Exp Neurobiol 2011; 20(1): 18-28
Published online March 31, 2011
© The Korean Society for Brain and Neural Sciences
Ilmin Kwon, Han Kyoung Choe, Gi Hoon Son and Kyungjin Kim*
Department of Biological Sciences, Seoul National University and the Brain Research Center for the 21st Century Frontier Program in Neuroscience,Seoul 151-742, Korea
Correspondence to: *To whom correspondence should be addressed.
Tel: 82-2-880-6694/872-9100, Fax: 82-2-872-9108
As a consequence of the Earth's rotation, almost all organisms experience day and night cycles within a 24-hr period. To adapt and synchronize biological rhythms to external daily cycles, organisms have evolved an internal time-keeping system. In mammals, the master circadian pacemaker residing in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus generates circadian rhythmicity and orchestrates numerous subsidiary local clocks in other regions of the brain and peripheral tissues. Regardless of their locations, these circadian clocks are cell-autonomous and self-sustainable, implicating rhythmic oscillations in a variety of biochemical and metabolic processes. A group of core clock genes provides interlocking molecular feedback loops that drive the circadian rhythm even at the single-cell level. In addition to the core transcription/translation feedback loops, post-translational modifications also contribute to the fine regulation of molecular circadian clocks. In this article, we briefly review the molecular mechanisms and post-translational modifications of mammalian circadian clock regulation. We also discuss the organization of and communication between central and peripheral circadian oscillators of the mammalian circadian clock.
Keywords: circadian pacemaker, SCN, feedback loop, mammalian circadian clock
Regardless of disparate phylogenetic origins and huge differences in complexity among species, organisms have evolved internal timing systems to adapt to the external day and night cycles. This daily time-keeping system is referred to as the 'circadian clock' from the Latin
The mammalian circadian clock system is composed of three basic components: input signals (environmental timing cue), a circadian oscillator (rhythm generator) and output signals (overt rhythm). In opaque animals, including mammals, the light signal is mainly detected by eyes. Behavioral studies using laboratory animals, such as mice, rats and hamsters, revealed that intact animals exhibit approximately 24 hr of circadian rhythm even in the absence of external time cues (e.g., constant darkness). These findings strongly support the existence of a 'master circadian oscillator', which generates intrinsic circadian rhythmicity (Fig. 1). In an effort to determine the master circadian pacemaker of mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus was anatomically found to be a direct target of the retinal fibers (Reppert and Weaver, 2002). More importantly, selective disruption of the SCN revealed a complete loss of circadian rhythmicity, whereas transplantation of an intact SCN to the mutant animal restored circadian rhythmicity (Stephan and Zucker, 1972; Ralph et al., 1990). Thus, the SCN is currently regarded as the master circadian oscillator where the circadian rhythm is generated. The circadian rhythm generated in the SCN is likely converted into neuronal or hormonal signals that affect the behavior, physiology and metabolic processes of entire animals (Schibler and Sassone-corsi, 2002).
In addition to the central pacemaker, numerous subsidiary circadian clocks exist outside the SCN in peripheral tissues (including the liver, heart, lung and muscle) and even in immortalized cell lines, which are kept
Most recent chronobiological studies have focused on the molecular basis of the circadian clock. Intensive studies have revealed that at least one internal autonomous circadian oscillator consisting of positive and negative elements of autoregulatory feedback loops is at the center of all examined circadian clocks. For example, the circadian clock of cyanobacteria is regulated by a cluster of three genes:
The mammalian circadian oscillator consists of a network of interlocking transcriptional-translational feedback loops that drive rhythmic expression of core clock components (Reppert and Weaver, 2002). The core clock components are established by genes whose protein products are necessary for the generation and maintenance of circadian rhythms within individual cells throughout the organism. As core components of the mammalian circadian molecular clock, CLOCK and BMAL1, which belong to the family of bHLH-PAS-containing transcription factors, constitute a positive feedback loop. CLOCK and BMAL1 form a heterodimer, which binds to the E-box cis-regulatory enhancer elements of their target genes, including
In addition to the feedback loops of transcription and translation, various post-translational modifications are also involved in the normal functioning of the circadian clockwork. A few hours would be sufficient for a molecular feedback loop to run a cycle by only transcriptional activation and following feedback repression. Therefore, if not for the significant delay between transcriptional activation and repression, 24-hr circadian periodicity would not be achieved. Increasing evidence supports the involvement of post-translational modifications for the required delay (Gallego and Virshup, 2007). Studies in molecular circadian clock machinery in various phyla have found several protein kinases involved in circadian regulation. The double-time (DBT) kinase was the first enzyme identified as an essential component of the
Phosphorylation is a prerequisite step for the recruitment of ubiquitin ligases and the subsequent degradation of PERs. In
