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PubMed1979

Spatial Distribution of Circadian Clock Phase in Aging Cultures of Neurospora crassa.

Neurospora crassa has been utilized extensively in the study of circadian clocks. Previously, the clock in this organism has been monitored by observing the morphological and biochemical changes occurring at the growing front of cultures grown on solid medium. A method has been developed for assaying the clock in regions of the culture behind the growing front, where no apparent morphological changes occur during the circadian cycle. Using this assay with Petri dish cultures that were 2 to 7 days old, the presence of a functional circadian clock not only at the growing front but in all other regions of the culture as well was demonstrated. Furthermore, the entire culture is not in the same phase, but shows a gradient of phases which is a function of the length of time the clock in a given part of the culture has been free-running. This gradient may be the result of a somewhat longer period of the oscillator behind the growing front compared to that at the growing front. The phase differences within a single culture of interconnected mycelium demonstrate the absence of total internal synchronization between adjacent regions of the hyphae under these conditions.

Journal Article↗
PubMed1998

Imbibition, but not release from stratification, sets the circadian clock in Arabidopsis seedlings.

Circadian rhythms in the abundance of the CAT2 catalase mRNA were not seen in etiolated seedlings but developed upon illumination. These circadian oscillations were preceded by a rapid and transient induction of CAT2 mRNA abundance that varied strikingly according to the timing (circadian phase) of the onset of illumination. This variation oscillated with a circadian periodicity of approximately 28 hr, indicating that the circadian oscillator is running in etiolated seedlings and regulates (gates) the induction of CAT2 by light. Moreover, because we assayed populations of seedlings, we infer that the individual clocks among populations of etiolated seedlings were synchronized before the onset of illumination. What developmental or environmental signals synchronized the clocks among seedlings? Varying the phase of the onset of illumination relative to release from stratification failed to affect the acute induction of CAT2, indicating that the temperature step from 4 to 22 degrees C associated with release from stratification did not reset the circadian clock. However, the acute induction of CAT2 mRNA varied with time after imbibition, demonstrating that imbibition provides a signal capable of resetting the circadian clock and of synchronizing the clocks among populations of seedlings.

Arabidopsis↗
PubMed2005

PSEUDO-RESPONSE REGULATOR 7 and 9 are partially redundant genes essential for the temperature responsiveness of the Arabidopsis circadian clock.

Environmental time cues, such as photocycles (light/dark) and thermocycles (warm/cold), synchronize (entrain) endogenous biological clocks to local time. Although much is known about entrainment of the Arabidopsis thaliana clock to photocycles, the determinants of thermoperception and entrainment to thermocycles are not known. The Arabidopsis PSEUDO-RESPONSE REGULATOR (PRR) genes, including the clock component TIMING OF CAB EXPRESSION 1/PRR1, are related to bacterial, fungal, and plant response regulators but lack the conserved Asp that is normally phosphorylated by an upstream sensory kinase. Here, we show that two PRR family members, PRR7 and PRR9, are partially redundant; single prr7-3 or prr9-1 mutants exhibit modest period lengthening, but the prr7-3 prr9-1 double mutant shows dramatic and more than additive period lengthening in the light and becomes arrhythmic in constant darkness. The prr7-3 prr9-1 mutant fails both to maintain an oscillation after entrainment to thermocycles and to reset its clock in response to cold pulses and thus represents an important mutant strongly affected in temperature entrainment in higher plants. We conclude that PRR7 and PRR9 are critical components of a temperature-sensitive circadian system. PRR7 and PRR9 could function in temperature and light input pathways or they could represent elements of an oscillator necessary for the clock to respond to temperature signals.

Alleles↗
PubMed2006

Arabidopsis FHY3 specifically gates phytochrome signaling to the circadian clock.

