Sirtuins and the circadian clock: bridging chromatin and metabolism

SIRT6 in Regulation of Mitochondrial Damage and Associated Cardiac Dysfunctions: A Possible Therapeutic Target for CVDs

Cardiovascular diseases (CVDs) can be described as a global health emergency imploring possible prevention strategies. Although the pathogenesis of CVDs has been extensively studied, the role of mitochondrial dysfunction in CVD development has yet to be investigated. Diabetic cardiomyopathy, ischemic-reperfusion injury, and heart failure are some of the CVDs resulting from mitochondrial dysfunction Recent evidence from the research states that any dysfunction of mitochondria has an impact on metabolic alteration, eventually causes the death of a healthy cell and therefore, progressively directing to the predisposition of disease. Cardiovascular research investigating the targets that both protect and treat mitochondrial damage will help reduce the risk and increase the quality of life of patients suffering from various CVDs. One such target, i.e., nuclear sirtuin SIRT6 is strongly associated with cardiac function. However, the link between mitochondrial dysfunction and SIRT6 concerning cardiovascular pathologies remains poorly understood. Although the Role of SIRT6 in skeletal muscles and cardiomyocytes through mitochondrial regulation has been well understood, its specific role in mitochondrial maintenance in cardiomyocytes is poorly determined. The review aims to explore the domain-specific function of SIRT6 in cardiomyocytes and is an effort to know how SIRT6, mitochondria, and CVDs are related.

Circadian-mediated regulation of cardiometabolic disorders and aging with time-restricted feeding

Circadian rhythms are present throughout biology, from the molecular level to complex behaviors such as eating and sleeping. They are driven by molecular clocks within cells, and different tissues can have unique rhythms. Circadian disruption can trigger obesity and other common metabolic disorders such as aging, diabetes, and cardiovascular disease, and circadian genes control metabolism. At an organismal level, feeding and fasting rhythms are key drivers of circadian rhythms. This underscores the bidirectional relationship between metabolism and circadian rhythms, and many metabolic disorders have circadian disruption or misalignment. Therefore, studying circadian rhythms may offer new avenues for understanding the etiology and management of obesity. This review describes how circadian rhythm dysregulation is linked with cardiometabolic disorders and how the lifestyle intervention of time-restricted feeding (TRF) regulates them. TRF reinforces feeding-fasting rhythms without reducing caloric intake and ameliorates metabolic disorders such as obesity and associated cardiac dysfunction, along with reducing inflammation. TRF optimizes the expression of genes and pathways related to normal metabolic function, linking metabolism with TRF's benefits and demonstrating the molecular link between metabolic disorders and circadian rhythms. Thus, TRF has tremendous therapeutic potential that could be easily adopted to reduce obesity-linked dysfunction and cardiometabolic disorders.

Redox Biology of Melatonin: Discriminating Between Circadian and Noncircadian Functions

Melatonin has not only to be seen as a regulator of circadian clocks. In addition to its chronobiotic functions, it displays other actions, especially in cell protection. This includes antioxidant, anti-inflammatory, and mitochondria-protecting effects. Although protection is also modulated by the circadian system, the respective actions of melatonin can be distinguished and differ with regard to dose requirements in therapeutic settings. It is the aim of this article to outline these differences in terms of function, signaling, and dosage. Focus has been placed on both the nexus and the dissecting properties between circadian and noncircadian mechanisms. This has to consider details beyond the classic view of melatonin's role, such as widespread synthesis in extrapineal tissues, formation in mitochondria, effects on the mitochondrial permeability transition pore, and secondary signaling, for example, via upregulation of sirtuins and by regulating noncoding RNAs, especially microRNAs. The relevance of these findings, the differences and connections between circadian and noncircadian functions of melatonin shed light on the regulation of inflammation, including macrophage/microglia polarization, damage-associated molecular patterns, avoidance of cytokine storms, and mitochondrial functions, with numerous consequences to antioxidative protection, that is, aspects of high actuality with regard to deadly viral and bacterial diseases. Antioxid. Redox Signal. 37, 704-725.

Screen for Small-Molecule Modulators of Circadian Rhythms Reveals Phenazine as a Redox-State Modifying Clockwork Tuner

A high-throughput cell-based screen identified redox-active small molecules that produce a period lengthening of the circadian rhythm. The strongest period lengthening phenotype was induced by a phenazine carboxamide (VU661). Comparison to two isomeric benzquinoline carboxamides (VU673 and VU164) shows the activity is associated with the redox modulating phenazine functionality. Furthermore, ex vivo cell analysis using optical redox ratio measurements shows the period lengthening phenotype to be associated with a shift to the NAD/FAD oxidation state of nicotinamide and flavine coenzymes.

Fueling genome maintenance: On the versatile roles of NAD+ in preserving DNA integrity

NAD⁺ is a versatile biomolecule acting as a master regulator and substrate in various cellular processes, including redox regulation, metabolism, and various signaling pathways. In this article, we concisely and critically review the role of NAD⁺ in mechanisms promoting genome maintenance. Numerous NAD⁺-dependent reactions are involved in the preservation of genome stability, the cellular DNA damage response, and other pathways regulating nucleic acid metabolism, such as gene expression and cell proliferation pathways. Of note, NAD⁺ serves as a substrate to ADP-ribosyltransferases, sirtuins, and potentially also eukaryotic DNA ligases, all of which regulate various aspects of DNA integrity, damage repair, and gene expression. Finally, we critically analyze recent developments in the field as well as discuss challenges associated with therapeutic actions intended to raise NAD⁺ levels.

