Internalization and degradation of the glutamate transporter GLT-1 in response to phorbol ester

The Regulation of GLT-1 Degradation Pathway by SIRT4

Glial cells give rise to glioblastoma multiform as a primary brain tumor. In glioblastomas, neurons are destroyed via excitotoxicity which is the accumulation of excess glutamate in synaptic cavity. Glutamate Transporter 1 (GLT-1) is the main transporter that absorbs the excessive glutamate. Sirtuin 4 (SIRT4) was shown to have a potential protective role against excitotoxicity in previous studies. In this study, the regulation of dynamic GLT-1 expression by SIRT4 was analyzed in glia (immortalized human astrocytes) and glioblastoma (U87) cells. The expression of GLT-1 dimers and trimers were reduced and the ubiquitination of GLT-1 was increased in glioblastoma cells when SIRT4 was silenced; however GLT-1 monomer was not affected. In glia cells, SIRT4 reduction did not affect GLT-1 monomer, dimer, trimer expression or the ubiquitination of GLT-1. The phosphorylation of Nedd4-2 and the expression of PKC did not change in glioblastoma cells when SIRT4 was silenced but increased in glia cells. We also showed that SIRT4 deacetylates PKC in glia cells. In addition, GLT-1 was shown to be deacetylated by SIRT4 which might be a priority for ubiquitination. Therefore, we conclude that GLT-1 expression is regulated differently in glia and glioblastoma cells. SIRT4 activators or inhibitors of ubiquitination may be used to prevent excitotoxicity in glioblastomas.

Trafficking of the glutamate transporter is impaired in LRRK2-related Parkinson's disease

The Excitatory Amino Acid Transporter 2 (EAAT2) accounts for 80% of brain glutamate clearance and is mainly expressed in astrocytic perisynaptic processes. EAAT2 function is finely regulated by endocytic events, recycling to the plasma membrane and degradation. Noteworthy, deficits in EAAT2 have been associated with neuronal excitotoxicity and neurodegeneration. In this study, we show that EAAT2 trafficking is impaired by the leucine-rich repeat kinase 2 (LRRK2) pathogenic variant G2019S, a common cause of late-onset familial Parkinson’s disease (PD). In LRRK2 G2019S human brains and experimental animal models, EAAT2 protein levels are significantly decreased, which is associated with elevated gliosis. The decreased expression of the transporter correlates with its reduced functionality in mouse LRRK2 G2019S purified astrocytic terminals and in Xenopus laevis oocytes expressing human LRRK2 G2019S. In LRRK2 G2019S knock-in mouse brain, the correct surface localization of the endogenous transporter is impaired, resulting in its interaction with a plethora of endo-vesicular proteins. Mechanistically, we report that pathogenic LRRK2 kinase activity delays the recycling of the transporter to the plasma membrane via Rabs inactivation, causing its intracellular re-localization and degradation. Taken together, our results demonstrate that pathogenic LRRK2 interferes with the physiology of EAAT2, pointing to extracellular glutamate overload as a possible contributor to neurodegeneration in PD.

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

The investigation of glutamate transporter 1 (GLT-1) degradation pathway in glioblastoma cells

Glioblastoma multiform is a primary brain tumor derived from glial cells. The aim of this study is to investigate how glutamate metabolism is regulated by glutamate transporter 1 (GLT-1) degradation pathway in glioblastoma and glial cell lines. The protein expression levels of GLT-1, total ubiquitin, protein kinase C (PKC) proteins involved in the GLT-1 degradation pathway were measured by the western blot technique. Additionally, in glial and glioblastoma cells, the level of glutamate accumulated in the medium and the lysates was measured with the glutamate assay. GLT-1 protein expression was increased significantly in glioblastoma cells. The expression levels of the PKC protein and total ubiquitin were found to be decreased in glioblastoma cells although not significantly. The glutamate accumulated in the medium and lysates of glioblastoma cells is reduced compared to glial cells. Further research regarding excitotoxicity in glioblastoma focusing on GLT-1 degradation or activation pathway may create new opportunities of drug and treatment development.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Acetylcholine Receptor Stimulation Activates Protein Kinase C Mediated Internalization of the Dopamine Transporter

The dopamine transporter (DAT) clears neurotransmitters from the extracellular space and serves as an important regulator of signal amplitude and duration at sites of dopamine release. Several different intracellular signaling pathways have been observed to modulate DAT activity through the regulation of the trafficking of the carriers to and from the cell surface. Acute activation of protein kinase C (PKC) by phorbol esters facilitates clathrin-dependent internalization of the DAT in a variety of model systems; however, the physiological stimuli and cell-surface receptor systems that activate PKC and regulate the DAT in dopamine neurons remain elusive. We report here that stimulation of M₁/M₅ muscarinic receptors in midbrain cultures decreases the ability of dopamine neurons to transport dopamine through DAT. Application of the cholinomimetic drug carbachol leads to a decrease in DAT activity in primary cultures while the M₁/M₅-specific antagonist, pirenzepine, blocks these effects. The M₃ antagonist, DAU 5884, does not affect, but a positive modulator of M₅, VU 0238429, enhances the loss of DAT function in response to carbachol and acetylcholine. These data implicate M₁/M₅ receptors on dopamine neurons in the modulation of DAT function. Bisindolylmaleimide, a PKC inhibitor, blocks the effects of carbachol stimulation on dopamine uptake, supporting a role for PKC in muscarinic receptor-mediated DAT internalization. Furthermore, as shown previously for PKC-induced internalization, downregulation of the DAT is dependent on both clathrin and dynamin. A G_q-specific inhibitor peptide also blocks the effects of carbachol on DAT in primary cultures, confirming G_q as the G-protein that couples M₁/M₅ receptors to PKC activation in these cells. In acute midbrain slices, biotinylation of cell-surface proteins revealed the loss of dopamine transport mediated by muscarinic receptor stimulation was, indeed, due to loss of membrane expression of the DAT in endogenous tissue. These data indicate that stimulation of cholinergic pathways can lead to modulation of dopamine through internalization of the DAT.

Endocytosis of the glutamate transporter 1 is regulated by laforin and malin: Implications in Lafora disease

Lafora disease (LD) is a fatal rare type of progressive myoclonus epilepsy that appears during early adolescence. The disease is caused by mutations in EPM2A or EPM2B genes, which encode laforin, a glucan phosphatase, and malin, an E3-ubiquitin ligase, respectively. Although the exact roles of laforin and malin are still not well understood, it is known that they work as a complex in which laforin recruits targets that will be ubiquitinated by malin. Recently, we suggested that the type of epilepsy that accompanies LD could be due to deficiencies in the function of the astrocytic glutamate transporter GLT-1. We described that astrocytes from LD mouse models presented decreased levels of GLT-1 at the plasma membrane, leading to increased levels of glutamate in the brain parenchyma. In this work, we present evidence indicating that in the absence of a functional laforin/malin complex (as in LD cellular models) there is an alteration in the ubiquitination of GLT-1, which could be the cause of the reduction in the levels of GLT-1 at the plasma membrane. On the contrary, overexpression of the laforin/malin complex promotes the retention of GLT-1 at the plasma membrane. This retention may be due to the direct ubiquitination of GLT-1 and/or to an opposite effect of this complex on the dynamics of the Nedd4.2-mediated endocytosis of the transporter. This work, therefore, presents new pieces of evidence on the regulation of GLT-1 by the laforin/malin complex, highlighting its value as a therapeutic target for the amelioration of the type of epilepsy that accompanies LD.

Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

Going the Extra (Synaptic) Mile: Excitotoxicity as the Road Toward Neurodegenerative Diseases

Excitotoxicity is a phenomenon that describes the toxic actions of excitatory neurotransmitters, primarily glutamate, where the exacerbated or prolonged activation of glutamate receptors starts a cascade of neurotoxicity that ultimately leads to the loss of neuronal function and cell death. In this process, the shift between normal physiological function and excitotoxicity is largely controlled by astrocytes since they can control the levels of glutamate on the synaptic cleft. This control is achieved through glutamate clearance from the synaptic cleft and its underlying recycling through the glutamate-glutamine cycle. The molecular mechanism that triggers excitotoxicity involves alterations in glutamate and calcium metabolism, dysfunction of glutamate transporters, and malfunction of glutamate receptors, particularly N-methyl-D-aspartic acid receptors (NMDAR). On the other hand, excitotoxicity can be regarded as a consequence of other cellular phenomena, such as mitochondrial dysfunction, physical neuronal damage, and oxidative stress. Regardless, it is known that the excessive activation of NMDAR results in the sustained influx of calcium into neurons and leads to several deleterious consequences, including mitochondrial dysfunction, reactive oxygen species (ROS) overproduction, impairment of calcium buffering, the release of pro-apoptotic factors, among others, that inevitably contribute to neuronal loss. A large body of evidence implicates NMDAR-mediated excitotoxicity as a central mechanism in the pathogenesis of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and epilepsy. In this review article, we explore different causes and consequences of excitotoxicity, discuss the involvement of NMDAR-mediated excitotoxicity and its downstream effects on several neurodegenerative disorders, and identify possible strategies to study new aspects of these diseases that may lead to the discovery of new therapeutic approaches. With the understanding that excitotoxicity is a common denominator in neurodegenerative diseases and other disorders, a new perspective on therapy can be considered, where the targets are not specific symptoms, but the underlying cellular phenomena of the disease.

SIRT4 prevents excitotoxicity via modulating glutamate metabolism in glioma cells

Excitotoxicity is the presence of excessive glutamate, which is normally taken up by glutamate transporters on astrocytes. Glutamate transporter 1 (GLT-1) is the major transporter on glia cells clearing more than 90% of the glutamate. Sirtuin 4 (SIRT4) is a mitochondrial sirtuin which is expressed in the brain. Previously, it was shown that loss of SIRT4 leads to a more severe reaction to kainic acid, an excitotoxic agent, and also decreased GLT-1 expression in the brain. In this study, we aimed to investigate whether overexpression of SIRT4 is protective against excitotoxicity in glia cells. We overexpressed SIRT4 in A172 glioma cell line and treated with kainic acid in order to induce excitotoxicity. We observed that SIRT4 overexpression increased the cell viability after kainic acid treatment. In addition, reduced glutamate was detected in glutamate assay with overexpression of SIRT4 after kainic acid treatment since SIRT4 decreased cell death by preventing excitotoxicity. Our results show that overexpression of SIRT4 increased the protein levels of GLT-1 and glutamate dehydrogenase (GDH) after kainic acid (KA) treatment so that excess glutamate can be absorbed. However, overexpression of SIRT4 decreased glutamine synthetase (GS) levels. These results demonstrate that, by inhibiting GS, SIRT4 prevents glutamine formation, which will be converted to glutamate in neurons. SIRT4 prevents excitotoxicity via upregulating glutamate metabolism. Finally, our results may show that SIRT4 might prevent excitotoxicity and related cell death via reducing GS expression and upregulating GLT-1 and GDH levels. Therefore, it is important to develop therapeutics against excitotoxicity through SIRT4-related pathways in the cell.

Role of Astrocytes in Manganese Neurotoxicity Revisited

Manganese (Mn) overexposure is a public health concern due to its widespread industrial usage and the risk for environmental contamination. The clinical symptoms of Mn neurotoxicity, or manganism, share several pathological features of Parkinson's disease (PD). Biologically, Mn is an essential trace element, and Mn in the brain is preferentially localized in astrocytes. This review summarizes the role of astrocytes in Mn-induced neurotoxicity, specifically on the role of neurotransmitter recycling, neuroinflammation, and genetics. Mn overexposure can dysregulate astrocytic cycling of glutamine (Gln) and glutamate (Glu), which is the basis for Mn-induced excitotoxic neuronal injury. In addition, reactive astrocytes are important mediators of Mn-induced neuronal damage by potentiating neuroinflammation. Genetic studies, including those with Caenorhabditis elegans (C. elegans) have uncovered several genes associated with Mn neurotoxicity. Though we have yet to fully understand the role of astrocytes in the pathologic changes characteristic of manganism, significant strides have been made over the last two decades in deciphering the role of astrocytes in Mn-induced neurotoxicity and neurodegeneration.

PKC-Mediated Modulation of Astrocyte SNAT3 Glutamine Transporter Function at Synapses in Situ

Astrocytes are glial cells that have an intimate physical and functional association with synapses in the brain. One of their main roles is to recycle the neurotransmitters glutamate and gamma-aminobutyric acid (GABA), as a component of the glutamate/GABA-glutamine cycle. They perform this function by sequestering neurotransmitters and releasing glutamine via the neutral amino acid transporter SNAT3. In this way, astrocytes regulate the availability of neurotransmitters and subsequently influence synaptic function. Since many plasma membrane transporters are regulated by protein kinase C (PKC), the aim of this study was to understand how PKC influences SNAT3 glutamine transport in astrocytes located immediately adjacent to synapses. We studied SNAT3 transport by whole-cell patch-clamping and fluorescence pH imaging of single astrocytes in acutely isolated brainstem slices, adjacent to the calyx of the Held synapse. Activation of SNAT3-mediated glutamine transport in these astrocytes was reduced to 77 ± 6% when PKC was activated with phorbol 12-myristate 13-acetate (PMA). This effect was very rapid (within ~20 min) and eliminated by application of bisindolylmaleimide I (Bis I) or 7-hydroxystaurosporine (UCN-01), suggesting that activation of conventional isoforms of PKC reduces SNAT3 function. In addition, cell surface biotinylation experiments in these brain slices show that the amount of SNAT3 in the plasma membrane is reduced by a comparable amount (to 68 ± 5%) upon activation of PKC. This indicates a role for PKC in dynamically controlling the trafficking of SNAT3 transporters in astrocytes in situ. These data demonstrate that PKC rapidly regulates the astrocytic glutamine release mechanism, which would influence the glutamine availability for adjacent synapses and control levels of neurotransmission.

Neuronal activity mediated regulation of glutamate transporter GLT-1 surface diffusion in rat astrocytes in dissociated and slice cultures

The astrocytic GLT‐1 (or EAAT2) is the major glutamate transporter for clearing synaptic glutamate. While the diffusion dynamics of neurotransmitter receptors at the neuronal surface are well understood, far less is known regarding the surface trafficking of transporters in subcellular domains of the astrocyte membrane. Here, we have used live‐cell imaging to study the mechanisms regulating GLT‐1 surface diffusion in astrocytes in dissociated and brain slice cultures. Using GFP‐time lapse imaging, we show that GLT‐1 forms stable clusters that are dispersed rapidly and reversibly upon glutamate treatment in a transporter activity‐dependent manner. Fluorescence recovery after photobleaching and single particle tracking using quantum dots revealed that clustered GLT‐1 is more stable than diffuse GLT‐1 and that glutamate increases GLT‐1 surface diffusion in the astrocyte membrane. Interestingly, the two main GLT‐1 isoforms expressed in the brain, GLT‐1a and GLT‐1b, are both found to be stabilized opposed to synapses under basal conditions, with GLT‐1b more so. GLT‐1 surface mobility is increased in proximity to activated synapses and alterations of neuronal activity can bidirectionally modulate the dynamics of both GLT‐1 isoforms. Altogether, these data reveal that astrocytic GLT‐1 surface mobility, via its transport activity, is modulated during neuronal firing, which may be a key process for shaping glutamate clearance and glutamatergic synaptic transmission. GLIA 2016;64:1252–1264

Regulation of glutamate transporter 1 (GLT-1) gene expression by cocaine self-administration and withdrawal

Downregulation of the astroglial glutamate transporter GLT-1 is observed in the nucleus accumbens (NAc) following administration of multiple drugs of abuse. The decrease in GLT-1 protein expression following cocaine self-administration is dependent on both the amount of cocaine self-administered and the length of withdrawal, with longer access to cocaine and longer withdrawal periods leading to greater decreases in GLT-1 protein. However, the mechanism(s) by which cocaine downregulates GLT-1 protein remains unknown. We used qRT-PCR to examine gene expression of GLT-1 splice isoforms (GLT-1A, GLT-1B) in the NAc, prelimbic cortex (PL) and basolateral amygdala (BLA) of rats, following two widely used models of cocaine self-administration: short-access (ShA) self-administration, and the long-access (LgA) self-administration/incubation model. While downregulation of GLT-1 protein is observed following ShA cocaine self-administration and extinction, this model did not lead to a change in GLT-1A or GLT-1B gene expression in any brain region examined. Forced abstinence following ShA cocaine self-administration also was without effect. In contrast, LgA cocaine self-administration and prolonged abstinence significantly decreased GLT-1A gene expression in the NAc and BLA, and significantly decreased GLT-1B gene expression in the PL. No change was observed in NAc GLT-1A gene expression one day after LgA cocaine self-administration, indicating withdrawal-induced decreases in GLT-1A mRNA. In addition, LgA cocaine self-administration and withdrawal induced hypermethylation of the GLT-1 gene in the NAc. These results indicate that a decrease in NAc GLT-1 mRNA is only observed after extended access to cocaine combined with protracted abstinence, and that epigenetic mechanisms likely contribute to this effect.