Circadian gating of light signaling limits the timing of maximum responsiveness to light to specific times of day. The fhy3 (for far-red elongated hypocotyl3) mutant of Arabidopsis thaliana is involved in independently gating signaling from a group of photoreceptors to an individual response. fhy3 shows an enhanced response to red light during seedling deetiolation. Analysis of two independent fhy3 alleles links enhanced inhibition of hypocotyl elongation in response to red light with an arrhythmic pattern of hypocotyl elongation. Both alleles also show disrupted rhythmicity of central-clock and clock-output gene expression in constant red light. fhy3 exhibits aberrant phase advances under red light pulses during the subjective day. Release-from-light experiments demonstrate clock disruption in fhy3 during the early part of the subjective day in constant red light, suggesting that FHY3 is important in gating red light signaling for clock resetting. The FHY3 gating function appears crucial in the early part of the day for the maintenance of rhythmicity under these conditions. However, unlike previously described Arabidopsis gating mutants that gate all light signaling, gating of direct red light-induced gene expression in fhy3 is unaffected. FHY3 appears to be a novel gating factor, specifically in gating red light signaling to the clock during daytime.

Arabidopsis↗
PubMed2001

A clock- and light-regulated gene that links the circadian oscillator to LHCB gene expression.

We have identified and characterized a novel tobacco gene, called ZGT (from the Chinese phrase zhong guang tiaokong, or clock and light controlled), that is regulated by the circadian clock and light. ZGT transcripts have alternate forms that are differentially expressed in different tissues. ZGT is expressed rhythmically in light/dark cycles and in constant light. Constitutive expression of ZGT sustains the expression of the clock-controlled LHCB1*1 gene in constant darkness, when it would normally dampen, but does not affect LHCB1*1 expression in constant light. ZGT expression is induced rapidly by light, and overexpression of ZGT increases the sensitivity of the circadian oscillator to brief light pulses. The ZGT promoter includes a G-box motif that is found in many light-regulated promoters in plants and is the same as the E box described for rhythmically regulated promoters of animal circadian clock genes. The ZGT promoter also includes "evening element" motifs that are correlated with circadian control of plant genes. We postulate that light- and clock-regulated expression of ZGT acts as a coupling agent between the central circadian oscillator and rhythmic LHCB1*1 expression and that it may function as a component in plant phototransduction pathways.

Alternative Splicing↗
PubMed2005

Molecular clocks and geological dates: cytochrome b of Anolis extremus substantially contradicts dating of Barbados emergence.

Even though molecular clocks vary in rate to some extent, they are widely used and very important in a range of evolutionary studies, not least in interpreting cause and colonization in phylogeography. Evolutionists may use island age and emergence to give the earliest possible date for colonization by a species and hence give the lower limit in a molecular clock calibration. The geology of the Lesser Antilles is well studied and Barbados, although composed of some ancient rocks, is thought to have emerged only about 1 million years ago (Ma). The cytochrome b mitochondrial gene is the most widely used gene in vertebrate phylogeography, and generally evolves at a rate of 1-2% per million years (Myr) for poikilothermic vertebrates. Divergence measured across almost all of this gene in the endemic anole (Anolis extremus) reveals a mean patristic distance of approximately 8.3% between this clade and its sister, together with distinct divergence and phylogeographical structure within Barbados. The divergence time, estimated by a range of procedures using four calibration points, is not in the least compatible with the proposed geological time of emergence of Barbados. Hence, either the molecular clock rate does not apply to the Barbadian anole population, or the geological dating of the emergence of Barbados is erroneous. The compatibility of geological times and molecular divergence of this complex on Martinique, together with relative rates tests comparing the rates on Barbados and Martinique, do not suggest atypical clock rates. The question of whether Barbados emerged much earlier than is currently thought, or whether the molecular clock assumptions are inappropriate, remains open.

Animals↗
PubMed2005

The suprachiasmatic nucleus controls the daily variation of plasma glucose via the autonomic output to the liver: are the clock genes involved?

