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PubMed2001

Origins of phytochrome-modulated Lhcb mRNA expression in seed plants.

The levels of Lhcb mRNA in higher plants are regulated by phytochrome, cryptochrome, and an endogenous circadian oscillator. To determine whether similar regulatory mechanisms operate in the ancient gymnosperm Ginkgo biloba, we measured Lhcb mRNA levels in seedlings in response to different light conditions. Removal of a diurnally oscillating light stimulus caused dampening of maximal Lhcb mRNA accumulation levels, with little change in periodicity. Although low fluence pulses of both red and blue light given to etiolated seedlings caused maximal accumulation of Lhcb mRNAs characteristic of the phasic/circadian response seen in flowering plants, the additional initial acute response seen in flowering plants was absent. The induction of Lhcb gene expression in both cases was at least partially reversible by far-red light, and appeared biphasic over a range of red fluences. Together, these data indicate that Lhcb genes in G. biloba appear to be regulated in a manner similar to that of flowering plants, whereas signaling and attenuation of mRNA levels through the photoreceptor systems and circadian clock show features distinct from those characterized to date. The implications for these findings are discussed in light of the evolution of circadian clock input signaling.

Arabidopsis Proteins↗
PubMed1983

Elevated riboflavin requirement for postphotoinductive events in sporulation of a trichoderma auxotroph.

A riboflavin auxotroph of Trichoderma required more riboflavin for blue-light-induced conidiation than for growth. Colonies transferred after illumination from 0.2 micromolar (limiting conidiation, not growth) to 3.5 micromolar riboflavin (nonlimiting), responded the same as those grown at 3.5 micromolar. The additional riboflavin is therefore not required as a photoreceptor. The data do not rule out the hypothesis that cryptochrome(s) is/are flavin(s).

Journal Article↗
PubMed1984

Photoregulation of Anthocyanin Synthesis : VIII. Effect of Light Pretreatments.

A comparative study of the spectral sensitivity of anthocyanin production in dark-grown and light-pretreated systems was carried out in Brassica oleracea L., Lycopersicon esculentum Mill., Secale cereale L. and Spirodela polyrrhiza L. Light pretreatments bring about an enhancement of the inductive, red-far red reversible response in all systems, a decrease of the continuous irradiation response in cabbage, rye, and tomato seedlings, and an enhancement of the continuous irradiation response in cabbage leaf disks. Light pretreatments also bring about a marked change in the spectral sensitivity of the continuous irradiation response. The different effect of light pretreatments on the photosensitivity of the response to short and long wavelength irradiations suggests that two photoreceptors, phytochrome and cryptochrome, may be involved in the photoregulation of anthocyanin production.

Journal Article↗
PubMed2005

Light regulates COP1-mediated degradation of HFR1, a transcription factor essential for light signaling in Arabidopsis.

Arabidopsis thaliana seedlings undergo photomorphogenesis in the light and etiolation in the dark. Long Hypocotyl in Far-Red 1 (HFR1), a basic helix-loop-helix transcription factor, is required for both phytochrome A-mediated far-red and cryptochrome 1-mediated blue light signaling. Here, we report that HFR1 is a short-lived protein in darkness and is degraded through a 26S proteasome-dependent pathway. Light, irrespective of its quality, enhances HFR1 protein accumulation via promoting its stabilization. We demonstrate that HFR1 physically interacts with Constitutive Photomorphogenesis 1 (COP1) and that COP1 exhibits ubiquitin ligase activity toward HFR1 in vitro. In addition, we show that COP1 is required for degradation of HFR1 in vivo. Furthermore, plants overexpressing a C-terminal 161-amino acid fragment of HFR1 (CT161) display enhanced photomorphogenesis, suggesting an autonomous function of CT161 in promoting light signaling. This truncated HFR1 gene product is more stable than the full-length HFR1 protein in darkness, indicating that the COP1-interacting N-terminal portion of HFR1 is essential for COP1-mediated destabilization of HFR1. These results suggest that light enhances HFR1 protein accumulation by abrogating COP1-mediated degradation of HFR1, which is necessary and sufficient for promoting light signaling. Additionally, our results substantiate the E3 ligase activity of COP1 and its critical role in desensitizing light signaling.

Arabidopsis↗
PubMed2005

Arabidopsis response regulators ARR3 and ARR4 play cytokinin-independent roles in the control of circadian period.

