The intermediate filament protein nestin is upregulated in response to cerebral ischemia; the significance of this, however, is incompletely understood. Here, we used transgenic mice that express green fluorescent protein (GFP) under control of the nestin promoter to characterize the fate of nestin-expressing cells up to 8 weeks following occlusion of the middle cerebral artery (MCAo) and reperfusion. The population of nestin-GFP+ cells increased in the ischemic lesion rim and core within 4 days, did not become TUNEL-positive and was detectable up to 8 weeks in the lesion scar. Nestin-GFP+ cells proliferated in situ and underwent approximately one round of cell division. They were not recruited in large numbers from the subventricular zone (SVZ) as indicated by absence of co-labeling with intracerebroventricularly injected dye DiI in the majority of nestin-GFP+ cells. Nestin-GFP+ cells expressed the chondroitin sulfate proteoglycan NG2 and nestin protein, but typically lacked mature astrocytic markers, i.e. glial fibrillary acidic protein (GFAP) or S100b. Vice versa, the majority of GFAP+ cells lacked nestin expression and surrounded the ischemic lesion by 4 days. The electrophysiological properties of nestin-GFP+ cells were analyzed in acute brain slices at different time points after MCAo/ reperfusion. Based on the membrane currents, two populations of nestin-GFP+ cells were identified. (1) A ‘complex’ current pattern was characterized by outwardly rectifying currents activated by depolarization and inward rectifying, inactivation currents induced with hyperpolarization. (2) The current profile of the second subpopulation was characterized by time and voltage-independent ‘passive’ currents. Recordings in acute slices from controls and sham-operated animals demonstrated that only about half of nestin-GFP+ cells displayed complex membrane properties and expressed AMPA receptors but lacked glutamate transporters similar to progenitor cells. In contrast, by 4 days after the insult all nestin-GFP+ cells expressed these properties (Figure 1). The absence of TUNEL-reaction in nestin-GFP+ cells and the fact that 24 hours following MCAo/ reperfusion, nestin-GFP+ cells were still detectable suggest that passive nestin-GFP+ cells had not undergone apoptosis or selective necrosis. We infer from our data that in response to the ischemic injury nestin-GFP+ cells with passive membrane properties transform into complex cells. We hypothesize that the change in physiological properties induced by the ischemic insult is directed toward a specific network function of nestin-expressing cells.
