How does the circadian clock send timing information to the brain?

This paper discusses circadian output in terms of the signaling mechanisms used by circadian pacemaker neurons. In mammals, the suprachiasmatic nucleus houses a clock controlling several rhythmic events. This nucleus contains one or more pacemaker circuits, and exhibits diversity in transmitter content and in axonal projections. In Drosophila, a comparable circadian clock is located among period -expressing neurons, a sub-set of which (called LN-vs) express the neuropeptide PDF. Genetic experiments indicate LN-vs are the primary pacemakers neurons controlling daily locomotion and that PDF is the principal circadian transmitter. Further definition of pacemaker properties in several model systems will provide a useful basis with which to describe circadian output mechanisms.     

Chlamydomonas reinhardtii as a new model system for studying the molecular basis of the circadian clock

The genome of the unicellular green alga Chlamydomonas reinhardtii has both plant-like and animal-like genes. It is of interest to know which types of clock genes this alga has. Recent forward and reverse genetic studies have revealed that its clock has both plant-like and algal clock components. In addition, since C. reinhardtii is a useful model organism also called "green yeast", the identification of clock genes will make C. reinhardtii a powerful model for studying the molecular basis of the eukaryotic circadian clock. In this review, we describe our forward genetic approach in C. reinhardtii and discuss some recent findings about its circadian clock.     

Genetics of Circadian Clocks

The isolation of circadian clock mutants in Neurospora crassa and Drosophila melanogaster have identified numerous genes whose function is necessary for the normal operation of the circadian clock. In Neurospora many of these mutants map to a single locus called frq, whose properties suggest that its gene product is intimately involved in clock function. In Drosophila mutations at the per locus also suggest a significant role for the product of this gene in the insect clock mechanism. The per gene has been cloned and its gene product identified as a proteoglycan, most likely a membrane protein involved in affecting the ionic or electrical properties of cells in which it is located. Future progress in elucidating the mechanisms of circadian clocks are likely to come from continued analysis of clock mutants, both at the genetic and molecular levels.     

Modulation of circadian clocks by nutrients and food factors

Daily activity rhythms that are dominated by internal clocks are called circadian rhythms. A central clock is located in the suprachiasmatic nucleus of the hypothalamus, and peripheral clocks are located in most mammalian peripheral cells. The central clock is entrained by light/dark cycles, whereas peripheral clocks are entrained by feeding cycles. The effects of nutrients on the central and peripheral clocks have been investigated during the past decade and much interaction between them has come to light. For example, a high-fat diet prolongs the period of circadian behavior, a ketogenic diet advances the onset of locomotor activity rhythms, and a high-salt diet advances the phase of peripheral molecular clocks. Moreover, some food factors such as caffeine, nobiletin, and resveratrol, alter molecular and/or behavioral circadian rhythms. Here, we review nutrients and food factors that modulate mammalian circadian clocks from the cellular to the behavioral level.     

Molecular oscillation behind the clockface

The earth rotates on its own axis while orbiting around the sun. This regular movement of the solar system results in cyclic changes of the light condition of the earth with a period of 24 h, although the lengths of daytime and nighttime depend on the latitude. The organisms living on the earth have evolved an internal time-measuring system called the "circadian clock," which ticks with a period of approximately 24 h in order to adapt to the environment and to anticipate the next cycle. The fact that most of existing organisms retain the circadian clock suggests that the clock-ownership must have been advantageous over non-ownership during their evolution. Here I will introduce the background of the research field of circadian rhythm and present an outline of this Special Review series, which is composed of three articles that review recent research into the molecular mechanisms of the three types of circadian clock systems in vertebrates.     

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.     

How many pieces to build a circadian clock?

