Evidence for allosteric regulation of pH-sensitive System A (SNAT2) and System N (SNAT5) amino acid transporter activity involving a conserved histidine residue

The SLC38A5/SNAT5 amino acid transporter: from pathophysiology to pro-cancer roles in the tumor microenvironment

SLC38A5/SNAT5 is a system N transporter that can mediate net inward or outward transmembrane fluxes of neutral amino acids coupled with Na+ (symport) and H+ (antiport). Its preferential substrates are not only amino acids with side chains containing amide (glutamine and asparagine) or imidazole (histidine) groups, but also serine, glycine, and alanine are transported by the carrier. Expressed in the pancreas, intestinal tract, brain, liver, bone marrow, and placenta, it is regulated at mRNA and protein levels by mTORC1 and WNT/β-catenin pathways, and it is sensitive to pH, nutritional stress, inflammation, and hypoxia. SNAT5 expression has been found to be altered in pathological conditions such as chronic inflammatory diseases, gestational complications, chronic metabolic acidosis, and malnutrition. Growing experimental evidence shows that SNAT5 is overexpressed in several types of cancer cells. Moreover, recently published results indicate that SNAT5 expression in stromal cells can support the metabolic exchanges occurring in the tumor microenvironment of asparagine-auxotroph tumors. We review the functional role of the SNAT5 transporter in pathophysiology and propose that, due to its peculiar operational and regulatory features, SNAT5 may play important pro-cancer roles when expressed either in neoplastic or in stromal cells of glutamine-auxotroph tumors.NEW & NOTEWORTHY The transporter SLC38A5/SNAT5 provides net influx or efflux of glutamine, asparagine, and serine. These amino acids are of particular metabolic relevance in several conditions. Changes in transporter expression or activity have been described in selected types of human cancers, where SNAT5 can mediate amino acid exchanges between tumor and stromal cells, thus providing a potential therapeutic target. This is the first review that recapitulates the characteristics and roles of the transporter in physiology and pathology.

Sodium-Dependent Neutral Amino Acid Transporter 2 Can Serve as a Tertiary Carrier for l-Type Amino Acid Transporter 1-Utilizing Prodrugs

Membrane transporters are the key determinants of the homeostasis of endogenous compounds in the cells and their exposure to drugs. However, the substrate specificities of distinct transporters can overlap. In the present study, the interactions of l-type amino acid transporter 1 (LAT1)-utilizing prodrugs with sodium-coupled neutral amino acid transporter 2 (SNAT2) were explored. The results showed that the cellular uptake of LAT1-utilizing prodrugs into a human breast cancer cell line, MCF-7 cells, was mediated via SNATs as the uptake was increased at higher pH (8.5), decreased in the absence of sodium, and inhibited in the presence of unselective SNAT-inhibitor, (α-(methylamino)isobutyric acid, MeAIB). Moreover, docking the compounds to a SNAT2 homology model (inward-open conformation) and further molecular dynamics simulations and the subsequent trajectory and principal component analyses confirmed the chemical features supporting the interactions of the studied compounds with SNAT2, which was found to be the main SNAT expressed in MCF-7 cells.

Low pH up-regulates interleukin-6 mRNA in L6-G8C5 rat skeletal muscle cells independent of pH sensing by SNAT2(SLC38A2) transporters

Exercise is known to create a transient, but potent increase in skeletal muscle expression of potentially anti-inflammatory myokine interleukin-6 (IL-6). This effect may be clinically important in managing chronic inflammatory states. It has previously been proposed that lactic acidosis following exercise promotes this IL-6 up-regulation, but the mechanism of this acidosis effect is unknown. Rat skeletal muscle cell line L6-G8C5 has been used previously to model metabolic effects of acidosis, sensing low pH through the resulting inhibition of amino acid transporter SNAT2(SLC38A2). Use of ionophore ionomycin to model the rise in intracellular Ca2+ concentration occurring in contracting muscle strongly up-regulates IL-6 mRNA in L6-G8C5 myotubes. This study used this model to test the hypothesis that low extracellular pH (7.1) enhances ionomycin-induced IL-6 mRNA up-regulation by inhibiting SNAT2. Incubation of L6-G8C5 myotubes for 6 h with 0.5 µM ionomycin at control pH (7.4) resulted in a 15-fold increase in IL-6 mRNA which was further enhanced (1.74-fold) at pH 7.1. In contrast low pH had no significant effect on IL-6 mRNA without ionomycin, nor on the IL-6 mRNA increase that was induced by cyclic stretch. Even though pH 7.1 halved the transport activity of SNAT2, alternative methods of SNAT2 inhibition (JNK inhibitor SP600125; SNAT2 antagonist MeAIB; or SNAT2 silencing with siRNA) did not mimic the enhancing effect of low pH on IL-6 mRNA. On the contrary, JNK inhibition blunted the effect of pH 7.1 with ionomycin, but had no effect at pH 7.4. It is concluded that low pH promotes Ca2+/ionomycin-induced up-regulation of IL-6 mRNA through a novel SNAT2-independent JNK-dependent pH-sensing pathway not previously described in this skeletal muscle model.

