CYP450s analysis across spiny lobster metamorphosis identifies a long sought missing link in crustacean development

Effects of molting on the expression of ecdysteroid responsive genes in the crustacean molting gland (Y-organ)

Ecdysteroid molting hormones coordinate arthropod growth and development. Binding of 20-hydroxyecdysone (20E) to ecdysteroid receptor EcR/RXR activates a cascade of nuclear receptor transcription factors that mediate tissue responses to hormone. Insect ecdysteroid responsive and Forkhead box class O (FOXO) transcription factor gene sequences were used to extract orthologs from blackback land crab (Gecarcinus lateralis) Y-organ (YO) transcriptome: Gl-Ecdysone Receptor (EcR), Gl-Broad Complex (Br-C), Gl-E74, Gl-Hormone Receptor 3 (HR3), Gl-Hormone Receptor 4 (HR4), Gl-FOXO, and Gl-Fushi tarazu factor-1 (Ftz-f1). Quantitative polymerase chain reaction quantified mRNA levels in tissues from intermolt animals and in YO of animals induced to molt by multiple limb autotomy (MLA) or eyestalk ablation (ESA). Gl-EcR, Gl-Retinoid X Receptor (RXR), Gl-Br-C, Gl-HR3, Gl-HR4, Gl-E74, Gl-E75, Gl-Ftz-f1, and Gl-FOXO were expressed in all 10 tissues, with Gl-Br-C, Gl-E74, Gl-E75, and Gl-HR4 mRNA levels in the YO lower than those in most of the other tissues. In MLA animals, molting had no effect on Gl-Br-C, Gl-E74, and Gl-Ftz-f1 mRNA levels and little effect on Gl-EcR, Gl-E75, and Gl-HR4 mRNA levels. Gl-HR3 and Gl-FOXO mRNA levels were increased during premolt stages, while Gl-RXR mRNA level was highest during intermolt and premolt stages and lowest at postmolt stage. In ESA animals, YO mRNA levels were not correlated with hemolymph ecdysteroid titers. ESA had no effect on Gl-EcR, Gl-E74, Gl-HR3, Gl-HR4, Gl-Ftz-f1, and Gl-FOXO mRNA levels, while Gl-RXR, Gl-Br-C, and Gl-E75 mRNA levels were decreased at 3 days post-ESA. These data suggest that transcriptional up-regulation of Gl-FOXO and Gl-HR3 contributes to increased YO ecdysteroidogenesis during premolt. By contrast, transcriptional regulation of ecdysteroid responsive genes and ecdysteroidogenesis were uncoupled in the YO of ESA animals.

Ontogeny of the Cytochrome P450 Superfamily in the Ornate Spiny Lobster (Panulirus ornatus)

Cytochrome P450s (CYP450s) are a versatile superfamily of enzymes known to undergo rapid evolution. They have important roles across growth and development pathways in crustaceans, although it is difficult to characterise orthologs between species due to their sequence diversity. Conserved CYP450s enzymes in crustaceans are those associated with ecdysteroidogenesis: synthesising and breaking down the active moult hormone, 20-hydroxyecdysone. The complex life cycle of the ornate spiny lobster, Panulirus ornatus, relies on moulting in order to grow and develop. Many of these diverse life stages have been analysed to establish a comprehensive transcriptomic database for this species. The transcripts putatively encoding for CYP450s were mapped using transcriptomic analysis and identified across growth and development stages. With the aid of phylogeny, 28 transcripts of 42 putative P. ornatus CYP450s were annotated, including the well conserved Halloween genes, which are involved in ecdysteroidogenesis. Expression patterns across the life stages determined that only a subset of the CYP450s can be detected in each life stage or tissue. Four Shed transcripts show overlapping expression between metamorphosis and adult tissues, suggesting pleotropic functions of the multiple Shed orthologs within P. ornatus.

