Identification and functional characterization of the first nucleobase transporter in mammals: implication in the species difference in the intestinal absorption mechanism of nucleobases and their analogs between higher primates and other mammals

Pseudogenization of the Slc23a4 gene is necessary for the survival of Xdh-deficient mice

In most patients with type 1 xanthinuria caused by mutations in the xanthine dehydrogenase gene (XDH), no clinical complications, except for urinary stones, are observed. In contrast, all Xdh(- / -) mice die due to renal failure before reaching adulthood at 8 weeks of age. Hypoxanthine or xanthine levels become excessive and thus toxic in Xdh(- / -) mice because enhancing the activity of hypoxanthine phosphoribosyl transferase (HPRT), which is an enzyme that uses hypoxanthine as a substrate, slightly increases the life span of these mice. In this study, we targeted the mouse intestinal sodium-dependent nucleobase transporter (SNBT) gene (Slc23a4), which is a pseudogene in humans. Hprt(high)Xdh(- / -)Slc23a4(- / -) mice had a longer life span and reached adulthood. The urinary xanthine excretion of these mice was 20-fold greater than that of patients with type 1 xanthinuria. The urinary hypoxanthine/xanthine ratio of Hprt(high)Xdh(- / -)Slc23a4(- / -) mice was lower than that of patients with type 1 xanthinuria. Hprt(high)Xdh(- / -)Slc23a4(- / -) mice exhibited renal impairment, accompanied by high plasma creatinine levels and anemia. Moreover, female Hprt(high)Xdh(- / -)Slc23a4(- / -) mice produced offspring that did not survive. In conclusion, for the first time, we established that Xdh(- / -) mice survive to adulthood.

Ascorbic acid transporter MmSLC23A2 functions to inhibit apoptosis via ROS scavenging in hard clam (Mercenaria mercenaria) under acute hypo-salinity stress

Solute carrier family 23 (SLC23) mediates cellular uptake of ascorbic acid, a crucial antioxidant protecting organisms against oxidative stress. Despite advances in understanding SLC23 in mammals, its physiological roles in bivalves remain poorly understood. Notably, euryhaline bivalves exhibit a significant expansion and positive selection of SLC23, highlighting the need for deeper investigation. Here, we identified 25 MmSLC23 in the hard clam genome. These genes predominantly cluster on chromosomes 3 and 14, with tandem duplications driving their expansion. All MmSLC23 localize to the plasma membrane, containing 9-14 transmembrane domains. Syntenic conservation of SLC23 was limited across order Venerida, with most expanded members being lineage-specific paralogs. Transcriptome analysis and fluorescence in situ hybridization revealed that MmSLC23 exhibited divergent expression patterns under acute and chronic salinity stress. Notably, RNA interference of MmSLC23A2 led to a significant reduction in intracellular ascorbic acid levels. Under acute hypo-salinity stress, increased ROS levels and elevated apoptosis rate were observed in MmSLC23A2 knockdown clams, as assessed by flow cytometry and transmission electron microscopy. These findings underscore the crucial role of SLC23 in mitigating oxidative damage and preventing premature apoptosis under acute salinity stress, offering new insights into the molecular mechanisms underlying the remarkable salinity adaptability of euryhaline bivalves.

The dimer of human SVCT1 is key for transport function

Humans and other primates lack the ability to synthesize the essential nutrient, Vitamin C, which is derived exclusively from the diet. Crucial for effective vitamin C uptake are the Na+ dependent Vitamin C transporters, SVCT1 and SVCT2, members of the nucleobase ascorbate transporter (NAT) family. SVCT1 and 2 actively transport the reduced form of Vitamin C, ascorbic acid, into key tissues. The recent structure of the mouse SVCT1 revealed the molecular basis of substrate binding and that, like the other structurally characterised members of the NAT family, it exists as a closely associated dimer. SVCT1 is likely to function via the elevator mechanism with the core domain of each protomer able to bind substrate and move through the membrane carrying the substrate across the membrane. Here we explored the function of a range of variants of the human SVCT1, revealing a range of residues involved in substrate selection and binding, and confirming the importance of the C-terminus in membrane localisation. Furthermore, using a dominant negative mutant we show that the dimer is essential for transport function, as previously seen in the fungal homologue, UapA. In addition, we show that a localisation deficient C-terminal truncation of SVCT1 blocks correct localisation of co-expressed, associated wildtype SVCT1. These results clearly show the importance of the dimer in both correct SVCT1 trafficking and transport activity.

SLC35G1 is a highly chloride-sensitive transporter responsible for the basolateral membrane transport in intestinal citrate absorption

The intestinal absorption of essential nutrients, especially those not readily biosynthesized, is a critical physiological process for maintaining homeostasis. Numerous studies have indicated that intestinal absorption is mediated by various membrane transporters. Citrate, a crucial bioactive compound produced as an intermediate in the Krebs cycle, is absorbed in the small intestine through carrier-mediated systems because of its high hydrophilicity. While the luminal absorption of citrate is mediated by Na+-dicarboxylate cotransporter 1 (NaDC1/SLC13A2), the mechanism governing the release of the transported citrate into the bloodstream remains unknown. Here, we explored the transporters responsible for intestinal citrate absorption at the basolateral membrane, focusing on highly expressed orphan transporters in the small intestine as candidates. Consequently, SLC35G1, originally identified as a partner of stromal interaction molecule 1, a cell surface transmembrane glycoprotein, was found to play a role in the intestinal absorption of citrate at the basolateral membrane. Furthermore, our results revealed that SLC35G1-mediated citrate transport was diminished by chloride ions at physiologically relevant extracellular concentrations. This suggests that SLC35G1, to our best knowledge, is the first transporter identified to be extremely sensitive to chloride ions among those functioning on the basolateral membrane of intestinal epithelial cells. This study provides valuable insights into the intestinal absorption of citrate and significantly contributes to elucidating the poorly understood molecular basis of the intestinal absorption system.

Mutational analysis of Phanerochaete chrysosporium´s purine transporter

We present here a mutational analysis of the purine transporter from Phanerochaete chrysosporium (PhZ), a member of the AzgA-like subfamily within the Nucleobase Ascorbate Transporters family. We identified key residues that determine its substrate specificity and transport efficiency. Thirteen PhZ mutants were generated and heterologously expressed in Aspergillus nidulans. The growth of mutant strains in the presence of purines and toxic analogues and the uptake rate of radiolabelled hypoxanthine were evaluated. Results revealed that ten mutants showed differences in transport compared to the wild-type PhZ: six mutants completely lost function, two exhibited decreased transport activity, and two showed increased hypoxanthine uptake. Subcellular localization and expression level analyses indicated that the differences in transport activity were not due to trafficking issues to the plasma membrane or protein stability. A three-dimensional model of PhZ, constructed with the artificial intelligence-based AlphaFold2 program, suggested that critical residues for transport are located in transmembrane segments and an internal helix. In the latter, the A418 residue was identified as playing a pivotal role in transport efficiency despite being far from the putative substrate binding site, as mutant A418V showed an increased initial uptake efficiency for the transporter´s physiological substrates. We also report that residue L124, which lies in the putative substrate binding site, plays a critical role in substrate transport, emerging as an additional determinant in the transport mechanism of this family of transporters. These findings underscore the importance of specific residues in AzgA-like transporters and enhance our understanding of the intricate mechanisms governing substrate specificity and transport efficiency within this family.

