Caenorhabditis elegans has a circadian clock

Ror homolog nhr-23 is essential for both developmental clock and circadian clock in C. elegans

Animals have internal clocks that generate biological rhythms. In mammals, clock genes such as Period form the circadian clock to generate approximately 24-h biological rhythms. In C. elegans, the clock gene homologs constitute the "developmental clock", which has an 8-h period during larval development to determine the timing of molting. Thus, the ancestral circadian clock has been believed to evolve into the oscillator with a shorter period in C. elegans. However, circadian rhythms have also been observed in adult C. elegans, albeit relatively weak. This prompts the question: if the clock gene homologs drive the developmental rhythm with 8-h period, which genes generate the circadian rhythms in C. elegans? In this study, we discovered that nhr-23, a homolog of the mammalian circadian clock gene Ror, is essential for circadian transcriptional rhythms in adult C. elegans. Interestingly, nhr-23 was also known to be essential for the molting clock. The bilaterian ancestral circadian clock genes might have evolved to function over multiple periods depending on developmental contexts rather than a single 8-h period in C. elegans.

Caenorhabditis elegans as a Promising Model Organism in Chronobiology

Circadian rhythms represent an adaptive feature, ubiquitously found in nature, which grants living beings the ability to anticipate daily variations in their environment. They have been found in a multitude of organisms, ranging from bacteria to fungi, plants, and animals. Circadian rhythms are generated by endogenous clocks that can be entrained daily by environmental cycles such as light and temperature. The molecular machinery of circadian clocks includes a transcriptional-translational feedback loop that takes approximately 24 h to complete. Drosophila melanogaster has been a model organism of choice to understand the molecular basis of circadian clocks. However, alternative animal models are also being adopted, each offering their respective experimental advantages. The nematode Caenorhabditis elegans provides an excellent model for genetics and neuro-behavioral studies, which thanks to its ease of use and manipulation, as well as availability of genetic data and mutant strains, is currently used as a novel model for circadian research. Here, we aim to evaluate C. elegans as a model for chronobiological studies, focusing on its strengths and weaknesses while reviewing the available literature. Possible zeitgebers (including light and temperature) are also discussed. Determining the molecular bases and the neural circuitry involved in the central pacemaker of the C. elegans' clock will contribute to the understanding of its circadian system, becoming a novel model organism for the study of diseases due to alterations of the circadian cycle.

Forward genetic approach for behavioral neuroscience using animal models

Forward genetics is a powerful approach to understand the molecular basis of animal behaviors. Fruit flies were the first animal to which this genetic approach was applied systematically and have provided major discoveries on behaviors including sexual, learning, circadian, and sleep-like behaviors. The development of different classes of model organism such as nematodes, zebrafish, and mice has enabled genetic research to be conducted using more-suitable organisms. The unprecedented success of forward genetic approaches was the identification of the transcription-translation negative feedback loop composed of clock genes as a fundamental and conserved mechanism of circadian rhythm. This approach has now expanded to sleep/wakefulness in mice. A conventional strategy such as dominant and recessive screenings can be modified with advances in DNA sequencing and genome editing technologies.

The Doubletime Homolog KIN-20 Mainly Regulates let-7 Independently of Its Effects on the Period Homolog LIN-42 in Caenorhabditis elegans

The Caenorhabditis elegans (C. elegans) heterochronic pathway, which regulates developmental timing, is thought to be an ancestral form of the circadian clock in other organisms. An essential member of this clock is the Period protein whose homolog, lin-42, in C. elegans is an important heterochronic gene. LIN-42 functions as a transcriptional repressor of multiple genes including the conserved lin-4 and let-7 microRNAs. Like other Period proteins, levels of LIN-42 oscillate throughout development. In other organisms this cycling is controlled in part by phosphorylation. KIN-20 is the C. elegans homolog of the Drosophila Period protein kinase Doubletime. Worms containing a large deletion in kin-20 have a significantly smaller brood size and develop slower than wild type C. elegans Here we analyze the effect of kin-20 on lin-42 phenotypes and microRNA expression. We find that kin-20 RNAi enhances loss-of-function lin-42 mutant phenotypes and that kin-20 mutant worms express lower levels of LIN-42 We also show that kin-20 is important for post-transcriptional regulation of mature let-7 and lin-4 microRNA expression. In addition, the increased level of let-7 found in lin-42(n1089) mutant worms is not maintained after kin-20 RNAi treatment. Instead, let-7 is further repressed when levels of kin-20 and lin-42 are both decreased. Altogether these results suggest that though kin-20 regulates lin-42 and let-7 microRNA, it mainly affects let-7 microRNA expression independently of lin-42 These findings further our understanding of the mechanisms by which these conserved circadian rhythmic genes interact to ultimately regulate rhythmic processes, developmental timing and microRNA biogenesis in C. elegans.

