A role for clock genes in sleep homeostasis

Effects of circadian misalignment on sleep in mice

Circadian rhythms and sleep-wake history determine sleep duration and intensity, and influence subsequent waking. Previous studies have shown that T cycles - light-dark (LD) cycles differing from 24 h - lead to acute changes in the daily amount and distribution of waking and sleep. However, little is known about the long-term effects of T cycles. Here we performed continuous 10 day recording of electroencephalography (EEG), locomotor activity and core body temperature in C57BL/6 mice under a T20 cycle, to investigate spontaneous sleep and waking at baseline compared with when the circadian clock was misaligned and then re-aligned with respect to the external LD cycle. We found that the rhythmic distribution of sleep was abolished during misalignment, while the time course of EEG slow wave activity (1–4 Hz) was inverted compared to baseline. Although the typical light-dark distribution of NREM sleep was re-instated when animals were re-aligned, slow wave activity during NREM sleep showed an atypical increase in the dark phase, suggesting a long-term effect of T cycles on sleep intensity. Our data show that circadian misalignment results in previously uncharacterised long-term effects on sleep, which may have important consequences for behaviour.

Phenotyping of PER3 variants reveals widespread effects on circadian preference, sleep regulation, and health

Period3 (Per3) is one of the most robustly rhythmic genes in humans and animals. It plays a significant role in temporal organisation in peripheral tissues. The effects of PER3 variants on many phenotypes have been investigated in targeted and genome-wide studies. PER3 variants, especially the human variable number tandem repeat (VNTR), associate with diurnal preference, mental disorders, non-visual responses to light, brain and cognitive responses to sleep loss/circadian misalignment. Introducing the VNTR into mice alters responses to sleep loss and expression of sleep homeostasis-related genes. Several studies were limited in size and some findings were not replicated. Nevertheless, the data indicate a significant contribution of PER3 to sleep and circadian phenotypes and diseases, which may be connected by common pathways. Thus, PER3-dependent altered light sensitivity could relate to high retinal PER3 expression and may contribute to altered brain response to light, diurnal preference and seasonal mood. Altered cognitive responses during sleep loss/circadian misalignment and changes to slow wave sleep may relate to changes in wake/activity-dependent patterns of hypothalamic gene expression involved in sleep homeostasis and neural network plasticity. Comprehensive characterisation of effects of clock gene variants may provide new insights into the role of circadian processes in health and disease.

Cross-species systems analysis identifies gene networks differentially altered by sleep loss and depression

To understand the transcriptomic organization underlying sleep and affective function, we studied a population of (C57BL/6J × 129S1/SvImJ) F2 mice by measuring 283 affective and sleep phenotypes and profiling gene expression across four brain regions. We identified converging molecular bases for sleep and affective phenotypes at both the single-gene and gene-network levels. Using publicly available transcriptomic datasets collected from sleep-deprived mice and patients with major depressive disorder (MDD), we identified three cortical gene networks altered by the sleep/wake state and depression. The network-level actions of sleep loss and depression were opposite to each other, providing a mechanistic basis for the sleep disruptions commonly observed in depression, as well as the reported acute antidepressant effects of sleep deprivation. We highlight one particular network composed of circadian rhythm regulators and neuronal activity-dependent immediate-early genes. The key upstream driver of this network, Arc, may act as a nexus linking sleep and depression. Our data provide mechanistic insights into the role of sleep in affective function and MDD.

Sleep homeostasis and the circadian clock: Do the circadian pacemaker and the sleep homeostat influence each other's functioning?

Sleep is regulated by a homeostatic and a circadian process. Together these two processes determine most aspects of sleep and related variables like sleepiness and alertness. The two processes are known to be able to work independently, but also to both influence sleep and sleep related variables in an additive or more complex manner. The question remains whether the two processes are directly influencing each other. The present review summarizes evidence from behavioural and electroencephalographic determined sleep, electrophysiology, gene knock out mouse models, and mathematical modelling to explore whether sleep homeostasis can influence circadian clock functioning and vice versa. There is a multitude of data available showing parallel action or influence of sleep homeostatic mechanisms and the circadian clock on several objective and subjective variables related to sleep and alertness. However, the evidence of a direct influence of the circadian clock on sleep homeostatic mechanisms is sparse and more research is needed, particularly applying longer sleep deprivations that include a second night. The strongest evidence of an influence of sleep homeostatic mechanisms on clock functioning comes from sleep deprivation experiments, demonstrating an attenuation of phase shifts of the circadian rhythm to light pulses when sleep homeostatic pressure is increased. The data suggest that the circadian clock is less susceptible to light when sleep pressure is high. The available data indicate that a strong central clock will induce periods of deep sleep, which in turn will strengthen clock function. Both are therefore important for health and wellbeing. Weakening of one will also hamper functioning of the other. Shift work and jet lag are situations where one tries to adapt to zeitgebers in a condition where sleep is compromised. Adaptation to zeitgebers may be improved by introducing nap schedules to reduce sleep pressure, and through that increasing clock susceptibility to light.