Biological rhythms represent a fundamental property of various living organisms. In particular, circadian rhythms, i.e. rhythms with a period close to 24 hours, help organisms to adapt to environmental daily rhythms. Although various factors can entrain or reset rhythms, they persist even in the absence of external timing cue, showing that their generation is endogenous. Indeed, the suprachiasmatic nucleus (SCN) of the hypothalamus is considered to be the main circadian clock in mammals. Isolated SCN neurons have been shown to display circadian rhythms, and in each cell, a set of genes, called "clock genes", are devoted to the generation and regulation of rhythms. Recently, it has become obvious that the clock located in the SCN is not homogenous, but is rather composed of multiple functional components somewhat reminiscent of its neurochemical organization. The significance and implications of these findings are still poorly understood but pave the way for future exciting studies. Here, current knowledge concerning these distinct neuronal populations and the ways through which synchronization could be achieved, as well as the potential role of neuropeptides in both photic and non-photic resetting of the clock, are summarized. Finally, we discuss the role of the SCN within the circadian system, which also includes oscillators located in various tissues and cell types.     

Circadian Clock-Regulated Expression of Phytochrome and Cryptochrome Genes in Arabidopsis

Many physiological and biochemical processes in plants exhibit endogenous rhythms with a period of about 24 h. Endogenous oscillators called circadian clocks regulate these rhythms. The circadian clocks are synchronized to the periodic environmental changes (e.g. day/night cycles) by specific stimuli; among these, the most important is the light. Photoreceptors, phytochromes, and cryptochromes are involved in setting the clock by transducing the light signal to the central oscillator. In this work, we analyzed the spatial, temporal, and long-term light-regulated expression patterns of the Arabidopsis phytochrome (PHYA to PHYE) and cryptochrome (CRY1 and CRY2) promoters fused to the luciferase (LUC(+)) reporter gene. The results revealed new details of the tissue-specific expression and light regulation of the PHYC and CRY1 and 2 promoters. More importantly, the data obtained demonstrate that the activities of the promoter::LUC(+) constructs, with the exception of PHYC::LUC(+), display circadian oscillations under constant conditions. In addition, it is shown by measuring the mRNA abundance of PHY and CRY genes under constant light conditions that the circadian control is also maintained at the level of mRNA accumulation. These observations indicate that the plant circadian clock controls the expression of these photoreceptors, revealing the formation of a new regulatory loop that could modulate gating and resetting of the circadian clock.     

Molecular mechanism of suppression of circadian rhythms by a critical stimulus

Circadian singularity behavior (also called suppression of circadian rhythms) is a phenomenon characterized by the abolishment of circadian rhythmicities by a critical stimulus. Here we demonstrate that both temperature step up and light pulse, stimuli that activate the expression of the Neurospora circadian clock gene frequency (frq), can trigger singularity behavior in this organism. The arrhythmicity is transient and is followed by the resumption of rhythm in randomly distributed phases. In addition, we show that induction of FRQ expression alone can trigger singularity behavior, indicating that FRQ is a state variable of the Neurospora circadian oscillator. Furthermore, mutations of frq lead to changes in the amplitude of FRQ oscillation, which determines the sensitivity of the clock to phase-resetting cues. Our results further suggest that the singularity behavior is due to the loss of rhythm in all cells. Together, these data suggest that the singularity behavior is due to a circadian negative feedback loop driven to a steady state after the critical treatment. After the initial arrhythmicity, cell populations are then desynchronized.     

生物节律影响巨核细胞发育和血小板产生的研究进展

自然界中生物体的生命活动、生活习性都存在着一定的周期性变化。生物昼夜节律的产生是以内源性的生物钟系统为基础的。生物钟不仅易受到外界环境的影响,而且可以通过调控一系列特定的下游基因的表达,影响生物体的生理生化过程。巨核细胞是生成血小板的前体细胞,经过分化、增殖、成熟和裂解,最终生成血小板。血小板是一种没有细胞核的特殊细胞,在生理性止血和器官修复上发挥着重要作用,同时参与血栓等多种疾病的发生。近几年借助现代分子生物学和细胞生物学手段,证实了哺乳动物的巨核细胞和血小板的生成呈现明显的周期性的变化,利用生物钟基因缺失模型进一步发现了生物钟基因对巨核细胞和血小板的影响。本文概述了生物节律对巨核细胞和血小板的影响,为进一步研究巨核细胞的发育和血小板生成机制提供了参考。     