Cancer cell metabolism connects epigenetic modifications to transcriptional regulation

Adaptation of cellular function with the nutrient environment is essential for survival. Failure to adapt can lead to cell death and/or disease. Indeed, energy metabolism alterations are a major contributing factor for many pathologies, including cancer, cardiovascular disease, and diabetes. In particular, a primary characteristic of cancer cells is altered metabolism that promotes survival and proliferation even in the presence of limited nutrients. Interestingly, recent studies demonstrate that metabolic pathways produce intermediary metabolites that directly influence epigenetic modifications in the genome. Emerging evidence demonstrates that metabolic processes in cancer cells fuel malignant growth, in part, through epigenetic regulation of gene expression programs important for proliferation and adaptive survival. In this review, recent progress toward understanding the relationship of cancer cell metabolism, epigenetic modification, and transcriptional regulation will be discussed. Specifically, the need for adaptive cell metabolism and its modulation in cancer cells will be introduced. Current knowledge on the emerging field of metabolite production and epigenetic modification will also be reviewed. Alterations of DNA (de)methylation, histone modifications, such as (de)methylation and (de)acylation, as well as chromatin remodeling, will be discussed in the context of cancer cell metabolism. Finally, how these epigenetic alterations contribute to cancer cell phenotypes will be summarized. Collectively, these studies reveal that both metabolic and epigenetic pathways in cancer cells are closely linked, representing multiple opportunities to therapeutically target the unique features of malignant growth.

Association of Sirtuin Gene Polymorphisms with Susceptibility to Coronary Artery Disease in a North Chinese Population

Aims

Coronary artery disease (CAD) represents the leading cause of death worldwide. Accumulating evidence also suggests that sirtuins (SIRTS) have been associated with CAD. The present study was aimed at investigating the association between 12 gene polymorphisms for SIRTs and the development of CAD in a Chinese population.

Materials and Methods

12 SNPs (rs12778366 (T > C), rs3758391 (T > C), rs3740051 (A > G), rs4746720 (C > T), rs7895833 (G > A), rs932658 (A > C) for SIRT1, rs2015 (G > T) for SIRT2, rs28365927 (G > A), rs11246020 (C > T) for SIRT3, rs350844 (G > A), rs350846 (G > C), and rs107251 (C > T) for SIRT6) were selected and assessed in a cohort of 509 CAD patients and 552 matched healthy controls for this study. Genomic DNA from whole blood was extracted, and the SNPs were assessed using MassARRAY method.

Results

TT genotype for rs3758391 and GG genotype for rs7895833 of SIRT1 were at higher risk of CAD, whereas the CC genotype for rs4746720 of SIRT1 was associated with a significantly decreased risk of CAD. The A allele of the rs28365927 of SIRT3 showed a significant decreased risk association with CAD patient group (P = 0.014). Significant difference in genotypes rs350844 (G > A) (P = 0.004), rs350846 (G > C) (P = 0.002), and rs107251 (C > T) (P ≤ 0.01) for SIRT6 was also found between the CAD patients and the healthy controls. Haplotype CTA significantly increased the risk of CAD (P = 0.000118, OR = 1.497, 95%CI = 1.218–1.840), while haplotype GCG significantly decreases the risk of CAD (P = 0.000414, OR = 1.131, 95%CI = 0.791–1.619).

Conclusions

The SNP rs28365927 in the SIRT3 gene and SNP rs350844, rs350846, and rs107251 in the SIRT6 gene present significant associations with CAD in a north Chinese population. Haplotype CTA and GCG generated by rs350846/rs107251/rs350844 in the SIRT6 might also increase and decrease the risk of CAD, respectively.

Treadmill Training Effect on the Myokines Content in Skeletal Muscles of Mice With a Metabolic Disorder Model

The effect of treadmill training loads on the content of cytokines in mice skeletal muscles with metabolic disorders induced by a 16 week high fat diet (HFD) was studied. The study included accounting the age and biorhythmological aspects. In the experiment, mice were used at the age of 4 and 32 weeks, by the end of the experiment—respectively 20 and 48 weeks. HFD feeding lasted 16 weeks. Treadmill training were carried out for last 4 weeks six times a week, the duration 60 min and the speed from 15 to 18 m/min. Three modes of loading were applied. The first subgroup was subjected to stress in the morning hours (light phase); the second subgroup was subjected to stress in the evening hours (dark phase); the third subgroup was subjected to loads in the shift mode (the first- and third-weeks treadmill training was used in the morning hours, the second and fourth treadmill training was used in the evening hours). In 20-week-old animals, the exercise effect does not depend on the training regime, however, in 48-week-old animals, the decrease in body weight in mice with the shift training regime was more profound. HFD affected muscle myokine levels. The content of all myokines, except for LIF, decreased, while the concentration of CLCX1 decreased only in young animals in response to HFD. The treadmill training caused multidirectional changes in the concentration of myokines in muscle tissue. The IL-6 content changed most profoundly. These changes were observed in all groups of animals. The changes depended to the greatest extent on the training time scheme. The effect of physical activity on the content of IL-15 in the skeletal muscle tissue was observed mostly in 48-week-old mice. In 20-week-old animals, physical activity led to an increase in the concentration of LIF in muscle tissue when applied under the training during the dark phase or shift training scheme. In the HFD group, this effect was significantly more pronounced. The content of CXCL1 did not change with the use of treadmill training in almost all groups of animals. Physical activity, introduced considering circadian rhythms, is a promising way of influencing metabolic processes both at the cellular and systemic levels, which is important for the search for new ways of correcting metabolic disorders.

