Discovery and Synthesis of Hydroxy-l-Proline Blockers of the Neutral Amino Acid Transporters SLC1A4 (ASCT1) and SLC1A5 (ASCT2)

As a conformationally restricted amino acid, hydroxy-l-proline is a versatile scaffold for the synthesis of diverse multi-functionalized pyrrolidines for probing the ligand binding sites of biological targets. With the goal to develop new inhibitors of the widely expressed amino acid transporters SLC1A4 and SLC1A5 (also known as ASCT1 and ASCT2), we synthesized and functionally screened synthetic hydroxy-l-proline derivatives using electrophysiological and radiolabeled uptake methods against amino acid transporters from the SLC1, SLC7, and SLC38 solute carrier families. We have discovered a novel class of alkoxy hydroxy-pyrrolidine carboxylic acids (AHPCs) that act as selective high-affinity inhibitors of the SLC1 family neutral amino acid transporters SLC1A4 and SLC1A5. AHPCs were computationally docked into a homology model and assessed with respect to predicted molecular orientation and functional activity. The series of hydroxyproline analogs identified here represent promising new agents to pharmacologically modulate SLC1A4 and SLC1A5 amino acid exchangers which are implicated in numerous pathophysiological processes such as cancer and neurological diseases.

Discovery of novel, potent and orally efficacious inhibitor of neutral amino acid transporter B0AT1 (SLC6A19)

Amino acid restriction by inhibition of neutral amino acid transporter, B⁰AT1 (SLC6A19) activity has been recently shown to improve glyceamic control by upregulating glucagon like peptide (GLP1) and fibroblast growth factor (FGF21) in mice. Hence, pharmacological inhibition of B⁰AT1 is expected to treat type-2 diabetes and related disorder. In this study, rationally designed trifluoromethyl sulfonyl derivatives were identified as novel, potent and orally bioavailable B⁰AT1 inhibitors. Compound 39 was found to be nanomolar potent (IC₅₀: 0.035 µM) B⁰AT1 inhibitor with excellent pharmacokinetic profile (%F: 66) in mice and efficacious in vivo in diet induced obese (DIO) mice model.

Crystal violet structural analogues identified by in silico drug repositioning present anti-Trypanosoma cruzi activity through inhibition of proline transporter TcAAAP069

BACKGROUND

Crystal violet (CV) was used for several years in blood banks to eliminate the parasite Trypanosoma cruzi in endemic areas in order to prevent transfusion-transmitted Chagas disease. One mechanism of action described for CV involves inhibition of proline uptake. In T. cruzi, proline is essential for host cell infection and intracellular differentiation among other processes, and can be obtained through the proline permease TcAAAP069.

METHODOLOGY/PRINCIPAL FINDINGS

CV inhibited proline transporter TcAAAP069 and parasites overexpressing this permease were 47-fold more sensitive to this compound than control parasites. Using CV as reference molecule, loratadine, cyproheptadine, olanzapine and clofazimine were identified as structurally related compounds to CV (structural analogues) by in silico drug repurposing through a similarity-based virtual screening protocol. All these already-approved drugs for clinical use inhibited TcAAAP069 activity with different efficacies and also presented trypanocidal action in epimastigotes, trypomastigotes and amastigotes of the Y, CL Brener and Dm28c T. cruzi strains. Finally, a synergistic effect between benznidazole and the CV chemical analogues was evidenced by combination and dose-reduction indexes values in epimastigotes and trypomastigotes of the Y strain.

CONCLUSIONS/SIGNIFICANCE

Loratadine, cyproheptadine and clofazimine inhibit TcAAAP069 proline transporter and also present trypanocidal effect against all T. cruzi life stages in strains from three different DTUs. These CV structural analogues could be a starting point to design therapeutic alternatives to treat Chagas disease by finding new indications for old drugs. This approach, called drug repurposing is a recommended strategy by the World Health Organization to treat neglected diseases, like Chagas disease, and combination therapy may improve the possibility of success of repositioned drugs.

Impact of atypical mitochondrial cyclic-AMP level in nephropathic cystinosis

Nephropathic cystinosis (NC) is a rare disease caused by mutations in the CTNS gene encoding for cystinosin, a lysosomal transmembrane cystine/H+ symporter, which promotes the efflux of cystine from lysosomes to cytosol. NC is the most frequent cause of Fanconi syndrome (FS) in young children, the molecular basis of which is not well established. Proximal tubular cells have very high metabolic rate due to the active transport of many solutes. Not surprisingly, mitochondrial disorders are often characterized by FS. A similar mechanism may also apply to NC. Because cAMP has regulatory properties on mitochondrial function, we have analyzed cAMP levels and mitochondrial targets in CTNS-/- conditionally immortalized proximal tubular epithelial cells (ciPTEC) carrying the classical homozygous 57-kb deletion (delCTNS-/-) or with compound heterozygous loss-of-function mutations (mutCTNS-/-). Compared to wild-type cells, cystinotic cells had significantly lower mitochondrial cAMP levels (delCTNS-/- ciPTEC by 56% ± 10.5, P < 0.0001; mutCTNS-/- by 26% ± 4.3, P < 0.001), complex I and V activities, mitochondrial membrane potential, and SIRT3 protein levels, which were associated with increased mitochondrial fragmentation. Reduction of complex I and V activities was associated with lower expression of part of their subunits. Treatment with the non-hydrolysable cAMP analog 8-Br-cAMP restored mitochondrial potential and corrected mitochondria morphology. Treatment with cysteamine, which reduces the intra-lysosomal cystine, was able to restore mitochondrial cAMP levels, as well as most other abnormal mitochondrial findings. These observations were validated in CTNS-silenced HK-2 cells, indicating a pivotal role of mitochondrial cAMP in the proximal tubular dysfunction observed in NC.

Inhibition of SNAT5 Induces Incretin-Responsive State From Incretin-Unresponsive State in Pancreatic β-Cells: Study of β-Cell Spheroid Clusters as a Model

β-Cell-β-cell interactions are required for normal regulation of insulin secretion. We previously found that formation of spheroid clusters (called K20-SC) from MIN6-K20 clonal β-cells lacking incretin-induced insulin secretion (IIIS) under monolayer culture (called K20-MC) drastically induced incretin responsiveness. Here we investigated the mechanism by which an incretin-unresponsive state transforms to an incretin-responsive state using K20-SC as a model. Glutamate production by glucose through the malate-aspartate shuttle and cAMP signaling, both of which are critical for IIIS, were enhanced in K20-SC. SC formed from β-cells deficient for aspartate aminotransferase 1, a critical enzyme in the malate-aspartate shuttle, exhibited reduced IIIS. Expression of the sodium-coupled neutral amino acid transporter 5 (SNAT5), which is involved in glutamine transport, was downregulated in K20-SC and pancreatic islets of normal mice but was upregulated in K20-MC and islets of rodent models of obesity and diabetes, both of which exhibit impaired IIIS. Inhibition of SNAT5 significantly increased cellular glutamate content and improved IIIS in islets of these models and in K20-MC. These results suggest that suppression of SNAT5 activity, which results in increased glutamate production, and enhancement of cAMP signaling endows incretin-unresponsive β-cells with incretin responsiveness.

Amino acid transporter SLC6A14 is a novel and effective drug target for pancreatic cancer

BACKGROUND AND PURPOSE

Pancreatic cancer is a solid tumour that is often fatal. Hence, there is an urgent need to identify new drug targets for this disease. Highly proliferating cancer cells have an increased demand for nutrients and, therefore, need to up-regulate selective amino acid transporters. Here, we investigated which amino acid transporters are up-regulated in pancreatic cancer and whether any of these transporters has potential as a drug target for this fatal disease.

EXPERIMENTAL APPROACH

The expression of amino acid transporters in pancreatic cancer was analysed using publicly available microarray datasets, and the findings with the transporter SLC6A14 were validated by mRNA and protein analysis. The potential of SLC6A14 as a drug target was evaluated using a pharmacological blocker in vitro and in vivo.

