Stopping the circadian pacemaker with inhibitors of protein synthesis

Parental uveitis causes elevated hair loss in offspring of C57BL/6J mice

Our previous study demonstrated that parental uveitis in a susceptible population can cause hair loss and increase the susceptibility to experimental autoimmune uveitis (EAU) in offspring. However, it is unclear whether parental uveitis affects the development of offspring in an EAU-moderate-susceptible population. Herein, moderate-susceptible C57BL/6J mice were immunized with inter-photoreceptor retinoid binding protein (IRBP) 651-670 to develop EAU and were kept together for mating. Gross examination and histopathological changes of the offspring gestated with parental uveitis were observed to evaluate the impact of parental uveitis on the development of the offspring. Differentially expressed genes (DEGs) were screened by RNA sequencing in the affected skin and eyeball of the offspring on postnatal day 27. Adult offspring were injected 75 μg IRBP_651-670 to evaluate their susceptibility to EAU. Gross examination in the offspring revealed hair loss on postnatal days 11-31. Histopathological observation showed increased melanin granules and hair follicles of skin in the affected offspring with hair loss. Gene Ontology (GO) analysis in the skin revealed differential expression of genes involved in the mitotic cell cycle, response to endogenous stimulus, hair follicle development, and hair cycle. The DEGs in the skin were predominately associated with the cell cycle and peroxisome proliferator-activated receptor (PPAR) signaling pathway. The GO enrichment analysis in the eyeball showed differential expression of genes involved in the nervous system development, camera-type eye photoreceptor cell differentiation, neuron projection morphogenesis, axon development, and calcium-induced calcium release activity; enriched pathways included the circadian entrainment and glutamatergic synapses. No increased susceptibility to EAU in offspring gestated from parental remitting EAU was observed at a low-dose 75 μg IRBP induction. These results suggested that parental uveitis in a moderate-susceptible population could affect the skin development and DEG profiles of skin and eyeball related to the response to endogenous stimulus, the PPAR signaling pathway, and glutamatergic synapse, which provides the molecular evidence to explain the influence of parental uveitis on offspring development.

Neural oscillations are a start toward understanding brain activity rather than the end

Does rhythmic neural activity merely echo the rhythmic features of the environment, or does it reflect a fundamental computational mechanism of the brain? This debate has generated a series of clever experimental studies attempting to find an answer. Here, we argue that the field has been obstructed by predictions of oscillators that are based more on intuition rather than biophysical models compatible with the observed phenomena. What follows is a series of cautionary examples that serve as reminders to ground our hypotheses in well-developed theories of oscillatory behavior put forth by theoretical study of dynamical systems. Ultimately, our hope is that this exercise will push the field to concern itself less with the vague question of "oscillation or not" and more with specific biophysical models that can be readily tested.

The eIF2α Kinase GCN2 Modulates Period and Rhythmicity of the Circadian Clock by Translational Control of Atf4

The integrated stress response (ISR) is activated in response to diverse stress stimuli to maintain homeostasis in neurons. Central to this process is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α). Here, we report a critical role for ISR in regulating the mammalian circadian clock. The eIF2α kinase GCN2 rhythmically phosphorylates eIF2α in the suprachiasmatic circadian clock. Increased eIF2α phosphorylation shortens the circadian period in both fibroblasts and mice, whereas reduced eIF2α phosphorylation lengthens the circadian period and impairs circadian rhythmicity in animals. Mechanistically, phosphorylation of eIF2α promotes mRNA translation of Atf4. ATF4 binding motifs are identified in multiple clock genes, including Per2, Per3, Cry1, Cry2, and Clock. ATF4 binds to the TTGCAGCA motif in the Per2 promoter and activates its transcription. Together, these results demonstrate a significant role for ISR in circadian physiology and provide a potential link between dysregulated ISR and circadian dysfunction in brain diseases.

Translational thresholds in a core circadian clock model

Organisms have evolved in a daily cyclic environment, developing circadian cell-autonomous clocks that temporally organize a wide range of biological processes. Translation is a highly regulated process mainly associated with the activity of microRNAs (miRNAs) at the translation initiation step that impacts on the molecular circadian clock dynamics. Recently, a molecular titration mechanism was proposed to explain the interactions between some miRNAs and their target mRNAs; new evidence also indicates that regulation by miRNA is a nonlinear process such that there is a threshold level of target mRNA below which protein production is drastically repressed. These observations led us to use a theoretical model of the circadian molecular clock to study the effect of miRNA-mediated translational thresholds on the molecular clock dynamics. We model the translational threshold by introducing a phenomenological Hill equation for the kinetics of PER translation and show how the parameters associated with translation kinetics affect the period, amplitude, and time delays between clock mRNA and clock protein expression. We show that our results are useful for analyzing experiments related to the translational regulation of negative elements of transcriptional-translational feedback loops. We also provide new elements for thinking about the translational threshold as a mechanism that favors the emergence of circadian rhythmicity, the tuning of the period-delay relationship and the cell capacity to control the protein oscillation amplitude with almost negligible changes in the mRNA amplitudes.

