Regulation of the Glycine Transporter GLYT1 by microRNAs.

The glycine transporter GLYT1 participates in inhibitory and excitatory neurotransmission by controlling the reuptake of this neuroactive substance from synapses. Over the past few years, microRNAs have emerged as potent negative regulators of gene expression. In this report, we investigate the possible regulation of GLYT1 by microRNAs. TargetScan software predicted the existence of multiple targets for microRNAs within the 3' UTR of the human GLYT1 (miR-7, miR-30, miR-96, miR-137 and miR-141), and as they are all conserved among mammalian orthologues, their effects on GLYT1 expression were determined experimentally. Dual reporter bioluminescent assays showed that only miR-96 and miR-137 down-regulated expression of the Renilla reporter fused to the 3' UTR of GLYT1. Mutations introduced into the target sequences blocked this inhibitory effect. Consistently, these two microRNAs downregulated the uptake of [3H]glycine into glial C6 cells, a cell line where GLYT1 is the main carrier for glycine. Moreover, the expression of endogenous GLYT1 in primary mixed cultures from rat spinal cord was decreased upon lentiviral expression of miR-96 and miR-137. Although the bulk of GLYT1 is glial, it is abundantly expressed in glycinergic neurons of the retina and in smaller amounts in glutamatergic neurons though the brain. Since miR-96 in the retina is strongly downregulated by light exposure, when rats were maintained in darkness for a few hours we observed a concomitant increase of GLYT1 expression, suggesting that at least miR-96 might be an important negative regulator of GLYT1 under physiological conditions.

Regulation of the voltage-dependent sodium channel NaV1.1 by AKT1.

The voltage-sensitive sodium channel NaV1.1 plays a critical role in regulating excitability of GABAergic neurons and mutations in the corresponding gene are associated to Dravet syndrome and other forms of epilepsy. The activity of this channel is regulated by several protein kinases. To identify novel regulatory kinases we screened a library of activated kinases and we found that AKT1 was able to directly phosphorylate NaV1.1. In vitro kinase assays revealed that the phosphorylation site was located in the C-terminal part of the large intracellular loop connecting domains I and II of NaV1.1, a region that is known to be targeted by other kinases like PKA and PKC. Electrophysiological recordings revealed that activated AKT1 strongly reduced peak Na+ currents and displaced the inactivation curve to more negative potentials in HEK-293 cell stably expressing NaV1.1. These alterations in current amplitude and steady-state inactivation were mimicked by SC79, a specific activator of AKT1, and largely reverted by triciribine, a selective inhibitor. Neurons expressing endogenous NaV1.1 in primary cultures were identified by expressing a fluorescent protein under the NaV1.1 promoter. There, we also observed a strong decrease in the current amplitude after addition of SC79, but small effects on the inactivation parameters. Altogether, we propose a novel mechanism that might regulate the excitability of neural networks in response to AKT1, a kinase that plays a pivotal role under physiological and pathological conditions, including epileptogenesis.

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.

Identification of novel regulatory partners of the glutamate transporter GLT-1.

We used proximity-dependent biotin identification (BioID) to find proteins that potentially interact with the major glial glutamate transporter, GLT-1, and we studied how these interactions might affect its activity. GTPase Rac1 was one protein identified, and interfering with its GTP/GDP cycle in mixed primary rat brain cultures affected both the clustering of GLT-1 at the astrocytic processes and the transport kinetics, increasing its uptake activity at low micromolar glutamate concentrations in a manner that was dependent on the effector kinase PAK1 and the actin cytoskeleton. Interestingly, the same manipulations had a different effect on another glial glutamate transporter, GLAST, inhibiting its activity. Importantly, glutamate acts through metabotropic receptors to stimulate the activity of Rac1 in astrocytes, supporting the existence of cross-talk between extracellular glutamate and the astrocytic form of the GLT-1 regulated by Rac1. CDC42EP4/BORG4 (a CDC42 effector) was also identified in the BioID screen, and it is a protein that regulates the assembly of septins and actin fibers, influencing the organization of the cytoskeleton. We found that GLT-1 interacts with septins, which reduces its lateral mobility at the cell surface. Finally, the G-protein subunit GNB4 dampens the activity of GLT-1, as revealed by its response to the activator peptide mSIRK, both in heterologous systems and in primary brain cultures. This effect occurs rapidly and thus, it is unlikely to depend on cytoskeletal dynamics. These novel interactions shed new light on the events controlling GLT-1 activity, thereby helping us to better understand how glutamate homeostasis is maintained in the brain.