Sonic hedgehog induces GLT-1 degradation via PKC delta to suppress its transporter activities

GLT-1 is mainly expressed in astrocytes and has a crucial role in glutamate uptake. Sonic hedgehog (SHH) can inhibit glutamate uptake and its pathway is activated in many brain diseases related with glutamate excitotoxicity. However, whether SHH regulates GLT-1 to affect glutamate uptake is not clear. Here, we use pharmacological and genetic methods to show that SHH induces GLT-1 degradation in astrocytes in a manner that is dependent on PKC delta (PKCδ) to regulate GLT-1 activities. GLT-1 protein levels are reduced as early as 2 hs in astrocytes after incubation with SHH, whereas its mRNA levels are not changed. This reduction is recapitulated when astrocytes are transfected with SmoA1, a constitutively active form of Smoothened (Smo), the mediator of SHH pathway. The reduction of GLT-1 and inhibition of aspartate current are not observed when staurosporine (STP) and BisindolylmaleimideII (BisII), agents known as PKC inhibitors, are present. Further, when PKCδ is knocked down in astrocytes, SHH cannot reduce GLT-1 protein levels. Therefore, SHH induces degradation of GLT-1 through PKCδ to regulate its activities.

Ligands of peroxisome proliferator-activated receptor-alpha promote glutamate transporter-1 endocytosis in astrocytes

Astrocytes, a stellate-shape glial population in the central nervous system (CNS), maintain glutamate homeostasis in adult CNS by undergoing glutamate uptake at the synapse through their glutamate transporter-1 (GLT-1). Peroxisome proliferator-activated receptor-α (PPARα) can be activated by endogenous saturated fatty acids to regulate astrocytic lipid metabolism and functions. However, it is unclear if PPARα can exert the regulatory action on GLT-1 expression in astrocytes. This study showed that treatment with palmitic acid (PA) and the other two PPARα agonists (GW 7647 and WY 14,643) caused no change in the morphology of astrocytes, whereas membranous GLT-1 protein levels in astrocytes were significantly decreased by PA and PPARα agonists. Through lentivirus-mediated overexpression of GLT-1 tagged with red fluorescent protein (GLT-1-RFP), we also observed that GLT-1-RFP puncta in the processes of astrocytes were inhibited by the PPARα agonists. This reduction was prevented by the addition of the PPARα antagonist, GW6471. GLT-1-RFP was co-localized to the early endosome marker-EEA1 in astrocytes treated with the PPARα agonists. Moreover, PPARα-induced inhibition in membranous GLT-1 expression was abolished by the addition of dynamin inhibitor (dynasore). Furthermore, the co-treatment of astrocytes with PPARα agonists and dynasore, or with PPARα agonists and protein kinase C (PKC) inhibitor bis-indolylmaleimide 1 (BIS1), prevented the endocytosis of GLT-1-RFP. Based on the results, we conclude that the PPARα agonists increased GLT-1 endocytosis in astrocytes possibly through the PKC signaling pathway. In addition, our findings provide important information of PPARα involvement in the downregulation of astrocytic glutamate uptake via the promoted GLT-1 endocytosis.

Regulation of glutamate transporter trafficking by Nedd4-2 in a Parkinson's disease model

Glutamate transporters play a key role in glutamate clearance and protect the central nervous system from glutamate excitotoxicity. Dysfunctional glutamate transporters contribute to the pathogenesis of Parkinson's disease (PD); however, the mechanisms that underlie the regulation of glutamate transporters in PD are still not well characterized. Here we report that Nedd4-2 mediates the ubiquitination of glutamate transporters in 1-methyl-4- phenylpyridinium (MPP⁺)-treated astrocytes and in the midbrain of 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP)-constructed PD model mice. Nedd4-2-mediated ubiquitination induces abnormal glutamate transporter trafficking between the membrane and cytoplasm and consequently decreases the expression and function of glutamate transporters in the membrane. Conversely, Nedd4-2 knockdown decreases glutamate transporter ubiquitination, promotes glutamate uptake and increases glutamate transporter expression in vitro and in vivo. We report for the first time that Nedd4-2 knockdown ameliorates movement disorders in PD mice and increases tyrosine hydroxylase expression in the midbrain and striatum of PD mice; Nedd4-2 knockdown also attenuates astrogliosis and reactive microgliosis in the MPTP model that may be associated with glutamate excitotoxicity. Furthermore, the SGK/PKC pathway is regulated downstream of Nedd4-2 in MPTP-treated mice. These findings indicate that Nedd4-2 may serve as a potential therapeutic target for the treatment of PD.

Pharmacologic inhibition of Hsp90 to prevent GLT-1 degradation as an effective therapy for epilepsy

Sha et al. report that Hsp90β, which is up-regulated in astrocytes of human epileptogenic tissue, interacts with GLT-1 and recruits it to 20S proteasome for degradation. The Hsp90 inhibitor 17AAG exhibits beneficial effects in a model of temporal lobe epilepsy.

The glutamate transporter GLT-1 is critical for the maintenance of low interstitial glutamate concentrations. Loss of GLT-1 is commonly observed in neurological disorders, including temporal lobe epilepsy (TLE). Despite the hypothesis that targeting the mechanisms of GLT-1 deficiency may be a novel strategy for treating drug-resistant epilepsy, the underlying molecular cascade remains largely unknown. Here, we show that Hsp90β is up-regulated in reactive astrocytes of the epileptic hippocampus in patients with TLE and mouse models of epilepsy. Inhibition of Hsp90, but not Hsp70, increased GLT-1 levels. Mechanistically, Hsp90β recruits GLT-1 to the 20S proteasome, thereby promoting GLT-1 degradation. Hsp90 inhibitor prevents GLT-1 degradation by disrupting the association between Hsp90β and GLT-1. Using a model of TLE, we demonstrated that long-term systemic administration of 17AAG dramatically suppressed spontaneous recurrent seizures and ameliorated astrogliosis. Overall, these results suggest that up-regulation of GLT-1 by inhibiting Hsp90β in reactive astrocytes may be a potential therapeutic target for the treatment of epilepsy and excitotoxicity.

Traumatic Brain Injury Induces Alterations in Cortical Glutamate Uptake without a Reduction in Glutamate Transporter-1 Protein Expression

We hypothesize that the primary mechanism for removal of glutamate from the extracellular space is altered after traumatic brain injury (TBI). To evaluate this hypothesis, we initiated TBI in adult male rats using a 2.0 atm lateral fluid percussion injury (LFPI) model. In the ipsilateral cortex and hippocampus, we found no differences in expression of the primary glutamate transporter in the brain (GLT-1) 24 h after TBI. In contrast, we found a decrease in glutamate uptake in the cortex, but not the hippocampus, 24 h after injury. Because glutamate uptake is potently regulated by protein kinases, we assessed global serine-threonine protein kinase activity using a kinome array platform. Twenty-five kinome array peptide substrates were differentially phoshorylated between LFPI and controls in the cortex, whereas 19 peptide substrates were differentially phosphorylated in the hippocampus (fold change ≥ ± 1.15). We identified several kinases as likely to be involved in acute TBI, including protein kinase B (Akt) and protein kinase C (PKC), which are well-characterized modulators of GLT-1. Exploratory studies using an inhibitor of Akt suggest selective activation of kinases in LFPI versus controls. Ingenuity pathway analyses of implicated kinases from our network model found apoptosis and cell death pathways as top functions in acute LFPI. Taken together, our data suggest diminished activity of glutamate transporters in the prefrontal cortex, with no changes in protein expression of the primary glutamate transporter GLT-1, and global alterations in signaling networks that include serine-threonine kinases that are known modulators of glutamate transport activity.

