Sci. Aging Knowl. Environ., 11 May 2005
Vol. 2005, Issue 19, p. pe13
[DOI: 10.1126/sageke.2005.19.pe13]

PERSPECTIVES

Keep Time, Stay Healthy

Gene Block

The author is in the Department of Biology at the University of Virginia, Charlottesville, VA 22903, USA. E-mail: gdb{at}virginia.edu

http://sageke.sciencemag.org/cgi/content/full/2005/19/pe13

Key Words: clock • circadian rhythm • obesity • metabolic syndrome • sleep-wake cycle

Introduction

Biological clocks, circadian clocks in particular, are present in nearly all organisms. These internal timers create rhythmic patterns in behavior and physiology that help an organism anticipate and cope with the changing environmental conditions associated with geophysical cycles.

Brain structures responsible for generating circadian rhythms in mammals have been identified. Localization was accomplished through the use of a wide range of experimental approaches, including ablation of portions of the brain through electrolytic lesions, measurements of in vivo metabolic rate, in vivo and in vitro electrophysiological recording, and transplantation (1). What emerged from these investigations was profound: the recognition that a small brain region at the base of the hypothalamus, above the optic chiasm, the suprachiasmatic nucleus (SCN), was essential for expression of the majority of mammalian behavioral and physiological rhythms. The identification of the SCN as a circadian clock stands as one of the few examples in neuroscience where brain function could be unambiguously mapped to a specific location. The discovery of identifiable mammalian "clock tissue" accelerated both the cellular and molecular analyses of biological clocks. What has emerged is a satisfying picture of the clockworks, circadian oscillators, competent at the single cell level, whose rhythms are generated by molecular feedback loops involving clock genes and their products (2). For a discussion of aging and mammalian clocks, see Pardee Review, "Up All Night," and "Good Timing."

It is indisputable that the development of an SCN-focused model, which emerged in the past quarter century, enabled productive research that addressed a wide range of questions about circadian timing systems, such as the identification of the photoreceptors and pathways through which the SCN is synchronized to local time by the retina (3). Perhaps inevitably, one shortcoming of this SCN-centric framework was that it drew experimental attention away from other potential sources of circadian rhythmicity, even though it was well recognized that some circadian oscillations remain following ablation of the SCN, including a rhythm in food anticipatory behavior (4).

Within the past 10 years, two lines of experimental investigation have altered, in substantial ways, our view about mammalian circadian timing. The first involved the unambiguous identification of robust circadian rhythmicity in other neural and nonneural tissues, with the demonstration of hormonal rhythmicity in the isolated retina (5) and subsequently with the identification of molecular circadian rhythms in neural (6) and nonneural tissues (7, 8). These discoveries led to the conclusion that, similar to the situation in invertebrates (9), circadian rhythm generation is relatively widespread in mammals. This recognition raised the intriguing question of whether many physiological functions throughout the body were regulated locally and "actively" by intrinsic circadian oscillators rather than by being driven directly by the SCN. It became useful to think of the mammalian temporal plan as more akin to a clock shop, with the SCN providing overall synchronization but with tissue-specific temporal regulation provided by local oscillators.

A second line of investigation that has prompted new thinking is the recognition that clock genes can also play nonclocklike roles. Perhaps most unexpected was the discovery in Drosophila that cocaine sensitivity ("drug craving") was eliminated in flies mutant for the period, clock, cycle, or doubletime genes (10). A related phenomenon has been observed in mammals (11), and both cases appear to involve clock genes in tissues outside the central pacemaker structures. Mutations in clock genes have also been implicated in carcinogenesis, because they can increase the probability of mice developing cancerous tumors (12).

Clock Mutations and Metabolic Syndrome

Given the discovery of autonomous circadian oscillators in many mammalian tissues, including those involved in energy metabolism, and the recognition of new and unforeseen roles for clock genes, it is perhaps not surprising that Bass and colleagues have recently reported that mice carrying the Clock gene mutation display metabolic disorders (13). This is an interesting discovery that takes on added importance in the context of other recent studies in humans and animal models on the role of sleep and wakefulness in the regulation of body weight, food preference, and metabolism.

