Galectin 9 is the sugar-regulated urate transporter/channel UAT

UAT, also designated galectin 9, is a multifunctional protein that can function as a urate channel/transporter, a regulator of thymocyte-epithelial cell interactions, a tumor antigen, an eosinophil chemotactic factor, and a mediator of apoptosis. We review the evidence that UAT is a transmembrane protein that transports urate, describe our molecular model for this protein, and discuss the evidence from epitope tag and lipid bilayer studies that support this model of the transporter. The properties of recombinant UAT are compared with those of urate transport into membrane vesicles derived from proximal tubule cells in rat kidney cortex. In addition, we review channel functions predicted by our molecular model that resulted in the novel finding that the urate channel activity is regulated by sugars and adenosine. Finally, the presence and possible functions of at least 4 isoforms of UAT and a closely related gene hUAT2 are discussed.     

Functional Analysis and Molecular Modeling of a Cloned Urate Transporter/Channel

Recombinant protein, designated UAT, prepared from a cloned rat renal cDNA library functions as a selective voltage-sensitive urate transporter/channel when fused with lipid bilayers. Since we previously suggested that UAT may represent the mammalian electrogenic urate transporter, UAT has been functionally characterized in the presence and absence of potential channel blockers, several of which are known to block mammalian electrogenic urate transport. Two substrates, oxonate (a competitive uricase inhibitor) and pyrazinoate, that inhibit renal electrogenic urate transport also block UAT activity. Of note, oxonate selectively blocks from the cytoplasmic side of the channel while pyrazinoate only blocks from the channel's extracellular face. Like oxonate, anti-uricase (an electrogenic transport inhibitor) also selectively blocks channel activity from the cytoplasmic side. Adenosine blocks from the extracellular side exclusively while xanthine blocks from both sides. These effects are consistent with newly identified regions of homology to uricase and the adenosine A1/A3 receptor in UAT and localize these homologous regions to the cytoplasmic and extracellular faces of UAT, respectively. Additionally, computer analyses identified four putative α-helical transmembrane domains, two β sheets, and blocks of homology to the E and B loops of aquaporin-1 within UAT. The experimental observations substantiate our proposal that UAT is the molecular representation of the renal electrogenic urate transporter and, in conjunction with computer algorithms, suggest a possible molecular structure for this unique channel. Received: 13 October 1998/Revised: 28 January 1999     

Localization and topology of a urate transporter/channel, a galectin, in epithelium-derived cells

Recombinant proteinproduced from a cDNA cloned in our laboratory (UAT) functions in lipidbilayers as a urate transporter/channel. Because UAT is a galectin, afamily of proteins presumed to be soluble, the localization andtopology of UAT were assessed in living cells. UAT was targeted toplasma membrane in multiple epithelium-derived cell lines and, inpolarized cells, was targeted to both apical and basolateral membranes.The amino and carboxy termini of UAT were both detected on thecytoplasmic side of plasma membranes, whereas cell surfacebiotinylation studies demonstrated that UAT is not merely a cytosolicmembrane-associated protein but contains at least one extracellulardomain. Madin-Darby canine kidney cells were shown both functionallyand immunologically to contain an apparent homolog of UAT; however,transfection with UAT did not modify urate uptake. Becausecoimmunoprecipitation studies revealed that UAT is capable of formingboth homo- and heteromultimers, it is proposed that monomers ofendogenous channels are in part replaced by monomers of the proteinexpressed subsequent to transfection, thereby maintaining constancy ofurate uptake at basal levels.

