On the Adaptive Significance of Circadian Clocks for Their Owners

Circadian rhythms are believed to be an evolutionary adaptation to daily environmental cycles resulting from Earth's rotation about its axis. A trait evolved through a process of natural selection is considered as adaptation; therefore, rigorous demonstration of adaptation requires evidence suggesting evolution of a trait by natural selection. Like any other adaptive trait, circadian rhythms are believed to be advantageous to living beings through some perceived function. Circadian rhythms are thought to confer advantage to their owners through scheduling of biological functions at appropriate time of daily environmental cycle (extrinsic advantage), coordination of internal physiology (intrinsic advantage), and through their role in responses to seasonal changes. So far, the adaptive value of circadian rhythms has been tested in several studies and evidence indeed suggests that they confer advantage to their owners. In this review, we have discussed the background for development of the framework currently used to test the hypothesis of adaptive significance of circadian rhythms. Critical examination of evidence reveals that there are several lacunae in our understanding of circadian rhythms as adaptation. Although it is well known that demonstrating a given trait as adaptation (or setting the necessary criteria) is not a trivial task, here we recommend some of the basic criteria and suggest the nature of evidence required to comprehensively understand circadian rhythms as adaptation. Thus, we hope to create some awareness that may benefit future studies in this direction. (Author correspondence: vsharma@jncasr.ac.in or vksharmas@gmail.com)     

How to trick mother nature into letting you fly around or stay up all night

Night shift work and rapid transmeridian travel result in a misalignment between circadian rhythms and the new times for sleep, wake, and work, which has health and safety implications for both the individual involved and the general public. Entrainment to the new sleep/wake schedule requires circadian rhythms to be phase-shifted, but this is often slow or impeded. The authors show superimposed light and melatonin PRCs to explain how to appropriately time these zeitgebers to promote circadian adaptation. They review studies in which bright light and melatonin were administered to try to counteract jet lag or to produce circadian adaptation to night work. They demonstrate how jet lag could be prevented entirely if rhythms are shifted before the flight using their preflight plan and discuss the combination of interventions that they now recommend for night shift workers.     

Clean thoughts about dirty genes

The mouse has joined the fruit fly, bread mold, and cyanobacteria as a tractable genetic system for studying mechanisms of circadian rhythms. The circadian rhythms of "knock-out" mice for specific clock genes, however, have demonstrated significant variability between laboratories. In this brief review, the authors discuss possible sources of this variability, focusing particularly on questions of modifier loci of circadian rhythms that vary between inbred mouse strains. They conclude with a short list of recommendations for researchers working on circadian rhythms in mixed-strain mice.     

Adaptation of the Hierarchical Factor Segmentation method to noisy activity data

ABSTRACT

The Hierarchical Factor Segmentation (HFS) method is a non-parametric statistical method for detection of the phase of a biological rhythm shown in an actogram. The detection accuracy of this method was measured on actograms showing only circadian rhythms with a constant ratio of signal to noise (S/N). In the present study, we generated 84 types of artificial actograms including circadian or circatidal rhythms by using three parameters: α/ρ, S/N and period length τ, and evaluated the effectiveness of our devised adaptation of the HFS method, the cycle-by-cycle adaptation. The results showed the effectiveness of the cycle-by-cycle adaptation was high even though S/N or τ was fluctuating through a whole actogram. These suggested that the cycle-by-cycle adaptation could be effectively applied to various kinds of rhythmic activity data. The C++ source code of the cycle-by-cycle adaptation is available on the website at https://github.com/KazukiSakura/cHFS.git.     

