Structure, function, and genomic organization of human Na(+)-dependent high-affinity dicarboxylate transporter

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.

Structural and functional implications of SLC13A3 and SLC9A6 mutations: an in silico approach to understanding intellectual disability

BACKGROUND

Intellectual disability (ID) is a condition that varies widely in both its clinical presentation and its genetic underpinnings. It significantly impacts patients' learning capacities and lowers their IQ below 70. The solute carrier (SLC) family is the most abundant class of transmembrane transporters and is responsible for the translocation of various substances across cell membranes, including nutrients, ions, metabolites, and medicines. The SLC13A3 gene encodes a plasma membrane-localized Na+/dicarboxylate cotransporter 3 (NaDC3) primarily expressed in the kidney, astrocytes, and the choroid plexus. In addition to three Na + ions, it brings four to six carbon dicarboxylates into the cytosol. Recently, it was discovered that patients with acute reversible leukoencephalopathy and a-ketoglutarate accumulation (ARLIAK) carry pathogenic mutations in the SLC13A3 gene, and the X-linked neurodevelopmental condition Christianson Syndrome is caused by mutations in the SLC9A6 gene, which encodes the recycling endosomal alkali cation/proton exchanger NHE6, also called sodium-hydrogen exchanger-6. As a result, there are severe impairments in the patient's mental capacity, physical skills, and adaptive behavior.

METHODS AND RESULTS

Two Pakistani families (A and B) with autosomal recessive and X-linked intellectual disorders were clinically evaluated, and two novel disease-causing variants in the SLC13A3 gene (NM 022829.5) and the SLC9A6 gene (NM 001042537.2) were identified using whole exome sequencing. Family-A segregated a novel homozygous missense variant (c.1478 C > T; p. Pro493Leu) in the exon-11 of the SLC13A3 gene. At the same time, family-B segregated a novel missense variant (c.1342G > A; p.Gly448Arg) in the exon-10 of the SLC9A6 gene. By integrating computational approaches, our findings provided insights into the molecular mechanisms underlying the development of ID in individuals with SLC13A3 and SLC9A6 mutations.

CONCLUSION

We have utilized in-silico tools in the current study to examine the deleterious effects of the identified variants, which carry the potential to understand the genotype-phenotype relationships in neurodevelopmental disorders.

Unique genetic architecture of CSF and brain metabolites pinpoints the novel targets for the traits of human wellness

Brain metabolism perturbation can contribute to traits and diseases. We conducted the first large-scale CSF and brain genome-wide association studies, which identified 219 independent associations (59.8% novel) for 144 CSF metabolites and 36 independent associations (55.6% novel) for 34 brain metabolites. Most of the novel signals (97.7% and 70.0% in CSF and brain) were tissue specific. We also integrated MWAS-FUSION approaches with Mendelian Randomization and colocalization to identify causal metabolites for 27 brain and human wellness phenotypes and identified eight metabolites to be causal for eight traits (11 relationships). Low mannose level was causal to bipolar disorder and as dietary supplement it may provide therapeutic benefits. Low galactosylglycerol level was found causal to Parkinson's Disease (PD). Our study expanded the knowledge of MQTL in central nervous system, provided insights into human wellness, and successfully demonstrates the utility of combined statistical approaches to inform interventions.

Genetic studies of paired metabolomes reveal enzymatic and transport processes at the interface of plasma and urine

The kidneys operate at the interface of plasma and urine by clearing molecular waste products while retaining valuable solutes. Genetic studies of paired plasma and urine metabolomes may identify underlying processes. We conducted genome-wide studies of 1,916 plasma and urine metabolites and detected 1,299 significant associations. Associations with 40% of implicated metabolites would have been missed by studying plasma alone. We detected urine-specific findings that provide information about metabolite reabsorption in the kidney, such as aquaporin (AQP)-7-mediated glycerol transport, and different metabolomic footprints of kidney-expressed proteins in plasma and urine that are consistent with their localization and function, including the transporters NaDC3 (SLC13A3) and ASBT (SLC10A2). Shared genetic determinants of 7,073 metabolite-disease combinations represent a resource to better understand metabolic diseases and revealed connections of dipeptidase 1 with circulating digestive enzymes and with hypertension. Extending genetic studies of the metabolome beyond plasma yields unique insights into processes at the interface of body compartments.

Genomics of perivascular space burden unravels early mechanisms of cerebral small vessel disease

Perivascular space (PVS) burden is an emerging, poorly understood, magnetic resonance imaging marker of cerebral small vessel disease, a leading cause of stroke and dementia. Genome-wide association studies in up to 40,095 participants (18 population-based cohorts, 66.3 ± 8.6 yr, 96.9% European ancestry) revealed 24 genome-wide significant PVS risk loci, mainly in the white matter. These were associated with white matter PVS already in young adults (N = 1,748; 22.1 ± 2.3 yr) and were enriched in early-onset leukodystrophy genes and genes expressed in fetal brain endothelial cells, suggesting early-life mechanisms. In total, 53% of white matter PVS risk loci showed nominally significant associations (27% after multiple-testing correction) in a Japanese population-based cohort (N = 2,862; 68.3 ± 5.3 yr). Mendelian randomization supported causal associations of high blood pressure with basal ganglia and hippocampal PVS, and of basal ganglia PVS and hippocampal PVS with stroke, accounting for blood pressure. Our findings provide insight into the biology of PVS and cerebral small vessel disease, pointing to pathways involving extracellular matrix, membrane transport and developmental processes, and the potential for genetically informed prioritization of drug targets.

Untargeted Metabolomics of Slc13a5 Deficiency Reveal Critical Liver-Brain Axis for Lipid Homeostasis

Though biallelic variants in SLC13A5 are known to cause severe encephalopathy, the mechanism of this disease is poorly understood. SLC13A5 protein deficiency reduces citrate transport into the cell. Downstream abnormalities in fatty acid synthesis and energy generation have been described, though biochemical signs of these perturbations are inconsistent across SLC13A5 deficiency patients. To investigate SLC13A5-related disorders, we performed untargeted metabolic analyses on the liver, brain, and serum from a Slc13a5-deficient mouse model. Metabolomic data were analyzed using the connect-the-dots (CTD) methodology and were compared to plasma and CSF metabolomics from SLC13A5-deficient patients. Mice homozygous for the Slc13a5^tm1b/tm1b null allele had perturbations in fatty acids, bile acids, and energy metabolites in all tissues examined. Further analyses demonstrated that for several of these molecules, the ratio of their relative tissue concentrations differed widely in the knockout mouse, suggesting that deficiency of Slc13a5 impacts the biosynthesis and flux of metabolites between tissues. Similar findings were observed in patient biofluids, indicating altered transport and/or flux of molecules involved in energy, fatty acid, nucleotide, and bile acid metabolism. Deficiency of SLC13A5 likely causes a broader state of metabolic dysregulation than previously recognized, particularly regarding lipid synthesis, storage, and metabolism, supporting SLC13A5 deficiency as a lipid disorder.

Consequences of NaCT/SLC13A5/mINDY deficiency: good versus evil, separated only by the blood-brain barrier

NaCT/SLC13A5 is a Na⁺-coupled transporter for citrate in hepatocytes, neurons, and testes. It is also called mINDY (mammalian ortholog of ‘I'm Not Dead Yet’ in Drosophila). Deletion of Slc13a5 in mice leads to an advantageous phenotype, protecting against diet-induced obesity, and diabetes. In contrast, loss-of-function mutations in SLC13A5 in humans cause a severe disease, EIEE25/DEE25 (early infantile epileptic encephalopathy-25/developmental epileptic encephalopathy-25). The difference between mice and humans in the consequences of the transporter deficiency is intriguing but probably explainable by the species-specific differences in the functional features of the transporter. Mouse Slc13a5 is a low-capacity transporter, whereas human SLC13A5 is a high-capacity transporter, thus leading to quantitative differences in citrate entry into cells via the transporter. These findings raise doubts as to the utility of mouse models to evaluate NaCT biology in humans. NaCT-mediated citrate entry in the liver impacts fatty acid and cholesterol synthesis, fatty acid oxidation, glycolysis, and gluconeogenesis; in neurons, this process is essential for the synthesis of the neurotransmitters glutamate, GABA, and acetylcholine. Thus, SLC13A5 deficiency protects against obesity and diabetes based on what the transporter does in hepatocytes, but leads to severe brain deficits based on what the transporter does in neurons. These beneficial versus detrimental effects of SLC13A5 deficiency are separable only by the blood-brain barrier. Can we harness the beneficial effects of SLC13A5 deficiency without the detrimental effects? In theory, this should be feasible with selective inhibitors of NaCT, which work only in the liver and do not get across the blood-brain barrier.

