ALX 5407: a potent, selective inhibitor of the hGlyT1 glycine transporter

Transport mechanism and pharmacology of the human GlyT1

The glycine transporter 1 (GlyT1) plays a crucial role in the regulation of both inhibitory and excitatory neurotransmission by removing glycine from the synaptic cleft. Given its close association with glutamate/glycine co-activated NMDA receptors (NMDARs), GlyT1 has emerged as a central target for the treatment of schizophrenia, which is often linked to hypofunctional NMDARs. Here, we report the cryo-EM structures of GlyT1 bound with substrate glycine and drugs ALX-5407, SSR504734, and PF-03463275. These structures, captured at three fundamental states of the transport cycle-outward-facing, occluded, and inward-facing-enable us to illustrate a comprehensive blueprint of the conformational change associated with glycine reuptake. Additionally, we identified three specific pockets accommodating drugs, providing clear insights into the structural basis of their inhibitory mechanism and selectivity. Collectively, these structures offer significant insights into the transport mechanism and recognition of substrate and anti-schizophrenia drugs, thus providing a platform to design small molecules to treat schizophrenia.

Impaired glycine neurotransmission causes adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is the most common form of spinal deformity, affecting millions of adolescents worldwide, but it lacks a defined theory of etiopathogenesis. Because of this, treatment of AIS is limited to bracing and/or invasive surgery after onset. Preonset diagnosis or preventive treatment remains unavailable. Here, we performed a genetic analysis of a large multicenter AIS cohort and identified disease-causing and predisposing variants of SLC6A9 in multigeneration families, trios, and sporadic patients. Variants of SLC6A9, which encodes glycine transporter 1 (GLYT1), reduced glycine-uptake activity in cells, leading to increased extracellular glycine levels and aberrant glycinergic neurotransmission. Slc6a9 mutant zebrafish exhibited discoordination of spinal neural activities and pronounced lateral spinal curvature, a phenotype resembling human patients. The penetrance and severity of curvature were sensitive to the dosage of functional glyt1. Administration of a glycine receptor antagonist or a clinically used glycine neutralizer (sodium benzoate) partially rescued the phenotype. Our results indicate a neuropathic origin for "idiopathic" scoliosis, involving the dysfunction of synaptic neurotransmission and central pattern generators (CPGs), potentially a common cause of AIS. Our work further suggests avenues for early diagnosis and intervention of AIS in preadolescents.

Functional crosstalk of the glycine transporter GlyT1 and NMDA receptors

NMDA-type glutamate receptors (NMDARs) constitute one of the main glutamate (Glu) targets in the central nervous system and are involved in synaptic plasticity, which is the molecular substrate of learning and memory. Hypofunction of NMDARs has been associated with schizophrenia, while overstimulation causes neuronal death in neurodegenerative diseases or in stroke. The function of NMDARs requires coincidental binding of Glu along with other cellular signals such as neuronal depolarization, and the presence of other endogenous ligands that modulate their activity by allosterism. Among these allosteric modulators are zinc, protons and Gly, which is an obligatory co-agonist. These characteristics differentiate NMDARs from other receptors, and their structural bases have begun to be established in recent years. In this review we focus on the crosstalk between Glu and glycine (Gly), whose concentration in the NMDAR microenvironment is maintained by various Gly transporters that remove or release it into the medium in a regulated manner. The GlyT1 transporter is particularly involved in this task, and has become a target of great interest for the treatment of schizophrenia since its inhibition leads to an increase in synaptic Gly levels that enhances the activity of NMDARs. However, the only drug that has completed phase III clinical trials did not yield the expected results. Notwithstanding, there are additional drugs that continue to be investigated, and it is hoped that knowledge gained from the recently published 3D structure of GlyT1 may allow the rational design of more effective new drugs.

SNAT2 is responsible for hyperosmotic induced sarcosine and glycine uptake in human prostate PC-3 cells

Solute carriers (SLC) are important membrane transport proteins in normal and pathophysiological cells. The aim was to identify amino acid SLC(s) responsible for uptake of sarcosine and glycine in prostate cancer cells and investigate the impact hereon of hyperosmotic stress. Uptake of ¹⁴C-sarcosine and ³H-glycine was measured in human prostate cancer (PC-3) cells cultured under isosmotic (300 mOsm/kg) and hyperosmotic (500 mOsm/kg) conditions for 24 h. Hyperosmotic culture medium was obtained by supplementing the medium with 200 mM of the trisaccharide raffinose. Amino acid SLC expression was studied using RT-PCR, real-time PCR, and western blotting. siRNA knockdown of SNAT2 was performed. Experiments were conducted in at least 3 independent cell passages. The uptake of Sar and Gly was increased approximately 8-ninefold in PC-3 cells after 24 h hyperosmotic culture. PAT1 mRNA and protein could not be detected, while SNAT2 was upregulated at the mRNA and protein level. Transfection with SNAT2-specific siRNA reduced V_max of Sar uptake from 2653 ± 38 to 513 ± 38 nmol mg protein^-1 min^-1, without altering the K_m value (3.19 ± 0.13 vs. 3.42 ± 0.71 mM), indicating that SNAT2 is responsible for at least 80% of Sar uptake in hyperosmotic cultured PC-3 cells. SNAT2 is upregulated in hyperosmotic stressed prostate cancer cells and SNAT2 is responsible for cellular sarcosine and glycine uptake in hyperosmotic cultured PC-3 cells. Sar is identified as a substrate for SNAT2, and this has physiological implications for understanding cellular solute transport in prostate cancer cells.