Visible light reduces C. elegans longevity

The transparent nematode Caenorhabditis elegans can sense UV and blue-violet light to alter behavior. Because high-dose UV and blue-violet light are not a common feature outside of the laboratory setting, we asked what role, if any, could low-intensity visible light play in C. elegans physiology and longevity. Here, we show that C. elegans lifespan is inversely correlated to the time worms were exposed to visible light. While circadian control, lite-1 and tax-2 do not contribute to the lifespan reduction, we demonstrate that visible light creates photooxidative stress along with a general unfolded-protein response that decreases the lifespan. Finally, we find that long-lived mutants are more resistant to light stress, as well as wild-type worms supplemented pharmacologically with antioxidants. This study reveals that transparent nematodes are sensitive to visible light radiation and highlights the need to standardize methods for controlling the unrecognized biased effect of light during lifespan studies in laboratory conditions.

Circadian rhythms identified in Caenorhabditis elegans by in vivo long-term monitoring of a bioluminescent reporter

Circadian rhythms are based on endogenous clocks that allow organisms to adjust their physiology and behavior by entrainment to the solar day and, in turn, to select the optimal times for most biological variables. Diverse model systems-including mice, flies, fungi, plants, and bacteria-have provided important insights into the mechanisms of circadian rhythmicity. However, the general principles that govern the circadian clock of Caenorhabditis elegans have remained largely elusive. Here we report robust molecular circadian rhythms in C elegans recorded with a bioluminescence assay in vivo and demonstrate the main features of the circadian system of the nematode. By constructing a luciferase-based reporter coupled to the promoter of the suppressor of activated let-60 Ras (sur-5) gene, we show in both population and single-nematode assays that C elegans expresses ∼24-h rhythms that can be entrained by light/dark and temperature cycles. We provide evidence that these rhythms are temperature-compensated and can be re-entrained after phase changes of the synchronizing agents. In addition, we demonstrate that light and temperature sensing requires the photoreceptors LITE and GUR-3, and the cyclic nucleotide-gated channel subunit TAX-2. Our results shed light on C elegans circadian biology and demonstrate evolutionarily conserved features in the circadian system of the nematode.

Life in a dark biosphere: a review of circadian physiology in "arrhythmic" environments

Most of the life with which humans interact is exposed to highly rhythmic and extremely predictable changes in illumination that occur with the daily events of sunrise and sunset. However, while the influence of the sun feels omnipotent to surface dwellers such as ourselves, life on earth is dominated, in terms of biomass, by organisms isolated from the direct effects of the sun. A limited understanding of what life is like away from the sun can be inferred from our knowledge of physiology and ecology in the light biosphere, but a full understanding can only be gained by studying animals from the dark biosphere, both in the laboratory and in their natural habitats. One of the least understood aspects of life in the dark biosphere is the rhythmicity of physiology and what it means to live in an environment of low or no rhythmicity. Here we describe methods that may be used to understand rhythmic physiology in the dark and summarise some of the studies of rhythmic physiology in "arrhythmic" environments, such as the poles, deep sea and caves. We review what can be understood about the adaptive value of rhythmic physiology on the Earth's surface from studies of animals from arrhythmic environments and what role a circadian clock may play in the dark.

