Long-lived alphaMUPA transgenic mice exhibit pronounced circadian rhythms

Robust biological rhythms have been shown to affect life span. Biological clocks can be entrained by two feeding regimens, restricted feeding (RF) and caloric restriction (CR). RF restricts the time of food availability, whereas CR restricts the amount of calories with temporal food consumption. CR is known to retard aging and extend life span of animals via yet-unknown pathways. We hypothesize that resetting the biological clock could be one possible mechanism by which CR extends life span. Because it is experimentally difficult to uncouple calorie reduction from temporal food consumption, we took advantage of the murine urokinase-like plasminogen activator (alphaMUPA) transgenic mice overexpressing a serine protease implicated in brain development and plasticity; they exhibit spontaneously reduced eating and increased life span. Quantitative real-time PCR analysis revealed that alphaMUPA mice exhibit robust expression of the clock genes mPer1, mPer2, mClock, and mCry1 but not mBmal1 in the liver. We also found changes in the circadian amplitude and/or phase of clock-controlled output systems, such as feeding behavior, body temperature, and enteric cryptdin expression. A change in the light-dark regimen led to modified clock gene expression and abrogated circadian patterns of food intake in wild-type (WT) and alphaMUPA mice. Consequently, food consumption of WT mice increased, whereas that of alphaMUPA mice remained the same, indicating that reduced food intake occurs upstream and independently of the biological clock. Thus we surmise that CR could lead to pronounced and synchronized biological rhythms. Because the biological clock controls mitochondrial, hormonal, and physiological parameters, system synchronicity could lead to extended life span.

Circadian clock molecules CLOCK and CRYs modulate fibrinolytic activity by regulating the PAI-1 gene expression

Disruptions of circadian rhythms are associated with the development of many disorders. However, whether a disruption of the circadian clock can cause anomalies of the hemostatic balance remains unknown. The present study examines coagulation and fibrinolytic activities in circadian clock mutants, a homozygous Clock mutant and Cry1/Cry2 double knockout (Cry1/2-deficient) mice. The euglobulin clot lysis time (ELT) showed circadian variations that peaked at 21:00 (early night) in wild-type mice, suggesting that fibrinolytic activity is lowest at this time. The ELT was continuously reduced in Clock mutants, while the ELT was significantly increased and did not differ between day and night (9:00 and 21:00) in Cry1/2-deficient mice. The prothrombin time (PT) and activated partial prothrombin time (APTT) were constant in all genotypes. To identify which factors cause the loss of ELT rhythm, we measured fibrinolytic parameters in Clock mutant and Cry1/2-deficient mice. The robust circadian fluctuation of plasma plasminogen activator inhibitor 1 (PAI-1) that peaked at early night was damped to trough levels in Clock mutant mice. On the other hand, PAI-1 levels in Cry1/2-deficient mice remained equivalent to the peak levels of those in wild-type mice at both 9:00 and 21:00. Circadian changes in plasma PAI-1 levels seemed to be regulated at the level of gene expression, because the plasma PAI-1 levels in Clock mutant and Cry1/2-deficient mice were closely correlated with the level of PAI-1 mRNA transcript in these mice. Plasma plasminogen and hepatic mRNA levels were not rhythmic in wild-type mice, and continuously higher in Clock mutant than in wild-type or Cry1/2-deficient mice. In contrast, the activity and mRNA levels of tissue type plasminogen activator (t-PA), plasma levels and mRNA levels of plasminogen, and plasma levels of alpha2 plasmin inhibitor (alpha2PI) in all genotypes were constant throughout the day. Coagulation parameters such as factor VII, factor X, prothrombin and fibrinogen remained constant throughout the day, and were not affected by clock gene mutations. These results suggest that circadian clock molecules play an important role in hemostatic balance by regulating the fibrinolytic systems.

Cryptochrome-1-dependent execution of programmed cell death induced by singlet oxygen in Arabidopsis thaliana

Programmed cell death (PCD) plays an important role during the life cycle of higher organisms. Although several regulatory mechanisms governing PCD are thought to be conserved in animals and plants, light-dependent cell death represents a form of PCD that is unique to plants. The light requirement of PCD has often been associated with the production of reactive oxygen species during photosynthesis. In support of this hypothesis, hydrogen peroxide and superoxide have been shown to be involved in triggering a PCD response. In the present work, we have used the conditional flu mutant of Arabidopsis to analyze the impact of another reactive oxygen species, singlet oxygen, on cell death. Unexpectedly, the light-dependent release of singlet oxygen alone is not sufficient to induce PCD of flu seedlings but has to act together with a second concurrent blue light reaction. This blue-light-specific trigger of PCD could not be attributed to a photosynthetic reaction or redox change within the chloroplast but to the activation of the blue light/UVA-specific photoreceptor cryptochrome. The singlet oxygen-mediated and cryptochrome-dependent cell death response differs in several ways from PCD triggered by hydrogen peroxide/superoxide.

Veela defines a molecular link between Cryptochrome and Timeless in the light-input pathway to Drosophila's circadian clock

Organisms use the daily cycles of light and darkness to synchronize their internal circadian clocks with the environment. Because they optimize physiological processes and behavior, properly synchronized circadian clocks are thought to be important for the overall fitness. In Drosophila melanogaster, the circadian clock is synchronized with the natural environment by light-dependent degradation of the clock protein Timeless, mediated by the blue-light photoreceptor Cryptochrome (Cry). Here we report identification of a genetic variant, Veela, which severely disrupts this process, because these genetically altered flies maintain behavioral and molecular rhythmicity under constant-light conditions that usually stop the clock. We show that the Veela strain carries a natural timeless allele (ls-tim), which encodes a less-light-sensitive form of Timeless in combination with a mutant variant of the F-box protein Jetlag. However, neither the ls-tim nor the jetlag genetic variant alone is sufficient to disrupt light input into the central pacemaker. We show a strong interaction between Veela and cryptochrome genetic variants, demonstrating that the Jetlag, Timeless, and Cry proteins function in the same pathway. Veela also reveals a function for the two natural variants of timeless, which differ in their sensitivity to light. In combination with the complex array of retinal and extraretinal photoreceptors known to signal light to the pacemaker, this previously undescribed molecular component of photic sensitivity mediated by the two Timeless proteins reveals that an unexpectedly rich complexity underlies modulation of this process.

A cryptochrome/photolyase class of enzymes with single-stranded DNA-specific photolyase activity

Photolyases and cryptochrome blue-light photoreceptors are evolutionarily related flavoproteins that perform distinct functions. Photolyases repair UV-damaged DNA in many species from bacteria to plants and animals. Cryptochromes regulate growth and development in plants and the circadian clock in animals. Recently, a new branch of the photolyase/cryptochrome family was identified. Members of this branch exhibited no or trace levels of DNA repair activity in vivo and in vitro and, therefore, were considered to be cryptochromes, and they were named cryptochrome-DASH. Here, we show that Cry-DASH proteins from bacterial, plant, and animal sources actually are photolyases with high degree of specificity for cyclobutane pyrimidine dimers in ssDNA.

