Age-dependent changes of the circadian system

Circadian Rhythm of Distal Skin Temperature in Healthy Older and Young Women and Its Relationship with Sleep-Wake Rhythm and Environmental Factors under Natural Living Conditions

Little is known about the healthy aging of the circadian timing system under natural living conditions. This study explores changes in the circadian rhythm of distal skin temperature (DST) with aging and relates these changes to sleep-wake timing and environmental influences. DST, sleep-wake timing, 24-h light exposure, and physical activity were measured and averaged over seven consecutive days using temperature sensors, actigraphy with a light meter, and sleep diaries in 35 healthy older women (60-79 years) and 30 young women (20-34 years). Circadian rhythm characteristics, describing strength (amplitude) and timing (acrophase) of the DST rhythm, were calculated using cosinor analysis. The older adults displayed an 18-19% smaller amplitude and a 66-73 min earlier acrophase (peak time) for DST rhythm than the young adults, indicating a weaker and phase-advanced DST rhythm. The phase advance for DST was not due to an earlier evening increase, but to a shorter nocturnal plateau period. Daytime light exposure inversely affected strength (amplitude) but not phasing of the DST rhythm in older adults. The DST rhythm was 3.5 times more advanced than the sleep-wake rhythm, showing an altered phase relationship (phase angle) between both rhythms with aging. The phase angle was more heterogeneous among older adults, showing differential aging. The phase advance for DST rhythm and the altered and heterogeneous phase relationship between DST and sleep-wake rhythms were not related to ambient light exposure and the physical activity of older adults. This suggests that healthy aging of the circadian system might be due to endogenous mechanisms such as an internal rearrangement rather than external influences.

Determinants of anemia among patients receiving cancer chemotherapy in Northwest Ethiopia

Background

Chemotherapy-induced anemia (CIA) is a hematologic complication that frequently affects patients with cancer undergoing chemotherapy. It is associated with worse treatment outcomes, higher rates of morbidity and mortality, worse quality of life, and higher healthcare costs. The incidence and predictors of CIA in Ethiopia, particularly in Northwest Ethiopian oncology centers, are poorly understood. This study was conducted at Northwest Ethiopian oncology centers to evaluate the incidence and determinants of chemotherapy-induced anemia in adult patients with cancer undergoing chemotherapy.

Methods

This 3-year hospital-based retrospective follow-up study included adult patients with cancer receiving chemotherapy between 2019 and 2021 at two oncology centers in Northwest Ethiopia. Data were collected from October to December 2021. A binary logistic regression model was used to select variables and determine the Crude Odds Ratio (COR). Variables with P-value < 0.2 were entered into the multivariable logistic regression and Adjusted odds ratio (AOR) with 95% Confidence intervals (CI) for variables with P-value < 0.05 were estimated to show determinants of chemotherapy-induced anemia among cancer patients who received chemotherapy.

Results

A total of 402 patients were included in the final analysis. The overall incidence of CIA was 75.4% (95% CI 70.7, 79.8). Older age [AOR = 1.8, 95% CI (1.4-3.5); P = 0.043], hematologic cancer [AOR = 3.7, 95% CI (3.2-5.7), P = 0.021], obesity [AOR = 3.4, 95% CI (2.3-6.9); P = 0.028], ≥6 chemotherapy cycles [AOR = 3.8, 95% CI (3.2-5.1), P = 0.019], cancer metastasis to bone [AOR = 2.9, 95% CI (1.2-4.7), P = 0.025] were statistically significant predictors of chemotherapy-induced anemia.

Conclusion

Chemotherapy-induced anemia persisted in a significant percentage of cancer patients. Chemotherapy-induced anemia developed in three-quarters of patients undergoing chemotherapy. Chemotherapy-induced anemia was significantly associated with older age, hematologic malignancy, obesity, a greater number of chemotherapy cycles, and cancer metastasis to bone. To lower the risk of morbidity related to anemia, patients with chemotherapy-induced anemia should be regularly evaluated and treated with appropriate treatment.

Circadian-driven tissue specificity is constrained under caloric restricted feeding conditions

Tissue specificity is a fundamental property of an organ that affects numerous biological processes, including aging and longevity, and is regulated by the circadian clock. However, the distinction between circadian-affected tissue specificity and other tissue specificities remains poorly understood. Here, using multi-omics data on circadian rhythms in mice, we discovered that approximately 35% of tissue-specific genes are directly affected by circadian regulation. These circadian-affected tissue-specific genes have higher expression levels and are associated with metabolism in hepatocytes. They also exhibit specific features in long-reads sequencing data. Notably, these genes are associated with aging and longevity at both the gene level and at the network module level. The expression of these genes oscillates in response to caloric restricted feeding regimens, which have been demonstrated to promote longevity. In addition, aging and longevity genes are disrupted in various circadian disorders. Our study indicates that the modulation of circadian-affected tissue specificity is essential for understanding the circadian mechanisms that regulate aging and longevity at the genomic level.

Chronobiological Study Designs

The chapter describes experimental designs for various chronobiological studies aimed at basic research and clinical trials, with an emphasis on circadian rhythms. In the first part, various methods of data collection, particularly longitudinal and transverse sampling and their relative merits, are discussed. Thereafter, specific methods and their constraints for monitoring marker rhythms are presented. Variables that are most effective in characterizing the endogenous pacemaker and those of clinical relevance are discussed. Besides melatonin and core body temperature rhythms, which are widely accepted as the gold standard for representing the circadian clock, rhythms of cortisol concentration, physical activity, sleep parameters and chronotypes are considered. The relevance of stable rhythms with appropriate internal and external phase relationships for health and wellbeing, as well as adverse effects of certain rhythm alterations are discussed. The last part describes two experimental designs that allow separating endogenous and exogenous components of biological rhythms, the constant routine and the forced desynchronization protocols.

Irisin/BDNF signaling in the muscle-brain axis and circadian system: A review

In mammals, the timing of physiological, biochemical and behavioral processes over a 24-h period is controlled by circadian rhythms. To entrain the master clock located in the suprachiasmatic nucleus of the hypothalamus to a precise 24-h rhythm, environmental zeitgebers are used by the circadian system. This is done primarily by signals from the retina via the retinohypothalamic tract, but other cues like exercise, feeding, temperature, anxiety, and social events have also been shown to act as non-photic zeitgebers. The recently identified myokine irisin is proposed to serve as an entraining non-photic signal of exercise. Irisin is a product of cleavage and modification from its precursor membrane fibronectin type Ⅲ domain-containing protein 5 (FNDC5) in response to exercise. Apart from well-known peripheral effects, such as inducing the "browning" of white adipocytes, irisin can penetrate the blood-brain barrier and display the effects on the brain. Experimental data suggest that FNDC5/irisin mediates the positive effects of physical activity on brain functions. In several brain areas, irisin induces the production of brain-derived neurotrophic factor (BDNF). In the master clock, a significant role in gating photic stimuli in the retinohypothalamic synapse for BDNF is suggested. However, the brain receptor for irisin remains unknown. In the current review, the interactions of physical activity and the irisin/BDNF axis with the circadian system are reconceptualized.