Bass and coinvestigators compared weight gain and metabolic indices in wild-type mice and those carrying a mutation of the Clock gene. The CLOCK transcription factor is a key component of the molecular circadian rhythm within SCN neurons. In constant darkness, Clock mutant mice exhibit unstable or long circadian periods, with activity often becoming arrhythmic over time (14). Clock mice also exhibit sloppy activity rhythms when exposed to light:dark cycles. Unlike wild-type animals that restrict most of their locomotor activity to the dark, Clock mutant mice exhibit increases in daytime activity and indistinct onsets of locomotor activity at dusk. Although most behavioral activity in Clock mice remains nocturnal, there is a significant increase in the amount of food intake during the daytime, leading to a nearly complete disappearance of the food-intake rhythm (Fig. 1). Associated with the loss of the feeding rhythm, Clock mutants exhibit weight gain that is modest when they are fed regular food but that becomes pronounced when they are fed high-fat food (Fig. 2). The weight gain is associated with an increase in food intake. Interestingly, the mRNAs of the regulatory peptides orexin, ghrelin, and CART (cocaine and amphetamine-regulated transcript) are present at lower than usual concentrations, and their rhythmicity is damped or absent. Clock mutant mice exhibit a breakdown in weight regulation. They exhibit hyperleptinemia (higher than usual serum concentrations of leptin), hyperlipidemia, hepatic steatosis (a condition in which fat droplets accumulate within liver cells), hyperglycemia, and hypoinsulinemia--in short, metabolic syndrome (see "Greasing Aging's Downward Slide"). It is not yet certain where the Clock mutation is exerting its effects. Alterations of circadian rhythmicity within the SCN could affect the function of tissues rhythmically regulated by the SCN. Alternatively, the presence of local oscillators within peripheral tissues and non-SCN brain regions raises the possibility of more regional action of the Clock mutation on body-weight regulation. Still another possibility, as discussed below, is that the effects of the Clock mutation are indirect, through alterations in the sleep-wake cycle.



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Fig. 1. Altered diurnal rhythm in feeding in Clock mutant mice. Different groups of adult wild-type and Clock mutant mice were maintained on a regular diet and food intake was measured during light (L) and dark (D) periods. Results shown are average food intake during light and dark periods as a percentage of total food intake (*, P < 0.001). [Figure reproduced, and legend reproduced in slightly modified form, from (13).]

 


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Fig. 2. Obesity in Clock mutant mice. (A) Energy intake. Average caloric intake over a 10-week period in male wild-type and Clock mutant mice. Wild-type and Clock mutant mice were provided ad libitum access to regular or high-fat chow for 10 weeks beginning at 6 weeks of age. Weekly food intake was analyzed in the two groups (*, P < 0.01). (B) Body weight. Body weights for the animals depicted in (A) after the 10-week study (*, P < 0.01). [Figure reproduced, and legend reproduced in slightly modified form, from (13).]

 
One unresolved issue is the relation of these findings with earlier studies showing that sleep loss in the rat leads to hyperphagia, that is, the ingestion of too much food (15). Insomuch as the Clock mutant mouse exhibits increased diurnal activity, one possibility is that some portion of the effects observed on eating behavior and weight gain is mediated through sleep loss. The current study did not explicitly measure sleep; however, an earlier study by the Turek laboratory (16) revealed that Clock mutant mice sleep 1 to 2 hours less than wild types when exposed to a 12-hour light, 12-hour dark cycle. This raises the issue of whether effects observed on metabolic activity may be related to sleep loss rather than to any direct dysregulation of metabolic processes.

Whatever the precise mechanism acting to affect metabolism in the Clock mouse, the issue of circadian breakdown and sleep loss is of substantial interest because of the similarity to the effects of sleep loss on weight gain and metabolic problems in humans. In a study by Eve van Cauter's group (17), 12 healthy male volunteers were studied to see how sleep loss affected the hormones that control appetite. Measurement of serum concentrations of leptin, a peptide produced by fat cells that promotes a feeling of satiety, revealed an 18% decrease in the sleep-deprived group. In contrast, there was a 28% increase in the concentration of the peptide ghrelin, a hormone produced primarily in the stomach that increases hunger (see "Opposites Detract" and "Growing Pains"). In addition, sleep-deprived subjects exhibited increased hunger and appetite, especially for calorie-dense foods with high carbohydrate content.

Conclusion

The Bass and co-workers study (13) is an interesting contribution in demonstrating that genetic mutations leading to altered circadian timing share characteristics with behaviorally induced alterations in the sleep-wake cycle. An important question remains about how these effects are mediated, whether the changes are an indirect result of sleep deprivation--due to the effects of altered SCN timing on the control of peripheral tissues--or whether the mutation acts directly upon the circadian oscillators in tissues controlling metabolic function. As always, nature will surprise us, most likely revealing regulation at many levels of circadian organization.


May 11, 2005
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Citation: G. Block, Keep Time, Stay Healthy. Sci. Aging Knowl. Environ. 2005 (19), pe13 (2005).








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