Endogenous oscillations in pathways of energy transduction as related to circadian rhythmicity and photoperiodic control

Interplay between cellular redox oscillations and circadian clocks

The circadian clock is a cellular timekeeping mechanism that helps organisms from bacteria to humans to organize their behaviour and physiology around the solar cycle. Current models for circadian timekeeping incorporate transcriptional/translational feedback loop mechanisms in the predominant model systems. However, recent evidence suggests that non-transcriptional oscillations such as metabolic and redox cycles may play a fundamental role in circadian timekeeping. Peroxiredoxins, an antioxidant protein family, undergo rhythmic oxidation on the circadian time scale in a variety of species, including bacteria, insects and mammals, but also in red blood cells, a naturally occurring, non-transcriptional system. The profound interconnectivity between circadian and redox pathways strongly suggests that a conserved timekeeping mechanism based on redox cycles could be integral to generating circadian rhythms.

Rethinking the clockwork: redox cycles and non-transcriptional control of circadian rhythms

Circadian rhythms are a hallmark of living organisms, observable in all walks of life from primitive bacteria to highly complex humans. They are believed to have evolved to co-ordinate the timing of biological and behavioural processes to the changing environmental needs brought on by the progression of day and night through the 24-h cycle. Most of the modern study of circadian rhythms has centred on so-called TTFLs (transcription-translation feedback loops), wherein a core group of 'clock' genes, capable of negatively regulating themselves, produce oscillations with a period of approximately 24 h. Recently, however, the prevalence of the TTFL paradigm has been challenged by a series of findings wherein circadian rhythms, in the form of redox reactions, persist in the absence of transcriptional cycles. We have found that circadian cycles of oxidation and reduction are conserved across all domains of life, strongly suggesting that non-TTFL mechanisms work in parallel with the canonical genetic processes of timekeeping to generate the cyclical cellular and behavioural phenotypes that we commonly recognize as circadian rhythms.

Sirtuins and pyridine nucleotides

The silencer information regulator (Sir) family of proteins has attracted much attention during the past decade due to its prominent role in metabolic homeostasis in mammals. The Sir1-4 proteins were first discovered in yeast as nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylases, which through a gene silencing effect promoted longevity. The subsequent discovery of a homologous sirtuin (Sirt) family of proteins in the mammalian systems soon led to the realization that these molecules have beneficial effects in metabolism- and aging-related diseases. Through their concerted functions in the central nervous system, liver, pancreas, skeletal muscle, and adipose tissue, they regulate the body's metabolism. Sirt1, -6, and -7 exert their functions, predominantly, through a direct effect on nuclear transcription of genes involved in metabolism, whereas Sirt3-5 reside in the mitochondrial matrix and regulate various enzymes involved in the tricarboxylic acid and urea cycles, oxidative phosphorylation, as well as reactive oxygen species production. An interesting aspect of the functionality of sirtuin involves their regulation by the circadian rhythm, which affects their function via cyclically regulating systemic NAD(+) availability, further establishing the link of these proteins to metabolism. In this review, we will discuss the relation of sirtuins to NAD(+) metabolism, their mechanism of function, and their role in metabolism and mitochondrial functions. In addition, we will describe their effects in the cardiovascular and central nervous systems.

Circadian tracking of nicotinamide cofactor levels in an immortalized suprachiasmatic nucleus cell line

Nicotinamide adenine dinucleotides can exhibit a daily rhythm in plants and regulate the activity of mammalian clock-like transcription factors in vitro. Because one such redox-sensitive transcription factor is present in the master circadian clock of the brain (the suprachiasmatic nuclei, SCN) and the SCN exhibits a characteristic daily rhythm in glucose usage, nicotinamide cofactors might be expected to influence, exhibit, and/or reflect biological rhythms in SCN cells. Therefore, cofactors were extracted from a model SCN cell line at 3 h intervals over 1-2 day periods and samples were analyzed by capillary electrophoresis with multiphoton excitation of fluorescence. Natively fluorescent reduced cofactors (nicotinamide adenine dinucleotide, NADH, and its phosphorylated form, NADPH) were assayed directly, and nonfluorescent oxidized cofactors (nicotinamide adenine dinucleotide, NAD, and its phosphorylated form, NADP) were enzymatically reduced to their fluorescent counterparts before analysis. In the first day after a synchronizing pulse of fetal bovine serum, a dramatic upregulation in cellular NADH content was observed, consistent with a response to serum insulin; this was accompanied by a smaller decrease in NADPH redox state, which may indicate scavenging of reactive oxygen species generated by increased cellular metabolism. However, when cells were investigated after these early phenomena had recovered or stabilized, no circadian NAD(P)(H) rhythms were observed. During these studies, the NADH/NAD(H) concentration ratio in SCN2.2 cells (0.13+/-0.03) was not high enough to activate clock-like transcription factors. Although the NADPH/NADP(H) concentration ratio was more appropriate (0.8+/-0.1), the intracellular NADPH concentration was < or = 0.7 mM, far too low for half-maximal DNA binding of clock-like transcription factors in vitro. Moreover, these concentration and ratio values represent cellular averages, and free cofactors should be much lower in the cell nucleus. Our data show that SCN2.2 cells maintain nearly constant circadian NAD(P)(H) levels, and that the previously reported in vitro relationship between clock-like transcription factors and NAD(P)(H) does not appear to be biologically relevant.

The molecular mechanism of circadian rhythms

The endogenous and ubiquitous circadian rhythms (CR) are not understood in molecular terms, in spite of recent interesting advances. They concern, on one hand, protein synthesis, of which a small fraction--possibly a single protein--, formed on cytosolic ribosomes, may be required on each 24 h cycle. On the other hand, the per gene, involved in the control of CR in drosophila has been found to direct the synthesis of 2 (or 3) proteoglycans. Several models have been put forward in order to explain CR generation. Considering the complexity of the cell's organization and the occurrence of partial arrhythmicity, CR generation might result from the integration of a few physiological and metabolic pathways normally involving at least one feed-back loop. Sequentiality would be inherent to the kinetics of both the metabolic pathways and translocators located in the membranes of the various compartments; as a consequence, the peaks of the oscillatory activities would be positioned at particular phase points relative to others. Proteoglycans (or other proteins modified post-transcriptionally) could be involved in the operation of rhythms in controlling not only some plasmalemma (as proposed by Yu et al., 1987) but also intracellular membranes. Finally, reversible enzyme modification occurring on each 24 h cycle could be critically important in circadian rhythms generation.

Pyridine nucleotides and redox-charge evolution during the induction of flowering in spinach leaves

In the long-day plant Spinacia oleracea changes in the pool size of pyridine nucleotides have been followed under different photoperiodic conditions. In short days (vegetative state), the dark and light phases of the cycle are characterized by specific reciprocal changes in NAD and NADP pool sizes. As a consequence, the ratios of NADH/NAD+NADH and NADPH/NADP+NADPH, which are respectively considered to represent the catabolic and anabolic state of metabolism, also show a characteristic pattern. Upon transfer to continuous light, i.e. during floral induction, a decrease in anabolic metabolism is paralleled by an increase in catabolic metabolism. In the floral state, both the catabolic and the anabolic couples of the pyridine nucleotides are considerably depressed, possibly reflecting the enhanced senescence of induced leaves. The results are discussed in relation to the involvment of the nucleotides in stoichiometric coupling of metabolic compartments at the cellular level in response to environmental signals.