of the IUNS 20 th Congress of Nutrition Chronobiology and Obesity : Interactions between Circadian Rhythms and Energy Regulation 1 – 3

Recent advances in the understanding of the molecular, genetic, neural, and physiologic basis for the generation and organization of circadian clocks in mammals have revealed profound bidirectional interactions between the circadian clock system and pathways critical for the regulation of metabolism and energy balance. The discovery that mice harboring a mutation in the core circadian gene circadian locomotor output cycles kaput (Clock) develop obesity and evidence of the metabolic syndrome represented a seminal moment for the field, clearly establishing a link between circadian rhythms, energy balance, and metabolism at the genetic level. Subsequent studies have characterized in great detail the depth and magnitude of the circadian clock’s crucial role in regulating body weight and other metabolic processes. Dietary nutrients have been shown to influence circadian rhythms at both molecular and behavioral levels; and many nuclear hormone receptors, which bind nutrients as well as other circulating ligands, have been observed to exhibit robust circadian rhythms of expression in peripheral metabolic tissues. Furthermore, the daily timing of food intake has itself been shown to affect body weight regulation in mammals, likely through, at least in part, regulation of the temporal expression patterns of metabolic genes. Taken together, these and other related findings have transformed our understanding of the important role of time, on a 24-h scale, in the complex physiologic processes of energy balance and coordinated regulation of metabolism. This research has implications for human metabolic disease and may provide unique and novel insights into the development of new therapeutic strategies to control and combat the epidemic of obesity. Adv. Nutr. 5: 312S–319S, 2014. Introduction Circadian rhythms, from the Latin circa dies for “about a day,” are fundamental biologic processes, nearly ubiquitous in living systems, that enable organisms to anticipate and prepare for predictable changes to the environment that occur as the earth rotates on its axis every 24 h. These rhythms are generated and sustained by a genetically encoded molecular pacemaker (i.e., the circadian clock) that synchronizes, or entrains, to daily cycles of light and darkness and imposes temporal organization to ongoing biochemical, molecular, and physiologic processes. This circadian clock system enables organisms to preferentially sequester reactions, pathways, and behaviors to particular times to optimize functioning and increase fitness. For example, cyanobacteria, single-celled organisms that derive energy from the sun, can use their circadian clock to synthesize, assemble, and activate the photosynthetic machinery immediately before sunrise tomaximize energy harvest during the day. Later in the day, the clock can direct the deactivation of photosynthesis to avoid inefficiency and energy wasting during darkness. In animals, including mammals, the circadian clock system performs a similar function in regulating cyclic energy harvest from the environment, because energy intake occurs in the context of daily feeding/fasting and sleep/wake cycles. Recent studies have revealed the profound interactions between the circadian clock system and energy regulation and metabolism at many levels of organization, which have substantial implications for human metabolic diseases, such as obesity and diabetes. This review summarizes key experimental findings linking the circadian clock to energy balance and metabolism in mice, beginningwith the initial description of the development of obesity andmetabolic syndrome inmice harboring amutation in the core circadian gene circadian locomotor output cycles kaput (Clock). 1 Published in a supplement to Advances in Nutrition. Presented at the International Union of Nutritional Sciences (IUNS) 20th International Congress of Nutrition (ICN) held in Granada, Spain, September 15–20, 2013. The IUNS and the 20th ICN wish to thank the California Walnut Commission and Mead Johnson Nutrition for generously providing educational grants to support the publication and distribution of proceedings from the 20th ICN. The contents of this supplement are solely the responsibility of the authors and do not necessarily represent official views of the IUNS. The supplement coordinators were Angel Gil, Ibrahim Elmadfa, and Alfredo Martinez. The supplement coordinators had no conflicts of interest to disclose. 2 K.C. S. acknowledges support for this work from training grants funded by NIH and Northwestern University (NIH T32 HL007909, NIH UL1 TL000150). F.W. T. acknowledges support for this work from research and training grants funded by NIH (NIH T32 HL007909, NIH P01 AG011412). 