Regulation of steroidogenesis in crayfish molting glands: involvement of protein synthesis

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.

TOR signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in the silkworm, Bombyx mori

The prothoracicotropic hormone (PTTH) is a stimulator of ecdysteroidogenesis in prothoracic gland of larval insects. Our recent studies showed that phosphoinositide 3-kinase (PI3K)/Akt signaling was involved in PTTH-stimulated ecdysteroidogenesis by Bombyx mori prothoracic glands. In the present study, downstream signaling of PI3K/Akt was further investigated. Results showed that PTTH rapidly enhanced the phosphorylation of translational repressor 4E-binding protein (4E-BP) and p70 ribosomal protein S6 kinase (S6K), two known downstream signaling targets of the target of rapamycin complex 1 (TORC1). PTTH stimulated 4E-BP phosphorylation in time- and dose-dependent manners. Injection of PTTH into day-6 last instar larvae greatly increased 4E-BP phosphorylation, verifying the in vitro effect. PTTH-stimulated 4E-BP phosphorylation was blocked by both LY294002 and wortmannin, indicating the involvement of PI3K. Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors (PD 98059 and U0126), did not inhibit PTTH-stimulated 4E-BP phosphorylation, implying that ERK signaling is not related to PTTH-stimulated 4E-BP phosphorylation. The phosphorylation of S6K was also stimulated by PTTH both in vitro and in vivo. PI3K signaling appears to be involved in PTTH-stimulated phosphorylation of S6K. Rapamycin, a specific inhibitor of mammalian TOR signaling attenuated PTTH-stimulated phosphorylation of 4E-BP and S6K of the glands, and greatly inhibited PTTH-stimulated ecdysteroidogenesis. Examination of gene expression levels of 4E-BP and S6K showed that PTTH inhibited mRNA levels of both 4E-BP and S6K, indicating that PTTH may exert its action at both the transcriptional and phosphorylation levels. These results suggest that PTTH/PI3K/TOR/4E-BP (S6K) signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in B. mori.

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.

Expression of crustacean (Callinectes sapidus) molt-inhibiting hormone in Escherichia coli: characterization of the recombinant peptide and assessment of its effects on cellular signaling pathways in Y-organs

A neuropeptide, molt-inhibiting hormone (MIH), negatively regulates the synthesis of ecdysteroid molting hormones by crustacean Y-organs. We report here the expression of blue crab (Callinectes sapidus) MIH in Escherichia coli. Bacteria were transformed with an expression plasmid containing a cDNA insert encoding MIH. After induction of protein synthesis, recombinant MIH (recMIH) was detected in the insoluble fraction of cell lysates. The insoluble recMIH was refolded and purified by reversed-phase high performance liquid chromatography (RP-HPLC). The refolded peptide was MIH-immunoreactive and comigrated with native MIH on RP-HPLC. Mass and CD spectral analyses showed the mass number and secondary structure of the recombinant peptide were as predicted for MIH. Bioassays showed recMIH dose-dependently suppresses ecdysteroid synthesis by Y-organs. The combined results suggest that recMIH is properly folded. In subsequent experiments, recMIH was used to assess cellular signaling pathways linked to MIH-mediated suppression of ecdysteroidogenesis. Incubation of Y-organs with recMIH produced an increase in intracellular cGMP content, but had no effect on intracellular cAMP. Further, a cGMP analog significantly suppressed ecdysteroid production, but neither cAMP analogs nor an activator of adenylyl cyclase had a detectable effect on ecdysteroidogenesis. The results are consistent with the hypothesis that MIH-induced suppression of ecdysteroidogenesis in Y-organs of C. sapidus is mediated by a cGMP second messenger. We anticipate recMIH will be a useful tool for additional studies of the cellular actions and physiological functions of MIH.

