Effects of aging on central and peripheral mammalian clocks

Identification of functional clock-controlled elements involved in differential timing of Per1 and Per2 transcription

It has been proposed that robust rhythmic gene expression requires clock-controlled elements (CCEs). Transcription of Per1 was reported to be regulated by the E-box and D-box in conventional reporter assays. However, such experiments are inconclusive in terms of how the CCEs and their combinations determine the phase of the Per1 gene. Whereas the phase of Per2 oscillation was found to be the most delayed among the three Period genes, the phase-delaying regions of the Per2 promoter remain to be determined. We therefore investigated the regulatory mechanism of circadian Per1 and Per2 transcription using an in vitro rhythm oscillation-monitoring system. We found that the copy number of the E-box might play an important role in determining the phase of Per1 oscillation. Based on real-time bioluminescence assays with various promoter constructs, we provide evidence that the non-canonical E-box is involved in the phase delay of Per2 oscillation. Transfection experiments confirmed that the non-canonical E-box could be activated by CLOCK/BMAL1. We also show that the D-box in the third conserved segment of the Per2 promoter generated high amplitude. Our experiments demonstrate that the copy number and various combinations of functional CCEs ultimately led to different circadian phases and amplitudes.

"Clocks" in the NAD World: NAD as a metabolic oscillator for the regulation of metabolism and aging

SIR2 (silent information regulator 2) proteins, now called "sirtuins," are an evolutionarily conserved family of NAD-dependent protein deacetylases/ADP-ribosyltransferases. Sirtuins have recently attracted major attention in the field of aging research, and it has been demonstrated that SIR2 and its orthologs regulate aging and longevity in yeast, worms, and flies. In mammals, the SIR2 ortholog SIRT1 coordinates important metabolic responses to nutritional availability in multiple tissues. Most recently, it has been demonstrated that SIRT1 regulates the amplitude and the duration of circadian gene expression through the interaction and the deacetylation of key circadian clock regulators, such as BMAL1 and PER2. More strikingly, we and others have discovered a novel circadian clock feedback loop in which both the rate-limiting enzyme in mammalian NAD biosynthesis, nicotinamide phosphoribosyltransferase (NAMPT), and NAD levels display circadian oscillations and modulate CLOCK:BMAL1-mediated circadian transcriptional regulation through SIRT1, demonstrating a new function of NAD as a "metabolic oscillator." These findings reveal a novel system dynamics of a recently proposed systemic regulatory network regulated by NAMPT-mediated NAD biosynthesis and SIRT1, namely, the NAD World. In the light of this concept, a new connection between physiological rhythmicity, metabolism, and aging will be discussed.

Impact of aging on diurnal expression patterns of CLOCK and BMAL1 in the mouse brain

Mammalian circadian rhythms are generated by a network of transcriptional and translational loops in the expression of a panel of clock genes in various brain and peripheral sites. Many of the output rhythms controlled by this system are significantly affected by ageing, although the mechanisms of age-related circadian dysfunction remain opaque. The aim of this study was to investigate the effect of aging on the daily oscillation of two clock gene proteins (CLOCK, BMAL1) in the mouse brain. Clock gene protein expression in the brain was measured by means of immunohistochemistry in groups of young (4 months) and older (16 months) mice sampled every 4h over a 24-h cycle. CLOCK and BMAL1 were constitutively expressed in the suprachiasmatic nucleus (SCN; the master circadian pacemaker) in young adult animals. We report novel rhythmic expression of CLOCK and BMAL1 in a number of extra-SCN sites in the young mouse brain, including the hippocampus, amygdala and the paraventricular, arcuate and dorsomedial nuclei of the hypothalamus. Aging altered the amplitude and/or phase of expression in these regions. These results indicate hitherto unreported expression patterns of CLOCK and BMAL1 in non-SCN brain circadian oscillators, and suggest that alterations of these patterns may contribute to age-related circadian dysfunction.

Effects of age on clock gene expression in the rhesus macaque pituitary gland

Recent studies have shown that circadian clock genes are expressed in various peripheral tissues, raising the possibility that multiple clocks regulate circadian physiology. To study clock gene expression in the rhesus macaque pituitary gland we used gene microarray data and found that the pituitary glands of young and old adult males express several components of the circadian clock (Per1, Per2, Cry1, Bmal1, Clock, Rev-erbalpha and Csnk1varepsilon). Semi-quantitative reverse-transcription polymerase chain reaction (sqRT-PCR) confirmed the presence of these core-clock genes and detected significant age-related differences in the expression of Per2. sqRT-PCR also showed differential expression of core-clock genes at two opposing time-points over the 24-h day, with greater expression of Per2 and Bmal1 (P<0.05) at 1300h as compared to 0100h. Immunohistochemistry revealed rhythmic expression of REV-ERBalpha in the pituitary glands of female macaques. These data provide evidence that the rhesus macaque pituitary gland expresses core-clock genes and their associated protein products in a 24-h rhythmic pattern, and that their expression is moderately impacted by aging processes.

Circadian clocks and antiaging: do non-aging microalgae like Euglena reveal anything?

Microalgae that divide symmetrically in all aspects do not age. While the evolutionary reason for this is obvious, little attention has been paid to the mechanistic explanations. A great deal of study involving many research fields would be needed to explain the mechanisms if we suppose that the immortality results from a lifelong sufficiency of defense from stress or from an essential part of counteracting age-accompanied damage accumulation. Additionally, little is known about the relationships between homeostasis and circadian clocks in antiaging, although each of these has been studied separately. Here, we present a conceptual generalization of those relationships, as suggested by evidence from non-aging microalgae, mainly Euglena. The circadian gating of mitosis and circadian temporal coordination may respectively reduce radiation- and disharmony-induced stress in which homeostasis cannot be involved, whereas circadian resistance rhythms may greatly help homeostatic defense from radiation- and metabolism-induced stress. We also briefly sketch mammalian aging research to compare the current status of knowledge with that of algal antiaging.

