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In vertebrate somitogenesis, the expression of segmentation clock genes oscillates and the oscillation is synchronized over nearby cells. Both experimental and theoretical studies have shown that the synchronization among cells is realized by intercellular interaction via Delta-Notch signaling. However, the following questions emerge: (i) During somitogenesis, dynamic rearrangement of relative cell positions is observed in the posterior presomitic mesoderm. Can a synchronized state be stably sustained under random cell movement? (ii) Experimental studies have reported that the synchronization of cells can be recovered in about 10 or fewer oscillation cycles after the complete loss of synchrony. However, such a quick recovery of synchronization is not possible according to previous theoretical models. In this paper, we first show by numerical modeling that synchronized oscillation can be sustained under random cell movement. We also find that for initial perturbation, the synchronization of cells is recovered much faster and it is for a wider range of reaction parameters than the case without cell movement. When the posterior presomitic mesoderm is rectangular, faster synchronization is achieved if ceils exchange their locations more with neighbors located along the longer side of the domain. Finally, we discuss that the enhancement of synchronization by random cell movement occurs in several different models for the oscillation of segmentation clock genes. zebrafish | somitogenesis | Delta-Notch | mathematical models doi/ 10.1073/pnas.0907122107

Phase-amplitude cross-frequency coupling (CFC) between theta (4-12 Hz) and gamma (30-100 Hz) oscillations occurs frequently in the hippocampus. However, it still remains unclear whether theta--gamma coupling has any functional significance. To address this issue, we studied CFC in local field potential oscillations recorded from the CA3 region of the dorsal hippocampus of rats as they learned to associate items with their spatial context. During the course of learning, the amplitude of the low gamma subband (30-60 Hz) became more strongly modulated by theta phase in CA3, and higher levels of theta--gamma modulation were maintained throughout overtraining sessions. Furthermore, the strength of theta--gamma coupling was directly correlated with the increase in performance accuracy during learning sessions. These findings suggest a role for hippocampal theta--gamma coupling in memory recall. associative memory | brain rhythms | local field potential doi/10.1073/pnas.0911331106

Understanding how nutrients affect gene expression will help us to understand the mechanisms controlling plant growth and development as a function of nutrient availability. Nitrate has been shown to serve as a signal for the control of gene expression in Arabidopsis. There is also evidence, on a gene-by-gene basis, that downstream products of nitrogen (N) assimilation such as glutamate (Glu) or glutamine (Gin) might serve as signals of organic N status that in turn regulate gene expression. To identify genome-wide responses to such organic N signals, Arabidopsis seedlings were transiently treated with ammonium nitrate in the presence or absence of MSX, an inhibitor of glutamine synthetase, resulting in a block of Glu/GIn synthesis. Genes that responded to organic N were identified as those whose response to ammonium nitrate treatment was blocked in the presence of MSX. We showed that some genes previously identified to be regulated by nitrate are under the control of an organic N-metabolite. Using an integrated network model of molecular interactions, we uncovered a subnetwork regulated by organic N that included CCA1 and target genes involved in N-assimilation. We validated some of the predicted interactions and showed that regulation of the master clock control gene CCA1 by Glu or a Glu-derived metabolite in turn regulates the expression of key N-assimilatory genes. Phase response curve analysis shows that distinct N-metabolites can advance or delay the CCA1 phase. Regulation of CCA 1 by organic N signals may represent a novel input mechanism for N-nutrients to affect plant circadian clock function. circadian | gene networks | glutamate | metabolism

Many social interactions rely upon mutual information exchange: one member of a pair changes in response to the other while at the same time producing actions that alter the behavior of the other. However, little is known about how such social processes are integrated in the brain. Here, we used a specially designed dual-electroencephalogram system and the conceptual framework of coordination dynamics to identify neural signatures of effective, real-time coordination between people and its breakdown or absence. High-resolution spectral analysis of electrical brain activity before and during visually mediated social coordination revealed a marked depression in occipital alpha and rolandic mu rhythms during social interaction that was independent of whether behavior was coordinated or not. In contrast, a pair of oscillatory components ([phi.sub.1] and [phi.sub.2]) located above right centroparietal cortex distinguished effective from ineffective coordination: increase of [phi.sub.1] favored independent behavior and increase of [phi.sub.2] favored coordinated behavior. The topography of the phi complex is consistent with neuroanatomical sources within the human mirror neuron system. A plausible mechanism is that the phi complex reflects the influence of the other on a person's ongoing behavior, with [phi.sub.1] expressing the inhibition of the human mirror neuron system and [phi.sub.2] its enhancement. brain oscillations | electroencephalography | mirror neuron system | phi rhythm | coordination dynamics