Interaction of the α7-nicotinic subunit with its human-specific duplicated dupα7 isoform in mammalian cells: Relevance in human inflammatory responses.

The α7 nicotinic receptor subunit and its partially duplicated human-specific dupα7 isoform are coexpressed in neuronal and non-neuronal cells. In these cells, α7 subunits form homopentameric α7 nicotinic acetylcholine receptors (α7-nAChRs) implicated in numerous pathologies. In immune cells, α7-nAChRs are essential for vagal control of inflammatory response in sepsis. Recent studies show that the dupα7 subunit is a dominant-negative regulator of α7-nAChR activity in Xenopus oocytes. However, its biological significance in mammalian cells, particularly immune cells, remains unexplored, as the duplicated form is indistinguishable from the original subunit in standard tests. Here, using immunocytochemistry, confocal microscopy, coimmunoprecipitation, FRET, flow cytometry, and ELISA, we addressed this challenge in GH4C1 rat pituitary cells and RAW264.7 murine macrophages transfected with epitope- and fluorescent protein-tagged α7 or dupα7. We used quantitative RT-PCR of dupα7 gene expression levels in peripheral blood mononuclear cells (PBMCs) from patients with sepsis to analyze its relationship with PBMC α7 mRNA levels and with serum concentrations of inflammatory markers. We found that a physical interaction between dupα7 and α7 subunits in both cell lines generates heteromeric nAChRs that remain mainly trapped in the endoplasmic reticulum. The dupα7 sequestration of α7 subunits reduced membrane expression of functional α7-nAChRs, attenuating their anti-inflammatory capacity in lipopolysaccharide-stimulated macrophages. Moreover, the PBMC's dupα7 levels correlated inversely with their α7 levels and directly with the magnitude of the patients' inflammatory state. These results indicate that dupα7 probably reduces human vagal anti-inflammatory responses and suggest its involvement in other α7-nAChR-mediated pathophysiological processes.

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.

Glycinergic transmission: glycine transporter GlyT2 in neuronal pathologies.

Glycinergic neurons are major contributors to the regulation of neuronal excitability, mainly in caudal areas of the nervous system. These neurons control fluxes of sensory information between the periphery and the CNS and diverse motor activities like locomotion, respiration or vocalization. The phenotype of a glycinergic neuron is determined by the expression of at least two proteins: GlyT2, a plasma membrane transporter of glycine, and VIAAT, a vesicular transporter shared by glycine and GABA. In this article, we review recent advances in understanding the role of GlyT2 in the pathophysiology of inhibitory glycinergic neurotransmission. GlyT2 mutations are associated to decreased glycinergic function that results in a rare movement disease termed hyperekplexia (HPX) or startle disease. In addition, glycinergic neurons control pain transmission in the dorsal spinal cord and their function is reduced in chronic pain states. A moderate inhibition of GlyT2 may potentiate glycinergic inhibition and constitutes an attractive target for pharmacological intervention against these devastating conditions.

Activity dependent internalization of the glutamate transporter GLT-1 mediated by ß-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 ß-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 ß-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.

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

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 (GSK3ß). This kinase is involved in mood stabilization, neurodegeneration and plasticity at excitatory and inhibitory synapses. The co-expression of GSK3ß 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 GSK3ß such as lithium chloride and TDZD-8. GSK3ß also increases the incorporation of 32Pi into GlyT1 and decreases that of GlyT2. The pharmacological inhibition of the endogenous GSK3ß in neuron cultures of brainstem and spinal cord leads to an opposite modulation of GlyT1 and GlyT2.Our results suggest that GSK3ß is important for stabilizing and/or controlling the expression of functional GlyTs on the neural cell surface.

