Crustacean molt-inhibiting hormone: Structure, function, and cellular mode of action

A brief and updated introduction to the neuroendocrine system of crustaceans

The neuroendocrine system of crustaceans is complex and regulates many processes, such as development, growth, reproduction, osmoregulation, behavior, and metabolism. Once stimulated, crustaceans' neuroendocrine tissues modulate the release of monoamines, ecdysteroids, and neuropeptides that can act as hormones or neurotransmitters. Over a few decades, research has unraveled some mechanisms governing these processes, substantially contributing to understanding crustacean physiology. More aspects of crustacean neuroendocrinology are being comprehended with molecular biology, transcriptome, and genomics analyses. Hence, these studies will also significantly enhance the ability to cultivate decapods, such as crabs and shrimps, used as human food sources. In this review, current knowledge on crustacean endocrinology is updated with new findings about crustacean hormones, focusing mainly on the main neuroendocrine organs and their hormones and the effects of these molecules regulating metabolism, growth, reproduction, and color adaptation. New evidence about vertebrate-type hormones found in crustaceans is included and discussed. Finally, this review may assist in understanding how the emerging chemicals of environmental concern can potentially impair and disrupt crustacean's endocrine functions and their physiology.

Effects of Different Sources of Culture Substrate on the Growth and Immune Performance of the Red Swamp Crayfish (Procambarus clarkii)

The substrate in the aquatic environment plays a crucial role in nutrient deposition and recovery for the growth of aquatic organisms. In order to optimize the culture medium of Procambarus Clarkii, culture media from different sources were selected in this study to explore their effects on the growth and immune performance of red swamp crayfish. The results showed that the weight gain rate (WGR), body length growth rate (BLGR) and specific growth rate (SGR) in group I2 were the highest, followed by group I1 and group I3. The WGR and SGR of crayfish in the I1 and I2 groups were significantly higher than those in the I3 group (p < 0.05). The activities of acid phosphatase (ACP), alkaline phosphatase (AKP) and superoxide dismutase (SOD) were the highest in group I2, followed by group I3, and the lowest in group I1. The expression trends in growth-related genes, nuclear hormone receptor (E75), molt-inhibiting hormone (MIH) and chitinase genes were similar, and the expression levels in the I2 group were higher than those in the I1 and I3 groups. It was noted that the expression levels of E75 and MIH genes in the I2 group were significantly higher than those in the I3 group (p < 0.05). α diversity analysis of 16S rRNA data showed that there was no statistically significant difference in the abundance of intestinal flora among the three culture substrate groups. The β diversity in the Xitangni group, crayfish Tangni group and Shuitangni group was significantly different. These changes in microbiota suggest that using different substrates to culture crayfish leads to differences in gut microbiota diversity. To sum up, the growth in crayfish and immune performance influenced by the culture substrate condition and aquatic breeding sediment substrates, rather than crab pool and paddy field pond sediment substrates, showed a better effect.

Sex-specific effects of propiconazole on the molting of the Chinese mitten crab (Eriocheir sinensis)

Given the inevitable exposure of Eriocheir sinensis (E. sinensis) to fungicides in rice-crab co-culture systems, understanding the potential effect of fungisides is important for practical application. Molting is a crucial development process of E. sinensis, which is regulated by endocrine system and genetic factors, and is susceptible to exogenous chemicals. However, the impact of fungicides application on the molting of E. sinensis have been rarely reported. In the present study, propiconazole, a widely used fungicide for rice disease management, was found to exert potential effects on the molting of E. sinensis at residual-related level in the rice-crab co-culture fields. After 14 days of short-term exposure to propiconazole, female crabs exhibited remarkably higher levels of hemolymph ecdysone than males. When the exposure was extended to 28 days, propiconazole markedly accelerated molt-inhibiting hormone expression by 3.3-fold, ecdysone receptor expression by 7.8-fold, and crustacean retinoid X receptor expression by 9.6-fold in male crabs, while it showed the opposite effect in females with suppressed gene expression. Propiconazole also induced the activity of N-acetylglucosaminidase in male crabs rather than females during the experiments. Our study suggests that propiconazole exerts sex-specific effects on the molting of E. sinensis. The impact of propiconazole application in the rice-crab co-culture systems remains more assessment to avoid affecting the growth of cultured E. sinensis.

Transcriptional and metabolomic investigation of the stress response in snow crab during simulated transport condition (Chionoecetes opilio)

The molecular mechanisms underlying the stress response are poorly described in crustaceans. This includes the snow crab (Chionoecetes opilio), a commercially important stenotherm species distributed throughout the northern hemisphere. A better understanding of the stress response in C. opilio is desperately needed for commercial and conservation purposes. The purpose of this study was to investigate the transcriptional and metabolomic response of C. opilio exposed to stressors. Crabs were randomly assigned to 24 or 72 h treatment groups where they were exposed to conditions simulating live transport (handling and air exposure). A control group was kept in cold (2 °C) and well‑oxygenated saltwater. The hepatopancreas of the crabs was sampled to perform RNA-sequencing and high-performance chemical isotope labeling metabolomics. Differential gene expression analyses showed that classic crustaceans' stress markers, such as crustacean hyperglycemic hormones and heat shock proteins, were overexpressed in response to stressors. Tyrosine decarboxylase was also up-regulated in stressed crabs, suggesting an implication of the catecholamines tyramine and octopamine in the stress response. Deregulated metabolites revealed that low oxygen was an important trigger in the stress response as intermediate metabolites of the tricarboxylic acid cycle (TCA) accumulated. Lactate, which accumulated unevenly between crabs could potentially be used to predict mortality. This study provides new information on how stressors affect crustaceans and provides a basis for the development of stress markers in C. opilio.

CasEcR and CasMIH Genes in the Blue Crab, Callinectes sapidus: A Temporal Evaluation and Melatonin Effects

Environmental cues synchronize endogenous rhythms of many physiological processes such as hormone synthesis and secretion. Little is known about the diurnal pattern of hormones and gene expression of the Callinectes sapidus molt cycle. We aimed to investigate in the eyestalk and hepatopancreas of premolt and intermolt C. sapidus the following parameters: 1) the diurnal expression of the ecdysteroid receptor CasEcR isoforms, and the molt inhibiting hormone CasMIH; 2) the diurnal hemolymph ecdysteroid and melatonin levels; and 3) melatonin effects on the transcripts of the above-mentioned genes in intermolt C. sapidus. Ecdysteroid levels were higher in the premolt than the intermolt animals at all time points evaluated (ZTs). Premolt crabs displayed a variation of ecdysteroid concentration between time points, with a reduction at ZT17. No difference in the melatonin level was seen in either molt stage or between stages. In the eyestalk of intermolt animals, CasEcR expression oscillated, with a peak at ZT9, and premolt crabs have a reduction at ZT9; CasMIH transcripts did not vary along 24 h in either molt stage. Moreover, the evaluated eyestalk genes were more expressed at ZT9 in the intermolt than the premolt crabs. In the hepatopancreas, CasEcR expression showed a peak at ZT9 in premolt crabs. Exogenous melatonin (10⁻⁷ mol/animal) reduced the expression of both genes in the eyestalk at ZT17. In the hepatopancreas, melatonin markedly increased the expression of the CasEcR gene at ZT9. Taken altogether, our results are pioneer in demonstrating the daily oscillation of gene expression associated to molt cycle stages, as well as the daily ecdysteroid and melatonin levels and the remarkable influence of melatonin on the molt cycle of C. sapidus.