3 Author disclosures: K. C. Summa and F. W. Turek, no conflicts of interest. * To whom correspondence should be addressed. E-mail: fturek@northwestern.edu 4 Abbreviations used: BMAL1, brain and muscle ARNT-like 1 (also known as ARNTL); CLOCK, circadian locomotor output cycles kaput; Cry2, Cryptochrome 2; DD, constant darkness; FA, fatty acid; HDAC3, histone deacetylase 3; HF, high-fat; LD, light:dark; Mntr1b, melatonin receptor 1B; NAD1, nicotinamide adenine dinucleotide; NCoR, nuclear receptor co-repressor; NES, night eating syndrome; NR, nuclear receptor; SCN, suprachiasmatic nucleus; SIRT1, sirtuin (silent mating type information regulation 2 homolog) 1. 312S ã2014 American Society for Nutrition. Adv. Nutr. 5: 312S–319S, 2014; doi:10.3945/an.113. 005132. at U N IV E R S IT Y O F Q U E E N S LA N D on N ovem er 0, 2014 advannutrition.org D ow nladed fom This genetic linkage between the core molecular oscillator and metabolic function has contributed to a growing interest in the relations between circadian rhythms and metabolism at biochemical and molecular levels, which has led to a number of important discoveries and advances in understanding the role of the circadian clock and its complex, bidirectional interactions with critical metabolic pathways. Next, the role of nutrient composition of the diet on circadian rhythms is discussed, as well as the finding that many nutrientand dietary ligand– binding nuclear hormone receptors in peripheral metabolic organs exhibit robust diurnal and circadian patterns of expression, potentially representing part of the interface linking diet and the circadian clock. In addition to dietary composition, the timing of food intake has recently emerged as an important factor mediating body weight regulation and the organization of metabolism in mice. Finally, evidence from clinical, observational, and epidemiologic studies in humans is presented, along with a discussion of the implications of the findings in mice for human health and metabolic disease, with particular emphasis on obesity. Current Status of Knowledge Genetic link between the clock and metabolism. Perhaps the initial evidence suggestive of a connection between circadian rhythms and energy balance came from epidemiologic studies describing increased risk of obesity, as defined by a BMI ˃30 kg/m, in shift workers compared with day workers [reviewed in (1)]. However, these studies remain complicated by a variety of other unhealthy behaviors that are common in shift workers, such as poor diet, lack of physical activity, and insufficient sleep. Therefore, the description of the first genetic evidence linking the circadian clock system to energy regulation and metabolism provided a critical step forward for the field, enabling a flurry of biochemical, genetic, molecular, and physiologic studies capable of addressing the underlying mechanisms and pathways. In the initial report, mice harboring a mutation in the core circadian gene Clock (termed Clock mutant mice) were fed a high-fat (HF) diet and observed to develop obesity at a young age, as well as a variety of metabolic and endocrine abnormalities consistent with the metabolic syndrome (2). In addition, the normal diurnal feeding rhythms present in mice were significantly blunted in the mutant mice: on a standard laboratory 12-h light:12-h dark (12:12 LD) cycle, nocturnal mice typically consume ~75–80% of their total daily calories during the dark phase; in contrast, Clock mutant mice consume ~50% (2). The Clock mutants were also shown to exhibit reduced overall expression levels and a blunted diurnal rhythm of orexin mRNA, a hypothalamic neuropeptide involved in energy regulation (2). Taken together, these results indicate profound metabolic dysfunction and energy imbalance in these mice that have a genetically defective circadian clock. A striking feature of the metabolic phenotype of the Clock mutant mice was the presence of hyperglycemia and hypoinsulinemia, a pattern suggestive of a defect along the insulin axis (2). Examination of isolated pancreatic islets, containing the insulin-secreting b-cells, from Clock mutant mice, as well as from mice carrying a null mutation of brain and muscle ARNT-like 1 (Bmal1), another core circadian clock gene that encodes the binding partner of the CLOCK protein, revealed profound defects in insulin secretion, both at basal levels and in response to glucose stimulation, compared with wildtype mice (3). Furthermore, mice lacking a functional circadian clock in the pancreatic islets were shown to develop frank diabetes at an early age due to insufficient insulin secretion (3). These mice were genetically engineered to eliminate the circadian clock machinery from the pancreatic islets alone, leaving normal clock function in remaining tissues intact. These results illustrate an important aspect of the circadian clock system: it acts cell-autonomously and exhibits specific functions limited to particular cell types and tissues. Those functions differ depending on the tissue, enabling the clock to regulate biologic processes in a tissue-specific manner, dependent on the needs of the individual tissue. For example, the clock in the liver has been shown to play a critical role in regulating