Clock genes, hair growth and aging

Hair follicles undergo continuous cycles of growth, involution and rest. This process, referred to as the hair growth cycle, has a periodicity of weeks to months. At the same time, skin and hair follicles harbor a functional circadian clock that regulates gene expression with a periodicity of approximately twenty four hours. In our recent study we found that circadian clock genes play a role in regulation of the hair growth cycle during synchronized hair follicle cycling, uncovering an unexpected connection between these two timing systems within skin. This work, therefore, indicates a role for circadian clock genes in a cyclical process of much longer periodicity than twenty four hours.

Circadian clock-coordinated hepatic lipid metabolism: only transcriptional regulation?

By regulating the metabolism of fatty acids, carbohydrates, and xenobiotic, the mammalian circadian clock plays a fundamental role on the liver physiology. At present, it is supposed that the circadian clock regulates metabolism mostly by regulating the expression of liver enzymes at the transcriptional level. However, recent evidences suggest that some signaling pathways synchronized by the circadian clock can also influence metabolism at a post-transcriptional level. In this context, we have recently shown that the circadian clock synchronizes the rhythmic activation of the IRE1alpha pathway in the endoplasmic reticulum. The absence of circadian clock perturbs this secondary clock, provokes deregulation of endoplasmic reticulum-localized enzymes, and leads to impaired lipid metabolism. We will describe here the additional pathways synchronized by the clock and discussed the influence of the circadian clock-controlled feeding rhythm on them.

Metabolism and circadian rhythms--implications for obesity

Obesity has become a serious public health problem and a major risk factor for the development of illnesses, such as insulin resistance and hypertension. Human homeostatic systems have adapted to daily changes in light and dark in a way that the body anticipates the sleep and activity periods. Mammals have developed an endogenous circadian clock located in the suprachiasmatic nuclei of the anterior hypothalamus that responds to the environmental light-dark cycle. Similar clocks have been found in peripheral tissues, such as the liver, intestine, and adipose tissue, regulating cellular and physiological functions. The circadian clock has been reported to regulate metabolism and energy homeostasis in the liver and other peripheral tissues. This is achieved by mediating the expression and/or activity of certain metabolic enzymes and transport systems. In return, key metabolic enzymes and transcription activators interact with and affect the core clock mechanism. In addition, the core clock mechanism has been shown to be linked with lipogenic and adipogenic pathways. Animals with mutations in clock genes that disrupt cellular rhythmicity have provided evidence for the relationship between the circadian clock and metabolic homeostasis. In addition, clinical studies in shift workers and obese patients accentuate the link between the circadian clock and metabolism. This review will focus on the interconnection between the circadian clock and metabolism, with implications for obesity and how the circadian clock is influenced by hormones, nutrients, and timed meals.

Circadian activity rhythms and mortality: the study of osteoporotic fractures

OBJECTIVES

To determine whether circadian activity rhythms are associated with mortality in community-dwelling older women.

DESIGN

Prospective study of mortality.

SETTING

A cohort study of health and aging.

PARTICIPANTS

Three thousand twenty-seven community-dwelling women from the Study of Osteoporotic Fractures cohort (mean age 84).

MEASUREMENTS

Activity data were collected using wrist actigraphy for a minimum of three 24-hour periods, and circadian activity rhythms were computed. Parameters of interest included height of activity peak (amplitude), midline estimating statistic of rhythm (mesor), strength of activity rhythm (robustness), and time of peak activity (acrophase). Vital status, with cause of death adjudicated through death certificates, was prospectively ascertained.

RESULTS

Over an average of 4.1 years of follow-up, there were 444 (14.7%) deaths. There was an inverse association between peak activity height and all-cause mortality rates, with higher mortality rates observed in the lowest activity quartile (hazard ratio (HR)=2.18, 95% confidence interval (CI)=1.63-2.92) than in the highest quartile after adjusting for age, clinic site, race, body mass index, cognitive function, exercise, instrumental activity of daily living impairments, depression, medications, alcohol, smoking, self-reported health status, married status, and comorbidities. A greater risk of mortality from all causes was observed for those in the lowest quartiles of mesor (HR=1.71, 95% CI=1.29-2.27) and rhythm robustness (HR=1.97, 95% CI=1.50-2.60) than for those in the highest quartiles. Greater mortality from cancer (HR=2.09, 95% CI=1.04-4.22) and stroke (HR=2.64, 95% CI=1.11-6.30) was observed for later peak activity (after 4:33 p.m.; >1.5 SD from mean) than for the mean peak range (2:50-4:33 p.m.).

CONCLUSION

Older women with weak circadian activity rhythms have higher mortality risk. If confirmed in other cohorts, studies will be needed to test whether interventions (e.g., physical activity, bright light exposure) that regulate circadian activity rhythms will improve health outcomes in older adults.

Circadian temperature variation and ageing

In the present paper, an attempt is made to summarize current knowledge concerning the daily body temperature rhythm and its age-dependent alterations. Homeostatic and circadian control mechanisms are considered. Special attention is paid to the circadian system, as the mechanisms of autonomic control are the topic of another contribution to this special issue. Also, the interactions of the core body temperature rhythm with other circadian functions are discussed in detail as they constitute an essential part of the internal temporal order of living systems and thus guarantee their optimal functioning. In the second part of the paper, age-dependent changes in the circadian body temperature rhythm and their putative causes, considering circadian and homeostatic components, are described. Consequences for health and fitness and some possibilities to prevent adverse effect are mentioned in the final section.

Neuroprotection by radical avoidance: search for suitable agents

Neurodegeneration is frequently associated with damage by free radicals. However, increases in reactive oxygen and nitrogen species, which may ultimately lead to neuronal cell death, do not necessarily reflect its primary cause, but can be a consequence of otherwise induced cellular dysfunction. Detrimental processes which promote free radical formation are initiated, e.g., by disturbances in calcium homeostasis, mitochondrial malfunction, and an age-related decline in the circadian oscillator system. Free radicals generated at high rates under pathophysiological conditions are insufficiently detoxified by scavengers. Interventions at the primary causes of dysfunction, which avoid secondary rises in radical formation, may be more efficient. The aim of such approaches should be to prevent calcium overload, to reduce mitochondrial electron dissipation, to support electron transport capacity, and to avoid circadian perturbations. L-theanine and several amphiphilic nitrones are capable of counteracting excitotoxicity and/or mitochondrial radical formation. Resveratrol seems to promote mitochondrial biogenesis. Mitochondrial effects of leptin include attenuation of electron leakage. Melatonin combines all the requirements mentioned, additionally regulates anti- and pro-oxidant enzymes and is, with few exceptions, very well tolerated. In this review, the perspectives, problems and limits of drugs are compared which may be suitable for reducing the formation of free radicals.

