Circadian basis of seasonal timing in higher vertebrates

Abstract

The two dominant environmental oscillations shape biology and survival of species: the day–night cycle and the succession of the seasons in the year. Organisms have adapted to anticipate these variations by evolving internal circadian (ca.- about, diem- day) and circannual clocks. The former enables the organisms to regulate physiological functions on a daily basis, and the latter on the annual basis. In mammals, the suprachiasmatic nuclei (SCN) of the anterior hypothalamus contain master pacemaker and orchestrate peripheral clocks in synchrony with the daily 24 h light-dark cycle, while in birds circadian pacemake is an interacting system principally located in the retina, pineal and the hypothalamus. In this mini review, we discuss the role of circadian clocks in regulation of seasonal timing in higher vertebrates, with reference to birds and mammals.     

To be or not to be rhythmic? A review of studies on organisms inhabiting constant environments

Abstract

Circadian clocks are endogenous time keeping mechanisms that drive near 24-h behavioural, physiological and metabolic rhythms in organisms. It is thought that organisms possess circadian clocks to facilitate coordination of essential biological events to the external day and night (extrinsic advantage) so as to enhance Darwinian fitness. However, on Earth, there are a number of habitats that are not subject to such robust daily cycling of geo-physical factors. Do organisms living under such conditions exhibit rhythmic behaviours that are driven by endogenous circadian clocks? We attempt to critically survey studies of rhythms (or the lack of them) in organisms living in a range of constant environments. Many such organisms do show rhythms in behaviour and/or physiological variables. We suggest that such presence of rhythms may be indicative of an underlying clock that facilitates, (a) internal synchrony among rhythms, and (b) temporal partitioning of incompatible cellular processes (intrinsic advantage). We then highlight reasons that limit our interpretations about the presence (or absence) of clocks in such organisms living under constant conditions, and suggest possible methods to conclusively test whether or not rhythms in these organisms are driven by endogenous circadian clocks with the hope that it may enhance our understanding of circadian clocks in organisms under constant environments.     

Circadian orchestration of developmental hormones in the insect, Rhodnius prolixus

This review presents a new perspective on the circadian regulation and functions of insect developmental hormones. In Rhodnius prolixus (Hemiptera), the brain neuropeptide prothoracicotropic hormone (PTTH) is released with a circadian rhythm that is controlled by paired photosensitive clocks in the brain. These clocks comprise the dorsal and lateral PER/TIM clock neurons known to regulate behavioral rhythms in Drosophila. Axons of PTTH and clock cells make close contact. Photosensitive PER/TIM clocks also reside in the paired prothoracic glands (PGs), which generate rhythmic synthesis and release of the ecdysteroid molting hormones. The PG clocks are entrained by both light and PTTH. These four clocks are coupled together by both nerves and hormones into a timing system whose primary regulated output is the circadian rhythm of ecdysteroids in the hemolymph. This complex timing system appears necessary to ensure circadian organization of the gene expression that is induced in target cells by ecdysteroids via circadian cycling of the nuclear ecdysteroid receptor (EcR). This multioscillator system serves to transduce 'the day outside' into endocrine rhythms that orchestrate 'the day inside'. It has many functional similarities with vertebrate circadian systems.     

Exploring the role of circadian clock gene and association with cancer pathophysiology

ABSTRACT

Most of the processes that occur in the mind and body follow natural rhythms. Those with a cycle length of about one day are called circadian rhythms. These rhythms are driven by a system of self-sustained clocks and are entrained by environmental cues such as light-dark cycles as well as food intake. In mammals, the circadian clock system is hierarchically organized such that the master clock in the suprachiasmatic nuclei of the hypothalamus integrates environmental information and synchronizes the phase of oscillators in peripheral tissues.

The circadian system is responsible for regulating a variety of physiological and behavioral processes, including feeding behavior and energy metabolism. Studies revealed that the circadian clock system consists primarily of a set of clock genes. Several genes control the biological clock, including BMAL1, CLOCK (positive regulators), CRY1, CRY2, PER1, PER2, and PER3 (negative regulators) as indicators of the peripheral clock.

Circadian has increasingly become an important area of medical research, with hundreds of studies pointing to the body’s internal clocks as a factor in both health and disease. Thousands of biochemical processes from sleep and wakefulness to DNA repair are scheduled and dictated by these internal clocks. Cancer is an example of health problems where chronotherapy can be used to improve outcomes and deliver a higher quality of care to patients.