KEY RESULTS

SLC6A14 was up-regulated several fold in patient-derived xenografts, primary tumour tissues and pancreatic cancer cells lines compared to normal pancreatic tissue or normal pancreatic epithelial cells. The magnitude of the up-regulation of SLC6A14 was the highest among the amino acid transporters examined. A pharmacological blocker of SLC6A14, α-methyltryptophan, induced amino acid starvation in pancreatic cancer cells and reduced the growth and proliferation of these cells, both in vitro and in vivo.

CONCLUSION AND IMPLICATIONS

The salient features of this study are that SLC6A14 is markedly up-regulated in pancreatic cancer and that pharmacological blockade of this transporter interferes with amino acid nutrition and reduces growth and proliferation of pancreatic cancer cells. These findings identify SLC6A14 as a novel druggable target for pancreatic cancer.

Loratadine and analogues: discovery and preliminary structure-activity relationship of inhibitors of the amino acid transporter B(0)AT2

B(0)AT2, encoded by the SLC6A15 gene, is a transporter for neutral amino acids that has recently been implicated in mood and metabolic disorders. It is predominantly expressed in the brain, but little is otherwise known about its function. To identify inhibitors for this transporter, we screened a library of 3133 different bioactive compounds. Loratadine, a clinically used histamine H1 receptor antagonist, was identified as a selective inhibitor of B(0)AT2 with an IC50 of 4 μM while being less active or inactive against several other members of the SLC6 family. Reversible inhibition of B(0)AT2 was confirmed by electrophysiology. A series of loratadine analogues were synthesized to gain insight into the structure-activity relationships. Our studies provide the first chemical tool for B(0)AT2.

Expression and substrate specificity of betaine/proline transporters suggest a novel choline transport mechanism in sugar beet

Proline transporters (ProTs) originally described as highly selective transporters for proline, have been shown to also transport glycinebetaine (betaine). Here we examined and compared the transport properties of Bet/ProTs from betaine accumulating (sugar beet, Amaranthus, and Atriplex,) and non-accumulating (Arabidopsis) plants. Using a yeast mutant deficient for uptake of proline and betaine, it was shown that all these transporters exhibited higher affinity for betaine than proline. The uptake of betaine and proline was pH-dependent and inhibited by the proton uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP). We also investigated choline transport by using a choline transport-deficient yeast mutant. Results revealed that these transporters exhibited a higher affinity for choline uptake rather than betaine. Uptake of choline by sugar beet BvBet/ProT1 was independent of the proton gradient and the inhibition by CCCP was reduced compared with that for uptake of betaine, suggesting different proton binding properties between the transport of choline and betaine. Additionally, in situ hybridization experiments revealed the localization of sugar beet BvBet/ProT1 in phloem and xylem parenchyma cells.

SLC6A14 (ATB0,+) protein, a highly concentrative and broad specific amino acid transporter, is a novel and effective drug target for treatment of estrogen receptor-positive breast cancer

SLC6A14, also known as ATB(0,+), is an amino acid transporter with unique characteristics. It transports 18 of the 20 proteinogenic amino acids. However, this transporter is expressed only at low levels in normal tissues. Here, we show that the transporter is up-regulated specifically in estrogen receptor (ER)-positive breast cancer, demonstrable with primary human breast cancer tissues and human breast cancer cell lines. SLC6A14 is an estrogen/ER target. The transport features of SLC6A14 include concentrative transport of leucine (an activator of mTOR), glutamine (an essential amino acid for nucleotide biosynthesis and substrate for glutaminolysis), and arginine (an essential amino acid for tumor cells), suggesting that ER-positive breast cancer cells up-regulate SLC6A14 to meet their increased demand for these amino acids. Consequently, treatment of ER-positive breast cancer cells in vitro with α-methyl-DL-tryptophan (α-MT), a selective blocker of SLC6A14, induces amino acid deprivation, inhibits mTOR, and activates autophagy. Prolongation of the treatment with α-MT causes apoptosis. Addition of an autophagy inhibitor (3-methyladenine) during α-MT treatment also induces apoptosis. These effects of α-MT are specific to ER-positive breast cancer cells, which express the transporter. The ability of α-MT to cause amino acid deprivation is significantly attenuated in MCF-7 cells, an ER-positive breast cancer cell line, when SLC6A14 is silenced with shRNA. In mouse xenograft studies, α-MT by itself is able to reduce the growth of the ER-positive ZR-75-1 breast cancer cells. These studies identify SLC6A14 as a novel and effective drug target for the treatment of ER-positive breast cancer.

Mechanism of leukotriene D4 inhibition of Na-alanine cotransport in intestinal epithelial cells

In a rabbit model of chronic intestinal inflammation, we previously demonstrated inhibition of neutral Na-amino acid cotransport. The mechanism of the inhibition was secondary to a decrease in the affinity for amino acid rather than the number of cotransporters. Since leukotriene (LT)D4 is known to be elevated in enterocytes during chronic intestinal inflammation, we used rat intestinal epithelial cell (IEC-18) monolayers to determine the mechanism of regulation of Na-alanine cotransport (alanine, serine, cysteine transporter 1: ASCT1) by LTD4. Na-alanine cotransport was inhibited by LTD4 in IEC-18 cells. The mechanism of inhibition of ASCT1 (solute carrier, SLC1A4) by LTD4 is secondary to a decrease in the affinity of the cotransporter for alanine without a significant change in cotransporter numbers and is not secondary to an alteration in the Na+ extruding capacity of the cells. Real-time quantitative PCR and Western blot analysis results indicate that ASCT1 message and protein levels are also unchanged in LTD4-treated IEC-18 cells. These results indicate that LTD4 inhibits Na-dependent neutral amino acid cotransport in IEC. The mechanism of inhibition is secondary to a decrease in the affinity for alanine, which is identical to that seen in villus cells from the chronically inflamed rabbit small intestine, where LTD4 levels are significantly increased.

Inhibitors of GlyT1 and GlyT2 differentially modulate inhibitory transmission

The chronic effects of glycine transporter 1 and 2 inhibitors (sarcosine and ALX-1393, respectively) on miniature inhibitory postsynaptic currents were studied in cultured spinal neurons. We found that sarcosine increased the frequency of overall miniature inhibitory postsynaptic currents without affecting the ratio of glycinergic, mixed and GABAergic miniature inhibitory postsynaptic currents, whereas ALX-1393 changed the ratio by increasing the proportions of GABAergic and mixed miniature inhibitory postsynaptic currents without affecting overall mIPSC frequency. We propose that inhibition of glycine transporter 1 by sarcosine increased overall mIPSC frequency via the activation of presynaptic glycine receptors, while inhibition of glycine transporter 2 by ALX-1393 changed the ratio of glycinergic, mixed and GABAergic miniature inhibitory postsynaptic currents by shifting the balance of inhibitory transmitters in vesicles towards gamma-aminobutyric acid.

Glycine transporters: crucial roles of pharmacological interest revealed by gene deletion

The functions of the high-affinity glycine transporters (GLYTs) in vivo have been revealed recently using gene-deletion studies. Results from studies of homozygous knockout mice have reinforced the idea that GLYTs might be specific clinical targets to modulate inhibitory glycine-mediated neurotransmission. In addition, molecular and behavioural analysis of heterozygous mice has confirmed the therapeutic potential of GLYT1 inhibitors in the treatment of several neurological and psychiatric disorders.

Modulators of the glycine site on NMDA receptors, D-serine and ALX 5407, display similar beneficial effects to clozapine in mouse models of schizophrenia

RATIONALE

Schizophrenia is characterized by disturbances in sensorimotor gating and attentional processes, which can be measured by prepulse inhibition (PPI) and latent inhibition (LI), respectively. Research has implicated dysfunction of neurotransmission at the NMDA-type glutamate receptor in this disorder.