Mutual influence of sleep and circadian clocks on physiology and cognition

The 24-h sleep-wake cycle is one of the most prominent outputs of the circadian clock system. At the same time, changes in sleep-wake behavior feedback on behavioral and physiological circadian rhythms, thus altering the coordination of the body's clock network. Sleep and circadian rhythm disruption have similar physiological endpoints including metabolic, cognitive, and immunologic impairments. This raises the question to which extent these phenomena are causally linked. In this review, we summarize different physiologic outcomes of sleep deprivation and mistimed sleep and discuss the experimental evidence for a mediating role of the circadian clock machinery in this context.

The endogenous circadian clock programs animals to eat at certain times of the 24-hour day: What if we ignore the clock?

The discovery of the molecular mechanisms underlying the circadian clock, which functions in virtually every cell throughout the body to coordinate biological processes to anticipate and better adapt to daily rhythmic changes in the environment, is one of the major biomedical breakthroughs in the 20th century. Twenty years after this breakthrough, the biomedical community is now at a new frontier to incorporate the circadian clock mechanisms into many areas of biomedical research, as studies continue to reveal an important role of the circadian clock in a wide range of biological functions and diseases. A forefront of this exciting area is the research of interactions between the clock and energy metabolism. In this review, we summarize animal and human studies linking disruptions of the circadian clock, either environmental or genetic, to metabolic dysfunctions associated with obesity, diabetes, and other metabolic disorders. We also discuss how these advances in circadian biology may pave the way to revolutionize clinical practice in the era of precision medicine.

Neuronal Mechanisms for Sleep/Wake Regulation and Modulatory Drive

Humans have been fascinated by sleep for millennia. After almost a century of scientific interrogation, significant progress has been made in understanding the neuronal regulation and functions of sleep. The application of new methods in neuroscience that enable the analysis of genetically defined neuronal circuits with unprecedented specificity and precision has been paramount in this endeavor. In this review, we first discuss electrophysiological and behavioral features of sleep/wake states and the principal neuronal populations involved in their regulation. Next, we describe the main modulatory drives of sleep and wakefulness, including homeostatic, circadian, and motivational processes. Finally, we describe a revised integrative model for sleep/wake regulation.

The after-hours circadian mutant has reduced phenotypic plasticity in behaviors at multiple timescales and in sleep homeostasis

Circadian clock is known to adapt to environmental changes and can significantly influence cognitive and physiological functions. In this work, we report specific behavioral, cognitive, and sleep homeostatic defects in the after hours (Afh) circadian mouse mutant, which is characterized by lengthened circadian period. We found that the circadian timing irregularities in Afh mice resulted in higher interval timing uncertainty and suboptimal decisions due to incapability of processing probabilities. Our phenotypic observations further suggested that Afh mutants failed to exhibit the necessary phenotypic plasticity for adapting to temporal changes at multiple time scales (seconds-to-minutes to circadian). These behavioral effects of Afh mutation were complemented by the specific disruption of the Per/Cry circadian regulatory complex in brain regions that govern food anticipatory behaviors, sleep, and timing. We derive statistical predictions, which indicate that circadian clock and sleep are complementary processes in controlling behavioral/cognitive performance during 24 hrs. The results of this study have pivotal implications for understanding how the circadian clock modulates sleep and behavior.

Motor-Activity Markers of Circadian Timekeeping Are Related to Ketamine's Rapid Antidepressant Properties

BACKGROUND

The rapid clinical antidepressant effects of the glutamatergic modulator ketamine may be due to its ability to restore synaptic plasticity and related effects on sleep-wake and circadian systems. Preclinical studies indicate that ketamine alters expression of circadian clock-associated molecules, and clinical studies of ketamine on plasticity-related biomarkers further suggest an association with sleep slow waves and sleep homeostasis.

METHODS

Wrist-activity monitors were used to examine the pharmacologic and rapid antidepressant effects of ketamine on markers of circadian timekeeping (amplitude and timing) in mood disorders. Circadian amplitude and timing of activity at baseline, postinfusion day 1 (D1), and day 3 (D3) were measured in 51 patients with major depressive disorder or bipolar disorder.

RESULTS

Compared with either placebo or baseline, a mood-independent decrease of the central circadian value (mesor) was present on D1 after ketamine treatment. Mood-associated circadian effects between rapid (D1) responders and nonresponders were found at baseline, D1, and D3. At baseline, a phase-advanced activity pattern and lower mesor distinguished subsequent responders from nonresponders. On D1, ketamine nonresponders had a lower mesor and a blunted 24-hour amplitude relative to baseline. On D3, patients with a persisting clinical response exhibited a higher amplitude and mesor compared with nonresponders.

CONCLUSIONS

The findings are the first to demonstrate an association between ketamine's clinical antidepressant effects and circadian timekeeping. The results suggest that traitlike circadian activity patterns indicate rapid mood response to ketamine, and that mediators of continuing ketamine-induced mood changes include altered timing and amplitude of the circadian system.