Molt-inhibiting hormone-mediated regulation of ecdysteroid synthesis in Y-organs of the crayfish (Procambarus clarkii): involvement of cyclic GMP and cyclic nucleotide phosphodiesterase

Crustacean molt-inhibiting hormone (MIH), a polypeptide secreted by the X-organ/sinus gland complex of the eyestalks, regulates molting by inhibiting the synthesis of ecdysteroids by Y-organs. Previous results indicate the biosynthetic activity of Y-organs is likely controlled not only by the level of hemolymphatic MIH, but also by the responsiveness of Y-organs to MIH. The present studies were conducted to (a) identify the second messenger that mediates MIH-induced suppression of ecdysteroidogenesis, and (b) assess the possible involvement of cyclic nucleotide phosphodiesterase (PDE) in determining the responsiveness of Y-organs to MIH. Adding 8-bromo cAMP or 8-bromo cGMP to incubation medium significantly suppressed ecdysteroid production by Y-organs of the crayfish (Procambarus clarkii). Incubating Y-organs with MIH produced a significant increase in glandular cGMP, but MIH had no effect on glandular cAMP. The composite data indicate that MIH-induced suppression of ecdysteroidogenesis in Y-organs of P. clarkii is mediated by cGMP. Subsequently, Y-organs from various stages of the molt cycle were incubated with MIH, 3-isobutyl-1-methylxanthine (IBMX, an inhibitor of PDE), or both. Y-Organs from middle and late premolt stages were poorly responsive to MIH alone. Including IBMX in the incubation medium enhanced the responsiveness of the Y-organs to MIH at these stages. Moreover, glandular PDE activity in the Y-organs at these stages was significantly higher than other stages. The combined results suggest that molt cycle-associated changes in PDE activity affect the ability of MIH to stimulate cGMP accumulation and suppress ecdysteroidogenesis in Y-organs of P. clarkii.

Regulation of protein synthesis in Y-organs of the blue crab (Callinectes sapidus): involvement of cyclic AMP

Paired Y-organs secrete ecdysteroid hormones that control cycles of growth and molting in crustaceans. Y-Organs are regulated, at least in part, by molt-inhibiting hormone (MIH), a polypeptide produced and released by the X-organ/sinus gland complex of the eyestalks. In the present studies, crab (Callinectes sapidus) Y-organs were incubated in vitro in the presence of [(35)S]methionine, and cyclic nucleotide analogs or experimental agents that influence the cAMP signaling pathway. In 4-hr incubations, 8-Br-cAMP and db-cAMP (but not 8-Br-cGMP) suppressed incorporation of [(35)S]methionine into Y-organ proteins; the effect of 8-Br-cAMP was concentration-dependent. Autoradiograms of radiolabeled Y-organ proteins separated on SDS-PAGE gels indicated the effect of 8-Br-cAMP was general (as opposed to selective) suppression of protein synthesis. Addition of both forskolin (an adenylyl cyclase activator) and 3-isobutyl-1-methylxanthine (a phosphodiesterase inhibitor) likewise suppressed incorporation of [(35)S]methionine into Y-organ proteins. Cycloheximide (a protein synthesis inhibitor) suppressed incorporation of [(35)S]methionine into Y-organ proteins and secretion of ecdysteroids. The combined results suggest that cAMP is involved in regulation of protein synthesis in C. sapidus Y-organs. We are currently investigating the link of protein synthesis to ecdysteroid production, and the possibility of cross-talk between cAMP and other cellular signaling pathways in Y-organs.

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.

Moult cycle-related changes in biological activity of moult-inhibiting hormone (MIH) and crustacean hyperglycaemic hormone (CHH) in the crab, Carcinus maenas. From target to transcript

The currently accepted model of moult control in crustaceans relies entirely on the hypothesis that moult-inhibiting hormone (MIH) and crustacean hyperglycaemic hormone (CHH) repress ecdysteroid synthesis of the target tissue (Y-organ) only during intermoult, and that changes in synthesis and/or release of these neurohormones are central to moult control. To further refine this model, we investigated the biological activities of these neuropeptides in the crab Carcinus maenas, at the target tissue, receptor and cellular level by bioassay (inhibition of ecdysteroid synthesis), radioligand (receptor) binding assays, and second messenger (cGMP) assays, at defined stages of the moult cycle. To investigate possible moult cycle-related changes in neuropeptide biosynthesis, steady-state transcript levels of both neuropeptide mRNAs were measured by quantitative RT-PCR, and stored neuropeptide levels in the sinus gland were quantified during intermoult and premoult. The results show that the most important level of moult control lies within the signalling machinery of the target tissue, that expression and biosynthesis of both neuropeptides is constant during the moult cycle, and are not central to the currently accepted model of moult control.

