Temperature as a universal resetting cue for mammalian circadian oscillators

Circadian rhythm as a therapeutic target

The circadian clock evolved in diverse organisms to integrate external environmental changes and internal physiology. The clock endows the host with temporal precision and robust adaptation to the surrounding environment. When circadian rhythms are perturbed or misaligned, as a result of jet lag, shiftwork or other lifestyle factors, adverse health consequences arise, and the risks of diseases such as cancer, cardiovascular diseases or metabolic disorders increase. Although the negative impact of circadian rhythm disruption is now well established, it remains underappreciated how to take advantage of biological timing, or correct it, for health benefits. In this Review, we provide an updated account of the circadian system and highlight several key disease areas with altered circadian signalling. We discuss environmental and lifestyle modifications of circadian rhythm and clock-based therapeutic strategies, including chronotherapy, in which dosing time is deliberately optimized for maximum therapeutic index, and pharmacological agents that target core clock components and proximal regulators. Promising progress in research, disease models and clinical applications should encourage a concerted effort towards a new era of circadian medicine.

Can chronopharmacology improve the therapeutic management of neurological diseases?

The importance of circadian rhythm dysfunctions in the pathophysiology of neurological diseases has been highlighted recently. Chronopharmacology principles imply that tailoring the timing of treatments to the circadian rhythm of individual patients could optimize therapeutic management. According to these principles, chronopharmacology takes into account the individual differences in patients' clocks, the rhythmic changes in the organism sensitivity to therapeutic and side effects of drugs, and the predictable time variations of disease. This review examines the current literature on chronopharmacology of neurological diseases focusing its scope on epilepsy, Alzheimer and Parkinson diseases, and neuropathic pain, even if other neurological diseases could have been analyzed. While the results of the studies discussed in this review point to a potential therapeutic benefit of chronopharmacology in neurological diseases, the field is still in its infancy. Studies including a sufficiently large number of patients and measuring gold standard markers of the circadian rhythmicity are still needed to evaluate the beneficial effect of administration times over the 24-hour day but also of clock modulating drugs.

The Circadian Clock and Viral Infections

The circadian clock controls several aspects of mammalian physiology and orchestrates the daily oscillations of biological processes and behavior. Our circadian rhythms are driven by an endogenous central clock in the brain that synchronizes with clocks in peripheral tissues, thereby regulating our immune system and the severity of infections. These rhythms affect the pharmacokinetics and efficacy of therapeutic agents and vaccines. The core circadian regulatory circuits and clock-regulated host pathways provide fertile ground to identify novel antiviral therapies. An increased understanding of the role circadian systems play in regulating virus infection and the host response to the virus will inform our clinical management of these diseases. This review provides an overview of the experimental and clinical evidence reporting on the interplay between the circadian clock and viral infections, highlighting the importance of virus-clock research.

The darkness and the light: diurnal rodent models for seasonal affective disorder

The development of animal models is a critical step for exploring the underlying pathophysiological mechanisms of major affective disorders and for evaluating potential therapeutic approaches. Although most neuropsychiatric research is performed on nocturnal rodents, differences in how diurnal and nocturnal animals respond to changing photoperiods, combined with a possible link between circadian rhythm disruption and affective disorders, has led to a call for the development of diurnal animal models. The need for diurnal models is most clear for seasonal affective disorder (SAD), a widespread recurrent depressive disorder that is linked to exposure to short photoperiods. Here, we briefly review what is known regarding the etiology of SAD and then examine progress in developing appropriate diurnal rodent models. Although circadian disruption is often invoked as a key contributor to SAD, a mechanistic understanding of how misalignment between endogenous circadian physiology and daily environmental rhythms affects mood is lacking. Diurnal rodents show promise as models of SAD, as changes in affective-like behaviors are induced in response to short photoperiods or dim-light conditions, and symptoms can be ameliorated by brief exposure to intervals of bright light coincident with activity onset. One exciting avenue of research involves the orexinergic system, which regulates functions that are disturbed in SAD, including sleep cycles, the reward system, feeding behavior, monoaminergic neurotransmission and hippocampal neurogenesis. However, although diurnal models make intuitive sense for the study of SAD and are more likely to mimic circadian disruption, their utility is currently hampered by a lack of genomic resources needed for the molecular interrogation of potential mechanisms.

A Tangled Threesome: Circadian Rhythm, Body Temperature Variations, and the Immune System

Simple Summary

In mammals, including humans, the body temperature displays a circadian rhythm and is maintained within a narrow range to facilitate the optimal functioning of physiological processes. Body temperature increases during the daytime and decreases during the nighttime thus influencing the expression of the molecular clock and the clock-control genes such as immune genes. An increase in body temperature (daytime, or fever) also prepares the organism to fight aggression by promoting the activation, function, and delivery of immune cells. Many factors may affect body temperature level and rhythm, including environment, age, hormones, or treatment. The disruption of the body temperature is associated with many kinds of diseases and their severity, thus supporting the assumed association between body temperature rhythm and immune functions. Recent studies using complex analysis suggest that circadian rhythm may change in all aspects (level, period, amplitude) and may be predictive of good or poor outcomes. The monitoring of body temperature is an easy tool to predict outcomes and maybe guide future studies in chronotherapy.

Abstract

The circadian rhythm of the body temperature (CRBT) is a marker of the central biological clock that results from multiple complex biological processes. In mammals, including humans, the body temperature displays a strict circadian rhythm and has to be maintained within a narrow range to allow optimal physiological functions. There is nowadays growing evidence on the role of the temperature circadian rhythm on the expression of the molecular clock. The CRBT likely participates in the phase coordination of circadian timekeepers in peripheral tissues, thus guaranteeing the proper functioning of the immune system. The disruption of the CRBT, such as fever, has been repeatedly described in diseases and likely reflects a physiological process to activate the molecular clock and trigger the immune response. On the other hand, temperature circadian disruption has also been described as associated with disease severity and thus may mirror or contribute to immune dysfunction. The present review aims to characterize the potential implication of the temperature circadian rhythm on the immune response, from molecular pathways to diseases. The origin of CRBT and physiological changes in body temperature will be mentioned. We further review the immune biological effects of temperature rhythmicity in hosts, vectors, and pathogens. Finally, we discuss the relationship between circadian disruption of the body temperature and diseases and highlight the emerging evidence that CRBT monitoring would be an easy tool to predict outcomes and guide future studies in chronotherapy.