Case Report: Compound Heterozygous Variants of SLC13A3 Identified in a Chinese Patient With Acute Reversible Leukoencephalopathy and α-Ketoglutarate Accumulation

Background: SLC13A3 gene encodes the Na⁺/dicarboxylate cotransporter 3 (NaDC3), which locates on the plasma membrane and is mainly expressed in kidney, astrocytes and the choroid plexus. It imports four to six carbon dicarboxylates together with three Na⁺ ions into the cytosol. Nowadays, pathogenic variants of SLC13A3 gene were found to cause acute reversible leukoencephalopathy and α-ketoglutarate accumulation (ARLIAK) in patients. Here, we report two novel SLC13A3 variants c.185C>T (p.T62M) and c.331C>T (p.R111^*) identified in a Chinese patient with ARLIAK. Case Presentation: The patient was a Chinese girl aged 13 years and 7 months old, who had acute, recurrent neurological deterioration during two febrile episodes. She presented with reversible leukoencephalopathy and increased urinary excretion of α-ketoglutarate. Genetic studies revealed compound heterozygous variants (c.185C>T, p.T62M, and c.331C>T, p.R111^*) in SLC13A3, which had not been reported previously. Conclusions: These findings expand the variant spectrum of SLC13A3, providing the basis for the further study of this rare disease.

Feeling the heat: source–sink mismatch as a mechanism underlying the failure of thermal tolerance

A mechanistic explanation for the tolerance limits of animals at high temperatures is still missing, but one potential target for thermal failure is the electrical signaling off cells and tissues. With this in mind, here I review the effects of high temperature on the electrical excitability of heart, muscle and nerves, and refine a hypothesis regarding high temperature-induced failure of electrical excitation and signal transfer [the temperature-dependent deterioration of electrical excitability (TDEE) hypothesis]. A central tenet of the hypothesis is temperature-dependent mismatch between the depolarizing ion current (i.e. source) of the signaling cell and the repolarizing ion current (i.e. sink) of the receiving cell, which prevents the generation of action potentials (APs) in the latter. A source–sink mismatch can develop in heart, muscles and nerves at high temperatures owing to opposite effects of temperature on source and sink currents. AP propagation is more likely to fail at the sites of structural discontinuities, including electrically coupled cells, synapses and branching points of nerves and muscle, which impose an increased demand of inward current. At these sites, temperature-induced source–sink mismatch can reduce AP frequency, resulting in low-pass filtering or a complete block of signal transmission. In principle, this hypothesis can explain a number of heat-induced effects, including reduced heart rate, reduced synaptic transmission between neurons and reduced impulse transfer from neurons to muscles. The hypothesis is equally valid for ectothermic and endothermic animals, and for both aquatic and terrestrial species. Importantly, the hypothesis is strictly mechanistic and lends itself to experimental falsification.

Compartmentalised acyl-CoA metabolism and roles in chromatin regulation

BACKGROUND

Many metabolites serve as important signalling molecules to adjust cellular activities and functions based on nutrient availability. Links between acetyl-CoA metabolism, histone lysine acetylation, and gene expression have been documented and studied over the past decade. In recent years, several additional acyl modifications to histone lysine residues have been identified, which depend on acyl-coenzyme A thioesters (acyl-CoAs) as acyl donors. Acyl-CoAs are intermediates of multiple distinct metabolic pathways, and substantial evidence has emerged that histone acylation is metabolically sensitive. Nevertheless, the metabolic sources of acyl-CoAs used for chromatin modification in most cases remain poorly understood. Elucidating how these diverse chemical modifications are coupled to and regulated by cellular metabolism is important in deciphering their functional significance.

SCOPE OF REVIEW

In this article, we review the metabolic pathways that produce acyl-CoAs, as well as emerging evidence for functional roles of diverse acyl-CoAs in chromatin regulation. Because acetyl-CoA has been extensively reviewed elsewhere, we will focus on four other acyl-CoA metabolites integral to major metabolic pathways that are also known to modify histones: succinyl-CoA, propionyl-CoA, crotonoyl-CoA, and butyryl-CoA. We also briefly mention several other acyl-CoA species, which present opportunities for further research; malonyl-CoA, glutaryl-CoA, 3-hydroxybutyryl-CoA, 2-hydroxyisobutyryl-CoA, and lactyl-CoA. Each acyl-CoA species has distinct roles in metabolism, indicating the potential to report shifts in the metabolic status of the cell. For each metabolite, we consider the metabolic pathways in which it participates and the nutrient sources from which it is derived, the compartmentalisation of its metabolism, and the factors reported to influence its abundance and potential nuclear availability. We also highlight reported biological functions of these metabolically-linked acylation marks. Finally, we aim to illuminate key questions in acyl-CoA metabolism as they relate to the control of chromatin modification.

MAJOR CONCLUSIONS

A majority of acyl-CoA species are annotated to mitochondrial metabolic processes. Since acyl-CoAs are not known to be directly transported across mitochondrial membranes, they must be synthesized outside of mitochondria and potentially within the nucleus to participate in chromatin regulation. Thus, subcellular metabolic compartmentalisation likely plays a key role in the regulation of histone acylation. Metabolite tracing in combination with targeting of relevant enzymes and transporters will help to map the metabolic pathways that connect acyl-CoA metabolism to chromatin modification. The specific function of each acyl-CoA may be determined in part by biochemical properties that affect its propensity for enzymatic versus non-enzymatic protein modification, as well as the various enzymes that can add, remove and bind each modification. Further, competitive and inhibitory effects of different acyl-CoA species on these enzymes make determining the relative abundance of acyl-CoA species in specific contexts important to understand the regulation of chromatin acylation. An improved and more nuanced understanding of metabolic regulation of chromatin and its roles in physiological and disease-related processes will emerge as these questions are answered.

Pharmacologic inducers of the uric acid exporter ABCG2 as potential drugs for treatment of gouty arthritis

Uric acid is the end product of purine catabolism and its plasma levels are maintained below its maximum solubility in water (6–7 mg/dl). The plasma levels are tightly regulated as the balance between the rate of production and the rate of excretion, the latter occurring in urine (kidney), bile (liver) and feces (intestinal tract). Reabsorption in kidney is also an important component of this process. Both excretion and reabsorption are mediated by specific transporters. Disruption of the balance between production and excretion leads to hyperuricemia, which increases the risk of uric acid crystallization as monosodium urate with subsequent deposition of the crystals in joints causing gouty arthritis. Loss-of-function mutations in the transporters that mediate uric acid excretion are associated with gout. The ATP-Binding Cassette exporter ABCG2 is important in uric acid excretion at all three sites: kidney (urine), liver (bile), and intestine (feces). Mutations in this transporter cause gout and these mutations occur at significant prevalence in general population. However, mutations that are most prevalent result only in partial loss of transport function. Therefore, if the expression of these partially defective transporters could be induced, the increased number of the transporter molecules would compensate for the mutation-associated decrease in transport function and hence increase uric acid excretion. As such, pharmacologic agents with ability to induce the expression of ABCG2 represent potentially a novel class of drugs for treatment of gouty arthritis.

