A Circadian Rhythm of Visual Sensitivity in the American Lobster, Homarus americanus

AbstractTo determine whether eyes of American lobsters (Homarus americanus) are more sensitive to light at night than during the day, electroretinograms were continuously recorded from 23 adult lobsters for at least 3 days (range: 3 to 9 days) in constant darkness. A green light-emitting diode, mounted 10 cm away from the eyes, was briefly flashed every 2 minutes to evoke the electroretinogram. The average increase in the response to a light flash, between the minimum during the subjective day and the maximum during the subjective night, was 105.6% ± 38.8%; and there was a statistically significant difference between day and night responses. This change in visual sensitivity took place while lobsters were held in constant darkness, suggesting that it was due to the influence of a circadian clock. The average period (tau) for the 10 animals that expressed significant circadian rhythms was 23.4 ± 0.8 hours. Previous studies have demonstrated that lobsters have circadian clocks that influence their locomotor activity; and the present data suggest that this is also true for their eyes, leading to an increase in their visual sensitivity at night, when they are typically most active.

Comparative transcriptome analysis reveals the impact of the daily rhythm on the hemolymph of the Chinese mitten crab (Eriocheir sinensis)

The daily rhythm affects a series of physiological functions in crustaceans. To study its effect on the physiological function in Eriocheir sinensis, a crustacean species of high economic value, we analyzed the hemolymph transcriptome during the daily rhythm by high-throughput sequencing. We sampled the hemolymph from crabs at four time points in a single day (06:00, 12:00, 18:00, and 24:00 h) and identified 3,01,661 and 1,03,998 transcripts and unigenes, respectively; some of the unigenes were annotated as core clock genes. Moreover, 15,564 differentially expressed genes (DEGs) were divided into nine different clusters. Functional enrichment analysis of DEGs indicated that the molting, metabolism, and immunity processes in E. sinensis were impacted by its daily rhythm. In addition, we mapped the DEGs involved in the daily entrainment pathway. To the best of our knowledge, this is the first comparative transcriptome analysis of crustacean hemolymph during the day-night cycle, and provides multi-level information for unraveling the finer regulatory effects of the daily cycle in crustaceans.

Daily rhythms in gene expression of the human parasite Schistosoma mansoni

Background

The consequences of the earth’s daily rotation have led to 24-h biological rhythms in most organisms. Even some parasites are known to have daily rhythms, which, when in synchrony with host rhythms, can optimise their fitness. Understanding these rhythms may enable the development of control strategies that take advantage of rhythmic vulnerabilities. Recent work on protozoan parasites has revealed 24-h rhythms in gene expression, drug sensitivity and the presence of an intrinsic circadian clock; however, similar studies on metazoan parasites are lacking. To address this, we investigated if a metazoan parasite has daily molecular oscillations, whether they reveal how these longer-lived organisms can survive host daily cycles over a lifespan of many years and if animal circadian clock genes are present and rhythmic. We addressed these questions using the human blood fluke Schistosoma mansoni that lives in the vasculature for decades and causes the tropical disease schistosomiasis.

Results

Using round-the-clock transcriptomics of male and female adult worms collected from experimentally infected mice, we discovered that ~ 2% of its genes followed a daily pattern of expression. Rhythmic processes included a stress response during the host’s active phase and a ‘peak in metabolic activity’ during the host’s resting phase. Transcriptional profiles in the female reproductive system were mirrored by daily patterns in egg laying (eggs are the main drivers of the host pathology). Genes cycling with the highest amplitudes include predicted drug targets and a vaccine candidate. These 24-h rhythms may be driven by host rhythms and/or generated by a circadian clock; however, orthologs of core clock genes are missing and secondary clock genes show no 24-h rhythmicity.

Conclusions

There are daily rhythms in the transcriptomes of adult S. mansoni, but they appear less pronounced than in other organisms. The rhythms reveal temporally compartmentalised internal processes and host interactions relevant to within-host survival and between-host transmission. Our findings suggest that if these daily rhythms are generated by an intrinsic circadian clock then the oscillatory mechanism must be distinct from that in other animals. We have shown which transcripts oscillate at this temporal scale and this will benefit the development and delivery of treatments against schistosomiasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01189-9.