Blood-to-Testis Transport of Ribavirin Involves Carrier-Mediated Processes at the Blood-Testis Barrier

Ribavirin, an antiretroviral agent targeting the hepatitis C virus, causes male reproductive toxicity. This study investigated the mechanism of ribavirin transport at the blood-testis barrier (BTB). In vivo mouse integration plot analysis after intravenous administration revealed that the net influx clearance of [3H]ribavirin in the testis was 3.6-fold greater than that of [14C]D-mannitol, a paracellular transport marker, implying transcellular transport of ribavirin across the BTB. Moreover, [3H]ribavirin uptake by TM4 cells, mouse-derived Sertoli cells, was time- and concentration-dependent, with a Km value of 2.49 mM. S-[(4-nitrophenyl)methyl]-6-thioinosine, an inhibitor of Na+-independent equilibrative nucleoside transporters (ENTs), strongly inhibited the [3H]ribavirin uptake by TM4 cells at 100 µM. Compared to the uptake of [3H]adenosine, a typical endogenous nucleoside, [3H]ribavirin uptake was relatively similar to ENT2 transport. [3H]Ribavirin uptake was also observed in mouse ENT2-expressing Xenopus laevis oocytes, and gene silencing via the transfection of ENT2 small interfering RNA significantly reduced the [3H]ribavirin transport into TM4 cells by 13%. Taken together, these results suggest that ENT2 partially contributes to ribavirin transport at the BTB.

Insight into the mechanism of H+-coupled nucleobase transport

Members of the nucleobase/ascorbic acid transporter (NAT) gene family are found in all kingdoms of life. In mammals, the concentrative uptake of ascorbic acid (vitamin C) by members of the NAT family is driven by the Na+ gradient, while the uptake of nucleobases in bacteria is powered by the H+ gradient. Here, we report the structure and function of PurTCp, a NAT family member from Colwellia psychrerythraea. The structure of PurTCp was determined to 2.80 Å resolution by X-ray crystallography. PurTCp forms a homodimer, and each protomer has 14 transmembrane segments folded into a transport domain (core domain) and a scaffold domain (gate domain). A purine base is present in the structure and defines the location of the substrate binding site. Functional studies reveal that PurTCp transports purines but not pyrimidines and that purine binding and transport is dependent on the pH. Mutation of a conserved aspartate residue close to the substrate binding site reveals the critical role of this residue in H+-dependent transport of purines. Comparison of the PurTCp structure with transporters of the same structural fold suggests that rigid-body motions of the substrate-binding domain are central for substrate translocation across the membrane.

Vitamin C transporter SVCT1 serves a physiological role as a urate importer: functional analyses and in vivo investigations

Uric acid, the end product of purine metabolism in humans, is crucial because of its anti-oxidant activity and a causal relationship with hyperuricemia and gout. Several physiologically important urate transporters regulate this water-soluble metabolite in the human body; however, the existence of latent transporters has been suggested in the literature. We focused on the Escherichia coli urate transporter YgfU, a nucleobase-ascorbate transporter (NAT) family member, to address this issue. Only SLC23A proteins are members of the NAT family in humans. Based on the amino acid sequence similarity to YgfU, we hypothesized that SLC23A1, also known as sodium-dependent vitamin C transporter 1 (SVCT1), might be a urate transporter. First, we identified human SVCT1 and mouse Svct1 as sodium-dependent low-affinity/high-capacity urate transporters using mammalian cell-based transport assays. Next, using the CRISPR-Cas9 system followed by the crossing of mice, we generated Svct1 knockout mice lacking both urate transporter 1 and uricase. In the hyperuricemic mice model, serum urate levels were lower than controls, suggesting that Svct1 disruption could reduce serum urate. Given that Svct1 physiologically functions as a renal vitamin C re-absorber, it could also be involved in urate re-uptake from urine, though additional studies are required to obtain deeper insights into the underlying mechanisms. Our findings regarding the dual-substrate specificity of SVCT1 expand the understanding of urate handling systems and functional evolutionary changes in NAT family proteins.

A Review on Experimental Models to Test Medicinal Plants on Postprandial Blood Glucose in Diabetes

Due to the gravity of postprandial hyperglycemia in the development of microvascular and macrovascular diseases in diabetics, many medicinal plants are tested to determine their effectiveness in glycemic control and the mechanisms of action of the products. Consequently, various diabetic models have been developed and enhanced over the years. The objective of this review is to describe some of the experimental models to study the effect of medicinal plants used to control postprandial hyperglycemia. Data was collected from PubMed, ScienceDirect, Scopus, and Google scholar (1953-2021). Fiftyseven (57) studies were included in this review article. Ten models were identified and described. For each model, we described the targets involved and their roles in postprandial blood glucose control. The experimental design and procedures described the targets such as an α-glucosidase enzyme, SGLT1, GLUT2, DPP-IV, Na+/K+ ATPase pump, or intestinal motility in the models, experiment design and procedures were described. This review will facilitate the selection of the most appropriate model for studying agents used to investigate postprandial blood glucose.

The evolution of vitamin C biosynthesis and transport in animals

Background

Vitamin C (VC) is an indispensable antioxidant and co-factor for optimal function and development of eukaryotic cells. In animals, VC can be synthesized by the organism, acquired through the diet, or both. In the single VC synthesis pathway described in animals, the penultimate step is catalysed by Regucalcin, and the last step by l-gulonolactone oxidase (GULO). The GULO gene has been implicated in VC synthesis only, while Regucalcin has been shown to have multiple functions in mammals.

Results

Both GULO and Regucalcin can be found in non-bilaterian, protostome and deuterostome species. Regucalcin, as here shown, is involved in multiple functions such as VC synthesis, calcium homeostasis, and the oxidative stress response in both Deuterostomes and Protostomes, and in insects in receptor-mediated uptake of hexamerin storage proteins from haemolymph. In Insecta and Nematoda, however, there is no GULO gene, and in the latter no Regucalcin gene, but species from these lineages are still able to synthesize VC, implying at least one novel synthesis pathway. In vertebrates, SVCT1, a gene that belongs to a family with up to five members, as here shown, is the only gene involved in the uptake of VC in the gut. This specificity is likely the result of a subfunctionalization event that happened at the base of the Craniata subphylum. SVCT-like genes present in non-Vertebrate animals are likely involved in both VC and nucleobase transport. It is also shown that in lineages where GULO has been lost, SVCT1 is now an essential gene, while in lineages where SVCT1 gene has been lost, GULO is now an essential gene.

Conclusions

The simultaneous study, for the first time, of GULO, Regucalcin and SVCTs evolution provides a clear picture of VC synthesis/acquisition and reveals very different selective pressures in different animal taxonomic groups.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02040-7.

Determination of total purine and free purine content in milk, soymilk, and enteral nutritional supplements to assist nutritional therapy for hyperuricemia and gout

In this study, we determined the purine contents in milk and soymilk, as protein-rich drinks, and in enteral nutritional supplements employed to ameliorate protein malnutrition in the elderly. Milk consumption is known to lower serum uric acid levels and to promote uric acid excretion. However, discrepant results have been reported regarding the effect of soymilk on serum uric acid levels. The purpose of this study was to quantify and compare the total purine contents and the contents of individual purines and pyrimidines by molecular type (nucleotides, nucleosides, and bases).