SLC13A3 variants cause acute reversible leukoencephalopathy and α-ketoglutarate accumulation

OBJECTIVE

SLC13A3 encodes the plasma membrane Na⁺ /dicarboxylate cotransporter 3, which imports inside the cell 4 to 6 carbon dicarboxylates as well as N-acetylaspartate (NAA). SLC13A3 is mainly expressed in kidney, in astrocytes, and in the choroid plexus. We describe two unrelated patients presenting with acute, reversible (and recurrent in one) neurological deterioration during a febrile illness. Both patients exhibited a reversible leukoencephalopathy and a urinary excretion of α-ketoglutarate (αKG) that was markedly increased and persisted over time. In one patient, increased concentrations of cerebrospinal fluid NAA and dicarboxylates (including αKG) were observed. Extensive workup was unsuccessful, and a genetic cause was suspected.

METHODS

Whole exome sequencing (WES) was performed. Our teams were connected through GeneMatcher.

RESULTS

WES analysis revealed variants in SLC13A3. A homozygous missense mutation (p.Ala254Asp) was found in the first patient. The second patient was heterozygous for another missense mutation (p.Gly548Ser) and an intronic mutation affecting splicing as demonstrated by reverse transcriptase polymerase chain reaction performed in muscle tissue (c.1016 + 3A > G). Mutations and segregation were confirmed by Sanger sequencing. Functional studies performed on HEK293T cells transiently transfected with wild-type and mutant SLC13A3 indicated that the missense mutations caused a marked reduction in the capacity to transport αKG, succinate, and NAA.

INTERPRETATION

SLC13A3 deficiency causes acute and reversible leukoencephalopathy with marked accumulation of αKG. Urine organic acids (especially αKG and NAA) and SLC13A3 mutations should be screened in patients presenting with unexplained reversible leukoencephalopathy, for which SLC13A3 deficiency is a novel differential diagnosis. ANN NEUROL 2019;85:385-395.

Succinate Accumulation Is Associated with a Shift of Mitochondrial Respiratory Control and HIF-1α Upregulation in PTEN Negative Prostate Cancer Cells

The idea of using metabolic aberrations as targets for diagnosis or therapeutic intervention has recently gained increasing interest. In a previous study, our group discovered intriguing differences in the oxidative mitochondrial respiration capacity of benign and prostate cancer (PCa) cells. In particular, we found that PCa cells had a higher total respiratory activity than benign cells. Moreover, PCa cells showed a substantial shift towards succinate-supported mitochondrial respiration compared to benign cells, indicating a re-programming of respiratory control. This study aimed to investigate the role of succinate and its main plasma membrane transporter NaDC3 (sodium-dependent dicarboxylate transporter member 3) in PCa cells and to determine whether targeting succinate metabolism can be potentially used to inhibit PCa cell growth. Using high-resolution respirometry analysis, we observed that ROUTINE respiration in viable cells and succinate-supported respiration in permeabilized cells was higher in cells lacking the tumor suppressor phosphatase and tensin-homolog deleted on chromosome 10 (PTEN), which is frequently lost in PCa. In addition, loss of PTEN was associated with increased intracellular succinate accumulation and higher expression of NaDC3. However, siRNA-mediated knockdown of NaDC3 only moderately influenced succinate metabolism and did not affect PCa cell growth. By contrast, mersalyl acid—a broad acting inhibitor of dicarboxylic acid carriers—strongly interfered with intracellular succinate levels and resulted in reduced numbers of PCa cells. These findings suggest that blocking NaDC3 alone is insufficient to intervene with altered succinate metabolism associated with PCa. In conclusion, our data provide evidence that loss of PTEN is associated with increased succinate accumulation and enhanced succinate-supported respiration, which cannot be overcome by inhibiting the succinate transporter NaDC3 alone.

D-Malate decreases renal content of α-ketoglutarate, a driving force of organic anion transporters OAT1 and OAT3, resulting in inhibited tubular secretion of phenolsulfonphthalein, in rats

d-Malate inhibits a Krebs cycle enzyme and the tubular transport of α-ketoglutarate, an intermediate of the Krebs cycle and the driving force for rat organic anion transporter 1 (rOAT1) and rOAT3 in the kidney. This study examined the effects of d-malate on the rat organic anion transport system. The uptake of 6-carboxyfluorescein by HEK293 cells expressing rOAT1 or rOAT3 was not affected by d-malate and l-malate. Up to 60 min after the intravenous injection of phenolsulfonphthalein (PSP), a typical substrate of the renal organic anion transporters, as a bolus to rats, 47.1% of the dose was recovered in the urine, and its renal clearance was estimated to be 8.60 ml/min/kg. d-Malate but not l-malate interfered with its renal excretion, resulting in the delayed elimination of PSP from plasma. No effect of d-malate was recognized on creatinine clearance or the expression level of rOAT3 in the kidney cortex. d-Malate increased the plasma concentration of α-ketoglutarate. In addition, the compound greatly stimulated the renal excretion of α-ketoglutarate, implying that d-malate inhibited its reabsorption. The content of α-ketoglutarate was significantly decreased in the kidney cortex of rats administered d-malate. Collectively, this study shows that d-malate abrogates the tubular secretion of PSP, and the reduction of the renal content of α-ketoglutarate was proposed to be one of the mechanisms. A relationship between the reabsorption of α-ketoglutarate and the basolateral uptake of organic anion in the kidney is suggested.

Genome-Wide Association Study with Targeted and Non-targeted NMR Metabolomics Identifies 15 Novel Loci of Urinary Human Metabolic Individuality

Genome-wide association studies with metabolic traits (mGWAS) uncovered many genetic variants that influence human metabolism. These genetically influenced metabotypes (GIMs) contribute to our metabolic individuality, our capacity to respond to environmental challenges, and our susceptibility to specific diseases. While metabolic homeostasis in blood is a well investigated topic in large mGWAS with over 150 known loci, metabolic detoxification through urinary excretion has only been addressed by few small mGWAS with only 11 associated loci so far. Here we report the largest mGWAS to date, combining targeted and non-targeted ¹H NMR analysis of urine samples from 3,861 participants of the SHIP-0 cohort and 1,691 subjects of the KORA F4 cohort. We identified and replicated 22 loci with significant associations with urinary traits, 15 of which are new (HIBCH, CPS1, AGXT, XYLB, TKT, ETNPPL, SLC6A19, DMGDH, SLC36A2, GLDC, SLC6A13, ACSM3, SLC5A11, PNMT, SLC13A3). Two-thirds of the urinary loci also have a metabolite association in blood. For all but one of the 6 loci where significant associations target the same metabolite in blood and urine, the genetic effects have the same direction in both fluids. In contrast, for the SLC5A11 locus, we found increased levels of myo-inositol in urine whereas mGWAS in blood reported decreased levels for the same genetic variant. This might indicate less effective re-absorption of myo-inositol in the kidneys of carriers. In summary, our study more than doubles the number of known loci that influence urinary phenotypes. It thus allows novel insights into the relationship between blood homeostasis and its regulation through excretion. The newly discovered loci also include variants previously linked to chronic kidney disease (CPS1, SLC6A13), pulmonary hypertension (CPS1), and ischemic stroke (XYLB). By establishing connections from gene to disease via metabolic traits our results provide novel hypotheses about molecular mechanisms involved in the etiology of diseases.

Human metabolism is influenced by genetic and environmental factors defining a person’s metabolic individuality. This individuality is linked to personal differences in the ability to react on metabolic challenges and in the susceptibility to specific diseases. By investigating how common variants in genetic regions (loci) affect individual blood metabolite levels, the substantial contribution of genetic inheritance to metabolic individuality has been demonstrated previously. Meanwhile, more than 150 loci influencing metabolic homeostasis in blood are known. Here we shift the focus to genetic variants that modulate urinary metabolite excretion, for which only 11 loci were reported so far. In the largest genetic study on urinary metabolites to date, we identified 15 additional loci. Most of the 26 loci also affect blood metabolite levels. This shows that the metabolic individuality seen in blood is also reflected in urine, which is expected when urine is regarded as “diluted blood”. Nonetheless, we also found loci that appear to primarily influence metabolite excretion. For instance, we identified genetic variants near a gene of a transporter that change the capability for renal re-absorption of the transporter’s substrate. Thus, our findings could help to elucidate molecular mechanisms influencing kidney function and the body’s detoxification capabilities.