Accuracy in cellular function has to be achieved despite random fluctuations (noise) in the concentrations of different molecular constituents inside and outside the cell. The circadian oscillator in cyanobacteria is an example of resilience to noise. This resilience could be either the consequence of intercellular communication or the intrinsic property of the built-in biochemical network. Here we investigate the intercellular coupling hypothesis. A short theoretical depiction of interacting noisy phase oscillators, confirmed by numerical simulations, allows us to discriminate the effect of coupling from noise. Experimentally, by studying the phase of concurrent populations of different initial phases, we evaluate a very small upper limit of the intercellular coupling strength. In addition, in situ entrainment experiments confirm our ability to detect a coupling of the circadian oscillator to an external force and to describe explicitly the dynamic change of the mean phase. We demonstrate, therefore, that the cyanobacterial clock stability is a built-in property as the intercellular coupling effect is negligible. biological clock | bioluminescence | dynamics | synechoccocus

The diurnal variation in the incidence of myocardial infarction and stroke may reflect an influence of the molecular clock and/or the time dependence of exposure to environmental stress. The circadian variation in blood pressure and heart rate is disrupted in mice, Bmal[1.sup.-/-], [Clock.sup.mut], and Npas[2.sup.mut], in which core clock genes are deleted or mutated. Although Bmal1 deletion abolishes the 24-h frequency in cardiovascular rhythms, a shorter ultradian rhythm remains. Sympathoadrenal function is disrupted in these mice, which reflects control of enzymes relevant to both synthesis (phenylethanolamine N-methyl transferase) and disposition (monoamine oxidase B and catechoI-O-methyl transferase) of catecholamines by the clock. Both timing and disruption or mutation of clock genes modulate the magnitude of both the sympathoadrenal and pressor but not the adrenocortical response to stress. Despite diurnal variation of catecholamines and corticosteroids, they are regulated differentially by the molecular clock. Furthermore, the clock may influence the time-dependent incidence of cardiovascular events by controlling the integration of selective asynchronous stress responses with an underlying circadian rhythm in cardiovascular function. catecholamine | clock

Time-delay circuitries in which a transcription factor processes independent input parameters can modulate NF-[kappa]B activation, manage quorum-sensing cross-talk, and control the circadian clock. We have constructed a synthetic mammalian gene network that processes four different input signals to control either immediate or time-delayed transcription of specific target genes. BirA-mediated ligation of biotin to a biotinylation signal-containing VP16 transactivation domain triggers heterodimerization of chimeric VP16 to a streptavidin-linked tetracycline repressor (TetR). At increasing biotin concentrations up to 20 nM, TetR-specific promoters are gradually activated (off to on, input signal 1), are maximally induced at concentrations between 20 nM and 10 [micro]M, and are adjustably shut off at biotin levels exceeding 10 [micro]M (on to off, input signal 2). These specific expression characteristics with a discrete biotin concentration window emulate a biotin-triggered bandpass filter. Removal of biotin from the culture environment (input signal 3) results in time-delayed transgene expression until the intracellular biotinylated VP16 pool is degraded. Because the TetR component of the chimeric transactivator retains its tetracycline responsiveness, addition of this antibiotic (input signal 4) overrides biotin control and immediately shuts off target gene expression. Biotin-responsive immediate, bandpass filter, and time-delay transcription characteristics were predicted by a computational model and have been validated in standard cultivation settings or biopharmaceutical manufacturing scenarios using trangenic CHO-K1 cell derivatives and have been confirmed in mice. Synthetic gene circuitries provide insight into structure-function correlations of native signaling networks and foster advances in gene therapy and biopharmaceutical manufacturing. biological circuit | biopharmaceutical manufacturing | gene switch | synthetic biology | biotin

