Glutamate transporter EAAT2 splice variants occur not only in ALS, but also in AD and controls

Amyotrophic Lateral Sclerosis: A Neurodegenerative Motor Neuron Disease With Ocular Involvement

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that causes degeneration of the lower and upper motor neurons and is the most prevalent motor neuron disease. This disease is characterized by muscle weakness, stiffness, and hyperreflexia. Patients survive for a short period from the onset of the disease. Most cases are sporadic, with only 10% of the cases being genetic. Many genes are now known to be involved in familial ALS cases, including some of the sporadic cases. It has also been observed that, in addition to genetic factors, there are numerous molecular mechanisms involved in these pathologies, such as excitotoxicity, mitochondrial disorders, alterations in axonal transport, oxidative stress, accumulation of misfolded proteins, and neuroinflammation. This pathology affects the motor neurons, the spinal cord, the cerebellum, and the brain, but recently, it has been shown that it also affects the visual system. This impact occurs not only at the level of the oculomotor system but also at the retinal level, which is why the retina is being proposed as a possible biomarker of this pathology. The current review discusses the main aspects mentioned above related to ALS, such as the main genes involved, the most important molecular mechanisms that affect this pathology, its ocular involvement, and the possible usefulness of the retina as a biomarker.

Astrocytes in Motor Neuron Diseases

Motor neuron disorders are highly debilitating and mostly fatal conditions for which only limited therapeutic options are available. To overcome this limitation and develop more effective therapeutic strategies, it is critical to discover the pathogenic mechanisms that trigger and sustain motor neuron degeneration with the greatest accuracy and detail. In the case of Amyotrophic Lateral Sclerosis (ALS), several genes have been associated with familial forms of the disease, whilst the vast majority of cases develop sporadically and no defined cause can be held responsible. On the contrary, the huge majority of Spinal Muscular Atrophy (SMA) occurrences are caused by loss-of-function mutations in a single gene, SMN1. Although the typical hallmark of both diseases is the loss of motor neurons, there is increasing awareness that pathological lesions are also present in the neighbouring glia, whose dysfunction clearly contributes to generating a toxic environment in the central nervous system. Here, ALS and SMA are sequentially presented, each disease section having a brief introduction, followed by a focussed discussion on the role of the astrocytes in the disease pathogenesis. Such a dissertation is substantiated by the findings that built awareness on the glial involvement and how the glial-neuronal interplay is perturbed, along with the appraisal of this new cellular site for possible therapeutic intervention.

The role of glutamate transporters in the pathophysiology of neuropsychiatric disorders

Altered glutamate transporter expression is a common feature of many neuropsychiatric conditions, including schizophrenia. Excitatory amino acid transporters (EAATs) are responsible for the reuptake of glutamate, preventing non-physiological spillover from the synapse. Postmortem studies have revealed significant dysregulation of EAAT expression in various brain regions at the cellular and subcellular level. Recent animal studies have also demonstrated a role for glutamate spillover as a mechanism of disease. In this review, we describe current evidence for the role of glutamate transporters in regulating synaptic plasticity and transmission. In neuropsychiatric conditions, EAAT splice variant expression is altered. There are changes in the localization of the transporters and disruption of the metabolic and structural protein network that supports EAAT activity. This results in aberrant neuroplasticity and excitatory signaling, contributing to the symptoms associated with neuropsychiatric disease. Understanding the complex functions of glutamate transporters will clarify the relevance of their role in the pathophysiology of neuropsychiatric disorders.

EAAT2 and the Molecular Signature of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a rapid and fatal neurodegenerative disease, primarily affecting upper and lower motor neurons. It is an extremely heterogeneous disease in both cause and symptom development, and its mechanisms of pathogenesis remain largely unknown. Excitotoxicity, a process caused by excessive glutamate signaling, is believed to play a substantial role, however. Excessive glutamate release, changes in postsynaptic glutamate receptors, and reduction of functional astrocytic glutamate transporters contribute to excitotoxicity in ALS. Here, we explore the roles of each, with a particular emphasis on glutamate transporters and attempts to increase them as therapy for ALS. Screening strategies have been employed to find compounds that increase the functional excitatory amino acid transporter EAAT2 (GLT1), which is responsible for the vast majority of glutamate clearance. One such compound, ceftriaxone, was recently tested in clinical trials but unfortunately did not modify disease course, though its effect on EAAT2 expression in patients was not measured.

Neuronal vs glial glutamate uptake: Resolving the conundrum

Neither normal brain function nor the pathological processes involved in neurological diseases can be adequately understood without knowledge of the release, uptake and metabolism of glutamate. The reason for this is that glutamate (a) is the most abundant amino acid in the brain, (b) is at the cross-roads between several metabolic pathways, and (c) serves as the major excitatory neurotransmitter. In fact most brain cells express glutamate receptors and are thereby influenced by extracellular glutamate. In agreement, brain cells have powerful uptake systems that constantly remove glutamate from the extracellular fluid and thereby limit receptor activation. It has been clear since the 1970s that both astrocytes and neurons express glutamate transporters. However the relative contribution of neuronal and glial transporters to the total glutamate uptake activity, however, as well as their functional importance, has been hotly debated ever since. The present short review provides (a) an overview of what we know about neuronal glutamate uptake as well as an historical description of how we got there, and (b) a hypothesis reconciling apparently contradicting observations thereby possibly resolving the paradox.

A comprehensive review of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late-onset fatal neurodegenerative disease affecting motor neurons with an incidence of about 1/100,000. Most ALS cases are sporadic, but 5–10% of the cases are familial ALS. Both sporadic and familial ALS (FALS) are associated with degeneration of cortical and spinal motor neurons. The etiology of ALS remains unknown. However, mutations of superoxide dismutase 1 have been known as the most common cause of FALS. In this study, we provide a comprehensive review of ALS. We cover all aspects of the disease including epidemiology, comorbidities, environmental risk factor, molecular mechanism, genetic factors, symptoms, diagnostic, treatment, and even the available supplement and management of ALS. This will provide the reader with an advantage of receiving a broad range of information about the disease.