Circadian regulation of diel vertical migration (DVM) and metabolism in Antarctic krill Euphausia superba

Antarctic krill (Euphausia superba) are high latitude pelagic organisms which play a key ecological role in the ecosystem of the Southern Ocean. To synchronize their daily and seasonal life-traits with their highly rhythmic environment, krill rely on the implementation of rhythmic strategies which might be regulated by a circadian clock. A recent analysis of krill circadian transcriptome revealed that their clock might be characterized by an endogenous free-running period of about 12-15 h. Using krill exposed to simulated light/dark cycles (LD) and constant darkness (DD), we investigated the circadian regulation of krill diel vertical migration (DVM) and oxygen consumption, together with daily patterns of clock gene expression in brain and eyestalk tissue. In LD, we found clear 24 h rhythms of DVM and oxygen consumption, suggesting a synchronization with photoperiod. In DD, the DVM rhythm shifted to a 12 h period, while the peak of oxygen consumption displayed a temporal advance during the subjective light phase. This suggested that in free-running conditions the periodicity of these clock-regulated output functions might reflect the shortening of the endogenous period observed at the transcriptional level. Moreover, differences in the expression patterns of clock gene in brain and eyestalk, in LD and DD, suggested the presence in krill of a multiple oscillator system. Evidence of short periodicities in krill behavior and physiology further supports the hypothesis that a short endogenous period might represent a circadian adaption to cope with extreme seasonal photoperiodic variability at high latitude.

Analysis of the circadian transcriptome of the Antarctic krill Euphausia superba

Antarctic krill (Euphausia superba) is a high latitude pelagic organism which plays a central role in the Southern Ocean ecosystem. E. superba shows daily and seasonal rhythms in physiology and behaviour, which are synchronized with the environmental cycles of its habitat. Recently, the main components of the krill circadian machinery have been identified and characterized. However, the exact mechanisms through which the endogenous timing system operates the control and regulation of the overt rhythms remains only partially understood. Here we investigate the involvement of the circadian clock in the temporal orchestration of gene expression by using a newly developed version of a krill microarray platform. The analysis of transcriptome data from krill exposed to both light-dark cycles (LD 18:6) and constant darkness (DD), has led to the identification of 1,564 putative clock-controlled genes. A remarkably large proportion of such genes, including several clock components (clock, period, cry2, vrille, and slimb), show oscillatory expression patterns in DD, with a periodicity shorter than 24 hours. Energy-storage pathways appear to be regulated by the endogenous clock in accordance with their ecological relevance in daily energy managing and overwintering. Our results provide the first representation of the krill circadian transcriptome under laboratory, free-running conditions.

Involvement of Melatonin in the Regulation of the Circadian System in Crayfish

Melatonin (MEL) is an ancient molecule, broadly distributed in nature from unicellular to multicellular species. MEL is an indoleamine that acts on a wide variety of cellular targets regulating different physiological functions. This review is focused on the role played by this molecule in the regulation of the circadian rhythms in crayfish. In these species, information about internal and external time progression might be transmitted by the periodical release of MEL and other endocrine signals acting through the pacemaker. We describe documented and original evidence in support of this hypothesis that also suggests that the rhythmic release of MEL contributes to the reinforcement of the temporal organization of nocturnal or diurnal circadian oscillators. Finally, we discuss how MEL might coordinate functions that converge in the performance of complex behaviors, such as the agonistic responses to establish social dominance status in Procambarus clarkii and the burrowing behavior in the secondary digging crayfish P. acanthophorus.

Functional characterization of the circadian clock in the Antarctic krill, Euphausia superba

Antarctic krill (Euphausia superba) is a key species in Southern Ocean ecosystem where it plays a central role in the Antarctic food web. Available information supports the existence of an endogenous timing system in krill enabling it to synchronize metabolism and behavior with an environment characterized by extreme seasonal changes in terms of day length, food availability, and surface ice extent. A screening of our transcriptome database “KrillDB” allowed us to identify the putative orthologues of 20 circadian clock components. Mapping of conserved domains and phylogenetic analyses strongly supported annotations of the identified sequences. Luciferase assays and co-immunoprecipitation experiments allowed us to define the role of the main clock components. Our findings provide an overall picture of the molecular mechanisms underlying the functioning of the endogenous circadian clock in the Antarctic krill and shed light on their evolution throughout crustaceans speciation. Interestingly, the core clock machinery shows both mammalian and insect features that presumably contribute to an evolutionary strategy to cope with polar environment’s challenges. Moreover, despite the extreme variability characterizing the Antarctic seasonal day length, the conserved light mediated degradation of the photoreceptor EsCRY1 suggests a persisting pivotal role of light as a Zeitgeber.