In this article, we will discuss knowledge about molecular mechanisms of the circadian clock and the role of clocks in physiology and pathophysiology of concerns.     

Cyclic Presence and Absence of Conspecifics Alters Circadian Clock Phase But Does Not Entrain the Locomotor Activity Rhythm of the Fruit Fly Drosophila melanogaster

Circadian clocks use a wide range of environmental cues, including cycles of light, temperature, food, and social interactions, to fine-tune rhythms in behavior and physiology. Although social cues have been shown to influence circadian clocks of a variety of organisms including the fruit fly Drosophila melanogaster, their mechanism of action is still unclear. Here, the authors report the results of their study aimed at investigating if daily cycles of presence and absence (PA) of conspecific male visitors are able to entrain the circadian locomotor activity rhythm of male hosts living under constant darkness (DD). The results suggest that PA cycles may not be able to entrain circadian locomotor activity rhythms of Drosophila. The outcome does not change when male hosts are presented with female visitors, suggesting that PA cycles of either sex may not be effective in bringing about stable entrainment of circadian clocks in D. melanogaster. However, in hosts whose clock phase has already been set by light/dark (LD) cycles, daily PA cycles of visitors can cause measurable change in the phase of subsequent free-running rhythms, provided that their circadian clocks are labile. Thus, the findings of this study suggest that D. melanogaster males may not be using cyclic social cues as their primary zeitgeber (time cue) for entrainment of circadian clocks, although social cues are capable of altering the phase of their circadian rhythms. (Author correspondence: vsharma@jncasr.ac.in, vksharmas@gmail.com)     

A randomized,double-blind and placebo-controlled crossover trial on the effect of l-ornithine ingestion on the human circadian clock

ABSTRACT

In mammals, daily physiological events are regulated by the circadian rhythm, which comprises two types of internal clocks: the central clock and peripheral clocks. Circadian rhythm plays an important role in maintaining physiological functions including the sleep-wake cycle, body temperature, metabolism and organ functions. Circadian rhythm disorder, which is caused, for example, by an irregular lifestyle or long-haul travel, increases the risk of developing disease; therefore, it is important to properly maintain the rhythm of the circadian clock. Food and the circadian clock system are known to be closely linked. Studies on rodents suggest that ingesting specific food ingredients, such as the flavonoid nobiletin, fish oil, the polyphenol resveratrol and the amino acid L-ornithine affects the circadian clock. However, there are few reports on the foods that affect these circadian clocks in humans. In this study, therefore, we examined whether L-ornithine affects the human central clock in a crossover design placebo-controlled human trial. In total, 28 healthy adults (i.e. ≥20 years) were randomly divided into two groups and completed the study protocol. In the 1st intake period, participants were asked to take either L-ornithine (400 mg) capsules or placebo capsules for 7 days. After 7 days’ interval, they then took the alternative test capsules for 7 days in the 2nd intake period. On the final day of each intake period, saliva was sampled at various time points in the dim light condition, and the concentration of melatonin was quantified to evaluate the phase of the central clock. The results revealed that dim light melatonin onset, a recognized marker of central circadian phase, was delayed by 15 min after ingestion of L-ornithine. Not only is this finding an indication that L-ornithine affects the human central clock, but it also demonstrates that the human central clock can be regulated by food ingredients.     

Metabolism as an integral cog in the mammalian circadian clockwork

Abstract

Circadian rhythms are an integral part of life. These rhythms are apparent in virtually all biological processes studies to date, ranging from the individual cell (e.g. DNA synthesis) to the whole organism (e.g. behaviors such as physical activity). Oscillations in metabolism have been characterized extensively in various organisms, including mammals. These metabolic rhythms often parallel behaviors such as sleep/wake and fasting/feeding cycles that occur on a daily basis. What has become increasingly clear over the past several decades is that many metabolic oscillations are driven by cell-autonomous circadian clocks, which orchestrate metabolic processes in a temporally appropriate manner. During the process of identifying the mechanisms by which clocks influence metabolism, molecular-based studies have revealed that metabolism should be considered an integral circadian clock component. The implications of such an interrelationship include the establishment of a vicious cycle during cardiometabolic disease states, wherein metabolism-induced perturbations in the circadian clock exacerbate metabolic dysfunction. The purpose of this review is therefore to highlight recent insights gained regarding links between cell-autonomous circadian clocks and metabolism and the implications of clock dysfunction in the pathogenesis of cardiometabolic diseases.     