Linking Depression to Epigenetics: Role of the Circadian Clock

The circadian clock governs multiple biological functions at the molecular level and plays an essential role in providing temporal diversity of behavior and physiology including neuronal activity. Studies spanning the past two decades have deciphered the molecular mechanisms of the circadian clock, which appears to operate as an essential interface in linking cellular metabolism to epigenetic control. Accumulating evidence illustrates that disruption of circadian rhythms through jet lag, shift work, and temporary irregular life-style could lead to depression-like symptoms. Remarkably, abnormal neuronal activity and depression-like behavior appear in animals lacking elements of the molecular clock. Recent studies demonstrate that neuronal and synaptic gene induction is under epigenetic control, and robust epigenetic remodeling is observed under depression and related psychiatric disorders. Thus, the intertwined links between the circadian clock and epigenetics may point to novel approaches for antidepressant treatments, epigenetic therapy, and chronotherapy. In this chapter we summarize how the circadian clock is involved in neuronal functions and depressive-like behavior and propose that potential strategies for antidepressant therapy by incorporating circadian genomic and epigenetic rewiring of neuronal signaling pathways.

Expanding the link between circadian rhythms and redox metabolism of epigenetic control

Circadian rhythms play a central role in physiological and metabolic processes. This is mostly achieved through rhythmic regulation of myriad genes via dynamic epigenome changes. Accumulating evidence indicates that oxidative stress and redox balance are under circadian control and feedback on the clock system. Circadian perturbations induce oxidative stress accumulation and disturb redox balance. Along with these changes, epigenomic landscape changes are a remarkable hallmark of clock disruption. This review aims to summarize evidence supporting the link between the circadian clock and redox metabolism, focusing on possible connections through epigenetic mechanisms.

The role of SIRT2 in vascular-related and heart-related diseases: A review

At present, cardiovascular disease is one of the important factors of human death, and there are many kinds of proteins involved. Sirtuins family proteins are involved in various physiological and pathological activities of the human body. Among them, there are more and more studies on the relationship between sirtuin2 (SIRT2) protein and cardiovascular diseases. SIRT2 can effectively inhibit pathological cardiac hypertrophy. The effect of SIRT2 on ischaemia‐reperfusion injury has different effects under different conditions. SIRT2 can reduce the level of reactive oxygen species (ROS), which may help to reduce the severity of diabetic cardiomyopathy. SIRT2 can affect a variety of cardiovascular diseases, energy metabolism and the ageing of cardiomyocytes, thereby affecting heart failure. SIRT2 also plays an important role in vascular disease. For endothelial cell damage used by oxidative stress, the role of SIRT2 is bidirectional, which is related to the degree of oxidative stress stimulation. When the degree of stimulation is small, SIRT2 plays a protective role, and when the degree of stimulation increases to a certain level, SIRT2 plays a negative role. In addition, SIRT2 is also involved in the remodelling of blood vessels and the repair of skin damage.

Sirtuins and the circadian clock interplay in cardioprotection: focus on sirtuin 1

Chronic disruption of circadian rhythms which include intricate molecular transcription-translation feedback loops of evolutionarily conserved clock genes has serious health consequences and negatively affects cardiovascular physiology. Sirtuins (SIRTs) are nuclear, cytoplasmic and mitochondrial histone deacetylases that influence the circadian clock with clock-controlled oscillatory protein, NAMPT, and its metabolite NAD+. Sirtuins are linked to the multi-organ protective role of melatonin, particularly in acute kidney injury and in cardiovascular diseases, where melatonin, via upregulation of SIRT1 expression, inhibits the apoptotic pathway. This review focuses on SIRT1, an NAD+-dependent class III histone deacetylase which counterbalances the intrinsic histone acetyltransferase activity of one of the clock genes, CLOCK. SIRT1 is involved in the development of cardiomyocytes, regulation of voltage-gated cardiac sodium ion channels via deacetylation, prevention of atherosclerotic plaque formation in the cardiovascular system, protection against oxidative damage and anti-thrombotic actions. Overall, SIRT1 has a see-saw effect on cardioprotection, with low levels being cardioprotective and higher levels leading to cardiac hypertrophy.

A Molecular Perspective on Sirtuin Activity

The protein acetylation of either the α-amino groups of amino-terminal residues or of internal lysine or cysteine residues is one of the major posttranslational protein modifications that occur in the cell with repercussions at the protein as well as at the metabolome level. The lysine acetylation status is determined by the opposing activities of lysine acetyltransferases (KATs) and lysine deacetylases (KDACs), which add and remove acetyl groups from proteins, respectively. A special group of KDACs, named sirtuins, that require NAD⁺ as a substrate have received particular attention in recent years. They play critical roles in metabolism, and their abnormal activity has been implicated in several diseases. Conversely, the modulation of their activity has been associated with protection from age-related cardiovascular and metabolic diseases and with increased longevity. The benefits of either activating or inhibiting these enzymes have turned sirtuins into attractive therapeutic targets, and considerable effort has been directed toward developing specific sirtuin modulators. This review summarizes the protein acylation/deacylation processes with a special focus on the current developments in the sirtuin research field.

Aerobic and resistance exercise training reverses age-dependent decline in NAD+ salvage capacity in human skeletal muscle

Aging decreases skeletal muscle mass and strength, but aerobic and resistance exercise training maintains skeletal muscle function. NAD⁺ is a coenzyme for ATP production and a required substrate for enzymes regulating cellular homeostasis. In skeletal muscle, NAD⁺ is mainly generated by the NAD⁺ salvage pathway in which nicotinamide phosphoribosyltransferase (NAMPT) is rate‐limiting. NAMPT decreases with age in human skeletal muscle, and aerobic exercise training increases NAMPT levels in young men. However, whether distinct modes of exercise training increase NAMPT levels in both young and old people is unknown. We assessed the effects of 12 weeks of aerobic and resistance exercise training on skeletal muscle abundance of NAMPT, nicotinamide riboside kinase 2 (NRK2), and nicotinamide mononucleotide adenylyltransferase (NMNAT) 1 and 3 in young (≤35 years) and older (≥55 years) individuals. NAMPT in skeletal muscle correlated negatively with age (r² = 0.297, P < 0.001, n = 57), and VO₂peak was the best predictor of NAMPT levels. Moreover, aerobic exercise training increased NAMPT abundance 12% and 28% in young and older individuals, respectively, whereas resistance exercise training increased NAMPT abundance 25% and 30% in young and in older individuals, respectively. None of the other proteins changed with exercise training. In a separate cohort of young and old people, levels of NAMPT, NRK1, and NMNAT1/2 in abdominal subcutaneous adipose tissue were not affected by either age or 6 weeks of high‐intensity interval training. Collectively, exercise training reverses the age‐dependent decline in skeletal muscle NAMPT abundance, and our findings highlight the value of exercise training in ameliorating age‐associated deterioration of skeletal muscle function.

