Egg-laying rhythm in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

Extensive research has been carried out to understand how circadian clocks regulate various physiological processes in organisms. The discovery of clock genes and the molecular clockwork has helped researchers to understand the possible role of these genes in regulating various metabolic processes. In Drosophila melanogaster, many studies have shown that the basic architecture of circadian clocks is multi-oscillatory. In nature, different neuronal subgroups in the brain of D. melanogaster have been demonstrated to control different circadian behavioural rhythms or different aspects of the same circadian rhythm. Among the circadian phenomena that have been studied so far in Drosophila, the egg-laying rhythm is unique, and relatively less explored. Unlike most other circadian rhythms, the egg-laying rhythm is rhythmic under constant light conditions, and the endogenous or free-running period of the rhythm is greater than those of most other rhythms. Although the clock genes and neurons required for the persistence of adult emergence and activity/rest rhythms have been studied extensively, those underlying the circadian egg-laying rhythm still remain largely unknown. In this review, we discuss our current understanding of the circadian egg-laying rhythm in D. melanogaster, and the possible molecular and physiological mechanisms that control the rhythmic output of the egg-laying process.     

The circadian conidiation rhythm inNeurospora crassa

The filamentous fungusNeurospora crassais one of the best organisms for analysing the molecular basis of the circadian rhythm observed in asexual spore formation, conidiation. Many clock mutants in which the circadian conidiation rhythm has different characteristics compared to those in the wild-type strain have been isolated since the early 1970s. With the cloning of one of these clock genes,frq, the molecular basis of the circadian clock inNeurosporahas become gradually clearer. Physiological and pharmacological studies have also contributed to our understanding of the physiological basis of the circadian clock inNeurospora. These studies strongly indicate that the circadian clock is based on or is closely related to a network of metabolic processes for cellular activities. Based on these studies, it may be possible to isolate new types of clock mutants which should contribute to a better understanding of the molecular basis of the circadian clock inNeurospora.     

Circadian expression of clock genes in human peripheral leukocytes

In mammals, behavioral and physiological processes display 24-h rhythms that are regulated by a circadian system. In the present study, we investigated the possibility that the expression of clock genes in peripheral leukocytes can be used to assess the circadian clock system. We found that Per1 and Per2 exhibit circadian oscillations in mRNA expression in mouse peripheral leukocytes. Furthermore, the rhythms of Per1 and Per2 mRNA expression in peripheral leukocytes are severely blunted in homozygous Cry1/2 double-deficient mice that are known to have an abolished biological clock. We have examined the circadian expression of clock genes in human leukocytes and found that Per1 mRNA exhibits a robust circadian expression while Per2 and Bmal1 mRNA showed weak rhythm. These observations suggest that monitoring Per1 mRNA expression in human leukocytes may be useful for investigating the function of the circadian system in physiological and pathophysiological states.     

A phase response curve for circannual rhythm in the varied carpet beetle <Emphasis Type="Italic">Anthrenus verbasci</Emphasis>

We know that entrainment, a stable phase relationship with an environmental cycle, must be established for a biological clock to function properly. Phase response curves (PRCs), which are plots of phase shifts that result as a function of the phase of a stimulus, have been created to examine the mode of entrainment. In circadian rhythms, single-light pulse PRCs have been obtained by giving a light pulse to various phases of a free-running rhythm under continuous darkness. This successfully explains the entrainment to light-dark cycles. Some organisms show circannual rhythms. In some of these, changes in photoperiod entrain the circannual rhythms. However, no single-pulse PRCs have been created. Here we show the PRC to a long-day pulse superimposed for 4 weeks over constant short days in the circannual pupation rhythm in the varied carpet beetle Anthrenus verbasci. Because the shape of that PRC closely resembles that of the Type 0 PRC with large phase shifts in circadian rhythms, we suggest that an oscillator having a common feature in the phase response with the circadian clock, produces a circannual rhythm.     