A High-Throughput Method for the Analysis of Larval Developmental Phenotypes in Caenorhabditis elegans

Caenorhabditis elegans postembryonic development consists of four discrete larval stages separated by molts. Typically, the speed of progression through these larval stages is investigated by visual inspection of the molting process. Here, we describe an automated method to monitor the timing of these discrete phases of C. elegans maturation, from the first larval stage through adulthood, using bioluminescence. The method was validated with a lin-42 mutant strain that shows delayed development relative to wild-type animals and with a daf-2 mutant that shows an extended second larval stage. This new method is inherently high-throughput and will finally allow dissecting the molecular machinery governing the speed of the developmental clock, which has so far been hampered by the lack of a method suitable for genetic screens.

Long-term imaging of circadian locomotor rhythms of a freely crawling C. elegans population

BACKGROUND

Locomotor activity is used extensively as a behavioral output to study the underpinnings of circadian rhythms. Recent studies have required a populational approach for the study of circadian rhythmicity in Caenorhabditis elegans locomotion.

NEW METHOD

We describe an imaging system for long-term automated recording and analysis of locomotion data of multiple free-crawling C. elegans animals on the surface of an agar plate. We devised image analysis tools for measuring specific features related to movement and shape to identify circadian patterns.

RESULTS

We demonstrate the utility of our system by quantifying circadian locomotor rhythms in wild-type and mutant animals induced by temperature cycles. We show that 13 °C:18 °C (12:12h) cycles are sufficient to entrain locomotor activity of wild-type animals, which persist but are rapidly damped during 13 °C free-running conditions. Animals with mutations in tax-2, a cyclic nucleotide-gated (CNG) ion channel, significantly reduce locomotor activity during entrainment and free-running.

COMPARISON WITH EXISTING METHOD(S)

Current methods for measuring circadian locomotor activity is generally restricted to recording individual swimming animals of C. elegans, which is a distinct form of locomotion from crawling behavior generally observed in the laboratory. Our system works well with up to 20 crawling adult animals, and allows for a detailed analysis of locomotor activity over long periods of time.

CONCLUSIONS

Our population-based approach provides a powerful tool for quantification of circadian rhythmicity of C. elegans locomotion, and could allow for a screening system of candidate circadian genes in this model organism.

Using circadian entrainment to find cryptic clocks

Three properties are most often attributed to the circadian clock: a ca. 24-h free-running rhythm, temperature compensation of the circadian rhythm, and its entrainment to zeitgeber cycles. Relatively few experiments, however, are performed under entrainment conditions. Rather, most chronobiology protocols concern constant conditions. We have turned this paradigm around and used entrainment to study the circadian clock in organisms where a free-running rhythm is weak or lacking. We describe two examples therein: Caenorhabditis elegans and Saccharomyces cerevisiae. By probing the system with zeitgeber cycles that have various structures and amplitudes, we can demonstrate the establishment of systematic entrained phase angles in these organisms. We conclude that entrainment can be utilized to discover hitherto unknown circadian clocks and we discuss the implications of using entrainment more broadly, even in model systems that show robust free-running rhythms.

Aging signaling pathways and circadian clock-dependent metabolic derangements

The circadian clock machinery orchestrates organism metabolism to ensure that development, survival, and reproduction are attuned to diurnal environmental variations. For unknown reasons, there is a decline in circadian rhythms with age, concomitant with declines in the overall metabolic tissue homeostasis and changes in the feeding behavior of aged organisms. This disruption of the relationship between the clock and the nutrient-sensing networks might underlie age-related diseases; overall, greater knowledge of the molecular mediators of and variations in clock networks during lifespan may shed light on the aging process and how it may be delayed. In this review we address the complex links between the circadian clock, metabolic (dys)functions, and aging in different model organisms.