The ageing epigenome and its rejuvenation

Ageing is characterized by the functional decline of tissues and organs and the increased risk of ageing-associated disorders. Several 'rejuvenating' interventions have been proposed to delay ageing and the onset of age-associated decline and disease to extend healthspan and lifespan. These interventions include metabolic manipulation, partial reprogramming, heterochronic parabiosis, pharmaceutical administration and senescent cell ablation. As the ageing process is associated with altered epigenetic mechanisms of gene regulation, such as DNA methylation, histone modification and chromatin remodelling, and non-coding RNAs, the manipulation of these mechanisms is central to the effectiveness of age-delaying interventions. This Review discusses the epigenetic changes that occur during ageing and the rapidly increasing knowledge of how these epigenetic mechanisms have an effect on healthspan and lifespan extension, and outlines questions to guide future research on interventions to rejuvenate the epigenome and delay ageing processes.

Circadian Clocks Make Metabolism Run

Most organisms adapt to the 24-h cycle of the Earth's rotation by anticipating the time of the day through light-dark cycles. The internal time-keeping system of the circadian clocks has been developed to ensure this anticipation. The circadian system governs the rhythmicity of nearly all physiological and behavioral processes in mammals. In this review, we summarize current knowledge stemming from rodent and human studies on the tight interconnection between the circadian system and metabolism in the body. In particular, we highlight recent advances emphasizing the roles of the peripheral clocks located in the metabolic organs in regulating glucose, lipid, and protein homeostasis at the organismal and cellular levels. Experimental disruption of circadian system in rodents is associated with various metabolic disturbance phenotypes. Similarly, perturbation of the clockwork in humans is linked to the development of metabolic diseases. We discuss recent studies that reveal roles of the circadian system in the temporal coordination of metabolism under physiological conditions and in the development of human pathologies.

NAD+ homeostasis in health and disease

The conceptual evolution of nicotinamide adenine dinucleotide (NAD⁺) from being seen as a simple metabolic cofactor to a pivotal cosubstrate for proteins regulating metabolism and longevity, including the sirtuin family of protein deacylases, has led to a new wave of scientific interest in NAD⁺. NAD⁺ levels decline during ageing, and alterations in NAD⁺ homeostasis can be found in virtually all age-related diseases, including neurodegeneration, diabetes and cancer. In preclinical settings, various strategies to increase NAD⁺ levels have shown beneficial effects, thus starting a competitive race to discover marketable NAD⁺ boosters to improve healthspan and lifespan. Here, we review the basics of NAD⁺ biochemistry and metabolism, and its roles in health and disease, and we discuss current challenges and the future translational potential of NAD⁺ research.

Circadian blueprint of metabolic pathways in the brain

The circadian clock is an endogenous, time-tracking system that directs multiple metabolic and physiological functions required for homeostasis. The master or central clock located within the suprachiasmatic nucleus in the hypothalamus governs peripheral clocks present in all systemic tissues, contributing to their alignment and ultimately to temporal coordination of physiology. Accumulating evidence reveals the presence of additional clocks in the brain and suggests the possibility that circadian circuits may feed back to these from the periphery. Here, we highlight recent advances in the communications between clocks and discuss how they relate to circadian physiology and metabolism.

Dual Deletion of the Sirtuins SIRT2 and SIRT3 Impacts on Metabolism and Inflammatory Responses of Macrophages and Protects From Endotoxemia

Sirtuin 2 (SIRT2) and SIRT3 are cytoplasmic and mitochondrial NAD-dependent deacetylases. SIRT2 and SIRT3 target proteins involved in metabolic, proliferation and inflammation pathways and have been implicated in the pathogenesis of neurodegenerative, metabolic and oncologic disorders. Both pro- and anti-inflammatory effects have been attributed to SIRT2 and SIRT3, and single deficiency in SIRT2 or SIRT3 had minor or no impact on antimicrobial innate immune responses. Here, we generated a SIRT2/3 double deficient mouse line to study the interactions between SIRT2 and SIRT3. SIRT2/3^−/− mice developed normally and showed subtle alterations of immune cell populations in the bone marrow, thymus, spleen, blood and peritoneal cavity that contained notably more anti-inflammatory B-1a cells and less NK cells. In vitro, SIRT2/3^−/− macrophages favored fatty acid oxidation (FAO) over glycolysis and produced increased levels of both proinflammatory and anti-inflammatory cytokines. In line with metabolic adaptation and increased numbers of peritoneal B-1a cells, SIRT2/3^−/− mice were robustly protected from endotoxemia. Yet, SIRT2/3 double deficiency did not modify endotoxin tolerance. Overall, these data suggest that sirtuins can act in concert or compensate each other for certain immune functions, a parameter to be considered for drug development. Moreover, inhibitors targeting multiple sirtuins developed for clinical purposes may be useful to treat inflammatory diseases.