Everything in moderation: Archaea as ‘non-extremophiles’

Well characterized and cultivated archaea are prokaryotic specialists that thrive in habitats of elevated temperature, low pH, high salinity, or strict anoxia. Recently, however, new groups of abundant, uncultivated archaea have been found to be widespread in more pedestrian biotopes, including marine plankton, terrestrial soils, lakes, marine and freshwater sediments, and in association with metazoa. Research efforts are presently focused on characterizing the physiology, biochemistry and genetics of these abundant and cosmopolitan but poorly understood archaea.     

Infection-related development in the rice blast fungus Magnaporthe grisea

Recent developments have been made in the identification of signal transduction pathways and gene products involved in the infection-related development of the rice blast fungus, Magnaporthe grisea. It has been established that cAMP-dependent and MAP kinase-mediated signaling are both critical for appressorium morphogenesis and function. These signaling pathways may act downstream of hydrophobin-mediated surface sensing by the growing germ tube. Several genes have been identified that are required for invasive growth of M. grisea including genes that allow adaptation of fungal metabolism to growth within plant tissues.     

MAP kinases in the mammalian circadian system--key regulators of clock function

Over the past 7 years, there has been spectacular progress in our understanding of the molecular basis of the circadian pacemaker in many species, from yeast to mammals. However, the biochemical signalling mechanisms that underpin synchronization of the clock to environmental cues are still poorly understood. Recently, attention has been focused on the role of mitogen-activated protein (MAP) kinase in biological timekeeping. It has been proposed that signal transduction via the MAP kinase cascades allows environmental information to be assimilated intracellularly within the circadian clock to produce changes in the phasing of clock gene expression, which, in turn, underlies clock-controlled phase-resetting of biological rhythms. This review examines the evidence for MAP kinase, particularly extracellular regulated kinases 1/2, involvement in the circadian clock and looks at the putative upstream regulators and downstream substrates of this signalling system.     

mPer2 antisense oligonucleotides inhibit mPER2 expression but not circadian rhythms of physiological activity in cultured suprachiasmatic nucleus neurons

Various day-night rhythms, observed at molecular, cellular, and behavioral levels, are governed by an endogenous circadian clock, predominantly functioning in the hypothalamic suprachiasmatic nucleus (SCN). A class of clock genes, mammalian Period (mPer), is known to be rhythmically expressed in SCN neurons, but the correlation between mPER protein levels and autonomous rhythmic activity in SCN neurons is not well understood. Therefore, we blocked mPer translation using antisense phosphothioate oligonucleotides (ODNs) for mPer1 and mPer2 mRNAs and examined the effects on the circadian rhythm of cytosolic Ca2+ concentration and action potentials in SCN slice cultures. Treatment with mPer2 ODNs (20microM for 3 days) but not randomized control ODNs significantly reduced mPER2 immunoreactivity (-63%) in the SCN. Nevertheless, mPer1/2 ODNs treatment inhibited neither action potential firing rhythms nor cytosolic Ca2+ rhythms. These suggest that circadian rhythms in mPER protein levels are not necessarily coupled to autonomous rhythmic activity in SCN neurons.     

Aging does not compromise in vitro oscillation of the suprachiasmatic nuclei but makes it more vulnerable to constant light

Circadian regulation of behavior worsens with age, however, the mechanism behind this phenomenon is still poorly understood. Specifically, it is not clear to what extend the ability of the circadian clock in the suprachiasmatic nuclei (SCN) to generate the rhythm is affected by aging. This study aimed to ascertain the effect of aging on the functioning of the SCN of mPer2^Luciferase mice under unnatural lighting conditions, such as constant light (LL). Under LL, which worsened the age-induced effect on behavioral rhythms, a marginal age-dependent effect on in vitro rhythmicity in explants containing the middle, but not the rostral/caudal, regions of the SCN was apparent; the proportion of mice in which middle-region SCN explants were completely arrhythmic or had an extremely long period (>30 h) was 47% in aged mice and 27% in adults. The results suggest that in some of the aged animals, LL may weaken the coupling among oscillators in specific sub-regions of the SCN, leaving other sub-regions better synchronized. In the standard light/dark cycle and in constant darkness, the SCN ability to produce bioluminescence rhythms in vitro was not compromised in aged mice although aging significantly affected their SCN-driven locomotor activity rhythms. Therefore, our results demonstrate that although age worsened the SCN output rhythm, the SCN molecular core clock mechanism itself was relatively resilient to aging in these same animals. The results suggest the involvement of pathways downstream of the core clock mechanism which are responsible for this phenomenon.     