Effects of molting on the expression of ecdysteroid biosynthesis genes in the Y-organ of the blackback land crab, Gecarcinus lateralis

A pair of Y-organs (YOs) synthesize ecdysteroids that initiate and coordinate molting processes in decapod crustaceans. The YO converts cholesterol to secreted products through a biosynthetic pathway involving a Rieske oxygenase encoded by Neverland (Nvd) and cytochrome P450 monooxygenases encoded by Halloween genes Spook (Spo; Cyp307a1), Phantom (Phm; Cyp306a1), Disembodied (Dib; Cyp302a1), and Shadow (Sad; Cyp315a1). NAD kinase (NADK) and 5-aminolevulinic acid synthase (ALAS) support ecdysteroid synthesis in insects. A 20-hydroxylase, encoded by Shed in decapods and Shade in insects, converts ecdysone to the active hormone 20-hydroxyecdysone (20E). 20E is inactivated by cytochrome P450 26-hydroxylase (Cyp18a1). Contigs encoding these eight proteins were extracted from a Gecarcinus lateralis YO transcriptome and their expression was quantified by quantitative polymerase chain reaction. mRNA levels of Gl-Spo and Gl-Phm were four orders of magnitude higher in YO than those in nine other tissues, while mRNA levels of Gl-NADK and Gl-ALAS were similar in all ten tissues. In G. lateralis induced to molt by multiple leg autotomy, YO mRNA levels of Gl-Nvd, Gl-Spo, Gl-Phm, Gl-NADK, and Gl-ALAS were highest in intermolt and premolt stages and lower in postmolt. Gl-Dib mRNA level was not affected by molt stage. mRNA level of Gl-Sad, which converts 2-deoxyecdysone to ecdysone, was higher in mid- and late premolt stages, when YO ecdysteroidogenic capacity is greatest. Gl-Cyp18a1 mRNA level was highest in intermolt, decreased in premolt stages, and was lowest in postmolt. In animals induced to molt by eyestalk ablation, YO mRNA levels of all eight genes were not correlated with increased hemolymph 20E titers. These results suggest that YO ecdysteroidogenic genes are differentially regulated at transcriptional and translational levels.

Effects of Vitamin D with Calcium and Associations of Mean 25-Hydroxyvitamin D Levels with 3-Year Change in Muscle Performance in Healthy Older Adults in the Boston STOP IT Trial

The purpose of this analysis was to assess whether (1) daily vitamin D₃ plus calcium supplementation vs. placebo or (2) the mean 25-hydroxyvitamin D [25(OH)D] level achieved during a 3-year trial was associated with muscle performance or balance in the Boston STOP IT study. Methods We conducted exploratory analyses in 386 men and women age 65 years and older who participated in the Boston STOP IT trial and had one or more muscle performance or balance assessments at baseline and 3 years. Participants were treated with 700 IU of vitamin D₃ plus 500 mg of calcium or with double placebo daily for 3 years. Plasma 25(OH)D was measured at baseline, 6, 12, 18, 24, and 36 months; muscle performance (timed walk, grip strength, and chair-rise) and two balance tests, the one-leg stand and tandem stand, were assessed at baseline and 3 years only. Results Supplementation with vitamin D3 and calcium had no favorable effect on any muscle performance measure. The 3-year mean 25(OH)D levels were 22.7 ± 6.3 (SD) in the placebo and 30.8 ± 7.5 ng/ml in the supplemented groups (p < 0.001). The 3-year mean 25(OH)D level was positively associated with change in one-leg stand time (p = 0.04), but not with the other measures. Conclusion Vitamin D3 and calcium supplementation had no favorable effect on muscle performance or balance in this relatively healthy older population. A higher 3-year mean 25(OH)D level may favor balance, as indicated by longer one-leg stand time, but this observation should be confirmed.

