cell-free systems often leads to non-functional proteins. We modified a previously designed cell-free system based on the protozoan Leishmania tarentolae, a parasite of the Moorish gecko Tarentola mauritanica, together with a species-independent translational sequences-based plasmid to produce human membrane proteins in 2 hours reaction time. We successfully established all four commonly used expression modes for cell-free synthesis of membrane proteins with a human organic anion transporter, SLC17A3, as a model membrane protein: (i) As precipitates without the addition of any hydrophobic environment, (ii) in the presence of detergents, (iii) with the addition of liposomes, and (iv) supplemented with nanodiscs. We utilized this adapted system to synthesize 22 human solute carriers from 20 different families. Our results demonstrate the capability of the Leishmania tarentolae cell-free system for the production of a huge variety of human solute carriers in the precipitate mode. Furthermore, purified SLC17A3 shows the formation of an oligomeric state.

Examining the Association of Rare Allelic Variants in Urate Transporters SLC22A11, SLC22A13, and SLC17A1 with Hyperuricemia and Gout.

Genetic variations in urate transporters play a significant role in determining human urate levels and have been implicated in developing hyperuricemia or gout. Polymorphism in the key urate transporters, such as ABCG2, URAT1, or GLUT9 was well-documented in the literature. Therefore in this study, our objective was to determine the frequency and effect of rare nonsynonymous allelic variants of SLC22A11, SLC22A13, and SLC17A1 on urate transport. In a cohort of 150 Czech patients with primary hyperuricemia and gout, we examined all coding regions and exon-intron boundaries of SLC22A11, SLC22A13, and SLC17A1 using PCR amplification and Sanger sequencing. For comparison, we used a control group consisting of 115 normouricemic subjects. To examine the effects of the rare allelic nonsynonymous variants on the expression, intracellular processing, and urate transporter protein function, we performed a functional characterization using the HEK293A cell line, immunoblotting, fluorescent microscopy, and site directed mutagenesis for preparing variants in vitro. Variants p.V202M (rs201209258), p.R343L (rs75933978), and p.P519L (rs144573306) were identified in the SLC22A11 gene (OAT4 transporter); variants p.R16H (rs72542450), and p.R102H (rs113229654) in the SLC22A13 gene (OAT10 transporter); and the p.W75C variant in the SLC17A1 gene (NPT1 transporter). All variants minimally affected protein levels and cytoplasmic/plasma membrane localization. The functional in vitro assay revealed that contrary to the native proteins, variants p.P519L in OAT4 (p ≤ 0.05), p.R16H in OAT10 (p ≤ 0.05), and p.W75C in the NPT1 transporter (p ≤ 0.01) significantly limited urate transport activity. Our findings contribute to a better understanding of (1) the risk of urate transporter-related hyperuricemia/gout and (2) uric acid handling in the kidneys.

Assessment of a bacterial 6-phytase in the diets of broiler chickens.

Two 21-d broiler experiments were conducted to assess the efficacy of a bacterial 6-phytase expressed in Aspergillus oryzae on growth performance, nutrient utilization, and intestinal molecular markers. Two hundred forty birds in 5 treatments (experiment 1) or 256 birds in 4 treatments (experiment 2) were used. The treatments included a negative control diet that was marginally deficient in P (NC) or NC plus tricalcium phosphate, 500, 1,000, or 2,000 phytase units/kg (experiment 1), and NC or NC plus monocalcium phosphate, 500 or 1,000 phytase units/kg (experiment 2). In both experiments, excreta were collected on d 19 to 21, whereas birds and feed were weighed and ileal digesta collected on d 21. For experiment 1, mucosa scraping was collected from the duodenum, jejunum, and ileum from all birds for quantification of expression level of gut level inflammatory cytokines, Toll-like receptors, and phosphate transporter (NaPi-IIb). In both experiments, tricalcium phosphate, monocalcium phosphate, and phytase supplementation improved (P < 0.05) weight gain and percentage tibia ash. Phosphorus and Ca retention and phytic acid disappearance improved (P < 0.05) with phytase supplementation (experiment 1) and there was an increase (P < 0.01) in Ca and P retention in response to phytase supplementation (experiment 2). Diets did not affect the expression of gut level cytokines, Toll-like receptors, or the mucin gene. Phytase supplementation tended to decrease (P < 0.10) the expression of NaPi-IIb. It was concluded from these studies that the bacterial 6-phytase was effective in enhancing growth of broilers receiving low-P diets as well as in increasing efficiency of P utilization and phytic acid degradation.