Reduction of Rapid Proliferating Tumour Cell Lines by Inhibition of the Specific Glycine Transporter GLYT1

Studies have highlighted the relevance of extracellular glycine and serine in supporting high growth rates of rapidly proliferating tumours. The present study analysed the role of the specific glycine transporter GLYT1 in supplying glycine to cancer cells and maintaining cell proliferation. GLYT1 knockdown in the rapidly proliferating tumour cell lines A549 and HT29 reduced the number of viable cells by approximately 30% and the replication rate presented a decrease of about 50% when compared to cells transfected with control siRNA. In contrast, when compared to control, GLYT1 siRNA had only a minimal effect on cell number of the slowly proliferating tumour cell line A498, reducing the number of viable cells by 7% and no significant difference was observed when analysing the replication rate between GLYT1 knockdown and control group. When utilising a specific GLYT1 inhibitor, ALX-5407, the doubling time of rapidly proliferating cells increased by about 8 h presenting a significant reduction in the number of viable cells after 96 h treatment when compared to untreated cells. Therefore, these results suggest that GLYT1 is required to maintain high proliferation rates in rapidly proliferating cancer cells and encourage further investigation of GLYT1 as a possible target in a novel therapeutic approach.

Dexamethasone acutely suppresses the anabolic SNAT2/SLC38A2 amino acid transporter protein in L6-G8C5 rat skeletal muscle cells

Chronic metabolic acidosis plays a role in cachexia by enhancing total proteolysis in skeletal muscle. Glucocorticoid also triggers proteolysis and plays a permissive role in the effect of acidosis. The System A amino acid transporter SNAT2/SLC38A2 is ubiquitously expressed in mammalian cells including muscle, performing Na⁺‐dependent active import of neutral amino acids, and is strongly inhibited by low pH. Exposure of rat skeletal muscle cell line L6‐G8C5 to low pH rapidly inhibits SNAT2 transport activity and enhances total proteolysis rate. Pharmacological inhibition or silencing of SNAT2 also enhances proteolysis. This study tests the hypothesis that the glucocorticoid dexamethasone (DEX), like low pH, inhibits SNAT2 activity in L6‐G8C5 myotubes, thus contributing to total proteolysis. Incubation with 500 nM DEX for 4 h reduced the System A amino acid transport rate to half the rate in control cultures. This inhibition depended on glucocorticoid receptor‐mediated gene transcription, but SNAT2 mRNA levels were unaffected by DEX. In contrast, the SNAT2 protein assessed by immunoblotting was significantly depleted. The co‐inhibitory effects of DEX and low pH on System A transport activity were additive in stimulating total proteolysis. In keeping with this mechanism, DEX’s inhibitory effect on SNAT2 transport activity was significantly blunted by the proteasome inhibitor MG132. Proof of principle was achieved in similar experiments using recombinant expression of a GFP‐tagged SNAT2 fusion protein in HEK293A cells. It is concluded that DEX acutely depletes the SNAT2 transporter protein, at least partly through proteasome‐dependent degradation of this functionally important transporter.

Interplay of Carbonic Anhydrase IX With Amino Acid and Acid/Base Transporters in the Hypoxic Tumor Microenvironment

Solid tumors are challenged with a hypoxic and nutrient-deprived microenvironment. Hence, hypoxic tumor cells coordinatively increase the expression of nutrient transporters and pH regulators to adapt and meet their bioenergetic and biosynthetic demands. Carbonic Anhydrase IX (CAIX) is a membrane-bound enzyme that plays a vital role in pH regulation in the tumor microenvironment (TME). Numerous studies have established the importance of CAIX in mediating tumor progression and metastasis. To understand the mechanism of CAIX in mediating tumor progression, we performed an unbiased proteomic screen to identify the potential interactors of CAIX in the TME using the proximity-dependent biotin identification (BioID) technique. In this review, we focus on the interactors from this BioID screen that are crucial for nutrient and metabolite transport in the TME. We discuss the role of transport metabolon comprising CAIX and bicarbonate transporters in regulating intra- and extracellular pH of the tumor. We also discuss the role of amino acid transporters that are high confidence interactors of CAIX, in optimizing favorable metabolic state for tumor progression, and give our perspective on the coordinative interplay of CAIX with the amino acid transporters in the hypoxic TME.

Protonation-Induced Dynamic Allostery in PDZ Domain: Evidence of Perturbation-Independent Universal Response Network

Dynamic allostery is a relatively new paradigm where certain external perturbations may lead to modulation of conformational dynamics at a distant part of a protein without significant changes in the overall structure. While most well-characterized examples of dynamic allostery involve binding with other entities like small molecules, peptides, or nucleic acids, in this work we demonstrate that chemical modifications like protonation may lead to significant dynamical allosteric response in a PDZ domain protein. Tuning the protonation states of two histidine residues (H317 and H372), we identify the allosteric pathways responsible for the dynamic response. Interestingly, the same set of residues that constitute the allosteric response network upon ligand binding seem to be responsible for protonation-induced dynamic allostery. Thus, we propose the existence of an inherent universal response network in signaling proteins, where the same set of residues can respond to varying types of external perturbations in terms of rearrangement of hydrogen-bonded network and redistribution of electrostatic interaction energies.