Effects of tryptophan photoproducts in the circadian timing system: searching for a physiological role for aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates adverse effects of dioxins, but its physiological role remains ambiguous. The similarity between AhR and canonical circadian clock genes suggests potential involvement of AhR in regulation of circadian timing. Photoproducts of tryptophan (TRP), including 6-formylindolo[3,2-b]carbazole (FICZ), have high affinity for AhR and are postulated as endogenous ligands. Although TRP photoproducts activate AhR signaling in vitro, their effects in vivo have not been investigated in mammals. Because TRP photoproducts may act as transducers of light, we examined their effects on the circadian clock. Intraperitoneal injection of TRP photoproducts or FICZ to C57BL/6J mice dose dependently induced AhR downstream targets, cytochrome P4501A1 (CYP1A1) and cytochrome P4501B1 mRNA expression, in liver. c-fos mRNA, a commonly used marker for light responses, was also induced with FICZ, and all responses were AhR dependent. A rat-immortalized suprachiasmatic nucleus (SCN) cell line, SCN 2.2, was used to examine the direct effect of TRP photoproducts on the molecular clock. Both TRP photoproducts and FICZ-increased CYP1A1 expression and prolonged FICZ incubation altered the circadian expression of clock genes (Per1, Cry1, and Cry2) in SCN 2.2 cells. Furthermore, FICZ inhibited glutamate-induced phase shifting of the mouse SCN electrical activity rhythm. Circadian light entrainment is critical for adjustment of the endogenous rhythm to environmental light cycle. Our results reveal a potential for TRP photoproducts to modulate light-dependent regulation of circadian rhythm through triggering of AhR signaling. This may lead to further understanding of toxicity of dioxins and the role of AhR in circadian rhythmicity.

Phototropin and light-signaling in phototropism

Blue-light-induced phototropism in higher plants is regulated by phototropin, which is a photoreceptor kinase that contains a flavin mononucleotide (FMN). Recently, it was found that this kinase is inhibited by the binding of the LOV2 (light-oxygen-voltage2) domain in the dark but that its activity is increased in the light by the release of the LOV2 domain. Phototropin-associated proteins have been identified, although the proteins that are phosphorylated by phototropin are still unknown. The asymmetrical auxin distribution caused by unilateral irradiation suggests that differential growth is induced by a difference in auxin-regulated gene expression between the shaded and illuminated sides of plant organs. Transcription-related factors, such as NPH4/ARF7, MSG2/IAA19 and SCF(TIR1), play key roles in this process.

Cryptochrome photoreceptors cry1 and cry2 antagonistically regulate primary root elongation in Arabidopsis thaliana

Cryptochromes are blue-light receptors controlling multiple aspects of plant growth and development. They are flavoproteins with significant homology to photolyases, but instead of repairing DNA they function by transducing blue light energy into a signal that can be recognized by the cellular signaling machinery. Here we report the effect of cry1 and cry2 blue light receptors on primary root growth in Arabidopsis thaliana seedlings, through analysis of both cryptochrome-mutant and cryptochrome-overexpressing lines. Cry1 mutant seedlings show reduced root elongation in blue light while overexpressing seedlings show significantly increased elongation as compared to wild type controls. By contrast, the cry2 mutation has the opposite effect on root elongation growth as does cry1, demonstrating that cry1 and cry2 act antagonistically in this response pathway. The site of cryptochrome signal perception is within the shoot, and the inhibitor of auxin transport, 1-N-naphthylphthalamic acid, abolishes the differential effect of cryptochromes on root growth, suggesting the blue-light signal is transmitted from the shoot to the root by a mechanism that involves auxin. Primary root elongation in blue light may thereby involve interaction between cryptochrome and auxin signaling pathways.

Phototropin involvement in the expression of genes encoding chlorophyll and carotenoid biosynthesis enzymes and LHC apoproteins in Chlamydomonas reinhardtii

Phototropin (PHOT) is a photoreceptor involved in a variety of blue-light-elicited physiological processes including phototropism, chloroplast movement and stomatal opening in plants. The work presented here tests whether PHOT is involved in expression of light-regulated genes in Chlamydomonas reinhardtii. When C. reinhardtii was transferred from the dark to very low-fluence rate white light, there was a substantial increase in the level of transcripts encoding glutamate-1-semialdehyde aminotransferase (GSAT), phytoene desaturase (PDS) and light-harvesting polypeptides (e.g. LHCBM6). Increased levels of these transcripts were also elicited by low-intensity blue light, and this blue-light stimulation was suppressed in three different RNAi strains that synthesize low levels of PHOT. The levels of GSAT and LHCBM6 transcripts also increased following exposure of algal cells to low-intensity red light (RL). The red-light-dependent increase in transcript abundance was not affected by the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, implying that the influence of RL on transcript accumulation was not controlled by cytoplasmic redox conditions, and that a red-light photoreceptor(s) may be involved in regulating the levels of transcripts from specific photosynthesis-related genes in C. reinhardtii. Interestingly, elevated GSAT and LHCBM6 transcript levels in RL were significantly reduced in the PHOT RNAi strains, which raises the possibility of co-action between blue and RL signaling pathways. Microarray experiments indicated that the levels of several transcripts for photosystem (PS) I and II polypeptides were also modulated by PHOT. These data suggest that, in C. reinhardtii, (i) PHOT is involved in blue-light-mediated changes in transcript accumulation, (ii) synchronization of the synthesis of chlorophylls (Chl), carotenoids, Chl-binding proteins and other components of the photosynthetic apparatus is achieved, at least in part, through PHOT-mediated signaling, and (iii) a red-light photoreceptor can also influence levels of certain transcripts associated with photosynthetic function, although its action requires normal levels of PHOT.

Genome-wide analysis of light sensing in Prochlorococcus

Prochlorococcus MED4 has, with a total of only 1,716 annotated protein-coding genes, the most compact genome of a free-living photoautotroph. Although light quality and quantity play an important role in regulating the growth rate of this organism in its natural habitat, the majority of known light-sensing proteins are absent from its genome. To explore the potential for light sensing in this phototroph, we measured its global gene expression pattern in response to different light qualities and quantities by using high-density Affymetrix microarrays. Though seven different conditions were tested, only blue light elicited a strong response. In addition, hierarchical clustering revealed that the responses to high white light and blue light were very similar and different from that of the lower-intensity white light, suggesting that the actual sensing of high light is mediated via a blue-light receptor. Bacterial cryptochromes seem to be good candidates for the blue-light sensors. The existence of a signaling pathway for the redox state of the photosynthetic electron transport chain was suggested by the presence of genes that responded similarly to red and blue light as well as genes that responded to the addition of DCMU [3-(3,4-dichlorophenyl)-1,1-N-N'-dimethylurea], a specific inhibitor of photosystem II-mediated electron transport.

Circadian Intraocular Pressure Rhythm Is Generated by Clock Genes

PURPOSE

The present study in a mouse model was undertaken to reveal the role of the circadian clock genes Cry1 and Cry2 in generation of 24-hour intraocular pressure (IOP) rhythm.

METHODS

IOP was measured at eight time points daily (circadian time [CT] 0, 3, 6, 9, 12, 15, 18, and 21 hours), using a microneedle method in four groups of C57BL/6J mice (groups 1 and 3, wild-type; groups 2 and 4, Cry-deficient [Cry1-/-Cry2-/-]). During the IOP measurements, mice in groups 1 and 2 were maintained in a 12-hour light-dark cycle (LD), whereas mice in groups 3 and 4 were kept in a constant darkness (DD) that started 24 to 48 hours before the measurements. Circadian IOP variations in each group were evaluated by one-way analysis of variance (ANOVA) and Scheffé tests.

RESULTS

In wild-type mice living in LD conditions, pressures measured in the light phase were significantly lower than those in the dark phase. This daily rhythm was maintained under DD conditions with low pressure in the subjective day and high pressure in the subjective night. In contrast, Cry-deficient mice did not show significant circadian changes in IOP, regardless of environmental light conditions.

CONCLUSIONS

These findings demonstrate that clock oscillatory mechanisms requiring the activity of core clock genes are essential for the generation of a circadian rhythm of intraocular pressure.