Circadian regulation of olfaction and an evolutionarily conserved, nontranscriptional marker in Caenorhabditis elegans

Circadian clocks provide a temporal structure to processes from gene expression to behavior in organisms from all phyla. Most clocks are synchronized to the environment by alternations of light and dark. However, many organisms experience only muted daily environmental cycles due to their lightless spatial niches (e.g., caves or soil). This has led to speculation that they may dispense with the daily clock. However, recent reports contradict this notion, showing various behavioral and molecular rhythms in Caenorhabditis elegans and in blind cave fish. Based on the ecology of nematodes, we applied low-amplitude temperature cycles to synchronize populations of animals through development. This entrainment regime reveals rhythms on multiple levels: in olfactory cued behavior, in RNA and protein abundance, and in the oxidation state of a broadly conserved peroxiredoxin protein. Our work links the nematode clock with that of other clock model systems; it also emphasizes the importance of daily rhythms in sensory functions that are likely to impact on organism fitness and population structure.

Daily variation in melatonin synthesis and arylalkylamine N-acetyltransferase activity in the nematode Caenorhabditis elegans

Melatonin influences circadian rhythms and seasonal behavioral changes in vertebrates; it is synthesized from serotonin by N-acetylation by arylalkylamine N-acetyltransferase (AA-NAT) and O-methylation by N-acetylserotonin methyltransferase. However, its physiology and function in invertebrate models are less understood. In this work, we studied daily variations in melatonin synthesis and AA-NAT activity in the nematode Caenorhabditis elegans. Under light-dark conditions (LD), a rhythmic pattern of melatonin levels was observed, with higher levels toward the middle of the night, peaking at zeitgeber time (ZT) 18, and with a minimum value around ZT0-6. AA-NAT activity showed a diurnal and circadian fluctuation with higher levels of activity during the early night, both under LD and constant darkness conditions. A peak was found around ZT12 and circadian time (CT) 12. In addition, we investigated whether this nocturnal AA-NAT activity is inhibited by light. Our results show that both white and blue light pulses significantly inhibited AA-NAT activity at ZT18. This work demonstrates the daily fluctuation of melatonin synthesis and AA-NAT activity in the adult nematode C. elegans. In summary, this study takes additional advantage of an extremely useful invertebrate model system, which has only recently been exploited for circadian studies.

C. elegans homologs of insect clock proteins: a tale of many stories

As a consequence of the Earth's axial rotation, organisms display daily recurring rhythms in behavior and biochemical properties, such as hormone titers. The neuronal system controlling such changes is best studied in the fruit fly Drosophila melanogaster. In the nematode worm Caenorhabditis elegans, most homologs of these genes function in the heterochronic pathway controlling the (timing of) developmental events. Recent data indicate that in the worm at least one of the genes involved in developmental timing is also active in circadian rhythm control, thereby opening up new perspectives on a central (neuronal) timer interfering with many processes. Also, new neuropeptidergic clock homologs have been identified in nematodes, supporting the idea of a broad range of clock-regulated targets. We will describe the current knowledge on homologous clock genes in C. elegans with a focus on the recently discovered pigment dispersing factor gene homologs. Similarities between developmental and daily timing are discussed.

Circadian rhythms in metabolic variables in Caenorhabditis elegans

Circadian rhythms govern a wide variety of physiological and metabolic functions in most organisms through neural networks, hormones and gene expression. In this work, we studied the circadian variation in metabolic variables of adult C. elegans such as food consumption, pharyngeal contractions, defecation and oxygen consumption. Feeding behavior was clearly rhythmic under LD conditions, with a non-significant trend under DD conditions. In addition, a daily and circadian variation in muscle contraction of the pharynx was observed. Oxygen consumption also showed a circadian fluctuation with a maximum in the middle of the night (a peak was found around ZT18/CT18). Furthermore, defecation behavior also showed a daily variation in the N2 strain (wild type). This work demonstrates that in the adult nematode C. elegans metabolic variables vary daily. In summary, our results will allow us to take full advantage of this widely used animal model (including research in genetics, ageing and developmental biology) for studies in Chronobiology.