Effect of glycine transporter 1 inhibition with bitopertin on parkinsonism and L-DOPA induced dyskinesia in the 6-OHDA-lesioned rat

Dyskinesia remains an unmet need in Parkinson's disease (PD). We have previously demonstrated that glycine transporter 1 (GlyT1) inhibition with ALX-5407 reduces dyskinesia and slightly improves parkinsonism in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. Here, we sought to determine the effect of bitopertin, a clinically-tested GlyT1 inhibitor, on parkinsonism and dyskinesia in the 6-hydroxydopamine (6-OHDA)-lesioned rat. To do so, we assessed the effect of bitopertin on parkinsonism as monotherapy and as adjunct to a low dose of L-3,4-dihydroxyphenylalanine (L-DOPA). We then assessed the efficacy of bitopertin on dyskinesia in the context of acute challenge and chronic administration studies. Lastly, we evaluated whether de novo treatment with bitopertin, started concurrently with L-DOPA, would diminish the development of dyskinesia. We discovered that bitopertin (0.3 mg/kg), when administered alone, reduced the severity of parkinsonism by 35% (P < 0.01). As adjunct to a low dose of L-DOPA, bitopertin (3 mg/kg) enhanced the anti-parkinsonian effect of L-DOPA by 36% (P < 0.05). Moreover, the acute addition of bitopertin (0.03 mg/kg) to L-DOPA reduced dyskinesia by 27% (P < 0.001), and there was no tolerance to the anti-dyskinetic benefit after 4 weeks of daily administration. Lastly, bitopertin (0.03 mg/kg) started concurrently with L-DOPA, also attenuated the development of dyskinesia, by 33% (P < 0.01), when compared to L-DOPA alone. Our results suggest that GlyT1 inhibition may simultaneously reduce parkinsonism and L-DOPA-induced dyskinesia and represents a novel approach to treat, possibly prevent, motor complications in PD.

Analysis of Binding Determinants for Different Classes of Competitive and Noncompetitive Inhibitors of Glycine Transporters

Glycine transporters are interesting therapeutic targets as they play significant roles in glycinergic and glutamatergic systems. The search for new selective inhibitors of particular types of glycine transporters (GlyT-1 and GlyT-2) with beneficial kinetics is hampered by limited knowledge about the spatial structure of these proteins. In this study, a pool of homology models of GlyT-1 and GlyT-2 in different conformational states was constructed using the crystal structures of related transporters from the SLC6 family and the recently revealed structure of GlyT-1 in the inward-open state, in order to investigate their binding sites. The binding mode of the known GlyT-1 and GlyT-2 inhibitors was determined using molecular docking studies, molecular dynamics simulations, and MM-GBSA free energy calculations. The results of this study indicate that two amino acids, Gly373 and Leu476 in GlyT-1 and the corresponding Ser479 and Thr582 in GlyT-2, are mainly responsible for the selective binding of ligands within the S1 site. Apart from these, one pocket of the S2 site, which lies between TM3 and TM10, may also be important. Moreover, selective binding of noncompetitive GlyT-1 inhibitors in the intracellular release pathway is affected by hydrophobic interactions with Ile399, Met382, and Leu158. These results can be useful in the rational design of new glycine transporter inhibitors with desired selectivity and properties in the future.

Glycinergic Modulation of Pain in Behavioral Animal Models

Animal models of human pain conditions allow for detailed interrogation of known and hypothesized mechanisms of pain physiology in awake, behaving organisms. The importance of the glycinergic system for pain modulation is well known; however, manipulation of this system to treat and alleviate pain has not yet reached the sophistication required for the clinic. Here, we review the current literature on what animal behavioral studies have allowed us to elucidate about glycinergic pain modulation, and the progress toward clinical treatments so far. First, we outline the animal pain models that have been used, such as nerve injury models for neuropathic pain, chemogenic pain models for acute and inflammatory pain, and other models that mimic painful human pathologies such as diabetic neuropathy. We then discuss the genetic approaches to animal models that have identified the crucial glycinergic machinery involved in neuropathic and inflammatory pain. Specifically, two glycine receptor (GlyR) subtypes, GlyRα1(β) and GlyRα3(β), and the two glycine transporters (GlyT), GlyT1 and GlyT2. Finally, we review the different pharmacological approaches to manipulating the glycinergic system for pain management in animal models, such as partial vs. full agonism, reversibility, and multi-target approaches. We discuss the benefits and pitfalls of using animal models in drug development broadly, as well as the progress of glycinergic treatments from preclinical to clinical trials.

An Overview of Glycine Transporter Subtype 1 Inhibitors Under Preclinical and Clinical Evaluation for the Treatment of Alcohol Abuse

Abstract:

Being a historical issue that withstands multiple societal control measures, alcohol abuse remains a major healthcare problem. Despite worldwide efforts to limit consumption and educate people about its effects, consumption rates remain unchanged. Alcohol abuse arises from chronic alcohol exposure-caused permanent synaptic plasticity changes in the brain. These manifest in life-threatening withdrawal symptoms and drive relapse even after detoxification and treatment. Since ethanol has multiple targets in the human brain, it warrants a multiapproach therapy; here we introduce the potential therapeutic effects of glycine transporter subtype 1 inhibitors. We have listed the various glycine transporter 1 inhibitors used in studies of alcoholism and how they influenced glycine release from rat hippocampus was demonstrated in a preliminary study. Glycine transporters modulate both glutamatergic and glycinergic pathways: (i) glutamatergic neurotransmission plays an important role in the development of chronic changes in alcoholism as daily alcohol administration was shown to increase N-methyl-D-aspartic acid receptor activity long-term, and (ii) ethanol has access to the dopaminergic reward system via glycine receptors, being an allosteric modulator of glycine receptors. This manuscript summarises the progress and development of glycine transporter 1 inhibitors, characterizing them by their mode of action, adverse effects, and discusses their clinical applicability. Furthermore, we highlight the progress in the latest clinical trials, outline currently applied treatment methods, and offer suggestions for implementing glycine transporter 1 inhibitors into the long-term treatment of alcohol abuse.

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

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

Effect of the glycine transporter 1 inhibitor ALX-5407 on dyskinesia, psychosis-like behaviours and parkinsonism in the MPTP-lesioned marmoset

Dyskinesia and psychosis are complications encountered in advanced Parkinson's disease (PD) following long-term therapy with L-3,4-dihydroxyphenylalanine (L-DOPA). Disturbances in the glutamatergic system have been associated with both dyskinesia and psychosis, making glutamatergic modulation a potential therapeutic approach for these. Treatments thus far have sought to dampen glutamatergic transmission, for example through blockade of N-methyl-D-aspartate (NMDA) receptors or modulation of metabotropic glutamate receptors 5. In contrast, activation of the glycine-binding site on NMDA receptors is required for their physiological response. Here, we investigated whether indirectly enhancing glutamatergic transmission through inhibition of glycine re-uptake would be efficacious in diminishing both dyskinesia and psychosis-like behaviours (PLBs) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned common marmoset. Six marmosets were rendered parkinsonian by MPTP injection. Following repeated administration of L-DOPA to induce dyskinesia and PLBs, they underwent acute challenges of the glycine transporter 1 (GlyT₁) inhibitor ALX-5407 (0.01, 0.1 and 1 mg/kg) or vehicle, in combination with L-DOPA, after which the severity of dyskinesia, PLBs and parkinsonian disability was evaluated. In combination with L-DOPA, ALX-5407 0.1 and 1 mg/kg significantly reduced the severity of dyskinesia, by 51% and 41% (both P < 0.001), when compared to vehicle. ALX-5407 0.01, 0.1 and 1 mg/kg also decreased the severity of global PLBs, by 25%, 51% and 38% (all P < 0.001), when compared to vehicle. The benefits on dyskinesia and PLBs were achieved without compromising the therapeutic effect of L-DOPA on parkinsonism. Our results suggest that GlyT₁ inhibition may be a novel strategy to attenuate dyskinesia and PLBs in PD, without interfering with L-DOPA anti-parkinsonian action.