Abnormal expressions of circadian-clock and circadian clock-controlled genes in the livers and kidneys of long-term, high-fat-diet-treated mice

OBJECTIVES

Physiological and behavioral circadian rhythmicities are exhibited by all mammals and are generated by intracellular levels of circadian oscillators, which are composed of transcriptional/translational feedback loops involving a set of circadian-clock genes, such as Clock, Per1-3, Cry1-2, Bmal1, Dbp, E4BP4 and CK1varepsilon. These circadian-clock genes play important roles in regulating circadian rhythms and also energy homeostasis and metabolism. Determining whether obesity induced by high-fat diet affected the expressions of circadian-clock genes and their related genes in peripheral tissues, was the main focus of this study. To address this issue, we fed male C57BL/6 mice a high-fat diet for 11 months to induce obesity, hyperglycemic, hypercholesterolemic and hyperinsulinemic symptoms, and used quantitative real-time reverse transcription-PCR to measure gene expression levels.

RESULTS

We found that the expressions of circadian-clock genes and circadian clock-controlled genes, including Per1-3, Cry1-2, Bmal1, Dbp, E4BP4, CK1varepsilon, PEPCK, PDK4 and NHE3, were altered in the livers and/or kidneys.

CONCLUSIONS

These results indicate that obesity induced by high-fat diet alters the circadian-clock system, and obesity and metabolic syndrome are highly correlated with the expressions of circadian-clock genes and their downstream, circadian clock-controlled genes.

"Clocks" in the NAD World: NAD as a metabolic oscillator for the regulation of metabolism and aging

SIR2 (silent information regulator 2) proteins, now called "sirtuins," are an evolutionarily conserved family of NAD-dependent protein deacetylases/ADP-ribosyltransferases. Sirtuins have recently attracted major attention in the field of aging research, and it has been demonstrated that SIR2 and its orthologs regulate aging and longevity in yeast, worms, and flies. In mammals, the SIR2 ortholog SIRT1 coordinates important metabolic responses to nutritional availability in multiple tissues. Most recently, it has been demonstrated that SIRT1 regulates the amplitude and the duration of circadian gene expression through the interaction and the deacetylation of key circadian clock regulators, such as BMAL1 and PER2. More strikingly, we and others have discovered a novel circadian clock feedback loop in which both the rate-limiting enzyme in mammalian NAD biosynthesis, nicotinamide phosphoribosyltransferase (NAMPT), and NAD levels display circadian oscillations and modulate CLOCK:BMAL1-mediated circadian transcriptional regulation through SIRT1, demonstrating a new function of NAD as a "metabolic oscillator." These findings reveal a novel system dynamics of a recently proposed systemic regulatory network regulated by NAMPT-mediated NAD biosynthesis and SIRT1, namely, the NAD World. In the light of this concept, a new connection between physiological rhythmicity, metabolism, and aging will be discussed.

Proximate mechanisms driving circadian control of neuroendocrine function: Lessons from the young and old

Circadian rhythms impact a variety of behavioral and physiological functions contributing to longevity and successful reproduction. In their natural environments, individuals of a species are faced with a multitude of challenges and the coordination of internal processes and behavior with external pressures has been hypothesized to be an important target of natural selection. Several lines of evidence from cyanobacteria, Drosophila, and plants provide strong support for an important role of the circadian clock in survival and reproductive success. Similarly in mammals, disruptions in circadian function markedly impact reproduction and lifespan. The present review discusses research outlining the proximate and ultimate mechanisms responsible for the central and peripheral control of the reproductive axis. Because precise temporal coordination of the endocrine system is particularly crucial for reproduction by females, the present overview focuses on the role of circadian timing in this sex.

INFLUENCE OF AGE ON THE INTERRELATION BETWEEN EEG FREQUENCY BANDS DURING NREM AND REM SLEEP

The age-dependence of temporal interrelations between distinct frequency bands of sleep EEG was investigated in a group of 59 healthy young and middle-aged males via cross correlation analysis. Based on global evaluation throughout the entire night, a highly significant decline of the delta/theta correlation with increasing age was found. A separate analysis for non-rapid eye movement (NREM) and rapid eye movement (REM) sleep revealed different changes with aging. During NREM sleep, the correlation between the delta and theta frequency bands decreased with increasing age. In contrast, during REM sleep, a stronger correlation became obvious between the theta, alpha, and beta frequency bands with increasing age, whereas the lower frequency components were not affected. These findings indicate that aging processes seem to interact with sleep EEG rhythms in a complex manner, where most conspicuous is a disintegration of the activities in the lower frequency range, both concerning the successive sleep cycles across the night and the micro-structure of NREM sleep.

The aging suprachiasmatic nucleus and cytokines: functional, molecular, and cellular changes in rodents

The aging process brings about a switch to a low-grade chronic inflammatory condition in the periphery and brain, a condition which may prime brain cells, including those of the hypothalamic suprachiasmatic nucleus (SCN). Little information is available, however, on the responses of the SCN to neuroinflammation and immune-related challenges, and such responses have not been hitherto investigated during aging. We here provide an overview of these issues and summarize data we obtained in the study of the SCN of young and aged mice. In particular, we analyzed: i) the electrophysiological properties of the SCN core (the retino-recipient region) in tissue slices; ii) expression and day/night variation of transcripts encoding the receptors for the cytokines interferon-gamma and tumor necrosis factor-alpha, as well as the expression of transcripts encoding the proteins "suppressors of cytokine signaling" SOCS1 and SOCS3, by means of quantitative real-time polymerase chain reaction; levels of mRNAs were correlated with neuronal activation, revealed by Fos induction, elicited in the SCN by intracerebroventricular injections of a mixture of interferon-gamma and tumor necrosis factor-alpha during the daytime and nighttime; and iii) response of astrocytes and microglia in the SCN to the same paradigm of cytokine administration. Marked changes of all the above-mentioned parameters were found in the aged SCN, indicating that the circadian pacemaker is a target of the aging process. In addition, the findings indicate that neurons and glial cells of the biological clock are sensitive to inflammatory signals, and that the response to such signals is altered during senescence.