Glutamate Neurotransmission in Rodent Models of Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and disability in people younger than 45 and is a significant public health concern. In addition to primary mechanical damage to cells and tissue, TBI involves additional molecular mechanisms of injury, termed secondary injury, that continue to evolve over hours, days, weeks, and beyond. The trajectory of recovery after TBI is highly unpredictable and in many cases results in chronic cognitive and behavioral changes. Acutely after TBI, there is an unregulated release of glutamate that cannot be buffered or cleared effectively, resulting in damaging levels of glutamate in the extracellular space. This initial loss of glutamate homeostasis may initiate additional changes in glutamate regulation. The excitatory amino acid transporters (EAATs) are expressed on both neurons and glia and are the principal mechanism for maintaining extracellular glutamate levels. Diffusion of glutamate outside the synapse due to impaired uptake may lead to increased extrasynaptic glutamate signaling, secondary injury through activation of cell death pathways, and loss of fidelity and specificity of synaptic transmission. Coordination of glutamate release and uptake is critical to regulating synaptic strength, long-term potentiation and depression, and cognitive processes. In this review, we will discuss dysregulation of extracellular glutamate and glutamate uptake in the acute stage of TBI and how failure to resolve acute disruptions in glutamate homeostatic mechanisms may play a causal role in chronic cognitive symptoms after TBI.

2,3,7,8-tetrachlorodibenzo-p-dioxin exposure influence the expression of glutamate transporter GLT-1 in C6 glioma cells via the Ca(2+) /protein kinase C pathway

The widespread environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is considered one of the most toxic dioxin-like compounds. Although epidemiological studies have shown that TCDD exposure is linked to some neurological and neurophysiological disorders, the underlying mechanism of TCDD-mediated neurotoxicity has remained unclear. Astrocytes are the most abundant cells in the nervous systems, and are recognized as the important mediators of normal brain functions as well as neurological, neurodevelopmental and neurodegenerative brain diseases. In this study, we investigated the role of TCDD in regulating the expression of glutamate transporter GLT-1 in astrocytes. TCDD, at concentrations of 0.1-100 nm, had no significantly harmful effect on the viability of C6 glioma cells. However, the expression of GLT-1 in C6 glioma cells was downregulated in a dose- and time-dependent manner. TCDD also caused activation of protein kinase C (PKC), as TCDD induced translocation of the PKC from the cytoplasm or perinuclear to the membrane. The translocation of PKC was inhibited by one Ca(2+) blocker, nifedipine, suggesting that the effects are triggered by the initial elevated intracellular concentration of free Ca(2+) . Finally, we showed that inhibition of the PKC activity reverses the TCDD-triggered reduction of GLT-1. In summary, our results suggested that TCDD exposure could downregulate the expression of GLT-1 in C6 via Ca(2+) /PKC pathway. The downregulation of GLT-1 might participate in TCDD-mediated neurotoxicity. Copyright © 2016 John Wiley & Sons, Ltd.

PKC-Dependent GlyT1 Ubiquitination Occurs Independent of Phosphorylation: Inespecificity in Lysine Selection for Ubiquitination

Neurotransmitter transporter ubiquitination is emerging as the main mechanism for endocytosis and sorting of cargo into lysosomes. In this study, we demonstrate PKC-dependent ubiquitination of three different isoforms of the glycine transporter 1 (GlyT1). Incubation of cells expressing transporter with the PKC activator phorbol ester induced a dramatic, time-dependent increase in GlyT1 ubiquitination, followed by accumulation of GlyT1 in EEA1 positive early endosomes. This occurred via a mechanism that was abolished by inhibition of PKC. GlyT1 endocytosis was confirmed in both retinal sections and primary cultures of mouse amacrine neurons. Replacement of only all lysines in the N-and C-termini to arginines prevented ubiquitination and endocytosis, displaying redundancy in the mechanism of ubiquitination. Interestingly, a 40–50% reduction in glycine uptake was detected in phorbol-ester stimulated cells expressing the WT-GlyT1, whereas no significant change was for the mutant protein, demonstrating that endocytosis participates in the reduction of uptake. Consistent with previous findings for the dopamine transporter DAT, ubiquitination of GlyT1 tails functions as sorting signal to deliver transporter into the lysosome and removal of ubiquitination sites dramatically attenuated the rate of GlyT1 degradation. Finally, we showed for the first time that PKC-dependent GlyT1 phosphorylation was not affected by removal of ubiquitination sites, suggesting separate PKC-dependent signaling events for these posttranslational modifications.

Conditional deletion of the glutamate transporter GLT-1 reveals that astrocytic GLT-1 protects against fatal epilepsy while neuronal GLT-1 contributes significantly to glutamate uptake into synaptosomes

GLT-1 (EAAT2; slc1a2) is the major glutamate transporter in the brain, and is predominantly expressed in astrocytes, but at lower levels also in excitatory terminals. We generated a conditional GLT-1 knock-out mouse to uncover cell-type-specific functional roles of GLT-1. Inactivation of the GLT-1 gene was achieved in either neurons or astrocytes by expression of synapsin-Cre or inducible human GFAP-CreERT2. Elimination of GLT-1 from astrocytes resulted in loss of ∼80% of GLT-1 protein and of glutamate uptake activity that could be solubilized and reconstituted in liposomes. This loss was accompanied by excess mortality, lower body weight, and seizures suggesting that astrocytic GLT-1 is of major importance. However, there was only a small (15%) reduction that did not reach significance of glutamate uptake into crude forebrain synaptosomes. In contrast, when GLT-1 was deleted in neurons, both the GLT-1 protein and glutamate uptake activity that could be solubilized and reconstituted in liposomes were virtually unaffected. These mice showed normal survival, weight gain, and no seizures. However, the synaptosomal glutamate uptake capacity (Vmax) was reduced significantly (40%). In conclusion, astrocytic GLT-1 performs critical functions required for normal weight gain, resistance to epilepsy, and survival. However, the contribution of astrocytic GLT-1 to glutamate uptake into synaptosomes is less than expected, and the contribution of neuronal GLT-1 to synaptosomal glutamate uptake is greater than expected based on their relative protein expression. These results have important implications for the interpretation of the many previous studies assessing glutamate uptake capacity by measuring synaptosomal uptake.

Ceftriaxone Protects Astrocytes from MPP(+) via Suppression of NF-κB/JNK/c-Jun Signaling

Ceftriaxone has been shown to attenuate the dopaminergic neuron death and alleviate behavioral disorders in Parkinson's disease models via upregulation of glutamate transporter-1 (GLT-1) and decreases in extracellular glutamate. However, details of how this neuroprotection occurs are uncertain. We hypothesized that cytoprotection by ceftriaxone in astrocytes exposed to 1-methyl-4-phenylpyridinium (MPP(+)) involves suppression of the NF-κB/JNK/c-Jun signaling pathway. Here, we observed a protective effect of ceftriaxone in primary astrocytes exposed to MPP(+). Ceftriaxone enhanced glutamate uptake and promoted primary astrocyte viability after MPP(+) exposure. Ceftriaxone enhances glutamate uptake via upregulation of GLT-1 in the plasma membrane, and alleviates MPP(+)-induced neurotoxicity via suppression of NF-κB/JNK/c-Jun signaling. Collectively, our data offer evidence that increased expression and function of GLT-1 are involved in the protective mechanism of ceftriaxone in astrocytes exposed to MPP(+) in vitro, and we offer insight into the potential therapeutic role of ceftriaxone in treatment of Parkinson's disease.

