A brief history of circadian time

Editorial: Parasites- the importance of time

The special edition of Parasite Immunology ‘Parasites—The importance of time’ embraces the intersection between three established research disciplines—parasitology, immunology, and circadian biology. Each of these research areas has a longstanding history littered with landmark discoveries with the intersect between the three bringing exciting findings and new questions and perhaps even a greater sense of awe in terms of how parasites have evolved to interact and live with their hosts.

Introduction to Biological Rhythms: A Brief History of Chronobiology and its Relevance to Parasite Immunology

Almost every living organism on Earth is exposed to a fluctuating environment e.g., light:dark cycles, food availability, seasonal photoperiods. Most species have therefore evolved internal timing mechanisms allowing them to anticipate these rhythmic environmental changes, obtaining a survival advantage. Circadian (24 h) rhythms, in particular, regulate multiple aspects of physiology, including sleep/wake activity, feeding rhythms, and immune function. Recent studies have identified circadian rhythms in symptoms of parasite infections, rhythms in parasite schizogony, and evidence that certain parasites can manipulate host rhythms. Furthermore, efficacy of anti-parasite medications can also be modulated by timing of drug administration. Understanding the interactions between host rhythms, parasite rhythms, and disease severity is crucial to fully understand how to combat infections and reduce pathology. The aim of this review is, therefore, to provide an introduction to the field of biological rhythms, give a brief history of chronobiology research, and discuss the relevance of biological rhythms to parasite immunology.

Circadian and Sleep Metabolomics Across Species

Under normal circadian function, metabolic control is temporally coordinated across tissues and behaviors with a 24-h period. However, circadian disruption results in negative consequences for metabolic homeostasis including energy or redox imbalances. Yet, circadian disruption has become increasingly prevalent within today's society due to many factors including sleep loss. Metabolic consequences of both have been revealed by metabolomics analyses of circadian biology and sleep. Specifically, two primary analytical platforms, mass spectrometry and nuclear magnetic resonance spectroscopy, have been used to study molecular clock and sleep influences on overall metabolic rhythmicity. For example, human studies have demonstrated the prevalence of metabolic rhythms in human biology, as well as pan-metabolome consequences of sleep disruption. However, human studies are limited to peripheral metabolic readouts primarily through minimally invasive procedures. For further tissue- and organism-specific investigations, a number of model systems have been studied, based upon the conserved nature of both the molecular clock and sleep across species. Here we summarize human studies as well as key findings from metabolomics studies using mice, Drosophila, and zebrafish. While informative, a limitation in existing literature is a lack of interpretation regarding dynamic synthesis or catabolism within metabolite pools. To this extent, future work incorporating isotope tracers, specific metabolite reporters, and single-cell metabolomics may provide a means of exploring dynamic activity in pathways of interest.

Identification of novel circadian transcripts in the zebrafish retina

High fecundity, transparent embryos for monitoring the rapid development of organs and the availability of a well-annotated genome has made zebrafish a model organism of choice for developmental biology and neurobiology. This vertebrate model, a favourite in chronobiology studies, shows striking circadian rhythmicity in behaviour. Here, we identify novel genes in the zebrafish genome, which are expressed in the zebrafish retina. We further resolve the expression pattern over time and tentatively assign specific novel transcripts to retinal bipolar cells of the inner nuclear layer. Using chemical ablation and free run experiments we segregate the transcripts that are rhythmic when entrained by light from those that show sustained oscillations in the absence of external cues. The transcripts reported here with rigorous annotation and specific functions in circadian biology provide the groundwork for functional characterisation of novel players in the zebrafish retinal clock.

Role of circadian rhythms in potassium homeostasis

It has been known for decades that urinary potassium excretion varies with a circadian pattern. In this review, we consider the historical evidence for this phenomenon and present an overview of recent developments in the field. Extensive evidence from the latter part of the past century clearly shows that circadian potassium excretion does not depend on endogenous aldosterone. Of note is the recent discovery that the expression of several renal potassium transporters varies with a circadian pattern that appears to be consistent with substantial clinical data regarding daily fluctuations in urinary potassium levels. We propose the circadian clock mechanism as a key regulator of renal potassium transporters, and consequently renal potassium excretion. Further investigation into the regulation mechanism of renal potassium transport by the circadian clock is warranted to increase our understanding of the clinical relevance of circadian rhythms to potassium homeostasis.

