A role for glutamate transporters in neurodegenerative diseases

Gene expression profiling in respond to TBT exposure in small abalone Haliotis diversicolor

In this study, we investigated the gene expression profiling of small abalone, Haliotis diversicolor by tributyltin (TBT) exposure using a cDNA microarray containing 2473 unique transcripts. Totally, 107 up-regulated genes and 41 down-regulated genes were found. For further investigation of candidate genes from microarray data and EST analysis, quantitative real-time PCR was performed at 6 h, 24 h, 48 h, 96 h and 192 h TBT exposure. 26 genes were found to be significantly differentially expressed in different time course, 3 of them were unknown. Some gene homologues like cellulose, endo-beta-1,4-glucanase, ferritin subunit 1 and thiolester containing protein II CG7052-PB might be the good biomarker candidate for TBT monitor. The identification of stress response genes and their expression profiles will permit detailed investigation of the defense responses of small abalone genes.

GLT-1 overexpression attenuates bladder nociception and local/cross-organ sensitization of bladder nociception

Glutamatergic pathways mediate transmission of pain. Strategies to reduce glutamatergic neurotransmission may have beneficial effects to mitigate nociception. Recent work revealed that overexpression of the astrocytic glutamate transporter (GLT-1) by transgenic or pharmacologic approaches produced a diminished visceral nociceptive response to colonic distension. The purpose of this study was to determine the effect of GLT-1 overexpression on the visceromotor response to bladder distension. Increased glutamate uptake activity produced by 1-wk ceftriaxone (CTX) treatment attenuated 60-64% the visceromotor response to graded bladder distension compared with vehicle-treated mice. One-hour pretreatment with selective GLT-1 antagonist dihydrokainate reversed the blunted visceromotor response to bladder distension produced by 1-wk CTX, suggesting that GLT-1 overexpression mediated the analgesic effect of CTX. Moreover, sensitization of the visceromotor response to bladder distension produced by local bladder irritation (acrolein) was also attenuated by 1-wk CTX treatment. A model of cross-organ sensitization of bladder visceromotor response to distension was next studied to determine whether increased expression of GLT-1 can mitigate colon to bladder sensitization. Intracolonic trinitrobenzene sulfonic acid (TNBS) administered 1 h before eliciting the visceromotor response to graded bladder distension produced a 75-138% increase in visceromotor response compared with animals receiving intracolonic vehicle. In marked contrast, animals treated with 1-wk CTX + intracolonic TNBS showed no enhanced visceromotor response compared with the 1-wk vehicle + intracolonic vehicle group. The study suggests that GLT-1 overexpression attenuates the visceromotor response to bladder distension and both local irritant-induced and cross-organ-sensitized visceromotor response to bladder distension.

BMAA neurotoxicity in Drosophila

We report the establishment of an in vivo model using the fruit fly Drosophila melanogaster to investigate the toxic effects of L-BMAA. We found that dietary intake of BMAA reduced the lifespan as well as the neurological functions of flies. Furthermore, we have developed an HPLC method to reliably detect both free and protein-bound BMAA in fly tissue extracts.

A glutamatergic component of lead toxicity in adult brain: the role of astrocytic glutamate transporters

Astroglial cells have a variety of roles in the central nervous system (CNS), providing a homeostasis for the proper functioning of neuronal cells. The classical view concerning the supportive role of astroglia towards associated neurons has to be extended. A great number of new evidences suggest that astrocytes interact closely with neurons being involved in the active control of neuronal activity and metabolism, forming with pre- and postsynaptic nerve terminals a tripartite synapse. Astrocytes control many aspects of brain function. Regulation of extracellular glutamate concentration, potentially neurotoxic neurotransmitter, is fundamental. Glial glutamate transporters system is of importance in protection against glutamate excitotoxicity and antioxidant defence system which is glutathione. When astrocytes fail to function properly, they influence the degree of neuronal damage. Thus, astrocytes are involved to a very great extent into numerous brain pathologies, including toxicity of heavy metals, like lead (Pb). Under pathological conditions they appear to express two opposite features: they are neuroprotective (until they can) or deleterious for neurons and may participate in neuronal damage. The very well known affinity of Pb to astroglia and the changes in glutamatergic transmission upon Pb toxicity, led us to discuss the role of astroglia and astrocytic glutamate transporters in the neurotoxicity of this metal. Our observations are viewed against a background of other results.

