Molt-inhibiting hormone stimulates vitellogenesis at advanced ovarian developmental stages in the female blue crab, Callinectes sapidus 2: novel specific binding sites in hepatopancreas and cAMP as a second messenger

Activation and characterization of G protein-coupled receptors for CHHs in the mud crab, Scylla paramamosain

Crustacean hyperglycemic hormone (CHH) superfamily peptides constitute a group of neurohormones, including the crustacean hyperglycemic hormone (CHH), molt-inhibiting hormone (MIH), and gonad-inhibiting hormone (GIH) or vitellogenesis-inhibiting hormone (VIH), which reportedly play an essential role in regulating various biological activities by binding to their receptors in crustaceans. Although bioinformatics analyses have identified G protein-coupled receptors (GPCRs) as potential CHH receptors, no validation through binding experiments has been carried out. This study employed a eukaryotic expression system, HEK293T cell transient transfection, and ligand-receptor interaction tests to identify the GPCRs of CHHs in the mud crab Scylla paramamosain. We found that four GPCRs (Sp-GPCR-A34-A37) were activated by their corresponding CHHs (Sp-CHH1-v1, Sp-MIH, Sp-VIH) in a dose-dependent manner. Of these, Sp-GPCR-A34 was exclusively activated by Sp-VIH; Sp-GPCR-A35 was activated by Sp-CHH1-v1 and Sp-VIH, respectively; Sp-GPCR-A36 was activated by Sp-CHH1-v1 and Sp-MIH; Sp-GPCR-A37 was exclusively activated by Sp-MIH. The half-maximal effective concentration (EC50) values for all CHHs/GPCRs pairs (both Ca2+ and cAMP signaling) were in the nanomolar range. Overall, our study provided hitherto undocumented evidence of the presence of G protein-coupled receptors of CHH in crustaceans, providing the foothold for further studies on the signaling pathways of CHHs and their corresponding GPCRs.

In silico analysis of crustacean hyperglycemic hormone family G protein-coupled receptor candidates

Ecdysteroid molting hormone synthesis is directed by a pair of molting glands or Y-organs (YOs), and this synthesis is inhibited by molt-inhibiting hormone (MIH). MIH is a member of the crustacean hyperglycemic hormone (CHH) neuropeptide superfamily, which includes CHH and insect ion transport peptide (ITP). It is hypothesized that the MIH receptor is a Class A (Rhodopsin-like) G protein-coupled receptor (GPCR). The YO of the blackback land crab, Gecarcinus lateralis, expresses 49 Class A GPCRs, three of which (Gl-CHHR-A9, -A10, and -A12) were provisionally assigned as CHH-like receptors. CrusTome, a transcriptome database assembled from 189 crustaceans and 12 ecdysozoan outgroups, was used to deorphanize candidate MIH/CHH GPCRs, relying on sequence homology to three functionally characterized ITP receptors (BNGR-A2, BNGR-A24, and BNGR-A34) in the silk moth, Bombyx mori. Phylogenetic analysis and multiple sequence alignments across major taxonomic groups revealed extensive expansion and diversification of crustacean A2, A24, and A34 receptors, designated CHH Family Receptor Candidates (CFRCs). The A2 clade was divided into three subclades; A24 clade was divided into five subclades; and A34 was divided into six subclades. The subclades were distinguished by conserved motifs in extracellular loop (ECL) 2 and ECL3 in the ligand-binding region. Eleven of the 14 subclades occurred in decapod crustaceans. In G. lateralis, seven CFRC sequences, designated Gl-CFRC-A2α1, -A24α, -A24β1, -A24β2, -A34α2, -A34β1, and -A34β2, were identified; the three A34 sequences corresponded to Gl-GPCR-A12, -A9, and A10, respectively. ECL2 in all the CFRC sequences had a two-stranded β-sheet structure similar to human Class A GPCRs, whereas the ECL2 of decapod CFRC-A34β1/β2 had an additional two-stranded β-sheet. We hypothesize that this second β-sheet on ECL2 plays a role in MIH/CHH binding and activation, which will be investigated further with functional assays.