Doxorubicin persistently rewires cardiac circadian homeostasis in mice

Circadian rhythms disruption can be the cause of chronic diseases. External cues, including therapeutic drugs, have been shown to modulate peripheral-circadian clocks. Since anthracycline cardiotoxicity is associated with loss of mitochondrial function and metabolic remodeling, we investigated whether the energetic failure induced by sub-chronic doxorubicin (DOX) treatment in juvenile mice was associated with persistent disruption of circadian regulators. Juvenile C57BL/6J male mice were subjected to a sub-chronic DOX treatment (4 weekly injections of 5 mg/kg DOX) and several cardiac parameters, as well as circadian-gene expression and acetylation patterns, were analyzed after 6 weeks of recovery time. Complementary experiments were performed with Mouse Embryonic Fibroblasts (MEFs) and Human Embryonic Kidney 293 cells. DOX-treated juvenile mice showed cardiotoxicity markers and persistent alterations of transcriptional- and signaling cardiac circadian homeostasis. The results showed a delayed influence of DOX on gene expression, accompanied by changes in SIRT1-mediated cyclic deacetylation. The mechanism behind DOX interference with the circadian clock was further studied in vitro, in which were observed alterations of circadian-gene expression and increased BMAL1 SIRT1-mediated deacetylation. In conclusion, DOX treatment in juvenile mice resulted in disruption of oscillatory molecular mechanisms including gene expression and acetylation profiles.

Liver regeneration in aged mice: new insights

The regenerative capacity of the liver after resection is reduced with aging. Recent studies on rodents revealed that both intracellular and extracellular factors are involved in the impairment of liver mass recovery during aging. Among the intracellular factors, age-dependent decrease of BubR1 (budding uninhibited by benzimidazole-related 1), YAP (Yes-associated protein) and SIRT1 (Sirtuin-1) have been associated to dampening of tissue reconstitution and inhibition of cell cycle genes following partial hepatectomy. Extra-cellular factors, such as age-dependent changes in hepatic stellate cells affect liver regeneration through inhibition of progenitor cells and reduction of liver perfusion. Furthermore, chronic release of pro-inflammatory proteins by senescent cells (SASP) affects cell proliferation suggesting that senescent cell clearance might improve tissue regeneration. Accordingly, young plasma restores liver regeneration in aged animals through autophagy re-establishment. This review will discuss how intracellular and extracellular factors cooperate to guarantee a proper liver regeneration and the possible causes of its impairment during aging. The possibility that an improvement of the liver regenerative capacity in elderly might be achieved through elimination of senescent cells via autophagy or by administration of direct mitogenic agents devoid of cytotoxicity will also be entertained.

At the Interface of Lifestyle, Behavior, and Circadian Rhythms: Metabolic Implications

Nutrient metabolism is under circadian regulation. Disruption of circadian rhythms by lifestyle and behavioral choices such as work schedules, eating patterns, and social jetlag, seriously impacts metabolic homeostasis. Metabolic dysfunction due to chronic misalignment of an organism's endogenous rhythms is detrimental to health, increasing the risk of obesity, metabolic and cardiovascular disease, diabetes, and cancer. In this paper, we review literature on recent findings on the mechanisms that communicate metabolic signals to circadian clocks and vice versa, and how human behavioral changes imposed by societal and occupational demands affect the physiological networks integrating peripheral clocks and metabolism. Finally, we discuss factors possibly contributing to inter-individual variability in response to circadian changes in the context of metabolic (dys)function.

Time of Exercise Specifies the Impact on Muscle Metabolic Pathways and Systemic Energy Homeostasis

While the timing of food intake is important, it is unclear whether the effects of exercise on energy metabolism are restricted to unique time windows. As circadian regulation is key to controlling metabolism, understanding the impact of exercise performed at different times of the day is relevant for physiology and homeostasis. Using high-throughput transcriptomic and metabolomic approaches, we identify distinct responses of metabolic oscillations that characterize exercise in either the early rest phase or the early active phase in mice. Notably, glycolytic activation is specific to exercise at the active phase. At the molecular level, HIF1α, a central regulator of glycolysis during hypoxia, is selectively activated in a time-dependent manner upon exercise, resulting in carbohydrate exhaustion, usage of alternative energy sources, and adaptation of systemic energy expenditure. Our findings demonstrate that the time of day is a critical factor to amplify the beneficial impact of exercise on both metabolic pathways within skeletal muscle and systemic energy homeostasis.

Deep sequencing and miRNA profiles in alcohol-induced neuroinflammation and the TLR4 response in mice cerebral cortex

Alcohol abuse can induce brain injury and neurodegeneration, and recent evidence shows the participation of immune receptors toll-like in the neuroinflammation and brain damage. We evaluated the role of miRNAs as potential modulators of the neuroinflammation associated with alcohol abuse and the influence of the TLR4 response. Using mice cerebral cortex and next-generation sequencing (NGS), we identified miRNAs that were differentially expressed in the chronic alcohol-treated versus untreated WT or TLR4-KO mice. We observed a differentially expression of miR-183 Cluster (C) (miR-96/-182/-183), miR-200a and miR-200b, which were down-regulated, while mirR-125b was up-regulated in alcohol-treated WT versus (vs.) untreated mice. These miRNAs modulate targets genes related to the voltage-gated sodium channel, neuron hyperexcitability (Nav1.3, Trpv1, Smad3 and PP1-γ), as well as genes associated with innate immune TLR4 signaling response (Il1r1, Mapk14, Sirt1, Lrp6 and Bdnf). Functional enrichment of the miR-183C and miR-200a/b family target genes, revealed neuroinflammatory pathways networks involved in TLR4 signaling and alcohol abuse. The changes in the neuroinflammatory targets genes associated with alcohol abuse were mostly abolished in the TLR4-KO mice. Our results show the relationship between alcohol intake and miRNAs expression and open up new therapeutically targets to prevent deleterious effects of alcohol on the brain.