Drosophila free-running rhythms require intercellular communication

Robust self-sustained oscillations are a ubiquitous characteristic of circadian rhythms. These include Drosophila locomotor activity rhythms, which persist for weeks in constant darkness (DD). Yet the molecular oscillations that underlie circadian rhythms damp rapidly in many Drosophila tissues. Although much progress has been made in understanding the biochemical and cellular basis of circadian rhythms, the mechanisms that underlie the differences between damped and self-sustaining oscillations remain largely unknown. A small cluster of neurons in adult Drosophila brain, the ventral lateral neurons (LN_vs), is essential for self-sustained behavioral rhythms and has been proposed to be the primary pacemaker for locomotor activity rhythms. With an LN_v-specific driver, we restricted functional clocks to these neurons and showed that they are not sufficient to drive circadian locomotor activity rhythms. Also contrary to expectation, we found that all brain clock neurons manifest robust circadian oscillations of timeless and cryptochrome RNA for many days in DD. This persistent molecular rhythm requires pigment-dispersing factor (PDF), an LN_v-specific neuropeptide, because the molecular oscillations are gradually lost when Pdf⁰¹ mutant flies are exposed to free-running conditions. This observation precisely parallels the previously reported effect on behavioral rhythms of the Pdf⁰¹ mutant. PDF is likely to affect some clock neurons directly, since the peptide appears to bind to the surface of many clock neurons, including the LN_vs themselves. We showed that the brain circadian clock in Drosophila is clearly distinguishable from the eyes and other rapidly damping peripheral tissues, as it sustains robust molecular oscillations in DD. At the same time, different clock neurons are likely to work cooperatively within the brain, because the LN_vs alone are insufficient to support the circadian program. Based on the damping results with Pdf⁰¹ mutant flies, we propose that LN_vs, and specifically the PDF neuropeptide that it synthesizes, are important in coordinating a circadian cellular network within the brain. The cooperative function of this network appears to be necessary for maintaining robust molecular oscillations in DD and is the basis of sustained circadian locomotor activity rhythms.     

Drosophila Free-Running Rhythms Require Intercellular Communication

Robust self-sustained oscillations are a ubiquitous characteristic of circadian rhythms. These include Drosophila locomotor activity rhythms, which persist for weeks in constant darkness (DD). Yet the molecular oscillations that underlie circadian rhythms damp rapidly in many Drosophila tissues. Although much progress has been made in understanding the biochemical and cellular basis of circadian rhythms, the mechanisms that underlie the differences between damped and self-sustaining oscillations remain largely unknown. A small cluster of neurons in adult Drosophila brain, the ventral lateral neurons (LN_vs), is essential for self-sustained behavioral rhythms and has been proposed to be the primary pacemaker for locomotor activity rhythms. With an LN_v-specific driver, we restricted functional clocks to these neurons and showed that they are not sufficient to drive circadian locomotor activity rhythms. Also contrary to expectation, we found that all brain clock neurons manifest robust circadian oscillations of timeless and cryptochrome RNA for many days in DD. This persistent molecular rhythm requires pigment-dispersing factor (PDF), an LN_v-specific neuropeptide, because the molecular oscillations are gradually lost when Pdf⁰¹ mutant flies are exposed to free-running conditions. This observation precisely parallels the previously reported effect on behavioral rhythms of the Pdf⁰¹ mutant. PDF is likely to affect some clock neurons directly, since the peptide appears to bind to the surface of many clock neurons, including the LN_vs themselves. We showed that the brain circadian clock in Drosophila is clearly distinguishable from the eyes and other rapidly damping peripheral tissues, as it sustains robust molecular oscillations in DD. At the same time, different clock neurons are likely to work cooperatively within the brain, because the LN_vs alone are insufficient to support the circadian program. Based on the damping results with Pdf⁰¹ mutant flies, we propose that LN_vs, and specifically the PDF neuropeptide that it synthesizes, are important in coordinating a circadian cellular network within the brain. The cooperative function of this network appears to be necessary for maintaining robust molecular oscillations in DD and is the basis of sustained circadian locomotor activity rhythms.     

Acute exposure to 2G phase shifts the rat circadian timing system

The circadian timing system (CTS) provides internal and external temporal coordination of an animal's physiology and behavior. In mammals, the generation and coordination of these circadian rhythms is controlled by a neural pacemaker, the suprachiasmatic nucleus (SCN), located within the hypothalamus. The pacemaker is synchronized to the 24 hour day by time cues (zeitgebers) such as the light/dark cycle. When an animal is exposed to an environment without time cues, the circadian rhythms maintain internal temporal coordination but exhibit a "free-running" condition in which the period length is determined by the internal pacemaker. Maintenance of internal and external temporal coordination are critical for normal physiological and psychological function in human and non-human primates. Exposure to altered gravitational environments has been shown to affect the amplitude, mean, and timing of circadian rhythms in species ranging from unicellular organisms to man. However, it has not been determined whether altered gravitational fields have a direct effect on the neural pacemaker, or affect peripheral physiological systems that express these circadian parameters. In previous studies, the ability of a stimulus to phase shift circadian rhythms was used to determine whether a stimulus has a direct effect on the neural pacemaker. The present experiment was performed in order to determine whether acute exposure to a hyperdynamic field could phase shift circadian rhythms.     