Extracellular microvesicles from astrocytes contain functional glutamate transporters: regulation by protein kinase C and cell activation

Glutamate transport through astrocytic excitatory amino-acid transporters (EAAT)-1 and EAAT-2 is paramount for neural homeostasis. EAAT-1 has been reported in secreted extracellular microvesicles (eMV, such as exosomes) and because the protein kinase C (PKC) family controls the sub-cellular distribution of EAATs, we have explored whether PKCs drive EAATs into eMV. Using rat primary astrocytes, confocal immunofluorescence and ultracentrifugation on sucrose gradient we here report that PKC activation by phorbol myristate acetate (PMA) reorganizes EAAT-1 distribution and reduces functional [³H]-aspartate reuptake. Western-blots show that EAAT-1 is present in eMV from astrocyte conditioned medium, together with NaK ATPase and glutamine synthetase all being further increased after PMA treatment. However, nanoparticle tracking analysis reveals that PKC activation did not change particle concentration. Functional analysis indicates that eMV have the capacity to reuptake [³H]-aspartate. In vivo, we demonstrate that spinal astrocytic reaction induced by peripheral nerve lesion (spared nerve injury, SNI) is associated with a phosphorylation of PKC δ together with a shift of EAAT distribution ipsilaterally. Ex vivo, spinal explants from SNI rats release eMV with an increased content of NaK ATPase, EAAT-1 and EAAT-2. These data indicate PKC and cell activation as important regulators of EAAT-1 incorporation in eMV, and raise the possibility that microvesicular EAAT-1 may exert extracellular functions. Beyond a putative role in neuropathic pain, this phenomenon may be important for understanding neural homeostasis and a wide range of neurological diseases associated with astrocytic reaction as well as non-neurological diseases linked to eMV release.

Upregulation of glutamate transporter GLT-1 by mTOR-Akt-NF-кB cascade in astrocytic oxygen-glucose deprivation

Excessive extracellular glutamate leads to neuronal death in central nervous system. Excitatory glutamate transporter subtype 2 (GLT-1) carries bulk of glutamate reuptake in cerebral ischemia. Although GLT-1 expression fluctuates during the period of ischemia, little is known about its regulatory mechanism. Here we show an up-regulation of GLT-1 via mammalian target of rapamycin (mTOR)-Akt-nuclear factor-кB (NF-кB) signaling cascade in oxygen glucose deprivation (OGD). We found that brief rapamycin treatment significantly increased GLT-1 expression in cultured astrocytes. Rapamycin increased phosphorylation of raptor at Ser792 and decreased phosphorylation of rictor at Thr1135, suggesting that both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) are involved in GLT-1 expression. This conclusion was further confirmed by raptor and rictor disruption experiments. Akt was activated by mTORC1 inhibition and required for GLT-1 expression because triciribine, a specific inhibitor of Akt, blocked the increase of GLT-1 expression. mTOR-Akt cascade then activated NF-кB and increased кB-motif-binding phosphoprotein (KBBP) expression and GLT-1 transcription. We next demonstrated that mTOR-Akt-NF-кB cascade was activated in OGD and subsequently caused the upregulation of GLT-1. Supporting evidence included: (1) inhibition of Akt or NF-кB occluded OGD-induced GLT-1 upregulation; (2) Raptor knock-down plus OGD did not add to the increase of GLT-1 expression; (3) Intact mTORC2 was required for GLT-1 enhancement. In summary, our data first showed that mTOR-Akt-NF-кB cascade played critical roles to up-regulate GLT-1 in OGD. This signaling cascade may work to promote glutamate uptake in brain ischemia and neurodegenerative diseases.

Inhibition of glycogen synthase kinase 3β activity with lithium prevents and attenuates paclitaxel-induced neuropathic pain

Paclitaxel (taxol) is a first-line chemotherapy-drug used to treat many types of cancers. Neuropathic pain and sensory dysfunction are the major toxicities, which are dose-limiting and significantly reduce the quality of life in patients. Two known critical spinal mechanisms underlying taxol-induced neuropathic pain are an increased production of pro-inflammatory cytokines including interleukin-1β (IL-1β) and suppressed glial glutamate transporter activities. In this study, we uncovered that increased activation of glycogen synthase kinase 3beta (GSK3β) in the spinal dorsal horn was concurrently associated with increased protein expressions of GFAP, IL-1β and a decreased protein expression of glial glutamate transporter 1 (GLT-1), as well as the development and maintenance of taxol-induced neuropathic pain. The enhanced GSK3β activities were supported by the concurrently decreased AKT and mTOR activities. The changes of all these biomarkers were basically prevented when animals received pre-emptive lithium (a GSK3β inhibitor) treatment, which also prevented the development of taxol-induced neuropathic pain. Further, chronic lithium treatment, which began on day 11 after the first taxol injection, reversed the existing mechanical and thermal allodynia induced by taxol. The taxol-induced increased GSK3β activities and decreased AKT and mTOR activities in the spinal dorsal horn were also reversed by lithium. Meanwhile, protein expressions of GLT-1, GFAP and IL-1β in the spinal dorsal horn were improved. Hence, suppression of spinal GSK3β activities is a key mechanism used by lithium to reduce taxol-induced neuropathic pain, and targeting spinal GSK3β is an effective approach to ameliorate GLT-1 expression and suppress the activation of astrocytes and IL-1β over-production in the spinal dorsal horn.

Histamine up-regulates astrocytic glutamate transporter 1 and protects neurons against ischemic injury

Astrocytic glutamate transporter 1 (GLT-1) is responsible for the majority of extracellular glutamate clearance and is essential for preventing excitotoxicity in the brain. Up-regulation of GLT-1 shows benefit effect on ischemia-induced neuronal damage. In present study, we examined the effect of histamine, a neurotransmitter or neuromodulator, on GLT-1 expression and function. In acute hippocampal slices, histamine selectively increased GLT-1 expression independent of neuronal activities. Similar up-regulation of GLT-1 was also observed after histamine treatment in pure cultured astrocytes, which was abolished by H1 receptor antagonist or PKC inhibitor. Cell surface biotinylation and whole-cell patch recordings of glutamate transporter current confirmed the up-regulation of functional GLT-1 following histamine exposure. Histamine treatment decreased the extracellular glutamate content and alleviated neuronal cell death induced by exogenous glutamate challenge. Moreover, we found a significant neuroprotective effect of histamine in brain slices after oxygen-glucose deprivation (OGD). In addition, histidine, the precursor of histamine, also showed neuroprotection against ischemic injury, which was accompanied by reversion of declined expression of GLT-1 in adult rats subjected to middle cerebral artery occlusion (MCAO). These neuroprotective effects of histamine/histidine were blocked by GLT-1 specific inhibitor dihydrokainate or H1 receptor antagonist. In summary, our results suggest that histamine up-regulates GLT-1 expression and function via astrocytic H1 receptors, thus resulting in neuroprotection against excitotoxicity and ischemic injury.

Functional modulation of the glutamate transporter variant GLT1b by the PDZ domain protein PICK1

The dominant glutamate transporter isoform in the mammalian brain, GLT1, exists as at least three splice variants, GLT1a, GLT1b, and GLT1c. GLT1b interacts with the scaffold protein PICK1 (protein interacting with kinase C1), which is implicated in glutamatergic neurotransmission via its regulatory effect on trafficking of AMPA-type glutamate receptors. The 11 extreme C-terminal residues specific for the GLT1b variant are essential for its specific interaction with the PICK1 PDZ domain, but a functional consequence of this interaction has remained unresolved. To identify a functional effect of PICK1 on GLT1a or GLT1b separately, we employed the Xenopus laevis expression system. GLT1a and GLT1b displayed similar electrophysiological properties and EC50 for glutamate. Co-expressed PICK1 localized efficiently to the plasma membrane and resulted in a 5-fold enhancement of the leak current in GLT1b-expressing oocytes with only a minor effect on [(3)H]glutamate uptake. Three different GLT1 substrates all caused a slow TBOA-sensitive decay in the membrane current upon prolonged application, which provides support for the leak current being mediated by GLT1b itself. Leak and glutamate-evoked currents in GLT1a-expressing oocytes were unaffected by PICK1 co-expression. PKC activation down-regulated GLT1a and GLT1b activity to a similar extent, which was not affected by co-expression of PICK1. In conclusion, PICK1 may not only affect glutamatergic neurotransmission by its regulatory effect on glutamate receptors but may also affect neuronal excitability via an increased GLT1b-mediated leak current. This may be particularly relevant in pathological conditions such as amyotrophic lateral sclerosis and cerebral hypoxia, which are associated with neuronal GLT1b up-regulation.