De novo transcriptome assemblies of red king crab (Paralithodes camtschaticus) and snow crab (Chionoecetes opilio) molting gland and eyestalk ganglia - Temperature effects on expression of molting and growth regulatory genes in adult red king crab

Red king crab (Paralithodes camtschaticus) and snow crab (Chionoecetes opilio) are deep-sea crustaceans widely distributed in the North Pacific and Northwest Atlantic Oceans. These giant predators have invaded the Barents Sea over the past decades, and climate-driven temperature changes may influence their distribution and abundance in the sub-Arctic region. Molting and growth in crustaceans are strongly affected by temperature, but the underlying molecular mechanisms are little known, particularly in cold-water species. Here, we describe multiple regulatory factors in the two high-latitude crabs by developing de novo transcriptomes from the molting gland (Y-organ or YO) and eye stalk ganglia (ESG), in addition to the hepatopancreas and claw muscle of red king crab. The Halloween genes encoding the ecdysteroidogenic enzymes were expressed in YO, and the ESG contained multiple neuropeptides, including molt-inhibiting hormone (MIH), crustacean hyperglycemic hormone (CHH), and ion-transport peptide (ITP). Both crabs expressed a diversity of growth-related factors, such as mTOR, AKT, Rheb and AMPKα, and stress-responsive factors, including multiple heat shock proteins (HSPs). Temperature effects on the expression of key regulatory genes were quantified by qPCR in adult red king crab males kept at 4 °C or 10 °C for two weeks during intermolt. The Halloween genes tended to be upregulated in YO at high temperature, while the ecdysteroid receptor and several growth regulators showed tissue-specific responses to elevated temperature. Constitutive and heat-inducible HSPs were expressed in an inverse temperature-dependent manner, suggesting that adult red king crabs can acclimate to increased water temperatures.

Identification and characterization of expression profiles of neuropeptides and their GPCRs in the swimming crab, Portunus trituberculatus

Neuropeptides and their G protein-coupled receptors (GPCRs) regulate multiple physiological processes. Currently, little is known about the identity of native neuropeptides and their receptors in Portunus trituberculatus. This study employed RNA-sequencing and reverse transcription-polymerase chain reaction (RT-PCR) techniques to identify neuropeptides and their receptors that might be involved in regulation of reproductive processes of P. trituberculatus. In the central nervous system transcriptome data, 47 neuropeptide transcripts were identified. In further analyses, the tissue expression profile of 32 putative neuropeptide-encoding transcripts was estimated. Results showed that the 32 transcripts were expressed in the central nervous system and 23 of them were expressed in the ovary. A total of 47 GPCR-encoding transcripts belonging to two classes were identified, including 39 encoding GPCR-A family and eight encoding GPCR-B family. In addition, we assessed the tissue expression profile of 33 GPCRs (27 GPCR-As and six GPCR-Bs) transcripts. These GPCRs were found to be widely expressed in different tissues. Similar to the expression profiles of neuropeptides, 20 of these putative GPCR-encoding transcripts were also detected in the ovary. This is the first study to establish the identify of neuropeptides and their GPCRs in P. trituberculatus, and provide information for further investigations into the effect of neuropeptides on the physiology and behavior of decapod crustaceans.

The genome of the European estuarine calanoid copepod Eurytemora affinis: Potential use in molecular ecotoxicology

In this study, we sequenced and assembled the genome of a European estuarine calanoid copepod using Oxford Nanopore PromethION and Illumina HiSeq 2500 platforms. The length of the assembled genome was 776.1 Mb with N50 = 474.9 kb (BUSCO 85.9%), and the genome consisted of 2473 contigs. A total of 18,014 genes were annotated and orthologous gene clusters were analyzed in comparison to other copepods. In addition, genome-wide identification of cytochrome P450s, glutathione S-transferases, and ATP-binding cassette transporters in E. affinis was performed to determine gene repertoire of these detoxification-related gene families. Results revealed the presence of species-specific gene inventories, indicating that these gene families have evolved through species-specific gene loss/expansion processes, possibly due to adaptation to different environmental stressors. Our study provides a new inventory of the European estuarine calanoid copepod E. affinis genome with emphasis on phase I, II, and III detoxification systems.