Molecular Characterization Related to Ovary Early Development Mechanisms after Eyestalk Ablation in Exopalaemon carinicauda

Eyestalk ablation is an effective method to promote ovarian development in crustaceans. Herein, we performed transcriptome sequencing of ovary and hepatopancreas tissues after eyestalk ablation in Exopalaemon carinicauda to identify genes related to ovarian development. Our analyses led to the identification of 97,383 unigenes and 190,757 transcripts, with an average N50 length of 1757 bp. In the ovary, four pathways related to oogenesis and three related to oocyte rapid growth were enriched. In the hepatopancreas, two vitellogenesis-associated transcripts were identified. Furthermore, short time-series expression miner (STEM) and gene ontology (GO) enrichment analyses revealed five terms related to gamete generation. In addition, two-color fluorescent in situ hybridization results suggested that dmrt1 might play a vital role in oogenesis during the early stage of ovarian development. Overall, our insights should support future studies focusing on investigating oogenesis and ovarian development in E. carinicauda.

Eyestalk neuropeptide identification in the female red deep-sea crab, Chaceon quinquedens

Eyestalk-derived neuropeptides, primarily the crustacean hyperglycemic hormone (CHH) neuropeptide family, regulate vitellogenesis in decapod crustaceans. The red deep-sea crab, Chaceon quinquedens, a cold-water species inhabiting depths between 200 and 1800 m, has supported a small fishery, mainly harvesting adult males in the eastern US for over 40 years. This study aimed to understand the role of eyestalk-neuropeptides in vitellogenesis in C. quinquedens with an extended intermolt stage. Chromatography shows two CHH and one MIH peak in the sinus gland, with a CHH2 peak area four times larger than CHH1. The cDNA sequence of MIH and CHH of C. quinquedens is isolated from the eyestalk ganglia, and the qPCR assay shows MIH is significantly higher only at ovarian stages 3 than 4 and 5. However, MIH transcript and its neuropeptides do differ between stages 1 and 3. While CHH transcripts remain constant, its neuropeptide levels are higher at stages 3 than 1. Additionally, transcriptomic analysis of the de novo eyestalk ganglia assembly at ovarian stages 1 and 3 found 28 eyestalk neuropeptides. A GIH/VIH or GSH/VSH belonging to the CHH family is absent in the transcriptome. Transcripts per million (TPM) values of ten neuropeptides increase by 1.3 to 2.0-fold at stage 3 compared to stage 1: twofold for Bursicon α, followed by CHH, AKH/corazonin-like, Pyrokinin, CCAP, Glycoprotein B, PDH1, and IDLSRF-like peptide, and 1.3-fold of allatostatin A and short NP-F. WXXXRamide, the only downregulated neuropeptide, decreases TPM by ∼ 2-fold at stage 3, compared to stage 1. Interestingly, neuroparsin with the highest TPM values remains the same in stages 1 and 3. The mandibular organ-inhibiting hormone is not found in de novo assembly. We report that CHH, MIH, and eight other neuropeptides may play a role in vitellogenesis in this species.

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.

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.

Chromosome-level genome assembly of the blue crab, Callinectes sapidus

The blue crab, Callinectes sapidus (Rathbun, 1896) is an economically, culturally, and ecologically important species found across the temperate and tropical North and South American Atlantic coast. A reference genome will enable research for this high-value species. Initial assembly combined 200× coverage Illumina paired-end reads, a 60× 8 kb mate-paired library, and 50× PacBio data using the MaSuRCA assembler resulting in a 985 Mb assembly with a scaffold N50 of 153 kb. Dovetail Chicago and HiC sequencing with the 3d DNA assembler and Juicebox assembly tools were then used for chromosome scaffolding. The 50 largest scaffolds span 810 Mb are 1.5-37 Mb long and have a repeat content of 36%. The 190 Mb unplaced sequence is in 3921 sequences over 10 kb with a repeat content of 68%. The final assembly N50 is 18.9 Mb for scaffolds and 9317 bases for contigs. Of arthropod BUSCO, ∼88% (888/1013) were complete and single copies. Using 309 million RNAseq read pairs from 12 different tissues and developmental stages, 25,249 protein-coding genes were predicted. Between C. sapidus and Portunus trituberculatus genomes, 41 of 50 large scaffolds had high nucleotide identity and protein-coding synteny, but 9 scaffolds in both assemblies were not clear matches. The protein-coding genes included 9423 one-to-one putative orthologs, of which 7165 were syntenic between the two crab species. Overall, the two crab genome assemblies show strong similarities at the nucleotide, protein, and chromosome level and verify the blue crab genome as an excellent reference for this important seafood species.