Theoretical investigation on models of circadian rhythms based on dimerization and proteolysis of PER and TIM

Circadian rhythms of physiology and behavior are widespread\break mechanisms in many organisms. The internal biological rhythms are driven by molecular clocks, which oscillate with a period nearly but not exactly 24 hours. Many classic models of circadian rhythms are based on a time-delayed negative feedback, suggested by the protein products inhibiting transcription of their own genes. In 1999, based on stabilization of PER upon dimerization, Tyson et al. [J. J. Tyson, C. I. Hong, C. D. Thron, B. Novak, Biophys. J. 77 (1999) 2411--2417] proposed a crucial positive feedback to the circadian oscillator. This idea was mathematically expressed in a three-dimensional model. By imposing assumptions that the dimerization reactions were fast and dimeric proteins were in rapid equilibrium, they reduced the model to a pair of nonlinear ordinary differential equations of mRNA and total protein concentrations. Then they used phase plane analysis tools to investigate circadian rhythms. In this paper, the original three-dimensional model is studied. We explore the existence of oscillations and their periods. Much attention is paid to investigate how the periods depend on model parameters. The numerical simulations are in good agreement with their reduced work.

Circadian PER2 protein oscillations do not persist in cycloheximide-treated mouse embryonic fibroblasts in culture

It is not known whether the endogenous mammalian core clock proteins sustain measurable oscillations in cells in culture where de novo translation is pharmacologically inhibited. We studied here the mammalian core clock protein PER2, which undergoes robust circadian oscillations in both abundance and phosphorylation. With a newly developed antibody that enables tracing the endogenous PER2 protein oscillations over circadian cycles with cultured mouse embryonic fibroblast cells, we provide evidence that PER2 does not persist noticeable circadian rhythms when translation is inhibited.

Ryanodine-sensitive intracellular Ca2+ channels are involved in the output from the SCN circadian clock

The suprachiasmatic nuclei (SCN) contain the major circadian clock responsible for generation of circadian rhythms in mammals. The time measured by the molecular circadian clock must eventually be translated into a neuronal firing rate pattern to transmit a meaningful signal to other tissues and organs in the animal. Previous observations suggest that circadian modulation of ryanodine receptors (RyR) is a key element of the output pathway from the molecular circadian clock. To directly test this hypothesis, we studied the effects of RyR activation and inhibition on real time expression of PERIOD2::LUCIFERASE, intracellular calcium levels and spontaneous firing frequency in mouse SCN neurons. Furthermore, we determined whether the RyR-2 mRNA is expressed with a daily variation in SCN neurons. We provide evidence that pharmacological manipulation of RyR in mice SCN neurons alters the free [Ca²⁺ ]_i in the cytoplasm and the spontaneous firing without affecting the molecular clock mechanism. Our data also show a daily variation in RyR-2 mRNA from single mouse SCN neurons with highest levels during the day. Together, these results confirm the hypothesis that RyR-2 is a key element of the circadian clock output from SCN neurons.

Lithium Leads to an Increased FRQ Protein Stability and to a Partial Loss of Temperature Compensation in the Neurospora Circadian Clock

In many organisms, the presence of lithium leads to an increase of the circadian period length. In Neurospora crassa, it was earlier found that lithium results in a decrease of overall growth and increased circadian periods. In this article, the authors show that lithium leads to a reduction of FRQ degradation with elevated FRQ levels and to a partial loss of temperature compensation. At a concentration of 13 mM lithium, FRQ degradation is reduced by about 60% while, surprisingly, the activity of the 20S proteasome remains unaffected. Experiments and model calculations have shown that the stability of FRQ is dependent on its phosphorylation state and that increased FRQ protein stabilities lead to increased circadian periods, consistent with the observed increase of the period when lithium is present. Because in Neurospora the proteasome activity is unaffected by lithium concentrations that lead to significant FRQ stabilization, it appears that lithium acts as an inhibitor of kinases that affect phosphorylation of FRQ and other proteins. A competition between Li+and Mg2+ions for Mg2+-binding sites may be a mechanism to how certain kinases are inhibited by Li+. A possible kinase in this respect is GSK-3, which in other organisms is known to be inhibited by lithium. The partial loss of temperature compensation in the presence of lithium can be understood as an increase in the overall activation energy of FRQ degradation. This increase in activation energy may be related to a reduction in FRQ phosphorylation so that more kinase activity, that is, higher temperature and longer times, is required to achieve the necessary amount of FRQ phosphorylation leading to turnover. Using a modified Goodwin oscillator as a semiquantitative model for the Neurospora clock, the effects of lithium can be described by adding lithium inhibitory terms of FRQ degradation to the model.

Non-transcriptional oscillators in circadian timekeeping

Circadian clocks have evolved as an adaptation to life on a rotating planet, and orchestrate rhythmic changes in physiology to match the time of day. For decades, cellular circadian rhythms were considered to solely result from feedback between the products of rhythmically expressed genes. These transcriptional/translational feedback loops (TTFLs) have been ubiquitously studied, and explain the majority of circadian outputs. In recent years, however, non-transcriptional processes were shown to be major contributors to circadian rhythmicity. These key findings have profound implications on our understanding of the evolution and mechanistic basis of cellular circadian timekeeping. This review summarises and discusses these results and the experimental and theoretical evidence of a possible relation between non-transcriptional oscillator (NTO) mechanisms and TTFL oscillations.

