Molecular and behavioral analysis of four period mutants in Drosophila melanogaster encompassing extreme short, novel long, and unorthodox arrhythmic types

Germline Editing of Drosophila Using CRISPR-Cas9-Based Cytosine and Adenine Base Editors

Target-AID, BE3, and ABE7.10 base editors fused to the catalytically modified Cas9 and xCas9(3.7) were tested for germline editing of the fruit fly Drosophila melanogaster. We developed a guide RNA-expressing construct, white-4gRNA, targeting splice sites in the white gene, an X-chromosome located gene. Using white-4gRNA flies and transgenic lines expressing Target-AID, BE3, and ABE7.10 base editors, we tested the efficiency of stable germline gene editing at three different temperatures. Classical Cas9 generating insertions/deletions by non-homologous end joining served as a reference. Our data indicate that gene editing is most efficient at 28°C, the highest temperature suitable for fruit flies. Finally, we created a new allele of the core circadian clock gene timeless using Target-AID. This base edited mutant allele timSS308-9FL had a disrupted circadian clock with a period of ∼29 h. The white-4gRNA expressing fly can be used to test new generations of base editors for future applications in Drosophila.

The Role of Glia Clocks in the Regulation of Sleep in Drosophila melanogaster

In Drosophila melanogaster, the pacemaker located in the brain plays the main role in maintaining circadian rhythms; however, peripheral oscillators including glial cells, are also crucial components of the circadian network. In the present study, we investigated an impact of oscillators located in astrocyte-like glia, the chiasm giant glia of the optic lobe, epithelial and subperineurial glia on sleep of Drosophila males. We described that oscillators located in astrocyte-like glia and chiasm giant glia are necessary to maintain daily changes in clock neurons arborizations, while those located in epithelial glia regulate amplitude of these changes. Finally, we showed that communication between glia and neurons through tripartite synapses formed by epithelial glia and, in effect, neurotransmission regulation plays important role in wake-promoting during the day.SIGNIFICANCE STATEMENT Circadian clock or pacemaker regulates many aspects of animals' physiology and behavior. The pacemaker is located in the brain and is composed of neurons. However, there are also additional oscillators, called peripheral clocks, which synchronize the main clock. Despite the critical role of glia in the clock machinery, little is known which type of glia houses peripheral oscillators and how they affect neuronal clocks. This study using Drosophila shows that oscillators in specific glia types maintain awakeness during the day by regulating the daily plasticity of clock neurons.

PERIOD Phosphoclusters Control Temperature Compensation of the Drosophila Circadian Clock

Ambient temperature varies constantly. However, the period of circadian pacemakers is remarkably stable over a wide-range of ecologically- and physiologically-relevant temperatures, even though the kinetics of most biochemical reactions accelerates as temperature rises. This thermal buffering phenomenon, called temperature compensation, is a critical feature of circadian rhythms, but how it is achieved remains elusive. Here, we uncovered the important role played by the Drosophila PERIOD (PER) phosphodegron in temperature compensation. This phosphorylation hotspot is crucial for PER proteasomal degradation and is the functional homolog of mammalian PER2 S478 phosphodegron, which also impacts temperature compensation. Using CRISPR-Cas9, we introduced a series of mutations that altered three Serines of the PER phosphodegron. While all three Serine to Alanine substitutions lengthened period at all temperatures tested, temperature compensation was differentially affected. S44A and S45A substitutions caused undercompensation, while S47A resulted in overcompensation. These results thus reveal unexpected functional heterogeneity of phosphodegron residues in thermal compensation. Furthermore, mutations impairing phosphorylation of the per s phosphocluster showed undercompensation, consistent with its inhibitory role on S47 phosphorylation. We observed that S47A substitution caused increased accumulation of hyper-phosphorylated PER at warmer temperatures. This finding was corroborated by cell culture assays in which S47A slowed down phosphorylation-dependent PER degradation at high temperatures, causing PER degradation to be excessively temperature-compensated. Thus, our results point to a novel role of the PER phosphodegron in temperature compensation through temperature-dependent modulation of the abundance of hyper-phosphorylated PER. Our work reveals interesting mechanistic convergences and differences between mammalian and Drosophila temperature compensation of the circadian clock.

Light and Temperature Synchronizes Locomotor Activity in the Linden Bug, Pyrrhocoris apterus

Circadian clocks are synchronized with the external environment by light and temperature. The effect of these cues on behavior is well-characterized in Drosophila, however, little is known about synchronization in non-model insect species. Therefore, we explored entrainment of locomotor activity by light and temperature in the linden bug Pyrrhocoris apterus (Heteroptera), an insect species with a strong seasonal response (reproductive diapause), which is triggered by both photoperiod and thermoperiod. Our results show that either light or temperature cycles are strong factors entraining P. apterus locomotor activity. Pyrrhocoris is able to be partially synchronized by cycles with temperature amplitude as small as 3°C and more than 50% of bugs is synchronized by 5°C steps. If conflicting zeitgebers are provided, light is the stronger signal. Linden bugs lack light-sensitive (Drosophila-like) cryptochrome. Notably, a high percentage of bugs is rhythmic even in constant light (LL) at intensity ∼400 lux, a condition which induces 100% arrhythmicity in Drosophila. However, the rhythmicity of bugs is still reduced in LL conditions, whereas rhythmicity remains unaffected in constant dark (DD). Interestingly, a similar phenomenon is observed after temperature cycles entrainment. Bugs released to constant thermophase and DD display weak rhythmicity, whereas strong rhythmicity is observed in bugs released to constant cryophase and DD. Our study describes the daily and circadian behavior of the linden bug as a response to photoperiodic and thermoperiodic entraining cues. Although the molecular mechanism of the circadian clock entrainment in the linden bug is virtually unknown, our study contributes to the knowledge of the insect circadian clock features beyond Drosophila research.

Locomotor activity patterns in three spider species suggest relaxed selection on endogenous circadian period and novel features of chronotype

We examined the circadian rhythms of locomotor activity in three spider species in the Family Theridiidae under light-dark cycles and constant darkness. Contrary to previous findings in other organisms, we found exceptionally high variability in endogenous circadian period both within and among species. Many individuals exhibited circadian periods much lower (19-22 h) or much higher (26-30 h) than the archetypal circadian period. These results suggest relaxed selection on circadian period as well as an ability to succeed in nature despite a lack of circadian resonance with the 24-h daily cycle. Although displaying similar entrainment waveforms under light-dark cycles, there were remarkable differences among the three species with respect to levels of apparent masking and dispersion of activity under constant dark conditions. These behavioral differences suggest an aspect of chronotype adapted to the particular ecologies of the different species.