Molting cycle-dependent expression of CYP4C15, a cytochrome P450 enzyme putatively involved in ecdysteroidogenesis in the crayfish, Orconectes limosus

A cytochrome P450 enzyme cDNA (CYP4C15) has been previously cloned from a cDNA library of crayfish steroidogenic glands (Y-organs). The conceptual translation of the CYP4C15 cDNA sequence was analyzed for regions of putative high antigenicity and a mixture of two synthetic peptides was chosen for the production of a specific polyclonal antibody. Western blot analysis on Y-organ subcellular fractions indicated an endoplasmic reticulum location of CYP4C15, in agreement with the structural feature of the predicted protein, i.e. the presence of a hydrophobic N-terminal segment. The protein is only expressed in Y-organs, thus showing a similar distribution to the corresponding mRNA. From this tissue specific expression, it has been postulated that CYP4C15 would play a role in ecdysteroid biosynthesis rather than detoxification and the variations of its expression during a molt cycle were carefully examined. CYP4C15 is not detectable in intermolt animals, expression levels are maximal during early premolt and decrease during late premolt. The results are discussed in relation to the variations of hemolymphatic ecdysteroid titers and steroidogenic capacities of the Y-organs during the molt cycle.

Cloning of a novel cytochrome P450 (CYP4C15) differentially expressed in the steroidogenic glands of an arthropod

Biosynthesis of ecdysteroids, arthropod steroid molting hormones, proceeds from dietary cholesterol through a complex and still incompletely elucidated pathway. Most of the known steps are catalyzed by cytochrome P450 enzymes (CYPs) but none of their genes has yet been identified. We have established a cDNA library of crayfish steroidogenic glands (Y organs). A full length CYP-cDNA was characterized containing a 1539 bp open reading frame encoding a predicted protein of 513 amino acid residues. This novel CYP was assigned to the CYP4 family and designated CYP4C15. Northern blots demonstrated predominant expression of this gene in the active molting glands, suggesting a role in ecdysteroid biosynthesis rather than detoxification.

Cloning and characterization of the L15 ribosomal protein gene homologue from the crayfish Orconectes limosus

A cDNA homologue of the large unit rat ribosomal protein L15 was cloned from an epidermal cDNA library of the crayfish Orconectes limosus, being the first crustacean ribosomal protein gene cloned to date. It contains 204 amino acids, deduced from the nucleotide sequence, and reveals 70-76% sequence identity with other arthropod and vertebrate ribosomal L15s. A single, abundant 0.7-kb mRNA transcript was constitutively expressed and revealed similar expression levels, among various adult crayfish tissues.

A transaldolase : An enzyme implicated in crab steroidogenesis

In arthropods, development is controlled by cholesterol-derived steroid hormones: the ecdysteroids. In vertebrates and insects, steroidogenesis is positively regulated and this is mediated by cAMP. In crustaceans, ecdysteroid biosynthesis by steroidogenic organs (Y-organs) is negatively regulated by a neuropeptide, the Molt Inhibiting Hormone (MIH). This neuropeptide-induced inhibition occurs via cyclic nucleotides and depends on protein synthesis. In the present work, we provide evidence that a major 36.2-kDa cytosolic protein (P36; pl: 6.8) from crab Y-organs is positively correlated with steroidogenic activity. On the basis of its amino acid sequence, P36 could be related to transaldolase, an enzyme of the pentose phosphate pathway which generates NADPH. In Y-organs, the enzymatic activity ofCarcinus transaldolase increases with steroidogenic activity, and MIH treatment decreases both synthesis and activity of transaldolase. Various transaldolases have been characterized in very distantly related groups, namely bacteria, yeasts, and humans. These enzymes are highly conserved and present strong structural homologies, interestingly the crab transaldolase is closest to that enzyme characterized in human cells.