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.

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.

Formation of ecdysteroids by Y-organs of the crab, Menippe mercenaria. I. Biosynthesis of 7-dehydrocholesterol in vivo

Y-organs of the xanthid crab, Menippe mercenaria, secrete ecdysteroid hormones in vitro, apparently both 3-dehydroecdysone and 25-deoxyecdysone. Studies were initiated on the biosynthetic path(s), in which cholesterol is converted to these ecdysteroids. Crabs were injected with [3H]cholesterol. Y-organs and hemolymph were removed 12 hr later and extracted directly and the extracts were analyzed by HPLC. Both polar and nonpolar sterols were surveyed. The only metabolite of cholesterol detectable in Y-organs was 7-dehydrocholesterol (identified by mass spectrometry). The total amount of 7-dehydrocholesterol and the amount that was labeled were generally greater than for cholesterol and were higher in Y-organs from de-eyestalked crabs than in those from intact crabs. Subcellular fractionation of the Y-organs showed that over 70% of total radioactivity was in cholesterol and 7-dehydrocholesterol of mitochondria and microsomes, distributed about equally between the two organellar fractions. In hemolymph, the only nonpolar sterols present were cholesterol and 7-dehydrocholesterol; the concentration ratio was 20:1. However, 7-dehydrocholesterol was not significantly labeled. Analyses of polar compounds revealed two prominent, uv-absorbing ecdysteroids which coeluted with the authentic standards, 3-dehydro-20-hydroxyecdysone and 25-deoxy-20-hydroxyecdysone (ponasterone A). The radioactivity profile showed, in addition, a third prominent peak that corresponded in retention time with 3-dehydroecdysone. These results indicate that the Y-organs in vivo form 7-dehydrocholesterol from cholesterol and convert the latter to secretion products without accumulation of other intermediates. At least two ecdysteroids are secreted and appear to be converted peripherally in this crab to their respective 20-hydroxy derivatives.

Regulation of Y-organ ecdysteroidogenesis by molt-inhibiting hormone in crabs: involvement of cyclic AMP-mediated protein synthesis

The crustacean neuropeptide, molt-inhibiting hormone (MIH), directly inhibits Y-organ ecdysteroidogenesis, an effect mediated by cyclic AMP (cAMP) and antagonized by calcium-calmodulin. We investigated regulation of Y-organ protein. RNA, and DNA syntheses by MIH, cAMP, and calcium in relation to steroidogenesis in vitro. Ecdysteroid production and [3H]leucine incorporation into protein were inhibited 50-60 and 80-90%, respectively, by MIH activity in eyestalk extracts (4 eyestalk equivalents), 10(-6) M forskolin, or a combination of 10(-2) M dibutyryl cAMP and 10(-4) M 3-isobutyl-1-methylxanthine (dbcAMP-IBMX). Calcium ionophore A23187 (10(-4) M) stimulated ecdysteroidogenesis two-fold, did not affect the relatively high basal (control) rate of protein synthesis, and reduced the inhibitory effects of forskolin on steroidogenesis and protein synthesis. Incorporation of [3H]uridine into RNA was unaffected by MIH, forskolin, or A23187 but was reduced 50% by dbcAMP-IBMX. Basal rates of [3H]thymidine incorporation into DNA were low and were not affected by treatments. The effects of MIH were specific; extracts of brain or muscle did not alter Y-organ steroidogenesis or protein synthesis, while muscle extract increased precursor incorporation into RNA. Eyestalk extract did not affect [3H]leucine incorporation into protein of brain, muscle, or gill. Cycloheximide (5 micrograms/ml) depressed protein synthesis 90% and steroidogenesis 60%, enhanced the inhibition induced by MIH, and blocked the stimulation of steroidogenesis induced by A23187; effects on basal steroidogenesis were evident after 1 hr. Actinomycin D (1 microgram/ml) depressed RNA synthesis 86% but did not alter basal, MIH-inhibited, or A23187-stimulated ecdysteroidogenesis during incubations. These results indicate that MIH suppresses Y-organ steroidogenesis in part by inhibiting protein synthesis at the translational level; the effect is mediated by cAMP. The stimulation of steroidogenesis by calcium, mediated by lowering cAMP, also appears to depend in part upon protein synthesis.