Development and validation of computational models for mammalian circadian oscillators

Circadian rhythms are endogenous rhythms with a cycle length of approximately 24 h. Rhythmic production of specific proteins within pacemaker structures is the basis for these physiological and behavioral rhythms. Prior work on mathematical modeling of molecular circadian oscillators has focused on the fruit fly, Drosophila melanogaster. Recently, great advances have been made in our understanding of the molecular basis of circadian rhythms in mammals. Mathematical models of the mammalian circadian oscillator are needed to piece together diverse data, predict experimental results, and help us understand the clock as a whole. Our objectives are to develop mathematical models of the mammalian circadian oscillator, generate and test predictions from these models, gather information on the parameters needed for model development, integrate the molecular model with an existing model of the influence of light and rhythmicity on human performance, and make models available in BioSpice so that they can be easily used by the general community. Two new mammalian models have been developed, and experimental data are summarized. These studies have the potential to lead to new strategies for resetting the circadian clock. Manipulations of the circadian clock can be used to optimize performance by promoting alertness and physiological synchronization.     

Rhythms,rhythmicity and aggression

The relationships between biological rhythms and human aggressive behavior are addressed and discussed in this article: First, circadian rhythms and aggression are considered. Studies of sleep/waking cycle disturbances in aggression are reported. Severe aggression is associated with profound changes in sleep architecture. Causal link is difficult to establish given that sleep disturbance and aggressive behavior could be the symptoms of the same disorder. Specific aggressive behavior developed during sleep is also described. In addition, hormonal circadian rhythm studies are reported. Thus, low cortisol levels, in particular low cortisol variability, are associated with aggressive behavior, suggesting an inhibitory role of cortisol. Testosterone has daily and seasonal fluctuations, but no link with aggression has been established. Neurophysiological underlying mechanisms are discussed in the last part of this article, with a focus on the relationship between brain rhythm and aggression. Increase of slow-wave EEG activities is observed in individuals with aggressive behavior. Epilepsy, as a disease of brain rhythm could be associated with aggressive behavior, in pre, post and inter ictal periodes. Incidence of aggression is not likely more prevalent in epileptic individuals compared to those with other neurological conditions. Ictal changes take the form of profound behavioral changes, including aggressive behavior which has been interpreted as the emergence of “archeical” or innate motor patterns. In this multidisciplinary approach, the main difficulty is the categorization of the differents types of aggression. Finally, taken together, these studies suggest that biological rhythms, especially circadian rhythms, could provide therapeutic benefits to human aggressive behavior. Biological rhythymicity seems to be a necessary permanent training offering interesting perspectives for the adaptation to changes in the field of aggression.     

The circadian timekeeping system of Drosophila

Daily rhythms in behavior, physiology and metabolism are controlled by endogenous circadian clocks. At the heart of these clocks is a circadian oscillator that keeps circadian time, is entrained by environmental cues such as light and activates rhythmic outputs at the appropriate time of day. Genetic and molecular analyses in Drosophila have revealed important insights into the molecules and mechanisms underlying circadian oscillator function in all organisms. In this review I will describe the intracellular feedback loops that form the core of the Drosophila circadian oscillator and consider how they are entrained by environmental light cycles, where they operate within the fly and how they are thought to control overt rhythms in physiology and behavior. I will also discuss where work remains to be done to give a comprehensive picture of the circadian clock in Drosophila and likely many other organisms.     

Seasonality and photoperiodism in fungi

This review gives a retrospective of what is known about photoperiodism in fungi, which is largely based on reports about seasonal spore concentrations. Relatively few species have been investigated under laboratory conditions, so that our knowledge whether seasonal reproduction in fungi is mainly a direct response to environmental conditions or whether it involves a photoperiodic machinery with memory capacities and a relationship to the circadian system is extremely limited. To form a basis for further experimental endeavors into fungal photoperiodism, we review the reports about endogenous rhythms and photobiology. Finally, we will look at the possibilities of using the fungal circadian model system of Neurospora crassa for future work on photoperiodism.     