Gender difference in circadian clock responses for social interaction with conspecific of the opposite-sex

Social cues are potent non-photic modulators of the circadian clock and play a vital role in resetting the endogenous clock. Several lines of evidence strongly suggest a functional link between olfactory cues and the circadian clock. However, there is a paucity of information on the effects of social interaction with the conspecifics of the opposite sex on the circadian clock. Hence, we studied the effect of social cues of sexually mature naïve opposite sex of the conspecific on the phase resetting of the circadian clock at various circadian times (CT) and molecular changes at the suprachiasmatic nuclei (SCN) and odor responsive structure in the brain of mice. Sexually naïve adult male and female free-running mice (designated as 'runners') were exposed to the conspecifics of the opposite-sex ('strangers') for 30 min at CT3, CT9, CT15, and CT21. Both male and female 'runners' exhibited a phase advance at CT3, delay at CT21, and no response at CT9. However, at CT15 only the male 'runners' exhibited phase advance but not the female 'runners'. Control mice did not elicit any significant phase shifts at all CTs. Social interactions with conspecifics of the opposite-sex up-regulated c-fos/C-FOS, omp in the olfactory bulb, per-1/PER-1 in the SCN, C-FOS, and PER-1 in the piriform cortex of both male and female runners at CT3. However, at CT15 up-regulation of variables only occurred in male but not in female runners. Together, the present investigation has shown the gender difference in circadian clock responses for social cues with conspecific of the opposite-sex in mice.

Distinct feedback actions of behavioural arousal to the master circadian clock in nocturnal and diurnal mammals

The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus provides a temporal pattern of sleep and wake that - like many other behavioural and physiological rhythms - is oppositely phased in nocturnal and diurnal animals. The SCN primarily uses environmental light, perceived through the retina, to synchronize its endogenous circadian rhythms with the exact 24 h light/dark cycle of the outside world. The light responsiveness of the SCN is maximal during the night in both nocturnal and diurnal species. Behavioural arousal during the resting period not only perturbs sleep homeostasis, but also acts as a potent non-photic synchronizing cue. The feedback action of arousal on the SCN is mediated by processes involving several brain nuclei and neurotransmitters, which ultimately change the molecular functions of SCN pacemaker cells. Arousing stimuli during the sleeping period differentially affect the circadian system of nocturnal and diurnal species, as evidenced by the different circadian windows of sensitivity to behavioural arousal. In addition, arousing stimuli reduce and increase light resetting in nocturnal and diurnal species, respectively. It is important to address further question of circadian impairments associated with shift work and trans-meridian travel not only in the standard nocturnal laboratory animals but also in diurnal animal models.

Wounding Induces Facultative Opn5-Dependent Circadian Photoreception in the Murine Cornea

Purpose

Autonomous molecular circadian clocks are present in the majority of mammalian tissues. These clocks are synchronized to phases appropriate for their physiologic role by internal systemic cues, external environmental cues, or both. The circadian clocks of the in vivo mouse cornea synchronize to the phase of the brain's master clock primarily through systemic cues, but ex vivo corneal clocks entrain to environmental light cycles. We evaluated the underlying mechanisms of this difference.

Methods

Molecular circadian clocks of mouse corneas were evaluated in vivo and ex vivo for response to environmental light. The presence of opsins and effect of genetic deletion of opsins were evaluated for influence on circadian photoresponses. Opn5-expressing cells were identified using Opn5^Cre;Ai14 mice and RT-PCR, and they were characterized using immunocytochemistry.

Results

Molecular circadian clocks of the cornea remain in phase with behavioral circadian locomotor rhythms in vivo but are photoentrainable in tissue culture. After full-thickness incision or epithelial debridement, expression of the opsin photopigment Opn5 is induced in the cornea in a subset of preexisting epithelial cells adjacent to the wound site. This induction coincides with conferral of direct, short-wavelength light sensitivity to the circadian clocks throughout the cornea.

Conclusions

Corneal circadian rhythms become photosensitive after wounding. Opn5 gene function (but not Opn3 or Opn4 function) is necessary for induced photosensitivity. These results demonstrate that opsin-dependent direct light sensitivity can be facultatively induced in the murine cornea.

Simple Kinetic Models in Molecular Chronobiology

Circadian rhythms are constituted by a complex dynamical system with intertwined feedback loops, molecular switches, and self-sustained oscillations. Mathematical modeling supports understanding available heterogeneous kinetic data, highlights basic mechanisms, and can guide experimental research. Here, we introduce the basic steps from a biological question to simple models providing insight into gene-regulatory mechanisms. We illustrate the general approach by three examples: modeling decay processes, clock-controlled genes, and self-sustained oscillations.

Diurnal and seasonal molecular rhythms in the human brain and their relation to Alzheimer disease

Diurnal and seasonal rhythms influence many aspects of human physiology including brain function. Moreover, altered diurnal and seasonal behavioral and physiological rhythms have been linked to Alzheimer's disease and related dementias (ADRD). Understanding the molecular basis for these links may lead to identification of novel targets to mitigate the negative impact of normal and abnormal diurnal and seasonal rhythms on ADRD or to alleviate the adverse consequences of ADRD on normal diurnal and seasonal rhythms. Diurnally and seasonally rhythmic gene expression and epigenetic modification in the human neocortex may be a key mechanism underlying these links. This chapter will first review the observed epidemiological links between normal and abnormal diurnal and seasonal rhythmicity, cognitive impairment, and ADRD. Then it will review normal diurnal and seasonal rhythms of brain epigenetic modification and gene expression in model organisms. Finally, it will review evidence for diurnal and seasonal rhythms of epigenetic modification and gene expression the human brain in aging, Alzheimer's disease, and other brain disorders.

Application of Single-Cell Bioluminescent Imaging to Monitor Circadian Rhythms of Individual Plant Cells

The bioluminescent reporter system is a powerful tool for the long-term monitoring of gene expression because of its noninvasive nature. Furthermore, in combination with high-sensitive imaging technology, spatiotemporal analysis on regulation and heterogeneity in gene expression is possible. We developed a single-cell bioluminescent imaging system for plants through a transient gene transfection by particle bombardment. By applying this system to a duckweed species, we succeeded in monitoring circadian rhythms of individual cells in an intact plant for over a week. Here we describe methods for gene transfection by particle bombardment and single-cell bioluminescence monitoring by a high-sensitive camera. This technique provides a platform for characterizing gene expression patterns of individual cells in the same tissue.

Time for Novel Strategies to Mitigate Cardiometabolic Risk in Shift Workers

Circadian disruption induced by shift work is robustly associated with obesity, diabetes, and cardiovascular disease in humans. Less well-known are the mechanisms underlying these associations, and the effectiveness of strategies to reduce cardiometabolic risk in the shift work population. In this review, the different ways in which shift work can deteriorate cardiometabolic health, and how to use this information to reflect on various risk-mitigating strategies, is discussed. While individual strategies appear promising in animal studies, the multifactorial disease risk in shift workers likely requires a multidisciplinary approach. Therefore, the need for individually-tailored combined lifestyle interventions, that could be essential in reducing cardiometabolic disorders in the large population of shift workers in our 24/7 society, is argued.