Circadian clocks in human red blood cells

Circadian (∼24 hour) clocks are fundamentally important for coordinated physiology in organisms as diverse as cyanobacteria and humans. All current models of the molecular circadian clockwork in eukaryotic cells are based on transcription-translation feedback loops. Non-transcriptional mechanisms in the clockwork have been difficult to study in mammalian systems. We circumvented these problems by developing novel assays using human red blood cells, which have no nucleus (or DNA) and therefore cannot perform transcription. Our results show that transcription is not required for circadian oscillations in humans, and that non-transcriptional events seem to be sufficient to sustain cellular circadian rhythms. Using red blood cells, we found that peroxiredoxins, highly conserved antioxidant proteins, undergo ∼24-hour redox cycles, which persist for many days under constant conditions (that is, in the absence of external cues). Moreover, these rhythms are entrainable (that is, tunable by environmental stimuli) and temperature-compensated, both key features of circadian rhythms. We anticipate that our findings will facilitate more sophisticated cellular clock models, highlighting the interdependency of transcriptional and non-transcriptional oscillations in potentially all eukaryotic cells.

Genome-wide analysis of light- and temperature-entrained circadian transcripts in Caenorhabditis elegans

Transcriptional profiling experiments identify light- and temperature-entrained circadian transcripts in C. elegans.

Most organisms have an endogenous circadian clock that is synchronized to environmental signals such as light and temperature. Although circadian rhythms have been described in the nematode Caenorhabditis elegans at the behavioral level, these rhythms appear to be relatively non-robust. Moreover, in contrast to other animal models, no circadian transcriptional rhythms have been identified. Thus, whether this organism contains a bona fide circadian clock remains an open question. Here we use genome-wide expression profiling experiments to identify light- and temperature-entrained oscillating transcripts in C. elegans. These transcripts exhibit rhythmic expression with temperature-compensated 24-h periods. In addition, their expression is sustained under constant conditions, suggesting that they are under circadian regulation. Light and temperature cycles strongly drive gene expression and appear to entrain largely nonoverlapping gene sets. We show that mutations in a cyclic nucleotide-gated channel required for sensory transduction abolish both light- and temperature-entrained gene expression, implying that environmental cues act cell nonautonomously to entrain circadian rhythms. Together, these findings demonstrate circadian-regulated transcriptional rhythms in C. elegans and suggest that further analyses in this organism will provide new information about the evolution and function of this biological clock.

Daily (circadian) rhythms in behavior and physiology allow organisms to adapt to periodic cues such as light and temperature associated with the rotation of the earth. Subsets of molecular components of the internal clock that drive these rhythms, as well as effector genes for behavioral outputs, also exhibit rhythmic expression in many organisms. While circadian rhythms in behavior have previously been described in the nematode Caenorhabditis elegans, no transcriptional rhythms or clock genes have been identified, leaving open the question of the nature of the clock in this organism. Here, we identify light- and temperature-entrained cycling genes in C. elegans via genome-wide transcriptional profiling. Transcripts showing circadian regulation (including expression with a 24-h period maintained upon removal of the entraining stimulus) and temperature compensation were identified. Light and temperature appear to entrain independent sets of genes. We also identify large sets of light- or temperature-driven genes. Mutations in a channel gene previously implicated in sensory transduction in a small set of sensory neurons abolish entrainment of gene expression by environmental signals. This work demonstrates the presence of circadian transcriptional rhythms in C. elegans, and provides the foundation for future investigations into the underlying mechanisms.

Timing of locomotor activity circadian rhythms in Caenorhabditis elegans

Circadian rhythms are driven by endogenous biological clocks and are synchronized to environmental cues. The chronobiological study of Caenorhabditis elegans, an extensively used animal model for developmental and genetic research, might provide fundamental information about the basis of circadian rhythmicity in eukaryotes, due to its ease of use and manipulations, as well as availability of genetic data and mutant strains. The aim of this study is to fully characterize the circadian rhythm of locomotor activity in C. elegans, as well as a means for genetic screening in this nematode and the identification of circadian mutants. We have developed an infrared method to measure locomotor activity in C. elegans and found that, under constant conditions, although inter-individual variability is present, circadian periodicity shows a population distribution of periods centered at 23.9±0.4 h and is temperature-compensated. Locomotor activity is entrainable by light-dark cycles and by low-amplitude temperature cycles, peaking around the night-day transition and day, respectively. In addition, lin-42(mg152) or lin-42(n1089) mutants (bearing a mutation in the lin-42 gene, homolog to the per gene) exhibit a significantly longer circadian period of 25.2±0.4 h or 25.6±0.5 h, respectively. Our results represent a complete description of the locomotor activity rhythm in C. elegans, with a methodology that allowed us to uncover three of the key features of circadian systems: entrainment, free-running and temperature compensation. In addition, abnormal circadian periods in clock mutants suggest a common molecular machinery responsible for circadian rhythmicity. Our analysis of circadian rhythmicity in C. elegans opens the possibility for further screening for circadian mutations in this species.

