Geldanamycin selectively reversed the apoptosis induced by the cytosolic forms of PrP mutants. A and B. Assays of mitochondrial dysfunction in the cells expressing various PrP constructs in the presence or absence of geldanamycin. HEK293 cells (A) and N2a cells (B) expressing various PrP constructs were treated with (GA) or without (Ctrl) geldanamycin, and the mitochondrial transmembrane transition was evaluated with a commercial kit (as described in the Materials and Methods). Apoptotic and living cells are green and red, respectively, by fluorescence microscopy. C and D. Quantification of the apoptotic cells. The number of total cells in a field of view was counted using ImageJ. The percentage of apoptotic cells was defined as the number of green cells vs. the total number of cells.