P2Y purinergic regulation of the glycine neurotransmitter transporters

GlyT1 Inhibition by NFPS Promotes Neuroprotection in Amyloid-β-Induced Alzheimer's Disease Animal Model

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the accumulation of amyloid-β, leading to N-methyl-D-aspartate (NMDA) receptor-dependent synaptic depression, spine elimination, and memory deficits. Glycine transporter type 1 (GlyT1) modulates glutamatergic neurotransmission via NMDA receptors (NMDAR), presenting a potential alternative therapeutic approach for AD. This study investigates the neuroprotective potential of GlyT1 inhibition in an amyloid-β-induced AD mouse model. C57BL/6 mice were treated with N-[3-([1,1-Biphenyl]-4-yloxy)-3-(4-fluorophenyl)propyl]-N-methylglycine (NFPS), a GlyT1 inhibitor, 24 h prior to intrahippocampal injection of amyloid-β. NFPS pretreatment prevented amyloid-β-induced cognitive deficits in short-term and long-term memory, evidenced by novel object recognition and spatial memory tasks. Moreover, NFPS pretreatment curbed microglial activation, astrocytic reactivity, and subsequent neuronal damage from amyloid-β injection. An extensive label-free quantitative UPLC-MSE proteomic analysis was performed on the hippocampi of mice treated with NFPS. In proteomics, KEGG enrichment analysis revealed increased in dopaminergic synapse, purine-containing compound biosynthetic process and long-term potentiation, and a reduction in Glucose catabolic process and glycolytic process pathways. The western blot analysis confirmed that NFPS treatment elevated BDNF levels, correlating with enhanced TRKB phosphorylation and mTOR activation. Moreover, NFPS treatment reduced the GluN2B expression after 6 h, which was associated with an increase on CaMKIV and CREB phosphorylation. Collectively, these findings demonstrate that GlyT1 inhibition by NFPS activates diverse neuroprotective pathways, enhancing long-term potentiation signaling and countering amyloid-β-induced hippocampal damage.

Role of palmitoylation on the neuronal glycine transporter GlyT2

The neuronal glycine transporter GlyT2 removes glycine from the synaptic cleft through active Na+, Cl-, and glycine cotransport contributing to the termination of the glycinergic signal as well as supplying substrate to the presynaptic terminal for the maintenance of the neurotransmitter content in synaptic vesicles. Patients with mutations in the human GlyT2 gene (SLC6A5), develop hyperekplexia or startle disease (OMIM 149400), characterized by hypertonia and exaggerated startle responses to trivial stimuli that may have lethal consequences in the neonates as a result of apnea episodes. Post-translational modifications in cysteine residues of GlyT2 are an aspect of structural interest we analyzed. Our study is compatible with a reversible and short-lived S-acylation in spinal cord membranes, detectable by biochemical and proteomics methods (acyl-Rac binding and IP-ABE) confirmed with positive and negative controls (palmitoylated and non-palmitoylated proteins). According to a short-lived modification, direct labeling using click chemistry was faint but mostly consistent. We have analyzed the physiological properties of a GlyT2 mutant lacking the cysteines with high prediction of palmitoylation and the mutant is less prone to be included in lipid rafts, an effect also observed upon treatment with the palmitoylation inhibitor 2-bromopalmitate. This work demonstrates there are determinants of lipid raft inclusion associated with the GlyT2 mutated cysteines, which are presumably modified by palmitoylation.

Reconstitution of GABA, Glycine and Glutamate Transporters

In contrast to water soluble enzymes which can be purified and studied while in solution, studies of solute carrier (transporter) proteins require both that the protein of interest is situated in a phospholipid membrane and that this membrane forms a closed compartment. An additional challenge to the study of transporter proteins has been that the transport depends on the transmembrane electrochemical gradients. Baruch I. Kanner understood this early on and first developed techniques for studying plasma membrane vesicles. This advanced the field in that the experimenter could control the electrochemical gradients. Kanner, however, did not stop there, but started to solubilize the membranes so that the transporter proteins were taken out of their natural environment. In order to study them, Kanner then had to find a way to reconstitute them (reinsert them into phospholipid membranes). The scope of the present review is both to describe the reconstitution method in full detail as that has never been done, and also to reveal the scientific impact that this method has had. Kanner's later work is not reviewed here although that also deserves a review because it too has had a huge impact.

The presynaptic glycine transporter GlyT2 is regulated by the Hedgehog pathway in vitro and in vivo

The identity of a glycinergic synapse is maintained presynaptically by the activity of a surface glycine transporter, GlyT2, which recaptures glycine back to presynaptic terminals to preserve vesicular glycine content. GlyT2 loss-of-function mutations cause Hyperekplexia, a rare neurological disease in which loss of glycinergic neurotransmission causes generalized stiffness and strong motor alterations. However, the molecular underpinnings controlling GlyT2 activity remain poorly understood. In this work, we identify the Hedgehog pathway as a robust controller of GlyT2 expression and transport activity. Modulating the activation state of the Hedgehog pathway in vitro in rodent primary spinal cord neurons or in vivo in zebrafish embryos induced a selective control in GlyT2 expression, regulating GlyT2 transport activity. Our results indicate that activation of Hedgehog reduces GlyT2 expression by increasing its ubiquitination and degradation. This work describes a new molecular link between the Hedgehog signaling pathway and presynaptic glycine availability.

By modulating the activation state of the Hedgehog pathway, de la Rocha-Muñoz et al demonstrate that Hedgehog signaling controls the expression and transport activity of the neuronal glycine transporter GlyT2. This work begins to reveal a potential link between the Hedgehog signaling pathway and presynaptic glycine availability.