New Drosophila Circadian Clock Mutants Affecting Temperature Compensation Induced by Targeted Mutagenesis of Timeless

Drosophila melanogaster has served as an excellent genetic model to decipher the molecular basis of the circadian clock. Two key proteins, PERIOD (PER) and TIMELESS (TIM), are particularly well explored and a number of various arrhythmic, slow, and fast clock mutants have been identified in classical genetic screens. Interestingly, the free running period (tau, τ) is influenced by temperature in some of these mutants, whereas τ is temperature-independent in other mutant lines as in wild-type flies. This, so-called "temperature compensation" ability is compromised in the mutant timeless allele "ritsu" (tim rit ), and, as we show here, also in the tim blind allele, mapping to the same region of TIM. To test if this region of TIM is indeed important for temperature compensation, we generated a collection of new mutants and mapped functional protein domains involved in the regulation of τ and in general clock function. We developed a protocol for targeted mutagenesis of specific gene regions utilizing the CRISPR/Cas9 technology, followed by behavioral screening. In this pilot study, we identified 20 new timeless mutant alleles with various impairments of temperature compensation. Molecular characterization revealed that the mutations included short in-frame insertions, deletions, or substitutions of a few amino acids resulting from the non-homologous end joining repair process. Our protocol is a fast and cost-efficient systematic approach for functional analysis of protein-coding genes and promoter analysis in vivo. Interestingly, several mutations with a strong temperature compensation defect map to one specific region of TIM. Although the exact mechanism of how these mutations affect TIM function is as yet unknown, our in silico analysis suggests they affect a putative nuclear export signal (NES) and phosphorylation sites of TIM. Immunostaining for PER was performed on two TIM mutants that display longer τ at 25°C and complete arrhythmicity at 28°C. Consistently with the behavioral phenotype, PER immunoreactivity was reduced in circadian clock neurons of flies exposed to elevated temperatures.

The Circadian Clock Improves Fitness in the Fruit Fly, Drosophila melanogaster

It is assumed that a properly timed circadian clock enhances fitness, but only few studies have truly demonstrated this in animals. We raised each of the three classical Drosophila period mutants for >50 generations in the laboratory in competition with wildtype flies. The populations were either kept under a conventional 24-h day or under cycles that matched the mutant’s natural cycle, i.e., a 19-h day in the case of per^s mutants and a 29-h day for per^l mutants. The arrhythmic per⁰ mutants were grown together with wildtype flies under constant light that renders wildtype flies similar arrhythmic as the mutants. In addition, the mutants had to compete with wildtype flies for two summers in two consecutive years under outdoor conditions. We found that wildtype flies quickly outcompeted the mutant flies under the 24-h laboratory day and under outdoor conditions, but per^l mutants persisted and even outnumbered the wildtype flies under the 29-h day in the laboratory. In contrast, per^s and per⁰ mutants did not win against wildtype flies under the 19-h day and constant light, respectively. Our results demonstrate that wildtype flies have a clear fitness advantage in terms of fertility and offspring survival over the period mutants and – as revealed for per^l mutants – this advantage appears maximal when the endogenous period resonates with the period of the environment. However, the experiments indicate that per^l and per^s persist at low frequencies in the population even under the 24-h day. This may be a consequence of a certain mating preference of wildtype and heterozygous females for mutant males and time differences in activity patterns between wildtype and mutants.

CRISPR/Cas9 Genome Editing Introduction and Optimization in the Non-model Insect Pyrrhocoris apterus

The CRISPR/Cas9 technique is widely used in experimentation with human cell lines as well as with other model systems, such as mice Mus musculus, zebrafish Danio reiro, and the fruit fly Drosophila melanogaster. However, publications describing the use of CRISPR/Cas9 for genome editing in non-model organisms, including non-model insects, are scarce. The introduction of this relatively new method presents many problems even for experienced researchers, especially with the lack of procedures to tackle issues concerning the efficiency of mutant generation. Here we present a protocol for efficient genome editing in the non-model insect species Pyrrhocoris apterus. We collected data from several independent trials that targeted several genes using the CRISPR/Cas9 system and determined that several crucial optimization steps led to a remarkably increased efficiency of mutant production. The main steps are as follows: the timing of embryo injection, the use of the heteroduplex mobility assay as a screening method, in vivo testing of sgRNA efficiency, and G0 germline mosaicism screening. The timing and the method of egg injections used here need to be optimized for other species, but other here-described optimization solutions can be applied immediately for genome editing in other insect species.

Systems and synthetic biology approaches in understanding biological oscillators

Background

Self-sustained oscillations are a ubiquitous and vital phenomenon in living systems. From primitive single-cellular bacteria to the most sophisticated organisms, periodicities have been observed in a broad spectrum of biological processes such as neuron firing, heart beats, cell cycles, circadian rhythms, etc. Defects in these oscillators can cause diseases from insomnia to cancer. Elucidating their fundamental mechanisms is of great significance to diseases, and yet challenging, due to the complexity and diversity of these oscillators.

Results

Approaches in quantitative systems biology and synthetic biology have been most effective by simplifying the systems to contain only the most essential regulators. Here, we will review major progress that has been made in understanding biological oscillators using these approaches. The quantitative systems biology approach allows for identification of the essential components of an oscillator in an endogenous system. The synthetic biology approach makes use of the knowledge to design the simplest, de novo oscillators in both live cells and cell-free systems. These synthetic oscillators are tractable to further detailed analysis and manipulations.

Conclusion

With the recent development of biological and computational tools, both approaches have made significant achievements.

Pharmacodynamic study on insomnia-curing effects of Shuangxia Decoction in Drosophila melanogaster

The present study aimed to establish a pharmacodynamic method using the pySolo software to explore the influence of freeze-dried powders of Shuangxia Decoction (SXD) on the sleep of normal Drosophila melanogaster and the Drosophila melanogaster whose sleep was divested by light. The dose-effect and the time-effect relationships of SXD on sleep were examined. The effect-onset concentration of SXD was 0.25%, the plateau appeared at the concentration of 2.5% and the total sleep time showed a downtrend when the concentration was greater than 2.5%. The sleep time was the longest on the fourth day after SXD was given. The fruit fly sleep deprivation model was repeated by light stimulation at night. The middle dosage group (2.5%) had the best insomnia-curing effect. In conclusion, using the pySolo software, an approach for the pharmacodynamics study was established with Drosophila melanogaster as a model organism to determine the insomnia-curing effects of the traditional Chinese medicine (TCM). Our results demonstrated the reliability of this method. The freeze-dried powders of SXD could effectively improve the sleep quality of Drosophila melanogaster.

The expression patterns of the clock genes period and timeless are affected by photoperiod in the Mediterranean corn stalk borer, Sesamia nonagrioides

To obtain clues to the link between the molecular mechanism of circadian and photoperiod clocks, we cloned two circadian clock genes, period (per) and timeless (tim) from the moth Sesamia nonagrioides, which undergoes facultative diapause controlled by photoperiod. Sequence analysis revealed a high degree of conservation among the compared insects fοr both genes. We also investigated the expression patterns of per and tim in brains of larvae growing under 16L:8D (long days), constant darkness (DD) and 10L:14D (short days) conditions by qPCR assays. The results showed that mRNA accumulations encoding both genes exhibited diel oscillations under different photoperiods. The oscillation of per and tim mRNA, under short-day photoperiod differed from long-day. The difference between long-day and short-day conditions in the pattern of mRNA levels of per and tim appears to distinguish photoperiodic conditions clearly and both genes were influenced by photoperiod in different ways. We infer that not all photoperiodic clocks of insects interact with circadian clocks in the same fashion. Our results suggest that transcriptional regulations of the both clock genes act in the diapause programing in S. nonagrioides. The expression patterns of these genes are affected by photoperiod but runs with 24 h by entrainment to daily environmental cues.