[18F]ALX5406: A Brain-Penetrating Prodrug for GlyT1-Specific PET Imaging

Selective inhibition of glycine transporter 1 (GlyT1) has emerged as a potential approach to alleviate N-methyl-d-aspartate receptor (NMDAR) hypofunction in patients with schizophrenia and cognitive decline. ALX5407 is a potent and selective inhibitor of GlyT1 derived from the metabolic intermediate sarcosine (N-methylglycine) that showed antipsychotic potential in a number of animal models. Whereas clinical application of ALX5407 is limited by adverse effects on motor performance and respiratory function, a suitably radiolabeled drug could represent a promising PET tracer for the visualization of GlyT1 in the brain. Herein, [¹⁸F]ALX5407 and the corresponding methyl ester, [¹⁸F]ALX5406, were prepared by alcohol-enhanced copper mediated radiofluorination and studied in vitro in rat brain slices and in vivo in normal rats. [¹⁸F]ALX5407 demonstrated accumulation consistent with the distribution of GlyT1 in in vitro autoradiographic studies but no brain uptake in μPET experiments in naı̈ve rats. In contrast, the methyl ester [¹⁸F]ALX5406 rapidly entered the brain and was enzymatically transformed into [¹⁸F]ALX5407, resulting in a regional accumulation pattern consistent with GlyT1 specific binding. We conclude that [¹⁸F]ALX5406 is a promising and easily accessible PET probe for preclinical in vivo imaging of GlyT1 in the brain.

Repurposing of glycine transport inhibitors for the treatment of erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) is a rare disease in which patients experience severe light sensitivity. It is caused by a deficiency of ferrochelatase (FECH), the last enzyme in heme biosynthesis (HBS). The lack of FECH causes accumulation of its photoreactive substrate protoporphyrin IX (PPIX) in patients' erythrocytes. Here, we explored an approach for the treatment of EPP by decreasing PPIX synthesis using small-molecule inhibitors directed to factors in the HBS pathway. We generated a FECH-knockout clone from K562 erythroleukemia cells, which accumulates PPIX and undergoes oxidative stress upon light exposure. We used these matched cell lines to screen a set of publicly available inhibitors of factors in the HBS pathway. Inhibitors of the glycine transporters GlyT1 and GlyT2 lowered levels of PPIX and markers of oxidative stress selectively in K562^11B4 cells, and in primary erythroid cultures from an EPP patient. Our findings open the door to investigation of glycine transport inhibitors for HBS disorders.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

A System for Assessing Dual Action Modulators of Glycine Transporters and Glycine Receptors

Reduced inhibitory glycinergic neurotransmission is implicated in a number of neurological conditions such as neuropathic pain, schizophrenia, epilepsy and hyperekplexia. Restoring glycinergic signalling may be an effective method of treating these pathologies. Glycine transporters (GlyTs) control synaptic and extra-synaptic glycine concentrations and slowing the reuptake of glycine using specific GlyT inhibitors will increase glycine extracellular concentrations and increase glycine receptor (GlyR) activation. Glycinergic neurotransmission can also be improved through positive allosteric modulation (PAM) of GlyRs. Despite efforts to manipulate this synapse, no therapeutics currently target it. We propose that dual action modulators of both GlyTs and GlyRs may show greater therapeutic potential than those targeting individual proteins. To show this, we have characterized a co-expression system in Xenopus laevis oocytes consisting of GlyT1 or GlyT2 co-expressed with GlyRα₁. We use two electrode voltage clamp recording techniques to measure the impact of GlyTs on GlyRs and the effects of modulators of these proteins. We show that increases in GlyT density in close proximity to GlyRs diminish receptor currents. Reductions in GlyR mediated currents are not observed when non-transportable GlyR agonists are applied or when Na⁺ is not available. GlyTs reduce glycine concentrations across different concentration ranges, corresponding with their ion-coupling stoichiometry, and full receptor currents can be restored when GlyTs are blocked with selective inhibitors. We show that partial inhibition of GlyT2 and modest GlyRα₁ potentiation using a dual action compound, is as useful in restoring GlyR currents as a full and potent single target GlyT2 inhibitor or single target GlyRα₁ PAM. The co-expression system developed in this study will provide a robust means for assessing the likely impact of GlyR PAMs and GlyT inhibitors on glycine neurotransmission.

Azetidine-based selective glycine transporter-1 (GlyT1) inhibitors with memory enhancing properties

A strategy to conformationally restrain a series of GlyT1 inhibitors identified potent analogs that exhibited slowly interconverting rotational isomers. Further studies to address this concern led to a series of azetidine-based inhibitors. Compound 26 was able to elevate CSF glycine levels in vivo and demonstrated potency comparable to Bitopertin in an in vivo rat receptor occupancy study. Compound 26 was subsequently shown to enhance memory in a Novel Object Recognition (NOR) behavioral study after a single dose of 0.03 mg/kg, and in a contextual fear conditioning (cFC) study after four QD doses of 0.01-0.03 mg/kg.

The Druggability of Solute Carriers

The transport of materials across membranes is a vital process for all aspects of cellular function, including growth, metabolism, and communication. Protein transporters are the molecular gates that control this movement and serve as key points of regulation for these processes, thus representing an attractive class of therapeutic targets. With more than 400 members, the solute carrier (SLC) membrane transport proteins are the largest family of transporters, yet, they are pharmacologically underexploited relative to other protein families and many of the available chemical tools possess suboptimal selectivity and efficacy. Fortuitously, there is increased interest in elucidating the physiological roles of SLCs as well as growing recognition of their therapeutic potential. This Perspective provides an overview of the SLC superfamily, including their biochemical and functional features, as well as their roles in various human diseases. In particular, we explore efforts and associated challenges toward drugging SLCs, as well as highlight opportunities for future drug discovery.

