The physiological period length of the human circadian clock in vivo is directly proportional to period in human fibroblasts

Background

Diurnal behavior in humans is governed by the period length of a circadian clock in the suprachiasmatic nuclei of the brain hypothalamus. Nevertheless, the cell-intrinsic mechanism of this clock is present in most cells of the body. We have shown previously that for individuals of extreme chronotype (“larks” and “owls”), clock properties measured in human fibroblasts correlated with extreme diurnal behavior.

Methodology/Principal Findings

In this study, we have measured circadian period in human primary fibroblasts taken from normal individuals and, for the first time, compared it directly with physiological period measured in vivo in the same subjects. Human physiological period length was estimated via the secretion pattern of the hormone melatonin in two different groups of sighted subjects and one group of totally blind subjects, each using different methods. Fibroblast period length was measured via cyclical expression of a lentivirally delivered circadian reporter. Within each group, a positive linear correlation was observed between circadian period length in physiology and in fibroblast gene expression. Interestingly, although blind individuals showed on average the same fibroblast clock properties as sighted ones, their physiological periods were significantly longer.

Conclusions/Significance

We conclude that the period of human circadian behaviour is mostly driven by cellular clock properties in normal individuals and can be approximated by measurement in peripheral cells such as fibroblasts. Based upon differences among sighted and blind subjects, we also speculate that period can be modified by prolonged unusual conditions such as the total light deprivation of blindness.     

Familial advanced sleep-phase syndrome: A short-period circadian rhythm variant in humans.

Biological circadian clocks oscillate with an approximately 24-hour period, are ubiquitous, and presumably confer a selective advantage by anticipating the transitions between day and night. The circadian rhythms of sleep, melatonin secretion and body core temperature are thought to be generated by the suprachiasmatic nucleus of the hypothalamus, the anatomic locus of the mammalian circadian clock. Autosomal semi-dominant mutations in rodents with fast or slow biological clocks (that is, short or long endogenous period lengths; tau) are associated with phase-advanced or delayed sleep-wake rhythms, respectively. These models predict the existence of familial human circadian rhythm variants but none of the human circadian rhythm disorders are known to have a familial tendency. Although a slight 'morning lark' tendency is common, individuals with a large and disabling sleep phase-advance are rare. This disorder, advanced sleep-phase syndrome, is characterized by very early sleep onset and offset; only two cases are reported in young adults. Here we describe three kindreds with a profound phase advance of the sleep-wake, melatonin and temperature rhythms associated with a very short tau. The trait segregates as an autosomal dominant with high penetrance. These kindreds represent a well-characterized familial circadian rhythm variant in humans and provide a unique opportunity for genetic analysis of human circadian physiology.     

Deleting the Arntl clock gene in the granular layer of the mouse cerebellum: impact on the molecular circadian clockwork

The suprachiasmatic nucleus houses the central circadian clock and is characterized by the timely regulated expression of clock genes. However, neurons of the cerebellar cortex also contain a circadian oscillator with circadian expression of clock genes being controlled by the suprachiasmatic nucleus. It has been suggested that the cerebellar circadian oscillator is involved in food anticipation, but direct molecular evidence of the role of the circadian oscillator of the cerebellar cortex is currently unavailable. To investigate the hypothesis that the circadian oscillator of the cerebellum is involved in circadian physiology and food anticipation, we therefore by use of Cre‐LoxP technology generated a conditional knockout mouse with the core clock gene Arntl deleted specifically in granule cells of the cerebellum, since expression of clock genes in the cerebellar cortex is mainly located in this cell type. We here report that deletion of Arntl heavily influences the molecular clock of the cerebellar cortex with significantly altered and arrhythmic expression of other central clock and clock‐controlled genes. On the other hand, daily expression of clock genes in the suprachiasmatic nucleus was unaffected. Telemetric registrations in different light regimes did not detect significant differences in circadian rhythms of running activity and body temperature between Arntl conditional knockout mice and controls. Furthermore, food anticipatory behavior did not differ between genotypes. These data suggest that Arntl is an essential part of the cerebellar oscillator; however, the oscillator of the granular layer of the cerebellar cortex does not control traditional circadian parameters or food anticipation.     