Glutamate transporter splice variant expression in an enriched pyramidal cell population in schizophrenia

Dysregulation of the glutamate transporters EAAT1 and EAAT2 and their isoforms have been implicated in schizophrenia. EAAT1 and EAAT2 expression has been studied in different brain regions but the prevalence of astrocytic glutamate transporter expression masks the more subtle changes in excitatory amino acid transporters (EAATs) isoforms in neurons in the cortex. Using laser capture microdissection, pyramidal neurons were cut from the anterior cingulate cortex of postmortem schizophrenia (n = 20) and control (n = 20) subjects. The messenger RNA (mRNA) levels of EAAT1, EAAT2 and the splice variants EAAT1 exon9skipping, EAAT2 exon9skipping and EAAT2b were analyzed by real time PCR (RT-PCR) in an enriched population of neurons. Region-level expression of these transcripts was measured in postmortem schizophrenia (n = 25) and controls (n = 25). The relationship between selected EAAT polymorphisms and EAAT splice variant expression was also explored. Anterior cingulate cortex pyramidal cell expression of EAAT2b mRNA was increased (P < 0.001; 67%) in schizophrenia subjects compared with controls. There was no significant change in other EAAT variants. EAAT2 exon9skipping mRNA was increased (P < 0.05; 38%) at region level in the anterior cingulate cortex with no significant change in other EAAT variants at region level. EAAT2 single-nucleotide polymorphisms were significantly associated with changes in EAAT2 isoform expression. Haloperidol decanoate-treated animals, acting as controls for possible antipsychotic effects, did not have significantly altered neuronal EAAT2b mRNA levels. The novel finding that EAAT2b levels are increased in populations of anterior cingulate cortex pyramidal cells further demonstrates a role for neuronal glutamate transporter splice variant expression in schizophrenia.

Astrocyte physiopathology: At the crossroads of intercellular networking, inflammation and cell death

Recent breakthroughs in neuroscience have led to the awareness that we should revise our traditional mode of thinking and studying the CNS, i.e. by isolating the privileged network of "intelligent" synaptic contacts. We may instead need to contemplate all the variegate communications occurring between the different neural cell types, and centrally involving the astrocytes. Basically, it appears that a single astrocyte should be considered as a core that receives and integrates information from thousands of synapses, other glial cells and the blood vessels. In turn, it generates complex outputs that control the neural circuitry and coordinate it with the local microcirculation. Astrocytes thus emerge as the possible fulcrum of the functional homeostasis of the healthy CNS. Yet, evidence indicates that the bridging properties of the astrocytes can change in parallel with, or as a result of, the morphological, biochemical and functional alterations these cells undergo upon injury or disease. As a consequence, they have the potential to transform from supportive friends and interactive partners for neurons into noxious foes. In this review, we summarize the currently available knowledge on the contribution of astrocytes to the functioning of the CNS and what goes wrong in various pathological conditions, with a particular focus on Amyotrophic Lateral Sclerosis, Alzheimer's Disease and ischemia. The observations described convincingly demonstrate that the development and progression of several neurological disorders involve the de-regulation of a finely tuned interplay between multiple cell populations. Thus, it seems that a better understanding of the mechanisms governing the integrated communication and detrimental responses of the astrocytes as well as their impact towards the homeostasis and performance of the CNS is fundamental to open novel therapeutic perspectives.

Association of rare variation in the glutamate receptor gene SLC1A2 with susceptibility to bipolar disorder and schizophrenia

The SLC1A2 gene encodes the excitatory amino acid transporter 2 (EAAT2). Glutamate is the major mediator of excitatory neurotransmission and EAAT2 is responsible for clearing the neurotransmitter from the synaptic cleft. Genetic variation in SLC1A2 has been implicated in a range of neurological and neuropsychiatric conditions including schizophrenia (SZ), autism and in core phenotypes of bipolar disorder (BD). The coding and putative regulatory regions of SLC1A2 gene were screened for variants using high resolution melting or sequenced in 1099 or in 32 BD subjects. Thirty-two variants were detected in the SLC1A2 gene. Fifteen potentially etiological variants were selected for genotyping in 1099 BD and 1095 control samples. Five amino acid changing variants were also genotyped in 630 participants suffering from SZ. None of the variants were found to be associated with BD or SZ or with the two diseases combined. However, two recurrent missense variants (rs145827578:G>A, p.(G6S); rs199599866:G>A, p.(R31Q)) and one recurrent 5′-untranslated region (UTR) variant (ss825678885:G>T) were detected in cases only. Combined analysis of the recurrent-case-only missense variants and of the case-only missense and 5′-UTR variants showed nominal evidence for association with the combined diseases (Fisher's P=0.019 and 0.0076). These findings are exploratory in nature and await replication in larger cohorts, however, they provide intriguing evidence that potentially functional rare variants in the SLC1A2 gene may confer susceptibility to psychotic disorders.

Genetic dys-regulation of astrocytic glutamate transporter EAAT2 and its implications in neurological disorders and manganese toxicity

Astrocytic glutamate transporters, the excitatory amino acid transporter (EAAT) 2 and EAAT1 (glutamate transporter 1 and glutamate aspartate transporter in rodents, respectively), are the main transporters for maintaining optimal glutamate levels in the synaptic clefts by taking up more than 90% of glutamate from extracellular space thus preventing excitotoxic neuronal death. Reduced expression and function of these transporters, especially EAAT2, has been reported in numerous neurological disorders, including amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, schizophrenia and epilepsy. The mechanism of down-regulation of EAAT2 in these diseases has yet to be fully established. Genetic as well as transcriptional dys-regulation of these transporters by various modes, such as single nucleotide polymorphisms and epigenetics, resulting in impairment of their functions, might play an important role in the etiology of neurological diseases. Consequently, there has been an extensive effort to identify molecular targets for enhancement of EAAT2 expression as a potential therapeutic approach. Several pharmacological agents increase expression of EAAT2 via nuclear factor κB and cAMP response element binding protein at the transcriptional level. However, the negative regulatory mechanisms of EAAT2 have yet to be identified. Recent studies, including those from our laboratory, suggest that the transcriptional factor yin yang 1 plays a critical role in the repressive effects of various neurotoxins, such as manganese (Mn), on EAAT2 expression. In this review, we will focus on transcriptional epigenetics and translational regulation of EAAT2.

Mechanisms of glutamate transport

L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.