The circadian clock and asthma

It is characteristic of asthma that symptoms worsen overnight, particularly in the early hours of the morning. Nocturnal symptoms in asthma are common and are an important indicator for escalation of treatment. An extensive body of research has demonstrated that nocturnal symptoms of cough and dyspnea are accompanied by circadian variations in airway inflammation and physiologic variables, including airflow limitation and airways hyper-responsiveness. The molecular apparatus that underpins circadian variations, controlled by so called 'clock' genes, has recently been characterised. Clock genes control circadian rhythms both centrally, in the suprachiasmatic nucleus of the brain and peripherally, within every organ of the body. Here, we will discuss how clock genes regulate circadian rhythms. We will focus particularly on the peripheral lung clock and the peripheral immune clock and discuss how these might relate to both the pathogenesis and treatment of asthma.

Genetic redundancy strengthens the circadian clock leading to a narrow entrainment range

Circadian clocks are internal timekeepers present in almost all organisms. Driven by a genetic network of highly conserved structure, they generate self-sustained oscillations that entrain to periodic external signals such as the 24 h light-dark cycle. Vertebrates possess multiple, functionally overlapping homologues of the core clock genes. Furthermore, vertebrate clocks entrain to a range of periods three times as narrow as that of other organisms. We asked whether genetic redundancies play a role in governing entrainment properties and analysed locomotor activity rhythms of genetically modified mice lacking one set of clock homologues. Exposing them to non-24 h light-dark cycles, we found that the mutant mice have a wider entrainment range than the wild types. Spectral analysis furthermore revealed nonlinear phenomena of periodically forced self-sustained oscillators for which the entrainment range relates inversely to oscillator amplitude. Using the forced oscillator model to explain the observed differences in entrainment range between mutant and wild-type mice, we sought to quantify the overall oscillator amplitude of their clocks from the activity rhythms and found that mutant mice have weaker circadian clocks than wild types. Our results suggest that genetic redundancy strengthens the circadian clock leading to a narrow entrainment range in vertebrates.

Circadian oscillators in eukaryotes

The biological clock, present in nearly all eukaryotes, has evolved such that organisms can adapt to our planet's rotation in order to anticipate the coming day or night as well as unfavorable seasons. As all modern high-precision chronometers, the biological clock uses oscillation as a timekeeping element. In this review, we describe briefly the discovery, historical development, and general properties of circadian oscillators. The issue of temperature compensation (TC) is discussed, and our present understanding of the underlying genetic and biochemical mechanisms in circadian oscillators are described with special emphasis on Neurospora crassa, mammals, and plants.

Photic and non-photic entrainment on daily rhythm of locomotor activity in goats

We studied the photic (L/D cycle) and non-photic (restricted feeding) entrainment on the patterns of daily rhythm of total locomotor activity in goats. Six female Maltese goats were subjected to three different artificial L/D cycles: 12/12 L/D, 12/12 D/L and constant light. During the 12/12 L/D and 12/12 D/L, food and water were available ad libitum. During constant light, animals were subjected to a restricted feeding treatment. Total activity was recorded by means of an actigraphy-based data logger (Actiwatch-Mini). Our results showed that goats exhibited clear daily rhythms of activity in 12/12 L/D cycle, 12/12 D/L cycle and constant light, although they showed FAA prior the feeding time during the restricted feeding treatment. Goats were diurnal, with activity consistently beginning promptly following the onset of light. Even when the L/D cycle was delayed by 12 h on some days, to the daily rhythm was re-established. During the constant light period, the onset of activity was linked to the time of food administration. Our study evidences two factors for the rhythm of total locomotor activity in goats: light stimuli (photic) and food access (non photic), strongly coupled to permit organisms the adaptive temporal coordination of behaviour with stable and unstable environmental periodicities.