Morphine induces ubiquitin-proteasome activity and glutamate transporter degradation

Glutamate transporters play a crucial role in physiological glutamate homeostasis, neurotoxicity, and glutamatergic regulation of opioid tolerance. However, how the glutamate transporter turnover is regulated remains poorly understood. Here we show that chronic morphine exposure induced posttranscriptional down-regulation of the glutamate transporter EAAC1 in C6 glioma cells with a concurrent decrease in glutamate uptake and increase in proteasome activity, which were blocked by the selective proteasome inhibitor MG-132 or lactacystin but not the lysosomal inhibitor chloroquin. At the cellular level, chronic morphine induced the PTEN (phosphatase and tensin homolog deleted on chromosome Ten)-mediated up-regulation of the ubiquitin E3 ligase Nedd4 via cAMP/protein kinase A signaling, leading to EAAC1 ubiquitination and proteasomal degradation. Either Nedd4 or PTEN knockdown with small interfering RNA prevented the morphine-induced EAAC1 degradation and decreased glutamate uptake. These data indicate that cAMP/protein kinase A signaling serves as an intracellular regulator upstream to the activation of the PTEN/Nedd4-mediated ubiquitin-proteasome system activity that is critical for glutamate transporter turnover. Under an in vivo condition, chronic morphine exposure also induced posttranscriptional down-regulation of the glutamate transporter EAAC1, which was prevented by MG-132, and transcriptional up-regulation of PTEN and Nedd4 within the spinal cord dorsal horn. Thus, inhibition of the ubiquitin-proteasome-mediated glutamate transporter degradation may be an important mechanism for preventing glutamate overexcitation and may offer a new strategy for treating certain neurological disorders and improving opioid therapy in chronic pain management.

Positive and negative regulation of EAAT2 by NF-kappaB: a role for N-myc in TNFalpha-controlled repression

The glutamate transporter gene, EAAT2/GLT-1, is induced by epidermal growth factor (EGF) and downregulated by tumor necrosis factor alpha (TNFalpha). While TNFalpha is generally recognized as a positive regulator of NF-kappaB-dependent gene expression, its ability to control transcriptional repression is not well characterized. Additionally, the regulation of NF-kappaB by EGF is poorly understood. Herein, we demonstrate that both TNFalpha-mediated repression and EGF-mediated activation of EAAT2 expression require NF-kappaB. We show that EGF activates NF-kappaB independently of signaling to IkappaB. Furthermore, TNFalpha can abrogate IKKbeta- and p65-mediated activation of EAAT2. Our results suggest that NF-kappaB can intrinsically activate EAAT2 and that TNFalpha mediates repression through a distinct pathway also requiring NF-kappaB. Consistently, we find that N-myc is recruited to the EAAT2 promoter with TNFalpha and that N-myc-binding sites are required for TNFalpha-mediated repression. Moreover, N-myc overexpression inhibits both basal and p65-induced activation of EAAT2. Our data highlight the remarkable specificity of NF-kappaB activity to regulate gene expression in response to diverse cellular signals and have implications for glutamate homeostasis and neurodegenerative disease.

Glutamate transporter cluster formation in astrocytic processes regulates glutamate uptake activity

Glutamate is the predominant excitatory neurotransmitter in the CNS, and it is removed from the synaptic cleft by sodium-dependent glutamate transport activity. Glutamate transporter-1 (GLT-1) is expressed predominantly in astroglial cells and is responsible for the largest proportion of glutamate transport in the adult forebrain. In the present study, we demonstrate the ability of endogenous and recombinant GLT-1 to form clusters in astrocytic processes and characterize the mobility and physiological importance of these clusters in the regulation of GLT-1 activity in the presence or absence of neurons. At the distal end of C6 glioma cell processes, GLT-1 clusters undergo rapid morphological changes in both shape and size, and these changes are inhibited by cytochalasin D treatment, suggesting that the morphogenesis of GLT-1 clusters is highly dependent on the actin network. Treatment of astrocytes with phorbol 12-myristate 13-acetate (PMA) quickly and preferentially decreases GLT-1 localization on the process membrane, leading to de novo generation of GLT-1 clusters along the process shaft. Pretreatment with the PKC inhibitor bisindolylmaleimide II (Bis II), with sucrose (0.4 m), or through the expression of a dominant-negative form of dynamin prevents PMA-induced GLT-1 internalization and cluster formation. In terms of glutamate transporter function, PMA treatment elicits a significant decrease in GLT-1 activity that is prevented by preexposure to either Bis II or hypertonic treatment. Together, these data indicate that GLT-1 trafficking and cluster formation in glial cell processes are dynamic events that play important roles in regulating glutamate uptake in astrocytes and glioma cells.