Impairment of neuronal glutamate uptake and modulation of the glutamate transporter GLT-1 induced by retinal ischemia

Excitotoxicity has been implicated in the retinal neuronal loss in several ocular pathologies including glaucoma. Dysfunction of Excitatory Amino Acid Transporters is often a key component of the cascade leading to excitotoxic cell death. In the retina, glutamate transport is mainly operated by the glial glutamate transporter GLAST and the neuronal transporter GLT-1. In this study we evaluated the expression of GLAST and GLT-1 in a rat model of acute glaucoma based on the transient increase of intraocular pressure (IOP) and characterized by high glutamate levels during the reperfusion that follows the ischemic event associated with raised IOP. No changes were reported in GLAST expression while, at neuronal level, a reduction of glutamate uptake and of transporter reversal-mediated glutamate release was observed in isolated retinal synaptosomes. This was accompanied by modulation of GLT-1 expression leading to the reduction of the canonical 65 kDa form and upregulation of a GLT-1-related 38 kDa protein. These results support a role for neuronal transporters in glutamate accumulation observed in the retina following an ischemic event and suggest the presence of a GLT-1 neuronal new alternative splice variant, induced in response to the detrimental stimulus.

Role of thiamine in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in elderly individuals and is associated with progressive neurodegeneration of the human neocortex. Thiamine levels and the activity of thiamine-dependent enzymes are reduced in the brains and peripheral tissues of patients with AD. Genetic studies have provided the opportunity to determine what proteins link thiamine to AD pathology (ie, transketolase, apolipoprotein E, α-1-antitrypsin, pyruvate dehydrogenase complex, p53, glycogen synthetase kinase-3β, c-Fos gene, the Sp1 promoter gene, and the poly(ADP-ribosyl) polymerase-1 gene). We reviewed the association between histopathogenesis and neurotransmitters to understand the relationship between thiamine and AD pathology. Oral thiamine trials have been shown to improve the cognitive function of patients with AD; however, absorption of thiamine is poor in elderly individuals. In the early stage of thiamine-deficient encephalopathy (Wernicke's encephalopathy), however, parental thiamine has been used successfully. Therefore, further studies are needed to determine the benefits of using parental thiamine as a treatment for AD.

Differential expression of the glutamate transporter GLT-1 in pancreas

The glutamate uptake transporter GLT-1 is best understood for its critical role in preventing brain seizures. Increasing evidence argues that GLT-1 also modulates, and is modulated by, metabolic processes that influence glucose homeostasis. To investigate further the potential role of GLT-1 in these regards, the authors examined GLT-1 expression in pancreas and found that mature multimeric GLT-1 protein is stably expressed in the pancreas of wild-type, but not GLT-1 knockout, mice. There are three primary functional carboxyl-terminus GLT-1 splice variants, called GLT-1a, b, and c. Brain and liver express all three variants; however, the pancreas expresses GLT-1a and GLT-1b but not GLT-1c. Quantitative real time-PCR further revealed that while GLT-1a is the predominant GLT-1 splice variant in brain and liver, GLT-1b is the most abundant splice variant expressed in pancreas. Confocal microscopy and immunohistochemistry showed that GLT-1a and GLT-1b are expressed in both islet β- and α-cells. GLT-1b was also expressed in exocrine ductal domains. Finally, glutamine synthetase was coexpressed with GLT-1 in islets, which suggests that, as with liver and brain, one possible role of GLT-1 in the pancreas is to support glutamine synthesis.

Exon-skipping splice variants of excitatory amino acid transporter-2 (EAAT2) form heteromeric complexes with full-length EAAT2

The glial transporter excitatory amino acid transporter-2 (EAAT2) is the main mediator of glutamate clearance in brain. The wild-type transporter (EAAT2wt) forms trimeric membrane complexes in which each protomer functions autonomously. Several EAAT2 variants are found in control and Alzheimer-diseased human brains; their expression increases with pathological severity. These variants might alter EAAT2wt-mediated transport by abrogating membrane trafficking, or by changing the configuration or functionality of the assembled transporter complex. HEK293 cells were transfected with EAAT2wt; EAAT2b, a C-terminal variant; or either of two exon-skipping variants: alone or in combination. Surface biotinylation studies showed that only the exon-7 deletion variant was not trafficked to the membrane when transfected alone, and that all variants could reach the membrane when co-transfected with EAAT2wt. Fluorescence resonance energy transfer (FRET) studies showed that co-transfected EAAT2wt and EAAT2 splice variants were expressed in close proximity. Glutamate transporter function was measured using a whole cell patch clamp technique, or by changes in membrane potential indexed by a voltage-sensitive fluorescent dye (FMP assay): the two methods gave comparable results. Cells transfected with EAAT2wt or EAAT2b showed glutamate-dependent membrane potential changes consistent with functional expression. Cells transfected with EAAT2 exon-skipping variants alone gave no response to glutamate. Co-transfection of EAAT2wt (or EAAT2b) and splice variants in various ratios significantly raised glutamate EC(50) and decreased Hill coefficients. We conclude that exon-skipping variants form heteromeric complexes with EAAT2wt or EAAT2b that traffic to the membrane but show reduced glutamate-dependent activity. This could allow glutamate to accumulate extracellularly and promote excitotoxicity.

Novel splice variants of the amyotrophic lateral sclerosis-associated gene VAPB expressed in human tissues

VAPB is a highly conserved integral membrane protein that is ubiquitously expressed in all eukaryotic organisms and located within the membranes of the endoplasmic reticulum (ER). The P56S missense mutation of the VAPB protein is linked to a hereditary form of amyotrophic lateral sclerosis (ALS8), and the pathogenesis of ALS8 has remained enigmatic. We report the cloning of five novel splice variants of the human VAPB gene, all of which are expressed at the mRNA level in the human nervous system. When transfected into human HEK293 or SH-SY5Y cells, two of these variants (VAPB-2 and VAPB-4,5) were readily detectable by immunoblotting whereas two variants (VAPB-3 and VAPB-3,4) became detectable after proteasomal inhibition, a condition commonly found in neurodegenerative diseases. Interestingly, one of these novel VAPB variants, VAPB-2, co-immunoprecipitated with wt-VAPB. However, so far none of these splice variants could be detected by immunoblotting of lysates from selected human tissues, suggesting that in vivo, the proteins translated from the variant VAPB mRNAs are quickly degraded or, alternatively, the expressed proteins are below detection limit of the available antibodies. We speculate that under conditions of proteasomal inhibition, as encountered in many neurodegenerative diseases including ALS, variant VAPB proteins might accumulate in affected cells and contribute to ALS pathogenesis.

