Cloning and functional identification of a neuronal glutamine transporter

Astrocyte-Neuron Interaction via the Glutamate-Glutamine Cycle and Its Dysfunction in Tau-Dependent Neurodegeneration

Astroglia constitute the largest group of glial cells and are involved in numerous actions that are critical to neuronal development and functioning, such as maintaining the blood-brain barrier, forming synapses, supporting neurons with nutrients and trophic factors, and protecting them from injury. These properties are deeply affected in the course of many neurodegenerative diseases, including tauopathies, often before the onset of the disease. In this respect, the transfer of essential amino acids such as glutamate and glutamine between neurons and astrocytes in the glutamate-glutamine cycle (GGC) is one example. In this review, we focus on the GGC and the disruption of this cycle in tau-dependent neurodegeneration. A profound understanding of the complex functions of the GGC and, in the broader context, searching for dysfunctions in communication pathways between astrocytes and neurons via GGC in health and disease, is of critical significance for the development of novel mechanism-based therapies for neurodegenerative disorders.

Exploring Amino Acid Transporters as Therapeutic Targets for Cancer: An Examination of Inhibitor Structures, Selectivity Issues, and Discovery Approaches

Amino acid transporters are abundant amongst the solute carrier family and have an important role in facilitating the transfer of amino acids across cell membranes. Because of their impact on cell nutrient distribution, they also appear to have an important role in the growth and development of cancer. Naturally, this has made amino acid transporters a novel target of interest for the development of new anticancer drugs. Many attempts have been made to develop inhibitors of amino acid transporters to slow down cancer cell growth, and some have even reached clinical trials. The purpose of this review is to help organize the available information on the efforts to discover amino acid transporter inhibitors by focusing on the amino acid transporters ASCT2 (SLC1A5), LAT1 (SLC7A5), xCT (SLC7A11), SNAT1 (SLC38A1), SNAT2 (SLC38A2), and PAT1 (SLC36A1). We discuss the function of the transporters, their implication in cancer, their known inhibitors, issues regarding selective inhibitors, and the efforts and strategies of discovering inhibitors. The goal is to encourage researchers to continue the search and development within the field of cancer treatment research targeting amino acid transporters.

Amino Acid Transporters Proteins Involved in the Glutamate-Glutamine Cycle and Their Alterations in Murine Models of Alzheimer's Disease

The brain's ability to integrate external stimuli and generate responses is highly complex. While these mechanisms are not completely understood, current evidence suggests that alterations in cellular metabolism and microenvironment are involved in some dysfunctions as complex as Alzheimer's disease. This pathology courses with defects in the establishment of chemical synapses, which is dependent on the production and supply of neurotransmitters like glutamate and its recycling through the glutamate-glutamine cycle. Alterations in the expression and function of the amino acid transporters proteins involved in this cycle have recently been reported in different stages of Alzheimer's disease. Most of these data come from patients in advanced stages of the disease or post-mortem, due to the ethical and technical limitations of human studies. Therefore, genetically modified mouse models have been an excellent tool to analyze metabolic and even behavioral parameters that are very similar to those that develop in Alzheimer's disease, even at presymptomatic stages. Hence, this paper analyzes the role of glutamate metabolism and its intercellular trafficking in excitatory synapses from different approaches using transgenic mouse models; such an analysis will contribute to our present understanding of AD.

Ca2+-regulated expression of high affinity methylaminoisobutryic acid transport in hippocampal neurons inhibited by riluzole and novel neuroprotective aminothiazoles

High affinity methylaminoisobutyric acid(MeAIB)/glutamine(Gln) transport activity regulated by neuronal firing occurs at the plasma membrane in mature rat hippocampal neuron-enriched cultures. Spontaneous Ca2+-regulated transport activity was similarly inhibited by riluzole, a benzothiazole anticonvulsant agent, and by novel naphthalenyl substituted aminothiazole derivatives such as SKA-378. Here, we report that spontaneous transport activity is stimulated by 4-aminopyridine (4-AP) and that phorbol-myristate acetate (PMA) increases high K+ stimulated transport activity that is inhibited by staurosporine. 4-AP-stimulated spontaneous and PMA-stimulated high K+-induced transport is not present at 7 days in vitro (DIV) and is maximal by DIV∼21. The relative affinity for MeAIB is similar for spontaneous and high K+-stimulated transport (Km ∼ 50 μM) suggesting that a single transporter is involved. While riluzole and SKA-378 inhibit spontaneous transport with equal potency (IC50 ∼ 1 μM), they exhibit decreased (∼3-5 X) potency for 4-AP-stimulated spontaneous transport. Interestingly, high K+-stimulated MeAIB transport displays lower and differential sensitivity to the two compounds. SKA-378-related halogenated derivatives of SKA-75 (SKA-219, SKA-377 and SKA-375) preferentially inhibit high K+-induced expression of MeAIB transport activity at the plasma membrane (IC50 < 25 μM), compared to SKA-75 and riluzole (IC50 > 100 μM). Ca2+-dependent spontaneous and high K+-stimulated MeAIB transport activity is blocked by ω-conotoxin MVIIC, ω-agatoxin IVA, ω-agatoxin TK (IC50 ∼ 500 nM) or cadmium ion (IC50 ∼ 20 μM) demonstrating that P/Q-type CaV channels that are required for activity-regulated presynaptic vesicular glutamate (Glu) release are also required for high-affinity MeAIB transport expression at the plasma membrane. We suggest that neural activity driven and Ca2+ dependent trafficking of the high affinity MeAIB transporter to the plasma membrane is a unique target to understand mechanisms of Glu/Gln recycling in synapses and acute neuroprotection against excitotoxic presynaptic Glu induced neural injury.

Importance of glutamine in synaptic vesicles revealed by functional studies of SLC6A17 and its mutations pathogenic for intellectual disability

Human mutations in the gene encoding the solute carrier (SLC) 6A17 caused intellectual disability (ID). The physiological role of SLC6A17 and pathogenesis of SLC6A17-based-ID were both unclear. Here, we report learning deficits in Slc6a17 knockout and point mutant mice. Biochemistry, proteomic, and electron microscopy (EM) support SLC6A17 protein localization in synaptic vesicles (SVs). Chemical analysis of SVs by liquid chromatography coupled to mass spectrometry (LC-MS) revealed glutamine (Gln) in SVs containing SLC6A17. Virally mediated overexpression of SLC6A17 increased Gln in SVs. Either genetic or virally mediated targeting of Slc6a17 reduced Gln in SVs. One ID mutation caused SLC6A17 mislocalization while the other caused defective Gln transport. Multidisciplinary approaches with seven types of genetically modified mice have shown Gln as an endogenous substrate of SLC6A17, uncovered Gln as a new molecule in SVs, established the necessary and sufficient roles of SLC6A17 in Gln transport into SVs, and suggested SV Gln decrease as the key pathogenetic mechanism in human ID.