Linkage analysis using whole exome sequencing data implicates SLC17A1, SLC17A3, TATDN2 and TMEM131L in type 1 diabetes in Kuwaiti families.

Type 1 diabetes (T1D) is characterized by the progressive destruction of pancreatic ß-cells, leading to insulin deficiency and lifelong dependency on exogenous insulin. Higher estimates of heritability rates in monozygotic twins, followed by dizygotic twins and sib-pairs, indicate the role of genetics in the pathogenesis of T1D. The incidence and prevalence of T1D are alarmingly high in Kuwait. Consanguineous marriages account for 50-70% of all marriages in Kuwait, leading to an excessive burden of recessive allele enrichment and clustering of familial disorders. Thus, genetic studies from this Arab region are expected to lead to the identification of novel gene loci for T1D. In this study, we performed linkage analyses to identify the recurrent genetic variants segregating in high-risk Kuwaiti families with T1D. We studied 18 unrelated Kuwaiti native T1D families using whole exome sequencing data from 86 individuals, of whom 37 were diagnosed with T1D. The study identified three potential loci with a LOD score of ≥ 3, spanning across four candidate genes, namely SLC17A1 (rs1165196:pT269I), SLC17A3 (rs942379: p.S370S), TATDN2 (rs394558:p.V256I), and TMEM131L (rs6848033:p.R190R). Upon examination of missense variants from these genes in the familial T1D dataset, we observed a significantly increased enrichment of the genotype homozygous for the minor allele at SLC17A3 rs56027330_p.G279R accounting for 16.2% in affected children from 6 unrelated Kuwaiti T1D families compared to 1000 genomes Phase 3 data (0.9%). Data from the NephQTL database revealed that the rs1165196, rs942379, rs394558, and rs56027330 SNPs exhibited genotype-based differential expression in either glomerular or tubular tissues. Data from the GTEx database revealed rs942379 and rs394558 as QTL variants altering the expression of TRIM38 and IRAK2 respectively. Global genome-wide association studies indicated that SLC17A1 rs1165196 and other variants from SLC17A3 are associated with uric acid concentrations and gout. Further evidence from the T1D Knowledge portal supported the role of shortlisted variants in T1D pathogenesis and urate metabolism. Our study suggests the involvement of SLC17A1, SLC17A3, TATDN2, and TMEM131L genes in familial T1D in Kuwait. An enrichment selection of genotype homozygous for the minor allele is observed at SLC17A3 rs56027330_p.G279R variant in affected members of Kuwaiti T1D families. Future studies may focus on replicating the findings in a larger T1D cohort and delineate the mechanistic details of the impact of these novel candidate genes on the pathophysiology of T1D.

A Na+-phosphate cotransporter homologue (SLC17A4 protein) is an intestinal organic anion exporter.

The SLC17 anion transporter family comprises nine members that transport various organic anions in membrane potential (Δψ)- and Cl(-)-dependent manners. Although the transport substrates and physiological relevance of the majority of the members have already been determined, little is known about SLC17A4 proteins known to be Na(+)-phosphate cotransporter homologue (NPT homologue). In the present study, we investigated the expression and transport properties of human SLC17A4 protein. Using specific antibodies, we found that a human NPT homologue is specifically expressed and present in the intestinal brush border membrane. Proteoliposomes containing the purified protein took up radiolabeled p-aminohippuric acid (PAH) in a Cl(-)-dependent manner at the expense of an electrochemical gradient of protons, especially Δψ, across the membrane. The Δψ- and Cl(-)-dependent PAH uptake was inhibited by diisothiocyanostilbene-2,2'-disulfonic acid and Evans blue, common inhibitors of SLC17 family members. cis-Inhibition studies revealed that various anionic compounds, such as hydrophilic nonsteroidal anti-inflammatory drugs, pravastatin, and urate inhibited the PAH uptake. Proteoliposomes took up radiolabeled urate, with the uptake having properties similar to those of PAH uptake. These results strongly suggested that the human NPT homologue acts as a polyspecific organic anion exporter in the intestines. Since SLC17A1 protein (NPT1) and SLC17A3 protein (NPT4) are responsible for renal urate extrusion, our results reveal the possible involvement of a NPT homologue in urate extrusion from the intestinal duct.

