Lactate transporter MCT1 in hepatic stellate cells promotes fibrotic collagen expression in nonalcoholic steatohepatitis

Circulating lactate is a fuel source for liver metabolism but may exacerbate metabolic diseases such as nonalcoholic steatohepatitis (NASH). Indeed, haploinsufficiency of lactate transporter monocarboxylate transporter 1 (MCT1) in mice reportedly promotes resistance to hepatic steatosis and inflammation. Here, we used adeno-associated virus (AAV) vectors to deliver thyroxin binding globulin (TBG)-Cre or lecithin-retinol acyltransferase (Lrat)-Cre to MCT1fl/fl mice on a choline-deficient, high-fat NASH diet to deplete hepatocyte or stellate cell MCT1, respectively. Stellate cell MCT1KO (AAV-Lrat-Cre) attenuated liver type 1 collagen protein expression and caused a downward trend in trichrome staining. MCT1 depletion in cultured human LX2 stellate cells also diminished collagen 1 protein expression. Tetra-ethylenglycol-cholesterol (Chol)-conjugated siRNAs, which enter all hepatic cell types, and hepatocyte-selective tri-N-acetyl galactosamine (GN)-conjugated siRNAs were then used to evaluate MCT1 function in a genetically obese NASH mouse model. MCT1 silencing by Chol-siRNA decreased liver collagen 1 levels, while hepatocyte-selective MCT1 depletion by AAV-TBG-Cre or by GN-siRNA unexpectedly increased collagen 1 and total fibrosis without effect on triglyceride accumulation. These findings demonstrate that stellate cell lactate transporter MCT1 significantly contributes to liver fibrosis through increased collagen 1 protein expression in vitro and in vivo, while hepatocyte MCT1 appears not to be an attractive therapeutic target for NASH.

Lactate transporter MCT1 in hepatic stellate cells promotes fibrotic collagen expression in nonalcoholic steatohepatitis

Circulating lactate is a fuel source for liver metabolism but may exacerbate metabolic diseases such as nonalcoholic steatohepatitis (NASH). Indeed, haploinsufficiency of lactate transporter monocarboxylate transporter 1 (MCT1) in mice reportedly promotes resistance to hepatic steatosis and inflammation. Here, we used adeno-associated virus (AAV) vectors to deliver thyroxin binding globulin (TBG)-Cre or lecithin-retinol acyltransferase (Lrat)-Cre to MCT1fl/fl mice on a choline deficient, high fat NASH diet to deplete hepatocyte or stellate cell MCT1, respectively. Stellate cell MCT1KO (AAV-Lrat-Cre) attenuated liver type 1 collagen protein expression and caused a downward trend in trichrome staining. MCT1 depletion in cultured human LX2 stellate cells also diminished collagen 1 protein expression. Tetra-ethylenglycol-cholesterol (Chol)-conjugated siRNAs, which enter all hepatic cell types, and hepatocyte-selective tri-N-acetyl galactosamine (GN)-conjugated siRNAs were then used to evaluate MCT1 function in a genetically obese NASH mouse model. MCT1 silencing by Chol-siRNA decreased liver collagen 1 levels, while hepatocyte-selective MCT1 depletion by AAV-TBG-Cre or by GN-siRNA unexpectedly increased collagen 1 and total fibrosis without effect on triglyceride accumulation. These findings demonstrate that stellate cell lactate transporter MCT1 significantly contributes to liver fibrosis through increased collagen 1 protein expression in vitro and in vivo, while hepatocyte MCT1 appears not to be an attractive therapeutic target for NASH.

Nucleocytoplasmic transport in C9orf72-mediated ALS/FTD

A GGGGCC hexanucleotide repeat expansion in C9orf72 is the most common genetic cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Recent studies indicate that disruption of nucleocytoplasmic transport pathways play a critical role in the pathogenesis of C9orf72-mediated ALS/FTD (C9-ALS). Here, we discuss mechanisms by which C9orf72 mutations cause nucleocytoplasmic transport deficits and contribute to disease pathogenesis. We review the current literature regarding nucleocytoplasmic transport disruption in C9-ALS, and discuss implications and directions for future research.

Transplanted glial restricted precursor cells improve neurobehavioral and neuropathological outcomes in a mouse model of neonatal white matter injury despite limited cell survival

OBJECTIVE

Neonatal white matter injury (NWMI) is the leading cause of cerebral palsy and other neurocognitive deficits in prematurely-born children, and no restorative therapies exist. Our objective was to determine the fate and effect of glial restricted precursor cell (GRP) transplantation in an ischemic mouse model of NWMI.

METHODS

Neonatal CD-1 mice underwent unilateral carotid artery ligation on postnatal-Day 5 (P5). At P22, intracallosal injections of either enhanced green fluorescent protein (eGFP) + GRPs or saline were performed in control and ligated mice. Neurobehavioral and postmortem studies were performed at 4 and 8 weeks post-transplantation.

RESULTS

GRP survival was comparable at 1 month but significantly lower at 2 months post-transplantation in NWMI mice compared with unligated controls. Surviving cells showed better migration capability in controls; however, the differentiation capacity of transplanted cells was similar in control and NWMI. Saline-treated NWMI mice showed significantly altered response in startle amplitude and prepulse inhibition (PPI) paradigms compared with unligated controls, while these behavioral tests were completely normal in GRP-transplanted animals. Similarly, there was significant increase in hemispheric myelin basic protein density, along with significant decrease in pathologic axonal staining in cell-treated NWMI mice compared with saline-treated NWMI animals.

INTERPRETATION

The reduced long-term survival and migration of transplanted GRPs in an ischemia-induced NWMI model suggests that neonatal ischemia leads to long-lasting detrimental effects on oligodendroglia even months after the initial insult. Despite limited GRP-survival, behavioral, and neuropathological outcomes were improved after GRP-transplantation. Our results suggest that exogenous GRPs improve myelination through trophic effects in addition to differentiation into mature oligodendrocytes.

Genetic deletion of the neuronal glutamate transporter, EAAC1, results in decreased neuronal death after pilocarpine-induced status epilepticus

Excitatory amino acid carrier 1 (EAAC1 also called EAAT3) is a Na(+)-dependent glutamate transporter expressed by both glutamatergic and GABAergic neurons. It provides precursors for the syntheses of glutathione and GABA and contributes to the clearance of synaptically released glutamate. Mice deleted of EAAC1 are more susceptible to neurodegeneration in models of ischemia, Parkinson's disease, and aging. Antisense knock-down of EAAC1 causes an absence seizure-like phenotype. Additionally, EAAC1 expression increases after chemonvulsant-induced seizures in rodent models and in tissue specimens from patients with refractory epilepsy. The goal of the present study was to determine if the absence of EAAC1 affects the sensitivity of mice to seizure-induced cell death. A chemoconvulsant dose of pilocarpine was administered to EAAC1(-/-) mice and to wild-type controls. Although EAAC1(-/-) mice experienced increased latency to seizure onset, no significant differences in behavioral seizure severity or mortality were observed. We examined EAAC1 immunofluorescence 24h after pilocarpine administration and confirmed that pilocarpine causes an increase in EAAC1 protein. Forty-eight hours after induction of seizures, cell death was measured in hippocampus and in cortex using Fluoro-Jade C. Surprisingly, there was ∼2-fold more cell death in area CA1 of wild-type mice than in the corresponding regions of the EAAC1(-/-) mice. Together, these studies indicate that absence of EAAC1 results in either a decrease in pilocarpine-induced seizures that is not detectable by behavioral criteria (surprising, since EAAC1 provides glutamate for GABA synthesis), or that the absence of EAAC1 results in less pilocarpine/seizure-induced cell death, possible explanations as discussed.