KaiA, KaiB, and KaiC clock proteins from cyanobacteria and ATP are sufficient to reconstitute the KaiC phosphorylation rhythm in vitro, whereas almost all gene promoters are under the control of the circadian clock. The mechanism by which the KaiC phosphorylation cycle drives global transcription rhythms is unknown. Here, we report that RpaA, a potential DNA-binding protein that acts as a cognate response regulator of the KaiC-interacting kinase SasA, mediates between KaiC phosphorylation and global transcription rhythms. Circadian transcription was severely attenuated in sasA (Synechococcus adaptive sensor A)- and rpaA (regulator of phycobilisome-associated)-mutant cells, and the phosphotransfer activity from SasA to RpaA changed dramatically depending on the circadian state of a coexisting Kai protein complex in vitro. We propose a model in which the SasA-RpaA two-component system mediates time signals from the enzymatic oscillator to drive genome-wide transcription rhythms in cyanobacteria. Moreover, our results indicate the presence of secondary output pathways from the clock to transcription control, suggesting that multiple pathways ensure a genome-wide circadian system. biological clock | response regulator | phosphorelay | Synechococcus

Pituitary growth hormone (GH)-secreting cells regulate growth and metabolism in animals and humans. To secrete highly ordered GH pulses (up to 1,000-fold rise in hormone levels in vivo), the pituitary GH cell population needs to mount coordinated responses to GH secretagogues, yet GH cells display an apparently heterogeneous scattered distribution in 2D histological studies. To address this paradox, we analyzed in 3D both positioning and signaling of GH cells using reconstructive, two-photon excitation microscopy to image the entire pituitary in GH-EGFP transgenic mice. Our results unveiled a homologous continuum of GH cells connected by adherens junctions that wired the whole gland and exhibited the three primary features of biological networks: robustness of architecture across lifespan, modularity correlated with pituitary GH contents and body growth, and connectivity with spatially stereotyped motifs of cell synchronization coordinating cell activity. These findings change our view of GH cells, from a collection of dispersed cells to a geometrically connected homotypic network of cells whose local morphology and connectivity can vary, to alter the timing of cellular responses to promote more coordinated pulsatile secretion. This large-scale 3D view of cell functioning provides a powerful approach to identify and understand other networks of endocrine cells that are thought to be scattered in situ. Many dispersed endocrine systems exhibit pulsatile outputs. We suggest that cell positioning and associated cell-cell connection mechanisms will be critical parameters that determine how well such systems can deliver a coordinated secretory pulse of hormone to their target tissues. biological rhythms | endocrinology | systems biology | connectivity | calcium

There is pronounced seasonal plasticity in the morphology of the neural circuits that regulate song behavior in adult songbirds, primarily in response to changes in plasma testosterone (T) levels. Most song nuclei have androgen receptors. Afferent input from the telencephalic nucleus HVc (also known as the 'high vocal center') is necessary for seasonal growth of the direct efferent target nuclei RA and area X. We asked here whether T-stimulated growth of HVc is sufficient to induce growth of its efferent nuclei. Intracerebral T implants were placed unilaterally near HVc or RA in photosensitive adult male white-crowned sparrows for one month. The T implant near HVc produced significant growth of the ipsilateral (but not contralateral) HVc, RA, and area X, and increased neuronal number in the ipsilateral HVc. The T implant near RA did not produce selective growth of ipsilateral RA, HVc, or area X. Intracerebral T implants did not elevate plasma T levels, nor did they stimulate growth of two peripheral androgen sensitive targets, the syrinx and the cloacal protuberance. These results suggest that seasonal growth of the adult song circuits results from T acting directly on HVc, which then stimulates the growth of RA and area X transynaptically.