Feature identification in circadian rhythms of mice strains using in vivo information

The objective of this work was to identify strain-specific characteristics from real-time measurements of circadian rhythms of two inbred mouse strains. In particular, heart rate, temperature, and activity data collected from A/J and C57BL/6J (B6) mice using telemetry are analyzed. The influence of activity on heart rate and temperature is minimized by correlation analysis followed by regression analysis. The correlation analysis is used to determine the length of the activity data filter that results in the best correlation between activity data and heart rate or temperature. After the activity data are filtered, they are used in regression analysis. The temperature and heart rate rhythms obtained as the intercepts of the regression analysis are interpreted as the zero-activity rhythms and consequently are good estimates of the circadian rhythms. The circadian temperature rhythms for the B6 mice follow a smoother cosine-like time waveform, whereas those for the A/J mice follow a more square-wave-like waveform. To quantify the difference between these two temperature rhythms, a feature based on Fourier analysis of the time-series data is used. Detrended fluctuation analysis is used to identify features in the heart rate rhythms. The results of this work show that the features for the circadian temperature and heart rate rhythms can be used as distinguishing characteristics of the A/J and B6 strains. This work provides the foundation for future studies directed at investigating the influence of chromosomal substitutions on the regulation of circadian rhythms in these two strains.     

The Effects of Arid Region Conditions on the Hormonal Status of Humans

The human hormonal status and its circadian and circannual rhythms that provide a background for human activities and diseases were studied under conditions of the extremely continental climate of the arid region with a long period of elevated ambient temperatures. Age- and sex-related features, biological rhythms (seasonal and circadian), and the dependence on the work specificity were studied for the activities and interrelations of the adrenal cortex, thyroid gland, and catecholamines. The peripheral blood plasma levels of the hormones and the urinary excretion of the hormone metabolites were determined by conventional methods. The findings show that the hormonal status depended on both the climatic and occupational conditions.     

Disruption of gene expression rhythms in mice lacking secretory vesicle proteins IA-2 and IA-2β

Insulinoma-associated protein (IA)-2 and IA-2β are transmembrane proteins involved in neurotransmitter secretion. Mice with targeted disruption of both IA-2 and IA-2β (double-knockout, or DKO mice) have numerous endocrine and physiological disruptions, including disruption of circadian and diurnal rhythms. In the present study, we have assessed the impact of disruption of IA-2 and IA-2β on molecular rhythms in the brain and peripheral oscillators. We used in situ hybridization to assess molecular rhythms in the hypothalamic suprachiasmatic nuclei (SCN) of wild-type (WT) and DKO mice. The results indicate significant disruption of molecular rhythmicity in the SCN, which serves as the central pacemaker regulating circadian behavior. We also used quantitative PCR to assess gene expression rhythms in peripheral tissues of DKO, single-knockout, and WT mice. The results indicate significant attenuation of gene expression rhythms in several peripheral tissues of DKO mice but not in either single knockout. To distinguish whether this reduction in rhythmicity reflects defective oscillatory function in peripheral tissues or lack of entrainment of peripheral tissues, animals were injected with dexamethasone daily for 15 days, and then molecular rhythms were assessed throughout the day after discontinuation of injections. Dexamethasone injections improved gene expression rhythms in liver and heart of DKO mice. These results are consistent with the hypothesis that peripheral tissues of DKO mice have a functioning circadian clockwork, but rhythmicity is greatly reduced in the absence of robust, rhythmic physiological signals originating from the SCN. Thus, IA-2 and IA-2β play an important role in the regulation of circadian rhythms, likely through their participation in neurochemical communication among SCN neurons.     

Circadian rhythm mutants of the prokaryoticSynechococcus RF-1

Synechococcus RF-1 established circadian rhythms in nitrogen fixation and leucine uptake when growing in a diurnal light/dark regimen. The rhythms persisted in subsequent uniform light/light conditions. In order to analyze the circadian rhythm at the genetic level, mutants were induced by N-methyl-N-nitro-N-nitrosoguanidine and then isolated by procedures with the circadian nitrogen-fixing rhythm as a selecton marker. Characterization of the mutants with respect to the circadian rhythm indicated that some mutants were abnormal only in the nitrogen-fixing rhythm, while some simultaneously lost the ability to establish the nitrogen-fixing and leucine-uptake rhythms. The physiological properties of the circadian rhythm were compared. The genetic potential of the mutants that were abnormal in both rhythms is emphasized.     

