Circadian regulation of the lark RNA-binding protein within identifiable neurosecretory cells

Accuracy of fruit-fly eclosion rhythms evolves by strengthening circadian gating rather than developmental fine-tuning

Fruit flies (Drosophila melanogaster) eclose from their pupae mainly around dawn. The timing of eclosion is thought to confer adaptive benefits to the organisms and thus shows remarkable accuracy. However, it is not clear what factors are involved in the evolution of such accuracy in natural populations. In this study, we examined the relative contributions of gating of eclosion by the circadian clock versus clock-independent developmental rates and light-induced responses in the eclosion phenotype exhibited by fly populations that have evolved greater accuracy in eclosion rhythms compared to controls. We compared variation in timing of transitions between early developmental stages (pupariation and pigmentation), overall development time under constant light conditions - where circadian clocks are dysfunctional - and eclosion profiles when developmental rate was manipulated using different larval densities in selected and control stocks. Our results showed that stocks that have evolved greater accuracy of eclosion rhythms due to artificial selection do not show reduced individual variation in pupariation and pigmentation time compared to controls, though they do exhibit lower variation in eclosion time. Selected stocks also did not show lower variation in eclosion time under constant light conditions in contrast to the greater accuracy seen under light-dark cycles. Moreover, manipulations of developmental rate by varying larval density and inducing eclosion by changing onset of light phase did not alter the eclosion profile of selected stocks as much as it did controls, since selected stocks largely restricted eclosion to the daytime. These results suggest that fly populations selected for greater accuracy have evolved accurate eclosion rhythms primarily by strengthening circadian gating of eclosion rather than due to fine-tuning of clock-independent developmental processes.This article has an associated First Person interview with the first author of the paper.

Cellular requirements for LARK in the Drosophila circadian system

RNA-binding proteins mediate posttranscriptional functions in the circadian systems of multiple species. A conserved RNA recognition motif (RRM) protein encoded by the lark gene is postulated to serve circadian output and molecular oscillator functions in Drosophila and mammals, respectively. In no species, however, has LARK been eliminated, in vivo, to determine the consequences for circadian timing. The present study utilized RNA interference (RNAi) techniques in Drosophila to decrease LARK levels in clock neurons and other cell types in order to evaluate the circadian functions of the protein. Knockdown of LARK in timeless (TIM)- or pigment dispersing factor (PDF)-containing clock cells caused a significant number of flies to exhibit arrhythmic locomotor activity, demonstrating a requirement for the protein in pacemaker cells. There was no obvious effect on PER protein cycling in lark interference (RNAi) flies, but a knockdown within the PDF neurons was associated with increased PDF immunoreactivity at the dorsal termini of the small ventral lateral neuronal (s-LNv) projections, suggesting an effect on neuropeptide release. The expression of lark RNAi in multiple neurosecretory cell populations demonstrated that LARK is required within pacemaker and nonpacemaker cells for the manifestation of normal locomotor activity rhythms. Interestingly, decreased LARK function in the prothoracic gland (PG), a peripheral organ containing a clock required for the circadian control of eclosion, was associated with weak population eclosion rhythms or arrhythmicity.

Molecular and genetic analysis of the Drosophila model of fragile X syndrome

The Drosophila genome contains most genes known to be involved in heritable disease. The extraordinary genetic malleability of Drosophila, coupled to sophisticated imaging, electrophysiology, and behavioral paradigms, has paved the way for insightful mechanistic studies on the causes of developmental and neurological disease as well as many possible interventions. Here, we focus on one of the most advanced examples of Drosophila genetic disease modeling, the Drosophila model of Fragile X Syndrome, which for the past decade has provided key advances into the molecular, cellular, and behavioral defects underlying this devastating disorder. We discuss the multitude of RNAs and proteins that interact with the disease-causing FMR1 gene product, whose function is conserved from Drosophila to human. In turn, we consider FMR1 mechanistic relationships in non-neuronal tissues (germ cells and embryos), peripheral motor and sensory circuits, and central brain circuits involved in circadian clock activity and learning/memory.

