Astrocytic glutamate transporter 1 (GLT1) deficient mice exhibit repetitive behaviors

Glutamatergic dysregulation is known to contribute to obsessive-compulsive disorder (OCD). Astrocytic glutamate transporter 1 (GLT1) is responsible for the majority of glutamate clearance. However, the role of GLT1 in OCD-like behavior remains unclear. Here, we found that astrocytic GLT1 deficient mice showed increased wheel running activity but reduced home cage activity. Notably, they exhibited elevated grooming/rearing time and increased repetitive behavior counts in contextual and cued fear conditioning. In addition, they showed increased rearing counts in the metabolic chamber, and also augmented rearing time and jumping counts in the open field test. Taken together, our findings suggest that astrocytic GLT1 deficiency promotes OCD-like repetitive behaviors.

Divergent roles of astrocytic versus neuronal EAAT2 deficiency on cognition and overlap with aging and Alzheimer's molecular signatures

The excitatory amino acid transporter 2 (EAAT2) is the major glutamate transporter in the brain expressed predominantly in astrocytes and at low levels in neurons and axonal terminals. EAAT2 expression is reduced in aging and sporadic Alzheimer's disease (AD) patients' brains. The role EAAT2 plays in cognitive aging and its associated mechanisms remains largely unknown. Here, we show that conditional deletion of astrocytic and neuronal EAAT2 results in age-related cognitive deficits. Astrocytic, but not neuronal EAAT2, deletion leads to early deficits in short-term memory and in spatial reference learning and long-term memory. Neuronal EAAT2 loss results in late-onset spatial reference long-term memory deficit. Neuronal EAAT2 deletion leads to dysregulation of the kynurenine pathway, and astrocytic EAAT2 deficiency results in dysfunction of innate and adaptive immune pathways, which correlate with cognitive decline. Astrocytic EAAT2 deficiency also shows transcriptomic overlaps with human aging and AD. Overall, the present study shows that in addition to the widely recognized astrocytic EAAT2, neuronal EAAT2 plays a role in hippocampus-dependent memory. Furthermore, the gene expression profiles associated with astrocytic and neuronal EAAT2 deletion are substantially different, with the former associated with inflammation and synaptic function similar to changes observed in human AD and gene expression changes associated with inflammation similar to the aging human brain.

Decreased glial and synaptic glutamate uptake in the striatum of HIV-1 gp120 transgenic mice

The mechanisms leading to the neurocognitive deficits in humans with immunodeficiency virus type 1 (HIV-1) are not well resolved. A number of cell culture models have demonstrated that the HIV-envelope glycoprotein 120 (gp120) decreases the reuptake of glutamate, which is necessary for learning, memory, and synaptic plasticity. However, the impact of brain HIV-1 gp120 on glutamate uptake systems in vivo remains unknown. Notably, alterations in brain glutamate uptake systems are implicated in a number of neurodegenerative and neurocognitive disorders. We characterized the kinetic properties of system XAG (sodium-dependent) and systems xc- (sodium-independent) [3H]-L-glutamate uptake in the striatum and hippocampus of HIV-1 gp120 transgenic mice, an established model of HIV neuropathology. We determined the kinetic constant Vmax (maximal velocity) and Km (affinity) of both systems XAG and xc- using subcellular preparations derived from neurons and glial cells. We show significant (30-35 %) reductions in the Vmax of systems XAG and xc- in both neuronal and glial preparations derived from the striatum, but not from the hippocampus of gp120 mice relative to wild-type (WT) controls. Moreover, immunoblot analysis showed that the protein expression of glutamate transporter subtype-1 (GLT-1), the predominant brain glutamate transporter, was significantly reduced in the striatum but not in the hippocampus of gp120 mice. These extensive and region-specific deficits of glutamate uptake likely contribute to the development and/or severity of HIV-associated neurocognitive disorders. Understanding the role of striatal glutamate uptake systems in HIV-1 gp120 may advance the development of new therapeutic strategies to prevent neuronal damage and improve cognitive function in HIV patients.