In order to drive tissue-specific rhythmic outputs, the master clock, located in the suprachiasmatic nucleus (SCN), is thought to reset peripheral oscillators via either chemical and hormonal cues or neural connections. Recently, the daily rhythm of plasma glucose (characterized by a peak before the onset of the activity period) has been shown to be directly driven by the SCN, independently of the SCN control of rhythmic feeding behaviour. Indeed, the daily variation in glucose was not impaired unless the scheduled feeding regimen (six-meal schedule) was associated with an SCN lesion. Here we show that the rhythmicity of both clock-gene mRNA expression in the liver and plasma glucose is not abolished under such a regular feeding schedule. Because the onset of the activity period and hyperglycemia are correlated with an increased sympathetic tonus, we investigated whether this autonomic branch is involved in the SCN control of plasma glucose rhythm and liver rhythmicity. Interestingly, hepatic sympathectomy combined with a six-meal feeding schedule resulted in a disruption of the plasma glucose rhythmicity without affecting the daily variation in clock-gene mRNA expression in the liver. Taking all these data together, we conclude that (i) the SCN needs the sympathetic pathway to the liver to generate the 24-h rhythm in plasma glucose concentrations, (ii) rhythmic clock-gene expression in the liver is not dependent on the sympathetic liver innervation and (iii) clock-gene rhythmicity in liver cells is not sufficient for sustaining a circadian rhythm in plasma glucose concentrations.

Animals↗
PubMed1989

Clock drawing in Alzheimer's disease. A novel measure of dementia severity.

We have tested a simple and reliable measure of visuospatial ability in Alzheimer patients--the Clock Drawing Test. To determine the usefulness of this measure, we asked 67 Alzheimer patients and 83 normal controls to draw the face of a clock reading the time of 2:45. Six independent observers blindly evaluated the results with ratings from 10 (best) to 1 (worst). The mean performance score of Alzheimer subjects was 4.9 +/- 2.7 compared to 8.7 +/- 1.1 for normal controls (P less than .001). Inter-rater reliability for the clocks drawn by Alzheimer patients was highly significant (r = 0.86; P less than .001), and there was relatively little overlap between ratings for Alzheimer patients and normal controls. Furthermore, correlations were highly significant (P less than .001) between the mean score of clock drawings and three independent global measures of dementia severity. Although the Clock Drawing Test is certainly not a definitive indicator of Alzheimer's disease, the test is easy to administer and provides a useful measure of dementia severity for both research and office settings where sophisticated neuropsychological testing is not available.

Aged↗
PubMed1989

Screening for Alzheimer's disease by clock drawing.

The fear of Alzheimer's disease for both patients and families is growing along with the increasing evidence of the disease itself. This study (N = 312) of the validity of the clock drawing test in screening patients with probable Alzheimer's disease was conducted in an active outpatient geriatric clinic. Clock drawings by patients with normal mental status or depression were essentially normal. Alzheimer's patients were unable to complete a normal clock and demonstrated five characteristically abnormal patterns. As a test for Alzheimer's disease, clock drawing had a sensitivity of 86.7% and a specificity of 92.7%. There was correct identification in 97.2% of normals. These findings indicate that the clock drawing test, an easily administered, low cost screening tool, can be useful to health care professionals in characterizing cognitive loss in a general geriatric clinic population.

Aged↗
PubMed1998

Differential effects of light and heat on the Drosophila circadian clock proteins PER and TIM.