The evidence for altered RNA metabolism in amyotrophic lateral sclerosis (ALS)

In this review, the role of aberrant RNA metabolism in ALS is examined, including the evidence that a majority of the genetic mutations observed in familial ALS (including mutations in TDP-43, FUS/TLS, SOD1, angiogenin (ANG) and senataxin (SETX)) can impact directly on either gene transcription, pre-mRNA splicing, ribonucleoprotein complex formation, transport, RNA translation or degradation. The evidence that perturbed expression or function of RNA binding proteins is causally related to the selective suppression of the low molecular weight subunit protein (NFL) steady state mRNA levels in degenerating motor neurons in ALS is examined. The discovery that mtSOD1, TDP-43 and 14-3-3 proteins, all of which form cytosolic aggregates in ALS, can each modulate the stability of NFL mRNA, suggests that a fundamental alteration in the interaction of mRNA species with key trans-acting binding factors has occurred in ALS. These observations lead directly to the hypothesis that ALS can be viewed as a disorder of RNA metabolism, thus providing a novel pathway for the development of molecular pharmacotherapies.

Caffeine consumption attenuates neurochemical modifications in the hippocampus of streptozotocin-induced diabetic rats

Type 1 diabetes can affect hippocampal function triggering cognitive impairment through unknown mechanisms. Caffeine consumption prevents hippocampal degeneration and memory dysfunction upon different insults and is also known to affect peripheral glucose metabolism. Thus we now characterized glucose transport and the neurochemical profile in the hippocampus of streptozotocin-induced diabetic rats using in vivo(1)H NMR spectroscopy and tested the effect of caffeine consumption thereupon. We found that hippocampal glucose content and transport were unaltered in diabetic rats, irrespective of caffeine consumption. However diabetic rats displayed alterations in their hippocampal neurochemical profile, which were normalized upon restoration of normoglycaemia, with the exception of myo-inositol that remained increased (36 +/- 5%, p < 0.01 compared to controls) likely reflecting osmolarity deregulation. Compared to controls, caffeine-consuming diabetic rats displayed increased hippocampal levels of myo-inositol (15 +/- 5%, p < 0.05) and taurine (23 +/- 4%, p < 0.01), supporting the ability of caffeine to control osmoregulation. Compared to controls, the hippocampus of diabetic rats displayed a reduced density of synaptic proteins syntaxin, synaptophysin and synaptosome-associated protein of 25 kDa (in average 18 +/- 1%, p < 0.05) as well increased glial fibrillary acidic protein (20 +/- 5%, p < 0.05), suggesting synaptic degeneration and astrogliosis, which were prevented by caffeine consumption. In conclusion, neurochemical alterations in the hippocampus of diabetic rats are not related to defects of glucose transport but likely reflect osmoregulatory adaptations caused by hyperglycemia. Furthermore, caffeine consumption affected this neurochemical adaptation to high glucose levels, which may contribute to its potential neuroprotective effects, namely preventing synaptic degeneration and astrogliosis.

Glutamate transporters and the excitotoxic path to motor neuron degeneration in amyotrophic lateral sclerosis

Responsible for the majority of excitatory activity in the central nervous system (CNS), glutamate interacts with a range of specific receptor and transporter systems to establish a functional synapse. Excessive stimulation of glutamate receptors causes excitotoxicity, a phenomenon implicated in both acute and chronic neurodegenerative diseases [e.g., ischemia, Huntington's disease, and amyotrophic lateral sclerosis (ALS)]. In physiology, excitotoxicity is prevented by rapid binding and clearance of synaptic released glutamate by high-affinity, Na(+)-dependent glutamate transporters and amplified by defects to the glutamate transporter and receptor systems. ALS pathogenetic mechanisms are not completely understood and characterized, but excitotoxicity has been regarded as one firm mechanism implicated in the disease because of data obtained from ALS patients and animal and cellular models as well as inferred by the documented efficacy of riluzole, a generic antiglutamatergic drug, has in patients. In this article, we critically review the several lines of evidence supporting a role for glutamate-mediated excitotoxicity in the death of motor neurons occurring in ALS, putting a particular emphasis on the impairment of the glutamate-transport system.

Enhanced expression of the high affinity glutamate transporter GLT-1 in C6 glioma cells delays tumour progression in rat

High grade gliomas are known to release excitotoxic concentrations of glutamate, a process thought to contribute to their malignant phenotype through enhanced autocrine stimulation of their proliferation and destruction of the surrounding nervous tissue. A model of C6 glioma cells in which expression of the high affinity glutamate transporter GLT-1 can be manipulated both in vivo and in vitro was used in order to investigate the consequences of increasing glutamate clearance on tumour progression. These cells were grafted in the striatum of Wistar rats and doxycycline was administered after validation of tumour development by magnetic resonance imaging. Both GLT-1 expression examined by immunohistochemistry and glutamate transport activity measured on synaptosomes appeared robustly increased in samples from doxycycline-treated animals. Moreover, these rats showed extended survival times as compared to vehicle-treated animals, an effect that was consistent with volumetric data revealing delayed tumour growth. As constitutive deficiency in glutamate clearance at the vicinity of brain tumours is well established, these data illustrate the potential benefit that could be obtained by enhancing glutamate transport by glioma cells in order to reduce their invasive behaviour.

The concentrations and distributions of three C-terminal variants of the GLT1 (EAAT2; slc1a2) glutamate transporter protein in rat brain tissue suggest differential regulation

The neurotransmitter glutamate is inactivated by cellular uptake; mostly catalyzed by the glutamate transporter GLT1 (slc1a2, excitatory amino acid transporter [EAAT2]) subtype which is expressed at high levels in brain astrocytes and at lower levels in neurons. Three coulombs-terminal variants of GLT1 exist (GLT1a, GLT1b and GLT1c). Their cellular distributions are currently being debated (that of GLT1b in particular). Here we have made antibodies to the variants and produced pure preparations of the individual variant proteins. The immunoreactivities of each variant per amount of protein were compared to that of total GLT1 immunoisolated from Wistar rat brains. At eight weeks of age GLT1a, GLT1b and GLT1c represented, respectively 90%+/-1%, 6+/-1% and 1%+/-0.5% (mean+/-SEM) of total hippocampal GLT1. The levels of all three variants were low at birth and increased towards adulthood, but GLT1a increased relatively more than the other two. At postnatal day 14 the levels of GLT1b and GLT1c relative to total GLT1 were, respectively, 1.7+/-0.1 and 2.5+/-0.1 times higher than at eight weeks. In tissue sections, antibodies to GLT1a gave stronger labeling than antibodies to GLT1b, but the distributions of GLT1a and GLT1b were similar in that both were predominantly expressed in astroglia, cell bodies as well as their finest ramifications. GLT1b was not detected in nerve terminals in normal brain tissue. The findings illustrate the need for quantitative measurements and support the notion that the importance of the variants may not be due to the transporter molecules themselves, but rather that their expression represents the activities of different regulatory pathways.