Purinergic-Glycinergic Interaction in Neurodegenerative and Neuroinflammatory Disorders of the Retina

Neurodegenerative–neuroinflammatory disorders of the retina seriously hamper human vision. In searching for key factors that contribute to the development of these pathologies, we considered potential interactions among purinergic neuromodulation, glycinergic neurotransmission, and microglia activity in the retina. Energy deprivation at cellular levels is mainly due to impaired blood circulation leading to increased release of ATP and adenosine as well as glutamate and glycine. Interactions between these modulators and neurotransmitters are manifold. First, P2Y purinoceptor agonists facilitate reuptake of glycine by glycine transporter 1, while its inhibitors reduce reverse-mode operation; these events may lower extracellular glycine levels. The consequential changes in extracellular glycine concentration can lead to parallel changes in the activity of NR1/NR2B type NMDA receptors of which glycine is a mandatory agonist, and thereby may reduce neurodegenerative events in the retina. Second, P2Y purinoceptor agonists and glycine transporter 1 inhibitors may indirectly inhibit microglia activity by decreasing neuronal or glial glycine release in energy-compromised retina. These inhibitions may have a role in microglia activation, which is present during development and progression of neurodegenerative disorders such as glaucomatous and diabetic retinopathies and age-related macular degeneration or loss of retinal neurons caused by thromboembolic events. We have hypothesized that glycine transporter 1 inhibitors and P2Y purinoceptor agonists may have therapeutic importance in neurodegenerative–neuroinflammatory disorders of the retina by decreasing NR1/NR2B NMDA receptor activity and production and release of a series of proinflammatory cytokines from microglial cells.

Structural Determinants of the Neuronal Glycine Transporter 2 for the Selective Inhibitors ALX1393 and ORG25543

The neuronal glycine transporter GlyT2 modulates inhibitory glycinergic neurotransmission by controlling the extracellular concentration of synaptic glycine and the supply of neurotransmitter to the presynaptic terminal. Spinal cord glycinergic neurons present in the dorsal horn diminish their activity in pathological pain conditions and behave as gate keepers of the touch-pain circuitry. The pharmacological blockade of GlyT2 reduces the progression of the painful signal to rostral areas of the central nervous system by increasing glycine extracellular levels, so it has analgesic action. O-[(2-benzyloxyphenyl-3-fluorophenyl)methyl]-l-serine (ALX1393) and N-[[1-(dimethylamino)cyclopentyl]methyl]-3,5-dimethoxy-4-(phenylmethoxy)benzamide (ORG25543) are two selective GlyT2 inhibitors with nanomolar affinity for the transporter and analgesic effects in pain animal models, although with deficiencies which preclude further clinical development. In this report, we performed a comparative ligand docking of ALX1393 and ORG25543 on a validated GlyT2 structural model including all ligand sites constructed by homology with the crystallized dopamine transporter from Drosophila melanogaster. Molecular dynamics simulations and energy analysis of the complex and functional analysis of a series of point mutants permitted to determine the structural determinants of ALX1393 and ORG25543 discrimination by GlyT2. The ligands establish simultaneous contacts with residues present in transmembrane domains 1, 3, 6, and 8 and block the transporter in outward-facing conformation and hence inhibit glycine transport. In addition, differential interactions of ALX1393 with the cation bound at Na1 site and ORG25543 with TM10 define the differential sites of the inhibitors and explain some of their individual features. Structural information about the interactions with GlyT2 may provide useful tools for new drug discovery.

Rescue of two trafficking-defective variants of the neuronal glycine transporter GlyT2 associated to hyperekplexia

Hyperekplexia is a rare sensorimotor syndrome characterized by pathological startle reflex in response to unexpected trivial stimuli for which there is no specific treatment. Neonates suffer from hypertonia and are at high risk of sudden death due to apnea episodes. Mutations in the human SLC6A5 gene encoding the neuronal glycine transporter GlyT2 may disrupt the inhibitory glycinergic neurotransmission and cause a presynaptic form of the disease. The phenotype of missense mutations giving rise to protein misfolding but maintaining residual activity could be rescued by facilitating folding or intracellular trafficking. In this report, we characterized the trafficking properties of two mutants associated with hyperekplexia (A277T and Y707C, rat numbering). Transporter molecules were partially retained in the endoplasmic reticulum showing increased interaction with the endoplasmic reticulum chaperone calnexin. One transporter variant had export difficulties and increased ubiquitination levels, suggestive of enhanced endoplasmic reticulum-associated degradation. However, the two mutant transporters were amenable to correction by calnexin overexpression. Within the search for compounds capable of rescuing mutant phenotypes, we found that the arachidonic acid derivative N-arachidonoyl glycine can rescue the trafficking defects of the two variants in heterologous cells and rat brain cortical neurons. N-arachidonoyl glycine improves the endoplasmic reticulum output by reducing the interaction transporter/calnexin, increasing membrane expression and improving transport activity in a comparable way as the well-established chemical chaperone 4-phenyl-butyrate. This work identifies N-arachidonoyl glycine as a promising compound with potential for hyperekplexia therapy.

Calcium-Dependent Regulation of the Neuronal Glycine Transporter GlyT2 by M2 Muscarinic Acetylcholine Receptors

The neuronal glycine transporter GlyT2 modulates inhibitory glycinergic neurotransmission and plays a key role in regulating nociceptive signal progression. The cholinergic system acting through muscarinic acetylcholine receptors (mAChRs) also mediates important regulations of nociceptive transmission being the M2 subtype the most abundantly expressed in the spinal cord. Here we studied the effect of M2 mAChRs stimulation on GlyT2 function co-expressed in a heterologous system with negligible levels of muscarinic receptor activity. We found GlyT2 is down-regulated by carbachol in a calcium-dependent manner. Different components involved in cell calcium homeostasis were analysed to establish a role in the mechanism of GlyT2 inhibition. GlyT2 down-regulation by carbachol was increased by thapsigargin and reduced by internal store depletion, although calcium release from endoplasmic reticulum or mitochondria had a minor role on GlyT2 inhibition. Our results are consistent with a GlyT2 sensitivity to intracellular calcium mobilized by M2 mAChRs in the subcortical area of the plasma membrane. A crucial role of the plasma membrane sodium calcium exchanger NCX is proposed.