An RNAi Screen To Identify Protein Phosphatases That Function Within the Drosophila Circadian Clock

Circadian clocks in eukaryotes keep time via cell-autonomous transcriptional feedback loops. A well-characterized example of such a transcriptional feedback loop is in Drosophila, where CLOCK-CYCLE (CLK-CYC) complexes activate transcription of period (per) and timeless (tim) genes, rising levels of PER-TIM complexes feed-back to repress CLK-CYC activity, and degradation of PER and TIM permits the next cycle of CLK-CYC transcription. The timing of CLK-CYC activation and PER-TIM repression is regulated posttranslationally, in part through rhythmic phosphorylation of CLK, PER, and TIM. Previous behavioral screens identified several kinases that control CLK, PER, and TIM levels, subcellular localization, and/or activity, but two phosphatases that function within the clock were identified through the analysis of candidate genes from other pathways or model systems. To identify phosphatases that play a role in the clock, we screened clock cell-specific RNA interference (RNAi) knockdowns of all annotated protein phosphatases and protein phosphatase regulators in Drosophila for altered activity rhythms. This screen identified 19 protein phosphatases that lengthened or shortened the circadian period by ≥1 hr (p ≤ 0.05 compared to controls) or were arrhythmic. Additional RNAi lines, transposon inserts, overexpression, and loss-of-function mutants were tested to independently confirm these RNAi phenotypes. Based on genetic validation and molecular analysis, 15 viable protein phosphatases remain for future studies. These candidates are expected to reveal novel features of the circadian timekeeping mechanism in Drosophila that are likely to be conserved in all animals including humans.

Unexpected Geographic Variability of the Free Running Period in the Linden Bug Pyrrhocoris apterus

Circadian clocks keep organisms in synchrony with external day-night cycles. The free running period (FRP) of the clock, however, is usually only close to—not exactly—24 h. Here, we explored the geographical variation in the FRP of the linden bug, Pyrrhocoris apterus, in 59 field-lines originating from a wide variety of localities representing geographically different environments. We have identified a remarkable range in the FRPs between field-lines, with the fastest clock at ~21 h and the slowest close to 28 h, a range comparable to the collections of clock mutants in model organisms. Similarly, field-lines differed in the percentage of rhythmic individuals, with a minimum of 13.8% and a maximum of 86.8%. Although the FRP correlates with the latitude and perhaps with the altitude of the locality, the actual function of this FRP diversity is currently unclear. With the recent technological progress of massive parallel sequencing and genome editing, we can expect remarkable progress in elucidating the genetic basis of similar geographic variants in P. apterus or in similar emerging model species of chronobiology.

Loss of Drosophila melanogaster TRPA1 Function Affects “Siesta” Behavior but Not Synchronization to Temperature Cycles

To maintain synchrony with the environment, circadian clocks use a wide range of cycling sensory cues that provide input to the clock (zeitgebers), including environmental temperature cycles (TCs). There is some knowledge about which clock neuronal groups are important for temperature synchronization, but we currently lack knowledge on the temperature receptors and their signaling pathways that feed temperature information to the (neuronal) clock. Since TRPA1 is a well-known thermosensor that functions in a range of temperature-related behaviors, and it is potentially expressed in clock neurons, we set out to test the putative role of TRPA1 in temperature synchronization of the circadian clock. We found that flies lacking TRPA1 are still able to synchronize their behavioral activity to TCs comparable to wild-type flies, both in 16°C : 25°C and 20°C : 29°C TCs. In addition, we found that flies lacking TRPA1 show higher activity levels during the middle of the warm phase of 20°C : 29°C TCs, and we show that this TRPA1-mediated repression of locomotor activity during the “siesta” is caused by a lack of sleep. Based on these data, we conclude that the TRPA1 channel is not required for temperature synchronization in this broad temperature range but instead is required to repress activity during the warm part of the day.

Casein kinase 1 promotes synchrony of the circadian clock network

Casein kinase 1, known as DOUBLETIME (DBT) in Drosophila melanogaster, is a critical component of the circadian clock that phosphorylates and promotes degradation of the PERIOD (PER) protein. However, other functions of DBT in circadian regulation are not clear, in part because severe reduction of dbt causes preadult lethality. Here we report the molecular and behavioral phenotype of a viable dbt(EY02910) loss-of-function mutant. We found that DBT protein levels are dramatically reduced in adult dbt(EY02910) flies, and the majority of mutant flies display arrhythmic behavior, with a few showing weak, long-period (∼32 h) rhythms. Peak phosphorylation of PER is delayed, and both hyper- and hypophosphorylated forms of the PER and CLOCK proteins are present throughout the day. In addition, molecular oscillations of the circadian clock are dampened. In the central brain, PER and TIM expression is heterogeneous and decoupled in the canonical clock neurons of the dbt(EY02910) mutants. We also report an interaction between dbt and the signaling pathway involving pigment dispersing factor (PDF), a synchronizing peptide in the clock network. These data thus demonstrate that overall reduction of DBT causes long and arrhythmic behavior, and they reveal an unexpected role of DBT in promoting synchrony of the circadian clock network.

Histamine-HisCl1 receptor axis regulates wake-promoting signals in Drosophila melanogaster

Histamine and its two receptors, histamine-gated chloride channel subunit 1 (HisCl1) and ora transientless (Ort), are known to control photoreception and temperature sensing in Drosophila. However, histamine signaling in the context of neural circuitry for sleep-wake behaviors has not yet been examined in detail. Here, we obtained mutant flies with compromised or enhanced histamine signaling and tested their baseline sleep. Hypomorphic mutations in histidine decarboxylase (HDC), an enzyme catalyzing the conversion from histidine to histamine, caused an increase in sleep duration. Interestingly, hisCl1 mutants but not ort mutants showed long-sleep phenotypes similar to those in hdc mutants. Increased sleep duration in hisCl1 mutants was rescued by overexpressing hisCl1 in circadian pacemaker neurons expressing a neuropeptide pigment dispersing factor (PDF). Consistently, RNA interference (RNAi)-mediated depletion of hisCl1 in PDF neurons was sufficient to mimic hisCl1 mutant phenotypes, suggesting that PDF neurons are crucial for sleep regulation by the histamine-HisCl1 signaling. Finally, either hisCl1 mutation or genetic ablation of PDF neurons dampened wake-promoting effects of elevated histamine signaling via direct histamine administration. Taken together, these data clearly demonstrate that the histamine-HisCl1 receptor axis can activate and maintain the wake state in Drosophila and that wake-activating signals may travel via the PDF neurons.