Ontogeny of circadian organization in the rat

The mammalian circadian system is orchestrated by a master pacemaker in the brain, but many peripheral tissues also contain independent or quasi-independent circadian oscillators. The adaptive significance of clocks in these structures must lie, in large part, in the phase relationships between the constituent oscillators and their micro- and macroenvironments. To examine the relationship between postnatal development, which is dependent on endogenous programs and maternal/environmental influences, and the phase of circadian oscillators, the authors assessed the circadian phase of pineal, liver, lung, adrenal, and thyroid tissues cultured from Period 1-luciferase (Per1-luc ) rat pups of various postnatal ages. The liver, thyroid, and pineal were rhythmic at birth, but the phases of their Per1-luc expression rhythms shifted remarkably during development. To determine if the timing of the phase shift in each tissue could be the result of changing environmental conditions, the behavior of pups and their mothers was monitored. The circadian phase of the liver shifted from the day to night around postnatal day (P) 22 as the pups nursed less during the light and instead ate solid food during the dark. Furthermore, the phase of Per1-luc expression in liver cultures from nursing neonates could be shifted experimentally from the day to the night by allowing pups access to the dam only during the dark. Peak Per1-luc expression also shifted from midday to early night in thyroid cultures at about P20, concurrent with the shift in eating times. The phase of Per1-luc expression in the pineal gland shifted from day to night coincident with its sympathetic innervation at around P5. Per1-luc expression was rhythmic in adrenal cultures and peaked around the time of lights-off throughout development; however, the amplitude of the rhythm increased at P25. Lung cultures were completely arrhythmic until P12 when the pups began to leave the nest. Taken together, the data suggest that the molecular machinery that generates circadian oscillations matures at different rates in different tissues and that the phase of at least some peripheral organs is malleable and may shift as the organ's function changes during development.

Aging in male primates: reproductive decline, effects of calorie restriction and future research potential

Although less dramatic than in females, male mammals experience decreasing reproductive function during aging. In primates, multiple facets of the hypothalamic-pituitary-gonadal axis show evidence of gradual age-related decline, including behavioral, neuroendocrine and endocrine alterations such as decreased testosterone levels, reduced circulating dehydroepiandrosterone sulfate (DHEAS) levels, increased numbers of sperm abnormalities, and a general decline in physiological responses. In this review we consider a range of age-related changes in males. These measures, including more subtle aging characteristics, are interesting additional indices for detecting the timing of age-related changes in behavioral, neuroendocrine, and endocrine responses. Evidence of potential effects of calorie restriction as an intervention in reproductive aging is also discussed. A discernable decline occurs in both metabolic and reproductive endocrine processes during male aging. This cascade of events includes neuroendocrine and behavioral changes; biomarkers such as circulating DHEAS also show clear age-related decline. The varied changes that occur during male aging are considered in the context of primate aging in general.

Age and oestrus cycle-related changes in glucocorticoid excretion and wheel-running activity in female mice carrying mutations in the circadian clock genes Per1 and Per2

In mammals, numerous physiological and behavioural functions are controlled by an endogenous circadian clock located in the suprachiasmatic nuclei (SCN). Within the SCN neurons, clock genes such as Per1 and Per2 interact in a molecular clockwork regulating the expression of hundreds of output genes. Through the timed release of humoral and neuronal signals, the rhythmicity of numerous biological processes, including reproductive behaviour, the oestrus cycle and endocrine parameters is controlled by the SCN. Mutations in Per genes in mice affect a wide array of physiological functions. Interestingly, most of these studies use only male animals, thus neglecting potential gender-specificities in clock function. In an attempt to broaden this perspective we have investigated the impact of Per1 and Per2 mutations on both glucocorticoid (GC) metabolite excretion and locomotor activity in relation to age and oestrus cycle stage of female mice. We show that the Per2 mutation dampens daily GC rhythms in young adult females. While locomotor activity does not vary along the different oestrus stages in Per2 mutant females, oestrus effects on GC excretion and locomotor activity are largely comparable between Per1 mutants and wild-type animals. 20 month-old, acyclic Per1 and wild-type females show reduced GC levels when compared to young adults while aged Per2 mutants retain their normal GC rhythmicity. Correlating with this, onsets of locomotor activity do not change with age in Per2 mutant females. Together, our data highlight specific roles for Per1 and Per2 in both the regulation of locomotor activity and endocrine functions in the female organism.

Live to the rhythm, slave to the rhythm

Circadian rhythms in health and disease have most often been described in terms of their phases and amplitudes, and how these respond to a single exposure to stimuli denoted as zeitgebers. The present paper argues that it is also important to consider the 24-h regularity in the repeated occurrence of the zeitgebers. The effect of the regularity of stimulation by light, melatonin, physical activity, body temperature, corticosteroids and feeding on synchronization within and between the central circadian clock and peripheral oscillators is discussed. In contrast to the phase shifts that can be recorded acutely after a single zeitgeber pulse, the effects of irregularly versus regularly timed zeitgeber can be studied only in long-term protocols and may develop slowly, which is a possible reason why they have received relatively little attention. Several observations indicate a reciprocal relation between the robustness of the endogenous circadian timing system and its dependency on regularly timed zeitgebers. Especially at old age and in disease, proper functioning of the circadian timing system may become more dependent on regularly timed exposure to zeitgeber stimuli. in such conditions, regularly timed exposure to zeitgeber appears to be highly important for health. After a concise introduction on inputs to the central and peripheral oscillators of the circadian timing system, the paper discusses the responses of the circadian timing system and health to (1) a chronic lack of zeitgeber stimuli; (2) fragmented or quasi-ultradian stimuli and (3) repeated phase shifts in stimuli. Subsequently, the specific relevance to aging is discussed, followed by an overview of the effects of experimentally imposed regularly timed stimuli. Finally, a possible mechanism for the gradually evolving effects of repeated regularly timed stimuli on the circadian timing system is proposed.