Dynamic switching mechanisms in LOV1 and LOV2 domains of plant phototropins

LOV domains are the light-sensitive portion of plant phototropins. They absorb light through a flavin cofactor, photochemically form a covalent bond between the chromophore and a cysteine residue in the protein, and proceed to mediate activation of an attached kinase domain. Although the photoreaction itself is now well-characterized experimentally and computationally, it is still unclear how the formation of the adduct leads to kinase activation. We have performed molecular dynamics simulations on the LOV1 domain of Chlamydomonas reinhardtii and the LOV2 domain of Avena sativa, both before and after the photoreaction, to answer this question. The extensive simulations, over 240 ns in duration, reveal significant differences in how the LOV1 and LOV2 domains respond to photoactivation. The simulations indicate that LOV1 activation is likely caused by a change in hydrogen bonding between protein and ligand that destabilizes a highly conserved salt bridge, whereas LOV2 activation seems to result from a change in the flexibility of a set of protein loops. Results of electrostatics calculations, principal component analysis, sequence alignments, and root mean-square deviation analysis corroborate the above findings.

Functional and expression analysis of Arabidopsis SPA genes during seedling photomorphogenesis and adult growth

The four members of the Suppressor of phyA-105 (SPA) gene family function to inhibit photomorphogenesis in dark- and light-grown seedlings. Additionally, SPA1-SPA4 regulate elongation growth of adult plants. In these processes, SPA2, SPA3 and SPA4 have overlapping but distinct functions. Here, we have further investigated the role of SPA1 which is partially masked by functional redundancy. We show that SPA1 represses not only red, far-red and blue light responses in a PHYA-dependent fashion, but also acts to suppress light signaling in darkness. We demonstrate that deletion-derivatives of SPA1 lacking the complete N-terminus or part of the kinase-like domain retain SPA1 function in light- and dark-grown seedlings, while deletion of the constitutive photomorphogenesis 1 (COP1)-interacting coiled-coil domain eliminates SPA1 activity. This suggests that the coiled-coil domain and the WD-repeat domain of SPA1 are sufficient for SPA1 function. An analysis of spa2 spa3 spa4 triple mutants demonstrates that SPA1, like SPA2, is sufficient for normal etiolation of dark-grown seedlings. In light-grown seedlings and adult plants, in contrast, SPA1 function is divergent from SPA2 function, with SPA1 playing the predominant role. Levels of SPA1, SPA3 and SPA4 transcript are increased by red, far-red and blue light, consistent with a role of these three SPA genes in light-grown seedlings. The abundance of SPA2 mRNA, in contrast, is not altered by light. Taken together, the analysis of SPA transcript levels suggests that differences in SPA gene expression patterns contribute to divergence in SPA1-SPA4 function.

The circadian rhythm of glucocorticoids is regulated by a gating mechanism residing in the adrenal cortical clock

In mammals, the master clock of the suprachiasmatic nuclei (SCN) and subordinate clocks found throughout the body coordinate circadian rhythms of behavior and physiology. We characterize the clock of the adrenal, an important endocrine gland that synchronizes physiological and metabolic rhythms. Clock gene expression was detected in the outer adrenal cortex prefiguring a role of the clock in regulating gluco- and mineral corticoid biogenesis. In Per2/Cry1 double mutant mice, which lack a circadian clock, hypothalamus/pituitary/adrenal axis regulation was defective. Organ culture and tissue transplantation suggest that the adrenal pacemaker gates glucocorticoid production in response to adrenocorticotropin (ACTH). In vivo the adrenal circadian clock can be entrained by light. Transcriptome profiling identified rhythmically expressed genes located at diverse nodes of steroid biogenesis that may mediate gating of the ACTH response by the adrenal clock.

JETLAG resets the Drosophila circadian clock by promoting light-induced degradation of TIMELESS

Organisms ranging from bacteria to humans synchronize their internal clocks to daily cycles of light and dark. Photic entrainment of the Drosophila clock is mediated by proteasomal degradation of the clock protein TIMELESS (TIM). We have identified mutations in jetlag-a gene coding for an F-box protein with leucine-rich repeats-that result in reduced light sensitivity of the circadian clock. Mutant flies show rhythmic behavior in constant light, reduced phase shifts in response to light pulses, and reduced light-dependent degradation of TIM. Expression of JET along with the circadian photoreceptor cryptochrome (CRY) in cultured S2R cells confers light-dependent degradation onto TIM, thereby reconstituting the acute response + of the circadian clock to light in a cell culture system. Our results suggest that JET is essential for resetting the clock by transmitting light signals from CRY to TIM.

Involvement of rice cryptochromes in de-etiolation responses and flowering

In order to elucidate the function of cryptochromes (cry) in rice, we have characterized all rice CRY genes, including OsCRY1a, OsCRY1b and OsCRY2. Our studies revealed that OsCRY1 genes were mainly expressed in the green plant tissue, while OsCRY2 gene expression was high in the coleoptile, flower and callus. Light treatment affected neither the expression of any of the OsCRY genes nor the stability of their transcripts. However, it was found that Oscry2 protein was negatively regulated by blue light. Moreover, the level of Oscry2 protein also decreased upon red light treatment, and this red light-dependent degradation was shown to be mediated by phytochrome B. Overexpression of OsCRY1 genes resulted in an increased responsiveness to blue light when measuring coleoptile growth inhibition. Moreover, growth of leaf sheaths and blades was also repressed more in OsCRY1 overexpressers than in wild type (WT) under blue light conditions. These results suggest that Oscry1s are responsible for regulating blue light-mediated de-etiolation in rice. In addition, OsCRY2 antisense transgenic rice flowered later than WT under both long-day and short-day conditions, indicating that Oscry2 is involved in the promotion of flowering time in rice.

Hypothermia modulates circadian clock gene expression in lizard peripheral tissues

The molecular mechanisms whereby the circadian clock responds to temperature changes are poorly understood. The ruin lizard Podarcis sicula has historically proven to be a valuable vertebrate model for exploring the influence of temperature on circadian physiology. It is an ectotherm that naturally experiences an impressive range of temperatures during the course of the year. However, no tools have been available to dissect the molecular basis of the clock in this organism. Here, we report the cloning of three lizard clock gene homologs (Period2, Cryptochrome1, and Clock) that have a close phylogenetic relationship with avian clock genes. These genes are expressed in many tissues and show a rhythmic expression profile at 29 degrees C in light-dark and constant darkness lighting conditions, with phases comparable to their mammalian and avian counterparts. Interestingly, we show that at low temperatures (6 degrees C), cycling clock gene expression is attenuated in peripheral clocks with a characteristic increase in basal expression levels. We speculate that this represents a conserved vertebrate clock gene response to low temperatures. Furthermore, these results bring new insight into the issue of whether circadian clock function is compatible with hypothermia.

The BMAL1 C terminus regulates the circadian transcription feedback loop

The circadian clock is driven by cell-autonomous transcription/translation feedback loops. The BMAL1 transcription factor is an indispensable component of the positive arm of this molecular oscillator in mammals. Here, we present a molecular genetic screening assay for mutant circadian clock proteins that is based on real-time circadian rhythm monitoring in cultured fibroblasts. By using this assay, we identified a domain in the extreme C terminus of BMAL1 that plays an essential role in the rhythmic control of E-box-mediated circadian transcription. Remarkably, the last 43 aa of BMAL1 are required for transcriptional activation, as well as for association with the circadian transcriptional repressor CRYPTOCHROME 1 (CRY1), depending on the coexistence of CLOCK protein. C-terminally truncated BMAL1 mutant proteins still associate with mPER2 (another protein of the negative feedback loop), suggesting that an additional repression mechanism may converge on the N terminus. Taken together, these results suggest that the C-terminal region of BMAL1 is involved in determining the balance between circadian transcriptional activation and suppression.