Sleep, Health, and Society

Biological needs for sleep are met by engaging in behaviors that are largely influenced by the environment, social norms and demands, and societal influences and pressures. Insufficient sleep duration and sleep disorders such as insomnia and sleep apnea are highly prevalent in the US population. This article outlines some of these downstream factors, including cardiovascular and metabolic disease risk, neurocognitive dysfunction, and mortality, as well as societal factors such as age, sex, race/ethnicity, and socioeconomics. This review also discusses societal factors related to sleep, such as globalization, health disparities, public policy, public safety, and changing patterns of use of technology.

Ketamine-Induced Glutamatergic Mechanisms of Sleep and Wakefulness: Insights for Developing Novel Treatments for Disturbed Sleep and Mood

Ketamine, a drug with rapid antidepressant effects and well-described effects on slow wave sleep (SWS), is a useful intervention for investigating sleep-wake mechanisms involved in novel therapeutics. The drug rapidly (within minutes to hours) reduces depressive symptoms in individuals with major depressive disorder (MDD) or bipolar disorder (BD), including those with treatment-resistant depression. Ketamine treatment elevates extracellular glutamate in the prefrontal cortex. Glutamate, in turn, plays a critical role as a proximal element in a ketamine-initiated molecular cascade that increases synaptic strength and plasticity, which ultimately results in rapidly improved mood. In MDD, rapid antidepressant response to ketamine is related to decreased waking as well as increased total sleep, SWS, slow wave activity (SWA), and rapid eye movement (REM) sleep. Ketamine also increases brain-derived neurotrophic factor (BDNF) levels. In individuals with MDD, clinical response to ketamine is predicted by low baseline delta sleep ratio, a measure of deficient early night production of SWS. Notably, there are important differences between MDD and BD that may be related to the effects of diagnosis or of mood stabilizers. Consistent with its effects on clock-associated molecules, ketamine alters the timing and amplitude of circadian activity patterns in rapid responders versus non-responders with MDD, suggesting that it affects mood-dependent central neural circuits. Molecular interactions between sleep homeostasis and clock genes may mediate the rapid and durable elements of clinical response to ketamine and its active metabolite.

Cell adhesion molecules and sleep

Cell adhesion molecules (CAMs) play essential roles in the central nervous system, where some families are involved in synaptic development and function. These synaptic adhesion molecules (SAMs) are involved in the regulation of synaptic plasticity, and the formation of neuronal networks. Recent findings from studies examining the consequences of sleep loss suggest that these molecules are candidates to act in sleep regulation. This review highlights the experimental data that lead to the identification of SAMs as potential sleep regulators, and discusses results supporting that specific SAMs are involved in different aspects of sleep regulation. Further, some potential mechanisms by which SAMs may act to regulate sleep are outlined, and the proposition that these molecules may serve as molecular machinery in the two sleep regulatory processes, the circadian and homeostatic components, is presented. Together, the data argue that SAMs regulate the neuronal plasticity that underlies sleep and wakefulness.

The Zfhx3-Mediated Axis Regulates Sleep and Interval Timing in Mice

An AT motif-dependent axis, modulated by the transcription factor Zfhx3, influences the circadian clock in mice. In particular, gain of function of Zfhx3 significantly shortens circadian rhythms and alters the transcriptional activity of an important class of neuropeptides that controls intercellular signaling in the suprachiasmatic nucleus (SCN) of the hypothalamus. The ZFHX3/AT axis revealed an important, largely cell-nonautonomous control of the circadian clock. Here, by studying the recently identified circadian mouse mutant Zfhx3^Sci/+, we identify significant effects on sleep homeostasis, a phenomenon that is outside the canonical circadian clock system and that is modulated by the activity of those neuropeptides at a circuit level. We show that the Zfhx3^Sci/+ mutation accelerates the circadian clock at both the hourly scale (i.e., advancing circadian rhythms) and the seconds-to-minutes scale (i.e., anticipating behavioral responses) in mice. The in vivo results are accompanied by a significant presence of sleep targets among protein-protein interactions of the Zfhx3^Sci/+-dependent network.

Circadian rhythms and attention deficit hyperactivity disorder: The what, the when and the why

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental condition characterised by impulsivity, inattention and hyperactivity. Aside from these core psychopathologies, sleep disturbances are found to be highly comorbid with ADHD, and indeed dysregulated sleep may contribute to some of the symptoms of the disorder. It is not clear how sleep disturbances come to be so common in ADHD, but one putative mechanism is through the circadian timekeeping system. This system underpins the generation of near 24-hour rhythms in a host of physiological, behavioural and psychological parameters, and is a key determinant of the sleep/wake cycle. In this paper we review the evidence for sleep and circadian rhythm disturbance in ADHD, examine the possible mechanistic links between these factors and the disorder and discuss future directions through which the circadian clock can be targetted for ADHD symptom relief.