OBJECTIVES

This study was conducted to examine whether compounds that enhance NMDA receptor (NMDAR) activity via glycine B site, D-serine and ALX 5407 (glycine transporter type 1 inhibitor), alter PPI and LI in the presence or absence of an NMDAR antagonist, MK-801.

METHODS

C57BL/6J mice were tested in a standard PPI paradigm with three prepulse intensities. LI was measured in a conditioned emotional response procedure by comparing suppression of drinking in response to a noise in mice that previously received 0 (non-preexposed) or 40 noise exposures (preexposed) followed by two or four noise-foot shock pairings.

RESULTS

Clozapine (3 mg/kg) and D-serine (600 mg/kg), but not ALX 5407, facilitated PPI. MK-801 dose dependently reduced PPI. The PPI disruptive effect of MK-801 (1 mg/kg) could be reversed by clozapine and ALX 5407, but not by D-serine. All the compounds were able to potentiate LI under conditions that disrupted LI in controls. MK-801 induced abnormal persistence of LI at a dose of 0.15 mg/kg. Clozapine, D-serine, and ALX 5407 were equally able to reverse persistent LI induced by MK-801.

CONCLUSIONS

D-Serine and ALX 5407 display similar effects to clozapine in PPI and LI mouse models, suggesting potential neuroleptic action. Moreover, the finding that agonists of NMDARs and clozapine can restore disrupted LI and disrupt persistent LI may point to a unique ability of the NMDA system to regulate negative and positive symptoms of schizophrenia.

Modulation of striatal dopamine release by glycine transport inhibitors

Traditional models of schizophrenia have focused primarily upon dopaminergic (DA) dysregulation. In contrast, more recent models focus on dysfunction of glutamatergic systems, acting particularly through N-methyl-D-aspartate (NMDA) receptors. NMDA receptors in brain are regulated by glycine, acting via a strychnine-insensitive regulatory site, and by glycine (GlyT1) transporters that maintain low glycine levels in the immediate vicinity of the NMDA receptor complex. The present study investigates the role of NMDA receptors in the modulation of striatal dopamine release in vitro, and of glycine transport inhibitors (GTIs) as potential psychotherapeutic agents in schizophrenia. In striatum, NMDA receptors exert dual excitatory/inhibitory effects, with inhibition reflecting activity of local GABAergic feedback regulation. We have previously demonstrated effectiveness of glycine in regulating [3H]DA release both in vivo and in vitro, consistent with its beneficial clinical effects. In the present study, similar effects were observed for the high-affinity GTI (+)N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy-)propyl]sarcosine (NFPS), and for a range of high-affinity GTIs with appropriate rank order of potency. In addition, (+)NFPS significantly stimulated NMDA-induced [3H]GABA release. Effects, of GTIs, were blocked by the glycine-site antagonists L689,560 and HA-966, and the GABA(B) antagonists phaclofen and CGP 52432, confirming the roles of both the NMDA-associated glycine-site and presynaptic GABA(B) receptors in NMDA receptor-mediated regulation of striatal DA release in vitro. Endogenous DA hyperactivity is associated with prominent positive symptoms in schizophrenia. The present results are consistent with recent clinical studies showing significant effectiveness of glycine-site agonists and GTIs in reduction of persistent positive, as well as negative, symptoms in schizophrenia.

The therapeutic potential of glycine transporter-1 inhibitors

While current antipsychotic medications are often efficacious for the positive symptoms of schizophrenia, there remains a critical need for compounds with improved tolerability and efficacy for the negative symptoms and cognitive dysfunction associated with this disease. There is a growing body of evidence suggesting that the potentiation of N -methyl-D-aspartate (NMDA) receptor function may be a useful approach for the treatment of schizophrenia. One proposed strategy for this potentiation is to increase synaptic levels of the neurotransmitter glycine by blocking the glycine transporter-1. Since glycine acts as a required co-agonist for the NMDA receptor complex; this approach allows an increase in the effectiveness of normal glutamatergic signalling at the NMDA receptor complex. Recent preclinical research, focused on the development and testing of novel glycine transporter-1 inhibitors, suggests that this approach may be feasible. Converging clinical evidence suggesting therapeutic efficacy following the potentiation of glycinergic activity further supports this approach. Clinical studies with novel glycine re-uptake inhibitors will provide critical information regarding the therapeutic utility and tolerability of this treatment for schizophrenia and other disorders associated with NMDA receptor hypofunction.

Novel glycine transporter type-2 reuptake inhibitors. Part 2: beta- and gamma-amino acid derivatives

Several beta- and gamma-amino acid derivatives were prepared as glycine transport inhibitors and their ability to block the uptake of [(14)C]-glycine in COS7 cells transfected with human glycine transporter-2 (hGlyT-2) were evaluated. A range of lipophilic side chains were tolerated in the beta-amino acid series (i.e., Ph, CH(2)Ph, CH(CH(3))(2), and CH(2)CH(CH(3))(2)). In the gamma-amino acid series, minimal differences in potency were observed between the alpha,beta-unsaturated analogs and the corresponding saturated derivatives. In both series, a 4-biphenyl or 4-phenoxyphenyl substituent appended to the urea or cyanogunaidine moiety was necessary for in vitro activity.

Inhibitors of the glycine transporter type-2 (GlyT-2): synthesis and biological activity of benzoylpiperidine derivatives

A series of benzoylpiperidine analogs related to 4a was prepared, and their ability to inhibit the uptake of [(14)C]-glycine in COS7 cells transfected with human glycine transporter type-2 (GlyT-2) was evaluated. Small structural changes to the benzoylpiperidine region of the molecule led to a significant decrease in GlyT-2 inhibitory activity. In contrast, the distal aryl ring was more tolerant to functional group modifications and could accommodate a variety of substitutes at the C-2 or C-3 positions. Comparable activities to 4a were obtained by replacing the anilino nitrogen with an ether linkage 27 or by exchanging the isopropoxy ether moiety with an isopropyl amino group 15. A distinct preference for a 2-carbon tether (n=1) was observed relative to the corresponding 3-carbon homolog (n=2).

Novel glycine transporter type-2 reuptake inhibitors. Part 1: alpha-amino acid derivatives

A variety of alpha-amino acid derivatives were prepared as glycine transport inhibitors and their ability to block the uptake of [(14)C]-glycine in COS7 cells transfected with human glycine transporter-2 (hGlyT-2) was evaluated. An array of substituents at the chiral center was studied and overall, L-phenylalanine was identified as the preferred amino acid residue. Compounds prepared from l-amino acids were more potent GlyT-2 inhibitors than analogs derived from the corresponding d-amino acids. Introducing an achiral amino acid such as glycine, or incorporating geminal substitution in the alpha-position, led to a significant reduction in GlyT-2 inhibitory properties.

Regulation of intracellular glycine as an organic osmolyte in early preimplantation mouse embryos

GLYT1, a glycine transporter belonging to the neurotransmitter transporter family, has recently been identified as a novel cell volume-regulatory mechanism in the earliest stages of the mouse preimplantation embryo. It apparently acts by regulating the steady-state intracellular concentration of glycine, which functions as an organic osmolyte in embryos, to balance external osmolarity and thus maintain cell volume. GLYT1 in embryos was the first mammalian organic osmolyte transporter identified that appears to function in cell volume control under conditions of normal osmolarity, rather than being a response to the stress of chronic hypertonicity. Its maximal rate of transport was shown to be regulated by osmolarity. However, it was not known whether this osmotic regulation of the rate of glycine transport is sufficient to account for the observed control of steady-state intracellular glycine levels as a function of osmolarity in embryos. Here, we show that the intracellular accumulation of glycine in embryos is a direct function of the rate of glycine uptake via GLYT1. In addition, we have shown that the rate of efflux, likely via the volume-regulated anion and organic osmolyte channel in embryos, is also under osmotic regulation and contributes substantially to the control of steady-state glycine concentrations. Together, control of both the rate of uptake and rate of efflux of glycine underlies the mechanism of osmotic regulation of the steady-state concentration of glycine and hence cell volume in early embryos.

