A case of multiple system atrophy with hyperglycinaemia due to a selective deficiency of glycine transporter mRNA

A patient presented with features of olivopontocerebellar atrophy and was found to have marked hyperglycinaemia. Severe atrophy of the cerebellum and brain stem was found at post-mortem, with numerous glial cytoplasmic inclusions (GCIs) in atrophic areas, characteristic of multiple system atrophy. In situ hybridization studies of the spinal cord demonstrated a selective reduction in expression of glycine transporter mRNA. We suggest that the resulting impairment of regulation of glycine concentrations at synaptic level resulted in excitotoxic damage to neurons.

Development of glycinergic transmission in organotypic cultures from auditory brain stem

We investigated whether glycinergic transmission develops organotypically in auditory brain stem cultures. Slices of the medial nucleus of the trapezoid body and the lateral superior olive were incubated in medium with a raised extracellular K+ concentration. As in vivo, glycine receptor alpha1 subunit immunoreactivity increased and became clustered on somata and proximal dendrites. Together with organotypic expression of glycine transporter GLYT2, this indicates that molecular components of glycinergic synapses form properly. In contrast, glycinergic synaptic currents did not develop as in vivo: after 7 days in vitro they were still similar to those at the time of culture preparation. We suggest that for organotypic development of glycine receptors and transporters, Ca2+ influx due to elevated K+ is sufficient. The development of functional synaptic transmission, however, may require patterned electrical activity.

[Glycine therapy of schizophrenia; its rationale and a review of clinical trials]

A major ground of dopamine hypothesis of schizophrenia is that all antipsychotics share dopamine D2 antagonistic activity. However, they are less effective in ameliorating the negative symptoms of schizophrenia. Recently, glutamate hypothesis of schizophrenia has been developed from the observation that phencyclidine, an NMDA receptor antagonist, induces a psychotic state closely resembling schizophrenia in normal individuals. Because glycine potentiates NMDA receptor activity, it has been tried as an adjunct to conventional antipsychotics. Most trials demonstrated a moderate improvement in negative symptoms. In this review we discussed the clinical usefulness of glycine therapy. First we described glutamate hypothesis of schizophrenia as a theoretical basis of glycine therapy. Then we reviewed clinical trials of glycine or other glycinergic agents (glycine receptor partial agonist, D-cycloserine, or the glycine prodrug milacemide). Although long-term side effects of glycine administration have not been fully investigated, glycine therapy could be a potential therapeutic tool for the negative symptoms of schizophrenia.

Ketamine inhibits monoamine transporters expressed in human embryonic kidney 293 cells

BACKGROUND

Ketamine has been characterized as having psychotomimetic and sympathomimetic effects. These symptoms have raised the possibility that ketamine affects monoaminergic neurotransmission. To elucidate the relation between ketamine and monoamine transporters, the authors constructed three cell lines that stably express the norepinephrine, dopamine, and serotonin transporters and investigated the effects of ketamine on these transporters.

METHODS

Human embryonic kidney cells were transfected using the Chen-Okayama method with the human norepinephrine, rat dopamine, and rat serotonin transporter cDNA subcloned into the eukaryotic expression vector. Using cells stably expressing these transporters, the authors investigated the effects of ketamine on the uptake of these compounds and compared them with those of pentobarbital.

RESULTS

Inhibition analysis showed that ketamine significantly inhibited the uptake of all three monoamine transporters in a dose-dependent manner. The Ki (inhibition constant) values of ketamine on the norepinephrine, dopamine, and serotonin transporters were 66.8 microM, 62.9 microM, and 162 microM, respectively. Pentobarbital, a typical general anesthetic agent with no psychotic symptoms, did not affect the uptake of monoamines, however. Further, neither the glycine transporter 1 nor the glutamate/aspartate transporter was affected by ketamine, indicating that ketamine preferentially inhibits monoamine transporters.

CONCLUSIONS

Ketamine inhibited monoamine transporters expressed in human embryonic kidney cells in a dose-dependent manner. This result suggests that the ketamine-induced inhibition of monoamine transporters might contribute to its psychotomimetic and sympathomimetic effects through potentiating monoaminergic neurotransmission.