Modelling the effect of phosphorylation on the circadian clock of Drosophila

It is by now well known that, at the molecular level, the core of the circadian clock of most living species is a negative feedback loop where some proteins inhibit their own transcription. However, it has recently been shown that post-translational processes, such as phosphorylations, are essential for a correct timing of the clock. Depending on which sites of a circadian protein are phosphorylated, different properties such as degradation, nuclear localization and repressing power can be altered. Furthermore, phosphorylation domains can be related in a positive way, giving rise to consecutive phosphorylations, or in a negative way, hindering phosphorylation at other domains. Here we present a simple mathematical model of a circadian protein having two mutually exclusive domains of phosphorylation. We show that the system has limit cycles that arise from a unique fixed point through a Hopf bifurcation. We find a set of parameters, with realistic values, for which the limit cycle has the same period as the wild type circadian oscillations of the fruit fly. The domains act as a switch, in the sense that alterations in their phosphorylation can alter the period of circadian oscillation in opposite ways, increasing or decreasing the period of the wild type oscillations. In particular, we show that our model is able to reproduce some of the experimental results found for switch-like phosphorylations of the PER protein of the circadian clock of the fly Drosophila melanogaster.

NEMO/NLK phosphorylates PERIOD to initiate a time-delay phosphorylation circuit that sets circadian clock speed

The speed of circadian clocks in animals is tightly linked to complex phosphorylation programs that drive daily cycles in the levels of PERIOD (PER) proteins. Using Drosophila, we identify a time-delay circuit based on hierarchical phosphorylation that controls the daily downswing in PER abundance. Phosphorylation by the NEMO/NLK kinase at the "per-short" domain on PER stimulates phosphorylation by DOUBLETIME (DBT/CK1δ/ɛ) at several nearby sites. This multisite phosphorylation operates in a spatially oriented and graded manner to delay progressive phosphorylation by DBT at other more distal sites on PER, including those required for recognition by the F box protein SLIMB/β-TrCP and proteasomal degradation. Highly phosphorylated PER has a more open structure, suggesting that progressive increases in global phosphorylation contribute to the timing mechanism by slowly increasing PER susceptibility to degradation. Our findings identify NEMO as a clock kinase and demonstrate that long-range interactions between functionally distinct phospho-clusters collaborate to set clock speed.

Ferritin overexpression in Drosophila glia leads to iron deposition in the optic lobes and late-onset behavioral defects

Cellular and organismal iron storage depends on the function of the ferritin protein complex in insects and mammals alike. In the central nervous system of insects, the distribution and relevance of ferritin remain unclear, though ferritin has been implicated in Drosophila models of Alzheimers' and Parkinsons' disease and in Aluminum-induced neurodegeneration. Here we show that transgene-derived expression of ferritin subunits in glial cells of Drosophila melanogaster causes a late-onset behavioral decline, characterized by loss of circadian rhythms in constant darkness and impairment of elicited locomotor responses. Anatomical analysis of the affected brains revealed crystalline inclusions of iron-loaded ferritin in a subpopulation of glial cells but not significant neurodegeneration. Although transgene-induced glial ferritin expression was well tolerated throughout development and in young flies, it turned disadvantageous at older age. The flies we characterize in this report contribute to the study of ferritin in the Drosophila brain and can be used to assess the contribution of glial iron metabolism in neurodegenerative models of disease.

RNA-interference knockdown of Drosophila pigment dispersing factor in neuronal subsets: the anatomical basis of a neuropeptide's circadian functions

Background

In animals, neuropeptide signaling is an important component of circadian timekeeping. The neuropeptide pigment dispersing factor (PDF) is required for several aspects of circadian activity rhythms in Drosophila.

Methodology/Principal Findings

Here we investigate the anatomical basis for PDF's various circadian functions by targeted PDF RNA-interference in specific classes of Drosophila neuron. We demonstrate that PDF is required in the ventro-lateral neurons (vLNs) of the central brain and not in the abdominal ganglion for normal activity rhythms. Differential knockdown of PDF in the large or small vLNs indicates that PDF from the small vLNs is likely responsible for the maintenance of free-running activity rhythms and that PDF is not required in the large vLNs for normal behavior. PDF's role in setting the period of free-running activity rhythms and the proper timing of evening activity under light:dark cycles emanates from both subtypes of vLN, since PDF in either class of vLN was sufficient for these aspects of behavior.

Conclusions/Significance

These results reveal the neuroanatomical basis PDF's various circadian functions and refine our understanding of the clock neuron circuitry of Drosophila.

Activating PER repressor through a DBT-directed phosphorylation switch

Protein phosphorylation plays an essential role in the generation of circadian rhythms, regulating the stability, activity, and subcellular localization of certain proteins that constitute the biological clock. This study examines the role of the protein kinase Doubletime (DBT), a Drosophila ortholog of human casein kinase I (CKI)ɛ/δ. An enzymatically active DBT protein is shown to directly phosphorylate the Drosophila clock protein Period (PER). DBT-dependent phosphorylation sites are identified within PER, and their functional significance is assessed in a cultured cell system and in vivo. The per^S mutation, which is associated with short-period (19-h) circadian rhythms, alters a key phosphorylation target within PER. Inspection of this and neighboring sequence variants indicates that several DBT-directed phosphorylations regulate PER activity in an integrated fashion: Alternative phosphorylations of two adjoining sequence motifs appear to be associated with switch-like changes in PER stability and repressor function.

Most proteins involved in circadian transcriptional feedback loops undergo reversible chemical modifications (called phosphorylation) that regulate their activity in a time-of-day–dependent manner. Doubletime (DBT), a Drosophila kinase, phosphorylates the circadian transcriptional repressor PERIOD (PER). Mutations of dbt shorten or lengthen the period of circadian behavioral rhythms, or abolish the rhythms altogether in flies. A mutation of the human ortholog of dbt, casein kinase I (CKI)δ, has been associated with certain forms of a heritable sleep disorder. The disorder may reflect altered activity of a human PER protein, as the syndrome can also be caused by mutation of a CKIɛ/δ phosphorylation site within PER2. In this study, we locate DBT-directed phosphorylation sites in the Drosophila PER protein, including a DBT target region of PER that was previously shown to regulate DBT activity. Two PER domains within this region appear to serve as alternative targets for DBT. Phosphorylation of the upstream domain seems to suppress phosphorylation elsewhere in the region, producing a stable PER protein with little activity as a transcriptional repressor. However, when phosphorylation of the upstream domain is blocked, downstream DBT targets appear to be phosphorylated, producing a highly active, but short-lived repressor. Our results suggest that ordered patterns of DBT-directed phosphorylation contribute to the timing of PER's function and disappearance, and thus influence the pace of the circadian clock.

Two phosphorylation domains inDrosophila PERIOD protein interact in a switch-like fashion with each other and the kinase DOUBLETIME to regulate PER's stability and activity as a transcriptional repressor in the circadian transcriptional feedback loop.

The phospho-occupancy of an atypical SLIMB-binding site on PERIOD that is phosphorylated by DOUBLETIME controls the pace of the clock

A common feature of animal circadian clocks is the progressive phosphorylation of PERIOD (PER) proteins, which is highly dependent on casein kinase Idelta/epsilon (CKIdelta/epsilon; termed DOUBLETIME [DBT] in Drosophila) and ultimately leads to the rapid degradation of hyperphosphorylated isoforms via a mechanism involving the F-box protein, beta-TrCP (SLIMB in Drosophila). Here we use the Drosophila melanogaster model system, and show that a key step in controlling the speed of the clock is phosphorylation of an N-terminal Ser (S47) by DBT, which collaborates with other nearby phosphorylated residues to generate a high-affinity atypical SLIMB-binding site on PER. DBT-dependent increases in the phospho-occupancy of S47 are temporally gated, dependent on the centrally located DBT docking site on PER and partially counterbalanced by protein phosphatase activity. We propose that the gradual DBT-mediated phosphorylation of a nonconsensus SLIMB-binding site establishes a temporal threshold for when in a daily cycle the majority of PER proteins are tagged for rapid degradation. Surprisingly, most of the hyperphosphorylation is unrelated to direct effects on PER stability. We also use mass spectrometry to map phosphorylation sites on PER, leading to the identification of a number of "phospho-clusters" that explain several of the classic per mutants.

