Crosstalk between circadian rhythmicity, mitochondrial dynamics and macrophage bactericidal activity

Dim light at night shifts microglia to a pro-inflammatory state after cerebral ischemia, altering stroke outcome in mice

Circadian rhythms are endogenous biological cycles that regulate physiology and behavior and are set to precisely 24-h by light exposure. Light at night (LAN) dysregulates physiology and function including immune response; a critical component that contributes to stroke pathophysiological progression of neuronal injury and may impair recovery from injury. The goal of this study is to explore the effects of dim LAN (dLAN) in a murine model of ischemic stroke to assess how nighttime lighting from hospital settings can affect stroke outcome. Further, this study sought to identify mechanisms underlying pathophysiological changes to immune response after circadian disruption. Male and female adult Swiss Webster (CFW) mice were subjected to transient or permanent focal cerebral ischemia, then were subsequently placed into either dark night conditions (LD) or one night of dLAN (5 lx). 24 h post-stroke, sensorimotor impairments and infarct sizes were quantified. A single night of dLAN following MCAO increased infarct size and sensorimotor deficits across both sexes and reduced survival in males after 24 h. Flow cytometry was performed to assess microglial phenotypes after MCAO, and revealed that dLAN altered the percentage of microglia that express pro-inflammatory markers (MHC II+ and IL-6) and microglia that express CD206 and IL-10 that likely contributed to poor ischemic outcomes. Following these results, microglia were reduced in the brain using Plexxikon 5622 (PLX 5622) a CSFR1 inhibitor, then the mice received an MCAO and were exposed to LD or dLAN conditions for 24 h. Microglial depletion by PLX5622 resulted in infarct sizes that were comparable between lighting conditions. This study provides supporting evidence that environmental lighting exacerbates ischemic injury and post-stroke mortality by a biological mechanism that exposure to dLAN causes a fundamental shift of activated microglial phenotypes from beneficial to detrimental at an early time point after stroke, resulting in irreversible neuronal death.

Circadian rhythms in solid organ transplantation

Circadian rhythms are daily cycles in physiology that can affect medical interventions. This review considers how these rhythms may relate to solid organ transplantation. It begins by summarizing the mechanism for circadian rhythm generation known as the molecular clock, and basic research connecting the clock to biological activities germane to organ acceptance. Next follows a review of clinical evidence relating time of day to adverse transplantation outcomes. The concluding section discusses knowledge gaps and practical areas where applying circadian biology might improve transplantation success.

Circadian rhythm regulation in the immune system

Circadian rhythms are a ubiquitous feature in nearly all living organisms, representing oscillatory patterns with a 24-h cycle that are widespread across various physiological processes. Circadian rhythms regulate a multitude of physiological systems, including the immune system. At the molecular level, most immune cells autonomously express clock-regulating genes, which play critical roles in regulating immune cell functions. These functions encompass migration, phagocytic activity, immune cell metabolism (such as mitochondrial structural function and metabolism), signalling pathway activation, inflammatory responses, innate immune recognition, and adaptive immune processes (including vaccine responses and pathogen clearance). The endogenous circadian clock orchestrates multifaceted rhythmicity within the immune system, optimizing immune surveillance and responsiveness; this bears significant implications for maintaining immune homeostasis and resilience against diseases. This work provides an overview of circadian rhythm regulation within the immune system.

Circadian immunity from bench to bedside: a practical guide

The immune system is built to counteract unpredictable threats, yet it relies on predictable cycles of activity to function properly. Daily rhythms in immune function are an expanding area of study, and many originate from a genetically based timekeeping mechanism known as the circadian clock. The challenge is how to harness these biological rhythms to improve medical interventions. Here, we review recent literature documenting how circadian clocks organize fundamental innate and adaptive immune activities, the immunologic consequences of circadian rhythm and sleep disruption, and persisting knowledge gaps in the field. We then consider the evidence linking circadian rhythms to vaccination, an important clinical realization of immune function. Finally, we discuss practical steps to translate circadian immunity to the patient's bedside.

Circadian regulation of liver function: from molecular mechanisms to disease pathophysiology

A wide variety of liver functions are regulated daily by the liver circadian clock and via systemic circadian control by other organs and cells within the gastrointestinal tract as well as the microbiome and immune cells. Disruption of the circadian system, as occurs during jetlag, shift work or an unhealthy lifestyle, is implicated in several liver-related pathologies, ranging from metabolic diseases such as obesity, type 2 diabetes mellitus and nonalcoholic fatty liver disease to liver malignancies such as hepatocellular carcinoma. In this Review, we cover the molecular, cellular and organismal aspects of various liver pathologies from a circadian viewpoint, and in particular how circadian dysregulation has a role in the development and progression of these diseases. Finally, we discuss therapeutic and lifestyle interventions that carry health benefits through support of a functional circadian clock that acts in synchrony with the environment.

The role of circadian clock-controlled mitochondrial dynamics in diabetic cardiomyopathy

Diabetes mellitus is a metabolic disease with a high prevalence worldwide, and cardiovascular complications are the leading cause of mortality in patients with diabetes. Diabetic cardiomyopathy (DCM), which is prone to heart failure with preserved ejection fraction, is defined as a cardiac dysfunction without conventional cardiac risk factors such as coronary heart disease and hypertension. Mitochondria are the centers of energy metabolism that are very important for maintaining the function of the heart. They are highly dynamic in response to environmental changes through mitochondrial dynamics. The disruption of mitochondrial dynamics is closely related to the occurrence and development of DCM. Mitochondrial dynamics are controlled by circadian clock and show oscillation rhythm. This rhythm enables mitochondria to respond to changing energy demands in different environments, but it is disordered in diabetes. In this review, we summarize the significant role of circadian clock-controlled mitochondrial dynamics in the etiology of DCM and hope to play a certain enlightening role in the treatment of DCM.