The Brain at High Altitude: From Molecular Signaling to Cognitive Performance

The brain requires over one-fifth of the total body oxygen demand for normal functioning. At high altitude (HA), the lower atmospheric oxygen pressure inevitably challenges the brain, affecting voluntary spatial attention, cognitive processing, and attention speed after short-term, long-term, or lifespan exposure. Molecular responses to HA are controlled mainly by hypoxia-inducible factors. This review aims to summarize the cellular, metabolic, and functional alterations in the brain at HA with a focus on the role of hypoxia-inducible factors in controlling the hypoxic ventilatory response, neuronal survival, metabolism, neurogenesis, synaptogenesis, and plasticity.

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Riluzole and novel naphthalenyl substituted aminothiazole derivatives prevent acute neural excitotoxic injury in a rat model of temporal lobe epilepsy

Epileptogenic seizures, or status epilepticus (SE), leads to excitotoxic injury in hippocampal and limbic neurons in the kainic acid (KA) animal model of temporal lobe epilepsy (TLE). Here, we have further characterized neural activity regulated methylaminoisobutryic acid (MeAIB)/glutamine transport activity in mature rat hippocampal neurons in vitro that is inhibited by riluzole (IC₅₀ = 1 μM), an anti-convulsant benzothiazole agent. We screened a library of riluzole derivatives and identified SKA-41 followed by a second screen and synthesized several novel chlorinated aminothiazoles (SKA-377, SKA-378, SKA-379) that are also potent MeAIB transport inhibitors in vitro, and brain penetrant following systemic administration. When administered before KA, SKA-378 did not prevent seizures but still protected the hippocampus and several other limbic areas against SE-induced neurodegeneration at 3d. When SKA-377 - 379, (30 mg/kg) were administered after KA-induced SE, acute neural injury in the CA3, CA1 and CA4/hilus was also largely attenuated. Riluzole (10 mg/kg) blocks acute neural injury. Kinetic analysis of SKA-378 and riluzoles' blockade of Ca²⁺-regulated MeAIB transport in neurons in vitro indicates that inhibition occurs via a non-competitive, indirect mechanism. Sodium channel Na_V1.6 antagonism blocks neural activity regulated MeAIB/Gln transport in vitro (IC₅₀ = 60 nM) and SKA-378 is the most potent inhibitor of Na_V1.6 (IC₅₀ = 28 μM) compared to Na_V1.2 (IC₅₀ = 118 μM) in heterologous cells. However, pharmacokinetic analysis suggests that sodium channel blockade may not be the predominant mechanism of neuroprotection here. Riluzole and our novel aminothiazoles are agents that attenuate acute neural hippocampal injury following KA-induced SE and may help to understand mechanisms involved in the progression of epileptic disease.

Impact of Inhibition of Glutamine and Alanine Transport on Cerebellar Glial and Neuronal Metabolism

The cerebellum, or “little brain”, is often overlooked in studies of brain metabolism in favour of the cortex. Despite this, anomalies in cerebellar amino acid homeostasis in a range of disorders have been reported. Amino acid homeostasis is central to metabolism, providing recycling of carbon backbones and ammonia between cell types. Here, we examined the role of cerebellar amino acid transporters in the cycling of glutamine and alanine in guinea pig cerebellar slices by inhibiting amino acid transporters and examining the resultant metabolism of [1-¹³C]d-glucose and [1,2-¹³C]acetate by NMR spectroscopy and LCMS. While the lack of specific inhibitors of each transporter makes interpretation difficult, by viewing results from experiments with multiple inhibitors we can draw inferences about the major cell types and transporters involved. In cerebellum, glutamine and alanine transfer is dominated by system A, blockade of which has maximum effect on metabolism, with contributions from System N. Inhibition of neural system A isoform SNAT1 by MeAIB resulted in greatly decreased metabolite pools and reduced net fluxes but showed little effect on fluxes from [1,2-¹³C]acetate unlike inhibition of SNAT3 and other glutamine transporters by histidine where net fluxes from [1,2-¹³C]acetate are reduced by ~50%. We interpret the data as further evidence of not one but several glutamate/glutamine exchange pools. The impact of amino acid transport inhibition demonstrates that the cerebellum has tightly coupled cells and that glutamate/glutamine, as well as alanine cycling, play a major role in that part of the brain.

Physiological synaptic activity and recognition memory require astroglial glutamine

Presynaptic glutamate replenishment is fundamental to brain function. In high activity regimes, such as epileptic episodes, this process is thought to rely on the glutamate-glutamine cycle between neurons and astrocytes. However the presence of an astroglial glutamine supply, as well as its functional relevance in vivo in the healthy brain remain controversial, partly due to a lack of tools that can directly examine glutamine transfer. Here, we generated a fluorescent probe that tracks glutamine in live cells, which provides direct visual evidence of an activity-dependent glutamine supply from astroglial networks to presynaptic structures under physiological conditions. This mobilization is mediated by connexin43, an astroglial protein with both gap-junction and hemichannel functions, and is essential for synaptic transmission and object recognition memory. Our findings uncover an indispensable recruitment of astroglial glutamine in physiological synaptic activity and memory via an unconventional pathway, thus providing an astrocyte basis for cognitive processes.

The authors present a fluorescent probe that tracks glutamine in live cells. They demonstrate the capabilities of the probe by providing direct visual evidence of an activity-dependent glutamine supply from astroglial networks to presynaptic structures under physiological conditions.

Unconventional Functions of Amino Acid Transporters: Role in Macropinocytosis (SLC38A5/SLC38A3) and Diet-Induced Obesity/Metabolic Syndrome (SLC6A19/SLC6A14/SLC6A6)

Amino acid transporters are expressed in mammalian cells not only in the plasma membrane but also in intracellular membranes. The conventional function of these transporters is to transfer their amino acid substrates across the lipid bilayer; the direction of the transfer is dictated by the combined gradients for the amino acid substrates and the co-transported ions (Na⁺, H⁺, K⁺ or Cl⁻) across the membrane. In cases of electrogenic transporters, the membrane potential also contributes to the direction of the amino acid transfer. In addition to this expected traditional function, several unconventional functions are known for some of these amino acid transporters. This includes their role in intracellular signaling, regulation of acid–base balance, and entry of viruses into cells. Such functions expand the biological roles of these transporters beyond the logical amino acid homeostasis. In recent years, two additional unconventional biochemical/metabolic processes regulated by certain amino acid transporters have come to be recognized: macropinocytosis and obesity. This adds to the repertoire of biological processes that are controlled and regulated by amino acid transporters in health and disease. In the present review, we highlight the unusual involvement of selective amino acid transporters in macropinocytosis (SLC38A5/SLC38A3) and diet-induced obesity/metabolic syndrome (SLC6A19/SLC6A14/SLC6A6).