SLC23A3 is a renal hypoxanthine transporter

LLC-PK1 renal cells show Na⁺-dependent and Na⁺-independent hypoxanthine uptake. While the latter is inhibited by adenine, neither are inhibited by xanthine. In rats, intestinal Na⁺-dependent hypoxanthine transporter Slc23a4 is not expressed in the kidney, and its action is inhibited by xanthine. This study aimed to clone Slc23a4-paralog SLC23A3 from the human kidney and investigate its hypoxanthine transport activity. We observed Na⁺-dependent 10 nM [³H]-hypoxanthine uptake in SLC23A3 RNA-injected Xenopus oocytes. Moreover, 100 μM xanthine did not inhibit Na⁺-independent 300 nM [³H]-hypoxanthine uptake, whereas 100 μM adenine did. These results confirm that SLC23A3 is a hypoxanthine transporter in the human kidney.

Uniport, Not Proton-Symport, in a Non-Mammalian SLC23 Transporter

SLC23 family members are transporters of either nucleobases or ascorbate. While the mammalian SLC23 ascorbate transporters are sodium-coupled, the non-mammalian nucleobase transporters have been proposed, but not formally shown, to be proton-coupled symporters. This assignment is exclusively based on in vivo transport assays using protonophores. Here, by establishing the first in vitro transport assay for this protein family, we demonstrate that a representative member of the SLC23 nucleobase transporters operates as a uniporter instead. We explain these conflicting assignments by identifying a critical role of uracil phosphoribosyltransferase, the enzyme converting uracil to UMP, in driving uracil uptake in vivo. Detailed characterization of uracil phosphoribosyltransferase reveals that the sharp reduction of uracil uptake in whole cells in presence of protonophores is caused by acidification-induced enzyme inactivation. The SLC23 family therefore consists of both uniporters and symporters in line with the structurally related SLC4 and SLC26 families that have previously been demonstrated to accommodate both transport modes as well.

Identification of New Specificity Determinants in Bacterial Purine Nucleobase Transporters based on an Ancestral Sequence Reconstruction Approach

The relation of sequence with specificity in membrane transporters is challenging to explore. Most relevant studies until now rely on comparisons of present-day homologs. In this work, we study a set of closely related transporters by employing an evolutionary, ancestral-reconstruction approach and reveal unexpected new specificity determinants. We analyze a monophyletic group represented by the xanthine-specific XanQ of Escherichia coli in the Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2 (NAT/NCS2) family. We reconstructed AncXanQ, the putative common ancestor of this clade, expressed it in E. coli K-12, and found that, in contrast to XanQ, it encodes a high-affinity permease for both xanthine and guanine, which also recognizes adenine, hypoxanthine, and a range of analogs. AncXanQ conserves all binding-site residues of XanQ and differs substantially in only five intramembrane residues outside the binding site. We subjected both homologs to rationally designed mutagenesis and present evidence that these five residues are linked with the specificity change. In particular, we reveal Ser377 of XanQ (Gly in AncXanQ) as a major determinant. Replacement of this Ser with Gly enlarges the specificity of XanQ towards an AncXanQ-phenotype. The ortholog from Neisseria meningitidis retaining Gly at this position is also a xanthine/guanine transporter with extended substrate profile like AncXanQ. Molecular Dynamics shows that the S377G replacement tilts transmembrane helix 12 resulting in rearrangement of Phe376 relative to Phe94 in the XanQ binding pocket. This effect may rationalize the enlarged specificity. On the other hand, the specificity effect of S377G can be masked by G27S or other mutations through epistatic interactions.

Transporter Specificity: A Tale of Loosened Elevator-Sliding

Elevator-type transporters are a group of proteins translocating nutrients and metabolites across cell membranes. Despite structural and functional differences, elevator-type transporters use a common mechanism of substrate translocation via reversible movements of a mobile core domain (the elevator), which includes the substrate binding site, along a rigid scaffold domain, stably anchored in the plasma membrane. How substrate specificity is determined in elevator transporters remains elusive. Here, I discuss how a recent report on the sliding elevator mechanism, seen under the context of genetic analysis of a prototype fungal transporter, sheds light on how specificity might be genetically modified. I propose that flexible specificity alterations might occur by 'loosening' of the sliding mechanism from tight coupling to substrate binding.

Model Analysis of the Apparent Saturation Kinetics of Purine Nucleobase Uptake in Cells co-Expressing Transporter and Metabolic Enzyme

PURPOSE

This study aims to understand the effect of salvage enzyme activity on the saturable kinetics of facilitated cellular uptake of purine nucleobase by developing a cellular kinetic model incorporating equilibrative nucleobase transporter 1 (ENBT1) and adenine phosphoribosyltransferase (APRT), with adenine as a model nucleobase.

METHODS

A cellular kinetic model incorporating the functions of ENBT1 and APRT was developed using Napp software and employed for model-based analysis of the cellular disposition of adenine.

RESULTS

Simulation analysis using the developed cellular kinetic model could account for the experimentally observed time-dependent changes in the K_m(app) value of adenine for ENBT1-mediated uptake. At a long experimental time, the model shows that uptake of adenine is rate-limited by APRT, enabling determination of the K_m value for APRT. At early time, the rate-limiting step for adenine uptake is ENBT1-mediated transport, enabling determination of the K_m value for ENBT1. Further simulations showed that the effect of experimental time on the K_m(app) value for ENBT1-mediated uptake is dependent on the APRT expression level.

CONCLUSION

Our findings indicate that both enzyme expression levels and experimental time should be considered when using cellular uptake studies to determine the K_m values of purine nucleobases for facilitated transporters.

Vitamin C Deficiency in the Young Brain-Findings from Experimental Animal Models

Severe and long-term vitamin C deficiency can lead to fatal scurvy, which is fortunately considered rare today. However, a moderate state of vitamin C (vitC) deficiency (hypovitaminosis C)-defined as a plasma concentration below 23 μM-is estimated to affect up to 10% of the population in the Western world, albeit clinical hallmarks in addition to scurvy have not been linked to vitC deficiency. The brain maintains a high vitC content and uniquely high levels during deficiency, supporting vitC's importance in the brain. Actions include both antioxidant and co-factor functions, rendering vitamin C deficiency likely to affect several targets in the brain, and it could be particularly significant during development where a high cellular metabolism and an immature antioxidant system might increase sensitivity. However, investigations of a non-scorbutic state of vitC deficiency and effects on the developing young brain are scarce. This narrative review provides a comprehensive overview of the complex mechanisms that regulate vitC homeostasis in vivo and in the brain in particular. Functions of vitC in the brain and the potential consequences of deficiency during brain development are highlighted, based primarily on findings from experimental animal models. Perspectives for future investigations of vitC are outlined.

Context-dependent Cryptic Roles of Specific Residues in Substrate Selectivity of the UapA Purine Transporter

Members of the ubiquitous Nucleobase Ascorbate Transporter (NAT) family are H⁺ or Na⁺ symporters specific for the cellular uptake of either purines and pyrimidines or L-ascorbic acid. Despite the fact that several bacterial and fungal members have been extensively characterised at a genetic, biochemical or cellular level, and crystal structures of NAT members from Escherichia coli and Aspergillus nidulans have been determined pointing to a mechanism of transport, we have little insight on how substrate selectivity is determined. Here, we present systematic mutational analyses, rational combination of mutations, and novel genetic screens that reveal cryptic context-dependent roles of partially conserved residues in the so-called NAT signature motif in determining the specificity of the UapA transporter of A. nidulans. We show that specific NAT signature motif substitutions, alone and in combinations with each other or with distant mutations in residues known to affect substrate selectivity, lead to novel UapA versions possessing variable transport capacities and specificities for nucleobases. In particular, we show that a UapA version including the quadruple mutation T405S/F406Y/A407S/Q408E in the NAT signature motif (UapA-SYSE) becomes incapable of purine transport, but gains a novel pyrimidine-related profile, which can be further altered to a more promiscuous purine/pyrimidine profile when combined with replacements at distantly located residues, especially at F528. Our results reveal that UapA specificity is genetically highly modifiable and allow us to speculate on how the elevator-type mechanism of transport might account for this flexibility.