Proteasome function is required for biological timing throughout the twenty-four hour cycle

Circadian clocks were, until recently, seen as a consequence of rhythmic transcription of clock components, directed by transcriptional/translational feedback loops (TTFLs). Oscillations of protein modification were then discovered in cyanobacteria. Canonical posttranslational signaling processes have known importance for clocks across taxa. More recently, evidence from the unicellular eukaryote Ostreococcus tauri revealed a transcription-independent, rhythmic protein modification shared in anucleate human cells. In this study, the Ostreococcus system reveals a central role for targeted protein degradation in the mechanism of circadian timing. The Ostreococcus clockwork contains a TTFL involving the morning-expressed CCA1 and evening-expressed TOC1 proteins. Cellular CCA1 and TOC1 protein content and degradation rates are analyzed qualitatively and quantitatively using luciferase reporter fusion proteins. CCA1 protein degradation rates, measured in high time resolution, feature a sharp clock-regulated peak under constant conditions. TOC1 degradation peaks in response to darkness. Targeted protein degradation, unlike transcription and translation, is shown to be essential to sustain TTFL rhythmicity throughout the circadian cycle. Although proteasomal degradation is not necessary for sustained posttranslational oscillations in transcriptionally inactive cells, TTFL and posttranslational oscillators are normally coupled, and proteasome function is crucial to sustain both.

Quantitative analysis of regulatory flexibility under changing environmental conditions

The circadian clock controls 24-h rhythms in many biological processes, allowing appropriate timing of biological rhythms relative to dawn and dusk. Known clock circuits include multiple, interlocked feedback loops. Theory suggested that multiple loops contribute the flexibility for molecular rhythms to track multiple phases of the external cycle. Clear dawn- and dusk-tracking rhythms illustrate the flexibility of timing in Ipomoea nil. Molecular clock components in Arabidopsis thaliana showed complex, photoperiod-dependent regulation, which was analysed by comparison with three contrasting models. A simple, quantitative measure, Dusk Sensitivity, was introduced to compare the behaviour of clock models with varying loop complexity. Evening-expressed clock genes showed photoperiod-dependent dusk sensitivity, as predicted by the three-loop model, whereas the one- and two-loop models tracked dawn and dusk, respectively. Output genes for starch degradation achieved dusk-tracking expression through light regulation, rather than a dusk-tracking rhythm. Model analysis predicted which biochemical processes could be manipulated to extend dusk tracking. Our results reveal how an operating principle of biological regulators applies specifically to the plant circadian clock.

Cellular analysis of a molluscan retinal biological clock

The eye of the opisthobranch mollusc Bulla gouldiana expresses a circadian rhythm in optic nerve impulse frequency. The circadian rhythm is generated among approximately 100 neurons at the base of the retina referred to as basal retinal neurons. These cells are electrically coupled to one another and fire spontaneous action potentials in synchrony. Basal retinal neurons recorded intracellularly exhibit a circadian rhythm in membrane potential that appears to be driven by a circadian modulation of membrane conductance. Membrane conductance is relatively high during the subjective night and decreases after subjective dawn. Recent experiments in our laboratory indicate that individual basal retinal neurons in culture can express circadian rhythms in membrane conductance. When completely isolated, these cells continue to show circadian conductance changes. These studies provide the first direct demonstration that individual neurons can act as circadian pacemakers. Although the precise details of the mechanism generating the circadian periodicity remain obscure, our research indicates that several transmembrane ionic fluxes are not involved in rhythm generation, but that a transmembrane Ca2+ flux is critical for entrainment. Both transcription and translation appear to play critical roles in generating the circadian cycle.

The genetic basis of the circadian clock: identification of frq and FRQ as clock components in Neurospora

Genetic approaches to the identification of clock components have succeeded in two model systems, Neurospora and Drosophila. In each organism, genes identified through screens for clock-affecting mutations (frq in Neurospora, per in Drosophila) have subsequently been shown to have characteristics of central clock components: (1) mutations in each gene can affect period length and temperature compensation, two canonical characteristics of circadian systems; (2) each gene regulates the timing of its own transcription in a circadian manner; and (3) in the case of frq, constitutively elevated expression will set the phase of the clock on release into normal conditions. Despite clear genetic and molecular similarities, however, the two genes are neither molecular nor temporal homologues. The timing of peak expression is distinct in the two genes, frq expression peaking after dawn and per expression peaking near midnight. Also, although expression of per from a constitutive promoter can rescue rhythmicity in a fly lacking the gene, constitutive expression of frq will not rescue rhythmicity in Neurospora frq-null strains, and in fact causes arrhythmicity when expressed in a wild-type strain. These data suggest that frq is and/or encodes a state variable of the circadian oscillator. Recent molecular genetic analyses of frq have shed light on the origin of temperature compensation and strongly suggest that this property is built into the oscillatory feedback loop rather than appended to it. It seems plausible that clocks are adjusted and reset through adjustments in central clock components such as frq, and, by extension, per.