Impaired glutamate transport in a mouse model of tau pathology in astrocytes

Filamentous tau inclusions in neurons and glia are neuropathological hallmarks of tauopathies. The discovery of microtubule-associated protein tau gene mutations that are pathogenic for a heterogenous group of neurodegenerative disorders, called frontotemporal dementia and parkinsonism linked to chromosome-17 (FTDP-17), directly implicate tau abnormalities in the onset/progression of disease. Although the role of tau pathology in neurons in disease pathogenesis is well accepted, the contribution of glial pathology is essentially unknown. We recently generated a transgenic (Tg) mouse model of tau pathology in astrocytes by expressing the human tau protein under the control of the glial fibrillary acidic protein (GFAP) promoter. Both wild-type and FTDP-17 mutant GFAP/tau Tg animals manifest an age-dependent accumulation of tau inclusions in astrocytes that resembles the pathology observed in human tauopathies. We further demonstrate that both strains of Tg mice manifest compromised motor function that correlates with altered expression of the glial glutamate-aspartate transporter and occurs before the development of tau pathology. Subsequently, the Tg mice manifest additional deficits in neuromuscular strength that correlates with reduced expression of glutamate transporter-1 (GLT-1) and occurs concurrent with tau inclusion pathology. Reduced GLT-1 expression was associated with a progressive decrease in sodium-dependent glutamate transport capacity. Reductions in GLT-1 expression were also observed in corticobasal degeneration, a tauopathy with prominent pathology in astrocytes. Less robust changes were observed in Alzheimer's disease in which neuronal tau pathology predominates. Thus, these Tg mice recapitulate features of astrocytic pathology observed in tauopathies and implicate a role for altered astrocyte function in the pathogenesis of these disorders.

Decreased metabolic response to visual stimulation in the superior colliculus of mice lacking the glial glutamate transporter GLT-1

During a specific task, an increase in glucose utilization anatomically restricted to the functionally activated region(s) is a landmark of brain physiology. While this response represents the biological bases for functional brain imaging, the underlying signalling pathway(s) are still not fully characterized. Recent evidence suggests that glial glutamate (re)uptake plays a key role. We provide evidence that the metabolic response to synaptic activation (i.e. enhancement of glucose uptake) is decreased in the superior colliculus during visual stimulation in young adult mice deficient in the glial glutamate transporter GLT-1. A similar reduction was not observed in the glial glutamate transporter GLAST-knockout mice. Consistent with our previous observation obtained in the somatosensory cortex, our data suggest that a metabolic crosstalk takes place between neurons and astrocytes in the adult brain which would be regulated by synaptic activity and mediated by GLT-1.

Altered expression of the glutamate transporter EAAT2b in neurological disease

Functional studies suggest that up to 95% of all glutamate transport is handled by the glutamate transporter EAAT2. Amino and C-terminal antibodies demonstrate that under normal conditions EAAT2 is specific to astrocytes. A truncated splice variant of EAAT2, known as EAAT2b, also has been identified in astrocytes and some neurons. In vitro studies suggest EAAT2b transports glutamate similar to EAAT2, although the contribution of EAAT2b to normal clearance of extracellular glutamate is unknown. To investigate EAAT2b biology in pathological conditions, we examined the cellular and regional distribution of EAAT2b in amyotrophic lateral sclerosis. Using epitope-specific, affinity purified antibodies, we found that EAAT2b tissue levels were increased by more than twofold in amyotrophic lateral sclerosis motor cortex, whereas EAAT2 levels were decreased by up to 95%. EAAT2b distribution in normal human cortex was largely confined to the neuropil-like EAAT2, with occasional faint neuronal expression. In contrast, amyotrophic lateral sclerosis motor cortex had an obvious qualitative increase in neuropil EAAT2b staining and a drastic increase in neuronal soma and dendritic EAAT2b immunostaining. Despite these increases in EAAT2b immunostaining, functional transporter studies demonstrated a large loss of EAAT2 function. These studies clearly document altered regulation and splicing of the dominant glutamate transporter EAAT2 under conditions of neurological stress.

Impaired spinal cord glutamate transport capacity and reduced sensitivity to riluzole in a transgenic superoxide dismutase mutant rat model of amyotrophic lateral sclerosis