Twelve-hour rhythms in transcript expression within the human dorsolateral prefrontal cortex are altered in schizophrenia

Twelve-hour (12 h) ultradian rhythms are a well-known phenomenon in coastal marine organisms. While 12 h cycles are observed in human behavior and physiology, no study has measured 12 h rhythms in the human brain. Here, we identify 12 h rhythms in transcripts that either peak at sleep/wake transitions (approximately 9 AM/PM) or static times (approximately 3 PM/AM) in the dorsolateral prefrontal cortex, a region involved in cognition. Subjects with schizophrenia (SZ) lose 12 h rhythms in genes associated with the unfolded protein response and neuronal structural maintenance. Moreover, genes involved in mitochondrial function and protein translation, which normally peak at sleep/wake transitions, peak instead at static times in SZ, suggesting suboptimal timing of these essential processes.

The circadian clock influences T cell responses to vaccination by regulating dendritic cell antigen processing

Dendritic cells play a key role in processing and presenting antigens to naïve T cells to prime adaptive immunity. Circadian rhythms are known to regulate many aspects of immunity; however, the role of circadian rhythms in dendritic cell function is still unclear. Here, we show greater T cell responses when mice are immunised in the middle of their rest versus their active phase. We find a circadian rhythm in antigen processing that correlates with rhythms in both mitochondrial morphology and metabolism, dependent on the molecular clock gene, Bmal1. Using Mdivi-1, a compound that promotes mitochondrial fusion, we are able to rescue the circadian deficit in antigen processing and mechanistically link mitochondrial morphology and antigen processing. Furthermore, we find that circadian changes in mitochondrial Ca²⁺ are central to the circadian regulation of antigen processing. Our results indicate that rhythmic changes in mitochondrial calcium, which are associated with changes in mitochondrial morphology, regulate antigen processing.

When the clock ticks wrong with COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a member of the coronavirus family that causes the novel coronavirus disease first diagnosed in 2019 (COVID-19). Although many studies have been carried out in recent months to determine why the disease clinical presentations and outcomes can vary significantly from asymptomatic to severe or lethal, the underlying mechanisms are not fully understood. It is likely that unique individual characteristics can strongly influence the broad disease variability; thus, tailored diagnostic and therapeutic approaches are needed to improve clinical outcomes. The circadian clock is a critical regulatory mechanism orchestrating major physiological and pathological processes. It is generally accepted that more than half of the cell-specific genes in any given organ are under circadian control. Although it is known that a specific role of the circadian clock is to coordinate the immune system's steady-state function and response to infectious threats, the links between the circadian clock and SARS-CoV-2 infection are only now emerging. How inter-individual variability of the circadian profile and its dysregulation may play a role in the differences noted in the COVID-19-related disease presentations, and outcome remains largely underinvestigated. This review summarizes the current evidence on the potential links between circadian clock dysregulation and SARS-CoV-2 infection susceptibility, disease presentation and progression, and clinical outcomes. Further research in this area may contribute towards novel circadian-centred prognostic, diagnostic and therapeutic approaches for COVID-19 in the era of precision health.

Shifting patterns of cellular energy production (adenosine triphosphate) over the day and key timings for the effect of optical manipulation

Mitochondria are optically responsive organelles producing energy for cell function via adenosine triphosphate (ATP). But ATP production appears to vary over the day. Here we use Drosophila melanogaster to reveal daily shifts in whole animal ATP production in a tight 24 hours' time series. We show a marked production peak in the morning that declines around midday and remains low through afternoon and night. ATP production can be improved with long wavelengths (>660 nm), but apparently not at all times. Hence, we treated flies with 670 nm light to reveal optimum times. Exposures at 670 nm resulted in a significant ATP increases and a shift in the ATP/adenosine diphosphate (ADP) ratio at 8.00 and 11.00, whilst application at other time points had no effect. Hence, light-induced ATP increases appear limited to periods when natural production is high. In summary, long wavelength influences on mitochondria are conserved across species from fly to human. Determining times for their administration to improve function in ageing and disease are of key importance. This study progresses this problem.

Circadian Control of Redox Reactions in the Macrophage Inflammatory Response

Significance: Macrophages are immune sentinels located throughout the body that function in both amplification and resolution of the inflammatory response. The circadian clock has emerged as a central regulator of macrophage inflammation. Reduction-oxidation (redox) reactions are central to both the circadian clock and macrophage function. Recent Advances: Circadian regulation of metabolism controls the macrophage inflammatory response, whereby disruption of the clock causes dysfunctional inflammation. Altering metabolism and reactive oxygen/nitrogen species (RONS) production rescues the inflammatory phenotype of clock-disrupted macrophages. Critical Issues: The circadian clock possesses many layers of regulation. Understanding how redox reactions coordinate clock function is critical to uncover the full extent of circadian regulation of macrophage inflammation. We provide insights into how circadian regulation of redox affects macrophage pattern recognition receptor signaling, immunometabolism, phagocytosis, and inflammasome activation. Future Directions: Many diseases associated with aberrant macrophage-derived inflammation exhibit time-of-day rhythms in disease symptoms and severity and are sensitive to circadian disruption. Macrophage function is highly dependent on redox reactions that signal through RONS. Future studies are needed to evaluate the extent of circadian control of macrophage inflammation, specifically in the context of redox signaling. Antioxid. Redox Signal. 37, 664-678.

Diurnal Differences in Intracellular Replication Within Splenic Macrophages Correlates With the Outcome of Pneumococcal Infection

Circadian rhythms affect the progression and severity of bacterial infections including those caused by Streptococcus pneumoniae, but the mechanisms responsible for this phenomenon remain largely elusive. Following advances in our understanding of the role of replication of S. pneumoniae within splenic macrophages, we sought to investigate whether events within the spleen correlate with differential outcomes of invasive pneumococcal infection. Utilising murine invasive pneumococcal disease (IPD) models, here we report that infection during the murine active phase (zeitgeber time 15; 15h after start of light cycle, 3h after start of dark cycle) resulted in significantly faster onset of septicaemia compared to rest phase (zeitgeber time 3; 3h after start of light cycle) infection. This correlated with significantly higher pneumococcal burden within the spleen of active phase-infected mice at early time points compared to rest phase-infected mice. Whole-section confocal microscopy analysis of these spleens revealed that the number of pneumococci is significantly higher exclusively within marginal zone metallophilic macrophages (MMMs) known to allow intracellular pneumococcal replication as a prerequisite step to the onset of septicaemia. Pneumococcal clusters within MMMs were more abundant and increased in size over time in active phase-infected mice compared to those in rest phase-infected mice which decreased in size and were present in a lower percentage of MMMs. This phenomenon preceded significantly higher levels of bacteraemia alongside serum IL-6 and TNF-α concentrations in active phase-infected mice following re-seeding of pneumococci into the blood. These data greatly advance our fundamental knowledge of pneumococcal infection by linking susceptibility to invasive pneumococcal infection to variation in the propensity of MMMs to allow persistence and replication of phagocytosed bacteria. These findings also outline a somewhat rare scenario whereby the active phase of an organism’s circadian cycle plays a seemingly counterproductive role in the control of invasive infection.

