Mitochondrial targeted cyclophilin D protects cells from cell death by peptidyl prolyl isomerization

Cyclophilin D (CyPD) is thought to sensitize opening of the mitochondrial permeability transition pore (mPTP) based on the findings that cyclosporin A (CsA), a pseudo-CyPD substrate, hyperpolarizes the mitochondrial membrane potential (Δψ) and inhibits apoptosis. We provide evidence that contrasts with this model. Using live cell imaging and two photon microscopy, we report that overexpression of CyPD desensitizes HEK293 and rat glioma C6 cells to apoptotic stimuli. By site-directed mutagenesis of CyPD that compromises peptidyl-prolyl cis-trans isomerase (PPIase) activity, we demonstrate that the mechanism involved in this protective effect requires PPIase activity. Furthermore, we show that, under resting conditions, Δψ is hyperpolarized in CyPD wild type-overexpressing cells but not in cells overexpressing mutant forms of CyPD that lack PPIase activity. Finally, in glutathione S-transferase (GST) pull-down assays, we demonstrate that CyPD binding to the adenine nucleotide translocator (ANT), which is considered to be the core component of the mPTP, is not affected by the loss of PPIase activity. Collectively, our data suggest that CyPD should be viewed as a cell survival-signaling molecule and indicate a protective role of CyPD against apoptosis that is mediated by one or more targets other than the ANT.