Nucleotides-Induced Changes in the Mechanical Properties of Living Endothelial Cells and Astrocytes, Analyzed by Atomic Force Microscopy

Endothelial cells and astrocytes preferentially express metabotropic P2Y nucleotide receptors, which are involved in the maintenance of vascular and neural function. Among these, P2Y₁ and P2Y₂ receptors appear as main actors, since their stimulation induces intracellular calcium mobilization and activates signaling cascades linked to cytoskeletal reorganization. In the present work, we have analyzed, by means of atomic force microscopy (AFM) in force spectroscopy mode, the mechanical response of human umbilical vein endothelial cells (HUVEC) and astrocytes upon 2MeSADP and UTP stimulation. This approach allows for simultaneous measurement of variations in factors such as Young’s modulus, maximum adhesion force and rupture event formation, which reflect the potential changes in both the stiffness and adhesiveness of the plasma membrane. The largest effect was observed in both endothelial cells and astrocytes after P2Y₂ receptor stimulation with UTP. Such exposure to UTP doubled the Young’s modulus and reduced both the adhesion force and the number of rupture events. In astrocytes, 2MeSADP stimulation also had a remarkable effect on AFM parameters. Additional studies performed with the selective P2Y₁ and P2Y₁₃ receptor antagonists revealed that the 2MeSADP-induced mechanical changes were mediated by the P2Y₁₃ receptor, although they were negatively modulated by P2Y₁ receptor stimulation. Hence, our results demonstrate that AFM can be a very useful tool to evaluate functional native nucleotide receptors in living cells.

Neurobiology of glycine transporters: From molecules to behavior

Glycine transporters (GlyTs) are Na⁺/Cl^--dependent neurotransmitter transporters, responsible for l-glycine uptake into the central nervous system. GlyTs are members of the solute carrier family 6 (SLC6) and comprise glycine transporter type 1 (SLC6A9; GlyT1) and glycine transporter type 2 (SLC6A5; Glyt2). GlyT1 and GlyT2 are expressed on both astrocytes and neurons, but their expression pattern in brain tissue is foremost related to neurotransmission. GlyT2 is markedly expressed in brainstem, spinal cord and cerebellum, where it is responsible for glycine uptake into glycinergic and GABAergic terminals. GlyT1 is abundant in neocortex, thalamus and hippocampus, where it is expressed in astrocytes, and involved in glutamatergic neurotransmission. Consequently, inhibition of GlyT1 transporters can modulate glutamatergic neurotransmission through NMDA receptors, suggesting an alternative therapeutic strategy. In this review, we focus on recent progress in the understanding of GlyTs role in brain function and in various diseases, such as epilepsy, hyperekplexia, neuropathic pain, drug addiction, schizophrenia and stroke, as well as in neurodegenerative disorders.

Update of P2Y receptor pharmacology: IUPHAR Review 27

Eight G protein-coupled P2Y receptor subtypes respond to extracellular adenine and uracil mononucleotides and dinucleotides. P2Y receptors belong to the δ group of rhodopsin-like GPCRs and contain two structurally distinct subfamilies: P2Y₁ , P2Y₂ , P2Y₄ , P2Y₆ , and P2Y₁₁ (principally G_q protein-coupled P2Y₁ -like) and P2Y_12-14 (principally G_i protein-coupled P2Y₁₂ -like) receptors. Brain P2Y receptors occur in neurons, glial cells, and vasculature. Endothelial P2Y₁ , P2Y₂ , P2Y₄ , and P2Y₆ receptors induce vasodilation, while smooth muscle P2Y₂ , P2Y₄ , and P2Y₆ receptor activation leads to vasoconstriction. Pancreatic P2Y₁ and P2Y₆ receptors stimulate while P2Y₁₃ receptors inhibits insulin secretion. Antagonists of P2Y₁₂ receptors, and potentially P2Y₁ receptors, are anti-thrombotic agents, and a P2Y₂ /P2Y₄ receptor agonist treats dry eye syndrome in Asia. P2Y receptor agonists are generally pro-inflammatory, and antagonists may eventually treat inflammatory conditions. This article reviews recent developments in P2Y receptor pharmacology (using synthetic agonists and antagonists), structure and biophysical properties (using X-ray crystallography, mutagenesis and modelling), physiological and pathophysiological roles, and present and potentially future therapeutic targeting.

E3 ubiquitin ligases LNX1 and LNX2 are major regulators of the presynaptic glycine transporter GlyT2

The neuronal glycine transporter GlyT2 is an essential regulator of glycinergic neurotransmission that recaptures glycine in presynaptic terminals to facilitate transmitter packaging in synaptic vesicles. Alterations in GlyT2 expression or activity result in lower cytosolic glycine levels, emptying glycinergic synaptic vesicles and impairing neurotransmission. Lack of glycinergic neurotransmission caused by GlyT2 loss-of-function mutations results in Hyperekplexia, a rare neurological disease characterized by generalized stiffness and motor alterations that may cause sudden infant death. Although the importance of GlyT2 in pathology is known, how this transporter is regulated at the molecular level is poorly understood, limiting current therapeutic strategies. Guided by an unbiased screening, we discovered that E3 ubiquitin ligase Ligand of Numb proteins X1/2 (LNX1/2) modulate the ubiquitination status of GlyT2. The N-terminal RING-finger domain of LNX1/2 ubiquitinates a cytoplasmic C-terminal lysine cluster in GlyT2 (K751, K773, K787 and K791), and this process regulates the expression levels and transport activity of GlyT2. The genetic deletion of endogenous LNX2 in spinal cord primary neurons causes an increase in GlyT2 expression and we find that LNX2 is required for PKC-mediated control of GlyT2 transport. This work identifies, to our knowledge, the first E3 ubiquitin-ligases acting on GlyT2, revealing a novel molecular mechanism that controls presynaptic glycine availability. Providing a better understanding of the molecular regulation of GlyT2 may help future investigations into the molecular basis of human disease states caused by dysfunctional glycinergic neurotransmission, such as hyperekplexia and chronic pain.

P2 receptor interaction and signalling cascades in neuroprotection

Nucleotides can contribute to the survival of different glial and neuronal models at the nervous system via activation of purinergic P2X and P2Y receptors. Their activation counteracts different proapoptotic events, such as excitotoxicity, mitochondrial impairment, oxidative stress and DNA damage, which concur to elicit cell loss in different processes of neurodegeneration and brain injury. Thus, it is frequent to find that different neuroprotective mediators converge in the activation of the same intracellular survival pathways to protect cells from death. The present review focuses on the role of P2Y₁ and P2Y₁₃ metabotropic receptors, and P2X7 ionotropic receptors to regulate the balance between survival and apoptosis. In particular, we analyze the intracellular pathways involved in the signaling of these nucleotide receptors to elicit survival, including calcium/PLC, PI3K/Akt/GSK3, MAPK cascades, and the expression of antioxidant and antiapoptotic genes. This review emphasizes the novel contribution of nucleotide receptors to maintain cell homeostasis through the regulation of MAP kinases and phosphatases. Unraveling the different roles found for nucleotide receptors in different models and cellular contexts may be crucial to delineate future therapeutic applications based on targeting nucleotide receptors for neuroprotection.