Role of OAT4 in Uptake of Estriol Precursor 16α-Hydroxydehydroepiandrosterone Sulfate Into Human Placental Syncytiotrophoblasts From Fetus

Estriol biosynthesis in human placenta requires the uptake of a fetal liver-derived estriol precursor, 16α-hydroxydehydroepiandrosterone sulfate (16α-OH DHEAS), by placental syncytiotrophoblasts at their basal plasma membrane (BM), which faces the fetal circulation. The aim of this work is to identify the transporter(s) mediating 16α-OH DHEAS uptake at the fetal side of syncytiotrophoblasts by using human placental BM-enriched vesicles and to examine the contribution of the putative transporter to estriol synthesis at the cellular level, using choriocarcinoma JEG-3 cells. Organic anion transporter (OAT)-4 and organic anion transporting polypeptide 2B1 proteins were enriched in human placental BM vesicles compared with crude membrane fraction. Uptake of [(3)H]16α-OH DHEAS by BM vesicles was partially inhibited in the absence of sodium but was significantly increased in the absence of chloride and after preloading glutarate. Uptake of [(3)H]16α-OH DHEAS by BM vesicles was significantly inhibited by OAT4 substrates such as dehydroepiandrosterone sulfate, estrone-3-sulfate, and bromosulfophthalein but not by cyclosporin A, tetraethylammonium, p-aminohippuric acid, or cimetidine. These characteristics of vesicular [(3)H]16α-OH DHEAS uptake are in good agreement with those of human OAT4-transfected COS-7 cells as well as forskolin-differentiated JEG-3 cells. Estriol secretion from differentiated JEG-3 cells was detected when the cells were incubated with 16α-OH DHEAS for 8 hours but was inhibited in the presence of 50 μM bromosulfophthalein. Our results indicate that OAT4 at the BM of human placental syncytiotrophoblasts plays a predominant role in the uptake of 16α-OH DHEAS for placental estriol synthesis.

Organic Anion Transporter 4-Mediated Transport of Olmesartan at Basal Plasma Membrane of Human Placental Barrier

Mechanisms regulating fetal transfer of olmesartan, an angiotensin-II receptor type 1 antagonist, are important as potential determinants of life-threatening adverse fetal effects. The purpose of this study was to examine the olmesartan transport mechanism through the basal plasma membrane (BM) of human syncytiotrophoblasts forming the placental barrier. Uptake of olmesartan by human placental BM vesicles was potently inhibited by dehydroepiandrosterone sulfate (DHEAS), estrone 3-sulfate, and bromosulfophthalein, which are all typical substrates of organic anion transporter (OAT) 4 localized at the BM of syncytiotrophoblasts, and was increased in the absence of chloride. In tetracycline-inducible OAT4-expressing cells, [(3) H]olmesartan uptake was increased by tetracycline treatment. Olmesartan uptake via OAT4 was concentration dependent with a Km of 20 μM, and was increased in the absence of chloride. [(3) H]Olmesartan efflux via OAT4 was also observed and was trans-stimulated by extracellular chloride and DHEAS. Thus, OAT4 mediates bidirectional transport of olmesartan and appears to regulate fetal transfer of olmesartan at the BM of syncytiotrophoblasts. Efflux transport of olmesartan via OAT4 from syncytiotrophoblasts to the fetal circulation might be facilitated in the presence of an inwardly directed physiological chloride gradient and extracellular DHEAS.

NaDC3 Induces Premature Cellular Senescence by Promoting Transport of Krebs Cycle Intermediates, Increasing NADH, and Exacerbating Oxidative Damage

High-affinity sodium-dependent dicarboxylate cotransporter 3 (NaDC3) is a key metabolism-regulating membrane protein responsible for transport of Krebs cycle intermediates. NaDC3 is upregulated as organs age, but knowledge regarding the underlying mechanisms by which NaDC3 modulates mammalian aging is limited. In this study, we showed that NaDC3 overexpression accelerated cellular senescence in young human diploid cells (MRC-5 and WI-38) and primary renal tubular cells, leading to cell cycle arrest in G1 phase and increased expression of senescent biomarkers, senescence-associated β-galactosidase and p16. Intracellular levels of reactive oxygen species, 8-hydroxy-2'-deoxyguanosine, malondialdehyde, and carbonyl were significantly enhanced, and activities of respiratory complexes I and III and ATP level were significantly decreased in NaDC3-infected cells. Stressful premature senescent phenotypes induced by NaDC3 were markedly ameliorated via treatment with the antioxidants Tiron and Tempol. High expression of NaDC3 caused a prominent increase in intracellular levels of Krebs cycle intermediates and NADH. Exogenous NADH and NAD(+) may aggravate and attenuate the aging phenotypes induced by NaDC3, respectively. These results suggest that NaDC3 can induce premature cellular senescence by promoting the transport of Krebs cycle intermediates, increasing generation of NADH and reactive oxygen species and leading to oxidative damage. Our results clarify the aging signaling pathway regulated by NaDC3.

Turbulent electrical activity at sharp-edged inexcitable obstacles in a model for human cardiac tissue

Wave propagation around various geometric expansions, structures, and obstacles in cardiac tissue may result in the formation of unidirectional block of wave propagation and the onset of reentrant arrhythmias in the heart. Therefore, we investigated the conditions under which reentrant spiral waves can be generated by high-frequency stimulation at sharp-edged obstacles in the ten Tusscher-Noble-Noble-Panfilov (TNNP) ionic model for human cardiac tissue. We show that, in a large range of parameters that account for the conductance of major inward and outward ionic currents of the model [fast inward Na+ current ( INa), L—type slow inward Ca2+ current ( ICaL), slow delayed-rectifier current ( IKs), rapid delayed-rectifier current ( IKr), inward rectifier K+ current ( IK1)], the critical period necessary for spiral formation is close to the period of a spiral wave rotating in the same tissue. We also show that there is a minimal size of the obstacle for which formation of spirals is possible; this size is ∼2.5 cm and decreases with a decrease in the excitability of cardiac tissue. We show that other factors, such as the obstacle thickness and direction of wave propagation in relation to the obstacle, are of secondary importance and affect the conditions for spiral wave initiation only slightly. We also perform studies for obstacle shapes derived from experimental measurements of infarction scars and show that the formation of spiral waves there is facilitated by tissue remodeling around it. Overall, we demonstrate that the formation of reentrant sources around inexcitable obstacles is a potential mechanism for the onset of cardiac arrhythmias in the presence of a fast heart rate.

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.

Determinants of substrate and cation transport in the human Na+/dicarboxylate cotransporter NaDC3

Metabolic intermediates, such as succinate and citrate, regulate important processes ranging from energy metabolism to fatty acid synthesis. Cytosolic concentrations of these metabolites are controlled, in part, by members of the SLC13 gene family. The molecular mechanism underlying Na(+)-coupled di- and tricarboxylate transport by this family is understood poorly. The human Na(+)/dicarboxylate cotransporter NaDC3 (SLC13A3) is found in various tissues, including the kidney, liver, and brain. In addition to citric acid cycle intermediates such as α-ketoglutarate and succinate, NaDC3 transports other compounds into cells, including N-acetyl aspartate, mercaptosuccinate, and glutathione, in keeping with its dual roles in cell nutrition and detoxification. In this study, we construct a homology structural model of NaDC3 on the basis of the structure of the Vibrio cholerae homolog vcINDY. Our computations are followed by experimental testing of the predicted NaDC3 structure and mode of interaction with various substrates. The results of this study show that the substrate and cation binding domains of NaDC3 are composed of residues in the opposing hairpin loops and unwound portions of adjacent helices. Furthermore, these results provide a possible explanation for the differential substrate specificity among dicarboxylate transporters that underpin their diverse biological roles in metabolism and detoxification. The structural model of NaDC3 provides a framework for understanding substrate selectivity and the Na(+)-coupled anion transport mechanism by the human SLC13 family and other key solute carrier transporters.

