Forskolin induces circadian gene expression of rPer1, rPer2 and dbp in mammalian rat-1 fibroblasts

Circadian synchronization differentially modifies cytokine-mediated transcriptomic remodeling and cell death in INS-1 cells and mouse islets

Perturbation of the β-cell circadian clock causes oxidative stress and secretory failure, and proinflammatory cytokines disrupt the β-cell core clock. We hypothesized that cytokine-mediated clock perturbation in β-cells depends on circadian synchronization status. Cytokine-mediated core clock mRNA expression in non-synchronized insulin-producing INS-1 cells were potentiated upon synchronization, which were differentially translated into alterations in protein levels. Synchronization sensitized INS-1 cells to cytokine-mediated cytotoxicity, associated with potentiation of NF-κB activity. Inhibition of NF-κB abrogated cytokine-mediated clock gene-expression independent of synchronization status and reversed cytokine-mediated period lengthening. In contrast, in murine islets, cytokines generally reduced core clock mRNA expression independently of synchronization status or NF-κB activity. Synchronization prevented cytokine-mediated cytotoxicity, but not NF-κB activity to a degree comparable to that of KINK-1, while alterations in islet rhythmicity were unaffected by NF-κB inhibition. In conclusion, circadian synchronization differentially modifies cytokine-mediated transcriptomic remodeling and cell death in INS-1 cells and murine islets, depending on NF-κB involvement.

Circadian rhythm defects in Prader-Willi syndrome neurons

Prader-Willi syndrome (PWS) is a neurodevelopmental disorder characterized by a spectrum of symptoms, including developmental delay, intellectual disability, and increased risk of autism. PWS is an imprinting disorder caused by the loss of paternal expression of critical genes in the 15q11.2-q13 region, including MAGEL2, SNRPN/SNURF, and SNORD116. PWS patients often suffer from various sleep disorders, including sleep-disordered breathing and central hypersomnolence. Mouse models of PWS also exhibit disruptions in circadian rhythms and sleep. In cultured cells, Magel2 was shown to regulate the expression of Bmal1 and Per2, two core clock genes involved in the circadian rhythm regulatory process. Here, we investigated the circadian clock function in neurons derived from dental pulp stem cells (DPSCs) of PWS patients and neurotypical controls. To study the circadian rhythms of PWS patients in vitro, we introduced the Per2 promoter-driven luciferase reporter (Per2:luc) to these DPSC cell lines to assess their circadian rhythm by bioluminescence. These Per2:luc cells were differentiated for 4 weeks to mature neuronal reporter cell lines, followed by kinetic measurements of luciferase activity over several days. We observed significant differences in circadian period length between PWS neurons and controls. Moreover, treatment with the small molecule longdaysin effectively lengthened the period length of PWS neurons with a shorter period length, as anticipated based on the mechanism of action of this compound. This work lays the foundation for a deeper understanding of PWS pathophysiology and represents a critical first step toward developing high-throughput assays for drug discovery targeting circadian and sleep dysfunction in PWS.

The time is now: accounting for time-of-day effects to improve reproducibility and translation of metabolism research

The constant expansion of the field of metabolic research has led to more nuanced and sophisticated understanding of the complex mechanisms that underlie metabolic functions and diseases. Collaborations with scientists of various fields such as neuroscience, immunology and drug discovery have further enhanced the ability to probe the role of metabolism in physiological processes. However, many behaviours, endocrine and biochemical processes, and the expression of genes, proteins and metabolites have daily ~24-h biological rhythms and thus peak only at specific times of the day. This daily variation can lead to incorrect interpretations, lack of reproducibility across laboratories and challenges in translating preclinical studies to humans. In this Review, we discuss the biological, environmental and experimental factors affecting circadian rhythms in rodents, which can in turn alter their metabolic pathways and the outcomes of experiments. We recommend that these variables be duly considered and suggest best practices for designing, analysing and reporting metabolic experiments in a circadian context.

Circadian rhythms in stem cells and their therapeutic potential

Circadian rhythms are present in almost all cells, but their existence in stem cells has remains not well established. Circadian clock appears to be closely associated with differentiated mature cells and rarely detected in immature embryonic stem cells. Recent evidence reveals the presence of circadian genes and rhythmic physiologic activities in stem cells as well as stem cell-derived extracellular vesicle (EV) characteristics. The circadian clock entails diverse physiologic and pathological mechanisms underlying cell fate. Integration of circadian rhythm to clinical applications, such as chronotherapy, chrono-biomarker, and environment modification, may facilitate therapeutic outcomes of stem cell-based regenerative medicine. Understanding circadian rhythms in stem cells can optimize stem cell-based therapies by determining the best times for harvesting and administering stem cells, thereby enhancing therapeutic efficacy. Further research into the circadian properties of stem cells will refine stem cell-based therapies, contributing to advancements in regenerative medicine.

Cyclin-dependent kinase 5 (Cdk5) activity is modulated by light and gates rapid phase shifts of the circadian clock

The circadian clock enables organisms to synchronize biochemical and physiological processes over a 24 hr period. Natural changes in lighting conditions, as well as artificial disruptions like jet lag or shift work, can advance or delay the clock phase to align physiology with the environment. Within the suprachiasmatic nucleus (SCN) of the hypothalamus, circadian timekeeping and resetting rely on both membrane depolarization and intracellular second-messenger signaling. Voltage-gated calcium channels (VGCCs) facilitate calcium influx in both processes, activating intracellular signaling pathways that trigger Period (Per) gene expression. However, the precise mechanism by which these processes are concertedly gated remains unknown. Our study in mice demonstrates that cyclin-dependent kinase 5 (Cdk5) activity is modulated by light and regulates phase shifts of the circadian clock. We observed that knocking down Cdk5 in the SCN of mice affects phase delays but not phase advances. This is linked to uncontrolled calcium influx into SCN neurons and an unregulated protein kinase A (PKA)-calcium/calmodulin-dependent kinase (CaMK)-cAMP response element-binding protein (CREB) signaling pathway. Consequently, genes such as Per1 are not induced by light in the SCN of Cdk5 knock-down mice. Our experiments identified Cdk5 as a crucial light-modulated kinase that influences rapid clock phase adaptation. This finding elucidates how light responsiveness and clock phase coordination adapt activity onset to seasonal changes, jet lag, and shift work.

Circadian regulation of endoplasmic reticulum calcium response in cultured mouse astrocytes

The circadian clock, an internal time-keeping system orchestrates 24 hr rhythms in physiology and behavior by regulating rhythmic transcription in cells. Astrocytes, the most abundant glial cells, play crucial roles in CNS functions, but the impact of the circadian clock on astrocyte functions remains largely unexplored. In this study, we identified 412 circadian rhythmic transcripts in cultured mouse cortical astrocytes through RNA sequencing. Gene Ontology analysis indicated that genes involved in Ca2+ homeostasis are under circadian control. Notably, Herpud1 (Herp) exhibited robust circadian rhythmicity at both mRNA and protein levels, a rhythm disrupted in astrocytes lacking the circadian transcription factor, BMAL1. HERP regulated endoplasmic reticulum (ER) Ca2+ release by modulating the degradation of inositol 1,4,5-trisphosphate receptors (ITPRs). ATP-stimulated ER Ca2+ release varied with the circadian phase, being more pronounced at subjective night phase, likely due to the rhythmic expression of ITPR2. Correspondingly, ATP-stimulated cytosolic Ca2+ increases were heightened at the subjective night phase. This rhythmic ER Ca2+ response led to circadian phase-dependent variations in the phosphorylation of Connexin 43 (Ser368) and gap junctional communication. Given the role of gap junction channel (GJC) in propagating Ca2+ signals, we suggest that this circadian regulation of ER Ca2+ responses could affect astrocytic modulation of synaptic activity according to the time of day. Overall, our study enhances the understanding of how the circadian clock influences astrocyte function in the CNS, shedding light on their potential role in daily variations of brain activity and health.