Phyto-inspired cyclic peptides derived from plant Pin-II type protease inhibitor reactive center loops for crop protection from insect pests

BackgroundNatural defence of plants against insect pests involves protease inhibitors (PIs) that interfere with insect digestive proteases. Pin-II type plant PIs are wound inducible upon insect damage and possess multiple inhibitory repeat domains that can inhibit trypsin and chymotrypsin-like proteases in the insect midgut. Yet, their agricultural ex-vivo application is limited due to large molecular size and environmental instability, which could be overcome by small peptides.MethodsBicyclic peptides were designed by grafting Pin-II PIs derived reactive center loop (RCL) on synthetic tris(bromomethyl)benzene scaffold. In vitro binding with trypsin-like proteases was evaluated by biochemical and biophysical assays, followed by molecular dynamics simulations. In vivo effects on two major lepidopteran insect pests, Helicoverpa armigera and Spodoptera litura were studied upon feeding with peptide treated leaves. Affinity based pull down assays were used to identify target proteins in insect gut.ResultsBicyclic RCLs showed ten-fold enhanced protease inhibition compared to their linear counterparts. They exhibited feeding deterrence and growth reduction of lepidopteran insects. Bicyclic peptides predominantly interact with midgut serine proteases. Possible binding modes involve simultaneous interaction with the active site and specificity-determining residues of insect gut trypsin.ConclusionBicyclic peptides are potent inhibitors of serine proteases in the insect midgut. They cause feeding aversion and larval growth retardation. Bi-domain cyclic peptides interact with two sites on trypsin, leading to enhanced efficacy over linear RCL peptides.General significanceBicyclic peptides mimic natural PIs by inhibiting insect proteases leading to growth reduction, thus, could be used as pest control molecules in agriculture.     

Insect quality control: synchronized sex,mating system,and biological rhythm

The sterile insect technique (SIT) is a method of eradicating insects by releasing mass-reared sterilized males into fields to reduce the hatchability of eggs laid by wild females that have mated with the sterile males. SIT requires mass-production of the target insect, and maintenance of the quality of the mass-reared insects. The most important factor is successful mating between wild females and sterile males because SIT depends on their synchronized copulation. Therefore, understanding the mating systems and fertilization processes of target insects is prerequisite. Insect behavior often has circadian rhythms that are controlled by a biological clock. However, very few studies of relationships between sterile insect quality and circadian rhythm have been performed compared with the amount of research on the mating ability of target insects. The timing of male copulation attempts with receptivity of females is key to successful mating between released males and wild females. Therefore, we should focus on the mechanisms controlling the timing of mating in target insects. On the other hand, in biological control projects, precise timing of the release of natural enemies to attack pest species is required because behavior of pests and control agents are affected by their circadian rhythms. Involving both chronobiologists and applied entomologists might produce novel ideas for sterile insect quality control by synchronized sex between mass-reared and wild flies, and for biological control agent quality by matching timing in activity between predator activity and prey behavior. Control of the biological clocks in sterile insects or biological control agents is required for advanced quality control of rearing insects.     

Evolution of circadian rhythms in Drosophila melanogaster populations reared in constant light and dark regimes for over 330 generations

Organisms are believed to have evolved circadian clocks as adaptations to deal with cyclic environmental changes, and therefore it has been hypothesized that evolution in constant environments would lead to regression of such clocks. However, previous studies have yielded mixed results, and evolution of circadian clocks under constant conditions has remained an unsettled topic of debate in circadian biology. In continuation of our previous studies, which reported persistence of circadian rhythms in Drosophila melanogaster populations evolving under constant light, here we intended to examine whether circadian clocks and the associated properties evolve differently under constant light and constant darkness. In this regard, we assayed activity-rest, adult emergence and oviposition rhythms of D. melanogaster populations which have been maintained for over 19 years (~330 generations) under three different light regimes – constant light (LL), light–dark cycles of 12:12 h (LD) and constant darkness (DD). We observed that while circadian rhythms in all the three behaviors persist in both LL and DD stocks with no differences in circadian period, they differed in certain aspects of the entrained rhythms when compared to controls reared in rhythmic environment (LD). Interestingly, we also observed that DD stocks have evolved significantly higher robustness or power of free-running activity-rest and adult emergence rhythms compared to LL stocks. Thus, our study, in addition to corroborating previous results of circadian clock evolution in constant light, also highlights that, contrary to the expected regression of circadian clocks, rearing in constant darkness leads to the evolution of more robust circadian clocks which may be attributed to an intrinsic adaptive advantage of circadian clocks and/or pleiotropic functions of clock genes in other traits.     