Differential regulation of the glutamate transporters GLT-1 and GLAST by GSK3ß.

The glutamate transporters GLAST and GLT-1 are mainly expressed in glial cells and regulate glutamate levels in the synapses. GLAST and GLT-1 are the targets of several signaling pathways. In this study we explore the possible functional interaction between these transporters and GSK3ß. This kinase is involved in multiple cellular processes including neuronal development and synaptic plasticity. To evaluate whether GLT-1 and GLAST were regulated by GSK3ß, we coexpressed these proteins in heterologous expression systems. In both COS-7 cells and Xenopus laevis oocytes, GSK3ß stimulated the activity of GLT-1 and reduced that of GLAST. These effects were associated with corresponding changes in the amounts of GLT-1 or GLAST in the plasma membrane. These effects were suppressed by inhibitors of GSK3ß or a catalytically inactive form of the kinase. GSK3ß also decreases the incorporation of (32)Pi into GLT-1 and increases GLAST phosphorylation. Pharmacological inhibition of endogenous GSK3ß in primary cultures of rat brain cortex also leads to a differential modulation of GLT-1 and GLAST. Our results suggest that constitutively active GSK3ß is important in controlling the expression of functional glutamate transporters on the plasma membrane. This regulation might be relevant in physiological and pathological conditions in which glutamate transporters and GSK3ß signaling are involved.

Expression of the System N transporter (SNAT5/SN2) during development indicates its plausible role in glutamatergic neurotransmission.

Solute neutral amino acid transporter 5 (SNAT5/SN2) is a member of the System N family, expressed in glial cells in the adult brain, able to transport glutamine, histidine or glycine among other substrates. Its tight association with synapses and its electroneutral mode of operation that allows the bidirectional movement of substrates, supports the idea that this transporter participates in the function of the glutamine-glutamate cycle between neurons and glia. Moreover, SNAT5/SN2 might contribute to the regulation of glycine concentration in glutamatergic synapses and, therefore, to the functioning of the N-methyl-d-aspartate (NMDA) subtype of glutamate receptors. Ontogenic maturation of these synapses occurs postnatally through the coordinate expression of a large number of receptors, transporters, structural and regulatory proteins that ensure the correct operation of the excitatory pathways in the central nervous system. Since the temporal pattern of expression of SNAT5/SN2 is unknown, we analyzed it by immunoblot and immunohistochemical techniques. Results indicate that the expression of SNAT5/SN2 is triggered between the second and third postnatal week in the cerebral cortex, in parallel to the expression of the vesicular glutamate transporter vGLUT1 and the glial glutamate transporter GLT1/EAAT2. In the cerebellum, this process occurs about one week later than in the cerebral cortex. Immunohistochemical staining of cortical sections shows that from postnatal day 14 to adulthood the transporter was expressed exclusively in glial cells. Our results are consistent with the idea that SNAT5/SN2 expression is coordinated with that of other proteins necessary for the operation of glutamatergic synapses and reinforce the existence of a regulatory cross-talk between neurons and glia that orchestrates the building up of these synapses.

Localization of the glycine transporter GLYT1 in glutamatergic synaptic vesicles.

We have previously shown the presence of the glycine transporter GLYT1 in glutamatergic terminals of the rat brain. In this study we present immunohistochemical and biochemical evidence indicating that GLYT1 is expressed not only at the plasma membrane of glutamatergic neurons, but also at synaptic vesicles. Confocal microscopy, immunoblots analysis of a highly purified synaptic vesicle fraction and immunoisolation of synaptic vesicles with anti-synaptophysin antibodies strongly suggested the presence of GLYT1 in synaptic vesicles. Moreover, direct observation with the electron microscope of purified vesicles immunoreacted with anti-GLYT1 and colloidal gold demonstrated that about 40% of the small vesicles of the purified vesicle fraction contained GLYT1. Double labeling for GLYT1 and synaptophysin of this vesicular fraction revealed that more of ninety percent of them were synaptic vesicles. Moreover, a significant part of the GLYT1 containing vesicles (86%) also contained the vesicular glutamate transporter vGLUT1, suggesting a functional role of GLYT1 in a subpopulation of glutamatergic vesicles.