GTRAP3-18 serves as a negative regulator of Rab1 in protein transport and neuronal differentiation

Glutamate transporter associated protein 3–18 (GTRAP3-18) is an endoplasmic reticulum (ER)-localized protein belonging to the prenylated rab-acceptor-family interacting with small Rab GTPases, which regulate intracellular trafficking events. Its impact on secretory trafficking has not been investigated. We report here that GTRAP3-18 has an inhibitory effect on Rab1, which is involved in ER-to-Golg trafficking. The effects on the early secretory pathway in HEK293 cells were: reduction of the rate of ER-to-Golgi transport of the vesicular stomatitis virus glycoprotein (VSVG), slowed accumulation of a Golgi marker plasmid in pre-Golgi structures after Brefeldin A treatment and inhibition of cargo concentration of the neuronal glutamate transporter excitatory amino-acid carrier 1 (EAAC1) into transpor complexes in HEK293 cells, an effect that could be completely reversed in the presence of an excess of Rab1. In accordance with the known role of Rab1 in neurite formation, overexpression of GTRAP3-18 significantly inhibited the length of outgrowing neurites in differentiated CAD cells. The inhibitory effect of GTRAP3-18 on neurite growth was rescued by co-expression with Rab1, supporting the conclusion that GTRAP 3-18 acted by inhibiting Rab1 action. Finally, we hypothesized that expression of GTRAP3-18 in the brain shoul be lower at stages of active synaptogenesis compared to early developmental stages. This was the case as expression of GTRAP3-18 declined from E17 to P0 and adult rat brains. Thus, we propose a model where protein trafficking and neuronal differentiation are directly linked by the interaction of Rab1 and its regulator GTRAP3-18.

Common molecular signature in SOD1 for both sporadic and familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating motor neuron degenerative disease whose etiology and pathogenesis remain poorly understood. Most cases of ALS ( approximately 90%) are sporadic (SALS), occurring in the absence of genetic associations. Approximately 20% of familial ALS (FALS) cases are due to known mutations in the copper, zinc superoxide dismutase (SOD1) gene. Molecular evidence for a common pathogenesis of SALS and FALS has remained elusive. Here we use covalent chemical modification to reveal an attribute of spinal cord SOD1 common to both SOD1-linked FALS and SALS, but not present in normal or disease-affected tissues from other neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases and spinal muscular atrophy, a non-ALS motor neuron disease. Biotinylation reveals a 32-kDa, covalently cross-linked SOD1-containing protein species produced not only in FALS caused by SOD1 mutation, but also in SALS. These studies use chemical modification as a novel tool for the detection of a disease-associated biomarker. Our results identify a shared molecular event involving a known target gene and suggest a common step in the pathogenesis between SALS and FALS.

Respiratory impairment in a mouse model of amyotrophic lateral sclerosis

Amyothrophic lateral sclerosis (ALS) is a progressive, lethal neuromuscular disease that is associated with the degeneration of cortical and spinal motoneurons, leading to atrophy of limb, axial, and respiratory muscles. Patients with ALS invariably develop respiratory muscle weakness and most die from pulmonary complications. Overexpression of superoxide dismutase 1 (SOD1) gene mutations in mice recapitulates several of the clinical and pathological characteristics of ALS and is therefore a valuable tool to study this disease. The present study is intended to evaluate an age-dependent progression of respiratory complications in SOD1(G93A) mutant mice. In each animal, baseline measurements of breathing pattern [i.e., breathing frequency and tidal volume (VT)], minute ventilation (VE), and metabolism (i.e., oxygen consumption and carbon dioxide production) were repeatedly sampled at variable time points between 10 and 20 wk of age with the use of whole-body plethysmographic chambers. To further characterize the neurodegeneration of breathing, VE was also measured during 5-min challenges of hypercapnia (5% CO(2)) and hypoxia (10% O(2)). At baseline, breathing characteristics and metabolism remained relatively unchanged from 10 to 14 wk of age. From 14 to 18 wk of age, there were significant (P < 0.05) increases in baseline VT, VE, and the ventilatory equivalent (VE/oxygen consumption). After 18 wk of age, there was a rapid decline in VE due to significant (P < 0.05) reductions in both breathing frequency and VT. Whereas little change in hypoxic VE responses occurred between 10 and 18 wk, hypercapnic VE responses were significantly (P < 0.05) elevated at 18 wk due to an augmented VT response. Like baseline breathing characteristics, hypercapnic VE responses also declined rapidly after 18 wk of age. The phenotypic profile of SOD1(G93A) mutant mice was apparently unique because similar changes in respiration and metabolism were not observed in SOD1 controls. The present results outline the magnitude and time course of respiratory complications in SOD1(G93A) mutant mice as the progression of disease occurs in this mouse model of ALS.

Regulation and dysregulation of glutamate transporters

Glutamate is the primary excitatory neurotransmitter in the central nervous system. During synaptic activity, glutamate is released into the synaptic cleft and binds to glutamate receptors on the pre- and postsynaptic membrane as well as on neighboring astrocytes in order to start a number of intracellular signaling cascades. To allow for an efficient signaling to occur, glutamate levels in the synaptic cleft have to be maintained at very low levels. This process is regulated by glutamate transporters, which remove excess extracellular glutamate via a sodium-potassium coupled uptake mechanism. When extracellular glutamate levels rise to about normal, glutamate overactivates glutamate receptors, triggering a multitude of intracellular events in the postsynaptic neuron, which ultimately results in neuronal cell death. This phenomenon is known as excitotoxicity and is the underlying mechanisms of a number of neurodegenerative diseases. A dysfunction of the glutamate transporter is thought to contribute to cell death during excitotoxicity. Therefore, efforts have been made to understand the regulation of glutamate transporter function. Transporter activity can be regulated in different ways, including through gene expression, transporter protein targeting and trafficking and through posttranslational modifications of the transporter protein. The identification of these mechanisms has helped to understand the role of glutamate transporters during pathology and will aid in the development of therapeutic strategies with the transporter as a desirable target.

Spinal glutamate uptake is critical for maintaining normal sensory transmission in rat spinal cord

Glutamate is a major excitatory neurotransmitter in primary afferent terminals and is critical for normal spinal excitatory synaptic transmission. However, little is known about the regulation of synaptically released glutamate in the spinal cord under physiologic conditions. The sodium-dependent, high-affinity glutamate transporters are the primary mechanism for the clearance of synaptically released glutamate. In the present study, we found that intrathecal injection of glutamate transporter blockers DL-threo-beta-benzyloxyaspartate (TBOA) and dihydrokainate produced significant and dose-dependent spontaneous nociceptive behaviors, such as licking, shaking, and caudally directed biting, phenomena similar to the behaviors caused by intrathecal glutamate receptor agonists. Intrathecal TBOA also led to remarkable hypersensitivity in response to thermal and mechanical stimuli. These behavioral responses could be significantly blocked by intrathecal injection of the NMDA receptor antagonists MK-801 and AP-5, the non-NMDA receptor antagonist CNQX or the nitric oxide synthase inhibitor L-NAME. In vivo microdialysis analysis showed short-term elevation of extracellular glutamate concentration in the spinal cord after intrathecal injection of TBOA. Furthermore, topical application of TBOA on the dorsal surface of the spinal cord resulted in a significant elevation of extracellular glutamate concentration demonstrated by in vivo glutamate voltametry. The present study indicates that defective spinal glutamate uptake caused by inhibition of glutamate transporters leads to excessive glutamate accumulation in the spinal cord. The latter results in persistent over-activation of synaptic glutamate receptors, producing spontaneous nociceptive behaviors and sensory hypersensitivity. Our results suggest that glutamate uptake through spinal glutamate transporters is critical for maintaining normal sensory transmission under physiologic conditions.