Neuronal expression of splice variants of "glial" glutamate transporters in brains afflicted by Alzheimer's disease: unmasking an intrinsic neuronal property

Anomalies in glutamate homeostasis may contribute to the pathological processes involved in Alzheimer's disease (AD). Glutamate released from neurons or glial cells is normally rapidly cleared by glutamate transporters, most of which are expressed at the protein level by glial cells. However, in some patho-physiological situations, expression of glutamate transporters that are normally considered to be glial types, appears to be evoked in populations of distressed neurons. This study analysed the expression of exon-skipping forms of the three predominant excitatory amino acid (glutamate) transporters (EAATs1-3) in brains afflicted with AD. We demonstrate by immunocytochemistry in temporal cortex, the expression of these proteins particularly in limited subsets of neurons, some of which appeared to be dys-morphic. Whilst the neuronal expression of the "glial" glutamate transporters EAAT1 and EAAT2 is frequently considered to represent the abnormal and ectopic expression of such transporters, we suggest this may be a misinterpretation, since neurons such as cortical pyramidal cells normally express abundant mRNA for these EAATs (but little if any EAAT protein expression). We hypothesize instead that distressed neurons in the AD brain can turn on the translation of pre-existent mRNA pools, or suppress the degradation of alternately spliced glutamate transporter protein, leading to the "unmasking" of, rather than evoked expression of "glial" glutamate transporters in stressed neurons.

Distinct expression and regulation of the glutamate transporter isoforms GLT-1a and GLT-1b in cultured astrocytes from a rat model of amyotrophic lateral sclerosis (hSOD1G93A)

Impaired glutamate uptake associated with accumulation of extracellular glutamate is a well-documented feature of amyotrophic lateral sclerosis (ALS) and related excitotoxicity is frequently proposed to participate in the progression of the disease. We herein characterised the expression and activity of the glutamate transporter glutamate transporter 1 (GLT-1) in cultured cortical astrocytes derived from a transgenic rat strain expressing an ALS-related mutated form of human superoxide dismutase 1 (hSOD1(G93A)). Measurements of d-[(3)H]-aspartate uptake velocity in the presence of selective glutamate transporter blockers demonstrated that astrocytes from the transgenic rats showed an impaired GLT-1 activity as compared to cells from wild-type animals. In addition, the density of GLT-1a mRNA in cells from hSOD1(G93A) animals appeared nearly 2-fold lower while the density of GLT-1b mRNA was nearly 2-fold higher. Besides, we observed that exposing the astrocytes from hSOD1(G93A) rats to the neuroprotective transmitter Peptide Histidine Isoleucine (PHI) for 24h caused a 4.5-fold increase in the GLT-1b mRNA level without influencing the expression of the other key isoform GLT-1a. This selective upregulation of GLT-1b by the neuropeptide was correlated with a significant increase in d-[(3)H]-aspartate uptake activity. The possibility to specifically regulate a single isoform of the high-affinity transporter GLT-1 is an unprecedented observation which sheds light on new perspectives for the pharmacological manipulation of glutamate transmission in diseases such as ALS.

Role of transition metals in the pathogenesis of amyotrophic lateral sclerosis

ALS (amyotrophic lateral sclerosis) is a devastating progressive neurodegenerative disorder resulting in selective degeneration of motor neurons in brain and spinal cord and muscle atrophy. In approx. 2% of all cases, the disease is caused by a mutation in the Cu,Zn-superoxide dismutase (SOD1) gene. The transition metals zinc and copper regulate SOD1 protein stability and activity, and disbalance of the homoeostasis of these metals has therefore been implicated in the pathogenesis of ALS. Recent data strengthen the hypothesis that these transition metals are excellent potential targets to develop an effective therapy for ALS.

Glutamate-induced glioma cell proliferation is prevented by functional expression of the glutamate transporter GLT-1

A tetracycline-dependent inducible system was used to achieve controlled expression of the glutamate transporter 1 (GLT-1) in C6 glioma cells. Non-induced cells show modest glutamate uptake and, in the presence of L-cystine, these cells tend to release substantial amounts of glutamate. Overnight exposure to doxycycline increased D-[3H]-aspartate uptake, reaching similar capacity as observed in cultured astrocytes. Efficient clearance of exogenously applied glutamate was evidenced in these cells, even in the presence of l-cystine. The addition of glutamate (100 microM) to the medium of non-induced cells significantly increased their proliferation rate, an effect that was blocked when the expression of GLT-1 was induced. This suggests that impaired glutamate uptake capacity in glioma cells indirectly contributes to their proliferation.

Amyotrophic lateral sclerosis: from current developments in the laboratory to clinical implications

Amyotrophic lateral sclerosis (ALS) is a late-onset progressive degeneration of motor neurons occurring both as a sporadic and a familial disease. The etiology of ALS remains unknown, but one fifth of instances are due to specific gene defects, the best characterized of which is point mutations in the gene coding for Cu/Zn superoxide dismutase (SOD1). Because sporadic and familial ALS affect the same neurons with similar pathology, it is hoped that understanding these gene defects will help in devising therapies effective in both forms. A wealth of evidence has been collected in rodents made transgenic for mutant SOD1, which represent the best available models for familial ALS. Mutant SOD1 likely induces selective vulnerability of motor neurons through a combination of several mechanisms, including protein misfolding, mitochondrial dysfunction, oxidative damage, cytoskeletal abnormalities and defective axonal transport, excitotoxicity, inadequate growth factor signaling, and inflammation. Damage within motor neurons is enhanced by noxious signals originating from nonneuronal neighboring cells, where mutant SOD1 induces an inflammatory response that accelerates disease progression. The clinical implication of these findings is that promising therapeutic approaches can be derived from multidrug treatments aimed at the simultaneous interception of damage in both motor neurons and nonmotor neuronal cells.

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and its dysregulation has been associated with multiple psychiatric and neurological disorders. However, investigation of this molecule has been complicated by its complex pattern of alternative splicing, including three coding isoforms and multiple 5'- and 3'-UTRs that may have a regulatory function. It is likely that these sequences permit modulation of EAAT2 expression with spatial, temporal and or activity-dependent specificity; however, few studies have attempted to delineate the function of these sequences. Additionally, there are problems with the use of antibodies to study protein localization, possibly due to posttranslational modification of critical amino acid residues. This review describes what is currently known about the regulation of EAAT2 mRNA and protein isoforms and concludes with a summary of studies showing dysregulation of EAAT2 in psychiatric and neurological disorders. EAAT2 has been either primarily or secondarily implicated in a multitude of neuropsychiatric diseases in addition to the normal physiology of learning and memory. Thus, this molecule represents an intriguing therapeutic target once we improve our understanding of how it is regulated under normal conditions.