Aging of the circadian system in zebrafish and the effects of melatonin on sleep and cognitive performance

Aging is a complex process involving intracellular changes and, notably, modifications in intercellular communications, required for coordinated responses to internal and external events. One of the potential reasons for such changes is an age-dependent failure of the integrating systems, including the circadian clock. Here we demonstrate that aging in a diurnal vertebrate, zebrafish (Danio rerio), is associated with major but selective circadian alterations. By 3-5 years of age, zebrafish have reduced amplitude and increased fragmentation of entrained circadian rhythms of activity, with fast desynchronization of the rhythms in the absence of environmental time cues. Aging in zebrafish is also associated with a reduction in the overall duration of nighttime sleep, followed by lower activity levels and a higher arousal threshold during the day. The production of the principal circadian hormone, melatonin, progressively declines during zebrafish aging. However, the ability of melatonin to acutely promote sleep and entrain circadian rhythms of activity remains robust until at least 4-5 years of age, consistent with the preserved levels of mRNA expression for melatonin receptors. Aged zebrafish have altered expression of the circadian genes zBmal1 and zPer1 but not zClock1. A lack of circadian time cues alters cognitive performance in aged more than in young zebrafish and this can be partially attenuated by daily melatonin administration. The advantages of zebrafish as a diurnal, small, prolific and genetically well-characterized vertebrate model provide new opportunities to clarify the intrinsic circadian factors involved in human aging and promote the search for prophylactic and treatment strategies.

Sleep and circadian rhythms: key components in the regulation of energy metabolism

In this review, we present evidence from human and animal studies to evaluate the hypothesis that sleep and circadian rhythms have direct impacts on energy metabolism, and represent important mechanisms underlying the major health epidemics of obesity and diabetes. The first part of this review will focus on studies that support the idea that sleep loss and obesity are "interacting epidemics." The second part will discuss recent evidence that the circadian clock system plays a fundamental role in energy metabolism at both the behavioral and molecular levels. These lines of research must be seen as in their infancy, but nevertheless, have provided a conceptual and experimental framework that potentially has great importance for understanding metabolic health and disease.

Orchestration of gene expression and physiology by the circadian clock

In mammals, the master circadian clock that drives many biochemical, physiological and behavioral rhythms is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Generation and maintenance of circadian rhythms rely on complex interlaced feedback loops based on transcriptional and posttranscriptional events involving clock genes and kinases. This clock serves the purpose to organize an organism's biochemistry on a 24 h time scale thereby avoiding interference between biochemical pathways and optimizing performance. Synchronization to environmental 24 h oscillations tunes physiological processes optimally with nature. In this review, I briefly describe the principle of the clock mechanism, its synchronization to the environment and consequences on health when the circadian clock is disrupted.

The interrelations among feeding, circadian rhythms and ageing

The master clock located in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus in the brain regulates circadian rhythms in mammals. Similar circadian oscillators have been found in peripheral tissues, such as the liver, intestine and retina. Life span has been previously linked independently to both circadian rhythms and caloric restriction (CR). The mechanisms by which CR attenuates ageing and extends life span are virtually unknown. It has recently been found that the alphaMUPA mice, transgenic mice that exhibit spontaneously reduced eating and live longer compared to their FVB/N wild-type control mice, show high amplitude, appropriately reset circadian rhythms. These pronounced rhythms were found both in clock gene expression in the liver and clock-controlled output systems, such as feeding time and body temperature. Furthermore, it was previously shown that CR could reset the central biological clock in the SCN. As the circadian clock in the SCN controls many physiological and biochemical systems, we suggest that appropriately reset peripheral rhythms could constitute an important mediator of longevity in calorically restricted animals. Thus, we suggest that three parameters, i.e., caloric restriction, circadian rhythms and life span, are interconnected. This surmise is novel, and we provide evidence to support it. Furthermore, we discuss other feeding regimens and their effects on circadian rhythms and/or life span.

Entrainment of the human circadian pacemaker to longer-than-24-h days

Entrainment of the circadian pacemaker to the light:dark cycle is necessary for rhythmic physiological functions to be appropriately timed over the 24-h day. Nonentrainment results in sleep, endocrine, and neurobehavioral impairments. Exposures to intermittent bright light pulses have been reported to phase shift the circadian pacemaker with great efficacy. Therefore, we tested the hypothesis that a modulated light exposure (MLE) with bright light pulses in the evening would entrain subjects to a light:dark cycle 1 h longer than their own circadian period (tau). Twelve subjects underwent a 65-day inpatient study. Individual subject's circadian period was determined in a forced desynchrony protocol. Subsequently, subjects were released into 30 longer-than-24-h days (daylength of tau + 1 h) in one of three light:dark conditions: (i) approximately 25 lux; (ii) approximately 100 lux; and (iii) MLE: approximately 25 lux followed by approximately 100 lux, plus two 45-min bright light pulses of approximately 9,500 lux near the end of scheduled wakefulness. We found that lighting levels of approximately 25 lux were insufficient to entrain all subjects tested. Exposure to approximately 100 lux was sufficient to entrain subjects, although at a significantly wider phase angle compared with baseline. Exposure to MLE was able to entrain the subjects to the imposed sleep-wake cycles but at a phase angle comparable to baseline. These results suggest that MLE can be used to entrain the circadian pacemaker to non-24-h days. The implications of these findings are important because they could be used to treat circadian misalignment associated with space flight and circadian rhythm sleep disorders such as shift-work disorder.