Recent advances in renal urate transport: characterization of candidate transporters indicated by genome-wide association studies.

Humans have higher serum uric acid levels than other mammalian species owing to the genetic silencing of the hepatic enzyme uricase that metabolizes uric acid into allantoin. Urate (the ionized form of uric acid) is generated from purine metabolism and it may provide antioxidant defense in the human body. Despite its potential advantage, sustained hyperuricemia has pathogenetic causes in gout and renal diseases, and putative roles in hypertension and cardiovascular diseases. Since the kidney plays a dominant role in maintaining plasma urate levels through the excretion process, it is important to understand the molecular mechanism of renal urate handling. Although the molecular identification of a kidney-specific urate/anion exchanger URAT1 in 2002 paved the way for successive identification of several urate transport-related proteins, the entire picture of effective renal urate handling in humans has not yet been clarified. Recently, several genome-wide association studies identified a substantial association between uric acid concentration and single nucleotide polymorphisms in at least ten genetic loci including eight transporter-coding genes. In 2008, we functionally characterized the facilitatory glucose transporter family member SLC2A9 (GLUT9), one of the candidate genes for urate handling, as a voltage-driven urate transporter URATv1 at the basolateral side of renal proximal tubules that comprises the main route of the urate reabsorption pathway, in tandem with URAT1 at the apical side. In this review, recent findings concerning these candidate molecules are presented.

Functional Characterization of the Novel and Specific Thyroid Hormone Transporter SLC17A4.

Background: A recent genome-wide association study identified the SLC17A4 locus associated with circulating free thyroxine (T4) concentrations. Human SLC17A4, being widely expressed in the gastrointestinal tract, was characterized as a novel triiodothyronine (T3) and T4 transporter. However, apart from the cellular uptake of T3 and T4, transporter characteristics are currently unknown. In this study, we delineated basic transporter characteristics of this novel thyroid hormone (TH) transporter. Methods: We performed a broad range of well-established TH transport studies in COS-1 cells transiently overexpressing SLC17A4. We studied cellular TH uptake in various incubation buffers, TH efflux, and the inhibitory effects of different TH metabolites and known inhibitors of other TH transporters on SLC17A4-mediated TH transport. Finally, we determined the effect of tunicamycin, a pharmacological inhibitor of N-linked glycosylation, and targeted mutations in Asn residues on SLC17A4 function. Results: SLC17A4 induced the cellular uptake of T3 and T4 by ∼4 times, and of reverse (r)T3 by 1.5 times over control cells. The uptake of T4 by SLC17A4 was Na+ and Cl- independent, stimulated by low extracellular pH, and reduced by various iodothyronines and metabolites thereof, particularly those that contain at least three iodine moieties irrespective of the presence of modification at the alanine side chain. None of the classical TH transporter inhibitors studied attenuated SLC17A4-mediated TH transport. SLC17A4 also facilitates the efflux of T3 and T4, and to a lesser extent of 3,3'-diiodothyronine (T2). Immunoblot studies on lysates of transfected cells cultured in absence or presence of tunicamycin indicated that SLC17A4 is subject to N-linked glycosylation. Complementary mutational studies identified Asn66, Asn75, and Asn90, which are located in extracellular loop 1, as primary targets. Conclusions: Our studies show that SLC17A4 facilitates the transport of T3 and T4, and less efficiently rT3 and 3,3'-T2. Further studies should reveal the physiological role of SLC17A4 in TH regulation.