Crustacean hyperglycemic hormone is synthesized in the eyestalk and brain of the crayfish Procambarus clarkii

Crustacean hyperglycemic hormone (CHH) is a neuropeptide that is synthesized, stored, and released by brain and eyestalk structures in decapods. CHH participates in the regulation of several mechanisms, including increasing the level of glucose in hemolymph. Although CHH mRNA levels have been quantified and the CHH protein has been localized in various structures of the crayfish P. clarkii, CHH synthesis has only been reported in the X-organ-sinus gland (XO-SG). Therefore, the aim of this study was to use in situ hybridization to determine whether CHH mRNA is located in other structures, including the putative pacemaker, eyestalk and brain, of crayfish P. clarkii at two times of day. CHH mRNA was observed in both the eyestalk and the brain of P. clarkii, indicating that CHH is synthesized in several structures in common with other crustaceans, possibly to provide metabolic support for these regions by increasing glucose levels.

Desensitization and recovery of crayfish photoreceptors. Dependency on circadian time, and pigment-dispersing hormone

In this work, we studied the characteristics of recovery from desensitization of the light-elicited current of crayfish. Applying a two-flash protocol, we found that the first flash triggers a current that activates with a noticeable latency, reaches a peak value, and thereafter decays along a single exponential time course. In comparison with the first-elicited current, the current elicited by the second flash not only presents an expected smaller peak current, depending on the time between flashes, but it also displays a different latency and decay time constant. Recovery of the first flash values of these current parameters depends on the circadian time at which the experiments are conducted, and on the presence of pigment-dispersing hormone. Our data also suggest the existence of distinctive desensitized states, whose induction depends on circadian time and the presence of pigment-dispersing hormone.

Characterization, localization and temporal expression of crustacean hyperglycemic hormone (CHH) in the behaviorally rhythmic peracarid crustaceans, Eurydice pulchra (Leach) and Talitrus saltator (Montagu)

Crustacean hyperglycemic hormone (CHH) has been extensively studied in decapod crustaceans where it is known to exert pleiotropic effects, including regulation of blood glucose levels. Hyperglycemia in decapods seems to be temporally gated to coincide with periods of activity, under circadian clock control. Here, we used gene cloning, in situ hybridization and immunohistochemistry to describe the characterization and localization of CHH in two peracarid crustaceans, Eurydice pulchra and Talitrus saltator. We also exploited the robust behavioral rhythmicity of these species to test the hypothesis that CHH mRNA expression would resonate with their circatidal (12.4h) and circadian (24h) behavioral phenotypes. We show that both species express a single CHH transcript in the cerebral ganglia, encoding peptides featuring all expected, conserved characteristics of other CHHs. E. pulchra preproCHH is an amidated 73 amino acid peptide N-terminally flanked by a short, 18 amino acid precursor related peptide (CPRP) whilst the T. saltator prohormone is also amidated but 72 amino acids in length and has a 56 residue CPRP. The localization of both was mapped by immunohistochemistry to the protocerebrum with axon tracts leading to the sinus gland and into the tritocerebrum, with striking similarities to terrestrial isopod species. We substantiated the cellular position of CHH immunoreactive cells by in situ hybridization. Although both species showed robust activity rhythms, neither exhibited rhythmic transcriptional activity indicating that CHH transcription is not likely to be under clock control. These data make a contribution to the inventory of CHHs that is currently lacking for non-decapod species.

Genome-wide profiling of 24 hr diel rhythmicity in the water flea, Daphnia pulex: network analysis reveals rhythmic gene expression and enhances functional gene annotation

Background

Marine and freshwater zooplankton exhibit daily rhythmic patterns of behavior and physiology which may be regulated directly by the light:dark (LD) cycle and/or a molecular circadian clock. One of the best-studied zooplankton taxa, the freshwater crustacean Daphnia, has a 24 h diel vertical migration (DVM) behavior whereby the organism travels up and down through the water column daily. DVM plays a critical role in resource tracking and the behavioral avoidance of predators and damaging ultraviolet radiation. However, there is little information at the transcriptional level linking the expression patterns of genes to the rhythmic physiology/behavior of Daphnia.