A novel mechanism for the control of circadian clock period by light

The free-running period expressed by circadian clocks in constant environmental conditions is history dependent, and one effect of entrainment of circadian clocks by light cycles is to cause long-lasting changes in the free-running period that are termed aftereffects. It has been suggested that aftereffects are a consequence of the particular phase relationships among constituent oscillators of the circadian system that are established by the entrainment. In a test of this hypothesis, it is shown that aftereffects of entrainment of the free-running rhythm of nerve impulse activity from the eye of Bulla gouldiana are completely unaffected by treatment with 12-, 24-, or 48-h pulses of the translation inhibitor cycloheximide or with 24-h pulses of the transcription inhibitor DRB (5,6-dichlorobenzimidazole riboside). These treatments reset the phase of circadian oscillators generally and those in the eye of B. gouldiana specifically. The absence of any effect of the treatments indicates that aftereffects are independent of oscillator phase. These results suggest that history-dependent changes in period result from a novel, long-lasting, and previously unrecognized mechanism of action of light on the circadian pacemaking system. Furthermore, the data indicate that aftereffects can persist in the absence of translation, transcription, or the continued cycling of the circadian system.

Protein synthesis is a required process for the optic lobe circadian clock in the cricket Gryllus bimaculatus

The effects of a translation inhibitor, cycloheximide (CHX), on the circadian neuronal activity rhythm of the optic lamina-medulla compound eye complex cultured in vitro were investigated in the cricket Gryllus bimaculatus. When the complex was treated with 10(-5) M CHX for 6 h, the rhythm exhibited a marked phase shift. The magnitude and direction of the phase shift were dependent on the phase at which the complex was treated with CHX; phase delays occurred during the late subjective day to early subjective night, whereas phase advances occurred around the late subjective night. Continuous application of CHX abolished circadian rhythms of both the spontaneous neuronal activity and the visually evoked response. However, it abolished neither the spontaneous activity nor the visually evoked response. As washed with fresh medium after CHX treatment, the rhythm soon reappeared and the subsequent phase was clearly correlated to the termination time of the treatment. These results suggest that protein synthesis is also involved in the cricket optic lobe circadian clock, and that the clock-related protein synthesis may be active during the late subjective day to subjective night.

Cellular circadian clocks in the pineal

Daily rhythms are a fundamental feature of all living organisms; most are synchronized by the 24 hr light/dark (LD) cycle. In most species, these rhythms are generated by a circadian system, and free run under constant conditions with a period close to 24 hr. To function properly the system needs a pacemaker or clock, an entrainment pathway to the clock, and one or more output signals. In vertebrates, the pineal hormone melatonin is one of these signals which functions as an internal time-keeping molecule. Its production is high at night and low during day. Evidence indicates that each melatonin producing cell of the pineal constitutes a circadian system per se in non-mammalian vertebrates. In addition to the melatonin generating system, they contain the clock as well as the photoreceptive unit. This is despite the fact that these cells have been profoundly modified from fish to birds. Modifications include a regression of the photoreceptive capacities, and of the ability to transmit a nervous message to the brain. The ultimate stage of this evolutionary process leads to the definitive loss of both the direct photosensitivity and the clock, as observed in the pineal of mammals. This review focuses on the functional properties of the cellular circadian clocks of non-mammalian vertebrates. How functions the clock? How is the photoreceptive unit linked to it and how is the clock linked to its output signal? These questions are addressed in light of past and recent data obtained in vertebrates, as well as invertebrates and unicellulars.

Keeping an eye on retinal clocks

Circadian pacemakers that drive rhythmicity in retinal function are found in both invertebrates and vertebrates. They have been localized to photoreceptors in molluscs, amphibians, and mammals. Like other circadian pacemakers, they entrain to light, oscillate based on a negative feedback between transcription and translation of clock genes, and control a variety of physiological and behavioral rhythms that often includes rhythmic melatonin production. As a highly organized and accessible tissue, the retina is particularly well suited for the study of the input-output pathways and the mechanism for rhythm generation. Impressive advances can now be expected as researchers apply new molecular techniques toward looking into the eye's clock.

Inhibitors of messenger RNA and protein synthesis affect differently serotonin arylalkylamine N-acetyltransferase activity in clock-controlled and non clock-controlled fish pineal

The pineal organ of fish contains photoreceptor cells. In some species (e.g., pike) each photoreceptor is a cellular circadian system which contains a photoreceptive unit, the clock and an output unit. In others (e.g., trout) the clock is lacking. The main rhythmic output of the pineal photoreceptor is melatonin, an internal 'zeitgeber' of the organisms. The nocturnal rise in melatonin secretion results from an increase in the activity of the arylalkylamine-N-acetyltransferase (AA-NAT) which converts serotonin to N-acetylserotonin. In the present study we investigated the effects of transcription and translation inhibitors on AA-NAT activity in pike and trout pineal organs in culture. Cycloheximide, anisomycin, and puromycin inhibited the rise in AA-NAT activity observed during the first 2, 4 or 6 h of the dark phase, in both species. Actinomycin D was active only in the pike. Six hours of treatment during the first half of the night induced inhibition of AA-NAT activity, providing that forskolin (an adenylyl cyclase stimulator) was present in the culture medium. When the treatment was run for 3, 6 or 12 h, starting at midday of a 12L/12D cycle, basal and forskolin-stimulated AA-NAT activity (measured at midnight) were dramatically reduced. Such a treatment had no effect on trout AA-NAT activity. It is concluded that: (1) the dark-induced rise in AA-NAT activity and melatonin secretion are dependent on newly synthesized protein in both pike and trout pineal; (2) AA-NAT regulation takes place at the translational and post-translational levels in both species; (3) AA-NAT regulation occurs also at the transcriptional level in the pike, but not in the trout; and (4) the cAMP-dependent activation of AA-NAT requires transcription in the pike, not in the trout. The presence of a cell population acting as a circadian clock in the pike pineal, but not in the trout pineal, can explain the difference between these two species. Thus, we suggest that the clock mechanism operates at the genetic level in these cells. Further comparative studies between clock-controlled and non-clock-controlled pineals might prove interesting to demonstrate the difference between these two regulatory pathways.