Blue light exposure-dependent improvement in robustness of circadian rest-activity rhythm in aged rats

The aging effects on circadian rhythms have diverse implications including changes in the pattern of rhythmic expressions, such as a wide fragmentation of the rhythm of rest-activity and decrease in amplitude of activity regulated by the suprachiasmatic nucleus (SCN). The study of blue light on biological aspects has received great current interest due, among some aspects, to its positive effects on psychiatric disorders in humans. This study aims to evaluate the effect of blue light therapy on the SCN functional aspects, through the evaluation of the rest-activity rhythm, in aging rats. For this, 33 sixteen-months-old male Wistar rats underwent continuous records of locomotor activity and were exposed to periods of 6 hours of blue light during the first half of the light phase (Zeitgeber times 0-6) for 14 days. After this, the rats were maintained at 12h:12h light:dark cycle to check the long-term effect of blue light for 14 days. Blue light repeated exposure showed positive effects on the rhythmic variables of locomotor activity in aged rats, particularly the increase in amplitude, elevation of rhythmic robustness, phase advance in acrophase, and greater consolidation of the resting phase. This effect depends on the presence of daily blue light exposure. In conclusion, our results indicate that blue light is a reliable therapy to reduce circadian dysfunctions in aged rats, but other studies assessing how blue light modulates the neural components to modulate this response are still needed.

Aryl hydrocarbon receptor (AhR) impairs circadian regulation: impact on the aging process

Circadian clocks control the internal sleep-wake rhythmicity of 24hours which is synchronized by the solar cycle. Circadian regulation of metabolism evolved about 2.5 billion years ago, i.e., the rhythmicity has been conserved from cyanobacteria and Archaea through to mammals although the mechanisms utilized have developed with evolution. While the aryl hydrocarbon receptor (AhR) is an evolutionarily conserved defence mechanism against environmental threats, it has gained many novel functions during evolution, such as the regulation of cell cycle, proteostasis, and many immune functions. There is robust evidence that AhR signaling impairs circadian rhythmicity, e.g., by interacting with the core BMAL1/CLOCK complex and disturbing the epigenetic regulation of clock genes. The maintenance of circadian rhythms is impaired with aging, disturbing metabolism and many important functions in aged organisms. Interestingly, it is known that AhR signaling promotes an age-related tissue degeneration, e.g., it is able to inhibit autophagy, enhance cellular senescence, and disrupt extracellular matrix. These alterations are rather similar to those induced by a long-term impairment of circadian rhythms. However, it is not known whether AhR signaling enhances the aging process by impairing circadian homeostasis. I will examine the experimental evidence indicating that AhR signaling is able to promote the age-related degeneration via a disruption of circadian rhythmicity.

Home-Cage Sipper Devices Reveal Age and Sex Differences in Ethanol Consumption Patterns

Free-choice paradigms such as two-bottle choice (2BC) are commonly used to characterize ethanol consumption and preference of rodent models used to study alcohol use disorder (AUD). However, these assays are limited by low temporal resolution that misses finer patterns of drinking behavior, including circadian drinking patterns that are known to vary with age and sex and are affected in AUD pathogenesis. Modern, cost-effective tools are becoming widely available that could elucidate these patterns, including open-source, Arduino-based home-cage sipper devices. We hypothesized that adaptation of these home-cage sipper devices would uncover distinct age- and sex-related differences in temporal drinking patterns. To test this hypothesis, we used the sipper devices in a continuous 2BC paradigm using water and ethanol (10%; v/v) for 14 days to measure drinking patterns of male and female adolescent (3-week), young adult (6-week), and mature adult (18-week) C57BL/6J mice. Daily grams of fluid consumption were manually recorded at the beginning of the dark cycle, while home-cage sipper devices continuously recorded the number of sips. Consistent with prior studies, females consumed more ethanol than males, and adolescent mice consumed the most out of any age group. Correlation analyses of manually recorded fluid consumption versus home-cage sipper activity revealed a statistically significant prediction of fluid consumption across all experimental groups. Sipper activity was able to capture subtle circadian differences between experimental groups, as well as distinct individual variation in drinking behavior among animals. Blood ethanol concentrations were significantly correlated with sipper data, suggesting that home-cage sipper devices can accurately determine individual timing of ethanol consumption. Overall, our studies show that augmenting the 2BC drinking paradigm with automated home-cage sipper devices can accurately measure ethanol consumption across sexes and age groups, revealing individual differences and temporal patterns of ethanol drinking behavior. Future studies utilizing these home-cage sipper devices will further dissect circadian patterns for age and sex relevant to the pathogenesis of AUD, as well as underlying molecular mechanisms for patterns in ethanol consumption.

Highlights

Female mice consume more ethanol than males in a continuous access paradigmAdolescent male and female mice consume more ethanol than young or mature adult miceAutomated home-cage sipper devices accurately measure ethanol consumptionDevices reveal sex- and age-dependent differences in circadian drinking patternsDevices reveal distinct individual variation in circadian drinking patterns.

Sex-dimorphic and age-dependent organization of 24-hour gene expression rhythms in humans

The circadian clock modulates human physiology. However, the organization of tissue-specific gene expression rhythms and how these depend on age and sex is not defined in humans. We combined data from the Genotype-Tissue Expression (GTEx) project with an algorithm that assigns circadian phases to 914 donors, by integrating temporal information from multiple tissues in each individual, to identify messenger RNA (mRNA) rhythms in 46 tissues. Clock transcripts showed conserved timing relationships and tight synchrony across the body. mRNA rhythms varied in breadth, covering global and tissue-specific functions, including metabolic pathways and systemic responses. The clock structure was conserved across sexes and age groups. However, overall gene expression rhythms were highly sex-dimorphic and more sustained in females. Rhythmic programs generally dampened with age across the body.

The Impact of Physical Activity on the Circadian System: Benefits for Health, Performance and Wellbeing

Circadian rhythms are an inherent property of all living systems and an essential part of the external and internal temporal order. They enable organisms to be synchronized with their periodic environment and guarantee the optimal functioning of organisms. Any disturbances, so-called circadian disruptions, may have adverse consequences for health, physical and mental performance, and wellbeing. The environmental light–dark cycle is the main zeitgeber for circadian rhythms. Moreover, regular physical activity is most useful. Not only does it have general favorable effects on the cardiovascular system, the energy metabolism and mental health, for example, but it may also stabilize the circadian system via feedback effects on the suprachiasmatic nuclei (SCN), the main circadian pacemaker. Regular physical activity helps to maintain high-amplitude circadian rhythms, particularly of clock gene expression in the SCN. It promotes their entrainment to external periodicities and improves the internal synchronization of various circadian rhythms. This in turn promotes health and wellbeing. In experiments on Djungarian hamsters, voluntary access to a running wheel not only stabilized the circadian activity rhythm, but intensive wheel running even reestablished the rhythm in arrhythmic individuals. Moreover, their cognitive abilities were restored. Djungarian hamsters of the arrhythmic phenotype in which the SCN do not generate a circadian signal not only have a diminished cognitive performance, but their social memory is also compromised. Voluntary wheel running restored these abilities simultaneously with the reestablishment of the circadian activity rhythm. Intensively exercising Syrian hamsters are less anxious, more resilient to social defeat, and show less defensive/submissive behaviors, i.e., voluntary exercise may promote self-confidence. Similar effects were described for humans. The aim of the present paper is to summarize the current knowledge concerning the effects of physical activity on the stability of the circadian system and the corresponding consequences for physical and mental performance.