Posttranslational regulation of the mammalian circadian clock by cryptochrome and protein phosphatase 5

The molecular oscillator that drives circadian rhythmicity in mammals obtains its near 24-h periodicity from posttranslational regulation of clock proteins. Activity of the major clock kinase casein kinase I (CKI) epsilon is regulated by inhibitory autophosphorylation. Here we show that protein phosphatase (PP) 5 regulates the kinase activity of CKIepsilon. We demonstrate that cryptochrome regulates clock protein phosphorylation by modulating the effect of PP5 on CKIepsilon. Like CKIepsilon, PP5 is expressed both in the master circadian clock in the suprachiasmatic nuclei and in peripheral tissues independent of the clock. Expression of a dominant-negative PP5 mutant reduces PER phosphorylation by CKIepsilon in vivo, and down-regulation of PP5 significantly reduces the amplitude of circadian cycling in cultured human fibroblasts. Collectively, these findings indicate that PP5, CKIepsilon, and cryptochrome dynamically regulate the mammalian circadian clock.

Photochemistry and photobiology of cryptochrome blue-light photopigments: the search for a photocycle

Cryptochromes are flavoproteins that exhibit high sequence and structural similarity to the light-dependent DNA-repair enzyme, photolyase. Cryptochromes have lost the ability to repair DNA; instead, they use the energy from near-UV/blue light to regulate a variety of growth and adaptive processes in organisms ranging from bacteria to humans. The photocycle of cryptochrome is not yet known, although it is hypothesized that it may share some similarity to that of photolyase, which utilizes light-driven electron transfer from the catalytic flavin chromophore. In this review, we present genetic evidence for the photoreceptive role of cryptochromes and discuss recent biochemical studies that have furthered our understanding of the cryptochrome photocycle. In particular, the role of the unique C-terminal domain in cryptochrome phototransduction is discussed.

Phototaxis in the cyanobacterium Synechocystis sp. PCC 6803: role of different photoreceptors

The second cyanobacterial phytochrome Cph2 from Synechocystis sp. PCC 6803 was suggested as a part of a light-stimulated signal transduction chain inhibiting movement toward blue light. Cph2 has the two bilin binding sites, cysteine-129 and cysteine-1022, that might be involved in sensing of red/far-red and blue light, respectively. Here, we present data on wavelength dependence of the phototaxis inhibition under blue light, indicating that Cph2 itself is the photoreceptor for this blue light response. We found that inhibition of blue-light phototaxis in wild-type cells occurred below the transition point of about 470 nm. Substitution of cysteine-1022 with valine led to photomovement of the cells toward blue light (cph2(-) mutant phenotype). Analysis of mutants lacking cysteine-129 in the N-terminal chromophore binding domain indicated that this domain is also important for Cph2 function or folding of the protein. Furthermore, putative blue-light and phytochrome-like photoreceptors encoded by the Synechocystis sp. PCC 6803 genome were inactivated in wild-type and cph2 knockout mutant background. Our results suggest that none of these potential photoreceptors interfere with Cph2 function, although inactivation of taxD1 as well as slr1694 encoding a BLUF protein led to cells that reversed the direction of movement under blue light illumination in mutant strains of cph2.

The Fern Adiantum capillus-veneris Lacks Stomatal Responses to Blue Light

We investigated the responses of stomata to light in the fern Adiantum capillus-veneris, a typical species of Leptosporangiopsida. Stomata in the intact leaves of the sporophytes opened in response to red light, but they did not open when blue light was superimposed on the red light. The results were confirmed in the isolated Adiantum epidermis. The red light-induced stomatal response was not affected by the mutation of phy3, a chimeric protein of phytochrome and phototropin in this fern. The lack of a blue light-specific stomatal response was observed in three other fern species of Leptosporangiopsida, i.e. Pteris cretica, Asplenium scolopendrium and Nephrolepis auriculata. Fusicoccin, an activator of the plasma membrane H(+)-ATPase, induced both stomatal opening and H(+) release in the Adiantum epidermis. Adiantum phototropin genes AcPHOT1 and AcPHOT2 were expressed in the fern guard cells. The transformation of an Arabidopsis phot1 phot2 double mutant, which lost blue light-specific stomatal opening, with AcPHOT1 restored the stomatal response to blue light. Taken together, these results suggest that ferns of Leptosporangiopsida lack a blue light-specific stomatal response, although the functional phototropin and plasma membrane H(+)-ATPase are present in this species.

[Blue light signaling in mosses]

Arabidopsis thaliana contains five identified blue light photoreceptors and at least one unidentified blue/UV-A light photoreceptor. Cryptochromes (CRY1 and CRY2) modulate photomorphological processes, flowering time, and circadian timing while phototrophins (PHOT1 and PHOT2) modulate phototropism, chloroplast movement, and stomatal opening. Flavins are the chromophores and absorb in the blue and UV-A range. Considerable information is known about the structure and mode of action of these photoreceptors. The moss Physcomitrella patens contains two identified cryptochromes (CRY1a and CRY1b) which modulate side branch formation and auxin metabolism. Blue light-induced chloroplast movement was mediated by four phototropins. Transduction of the blue/UV-A stimulus does involve calcium signaling in moss cells.

Functional and signaling mechanism analysis of rice CRYPTOCHROME 1

Cryptochromes (CRY) are blue-light photoreceptors that mediate various light responses, such as inhibition of hypocotyl elongation, enhancement of cotyledon expansion, anthocyanin accumulation and stomatal opening in Arabidopsis. The signaling mechanism of Arabidopsis CRY is mediated through direct interaction with COP1, a negative regulator of photomorphogenesis. CRY has now been characterized in tomato, pea, moss and fern, but its function in monocots is largely unknown. Here we report the function and basic signaling mechanism of rice cryptochrome 1 (OsCRY1). Overexpresion of OsCRY1b resulted in a blue light-dependent short hypcotyl phenotype in Arabidopsis, and a short coleoptile, leaf sheath and leaf blade phenotype in rice (Oryza sativa). On fusion with beta-glucuronidase (GUS), the C-terminal domain of either OsCRY1a (OsCCT1a) or OsCRY1b (OsCCT1b) mediated a constitutive photomorphogenic (COP) phenotype in both Arabidopsis and rice, whereas OsCCT1b mutants corresponding to missense mutations in previously described Arabidopsis cry1 alleles failed to confer a COP phenotype. Yeast two-hybrid and subcellular co-localization studies demonstrated that OsCRY1b interacted physically with rice COP1 (OsCOP1). From these results, we conclude that OsCRY1 is implicated in blue-light inhibition of coleoptile and leaf elongation during early seedling development in rice, and that the signaling mechanism of OsCRY1 involves direct interaction with OsCOP1.

Circadian rhythms in murine pups develop in absence of a functional maternal circadian clock

A genetic approach was used to investigate whether the emergence of circadian rhythms in murine pups is dependent on a functional maternal clock. Arrhythmic females bearing either the mPer1Brdm1/Per2Brdm1 or mPer2Brdm1/Cry1-/- double-mutant genotype were crossed with wild-type males under constant darkness. The heterozygous offspring have the genetic constitution for a functional circadian clock. Individual pups born to arrhythmic mPer1Brdm1/Per2Brdm1 and mPer2Brdm1/Cry1-/- mothers in constant darkness without external zeitgeber developed normal circadian rhythms, but their clocks were less synchronized to each other compared to wild-type animals. These findings indicate that development of circadian rhythms does not depend on a functional circadian clock in maternal tissue, extending previous findings obtained from pups born to SCN-lesioned mothers.

