Demonstration of protein kinase C activity in crustacean Y-organs, and partial definition of its role in regulation of ecdysteroidogenesis

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.

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.

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.

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.

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.

Co-expressed peJK genes of lobster (Jasus edwardsii)

Previous studies have shown that the two novel genes of southern rock lobster (Jasus edwardsii) named peJK2 and peJK3 are implicated in eyestalk hormonal regulation of the lobster moult cycle. Northern blot, in situ hybridisation studies and sequence analyses showed that their putative products might be transmembrane proteins associated with cell signal transduction of hormonal signals in the eyestalk during the intermoult phase of the moult cycle. The aim of this study was to analyse coexpression of peJK genes in different J. edwardsii tissues. Using reverse transcriptase–polymerase chain reaction (RT-PCR), the expression of peJK genes was analysed in seven different tissues (eyestalk, brain, epidermis, hepatopancreas, gill, muscle and heart) of an intermoult lobster. During RT-PCR analysis, a novel sequence was isolated, and was named peJK4. It shares 88% and 86% sequence identity with peJK2 and peJK3 respectively. The peJK2 and peJK4 genes are expressed in all tested tissues. Sequence analyses of the predicted peJK2 and peJK4 proteins revealed two common signal transduction motifs, transmembrane helices and protein kinase C. These results showed that the peJK genes of J. edwardsii are a complex group of genes and possibly involved in different signal transduction pathways.

Trimeric G proteins in crustacean (Callinectes sapidus) Y-organs: occurrence and functional link to protein synthesis

Crustacean Y-organs produce ecdysteroid molting hormones. Regulation of ecdysteroidogenesis appears to be complex, involving regulatory ligands (including but not limited to molt-inhibiting hormone, an eyestalk neurohormone) and the capacity of the Y-organs to respond to those ligands. Available data indicate cell signaling pathways involving cAMP, cGMP, or both may be involved in regulation of Y-organ function. Trimeric G proteins link receptor occupancy to regulation of intracellular cAMP levels. In studies reported here, we have assessed the occurrence of G proteins in blue crab (Callinectes sapidus) Y-organs, and the link of G proteins to Y-organ function. Bacterial toxin-catalyzed ADP-ribosylation revealed a PTX-sensitive (alpha i-like) protein in Y-organ membranes, but failed to reveal a CTX-sensitive (alpha s-like) protein in Y-organ membranes. Western blotting with primary antibodies raised against conserved regions of mammalian G proteins detected an alpha i-immunoreactive protein (approximately 40 kDa) and two alpha s-immunoreactive proteins (approximately 50 and approximately 57 kDa) in Y-organ membrane preparations. Incubation of Y-organ membrane fractions with cholera toxin significantly suppressed incorporation of [35S]-methionine into TCA-precipitable Y-organ proteins, but had no detectable effect on ecdysteroidogenesis in short-term (6 h) incubations. The combined results indicate that C. sapidus Y-organs possess both Gi and Gs proteins, and that alpha s is functionally linked to regulation of glandular protein synthesis.

Molt-inhibiting hormone mRNA levels and ecdysteroid titer during a molt cycle of the blue crab, Callinectes sapidus

Synthesis of ecdysteroid molting hormones by crustacean Y-organs is believed to be regulated by a neuropeptide, molt-inhibiting hormone (MIH), produced in eyestalk neural ganglia. In the present study, steady-state MIH mRNA and hemolymph ecdysteroid levels were determined by Northern blot and radioimmunoassay, respectively, during the molt cycle of the blue crab, Callinectes sapidus. The level of MIH mRNA dropped steadily during premolt (D1-D4), reaching a minimum in D3/D4, then increased by 10-fold in postmolt (A/B) and remained elevated during intermolt (C4). These stage-specific changes in MIH mRNA levels were accompanied by significant fluctuations in the hemolymph ecdysteroid titer. The ecdysteroid titer increased steadily to a peak of 377.0 ng/ml in D3 of premolt, then dropped to 120.0 ng/ml in D4 (just prior to molting), and was low during postmolt (A/B, 4.4 ng/ml) and intermolt (C4, 3.3 ng/ml). The results represent the first report of developmental changes in MIH gene expression and are generally consistent with the hypothesis that MIH negatively regulates ecdysteroid synthesis in crustaceans.

Physiological role of 3-hydroxykynurenine and xanthurenic acid upon crustacean molting

Studies with crabs (Charybdis japonica) and crayfish (Procambarus clarkii) revealed that the tryptophan metabolites, 3-hydroxy-L-kynurenine (3-OH-K) and xanthurenic acid (XA), common secretory products of the X-organ-sinus gland complex of eyestalks from several decapods, regulated the molting of crustaceans in species-nonspecific fashion. Injection of 3-OH-K to the eyestalk-ablated crayfish delayed the onset of the first molt and lengthened the interval between the first and second molts. These lines of evidence were in accord with previous accounts of the so-called "molt inhibiting hormone" (MIH) effect. Removal of eyestalks caused a change in the conversion capacity of exogenous 3-OH-K to XA in the hemolymph. The peak in transformation capacity was followed by a peak in the titer of 20-hydroxyecdysone or molting hormone. Moreover, the seasonal profiles of the XA and ecdysone titers in Charybdis japonica exhibited a staggered relationship in the tissues tested. The ratio of XA to 3-OH-K, which is expected to indicate the apparent 3-OH-Kase activity, fluctuated seasonally and locally. When the Y-organ with the adhering tissues (Y-organ complex or YOC) was incubated during the period of high XA titer, the YOC produced 100 times more ecdysone than before incubation. It is suggested that ecdysteroidogenesis in situ was suppressed during this period by XA, but incubation of the YOC lead to a dramatic acceleration in ecdysone synthesis by overriding this inhibitory effect. XA profoundly repressed ecdysteroidogenesis in the YOC culture. Thus, XA is the ecdysone biosynthesis inhibitor (EBI) and 3-OH-K the precursor in crustaceans. An interfering effect of XA to a biocatalyst cytochrome P-450 system was postulated for the inhibition mechanism of ecdysteroidogenesis.

Inhibition of insect juvenile hormone synthesis by phorbol 12-myristate 13-acetate

The synthesis of insect juvenile hormone III (JH III) by isolated corpora allata of the cockroach Diploptera punctata incubated in vitro is inhibited by phorbol 12-myristate 13-acetate (PMA), phorbol 12,13-dibutyrate and 1-oleyl-2-acetylglycerol. 4 alpha-Phorbol 12,13-didecanoate and diolein are inactive. The inhibitory effect of phorbol 12-myristate 13-acetate is fully reversed by 2E,6E-farnesol or by 2E,6E-farnesoic acid. It is highest in corpora allata that are past their peak in secretory activity or that have been inhibited by injections of 20-hydroxyecdysone. This effect of phorbol esters implicates protein kinase C in the regulation of insect corpus allatum activity.