Time-course RNASeq of Camponotus floridanus forager and nurse ant brains indicate links between plasticity in the biological clock and behavioral division of labor

Background

Circadian clocks allow organisms to anticipate daily fluctuations in their environment by driving rhythms in physiology and behavior. Inter-organismal differences in daily rhythms, called chronotypes, exist and can shift with age. In ants, age, caste-related behavior and chronotype appear to be linked. Brood-tending nurse ants are usually younger individuals and show “around-the-clock” activity. With age or in the absence of brood, nurses transition into foraging ants that show daily rhythms in activity. Ants can adaptively shift between these behavioral castes and caste-associated chronotypes depending on social context. We investigated how changes in daily gene expression could be contributing to such behavioral plasticity in Camponotus floridanus carpenter ants by combining time-course behavioral assays and RNA-Sequencing of forager and nurse brains.

Results

We found that nurse brains have three times fewer 24 h oscillating genes than foragers. However, several hundred genes that oscillated every 24 h in forager brains showed robust 8 h oscillations in nurses, including the core clock genes Period and Shaggy. These differentially rhythmic genes consisted of several components of the circadian entrainment and output pathway, including genes said to be involved in regulating insect locomotory behavior. We also found that Vitellogenin, known to regulate division of labor in social insects, showed robust 24 h oscillations in nurse brains but not in foragers. Finally, we found significant overlap between genes differentially expressed between the two ant castes and genes that show ultradian rhythms in daily expression.

Conclusion

This study provides a first look at the chronobiological differences in gene expression between forager and nurse ant brains. This endeavor allowed us to identify a putative molecular mechanism underlying plastic timekeeping: several components of the ant circadian clock and its output can seemingly oscillate at different harmonics of the circadian rhythm. We propose that such chronobiological plasticity has evolved to allow for distinct regulatory networks that underlie behavioral castes, while supporting swift caste transitions in response to colony demands. Behavioral division of labor is common among social insects. The links between chronobiological and behavioral plasticity that we found in C. floridanus, thus, likely represent a more general phenomenon that warrants further investigation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08282-x.

CircadiOmic medicine and aging

The earth displays daily, seasonal and annual environmental cycles that have led to evolutionarily adapted ultradian, circadian and infradian rhythmicities in the entire biosphere. All biological organisms must adapt to these cycles that synchronize the function of their circadiome. The objective of this review is to discuss the latest knowledge regarding the role of circadiomics in health and aging. The biological timekeepers are responsive to the environmental cues at microsecond to seasonal time-scales and act with precision of a clock machinery. The robustness of these rhythms is essential to normal daily function of cells, tissues and organs. Mis-alignment of circadian rhythms makes the individual prone to aging, sleep disorders, cancer, diabetes, and neuro-degenerative diseases. Circadian and CircadiOmic medicine are emerging fields that leverage our in-depth understanding of health issues, that arise as a result of disturbances in circadian rhythms, towards establishing better therapeutic approaches in personalized medicine and for geroprotection.

Perception, magnitude, and implications of cancer-related fatigue in breast cancer survivors: Study from a developing country

Background

We have analyzed perceptions, magnitude, interventions adopted, and overall implications of cancer-related fatigue (CRF) in breast cancer survivors (BCSs).

Methodology

BCSs who attended follow-up clinic at our institute between January and June 2018 were asked to fill a questionnaire focused on assessing an individual's perception, severity, potential causes, implications on quality of life, and measures taken to deal with CRF.

Results

Sixty-five patients were included. Fifty-four (83%) had undergone surgery, 59 (91%) chemotherapy, 43 (66%) radiation therapy, and 36 (55%) hormonal/targeted therapy. Sixty-two (95%) patients experienced any grade CRF. Fifty-five (85%) patients experienced moderate to severe CRF affecting work (58%) and activities of daily living (27%). CRF was perceived as generalized weakness by 54 (83%) patients, diminished concentration/attention span by 24 (37%) patients, decreased motivation and interest in usual activities by 29 (45%) patients, and emotional labiality by 16 (25%) patients. Fifty-six patients (86%) believed that fatigue was due to the effect of cancer treatment on the body, while only 8 (12%) attributed it to underlying cancer. CRF had negative impact on mood, daily activities, interpersonal relationships, and professional work in 40 (62%), 39 (60%), 13 (20%), and 10 (15%) patients, respectively. Measures taken to overcome CRF were increased physical exercise, psychosocial interventions, mind-body interventions, and pharmacological interventions in 32 (49%), 8 (12%), 28 (43), and 17 (26%) patients, respectively. Thirty-nine (60%) patients reported persistence of CRF after completion of treatment while it took up to 6 months, 6-12 months, and more than 12 months for resolution of CRF in 13, 10, and 3 patients, respectively.

Conclusion

Development and persistence of CRF remains a major health concern, and current interventions are not able to mitigate this problem. Further research in this field is warranted.

Exploring the multifunctional role of melatonin in regulating autophagy and sleep to mitigate Alzheimer's disease neuropathology

Melatonin (MLT) is a neurohormone that is regulated by the circadian clock and plays multifunctional roles in numerous neurodegenerative disorders, such as Alzheimer's disease (AD). AD is the most common form of dementia and is associated with the degradation of axons and synapses resulting in memory loss and cognitive impairment. Despite extensive research, there is still no effective cure or specific treatment to prevent the progression of AD. The pathogenesis of AD involves atrophic alterations in the brain that also result in circadian alterations, sleep disruption, and autophagic dysfunction. In this scenario, MLT and autophagy play a central role in removing the misfolded protein aggregations. MLT also promotes autophagy through inhibiting methamphetamine toxicity to protect against neuronal cell death in AD brain. Besides, MLT plays critical roles as either a pro-autophagic indicator or anti-autophagic regulator depending on the phase of autophagy. MLT also has antioxidant properties that can counteract mitochondrial damage, oxidative stress, and apoptosis. Aging, a major risk factor for AD, can change sleep patterns and sleep quality, and MLT can improve sleep quality through regulating sleep cycles. The primary purpose of this review is to explore the putative mechanisms of the beneficial effects of MLT in AD patients. Furthermore, we also summarize the findings from preclinical and clinical studies on the multifunctional roles of MLT on autophagic regulation, the control of the circadian clock-associated genes, and sleep regulation.

Changes in sleep EEG with aging in humans and rodents

Sleep is one of the most ubiquitous but also complex animal behaviors. It is regulated at the global, systems level scale by circadian and homeostatic processes. Across the 24-h day, distribution of sleep/wake activity differs between species, with global sleep states characterized by defined patterns of brain electric activity and electromyography. Sleep patterns have been most intensely investigated in mammalian species. The present review begins with a brief overview on current understandings on the regulation of sleep, and its interaction with aging. An overview on age-related variations in the sleep states and associated electrophysiology and oscillatory events in humans as well as in the most common laboratory rodents follows. We present findings observed in different studies and meta-analyses, indicating links to putative physiological changes in the aged brain. Concepts requiring a more integrative view on the role of circadian and homeostatic sleep regulatory mechanisms to explain aging in sleep are emerging.

