Plant cell responses to cold are all about timing

Changes in temperature present the cells of plants with particular challenges. Fortunately, many changes in temperature can be anticipated due to the rhythms of day/night and the seasons. To anticipate changes in the environment most organisms have a circadian clock to optimize daily and seasonal timing of gene expression, metabolism, physiology and cell biology. Circadian clocks comprised positive and negative feedback loops which ensure an internal period of approximately 24 hours. We describe the role of the circadian clock in modulating cellular cold signalling networks to prepare the cell for the onset of winter.     

Disrupted seasonal biology impacts health,food security and ecosystems

The rhythm of life on earth is shaped by seasonal changes in the environment. Plants and animals show profound annual cycles in physiology, health, morphology, behaviour and demography in response to environmental cues. Seasonal biology impacts ecosystems and agriculture, with consequences for humans and biodiversity. Human populations show robust annual rhythms in health and well-being, and the birth month can have lasting effects that persist throughout life. This review emphasizes the need for a better understanding of seasonal biology against the backdrop of its rapidly progressing disruption through climate change, human lifestyles and other anthropogenic impact. Climate change is modifying annual rhythms to which numerous organisms have adapted, with potential consequences for industries relating to health, ecosystems and food security. Disconcertingly, human lifestyles under artificial conditions of eternal summer provide the most extreme example for disconnect from natural seasons, making humans vulnerable to increased morbidity and mortality. In this review, we introduce scenarios of seasonal disruption, highlight key aspects of seasonal biology and summarize from biomedical, anthropological, veterinary, agricultural and environmental perspectives the recent evidence for seasonal desynchronization between environmental factors and internal rhythms. Because annual rhythms are pervasive across biological systems, they provide a common framework for trans-disciplinary research.     

Signaling networks in the plant circadian system

Significant advances have been made during the past year in the genetic and molecular dissection of the plant circadian system. Several proteins involved in circadian clock regulation have been identified and the way that their interactions contribute to temporal organization is starting to emerge. In addition, genomic approaches have identified hundreds of genes under clock control, providing a molecular basis to our understanding of how the clock coordinates plant physiology and development with daily and seasonal environmental cycles.     

Unraveling the mechanisms of the vertebrate circadian clock: zebrafish may light the way

Most organisms display oscillations of approximately 24 hours in their physiology. In higher organisms, these circadian oscillations in biochemical and physiological processes ultimately control complex behavioral rhythms that allow an organism to thrive in its natural habitat. Daily and seasonal light cycles are mainly responsible for keeping the circadian system properly aligned with the environment. The molecular mechanisms responsible for the control of the circadian clock have been explored in a number of systems. Interestingly, the circadian oscillations that are responsive to environmental stimuli are present very early during development. This review focuses on the advantages of using the zebrafish to study the development of the vertebrate circadian system and light-dependent signaling to the clock.     

Periodicity and time trends in the prevalence of total births and conceptions with congenital malformations among Jews and Muslims in Israel, 1999-2006: a time series study of 823,966 births

BACKGROUND Congenital malformations (CMs) are a leading cause of infant disability. Geophysical patterns such as 2-year, yearly, half-year, 3-month, and lunar cycles regulate much of the temporal biology of all life on Earth and may affect birth and birth outcomes in humans. Therefore, the aim of this study was to evaluate and compare trends and periodicity in total births and CM conceptions in two Israeli populations. METHODS Poisson nonlinear models (polynomial) were applied to study and compare trends and geophysical periodicity cycles of weekly births and weekly prevalence rate of CM (CMPR), in a time-series design of conception date within and between Jews and Muslims. The population included all live births and stillbirths (n = 823,966) and CM (three anatomic systems, eight CM groups [n = 2193]) in Israel during 2000 to 2006. Data were obtained from the Ministry of Health. RESULTS We describe the trend and periodicity cycles for total birth conceptions. Of eight groups of CM, periodicity cycles were statistically significant in four CM groups for either Jews or Muslims. Lunar month and biennial periodicity cycles not previously investigated in the literature were found to be statistically significant. Biennial cycle was significant in total births (Jews and Muslims) and syndactyly (Muslims), whereas lunar month cycle was significant in total births (Muslims) and atresia of small intestine (Jews). CONCLUSION We encourage others to use the method we describe as an important tool to investigate the effects of different geophysical cycles on human health and pregnancy outcomes, especially CM, and to compare between populations.     

