Molecular cloning, chromosomal organization, and functional characterization of a sodium-dicarboxylate cotransporter from mouse kidney

Cryo-EM structures of the human NaS1 and NaDC1 transporters revealed the elevator transport and allosteric regulation mechanism

The solute carrier 13 (SLC13) family comprises electrogenic sodium ion-coupled anion cotransporters, segregating into sodium ion-sulfate cotransporters (NaSs) and sodium ion-di- and-tricarboxylate cotransporters (NaDCs). NaS1 and NaDC1 regulate sulfate homeostasis and oxidative metabolism, respectively. NaS1 deficiency affects murine growth and fertility, while NaDC1 affects urinary citrate and calcium nephrolithiasis. Despite their importance, the mechanisms of substrate recognition and transport remain insufficiently characterized. In this study, we determined the cryo-electron microscopy structures of human NaS1, capturing inward-facing and combined inward-facing/outward-facing conformations within a dimer both in apo and sulfate-bound states. In addition, we elucidated NaDC1's outward-facing conformation, encompassing apo, citrate-bound, and N-(p-amylcinnamoyl) anthranilic acid (ACA) inhibitor-bound states. Structural scrutiny illuminates a detailed elevator mechanism driving conformational changes. Notably, the ACA inhibitor unexpectedly binds primarily anchored by transmembrane 2 (TM2), Loop 10, TM11, and TM6a proximate to the cytosolic membrane. Our findings provide crucial insights into SLC13 transport mechanisms, paving the way for future drug design.

Breaking the Cycle of Recurrent Calcium Stone Disease

Calcium stones are common and recurrent in nature, yet few therapeutic tools are available for secondary prevention. Personalized approaches for stone prevention have been informed by 24-hour urine testing to guide dietary and medical interventions. However, current evidence is conflicting about whether an approach guided by 24-hour urine testing is more effective than a generic one. The available medications for stone prevention, namely thiazide diuretics, alkali, and allopurinol, are not always prescribed consistently, dosed correctly, or tolerated well by patients. New treatments on the horizon hold the promise of preventing calcium oxalate stones by degrading oxalate in the gut, reprogramming the gut microbiome to reduce oxalate absorption, or knocking down expression of enzymes involved in hepatic oxalate production. New treatments are also needed to target Randall's plaque, the root cause of calcium stone formation.

Role of plasma membrane dicarboxylate transporters in the uptake and toxicity of diglycolic acid, a metabolite of diethylene glycol, in human proximal tubule cells

Diethylene glycol (DEG) mass poisonings have resulted from ingestion of pharmaceuticals mistakenly adulterated with DEG, typically leading to proximal tubular necrosis and acute kidney injury. The metabolite, diglycolic acid (DGA) accumulates greatly in kidney tissue and its direct administration results in toxicity identical to that in DEG-treated rats. DGA is a dicarboxylic acid, similar in structure to metabolites like succinate. These studies have assessed the mechanism for cellular accumulation of DGA, specifically whether DGA is taken into primary cultures of human proximal tubule (HPT) cells via sodium dicarboxylate transporters (NaDC-1 or NaDC-3) like those responsible for succinate uptake. When HPT cells were cultured on membrane inserts, sodium dependent succinate uptake was observed from both apical and basolateral directions. Pretreatment with the NaDC-1 inhibitor N-(p-amylcinnamoyl)anthranilic acid (ACA) markedly reduced apical uptakes of both succinate and DGA. Basolateral uptake of both succinate and DGA were decreased similarly following combined treatment with ACA and the NaDC-3 inhibitor 2,3-dimethylsuccinate. When the cells were pre-treated with siRNA to knockdown NaDC-1 function, apical uptake of succinate and toxicity of apically applied DGA were reduced, while the reduction in basolateral succinate uptake and basolateral DGA toxicity was marginal with NaDC-3 knockdown. DGA reduced apical uptake of succinate, but not basolateral uptake. This study confirmed that primary HPT cells retain sodium dicarboxylate transport functionality and that DGA was taken up by these transporters. This study identified NaDC-1 as a likely and NaDC-3 as a possible molecular target to reduce uptake of this toxic metabolite by the kidney.

