Light induction of the clock-controlled geneccg-1 is not transduced through the circadian clock inNeurospora crassa

Ambient light and the circadian clock have been shown to be capable of acting either independently or in an interrelated fashion to regulate the expression of conidiation in the ascomycete fungusNeurospora crassa. Recently several molecular correlates of the circadian clock have been identified in the form of the morning-specific clock-controlled genesccg-1 andccg-2. In this paper we report studies on the regulation ofccg-1, an abundantly expressed gene displaying complex regulation. Consistent with an emerging consensus for clock-controlled genes and conidiation genes inNeurospora, we report thatccg-1 expression is induced by light, and show that this induction is independent of the direct effects of light on the circadian clock. Although circadian regulation of the gene is lost in strains lacking a functional clock, expression ofccg-1 is still not constitutive, but rather fluctuates in concert with changes in developmental potential seen in such strains. Light induction ofccg-1 requires the products of theNeurospora wc-1 andwc-2 genes, but surprisingly the requirement forwc-2 is suppressed in conditional mutants ofcot-1, a gene that encodes a cAMP-dependent protein kinase. These data provide insight into a complex regulatory web, involving at least circadian clock control, light control, metabolic control, and very probably developmental regulation, that governs the expression ofccg-1.     

Light induction of the clock-controlled geneccg-1 is not transduced through the circadian clock inNeurospora crassa

Ambient light and the circadian clock have been shown to be capable of acting either independently or in an interrelated fashion to regulate the expression of conidiation in the ascomycete fungusNeurospora crassa. Recently several molecular correlates of the circadian clock have been identified in the form of the morning-specific clock-controlled genesccg-1 andccg-2. In this paper we report studies on the regulation ofccg-1, an abundantly expressed gene displaying complex regulation. Consistent with an emerging consensus for clock-controlled genes and conidiation genes inNeurospora, we report thatccg-1 expression is induced by light, and show that this induction is independent of the direct effects of light on the circadian clock. Although circadian regulation of the gene is lost in strains lacking a functional clock, expression ofccg-1 is still not constitutive, but rather fluctuates in concert with changes in developmental potential seen in such strains. Light induction ofccg-1 requires the products of theNeurospora wc-1 andwc-2 genes, but surprisingly the requirement forwc-2 is suppressed in conditional mutants ofcot-1, a gene that encodes a cAMP-dependent protein kinase. These data provide insight into a complex regulatory web, involving at least circadian clock control, light control, metabolic control, and very probably developmental regulation, that governs the expression ofccg-1.     

The interplay of light and the circadian clock. Independent dual regulation of clock-controlled gene ccg-2(eas).

Ambient light is the major agent mediating entrainment of circadian rhythms and is also a major factor influencing development and morphogenesis. We show that in Neurospora crassa the expression of clock-controlled gene 2 (ccg-2), a gene under the control of the circadian clock and allelic to the developmental gene easy wettable (eas), is regulated by light in wild-type strains. Light elicits a direct and important physiological effect on ccg-2(eas) expression as demonstrated using several mutant Neurospora strains. In white collar mutants (wc-1 and wc-2) that are "blind" to blue light, ccg-2(eas) mRNA shows no variation following illumination with saturating light. By contrast, ccg-2(eas) mRNA is photoinduced in clock-null strains such as frequency (bd;frq). The results in the clock mutants show that an intact circadian oscillator is not required for light induction of ccg-2(eas). Thus, ccg-2(eas) is subject to a dual regulation that involves separable regulation by light and circadian rhythm.     

A genetic selection for circadian output pathway mutations in Neurospora crassa

In most organisms, circadian oscillators regulate the daily rhythmic expression of clock-controlled genes (ccgs). However, little is known about the pathways between the circadian oscillator(s) and the ccgs. In Neurospora crassa, the frq, wc-1, and wc-2 genes encode components of the frq-oscillator. A functional frq-oscillator is required for rhythmic expression of the morning-specific ccg-1 and ccg-2 genes. In frq-null or wc-1 mutant strains, ccg-1 mRNA levels fluctuate near peak levels over the course of the day, whereas ccg-2 mRNA remains at trough levels. The simplest model that fits the above observations is that the frq-oscillator regulates a repressor of ccg-1 and an activator of ccg-2. We utilized a genetic selection for mutations that affect the regulation of ccg-1 and ccg-2 by the frq-oscillator. We find that there is at least one mutant strain, COP1-1 (circadian output pathway derived from ccg-1), that has altered expression of ccg-1 mRNA, but normal ccg-2 expression levels. However, the clock does not appear to simply regulate a repressor of ccg-1 and an activator of ccg-2 in two independent pathways, since in our selection we identified three mutant strains, COP1-2, COP1-3, and COP1-4, in which a single mutation in each strain affects the expression levels and rhythmicity of both ccg-1 and ccg-2.     