Chemical hypoxia facilitates alternative splicing of EAAT2 in presymptomatic APP23 transgenic mice

Hypoxia is one of the major common components of vascular risk factors for pathogenesis of Alzheimer's disease. This study investigated the possible relationship between hypoxia and alternative splicing of the excitatory amino acid transporter 2 (EAAT2) in a transgenic model for Alzheimer's disease. We used an APP23 mouse model prior to amyloid deposition and subjected it to chemical hypoxia treatment as induced by 3-nitropropionic acid. One hour after administration of 3-nitropropionic acid changes in the expression of the 5'-splice forms mEAAT2/5UT3, mEAAT2/5UT4, and mEAAT2/5UT5 were found in the frontal cortex, hippocampus and cerebellum of the APP23 model. In untreated APP23 animals the expression of EAAT2 splice variants was unchanged. Our results demonstrate that hypoxia facilitates alternative splicing of EAAT2 in the APP23 model. This may be a molecular mechanism linking vascular factors to early pathophysiology of Alzheimer's disease.

Altered in-vitro and in-vivo expression of glial glutamate transporter-1 following exposure to cerebrospinal fluid of amyotrophic lateral sclerosis patients

Our earlier studies have shown that cerebrospinal fluid (CSF) of amyotrophic lateral sclerosis (ALS) patients causes death of motor neurons, both in in-vitro as well as in-vivo. There was an aberrant phosphorylation of neurofilaments in cultured spinal cord neurons of chick and rats following exposure to CSF of ALS patients (ALS-CSF). Other features of neurodegeneration, such as swollen neuronal soma and beading of neurites were also observed. In neonatal rat pups exposed to ALS-CSF, we observed phosphorylated neurofilaments in the soma of spinal motor neurons in addition to the increased lactate dehydrogenase activity and reactive astrogliosis. The present study examines the effect of ALS-CSF on the expression of glial glutamate transporter (GLT-1) in embryonic rat spinal cord cultures as well as in spinal astrocytes of neonatal rats. Immunostaining suggested a decrease in the expression of GLT-1 by astrocytes both in culture and in-vivo following exposure to ALS-CSF. Quantification of Western blots confirmed the decreased expression of GLT-1. Our results provide evidence that toxic factor(s) present in ALS-CSF depletes GLT-1 expression. This could lead to an increased level of glutamate in the synaptic pool causing excitotoxicity to motor neurons, possibly by triggering the 'glutamate-mediated toxicity-pathway'.

Analysis of multiple exon-skipping mRNA splice variants using SYBR Green real-time RT-PCR

Fluorescence-based PCR techniques are becoming an increasingly popular method for measuring low-abundance alternatively spliced mRNA transcripts. The dynamic range of real-time RT-PCR affords high sensitivity for the measurement of gene expression, but this mandates the need for strict controls to ensure assay validity. Primer design, reverse transcription, and cycling conditions need to be optimized to ensure an accurate and reproducible assay. Here, we describe a procedure for creating a cost effective and reliable method for the absolute quantification of several exon-skipping variants of human excitatory amino acid transporter-2 (EAAT2). We show that the cycling conditions can be adjusted to increase the specificity of primers that span exon-exon junctions, and that differences in the reverse transcription reaction can be minimized. Standard curves are stable and produce accurate absolute copy number data. We report that exon-skipping transcripts, EAAT2Delta7 and EAAT2Delta9, account for 5.8% of EAAT2 mRNA in autopsy human neocortex.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

A quantitative regional expression profile of EAAT2 known and novel splice variants reopens the question of aberrant EAAT2 splicing in disease

The glutamate transporter 1 (GLT1) in rodents, or EAAT2 in humans, is alternatively spliced in a complex manner including the use of multiple 5' and 3' untranslated regions and several coding variants. We used quantitative RT-PCR to profile these splice variants in human and rat brain. We also used RT-PCR and Northern blotting to demonstrate that a novel isoform of GLT1b has an approximately 11kb 3' UTR extending through intron 9, exon 10 and approximately 5kb into the 3' untranslated region of GLT1. However, our most important finding concerns an aberrant transcript lacking exon 9, which contains a motif permitting translocation from the endoplasmic reticulum. This variant had previously been associated with amyotrophic lateral sclerosis until several groups reported high levels in normal brain tissue. In contrast, our data shows that this aberrant transcript is present at 0.1-0.2% of the major EAAT2 isoforms.

Loss of the astrocyte glutamate transporter GLT1 modifies disease in SOD1G93A mice

Recent studies have highlighted the role of astrocytes in the development of motor neuron disease in animal models. The astrocyte glutamate transporter GLT1 is responsible for a significant portion of glutamate transport from the synaptic cleft; regulating synaptic transmission and preventing glutamate excitotoxicity. While previous studies have demonstrated reductions in GLT1 with SOD1-mediated disease progression, it is not well established whether a reduction in this astrocyte-specific transporter alters the pathobiology of motor neuron degeneration in the SOD1(G93A) mouse. In order to address this possible astrocyte-specific influence, we crossed the SOD1(G93A) mouse line with a mouse heterozygous for GLT1 (GLT1+/-) exhibiting a significant reduction in transporter protein. Mice that carried both the SOD1 mutation and a reduced amount of GLT1 (SOD1(G93A)/GLT1+/-) exhibited an increase in the rate of motor decline accompanied by earlier motor neuron loss when compared with SOD1(G93A) mice. A modest reduction in survival was also noted in these mice. Dramatic losses of the GLT1 protein and reduced glutamate transport in the lumbar spinal cords of the SOD1(G93A)/GLT1+/- animals were also observed. GLT1 was not significantly changed in cortices from these animals suggesting that the effect of mutant SOD1 on GLT1 production/function was largely targeted to spinal cord rather than cortical astrocytes. This study suggests that astrocytes, and the astrocyte glutamate transporter GLT1, play a role in modifying disease progression and motor neuron loss in this model.