Does a high-fat diet affect the circadian clock, or is it the other way around? A systematic review

This paper reviews studies that addressed the influence of diet on circadian rhythmicity in mice and, in turn, circadian clock chronodisruption and its role in the development of metabolic disorders. Studies from the past 14 years were selected via a systematic search conducted using the PubMed electronic database. After applying the inclusion and exclusion criteria, 291 studies were selected, of which 13 were chosen using the following inclusion criteria: use of a high-fat diet for mice, evaluation of clock gene expression, and the association between chronodisruption and lipid metabolism disorders. These studies reported changes in animals' biological clock when they developed metabolic disorders by consuming a high-fat diet. It was also evident that some clock gene mutations or deletions triggered metabolic changes. Disturbances of clock gene machinery may play important roles in lipid metabolism and the development of atherosclerotic processes. However, many metabolic processes also affect the function of clock genes and circadian systems. In summary, this review's results may provide new insights into the reciprocal regulation of energy homeostasis and the biological clock.

Frozen? Let it go to reset circadian rhythms

The molecular events in response to severe hyperthermia are not fully understood, and research has focused mainly on the effects of cooling at temperatures between 28°C and 35°C. In a new study, Fischl et al have analysed human cardiomyocytes at lower temperatures (8°C, 18°C and 28°C) and identified a novel mechanism by which hypothermia synchronises the circadian clock: cooling induces nuclear accumulation of transcripts that encode negative regulators of the circadian clock, which are released into the cytoplasm upon rewarming allowing synthesis of specific clock proteins.

Night-restricted feeding improves locomotor activity rhythm and modulates nutrient utilization to accelerate growth in rabbits

An unfavorable lifestyle disrupts the circadian rhythm, leading to metabolic dysfunction in adult humans and animals. Increasing evidence suggests that night-restricted feeding (NRF) can effectively prevent ectopic fat deposition caused by circadian rhythm disruption, and reduce the risk of metabolic diseases. However, previous studies have mainly focused on the prevention of obesity in adults by regulating dietary patterns, whereas limited attention has been paid to the effect of NRF on metabolism during growth and development. Here, we used weaning rabbits as models and found that NRF increased body weight gain without increasing feed intake, and promoted insulin-mediated protein synthesis through the mTOR/S6K pathway and muscle formation by upregulating MYOG. NRF improved the circadian clock, promoted PDH-regulated glycolysis and CPT1B-regulated fatty-acid β-oxidation, and reduced fat content in the serum and muscles. In addition, NRF-induced body temperature oscillation might be partly responsible for the improvement in the circadian clock and insulin sensitivity. Time-restricted feeding could be used as a nondrug intervention to prevent obesity and accelerate growth in adolescents.

Natural food intake patterns have little synchronizing effect on peripheral circadian clocks

Background

Circadian rhythms across mammalian tissues are coordinated by a master clock in the suprachiasmatic nucleus (SCN) that is principally entrained by light-dark cycles. Prior investigations have shown, however, that time-restricted feeding (TRF)—daily alternation of fasting and food availability—synchronizes peripheral clocks independent of the light-dark cycle and of the SCN. This has led to the idea that downstream peripheral clocks are entrained indirectly by food intake rhythms. However, TRF is not a normal eating pattern, and it imposes non-physiologic long fasts that rodents do not typically experience. Therefore, we tested whether normal feeding patterns can phase-shift or entrain peripheral tissues by measuring circadian rhythms of the liver, kidney, and submandibular gland in mPer2^Luc mice under different food schedules.

Results

We employed home cage feeders to first measure ad libitum food intake and then to dispense 20-mg pellets on a schedule mimicking that pattern. In both conditions, PER2::LUC bioluminescence peaked during the night as expected. Surprisingly, shifting the scheduled feeding by 12 h advanced peripheral clocks by only 0–3 h, much less than predicted from TRF protocols. To isolate the effects of feeding from the light-dark cycle, clock phase was then measured in mice acclimated to scheduled feeding over the course of 3 months in constant darkness. In these conditions, peripheral clock phases were better predicted by the rest-activity cycle than by the food schedule, contrary to expectation based on TRF studies. At the end of both experiments, mice were exposed to a modified TRF with food provided in eight equally sized meals over 12 h. In the light-dark cycle, this advanced the phase of the liver and kidney, though less so than in TRF with ad libitum access; in darkness, this entrained the liver and kidney but had little effect on the submandibular gland or the rest-activity cycle.

Conclusions

These data suggest that natural feeding patterns can only weakly affect circadian clocks. Instead, in normally feeding mice, the central pacemaker in the brain may set the phase of peripheral organs via pathways that are independent of feeding behavior.

Alternative splicing coupled mRNA decay shapes the temperature-dependent transcriptome

Mammalian body temperature oscillates with the time of the day and is altered in diverse pathological conditions. We recently identified a body temperature‐sensitive thermometer‐like kinase, which alters SR protein phosphorylation and thereby globally controls alternative splicing (AS). AS can generate unproductive variants which are recognized and degraded by diverse mRNA decay pathways—including nonsense‐mediated decay (NMD). Here we show extensive coupling of body temperature‐controlled AS to mRNA decay, leading to global control of temperature‐dependent gene expression (GE). Temperature‐controlled, decay‐inducing splicing events are evolutionarily conserved and pervasively found within RNA‐binding proteins, including most SR proteins. AS‐coupled poison exon inclusion is essential for rhythmic GE of SR proteins and has a global role in establishing temperature‐dependent rhythmic GE profiles, both in mammals under circadian body temperature cycles and in plants in response to ambient temperature changes. Together, these data identify body temperature‐driven AS‐coupled mRNA decay as an evolutionary ancient, core clock‐independent mechanism to generate rhythmic GE.

Metabolic rivalry: circadian homeostasis and tumorigenesis

Circadian rhythms govern a large array of physiological and metabolic functions. Perturbations of the daily cycle have been linked to elevated risk of developing cancer as well as poor prognosis in patients with cancer. Also, expression of core clock genes or proteins is remarkably attenuated particularly in tumours of a higher stage or that are more aggressive, possibly linking the circadian clock to cellular differentiation. Emerging evidence indicates that metabolic control by the circadian clock underpins specific hallmarks of cancer metabolism. Indeed, to support cell proliferation and biomass production, the clock may direct metabolic processes of cancer cells in concert with non-clock transcription factors to control how nutrients and metabolites are utilized in a time-specific manner. We hypothesize that the metabolic switch between differentiation or stemness of cancer may be coupled to the molecular clockwork. Moreover, circadian rhythms of host organisms appear to dictate tumour growth and proliferation. This Review outlines recent discoveries of the interplay between circadian rhythms, proliferative metabolism and cancer, highlighting potential opportunities in the development of future therapeutic strategies.

Dual-Color Single-Cell Imaging of the Suprachiasmatic Nucleus Reveals a Circadian Role in Network Synchrony

The suprachiasmatic nucleus (SCN) acts as a master pacemaker driving circadian behavior and physiology. Although the SCN is small, it is composed of many cell types, making it difficult to study the roles of particular cells. Here we develop bioluminescent circadian reporter mice that are Cre dependent, allowing the circadian properties of genetically defined populations of cells to be studied in real time. Using a Color-Switch PER2::LUCIFERASE reporter that switches from red PER2::LUCIFERASE to green PER2::LUCIFERASE upon Cre recombination, we assess circadian rhythms in two of the major classes of peptidergic neurons in the SCN: AVP (arginine vasopressin) and VIP (vasoactive intestinal polypeptide). Surprisingly, we find that circadian function in AVP neurons, not VIP neurons, is essential for autonomous network synchrony of the SCN and stability of circadian rhythmicity.