The mammalian circadian system: models and physiology

Mammalian circadian rhythms have been studied in great detail using primarily two different methods. One method is usually referred to as the formal analysis of rhythms. Its goal is to describe the properties of both rhythms and their underlying mechanisms, and it aims at the development of adequate mathematical models of the circadian system. The other method is the physiological analysis of the mechanisms that generate and entrain rhythms. Its goal is the identification of the anatomical components of the circadian system and the elucidation at a cellular and molecular level of how these components work. This paper reviews how the formal analysis of circadian systems, primarily in rodents, set the agenda for physiological studies, and the degree to which this agenda has been fulfilled. It then discusses how physiological analyses of the system have helped to redefine issues such as the nature and identity of the pacemaker, the nature of the entrainment process, the roles of photic and nonphotic cues, and the role of feedback in the circadian system. The continued commerce between these two approaches has led to a sophisticated appreciation of the complexities and subtleties of circadian organization in mammals. The further integration of formal and physiological analyses remains a challenging goal for the future.     

Circadian organization of a subarctic rodent, the northern red-backed vole (Clethrionomys rutilus)

Arctic and subarctic environments are exposed to extreme light: dark (LD) regimes, including periods of constant light (LL) and constant dark (DD) and large daily changes in day length, but very little is known about circadian rhythms of mammals at high latitudes. The authors investigated the circadian rhythms of a subarctic population of northern red-backed voles (Clethrionomys rutilus). Both wild-caught and third-generation laboratory-bred animals showed predominantly nocturnal patterns of wheel running when exposed to a 16:8 LD cycle. In LL and DD conditions, animals displayed large phenotypic variation in circadian rhythms. Compared to wheel-running rhythms under a 16:8 LD cycle, the robustness of circadian activity rhythms decreased among all animals tested in LL and DD (i.e., decreased chi-squared periodogram waveform amplitude). A large segment of the population became noncircadian (60% in DD, 72% in LL) within 8 weeks of exposure to constant lighting conditions, of which the majority became ultradian, with a few individuals becoming arrhythmic, indicating highly labile circadian organization. Wild-caught and laboratory-bred animals that remained circadian in wheel running displayed free-running periods between 23.3 and 24.8 h. A phase-response curve to light pulses in DD showed significant phase delays at circadian times 12 and 15, indicating the capacity to entrain to rapidly changing day lengths at high latitudes. Whether this phenotypic variation in circadian organization, with circadian, ultradian, and arrhythmic wheel-running activity patterns in constant lighting conditions, is a novel adaptation to life in the arctic remains to be elucidated.     

Modeling the emergence of circadian rhythms in a clock neuron network

Circadian rhythms in pacemaker cells persist for weeks in constant darkness, while in other types of cells the molecular oscillations that underlie circadian rhythms damp rapidly under the same conditions. Although much progress has been made in understanding the biochemical and cellular basis of circadian rhythms, the mechanisms leading to damped or self-sustained oscillations remain largely unknown. There exist many mathematical models that reproduce the circadian rhythms in the case of a single cell of the Drosophila fly. However, not much is known about the mechanisms leading to coherent circadian oscillation in clock neuron networks. In this work we have implemented a model for a network of interacting clock neurons to describe the emergence (or damping) of circadian rhythms in Drosophila fly, in the absence of zeitgebers. Our model consists of an array of pacemakers that interact through the modulation of some parameters by a network feedback. The individual pacemakers are described by a well-known biochemical model for circadian oscillation, to which we have added degradation of PER protein by light and multiplicative noise. The network feedback is the PER protein level averaged over the whole network. In particular, we have investigated the effect of modulation of the parameters associated with (i) the control of net entrance of PER into the nucleus and (ii) the non-photic degradation of PER. Our results indicate that the modulation of PER entrance into the nucleus allows the synchronization of clock neurons, leading to coherent circadian oscillations under constant dark condition. On the other hand, the modulation of non-photic degradation cannot reset the phases of individual clocks subjected to intrinsic biochemical noise.