Circadian Dysfunction Reduces Lifespan in Drosophila melanogaster

Circadian clocks regulate physiological and behavioral processes in a wide variety of organisms, and any malfunction in these clocks can cause significant health problems. In this paper, we report the results of our study on the physiological consequences of circadian dysfunction (malfunctioning of circadian clocks) in two wild‐type populations of fruit flies (Drosophila melanogaster). We assayed locomotor activity behavior and lifespan among adult flies kept under constant dark (DD) conditions of the laboratory, wherein they were categorized as rhythmic if their activity/rest schedules followed circadian (approximately 24 h) patterns, and as arrhythmic if their activity/rest schedules did not display any pattern. The rhythmic flies from both populations lived significantly longer than the arrhythmic ones. Based on these results, we conclude that circadian dysfunction is deleterious, and proper functioning of circadian clocks is essential for the physiological well being of D. melanogaster.     

Insect circadian clocks: is it all in their heads?

Circadian rhythms are ubiquitous in living organisms, synchronizing life functions at the biochemical, physiological, and behavioral levels. The rhythm-generating mechanisms, collectively known as circadian clocks, are not fully understood in any organism. Research in the fruit fly Drosophila has led to the identification of several clock genes that are involved in the function of the brain-centered clock, which controls behavioral rhythms of adult flies. With the use of clock genes as markers, putative circadian clocks were mapped in the fly peripheral organs and shown to be independent from clocks located in the brain. A homologue of fruit fly period gene has been identified in moths and other insects, allowing investigations of this gene's role in known insect rhythms. This approach may increase our understanding of how circadian clocks are organized into the circadian system that orchestrates temporal integration of life processess in insects.     

Peripheral clocks and their role in circadian timing: insights from insects.

Impressive advances have been made recently in our understanding of the molecular basis of the cell-autonomous circadian feedback loop; however, much less is known about the overall organization of the circadian systems. How many clocks tick in a multicellular animal, such as an insect, and what are their roles and the relationships between them? Most attempts to locate clock-containing tissues were based on the analysis of behavioural rhythms and identified brain-located timing centres in a variety of animals. Characterization of several essential clock genes and analysis of their expression patterns revealed that molecular components of the clock are active not only in the brain, but also in many peripheral organs of Drosophila and other insects as well as in vertebrates. Subsequent experiments have shown that isolated peripheral organs can maintain self-sustained and light sensitive cycling of clock genes in vitro. This, together with earlier demonstrations that physiological output rhythms persist in isolated organs and tissues, provide strong evidence for the existence of functionally autonomous local circadian clocks in insects and other animals. Circadian systems in complex animals may include many peripheral clocks with tissue-specific functions and a varying degree of autonomy, which seems to be correlated with their sensitivity to external entraining signals.     

BIOLOGICAL TIMING AND THE CLOCK METAPHOR: OSCILLATORY AND HOURGLASS MECHANISMS

Living organisms have developed a multitude of timing mechanisms— “biological clocks.” Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations—which keep time with environmental periodicities—as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly reviewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which “dependent variables” are triggered play an important role. (Chronobiology International, 18(3), 329–369, 2001)     

Daily Changes in Ultraviolet Light Levels Can Synchronize the Circadian Clock of Bumblebees (Bombus terrestris)