Circadian (approximately 24-h) rhythms are governed by endogenous biochemical oscillators (clocks) that in a wide variety of organisms can be phase shifted (i.e., delayed or advanced) by brief exposure to light and changes in temperature. However, how changes in temperature reset circadian timekeeping mechanisms is not known. To begin to address this issue, we measured the effects of short-duration heat pulses on the protein and mRNA products from the Drosophila circadian clock genes period (per) and timeless (tim). Heat pulses at all times in a daily cycle elicited dramatic and rapid decreases in the levels of PER and TIM proteins. PER is sensitive to heat but not light, indicating that individual clock components can markedly differ in sensitivity to environmental stimuli. A similar resetting mechanism involving delays in the per-tim transcriptional-translational feedback loop likely underlies the observation that when heat and light signals are administered in the early night, they both evoke phase delays in behavioral rhythms. However, whereas previous studies showed that the light-induced degradation of TIM in the late night is accompanied by stable phase advances in the temporal regulation of the PER and TIM biochemical rhythms, the heat-induced degradation of PER and TIM at these times in a daily cycle results in little, if any, long-term perturbation in the cycles of these clock proteins. Rather, the initial heat-induced degradation of PER and TIM in the late night is followed by a transient and rapid increase in the speed of the PER-TIM temporal program. The net effect of these heat-induced changes results in an oscillatory mechanism with a steady-state phase similar to that of the unperturbed control situation. These findings can account for the lack of apparent steady-state shifts in Drosophila behavioral rhythms by heat pulses applied in the late night and strongly suggest that stimulus-induced changes in the speed of circadian clocks can contribute to phase-shifting responses.

Animals↗
PubMed2001

Circadian clock-specific roles for the light response protein WHITE COLLAR-2.

To understand the role of white collar-2 in the Neurospora circadian clock, we examined alleles of wc-2 thought to encode partially functional proteins. We found that wc-2 allele ER24 contained a conservative mutation in the zinc finger. This mutation results in reduced levels of circadian rhythm-critical clock gene products, frq mRNA and FRQ protein, and in a lengthened period of the circadian clock. In addition, this mutation altered a second canonical property of the clock, temperature compensation: as temperature increased, period length decreased substantially. This temperature compensation defect correlated with a temperature-dependent increase in overall FRQ protein levels, with the relative increase being greater in wc-2 (ER24) than in wild type, while overall frq mRNA levels were largely unaltered by temperature. We suggest that this temperature-dependent increase in FRQ levels partially rescues the lowered levels of FRQ resulting from the wc-2 (ER24) defect, yielding a shorter period at higher temperatures. Thus, normal activity of the essential clock component WC-2, a positive regulator of frq, is critical for establishing period length and temperature compensation in this circadian system.

Amino Acid Sequence↗
PubMed2004

Direct association between mouse PERIOD and CKIepsilon is critical for a functioning circadian clock.

The mPER1 and mPER2 proteins have important roles in the circadian clock mechanism, whereas mPER3 is expendable. Here we examine the posttranslational regulation of mPER3 in vivo in mouse liver and compare it to the other mPER proteins to define the salient features required for clock function. Like mPER1 and mPER2, mPER3 is phosphorylated, changes cellular location, and interacts with other clock proteins in a time-dependent manner. Consistent with behavioral data from mPer2/3 and mPer1/3 double-mutant mice, either mPER1 or mPER2 alone can sustain rhythmic posttranslational events. However, mPER3 is unable to sustain molecular rhythmicity in mPer1/2 double-mutant mice. Indeed, mPER3 is always cytoplasmic and is not phosphorylated in the livers of mPer1-deficient mice, suggesting that mPER3 is regulated by mPER1 at a posttranslational level. In vitro studies with chimeric proteins suggest that the inability of mPER3 to support circadian clock function results in part from lack of direct and stable interaction with casein kinase Iepsilon (CKIepsilon). We thus propose that the CKIepsilon-binding domain is critical not only for mPER phosphorylation but also for a functioning circadian clock.

Animals↗
PubMed2006

Circadian clock coordinates cancer cell cycle progression, thymidylate synthase, and 5-fluorouracil therapeutic index.