A High-Density Genetic Map and QTL Fine Mapping for Growth- and Sex-Related Traits in Red Swamp Crayfish (Procambarus clarkii)

Red swamp crayfish (Procambarus clarkii) is a commercially important species in global aquaculture and most successfully invasive freshwater shrimp in China. In order to determine the genetic basis of growth- and sex-related traits, a high-density genetic linkage map was constructed using 2b-RAD sequencing technology in a full-sib family. The consensus map contains 4,878 SNP markers assigned to 94 linkage groups (LGs) and spanned 6,157.737 cM with an average marker interval of 1.26 cM and 96.93% genome coverage. The quantitative trait locus (QTL) mapping for growth and sex traits was performed for the first time. QTL mapping uncovers 28 QTLs for growth-related traits in nine LGs, explaining 7.9-14.4% of the phenotypic variation, and identifies some potential candidate growth-related genes such as mih, lamr, golgb1, nurf301, and tbcd1 within the QTL intervals. A single major locus for sex determination was revealed in LG20 that explains 59.3-63.7% of the phenotypic variations. Some candidate sex-related genes, such as vps4bl, ssrf, and acot1, were identified in the QTL intervals and found to be differentially expressed in the muscle tissues between the females and the males. Furthermore, the identified SNPs were revealed to be female heterozygotes, suggesting that red swamp crayfish might have the female heterogametic ZZ/ZW sex determination system. The present study provides a valuable resource for marker-assisted selection and genetic improvement and for further genetic and genomic research in red swamp crayfish.

Calcium signaling and regulation of ecdysteroidogenesis in crustacean Y-organs

Crustacean Y-organs secrete ecdysteroid molting hormones. Ecdysteroids are released in increased amount during premolt, circulate in hemolymph, and stimulate the events in target cells that lead to molting. During much of the molting cycle, ecdysteroid production is suppressed by molt-inhibiting hormone (MIH), a peptide neurohormone produced in the eyestalks. The suppressive effect of MIH is mediated by a cyclic nucleotide second messenger. A decrease in circulating MIH is associated with an increase in the hemolymphatic ecdysteroid titer during pre-molt. Nevertheless, it has long been hypothesized that a positive regulatory signal or stimulus is also involved in promoting ecdysteroidogenensis during premolt. Data reviewed here are consistent with the hypothesis that an intracellular Ca²⁺ signal provides that stimulus. Pharmacological agents that increase intracellular Ca²⁺ in Y-organs promote ecdysteroidogenesis, while agents that lower intracellular Ca²⁺ or disrupt Ca²⁺ signaling suppress ecdysteroidogenesis. Further, an increase in the hemolymphatic ecdysteroid titer after eyestalk ablation or during natural premolt is associated with an increase in intracellular free Ca²⁺ in Y-organ cells. Several lines of evidence suggest elevated intracellular calcium is linked to enhanced ecdysteroidogenesis through activation of Ca²⁺/calmodulin dependent cyclic nucleotide phosphodiesterase, thereby lowering intracellular cyclic nucleotide second messenger levels and promoting ecdysteroidogenesis. Results of transcriptomic studies show genes involved in Ca²⁺ signaling are well represented in Y-organs. Several recent studies have focused on Ca²⁺ transport proteins in Y-organs. Complementary DNAs encoding a plasma membrane Ca²⁺ ATPase (PMCA) and a sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA) have been cloned from crab Y-organs. The relative abundance of PMCA and SERCA transcripts in Y-organs is elevated during premolt, a time when Ca²⁺ levels in Y-organs are likewise elevated. The results are consistent with the notion that these transport proteins act to maintain the Ca²⁺ gradient across the cell membrane and re-set the cell for future Ca²⁺ signals.

Understanding molt control switches: Transcriptomic and expression analysis of the genes involved in ecdysteroidogenesis and cholesterol uptake pathways in the Y-organ of the blue crab, Callinectes sapidus

The crustacean molting process is regulated by an interplay of hormones produced by the eyestalk ganglia and Y-organs (YO). Molt-inhibiting hormone and crustacean hyperglycemic hormone released by the sinus gland of the eyestalk ganglia (EG) inhibit the synthesis and secretion of ecdysteroid by the YO, hence regulating hemolymph levels during the molt cycle. The purpose of this study is to investigate the ecdysteroidogenesis pathway, specifically genes linked to changes in ecdysteroid levels occurring at early premolt (ePM). To this end, a reference transcriptome based on YO, EG, and hepatopancreas was de novo assembled. Two genes (cholesterol 7-desaturase Neverland and cytochrome p450 307a1-like Spook) involved in ecdysteroidogenesis were identified from the YO transcriptome using sequence comparisons and transcript abundance. Two other candidates, Hormone receptor 4 and probable cytochrome p450 49a1 potentially involved in ecdysteroidogenesis were also identified. Since cholesterol is the ecdysteroid precursor, a putative cholesterol carrier (Apolipoprotein D-like) was also examined to understand if cholesterol uptake coincided with the increase in the ecdysteroid levels at the ePM stage. The expression level changes of the five candidate genes in the YO were compared between intermolt (IM) and induced ePM (iePM) stages using transcriptomic analysis. Expression analysis using qPCR were carried out at IM, iePM, and normal ePM. The increase in Spook and Neverland expression in the YO at the ePM was accompanied by a concomitant rise in ecdysteroid levels. The data obtained from iePM stage were congruent with those obtained from the normal ePM stage of intact control animals. The present findings support the role of Halloween genes in the ecdysteroidogenesis and molt cycle in the blue crab, Callinectes sapidus.

Biological insights into the rapid tissue regeneration of freshwater crayfish and crustaceans

The freshwater crayfish is capable of regenerating limbs, following autotomy, injury and predation. In arthropod species, regeneration and moulting are two processes linked and strongly regulated by ecdysone. The regeneration of crayfish limbs is divided into wound healing, blastema formation, cellular reprogramming and tissue patterning. Limb blastema cells undergo proliferation, dedifferentiation and redifferentiation. A limb bud, containing folded segments of the regenerating limb, is encased within a cuticular sheath. The functional limb regenerates, in proecdysis, in two to three consecutive moults. Rapid tissue growth is regulated by hormones, limb nerves and local cells. The TGF-β/activin signalling pathway has been determined in the crayfish, P. fallax f. virginalis, and is suggested as a potential regulator of tissue regeneration. In this review article, we discuss current understanding of tissue regeneration in the crayfish and various crustaceans. A thorough understanding of the cellular, genetic and molecular pathways of these biological processes is promising for the development of therapeutic applications for a wide array of diseases in regenerative medicine.

Targeted Top-Down Mass Spectrometry for the Characterization and Tissue-Specific Functional Discovery of Crustacean Hyperglycemic Hormones (CHH) and CHH Precursor-Related Peptides in Response to Low pH Stress

Crustacean hyperglycemic hormones (CHHs) are a family of neuropeptides that were discovered in multiple tissues in crustaceans, but the function of most isoforms remains unclear. Functional discovery often requires comprehensive qualitative profiling and quantitative analysis. The conventional enzymatic digestion method has several limitations, such as missing post-translational modification (PTM) information, homology interference, and incomplete sequence coverage. Herein, by using a targeted top-down method, facilitated by higher sensitivity instruments and hybrid fragmentation modes, we achieved the characterization of two CHH isoforms from the sinus glands (SG-CHH) and the pericardial organs (PO-CHH) from the Atlantic blue crab, Callinectes sapidus, with improved sequence coverage compared to earlier studies. In this study, both label-free and isotopic labeling approaches were adopted to monitor the response of CHHs and CHH precursor-related peptide (CPRP) under low pH stress. The identical trends of CPRP and CHH expression indicated that CPRP could serve as an ideal probe in tracking the CHH expression level changes, which would greatly simplify the quantitative analysis of large peptides. Furthermore, the distinct patterns of changes in the expression of CHHs in the SG and the PO suggested their tissue-specific functions in the regulation of low pH stress. Ion mobility-mass spectrometry (IM-MS) was also employed in this study to provide conformation analysis of both CHHs and CPRPs from different tissues.