Toxicity of Familial ALS-Linked SOD1 Mutants from Selective Recruitment to Spinal Mitochondria

One cause of amyotrophic lateral sclerosis (ALS) is mutation in ubiquitously expressed copper/zinc superoxide dismutase (SOD1), but the mechanism of toxicity to motor neurons is unknown. Multiple disease-causing mutants, but not wild-type SOD1, are now demonstrated to be recruited to mitochondria, but only in affected tissues. This is independent of the copper chaperone for SOD1 and dismutase activity. Highly preferential association with spinal cord mitochondria is seen in human ALS for a mutant SOD1 that accumulates only to trace cytoplasmic levels. Despite variable proportions that are successfully imported, nearly constant amounts of SOD1 mutants and covalently damaged adducts of them accumulate as apparent import intermediates and/or are tightly aggregated or crosslinked onto integral membrane components on the cytoplasmic face of those mitochondria. These findings implicate damage from action of spinal cord-specific factors that recruit mutant SOD1 to spinal mitochondria as the basis for their selective toxicity in ALS.

Evidence of neuronal excitatory amino acid carrier 1 expression in rat dorsal root ganglion neurons and their central terminals

The expression and distribution of the neuronal glutamate transporter, excitatory amino acid carrier-1 (EAAC1), are demonstrated in the dorsal root ganglion neurons and their central terminals. Reverse transcriptase-polymerase chain reaction shows expression of EAAC1 mRNA in the dorsal root ganglion. Immunoblotting analysis further confirms existence of EAAC1 protein in this region. Immunocytochemistry reveals that approximately 46.6% of the dorsal root ganglion neurons are EAAC1-positive. Most EAAC1-positive neurons are small and around 250-750 microm2 in surface area, and some co-label with calcitonin gene-related peptide (CGRP) or isolectin IB4. In the spinal cord, EAAC-1 immunoreactive small dot- or patch-like structures are mainly localized in the superficial dorsal horn, and some are positive for CGRP or labeled by isolectin IB4. Unilateral dorsal rhizotomy experiments further show that EAAC1 immunoreactivity is less intense in superficial dorsal horn on the side ipsilateral to the dorsal rhizotomy than on the contralateral side. The results indicate the presence of EAAC1 in the dorsal root ganglion neurons and their central terminals. Our findings suggest that EAAC1 might play an important role in transmission and modulation of nociceptive information via the regulation of pre-synaptically released glutamate.

A randomized, placebo-controlled trial of topiramate in amyotrophic lateral sclerosis

OBJECTIVE

To determine if long-term topiramate therapy is safe and slows disease progression in patients with ALS.

METHODS

A double-blind, placebo-controlled, multicenter randomized clinical trial was conducted. Participants with ALS (n = 296) were randomized (2:1) to receive topiramate (maximum tolerated dose up to 800 mg/day) or placebo for 12 months. The primary outcome measure was the rate of change in upper extremity motor function as measured by the maximum voluntary isometric contraction (MVIC) strength of eight arm muscle groups. Secondary endpoints included safety and the rate of decline of forced vital capacity (FVC), grip strength, ALS functional rating scale (ALSFRS), and survival.

RESULTS

Patients treated with topiramate showed a faster decrease in arm strength (33.3%) during 12 months (0.0997 vs 0.0748 unit decline/month, p = 0.012). Topiramate did not significantly alter the decline in FVC and ALSFRS or affect survival. Topiramate was associated with an increased frequency of anorexia, depression, diarrhea, ecchymosis, nausea, kidney calculus, paresthesia, taste perversion, thinking abnormalities, weight loss, and abnormal blood clotting (pulmonary embolism and deep venous thrombosis).

CONCLUSIONS

At the dose studied, topiramate did not have a beneficial effect for patients with ALS. High-dose topiramate treatment was associated with a faster rate of decline in muscle strength as measured by MVIC and with an increased risk for several adverse events in patients with ALS. Given the lack of efficacy and large number of adverse effects, further studies of topiramate at a dose of 800 mg or maximum tolerated dose up to 800 mg/day are not warranted.

Modulation of spinal nociceptive processing through the glutamate transporter GLT-1

GLT-1 is the predominant glutamate transporter in most brain regions and therefore plays a major role in terminating synaptic transmission and protecting neurons from glutamate neurotoxicity. In the present study we assessed (i) the regulation of GLT-1 expression in the spinal cord after peripheral nociceptive stimulation and (ii) the nociceptive behavior of rats following inhibition or transient knockdown of spinal GLT-1. Formalin injection into one hindpaw caused a rapid transient upregulation of GLT-1 protein expression in the spinal cord which did not occur when rats were pretreated with morphine (10 mg/kg, i.p.) suggesting that the nociceptive input specifically caused the increase of GLT-1 transcription. Inhibition of GLT-1 by the transportable inhibitor trans-pyrrolidine-2,4-dicarboxylic acid resulted in a significant reduction of nociceptive behavior in the rat formalin assay. Similar results were obtained with a transient reduction of GLT-1 protein expression by antisense oligonucleotides. These data suggest that inhibition of GLT-1 activity or expression reduces excitatory synaptic efficacy and thereby nociception. Mechanisms that might explain this phenomenon may include activation of inhibitory metabotropic glutamate receptors, postsynaptic desensitization or disturbance of glutamate recycling.

Altered expressions of glutamate transporter subtypes in rat model of neonatal cerebral hypoxia-ischemia

Glutamate transporters are essential for maintaining the extracellular levels of glutamate at synaptic clefts and are regulated developmentally in a subtype-specific manner. We investigated chronological changes of immunoreactivities for glial glutamate transporters GLAST and GLT-1 and a neuronal glutamate transporter, EAAC1, in postnatal 7-day-old rat neocortices and hippocampi at 12, 24, 48 and 72 h after hypoxia-ischemia. Glutamate transporter subtypes are differentially expressed in the ischemic core and the boundary area of the neonatal rat brain with hypoxia-ischemia. Expressions of these glutamate transporters decreased in the ischemic core at 12 h, then immunoreactivities for GLAST and GLT-1 were recovered at the hippocampus. This was accompanied by a GFAP-positive gliosis at 72 h, whereas these immunoreactivities were reduced at the neocortex in the ischemic core. Glial glutamate transporters, especially GLAST, were noted in some astrocytes appearing as apoptosis as well as shrunken pyramidal neurons mainly in the boundary area of the neocortex. Increased perikaryal expression of EAAC1 was associated with that of MAP2 at the border of the boundary area. These temporal and regional expressions of glutamate transporters may contribute towards understanding the excitotoxic cell death mechanism in hypoxic-ischemic encephalopathy during the perinatal period.

Histological evidence of protein aggregation in mutant SOD1 transgenic mice and in amyotrophic lateral sclerosis neural tissues

The mechanisms leading to neurodegeneration in ALS (amyotrophic lateral sclerosis) are not well understood, but cytosolic protein aggregates appear to be common in sporadic and familial ALS as well as transgenic mouse models expressing mutant Cu/Zn superoxide dismutase (SOD1). In this study, we systematically evaluated the presence of these aggregates in three different mouse models (G93A, G85R, and G37R SOD1) and compared these aggregates to those seen in cases of sporadic and familial ALS. Inclusions and loss of motor neurons were observed in spinal cords of all of these three mutant transgenic lines. Since a copper-mediated toxicity hypothesis has been proposed to explain the cytotoxic gain-of-function of mutant SOD1, we sought to determine the involvement of the copper chaperone for SOD1 (CCS) in the formation of protein aggregates. Although all aggregates contained CCS, SOD1 was not uniformly found in the inclusions. Similarly, CCS-positive skein-like inclusions were rarely seen in ALS neurons. These studies do not provide strong evidence for a causal role of CCS in aggregate formation, but they do suggest that protein aggregation is a common event in all animal models of the disease. Selected proteins, such as the glutamate transporter GLT-1, were not typically observed within the inclusions. Most inclusions were positively stained with antibodies recognizing ubiquitin, proteasome, Hsc70 in transgenic lines, and some Hsc70-positive inclusions were detected in sporadic ALS cases. Overall, these observations suggest that inclusions might be sequestered into ubiquitin-proteasome pathway and some chaperone proteins such as Hsc70 may be involved in formation and/or degradation of these inclusions.