Sleep as spatiotemporal integration of biological processes that evolved to periodically reinforce neurodynamic and metabolic homeostasis: The 2m3d paradigm of sleep

Sleep continues to perplex scientists and researchers. Despite decades of sleep research, we still lack a clear understanding of the biological functions and evolution of sleep. In this review, we will examine sleep from a functional and phylogenetic perspective and describe some important conceptual gaps in understanding sleep. Classical theories of the biology and evolution of sleep emphasize sensory activation, energy balance, and metabolic homeostasis. Advances in electrophysiology, functional neuroimaging, and neuroplasticity allow us to view sleep within the framework of neural dynamics. With this paradigm shift, we have come to realize the importance of neurodynamic homeostasis in shaping the biology of sleep. Evidently, animals sleep to achieve neurodynamic and metabolic homeostasis. We are not aware of any framework for understanding sleep where neurodynamic, metabolic, homeostatic, chronophasic, and afferent variables are all taken into account. This motivated us to propose the two-mode three-drive (2m3d) paradigm of sleep. In the 2m3d paradigm, local neurodynamic/metabolic (N/M) processes switch between two modes-m0 and m1-in response to three drives-afferent, chronophasic, and homeostatic. The spatiotemporal integration of local m0/m1 operations gives rise to the global states of sleep and wakefulness. As a framework of evolution, the 2m3d paradigm allows us to view sleep as a robust adaptive strategy that evolved so animals can periodically reinforce neurodynamic and metabolic homeostasis while remaining sensitive to their internal and external environment.

Altered Sleep Homeostasis in Rev-erbα Knockout Mice

STUDY OBJECTIVES

The nuclear receptor REV-ERBα is a potent, constitutive transcriptional repressor critical for the regulation of key circadian and metabolic genes. Recently, REV-ERBα's involvement in learning, neurogenesis, mood, and dopamine turnover was demonstrated suggesting a specific role in central nervous system functioning. We have previously shown that the brain expression of several core clock genes, including Rev-erbα, is modulated by sleep loss. We here test the consequences of a loss of REV-ERBα on the homeostatic regulation of sleep.

METHODS

EEG/EMG signals were recorded in Rev-erbα knockout (KO) mice and their wild type (WT) littermates during baseline, sleep deprivation, and recovery. Cortical gene expression measurements after sleep deprivation were contrasted to baseline.

RESULTS

Although baseline sleep/wake duration was remarkably similar, KO mice showed an advance of the sleep/wake distribution relative to the light-dark cycle. After sleep onset in baseline and after sleep deprivation, both EEG delta power (1-4 Hz) and sleep consolidation were reduced in KO mice indicating a slower increase of homeostatic sleep need during wakefulness. This slower increase might relate to the smaller increase in theta and gamma power observed in the waking EEG prior to sleep onset under both conditions. Indeed, the increased theta activity during wakefulness predicted delta power in subsequent NREM sleep. Lack of Rev-erbα increased Bmal1, Npas2, Clock, and Fabp7 expression, confirming the direct regulation of these genes by REV-ERBα also in the brain.

CONCLUSIONS

Our results add further proof to the notion that clock genes are involved in sleep homeostasis. Because accumulating evidence directly links REV-ERBα to dopamine signaling the altered homeostatic regulation of sleep reported here are discussed in that context.

Deletion of the Snord116/SNORD116 Alters Sleep in Mice and Patients with Prader-Willi Syndrome

STUDY OBJECTIVES

Sleep-wake disturbances are often reported in Prader-Willi syndrome (PWS), a rare neurodevelopmental syndrome that is associated with paternally-expressed genomic imprinting defects within the human chromosome region 15q11-13. One of the candidate genes, prevalently expressed in the brain, is the small nucleolar ribonucleic acid-116 (SNORD116). Here we conducted a translational study into the sleep abnormalities of PWS, testing the hypothesis that SNORD116 is responsible for sleep defects that characterize the syndrome.

METHODS

We studied sleep in mutant mice that carry a deletion of Snord116 at the orthologous locus (mouse chromosome 7) of the human PWS critical region (PWScr). In particular, we assessed EEG and temperature profiles, across 24-h, in PWScr (m+/p-) heterozygous mutants compared to wild-type littermates. High-resolution magnetic resonance imaging (MRI) was performed to explore morphoanatomical differences according to the genotype. Moreover, we complemented the mouse work by presenting two patients with a diagnosis of PWS and characterized by atypical small deletions of SNORD116. We compared the individual EEG parameters of patients with healthy subjects and with a cohort of obese subjects.

RESULTS

By studying the mouse mutant line PWScr(m+/p-), we observed specific rapid eye movement (REM) sleep alterations including abnormal electroencephalograph (EEG) theta waves. Remarkably, we observed identical sleep/EEG defects in the two PWS cases. We report brain morphological abnormalities that are associated with the EEG alterations. In particular, mouse mutants have a bilateral reduction of the gray matter volume in the ventral hippocampus and in the septum areas, which are pivotal structures for maintaining theta rhythms throughout the brain. In PWScr(m+/p-) mice we also observed increased body temperature that is coherent with REM sleep alterations in mice and human patients.