Light and temperature are potent environmental signals used to synchronize the circadian oscillator with external time and photoperiod. Phytochrome and cryptochrome photoreceptors integrate light quantity and quality to modulate the pace and phase of the clock. PHYTOCHROME B (phyB) controls period length in red light as well as the phase of the clock in white light. phyB interacts with ARABIDOPSIS RESPONSE REGULATOR4 (ARR4) in a light-dependent manner. Accordingly, we tested ARR4 and other members of the type-A ARR family for roles in clock function and show that ARR4 and its closest relative, ARR3, act redundantly in the Arabidopsis thaliana circadian system. Loss of ARR3 and ARR4 lengthens the period of the clock even in the absence of light, demonstrating that they do so independently of active phyB. In addition, in white light, arr3,4 mutants show a leading phase similar to phyB mutants, suggesting that circadian light input is modulated by the interaction of phyB with ARR4. Although type-A ARRs are involved in cytokinin signaling, the circadian defects appear to be independent of cytokinin, as exogenous cytokinin affects the phase but not the period of the clock. Therefore, ARR3 and ARR4 are critical for proper circadian period and define an additional level of regulation of the circadian clock in Arabidopsis.

Arabidopsis↗
PubMed2001

An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans.

1. Non-image forming, irradiance-dependent responses mediated by the human eye include synchronisation of the circadian axis and suppression of pineal melatonin production. The retinal photopigment(s) transducing these light responses in humans have not been characterised. 2. Using the ability of light to suppress nocturnal melatonin production, we aimed to investigate its spectral sensitivity and produce an action spectrum. Melatonin suppression was quantified in 22 volunteers in 215 light exposure trials using monochromatic light (30 min pulse administered at circadian time (CT) 16-18) of different wavelengths (lambda(max) 424, 456, 472, 496, 520 and 548 nm) and irradiances (0.7-65.0 microW cm(-2)). 3. At each wavelength, suppression of plasma melatonin increased with increasing irradiance. Irradiance-response curves (IRCs) were fitted and the generated half-maximal responses (IR(50)) were corrected for lens filtering and used to construct an action spectrum. 4. The resulting action spectrum showed unique short-wavelength sensitivity very different from the classical scotopic and photopic visual systems. The lack of fit (r(2) < 0.1) of our action spectrum with the published rod and cone absorption spectra precluded these photoreceptors from having a major role. Cryptochromes 1 and 2 also had a poor fit to the data. Fitting a series of Dartnall nomograms generated for rhodopsin-based photopigments over the lambda(max) range 420-480 nm showed that rhodopsin templates between lambda(max) 457 and 462 nm fitted the data well (r(2) > or =0.73). Of these, the best fit was to the rhodopsin template with lambda(max) 459 nm (r(2) = 0.74). 5. Our data strongly support a primary role for a novel short-wavelength photopigment in light-induced melatonin suppression and provide the first direct evidence of a non-rod, non-cone photoreceptive system in humans.

Adult↗
PubMed2006

Role of calcium in the gating of isoproterenol-induced arylalkylamine N-acetyltransferase gene expression in the mouse pineal gland.

Melatonin and its autonomic regulation serve important physiological functions. We recently demonstrated that stimulation of beta-adrenergic receptors only increases nighttime arylalkylamine N-acetyltransferase (Aa-Nat, the rate-limiting enzyme in melatonin synthesis) mRNA levels in mouse pineal gland in vitro, which suggests that pineal clocks may gate Aa-Nat gene expression. In the present study, our data reveal that cAMP analog increased Aa-Nat at any time of day but only in the presence of ionomycin. Using Fura-2AM in ratiometric calcium measurements, we show that isoproterenol stimulation increased intracellular free calcium levels at night, contrary to previous reports. Further, intra- or extracellular calcium depletion suppressed the isoproterenol-induced calcium responses as well as Aa-Nat gene expression. These results suggest calcium may be a critical factor in isoproterenol-induced Aa-Nat gene expression, which may be limited in the daytime. We also found that basal intracellular calcium levels were lower during the night and responses to isoproterenol and KCl depolarization were more robust. In addition, pineals of Cryptochrome mutant mice exhibited no significant difference between day and nighttime basal calcium or isoproterenol response. Together, these results suggest that basal calcium levels in the pineal may be controlled by the endogenous pineal clock, which may influence calcium dynamics, cellular homeostasis and sensitivity to external stimulation. Although the mechanism underlying Aa-Nat gene expression has been well studied, the role of calcium as a link between the pineal clock and Aa-Nat gene expression has been underestimated in rodent pineals.

Adrenergic beta-Agonists↗
PubMed2001

Molecular mechanisms of morning onset of myocardial infarction.