We characterized synaptosomal glutamate transport activity in a recently developed transgenic rat model of amyotrophic lateral sclerosis (ALS) overexpressing the G93A Cu(2+)/Zn(2+) superoxide dismutase (SOD1) mutation. Using spinal cord synaptosomes, a significant reduction (43%) in the maximal velocity for high-affinity, Na(+)-dependent glutamate uptake was observed at disease end stage in G93A rats compared with age-matched controls. Similarly, a 27% reduction in maximum velocity (V(max)) was measured at disease onset, but no difference in spinal cord V(max) values were observed with presymptomatic animals compared with controls. In comparison, we observed no differences in the V(max) for glutamate clearance at disease end stage with synaptosomes from cortex, hippocampus, striatum, cerebellum, and brainstem, indicating a specific deficit in the spinal cord. The pharmacological sensitivity of spinal cord uptake to dihydrokainate suggests that the GLT-1 (glutamate transporter-1) subtype primarily mediates the transport activity. Expression analysis revealed a loss of GLT-1 as well as qualitative changes in GLAST (glutamate/aspartate transporter) but no measurable changes in EAAC1 (excitatory amino acid carrier 1) in spinal cord of end-stage G93A rats, indicating that deficits in glutamate transporters in this rat model may be glial specific. Riluzole, a neuroprotective agent used clinically to slow the progression of ALS, produced an enhancement of spinal cord synaptosomal glutamate uptake in control animals and early-stage disease G93A rats, but this effect was lost in end-stage animals. Altered expression of astroglial glutamate transporters accompanied by reduced capacity for spinal cord clearance of extracellular glutamate in the G93A SOD1 transgenic rat may account for a dampened effect of riluzole to enhance glutamate uptake at end-stage disease.

Human glioma cells and undifferentiated primary astrocytes that express aberrant EAAT2 mRNA inhibit normal EAAT2 protein expression and prevent cell death

Abnormal splicing of astroglial glutamate transporter EAAT2 mRNA has been suggested to account for the loss of EAAT2 protein in amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD). We have identified several clones of human U251 glioma cells which express varying amounts of aberrantly spliced EAAT2 mRNA; these clones do not express any detectable EAAT2 protein. When the wild-type EAAT2 cDNA was expressed in each of these clones, we found that the amount of EAAT2 protein inversely correlated with the levels of endogenous aberrant EAAT2 mRNA. We also observed that ectopic expression of normal EAAT2 protein is toxic to U251 cells as well as to undifferentiated primary astrocytes. We conclude that expression of aberrant EAAT2 mRNA may be one possible mechanism to repress normal EAAT2 protein expression. The implication of this study for the mechanisms of EAAT2 protein loss in ALS and AD is discussed.

Oxidative stress and disturbed glutamate transport in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a hereditary motor neuron disease, and three clinical subtypes of autosomal recessive SMA, including Werdnig Hoffmann disease (type 1), have been shown to be induced by deletion within the same genes. In order to clarify the pathogenesis of motor neuron degeneration in SMA, we immunohistochemically examine the expressions of oxidative stress-related materials (oxidative products) and glutamate transporters, which can prevent glutamate neurotoxicity, in five autopsy cases of SMA type 1. Age-matched controls did not show any deposition of oxidative products in the brain. In contrast, the abnormal deposition of 4-hydroxy-2-nonenal-modified protein, a product of membrane lipid oxidation, was observed in the spinal motor neurons in three cases, although the motor neurons did not show an increase of nitrotyrosine, which was observed in adult-onset amyotrophic lateral sclerosis. In addition, the nuclei of neurons and glial cells in the precentral gyrus, thalamus or cerebellar cortex were immunoreactive for 8-hydroxy-2'-deoxyguanosine in two cases, which was one of the most commonly used markers for oxidative DNA damage. Regarding glial glutamate transporters, three of five cases of SMA type 1 showed a reduction in immunoreactivity for excitatory amino acid transporter-1 (GLAST) in the ventrolateral nucleus of the thalamus, in which there was neither neuronal loss nor gliosis in routine histochemistry. One case, having mechanical ventilation, demonstrated a reduced expression of another glial glutamate transporter (GLT-1) throughout the central nervous system. These data suggest that oxidative stress and disturbed glutamate transport can partly be involved in the motor neuron devastation and/or latent thalamic degeneration in SMA type 1.

A new neuromodulatory pathway with a glial contribution mediated via A(2a) adenosine receptors

A low concentration (10 nM) of adenosine potentiated hippocampal neuronal activity via A(2a) adenosine receptors without affecting presynaptic glutamate release or postsynaptic glutamatergic conductance. Adenosine inhibited glutamate uptake through the glial glutamate transporter, GLT-1, via A(2a) adenosine receptors. In addition, adenosine stimulated GLT-1-independent glutamate release from astrocytes, possibly in response to a rise in intracellular Ca(2+), via A(2a) adenosine receptors involving PKA activation. Those adenosine actions could lead to an increase in synaptic glutamate concentrations responsible for the potentiation of hippocampal neuronal activity. The results of the present study thus represent a novel neuromodulatory pathway with a glial contribution, bearing both inhibition of GLT-1 function and stimulation of glial glutamate release, as mediated via A(2a) adenosine receptors.