SLC26A6 and NADC‑1: Future direction of nephrolithiasis and calculus‑related hypertension research (Review)

Nephrolithiasis is the most common type of urinary system disease in developed countries, with high morbidity and recurrence rates. Nephrolithiasis is a serious health problem, which eventually leads to the loss of renal function and is closely related to hypertension. Modern medicine has adopted minimally invasive surgery for the management of kidney stones, but this does not resolve the root of the problem. Thus, nephrolithiasis remains a major public health issue, the causes of which remain largely unknown. Researchers have attempted to determine the causes and therapeutic targets of kidney stones and calculus‑related hypertension. Solute carrier family 26 member 6 (SLC26A6), a member of the well‑conserved solute carrier family 26, is highly expressed in the kidney and intestines, and it primarily mediates the transport of various anions, including OXa^2‑, HCO₃^‑, Cl^‑ and SO₄^2‑, amongst others. Na⁺‑dependent dicarboxylate‑1 (NADC‑1) is the Na⁺‑carboxylate co‑transporter of the SLC13 gene family, which primarily mediates the co‑transport of Na⁺ and tricarboxylic acid cycle intermediates, such as citrate and succinate, amongst others. Studies have shown that Ca²⁺ oxalate kidney stones are the most prevalent type of kidney stones. Hyperoxaluria and hypocitraturia notably increase the risk of forming Ca²⁺ oxalate kidney stones, and the increase in succinate in the juxtaglomerular device can stimulate renin secretion and lead to hypertension. Whilst it is known that it is important to maintain the dynamic equilibrium of oxalate and citrate in the kidney, the synergistic molecular mechanisms underlying the transport of oxalate and citrate across kidney epithelial cells have undergone limited investigations. The present review examines the results from early reports studying oxalate transport and citrate transport in the kidney, describing the synergistic molecular mechanisms of SLC26A6 and NADC‑1 in the process of nephrolithiasis formation. A growing body of research has shown that nephrolithiasis is intricately associated with hypertension. Additionally, the recent investigations into the mediation of succinate via regulation of the synergistic molecular mechanism between the SLC26A6 and NADC‑1 transporters is summarized, revealing their functional role and their close association with the inositol triphosphate receptor‑binding protein to regulate blood pressure.

Expression of sodium-dependent dicarboxylate transporter 1 (NaDC1/SLC13A2) in normal and neoplastic human kidney

Regulated dicarboxylate transport is critical for acid-base homeostasis, prevention of calcium nephrolithiasis, regulation of collecting duct sodium chloride transport, and the regulation of blood pressure. Although luminal dicarboxylate reabsorption via NaDC1 (SLC13A2) is believed to be the primary mechanism regulating renal dicarboxylate transport, the specific localization of NaDC1 in the human kidney is currently unknown. This study's purpose was to determine NaDC1's expression in normal and neoplastic human kidneys. Immunoblot analysis demonstrated NaDC1 expression with an apparent molecular weight of ~61 kDa. Immunohistochemistry showed apical NaDC1 immunolabel in the proximal tubule of normal human kidney tissue; well-preserved proximal tubule brush border was clearly labeled. Apical NaDC1 expression was evident throughout the entire proximal tubule, including the initial proximal convoluted tubule, as identified by origination from the glomerular tuft, and extending through the terminal of the proximal tubule, the proximal straight tubule in the outer medulla. We confirmed proximal tubule localization by colocalization with the proximal tubule specific protein, NBCe1. NaDC1 immunolabel was not detected other than in the proximal tubule. In addition, NaDC1 immunolabel was not detected in tumors of presumed proximal tubule origin, clear cell and papillary renal cell carcinoma, or in tumors of nonproximal tubule origin, oncocytoma and chromophobe carcinoma. In summary, 1) in the human kidney, apical NaDC1 immunolabel is present throughout the entire proximal tubule, and is not detectable in other renal cells; and 2) NaDC1 immunolabel is not present in renal tumors. These studies provide important information regarding NaDC1's role in human dicarboxylate metabolism.

Structure-Based Identification of Inhibitors for the SLC13 Family of Na(+)/Dicarboxylate Cotransporters

In mammals, citric acid cycle intermediates play a key role in regulating various metabolic processes, such as fatty acid synthesis and glycolysis. Members of the sodium-dependent SLC13 transporter family mediate the transport of di- and tricarboxylates into cells. SLC13 family members have been implicated in lifespan extension and resistance to high-fat diets; thus, they are emerging drug targets for aging and metabolic disorders. We previously characterized key structural determinants of substrate and cation binding for the human NaDC3/SLC13A3 transporter using a homology model. Here, we combine computational modeling and virtual screening with functional and biochemical testing, to identify nine previously unknown inhibitors for multiple members of the SLC13 family from human and mouse. Our results reveal previously unknown substrate selectivity determinants for the SLC13 family, including key residues that mediate ligand binding and transport, as well as promiscuous and specific SLC13 small molecule ligands. The newly discovered ligands can serve as chemical tools for further characterization of the SLC13 family or as lead molecules for the future development of potent inhibitors for the treatment of metabolic diseases and aging. Our results improve our understanding of the structural components that are important for substrate specificity in this physiologically important family as well as in other structurally related transport systems.

