Circadian clocks and metabolism: the nutrient-sensing AKT and TOR pathways make the link

The importance of time in nutrient regulation: a case study with spotted-wing Drosophila (Drosophila suzukii)

Introduction

The ability of living organisms to acquire the nutrients needed to carry out required physiological functions has important consequences for fitness. However, an organism must not simply meet the requirements for individual nutrients, but must ingest an optimal balance of multiple nutrients. Despite this, animals rarely consume truly balanced resources, and instead commonly feed selectively across multiple unbalanced resources to reach an optimal balance, i.e., intake target. Nutritional research has predominantly focused on the behavioral strategies employed during nutrient regulation, as well as the fitness consequence of failing to meet intake targets, but little work has been done on the temporal aspects of this process. For instance, within what timeframe must organisms reach their intake target before a fitness cost is incurred? Hours, days, weeks?

Methods

In this study, we investigated how nutrient regulation interval impacts consumption and performance in adult female spotted-wing Drosophila (Drosophila suzukii). Females were constrained to either a protein- orcarbohydrate-biased diet over different time intervals and at different schedules, while control flies were constrained to one diet for the entire feeding period.

Results

Regulation interval had a significant impact on feeding behavior and consumption. Total consumption was highest on the shorter interval treatments, where diets were alternated more frequently, and declined as the interval period increased. The relative consumption of both diets was statistically-different across intervals and was higher for the carbohydrate-biased diet. Consumption of the protein-biased diet was more variable across intervals and was more strongly impacted by the daily timing of diet switches. Performance data showed that shorter regulation intervals led to longer fly lifespans, a result commonly observed in studies exploring the impacts of diet macronutrient ratio variability on performance.

Discussion

These results show that the temporal aspects of nutrition, such as feeding intervals and the timing of resource availability, can have strong impacts on feeding behavior, nutrient regulation, and fitness. These results provide an insight into how consumers may deal with changes in host phenology, the availability of hosts, and changes in nutrient availability within hosts. Understanding these mechanisms will be important for predicting responses to changes in nutrient cycling and resource availability mediated by natural and anthropogenic habitat modifications, such as global climate change.

It's about time; divergent circadian clocks in livers of mice and naked mole-rats

Light is the key regulator of circadian clock, the time-keeping system synchronizing organism physiology and behavior with environmental day and night conditions. In its natural habitat, the strictly subterranean naked mole-rat (Heterocephalus glaber) has lived in a light-free environment for millennia. We questioned if this species retains a circadian clock and if the patterns of this clock and concomitant rhythms differed in liver tissue from mice and naked mole-rats. As expected, in mice, the various circadian clock genes peaked at different times of the day; the Period gene (Per) group peaked in the evening, whereas Brain and Muscle ARNT-like1 (Bmal1) gene peaked in the morning; this phase shift is considered to be fundamental for circadian clock function. In sharp contrast, in the naked mole-rat both Per1 and Per2, as well as Bmal1, peaked at the same time in the morning-around ZT2-suggesting the organization of the molecular circadian oscillator was different. Moreover, gene expression rhythms associated with glucose metabolism and mTOR signaling also differed between the species. Although the activity of mTORC1 was lower, while that of mTORC2 was higher in naked mole-rat livers compared to mice, unlike that of mice where the expression profiles of glucose metabolism genes were not synchronized, these were highly synchronized in naked mole-rats and likely linked to their use of feeding times at zeitgebers.

Time-restricted feeding regulates molecular mechanisms with involvement of circadian rhythm to prevent metabolic diseases

Lifestyle and genetic perturbation of circadian rhythm can trigger the incidence and severity of metabolic diseases. Time-restricted feeding (TRF) regulates the circadian rhythm of food intake that protects against metabolic disorders induced by adverse nutrient intake. TRF also executes host metabolism from nutrient availability to optimize nutrient utilization. Circadian clock and nutrient-sensing pathways coordinate to regulate metabolic health through the feeding/fasting cycle. Concurrently, TRF imposes diurnal rhythm in nutrient utilization, thereby preserving cellular homeostasis. However, modulation of daily feeding and fasting periods calibrates the circadian clock, which protects against the lethal effects of nutrient imbalance on metabolism. Therefore, TRF also improves and restores metabolic rhythms that ultimately lead to better fitness by reversing the alteration in genotype-specific gene expression. The aim of this review was to summarize that TRF is an emerging dietary approach that maintains robust circadian rhythms in support of a steady daily feeding and fasting cycle. TRF also encourages the coordination between circadian clock components and nutrient-sensing pathways via molecular effectors that exert a protective role in the prevention of metabolic diseases.