Glycine Transporters and Its Coupling with NMDA Receptors

Glycine plays two roles in neurotransmission. In caudal areas like the spinal cord and the brainstem, it acts as an inhibitory neurotransmitter, but in all regions of the CNS, it also works as a co-agonist with L-glutamate at N-methyl-D-aspartate receptors (NMDARs). The glycine fluxes in the CNS are regulated by two specific transporters for glycine, GlyT1 and GlyT2, perhaps with the cooperation of diverse neutral amino acid transporters like Asc-1 or SNAT5/SN2. While GlyT2 and Asc-1 are neuronal proteins, GlyT1 and SNAT5 are mainly astrocytic, although neuronal forms of GlyT1 also exist. GlyT1 has attracted considerable interest from the medical community and the pharmaceutical industry since compelling evidence indicates a clear association with the functioning of NMDARs, whose activity is decreased in various psychiatric illnesses. By controlling extracellular glycine, transporter inhibitors might potentiate the activity of NMDARs without activating excitotoxic processes. Physiologically, GlyT1 is a central actor in the cross talk between glutamatergic, glycinergic, dopaminergic, and probably other neurotransmitter systems. Many of these relationships begin to be unraveled by studies performed in recent years using genetic and pharmacological models. These studies are also clarifying the interactions between glycine, glycine transporters, and other co-agonists of the glycine site of NMDARs like D-serine. These findings are also relevant to understand the pathophysiology of devastating diseases like schizophrenia, depression, anxiety, epilepsy, stroke, and chronic pain.

Activity dependent internalization of the glutamate transporter GLT-1 requires calcium entry through the NCX sodium/calcium exchanger

GLT-1 is the main glutamate transporter in the brain and its trafficking controls its availability at the cell surface, thereby shaping glutamatergic neurotransmission under physiological and pathological conditions. Extracellular glutamate is known to trigger ubiquitin-dependent GLT-1 internalization from the surface of the cell to the intracellular compartment, yet here we show that internalization also requires the participation of calcium ions. Consistent with previous studies, the addition of glutamate (1 mM) to mixed primary cultures (containing neurons and astrocytes) promotes GLT-1 internalization, an effect that was suppressed in the absence of extracellular Ca²⁺. The pathways of Ca²⁺ mobilization by astrocytes were analyzed in these mixed cultures using the genetically encoded calcium sensor GCaMP6f. A complex pattern of calcium entry was activated by glutamate, with a dramatic and rapid rise in the intracellular Ca²⁺ concentration partially driven by glutamate transporters, especially in the initial stages after exposure to glutamate. The Na⁺/Ca²⁺ exchanger (NCX) plays a dominant role in this Ca²⁺ mobilization and its blockade suppresses the glutamate induced internalization of GLT-1, both in astrocytes and in a more straightforward experimental system like HEK293 cells transiently transfected with GLT-1. This regulatory mechanism might be relevant to control the amount of GLT-1 transporter at the cell surface in conditions like ischemia or traumatic brain injury, where extracellular concentrations of glutamate are persistently elevated and they promote rapid Ca²⁺ mobilization.

P2X receptors up-regulate the cell-surface expression of the neuronal glycine transporter GlyT2

Glycinergic inhibitory neurons of the spinal dorsal horn exert critical control over the conduction of nociceptive signals to higher brain areas. The neuronal glycine transporter 2 (GlyT2) is involved in the recycling of synaptic glycine from the inhibitory synaptic cleft and its activity modulates intra and extracellular glycine concentrations. In this report we show that the stimulation of P2X purinergic receptors with βγ-methylene adenosine 5'-triphosphate induces the up-regulation of GlyT2 transport activity by increasing total and plasma membrane expression and reducing transporter ubiquitination. We identified the receptor subtypes involved by combining pharmacological approaches, siRNA-mediated protein knockdown, and dorsal root ganglion cell enrichment in brainstem and spinal cord primary cultures. Up-regulation of GlyT2 required the combined stimulation of homomeric P2X₃ and P2X₂ receptors or heteromeric P2X_2/3 receptors. We measured the spontaneous glycinergic currents, glycine release and GlyT2 uptake concurrently in response to P2X receptor agonists, and showed that the impact of P2X3 receptor activation on glycinergic neurotransmission involves the modulation of GlyT2 expression or activity. The recognized pro-nociceptive action of P2X₃ receptors suggests that the fine-tuning of GlyT2 activity may have consequences in nociceptive signal conduction.

An Update on P2Y13 Receptor Signalling and Function

The distribution of nucleotide P2Y receptors across different tissues suggests that they fulfil key roles in a number of physiological and pathological conditions. P2Y₁₃ is one of the latest P2Y receptors identified, a novel member of the Gi-coupled P2Y receptor subfamily that responds to ADP, together with P2Y₁₂ and P2Y₁₄. Pharmacological studies drew attention to this new ADP receptor, with a pharmacology that overlaps that of P2Y₁₂ receptors but with unique features and roles. The P2RY12-14 genes all reside on human chromosome 3 at 3q25.1 and their strong sequence homology supports their evolutionary origin through gene duplication. Polymorphisms of P2Y₁₃ receptors have been reported in different human populations, yet their consequences remain unknown. The P2Y₁₃ receptor is versatile in its signalling, extending beyond the canonical signalling of a Gi-coupled receptor. Not only can it couple to different G proteins (Gs/Gq) but the P2Y₁₃ receptor can also trigger several intracellular pathways related to the activation of MAPKs (mitogen-activated protein kinases) and the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3 axis. Moreover, the availability of P2Y₁₃ receptor knockout mice has highlighted the specific functions in which it is involved, mainly in the regulation of cholesterol and glucose metabolism, bone homeostasis and aspects of central nervous system function like pain transmission and neuroprotection. This review summarizes our current understanding of this elusive receptor, not only at the pharmacological and molecular level but also, in terms of its signalling properties and specific functions, helping to clarify the involvement of P2Y₁₃ receptors in pathological situations.