Effects of chronic lithium administration on renal acid excretion in humans and rats

Lithium therapy's most common side effects affecting the kidney are nephrogenic diabetes insipidus (NDI) and chronic kidney disease. Lithium may also induce a distal renal tubular acidosis. This study investigated the effect of chronic lithium exposure on renal acid–base homeostasis, with emphasis on ammonia and citrate excretion. We compared 11 individuals on long‐term lithium therapy with six healthy individuals. Under basal conditions, lithium‐treated individuals excreted significantly more urinary ammonia than did control subjects. Following an acute acid load, urinary ammonia excretion increased approximately twofold above basal rates in both lithium‐treated and control humans. There were no significant differences between lithium‐treated and control subjects in urinary pH or urinary citrate excretion. To elucidate possible mechanisms, rats were randomized to diets containing lithium or regular diet for 6 months. Similar to humans, basal ammonia excretion was significantly higher in lithium‐treated rats; in addition, urinary citrate excretion was also significantly greater. There were no differences in urinary pH. Expression of the critical ammonia transporter, Rhesus C Glycoprotein (Rhcg), was substantially greater in lithium‐treated rats than in control rats. We conclude that chronic lithium exposure increases renal ammonia excretion through mechanisms independent of urinary pH and likely to involve increased collecting duct ammonia secretion via the ammonia transporter, Rhcg.

This study investigated the effect of chronic lithium exposure on renal acid–base homeostasis, with emphasis on ammonia and citrate excretion. Chronic lithium exposure increases renal ammonia excretion through mechanisms independent of urinary pH and likely to involve increased collecting duct ammonia secretion via the ammonia transporter, Rhcg.

Functional characterization of a Na+-dependent dicarboxylate transporter from Vibrio cholerae

VcINDY, a bacterial homolog of transporters implicated in lifespan in fruit flies and insulin resistance in mammals, is a high affinity, electrogenic, Na⁺-dependent dicarboxylate transporter.

The SLC13 transporter family, whose members play key physiological roles in the regulation of fatty acid synthesis, adiposity, insulin resistance, and other processes, catalyzes the transport of Krebs cycle intermediates and sulfate across the plasma membrane of mammalian cells. SLC13 transporters are part of the divalent anion:Na⁺ symporter (DASS) family that includes several well-characterized bacterial members. Despite sharing significant sequence similarity, the functional characteristics of DASS family members differ with regard to their substrate and coupling ion dependence. The publication of a high resolution structure of dimer VcINDY, a bacterial DASS family member, provides crucial structural insight into this transporter family. However, marrying this structural insight to the current functional understanding of this family also demands a comprehensive analysis of the transporter’s functional properties. To this end, we purified VcINDY, reconstituted it into liposomes, and determined its basic functional characteristics. Our data demonstrate that VcINDY is a high affinity, Na⁺-dependent transporter with a preference for C₄- and C₅-dicarboxylates. Transport of the model substrate, succinate, is highly pH dependent, consistent with VcINDY strongly preferring the substrate’s dianionic form. VcINDY transport is electrogenic with succinate coupled to the transport of three or more Na⁺ ions. In contrast to succinate, citrate, bound in the VcINDY crystal structure (in an inward-facing conformation), seems to interact only weakly with the transporter in vitro. These transport properties together provide a functional framework for future experimental and computational examinations of the VcINDY transport mechanism.

Sodium-sulfate/carboxylate cotransporters (SLC13)

The SLC13 gene family is comprised of five sequence related proteins that are found in animals, plants, yeast and bacteria. Proteins encoded by the SLC13 genes are divided into the following two groups of transporters with distinct anion specificities: the Na(+)-sulfate (NaS) cotransporters and the Na(+)-carboxylate (NaC) cotransporters. Members of this gene family (in ascending order) are: SLC13A1 (NaS1), SLC13A2 (NaC1), SLC13A3 (NaC3), SLC13A4 (NaS2) and SLC13A5 (NaC2). SLC13 proteins encode plasma membrane polypeptides with 8-13 putative transmembrane domains, and are expressed in a variety of tissues. They are all Na(+)-coupled symporters with strong cation preference for Na(+), and insensitive to the stilbene 4, 4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS). Their Na(+):anion coupling ratio is 3:1, indicative of electrogenic properties. They have a substrate preference for divalent anions, which include tetra-oxyanions for the NaS cotransporters or Krebs cycle intermediates (including mono-, di- and tricarboxylates) for the NaC cotransporters. This review will describe the molecular and cellular mechanisms underlying the biochemical, physiological and structural properties of the SLC13 gene family.