     

Human Sodium Phosphate Transporter 4 (hNPT4/SLC17A3) as a Common Renal Secretory Pathway for Drugs and Urate

The evolutionary loss of hepatic urate oxidase (uricase) has resulted in humans with elevated serum uric acid (urate). Uricase loss may have been beneficial to early primate survival. However, an elevated serum urate has predisposed man to hyperuricemia, a metabolic disturbance leading to gout, hypertension, and various cardiovascular diseases. Human serum urate levels are largely determined by urate reabsorption and secretion in the kidney. Renal urate reabsorption is controlled via two proximal tubular urate transporters: apical URAT1 (SLC22A12) and basolateral URATv1/GLUT9 (SLC2A9). In contrast, the molecular mechanism(s) for renal urate secretion remain unknown. In this report, we demonstrate that an orphan transporter hNPT4 (human sodium phosphate transporter 4; SLC17A3) was a multispecific organic anion efflux transporter expressed in the kidneys and liver. hNPT4 was localized at the apical side of renal tubules and functioned as a voltage-driven urate transporter. Furthermore, loop diuretics, such as furosemide and bumetanide, substantially interacted with hNPT4. Thus, this protein is likely to act as a common secretion route for both drugs and may play an important role in diuretics-induced hyperuricemia. The in vivo role of hNPT4 was suggested by two hyperuricemia patients with missense mutations in SLC17A3. These mutated versions of hNPT4 exhibited reduced urate efflux when they were expressed in Xenopus oocytes. Our findings will complete a model of urate secretion in the renal tubular cell, where intracellular urate taken up via OAT1 and/or OAT3 from the blood exits from the cell into the lumen via hNPT4.     

Expression,Purification, and Structural Insights for the Human Uric Acid Transporter,GLUT9, Using the Xenopus laevis Oocytes System

The urate transporter, GLUT9, is responsible for the basolateral transport of urate in the proximal tubule of human kidneys and in the placenta, playing a central role in uric acid homeostasis. GLUT9 shares the least homology with other members of the glucose transporter family, especially with the glucose transporting members GLUT1-4 and is the only member of the GLUT family to transport urate. The recently published high-resolution structure of XylE, a bacterial D-xylose transporting homologue, yields new insights into the structural foundation of this GLUT family of proteins. While this represents a huge milestone, it is unclear if human GLUT9 can benefit from this advancement through subsequent structural based targeting and mutagenesis. Little progress has been made toward understanding the mechanism of GLUT9 since its discovery in 2000. Before work can begin on resolving the mechanisms of urate transport we must determine methods to express, purify and analyze hGLUT9 using a model system adept in expressing human membrane proteins. Here, we describe the surface expression, purification and isolation of monomeric protein, and functional analysis of recombinant hGLUT9 using the Xenopus laevis oocyte system. In addition, we generated a new homology-based high-resolution model of hGLUT9 from the XylE crystal structure and utilized our purified protein to generate a low-resolution single particle reconstruction. Interestingly, we demonstrate that the functional protein extracted from the Xenopus system fits well with the homology-based model allowing us to generate the predicted urate-binding pocket and pave a path for subsequent mutagenesis and structure-function studies.     

Plasma urate level is directly regulated by a voltage-driven urate efflux transporter URATv1 (SLC2A9) in humans

Hyperuricemia is a significant factor in a variety of diseases, including gout and cardiovascular diseases. Although renal excretion largely determines plasma urate concentration, the molecular mechanism of renal urate handling remains elusive. Previously, we identified a major urate reabsorptive transporter, URAT1 (SLC22A12), on the apical side of the renal proximal tubular cells. However, it is not known how urate taken up by URAT1 exits from the tubular cell to the systemic circulation. Here, we report that a sugar transport facilitator family member protein GLUT9 (SLC2A9) functions as an efflux transporter of urate from the tubular cell. GLUT9-expressed Xenopus oocytes mediated saturable urate transport (K(m): 365+/-42 microm). The transport was Na(+)-independent and enhanced at high concentrations of extracellular potassium favoring negative to positive potential direction. Substrate specificity and pyrazinoate sensitivity of GLUT9 was distinct from those of URAT1. The in vivo role of GLUT9 is supported by the fact that a renal hypouricemia patient without any mutations in SLC22A12 was found to have a missense mutation in SLC2A9, which reduced urate transport activity in vitro. Based on these data, we propose a novel model of transcellular urate transport in the kidney; urate [corrected] is taken up via apically located URAT1 and exits the cell via basolaterally located GLUT9, which we suggest be renamed URATv1 (voltage-driven urate transporter 1).     