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.

Cell surface turnover of the glutamate transporter GLT-1 is mediated by ubiquitination/deubiquitination.

The main glutamate transporter in the brain, GLT-1, mediates glutamatergic neurotransmission in both physiological and pathological conditions. GLT-1 activity is controlled by both constitutive and regulated trafficking, and although recent evidence indicates that the turnover of this protein in the plasma membrane is accelerated by protein kinase C via an ubiquitin-dependent process, the mechanisms driving the constitutive trafficking of GLT-1 remain unexplored. Here, we used a heterologous system and primary astrocytes to investigate the turnover of GLT-1 and the role of ubiquitin attachment in this process. We show that GLT-1 is endocytosed constitutively in a clathrin-dependent manner, recycling the transporter into endosomes containing EEA1 and Rab4, a marker of rapidly recycling endosomes, and not Rab11 or Rab7, markers of the slow recycling and late endosomal compartments, respectively. We also show that this process is dependent on ubiquitination, because the inhibitor of the ubiquitin-activating enzyme E1, 4[4-(5-nitro-furan-2-ylmethylene)-3,5-dioxo-pyrazolidin-1-yl]-benzoic acid ethyl ester, promotes the retention of GLT-1 at the plasma membrane. Moreover, site-directed mutagenesis demonstrated the involvement of lysines 517 and 526 of GLT-1 in the constitutive internalization of the transporter. The translocation of GLT-1 from the recycling endosomes to the plasma membrane was blocked by LDN-57444, a specific inhibitor to the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase-L1, but not by an inhibitor of the related DUB ubiquitin C-terminal hydrolase-L3, supporting the existence of specific ubiquitination/deubiquitination cycles that ensure the correct concentrations of GLT-1 at the cell surface.

Protein kinase C (PKC)-promoted endocytosis of glutamate transporter GLT-1 requires ubiquitin ligase Nedd4-2-dependent ubiquitination but not phosphorylation.

Glutamate transporter-1 (GLT-1) is the main glutamate transporter in the central nervous system, and its concentration severely decreases in neurodegenerative diseases. The number of transporters in the plasma membrane reflects the balance between their insertion and removal, and it has been reported that the regulated endocytosis of GLT-1 depends on its ubiquitination triggered by protein kinase C (PKC) activation. Here, we identified serine 520 of GLT-1 as the primary target for PKC-dependent phosphorylation, although elimination of this serine did not impair either GLT-1 ubiquitination or endocytosis in response to phorbol esters. In fact, we present evidence indicating that the ubiquitin ligase Nedd4-2 mediates the PKC-dependent ubiquitination and down-regulation of GLT-1. Overexpression of Nedd4-2 increased the ubiquitination of the transporter and promoted its degradation. Moreover, phorbol myristate acetate enhanced Nedd4-2 phosphorylation and the formation of GLT-1·Nedd4-2 complexes, whereas siRNA knockdown of Nedd4-2 prevented ubiquitination, endocytosis, and the concomitant decrease in GLT-1 activity triggered by PKC activation. These results indicate that GLT-1 endocytosis is independent of its phosphorylation and that Nedd4-2 mediates PKC-dependent down-regulation of the transporter.

Expression of the SNAT2 amino acid transporter during the development of rat cerebral cortex.

The sodium-coupled neutral amino acid transporter 2 (SNAT2) is a protein that is expressed ubiquitously in mammalian tissues and that displays Na(+), voltage and pH dependent activity. This transporter mediates the passage of small zwitterionic amino acids across the cell membrane and regulates the cell homeostasis and its volume. We have examined the expression of SNAT2 mRNA and protein during the development of the rat cerebral cortex, from gestation through the postnatal stages to adulthood. Our data reveal that SNAT2 mRNA and protein expression is higher during embryogenesis, while it subsequently diminishes during postnatal development. Moreover, during embryonic period SNAT2 colocalizes with the radial glial cells marker GLAST, while in postnatal period it is mainly detected in neuronal dendrites. These findings suggest a relevant role for amino acid transport through SNAT2 in the developing embryonic brain.