Each year more than 250,000 infants in the United States are exposed to artificial lighting in hospital nurseries with little consideration given to environmental lighting cycles. Essential in determining whether environmental lighting cycles need to be considered in hospital nurseries is identifying when the infant's endogenous circadian clock becomes responsive to light. Using a non-human primate model of the developing human, we examined when the circadian clock, located in the hypothalamic suprachiasmatic nuclei (SCN), becomes responsive to light. Preterm infant baboons of different ages were exposed to light (5,000 lux) at night, and then changes in SCN metabolic activity and gene expression were assessed. After exposure to bright light at night, robust increases in SCN metabolic activity and gene expression were seen at ages that were equivalent to human infants at 24 weeks after conception. These data provide direct evidence that the biological clock of very premature primate infants is responsive to light.

Circadian rhythms control many physiological activities. The environmental entrainment of rhythms involves the immediate responses of dock components. Levels of the dock protein FRQ were measured in Neurospora at various temperatures; at higher temperatures, the amount of FRQ oscillated around higher levels. Absolute FRQ amounts thus identified different times at different temperatures, so temperature shifts corresponded to shifts in dock time without immediate synthesis or turnover of components. Moderate temperature changes could dominate light-to-dark shifts in the influence of circadian timing. Temperature regulation of dock components could explain temperature resetting of rhythms and how single transitions can initiate rhythmicity from characteristic circadian phases., Common elements are emerging in the molecular mechanisms of circadian rhythms and in the ways that these mechanisms respond to environmental cues, including light and temperature (1-6). Phase resetting by [...]

The circadian oscillator generates a rhythmic output with a period of about 24 hours. Despite extensive studies in several model systems, the biochemical mode of action has not yet been demonstrated for any of its components. Here, the Drosophila CLOCK protein was shown to induce transcription of the circadian rhythm genes period and timeless. dCLOCK functioned as a heterodimer with a Drosophila homolog of BMAL1. These proteins acted through an E-box sequence in the period promoter. The timeless promoter contains an 18-base pair element encompassing an E-box, which was sufficient to confer dCLOCK responsiveness to a reporter gene. PERIOD and TIMELESS proteins blocked dCLOCK's ability to transactivate their promoters via the E-box. Thus, dCLOCK drives expression of period and timeless, which in turn inhibit dCLOCK's activity and close the circadian loop., In animals, plants, or prokaryotes, activities such as locomotion or photosynthesis do not occur with equal probability throughout the 24-hour day but are organized by an endogenous circadian oscillator. The [...]

Animals engage in a startlingly diverse array of behaviours that depend critically on the time of day or the ability to time short intervals. Timing intervals on the scale of many hours to around a day is mediated by the circadian timing system, while in the range of seconds to hours a different system, known as interval timing, is used. Recent research has illuminated some of the neural mechanisms underlying the 'internal clocks' of these two different timing systems in both animals and humans. Therapeutic applications for humans with impairments in either timing system may ultimately result from these endeavours.

Hibernation patterns were monitored continuously for 2.5 years in female squirrels that were neurologically intact or in which the hypothalamic suprachiasmatic nucleus (SCN) was completely ablated (SCNx). The number of hibernation bouts in SCNx squirrels increased by 159%, total hibernation time increased by 58%, and periodic arousals from hibernation were 47% longer in SCNx than in control squirrels; the duration of individual torpor bouts was 2 days shorter and far more variable in SCNx than in control animals. Some SCNx squirrels cycled through bouts of torpor continuously for nearly 2 years. The SCN appears to be part of the mechanism that controls the duration of the hibernation season and the temporal structure of individual torpor bouts.

Prothoracicotropic hormone (PTTH) is the central cerebral neurohormone in insect development. Its release has been believed for decades to be confined to one (or two) critical moments early in each developmental stage at which time it triggers prolonged activation of the prothoracic glands to synthesize and release the steroid molting hormones (ecdysteroids), which elicit developmental responses in target tissues. We used an in vitro assay for PTTH released from excised brains of the bug Rhodnius prolixus and report that release of PTTH does occur at the expected time on day 6, but that this release is merely the first in a daily rhythm of release that continues throughout most of the 21 days of larval-adult development. This finding, together with reports of circadian control of ecdysteroid synthesis and titer throughout this time, raises significant challenges to several features of the current understanding of the hormonal control of insect development. New questions are raised concerning the function(s) of PTTH, its relationship with the prothoracic glands, and the significance of circadian rhythmicity throughout this endocrine axis. The significance of the reported observations derives from the set of entirely new questions they raise concerning the regulation of insect development.