Results

Here we analyzed genome-wide temporal transcriptional patterns from Daphnia pulex collected over a 44 h time period under a 12:12 LD cycle (diel) conditions using a cosine-fitting algorithm. We used a comprehensive network modeling and analysis approach to identify novel co-regulated rhythmic genes that have similar network topological properties and functional annotations as rhythmic genes identified by the cosine-fitting analyses. Furthermore, we used the network approach to predict with high accuracy novel gene-function associations, thus enhancing current functional annotations available for genes in this ecologically relevant model species. Our results reveal that genes in many functional groupings exhibit 24 h rhythms in their expression patterns under diel conditions. We highlight the rhythmic expression of immunity, oxidative detoxification, and sensory process genes. We discuss differences in the chronobiology of D. pulex from other well-characterized terrestrial arthropods.

Conclusions

This research adds to a growing body of literature suggesting the genetic mechanisms governing rhythmicity in crustaceans may be divergent from other arthropod lineages including insects. Lastly, these results highlight the power of using a network analysis approach to identify differential gene expression and provide novel functional annotation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2998-2) contains supplementary material, which is available to authorized users.

Circadian Control of Antibacterial Immunity: Findings from Animal Models

Most of the biological functions, including the immune system, are linked to circadian rhythms in living organisms. Changes occurring to biological parameters as the result of these circadian rhythms can therefore affect the outcome of a disease. For decades, model organisms have proven to be a great tool to understanding biological mechanisms such as circadian cycle and immunity. In this review, we created an inventory of the use of model organisms in order to decipher the relation between circadian rhythms and antibacterial immunity.

Identification, Characterization, and Diel Pattern of Expression of Canonical Clock Genes in Nephrops norvegicus (Crustacea: Decapoda) Eyestalk

The Norway lobster, Nephrops norvegicus, is a burrowing decapod with a rhythmic burrow emergence (24 h) governed by the circadian system. It is an important resource for European fisheries and its behavior deeply affects its availability. The current knowledge of Nephrops circadian biology is phenomenological as it is currently the case for almost all crustaceans. In attempt to elucidate the putative molecular mechanisms underlying circadian gene regulation in Nephrops, we used a transcriptomics approach on cDNA extracted from the eyestalk, a structure playing a crucial role in controlling behavior of decapods. We studied 14 male lobsters under 12–12 light-darkness blue light cycle. We used the Hiseq 2000 Illumina platform to sequence two eyestalk libraries (under light and darkness conditions) obtaining about 90 millions 100-bp paired-end reads. Trinity was used for the de novo reconstruction of transcriptomes; the size at which half of all assembled bases reside in contigs (N50) was equal to 1796 (light) and 2055 (darkness). We found a list of candidate clock genes and focused our attention on canonical ones: timeless, period, clock and bmal1. The cloning of assembled fragments validated Trinity outputs. The putative Nephrops clock genes showed high levels of identity (blastx on NCBI) with known crustacean clock gene homologs such as Eurydice pulchra (period: 47%, timeless: 59%, bmal1: 79%) and Macrobrachium rosenbergii (clock: 100%). We also found a vertebrate-like cryptochrome 2. RT-qPCR showed that only timeless had a robust diel pattern of expression. Our data are in accordance with the current knowledge of the crustacean circadian clock, reinforcing the idea that the molecular clockwork of this group shows some differences with the established model in Drosophila melanogaster.

Putative pacemakers in the eyestalk and brain of the crayfish Procambarus clarkii show circadian oscillations in levels of mRNA for crustacean hyperglycemic hormone

Crustacean hyperglycemic hormone (CHH) synthesizing cells in the optic lobe, one of the pacemakers of the circadian system, have been shown to be present in crayfish. However, the presence of CHH in the central brain, another putative pacemaker of the multi-oscillatory circadian system, of this decapod and its circadian transcription in the optic lobe and brain have yet to be explored. Therefore, using qualitative and quantitative PCR, we isolated and cloned a CHH mRNA fragment from two putative pacemakers of the multi-oscillatory circadian system of Procambarus clarkii, the optic lobe and the central brain. This CHH transcript synchronized to daily light-dark cycles and oscillated under dark, constant conditions demonstrating statistically significant daily and circadian rhythms in both structures. Furthermore, to investigate the presence of the peptide in the central brain of this decapod, we used immunohistochemical methods. Confocal microscopy revealed the presence of CHH-IR in fibers and cells of the protocerebral and tritocerebal clusters and neuropiles, particularly in some neurons located in clusters 6, 14, 15 and 17. The presence of CHH positive neurons in structures of P. clarkii where clock proteins have been reported suggests a relationship between the circadian clockwork and CHH. This work provides new insights into the circadian regulation of CHH, a pleiotropic hormone that regulates many physiological processes such as glucose metabolism and osmoregulatory responses to stress.