Shiftwork and Light at Night Negatively Impact Molecular and Endocrine Timekeeping in the Female Reproductive Axis in Humans and Rodents

Shiftwork, including work that takes place at night (nightshift) and/or rotates between day and nightshifts, plays an important role in our society, but is associated with decreased health, including reproductive dysfunction. One key factor in shiftwork, exposure to light at night, has been identified as a likely contributor to the underlying health risks associated with shiftwork. Light at night disrupts the behavioral and molecular circadian timekeeping system, which is important for coordinated timing of physiological processes, causing mistimed hormone release and impaired physiological functions. This review focuses on the impact of shiftwork on reproductive function and pregnancy in women and laboratory rodents and potential underlying molecular mechanisms. We summarize the negative impact of shiftwork on female fertility and compare these findings to studies in rodent models of light shifts. Light-shift rodent models recapitulate several aspects of reproductive dysfunction found in shift workers, and their comparison with human studies can enable a deeper understanding of physiological and hormonal responses to light shifts and the underlying molecular mechanisms that may lead to reproductive disruption in human shift workers. The contributions of human and rodent studies are essential to identify the origins of impaired fertility in women employed in shiftwork.

Peroxisome Proliferator-Activated Receptors as Molecular Links between Caloric Restriction and Circadian Rhythm

The circadian rhythm plays a chief role in the adaptation of all bodily processes to internal and environmental changes on the daily basis. Next to light/dark phases, feeding patterns constitute the most essential element entraining daily oscillations, and therefore, timely and appropriate restrictive diets have a great capacity to restore the circadian rhythm. One of the restrictive nutritional approaches, caloric restriction (CR) achieves stunning results in extending health span and life span via coordinated changes in multiple biological functions from the molecular, cellular, to the whole-body levels. The main molecular pathways affected by CR include mTOR, insulin signaling, AMPK, and sirtuins. Members of the family of nuclear receptors, the three peroxisome proliferator-activated receptors (PPARs), PPARα, PPARβ/δ, and PPARγ take part in the modulation of these pathways. In this non-systematic review, we describe the molecular interconnection between circadian rhythm, CR-associated pathways, and PPARs. Further, we identify a link between circadian rhythm and the outcomes of CR on the whole-body level including oxidative stress, inflammation, and aging. Since PPARs contribute to many changes triggered by CR, we discuss the potential involvement of PPARs in bridging CR and circadian rhythm.

Longitudinal Characterization and Biomarkers of Age and Sex Differences in the Decline of Spatial Memory

The current longitudinal study examined factors (sex, physical function, response to novelty, ability to adapt to a shift in light/dark cycle, brain connectivity), which might predict the emergence of impaired memory during aging. Male and female Fisher 344 rats were tested at 6, 12, and 18 months of age. Impaired spatial memory developed in middle-age (12 months), particularly in males, and the propensity for impairment increased with advanced age. A reduced response to novelty was observed over the course of aging, which is inconsistent with cross-sectional studies. This divergence likely resulted from differences in the history of environmental enrichment/impoverishment for cross-sectional and longitudinal studies. Animals that exhibited lower level exploration of the inner region on the open field test exhibited better memory at 12 months. Furthermore, males that exhibited a longer latency to enter a novel environment at 6 months, exhibited better memory at 12 months. For females, memory at 12 months was correlated with the ability to behaviorally adapt to a shift in light/dark cycle. Functional magnetic resonance imaging of the brain, conducted at 12 months, indicated that the decline in memory was associated with altered functional connectivity within different memory systems, most notably between the hippocampus and multiple regions such as the retrosplenial cortex, thalamus, striatum, and amygdala. Overall, some factors, specifically response to novelty at an early age and the capacity to adapt to shifts in light cycle, predicted spatial memory in middle-age, and spatial memory is associated with corresponding changes in brain connectivity. We discuss similarities and differences related to previous longitudinal and cross-sectional studies, as well as the role of sex differences in providing a theoretical framework to guide future longitudinal research on the trajectory of cognitive decline. In addition to demonstrating the power of longitudinal studies, these data highlight the importance of middle-age for identifying potential predictive indicators of sexual dimorphism in the trajectory in brain and cognitive aging.

Resetting the Aging Clock: Implications for Managing Age-Related Diseases

Worldwide, individuals are living longer due to medical and scientific advances, increased availability of medical care and changes in public health policies. Consequently, increasing attention has been focused on managing chronic conditions and age-related diseases to ensure healthy aging. The endogenous circadian system regulates molecular, physiological and behavioral rhythms orchestrating functional coordination and processes across tissues and organs. Circadian disruption or desynchronization of circadian oscillators increases disease risk and appears to accelerate aging. Reciprocally, aging weakens circadian function aggravating age-related diseases and pathologies. In this review, we summarize the molecular composition and structural organization of the circadian system in mammals and humans, and evaluate the technological and societal factors contributing to the increasing incidence of circadian disorders. Furthermore, we discuss the adverse effects of circadian dysfunction on aging and longevity and the bidirectional interactions through which aging affects circadian function using examples from mammalian research models and humans. Additionally, we review promising methods for managing healthy aging through behavioral and pharmacological reinforcement of the circadian system. Understanding age-related changes in the circadian clock and minimizing circadian dysfunction may be crucial components to promote healthy aging.

Aging and the clock: Perspective from flies to humans

Endogenous circadian oscillators regulate molecular, cellular and physiological rhythms, synchronizing tissues and organ function to coordinate activity and metabolism with environmental cycles. The technological nature of modern society with round-the-clock work schedules and heavy reliance on personal electronics has precipitated a striking increase in the incidence of circadian and sleep disorders. Circadian dysfunction contributes to an increased risk for many diseases and appears to have adverse effects on aging and longevity in animal models. From invertebrate organisms to humans, the function and synchronization of the circadian system weakens with age aggravating the age-related disorders and pathologies. In this review, we highlight the impacts of circadian dysfunction on aging and longevity and the reciprocal effects of aging on circadian function with examples from Drosophila to humans underscoring the highly conserved nature of these interactions. Additionally, we review the potential for using reinforcement of the circadian system to promote healthy aging and mitigate age-related pathologies. Advancements in medicine and public health have significantly increased human life span in the past century. With the demographics of countries worldwide shifting to an older population, there is a critical need to understand the factors that shape healthy aging. Drosophila melanogaster, as a model for aging and circadian interactions, has the capacity to facilitate the rapid advancement of research in this area and provide mechanistic insights for targeted investigations in mammals.