Charge translocation during cosubstrate binding in the Na+/proline transporter of E.coli

Charge translocation associated with the activity of the Na(+)/proline cotransporter PutP of Escherichia coli was analyzed for the first time. Using a rapid solution exchange technique combined with a solid-supported membrane (SSM), it was demonstrated that Na(+)and/or proline individually or together induce a displacement of charge. This was assigned to an electrogenic Na(+)and/or proline binding process at the cytoplasmic face of the enzyme with a rate constant of k>50s(-1) which preceeds the rate-limiting step. Based on the kinetic analysis of our electrical signals, the following characteristics are proposed for substrate binding in PutP. (1) Substrate binding is electrogenic not only for Na(+), but also for the uncharged cosubstrate proline. The charge displacement associated with the binding of both substrates is of comparable size and independent of the presence of the respective cosubstrate. (2) Both substrates can bind individually to the transporter. Under physiological conditions, an ordered binding mechanism prevails, while at sufficiently high concentrations, each substrate can bind in the absence of the other. (3) Both substrate binding sites interact cooperatively with each other by increasing the affinity and/or the speed of binding of the respective cosubstrate. (4) Proline binding proceeds in a two-step process: low affinity (approximately 1mM) electroneutral substrate binding followed by a nearly irreversible electrogenic conformational transition.

Effects of typical and atypical antipsychotics on human glycine transporters

Augmentation strategy in the treatment of schizophrenia with the NMDA receptor co-agonist glycine has demonstrated significant improvement in patient symptoms. Interestingly, the therapeutic efficacy of glycine was more consistent among patients that were not co-administered clozapine suggesting that clozapine modulates glycine levels in brain. Since cerebral glycine concentration in the vicinity of NMDA receptors is thought to be controlled by the glia expressed glycine transporter type 1 (GlyT1), the effects of several typical and atypical antipsychotics on glycine uptake were examined in human placenta choriocarcinoma (JAR) cells expressing human GlyT1a. The selectivity of these compounds was investigated by measuring their inhibitory potency at the closely related glycine transporter type 2 (GlyT2). Typical antipsychotics haloperidol, thioridazine and chlorpromazine non-selectively inhibited [(14)C]glycine uptake mediated by GlyT1a and GlyT2 with potency of 9-21 microM. The atypical antipsychotic, clozapine antagonized glycine transport by human GlyT1a with an IC(50) of 100 microM and was weaker at recombinant GlyT2. Its main metabolites, N-desmethylclozapine and clozapine N-oxide were very weak inhibitors at all glycine transporters. Similarly, olanzapine did not potently block GlyT1a- and GlyT2-mediated uptake. Detailed kinetic analysis of hGlyT1a in the presence and absence of haloperidol and clozapine revealed that both drugs were not competitive inhibitors of glycine uptake. Data also indicated that these compounds did not interact with the Na(+) and Cl(-) sites of hGlyT1a. Our results have revealed the existence of an inhibitory interaction between some antipsychotics and hGlyT1a and raise the possibility that these drugs could interact with GlyT1 function at therapeutic doses.

2-(Aminomethyl)-benzamide-based glycine transporter type-2 inhibitors

Structure-activity studies on benzamide 1 obtained from library screening led to the discovery of a novel series of potent and selective glycine transporter type-2 inhibitors.

Molecular pharmacology of glutamate transporters, EAATs and VGLUTs

L-Glutamate serves as a major excitatory neurotransmitter in the mammalian central nervous system (CNS) and is stored in synaptic vesicles by an uptake system that is dependent on the proton electrochemical gradient (VGLUTs). Following its exocytotic release, glutamate activates fast-acting, excitatory ionotropic receptors and slower-acting metabotropic receptors to mediate neurotransmission. Na+-dependent glutamate transporters (EAATs) located on the plasma membrane of neurons and glial cells rapidly terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels. Thus far, five Na+-dependent glutamate transporters (EAATs 1-5) and three vesicular glutamate transporters (VGLUTs 1-3) have been identified. Examination of EAATs and VGLUTs in brain preparations and by heterologous expression of the various cloned subtypes shows these two transporter families differ in many of their functional properties including substrate specificity and ion requirements. Alterations in the function and/or expression of these carriers have been implicated in a range of psychiatric and neurological disorders. EAATs have been implicated in cerebral stroke, epilepsy, Alzheimer's disease, HIV-associated dementia, Huntington's disease, amyotrophic lateral sclerosis (ALS) and malignant glioma, while VGLUTs have been implicated in schizophrenia. To examine the physiological role of glutamate transporters in more detail, several classes of transportable and non-transportable inhibitors have been developed, many of which are derivatives of the natural amino acids, aspartate and glutamate. This review summarizes the development of these indispensable pharmacological tools, which have been critical to our understanding of normal and abnormal synaptic transmission.

The synthesis and SAR of 2-arylsulfanyl-phenyl piperazinyl acetic acids as glyT-1 inhibitors

Elevation of glycine levels and activation of the NMDA receptor by inhibition of the glycine transporter 1 (GlyT-1) is a potential strategy for the treatment of schizophrenia. A novel series of GlyT-1 inhibitors have been identified containing the 2-arylsulfanyl-phenylpiperazine motif. The most prominent member of this series, (R)-4-[5-chloro-2-(4-methoxy-phenylsulfanyl)-phenyl]-2-methyl-piperazin-1-yl-acetic acid (31) is a potent glycine transporter-1 inhibitor (IC(50)=150 nM), which elevated glycine levels in rat ventral hippocampus as measured by microdialysis in vivo at doses of 1.2-4.6 mg/kg s.c.

A novel physiological mechanism of glycine-induced immunomodulation: Na+-coupled amino acid transporter currents in cultured brain macrophages

Glycine is known to modulate immune cell responses. However, the physiological mechanisms underlying inhibitory effects of glycine on macrophages are not well understood. Here we show that glycine is capable of inducing inward currents in brain macrophages (microglia). In contrast to glycine, the glycine receptor agonist taurine failed to elicit currents. Glycine-evoked currents of brain macrophages were unaffected by strychnine, Cl(-)-free extracellular solution, N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl])sarcosine (NFPS) and amoxapine, but were abolished upon omission of extracellular Na(+). Furthermore, glycine caused increases in the intracellular Na(+) concentration and pronounced membrane depolarization. Glycine-evoked depolarization was Na(+) dependent and occurred independently of the intracellular Cl(-) concentration. Similarly to glycine, glutamine and alpha-(methylamino)isobutyric acid (MeAIB) elicited inward currents in brain macrophages. In the presence of either glutamine or MeAIB, glycine-induced currents were inhibited. It is concluded that neither functional glycine receptors nor glycine transporters are expressed in brain macrophages. We suggest that glycine mediates its effects by activation of system A Na(+)-coupled neutral amino acid transporters.