Differential synaptic localization of GluR2 and EAAC1 in the macaque monkey entorhinal cortex: a postembedding immunogold study

The synaptic distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit GluR2 and neuronal glutamate transporter subunit EAAC1 were studied using immunogold in layer II of the macaque monkey entorhinal cortex. Immunoreactivity for EAAC1 and GluR2 was frequent at asymmetric synapses and their associated membrane. The synaptic localization of EAAC1 differed considerably from that of GluR2, in that GluR2 immunolabelling was most commonly located within the postsynaptic density, but EAAC1 localization was more heterogeneous and was predominant at the edge of postsynaptic densities and perisynaptic zones. Since EAAC1 may play an important role in clearing glutamate from the synaptic cleft and intercellular spaces, the high perisynaptic expression of EAAC1 in these neurons could presumably offer a powerful mechanism through which high concentrations of glutamate could be efficiently removed from the synapses following release and interaction with glutamate receptors. The distribution of EAAC1 may also offer protection for these neurons against excessive glutamatergic stimuli that may occur under certain pathological conditions.

The expression of glutamate transporter GLT-1 in the rat cerebral cortex is down-regulated by the antipsychotic drug clozapine

We show here that clozapine, a beneficial antipsychotic, down-regulates the expression of the glutamate transporter GLT-1 in the rat cerebral cortex, thereby reducing glutamate transport and raising extracellular glutamate levels. Clozapine treatment (25--35 mg kg(-1) day(-1) orally) reduced GLT-1 immunoreactivity in several brain regions after 3 weeks; this effect was most prominent after 9 weeks and most evident in the frontal cortex. GLT-1 protein levels were reduced in the cerebral cortex of treated rats compared with controls and were more severely affected in the anterior (71.9 +/- 4.5%) than in the posterior (53.2 +/- 15.4%) cortex. L-[(3)H]-glutamate uptake in Xenopus laevis oocytes injected with mRNA extracted from the anterior cerebral cortex of rats treated for 9 weeks was remarkably reduced (to 30.6 +/- 8.6%) as compared to controls. In addition, electrophysiological recordings from oocytes following application of glutamate revealed a strong reduction in glutamate uptake currents (46.3 +/- 10.2%) as compared to controls. Finally, clozapine treatment led to increases in both the mean basal (8.1 +/- 0.7 microM) and the KCl-evoked (28.7 +/- 7.7 microM) output of glutamate that were 3.1 and 3.5, respectively, higher than in control rats. These findings indicate that clozapine may potentiate glutamatergic synaptic transmission by regulating glutamate transport.

Antisense knockdown of the glial glutamate transporter GLT-1, but not the neuronal glutamate transporter EAAC1, exacerbates transient focal cerebral ischemia-induced neuronal damage in rat brain

Transient focal cerebral ischemia leads to extensive neuronal damage in cerebral cortex and striatum. Normal functioning of glutamate transporters clears the synaptically released glutamate to prevent excitotoxic neuronal death. This study evaluated the functional role of the glial (GLT-1) and neuronal (EAAC1) glutamate transporters in mediating ischemic neuronal damage after transient middle cerebral artery occlusion (MCAO). Transient MCAO in rats infused with GLT-1 antisense oligodeoxynucleotides (ODNs) led to increased infarct volume (45 +/- 8%; p < 0.05), worsened neurological status, and increased mortality rate, compared with GLT-1 sense/random ODN-infused controls. Transient MCAO in rats infused with EAAC1 antisense ODNs had no significant effect on any of these parameters. This study suggests that GLT-1, but not EAAC1, knockdown exacerbates the neuronal death and thus neurological deficit after stroke.

Modulation of the neuronal glutamate transporter EAAC1 by the interacting protein GTRAP3-18

Excitatory amino-acid carrier 1 (EAAC1) is a high-affinity Na+-dependent L-glutamate/D,L-aspartate cell-membrane transport protein. It is expressed in brain as well as several non-nervous tissues. In brain, EAAC1 is the primary neuronal glutamate transporter. It has a polarized distribution in cells and mainly functions perisynaptically to transport glutamate from the extracellular environment. In the kidney it is involved in renal acidic amino-acid re-absorption and amino-acid metabolism. Here we describe the identification and characterization of an EAAC1-associated protein, GTRAP3-18. Like EAAC1, GTRAP3-18 is expressed in numerous tissues. It localizes to the cell membrane and cytoplasm, and specifically interacts with carboxy-terminal intracellular domain of EAAC1. Increasing the expression of GTRAP3-18 in cells reduces EAAC1-mediated glutamate transport by lowering substrate affinity. The expression of GTRAP3-18 can be upregulated by retinoic acid, which results in a specific reduction of EAAC1-mediated glutamate transport. These studies show that glutamate transport proteins can be regulated potently and that GTRAP can modulate the transport functions ascribed to EAAC1. GTRAP3-18 may be important in regulating the metabolic function of EAAC1.

Modulation of the neuronal glutamate transporter EAAT4 by two interacting proteins

Glutamate is the main excitatory neurotransmitter in the mammalian central nervous system and is removed from the synaptic cleft by sodium-dependent glutamate transporters. To date, five distinct glutamate transporters have been cloned from animal and human tissue: GLAST (EAAT1), GLT-1 (EAAT2), EAAC1 (EAAT3), EAAT4, and EAAT5 (refs 1-5). GLAST and GLT-1 are localized primarily in astrocytes, whereas EAAC1 (refs 8, 9), EAAT4 (refs 9-11) and EAAT5 (ref 5) are neuronal. Studies of EAAT4 and EAAC1 indicate an extrasynaptic localization on perisynaptic membranes that are near release sites. This localization facilitates rapid glutamate binding, and may have a role in shaping the amplitude of postsynaptic responses in densely packed cerebellar terminals. We have used a yeast two-hybrid screen to identify interacting proteins that may be involved in regulating EAAT4--the glutamate transporter expressed predominately in the cerebellum--or in targeting and/or anchoring or clustering the transporter to the target site. Here we report the identification and characterization of two proteins, GTRAP41 and GTRAP48 (for glutamate transporter EAAT4 associated protein) that specifically interact with the intracellular carboxy-terminal domain of EAAT4 and modulate its glutamate transport activity.

Microglial response to the neurotoxicity of 6-hydroxydopamine in neonatal rat cerebellum

Depletion of noradrenaline in newborn rats by 6-hydroxydopamine (6-OHDA) affects the postnatal development and reduces the granular cell area in the neocerebellum (lobules V-VII). During the first postnatal month, Bergmann glial fibers guide the migration of immature granule cells to the internal granule cell layer. Microglia and Bergmann glia may play an important role in this process, but the exact mechanism behind this phenomenon is not known. We studied the effect of systemic administration of 6-OHDA on the expression and localization on microglia and Bergmann glia in the neonatal cerebellum by immunohistochemistry. In the neocerebellum, 6-OHDA treatment caused a significant increase in the number of activated microglia. The increase was observed mainly in the granule cell layer and the cerebellar medulla. Bergmann glial cells in treated brains were abnormally located, did not form intimate associations with Purkinje cells, and the glial fibers were structurally different. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of granule cells, and abnormal migration may be the result of Bergmann glia destruction and the activation of microglia. Activated microglia in the granule cell layer may be used as a marker for an injured cerebellar area.