Although the peripheral oscillators share the same basic molecular components with the central pacemaker, the peripheral clocks are thought to be less self-sustainable while the master pacemaker is indispensible for rhythm generation in peripheral clocks. Indeed, circadian rhythms in peripheral clocks of liver, lung and skeletal muscle are damped in two to seven days, whereas the SCN exhibits robust rhythmicity up to 30 days
The roles of such self-sustainable peripheral oscillators are still unknown. The adrenal local clock is a good example of a peripheral clock. The adrenal cortex is well known for synthesis and secretion of the steroid hormone glucocorticoid (GC). This hormone is secreted in a circadian rhythmic pattern and is implicated in a wide variety of biological processes, including stress response, growth, reproduction and immune response (Buckingham, 2006). Interestingly, injection of a synthetic GC, dexamethasone (DEX), induced a phase shift of circadian rhythmicity in mouse liver, while the central clock in the SCN was barely affected (Balsalobre et al., 2000), suggesting that GC is also important for the synchronization of circadian rhythmicity. The SCN has been reported to directly modulate GC secretion
The food-entrainable oscillator (FEO) is another good example of a SCN-independent local clock. Unlike the central circadian oscillator, the mammalian peripheral circadian oscillators are not entrained by light. Instead, feeding time and/or hormone stimulation can be a dominant zeitgeber for several peripheral circadian clocks (Balsalobre et al., 2000; Stokkan et al., 2001). SCN-disrupted animals were reported to still be able to respond to rhythmic food availability (Krieger et al., 1977). In addition, SCN lesions do not abolish food-anticipatory behaviors in rats (Stephan et al., 1979). When the feeding was restricted to a certain time, laboratory animals showed a food anticipatory activity (FAA), namely increased locomotor activity prior to food presentation. Thus, mammals surprisingly can exhibit FAA despite the presence of SCN lesions. Another local clock in stomach gland oxyntic cells that rhythmically secrete ghrelin, a hormone stimulating hunger, was reported recently to play a role in mouse FAA (LeSauter et al., 2009). In these cells, rhythmic expression of ghrelin was controlled by circadian clock machinery, and loss of ghrelin receptors resulted in diminished FAA. However, the mechanisms by which the gastric local clocks and their products act on the central nervous system to generate food-anticipation behaviors remain unclear.
Circadian oscillators also exist in various areas of the brain outside of the SCN. Several transgenic animal models have been developed whose circadian rhythmicity can be measured in a real time-enabled evaluation of the molecular properties of these extra-SCN brain clocks. For example, 14 of 27
The extra-SCN brain clocks are also associated with a reward system (Dibner et al., 2010). This reward system is closely related to the survival of individual organisms, and accessibility to food and water can be a primary reward. Rewards can influence the higher brain functions of an organism, including behaviors, learning capability and mood. Several groups recently described the relationship between the brain reward system and the circadian clock. For example, mice lacking
Circadian clock research during recent decades has focused on the elucidation of the molecular mechanisms involved in regulating these processes. However, studies have become more diverse, concentrating on the understanding of functional roles of molecular circadian clocks in the regulation of physiology, behaviors and metabolism. In particular, the circadian clock is associated with various metabolic processes. In unicellular organisms such as cyanobacteria, the circadian clock system segregates two essential metabolic processes, nitrogen fixation and photosynthesis, which are incompatible with each other (Tu and McKnight, 2006). By separating nitrogen fixation and photosynthesis between night and day, respectively, cyanobacteria can resolve the incompatibility of these two key processes. Even in mammals, expression of many metabolic genes is tissue-specifically regulated by circadian clockwork (Panda et al., 2002). In addition, nuclear receptors involved in lipid and carbohydrate metabolism are expressed in a circadian manner in various tissues, including liver, skeletal muscle and white and brown adipose tissues (Yang et al., 2006). In a more recent study, the core clock gene
The circadian clock is also implicated in mental disorders, such as bipolar disorder and depression. The mania-like behavior is defined as a state of abnormal mood, arousal and energy levels. Recently, mice bearing a mutant form of CLOCK were reported to exhibit mania-like behavior (Roybal et al., 2007). The
The most important development in the history of circadian clock research was the recognition of the development of animal models harboring mutations in core clock genes. Using these animals, investigators have identified the functions and implications of specific core clock components in various biological processes and diseases. Although most of the components of molecular clock loops and their interlocking networks are relatively well established using these animal models, several unsolved questions, particularly in terms of the human circadian clock system, still remain. In fact, the circadian clock is closely related to the everyday lives of humans. For example, travel to a different time zone results in fatigue, mild depression and sleep disorders. These symptoms are generally called jet lag and are caused by desynchronization of the internal body clock to day/night cycles. Moreover, tumorsuppressive actions of the molecular clock component