The impact of D-cycloserine and sarcosine on in vivo frontal neural activity in a schizophrenia-like model

BACKGROUND

N-methyl-D-aspartate receptor (NMDAR) hypofunction has been proposed to underlie the pathogenesis of schizophrenia. Specifically, reduced function of NMDARs leads to altered balance between excitation and inhibition which further drives neural network malfunctions. Clinical studies suggested that NMDAR modulators (glycine, D-serine, D-cycloserine and glycine transporter inhibitors) may be beneficial in treating schizophrenia patients. Preclinical evidence also suggested that these NMDAR modulators may enhance synaptic NMDAR function and synaptic plasticity in brain slices. However, an important issue that has not been addressed is whether these NMDAR modulators modulate neural activity/spiking in vivo.

METHODS

By using in vivo calcium imaging and single unit recording, we tested the effect of D-cycloserine, sarcosine (glycine transporter 1 inhibitor) and glycine, on schizophrenia-like model mice.

RESULTS

In vivo neural activity is significantly higher in the schizophrenia-like model mice, compared to control mice. D-cycloserine and sarcosine showed no significant effect on neural activity in the schizophrenia-like model mice. Glycine induced a large reduction in movement in home cage and reduced in vivo brain activity in control mice which prevented further analysis of its effect in schizophrenia-like model mice.

CONCLUSIONS

We conclude that there is no significant impact of the tested NMDAR modulators on neural spiking in the schizophrenia-like model mice.

Discovery of GlyT2 Inhibitors Using Structure-Based Pharmacophore Screening and Selectivity Studies by FEP+ Calculations

In recent years, mammalian Glycine transporter 2 (GlyT2) has emerged as a promising target for the development of compounds against chronic pain states. In our current work, we discovered a new set of promising hits that inhibit the glycine transporter at nano- and micromolar activity and have excellent selectivity over GlyT1 (as shown by in vitro studies) using a newly designed virtual screening (VS) protocol that combines a structure-based pharmacophore and docking screens with a success rate of 75%. Furthermore, the free energy perturbation calculations and molecular dynamics (MD) studies revealed the GlyT2 amino acid residues critical for the binding and selectivity of both Glycine and our Hit1 compound. The FEP+ results well-matched with the available literature mutational data proving the quality of the generated GlyT2 structure. On the basis of these results, we propose that our hit compounds may lead to new chronic pain agents to address unmet and challenging clinical needs.

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.

GlyT1 determines the glycinergic phenotype of amacrine cells in the mouse retina

The amino acid glycine acts as a neurotransmitter at both inhibitory glycinergic and excitatory glutamatergic synapses predominantly in caudal regions of the central nervous system but also in frontal brain regions and the retina. After its presynaptic release and binding to postsynaptic receptors at caudal glycinergic synapses, two high-affinity glycine transporters GlyT1 and GlyT2 remove glycine from the extracellular space. Glycinergic neurons express GlyT2, which is essential for the presynaptic replenishment of the transmitter, while glial-expressed GlyT1 was shown to control the extracellular glycine concentration. Here we show that GlyT1 expressed by glycinergic amacrine cells of the retina does not only contribute to the control of the extracellular glycine concentration in the retina but is also essential for the maintenance of the glycinergic transmitter phenotype of this cell population. Specifically, loss of GlyT1 from the glycinergic AII amacrine cells impairs AII-mediated glycinergic neurotransmission and alters regulation of the extracellular glycine concentration, without changes in the overall distribution and/or size of glycinergic synapses. Taken together, our results suggest that GlyT1 expressed by amacrine cells in the retina combines functions covered by neuronal GlyT2 and glial GlyT1 at caudal glycinergic synapses.

Dysregulated Glycine Signaling Contributes to Increased Impulsivity during Protracted Alcohol Abstinence

Persons with alcoholism who are abstinent exhibit persistent impairments in the capacity for response inhibition, and this form of impulsivity is significantly associated with heightened relapse risk. Brain-imaging studies implicate aberrant prefrontal cortical function in this behavioral pathology, although the underlying mechanisms are not understood. Here we present evidence that deficient activation of glycine and serine release in the ventral medial prefrontal cortex (vmPFC) contributes to increased motor impulsivity during protracted abstinence from long-term alcohol exposure. Levels of 12 neurotransmitters were monitored in the rat vmPFC during the performance of a challenging variant of the five-choice serial reaction time task (5-CSRTT) in which alcohol-exposed rats exhibit excessive premature responding. Following long-term ethanol exposure, rats showed blunted task-related recruitment of vmPFC glycine and serine release, and the loss of an inverse relationship between levels of these neurotransmitters and premature responding normally evident in alcohol-naive subjects. Intra-vmPFC administration of the glycine transport inhibitor ALX5407 prevented excessive premature responding by alcohol-exposed rats, and this was reliant on NMDA glycine site availability. Alcohol-exposed rats and controls did not differ in their premature responding and glycine and serine levels in vmPFC during the performance of the standard 5-CSRTT. Collectively, these findings provide novel insight into cortical neurochemical mechanisms contributing to increased impulsivity following long-term alcohol exposure and highlight the NMDA receptor coagonist site as a potential therapeutic target for increased impulsivity that may contribute to relapse risk.SIGNIFICANCE STATEMENT Persons with alcoholism demonstrate increased motor impulsivity during abstinence; however, the neuronal mechanisms underlying these behavioral effects remain unknown. Here, we took advantage of an animal model that shows deficiencies in inhibitory control following prolonged alcohol exposure to investigate the neurotransmitters that are potentially responsible for dysregulated motor impulsivity following long-term alcohol exposure. We found that increased motor impulsivity is associated with reduced recruitment of glycine and serine neurotransmitters in the ventromedial prefrontal cortex (vmPFC) cortex in rats following long-term alcohol exposure. Administration of glycine transport inhibitor ALX5407 in the vmPFC alleviated deficits in impulse control.

Differential regulation of NMDA receptors by d-serine and glycine in mammalian spinal locomotor networks

We provide evidence that NMDARs within murine spinal locomotor networks determine the frequency and amplitude of ongoing locomotor-related activity in vitro and that NMDARs are regulated by d-serine and glycine in a synapse-specific and activity-dependent manner. In addition, glycine transporter-1 is shown to be an important regulator of NMDARs during locomotor-related activity. These results show how excitatory transmission can be tuned to diversify the output repertoire of spinal locomotor networks in mammals.