Inhibition of cyclooxygenase-2 protects motor neurons in an organotypic model of amyotrophic lateral sclerosis

The pathogenesis of motor neuron loss in amyotrophic lateral sclerosis (ALS) is thought to involve both glutamate-mediated excitotoxicity and oxidative damage due to the accumulation of free radicals and other toxic molecules. Cyclooxygenase-2 (COX-2) may play a key role in these processes by producing prostaglandins, which trigger astrocytic glutamate release, and by inducing free radical formation. We tested the effects of COX-2 inhibition in an organotypic spinal cord culture model of ALS. The COX-2 inhibitor (SC236) provided significant protection against loss of spinal motor neurons in this system, suggesting that it may be useful in the treatment of ALS.

Calcium channel agonists and dystonia in the mouse

Systemic administration of the L-type calcium channel agonists +/-Bay K 8644 or FPL 64176 causes a characteristic pattern of motor dysfunction in normal C57BL/6J mice that resembles generalized dystonia. There is no associated change in the electroencephalogram, confirming that the motor disorder does not reflect epileptic seizures. However, the electromyogram reveals an increase in baseline motor unit activity with prolonged phasic discharges consistent with dystonia. The duration and severity of dystonia is dependent on the dose administered and the age of the animal at testing. The effects are transient, with the return of normal motor behavior 1-4 hours after treatment. Similar effects can be provoked by intracerebral administration of small amounts of the drugs, indicating a centrally mediated response. Dystonia can be attenuated by co-administration of dihydropyridine L-type calcium channel antagonists (nifedipine, nimodipine, and nitrendipine) but not by non-dihydropyridine antagonists (diltiazem, verapamil, and flunarizine). These results implicate abnormal function of L-type calcium channels in the expression of dystonia in this model.

Glial glutamate transporter GLT-1 down-regulation precedes delayed neuronal death in gerbil hippocampus following transient global cerebral ischemia

Glial (GLT-1 and GLAST) and neuronal (EAAC1) high-affinity transporters mediate the sodium dependent glutamate reuptake in mammalian brain. Their dysfunction leads to neuronal damage by allowing glutamate to remain in the synaptic cleft for a longer duration. The purpose of the present study is to understand their contribution to the ischemic delayed neuronal death seen in gerbil hippocampus following transient global cerebral ischemia. The protein levels of these three transporters were studied by immunoblotting as a function of reperfusion time (6 h to 7 days) following a 10 min occlusion of bilateral common carotid arteries in gerbils. In the vulnerable hippocampus, there was a significant decrease in the protein levels of GLT-1 (by 36-46%, P < 0.05; between 1 and 3 days of reperfusion) and EAAC1 (by 42-68%, P < 0.05; between 1 and 7 days of reperfusion). Histopathological evaluation showed no neuronal loss up to 2 days of reperfusion but an extensive neuronal loss (by approximately 84%, P < 0.01) at 7 days of reperfusion in the hippocampal CA1 region. The time frame of GLT-1 dysfunction (1-3 days of reperfusion) precedes the initiation of delayed neuronal death (2-3 days of reperfusion). This suggests GLT-1 dysfunction as a contributing factor for the hippocampal neuronal death following transient global cerebral ischemia. Furthermore, decreased EAAC1 levels may contribute to GABAergic dysfunction and excitatory/inhibitory imbalance following transient global ischemia.

Epidermal growth factor receptor agonists increase expression of glutamate transporter GLT-1 in astrocytes through pathways dependent on phosphatidylinositol 3-kinase and transcription factor NF-kappaB

The glial glutamate transporter GLT-1 may be the predominant Na(+)-dependent glutamate transporter in forebrain. Expression of GLT-1 correlates with astrocyte maturation in vivo and increases during synaptogenesis. In astrocyte cultures, GLT-1 expression parallels differentiation induced by cAMP analogs or by coculturing with neurons. Molecule(s) secreted by neuronal cultures contribute to this induction of GLT-1, but little is known about the signaling pathways mediating this regulation. In the present study, we determined whether growth factors previously implicated in astrocyte differentiation regulate GLT-1 expression. Of the six growth factors tested, two [epidermal growth factor (EGF) and transforming growth factor-alpha] induced expression of GLT-1 protein in cultured astrocytes. Induction of GLT-1 protein was accompanied by an increase in mRNA and in the V(max) for Na(+)-dependent glutamate transport activity. The effects of dibutyryl-cAMP and EGF were additive but were independently blocked by inhibitors of protein kinase A or protein tyrosine kinases, respectively. The induction of GLT-1 in both EGF- and dibutyryl-cAMP-treated astrocytes was blocked by inhibitors targeting phosphatidylinositol 3-kinase (PI3K) or the nuclear transcription factor-kappaB. Furthermore, transient transfection of astrocyte cultures with a constitutively active PI3K construct was sufficient to induce expression of GLT-1. These data suggest that independent but converging pathways mediate expression of GLT-1. Although an EGF receptor-specific antagonist did not block the effects of neuron-conditioned medium, the induction of GLT-1 by neuron-conditioned medium was completely abolished by inhibition of PI3K or nuclear factor-kappaB. EGF also increased expression of GLT-1 in spinal cord organotypic cultures. Together, these data suggest that activation of specific signaling pathways with EGF-like molecules may provide a novel approach for limiting excitotoxic brain injury.

Differential synaptic localization of the glutamate transporter EAAC1 and glutamate receptor subunit GluR2 in the rat hippocampus

EAAC1, a neuron-specific glutamate transporter, is likely to play an important role in the regulation of glutamate levels in the synaptic cleft. Ultrastructural studies have demonstrated that the glutamate receptor subunit proteins (e.g., GluR2) are frequently preferentially located at the postsynaptic density of asymmetric synapses. While the glutamate/glutamate receptor interaction is likely to be influenced by the activity and location of the transporter molecules, the spatial localization of the transporter molecules relative to the receptor molecules is not well delineated. Thus, we analyzed the cellular, ultrastructural, and synaptic distribution of EAAC1 in the context of the distribution of the AMPA receptor subunit GluR2 in the hippocampus. While GluR2 and EAAC1 are both present in hippocampal projection neurons, their intracellular distribution patterns differ. Both GluR2 and EAAC1 are present in the dendritic membranes and cytoplasm; however EAAC1 has a distinctive punctate distribution in the dendrite compared to the more diffuse labeling reflected by GluR2. Pre-embedding ultrastructural studies also revealed cytoplasmic and membrane-associated pools of EAAC1 within dendritic shafts and spines, as well as in a subset of axonal profiles and terminals. Postembedding double label immunogold localization demonstrated a similar intraneuronal distribution, but in addition showed that membrane-associated EAAC1 is not intermingled with GluR2 within the synaptic complex, but in contrast is primarily located perisynaptically, often immediately outside the synaptic specialization. In addition, there is a significant presynaptic pool of EAAC1, whereas GluR2 is essentially absent from the pre-synaptic profile. Thus, membrane-associated EAAC1 within the synaptic region is ideally situated to restrict the site of action of glutamate with respect to ionotropic receptors to the synaptic cleft, as well as regulate glutamate levels in the perisynaptic and presynaptic domains, the ultrastructural sites that have been associated with metabotropic receptor localization.