Inhibition of glial glutamate transporter GLT-1 augments brain edema after transient focal cerebral ischemia in mice

Excessively released glutamate is neurotoxic. Glutamate transporters maintain the extracellular level of glutamate by uptake into glia or neurons. We examined the role of GLT-1, a glial glutamate transporter, in brain damage resulting from transient focal ischemia in mice. Heterozygous gene deletion of GLT-1 significantly augmented brain swelling resulting from 1 h of middle cerebral artery occlusion and 24 h reperfusion. In addition, this gene deletion significantly increased brain water contents in ischemic hemisphere at 6 h after reperfusion. Moreover, intraperitoneal injection of dihydrokainate (10 mg/kg), a specific inhibitor of GLT-1, augmented brain swelling. These data suggest that GLT-1 limits brain edema resulting from ischemia.

Decreased expression of glutamate transporters in genetic absence epilepsy rats before seizure occurrence

In absence epilepsy, epileptogenic processes are suspected of involving an imbalance between GABAergic inhibition and glutamatergic excitation. Here, we describe alteration of the expression of glutamate transporters in rats with genetic absence (the Genetic Absence Epilepsy Rats from Strasbourg: GAERS). In these rats, epileptic discharges, recorded in the thalamo-cortical network, appear around 40 days after birth. In adult rats no alteration of the protein expression of the glutamate transporters was observed. In 30-day-old GAERS protein levels (quantified by western blot) were lower in the cortex by 21% and 35% for the glial transporters GLT1 and GLAST, respectively, and by 32% for the neuronal transporter EAAC1 in the thalamus compared to control rats. In addition, the expression and activity of GLAST were decreased by 50% in newborn GAERS cortical astrocytes grown in primary culture. The lack of modification of the protein levels of glutamatergic transporters in adult epileptic GAERS, in spite of mRNA variations (quantified by RT-PCR), suggests that they are not involved in the pathogeny of spike-and-wave discharges. In contrast, the alteration of glutamate transporter expression, observed before the establishment of epileptic discharges, could reflect an abnormal maturation of the glutamatergic neurone-glia circuitry.

Focal loss of the glutamate transporter EAAT2 in a transgenic rat model of SOD1 mutant-mediated amyotrophic lateral sclerosis (ALS)

Transgenic overexpression of Cu(+2)/Zn(+2) superoxide dismutase 1 (SOD1) harboring an amyotrophic lateral sclerosis (ALS)-linked familial genetic mutation (SOD1(G93A)) in a Sprague-Dawley rat results in ALS-like motor neuron disease. Motor neuron disease in these rats depended on high levels of mutant SOD1 expression, increasing from 8-fold over endogenous SOD1 in the spinal cord of young presymptomatic rats to 16-fold in end-stage animals. Disease onset in these rats was early, approximately 115 days, and disease progression was very rapid thereafter with affected rats reaching end stage on average within 11 days. Pathological abnormalities included vacuoles initially in the lumbar spinal cord and subsequently in more cervical areas, along with inclusion bodies that stained for SOD1, Hsp70, neurofilaments, and ubiquitin. Vacuolization and gliosis were evident before clinical onset of disease and before motor neuron death in the spinal cord and brainstem. Focal loss of the EAAT2 glutamate transporter in the ventral horn of the spinal cord coincided with gliosis, but appeared before motor neuron/axon degeneration. At end-stage disease, gliosis increased and EAAT2 loss in the ventral horn exceeded 90%, suggesting a role for this protein in the events leading to cell death in ALS. These transgenic rats provide a valuable resource to pursue experimentation and therapeutic development, currently difficult or impossible to perform with existing ALS transgenic mice.

Impaired glutamate uptake in the R6 Huntington's disease transgenic mice

Huntington's disease (HD) is a late-onset neurodegenerative disease for which the mutation is CAG/polyglutamine repeat expansion. The R6 mouse lines expressing the HD mutation develop a movement disorder that is preceded by the formation of neuronal polyglutamine aggregates. The phenotype is likely caused by a widespread neuronal dysfunction, whereas neuronal cell death occurs late and is very selective. We show that a decreased mRNA level of the major astroglial glutamate transporter (GLT1) in the striatum and cortex of these mice is accompanied by a concomitant decrease in glutamate uptake. In contrast, the expression of the glutamate transporters, GLAST and EAAC1, remain unchanged. The mRNA level of the astroglial enzyme glutamine synthetase is also decreased. These changes in expression occur prior to any evidence of neurodegeneration and suggest that a defect in astrocytic glutamate uptake may contribute to the phenotype and neuronal cell death in HD.