Characterization of Shed genes encoding ecdysone 20-monooxygenase (CYP314A1) in the Y-organ of the blackback land crab, Gecarcinus lateralis

Molting in decapod crustaceans is controlled by ecdysteroid hormones synthesized and secreted by the molting gland, or Y-organ (YO). Halloween genes encode cytochrome P450 enzymes in the ecdysteroid synthetic pathway. The current paradigm is that YOs secrete an inactive precursor (e.g., ecdysone or E), which is hydroxylated at the #20 carbon to form an active hormone (20-hydroxyecdysone or 20E) by a mitochonrial 20-monooxygenase (CYP314A1) in peripheral tissues. 20-Monooxygenase is encoded by Shed in decapods and Shade in insects. We used eastern spiny lobster Shed sequences to extract six orthologs in the G. lateralis YO transcriptome. Phylogenetic analysis of the deduced amino acid sequences from six decapod species organized the Sheds into seven classes (Sheds 1-7), resulting in the assignment of the G. lateralis Sheds to Gl-Shed1, 2, 4A, 4B, 5A, and 5B. The mRNA levels of the six Gl-Sheds in the YO of intermolt animals were comparable to those in nine other tissues that included hepatopancreas and muscle. qPCR was used to compare the effects of molt induction by multiple leg autotomy (MLA) and eyestalk ablation (ESA) on Gl-Shed mRNA levels in the YO. Molt stage had little effect on Gl-Shed1 and Gl-Shed5B expression in the YO of MLA animals. Gl-Shed5A was expressed at the highest mRNA levels in the YO and was significantly increased during early and mid premolt stages. By contrast, ESA ± SB431542 had no effect on Gl-Shed expression at 1, 3, 5, and 7 days post-ESA. SB431542, which inhibits Transforming Growth Factor-β/activin signaling and blocks YO commitment, decreased Gl-Shed2 and Gl-Shed4A mRNA levels at 14 days post-ESA. A targeted metabolomic analysis showed that YOs cultured in vitro secreted E and 20E to the medium. These data suggest that the YO expresses 20-monooygenases that can convert E to 20E, which may contribute to the increase in active hormone in the hemolymph during premolt.

Diversity and evolution of the P450 family in arthropods

The P450 family (CYP genes) of arthropods encodes diverse enzymes involved in the metabolism of foreign compounds and in essential endocrine or ecophysiological functions. The P450 sequences (CYPome) from 40 arthropod species were manually curated, including 31 complete CYPomes, and a maximum likelihood phylogeny of nearly 3000 sequences is presented. Arthropod CYPomes are assembled from members of six CYP clans of variable size, the CYP2, CYP3, CYP4 and mitochondrial clans, as well as the CYP20 and CYP16 clans that are not found in Neoptera. CYPome sizes vary from two dozen genes in some parasitic species to over 200 in species as diverse as collembolans or ticks. CYPomes are comprised of few CYP families with many genes and many CYP families with few genes, and this distribution is the result of dynamic birth and death processes. Lineage-specific expansions or blooms are found throughout the phylogeny and often result in genomic clusters that appear to form a reservoir of catalytic diversity maintained as heritable units. Among the many P450s with physiological functions, six CYP families are involved in ecdysteroid metabolism. However, five so-called Halloween genes are not universally represented and do not constitute the unique pathway of ecdysteroid biosynthesis. The diversity of arthropod CYPomes has only partially been uncovered to date and many P450s with physiological functions regulating the synthesis and degradation of endogenous signal molecules (including ecdysteroids) and semiochemicals (including pheromones and defense chemicals) remain to be discovered. Sequence diversity of arthropod P450s is extreme, and P450 sequences lacking the universally conserved Cys ligand to the heme have evolved several times. A better understanding of P450 evolution is needed to discern the relative contributions of stochastic processes and adaptive processes in shaping the size and diversity of CYPomes.