Immunoreactivities to three circadian clock proteins in two ground crickets suggest interspecific diversity of the circadian clock structure

The closely related crickets Dianemobius nigrofasciatus and Allonemobius allardi exhibit similar circadian rhythms and photoperiodic responses, suggesting that they possess similar circadian and seasonal clocks. To verify this assumption, antisera to Period (PER), Doubletime (DBT), and Cryptochrome (CRY) were used to visualize circadian clock neurons in the cephalic ganglia. Immunoreactivities referred to as PER-ir, DBT-ir, and CRY-ir were distributed mainly in the optic lobes (OL), pars intercerebralis (PI), dorsolateral protocerebrum, and the subesophageal ganglion (SOG). A system of immunoreactive cells in the OL dominates in D. nigrofasciatus, while immunoreactivities in the PI and SOG prevail in A. allardi. Each OL of D. nigrofasciatus contains 3 groups of cells that coexpress PER-ir and DBT-ir and send processes over the frontal medulla face to the inner lamina surface, suggesting functional linkage to the compound eye. Only 2 pairs of PER-ir cells (no DBT-ir) were found in the OL of A. allardi. Several groups of PER-ir cells occur in the brain of both species. The PI also contains DBT-ir and CRY-ir cells, but in A. allardi, most of the DBT-ir is confined to the SOG. Most immunoreactive cells in the PI and in the dorsolateral brain send their fibers to the contralateral corpora cardiaca and corpora allata. The proximity and, in some cases, proven identity of the PER-ir, DBT-ir, and CRY-ir perikarya are consistent with presumed interactions between the examined clock components. The antigens were always found in the cytoplasm, and no diurnal oscillations in their amounts were detected. The photoperiod, which controls embryonic diapause, the rate of larval development, and the wing length of crickets, had no discernible effect on either distribution or the intensity of the immunostaining.

Absorption and fluorescence spectroscopic characterization of cryptochrome 3 from Arabidopsis thaliana

The blue light photoreceptor cryptochrome 3 (cry3) from Arabidopsis thaliana was characterized at room temperature in vitro in aqueous solution by optical absorption and emission spectroscopic studies. The protein non-covalently binds the chromophores flavin adenine dinucleotide (FAD) and N5,N10-methenyl-5,6,7,8-tetrahydrofolate (MTHF). In the dark-adapted state of cry3, the bound FAD is present in the oxidized form (FAD(ox), ca. 38.5%), in the semiquinone form (FADH., ca. 5%), and in the fully reduced neutral form (FAD(red)H2) or fully reduced anionic form (FAD(red)H-, ca. 55%). Some amount of FAD (ca. 1.5%) in the oxidized state remains unbound probably caused by chromophore release and/or denaturation. Förster-type energy transfer from MTHF to FAD(ox) is observed. Photo-excitation reversibly modifies the protein conformation causing a slight rise of the MTHF absorption strength and an increase of the MTHF fluorescence efficiency (efficient protein conformation photo-cycle). Additionally there occurs reversible reduction of bound FAD(ox) to FAD(red)H2 (or FAD(red)H-, FAD(ox) photo-cycle of moderate efficiency), reversible reduction of FADH. to FAD(red)H2 (or FAD(red)H-, FADH. photo-cycle of high efficiency), and modification of re-oxidable FAD(red)H2 (or FAD(red)H-) to permanent FAD(red)H2 (or FAD(red)H-) with low quantum efficiency. Photo-excitation of MTHF causes the reversible formation of a MTHF species (MTHF', MTHF photo-cycle, moderate quantum efficiency) with slow recovery to the initial dark state, and also the formation of an irreversible photoproduct (MTHF'').

Kinetic measurement of transient dimerization and dissociation reactions of Arabidopsis phototropin 1 LOV2 domain

Photochemical reaction of a plant blue-light photoreceptor, Arabidopsis phototropin 1-LOV (light-oxygen-voltage sensing) domain 2, was studied with a view to the diffusion coefficients (D) using the pulsed-laser-induced transient grating method. Although the reaction dynamics completes at a rate of several microseconds as long as it is monitored by the absorption change, the diffusion coefficient was found to be time-dependent in a time range of submilliseconds to seconds. The observed signal can be analyzed by the two-state model, which includes the D-value decrease from D of the reactant (9.8 +/- 0.4) x 10(-11) m2/s to D of the product (8.0 +/- 0.4) x 10(-11) m2/s. The D-value of the reactant implies that the dominant form in the ground state of phototropin 1 LOV2 is the monomeric form in a concentration range of 50-200 microM. According to the Stokes-Einstein relationship, the D-change can be explained by a volume increase of 1.8 times. Furthermore, the rate of the D-change was roughly proportional to the concentration of the sample. These two observations indicate that the LOV2 domain transiently forms a dimer upon photoexcitation. When the sample concentration is increased (>180 microM), a new signal component appears within a few milliseconds. This signal represents a D increase from 8.0 x 10(-11) m2/s to 9.8 x 10(-11) m2/s with a time constant of 300 micros. The completely opposite D-change from that observed in a lower concentration, as well as the concentration dependence, implies that a dimer is formed in the ground state in a higher concentration range, even though the fraction of the dimer is still minor in this range. This dimer is photodissociated, with a time constant of 300 micros. This research clearly shows that the time-resolved diffusion measurement is a very powerful tool for detecting spectrally silent association/dissociation processes during chemical reactions. The photoreaction of the LOV2 domain is discussed.

Twenty-four-hour rhythmic gene expression in the rhesus macaque adrenal gland

The suprachiasmatic nucleus plays a key role in the circadian secretion of adrenocortical hormones. However, there is evidence from mouse studies that components of the circadian clock are also expressed within the adrenal gland itself. In the present study we performed genome-wide expression profiling to determine whether the adrenal gland of rhesus monkeys shows temporal gene expression across a 24-h period. We identified 322 transcripts with rhythmic patterns of expression and found that the phase distribution of cycling transcripts varied across the day, with more genes showing activation during the night. We classified the transcripts by their function and clustered them according to their participation in common biochemical pathways: 1) catecholamine synthesis and reuptake; 2) cholesterol cleavage and dehydroepiandrosterone sulfate synthesis; 3) protein synthesis and turnover; and 4) the circadian clock mechanism. In an additional experiment, we assessed the expression of various clock genes at two time points, 12 h apart. We found that expression of Bmal1 and Cry1 was higher at 1300 h, or zeitgeber time 6, whereas expression of Per1 was higher at 0100 h (zeitgeber time 18). Expression levels of Rev-erbalpha were higher at 0100 h than at 1300 h (P<0.05), and immunohistochemistry revealed a strong expression of this transcription factor specifically in chromaffin cells of the adrenal medulla. Taken together, the data indicate that the primate adrenal gland shows rhythmic expression of genes associated with cell biology and synthesis of steroids and catecholamines. Moreover, they strongly imply the existence of an intrinsic circadian clock.

Cryptochrome 1 from Brassica napus is up-regulated by blue light and controls hypocotyl/stem growth and anthocyanin accumulation

Cryptochromes are blue/ultraviolet-A light sensing photoreceptors involved in regulating various growth and developmental responses in plants. Investigations on the structure and functions of cryptochromes in plants have been largely confined to Arabidopsis (Arabidopsis thaliana), tomato (Lycopersicon esculentum), and pea (Pisum sativum). We report here the characterization of the cryptochrome 1 gene from Brassica napus (BnCRY1), an oilseed crop, and its functional validation in transgenics. The predicted BnCRY1 protein sequence shows a high degree of sequence identity (94%) to Arabidopsis CRY1. A semiquantitative reverse transcription-polymerase chain reaction and the western-blot analysis revealed that blue light up-regulates its transcript and protein levels in young seedlings. The BnCRY1 promoter harbors conventional light-responsive cis-acting elements, which presumably impart light activation to the GUS (beta-glucuronidase) reporter gene expressed in Arabidopsis. Although the BnCRY1 transcript could be detected in all the tissues examined, its protein was virtually undetectable in mature leaves and the root, indicating a tissue-specific translational control or protein turnover. The antisense-BnCRY1 Brassica transgenic seedlings accumulated negligible levels of CRY1 protein and displayed an elongated hypocotyl when grown under continuous white or blue light (but not under red or far-red light); the accumulation of anthocyanins was also reduced significantly. The adult transformants were also found to be tall when grown under natural light environment in a containment facility without any artificial illumination. These data provide functional evidence for a role of blue light up-regulated cry1 in controlling photomorphogenesis in Brassica species.