Depletion of Intracellular Glutamine Pools Triggers Toxoplasma gondii Stage Conversion in Human Glutamatergic Neurons

Toxoplasma gondii chronically infects the brain as latent cysts containing bradyzoites and causes various effects in the host. Recently, the molecular mechanisms of cyst formation in the mouse brain have been elucidated, but those in the human brain remain largely unknown. Here, we show that abnormal glutamine metabolism caused by both interferon-γ (IFN-γ) stimulation and T. gondii infection induce cyst formation in human neuroblastoma cells regardless of the anti-T. gondii host factor nitric oxide (NO) level or Indoleamine 2,3-dioxygenase-1 (IDO1) expression. IFN-γ stimulation promoted intracellular glutamine degradation in human neuronal cells. Additionally, T. gondii infection inhibited the mRNA expression of the host glutamine transporters SLC38A1 and SLC38A2. These dual effects led to glutamine starvation and triggered T. gondii stage conversion in human neuronal cells. Furthermore, these mechanisms are conserved in human iPSC-derived glutamatergic neurons. Taken together, our data suggest that glutamine starvation in host cells is an important trigger of T. gondii stage conversion in human neurons.

NMDARs, Coincidence Detectors of Astrocytic and Neuronal Activities

Synaptic plasticity is an extensively studied cellular correlate of learning and memory in which NMDARs play a starring role. One of the most interesting features of NMDARs is their ability to act as a co-incident detector. It is unique amongst neurotransmitter receptors in this respect. Co-incident detection is possible because the opening of NMDARs requires membrane depolarisation and the binding of glutamate. Opening of NMDARs also requires a co-agonist. Although the dynamic regulation of glutamate and membrane depolarization have been well studied in coincident detection, the role of the co-agonist site is unexplored. It turns out that non-neuronal glial cells, astrocytes, regulate co-agonist availability, giving them the ability to influence synaptic plasticity. The unique morphology and spatial arrangement of astrocytes at the synaptic level affords them the capacity to sample and integrate information originating from unrelated synapses, regardless of any pre-synaptic and post-synaptic commonality. As astrocytes are classically considered slow responders, their influence at the synapse is widely recognized as modulatory. The aim herein is to reconsider the potential of astrocytes to participate directly in ongoing synaptic NMDAR activity and co-incident detection.

The Glutamine Transporter Slc38a1 Regulates GABAergic Neurotransmission and Synaptic Plasticity

GABA signaling sustains fundamental brain functions, from nervous system development to the synchronization of population activity and synaptic plasticity. Despite these pivotal features, molecular determinants underscoring the rapid and cell-autonomous replenishment of the vesicular neurotransmitter GABA and its impact on synaptic plasticity remain elusive. Here, we show that genetic disruption of the glutamine transporter Slc38a1 in mice hampers GABA synthesis, modifies synaptic vesicle morphology in GABAergic presynapses and impairs critical period plasticity. We demonstrate that Slc38a1-mediated glutamine transport regulates vesicular GABA content, induces high-frequency membrane oscillations and shapes cortical processing and plasticity. Taken together, this work shows that Slc38a1 is not merely a transporter accumulating glutamine for metabolic purposes, but a key component regulating several neuronal functions.

The Transcriptomic Analysis of NSC-34 Motor Neuron-Like Cells Reveals That Cannabigerol Influences Synaptic Pathways: A Comparative Study with Cannabidiol

More than 120 cannabinoids were isolated from Cannabis sativa. In particular, Cannabidiol (CBD) and Cannabigerol (CBG) represent the two most studied non-psychoactive cannabinoids. However, CBG is less studied and less data are available on its biological properties and influence on synaptic transmission. On the contrary, CBD is already known to modulate brain excitatory glutamate, inhibitory γ-aminobutyric acid (GABA) and dopamine neurotransmission. In this study, using Next-Generation Sequencing (NGS) technology, we evaluated how CBG (1 or 5 µM) and CBD (1 or 5 µM) influence the transcriptome of the main neurotransmission pathways in NSC-34 motor neuron-like cells. At first, we evaluated that CBG and CBD were not cytotoxic and decreased the expression of pro-apoptotic genes. CBG and CBD are able to influence the expression of the genes involved in glutamate, GABA and dopamine signaling. Interestingly, the transcriptional changes induced by CBG were similar compared to CBD.

D-Serine Signaling and NMDAR-Mediated Synaptic Plasticity Are Regulated by System A-Type of Glutamine/D-Serine Dual Transporters

D-serine is a physiologic coagonist of NMDA receptors (NMDARs) required for synaptic plasticity, but mechanisms that terminate D-serine signaling are unclear. In particular, the identity of unidirectional plasma membrane transporters that mediate D-serine reuptake has remained elusive. We report that D-serine and glutamine share the same neuronal transport system, consisting of the classic system A transporters Slc38a1 and Slc38a2. We show that these transporters are not saturated with glutamine in vivo and regulate the extracellular levels of D-serine and NMDAR activity. Glutamine increased the NMDAR-dependent long-term potentiation and the isolated NMDAR potentials at the Schaffer collateral-CA1 synapses, but without affecting basal neurotransmission in male mice. Glutamine did not increase the NMDAR potentials in slices from serine racemase knock-out mice, which are devoid of D-serine, indicating that the effect of glutamine is caused by outcompeting D-serine for a dual glutamine-D-serine transport system. Inhibition of the system A reduced the uptake of D-serine in synaptosomes and neuronal cultures of mice of either sex, while increasing the extracellular D-serine concentration in slices and in vivo by microdialysis. When compared with Slc38a2, the Slc38a1 transporter displayed more favorable kinetics toward the D-enantiomer. Biochemical experiments with synaptosomes from Slc38a1 knock-down mice of either sex further support its role as a D-serine reuptake system. Our study identifies the first concentrative and electrogenic transporters mediating D-serine reuptake in vivo In addition to their classical role in the glutamine-glutamate cycle, system A transporters regulate the synaptic turnover of D-serine and its effects on NMDAR synaptic plasticity.SIGNIFICANCE STATEMENT Despite the plethora of roles attributed to D-serine, the regulation of its synaptic turnover is poorly understood. We identified the system A transporters Slc38a1 and Slc38a2 as the main pathway for neuronal reuptake of D-serine. These transporters are not saturated with glutamine in vivo and provide an unexpected link between the serine shuttle pathway, responsible for regulating D-serine synaptic turnover, and the glutamine-glutamate cycle. Our observations suggest that Slc38a1 and Slc38a2 have a dual role in regulating neurotransmission. In addition to their classical role as the glutamine providers, the system A transporters regulate extracellular D-serine and therefore affect NMDAR-dependent synaptic plasticity. Higher glutamine export from astrocytes would increase extracellular D-serine, providing a feedforward mechanism to increase synaptic NMDAR activation.