Localization of the calcium-regulated citrate transport process in proximal tubule cells

Urinary citrate is an important inhibitor of calcium-stone formation. Most of the citrate reabsorption in the proximal tubule is thought to occur via a dicarboxylate transporter NaDC1 located in the apical membrane. OK cells, an established opossum kidney proximal tubule cell line, transport citrate but the characteristics change with extracellular calcium such that low calcium solutions stimulate total citrate transport as well as increase the apparent affinity for transport. The present studies address several fundamental properties of this novel process: the polarity of the transport process, the location of the calcium-sensitivity and whether NaDC1 is present in OK cells. OK cells grown on permeable supports exhibited apical >basolateral citrate transport. Apical transport of both citrate and succinate was sensitive to extracellular calcium whereas basolateral transport was not. Apical calcium, rather than basolateral, was the predominant determinant of changes in transport. Also 2,3-dimethylsuccinate, previously identified as an inhibitor of basolateral dicarboxylate transport, inhibited apical citrate uptake. Although the calcium-sensitive transport process in OK cells is functionally not typical NaDC1, NaDC1 is present in OK cells by Western blot and PCR. By immunolocalization studies, NaDC1 was predominantly located in discrete apical membrane or subapical areas. However, by biotinylation, apical NaDC1 decreases in the apical membrane with lowering calcium. In sum, OK cells express a calcium-sensitive/regulated dicarboxylate process at the apical membrane which responds to variations in apical calcium. Despite the functional differences of this process compared to NaDC1, NaDC1 is present in these cells, but predominantly in subapical vesicles.

A microstructural model of reentry arising from focal breakthrough at sites of source-load mismatch in a central region of slow conduction

Regions of cardiac tissue that have a combination of focal activity and poor, heterogeneous gap junction coupling are often considered to be arrhythmogenic; however, the relationship between the properties of the cardiac microstructure and patterns of abnormal propagation is not well understood. The objective of this study was to investigate the effect of microstructure on the initiation of reentry from focal stimulation inside a poorly coupled region embedded in more well-coupled tissue. Two-dimensional discrete computer models of ventricular monolayers (1 × 1 cm) were randomly generated to represent heterogeneity in the cardiac microstructure. A small, central poorly coupled patch (0.40 × 0.40 cm) was introduced to represent the site of focal activity. Simulated unipolar electrogram recordings were computed at various points in the tissue. As the gap conductance of the patch decreased, conduction slowed and became increasingly complex, marked by fractionated electrograms with reduced amplitude. Near the limit of conduction block, isolated breakthrough sites occurred at single cells along the patch boundary and were marked by long cell-to-cell delays and negative deflections on electrogram recordings. The strongest determinant of the site of wavefront breakthrough was the connectivity of the brick wall architecture, which enabled current flow through small regions of overlapping cells to drive propagation into the well-coupled zone. In conclusion, breakthroughs at the size scale of a single cell can occur at the boundary of source-load mismatch allowing focal activations from slow conducting regions to produce reentry. These breakthrough regions, identifiable by distinct asymmetric, reduced amplitude electrograms, are sensitive to tissue architecture and may be targets for ablation.

The Role of INDY in Metabolic Regulation

Reduced expression of the Indy (I'm Not Dead Yet) gene in D. melanogaster and C. elegans extends longevity. Indy and its mammalian homolog mINDY (Slc13a5, NaCT) are transporters of TCA cycle intermediates, mainly handling the uptake of citrate via the plasma membrane into the cytosol. Deletion of mINDY in mice leads to significant metabolic changes akin to caloric restriction, likely caused by reducing the effects of mINDY-imported citrate on fatty acid and cholesterol synthesis, glucose metabolism and ß-oxidation. This review will provide an overview on different mammalian SLC1 3 family members with a focus on mINDY (SLCl3A5) in glucose and energy metabolism and will highlight the role of mINDY as a putative therapeutic target for the treatment of obesity, non-alcoholic fatty liver disease and type 2 diabetes.

Sodium-coupled dicarboxylate and citrate transporters from the SLC13 family

The SLC13 family in humans and other mammals consists of sodium-coupled transporters for anionic substrates: three transporters for dicarboxylates/citrate and two transporters for sulfate. This review will focus on the di- and tricarboxylate transporters: NaDC1 (SLC13A2), NaDC3 (SLC13A3), and NaCT (SLC13A5). The substrates of these transporters are metabolic intermediates of the citric acid cycle, including citrate, succinate, and α-ketoglutarate, which can exert signaling effects through specific receptors or can affect metabolic enzymes directly. The SLC13 transporters are important for regulating plasma, urinary and tissue levels of these metabolites. NaDC1, primarily found on the apical membranes of renal proximal tubule and small intestinal cells, is involved in regulating urinary levels of citrate and plays a role in kidney stone development. NaDC3 has a wider tissue distribution and high substrate affinity compared with NaDC1. NaDC3 participates in drug and xenobiotic excretion through interactions with organic anion transporters. NaCT is primarily a citrate transporter located in the liver and brain, and its activity may regulate metabolic processes. The recent crystal structure of the Vibrio cholerae homolog, VcINDY, provides a new framework for understanding the mechanism of transport in this family. This review summarizes current knowledge of the structure, function, and regulation of the di- and tricarboxylate transporters of the SLC13 family.

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.

Lithium citrate reduces excessive intra-cerebral N-acetyl aspartate in Canavan disease

Our group has previously reported the first clinical application of lithium in a child affected by Canavan disease. In this study, we aimed to assess the effects of lithium on N-acetyl aspartate (NAA) as well as other end points in a larger cohort. Six patients with clinical, laboratory and genetic confirmation of Canavan disease were recruited and underwent treatment with lithium. The battery of safety and efficacy testing performed before and after sixty days of treatment included Gross Motor Function Testing (GMFM), Magnetic Resonance Imaging (MRI) Proton Magnetic Spectroscopy (H-MRS) as well as blood work. The medication was safe without any clinical or laboratory evidence for toxicity. Parental reports indicated improvement in alertness and social interactions. GMFM did not show statistically significant improvement in motor development. H-MRS documented an overall drop in NAA which was statistically significant in the basal ganglia. T1 measurements recorded on MRI studies suggested a mild improvement in myelination in the frontal white matter after treatment. Diffusion Tensor Imaging was available in two patients and suggested micro-structural improvement in the corpus callosum. The results suggest that lithium administration may be beneficial in patients with Canavan disease.

Low-affinity excitatory amino acid uptake in hippocampal astrocytes: a possible role of Na+/dicarboxylate cotransporters

The excitatory amino acid transporters (EAATs) underlie the so-called "high affinity" uptake of glutamate, which is well characterized. In contrast, the "low-affinity" uptake of glutamate remains poorly defined, and it has been discussed whether it may represent a mere in vitro artifact. Here we have visualized "low-affinity" excitatory amino acid uptake sites by incubating rat hippocampal slices with the glutamate analogue D-aspartate in the presence of PMB-TBOA, which blocks the EAATs. After fixation of the slices, D-aspartate taken up into the tissue was localized with the use of light microscopic immunoperoxidase and electron microscopic immunogold methods, exploiting highly specific antibodies against D-aspartate. PMB-TBOA blocked uptake of both low and high exogenous D-aspartate concentrations (0.01-1.0 mM) into nerve terminals, as well as the uptake of 0.01 mM D-aspartate into astrocytes. Interestingly, there was a residual PMB-TBOA insensitive uptake of D-aspartate in astrocytes at higher exogenous D-aspartate concentrations (0.05-1.0 mM), strongly suggesting that astrocytes have "low-affinity" uptake sites for excitatory amino acid. The PMB-TBOA insensitive D-aspartate uptake in astrocytes was sodium dependent and inhibited by succinate and to certain extent by homocysteate, but not by cystine or DIDS. We suggest that excitatory amino acid is transported into astrocytes in a "low-affinity" fashion by sodium/dicarboxylate transporters.

gamma-Hydroxybutyrate binds to the synaptic site recognizing succinate monocarboxylate: a new hypothesis on astrocyte-neuron interaction via the protonation of succinate