Periodicity and time trends in the prevalence of total births and conceptions with congenital malformations among Jews and Muslims in Israel, 1999‐2006: A time series study of 823,966 births

BACKGROUND Congenital malformations (CMs) are a leading cause of infant disability. Geophysical patterns such as 2‐year, yearly, half‐year, 3‐month, and lunar cycles regulate much of the temporal biology of all life on Earth and may affect birth and birth outcomes in humans. Therefore, the aim of this study was to evaluate and compare trends and periodicity in total births and CM conceptions in two Israeli populations. METHODS Poisson nonlinear models (polynomial) were applied to study and compare trends and geophysical periodicity cycles of weekly births and weekly prevalence rate of CM (CMPR), in a time‐series design of conception date within and between Jews and Muslims. The population included all live births and stillbirths (n = 823,966) and CM (three anatomic systems, eight CM groups [n = 2193]) in Israel during 2000 to 2006. Data were obtained from the Ministry of Health. RESULTS We describe the trend and periodicity cycles for total birth conceptions. Of eight groups of CM, periodicity cycles were statistically significant in four CM groups for either Jews or Muslims. Lunar month and biennial periodicity cycles not previously investigated in the literature were found to be statistically significant. Biennial cycle was significant in total births (Jews and Muslims) and syndactyly (Muslims), whereas lunar month cycle was significant in total births (Muslims) and atresia of small intestine (Jews). CONCLUSION We encourage others to use the method we describe as an important tool to investigate the effects of different geophysical cycles on human health and pregnancy outcomes, especially CM, and to compare between populations. Birth Defects Research (Part A) 2012. © 2012 Wiley Periodicals, Inc.     

The Drosophila circadian clock: what we know and what we don't know.

Circadian rhythms are regulated by endogenous body clocks, which are formed by rhythmic cycles of clock gene expression. Almost all reviews of the Drosophila circadian clock state that the intracellular oscillator is based on a simple negative feedback loop. However, not many 'simple' feedback loops in biology last for 24 h. Instead, the Drosophila clock is a series of precisely timed steps that are deliberately slow. In this paper, I will discuss the current model for how the Drosophila clock is regulated, and ask what questions remain to be answered.     

Mammalian circadian signaling networks and therapeutic targets

Virtually all cells in the body have an intracellular clockwork based on a negative feedback mechanism. The circadian timekeeping system in mammals is a hierarchical multi-oscillator network, with the suprachiasmatic nuclei (SCN) acting as the central pacemaker. The SCN synchronizes to daily light-dark cycles and coordinates rhythmic physiology and behavior. Synchronization in the SCN and at the organismal level is a key feature of the circadian clock system. In particular, intercellular coupling in the SCN synchronizes neuron oscillators and confers robustness against perturbations. Recent advances in our knowledge of and ability to manipulate circadian rhythms make available cell-based clock models, which lack strong coupling and are ideal for target discovery and chemical biology.     

Reproductive cycles in tropical intertidal gastropods are timed around tidal amplitude cycles

Reproduction in iteroparous marine organisms is often timed with abiotic cycles and may follow lunar, tidal amplitude, or daily cycles. Among intertidal marine invertebrates, decapods are well known to time larval release to coincide with large amplitude nighttime tides, which minimizes the risk of predation. Such bimonthly cycles have been reported for few other intertidal invertebrates. We documented the reproduction of 6 gastropod species from Panama to determine whether they demonstrate reproductive cycles, whether these cycles follow a 2‐week cycle, and whether cycles are timed so that larval release occurs during large amplitude tides. Two of the species (Crepidula cf. marginalis and Nerita scabricosta) showed nonuniform reproduction, but without clear peaks in timing relative to tidal or lunar cycles. The other 4 species show clear peaks in reproduction occurring every 2 weeks. In 3 of these species (Cerithideopsis carlifornica var. valida, Littoraria variegata, and Natica chemnitzi), hatching occurred within 4 days of the maximum amplitude tides. Siphonaria palmata exhibit strong cycles, but reproduction occurred during the neap tides. Strong differences in the intensity of reproduction of Cerithideopsis carlifornica, and in particular, Littoraria variegata, between the larger and smaller spring tides of a lunar month indicate that these species time reproduction with the tidal amplitude cycle rather than the lunar cycle. For those species that reproduce during both the wet and dry seasons, we found that reproductive timing did not differ between seasons despite strong differences in temperature and precipitation. Overall, we found that most (4/6) species have strong reproductive cycles synchronized with the tidal amplitude cycle and that seasonal differences in abiotic factors do not alter these cycles.     