SLC2A9 is a high-capacity urate transporter in humans

Background

Serum uric acid levels in humans are influenced by diet, cellular breakdown, and renal elimination, and correlate with blood pressure, metabolic syndrome, diabetes, gout, and cardiovascular disease. Recent genome-wide association scans have found common genetic variants of SLC2A9 to be associated with increased serum urate level and gout. The SLC2A9 gene encodes a facilitative glucose transporter, and it has two splice variants that are highly expressed in the proximal nephron, a key site for urate handling in the kidney. We investigated whether SLC2A9 is a functional urate transporter that contributes to the longstanding association between urate and blood pressure in man.

Methods and Findings

We expressed both SLC2A9 splice variants in Xenopus laevis oocytes and found both isoforms mediate rapid urate fluxes at concentration ranges similar to physiological serum levels (200–500 μM). Because SLC2A9 is a known facilitative glucose transporter, we also tested whether glucose or fructose influenced urate transport. We found that urate is transported by SLC2A9 at rates 45- to 60-fold faster than glucose, and demonstrated that SLC2A9-mediated urate transport is facilitated by glucose and, to a lesser extent, fructose. In addition, transport is inhibited by the uricosuric benzbromarone in a dose-dependent manner (K _i = 27 μM). Furthermore, we found urate uptake was at least 2-fold greater in human embryonic kidney (HEK) cells overexpressing SLC2A9 splice variants than nontransfected kidney cells. To confirm that our findings were due to SLC2A9, and not another urate transporter, we showed that urate transport was diminished by SLC2A9-targeted siRNA in a second mammalian cell line. In a cohort of men we showed that genetic variants of SLC2A9 are associated with reduced urinary urate clearance, which fits with common variation at SLC2A9 leading to increased serum urate. We found no evidence of association with hypertension (odds ratio 0.98, 95% confidence interval [CI] 0.9 to 1.05, p > 0.33) by meta-analysis of an SLC2A9 variant in six case–control studies including 11,897 participants. In a separate meta-analysis of four population studies including 11,629 participants we found no association of SLC2A9 with systolic (effect size −0.12 mm Hg, 95% CI −0.68 to 0.43, p = 0.664) or diastolic blood pressure (effect size −0.03 mm Hg, 95% CI −0.39 to 0.31, p = 0.82).

Conclusions

This study provides evidence that SLC2A9 splice variants act as high-capacity urate transporters and is one of the first functional characterisations of findings from genome-wide association scans. We did not find an association of the SLC2A9 gene with blood pressure in this study. Our findings suggest potential pathogenic mechanisms that could offer a new drug target for gout.     

Apical voltage-driven urate efflux transporter NPT4 in renal proximal tubule

Uric acid (urate) is the end product of purine metabolism in humans. Human kidneys reabsorb a large proportion of filtered urate. This extensive renal reabsorption, together with the fact that humans do not possess uricase that catalyzes the biotransformation of urate into allantoin, results in a higher plasma urate concentration in humans compared to other mammals. A major determinant of plasma urate concentration is renal excretion as a function of the balance between reabsorption and secretion. We previously identified that renal urate absorption in proximal tubular epithelial cells occurs mainly via apical urate/anion exchanger, URAT1/SLC22A12, and by facilitated diffusion along the trans-membrane potential gradient by the basolateral voltage-driven urate efflux transporter, URATv1/SLC2A9/GLUT9. In contrast, the molecular mechanism by which renal urate secretion occurs remains elusive. Recently, we reported a newly characterized human voltage-driven drug efflux transporter, hNPT4/SLC17A3, which functions as a urate exit pathway located at the apical side of renal proximal tubules. This transporter protein has been hypothesized to play an important role with regard to net urate efflux. An in vivo role of hNPT4 is supported by the fact that missense mutations in SLC17A3 present in hyperuricemia patients with urate underexcretion abolished urate efflux capacity in vitro. Herein, we report data demonstrating that loop diuretics and thiazide diuretics substantially interact with hNPT4. These data provide molecular evidence for loop and thiazide-diuretics-induced hyperuricemia. Thus, we propose that hNPT4 is an important transepithelial proximal tubular transporter that transports diuretic drugs and operates functionally with basolateral organic anion transporters 1/3 (OAT1/OAT3).     