Fragile X mental retardation protein is required for programmed cell death and clearance of developmentally-transient peptidergic neurons

Fragile X syndrome (FXS), caused by loss of fragile X mental retardation 1 (FMR1) gene function, is the most common heritable cause of intellectual disability and autism spectrum disorders. The FMR1 product (FMRP) is an RNA-binding protein best established to function in activity-dependent modulation of synaptic connections. In the Drosophila FXS disease model, loss of functionally-conserved dFMRP causes synaptic overgrowth and overelaboration in pigment dispersing factor (PDF) peptidergic neurons in the adult brain. Here, we identify a very different component of PDF neuron misregulation in dfmr1 mutants: the aberrant retention of normally developmentally-transient PDF tritocerebral (PDF-TRI) neurons. In wild-type animals, PDF-TRI neurons in the central brain undergo programmed cell death and complete, processive clearance within days of eclosion. In the absence of dFMRP, a defective apoptotic program leads to constitutive maintenance of these peptidergic neurons. We tested whether this apoptotic defect is circuit-specific by examining crustacean cardioactive peptide (CCAP) and bursicon circuits, which are similarly developmentally-transient and normally eliminated immediately post-eclosion. In dfmr1 null mutants, CCAP/bursicon neurons also exhibit significantly delayed clearance dynamics, but are subsequently eliminated from the nervous system, in contrast to the fully persistent PDF-TRI neurons. Thus, the requirement of dFMRP for the retention of transitory peptidergic neurons shows evident circuit specificity. The novel defect of impaired apoptosis and aberrant neuron persistence in the Drosophila FXS model suggests an entirely new level of "pruning" dysfunction may contribute to the FXS disease state.

Posttranscriptional mechanisms in controlling eukaryotic circadian rhythms

The circadian clock is essential in almost all living organisms to synchronise biochemical, metabolic, physiological and behavioural cycles to daily changing environmental factors. In a highly conserved fashion, the circadian clock is primarily controlled by multiple positive and negative molecular circuitries that control gene expression. More recently, research in Neurospora and other eukaryotes has uncovered the involvement of additional regulatory components that operate at the posttranslational level to fine tune the circadian system. Though it remains poorly understood, a growing body of evidence has shown that posttranscriptional regulation controls the expression of both circadian oscillator and output gene transcripts at a number of different steps. This regulation is crucial for driving and maintaining robust circadian rhythms. Here we review recent advances in circadian rhythm research at the RNA level.

Circadian organization of behavior and physiology in Drosophila

Circadian clocks organize behavior and physiology to adapt to daily environmental cycles. Genetic approaches in the fruit fly, Drosophila melanogaster, have revealed widely conserved molecular gears of these 24-h timers. Yet much less is known about how these cell-autonomous clocks confer temporal information to modulate cellular functions. Here we discuss our current knowledge of circadian clock function in Drosophila, providing an overview of the molecular underpinnings of circadian clocks. We then describe the neural network important for circadian rhythms of locomotor activity, including how these molecular clocks might influence neuronal function. Finally, we address a range of behaviors and physiological systems regulated by circadian clocks, including discussion of specific peripheral oscillators and key molecular effectors where they have been described. These studies reveal a remarkable complexity to circadian pathways in this "simple" model organism.

The Circadian Control of Eclosion

Eclosion is the stage in development when the adult insect emerges from the shell of its old cuticle. The sequence of behaviors necessary for eclosion is coordinated by an integrated system of hormones and is activated by hormones that relay developmental readiness. The circadian clock, which controls the timing of behaviors such as the rest: activity rhythm of adult insects, also controls eclosion timing. A number of groups are actively investigating the mechanisms by which the circadian clock restricts or gates eclosion to a particular time of day. Data from these studies are beginning to reveal details of the molecular and physiological basis of the eclosion rhythm.

Altered LARK expression perturbs development and physiology of the Drosophila PDF clock neurons

The LARK RNA-binding protein (RBP) has well documented roles in the circadian systems of Drosophila and mammals. Recent studies have demonstrated that the Drosophila LARK RBP is associated with many mRNA targets, in vivo, including those that regulate either neurophysiology or development of the nervous system. In the present study, we have employed conditional expression techniques to distinguish developmental and physiological functions of LARK for a defined class of neurons: the Pigment-Dispersing Factor (PDF)-containing LNv clock neurons. We found that increased LARK expression during development dramatically alters the small LNv class of neurons with no obvious effects on the large LNv cells. Conversely, conditional expression of LARK at the adult stage results in altered clock protein rhythms and circadian locomotor activity, even though neural morphology is normal in such animals. Electrophysiological analyses at the larval neuromuscular junction indicate a role for LARK in regulating neuronal excitability. Altogether, our results demonstrate that LARK activity is critical for neuronal development and physiology.

