Mechanism of cell death caused by complex I defects in a rat dopaminergic cell line

Defects in the proton-translocating NADH-quinone oxidoreductase (complex I) of mammalian mitochondria are linked to neurodegenerative disorders. The mechanism leading to cell death elicited by complex I deficiency remains elusive. We have shown that expression of a rotenone-insensitive yeast NADH-quinone oxidoreductase (Ndi1) can rescue mammalian cells from complex I dysfunction. By using the Ndi1 enzyme, we have investigated the key events in the process of cell death using a rat dopaminergic cell line, PC12. We found that complex I inhibition provokes the following events: 1) activation of specific kinase pathways; 2) release of mitochondrial proapoptotic factors, apoptosis inducing factor, and endonuclease G. AS601245, a kinase inhibitor, exhibited significant protection against these apoptotic events. The traditional caspase pathway does not seems to be involved because caspase 3 activation was not observed. Our data suggest that overproduction of reactive oxygen species (ROS) caused by complex I inhibition is responsible for triggering the kinase activation, for the release of the proapoptotic factors, and then for cell death. Nearly perfect prevention of apoptotic cell death by Ndi1 agrees with our earlier observation that the presence of Ndi1 diminishes rotenone-induced ROS generation from complex I. In fact, this study demonstrated that Ndi1 keeps the redox potential high even in the presence of rotenone. Under these conditions, ROS formation by complex I is known to be minimal. Possible use of our cellular model is discussed with regard to development of therapeutic strategies for neurodegenerative diseases caused by complex I defects.