Molecular cloning, mRNA expression, and immunocytochemical localization of a putative blue-light photoreceptor CRY4 in the chicken pineal gland

In non-mammalian vertebrates, the pineal gland contains an endogenous circadian oscillator and serves as a photosensitive neuroendocrinal organ. To better understand the pineal phototransduction mechanism, we focused on the chicken putative blue-light photoreceptive molecule, Cryptochrome4 (cCRY4). Here we report the molecular cloning of pineal cCry4 cDNA, the in vivo expression of cCry4 mRNA, and the detection of cCRY4 protein. cCry4 is transcribed in a wide variety of chick tissues out of which the pineal gland and retina contain high levels of cCry4 mRNA. In the pineal gland, under 12 h light : 12 h dark cycles, the levels of both cCry4 mRNA and cCRY4 protein showed diurnal changes, and in cultured chick pineal cells, the cCry4 mRNA level was not only up-regulated by light but also controlled by circadian signals. Immunoblot analysis with a cCRY4-specific antibody detected cCRY4 in a soluble fraction of the pineal lysate. Immunocytochemistry revealed that cCRY4 was expressed in many parenchymal cells and a limited number of stromal cells. These cCRY4 features strikingly contrast with those of the chick pineal photoreceptor pinopsin, suggesting a possible temporal and/or spatial duplicity of the pineal photoreceptive system, the opsin- and CRY-based mechanisms.

Blue-light-induced changes in Arabidopsis cryptochrome 1 probed by FTIR difference spectroscopy

Cryptochromes are blue-light photoreceptors that regulate a variety of responses in animals and plants, including circadian entrainment in Drosophila and photomorphogenesis in Arabidopsis. They comprise a photolyase homology region (PHR) of about 500 amino acids and a C-terminal extension of varying length. In the PHR domain, flavin adenine dinucleotide (FAD) is noncovalently bound. The presence of a second chromophore, such as methenyltetrahydrofolate, in animal and plant cryptochromes is still under debate. Arabidopsis cryptochrome 1 (CRY1) has been intensively studied with regard to function and interaction of the protein in vivo and in vitro. However, little is known about the pathway from light absorption to signal transduction on the molecular level. We investigated the full-length CRY1 protein by Fourier transform infrared (FTIR) and UV/vis difference spectroscopy. Starting from the fully oxidized state of the chromophore FAD, a neutral flavoprotein radical is formed upon illumination in the absence of any exogenous electron donor. A preliminary assignment of the chromophore bands is presented. The FTIR difference spectrum reveals only moderate changes in secondary structure of the apoprotein in response to the photoreduction of the chromophore. Deprotonation of an aspartic or glutamic acid, probably D396, accompanies radical formation, as is deduced from the negative band at 1734 cm(-)(1) in D(2)O. The main positive band at 1524 cm(-)(1) in the FTIR spectrum shows a strong shift to lower frequencies as compared to other flavoproteins. Together with the unusual blue-shift of the absorption in the visible range to 595 nm, this clearly distinguishes the radical form of CRY1 from those of structurally highly homologous DNA photolyases. As a consequence, the direct comparison of cryptochrome to photolyase in terms of photoreactivity and mechanism has to be made with caution.

Drosophila CRYPTOCHROME is a circadian transcriptional repressor

BACKGROUND

Although most circadian clock components are conserved between Drosophila and mammals, the roles assigned to the CRYPTOCHROME (CRY) proteins are very different: Drosophila CRY functions as a circadian photoreceptor, whereas mammalian CRY proteins (mCRY1 and 2) are transcriptional repressors essential for molecular clock oscillations.

RESULTS

Here we demonstrate that Drosophila CRY also functions as a transcriptional repressor. We found that RNA levels of genes directly activated by the transcription factors CLOCK (CLK) and CYCLE (CYC) are derepressed in cry(b) mutant eyes. Conversely, while overexpression of CRY and PERIOD (PER) in the eye repressed CLK/CYC activity, neither PER nor CRY repressed individually. Drosophila CRY also repressed CLK/CYC activity in cell culture. Repression by CRY appears confined to peripheral clocks, since neither cry(b) mutants nor overexpression of PER and CRY together in pacemaker neurons significantly affected molecular or behavioral rhythms. Increasing CLK/CYC activity by removing two repressors, PER and CRY, led to ectopic expression of the timeless clock gene, similar to overexpression of Clk itself.

CONCLUSIONS

Drosophila CRY functions as a transcriptional repressor required for the oscillation of peripheral circadian clocks and for the correct specification of clock cells.

Signaling mediated by the dopamine D2 receptor potentiates circadian regulation by CLOCK:BMAL1

Environmental cues modulate a variety of intracellular pathways whose signaling is integrated by the molecular mechanism that constitutes the circadian clock. Although the essential gears of the circadian machinery have been elucidated, very little is known about the signaling systems regulating it. Here, we report that signaling mediated by the dopamine D2 receptor (D2R) enhances the transcriptional capacity of the CLOCK:BMAL1 complex. This effect involves the mitogen-activated protein kinase transduction cascade and is associated with a D2R-induced increase in the recruiting and phosphorylation of the transcriptional coactivator cAMP-responsive element-binding protein (CREB) binding protein. Importantly, CLOCK:BMAL1-dependent activation and light-inducibility of mPer1 gene transcription is drastically dampened in retinas of D2R-null mice. Because dopamine is the major catecholamine in the retina, central for the neural adaptation to light, our findings establish a physiological link among photic input, dopamine signaling, and the molecular clock machinery.

Functional analysis of circadian pacemaker neurons in Drosophila melanogaster

The molecular mechanisms of circadian rhythms are well known, but how multiple clocks within one organism generate a structured rhythmic output remains a mystery. Many animals show bimodal activity rhythms with morning (M) and evening (E) activity bouts. One long-standing model assumes that two mutually coupled oscillators underlie these bouts and show different sensitivities to light. Three groups of lateral neurons (LN) and three groups of dorsal neurons govern behavioral rhythmicity of Drosophila. Recent data suggest that two groups of the LN (the ventral subset of the small LN cells and the dorsal subset of LN cells) are plausible candidates for the M and E oscillator, respectively. We provide evidence that these neuronal groups respond differently to light and can be completely desynchronized from one another by constant light, leading to two activity components that free-run with different periods. As expected, a long-period component started from the E activity bout. However, a short-period component originated not exclusively from the morning peak but more prominently from the evening peak. This reveals an interesting deviation from the original Pittendrigh and Daan (1976) model and suggests that a subgroup of the ventral subset of the small LN acts as "main" oscillator controlling M and E activity bouts in Drosophila.

Evidence for a putative antennal clock in Mamestra brassicae: molecular cloning and characterization of two clock genes--period and cryptochrome-- in antennae

Circadian rhythms are generated by endogenous circadian clocks, organized in central and peripheral clocks. An antennal peripheral clock has been demonstrated to be necessary and sufficient to generate Drosophila olfactory rhythms in response to food odours. As moth pheromonal communication has been demonstrated to follow daily rhythms, we thus investigated the occurence of a putative antennal clock in the noctuid Mamestra brassicae. From moth antennae, we isolated two full-length cDNAs encoding clock genes, period and cryptochrome, which appeared to be expressed throughout the body. In the antennae, expression of both transcripts was restricted to cells that likely represent olfactory sensory neurones. Our results suggest the occurence of a putative antennal clock that could participate in the pheromonal communication rhythms observed in vivo.