Cold-induced chromatin compaction and nuclear retention of clock mRNAs resets the circadian rhythm

Cooling patients to sub‐physiological temperatures is an integral part of modern medicine. We show that cold exposure induces temperature‐specific changes to the higher‐order chromatin and gene expression profiles of human cells. These changes are particularly dramatic at 18°C, a temperature synonymous with that experienced by patients undergoing controlled deep hypothermia during surgery. Cells exposed to 18°C exhibit largely nuclear‐restricted transcriptome changes. These include the nuclear accumulation of mRNAs encoding components of the negative limbs of the core circadian clock, most notably REV‐ERBα. This response is accompanied by compaction of higher‐order chromatin and hindrance of mRNPs from engaging nuclear pores. Rewarming reverses chromatin compaction and releases the transcripts into the cytoplasm, triggering a pulse of negative limb gene proteins that reset the circadian clock. We show that cold‐induced upregulation of REV‐ERBα is sufficient to trigger this reset. Our findings uncover principles of the cellular cold response that must be considered for current and future applications involving therapeutic deep hypothermia.

Using Circadian Rhythm Patterns of Continuous Core Body Temperature to Improve Fertility and Pregnancy Planning

Objective

Review relationships among circadian clocks, core body temperature (CBT), and fertility in women.

Methods

Scoping literature review.

Results

Circadian clocks are a ubiquitous adaptation to the most predictable environmental events - the daily cycles of light and dark. Core body temperature (CBT) also follows a circadian rhythm. Additionally, CBT is tightly controlled by a combination of neuronal circuits that begin in the hypothalamus and involve many other portions of the brain as well as a wide range of peripheral mechanisms. In women with normal reproductive function, the diurnal temperature pattern for CBT is strongly influenced by the menstrual cycle of reproductive hormones, primarily estradiol and progesterone, which modulate the activity of hypothalamic neural circuits involved in body temperature control, resulting in an infradian CBT rhythm.

Conclusions

Analysis of CBT via continuous recording reveals patterns in the interactions of circadian and infradian CBT rhythms capable of accurately predicting the fertility window and hormonal patterns suggesting oligo-ovulation and subfertility. New wearable technologies can facilitate employment of hormone-associated changes in CBT for pregnancy planning and offer clinical insight to infertility and menopause.

Suprachiasmatic VIP neurons are required for normal circadian rhythmicity and comprised of molecularly distinct subpopulations

The hypothalamic suprachiasmatic (SCN) clock contains several neurochemically defined cell groups that contribute to the genesis of circadian rhythms. Using cell-specific and genetically targeted approaches we have confirmed an indispensable role for vasoactive intestinal polypeptide-expressing SCN (SCN^VIP) neurons, including their molecular clock, in generating the mammalian locomotor activity (LMA) circadian rhythm. Optogenetic-assisted circuit mapping revealed functional, di-synaptic connectivity between SCN^VIP neurons and dorsomedial hypothalamic neurons, providing a circuit substrate by which SCN^VIP neurons may regulate LMA rhythms. In vivo photometry revealed that while SCN^VIP neurons are acutely responsive to light, their activity is otherwise behavioral state invariant. Single-nuclei RNA-sequencing revealed that SCN^VIP neurons comprise two transcriptionally distinct subtypes, including putative pacemaker and non-pacemaker populations. Altogether, our work establishes necessity of SCN^VIP neurons for the LMA circadian rhythm, elucidates organization of circadian outflow from and modulatory input to SCN^VIP cells, and demonstrates a subpopulation-level molecular heterogeneity that suggests distinct functions for specific SCN^VIP subtypes.

Cell groups in the hypothalamic suprachiasmatic clock contribute to the genesis of circadian rhythms. The authors identified two populations of vasoactive intestinal polypeptide-expressing neurons in the suprachiasmatic nucleus which regulate locomotor circadian rhythm in mice.

Coupled network of the circadian clocks: a driving force of rhythmic physiology

The circadian system is composed of coupled endogenous oscillators that allow living beings, including humans, to anticipate and adapt to daily changes in their environment. In mammals, circadian clocks form a hierarchically organized network with a 'master clock' located in the suprachiasmatic nucleus of the hypothalamus, which ensures entrainment of subsidiary oscillators to environmental cycles. Robust rhythmicity of body clocks is indispensable for temporally coordinating organ functions, and the disruption or misalignment of circadian rhythms caused for instance by modern lifestyle is strongly associated with various widespread diseases. This review aims to provide a comprehensive overview of our current knowledge about the molecular architecture and system-level organization of mammalian circadian oscillators. Furthermore, we discuss the regulatory roles of peripheral clocks for cell and organ physiology and their implication in the temporal coordination of metabolism in human health and disease. Finally, we summarize methods for assessing circadian rhythmicity in humans.

Connectomics Analysis Reveals First-, Second-, and Third-Order Thermosensory and Hygrosensory Neurons in the Adult Drosophila Brain

Animals exhibit innate and learned preferences for temperature and humidity-conditions critical for their survival and reproduction. Leveraging a whole-brain electron microscopy volume, we studied the adult Drosophila melanogaster circuitry associated with antennal thermo- and hygrosensory neurons. We have identified two new target glomeruli in the antennal lobe, in addition to the five known ones, and the ventroposterior projection neurons (VP PNs) that relay thermo- and hygrosensory information to higher brain centers, including the mushroom body and lateral horn, seats of learned and innate behavior. We present the first connectome of a thermo- and hygrosensory neuropil, the lateral accessory calyx (lACA), by reconstructing neurons downstream of heating- and cooling-responsive VP PNs. A few mushroom body-intrinsic neurons solely receive thermosensory input from the lACA, while most receive additional olfactory and thermo- and/or hygrosensory PN inputs. Furthermore, several classes of lACA-associated neurons form a local network with outputs to other brain neuropils, suggesting that the lACA serves as a hub for thermo- and hygrosensory circuitry. For example, DN1a neurons link thermosensory PNs in the lACA to the circadian clock via the accessory medulla. Finally, we survey strongly connected downstream partners of VP PNs across the protocerebrum; these include a descending neuron targeted by dry-responsive VP PNs, meaning that just two synapses might separate hygrosensory inputs from motor circuits. These data provide a comprehensive first- and second-order layer analysis of Drosophila thermo- and hygrosensory systems and an initial survey of third-order neurons that could directly modulate behavior.