Circadian oscillations of neuropeptide expression in the human biological clock

The mammalian suprachiasmatic nucleus is the principal component of a neural timing system implicated in the temporal organization of circadian and seasonal processes. The present study was performed to analyze the circadian profiles of two major neuropeptidergic cell groups in the human suprachiasmatic nucleus. To that end the brains of 40 human subjects collected at autopsy were investigated. The populations of arginine vasopressin- and vasoactive intestinal polypeptide-expressing neurons, located in the shell and core of the suprachiasmatic nucleus, respectively, showed marked circadian rhythms with an asymmetrical, bimodal waveform. Time series analysis revealed that these circadian cycles in neuronal activity could be described by a composite model consisting of a nonlinear periodic function, with mono- and diphasic cycles. The findings suggest that the 24-h biosynthesis of neuropeptides in the human suprachiasmatic nucleus, being part of the neural output pathway of the clock, is driven by a complex pacemaker system consisting of coupled nonlinear oscillators, in accordance with a multioscillator model of circadian timekeeping.Abbreviations AIC Akaikie's information criterion - ARMA autoregressive moving average - AVP arginine vasopressin - c-fos immediate early gene - Per period gene - SCN suprachiasmatic nucleus - VIP vasoactive intestinal polypeptide     

Extracellular low pH affects circadian rhythm expression in human primary fibroblasts

Circadian rhythm is a fundamental biological system involved in the regulation of various physiological functions. However, little is known about a nature or function of circadian clock in human primary cells. In the present study, we have applied in vitro real time circadian rhythm monitoring to study human clock properties using primary skin fibroblasts. Among factors that affect human physiology, slightly lower extracellular pH was chosen to test its effects on circadian rhythm expression. We established human primary fibroblast cultures obtained from three healthy subjects, stably delivered a circadian reporter gene Bmal1-luciferase, and recorded circadian rhythms in the culture medium at pH 7.2 and 6.7. At pH 7.2, robust and sustained circadian rhythms were observed with average period length 24.47 ± 0.03 h. Such rhythms were also found at pH 6.7; however, period length was significantly shortened to 22.60 ± 0.20, amplitude was increased, and damping rate was decreased. The effect of exposure to low pH on the period length was reversible. The shortened period was unlikely caused by factors affecting cell viability because cell morphology and MTT assay showed no significant difference between the two conditions. In summary, our results showed that the circadian rhythm expression is affected at pH 6.7 in human primary fibroblasts without affecting cell viability.     

Caffeine lengthens circadian rhythms in mice

Although caffeine alters sleep in many animals, whether or not it affects mammalian circadian clocks remains unknown. Here, we found that incubating cultured mammalian cell lines, human osteosarcoma U2OS cells and mouse fibroblast NIH3T3 cells, with caffeine lengthened the period of circadian rhythms. Adding caffeine to ex vivo cultures also lengthened the circadian period in mouse liver explants from Per2::Luciferase reporter gene knockin mice, and caused a phase delay in brain slices containing the suprachiasmatic nucleus (SCN), where the central circadian clock in mammals is located. Furthermore, chronic caffeine consumption ad libitum for a week delayed the phase of the mouse liver clock in vivo under 12 h light–dark conditions and lengthened the period of circadian locomotor rhythms in mice under constant darkness. Our results showed that caffeine alters circadian clocks in mammalian cells in vitro and in the mouse ex vivo and in vivo.     