OS-2 Mitogen Activated Protein Kinase Regulates the Clock-Controlled Gene ccg-1 in Neurospora crassa

OS-2 MAP kinase is involved in osmoadaptation in Neurospora crassa. Clock-controlled genes ccg-1, bli-3, and con-10 were induced by osmotic stress in an OS-2 dependent manner. In contrast, osmotic stress did not affect the expression of clock genes frq, wc-1, and wc-2 or of clock-controlled genes ccg-2 and bli-4. These results suggest that OS-2 participates in the regulation of certain circadian-clock output genes.     

Entrainment of eclosion and preliminary ontogeny of circadian clock gene expression in the flesh fly,Sarcophaga crassipalpis

Timing of circadian activities is controlled by rhythmic expression of clock genes in pacemaker neurons in the insect brain. Circadian behavior and clock gene expression can entrain to both thermoperiod and photoperiod but the availability of such cues, the organization of the brain, and the need for circadian behavior change dramatically during the course of insect metamorphosis. We asked whether photoperiod or thermoperiod entrains the clock during pupal and pharate adult stages by exposing flies to different combinations of thermoperiod and photoperiod and observing the effect on the timing of adult eclosion. This study used qRT-PCR to examine how entrainment and expression of circadian clock genes change during the course of development in the flesh fly, Sarcophaga crassipalpis. Thermoperiod entrains expression of period and controls the timing of adult eclosion, suggesting that the clock gene period may be upstream of the eclosion pathway. Rhythmic clock gene expression is evident in larvae, appears to cease during the early pharate adult stage, and resumes again by the time of adult eclosion. Our results indicate that both patterns of clock gene expression and the cues to which the clock entrains are dynamic and respond to different environmental signals at different developmental stages in S. crassipalpis.     

Two circadian timing circuits in Neurospora crassa cells share components and regulate distinct rhythmic processes

In Neurospora crassa, FRQ, WC-1, and WC-2 proteins comprise the core circadian FRQ-based oscillator that is directly responsive to light and drives daily rhythms in spore development and gene expression. However, physiological and biochemical studies have demonstrated the existence of additional oscillators in the cell that function in the absence of FRQ (collectively termed FRQ-less oscillators [FLOs]). Whether or not these represent temperature-compensated, entrainable circadian oscillators is not known. The authors previously identified an evening-peaking gene, W06H2 (now called clock-controlled gene 16 [ccg-16]), which is expressed with a robust daily rhythm in cells that lack FRQ protein, suggesting that ccg-16 is regulated by a FLO. In this study, the authors provide evidence that the FLO driving ccg-16 rhythmicity is a circadian oscillator. They find that ccg-16 rhythms are generated by a temperature-responsive, temperature-compensated circadian FLO that, similar to the FRQ-based oscillator, requires functional WC-1 and WC-2 proteins for activity. They also find that FRQ is not essential for rhythmic WC-1 protein levels, raising the possibility that this WCFLO is involved in the generation of WC-1 rhythms. The results are consistent with the presence of 2 circadian oscillators within Neurospora cells, which the authors speculate may interact with each other through the shared WC proteins.     

Disrupting Circadian Homeostasis of Sympathetic Signaling Promotes Tumor Development in Mice

Background

Cell proliferation in all rapidly renewing mammalian tissues follows a circadian rhythm that is often disrupted in advanced-stage tumors. Epidemiologic studies have revealed a clear link between disruption of circadian rhythms and cancer development in humans. Mice lacking the circadian genes Period1 and 2 (Per) or Cryptochrome1 and 2 (Cry) are deficient in cell cycle regulation and Per2 mutant mice are cancer-prone. However, it remains unclear how circadian rhythm in cell proliferation is generated in vivo and why disruption of circadian rhythm may lead to tumorigenesis.

