Expression of glutamate transporters GLT-1 and GLAST in different regions of rat brain during the course of experimental autoimmune encephalomyelitis

Abstract

Demyelination and oligodendroglial cell death accompanied by axonal injury are dominating features of multiple sclerosis (MS) a chronic demyelinating disease of the CNS. Accumulation of extracellular glutamate, observed during MS, is implicated in excitotoxic injury of nerve and oligodendroglial cells as a result of over-activation of glutamate receptors. The appropriate concentration of extracellular glutamate is maintained by glutamate transporters, the most predominant of which is glial transporter GLT-1 (excitatory amino acid transporter (EAAT) 2). The aim of this study is to determine the time-course of GLT-1 and glutamate-aspartate transporter (GLAST) expression in forebrain and cerebellum of rats subjected to experimental autoimmune encephalomyelitis (EAE). Our findings revealed that: (1) GLT-1 mRNA and to a lower extent GLAST mRNA are overexpressed in forebrain and cerebellum of EAE rats (2) expression of GLT-1 transporter mRNA shows a similar temporal pattern throughout the course of EAE in both structures examined, and is closely correlated with the appearance of neurological symptoms; and (3) the expression of GLT-1 and GLAST protein does not mirror mRNA changes during EAE and exhibits a differential spatial pattern. The protein levels of GLT-1 in cerebellum and GLAST in both structures are significantly reduced just before the acute phase and later during the recovery. The results imply that transcriptional up-regulation of the GLT-1 gene occurs early in both the forebrain and the cerebellum of the EAE rat model. This up-regulation is associated with the severity of symptoms but tends to precede the onset of maximal neurological deficits. The observations confirm the involvement of glutamate in the pathogenesis of EAE and provide an indication of the protective role of this glutamate transporter. However, changes in protein expression of both transporters suggest the existence of post-translational disturbances or the influence of regulating factors connecting with EAE conditions that may lead to the insufficient protection against glutamate excitotoxicity.