A model for circadian rhythms in Drosophila incorporating the formation of a complex between the PER and TIM proteins

The authors present a model for circadian oscillations of the Period (PER) and Timeless (TIM) proteins in Drosophila. The model for the circadian clock is based on multiple phosphorylation of PER and TIM and on the negative feedback exerted by a nuclear PER-TIM complex on the transcription of the per and tim genes. Periodic behavior occurs in a large domain of parameter space in the form of limit cycle oscillations. These sustained oscillations occur in conditions corresponding to continuous darkness or to entrainment by light-dark cycles and are in good agreement with experimental observations on the temporal variations of PER and TIM and of per and tim mRNAs. Birhythmicity (coexistence of two periodic regimes) and aperiodic oscillations (chaos) occur in a restricted range of parameter values. The results are compared to the predictions of a model based on the sole regulation by PER. Both the formation of a complex between PER and TIM and protein phosphorylation are found to favor oscillatory behavior. Determining how the period depends on several key parameters allows us to test possible molecular explanations proposed for the altered period in the per(l) and per(s) mutants. The extended model further allows the construction of phase-response curves based on the light-induced triggering of TIM degradation. These curves, established as a function of both the duration and magnitude of the effect of a light pulse, match the phase-response curves obtained experimentally in the wild type and per(s) mutant of Drosophila.

The cyclin-dependent kinase (cdk) inhibitors, olomoucine and roscovitine, alter the expression of a molluscan circadian pacemaker

1. In this study, we determined the effects of the cyclin-dependent kinase (cdk) inhibitors, olomoucine and roscovitine, on the circadian rhythm of optic nerve impulse activity recorded from the eye of the marine snail Bulla gouldiana. 2. We found that olomoucine lengthened period and altered circadian phase in a dose-dependent manner without appreciably affecting gene transcription or translation. We also found that the more specific cdk inhibitor, roscovitine, was approximately 10-fold more effective in lengthening circadian period, while the inactive analogue, iso-olomoucine, was ineffective. 3. The current results, along with previous results from our laboratory, are consistent with the hypothesis that the biochemical mechanism responsible for generating the ocular circadian rhythm in B. gouldiana is related to the biochemical mechanism that regulates the eukaryotic cell division cycle, i.e., by modulation of the activity of protein kinases belonging to the cdk family.

Genetics and molecular analysis of circadian rhythms

The first part of this review summarizes the two best understood aspects of the two best understood circadian systems, the feedback oscillators of Neurospora and Drosophila, concentrating on what we know about the frequency (frq), period (per) and timeless (tim) genes. In the second part, the general circadian genetic and molecular literature is surveyed, with an eye to describing what is known from a variety of systems about input to the oscillator (entrainment), and how the oscillator might work and be temperature compensated, in emerging systems including Synechococcus, Gonyaulax, Arabidopsis, hamsters, and mice. Finally, the conversation of the molecular components of clocks is analyzed: both frq and per are widely conserved in their respective phylogenetic classes. Pharmacological data suggests that most other organisms use a day-phased oscillator of the type seen in Neurospora rather than a night-phased oscillator such as in Drosophila.

Evidence for a central role of transcription in the timing mechanism of a circadian clock

The retinal circadian clock in the isolated in vitro eye of the marine mollusc Bulla gouldiana exhibits a phase-dependent requirement for transcription. The transcription-sensitive phase extends through most of the subjective day and therefore is substantially longer than the previously reported translation-sensitive phase. Lower concentrations of transcription inhibitors yield a significant dose-dependent lengthening of circadian period. Clock motion can be stopped by a high concentration of the transcription inhibitor 5,6-dichlorobenz-imidazole riboside (DRB) when applied during the sensitive phase; after withdrawal of the inhibitor, motion resumes from the phase at which it was stopped. In a double-pulse experiment, phase shifts to light pulses applied after DRB pulses, and not during the translation-sensitive phase, indicate that the inhibition of transcription has immediate effects on the phase of the clock. These data suggest that DRB-induced phase shifts are independent of translation, which implies that the rate of transcription itself plays a significant role in the mechanism underlying the generation of the circadian cycle.

