Reconsidering mammalian circadian organization.

The genomic basis of temporal niche evolution in a diurnal rodent

Circadian rhythms are prevalent on Earth and temporally organize behaviour and physiology of organisms to occur in species-specific 'temporal niches'. However, species differ in how strictly individuals are controlled by their circadian clock, suggesting that it may offer a selective advantage for an individual to extend its temporal niche under certain circumstances, for example during stressful environmental conditions. A potential mechanism controlling temporal niche adherence involves the evolutionary capacitor and chaperon protein HSP90, known to assist the proper folding of important signalling molecules. If HSP90 becomes rate limiting (e.g., under environmental stress) hidden genetic variation will be expressed, producing novel and potentially beneficial phenotypes for the individual. While this role of HSP90 is well established for morphological traits, we show here that it extends to regulation of temporal behavioural patterns. We show that within a small subset of clock neurons in the fly brain, HSP83, the fly homologue of HSP90, mitigates inter-individual behavioural variability. We provide evidence for the requirement of HSP83 for efficient transcription of the gene encoding the circadian neuropeptide Pigment Dispersing Factor (PDF), and for correct PDF accumulation in central clock neurons. Strikingly, Hsp83 mutants affect synchronized oscillations of the clock protein PERIOD (PER) in subsets of circadian clock neurons in the same way as flies without PDF, further supporting a role of Hsp83 in regulating Pdf. Our findings therefore provide a mechanistic explanation for HSP83 function in regulation of behavioural variability, and offer an explanation for how to restrict temporal niche extension to stressful environmental conditions.

Abstract: There is now little doubt that disrupted day/night (circadian) time structures are involved in the initiation and promotion of neoplastic disease. It has been established that the incidence of breast cancer, colorectal cancer, and prostate cancer is increased as a result of nocturnal exposure to light and circadian function disruption and that cancer patient survival is diminished. So the question is: what public health measures can be implemented to minimize these health hazards? In addition, untreated cancer patients experience the symptom cluster of brief, interrupted, and poor nighttime sleep; depressed mood/anxiety; daytime fatigue/lethargy; and anorexia/early satiety/diminished taste sensation – each of which is virtually pathognomonic of a disrupted circadian temporal organization. Direct measurements of patients' activities and their timing and intensity using actigraphy reveal that untreated cancer patients experience severe deterioration in the robustness (amplitude) and day-to-day phase stability of their daily rest/activity rhythms – and one of the most personal and socially destructive results of such circadian disorientation is unplanned, unwanted, and avoidable temporal isolation from family, friends, and society. Thus, therapeutic manipulation of the circadian clock is a powerful tool for improving cancer patients' quality of life (QOL), making life more worth living and perhaps prolonging higher quality survival. We herein take what we have learned to design and execute strategies shown to be beneficial and carefully measure cancer patient benefits. Outcome measures include indices that describe our ability to enhance/maintain circadian organization and orientation, diminish the above-mentioned symptoms, and improve QOL and survival prolongation. We herein implement a suite of noninvasive, riskless and costless, largely behavior-based circadian rhythm entrainment and disturbance avoidance techniques for widespread everyday use by cancer patients and survivors together with real-time actigraphic monitoring, continuous electronic feedback, and positive reinforcement of these simple temporal tuning interventions. Keywords: actigraphy, circadian clock, circadian rhythm, circadian therapy, nighttime sleep, daytime activity, fatigue, depression, anxiety, functional status, quality of life, survival, 24-hour activity monitoring

The circadian system temporally organizes physiology and behavior throughout the 24-h day. At the core of this organization lies a network of multiple circadian oscillators located within the central nervous system as well as in virtually every peripheral organ. These oscillators define a 24-h temporal landscape of mutually interacting circadian rhythms that is known as the temporal niche of a species. This temporal niche is constituted by the collective phases of all biological rhythms emerging from this multi-oscillatory system. We review evidence showing that under different environmental conditions, this system can adopt different harmonic configurations. Thus, the classic chronobiological approach of searching for "the" circadian phase of an animal-typically by studying circadian rhythms of locomotor activity-represents a narrow look into the circadian system of an animal. We propose that the study of hormonal rhythms may lead to a more insightful assessment of a species' temporal niche.