Signaling Pathways That Regulate the Crustacean Molting Gland

A pair of Y-organs (YOs) are the molting glands of decapod crustaceans. They synthesize and secrete steroid molting hormones (ecdysteroids) and their activity is controlled by external and internal signals. The YO transitions through four physiological states over the molt cycle, which are mediated by molt-inhibiting hormone (MIH; basal state), mechanistic Target of Rapamycin Complex 1 (mTORC1; activated state), Transforming Growth Factor-β (TGFβ)/Activin (committed state), and ecdysteroid (repressed state) signaling pathways. MIH, produced in the eyestalk X-organ/sinus gland complex, inhibits the synthesis of ecdysteroids. A model for MIH signaling is organized into a cAMP/Ca²⁺-dependent triggering phase and a nitric oxide/cGMP-dependent summation phase, which maintains the YO in the basal state during intermolt. A reduction in MIH release triggers YO activation, which requires mTORC1-dependent protein synthesis, followed by mTORC1-dependent gene expression. TGFβ/Activin signaling is required for YO commitment in mid-premolt. The YO transcriptome has 878 unique contigs assigned to 23 KEGG signaling pathways, 478 of which are differentially expressed over the molt cycle. Ninety-nine contigs encode G protein-coupled receptors (GPCRs), 65 of which bind a variety of neuropeptides and biogenic amines. Among these are putative receptors for MIH/crustacean hyperglycemic hormone neuropeptides, corazonin, relaxin, serotonin, octopamine, dopamine, allatostatins, Bursicon, ecdysis-triggering hormone (ETH), CCHamide, FMRFamide, and proctolin. Contigs encoding receptor tyrosine kinase insulin-like receptor, epidermal growth factor (EGF) receptor, and fibroblast growth factor (FGF) receptor and ligands EGF and FGF suggest that the YO is positively regulated by insulin-like peptides and growth factors. Future research should focus on the interactions of signaling pathways that integrate physiological status with environmental cues for molt control.

De novo transcriptome assembly and functional annotation for Y-organs of the blue crab (Callinectes sapidus), and analysis of differentially expressed genes during pre-molt

Blue crabs (Callinectes sapidus) undergo incremental growth involving the shedding (molting) of the old exoskeleton, and subsequent expansion and re-calcification of the newly synthesized one. The cellular events that lead to molting are triggered by steroid hormones termed ecdysteroids released from Y-organs, paired endocrine glands located in the anterior cephalothorax. The regulatory pathways leading to increased synthesis and release of ecdysteroids are not fully understood, and no transcriptome has yet been published for blue crab Y-organs. Here we report de novo transcriptome assembly and annotation for adult blue crab Y-organs, and differential gene expression (DGE) analysis between Y-organs of intermolt and premolt crabs. After trimming and quality assessment, a total of 91,819,458 reads from four cDNA libraries were assembled using Trinity to form the reference transcriptome. Trinity produced a total of 171,530 contigs coding for 150,388 predicted genes with an average contig length of 613 and an N50 of 940. Of these, TransDecoder predicted 31,661 open reading frames (ORFs), and 10,210 produced non-redundant blastx results through Trinotate annotation. Genes involved in multiple cell signaling pathways, including Ca²⁺ signaling, cGMP signaling, cAMP signaling, and mTOR signaling were present in the annotated reference transcriptome. DGE analysis showed in premolt Y-organs up-regulated genes involved in energy production, cholesterol metabolism, and exocytosis. The results provide insights into the transcriptome of blue crab Y-organs during a natural (rather than experimentally induced) molting cycle, and constitute a step forward in understanding the cellular mechanisms that underlie stage-specific changes in the synthesis and secretion of ecdysteroids by Y-organs.

The Crustacean Hyperglycemic Hormone Superfamily: Progress Made in the Past Decade

Early studies recognizing the importance of the decapod eyestalk in the endocrine regulation of crustacean physiology-molting, metabolism, reproduction, osmotic balance, etc.-helped found the field of crustacean endocrinology. Characterization of putative factors in the eyestalk using distinct functional bioassays ultimately led to the discovery of a group of structurally related and functionally diverse neuropeptides, crustacean hyperglycemic hormone (CHH), molt-inhibiting hormone (MIH), gonad-inhibiting hormone (GIH) or vitellogenesis-inhibiting hormone (VIH), and mandibular organ-inhibiting hormone (MOIH). These peptides, along with the first insect member (ion transport peptide, ITP), constitute the original arthropod members of the crustacean hyperglycemic hormone (CHH) superfamily. The presence of genes encoding the CHH-superfamily peptides across representative ecdysozoan taxa has been established. The objective of this review is to, aside from providing a general framework, highlight the progress made during the past decade or so. The progress includes the widespread identification of the CHH-superfamily peptides, in particular in non-crustaceans, which has reshaped the phylogenetic profile of the superfamily. Novel functions have been attributed to some of the newly identified members, providing exceptional opportunities for understanding the structure-function relationships of these peptides. Functional studies are challenging, especially for the peptides of crustacean and insect species, where they are widely expressed in various tissues and usually pleiotropic. Progress has been made in deciphering the roles of CHH, ITP, and their alternatively spliced counterparts (CHH-L, ITP-L) in the regulation of metabolism and ionic/osmotic hemostasis under (eco)physiological, developmental, or pathological contexts, and of MIH in the stimulation of ovarian maturation, which implicates it as a regulator for coordinating growth (molt) and reproduction. In addition, experimental elucidation of the steric structure and structure-function relationships have given better understanding of the structural basis of the functional diversification and overlapping among these peptides. Finally, an important finding was the first-ever identification of the receptors for this superfamily of peptides, specifically the receptors for ITPs of the silkworm, which will surely give great impetus to the functional study of these peptides for years to come. Studies regarding recent progress are presented and synthesized, and prospective developments remarked upon.

Hormonal control of the crustacean molting gland: Insights from transcriptomics and proteomics

Endocrine control of molting in decapod crustaceans involves the eyestalk neurosecretory center (X-organ/sinus gland complex), regenerating limbs, and a pair of Y-organs (YOs), as molting is induced by eyestalk ablation or multiple leg autotomy and suspended in early premolt by limb bud autotomy. Molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH), produced in the X-organ/sinus gland complex, inhibit the YO. The YO transitions through four physiological states over the molt cycle: basal in intermolt; activated in early premolt; committed in mid- and late premolt; and repressed in postmolt. We assembled the first comprehensive YO transcriptome over the molt cycle in the land crab, Gecarcinus lateralis, showing that as many as 23 signaling pathways may interact in controlling ecdysteroidogenesis. A proposed model of the MIH/cyclic nucleotide pathway, which maintains the basal YO, consists of cAMP/Ca²⁺ triggering and nitric oxide (NO)/cGMP summation phases. Mechanistic target of rapamycin (mTOR) signaling is required for YO activation in early premolt and affects the mRNA levels of thousands of genes. Transforming Growth Factor-β (TGFβ)/Activin signaling is required for YO commitment in mid-premolt and high ecdysteroid titers at the end of premolt may trigger YO repression. The G. lateralis YO expresses 99 G protein-coupled receptors, three of which are putative receptors for MIH/CHH. Proteomic analysis shows the importance of radical oxygen species scavenging, cytoskeleton, vesicular secretion, immune response, and protein homeostasis and turnover proteins associated with YO function over the molt cycle. In addition to eyestalk ganglia, MIH mRNA and protein are present in brain, optic nerve, ventral nerve cord, and thoracic ganglion, suggesting that they are secondary sources of MIH. Down-regulation of mTOR signaling genes, in particular Ras homolog enriched in brain or Rheb, compensates for the effects of elevated temperature in the YO, heart, and eyestalk ganglia in juvenile Metacarcinus magister. Rheb expression increases in the activated and committed YO. These data suggest that mTOR plays a central role in mediating molt regulation by physiological and environmental factors.

Impacts of Methyl Farnesoate and 20-Hydroxyecdysone on Larval Mortality and Metamorphosis in the Kuruma Prawn Marsupenaeus japonicus

Physiological functions of juvenile hormone (JH) and molting hormone have been demonstrated in insects. JH, molting hormone and their mimics (insect growth regulators, IGRs) show endocrine-disrupting effects not only on target pest insects but also on other arthropod species such as crustaceans. However, little is known about the endocrine-disrupting effects of IGRs on benthic crustaceans. In this study, laboratory experiments were conducted to investigate effects of representative innate JH in crustaceans (methyl farnesoate, MF) and molting hormone (20-hydroxyecdysone, 20E, active form of ecdysteroid) on larval stages of the kuruma prawn Marsupenaeus japonicus, which is a decapod crustacean living in warm seawater. Larval development of kuruma prawn progresses in the order of nauplius, zoea, mysis, and then post-larvae with molting and metamorphosis, but it is unknown whether both MF and 20E have crucial roles in metamorphosis and molting of this species. Treatments of either MF or 20E on shrimp larvae were attempted at each developmental stage and those effects were validated. In terms of EC50 values between mortality and metamorphosis, there were apparent differences in the transition from nauplius to zoea (MF: 7.67 and 0.12 μM; 20E: 3.84 and 0.06 μM in survival and metamorphic rates, respectively). In contrast, EC50 values in MF and 20E treatments showed high consistency in the transitions between zoea to mysis (EC50 values for survival; MF: 1.25 and 20E: 0.22 μM), and mysis to post-larvae (EC50 values for survival; MF: 0.65 and 20E: 0.46 μM). These data suggest that nauplius has strong resistance against exposure to MF and 20E. Moreover, both chemicals induced high mortality triggered by the disruption of molting associated with metamorphosis. To our knowledge, this is the first experimental evidence that investigates in vivo physiological functions of MF and 20E in the larval stages of kuruma prawn, shedding light on not only ecotoxicological impacts of IGRs released into nature, but also endocrine mechanisms underlying larval development with metamorphosis in benthic decapod crustaceans.