Slc38a1 Conveys Astroglia-Derived Glutamine into GABAergic Interneurons for Neurotransmitter GABA Synthesis

GABA signaling is involved in a wide range of neuronal functions, such as synchronization of action potential firing, synaptic plasticity and neuronal development. Sustained GABA signaling requires efficient mechanisms for the replenishment of the neurotransmitter pool of GABA. The prevailing theory is that exocytotically released GABA may be transported into perisynaptic astroglia and converted to glutamine, which is then shuttled back to the neurons for resynthesis of GABA-i.e., the glutamate/GABA-glutamine (GGG) cycle. However, an unequivocal demonstration of astroglia-to-nerve terminal transport of glutamine and the contribution of astroglia-derived glutamine to neurotransmitter GABA synthesis is lacking. By genetic inactivation of the amino acid transporter Solute carrier 38 member a1 (Slc38a1)-which is enriched on parvalbumin+ GABAergic neurons-and by intraperitoneal injection of radiolabeled acetate (which is metabolized to glutamine in astroglial cells), we show that Slc38a1 mediates import of astroglia-derived glutamine into GABAergic neurons for synthesis of GABA. In brain slices, we demonstrate the role of Slc38a1 for the uptake of glutamine specifically into GABAergic nerve terminals for the synthesis of GABA depending on demand and glutamine supply. Thus, while leaving room for other pathways, our study demonstrates a key role of Slc38a1 for newly formed GABA, in harmony with the existence of a GGG cycle.

Mechanisms of Regulation of Transporters of Amino Acid Absorption in Inflammatory Bowel Diseases

Intestinal absorption of dietary amino acids/peptides is essential for protein homeostasis, which in turn is crucial for maintaining health as well as restoration of health from significant diseases. Dietary amino acids/peptides are absorbed by unique transporter processes present in the brush border membrane of absorptive villus cells, which line the entire length of the intestine. To date, the only nutrient absorptive system described in the secretory crypt cells in the mammalian intestine is the one that absorbs the amino acid glutamine. Majority of the amino acid transporters are sodium dependent and therefore require basolateral membrane Na-K-ATPase to maintain an efficient transcellular Na gradient for their activity. These transport processes are tightly regulated by various cellular and molecular mechanisms that facilitate their optimal activity during normal physiological processes. Malabsorption of amino acids, recently described in pathophysiological states such as in inflammatory bowel disease (IBD), is undoubtedly responsible for the debilitating symptoms of IBD such as malnutrition, weight loss and ultimately a failure to thrive. Also recently, in vivo models of IBD and in vitro studies have demonstrated that specific immune-inflammatory mediators/pathways regulate specific amino acid transporters. This provides possibilities to derive novel nutrition and immune-based treatment options for conditions such as IBD. © 2020 American Physiological Society. Compr Physiol 10:673-686, 2020.

Glutamate Transporters and Mitochondria: Signaling, Co-compartmentalization, Functional Coupling, and Future Directions

In addition to being an amino acid that is incorporated into proteins, glutamate is the most abundant neurotransmitter in the mammalian CNS, the precursor for the inhibitory neurotransmitter γ-aminobutyric acid, and one metabolic step from the tricarboxylic acid cycle intermediate α-ketoglutarate. Extracellular glutamate is cleared by a family of Na⁺-dependent transporters. These transporters are variably expressed by all cell types in the nervous system, but the bulk of clearance is into astrocytes. GLT-1 and GLAST (also called EAAT2 and EAAT1) mediate this activity and are extremely abundant proteins with their expression enriched in fine astrocyte processes. In this review, we will focus on three topics related to these astrocytic glutamate transporters. First, these transporters co-transport three Na⁺ ions and a H⁺ with each molecule of glutamate and counter-transport one K⁺; they are also coupled to a Cl^- conductance. The movement of Na⁺ is sufficient to cause profound astrocytic depolarization, and the movement of H⁺ is linked to astrocytic acidification. In addition, the movement of Na⁺ can trigger the activation of Na⁺ co-transporters (e.g. Na⁺-Ca²⁺ exchangers). We will describe the ways in which these ionic movements have been linked as signals to brain function and/or metabolism. Second, these transporters co-compartmentalize with mitochondria, potentially providing a mechanism to supply glutamate to mitochondria as a source of fuel for the brain. We will provide an overview of the proteins involved, discuss the evidence that glutamate is oxidized, and then highlight some of the un-resolved issues related to glutamate oxidation. Finally, we will review evidence that ischemic insults (stroke or oxygen/glucose deprivation) cause changes in these astrocytic mitochondria and discuss the ways in which these changes have been linked to glutamate transport, glutamate transport-dependent signaling, and altered glutamate metabolism. We conclude with a broader summary of some of the unresolved issues.

SLC38A10 (SNAT10) is Located in ER and Golgi Compartments and Has a Role in Regulating Nascent Protein Synthesis

The solute carrier (SLC) family-38 of transporters has eleven members known to transport amino acids, with glutamine being a common substrate for ten of them, with SLC38A9 being the exception. In this study, we examine the subcellular localization of SNAT10 in several independent immortalized cell lines and stem cell-derived neurons. Co-localization studies confirmed the SNAT10 was specifically localized to secretory organelles. SNAT10 is expressed in both excitatory and inhibitory neurons in the mouse brain, predominantly in the endoplasmic reticulum, and in the Golgi apparatus. Knock-down experiments of SNAT10, using Slc38a10-specific siRNA in PC12 cells reduced nascent protein synthesis by more than 40%, suggesting that SNAT10 might play a role in signaling pathways that regulate protein synthesis, and may act as a transceptor in a similar fashion to what has been shown previously for SLC38A2 (SNAT2) and SNAT9(SLC38A9).

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'.