Transmission pathways and mediators as the basis for clinical pharmacology of pain

INTRODUCTION

Mediators in pain transmission are the targets of a multitude of different analgesic pharmaceuticals. This review explores the most significant mediators of pain transmission as well as the pharmaceuticals that act on them. Areas covered: The review explores many of the key mediators of pain transmission. In doing so, this review uncovers important areas for further research. It also highlights agents with potential for producing novel analgesics, probes important interactions between pain transmission pathways that could contribute to synergistic analgesia, and emphasizes transmission factors that participate in transforming acute injury into chronic pain. Expert commentary: This review examines current pain research, particularly in the context of identifying novel analgesics, highlighting interactions between analgesic transmission pathways, and discussing factors that may contribute to the development of chronic pain after an acute injury.

Activity dependent internalization of the glutamate transporter GLT-1 mediated by β-arrestin 1 and ubiquitination

GLT-1 is the main glutamate transporter in the brain and undergoes trafficking processes that control its concentration on the cell surface thereby shaping glutamatergic neurotransmission. We have investigated how the traffic of GLT-1 is regulated by transporter activity. We report that internalization of GLT-1 from the cell surface is accelerated by transportable substrates like glutamate or aspartate, as well as by the transportable inhibitor L-trans-2,4-PDC, but not by the non-substrate inhibitor WAY 213613 in primary mixed cultures and in transiently transfected HEK293 cells. Analysis of the mechanism of endocytosis in HEK293 cells revealed that glutamate promoted the association with the transporter of the adaptor protein β-arrestin and the ubiquitin ligase Nedd4-2. The addition of glutamate is accompanied by an increase in the transporter ubiquitination, and the internalization is suppressed by an ubiquitination inhibitor (PYR41), and in a mutant defective in C-terminal lysines. The glutamate triggered endocytosis was also suppressed by siRNA for β-arrestin. This regulatory mechanism might be relevant in controlling the amount of transporter on the cell surface in conditions such as ischemia or traumatic brain injury, where extracellular concentrations of glutamate are persistently elevated.

Neuroprotection Mediated by P2Y13 Nucleotide Receptors in Neurons

ADP-specific P2Y₁₃ receptor constitutes one of the most recently identified nucleotide receptor and the understanding of their physiological role is currently under investigation. Cerebellar astrocytes and granule neurons provide excellent models to study P2Y₁₃ expression and function since the first identification of ADP-evoked calcium responses not attributable to the related P2Y₁ receptor was performed in these cell populations. In this regard, all responses induced by ADP analogues in astrocytes resulted to be Gi-coupled activities mediated by P2Y₁₃ instead of P2Y₁ receptors. Similarly, both glycogen synthase kinase-3 (GSK3) and ERK1/2 signaling triggered by 2MeSADP in cerebellar granule neurons were also dependent on Gi-coupled receptors, and mediated by PI3K activity. In granule neurons, P2Y₁₃ receptor was specifically coupled to the main neuronal survival PI3K/Akt-cascade targeting GSK3 phosphorylation. GSK3 inhibition led to nuclear translocation of transcriptional targets, including β-catenin and Nrf2. The activation of the Nrf2/heme oxygenase-1 (HO-1) axis was responsible for the prosurvival effect against oxidative stress. In addition, P2Y₁₃-mediated ERK1/2 signaling in granule neurons also triggered activation of transcription factors, such as CREB, which underlined the antiapoptotic action against glutamate-induced excitotoxicity. Finally, a novel signaling mechanism has been recently described for a P2Y₁₃ receptor in granule neurons that involved the expression of a dual protein phosphatase, DUSP2. This activity contributed to regulate MAPK activation after genotoxic stress. In conclusion, P2Y₁₃ receptors harbored in cerebellar astrocytes and granule neurons exhibit specific signaling properties that link them to specialized functions at the level of neuroprotection and trophic activity in both cerebellar cell populations.

Molecular basis of the dominant negative effect of a glycine transporter 2 mutation associated with hyperekplexia

Hyperekplexia or startle disease is a rare clinical syndrome characterized by an exaggerated startle in response to trivial tactile or acoustic stimuli. This neurological disorder can have serious consequences in neonates, provoking brain damage and/or sudden death due to apnea episodes and cardiorespiratory failure. Hyperekplexia is caused by defective inhibitory glycinergic neurotransmission. Mutations in the human SLC6A5 gene encoding the neuronal GlyT2 glycine transporter are responsible for the presynaptic form of the disease. GlyT2 mediates synaptic glycine recycling, which constitutes the main source of releasable transmitter at glycinergic synapses. Although the majority of GlyT2 mutations detected so far are recessive, a dominant negative mutant that affects GlyT2 trafficking does exist. In this study, we explore the properties and structural alterations of the S512R mutation in GlyT2. We analyze its dominant negative effect that retains wild-type GlyT2 in the endoplasmic reticulum (ER), preventing surface expression. We show that the presence of an arginine rather than serine 512 provoked transporter misfolding, enhanced association to the ER-chaperone calnexin, altered association with the coat-protein complex II component Sec24D, and thereby impeded ER exit. The S512R mutant formed oligomers with wild-type GlyT2 causing its retention in the ER. Overexpression of calnexin rescued wild-type GlyT2 from the dominant negative effect of the mutant, increasing the amount of transporter that reached the plasma membrane and dampening the interaction between the wild-type and mutant GlyT2. The ability of chemical chaperones to overcome the dominant negative effect of the disease mutation on the wild-type transporter was demonstrated in heterologous cells and primary neurons.