Glucose is a pH-dependent motor for sperm beat frequency during early activation

To reach the egg in the ampulla, sperm have to travel along the female genital tract, thereby being dependent on external energy sources and substances to maintain and raise the flagellar beat. The vaginal fluid is rich in lactate, whereas in the uterine fluid glucose is the predominant substrate. This evokes changes in the lactate content of sperm as well as in the intracellular pH (pH(i)) since sperm possess lactate/proton co-transporters. It is well documented that glycolysis yields ATP and that HCO(3)- is a potent factor in the increase of beat frequency. We here show for the first time a pathway that connects both parts. We demonstrate a doubling of beat frequency in the mere presence of glucose. This effect can reversibly be blocked by 2-deoxy-D-glucose, dichloroacetate and aminooxyacetate, strongly suggesting that it requires both glycolysis and mitochondrial oxidation of glycolytic end products. We show that the glucose-mediated acceleration of flagellar beat and ATP production are hastened by a pH(i) ≥7.1, whereas a pH(i) ≤7.1 leaves both parameters unchanged. Since we observed a diminished rise in beat frequency in the presence of specific inhibitors against carbonic anhydrases, soluble adenylyl cyclase and protein kinase, we suggest that the glucose-mediated effect is linked to CO(2) hydration and thus the production of HCO(3)- by intracellular CA isoforms. In summary, we propose that, in sperm, glycolysis is an additional pH(i)-dependent way to produce HCO(3)-, thus enhancing sperm beat frequency and contributing to fertility.

Identification of a gene encoding a transporter essential for utilization of C4 dicarboxylates in Corynebacterium glutamicum

The Corynebacterium glutamicum R genome contains a total of eight genes encoding proteins with sequence similarity to C4-dicarboxylate transporters identified from other bacteria. Three of the genes encode proteins within the dicarboxylate/amino acid:cation symporter (DAACS) family, another three encode proteins within the tripartite ATP-independent periplasmic transporter family, and two encode proteins within the divalent anion:Na+ symporter (DASS) family. We observed that a mutant strain deficient in one of these genes, designated dcsT, of the DASS family did not aerobically grow on the C4 dicarboxylates succinate, fumarate, and malate as the sole carbon sources. Mutant strains deficient in each of the other seven genes grew as well as the wild-type strain under the same conditions, although one of these genes is a homologue of dctA of the DAACS family, involved in aerobic growth on C4 dicarboxylates in various bacteria. The utilization of C4 dicarboxylates was markedly enhanced by overexpression of the dcsT gene. We confirmed that the uptake of [13C]labeled succinate observed for the wild-type cells was hardly detected in the dcsT-deficient mutant but was markedly enhanced in a dcsT-overexpressing strain. These results suggested that in C. glutamicum, the uptake of C4 dicarboxylates for aerobic growth was mainly mediated by the DASS transporter encoded by dcsT. The expression level of the dcsT gene transiently increased in the early exponential phase during growth on nutrient-rich medium. This expression was enhanced by the addition of succinate in the mid-exponential phase and was repressed by the addition of glucose in the early exponential phase.

Generation and characterization of sodium-dicarboxylate cotransporter-deficient mice

The sodium-dependent dicarboxylate cotransporter (NaDC1) has a proposed function of reabsorbing various Krebs cycle intermediates in the kidney and the small intestine. Since Krebs cycle intermediates have been suggested to be important for renal cell survival and recovery after hypoxia and reoxygenation, the transporter may play a role in the recovery of the kidney. Additionally, mutations in the transporter homolog in Drosophila led to fly longevity which was thought to be similar to that induced by caloric restriction (CR). To clarify the role of the sodium dicarboxylate cotransporter in vivo we generated cotransporter-deficient mice. These knockout mice excreted significantly higher amounts of various Krebs cycle intermediates in their urine; thus confirming the proposed function to reabsorb these metabolic intermediates in the kidney. No other phenotypic change was identified in these mice, however. Transporter deficiency did not affect renal function under normal physiological conditions, nor did it have an effect on renal damage and recovery from ischemic injury. Additionally, the absence of the transporter did not lead to metabolic or physiological changes associated with CR. Our results suggest that although the sodium dicarboxylate cotransporter is involved in regulating levels of various Krebs cycle intermediates in the kidney, impaired uptake of these intermediates does not significantly affect renal function under normal or ischemic stress.