The Drosophila FMRP and LARK RNA-binding proteins function together to regulate eye development and circadian behavior

Fragile X syndrome (FXS) is the most common form of hereditary mental retardation. FXS patients have a deficit for the fragile X mental retardation protein (FMRP) that results in abnormal neuronal dendritic spine morphology and behavioral phenotypes, including sleep abnormalities. In a Drosophila model of FXS, flies lacking the dfmr1 protein (dFMRP) have abnormal circadian rhythms apparently as a result of altered clock output. In this study, we present biochemical and genetic evidence that dFMRP interacts with a known clock output component, the LARK RNA-binding protein. Our studies demonstrate physical interactions between dFMRP and LARK, that the two proteins are present in a complex in vivo, and that LARK promotes the stability of dFMRP. Furthermore, we show genetic interactions between the corresponding genes indicating that dFMRP and LARK function together to regulate eye development and circadian behavior.

The LARK RNA-binding protein selectively regulates the circadian eclosion rhythm by controlling E74 protein expression

Despite substantial progress in defining central components of the circadian pacemaker, the output pathways coupling the clock to rhythmic physiological events remain elusive. We previously showed that LARK is a Drosophila RNA-binding protein which functions downstream of the clock to mediate behavioral outputs. To better understand the roles of LARK in the circadian system, we sought to identify RNA molecules associated with it, in vivo, using a three-part strategy to (1) capture RNA ligands by immunoprecipitation, (2) visualize the captured RNAs using whole-genome microarrays, and (3) identify functionally relevant targets through genetic screens. We found that LARK is associated with a large number of RNAs, in vivo, consistent with its broad expression pattern. Overexpression of LARK increases protein abundance for certain targets without affecting RNA level, suggesting a translational regulatory role for the RNA-binding protein. Phenotypic screens of target-gene mutants have identified several with rhythm-specific circadian defects, indicative of effects on clock output pathways. In particular, a hypomorphic mutation in the E74 gene, E74(BG01805), was found to confer an early-eclosion phenotype reminiscent of that displayed by a mutant with decreased LARK gene dosage. Molecular analyses demonstrate that E74A protein shows diurnal changes in abundance, similar to LARK. In addition, the E74(BG01805) allele enhances the lethal phenotype associated with a lark null mutation, whereas overexpression of LARK suppresses the early eclosion phenotype of E74(BG01805), consistent with the idea that E74 is a target, in vivo. Our results suggest a model wherein LARK mediates the transfer of temporal information from the molecular oscillator to different output pathways by interacting with distinct RNA targets.

Expression analysis of two types of transcripts from circadian output gene lark in Bombyx mori

We analyzed expression patterns of lark (Bmlark) in Bombyx mori. Southern blot analysis demonstrated that Bmlark was a single copy gene. Northern blot analyses revealed two types of Bmlark transcripts, one being of 1.38 kb (Bmlark-PA) and the other of 0.85 kb (Bmlark-PB). Both transcripts were detected in the eggs, larval and adult heads, testes, ovaries and flight muscles. Both types of transcripts are constitutively expressed with no clear rhythmicity in the adult heads under light:dark (LD) cycles but the amount of the Bmlark-PA transcript was twice as much as that of the Bmlark-PB transcript in the adult heads throughout a day. Real-Time PCR assays also indicated constant expressions of the two types of Bmlark in the pupal brains under LD12:12 and LD16:8.

Identification and expression pattern of Bmlark, a homolog of the Drosophila gene lark in Bombyx mori

Two Bombyx mori isoforms of the gene lark, which is shown to play an important role in Drosophila circadian rhythms, were identified and named Bmlark-PA and Bmlark-PB, respectively. Bmlark-PA consists of 5 exons and encodes a protein of 343 amino acid residues which contains 3 functional domains: two RRM (RNA recognization motif) domains and an RTZF (retroviral-type zinc finger) and shares 72% identity with the Drosophila gene lark at the amino acid level. Bmlark-PB lacks the sequence between 118 and 791 nt of Bmlark-PA and codes for a protein of 68 amino acid residues, which contains no distinct functional domains. Alignments of the cDNAs of Bmlark to the genomic draft sequence of B. mori showed that the gene Bmlark had a single copy in the genome, suggesting that an alternative splicing mechanism occurs in the gene Bmlark. RT-PCR analysis indicated that Bmlark-PA was expressed only in late pupae and adult but Bmlark-PB was broadly expressed in many tissues and throughout the developmental stages from embryo to adult.