Involvement of sodium-coupled neutral amino acid transporters (system A) in l-proline transport in the rat retinal pericytes

The retinal pericytes contribute to the supply of collagen to the basement membrane, and thus, form the structural support of the blood-retinal barrier. Since l-proline (L-Pro) is a major component of collagen, the uptake of L-Pro is an important process for the synthesis of collagen. This study was aimed to elucidate L-Pro transport mechanism(s) in the retinal pericytes. The transport of [³H]L-Pro was evaluated in the conditionally immortalized rat retinal pericyte cell line, TR-rPCT1 cells. The expression of the candidate transporter was examined by qualitative/quantitative reverse transcription-polymerase chain reaction, immunoblot analysis, and immunostaining. The [³H]L-Pro uptake by TR-rPCT1 cells showed Na⁺-dependence, Cl^--independence, and concentration-dependence with a K_m of 810 μM. The substrates for system A, such as 2-(methylamino)isobutyric acid (MeAIB), significantly inhibited the L-Pro uptake, suggesting the involvement of system A in the uptake of L-Pro. Among the subtypes of system A, the mRNA expression levels of ATA2 were the highest in TR-rPCT1 cells. Immunostaining analysis of the isolated rat retinal capillaries containing pericytes indicated the protein expression of ATA2 in retinal pericytes. In conclusion, it is suggested that ATA2, at least in part, is involved in the transport of L-Pro in the retinal pericytes.

Multifaceted regulation of the system A transporter Slc38a2 suggests nanoscale regulation of amino acid metabolism and cellular signaling

Amino acids are essential for cellular protein synthesis, growth, metabolism, signaling and in stress responses. Cell plasma membranes harbor specialized transporters accumulating amino acids to support a variety of cellular biochemical pathways. Several transporters for neutral amino acids have been characterized. However, Slc38a2 (also known as SA1, SAT2, ATA2, SNAT2) representing the classical transport system A activity stands in a unique position: Being a secondarily active transporter energized by the electrochemical gradient of Na⁺, it creates steep concentration gradients for amino acids such as glutamine: this may subsequently drive the accumulation of additional neutral amino acids through exchange via transport systems ASC and L. Slc38a2 is ubiquitously expressed, yet in a cell-specific manner. In this review, we show that Slc38a2 is regulated at the transcriptional and translational levels as well as by ions and proteins through direct interactions. We describe how Slc38a2 senses amino acid availability and passes this onto intracellular signaling pathways and how it regulates protein synthesis, cellular proliferation and apoptosis through the mechanistic (mammalian) target of rapamycin (mTOR) and general control nonderepressible 2 (GCN2) pathways. Furthermore, we review how this extensively regulated transporter contributes to cellular osmoadaptation and how it is regulated by endoplasmic reticulum stress and various hormonal stimuli to promote cellular metabolism, cellular signaling and cell survival. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Effects of Sodium and Amino Acid Substrate Availability upon the Expression and Stability of the SNAT2 (SLC38A2) Amino Acid Transporter

The SNAT2 (SLC38A2) System A amino acid transporter mediates Na⁺-coupled cellular uptake of small neutral α-amino acids (AAs) and is extensively regulated in response to humoral and nutritional cues. Understanding the basis of such regulation is important given that AA uptake via SNAT2 has been linked to activation of mTORC1; a major controller of many important cellular processes including, for example, mRNA translation, lipid synthesis, and autophagy and whose dysregulation has been implicated in the development of cancer and conditions such as obesity and type 2 diabetes. Extracellular AA withdrawal induces an adaptive upregulation of SNAT2 gene transcription and SNAT2 protein stability but, as yet, the sensing mechanism(s) that initiate this response remain poorly understood although interactions between SNAT2 and its substrates may play a vital role. Herein, we have explored how changes in substrate (AA and Na⁺) availability impact upon the adaptive regulation of SNAT2 in HeLa cells. We show that while AA deprivation induces SNAT2 gene expression, this induction was not apparent if extracellular Na⁺ was removed during the AA withdrawal period. Furthermore, we show that the increase in SNAT2 protein stability associated with AA withdrawal is selectively repressed by provision of SNAT2 AA substrates (N-methylaminoisobutyric acid and glutamine), but not non-substrates. This stabilization and substrate-induced repression were critically dependent upon the cytoplasmic N-terminal tail of SNAT2 (containing lysyl residues which are putative targets of the ubiquitin-proteasome system), because “grafting” this tail onto SNAT5, a related SLC38 family member that does not exhibit adaptive regulation, confers substrate-induced changes in stability of the SNAT2-5 chimeric transporter. In contrast, expression of SNAT2 in which the N-terminal lysyl residues were mutated to alanine rendered the transporter stable and insensitive to substrate-induced changes in protein stability. Intriguingly, SNAT2 protein stability was dramatically reduced in the absence of extracellular Na⁺ irrespective of whether substrate AAs were present or absent. Our findings indicate that the presence of extracellular Na⁺ (and potentially its binding to SNAT2) may be crucial for not only sensing SNAT2 AA occupancy and consequently for initiating the adaptive response under AA insufficient conditions, but for enabling substrate-induced changes in SNAT2 protein stability.

The neuronal and astrocytic protein SLC38A10 transports glutamine, glutamate, and aspartate, suggesting a role in neurotransmission

In brain cells, glutamine transporters are vital to monitor and control the levels of glutamate and GABA. There are 11 members of the SLC38 family of amino acid transporters of which eight have been functionally characterized. Here, we report the first histological and functional characterization of the previously orphan member, SLC38A10. We used pairwise global sequence alignments to determine the sequence identity between the SLC38 family members. SLC38A10 was found to share 20–25% transmembrane sequence identity with several family members, and was predicted to have 11 transmembrane helices. SLC38A10 immunostaining was abundant in mouse brain using a custom‐made anti‐SLC38A10 antibody and colocalization of SLC38A10 immunoreactivity with markers for neurons and astrocytes was detected. Using Xenopus laevis oocytes overexpressing SLC38A10, we show that SLC38A10 mediates bidirectional transport of l‐glutamine, l‐alanine, l‐glutamate, and d‐aspartate, and efflux of l‐serine. This profile mostly resembles system A members of the SLC38 family. In conclusion, the bidirectional transport of glutamine, glutamate, and aspartate by SLC38A10, and the immunostaining detected in neurons and astrocytes, suggest that SLC38A10 plays a role in pathways involved in neurotransmission.

