SummaryCellular life emerged ~3.7 billion years ago. With scant exception, terrestrial organisms have evolved under predictable daily cycles due to the Earth’s rotation. The advantage conferred upon organisms that anticipate such environmental cycles has driven the evolution of endogenous circadian rhythms that tune internal physiology to external conditions. The molecular phylogeny of mechanisms driving these rhythms has been difficult to dissect because identified clock genes and proteins are not conserved across the domains of life: Bacteria, Archaea and Eukaryota. Here we show that oxidation-reduction cycles of peroxiredoxin proteins constitute a universal marker for circadian rhythms in all domains of life, by characterising their oscillations in a variety of model organisms. Furthermore, we explore the interconnectivity between these metabolic cycles and transcription-translation feedback loops of the clockwork in each system. Our results suggest an intimate co-evolution of cellular time-keeping with redox homeostatic mechanisms following the Great Oxidation Event ~2.5 billion years ago.

Temporally complex, circadian programming of the transcriptome in a peripheral organ is imposed across a wide range of core cellular functions and is dependent on an interaction between intrinsic, tissue-specific factors and extrinsic regulation by the SCN central pacemaker.

Circadian rhythms are essential to health. Their disruption is associated with metabolic diseases in experimental animals and man. Local metabolic rhythms represent an output of tissue-based circadian clocks. Attempts to define how local metabolism is temporally coordinated have focused on gene expression by defining extensive and divergent "circadian transcriptomes" involving 5%-10% of genes assayed. These analyses are inevitably incomplete, not least because metabolic coordination depends ultimately upon temporal regulation of proteins. We therefore conducted a systematic analysis of a mammalian "circadian proteome." Our analysis revealed that up to 20% of soluble proteins assayed in mouse liver are subject to circadian control. Many of these circadian proteins are novel and cluster into discrete phase groups so that the liver's enzymatic profile contrasts dramatically between day and night. Unexpectedly, almost half of the cycling proteins lack a corresponding cycling transcript, as determined by quantitative PCR, microarray, or both and revealing for the first time the extent of posttranscriptional mechanisms as circadian control points. The circadian proteome includes rate-limiting factors in vital pathways, including urea formation and sugar metabolism. These findings provide a new perspective on the extensive contribution of circadian programming to hepatic physiology.

Circadian and other natural clock-like endogenous rhythms may have evolved to anticipate regular temporal changes in the environment. We report that a mutation in the circadian clock gene timeless in Drosophila melanogaster has arisen and spread by natural selection relatively recently in Europe. We found that, when introduced into different genetic backgrounds, natural and artificial alleles of the timeless gene affect the incidence of diapause in response to changes in light and temperature. The natural mutant allele alters an important life history trait that may enhance the fly's adaptation to seasonal conditions.

Circadian clocks have evolved to synchronize physiology, metabolism and behaviour to the 24-h geophysical cycles of the Earth. Drosophila melanogaster's rhythmic locomotor behaviour provides the main phenotype for the identification of higher eukaryotic clock genes. Under laboratory light-dark cycles, flies show enhanced activity before lights on and off signals, and these anticipatory responses have defined the neuronal sites of the corresponding morning (M) and evening (E) oscillators. However, the natural environment provides much richer cycling environmental stimuli than the laboratory, so we sought to examine fly locomotor rhythms in the wild. Here we show that several key laboratory-based assumptions about circadian behaviour are not supported by natural observations. These include the anticipation of light transitions, the midday 'siesta', the fly's crepuscular activity, its nocturnal behaviour under moonlight, and the dominance of light stimuli over temperature. We also observe a third major locomotor component in addition to M and E, which we term 'A' (afternoon). Furthermore, we show that these natural rhythm phenotypes can be observed in the laboratory by using realistic temperature and light cycle simulations. Our results suggest that a comprehensive re-examination of circadian behaviour and its molecular readouts under simulated natural conditions will provide a more authentic interpretation of the adaptive significance of this important rhythmic phenotype. Such studies should also help to clarify the underlying molecular and neuroanatomical substrates of the clock under natural protocols.

The threonine-glycine (Thr-Gly) encoding repeat within the clock gene period of Drosophila melanogaster is polymorphic in length. The two major variants (Thr-Gly)17 and (Thr-Gly)20 are distributed as a highly significant latitudinal cline in Europe and North Africa. Thr-Gly length variation from both wild-caught and transgenic individuals is related to the flies' ability to maintain a circadian period at different temperatures. This phenomenon provides a selective explanation for the geographical distribution of Thr-Gly lengths and gives a rare glimpse of the interplay between molecular polymorphism, behavior, population biology, and natural selection.

Summary Neuroactive metabolites of the kynurenine pathway (KP) of tryptophan degradation have been implicated in the pathophysiology of neurodegenerative disorders, including Huntington’s disease (HD) [1]. A central hallmark of HD is neurodegeneration caused by a polyglutamine expansion in the huntingtin (htt) protein [2]. Here we exploit a transgenic Drosophila melanogaster model of HD to interrogate the therapeutic potential of KP manipulation. We observe that genetic and pharmacological inhibition of kynurenine 3-monooxygenase (KMO) increases levels of the neuroprotective metabolite kynurenic acid (KYNA) relative to the neurotoxic metabolite 3-hydroxykynurenine (3-HK) and ameliorates neurodegeneration. We also find that genetic inhibition of tryptophan 2,3-dioxygenase (TDO), the first and rate-limiting step in the pathway, leads to a similar neuroprotective shift toward KYNA synthesis. Importantly, we demonstrate that the feeding of KYNA and 3-HK to HD model flies directly modulates neurodegeneration, underscoring the causative nature of these metabolites. This study provides the first genetic evidence that inhibition of KMO and TDO activity protects against neurodegenerative disease in an animal model, indicating that strategies targeted at two key points within the KP may have therapeutic relevance in HD, and possibly other neurodegenerative disorders.

Properties of the circadian and annual timing systems are expected to vary systematically with latitude on the basis of different annual light and temperature patterns at higher latitudes, creating specific selection pressures. We review literature with respect to latitudinal clines in circadian phenotypes as well as in polymorphisms of circadian clock genes and their possible association with annual timing. The use of latitudinal (and altitudinal) clines in identifying selective forces acting on biological rhythms is discussed, and we evaluate how these studies can reveal novel molecular and physiological components of these rhythms.