Manganese toxicity in the central nervous system: the glutamine/glutamate-γ-aminobutyric acid cycle

Manganese (Mn) is an essential trace element that is required for maintaining proper function and regulation of numerous biochemical and cellular reactions. Despite its essentiality, at excessive levels Mn is toxic to the central nervous system (CNS). Increased accumulation of Mn in specific brain regions, such as the substantia nigra, globus pallidus and striatum, triggers neurotoxicity resulting in a neurological brain disorder, termed manganism. Mn has been also implicated in the pathophysiology of several other neurodegenerative diseases. Its toxicity is associated with disruption of the glutamine (Gln)/glutamate (Glu)-γ-aminobutyric acid (GABA) cycle (GGC) between astrocytes and neurons, thus leading to changes in Glu-ergic and/or GABAergic transmission and Gln metabolism. Here we discuss the common mechanisms underlying Mn-induced neurotoxicity and their relationship to CNS pathology and GGC impairment.

Role of astrocytes in manganese mediated neurotoxicity

Astrocytes are responsible for numerous aspects of metabolic support, nutrition, control of the ion and neurotransmitter environment in central nervous system (CNS). Failure by astrocytes to support essential neuronal metabolic requirements plays a fundamental role in the pathogenesis of brain injury and the ensuing neuronal death. Astrocyte-neuron interactions play a central role in brain homeostasis, in particular via neurotransmitter recycling functions. Disruption of the glutamine (Gln)/glutamate (Glu) -γ-aminobutyric acid (GABA) cycle (GGC) between astrocytes and neurons contributes to changes in Glu-ergic and/or GABA-ergic transmission, and is associated with several neuropathological conditions, including manganese (Mn) toxicity. In this review, we discuss recent advances in support of the important roles for astrocytes in normal as well as neuropathological conditions primarily those caused by exposure to Mn.

Flotillins regulate membrane mobility of the dopamine transporter but are not required for its protein kinase C dependent endocytosis

Flotillins were proposed to mediate clathrin-independent endocytosis, and recently, flotillin-1 was implicated in the protein kinase C (PKC)-triggered endocytosis of the dopamine transporter (DAT). Since endocytosis of DAT was previously shown to be clathrin-mediated, we re-examined the role of clathrin coat proteins and flotillin in DAT endocytosis using DAT tagged with the hemagglutinin epitope (HA) in the extracellular loop and a quantitative HA antibody uptake assay. Depletion of flotillin-1, flotillin-2 or both flotillins together by small interfering RNAs (siRNAs) did not inhibit PKC-dependent internalization and degradation of HA-DAT. In contrast, siRNAs to clathrin heavy chain and μ2 subunit of clathrin adaptor complex AP-2 as well as a dynamin inhibitor Dyngo-4A significantly decreased PKC-dependent endocytosis of HA-DAT. Similarly, endocytosis and degradation of DAT that is not epitope-tagged were highly sensitive to the clathrin siRNAs and dynamin inhibition but were not affected by flotillin knockdown. Very little co-localization of DAT with flotillins was observed in cells ectopically expressing DAT and in cultured mouse dopaminergic neurons. Depletion of flotillins increased diffusion rates of HA-DAT in the plasma membrane, suggesting that flotillin-organized microdomains may regulate the lateral mobility of DAT. We propose that clathrin-mediated endocytosis is the major pathway of PKC-dependent internalization of DAT, and that flotillins may modulate functional association of DAT with plasma membrane rafts rather than mediate DAT endocytosis.

Astrocytic group I mGluR-dependent potentiation of astrocytic glutamate and potassium uptake

One of the most important functions of astrocytes is removal of glutamate released during synaptic transmission. Surprisingly, the mechanisms by which astrocyte glutamate uptake is acutely modulated remain to be clarified. Astrocytes express metabotropic glutamate receptors (mGluRs) and other G protein-coupled receptors (GPCRs), which are activated during neuronal activity. Here, we test the hypothesis that astrocytic group I mGluRs acutely regulate glutamate uptake by astrocytes in situ. This hypothesis was tested in acute mouse hippocampal slices. Activation of astrocytic mGluRs, using a tetanic high-frequency stimulus (HFS) applied to Schaffer collaterals, led to potentiation of the amplitude of the synaptically evoked glutamate transporter currents (STCs) and associated charge transfer without changes in kinetics. Similar potentiation of STCs was not observed in the presence of group I mGluR antagonists or the PKC inhibitor, PKC 19-36, suggesting that HFS-induced potentiation of astrocyte glutamate uptake is astrocytic group I mGluR and PKC dependent. Pharmacological stimulation of a transgenic GPCR (MrgA1R), expressed exclusively in astrocytes, also potentiated STC amplitude and charge transfer, albeit quicker and shorter lasting compared with HFS-induced potentiation. The amplitude of the slow, inward astrocytic current due to potassium (K(+)) influx was also enhanced following activation of the endogenous mGluRs or the astrocyte-specific MrgA1 Gq GPCRs. Taken together, these findings suggest that astrocytic group I mGluR activation has a synergistic, modulatory effect on the uptake of glutamate and K(+).

The rates of postmortem proteolysis of glutamate transporters differ dramatically between cells and between transporter subtypes

Glutamate transporters (GLT-1, GLAST, EAAC1) limit the actions of excitatory amino acids. Because a disturbed transporter operation can cause or aggravate neurological diseases, transporters are of considerable neuropathological interest. Human samples, however, are seldom obtained fresh. Here, we used mice brains to study how fast glutamate transporters are degraded after death. Immunoblots showed that terminal GLT-1 epitopes (within residues 1-26 and 518-573) had mostly disappeared after 24 hr. GLAST termini (1-25 and 522-543) degraded slightly slower. In contrast, epitopes within central parts of GLT-1 (493-508) and the EAAC1 C-terminus (510-523) were readily detectable after 72 hr. The decline in immunoreactivity of the GLT-1 and GLAST termini was also seen in tissue sections, but proteolysis did not happen synchronously in all cells. At 24 hr, scattered cells remained strongly immunopositive, while the majority of cells were completely immunonegative. GLAST and GLT-1 co-localized in neocortical tissue, but at 12 hr, many GLAST-positive cells had lost the GLT-1 termini. The uneven disappearance of labeling was not observed with the antibodies to GLT-1 residues 493-508. The immunoreactivity to this epitope correlated better with the reported glutamate uptake activity. Thus, postmortem delay may affect epitopes differently, possibly causing erroneous conclusions about relative expression levels.

Mechanism of Mn(II)-mediated dysregulation of glutamine-glutamate cycle: focus on glutamate turnover

Manganese (Mn) has been implicated in the impairment of the glutamate-glutamine cycling (GGC) by deregulation of Glu and glutamine (Gln) turnover in astrocytes. Here, we have examined possible mechanisms involved in the Mn(II)-mediated disruption of Glu turnover, including those related to protein degradation, such as the proteasomal and lysosomal machinery. Our study revealed that lysosome but not proteasomal inhibition is responsible for down-regulation of the Glu transporter after Mn(II) treatment. Because protein kinase C (PKC) activation leads to the down-regulation of Glu carriers, and Mn(II) increases PKC activity, we hypothesized that the PKC signaling contributes to the Mn(II)-mediated disruption of Glu turnover. Our results show that PKC activation causes a decrease in Glu uptake and that inhibition of PKC reverses Mn(II)-dependent down-regulation of Glu influx as well as glutamate transporter 1 (GLT1) and glutamate-aspartate transporter (GLAST) protein level. Co-immunoprecipitation studies show association of GLT1 with the PKCδ and PKCα isoforms and Mn(II)-induced specific increase in PKCδ-GLT1 interaction. In addition, astrocytes transfected with shRNA against PKCδ show decreased sensitivity to Mn(II) compared with those transfected with control shRNA or shRNA targeted against PKCα. Taken together, these findings demonstrate that PKCδ signaling is involved in the Mn(II)-induced deregulation of Glu turnover in astrocytes.