Genetic and genomic analysis of oxygen consumption in mice

We estimated genetic parameters for oxygen consumption (OC), OC per metabolic body weight (OCMBW) and body weight at three through 8 weeks of age in divergently selected mice populations, with an animal model considering maternal genetic, common litter environmental and cytoplasmic inheritance effects. Cytoplasmic inheritance was considered based on maternal lineage information. With respect to OC, estimated direct heritability was moderate (0.32) and the estimated proportion of the variance of cytoplasmic inheritance effects to the phenotypic variance was very low (0.01), implying that causal genes for OC could be located on autosomes. To assess this hypothesis, we attempted to identify possible candidate causal genes through selective signature detection with the results of pooled whole-genome resequencing using pooled DNA samples from high and low OC mice. We made a list of possible candidate causal genes for OC, including those relating to electron transport chain and ATP-binding proteins (Ndufa12, Sdhc, Atp10b, etc.), Prr16 encoding Largen protein, Cry1 encoding a key component of the circadian core oscillator and so on. The results, although careful interpretation must be required, could contribute to elucidate the genetic mechanism of OC, an indicator for maintenance energy requirement, and therefore feed efficiency.

The circadian rhythm of viruses and its implications on susceptibility to infection

INTRODUCTION

Circadian genes have an impact on multiple hormonal, metabolic, and immunological pathways and have recently been implicated in some infectious diseases.

AREAS COVERED

We review aspects related to the current knowledge about circadian rhythm and viral infections, their consequences, and the potential therapeutic options.

EXPERT OPINION

Expert opinion: In order to address a problem, it is necessary to know the topic in depth. Although in recent years there has been a growing interest in the role of circadian rhythms, many relevant questions remain to be resolved. Thus, the mechanisms linking the circadian machinery against viral infections are poorly understood. In a clear approach to personalized precision medicine, in order to treat a disease in the most appropriate phase of the circadian rhythm, and in order to achieve the optimal efficacy, it is highly recommended to carry out studies that improve the knowledge about the circadian rhythm.

Circadian rhythms in immunity and host-parasite interactions

The mammalian immune system adheres to a 24 h circadian schedule, exhibiting daily rhythmic patterns in homeostatic immune processes, such as immune cell trafficking, as well as the inflammatory response to infection. These diurnal rhythms are driven by endogenous molecular clocks within immune cells which are hierarchically coordinated by a light-entrained central clock in the suprachiasmatic nucleus of the hypothalamus and responsive to local rhythmic cues including temperature, hormones and feeding time. Circadian control of immunity may enable animals to anticipate daily pathogenic threat from parasites and gate the magnitude of the immune response, potentially enhancing fitness. However, parasites also strive for optimum fitness and some may have co-evolved to benefit from host circadian timing mechanisms, possibly via the parasites' own intrinsic molecular clocks. In this review, we summarize the current knowledge surrounding the influence of the circadian clock on the mammalian immune system and the host-parasitic interaction. We also discuss the potential for chronotherapeutic strategies in the treatment of parasitic diseases.

Macrophage Meets the Circadian Clock: Implication of the Circadian Clock in the Role of Macrophages in Acute Lower Respiratory Tract Infection

The circadian rhythm is a biological system that creates daily variations of physiology and behavior with a 24-h cycle, which is precisely controlled by the molecular circadian clock. The circadian clock dominates temporal activity of physiological homeostasis at the molecular level, including endocrine secretion, metabolic, immune response, coupled with extrinsic environmental cues (e.g., light/dark cycles) and behavioral cues (e.g., sleep/wake cycles and feeding/fasting cycles). The other side of the clock is that the misaligned circadian rhythm contributes to the onset of a variety of diseases, such as cancer, metabolic diseases, and cardiovascular diseases, the acceleration of aging, and the development of systemic inflammation. The role played by macrophages is a key mediator between circadian disruption and systemic inflammation. At the molecular level, macrophage functions are under the direct control of the circadian clock, and thus the circadian misalignment remodels the phenotype of macrophages toward a ‘killer’ mode. Remarkably, the inflammatory macrophages induce systemic and chronic inflammation, leading to the development of inflammatory diseases and the dampened immune defensive machinery against infectious diseases such as COVID-19. Here, we discuss how the circadian clock regulates macrophage immune functions and provide the potential risk of misaligned circadian rhythms against inflammatory and infectious diseases.

Roles of clock genes in the pathogenesis of Parkinson's disease

Parkinson's disease (PD) is a common motor disorder that has become increasingly prevalent in the ageing population. Recent works have suggested that circadian rhythms disruption is a common event in PD patients. Clock genes regulate the circadian rhythm of biological processes in eukaryotic organisms, but their roles in PD remain unclear. Despite this, several lines of evidence point to the possibility that clock genes may have a significant impact on the development and progression of the disease. This review aims to consolidate recent understanding of the roles of clock genes in PD. We first summarized the findings of clock gene expression and epigenetic analyses in PD patients and animal models. We also discussed the potential contributory role of clock gene variants in the development of PD and/or its symptoms. We further reviewed the mechanisms by which clock genes affect mitochondrial dynamics as well as the rhythmic synthesis and secretion of endocrine hormones, the impairment of which may contribute to the development of PD. Finally, we discussed the limitations of the currently available studies, and suggested future potential studies to deepen our understanding of the roles of clock genes in PD pathogenesis.