Analysis of locomotor activity rhythms in Drosophila

The genetic, molecular and anatomical dissection of the circadian clock in Drosophila and other higher organisms relies on the quantification of rhythmic phenotypes. Here, we introduce the methods currently in use in our laboratories for the analysis of fly locomotor activity rhythms. This phenotype provides a relatively simple, automated, efficient, reliable and robust output for the circadian clock. Thus it is not surprising that it is the preferred readout for measuring rhythmicity under a variety of conditions for most fly clock laboratories. The procedure requires at least 10 days of data collection and several days for analysis. In this protocol we advise on fly maintenance and on experimental design when studying the genetics of behavioral traits. We describe the setup for studying locomotor activity rhythms in the fruit fly and we introduce the statistical methods in use in our laboratories for the analysis of periodic data.

Circadian transcription depends on limiting amounts of the transcription co-activator nejire/CBP

The circadian clock orchestrates physiological and behavioral activities, including metabolism, neuronal activity, and cell proliferation in synchrony with the environmental cycle of day and night. Here we show that the Drosophila ortholog of the CBP/p300 family of transcription co-activators, nejire (nej), is an intrinsic component of the circadian clock that performs regulatory functions for circadian controlled transcription. Screening of overexpression mutants revealed that gain of nej function was associated with a loss of behavioral and molecular rhythms. Overexpression of NEJ suppresses the long period phenotype of a mutation in the clock gene period (per). NEJ physically interacts through two binding sites with CLOCK and the CLOCK. CYCLE (CLK.CYC) complex. Induction of CLK.CYC-dependent transcripts upon induction of nej expression from a heat-shock promoter showed that NEJ is limiting. Reduced CLK.CYC-mediated transcription in a nej hypomorphic mutant indicates an essential function of NEJ/CBP for CLK.CYC activity and a regulation of circadian transcription by availability of the co-activator. Competition for recruitment of NEJ/CBP provides a potential mechanism for cross-talk between circadian transcription and other CBP-dependent physiological processes.

Functional role of CREB-binding protein in the circadian clock system of Drosophila melanogaster

Rhythmic histone acetylation underlies the oscillating expression of clock genes in the mammalian circadian clock system. Cellular factors that contain histone acetyltransferase and histone deacetylase activity have been implicated in these processes by direct interactions with clock genes, but their functional relevance remains to be assessed by use of appropriate animal models. Here, using transgenic fly models, we show that CREB-binding protein (CBP) participates in the transcriptional regulation of the Drosophila CLOCK/CYCLE (dCLK/CYC) heterodimer. CBP knockdown in pigment dispersing factor-expressing cells lengthens the period of adult locomotor rhythm with the prolonged expression of period and timeless genes, while CBP overexpression in timeless-expressing cells causes arrhythmic circadian behaviors with the impaired expression of these dCLK/CYC-induced clock genes. In contrast to the mammalian circadian clock system, CBP overexpression attenuates the transcriptional activity of the dCLK/CYC heterodimer in cultured cells, possibly by targeting the PER-ARNT-SIM domain of dCLK. Our data suggest that the Drosophila circadian clock system has evolved a distinct mechanism to tightly regulate the robust transcriptional potency of the dCLK/CYC heterodimer.

Even a stopped clock tells the right time twice a day: circadian timekeeping in Drosophila

"Even a stopped clock tells the right time twice a day, and for once I'm inclined to believe Withnail is right. We are indeed drifting into the arena of the unwell... What we need is harmony. Fresh air. Stuff like that" "Bruce Robinson (1986, ref. 1)". Although a stopped Drosophila clock probably does not tell the right time even once a day, recent findings have demonstrated that accurate circadian time-keeping is dependent on harmony between groups of clock neurons within the brain. Furthermore, when harmony between the environment and the endogenous clock is lost, as during jet lag, we definitely feel unwell. In this review, we provide an overview of the current understanding of circadian rhythms in Drosophila, focussing on recent discoveries that demonstrate how approximately 100 neurons within the Drosophila brain control the behaviour of the whole fly, and how these rhythms respond to the environment.

A proposal for robust temperature compensation of circadian rhythms

The internal circadian rhythms of cells and organisms coordinate their physiological properties to the prevailing 24-h cycle of light and dark on earth. The mechanisms generating circadian rhythms have four defining characteristics: they oscillate endogenously with period close to 24 h, entrain to external signals, suffer phase shifts by aberrant pulses of light or temperature, and compensate for changes in temperature over a range of 10 degrees C or more. Most theoretical descriptions of circadian rhythms propose that the underlying mechanism generates a stable limit cycle oscillation (in constant darkness or dim light), because limit cycles quite naturally possess the first three defining properties of circadian rhythms. On the other hand, the period of a limit cycle oscillator is typically very sensitive to kinetic rate constants, which increase markedly with temperature. Temperature compensation is therefore not a general property of limit cycle oscillations but must be imposed by some delicate balance of temperature dependent effects. However, "delicate balances" are unlikely to be robust to mutations. On the other hand, if circadian rhythms arise from a mechanism that concentrates sensitivity into a few rate constants, then the "balancing act" is likely to be more robust and evolvable. We propose a switch-like mechanism for circadian rhythms that concentrates period sensitivity in just two parameters, by forcing the system to alternate between a stable steady state and a stable limit cycle.

Monitoring and analyzing Drosophila circadian locomotor activity

In the 1970s, the intriguing discovery of autonomous circadian rhythmicity at the behavioral level in Drosophila set the starting point for one of the most remarkably rapid advancements in the understanding of the genetic and molecular bases of a complex behavioral trait. To this end, the design of appropriate electronic devices, apt to continuously monitor behavioral activity, has proven to be fundamental to such progress. In particular, most of the mutational screens performed to date in the search for genes involved in circadian rhythmicity were based on monitoring Drosophila mutants for alterations in the circadian pattern of locomotor activity. Many different experimental paradigms, based on the use of circadian locomotor activity monitors, have been developed. Experiments can be designed to determine (1) the natural period, (2) the capacity to adapt to day-night cycles with photoperiods of differing length, and (3) the phase of the circadian activity cycles with respect to the entraining stimulus. Here we describe some of the rationale and the steps required to set up experiments to monitor circadian locomotor activity in Drosophila. Suggestions for the statistical analysis of the data obtained in such experiments are also provided.