The Biological Clock in Gray Mouse Lemur: Adaptive, Evolutionary and Aging Considerations in an Emerging Non-human Primate Model

Circadian rhythms, which measure time on a scale of 24 h, are genetically generated by the circadian clock, which plays a crucial role in the regulation of almost every physiological and metabolic process in most organisms. This review gathers all the available information about the circadian clock in a small Malagasy primate, the gray mouse lemur (Microcebus murinus), and reports 30 years data from the historical colony at Brunoy (France). Although the mouse lemur has long been seen as a "primitive" species, its clock displays high phenotypic plasticity, allowing perfect adaptation of its biological rhythms to environmental challenges (seasonality, food availability). The alterations of the circadian timing system in M. murinus during aging show many similarities with those in human aging. Comparisons are drawn with other mammalian species (more specifically, with rodents, other non-human primates and humans) to demonstrate that the gray mouse lemur is a good complementary and alternative model for studying the circadian clock and, more broadly, brain aging and pathologies.

Phase Shifts to a Moderate Intensity Light Exposure in Older Adults: A Preliminary Report

Age-related disrupted sleep has been associated with modifications in the timing of endogenous circadian rhythms. There are studies suggesting a decreased sensitivity of the aging circadian pacemaker to light. In this study, we aimed to test whether a moderate illuminance light stimulus would produce significant phase shifts in older adults, and whether those would fall in a range consistent with the illuminance response curve (IRC) we previously reported in older adults. We conducted an 8-day in-patient study with 12 healthy older participants (mean age [SD], 58.3 [4.2] y, 5 females). A 6.5-h polychromatic white light exposure with a target illuminance of 270 lux was administered beginning in the early biological night. Before and after the light exposure, a circadian phase estimation procedure was carried out, with plasma melatonin data used as a circadian phase marker. The mean phase delay produced by the light stimulus was 1.78 ± 0.77 h. Ten of the 12 phase delay shifts at ~270 lux fell within the 95% predictive interval of the model fit to data from our previous IRC in older adults. This finding demonstrates that the circadian system of healthy older adults is sensitive to the phase-delaying effects of a moderate intensity light stimulus. The magnitude of the observed phase shifts is consistent with the previously described IRC in older adults.

Gene expression profiling of the SCN in young and old rhesus macaques

In mammals, the suprachiasmatic nucleus (SCN) is the location of a master circadian pacemaker. It receives photic signals from the environment via the retinal hypothalamic tract, which play a key role in synchronizing the body's endogenously generated circadian rhythms with the 24-h rhythm of the environment. Therefore, it is plausible that age-related changes within the SCN contribute to the etiology of perturbed activity-rest cycles that become prevalent in humans during aging. To test this hypothesis, we used gene arrays and quantitative RT-PCR to profile age-related gene expression changes within the SCN of male rhesus macaques - a pragmatic translational animal model of human aging, which similarly displays an age-related attenuation of daytime activity levels. As expected, the SCN showed high expression of arginine vasopressin, vasoactive intestinal polypeptide, calbindin and nuclear receptor subfamily 1, group D, member 1 (NR1D1) (also known as reverse strand of ERBA (REV-ERBα), both at the mRNA and protein level. However, no obvious difference was detected between the SCNs of young (7-12 years) and old animals (21-26 years), in terms of the expression of core clock genes or genes associated with SCN signaling and neurotransmission. These data demonstrate the resilience of the primate SCN to normal aging, at least at the transcriptional level and, at least in males, suggest that age-related disruption of activity-rest cycles in humans may instead stem from changes within other components of the circadian system, such as desynchronization of subordinate oscillators in other parts of the body.

Voluntary exercise stabilizes photic entrainment of djungarian hamsters (Phodopus sungorus) with a delayed activity onset

The Djungarian hamsters of our breeding colony show unstable daily activity patterns when kept under standard laboratory conditions. Moreover, part of them develops a delayed activity onset (DAO) or an arrhythmic phenotype. In former studies, we have shown that the system of photic entrainment works at its limits. If the period length (tau) increases, which is the case in DAO hamsters, the light-induced phase advances are too small to compensate the daily delay of the activity rhythm caused by tau being longer than 24 h. Accordingly, under natural conditions, there must be further (environmental) factors to enable a stable entrainment. One of these may be the higher level of motor activity. Animals must cover long distances to search for food, sexual partners and others. In the laboratory, hamsters are kept singly in small cages. This does restrict animals' options for motor activity. Also, there is less need for moving around as the hamsters are fed ad libitum. In the present study, a series of experiments was performed to investigate the putative effect of the activity level. To begin with, wild type (WT) and DAO animals were given access to running wheels. 50% of DAO hamsters developed a WT activity pattern. As the main reason for the DAO phenomenon is their long tau together with a too weak photic phase response, the effect of wheel running on these parameters was investigated in further experiments. With higher activity level, tau decreased in WT hamsters but increased in DAO animals even though the increase for the activity onset was only close to significance. Moreover, the photic phase responses were weaker though significant only for the activity offset of DAO hamsters. Based on the assumptions that running wheel activity will affect the phase response and/or the free running period, the results of the present paper do not provide an explanation for why part of DAO hamsters developed a WT phenotype when they had access to running wheels. Obviously, mechanisms downstream from the suprachiasmatic nuclei must be taken into account when investigating the stabilizing, improving circadian entrainment effect of motor activity.

Sleep disorders in the elderly: a growing challenge

In contrast to newborns, who spend 16-20 h in sleep each day, adults need only about sleep daily. However, many elderly may struggle to obtain those 8 h in one block. In addition to changes in sleep duration, sleep patterns change as age progresses. Like the physical changes that occur during old age, an alteration in sleep pattern is also a part of the normal ageing process. As people age, they tend to have a harder time falling asleep and more trouble staying asleep. Older people spend more time in the lighter stages of sleep than in deep sleep. As the circadian mechanism in older people becomes less efficient, their sleep schedule is shifted forward. Even when they manage to obtain 7 or 8 h sleep, they wake up early, as they have gone to sleep quite early. The prevalence of sleep disorders is higher among older adults. Loud snoring, which is more common in the elderly, can be a symptom of obstructive sleep apnoea, which puts a person at risk for cardiovascular diseases, headaches, memory loss, and depression. Restless legs syndrome and periodic limb movement disorder that disrupt sleep are more prevalent in older persons. Other common medical problems of old age such as hypertension diabetes mellitus, renal failure, respiratory diseases such as asthma, immune disorders, gastroesophageal reflux disease, physical disability, dementia, pain, depression, and anxiety are all associated with sleep disturbances.

Insomnia in the Ageing Population: Characterisation and Non-Pharmacological Treatment Strategies

Sleep problems represent a worldwide health concern among older adults, with an increasing prevalence of multimorbid conditions and a decreased quality of life. However, most elderly patients are not correctly diagnosed due to numerous confounding variables (e.g., medical and psychiatric disorders, polypharmacy, and psychosocial factors) affecting sleep and the confusion regarding the differential diagnosis in older adults between normal changes in sleep pattern as a result of ageing and sleep disorders. There are normal changes to the sleep architecture throughout the lifespan, and sleep disorders are not part of the ageing process; however, there are several sleep disorders that affect older adults. The most prevalent sleep disorder is insomnia, which is found in different forms and affects approximately 30–50% of the older adult population. The treatment strategies for sleep disorders are multivariate, with prescriptions of pharmacological treatments being the most common method among healthcare professionals; however, there is strong evidence that non-pharmacological treatments have better long-term effects. The aim of this review is to explain the difference between sleep disorders and sleep alterations as a result of ageing, to characterise insomnia in older adults, and, finally, to present the different effective non-pharmacological possibilities, accompanied by evidence, for the treatment of insomnia in older adults.

