The Islet and Metabolism Keep Time (2015)

The Servier-IGIS Meetings have been held yearly since 2000 in St Jean-Cap Ferrat in southern France, with a focus on insulin secretion and β-cell biology, and their role in the development of diabetes. The 16th IGIS symposium on 9–11 April 2015 focused on circadian clock regulation and its impact on metabolism, insulin secretion and the islet. The 2015 symposium was an extension of last year's symposium that focused on neural orchestration of metabolism and islet function 1. Clock regulation is a beautiful example of the importance of the central control of key metabolic functions. The rotation of the earth around its own axis imposes daylight cycles on every living species, and this has led to the evolution of rhythmic physiological processes to anticipate, and maximally benefit from, this alternate exposure to sunlight. Such cyclic processes include the alternation of sleep and wake periods, but also metabolic pathways controlling lipid storage and oxidation, or glycolysis and gluconeogenesis, which are at the basis of glucose and energy homeostasis. To control these processes, a hierarchical clock system has evolved, with a master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus, and peripheral clocks present in almost every cell of the body. The master clock of the SCN has a period of approx. 24 hours but is continuously reset to a 24-hour cycle by daylight. Its role is to ensure synchronization of the peripheral clocks, which are also responsive to other cues, in particular feeding-fasting cycles. A major question in the context of the pathophysiology of metabolic diseases is whether deregulation of the master clock or the misalignment of the master and peripheral clocks can lead to metabolic diseases. Thus, the overall question that was discussed during the 16th IGIS meeting was the possibility that obesity, diabetes and cardiovascular diseases could be caused by deregulation of the circadian clock system, caused in particular by our changing environment characterized by exposure to artificial light at night, shift work, insufficient sleep or almost unlimited access to food, which can be consumed anytime. To address these questions invited speakers covered several aspects of circadian clock physiology, including the basic molecular mechanisms of the cellular clocks, the relationship between the central clock of the suprachiasmatic nucleus and those of peripheral organs, and how the core clock components control transcription of rhythmic genes. An important aspect was the discussion of how the clock function could be modified by nutrients acting as epigenetic regulators of gene expression. This was followed by discussions of the pathophysiology of the clock and of the evidence that its misalignment with the environment can trigger insulin resistance, hypertension, obesity and diabetes. The rhythmicity of the circadian clock is ensured by a group of transcription factors that form an autoregulatory loop. In this system, the transcription factors CLOCK and BMAL1 activate the expression of the Period and Cryptochrome genes, which act as negative regulators of CLOCK and BMAL1 and of their own expression. These genes are expressed ubiquitously and control rhythmic gene expression in almost all cell types and it has indeed been demonstrated that peripheral cells have their own independent, fully operative clock system. In liver cells, the core clock controls the transcription of thousands of genes. As reported by Takahashi, chromatin immunopurification, followed by sequencing of the immuno-isolated DNA fragments, revealed a rhythmic binding of CLOCK-BMAL1 on cis-regulatory elements and an inverse rhythm of Per and Cry binding. This circadian binding of CLOCK-BMAL1 is associated with the recruitment of RNA polymerase II and rhythmic posttranslational modifications of histones, ensuring, at the genome-wide level, circadian chromatin remodeling gene transcription. An interesting integrator of circadian rhythms and metabolism is Rev-erbα, a transcription factor that is part of the negative arm of the core clock. Lazar described the way Rev-erbα has a general repressive function on the expression of the core clock genes by interacting with the transcription factor RORα in every tissue. However, Rev-erbα also controls, in a tissue-specific manner, the expression of a large number of metabolic genes. This tissue specificity depends on the recruitment by Rev-erbα of a specific corepressor and of the histone deacetylase HADC3. As Rev-erbα activity is rhythmically regulated by the circadian production of its co-factor, heme, transcriptional regulation by this factor is a beautiful example of the intricate mechanisms linking rhythmic transcription to the control of metabolism. Another level of circadian clock regulation is brought about by posttranslational modifications of histones, which control transcriptional activity, and are modified by changes in metabolic fluxes (Sassone-Corsi). As an example, enhancing glucose utilization increases the production of acetyl-CoA, which favors acetylation of histones and hence increased transcriptional activity. On the other hand, histone deacetylation to silence gene expression is controlled by SIRT1 and SIRT6, two deacetylases that are activated by NAD+, a co-factor produced according to a circadian rhythm. Multiple other posttranslational modifications, directly related to the energy status of the cells, can also control clock function. This is the case for histone methylation, which depends on the production of S-adenosyl-methionine; for the O-glycosylation of CLOCK, BMAL or Per 2, which requires N-acetyl-glucosamine production from glucose; or the phosphorylation of CRY by the energy sensor AMP-dependent protein kinases. These multiple regulations provide an essential link between circadian transcriptional activity and nutrient availability. Peripheral organs and cells have their own clock, which functions even when cells are kept in vitro. This poses the question of the relationship between the central clock of the SCN and these peripheral oscillators. Schibler has developed a highly sensitive monitoring system to measure luciferase reporter activity expressed in different organs in control mice or in SCN-lesioned mice. These studies showed that in the absence of SCN, the clock of peripheral tissues continues to oscillate but that the phase coherence between different organs is lost. However, in individual organs every cell preserves phase coherence, indicating that intercellular communications maintain synchrony at the organ level. This luminescence monitoring system was also used to show that feeding and fasting cycles can re-set the peripheral clocks but that the SCN was largely insensitive to these nutritional cues. Interestingly, Reddy reported that non-transcriptional oscillators, which involve redox-sensitive peroxyredoxins, are able to maintain 24-hour cycles based purely on biochemical reactions. These redox clocks are present in bacteria, insects and mammals, and in red blood cells where they maintain circadian oscillations in ATP levels. As many aspects of the transcriptional core clock gene functions are regulated by intracellular redox levels, it is speculated that both redox and transcriptional cycling processes interact to continuously optimize cellular function over a 24-hour period. What is the role of the circadian clock in controlling physiological functions? Can deregulation of the rhythms induce pathologies? Could this knowledge help improve drug treatment of disease? Whether a proper alignment of the circadian clock and lifestyle is required to maintain health is a central question in chronobiology. Although robust, the circadian activity of the SCN shows plasticity. This group of approx. 