CONCLUSIONS

Our study indicates that paternally expressed Snord116 is involved in the 24-h regulation of sleep physiological measures, suggesting that it is a candidate gene for the sleep disturbances that most individuals with PWS experience.

Cross-talk between circadian clocks, sleep-wake cycles, and metabolic networks: Dispelling the darkness

Integration of knowledge concerning circadian rhythms, metabolic networks, and sleep‐wake cycles is imperative for unraveling the mysteries of biological cycles and their underlying mechanisms. During the last decade, enormous progress in circadian biology research has provided a plethora of new insights into the molecular architecture of circadian clocks. However, the recent identification of autonomous redox oscillations in cells has expanded our view of the clockwork beyond conventional transcription/translation feedback loop models, which have been dominant since the first circadian period mutants were identified in fruit fly. Consequently, non‐transcriptional timekeeping mechanisms have been proposed, and the antioxidant peroxiredoxin proteins have been identified as conserved markers for 24‐hour rhythms. Here, we review recent advances in our understanding of interdependencies amongst circadian rhythms, sleep homeostasis, redox cycles, and other cellular metabolic networks. We speculate that systems‐level investigations implementing integrated multi‐omics approaches could provide novel mechanistic insights into the connectivity between daily cycles and metabolic systems.

Sleep-Wake Regulation and Its Impact on Working Memory Performance: The Role of Adenosine

The sleep-wake cycle is regulated by a fine-tuned interplay between sleep-homeostatic and circadian mechanisms. Compelling evidence suggests that adenosine plays an important role in mediating the increase of homeostatic sleep pressure during time spent awake and its decrease during sleep. Here, we summarize evidence that adenosinergic mechanisms regulate not only the dynamic of sleep pressure, but are also implicated in the interaction of homeostatic and circadian processes. We review how this interaction becomes evident at several levels, including electrophysiological data, neuroimaging studies and behavioral observations. Regarding complex human behavior, we particularly focus on sleep-wake regulatory influences on working memory performance and underlying brain activity, with a specific emphasis on the role of adenosine in this interplay. We conclude that a change in adenosinergic mechanisms, whether exogenous or endogenous, does not only impact on sleep-homeostatic processes, but also interferes with the circadian timing system.

The two-process model of sleep regulation: a reappraisal

In the last three decades the two-process model of sleep regulation has served as a major conceptual framework in sleep research. It has been applied widely in studies on fatigue and performance and to dissect individual differences in sleep regulation. The model posits that a homeostatic process (Process S) interacts with a process controlled by the circadian pacemaker (Process C), with time-courses derived from physiological and behavioural variables. The model simulates successfully the timing and intensity of sleep in diverse experimental protocols. Electrophysiological recordings from the suprachiasmatic nuclei (SCN) suggest that S and C interact continuously. Oscillators outside the SCN that are linked to energy metabolism are evident in SCN-lesioned arrhythmic animals subjected to restricted feeding or methamphetamine administration, as well as in human subjects during internal desynchronization. In intact animals these peripheral oscillators may dissociate from the central pacemaker rhythm. A sleep/fast and wake/feed phase segregate antagonistic anabolic and catabolic metabolic processes in peripheral tissues. A deficiency of Process S was proposed to account for both depressive sleep disturbances and the antidepressant effect of sleep deprivation. The model supported the development of novel non-pharmacological treatment paradigms in psychiatry, based on manipulating circadian phase, sleep and light exposure. In conclusion, the model remains conceptually useful for promoting the integration of sleep and circadian rhythm research. Sleep appears to have not only a short-term, use-dependent function; it also serves to enforce rest and fasting, thereby supporting the optimization of metabolic processes at the appropriate phase of the 24-h cycle.

Impact of Sleep and Circadian Disruption on Energy Balance and Diabetes: A Summary of Workshop Discussions

A workshop was held at the National Institute for Diabetes and Digestive and Kidney Diseases with a focus on the impact of sleep and circadian disruption on energy balance and diabetes. The workshop identified a number of key principles for research in this area and a number of specific opportunities. Studies in this area would be facilitated by active collaboration between investigators in sleep/circadian research and investigators in metabolism/diabetes. There is a need to translate the elegant findings from basic research into improving the metabolic health of the American public. There is also a need for investigators studying the impact of sleep/circadian disruption in humans to move beyond measurements of insulin and glucose and conduct more in-depth phenotyping. There is also a need for the assessments of sleep and circadian rhythms as well as assessments for sleep-disordered breathing to be incorporated into all ongoing cohort studies related to diabetes risk. Studies in humans need to complement the elegant short-term laboratory-based human studies of simulated short sleep and shift work etc. with studies in subjects in the general population with these disorders. It is conceivable that chronic adaptations occur, and if so, the mechanisms by which they occur needs to be identified and understood. Particular areas of opportunity that are ready for translation are studies to address whether CPAP treatment of patients with pre-diabetes and obstructive sleep apnea (OSA) prevents or delays the onset of diabetes and whether temporal restricted feeding has the same impact on obesity rates in humans as it does in mice.