Comparative approaches to understanding thyroid hormone regulation of neurogenesis

Thyroid hormone (TH) signalling, an evolutionary conserved pathway, is crucial for brain function and cognition throughout life, from early development to ageing. In humans, TH deficiency during pregnancy alters offspring brain development, increasing the risk of cognitive disorders. How TH regulates neurogenesis and subsequent behaviour and cognitive functions remains a major research challenge. Cellular and molecular mechanisms underlying TH signalling on proliferation, survival, determination, migration, differentiation and maturation have been studied in mammalian animal models for over a century. However, recent data show that THs also influence embryonic and adult neurogenesis throughout vertebrates (from mammals to teleosts). These latest observations raise the question of how TH availability is controlled during neurogenesis and particularly in specific neural stem cell populations. This review deals with the role of TH in regulating neurogenesis in the developing and the adult brain across different vertebrate species. Such evo-devo approaches can shed new light on (i) the evolution of the nervous system and (ii) the evolutionary control of neurogenesis by TH across animal phyla. We also discuss the role of thyroid disruptors on brain development in an evolutionary context.

Scheduled feeding restores memory and modulates c-Fos expression in the suprachiasmatic nucleus and septohippocampal complex

Disruptions in circadian timing impair spatial memory in humans and rodents. Circadian-arrhythmic Siberian hamsters (Phodopus sungorus) exhibit substantial deficits in spatial working memory as assessed by a spontaneous alternation (SA) task. The present study found that daily scheduled feeding rescued spatial memory deficits in these arrhythmic animals. Improvements in memory persisted for at least 3 weeks after the arrhythmic hamsters were switched back to ad libitum feeding. During ad libitum feeding, locomotor activity resumed its arrhythmic state, but performance on the SA task varied across the day with a peak in daily performance that corresponded to the previous daily window of food anticipation. At the end of scheduled feeding, c-Fos brain mapping revealed differential gene expression in entrained versus arrhythmic hamsters in the suprachiasmatic nucleus (SCN) that paralleled changes in the medial septum and hippocampus, but not in other neural structures. These data show that scheduled feeding can improve cognitive performance when SCN timing has been compromised, possibly by coordinating activity in the SCN and septohippocampal pathway.

Physiological links of circadian clock and biological clock of aging

Circadian rhythms orchestrate biochemical and physiological processes in living organisms to respond the day/night cycle. In mammals, nearly all cells hold self-sustained circadian clocks meanwhile couple the intrinsic rhythms to systemic changes in a hierarchical manner. The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master pacemaker to initiate daily synchronization according to the photoperiod, in turn determines the phase of peripheral cellular clocks through a variety of signaling relays, including endocrine rhythms and metabolic cycles. With aging, circadian desynchrony occurs at the expense of peripheral metabolic pathologies and central neurodegenerative disorders with sleep symptoms, and genetic ablation of circadian genes in model organisms resembled the aging-related features. Notably, a number of studies have linked longevity nutrient sensing pathways in modulating circadian clocks. Therapeutic strategies that bridge the nutrient sensing pathways and circadian clock might be rational designs to defy aging.

Progressive gene dose-dependent disruption of the methamphetamine-sensitive circadian oscillator-driven rhythms in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is a progressive genetic neurodegenerative disorder characterised by motor and cognitive deficits, as well as sleep and circadian abnormalities. In the R6/2 mouse, a fragment model of HD, rest-activity rhythms controlled by the suprachiasmatic nucleus disintegrate completely by 4months of age. Rhythms driven by a second circadian oscillator, the methamphetamine-sensitive circadian oscillator (MASCO), are disrupted even earlier, and cannot be induced after 2months of age. Here, we studied the effect of the HD mutation on the expression of MASCO-driven rhythms in a more slowly developing, genetically relevant mouse model of HD, the Q175 'knock-in' mouse. We induced expression of MASCO output by administering low dose methamphetamine (0.005%) chronically via the drinking water. We measured locomotor activity in constant darkness in wild-type and Q175 mice at 2 (presymptomatic), 6 (early symptomatic), and 12 (symptomatic) months of age. At 2months, all mice expressed MASCO-driven rhythms, regardless of genotype. At older ages, however, there was a progressive gene dose-dependent deficit in MASCO output in Q175 mice. At 6months of age, these rhythms could be observed in only 45% of heterozygous and 15% of homozygous mice. By 1year of age, 90% of homozygous mice had an impaired MASCO output. There was also an age-dependent disruption of MASCO output seen in wild-type mice. The fact that the progressive deficit in MASCO-driven rhythms in Q175 mice is HD gene dose-dependent suggests that, whatever its role in humans, abnormalities in MASCO output may contribute to the HD circadian phenotype.

Insomnia and risk of dementia in older adults: Systematic review and meta-analysis

There are cross-sectional evidences of an association between sleep disorders and cognitive impairment on older adults. However, there are no consensus by means of longitudinal studies data on the increased risk of developing dementia related to insomnia. We conduct a systematic review and meta-analysis to evaluate the risk of incident all-cause dementia in individuals with insomnia in population-based prospective cohort studies. Five studies of 5.242 retrieved references were included in the meta-analysis. We used the generic inverse variance method with a random effects model to calculate the pooled risk of dementia in older adults with insomnia. We assessed heterogeneity in the meta-analysis by means of the Q-test and I2 index. Study quality was assessed with the Newcastle-Ottawa Scale The results showed that Insomnia was associated with a significant risk of all-cause dementia (RR = 1.53 CI95% (1.07-2.18), z = 2.36, p = 0.02). There was evidence for significant heterogeneity in the analysis (q-value = 2.4, p < 0.001 I2 = 82%). Insomnia is associated with an increased risk for dementia. This results provide evidences that future studies should investigate dementia prevention among elderly individuals through screening and proper management of insomnia.

Circadian Characteristics of Older Adults and Aerobic Capacity

BACKGROUND

Alteration of circadian rhythmicity with aging might depend on physical aerobic capacity.

METHODS

Three groups of participants were established based on their peak oxygen consumption (Group 1 < 20mL/min/kg; Group 2 > 20mL/min/kg and <30mL/min/kg; Group 3 > 30mL/min/kg). Each participant had an individual evaluation of their circadian rhythmicity characteristics through two well-known circadian rhythms: core temperature and rest/activity cycles. Nocturnal sleep was also recorded using actimetry and diurnal vigilance tested in a car driving simulator.

RESULTS

The amplitude of the oral temperature fluctuations for Group 1 is significantly lower (p < .05) than that of Group 3. Group 2 (p < .01) and Group 3 (p < .05) were significantly more active during the day than Group 1. The index of inactivity during the night for Groups 2 (p < .05) and 3 (p < .01) was higher than Group 1. Results of the car driving simulation showed that for Group 1, the number of lane crossings was significantly higher than Groups 2 (p < .01) and 3 (p < .01). In addition, diurnal vigilance was lower in Group 1.