Characterization of the 5' region of the rat brain glycine transporter GLYT2 gene: identification of a novel isoform

We have identified two alternative 5' ends for the GLYT2 glycine transporter in rat brain DNA by using rapid amplification of cDNA ends (RACE) analysis. Study of the genomic DNA revealed that the isoform diversity is generated by alternative use of exons 1a or 1b, respectively. Upon translation, the mRNA corresponding to the novel isoform GLYT2b would yield a protein five amino acids longer than the previously characterized isoform GLYT2a. Both forms display similar regional distribution and kinetics characteristics. However, whereas GLYT2a is able to actively accumulate glycine into transfected COS cells, GLYT2b seems only to exchange (or release) glycine.

Cloning of a functional vesicular GABA and glycine transporter by screening of genome databases

The unc-47 locus of Caenorhabditis elegans has been suggested to encode a synaptic vesicle GABA transporter. Here we used hydropathy plot analysis to identify a candidate vesicular GABA transporter in genomic sequences derived from a region of the physical map comprising unc-47. A mouse homologue was identified and cloned from EST database information. In situ hybridization in rat brain revealed codistribution with both GABAergic and glycinergic neuronal markers. Moreover, expression in COS-7 and PC12 cells induced an intracellular, glycine-sensitive GABA uptake activity. These observations, consistent with previous data on GABA and glycine uptake by synaptic vesicles, demonstrate that the mouse clone encodes a vesicular inhibitory amino acid transporter.

The glycine transporter GLYT2 is a reliable marker for glycine-immunoreactive neurons

The glycine transporter GLYT2 is present in neurons of the spinal cord, the brain stem and the cerebellum. This localization is similar to that of glycine immunoreactivity, suggesting a causal relationship between GLYT2 expression and glycine distribution. In this report, we analyzed if such a relationship does exist by using neuronal cultures derived from embryonic spinal cord. GLYT2 was synthesized in a small subpopulation of neurons where it was targeted both to dendrites and to axons, being the axonal content higher than the dendritic one. At early stages in the development of cultured spinal neurons, the highest GLYT2 levels were found in the axonal growth cones. As the culture matured, immunoreactivity extended to the axonal shaft. Double-immunofluorescence experiments indicated a perfect co-localization of GLYT2 and glycine immunoreactivity in cultured neurons. Moreover, the concentration of glycine into neurons expressing GLYT2 was proportional to the concentration of the transporter. This observation was reproduced in GLYT2-transfected COS cells. These evidences indicate that the high content of glycine observed in some neurons in culture is indeed achieved by the concentrative task performed by GLYT2, and that GLYT2 can be used as a reliable marker for identification of glycine-enriched neurons.

Characterization of glycine release mediated by glycine transporter 1 stably expressed in HEK-293 cells

We constructed a cell line which stably expresses glycine transporter 1 (GlyT1) proteins. The cell line showed significant [14C]glycine uptake and could keep steep glycine concentration gradient between intracellular and extracellular space (in > out). Using this cell line, we investigated glycine release mediated by this transporter. The [14C]glycine release was enhanced by extracellular glycine and sarcosine, a selective inhibitor of the transporter, in a dose-dependent manner. In addition, the replacement of extracellular Na+ with Li+ or extracellular Cl- with acetate- markedly increased the release. Furthermore, we investigated the effects of extracellular Ca2+ and K+. The removal of these ions also showed enhancement of the release. These results suggest that glycine transporter 1 protein, which might be involved in the NMDA receptor neurotransmission, can release glycine into the extracellular space in the vicinity of synapses, and that the release might be influenced by the extracellular substrate concentration and ion composition in the synaptic cleft.