TGFbeta trophic factors differentially modulate motor axon outgrowth and protection from excitotoxicity

Transforming growth factor (TGF) beta-like trophic factors have been shown to be protective in acute neuronal injury paradigms. In the current study, we analyzed and compared members of this growing family, including glial cell line-derived neurotrophic factor (GDNF), neurturin, nodal, persephin, and TGFbeta1, for protection against chronic glutamate toxicity. In parallel, we developed a organotypic spinal cord culture system to study the ability of these factors to promote motor axon outgrowth across white matter. Using these systems, we were able to differentiate the neuroprotective effect of the TGFbeta-like factors from their motor axon outgrowth-promoting activity. GDNF, neurturin, persephin, nodal, and TGFbeta1 all protected against excitotoxic motor neuron degeneration. Low amounts of GDNF (1 ng/ml) and high concentrations of neurturin induced vigorous motor axon outgrowth. In contrast, nodal, persephin, and TGFbeta1 did not induce motor axon outgrowth. Both GDNF and neurturin bind to Ret receptor complexes and were capable of activating the MAP kinase pathway. A specific inhibitor of MAP kinase kinase, PD98059, inhibited the motor axon outgrowth-promoting activity of the GDNF but not the neuroprotective activity. Similarly, the specific PI3K inhibitors, LY294002 and wortmannin, were able to inhibit the promotion of motor axon outgrowth by GDNF, but did not affect neuroprotective activity. Our results suggest that the neurite outgrowth-promoting effect of GDNF is mediated through the PI3K and MAP kinase pathways. The neuroprotective effect of GDNF appears to be through a separate pathway.

Calcium channel activation and self-biting in mice

The L type calcium channel agonist (+/-)Bay K 8644 has been reported to cause characteristic motor abnormalities in adult mice. The current study shows that administration of this drug can also cause the unusual phenomenon of self-injurious biting, particularly when given to young mice. Self-biting is provoked by injecting small quantities of (+/-)Bay K 8644 directly into the lateral ventricle of the brain, suggesting a central effect of the drug. Similar behaviors can be provoked by administration of another L type calcium channel agonist, FPL 64176. The self-biting provoked by (+/-)Bay K 8644 can be inhibited by pretreating the mice with dihydropyridine L type calcium channel antagonists such as nifedipine, nimodipine, or nitrendipine. However, self-biting is not inhibited by nondihydropyridine antagonists including diltiazem, flunarizine, or verapamil. The known actions of (+/-)Bay K 8644 as an L type calcium channel agonist, the reproduction of similar behavior with another L type calcium channel agonist, and the protection afforded by certain L type calcium channel antagonists implicate calcium channels in the mediation of the self-biting behavior. This phenomenon provides a model for studying the neurobiology of this unusual behavior.

Compromised glutamate transport in human glioma cells: reduction-mislocalization of sodium-dependent glutamate transporters and enhanced activity of cystine-glutamate exchange

Elevated levels of extracellular glutamate ([Glu](o)) can induce seizures and cause excitotoxic neuronal cell death. This is normally prevented by astrocytic glutamate uptake. Neoplastic transformation of human astrocytes causes malignant gliomas, which are often associated with seizures and neuronal necrosis. Here, we show that Na(+)-dependent glutamate uptake in glioma cell lines derived from human tumors (STTG-1, D-54MG, D-65MG, U-373MG, U-251MG, U-138MG, and CH-235MG) is up to 100-fold lower than in astrocytes. Immunohistochemistry and subcellular fractionation show very low expression levels of the astrocytic glutamate transporter GLT-1 but normal expression levels of another glial glutamate transporter, GLAST. However, in glioma cells, essentially all GLAST protein was found in cell nuclei rather than the plasma membrane. Similarly, brain tissues from glioblastoma patients also display reduction of GLT-1 and mislocalization of GLAST. In glioma cell lines, over 50% of glutamate transport was Na(+)-independent and mediated by a cystine-glutamate exchanger (system x(c)(-)). Extracellular L-cystine dose-dependently induced glutamate release from glioma cells. Glutamate release was enhanced by extracellular glutamine and inhibited by (S)-4-carboxyphenylglycine, which blocked cystine-glutamate exchange. These data suggest that the unusual release of glutamate from glioma cells is caused by reduction-mislocalization of Na(+)-dependent glutamate transporters in conjunction with upregulation of cystine-glutamate exchange. The resulting glutamate release from glioma cells may contribute to tumor-associated necrosis and possibly to seizures in peritumoral brain tissue.

Preclinical testing of neuroprotective neurotrophic factors in a model of chronic motor neuron degeneration

Many neurotrophic factors have been shown to enhance survival of embryonic motor neurons or affect their response to injury. Few studies have investigated the potential effects of neurotrophic factors on more mature motor neurons that might be relevant for neurodegenerative diseases. Using organotypic spinal cord cultures from postnatal rats, we have demonstrated that insulin-like growth factor-I (IGF-I) and glial-derived neurotrophic factor (GDNF) significantly increase choline acetyltransferase (ChAT) activity, but brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4/5), and neurotrophin-3 (NT-3) do not. Surprisingly, ciliary neurotrophic factor (CNTF) actually reduces ChAT activity compared to age-matched control cultures. Neurotrophic factors have also been shown to alter the sensitivity of some neurons to glutamate neurotoxicity, a postulated mechanism of injury in the neurodegenerative disease, amyotrophic lateral sclerosis (ALS). Incubation of organotypic spinal cord cultures in the presence of the glutamate transport inhibitor threo-hydroxyaspartate (THA) reproducibly causes death of motor neurons which is glutamate-mediated. In this model of motor neuron degeneration, IGF-I, GDNF, and NT-4/5 are potently neuroprotective, but BDNF, CNTF, and NT-3 are not. The organotypic glutamate toxicity model appears to be the best preclinical predictor to date of success in human clinical trials in ALS.

Expression of the GLT-1 subtype of Na+-dependent glutamate transporter: pharmacological characterization and lack of regulation by protein kinase C

Several subtypes of Na+-dependent glutamate transporters have been pharmacologically differentiated in brain tissues. Five distinct cDNA clones that express Na+-dependent glutamate transport activity have been isolated. One goal of the current study was to compare the pharmacological properties of the rat GLT-1 subtype of transporter to those identified previously using rat brain tissues. To accomplish this goal, GLT-1 was stably transfected into two different cell lines that express low levels of endogenous transport activity (MCB and L-M (TK-)). Several clones stably transfected with GLT-1 were isolated. In each cell line, Na+-dependent glutamate transport activity was saturable with similar Km values (19 and 37 microM). The pharmacological properties of GLT-1-mediated transport in these cell lines paralleled those observed for the predominant pharmacology observed in cortical crude synaptosomes. These data are consistent with other lines of evidence that suggest that GLT-1 may be sufficient to explain most of the Na+-dependent glutamate transport activity in cortical synaptosomes. Although recent studies using HeLa cells have suggested that GLT-1 can be rapidly up-regulated by activation of protein kinase C (PKC), modulation of PKC or phosphatase activity had no effect on GLT-1-mediated activity in these transfected cell lines. To determine if GLT-1 regulation by PKC is cell-specific, HeLa cells, which endogenously express the EAAC1 subtype of transporter, were stably transfected with GLT-1. Although EAAC1-mediated activity was increased by activation of PKC, we found no evidence for regulation of GLT-1. Despite the present findings, GLT-1 activity may be regulated by PKC under certain conditions.

Cultured motor neurons possess calcium-permeable AMPA/kainate receptors

We examined the biology of AMPA/kainate-induced motor neuron degeneration using dissociated spinal cord cultures and motor neuron-specific antibodies which enable characterization of individual motor neurons in culture. Cobalt, which is thought to pass through Ca2+-permeable AMPA/kainate receptors following kainate exposure, labeled motor neurons in spinal cord cultures. The analysis of AMPA subunit distribution in dissociated motor neurons revealed a unique pattern of glutamate receptor (GluR) subunits in those cells; the GluR1 subunit was found in all spinal cord neurons, but the GluR2 subunit was not found in identified dissociated motor neurons. These data suggest that selective sensitivity of motor neurons to non-NMDA receptor activation is due, at least in part, to the presence of Ca2+-permeable AMPA/kainate receptors.

