A genetically encoded tool for manipulation of NADP+/NADPH in living cells

Summary Circadian (~24 hour) clocks are fundamentally important for coordinated physiology in organisms as diverse as cyanobacteria and humans. All current models of the clockwork in eukaryotic cells are based on transcription-translation feedback loops. Non-transcriptional mechanisms in the clockwork have been difficult to study in mammalian systems. We circumvented these problems by developing novel assays using human red blood cells (RBCs), which have no nucleus (or DNA), and therefore cannot perform transcription. Our results show that transcription is, in fact, not required for circadian oscillations in humans, and that non-transcriptional events appear sufficient to sustain cellular circadian rhythms. Using RBCs, we found that peroxiredoxins, highly conserved antioxidant proteins, undergo ~24 hour redox cycles, which persist for many days under constant conditions (i.e. in the absence of external cues). Moreover, these rhythms are entrainable (i.e. tunable by environmental stimuli), and temperature-compensated, both key features of circadian rhythms. We anticipate our findings will facilitate more sophisticated cellular clock models, highlighting the interdependency of transcriptional and non-transcriptional oscillations in potentially all eukaryotic cells.

Two human cell lines (termed rho 0), which had been completely depleted of mitochondrial DNA (mtDNA) by long-term exposure to ethidium bromide, were found to be dependent on uridine and pyruvate for growth because of the absence of a functional respiratory chain. Loss of either of these two metabolic requirements was used as a selectable marker for the repopulation of rho 0 cells with exogenous mitochondria by complementation. Transformants obtained with various mitochondrial donors exhibited a respiratory phenotype that was in most cases distinct from that of the rho 0 parent or the donor, indicating that the genotypes of the mitochondrial and nuclear genomes as well as their specific interactions play a role in the respiratory competence of a cell.

1. The concentrations of the oxidized and reduced substrates of the lactate-, beta-hydroxybutyrate- and glutamate-dehydrogenase systems were measured in rat livers freeze-clamped as soon as possible after death. The substrates of these dehydrogenases are likely to be in equilibrium with free NAD(+) and NADH, and the ratio of the free dinucleotides can be calculated from the measured concentrations of the substrates and the equilibrium constants (Holzer, Schultz & Lynen, 1956; Bücher & Klingenberg, 1958). The lactate-dehydrogenase system reflects the [NAD(+)]/[NADH] ratio in the cytoplasm, the beta-hydroxybutyrate dehydrogenase that in the mitochondrial cristae and the glutamate dehydrogenase that in the mitochondrial matrix. 2. The equilibrium constants of lactate dehydrogenase (EC 1.1.1.27), beta-hydroxybutyrate dehydrogenase (EC 1.1.1.30) and malate dehydrogenase (EC 1.1.1.37) were redetermined for near-physiological conditions (38 degrees ; I0.25). 3. The mean [NAD(+)]/[NADH] ratio of rat-liver cytoplasm was calculated as 725 (pH7.0) in well-fed rats, 528 in starved rats and 208 in alloxan-diabetic rats. 4. The [NAD(+)]/[NADH] ratio for the mitochondrial matrix and cristae gave virtually identical values in the same metabolic state. This indicates that beta-hydroxybutyrate dehydrogenase and glutamate dehydrogenase share a common pool of dinucleotide. 5. The mean [NAD(+)]/[NADH] ratio within the liver mitochondria of well-fed rats was about 8. It fell to about 5 in starvation and rose to about 10 in alloxan-diabetes. 6. The [NAD(+)]/[NADH] ratios of cytoplasm and mitochondria are thus greatly different and do not necessarily move in parallel when the metabolic state of the liver changes. 7. The ratios found for the free dinucleotides differ greatly from those recorded for the total dinucleotides because much more NADH than NAD(+) is protein-bound. 8. The bearing of these findings on various problems, including the following, is discussed: the number of NAD(+)-NADH pools in liver cells; the applicability of the method to tissues other than liver; the transhydrogenase activity of glutamate dehydrogenase; the physiological significance of the difference of the redox states of mitochondria and cytoplasm; aspects of the regulation of the redox state of cell compartments; the steady-state concentration of mitochondrial oxaloacetate; the relations between the redox state of cell compartments and ketosis.