Effects of ageing on the fine distribution of the circadian CLOCK protein in reticular formation neurons

Many biochemical, physiological and behavioural processes, from bacteria to human, exhibit roughly 24 h cyclic oscillations defined as circadian rhythms. However, during ageing, numerous aspects of the circadian biology undergo alterations; in particular, the sleep pattern changes, with more frequent awakenings and shorter sleep time. The basic mechanism of the circadian clock relies on intracellular molecular pathways involving interlocking transcriptional/translational feedback loops, and CLOCK protein, a transcription factor, is essential for normal circadian rhythms. In this study, the fine distribution of CLOCK protein has been analysed, in adult and old rats, at different phases of the daily cycle in the neurons of the medullary reticular formation, involved in the control of the sleep-wake cycle. The results demonstrate quali-quantitative modifications of CLOCK protein in the neurons of old animals, suggesting that such a deregulation of the intracellular clock mechanism may play some role in the degeneration of the sleep-wake circadian cycle.

Disturbance and strategies for reactivation of the circadian rhythm system in aging and Alzheimer's disease

Circadian rhythm disturbances, such as sleep disorders, are frequently seen in aging and are even more pronounced in Alzheimer's disease (AD). Alterations in the biological clock, the suprachiasmatic nucleus (SCN), and the pineal gland during aging and AD are considered to be the biological basis for these circadian rhythm disturbances. Recently, our group found that pineal melatonin secretion and pineal clock gene oscillation were disrupted in AD patients, and surprisingly even in non-demented controls with the earliest signs of AD neuropathology (neuropathological Braak stages I-II), in contrast to non-demented controls without AD neuropathology. Furthermore, a functional disruption of the SCN was observed from the earliest AD stages onwards, as shown by decreased vasopressin mRNA, a clock-controlled major output of the SCN. The observed functional disconnection between the SCN and the pineal from the earliest AD stage onwards seems to account for the pineal clock gene and melatonin changes and underlies circadian rhythm disturbances in AD. This paper further discusses potential therapeutic strategies for reactivation of the circadian timing system, including melatonin and bright light therapy. As the presence of melatonin MT1 receptor in the SCN is extremely decreased in late AD patients, supplementary melatonin in the late AD stages may not lead to clear effects on circadian rhythm disorders.

Mouse model of diffuse brain damage following anoxia, evaluated by a new assay of generalized arousal

Diffuse brain damage following anoxia due to cardiac failure, drowning, carbon monoxide exposure or other accidents constitutes a major medical problem. We have created a novel mouse model using the breathing of pure nitrogen, followed by a recently developed assay that reflects an operational definition of generalized arousal. The operational definition is precise, complete, and leads to quantitative, physical measures in a genetically tractable animal. Exposure to pure nitrogen for controlled periods had a surprising bifurcate effect: about half the mice survived with neurological measures that were virtually normal while the other half died. The new assay detected behavioral deficits unrevealed by neurological screening. Two important features of the results were that (i) deficits were not equal across the circadian cycle, and (ii) deficits were not equal across all the measures within the operational definition of arousal. Specific voluntary motor measurements were decreased in a manner that depended on the phase of the circadian cycle. Sensory responses were also decreased, with an emphasis on vertical movement responses; but, interestingly, fear learning was not damaged. This study establishes the first useful approach to diffuse brain damage in a genetically tractable animal. The model and its outcome measurements will be useful during future attempts at amelioration of acquired neurological disabilities following hypoxic-ischemic injuries.

c-Jun N-terminal kinase inhibitor SP600125 modulates the period of mammalian circadian rhythms

Circadian rhythms are endogenous cycles with periods close to, but not exactly equal to, 24 h. In mammals, circadian rhythms are generated in the suprachiasmatic nucleus (SCN) of the hypothalamus as well as several peripheral cell types, such as fibroblasts. Protein kinases are key regulators of the circadian molecular machinery. We investigated the role of the c-Jun N-terminal kinases (JNK), which belong to the mitogen-activated protein kinases family, in the regulation of circadian rhythms. In rat-1 fibroblasts, the p46 kDa, but not the p54 kDa, isoforms of JNK expressed circadian rhythms in phosphorylation. The JNK-inhibitor SP600125 dose-dependently extended the period of Period1-luciferase rhythms in rat-1 fibroblasts from 24.23+/-0.17-31.48+/-0.07 h. This treatment also dose-dependently delayed the onset of the bioluminescence rhythms. The effects of SP600125 on explant cultures from Period1-luciferase transgenic mice and Period2(Luciferase) knockin mice appeared tissue-specific. SP600125 lengthened the period in SCN, pineal gland, and lung explants in Period1-luciferase and Period2(Luciferase) mice. However, in the kidneys circadian rhythms were abolished in Period1-luciferase, while circadian rhythms were not affected by SP600125 treatment in Period2(Luciferase) mice. Valproic acid, already known to affect period length, enhanced JNK phosphorylation and, as predicted, shortened the period of the Period1-bioluminescence rhythms in rat-1 fibroblasts. In conclusion, our results showed that SP600125 treatment, as well as valproic acid, alters JNK phosphorylation levels, and modulates the period length in various tissues. We conclude that JNK phosphorylation levels may help to set the period length of mammalian circadian rhythms.

Resetting of central and peripheral circadian oscillators in aged rats

The mammalian circadian timing system is affected by aging. Analysis of the suprachiasmatic nucleus (SCN) and of other circadian oscillators reveals age-related changes which are most profound in extra-SCN tissues. Some extra-SCN oscillators appear to stop oscillating in vivo or display altered phase relationships. To determine whether the dynamic behavior of circadian oscillators is also affected by aging we studied the resetting behavior of the Period1 transcriptional rhythm of peripheral and central oscillators in response to a 6h advance or delay in the light schedule. We employed a transgenic rat with a luciferase reporter to allow for real-time measurements of transcriptional rhythmicity. While phase resetting in the SCN following an advance or a delay of the light cycle appears nearly normal in 2-year-old rats, resynchronization of the liver was seriously disrupted. In addition, the arcuate nucleus and pineal gland exhibited faster resetting in aged rats relative to 4-8-month-old controls. The consequences of these deficits are unknown, but may contribute to organ and brain diseases in the aged as well as the health problems that are common in older shift-workers.