The life-extending gene Indy encodes an exchanger for Krebs-cycle intermediates

A longevity gene called Indy (for 'I'm not dead yet'), with similarity to mammalian genes encoding sodium-dicarboxylate cotransporters, was identified in Drosophila melanogaster. Functional studies in Xenopus oocytes showed that INDY mediates the flux of dicarboxylates and citrate across the plasma membrane, but the specific transport mechanism mediated by INDY was not identified. To test whether INDY functions as an anion exchanger, we examined whether substrate efflux is stimulated by transportable substrates added to the external medium. Efflux of [14C]citrate from INDY-expressing oocytes was greatly accelerated by the addition of succinate to the external medium, indicating citrate-succinate exchange. The succinate-stimulated [14C]citrate efflux was sensitive to inhibition by DIDS (4,4'-di-isothiocyano-2,2'-disulphonic stilbene), as demonstrated previously for INDY-mediated succinate uptake. INDY-mediated efflux of [14C]citrate was also stimulated by external citrate and oxaloacetate, indicating citrate-citrate and citrate-oxaloacetate exchange. Similarly, efflux of [14C]succinate from INDY-expressing oocytes was stimulated by external citrate, alpha-oxoglutarate and fumarate, indicating succinate-citrate, succinate-alpha-oxoglutarate and succinate-fumarate exchange respectively. Conversely, when INDY-expressing Xenopus oocytes were loaded with succinate and citrate, [14C]succinate uptake was markedly stimulated, confirming succinate-succinate and succinate-citrate exchange. Exchange of internal anion for external citrate was markedly pH(o)-dependent, consistent with the concept that citrate is co-transported with a proton. Anion exchange was sodium-independent. We conclude that INDY functions as an exchanger of dicarboxylate and tricarboxylate Krebs-cycle intermediates. The effect of decreasing INDY activity, as in the long-lived Indy mutants, may be to alter energy metabolism in a manner that favours lifespan extension.

Ala-504 is a determinant of substrate binding affinity in the mouse Na(+)/dicarboxylate cotransporter

The Na(+)/dicarboxylate cotransporters from mouse (mNaDC1) and rabbit (rbNaDC1) differ in their ability to handle adipate, a six-carbon terminal dicarboxylic acid. The mNaDC1 and rbNaDC1 amino acid sequences are 75% identical. The rbNaDC1 does not transport adipate and only succinate produced inward currents under two-electrode voltage clamp. In contrast, oocytes expressing mNaDC1 had adipate-dependent inward currents that were about 60% of those induced by succinate. In order to identify domains involved in adipate transport, we examined the functional properties of a series of chimeric transporters made between mouse and rabbit NaDC1. We find that multiple transmembrane helices (TM), particularly TM 8, 9, and 10, are involved in adipate transport. In TM 10 there is only one amino acid difference between the two proteins, corresponding to Ala-504 in mouse and Ser-512 in rabbit NaDC1. The mNaDC1-A504S mutant had decreased adipate-dependent currents relative to succinate-dependent currents and an increase in the K(0.5) for both succinate and glutarate. We conclude that multiple amino acids from TM 8, 9 and 10 contribute to the transport of adipate in NaDC1. Furthermore, Ala-504 in TM 10 is an important determinant of K(0.5) for both adipate and succinate.

Role of conserved prolines in the structure and function of the Na+/dicarboxylate cotransporter 1, NaDC1

The Na+/dicarboxylate cotransporter 1 (NaDC1) is a low-affinity transporter for citric acid cycle intermediates such as succinate and citrate. The sequence of NaDC1 contains a number of conserved proline residues in predicted transmembrane helices (TMs) 7 and 10. These transmembrane domains are of particular importance because they may be involved in determining the substrate or cation-binding affinity in NaDC1. Four conserved proline residues in TMs 7 and 10 of rabbit NaDC1 were replaced with alanine to promote ideal alpha helix or glycine to promote free conformation, and the mutant transporters were expressed in the HRPE cell line. Mutations of prolines in TM 10 produced decreased protein expression and activity, whereas mutations of prolines in TM 7 completely abolished protein expression and activity. The chemical chaperone glycerol was found to improve the expression of the Pro-351 mutants in TM 7, suggesting that these mutants had defects in trafficking. The inactive mutant transporters at position 351 could also be rescued by the addition of a proline at a second site. For example, the P351A-F347P mutant had restored activity, although its substrate specificity was altered. We conclude that, in TM 7, Pro-327 may be of particular importance in the function of the transporter, whereas Pro-351 may affect protein targeting. The prolines in TM 10, at positions 523 and 524, may not be directly involved in the transporter function but may be necessary for maintaining structure.