Ghrelin Restores the Disruption of the Circadian Clock in Steatotic Liver

Obese mice demonstrate disruption of the circadian clock and feeding cycle. Circulating ghrelin, a hormone secreted mainly by gastric X/Alike cells, is significantly reduced in obese humans and animals. Here, we examined whether ghrelin improves the disruption of the circadian rhythm in steatotic hepatocytes and liver. The effects of ghrelin on hepatic circadian clock genes were studied in steatotic hepatocytes and liver of mice fed a high-fat diet (HFD) for 12 weeks. The circadian clock of cultured hepatocytes was synchronized by treatment with 100 nM dexamethasone for 1 h. Ghrelin was administrated to the cultured hepatocytes (10-8 M) or to mice at a dose of 11 nmol/kg/d for two weeks via a subcutaneous minipump. The mRNA and protein levels of core clock genes were analyzed. Steatosis significantly blunted the circadian pattern of clock genes such as Bmal1, Clock, and Per in cultured hepatocytes and liver. Treatment with ghrelin markedly restored the daily rhythm of the clock genes, with a robust oscillation between peak and trough in cultured hepatocytes isolated from obese mice. It also increased the abundance and expression amplitude of clock genes in steatotic liver, causing the peak of Clock to shift to the dark period and the peak of Per2 to shift to the light period compared with the control groups. Deletion of GHSR1a further deteriorated the derangement of clock gene patterns in obese mice. Ghrelin significantly increased the oscillations of mTOR/S6 signaling. We demonstrate that ghrelin restored the derangement of the circadian rhythm in steatotic liver via mTOR signaling.

Lysosomes Mediate Benefits of Intermittent Fasting in Cardiometabolic Disease: The Janitor Is the Undercover Boss

Adaptive responses that counter starvation have evolved over millennia to permit organismal survival, including changes at the level of individual organelles, cells, tissues, and organ systems. In the past century, a shift has occurred away from disease caused by insufficient nutrient supply toward overnutrition, leading to obesity and diabetes, atherosclerosis, and cardiometabolic disease. The burden of these diseases has spurred interest in fasting strategies that harness physiological responses to starvation, thus limiting tissue injury during metabolic stress. Insights gained from animal and human studies suggest that intermittent fasting and chronic caloric restriction extend lifespan, decrease risk factors for cardiometabolic and inflammatory disease, limit tissue injury during myocardial stress, and activate a cardioprotective metabolic program. Acute fasting activates autophagy, an intricately orchestrated lysosomal degradative process that sequesters cellular constituents for degradation, and is critical for cardiac homeostasis during fasting. Lysosomes are dynamic cellular organelles that function as incinerators to permit autophagy, as well as degradation of extracellular material internalized by endocytosis, macropinocytosis, and phagocytosis. The last decade has witnessed an explosion of knowledge that has shaped our understanding of lysosomes as central regulators of cellular metabolism and the fasting response. Intriguingly, lysosomes also store nutrients for release during starvation; and function as a nutrient sensing organelle to couple activation of mammalian target of rapamycin to nutrient availability. This article reviews the evidence for how the lysosome, in the guise of a janitor, may be the "undercover boss" directing cellular processes for beneficial effects of intermittent fasting and restoring homeostasis during feast and famine. © 2018 American Physiological Society. Compr Physiol 8:1639-1667, 2018.

Optogenetic interrogation reveals separable G-protein-dependent and -independent signalling linking G-protein-coupled receptors to the circadian oscillator

BACKGROUND

Endogenous circadian oscillators distributed across the mammalian body are synchronised among themselves and with external time via a variety of signalling molecules, some of which interact with G-protein-coupled receptors (GPCRs). GPCRs can regulate cell physiology via pathways originating with heterotrimeric G-proteins or β-arrestins. We applied an optogenetic approach to determine the contribution of these two signalling modes on circadian phase.