NAD metabolism and the SLC34 family: evidence for a liver-kidney axis regulating inorganic phosphate

The solute carrier 34 (SLC34) family of membrane transporters is a major contributor to Pi homeostasis. Many factors are involved in regulating the SLC34 family. The roles of the bone mineral metabolism factors parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) in Pi homeostasis are well studied. Intracellular Pi is thought to be involved in energy metabolism, such as ATP production. Under certain conditions of altered energy metabolism, plasma Pi concentrations are affected by the regulation of a Pi shift into cells or release from the tissues. We recently investigated the mechanism of hepatectomy-related hypophosphatemia, which is thought to involve an unknown phosphaturic factor. Hepatectomy-related hypophosphatemia is due to impaired nicotinamide adenine dinucleotide (NAD) metabolism through its effects on the SLC34 family in the liver-kidney axis. The oxidized form of NAD, NAD⁺, is an essential cofactor in various cellular biochemical reactions. Levels of NAD⁺ and its reduced form NADH vary with the availability of dietary energy and nutrients. Nicotinamide phosphoribosyltransferase (Nampt) generates a key NAD⁺ intermediate, nicotinamide mononucleotide, from nicotinamide and 5-phosphoribosyl 1-pyrophosphate. The liver, an important organ of NAD metabolism, is thought to release metabolic products such as nicotinamide and may control NAD metabolism in other organs. Moreover, NAD is an important regulator of the circadian rhythm. Liver-specific Nampt-deficient mice and heterozygous Nampt mice have abnormal daily plasma Pi concentration oscillations. These data indicate that NAD metabolism in the intestine, liver, and kidney is closely related to Pi metabolism through the SLC34 family. Here, we review the relationship between the SLC34 family and NAD metabolism based on our recent studies.

Interplay between Microbes and the Circadian Clock

Circadian rhythms influence virtually all life forms on our planet, a notion that opens the question on how the circadian cycles of individual organisms may interplay with each other. In mammals, a potentially dangerous environmental stress is represented by encounters with infectious agents. Microbial attack is a major risk for organismal homeostasis and therefore needs to be efficiently counteracted by mechanisms implemented by the host immune system. Accumulating evidence shows that the immune system may anticipate an emerging pathogenic exposure through an enhanced inflammatory state. Notably, the circadian clock orchestrates these anticipatory responses to fluctuating conditions in the external world. In this article, we review the current knowledge about the relationship between the circadian clock and pathogenic infections. We discuss the role of the circadian clock against infection and specific pathogens, the core clock proteins involved in the defense mechanisms, and the specific tissue or cell type in which they function to counteract the infection. Finally, circadian oscillations in the gut microbiome composition and its possible role in protecting against foodborne pathogen colonization are presented.

The NAD+/PARP1/SIRT1 Axis in Aging

NAD+ levels decline with age in diverse animals from Caenorhabditis elegans to mice. Raising NAD+ levels by dietary supplementation with NAD+ precursors, nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN), improves mitochondrial function and muscle and neural and melanocyte stem cell function in mice, as well as increases murine life span. Decreased NAD+ levels with age reduce SIRT1 function and reduce the mitochondrial unfolded protein response, which can be overcome by NR supplementation. Decreased NAD+ levels cause NAD+-binding protein DBC1 to form a complex with PARP1, inhibiting poly(adenosine diphosphate-ribose) polymerase (PARP) catalytic activity. Old mice have increased amounts of DBC1-PARP1 complexes, lower PARP activity, increased DNA damage, and reduced nonhomologous end joining and homologous recombination repair. DBC1-PARP1 complexes in old mice can be broken by increasing NAD+ levels through treatment with NMN, reducing DNA damage and restoring PARP activity to youthful levels. The mechanism of declining NAD+ levels and its fundamental importance to aging are yet to be elucidated. There is a correlation of PARP activity with mammalian life span that suggests that NAD+/SIRT1/PARP1 may be more significant than the modest effects on life span observed for NR supplementation in old mice. The NAD+/PARP1/SIRT1 axis may link NAD+ levels and DNA damage with the apparent epigenomic DNA methylation clocks that have been described.

Identification of a Tissue-Restricted Isoform of SIRT1 Defines a Regulatory Domain that Encodes Specificity

The conserved NAD⁺-dependent deacylase SIRT1 plays pivotal, sometimes contrasting, roles in diverse physiological and pathophysiological conditions. In this study, we uncover a tissue-restricted isoform of SIRT1 (SIRT1-ΔE2) that lacks exon 2 (E2). Candidate-based screening of SIRT1 substrates demonstrated that the domain encoded by this exon plays a key role in specifying SIRT1 protein-protein interactions. The E2 domain of SIRT1 was both necessary and sufficient for PGC1α binding, enhanced interaction with p53, and thus downstream functions. Since SIRT1-FL and SIRT1-ΔE2 were found to have similar intrinsic catalytic activities, we propose that the E2 domain tethers specific substrate proteins. Given the absence of SIRT1-ΔE2 in liver, our findings provide insight into the role of the E2 domain in specifying "metabolic functions" of SIRT1-FL. Identification of SIRT1-ΔE2 and the conserved specificity domain will enhance our understanding of SIRT1 and guide the development of therapeutic interventions.