Motor neuron degeneration is attenuated in bax-deficient neurons in vitro

Apoptosis plays a major role in motor neuron survival during developmental cell death, after axotomy, and in motor neuron diseases. Bax is the first member of the bcl-2 family shown to promote apoptosis. In the present study, we used the bax-deficient mouse model to determine the role of bax in motor neuron survival in vitro by using dissociated spinal cord cultures. This system enables the maturation of individual motor neurons in a controlled in vitro environment. Motor neurons were identified by using the antineurofilament antibody SMI-32 and the antitranscription factor antibody Islet1. Both antibodies labeled large motor neurons in wild-type and bax-null cultures. Differentiated wild-type cultures exhibited a reduction in long-term cultures of two- and fivefold in the number of SMI-32- and Islet1-positive cells, respectively. The reduction in the number of motor neurons was attenuated in bax -/- cultures. Bax deficiency also attenuated serum withdrawal- and kainate-induced apoptosis in motor neurons. For comparison, necrotic cell death led to significant motor neuron cell death in both wild-type and bax -/- cultures. In addition, bax deficiency did not induce proliferation of motor neuron precursors in vitro. This study indicates for the first time that bax has a dominant role in the survival of long-term cultured motor neurons.

Changes in expression of neuronal and glial glutamate transporters in rat hippocampus following kainate-induced seizure activity

The expression of excitatory amino acid transporters (EAATs) in rat hippocampus was studied following kainic acid-induced seizure activity in vivo and in hippocampal slice cultures. Protein and mRNA levels of the glial (EAAT2) and neuronal (EAAT3) transporters were determined with affinity-purified antibodies and oligonucleotide probes, respectively. Kainate treatment decreased EAAT3 immunoreactivity in stratum lacunosum moleculare within 4 h of seizure onset. Upon pyramidal cell death (5 days after kainate treatment), EAAT3 immunoreactivity in stratum pyramidale of CA1 and in stratum lacunosum moleculare was almost completely eliminated. The rapid effect of kainate on EAAT3 expression was confirmed by in situ hybridization; EAAT3 mRNA levels were decreased in CA1 and CA3 regions within 4-8 h of seizure onset. Kainate treatment had an opposite effect on levels and expression of EAAT2. Developmental studies indicated that the rapid regulation of transporter expression was not observed in rats younger than 21 days, an observation congruent with previous reports regarding the resistance of young rats to kainate. In hippocampal organotypic cultures, which lack a major excitatory input from the entorhinal cortex, kainate produced a slow decrease in [3H]d-aspartate uptake. This study indicates that an early effect of kainate treatment consists of down-regulation of the neuronal transporter EAAT3 in restricted hippocampal regions, together with a modest increase in the expression of the glial transporter EAAT2. Differential regulation of neuronal and glial glutamate transporters may thus play a role in kainate-induced seizure, neurotoxicity and neuronal plasticity.

The copper chaperone CCS is abundant in neurons and astrocytes in human and rodent brain

Copper trafficking in mammalian cells is highly regulated. CCS is a copper chaperone that donates copper to the antioxidant enzyme copper/zinc superoxide dismutase 1 (SOD 1). Mutations of SOD1 are responsible for approximately 20% of familial amyotrophic lateral sclerosis (FALS). Monospecific antibodies were generated to evaluate the localization and cellular distribution of this copper chaperone in human and mouse brain as well as other organs. CCS is found to be ubiquitously expressed by multiple tissues and is present in particularly high concentrations in kidney and liver. In brain and spinal cord, CCS was found throughout the neuropil, with expression largely confined to neurons and some astrocytes. Like SOD1, CCS immunoreactivity was intense in Purkinje cells, deep cerebellar neurons, and pyramidal cortical neurons, whereas in spinal cord, CCS was highly expressed in motor neurons. In cortical neurons, CCS was present in the soma and proximal dendrites, as well as some axons. Although the distribution of CCS paralleled that of SOD1, there was a 12-30-fold molar excess of SOD1 over CCS. That both SOD1 and CCS are present, together, in cells that degenerate in ALS also emphasizes the potential role of CCS in mutant SOD1-mediated toxicity.

Influence of age and strain on striatal dopamine loss in a genetic mouse model of Lesch-Nyhan disease

Lesch-Nyhan disease is a neurogenetic disorder caused by deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Affected individuals exhibit a characteristic pattern of neurological and behavioral features attributable in part to dysfunction of basal ganglia dopamine systems. In the current studies, striatal dopamine loss was investigated in five different HPRT-deficient strains of mice carrying one of two different HPRT gene mutations. Caudoputamen dopamine concentrations were significantly reduced in all five of the strains, with deficits ranging from 50.7 to 61.1%. Mesolimbic dopamine was significantly reduced in only three of the five strains, with a range of 31.6-38.6%. The reduction of caudoputamen dopamine was age dependent, emerging between 4 and 12 weeks of age. Tyrosine hydroxylase and aromatic amino acid decarboxylase, two enzymes responsible for the synthesis of dopamine, were reduced by 22.4-37.3 and 22.2-43.1%, respectively. These results demonstrate that HPRT deficiency is strongly associated with a loss of basal ganglia dopamine. The magnitude of dopamine loss measurable is dependent on the genetic background of the mouse strain used, the basal ganglia subregion examined, and the age of the animals at assessment.

Molecular cloning and expression of the rat EAAT4 glutamate transporter subtype

Glutamate transport is a primary mechanism for the synaptic inactivation of glutamate. Excitatory amino acid transporter 4 (EAAT4) is a novel glutamate transporter with properties of a ligand-gated chloride channel that was recently cloned from human brain. Here we report the cloning of rat EAAT4 (rEAAT4) cDNA from rat cerebellum. The nucleotide sequence of rEAAT4 was 88% identical to the human sequence, and the predicted peptide was 89% identical to the human protein. The transport activity encoded by rEAAT4 has high affinity for L-glutamate. In Xenopus laevis oocytes expressing rEAAT4, L-glutamate and other transporter substrates elicited a current predominantly carried by chloride ions. Like human EAAT4, the rEAAT4 mRNA was largely restricted to cerebellar Purkinje cells; the rEAAT4 protein was localized to Purkinje cell somas and dendrites.

The genetic and molecular mechanisms of motor neuron disease

Significant progress has been made in the identification of genes and chromosomal loci associated with several types of motor neuron disease. Of particular interest is recent work on the pathogenic mechanisms underlying these diseases, especially studies in in vitro model systems and in transgenic and gene-targeted mice.

Glutamate transport in cultures from developing avian cerebellum: presence of GLT-1 immunoreactivity in Purkinje neurons

Immunocytochemical studies indicated that Purkinje cells cultured from chick embryonic cerebellum (embryonic day 8) strongly express a glutamate transporter EAAT2 cloned from human brain (GLT-1 in rat brain). At both 7 days and 14 days in culture, Purkinje neurons accumulated 1 microM [3H]L-glutamate via a potent "high-affinity" transport system that could be inhibited by D- and L-threo-3-hydroxyaspartate (D- and L-t-3OHA) and by L-trans-pyrrolidine-2,4-dicarboxylate (L-t-PDC). The order of potency of the three inhibitors was L-t-PDC approximately L-t-3OHA > D-t-30HA. Only the value of IC50 (concentration causing 50% inhibition) for D-t-3OHA significantly changed between 7 days (116 microM) and 14 days in culture (40 microM). All nH approximately 1, except in the case of the inhibition by D-t-3OHA at 14 days in culture (nH = 0.57), indicating the possible appearance of heterogeneity of the transport sites at later stages of culturing. Chronic inhibition of L-glutamate transport by L-t-PDC resulted in major changes in the morphology of Purkinje cells; particularly, the neurites almost completely regressed.