Lactobacillus gasseri PA-3 directly incorporates purine mononucleotides and utilizes them for growth

Lactococcus lactis has been reported unable to directly incorporate mononucleotides but instead requires their external dephosphorylation by nucleotidases to the corresponding nucleosides prior to their incorporation. Although Lactobacillus gasseri PA-3 (PA-3), a strain of lactic acid bacteria, has been found to incorporate purine mononucleotides such as adenosine 5'-monophosphate (AMP), it remains unclear whether these bacteria directly incorporate these mononucleotides or incorporate them after dephosphorylation to the corresponding nucleosides. This study evaluated whether PA-3 incorporated radioactively-labeled mononucleotides in the presence or absence of the 5'-nucleotidase inhibitor α,β-methylene ADP (APCP). PA-3 took up ¹⁴C-AMP in the presence of APCP, as well as incorporating ³²P-AMP. Furthermore, radioactivity was detected in the RNA/DNA of bacterial cells cultured in the presence of ³²P-AMP. Taken together, these findings indicated that PA-3 incorporated purine mononucleotides directly rather than after their dephosphorylation to purine nucleosides and that PA-3 utilizes these purine mononucleotides in the synthesis of RNA and DNA. Although additional studies are required to identify purine mononucleotide transporters in PA-3, this study is the first to show that some lactic acid bacteria directly incorporate purine mononucleotides and use them for growth.

Species-dependent patterns of incorporation of purine mononucleotides and nucleosides by lactic acid bacteria

Although most lactic acid bacteria do not directly incorporate purine nucleotides, the strain Lactobacillus gasseri PA-3 was found to incorporate purine mononucleotides. To determine whether the direct uptake of purine mononucleotides is dependent on the species or strain of lactic acid bacteria, incorporation of purine mononucleotides was assessed in L. gasseri, Lactcoccus lactis sbsp. lactis, Streptococcus thermophilus and other species of lactic acid bacteria. Each bacterial strain was incubated with ³²P-AMP or ¹⁴C-adenosine and the incorporation of each purine was evaluated by measuring their radioactivity. All investigated strains of L. gasseri incorporated ³²P-AMP, whereas strains of S. thermophilus and most strains of L. lactis did not. Incorporation of ³²P-AMP into strains of Pediococcus was dependent on the strain or species of that genus of bacteria. All investigated strains, except for one strain of L. gasseri, incorporated ¹⁴C-adenosine, with S. thermophilus, L. lactis and Pediococcus generally displaying greater incorporation of ¹⁴C-adenosine than L. gasseri. Although most lactic acid bacteria such as S. thermophiles and L. lactis do not incorporate purine mononucleotides, some species such as L. gasseri directly incorporate purine mononucleotides. These findings indicate that the preferential incorporation of purine mononucleotides or nucleosides by lactic acid bacteria is dependent on the species or strain.

Purine and pyrimidine transporters of pathogenic protozoa - conduits for therapeutic agents

Purines and pyrimidines are essential nutrients for any cell. Most organisms are able to synthesize their own purines and pyrimidines, but this ability was lost in protozoans that adapted to parasitism, leading to a great diversification in transporter activities in these organisms, especially for the acquisition of amino acids and nucleosides from their hosts throughout their life cycles. Many of these transporters have been shown to have sufficiently different substrate affinities from mammalian transporters, making them good carriers for therapeutic agents. In this review, we summarize the knowledge obtained on purine and pyrimidine activities identified in protozoan parasites to date and discuss their importance for the survival of these parasites and as drug carriers, as well as the perspectives of developments in the field.

Xanthine oxidoreductase knockout mice with high HPRT activity were not rescued by NAD+ replenishment

Although xanthinuria is nonfatal in human, xanthine oxidoreductase knockout (Xor-KO) mice have only a short lifespan. Hypoxanthine phosphoribosyltransferase activity (HPRT) in human and wild mice is higher than in laboratory mice. The aim of this study was to investigate the underlying mechanisms that give rise to the longer lifespan of high-HPRT/Xor-KO mice. Before Xor-KO mice die, urinary excretion of hypoxanthine increased with a corresponding decrease in excretion of xanthine. The switch of excretion from xanthine to hypoxanthine might be a cause of death for Xor-KO mice, suggesting inhibition of NAD⁺-dependent IMP dehydrogenase. Because hypoxanthine inhibits the synthesis of nicotinamide mononucleotide (NMN), a precursor of NAD⁺, the accumulation of hypoxanthine in Xor-KO mice may cause a depletion in the levels of NAD⁺. Moreover, urinary excretion of urate in high-HPRT/Uox-KO/Xor-KO mice means urate derived from gut microbiota is absorbed by the intestine. Likewise, over excretion of oxypurine in mice may be caused by intestinal absorption of oxypurine. For NAD⁺ replenishment, oral supplementation with 1% L-tryptophan, an alternative precursor of NAD⁺, resulted in a recovery of body weight gain in high-HPRT/Uox-KO/Xor-KO mice. In conclusion, the death of Xor-KO mice by renal failure seems to be caused by a depletion in NAD⁺ levels due to the intracellular accumulation of hypoxanthine. NAD⁺ replenishment by oral supplementation of NMN or tryptophan was complicated by the effect of gut microbiota and failed to rescue high-HPRT/Xor-KO mice. The attenuation of intestinal absorption of oxypurines seems to be necessary to avoid hypoxanthine accumulation and over excretion of oxypurine.

Identification of the amino acid residue responsible for the myricetin sensitivity of human proton-coupled folate transporter

Human proton-coupled folate transporter (hPCFT/SLC46A1) has recently been found to be inhibited by myricetin by a sustained mechanism, raising a concern that the inhibition might lead to malabsorption of folates in the intestine, where hPCFT works for their epithelial uptake. However, rat PCFT (rPCFT) has more recently been found not to be inhibited by myricetin. Prompted by this finding, we attempted to determine the amino acid residue involved in that by analyses comparing between hPCFT and rPCFT. In the initial analysis, chimeric constructs prepared from hPCFT and rPCFT were examined for myricetin sensitivity to determine the hPCFT segment involved in the sensitivity. Focusing on the thereby determined segment from 83rd to 186th amino acid residue, hPCFT mutants having a designated amino acid residue replaced with its counterpart in rPCFT were prepared for the subsequent analysis. Among them, only G158N-substituted hPCFT was found to be transformed to be insensitive to myricetin and, accordingly, oppositely N158G-substituted rPCFT was transformed to be sensitive to myricetin. These results indicate the critical role of Gly¹⁵⁸ in the myricetin sensitivity of hPCFT. This finding would help advance the elucidation of the mechanism of the myricetin-induced inhibition of hPCFT and manage the potential risk arising from that.