Time-dependent changes in proliferation, DNA damage and clock gene expression in hepatocellular carcinoma and healthy liver of a transgenic mouse model

Hepatocellular carcinoma (HCC) is highly resistant to anticancer therapy and novel therapeutic strategies are needed. Chronotherapy may become a promising approach because it may improve the efficacy of antimitotic radiation and chemotherapy by considering timing of treatment. To date little is known about time-of-day dependent changes of proliferation and DNA damage in HCC. Using transgenic c-myc/transforming growth factor (TGFα) mice as HCC animal model, we immunohistochemically demonstrated Ki67 as marker for proliferation and γ-H2AX as marker for DNA damage in HCC and surrounding healthy liver (HL). Core clock genes (Per1, Per2, Cry1, Cry2, Bmal 1, Rev-erbα and Clock) were examined by qPCR. Data were obtained from samples collected ex vivo at four different time points and from organotypic slice cultures (OSC). Significant differences were found between HCC and HL. In HCC, the number of Ki67 immunoreactive cells showed two peaks (ex vivo: ZT06 middle of day and ZT18 middle of night; OSC: CT04 and CT16). In ex vivo samples, the number of γ-H2AX positive cells in HCC peaked at ZT18 (middle of the night), while in OSC their number remained high during subjective day and night. In both HCC and HL, clock gene expression showed a time-of-day dependent expression ex vivo but no changes in OSC. The expression of Per2 and Cry1 was significantly lower in HCC than in HL. Our data support the concept of chronotherapy of HCC. OSC may become useful to test novel cancer therapies.

Clock Protein Bmal1 and Nrf2 Cooperatively Control Aging or Oxidative Response and Redox Homeostasis by Regulating Rhythmic Expression of Prdx6

Many disorders of aging, including blinding-diseases, are associated with deficiency of brain and muscle arnt-like protein 1 (Bmal1) and, thereby, dysregulation of antioxidant-defense pathway. However, knowledge is limited regarding the role of Bmal1 regulation of antioxidant-pathway in the eye lens/lens epithelial cells (LECs) at the molecular level. We found that, in aging human (h)LECs, a progressive decline of nuclear factor erythroid 2-related factor 2 (Nrf2)/ARE (antioxidant response element)-mediated antioxidant genes was connected to Bmal1-deficiency, leading to accumulation of reactive oxygen species (ROS) and cell-death. Bmal1-depletion disrupted Nrf2 and expression of its target antioxidant genes, like Peroxiredoxin 6 (Prdx6). DNA binding and transcription assays showed that Bmal1 controlled expression by direct binding to E-Box in Prdx6 promoter to regulate its transcription. Mutation at E-Box or ARE reduced promoter activity, while disruption of both sites diminished the activity, suggesting that both sites were required for peak Prdx6-transcription. As in aging hLECs, ROS accumulation was increased in Bmal1-deficient cells and the cells were vulnerable to death. Intriguingly, Bmal1/Nrf2/Prdx6 and PhaseII antioxidants showed rhythmic expression in mouse lenses in vivo and were reciprocally linked to ROS levels. We propose that Bmal1 is pivotal for regulating oxidative responses. Findings also reveal a circadian control of antioxidant-pathway, which is important in combating lens/LECs damage induced by aging or oxidative stress.

A role for alternative splicing in circadian control of exocytosis and glucose homeostasis

The circadian clock is encoded by a negative transcriptional feedback loop that coordinates physiology and behavior through molecular programs that remain incompletely understood. Here, we reveal rhythmic genome-wide alternative splicing (AS) of pre-mRNAs encoding regulators of peptidergic secretion within pancreatic β cells that are perturbed in Clock-/- and Bmal1-/- β-cell lines. We show that the RNA-binding protein THRAP3 (thyroid hormone receptor-associated protein 3) regulates circadian clock-dependent AS by binding to exons at coding sequences flanking exons that are more frequently skipped in clock mutant β cells, including transcripts encoding Cask (calcium/calmodulin-dependent serine protein kinase) and Madd (MAP kinase-activating death domain). Depletion of THRAP3 restores expression of the long isoforms of Cask and Madd, and mimicking exon skipping in these transcripts through antisense oligonucleotide delivery in wild-type islets reduces glucose-stimulated insulin secretion. Finally, we identify shared networks of alternatively spliced exocytic genes from islets of rodent models of diet-induced obesity that significantly overlap with clock mutants. Our results establish a role for pre-mRNA alternative splicing in β-cell function across the sleep/wake cycle.

A Robust and Self-Sustained Peripheral Circadian Oscillator Reveals Differences in Temperature Compensation Properties with Central Brain Clocks

Circadian clocks are characterized by three properties: they run in constant conditions with a period of ∼24 h, synchronize to the environmental cycles of light and temperature, and are temperature compensated, meaning they do not speed up with temperature. Central brain clocks regulate daily activity rhythms, whereas peripheral clocks are dispersed throughout the body of insects and vertebrates. Using a set of luciferase reporter genes, we show that Drosophila peripheral clocks are self-sustained but over-compensated, i.e., they slow down with increasing temperature. In contrast, central clock neurons in the fly brain, both in intact flies and in cultured brains, show accurate temperature compensation. Although this suggests that neural network properties contribute to temperature compensation, the circadian neuropeptide Pigment Dispersing Factor (PDF) is not required for temperature-compensated oscillations in brain clock neurons. Our findings reveal a fundamental difference between central and peripheral clocks, which likely also applies for vertebrate clocks.

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Drosophila halteres contain a robust circadian oscillator

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Circadian clocks of halteres and antennae are over-compensated

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The Drosophila central brain clock is temperature compensated

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The neuropeptide PDF is not required for temperature compensation of clock neurons

Circuit-level analysis identifies target genes of sex steroids in ewe seasonal breeding

Thyroid hormone (TH) and estradiol (E2) direct seasonal switches in ovine reproductive physiology. In sheep, as in other mammals and birds, control of thyrotropin (TSH) production by the pars tuberalis (PT) links photoperiod responsiveness to seasonal breeding. PT-derived TSH governs opposite seasonal patterns of the TH deiodinases Dio2/Dio3 expression in tanycytes of the neighboring medio-basal hypothalamus (MBH), which explain the key role of TH. We recently used RNA-Seq to identify seasonal markers in the MBH and define the impact of TH. This impact was found to be quite limited, in terms of number of target genes, and very restricted with regards to neuroanatomical location, as TH specifically impacts genes expressed in tanycytes and hypothalamus, not in the PT. Here we address the impact of E2 on these seasonal markers, which are specifically expressed in either PT, tanycytes or hypothalamus. We also investigate if progesterone (P4) may be involved in timing the seasonal transition to anestrus. Our analysis provides circuit-level insights into the impact of sex steroids on the ewe seasonal breeding cycle. First, seasonal gene expression in the PT is independent of the sex steroid status. The fact that seasonal gene expression in the PT is also TH-independent strengthens the view that the PT is a circannual timer. Second, select tanycytic markers display some level of responsiveness to E2 and P4, which indicates another potential level of feedback control by sex steroids. Third, Kiss1 neurons of the arcuate nucleus are responsive to both TH and E2, which places them at the crossroads of photoperiodic transduction pathway and sex steroid feedback. This provides strong support to the concept that these Kiss1 neurons are pivotal to the long-recognized "seasonal switch in the ability of E2 to exert negative feedback", which drives seasonal breeding.