Succinate (SUC), a citrate (CIT) cycle intermediate, and carbenoxolone (CBX), a gap junction inhibitor, were shown to displace [3H]gamma-hydroxybutyrate ([3H]GHB), which is specifically bound to sites present in synaptic membrane subcellular fractions of the rat forebrain and the human nucleus accumbens. Elaboration on previous work revealed that acidic pH-induced specific binding of [3H]SUC occurs, and it has been shown to have a biphasic displacement profile distinguishing high-affinity (K(i,SUC) = 9.1 +/- 1.7 microM) and low-affinity (K(i,SUC) = 15 +/- 7 mM) binding. Both high- and low- affinity sites were characterized by the binding of GHB (K(i,GHB) = 3.9 +/- 0.5 microM and K(i,GHB) = 5.0 +/- 2.0 mM) and lactate (LAC; K(i,LAC) = 3.9 +/- 0.5 microM and K(i,LAC) = 7.7 +/- 0.9 mM). Ligands, including the hemiester ethyl-hemi-SUC, and the gap junction inhibitors flufenamate, CBX, and the GHB binding site-selective NCS-382 interacted with the high-affinity site (in microM: K(i,EHS) = 17 +/- 5, K(i,FFA) = 24 +/- 13, K(i,CBX) = 28 +/- 9, K(i,NCS-382) = 0.8 +/- 0.1 microM). Binding of the Na+,K+-ATPase inhibitor ouabain, the proton-coupled monocarboxylate transporter (MCT)-specific alpha-cyano-hydroxycinnamic acid (CHC), and CIT characterized the low-affinity SUC binding site (in mM: K(i,ouabain) = 0.13 +/- 0.05, K(i,CHC) = 0.32 +/- 0.07, K(i,CIT) = 0.79 +/- 0.20). All tested compounds inhibited [3H]SUC binding in the human nucleus accumbens and had K(i) values similar to those observed in the rat forebrain. The binding process can clearly be recognized as different from synaptic and mitochondrial uptake or astrocytic release of SUC, GHB, and/or CIT by its unique GHB selectivity. The transient decrease of extracellular SUC observed during epileptiform activity suggested that the function of the synaptic target recognizing protonated succinate monocarboxylate may vary under different (patho)physiological conditions. Furthermore, we put forward a hypothesis on the synaptic activity-regulated signaling between astrocytes and neurons via SUC protonation.

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.

Neurodegeneration and chronic renal failure in methylmalonic aciduria--a pathophysiological approach

In the last decades the survival of patients with methylmalonic aciduria has been improved. However, the overall outcome of affected patients remains disappointing. The disease course is often complicated by acute life-threatening metabolic crises, which can result in multiple organ failure or even death, resembling primary defects of mitochondrial energy metabolism. Biochemical abnormalities during metabolic derangement, such as metabolic acidosis, ketonaemia/ketonuria, lactic acidosis, hypoglycaemia and hyperammonaemia, suggest mitochondrial dysfunction. In addition, long-term complications such as chronic renal failure and neurological disease are frequently found. Neuropathophysiological studies have focused on various effects caused by accumulation of putatively toxic organic acids, the so-called 'toxic metabolite' hypothesis. In previous studies, methylmalonate (MMA) has been considered as the major neurotoxin in methylmalonic aciduria, whereas more recent studies have highlighted a synergistic inhibition of mitochondrial energy metabolism (pyruvate dehydrogenase complex, tricarboxylic acid cycle, respiratory chain, mitochondrial salvage pathway of deoxyribonucleoside triphosphate (dNTP)) induced by propionyl-CoA, 2-methylcitrate and MMA as the key pathomechanism of inherited disorders of propionate metabolism. Intracerebral accumulation of toxic metabolites ('trapping' hypothesis') is considered a biochemical risk factor for neurodegeneration. Secondary effects of mitochondrial dysfunction, such as oxidative stress and impaired mtDNA homeostasis, contribute to pathogenesis of these disorders. The underlying pathomechanisms of chronic renal insufficiency in methylmalonic acidurias are not yet understood. We hypothesize that renal and cerebral pathomechanisms share some similarities, such as an involvement of dicarboxylic acid transport. This review aims to give a comprehensive overview on recent pathomechanistic concepts for methylmalonic acidurias.

Biochemistry and bioenergetics of glutaryl-CoA dehydrogenase deficiency

Glutaryl-CoA dehydrogenase (GCDH) is a central enzyme in the catabolic pathway of L-tryptophan, L-lysine, and L-hydroxylysine which catalyses the oxidative decarboxylation of glutaryl-CoA to crotonyl-CoA and CO2. Glutaryl-CoA dehydrogenase deficiency (GDD) is an autosomal recessive disease characterized by the accumulation of glutaric and 3-hydroxyglutaric acids in tissues and body fluids. Untreated patients commonly present with severe striatal degeneration during encephalopathic crises. Previous studies have highlighted primary excitotoxicity as a trigger of striatal degeneration. The aim of this PhD study was to investigate in detail tissue-specific bioenergetic and biochemical parameters of GDD in vitro, post mortem, and in Gcdh-/- mice. The major bioenergetic finding was uncompetitive inhibition of alpha-ketoglutarate dehydrogenase complex by glutaryl-CoA. It is suggested that a synergism of primary and secondary excitotoxic effects in concert with age-related physiological changes in the developing brain underlie acute and chronic neurodegenerative changes in GDD patients. The major biochemical findings were highly elevated cerebral concentrations of glutaric and 3-hydroxyglutaric acid despite low permeability of the blood-brain barrier for these dicarboxylic acids. It can be postulated that glutaric and 3-hydroxyglutaric acids are synthesized de novo and subsequently trapped in the brain. In this light, neurological disease in GDD is not 'transported' to the brain in analogy with phenylketonuria or hepatic encephalopathy as suggested previously but is more likely to be induced by the intrinsic biochemical properties of the cerebral tissue and the blood-brain barrier.

Transport of nicotinate and structurally related compounds by human SMCT1 (SLC5A8) and its relevance to drug transport in the mammalian intestinal tract

PURPOSE. To examine the involvement of human SMCT1, a Na+-coupled transporter for short-chain fatty acids, in the transport of nicotinate/structural analogs and monocarboxylate drugs, and to analyze its expression in mouse intestinal tract.

MATERIALS AND METHODS

We expressed human SMCT1 in X. laevis oocytes and monitored its function by [14C]nicotinate uptake and substrate-induced inward currents. SMCT1 expression in mouse intestinal tract was examined by immunofluorescence.

RESULTS

[14C]Nicotinate uptake was several-fold higher in SMCT1-expressing oocytes than in water-injected oocytes. The uptake was inhibited by short-chain/medium-chain fatty acids and various structural analogs of nicotinate. Exposure of SMCT1-expressing oocytes to nicotinate induced Na+-dependent inward currents. Measurements of nicotinate flux and associated charge transfer into oocytes suggest a Na+:nicotinate stoichiometry of 2:1. Monocarboxylate drugs benzoate, salicylate, and 5-aminosalicylate are also transported by human SMCTI. The transporter is expressed in the small intestine as well as colon, and the expression is restricted to the lumen-facing apical membrane of intestinal and colonic epithelial cells.

CONCLUSIONS

Human SMCTI transports not only nicotinate and its structural analogs but also various monocarboxylate drugs. The transporter is expressed on the luminal membrane of the epithelial cells lining the intestinal tract. SMCT1 may participate in the intestinal absorption of monocarboxylate drugs.

