Glycine transporters

Overview: Plasma membrane located glycine transporters (provisional nomenclature) are members of the solute carrier family 6 (SLC6) of sodium- and chloride-dependent neurotransmitter receptor transporters that includes the monoamine and GABA transporters (Chen et al., 2004). The members of this superfamily share a structural motif of 12 putative TM segments (Palacín et al., 1998) that has been confirmed by the crystal structure of a bacterial homolog of the Na⁺/Cl^--dependent neurotransmitter transporters from Aquiflex aeolicus (LeuT_Aa) (Yamashita et al., 2005). Two gene products, GlyT1 and GlyT2, are known that give rise to transporters that are predominantly located on glia and neurones respectively. Five variants of GlyT1 (a, b, c, d & e) differing in their N- and C-termini are generated by alternative promoter usage and splicing, and three splice variants of GlyT2 (a, b & c) have also been identified (see Supplisson and Roux, 2002; Eulenburg et al., 2005; Betz et al., 2006; Gomeza et al., 2006 for reviews). GlyT1 transporter isoforms expressed in glia surrounding glutamatergic synapses regulate synaptic glycine concentrations influencing NMDA receptor-mediated neurotransmission (Bergeron et al., 1998; Gabernet et al., 2005), but also are important, in early neonatal life, for regulating glycine concentrations at inhibitory glycinergic synapses (Gomeza et al., 2003a). Homozygous mice engineered to totally lack GlyT1 exhibit severe respiratory and motor deficiencies due to hyperactive glycinergic signalling and die within the first postnatal day (Gomeza et al., 2003a; Tsai et al., 2004). Disruption of GlyT1 restricted to forebrain neurones is associated with enhancement of EPSCs mediated by NMDA receptors and behaviours that are suggestive of a promnesic action (Yee et al., 2006). GlyT2 transporters localized on the axons and boutons of glycinergic neurones appear crucial for efficient transmitter loading of synaptic vesicles but may not be essential for the termination of inhibitory neurotransmission (Gomeza et al., 2003b; Rousseau et al., 2008). Mice in which GlyT2 has been deleted develop a fatal hyperekplexia phenotype during the second postnatal week (Gomeza et al., 2003b) and mutations in the human gene encoding GlyT2 (SLC6A5) have been identified in patients with hyperekplexia (reviewed by Harvey et al., 2008). A structurally and functionally distinct vesicular transporter [VGAT/VIAAT (ENSG00000101438); McIntire et al., 1997; Sagne et al., 1997], subject to inhibition by vigabatrin, is responsible for concentrating glycine (and GABA) within synaptic vesicles.

Nomenclature	GlyT1	GlyT2
Other names	SLC6A9	SLC6A5
Ensembl ID	ENSG00000117413	ENSG00000165970
Endogenous substrates	Glycine	Glycine
Selective inhibitors (IC50)	(R)-NFPS (ALX 5407) (0.8–3 nM), SSR103800 (2 nM), N-methy-SSR504734 (2.5 nM), NFPS (3 nM), LY2365109 (16 nM), SSR504734 (18–314 nM), NPTS (37 nM), Org 24598	ALX 1393, ALX 1405, Org 25543 (20 nM)
Radioligands (KD)	[3H]-(R)-NPTS (1 nM), [35S]ACPPB (2 nM), [3H]-N-methyl-SSR504734 (3.3–8.1 nM), [3H]-NFPS (7–21 nM),	–
Stoichiometry	2 Na+ : 1 Cl- : 1 glycine	3 Na+ : 1 Cl- : 1 glycine

In addition to the inhibitors listed, sarcosine is a selective transportable inhibitor of GlyT1, but has no affect on GlyT2. This difference has been attributed to a single glycine residue in transmembrane domain 6 (serine residue in GlyT2) (Vandenberg et al., 2007). Inhibition of GLYT1 by the sarcosine derivatives NFPS, NPTS and Org24598 is non-competitive (Mallorga et al., 2003; Mezler et al., 2008). IC₅₀ values for Org 24598 reported in the literature vary, most likely due to differences in assay conditions (Brown et al., 2001; Mallorga et al., 2003). The tricyclic antidepressant amoxapine weakly inhibits GlyT2 (IC₅₀ 92 µM) with approximately 10-fold selectivity over GlyT1 (Nunez et al., 2000). The endogenous lipids arachidonic acid and anandamide exert opposing effects upon GlyT1a, inhibiting (IC₅₀∼2 µM) and potentiating (EC₅₀∼13 µM) transport currents respectively (Pearlman et al., 2003). N-arachidonyl-glycine has recently been described as a non-competitive inhibitor of GlyT2a, but not GlyT1b (Wiles et al., 2006). Protons (Aubrey et al., 2000) and Zn²⁺ (Ju et al., 2004) act as non-competitive inhibitors of GlyT1b, with IC₅₀ values of ∼100 nM and ∼10 µM respectively, but neither ion affects GlyT2 (reviewed by Vandenberg et al., 2004).