Amyotrophic lateral sclerosis as a complex genetic disease

In complex diseases like ALS, there are multiple genetic and environmental factors all contributing to disease liability. The genetic factors causing susceptibility to developing ALS can be considered a spectrum from single genes with large effect sizes causing classical Mendelian ALS, to genes of smaller effect, producing apparently sporadic disease. We examine the statistical genetic principles that underpin this model and review what is known about ALS as a disease with complex genetics.

Mitochondrial involvement in amyotrophic lateral sclerosis: trigger or target?

Despite numerous reports demonstrating mitochondrial abnormalities associated with amyotrophic lateral sclerosis (ALS), the role of mitochondrial dysfunction in the disease onset and progression remains unknown. The intrinsic mitochondrial apoptotic program is activated in the central nervous system of mouse models of ALS harboring mutant superoxide dismutase 1 protein. This is associated with the release of cytochrome-c from the mitochondrial intermembrane space and mitochondrial swelling. However, it is unclear if the observed mitochondrial changes are caused by the decreasing cellular viability or if these changes precede and actually trigger apoptosis. This article discusses the current evidence for mitochondrial involvement in familial and sporadic ALS and concludes that mitochondria is likely to be both a trigger and a target in ALS and that their demise is a critical step in the motor neuron death.

Astrocyte dysfunction in neurological disorders: a molecular perspective

Recent work on glial cell physiology has revealed that glial cells, and astrocytes in particular, are much more actively involved in brain information processing than previously thought. This finding has stimulated the view that the active brain should no longer be regarded solely as a network of neuronal contacts, but instead as a circuit of integrated, interactive neurons and glial cells. Consequently, glial cells could also have as yet unexpected roles in the diseased brain. An improved understanding of astrocyte biology and heterogeneity and the involvement of these cells in pathogenesis offers the potential for developing novel strategies to treat neurological disorders.

Post-translational regulation of EAAT2 function by co-expressed ubiquitin ligase Nedd4-2 is impacted by SGK kinases

The human excitatory amino acid transporter (EAAT)2 is the major glutamate carrier in the mammalian CNS. Defective expression of the transporter results in neuroexcitotoxicity that may contribute to neuronal disorders such as amyotrophic lateral sclerosis (ALS). The serum and glucocorticoid inducible kinase (SGK) 1 is expressed in the brain and is known to interact with the ubiquitin ligase Nedd4-2 to modulate membrane transporters and ion channels. The present study aimed to investigate whether SGK isoforms and the related kinase, protein kinase B (PKB), regulate EAAT2. Expression studies in Xenopus oocytes demonstrated that glutamate-induced inward current (IGLU) was stimulated by co-expression of SGK1, SGK2, SGK3 or PKB. IGLU is virtually abolished by Nedd4-2, an effect abrogated by additional co-expression of either kinase. The kinases diminish the effect through Nedd4-2 phosphorylation without altering Nedd4-2 protein abundance. SGKs increase the transporter maximal velocity without significantly affecting substrate affinity. Similar to glutamate-induced currents, [3H] glutamate uptake and cell surface abundance of the transporter were increased by the SGK isoforms and down-regulated by the ubiquitin ligase Nedd4-2. In conclusion, all three SGK isoforms and PKB increase EAAT2 activity and plasma membrane expression and thus, may participate in the regulation of neuroexcitability.

Increased glutamine synthetase but normal EAAT2 expression in platelets of ALS patients

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease and glutamate excitotoxicity has been implicated in its pathogenesis. Platelets contain a glutamate uptake system and express components of the glutamate-glutamine cycle, such as the predominant glial excitatory amino acid transporter 2 (EAAT2). In several neurological diseases platelets have proven to be systemic markers for the disease. We compared properties of key components of the glutamate-glutamine cycle in blood platelets of ALS patients and healthy controls. Platelets were analyzed for (3)H-glutamate uptake in the presence or absence of thrombin and for EAAT2 and glutamine synthetase protein expression by Western blotting. Platelets of ALS patients showed a 37% increase in expression of glutamine synthetase, but normal expression of glutamate transporter EAAT2. Glutamate uptake in resting or thrombin-stimulated platelets did not differ significantly between platelets from ALS patients and controls. Thrombin-stimulation resulted in about a seven-fold increase in glutamate uptake. Our data suggest that glutamine synthetase may be a peripheral marker of ALS and encourage further investigation into the role of this enzyme in ALS.

A novel alternative splicing form of excitatory amino acid transporter 1 is a negative regulator of glutamate uptake

Abstract EAAT1 is a major glutamate transporter in the CNS and is required for normal neurotransmission and neuroprotection from excitotoxicity. In the present study, we have identified a novel form of the human EAAT1, named here as EAAT1ex9skip, which lacks the entire exon 9. Quantitative PCR analysis indicates that this variant is expressed throughout the CNS, both in grey matter and axonal tracts, at levels ranging between 10% and 20% of the full-length EAAT1 form. When expressed in HEK293 cells, EAAT1ex9skip mRNA is translated into a truncated protein localized in the endoplasmic reticulum. EAAT1ex9skip has no functional glutamate uptake activity but instead, exerts a dominant negative effect over full-length EAAT1 function. In turn, co-expression of full-length EAAT1 and EAAT1ex9skip variants reduces the insertion of the former into the plasma membrane. Together, these results indicate that the EAAT1ex9skip splice variant is a negative regulator of full-length EAAT1 function in the human brain.

Impaired metabotropic glutamate receptor/phospholipase C signaling pathway in the cerebral cortex in Alzheimer's disease and dementia with Lewy bodies correlates with stage of Alzheimer's-disease-related changes

The aim of the present work was to analyze the status of metabotropic glutamate receptors (mGluRs) in the frontal cortex (area 8) from ten cases with common form DLB (cDLB) and eleven cases with pure AD in comparison with five age-matched controls. mGluRs, determined by radioligand binding assays, were significantly decreased in cerebral cortex in cDLB. This decrease was already present in cases with early AD changes not involving the frontal cortex, but dramatically correlated with AD neuropathological changes, at its greatest in isocortical stages, which was associated with a decrease in the expression levels of mGluR1 detected by Western blotting. Moreover, mGluRs analyzed in pure AD were lower than those obtained in cDLB and also correlated with progression of illness. On the other hand, the expression levels of phospholipase Cbeta1 (PLCbeta1) isoform, which is the effector of group I mGluRs, was decreased in parallel in cDLB cases. Finally, the PLCbeta1 decrease was associated with reduced GTP- and l-glutamate-stimulated PLC activity in both cDLB and AD cases. These results show that group I mGluRs/PLC signaling are down-regulated and desensitized in the frontal cortex in cDLB and AD cases and that these modifications worsen with progression of AD changes in the cerebral neocortex. Therefore, group I mGluR dysfunction may be implicated in the pathogenesis of cognitive impairment and dementia in common form of DLB and pure AD.