Social jetlag is associated with an increased likelihood of having depressive symptoms among the Japanese working population: the Furukawa Nutrition and Health Study

STUDY OBJECTIVES

Social jetlag, a mismatch between biological and social timing, has been reported to be associated with depressive symptoms among general population. However, evidence on this association is lacking among non-shift workers, who are under pressure to adapt themselves to a work schedule. We investigated the cross-sectional association of social jetlag with depressive symptoms among Japanese non-shift workers.

METHODS

This study included 1,404 employees, aged 18-78 years, who completed a study questionnaire at a periodic health checkup. Social jetlag was calculated as the absolute value of the difference in the midpoint of sleep times between weekdays and weekends. Depressive symptoms were assessed using the Center for Epidemiologic Studies Depression Scale. Multivariable logistic regression was used to estimate the odds ratio (OR) with adjustments for potential confounders including diet and chronotype.

RESULTS

Of the study participants, 63.5%, 28.4%, and 8.1% had less than 1 hour, 1 to less than 2 hours, and at least 2 hours of social jetlag, respectively. Greater social jetlag was significantly associated with an increased likelihood of having depressive symptoms. The multivariable-adjusted OR (95% confidence interval) were 1.30 (0.95 to 1.78) and 2.14 (1.26-3.62) for 1 to less than 2 hour and at least 2 hours compared to less than 1 hour of social jetlag. The association between social jetlag and depressive symptoms appeared to be linear, according to restricted cubic spline regression.

CONCLUSION

Results suggest that greater social jetlag is associated with an increased likelihood of having depressive symptoms among non-shift workers.

Genome-wide circadian regulation: A unique system for computational biology

Circadian rhythms are 24-hour oscillations affecting an organism at multiple levels from gene expression all the way to tissues and organs. They have been observed in organisms across the kingdom of life, spanning from cyanobacteria to humans. In mammals, the master circadian pacemaker is located in the hypothalamic suprachiasmatic nuclei (SCN) in the brain where it synchronizes the peripheral oscillators that exist in other tissues. This system regulates the circadian activity of a large part of the transcriptome and recent findings indicate that almost every cell in the body has this clock at the molecular level. In this review, we briefly summarize the different factors that can influence the circadian transcriptome, including light, temperature, and food intake. We then summarize recently identified general principles governing genome-scale circadian regulation, as well as future lines of research. Genome-scale circadian activity represents a fascinating study model for computational biology. For this purpose, systems biology methods are promising exploratory tools to decode the global regulatory principles of circadian regulation.

Wearable Sensors for COVID-19: A Call to Action to Harness Our Digital Infrastructure for Remote Patient Monitoring and Virtual Assessments

The COVID-19 pandemic has brought into sharp focus the need to harness and leverage our digital infrastructure for remote patient monitoring. As current viral tests and vaccines are slow to emerge, we see a need for more robust disease detection and monitoring of individual and population health, which could be aided by wearable sensors. While the utility of this technology has been used to correlate physiological metrics to daily living and human performance, the translation of such technology toward predicting the incidence of COVID-19 remains a necessity. When used in conjunction with predictive platforms, users of wearable devices could be alerted when changes in their metrics match those associated with COVID-19. Anonymous data localized to regions such as neighborhoods or zip codes could provide public health officials and researchers a valuable tool to track and mitigate the spread of the virus, particularly during a second wave. Identifiable data, for example remote monitoring of cohorts (family, businesses, and facilities) associated with individuals diagnosed with COVID-19, can provide valuable data such as acceleration of transmission and symptom onset. This manuscript describes clinically relevant physiological metrics which can be measured from commercial devices today and highlights their role in tracking the health, stability, and recovery of COVID-19+ individuals and front-line workers. Our goal disseminating from this paper is to initiate a call to action among front-line workers and engineers toward developing digital health platforms for monitoring and managing this pandemic.

NAD+ Controls Circadian Reprogramming through PER2 Nuclear Translocation to Counter Aging

Disrupted sleep-wake and molecular circadian rhythms are a feature of aging associated with metabolic disease and reduced levels of NAD+, yet whether changes in nucleotide metabolism control circadian behavioral and genomic rhythms remains unknown. Here, we reveal that supplementation with the NAD+ precursor nicotinamide riboside (NR) markedly reprograms metabolic and stress-response pathways that decline with aging through inhibition of the clock repressor PER2. NR enhances BMAL1 chromatin binding genome-wide through PER2K680 deacetylation, which in turn primes PER2 phosphorylation within a domain that controls nuclear transport and stability and that is mutated in human advanced sleep phase syndrome. In old mice, dampened BMAL1 chromatin binding, transcriptional oscillations, mitochondrial respiration rhythms, and late evening activity are restored by NAD+ repletion to youthful levels with NR. These results reveal effects of NAD+ on metabolism and the circadian system with aging through the spatiotemporal control of the molecular clock.

"How does Austria sleep?" self-reported sleep habits and complaints in an online survey

During the past years, the prevalence of sleep problems has been increasing steadily in industrial societies and represents a major social and socioeconomic burden. The situation in Austria was last evaluated in 2007 by Zeitlhofer and colleagues in a representative sample of 1000 participants. In the current study, we sought to evaluate the sleep behaviour of the Austrian population in an ongoing online survey, in which we have collected data from 986 participants (66% women, mean age 40.9 ± 16.4 years) between March 2018 and May 2019. Sleep duration was appropriate in 52% of the respondents (i.e. 7-9 h per night). However, we found an alarmingly high number of self-reported sleep problems (46%), and only 31% of the participants classified themselves as "good sleepers" using a validated self-report questionnaire (Pittsburgh Sleep Quality Index, PSQI). Furthermore, many participants reported suffering from sleep problems for a very long time (86% > 6 months; 37% > 5 years) suggesting that currently available treatment options are either ineffective or not employed. Possible reasons for sleep problems could include irregular sleep-wake cycles, increased perceived stress levels, and the use of electronic devices just before sleep.

Emerging Roles of Translational Control in Circadian Timekeeping

A large part of mammalian physiology and behaviour shows regular daily variations. This temporal organisation is driven by the activity of an endogenous circadian clock, whose molecular basis consists of diurnal waves in gene expression. Circadian transcription is the major driver of these rhythms, yet post-transcriptional mechanisms, some of which occur in response to systemic cues and in a tissue-specific fashion, have central roles in ultimately establishing the oscillatory gene expression programme as well. Regulatory control that occurs at the level of translation is emerging as an important player in the generation and modulation of protein accumulation rhythms. As a mechanism, translation lies at a privileged position to integrate genetically encoded rhythmic signals with other, external and internal stimuli, including nutrient-derived cues. In this review, we summarise our current knowledge of how diurnal control of translation affects both bulk protein levels and gene-specific protein biosynthesis. We discuss mechanisms of regulation, in particular with regard to the complex interplay between circadian cycles and feeding/fasting cycles, as well as emerging roles for upstream open reading frames in clock control.