An automated tracking system for Caenorhabditis elegans locomotor behavior and circadian studies application

Automation of simple behavioral patterns, such as locomotor activity, is fundamental for pharmacological and genetic screening studies. Recently, circadian behaviors in locomotor activity and stress responses were reported in the nematode Caenorhabditis elegans, a well-known model in genetics and developmental studies. Here we present a new method for long-term recordings of C. elegans (as well as other similar-sized animals) locomotor activity based on an infrared microbeam scattering. Individual nematodes were cultured in a 96-well microtiter plate; we tested L15, CeMM and E. coli liquid cultures in long-term activity tracking experiments, and found CeMM to be the optimal medium. Treatment with 0.2% azide caused an immediate decrease in locomotor activity as recorded with our system. In addition to the validation of the method (including hardware and software details), we report its application in chronobiological studies. Circadian rhythms in animals entrained to light-dark and constant dark conditions (n=48 and 96 worms, respectively) at 16 degrees C, were analyzed by LS periodograms. We obtained a 24.2+/-0.44 h period (52% of significantly rhythmic animals) in LD, and a 23.1+/-0.40 h period (37.5% of significantly rhythmic animals) under DD. The system is automateable using microcontrollers, of low-cost construction and highly reproducible.

Intrinsic and extrinsic regulators of developmental timing: from miRNAs to nutritional cues

A fundamental challenge in biology is to understand the reproducibility of developmental programs between individuals of the same metazoan species. This developmental precision reflects the meticulous integration of temporal control mechanisms with those that specify other aspects of pattern formation, such as spatial and sexual information. The cues that guide these developmental events are largely intrinsic to the organism but can also include extrinsic inputs, such as nutrition or temperature. This review discusses the well-characterized developmental timing mechanism that patterns the C. elegans epidermis. Components of this pathway are conserved, and their links to developmental time control in other species are considered, including the temporal patterning of the fly nervous system. Particular attention is given to the roles of miRNAs in developmental timing and to the emerging mechanisms that link developmental programs to nutritional cues.

Neuronal substrates of complex behaviors in C. elegans

A current challenge in neuroscience is to bridge the gaps between genes, proteins, neurons, neural circuits, and behavior in a single animal model. The nematode Caenorhabditis elegans has unique features that facilitate this synthesis. Its nervous system includes exactly 302 neurons, and their pattern of synaptic connectivity is known. With only five olfactory neurons, C. elegans can dynamically respond to dozens of attractive and repellent odors. Thermosensory neurons enable the nematode to remember its cultivation temperature and to track narrow isotherms. Polymodal sensory neurons detect a wide range of nociceptive cues and signal robust escape responses. Pairing of sensory stimuli leads to long-lived changes in behavior consistent with associative learning. Worms exhibit social behaviors and complex ultradian rhythms driven by Ca(2+) oscillators with clock-like properties. Genetic analysis has identified gene products required for nervous system function and elucidated the molecular and neural bases of behaviors.

Caenorhabditis elegans opens up new insights into circadian clock mechanisms

The roundworm, Caenorhabditis elegans, is known to carry homologues of clock genes such as per (=period) and tim (=timeless), which constitute the core of the circadian clock in Drosophila and mammals: lin-42 and tim-1. Analyses using WormBase (C. elegans gene database) have identified with relatively high identity analogous of the clock genes recognized in Drosophila and mammals, with the notable exception of cry (=cryptochrome), which is lacking in C. elegans. All of these C. elegans cognates of the clock genes appear to belong to members of the PAS-superfamily and to participate in development or responsiveness to the environment but apparently are not involved in the C. elegans circadian clock. Nevertheless, C. elegans exhibits convincing circadian rhythms in locomotor behavior in the adult stage and in resistance to hyperosmotic stress in starved larvae (L1) after hatching, indicating that it has a circadian clock with a core design entirely different from that of Drosophila and mammals. Here two possibilities are considered. First, the core of the C. elegans circadian clock includes transcriptional/translational feedback loops between genes and their protein products that are entirely different from those of Drosophila and mammals. Second, a more basic principle such as homeostasis governs the circadian cellular physiology, and was established primarily to minimize the accumulation of DNA damage in response to an environment cycling at 24 h intervals.