Circadian control of polycyclic aromatic hydrocarbon-induced dysregulation of endothelial tight junctions and mitochondrial bioenergetics

The study evaluates the impact of environmental toxicants, such as polycyclic aromatic hydrocarbons (PAHs), on circadian regulations and functions of brain endothelial cells, which form the main structural element of the blood-brain barrier (BBB). PAH are lipophilic and highly toxic environmental pollutants that accumulate in human and animal tissues. Environmental factors related to climate change, such as an increase in frequency and intensity of wildfires or enhanced strength of hurricanes or tropical cyclones, may lead to redistribution of these toxicants and enhanced human exposure. These natural disasters are also associated with disruption of circadian rhythms in affected populations, linking increased exposure to environmental toxicants to alterations of circadian rhythm pathways. Several vital physiological processes are coordinated by circadian rhythms, and disruption of the circadian clock can contribute to the development of several diseases. The blood-brain barrier (BBB) is crucial for protecting the brain from blood-borne harmful substances, and its integrity is influenced by circadian rhythms. Exposure of brain endothelial cells to a human and environmentally-relevant PAH mixture resulted in dose-dependent alterations of expression of critical circadian modulators, such as Clock, Bmal1, Cry1/2, and Per1/2. Moreover, silencing of the circadian Clock gene potentiated the impact of PAHs on the expression of the main tight junction genes and proteins (namely, claudin-5, occludin, JAM-2, and ZO-2), as well as mitochondrial bioenergetics. Findings from this study contribute to a better understanding of pathological influence of PAH-induced health effects, especially those related to circadian rhythm disruption.

Circadian rhythms of macrophages are altered by the acidic tumor microenvironment

Tumor-associated macrophages (TAMs) are prime therapeutic targets due to their pro-tumorigenic functions, but varying efficacy of macrophage-targeting therapies highlights our incomplete understanding of how macrophages are regulated within the tumor microenvironment (TME). The circadian clock is a key regulator of macrophage function, but how circadian rhythms of macrophages are influenced by the TME remains unknown. Here, we show that conditions associated with the TME such as polarizing stimuli, acidic pH, and lactate can alter circadian rhythms in macrophages. While cyclic AMP (cAMP) has been reported to play a role in macrophage response to acidic pH, our results indicate pH-driven changes in circadian rhythms are not mediated solely by cAMP signaling. Remarkably, circadian disorder of TAMs was revealed by clock correlation distance analysis. Our data suggest that heterogeneity in circadian rhythms within the TAM population level may underlie this circadian disorder. Finally, we report that circadian regulation of macrophages suppresses tumor growth in a murine model of pancreatic cancer. Our work demonstrates a novel mechanism by which the TME influences macrophage biology through modulation of circadian rhythms.

Fibroblasts as an in vitro model of circadian genetic and genomic studies

Bipolar disorder (BD) is a heritable disorder characterized by shifts in mood that manifest in manic or depressive episodes. Clinical studies have identified abnormalities of the circadian system in BD patients as a hallmark of underlying pathophysiology. Fibroblasts are a well-established in vitro model for measuring circadian patterns. We set out to examine the underlying genetic architecture of circadian rhythm in fibroblasts, with the goal to assess its contribution to the polygenic nature of BD disease risk. We collected, from primary cell lines of 6 healthy individuals, temporal genomic features over a 48 h period from transcriptomic data (RNA-seq) and open chromatin data (ATAC-seq). The RNA-seq data showed that only a limited number of genes, primarily the known core clock genes such as ARNTL, CRY1, PER3, NR1D2 and TEF display circadian patterns of expression consistently across cell cultures. The ATAC-seq data identified that distinct transcription factor families, like those with the basic helix-loop-helix motif, were associated with regions that were increasing in accessibility over time. Whereas known glucocorticoid receptor target motifs were identified in those regions that were decreasing in accessibility. Further evaluation of these regions using stratified linkage disequilibrium score regression analysis failed to identify a significant presence of them in the known genetic architecture of BD, and other psychiatric disorders or neurobehavioral traits in which the circadian rhythm is affected. In this study, we characterize the biological pathways that are activated in this in vitro circadian model, evaluating the relevance of these processes in the context of the genetic architecture of BD and other disorders, highlighting its limitations and future applications for circadian genomic studies.

Circadian rhythms of macrophages are altered by the acidic pH of the tumor microenvironment

Macrophages are prime therapeutic targets due to their pro-tumorigenic and immunosuppressive functions in tumors, but the varying efficacy of therapeutic approaches targeting macrophages highlights our incomplete understanding of how the tumor microenvironment (TME) can influence regulation of macrophages. The circadian clock is a key internal regulator of macrophage function, but how circadian rhythms of macrophages may be influenced by the tumor microenvironment remains unknown. We found that conditions associated with the TME such as polarizing stimuli, acidic pH, and elevated lactate concentrations can each alter circadian rhythms in macrophages. Circadian rhythms were enhanced in pro-resolution macrophages but suppressed in pro-inflammatory macrophages, and acidic pH had divergent effects on circadian rhythms depending on macrophage phenotype. While cyclic AMP (cAMP) has been reported to play a role in macrophage response to acidic pH, our results indicate that pH-driven changes in circadian rhythms are not mediated solely by the cAMP signaling pathway. Remarkably, clock correlation distance analysis of tumor-associated macrophages (TAMs) revealed evidence of circadian disorder in TAMs. This is the first report providing evidence that circadian rhythms of macrophages are altered within the TME. Our data further suggest that heterogeneity in circadian rhythms at the population level may underlie this circadian disorder. Finally, we sought to determine how circadian regulation of macrophages impacts tumorigenesis, and found that tumor growth was suppressed when macrophages had a functional circadian clock. Our work demonstrates a novel mechanism by which the tumor microenvironment can influence macrophage biology through altering circadian rhythms, and the contribution of circadian rhythms in macrophages to suppressing tumor growth.

Effects of (Poly)phenols on Circadian Clock Gene-Mediated Metabolic Homeostasis in Cultured Mammalian Cells: A Scoping Review

Circadian clocks regulate metabolic homeostasis. Disruption to our circadian clocks, by lifestyle behaviors such as timing of eating and sleeping, has been linked to increased rates of metabolic disorders. There is now considerable evidence that selected dietary (poly)phenols, including flavonoids, phenolic acids and tannins, may modulate metabolic and circadian processes. This review evaluates the effects of (poly)phenols on circadian clock genes and linked metabolic homeostasis in vitro, and potential mechanisms of action, by critically evaluating the literature on mammalian cells. A systematic search was conducted to ensure full coverage of the literature and identified 43 relevant studies addressing the effects of (poly)phenols on cellular circadian processes. Nobiletin and tangeretin, found in citrus, (-)-epigallocatechin-3-gallate from green tea, urolithin A, a gut microbial metabolite from ellagitannins in fruit, curcumin, bavachalcone, cinnamic acid, and resveratrol at low micromolar concentrations all affect circadian molecular processes in multiple types of synchronized cells. Nobiletin emerges as a putative retinoic acid-related orphan receptor (RORα/γ) agonist, leading to induction of the circadian regulator brain and muscle ARNT-like 1 (BMAL1), and increased period circadian regulator 2 (PER2) amplitude and period. These effects are clear despite substantial variations in the protocols employed, and this review suggests a methodological framework to help future study design in this emerging area of research.