Role of glial and neuronal glycine transporters in the control of glycinergic and glutamatergic synaptic transmission in lamina X of the rat spinal cord

Using whole cell voltage clamp recordings from lamina X neurones in rat spinal cord slices, we investigated the effect of glycine transporter (GlyT) antagonists on both glycinergic inhibitory postsynaptic current (IPSCs) and glutamatergic excitatory postsynaptic current (EPSCs). We used ORG 24598 and ORG 25543, selective antagonists of the glial GlyT (GlyT1) and neuronal GlyT (GlyT2), respectively. In rats (P12-P16) and in the presence of kynurenic acid, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and bicuculline, ORG 24598 and ORG 25543 applied individually at a concentration of 10 microm induced a mean inward current of -10/-50 pA at -60 mV and increased significantly the decay time constants of miniature (mIPSCs), spontaneous (sIPSCs) and electrically evoked glycinergic (eIPSCs) inhibitory postsynaptic currents. ORG 25543, but not ORG 24598, decreased the frequency of mIPSCs and sIPSCs. Replacing extracellular sodium with N-methyl-d-glucamine or superfusing the slice with micromolar concentrations of glycine also increased the decay time constant of glycinergic IPSCs. By contrast, the decay time constant, amplitude and frequency of miniature GABAergic IPSCs recorded in the presence of strychnine were not affected by ORG 24598 and ORG 25543. In the presence of strychnine, bicuculline and CNQX, we recorded electrically evoked NMDA receptor-mediated EPSCs (eEPSCs). eEPSCs were suppressed by 30 micromd-2-amino-5-phosphonovalerate (APV), an antagonist of the NMDA receptor, and by 30 microm dichlorokynurenic acid (DCKA), an antagonist of the glycine site of the NMDA receptor. Glycine (1-5 microm) and d-serine (10 microm) increased the amplitude of eEPSCs whereas l-serine had no effect. ORG 24598 and ORG 25543 increased significantly the amplitude of NMDA receptor-mediated eEPSCs without affecting the amplitude of non-NMDA receptor-mediated eEPSCs. We conclude that blocking glial and/or neuronal glycine transporters increased the level of glycine in spinal cord slices, which in turn prolonged the duration of glycinergic synaptic current and potentiated the NMDA-mediated synaptic response.

Blockade of glycine transporter-1 (GLYT-1) potentiates NMDA receptor-mediated synaptic transmission in hypoglossal motorneurons

NMDA receptor (NMDAR)-mediated spontaneous miniature excitatory postsynaptic currents (mEPSCs) are potentiated by exogenously applied glycine. In this study, we have investigated the effect of blocking glycine uptake on NMDAR-mediated responses from hypoglossal motorneurons (HMs) of rats. We have used N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-propyl]sarcosine (NFPS; 500 nM), an antagonist of glycine transporter-1 (GLYT1), to study the effect of blocking endogenous glycine uptake on NMDAR-mediated synaptic transmission. We show that the charge transfer of NMDAR-mediated mEPSCs was enhanced after NFPS application in neonate (P2-4) and juvenile rats (P8-11), but this enhancement was statistically significant only in the former group. Spontaneous and evoked EPSCs showed a significant increase in NMDAR-mediated charge transfer in both neonates and juveniles. The greater increase observed in spontaneous EPSCs may be due to increased release of glycine from glycinergic terminals in the absence of tetrodotoxin (TTX). Brief application of NMDA onto HMs showed that extrasynaptic NMDARs may be potentiated by NFPS only in the presence of extracellularly applied glycine. Immunohistochemistry of GLYT1 and -2 shows labeling throughout the hypoglossal nucleus. GLYT1 labeling is diffuse and becomes more intense and uniform during development consistent with its glial localization. In contrast, GLYT2 labeling is intense throughout the nucleus and increases in intensity with age. Our results demonstrate the glycine binding site of the NMDAR is not saturated in the brain stem slice during the first 2 wk of development. We suggest that modulation of glycine concentration by GLYT1 is an important mechanism to regulate NMDAR-mediated synaptic transmission.

Glycine transporter type 1 blockade changes NMDA receptor-mediated responses and LTP in hippocampal CA1 pyramidal cells by altering extracellular glycine levels

Long-term potentiation (LTP) in the hippocampal CA1 region requires the activation of NMDA receptors (NMDARs). NMDAR activation in turn requires membrane depolarization as well as the binding of glutamate and its coagonist glycine. Previous pharmacological studies suggest that the glycine transporter type 1 (GlyT1) maintains subsaturating concentrations of glycine at synaptic NMDARs. Antagonists of GlyT1 increase levels of glycine in the synaptic cleft and, like direct glycine site agonists, can augment NMDAR currents and NMDAR-mediated functions such as LTP. In addition, stimulation of the glycine site initiates signalling through the NMDAR complex, priming the receptors for clathrin-dependent endocytosis. We have used a new potent GlyT1 antagonist, CP-802,079, with whole-cell patch-clamp recordings in acute rat hippocampal slices to determine the effect of GlyT1 blockade on LTP. Reverse microdialysis experiments in the hippocampus of awake, freely moving rats, showed that this drug elevated only the extracellular concentration of glycine. We found that CP-802,079, sarcosine and glycine significantly increased the amplitude of the NMDAR currents and LTP. In contrast, application of higher concentrations of CP-802,079 and glycine slightly reduced NMDAR currents and did not increase LTP. Overall, these data suggest that the level of glycine present in the synaptic cleft tightly regulates the NMDAR activity. This level is kept below the 'set point' of the NMDAR internalization priming mechanism by the presence of GlyT1-dependent uptake.

Glycine transporter I inhibitor, N-methylglycine (sarcosine), added to antipsychotics for the treatment of schizophrenia

BACKGROUND

Hypofunction of N-methyl-D-aspartate glutamate receptor had been implicated in the pathophysiology of schizophrenia. Treatment with D-serine or glycine, endogenous full agonists of the glycine site of N-methyl-D-aspartate receptor, or D-cycloserine, a partial agonist, improve the symptoms of schizophrenia. N-methylglycine (sarcosine) is an endogenous antagonist of glycine transporter-1, which potentiates glycine's action on N-methyl-D-aspartate glycine site and can have beneficial effects on schizophrenia.

METHODS

Thirty-eight schizophrenic patients were enrolled in a 6-week double-blind, placebo-controlled trial of sarcosine (2 g/d), which was added to their stable antipsychotic regimens. Twenty of them received risperidone. Measures of clinical efficacy and side effects were determined every other week.

RESULTS

Patient who received sarcosine treatment revealed significant improvements in their positive, negative, cognitive, and general psychiatric symptoms. Similar therapeutic effects were observed when only risperidone-treated patients were analyzed. Sarcosine was well-tolerated, and no significant side effect was noted.

CONCLUSIONS

Sarcosine treatment can benefit schizophrenic patients treated by antipsychotics including risperidone. The significant improvement with the sarcosine further supports the hypothesis of N-methyl-D-aspartate receptor hypofunction in schizophrenia. Glycine transporter-1 is a novel target for the pharmacotherapy to enhance N-methyl-D-aspartate function.

Irreversible Inhibition of [3H]Glycine Transport into Channel Catfish Erythrocytes by Thiol Group Modifiers

Erythrocytes can take up amino acids from the blood by using a variety of transport systems. GLYT is a key transport protein in the plasma membrane responsible for the Na(2+)-dependent uptake of glycine needed for glutathione biosynthesis. Certain cysteine-specific compounds, particularly mercuric chloride and 5,5'-dithiobis(2-nitrobenzoate), irreversibly inhibited the [(3)H]glycine transport via GLYT by red blood cells isolated from channel catfish. Bimolecular rate constants (k(2)) of 0.556 (mmol/l)(-1) min(-1) and 0.032 (mmol/l)(-1) min(-1), respectively, were calculated for the two inhibitors. Addition of 2-mercaptoethanol 1 min after the initiation of inactivation by mercuric chloride stopped further inactivation, but did not reverse the inhibition. The presence of glycine, but not Na(+) ions, during the preincubation of the cells with each inhibitor markedly reduced the degree of inhibition. Thus cysteinyl residues within the transport protein appear to be vital for the binding and uptake of glycine by channel catfish erythrocytes.