Invisible endocrine disruption and its mechanisms: A current review

The research on impacts of environmental chemicals on crustacean molting dates back to the 1970s when ground-breaking studies investigated the disruption of molting in Crustacea by organochlorines. With the emergence of a new scientific inquiry, termed environmental endocrine disruption, in the early 1990s, increasing attention has been attracted to the possibility that environmental chemicals capable of wreaking havoc on sex steroid-regulated processes in vertebrates can also adversely affect ecdysteroid-mediated processes, e.g. molting, in crustaceans. Given the fact that many molting-disrupting chemicals accumulate in crustacean tissues and that the effect on molting is not readily visible in the field, the disruption of molting by environmental chemicals has been dubbed the invisible endocrine disruption. In recent years, much advancement has been made in both the documentation of the phenomenon of molting disruption and the search for mechanisms, by which molting disruption occurs. This review provides an overview of the current status of the field of invisible endocrine disruption, and perspectives on future directions are also presented.

Calmodulin inhibition as a mode of action of antifungal imidazole pharmaceuticals in non-target organisms

To improve assessment of risks associated with pharmaceutical contamination of the environment, it is crucial to understand effects and mode of action of drugs in non-target species. The evidence is accumulating that species with well-conserved drug targets are prone to be at risk when exposed to pharmaceuticals. An interesting group of pharmaceuticals released into the environment is imidazoles, antifungal agents with inhibition of ergosterol synthesis as a primary mode of action in fungi. However, imidazoles have also been identified as competitive antagonists of calmodulin (CaM), a calcium-binding protein with phylogenetically conserved structure and function. Therefore, imidazoles would act as CaM inhibitors in various organisms, including those with limited capacity to synthesize sterols, such as arthropods. We hypothesized that effects observed in crustaceans exposed to imidazoles are related to the CaM inhibition and CaM-dependent nitric oxide (NO) synthesis. To test this hypothesis, we measured (i) CaM levels and its gene expression, (ii) NO accumulation and (iii) gene expression of NO synthase (NOS1 and NOS2), in the cladoceran Daphnia magna exposed to miconazole, a model imidazole drug. Whereas significantly increased CaM gene expression and its cellular allocation were observed, supporting the hypothesized mode of action, no changes occurred in either NO synthase expression or NO levels in the exposed animals. These findings suggest that CaM inhibition by miconazole leads to protein overexpression that compensates for the loss in the protein activity, with no measurable downstream effects on NO pathways. The inhibition of CaM in D. magna may have implications for effect assessment of exposure to mixtures of imidazoles in aquatic non-target species.

Transcriptomic analysis of gills provides insights into the molecular basis of molting in Chinese mitten crab (Eriocheir sinensis)

Chinese mitten crab (Eriocheir sinensis) is an economically important freshwater aquaculture species and is a model species for research on the mechanism of molting. This study aimed to identify important candidate genes associated with the molting process and to determine the role of gills in the regulation of molting with the help of transcriptomic analysis. The transcriptomes of crabs at different molting stages—postmolt (PoM), intermolt (InM), premolt (PrM) and ecdysis (E)—were de novo assembled to generate 246,232 unigenes with a mean length of 851 bp. A total of 86,634 unigenes (35.18% of the total unigenes) were annotated against reference databases. Significantly upregulated genes were identified in postmolt compared to intermolt (1,475), intermolt compared to premolt (65), premolt compared to ecdysis (1,352), and ecdysis compared to postmolt (153), and the corresponding numbers of downregulated genes were 1,276, 32, 1,573 and 171, respectively. Chitin synthase, endochitinase, chitinase A, chitinase 3, chitinase 6 and chitin deacetylase 1 were upregulated during the postmolt and ecdysis stages, while phosphoglucomutase 3 (PGM3), glucosamine 6-phosphate deaminase (GNPDA) and glucosamine glycoside hydrolase (nagZ) were upregulated during the intermolt and premolt stages compared to the other stages. The upregulated genes were enriched in several lipid-related metabolic pathways, such as “fatty acid elongation”, “glycerophospholipid metabolism” and “sulfur metabolism”. Meanwhile, three signaling pathways, including the “phosphatidylinositol signaling system”, the “calcium signaling pathway” and the “GnRH signaling pathway” were also enriched. Tetraspanin-18, an important effector gene in the lysosomal pathway involved in cell apoptosis, up-regulate with the beginning of molting (in premolt stage) and reach the top in the ecdysis stage, and barely expressed in the intermolt stage. The expression variations in the tetraspanin-18 gene indicated that it may play an important role in the beginning of molting cycle, which might be regulated by the stress of salinity. This study revealed that the gills could participate in chitin degradation, in reestablishment of the exoskeleton and the signaling process. Based on transcriptomic analysis of the gills, we not only explored novel molecular mechanisms of molting in E. sinensis but also acquired foundational genetic data for E. sinensis.

Multifaceted Mass Spectrometric Investigation of Neuropeptide Changes in Atlantic Blue Crab, Callinectes sapidus, in Response to Low pH Stress

The decrease of pH level in the water affects animals living in aquatic habitat, such as crustaceans. The molecular mechanisms enabling these animals to survive this environmental stress remain unknown. To understand the modulatory function of neuropeptides in crustaceans when encountering drops in pH level, we developed and implemented a multifaceted mass spectrometric platform to investigate the global neuropeptide changes in response to water acidification in the Atlantic blue crab, Callinectes sapidus. Neural tissues were collected at different incubation periods to monitor dynamic changes of neuropeptides under different stress conditions occurring in the animal. Neuropeptide families were found to exhibit distinct expression patterns in different tissues and even each isoform had its specific response to the stress. Circulating fluid in the crabs (hemolymph) was also analyzed after 2-h exposure to acidification, and together with results from tissue analysis, enabled the discovery of neuropeptides participating in the stress accommodation process as putative hormones. Two novel peptide sequences were detected in the hemolymph that appeared to be involved in the stress-related regulation in the crabs.

Modulation of complex coordinated molecular signaling by 5HT and a cocktail of inhibitors leads to ovarian maturation of Penaeus monodon in captivity

In aquaculture practices, prawn cultivation holds the major share and Penaeus monodon is the main species cultured. The decline in production of P. monodon is mainly due to the limited availability of domesticated broodstock, which is attributed to its reproductive cycle, controlled by complex coordinated signaling mechanisms. Unilateral eyestalk ablation of domesticated females held in captivity is done to induce ovary development, which has certain disadvantages, including a high rate of mortality. Thus, developing alternative techniques for eyestalk ablation in captive broodstock is necessary to induce maturation of ovary. This study exemplifies the role of 5HT along with a cocktail of inhibitors (U0126, Rp-cAMP, and LY294002) in inducing ovarian maturation. In this study, inhibition of pERK by U0126 inhibited vitellogenesis-inhibiting hormone (VIH), which in turn led to the overexpression of vitellogenin. 5HT induces steroidogenesis (estradiol-17β) through induction of the gonadotropin-releasing hormone by activating calcium-calmodulin signaling. Steroidogenesis is also aided by synthesis of StAR protein. Estradiol-17β stimulates the formation of the maturation-promoting factor (MPF) complex by cdc25 activation and Myt1 inactivation. LY294002 aids in keeping cdc25 activated by inhibiting calcium-calmodulin induced phosphorylation of Akt which is a negative regulator of mitogen-activated protein kinases. VIH induced activation of Myt1, through protein kinase A (PKA), was inhibited by Rp-cAMP which inhibits adenylate cyclase, thus stabilizing the activated MPF complex. To conclude, the coordinated effect of inhibitors and 5HT accelerates the development of ovary from previtellogenic to matured oocytes, yielding high quality and quantity larvae compared with eyestalk-ablated P. monodon.