Neural circuits underlying circadian behavior in Drosophila melanogaster

Circadian clocks include control systems for organizing daily behavior. Such a system consists of a time-keeping mechanism (the clock or pacemaker), input pathways for entraining the clock, and output pathways for producing overt rhythms in behavior and physiology. In Drosophila melanogaster, as in mammals, neural circuits play vital roles in all three functional subdivisions of the circadian system. Regarding the pacemaker, multiple clock neurons, each with cell-autonomous pacemaker capability, are coupled to each other in a network. The outputs of different sets of clock neurons in this network combine to produce the normal bimodal pattern of locomotor activity observed in Drosophila. Regarding input, multiple sensory modalities (including light, temperature, and pheromones) use their own circuitry to entrain the clock. Regarding output, distinct circuits are likely involved for controlling the timing of eclosion and for generating the locomotor activity rhythms. This review summarizes work on all of these circadian circuits, and discusses the broader utility of studying the fly's circadian system.

Genetic and biochemical strategies for identifying Drosophila genes that function in circadian control

Explicit biochemical models have been elaborated for the circadian oscillators of cyanobacterial, fungal, insect, and mammalian species. In contrast, much remains to be learned about how such circadian oscillators regulate rhythmic physiological processes. This article summarizes contemporary genetic and biochemical strategies that are useful for identifying gene products that have a role in circadian control.

The neuroarchitecture of the circadian clock in the brain of Drosophila melanogaster

Neuroethologists try to assign behavioral functions to certain brain centers, if possible down to individual neurons and to the expression of specific genes. This approach has been successfully applied for the control of circadian rhythmic behavior in the fruit fly Drosophila melanogaster. Several so-called "clock genes" are expressed in specific neurons in the lateral and dorsal brain where they generate cell-autonomous molecular circadian oscillations. These clusters are connected with each other and contribute differentially to the control of behavioral rhythmicity. This report reviews the latest work on characterizing individual circadian pacemaker neurons in the fruit fly's brain that control activity and pupal eclosion, leading to the questions by which neuronal pathways they are synchronized to the external light-dark cycle, and how they impose periodicity on behavior.

Neural organization of the circadian system of the cockroach Leucophaea maderae

The cockroach Leucophaea maderae was the first animal in which lesion experiments localized an endogenous circadian clock to a particular brain area, the optic lobe. The neural organization of the circadian system, however, including entrainment pathways, coupling elements of the bilaterally distributed internal clock, and output pathways controlling circadian locomotor rhythms are only recently beginning to be elucidated. As in flies and other insect species, pigment-dispersing hormone (PDH)-immunoreactive neurons of the accessory medulla of the cockroach are crucial elements of the circadian system. Lesions and transplantation experiments showed that the endogeneous circadian clock of the brain resides in neurons associated with the accessory medulla. The accessory medulla is organized into a nodular core receiving photic input, and into internodular and peripheral neuropil involved in efferent output and coupling input. Photic entrainment of the clock through compound eye photoreceptors appears to occur via parallel, indirect pathways through the medulla. Light-like phase shifts in circadian locomotor activity after injections of gamma-aminobutyric acid (GABA)- or Mas-allatotropin into the vicinity of the accessory medulla suggest that both substances are involved in photic entrainment Extraocular, cryptochrome-based photoreceptors appear to be present in the optic lobe, but their role in photic entrainment has not been examined. Pigment-dispersing hormone-immunoreactive neurons provide efferent output from the accessory medulla to several brain areas and to the peripheral visual system. Pigment-dispersing hormone-immunoreactive neurons, and additional heterolateral neurons are, furthermore, involved in bilateral coupling of the two pacemakers. The neuronal organization, as well as the prominent involvement of GABA and neuropeptides, shows striking similarities to the organization of the suprachiasmatic nucleus, the circadian clock of the mammalian brain.

Targeted ablation of CCAP neuropeptide-containing neurons of Drosophila causes specific defects in execution and circadian timing of ecdysis behavior