Transmembrane helices 3 and 4 are involved in substrate recognition by the Na+/dicarboxylate cotransporter, NaDC1

The Na(+)/dicarboxylate cotransporters (NaDC1) from mouse (m) and rabbit (rb) differ in their ability to handle glutarate. Substrate-dependent inward currents, measured using two-electrode voltage clamp, were similar for glutarate and succinate in Xenopus oocytes expressing mNaDC1. In contrast, currents evoked by glutarate in rbNaDC1 were only about 5% of the succinate-dependent currents. To identify domains involved in glutarate transport, we constructed a series of chimeric transporters between mouse and rabbit NaDC1. Although residues found in multiple transmembrane helices (TM) participate in glutarate transport, the most important contribution is made by TM 3 and 4 and the associated loops. The R(M3-4) chimera, consisting of rbNaDC1 with substitution of TM 3-4 from mNaDC1, had a decreased K(0.5)(glutarate) of 4 mM compared with 15 mM in wild-type rbNaDC1 without any effect on K(0.5)(succinate). The chimeras were also characterized using dual-label competitive uptakes with (14)C-glutarate and (3)H-succinate to calculate the transport specificity ratio (TSR), a measure of relative catalytic efficiency with the two substrates. The TSR analysis provides evidence for functional coupling in the transition state between TM 3 and 4. We conclude that TM 3 and 4 contain amino acid residues that are important determinants of substrate specificity and catalytic efficiency in NaDC1.

Molecular properties of the SLC13 family of dicarboxylate and sulfate transporters

The SLC13 gene family consists of five members in humans, with corresponding orthologs from different vertebrate species. All five genes code for sodium-coupled transporters that are found on the plasma membrane. Two of the transporters, NaS1 and NaS2, carry substrates such as sulfate, selenate and thiosulfate. The other members of the family (NaDC1, NaDC3, and NaCT) are transporters for di- and tri-carboxylates including succinate, citrate and alpha-ketoglutarate. The SLC13 transporters from vertebrates are electrogenic and they produce inward currents in the presence of sodium and substrate. Substrate-independent leak currents have also been described. Structure-function studies have identified the carboxy terminal half of these proteins as the most important for determining function. Transmembrane helices 9 and 10 may form part of the substrate permeation pathway and participate in conformational changes during the transport cycle. This review also discusses new members of the SLC13 superfamily that exhibit both sodium-dependent and sodium-independent transport mechanisms. The Indy protein from Drosophila, involved in determining lifespan, and the plant vacuolar malate transporter are both sodium-independent dicarboxylate transporters, possibly acting as exchangers. The purpose of this review is to provide an update on new advances in this gene family, particularly on structure-function studies and new members of the family.

Transport characteristics of N ‐acetyl‐ l ‐aspartate in rat astrocytes: involvement of sodium‐coupled high‐affinity carboxylate transporter NaC3/NaDC3‐mediated transport system

We investigated in the present study the transport characteristics of N-acetyl-L-aspartate in primary cultures of astrocytes from rat cerebral cortex and the involvement of NA+-coupled high-affinity carboxylate transporter NaC3 (formerly known as NaDC3) responsible for N-acetyl-L-aspartate transport. N-acetyl-L-aspartate transport was NA+-dependent and saturable with a Michaelis-Menten constant (Km) of approximately 110 microm. NA+-activation kinetics revealed that the NA+ to-N-acetyl-L-aspartate stoichiometry was 3 : 1 and concentration of Na+ necessary for half-maximal transport (KNA m) was 70 mm. NA+-dependent N-acetyl-L-aspartate transport was competitively inhibited by succinate with an inhibitory constant (Ki) of 14.7 microm, which was comparable to the Km value of NA+-dependent succinate transport (29.4 microm). L-aspartate also inhibited NA+-dependent [14C]N-acetyl-L-aspartate transport with relatively low affinity (Ki = 2.2 mm), whereas N-acetyl-L-aspartate was not able to inhibit NA+-dependent aspartate transport in astrocytes. In addition, Li+ was found to have a significant inhibitory effect on the NA+-dependent N-acetyl-L-aspartate transport in a concentration-dependent manner. Furthermore, RT-PCR and western blot analyses revealed that NaC3 is expressed in primary cultures of astrocytes. Taken collectively, these results indicate that NaC3 expressed in rat cerebrocortical astrocytes is responsible for NA+-dependent N-acetyl-L-aspartate transport. This transporter is likely to be an essential prerequisite for the metabolic role of N-acetyl-L-aspartate in the process of myelination.

Relevance of NAC-2, an Na+-coupled citrate transporter, to life span, body size and fat content in Caenorhabditis elegans