Glutamine/Glutamate Transporters in Glial Cells: Much More Than Participants of a Metabolic Shuttle

Glial glutamine and glutamate transporters play an important role in glial/neuronal interactions. An excellent model to establish the role of these membrane proteins is the cerebellum. The most abundant glutamatergic synapse in the central nervous system is present in the molecular layer of the cerebellar cortex, and it is entirely wrapped by Bergmann glial cells. The recycling of glutamate involves glutamate and glutamine transporters enriched in these radial glial processes. The functional properties of amino acid glial transporters allow, in an activity-dependent manner, the conformation of protein complexes important for the adequate support of glutamatergic neurotransmission. A detailed description of the most important features of glial glutamate and glutamine transporters follows, and a working model of the molecular mechanisms by which these glutamate and glutamine binding proteins interact, and by these means might modulate cerebellar glutamatergic transactions, is presented.

Identification of a Disulfide Bridge in Sodium-Coupled Neutral Amino Acid Transporter 2(SNAT2) by Chemical Modification

Sodium-coupled neutral amino acid transporter 2 (SNAT2) belongs to solute carrier 38 (SLC38) family of transporters, which is ubiquitously expressed in mammalian tissues and mediates transport of small, neutral amino acids, exemplified by alanine(Ala, A). Yet structural data on SNAT2, including the relevance of intrinsic cysteine residues on structure and function, is scarce, in spite of its essential roles in many tissues. To better define the potential of intrinsic cysteines to form disulfide bonds in SNAT2, mutagenesis experiments and thiol-specific chemical modifications by N-ethylmaleimide (NEM) and methoxy-polyethylene glycol maleimide (mPEG-Mal, MW 5000) were performed, with or without the reducing regent dithiothreitol (DTT) treatment. Seven single mutant transporters with various cysteine (Cys, C) to alanine (Ala, A) substitutions, and a C245,279A double mutant were introduced to SNAT2 with a hemagglutinin (HA) tag at the C-terminus. The results showed that the cells expressing C245A or C279A were labeled by one equivalent of mPEG-Mal in the presence of DTT, while wild-type or all the other single Cys to Ala mutants were modified by two equivalents of mPEG-Mal. Furthermore, the molecular weight of C245,279A was not changed in the presence or absence of DTT treatment. The results suggest a disulfide bond between Cys245 and Cys279 in SNAT2 which has no effect on cell surface trafficking, as well as transporter function. The proposed disulfide bond may be important to delineate proximity in the extracellular domain of SNAT2 and related proteins.

Allosteric sites can be identified based on the residue-residue interaction energy difference

Allosteric drugs act at a distance to regulate protein functions. They have several advantages over conventional orthosteric drugs, including diverse regulation types and fewer side effects. However, the rational design of allosteric ligands remains a challenge, especially when it comes to the identification allosteric binding sites. As the binding of allosteric ligands may induce changes in the pattern of residue-residue interactions, we calculated the residue-residue interaction energies within the allosteric site based on the molecular mechanics generalized Born surface area energy decomposition scheme. Using a dataset of 17 allosteric proteins with structural data for both the apo and the ligand-bound state available, we used conformational ensembles generated by molecular dynamics simulations to compute the differences in the residue-residue interaction energies in known allosteric sites from both states. For all the known sites, distinct interaction energy differences (>25%) were observed. We then used CAVITY, a binding site detection program to identify novel putative allosteric sites in the same proteins. This yielded a total of 31 "druggable binding sites," of which 21 exhibited >25% difference in residue interaction energies, and were hence predicted as novel allosteric sites. Three of the predicted allosteric sites were supported by recent experimental studies. All the predicted sites may serve as novel allosteric sites for allosteric ligand design. Our study provides a computational method for identifying novel allosteric sites for allosteric drug design.

Mechanism of Fine-tuning pH Sensors in Proprotein Convertases: IDENTIFICATION OF A pH-SENSING HISTIDINE PAIR IN THE PROPEPTIDE OF PROPROTEIN CONVERTASE 1/3

The propeptides of proprotein convertases (PCs) regulate activation of cognate protease domains by sensing pH of their organellar compartments as they transit the secretory pathway. Earlier experimental work identified a conserved histidine-encoded pH sensor within the propeptide of the canonical PC, furin. To date, whether protonation of this conserved histidine is solely responsible for PC activation has remained unclear because of the observation that various PC paralogues are activated at different organellar pH values. To ascertain additional determinants of PC activation, we analyzed PC1/3, a paralogue of furin that is activated at a pH of ∼5.4. Using biophysical, biochemical, and cell-based methods, we mimicked the protonation status of various histidines within the propeptide of PC1/3 and examined how such alterations can modulate pH-dependent protease activation. Our results indicate that whereas the conserved histidine plays a crucial role in pH sensing and activation of this protease an additional histidine acts as a "gatekeeper" that fine-tunes the sensitivity of the PC1/3 propeptide to facilitate the release inhibition at higher proton concentrations when compared with furin. Coupled with earlier analyses that highlighted the enrichment of the amino acid histidine within propeptides of secreted eukaryotic proteases, our work elucidates how secreted proteases have evolved to exploit the pH of the secretory pathway by altering the spatial juxtaposition of titratable groups to regulate their activity in a spatiotemporal fashion.