Control of NMDA receptor activation by a glycine transporter co-expressed in Xenopus oocytes

We present evidence that membrane transporters can control the membrane receptor's agonist concentration in restricted extracellular spaces of a biological model. The model is constructed by co-expressing glycine/Na/Cl cotransporters (GLYT1b) and NMDA receptors (NMDARs) (composed of the subunits NR1 and NR2A or NR2B) in Xenopus oocytes. We use the high-affinity glycine site of the NMDARs as a sensor of the actual juxtamembrane glycine concentration. We show that glycine uptake by GLYT1b dramatically reduces NMDAR currents by reducing the glycine concentration in extracellular spaces in which diffusion is restricted. This effect appears only in oocytes in which GLYT1b and NMDAR are co-expressed. It is Na+- and voltage-dependent, and is abolished when Na+ is replaced by Li+ and when glycine is replaced by D-serine (a coagonist of the NMDAR that is not transported by GLYT1b). These results demonstrate the ability of the GLYT transporter to reduce glycine concentration at the level of NMDARs in restricted diffusion spaces. This observation could account for a prevalent role of membrane transporters in the modulation of synapse transmission in the CNS. From a more general point of view, our results draw attention to possible significant discrepancies between local concentrations at the level of substrate targets in biological membranes and their concentration in the bulk solution when membrane transporters are present.

Neuronal dependency of the glycine transporter GLYT1 expression in glial cells

Two membrane-localized transporter proteins (GLYT1 and GLYT2) are responsible for removal of extracellular glycine in the mammalian CNS. Whereas GLYT1 seems to be expressed mainly in glial cells, GLYT2 is neuronal. The highest concentrations of both transporters are found in glycinergic areas of the nervous system. The expression of these proteins may be under regulatory control. We demonstrate here that GLYT1 is not expressed in pure glial cultures, but it is expressed by diverse types of glial cells in mixed neuronal/glial cultures. In these mixed cultures, the glial expression of GLYT1 is down-regulated after selective elimination of the neurons. The absence of expression in pure glial cultures and the observed reduction in the GLYT1 expression after neuronal loss support the existence of a regulatory cross-talk between neurons and glia to initiate and sustain the glial expression of GLYT1.

Molecular biology of glycinergic neurotransmission

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem of vertebrates. Glycine is accumulated into synaptic vesicles by a proton-coupled transport system and released to the synaptic cleft after depolarization of the presynaptic terminal. The inhibitory action of glycine is mediated by pentameric glycine receptors (GlyR) that belong to the ligand-gated ion channel superfamily. The synaptic action of glycine is terminated by two sodium- and chloride-coupled transporters, GLYT1 and GLYT2, located in the glial plasma membrane and in the presynaptic terminals, respectively. Dysfunction of inhibitory glycinergic neurotransmission is associated with several forms of inherited mammalian myoclonus. In addition, glycine could participate in excitatory neurotransmission by modulating the activity of the NMDA subtype of glutamate receptor. In this article, we discuss recent progress in our understanding of the molecular mechanisms that underlie the physiology and pathology of glycinergic neurotransmission.

An ultrastructural study of the glycine transporter GLYT2 and its association with glycine in the superficial laminae of the rat spinal dorsal horn

The glycine transporter GLYT2 is present in axonal boutons throughout the spinal cord, and its laminar distribution matches that of glycine-enriched axons, which are presumed to be glycinergic. In order to determine whether boutons which possess GLYT2 are glycine-enriched, we have carried out pre-embedding immunocytochemistry with antibody raised against GLYT2, and combined this with post-embedding detection of glycine, in the rat. GLYT2 immunoreactivity was present in boutons which formed symmetrical axodendritic, axosomatic or axoaxonic synapses, and was often seen in peripheral axons of type II synaptic glomeruli. One hundred and fifty GLYT2-immunoreactive boutons were analysed quantitatively, and in 142 (94.6%) of these the density of gold particles representing glycine-like immunoreactivity exceeded the background level (over presumed glutamatergic boutons) by at least a factor of two. Within immunoreactive boutons, the GLYT2 reaction product was associated with the plasma membrane, but often appeared as discrete clumps and was generally excluded from the region of the active sites of synapses. These results confirm that GLYT2 is associated with glycine-enriched axonal boutons in the superficial dorsal horn. They also suggest that GLYT2 is unevenly distributed on the plasma membrane of these boutons, and raise the possibility that it may be excluded from synaptic clefts.