Presynaptic control of glycine transporter 2 (GlyT2) by physical and functional association with plasma membrane Ca2+-ATPase (PMCA) and Na+-Ca2+ exchanger (NCX)

Fast inhibitory glycinergic transmission occurs in spinal cord, brainstem, and retina to modulate the processing of motor and sensory information. After synaptic vesicle fusion, glycine is recovered back to the presynaptic terminal by the neuronal glycine transporter 2 (GlyT2) to maintain quantal glycine content in synaptic vesicles. The loss of presynaptic GlyT2 drastically impairs the refilling of glycinergic synaptic vesicles and severely disrupts neurotransmission. Indeed, mutations in the gene encoding GlyT2 are the main presynaptic cause of hyperekplexia in humans. Here, we show a novel endogenous regulatory mechanism that can modulate GlyT2 activity based on a compartmentalized interaction between GlyT2, neuronal plasma membrane Ca(2+)-ATPase (PMCA) isoforms 2 and 3, and Na(+)/Ca(2+)-exchanger 1 (NCX1). This GlyT2·PMCA2,3·NCX1 complex is found in lipid raft subdomains where GlyT2 has been previously found to be fully active. We show that endogenous PMCA and NCX activities are necessary for GlyT2 activity and that this modulation depends on lipid raft integrity. Besides, we propose a model in which GlyT2·PMCA2-3·NCX complex would help Na(+)/K(+)-ATPase in controlling local Na(+) increases derived from GlyT2 activity after neurotransmitter release.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

Glycine transporters GlyT1 and GlyT2 are differentially modulated by glycogen synthase kinase 3β

Inhibitory glycinergic neurotransmission is terminated by the specific glycine transporters GlyT1 and GlyT2 which actively reuptake glycine from the synaptic cleft. GlyT1 is associated with both glycinergic and glutamatergic pathways, and is the main regulator of the glycine levels in the synapses. GlyT2 is the main supplier of glycine for vesicle refilling, a process that is vital to preserve the quantal glycine content in synaptic vesicles. Therefore, to control glycinergic neurotransmission efficiently, GlyT1 and GlyT2 activity must be regulated by diverse neuronal and glial signaling pathways. In this work, we have investigated the possible functional modulation of GlyT1 and GlyT2 by glycogen synthase kinase 3 (GSK3β). This kinase is involved in mood stabilization, neurodegeneration and plasticity at excitatory and inhibitory synapses. The co-expression of GSK3β with GlyT1 or GlyT2 in COS-7 cells and Xenopus laevis oocytes, leads to inhibition and stimulation of GlyT1 and GlyT2 activities, respectively, with a decrease of GlyT1, and an increase in GlyT2 levels at the plasma membrane. The specificity of these changes is supported by the antagonism exerted by a catalytically inactive form of the kinase and through inhibitors of GSK3β such as lithium chloride and TDZD-8. GSK3β also increases the incorporation of 32Pi into GlyT1 and decreases that of GlyT2. The pharmacological inhibition of the endogenous GSK3β in neuron cultures of brainstem and spinal cord leads to an opposite modulation of GlyT1 and GlyT2.Our results suggest that GSK3β is important for stabilizing and/or controlling the expression of functional GlyTs on the neural cell surface.

Sarcosine preconditioning induces ischemic tolerance against global cerebral ischemia

Brain ischemic tolerance is an endogenous protective mechanism activated by a preconditioning stimulus that is closely related to N-methyl-d-aspartate receptor (NMDAR). Glycine transporter type 1 (GlyT-1) inhibitors potentiate NMDAR and suggest an alternative strategy for brain preconditioning. The aim of this work was to evaluate the effects of brain preconditioning induced by sarcosine, a GlyT-1 inhibitor, against global cerebral ischemia and its relation to NMDAR. Sarcosine was administered over 7 days (300 or 500 mg/kg/day, ip) before the induction of a global cerebral ischemia model in Wistar rats (male, 8-week-old). It was observed that sarcosine preconditioning reduced cell death in rat hippocampi submitted to cerebral ischemia. Hippocampal levels of glycine were decreased in sarcosine-treated animals, which was associated with a reduction of [(3)H] glycine uptake and a decrease in glycine transporter expression (GlyT-1 and GlyT-2). The expression of glycine receptors and the NR1 and NR2A subunits of NMDAR were not affected by sarcosine preconditioning. However, sarcosine preconditioning reduced the expression of the NR2B subunits of NMDAR. In conclusion, these data demonstrate that sarcosine preconditioning induces ischemic tolerance against global cerebral ischemia and this neuroprotective state is associated with changes in glycine transport and reduction of NR2B-containing NMDAR expression.

Inhibitory glycinergic neurotransmission in the mammalian auditory brainstem upon prolonged stimulation: short-term plasticity and synaptic reliability

Short-term plasticity plays a key role in synaptic transmission and has been extensively investigated for excitatory synapses. Much less is known about inhibitory synapses. Here we analyze the performance of glycinergic connections between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO) in the auditory brainstem, where high spike rates as well as fast and precise neurotransmission are hallmarks. Analysis was performed in acute mouse slices shortly after hearing onset (postnatal day (P)11) and 8 days later (P19). Stimulation was done at 37°C with 1–400 Hz for 40 s. Moreover, in a novel approach named marathon experiments, a very prolonged stimulation protocol was employed, comprising 10 trials of 1-min challenge and 1-min recovery periods at 50 and 1 Hz, respectively, thus lasting up to 20 min and amounting to >30,000 stimulus pulses. IPSC peak amplitudes displayed short-term depression (STD) and synaptic attenuation in a frequency-dependent manner. No facilitation was observed. STD in the MNTB-LSO connections was less pronounced than reported in the upstream calyx of Held-MNTB connections. At P11, the STD level and the failure rate were slightly lower within the ms-to-s range than at P19. During prolonged stimulation periods lasting 40 s, P19 connections sustained virtually failure-free transmission up to frequencies of 100 Hz, whereas P11 connections did so only up to 50 Hz. In marathon experiments, P11 synapses recuperated reproducibly from synaptic attenuation during all recovery periods, demonstrating a robust synaptic machinery at hearing onset. At 26°C, transmission was severely impaired and comprised abnormally high amplitudes after minutes of silence, indicative of imprecisely regulated vesicle pools. Our study takes a fresh look at synaptic plasticity and stability by extending conventional stimulus periods in the ms-to-s range to minutes. It also provides a framework for future analyses of synaptic plasticity.