Methodology/Principal Findings

Mice lacking Per1 and 2, Cry1 and 2, or one copy of Bmal1, all show increased spontaneous and radiation-induced tumor development. The neoplastic growth of Per-mutant somatic cells is not controlled cell-autonomously but is dependent upon extracellular mitogenic signals. Among the circadian output pathways, the rhythmic sympathetic signaling plays a key role in the central-peripheral timing mechanism that simultaneously activates the cell cycle clock via AP1-controlled Myc induction and p53 via peripheral clock-controlled ATM activation. Jet-lag promptly desynchronizes the central clock-SNS-peripheral clock axis, abolishes the peripheral clock-dependent ATM activation, and activates myc oncogenic potential, leading to tumor development in the same organ systems in wild-type and circadian gene-mutant mice.

Conclusions/Significance

Tumor suppression in vivo is a clock-controlled physiological function. The central circadian clock paces extracellular mitogenic signals that drive peripheral clock-controlled expression of key cell cycle and tumor suppressor genes to generate a circadian rhythm in cell proliferation. Frequent disruption of circadian rhythm is an important tumor promoting factor.     

Microcystin-LR regulates circadian clock and antioxidant gene expression in cultured rat cardiomyocytes

Background

Microcystins are waterborne environmental toxins that induce oxidative stress and cause injuries in the heart. On the other hand, many physiological processes, including antioxidant defense, are under precise control by the mammalian circadian clock.

Results

In the present study, we evaluated the effect of microcystin-LR (MC-LR) on the rhythmic expression patterns of circadian and antioxidant genes in rat cardiomyocytes using the serum shock technique. We found that a non-toxic dose (10 μm) of MC-LR decreased the amplitudes of rhythmic patterns of clock genes, while it increased the expression levels of antioxidant genes.

Conclusions

Our results indicate an influence of MC-LR on the circadian clock system and clock-controlled antioxidant genes, which will shed some light on the explanation of heart toxicity induced by MC-LR from the viewpoint of chronobiology.

     

Free Access to a Running-Wheel Advances the Phase of Behavioral and Physiological Circadian Rhythms and Peripheral Molecular Clocks in Mice

Behavioral and physiological circadian rhythms are controlled by endogenous oscillators in animals. Voluntary wheel-running in rodents is thought to be an appropriate model of aerobic exercise in humans. We evaluated the effects of chronic voluntary exercise on the circadian system by analyzing temporal profiles of feeding, core body temperature, plasma hormone concentrations and peripheral expression of clock and clock-controlled genes in mice housed under sedentary (SED) conditions or given free access to a running-wheel (RW) for four weeks. Voluntary wheel-running activity advanced the circadian phases of increases in body temperature, food intake and corticosterone secretion in the mice. The circadian expression of clock and clock-controlled genes was tissue- and gene-specifically affected in the RW mice. The temporal expression of E-box-dependent circadian clock genes such as Per1, Per2, Nr1d1 and Dbp were slightly, but significantly phase-advanced in the liver and white adipose tissue, but not in brown adipose tissue and skeletal muscle. Peak levels of Per1, Per2 and Nr1d1 expression were significantly increased in the skeletal muscle of RW mice. The circadian phase and levels of hepatic mRNA expression of the clock-controlled genes that are involved in cholesterol and fatty acid metabolism significantly differed between SED and RW mice. These findings indicated that endogenous clock-governed voluntary wheel-running activity provides feedback to the central circadian clock that systemically governs behavioral and physiological rhythms.     

Reactive oxygen species can modulate circadian phase and period in Neurospora crassa

Reactive oxygen species (ROS) may serve as signals coupling metabolism to other cell functions. In addition to being by-products of normal metabolism, they are generated at elevated levels under environmental stress situations. We analyzed how reactive oxygen species affect the circadian clock in the model organism Neurospora crassa. In light/dark cycles, an increase in the levels of reactive oxygen species advanced the phase of both the conidiation rhythm and the expression of the clock gene frequency. Our results indicate a dominant role of the superoxide anion in the control of the phase. Elevation of superoxide production resulted in the activation of protein phosphatase 2A, a regulator of the positive element of the circadian clock. Our data indicate that even under nonstress conditions, reactive oxygen species affect circadian timekeeping. Reduction of their basal levels results in a delay of the phase in light/dark cycles and a longer period under constant conditions. We show that under entrained conditions the phase depends on the temperature and reactive oxygen species contribute to this effect. Our results suggest that the superoxide anion is an important factor controlling the circadian oscillator and is able to reset the clock most probably by activating protein phosphatase 2A, thereby modulating the activity of the White Collar complex.