Recent data about the role of hypothalamic suprachiasmatic nucleus in circadian organization of physiological functions

The suprachiasmatic nucleus is the primary circadian pacemaker in mammals. In turn, the suprachiasmatic nucleus influences circadian physiology, endocrinology and behavior via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus circadian pacemaker may play an important role in psychiatric disorders and in psychotherapeutic drugs effect. In this review, we summarize data about the suprachiasmatic nuclei anatomy, physiology and pharmacological sensitivity.     

Expression of clock genes in human peripheral blood mononuclear cells throughout the sleep/wake and circadian cycles

The rhythmic expression of circadian clock genes in the neurons of the suprachiasmatic nucleus (SCN) underlies the manifestation of endogenous circadian rhythmicity in behavior and physiology. Recent evidence demonstrating rhythmic clock gene expression in non-SCN tissues suggests that functional clocks exist outside the central circadian pacemaker of the brain. In this investigation, the nature of an oscillator in peripheral blood mononuclear cells (PBMCs) is evaluated by assessing clock gene expression throughout both a typical sleep/wake cycle (LD) and during a constant routine (CR). Six healthy men and women aged (mean±SEM) 23.7±1.6 yrs participated in this five-day investigation in temporal isolation. Core body temperature and plasma melatonin concentration were measured as markers of the central circadian pacemaker. The expression of HPER1, HPER2, and HBMAL1 was quantified in PBMCs sampled throughout an uninterrupted 72 h period. The core body temperature minimum and the midpoint of melatonin concentration measured during the CR occurred 2:17±0:20 and 3:24 ±0:09 h before habitual awakening, respectively, and were well aligned to the sleep/wake cycle. HPER1 and HPER2 expression in PBMCs demonstrated significant circadian rhythmicity that peaked early after wake-time and was comparable under LD and CR conditions. HBMAL1 expression was more variable, and peaked in the middle of the wake period under LD conditions and during the habitual sleep period under CR conditions. For the first time, bi-hourly sampling over three consecutive days is used to compare clock gene expression in a human peripheral oscillator under different sleep/wake conditions.     

Circadiane Uhren und Energiemetabolismus

Chrononutrition – circadian clocks and energy metabolism Genetically encoded endogenous clocks regulate 24‐hour rhythms of physiology and behavior. A central pacemaker residing in the suprachiasmatic nucleus synchronizes peripheral clocks found in all tissues with each other and with the external day‐night cycle. One function of circadian clocks is the regulation of energy metabolism via rhythmic activation of tissue‐specific clock‐controlled genes. In the liver, genes involved in glucose and lipid metabolism are regulated in this fashion, while in adipocytes, fatty acid release and adipokine secretion are controlled by the circadian clock. Disruption of circadian rhythms as seen, for example, in shift workers promotes the development of metabolic disorders such as obesity and type‐2 diabetes.     

Emergence of circadian and photoperiodic system level properties from interactions among pacemaker cells

Daily patterns of behavior and physiology in animals in temperate zones often differ substantially between summer and winter. In mammals, this may be a direct consequence of seasonal changes of activity of the suprachiasmatic nucleus (SCN). The purpose of this study was to understand such variation on the basis of the interaction between pacemaker neurons. Computer simulation demonstrates that mutual electrical activation between pacemaker cells in the SCN, in combination with cellular electrical activation by light, is sufficient to explain a variety of circadian phenomena including seasonal changes. These phenomena are: self-excitation, that is, spontaneous development of circadian rhythmicity in the absence of a light-dark cycle; persistent rhythmicity in constant darkness, and loss of circadian rhythmicity in pacemaker output in constant light; entrainment to light-dark cycles; aftereffects of zeitgeber cycles with different periods; adjustment of the circadian patterns to day length; generation of realistic phase response curves to light pulses; and relative independence from day-to-day variation in light intensity. In the model, subsets of cells turn out to be active at specific times of day. This is of functional importance for the exploitation of the SCN to tune specific behavior to specific times of day. Thus, a network of on-off oscillators provides a simple and plausible construct that behaves as a clock with readout for time of day and simultaneously as a clock for all seasons.     