Effects of long-term continuous exposure to light on memory and anxiety in mice

The studies on the relationship between the light/dark cycle and memory function mostly used protocols of acute disruption of the circadian rhythm. The aim of the present study is to verify the effects of long-term continuous exposure to light on memory, anxiety and motor parameters of mice tested in the plus-maze discriminative avoidance task. Mice were conditioned to choose between the two enclosed arms (one aversive and one non-aversive) while avoiding the open arms of a modified elevated plus-maze apparatus. Memory was evaluated by the time spent in the aversive enclosed arm, anxiety was evaluated by the time spent in the open arms and locomotor behavior was evaluated by number of entries in the arms of the maze. The results showed that long-term (35-42 days) continuous light exposure did not modify memory or anxiety parameters but increased locomotor activity. While the increase in locomotor behavior is in line with previous studies, the unexpected absence of alterations in memory and anxiety (reported to be influenced by the circadian rhythm) is discussed.

Gene duplication and complex circadian clocks in mammals

The circadian clock arose early in the evolution of life to enable organisms to adapt to the cycle of day and night. Recently, the extent and importance of circadian regulation of behaviour and physiology has come to be more fully realized. Core molecular cogs of circadian oscillators appear to have been largely conserved between such diverse organisms as Drosophila melanogaster and mammals. However, gene duplication events have produced multiple copies of many clock genes in mammals. Recent studies suggest that genome duplication has lead to increased circadian complexity and local tissue regulation. This has important implications for temporal regulation of behaviour via multiple clocks in the central nervous system, and also extends to the local physiology of major body organs and tissues.

Circadian clocks and natural antisense RNA

Eukaryotes regulate gene expression in a number of different ways. On a daily and seasonal timescale, the orchestration of gene expression is to a large extent governed by circadian clocks. These endogenous timekeepers enable organisms to prepare for predictable environmental conditions from one day to the next and thus allow adaptation to a given temporal niche. In general, circadian clocks have been shown to employ the classical transcriptional and posttranscriptional control mechanisms to generate rhythmicity. However, the discovery of antisense clock gene transcripts suggests that mechanisms of gene regulation operating through antisense RNA may also be integral to the circadian clockwork. Following a brief history of the impact of genetic and molecular techniques in aiding our understanding of circadian clocks, this review concentrates on the few examples of antisense clock gene transcripts so far investigated and their effect on circadian timing.

Circadian modulation of learning and memory in fear-conditioned mice

Endogenous processes referred to as circadian oscillators generate many of the daily rhythms in physiology and behavior of a variety of animals including humans. We investigated the possible circadian regulation of acquisition, recall and extinction in two strains of mice (C-57/6J and C-3H). Mice were trained in either the day or night with a tone and context fear conditioning protocol. The mice were then tested over the course of several days for their ability to recall the training. When comparing the performance of animals in the day and night, the mice acquired the conditioning faster in the day than in the night. Furthermore, the recall for context and tone consistently peaked during the day for at least 3 days after training, irrespective of the time of training. Finally, the loss of this training (or extinction) exhibited a rhythm in that mice trained in night exhibited a greater degree of extinction than mice trained in the day. For all of these rhythms in acquisition, recall, and extinction the phase of the rhythm was controlled by the prior light-dark (LD) cycle. When we reversed the phase of the LD cycle, the phase of the rhythm also reversed. Importantly, all three of the rhythms also continued in constant darkness demonstrating the endogenous, and presumably circadian nature, of the rhythms.

Respiratory oscillations in yeast: clock-driven mitochondrial cycles of energization

Respiratory oscillations in continuous yeast cultures can be accounted for by cyclic energization of mitochondria, dictated by the demands of a temperature-compensated ultradian clock with a period of 50 min. Inner mitochondrial membranes show both ultrastructural modifications and electrochemical potential changes. Electron transport components (NADH and cytochromes c and c oxidase) show redox state changes as the organisms cycle between their energized and de-energized phases. These regular cycles are transiently perturbed by uncouplers of energy conservation, with amplitudes more affected than period; that the characteristic period is restored after only one prolonged cycle, indicates that mitochondrial energy generation is not part of the clock mechanism itself, but is responding to energetic requirement.