Abstracts

Rationale: Most patients with either tuberous sclerosis or hippocampal sclerosis suffer from epilepsy. Removal of tubers or hippocampus from brain typically results in amelioration or freedom of seizure in those patients. Dramatic neuronal loss has only been seen in sclerotic hippocampal tissue but not in brain tuber tissue. In our studies, we have observed dramatic changes in morphology, immunohistochemical staining and electrophysiological properties of astrocytes in resected cortical specimens from both patients with tuberous sclerosis or hippocampal sclerosis. We propose that astrocytic changes may be an important contributor to human epileptogenesis. Methods: Surgical cortical tuber specimens (14 samples) were obtained from pediatric patients with refractory TSC-related epilepsy. Human sclerotic hippocampal tissue (8 samples) was obtained from patients with mesial temporal sclerosis with refractory epilepsy, who also underwent resection of the epileptogenic zone. Whole-cell patch-clamp recording of astrocytes was done on the 180 to 200 µm brain tissue slices. After recording, the brain slices were fixed for the further immunohistochemical study. Results: Both normal and abnormal appearance neurons were observed in human tuber (n=6). Long, radial and non-branched fibrous-like astrocytes were the major glial constituent of human tubers (n=14). Protoplasmic astrocytes can only be seen in the tuber surrounding tissues. Immunohistochemical expression of GFAP and phospho-S6 in the astrocytes was enhanced in human tuberous sclerotic tissue. Glutamine transporters (EAAT1/GLAST and EAAT2/GLT1) were reduced in human tubers. Glutamate transporter and potassium uptake current was dramatically reduced in the fibrous-like astrocytes in human tubers. Long, radial and non-branched fibrous-like astrocytes could be seen in human sclerotic hippocampus (n=7). Some thick, branched astrocytes were also present in the sclerotic hippocampus and near sclerotic tissue (“border zone”) (n=9). Those astrocytes are GFAP positive and showed passive current-voltage (IV) pattern. Those astrocytes showed reduced function for glutamate and potassium clearance. Glial cells that have complex IV pattern (complex cells) could also be observed in the sclerotic hippocampus. However, most of the complex cells are NG2 positive (8/9 cells). “Spongiform” protoplasmic astrocytes could not be observed in human sclerotic tissue. Conclusions: Morphological, immunohistochemical and electrophysiological changes of astrocytes have been observed in the human tuberous sclerosis and sclerotic hippocampus. Those 'sclerotic' astrocytes have shown impaired glutamate and potassium uptake, which might contribute to epileptogenesis.