A Circadian Rhythm in Mice that is Unaffected by the Period Mutation at Clock

Circadian rhythms of locomotor behavior were induced in mice using chronic d-amphetamine. The periods of the rhythms at various doses of amphetamine were unaffected by a mutation at the clock locus that lengthens circadian period. Amphetamine-induced rhythms were superimposed on the hostÕs natural circadian rhythm that is driven by the pacemaker in the suprachiasmatic nucleus. The results demonstrate the existence of an alternative mechanism for generating circadian rhythms that does not require the activity of canonical clock genes.     

The role of {beta}-TrCP1 and {beta}-TrCP2 in circadian rhythm generation by mediating degradation of clock protein PER2

The mammalian circadian clock proteins undergo a daily cycle of accumulation followed by phosphorylation and degradation. The mechanism by which clock proteins undergo degradation has not been fully understood. Circadian clock protein PERIOD2 (PER2) is shown to be the potential target of F-box protein beta-TrCP1, a component of ubiquitin E3 ligase. Here, we show that beta-TrCP2 as well as beta-TrCP1 target PER2 protein in vitro. We also identified beta-TrCP binding site (m2) of PER2 being recognized by both beta-TrCP1 and beta-TrCP2. Luciferase-PER2 fusion system revealed that m2 site was responsible for the stability of PER2. The role of beta-TrCP1 and beta-TrCP2 in circadian rhythm generation was analysed by real-time reporter assay revealing that siRNA-mediated suppressions of beta-TrCP1 and/or beta-TrCP2 attenuate circadian oscillations in NIH3T3 cell. beta-TrCP1-deficient mice, however, showed normal period length, light-induced phase-shift response in behaviour and normal expression of PER2, suggesting that beta-TrCP1 is dispensable for the central clock in the suprachiasmatic nucleus. Our study indicates that beta-TrCP1 and beta-TrCP2 were involved in the cell autonomous circadian rhythm generation in culture cells, although the role of beta-TrCP2 in the central clock in the suprachiasmatic nucleus remains to be elucidated.     

Estrogen directly modulates circadian rhythms of PER2 expression in the uterus

Fluctuations in circulating estrogen and progesterone levels associated with the estrous cycle alter circadian rhythms of physiology and behavior in female rodents. Endogenously applied estrogen shortens the period of the locomotor activity rhythm in rodents. We recently found that estrogen implants affect Period (Per) gene expression in the suprachiasmatic nucleus (SCN; central clock) and uterus of rats in vivo. To explore whether estrogen directly influences the circadian clock in the SCN and/or tissues of the reproductive system, we examined the effects of 17beta-estradiol (E(2)) on PER2::LUCIFERASE (PER2::LUC) expression in tissue explant cultures from ovariectomized PER2::LUC knockin mice. E(2) applied to explanted cultures shortened the period of rhythmic PER2::LUC expression in the uterus but did not change the period of PER2::LUC expression in the SCN. Raloxifene, a selective estrogen receptor modulator and known E(2) antagonist in uterine tissues, attenuated the effect of E(2) on the period of the PER2::LUC rhythm in the uterus. These data indicate that estrogen directly affects the timing of the molecular clock in the uterus via an estrogen receptor-mediated response.     

Expression of Clock Genes in Human Peripheral Blood Mononuclear Cells throughout the Sleep/Wake and Circadian Cycles