Endogenous circadian clocks are synchronized to the 24-h day by external zeitgebers such as daily light and temperature cycles. Bumblebee foragers show diurnal rhythms under daily light:dark cycles and short-period free-running circadian rhythms in constant light conditions in the laboratory. In contrast, during the continuous light conditions of the arctic summer, they show robust 24-h rhythms in their foraging patterns, meaning that some external zeitgeber must entrain their circadian clocks in the presence of constant light. Although the sun stays above the horizon for weeks during the arctic summer, the light quality, especially in the ultraviolet (UV) range, exhibits pronounced daily changes. Since the photoreceptors and photopigments that synchronize the circadian system of bees are not known, we tested if the circadian clocks of bumblebees (Bombus terrestris) can be entrained by daily cycles in UV light levels. Bumblebee colonies were set up in the laboratory and exposed to 12?h:12?h UV?+?:UV? cycles in otherwise continuous lighting conditions by placing UV filters on their foraging arenas for 12?h each day. The activity patterns of individual bees were recorded using fully automatic radiofrequency identification (RFID). We found that colonies manipulated in such a way showed synchronized 24-h rhythms, whereas simultaneously tested control colonies with no variation in UV light levels showed free-running rhythms instead. The results of our study show that bumblebee circadian rhythms can indeed be synchronized by daily cycles in ambient light spectral composition. (Author correspondence: r.stanewsky@qmul.ac.uk)     

Correlated changes in circadian clocks in response to selection for faster pre-adult development in fruit flies Drosophila melanogaster

Although, circadian clocks are believed to be involved in the regulation of life-history traits such as pre-adult development time and lifespan in fruit flies Drosophila melanogaster, there is very little unequivocal evidence either to support or refute this. Here we report the results of a long-term study aimed at examining the role of circadian clocks in the temporal regulation of pre-adult development in D. melanogaster. We employed laboratory selection protocol for faster pre-adult development on four large, outbred, random mating populations of Drosophila. We assayed pre-adult development time and circadian period of locomotor activity rhythm of these flies at regular intervals of 5–10 generations. After 50 generations of selection, the overall egg-to-adult duration in the selected stocks was reduced by ~29 h (~12.5 %) relative to controls, with the selected populations showing a concurrent reduction in time taken to hatching, pupation and wing pigmentation, by ~2, ~16, and ~25.2 h, respectively. Furthermore, selected populations showed a concomitant reduction in the circadian period of locomotor activity rhythm, implying that circadian clocks and development time are correlated. Thus, our study provides the first ever unequivocal evidence for the evolution of circadian clocks as a correlated response to selection for faster pre-adult development, suggesting that circadian clocks and development are linked in fruit flies D. melanogaster.     

PACAP and PDF signaling in the regulation of mammalian and insect circadian rhythms

Endogenous circadian clocks are inherent to all living organisms. They are needed to guarantee successful life since they regulate very important biological processes such as behavior and reproduction. Secretin-like G-protein coupled receptors are very important factors in the signal transduction pathways of circadian clocks. In this review, we will focus on the role of two secretin-like signaling pathways that play an important role in the regulation of the mammalian and the insect clock, respectively: the pituitary adenylate cyclase-activating polypeptide (PACAP) and pigment dispersing factor (PDF) signaling pathways. Both pathways are most likely related although their function in the biological clock differs.     

Altered Phase-Relationship between Peripheral Oscillators and Environmental Time in Cry1 or Cry2 Deficient Mouse Models for Early and Late Chronotypes

The mammalian circadian system is composed of a light-entrainable central clock in the suprachiasmatic nuclei (SCN) of the brain and peripheral clocks in virtually any other tissue. It allows the organism to optimally adjust metabolic, physiological and behavioral functions to the physiological needs it will have at specific time of the day. According to the resonance theory, such rhythms are only advantageous to an organism when in tune with the environment, which is illustrated by the adverse health effects originating from chronic circadian disruption by jetlag and shift work. Using short-period Cry1 and long-period Cry2 deficient mice as models for morningness and eveningness, respectively, we explored the effect of chronotype on the phase relationship between the central SCN clock and peripheral clocks in other organs. Whereas the behavioral activity patterns and circadian gene expression in the SCN of light-entrained Cry1^-/- and Cry2^-/- mice largely overlapped with that of wild type mice, expression of clock and clock controlled genes in liver, kidney, small intestine, and skin was shown to be markedly phase-advanced or phase-delayed, respectively. Likewise, circadian rhythms in urinary corticosterone were shown to display a significantly altered phase relationship similar to that of gene expression in peripheral tissues. We show that the daily dissonance between peripheral clocks and the environment did not affect the lifespan of Cry1^-/- or Cry2^-/- mice. Nonetheless, the phase-shifted peripheral clocks in light-entrained mice with morningness and eveningness-like phenotypes may have implications for personalized preventive and therapeutic (i.e. chronomodulation-based) health care for people with early and late chronotypes.