Late appearance of glutamate transporter defects in a murine model of ALS–parkinsonism dementia complex

Excitotoxicity has been widely hypothesized to play a major role in various neurodegenerative diseases. We have used a mouse model of ALS–parkinsonism dementia complex (ALS–PDC) of the Western Pacific to explore this hypothesis. Mice fed washed cycad flour, the major epidemiological link to ALS–PDC, showed significant and progressive motor, cognitive, and sensory behavioural deficits [Wilson, J.M., Khabazian, I., Wong, M.C., Seyedalikhani, A., Bains, J.S., Pasqualotto, B.A., Williams, D.E., Andersen, R.J., Simpson, R.J., Smith, R., Craig, U.K., Kurland, L.T., Shaw, C.A., 2002. Behavioral and neurological correlates of ALS-parkinsonism dementia complex in adult mice fed washed cycad flour. Neuromol. Med. 1 (3), 207–221]. In addition, glutamate transporter (GLT-1/EAAT2) levels measured by immunohistochemistry with antibodies specific for two glial glutamate transporter splice variants (GLT-1α and GLT-1B) were significantly down-regulated showing a ‘patchy’ loss of antibody label centered on blood vessels [Wilson, J.M., Khabazian, I., Pow, D.V., Craig, U.K., Shaw, C.A., 2003. Decrease in glial glutamate transporter variants and excitatory amino acid receptor down-regulation in a murine model of ALS-PDC. Neuromol. Med. 3 (2), 105–118]. Receptor binding assays showed decreased NMDA and AMPA receptor levels combined with increased GABAA receptor levels in various CNS regions. The alterations in GLT-1 variants and the ionotropic receptors are consistent with an increased level of extracellular glutamate. The interaction between environmental toxicity and genetic susceptibility was also tested using mice expressing various Apolipoprotein E (ApoE) genotypes. Mice lacking the ApoE gene showed relative resistance to cycad-induced toxicity as measured by GLT-1B labeling, but all mice expressing the human ApoE isoforms showed a similar loss of GLT-1B. We have further shown that an isolated cycad toxin (β-sitosterol-β-d-glucoside, BSSG), previously shown to release glutamate in vitro [Wilson, J.M., Khabazian, I., Wong, M.C., Seyedalikhani, A., Bains, J.S., Pasqualotto, B.A., Williams, D.E., Andersen, R.J., Simpson, R.J., Smith, R., Craig, U.K., Kurland, L.T., Shaw, C.A., 2002. Behavioral and neurological correlates of ALS-parkinsonism dementia complex in adult mice fed washed cycad flour. Neuromol. Med. 1 (3), 207–221], can be directly toxic to motor neurons in vivo [Wilson, J.M., Petrik, M.S., Moghadasian, M.H., Shaw, C.A., 2005. Examining the interaction of apo E and neurotoxicity on a murine model of ALS-PDC. Can. J. Physiol. Pharmacol. 83 (2), 131–141]. However, BSSG-fed mice did not show altered GLT-1B labeling in the spinal cord suggesting that an initial excitotoxic mechanism may not be responsible for the final neuronal loss observed. While glutamate-mediated excitotoxicity is likely involved in the outcomes following cycad/BSSG exposure, the precise location in the cascade of events ultimately leading to neuronal death remains to be determined.