The circadian clock controls the physiological function of tissues through the regulation of thousands of genes in a cell‐type‐specific manner. The core cellular circadian clock is a transcription–translation negative feedback loop, which can recruit epigenetic regulators to facilitate temporal control of gene expression. Histone methyltransferase, mixed lineage leukemia gene 3 (MLL3) was reported to be required for the maintenance of circadian oscillations in cultured cells. Here, we test the role of MLL3 in circadian organization in whole animals. Using mice expressing catalytically inactive MLL3, we show that MLL3 methyltransferase activity is in fact not required for circadian oscillations in vitro in a range of tissues, nor for the maintenance of circadian behavioral rhythms in vivo. In contrast to a previous report, loss of MLL3‐dependent methylation did not affect the global levels of H3K4 methylation in liver, indicating substantial compensation from other methyltransferases. Furthermore, we found little evidence of genomic repositioning of H3K4me3 marks. We did, however, observe repositioning of H3K4me1 from intronic regions to intergenic regions and gene promoters; however, there were no changes in H3K4me1 mark abundance around core circadian clock genes. Output functions of the circadian clock, such as control of inflammation, were largely intact in MLL3‐methyltransferase‐deficient mice, although some gene‐specific changes were observed, with sexually dimorphic loss of circadian regulation of specific cytokines. Taken together, these observations indicate that MLL3‐directed histone methylation is not essential for core circadian clock function; however, it may influence the inflammatory response.

Temporal niche switching and reduced nest attendance in response to heat dissipation limits in lactating common voles (Microtus arvalis)

The mammalian circadian system synchronizes daily timing of activity and rest with the environmental light-dark cycle. Although the underlying molecular oscillatory mechanism is well studied, factors that influence phenotypic plasticity in daily activity patterns (temporal niche switching, chronotype) are presently unknown. Molecular evidence suggests that metabolism may influence the circadian molecular clock, but evidence at the level of the organism is lacking. Here we show that a metabolic challenge by cold and hunger induces diurnality in otherwise nocturnal mice. Lowering ambient temperature changes the phase of circadian light-dark entrainment in mice by increasing daytime and decreasing nighttime activity. This effect is further enhanced by simulated food shortage, which identifies metabolic balance as the underlying common factor influencing circadian organization. Clock gene expression analysis shows that the underlying neuronal mechanism is downstream from or parallel to the main circadian pacemaker (the hypothalamic suprachiasmatic nucleus) and that the behavioral phenotype is accompanied by phase adjustment of peripheral tissues. These findings indicate that nocturnal mammals can display considerable plasticity in circadian organization and may adopt a diurnal phenotype when energetically challenged. Our previously defined circadian thermoenergetics hypothesis proposes that such circadian plasticity, which naturally occurs in nocturnal mammals, reflects adaptive maintenance of energy balance. Quantification of energy expenditure shows that diurnality under natural conditions reduces thermoregulatory costs in small burrowing mammals like mice. Metabolic feedback on circadian organization thus provides functional benefits by reducing energy expenditure. Our findings may help to clarify relationships between sleep-wake patterns and metabolic phenotypes in humans.

Aging significantly impacts circadian timing in mammals. The amplitude and precision of behavioral, endocrine, and metabolic rhythms decline with age. This is accompanied with an age-related decline in the amplitude of central pacemaker output, although the molecular clock in the suprachiasmatic nucleus exhibit robust oscillation. Peripheral clocks also exhibit robust oscillation during aging, when extensive reprogramming of other genes' expression rhythms occurs in peripheral tissues. The age-related dissociation between the molecular clock and downstream rhythms in both central and peripheral tissues indicates that mechanisms other than the molecular clock are involved in mediating the impact the aging on circadian organization. In this article, findings are reviewed on the impact of aging on circadian timing functions, and the potential role of increased inflammatory response in age-related changes in circadian organization is highlighted.

GW182 Controls Drosophila Circadian Behavior and PDF-Receptor Signaling

Cell physiology is regulated along the 24-hour timescale by a circadian clock, which is comprised of interconnected molecular loops involving at least nine genes. The cellular clocks are coordinated by the suprachiasmatic nucleus, a hypothalamic pacemaker that also helps the organism adjust to environmental cycles. The rest-activity rhythm is a reliable marker of the circadian system function in both rodents and humans. This circadian organization is responsible for predictable changes in the tolerability and efficacy of anticancer agents, and possibly also may be involved in tumor promotion or growth. Expected least toxic times of chemotherapy were extrapolated from experimental models to human subjects with reference to the rest-activity cycle. The clinical relevance of the chronotherapy principle (i.e., treatment administration as a function of rhythms) has been investigated previously in randomized multicenter trials. In the current study, chronotherapeutic schedules were used to safely document activity of the combination of oxaliplatin, 5-fluorouracil, and leucovorin against metastatic colorectal carcinoma and to establish new medicosurgical management for this disease, and were reported to result in unprecedented long-term survival. Chronotherapy concepts appear to offer further potential to improve current cancer treatment options as well as to optimize the development of new anticancer or supportive agents.