Loss of responsiveness to melatonin in the aging mouse suprachiasmatic nucleus

Melatonin modulates circadian rhythms via the hypothalamic suprachiasmatic nucleus (SCN). One of the most robust assays for SCN melatonin receptor activation in mice is the inhibition of PACAP-induced phosphorylation of the transcription factor Ca(2+)/cAMP responsive element binding protein (CREB). To assess the effect of aging on responsiveness to melatonin, SCN slices from mice of different ages were prepared and treated with PACAP alone or PACAP plus melatonin. CREB phosphorylation state was assessed by immunohistochemistry. In SCN slices from young (2-4-month-old) mice, melatonin reduced the level of phospho-CREB immunoreactivity following PACAP treatment in a dose-dependent manner. In SCN slices from aged mice (19-22 months of age), PACAP alone induced comparable levels of phospho-CREB, but melatonin treatment failed to inhibit the PACAP-induced CREB phosphorylation. The results indicate an age-related loss of sensitivity to melatonin in the SCN. The findings are discussed in the context of the impact of endogenous and exogenous melatonin on sleep in elderly humans.

New reporter system for Per1 and Bmal1 expressions revealed self-sustained circadian rhythms in peripheral tissues

A new reporter system for monitoring expressions of two clock genes, Per1 and Bmal1, from a single tissue in culture was developed in mice. Reporters are Vargula hilgendorfii luciferase (VL) and firefly luciferase (FL), whose activities are increased in parallel with Per1 and Bmal1 expressions, respectively. Formal properties of the circadian system in transgenic mice are indistinguishable from those in wild-type animals. Circadian rhythms in Per1-VL and Bmal1-FL in the suprachiasmatic nucleus (SCN) were robust and anti-phasic, although they were phase delayed by 4-8 h as compared with circadian rhythms in respective transcript levels in vivo. In peripheral tissues such as liver, circadian rhythms in Bmal1-FL persisted for more than 3 weeks. In the course of prolonged culture, circadian rhythms apparently damped out, but were restored immediately by refreshment of the culture medium. Restoration of the circadian rhythm is unlikely to be due to resetting of desynchronized population oscillation, because peripheral circadian rhythms did not show a type 0 phase response curve (PRC) for medium refreshment, a requirement for instantaneous resetting of circadian oscillation. Long-term persistence of circadian oscillation in spite of external perturbations supports an idea that circadian oscillations in peripheral tissues are self-sustained.

A Drosophila model for age-associated changes in sleep:wake cycles

One of the most consistent behavioral changes that occurs with age in humans is the loss of sleep consolidation. This can be quite disruptive and yet little is known about its underlying basis. To better understand the effects of aging on sleep:wake cycles, we sought to study this problem in Drosophila melanogaster, a powerful system for research on aging and behavior. By assaying flies of different ages as well as monitoring individual flies constantly over the course of their lifetime, we found that the strength of sleep:wake cycles decreased and that sleep became more fragmented with age in Drosophila. These changes in sleep:wake cycles became faster or slower with manipulations of ambient temperature that decreased or increased lifespan, respectively, demonstrating that they are a function of physiological rather than chronological age. The effect of temperature on lifespan was not mediated by changes in overall activity level or sleep amount. Flies treated with the oxidative stress-producing reagent paraquat showed a breakdown of sleep:wake cycles similar to that seen with aging, leading us to propose that the accumulation of oxidative damage with age contributes to the changes in rhythm and sleep. Together, these findings establish Drosophila as a valuable model for studying age-associated sleep fragmentation and breakdown of rhythm strength, and indicate that these changes in sleep:wake cycles are an integral part of the physiological aging process.

Serine 714 might be implicated in the regulation of the phosphorylation in other areas of mPer1 protein

The phosphorylation of mPer proteins may play important roles in the mechanism of the circadian clock via changes in subcellular localization and degradation. However, the mechanism has remained unclear. Previously, we identified three putative casein kinase (CK)1epsilon phosphorylation motif clusters in mPer1. In this work, we examined the role of the phosphorylation of serine residue, Ser(S)714, in mPer1. mPer1 S[714-726]A mutant, in which potential phosphorylation serine residues replaced by alanine residues, is rapidly phosphorylated compared with wild-type mPer1 by CK1epsilon. Coexpression with S[714]G mutant of mPer1 advanced phase of circadian expression of mPer2-luc expression, which was monitored by in vitro bioluminescence system. This result showed that the mPER1 S[714]G mutation affects circadian core oscillator. Considering these, it seems that Ser 714 might be involved in the regulation of the phosphorylation of other sites in mPer1 by CK1epsilon.

Effects of preparation time on phase of cultured tissues reveal complexity of circadian organization

The phases of central (SCN) and peripheral circadian oscillators are held in specific relationships under LD cycles but, in the absence of external rhythmic input, may damp or drift out of phase with each other. Rats exposed to prolonged constant light become behaviorally arrhythmic, perhaps as a consequence of dissociation of phases among SCN cells. The authors asked whether individual central and peripheral circadian oscillators were rhythmic in LL-treated arrhythmic rats and, if rhythmic, what were the phase relationships between them. The authors prepared SCN, pineal gland, pituitary, and cornea cultures from transgenic Period1-luciferaserats whose body temperature and locomotor activity were arrhythmic and from several groups of rhythmic rats held in LD, DD, and short-term LL. The authors measured mPer1gene expression by recording light output with sensitive photomultipliers. Most of the cultures from all groups displayed circadian rhythms. This could reflect persistent rhythmicity in vivo prior to culture or, alternatively, rhythmicity that may have been initiated by the culture procedure. To test this, the authors cultured tissues at 2 different times 12 h apart and asked whether phase of the rhythm was related to culture time. The pineal, pituitary, and SCN cultures showed partial or complete dependence of phase on culture time, while peak phases of the cornea cultures were independent of culture time in rhythmic rats and were randomly distributed regardless of culture time in arrhythmic animals. These results suggest that in behaviorally arrhythmic rats, oscillators in the pineal, pituitary, and SCN had been arrhythmic or severely damped in vivo, while the cornea oscillator was free running. The peak phases of the SCN cultures were particularly sensitive to some aspect of the culture procedure since rhythmicity of SCN cultures from robustly rhythmic LD-entrained rats was strongly influenced when the procedure was carried out at any time except the 2nd half of the day.