Glycine transporters as novel therapeutic targets in schizophrenia, alcohol dependence and pain

Glycine transporters are endogenous regulators of the dual functions of glycine, which acts as a classical inhibitory neurotransmitter at glycinergic synapses and as a modulator of neuronal excitation mediated by NMDA (N-methyl-D-aspartate) receptors at glutamatergic synapses. The two major subtypes of glycine transporters, GlyT1 and GlyT2, have been linked to the pathogenesis and/or treatment of central and peripheral nervous system disorders, including schizophrenia and related affective and cognitive disturbances, alcohol dependence, pain, epilepsy, breathing disorders and startle disease (also known as hyperekplexia). This Review examines the rationale for the therapeutic potential of GlyT1 and GlyT2 inhibition, and surveys the latest advances in the biology of glycine reuptake and transport as well as the drug discovery and clinical development of compounds that block glycine transporters.

Na+/K+-ATPase is a new interacting partner for the neuronal glycine transporter GlyT2 that downregulates its expression in vitro and in vivo

The neuronal glycine transporter GlyT2 plays a fundamental role in the glycinergic neurotransmission by recycling the neurotransmitter to the presynaptic terminal. GlyT2 is the main supplier of glycine for vesicle refilling, a process that is absolutely necessary to preserve quantal glycine content in synaptic vesicles. Alterations in GlyT2 activity modify glycinergic neurotransmission and may underlie several neuromuscular disorders, such as hyperekplexia, myoclonus, dystonia, and epilepsy. Indeed, mutations in the gene encoding GlyT2 are the main presynaptic cause of hyperekplexia in humans and produce congenital muscular dystonia type 2 (CMD2) in Belgian Blue cattle. GlyT2 function is strictly coupled to the sodium electrochemical gradient actively generated by the Na+/K+-ATPase (NKA). GlyT2 cotransports 3Na+/Cl-/glycine generating large rises of Na+ inside the presynaptic terminal that must be efficiently reduced by the NKA to preserve Na+ homeostasis. In this work, we have used high-throughput mass spectrometry to identify proteins interacting with GlyT2 in the CNS. NKA was detected as a putative candidate and through reciprocal coimmunoprecipitations and immunocytochemistry analyses the association between GlyT2 and NKA was confirmed. NKA mainly interacts with the raft-associated active pool of GlyT2, and low and high levels of the specific NKA ligand ouabain modulate the endocytosis and total expression of GlyT2 in neurons. The ouabain-mediated downregulation of GlyT2 also occurs in vivo in two different systems: zebrafish embryos and adult rats, indicating that this NKA-mediated regulatory mechanism is evolutionarily conserved and may play a relevant role in the physiological control of inhibitory glycinergic neurotransmission.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

Rapid, activity-independent turnover of vesicular transmitter content at a mixed glycine/GABA synapse

The release of neurotransmitter via the fusion of transmitter-filled, presynaptic vesicles is the primary means by which neurons relay information. However, little is known regarding the molecular mechanisms that supply neurotransmitter destined for vesicle filling, the endogenous transmitter concentrations inside presynaptic nerve terminals, or the dynamics of vesicle refilling after exocytosis. We addressed these issues by recording from synaptically coupled pairs of glycine/GABA coreleasing interneurons (cartwheel cells) of the mouse dorsal cochlear nucleus. We find that the plasma membrane transporter GlyT2 and the intracellular enzyme glutamate decarboxylase supply the majority of glycine and GABA, respectively. Pharmacological block of GlyT2 or glutamate decarboxylase led to rapid and complete rundown of transmission, whereas increasing GABA synthesis via intracellular glutamate uncaging dramatically potentiated GABA release within 1 min. These effects were surprisingly independent of exocytosis, indicating that prefilled vesicles re-equilibrated upon acute changes in cytosolic transmitter. Titration of cytosolic transmitter with postsynaptic responses indicated that endogenous, nonvesicular glycine/GABA levels in nerve terminals are 5-7 mm, and that vesicular transport mechanisms are not saturated under basal conditions. Thus, cytosolic transmitter levels dynamically set the strength of inhibitory synapses in a release-independent manner.

Calnexin-assisted biogenesis of the neuronal glycine transporter 2 (GlyT2)

The neuronal transporter GlyT2 is a polytopic, 12-transmembrane domain, plasma membrane glycoprotein involved in the removal and recycling of synaptic glycine from inhibitory synapses. Mutations in the human GlyT2 gene (SLC6A5) that cause deficient glycine transport or defective GlyT2 trafficking are the second most common cause of hyperekplexia or startle disease. In this study we examined several aspects of GlyT2 biogenesis that involve the endoplasmic reticulum chaperone calnexin (CNX). CNX binds transiently to an intermediate under-glycosylated transporter precursor and facilitates GlyT2 processing. In cells expressing GlyT2, transporter accumulation and transport activity were attenuated by siRNA-mediated CNX knockdown and enhanced by CNX overexpression. GlyT2 binding to CNX was mediated by glycan and polypeptide-based interactions as revealed by pharmacological approaches and the behavior of GlyT2 N-glycan-deficient mutants. Moreover, transporter folding appeared to be stabilized by N-glycans. Co-expression of CNX and a fully non-glycosylated mutant rescues glycine transport but not mutant surface expression. Hence, CNX discriminates between different conformational states of GlyT2 displaying a lectin-independent chaperone activity. GlyT2 wild-type and mutant transporters were finally degraded in the lysosome. Our findings provide further insight into GlyT2 biogenesis, and a useful framework for the study of newly synthesized GlyT2 transporters bearing hyperekplexia mutations.