Localization of the central rhythm generator (CRG) of spontaneous consummatory licking was studied in freely moving rats by microinjection of tetrodotoxin (TTX) into the pontine reticular formation. Maximum suppression of spontaneous water consumption was elicited by TTX (1 ng) blockade of the oral part of the nucleus reticularis giganto-cellularis (NRG), whereas TTX injections into more caudal or rostral locations caused significantly weaker disruption of drinking. To verify the assumption that TTX blocked the proper CRG of licking rather than some relay in its output, spontaneously drinking thirsty rats were intracranially stimulated via electrodes chronically implanted into the oral part of the NRG. Lick-synchronized stimulation (a 100-ms train of 0.1-ms-wide rectangular pulses at 100 Hz and 25-150 [Mu]A) applied during continuous licking (after eight regular consecutive licks) caused a phase shift of licks emitted after stimulus delivery. The results suggest that the stimulation has reset the CRG of licking without changing its frequency. The reset-inducing threshold current was lowest during the tongue retraction and highest during the tongue protrusion period of the lick cycle. It is concluded that the CRG of licking is located in the oral part of NRG.

Substantial progress has been made in understanding the genetic basis of temperature-compensated circadian clocks. Ultradian rhythms, with a period shorter than 24 h, are at least as widespread as circadian rhythms. We have initiated genetic analysis of defecation behavior, which is controlled by an ultradian clock in Caenorhabditis elegans. The defecation motor program is activated every 45 see, and this rhythm is temperature compensated. We describe mutations in 12 genes that either shorten or lengthen the cycle period. We find that most of these mutations also disrupt temperature compensation, suggesting that this process is an integral part of the clock. These genes open the way for molecular genetic dissection of this ultradian clock.

The discovery of a mouse gene responsible for daily cycles of activity ushers in a new era in mammalian genetics Most time-keeping systems are based on the sun, reflecting age-old [...]

IN 1729 THE FRENCH ASTRONOMER JEAN-Jacques de Mairan tested a phenomenon that had intrigued observers since ancient times: the leaves of certain plants regularly open in the daytime and then [...]

The hypothesis that the biological clock is based on proteins and the events regulating protein synthesis was investigated. The study protocol involved the treatment of the ocular circadian pacemaker of the the mollusc Bulla gouldiana with the protein synthesis inhibitors anisomycin and cycloheximide. The results showed that the free-running period of the circadianrhythm was significantly lenghtened. Furthermore, long pulse treatments of various lengths affected the rhythm in a phase-dependent manner. These results indicate a role for protein synthesis in the regulation of the circadian pacemaker.

The location of neurons generating the rhythm of breathing in mammals is unknown. By microsection of the neonatal rat brainstem in vitro, a limited region of the ventral medulla (the pre-Botzinger Complex) that contains neurons essential for rhythmogenesis was identified. Rhythm generation was eliminated by removal of only this region. Medullary slices containing the pre-Botzinger Complex generated respiratory-related oscillations similar to those generated by the whole brainstem in vitro, and neurons with voltage-dependent pacemaker-like properties were identified in this region. Thus, the respiratory rhythm in the mammalian neonatal nervous system may result from a population of conditional bursting pacemaker neurons in the pre-Botzinger Complex., THE RHYTHM OF BREATHING ANIMATES mammalian life, and the source of this rhythm, the noeud vital [1], is unknown. The basic oscillator lies within the brainstem, but technical limitations of [...]

