Abstracts

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder resulting from mutations in one of two genes, TSC[sub]1[/sub] or TSC[sub]2[/sub]. It has been reported that TSC[sub]1[/sub] knockout mice exhibit decreased GLT-1 and GLAST protein and progressive epilepsy (Uhlmann EJ., et al, 2002, Wong M., 2003). We have examined astrocytic glutamate transport in resected human cerebral cortical tubers from epileptic TSC patients. Astrocytic glutamate transport in human cortical tuber and surrounding tissue from TSC patients was studied using whole-cell patch-clamp recording and immunohistochemical techniques. D-aspartic acid was used to detect glutamate transporter current. Giant cells and large tuber astrocytes were prominent cell types in cortical tubers of human TSC. Giant cells were characterized by: immunostaining with vimentin (100%) and nestin (75%); high level of expression of glutamine synthetase; and minimal GFAP expression. Electrophysiologically, giant cells had large membrane capacitance (C[sub]m[/sub]) and high membrane input resistance (R[sub]in[/sub]). Small glutamate transporter currents (GluTCs) were detected in the giant cells.
Astrocytes in the tuber were larger than astrocytes in non-TSC tissue. They had radial oriented processes without lamellapodia-like protrusions. There was few dye coupling between them. Tuberous astrocytes expressed high levels of GFAP and vimentin immunostaining, and had no detectable or minimal GluTCs. Astrocytes in the tissue surrounding tubers had similar passive electrophysiological properties as control astrocytes from non-tuberous human cortical tissue. However, GluTCs were absent in the surrounding tissue astrocytes.
Spontaneous electrical oscillations were recorded in both giant cells and astrocytes from tubers. Such oscillations have never been observed in our astrocyte recordings in non-TSC control tissue and rarely seen in the sclerotic hippocampus. The frequency of the oscillations was similar to the spontaneous rhythmic neuronal oscillation induced by 4-AP in rodent brain slices. It was reduced using 5 mM magnesium or furosemide. Oscillations were also found in TSC astrocytes from surrounding tissue, suggesting that excitability was also enhanced in surrounding tissue. Human TSC astrocytic glutamate transport function is dramatically decreased in both cortical tubers and the immediately surrounding tissue. Both tubers and surrounding tissue demonstrate abnormal epileptiform oscillations.[figure1] (Supported by NIH R21, K08
Parents Against Childhood Epilepsy.)