Modern society characterized by a 24/7 lifestyle leads to misalignment between environmental cycles and endogenous circadian rhythms. Persisting circadian misalignment leads to deleterious effects on health and healthspan. However, the underlying mechanism remains not fully understood. Here, we subjected adult, wild-type mice to distinct chronic jet-lag paradigms, which showed that long-term circadian misalignment induced significant early mortality. Non-biased RNA sequencing analysis using liver and kidney showed marked activation of gene regulatory pathways associated with the immune system and immune disease in both organs. In accordance, we observed enhanced steatohepatitis with infiltration of inflammatory cells. The investigation of senescence-associated immune cell subsets from the spleens and mesenteric lymph nodes revealed an increase in PD-1 + CD44 high CD4 T cells as well as CD95 + GL7 + germinal center B cells, indicating that the long-term circadian misalignment exacerbates immune senescence and consequent chronic inflammation. Our results underscore immune homeostasis as a pivotal interventional target against clock-related disorders. From the cellular to the organismal levels, circadian clocks regulate various essential biological processes to enable anticipation of and adaptation to the daily environmental changes from Earth rotation 1. Modernization of our society is accompanied by a dramatic change in human lifestyle, with unprecedented increases in, for example, night shift work and nocturnal feeding/recreational activities 2. Recent epidemiological studies have revealed shift workers as being at a higher risk of various diseases, such as mood disorders, metabolic syndrome, cardiovascular disease, and some types of cancers, suggesting that the misalignment between environmental cycles and endogenous circadian clocks exacerbates systemic pathological consequences 3-8. However, the pathophysiological mechanisms underlying the deleterious effects of long-term circadian misalignment in health and healthspan remain unclear. Recent studies have investigated the perturbation of circadian systems by environmental and/or genetic manipulation in animal models 9,10. For example, Davidson et al. reported that an experimental model of environmental perturbation induced by the scheduled shifts of light-dark cycles-called chronic jet-lag (CJL)-for 8 weeks using aged mice (27-31 months old) showed the mortality rate to be higher in the phase advance condition (6-hour phase advance every 7 days) than in the phase delay (6-hour phase delay every 7 days) condition or control LD condition 9. These studies principally investigated the acute or subacute (for up to a few months) effects of circadian misalignment; it thus remains uncertain how long-term perturbation of environmental light-dark cycle induces physiological transformation and pathological consequences.

The circadian clock regulates behavioural and physiological processes in a 24-h cycle. The nuclear receptors REV-ERBα and REV-ERBβ are involved in the cell-autonomous circadian transcriptional/translational feedback loops as transcriptional repressors. A number of studies have also demonstrated a pivotal role of REV-ERBs in regulation of metabolic, neuronal, and inflammatory functions including bile acid metabolism, lipid metabolism, and production of inflammatory cytokines. Given the multifunctional role of REV-ERBs, it is important to elucidate the mechanism through which REV-ERBs exert their functions. To this end, we established a Rev-erbα / Rev-erbβ double-knockout mouse embryonic stem (ES) cell model and analyzed the circadian clock and clock-controlled output gene expressions. A comprehensive mRNA-seq analysis revealed that the double knockout of both Rev-erbα and Rev-erbβ does not abrogate expression rhythms of E-box-regulated core clock genes but drastically changes a diverse set of other rhythmically-expressed output genes. Of note, REV-ERBα/ β deficiency does not compromise circadian expression rhythms of PER2, while REV-ERB target genes, Bmal1 and Npas2 , are significantly upregulated. This study highlight the relevance of REV-ERBs as pivotal output mediators of the mammalian circadian clock.

Purpose To investigate circadian clock oscillation and circadian global gene expression in cultured human corneal endothelial cells (cHCECs) to elucidate and assess the potential function of circadian regulation in HCECs. Methods In this study, we introduced a circadian bioluminescence reporter, Bmal1:luciferase ( Bmal1:luc ), into cHCECs and subsequently monitored real-time bioluminescence rhythms. RNA-sequencing data analysis was then performed using sequential time-course samples of the cHCECs to obtain a comprehensive understanding of the circadian gene expression rhythms. The potential relevance of rhythmically expressed genes was then assessed by systematic approaches using functional clustering and individual gene annotations. Results Bmal1:luc bioluminescence exhibited clear circadian oscillation in the cHCECs. The core clock genes and clock-related genes showed high-amplitude robust circadian messenger RNA (mRNA) expression rhythms in cHCECs after treatment with dexamethasone, and 329 genes that exhibited circadian mRNA expression rhythms were identified (i.e., genes involved in various physiological processes including glycolysis, mitochondrial function, antioxidative systems, hypoxic responses, apoptosis, and extracellular matrix regulation, which represent the physiological functions of HCECs). Conclusions Our findings revealed that cHCECs have a robust and functional circadian clock, and our discovery that a large number of genes exhibit circadian mRNA expression rhythms in cHCECs suggests a potential contribution of circadian regulation to fine-tune HCEC functions for daily changes in the environment.

Rhythmic incremental growth lines occur in dental hard tissues of vertebrates, and dentinogenesis in rodent incisors is suggested to be controlled by the 24-hr circadian clock. Rodent incisors continue to grow throughout the animal's life; however, similar to human teeth, rodent molars stop growing after crown formation. This similarity suggests that the mouse molar is an excellent model to understand the molecular mechanisms underlying growth of human teeth. However, not much is known about the rhythmic dentinogenesis in mouse molars. Here, we investigated the incremental growth lines in mouse molar dentin using tetracycline as the growth marker. The incremental growth lines were observed to be generated at approximately 8-hr intervals in wild-type mice housed under 12:12 hr light-dark conditions. Moreover, the 8-hr rhythmic increments persisted in the wild-type and Bmal1 −/− mice housed in constant darkness, where Bmal1 −/− mice become behaviorally arrhythmic. These results revealed that the dentinogenesis in mouse molars underlie the ultradian rhythms with around 8-hr periodicity. Further, the circadian clock does not seem to be involved in this process, providing new insight into the mechanisms involved in the tooth growth.

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