Constitutive endocytosis and turnover of the neuronal glycine transporter GlyT2 is dependent on ubiquitination of a C-terminal lysine cluster

Inhibitory glycinergic neurotransmission is terminated by sodium and chloride-dependent plasma membrane glycine transporters (GlyTs). The mainly glial glycine transporter GlyT1 is primarily responsible for the completion of inhibitory neurotransmission and the neuronal glycine transporter GlyT2 mediates the reuptake of the neurotransmitter that is used to refill synaptic vesicles in the terminal, a fundamental role in the physiology and pathology of glycinergic neurotransmission. Indeed, inhibitory glycinergic neurotransmission is modulated by the exocytosis and endocytosis of GlyT2. We previously reported that constitutive and Protein Kinase C (PKC)-regulated endocytosis of GlyT2 is mediated by clathrin and that PKC accelerates GlyT2 endocytosis by increasing its ubiquitination. However, the role of ubiquitination in the constitutive endocytosis and turnover of this protein remains unexplored. Here, we show that ubiquitination of a C-terminus four lysine cluster of GlyT2 is required for constitutive endocytosis, sorting into the slow recycling pathway and turnover of the transporter. Ubiquitination negatively modulates the turnover of GlyT2, such that increased ubiquitination driven by PKC activation accelerates transporter degradation rate shortening its half-life while decreased ubiquitination increases transporter stability. Finally, ubiquitination of GlyT2 in neurons is highly responsive to the free pool of ubiquitin, suggesting that the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase-L1 (UCHL1), as the major regulator of neuronal ubiquitin homeostasis, indirectly modulates the turnover of GlyT2. Our results contribute to the elucidation of the mechanisms underlying the dynamic trafficking of this important neuronal protein which has pathological relevance since mutations in the GlyT2 gene (SLC6A5) are the second most common cause of human hyperekplexia.

A novel dominant hyperekplexia mutation Y705C alters trafficking and biochemical properties of the presynaptic glycine transporter GlyT2

Hyperekplexia or startle disease is characterized by an exaggerated startle response, evoked by tactile or auditory stimuli, producing hypertonia and apnea episodes. Although rare, this orphan disorder can have serious consequences, including sudden infant death. Dominant and recessive mutations in the human glycine receptor (GlyR) α1 gene (GLRA1) are the major cause of this disorder. However, recessive mutations in the presynaptic Na(+)/Cl(-)-dependent glycine transporter GlyT2 gene (SLC6A5) are rapidly emerging as a second major cause of startle disease. In this study, systematic DNA sequencing of SLC6A5 revealed a new dominant GlyT2 mutation: pY705C (c.2114A→G) in transmembrane domain 11, in eight individuals from Spain and the United Kingdom. Curiously, individuals harboring this mutation show significant variation in clinical presentation. In addition to classical hyperekplexia symptoms, some individuals had abnormal respiration, facial dysmorphism, delayed motor development, or intellectual disability. We functionally characterized this mutation using molecular modeling, electrophysiology, [(3)H]glycine transport, cell surface expression, and cysteine labeling assays. We found that the introduced cysteine interacts with the cysteine pair Cys-311-Cys-320 in the second external loop of GlyT2. This interaction impairs transporter maturation through the secretory pathway, reduces surface expression, and inhibits transport function. Additionally, Y705C presents altered H(+) and Zn(2+) dependence of glycine transport that may affect the function of glycinergic neurotransmission in vivo.

P2 receptors for extracellular nucleotides in the central nervous system: role of P2X7 and P2Y₂ receptor interactions in neuroinflammation

Extracellular nucleotides induce cellular responses in the central nervous system (CNS) through the activation of ionotropic P2X and metabotropic P2Y nucleotide receptors. Activation of these receptors regulates a wide range of physiological and pathological processes. In this review, we present an overview of the current literature regarding P2X and P2Y receptors in the CNS with a focus on the contribution of P2X7 and P2Y(2) receptor-mediated responses to neuroinflammatory and neuroprotective mechanisms.

Endocytosis of the neuronal glycine transporter GLYT2: role of membrane rafts and protein kinase C-dependent ubiquitination

Glycinergic neurotransmission is terminated by sodium- and chloride-dependent plasma membrane transporters. The neuronal glycine transporter 2 (GLYT2) supplies the terminal with substrate to refill synaptic vesicles containing glycine. This crucial process is defective in human hyperekplexia, a condition that can be caused by mutations in GLYT2. Inhibitory glycinergic neurotransmission is modulated by the GLYT2 exocytosis/endocytosis equilibrium, although the mechanisms underlying the turnover of this transporter remain elusive. We studied GLYT2 internalization pathways and the role of ubiquitination and membrane raft association of the transporter in its endocytosis. Using pharmacological tools, dominant-negative mutants and small-interfering RNAs, we show that the clathrin-mediated pathway is the primary mechanism for constitutive and regulated GLYT2 endocytosis in heterologous cells and neurons. We show that GLYT2 is constitutively internalized from cell surface lipid rafts, remaining associated with rafts in subcellular recycling structures. Protein kinase C (PKC) negatively modulates GLYT2 via rapid and dynamic redistribution of GLYT2 from raft to non-raft membrane subdomains and increasing ubiquitinated GLYT2 endocytosis. This biphasic mechanism is a versatile means to modulate GLYT2 behavior and hence, inhibitory glycinergic neurotransmission. These findings may reveal new therapeutic targets to address glycinergic pathologies associated with alterations in GLYT2 trafficking.

Molecular basis of the differential interaction with lithium of glycine transporters GLYT1 and GLYT2

Glycine synaptic levels are controlled by glycine transporters (GLYTs) catalyzing Na(+)/Cl(-)/glycine cotransport. GLYT1 displays a 2:1 :1 stoichiometry and is the main regulator of extracellular glycine concentrations. The neuronal GLYT2, with higher sodium coupling (3:1 :1), supplies glycine to the pre-synaptic terminal to refill synaptic vesicles. In this work, using structural homology modelling and molecular dynamics simulations of GLYTs, we predict the conservation of the two sodium sites present in the template (leucine transporter from Aquifex aeolicus), and confirm its use by mutagenesis and functional analysis. GLYTs Na1 and Na2 sites show differential cation selectivity, as inferred from the action of lithium, a non-transport-supporting ion, on Na(+)-site mutants. GLYTs lithium responses were unchanged in Na1-site mutants, but abolished or inverted in mutants of Na2 site, which binds lithium in the presence of low sodium concentrations and therefore, controls lithium responses. Here, we report, for the first time, that lithium exerts opposite actions on GLYTs isoforms. Glycine transport by GLYT1 is inhibited by lithium whereas GLYT2 transport is stimulated, and this effect is more evident at increased glycine concentrations. In contrast to GLYT1, high and low affinity lithium-binding processes were detected in GLYT2.