Structure and expressions of two circadian clock genes, period and timeless in the commercial silkmoth, Bombyx mori

We cloned two circadian clock genes period (Bmper) and timeless (Bmtim) from the commercial silkmoth, Bombyx mori. Sequence analysis revealed a high degree of conservation among insects for both genes. BmPER predicted from the DNA sequence is a polypeptide of 1, 113 amino acids with functional domains such as PAS, PAC, nuclear localization signal (NLS) and cytoplasmic localization domain (CLD). Deduced BmTIM consists of 997 amino acids with PER interaction site (PIS) as well as NLS and CLD. Southern blot analyses revealed that Bmper and Bmtim are single copy genes. Northern blot analysis demonstrated that Bmper and Bmtim are expressed both in the head and peripheral tissues. We also examined temporal profiles of Bmper and Bmtim expressions in the head, flight muscle, testis and antenna of adult males under LD12:12 and LD16:8 by Real-Time PCR assays. Our data show that photoperiod differentially affects the temporal expression patterns of Bmper and Bmtim. The mRNA expression of Bmper and Bmtim in the head had a phase lead under LD12:12 compared to that under LD16:8, whereas photoperiod did not affect expression patterns in peripheral tissues relative to light-on. Photoperiod affected not only the phase relationship but also the expression level. In the testis and antenna, the level of transcription of Bmtim was low in LD12:12 but high in LD16:8. The daily differences in amplitudes of the Bmper and Bmtim expression rhythms were 2-fold in the head and 1.5-2.5 folds in the peripheral tissues examined.

Natural allelic variation in the temperature-compensation mechanisms of the Arabidopsis thaliana circadian clock

Temperature compensation is a defining feature of circadian oscillators, yet no components contributing to the phenomenon have been identified in plants. We tested 27 accessions of Arabidopsis thaliana for circadian leaf movement at a range of constant temperatures. The accessions showed varying patterns of temperature compensation, but no clear associations to the geographic origin of the accessions could be made. Quantitative trait loci (QTL) were mapped for period and amplitude of leaf movement in the Columbia by Landsberg erecta (CoL) and Cape Verde Islands by Landsberg erecta (CvL) recombinant inbred lines (RILs) at 12 degrees , 22 degrees , and 27 degrees . Six CvL and three CoL QTL were located for circadian period. All of the period QTL were temperature specific, suggesting that they may be involved in temperature compensation. The flowering-time gene GIGANTEA and F-box protein ZEITLUPE were identified as strong candidates for two of the QTL on the basis of mapping in near isogenic lines (NILs) and sequence comparison. The identity of these and other candidates suggests that temperature compensation is not wholly determined by the intrinsic properties of the central clock proteins in Arabidopsis, but rather by other genes that act in trans to alter the regulation of these core proteins.

The neuropeptide pigment-dispersing factor coordinates pacemaker interactions in the Drosophila circadian system

In Drosophila, the neuropeptide pigment-dispersing factor (PDF) is required to maintain behavioral rhythms under constant conditions. To understand how PDF exerts its influence, we performed time-series immunostainings for the PERIOD protein in normal and pdf mutant flies over 9 d of constant conditions. Without pdf, pacemaker neurons that normally express PDF maintained two markers of rhythms: that of PERIOD nuclear translocation and its protein staining intensity. As a group, however, they displayed a gradual dispersion in their phasing of nuclear translocation. A separate group of non-PDF circadian pacemakers also maintained PERIOD nuclear translocation rhythms without pdf but exhibited altered phase and amplitude of PERIOD staining intensity. Therefore, pdf is not required to maintain circadian protein oscillations under constant conditions; however, it is required to coordinate the phase and amplitude of such rhythms among the diverse pacemakers. These observations begin to outline the hierarchy of circadian pacemaker circuitry in the Drosophila brain.

Essentials of Sleep Recordings in Drosophila: Moving Beyond Sleep Time

The power of Drosophila genetics can be used to facilitate the molecular dissection of sleep regulatory mechanisms. While evaluating total sleep time and homeostatic processes provides valuable information, other variables, such as sleep latency, sleep bout duration, sleep cycle length, and the time of day when the longest sleep bout is initiated, should also be used to explore the nature of a genetic lesion on sleep regulatory processes. Each of these variables requires that the recording interval used to identify periods of sleep and waking be determined accurately and empirically. This article describes the procedures for recording sleep in Drosophila and associated methodological constraints. In addition, it provides results from a normative data set of 1037 Canton-S female flies and 639 male flies to illustrate the nature and variability of sleep variables that one can extract from 24 h of data collection in Drosophila.

Genetics and molecular biology of rhythms in Drosophila and other insects

Application of generic variants (Sections II-IV, VI, and IX) and molecular manipulations of rhythm-related genes (Sections V-X) have been used extensively to investigate features of insect chronobiology that might not have been experimentally accessible otherwise. Most such tests of mutants and molecular-genetic xperiments have been performed in Drosophila melanogaster. Results from applying visual-system variants have revealed that environmental inputs to the circadian clock in adult flies are mediated by external photoreceptive structures (Section II) and also by direct light reception chat occurs in certain brain neurons (Section IX). The relevant light-absorbing molecuLes are rhodopsins and "blue-receptive" cryptochrome (Sections II and IX). Variations in temperature are another clock input (Section IV), as has been analyzed in part by use of molecular techniques and transgenes involving factors functioning near the heart of the circadian clock (Section VIII). At that location within the fly's chronobiological system, approximately a half-dozen-perhaps up to as many as 10-clock genes encode functions that act and interact to form the circadian pacemaker (Sections III and V). This entity functions in part by transcriptional control of certain clock genes' expressions, which result in the production of key proteins that feed back negatively to regulate their own mRNA production. This occurs in part by interactions of such proteins with others that function as transcriptional activators (Section V). The implied feedback loop operates such that there are daily variations in the abundances of products put out by about one-half of the core clock genes. Thus, the normal expression of these genes defines circadian rhythms of their own, paralleling the effects of mutations at the corresponding genetic loci (Section III), which are to disrupt or apparently eliminate clock functioning. The fluctuations in the abundance of gene products are controlled transciptionally and posttranscriptionally. These clock mechanisms are being analyzed in ways that are increasingly complex and occasionally obscure; not all panels of this picture are comprehensive or clear, including problems revolving round the biological meaning or a given features of all this molecular cycling (Section V). Among the complexities and puzzles that have recently arisen, phenomena that stand out are posttranslational modifications of certain proteins that are circadianly regulated and regulating; these biochemical events form an ancillary component of the clock mechanism, as revealed in part by genetic identification of Factors (Section III) that turned out to encode protein kinases whose substrates include other pacemaking polypeptides (Section V). Outputs from insect circadian clocks have been long defined on formalistic and in some cases concrete criteria, related to revealed rhythms such as periodic eclosion and daily fluctuations of locomotion (Sections II and III). Based on the reasoning that if clock genes can regulate circadian cyclings of their own products, they can do the same for genes that function along output pathways; thus clock-regulated genes have been identified in part by virtue of their products' oscillations (Section X). Those studied most intensively have their expression influenced by circadian-pacemaker mutations. The clock-regulated genes discovered on molecular criteria have in some instances been analyzed further in their mutant forms and found to affect certain features of overt whole-organismal rhythmicity (Sections IV and X). Insect chronogenetics touches in part on naturally occurring gene variations that affect biological rhythmicity or (in some cases) have otherwise informed investigators about certain features of the organism's rhythm system (Section VII). Such animals include at least a dozen insect species other than D. melanogaster in which rhythm variants have been encountered (although usually not looked for systematically). The chronobiological "system" in the fruit fly might better be graced with a plural appellation because there is a myriad of temporally related phenomena that have come under the sway of one kind of putative rhythm variant or the other (Section IV). These phenotypes, which range well beyond the bedrock eclosion and locomotor circadian rhythms, unfortunately lead to the creation of a laundry list of underanalyzed or occult phenomena that may or may not be inherently real, whether or not they might be meaningfully defective under the influence of a given chronogenetic variant. However, such mutants seem to lend themselves to the interrogation of a wide variety of time-based attributes-those that fall within the experimental confines of conventionally appreciated circadian rhythms (Sections II, III, VI, and X); and others that consist of 24-hr or nondaily cycles defined by many kinds of biological, physiological, or biochemical parameters (Section IV).