Positive N-methyl-d-aspartate receptor modulation by selective glycine transporter-1 inhibition in the rat dorsal spinal cord in vivo

In this study we have employed the selective glycine transporter-1 (GlyT-1) and GlyT-2 transporter inhibitors R-(-)-N-methyl-N-[3-[(4-trifluoromethyl)phenoxy]-3-phenyl-propyl]glycine (1:1) lithium salt (Org 24598) and 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (Org 25543), respectively, and microdialysis perfusion to determine the effect of GlyT transporter inhibition on extracellular amino acid concentrations in the lumbar dorsal spinal cord of the halothane-anaesthetised rat. Reverse dialysis of Org 24598 (0.1-10 microM) induced a concentration-related increase in extracellular glycine accompanied by a progressive increase in citrulline, but not aspartate, glutamate or GABA, efflux. Org 25543 (10 microM) by the same route induced a similar increase in glycine levels without affecting the efflux of other amino acids quantified. To test the hypothesis that the increase in citrulline efflux resulted from activation of the N-methyl-D-aspartate receptor (NMDA-R)/nitric oxide synthase (NOS) signalling cascade, the sensitivity was determined of GlyT-1 inhibition-induced effects to NMDA-R antagonism or NOS inhibition. Co-administration by reverse dialysis of the selective NMDA-R channel blocker MK-801 (0.5 mM) or the selective antagonist of the strychnine-insensitive glycine site, 7-chlorokynurenic acid (1 mM), with Org 24598 (10 microM) did not affect the uptake inhibition-induced increase in glycine efflux, but did significantly attenuate the increase in extracellular citrulline. Similarly, co-administration with Org 24598 of the isoform non-selective and selective neuronal NOS inhibitors Nomega-nitro-L-arginine methyl ester (1 mM) or 1-(2-trifluoromethylphenyl)imidazole (0.2 mM), respectively, prevented Org 24598-induced citrulline efflux with no effect on increased glycine efflux. These data provide evidence that the observed increased in extracellular citrulline is a consequence of positive modulation of NMDA-R, secondary to increased extracellular glycine and support a protective role for GlyT-1 against fluctuations in extracellular glycine uptake at glutamatergic synapses in the dorsal spinal cord. Such a mechanism could be important to NMDA-R-mediated synaptic plasticity in the spinal cord and be of relevance to the clinical usage of GlyT-1 inhibitors.

Prepulse inhibition deficits of the startle reflex in neonatal ventral hippocampal-lesioned rats: reversal by glycine and a glycine transporter inhibitor

BACKGROUND

Neonatal ventral hippocampal (NVH) lesions in rats induce behavioral abnormalities at adulthood thought to simulate some aspects of the positive, negative, and cognitive deficits classically observed in schizophrenic patients. Such lesions induce a postpubertal emergence of prepulse inhibition (PPI) deficits of the startle reflex reminiscent of the sensorimotor gating deficits observed in a majority of schizophrenic patients. To study the potential involvement of the glycinergic neurotransmission in such deficits, we investigated the capacity of glycine (an obligatory N-methyl-D-aspartate [NMDA] receptor co-agonist) and ORG 24598 (a selective glycine transporter 1 inhibitor) to reverse NVH lesion-induced PPI deficits in rats.

METHODS

Ibotenic acid was injected bilaterally into the ventral hippocampus of 7-day-old pups. Prepulse inhibition of the startle reflex was measured at adulthood.

RESULTS

Glycine (.8 and 1.6 g/kg IP) and ORG 24598 (10 mg/kg IP) fully and partially reversed lesion-induced PPI deficits, respectively.

CONCLUSIONS

These findings confirm that an impaired glutamatergic neurotransmission may be responsible for PPI deficits exhibited by NVH-lesioned rats and support the hypoglutamatergic hypothesis of schizophrenia. They also suggest that drugs acting either directly at the NMDA receptor glycine site or indirectly on the glycine transporter 1 could offer promising targets for the development of novel therapies for schizophrenia.

The glycine neurotransmitter transporter GLYT1 is an organic osmolyte transporter regulating cell volume in cleavage-stage embryos

Cells subjected to sustained high osmolarity almost universally respond by accumulating compatible organic osmolytes that, in contrast to inorganic ions, are not deleterious even at high intracellular concentrations. Their accumulation from the external environment by known organic osmolyte transporters, such as the four identified in mammals, occurs only slowly in response to sustained high osmolarity, by synthesis of new transporter proteins. Most cells, however, are not subject to high or varying osmolarity, and it is not clear whether organic osmolytes are generally required at normal osmolarities or how they are regulated. The fertilized egg of the mouse is protected in the oviduct from perturbations in osmolarity. However, deleterious effects of osmotic stress were evident in vitro even at normal oviductal osmolarity. Glycine was found to protect development, indicating that early mouse embryos may use glycine as an organic osmolyte at physiological osmolarity. We have now found that GLYT1, a glycine transporter of the neurotransmitter transporter gene family, functions as the organic osmolyte transporter that mediates the osmotically regulated accumulation of glycine and regulates cell volume in early embryos. Furthermore, osmotic stimulation of GLYT1 transport was immediate, without a requirement for protein synthesis, implying regulation different from known organic osmolyte transporters. Thus, GLYT1 appears to have a previously unidentified role as an organic osmolyte transporter that functions in acute organic osmolyte and volume homeostasis near normal osmolarity.

Effects of endogenous agonists, glycine and D-serine, on in vivo specific binding of [11C]L-703,717, a PET radioligand for the glycine-binding site of NMDA receptors

A positron-emitter (carbon-11) labeled antagonist for the glycine-binding site of NMDA receptors, [(11)C]L-703,717, has a unique in vivo binding characteristic, in which it preferentially binds to cerebellar-specific NMDA receptors consisting of a GluRepsilon3 subunit and eventually accumulates in rodent cerebellum under in vivo conditions, but not under in vitro conditions. In order to understand the in vivo-specific site and subunit localization of this radioligand, we examined the effect of the endogenous glycine site agonists, glycine and D-serine, on in vivo [(11)C]L-703,717 binding. An increase in extracellular glycine concentration by treatment with a glycine transporter 1 (GlyT1)-selective inhibitor, NFPS ethyl ester, significantly decreased the cerebellar localization of [(11)C]L-703,717 in rats. D-serine is known to be concentrated in mammalian forebrain regions. The lack of D-serine detection in the cerebellum may be due to the fact that it has the highest enzymatic activity of D-amino acid oxidase (DAO). It was found that the cerebellar localization of [(11)C]L-703,717 is greatly diminished in mutant mice lacking DAO, in which D-serine content in the cerebellum is drastically increased from a nondetectable level in normal mice. These studies indicate that [(11)C]L-703,717 is susceptible to inhibition by glycine site agonists in its in vivo binding, and suggest that regional differences in inhibitions by endogenous agonists may be a crucial factor in the site- and subunit-specific binding of this glycine-site antagonist.

Pharmacology and expression analysis of glycine transporter GlyT1 with [3H]-(N-[3-(4'-fluorophenyl)-3-(4'phenylphenoxy)propyl])sarcosine

In the central nervous system, re-uptake of the neurotransmitter glycine is mediated by two different glycine transporters, GlyT1 and GlyT2. GlyT2 is found in brainstem and spinal cord, whereas GlyT1 is expressed in rat forebrain regions where it is responsible for most glycine transport activity. Initially, GlyT1 and GlyT2 were pharmacologically differentiated by sarcosine, a weak selective inhibitor of GlyT1. The recently described selective and potent GlyT1 antagonist, NFPS/ALX-5407 provided an important additional tool to further characterize GlyT1 pharmacology. In the present study, we have radiolabeled the racemic form of NFPS (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl])sarcosine (also known as ALX-5407) to investigate its interaction with GlyT1, as well as define GlyT1 expression in the rat central nervous system. Kinetic studies indicated that [3H]NFPS binds rapidly to rat forebrain membranes and dissociates with a t(1/2) of 28 +/- 5 min. [3H]NFPS labeled a saturable population of sites in rat forebrain with a Kd of 7.1+/-1.3 nM and a B(max) of 3.14 +/- 0.26 pmol/mg protein. Bound [3H]NFPS was fully and potently displaced by unlabeled NFPS, whereas glycine and sarcosine were weak, Na+-dependent inhibitors with IC50 of 1,008 and 190 microM, respectively. Additional saturation experiments indicated that glycine and sarcosine were non-competitive antagonists of [3H]NFPS binding. Functional studies revealed that NFPS was a non-competitive inhibitor of [3H]glycine uptake and does not interact with Na+ and Cl- binding sites of GlyT1. Overall, this work shows that [3H]NFPS is a valuable tool in studying GlyT1 expression and pharmacology and that NFPS interacts with GlyT1 at a site different from the transporter translocation and ion binding sites.