Twist1 Is a TNF-Inducible Inhibitor of Clock Mediated Activation of Period Genes

Background

Activation of the immune system affects the circadian clock. Tumor necrosis factor (TNF) and Interleukin (IL)-1β inhibit the expression of clock genes including Period (Per) genes and the PAR-bZip clock-controlled gene D-site albumin promoter-binding protein (Dbp). These effects are due to cytokine-induced interference of E-box mediated transcription of clock genes. In the present study we have assessed the two E-box binding transcriptional regulators Twist1 and Twist2 for their role in cytokine induced inhibition of clock genes.

Methods

The expression of the clock genes Per1, Per2, Per3 and of Dbp was assessed in NIH-3T3 mouse fibroblasts and the mouse hippocampal neuronal cell line HT22. Cells were treated for 4h with TNF and IL-1β. The functional role of Twist1 and Twist2 was assessed by siRNAs against the Twist genes and by overexpression of TWIST proteins. In luciferase (luc) assays NIH-3T3 cells were transfected with reporter gene constructs, which contain a 3xPer1 E-box or a Dbp E-box. Quantitative chromatin immunoprecipitation (ChIP) was performed using antibodies to TWIST1 and CLOCK, and the E-box consensus sequences of Dbp (CATGTG) and Per1 E-box (CACGTG).

Results

We report here that siRNA against Twist1 protects NIH-3T3 cells and HT22 cells from down-regulation of Period and Dbp by TNF and IL-1β. Overexpression of Twist1, but not of Twist2, mimics the effect of the cytokines. TNF down-regulates the activation of Per1-3xE-box-luc, the effect being prevented by siRNA against Twist1. Overexpression of Twist1, but not of Twist2, inhibits Per1-3xE-box-luc or Dbp-E-Box-luc activity. ChIP experiments show TWIST1 induction by TNF to compete with CLOCK binding to the E-box of Period genes and Dbp.

Conclusion

Twist1 plays a pivotal role in the TNF mediated suppression of E-box dependent transactivation of Period genes and Dbp. Thereby Twist1 may provide a link between the immune system and the circadian timing system.

In Vivo Imaging of the Central and Peripheral Effects of Sleep Deprivation and Suprachiasmatic Nuclei Lesion on PERIOD-2 Protein in Mice

STUDY OBJECTIVES

That sleep deprivation increases the brain expression of various clock genes has been well documented. Based on these and other findings we hypothesized that clock genes not only underlie circadian rhythm generation but are also implicated in sleep homeostasis. However, long time lags have been reported between the changes in the clock gene messenger RNA levels and their encoded proteins. It is therefore crucial to establish whether also protein levels increase within the time frame known to activate a homeostatic sleep response. We report on the central and peripheral effects of sleep deprivation on PERIOD-2 (PER2) protein both in intact and suprachiasmatic nuclei-lesioned mice.

DESIGN

In vivo and in situ PER2 imaging during baseline, sleep deprivation, and recovery.

SETTINGS

Mouse sleep-recording facility.

PARTICIPANTS

Per2::Luciferase knock-in mice.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

Six-hour sleep deprivation increased PER2 not only in the brain but also in liver and kidney. Remarkably, the effects in the liver outlasted those observed in the brain. Within the brain the increase in PER2 concerned the cerebral cortex mainly, while leaving suprachiasmatic nuclei (SCN) levels unaffected. Against expectation, sleep deprivation did not increase PER2 in the brain of arrhythmic SCN-lesioned mice because of higher PER2 levels in baseline. In contrast, liver PER2 levels did increase in these mice similar to the sham and partially lesioned controls.

CONCLUSIONS

Our results stress the importance of considering both sleep-wake dependent and circadian processes when quantifying clock-gene levels. Because sleep deprivation alters PERIOD-2 in the brain as well as in the periphery, it is tempting to speculate that clock genes constitute a common pathway mediating the shared and well-known adverse effects of both chronic sleep loss and disrupted circadian rhythmicity on metabolic health.

How does healthy aging impact on the circadian clock?

Circadian rhythms are recurring patterns in a host of physiological and other parameters that recur with periods of near 24 h. These rhythms reflect the temporal organization of an organism's homeostatic control systems and as such are key processes in ensuring optimal physiological performance. Dysfunction of circadian processes is linked with adverse health conditions. In this review we highlight the evidence that normal, healthy aging is associated with changes in the circadian system; we examine the molecular mechanisms through which such changes may arise, discuss whether more robust circadian function is a predictor of longevity and highlight the role of circadian rhythms in age-related diseases. Overall, the literature shows that aging is associated with marked changes in circadian processes, both at the behavioral and molecular levels, and the molecular mechanisms through which such changes arise remain to be elucidated, but may involve inflammatory process, redox homeostasis and epigenetic modifications. Understanding the nature of age-related circadian dysfunction will allow for the design of chronotherapeutic intervention strategies to attenuate circadian dysfunction and thus improve health and quality of life.

