Metabolism and the circadian clock converge

Abstract

Circadian rhythms occur in almost all species and control vital aspects of our physiology, from sleeping and waking to neurotransmitter secretion and cellular metabolism. Epidemiological studies from recent decades have supported a unique role for circadian rhythm in metabolism. As evidenced by individuals working night or rotating shifts, but also by rodent models of circadian arrhythmia, disruption of the circadian cycle is strongly associated with metabolic imbalance. Some genetically engineered mouse models of circadian rhythmicity are obese and show hallmark signs of the metabolic syndrome. Whether these phenotypes are due to the loss of distinct circadian clock genes within a specific tissue versus the disruption of rhythmic physiological activities (such as eating and sleeping) remains a cynosure within the fields of chronobiology and metabolism. Becoming more apparent is that from metabolites to transcription factors, the circadian clock interfaces with metabolism in numerous ways that are essential for maintaining metabolic homeostasis.

Figure 1.

Basic characteristics of circadian rhythms. Circadian rhythms have a period of ∼24 h. A: the amplitude of various biological processes, such as eating, locomotion, gene expression, etc., vary considerably across organisms and physiological events, as does the phase of the rhythm. B: actograms are used to depict the rhythm of an organism over the 24-h cycle and typically consist of digitized activity values that are presented as a double plot, where a line contains data for that day as well as the proceeding day. In this example, the red arrow denotes the activity of a mouse in a 12-h light/12-h dark cycle, while the blue arrow denotes the activity of the animal as a result of a switch to constant darkness (DD). Such activity in DD is referred to as “free running.” C: the circadian clock depends in part on negative transcriptional/translational feedback mechanisms in which proteins participate in the production of their own negative-feedback regulators. D: the positive and negative regulators in the core clock system. Positive factors TOC (Timing of Cab Expression1), WC-1 (White Collar-1) and WC-2 (White Collar-2), CLK (CLOCK), CYC (Cycle), CLK, BMAL1 (Brain and Muscle Arnt-like Protein-1) regulate the transcription of their own negative regulators, CCA1 (Circadian Clock-Associated1), LHY (Late Elongated Hypocotyl), FRQ (Frequency), PER (Period), TIM (Timeless) and PER, CRY (Cryptochrome). The bacterial system does not rely on a negative transcriptional feedback mechanism but rather relies on three Kai proteins. The rhythmicity governing this system relies on rhythmic phosphorylation and can be recapitulated in vitro by the combination of the three Kai proteins and ATP (164).

Figure 2.

Additional inputs to the core circadian loop. The basic positive and negative transcriptional feedback loop of the circadian clock is elaborate, with external loops and posttranslational modifications, such as phosphorylation contributing to maintenance of the core oscillatory players. 5′ AMP-activated protein kinase (AMPK) and casein kinase I epsilon (CKIϵ) contribute to phosphorylation and degradation of the CRY and PER proteins, respectively, thus regulating the negative-feedback potential of these proteins on the CLOCK:BMAL1 complex. The transcription of Bmal1 is negatively regulated by one of its own gene targets, Rev-erbα, the process of which controls the amount of BMAL1 protein available for CLOCK binding. Retinoic acid receptor-related orphan receptor alpha (RORα) exerts opposite effects to REV-ERBα on the Bmal1 promoter.

Figure 3.

Main outputs of the SCN as revealed by anterograde labeling experiments. SCN neurons project to numerous other regions of the central nervous system, the large majority of which are hypothalamic. SCN efferents include the paraventricular thalamic nucleus, the subparaventricular zone, the hypothalamic paraventricular and dorsomedial nuclei, the tuberomammillary nucleus, and the medial preoptic area. The SCN communicates to the periphery in part via humoral signals released by other tissues of the central nervous system and via the sympathetic and parasympathetic nervous system. Some SCN-targeted peripheral tissues include white and brown adipose tissue, the gallbladder, the thyroid gland, the kidney, the spleen, the adrenal gland, the thyroid, the liver, the pancreas, and the submandibular gland (1, 13, 207, 239).

Figure 4.

The sirtuins link circadian rhythmicity to metabolism. The SIRT1:CLOCK:BMAL1 complex drives expression of Nampt, the rate-limiting enzyme in the salvage pathway for SIRT1's own cofactor, NAD⁺. The two loops depicted in the figure demonstrate the mechanisms of both the transcriptional feedback circuit as well as the enzymatic feedback circuit.

Figure 5.

Circadian clock proteins engage in additional functions when they collaborate with proteins outside of the core clock machinery. While CLOCK:BMAL1 activate numerous clock output genes, CRY1 and PER proteins bind to other nuclear receptors or intracellular proteins to regulate disparate functions such as adipogenesis and gluconeogenesis.

Figure 6.

Possible molecular mechanisms observed for the salt-sensitive hypertension observed in Cry1/Cry2 double-knockout animals. Elevated DBP-mediated gene expression may be responsible in the Cry1/Cry2 double-knockout animals for the elevated Hsd3b6 observed in these animals.

Figure 7.

Coordination between large and small loops is necessary for circadian physiology. Neurons of the SCN undergo oscillations in depolarization, activation of the ERK/MAPK cascade and CREB activity, as well as oscillations in gene expression. In addition, several small molecules have been shown to oscillate, including cAMP and calcium. While the SCN indirectly controls the oscillation of humoral factors coming from other tissues such as the pineal and adrenal cortex, continuity within the SCN is mediated by loops within VIP and AVP-expressing neurons. Other tissues also maintain circadian output via positive- and negative-feedback loops within cells that make up different compartments of the tissue. The arcuate nucleus is one such example, where NPY/AgRP-expressing and POMC-expressing neurons communicate in an oscillatory way to convey signals to the body after ghrelin release from the periphery and binding in the region. Oscillations in centrally released humoral factors control the circadian release of factors from the periphery, such as ghrelin, leptin, insulin, and glucose, and these in turn regulate brain function. Molecular oscillations within individual neurons of the pineal include those of cAMP response element binding protein (CREB) activity; its negative regulator, ICER; and the rate-limiting enzyme in melatonin biosynthesis, AANAT. Melatonin, which is released in a circadian fashion from the pineal, is involved in feedback regulation of the SCN where melatonin receptors are abundantly expressed.