The glutamate and neutral amino acid transporter family: physiological and pharmacological implications

The solute carrier family 1 (SLC1) is composed of five high affinity glutamate transporters, which exhibit the properties of the previously described system XAG-, as well as two Na+-dependent neutral amino acid transporters with characteristics of the so-called "ASC" (alanine, serine and cysteine). The SLC1 family members are structurally similar, with almost identical hydropathy profiles and predicted membrane topologies. The transporters have eight transmembrane domains and a structure reminiscent of a pore loop between the seventh and eighth domains [Neuron 21 (1998) 623]. However, each of these transporters exhibits distinct functional properties. Glutamate transporters mediate transport of L-Glu, L-Asp and D-Asp, accompanied by the cotransport of 3 Na+ and one 1 H+, and the countertransport of 1 K+, whereas ASC transporters mediate Na+-dependent exchange of small neutral amino acids such as Ala, Ser, Cys and Thr. Given the high concentrating capacity provided by the unique ion coupling pattern of glutamate transporters, they play crucial roles in protecting neurons against glutamate excitotoxicity in the central nervous system (CNS). The regulation and manipulation of their function is a critical issue in the pathogenesis and treatment of CNS disorders involving glutamate excitotoxicity. Loss of function of the glial glutamate transporter GLT1 (SLC1A2) has been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS), resulting in damage of adjacent motor neurons. The importance of glial glutamate transporters in protecting neurons from extracellular glutamate was further demonstrated in studies of the slc1A2 glutamate transporter knockout mouse. The findings suggest that therapeutic upregulation of GLT1 may be beneficial in a variety of pathological conditions. Selective inhibition of the neuronal glutamate transporter EAAC1 (SLC1A1) but not the glial glutamate transporters may be of therapeutic interest, allowing blockage of glutamate exit from neurons due to "reversed glutamate transport" of EAAC1, which will occur during pathological conditions, such as during ischemia after a stroke.

Glycine transporter isoforms in the mammalian central nervous system: structures, functions and therapeutic promises

The amino acid glycine (Gly) serves as a neurotransmitter at excitatory and inhibitory synapses in the mammalian central nervous system. Gly concentrations at post-synaptic neurotransmitter receptors are regulated by Na+/Cl(-)-dependent Gly transporters, which are expressed in neurons and in glial cells. Recent evidence suggests that these transporters are promising targets for the treatment of psychiatric and neurological disorders, such as schizophrenia and pain. Here, recent research on the structure, regulation and pharmacology of mammalian Gly transporters is reviewed.

The glycine transporter type 1 inhibitor N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine potentiates NMDA receptor-mediated responses in vivo and produces an antipsychotic profile in rodent behavior

Glycine acts as a necessary coagonist for glutamate at the NMDA receptor (NMDAR) complex by binding to the strychnine-insensitive glycine-B binding site on the NR1 subunit. The fact that glycine is normally found in the brain and spinal cord at concentrations that exceed those required to saturate this site has led to the speculation that glycine normally saturates NMDAR-containing synapses in vivo. However, additional lines of evidence suggest that synaptic glycine may be efficiently regulated in synaptic areas by the glycine transporter type 1 (GlyT1). The recent description of a potent and selective GlyT1 inhibitor (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine [NFPS]) provides a tool for evaluation of the hypothesis that inhibition of GlyT1 may increase synaptic glycine and thereby potentiate NMDAR function in vivo. In the present study, we found that (+)-NFPS demonstrated >10-fold greater activity in an in vitro functional glycine reuptake assay relative to the racemic compound. In vivo, (+/-)-NFPS significantly enhanced long-term potentiation in the hippocampal dentate gyrus induced by high-frequency electrical stimulation of the afferent perforant pathway. Furthermore, (+)-NFPS induced a pattern of c-Fos immunoreactivity comparable with the atypical antipsychotic clozapine and enhanced prepulse inhibition of the acoustic startle response in DBA/2J mice, a strain with low basal levels of prepulse inhibition. Collectively, these data suggest that selective inhibition of GlyT1 can enhance NMDAR-sensitive activity in vivo and also support the idea that GlyT1 may represent a novel target for developing therapeutics to treat disorders associated with NMDAR hypofunction.

[3H]-(R)-NPTS, a radioligand for the type 1 glycine transporter

The synthesis of NPTS, 6, a potent inhibitor of the type 1 glycine transporter (GlyT1) is described, as well as preparation of 6 in optically active and tritiated form for use as a radioligand for affinity displacement assay of GlyT1.

The glycine transporter-1 inhibitors NFPS and Org 24461: a pharmacological study

The in vitro and in vivo pharmacology of two glycine transporter-1 (GlyT1) inhibitors, N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)-propyl]sarcosine (NFPS) and R,S-(+/-)N-methyl-N-[(4-trifluoromethyl)phenoxy]-3-phenyl-propylglycine (Org 24461), was studied. NFPS and Org 24461 inhibited the uptake of [3H]glycine in hippocampal synaptosomal preparation with IC(50) values of 0.022 and 2.5 microM. Neither NFPS nor Org 24461 (0.1 microM) showed significant binding to alpha-1, alpha-2, and beta-adrenoceptors, D(1) and D(2) dopamine receptors, and 5-HT(1A) and 5-HT(2A) serotonin receptors in membranes prepared from rat brain or to cloned 5-HT(6) and 5-HT(7) receptors. At 10 microM concentrations, binding affinity was measured for NFPS to 5-HT(2A) and 5-HT(2C) serotonin receptors and alpha-2 adrenoceptors and for NFPS and Org 24461 to 5-HT(7) serotonin receptors. Glycine (0.1 mM) and sarcosine (5 mM) increased [3H]glycine efflux from superfused rat hippocampal slices preloaded with [3H]glycine. NFPS and Org 24461 (0.1 mM) did not influence [3H]glycine efflux, however, they inhibited glycine-induced [3H]glycine release. These findings indicate that NFPS and Org 24461 selectively inhibit glycine uptake without being substrates of the transporter protein. Several antipsychotic tests were used to characterize antipsychotic effects of NFPS and Org 24461 in vivo. These compounds did not alter apomorphine-induced climbing and stereotypy in a dose of 10 mg/kg p.o. in mice and did not induce catalepsy in a dose of 10 mg/kg i.p. in rats. The ID(50) values of NFPS were 21.4 mg/kg and higher than 30 mg/kg i.p. for inhibition of phencyclidine (PCP)- and D-amphetamine-induced hypermotility in mice and these values were 2.5 and 8.6 mg/kg i.p. for Org 24461. NFPS and Org 24461 did not exhibit anxiolytic effects in light-dark test in mice, in the meta-chlorophenylpiperazine (mCPP)-induced anxiety test (minimal effective dose or MED was higher than 3 mg/kg i.p.) and in the Vogel conflict drinking test in rats (MED was higher than 10 mg/kg i.p.). Both NFPS and Org 24461 (1-10 mg/kg i.p.) reversed PCP-induced changes in EEG power spectra in conscious rats. These data support the view that GlyT1 inhibitors may have potential importance in treatment of negative symptoms of schizophrenia.