Analysis of the transmembrane topology of the glycine transporter GLYT1

A theoretical 12-transmembrane segment model based on the hydrophobic moment has been proposed for the transmembrane topology of the glycine transporter GLYT1 and all other members of the sodium- and chloride-dependent transporter family. We tested this model by introducing N-glycosylation sites along the GLYT1 sequence as reporter for an extracellular localization and by an in vitro transcription/translation assay that allows the analysis of the topogenic properties of different segments of the protein. The data reported herein are compatible with the existence of 12 transmembrane segments, but support a rearrangement of the first third of the protein. Contrary to prediction, hydrophobic domain 1 seems not to span the membrane, and the loop connecting hydrophobic domains 2 and 3, formerly believed to be intracellular, appears to be extracellularly located. In agreement with the theoretical model, we provide evidence for the extracellular localization of loops between hydrophobic segments 5 and 6, 7 and 8, 9 and 10, and 11 and 12.

Quantitative gene expression of two types of glycine transporter in the rat central nervous system

Nuclease protection assays were performed to determine the levels of gene expression for the glycine transporters (GLY(T)), GLY(T)-1a, GLY(T)-1b and GLY(T)-2 in select regions of rat CNS. Results showed regional differences in GLY(T)-1a and GLY(T)-1b gene expression throughout the central nervous system (CNS) whereas GLY(T)-2 was predominantly expressed in the caudal brain. Although the distribution of GLY(T) correlates with the regional distribution of the glycine and N-methyl-D-aspartate receptor, areas shown to be highly susceptible to acute ischemia consistently showed low levels of GLY(T) mRNAs.

Developmental expression of the glycine transporters GLYT1 and GLYT2 in mouse brain

Using immunocytochemical localization, the distribution of the glycine transporters GLYT1 and GLYT2 in the developing mouse brain was studied. GLYT1 and GLYT2 immunoreactivity begins during the period of fiber outgrow and synaptogenesis. GLYT2 is first expressed in spinal and spinothalamic white matter and is followed by the expression of synaptophysin. In the postnatal stages, GLYT2 staining in the white matter disappears, and a punctuated pattern in the gray matter emerges. In contrast, in the fetal brain GLYT1 immunoreactivity coincides with gray matter neuropil and processes of radial glia. GLYT1 is distributed over a much wider area of the brain than GLYT2. However, the distribution of these two GLYTs implies that GLYT1 and GLYT2 operate in concert within the area where both are present. At the day 12 embryo stage, GLYT1 antibodies stain the liver, and later they also react with the pancreas and the gastroduodenal junction. No other organs exhibit significant GLYT1 immunoreactivity. We additionally observed the presence of GLYT1 in rat fetal cerebral cortex and hippocampus, which was not detected in fetal mouse brain. Moreover, GLYT1 immunoreactivity was found in the mouse floor plate and the ventral commissure but was not present in the same regions in rats. These findings suggest possible differences in the expression of GLYT1 between these two species.

Modulation of a recombinant glycine transporter (GLYT1b) by activation of protein kinase C

Treatment of human embryonic kidney cells (HEK 293 cells) expressing the mouse glycine transporter 1 (GLYT1b) with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) decreased specific [3H]glycine uptake. This down-regulation resulted from a reduction of the maximal transport rate and was blocked by the PKC inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) and staurosporine. The inhibitory effect of PMA treatment was also observed after removing all five predicted phosphorylation sites for PKC in GLYT1b by site-directed mutagenesis. These data indicate that glycine transport by GLYT1b is modulated by PKC activation; however, this regulation may involve indirect phosphorylation mechanisms.