RESULTS

We employed a photopigment (JellyOp) that activates Gαs signalling with better selectivity and higher sensitivity than available alternatives, and a point mutant of this pigment (F112A) biased towards β-arrestin signalling. When expressed in fibroblasts, both native JellyOp and the F112A arrestin-biased mutant drove light-dependent phase resetting in the circadian clock. Shifts induced by the two opsins differed in their circadian phase dependence and the degree to which they were associated with clock gene induction.

CONCLUSIONS

Our data imply separable G-protein and arrestin inputs to the mammalian circadian clock and establish a pair of optogenetic tools suitable for manipulating Gαs- and β-arrestin-biased signalling in live cells.

Aging and Autophagic Function Influences the Progressive Decline of Adult Drosophila Behaviors

Multiple neurological disorders are characterized by the abnormal accumulation of protein aggregates and the progressive impairment of complex behaviors. Our Drosophila studies demonstrate that middle-aged wild-type flies (WT, ~4-weeks) exhibit a marked accumulation of neural aggregates that is commensurate with the decline of the autophagy pathway. However, enhancing autophagy via neuronal over-expression of Atg8a (Atg8a-OE) reduces the age-dependent accumulation of aggregates. Here we assess basal locomotor activity profiles for single- and group-housed male and female WT flies and observed that only modest behavioral changes occurred by 4-weeks of age, with the noted exception of group-housed male flies. Male flies in same-sex social groups exhibit a progressive increase in nighttime activity. Infrared videos show aged group-housed males (4-weeks) are engaged in extensive bouts of courtship during periods of darkness, which is partly repressed during lighted conditions. Together, these nighttime courtship behaviors were nearly absent in young WT flies and aged Atg8a-OE flies. Previous studies have indicated a regulatory role for olfaction in male courtship partner choice. Coincidently, the mRNA expression profiles of several olfactory genes decline with age in WT flies; however, they are maintained in age-matched Atg8a-OE flies. Together, these results suggest that middle-aged male flies develop impairments in olfaction, which could contribute to the dysregulation of courtship behaviors during dark time periods. Combined, our results demonstrate that as Drosophila age, they develop early behavior defects that are coordinate with protein aggregate accumulation in the nervous system. In addition, the nighttime activity behavior is preserved when neuronal autophagy is maintained (Atg8a-OE flies). Thus, environmental or genetic factors that modify autophagic capacity could have a positive impact on neuronal aging and complex behaviors.

Epigenetic control and the circadian clock: linking metabolism to neuronal responses

Experimental and epidemiological evidence reveal the profound influence that industrialized modern society has imposed on human social habits and physiology during the past 50 years. This drastic change in life-style is thought to be one of the main causes of modern diseases including obesity, type 2 diabetes, mental illness such as depression, sleep disorders, and certain types of cancer. These disorders have been associated to disruption of the circadian clock, an intrinsic time-keeper molecular system present in virtually all cells and tissues. The circadian clock is a key element in homeostatic regulation by controlling a large array of genes implicated in cellular metabolism. Importantly, intimate links between epigenetic regulation and the circadian clock exist and are likely to prominently contribute to the plasticity of the response to the environment. In this review, we summarize some experimental and epidemiological evidence showing how environmental factors such as stress, drugs of abuse and changes in circadian habits, interact through different brain areas to modulate the endogenous clock. Furthermore we point out the pivotal role of the deacetylase silent mating-type information regulation 2 homolog 1 (SIRT1) as a molecular effector of the environment in shaping the circadian epigenetic landscape.

BMAL1-dependent regulation of the mTOR signaling pathway delays aging

The circadian clock, an internal time-keeping system, has been linked with control of aging, but molecular mechanisms of regulation are not known. BMAL1 is a transcriptional factor and core component of the circadian clock; BMAL1 deficiency is associated with premature aging and reduced lifespan. Here we report that activity of mammalian Target of Rapamycin Complex 1 (mTORC1) is increased upon BMAL1 deficiency both in vivo and in cell culture. Increased mTOR signaling is associated with accelerated aging; in accordance with that, treatment with the mTORC1 inhibitor rapamycin increased lifespan of Bmal1-/- mice by 50%. Our data suggest that BMAL1 is a negative regulator of mTORC1 signaling. We propose that the circadian clock controls the activity of the mTOR pathway through BMAL1-dependent mechanisms and this regulation is important for control of aging and metabolism.