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.

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.

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.

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.

Mechanisms for type-II vitellogenesis-inhibiting hormone suppression of vitellogenin transcription in shrimp hepatopancreas: Crosstalk of GC/cGMP pathway with different MAPK-dependent cascades

Vitellogenesis is the process of yolk formation via accumulating vitellin (Vn) with nutrients in the oocytes. Expression of vitellogenin (Vg), the precursor of Vn, is one of the indicators for the start of vitellogenesis. In Pacific white shrimp (Litopenaeus vannamei), the type-II vitellogenesis-inhibiting hormone (VIH-2) effectively suppresses hepatopancreatic Vg mRNA expression. In this study, we demonstrate the increasing transcript levels of hepatopancreatic Vg during L. vannamei ovarian development, suggesting that the hepatopancreas-derived Vg/Vn may also contribute to vitellogenesis in this species. Using a combination of in vivo injections and in vitro primary cell cultures, we provide evidences that the inhibition of VIH-2 on hepatopancreatic Vg gene expression is mediated through a functional coupling of the GC/cGMP pathway with different MAPK-dependent cascades in female shrimp. In VIH-2 signaling, the NO-independent GC/cGMP/PKG cascades were upstream of the MAPKs. Activations of the MAPK signal by VIH-2 include the phosphorylation of JNK and the mRNA/protein expression of P³⁸MAPK. Additionally, the cAMP/PKA pathway is another positive intracellular signal for hepatopancreatic Vg mRNA expression but is independent of its VIH-2 regulation. Our findings establish a model for the signal transduction mechanism of Vg regulation by VIH and shed light on the biological functions and signaling of the CHH family in crustaceans.

Localization and expression of molt-inhibiting hormone and nitric oxide synthase in the central nervous system of the green shore crab, Carcinus maenas, and the blackback land crab, Gecarcinus lateralis

In decapod crustaceans, molting is controlled by the pulsatile release of molt-inhibiting hormone (MIH) from neurosecretory cells in the X-organ/sinus gland (XO/SG) complex in the eyestalk ganglia (ESG). A drop in MIH release triggers molting by activating the molting gland or Y-organ (YO). Post-transcriptional mechanisms ultimately control MIH levels in the hemolymph. Neurotransmitter-mediated electrical activity controls Ca²⁺-dependent vesicular release of MIH from the SG axon terminals, which may be modulated by nitric oxide (NO). In green shore crab, Carcinus maenas, nitric oxide synthase (NOS) protein and NO are present in the SG. Moreover, C. maenas are refractory to eyestalk ablation (ESA), suggesting other regions of the nervous system secrete sufficient amounts of MIH to prevent molting. By contrast, ESA induces molting in the blackback land crab, Gecarcinus lateralis. Double-label immunofluorescence microscopy and quantitative polymerase chain reaction were used to localize and quantify MIH and NOS proteins and transcripts, respectively, in the ESG, brain, and thoracic ganglion (TG) of C. maenas and G. lateralis. In ESG, MIH- and NOS-immunopositive cells were closely associated in the SG of both species; confocal microscopy showed that NOS was localized in cells adjacent to MIH-positive axon terminals. In brain, MIH-positive cells were located in a small number of cells in the olfactory lobe; no NOS immunofluorescence was detected. In TG, MIH and NOS were localized in cell clusters between the segmental nerves. In G. lateralis, Gl-MIH and Gl-crustacean hyperglycemic hormone (CHH) mRNA levels were ~10⁵-fold higher in ESG than in brain or TG of intermolt animals, indicating that the ESG is the primary source of these neuropeptides. Gl-NOS and Gl-elongation factor (EF2) mRNA levels were also higher in the ESG. Molt stage had little or no effect on CHH, NOS, NOS-interacting protein (NOS-IP), membrane Guanylyl Cyclase-II (GC-II), and NO-independent GC-III expression in the ESG of both species. By contrast, MIH and NO receptor GC-I beta subunit (GC-Iβ) transcripts were increased during premolt and postmolt stages in G. lateralis, but not in C. maenas. MIH immunopositive cells in the brain and TG may be a secondary source of MIH; the release of MIH from these sources may contribute to the difference between the two species in response to ESA. The MIH-immunopositive cells in the TG may be the source of an MIH-like factor that mediates molt inhibition by limb bud autotomy. The association of MIH- and NOS-labeled cells in the ESG and TG suggests that NO may modulate MIH release. A model is proposed in which NO-dependent activation of GC-I inhibits Ca²⁺-dependent fusion of MIH vesicles with the nerve terminal membrane; the resulting decrease in MIH activates the YO and the animal enters premolt.