Ecdysis triggering hormone modulates molt behaviour in the redclaw crayfish Cherax quadricarinatus, providing a mechanistic evidence for conserved function in molt regulation across Pancrustacea

Molting enables growth and development across ecdysozoa. The molting process is strictly controlled by hormones - ecdysteroids. Ecdysteroidogenesis occurs in theprothoracic glands and stimulated by prothoracicotropic hormone in insects, while it ensues in the Y-organ and regulated by the molt inhibiting hormone in crustaceans. A peak in ecdysteroids in the hemolymph induces a cascade of multiple neuropeptides including Ecdysis Triggering Hormone (ETH) and Corazonin. The role of ETH is well defined in controlling the molt process in insects, but it is yet to be defined in crustaceans. In this study, we investigated the behavioral response of intermolt crayfish to ETH and Corazonin injections as well as the impact of ETH on the molt period using in vivo assays. Injection of Corazonin and ETH resulted in a clear and immediate eye twitching response to these two neuropeptides. The Corazonin injection induced eye twitching in slow and asynchronous manner, while ETH injection caused eye twitching in a relatively fast and synchronous way. A single injection of ETH to crayfish resulted in a remarkable prolong molt period, at twice the normal molting cycle, suggesting that ETH plays a key role in controlling the molt cycle in decapod crustaceans. Given the key significance of ETH in molt regulation and its plausible application in pest control, we characterized ETH across the pancrustacean orders. Bioinformatic analysis shows the mature ETH sequence is identical in all studied decapod species. ETH can be classified into specific groups based on the associated motif in each insect order and shows an insect motif -KxxPRx to be conserved in crustaceans.

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.

Expression and functional analysis of cytochrome P450 genes in the wolf spider Pardosa pseudoannulata under cadmium stress

Cytochrome P450 enzymes (CYPs), encoded by Halloween genes, mediate the biosynthesis of molting hormone, ecdysteroids, in arthropods. In this report, the effect of heavy metal cadmium (Cd) stress on the expression of cytochrome P450 genes in the wolf spider Pardosa pseudoannulata was analyzed. The results showed the expression levels of genes encoding for Cd transporters including ABC transporters, zinc transporters, calcium channel proteins and calcium binding proteins were inhibited or induced by Cd stress. In addition, the increase in metallothionein (MT) content and glutathione peroxidase (GPX) activity and decrease in total acetylcholine esterase (AChE) activity were also detected. Apparently, these detoxification methods did not completely protect the spider from the cytotoxicity of Cd stress. Increased mortality of P. pseudoannulata was observed when they were under Cd tress. In total 569 CYP genes belonging to 62 CYP subfamilies were obtained from P. pseudoannulata RNA-seq databases. BlaxtX analysis showed that 150, 161, 11, and 40 CYP genes were similar to the genes dib, phm, sad and shd, respectively, which are thought to catalyze the biosynthesis of ecdysteroids. Gene expression analysis suggested that 25 dib encoding genes, 27 phm encoding genes, 2 sad encoding genes, and 6 shd encoding genes were differentially expressed in TS2 vs. S2 comparison (Cd-treated 2nd instar spider vs. 2nd instar spider), respectively. There were 70 dib, 70 phm and 19 shd encoding genes either upregulated or downregulated, while 3 sad encoding genes were upregulated in TS5 vs. S5 (Cd-treated 5nd instar spider vs. 5nd instar spider). Genes related to heme binding and essential for activating the CYPs were also differentially expressed. Expression levels of cuticle related genes were significant differentially expressed, implying the changes in activities of chitin synthases and chitinase. Therefore we assume that unsuccessful molting process may occur on P. pseudoannulata due to influenced ecdysteroids levels, thus increasing mortality of spider.