The role of glutamine in neurogenesis promoted by the green tea amino acid theanine in neural progenitor cells for brain health

The green tea amino acid theanine is abundant in green tea rather than black and oolong teas, which are all made of the identical tea plant "Chanoki" (Camellia sinensis). Theanine has a molecular structure close to glutamine (GLN) compared to glutamic acid (Glu), in terms of the absence of a free carboxylic acid moiety from the gamma carbon position. Theanine efficiently inhibits [³H]GLN uptake without affecting [³H]Glu uptake in rat brain synaptosomes. In contrast to GLN, however, theanine markedly stimulates the abilities to replicate and to commit to a neuronal lineage following prolonged exposure in cultured neural progenitor cells (NPCs) prepared from embryonic and adult rodent brains. Upregulation of transcript expression is found for one of the GLN transporter isoforms, Slc38a1, besides the promotion of both proliferation and neuronal commitment along with acceleration of the phosphorylation of mechanistic target of rapamycin (mTOR) and relevant downstream proteins, in murine NPCs cultured with theanine. Stable overexpression of Slc38a1 similarly facilitates both cellular replication and neuronal commitment in pluripotent embryonic carcinoma P19 cells. In P19 cells with stable overexpression of Slc38a1, marked phosphorylation is seen for mTOR and downstream proteins in a manner insensitive to further additional phosphorylation by theanine. Taken together, theanine would exhibit a novel pharmacological property to up-regulate Slc38a1 expression for activation of the intracellular mTOR signaling pathway required for neurogenesis after sustained exposure in undifferentiated NPCs in the brain. In this review, a novel neurogenic property of the green tea amino acid theanine is summarized for embryonic and adult neurogenesis with a focus on the endogenous amino acid GLN on the basis of our accumulating evidence to date.

Insufficient glutamine synthetase activity during synaptogenesis causes spatial memory impairment in adult mice

Glutamatergic synapses constitute a major excitatory neurotransmission system and are regulated by glutamate/glutamine (Gln) cycling between neurons and astrocytes. Gln synthetase (GS) produced by astrocytes plays an important role in maintaining the cycle. However, the significance of GS during synaptogenesis has not been clarified. GS activity and expression significantly increase from postnatal day (PD) 7 to 21, and GS is expressed prior to glial fibrillary acidic protein (GFAP) and is more abundant than GFAP throughout synaptogenesis. These observations suggest that GS plays an important role in synaptogenesis. We investigated this by inhibiting GS activity in neonatal mice and assessed the consequences in adult animals. Lower expression levels of GS and GFAP were found in the CA3 region of the hippocampus but not in the CA1 region. Moreover, synaptic puncta and glutamatergic neurotransmission were also decreased in CA3. Behaviorally, mice with inhibited GS during synaptogenesis showed spatial memory-related impairment as adults. These results suggest that postnatal GS activity is important for glutamatergic synapse development in CA3.

The brain in flux: Genetic, physiologic, and therapeutic perspectives on transporters in the CNS

The brain has specific properties that make it uniquely dependent upon transporters. This is the 3rd edition of a biennial special issue that originates from a scientific meeting devoted to studies of transporters and their relationship to brain function and to neurodevelopmental, neurologic, and psychiatric disorders. The field continues to rapidly evolve with advances in studies of structure that inform mechanism, with genetic analyses in humans revealing surprising aspects of biology, and with integrated cellular to whole animal analyses of the role of transporters in their control of physiology and pathophysiology. This special issue includes a sampling of review articles that address timely questions of the field followed by several primary research articles.

LPAR5, GNAT3 and partial amino acid transporters messenger RNA expression patterns in digestive tracts, metabolic organs and muscle tissues of growing goats

Sufficient amino acid (AA) transport is essential to ensure the normal physiological function and growth of growing animals. The processes of AA sensing and transport in humans and murine animals, but rarely in goats, have been arousing great interest recently. This study was conducted to investigate the messenger RNA expression patterns of lysophosphatidic acid receptor 5 (LPAR5), guanine nucleotide-binding protein α-transducing 3 (GNAT3) and important partial AA transporters in digestive tracts, metabolic organs and muscles of growing goats. The results showed that these genes were widely expressed in goats, and had different expression patterns. LPAR5, GNAT3, solute carrier (SLC38A2), SLC7A7, SLC7A1 and SLC3A1 were rarely expressed in the rumen, but were highly expressed in the abomasum and intestine which are the main sites of AA absorption. GNAT3, SLC38A1, SLC38A2, SLC6A19, SLC7A7 and SLC7A1 showed comparatively high expression in the pancreas and the vital digestive glands, and the relatively high expression of these nine genes were noted in the tibialis posterior, the active muscle in energy metabolism. The correlation analysis showed that there were certain positive correlation among most genes. The current results indicate that the AA sensing and transport occur extensively in the abomasum and small intestine, metabolic organs and muscle tissues of ruminants, and that related genes have tissue specificity.

Control of amino acid transport into Chinese hamster ovary cells

Amino acid transporters (AATs) represent a key interface between the cell and its environment, critical for all cellular processes: Energy generation, redox control, and synthesis of cell and product biomass. However, very little is known about the activity of different functional classes of AATs in Chinese hamster ovary (CHO) cells, how they support cell growth and productivity, and the potential for engineering their activity and/or the composition of amino acids in growth media to improve CHO cell performance in vitro. In this study, we have comparatively characterized AAT expression in untransfected and monoclonal antibody (MAb)-producing CHO cells using transcriptome analysis by RNA-seq, and mechanistically dissected AAT function using a variety of transporter-specific chemical inhibitors, comparing their effect on cell proliferation, recombinant protein production, and amino acid transport. Of a possible 56 mammalian plasma membrane AATs, 16 AAT messenger RNAs (mRNAs) were relatively abundant across all CHO cell populations. Of these, a subset of nine AAT mRNAs were more abundant in CHO cells engineered to produce a recombinant MAb. Together, upregulated AATs provide additional supply of specific amino acids overrepresented in MAb biomass compared to CHO host cell biomass, enable transport of synthetic substrates for glutathione synthesis, facilitate transport of essential amino acids to maintain active protein synthesis, and provide amino acid substrates for coordinated antiport systems to maintain supplies of proteinogenic and essential amino acids.

Crystal structure of arginine-bound lysosomal transporter SLC38A9 in the cytosol-open state

Recent advances in understanding intracellular amino acid transport and mechanistic target of rapamycin complex 1 (mTORC1) signaling shed light on solute carrier 38, family A member 9 (SLC38A9), a lysosomal transporter responsible for the binding and translocation of several essential amino acids. Here we present the first crystal structure of SLC38A9 from Danio rerio in complex with arginine. As captured in the cytosol-open state, the bound arginine was locked in a transitional state stabilized by transmembrane helix 1 (TM1) of drSLC38A9, which was anchored at the groove between TM5 and TM7. These anchoring interactions were mediated by the highly conserved WNTMM motif in TM1, and mutations in this motif abolished arginine transport by drSLC38A9. The underlying mechanism of substrate binding is critical for sensitizing the mTORC1 signaling pathway to amino acids and for maintenance of lysosomal amino acid homeostasis. This study offers a first glimpse into a prototypical model for SLC38 transporters.