Ecdysteroids regulate the levels of Molt-Inhibiting Hormone (MIH) expression in the blue crab, Callinectes sapidus

Arthropod molt is coordinated through the interplay between ecdysteroids and neuropeptide hormones. In crustaceans, changes in the activity of Y-organs during the molt cycle have been regulated by molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH). Little has been known of the mode of direct effects of ecdysteroids on the levels of MIH and CHH in the eyestalk ganglia during the molt cycle. This study focused on a putative feedback of ecdysteroids on the expression levels of MIH transcripts using in vitro incubation study with ecdysteroids and in vivo RNAi in the blue crab, Callinectes sapidus. Our results show a specific expression of ecdysone receptor (EcR) in which EcR1 is the major isoform in eyestalk ganglia. The initial elevation of MIH expression at the early premolt stages is replicated by in vitro incubations of eyestalk ganglia with ecdysteroids that mimic the intrinsic conditions of D₀ stage: the concentration (75 ng/ml) and composition (ponasterone A and 20-hydroxyecdysone at a 3:1 (w:w) ratio). Additionally, multiple injections of EcR1-dsRNA reduce MIH expression by 67%, compared to the controls. Our data provide evidence on a putative feedback mechanism of hormonal regulation during molting cycle, specifically how the molt cycle is repeated during the life cycle of crustaceans. The elevated concentrations of ecdysteroids at early premolt stage may act positively on the levels of MIH expression in the eyestalk ganglia. Subsequently, the increased MIH titers in the hemolymph at postmolt would inhibit the synthesis and release of ecdysteroids by Y-organs, resulting in re-setting the subsequent molt cycle.

Changes in ecdysteroid levels and expression patterns of ecdysteroid-responsive factors and neuropeptide hormones during the embryogenesis of the blue crab, Callinectes sapidus

Embryogenesis requires the involvement and coordination of multiple networks of various genes, according to a timeline governing development. Crustacean embryogenesis usually includes the first molt, a process that is known to be positively controlled by ecdysteroids. We determined the amounts of ecdysteroids, as well as other related factors: the ecdysone receptor (CasEcR), the retinoid X receptor (CasRXR), the molt-inhibiting hormone (CasMIH), and crustacean hyperglycemic hormone (CasCHH) during the ovarian and embryonic developments of Callinectes sapidus. In summary, the ovaries at stages 1-4 have expression levels of maternal CasEcR and CasRXR 10-50 times higher than levels seen in embryos at the yolk stage. This large difference in the amount of the these factors in C. sapidus ovaries suggests that these maternal ecdysteroid-responsive factors may be utilized at the initiation of embryogenesis. During embryogenesis, the changes in total ecdysteroids and levels of CasEcR and CasRXR expression are similar to those observed in juvenile molts. The full-length cDNA sequence of the C. sapidus BTB domain protein (CasBTBDP) initially isolated from Y-organ cDNA, contains only Broad-Complex, Tramtrack, and Bric a brac (BTB) domains. The levels of CasBTBDP are kept constant throughout embryogenesis. The expression profiles of CasMIH and CasCHH are similar to the titers of ecdysteroids. However, the timing of their appearance is followed by increases in CasEcRs and CasRXRs, implying that the expressions of these neuropeptides may be influenced by ecdysteroids. Moreover, the ecdysteroid profile during embryogenesis may track directly with the timing of organogenesis of Y-organs and their activity. Our work reports, for first time, the observed expression and changes of ecdysteroid-responsive factors, along with CasCHH and CasMIH, during embryogenesis in the crustacean C. sapidus.