Use of antisense oligodeoxynucleotides to inhibit expression of glutamate transporter subtypes

Antisense ODN are highly effective tools to selectively inhibit synthesis of glutamate transporter subtypes. They have been used to evaluate the biology of individual transporter subtypes in vivo and in vitro. Appropriate use of antisense ODN, however, requires a number of important controls to validate the specificity of their effects. Ultimately, the efficacy at inhibiting the synthesis of transporters proteins reflects several variables: the actual efficacy of the antisense to directly inhibit synthesis, the rate of degradation of the ODN, the rate of intracellular penetration of the ODN, the rate of new protein synthesis for the transporter subtype, and the actual penetration of the ODN into CNS tissue. The methods described in this chapter will help one to optimize each of these parameters.

Regional and cellular expression of glial (GLT1) and neuronal (EAAC1) glutamate transporter proteins in ovine fetal brain

Glutamate transport is a primary mechanism for regulating extracellular levels of glutamate which can have either neurotrophic or neurotoxic effects in the developing brain, depending on its concentration. Using immunoblotting and immunocytochemistry, we tested the hypotheses that expression of neuronal and glial glutamate transporter proteins was regionally and temporally regulated in the developing ovine brain and that expression of the glial isoform early in development was not cell-type specific. Immunoblots for the neuronal glutamate transporter EAAC1 revealed a major band of immunoreactivity at 69,000 nmol. wt, whereas glial glutamate transporter-1 (GLT1) immunoreactivity was observed as 73,000 and 146,000 mol. wt proteins. EAAC1 and GLT1 are regulated differently during development, with EAAC1 immunoreactivity being most abundant at 60 and 71 days completed gestation (term=145 days) and dissipating thereafter, while GLT1 immunoreactivity was most abundant at 136 days gestation. By immunocytochemistry EAAC1 expression is neuronal throughout gestation with intense labelling of dendrites within the telencephalon evident at 60 days. Neuropil, neuronal cell bodies and processes are EAAC1-immunoreactive throughout gestation with no evidence of astrocytic or oligodendroglial immunoreactivity. In contrast, GLT1 is expressed by neuronal and non-neuronal cell types during midgestation with astrocyte selectivity developing by 136 days. During midgestation, GLT1 is transiently expressed in neurons of the subplate, cranial nerve nuclei, basal ganglia, and cerebellar cortex. The major finding of this study, that GLT1 is transiently expressed in various neuronal populations at midgestation demonstrates that the cell-type specificity of the GLT1 phenotype is developmentally regulated and depends on brain maturity.

Arachidonic acid activates a proton current in the rat glutamate transporter EAAT4

The excitatory amino acid transporter EAAT4 is expressed predominantly in Purkinje neurons in the rat cerebellum (1-3), and it participates in postsynaptic reuptake of glutamate released at the climbing fiber synapse (4). Transporter-mediated currents in Purkinje neurons are increased more than 3-fold by arachidonic acid, a second messenger that is liberated following depolarization-induced Ca2+ activation of phospholipase A2 (5). In this study we demonstrate that application of arachidonic acid to oocytes expressing rat EAAT4 increased glutamate-induced currents to a similar extent. However, arachidonic acid did not cause an increase in the rate of glutamate transport or in the chloride current associated with glutamate transport but rather activated a proton-selective conductance. These data reveal a novel action of arachidonate on a glutamate transporter and suggest a mechanism by which synaptic activity may decrease intracellular pH in neurons where this transporter is localized.

Neuronal expression of the glutamate transporter GLT-1 in hippocampal microcultures

To address the question of the relative contributions of glial and neuronal glutamate transport in the vertebrate CNS, we studied the distribution of forebrain glutamate transporters in rat hippocampal microcultures, a preparation in which physiological functions of glutamate transporters have been well characterized. Two of the three transporters, GLAST (EAAT1) and EAAC1 (EAAT3), are localized to microculture glia and neurons, respectively, as expected. However, we find strong immunoreactivity for the third glutamate transporter GLT-1 (EAAT2), a putatively glial transporter, in microculture neurons and in a small subset of microculture glia. Indistinguishable immunohistochemical staining patterns for GLT-1 were obtained with antibodies directed against both the N terminal and C terminal of the GLT-1 protein. Double-labeling experiments suggest that neuronal GLT-1 protein is primarily localized to the dendrites of excitatory neurons. Neuronal electrogenic transport currents in response to D-aspartate applications were occluded by the selective GLT-1 inhibitor dihydrokainate. In contrast, glia exhibited a larger transporter current density than did neurons, and the glial transport current was less sensitive to dihydrokainate. Neuronal transport currents were potentiated less than were glial currents when the chaotropic anion thiocyanate was substituted for gluconate in the whole-cell recording pipette, consistent with the previously reported lower anion permeability of EAAC1 and GLT-1 compared with that of GLAST. After microculture glia were rendered nonviable, excitatory autaptic currents (EACs) were prolonged in the presence of dihydrokainate, suggesting that neuronal GLT-1 is capable of participating in the clearance of synaptically released glutamate. Our results suggest that the initially proposed characterization of GLT-1 as a purely glial transporter is too simplistic and that under certain conditions functional GLT-1 protein can be expressed in brain neurons. The study suggests that changes in GLT-1 levels that occur with pathology or experimental manipulations cannot be assumed to be glial.

Structural selectivity and molecular nature of L-glutamate transport in cultured human fibroblasts

Uptake of L-[3H]glutamate by monolayers of fibroblasts cultured from human embryonic skin has been studied in the presence of several nonradioactive structural analogs of glutamate and aspartate. Results have suggested that the structural specificites of glutamate transporters in cultured human fibroblasts are similar to those of glutamate transporters in the mammalian brain. Only subtle differences have been detected: in the mammalian cerebral cortex, enantiomers of threo-3-hydroxyaspartate are almost equipotent as inhibitors of L-[3H]glutamate uptake while, in human fibroblasts, the D-isomer has been found to be an order of magnitude less potent than the corresponding L-isomer. Kinetic analysis of a model in which substrates are recognized by the glutamate transporter binding site(s) as both alpha- and beta-amino acids indicated that such a mechanism cannot explain the apparent negative cooperativity characterizing the effects of D- and L-aspartate. Molecular modeling has been used to estimate the optimum conformation of L-glutamate as it interacts with the transporter(s). Flow cytometry has indicated that all fibroblasts in culture express at least moderate levels of four glutamate transporters cloned from human brain. Small subpopulations (< 3%) of cells, however, were strongly labeled with antibodies against EAAT1 (GLAST) and EAAT2 (GLT-1) transporters. We conclude that these two transporters--known to be strongly expressed in brain tissue--can be principally responsible for the "high affinity" transport of glutamate also in nonneural cells.

Traumatic brain injury down-regulates glial glutamate transporter (GLT-1 and GLAST) proteins in rat brain

Excess activation of NMDA receptors is felt to participate in secondary neuronal damage after traumatic brain injury (TBI). Increased extracellular glutamate is active in this process and may result from either increased release or decreased reuptake. The two high-affinity sodium-dependent glial transporters [glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST)] mediate the bulk of glutamate transport. We studied the protein levels of GLT-1 and GLAST in the brains of rats after controlled cortical impact-induced TBI. With use of subtype-specific antibodies, GLT-1 and GLAST proteins were quantitated by immunoblotting in the ipsilateral and contralateral cortex at 2, 6, 24, 72, and 168 h after the injury. Sham-operated rats served as control. TBI resulted in a significant decrease in GLT-1 (by 20-45%; p < 0.05) and GLAST (by 30-50%; p < 0.05) protein levels between 6 and 72 h after the injury. D-[3H]Aspartate binding also decreased significantly (by 30-50%; p < 0.05) between 6 and 72 h after the injury. Decreased glial glutamate transporter function may contribute to the increased extracellular glutamate that may mediate the excitotoxic neuronal damage after TBI. This is a first report showing altered levels of glutamate transporter proteins after TBI.