In vivo effects of serotonin and fluoxetine on cardio-ventilatory functions in the shore crab Carcinus maenas (L. 1758)

Serotonin (5-HT) takes a key position in regulating vital functions, such as cardio-ventilatory activity, locomotion and behaviour. Selective serotonin reuptake inhibitors (SSRIs) modulate the serotonergic system and thus affect these functions. Rhythmic behaviours, such as cardio-ventilatory activity, are controlled by central pattern generators, which in turn are regulated by 5-HT. In crustaceans, 5-HT also regulates the synthesis and secretion of crustacean hyperglycaemic hormone, a pleiotropic hormone involved in the mobilisation and release of glucose into the haemolymph, thus stimulating the animal's activity. As a matter of consequence, SSRIs may affect cardio-ventilatory activity. In order to examine how the SSRIs affect fundamental physiological parameters based on rhythmic behaviours in decapods, cardio-respiratory activity in the shore crab Carcinus maenas was assessed after pericardial injection of a single dose of either 0.5 μM, 0.75 μM or 1 μM fluoxetine, respectively. Simultaneous recordings of heart and scaphognathite movements in both brachial chambers were conducted by measuring impedance changes in the respective body compartments. Injection of 5-HT had an immediate effect on cardio-ventilatory activities and strongly upregulated both cardiac and ventilatory activities. Fluoxetine showed similar effects, entailing moderate tachycardia and increased ventilation rates. Compared to 5-HT, these effects were delayed in time and much less pronounced. Metabolism of fluoxetine into the active compound nor-fluoxetine might account for the delayed action, whereas compensatory regulation of cardio-ventilatory frequencies and amplitudes are likely to explain the attenuation of the responses compared to the strong and immediate increase by 5-HT. Overall, the results suggest increased 5-HT levels in invertebrates following fluoxetine exposure, which are able to disturb physiological functions regulated by 5-HT, such as cardiac and respiratory activity.

Penaeid shrimp genome provides insights into benthic adaptation and frequent molting

Crustacea, the subphylum of Arthropoda which dominates the aquatic environment, is of major importance in ecology and fisheries. Here we report the genome sequence of the Pacific white shrimp Litopenaeus vannamei, covering ~1.66 Gb (scaffold N50 605.56 Kb) with 25,596 protein-coding genes and a high proportion of simple sequence repeats (>23.93%). The expansion of genes related to vision and locomotion is probably central to its benthic adaptation. Frequent molting of the shrimp may be explained by an intensified ecdysone signal pathway through gene expansion and positive selection. As an important aquaculture organism, L. vannamei has been subjected to high selection pressure during the past 30 years of breeding, and this has had a considerable impact on its genome. Decoding the L. vannamei genome not only provides an insight into the genetic underpinnings of specific biological processes, but also provides valuable information for enhancing crustacean aquaculture.

The Pacific white shrimp Litopenaeus vannamei is an important aquaculture species and a promising model for crustacean biology. Here, the authors provide a reference genome assembly, and show that gene expansion is involved in the regulation of frequent molting as well as benthic adaptation of the shrimp.

The crustacean ecdysone cassette: A gatekeeper for molt and metamorphosis

Arthropods have long been utilized as models to explore molecular function, and the findings derived from them can be applied throughout metazoa, including as a basis for medical research. This has led to the adoption of many representative insect models beyond Drosophila, as each lends its own unique perspective to questions in endocrinology and genetics. However, non-insect arthropods are yet to be realised for the potential insight they may provide in such studies. The Crustacea are among the most ancient arthropods from which insects descended, comprising a huge variety of life histories and ecological roles. Of the events in a typical crustacean development, metamorphosis is perhaps the most ubiquitous, challenging and highly studied. Despite this, our knowledge of the endocrinology which underpins metamorphosis is rudimentary at best; although several key molecules have been identified and studied in depth, the link between them is quite nebulous and leans heavily on well-explored insect models, which diverged from the Pancrustacea over 450 million years ago. As omics technologies become increasingly accessible, they bring the prospect of explorative molecular research which will allow us to uncover components and pathways unique to crustaceans. This review reconciles known components of crustacean metamorphosis and reflects on our findings in insects to outline a future search space, with focus given to the ecdysone cascade. To expand our knowledge of this ubiquitous endocrine system not only aids in our understanding of crustacean metamorphosis, but also provides a deeper insight into the adaptive capacity of arthropods throughout evolution.

Precursors of neuropeptides and peptide hormones in the genomes of tardigrades

Tardigrades are a key group for understanding the evolution of the Ecdysozoa, a large clade of molting animals that also includes arthropods and nematodes. However, little is known about most aspects of their basic biology. Neuropeptide and peptide hormone signaling has been extensively studied in arthropods and nematodes (particularly regarding their roles in molting in arthropods), but very little is known about neuropeptide signaling in other ecdysozoans. In this work, different strategies were used to search for neuropeptide and peptide hormone precursors in the genomes of the tardigrades Hypsibius dujardini and Ramazzottius varieornatus. In general, there is a remarkable similarity in the complement of neuropeptides and their sequences between tardigrades and arthropods. The precursors found in tardigrades included homologs of achatin, allatostatins A, B and C, allatotropin, calcitonin, CCHamide, CCRFa, corazonin, crustacean cardioactive peptide, diuretic hormone 31, diuretic hormone 44, ecdysis triggering hormone, eclosion hormone, gonadotropin-releasing hormone (GnRH), GSEFLamide, insulin-like peptides, ion transport peptide, kinin, neuropeptide F, orcokinin, pigment dispersing hormone, proctolin, pyrokinin, RYamide, short neuropeptide F, sulfakinin, tachykinin, trissin and vasopressin. In most cases, homologs of known cognate receptors for each neuropeptide family could only be identified when the precursors were also present in the genome, further supporting their identification. Some neuropeptide precursor genes have undergone several duplications in tardigrades, including allatostatin A and C, corazonin, GnRH, eclosion hormone, sulfakinin and trissin. Furthermore, four novel families of candidate neuropeptide precursors were identified (two of which could also be found in several arthropods). To the best of my knowledge, this work represents the first genome-wide search for neuropeptide precursors in any ecdysozoan species outside arthropods and nematodes, and is a necessary first step towards understanding neuropeptide function in tardigrades.

Tissue distribution and biochemical characteristics of receptors for sinus gland peptide VII as a crustacean hyperglycemic hormone and vitellogenesis-inhibiting hormone of the kuruma prawn, Marsupenaeus japonicus

Crustacean hyperglycemic hormone (CHH) and vitellogenesis-inhibiting hormone (VIH) belong to the CHH family, a neuropeptide superfamily conserved in ecdysozoans. To date, no receptor for the CHH family peptides has been identified in crustaceans. Here, we used a CHH family isoform, Mj-sinus gland peptide (SGP)-VII, as a representative of CHH and VIH in order to determine its target tissues and obtain biochemical information regarding its receptor in the kuruma prawn Marsupenaeus japonicus (Crustacea, Decapoda). An in vitro binding assay using a radiolabeled recombinant Mj-SGP-VII and tissue membranes showed that ligand-receptor binding was specific and dissociable. Six tissues, including the hepatopancreas, gill, heart, skeletal muscle, hindgut, and ovary, were identified as the main targets for Mj-SGP-VII. Scatchard analysis of these six tissues determined the dissociation constant and maximum binding capacity values as K_d = 0.86-3.6 nM and B_max = 102-915 fmol/mg protein, respectively. Of these six tissues, the hepatopancreas, heart, and ovary showed changes in the levels of ligand-binding after the elimination of endogenous ligands by eyestalk ablation. In the hepatopancreas, an increase in the amount of ligand-binding was observed after eyestalk ablation, independent of gender, which appears to be associated with hypoglycemia caused by the treatment. The change observed in the hepatopancreas was due to the increase in the ligand-binding capacity, but not in the ligand-binding affinity, of the receptors. Furthermore, chemical cross-linking analysis demonstrated the presence of target tissue-specific receptors for Mj-SGP-VII with molecular masses of 34-62 kDa. Collectively, the present data provided important information on tissue distribution, temporal changes in expression level, and molecular mass, for the identification and characterization of receptors for CHH family peptides in crustaceans.