CONCLUSIONS

The biological clock seems to be enhanced in older participants with a higher level of physical capacity.

The Circadian Clock in the Regulation of Renal Rhythms

Since the kidney is integral to maintenance of fluid and ion homeostasis, and therefore blood pressure regulation, its proper function is paramount. Circadian fluctuations in blood pressure, renal blood flow, glomerular filtration rate, and sodium and water excretion have been documented for decades, if not longer. Recent studies on the role of circadian clock proteins in the regulation of a variety of renal transport genes suggest that the molecular clock in the kidney controls circadian fluctuations in renal function. The circadian clock appears to be a critical regulator of renal function with important implications for the treatment of renal pathologies, which include chronic kidney disease and hypertension. The development, regulation, and mechanism of the kidney clock are reviewed here.

Integrative neurobiology of metabolic diseases, neuroinflammation, and neurodegeneration

Alzheimer's disease (AD) is a complex, multifactorial disease with a number of leading mechanisms, including neuroinflammation, processing of amyloid precursor protein (APP) to amyloid β peptide, tau protein hyperphosphorylation, relocalization, and deposition. These mechanisms are propagated by obesity, the metabolic syndrome and type-2 diabetes mellitus. Stress, sedentariness, dietary overconsumption of saturated fat and refined sugars, and circadian derangements/disturbed sleep contribute to obesity and related metabolic diseases, but also accelerate age-related damage and senescence that all feed the risk of developing AD too. The complex and interacting mechanisms are not yet completely understood and will require further analysis. Instead of investigating AD as a mono- or oligocausal disease we should address the disease by understanding the multiple underlying mechanisms and how these interact. Future research therefore might concentrate on integrating these by “systems biology” approaches, but also to regard them from an evolutionary medicine point of view. The current review addresses several of these interacting mechanisms in animal models and compares them with clinical data giving an overview about our current knowledge and puts them into an integrated framework.

Time-place learning over a lifetime: absence of memory loss in trained old mice

Time–place learning (TPL) offers the possibility to study the functional interaction between cognition and the circadian system with aging. With TPL, animals link biological significant events with the location and the time of day. This what–where–when type of memory provides animals with an experience-based daily schedule. Mice were tested for TPL five times throughout their lifespan and showed (re)learning from below chance level at the age of 4, 7, 12, and 18 mo. In contrast, at the age of 22 mo these mice showed preservation of TPL memory (absence of memory loss), together with deficiencies in the ability to update time-of-day information. Conversely, the majority of untrained (naïve) mice at 17 mo of age were unable to acquire TPL, indicating that training had delayed TPL deficiencies in the mice trained over lifespan. Two out of seven naïve mice, however, compensated for correct performance loss by adapting an alternative learning strategy that is independent of the age-deteriorating circadian system and presumably less cognitively demanding. Together, these data show the age-sensitivity of TPL, and the positive effects of repeated training over a lifetime. In addition, these data shed new light on aging-related loss of behavioral flexibility to update time-of-day information.

Relationship between perceived sleep and polysomnography in older adult patients

BACKGROUND AND AIMS

Aging is a multifactorial process that elicits changes in the duration and quality of sleep. Polysomnography is considered to be the standard examination for the analysis of sleep and consists of the simultaneous recording of selected physiological variables during sleep.

OBJECTIVE

The objective of this study was to use polysomnography to compare sleep reported by senior citizens.

METHODS

We selected 40 patients, both male and female, with ages ranging from 64 to 89 years from the Center for the Study of Aging at the Federal University of São Paulo. Patients answered questions about sleep on the Comprehensive Geriatric Assessment and underwent polysomnography.

RESULTS

The results were compared, and agreement between perceived sleep and polysomnography was found in several areas. There was an association between difficulty sleeping and sleep onset latency (p=0.015), waking up at night with sleep onset latency (p=0.005), total sleep time with daytime sleepiness (0.005) and snoring (0.027), sleep efficiency with sleepiness (0.004), snoring (0.033) and pause in breathing (p=0.024), awakenings with snoring (p=0.012) and sleep apnea with pauses in breathing (p=0.001).

CONCLUSION

These results suggest that the older adult population have a good perception of their sleep. The questionnaires aimed at this population should be used as an alternative to polysomnography.

Behavior in the elevated plus maze is differentially affected by testing conditions in rats under and over three weeks of age

The late postnatal period in rats is marked by numerous changes in perceptual and cognitive abilities. As such, age-related variation in cognitive test performance might result in part from disparate sensitivities to environmental factors. To better understand how testing conditions might interact with age, we assessed anxiety behavior on an elevated plus maze (EPM) in juvenile rats around 3 weeks of age under diverse testing conditions. Plasma corticosterone and neuronal activation patterns in the forebrain were examined after maze exposure. We found that anxiety was differentially expressed during different stages of late postnatal development. Bright illumination and morning testing encouraged greatest open arm exploration on the EPM in younger animals, while older rats explored open areas more under dim illumination in the morning compared to bright illumination in the afternoon/evening. Older rats exhibited higher plasma corticosterone levels at baseline compared to younger rats; however, this trend was reversed for post-testing corticosterone. Additionally, post-testing corticosterone levels were inversely related to time of testing. Compared to testing in the morning, EPM exposure in the afternoon/evening elicited greater neuronal Arc expression in the amygdala. Arc expression in the amygdala after morning testing was greater at P22–24 than P17–19. In layer 2/3 of primary visual cortex, Arc expression was elevated in younger animals and age interacted with time of testing to produce opposing effects at P17–19 and P22–24. These data suggest that age-related differences in anxiety-associated behavior during the late postnatal period are due in part to changes in light sensitivity and emergence of a circadian cycle for corticosterone. The findings illustrate that late postnatal behavioral development in rodents is a complex orchestration of changes in neural systems involved in perception, cognition, affect and homeostatic regulation.

Buying time: a rationale for examining the use of circadian rhythm and sleep interventions to delay progression of mild cognitive impairment to Alzheimer's disease

As of 2010, the worldwide economic impact of dementia was estimated at $604 billion USD; and without discovery of a cure or effective interventions to delay disease progression, dementia's annual global economic impact is expected to surpass $1 trillion USD as early as 2030. Alzheimer's disease (AD) is the leading cause of dementia accounting for over 75% of all cases. Toxic accumulation of amyloid beta (Aβ), either by overproduction or some clearance failure, is thought to be an underlying mechanism of the neuronal cell death characteristic of AD-though this amyloid hypothesis has been increasingly challenged in recent years. A compelling alternative hypothesis points to chronic neuroinflammation as a common root in late-life degenerative diseases including AD. Apolipoprotein-E (APOE) genotype is the strongest genetic risk factor for AD: APOE-ε4 is proinflammatory and individuals with this genotype accumulate more Aβ, are at high risk of developing AD, and almost half of all AD patients have at least one ε4 allele. Recent studies suggest a bidirectional relationship exists between sleep and AD pathology. Sleep may play an important role in Aβ clearance, and getting good quality sleep vs. poor quality sleep might reduce the AD risk associated with neuroinflammation and the ε4 allele. Taken together, these findings are particularly important given the sleep disruptions commonly associated with AD and the increased burden disrupted sleep poses for AD caregivers. The current review aims to: (1) identify individuals at high risk for dementia who may benefit most from sleep interventions; (2) explore the role poor sleep quality plays in exacerbating AD type dementia; (3) examine the science of sleep interventions to date; and (4) provide a road map in pursuit of comprehensive sleep interventions, specifically targeted to promote cognitive function and delay progression of dementia.