Crustacean larval factor shares structural characteristics with the insect-specific follicle cell protein

Literature on the cuticle formation in larval stages of the diverse group of decapod crustaceans is lacking, as opposed to a wealth of knowledge in several insect groups. Here we provide the first glimpse of the cuticular organisation in larvae of the eastern spiny lobster Sagmariasus verreauxi. A bioinformatic approach applied to S. verreauxi transcriptome through metamorphosis identified for the first time a small secreted protein with multiple isoforms that is highly expressed in crustacean larvae. This protein, named crustacean larval factor (Clf) shares structural characteristics with insect follicle cell protein 3 (FCP3), an insect-specific, rapidly evolving protein, with spatial-temporal regulated expression that is restricted to follicular cells during the production of the vitellin coat. Furthermore, we identified the FCP3 domain in additional structural proteins in multiple arthropod groups. Recombinant Clf inhibited in vitro calcium carbonate crystalline precipitation, in keeping with the finding that the spiny lobster larval cuticle is mainly composed of amorphous calcium carbonate. In addition, the recombinant Clf was shown to bind chitosan. Taken together, this research identifies two novel structural domains with lineage-specific expansion across arthropods. In crustaceans, Clf is found predominantly in larvae and the spatial-temporal regulated FCP3 factor occurs as a domain identified in multiple structural proteins across arthropods. Given the shared ten cysteines backbone between the Clf and FCP domains, a shared evolution is suggested and should be further explored.

Mining insect genomes for functionally affiliated genes

Several hundred insect genome assemblies are already publicly available, and this total grows on a weekly basis. A major challenge now confronting insect science is how best to use genomic data to improve our understanding of insect biology. We consider a framework for genome analysis based on functional affiliation, that is, groups of genes involved in the same biological process or pathway, and explore how such an approach furthers our understanding of several aspects of insect phenotype. We anticipate that this approach will prove useful for future research across the breadth of insect studies, whatever organism or trait it involves.

Characterization of G-protein coupled receptors from the blackback land crab Gecarcinus lateralis Y organ transcriptome over the molt cycle

BACKGROUND

G-protein coupled receptors (GPCRs) are ancient, ubiquitous, constitute the largest family of transducing cell surface proteins, and are integral to cell communication via an array of ligands/neuropeptides. Molt inhibiting hormone (MIH) is a key neuropeptide that controls growth and reproduction in crustaceans by regulating the molt cycle. It inhibits ecdysone biosynthesis by a pair of endocrine glands (Y-organs; YOs) through binding a yet uncharacterized GPCR, which triggers a signalling cascade, leading to inhibition of the ecdysis sequence. When MIH release stops, ecdysone is synthesized and released to the hemolymph. A peak in ecdysone titer is followed by a molting event. A transcriptome of the blackback land crab Gecarcinus lateralis YOs across molt was utilized in this study to curate the list of GPCRs and their expression in order to better assess which GPCRs are involved in the molt process.

RESULTS

Ninety-nine G. lateralis putative GPCRs were obtained by screening the YO transcriptome against the Pfam database. Phylogenetic analysis classified 49 as class A (Rhodopsin-like receptor), 35 as class B (Secretin receptor), and 9 as class C (metabotropic glutamate). Further phylogenetic analysis of class A GPCRs identified neuropeptide GPCRs, including those for Allatostatin A, Allatostatin B, Bursicon, CCHamide, FMRFamide, Proctolin, Corazonin, Relaxin, and the biogenic amine Serotonin. Three GPCRs clustered with recently identified putative CHH receptors (CHHRs), and differential expression over the molt cycle suggests that they are associated with ecdysteroidogenesis regulation. Two putative Corazonin receptors showed much higher expression in the YOs compared with all other GPCRs, suggesting an important role in molt regulation.

CONCLUSIONS

Molting requires an orchestrated regulation of YO ecdysteroid synthesis by multiple neuropeptides. In this study, we curated a comprehensive list of GPCRs expressed in the YO and followed their expression across the molt cycle. Three putative CHH receptors were identified and could include an MIH receptor whose activation negatively regulates molting. Orthologs of receptors that were found to be involved in molt regulation in insects were also identified, including LGR3 and Corazonin receptor, the latter of which was expressed at much higher level than all other receptors, suggesting a key role in YO regulation.

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.