Circadian and Sleep Metabolomics Across Species

Under normal circadian function, metabolic control is temporally coordinated across tissues and behaviors with a 24-h period. However, circadian disruption results in negative consequences for metabolic homeostasis including energy or redox imbalances. Yet, circadian disruption has become increasingly prevalent within today's society due to many factors including sleep loss. Metabolic consequences of both have been revealed by metabolomics analyses of circadian biology and sleep. Specifically, two primary analytical platforms, mass spectrometry and nuclear magnetic resonance spectroscopy, have been used to study molecular clock and sleep influences on overall metabolic rhythmicity. For example, human studies have demonstrated the prevalence of metabolic rhythms in human biology, as well as pan-metabolome consequences of sleep disruption. However, human studies are limited to peripheral metabolic readouts primarily through minimally invasive procedures. For further tissue- and organism-specific investigations, a number of model systems have been studied, based upon the conserved nature of both the molecular clock and sleep across species. Here we summarize human studies as well as key findings from metabolomics studies using mice, Drosophila, and zebrafish. While informative, a limitation in existing literature is a lack of interpretation regarding dynamic synthesis or catabolism within metabolite pools. To this extent, future work incorporating isotope tracers, specific metabolite reporters, and single-cell metabolomics may provide a means of exploring dynamic activity in pathways of interest.

Conceptual Models of Entrainment, Jet Lag, and Seasonality

Understanding entrainment of circadian rhythms is a central goal of chronobiology. Many factors, such as period, amplitude, Zeitgeber strength, and daylength, govern entrainment ranges and phases of entrainment. We have tested whether simple amplitude-phase models can provide insight into the control of entrainment phases. Using global optimization, we derived conceptual models with just three free parameters (period, amplitude, and relaxation rate) that reproduce known phenotypic features of vertebrate clocks: phase response curves (PRCs) with relatively small phase shifts, fast re-entrainment after jet lag, and seasonal variability to track light onset or offset. Since optimization found multiple sets of model parameters, we could study this model ensemble to gain insight into the underlying design principles. We found complex associations between model parameters and entrainment features. Arnold onions of representative models visualize strong dependencies of entrainment on periods, relative Zeitgeber strength, and photoperiods. Our results support the use of oscillator theory as a framework for understanding the entrainment of circadian clocks.

Circadian rhythmicity of body temperature and metabolism

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

A microfluidic approach for experimentally modelling the intercellular coupling system of a mammalian circadian clock at single-cell level

In mammals, it is believed that the intercellular coupling mechanism between neurons in the suprachiasmatic nucleus (SCN) confers robustness and distinguishes the central clock from peripheral circadian oscillators. Current in vitro culturing methods used in Petri dishes to study intercellular coupling by exogenous factors invariably cause perturbations, such as simple media changes. Here, we design a microfluidic device to quantitatively study the intercellular coupling mechanism of circadian clock at the single cell level, and demonstrate that vasoactive intestinal peptide (VIP) induced coupling in clock mutant Cry1-/- mouse adult fibroblasts engineered to express the VIP receptor, VPAC2, is sufficient to synchronize and maintain robust circadian oscillations. Our study provides a proof-of-concept platform to reconstitute the intercellular coupling system of the central clock using uncoupled, single fibroblast cells in vitro, to mimic SCN slice cultures ex vivo and mouse behavior in vivo phenotypically. Such a versatile microfluidic platform may greatly facilitate the studies of intercellular regulation networks, and provide new insights into the coupling mechanisms of 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.

Melatonin rhythm and other outputs of the master circadian clock in the desert goat (Capra hircus) are entrained by daily cycles of ambient temperature

In desert areas, mammals such as camel and goat are exposed to harsh environmental conditions. The ambient temperature (Ta) cycles have been shown to entrain the circadian clock in the camel. In the present work, we assumed that, in the goat living in a desert biotope, Ta cycles would have the same synchronizing effect on the central clock. Therefore, the effects of Ta cycles on body temperature (Tb), locomotor activity (LA) and melatonin (Mel) rhythms as outputs of the master circadian clock have been studied. The study was performed on bucks kept first under constant conditions of total darkness (DD) and constant Ta, then maintained under DD conditions but exposed to Ta cycles with heat period during subjective day and cold period during subjective night. Finally, the Ta cycles were reversed with highest temperatures during the subjective night and the lowest temperatures during the subjective day. Under constant conditions, the circadian rhythms of Tb and LA were free running with an endogenous period of 25.3 and 25.0 hours, respectively. Ta cycles entrained the rhythms of Tb and LA to a period of exactly 24.0 hours; while when reversed, the Ta cycles led to an inversion of Tb and LA rhythms. Similarly, Ta cycles were also able to entrain Mel rhythm, by adjusting its secretion to the cooling phase before and after Ta cycles inversion. All together, these results show that the Ta cycles entrain the master circadian clock in the goat.

Gut Microbial Metabolites Induce Changes in Circadian Oscillation of Clock Gene Expression in the Mouse Embryonic Fibroblasts

Circadian rhythm is an endogenous oscillation of about 24-h period in many physiological processes and behaviors. This daily oscillation is maintained by the molecular clock machinery with transcriptional-translational feedback loops mediated by clock genes including Period2 (Per2) and Bmal1. Recently, it was revealed that gut microbiome exerts a significant impact on the circadian physiology and behavior of its host; however, the mechanism through which it regulates the molecular clock has remained elusive. 3-(4-hydroxyphenyl)propionic acid (4-OH-PPA) and 3-phenylpropionic acid (PPA) are major metabolites exclusively produced by Clostridium sporogenes and may function as unique chemical messengers communicating with its host. In the present study, we examined if two C. sporogenes-derived metabolites can modulate the oscillation of mammalian molecular clock. Interestingly, 4-OH-PPA and PPA increased the amplitude of both PER2 and Bmal1 oscillation in a dosedependent manner following their administration immediately after the nadir or the peak of their rhythm. The phase of PER2 oscillation responded differently depending on the mode of administration of the metabolites. In addition, using an organotypic slice culture ex vivo, treatment with 4-OH-PPA increased the amplitude and lengthened the period of PER2 oscillation in the suprachiasmatic nucleus and other tissues. In summary, two C. sporogenes-derived metabolites are involved in the regulation of circadian oscillation of Per2 and Bmal1 clock genes in the host's peripheral and central clock machineries.

Melatonin Relations with Energy Metabolism as Possibly Involved in Fatal Mountain Road Traffic Accidents

Previous results evidenced acute exposure to high altitude (HA) weakening the relation between daily melatonin cycle and the respiratory quotient. This review deals with the threat extreme environments pose on body time order, particularly concerning energy metabolism. Working at HA, at poles, or in space challenge our ancestral inborn body timing system. This conflict may also mark many aspects of our current lifestyle, involving shift work, rapid time zone crossing, and even prolonged office work in closed buildings. Misalignments between external and internal rhythms, in the short term, traduce into risk of mental and physical performance shortfalls, mood changes, quarrels, drug and alcohol abuse, failure to accomplish with the mission and, finally, high rates of fatal accidents. Relations of melatonin with energy metabolism being altered under a condition of hypoxia focused our attention on interactions of the indoleamine with redox state, as well as, with autonomic regulations. Individual tolerance/susceptibility to such interactions may hint at adequately dealing with body timing disorders under extreme conditions.