Characterization and Regulation of the Amino Acid Transporter SNAT2 in the Small Intestine of Piglets

The sodium-dependent neutral amino acid transporter 2 (SNAT2), which has dual transport/receptor functions, is well documented in eukaryotes and some mammalian systems, but has not yet been verified in piglets. The objective of this study was to investigate the characteristics and regulation of SNAT2 in the small intestine of piglets. The 1,521-bp porcine full cDNA sequence of SNAT2 (KC769999) from the small intestine of piglets was cloned. The open reading frame of cDNA encodes 506 deduced amino acid residues with a calculated molecular mass of 56.08 kDa and an isoelectric point (pI) of 7.16. Sequence alignment and phylogenetic analysis revealed that SNAT2 is highly evolutionarily conserved in mammals. SNAT2 mRNA can be detected in the duodenum, jejunum and ileum by real-time quantitative PCR. During the suckling period from days 1 to 21, the duodenum had the highest abundance of SNAT2 mRNA among the three segments of the small intestine. There was a significant decrease in the expression of SNAT2 mRNA in the duodenal and jejunal mucosa and in the expression of SNAT2 protein in the jejunal and ileal mucosa on day 1 after weaning (P < 0.05). Studies with enterocytes in vitro showed that amino acid starvation and supplementation with glutamate, arginine or leucine enhanced, while supplementation with glutamine reduced, SNAT2 mRNA expression (P < 0.05). These results regarding the characteristics and regulation of SNAT2 should help to provide some information to further clarify its roles in the absorption of amino acids and signal transduction in the porcine small intestine.

Involvement of Histidine Residue His382 in pH Regulation of MCT4 Activity

Monocarboxylate transporter 4 (MCT4) is a pH-dependent bi-directional lactate transporter. Transport of lactate via MCT4 is increased by extracellular acidification. We investigated the critical histidine residue involved in pH regulation of MCT4 function. Transport of lactate via MCT4 was measured by using a Xenopus laevis oocyte expression system. MCT4-mediated lactate transport was inhibited by Zn2+ in a pH physiological condition but not in an acidic condition. The histidine modifier DEPC (diethyl pyrocarbonate) reduced MCT4 activity but did not completely inactivate MCT4. After treatment with DEPC, pH regulation of MCT4 function was completely knocked out. Inhibitory effects of DEPC were reversed by hydroxylamine and suppressed in the presence of excess lactate and Zn2+. Therefore, we performed an experiment in which the extracellular histidine residue was replaced with alanine. Consequently, the pH regulation of MCT4-H382A function was also knocked out. Our findings demonstrate that the histidine residue His382 in the extracellular loop of the transporter is essential for pH regulation of MCT4-mediated substrate transport activity.

Expression of glutamine transporter isoforms in cerebral cortex of rats with chronic hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric disorder that occurs due to acute and chronic liver diseases, the hallmark of which is the increased levels of ammonia and subsequent alterations in glutamine synthesis, i.e. conditions associated with the pathophysiology of HE. Under physiological conditions, glutamine is fundamental for replenishment of the neurotransmitter pools of glutamate and GABA. The different isoforms of glutamine transporters play an important role in the transfer of this amino acid between astrocytes and neurons. A disturbance in the GABA biosynthetic pathways has been described in bile duct ligated (BDL) rats, a well characterized model of chronic HE. Considering that glutamine is important for GABA biosynthesis, altered glutamine transport and the subsequent glutamate/GABA-glutamine cycle efficacy might influence these pathways. Given this potential outcome, the aim of the present study was to investigate whether the expression of the glutamine transporters SAT1, SAT2, SN1 and SN2 would be affected in chronic HE. We verified that mRNA expression of the neuronal glutamine transporters SAT1 and SAT2 was found unaltered in the cerebral cortex of BDL rats. Similarly, no changes were found in the mRNA level for the astrocytic transporter SN1, whereas the gene expression of SN2 was increased by two-fold in animals with chronic HE. However, SN2 protein immuno-reactivity did not correspond with the increase in gene transcription since it remained unaltered. These data indicate that the expression of the glutamine transporter isoforms is unchanged during chronic HE, and thus likely not to participate in the pathological mechanisms related to the imbalance in the GABAergic neurotransmitter system observed in this neurologic condition.

Glutamate metabolism and HIV-associated neurocognitive disorders

HIV-1 infection can lead to neurocognitive impairment collectively known as HIV-associated neurocognitive disorders (HAND). Although combined antiretroviral treatment (cART) has significantly ameliorated HIV's morbidity and mortality, persistent neuroinflammation and neurocognitive dysfunction continue. This review focuses on the current clinical and molecular evidence of the viral and host factors that influence glutamate-mediated neurotoxicity and neuropathogenesis as an important underlying mechanism during the course of HAND development. In addition, discusses potential pharmacological strategies targeting the glutamatergic system that may help prevent and improve neurological outcomes in HIV-1-infected subjects.