Melatonin regulates aging and neurodegeneration through energy metabolism, epigenetics, autophagy and circadian rhythm pathways

Brain aging is linked to certain types of neurodegenerative diseases and identifying new therapeutic targets has become critical. Melatonin, a pineal hormone, associates with molecules and signaling pathways that sense and influence energy metabolism, autophagy, and circadian rhythms, including insulin-like growth factor 1 (IGF-1), Forkhead box O (FoxOs), sirtuins and mammalian target of rapamycin (mTOR) signaling pathways. This review summarizes the current understanding of how melatonin, together with molecular, cellular and systemic energy metabolisms, regulates epigenetic processes in the neurons. This information will lead to a greater understanding of molecular epigenetic aging of the brain and anti-aging mechanisms to increase lifespan under healthy conditions.

Synchronizing an aging brain: can entraining circadian clocks by food slow Alzheimer's disease?

Alzheimer's disease (AD) is a global epidemic. Unfortunately, we are still without effective treatments or a cure for this disease, which is having devastating consequences for patients, their families, and societies around the world. Until effective treatments are developed, promoting overall health may hold potential for delaying the onset or preventing neurodegenerative diseases such as AD. In particular, chronobiological concepts may provide a useful framework for identifying the earliest signs of age-related disease as well as inexpensive and noninvasive methods for promoting health. It is well reported that AD is associated with disrupted circadian functioning to a greater extent than normal aging. However, it is unclear if the central circadian clock (i.e., the suprachiasmatic nucleus) is dysfunctioning, or whether the synchrony between the central and peripheral clocks that control behavior and metabolic processes are becoming uncoupled. Desynchrony of rhythms can negatively affect health, increasing morbidity and mortality in both animal models and humans. If the uncoupling of rhythms is contributing to AD progression or exacerbating symptoms, then it may be possible to draw from the food-entrainment literature to identify mechanisms for re-synchronizing rhythms to improve overall health and reduce the severity of symptoms. The following review will briefly summarize the circadian system, its potential role in AD, and propose using a feeding-related neuropeptide, such as ghrelin, to synchronize uncoupled rhythms. Synchronizing rhythms may be an inexpensive way to promote healthy aging and delay the onset of neurodegenerative disease such as AD.

Nothobranchius as a model for aging studies. A review

In recent decades, the increase in human longevity has made it increasingly important to expand our knowledge on aging. To accomplish this, the use of animal models is essential, with the most common being mouse (phylogenetically similar to humans, and a model with a long life expectancy) and Caenorhabditis elegans (an invertebrate with a short life span, but quite removed from us in evolutionary terms). However, some sort of model is needed to bridge the differences between those mentioned above, achieving a balance between phylogenetic distance and life span. Fish of the genus Nothobranchius were suggested 10 years ago as a possible alternative for the study of the aging process. In the meantime, numerous studies have been conducted at different levels: behavioral (including the study of the rest-activity rhythm), populational, histochemical, biochemical and genetic, among others, with very positive results. This review compiles what we know about Nothobranchius to date, and examines its future prospects as a true alternative to the classic models for studies on aging.

Differential modulation of clock gene expression in the suprachiasmatic nucleus, liver and heart of aged mice

Studies on the molecular clockwork during aging have been hitherto addressed to core clock genes. These previous investigations indicate that circadian profiles of core clock gene expression at an advanced age are relatively preserved in the master circadian pacemaker and the hypothalamic suprachiasmatic nucleus (SCN), and relatively impaired in peripheral tissues. It remains to be clarified whether the effects of aging are confined to the primary loop of core clock genes, or also involve secondary clock loop components, including Rev-erbα and the clock-controlled genes Dbp and Dec1. Using quantitative real-time RT-PCR, we here report a comparative analysis of the circadian expression of canonical core clock genes (Per1, Per2, Cry1, Cry2, Clock and Bmal1) and non-core clock genes (Rev-erbα, Dbp and Dec1) in the SCN, liver, and heart of 3month-old vs 22month-old mice. The results indicate that circadian clock gene expression is significantly modified in the SCN and peripheral oscillators of aged mice. These changes are not only highly tissue-specific, but also involve different clock gene loops. In particular, we here report changes of secondary clock loop components in the SCN, changes of the primary clock loop in the liver, and minor changes of clock gene expression in the heart of aged mice. The present findings outline a track to further understanding of the role of primary and secondary clock loop components and their crosstalk in the impairment of circadian output which characterizes aging.

The trouble with circadian clock dysfunction: multiple deleterious effects on the brain and body

This review consolidates research employing human correlational and experimental work across brain and body with experimental animal models to provide a more complete representation of how circadian rhythms influence almost all aspects of life. In doing so, we will cover the morphological and biochemical pathways responsible for rhythm generation as well as interactions between these systems and others (e.g., stress, feeding, reproduction). The effects of circadian disruption on the health of humans, including time of day effects, cognitive sequelae, dementia, Alzheimer's disease, diet, obesity, food preferences, mood disorders, and cancer will also be discussed. Subsequently, experimental support for these largely correlational human studies conducted in non-human animal models will be described.

Circadian aspects of energy metabolism and aging

Life span extension has been a goal of research for several decades. Resetting circadian rhythms leads to well being and increased life span, while clock disruption is associated with increased morbidity accelerated aging. Increased longevity and improved health can be achieved by different feeding regimens that reset circadian rhythms and may lead to better synchrony in metabolism and physiology. This review focuses on the circadian aspects of energy metabolism and their relationship with aging in mammals.

Thyroid hormone signaling and homeostasis during aging

Studies in humans and in animal models show negative correlations between thyroid hormone (TH) levels and longevity. TH signaling is implicated in maintaining and integrating metabolic homeostasis at multiple levels, notably centrally in the hypothalamus but also in peripheral tissues. The question is thus raised of how TH signaling is modulated during aging in different tissues. Classically, TH actions on mitochondria and heat production are obvious candidates to link negative effects of TH to aging. Mitochondrial effects of excess TH include reactive oxygen species and DNA damage, 2 factors often considered as aging accelerators. Inversely, caloric restriction, which can retard aging from nematodes to primates, causes a rapid reduction of circulating TH, reducing metabolism in birds and mammals. However, many other factors could link TH to aging, and it is these potentially subtler and less explored areas that are highlighted here. For example, effects of TH on membrane composition, inflammatory responses, stem cell renewal and synchronization of physiological responses to light could each contribute to TH regulation of maintenance of homeostasis during aging. We propose the hypothesis that constraints on TH signaling at certain life stages, notably during maturity, are advantageous for optimal aging.