Specific Residues in a Purine Transporter Are Critical for Dimerization, ER Exit, and Function

Transporters are transmembrane proteins that mediate the selective translocation of solutes across biological membranes. Recently, we have shown that specific interactions with plasma membrane phospholipids are essential for the formation and/or stability of functional dimers of the purine transporter UapA, a prototypic eukaryotic member of the ubiquitous nucleobase ascorbate transporter (NAT) family. Here, we provide strong evidence that distinct interactions of UapA with membrane lipids are essential for ab initio formation of functional dimers in the ER, or ER exit and further subcellular trafficking. Through genetic screens, we identify mutations that restore defects in dimer formation and/or trafficking. Suppressors of defective dimerization restore ab initio formation of UapA dimers in the ER. Most of these suppressors are located in the movable core domain, but also in the core-dimerization interface and in residues of the dimerization domain exposed to lipids. Molecular dynamics suggest that the majority of suppressors stabilize interhelical interactions in the core domain and thus assist the formation of functional UapA dimers. Among suppressors restoring dimerization, a specific mutation, T401P, was also isolated independently as a suppressor restoring trafficking, suggesting that stabilization of the core domain restores function by sustaining structural defects caused by the abolishment of essential interactions with specific lipids. Importantly, the introduction of mutations topologically equivalent to T401P into a rat homolog of UapA, namely rSNBT1, permitted the functional expression of a mammalian NAT in Aspergillus nidulans Thus, our results provide a potential route for the functional expression and manipulation of mammalian transporters in the model Aspergillus system.

A simple ex vivo bioassay for 5-FU transport into healthy buccal mucosal cells

PURPOSE

Fluorouracil (5-FU), a chemotherapeutic agent widely used in the treatment of numerous common malignancies, causes oral mucositis in a proportion of patients. The contribution of drug transport processes to the development of this toxicity is currently unknown. This work aimed to establish and optimise a simple phenotyping assay for 5-FU uptake into primary buccal mucosal cells (BMC).

METHODS

The uptake kinetics of radiolabelled 5-FU were determined in pooled BMC freshly collected from healthy volunteers. The inter- and intra-individual variability in 5-FU uptake was then assessed across a cohort that included both healthy volunteers and cancer patients.

RESULTS

5-FU uptake into pooled primary BMC was both time and concentration dependent. An Eadie-Hofstee analysis suggested two components; a high-affinity (K_M = 3.3 µM) low-capacity ([Formula: see text] = 57.8 pmol min^-1 10⁵ viable cells^-1) transporter, and a high-capacity ([Formula: see text] = 1230 pmol min^-1 10⁵ viable cells^-1) low-affinity (K_M = 3932 µM) transporter. There was 180-fold variation in the rate of 5-FU uptake into BMC (0.10-17.86 pmol min^-1 10⁵ viable cells^-1) across the 34 subjects (healthy participants N = 24, cancer patients N = 10). Notably, retesting of a subset of these participants (N = 16) multiple times over a period of up to 140 days demonstrated poor stability of the uptake phenotype within individuals.

CONCLUSION

The uptake of 5-FU into healthy oral mucosal cells is a highly variable process facilitated by membrane transporters at pharmacologically relevant concentrations. This bioassay is simple, minimally invasive, and suitable for phenotypic analysis of drug transport in healthy primary cells.

Heterologous expression of the mammalian sodium-nucleobase transporter rSNBT1 in Leishmania tarentolae

Recombinant expression systems for mammalian membrane transport proteins are often limited by insufficient yields to support structural studies, inadequate post-translational processing and problems related with improper membrane targeting or cytotoxicity. Use of alternative expression systems and optimization of expression/purification protocols are constantly needed. In this work, we explore the applicability of the laboratory strain LEXSY of the ancient eukaryotic microorganism Leishmania tarentolae as a new expression system for mammalian nucleobase permeases of the NAT/NCS2 (Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2) family. We achieved the heterologous expression of the purine-pyrimidine permease rSNBT1 from Rattus norvegicus (tagged at C-terminus with a red fluorescent protein), as confirmed by confocal microscopy and biochemical analysis of the subcellular fractions enriched in membrane proteins. The cDNA of rSNBT1 has been subcloned in a pLEXSY-sat-mrfp1vector and used to generate transgenic L. tarentolae-rsnbt1-mrfp1 strains carrying the pLEXSY-sat-rsnbt1-mrfp1 plasmid either episomally or integrated in the chromosomal DNA. The chimeric transporter rSNBT1-mRFP1 is targeted to the ER and the plasma membrane of the L. tarentolae promastigotes. The transgenic strains are capable of transporting nucleobases that are substrates of rSNBT1 but also of the endogenous L. tarentolae nucleoside/nucleobase transporters. A dipyridamole-resistant Na⁺-dependent fraction of uptake is attributed to the exogenously expressed rSNBT1.

Characterization of 6-Mercaptopurine Transport by the SLC43A3-Encoded Nucleobase Transporter

6-Mercaptopurine (6-MP) is a nucleobase analog used in the treatment of acute lymphoblastic leukemia and inflammatory bowel disorders. However, the mechanisms underlying its transport into target cells have remained elusive. The protein encoded by SLC43A3_1 [equilibrative nucleobase transporter 1 (ENBT1)] has recently been shown to transport endogenous nucleobases. A splice variant (SLC43A3_2), encoding a protein with 13 additional amino acids in the first extracellular loop, is also expressed but its function is unknown. We hypothesized that 6-MP is a substrate for both variants of ENBT1. Human embryonic kidney 293 (HEK293) cells (lacking endogenous ENBT1 activity) were transfected with each of the coding region variants of SLC43A3. ENBT1 function was assessed via the rate of flux of [3H]adenine and [14C]6-MP across the plasma membrane. Both SLC43A3 variants encoded proteins with similar functional properties. [14C]6-MP and [3H]adenine had K m values (±S.D.) of 163 ± 126 and 37 ± 26 µM, respectively, for this system. Decynium-22, 6-thioguanine, and 6-methylmercaptopurine inhibited 6-MP uptake with K i values of 1.0 ± 0.4, 67 ± 30, and 73 ± 20 µM, respectively. ENBT1 also mediated adenine-sensitive efflux of 6-MP from the SLC43A3-HEK293 cells. MRP4 also contributed to the efflux of 6-MP in this model, but was less efficient than ENBT1 in this regard. Furthermore, transfection of HEK293 cells with SLC43A3 increased the sensitivity of the cells to the cytotoxic effects of 6-MP by more than 7-fold. Thus, both variants of ENBT1 are key players in the transfer of 6-MP into and out of cells, and changes in SLC43A3 expression impact 6-MP cytotoxicity.

Preliminary Evidence for Enhanced Thymine Absorption: A Putative New Phenotype Associated With Fluoropyrimidine Toxicity in Cancer Patients

BACKGROUND

Chemotherapy for colorectal, head and neck, and breast cancer continues to rely heavily on 5-fluorouracil and its oral prodrug capecitabine. Associations of serious fluoropyrimidine adverse effects have focused on inherited deficiency of the catabolic enzyme, dihydropyrimidine dehydrogenase. However, abnormal dihydropyrimidine dehydrogenase activity accounts for only about one-third of observed toxicity cases. Thus, the cause of most fluorouracil toxicity cases remains unexplained.

METHODS

For this small cohort study, thymine (THY) 250 mg was administered orally to 6 patients who had experienced severe toxicity during treatment with 5FU or capecitabine. Plasma and urine were analyzed for THY and its catabolites dihydrothymine (DHT) and β-ureidoisobutyrate.