Cell surface turnover of the glutamate transporter GLT-1 is mediated by ubiquitination/deubiquitination

The main glutamate transporter in the brain, GLT-1, mediates glutamatergic neurotransmission in both physiological and pathological conditions. GLT-1 activity is controlled by both constitutive and regulated trafficking, and although recent evidence indicates that the turnover of this protein in the plasma membrane is accelerated by protein kinase C via an ubiquitin-dependent process, the mechanisms driving the constitutive trafficking of GLT-1 remain unexplored. Here, we used a heterologous system and primary astrocytes to investigate the turnover of GLT-1 and the role of ubiquitin attachment in this process. We show that GLT-1 is endocytosed constitutively in a clathrin-dependent manner, recycling the transporter into endosomes containing EEA1 and Rab4, a marker of rapidly recycling endosomes, and not Rab11 or Rab7, markers of the slow recycling and late endosomal compartments, respectively. We also show that this process is dependent on ubiquitination, because the inhibitor of the ubiquitin-activating enzyme E1, 4[4-(5-nitro-furan-2-ylmethylene)-3,5-dioxo-pyrazolidin-1-yl]-benzoic acid ethyl ester, promotes the retention of GLT-1 at the plasma membrane. Moreover, site-directed mutagenesis demonstrated the involvement of lysines 517 and 526 of GLT-1 in the constitutive internalization of the transporter. The translocation of GLT-1 from the recycling endosomes to the plasma membrane was blocked by LDN-57444, a specific inhibitor to the deubiquitinating enzyme (DUB) ubiquitin C-terminal hydrolase-L1, but not by an inhibitor of the related DUB ubiquitin C-terminal hydrolase-L3, supporting the existence of specific ubiquitination/deubiquitination cycles that ensure the correct concentrations of GLT-1 at the cell surface.

Protein kinase C (PKC)-promoted endocytosis of glutamate transporter GLT-1 requires ubiquitin ligase Nedd4-2-dependent ubiquitination but not phosphorylation

Glutamate transporter-1 (GLT-1) is the main glutamate transporter in the central nervous system, and its concentration severely decreases in neurodegenerative diseases. The number of transporters in the plasma membrane reflects the balance between their insertion and removal, and it has been reported that the regulated endocytosis of GLT-1 depends on its ubiquitination triggered by protein kinase C (PKC) activation. Here, we identified serine 520 of GLT-1 as the primary target for PKC-dependent phosphorylation, although elimination of this serine did not impair either GLT-1 ubiquitination or endocytosis in response to phorbol esters. In fact, we present evidence indicating that the ubiquitin ligase Nedd4-2 mediates the PKC-dependent ubiquitination and down-regulation of GLT-1. Overexpression of Nedd4-2 increased the ubiquitination of the transporter and promoted its degradation. Moreover, phorbol myristate acetate enhanced Nedd4-2 phosphorylation and the formation of GLT-1·Nedd4-2 complexes, whereas siRNA knockdown of Nedd4-2 prevented ubiquitination, endocytosis, and the concomitant decrease in GLT-1 activity triggered by PKC activation. These results indicate that GLT-1 endocytosis is independent of its phosphorylation and that Nedd4-2 mediates PKC-dependent down-regulation of the transporter.

Retinoic acid mediates the expression of glutamate transporter-1 in rat astrocytes through genomic RXR action and non-genomic protein kinase C signaling pathway

Astrocytic glutamate transporter-1 (GLT-1) is responsible for 90% of forebrain glutamate uptake in the adult CNS. Retinoic acid (RA) is a potent regulator of neural cell differentiation and neuronal maturation in the developing CNS through activation of RA receptors/retinoic X receptors (RXRs) or non-genomic mechanisms. Although rat GLT-1 contains several RXR binding regions, RA-triggered RXR mechanisms regulating GLT-1 expression remain unknown. RA applied at submicromolar concentrations for 24 h significantly reduced GLT-1 mRNA and membrane levels in astrocytes and dibutyryl cAMP (dbcAMP)-primed astrocytes. An RXR agonist reduced astrocytic GLT-1 mRNA expression, whereas an RXR antagonist blocked the effects of RA on the reduction of astrocytic GLT-1 mRNA expression. Electrophoresis motility shift assay indicated that RA-treatment increased astrocytic RXR-DNA binding activity. RA-induced reduction in GLT-1 mRNA expression was also observed in dbcAMP-primed astrocytes. Through lentivirus-mediated astrocytic over-expression of rat GLT-1, levels of GLT-1 in the processes of dbcAMP-treated astrocytes were attenuated by exposure to RA. The protein kinase C inhibitor, Bis I, restored GLT-1 distribution in the processes of RA-treated dbcAMP-primed astrocytes. These results suggest that RA reduces astrocytic GLT-1 levels through both RXR-mediated inhibition at the transcriptional level and triggering activation of protein kinase C which reduces cell surface GLT-1 levels.

Co-compartmentalization of the astroglial glutamate transporter, GLT-1, with glycolytic enzymes and mitochondria

Efficient excitatory transmission depends on a family of transporters that use the Na(+)-electrochemical gradient to maintain low synaptic concentrations of glutamate. These transporters consume substantial energy in the spatially restricted space of fine astrocytic processes. GLT-1 (EAAT2) mediates the bulk of this activity in forebrain. To date, relatively few proteins have been identified that associate with GLT-1. In the present study, GLT-1 immunoaffinity isolates were prepared from rat cortex using three strategies and analyzed by liquid chromatography-coupled tandem mass spectrometry. In addition to known interacting proteins, the analysis identified glycolytic enzymes and outer mitochondrial proteins. Using double-label immunofluorescence, GLT-1 was shown to colocalize with the mitochondrial matrix protein, ubiquinol-cytochrome c reductase core protein 2 or the inner mitochondrial membrane protein, ADP/ATP translocase, in rat cortex. In biolistically transduced hippocampal slices, fluorescently tagged GLT-1 puncta overlapped with fluorescently tagged mitochondria along fine astrocytic processes. In a Monte Carlo-type computer simulation, this overlap was significantly more frequent than would occur by chance. Furthermore, fluorescently tagged hexokinase-1 overlapped with mitochondria or GLT-1, strongly suggesting that GLT-1, mitochondria, and the first step in glycolysis are cocompartmentalized in astrocytic processes. Acute inhibition of glycolysis or oxidative phosphorylation had no effect on glutamate uptake in hippocampal slices, but simultaneous inhibition of both processes significantly reduced transport. Together with previous results, these studies show that GLT-1 cocompartmentalizes with Na(+)/K(+) ATPase, glycolytic enzymes, and mitochondria, providing a mechanism to spatially match energy and buffering capacity to the demands imposed by transport.

Protein kinase C-mediated phosphorylation of a single serine residue on the rat glial glutamine transporter SN1 governs its membrane trafficking

Molecular mechanisms involved in the replenishment of the fast neurotransmitters glutamate and GABA are poorly understood. Glutamine sustains their generation. However, glutamine formation from the recycled transmitters is confined to glial processes and requires facilitators for its translocation across the glial and neuronal membranes. Indeed, glial processes are enriched with the system N transporter SN1 (Slc38a3), which, by bidirectional transport, maintains steady extracellular glutamine levels and thereby furnishes neurons with the primary precursor for fast neurotransmitters. We now demonstrate that SN1 is phosphorylated by protein kinase Cα (PKCα) and PKCγ. Electrophysiological characterization shows that phosphorylation reduces V(max) dramatically, whereas no significant effects are seen on the K(m). Phosphorylation occurs specifically at a single serine residue (S52) in the N-terminal rat (Rattus norvegicus) SN1 and results in sequestration of the protein into intracellular reservoirs. Prolonged activation of PKC results in partial degradation of SN1. These results provide the first demonstration of phosphorylation of SN1 and regulation of its activity at the plasma membrane. Interestingly, membrane trafficking of SN1 resembles that of the glutamate transporter GLT and the glutamate-aspartate transporter GLAST: it involves the same PKC isoforms and occurs in the same glial processes. This suggests that the glutamate/GABA-glutamine cycle may be modified at two key points by similar signaling events and unmasks a prominent role for PKC-dependent phosphorylation. Our data suggest that extracellular glutamine levels may be fine-tuned by dynamic regulation of glial SN1 activity, which may impact on transmitter generation, contribute to defining quantal size, and have profound effects on synaptic plasticity.