Insect growth and metamorphosis is punctuated by molts, during which a new cuticle is produced. Every molt culminates in ecdysis, the shedding of the remains of the old cuticle. Both the timing of ecdysis relative to the molt and the actual execution of this vital insect behavior are under peptidergic neuronal control. Based on studies in the moth, Manduca sexta, it has been postulated that the neuropeptide Crustacean cardioactive peptide (CCAP) plays a key role in the initiation of the ecdysis motor program. We have used Drosophila bearing targeted ablations of CCAP neurons (CCAP KO animals) to investigate the role of CCAP in the execution and circadian regulation of ecdysis. CCAP KO animals showed specific defects at ecdysis, yet the severity and nature of the defects varied at different developmental stages. The majority of CCAP KO animals died at the pupal stage from the failure of pupal ecdysis, whereas larval ecdysis and adult eclosion behaviors showed only subtle defects. Interestingly, the most severe failure seen at eclosion appeared to be in a function required for abdominal inflation, which could be cardioactive in nature. Although CCAP KO populations exhibited circadian eclosion rhythms, the daily distribution of eclosion events (i.e., gating) was abnormal. Effects on the execution of ecdysis and its circadian regulation indicate that CCAP is a key regulator of the behavior. Nevertheless, an unexpected finding of this work is that the primary functions of CCAP as well as its importance in the control of ecdysis behaviors may change during the postembryonic development of 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).

Invited review: Sleeping flies don't lie: the use of Drosophila melanogaster to study sleep and circadian rhythms

During the past century, flies thoroughly proved their value as an animal model for the study of the genetics of development and basic cell processes. During the past three decades, they have also been extensively used to study the genetics of behavior. For both circadian rhythms and for sleep, flies are helping us to understand the genetic mechanisms that underlie these complex behaviors. Since 1971, discoveries in the fly have led the way to a number of significant discoveries, establishing a mechanistic framework that is now known to be conserved in the mammalian clock. The highlights of this history are described. For sleep, the use of the fly as a model is relatively new, that is, only within the past 2 yr. Nonetheless, studies have already established that two transcription factors alter rest and rest homeostasis. The implications of these advances for the future of sleep research are summarized.

Circadian control of eclosion: interaction between a central and peripheral clock in Drosophila melanogaster

Drosophila melanogaster display overt circadian rhythms in rest:activity behavior and eclosion. These rhythms have an endogenous period of approximately 24 hr and can adjust or "entrain" to environmental inputs such as light. Circadian rhythms depend upon a functioning molecular clock that includes the core clock genes period and timeless (reviewed in and ). Although we know that a clock in the lateral neurons (LNs) of the brain controls rest:activity rhythms, the cellular basis of eclosion rhythms is less well understood. We show that the LN clock is insufficient to drive eclosion rhythms. We establish that the prothoracic gland (PG), a tissue required for fly development, contains a functional clock at the time of eclosion. This clock is required for normal eclosion rhythms. However, both the PG clock function and eclosion rhythms require the presence of LNs. In addition, we demonstrate that pigment-dispersing factor (PDF), a neuropeptide secreted from LNs, is necessary for the PG clock and eclosion rhythms. Unlike other clocks in the fly periphery, the PG is similar to mammalian peripheral oscillators because it depends upon input, including PDF, from central pacemaker cells. This is the first report of a peripheral clock necessary for a circadian event.

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.

Downloading central clock information in Drosophila

Pigment-dispersing factor (PDF) neuropeptide is an important neurochemical that carries circadian timing information originating from the central oscillator in Drosophila. Several core-clock factors function as upstream pdf regulators; the dClock and cycle genes control pdf transcription, whereas the period and timeless genes regulate post-translational processes of PDF via unknown mechanisms. For a downstream neural path, PDF most likely acts as a local modulator, which binds to its receptors that are possibly linked to Ras/MAPK signaling pathways. PDF receptor-containing cells seem to localize in the vicinity of nerve terminals from pace-making neurons. Although PDF is likely to be a principal clock-output factor, our recent evidence predicts the presence of other neuropeptides with rhythm-relevant functions. Furthermore, recent microarray screens have identified numerous potential clock-controlled genes, suggesting that diverse physiological processes might be affected by the biological clock system.

Neuropeptides in the nervous system of Drosophila and other insects: multiple roles as neuromodulators and neurohormones