Activation of N-methyl-d-aspartate receptors (NMDARs) requires the binding of a coagonist, either d-serine or glycine, in addition to glutamate. Changes in occupancy of the coagonist binding site are proposed to modulate neural networks including those controlling swimming in frog tadpoles. Here, we characterize regulation of the NMDAR coagonist binding site in mammalian spinal locomotor networks. Blockade of NMDARs by d(−)-2-amino-5-phosphonopentanoic acid (d-APV) or 5,7-dichlorokynurenic acid reduced the frequency and amplitude of pharmacologically induced locomotor-related activity recorded from the ventral roots of spinal-cord preparations from neonatal mice. Furthermore, d-APV abolished synchronous activity induced by blockade of inhibitory transmission. These results demonstrate an important role for NMDARs in murine locomotor networks. Bath-applied d-serine enhanced the frequency of locomotor-related but not disinhibited bursting, indicating that coagonist binding sites are saturated during the latter but not the former mode of activity. Depletion of endogenous d-serine by d-amino acid oxidase or the serine-racemase inhibitor erythro-β-hydroxy-l-aspartic acid (HOAsp) increased the frequency of locomotor-related activity, whereas application of l-serine to enhance endogenous d-serine synthesis reduced burst frequency, suggesting a requirement for d-serine at a subset of synapses onto inhibitory interneurons. Consistent with this, HOAsp was ineffective during disinhibited activity. Bath-applied glycine (1–100 µM) failed to alter locomotor-related activity, whereas ALX 5407, a selective inhibitor of glycine transporter-1 (GlyT1), enhanced burst frequency, supporting a role for GlyT1 in NMDAR regulation. Together these findings indicate activity-dependent and synapse-specific regulation of the coagonist binding site within spinal locomotor networks, illustrating the importance of NMDAR regulation in shaping motor output.

NEW & NOTEWORTHY We provide evidence that NMDARs within murine spinal locomotor networks determine the frequency and amplitude of ongoing locomotor-related activity in vitro and that NMDARs are regulated by d-serine and glycine in a synapse-specific and activity-dependent manner. In addition, glycine transporter-1 is shown to be an important regulator of NMDARs during locomotor-related activity. These results show how excitatory transmission can be tuned to diversify the output repertoire of spinal locomotor networks in mammals.

Inhibition of glycine transporter-1 in the dorsal vagal complex improves metabolic homeostasis in diabetes and obesity

Impaired glucose homeostasis and energy balance are integral to the pathophysiology of diabetes and obesity. Here we show that administration of a glycine transporter 1 (GlyT1) inhibitor, or molecular GlyT1 knockdown, in the dorsal vagal complex (DVC) suppresses glucose production, increases glucose tolerance and reduces food intake and body weight gain in healthy, obese and diabetic rats. These findings provide proof of concept that GlyT1 inhibition in the brain improves glucose and energy homeostasis. Considering the clinical safety and efficacy of GlyT1 inhibitors in raising glycine levels in clinical trials for schizophrenia, we propose that GlyT1 inhibitors have the potential to be repurposed as a treatment of both obesity and diabetes.

Synthesis and Biological Evaluation of N-((1-(4-(Sulfonyl)piperazin-1-yl)cycloalkyl)methyl)benzamide Inhibitors of Glycine Transporter-1

We previously disclosed the discovery of rationally designed N-((1-(4-(propylsulfonyl)piperazin-1-yl)cycloalkyl)methyl)benzamide inhibitors of glycine transporter-1 (GlyT-1), represented by analogues 10 and 11. We describe herein further structure-activity relationship exploration of this series via an optimization strategy that primarily focused on the sulfonamide and benzamide appendages of the scaffold. These efforts led to the identification of advanced leads possessing a desirable balance of excellent in vitro GlyT-1 potency and selectivity, favorable ADME and in vitro pharmacological profiles, and suitable pharmacokinetic and safety characteristics. Representative analogue (+)-67 exhibited robust in vivo activity in the cerebral spinal fluid glycine biomarker model in both rodents and nonhuman primates. Furthermore, rodent microdialysis experiments also demonstrated that oral administration of (+)-67 significantly elevated extracellular glycine levels within the medial prefrontal cortex (mPFC).

Time-dependent impact of glutamatergic modulators on the promnesiant effect of 5-HT6R blockade on mice recognition memory

Selective antagonists at serotonin 5-HT₆ receptors (5-HT₆R) improve memory performance in rodents and are currently under clinical investigations. If blockade of 5-HT₆R is known to increase glutamate release, only two studies have so far demonstrated an interaction between 5-HT₆R and glutamate transmission, but both, using the non-competitive NMDA antagonist MK-801, insensitive to variations of glutamate concentrations. In a place recognition task, we investigated here in mice the role of glutamate transmission in the beneficial effects of 5-HT₆R blockade (SB-271046). Through the use of increasing intervals (2, 4 and 6h) between acquisition and retrieval, we investigated the time-dependent impact of two different glutamatergic modulators. NMDAR-dependant glutamate transmission (NMDA Receptors) was either blocked by the competitive antagonist at NMDAR, CGS 19755, or potentiated by the glycine transporter type 1 (GlyT1) inhibitor, NFPS. Results showed that neither SB-271046, nor CGS 19755, nor NFPS, alter behavioural performances after short intervals, i.e. when control mice displayed significant memory performances (2h and 4h) (respectively 10, 3, and 0.625mg.kg^-1). Conversely, with the 6h-interval, a situation in which spontaneous forgetting is observed in control mice, SB-271046 improved recognition memory performances. This beneficial effect was prevented when co-administered with either CGS 19755 or NFPS, which themselves had no effect. Interestingly, a dose-dependent effect was observed with NFPS, with promnesic effect observed at lower dose (0.156mg.kg^-1) when administrated alone, whereas it did no modify promnesic effect of SB-271046. These results demonstrate that promnesiant effect induced by 5-HT₆R blockade is sensitive to the competitive blockade of NMDAR and underline the need of a fine adjustment of the inhibition of GlyT1. Overall, our findings support the idea of a complex crosstalk between serotonergic and glutamatergic systems in the promnesic properties of 5-HT₆R antagonists.