Post-translational regulation of circadian transcriptional CLOCK(NPAS2)/BMAL1 complex by CRYPTOCHROMES

Mammalian CLOCK(NPAS2), BMAL1 and CRYPTOCHROMEs are core components of the circadian oscillatory mechanism. The active CLOCK/BMAL1 or NPAS2/BMAL1 complexes regulate expression of numerous genes including two Cryptochromes. The products of these genes, CRY1 and CRY2, in turn repress CLOCK/BMAL1 transcriptional activity by an unknown mechanism. We have examined the effect of CRYPTOCHROMEs on posttranslational modifications and intracellular distribution of endogenous and ectopically expressed CLOCK(NPAS2) and BMAL1 proteins. We found that ectopic coexpression with CRY led to stabilization and nuclear accumulation of unphosphorylated forms of the proteins, which directly correlated with the inhibition of their transcriptional activity. This effect was CRY-specific, as other known repressors of CLOCK/BMAL1 and NPAS2/ BMAL1 transcriptional activity were not able to induce similar effects. CRYs had no effect on CLOCK(NPAS2)/BMAL1 complex formation or its ability to bind DNA. Altogether, these results demonstrate that CRYs regulate the functional activity of circadian transcriptional complex at the posttranslational level. Importantly, the posttranslational modifications and intracellular distribution of CLOCK and BMAL1 proteins were critically impaired in the tissues of mice with targeted disruption of both Cry genes, thus confirming the suggested role of CRY in clock function in vivo. Based on these findings we propose a modified model of the circadian transcriptional control, which implies CRY-mediated periodic rotation of transcriptionally active and inactive forms of CLOCK/BMAL1 on the promoter. This model provides mechanistic explanation for previously reported dual functional activity of CLOCK/BMAL1 and highlights the involvement of the circadian system in modulating the organism's response to various types of genotoxic stress, including chemotherapy and radiation.

Arabidopsis SHORT HYPOCOTYL UNDER BLUE1 contains SPX and EXS domains and acts in cryptochrome signaling

Photomorphogenesis is regulated by red/far-red light-absorbing phytochromes and blue/UV-A light-absorbing cryptochromes. We isolated an Arabidopsis thaliana blue light mutant, short hypocotyl under blue1 (shb1), a knockout allele. However, shb1-D, a dominant allele, exhibited a long-hypocotyl phenotype under red, far-red, and blue light. The phenotype conferred by shb1-D was caused by overaccumulation of SHB1 transcript and recapitulated by overexpression of SHB1 in Arabidopsis. Therefore, SHB1 acts in cryptochrome signaling but overexpression may expand its signaling activity to red and far-red light. Consistent with this, overexpression of SHB1 enhanced the expression of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) under red light. PIF4 appears to specifically mediate SHB1 regulation of hypocotyl elongation and CHLOROPHYLL a/b BINDING PROTEIN3 or CHALCONE SYNTHASE expression under red light. Overexpression of SHB1 also promoted proteasome-mediated degradation of phytochrome A and hypocotyl elongation under far-red light. Under blue light, shb1 suppressed LONG HYPOCOTYL IN FAR-RED LIGHT1 (HFR1) expression and showed several deetiolation phenotypes similar to hfr1-201. However, the hypocotyl and cotyledon-opening phenotypes of shb1 were opposite to those of hfr1-201, and HFR1 acts downstream of SHB1. SHB1 encodes a nuclear and cytosolic protein that has motifs homologous with SYG1 protein family members. Therefore, our studies reveal a signaling step in regulating cryptochrome- and possibly phytochrome-mediated light responses.

Circadian clockwork machinery in neural retina: evidence for the presence of functional clock components in photoreceptor-enriched chick retinal cell cultures

PURPOSE

Circadian clocks in retinas regulate a variety of biochemical and physiological processes. Retinal neurons, particularly photoreceptor cells, are thought to contain autonomous circadian clocks that control iodopsin expression, cFos expression, cAMP levels, and melatonin synthesis. Photoreceptor-enriched cell cultures prepared from chick embryo retina and entrained to a daily light-dark (LD) cycle exhibit circadian rhythms of cAMP levels and the activity of arylalkylamine N-acetyltransferase (AANAT), a key regulatory enzyme in melatonin synthesis. The present study was conducted to investigate the expression of circadian clockwork machinery comprised of clock genes; a clock-controlled gene, Aanat; and a clock output, melatonin, in the photoreceptor-enriched cultured retinal cells.

METHODS

Photoreceptor-enriched cell cultures were prepared from E6 neural retinas and incubated under 14 h:10 h light-dark cycle (LD) of illumination for 8 days and then transferred to constant (24 h/day) darkness (DD). Cells were collected every 4 h in LD and DD, and RNA was isolated. cDNA was prepared from each sample and transcripts of clock genes and Aanat were measured using real-time polymerase chain reaction (PCR). Melatonin release into the culture medium was assayed by HPLC with fluorescence detection at intervals of 3 h in LD and DD.

RESULTS

Cultured neural retina cells exposed to a light-dark cycle showed rhythmic expression of clock genes. Bmal1 and Npas2 (also known as Mop4) peaked late in the day in LD and in DD. Clock mRNA was high at night in LD, but arrhythmic in DD. Cry1 and Per2 transcripts increased rapidly in the early morning and were low at night. The rhythm of Per2 was reduced in amplitude in constant darkness (DD). Levels of Cry1 and Per2 transcripts were stimulated by light exposure at night. Melatonin release and Aanat mRNA were low during the day and high at night. Rhythmic expression of clock genes and Aanat was not observed in cultures not exposed to a LD cycle but treated otherwise identically to cultures described above.

CONCLUSIONS

Photoreceptor-enriched cell cultures derived from chick embryo neural retina contain a complete circadian clockwork system that is entrained by the light-dark cycle, and has a core timekeeping mechanism and circadian output in the form of melatonin synthesis.

The Phycomyces madA gene encodes a blue-light photoreceptor for phototropism and other light responses

Phycomyces blakesleeanus is a filamentous zygomycete fungus that produces striking elongated single cells that extend up to 10 cm into the air, with each such sporangiophore supporting a sphere containing the spores for dispersal. This organism has served as a model for the detection of environmental signals as diverse as light, chemicals, touch, wind, gravity, and adjacent objects. In particular, sporangiophore growth is regulated by light, and it exhibits phototropism by bending toward near-UV and blue wavelengths and away from far-UV wavelengths in a manner that is physiologically similar to plant phototropic responses. The Phycomyces madA mutants were first isolated more than 40 years ago, and they exhibit reduced sensitivity to light. Here, we identify two (duplicated) homologs in the White Collar 1 family of blue-light photoreceptors in Phycomyces. We describe that the madA mutant strains contain point mutations in one of these genes and that these mutations cosegregate with a defect in phototropism after genetic crosses. Thus, the phototropic responses of fungi through madA and plants through phototropin rely on diverse proteins; however, these proteins share a conserved flavin-binding domain for photon detection.

Functional evolution of the photolyase/cryptochrome protein family: importance of the C terminus of mammalian CRY1 for circadian core oscillator performance

Cryptochromes (CRYs) are composed of a core domain with structural similarity to photolyase and a distinguishing C-terminal extension. While plant and fly CRYs act as circadian photoreceptors, using the C terminus for light signaling, mammalian CRY1 and CRY2 are integral components of the circadian oscillator. However, the function of their C terminus remains to be resolved. Here, we show that the C-terminal extension of mCRY1 harbors a nuclear localization signal and a putative coiled-coil domain that drive nuclear localization via two independent mechanisms and shift the equilibrium of shuttling mammalian CRY1 (mCRY1)/mammalian PER2 (mPER2) complexes towards the nucleus. Importantly, deletion of the complete C terminus prevents mCRY1 from repressing CLOCK/BMAL1-mediated transcription, whereas a plant photolyase gains this key clock function upon fusion to the last 100 amino acids of the mCRY1 core and its C terminus. Thus, the acquirement of different (species-specific) C termini during evolution not only functionally separated cryptochromes from photolyase but also caused diversity within the cryptochrome family.