How sleep and wakefulness influence circadian rhythmicity: effects of insufficient and mistimed sleep on the animal and human transcriptome

The mammalian circadian system is a multi-oscillator, hierarchically organised system where a central pacemaker synchronises behavioural, physiological and gene expression rhythms in peripheral tissues. Epidemiological studies show that disruption of this internal synchronisation by short sleep and shift work is associated with adverse health outcomes through mechanisms that remain to be elucidated. Here, we review recent animal and human studies demonstrating the profound effects of insufficient and mistimed sleep on the rhythms of gene expression in central and peripheral tissues. In mice, sleep restriction leads to an ~80% reduction in circadian transcripts in the brain and profound disruption of the liver transcriptome. In humans, sleep restriction leads to a 1.9% reduction in circadian transcripts in whole blood, and when sleep is displaced to the daytime, 97% of rhythmic genes become arrhythmic and one-third of all genes show changes in temporal expression profiles. These changes in mice and humans include a significant reduction in the circadian regulation of transcription and translation and core clock genes in the periphery, while at the same time rhythms within the suprachiasmatic nucleus are not disrupted. Although the physiological mediators of these sleep disruption effects on the transcriptome have not been established, altered food intake, changes in hormones such as cortisol, and changes in body and brain temperature may play important roles. Processes and molecular pathways associated with these disruptions include metabolism, immune function, inflammatory and stress responses, and point to the molecular mechanisms underlying the established adverse health outcomes associated with short sleep duration and shift work, such as metabolic syndrome and cancer.

Sustained sleep fragmentation induces sleep homeostasis in mice

STUDY OBJECTIVES

Sleep fragmentation (SF) is an integral feature of sleep apnea and other prevalent sleep disorders. Although the effect of repetitive arousals on cognitive performance is well documented, the effects of long-term SF on electroencephalography (EEG) and molecular markers of sleep homeostasis remain poorly investigated. To address this question, we developed a mouse model of chronic SF and characterized its effect on EEG spectral frequencies and the expression of genes previously linked to sleep homeostasis including clock genes, heat shock proteins, and plasticity-related genes.

DESIGN

N/A.

SETTING

Animal sleep research laboratory.

PARTICIPANTS

Sixty-six C57BL6/J adult mice.

INTERVENTIONS

Instrumental sleep disruption at a rate of 60/h during 14 days.

MEASUREMENTS AND RESULTS

Locomotor activity and EEG were recorded during 14 days of SF followed by recovery for 2 days. Despite a dramatic number of arousals and decreased sleep bout duration, SF minimally reduced total quantity of sleep and did not significantly alter its circadian distribution. Spectral analysis during SF revealed a homeostatic drive for slow wave activity (SWA; 1-4 Hz) and other frequencies as well (4-40 Hz). Recordings during recovery revealed slow wave sleep consolidation and a transient rebound in SWA, and paradoxical sleep duration. The expression of selected genes was not induced following chronic SF.

CONCLUSIONS

Chronic SF increased sleep pressure confirming that altered quality with preserved quantity triggers core sleep homeostasis mechanisms. However, it did not induce the expression of genes induced by sleep loss, suggesting that these molecular pathways are not sustainably activated in chronic diseases involving SF.

Genetic dissection of sleep homeostasis

Sleep is a complex behavior both in its manifestation and regulation, that is common to almost all animal species studied thus far. Sleep is not a unitary behavior and has many different aspects, each of which is tightly regulated and influenced by both genetic and environmental factors. Despite its essential role for performance, health, and well-being, genetic mechanisms underlying this complex behavior remain poorly understood. One important aspect of sleep concerns its homeostatic regulation, which ensures that levels of sleep need are kept within a range still allowing optimal functioning during wakefulness. Uncovering the genetic pathways underlying the homeostatic aspect of sleep is of particular importance because it could lead to insights concerning sleep's still elusive function and is therefore a main focus of current sleep research. In this chapter, we first give a definition of sleep homeostasis and describe the molecular genetics techniques that are used to examine it. We then provide a conceptual discussion on the problem of assessing a sleep homeostatic phenotype in various animal models. We finally highlight some of the studies with a focus on clock genes and adenosine signaling molecules.

Sleep, recovery, and metaregulation: explaining the benefits of sleep

A commonly held view is that extended wakefulness is causal for a broad spectrum of deleterious effects at molecular, cellular, network, physiological, psychological, and behavioral levels. Consequently, it is often presumed that sleep plays an active role in providing renormalization of the changes incurred during preceding waking. Not surprisingly, unequivocal empirical evidence supporting such a simple bi-directional interaction between waking and sleep is often limited or controversial. One difficulty is that, invariably, a constellation of many intricately interrelated factors, including the time of day, specific activities or behaviors during preceding waking, metabolic status and stress are present at the time of measurement, shaping the overall effect observed. In addition to this, although insufficient or disrupted sleep is thought to prevent efficient recovery of specific physiological variables, it is also often difficult to attribute specific changes to the lack of sleep proper. Furthermore, sleep is a complex phenomenon characterized by a multitude of processes, whose unique and distinct contributions to the purported functions of sleep are difficult to determine, because they are interrelated. Intensive research effort over the last decades has greatly progressed current understanding of the cellular and physiological processes underlying the regulation of vigilance states. Notably, it also highlighted the infinite complexity within both waking and sleep, and revealed a number of fundamental conceptual and technical obstacles that need to be overcome in order to fully understand these processes. A promising approach could be to view sleep not as an entity, which has specific function(s) and is subject to direct regulation, but as a manifestation of the process of metaregulation, which enables efficient moment-to-moment integration between internal and external factors, preceding history and current homeostatic needs.