Dysregulated cellular proliferation is a characteristic property of cancer. We show that, despite this fact, cancers maintain high amplitude, circadian rhythms in their growth, DNA synthesis, and mitosis. These patterns are accompanied by the daily traverse of BMAL-1 protein between the cytoplasm, where it is produced, and nucleus, where it influences timing of cancer cell proliferation. This core clock gene product gates cancer cell proliferation by coordinating clock-controlled proteins, thymidylate synthase [thymidylate synthase activity (TSA) cell DNA replication], WEE-1 (cell mitosis), and vascular endothelial growth factor (growth). 5-Fluorouracil (5-FU)-induced host bone marrow and gut toxicity and tumor shrinkage following administration at six equispaced times of day allowed determination of circadian relationships among tumor growth, relevant clock, and clock-controlled proteins and dependence of 5-FU target availability (TSA) in normal and cancer tissues and resultant 5-FU toxic-therapeutic index. The time of day (hours after lights on) of low TSA in each tissue and tumor is respectively associated with greatest toxicity to that tissue and greatest tumor shrinkage. 5-FU treatment near daily awakening results in least damage to bone marrow and gut, greatest antitumor effect, and best survival. This time of day is associated with maximum tumor nuclear BMAL-1 and total cell WEE-1 protein. The described chain of events, for the first time, links cancer cell clock proteins, cancer cell DNA synthesis, proliferation, TSA, and 5-FU toxic-therapeutic index, explaining the dependence of cancer outcome on circadian timing of 5-FU.

ARNTL Transcription Factors↗
PubMed2003

Alterations of circadian expressions of clock genes in Dahl salt-sensitive rats fed a high-salt diet.

In mammals, behavioral and physiologic processes display 24-hour rhythms that are regulated by a circadian system consisting of central and peripheral oscillators. Because various cardiovascular functions show diurnal variations and abnormal patterns of circadian blood pressure variation carry a high risk of cardiovascular complications, we investigated whether the expression of clock genes is altered in an animal model of hypertension. In Dahl salt-sensitive rats fed a high-salt (4% NaCl) diet for 6 weeks (DS-H), radiotelemetry monitoring showed increased amplitude of circadian variations in blood pressure. The ratio of heart weight to body weight and the ratio of kidney weight to body weight were higher in DS-H. Echocardiographic data showed that the wall thickness of the left ventricle was greater in DS-H. Northern blot analysis and single cosinor analysis revealed that the amplitudes of circadian expression changes of the clock genes (mPer2, Bmal1, and dbp) in the heart, liver, and kidney were significantly decreased in DS-H rats compared with a normal-salt-diet group, except for Bmal1 in the liver. The circadian expression changes of plasminogen activator inhibitor-1, a clock-regulated gene, were attenuated in the hearts of DS-H. The present results demonstrate that DS-H show altered circadian expression of peripheral clock genes. Detailed analyses of the relation between circadian expression of clock genes and blood pressure regulation might reveal a role for chronologic therapy of cardiovascular disease.

ARNTL Transcription Factors↗
PubMed2005

The performance of the Clock Drawing Test in elderly medical inpatients: does it have utility in the identification of delirium?

The Clock Drawing Test is an often-used test for the detection of cognitive impairment, but the few studies that have evaluated its utility in delirium have produced rather inconsistent results. In a longitudinal study of delirium in elderly medical inpatients, we have investigated the relationships between the Clock Drawing Test, the presence and severity of delirium, and cognitive impairment. Using mixed linear model analysis we found that cognitive impairment was the major factor associated with low Clock Drawing Test scores (P < .0001): neither the presence nor the severity of delirium had additional significant effect on the Clock Drawing Test. Thus, we conclude that although the Clock Drawing Test is a good detector of cognitive impairment, it is not a suitable tool for detection of delirium in elderly medical inpatients.

Aged↗
PubMed2006

Diurnal rhythmicity of the clock genes Per1 and Per2 in the rat ovary.