20,000 neurons synchronizes the activity of peripheral clocks but can be modulated by both external and internal signals. Light is the principal cue to set the rhythmicity of SCN, and this connection with external light is ensured by photoreceptors of the retina and nervous connections to the SCN. This provides a means to adapt the master clock to changes in time zones, but this also subjects it to influences that had not been anticipated during evolution, i.e. the almost continuous exposure to light as experienced in our modern cities, but also imposed by shift work, or associated with insufficient sleep. The consequence of these non-rhythmic light exposures is to dampen the synchronicity of the neurons of the SCN and of the signal that emanates from this structure. Experiments in animals as well as epidemiological studies in humans provide strong support for a role of deregulation of the master clock in the development of obesity and diabetes (Meijer). The SCN is also regulated by hormones (Challet). Melatonin, secreted by the pineal gland during the night, controls the resetting of the master clock or its adaptation to changes in day/light cycles or seasonal light variations. The SCN also expresses specific receptors for insulin, leptin, and ghrelin. This indicates that these feeding-regulated hormones can regulate the master clock. Misalignment of light entrainment of the clock and of these feeding-related signals, as caused by eating at unanticipated clock time, has a negative influence on energy metabolism, causing obesity and insulin resistance. One of the mechanisms by which this could happen is through alteration in the circadian release of glucocorticoids, which synchronize the clocks or peripheral tissues, including those of adipocytes and liver. Thus, as discussed by Meijer, Scott and Westerterp-Plantenga, all conditions that affect the activity of the SCN may lead to pathophysiological deregulations, including hypertension, dyslipidemia, obesity, and type 2 diabetes. However, although the influence of continuous light or sleep deprivation on energy metabolism has been studied rigorously in animal models, less is known about the impact of these conditions on human health, even though existing evidence supports a similar impact. Nevertheless, it must be remembered that physiology is the art of the organism to adapt to even extreme situations, and to return to a basal state when the environmental conditions normalize. Thus, the extent to which the human body is able to cope with changes in these external cues while maintaining health, and whether the physiological deregulations induced by sleep deprivation or shift work are reversible, is not known. But the genetic architecture of the individual probably also conditions the development of permanent metabolic deregulations. The islet of Langerhans in the pancreas is central among the many organs involved in metabolic regulations and in diabetes development. Oscillations in insulin secretion have long been appreciated, as have been their dysregulation in type 2 diabetes. The circadian clock machinery is thus present in the islet. Circadian rhythmicity in oscillations of insulin secretion is now regarded as central in islet physiology, and core clock genes are identified as diabetes susceptibility genes. Disrupting central as well as peripheral clocks leads to loss of circadian rhythms in insulin secretion and deterioration of glucose tolerance. The early demonstration in 2005 that homozygous global Clock mutants develop obesity, hyperglycemia and hypoinsulinemia, along with a broad range of metabolic dysregulations, was a breakthrough in the field 2 that was largely confirmed by other models, especially mutant of the major Clock counterpart, Bmal1 (Bass). Selective ablation of the clock in the endocrine pancreas resulted in a similar form of diabetes with decreased secretion of insulin. Analysis of rhythmically regulated genes in the pancreas shows that the β-cell clock controls nutrient and cAMP-induced exocytosis. Quesada showed that Rev-Erbα expression is circadian in islet α- and β-cells and regulates glucagon and insulin secretion, at least in part by modulating the expression of exocytotic proteins. The role of Rev-Erbα in endocrine islet cells paves the way to the study of agonists that modulate its activity. Beyond impacting on different aspects of insulin secretion, the islet clock also regulates β-cell homeostasis through β-cell response to oxidative and endoplasmic reticulum stress (Matveyenko, Yechoor). A circadian regulation of mitochondrial OXPHOS has been evidenced, and impaired hyperpolarization of the inner mitochondrial membrane as well as upregulation of UCP2 are seen in BMAL 1 knockout mice. Beyond the key role of circadian regulation in metabolic functions, another example has been given of the role of circadian regulation in controlling the immune responses in an autoimmune model of type 1 diabetes. Type 1 diabetes in the NOD mouse is associated with gene variants located in Idd6 on distal chromosome 6, one of which is ARNTL2, also known as BMAL2, a duplicated BMAL1 gene. This gene controls the expression of interleukin 21 and T lymphocyte proliferation in mice, which may participate in the destruction of β-cells (Rogner). Translational implications of circadian physiology are as yet far from clinical applications, even in fields in which circadian regulation is likely to be central, such as in pharmacology (FitzGerald). Targets of a majority of drugs show circadian oscillations, while many of them have a short half-life. Drug transporters as well as drug metabolism show circadian variations. This has received little consideration in clinical practice, although this may be determinant for the efficacy of most drugs. This is especially important in metabolic diseases, in which disruption of circadian clock should now be regarded as a major environmental clue in their development. We are grateful to Servier, Paris, for their long-term commitment to the IGIS project. We are indebted to Laurence Alliot for her leadership and support of the IGIS Symposia, and we thank Martine Zeitoun and Catriona Donagh (Servier) for their outstanding logistical and editorial support. Alain Ktorza (Servier), as Secretary of the IGIS Board, provides us with invaluable support and counsel; our sincere thanks to him. The authors have no conflicts of interest regarding this editorial.