Evidence for Altered Glutamine Metabolism in Human Immunodeficiency Virus Type 1 Infected Primary Human CD4+ T Cells

Glutamine is a conditionally essential amino acid that is an important metabolic resource for proliferating tissues by acting as a proteinogenic amino acid, a nitrogen donor for biosynthetic reactions and as a substrate for the citric acid or tricarboxylic acid cycle. The human immunodeficiency virus type 1 (HIV-1) productively infects activated CD4⁺ T cells that are known to require glutamine for proliferation and for carrying out effector functions. As a virus, HIV-1 is furthermore entirely dependent on host metabolism to support its replication. In this study, we compared HIV-1 infected with uninfected activated primary human CD4⁺ T cells with regard to glutamine metabolism. We report that glutamine concentrations are elevated in HIV-1-infected cells and that glutamine is important to support HIV-1 replication, although the latter is closely linked to the glutamine dependency of cell survival. Metabolic tracer experiments showed that entry of glutamine-derived carbon into the citric acid cycle is unaffected by HIV-1 infection, but that there is an increase in the secretion of glutamine-derived glutamic acid from HIV-1-infected cells. Western blotting of key enzymes that metabolize glutamine revealed marked differences in the expression of glutaminase isoforms, KGA and CAG, as well as the PPAT enzyme that targets glutamine-derived nitrogen toward nucleotide synthesis. Altogether, this demonstrates that infection of CD4⁺ T cells with HIV-1 leads to considerable changes in the cellular glutamine metabolism.

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.

The gene expression of the neuronal protein, SLC38A9, changes in mouse brain after in vivo starvation and high-fat diet

SLC38A9 is characterized as a lysosomal component of the amino acid sensing Ragulator-RAG GTPase complex, controlling the mechanistic target of rapamycin complex 1 (mTORC1). Here, immunohistochemistry was used to map SLC38A9 in mouse brain and staining was detected throughout the brain, in cortex, hypothalamus, thalamus, hippocampus, brainstem and cerebellum. More specifically, immunostaining was found in areas known to be involved in amino acid sensing and signaling pathways e.g. piriform cortex and hypothalamus. SLC38A9 immunoreactivity co-localized with both GABAergic and glutamatergic neurons, but not with astrocytes. SLC38A9 play a key role in the mTORC1 pathway, and therefore we performed in vivo starvation and high-fat diet studies, to measure gene expression alterations in specific brain tissues and in larger brain regions. Following starvation, Slc38a9 was upregulated in brainstem and cortex, and in anterior parts of the brain (Bregma 3.2 to -2.1mm). After high-fat diet, Slc38a9 was specifically upregulated in hypothalamus, while overall downregulation was noticed throughout the brain (Bregma 3.2 to -8.6mm).

SLC38A1 promotes proliferation and migration of human colorectal cancer cells

Current studies have demonstrated that SLC38A1 proteins play a causal role in neoplastic cell transformation. The twofold aim of this study was to provide insight into whether a variance in the expression of SLC38A1 exists between human colorectal cancer and healthy human tissues and to determine how silencing or overexpressing the SLC38A1 gene could affect the proliferation, viability and migration of colorectal cancer cells. Immunohistochemical staining was used to analyze the expression of SLC38A1 in colorectal cancer tissues and adjacent normal mucosa in 77 patients who underwent surgical resection. The expression of SLC38A1 in colorectal cancer tissues and cell lines was detected using RT-PCR and Western blotting. Two colorectal cancer cell lines SW480 and HCT116 were used to examine whether silencing SLC38A1 with siRNA and overexpressing SLC38A1 with shRNA could affect cell viability and migration. As a result, the SLC38A1 protein was very low or undetectable in the normal colon mucosa. In contrast, strong staining of SLC38A1 protein was found in the cytoplasm in 79.2% colorectal cancer samples. More pronounced SLC38A1 expression in colorectal cancer tissues was significantly associated with tumor node metastasis (TNM) stage. Inhibition of SLC38A1 reduced tumour growth and suppressed proliferation and migration of SW480 cells. In contrast, overexpression of SLC38A1 had the opposite effects on HCT116 cells. SLC38A1 is overexpressed in colorectal cancer, which suggests that it is associated with tumour progression. These results encourage the exploration of SLC38A1 as a target for intervention in colorectal cancer.

Amino acid transporter SLC38A3 promotes metastasis of non-small cell lung cancer cells by activating PDK1

BACKGROUND

Tumor metastasis is a finely-tuned pathological process coupled to metabolic reprogramming that includes both glutamine and glucose. The solute carrier SLC38A3, a member of amino acid/polyamine/organocation (APC) superfamily, is an l-glutamine transporter. It is not clear whether SLC38A3 involves the metastasis of NSCLC (non small cell lung cancer).

METHODS

The scratch test and transwell assay were used to determine the ability of NSCLC to migrate. Cellular amino acids content was determined by mass spectrometry. The cellular response to glutamine/histidine deficiency was evaluated in A549 cells. The expression of SLC38A3 was assayed in clinical NSCLC and paratumor tissues by histoimmunochemistry staining. A nude mouse model of NSCLC metastasis was developed by tail vein injection of tumor cells.

RESULTS

SLC38A3 was upregulated in metastatic NSCLC cells and its expression was correlated with prognosis of NSCLC patients. SLC38A3 overexpression promoted epithelial - mesenchymal transition (EMT) and migration of HCC827 and A549 human lung adenocarcinoma cells, and accelerated tumor metastasis in mice. We found that SLC38A3 decreased the cellular concentrations of glutamine and histidine, and the deficiency of glutamine or histidine activated PDK1/AKT signaling that in turn, triggered NSCLC metastasis.

CONCLUSIONS

SLC38A3 activated PDK1/AKT signaling and promoted metastasis of NSCLC through regulating glutamine and histidine transport, suggesting SLC38A3 as a potential therapeutic target for treatment of NSCLC.