Fibroblasts as an in vitro model of circadian genetic and genomic studies: A temporal analysis

Bipolar disorder (BD) is a heritable disorder characterized by shifts in mood that manifest in manic or depressive episodes. Clinical studies have identified abnormalities of the circadian system in BD patients as a hallmark of underlying pathophysiology. Fibroblasts are a well-established in vitro model for measuring circadian patterns. We set out to examine the underlying genetic architecture of circadian rhythm in fibroblasts, with the goal to assess its contribution to the polygenic nature of BD disease risk. We collected, from primary cell lines of 6 healthy individuals, temporal genomic features over a 48 hour period from transcriptomic data (RNA-seq) and open chromatin data (ATAC-seq). The RNA-seq data showed that only a limited number of genes, primarily the known core clock genes such as ARNTL, CRY1, PER3, NR1D2 and TEF display circadian patterns of expression consistently across cell cultures. The ATAC-seq data identified that distinct transcription factor families, like those with the basic helix-loop-helix motif, were associated with regions that were increasing in accessibility over time. Whereas known glucocorticoid receptor target motifs were identified in those regions that were decreasing in accessibility. Further evaluation of these regions using stratified linkage disequilibrium score regression (sLDSC) analysis failed to identify a significant presence of them in the known genetic architecture of BD, and other psychiatric disorders or neurobehavioral traits in which the circadian rhythm is affected. In this study, we characterize the biological pathways that are activated in this in vitro circadian model, evaluating the relevance of these processes in the context of the genetic architecture of BD and other disorders, highlighting its limitations and future applications for circadian genomic studies.

Circadian control of cisplatin-DNA adduct repair and apoptosis in culture cells

Cisplatin, a widely prescribed chemotherapeutic agent for treating solid tumors, induces DNA adducts and activates cellular defense mechanisms, including DNA repair, cell cycle checkpoint control, and apoptosis. Considering the circadian rhythmicity displayed by most chemotherapeutic agents and their varying therapeutic efficacy based on treatment timing, our study aimed to investigate whether the circadian clock system influences the DNA damage responses triggered by cisplatin in synchronized cells. We examined the DNA damage responses in circadian-synchronized wild-type mouse embryonic fibroblasts (WT-MEF; clock-proficient cells), cryptochrome1 and 2 double knock-out MEF (CRYDKO; clock-deficient cells), and mouse hepatocarcinoma Hepa1c1c7 cells. Varying the treatment time resulted in a significant difference in the rate of platinum-DNA adduct removal specifically in circadian-synchronized WT-MEF, while CRYDKO did not exhibit such variation. Moreover, diurnal variation in other DNA damage responses, such as cell cycle checkpoint activity indicated by p53 phosphorylation status and apoptosis measured by DNA break frequency, was observed only in circadian-synchronized WT-MEF, not in CRYDKO or mouse hepatocarcinoma Hepa1c1c7 cells. These findings highlight that the DNA damage responses triggered by cisplatin are indeed governed by circadian control exclusively in clock-proficient cells. This outcome bears potential implications for enhancing or devising chronotherapy approaches for cancer patients.

Artificial induction of circadian rhythm by combining exogenous BMAL1 expression and polycomb repressive complex 2 inhibition in human induced pluripotent stem cells

Understanding the physiology of human-induced pluripotent stem cells (iPSCs) is necessary for directed differentiation, mimicking embryonic development, and regenerative medicine applications. Pluripotent stem cells (PSCs) exhibit unique abilities such as self-renewal and pluripotency, but they lack some functions that are associated with normal somatic cells. One such function is the circadian oscillation of clock genes; however, whether or not PSCs demonstrate this capability remains unclear. In this study, the reason why circadian rhythm does not oscillate in human iPSCs was examined. This phenomenon may be due to the transcriptional repression of clock genes resulting from the hypermethylation of histone H3 at lysine 27 (H3K27), or it may be due to the low levels of brain and muscle ARNT-like 1 (BMAL1) protein. Therefore, BMAL1-overexpressing cells were generated and pre-treated with GSK126, an inhibitor of enhancer of zest homologue 2 (EZH2), which is a methyltransferase of H3K27 and a component of polycomb repressive complex 2. Consequently, a significant circadian rhythm following endogenous BMAL1, period 2 (PER2), and other clock gene expression was induced by these two factors, suggesting a candidate mechanism for the lack of rhythmicity of clock gene expression in iPSCs.

Input integration by the circadian clock exhibits nonadditivity and fold-change detection

Circadian clocks are synchronized by external timing cues to align with one another and the environment. Various signaling pathways have been shown to independently reset the phase of the clock. However, in the body, circadian clocks are exposed to a multitude of potential timing cues with complex temporal dynamics, raising the question of how clocks integrate information in response to multiple signals. To investigate different modes of signal integration by the circadian clock, we used Circa-SCOPE, a method we recently developed for high-throughput phase resetting analysis. We found that simultaneous exposure to different combinations of known pharmacological resetting agents elicits a diverse range of responses. Often, the response was nonadditive and could not be readily predicted by the response to the individual signals. For instance, we observed that dexamethasone is dominant over other tested inputs. In the case of signals administered sequentially, the background levels of a signal attenuated subsequent resetting by the same signal, but not by signals acting through a different pathway. This led us to examine whether the circadian clock is sensitive to relative rather than absolute levels of the signal. Importantly, our analysis revealed the involvement of a signal-specific fold-change detection mechanism in the clock response. This mechanism likely stems from properties of the signaling pathway that are upstream to the clock. Overall, our findings elucidate modes of input integration by the circadian clock, with potential relevance to clock resetting under both physiological and pathological conditions.

Oscillations of the circadian clock protein, BMAL-1, align to daily cycles of mechanical stimuli: a novel means to integrate biological time within predictive in vitro model systems

Purpose

In vivo, the circadian clock drives 24-h rhythms in human physiology. Isolated cells in vitro retain a functional clockwork but lack necessary timing cues resulting in the rapid loss of tissue-level circadian rhythms. This study tests the hypothesis that repeated daily mechanical stimulation acts as a timing cue for the circadian clockwork. The delineation and integration of circadian timing cues into predictive in vitro model systems, including organ-on-a-chip (OOAC) devices, represent a novel concept that introduces a key component of in vivo physiology into predictive in vitro model systems.

Methods

Quiescent bovine chondrocytes were entrained for 3 days by daily 12-h bouts of cyclic biaxial tensile strain (10%, 0.33 Hz, Flexcell) before sampling during free-running conditions. The core clock protein, BMAL-1, was quantified from normalised Western Blot signal intensity and the temporal oscillations characterised by Cosinor linear fit with 24-h period.

Results

Following entrainment, the cell-autonomous oscillations of the molecular clock protein, BMAL-1, exhibited circadian (24 h) periodicity (p < 0.001) which aligned to the diurnal mechanical stimuli. A 6-h phase shift in the mechanical entrainment protocol resulted in an equivalent shift of the circadian clockwork. Thus, repeated daily mechanical stimuli synchronised circadian rhythmicity of chondrocytes in vitro.

Conclusion

This work demonstrates that daily mechanical stimulation can act as a timing cue that is sufficient to entrain the peripheral circadian clock in vitro. This discovery may be exploited to induce and sustain circadian physiology within into predictive in vitro model systems, including OOAC systems. Integration of the circadian clock within these systems will enhance their potential to accurately recapitulate human diurnal physiology and hence augment their predictive value as drug testing platforms and as realistic models of human (patho)physiology.

Supplementary Information

The online version contains supplementary material available at 10.1007/s44164-022-00032-x.