Mitochondrial-epigenetic crosstalk in environmental toxicology

Crosstalk between the nuclear epigenome and mitochondria, both in normal physiological function and in responses to environmental toxicant exposures, is a developing sub-field of interest in environmental and molecular toxicology. The majority (∼99%) of mitochondrial proteins are encoded in the nuclear genome, so programmed communication among nuclear, cytoplasmic, and mitochondrial compartments is essential for maintaining cellular health. In this review, we will focus on correlative and mechanistic evidence for direct impacts of each system on the other, discuss demonstrated or potential crosstalk in the context of chemical insult, and highlight biological research questions for future study. We will first review the two main signaling systems: nuclear signaling to the mitochondria [anterograde signaling], best described in regulation of oxidative phosphorylation (OXPHOS) and mitochondrial biogenesis in response to environmental signals received by the nucleus, and mitochondrial signals to the nucleus [retrograde signaling]. Both signaling systems can communicate intracellular energy needs or a need to compensate for dysfunction to maintain homeostasis, but both can also relay inappropriate signals in the presence of dysfunction in either system and contribute to adverse health outcomes. We will first review these two signaling systems and highlight known or biologically feasible epigenetic contributions to both, then briefly discuss the emerging field of epigenetic regulation of the mitochondrial genome, and finally discuss putative "crosstalk phenotypes", including biological phenomena, such as caloric restriction, maintenance of stemness, and circadian rhythm, and states of disease or loss of function, such as cancer and aging, in which both the nuclear epigenome and mitochondria are strongly implicated.

Sirtuin 2 Deficiency Increases Bacterial Phagocytosis by Macrophages and Protects from Chronic Staphylococcal Infection

Sirtuin 2 (SIRT2) is one of the seven members of the family of NAD⁺-dependent histone deacetylases. Sirtuins target histones and non-histone proteins according to their subcellular localization, influencing various biological processes. SIRT2 resides mainly in the cytoplasm and regulates cytoskeleton dynamics, cell cycle, and metabolic pathways. As such, SIRT2 has been implicated in the pathogenesis of neurodegenerative, metabolic, oncologic, and chronic inflammatory disorders. This motivated the development of SIRT2-directed therapies for clinical purposes. However, the impact of SIRT2 on antimicrobial host defense is largely unknown. Here, we address this question using SIRT2 knockout mice. We show that SIRT2 is the most highly expressed sirtuin in myeloid cells, especially macrophages. SIRT2 deficiency does not affect immune cell development and marginally impacts on intracellular signaling and cytokine production by splenocytes and macrophages. However, SIRT2 deficiency enhances bacterial phagocytosis by macrophages. In line with these observations, in preclinical models, SIRT2 deficiency increases survival of mice with chronic staphylococcal infection, while having no effect on the course of toxic shock syndrome toxin-1, LPS or TNF-induced shock, fulminant Escherichia coli peritonitis, sub-lethal Klebsiella pneumoniae pneumonia, and chronic candidiasis. Altogether, these data support the safety profile of SIRT2 inhibitors under clinical development in terms of susceptibility to infections.

Circadian and Feeding Rhythms Orchestrate the Diurnal Liver Acetylome

Lysine acetylation is involved in various biological processes and is considered a key reversible post-translational modification in the regulation of gene expression, enzyme activity, and subcellular localization. This post-translational modification is therefore highly relevant in the context of circadian biology, but its characterization on the proteome-wide scale and its circadian clock dependence are still poorly described. Here, we provide a comprehensive and rhythmic acetylome map of the mouse liver. Rhythmic acetylated proteins showed subcellular localization-specific phases that correlated with the related metabolites in the regulated pathways. Mitochondrial proteins were over-represented among the rhythmically acetylated proteins and were highly correlated with SIRT3-dependent deacetylation. SIRT3 activity being nicotinamide adenine dinucleotide (NAD)+ level-dependent, we show that NAD+ is orchestrated by both feeding rhythms and the circadian clock through the NAD+ salvage pathway but also via the nicotinamide riboside pathway. Hence, the diurnal acetylome relies on a functional circadian clock and affects important diurnal metabolic pathways in the mouse liver.

Interplay between Metabolism and Epigenetics: A Nuclear Adaptation to Environmental Changes

The physiological identity of every cell is maintained by highly specific transcriptional networks that establish a coherent molecular program that is in tune with nutritional conditions. The regulation of cell-specific transcriptional networks is accomplished by an epigenetic program via chromatin-modifying enzymes, whose activity is directly dependent on metabolites such as acetyl-coenzyme A, S-adenosylmethionine, and NAD+, among others. Therefore, these nuclear activities are directly influenced by the nutritional status of the cell. In addition to nutritional availability, this highly collaborative program between epigenetic dynamics and metabolism is further interconnected with other environmental cues provided by the day-night cycles imposed by circadian rhythms. Herein, we review molecular pathways and their metabolites associated with epigenetic adaptations modulated by histone- and DNA-modifying enzymes and their responsiveness to the environment in the context of health and disease.

Circadian Reprogramming in the Liver Identifies Metabolic Pathways of Aging

The process of aging and circadian rhythms are intimately intertwined, but how peripheral clocks involved in metabolic homeostasis contribute to aging remains unknown. Importantly, caloric restriction (CR) extends lifespan in several organisms and rewires circadian metabolism. Using young versus old mice, fed ad libitum or under CR, we reveal reprogramming of the circadian transcriptome in the liver. These age-dependent changes occur in a highly tissue-specific manner, as demonstrated by comparing circadian gene expression in the liver versus epidermal and skeletal muscle stem cells. Moreover, de novo oscillating genes under CR show an enrichment in SIRT1 targets in the liver. This is accompanied by distinct circadian hepatic signatures in NAD+-related metabolites and cyclic global protein acetylation. Strikingly, this oscillation in acetylation is absent in old mice while CR robustly rescues global protein acetylation. Our findings indicate that the clock operates at the crossroad between protein acetylation, liver metabolism, and aging.

mPer1 promotes morphine-induced locomotor sensitization and conditioned place preference via histone deacetylase activity

RATIONALE

Previous studies have shown that repeated exposure to drugs of abuse is associated with changes in clock genes expression and that mice strains with various mutations in clock genes show alterations in drug-induced behaviors.

OBJECTIVE

The objective of this study is to characterize the role of the clock gene mPer1 in the development of morphine-induced behaviors and a possible link to histone deacetylase (HDAC) activity.

