Cellular Timekeeping: It's Redox o'Clock

Abstract

Mounting evidence in recent years supports the extensive interaction between the circadian and redox systems. The existence of such a relationship is not surprising because most organisms, be they diurnal or nocturnal, display daily oscillations in energy intake, locomotor activity, and exposure to exogenous and internally generated oxidants. The transcriptional clock controls the levels of many antioxidant proteins and redox-active cofactors, and, conversely, the cellular redox poise has been shown to feed back to the transcriptional oscillator via redox-sensitive transcription factors and enzymes. However, the circadian cycles in the S-sulfinylation of the peroxiredoxin (PRDX) proteins constituted the first example of an autonomous circadian redox oscillation, which occurred independently of the transcriptional clock. Importantly, the high phylogenetic conservation of these rhythms suggests that they might predate the evolution of the transcriptional oscillator, and therefore could be a part of a primordial circadian redox/metabolic oscillator. This discovery forced the reappraisal of the dogmatic transcription-centered view of the clockwork and opened a new avenue of research. Indeed, the investigation into the links between the circadian and redox systems is still in its infancy, and many important questions remain to be addressed.

Figure 1.

Timeline of the key experiments performed in the field of nontranscriptional oscillators. The timeline begins with the first chronobiology meeting at the Cold Spring Harbor Symposia for Quantitative Biology, where the foundations of the field were laid. The next five decades yielded exciting discoveries in the area of nontranscriptional oscillators, culminating with the finding of the cyanobacterial KaiC oscillator and the autonomous redox oscillations in peroxiredoxins in red blood cells (RBCs). GSH, glutathione.

Figure 2.

Peroxiredoxins (PRDXs) are the primary H₂O₂ scavengers. The catalytic loop of 2-Cys PRDXs begins with the reaction between the reactive peroxidatic cysteine residues (CysP) and an incoming peroxide molecule (H₂O₂). This reaction results in the oxidation of CysP to a sulfenic acid (-SOH) intermediate and the detoxification of H₂O₂ to water. The sulfenic acid intermediate is relatively unstable and can continue its reaction route either via the catalytic or the inactivation loop. In the first case, the sulfenic acid intermediate reacts with another conserved residue, termed the resolving cysteine (CysR), originating from a second PRDX monomer to form an intramolecular disulfide bond. The thioredoxin (TRX) system acts to recycle the disulfide form of the enzyme via the action of TRX and TrxR, with electrons originating from nicotinamide adenine dinucleotide phosphate (NADPH). Occasionally, the sulfenic acid intermediate reacts with a second peroxide molecule, which results in the hyperoxidation to sulfinic acid (SO₂H). This form of the enzyme is slowly recycled back to the active thiolate form by the enzyme sulfiredoxin (SRX) in an ATP-consuming reaction. The inactivation loop is considerably slower than the catalytic loop, which allows the accumulation of overoxidized PRDXs. The proportion of PRDX molecules entering the two pathways is dictated by intrinsic factors (e.g., structural characteristics) and dynamic parameters (the local H₂O₂ concentration).

Figure 3.

Peroxiredoxins (PRDXs) are mediators of reactive oxygen species (ROS) signaling. Schematic representation of the three main models for PRDXs signaling. The redox relay model: In this model, the oxidized PRDXs transfer oxidative equivalents to protein targets. This transfer reaction proceeds via the formation of a mixed disulfide intermediate between PRDXs and the client protein. The floodgate model: This model postulates that the inactivation of PRDXs via overoxidation allows the local accumulation of H₂O₂, which can then react with other cellular targets. The thioredoxin (TRX) redirection model: This model proposes that the inactivation of PRDXs allows the accumulation of active TRX, which can then reduce other cellular targets. Note, that the redox relay model could occur under normal operational conditions when the PRDX and TRX loops are coupled. The floodgate and TRX redirection models are dependent on the uncoupling of the PRDX-TRX loop via the overoxidation of the peroxidases.

Figure 4.

The unifying model of the transcriptional and metabolic oscillators—coupled oscillators. In the cell, the metabolic and transcriptional oscillators are coupled to generate a robust timekeeper, which is shielded from both metabolic and transcriptional perturbations. The coupling of the two oscillators occurs via the various known and potentially many unknown loops, which are individually dispensable but confer a high degree of flexibility to the clock. The high degree of interplay between the two oscillators in nucleated systems makes it extremely challenging to tease apart the two systems. However, manipulations of the coupling loops have provided ample evidence that the two oscillators are tightly connected. NADPH, Nicotinamide adenine dinucleotide phosphate; PRDX, peroxiredoxin; HIF, hypoxia-inducible factor; CO, carbon monoxide; GSH, glutathione; ROS, reactive oxygen species; SIRT, sirtuin.