Circadian regulation of digestive and metabolic tissues

The circadian clock is a self-sustained molecular timekeeper that drives 24-h (circadian) rhythms in animals. The clock governs important aspects of behavior and physiology including wake/sleep activity cycles that regulate the activity of metabolic and digestive systems. Light/dark cycles (photoperiod) and cycles in the time of feeding synchronize the circadian clock to the surrounding environment, providing an anticipatory benefit that promotes digestive health. The availability of animal models targeting the genetic components of the circadian clock has made it possible to investigate the circadian clock's role in cellular functions. Circadian clock genes have been shown to regulate the physiological function of hepatocytes, gastrointestinal cells, and adipocytes; disruption of the circadian clock leads to the exacerbation of liver diseases and liver cancer, inflammatory bowel disease and colorectal cancer, and obesity. Previous findings provide strong evidence that the circadian clock plays an integral role in digestive/metabolic disease pathogenesis, hence, the circadian clock is a necessary component in metabolic and digestive health and homeostasis. Circadian rhythms and circadian clock function provide an opportunity to improve the prevention and treatment of digestive and metabolic diseases by aligning digestive system tissue with the 24-h day.

Circadian clock gene BMAL1 inhibits the proliferation and tumor-formation ability of nasopharyngeal carcinoma cells and increases the sensitivity of radiotherapy

BMAL1 is a core circadian clock gene that is expressed rhythmically in a variety of tumor cells and is related to cancer cell proliferation and chemoradiotherapy sensitivity. Radiotherapy plays an important role in the treatment of nasopharyngeal carcinoma (NPC). However, the rhythmicity of BMAL1 in NPC, as well as its precise role in radiotherapy, remains unclear. We assessed changes in BMAL1 expression over 48 h in NPC cells and normal nasopharyngeal epithelial cells NP69 using real-time quantitative polymerase chain reaction (RT-PCR) and western blotting (WB). Then, we induced the overexpression and knocked-down the levels of BMAL1 in NPC cells, and subsequently used Cell Counting Kit-8 assays to assess the proliferation of NPC cells. Xenograft tumour growth was used to evaluate the effect of BMAL1 in vivo. Immunohistochemical staining was used to detect the expression of BMAL1 protein in transplanted tumors. Gene Set Enrichment Analysis (GSEA) was performed to explore the biological signaling pathway. Finally, RT-PCR and WB were used to detect the expressions of BMAL1, p53 and p21. The results showed that the mRNA expression levels of circadian clock gene BMAL1 fluctuated rhythmically with time, and the expression levels of BMAL1 also changed depending on the protein levels in NPC and NP69 cells. Overexpression of BMAL1 inhibited the proliferation of NPC cells, while knockdown BMAL1 had the opposite effects. In a xenograft model, we observed that the upregulation of BMAL1 inhibited tumor growth and enhanced the sensitivity of NPC cells to radiotherapy. Ultimately, the downregulation of BMAL1 promoted tumor growth and decreased radiosensitivity. GSEA analysis suggested that BMAL1 significantly affected the p53 pathway. Overexpression of BMAL1 promoted the expression of p53 and p21, while the knockdown of BMAL1 inhibited the expression of p53 and p21. We speculate that BMAL1 has the potential to be a prognostic biomarker and therapeutic target for NPC.

Physical Interaction between Cyclin-Dependent Kinase 5 (CDK5) and Clock Factors Affects the Circadian Rhythmicity in Peripheral Oscillators

Circadian rhythms are self-sustained oscillators with a period of 24 h that is based on the output of transcriptional and post-translational feedback loops. Phosphorylation is considered one of the most important post-translational modifications affecting rhythmicity from cyanobacteria to mammals. For example, the lack of cyclin-dependent kinase 5 (CDK5) shortened the period length of the circadian oscillator in the Suprachiasmatic Nuclei (SCN) of mice via the destabilization of the PERIOD 2 (PER2) protein. Here, we show that CDK5 kinase activity and its interaction with clock components, including PER2 and CLOCK, varied over time in mouse embryonic fibroblast cells. Furthermore, the deletion of Cdk5 from cells resulted in a prolonged period and shifted the transcription of clock-controlled genes by about 2 to 4 h with a simple delay of chromatin binding of ARNTL (BMAL1) CLOCK. Taken together, our data indicate that CDK5 is critically involved in regulating the circadian clock in vitro at the molecular level.

Circadian Regulation of Retinal Pigment Epithelium Function

The retinal pigment epithelium (RPE) is a single layer of cells located between the choriocapillaris vessels and the light-sensitive photoreceptors in the outer retina. The RPE performs physiological processes necessary for the maintenance and support of photoreceptors and visual function. Among the many functions performed by the RPE, the timing of the peak in phagocytic activity by the RPE of the photoreceptor outer segments that occurs 1-2 h. after the onset of light has captured the interest of many investigators and has thus been intensively studied. Several studies have shown that this burst in phagocytic activity by the RPE is under circadian control and is present in nocturnal and diurnal species and rod and cone photoreceptors. Previous investigations have demonstrated that a functional circadian clock exists within multiple retinal cell types and RPE cells. However, the anatomical location of the circadian controlling this activity is not clear. Experimental evidence indicates that the circadian clock, melatonin, dopamine, and integrin signaling play a key role in controlling this rhythm. A series of very recent studies report that the circadian clock in the RPE controls the daily peak in phagocytic activity. However, the loss of the burst in phagocytic activity after light onset does not result in photoreceptor or RPE deterioration during aging. In the current review, we summarized the current knowledge on the mechanism controlling this phenomenon and the physiological role of this peak.

Cell-Type-Specific Circadian Bioluminescence Rhythms in Dbp Reporter Mice

Circadian rhythms are endogenously generated physiological and molecular rhythms with a cycle length of about 24 h. Bioluminescent reporters have been exceptionally useful for studying circadian rhythms in numerous species. Here, we report development of a reporter mouse generated by modification of a widely expressed and highly rhythmic gene encoding D-site albumin promoter binding protein (Dbp). In this line of mice, firefly luciferase is expressed from the Dbp locus in a Cre recombinase-dependent manner, allowing assessment of bioluminescence rhythms in specific cellular populations. A mouse line in which luciferase expression was Cre-independent was also generated. The Dbp reporter alleles do not alter Dbp gene expression rhythms in liver or circadian locomotor activity rhythms. In vivo and ex vivo studies show the utility of the reporter alleles for monitoring rhythmicity. Our studies reveal cell-type-specific characteristics of rhythms among neuronal populations within the suprachiasmatic nuclei ex vivo. In vivo studies show Dbp-driven bioluminescence rhythms in the liver of Albumin-Cre;Dbp^KI/+ "liver reporter" mice. After a shift of the lighting schedule, locomotor activity achieved the proper phase relationship with the new lighting cycle more rapidly than hepatic bioluminescence did. As previously shown, restricting food access to the daytime altered the phase of hepatic rhythmicity. Our model allowed assessment of the rate of recovery from misalignment once animals were provided with food ad libitum. These studies confirm the previously demonstrated circadian misalignment following environmental perturbations and reveal the utility of this model for minimally invasive, longitudinal monitoring of rhythmicity from specific mouse tissues.

PER2 mediates CREB-dependent light induction of the clock gene Per1

Light affects many physiological processes in mammals such as entrainment of the circadian clock, regulation of mood, and relaxation of blood vessels. At the molecular level, a stimulus such as light initiates a cascade of kinases that phosphorylate CREB at various sites, including serine 133 (S133). This modification leads CREB to recruit the co-factor CRCT1 and the histone acetyltransferase CBP to stimulate the transcription of genes containing a CRE element in their promoters, such as Period 1 (Per1). However, the details of this pathway are poorly understood. Here we provide evidence that PER2 acts as a co-factor of CREB to facilitate the formation of a transactivation complex on the CRE element of the Per1 gene regulatory region in response to light or forskolin. Using in vitro and in vivo approaches, we show that PER2 modulates the interaction between CREB and its co-regulator CRTC1 to support complex formation only after a light or forskolin stimulus. Furthermore, the absence of PER2 abolished the interaction between the histone acetyltransferase CBP and CREB. This process was accompanied by a reduction of histone H3 acetylation and decreased recruitment of RNA Pol II to the Per1 gene. Collectively, our data show that PER2 supports the stimulus-dependent induction of the Per1 gene via modulation of the CREB/CRTC1/CBP complex.