Glossary

Abbreviations:

ACPPB

(S)-2-amino-4-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide

ALX 1393

O-[2-benzyloxyphenyl-3-flurophenyl]methyl-L-serine

ALX 1405

structure not available

LY2365109

{[2-(4-benzo[1,3]dioxol-5-yl-2-tert-butylphenoxy)ethyl]-methylamino}-acetic acid

NFPS

N-[3-(4′-fluorophenyl)-3-(4′-phenylphenoxy)propyl]sarcosine

NPTS

(N-[3-phenyl-3-(4′-(4-toluoyl) phenoxy)propyl]sarcosine

Org 24598

R-(-)-N-[3-[(4-triflouromethyl)phenoxy]-3-phenyl-propylglycine

Org 25543

4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopentyl) methyl] benzamide

SSR103800

structure not available

SSR504734

2-chloro-[N-(S)-phenyl[(2S)-piperidin-2-yl]methyl]-3-trifluoromethyl benzamide

Further Reading

Aragón C, Lopez-Córcuera B (2005). Glycine transporters: crucial roles of pharmacological interest revealed by gene deletion. Trends Pharmacol Sci26: 283–286.

Betz H, Gomeza J, Scholze P, Eulenberg V (2006). Glycine transporters: essential regulators of synaptic transmission. J Neurochem97: 1600–1610.

Bridges TM, Williams R, Lindsley CW (2008). Design of potent GlyT1 inhibitors: in vitro and in vivo profiles. Curr Opin Mol Ther10: 591–601.

Chen N-H, Reith MEA, Quick MW (2004). Synaptic uptake and beyond: the sodium and chloride dependent neurotransmitter transporter family SLC6. Pflügers Arch447: 519–531.

Dohi T, Morita K, Kitayama T, Motoyama N, Morioka N (2009). Glycine transporter inhibitors as a novel drug discovery strategy for neuropathic pain. Pharmacol Ther123: 54–79.

Eulenburg V, Armsen W, Betz H, Gomeza J (2005). Glycine transporters: essential regulators of neurotransmission. Trends Biochem Sci30: 325–333.

Gomeza J, Armsen W, Betz H, Eulenburg V (2006). Lessons from the knocked-out glycine transporters. Handb Exp Pharmacol175: 457–483.

Harsing LG Jr, Juranyi Z, Gacsalyi I, Tapolcsanyi P, Czompa A, Matyus P (2006). Glycine transporter type-1 and its inhibitors. Curr Med Chem13: 1017–1044.

Harvey RJ, Topf M, Harvey K, Rees MI (2008).The genetics of hyperekplexia: more than startle! Trends Genet24: 439–447.

Javitt DC (2009). Glycine transport inhibitors for the treatment of schizophrenia: Symptom and disease modification. Curr Opin Drug Discov Devel 12:468–478.

Lechner SM (2006). Glutamate-based therapeutic approaches: inhibitors of glycine transport. Curr Opin Pharmacol6: 75–81.

Palacín M, Estévez R, Bertran J, Zorano A (1998). Molecular biology of mammalian plasma membrane amino acid transporters. Physiol Rev78: 969–1054.

Supplisson S, Roux MJ (2002). Why glycine transporters have different stoichiometries. FEBS Lett529: 93–101.

Sur C, Kinney GG (2007). Glycine transporter 1 inhibitors and modulation of NMDA receptor-mediated excitatory neurotransmission. Curr Drug Targets8: 643–649.

Vandenberg RJ, Ju P, Aubrey KR, Ryan RM, Mitrovic AD (2004). Allosteric modulation of neurotransmitter transporters at excitatory synapses. Eur J Pharm Sci23: 1–11.

Zafra F, Giménez C (2008). Glycine transporters and synaptic function. IUBMB Life 60:810–817.