Abstracts

Chondroitin sulphate proteoglycan expressing cells (NG2 cells) comprise a population of unique glial cells in the adult CNS. They are differentiated from other types of glia by their immunophenotype. NG2 cells have traditionally been considered as oligodendrocyte precursors, although their abundance and direct synaptic connections (in rodents) suggests that they not only function as progenitors but also may actively participate in neuron-glia interactions in healthy and pathologic brain. We sought to characterize NG2 cells in the human TLE brain. Hippocampi and neocortex resected from patients with intractable TLE (n=42; 29 with HS and 13 without HS) were studied with immunohistochemistry combined with confocal and electron microscopy, electrophysiology (patch-clamp technique on brain slices), and Western blotting for detection of NG2 cells andother glial cell subtypes. Pilocarpine and kainic acid mouse models of epilepsy were used for comparative purposes. NG2 cells were detected in all areas of hippocampus, as well as in neocortex. Immunolabelling was higher in white matter than gray matter. Using electron microscopy and patch-clamp techniques, we detected that human NG2 cells have synaptic contacts, as well as postsynaptic excitatory currents (4/6 cells studied). All recorded NG2 cells revealed complex electrophysiological current features that differentiated them from [ldquo]passive[rdquo] astrocytes. Two types of NG2 cells were determined: multipolar stellate, predominating type revealed S100[beta] expression, while spindle-like bipolar cells expressed a basal level of nestin and vimentin but lack S100[beta].
In [sim]30% of specimens in both pathologies, multipolar NG2-cells revealed unusually high levels of nestin and vimentin IR . Such nestin/vimentin+ NG2 reactive cells were observed mainly in dentate gyrus and rarely in CA3. In [sim] 15% of cases examined, proliferationof these reactive NG2 cells was found in dentate gyrus, as detected by Ki-67 labelling. Reactive NG2 cells occupied focal areas in the hippocampus or neocortex; they were usually found in parallel with reactive astrocytes, but not with reactive microglial cells. In kainic acid and pilocarpine mouse models of epilepsy, reactive NG-2 cells were visulaized, based on morphological alterations and increased NG2 IR. However, colocalization of nestin and/or vimentin was never observed in these cells, either acutely (up to 7 days) or chronically (up to 4 months). NG2 cells in the human epileptic brain comprise at least two populations of cells. The bipolar cells are likely oligodendrocyte progenitors that are preserved in adult human brain. Multipolar cells make synaptic connections with neurons and become [quot]reactive[quot] in both HS and non-HS TLE. How these cells participate in hippocampal pathology in epilepsy remains to be determined. (Supported by Klingenstein Foundation, NIH R21 NS 42334, Parents Against Childhood Epilepsy (GM))