Apical Voltage-Driven Urate Efflux Transporter NPT4 in Renal Proximal Tubule

Uric acid (urate) is the end product of purine metabolism in humans. Human kidneys reabsorb a large proportion of filtered urate. This extensive renal reabsorption, together with the fact that humans do not possess uricase that catalyzes the biotransformation of urate into allantoin, results in a higher plasma urate concentration in humans compared to other mammals. A major determinant of plasma urate concentration is renal excretion as a function of the balance between reabsorption and secretion. We previously identified that renal urate absorption in proximal tubular epithelial cells occurs mainly via apical urate/anion exchanger, URAT1/SLC22A12, and by facilitated diffusion along the trans-membrane potential gradient by the basolateral voltage-driven urate efflux transporter, URATv1/SLC2A9/GLUT9. In contrast, the molecular mechanism by which renal urate secretion occurs remains elusive. Recently, we reported a newly characterized human voltage-driven drug efflux transporter, hNPT4/SLC17A3, which functions as a urate exit pathway located at the apical side of renal proximal tubules. This transporter protein has been hypothesized to play an important role with regard to net urate efflux. An in vivo role of hNPT4 is supported by the fact that missense mutations in SLC17A3 present in hyperuricemia patients with urate underexcretion abolished urate efflux capacity in vitro. Herein, we report data demonstrating that loop diuretics and thiazide diuretics substantially interact with hNPT4. These data provide molecular evidence for loop and thiazide-diuretics-induced hyperuricemia. Thus, we propose that hNPT4 is an important transepithelial proximal tubular transporter that transports diuretic drugs and operates functionally with basolateral organic anion transporters 1/3 (OAT1/OAT3).     

Reconstitution of hepatic uricase in planar lipid bilayer reveals a functional organic anion channel

Rat renal proximal tubule cell membranes have been reported to contain uricase-like proteins that function as electrogenic urate transporters. Although uricase, per se, has only been detected within peroxisomes in rat liver (where it functions as an oxidative enzyme) this protein has been shown to function as a urate transport protein when inserted into liposomes. Since both the uricase-like renal protein and hepatic uricase can transport urate, reconstitution studies were performed to further characterize the mechanism by which uricase may function as a transport protein. Ion channel activity was evaluated in planar lipid bilayers before and after fusion of uricase-containing proteoliposomes. In the presence of symmetrical solutions of urate and KCl, but absence of uricase, no current was generated when the voltage was ramped between ±100 mV. Following fusion of uricase with the bilayer, single channel activity was evident: the reconstituted channel rectified with a mean slope conductance of 8 pS, displayed voltage sensitivity, and demonstrated a marked selectivity for urate relative to K⁺ and Cl^–. The channel was more selective to oxonate, an inhibitor of both enzymatic uricase activity and urate transport, than urate and it was equally selective to urate and pyrazinoate, an inhibitor of urate transport. With time, pyrazinoate blocked both its own movement and the movement of urate through the channel. Channel activity was also blocked by the IgG fraction of a polyclonal antibody to affinity purified pig liver uricase. These studies demonstrate that a highly selective, voltage dependent organic anion channel is formed when a purified preparation of uricase is reconstituted in lipid bilayers.This work was supported in part by the G. Harold and Leila Y. Mathers Charitable Foundation (E.L.P. and R.D.L.), the Irma T. Hirschl Trust (R.D.L.), National Institutes of Health grant DK08419 (B.A.K.) and a Grant-in-Aid from the American Heart Association, N.Y.C. Affiliate (R.G.A.).     