We recently isolated a novel bHLH/PAS protein, CLIF (cycle like factor), by yeast two-hybrid screening of human umbilical endothelial cell cDNA library. CLIF is preferentially expressed in endothelial and neuronal cells. Because CLIF is expressed in vascular endothelial cells and forms a heterodimer with CLOCK, the key transcription factor controlling the circadian rhythm, we hypothesized that CLIF regulates the circadian oscillation of PAI-1 gene expression in endothelial cells. Northern blot analysis of mouse organs showed circadian oscillations of PAI-I mRNA levels. In addition, the clock-related genes also showed circadian oscillation in peripheral tissues. In endothelial cells, the heterodimer of CLIF and CLOCK upregulated the PAI-1 gene expression through E-box sites. Furthermore, Period and Cryptochrome, which are negative regulators in the feedback loop of the biological clock, inhibited PAI-1 promoter activation by the CLOCK:CLIF heterodimer. These results suggest that the peripheral tissues have their own biological clock and CLIF regulates the circadian oscillation of PAI-1 gene expression in endothelial cells. This study suggests a novel molecular mechanism of the morning onset of myocardial infarction. Here we review our recent work and literature.

ARNTL Transcription Factors↗
PubMed2002

Phytochromes control photomorphogenesis by differentially regulated, interacting signaling pathways in higher plants.

In this review the kinetic properties of both phytochrome A and B measured by in vivo spectroscopy in Arabidopsis are described. Inactivation of phyA is mediated by destruction and that of phyB by fast dark reversion. Recent observations, describing a complex interaction network of various phytochromes and cryptochromes, are also discussed. The review describes recent analysis of light-dependent nuclear translocation of phytochromes and genetic and molecular dissection of phyA- and phyB-mediated signal transduction. After nuclear transport, both phyA- and phyB-mediated signal transduction probably include the formation of light-dependent transcriptional complexes. Although this hypothesis is quite attractive and probably true for some responses, it cannot account for the complex network of phyA-mediated signaling and the interaction with the circadian clock. In addition, the biological function of phytochromes localized in the cytosol remains to be elucidated.

Morphogenesis↗
PubMed2000

Genetics of the mammalian circadian system: Photic entrainment, circadian pacemaker mechanisms, and posttranslational regulation.

During the past four years, significant progress has been made in identifying the molecular components of the mammalian circadian clock system. An autoregulatory transcriptional feedback loop similar to that described in Drosophila appears to form the core circadian rhythm generating mechanism in mammals. Two basic helix-loop-helix (bHLH) PAS (PER-ARNT-SIM) transcription factors, CLOCK and BMAL1, form the positive elements of the system and drive transcription of three Period and two Cryptochrome genes. The protein products of these genes are components of a negative feedback complex that inhibits CLOCK and BMAL1 to close the circadian loop. In this review, we focus on three aspects of the circadian story in mammals: the genetics of the photic entrainment pathway; the molecular components of the circadian pacemaker in the hypothalamic suprachiasmatic nucleus; and the role of posttranslational regulation of circadian elements. A molecular description of the mammalian circadian system has revealed that circadian oscillations may be a fundamental property of many cells in the body and that a circadian hierarchy underlies the temporal organization of animals.

Animals↗
PubMed2000

Involvement of the period gene in developmental time-memory: effect of the perShort mutation on phase shifts induced by light pulses delivered to Drosophila larvae.

Phases of circadian locomotor activity rhythms of adult Drosophila reared in constant darkness have been shown to be set by a light stimulus delivered as early as the first-instar larval stage. This implies that a circadian clock functions continuously throughout postembryonic development. The clock genes period (per) and timeless (tim) are expressed cyclically in the larval central nervous system of Drosophila, and daily oscillations of per expression persist throughout metamorphosis in a group of cells, which gives rise to the pacemaker cells underlying locomotor activity rhythms of adults. Therefore, PER and TIM cyclings in these neurons may be responsible for the phenomenon of "larval time-memory." In the absence of any evidence for the involvement of these genes in such a developmental clock, and because circadian-pacemaker functions are underanalyzed in terms of the functions during development, the authors tested the time-memory of a fast-clock period mutant. They show that dark-reared perS mutant individuals as well as wild-type flies can be entrained as larvae and that a brief light pulse given to such entrained larvae can induce phase shifts in animals of either genotype. However, the direction and magnitude of phase shifts were different between wild type and perS, suggesting that a clock under the control of period gene participates in the regulation of developmental time-memory. The authors show that the relevant clock can be entrained by two light input pathways, one involving the phospholipase C encoded by the norpA gene, the other mediated by the blue-light receptor cryptochrome. Phase shifts of molecular oscillations during the larval stage were smaller than those measured by adult behavior, suggesting molecularly transient responses during development.