D-Serine and Glycine Differentially Control Neurotransmission during Visual Cortex Critical Period

N-methyl-D-aspartate receptors (NMDARs) play a central role in synaptic plasticity. Their activation requires the binding of both glutamate and d-serine or glycine as co-agonist. The prevalence of either co-agonist on NMDA-receptor function differs between brain regions and remains undetermined in the visual cortex (VC) at the critical period of postnatal development. Here, we therefore investigated the regulatory role that d-serine and/or glycine may exert on NMDARs function and on synaptic plasticity in the rat VC layer 5 pyramidal neurons of young rats. Using selective enzymatic depletion of d-serine or glycine, we demonstrate that d-serine and not glycine is the endogenous co-agonist of synaptic NMDARs required for the induction and expression of Long Term Potentiation (LTP) at both excitatory and inhibitory synapses. Glycine on the other hand is not involved in synaptic efficacy per se but regulates excitatory and inhibitory neurotransmission by activating strychnine-sensitive glycine receptors, then producing a shunting inhibition that controls neuronal gain and results in a depression of synaptic inputs at the somatic level after dendritic integration. In conclusion, we describe for the first time that in the VC both D-serine and glycine differentially regulate somatic depolarization through the activation of distinct synaptic and extrasynaptic receptors.

Multiple functions of neuronal plasma membrane neurotransmitter transporters

Removal from receptors of neurotransmitters just released into synapses is one of the major steps in neurotransmission. Transporters situated on the plasma membrane of nerve endings and glial cells perform the process of neurotransmitter (re)uptake. Because the density of transporters in the membranes can fluctuate, transporters can determine the transmitter concentrations at receptors, thus modulating indirectly the excitability of neighboring neurons. Evidence is accumulating that neurotransmitter transporters can exhibit multiple functions. Being bidirectional, neurotransmitter transporters can mediate transmitter release by working in reverse, most often under pathological conditions that cause ionic gradient dysregulations. Some transporters reverse to release transmitters, like dopamine or serotonin, when activated by 'indirectly acting' substrates, like the amphetamines. Some transporters exhibit as one major function the ability to capture transmitters into nerve terminals that perform insufficient synthesis. Transporter activation can generate conductances that regulate directly neuronal excitability. Synaptic and non-synaptic transporters play different roles. Cytosolic Na(+) elevations accompanying transport can interact with plasmalemmal or/and mitochondrial Na(+)/Ca(2+) exchangers thus generating calcium signals. Finally, neurotransmitter transporters can behave as receptors mediating releasing stimuli able to cause transmitter efflux through multiple mechanisms. Neurotransmitter transporters are therefore likely to play hitherto unknown roles in multiple therapeutic treatments.

Discovery of 4-benzoylpiperidine and 3-(piperidin-4-yl)benzo[d]isoxazole derivatives as potential and selective GlyT1 inhibitors

Two series of potential and selective GlyT1 inhibitors were discovered by bioisosteric replacement.23qwas effective on chronic PCP-treated schizophrenia-like behavioral models.

Glycine transport inhibitors for the treatment of pain

Opioids, local anesthetics, anticonvulsant drugs, antidepressants, and non-steroidal anti-inflammatory drugs (NSAIDs) are used to provide pain relief but they do not provide adequate pain relief in a large proportion of chronic pain patients and are often associated with unacceptable side effects. Inhibitory glycinergic neurotransmission is impaired in chronic pain states, and this provides a novel target for drug development. Inhibitors of the glycine transporter 2 (GlyT2) enhance inhibitory neurotransmission and show particular promise for the treatment of neuropathic pain. N-arachidonyl-glycine (NAGly) is an endogenous lipid that inhibits glycine transport by GlyT2 and also shows potential as an analgesic, which may be further exploited in drug development. In this review we discuss the role of glycine neurotransmission in chronic pain and future prospects for the use of glycine transport inhibitors in the treatment of pain.

3-Amido-3-aryl-piperidines: A Novel Class of Potent, Selective, and Orally Active GlyT1 Inhibitors

3-Amido-3-aryl-piperidines were discovered as a novel structural class of GlyT1 inhibitors. The structure-activity relationship, which was developed, led to the identification of highly potent compounds exhibiting excellent selectivity against the GlyT2 isoform, drug-like properties, and in vivo activity after oral administration.

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.

SSR504734 enhances basal expression of prepulse inhibition but exacerbates the disruption of prepulse inhibition by apomorphine

RATIONALE

Inhibition of glycine transporter 1 (GlyT1) elevates extracellular glycine and can thus increase N-methyl-D-aspartate receptor (NMDAR) excitability in the brain. The potent GlyT1 inhibitor, SSR504734, has also been shown to potentiate the behavioral effects of direct and indirect dopamine agonists. Thus, an acute systemic dose of SSR504734 was sufficient to exacerbate the motor-stimulant effect of the dopamine releaser amphetamine in C57BL/6 mice, even though SSR504734 alone exerted no significant effect on motor activity.

OBJECTIVES

Here, we explore if SSR504734 might modulate dopamine-dependent sensory gating in the paradigm of prepulse inhibition (PPI) of the acoustic startle reflex.

METHODS

Experiment 1 characterized the effect of SSR504734 (10 and 30 mg/kg i.p.) on PPI expression when administered alone. Experiments 2 and 3 investigated the impact of SSR504734 when administered in conjunction with the dopamine receptor agonist, apomorphine (1 and 2 mg/kg s.c.), which is known to reliably disrupt PPI.

RESULTS

When administered alone, acute SSR504734 enhanced PPI only at 30 mg/kg--a dose that has been shown to improve cognitive functions including working memory, which has been linked to enhanced NMDAR function resulting from the elevation of extracellular glycine. However, this effect did not allow SSR504734 to antagonize the PPI-disruptive effect of apomorphine. At the lower dose of 10 mg/kg--that was insufficient to enhance PPI when administered alone--SSR504734 even exacerbated the deleterious effect of apomorphine on PPI.

CONCLUSIONS

The therapeutic potential of GlyT1 inhibition against distinct behavioral/cognitive deficiency might require different magnitudes of GlyT1 inhibition.

Lipid inhibitors of high affinity glycine transporters: identification of a novel class of analgesics

Glycine plays a key role in regulating inhibitory neurotransmission in the spinal cord and concentrations of glycine in the CNS are regulated by two subtypes of high affinity glycine transporters, GlyT1 and GlyT2. In this mini review we will discuss a series of lipid inhibitors of GlyT2 that show promise as analgesics in the treatment of neuropathic and inflammatory pain. N-arachidonyl-glycine inhibits the rate of transport by GlyT2, but has very little or no activity on GlyT1. We will discuss structure-activity studies of the actions of related lipids on GlyT2 and also the characterization of a more potent lipid inhibitor of GlyT2, oleoyl-l-carnitine. Both N-arachidonyl-glycine and oleoyl-l-carnitine show specificity for GlyT2 over GlyT1, which has allowed the use of chimeric GlyT1/GlyT2 transporters to begin characterizing the molecular basis for specificity and mechanism of action of these lipid inhibitors. Although our understanding of the molecular basis for lipid inhibition is still in its infancy, it appears that extracellular loop 4 of GlyT2 plays an important role in the inhibitory mechanism.