Stimulation of molt by RNA interference of the molt-inhibiting hormone in the crayfish Cherax quadricarinatus

In crustaceans, molting is known to be under the control of neuropeptide hormones synthesized and secreted from the eyestalk ganglia. While the role of molt-inhibiting hormone (MIH) in regulating molting has been described in several species using classical methods, an in vivo specific MIH targeted manipulation has not been described yet. In the present study, an MIH cDNA was isolated and sequenced from the eyestalk ganglia of the Australian freshwater red claw crayfish Cherax quadricarinatus (Cq) by 5' and 3' RACE. We analyzed the putative Cq-MIH based on sequence homology, a three dimensional structure model and transcript's tissue specificity. We further examined the involvement of Cq-MIH in the control of molt in the crayfish through RNAi by in vivo injections of Cq-MIH double-stranded RNA, which resulted in, similarly to eyestalk ablation, acceleration of molt cycles. This acceleration was reflected by a significant reduction (up to 32%) in molt interval and an increased rate in molt mineralization index (MMI), which correlated with the induction of ecdysteroid hormones compared to control. Altogether, this study provides a proof of function for the involvement of the Cq-MIH gene in molt regulation in the crayfish.

The CHH-superfamily of multifunctional peptide hormones controlling crustacean metabolism, osmoregulation, moulting, and reproduction

Apart from providing an up-to-date review of the literature, considerable emphasis was placed in this article on the historical development of the field of "crustacean eyestalk hormones". A role of the neurosecretory eyestalk structures of crustaceans in endocrine regulation was recognized about 80 years ago, but it took another half a century until the first peptide hormones were identified. Following the identification of crustacean hyperglycaemic hormone (CHH) and moult-inhibiting hormone (MIH), a large number of homologous peptides have been identified to this date. They comprise a family of multifunctional peptides which can be divided, according to sequences and precursor structure, into two subfamilies, type-I and -II. Recent results on peptide sequences, structure of genes and precursors are described here. The best studied biological activities include metabolic control, moulting, gonad maturation, ionic and osmotic regulation and methyl farnesoate synthesis in mandibular glands. Accordingly, the names CHH, MIH, and GIH/VIH (gonad/vitellogenesis-inhibiting hormone), MOIH (mandibular organ-inhibiting hormone) were coined. The identification of ITP (ion transport peptide) in insects showed, for the first time, that CHH-family peptides are not restricted to crustaceans, and data mining has recently inferred their occurrence in other ecdysozoan clades as well. The long-held tenet of exclusive association with the eyestalk X-organ-sinus gland tract has been challenged by the finding of several extra nervous system sites of expression of CHH-family peptides. Concerning mode of action and the question of target tissues, second messenger mechanisms are discussed, as well as binding sites and receptors. Future challenges are highlighted.

Cloning of an insulin-like androgenic gland factor (IAG) from the blue crab, Callinectes sapidus: implications for eyestalk regulation of IAG expression

In malacostracan crustaceans, sex differentiation is uniquely regulated by a hormone secreted by the male-specific androgenic gland (AG). An isopod AG hormone was the first to be structurally elucidated and was found to belong to the insulin superfamily of proteins. Recently, it has been found that the AGs of several decapod crustaceans express insulin-like androgenic gland factors (IAGs), whose function is believed to be similar to that of the isopod AG hormone. Here we report the isolation from the blue crab Callinectes sapidus of the full-length cDNA encoding a candidate insulin-like AG hormone, termed Cas-IAG. The predicted protein Cas-IAG was encoded as a precursor consisting of a signal peptide, the B chain, the C peptide, and the A chain in that order. While the AG was the main source of Cas-IAG expression, as found in other decapod species, the hepatopancreas of male Callinectes sapidus crabs displayed minor Cas-IAG expression. Eyestalk ablation confirmed the presence of a possible endocrine axis between the eyestalk ganglia and the AG, implying that Cas-IAG expression is negatively regulated by (a) substance(s) present in the eyestalk ganglia.