Contributions of system A subtypes to α-methylaminoisobutyric acid uptake by placental microvillous membranes of human and rat

System A consists of three subtypes, sodium-coupled neutral amino acid transporter 1 (SNAT1), SNAT2, and SNAT4, which are all expressed in the placenta. The aim of this study was to evaluate the contributions of each of the three subtypes to total system A-mediated uptake in placental MVM of human and rat, using betaine and L-arginine as subtype-selective inhibitors of SNAT2 and SNAT4, respectively. Appropriate concentrations of betaine and L-arginine for subtype-selective inhibition in SNAT-overexpressing cells were identified. It was found that 10 mM betaine specifically and almost completely inhibited human and rat SNAT2-mediated [¹⁴C]α-methylaminoisobutyric acid ([¹⁴C]MeAIB) uptake, while 5 mM L-arginine specifically and completely inhibited [³H]glycine uptake via human SNAT4, as well as [¹⁴C]MeAIB uptake via rat SNAT4. In both human and rat placental MVM vesicles, sodium-dependent uptake of [¹⁴C]MeAIB was almost completely inhibited by 20 mM unlabeled MeAIB. L-Arginine (5 mM) partly inhibited the uptake in humans, but hardly affected that in rats. Betaine (10 mM) partly inhibited the uptake in rats, but hardly affected it in humans. These results suggest that SNAT1 is most likely the major contributor to system A-mediated MeAIB uptake by human and rat MVM vesicles and that the remaining uptake is mainly mediated by SNAT4 in humans and SNAT2 in rats. Thus, inhibition studies using betaine and L-arginine are useful to characterize the molecular mechanisms of system A-mediated transport.

Various plus unique: Viral protein U as a plurifunctional protein for HIV-1 replication

Human immunodeficiency virus type 1 (HIV-1), the causative agent of acquired immunodeficiency syndrome, encodes four accessory genes, one of which is viral protein U (Vpu). Recently, the study of Vpu has been of great interest. For instance, various cellular proteins are degraded (e.g. CD4) and down-modulated (e.g. tetherin) by Vpu. Vpu also antagonizes the function of tetherin and inhibits NF-κB. Moreover, Vpu is a viroporin forming ion channels and may represent a promising target for anti-HIV-1 drugs. In this review, we summarize the domains/residues that are responsible for Vpu's functions, describe the current understanding of the role of Vpu in HIV-1-infected cells, and review the effect of Vpu on HIV-1 in replication and pathogenesis. Future investigations that simultaneously assess a combination of Vpu functions are required to clearly delineate the most important functions for viral replication. Impact statement Viral protein U (Vpu) is a unique protein encoded by human immunodeficiency virus type 1 (HIV-1) and related lentiviruses, playing multiple roles in viral replication and pathogenesis. In this review, we briefly summarize the most up-to-date knowledge of HIV-1 Vpu.

N-Glycosylation influences transport, but not cellular trafficking, of a neuronal amino acid transporter SNAT1

SNAT1 is a system N/A neutral amino acid transporter that primarily expresses in neurons and mediates the transport of l-glutamine (Gln). Gln is an important amino acid involved in multiple cellular functions and also is a precursor for neurotransmitters, glutamate and GABA. In the present study, we demonstrated that SNAT1 is an N-glycoprotein expressed in neurons. We identified three glycosylation sites at asparagine residues 251, 257 and 310 in SNAT1 protein, and that the first two are the primary sites. The biotinylation and confocal immunofluorescence analysis showed that the glycosylation-impaired mutants and deglycosylated SNAT1 were equally capable of expressing on the cell surface. However, l-Gln and 3H-labeled methyl amino isobutyrate (MeAIB) was significantly compromised in N-glycosylation-impaired mutants and deglycosylated SNAT1 when compared with the wild-type control. Taken together, these results suggest that SNAT1 is an N-glycosylated protein with three de novo glycosylation sites and N-glycosylation of SNAT1 may play an important role in the transport of substrates across the cell membrane.

Prefrontal changes in the glutamate-glutamine cycle and neuronal/glial glutamate transporters in depression with and without suicide

There are indications for changes in glutamate metabolism in relation to depression or suicide. The glutamate-glutamine cycle and neuronal/glial glutamate transporters mediate the uptake of the glutamate and glutamine. The expression of various components of the glutamate-glutamine cycle and the neuronal/glial glutamate transporters was determined by qPCR in postmortem prefrontal cortex. The anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC) were selected from young MDD patients who had committed suicide (MDD-S; n = 17), from MDD patients who died of non-suicide related causes (MDD-NS; n = 7) and from matched control subjects (n = 12). We also compared elderly depressed patients who had not committed suicide (n = 14) with matched control subjects (n = 22). We found that neuronal located components (EAAT3, EAAT4, ASCT1, SNAT1, SNAT2) of the glutamate-glutamine cycle were increased in the ACC while the astroglia located components (EAAT1, EAAT2, GLUL) were decreased in the DLPFC of MDD-S patients. In contrast, most of the components in the cycle were increased in the DLPFC of MDD-NS patients. In conclusion, the glutamate-glutamine cycle - and thus glutamine transmission - is differentially affected in depressed suicide patients and depressed non-suicide patients in an area specific way.

In vivo N-15 MRS study of glutamate metabolism in the rat brain

In vivo ¹⁵N MRS has made a unique contribution to kinetic studies of the individual pathways that control glutamate flux in the rat brain. This review covers the following topics: (1) the advantages and limitations of in vivo ¹⁵N MRS and its indirect detection through coupled ¹H; (2) kinetic methods; (3) major findings from our and other laboratories in the areas: (a) the uptake of the neurotransmitter glutamate from the extracellular fluid into glia; (b) the metabolism of glutamate to glutamine; (c) glutamine transport to the extracellular fluid; (d) hydrolysis of neuronal glutamine to glutamate; and (e) contribution of transamination from leucine to replenish the glutamate nitrogen. In vivo glutamine synthetase activities measured at several levels of hyperammonemia showed that this enzyme becomes saturated at blood ammonia concentration >0.9 μmol/g, and causes the elevation of brain ammonia. Implications of the results for the cause of hyperammonemic encephalopathy are discussed. Leucine provides >25% of glutamate nitrogen. An intriguing possibility that supplementing leucine may restore cognitive function after brain injury is discussed. Finally, some characteristics of ¹⁵N MRS that may facilitate the future application of this technique to the study of the human brain at 4 or 7 T are described.

Attacking the Supply Lines: HIV-1 Restricts Alanine Uptake to Prevent T Cell Activation

HIV commonly escapes host antiviral immunity by downregulating cell-surface immunoreceptors. In a recent issue of Cell Host & Microbe, Matheson et al. (2015) systematically examined how HIV-1 infection remodels the T cell surface and identified serine carriers SERINC3/5 and alanine transporter SNAT1 as targets of HIV-1 Nef and Vpu, respectively.

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.