Aging disrupts circadian gene regulation and function in macrophages

Aging is characterized by an increased vulnerability to infection and the development of inflammatory diseases, such as atherosclerosis, frailty, cancer and neurodegeneration. Here, we find that aging is associated with the loss of diurnally rhythmic innate immune responses, including monocyte trafficking from bone marrow to blood, response to lipopolysaccharide and phagocytosis. This decline in homeostatic immune responses was associated with a striking disappearance of circadian gene transcription in aged compared to young tissue macrophages. Chromatin accessibility was significantly greater in young macrophages than in aged macrophages; however, this difference did not explain the loss of rhythmic gene transcription in aged macrophages. Rather, diurnal expression of Kruppel-like factor 4 (Klf4), a transcription factor (TF) well established in regulating cell differentiation and reprogramming, was selectively diminished in aged macrophages. Ablation of Klf4 expression abolished diurnal rhythms in phagocytic activity, recapitulating the effect of aging on macrophage phagocytosis. Examination of individuals harboring genetic variants of KLF4 revealed an association with age-dependent susceptibility to death caused by bacterial infection. Our results indicate that loss of rhythmic Klf4 expression in aged macrophages is associated with disruption of circadian innate immune homeostasis, a mechanism that may underlie age-associated loss of protective immune responses.

Time-restricted feeding during the inactive phase abolishes the daily rhythm in mitochondrial respiration in rat skeletal muscle

Shift-workers show an increased incidence of type 2 diabetes mellitus (T2DM). A possible mechanism is the disruption of the circadian timing of glucose homeostasis. Skeletal muscle mitochondrial function is modulated by the molecular clock. We used time-restricted feeding (TRF) during the inactive phase to investigate how mistimed feeding affects muscle mitochondrial metabolism. Rats on an ad libitum (AL) diet were compared to those that could eat only during the light (inactive) or dark (active) phase. Mitochondrial respiration, metabolic gene expressions, and metabolite concentrations were determined in the soleus muscle. Rats on AL feeding or dark-fed TRF showed a clear daily rhythm in muscle mitochondrial respiration. This rhythm in mitochondrial oxidative phosphorylation capacity was abolished in light-fed TRF animals and overall 24h respiration was lower. The expression of several genes involved in mitochondrial biogenesis and the fission/fusion machinery was altered in light-fed animals. Metabolomics analysis indicated that light-fed animals had lost rhythmic levels of α-ketoglutarate and citric acid. Contrastingly, lipidomics showed that light-fed animals abundantly gained rhythmicity in levels of triglycerides. Furthermore, while the RER shifted entirely with the food intake in the light-fed animals, many measured metabolic parameters (e.g., activity and mitochondrial respiration) did not strictly align with the shifted timing of food intake, resulting in a mismatch between expected metabolic supply/demand (as dictated by the circadian timing system and light/dark-cycle) and the actual metabolic supply/demand (as dictated by the timing of food intake). These data suggest that shift-work impairs mitochondrial metabolism and causes metabolic inflexibility, which can predispose to T2DM.

The involvement of host circadian clocks in the regulation of the immune response to parasitic infections in mammals

Circadian rhythms are recurring variations of physiology with a period of ~24 hours, generated by circadian clocks located throughout the body. Studies have shown a circadian regulation of many aspects of immunity. Immune cells have intrinsic clock mechanisms, and innate and adaptive immune responses - such as leukocyte migration, magnitude of inflammation, cytokine production and cell differentiation - are under circadian control. This circadian regulation has consequences for infections including parasitic infections. In the context of Leishmania infection, the circadian clock within host immune cells modulates the magnitude of the infection and the inflammatory response triggered by the parasite. As for malaria, rhythms within the immune system were shown to impact the developmental cycles of Plasmodium parasites within red blood cells. Further, host circadian rhythms impact infections by multicellular parasites; for example, infection with helminth Trichuris muris shows different kinetics of worm expulsion depending on time of day of infection, a variation that depends on the dendritic cell clock. Although the research on the circadian control of immunity in the context of parasitic infections is in its infancy, the research reviewed here suggests a crucial involvement of host circadian rhythms in immunity on the development and progression of parasitic infections.

The Circadian Clock Protein BMAL1 Acts as a Metabolic Sensor In Macrophages to Control the Production of Pro IL-1β

The transcription factor BMAL1 is a clock protein that generates daily or circadian rhythms in physiological functions including the inflammatory response of macrophages. Intracellular metabolic pathways direct the macrophage inflammatory response, however whether the clock is impacting intracellular metabolism to direct this response is unclear. Specific metabolic reprogramming of macrophages controls the production of the potent pro-inflammatory cytokine IL-1β. We now describe that the macrophage molecular clock, through Bmal1, regulates the uptake of glucose, its flux through glycolysis and the Krebs cycle, including the production of the metabolite succinate to drive Il-1β production. We further demonstrate that BMAL1 modulates the level and localisation of the glycolytic enzyme PKM2, which in turn activates STAT3 to further drive Il-1β mRNA expression. Overall, this work demonstrates that BMAL1 is a key metabolic sensor in macrophages, and its deficiency leads to a metabolic shift of enhanced glycolysis and mitochondrial respiration, leading to a heightened pro-inflammatory state. These data provide insight into the control of macrophage driven inflammation by the molecular clock, and the potential for time-based therapeutics against a range of chronic inflammatory diseases.

Circadian immune circuits

Immune responses are gated to protect the host against specific antigens and microbes, a task that is achieved through antigen- and pattern-specific receptors. Less appreciated is that in order to optimize responses and to avoid collateral damage to the host, immune responses must be additionally gated in intensity and time. An evolutionary solution to this challenge is provided by the circadian clock, an ancient time-keeping mechanism that anticipates environmental changes and represents a fundamental property of immunity. Immune responses, however, are not exclusive to immune cells and demand the coordinated action of nonhematopoietic cells interspersed within the architecture of tissues. Here, we review the circadian features of innate immunity as they encompass effector immune cells as well as structural cells that orchestrate their responses in space and time. We finally propose models in which the central clock, structural elements, and immune cells establish multidirectional circadian circuits that may shape the efficacy and strength of immune responses and other physiological processes.