RESULTS

Of the 6 patients, 2 had decreased THY elimination and raised urinary THY recovery consistent with inherited partial dihydropyrimidine dehydrogenase deficiency, confirmed by DPYD sequencing. Unexpectedly, 3 patients displayed grossly raised plasma THY concentrations but normal elimination profiles (compared with a normal range for healthy volunteers previously published by the authors). DPYD and DPYS sequencing of these 3 patients did not reveal any significant loss-of-activity allelic variants. The authors labeled the phenotype in these 3 patients as "enhanced thymine absorption". Only 1 of the 6 cases of toxicity had a normal postdose plasma profile for THY and its catabolites. Postdose urine collections from all 6 patients had THY/DHT urinary ratios above 4.0, clearly separated from the ratios in healthy subjects that were all below 3.0.

CONCLUSIONS

This small cohort provided evidence for a hypothesis that fluorouracil toxicity cases may include a previously undescribed pyrimidine absorption variant, "enhanced thymine absorption," and elevated THY/DHT ratios in urine may predict fluorouracil toxicity. A prospective study is currently being conducted.

NAT/NCS2-hound: a webserver for the detection and evolutionary classification of prokaryotic and eukaryotic nucleobase-cation symporters of the NAT/NCS2 family

Nucleobase transporters are important for supplying the cell with purines and/or pyrimidines, for controlling the intracellular pool of nucleotides, and for obtaining exogenous nitrogen/carbon sources for metabolism. Nucleobase transporters are also evaluated as potential targets for antimicrobial therapies, since several pathogenic microorganisms rely on purine/pyrimidine salvage from their hosts. The majority of known nucleobase transporters belong to the evolutionarily conserved and ubiquitous nucleobase-ascorbate transporter/nucleobase-cation symporter-2 (NAT/NCS2) protein family. Based on a large-scale phylogenetic analysis that we performed on thousands of prokaryotic proteomes, we developed a webserver that can detect and distinguish this family of transporters from other homologous families that recognize different substrates. We can further categorize these transporters to certain evolutionary groups with distinct substrate preferences. The webserver scans whole proteomes and graphically displays which proteins are identified as NAT/NCS2, to which evolutionary groups and subgroups they belong to, and which conserved motifs they have. For key subgroups and motifs, the server displays annotated information from published crystal-structures and mutational studies pointing to key functional amino acids that may help experts assess the transport capability of the target sequences. The server is 100% accurate in detecting NAT/NCS2 family members. We also used the server to analyze 9,109 prokaryotic proteomes and identified Clostridia, Bacilli, β- and γ-Proteobacteria, Actinobacteria, and Fusobacteria as the taxa with the largest number of NAT/NCS2 transporters per proteome. An analysis of 120 representative eukaryotic proteomes also demonstrates the server's capability of correctly analyzing this major lineage, with plants emerging as the group with the highest number of NAT/NCS2 members per proteome.

Urate transport function of rat sodium-dependent nucleobase transporter 1

Sodium-dependent nucleobase transporter 1 (SNBT1) is a nucleobase-specific transporter identified in our recent study. In an attempt to search for its potential substrates other than nucleobases in this study, we could successfully find urate, a metabolic derivative of purine nucleobases, as a novel substrate, as indicated by its specific Na+ -dependent and saturable transport, with a Michaelis constant of 433 μmol/L, by rat SNBT1 (rSNBT1) stably expressed in Madin-Darby canine kidney II cells. However, urate uptake was observed only barely in the everted tissue sacs of the rat small intestine, in which rSNBT1 operates for nucleobase uptake. These findings suggested that urate undergoes a futile cycle, in which urate transported into epithelial cells is immediately effluxed back by urate efflux transporters, in the small intestine. In subsequent attempts to examine that possibility, such a futile urate cycle was demonstrated in the human embryonic kidney 293 cell line as a model cell system, where urate uptake induced by transiently introduced rSNBT1 was extensively reduced by the co-introduction of rat breast cancer resistance protein (rBCRP), a urate efflux transporter present in the small intestine. However, urate uptake was not raised in the presence of Ko143, a BCRP inhibitor, in the everted intestinal tissue sacs, suggesting that some other transporter might also be involved in urate efflux. The newly found urate transport function of SNBT1, together with the suggested futile urate cycle in the small intestine, should be of interest for its evolutional and biological implications, although SNBT1 is genetically deficient in humans.

Evolution of substrate specificity in the Nucleobase-Ascorbate Transporter (NAT) protein family

L-ascorbic acid (vitamin C) is an essential metabolite in animals and plants due to its role as an enzyme co-factor and antioxidant activity. In most eukaryotic organisms, L-ascorbate is biosynthesized enzymatically, but in several major groups, including the primate suborder Haplorhini, this ability is lost due to gene truncations in the gene coding for L-gulonolactone oxidase. Specific ascorbate transporters (SVCTs) have been characterized only in mammals and shown to be essential for life. These belong to an extensively studied transporter family, called Nucleobase-Ascorbate Transporters (NAT). The prototypic member of this family, and one of the most extensively studied eukaryotic transporters, is UapA, a uric acid-xanthine/H⁺ symporter in the fungus Aspergillus nidulans. Here, we investigate molecular aspects of NAT substrate specificity and address the evolution of ascorbate transporters apparently from ancestral nucleobase transporters. We present a phylogenetic analysis, identifying a distinct NAT clade that includes all known L-ascorbate transporters. This clade includes homologues only from vertebrates, and has no members in non-vertebrate or microbial eukaryotes, plants or prokaryotes. Additionally, we identify within the substrate-binding site of NATs a differentially conserved motif, which we propose is critical for nucleobase versus ascorbate recognition. This conclusion is supported by the amino acid composition of this motif in distinct phylogenetic clades and mutational analysis in the UapA transporter. Together with evidence obtained herein that UapA can recognize with extremely low affinity L-ascorbate, our results support that ascorbate-specific NATs evolved by optimization of a sub-function of ancestral nucleobase transporters.

The porcine translational research database: a manually curated, genomics and proteomics-based research resource

BACKGROUND

The use of swine in biomedical research has increased dramatically in the last decade. Diverse genomic- and proteomic databases have been developed to facilitate research using human and rodent models. Current porcine gene databases, however, lack the robust annotation to study pig models that are relevant to human studies and for comparative evaluation with rodent models. Furthermore, they contain a significant number of errors due to their primary reliance on machine-based annotation. To address these deficiencies, a comprehensive literature-based survey was conducted to identify certain selected genes that have demonstrated function in humans, mice or pigs.

RESULTS

The process identified 13,054 candidate human, bovine, mouse or rat genes/proteins used to select potential porcine homologs by searching multiple online sources of porcine gene information. The data in the Porcine Translational Research Database (( http://www.ars.usda.gov/Services/docs.htm?docid=6065 ) is supported by >5800 references, and contains 65 data fields for each entry, including >9700 full length (5' and 3') unambiguous pig sequences, >2400 real time PCR assays and reactivity information on >1700 antibodies. It also contains gene and/or protein expression data for >2200 genes and identifies and corrects 8187 errors (gene duplications artifacts, mis-assemblies, mis-annotations, and incorrect species assignments) for 5337 porcine genes.

CONCLUSIONS

This database is the largest manually curated database for any single veterinary species and is unique among porcine gene databases in regard to linking gene expression to gene function, identifying related gene pathways, and connecting data with other porcine gene databases. This database provides the first comprehensive description of three major Super-families or functionally related groups of proteins (Cluster of Differentiation (CD) Marker genes, Solute Carrier Superfamily, ATP binding Cassette Superfamily), and a comparative description of porcine microRNAs.