Allosteric modulators for the treatment of schizophrenia: targeting glutamatergic networks

Schizophrenia is a highly debilitating mental disorder which afflicts approximately 1% of the global population. Cognitive and negative deficits account for the lifelong disability associated with schizophrenia, whose symptoms are not effectively addressed by current treatments. New medicines are needed to treat these aspects of the disease. Neurodevelopmental, neuropathological, genetic, and behavioral pharmacological data indicate that schizophrenia stems from a dysfunction of glutamate synaptic transmission, particularly in frontal cortical networks. A number of novel pre- and postsynaptic mechanisms affecting glutamatergic synaptic transmission have emerged as viable targets for schizophrenia. While developing orthosteric glutamatergic agents for these targets has proven extremely difficult, targeting allosteric sites of these targets has emerged as a promising alternative. From a medicinal chemistry perspective, allosteric sites provide an opportunity of finding agents with better drug-like properties and greater target specificity. Furthermore, allosteric modulators are better suited to maintaining the highly precise temporal and spatial aspects of glutamatergic synaptic transmission. Herein, we review neuropathological and genomic/genetic evidence underscoring the importance of glutamate synaptic dysfunction in the etiology of schizophrenia and make a case for allosteric targets for therapeutic intervention. We review progress in identifying allosteric modulators of AMPA receptors, NMDA receptors, and metabotropic glutamate receptors, all with the aim of restoring physiological glutamatergic synaptic transmission. Challenges remain given the complexity of schizophrenia and the difficulty in studying cognition in animals and humans. Nonetheless, important compounds have emerged from these efforts and promising preclinical and variable clinical validation has been achieved.

Modulation of NMDA receptor function as a treatment for schizophrenia

Schizophrenia is a devastating mental illness that afflicts nearly 1% of the world's population. Currently available antipsychotics treat positive symptoms, but are largely ineffective at addressing negative symptoms and cognitive dysfunction. Thus, improved pharmacotherapies that treat all aspects of the disease remain a critical unmet need. There is mounting evidence that links NMDA receptor hypofunction and the expression of schizophrenia, and numerous drug discovery programs have developed agents that directly or indirectly potentiate NMDA receptor-mediated neurotransmission. Several compounds have emerged that show promise for treating all symptom sub-domains in both preclinical models and clinical studies, and we will review recent developments in many of these areas.

Oleoyl-L-carnitine inhibits glycine transport by GlyT2

BACKGROUND AND PURPOSE

Concentrations of extracellular glycine in the CNS are regulated by two Na(+)/Cl(-) -dependent glycine transporters, GlyT1 and GlyT2. Selective inhibitors of GlyT1 have been developed for the treatment of schizophrenia, whilst selective inhibitors of GlyT2 are analgesic in animal models of pain. We have assessed a series of endogenous lipids as inhibitors of GlyT1 and GlyT2.

EXPERIMENTAL APPROACH

Human GlyT1 and GlyT2 were expressed in Xenopus laevis oocytes, and the inhibitory actions of a series of acylcarnitines on glycine transport were measured using electrophysiological techniques.

KEY RESULTS

Oleoyl-L-carnitine inhibited glycine transport by GlyT2, with an IC(50) of 340 nM, which is 15-fold more potent than the previously identified lipid inhibitor N-arachidonyl-glycine. Oleoyl-L-carnitine had a slow onset of inhibition and a slow washout. Using a series of chimeric GlyT1/2 transporters and point mutant transporters, we have identified an isoleucine residue in extracellular loop 4 of GlyT2 that conferred differences in sensitivity to oleoyl-L-carnitine between GlyT2 and GlyT1.

CONCLUSIONS AND IMPLICATIONS

Oleoyl-L-carnitine is a potent non-competitive inhibitor of GlyT2. Previously identified GlyT2 inhibitors show potential as analgesics and the identification of oleoyl-L-carnitine as a novel GlyT2 inhibitor may lead to new ways of treating pain.

Probing the modulation of acute ethanol intoxication by pharmacological manipulation of the NMDAR glycine co-agonist site

BACKGROUND

Stimulating the glycine(B) binding site on the N-methyl-d-aspartate ionotropic glutamate receptor (NMDAR) has been proposed as a novel mechanism for modulating behavioral effects of ethanol (EtOH) that are mediated via the NMDAR, including acute intoxication. Here, we pharmacologically interrogated this hypothesis in mice.

METHODS

Effects of systemic injection of the glycine(B) agonist, d-serine, the GlyT-1 glycine transporter inhibitor, ALX-5407, and the glycine(B) antagonist, L-701,324, were tested for the effects on EtOH-induced ataxia, hypothermia, and loss of righting reflex (LORR) duration in C57BL/6J (B6) and 129S1/SvImJ (S1) inbred mice. Effects of the glycine(B) partial agonist, d-cycloserine (DCS), the GlyT-1 inhibitor, N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS), and the glycine(B) antagonist, 5,7-dichlorokynurenic (DCKA), on EtOH-induced LORR duration were also tested. Interaction effects on EtOH-induced LORR duration were examined via combined treatment with d-serine and ALX-5407, d-serine and MK-801, d-serine and L-701,324, as well as L-701,324 and ALX-5407, in B6 mice, and d-serine in GluN2A and PSD-95 knockout mice. The effect of dietary depletion of magnesium (Mg), an element that interacts with the glycine(B) site, was also tested.

RESULTS

Neither d-serine, DCS, ALX-5407, nor NFPS significantly affected EtOH intoxication on any of the measures or strains studied. L-701,324, but not DCKA, dose-dependently potentiated the ataxia-inducing effects of EtOH and increased EtOH-induced (but not pentobarbital-induced) LORR duration. d-serine did not have interactive effects on EtOH-induced LORR duration when combined with ALX-5407. The EtOH-potentiating effects of L-701,324, but not MK-801, on LORR duration were prevented by d-serine, but not ALX-5407. Mg depletion potentiated LORR duration in B6 mice and was lethal in a large proportion of S1 mice.