Circa-SCOPE: high-throughput live single-cell imaging method for analysis of circadian clock resetting

Circadian clocks are self-sustained and cell-autonomous oscillators. They respond to various extracellular cues depending on the time-of-day and the signal intensity. Phase Transition Curves (PTCs) are instrumental in uncovering the full repertoire of responses to a given signal. However, the current methodologies for reconstructing PTCs are low-throughput, laborious, and resource- and time-consuming. We report here the development of an efficient and high throughput assay, dubbed Circadian Single-Cell Oscillators PTC Extraction (Circa-SCOPE) for generating high-resolution PTCs. This methodology relies on continuous monitoring of single-cell oscillations to reconstruct a full PTC from a single culture, upon a one-time intervention. Using Circa-SCOPE, we characterize the effects of various pharmacological and blood-borne resetting cues, at high temporal resolution and a wide concentration range. Thus, Circa-SCOPE is a powerful tool for comprehensive analysis and screening for circadian clocks' resetting cues, and can be valuable for basic as well as translational research.

Spermidine resets circadian clock phase in NIH3T3 cells

Irregular light-dark cycles desynchronize the circadian clock via hormonal and neuronal pathways and increase the risk of various diseases. This study demonstrated that a single pulse of spermidine-a polyamine-strongly induced circadian phase advances in the presence or absence of dexamethasone (a synthetic glucocorticoid) in NIH3T3 cells transfected with the Bmal1 promotor-driven luciferase reporter gene. The spermidine-induced phase advances were 2-3 fold greater than were the dexamethasone-induced shifts. The phase resetting effect of spermidine occurred in a dose- and time-dependent manner and was not blocked by RU486, an antagonist of glucocorticoid receptors. Spermidine treatment modulated the expression of clock genes within 60 min, which was sooner than changes in the expression of autophagy-related genes. These findings suggested that spermidine is a potent modulator of the circadian phase, acting through glucocorticoid receptor-independent pathways, and may be useful for treating diseases related to circadian desynchrony.

Oncogenic and Circadian Effects of Small Molecules Directly and Indirectly Targeting the Core Circadian Clock

Circadian rhythms are essential for controlling the cell cycle, cellular proliferation, and apoptosis, and hence are tightly linked to cell fate. Several recent studies have used small molecules to affect circadian oscillations; however, their concomitant cellular effects were not assessed, and they have not been compared under similar experimental conditions. In this work, we use five molecules, grouped into direct versus indirect effectors of the circadian clock, to modulate periods in a human osteosarcoma cell line (U2OS) and determine their influences on cellular behaviors, including motility and colony formation. Luciferase reporters, whose expression was driven via Bmal1- or Per2-promoters, were used to facilitate the visualization and quantitative analysis of circadian oscillations. We show that all molecules increase or decrease the circadian periods of Bmal1 and Per2 in a dose-dependent manner, but period length does not correlate with the extent of cell migration or proliferation. Nonetheless, molecules that affected circadian oscillations to a greater degree resulted in substantial influence on cellular behaviors (ie, motility and colony formation), which may also be attributable to noncircadian targets. Furthermore, we find that the ability and extent to which the molecules are able to affect oscillations is independent of whether they are direct or indirect modulators. Because of the numerous connections and feedback between the circadian clock and other pathways, it is important to consider the effects of both in assessing these and other compounds.

Highly sensitive and multiplexed one-step RT-qPCR for profiling genes involved in the circadian rhythm using microparticles

Given the growing interest in molecular diagnosis, highly extensive and selective detection of genetic targets from a very limited amount of samples is in high demand. We demonstrated the highly sensitive and multiplexed one-step RT-qPCR platform for RNA analysis using microparticles as individual reactors. Those particles are equipped with a controlled release system of thermo-responsive materials, and are able to capture RNA targets inside. The particle-based assay can successfully quantify multiple target RNAs from only 200 pg of total RNA. The assay can also quantify target RNAs from a single cell with the aid of a pre-concentration process. We carried out 8-plex one-step RT-qPCR using tens of microparticles, which allowed extensive mRNA profiling. The circadian cycles were shown by the multiplex one-step RT-qPCR in human cell and human hair follicles. Reliable 24-plex one-step RT-qPCR was developed using a single operation in a PCR chip without any loss of performance (i.e., selectivity and sensitivity), even from a single hair. Many other disease-related transcripts can be monitored using this versatile platform. It can also be used non–invasively for samples obtained in clinics.

High-throughput measurement of fibroblast rhythms reveals genetic heritability of circadian phenotypes in diversity outbred mice and their founder strains

Circadian variability is driven by genetics and Diversity Outbred (DO) mice is a powerful tool for examining the genetics of complex traits because their high genetic and phenotypic diversity compared to conventional mouse crosses. The DO population combines the genetic diversity of eight founder strains including five common inbred and three wild-derived strains. In DO mice and their founders, we established a high-throughput system to measure cellular rhythms using in vitro preparations of skin fibroblasts. Among the founders, we observed strong heritability for rhythm period, robustness, phase and amplitude. We also found significant sex and strain differences for these rhythms. Extreme differences in period for molecular and behavioral rhythms were found between the inbred A/J strain and the wild-derived CAST/EiJ strain, where A/J had the longest period and CAST/EiJ had the shortest. In addition, we measured cellular rhythms in 329 DO mice, which displayed far greater phenotypic variability than the founders—80% of founders compared to only 25% of DO mice had periods of ~ 24 h. Collectively, our findings demonstrate that genetic diversity contributes to phenotypic variability in circadian rhythms, and high-throughput characterization of fibroblast rhythms in DO mice is a tractable system for examining the genetics of circadian traits.

Modulation of cellular circadian clocks by triterpenoids

Many living organisms on earth have clock systems in their body. It has increasingly become clear that a disturbance in the internal clocks has negative effects on our body. Terpenes are organic compounds found in various plants that are reported to have several pharmacological actions. In this study, we focused on commercially available 27 triterpenoids and evaluated their influence on the circadian rhythm of human U2OS cells and mouse NIH3T3 cells. The expression level of Per2, one of the core clock genes, was measured using luminescent reporters over the time period of a few days. We found that 8 triterpenoids reset the phase of the circadian clocks. Representative compounds were corosolic acid, cucurbitacin B, and celastrol; similar effects were also confirmed with some structural analogues of cucurbitacin B and celastrol. These compounds shifted the phase bilaterally depending on the stimulus timing and also acted as synchronizers in desynchronized cells. The effective concentrations of cucurbitacin B and celastrol were less than 0.5 μM. In addition, cucurbitacin B and celastrol were also found to be effective in tissue explants in mice. Furthermore, celastrol dose-dependently shortened the period length of NIH3T3 cells. Some of these compounds are found in edible and medicinal plants and may help regulate our circadian clocks in everyday life.

Minimally Invasive Ways of Determining Circadian Rhythms in Humans

Circadian rhythm exerts a critical role in mammalian health and disease. A malfunctioning circadian clock can be a consequence, as well as the cause of several pathophysiologies. Clinical therapies and research may also be influenced by the clock. Since the most suitable manner of revealing this rhythm in humans is not yet established, we discuss existing methods and seek to determine the most feasible ones.