Characterization, expression patterns of molt-inhibiting hormone gene of Macrobrachium nipponense and its roles in molting and growth

The oriental river prawn, Macrobrachium nipponense, is an important commercial aquaculture resource in China. In order to overwinter, M. nipponense displays decreased physiological activity and less consumption of energy. Sudden warming would trigger molting and cause an extensive death, resulting in huge economic losses. Therefore, it is of great practical significance to study the molting mechanism of oriental river prawns. Molt-inhibiting hormone gene (MIH) plays a major role in regulating molting in crustaceans. In this study, a full length MIH cDNA of M. nipponense (Mn-MIH) was cloned from the eyestalk. The total length of the Mn-MIH was 925 bp, encoding a protein of 119 amino acids. Tissue distribution analysis showed that Mn-MIH was highly expressed in the eyestalk, and that it had relatively low expression in gill, ovary, and abdominal ganglion. Mn-MIH was detected in all developmental stages, and changed regularly in line with the molting cycle of the embryo and larva. Mn-MIH varied in response to the molting cycle, suggesting that Mn-MIH negatively regulates ecdysteroidogenesis. Mn-MIH inhibition by RNAi resulted in a significant acceleration of molting cycles in both males and females, confirming the inhibitory role of MIH in molting. After long-term RNAi males, but not females had significant weight gain, confirming that Mn-MIH plays an important role in growth of M. nipponense. Our work contributes to a better understanding of the role of Mn-MIH in crustacean molting and growth.

Molecular cloning and characterization of a sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) from Y-organs of the blue crab (Callinectes sapidus)

Existing data indicate that a Ca²⁺ signal stimulates ecdysteroid hormone production by crustacean molting glands (Y-organs). Ca²⁺ signaling is dependent on a tightly regulated Ca²⁺ gradient, with intracellular free Ca²⁺ maintained at a low basal level (typically sub-micromolar). This is achieved through the action of proteins intrinsic to the plasma membrane and the membranes of organelles. One such protein, the sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA), pumps Ca²⁺ from cytosol to the lumen of the endoplasmic reticulum. As a step toward understanding Ca²⁺-mediated regulation of ecdysteroidogenesis, we have begun investigating Ca²⁺ transport proteins in Y-organs. In studies reported here, we used a PCR-based strategy to clone from Y-organs of the blue crab (Callinectes sapidus) a cDNA encoding a putative SERCA protein. The cloned Cas-SERCA cDNA (3806 bp) includes a 3057-bp open reading frame that encodes a 1019-residue protein (Cas-SERCA). The conceptually translated protein has a predicted molecular mass of 111.42 × 10³ and contains all signature domains of an authentic SERCA, including ten transmembrane domains and a phosphorylation site at aspartate 351. A homology model of Cas-SERCA closely resembles models of related SERCA proteins. Phylogenetic analysis shows Cas-SERCA clusters with SERCA proteins from other arthropods. An assessment of tissue distribution indicates the Cas-SERCA transcript is widely distributed across tissues. Studies using quantitative PCR showed Cas-SERCA transcript abundance increased significantly in Y-organs activated by eyestalk ablation, a pattern consistent with the hypothesis that Cas-SERCA functions to maintain Ca²⁺ homeostasis in Y-organs.

CRISPR/Cas9-mediated deletion of EcMIH shortens metamorphosis time from mysis larva to postlarva of Exopalaemon carinicauda

The recently emerged CRISPR/Cas9 technology is the most flexible means to produce targeted mutations at the genomic loci in a variety of organisms. In Crustaceans, molt-inhibiting hormone (MIH) is an important negative-regulatory factor and plays a key role in suppressing the molting process. However, whether precise disruption of MIH in crustacean can be achieved and successfully used to improve the development and growth has not been proved. In this research, the complementary DNA (cDNA) and genomic DNA, including flanking regions of the MIH gene (EcMIH) of ridgetail white prawn Exopalaemon carinicauda, were cloned and sequenced. Sequence analysis revealed that EcMIH was composed of three exons and two introns. Analysis by RT-PCR showed that EcMIH mainly expressed in eyestalks. During different development periods, EcMIH was highest in juvenile stage and extremely low in others but adult prawns eyestalks. In addition, we applied CRISPR/Cas9 technology to generate EcMIH knock-out (KO) prawns and then analyzed the changes in their phenotypes. We efficiently generated 12 EcMIH-KO prawns out of 250 injected one-cell stage embryos and the mutant rate reached 4.8% after embryo injection with one sgRNA targeting the second exon of EcMIH. The EcMIH-KO prawns exhibited increased the body length and shortened the metamorphosis time of larvae from mysis larva to postlarva. Meanwhile, EcMIH-KO did not cause the health problems such as early stage death or deformity. In conclusion, we successfully obtained EcMIH gene and generated EcMIH-KO prawns using CRISPR/Cas9 technology. This study will certainly lead to a wide application prospect of MIH gene in prawns breeding.

Transcriptomic analysis of crustacean molting gland (Y-organ) regulation via the mTOR signaling pathway

The intermolt crustacean Y-organ (YO) maintains a basal state mediated by pulsatile release of molt inhibiting hormone (MIH), a neuropeptide produced in the eyestalk ganglia, inhibiting YO ecdysteroidogenesis. Reduction of MIH results in YO activation and the animal enters premolt. In the crab, Gecarcinus lateralis, molting was induced by eyestalk ablation (ESA). ESA animals were injected with either rapamycin, an mTOR inhibitor, or DMSO vehicle at Day 0. YOs were harvested at 1, 3, and 7 days post-ESA and processed for high throughput RNA sequencing. ESA-induced increases in mRNA levels of mTOR signaling genes (e.g., mTOR, Rheb, TSC1/2, Raptor, Akt, and S6 kinase) declined following rapamycin treatment. In concert with mTOR inhibition, mRNA levels of ecdysteroid biosynthesis genes (e.g., Nvd, Spo, Sad, Dib, and Phm) were decreased and accompanied by a decrease in hemolymph ecdysteroid titer. By contrast, rapamycin increased the mRNA level of FKBP12, the rapamycin-binding protein, as well as the mRNA levels of genes associated with Wnt and insulin-like growth factor signaling pathways. Many MIH and transforming growth factor-β signaling genes were down regulated in ESA animals. These results indicate that mTOR activity either directly or indirectly controls transcription of genes that drive activation of the YO.

Transcriptomic analysis of the hepatopancreas induced by eyestalk ablation in shrimp, Litopenaeus vannamei

Although eyestalk ablation (ESA) is currently considered the most effective method to facilitate molting and maturation, its physiological responses are still not clearly explained in decapod crustaceans. In this study, we analyzed the hepatopancreatic transcriptomes of Litopenaeus vannamei after ESA using the Illumina Miseq platform. After screening 53,029 contigs with high cutoff values (fold change>|10|; P-value<0.05; RPKM>1), we were able to identify 105 differentially expressed genes (DEGs), of which 100 were up-regulated and five were down-regulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that many DEGs were involved in the synthetic pathways for glycerol and trehalose, which are known to function as the major protectants under conditions of low temperature and osmotic stress in arthropods. Additional analysis of the other DEGs enabled us to classify them in four categories: immunity; cellular trafficking; transcriptional regulation; molting and maturation. Many DEGs were involved in immunity and stress responses, in particular the proPO activation system, which is the major immune and wound-healing system in arthropods. In addition to immunity and stress responses, we were also able to identify DEGs involved in molting and maturation processes (e.g., group I chitinase), as well as those involved in hormone metabolism and trafficking. Collectively, based on the transcriptomic analysis, ESA causes not only stress and immune responses, but also molting and maturation in L. vannamei. The DEGs identified in this study could be useful markers to understand the physiological responses that ESA induces in shrimp, such as molting, maturation, and immunity.