Alterations in mitochondrial morphology as a key driver of immunity and host defence

Mitochondria are dynamic organelles whose architecture changes depending on the cell's energy requirements and other signalling events. These structural changes are collectively known as mitochondrial dynamics. Mitochondrial dynamics are crucial for cellular functions such as differentiation, energy production and cell death. Importantly, it has become clear in recent years that mitochondrial dynamics are a critical control point for immune cell function. Mitochondrial remodelling allows quiescent immune cells to rapidly change their metabolism and become activated, producing mediators, such as cytokines, chemokines and even metabolites to execute an effective immune response. The importance of mitochondrial dynamics in immunity is evident, as numerous pathogens have evolved mechanisms to manipulate host cell mitochondrial remodelling in order to promote their own survival. In this review, we comprehensively address the roles of mitochondrial dynamics in immune cell function, along with modulation of host cell mitochondrial morphology during viral and bacterial infections to facilitate either pathogen survival or host immunity. We also speculate on what the future may hold in terms of therapies targeting mitochondrial morphology for bacterial and viral control.

Post-transcriptional circadian regulation in macrophages organizes temporally distinct immunometabolic states

Our core timekeeping mechanism, the circadian clock, plays a vital role in immunity. Although the mechanics of circadian control over the immune response is generally explained by transcriptional activation or repression derived from this clock's transcription-translation negative-feedback loop, research suggests that some regulation occurs beyond transcriptional activity. We comprehensively profiled the transcriptome and proteome of murine bone marrow-derived macrophages and found that only 15% of the circadian proteome had corresponding oscillating mRNA, suggesting post-transcriptional regulation influences macrophage clock regulatory output to a greater extent than any other tissue previously profiled. This regulation may be explained by the robust temporal enrichment we identified for proteins involved in degradation and translation. Extensive post-transcriptional temporal-gating of metabolic pathways was also observed and further corresponded with daily variations in ATP production, mitochondrial morphology, and phagocytosis. The disruption of this circadian post-transcriptional metabolic regulation impaired immune functionality. Our results demonstrate that cell-intrinsic post-transcriptional regulation is a primary driver of circadian output in macrophages and that this regulation, particularly of metabolic pathways, plays an important role in determining their response to immune stimuli.

It's time to think about circadian rhythms

We here introduce a Review Series focussing on the important influences circadian rhythms have on immune responses. The three reviews in this series, expertly curated by Rachel Edgar, discuss how the cyclic oscillations in our cellular clock affect the innate and adaptive immune response, and how interactions with the intestinal microbiota, themselves subject to daily oscillations, also influence immune responses. As we understand more about these mechanisms, by which chronobiology contributes to immunology, it is becoming increasingly clear that they have important functions in maintaining health, influence autoimmunity and may contribute to the effectiveness of vaccinations.

Do not neglect the role of circadian rhythm in muscle atrophy

In addition to its role in movement, human skeletal muscle also plays important roles in physiological activities related to metabolism and the endocrine system. Aging and disease onset and progression can induce the reduction of skeletal muscle mass and function, thereby exacerbating skeletal muscle atrophy. Recent studies have confirmed that skeletal muscle atrophy is mainly controlled by the balance between protein synthesis and degradation, the activation of satellite cells, and mitochondrial quality in skeletal muscle. Circadian rhythm is an internal rhythm related to an organism's adaptation to light-dark or day-night cycles of the planet, and consists of a core biological clock and a peripheral biological clock. Skeletal muscle, as the most abundant tissue in the human body, is an essential part of the peripheral biological clock in humans. Increasing evidence has confirmed that maintaining a normal circadian rhythm can be beneficial for increasing protein content, improving mitochondrial quality, and stimulating regeneration and repairing of cells in skeletal muscle to prevent or alleviate skeletal muscle atrophy. In this review, we summarize the roles and underlying mechanisms of circadian rhythm in delaying skeletal muscle atrophy, which will provide a theoretical reference for incorporating aspects of circadian rhythm to the prevention and treatment of skeletal muscle atrophy.

Innate Rhythms: Clocks at the Center of Monocyte and Macrophage Function

The circadian cycle allows organisms to track external time of day and predict/respond to changes in the external environment. In higher order organisms, circadian rhythmicity is a central feature of innate and adaptive immunity. We focus on the role of the molecular clock and circadian rhythmicity specifically in monocytes and macrophages of the innate immune system. These cells display rhythmicity in their internal functions, such as metabolism and inflammatory mediator production as well as their external functions in pathogen sensing, phagocytosis, and migration. These inflammatory mediators are of clinical interest as many are therapeutic targets in inflammatory disease such as cardiovascular disease, diabetes, and rheumatoid arthritis. Moreover, circadian rhythm disruption is closely linked with increased prevalence of these conditions. Therefore, understanding the mechanisms by which circadian disruption affects monocyte/macrophage function will provide insights into novel therapeutic opportunities for these chronic inflammatory diseases.

For Whom the Clock Ticks: Clinical Chronobiology for Infectious Diseases

The host defense against pathogens varies among individuals. Among the factors influencing host response, those associated with circadian disruptions are emerging. These latter depend on molecular clocks, which control the two partners of host defense: microbes and immune system. There is some evidence that infections are closely related to circadian rhythms in terms of susceptibility, clinical presentation and severity. In this review, we overview what is known about circadian rhythms in infectious diseases and update the knowledge about circadian rhythms in immune system, pathogens and vectors. This heuristic approach opens a new fascinating field of time-based personalized treatment of infected patients.

Regulatory T cells confer a circadian signature on inflammatory arthritis

The circadian clock is an intrinsic oscillator that imparts 24 h rhythms on immunity. This clock drives rhythmic repression of inflammatory arthritis during the night in mice, but mechanisms underlying this effect are not clear. Here we show that the amplitude of intrinsic oscillators within macrophages and neutrophils is limited by the chronic inflammatory environment, suggesting that rhythms in inflammatory mediators might not be a direct consequence of intrinsic clocks. Anti-inflammatory regulatory T (Treg) cells within the joints show diurnal variation, with numbers peaking during the nadir of inflammation. Furthermore, the anti-inflammatory action of Treg cells on innate immune cells contributes to the night-time repression of inflammation. Treg cells do not seem to have intrinsic circadian oscillators, suggesting that rhythmic function might be a consequence of external signals. These data support a model in which non-rhythmic Treg cells are driven to rhythmic activity by systemic signals to confer a circadian signature to chronic arthritis.