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

Remodeling of the Host Cell Plasma Membrane by HIV-1 Nef and Vpu: A Strategy to Ensure Viral Fitness and Persistence

The plasma membrane protects the cell from its surroundings and regulates cellular communication, homing, and metabolism. Not surprisingly, the composition of this membrane is highly controlled through the vesicular trafficking of proteins to and from the cell surface. As intracellular pathogens, most viruses exploit the host plasma membrane to promote viral replication while avoiding immune detection. This is particularly true for the enveloped human immunodeficiency virus (HIV), which assembles and obtains its lipid shell directly at the plasma membrane. HIV-1 encodes two proteins, negative factor (Nef) and viral protein U (Vpu), which function primarily by altering the quantity and localization of cell surface molecules to increase virus fitness despite host antiviral immune responses. These proteins are expressed at different stages in the HIV-1 life cycle and employ a variety of mechanisms to target both unique and redundant surface proteins, including the viral receptor CD4, host restriction factors, immunoreceptors, homing molecules, tetraspanins and membrane transporters. In this review, we discuss recent progress in the study of the Nef and Vpu targeting of host membrane proteins with an emphasis on how remodeling of the cell membrane allows HIV-1 to avoid host antiviral immune responses leading to the establishment of systemic and persistent infection.

Anaplerosis for Glutamate Synthesis in the Neonate and in Adulthood

A central task of the tricarboxylic acid (TCA, Krebs, citric acid) cycle in brain is to provide precursors for biosynthesis of glutamate, GABA, aspartate and glutamine. Three of these amino acids are the partners in the intricate interaction between astrocytes and neurons and form the so-called glutamine-glutamate (GABA) cycle. The ketoacids α-ketoglutarate and oxaloacetate are removed from the cycle for this process. When something is removed from the TCA cycle it must be replaced to permit the continued function of this essential pathway, a process termed anaplerosis. This anaplerotic process in the brain is mainly carried out by pyruvate carboxylation performed by pyruvate carboxylase. The present book chapter gives an introduction and overview into this carboxylation and additionally anaplerosis mediated by propionyl-CoA carboxylase under physiological conditions in the adult and in the developing rodent brain. Furthermore, examples are given about pathological conditions in which anaplerosis is disturbed.

The Glutamine Transporters and Their Role in the Glutamate/GABA-Glutamine Cycle

Glutamine is a key amino acid in the CNS, playing an important role in the glutamate/GABA-glutamine cycle (GGC). In the GGC, glutamine is transferred from astrocytes to neurons, where it will replenish the inhibitory and excitatory neurotransmitter pools. Different transporters participate in this neural communication, i.e., the transporters responsible for glutamine efflux from astrocytes and influx into the neurons, such as the members of the SNAT, LAT, y⁺LAT, and ASC families of transporters. The SNAT family consists of the transporter isoforms SNAT3 and SNAT5 that are related to efflux from the astrocytic compartment, and SNAT1 and SNAT2 that are associated with glutamine uptake into the neuronal compartment. The isoforms SNAT7 and SNAT8 do not have their role completely understood, but they likely also participate in the GGC. The isoforms LAT2 and y⁺LAT2 facilitate the exchange of neutral amino acids and cationic amino acids (y⁺LAT2 isoform) and have been associated with glutamine efflux from astrocytes. ASCT2 is a Na⁺-dependent antiporter, the participation of which in the GGC also remains to be better characterized. All these isoforms are tightly regulated by transcriptional and translational mechanisms, which are induced by several determinants such as amino acid deprivation, hormones, pH, and the activity of different signaling pathways. Dysfunctional glutamine transporter activity has been associated with the pathophysiological mechanisms of certain neurologic diseases, such as Hepatic Encephalopathy and Manganism. However, there might also be other neuropathological conditions associated with an altered GGC, in which glutamine transporters are dysfunctional. Hence, it appears to be of critical importance that the physiological and pathological aspects of glutamine transporters are thoroughly investigated.

Sex hormones and macronutrient metabolism

The biological differences between males and females are determined by a different set of genes and by a different reactivity to environmental stimuli, including the diet, in general. These differences are further emphasized and driven by the exposure to a different hormone flux throughout the life.

These differences have not been taken into appropriate consideration by the scientific community. Nutritional sciences are not immune from this “bias” and when nutritional needs are concerned, females are considered only when pregnant, lactating or when their hormonal profile is returning back to “normal,” i.e., to the male-like profile.

The authors highlight some of the most evident differences in aspects of biology that are associated with nutrition.

This review presents and describes available data addressing differences and similarities of the “reference man” vs. the “reference woman” in term of metabolic activity and nutritional needs. According to this assumption, available evidences of sex-associated differences of specific biochemical pathways involved in substrate metabolism are reported and discussed. The modulation by sexual hormones affecting glucose, amino acid and protein metabolism and the metabolization of nutritional fats and the distribution of fat depots, is considered targeting a tentative starting up background for a gender concerned nutritional science.

Loss of function mutation of the Slc38a3 glutamine transporter reveals its critical role for amino acid metabolism in the liver, brain, and kidney

Glutamine, the most abundant amino acid in mammals, is critical for cell and organ functions. Its metabolism depends on the ability of cells to take up or release glutamine by transporters located in the plasma membrane. Several solute carrier (SLC) families transport glutamine, but the SLC38 family has been thought to be mostly responsible for glutamine transport. We demonstrate that despite the large number of glutamine transporters, the loss of Snat3/Slc38a3 glutamine transporter has a major impact on the function of organs expressing it. Snat3 mutant mice were generated by N-ethyl-N-nitrosurea (ENU) mutagenesis and showed stunted growth, altered amino acid levels, hypoglycemia, and died around 20 days after birth. Hepatic concentrations of glutamine, glutamate, leucine, phenylalanine, and tryptophan were highly reduced paralleled by downregulation of the mTOR pathway possibly linking reduced amino acid availability to impaired growth and glucose homeostasis. Snat3-deficient mice had altered urea levels paralleled by dysregulation of the urea cycle, gluconeogenesis, and glutamine synthesis. Mice were ataxic with higher glutamine but reduced glutamate and gamma-aminobutyric acid (GABA) levels in brain consistent with a major role of Snat3 in the glutamine-glutamate cycle. Renal ammonium excretion was lower, and the expression of enzymes and amino acid transporters involved in ammoniagenesis were altered. Thus, SNAT3 is a glutamine transporter required for amino acid homeostasis and determines critical functions in various organs. Despite the large number of glutamine transporters, loss of Snat3 cannot be compensated, suggesting that this transporter is a major route of glutamine transport in the liver, brain, and kidney.