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Review
. 2011:76:63-72.
doi: 10.1101/sqb.2011.76.010546. Epub 2011 Sep 2.

Circadian clocks in fuel harvesting and energy homeostasis

K M Ramsey  1 , J Bass
Affiliations
Review

Circadian clocks in fuel harvesting and energy homeostasis

K M Ramsey et al. Cold Spring Harb Symp Quant Biol. 2011.

Abstract

Circadian systems have evolved in plants, eubacteria, neurospora, and the metazoa as a mechanism to optimize energy acquisition and storage in synchrony with the rotation of the Earth on its axis. In plants, circadian clocks drive the expression of genes involved in oxygenic photosynthesis during the light and nitrogen fixation during the dark, repeating this cycle each day. In mammals, the core clock in the suprachiasmatic nucleus (SCN) functions to entrain extra-SCN and peripheral clocks to the light cycle, including regions central to energy homeostasis and sleep, as well as peripheral tissues involved in glucose and lipid metabolism. Tissue-specific gene targeting has shown a primary role of clock genes in endocrine pancreas insulin secretion, indicating that local clocks play a cell-autonomous role in organismal homeostasis. A present focus is to dissect the consequences of clock disruption on modulation of nuclear hormone receptor signaling and on posttranscriptional regulation of intermediary metabolism. Experimental genetic studies have pointed toward extensive interplay between circadian and metabolic systems and offer a means to dissect the impact of local tissue molecular clocks on fuel utilization across the sleep-wake cycle.

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Figures

Figure 1
Figure 1
Interplay between clock, environment, and nutrient availability. The main pacemaker clock within the suprachiasmatic nucleus (SCN) receives photic input from the environment and synchronizes downstream extra-SCN and peripheral metabolic oscillators to optimize energy storage and utilization pathways (listed as physiological outputs). The daily cycles of fasting and feeding entrain the peripheral clocks, perhaps through changes in glucose, fatty acids, NAD/NADH and AMP/ATP ratios, or hormones (peptidergic or steroidal). Changes in food availability, such as in conditions of high-fat feeding, caloric restriction, and restricted feeding, also affect the clock. Although restricted feediing entrains just peripheral clocks, a hypocaloric diet (caloric restriction) has been shown to entrain both peripheral and SCN clocks. Thus, the circadian network optimizes the timing of downstream metabolic processes to coordinate with daily light–dark cycles and the nutrient millieu.
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