Quantitative two-dimensional gel electrophoretic analysis of clock-controlled proteins in cultured chick pineal cells: circadian regulation of tryptophan hydroxylase

The progression of the circadian oscillator through its cycle and the circadian rhythm of melatonin production in dissociated chick pineal cultures both require daily de novo protein synthesis during defined circadian phases. To identify specific proteins involved in these two processes, we have performed a quantitative two-dimensional polyacrylamide gel electrophoretic screen of proteins that are synthesized at different times of the day in chick pineal cell cultures. Out of approximately 700 proteins analyzed, we have identified several proteins whose levels of 35S incorporation oscillate in a light/dark cycle. One protein of 56 kDa, pI 6 (p56) undergoes a diurnal oscillation that parallels the melatonin rhythm, reaching a peak early in the night and falling to minimal levels during the day. A second protein of 22 kDa, pI 4.5 (p22) also expresses a diurnal rhythm in 35S incorporation; however, it peaks at the end of the night. The oscillations of both proteins persist, with a reduced amplitude, in constant darkness. Furthermore, the phases of the p56 and p22 rhythms are regulated by the light/dark cycle. Both p56 and p22 appear to be under direct control of the chick pineal circadian oscillator, and therefore can be described as "clock-controlled proteins." We have identified p56 as tryptophan hydroxylase by microsequencing and western blotting. Chick pineal tryptophan hydroxylase also expresses a 24-h oscillation in abundance both in vitro and in vivo. The rhythm in tryptophan hydroxylase expression represents a newly discovered level of regulation of the melatonin synthesis pathway by the circadian clock in chick pineal cells.

Resetting the Drosophila clock by photic regulation of PER and a PER-TIM complex

Circadian clocks can be reset by light stimulation. To investigate the mechanism of this phase shifting, the effects of light pulses on the protein and messenger RNA products of the Drosophila clock gene period (per) were measured. Photic stimuli perturbed the timing of the PER protein and messenger RNA cycles in a manner consistent with the direction and magnitude of the phase shift. In addition, the recently identified clock protein TIM (for timeless) interacted with PER in vivo, and this association was rapidly decreased by light. This disruption of the PER-TIM complex in the cytoplasm was accompanied by a delay in PER phosphorylation and nuclear entry and disruption in the nucleus by an advance in PER phosphorylation and disappearance. These results suggest a mechanism for how a unidirectional environmental signal elicits a bidirectional clock response.

The use of a reversible transcription inhibitor, DRB, to investigate the involvement of specific proteins in the ocular circadian system of Aplysia

Previously, the effects of 2-h treatments with the reversible transcription inhibitor 5,6-dichloro-1-beta-D-ribobenzimidazole (DRB) on the phase of the circadian rhythm in the eye of Aplysia californica were studied. Here we report a study of the effects of DRB on protein synthesis and a more detailed investigation of the effects of DRB on the phase of the circadian rhythm. Treatments of DRB for 30 min reduced the rate of transcription to about 30% of control values, and this inhibition reversed completely within 2 h after the end of the treatment. A phase-response curve was obtained for 30-min treatments of DRB. Shorter (30 min) treatments with DRB produced phase shifts comparable to those produced by treatments with DRB for 2 h. The phase-response curve obtained using 30-min treatments of DRB was similar to one obtained using 2-h treatments with respect to the phase at which DRB exerts its maximum effect on the rhythm (around circadian time [CT] 6). However, some aspects of the two phase-response curves were different. The effect of DRB on the phase of the rhythm appeared rapidly after removal of DRB treatments given during CT 22-6, but the effects of DRB on the phase of the rhythm appeared more slowly (approximately 10 h) after the treatments given during CT 6-12. Because the effects of DRB on the phase of the overt rhythm appear to be rapid at a particular phase, it is very likely that DRB affects the phase of the rhythm by altering the synthesis of proteins during or shortly after the treatment. Thus we searched for proteins whose synthesis was altered by DRB. Incorporation of labeled amino acids into 2 proteins was found to be altered during the DRB treatment, whereas 15 proteins were affected after the DRB treatment. Among the proteins affected during or shortly after the DRB treatment were four previously identified proteins affected by other treatments that can shift the phase of the eye circadian rhythm. These four proteins are worthy of further study as possible candidates for components of the circadian oscillator.

The genetic and molecular dissection of a prototypic circadian system

A great deal is known about this archetypal circadian system, and it is likely that Neurospora will represent the first circadian system in which it will be possible to provide a complete description of the flow of information from the photoreceptor, through the components of oscillator, out to a terminal aspect of regulation. In Neurospora the strongest case has been made for there being a state variable of clock identified (Hall, 1995), it has now been shown that light resetting of the clock is mediated by the rapid light induction of the gene encoding this state variable, and a number of defined clock-regulated output genes have been identified, in two of which the clock-specific parts of the promoters have been localized. In addition to the importance of these factoids themselves, our efforts towards understanding of this system has allowed the development of tools and paradigms (e.g. Loros et al., 1989; Loros and Dunlap, 1991; Aronson et al., 1994a) that will help to pave the way for proving the identity of clock components in more complex systems, for understanding how clocks are regulated by entraining factors, and for showing how time information eventually is used to regulate the behaviors of clock cells, and of whole organisms.