At the pulse of time: protein interactions determine the pace of circadian clocks

Circadian clocks, internal timekeepers that generate a daily rhythmicity, help organisms to be prepared for periodic environmental changes of light and temperature. These molecular clocks are transcriptional feedback loops that generate 24-h oscillations in the abundance of clock proteins. For the maintenance of this rhythm inside the core clockwork and for its transmission to downstream genes the clock proteins additionally rely on post-transcriptional and post-translational mechanisms. Thus clock proteins engage in a variety of interactions with DNA, RNA and other proteins. Based on the model organisms Drosophila melanogaster and Arabidopsis thaliana molecular principles of circadian clocks are discussed in this review.

Developmental Timing in C. elegans Is Regulated by kin-20 and tim-1, Homologs of Core Circadian Clock Genes

In Caenorhabditis elegans, heterochronic genes constitute a developmental timer that specifies temporal cell fate selection. The heterochronic gene lin-42 is the C. elegans homolog of Drosophila and mammalian period, key regulators of circadian rhythms, which specify changes in behavior and physiology over a 24 hr day/night cycle. We show a role for two other circadian gene homologs, tim-1 and kin-20, in the developmental timer. Along with lin-42, tim-1 and kin-20, the C. elegans homologs of the Drosophila circadian clock genes timeless and doubletime, respectively, are required to maintain late-larval identity and prevent premature expression of adult cell fates. The molecular parallels between circadian and developmental timing pathways suggest the existence of a conserved molecular mechanism that may be used for different types of biological timing.

Une vie de ver

Despite its relative anatomic simplicity, the nematode Caenorhabditis elegans (C. elegans) is a complex multicellular organism. In this review, we describe studies that have contributed to a better understanding of certain aspects of the worm's physiology. We focus on the cellular and molecular basis of the interaction between C. elegans and its environment, including its sensory capacities, the intrinsic biological clock that governs the speed of its life, and on some of the factors that control its life span. We also outline very recent findings that have demonstrated the existence of an innate immune system in C. elegans. Finally, we highlight a number of novel techniques that are transforming the worm from a largely genetic model system into an attractive organism for functional genomic studies.

Getting (chromosomes) loaded--a new role for timeless

A recent study in C. elegans reveals an unanticipated link between sister chromatid cohesion and the TIM-1 protein, a homolog of the Drosophila circadian rhythm protein TIMELESS. The phenotypes of tim-1 mutants suggest that cohesin subunits load onto chromosomes in a stepwise manner. Whether TIM-1 is also involved in circadian rhythms is discussed.

Life before the clock: modeling circadian evolution

A feedback loop that functions via transcription and translation is thought to be the mechanistic core of circadian rhythmicity. Numerous modeling efforts incorporate the identified components and their modifications to recreate the circadian clock in computer simulations. Several issues remain problematic, including the lack of precise quantitative kinetics and the likely existence of additional, as-yet-undiscovered components. Even without these complications, models and flow charts of the circadian system have reached high complexity. They attempt to reconcile all observations without violating current views and concepts. In this article, the authors consider the mechanisms that may have preceded the circadian system in evolution. Given that cellular metabolism and biochemistry were presumably already interconnected in cascading feedback reactions prior to the appendage of the transcription/translation feedback loop, a coordinated response to exogenous changes would be advantageous over unsystematic responses. The authors hypothesize that those mechanisms that allowed synchronization in spite of metabolic complexity form the basis for the evolution of circadian properties and are as fundamental to the circadian system as the transcriptional/translational feedback loop.