Animals↗
PubMed2000

Physiological response properties of cat retinal ganglion cells projecting to suprachiasmatic nucleus.

The aim of this experiment was to characterize the physiological properties of cat retinal ganglion cells that project to the suprachiasmatic nucleus (SCN). Retrogradely labeled SCN-projecting ganglion cells were recorded extracellularly in vitro. For the first time, this study provides crucial information on visual response properties of ganglion cells in the entrainment circuitry. All recorded cells gave sustained responses (n = 9). Although most of the cells (n = 8) had an "on" center receptive field, one cell showed "on-off" center receptive field properties. The range of receptive field sizes was 2 to 5 deg. For most of the cells tested, the spectral wavelength that evoked peak responses was 500 nm (3 out of 5 cells). All recorded cells (n = 9) preferred still or extremely slow-moving stimuli (3.3 deg/s). These results indicate that cat SCN-projecting cells receive inputs from conventional photoreceptors. The hypothesis that both conventional and cryptochromic photoreceptors are involved in transferring photic signals is discussed.

Animals↗
PubMed2000

The period E-box is sufficient to drive circadian oscillation of transcription in vivo.

The minimum element from the Drosophila period promoter capable of driving in vivo cycling mRNA is the 69 bp circadian regulatory sequence (CRS). In cell culture, an 18 bp E-box element from the period promoter is regulated by five genes that are involved in the regulation of circadian expression in flies. This E-box is a target for transcriptional activation by bHLH-PAS proteins dCLOCK (dCLK) and CYCLE (CYC), this activation is inhibited by PERIOD (PER) and TIMELESS (TIM) together, and inhibition of dCLK/CYC by PER and TIM is blocked by CRYPTOCHROME (CRY) in the presence of light. Here, the same 18 bp E-box region generated rhythmic expression of luciferase in flies under both light-dark cycling and constant conditions. Flies heterozygous for the Clke(jrk) mutation maintained rhythmic expression from the E-box although at a lower level than wild type. Homozygous mutant Clk(jrk) animals had drastically lowered and arrhythmic expression. In a per01 background, expression from the E-box was high and not rhythmic. Transcription mediated by the per E-box was restricted to the same spatial pattern as the CRS. The per E-box DNA element and cognate binding proteins can confer per-like temporal and spatial expression. This demonstrates in vivo that the known circadian genes that form the core of the circadian oscillator in Drosophila integrate their activities at a single DNA element.

Animals↗
PubMed2003

A suite of photoreceptors entrains the plant circadian clock.

Circadian rhythms in plants are relatively robust, as they are maintained both in constant light of high fluence rates and in darkness. Plant circadian clocks exhibit the expected modes of photoentrainment, including period modulation by ambient light and phase resetting by brief light pulses. Several of the phytochrome and cryptochrome photoreceptors responsible have been studied in detail. This review concentrates on the resulting patterns of entrainment and on the multiple proposed mechanisms of light input to the circadian oscillator components.

Circadian Rhythm↗
PubMed2004

Distribution of circadian clock-related proteins in the cephalic nervous system of the silkworm, Bombyx mori.

In the circadian timing systems, input pathways transmit information on the diurnal environmental changes to a core oscillator that generates signals relayed to the body periphery by output pathways. Cryptochrome (CRY) protein participates in the light perception; period (PER), Cycle (CYC), and Doubletime (DBT) proteins drive the core oscillator; and arylalkylamines are crucial for the clock output in vertebrates. Using antibodies to CRY, PER, CYC, DBT, and arylalkylamine N-acetyltransferase (aaNAT), the authors examined neuronal architecture of the circadian system in the cephalic ganglia of adult silkworms. The antibodies reacted in the cytoplasm, never in the nuclei, of specific neurons. A cluster of 4 large Ia(1) neurons in each dorsolateral protocerebrum, a pair of cells in the frontal ganglion, and nerve fibers in the corpora cardiaca and corpora allata were stained with all antibodies. The intensity of PER staining in the Ia(1) cells and in 2 to 4 adjacent small cells oscillated, being maximal late in subjective day and minimal in early night. No other oscillations were detected in any cell and with any antibody. Six small cells in close vicinity to the Ia(1) neurons coexpressed CYC-like and DBT-like, and 4 to 5 of them also coexpressed aaNATlike immunoreactivity; the PER- and CRY-like antigens were each present in separate groups of 4 cells. The CYC- and aaNAT-like antigens were further colocalized in small groups of neurons in the pars intercerebralis, at the venter of the optic tract, and in the subesophageal ganglion. Remaining antibodies reacted with similarly positioned cells in the pars intercerebralis, and the DBT antibody also reacted with the cells in the subesophageal ganglion, but antigen colocalizations were not proven. The results imply that key components of the silkworm circadian system reside in the Ia(1) neurons and that additional, hierarchically arranged oscillators contribute to overt pacemaking. The retrocerebral neurohemal organs seem to serve as outlets transmitting central neural oscillations to the hemolymph. The frontal ganglion may play an autonomous function in circadian regulations. The colocalization of aaNAT- and CYC-like antigens suggests that the enzyme is functionally linked to CYC as in vertebrates and that arylalkylamines are involved in the insect output pathway.