Circadian rhythm generation, expression and entrainment in a molluscan model system

The Bulla ocular pacemaker provides remarkable opportunities for cellular study of circadian pacemaker systems. The demonstration of circadian oscillations within individual neurons maintained in culture provides us with a first occasion to study the biophysical and biochemical properties of bona fide neuronal circadian pacemakers. The ocular clock is robust and shares formal similarity with other circadian systems. The development of molecular techniques that can be applied to single neurons should allow research on the Bulla retina to continue to progress towards a molecular analysis of circadian timekeeping.

Molecular control of circadian rhythms

Circadian rhythms are virtually ubiquitous in eukaryotes and have been shown to exist even in some prokaryotes. The generally accepted view is that these rhythms are generated by an endogenous clock. Recent progress, especially in the Drosophila, Neurospora and mouse systems, has revealed new clock components and mechanisms. These include the mouse clock gene, the Drosophila timeless gene, and the role of light in Neurospora.

A model for circadian oscillations in the Drosophila period protein (PER)

The mechanism of circadian oscillations in the period protein (PER) in Drosophila is investigated by means of a theoretical model. Taking into account recent experimental observations, the model for the circadian clock is based on multiple phosphorylation of PER and on the negative feedback exerted by PER on the transcription of the period (per) gene. This minimal biochemical model provides a molecular basis for circadian oscillations of the limit cycle type. During oscillations, the peak in per mRNA precedes by several hours the peak in total PER protein. The results support the view that multiple PER phosphorylation introduces times delays which strengthen the capability of negative feedback to produce oscillations. The analysis shows that the rhythm only occurs in a range bounded by two critical values of the maximum rate of PER degradation. A similar result is obtained with respect to the rate of PER transport into the nucleus. The results suggest a tentative explanation for the altered period of per mutants, in terms of variations in the rate of PER degradation.

The circadian clock: from molecules to behaviour

Circadian rhythms are a cardinal feature of living organisms. The stereotypical organization of homeostatic, endocrine and behavioural variables around the 24-hour cycle constitutes one of the most conserved attributes among species. It is now well established that circadian rhythmicity is not a learned behaviour, but is genetically transmitted and therefore subject to genetic manipulations. Recent advances in the circadian field have demonstrated that circadian oscillations are cell autonomous, that the circadian mechanism operates through a negative feedback loop and that a growing number of genes is under circadian control. Furthermore, single-gene mutations have been isolated in mammals that have profound effects on circadian behaviour. The production and mapping of one of these mutations in the mouse, an organism about which there exists a wealth of genetic information, should accelerate the elucidation of the molecular events involved in the generation of circadian rhythms in mammals.

Identification of three proteins in the eye of Aplysia, whose synthesis is altered by serotonin (5-HT). Possible involvement of these proteins in the ocular circadian system

Previous results using translation inhibitors in the ocular circadian system of Aplysia suggest that protein synthesis may be involved in the light and serotonin (5-HT) entrainment pathways or perhaps in the circadian oscillator. Proteins have been previously identified whose synthesis was altered by treatments of light capable of perturbing the phase of the circadian rhythm in the eye of Aplysia. We extended these studies by investigating the effects of other treatments that perturb the ocular circadian rhythm on protein synthesis. 5-HT altered the synthesis of nine proteins. Interestingly, five of the proteins affected by treatments with 5-HT were previously shown to be affected by treatments with light. Four of the proteins affected by treatments with 5-HT were also affected by treatments with analogs of cAMP, a treatment which mimics the effects of 5-HT on the ocular circadian rhythm. To identify the cellular function of some of these proteins, we obtained their partial amino acid sequences. Based on these sequences and additional characterizations, a 78-kDa, pI 5.6 Aplysia protein appears to be glucose-regulated protein 78/binding protein, and a 36-kDa, pI 5.7 Aplysia protein appears to be porin/voltage-dependent anion channel. Heat shock experiments on Aplysia eyes revealed that yet another one of the Aplysia proteins (70 kDa) affected by 5-HT appears to be a heat-inducible member (heat shock protein 70) of the family of heat shock proteins. These findings suggest that these three identified proteins, together or individually, may be involved in some way in the regulation of the timing of the circadian oscillator in the eye of Aplysia.

Calcium plays a central role in phase shifting the ocular circadian pacemaker of Aplysia

The eye of the marine mollusk Aplysia californica contains an oscillator that drives a circadian rhythm of spontaneous compound action potentials in the optic nerve. Both light and serotonin are known to influence the phase of this ocular rhythm. The aim of the present study was to evaluate the role of extracellular calcium in both light and serotonin-mediated phase shifts. Low calcium treatments were found to cause phase shifts which resembled those produced by the transmitter serotonin. However, unlike serotonin, low calcium neither increased ocular cAMP levels nor could these phase shifts be prevented by increasing extracellular potassium concentration. Low calcium-induced phase shifts were prevented by the simultaneous application of the translational inhibitor anisomycin and low calcium treatment resulted in changes in [35S]methionine incorporation into several proteins as measured by a two-dimensional electrophoresis gel analysis. Finally, light treatments failed to produce phase shifts in the presence of low calcium or the calcium channel antagonist nickel chloride. These results are consistent with a model in which serotonin phase shifts the ocular pacemaker by decreasing a transmembrane calcium flux through membrane hyperpolarization while light-induced phase shifts are mediated by an increase in calcium flux.