Involvement of cAMP and calmodulin in endocytic yolk uptake during Xenopus laevis oogenesis

The aim of the present study was to show the participation and physiological role of calmodulin (CaM) and cAMP during vitellogenin endocytic uptake in the amphibian Xenopus laevis. The results showed a differential distribution of CaM in the ovary follicles during oogenesis. The CaM intracellular localization was not affected by gap junction's downregulation and CaM inhibition did not completely abolished the endocytic activity of oocytes. We showed that cAMP was able to completely rescue the endocytic competence in follicles in which gap junctional communication had been disrupted by octanol. Moreover cAMP was capable of restoring oocyte endocytic capability in the presence of octanol and stelazine, a CaM inhibitor. We propose that, in Vtg uptake regulation, cAMP is upstream of CaM during the endocytic signalling pathway.

Crustacean hyperglycemic hormone (CHH) neuropeptidesfamily: Functions, titer, and binding to target tissues

The removal of the eyestalk (s) induces molting and reproduction promoted the presence of regulatory substances in the eyestalk (ES), particularly medulla terminalis X-organ and the sinus gland (MTXO-SG). The PCR-based cloning strategies have allowed for isolating a great number of cDNAs sequences of crustacean hyperglycemic hormone (CHH) neuropeptides family from the eyestalk and non-eyestalk tissues, e.g., pericardial organs and fore- and hindguts. However, the translated corresponding neuropeptides in these tissues, their circulating concentrations, the mode of actions, and specific physiological functions have not been well described. The profiles of CHH neuropeptides present in the MTXO-SG may differ among decapod crustacean species, but they can be largely divided into two sub-groups on the basis of structural homology: (1) CHH and (2) molt-inhibiting hormone (MIH)/mandibular organ-inhibiting hormone (MOIH)/vitellogenesis/gonad-inhibiting hormone (V/GIH). CHH typically elevating the level of circulating glucose from animals under stressful conditions (hyper- and hypothermia, hypoxia, and low salinity) has multiple target tissues and functions such as ecdysteroidogenesis, osmoregulation, and vitellogenesis. Recently, MIH, known for exclusively suppressing ecdysteroidogenesis in Y-organs, is also reported to have an additional role in vitellogenesis of adult female crustacean species, suggesting that some CHH neuropeptides may acquire an extra regulatory role in reproduction at adult stage. This paper reviews the regulatory roles of CHH and MIH at the levels of specific functions, temporal and spatial expression, titers, their binding sites on the target tissues, and second messengers from two crab species: the blue crab, Callinectes sapidus, and the European green crab, Carcinus maenas. It further discusses the diverse regulatory roles of these neuropeptides and the functional plasticity of these neuropeptides in regard to life stage and species-specific physiology.

Molt-inhibiting hormone stimulates vitellogenesis at advanced ovarian developmental stages in the female blue crab, Callinectes sapidus 1: an ovarian stage dependent involvement

To understand the hormonal coordination of the antagonism between molting and reproduction in crustaceans, the terminally anecdysial mature female Callinectes sapidus was used as a model. The regulatory roles of crustacean hyperglycemic hormone (CHH) and molt-inhibiting hormone (MIH) in vitellogenesis were examined. A competitive specific RIA was used to measure the levels of MIH and CHH in the hemolymphs of mature females at pre- and mid- vitellogenic stages, and their effects on vitellogenesis at early (early 2, E2) and mid vitellogenesis (3) stages were determined in vitro. A hepatopancreas fragments incubation system was developed and the levels of vitellogenin (VtG), as well as VtG mRNA and heterogeneous nuclear (hn)VtG RNA were determined using RIA or QPCR, respectively. MIH titers were four times higher at mid-vitellogenesis than at pre-vitellogenesis, while CHH levels in the hemolymph were constant. In the in vitro incubation experiments, MIH increased both VtG mRNA levels and secretion at ovarian stage 3. At stage E2, however, MIH resulted in a mixed response: downregulation of VtG mRNA and upregulation of hnVtG RNA. CHH had no effect on any of the parameters. Actinomycin D blocked the stimulatory effects of MIH in stage 3 animals on VtG mRNA and VtG, while cycloheximide attenuated only VtG levels, confirming the MIH stimulatory effect at this stage. MIH is a key endocrine regulator in the coordination of molting and reproduction in the mature female C. sapidus, which simultaneously inhibits molt and stimulates vitellogenesis.