Transcriptomic information from Pacific white shrimp (Litopenaeus vannamei) ovary and eyestalk, and expression patterns for genes putatively involved in the reproductive process

The increased use of massive sequencing technologies has enabled the identification of several genes known to be involved in different mechanisms associated with reproduction that so far have only been studied in vertebrates and other model invertebrate species. In order to further investigate the genes involved in Litopenaeus vannamei reproduction, cDNA and SSH libraries derived from female eyestalk and gonad were produced, allowing the identification of expressed sequences tags (ESTs) that potentially have a role in the regulation of gonadal maturation. In the present study, different transcripts involved in reproduction were identified and a number of them were characterized as full-length. These transcripts were evaluated in males and females in order to establish their tissue expression profiles during developmental stages (juvenile, subadult and adult), and in the case of females, their possible association with gonad maturation was assessed through expression analysis of vitellogenin. The results indicated that the expression of vitellogenin receptor (vtgr) and minichromosome maintenance (mcm) family members in the female gonad suggest an important role during previtellogenesis. Additionally, the expression profiles of genes such as famet, igfbp and gpcr in brain tissues suggest an interaction between the insulin/insulin-like growth factor signaling pathway (IIS) and methyl farnesoate (MF) biosynthesis for control of reproduction. Furthermore, the specific expression pattern of farnesoic acid O-methyltransferase suggests that final synthesis of MF is carried out in different target tissues, where it is regulated by esterase enzymes under a tissue-specific hormonal control. Finally, the presence of a vertebrate type steroid receptor in hepatopancreas and intestine besides being highly expressed in female gonads, suggest a role of that receptor during sexual maturation.

Ovarian Transcriptome Analysis of Vitellogenic and Non-Vitellogenic Female Banana Shrimp (Fenneropenaeus merguiensis)

The banana shrimp (Fenneropenaeus merguiensis) is one of the most commercially important penaeid species in the world. Its numbers are declining in the wild, leading to a loss of broodstock for farmers of the shrimp and a need for more successful breeding programs. However, the molecular mechanism of the genes involved in this shrimp’s ovarian maturation is still unclear. Consequently, we compared transcriptomic profiles of ovarian tissue from females in both the vitellogenic stage and the non-vitellogenic stage. Using RNA-Seq technology to prepare the transcriptome libraries, a total of 12,187,412 and 11,694,326 sequencing reads were acquired from the non-vitellogenic and vitellogenic stages respectively. The analysis of the differentially expressed genes identified 1,025 which were significantly differentially expressed between the two stages, of which 694 were up-regulated and 331 down-regulated. Four genes putatively involved in the ovarian maturation pathway were chosen for validation by quantitative real-time PCR (RT-qPCR). The data from this study provided information about gene expression in ovarian tissue of the banana shrimp which could be useful for a better understanding of the regulation of this species’ reproductive cycle.

A Comparison of Resources for the Annotation of a De Novo Assembled Transcriptome in the Molting Gland (Y-Organ) of the Blackback Land Crab, Gecarcinus lateralis

Next-generation sequencing technologies are revolutionizing crustacean biology. De novo assembly of RNA sequencing (RNA-seq) data allows researchers to catalog and quantify genes expressed in tissues of a species that lacks a complete genome sequence. RNA-seq has become an important tool for understanding phenotypic plasticity and the responses of organisms to environmental cues. However, there are challenges with identification of assembled contiguous sequences (contigs) without a reference genome. Thus, the selection of resources for annotating contigs is critical for the downstream analysis of gene functions. A de novo-assembled transcriptome of the Gecarcinus lateralis molting gland, or Y-organ (YO), was used to compare two functional annotation packages, Trinotate and Blast2GO. The assembled transcriptome contained 229,278 contigs derived from YOs from animals in intermolt, premolt (early, mid, and late), and postmolt stages. Gene identification using BLAST against four databases and functional annotation using Gene Ontologies were conducted. The analysis revealed two major limitations of de novo assembly: (1) assembly using Trinity generates redundant contigs and (2) transcripts that encode protein isoforms are not distinguished with current computational tools. It is recommended that the NCBI Non-Redundant, SwissProt, TrEMBL, and Uniref90 databases be used to maximize gene identification. Trinotate is preferred for assigning functions to identified genes, as the package uses multiple databases for annotation. The differences between packages to generate Gene Ontology (GO) terms are attributed to the databases used for inputs: Trinotate uses both Pfam and BLAST databases, while Blast2GO uses only the BLAST database. InterProScan was used to verify the GO terms assigned via BLAST. A comprehensive annotation of de novo assembled transcriptome is necessary for the downstream analysis of differentially expressed transcripts in the YO over the molt cycle.

Dopamine is a key regulator in the signalling pathway underlying predator-induced defences in Daphnia

The waterflea Daphnia is a model to investigate the genetic basis of phenotypic plasticity resulting from one differentially expressed genome. Daphnia develops adaptive phenotypes (e.g. morphological defences) thwarting predators, based on chemical predator cue perception. To understand the genomic basis of phenotypic plasticity, the description of the precedent cellular and neuronal mechanisms is fundamental. However, key regulators remain unknown. All neuronal and endocrine stimulants were able to modulate but not induce defences, indicating a pathway of interlinked steps. A candidate able to link neuronal with endocrine responses is the multi-functional amine dopamine. We here tested its involvement in trait formation in Daphnia pulex and Daphnia longicephala using an induction assay composed of predator cues combined with dopaminergic and cholinergic stimulants. The mere application of both stimulants was sufficient to induce morphological defences. We determined dopamine localization in cells found in close association with the defensive trait. These cells serve as centres controlling divergent morphologies. As a mitogen and sclerotization agent, we anticipate that dopamine is involved in proliferation and structural formation of morphological defences. Furthermore, dopamine pathways appear to be interconnected with endocrine pathways, and control juvenile hormone and ecdysone levels. In conclusion, dopamine is suggested as a key regulator of phenotypic plasticity.

Role of Halloween genes in ecdysteroids biosynthesis of the swimming crab (Portunus trituberculatus): Implications from RNA interference and eyestalk ablation

Molting, including metamorphosis molting in arthropods are controlled by the ecdysteroids that are synthesized and secreted by the crustacean Y-organ (YO) or the insect prothoracic gland (PG). The Halloween genes encoding the enzymes mainly involved in the biosynthesis of ecdysteroids are well studied in insects but not in crustaceans. Given the importance of Halloween genes in ecdysteroids biosynthesis, we have previously reported the cDNA cloning of disembodied (Dib) in P. trituberculatus. Here, cDNA sequences of another two Halloween genes, Spook (Spo) and Shadow (Sad), were further identified and characterized. The predicted amino acid sequences for these two Halloween genes of Portunus trituberculatus were compared to those of several other arthropods, and several typical domains of the cytochrome P450 mono-oxygenase (CYP) were identified. Similar to the tissue distribution of Dib, the Spo and Sad also showed high specificity to the YO. RNA interference (RNAi) of these 3 genes indicated they all play essential role in ecdysteroids biosynthesis. To investigate the relationships of the Halloween genes to the eyestalk neuropeptides such as molt-inhibiting hormone (MIH), effects of eyestalk ablation (ESA) on the expression of Dib, Spo and Sad were detected. Expression of Dib and Sad, but not Spo, was significantly induced by ESA. The result indicated that the inhibition of MIH in ecdysteroids biosynthesis may be partly through the transcriptional regulation of certain Halloween genes, such as Dib and Sad, while the Spo might not be the target for MIH signal.

