Circadian clocks in human red blood cells

Abstract

SummaryCircadian (~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.

Circadian transcriptional regulation by the posttranslational oscillator without de novo clock gene expression in Synechococcus

“…4). The coordination of the posttranslational oscillator and the transcription/translation feedback loops has recently also been suggested to occur in the mammalian (8) and Ostreococcus circadian systems (9). In both cases, transcription/ translation feedback loops regulated by transcription-related proteins (such as the CLOCK and BMAL proteins in mammals and the CCA1 and TOC1 proteins in Ostreococcus) are important in driving the transcriptional rhythms.…”

“…In both cases, transcription/ translation feedback loops regulated by transcription-related proteins (such as the CLOCK and BMAL proteins in mammals and the CCA1 and TOC1 proteins in Ostreococcus) are important in driving the transcriptional rhythms. They are probably coupled to posttranslational oscillators driving the rhythmic oxidation/oligomerization of PRX, which is sustained without de novo transcription/translation (8,9). It is noteworthy that the Ostreococcus circadian system, in particular, shares some important properties with that of Synechococcus: (i) high-amplitude transcription cycles are observed under LL; (ii) general transcription activity, including the de novo expression of known clock genes, is strongly inhibited in the dark; and (iii) some posttranslational oscillations are sustained to keep time under DD, so that the phase of the transcriptional rhythms can be adjusted after light returns (3,8).…”

“…In eukaryotic model organisms, the core process that generates and maintains self-sustaining circadian oscillations is reported to be driven by transcription/translation feedback loops (7). However, it was recently demonstrated in the pico-eukaryotic alga Ostreococcus tauri and human red blood cells that the oxidation of 2-Cys peroxiredoxin (PRX) proteins undergo ≈24-h modification cycles without transcription (8,9). Therefore, we now require a more general understanding of the mechanisms by which posttranslational oscillators control overt physiological rhythms, such as transcription cycles.…”

Horloges circadiennes et métabolisme : intégration des signaux métaboliques et environnementaux

“…Enfin, le statut d'oxydoréduction de la cellule influence le fonctionnement de l'horloge au travers du rapport NAD(P) + /NAD(P)H, dont l'augmentation se traduit par une diminution de l'interaction de CLOCK/BMAL1 avec l'ADN [31]. Plus récemment, des oscillations circadiennes des peroxyrédo-xines, protéines anti-oxydantes, ont été mises en évidence dans les globules rouges matures dépourvus de noyau, indépendamment donc de la transcription [32]. L'état de multimérisation et d'activité de ces protéines suit un rythme circadien qui pourrait être entraîné par les variations d'ATP produit par la glycolyse.…”

Rythmes du vivant : des horloges pour tous les temps

“…Plusieurs observations montrent que d'autres mécanismes, qui ne sont pas fondés sur une régula-tion de l'expression génétique, peuvent également être à l'origine de rythmes circadiens. Ces observations ont tout d'abord porté sur les cyanobactéries, et viennent d'être étendues aux érythrocytes humains [6] ainsi qu'à l'algue Ostreococcus tauri [7].…”