We have cloned and functionally characterized an Na+-coupled citrate transporter from Caenorhabditis elegans (ceNAC-2). This transporter shows significant sequence homology to Drosophila Indy and the mammalian Na+-coupled citrate transporter NaCT (now known as NaC2). When heterologously expressed in a mammalian cell line or in Xenopus oocytes, the cloned ceNAC-2 mediates the Na+-coupled transport of various intermediates of the citric acid cycle. However, it transports the tricarboxylate citrate more efficiently than dicarboxylates such as succinate, a feature different from that of ceNAC-1 (formerly known as ceNaDC1) and ceNAC-3 (formerly known as ceNaDC2). The transport process is electrogenic, as evidenced from the substrate-induced inward currents in oocytes expressing the transporter under voltage-clamp conditions. Expression studies using a reporter-gene fusion method in transgenic C. elegans show that the gene is expressed in the intestinal tract, the organ responsible for not only the digestion and absorption of nutrients but also for the storage of energy in this organism. Functional knockdown of the transporter by RNAi (RNA interference) not only leads to a significant increase in life span, but also causes a significant decrease in body size and fat content. The substrates of ceNAC-2 play a critical role in metabolic energy production and in the biosynthesis of cholesterol and fatty acids. The present studies suggest that the knockdown of these metabolic functions by RNAi is linked to an extension of life span and a decrease in fat content and body size.

Functional features and genomic organization of mouse NaCT, a sodium-coupled transporter for tricarboxylic acid cycle intermediates

In the present study, we report on the molecular cloning and functional characterization of mouse NaCT (Na+-coupled citrate transporter), the mouse orthologue of Drosophila Indy. Mouse NaCT consists of 572 amino acids and is highly similar to rat and human NaCTs in primary sequence. The mouse nact gene coding for the transporter is approx. 23 kb long and consists of 12 exons. When expressed in mammalian cells, the cloned transporter mediates the Na+-coupled transport of citrate and succinate. Competition experiments reveal that mouse NaCT also recognizes other tricarboxylic acid cycle intermediates such as malate, fumarate and 2-oxo-glutarate as excellent substrates. The Michaelis-Menten constant for the transport process is 38+/-5 mM for citrate and 37+/-6 mM for succinate at pH 7.5. The transport process is electrogenic and exhibits an obligatory requirement for Na+. Na+-activation kinetics indicates that multiple Na+ ions are involved in the activation process. Extracellular pH has a differential effect on the transport function of mouse NaCT depending on whether the transported substrate is citrate or succinate. The Michaelis-Menten constants for these substrates are also influenced markedly by pH. When examined in the Xenopus laevis oocyte expression system with the two-microelectrode voltage-clamp technique, the transport process mediated by mouse NaCT is electrogenic. The charge-to-substrate ratio is 1 for citrate and 2 for succinate. The most probable transport mechanism predicted by these studies involves the transport of citrate as a tervalent anion and succinate as a bivalent anion with a fixed Na+/substrate stoichiometry of 4:1. The present study provides the first unequivocal evidence for the electrogenic nature of mammalian NaCT.

OKP cells express the Na-dicarboxylate cotransporter NaDC-1

Urinary citrate concentration, a major factor in the formation of kidney stones, is primarily determined by its rate of reabsorption in the proximal tubule. Citrate reabsorption is mediated by the Na-dicarboxylate cotransporter-1 (NaDC-1). The opossum kidney (OKP) cell line possesses many characteristics of the renal proximal tubule. The OKP NaDC-1 (oNaDC-1) cDNA was cloned and encodes a 2.4-kb mRNA. When injected into Xenopus oocytes, the cotransporter is expressed and demonstrates Na-coupled citrate transport with a stoichiometry of >or=3 Na:1 citrate, specificity for di- and tricarboxylates, pH-dependent citrate transport, and pH-independent succinate transport, all characteristics of the other NaDC-1 orthologs. Chronic metabolic acidosis increases proximal tubule citrate reabsorption, leading to profound hypocitraturia and an increased risk for stone formation. Under the conditions studied, endogenous OKP NaDC-1 mRNA abundance is not regulated by changes in media pH. In OKP cells transfected with a green fluorescent protein-oNaDC-1 construct, however, media acidification increases Na-dependent citrate uptake, demonstrating posttranscriptional acid regulation of NaDC-1 activity.

Transport of organic anions across the basolateral membrane of proximal tubule cells

Renal proximal tubules secrete diverse organic anions (OA) including widely prescribed anionic drugs. Here, we review the molecular properties of cloned transporters involved in uptake of OA from blood into proximal tubule cells and provide extensive lists of substrates handled by these transport systems. Where tested, transporters have been immunolocalized to the basolateral cell membrane. The sulfate anion transporter 1 (sat-1) cloned from human, rat and mouse, transported oxalate and sulfate. Drugs found earlier to interact with sulfate transport in vivo have not yet been tested with sat-1. The Na(+)-dicarboxylate cotransporter 3 (NaDC-3) was cloned from human, rat, mouse and flounder, and transported three Na(+) with one divalent di- or tricarboxylate, such as citric acid cycle intermediates and the heavy metal chelator 2,3-dimercaptosuccinate (succimer). The organic anion transporter 1 (OAT1) cloned from several species was shown to exchange extracellular OA against intracellular alpha-ketoglutarate. OAT1 translocated, e.g., anti-inflammatory drugs, antiviral drugs, beta-lactam antibiotics, loop diuretics, ochratoxin A, and p-aminohippurate. Several OA, including probenecid, inhibited OAT1. Human, rat and mouse OAT2 transported selected anti-inflammatory and antiviral drugs, methotrexate, ochratoxin A, and, with high affinities, prostaglandins E(2) and F(2alpha). OAT3 cloned from human, rat and mouse showed a substrate specificity overlapping with that of OAT1. In addition, OAT3 interacted with sulfated steroid hormones such as estrone-3-sulfate. The driving forces for OAT2 and OAT3, the relative contributions of all OA transporters to, and the impact of transporter regulation by protein kinases on renal drug excretion in vivo must be determined in future experiments.