CONCLUSIONS

Glycine(B) site activation failed to produce the hypothesized reduction in EtOH intoxication across a range of measures and genetic strains, but blockade of the glycine(B) site potentiated EtOH intoxication. These data suggest endogenous activity at the glycine(B) opposes EtOH intoxication, but it may be difficult to pharmacologically augment this action, at least in nondependent subjects, perhaps because of physiological saturation of the glycine(B) site.

Glycine transport inhibitors in the treatment of schizophrenia

Schizophrenia is a severe neuropsychiatric disorder without adequate current treatment. Recent theories of schizophrenia focus on disturbances of glutamatergic neurotransmission particularly at N-methyl-D-aspartate (NMDA)-type glutamate receptors. NMDA receptors are regulated in vivo by the amino acids glycine and D-serine. Glycine levels, in turn, are regulated by glycine type I (GlyT1) transporters, which serve to maintain low subsaturating glycine levels in the vicinity of the NMDA receptor. A proposed approach to treatment of schizophrenia, therefore, is inhibition of GlyT1-mediated transport. Over the past decade, several well tolerated, high affinity GlyT1 inhibitors have been developed and shown to potentiate NMDA receptor-mediated neurotransmission in animal models relevant to schizophrenia. In addition, clinical trials have been conducted with sarcosine (N-methylglycine), a naturally occurring GlyT1 inhibitor, and with the high affinity compound RG1678. Although definitive trials remain ongoing, encouraging results to date have been reported.

Modeling resilience to schizophrenia in genetically modified mice: a novel approach to drug discovery

Complex psychiatric disorders, such as schizophrenia, arise from a combination of genetic, developmental, environmental and social factors. These vulnerabilities can be mitigated by adaptive factors in each of these domains engendering resilience. Modeling resilience in mice using transgenic approaches offers a direct path to intervention, as resilience mutations point directly to therapeutic targets. As prototypes for this approach, we discuss the three mouse models of schizophrenia resilience, all based on modulating glutamatergic synaptic transmission. This motivates the broader development of schizophrenia resilience mouse models independent of specific pathophysiological hypotheses as a strategy for drug discovery. Three guiding validation criteria are presented. A resilience-oriented approach should identify pharmacologically tractable targets and in turn offer new insights into pathophysiological mechanisms.

Glycine transporter-1 inhibition preceding extinction training inhibits reacquisition of cocaine seeking

Cognitive enhancers that act by increasing glycine transmission might be useful adjuncts to cocaine-cue extinction training to deter relapse. The study investigated the effects of combining treatments of the glycine transporter-1 (GlyT-1) inhibitor, Org24598, with extinction training on the subsequent reacquisition of cocaine self-administration. Squirrel monkeys and rats were trained to self-administer cocaine under a second-order schedule of intravenous drug injection in which responding was maintained by cocaine injections and a cocaine-paired visual stimulus. During three weekly extinction sessions, saline was substituted for cocaine but responding still produced the cocaine-paired stimulus. Subjects were treated with Org24598 or vehicle, either before or after each extinction session. One week later, cocaine injections were restored, and reacquisition of cocaine self-administration was evaluated over 15 sessions. Compared with vehicle, administration of Org24598 (1.0 mg/kg in monkeys; 3.0 or 7.5 mg/kg in rats) before each extinction session significantly inhibited reacquisition of cocaine self-administration in each species. In contrast, administration of Org24598 (1.0 mg/kg in monkeys) following, rather than preceding, each extinction session did not affect reacquisition compared with vehicle. When extinction training was replaced by cocaine self-administration or abstinence control conditions, treatment with the same doses of Org24598 resulted in reacquisition that was significantly more rapid than the reacquisition observed when Org24598 was administered before extinction training sessions. The results support the potential clinical utility of GlyT-1 inhibitor pretreatments combined with cocaine-cue extinction training to inhibit relapse.

Glutamatergic Approaches for the Treatment of Schizophrenia

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and plays a key role in most aspects of normal brain function including cognition, learning and memory. Dysfunction of glutamatergic neurotransmission has been implicated in a number of neurological and psychiatric disorders with a growing body of evidence suggesting that hypofunction of glutamatergic neurotransmission via the N-methyl-d-aspartate (NMDA) receptor plays an important role in the pathophysiology of schizophrenia. It thus follows that potentiation of NMDA receptor function via pharmacological manipulation may provide therapeutic utility for the treatment of schizophrenia and a number of different approaches are currently being pursued by the pharmaceutical industry with this aim in mind. These include strategies that target the glycine/d-serine site of the NMDA receptor (glycine transporter GlyT1, d-serine transporter ASC-1 and d-amino acid oxidase (DAAO) inhibitors) together with those aimed at enhancing glutamatergic neurotransmission via modulation of AMPA receptor and metabotropic glutamate receptor function. Such efforts are now beginning to bear fruit with compounds such as the GlyT1 inhibitor RG1678 and mGlu2 agonist LY2140023 proving to have clinical meaningful effects in phase II clinical trials. While more studies are required to confirm long-term efficacy, functional outcome and safety in schizophrenic agents, these agents hold real promise for addressing unmet medical needs, in particular refractory negative and cognitive symptoms, not currently addressed by existing antipsychotic agents.

Synaptic and extrasynaptic NMDA receptors are gated by different endogenous coagonists

N-methyl-d-aspartate receptors (NMDARs) are located in neuronal cell membranes at synaptic and extrasynaptic locations, where they are believed to mediate distinct physiological and pathological processes. Activation of NMDARs requires glutamate and a coagonist whose nature and impact on NMDAR physiology remain elusive. We report that synaptic and extrasynaptic NMDARs are gated by different endogenous coagonists, d-serine and glycine, respectively. The regionalized availability of the coagonists matches the preferential affinity of synaptic NMDARs for d-serine and extrasynaptic NMDARs for glycine. Furthermore, glycine and d-serine inhibit NMDAR surface trafficking in a subunit-dependent manner, which is likely to influence NMDARs subcellular location. Taking advantage of this coagonist segregation, we demonstrate that long-term potentiation and NMDA-induced neurotoxicity rely on synaptic NMDARs only. Conversely, long-term depression requires both synaptic and extrasynaptic receptors. Our observations provide key insights into the operating mode of NMDARs, emphasizing functional distinctions between synaptic and extrasynaptic NMDARs in brain physiology.