Mitochondria: An Integrative Hub Coordinating Circadian Rhythms, Metabolism, the Microbiome, and Immunity

There is currently some understanding of the mechanisms that underpin the interactions between circadian rhythmicity and immunity, metabolism and immune response, and circadian rhythmicity and metabolism. In addition, a wealth of studies have led to the conclusion that the commensal microbiota (mainly bacteria) within the intestine contributes to host homeostasis by regulating circadian rhythmicity, metabolism, and the immune system. Experimental studies on how these four biological domains interact with each other have mainly focused on any two of those domains at a time and only occasionally on three. However, a systematic analysis of how these four domains concurrently interact with each other seems to be missing. We have analyzed current evidence that signposts a role for mitochondria as a key hub that supports and integrates activity across all four domains, circadian clocks, metabolic pathways, the intestinal microbiota, and the immune system, coordinating their integration and crosstalk. This work will hopefully provide a new perspective for both hypothesis-building and more systematic experimental approaches.

The clock gene Bmal1 inhibits macrophage motility, phagocytosis, and impairs defense against pneumonia

The circadian clock regulates many aspects of immunity. Bacterial infections are affected by time of day, but the mechanisms involved remain undefined. Here we show that loss of the core clock protein BMAL1 in macrophages confers protection against pneumococcal pneumonia. Infected mice show both reduced weight loss and lower bacterial burden in circulating blood. In vivo studies of macrophage phagocytosis reveal increased bacterial ingestion following Bmal1 deletion, which was also seen in vitro. BMAL1-/- macrophages exhibited marked differences in actin cytoskeletal organization, a phosphoproteome enriched for cytoskeletal changes, with reduced phosphocofilin and increased active RhoA. Further analysis of the BMAL1-/- macrophages identified altered cell morphology and increased motility. Mechanistically, BMAL1 regulated a network of cell movement genes, 148 of which were within 100 kb of high-confidence BMAL1 binding sites. Links to RhoA function were identified, with 29 genes impacting RhoA expression or activation. RhoA inhibition restored the phagocytic phenotype to that seen in control macrophages. In summary, we identify a surprising gain of antibacterial function due to loss of BMAL1 in macrophages, associated with a RhoA-dependent cytoskeletal change, an increase in cell motility, and gain of phagocytic function.

Chronobiological Influence Over Cardiovascular Function: The Good, the Bad, and the Ugly

Essentially all biological processes fluctuate over the course of the day, observed at cellular (eg, transcription, translation, and signaling), organ (eg, contractility and metabolism), and whole-body (eg, physical activity and appetite) levels. It is, therefore, not surprising that both cardiovascular physiology (eg, heart rate and blood pressure) and pathophysiology (eg, onset of adverse cardiovascular events) oscillate during the 24-hour day. Chronobiological influence over biological processes involves a complex interaction of factors that are extrinsic (eg, neurohumoral factors) and intrinsic (eg, circadian clocks) to cells. Here, we focus on circadian governance of 6 fundamentally important processes: metabolism, signaling, electrophysiology, extracellular matrix, clotting, and inflammation. In each case, we discuss (1) the physiological significance for circadian regulation of these processes (ie, the good); (2) the pathological consequence of circadian governance impairment (ie, the bad); and (3) whether persistence/augmentation of circadian influences contribute to pathogenesis during distinct disease states (ie, the ugly). Finally, the translational impact of chronobiology on cardiovascular disease is highlighted.

Circadian rhythms in innate immunity and stress responses

Circadian clocks are a common feature of life on our planet, allowing physiology and behaviour to be adapted to recurrent environmental fluctuation. There is now compelling evidence that disturbance of circadian coherence can severely undermine mental and physical health, as well as exacerbate pre-existing pathology. Common molecular design principles underpin the generation of cellular circadian rhythms across the kingdoms, and in animals, the genetic components are extremely well conserved. In mammals, the circadian timing mechanism is present in most cell types and establishes local cycles of gene expression and metabolic activity. These distributed tissue clocks are normally synchronized by a central pacemaker, the suprachiasmatic nuclei (SCN), located in the hypothalamus. Nevertheless, most clocks of the body remain responsive to non-SCN-derived hormonal and metabolic cues (for example, re-alignment of liver clocks to altered meal patterning). It has been demonstrated that the clock is an influential regulator of energy metabolism, allowing key pathways to be tuned across the 24-hr cycle as metabolic requirements fluctuate. Furthermore, clock components, including Cryptochrome and Rev-Erb proteins, have been identified as essential modulators of the innate immune system and inflammatory responses. Studies have also revealed that these proteins regulate glucocorticoid receptor function, a major drug target and crucial regulator of inflammation and metabolism.

A day in the life of mitochondria reveals shifting workloads

Mitochondria provide energy for cellular function. We examine daily changing patterns of mitochondrial function and metabolism in Drosophila in vivo in terms of their complex (I-IV) activity, ATP production, glycolysis, and whole fly respiration in the morning, afternoon and night. Complex activity and respiration showed significant and unexpected variation, peaking in the afternoon. However, ATP levels by contrast are >40% greater in the morning and lowest at night when glycolysis peaks. Complex activity modulation was at the protein level with no evidence for differential transcription over the day. Timing differences between increased ATP production and peaks of complex activity may result from more efficient ATP production early in the day leaving complex activity with spare capacity. Optical stimulation of mitochondria is only possible in the mornings when there is such spare capacity. These results provide first evidence of shifts in cellular energy capacity at the organism level. Understanding their translation may be significant to the chosen timing of energy demanding interventions to improve function and health.

