Complementation of mitochondrial electron transport chain by manipulation of the NAD+/NADH ratio
Taking control of cellular NAD+ concentrations Cellular concentrations of the nicotinamide adenine dinucleotide (NAD+ are critical for proper metabolism and are often altered in aging and disease. To enable better understanding of these processes, Titov et al. altered the concentration of NAD+ in particular cellular compartments. They did this through expression of a bacterial enzyme targeted to specific compartments of human cells in culture. Their experiments emphasize the important role of the electron transport chain in redox transfer of electrons to NADH, rather than proton pumping, in mitochondrial pathogenesis. Science, this issue p. 231 The process of borrowing a bacterial enzyme to control nicotinamide adenine dinucleotide concentrations in mammalian cells is discussed. A decline in electron transport chain (ETC) activity is associated with many human diseases. Although diminished mitochondrial adenosine triphosphate production is recognized as a source of pathology, the contribution of the associated reduction in the ratio of the amount of oxidized nicotinamide adenine dinucleotide (NAD+ to that of its reduced form (NADH) is less clear. We used a water forming NADH oxidase from Lactobacillus brevis (LbNOX) as a genetic tool for inducing a compartment specific increase of the NAD+/NADH ratio in human cells. We used LbNOX to demonstrate the dependence of key metabolic fluxes, gluconeogenesis, and signaling on the cytosolic or mitochondrial NAD+/NADH ratios. Expression of LbNOX in the cytosol or mitochondria ameliorated proliferative and metabolic defects caused by an impaired ETC. The results underscore the role of reductive stress in mitochondrial pathogenesis and demonstrate the utility of targeted LbNOX for direct, compartment specific manipulation of redox state.
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