The SLC38 family of sodium-amino acid co-transporters

Transporters of the SLC38 family are found in all cell types of the body. They mediate Na(+)-dependent net uptake and efflux of small neutral amino acids. As a result they are particularly expressed in cells that grow actively, or in cells that carry out significant amino acid metabolism, such as liver, kidney and brain. SLC38 transporters occur in membranes that face intercellular space or blood vessels, but do not occur in the apical membrane of absorptive epithelia. In the placenta, they play a significant role in the transfer of amino acids to the foetus. Members of the SLC38 family are highly regulated in response to amino acid depletion, hypertonicity and hormonal stimuli. SLC38 transporters play an important role in amino acid signalling and have been proposed to act as transceptors independent of their transport function. The structure of SLC38 transporters is characterised by the 5 + 5 inverted repeat fold, which is observed in a wide variety of transport proteins.

Capturing cooperative interactions with the PSI-MI format

The complex biological processes that control cellular function are mediated by intricate networks of molecular interactions. Accumulating evidence indicates that these interactions are often interdependent, thus acting cooperatively. Cooperative interactions are prevalent in and indispensible for reliable and robust control of cell regulation, as they underlie the conditional decision-making capability of large regulatory complexes. Despite an increased focus on experimental elucidation of the molecular details of cooperative binding events, as evidenced by their growing occurrence in literature, they are currently lacking from the main bioinformatics resources. One of the contributing factors to this deficiency is the lack of a computer-readable standard representation and exchange format for cooperative interaction data. To tackle this shortcoming, we added functionality to the widely used PSI-MI interchange format for molecular interaction data by defining new controlled vocabulary terms that allow annotation of different aspects of cooperativity without making structural changes to the underlying XML schema. As a result, we are able to capture cooperative interaction data in a structured format that is backward compatible with PSI-MI–based data and applications. This will facilitate the storage, exchange and analysis of cooperative interaction data, which in turn will advance experimental research on this fundamental principle in biology.

Database URL:http://psi-mi-cooperativeinteractions.embl.de/

Evolutionary origin of amino acid transporter families SLC32, SLC36 and SLC38 and physiological, pathological and therapeutic aspects

About 25% of all solute carriers (SLCs) are likely to transport amino acids as their primary substrate. One of the major phylogenetic clusters of amino acid transporters from the SLC family is the β-family, which is part of the PFAM APC clan. The β-family includes three SLC families, SLC32, SLC36 and SLC38 with one, four and eleven members in humans, respectively. The most well characterized genes within these families are the vesicular inhibitory amino acid transporter (VIAAT, SLC32A1), PAT1 (SLC36A1), PAT2 (SLC36A2), PAT4 (SLC36A4), SNAT1 (SLC38A1), SNAT2 (SLC38A2), SNAT3 (SLC38A3), and SNAT4 (SLC38A4). Here we review the structural characteristics and functional role of these transporters. We also mined the complete protein sequence datasets for nine different genomes to clarify the evolutionary history of the β-family of transporters. We show that all three main branches of the this family are found as far back as green algae suggesting that genes from these families existed in the early eukaryote before the split of animals and plants and that they are present in most animal species. We also address the potential of further drug development within this field highlighting the important role of these transporters in neurotransmission and transport of amino acids as nutrients.

Combined walking exercise and alkali therapy in patients with CKD4-5 regulates intramuscular free amino acid pools and ubiquitin E3 ligase expression

Muscle-wasting in chronic kidney disease (CKD) arises from several factors including sedentary behaviour and metabolic acidosis. Exercise is potentially beneficial but might worsen acidosis through exercise-induced lactic acidosis. We studied the chronic effects of exercise in CKD stage 4-5 patients (brisk walking, 30 min, 5 times/week), and non-exercising controls; each group receiving standard oral bicarbonate (STD), or additional bicarbonate (XS) (Total n = 26; Exercising + STD n = 9; Exercising +XS n = 6; Control + STD n = 8; Control + XS n = 3). Blood and vastus lateralis biopsies were drawn at baseline and 6 months. The rise in blood lactate in submaximal treadmill tests was suppressed in the Exercising + XS group. After 6 months, intramuscular free amino acids (including the branched chain amino acids) in the Exercising + STD group showed a striking chronic depletion. This did not occur in the Exercising + XS group. The effect in Exercising + XS patients was accompanied by reduced transcription of ubiquitin E3-ligase MuRF1 which activates proteolysis via the ubiquitin-proteasome pathway. Other anabolic indicators (Akt activation and suppression of the 14 kDa actin catabolic marker) were unaffected in Exercising + XS patients. Possibly because of this, overall suppression of myofibrillar proteolysis (3-methylhistidine output) was not observed. It is suggested that alkali effects in exercisers arose by countering exercise-induced acidosis. Whether further anabolic effects are attainable on combining alkali with enhanced exercise (e.g. resistance exercise) merits further investigation.

Considering protonation as a posttranslational modification regulating protein structure and function

Posttranslational modification is an evolutionarily conserved mechanism for regulating protein activity, binding affinity, and stability. Compared with established posttranslational modifications such as phosphorylation or ubiquitination, posttranslational modification by protons within physiological pH ranges is a less recognized mechanism for regulating protein function. By changing the charge of amino acid side chains, posttranslational modification by protons can drive dynamic changes in protein conformation and function. Addition and removal of a proton is rapid and reversible and, in contrast to most other posttranslational modifications, does not require an enzyme. Signaling specificity is achieved by only a minority of sites in proteins titrating within the physiological pH range. Here, we examine the structural mechanisms and functional consequences of proton posttranslational modification of pH-sensing proteins regulating different cellular processes.