Daily variation in macrophage phagocytosis is clock-independent and dispensable for cytokine production

Innate immune responses vary in a circadian manner, and more recent investigations aim to understand the underlying molecular mechanisms. Cytokine production varies significantly over the course of a day depending on the time of stimulation by pathogens or Toll-like receptor ligands, and multiple signaling pathways linked to the cell-autonomous circadian clock modulate innate immunity. Recognition of foreign material, especially by innate immune cells, engages a myriad of receptors, which trigger inflammatory responses, as well as endocytosis and degradation and/or processing for antigen presentation. Because of the close connection between particle engulfment and inflammation, it has been proposed that phagocytic uptake may drive cytokine production in phagocytes. Here we show that bacterial particle ingestion by mouse peritoneal macrophages displays temporal variation, but is independent of the cell-intrinsic circadian clock in an ex vivo setting. Although cytokine production is dependent on phagocytosis, uptake capacity across 12 hr does not translate into 24-hr rhythms in cytokine production. In vivo, time-of-day variations in phagocytic capacity are not found, whereas a time of day and clock-dependent cytokine response is maintained. These data show that efficiency of bacterial phagocytosis and the 24-hr rhythmicity of cytokine production by macrophages are independent of one another and should be studied separately.

The estrogen-macrophage interplay in the homeostasis of the female reproductive tract

BACKGROUND

Estrogens are known to orchestrate reproductive events and to regulate the immune system during infections and following tissue damage. Recent findings suggest that, in the absence of any danger signal, estrogens trigger the physiological expansion and functional specialization of macrophages, which are immune cells that populate the female reproductive tract (FRT) and are increasingly being recognized to participate in tissue homeostasis beyond their immune activity against infections. Although estrogens are the only female gonadal hormones that directly target macrophages, a comprehensive view of this endocrine-immune communication and its involvement in the FRT is still missing.

OBJECTIVE AND RATIONALE

Recent accomplishments encourage a revision of the literature on the ability of macrophages to respond to estrogens and induce tissue-specific functions required for reproductive events, with the aim to envision macrophages as key players in FRT homeostasis and mediators of the regenerative and trophic actions of estrogens.

SEARCH METHODS

We conducted a systematic search using PubMed and Ovid for human, animal (rodents) and cellular studies published until 2018 on estrogen action in macrophages and the activity of these cells in the FRT.

OUTCOMES

Our search identified the remarkable ability of macrophages to activate biochemical processes in response to estrogens in cell culture experiments. The distribution at specific locations, interaction with selected cells and acquisition of distinct phenotypes of macrophages in the FRT, as well as the cyclic renewal of these properties at each ovarian cycle, demonstrate the involvement of these cells in the homeostasis of reproductive events. Moreover, current evidence suggests an association between estrogen-macrophage signaling and the generation of a tolerant and regenerative environment in the FRT, although a causative link is still missing.

WIDER IMPLICATIONS

Dysregulation of the functions and estrogen responsiveness of FRT macrophages may be involved in infertility and estrogen- and macrophage-dependent gynecological diseases, such as ovarian cancer and endometriosis. Thus, more research is needed on the physiology and pharmacological control of this endocrine-immune interplay.

Circadian rhythms and exercise - re-setting the clock in metabolic disease

Perturbed diurnal rhythms are becoming increasingly evident as deleterious events in the pathology of metabolic diseases. Exercise is well characterized as a crucial intervention in the prevention and treatment of individuals with metabolic diseases. Little is known, however, regarding optimizing the timing of exercise bouts in order to maximize their health benefits. Furthermore, exercise is a potent modulator of skeletal muscle metabolism, and it is clear that skeletal muscle has a strong circadian profile. In humans, mitochondrial function peaks in the late afternoon, and the circadian clock might be inherently impaired in myotubes from patients with metabolic disease. Timing exercise bouts to coordinate with an individual's circadian rhythms might be an efficacious strategy to optimize the health benefits of exercise. The role of exercise as a Zeitgeber can also be used as a tool in combating metabolic disease. Shift work is known to induce acute insulin resistance, and appropriately timed exercise might improve health markers in shift workers who are at risk of metabolic disease. In this Review, we discuss the literature regarding diurnal skeletal muscle metabolism and the interaction with exercise bouts at different times of the day to combat metabolic disease.

Circadian clock protein BMAL1 regulates IL-1β in macrophages via NRF2

A variety of innate immune responses and functions are dependent on time of day, and many inflammatory conditions are associated with dysfunctional molecular clocks within immune cells. However, the functional importance of these innate immune clocks has yet to be fully characterized. NRF2 plays a critical role in the innate immune system, limiting inflammation via reactive oxygen species (ROS) suppression and direct repression of the proinflammatory cytokines, IL-1β and IL-6. Here we reveal that the core molecular clock protein, BMAL1, controls the mRNA expression of Nrf2 via direct E-box binding to its promoter to regulate its activity. Deletion of Bmal1 decreased the response of NRF2 to LPS challenge, resulting in a blunted antioxidant response and reduced synthesis of glutathione. ROS accumulation was increased in Bmal1-/- macrophages, facilitating accumulation of the hypoxic response protein, HIF-1α. Increased ROS and HIF-1α levels, as well as decreased activity of NRF2 in cells lacking BMAL1, resulted in increased production of the proinflammatory cytokine, IL-1β. The excessive prooxidant and proinflammatory phenotype of Bmal1-/- macrophages was rescued by genetic and pharmacological activation of NRF2, or through addition of antioxidants. Our findings uncover a clear role for the molecular clock in regulating NRF2 in innate immune cells to control the inflammatory response. These findings provide insights into the pathology of inflammatory conditions, in which the molecular clock, oxidative stress, and IL-1β are known to play a role.

Clocking in to immunity

Circadian rhythms are a ubiquitous feature of virtually all living organisms, regulating a wide diversity of physiological systems. It has long been established that the circadian clockwork plays a key role in innate immune responses, and recent studies reveal that several aspects of adaptive immunity are also under circadian control. We discuss the latest insights into the genetic and biochemical mechanisms linking immunity to the core circadian clock of the cell and hypothesize as to why the immune system is so tightly controlled by circadian oscillations. Finally, we consider implications for human health, including vaccination strategies and the emerging field of chrono-immunotherapy.