The excitatory amino acid transporter-2 induces apoptosis and decreases glioma growth in vitro and in vivo

Accumulating evidence suggests that glutamate plays a key role in the proliferation and invasion of glioblastoma tumors. Astrocytic tumors have been shown to release glutamate at high levels, which may stimulate tumor cell proliferation and motility via activation of glutamate receptors. Excess glutamate has also been found to facilitate tumor invasion by causing excitotoxic damage to normal brain thereby paving a pathway for tumor migration. Results from tissue microarray analyses showed decreased excitatory amino acid transporter-2 (EAAT-2) expression in high-grade glial tumors compared with low-grade astrocytomas and normal brain. EAAT-2 expression was inversely correlated with tumor grade, implicating its potential role in glial tumor progression, which was reflected by an undetectable level of EAAT-2 protein in glioma cell lines. In this study, we sought to investigate the effect of reconstituted EAAT-2 on glioma cell growth in vitro and in vivo by adenoviral-mediated gene transfer. Infection of glioma cells with Ad-EAAT-2 resulted in a physiologic level of functional EAAT-2, and a subsequent dose-dependent reduction in cell proliferation in all glioma cell lines tested compared with controls. Interestingly, results from analyses of Annexin V staining, detection of poly(ADP-ribose)polymerase cleavage and caspase-3 activation all indicated that Ad-EAAT-2 infection elicited apoptosis in glioma cells. Ex vivo experiments in nude mice showed a total suppression of tumor growth at sites that received Ad-EAAT-2-infected cells. Collectively, our results uncovered a new function of EAAT-2 in controlling glioma proliferation. Further studies will improve our knowledge of the role of glutamate in glioma growth and may provide useful prognostic information and alternative therapeutic targets for the treatment of glioma.

Multiple 5′-splice variants of the rat glutamate transporter-1

In most brain areas, uptake of extracellular glutamate predominantly occurs through the glutamate transporter subtype, glutamate transporter-1 (GLT-1), which is enriched in astroglia. Here, we report the identification of five splice variants of the 5'-leader sequence of rat GLT-1 which contain varying numbers of upstream open reading frames and encode putative GLT-1 proteins with two distinct N-terminal modifications. We further demonstrate that the identified rat 5'-GLT-1 splice variants are expressed in a brain region-specific manner. Our findings point to potential influences of RNA splicing on glial glutamate transport in the intact and injured rat brain.

A role for astrocytes in motor neuron loss in amyotrophic lateral sclerosis

A strong glial reaction typically surrounds the affected upper and lower motor neurons and degenerating descending tracts of ALS patients. Reactive astrocytes in ALS contain protein inclusions, express inflammatory makers such as the inducible forms of nitric oxide synthase (iNOS) and cyclooxygenase (COX-2), display nitrotyrosine immunoreactivity and downregulate the glutamate transporter EAAT2. In this review, we discuss the evidence sustaining an active role for astrocytes in the induction and propagation of motor neuron loss in ALS. Available evidence supports the view that glial activation could be initiated by proinflammatory mediators secreted by motor neurons in response to injury, axotomy or muscular pathology. In turn, reactive astrocytes produce nitric oxide and peroxynitrite, which cause mitochondrial damage in cultured neurons and trigger apoptosis in motor neurons. Astrocytes may also contribute to the excitotoxic damage of motor neurons by decreasing glutamate transport or actively releasing the excitotoxic amino acid. In addition, reactive astrocytes secrete pro-apoptotic mediators, such as nerve growth factor (NGF) or Fas-ligand, a mechanism that may serve to eliminate vulnerable motor neurons. The comprehensive understanding of the interactions between motor neurons and glia in ALS may lead to a more accurate theory of the pathogenesis of the disease.

Mitochondrial proteomic analysis of a cell line model of familial amyotrophic lateral sclerosis

Mutations in copper-zinc superoxide dismutase (SOD1) have been linked to a subset of familial amytrophic lateral sclerosis (fALS), a fatal neurodegenerative disease characterized by progressive motor neuron death. An increasing amount of evidence supports that mitochondrial dysfunction and apoptosis activation play a critical role in the fALS etiology, but little is known about the mechanisms by which SOD1 mutants cause the mitochondrial dysfunction and apoptosis. In this study, we use proteomic approaches to identify the mitochondrial proteins that are altered in the presence of a fALS-causing mutant G93A-SOD1. A comprehensive characterization of mitochondrial proteins from NSC34 cells, a motor neuron-like cell line, was achieved by two independent proteomic approaches. Four hundred seventy unique proteins were identified in the mitochondrial fraction collectively, 75 of which are newly discovered proteins that previously had only been reported at the cDNA level. Two-dimensional gel electrophoresis was subsequently used to analyze the differences between the mitochondrial proteomes of NSC34 cells expressing wild-type and G93A-SOD1. Nine and 36 protein spots displayed elevated and suppressed abundance respectively in G93A-SOD1-expressing cells. The 45 spots were identified by MS, and they include proteins involved in mitochondrial membrane transport, apoptosis, the respiratory chain, and molecular chaperones. In particular, alterations in the post-translational modifications of voltage-dependent anion channel 2 (VDAC2) were found, and its relevance to regulating mitochondrial membrane permeability and activation of apoptotic pathways is discussed. The potential role of other proteins in the mutant SOD1-mediated fALS is also discussed. This study has produced a short list of mitochondrial proteins that may hold the key to the mechanisms by which SOD1 mutants cause mitochondrial dysfunction and neuronal death. It has laid the foundation for further detailed functional studies to elucidate the role of particular mitochondrial proteins, such as VDAC2, in the pathogenesis of familial ALS.

Reversal or reduction of glutamate and GABA transport in CNS pathology and therapy

A dysfunction of amino acid neurotransmitter transporters occurs in a number of central nervous system disorders, including stroke, epilepsy, cerebral palsy and amyotrophic lateral sclerosis. This dysfunction can comprise a reversal of transport direction, leading to the release of neurotransmitter into the extracellular space, or an alteration in transporter expression level. This review analyses the role of glutamate and GABA transporters in the pathogenesis and therapy of a number of acute and chronic neurological disorders.

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.