Glutamate transporters

This research theme focuses on the role of glutamate transporters in neurological disorders. The different glutamate transporters that are being studied comprise system xc- or the cystine/glutamate antiporter, the Na+/K+-dependent glutamate reuptake transporters (EAATs) and the vesicular glutamate transporters (VGLUTs).

Figure: Glutamate (Glu) is loaded into presynaptic vesicles by vesicular glutamate transporters (VGLUTs). After its release into the synaptic cleft, glutamate is cleared by high-affinity Na+/K+-dependent glutamate transporters (EAATs). Glutamate can be released into the extrasynaptic space via system xc- or the cystine/glutamate antiporter. This antiporter imports one cystine molecule in exchange for one glutamate molecule.


Recently, most of our projects focus on system xc-, with xCT as specific subunit. This antiporter imports one cystine molecule into the glial cell in exchange for one glutamate molecule. Cystine is intracellularly reduced to cysteine, the rate-limiting building block in the synthesis of glutathione. As such, increased activity of system xc- can protect cells against oxidative stress. Yet, this increased uptake of cystine is coupled to increased release of glutamate and can consequently result in toxic extracellular glutamate levels. The involvement of this antiporter in neurological disorders that are commonly characterized by increased oxidative stress as well as excitotoxic damage, can thus be dual.

System xc- in the brain: screening of antibodies

PIs: Ann Massie, Ilse Smolders
PhD student: Joeri Van Liefferinge
Collaborators: Lut Arckens (K.U.Leuven), Niels Danbolt (University of Oslo, Norway), Joeri Aerts (VUB), Emmanuel Hermans (UCL), Hideyo Sato (Yamagata University, Japan)

It is common knowledge that the antibodies that are being used for the detection of system xc- in immunohistochemistry/immunocytochemistry are not always specifically binding xCT, the specific subunit of system xc-. As such, the regional as well as cellular distribution of system xc- in the brain is still questionable. We therefore decided to screen a large battery of home-made as well as commercial antibodies for their specificity in the most common immunological techniques, i.e. immunohisto(cyto)chemistry, Western blotting and FACS analysis, by using xCT knockout tissue as negative control. Using the appropriate antibody combined with the appropriate protocol, we will perform a regional/cellular distribution study.

System xc- and Parkinson’s disease

PIs: Ann Massie, Yvette Michotte
PhD-student: Eduard Bentea
Collaborators: Charles Meshul (Portland VA Medical Center, USA); Veerle Baeckelandt (K.U.Leuven); Hideyo Sato (Yamagata University, Japan)

We recently reported increased expression levels of xCT, the specific subunit of system xc-, in the striatum of the 6-OHDA hemi-Parkinson rat model (Massie et al., 2008). To further understand the functional meaning of this upregulation, we investigated the susceptibility of xCT knockout mice, i.e. mice that do not express functional system xc-, for 6-OHDA-induced neurodegeneration and we observed a significant neuroprotection in these mice compared to wildtype littermates (Massie et al., 2011). These data suggest that inhibition of system xc- might be a neuroprotective strategy for the treatment of Parkinson’s disease. Currently, we are using two other mouse models for PD to confirm our observations in the 6-OHDA model, i.e. the chronic, progressive MPTP model and the lactacystin model. Moreover, in the latter model we will be investigating the link between glutamate toxicity and alpha-synuclein aggregation.
All glutamate transporters subtypes as well as the proteins that regulate their expression levels are being studied in tissue of PD patients.

System xc- and epilepsy/epileptogenesis

PI: Ilse Smolders, Ann Massie
PhD student: Joeri Van Liefferinge
Collaborators: Hideyo Sato (Yamagata University, Japan), Jan Lewerenz (University Hospital Ulm, Germany)

We have shown that the threshold for limbic seizures is higher in mice lacking xCT, the specific subunit of system xc-, compared to wildtype littermates. This decreased susceptibility was suggested to be related to the decreased hippocampal extracellular glutamate levels in xCT knockout mice (De Bundel et al., 2011). Moreover, we recently discovered increased xCT expression levels in hippocampal samples of epileptic patients (Lewerenz et al, submitted). Currently, we are investigating whether system xc- might be involved in the process of epileptogenesis by using several rodent models for epilepsy.

System xc- in multiple sclerosis

Co-PI: Ann Massie, Jacques Dekeyser, Bart Rombaut
PhD student: Ellen Merckx
Collaborators: Jan Lewerenz (University Hospital Ulm, Germany), Pam Maher (The Salk Institute, USA), Axel Methner (University of Düsseldorf, Germany), Thomas Michiels (UCL), Emmanuel Hermans (UCL)

Ample evidence points towards an involvement of glutamate excitotoxicity in the pathogenesis of multiple sclerosis (MS). Indeed, the focal lesions in the central nervous system (CNS) are characterized by inflammation, demyelination and infiltration of immune cells. Whereas inflammation and demyelination can be the result of glutamate toxicity, the infiltrating immune cells could be a possible source of glutamate. The aim of this project is to unveil the involvement of system xc- in the pathogenesis of MS. We will thereby distinguish between system xc- present on the glial cells of the CNS and system xc- present on the infiltrating immune cells, as it has been shown that activation of monocytes can induce glutamate release through system xc- (Pampliega et al., 2011).

System xc- in major depression

Co-PI: Ilse Smolders, Ann Massie
PhD student: Thomas Demuyser
Collaborators: Mia Lindskog (Karolinska Institute, Sweden)

In the field of depression, the glutamate hypothesis is currently one of the most exciting avenues. Monoaminergic imbalance is still proposed as a causative factor, whereas the resulting abnormal glutamatergic transmission critically mediates emotional changes. With this project we investigate the possible involvement of glutamate transporters, including system xc-, in depression.

 

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