Proinflammatory Cytokines Perturb Mouse and Human Pancreatic Islet Circadian Rhythmicity and Induce Uncoordinated β-Cell Clock Gene Expression via Nitric Oxide, Lysine Deacetylases, and Immunoproteasomal Activity

Pancreatic β-cell-specific clock knockout mice develop β-cell oxidative-stress and failure, as well as glucose-intolerance. How inflammatory stress affects the cellular clock is under-investigated. Real-time recording of Per2:luciferase reporter activity in murine and human pancreatic islets demonstrated that the proinflammatory cytokine interleukin-1β (IL-1β) lengthened the circadian period. qPCR-profiling of core clock gene expression in insulin-producing cells suggested that the combination of the proinflammatory cytokines IL-1β and interferon-γ (IFN-γ) caused pronounced but uncoordinated increases in mRNA levels of multiple core clock genes, in particular of reverse-erythroblastosis virus α (Rev-erbα), in a dose- and time-dependent manner. The REV-ERBα/β agonist SR9009, used to mimic cytokine-mediated Rev-erbα induction, reduced constitutive and cytokine-induced brain and muscle arnt-like 1 (Bmal1) mRNA levels in INS-1 cells as expected. SR9009 induced reactive oxygen species (ROS), reduced insulin-1/2 (Ins-1/2) mRNA and accumulated- and glucose-stimulated insulin secretion, reduced cell viability, and increased apoptosis levels, reminiscent of cytokine toxicity. In contrast, low (<5,0 μM) concentrations of SR9009 increased Ins-1 mRNA and accumulated insulin-secretion without affecting INS-1 cell viability, mirroring low-concentration IL-1β mediated β-cell stimulation. Inhibiting nitric oxide (NO) synthesis, the lysine deacetylase HDAC3 and the immunoproteasome reduced cytokine-mediated increases in clock gene expression. In conclusion, the cytokine-combination perturbed the intrinsic clocks operative in mouse and human pancreatic islets and induced uncoordinated clock gene expression in INS-1 cells, the latter effect associated with NO, HDAC3, and immunoproteasome activity.

Cardiac circadian rhythms in time and space: The future is in 4D

The circadian clock synchronizes the body into 24-h cycles, thereby anticipating variations in tissue-specific diurnal tasks, such as response to increased cardiac metabolic demand during the active period of the day. As a result, blood pressure, heart rate, cardiac output, and occurrence of fatal cardiovascular events fluctuate in a diurnal manner. The heart contains different cell types that make up and reside in an environment of biochemical, mechanical, and topographical signaling. Cardiac architecture is essential for proper heart development as well as for maintenance of cell homeostasis and tissue repair. In this review, we describe the possibilities of studying circadian rhythmicity in the heart by using advanced in vitro systems that mimic the native cardiac 3D microenvironment which can be tuned in time and space. Harnessing the knowledge that originates from those in vitro models could significantly improve innovative cardiac modeling and regenerative strategies.

Circulating Exosomal miRNAs Signal Circadian Misalignment to Peripheral Metabolic Tissues

Night shift work increases risk of metabolic disorders, particularly obesity and insulin resistance. While the underlying mechanisms are unknown, evidence points to misalignment of peripheral oscillators causing metabolic disturbances. A pathway conveying such misalignment may involve exosome-based intercellular communication. Fourteen volunteers were assigned to a simulated day shift (DS) or night shift (NS) condition. After 3 days on the simulated shift schedule, blood samples were collected during a 24-h constant routine protocol. Exosomes were isolated from the plasma samples from each of the blood draws. Exosomes were added to naïve differentiated adipocytes, and insulin-induced pAkt/Akt expression changes were assessed. ChIP-Seq analyses for BMAL1 protein, mRNA microarrays and exosomal miRNA arrays combined with bioinformatics and functional effects of agomirs and antagomirs targeting miRNAs in NS and DS exosomal cargo were examined. Human adipocytes treated with exosomes from the NS condition showed altered Akt phosphorylation responses to insulin in comparison to those treated with exosomes from the DS condition. BMAL1 ChIP-Seq of exosome-treated adipocytes showed 42,037 binding sites in the DS condition and 5538 sites in the NS condition, with a large proportion of BMAL1 targets including genes encoding for metabolic regulators. A significant and restricted miRNA exosomal signature emerged after exposure to the NS condition. Among the exosomal miRNAs regulated differentially after 3 days of simulated NS versus DS, proof-of-concept validation of circadian misalignment signaling was demonstrated with hsa-mir-3614-5p. Exosomes from the NS condition markedly altered expression of key genes related to circadian rhythm in several cultured cell types, including adipocytes, myocytes, and hepatocytes, along with significant changes in 29 genes and downstream gene network interactions. Our results indicate that a simulated NS schedule leads to changes in exosomal cargo in the circulation. These changes promote reduction of insulin sensitivity of adipocytes in vitro and alter the expression of core clock genes in peripheral tissues. Circulating exosomal miRNAs may play an important role in metabolic dysfunction in NS workers by serving as messengers of circadian misalignment to peripheral tissues.

Chemical modulation of circadian rhythms and assessment of cellular behavior via indirubin and derivatives

Circadian rhythms are critical regulators of many physiological and behavioral functions. The use and abilities of small molecules to affect oscillations have recently received significant attention. These manipulations can be reversible and tunable, and have been used to study various biological mechanisms and molecular properties. Here, we outline procedures for assessment of cellular circadian changes following treatment with small molecules, using luminescent reporters. We describe reporter generation, luminometry experiments, and data analysis. Protocols for studies of accompanying effects on cells, including motility, viability, and anchorage-independent proliferation assays are also presented. As examples, we use indirubin-3'-oxime and two derivatives, 5-iodo-indirubin-3'-oxime and 5-sulfonic acid-indirubin-3'-oxime. In this case study, we analyze effects of these compounds on Bmal1 and Per2 (positive and negative core circadian elements) oscillations and provide step-by-step protocols for data analysis, including removal of trends from raw data, period estimations, and statistical analysis. The reader is provided with detailed protocols, and guidance regarding selection of and alternative approaches.

Replicative senescent human cells possess altered circadian clocks with a prolonged period and delayed peak-time

Over the last decade, a wide array of evidence has been accumulated that disruption of circadian clock is prone to cause age-related diseases and premature aging. On the other hand, aging has been identified as one of the risk factors linked to the alteration of circadian clock. These evidences suggest that the processes of aging and circadian clock feedback on each other at the animal level. However, at the cellular level, we recently revealed that the primary fibroblast cells derived from Bmal1^-/- mouse embryo, in which circadian clock is completely disrupted, do not demonstrate the acceleration of cellular aging, i.e., cellular senescence. In addition, little is known about the impact of cellular senescence on circadian clock. In this study, we show for the first time that senescent cells possess a longer circadian period with delayed peak-time and that the variability in peak-time is wider in the senescent cells compared to their proliferative counterparts, indicating that senescent cells show alterations of circadian clock. We, furthermore, propose that investigation at cellular level is a powerful and useful approach to dissect molecular mechanisms of aging in the circadian clock.

Time after time: circadian clock regulation of intestinal stem cells

Daily fluctuations in animal physiology, known as circadian rhythms, are orchestrated by a conserved molecular timekeeper, known as the circadian clock. The circadian clock forms a transcription-translation feedback loop that has emerged as a central biological regulator of many 24-h processes. Early studies of the intestine discovered that many digestive functions have a daily rhythm and that intestinal cell production was similarly time-dependent. As genetic methods in model organisms have become available, it has become apparent that the circadian clock regulates many basic cellular functions, including growth, proliferation, and differentiation, as well as cell signalling and stem cell self-renewal. Recent connections between circadian rhythms and immune system function, and between circadian rhythms and microbiome dynamics, have also been revealed in the intestine. These processes are highly relevant in understanding intestinal stem cell biology. Here we describe the circadian clock regulation of intestinal stem cells primarily in two model organisms: Drosophila melanogaster and mice. Like all cells in the body, intestinal stem cells are subject to circadian timing, and both cell-intrinsic and cell-extrinsic circadian processes contribute to their function.