Vitamin C Transporters in Cancer: Current Understanding and Gaps in Knowledge

Sufficient uptake and whole body distribution of vitamin C (ascorbate) is essential for many biochemical processes, including some that are vital for tumor growth and spread. Uptake of ascorbate into cancer cells is modulated by availability, tumor blood flow, tissue diffusion parameters, and ascorbate transport proteins. Uptake into cells is mediated by two families of transport proteins, namely, the solute carrier gene family 23, consisting of sodium-dependent vitamin C transporters (SVCTs) 1 and 2, and the SLC2 family of glucose transporters (GLUTs). GLUTs transport the oxidized form of the vitamin, dehydroascorbate (DHA), which is present at negligible to low physiological levels. SVCT1 and 2 are capable of accumulating ascorbate against a concentration gradient from micromolar concentrations outside to millimolar levels inside of cells. Investigating the expression and regulation of SVCTs in cancer has only recently started to be included in studies focused on the role of ascorbate in tumor formation, progression, and response to therapy. This review gives an overview of the current, limited knowledge of ascorbate transport across membranes, as well as tissue distribution, gene expression, and the relevance of SVCTs in cancer. As tumor ascorbate accumulation may play a role in the anticancer activity of high dose ascorbate treatment, further research into ascorbate transport in cancer tissue is vital.

Functional characterization of two novel purine transporters from the Basidiomycota Phanerochaete chrysosporium

Purine transporters as substrate entry points in organisms, are involved in a number of cellular processes such as nitrogen source uptake, energy metabolism and synthesis of nucleic acids. In this study, two nucleobase transporter genes (phZ, phU) from Phanerochaete chrysosporium were cloned, identified, and functionally characterized. Our results show that PhZ is a transporter of adenine and hypoxanthine, and a protein belonging to the AzgA-like family, whilst PhU belongs to the NAT/NCS2 family, transporting xanthine and uric acid. No other sequences belonging to these families were detected in P. chrysosporium's genome. Phylogenetic analyses show that AzgA-like sequences form monophyletic groups for each major lineage (Ascomycota, Basidiomycota and Zygomycota). In contrast, Ascomycota and Basidiomycota NAT/NCS2 sequences do not form monophyletic groups and several copies of this protein are distributed across the tree. Expression of phU was significantly downregulated in the presence of a primary source like ammonium, and enhanced if purines were present or if the mycelium was nitrogen starved. phZ was clearly induced by its substrates (hypoxanthine, adenine), very lightly induced by xanthine, suppressed by urea and amino acids and expressed at a basal level when uric acid or ammonium was the nitrogen source or when the mycelium was starved for nitrogen. In order to perform substrate analyses, both P. chrysosporium proteins (PhZ, PhU) were expressed in Aspergillus nidulans. Epifluorescent microscopy showed that under inducing conditions, PhZ-GFP and PhU-GFP were present at the plasma membrane of A. nidulans transformed strains, and were internalized in repressed conditions. Our results suggest that in the white-rot fungus P. chrysosporium, phU has a catabolic role and phZ, (less dependent of the nitrogen source), plays a key role in purine acquisition to provide biosynthetic components. These are the first purine transporters characterized in Basidiomycota.

A Targeted Metabolomics Assay to Measure Eight Purines in the Diet of Common Bottlenose Dolphins, Tursiops truncatus

Bottlenose dolphins managed under human care, human beings and Dalmatian dogs are prone to forming urate uroliths. Limiting dietary purine intake limits urate urolith formation in people and dogs because purines are metabolized to uric acid, which is excreted in urine. Managed dolphins develop ammonium urate nephroliths, whereas free-ranging dolphins do not. Free-ranging dolphins consume live fish, whereas managed dolphins consume different species that have been stored frozen and thawed. Differences in the purine content of fish consumed by dolphins under human care versus in the wild may be responsible for the difference in urolith prevalence. Commercially available purine assays measure only four purines, but reported changes in purines during frozen storage suggest that a wider range of metabolites should be measured when comparing fresh and stored fish. A method using high performance liquid chromatography with tandem mass spectrometry was developed to quantify eight purine metabolites in whole fish and squid commonly consumed by dolphins. The coefficient of variation within and among days was sometimes high for purines present in small amounts but was acceptable (≤ 25%) for guanine, hypoxanthine, and inosine, which were present in high concentrations. This expanded assay identified a total purine content up to 2.5 times greater than the total that would be quantified if only four purines were measured. Assuming additional purines are absorbed, these results suggest that additional purine metabolites should be measured to better understand the associated risk when fish or other purine-rich foods are consumed by people or animals prone to developing uroliths.

A Targeted Metabolomics Assay to Measure Eight Purines in the Diet of Common Bottlenose Dolphins, Tursiops truncatus

Bottlenose dolphins managed under human care, human beings and Dalmatian dogs are prone to forming urate uroliths. Limiting dietary purine intake limits urate urolith formation in people and dogs because purines are metabolized to uric acid, which is excreted in urine. Managed dolphins develop ammonium urate nephroliths, whereas free-ranging dolphins do not. Free-ranging dolphins consume live fish, whereas managed dolphins consume different species that have been stored frozen and thawed. Differences in the purine content of fish consumed by dolphins under human care versus in the wild may be responsible for the difference in urolith prevalence. Commercially available purine assays measure only four purines, but reported changes in purines during frozen storage suggest that a wider range of metabolites should be measured when comparing fresh and stored fish. A method using high performance liquid chromatography with tandem mass spectrometry was developed to quantify eight purine metabolites in whole fish and squid commonly consumed by dolphins. The coefficient of variation within and among days was sometimes high for purines present in small amounts but was acceptable (≤ 25%) for guanine, hypoxanthine, and inosine, which were present in high concentrations. This expanded assay identified a total purine content up to 2.5 times greater than the total that would be quantified if only four purines were measured. Assuming additional purines are absorbed, these results suggest that additional purine metabolites should be measured to better understand the associated risk when fish or other purine-rich foods are consumed by people or animals prone to developing uroliths.

Structure of eukaryotic purine/H(+) symporter UapA suggests a role for homodimerization in transport activity

The uric acid/xanthine H⁺ symporter, UapA, is a high-affinity purine transporter from the filamentous fungus Aspergillus nidulans. Here we present the crystal structure of a genetically stabilized version of UapA (UapA-G411V_Δ1–11) in complex with xanthine. UapA is formed from two domains, a core domain and a gate domain, similar to the previously solved uracil transporter UraA, which belongs to the same family. The structure shows UapA in an inward-facing conformation with xanthine bound to residues in the core domain. Unlike UraA, which was observed to be a monomer, UapA forms a dimer in the crystals with dimer interactions formed exclusively through the gate domain. Analysis of dominant negative mutants is consistent with dimerization playing a key role in transport. We postulate that UapA uses an elevator transport mechanism likely to be shared with other structurally homologous transporters including anion exchangers and prestin.

UapA is a uric acid/xanthine H⁺ symporter from a filamentous fungus. Here, the authors solve the crystal structure of the transporter in complex with xanthine revealing it to be a dimer, and this homodimerisation is proposed to be important for function.