Transcriptome analysis of the synganglion from the honey bee mite, Varroa destructor and RNAi knockdown of neural peptide targets

Varroa mites (Varroa destructor) and the viruses that they transmit are one of the major contributing factors to the global honey bee crisis. Gene products within the nervous system are the targets of all the insecticides currently used to control Varroa but there is a paucity of transcriptomic data available for Varroa neural tissues. A cDNA library from the synganglia ("brains") of adult female Varroa was constructed and 600 ESTs sequenced and analysed revealing several current and potential druggable targets. Contigs coding for the deformed wing virus (DWV) variants V. destructor virus-1 (VDV-1) and the recombinant (VDV-1DVD) were present in the synganglion library. Negative-sense RNA-specific PCR indicated that VDV-1 replicates in the Varroa synganglion and all other tissues tested, but we could not detect DWV replicating in any Varroa tissue. Two neuropeptides were identified in the synganlion EST library: a B-type allatostatin and a member of the crustacean hyperglycaemic hormone (CHH) superfamily. Knockdown of the allatostatin or the CHH-like gene by double-stranded RNA-interference (dsRNAi) resulted in 85% and 55% mortality, respectively, of Varroa. Here, we present the first transcriptomic survey in Varroa and demonstrate that neural genes can be targeted by dsRNAi either for genetic validation of putative targets during drug discovery programmes or as a potential control measure in itself.

Transcriptome analysis of the molting gland (Y-organ) from the blackback land crab, Gecarcinus lateralis

In decapod crustaceans, arthropod steroid hormones or ecdysteroids regulate molting. These hormones are synthesized and released from a pair of molting glands called the Y-organs (YO). Cyclic nucleotide, mTOR, and TGFβ/Smad signaling pathways mediate molt cycle-dependent phase transitions in the YO. To further identify the genes involved in the regulation of molting, a YO transcriptome was generated from three biological replicates of intermolt blackback land crab, Gecarcinus lateralis. Illumina sequencing of cDNA libraries generated 227,811,829 100-base pair (bp) paired-end reads; following trimming, 90% of the reads were used for further analyses. The trimmed reads were assembled de novo using Trinity software to generate 288,673 contigs with a mean length of 872 bp and a median length of 1842 bp. Redundancy among contig sequences was reduced by CD-HIT-EST, and the output constituted the baseline transcriptome database. Using Bowtie2, 92% to 93% of the reads were mapped back to the transcriptome. Individual contigs were annotated using BLAST, HMMER, TMHMM, SignalP, and Trinotate, resulting in assignments of 20% of the contigs. Functional and pathway annotations were carried out via gene ontology (GO) and KEGG orthology (KO) analyses; 58% and 44% of the contigs with BLASTx hits were assigned to GO and KO terms, respectively. The gene expression profile was similar to a crayfish YO transcriptome database, and the relative abundance of each contig was highly correlated among the three G. lateralis replicates. Signal transduction pathway orthologs were well represented, including those in the mTOR, TGFβ, cyclic nucleotide, MAP kinase, calcium, VEGF, phosphatidylinositol, ErbB, Wnt, Hedgehog, Jak-STAT, and Notch pathways.

Structure-activity relationship of crustacean peptide hormones

In crustaceans, various physiological events, such as molting, vitellogenesis, and sex differentiation, are regulated by peptide hormones. To understanding the functional sites of these hormones, many structure-activity relationship (SAR) studies have been published. In this review, the author focuses the SAR of crustacean hyperglycemic hormone-family peptides and androgenic gland hormone and describes the detailed results of our and other research groups. The future perspectives will be also discussed.

Differential regulation of hepatopancreatic vitellogenin (VTG) gene expression by two putative molt-inhibiting hormones (MIH1/2) in Pacific white shrimp (Litopenaeus vannamei)

Molt-inhibiting hormone (MIH), a peptide member of the crustacean hyperglycemic hormone (CHH) family, is commonly considered as a negative regulator during the molt cycle in crustaceans. Phylogenetic analysis of CHH family peptides in penaeidae shrimps suggested that there is no significant differentiation between MIH and vitellogenesis-inhibiting hormone (VIH, another peptide member of CHH family), by far the most potent negative regulator of crustacean vitellogenesis known. Thus, MIH may also play a role in regulating vitellogenesis. In this study, two previously reported putative MIHs (LivMIH1 and LivMIH2) in the Pacific white shrimp (Litopenaeus vannamei) were expressed in Escherichia coli, purified by immobilized metal ion affinity chromatography (IMAC) and further confirmed by western blot. Regulation of vitellogenin (VTG) mRNA expression by recombinant LivMIH1 and LivMIH2 challenge was performed by both in vitro hepatopancreatic primary cells culture and in vivo injection approaches. In in vitro primary culture of shrimp hepatopancreatic cells, only LivMIH2 but not LivMIH1 administration could improve the mRNA expression of VTG. In in vivo injection experiments, similarly, only LivMIH2 but not LivMIH1 could stimulate hepatopancreatic VTG gene expression and induce ovary maturation. Our study may provide evidence for one isoform of MIH (MIH2 in L. vannamei) may serve as one of the mediators of the physiological progress of molting and vitellogenesis. Our study may also give new insight in CHH family peptides regulating reproduction in crustaceans, in particular penaeidae shrimps.

Cadmium inhibits molting of the freshwater crab Sinopotamon henanense by reducing the hemolymph ecdysteroid content and the activities of chitinase and N-acetyl-β-glucosaminidase in the epidermis

Molting is an essential process during the growth of crustaceans, which is coordinated by ecdysteroids secreted by the Y-organ, molting inhibiting hormone secreted by the X-organ sinus-gland complex, as well as chitinase and N-acetyl-β-glucosaminidase synthesized by the epidermis. Cadmium is one of the toxic metals in the aquatic environment. However, the endocrine effects of cadmium on the molting of freshwater crabs and the underlying mechanisms are unknown. To investigate these, freshwater crabs (Sinopotamon henanense) were acutely exposed to 0, 7.25, 14.5 and 29 mg/l Cd for 3, 4, 5 days or in some experiments for 4 days after eyestalk-ablation. The concentration of hemolymph ecdysone and the activities of the molting enzymes chitinase and NAG were measured. Histological changes in the epidermal tissues were documented. Our results showed that eyestalk ablation increased the ecdysteroid content as well as the activities of chitinase and NAG, which were inhibited by cadmium in a concentration-dependent manner; histological examinations demonstrated that eyestalk ablation produced storage particles in the epidermal tissues, which was also reduced by cadmium in a concentration-dependent manner. Our data suggest that cadmium disrupts endocrine function through inhibiting the secretion of ecdysteroids by the Y-organ and altering with the regulation of chitinase and NAG activity in the epidermis. This work provides new insights into the mechanisms underlying the molting inhibition effect of cadmium on the crabs.

Identification and characterization of receptors for ion transport peptide (ITP) and ITP-like (ITPL) in the silkworm Bombyx mori

Ion transport peptide (ITP) and its alternatively spliced variant, ITP-like (ITPL), are insect peptides that belong to the crustacean hyperglycemic hormone family. These peptides modulate the homeostatic mechanisms for regulating energy metabolism, molting, and reproduction and are specifically conserved in ecdysozoans. Many of the details of the molecular mechanisms by which crustacean hyperglycemic hormone family peptides exert pleiotropy remain to be elucidated, including characterization of their receptors. Here we identified three Bombyx mori orphan neuropeptide G protein-coupled receptors (BNGRs), BNGR-A2, -A24, and -A34, as receptors for ITP and ITPL (collectively referred to as ITPs). BNGR-A2 and -A34 and BNGR-A24 respond to recombinant ITPs, respectively, with EC50 values of 1.1-2.6 × 10(-8) M, when expressed in a heterologous expression system. These three candidate BNGRs are expressed at larval B. mori tissues targeted by ITPs, with cGMP elevation observed after exposure to recombinant ITPs. ITPs also increased the cGMP level in B. mori ovary-derived BmN cells via membrane-bound and soluble guanylyl cyclases. The simultaneous knockdown of bngr-A2 and -A34 significantly decreased the response of BmN cells to ITP, whereas knockdown of bngr-A24 led to decreased responses to ITPL. Conversely, transient expression of bngr-A24 potentiated the response of BmN cells to ITPL. An in vitro binding assay showed direct interaction between ITPs and heterologously expressed BNGRs in a ligand-receptor-specific manner. Taken together, these data demonstrate that BNGR-A2 and -A34 are ITP receptors and that BNGR-A24 is an ITPL receptor in B. mori.