Critical Roles of Two Hydrophobic Residues within Human Glucose Transporter 9 (hSLC2A9) in Substrate Selectivity and Urate Transport

High blood urate levels (hyperuricemia) have been found to be a significant risk factor for cardiovascular diseases and inflammatory arthritis, such as hypertension and gout. Human glucose transporter 9 (hSLC2A9) is an essential protein that mainly regulates urate/hexose homeostasis in human kidney and liver. hSLC2A9 is a high affinity-low capacity hexose transporter and a high capacity urate transporter. Our previous studies identified a single hydrophobic residue in trans-membrane domain 7 of class II glucose transporters as a determinant of fructose transport. A mutation of isoleucine 335 to valine (I355V) in hSLC2A9 can reduce fructose transport while not affecting glucose fluxes. This current study demonstrates that the I335V mutant transports urate similarly to the wild type hSLC2A9; however, Ile-335 is necessary for urate/fructose trans-acceleration exchange to occur. Furthermore, Trp-110 is a critical site for urate transport. Two structural models of the class II glucose transporters, hSLC2A9 and hSLC2A5, based on the crystal structure of hSLC2A1 (GLUT1), reveal that Ile-335 (or the homologous Ile-296 in hSLC2A5) is a key component for protein conformational changes when the protein translocates substrates. The hSLC2A9 model also predicted that Trp-110 is a crucial site that could directly interact with urate during transport. Together, these studies confirm that hSLC2A9 transports both urate and fructose, but it interacts with them in different ways. Therefore, this study advances our understanding of how hSLC2A9 mediates urate and fructose transport, providing further information for developing pharmacological agents to treat hyperuricemia and related diseases, such as gout, hypertension, and diabetes.     

Type 1 Sodium-dependent Phosphate Transporter (SLC17A1 Protein) Is a Cl?-dependent Urate Exporter

SLC17A1 protein (NPT1) is the first identified member of the SLC17 phosphate transporter family and mediates the transmembrane cotransport of Na⁺/P_i in oocytes. Although this protein is believed to be a renal polyspecific anion exporter, its transport properties are not well characterized. Here, we show that proteoliposomes containing purified SLC17A1 transport various organic anions such as p-aminohippuric acid and acetylsalicylic acid (aspirin) in an inside positive membrane potential (Δψ)-dependent manner. We found that NPT1 also transported urate. The uptake characteristics were similar to that of SLC17 members in its Cl⁻ dependence and inhibitor sensitivity. When arginine 138, an essential amino acid residue for members of the SLC17 family such as the vesicular glutamate transporter, was specifically mutated to alanine, the resulting mutant protein was inactive in Δψ-dependent anion transport. Heterologously expressed and purified human NPT1 carrying the single nucleotide polymorphism mutation that is associated with increased risk of gout in humans exhibited 32% lower urate transport activity compared with the wild type protein. These results strongly suggested that NPT1 is a Cl⁻-dependent polyspecific anion exporter involved in urate excretion under physiological conditions.     

The Role of Auxiliary Subunits for the Functional Diversity of Voltage‐Gated Calcium Channels

Voltage‐gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore‐forming α₁ subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice‐variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre‐existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein–protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein–protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity. J. Cell. Physiol. 999: 00–00, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc. J. Cell. Physiol. 230: 2019–2031, 2015. © 2015 Wiley Periodicals, Inc.     

The effect of female hormones upon urate transport systems in the mouse kidney

Increased levels of serum urate in postmenopausal women are thought to be caused by a change in renal urate elimination associated with the loss of female hormones. In this study, we investigated the regulation of renal urate transporter expression by female hormones using ovariectomized mice with or without hormone replacement. Estradiol suppressed the protein levels of urate reabsorptive transporters urate transporter 1 and glucose transporter 9 (Urat1 and Glut9), and that of urate efflux transporter ATP-binding cassette sub-family G member 2 (Abcg2). Progesterone suppressed protein levels of sodium-coupled monocarboxylate transporter 1 (Smct1). However, neither estradiol nor progesterone influenced the respective levels of mRNA.     

