In vitrosecretion of ecdysteroid by Y-organ during molt cycle and evidence for secretion of 3-dehydroecdysone in the giant freshwater prawn,Macrobrachium rosenbergii(Crustacea: Decapoda: Caridea)

Dual roles of CYP302A1 in regulating ovarian maturation and molting in Macrobrachium nipponense

CYP302A1 is a member of the Halloween genes in the cytochrome P450 supergene family, which play an important regulatory role in the synthesis of 20-hydroxyecdysone (20E) in crustaceans and insects. In this study, we found that the Mn-CYP302A1sequence included typical CYP450 conserved domains. Phylogenic showed that it is closely related to crustaceans and insects. q-PCR analysis indicated that Mn-CYP302A1 was highly expressed in the ovaries and peaked before ovarian maturation. Mn-CYP302A1 expression was higher at the post-larval stage of day 15 than at other stages of embryogenesis. In situ hybridization indicated that Mn-CYP302A1 was mainly distributed in the nucleus, yolk granules, cell membrane and cytoplasm To further establish the function of CYP302A1, a 21-day RNA interference experiment was conducted. On day 16, the Gonad Somatic Index of the control group and the experimental group showed significant differences, with GSI of 11.72% in the control group and 3.21% in the experimental group. The cumulative proportion of the second entry into stage O-Ⅲ was 100% in the control group, while it was 41.67% in the experimental group on day 21. The ecdysone content was 8.91nmol/L in the control group and 6.11nmol/L in the experimental group on day 9. A significant difference in the molting proportion between the control group and the experimental group was also observed (49% in the control group and 34% in the experimental group) on day 16. Statistical results showed that the average molting cycle of the control group was 14.5 days, while that of the experimental group was 16.5 days. However, the morphological structure of ovarian tissue did not abnormal change. Therefore, the results of this study suggest that Mn-CYP302A1 can promote ovarian maturation and molting in female M. nipponense.

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.

Light and electron microscopic studies on the Y organ of the freshwater crab Travancoriana schirnerae

The fine structure of the premoult Y organ in the freshwater crab Travancoriana schirnerae revealed elliptical epithelial gland cells with large, eccentric, multinucleolated nuclei and ample cytoplasm. The cytoplasm showed numerous polymorphic mitochondria with tubular cristae, highly anastomosed tubules and vesicles of smooth endoplasmic reticulum (SER), rich free ribosomes, small amounts of cisternae of rough endoplasmic reticulum (RER), microtubules and was devoid of Golgi complexes. Mitochondria were of two types the more abundant micromitochondria with electron dense matrix and the less abundant macromitochondria with moderately dense matrix. The tubular SER was particularly concentrated towards the basal region of the cell, intermingled with mitochondria and dense patches of free ribosomes while the vesicular SER lie close to the lateral plasma membrane. Large vesicles with flocculent substances, a few electron dense granules and multivesicular bodies could also be noticed in the gland cell cytoplasm. Aggregations of microvesicles which appeared close to the lateral plasma membrane, in association with dilated SER cisternae and microtubules, possibly suggest the intercellular exchange of substances. The plasma membrane beneath the basal lamina was composed of invaginations and the apical surface possessed numerous microvilli which serve to increase the surface area for metabolic exchange. Towards the apical region, the lateral plasma membrane of adjacent cells was linked by tight junctions. The presence of extraordinarily abundant tubular SER, high proportion of mitochondria with tubular cristae and rich free ribosomes could well be elucidated in favour of steroid production by the gland cells.

Ecdysteroid metabolism in crustaceans

The molting gland, or Y-organ (YO), is the primary site for ecdysteroid synthesis in decapod crustaceans. Ecdysteroid biosynthesis is divided into two stages: (1) conversion of cholesterol to 5β-diketol and (2) conversion of 5β-diketol to secreted products. Stage 1 involves the conversion of cholesterol to 7-dehydrocholesterol (7DC) by 7,8-dehydrogenase, the "Black Box" reactions involving 3-oxo-Δ(4) intermediates, and the conversion of Δ(4)-diketol to 5β-diketol by 5β[H]-reductase. The stage 2 reactions generate four major products, depending on species: ecdysone, 3-dehydroecdysone (3DE), 25-deoxyecdysone (25dE), and 3-dehydro-25-deoxyecdysone (3D25dE). Peripheral tissues convert these compounds to the active hormones 20-hydroxyecdysone (20E) and ponasterone A (25-deoxy-20-hydroxyecdysone or 25d20E). The hydroxylations at C25, C22, C2, and C20 are catalyzed by cytochrome P-450 mono-oxygenases, which are encoded by the Halloween genes Phantom, Disembodied, Shadow, and Shade, respectively, in insects. Orthologs of these genes are present in the Daphnia genome and a cDNA encoding Phantom has been cloned from prawn. Inactivation involves conversion of ecdysteroids to polar metabolites and/or conjugates, which are eliminated in the urine and feces. The antennal gland is the major route for excretion of ecdysteroids synthesized by the YO. The hepatopancreas eliminates ingested ecdysteroids by forming apolar conjugates. The concentrations of ecdysteroids vary over the molt cycle and are determined by the combined effects biosynthesis, metabolism, and excretion.

Regulation of ecdysteroid secretion from the Y-organ by molt-inhibiting hormone in the American crayfish, Procambarus clarkii

In crustaceans, molt-inhibiting hormone (MIH) has been proposed to regulate molting by inhibiting the secretion of ecdysteroids from the Y-organ. Thus, MIH titer in the hemolymph should be inversely related to ecdysteroid titers during the molt cycle. However, it has not been demonstrated whether the MIH titer in the hemolymph changes during the molt cycle. The purpose of this study was to determine the changes in the MIH titers in the hemolymph during the molt cycle of the American crayfish, Procambarus clarkii, and to discuss the role of MIH in regulation of molting. As predicted by the hypothesis, the hemolymph MIH titer was high at the intermolt stage when the hemolymph ecdysteroid titer was low, and the MIH titer decreased to a basal level at the early premolt stage when the hemolymph ecdysteroid titer began to increase slightly. At the middle premolt stage when the hemolymph ecdysteroid titer increased, the MIH titer was restored to a level as high as that during the intermolt stage. This is in contradiction to the hypothesis. However, the Y-organs at this stage scarcely responded to MIH both in vitro and in vivo. The present findings suggest that ecdysteroid secretion from the Y-organ may be regulated not only by changes in the hemolymph MIH titer, but also by changes in the responsiveness of the Y-organ to MIH.