Circadian rhythms are generated by endogenous clocks in the central brain oscillator, the suprachiasmatic nucleus, and peripheral tissues. The molecular basis for the circadian clock consists of a number of genes and proteins that form transcriptional/translational feedback loops. In the mammalian gonads, clock genes have been reported in the testes, but the expression pattern is developmental rather than circadian. Here we investigated the daily expression of the two core clock genes, Per1 and Per2, in the rat ovary using real-time RT-PCR, in situ hybridization histochemistry, and immunohistochemistry. Both Per1 and Per2 mRNA displayed a statistically significant rhythmic oscillation in the ovary with a period of 24 h in: 1) a group of rats during proestrus and estrus under 12-h light,12-h dark cycles; 2) a second group of rats representing a mixture of all 4 d of the estrous cycle under 12-h light,12-h dark conditions; and 3) a third group of rats representing a mixture of all 4 d of estrous cycle during continuous darkness. Per1 mRNA was low at Zeitgeber time 0-2 and peaked at Zeitgeber time 12-14, whereas Per2 mRNA was delayed by approximately 4 h relative to Per1. By in situ hybridization histochemistry, Per mRNAs were localized to steroidogenic cells in preantral, antral, and preovulatory follicles; corpora lutea; and interstitial glandular tissue. With newly developed antisera, we substantiated the expression of Per1 and Per2 in these cells by single/double immunohistochemistry. Furthermore, we visualized the temporal intracellular movements of PER1 and PER2 proteins. These findings suggest the existence of an ovarian circadian clock, which may play a role both locally and in the hypothalamo-pituitary-ovarian axis.

Animals↗
PubMed2005

Chronic treatment with prednisolone represses the circadian oscillation of clock gene expression in mouse peripheral tissues.

Although altered homeostatic regulation, including disturbance of 24-h rhythms, is often observed in the patients undergoing glucocorticoid therapy, the mechanisms underlying the disturbance remains poorly understood. We report here that chronic treatment with a synthetic glucocorticoid, prednisolone (PSL), can cause alteration of circadian clock function at molecular level. Treatment of cultured hepatic cells (HepG2) with PSL induced expression of Period1 (Per1), and the PSL treatment also attenuated the serum-induced oscillations in the expression of Period2 (Per2), Rev-erbalpha, and Bmal1 mRNA in HepG2 cells. Because the attenuation of clock gene oscillations was blocked by pretreating the cells with a Per1 antisense phosphothioate oligodeoxynucleotide, the extensive expression of Per1 induced by PSL may have resulted in the reduced amplitude of other clock gene oscillations. Continuous administration of PSL into mice constitutively increased the Per1 mRNA levels in liver and skeletal muscle, which seems to attenuate the oscillation in the expressions of Per2, Rev-erbalpha, and Bmal1. However, a single daily administration of PSL at the time of day corresponding to acrophase of endogenous glucocorticoid levels had little effect on the rhythmic expression of clock genes. These results suggest a possible pharmacological action by PSL on the core circadian oscillation mechanism and indicate the possibility that the alteration of clock function induced by PSL can be avoided by optimizing the dosing schedule.

ARNTL Transcription Factors↗
PubMed2002

Larval optic nerve and adult extra-retinal photoreceptors sequentially associate with clock neurons during Drosophila brain development.

The visual system is one of the input pathways for light into the circadian clock of the Drosophila brain. In particular, extra-retinal visual structures have been proposed to play a role in both larval and adult circadian photoreception. We have analyzed the interactions between extra-retinal structures of the visual system and the clock neurons during brain development. We first show that the larval optic nerve, or Bolwig nerve, already contacts clock cells (the lateral neurons) in the embryonic brain. Analysis of visual system-defective genotypes showed that the absence of the afferent Bolwig nerve resulted in a severe reduction of the lateral neurons dendritic arborization, and that the inhibition of nerve activity induced alterations of the dendritic morphology. During wild-type development, the loss of a functional Bolwig nerve in the early pupa was also accompanied by remodeling of the arborization of the lateral neurons. Approximately 1.5 days later, visual fibers that came from the Hofbauer-Buchner eyelet, a putative photoreceptive organ for the adult circadian clock, were seen contacting the lateral neurons. Both types of extra-retinal photoreceptors expressed rhodopsins RH5 and RH6, as well as the norpA-encoded phospholipase C. These data strongly suggest a role for RH5 and RH6, as well as NORPA, signaling in both larval and adult extra-retinal circadian photoreception. The Hofbauer-Buchner eyelet therefore does not appear to account for the previously described norpA-independent light input to the adult clock. This supports the existence of yet uncharacterized photoreceptive structures in Drosophila.

Animals↗