A convenient and precise mass spectrometric method for measurement of the deamidation rates of glutaminyl and asparaginyl residues in peptides and proteins has been developed; the rates of deamidation of 306 asparaginyl sequences in model peptides at pH 7.4, 37.0 [degrees] C, 0.15 M Tris-HCl buffer have been determined; a library of 913 amide-containing peptides for use by other investigators in similar studies has been established; and, by means of simultaneous deamidation rate measurements of rabbit muscle aldolase and appropriate model peptides in the same solutions, the use of this method for quantitative measurement of the relative effects of primary, secondary, tertiary, and quaternary protein structure on deamidation rates has been demonstrated. The measured rates are discussed with respect to the hypothesis that glutaminyl and asparaginyl residues serve, through deamidation, as molecular timers of biological events. deamidation | biological clocks | peptides | mass spectrometry

As a grandfather clock keeps time with an oscillating pendulum, the 24-hour rhythm of the biological clock is also maintained by oscillations--in this case by oscillating levels of proteins. But [...]

Mice lacking mCry1 and mCry2 are behaviorally arrhythmic. As shown here, cyclic expression of the clock genes mPer7 and mPer2 (mammalian Period genes 1 and 2) in the suprachiasmatic nucleus and peripheral tissues is abolished and mPer7 and mPer2 mRNA levels are constitutively high. These findings indicate that the biological dock is eliminated in the absence of both mCRY1 and mCRY2 (mammalian cryptochromes 1 and 2) and support the idea that mammalian CRY proteins act in the negative limb of the circadian feedback loop. The mCry double-mutant mice retain the ability to have mPer7 and mPer2 expression induced by a brief light stimulus known to phase-shift the biological dock in wild-type animals. Thus, mCRY1 and mCRY2 are dispensable for light-induced phase shifting of the biological clock., Many physiological and behavioral systems are controlled by an internal self-sustaining molecular oscillating mechanism with a periodicity of approximately 24 hours, known as the biological clock. The core oscillator consists [...]

Regulation of circadian period in humans was thought to differ from that of other species, with the period of the activity rhythm reported to range from 13 to 65 hours (median 25.2 hours) and the period of the body temperature rhythm reported to average 25 hours in adulthood, and to shorten with age. However, those observations were based on studies of humans exposed to light levels sufficient to confound circadian period estimation. Precise estimation of the periods of the endogenous circadian rhythms of melatonin, core body temperature, and cortisol in healthy young and older individuals living in carefully controlled lighting conditions has now revealed that the intrinsic period of the human circadian pacemaker averages 24.18 hours in both age groups, with a tight distribution consistent with other species. These findings have important implications for understanding the pathophysiology of disrupted sleep in older people., Natural selection has favored endogenous circadian rhythmicity that, in the absence of periodic synchronizing cues from the environment, persists with an intrinsic period close to that of Earth's rotation in [...]

The behavior of humans, like that of other animals, is characterized by precise 24-hour cycles of rest and activity, sleep and wakefulness. These cycles, termed a circadian rhythm, represent a [...]

Diel Periodicity of Photosynthesis in Polar Phytoplankton: Influence on Primary Production THE SOUTHERN OCEAN SUPPORTS large populations of zooplankton, marine mammals, and seabirds (1) and is the site of approximately [...]

The nature of the clock genes that control daily fluctuations in an organism's behaviour and physiology is ill understood. Four new studies investigate the circadian oscillator and show how genes may make the clock tick in mammals and flies. It is shown that the circadian clock comprises an input pathway, a central pacemaker and an output pathway.

Circadian rhythms and the cellular oscillators that underlie them are ubiquitous---and for good reason. For most organisms, dawn means food (either fixing carbon or hunting prey), predation, and changes in [...]

The molecular clock not only controls the rhythmic expression of genes with physiological roles, but also regulates the creation of ribosomes--molecular machines that translate messenger RNA into protein. Frederic Gachon [...]

For male and female ring doves, which incubate their eggs in alternating shifts, domestic bliss is a matter of timing. Males arrive for their stint in the morning and give [...]

Cryptochrome, a light-absorbing molecule first discovered in plants, apparently helps light to set the daily clocks of Arabidopsis, fruit flies, and mice You arrive in a new time zone, go [...]

Biological clocks are turning up all over, and in the most unexpected places (see p. 1560). But they all typically keep to a 24-hour schedule, which is logical because it [...]