A spontaneous mutation in the nicotinamide nucleotide transhydrogenase gene of C57BL/6J mice results in mitochondrial redox abnormalities

摘要

NADPH is the reducing agent for mitochondrial H_2O_2 detoxification systems. Nicotinamide nucleotide transhydrogenase (NNT), an integral protein located in the inner mitochondrial membrane, contributes to an elevated mitochondrial NADPH/NADP~+ ratio. This enzyme catalyzes the reduction of NADP~+ at the expense of NADH oxidation and H~+ reentry to the mitochondrial matrix. A spontaneous Nnt mutation in C57BL/6J (B6J-Nnt~(MUT)) mice arose nearly 3 decades ago but was only discovered in 2005. Here, we characterize the consequences of the Nnt mutation on the mitochondrial redox functions of B6J-Nnt~(MUT) mice. Liver mitochondria were isolated both from an Nnt wild-type C57BL/6 substrain (B6JUnib-Nnt~W) and from B6J-Nnt~(MUT) mice. The functional evaluation of respiring mitochondria revealed major redox alterations in B6J~Nnt~(MUT) mice, including an absence of transhydrogenation between NAD and NADP, higher rates of H_2O_2 release, the spontaneous oxidation of NADPH, the poor ability to metabolize organic peroxide, and a higher susceptibility to undergo Ca~(2+)-induced mitochondrial permeability transition. In addition, the mitochondria of B6J-Nnt~(MUT) mice exhibited increased oxidized/reduced glutathione ratios as compared to B6JUnib-Nnt~W mice. Nonetheless, the maximal activity of NADP-dependent isocitrate dehydrogenase, which is a coexisting source of mitochondrial NADPH, was similar between both groups. Altogether, our data suggest that NNT functions as a high-capacity source of mitochondrial NADPH and that its functional loss due to the Nnt mutation results in mitochondrial redox abnormalities, most notably a poor ability to sustain NADP and glutathione in their reduced states. In light of these alterations, the potential drawbacks of using B6J-Nnt~(MUT) mice in biomedical research should not be overlooked.