METHODS

In Per1 ^Brdm1 null mutant mice and wild-type (WT) littermates, we examined whether there were any differences in the development of morphine antinociception, tolerance to antinociception, withdrawal, sensitization to locomotion, and conditioned place preference (CPP).

RESULTS

Per1 ^Brdm1 mutant mice did not show any difference in morphine antinociception, tolerance development, nor in physical withdrawal signs precipitated by naloxone administration compared to WT. However, morphine-induced locomotor sensitization and CPP were significantly impaired in Per1 ^Brdm1 mutant mice. Because a very similar dissociation between tolerance and dependence vs. sensitization and CPP was recently observed after the co-administration of morphine and the HDAC inhibitor sodium butyrate (NaBut), we studied a possible link between mPer1 and HDAC activity. As opposed to WT controls, Per1 ^Brdm1 mutant mice showed significantly enhanced striatal global HDAC activity within the striatum when exposed to a locomotor-sensitizing morphine administration regimen. Furthermore, the administration of the HDAC inhibitor NaBut restored the ability of morphine to promote locomotor sensitization and reward in Per1 ^Brdm1 mutant mice.

CONCLUSIONS

Our results reveal that although the mPer1 gene does not alter morphine-induced antinociception nor withdrawal, it plays a prominent role in the development of morphine-induced behavioral sensitization and reward via inhibitory modulation of striatal HDAC activity. These data suggest that PER1 inhibits deacetylation to promote drug-induced neuroplastic changes.

Desynchronization of Circadian Clocks in Cancer: A Metabolic and Epigenetic Connection

Circadian clocks are innate oscillators that drive daily rhythms in metabolism, physiology, and behavior. 24-h rhythms in gene expression, driven by core clock transcription factors, reflect the epigenetic state of the cell, which in turn is dictated by the metabolic environment. Cancer cells alter their metabolic state and gene expression and therefore are likely to tweak circadian clock function in their favor. Over the past decade, we have witnessed an extraordinary increase in systems-level studies that suggest intricate mechanistic links between the cellular metabolome and the circadian epigenome. In parallel, reprogramming of cellular clock function in cancers is increasingly evident and the role of clock genes in the development of hematological tumors, as well as their pathophysiological effects on tissues distal to the tumor, has been described. Furthermore, the interplay between components of the circadian clock, metabolic enzymes, and oncogenes is starting to be better understood, such as the close association between overexpression of the Myc oncogene and perturbation of circadian and metabolic rhythms, thus opening new avenues to treat cancers. This review article explores current knowledge on the circadian metabolome and the molecular pathways they control, with a focus on their involvement in the development of hematopoietic malignancies.

Cofactors As Metabolic Sensors Driving Cell Adaptation in Physiology and Disease

Chromatin architectures and epigenetic fingerprint regulation are fundamental for genetically determined biological processes. Chemical modifications of the chromatin template sensitize the genome to intracellular metabolism changes to set up diverse functional adaptive states. Accumulated evidence suggests that the action of epigenetic modifiers is sensitive to changes in dietary components and cellular metabolism intermediates, linking nutrition and energy metabolism to gene expression plasticity. Histone posttranslational modifications create a code that acts as a metabolic sensor, translating changes in metabolism into stable gene expression patterns. These observations support the notion that epigenetic reprograming-linked energy input is connected to the etiology of metabolic diseases and cancer. In the present review, we introduce the role of epigenetic cofactors and their relation with nutrient intake and we question the links between epigenetic regulation and the development of metabolic diseases.

Spatial dynamics of SIRT1 and the subnuclear distribution of NADH species

Sirtuin 1 (SIRT1) is an NAD⁺-dependent deacetylase that functions as metabolic sensor of cellular energy and modulates biochemical pathways in the adaptation to changes in the environment. SIRT1 substrates include histones and proteins related to enhancement of mitochondrial function as well as antioxidant protection. Fluctuations in intracellular NAD⁺ levels regulate SIRT1 activity, but how SIRT1 enzymatic activity impacts on NAD⁺ levels and its intracellular distribution remains unclear. Here, we show that SIRT1 determines the nuclear organization of protein-bound NADH. Using multiphoton microscopy in live cells, we show that free and bound NADH are compartmentalized inside of the nucleus, and its subnuclear distribution depends on SIRT1. Importantly, SIRT6, a chromatin-bound deacetylase of the same class, does not influence NADH nuclear localization. In addition, using fluorescence fluctuation spectroscopy in single living cells, we reveal that NAD⁺ metabolism in the nucleus is linked to subnuclear dynamics of active SIRT1. These results reveal a connection between NAD⁺ metabolism, NADH distribution, and SIRT1 activity in the nucleus of live cells and pave the way to decipher links between nuclear organization and metabolism.

Clock-genes and mitochondrial respiratory activity: Evidence of a reciprocal interplay

In the past few years mounting evidences have highlighted the tight correlation between circadian rhythms and metabolism. Although at the organismal level the central timekeeper is constituted by the hypothalamic suprachiasmatic nuclei practically all the peripheral tissues are equipped with autonomous oscillators made up by common molecular clockworks represented by circuits of gene expression that are organized in interconnected positive and negative feed-back loops. In this study we exploited a well-established in vitro synchronization model to investigate specifically the linkage between clock gene expression and the mitochondrial oxidative phosphorylation (OxPhos). Here we show that synchronized cells exhibit an autonomous ultradian mitochondrial respiratory activity which is abrogated by silencing the master clock gene ARNTL/BMAL1. Surprisingly, pharmacological inhibition of the mitochondrial OxPhos system resulted in dramatic deregulation of the rhythmic clock-gene expression and a similar result was attained with mtDNA depleted cells (Rho0). Our findings provide a novel level of complexity in the interlocked feedback loop controlling the interplay between cellular bioenergetics and the molecular clockwork. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.