The system N transporter SN2 doubles as a transmitter precursor furnisher and a potential regulator of NMDA receptors

Activation of NMDA receptor requires two co-agonists, glutamate and glycine. Despite its intrinsic role in brain functions molecular mechanisms involved in glutamate replenishment and identification of the origin of glycine have eluded characterization. We have performed direct measurements of glycine flux by SN2 (Slc38a5; also known as SNAT5), executed extensive electrophysiological characterization as well as implemented ratiometric analyses to show that SN2 transport resembles SN1 in mechanism but differ in functional implications. We report that rat SN2 mediates electroneutral and bidirectional transport of glutamine and glycine at perisynaptic astroglial membranes. Sophisticated coupled and uncoupled movements of H(+) differentially associate with glutamine and glycine transport by SN2 and regulate pH(i) and the release mode of the transporter. Consequently, SN2 doubles as a transmitter precursor furnisher and a potential regulator of NMDA receptors.

Inner blood-retinal barrier mediates l-isomer-predominant transport of serine

D-serine, a coagonist for N-methyl-D-aspartate-type glutamate receptors, which mediate visual signal transmission, is thought to be generated from L-serine via serine racemase in the retina. However, the source of L-serine and D-serine in the retina are yet to be determined. The purpose of the present study was to investigate the characteristics of the blood-to-retina transport of serine at the inner blood-retinal barrier (BRB). In vivo study revealed the blood-to-retina transport of [(3) H]L-serine with an influx clearance of 49.9 μL/(min·g retina), which is greater than that of [(3) H]D-serine. This was consistent with the L-isomer-predominant uptake of serine by conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells), an in vitro inner BRB model. [(3) H]L-Serine and [(3) H]D-serine uptake by TR-iBRB2 cells took place in an Na(+)-dependent and a concentration-dependent manner with Michaelis constant values of 97.5 μM and 9.63 mM, respectively. The uptake process of [(3) H]L-serine and [(3) H]D-serine was significantly inhibited by system ASC (alanine-serine-cysteine) substrates. Polymerase chain reaction analysis and immunocytochemistry revealed the expression of ASC transporters ASCT1 and ASCT2 in TR-iBRB2 cells. These results suggest that the system ASC at the inner BRB is a potent pathway for supplying serine in the form of the L-isomer from the circulating blood to the retina.

The C-terminal domain of the neutral amino acid transporter SNAT2 regulates transport activity through voltage-dependent processes

SNAT (sodium-coupled neutral amino acid transporter) 2 belongs to the SLC38 (solute carrier 38) family of solute transporters. Transport of one amino acid molecule into the cell is driven by the co-transport of one Na(+) ion. The functional significance of the C-terminus of SNAT2, which is predicted to be located in the extracellular space, is currently unknown. In the present paper, we removed 13 amino acid residues from the SNAT2 C-terminus and studied the effect of this deletion on transporter function. The truncation abolished amino acid transport currents at negative membrane potentials (<0 mV), as well as substrate uptake. However, transport currents were observed at positive membrane potentials demonstrating that transport was accelerated while the driving force decreased. Membrane expression levels were normal in the truncated transporter. SNAT2(Del C-ter) (13 residues deleted from the C-terminus) showed 3-fold higher apparent affinity for alanine, and 2-fold higher Na(+) affinity compared with wild-type SNAT2, suggesting that the C-terminus is not required for high-affinity substrate and Na(+) interaction with SNAT2. The pH sensitivity of amino acid transport was retained partially after the truncation. In contrast with the truncation after TM (transmembrane domain) 11, the deletion of TM11 resulted in an inactive transporter, most probably due to a defect in cell surface expression. Taken together, the results demonstrate that the C-terminal domain of SNAT2 is an important voltage regulator that is required for a normal amino acid translocation process at physiological membrane potentials. However, the C-terminus appears not to be involved in the regulation of membrane expression.

Tertiary active transport of amino acids reconstituted by coexpression of System A and L transporters in Xenopus oocytes

The System L transporter facilitates cellular import of large neutral amino acids (AAs) such as Leu, a potent activator of the intracellular target of rapamycin (TOR) pathway, which signals for cell growth. System L is an AA exchanger, proposed to accumulate certain AAs by coupling to dissipation of concentration gradient(s) of exchange substrates generated by secondary active AA transporters such as System A (SNAT2). We addressed the hypothesis that this type of coupling (termed tertiary active transport) acts as an indirect mechanism to extend the range of AA stimulating TOR to those transported by both Systems A and L (e.g., Gln) through downstream enhancement of Leu accumulation. System A overexpression enabled Xenopus oocytes to accumulate substrate AAs (notably Ser, Gln, Ala, Pro, Met; totaling 2.6 nmol/oocyte) from medium containing a physiological AA mixture at plasma concentrations. Net accumulation of System L (4F2hc-xLAT1) substrates from this medium by System L-overexpressing oocytes was increased by 90% (from 0.7 to 1.35 nmol/oocyte; mainly Leu, Ile) when Systems A and L were coexpressed, coincident with a decline in accumulation of specific System A substrates (Gln, Ser, Met), as expected if the latter were also System L substrates and functional coupling of the transport Systems occurred. AA flux coupling was confirmed as trans-stimulation of Leu influx in System L-expressing oocytes by Gln injection (0.5 nmol/oocyte). The observed changes in Leu accumulation are sufficient to activate the TOR pathway in oocytes, although intracellular AA metabolism limits the potential for AA accumulation by tertiary active transport in this system.