Conformationally sensitive residues in transmembrane domain 9 of the Na+/dicarboxylate co-transporter

The Na(+)/dicarboxylate co-transporter, NaDC-1, couples the transport of sodium and Krebs cycle intermediates, such as succinate and citrate. Previous studies identified two functionally important amino acids, Glu-475 and Cys-476, located in transmembrane domain (TMD) 9 of NaDC-1. In the present study, each amino acid in TMD-9 was mutated to cysteine, one at a time, and the accessibility of the membrane-impermeant reagent [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) to the replacement cysteines was determined. Cysteine substitution was tolerated at all but five of the sites: the A461C mutant was not present at the plasma membrane, whereas the F473C, T474C, E475C, and N479C mutants were inactive proteins located on the plasma membrane. Cysteine substitution of four residues found near the extracellular surface of TMD-9 (Ser-478, Ala-480, Ala-481, and Thr-482) resulted in proteins that were sensitive to inhibition by MTSET. The accessibility of MTSET to the four substituted cysteines was highest in the presence of the transported cations, sodium or lithium, and low in choline. The four mutants also exhibited substrate protection of MTSET accessibility. The MTSET accessibility to S478C, A481C, and A480C was independent of voltage. In contrast, T482C was more accessible to MTSET in choline buffer at negative holding potentials, but there was no effect of voltage in sodium buffer. In conclusion, TMD-9 may be involved in transducing conformational changes between the cation-binding sites and the substrate-binding site in NaDC-1, and it may also form part of the translocation pathway through the transporter.

Cloning and functional characterization of a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain

Neurons contain a high-affinity Na(+)/dicarboxylate cotransporter for absorption of neurotransmitter precursor substrates, such as alpha-ketoglutarate and malate, which are subsequently metabolized to replenish pools of neurotransmitters, including glutamate. We have isolated the cDNA coding for a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain, called mNaDC-3. The mRNA coding for mNaDC-3 is found in brain and choroid plexus as well as in kidney and liver. The mNaDC-3 transporter has a broad substrate specificity for dicarboxylates, including succinate, alpha-ketoglutarate, fumarate, malate, and dimethylsuccinate. The transport of citrate is relatively insensitive to pH, but the transport of succinate is inhibited by acidic pH. The Michaelis-Menten constant for succinate in mNaDC-3 is 140 microM in transport assays and 16 microM at -50 mV in two-electrode voltage clamp assays. Transport is dependent on sodium, although lithium can partially substitute for sodium. In conclusion, mNaDC-3 likely codes for the high-affinity Na(+)/dicarboxylate cotransporter in brain, and it has some unusual electrical properties compared with the other members of the family.

Topology of the Na(+)/dicarboxylate cotransporter: the N-terminus and hydrophilic loop 4 are located intracellularly

The current secondary structure model of the Na(+)/dicarboxylate cotransporter, NaDC-1, contains 11 transmembrane domains. The model is based on hydropathy analysis and the extracellular location of the carboxy terminus, which contains an N-glycosylation site. In this study, the model was further tested using indirect immunofluorescence of COS-7 cells. The Flag epitope tag (DYKDDDDK) was fused to the amino terminus of NaDC-1 (Flag-NaDC-1), and a monoclonal antibody against the Flag epitope was used to determine the location of the N-terminus. Hydrophilic loop 4 of NaDC-1 was identified using polyclonal antibodies raised against a fusion protein containing amino acids 164--233 of NaDC-1. The expression of NaDC-1 and Flag-NaDC-1 in COS-7 cells was confirmed by functional assays of succinate transport and by Western blots of cell surface biotinylated proteins. Immunofluorescent labeling of cells expressing both NaDC-1 and Flag-NaDC-1 required permeabilization of the plasma membranes with digitonin whereas no immunofluorescence was visible in intact cells. The results of this study show that both the N-terminus and hydrophilic loop 4 of NaDC-1 are located intracellularly, which supports the current model of NaDC-1 structure.