Circadian factor BMAL1 in histaminergic neurons regulates sleep architecture

Circadian clocks allow anticipation of daily environmental changes. The suprachiasmatic nucleus (SCN) houses the master clock, but clocks are also widely expressed elsewhere in the body. Although some peripheral clocks have established roles, it is unclear what local brain clocks do. We tested the contribution of one putative local clock in mouse histaminergic neurons in the tuberomamillary nucleus to the regulation of the sleep-wake cycle. Histaminergic neurons are silent during sleep, and start firing after wake onset; the released histamine, made by the enzyme histidine decarboxylase (HDC), enhances wakefulness. We found that hdc gene expression varies with time of day. Selectively deleting the Bmal1 (also known as Arntl or Mop3) clock gene from histaminergic cells removes this variation, producing higher HDC expression and brain histamine levels during the day. The consequences include more fragmented sleep, prolonged wake at night, shallower sleep depth (lower nonrapid eye movement [NREM] δ power), increased NREM-to-REM transitions, hindered recovery sleep after sleep deprivation, and impaired memory. Removing BMAL1 from histaminergic neurons does not, however, affect circadian rhythms. We propose that for mammals with polyphasic/nonwake consolidating sleep, the local BMAL1-dependent clock directs appropriately timed declines and increases in histamine biosynthesis to produce an appropriate balance of wake and sleep within the overall daily cycle of rest and activity specified by the SCN.

Polymorphisms in the circadian expressed genes PER3 and ARNTL2 are associated with diurnal preference and GNβ3 with sleep measures

Sleep and circadian rhythms are intrinsically linked, with several sleep traits, including sleep timing and duration, influenced by both sleep homeostasis and the circadian phase. Genetic variation in several circadian genes has been associated with diurnal preference (preference in timing of sleep), although there has been limited research on whether they are associated with other sleep measurements. We investigated whether these genetic variations were associated with diurnal preference (Morningness-Eveningness Questionnaire) and various sleep measures, including: the global Pittsburgh Sleep Quality index score; sleep duration; and sleep latency and sleep quality. We genotyped 10 polymorphisms in genes with circadian expression in participants from the G1219 sample (n = 966), a British longitudinal population sample of young adults. We conducted linear regressions using dominant, additive and recessive models of inheritance to test for associations between these polymorphisms and the sleep measures. We found a significant association between diurnal preference and a polymorphism in period homologue 3 (PER3) (P < 0.005, recessive model) and a novel nominally significant association between diurnal preference and a polymorphism in aryl hydrocarbon receptor nuclear translocator-like 2 (ARNTL2) (P < 0.05, additive model). We found that a polymorphism in guanine nucleotide binding protein beta 3 (GNβ3) was associated significantly with global sleep quality (P < 0.005, recessive model), and that a rare polymorphism in period homologue 2 (PER2) was associated significantly with both sleep duration and quality (P < 0.0005, recessive model). These findings suggest that genes with circadian expression may play a role in regulating both the circadian clock and sleep homeostasis, and highlight the importance of further studies aimed at dissecting the specific roles that circadian genes play in these two interrelated but unique behaviours.

Contribution of transcriptional and translational mechanisms to the recovery aspect of sleep regulation

Sleep parallels brain functioning and mental health. Neuronal activity during wakefulness leads to a subsequent increase in sleep intensity as measured using electroencephalographic slow-wave activity (SWA; index of neuronal synchrony in the low-frequency range). Wakefulness, and particularly prolonged wakefulness, also drives important changes in brain gene expression and changes in protein regulation. The role of these two cellular mechanisms in sleep-wake regulation has typically been studied independently, and their exact contribution to SWA remains poorly defined. In this review, we highlight that many transcriptional pathways driven by sleep deprivation are associated to protein regulation. We first describe the relationship between cytokines, clock genes, and markers of sleep need with an emphasis on transcriptional processes. Observations regarding the role of protein metabolism in sleep-wake regulation are then depicted while presenting interconnections between transcriptional and translational responses driven by sleep loss. Lastly, a manner by which this integrated response can feed back on neuronal network activity to determine sleep intensity is proposed. Overall, the literature supports that a complex cross-talk between transcriptional and translational regulation during prolonged wakefulness drives the changes in sleep intensity as a function of the sleep/wake history.