Inhibition of the Na+/dicarboxylate cotransporter by anthranilic acid derivatives

The Na(+)/dicarboxylate cotransporter NaDC1 absorbs citric acid cycle intermediates from the lumen of the small intestine and kidney proximal tubule. No effective inhibitor has been identified yet, although previous studies showed that the nonsteroidal anti-inflammatory drug, flufenamate, inhibits the human (h) NaDC1 with an IC(50) value of 2 mM. In the present study, we have tested compounds related in structure to flufenamate, all anthranilic acid derivatives, as potential inhibitors of hNaDC1. We found that N-(p-amylcinnamoyl) anthranilic acid (ACA) and 2-(p-amylcinnamoyl) amino-4-chloro benzoic acid (ONO-RS-082) are the most potent inhibitors with IC(50) values lower than 15 microM, followed by N-(9-fluorenylmethoxycarbonyl)-anthranilic acid (Fmoc-anthranilic acid) with an IC(50) value of approximately 80 microM. The effects of ACA on NaDC1 are not mediated through a change in transporter protein abundance on the plasma membrane and seem to be independent of its effect on phospholipase A(2) activity. ACA acts as a slow inhibitor of NaDC1, with slow onset and slow reversibility. Both uptake activity and efflux are inhibited by ACA. Other Na(+)/dicarboxylate transporters from the SLC13 family, including hNaDC3 and rbNaDC1, were also inhibited by ACA, ONO-RS-082, and Fmoc-anthranilic acid, whereas the Na(+)/citrate transporter (hNaCT) is much less sensitive to these compounds. The endogenous sodium-dependent succinate transport in Caco-2 cells is also inhibited by ACA. In conclusion, ACA and ONO-RS-082 represent promising lead compounds for the development of specific inhibitors of the Na(+)/dicarboxylate cotransporters.

Membrane topology structure of human high-affinity, sodium-dependent dicarboxylate transporter

High-affinity, sodium-dependent dicarboxylate transporter (NaDC3) is responsible for transport of Krebs cycle intermediates and may involve in regulation of aging and life span. Hydropathy analysis predicts that NaDC3 contains 11 or 12 hydrophobic transmembrane (TM) domains. However, the actual membrane topological structure of NaDC3 remains unknown. In this study, confocal immunofluorescence microscopy and membrane biotinylation of epitope-tagged N and C termini of NaDC3 provide evidence of an extracellular C terminus and an intracellular N terminus, indicating an odd number of transmembrane regions. The position of hydrophilic loops within NaDC3 was identified with antibodies against the loops domains combined with cysteine accessibility methods. A confocal image of membrane localization and transport activity assay of the cysteine insertion mutants show behavior similar to that of wild-type NaDC3 in transfected HEK293 cells, suggesting that these mutants retain a native protein configuration. We find that NaDC3 contains 11 transmembrane helices. The loops 1, 3, 5, 7, and 9 face the extracellular side, and loops 2, 4, 6, and 10 face the cytoplasmic side. A re-entrant loop-like structure between TM8 and TM9 may protrude into the membrane. Our results support the topography of 11 transmembrane domains with an extracellular C terminus and an intracellular N terminus of NaDC3, and for the first time provide experimental evidence for a novel topological model for NaDC3.

Organic anion transporters of the SLC22 family: biopharmaceutical, physiological, and pathological roles

The human organic anion transporters OAT1, OAT2, OAT3, OAT4 and URAT1 belong to a family of poly-specific transporters mainly located in kidneys. Selected OATs occur also in liver, placenta, and brain. OATs interact with endogenous metabolic end products such as urate and acidic neutrotransmitter metabolites, as well as with a multitude of widely used drugs, including antibiotics, antihypertensives, antivirals, anti-inflammatory drugs, diuretics and uricosurics. Thereby, OATs play an important role in renal drug elimination and have an impact on pharmacokinetics. In this review we focus on the interaction of human OATs with drugs. We report the affinities of human OATs for drug classes and compare the putative importance of individual OATs for renal drug excretion. The role of OATs as sites of drug-drug interaction and mediators cell toxicity, their gender-dependent regulation in health and diseased states, and the possible impact of single nucleotide polymorphisms are also dealt with.

Modulation of succinate transport in Hep G2 cell line by PKC

The cellular uptake of the tricarboxylic acid cycle (TCA) intermediates is very important for cellular metabolism. However, the transport pathways for these intermediates in liver cells are not well characterized. We have examined the transport of succinate and citrate in the human hepatoma cell line Hep G2 and found that it exhibited a higher rate of succinate compared to citrate transport, which was sodium dependent. Comparison of the transport properties of Hep G2 to that of human retinal pigment epithelial (HRPE) cells transfected with human sodium dicarboxylate transporters, hNaDC-1, hNaDC-3, and hNaCT indicated that Hep G2 cells express a combination of hNaDC-3 and hNaCT. Short period activation of protein kinase C (PKC) by phorbol 12-myristate, 13-acetate (PMA) and alpha-adrenergic receptor agonist, phenylephrine (PE), downregulated sodium-dependent succinate transport presumably via hNaDC-3. The inhibition by PMA was partially prevented by cytochalasin D, suggesting that PKC reduces the hNaDC-3 activity, at least in part, by increased endocytosis. In contrast, activation of PKA by both forskolin and epidermal growth factor (EGF) had no effect on succinate transport. Our results suggest that Hep G2 cells provide a useful model for studies of di- and tricarboxylate regulation of human liver.

A Specific Dileucine Motif Is Required for the GGA-dependent Entry of Newly Synthesized Insulin-responsive Aminopeptidase into the Insulin-responsive Compartment

In muscle and adipose cells, the insulin-responsive aminopeptidase (IRAP) is localized to intracellular storage sites and undergoes insulin-dependent redistribution to the cell surface. Following expression, the newly synthesized IRAP protein traffics to the perinuclear insulin-sensitive compartment and acquires insulin sensitivity 6-9 h following biosynthesis. Knockdown of GGA1 by RNA interference prevented IRAP from entering, but not exiting, the insulin-responsive compartment. Mutation of the dileucine motif at positions 76 and 77 (EGFP-IRAP/AA(76,77)), but not the dileucine motif at positions 53 and 54, resulted in the rapid default of the reporter to the cell surface beginning at 3 h following biosynthesis. Alanine substitution of 9 residues amino- or carboxyl-terminal to LL(76,77) did not perturb basal intracellular sequestration or abrogate insulin-stimulated IRAP translocation. Moreover, a dominant interfering GGA mutant (VHS-GAT) potently inhibited insulin-stimulated translocation of EGFP-IRAP/WT but did not block the constitutive exocytotic trafficking of EGFP-IRAP/AA(76,77). In addition, the EGFP-IRAP/WT and EGFP-IRAP/AA(76,77) constructs occupied morphologically distinct tubulovesicular compartments in the perinuclear region. Taken together, these data indicate that LL(76,77) functions during the GGA-dependent sorting of newly made IRAP into the insulin-responsive storage compartment.

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.

Identification of basolateral membrane targeting signal of human sodium-dependent dicarboxylate transporter 3

Sodium-dependent dicarboxylate transporters (NaDC) include low-affinity NaDC1 and high-affinity NaDC3. Despite high similarities structurally and functionally, both are localized to opposite surfaces of renal tubular cells. The molecular mechanisms and localization signals leading to this polarized distribution remain unknown. In this study, distribution of NaDC3 in human kidney tissue was firstly observed by immunohistochemistry and immunofluorescence. Then, EGFP-fused wild-type, NH2- and COOH-terminal deletion and point mutants of NaDC3, and chimera between NaDC3 and NaDC1, were generated and transfected into polarized renal cells lines, LLC-PK1 and MDCK. Their subcellular localizations were analyzed by laser confocal microscopy. Immunolocalization results revealed that NaDC3 was expressed at basolateral membrane of human renal proximal tubular epithelia. Confocal examinations showed that wild-type NaDC3 was targeted to the basolateral membrane of MDCK and LLC-PK1. Deletion mutations indicated that the basolateral targeting signal of NaDC3 located within a short sequence AKKVWSARR of its amino-terminal cytoplasmic domain. Addition of this sequence could redirect apical NaDC1 to the basolateral membrane of LLC-PK1. Point mutagenesis revealed that mutation of either of two hydrophobic amino acids V and W in this short sequence largely redirected NaDC3 to both apical and basolateral surfaces of LLC-PK, indicating that the two hydrophobic amino acids are critical for the basolateral targeting of NaDC3. Our studies provide direct evidence of the localization of NaDC3 at the basolateral membrane of human renal proximal tubule cells and identify a di-hydrophobic amino acid motif VW as basolateral localization signal in the N-terminal cytoplasmic domain of NaDC3.