Going beyond the limits: effect of clock disruption on human health

Abstract

Synchronisation of organisms’ physiology and behaviour with the external environment is necessary for survival and reproductive fitness. This is critical for human health also. In the past, humans were exposed to predictable natural day and night cycles that allowed the internal clock to synchronise the daily rhythms in physiology and behaviour with the external environment. However, the industrial revolution has made us a 24*7 society and forced the extension of day into night via adoption of artificial light in our lives. This has altered the perception of day and night and made it difficult for the biological processes to synchronise. Such weak synchronisation can be seen in different physiological and behavioural functions that are under circadian control, such as sleep–wake behaviour, melatonin and cortisol rhythms, core body temperature cycle, etc. This also influences the regulatory mechanism at cell and gene levels. Circadian disruption has resulted in increasing incidences of certain cancers, metabolic dysfunction and mood disorders. Several evidence suggest that exposure to aberrant light alters the brain functions that regulate emotion and mood. The present discussion focuses on understanding the effect of circadian disruption on human health, and its various aspects.     

Host phenology regulates parasite–host demographic cycles and eco‐evolutionary feedbacks

Parasite–host interactions can drive periodic population dynamics when parasites overexploit host populations. The timing of host seasonal activity, or host phenology, determines the frequency and demographic impact of parasite–host interactions, which may govern whether parasites sufficiently overexploit hosts to drive population cycles. We describe a mathematical model of a monocyclic, obligate‐killer parasite system with seasonal host activity to investigate the consequences of host phenology on host–parasite dynamics. The results suggest that parasites can reach the densities necessary to destabilize host dynamics and drive cycling as they adapt, but only in some phenological scenarios such as environments with short seasons and synchronous host emergence. Furthermore, only parasite lineages that are sufficiently adapted to phenological scenarios with short seasons and synchronous host emergence can achieve the densities necessary to overexploit hosts and produce population cycles. Host‐parasite cycles also generate an eco‐evolutionary feedback that slows parasite adaptation to the phenological environment as rare advantageous phenotypes can be driven extinct due to a population bottleneck depending on when they are introduced in the cycle. The results demonstrate that seasonal environments can drive population cycling in a restricted set of phenological patterns and provide further evidence that the rate of adaptive evolution depends on underlying ecological dynamics.     

Foundations of circadian medicine

The circadian clock is an evolutionarily highly conserved endogenous timing program that structures physiology and behavior according to the time of day. Disruption of circadian rhythms is associated with many common pathologies. The emerging field of circadian medicine aims to exploit the mechanisms of circadian physiology and clock–disease interaction for clinical diagnosis, treatment, and prevention. In this Essay, we outline the principle approaches of circadian medicine, highlight the development of the field in selected areas, and point out open questions and challenges. Circadian medicine has unambiguous health benefits over standard care but is rarely utilized. It is time for clock biology to become an integrated part of translational research.

The emerging field of circadian medicine implements and translates findings from clock biology to improve health. Circadian medicine has clear health benefits over standard care but is rarely used owing to a lack of systematic and mechanistic evidence and overarching concepts. This Essay explains the principles of circadian medicine and highlights future approaches to promote its dissemination.     

Emergence of circadian and photoperiodic system level properties from interactions among pacemaker cells

Daily patterns of behavior and physiology in animals in temperate zones often differ substantially between summer and winter. In mammals, this may be a direct consequence of seasonal changes of activity of the suprachiasmatic nucleus (SCN). The purpose of this study was to understand such variation on the basis of the interaction between pacemaker neurons. Computer simulation demonstrates that mutual electrical activation between pacemaker cells in the SCN, in combination with cellular electrical activation by light, is sufficient to explain a variety of circadian phenomena including seasonal changes. These phenomena are: self-excitation, that is, spontaneous development of circadian rhythmicity in the absence of a light-dark cycle; persistent rhythmicity in constant darkness, and loss of circadian rhythmicity in pacemaker output in constant light; entrainment to light-dark cycles; aftereffects of zeitgeber cycles with different periods; adjustment of the circadian patterns to day length; generation of realistic phase response curves to light pulses; and relative independence from day-to-day variation in light intensity. In the model, subsets of cells turn out to be active at specific times of day. This is of functional importance for the exploitation of the SCN to tune specific behavior to specific times of day. Thus, a network of on-off oscillators provides a simple and plausible construct that behaves as a clock with readout for time of day and simultaneously as a clock for all seasons.     