Time-restricted feeding without reducing caloric intake prevents metabolic diseases in mice fed a high-fat diet

While diet-induced obesity has been exclusively attributed to increased caloric intake from fat, animals fed a high-fat diet (HFD) ad libitum (ad lib) eat frequently throughout day and night, disrupting the normal feeding cycle. To test whether obesity and metabolic diseases result from HFD or disruption of metabolic cycles, we subjected mice to either ad lib or time-restricted feeding (tRF) of a HFD for 8 hr per day. Mice under tRF consume equivalent calories from HFD as those with ad lib access yet are protected against obesity, hyperinsulinemia, hepatic steatosis, and inflammation and have improved motor coordination. The tRF regimen improved CREB, mTOR, and AMPK pathway function and oscillations of the circadian clock and their target genes' expression. These changes in catabolic and anabolic pathways altered liver metabolome and improved nutrient utilization and energy expenditure. We demonstrate in mice that tRF regimen is a nonpharmacological strategy against obesity and associated diseases.

TRIB1 constitutes a molecular link between regulation of sleep and lipid metabolism in humans

Epidemiological studies show association between sleep duration and lipid metabolism. In addition, inactivation of circadian genes induces insulin resistance and hyperlipidemia. We hypothesized that sleep length and lipid metabolism are partially controlled by the same genes. We studied the association of total sleep time (TST) with 60 genetic variants that had previously been associated with lipids. The analyses were performed in a Finnish population-based sample (N = 6334) and replicated in 2189 twins. Finally, RNA expression from mononuclear leucocytes was measured in 10 healthy volunteers before and after sleep restriction. The genetic analysis identified two variants near TRIB1 gene that independently contributed to both blood lipid levels and to TST (rs17321515, P = 8.92(*)10(-5), Bonferroni corrected P = 0.0053, β = 0.081 h per allele; rs2954029, P = 0.00025, corrected P = 0.015, β = 0.076; P<0.001 for both variants after adjusting for blood lipid levels or body mass index). The finding was replicated in the twin sample (rs17321515, P = 0.022, β = 0.063; meta-analysis of both samples P = 8.1(*)10(-6), β = 0.073). After the experimentally induced sleep restriction period TRIB1 expression increased 1.6-fold and decreased in recovery phase (P = 0.006). In addition, a negative correlation between TRIB1 expression and slow wave sleep was observed in recovery from sleep restriction. These results show that allelic variants of TRIB1 are independently involved in regulation of lipid metabolism and sleep. The findings give evidence for the pleiotropic nature of TRIB1 and may reflect the shared roots of sleep and metabolism. The shared genetic background may at least partially explain the mechanism behind the well-established connection between diseases with disrupted metabolism and sleep.

Role of TOR signaling in aging and related biological processes in Drosophila melanogaster

Extensive studies in model organisms in the last few decades have revealed that aging is subject to profound genetic influence. The conserved nutrient sensing TOR (Target of Rapamycin) pathway is emerging as a key regulator of lifespan and healthspan in various species from yeast to mammals. The TOR signaling pathway plays a critical role in determining how a eukaryotic cell or a cellular system co-ordinates its growth, development and aging in response to constant changes in its surrounding environment? TOR integrates signals originating from changes in growth factors, nutrient availability, energy status and various physiological stresses. Each of these inputs is specialized to sense particular signal(s), and conveys it to the TOR complex which in turn relays the signal to downstream outputs to appropriately respond to the environmental changes. These outputs include mRNA translation, autophagy, transcription, metabolism, cell survival, proliferation and growth amongst a number of other cellular processes, some of which influence organismal lifespan. Here we review the contribution of the model organism Drosophila in the understanding of TOR signaling and the various biological processes it modulates that may impact on aging. Drosophila was the first organism where the nutrient dependent effects of the TOR pathway on lifespan were first uncovered. We also discuss how the nutrient-sensing TOR pathway appears to be critically important for mediating the longevity effects of dietary restriction (DR), a potent environmental method of lifespan extension by nutrient limitation. Identifying the molecular mechanisms that modulate lifespan downstream of TOR is being intensely investigated and there is hope that these are likely to serve as potential targets for amelioration of age-related diseases and enhance healthful lifespan extension in humans.