Inhibition of SNAT2 by metabolic acidosis enhances proteolysis in skeletal muscle

Insulin resistance is a major cause of muscle wasting in patients with ESRD. Uremic metabolic acidosis impairs insulin signaling, which normally suppresses proteolysis. The low pH may inhibit the SNAT2 l-Glutamine (L-Gln) transporter, which controls protein synthesis via amino acid-dependent insulin signaling through mammalian target of rapamycin (mTOR). Whether SNAT2 also regulates signaling to pathways that control proteolysis is unknown. In this study, inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate or metabolic acidosis (pH 7.1) depleted intracellular L-Gln and stimulated proteolysis in cultured L6 myotubes. At pH 7.1, inhibition of the proteasome led to greater depletion of L-Gln, indicating that amino acids liberated by proteolysis sustain L-Gln levels when SNAT2 is inhibited by acidosis. Acidosis shifted the dose-response curve for suppression of proteolysis by insulin to the right, confirming that acid increases proteolysis by inducing insulin resistance. Blocking mTOR or phosphatidylinositol-3-kinase (PI3K) increased proteolysis, indicating that both signaling pathways are involved in its regulation. When both mTOR and PI3K were inhibited, methylaminoisobutyrate or acidosis did not stimulate proteolysis further. Moreover, partial silencing of SNAT2 expression in myotubes and myoblasts with small interfering RNA stimulated proteolysis and impaired insulin signaling through PI3K. In conclusion, SNAT2 not only regulates mTOR but also regulates proteolysis through PI3K and provides a link among acidosis, insulin resistance, and protein wasting in skeletal muscle cells.

Distinct sensor pathways in the hierarchical control of SNAT2, a putative amino acid transceptor, by amino acid availability

Mammalian nutrient sensors are novel targets for therapeutic intervention in disease states such as insulin resistance and muscle wasting; however, the proteins responsible for this important task are largely uncharacterized. To address this issue we have dissected an amino acid (AA) sensor/effector regulon that controls the expression of the System A amino acid transporter SNAT2 in mammalian cells, a paradigm nutrient-responsive process, and found evidence for the convergence of at least two sensor/effector pathways. During AA withdrawal, JNK is activated and induces the expression of SNAT2 in L6 myotubes by stimulating an intronic nutrient-sensitive domain. A sensor for large neutral AA (e.g. Tyr, Gln) inhibits JNK activation and SNAT2 up-regulation. Additionally, shRNA and transporter chimeras demonstrate that SNAT2 provides a repressive signal for gene transcription during AA sufficiency, thus echoing AA sensing by transceptor (transporter-receptor) orthologues in yeast (Gap1/Ssy1) and Drosophila (PATH). Furthermore, the SNAT2 protein is stabilized during AA withdrawal.

Acidosis-sensing glutamine pump SNAT2 determines amino acid levels and mammalian target of rapamycin signalling to protein synthesis in L6 muscle cells

Wasting of lean tissue as a consequence of metabolic acidosis is a serious problem in patients with chronic renal failure. A possible contributor is inhibition by low pH of the System A (SNAT2) transporter, which carries the amino acid L-glutamine (L-Gln) into muscle cells. The aim of this study was to determine the effect of selective SNAT2 inhibition on intracellular amino acid profiles and amino acid-dependent signaling through mammalian target of rapamycin in L6 skeletal muscle cells. Inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate, metabolic acidosis (pH 7.1), or silencing SNAT2 expression with small interfering RNA all depleted intracellular L-Gln. SNAT2 inhibition also indirectly depleted other amino acids whose intracellular concentrations are maintained by the L-Gln gradient across the plasma membrane, notably the anabolic amino acid L-leucine. Consequently, SNAT2 inhibition strongly impaired signaling through mammalian target of rapamycin to ribosomal protein S6 kinase, ribosomal protein S6, and 4E-BP1, leading to impairment of protein synthesis comparable with that induced by rapamycin. It is concluded that even though SNAT2 is only one of several L-Gln transporters in muscle, it may determine intracellular anabolic amino acid levels, regulating the amino acid signaling that affects protein mass, nucleotide/nucleic acid metabolism, and cell growth.

Evidence for NL1-independent nuclear translocation of the mineralocorticoid receptor

In the absence of hormone, corticosteroid receptors are primarily located in the cytoplasm, and they rapidly accumulate in the nucleus (t0.5 = 5 min) upon ligand binding. It is generally believed that the dissociation of hsp90 from the receptor is an absolute requirement for allowing its nuclear translocation. However, recent evidence suggests that hsp90 may remain associated with the glucocorticoid receptor during this process, and thus, the receptor nuclear localization signal (NLS) is not obscured by its presence. To determine the requirements for mineralocorticoid receptor (MR) nuclear transport, it was first shown that in rat kidney collecting duct cells, nuclear localization of MR in the presence of aldosterone was complete in 10 min. Although the hsp90 inhibitor radicicol delayed nuclear translocation, it did not prevent complete nuclear accumulation of MR at longer incubation times (t0.5 = 30-40 min). MR carbamylation generates a non-steroid-transformed receptor that, in contrast to native MR, is very stable in cell-free systems. In contrast to the full nuclear translocation of aldosterone-transformed MR, only a fraction of the carbamylated MR became nuclear in digitonin-permeabilized cells even though its NLS is exposed. Furthermore, while preincubation of permeabilized cells with NL1 peptide or anti-NL1 antibody fully inhibited the nuclear translocation of NL1-tagged albumin, neither treatment fully inhibited MR nuclear translocation. We postulate that there are at least two possible mechanisms for MR nuclear translocation. One of them is hsp90- and NL1-dependent, and the other functions in a manner that is independent of the classical pathway.