New insights into ancient seasonal life timers

Organisms must adapt to seasonal changes in the environment and time their physiology accordingly. In vertebrates, the annual change in photoperiod is often critical for entraining the neuroendocrine pathways, which drive seasonal metabolic and reproductive cycles. These cycles depend on thyroid hormone (TH), reflecting its ancestral role in metabolic control. Recent studies reveal that - in mammals and birds - TH effects are mediated by the hypothalamus. Photoperiodic manipulations alter hypothalamic TH availability by regulating the expression of TH deiodinases (DIO). In non-mammalian vertebrates, light acts through extraretinal, 'deep brain' photoreceptors, and the eyes are not involved in seasonal photoperiodic responses. In mammals, extraretinal photoreceptors have been lost, and the nocturnal melatonin signal generated from the pineal gland has been co-opted to provide the photoperiodic message. Pineal function is phased to the light-dark cycle by retinal input, and photoperiodic changes in melatonin secretion control neuroendocrine pathway function. New evidence indicates that these comparatively divergent photosensensory mechanisms re-converge in the pars tuberalis of the pituitary, lying beneath the hypothalamus. In all vertebrates studied, the pars tuberalis secretes thyrotrophin in a light- or melatonin-sensitive manner, to act on neighbouring hypothalamic DIO expressing cells. Hence, an ancient and fundamentally conserved brain thyroid signalling system governs seasonal biology in vertebrates.     

Circadian clock gene expression in the coral Favia fragum over diel and lunar reproductive cycles

Natural light cycles synchronize behavioral and physiological cycles over varying time periods in both plants and animals. Many scleractinian corals exhibit diel cycles of polyp expansion and contraction entrained by diel sunlight patterns, and monthly cycles of spawning or planulation that correspond to lunar moonlight cycles. The molecular mechanisms for regulating such cycles are poorly understood. In this study, we identified four molecular clock genes (cry1, cry2, clock and cycle) in the scleractinian coral, Favia fragum, and investigated patterns of gene expression hypothesized to be involved in the corals' diel polyp behavior and lunar reproductive cycles. Using quantitative PCR, we measured fluctuations in expression of these clock genes over both diel and monthly spawning timeframes. Additionally, we assayed gene expression and polyp expansion-contraction behavior in experimental corals in normal light:dark (control) or constant dark treatments. Well-defined and reproducible diel patterns in cry1, cry2, and clock expression were observed in both field-collected and the experimental colonies maintained under control light:dark conditions, but no pattern was observed for cycle. Colonies in the control light:dark treatment also displayed diel rhythms of tentacle expansion and contraction. Experimental colonies in the constant dark treatment lost diel patterns in cry1, cry2, and clock expression and displayed a diminished and less synchronous pattern of tentacle expansion and contraction. We observed no pattern in cry1, cry2, clock, or cycle expression correlated with monthly spawning events suggesting these genes are not involved in the entrainment of reproductive cycles to lunar light cycles in F. fragum. Our results suggest a molecular clock mechanism, potentially similar to that in described in fruit flies, exists within F. fragum.     

Understanding circadian rhythmicity in Neurospora crassa: from behavior to genes and back again

Circadian clocks have been described in organisms ranging in complexity from unicells to mammals, in which they function to control daily rhythms in cellular activities and behavior. The significance of a detailed understanding of the clock can be appreciated by its ubiquity and its established involvement in human physiology, including endocrine function, sleep/wake cycles, psychiatric illness, and drug tolerances and effectiveness. Because the clock in all organisms is assembled within the cell and clock mechanisms are evolutionarily conserved, simple eukaryotes provide appropriate experimental systems for dissecting the clock. Significant progress has been made in deciphering the circadian system in Neurospora crassa using both genetic and molecular approaches, and Neurospora has contributed greatly to our understanding of (1) the feedback cycle that comprises a circadian oscillator, (2) the mechanisms by which the clock is kept in synchrony with the environment, and (3) the genes that reside in rhythmic output pathways. Importantly, the lessons learned in Neurospora are relevant to our understanding of clocks in higher eukaryotes.