Response to Stimulations Inducing Circadian Rhythm in Human Induced Pluripotent Stem Cells

Regenerative medicine and disease modeling are expanding rapidly, through the development of human-induced pluripotent stem cells (hiPSCs). Many exogeneous supplements are often used for the directed differentiation of hiPSCs to specific lineages, such as chemicals and hormones. Some of these are known to synchronize the circadian clock, like forskolin (Frk) and dexamethasone (Dex); however, the response to these stimulations has not been fully elucidated for hiPSCs. In this study, we examined the response of clock genes to synchronizing stimulation, and compared it with fully differentiated cells, U2OS, and fibroblasts. The expression of clock genes did not show circadian rhythms in hiPSCs with Frk and Dex, which could be due to the significantly low levels of BMAL1. On the other hand, a circadian-like rhythm of D-box binding protein (DBP) expression was observed in hiPSCs by culturing them in an environment with a simulated body temperature. However, the inhibition of temperature-inducible factors, which are involved in temperature rhythm-induced synchronization, could not repress the expression of such rhythms, while the inhibition of HIF-1α significantly repressed them. In summary, we suggest that clock genes do not respond to the synchronizing agents in hiPSCs; instead, a unique circadian-like rhythm is induced by the temperature rhythm.

Circadian expression of Glycoprotein 2 (Gp2) gene is controlled by a molecular clock in mouse Peyer's patches

The expression levels of many cell-surface proteins vary with the time of day. Glycoprotein 2 (Gp2), specifically expressed on the apical surface of M cells in Peyer's patches, functions as a transcytotic receptor for mucosal antigens. We report that cAMP response element-binding protein (CREB) regulates the transcription of the Gp2 gene, thereby generating the circadian change in its expression in mouse Peyer's patches. The transcytotic receptor activity of Gp2 was increased during the dark phase when the Gp2 protein abundance increased. Rhythmic expression of clock gene mRNA was observed in mouse Peyer's patches, and expression levels of Gp2 mRNA also exhibited circadian oscillation, with peak levels during the early dark phase. The promoter region of the mouse Gp2 gene contains several cAMP response elements (CREs). Chromatin immunoprecipitation assays revealed that CREB bound to the CREs in the Gp2 gene in Peyer's patches. Forskolin, which promotes CREB phosphorylation, increased the transcription of the Gp2 gene in Peyer's patches. As phosphorylation of CREB protein was increased when Gp2 gene transcription was activated, CREB may regulate the rhythmic expression of Gp2 mRNA in Peyer's patches. These findings suggest that intestinal immunity is controlled by the circadian clock system.

Restoring circadian synchrony in vitro facilitates physiological responses to environmental chemicals

BACKGROUND

The growing requirement of hazard and risk assessment of environmental chemicals and the efforts to minimize animal testing, increases the demand for innovative and predictive in vitro test systems in toxicology, reflecting the physiological conditions of human nature. Here, an elemental factor regulating a variety of physiological processes is the day-night rhythm. This circadian rhythm, describing a biological oscillation with a 24-h period is hardly acknowledged in toxicology and test method development. Whilst, in animals or humans the entire organism exhibits a rigorous cellular circadian synchrony, in conventional in vitro systems each cell follows its own rhythm, due to the absence of appropriate synchronizing signals.

OBJECTIVE

Here we investigated whether circadian synchronization of human cells in an in vitro system improves the cellular response and, thus, increases the sensitivity of the test system. Since the circadian regulation of metabolism is particularly well understood, and dioxin and dioxin-like compounds are of major concern for environmental health we focused on the ubiquitous drug metabolizing detoxification system mediated by the aryl hydrocarbon receptor (AHR).

METHODS

To this end, we applied various prototypical AHR activators onto different human cell lines under non-synchronized or circadian synchronized conditions and determined the dose response on representative endogenous target genes.

RESULTS

Remarkably, the cellular response dynamic upon chemical treatment was substantially enhanced in circadian synchronized cells and followed a rhythmic expression pattern. This broader dynamic range was associated with a strikingly higher induction of AHR target genes and the corresponding enzymatic activity, thereby rather mimicking the in vivo situation.

CONCLUSION

Our findings indicate that a synchronized circadian rhythm in a cell culture based test system can improve the physiological relevance of an appropriate in vitro method by reflecting the biological in vivo situation more closely. Accordingly, it is a promising tool to facilitate the wide acceptance of in vitro methods in the field of regulatory toxicology and to further optimize the toxicological assessment of environmental chemicals.

The core clock gene, Bmal1, and its downstream target, the SNARE regulatory protein secretagogin, are necessary for circadian secretion of glucagon-like peptide-1

OBJECTIVES

The incretin hormone glucagon-like peptide-1 (GLP-1) is secreted from intestinal L-cells upon nutrient intake. While recent evidence has shown that GLP-1 is released in a circadian manner in rats, whether this occurs in mice and if this pattern is regulated by the circadian clock remain to be elucidated. Furthermore, although circadian GLP-1 secretion parallels expression of the core clock gene Bmal1, the link between the two remains largely unknown. Secretagogin (Scgn) is an exocytotic SNARE regulatory protein that demonstrates circadian expression and is essential for insulin secretion from β-cells. The objective of the current study was to establish the necessity of the core clock gene Bmal1 and the SNARE protein SCGN as essential regulators of circadian GLP-1 secretion.

METHODS

Oral glucose tolerance tests were conducted at different times of the day on 4-hour fasted C57BL/6J, Bmal1 wild-type, and Bmal1 knockout mice. Mass spectrometry, RNA-seq, qRT-PCR and/or microarray analyses, and immunostaining were conducted on murine (m) and human (h) primary L-cells and mGLUTag and hNCI-H716 L-cell lines. At peak and trough GLP-1 secretory time points, the mGLUTag cells were co-stained for SCGN and a membrane-marker, ChIP was used to analyze BMAL1 binding sites in the Scgn promoter, protein interaction with SCGN was tested by co-immunoprecipitation, and siRNA was used to knockdown Scgn for GLP-1 secretion assay.

RESULTS

C57BL/6J mice displayed a circadian rhythm in GLP-1 secretion that peaked at the onset of their feeding period. Rhythmic GLP-1 release was impaired in Bmal1 knockout (KO) mice as compared to wild-type controls at the peak (p < 0.05) but not at the trough secretory time point. Microarray identified SNARE and transport vesicle pathways as highly upregulated in mGLUTag L-cells at the peak time point of GLP-1 secretion (p < 0.001). Mass spectrometry revealed that SCGN was also increased at this time (p < 0.001), while RNA-seq, qRT-PCR, and immunostaining demonstrated Scgn expression in all human and murine primary L-cells and cell lines. The mGLUTag and hNCI-H716 L-cells exhibited circadian rhythms in Scgn expression (p < 0.001). The ChIP analysis demonstrated increased binding of BMAL1 only at the peak of Scgn expression (p < 0.01). Immunocytochemistry showed the translocation of SCGN to the cell membrane after stimulation at the peak time point only (p < 0.05), while CoIP showed that SCGN was pulled down with SNAP25 and β-actin, but only the latter interaction was time-dependent (p < 0.05). Finally, Scgn siRNA-treated cells demonstrated significantly blunted GLP-1 secretion (p < 0.01) in response to stimulation at the peak time point only.

CONCLUSIONS

These data demonstrate, for the first time, that mice display a circadian pattern in GLP-1 secretion, which is impaired in Bmal1 knockout mice, and that Bmal1 regulation of Scgn expression plays an essential role in the circadian release of the incretin hormone GLP-1.