Neuropeptides in insects act as neuromodulators in the central and peripheral nervous system and as regulatory hormones released into the circulation. The functional roles of insect neuropeptides encompass regulation of homeostasis, organization of behaviors, initiation and coordination of developmental processes and modulation of neuronal and muscular activity. With the completion of the sequencing of the Drosophila genome we have obtained a fairly good estimate of the total number of genes encoding neuropeptide precursors and thus the total number of neuropeptides in an insect. At present there are 23 identified genes that encode predicted neuropeptides and an additional seven encoding insulin-like peptides in Drosophila. Since the number of G-protein-coupled neuropeptide receptors in Drosophila is estimated to be around 40, the total number of neuropeptide genes in this insect will probably not exceed three dozen. The neuropeptides can be grouped into families, and it is suggested here that related peptides encoded on a Drosophila gene constitute a family and that peptides from related genes (orthologs) in other species belong to the same family. Some peptides are encoded as multiple related isoforms on a precursor and it is possible that many of these isoforms are functionally redundant. The distribution and possible functions of members of the 23 neuropeptide families and the insulin-like peptides are discussed. It is clear that each of the distinct neuropeptides are present in specific small sets of neurons and/or neurosecretory cells and in some cases in cells of the intestine or certain peripheral sites. The distribution patterns vary extensively between types of neuropeptides. Another feature emerging for many insect neuropeptides is that they appear to be multifunctional. One and the same peptide may act both in the CNS and as a circulating hormone and play different functional roles at different central and peripheral targets. A neuropeptide can, for instance, act as a coreleased signal that modulates the action of a classical transmitter and the peptide action depends on the cotransmitter and the specific circuit where it is released. Some peptides, however, may work as molecular switches and trigger specific global responses at a given time. Drosophila, in spite of its small size, is now emerging as a very favorable organism for the studies of neuropeptide function due to the arsenal of molecular genetics methods available.

Integration of endocrine signals that regulate insect ecdysis

The extremely large number of insects and members of allied groups alive today suggests that molting--shedding of an old cuticle--may be one of the most commonly performed behaviors on our planet. Removal of an old cuticle in insects is associated with stereotyped, species-specific patterns of behavior referred to as ecdysis. It has been recognized for decades that the initiation of ecdysis is under hormonal control, but until recently many of the key peptides that regulate ecdysis were unknown. The report in 1996 of a new ecdysis-triggering hormone (ETH) sparked an era of significant advances in our understanding of the regulation of molting. This article summarizes the current model of peptide regulation of ecdysis, a model that is based on a positive feedback loop between ETH and a brain peptide, eclosion hormone. Then the relationship of these regulatory peptides to the neural circuitry that is the ultimate driver of the behavior are described. Because insects can undergo both status quo (larval-larval) and metamorphic (larval-pupal and pupal-adult) molts, differences in ecdysis behavior at different life stages are described and potential sources of these differences are identified. Most of the work described is based on studies of ecdysis in the hawkmoth, Manduca sexta, but results from studies of ecdysis in the fruit fly Drosophila melanogaster are also discussed.

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.

Genetic analysis of functional domains within the Drosophila LARK RNA-binding protein

LARK is an essential Drosophila RNA-binding protein of the RNA recognition motif (RRM) class that functions during embryonic development and for the circadian regulation of adult eclosion. LARK protein contains three consensus RNA-binding domains: two RRM domains and a retroviral-type zinc finger (RTZF). To show that these three structural domains are required for function, we performed a site-directed mutagenesis of the protein. The analysis of various mutations, in vivo, indicates that the RRM domains and the RTZF are required for wild-type LARK functions. RRM1 and RRM2 are essential for viability, although interestingly either domain can suffice for this function. Remarkably, mutation of either RRM2 or the RTZF results in the same spectrum of phenotypes: mutants exhibit reduced viability, abnormal wing and mechanosensory bristle morphology, female sterility, and flightlessness. The severity of these phenotypes is similar in single mutants and double RRM2; RTZF mutants, indicating a lack of additivity for the mutations and suggesting that RRM2 and the RTZF act together, in vivo, to determine LARK function. Finally, we show that mutations in RRM1, RRM2, or the RTZF do not affect the circadian regulation of eclosion, and we discuss possible interpretations of these results. This genetic analysis demonstrates that each of the LARK structural domains functions in vivo and indicates a pleiotropic requirement for both the LARK RRM2 and RTZF domains.

Insect photoperiodism and circadian clocks: models and mechanisms

Photoperiodic clocks allow organisms to predict the coming season. In insects, the seasonal adaptive response mainly takes the form of diapause. The extensively studied photoperiodic clock in insects was primarily characterized by a "black-box" approach, resulting in numerous cybernetic models. This is in contrast with the circadian clock, which has been dissected pragmatically at the molecular level, particularly in Drosophila. Unfortunately, Drosophila melanogaster, the favorite model organism for circadian studies, does not demonstrate a pronounced seasonal response, and consequently molecular analysis has not progressed in this area. In the current article, the authors explore different ways in which identified molecular components of the circadian pacemaker may play a role in photoperiodism. Future progress in understanding the Drosophila circadian pacemaker, particularly as further output components are identified, may provide a direct link between the clock and photoperiodism. In addition, with improved molecular tools, it is now possible to turn to other insects that have a more dramatic photoperiodic response.