Identification of a new urate and high affinity nicotinate transporter, hOAT10 (SLC22A13)

The orphan transporter hORCTL3 (human organic cation transporter like 3; SLC22A13) is highly expressed in kidneys and to a weaker extent in brain, heart, and intestine. hORCTL3-expressing Xenopus laevis oocytes showed uptake of [(3)H]nicotinate, [(3)H]p-aminohippurate, and [(14)C]urate. Hence, hORCTL3 is an organic anion transporter, and we renamed it hOAT10. [(3)H]Nicotinate transport by hOAT10 into X. laevis oocytes and into Caco-2 cells was saturable with Michaelis constants (K(m)) of 22 and 44 microm, respectively, suggesting that hOAT10 may be the molecular equivalent of the postulated high affinity nicotinate transporter in kidneys and intestine. The pH dependence of hOAT10 suggests p-aminohippurate(-)/OH(-), urate(-)/OH(-), and nicotinate(-)/OH(-) exchange as possible transport modes. Urate inhibited [(3)H]nicotinate transport by hOAT10 with an IC(50) value of 759 microm, assuming that hOAT10 represents a low affinity urate transporter. hOAT10-mediated [(14)C]urate uptake was elevated by an exchange with l -lactate, pyrazinoate, and nicotinate. Surprisingly, we have detected urate(-)/glutathione exchange by hOAT10, consistent with an involvement of hOAT10 in the renal glutathione cycle. Uricosurics, diuretics, and cyclosporine A showed substantial interactions with hOAT10, of which cyclosporine A enhanced [(14)C]urate uptake, providing the first molecular evidence for cyclosporine A-induced hyperuricemia.     

Novel URAT1 mutations caused acute renal failure after exercise in two Chinese families with renal hypouricemia

Renal hypouricemia (RHUC), as an infrequent hereditary disease, is associated with severe complications such as exercise-induced acute renal failure (EIARF). Loss-of-function mutations in urate transporter gene URAT1 (Type 1) and in glucose transporter gene GLUT9 (Type 2) are major causes of this disorder. In this study, URAT1 and GLUT9 were screened in two uncorrelated families from mainland China and a total of five mutations were identified in exons, including two novel heterozygous URAT1 mutations. In four members of the first family, c.151delG (p.A51fsX64) in exon 1 was detected, which resulted in a frameshift and truncated the original 553-residue-protein to 63 amino acid protein. A missense mutation c.C1546A (p.P516T) in exon 9 in GLUT9 was revealed in the second family, which caused a functional protein substitution at codon 516. These two novel mutations were neither identified in the subsequent scanning of 200 ethnically matched healthy control subjects with normal serum UA level nor in a 1000 genome project database. Thus our report identifies two novel loss-of-function mutations (c.151delG in URAT1 and p.P516T in GLUT9) which cause RHUC and renal dysfunction in two independent RHUC pedigrees.     

ABCG2 is a High-Capacity Urate Transporter and its Genetic Impairment Increases Serum Uric Acid Levels in Humans

The ATP-binding cassette, subfamily G, member 2 (ABCG2/BCRP) gene encodes a well-known transporter, which exports various substrates including nucleotide analogs such as 3′-azido-3′-deoxythymidine (AZT). ABCG2 is also located in a gout-susceptibility locus (MIM 138900) on chromosome 4q, and has recently been identified by genome-wide association studies to relate to serum uric acid (SUA) and gout. Becuase urate is structurally similar to nucleotide analogs, we hypothesized that ABCG2 might be a urate exporter. To demonstrate our hypothesis, transport assays were performed with membrane vesicles prepared from ABCG2-overexpressing cells. Transport of estrone-3-sulfate (ES), a typical substrate of ABCG2, is inhibited by urate as well as AZT and ES. ATP-dependent transport of urate was then detected in ABCG2-expressing vesicles but not in control vesicles. Kinetic analysis revealed that ABCG2 is a high-capacity urate transporter that maintained its function even under high-urate concentration. The calculated parameters of ABCG2-mediated transport of urate were a Km of 8.24 ± 1.44 mM and a Vmax of 6.96 ± 0.89 nmol/min per mg of protein. Moreover, the quantitative trait locus (QTL) analysis performed in 739 Japanese individuals revealed that a dysfunctional variant of ABCG2 increased SUA as the number of minor alleles of the variant increased (p = 6.60 × 10^?5). Because ABCG2 is expressed on the apical membrane in several tissues, including kidney, intestine, and liver, these findings indicate that ABCG2, a high-capacity urate exporter, has a physiological role of urate homeostasis in the human body through both renal and extrarenal urate excretion.     