Regional distribution and developmental variation of the glycine transporters GLYT1 and GLYT2 in the rat CNS

The high-affinity glycine transporter in neurons and glial cells is the primary means of inactivating synaptic glycine. Previous molecular cloning studies have indicated heterogeneity of glycine transporters in the CNS. Here the distribution of glycine transporter GLYT1 and GLYT2 transcripts and proteins in different regions and developmental stages of the rat brain were analysed by Northern, Western and in situ hybridization techniques. Sequence-specific riboprobes and two specific antibodies raised against fusion proteins were used, containing either 76 or 193 amino acids of the C or N terminus of the GLYT1 and GLYT2 transporters respectively. High levels of GLYT1 transcripts were found in the spinal cord, brainstem and cerebellum, and moderate levels in forebrain regions such as the cortex or hippocampus. GLYT2 transcripts are restricted to the spinal cord, brainstem and cerebellum. The onset of both GLYT1 and GLYT2 expression in the brainstem occurred in late fetal life, and full expression of these proteins was observed before weaning. There was a stepwise increase in the levels of mRNA and protein for these two transporters, reaching a maximum by the second postnatal week, followed by a slight decrease until adult values were reached by the fourth postnatal week. These data reveal interesting parallelism between the distribution of different glycine transporters and glycine receptor subunits, and suggest discrete roles for distinct glycine transporters.

Glycine transporters are differentially expressed among CNS cells

Glycine is the major inhibitory neurotransmitter in the spinal cord and brainstem and is also required for the activation of NMDA receptors. The extracellular concentration of this neuroactive amino acid is regulated by at least two glycine transporters (GLYT1 and GLYT2). To study the localization and properties of these proteins, sequence-specific antibodies against the cloned glycine transporters have been raised. Immunoblots show that the 50-70 kDa band corresponding to GLYT1 is expressed at the highest concentrations in the spinal cord, brainstem, diencephalon, and retina, and, in a lesser degree, to the olfactory bulb and brain hemispheres, whereas it is not detected in peripheral tissues. Pre-embedding light and electron microscopic immunocytochemistry show that GLYT1 is expressed in glial cells around both glycinergic and nonglycinergic neurons except in the retina, where it is expressed by amacrine neurons, but not by glia. The expression of a 90-110 kDa band corresponding to GLYT2 is restricted to the spinal cord, brain-stem, and cerebellum; in addition, very low levels occur in the diencephalon. GLYT2 is found in presynaptic elements of neurons thought to be glycinergic. However, in the cerebellum, GLYT2 is expressed both in terminal boutons and in glial elements. The physiological consequences of the regional and cellular distributions of these two proteins as well as the possibility of the existence of an unidentified neuronal form of GLYT1 are discussed.

The role of N-glycosylation in the targeting and activity of the GLYT1 glycine transporter

To elucidate the role of N-glycosylation in the function of the high affinity glycine transporter GLYT1, we have investigated the effect of the glycosylation inhibitor tunicamycin as well as the effect of the disruption of the putative glycosylation sites by site-directed mutagenesis. SDS-polyacrylamide gel electrophoresis of proteins from GLYT1-transfected COS cells reveals a major band of 80-100 kDa and a minor one of 57 kDa. Treatment with tunicamycin produces a 40% inhibition in transport activity and a decrease in the intensity of the 80-100-kDa band, whereas the 57-kDa band decreases in size to yield a 47-kDa protein corresponding to the unglycosylated form of the transporter. Simultaneous mutation of Asn-169, Asn-172, Asn-182, and Asn-188 to Gln also produces the 47-kDa form of the protein, indicating that there are no additional sites for N-glycosylation. Progressive mutation of the potential glycosylation sites produces a progressive decrease in transport activity and in size of the protein, indicating that the four putative glycosylation sites are actually glycosylated. N-Glycosylation of the GLYT1 is not indispensable for the transport activity itself, as demonstrated by enzymatic deglycosylation of the transporter. Analysis of surface proteins by biotinylation and by immunofluorescence demonstrates that a significant portion of the unglycosylated GLYT1 mutant remains in the intracellular compartment. This suggests that the carbohydrate moiety of glycine transporter GLYT1 is necessary for the proper trafficking of the protein to the plasma membrane.