The rhythmic expression of circadian clock genes in the neurons of the suprachiasmatic nucleus (SCN) underlies the manifestation of endogenous circadian rhythmicity in behavior and physiology. Recent evidence demonstrating rhythmic clock gene expression in non‐SCN tissues suggests that functional clocks exist outside the central circadian pacemaker of the brain. In this investigation, the nature of an oscillator in peripheral blood mononuclear cells (PBMCs) is evaluated by assessing clock gene expression throughout both a typical sleep/wake cycle (LD) and during a constant routine (CR). Six healthy men and women aged (mean±SEM) 23.7±1.6 yrs participated in this five‐day investigation in temporal isolation. Core body temperature and plasma melatonin concentration were measured as markers of the central circadian pacemaker. The expression of HPER1, HPER2, and HBMAL1 was quantified in PBMCs sampled throughout an uninterrupted 72 h period. The core body temperature minimum and the midpoint of melatonin concentration measured during the CR occurred 2:17±0:20 and 3:24 ±0:09 h before habitual awakening, respectively, and were well aligned to the sleep/wake cycle. HPER1 and HPER2 expression in PBMCs demonstrated significant circadian rhythmicity that peaked early after wake‐time and was comparable under LD and CR conditions. HBMAL1 expression was more variable, and peaked in the middle of the wake period under LD conditions and during the habitual sleep period under CR conditions. For the first time, bi‐hourly sampling over three consecutive days is used to compare clock gene expression in a human peripheral oscillator under different sleep/wake conditions.     

Long-Lasting Effects of Sepsis on Circadian Rhythms in the Mouse

Daily patterns of activity and physiology are termed circadian rhythms and are driven primarily by an endogenous biological timekeeping system, with the master clock located in the suprachiasmatic nucleus. Previous studies have indicated reciprocal relationships between the circadian and the immune systems, although to date there have been only limited explorations of the long-term modulation of the circadian system by immune challenge, and it is to this question that we addressed ourselves in the current study. Sepsis was induced by peripheral treatment with lipopolysaccharide (5 mg/kg) and circadian rhythms were monitored following recovery. The basic parameters of circadian rhythmicity (free-running period and rhythm amplitude, entrainment to a light/dark cycle) were unaltered in post-septic animals compared to controls. Animals previously treated with LPS showed accelerated re-entrainment to a 6 hour advance of the light/dark cycle, and showed larger phase advances induced by photic stimulation in the late night phase. Photic induction of the immediate early genes c-FOS, EGR-1 and ARC was not altered, and neither was phase-shifting in response to treatment with the 5-HT-1a/7 agonist 8-OH-DPAT. Circadian expression of the clock gene product PER2 was altered in the suprachiasmatic nucleus of post-septic animals, and PER1 and PER2 expression patterns were altered also in the hippocampus. Examination of the suprachiasmatic nucleus 3 months after treatment with LPS showed persistent upregulation of the microglial markers CD-11b and F4/80, but no changes in the expression of various neuropeptides, cytokines, and intracellular signallers. The effects of sepsis on circadian rhythms does not seem to be driven by cell death, as 24 hours after LPS treatment there was no evidence for apoptosis in the suprachiasmatic nucleus as judged by TUNEL and cleaved-caspase 3 staining. Overall these data provide novel insight into how septic shock exerts chronic effects on the mammalian circadian system.     

Differential effects of two period genes on the physiology and proteomic profiles of mouse anterior tibialis muscles

The molecular components that generate and maintain circadian rhythms of physiology and behavior in mammals are present both in the brain (suprachiasmatic nucleus; SCN) and in peripheral tissues. Examination of mice with targeted disruptions of either mPer1 or mPer2 has shown that these two genes have key roles in the SCN circadian clock. Here we show that loss of the clock gene mPer2 affects forced locomotor performance in mice without altering muscle contractility. A proteomic analysis revealed that the anterior tibialis muscles of the mPer2 knockout mice had higher levels of glycolytic enzymes such as triose phosphate isomerase and enolase than those of either the wild type or mPer1 knockout mice. In addition, the level of expression of HSP90 in the mPer2 mutant mice was also significantly higher than in wildtype mice. These results suggest that the reduced locomotor endurance of the mPer2 knockout mice reflects a greater dependence on anaerobic metabolism under stress conditions, and that the two canonical clock genes, mPer1 and mPer2, play distinct roles in the physiology of skeletal muscle.