Catálogo de publicaciones - libros

The circadian clock controls a large variety of neuronal, endocrine, behavioral and physiological responses in mammals. This control is exerted in large part at the transcriptional level on genes expressed in a cyclic manner. A highly specialized transcriptional machinery based on clock regulatory factors organized in feedback autoregulatory loops governs a significant portion of the genome. These oscillations in gene expression are paralleled by critical events of chromatin remodeling that appear to provide plasticity to circadian regulation. Specifically, the NAD-dependent deacetylases SIRT1 and SIRT6 have been linked to circadian control of gene expression. This and additional accumulating evidence shows that the circadian epigenome appears to share intimate links with cellular metabolic processes and has remarkable plasticity, showing reprogramming in response to nutritional challenges. In addition to SIRT1 and SIRT6, a number of chromatin remodelers have been implicated in clock control, including the histone H3K4 tri-methyltransferase MLL1. Deciphering the molecular mechanisms that link metabolism, epigenetic control and circadian responses will provide valuable insights towards innovative strategies of therapeutic intervention.

The circadian clock mechanism in animals involves an autoregulatory transcriptional feedback loop in which CLOCK and BMAL1 activate the transcription of the and genes. The PERIOD and CRYPTOCHROME proteins then feed back and repress their own transcription by interaction with CLOCK and BMAL1. We have studied the biochemistry of the CLOCK:BMAL1 transcriptional activator complex using structural biology as well as the genomic targets of CLOCK and BMAL1 using ChIP-seq methods. We describe the dynamics of the core circadian clock transcriptional system. CLOCK and BMAL1 interact with the regulatory regions of thousands of genes. The gene network and dynamics of the system will be discussed. A mechanistic description of the core circadian clock mechanism should promote our understanding of how the circadian clock system influences behavior, physiology and behavioral disorders.

Circadian clocks are biologic oscillators present in all photosensitive species that produce 24-h cycles in the transcription of rate-limiting metabolic enzymes in anticipation of the light–dark cycle. In mammals, the clock drives energetic cycles to maintain physiologic constancy during the daily switch in behavioral (sleep/wake) and nutritional (fasting/feeding) states. A molecular connection between circadian clocks and tissue metabolism was first established with the discovery that 24-h transcriptional rhythms are cell-autonomous and self-sustained in cultured fibroblasts, and that clocks are present in most tissues and comprise a robust temporal network throughout the body. A central question remains: how do circadian transcriptional programs integrate physiologic systems within individual cells of the intact animal and how does the ensemble of local clocks align temporal harmonics in the organism with the environment? Our approach to studies of metabolic regulation by the molecular clock began with analyses of metabolic pathologies in circadian mutant animals, experiments that first became possible with the cloning of the clock genes in the late 1990s. A paradox in our early studies was that the effects of circadian clock disruption were both nutrient- and time-dependent, so that, under fed conditions, animals exhibited diabetes whereas during fasting, they decompensated and died. Application of a broad range of tissue-specific genetic and biochemical approaches has now begun to provide mechanistic insight into the circadian control of metabolism.

Drosophila is a powerful system for the molecular analysis of circadian clocks, providing the first account of how such a clock is generated. It is also proving to be an excellent model to dissect the neural basis of circadian behavior. In addition, clocks are located in peripheral tissues in flies, but much less is known about these clocks and about the physiological processes they control. This chapter describes the use of Drosophila for understanding the circadian control of metabolism. While a clock in the fat body is critical for metabolic function, it is clear that neuronal clocks are also involved. Indeed, synchrony between these clocks is important for reproductive fitness. A complex interplay between circadian and metabolic signals is indicated by the finding that metabolic pathways can even impact rest:activity rhythms controlled by the brain clock. Drosophila may be an optimal system to dissect the nature of these interactions and their importance for organismal fitness and life span.

Circadian clocks control thousands of genes, which ultimately generate rhythms in signaling pathways, metabolism, tissue physiology and behavior. Although rhythmic transcription plays a critical role in generating these rhythmic gene expression patterns, recent evidence has shown that post-transcriptional mechanisms are also important. Here we describe studies showing that regulation of mRNA poly(A) tail length is under circadian control and that these changes contribute to rhythmic protein expression independently of transcription. Nocturnin, a circadian deadenylase that shortens poly(A) tails, contributes to this type of circadian post-transcriptional regulation. The importance of tail-shortening by Nocturnin is evident from the phenotype of mice lacking Nocturnin, which exhibit resistance to diet-induced obesity and other metabolic changes.