Vieillissement et rythmes biologiques chez les primates

All living organisms exhibit rhythmic activities in a wide variety of endocrine and behavioural parameters. These biological rhythms are endogenously generated by a circadian clock, and they are entrained by cyclic variations of environmental factors called synchronizers. Aging is associated with changes in amplitude and temporal organization of many daily and seasonal rhythms. In humans, daily rhythms of sleep, thermoregulation and hormonal secretion are severely altered with aging. Except in humans, studies on primates are scarce. However, age-related effects on biological rhythms are relatively consistent among primate species studied to date, including humans. Therefore, non human primates are of valuable use for such investigations. Most studies have been performed on the Rhesus macaque (longevity 35-40 years) and on the gray mouse lemur (longevity 10-12 years). Like in humans, the rest-activity rhythm becomes fragmented in aged primates, and shows an increased activity during the resting period. Aging induces a decrease in amplitude of the body temperature rhythm and an increase in energy consumption. Various hormonal secretions exhibit a decrease with aging, but the rhythmic components of these declines have not always been depicted. Moreover, changes (amplitude or phase) in daily variations depended of the hormonal secretion tested. Taken together, these results suggest that the biological clock in the brain would be a primary target of aging. The main central clock is located in the suprachiasmatic nucleus of the hypothalamus whose endogenous oscillations are entrained by light. In this brain structure, cellular function and sensitivity to light show drastic changes with age in the mouse lemur. The precise knowledge of age-related alterations of biological rhythms in primates can have important consequences on the development of new treatments to maintain or restore biological rhythmicity in the elderly.

Cellular senescence impairs circadian expression of clock genes in vitro and in vivo

Circadian rhythms are regulated by a set of clock genes that form transcriptional feedback loops and generate circadian oscillation with a 24-hour cycle. Aging alters a broad spectrum of physiological, endocrine, and behavioral rhythms. Although recent evidence suggests that cellular aging contributes to various age-associated diseases, its effects on the circadian rhythms have not been examined. We report here that cellular senescence impairs circadian rhythmicity both in vitro and in vivo. Circadian expression of clock genes in serum-stimulated senescent cells was significantly weaker compared with that in young cells. Introduction of telomerase completely prevented this reduction of clock gene expression associated with senescence. Stimulation by serum activated the cAMP response element-binding protein, but the activation of this signaling pathway was significantly weaker in senescent cells. Treatment with activators of this pathway effectively restored the impaired clock gene expression of senescent cells. When young cells were implanted into young mice or old mice, the implanted cells were effectively entrained by the circadian rhythm of the recipients. In contrast, the entrainment of implanted senescent cells was markedly impaired. These results suggest that senescence decreases the ability of cells to transmit circadian signals to their clocks and that regulation of clock gene expression may be a novel strategy for the treatment of age-associated impairment of circadian rhythmicity.

Molecular mechanism of cell-autonomous circadian gene expression of Period2, a crucial regulator of the mammalian circadian clock

Although circadian transcription of Period2 (Per2) is fundamental for the generation of circadian rhythm, the molecular mechanism remains unclear. Here we report that cell-autonomous circadian transcription of Per2 is driven by two transcriptional elements, one for rhythm generation and the other for phase control. The former contains the E-box-like sequence (CACGTT) that is sufficient and indispensable to drive oscillation, and indeed circadian transcription factors site-specifically bind to it. Furthermore, the nature of this atypical E-box is different from that of the classical circadian E-box. The current feedback loop model is based mainly on Period1. Our results provide not only compelling evidence in support of this model but also an explanation for a general basic mechanism to produce various patterns in the phase and amplitude of cell-autonomous circadian gene expression.

Clock genes of mammalian cells: Practical implications in tissue culture

The clock genes family is expressed by all the somatic cells driving central and peripheral circadian rhythms through transcription/translation feedback loops. The circadian clock provides a local time for a cell and a way to integrate the normal environmental changes to smoothly adapt the cellular machinery to new conditions. The central circadian rhythm is retained in primary cultures by neurons of the suprachiasmatic nuclei. The peripheral circadian rhythms of the other somatic cells are progressively dampened down up to loss unless neuronal signals of the central clock are provided for re-entrainment. Under typical culture conditions (obscurity, 37 +/- 1 degrees C, 5-7% CO(2)), freshly explanted peripheral cells harbor chaotic expression of clock genes for 12-14 h and loose, coordinated oscillating patterns of clock components. Cells of normal or cancerous phenotypes established in culture harbor low levels of clock genes idling up to the re-occurrence of new synchronizer signals. Synchronizers are physicochemical cues (like thermic oscillations, short-term exposure to high concentrations of serum or single medium exchange) able to re-induce molecular oscillations of clock genes. The environmental synchronizers are integrated by response elements located in the promoter region of period genes that drive the central oscillator complex (CLOCK:BMAL1 and NPAS2:BMAL1 heterodimers). Only a few cell lines from different species and lineages have been tested for the existence or the functioning of a circadian clockwork. The best characterized cell lines are the immortalized SCN2.2 neurons of rat suprachiasmatic nuclei for the central clock and the Rat-1 fibroblasts or the NIH/3T3 cells for peripheral clocks. Isolation methods of fragile cell phenotypes may benefit from research on the biological clocks to design improved tissue culture media and new bioassays to diagnose pernicious consequences for health of circadian rhythm alterations.

Aging human circadian rhythms: conventional wisdom may not always be right

This review discusses the ways in which the circadian rhythms of older people are different from those of younger adults. After a brief discussion of clinical issues, the review describes the conventional wisdom regarding age-related changes in circadian rhythms. These can be summarized as four assertions regarding what happens to people as they get older: 1) the amplitude of their circadian rhythms reduces, 2) the phase of their circadian rhythms becomes earlier, 3) their natural free-running period (tau) shortens, and 4) their ability to tolerate abrupt phase shifts (e.g., from jet travel or night work) worsens. The review then discusses the empirical evidence for and against these assertions and discusses some alternative explanations. The conclusions are that although older people undoubtedly have earlier circadian phases than younger adults, and have more trouble coping with shift work and jet lag, evidence for the assertions about rhythm amplitude and tau are, at best, mixed.