Genetic analysis of the circadian system in Drosophila melanogaster and mammals

The fruit fly, Drosophila melanogaster, has been a grateful object for circadian rhythm researchers over several decades. Behavioral, genetic, and molecular studies helped to reveal the genetic bases of circadian time keeping and rhythmic behaviors. Contrary, mammalian rhythm research until recently was mainly restricted to descriptive and physiologic approaches. As in many other areas of research, the surprising similarity of basic biologic principles between the little fly and our own species, boosted the progress of unraveling the genetic foundation of mammalian clock mechanisms. Once more, not only the basic mechanisms, but also the molecules involved in establishing our circadian system are taken or adapted from the fly. This review will try to give a comparative overview about the two systems, highlighting similarities as well as specifics of both insect and murine clocks.

Evolutionary behavioral genetics in Drosophila

Behavioral genes have a special evolutionary interest because they are potentially involved in speciation and in many forms of adaptation. Dozens of loci affecting different aspects of behavior have been already identified and cloned in Drosophila. Some of these genes determine variation in such ethologically complex phenotypes as the male "love song" that is produced during courtship and the locomotor "sleep-wake" activity cycles that are controlled by the circadian clock. Although the evolutionary analysis of most behavioral genes in Drosophila is relatively new, it has already given important insights into the forces shaping the molecular variation at these loci and their functional consequences.

Population dynamics of clock-controlled biological species: models and why circadian rhythms are circadian

Conventional models of population dynamics of biological species do not take into account the fact that activity of most living organisms is under control of biological clocks, which are oscillators with periods near 1 day, 1 year, and so on. Here proposed are new categories of population dynamics models which are combinations of Lotka-Volterra type of equations (as a simplest example; in fact, an arbitrary type may be used) and equations of the biological oscillators. Such models may be particularly useful in addressing to those problems which are inaccessible with conventional models. A simple example of such models with distributed clock periods is proposed and studied numerically and analytically to consider why periods of biological rhythms are not precisely one day or one year, etc., as is often questioned. For this purpose, a notion of survival index is introduced to measure the degree of success in survival. It turns out that clock periods equal or close to that of the milieu are not necessarily advantageous for survival and can even lead to extinction; survival is then most successful at the edge of entrainment. This may provide a clue to answer the question. It is also found that survival is difficult for those species which cannot be entrained by the environmental cycle.

Short-day and long-day expression patterns of genes involved in the flesh fly clock mechanism: period, timeless, cycle and cryptochrome

Though our knowledge of the molecular details of the circadian clock has advanced rapidly, the functional elements of the photoperiodic clock remain largely unknown. As a first step to approach this issue, we report here the sequences and expression patterns of period (per), timeless (tim), cycle (cyc) and cryptochrome (cry) mRNAs in the flesh fly Sarcophaga crassipalpis. Nucleotide and deduced amino acid sequences of the genes in S. crassipalpis show high similarity to homologous genes in other insects that have been investigated. S. crassipalpis TIM has a unique C-terminus that contains a poly Q region. A diel rhythmicity of per and tim mRNA abundance was detected in the adult heads (peak during scotophase), while cry and cyc mRNA abundance remained fairly constant throughout. The abundance of cyc mRNA was quite low when compared to per, tim and cry mRNA. Rearing temperature affected the amount of per and tim mRNAs: abundance of per mRNA increased at 20 degrees C when compared to 25 degrees C, but that of tim mRNA decreased. Photoperiod influenced the expression patterns of per and tim mRNA: the peak of per mRNA expression shifted in concert with onset of the scotophase, while a shift in tim mRNA expression was less pronounced. The amplitude of tim mRNA was severely dampened under long daylength, but that of per mRNA was not affected. These distinct patterns of expression suggest that this information could be used to determine photoperiodic responses such as diapause.

Giant neuron pathway neurophysiological activity in per(0) mutants of Drosophila melanogaster

In Drosophila melanogaster, the clock gene period (per) has a clearly defined role in the molecular machinery involved in generating free-running circadian rhythms. per mutations also influence rhythms in the Drosophila love song and in the ultradian timescale. The relationship between these two phenomena has so far escaped satisfactory explanation. Here we analyzed the neurophysiological activity of the giant fiber neural pathway in per(0) flies. Under constant light, and at relatively low stimulation frequencies (1-2 Hz), per(01) flies habituate significantly earlier than they do under 12 h light-dark cycles. The results suggest an involvement of per in phenomena of short-term neural plasticity.

Identification of circadian-clock-regulated enhancers and genes of Drosophila melanogaster by transposon mobilization and luciferase reporting of cyclical gene expression

A new way was developed to isolate rhythmically expressed genes in Drosophila by modifying the classic enhancer-trap method. We constructed a P element containing sequences that encode firefly luciferase as a reporter for oscillating gene expression in live flies. After generation of 1176 autosomal insertion lines, bioluminescence screening revealed rhythmic reporter-gene activity in 6% of these strains. Rhythmically fluctuating reporter levels were shown to be altered by clock mutations in genes that specify various circadian transcription factors or repressors. Intriguingly, rhythmic luminescence in certain lines was affected by only a subset of the pacemaker mutations. By isolating genes near 13 of the transposon insertions and determining their temporal mRNA expression pattern, we found that four of the loci adjacent to the trapped enhancers are rhythmically expressed. Therefore, this approach is suitable for identifying genetic loci regulated by the circadian clock. One transposon insert caused a mutation in the rhythmically expressed gene numb. This novel numb allele, as well as previously described ones, was shown to affect the fly's rhythm of locomotor activity. In addition to its known role in cell fate determination, this gene and the phosphotyrosine-binding protein it encodes are likely to function in the circadian system.