生物节律影响巨核细胞发育和血小板产生的研究进展

自然界中生物体的生命活动、生活习性都存在着一定的周期性变化。生物昼夜节律的产生是以内源性的生物钟系统为基础的。生物钟不仅易受到外界环境的影响,而且可以通过调控一系列特定的下游基因的表达,影响生物体的生理生化过程。巨核细胞是生成血小板的前体细胞,经过分化、增殖、成熟和裂解,最终生成血小板。血小板是一种没有细胞核的特殊细胞,在生理性止血和器官修复上发挥着重要作用,同时参与血栓等多种疾病的发生。近几年借助现代分子生物学和细胞生物学手段。证实了哺乳动物的巨核细胞和血小板的生成呈现明显的周期性的变化,利用生物钟基因缺失模型进一步发现了生物钟基因对巨核细胞和血小板的影响。本文概述了生物节律对巨核细胞和血小板的影响,为进一步研究巨核细胞的发育和血小板生成机制提供了参考。     

Kernel Architecture of the Genetic Circuitry of the Arabidopsis Circadian System

A wide range of organisms features molecular machines, circadian clocks, which generate endogenous oscillations with ~24 h periodicity and thereby synchronize biological processes to diurnal environmental fluctuations. Recently, it has become clear that plants harbor more complex gene regulatory circuits within the core circadian clocks than other organisms, inspiring a fundamental question: are all these regulatory interactions between clock genes equally crucial for the establishment and maintenance of circadian rhythms? Our mechanistic simulation for Arabidopsis thaliana demonstrates that at least half of the total regulatory interactions must be present to express the circadian molecular profiles observed in wild-type plants. A set of those essential interactions is called herein a kernel of the circadian system. The kernel structure unbiasedly reveals four interlocked negative feedback loops contributing to circadian rhythms, and three feedback loops among them drive the autonomous oscillation itself. Strikingly, the kernel structure, as well as the whole clock circuitry, is overwhelmingly composed of inhibitory, rather than activating, interactions between genes. We found that this tendency underlies plant circadian molecular profiles which often exhibit sharply-shaped, cuspidate waveforms. Through the generation of these cuspidate profiles, inhibitory interactions may facilitate the global coordination of temporally-distant clock events that are markedly peaked at very specific times of day. Our systematic approach resulting in experimentally-testable predictions provides insights into a design principle of biological clockwork, with implications for synthetic biology.     

Recording and Analysis of Circadian Rhythms in Running-wheel Activity in Rodents

When rodents have free access to a running wheel in their home cage, voluntary use of this wheel will depend on the time of day^1-5. Nocturnal rodents, including rats, hamsters, and mice, are active during the night and relatively inactive during the day. Many other behavioral and physiological measures also exhibit daily rhythms, but in rodents, running-wheel activity serves as a particularly reliable and convenient measure of the output of the master circadian clock, the suprachiasmatic nucleus (SCN) of the hypothalamus. In general, through a process called entrainment, the daily pattern of running-wheel activity will naturally align with the environmental light-dark cycle (LD cycle; e.g. 12 hr-light:12 hr-dark). However circadian rhythms are endogenously generated patterns in behavior that exhibit a ~24 hr period, and persist in constant darkness. Thus, in the absence of an LD cycle, the recording and analysis of running-wheel activity can be used to determine the subjective time-of-day. Because these rhythms are directed by the circadian clock the subjective time-of-day is referred to as the circadian time (CT). In contrast, when an LD cycle is present, the time-of-day that is determined by the environmental LD cycle is called the zeitgeber time (ZT).Although circadian rhythms in running-wheel activity are typically linked to the SCN clock^6-8, circadian oscillators in many other regions of the brain and body^9-14 could also be involved in the regulation of daily activity rhythms. For instance, daily rhythms in food-anticipatory activity do not require the SCN^15,16 and instead, are correlated with changes in the activity of extra-SCN oscillators^17-20. Thus, running-wheel activity recordings can provide important behavioral information not only about the output of the master SCN clock, but also on the activity of extra-SCN oscillators. Below we describe the equipment and methods used to record, analyze and display circadian locomotor activity rhythms in laboratory rodents.