Splice variants of the glutamate transporter GLT1 form hetero-oligomers that interact with PSD-95 and NMDA receptors.

The glutamate transporter GLT1 is expressed in at least two isoforms, GLT1a and GLT1b, which differ in their C termini. As GLT1 is an oligomeric protein, we have investigated whether GLT1a and GLT1b might associate as hetero-oligomers. Differential tagging (HA-GLT1a and YFP-GLT1b) revealed that these isoforms form complexes that could be immunoprecipitated when co-expressed in heterologous systems. The association of GLT1a and GLT1b was also observed in mixed primary cultures of rat brain and in the adult rat brain, where specific antibodies for GLT1a immunoprecipitated GLT1b and vice versa. Dual immunofluorescence in mixed cultures demonstrated the partial co-localization of both isoforms in neurons and in glial cells. Because GLT1b interacts with an organizer of post-synaptic densities, PSD-95, we examined the capacity of GLT1a to associate with this protein. GLT1a was immunoprecipitated from the rat brain in protein complexes that contained not only GLT1b but also PSD-95 and NMDAR. The interaction between GLT1a with PSD-95 and NMDAR was reproduced in transfected COS7 cells and it appears to be indirect as it requires the presence of GLT1b. These results indicate that the major isoform of the glutamate transporter, GLT1a, can acquire the capacity to interact with PDZ proteins through its inclusion in hetero-oligomers containing GLT1b.

Constitutive and regulated endocytosis of the glycine transporter GLYT1b is controlled by ubiquitination.

The glycine transporter GLYT1 regulates both glycinergic and glutamatergic neurotransmission by controlling the reuptake of glycine at synapses. Trafficking of GLYT1 to and from the cell surface is critical for its function. Activation of PKC down-regulates the activity of GLYT1 through a mechanism that has so far remained uncharacterized. Here we show that GLYT1b undergoes fast constitutive endocytosis that is accelerated by phorbol esters. Both constitutive and regulated endocytosis occur through a dynamin 2- and clathrin-dependent pathway, accumulating in the transporter in transferrin-containing endosomes. A chimera with the extracellular and transmembrane domains of the nerve growth factor receptor and the COOH-terminal tail of GLYT1 was efficiently internalized through this clathrin pathway, suggesting the presence of molecular determinants for GLYT1b endocytosis in its COOH-terminal tail. Extensive site-directed mutagenesis in this region of the chimera highlighted the involvement of lysine residues in its internalization. In the context of the full-length transporter, lysine 619 played a prominent role in both the constitutive and phorbol 12-myristate 13-acetate-induced endocytosis of GLYT1b, suggesting the involvement of ubiquitin modification of GLYT1b during the internalization process. Indeed, we show that GLYT1b undergoes ubiquitination and that this process is stimulated by phorbol 12-myristate 13-acetate. In addition, this endocytosis is impaired in an ubiquitination-deficient cell line, further evidence that constitutive and regulated endocytosis of GLYT1b is ubiquitin-dependent. It remains to be determined whether GLYT1b recycling might be affected in pathologies involving alterations to the ubiquitin system, thereby interfering with its influence on inhibitory and excitatory neurotransmission.

Glycine transporters and synaptic function.

Glycine is an inhibitory neurotransmitter that is mainly active in the caudal areas of the CNS. However, glycine also participates in excitatory neurotransmission since it is a co-agonist of the NMDA subtype of glutamate receptors. The concentration of glycine at synapses is mainly controlled by two sodium and chloride dependent transporters, GLYT1 and GLYT2, proteins that display a complementary distribution and activity in the nervous system. Our understanding of the physiological role of these transporters has advanced recently, thanks to the development of specific inhibitors and the generation of mice defective in the corresponding genes. In addition, the three-dimensional resolution of the structure of a bacterial homologue has shed light on the mechanisms of glycine transport. It is likely that this knowledge will prove to be useful for the development of drugs with antipsychotic, procognitive or analgesic properties.