Dual role of the CLOCK/BMAL1 circadian complex in transcriptional regulation

The basic helix-loop-helix (bHLH) -PAS domain containing transcription factors CLOCK and BMAL1 are two major components of the circadian molecular oscillator. It is known that the CLOCK/BMAL1 complex positively regulates the activity of E-box containing promoters. Here we demonstrate that the CLOCK/BMAL1 complex can also suppress the activity of some promoters upon its interaction with CRYPTOCHROME (CRY). Such a dual function of the circadian transcriptional complex provides a mechanistic explanation for the unpredicted pattern of circadian gene expression in the tissues of Bmal1 null mice. We speculate that the switch from transcriptional activation to transcriptional repression may provide a highly efficient mechanism for circadian control of gene expression. We also show that CLOCK/BMAL1 can interfere with promoter regulation by other, non-circadian, transcription factors including N-MYC and ETS, leading to attenuation or abrogation of transcription of CLOCK/BMAL1-controlled stress-induced genes. We propose that, based upon these results, both circadian repression and activation of the transcription of different target genes are required for circadian responses to various external stimuli, including genotoxic stress induced by anticancer treatment.

Photochemically induced dynamic nuclear polarization in a C450A mutant of the LOV2 domain of the Avena sativa blue-light receptor phototropin

Phototropin is a blue-light receptor involved in the phototropic response of higher plants. The photoreceptor comprises a protein kinase domain and two structurally similar flavin-mononucleotide (FMN) binding domains designated LOV1 and LOV2. Blue-light irradiation of recombinant LOV2 domains induces the formation of a covalent adduct of the thiol group of a functional cysteine in the cofactor-binding pocket to C(4a) of the FMN. Cysteine-to-alanine mutants of LOV domains are unable to form that adduct but generate an FMN radical upon illumination. The recombinant C450A mutant of the LOV2 domain of Avena sativa phototropin was reconstituted with universally and site-selectively (13)C-labeled FMN and the (13)C NMR signals were unequivocally assigned. (13)C NMR spectra were acquired in darkness and under blue-light irradiation. The chemical shifts and the coupling patterns of the signals were not affected by irradiation. However, under blue-light exposure, exceptionally strong nuclear-spin polarization was developed in the resonances belonging to certain carbons of the FMN's isoalloxazine moiety. An enhancement of the NMR absorption was observed for the signals of C(5a), C(7), and C(9). NMR lines in emission were detected for the signals belonging to C(2), C(4), C(4a), C(6), C(8), and C(9a). The signal of C(10a) remained in absorption but was slightly attenuated. In contrast, the intensities of the NMR signals belonging to the carbons of the ribityl side chain of FMN were not affected by light. The observation of spin-polarized (13)C-nuclei in the NMR spectra of the mutant LOV2 domain is clear evidence for radical-pair intermediates in the reaction steps following optical sample excitation.

Photolyases and cryptochromes: common mechanisms of DNA repair and light-driven signaling?

DNA photolyases are extremely efficient light-driven DNA repair enzymes that use the energy of a blue-light photon to 'inject' an electron onto UV-damaged DNA, catalyzing the splitting of mutagenic pyrimidine dimers. By contrast, cryptochromes use blue light to trigger signaling cascades in multicellular organisms, fungi and several prokaryotes. Despite these functional differences, both protein families arose from a common ancestor and share many similarities, such as the overall protein fold, the presence of antenna chromophores and the use of flavin adenine dinucleotide (FAD) as the primary reactive group. Several significant advances in the biophysical and structural characterization of photolyases and cryptochromes are now revealing the details of how light-driven redox reactions can be used for such seemingly different purposes.

Light signals and flowering

Physiological studies over a long period have shown that light acts to regulate flowering through the three main variables of quality, quantity, and duration. Intensive molecular genetic and genomic studies with the model plant Arabidopsis have given considerable insight into the mechanisms involved, particularly with regard to quality and photoperiod. For photoperiodism light, acting through phytochromes and cryptochromes, the main photomorphogenetic photoreceptors, acts to entrain and interact with a circadian rhythm of CONSTANS (CO) expression leading to transcription of the mobile floral integrator, FLOWERING LOCUS T (FT). The action of phytochromes and cryptochromes in photoperiodism is augmented by ZEITLUPE (ZTL) and FLAVIN-BINDING, KELCH REPEAT, F-BOX (FKF1) acting as accessory photoreceptors on entrainment and interaction, respectively. Light quality acts independently of the circadian system through Phytochromes B, D, and E to regulate FT. Light quantity effects, on the other hand, are still incompletely understood but are likely to be linked either directly or indirectly to patterns of assimilate partitioning and resource utilization within the plant.

The rat circadian clockwork and its photoperiodic entrainment during development

The mammalian circadian pacemaker is located in the suprachiasmatic nucleus (SCN), which is composed of dorsomedial (dm) and ventrolateral (vl) regions. The molecular clockwork responsible for the SCN rhythmicity consists of clock genes and their transcriptional-translational feedback loops. The rat SCN rhythmicity and clockwork are affected by the photoperiod. The aim of this study was to elucidate development of the rat SCN rhythmicity, namely of the rhythmicity of the dm- and vl-SCN and of expression of clock genes and to ascertain when the photoperiod starts to affect the SCN rhythmicity. Rhythmicity of the dm-SCN, measured as the rhythm in spontaneous c-FOS production, developed earlier than that of the vl-SCN, which was measured as the rhythm in c-FOS photoinduction. However, photoperiodic affection of the rhythmicity occurred earlier in the vl-SCN than in the dm-SCN. From the 4 clock genes (Per1, Per2, Cry1 and Bmal1) studied, the expression of Bmal1 and Per1 was rhythmic already in 1-day-old rats; at this age, the Per2 mRNA rhythm just started to form and no rhythm in Cry1 expression was detected. After the second postnatal day, all 4 genes were expressed in a rhythmic manner. Thereafter, the rhythms matured gradually via increasing amplitude. Per1 and Per2 mRNA rhythms started to be affected by the photoperiod at the 10th postnatal day. The data suggest that the rhythms in clock genes expression in the rat SCN develop mostly postnatally. The molecular clockwork may start to be photoperiod-dependent around the 10th postnatal day.

Purification and Characterization of DNA Photolyases

Members of the photolyase/cryptochrome family of blue-light photoreceptors are monomeric proteins of 50-70 kDa that contain two noncovalently bound chromophores/cofactors: either folate or deazaflavin, which act as a photoantenna, and a two electron-reduced FAD, which acts as a catalytic cofactor. DNA photolyases bind their substrates with high affinity and specificity and subsequently use blue light as a cosubstrate for the in situ conversion of ultraviolet-induced cyclobutane pyrimidine dimers and (6-4) photoproducts to canonical bases, thereby restoring the integrity of DNA. The determinants for binding, as well as the mechanism of the photolysis reaction, have been studied extensively using highly purified enzyme. In contrast, neither the substrate nor the reaction catalyzed by the closely related cryptochromes has been identified. This chapter describes methods used to purify DNA photolyases from a variety of organisms using an Escherichia coli overexpression system, as well as the properties of the purified enzymes and some of the assays commonly used to study DNA binding and repair by these enzymes in vitro.