Functional architecture of the CFTR chloride channel

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) family of membrane transport proteins. CFTR is unique among ABC proteins in that it functions not as an active transporter but as an ATP-gated Cl^? channel. As an ion channel, the function of the CFTR transmembrane channel pore that mediates Cl^? movement has been studied in great detail. On the other hand, only low resolution structural data is available on the transmembrane parts of the protein. The structure of the channel pore has, however, been modeled on the known structure of active transporter ABC proteins. Currently, significant barriers exist to building a unified view of CFTR pore structure and function. Reconciling functional data on the channel with indirect structural data based on other proteins with very different transport functions and substrates has proven problematic. This review summarizes current structural and functional models of the CFTR Cl^? channel pore, including a comprehensive review of previous electrophysiological investigations of channel structure and function. In addition, functional data on the three-dimensional arrangement of pore-lining helices, as well as contemporary hypotheses concerning conformational changes in the pore that occur during channel opening and closing, are discussed. Important similarities and differences between different models of the pore highlight current gaps in our knowledge of CFTR structure and function. In order to fill these gaps, structural and functional models of the membrane-spanning pore need to become better integrated.     

Recent developments in our understanding of the renal basis of hyperuricemia and the development of novel antihyperuricemic therapeutics

Although dietary, genetic, or disease-related excesses in urate production may contribute to hyperuricemia, impaired renal excretion of uric acid is the dominant cause of hyperuricemia in the majority of patients with gout. The aims of this review are to highlight exciting and clinically pertinent advances in our understanding of how uric acid is reabsorbed by the kidney under the regulation of urate transporter (URAT)1 and other recently identified urate transporters; to discuss urate-lowering agents in clinical development; and to summarize the limitations of currently available antihyperuricemic drugs. The use of uricosuric drugs to treat hyperuricemia in patients with gout is limited by prior urolothiasis or renal dysfunction. For this reason, our discussion focuses on the development of the novel xanthine oxidase inhibitor febuxostat and modified recombinant uricase preparations.     

CATP-6, a C. elegans Ortholog of ATP13A2 PARK9, Positively Regulates GEM-1, an SLC16A Transporter

In previous work, we found that gain-of-function mutations that hyperactivate GEM-1 (an SLC16A transporter protein) can bypass the requirement for GON-2 (a TRPM channel protein) during the initiation of gonadogenesis in C. elegans. Consequently, we proposed that GEM-1 might function as part of a Mg²⁺ uptake pathway that functions in parallel to GON-2. In this study, we report that CATP-6, a C. elegans ortholog of the P5B ATPase, ATP13A2 (PARK9), is necessary for gem-1 gain-of-function mutations to suppress the effects of gon-2 inactivation. One possible explanation for this observation is that GEM-1 serves to activate CATP-6, which then functions as a Mg²⁺ transporter. However, we found that overexpression of GEM-1 can alleviate the requirement for CATP-6 activity, suggesting that CATP-6 probably acts as a non-essential upstream positive regulator of GEM-1. Our results are consistent with the notion that P5B ATPases govern intracellular levels of Mg²⁺ and/or Mn²⁺ by regulating the trafficking of transporters and other proteins associated with the plasma membrane.