The representation of temporal information in perception and motor control

The representation of temporal information can be examined from both a neurological and a computational perspective. Recent evidence suggests that two subcortical structures, the cerebellum and basal ganglia, play a critical role in the timing of both movement and perception. At a computational level, models of an internal clock have been developed in which timing is based on either endogeneous oscillatory processes or distributed interval-based representations derived from relatively slow physiological processes.     

Daily rhythms are retained both in spontaneously developed sarcomas and in xenografts grown in immunocompromised SCID mice

The circadian clock generates and regulates many daily physiological, metabolic and behavioral rhythms as well as acute responses to various types of stresses including those induced by anticancer treatment. It has been proposed that modulatory function of the clock may be used for improving the therapeutic efficacy of established anti-cancer treatments. In order to rationally exploit this mechanism, more information is needed to fully characterize the functional status of the molecular clock in tumors of different cellular origin; however, the data describing tumor clocks are still inconsistent. Here we tested the status of clock in two models of tumors derived from connective tissue: sarcomas spontaneously developed in p53-deficient mice and human fibrosarcoma cells grown as xenografts in immunocompromised severe combined immunodeficient (SCID) mice. We show that both types of tumors retain a functional clock, which is synchronized in phase with normal tissues. We also show that spontaneously developed tumors are not only oscillating in the context of an organism where they receive hormonal and metabolic signals but continue oscillating ex vivo in tissue explants demonstrating that tumors have functional clocks capable of timing all their functions. We also provide evidence that similar to liver, tumors can be synchronized by food availability independent of the central pacemaker in the suprachiasmatic nuclei (SCN). These data provide the basis for the design of anticancer therapies that take into account the circadian metabolic and physiological patterns of both the tumor and normal tissues.     

Review of the Dual-Clock Control of Tidal Rhythms and the Hypothesis that the Same Clock Governs Both Circatidal and Circadian Rhythms

Discoveries first published in 1986 did not fit the de rigueur working hypothesis that the clocks governing tide-associated rhythms had a fundamental period of 12.4 h, a value equal to the average interval between successive tides on most coastlines of the world. To explain the results a dual-clock schema was fashioned that envisioned two clocks, strongly coupled together 180° antiphase, each running at a basic rate of 24.8 h (the interval of a lunar day), as the driving agents of tide-associated rhythms (details are given in the text). This elaboration has been named the circalunidian-clock hypothesis, a hypocorism used in some armchair ruminations back in 1973. In the decade since 1986, a goodly amount of evidence has been garnered that is consistent with this hypothesis—suggesting that first-call divination appears to have been visionary. Acceptance of this hypothesis leads to further cerebration that a 24.8-h clock, its circa periods in constant conditions, and other properties—which fully overlap with our perception of the circadian clock that drives daily rhythms—may indicate that circadian and circalunidan timepieces are not different entities. The known properties of both daily and lunar clock-types are compared and contrasted, and, with the exception of one feature (for which there is at least a philosophical explanation), it is concluded that the same clock that drives tidal rhythms could also motor daily rhythms, i.e., there may be no such thing as a 12.4-h horologue.     

Two-component signal transduction systems in eukaryotic microorganisms

Conserved signal transduction pathways that use phosphorelay from histidine kinases through an intermediate transfer protein (H2) to response regulators have been found in a variety of eukaryotic microorganisms. Several of these pathways are linked to mitogen-activated protein kinase cascades. These networks control different physiological responses including osmoregulation, cAMP levels and cellular morphogenesis.