The Drosophila double-timeS mutation delays the nuclear accumulation of period protein and affects the feedback regulation of period mRNA

The Drosophila double-time (dbt) gene, which encodes a protein similar to vertebrate epsilon and delta isoforms of casein kinase I, is essential for circadian rhythmicity because it regulates the phosphorylation and stability of period (per) protein. Here, the circadian phenotype of a short-period dbt mutant allele (dbt(S)) was examined. The circadian period of the dbt(S) locomotor activity rhythm varied little when tested at constant temperatures ranging from 20 to 29 degrees C. However, per(L);dbt(S) flies exhibited a lack of temperature compensation like that of the long-period mutant (per(L)) flies. Light-pulse phase-response curves were obtained for wild-type, the short-period (per(S)), and dbt(S) genotypes. For the per(S) and dbt(S) genotypes, phase changes were larger than those for wild-type flies, the transition period from delays to advances was shorter, and the light-insensitive period was shorter. Immunohistochemical analysis of per protein levels demonstrated that per protein accumulates in photoreceptor nuclei later in dbt(S) than in wild-type and per(S) flies, and that it declines to lower levels in nuclei of dbt(S) flies than in nuclei of wild-type flies. Immunoblot analysis of per protein levels demonstrated that total per protein accumulation in dbt(S) heads is neither delayed nor reduced, whereas RNase protection analysis demonstrated that per mRNA accumulates later and declines sooner in dbt(S) heads than in wild-type heads. These results suggest that dbt can regulate the feedback of per protein on its mRNA by delaying the time at which it is translocated to nuclei and altering the level of nuclear PER during the declining phase of the cycle.

Multiple amidated neuropeptides are required for normal circadian locomotor rhythms in Drosophila

In Drosophila, the amidated neuropeptide pigment dispersing factor (PDF) is expressed by the ventral subset of lateral pacemaker neurons and is required for circadian locomotor rhythms. Residual rhythmicity in pdf mutants likely reflects the activity of other neurotransmitters. We asked whether other neuropeptides contribute to such auxiliary mechanisms. We used the gal4/UAS system to create mosaics for the neuropeptide amidating enzyme PHM; amidation is a highly specific and widespread modification of secretory peptides in Drosophila. Three different gal4 drivers restricted PHM expression to different numbers of peptidergic neurons. These mosaics displayed aberrant locomotor rhythms to degrees that paralleled the apparent complexity of the spatial patterns. Certain PHM mosaics were less rhythmic than pdf mutants and as severe as per mutants. Additional gal4 elements were added to the weakly rhythmic PHM mosaics. Although adding pdf-gal4 provided only partial improvement, adding the widely expressed tim-gal4 largely restored rhythmicity. These results indicate that, in Drosophila, peptide amidation is required for neuropeptide regulation of behavior. They also support the hypothesis that multiple amidated neuropeptides, acting upstream, downstream, or in parallel to PDF, help organize daily locomotor rhythms.

The Drosophila double-timeS mutation delays the nuclear accumulation of period protein and affects the feedback regulation of period mRNA

The Drosophila double-time (dbt) gene, which encodes a protein similar to vertebrate epsilon and delta isoforms of casein kinase I, is essential for circadian rhythmicity because it regulates the phosphorylation and stability of period (per) protein. Here, the circadian phenotype of a short-period dbt mutant allele (dbt(S)) was examined. The circadian period of the dbt(S) locomotor activity rhythm varied little when tested at constant temperatures ranging from 20 to 29 degrees C. However, per(L);dbt(S) flies exhibited a lack of temperature compensation like that of the long-period mutant (per(L)) flies. Light-pulse phase-response curves were obtained for wild-type, the short-period (per(S)), and dbt(S) genotypes. For the per(S) and dbt(S) genotypes, phase changes were larger than those for wild-type flies, the transition period from delays to advances was shorter, and the light-insensitive period was shorter. Immunohistochemical analysis of per protein levels demonstrated that per protein accumulates in photoreceptor nuclei later in dbt(S) than in wild-type and per(S) flies, and that it declines to lower levels in nuclei of dbt(S) flies than in nuclei of wild-type flies. Immunoblot analysis of per protein levels demonstrated that total per protein accumulation in dbt(S) heads is neither delayed nor reduced, whereas RNase protection analysis demonstrated that per mRNA accumulates later and declines sooner in dbt(S) heads than in wild-type heads. These results suggest that dbt can regulate the feedback of per protein on its mRNA by delaying the time at which it is translocated to nuclei and altering the level of nuclear PER during the declining phase of the cycle.

Multiple amidated neuropeptides are required for normal circadian locomotor rhythms in Drosophila

In Drosophila, the amidated neuropeptide pigment dispersing factor (PDF) is expressed by the ventral subset of lateral pacemaker neurons and is required for circadian locomotor rhythms. Residual rhythmicity in pdf mutants likely reflects the activity of other neurotransmitters. We asked whether other neuropeptides contribute to such auxiliary mechanisms. We used the gal4/UAS system to create mosaics for the neuropeptide amidating enzyme PHM; amidation is a highly specific and widespread modification of secretory peptides in Drosophila. Three different gal4 drivers restricted PHM expression to different numbers of peptidergic neurons. These mosaics displayed aberrant locomotor rhythms to degrees that paralleled the apparent complexity of the spatial patterns. Certain PHM mosaics were less rhythmic than pdf mutants and as severe as per mutants. Additional gal4 elements were added to the weakly rhythmic PHM mosaics. Although adding pdf-gal4 provided only partial improvement, adding the widely expressed tim-gal4 largely restored rhythmicity. These results indicate that, in Drosophila, peptide amidation is required for neuropeptide regulation of behavior. They also support the hypothesis that multiple amidated neuropeptides, acting upstream, downstream, or in parallel to PDF, help organize daily locomotor rhythms.

Light-dependent interaction between Drosophila CRY and the clock protein PER mediated by the carboxy terminus of CRY

BACKGROUND

The biological clock synchronizes the organism with the environment, responding to changes in light and temperature. Drosophila CRYPTOCHROME (CRY), a putative circadian photoreceptor, has previously been reported to interact with the clock protein TIMELESS (TIM) in a light-dependent manner. Although TIM dimerizes with PERIOD (PER), no association between CRY and PER has previously been revealed, and aspects of the light dependence of the TIM/CRY interaction are still unclear.

RESULTS

Behavioral analysis of double mutants of per and cry suggested a genetic interaction between the two loci. To investigate whether this was reflected in a physical interaction, we employed a yeast-two-hybrid system that revealed a dimerization between PER and CRY. This was further supported by a coimmunoprecipitation assay in tissue culture cells. We also show that the light-dependent nuclear interactions of PER and TIM with CRY require the C terminus of CRY and may involve a trans-acting repressor.

CONCLUSIONS

This study shows that, as in mammals, Drosophila CRY interacts with PER, and, as in plants, the C terminus of CRY is involved in mediating light responses. A model for the light dependence of CRY is discussed.

Molecular components of the circadian system in Drosophila

Much of our current understanding of how circadian rhythms are generated is based on work done with Drosophila melanogaster. Molecular mechanisms used to assemble an endogenous clock in this organism are now known to underlie circadian rhythms in many other species, including mammals. The genetic amenability of Drosophila has led to the identification of some genes that encode components of the clock (so-called clock genes) and others that either link the clock to the environment or act downstream of it. The clock provides time-of-day cues by regulating levels of specific gene products such that they oscillate with a circadian rhythm. The mechanisms that synchronize these oscillations to light are understood to some extent. However, there are still large gaps in our knowledge, in particular with respect to the mechanisms used by the clock to control overt rhythms. It has, however, become clear that in addition to the brain clock, autonomous or semi-autonomous clocks occur in peripheral tissues where they confer circadian regulation on specific functions.

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.