PMC

Genetic or pharmacological targeting of CK1e accelerates re‐entrainment to shifts in LD cycle. Rapid Entrainment of Behavioral and Physiological Rhythms to Phase Shifts in CK1ϵ−/− Mice. A–D: WT and CK1ϵ−/− mice were subjected to a 6 hours advance (A and B) or 12 hours delay (C and D) of the LD cycle. As shown by representative actogram records of wheel‐running locomotor activity (A and C) or group analysis of activity onset (B and D), CK1ϵ−/− mice entrained to the new LD phase significantly faster than WT mice. Shading indicates lights‐off. **p < 0.01, repeated‐measures two‐way ANOVA. E–F: WT mice were implanted with an osmotic minipump containing either vehicle or the selective CK1e inhibitor, PF4800467. Two days post‐implantation, the mice were subjected to a 6hr phase advance of the LD cycle (E), and daily onset of activity was determined each day (F). Mice treated with PF4800567 exhibited a significant acceleration in the time required to re‐entrain locomotor activity rhythms to the new LD cycle. Shading indicates lights‐off; red circles indicate the timing of pump implantation. **p < 0.01, repeated‐measures two‐way ANOVA. Adapted from .

Selective advantage is conferred by resonance between internal clocks and environmental cycles. Evidence for the inherent benefit of circadian resonance. A–C: Competition in mixed cultures of S. elongatus. A: Free running periods (FRP) of wild type and circadian mutant strains. B: Growth of each strain in individual cultures under LD 11:11 and LD 15:15 C: Competition kinetics in mixed cultures of wild type and mutant strains under LD 11:11 and LD 15:15. Data plotted as proportion of mutant strain in mixed versus the estimated number of generations. Strains whose FRP match the LD cycle outcompete the wild type strain, conversely strains whose FRP is dissonant with the LD cycle are at a competitive disadvantage . D: Two representative A. thaliana circadian mutant strains (toc‐1, FRP 20.7 h; ztl‐27, FRP 27.1–32.5 h) from a competition experiment. Lower mortality is observed in the line whose endogenous clock matches the light environment . E: Survival plots from wild type (FRP 24 h, filled circles), heterozygous tau (FRP 22 h, triangles) and homozygous tau mutant (FRP 20 h, open circles) Mesocricetus auratus maintained under LD 14:10. Mean survival times for wild type, hetero‐ and homozygous strains are 17.5, 10.9, and 15.8 months respectively .

Entrainment to non‐resonant LD cycles leads to an altered phase relationship between behavioral and environmental cycles. A–B: Double plotted actograms of wheel‐running activity under LD (12:12 hours, as denoted by black and white bars, blue boxes indicate light phase in (A) and DD (arrow indicates release into DD). Short vertical bars represent bouts of wheel running activity. Circadian clocks have an inherent cycle length of ∼24 hours, to match with the day/night cycle of the Earth. Variations in period length of the clock in different organisms (or tissues) can be important in dictating subtle differences in the phase relationship between the exogenous zeitgebers (e.g. solar time) and internal physiological or behavioral rhythms . However, as internal and environmental cycles diverge stable yet inappropriate phase alignment can occur – like a watch that runs too fast or too slow; even if it is reset each morning, by the afternoon it is no longer accurate. This is evidenced in wheel‐running activity records of long‐running Afterhours mutant mice (A, ) and short‐running tau mutant hamsters (B, ) housed under 24 hours LD cycles. In both cases, a stable entrainment is achieved, yet the onset of activity is significantly delayed (A) or advanced (B) relative to the light to dark transition (schematic shown in C).

Consolidation of internal and external entraining signals to reinforce internal synchrony. A: The master SCN clock is powerfully synchronized by light, the most conspicuous and predictable fluctuating environmental factor. The SCN imparts this information to the rest of the body, via neural contacts to other brain clocks, and certain peripheral tissues. Importantly, alignment to the SCN rhythm is reinforced across the body through complimentary and consolidating rhythms in hormone release (most notably, melatonin from the pineal, and glucocorticoids from the adrenals), body temperature, and feeding behavior. Further consolidation will come from subordinate brain or peripheral tissue clocks, whose activity is both entrained by the SCN, but also propagates the rhythm by influencing rhythmic physiology (e.g. arousal pathways in the brain) or the rhythmic production of secreted factors which themselves can feedback and influence the phase or amplitude of the clockwork. Therefore when properly aligned, the circadian system exists in a state of resonance whereby internal clocks are constantly reinforced by our environment and behavior. B: Many aspects of modern life undermine the temporal stability and consolidation of internal and external zeitgebers. Should zeitgebers arrive at a phase interval that is highly variable or that does not match that of the clock, biological clock‐driven rhythms would be continually phase reset and be unable to achieve the optimal alignment with the environment. This would undermine the predictive ability of the clock, whereby biological processes become only passively responsive to environmental change, or worse unable to adequately respond due to misalignment.

The molecular clockwork in mammals. In mammals, the molecular basis of circadian timing involves a transcriptional/translational/post‐translational feedback loop centred on the transcriptional activators CLOCK (or NPAS2) and BMAL1 and repressors PERIOD (PER1,2,3) and CRYPTOCHROME (CRY1,2). CLOCK and BMAL1 heterodimers bind to E‐box recognition sites to drive transcription of Period (Per1/2/3) and Cryptochrome (Cry1/2) , , , , , . PER and CRY proteins subsequently interact to form the core of a repressive complex, which inhibits BMAL1:CLOCK activity and expedites their clearance. Subsequent reduction of transcriptional activity at the Per and Cry loci, combined with active targeting of PER and CRY proteins for ubiquitination and degeneration (by TrCPβ and FXBL3/21, respectively) attenuates this repressive arm of the clock . CLOCK and BMAL1 concentrations rise once again to perpetuate the cycle. This core loop is influenced by auxiliary feedback loops, such as that involving the nuclear hormone receptors REV‐ERBα and RORα/β/γ , . Circadian proteins are also highly regulated by kinases and phosphatases which affect their stability, turnover, and sub‐cellular localization. Critically of course, components of the clock do not only drive their own expression cycles, but also impose rhythmic expression profiles onto a vast array of target genes through transcriptional enhancer element activity (e.g. via E‐Box and RORE elements), rhythmic chromatin modification (e.g. via CLOCK driven acetylation), and interaction with other non‐clock transcription factors (e.g. PER inhibition of PPARγ). In this way, transcriptional and biochemical processes within the cell are temporally coordinated across the cycle. Finally, many circadian factors are also responsive to systemic factors, which drive local cellular signaling pathways to achieve global synchronization of circadian phase.