The expression of the glial glutamate transporter protein EAAT2 in motor neuron disease: an immunohistochemical study

Emerging evidence suggests that a disturbance of the glutamate neurotransmitter system may be a contributory factor to motor neuron injury in motor neuron disease. Previous autoradiographic and immunoblotting studies have suggested that there may be reduced expression of glutamate transporter proteins in pathologically affected areas of the CNS in motor neuron disease. This study further explores the possible alteration in expression of the excitatory amino acid transporter protein EAAT2 in MND, by examining the protein expression in situ, in frozen sections, using immunohistochemistry. The aim of the study was to compare the distribution and density of EAAT2 in the motor cortex and spinal cord of MND cases (n = 16) compared with neurologically normal controls (n = 12), matched for relevant parameters. A novel, previously characterized, monoclonal antibody to EAAT2 was employed. EAAT2 immunoreactivity in motor neuron disease and control cases was compared using relative optical density measurements generated by computerized image analysis. In the motor cortex, EAAT2 immunoreactivity was laminated comprising a superficial intense band (corresponding to layers 1 and 2); a paler middle band (layer 3 and part of 5) and a more intense deep layer (layers 5 and 6). In the spinal cord, the ventral horn showed strong immunoreactivity with dense perisomatic staining around motor neuron cell bodies, the substantia gelatinosa showed moderate diffuse staining and the intermediate spinal laminae showed weak staining. This general pattern of immunoreactivity was preserved in the motor neuron disease cases. However, in the motor neuron disease cases compared with controls, the optical density values for EAAT2 immunoreactivity were significantly reduced in all grey matter regions of the lumbar spinal cord (P < 0.001) and were increased in the middle laminae of the motor cortex (P < 0.05). This study indicates that glutamate transporter pathology in motor neuron disease may be a more complex phenomenon than previously recognized.