Impact of noise exposure on the circadian clock in the auditory system

Circadian rhythms control the timing of all bodily functions, and misalignment in the rhythms can cause various diseases. Moreover, circadian rhythms are highly conserved and are regulated by a transcriptional-translational feedback loop of circadian genes that has a periodicity of approximately 24 h. The cochlea and the inferior colliculus (IC) have been shown to possess an autonomous and self-sustained circadian system as demonstrated by recording, in real time, the bioluminescence from PERIOD2::LUCIFERASE (PER2::LUC) mice. The cochlea and IC both express the core clock genes, Per1, Per2, Bmal1, and Rev-Erbα, where RNA abundance is rhythmically distributed with a 24 h cycle. Noise exposure alters clock gene expression in the cochlea and the IC after noise stimulation, although in different ways. These findings highlight the importance of circadian responses in the cochlea and the IC and emphasize the importance of circadian mechanisms for understanding the differences in central and peripheral auditory function and the subsequent molecular changes that occur after daytime (inactive phase) or nighttime (active phase) noise trauma.

The Cancer Clock Is (Not) Ticking: Links between Circadian Rhythms and Cancer

Circadian rhythms regulate many physiological and behavioral processes, including sleep, metabolism and cell division, which have a 24-h oscillation pattern. Rhythmicity is generated by a transcriptional–translational feedback loop in individual cells, which are synchronized by the central pacemaker in the brain and external cues. Epidemiological and clinical studies indicate that disruption of these rhythms can increase both tumorigenesis and cancer progression. Environmental changes (shift work, jet lag, exposure to light at night), mutations in circadian regulating genes, and changes to clock gene expression are recognized forms of disruption and are associated with cancer risk and/or cancer progression. Experimental data in animals and cell cultures further supports the role of the cellular circadian clock in coordinating cell division and DNA repair, and disrupted cellular clocks accelerate cancer cell growth. This review will summarize studies linking circadian disruption to cancer biology and explore how such disruptions may be further altered by common characteristics of tumors including hypoxia and acidosis. We will highlight how circadian rhythms might be exploited for cancer drug development, including how delivery of current chemotherapies may be enhanced using chronotherapy. Understanding the role of circadian rhythms in carcinogenesis and tumor progression will enable us to better understand causes of cancer and how to treat them.

The importance of determining circadian parameters in pharmacological studies

In mammals, most molecular and cellular processes show circadian changes, leading to daily variations in physiology and ultimately in behaviour. Such daily variations induce a temporal coordination of processes that is essential to ensure homeostasis and health. Thus, it is of no surprise that pharmacokinetics (PK) and pharmacodynamics (PD) of many drugs are also subject to circadian variations, profoundly affecting their efficacy and tolerability. Understanding how circadian rhythms influence drug PK, PD, and toxicity might significantly improve treatment efficacy and decrease related side effects. Therefore, it is essential to take circadian variations into account and to determine circadian parameters in pharmacological studies, especially when drugs have a short half-life or target rhythmic processes. This review provides an overview of the current knowledge on circadian rhythms and their relevance to the field of pharmacology. Methodologies to evaluate circadian rhythms in vitro, in rodent models and in humans, from experimental to computational approaches, are described and discussed. Lastly, we aim at alerting the scientific, medical, and regulatory communities to the relevance of the physiological time, as a key parameter to be considered when designing pharmacological studies. This will eventually lead to more successful preclinical and clinical trials and pave the way to a more personalized treatment to the benefit of the patients.

New Insights Into the Circadian Rhythm and Its Related Diseases

Circadian rhythms (CR) are a series of endogenous autonomous oscillators generated by the molecular circadian clock which acting on coordinating internal time with the external environment in a 24-h daily cycle. The circadian clock system is a major regulatory factor for nearly all physiological activities and its disorder has severe consequences on human health. CR disruption is a common issue in modern society, and researches about people with jet lag or shift works have revealed that CR disruption can cause cognitive impairment, psychiatric illness, metabolic syndrome, dysplasia, and cancer. In this review, we summarized the synchronizers and the synchronization methods used in experimental research, and introduced CR monitoring and detection methods. Moreover, we evaluated conventional CR databases, and analyzed experiments that characterized the underlying causes of CR disorder. Finally, we further discussed the latest developments in understanding of CR disruption, and how it may be relevant to health and disease. Briefly, this review aimed to synthesize previous studies to aid in future studies of CR and CR-related diseases.

Microcurrent stimulation activates the circadian machinery in mice

The circadian rhythm, which regulates various body functions, is transcriptionally controlled by a series of clock gene clusters. The clock genes are related to the pathology of various kinds of diseases, which in turn, is related to aging. Aging in humans is a worldwide problem; it induces sleep disorders and disruption of the circadian rhythm. It also decreases ocular vision and appetite and weakens the synchronization of clock genes by light and food. Therefore, a simple method for the synchronization of clock genes in the body is required. In this study, the influence of microcurrent stimulation (MCS) on the circadian machinery in wild-type (WT) and Clock mutant (Clk/Clk) mice was investigated. MCS induced Per1 mRNA expression in cultured mouse astrocytes; cAMP response element (CRE) in the Per1 mouse promoter was found to be important for the induction of Per1 mRNA. In addition, MCS increased the Per1 mRNA levels in mouse livers and caused the phase advance of the Per1 expression rhythm. The protein expression rhythm of phosphor-cAMP response element-binding protein (pCREB) was altered and the phase of expression of pCREB protein advanced. Finally, the influence of MCS on the locomotor activity rhythm in WT and Clk/Clk mice was investigated. MCS caused the phase advance of the locomotor activity rhythm in WT and Clk/Clk mice. The results of this study indicate that MCS activated the clock machinery in mice; MCS may thus improve the quality of new treatment modalities in the future.

Ultraviolet-C (UVC) ray acts as a synchronizing cue for circadian rhythm control in murine fibroblast

Ultraviolet-C (UVC) electromagnetic radiation is the most damaging type of the UV radiation and causes many cellular and physiological responses. UVC has been using for sterilization and disinfection, and the risk of exposure to the UVC is increasing. Here, we determined the effect of the UVC on the cellular circadian clock system. UVC irradiation synchronized the biological clock system and induced time-dependent expression of clock genes including Clock, Cry1, and Per1. The rhythmic expression of clock genes is also followed by time-dependent mRNA degradation or non-canonical translation initiation of clock genes. Furthermore, we show a translocation of PERIOD1 (PER1) protein after UVC irradiation, which mediates the rhythmic feedback loop of clock genes. Our results suggest that UVC can synchronize the circadian clock system, and induces rhythmic expression of clock genes via time-dependent transcription, post-transcription, and post-translational modification.

Molecular and Cellular Networks in The Suprachiasmatic Nuclei

Why do we experience the ailments of jetlag when we travel across time zones? Why is working night-shifts so detrimental to our health? In other words, why can’t we readily choose and stick to non-24 h rhythms? Actually, our daily behavior and physiology do not simply result from the passive reaction of our organism to the external cycle of days and nights. Instead, an internal clock drives the variations in our bodily functions with a period close to 24 h, which is supposed to enhance fitness to regular and predictable changes of our natural environment. This so-called circadian clock relies on a molecular mechanism that generates rhythmicity in virtually all of our cells. However, the robustness of the circadian clock and its resilience to phase shifts emerge from the interaction between cell-autonomous oscillators within the suprachiasmatic nuclei (SCN) of the hypothalamus. Thus, managing jetlag and other circadian disorders will undoubtedly require extensive knowledge of the functional organization of SCN cell networks. Here, we review the molecular and cellular principles of circadian timekeeping, and their integration in the multi-cellular complexity of the SCN. We propose that new, in vivo imaging techniques now enable to address these questions directly in freely moving animals.