Glycine tranporter-1 blockade potentiates NMDA-mediated responses in rat prefrontal cortical neurons in vitro and in vivo

The N-methyl-D-aspartate (NMDA) receptor (NMDA-R) has pivotal roles in neural development, learning, memory, and synaptic plasticity. Functional impairment of NMDA-R has been implicated in schizophrenia. NMDA-R activation requires glycine to act on the glycine-B (GlyB) site of the NMDA-R as an obligatory co-agonist with glutamate. Extracellular glycine near NMDA-R is regulated effectively by a glial glycine transporter (GlyT1). Using whole-cell voltage-clamp recordings in prefrontal cortex (PFC) slices, we have shown that exogenous GlyB site agonists glycine and D-serine, or a specific GlyT1 inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) in the presence of exogenous glycine (10 microM), potentiated synaptically evoked NMDA excitatory postsynaptic currents (EPSCs) in vitro. Furthermore, in urethan-anesthetized rats, microiontophoretic NMDA pulses excite single PFC neurons. When these responses were blocked by approximately 50% to approximately 90% on continuous iontophoretic application of the GlyB site, antagonist (+)HA-966, intravenous NFPS (5 mg/kg), or a GlyB site agonist D-serine (50 mg/kg iv) reversed this (+)HA-966 block. NFPS may elevate endogenous glycine levels sufficiently to displace (+)HA-966 from the GlyB sites of the NMDA-R, thus enabling reactivation of the NMDA-Rs by iontophoretic NMDA applications. D-Serine (50-100 mg/kg iv) or NFPS (1-2 mg/kg iv) alone also augmented NMDA-evoked excitatory responses. These data suggest that direct GlyB site stimulation by D-serine, or blockade of GLYT1 to elevate endogenous glycine to act on unsaturated GlyB sites on NMDA-Rs, potentiated NMDA-R-mediated firing responses in rat PFC. Hence, blockade of GlyT1 to elevate glycine near the NMDA-R may activate hypofunctional NMDA-R, which has been implicated to play a critical role in the pathophysiology of schizophrenia.

Functional characterization of monocarboxylic acid, large neutral amino acid, bile acid and peptide transporters, and P-glycoprotein in MDCK and Caco-2 cells

Bidirectional transport studies were conducted to determine whether substrates of five intestinal transporters showed carrier-mediated asymmetric transport across MDCK (Madin-Darby canine kidney) cell monolayers grown under standard conditions. Drug concentrations were quantitated using liquid scintillation counting, liquid chromatography/mass spectrometry/mass spectrometry, or liquid chromatography/mass spectrometry. In the presence of a pH gradient, benzoic acid exhibited net apical-to-basolateral transport, with apparent permeability ratios (apical-to-basolateral permeability/basolateral-to-apical permeability) ranging from 14 to 25. The addition of valproic acid reduced the permeability ratio by 70-90%. Cephalexin transport also exhibited net absorption in the presence of a pH gradient, with apparent permeability ratios ranging from 14 to 71, depending on growth conditions. Radiolabeled phenylalanine exhibited a low level of carrier-mediated absorption with an apparent permeability ratio of 1.8 that was reduced to 1.0 in the presence of unlabeled L-phenylalanine. Taurocholic acid did not exhibit carrier-mediated absorption. Cyclosporine and fexofenadine exhibited P-glycoprotein-mediated efflux from both MDCK and Caco-2 cells, which was more sensitive to inhibition in MDCK cells. These results suggest that although MDCK cell monolayers may be a useful model for evaluating transport by the absorptive monocarboxylic acid and peptide transporters and the efflux transporter, P-glycoprotein, they are not useful for predicting large neutral amino acid or bile acid transport in the intestine.

Inhibition of Na(+)-dependent transporters in cystine-loaded human renal cells: electrophysiological studies on the Fanconi syndrome of cystinosis

Cystinosis is the most common cause of the renal Fanconi syndrome in children, leading to severe electrolyte disturbances and growth failure. A defective lysosomal transporter, cystinosin, results in intralysosomal accumulation of cystine. Loading cells with cystine dimethyl ester (CDME) is the only available model for this disease. This model was used to present electrophysiologic studies on immortalized human kidney epithelial (IHKE-1) cells that had been derived from the proximal tubule with the slow whole-cell patch clamp technique. Basal membrane voltages (V(m)) of IHKE-1 cells were -30.7 +/- 0.4 mV (n = 151). CDME concentration-dependently altered V(m) with an initial depolarization (2.7 +/- 0.2 mV;n = 76; 1 mM CDME) followed by a more pronounced hyperpolarization (-9.9 +/- 1.0 mV;n = 49). Three Na(+)-dependent transporters were examined. Alanine (1 mM) depolarized IHKE-1 cells by 17.6 +/- 0.7 mV (n = 59), and phosphate (1.8 mM) depolarized by 9.7 +/- 1.1 mV (n = 18). Acidification of IHKE-1 cells with propionate (20 mM) resulted in a depolarization of V(m) by 7.1 +/- 0.3 mV (n = 21) followed by a repolarization by 2.9 +/- 0.3 mV/min (n = 17), reflecting Na(+)/H(+)-exchanger activity. Acute addition of 1 mM CDME did not alter the alanine- and propionate-induced changes in V(m), but it reduced the phosphate-induced depolarization by 37 +/- 9% (n = 10). Incubation with 1 mM CDME reduced the activity of all three transporters. Depolarizations by alanine and phosphate and the repolarization after propionate were inhibited by 57 +/- 4% (n =30), 45 +/- 9% (n = 9), and 78 +/- 15% (n = 8), respectively. In conclusion, this study demonstrates that CDME acutely alters V(m) of IHKE-1 cells and that at least three Na(+)-dependent transporters are inhibited, the Na(+)-phosphate cotransporter most sensitively. This might suggest that phosphate depletion and dissipation of the Na(+)-gradient are involved in the development of the Fanconi syndrome of cystinosis.

N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) is a selective persistent inhibitor of glycine transport

1. The regulation of glycine concentrations within excitatory synapses is poorly understood and it has been proposed that the GLYT1 subtypes of glycine transporters play a critical role in determining resting concentrations of glycine. Selective GLYT1 inhibitors may provide pharmacological tools to probe the dynamics of synaptic glycine concentrations, which may influence the activation properties of NMDA receptor activity. 2. We have characterized the selectivity and mechanism of action of the glycine transport inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS). The glycine transporters, GLYT1a, b and c and GLYT2a were expressed in Xenopus laevis oocytes and two electrode voltage clamp techniques and radiolabelled (3)H-glycine flux measurements were used to characterize the effects of NFPS on glycine transport. 3. NFPS inhibits glycine transport by the GLYT1a, b and c subtypes of glycine transporters, but has no effect on the GLYT2a subtype of transporter. We show that NFPS does not attain its specificity via an interaction with the Na(+), Cl(-) or glycine site, nor does it act at an intracellular site. NFPS inhibition of glycine transport is time and concentration dependent and inhibition of transport by NFPS persists after washout of NFPS from the bath solution, which suggests that inhibition by NFPS is long lasting.

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

High-affinity glycine transport in neurons and glial cells is a primary means of inactivating synaptic glycine. We have synthesized a potent selective inhibitor of glycine transporter 1 (GlyT1), and characterized its activity using a quail fibroblast cell line (QT6). The glycine transporters GlyT1A, GlyT1B, GlyT1C, and GlyT2 were stably expressed in QT6 cells. The transporters expressed in these cells exhibited appropriate characteristics as described previously for these genes: Na(+)/Cl(-) dependence, appropriate K(m) values for glycine uptake, and appropriate pharmacology, as defined in part by the ability of N-methyl glycine (sarcosine) to competitively inhibit glycine transport. Furthermore, the characteristics of the transporters in the cell lines recapitulate the characteristics of glycine transporters observed in tissue preparations. We developed a sarcosine derivative, (R)-(N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl])sarcosine (ALX 5407), and examined its activity against the cloned glycine transporters. ALX 5407 completely inhibited glycine transport in the GlyT1 cells, with an IC(50) value of 3 nM, but had little or no activity at the human GlyT2 transporter, at other binding sites for glycine, or at other neurotransmitter transporters. The inhibition of glycine transport was essentially irreversible. ALX 5407 represents a novel tool in the investigation of N-methyl-D-aspartate-receptor function. This class of drug may lead to novel therapies in the treatment of schizophrenia.