Functional identification of SLC43A3 as an equilibrative nucleobase transporter involved in purine salvage in mammals

The purine salvage pathway plays a major role in the nucleotide production, relying on the supply of nucleobases and nucleosides from extracellular sources. Although specific transporters have been suggested to be involved in facilitating their transport across the plasma membrane in mammals, those which are specifically responsible for utilization of extracellular nucleobases remain unknown. Here we present the molecular and functional characterization of SLC43A3, an orphan transporter belonging to an amino acid transporter family, as a purine-selective nucleobase transporter. SLC43A3 was highly expressed in the liver, where it was localized to the sinusoidal membrane of hepatocytes, and the lung. In addition, SLC43A3 expressed in MDCKII cells mediated the uptake of purine nucleobases such as adenine, guanine, and hypoxanthine without requiring typical driving ions such as Na(+) and H(+), but it did not mediate the uptake of nucleosides. When SLC43A3 was expressed in APRT/HPRT1-deficient A9 cells, adenine uptake was found to be low. However, it was markedly enhanced by the introduction of SLC43A3 with APRT. In HeLa cells, knock-down of SLC43A3 markedly decreased adenine uptake. These data suggest that SLC43A3 is a facilitative and purine-selective nucleobase transporter that mediates the cellular uptake of extracellular purine nucleobases in cooperation with salvage enzymes.

The nucleobase adenine as a signalling molecule in the kidney

In 2002, the first receptor activated by the nucleobase adenine was discovered in rats. In the past years, two adenine receptors (AdeRs) in mice and one in Chinese hamsters, all of which belong to the family of G protein-coupled receptors (GPCRs), were cloned and pharmacologically characterized. Based on the nomenclature for other purinergic receptor families (P1 for adenosine receptors and P2 for nucleotide, e.g. ATP, receptors), AdeRs were designated P0 receptors. Pharmacological data indicate the existence of G protein-coupled AdeRs in pigs and humans as well; however, those have not been cloned so far. Current data suggest a role for adenine and AdeRs in renal proximal tubules. Furthermore, AdeRs are suggested to be functional counterplayers of vasopressin in the collecting duct system, thus exerting diuretic effects. We are only at the beginning of understanding the significance of this new class of purinergic receptors, which might become future drug targets.

Nucleobase and nucleoside transport and integration into plant metabolism

Nucleotide metabolism is an essential process in all living organisms. Besides newly synthesized nucleotides, the recycling (salvage) of partially degraded nucleotides, i.e., nucleosides and nucleobases serves to keep the homeostasis of the nucleotide pool. Both types of metabolites are substrates of at least six families of transport proteins in Arabidopsis thaliana (Arabidopsis) with a total of 49 members. In the last years several members of such transport proteins have been analyzed allowing to present a more detailed picture of nucleoside and nucleobase transport and the physiological function of these processes. Besides functioning in nucleotide metabolism it turned out that individual members of the before named transporters exhibit the capacity to transport a wide range of different substrates including vitamins and phytohormones. The aim of this review is to summarize the current knowledge on nucleobase and nucleoside transport processes in plants and integrate this into nucleotide metabolism in general. Thereby, we will focus on those proteins which have been characterized at the biochemical level.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Functional identification of the hypoxanthine/guanine transporters YjcD and YgfQ and the adenine transporters PurP and YicO of Escherichia coli K-12

The evolutionarily broad family nucleobase-cation symporter-2 (NCS2) encompasses transporters that are conserved in binding site architecture but diverse in substrate selectivity. Putative purine transporters of this family fall into one of two homology clusters: COG2233, represented by well studied xanthine and/or uric acid permeases, and COG2252, consisting of transporters for adenine, guanine, and/or hypoxanthine that remain unknown with respect to structure-function relationships. We analyzed the COG2252 genes of Escherichia coli K-12 with homology modeling, functional overexpression, and mutagenesis and showed that they encode high affinity permeases for the uptake of adenine (PurP and YicO) or guanine and hypoxanthine (YjcD and YgfQ). The two pairs of paralogs differ clearly in their substrate and ligand preferences. Of 25 putative inhibitors tested, PurP and YicO recognize with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purine, whereas YjcD and YgfQ recognize 1-methylguanine, 8-azaguanine, 6-thioguanine, and 6-mercaptopurine and do not recognize any of the PurP ligands. Furthermore, the permeases PurP and YjcD were subjected to site-directed mutagenesis at highly conserved sites of transmembrane segments 1, 3, 8, 9, and 10, which have been studied also in COG2233 homologs. Residues irreplaceable for uptake activity or crucial for substrate selectivity were found at positions occupied by similar role amino acids in the Escherichia coli xanthine- and uric acid-transporting homologs (XanQ and UacT, respectively) and predicted to be at or around the binding site. Our results support the contention that the distantly related transporters of COG2233 and COG2252 use topologically similar side chain determinants to dictate their function and the distinct purine selectivity profiles.

The N-terminal basolateral targeting signal unlikely acts alone in the differential trafficking of membrane transporters in MDCK cells

We have shown previously, using confocal imaging and transport assays, that the N-terminus of sodium-dependent vitamin C transporter 2 (SVCT2) can redirect apical SVCT1 to the basolateral membrane. Here, the SVCT model was used to further characterize the basolateral targeting peptide signal. Both the length (31 amino acids) and sequence accuracy of the N-terminus of SVCT2 were found to be important in basolateral targeting activity, suggesting a structural requirement. However, the N-terminal basolateral targeting sequence did not appear to act alone, based on analyses of heterologous chimeras. Although diverse N-terminal basolateral targeting signals from multipass membrane proteins can all redirect apical protein from the same gene family to the basolateral membrane, none of the N-terminal basolateral targeting signals can redirect the transmembrane and C-terminal regions from a different gene family. Instead, the presence of these heterologous N-terminal basolateral targeting signals affected the trafficking of otherwise apical protein, causing their accumulation in a stable tubulin-like non-actin structure. Nontargeting N-terminal sequences had no effect. Similar protein retention was observed previously and in this study when the C-terminus of apical or basolateral protein was mutated. These results suggest that the N- and C-termini interact, directly or indirectly, within each gene family for basolateral targeting. Circular dichroism and two-dimensional nuclear magnetic resonance analyses both found a lack of regular secondary structure in the conserved N-terminus of SVCT2, consistent with the presence of partner(s) in the targeting unit. Our finding, a departure from the prevailing single-peptide motif model, is consistent with the evolution of basolateral transporters from the corresponding apical genes. The interaction among the N-terminus, its partner(s), and the cellular basolateral targeting machinery needs to be further elucidated.

Gene expression analysis of zebrafish melanocytes, iridophores, and retinal pigmented epithelium reveals indicators of biological function and developmental origin

In order to facilitate understanding of pigment cell biology, we developed a method to concomitantly purify melanocytes, iridophores, and retinal pigmented epithelium from zebrafish, and analyzed their transcriptomes. Comparing expression data from these cell types and whole embryos allowed us to reveal gene expression co-enrichment in melanocytes and retinal pigmented epithelium, as well as in melanocytes and iridophores. We found 214 genes co-enriched in melanocytes and retinal pigmented epithelium, indicating the shared functions of melanin-producing cells. We found 62 genes significantly co-enriched in melanocytes and iridophores, illustrative of their shared developmental origins from the neural crest. This is also the first analysis of the iridophore transcriptome. Gene expression analysis for iridophores revealed extensive enrichment of specific enzymes to coordinate production of their guanine-based reflective pigment. We speculate the coordinated upregulation of specific enzymes from several metabolic pathways recycles the rate-limiting substrate for purine synthesis, phosphoribosyl pyrophosphate, thus constituting a guanine cycle. The purification procedure and expression analysis described here, along with the accompanying transcriptome-wide expression data, provide the first mRNA sequencing data for multiple purified zebrafish pigment cell types, and will be a useful resource for further studies of pigment cell biology.