A standardized method to assess the endogenous activity and the light-response of the retinal clock in mammals

Purpose

The bioluminescence reporter PER2::Luciferase (PER2::Luc) provides a powerful tool to study the regulation of biological clocks in explant tissues, including the retinal clock. However, the establishment of a standardized procedure to replicate experimental conditions and to enable meaningful comparisons between findings from different studies is still lacking. In addition, different parameters may affect the retinal circadian bioluminescence signal and its dynamic in in vitro assays. In the present study, we first evaluated the effect of sex and age on the main parameters of the mouse retinal clock. We then examined the impact of medium change on PER2::Luc rhythm and compared two light stimulation protocols of the retinal clock.

Methods

In a first set of experiments, retinal explants from both male and female Per2^Luc mice of different ages (1 to 8 months) are cultured and the period, phase, amplitude, and rhythmic power of PER2::Luc oscillations are analyzed. In a second set of experiments, we quantified the effect of a medium change done after 4, 6, 8, 9, or 10 days of culture on the phase and period of retinal explants. Finally, we compared the phase shift and the period change resulting from two methods of light stimulations of retinal explants: the first involved the transfer of the cultured tissues from the Lumicycle into a light stimulation chamber, while the second used a light delivery apparatus embedded in the Lumicycle.

Results

We do not observe any sex-dependent effects on the amplitude, period, phase, and rhythmic power of the in vitro retinal PER2::Luc oscillations in animals aged of 2 to 3 months. The most remarkable effect of age is on the amplitude of PER2::Luc oscillations that significantly decrease from 1 to 4-5 months, whereas the endogenous period and rhythmic power increase slightly until 2 to 3 months and then do not change until 8 months. The phase is not affected by age. We then show that a medium change occurring after 4 days of culture does not alter the phase of PER2::Luc rhythm by comparison with day 0, whereas a medium change done after 6, 8, 9, or 10 days in culture advances the phase and lengthens the period. Finally, we observe that the physical displacement of the culture dishes containing retinal explants, even in complete darkness, induces a strong phase shift of PER2::Luc oscillations.

Conclusions

Our work shows that the retina cultures are particularly sensitive to some aspects of the culture procedure, and it provides an accurate standard protocol to avoid biases due to artifactually induced phase shifts resulting from the medium change or physical displacement.

Isolation and Structure Determination of a Heat Shock Protein Inducer, Asparagus-Derived Proline-Containing 3-Alkyldiketopiperazines (Asparaprolines), From a Standardized Extract of Asparagus officinalis Stem

ETAS® has been developed from the stems of Asparagus officinalis L. as a functional ingredient for nutraceuticals. ETAS possesses heat shock protein 70 (HSP70) induction activity and may contribute to maintenance and improvement of health. Here, 3 compounds (1, 2, 3) were isolated from ETAS. The structures of 1, 2, and 3 were deduced by HREIMS and NMR spectroscopic data, and the compounds were identified as cyclo(l-Phe-l-Pro), cyclo(l-Tyr-l-Pro), and cyclo(l-Leu-l-Pro), respectively. Each compound contained a diketopiperazine ring derived from proline with an alkyl group at C-3; thus, we termed them asparagus-derived proline-containing 3-alkyldiketopiperazines (Asparaprolines). In an HSP70 mRNA induction assay in HL-60 cells, Asparaprolines significantly enhanced the expression of HSP70 mRNA compared with a control. To our knowledge, these results demonstrate for the first time that proline-containing diketopiperazines derived from natural amino acids exhibit HSP70 mRNA induction activity.

A Conserved Kinase-Based Body-Temperature Sensor Globally Controls Alternative Splicing and Gene Expression

Homeothermic organisms maintain their core body temperature in a narrow, tightly controlled range. Whether and how subtle circadian oscillations or disease-associated changes in core body temperature are sensed and integrated in gene expression programs remain elusive. Furthermore, a thermo-sensor capable of sensing the small temperature differentials leading to temperature-dependent sex determination (TSD) in poikilothermic reptiles has not been identified. Here, we show that the activity of CDC-like kinases (CLKs) is highly responsive to physiological temperature changes, which is conferred by structural rearrangements within the kinase activation segment. Lower body temperature activates CLKs resulting in strongly increased phosphorylation of SR proteins in vitro and in vivo. This globally controls temperature-dependent alternative splicing and gene expression, with wide implications in circadian, tissue-specific, and disease-associated settings. This temperature sensor is conserved across evolution and adapted to growth temperatures of diverse poikilotherms. The dynamic temperature range of reptilian CLK homologs suggests a role in TSD.

Why Lungs Keep Time: Circadian Rhythms and Lung Immunity

Circadian rhythms are daily cycles in biological function that are ubiquitous in nature. Understood as a means for organisms to anticipate daily environmental changes, circadian rhythms are also important for orchestrating complex biological processes such as immunity. Nowhere is this more evident than in the respiratory system, where circadian rhythms in inflammatory lung disease have been appreciated since ancient times. In this focused review we examine how emerging research on circadian rhythms is being applied to the study of fundamental lung biology and respiratory disease. We begin with a general introduction to circadian rhythms and the molecular circadian clock that underpins them. We then focus on emerging data tying circadian clock function to immunologic activities within the respiratory system. We conclude by considering outstanding questions about biological timing in the lung and how a better command of chronobiology could inform our understanding of complex lung diseases.

Liver gene regulatory networks: Contributing factors to nonalcoholic fatty liver disease

Metabolic diseases such as nonalcoholic fatty liver disease (NAFLD) result from complex interactions between intrinsic and extrinsic factors, including genetics and exposure to obesogenic environments. These risk factors converge in aberrant gene expression patterns in the liver, which are underlined by altered cis-regulatory networks. In homeostasis and in disease states, liver cis-regulatory networks are established by coordinated action of liver-enriched transcription factors (TFs), which define enhancer landscapes, activating broad gene programs with spatiotemporal resolution. Recent advances in DNA sequencing have dramatically expanded our ability to map active transcripts, enhancers and TF cistromes, and to define the 3D chromatin topology that contains these elements. Deployment of these technologies has allowed investigation of the molecular processes that regulate liver development and metabolic homeostasis. Moreover, genomic studies of NAFLD patients and NAFLD models have demonstrated that the liver undergoes pervasive regulatory rewiring in NAFLD, which is reflected by aberrant gene expression profiles. We have therefore achieved an unprecedented level of detail in the understanding of liver cis-regulatory networks, particularly in physiological conditions. Future studies should aim to map active regulatory elements with added levels of resolution, addressing how the chromatin landscapes of different cell lineages contribute to and are altered in NAFLD and NAFLD-associated metabolic states. Such efforts would provide additional clues into the molecular factors that trigger this disease. This article is categorized under: Biological Mechanisms > Metabolism Biological Mechanisms > Regulatory Biology Laboratory Methods and Technologies > Genetic/Genomic Methods.