BACKGROUND Cell physiology is regulated along the 24-hour timescale by a circadian clock, which is comprised of interconnected molecular loops involving at least nine genes. The cellular clocks are coordinated by the suprachiasmatic nucleus, a hypothalamic pacemaker that also helps the organism adjust to environmental cycles. The rest-activity rhythm is a reliable marker of the circadian system function in both rodents and humans. This circadian organization is responsible for predictable changes in the tolerability and efficacy of anticancer agents, and possibly also may be involved in tumor promotion or growth. METHODS Expected least toxic times of chemotherapy were extrapolated from experimental models to human subjects with reference to the rest-activity cycle. The clinical relevance of the chronotherapy principle (i.e., treatment administration as a function of rhythms) has been investigated previously in randomized multicenter trials. RESULTS In the current study, chronotherapeutic schedules were used to safely document activity of the combination of oxaliplatin, 5-fluorouracil, and leucovorin against metastatic colorectal carcinoma and to establish new medicosurgical management for this disease, and were reported to result in unprecedented long-term survival. CONCLUSIONS Chronotherapy concepts appear to offer further potential to improve current cancer treatment options as well as to optimize the development of new anticancer or supportive agents. Cancer 2003;97:155–69. © 2003 American Cancer Society. DOI 10.1002/cncr.11040

A crucial property of circadian clocks is the ability to regulate the shape of an oscillation over its cycle length (waveform) appropriately, thus enhancing Darwinian fitness. Many studies over the past decade have revealed interesting ways in which the waveform of rodent behavior could be manipulated, one of which is that the activity bout bifurcates under environments that have 2 light/dark cycles within one 24-h day (LDLD). It has been observed that such unique, although unnatural, environments reveal acute changes in the circadian clock network. However, although adaptation of waveforms to different photoperiods is well studied, modulation of waveforms under LDLD has received relatively less attention in research on insect rhythms. Therefore, we undertook this study to ask the following questions: what is the extent of waveform plasticity that Drosophila melanogaster exhibits, and what are the neuronal underpinnings of such plasticity under LDLD? We found that the activity/rest rhythms of wild-type flies do not bifurcate under LDLD. Instead, they show similar but significantly different behavior from that under a long-day LD cycle. This behavior is accompanied by differences in the organization of the circadian neuronal network, which include changes in waveforms of a core clock component and an output molecule. In addition, to understand the functional significance of such variations in the waveform, we examined laboratory selected populations that exhibit divergent eclosion chronotypes (and therefore, waveforms). We found that populations selected for predominant eclosion in an evening window (late chronotypes) showed reduced amplitude plasticity and increased phase plasticity of activity/rest rhythms. This, we argue, is reflective of divergent evolution of circadian neuronal network organization in our laboratory selected flies.

The functional changes of the circadian system organization in aging

Daily and circadian expression of neuropeptides in the suprachiasmatic nuclei of nocturnal and diurnal rodents

Circadian organization means the way in which the entire circadian system above the cellular level is put together physically and the principles and rules that determine the interactions among its component parts which produce overt rhythms of physiology and behavior. Understanding this organization and its evolution is of practical importance as well as of basic interest. The first major problem that we face is the difficulty of making sense of the apparently great diversity that we observe in circadian organization of diverse vertebrates. Some of this diversity falls neatly into place along phylogenetic lines leading to firm generalizations: i) in all vertebrates there is a "circadian axis" consisting of the retinas, the pineal gland and the suprachiasmatic nucleus (SCN), ii) in many non-mammalian vertebrates of all classes (but not in any mammals) the pineal gland is both a photoreceptor and a circadian oscillator, and iii) in all non-mammalian vertebrates (but not in any mammals) there are extraretinal (and extrapineal) circadian photoreceptors. An interesting explanation of some of these facts, especially the differences between mammals and other vertebrates, can be constructed on the assumption that early in their evolution mammals passed through a "nocturnal bottleneck". On the other hand, a good deal of the diversity among the circadian systems of vertebrates does not fall neatly into place along phylogenetic lines. In the present review we will consider how we might better understand such "phylogenetically incoherent" diversity and what sorts of new information may help to further our understanding of the evolution of circadian organization in vertebrates.