The molecular basis for winter depression

A pulse of light is capable of inducing the circadian phase-dependent gene expression in neurons. The phase or amplitude of the circadian rhythms can be modulated by critically timed exposures to light. A significant heritability for the light-induced responses has been observed in hamsters. In humans, light has been used for treatment of the light-dependent winter depressive disorder. A genetic predisposition for high responsiveness to light may occur in patients with winter depression. The altered gene expression induced by light may account for a unique sensitivity to light and mediate the anti-depressant effect of light treatment.

Negative feedback defining a circadian clock: autoregulation of the clock gene frequency

The frequency (frq) locus of Neurospora crassa was originally identified in searches for loci encoding components of the circadian clock. The frq gene is now shown to encode a central component in a molecular feedback loop in which the product of frq negatively regulated its own transcript, which resulted in a daily oscillation in the amount of frq transcript. Rhythmic messenger RNA expression was essential for overt rhythmicity in the organism and no amount of constitutive expression rescued normal rhythmicity in frq loss-of-function mutants. Step reductions in the amount of FRQ-encoding transcript set the clock to a specific and predicted phase. These results establish frq as encoding a central component in a circadian oscillator.

An inhibitor of protein phosphorylation stops the circadian oscillator and blocks light-induced phase shifting in Gonyaulax polyedra

The expression of circadian rhythmicity in Gonyaulax polyedra is strikingly altered by an inhibitor of protein phosphorylation. The effects of 6-dimethylaminopurine (6-DMAP), known to reversibly block cell division in many systems through inhibition of protein kinase activity, are described here for Gonyaulax. Its action appears to be exclusively tonic in nature; in cells continuously exposed, the period is lengthened in a concentration-dependent fashion. Shorter treatments at a higher concentration of 6-DMAP (5 mM) apparently stop the circadian oscillator, but reversibly so, since the rhythm resumes after drug removal with a phase delay approximately equal to the duration of the treatment. Pulses of the inhibitor are effective in causing phase delays at all times of the circadian cycle. In addition, 6-DMAP completely blocks light-induced phase advances and is effective in inhibiting many Gonyaulax protein kinases in vitro.

Phase shifting of the circadian clock by induction of the Drosophila period protein

Virtually all organisms manifest circadian (24-hour) rhythms, governed by an ill-defined endogenous pacemaker or clock. Several lines of evidence suggest that the Drosophila melanogaster period gene product PER is a clock component. If PER were central to the time-keeping mechanism, a transient increase in its concentration would cause a stable shift in the phase of the clock. Therefore, transgenic flies bearing a heat-inducible copy of PER were subjected to temperature pulses. This treatment caused long-lasting phase shifts in the locomotor activity circadian rhythm, a result that supports the contention that PER is a bona fide clock component.

PAS is a dimerization domain common to Drosophila period and several transcription factors

Mutations in the period gene product (PER) can shorten or lengthen the circadian rhythms of Drosophila melanogaster, but its biochemical activity has not been established. PER contains a motif of approximately 270 amino acids whose function is unknown (termed PAS) and which is also present in three transcription factors of the basic-helix-loop-helix (bHLH) type, in the D. melanogaster single-minded gene product (SIM), and in both subunits of the mammalian dioxin receptor complex. We show here that the PER PAS functions in vitro as a novel protein dimerization motif and that it can mediate associations between different members of the PAS protein family. The dimerization efficiency is decreased by several missense mutations in the PAS domain, including the original perL mutation, which lengthens circadian periods from 24 h to 29 h (ref. 1). The results indicate that the PAS domain may function as a dimerization domain in both SIM and the dioxin receptor complex, and that PER may regulate circadian gene transcription partly by interacting with the PAS domain of bHLH--PAS-containing transcription factors.

The role of extracellular calcium in generating and in phase-shifting the Bulla ocular circadian rhythm

Since extracellular calcium is known to be involved in the entrainment of the circadian pacemaker in the retina of Bulla gouldiana, we have assessed the requirement for extracellular calcium in the generation of the circadian rhythm. To enable us to assay the state of the pacemaker during low-calcium treatment, which often obscures rhythmicity, long-duration pulses of low-calcium artificial seawater (no added calcium, 10 mM EGTA, calculated calcium concentration = 4.5 x 10(-10) M) were applied, and the phase of the subsequent rhythm was measured. Pulse treatments started at zeitgeber time (ZT) 6, and durations ranged from 4 to 72 hr. Although no phase shifts followed pulses ending before the next projected dawn (ZT 24), phase delays of up to 4 hr followed pulses ending after projected dawn, and delays of up to 8 hr followed pulses spanning two dawns. Some activity records exhibited unequivocal circadian rhythmicity during the long low-calcium treatments, with phases and periods similar to untreated control eye records; this finding suggests that the phase delays observed following long low-calcium pulses are attributable to the pulsatile nature of the treatment. These data suggest that extracellular calcium is not an essential requirement for the pacemaker in generating the circadian rhythm.