Circadian rhythms in mitochondrial respiration

Many physiological processes are regulated with a 24-h periodicity to anticipate the environmental changes of daytime to nighttime and vice versa. These 24-h regulations, commonly termed circadian rhythms, among others control the sleep-wake cycle, locomotor activity and preparation for food availability during the active phase (daytime for humans and nighttime for nocturnal animals). Disturbing circadian rhythms at the organ or whole-body level by social jetlag or shift work, increases the risk to develop chronic metabolic diseases such as type 2 diabetes mellitus. The molecular basis of this risk is a topic of increasing interest. Mitochondria are essential organelles that produce the majority of energy in eukaryotes by converting lipids and carbohydrates into ATP through oxidative phosphorylation. To adapt to the ever-changing environment, mitochondria are highly dynamic in form and function and a loss of this flexibility is linked to metabolic diseases. Interestingly, recent studies have indicated that changes in mitochondrial morphology (i.e., fusion and fission) as well as generation of new mitochondria are dependent on a viable circadian clock. In addition, fission and fusion processes display diurnal changes that are aligned to the light/darkness cycle. Besides morphological changes, mitochondrial respiration also displays diurnal changes. Disturbing the molecular clock in animal models leads to abrogated mitochondrial rhythmicity and altered respiration. Moreover, mitochondrial-dependent production of reactive oxygen species, which plays a role in cellular signaling, has also been linked to the circadian clock. In this review, we will summarize recent advances in the study of circadian rhythms of mitochondria and how this is linked to the molecular circadian clock.

Circadian Clearance of a Fungal Pathogen from the Lung Is Not Based on Cell-intrinsic Macrophage Rhythms

Circadian rhythms govern immune cell function, giving rise to time-of-day variation in the recognition and clearance of bacterial or viral pathogens; to date, however, no such regulation of the host-fungal interaction has been described. In this report, we use murine models to explore circadian control of either fungal-macrophage interactions in vitro or pathogen clearance from the lung in vivo. First, we show that expression of the important fungal pattern recognition receptor Dectin-1 ( clec7a), from either bone marrow-derived or peritoneum-derived macrophages, is not under circadian regulation at either the level of transcript or cell surface protein expression. Consistent with this finding, the phagocytic activity of macrophages in culture against spores of the pathogen Aspergillus fumigatus also did not vary over time. To account for the multiple cell types and processes that may be coordinated in a circadian fashion in vivo, we examined the clearance of A. fumigatus from the lungs of immunocompetent mice. Interestingly, animals inoculated at night demonstrated a 2-fold enhancement in clearance compared with animals inoculated in the morning. Taken together, our data suggest that while molecular recognition of fungi by immune cells may not be circadian, other processes in vivo may still allow for time-of-day differences in fungal clearance from the lung.

Participation of 14-3-3ε and 14-3-3ζ proteins in the phagocytosis, component of cellular immune response, in Aedes mosquito cell lines

BACKGROUND

Better knowledge of the innate immune system of insects will improve our understanding of mosquitoes as potential vectors of diverse pathogens. The ubiquitously expressed 14-3-3 protein family is evolutionarily conserved from yeast to mammals, and at least two isoforms of 14-3-3, the ε and ζ, have been identified in insects. These proteins have been shown to participate in both humoral and cellular immune responses in Drosophila. As mosquitoes of the genus Aedes are the primary vectors for arboviruses, causing several diseases such as dengue fever, yellow fever, Zika and chikungunya fevers, cell lines derived from these mosquitoes, Aag-2 from Aedes aegypti and C6/36 HT from Aedes albopictus, are currently used to study the insect immune system. Here, we investigated the role of 14-3-3 proteins (ε and ζ isoform) in phagocytosis, the main cellular immune responses executed by the insects, using Aedes spp. cell lines.

RESULTS

We evaluated the mRNA and protein expression of 14-3-3ε and 14-3-3ζ in C6/36 HT and Aag-2 cells, and demonstrated that both proteins were localised in the cytoplasm. Further, in C6/36 HT cells treated with a 14-3-3 specific inhibitor we observed a notable modification of cell morphology with filopodia-like structure caused through cytoskeleton reorganisation (co-localization of 14-3-3 proteins with F-actin), more importantly the decrease in Salmonella typhimurium, Staphylococcus aureus and E. coli phagocytosis and reduction in phagolysosome formation. Additionally, silencing of 14-3-3ε and 14-3-3ζ expression by mean of specific DsiRNA confirmed the decreased phagocytosis and phagolysosome formation of pHrodo labelled E. coli and S. aureus bacteria by Aag-2 cells.

CONCLUSION

The 14-3-3ε and 14-3-3ζ proteins modulate cytoskeletal remodelling, and are essential for phagocytosis of Gram-positive and Gram-negative bacteria in Aedes spp. cell lines.

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.

The Circadian Nature of Mitochondrial Biology

Circadian clocks orchestrate the daily changes in physiology and behavior of light-sensitive organisms. These clocks measure about 24 h and tick in a self-sustained and cell-autonomous manner. Mounting evidence points toward a tight intertwining between circadian clocks and metabolism. Although various aspects of circadian control of metabolic functions have been extensively studied, our knowledge regarding circadian mitochondrial function is rudimentary. In this review, we will survey the current literature related to the circadian nature of mitochondrial biology: from mitochondrial omics studies (e.g., proteome, acetylome, and lipidome), through dissection of mitochondrial morphology, to analyses of mitochondrial processes such as nutrient utilization and respiration. We will describe potential mechanisms that are implicated in circadian regulation of mitochondrial functions in mammals and discuss the possibility of a mitochondrial-autonomous oscillator.

Circadian Clocks in the Hematologic System

The hematologic system performs a number of essential functions, including oxygen transport, the execution of the immune response against tumor cells and invading pathogens, and hemostasis (blood clotting). These roles are performed by erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets), respectively. Critically, circadian rhythms are evident in the function of all 3 cell types. In this review, we describe these oscillations, explore their mechanistic bases, and highlight their key implications. Since erythrocytes are anucleate, circadian rhythms in these cells testify to the existence of a nontranscriptional circadian clock. From a clinical perspective, leukocyte rhythms could underlie daily variation in the severity of allergic reactions, the symptoms of chronic inflammatory diseases, and the body’s response to infection, while the rhythmic properties of thrombocytes may explain daily fluctuations in the incidence of heart attack and stroke. Consequently, the efficacy of treatments for these conditions is likely to depend on the timing of their administration. Last, we outline preliminary evidence that circadian disruption in the hematologic system could contribute to the deleterious effects of poor diet, shift work, and alcohol abuse on human health.