Induction of the immediate early gene c-jun in human spinal cord in amyotrophic lateral sclerosis with concomitant loss of NMDA receptor NR-1 and glycine transporter mRNA

The aetiology of the sporadic form of amyotrophic lateral sclerosis (ALS) is poorly understood although abnormalities in glutamate and glycine transport have been implicated which both could contribute to a neurodegenerative process mediated through the N-methyl-D-aspartate (NMDA) receptor. In this study we have used in situ hybridization to investigate whether any changes in the expression of NMDA receptors, the glycine transporter or glutamate-mediated injury responses are detectable in ALS. Two immediate early genes were investigated as markers of neuronal injury responses, c-jun and zif-268, both constitutively expressed in the spinal cord. Levels of c-jun mRNA were most abundant in intermediate grey and layer IX of the ventral horn containing motor neurones. This pattern was markedly changed in ALS with large increases (2-3 fold) in c-jun mRNA occurring in dorsal and ventral horn. The marked increase in c-jun mRNA was also substantiated by slot blot analysis of tissue homogenates of spinal cord and a parallel induction of zif-268 mRNA was also seen. NMDA receptor NR-1 mRNA was widely distributed in control spinal cord with the highest concentrations occurring in layers IX, X, intermediate grey and dorsal horn. The ALS cases showed a selective decrease in the level of NR-1 mRNA in the ventral region (50%) whilst no significant decrease was detected in the dorsal region. Quantitation of tissue homogenates with dorsal and ventral regions combined also yielded a significant decrease of 40% which supports the analysis from in situ hybridization densitometry.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene structure and glial expression of the glycine transporter GlyT1 in embryonic and adult rodents

Na+/Cl(-)-dependent glycine transporters are crucial for the termination of neurotransmission at glycinergic synapses. Two different glycine transporter genes, GlyT1 and GlyT2, have been described. Several isoforms differing in their 5' ends originate from the GlyT1 gene. We have determined the genomic structure of the murine GlyT1 gene to elucidate the genetic basis underlying the different isoforms. Analysis of cDNA 5'-ends revealed that the GlyT1a and 1b/1c mRNAs are transcribed from two different promoters. During murine embryonic development GlyT1 mRNAs were detectable by RNase protection assays as early as embryonic day E9 and reached maximal levels between E13 and E15. In situ hybridization revealed GlyT1 expression in the developing spinal cord mainly in the ventral part of the ventricular zone at E12. At later stages (E15) transcripts were also found in the lateral half of the basal and intermediate gray matter. In contrast, the second glycine transporter gene GlyT2 displayed a completely different expression pattern. At E11 it is expressed in the mantle zone, and at later stages throughout the ventral horns. In the adult rat brain and spinal cord, GlyT1 hybridization signals were found exclusively in glial cells. Our data indicate that GlyT1 is an early marker of neural development and encodes glia-specific transporter proteins.

Localization of glycine neurotransmitter transporter (GLYT2) reveals correlation with the distribution of glycine receptor

We studied by immunocytochemical localization, the glycine neurotransmitter transporter (GLYT2) in mouse brain, using polyclonal antibodies raised against recombinant N-terminus and loop fusion proteins. Western analysis and immunocytochemistry of mouse brain frozen sections revealed caudal-rostral gradient of GLYT2 distribution with massive accumulation in the spinal cord, brainstem, and less in the cerebellum. Immunoreactivity was detected in processes with varicosities but not cell bodies. A correlation was observed between the pattern we obtained and previously reported strychnine binding studies. The results indicate that GLYT2 is involved in the termination of glycine neurotransmission accompanying the glycine receptor at the classic inhibitory system in the hindbrain.

Regulation by phorbol esters of the glycine transporter (GLYT1) in glioblastoma cells

The high-affinity glycine transporter in neurons and glial cells is the primary means of inactivating synaptic glycine. The effects of 12-O-tetradecanoylphorbol ester (TPA), a potent activator of protein kinase C (PKC), on the high-affinity Na(+)-dependent glycine transport were investigated in C6 cells, a cell line of glial origin. Incubation of C6 cells with TPA led to concentration- and time-dependent decrease in the glycine transport that could be completely suppressed by the addition of the PKC inhibitor staurosporine. The TPA effect could be mimicked by oleoylacetylglycerol and exogenous phospholipase C. Northern and Western blot analysis indicate that C6 cells express the GLYT1 glycine transporter. Incubation of COS cells transiently transfected with a full-length clone of the GLYT1 transporter in the presence of TPA, produces a decrease in glycine uptake.