Astakine 2--the dark knight linking melatonin to circadian regulation in crustaceans

Daily, circadian rhythms influence essentially all living organisms and affect many physiological processes from sleep and nutrition to immunity. This ability to respond to environmental daily rhythms has been conserved along evolution, and it is found among species from bacteria to mammals. The hematopoietic process of the crayfish Pacifastacus leniusculus is under circadian control and is tightly regulated by astakines, a new family of cytokines sharing a prokineticin (PROK) domain. The expression of AST1 and AST2 are light-dependent, and this suggests an evolutionarily conserved function for PROK domain proteins in mediating circadian rhythms. Vertebrate PROKs are transmitters of circadian rhythms of the suprachiasmatic nucleus (SCN) in the brain of mammals, but the mechanism by which they function is unknown. Here we demonstrate that high AST2 expression is induced by melatonin in the brain. We identify RACK1 as a binding protein of AST2 and further provide evidence that a complex between AST2 and RACK1 functions as a negative-feedback regulator of the circadian clock. By DNA mobility shift assay, we showed that the AST2-RACK1 complex will interfere with the binding between BMAL1 and CLK and inhibit the E-box binding activity of the complex BMAL1-CLK. Finally, we demonstrate by gene knockdown that AST2 is necessary for melatonin-induced inhibition of the complex formation between BMAL1 and CLK during the dark period. In summary, we provide evidence that melatonin regulates AST2 expression and thereby affects the core clock of the crustacean brain. This process may be very important in all animals that have AST2 molecules, i.e. spiders, ticks, crustaceans, scorpions, several insect groups such as Hymenoptera, Hemiptera, and Blattodea, but not Diptera and Coleoptera. Our findings further reveal an ancient evolutionary role for the prokineticin superfamily protein that links melatonin to direct regulation of the core clock gene feedback loops.

Most living organisms are able to sense the time and in particular time of day by their internal clocks. So-called clock proteins control these internal clockworks. BMAL1 and CLK are two important clock proteins, which together form a complex that serves as a transcription factor and controls the production of diurnal proteins. These diurnal proteins, in turn, inhibit the formation of clock proteins so that the concentration of the different proteins in the cell oscillates back and forth throughout the day. External factors may affect the balance of clock proteins, and one such important factor is light. Melatonin is a darkness hormone produced in the brain of most animals during the night, and here we show that melatonin controls the formation of a protein named AST2 in crayfish. AST2 belongs to a group of proteins found in many arthropods, such as spiders, scorpions, crustaceans, and some insects, whose function has been unknown until now. Now we demonstrate that AST2 is induced by melatonin at night and then functions in the internal biological clock by preventing BMAL1 and CLK to form a complex. In this way, AST2 acts as a link between melatonin and the internal biological clock.

Daily Rhythms of PERIOD protein in the eyestalk of the American lobster, Homarus americanus

The daily rhythm of PERIOD protein (PER) expression is an integral component of the circadian clock, which is found among a broad range of animal species including fruit flies, marine mollusks and even humans. The use of antibodies directed against PER has provided a helpful tool in the discovery of PER homologues and the labeling of putative pacemaker cells, especially in animals for which an annotated genome is not readily available. In this study, DrosophilaPER antibodies were used to probe for PER in the American lobster, Homarus americanus. This species exhibits robust endogenous circadian rhythms but the circadian clock has yet to be located or characterized. PER was detected in the eyestalks of the lobster but not in the brain. Furthermore, a significant effect of the LD cycle on daily PER abundance was identified, and PER was significantly more abundant at mid dark than in early light or mid light hours. Our results suggest that PER is a part of the molecular machinery of the circadian clock located in the eyestalk of the lobster.