ARNTL Transcription Factors↗
PubMed2007

Hofbauer-Buchner eyelet affects circadian photosensitivity and coordinates TIM and PER expression in Drosophila clock neurons.

Extraretinal photoreception is a common input route for light resetting signals into the circadian clock of animals. In Drosophila melanogaster, substantial circadian light inputs are mediated via the blue light photoreceptor CRYPTOCHROME (CRY) expressed in clock neurons within the brain. The current model predicts that, upon light activation, CRY interacts with the clock proteins TIMELESS (TIM) and PERIOD (PER), thereby inducing their degradation, which in turn leads to a resetting of the molecular oscillations within the circadian clock. Here the authors investigate the function of another putative extraretinal circadian photoreceptor, the Hofbauer-Buchner eyelet (H-B eyelet), located between the retina and the medulla in the fly optic lobes. Blocking synaptic transmission between the H-B eyelet and its potential target cells, the ventral circadian pacemaker neurons, impaired the flies' ability to resynchronize their behavior under jet-lag conditions in the context of nonfunctional retinal photoreception and a mutation in the CRY-encoding gene. The same manipulation also affected synchronized expression of the clock proteins TIM and PER in different subsets of the clock neurons. This shows that synaptic communication between the H-B eyelet and clock neurons contributes to synchronization of molecular and behavioral rhythms and confirms that the H-B eyelet functions as a circadian photoreceptor. Blockage of synaptic transmission from the H-B eyelet in the presence of functional compound eyes and the absence of CRY also results in increased numbers of flies that are unable to synchronize to extreme photoperiods, supplying independent proof for the role of the H-B eyelet as a circadian photoreceptor.

Animals↗
PubMed2001

Molecular regulation of circadian rhythms in Drosophila and mammals.

Through the use of genetically amenable model systems, we have begun to form a relatively clear idea as to the molecular mechanisms that constitute a functioning circadian clock. It is now known that mechanisms that underlie overt rhythms are conserved across species. At the basic core of the clock lies a transcriptional/translational feedback loop. The primary components of this loop are called clock genes and are similar for the fruit fly, Drosophila melanogaster, and mammalian systems. However, many questions regarding their regulation remain unanswered. In addition to their localization in brain areas associated with pacemaking function, clock genes are also found in peripheral tissues where their presence may confer circadian regulation upon local, tissue-specific functions. The light-dark cycle is the primary environmental stimulus for the synchronization of the circadian clock. In Drosophila, light is known to induce the degradation of a clock component resulting in the synchronization of the core clock mechanism. Photic signals are transmitted to the clock, at least in part, by the blue light photoreceptor cryptochrome. Although expression of several mammalian clock gene products is also altered in response to light, the photoreceptor(s) involved have not yet been defined.

Animals↗
PubMed2001

Intracellular trafficking of photoreceptors during light-induced signal transduction in plants.

Plants monitor changes in the ambient light environment by highly specialised photoreceptors, which include the red/far-red photoreversible phytochromes, the blue-light-absorbing cryptochromes and phototropin and the so-far-unidentified UVB photoreceptor(s). Light easily penetrates plant organs/tissues and reaches even the subcellular compartments of various cell types. Therefore, it is not surprising that the determination of the intracellular localisation of photoreceptors has been, for many years, a major, and often controversial, subject of plant photobiology and cell biology research. Phototropin, one of the blue-light photoreceptors of higher plants, controls phototropism by monitoring the direction of light, and it is localised in or at the plasmalemma. In contrast, the subcellular localisation of phytochromes changes dynamically and exhibits a very complex pattern. These photoreceptors are localised in the cytosol in dark- grown tissues. Irradiation, however, induces import of phytochromes into the nucleus. The import occurs in a light-quality- and light-quantity-dependent fashion and, as such, seems to be unique to higher plants. Light-induced accumulation of phytochromes in the nuclei correlates well with various physiological responses mediated by these photoreceptors. These observations indicate that light-dependent intracellular redistribution of phytochrome photoreceptors is one of the major regulatory steps in photomorphogenesis.

Darkness↗