Functional and molecular identification of sodium-coupled dicarboxylate transporters in rat primary cultured cerebrocortical astrocytes and neurons

Na+-coupled carboxylate transporters (NaCs) mediate the uptake of tricarboxylic acid cycle intermediates in mammalian tissues. Of these transporters, NaC3 (formerly known as Na+-coupled dicarboxylate transporter 3, NaDC3/SDCT2) and NaC2 (formerly known as Na+-coupled citrate transporter, NaCT) have been shown to be expressed in brain. There is, however, little information available on the precise distribution and function of both transporters in the CNS. In the present study, we investigated the functional characteristics of Na+-dependent succinate and citrate transport in primary cultures of astrocytes and neurons from rat cerebral cortex. Uptake of succinate was Na+ dependent, Li+ sensitive and saturable with a Michaelis constant (Kt) value of 28.4 microM in rat astrocytes. Na+ activation kinetics revealed that the Na+ to succinate stoichiometry was 3:1 and the concentration of Na+ necessary for half-maximal transport was 53 mM. Although uptake of citrate in astrocytes was also Na+ dependent and saturable, its Kt value was significantly higher (approximately 1.2 mM) than that of succinate. Unlabeled succinate (2 mM) inhibited Na+-dependent [14C]succinate (18 microM) and [14C]citrate (4.5 microM) transport completely, whereas unlabeled citrate inhibited Na+-dependent [14C]succinate uptake more weakly. Interestingly, N-acetyl-L-aspartate, which is the second most abundant amino acid in the nervous system, also completely inhibited Na+-dependent succinate transport in rat astrocytes. The inhibition constant (Ki) for the inhibition of [14C]succinate uptake by unlabeled succinate, N-acetyl-L-aspartate and citrate was 15.9, 155 and 764 microM respectively. In primary cultures of neurons, uptake of citrate was also Na+ dependent and saturable with a Kt value of 16.2 microM, which was different from that observed in astrocytes, suggesting that different Na+-dependent citrate transport systems are expressed in neurons and astrocytes. RT-PCR and immunocytochemistry revealed that NaC3 and NaC2 are expressed in cerebrocortical astrocytes and neurons respectively. These results are in good agreement with our previous reports on the brain distribution pattern of NaC2 and NaC3 mRNA using in situ hybridization. This is the first report of the differential expression of different NaCs in astrocytes and neurons. These transporters might play important roles in the trafficking of tricarboxylic acid cycle intermediates and related metabolites between glia and neurons.

Functional characterization of Na+-coupled citrate transporter NaC2/NaCT expressed in primary cultures of neurons from mouse cerebral cortex

Neurons are known to express a high-affinity Na+ -coupled dicarboxylate transporter(s) for uptake of tricarboxylic acid cycle intermediates, such as alpha-ketoglutarate and malate, which are precursors for neurotransmitters including glutamate and gamma-aminobutyric acid. There is, however, little information available on the molecular identity of the transporters responsible for this uptake process in neurons. In the present study, we investigated the characteristics of Na+ -dependent citrate transport in primary cultures of neurons from mouse cerebral cortex and established the molecular identity of this transport system as the Na+ -coupled citrate transporter (NaC2/NaCT). Reverse transcriptase (RT)-PCR and immunocytochemical analyses revealed that only NaC2/NaCT was expressed in mouse cerebrocortical neurons but not in astrocytes. Uptake of citrate in neurons was Na+ -dependent, Li+ -sensitive, and saturable with the Kt value of 12.3 microM. This Kt value was comparable with that in the case of Na+ -dependent succinate transport (Kt = 9.2 microM). Na+ -activation kinetics revealed that the Na+ -to-citrate stoichiometry was 3.4:1 and concentration of Na+ necessary for half-maximal activation (K0.5(Na)) was 45.7 mM. Na+ -dependent uptake of [14C]citrate (18 microM) was significantly inhibited by unlabeled citrate as well as dicarboxylates such as succinate, malate, fumarate, and alpha-ketoglutarate. This is the first report demonstrating the molecular identity of the Na+ -coupled di/tricarboxylate transport system expressed in neurons as NaC2/NaCT, which can transport the tricarboxylate citrate as well as dicarboxylates such as succinate, alpha-ketoglutarate, and malate.

Intracerebral accumulation of glutaric and 3-hydroxyglutaric acids secondary to limited flux across the blood-brain barrier constitute a biochemical risk factor for neurodegeneration in glutaryl-CoA dehydrogenase deficiency

Glutaric acid (GA) and 3-hydroxyglutaric acids (3-OH-GA) are key metabolites in glutaryl co-enzyme A dehydrogenase (GCDH) deficiency and are both considered to be potential neurotoxins. As cerebral concentrations of GA and 3-OH-GA have not yet been studied systematically, we investigated the tissue-specific distribution of these organic acids and glutarylcarnitine in brain, liver, skeletal and heart muscle of Gcdh-deficient mice as well as in hepatic Gcdh-/- mice and in C57Bl/6 mice following intraperitoneal loading. Furthermore, we determined the flux of GA and 3-OH-GA across the blood-brain barrier (BBB) using porcine brain microvessel endothelial cells. Concentrations of GA, 3-OH-GA and glutarylcarnitine were significantly elevated in all tissues of Gcdh-/- mice. Strikingly, cerebral concentrations of GA and 3-OH-GA were unexpectedly high, reaching similar concentrations as those found in liver. In contrast, cerebral concentrations of these organic acids remained low in hepatic Gcdh-/- mice and after intraperitoneal injection of GA and 3-OH-GA. These results suggest limited flux of GA and 3-OH-GA across the BBB, which was supported in cultured porcine brain capillary endothelial cells. In conclusion, we propose that an intracerebral de novo synthesis and subsequent trapping of GA and 3-OH-GA should be considered as a biochemical risk factor for neurodegeneration in GCDH deficiency.

Differential expression of components of the cardiomyocyte adrenomedullin/intermedin receptor system following blood pressure reduction in nitric oxide-deficient hypertension

Adrenomedullin (AM) and intermedin (IMD; adrenomedulln-2) are vasodilator peptides related to calcitonin gene-related peptide (CGRP). The actions of these peptides are mediated by the calcitonin receptor-like receptor (CLR) in association with one of three receptor activity-modifying proteins. CGRP is selective for CLR/receptor activity modifying protein (RAMP)1, AM for CLR/RAMP2 and -3, and IMD acts at both CGRP and AM receptors. In a model of pressure overload induced by inhibition of nitric-oxide synthase, up-regulation of AM was observed previously in cardiomyocytes demonstrating a hypertrophic phenotype. The current objective was to examine the effects of blood pressure reduction on cardiomyocyte expression of AM and IMD and their receptor components. Nomega-nitro-L-arginine methyl ester (L-NAME) (35 mg/kg/day) was administered to rats for 8 weeks, with or without concurrent administration of hydralazine (50 mg/kg/day) and hydrochlorothiazide (7.5 mg/kg/day). In left ventricular cardiomyocytes from L-NAME-treated rats, increases (-fold) in mRNA expression were 1.6 (preproAM), 8.4 (preproIMD), 3.4 (CLR), 4.1 (RAMP1), 2.8 (RAMP2), and 4.4 (RAMP3). Hydralazine/hydrochlorothiazide normalized systolic blood pressure (BP) and abolished mRNA up-regulation of hypertrophic markers sk-alpha-actin and BNP and of preproAM, CLR, RAMP2, and RAMP3 but did not normalize cardiomyocyte width nor preproIMD or RAMP1 mRNA expression. The robust increase in IMD expression indicates an important role for this peptide in the cardiac pathology of this model but, unlike AM, IMD is not associated with pressure overload upon the myocardium. The concordance of IMD and RAMP1 up-regulation indicates a CGRP-type receptor action; considering also a lack of response to BP reduction, IMD may, like CGRP, have an anti-ischemic function.