Daily (circadian) clocks have evolved to coordinate behaviour and physiology around the 24-h day. Most models of the eukaryotic circadian oscillator have focused principally on transcription/translation feedback loop (TTFL) mechanisms, with accessory cytosolic loops that connect them to cellular physiology. Recent work, however, questions the absolute necessity of transcription-based oscillators for circadian rhythmicity. The recent discovery of reduction-oxidation cycles of peroxiredoxin proteins, which persist even in the absence of transcription, have prompted a reappraisal of current clock models in disparate organisms. A novel mechanism based on metabolic cycles may underlie circadian transcriptional and cytosolic rhythms, making it difficult to know where one oscillation ends and the other begins.

Mammalian circadian and metabolic physiologies are intertwined, and the nuclear Rev-erbα is a key transcriptional link between them. Rev-erbα, and the highly related Rev-erbβ, are potent transcriptional repressors that are required for the function of the core mammalian molecular clock. The Rev-erbs are also critical regulators of clock output in metabolic cells and tissues. This chapter focuses on the physiological functions of Rev-erbα and β in regulating circadian rhythms and metabolism in mammalian tissues.

Inter-oscillator communication modulates and sustains the circadian locomotor rhythms in flies and rodent animal models. In Drosophila, the multi-oscillator network that controls sleep-wake cycles includes about 150 clock neurons. A subset of lateral neurons (LNs) expressing the Pigment-dispersing factor (PDF) appears to act as a master clock in constant darkness (DD). In light–dark (LD) cycles, flies show a bimodal distribution of their activity, and the PDF-expressing LNs play a major role in the control of the morning bout of activity. In contrast, a subset of PDF-negative LNs can generate evening activity in the absence of other functional oscillators. How these oscillators interact in a fully functional network to shape the sleep-wake cycle remains debated. The PDF neurons strongly influence the PDF-negative ones in DD and, to a lesser extent, in LD. The extent of hierarchy depends on environmental conditions and the way the dominance of PDF neurons is exerted on the different types of PDF-negative neurons is unclear. The recent discovery of light- and temperature-dependent oscillators in the dorsal neurons (DNs) sheds new light on the circuits that control the Drosophila diurnal behavior and its adaptation to environmental changes.

A biological “circadian” clock governs nearly all aspects of mammalian behavior and physiology. This control extends from activities of entire organ systems down to individual cells, all of which contain autonomous molecular clocks. Under this control, a significant fraction of the cellular metabolome—the collection of all small-molecule metabolites—varies in abundance according to time of day. Comparing the rhythmic expression of transcripts, proteins, and metabolites has yielded valuable insights into clock-controlled physiological mechanisms. In the future, their analysis could provide a glimpse of instantaneous clock phase, even providing notions of clock time based upon molecules within a single breath. Such knowledge could be important for disease diagnosis and for chronopharmacology.

The circadian activity of the hypothalamic-pituitary-adrenal (HPA) axis is made up from an underlying oscillatory rhythm of ACTH and glucocorticoid pulses that vary in amplitude but not frequency over the 24 h. This oscillatory activity is not the result of a hypothalamic oscillator but emerges as a natural consequence of the feedforward:feedback interaction between the pituitary corticotropes and the glucocorticoid-secreting cells of the adrenal cortex. This oscillatory activity has resulted in adaptations in the way tissues read their ‘digital’ ligand signal. The adrenal cortex is relatively insensitive to constant signals of ACTH but responds briskly to the equivalent amount of ACTH administered in a pulsatile manner. Similarly glucocorticoid-responsive tissues such as the brain and the liver are able to read the oscillating signals of cortisol or corticosterone secretion, with differential biochemical and functional responses to different patterns of ligand presentation. During a prolonged acute stress there is a major change in the pituitary-adrenal relationship, with a marked increase in the sensitivity of the adrenal to small changes in ACTH, so that following cardiac surgery small oscillations in ACTH result in massive swings in cortisol. This response appears to be due to a change both in the ACTH signalling pathway and in the endogenous activators and inhibitors of glucocorticoid synthesis.