Insulin increases glutamate transporter GLT1 in cultured astrocytes

The astroglial cell-specific glutamate transporter subtype 2 (excitatory amino acid transporter 2, GLT1) plays an important role in excitotoxicity that develops after damage to the central nervous system (CNS) is incurred. Both the protein kinase C signaling pathway and the epidermal growth factor (EGF) pathway have been suggested to participate in the modulation of GLT1, but the modulatory mechanisms of GLT1 expression are not fully understood. In the present study, we aimed to evaluate the effects of insulin on GLT1 expression. We found that short-term stimulation of insulin led to the upregulation of both total and surface expressions of GLT1. Akt phosphorylation increased after insulin treatment, and triciribine, the inhibitor of Akt phosphorylation, significantly inhibited the effects of insulin. We also found that the upregulation of GLT1 expression correlated with increased kappa B motif-binding phosphoprotein (KBBP) and GLT1 mRNA levels. Our results suggest that insulin may modulate the expression of astrocytic GLT1, which might play a role in reactive astrocytes after CNS injuries.

Glutamine synthetase gene expression and glutamate transporters in C6-glioma cells

Glutamine synthetase (GS) is the major glutamate-forming enzyme of vertebrae and is accepted to be a marker of astroglial cells. Maturation of astroglial cells is characterized by an increase in GS activity, and the regulation of this enzyme is the topic of many publications. The amino acid glutamate is the major excitatory neurotransmitter in the brain and mediates normal excitatory synaptic transmission by interaction with postsynaptic receptors. Glutamate also acts as a potent neurotoxin when present at high concentration. Glutamate neurotoxicity plays an important role in the pathophysiology of many neurological disorders, such as Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. In the normal condition, L-glutamate is predominantly taken up, metabolized and recycled by astrocytes through the glutamate transporters (GLAST/GLT1) and glutamine synthetase (GS) catalytic activity. Because of the fundamental role of these glutamate transporters and the glutamine synthetase enzyme in controlling cerebral glutamate level, regulation of GS and studying of the glutamate transporters in glial cells is important. Astrocytes are supportive cells and act as the site of detoxification of glutamate in the brain. However, their isolation from the brain is a tedious, costly and time consuming procedure. On the other hand, the C6-glioma cells are readily available on the market. They are well characterized and have been a useful model for CNS glia in many laboratories. For this study, we used the C6-glioma cell line as a model system. We examined the presence or absence of glial specific glutamate transporters (GLTI and GLAST) in C6-glioma cells, which was done by immunocytochemistry. We also examined glutamine synthetase gene expression in these cells by treatment of the C6-glioma cells with estrogen (17ß estradiol). The findings from this study provide useful information about C6-glioma cells which makes the study of the CNS tremendously inexpensive.

Strategies for treatment in Alexander disease

Alexander disease is a rare and generally fatal disorder of the CNS, originally classified among the leukodystrophies because of the prominent myelin deficits found in young patients. The most common form of this disease affects infants, who often have profound mental retardation and a variety of developmental delays, but later onset forms also occur, sometimes with little or no white matter pathology at all. The pathological hallmark of Alexander disease is the inclusion body, known as Rosenthal fiber, within the cell bodies and processes of astrocytes. Recent genetic studies identified heterozygous missense mutations in glial fibrillary acidic protein (GFAP), the major intermediate filament protein in astrocytes, as the cause of nearly all cases of Alexander disease. These studies have transformed our view of this disorder and opened new directions for investigation and clinical practice, particularly with respect to diagnosis. Mechanisms by which expression of mutant forms of glial fibrillary acidic protein (GFAP) lead to the pleiotropic manifestations of disease (afflicting cell types beyond the ones expressing the mutant gene) are slowly coming into focus. Ideas are beginning to emerge that suggest several compelling therapeutic targets for interventions that might slow or arrest the evolution of the disease. This review will outline the rationale for pursuing these strategies, and highlight some of the critical issues that must be addressed in the planning of future clinical trials.

Glutamate transporter coupling to Na,K-ATPase

Deactivation of glutamatergic signaling in the brain is mediated by glutamate uptake into glia and neurons by glutamate transporters. Glutamate transporters are sodium-dependent proteins that putatively rely indirectly on Na,K-ATPases to generate ion gradients that drive transmitter uptake. Based on anatomical colocalization, mutual sodium dependency, and the inhibitory effects of the Na,K-ATPase inhibitor ouabain on glutamate transporter activity, we postulated that glutamate transporters are directly coupled to Na,K-ATPase and that Na,K-ATPase is an essential modulator of glutamate uptake. Na,K-ATPase was purified from rat cerebellum by tandem anion exchange and ouabain affinity chromatography, and the cohort of associated proteins was characterized by mass spectrometry. The alpha1-alpha 3 subunits of Na,K-ATPase were detected, as were the glutamate transporters GLAST and GLT-1, demonstrating that glutamate transporters copurify with Na,K-ATPases. The link between glutamate transporters and Na,K-ATPase was further established by coimmunoprecipitation and colocalization. Analysis of the regulation of glutamate transporter and Na,K-ATPase activities was assessed using [(3)H]D-aspartate, [(3)H]L-glutamate, and rubidium-86 uptake into synaptosomes and cultured astrocytes. In synaptosomes, ouabain produced a dose-dependent inhibition of glutamate transporter and Na,K-ATPase activities, whereas in astrocytes, ouabain showed a bimodal effect whereby glutamate transporter activity was stimulated at 1 microm ouabain and inhibited at higher concentrations. The effects of protein kinase inhibitors on [(3)H]D-aspartate uptake indicated the selective involvement of Src kinases, which are probably a component of the Na,K-ATPase/glutamate transporter complex. These findings demonstrate that glutamate transporters and Na,K-ATPases are part of the same macromolecular complexes and operate as a functional unit to regulate glutamatergic neurotransmission.

The four major N- and C-terminal splice variants of the excitatory amino acid transporter GLT-1 form cell surface homomeric and heteromeric assemblies

The L-glutamate transporter GLT-1 is an abundant central nervous system (CNS) membrane protein of the excitatory amino acid transporter (EAAT) family that controls extracellular L-glutamate levels and is important in limiting excitotoxic neuronal death. Using reverse transcription-polymerase chain reaction, we have determined that four mRNAs encoding GLT-1 exist in mouse brain, with the potential to encode four GLT-1 isoforms that differ in their N and C termini. We expressed all four isoforms (termed MAST-KREK, MPK-KREK, MAST-DIETCI, and MPK-DIETCI according to amino acid sequence) in a range of cell lines and primary astrocytes and show that each isoform can reach the cell surface. In transfected human embryonic kidney (HEK) 293 or COS-7 cells, all four isoforms support high-affinity sodium-dependent L-glutamate uptake with identical pharmacological and kinetic properties. Inserting a viral epitope (tagged with V5, hemagglutinin, or FLAG) into the second extracellular domain of each isoform allowed coimmunoprecipitation and time-resolved Förster resonance energy transfer (tr-FRET) studies using transfected HEK-293 cells. Here we show for the first time that each of the four isoforms is able to combine to form homomeric and heteromeric assemblies, each of which is expressed at the cell surface of primary astrocytes. After activation of protein kinase C by phorbol ester, V5-tagged GLT-1 is rapidly removed from the cell surface of HEK-293 cells and degraded. This study provides direct biochemical evidence for oligomeric assembly of GLT-1 and reports the development of novel tools to provide insight into the trafficking of GLT-1.

Ubiquitination-mediated internalization and degradation of the astroglial glutamate transporter, GLT-1

Sodium-dependent glutamate uptake is essential for limiting excitotoxicity, and dysregulation of this process has been implicated in a wide array of neurological disorders. The majority of forebrain glutamate uptake is mediated by the astroglial glutamate transporter, GLT-1. We and others have shown that this transporter undergoes endocytosis and degradation in response to activation of protein kinase C (PKC), however, the mechanisms involved remain unclear. In the current study, transfected C6 glioma cells or primary cortical cultures were used to show that PKC activation results in incorporation of ubiquitin into GLT-1 immunoprecipitates. Mutation of all 11 lysine residues in the amino and carboxyl-terminal domains to arginine (11R) abolished this signal. Selective mutation of the seven lysine residues in the carboxyl terminus (C7K-R) did not eliminate ubiquitination, but it completely blocked PKC-dependent internalization and degradation. Two families of variants of GLT-1 were prepared with various lysine residues mutated to arginine. Analyses of these constructs indicated that redundant lysine residues in the carboxyl terminus were sufficient for the appearance of ubiquitinated product and degradation of GLT-1. Together these data define a novel mechanism by which the predominant forebrain glutamate transporter can be rapidly targeted for degradation.