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Sea cucumber Apostichopus japonicus displays the typical circadian rhythms. This present study investigated the molecular regulation of clock genes, as well as monoamines and melatonin, in multiple tissues of A. japonicus, responding to the photoperiod. In order to determine their pivotal role in circadian rhythms, the crucial clock genes, namely AjClock, AjArnt1, AjCry1, and AjTimeless, were identified and a comprehensive analysis of their expressions across various tissues in adult A. japonicus was conducted, revealing the potential existence of central and peripheral oscillators. Results demonstrated that the tissues of polian vesicle and nerve ring exhibited significant clock gene expression associated with the orchestration of circadian regulation, and that environmental light fluctuations exerted influence on the expression of these clock genes. However, a number of genes, such as AjArnt1 and AjCry1, maintained their circadian rhythmicity even under continuous light conditions. Moreover, we further investigated the circadian patterns of melatonin (MT), serotonin (5-HT), and dopamine (DA) secretion in A. japonicus, data that underscored the tissue-specific regulatory differences and the inherent adaptability to dynamic light environments. Collectively, these findings will provide the molecular mechanisms controlling the circadian rhythm in echinoderms and the candidate tissues playing the role of central oscillators in sea cucumbers., Competing Interests: Declaration of Competing Interest All authors disclosed no relevant relationships., (Copyright © 2023. Published by Elsevier Inc.)

The sea cucumber Apostichopus japonicus is an economically important marine species in China, and understanding the mechanisms underlying its gonad development is crucial for successful reproduction and breeding. In this study, we performed transcriptome comparisons and analyses of A. japonicus gonadal and non-gonadal tissues to identify genes and molecular pathways associated with gonadal development. We also supplemented the annotation of the A. japonicus genome. Collectively, results revealed a total of 941 ovary-specific genes and 2499 testis-specific genes through different expression analysis and WGCNA analysis. The most enriched pathways in ovary and testis were "DNA replication" and "purine metabolism", respectively. Additionally, we identified key candidate gene modules that control gonad development and germ cell maturation, with CDT1 and DYNC2LI1 serving as hub genes. Our findings provide important insights into the gonadal development system of A. japonicus and offer valuable references for further research on reproductive biology in this marine invertebrate species., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Inc.)

Simple Summary: Apostichopus japonicus is a temperate marine invertebrate and an economically important aquatic echinoderm species in China. As a benthic organism, sea cucumbers feed on small benthic particulate matter and are easily affected by pollutants. Therefore, for conservation purposes, it is important to elucidate the effects of specific pollutants on these animals. Bisphenol A (BPA) has been identified as an endocrine disruptor that functions as an estrogen analog and typically causes reproductive toxicity by interfering with the endocrine system. We identified the G protein-coupled estrogen receptor 1 in A. japonicus and found that both BPA and estradiol could activate the estrogen-receptor-mediated mitogen-activated protein kinase signaling pathway. Using biological approaches, we have confirmed the estrogen-analog bioactivity of BPA on sea cucumbers by direct interaction with the estrogen receptor. We also determined that BPA affects the reproduction of sea cucumbers by metabolic disruption in ovary tissue. The sea cucumber, Apostichopus japonicus, is a marine benthic organism that feeds on small benthic particulate matter and is easily affected by pollutants. Bisphenol A (BPA, 4,4′-isopropylidenediphenol) has been identified as an endocrine disruptor. It is ubiquitously detectable in oceans and affects a variety of marine animals. It functions as an estrogen analog and typically causes reproductive toxicity by interfering with the endocrine system. To comparatively analyze the reproductive effects of estradiol (E2) and BPA on sea cucumbers, we identified a G protein-coupled estrogen receptor 1 (GPER1) in A. japonicus and investigated its effects on reproduction. The results showed that BPA and E2 exposure activated A. japonicus AjGPER1, thereby mediating the mitogen-activated protein kinase signaling pathways. High-level expression of AjGPER1 in the ovarian tissue was confirmed by qPCR. Furthermore, metabolic changes were induced by 100 nM (22.83 μg/L) BPA exposure in the ovarian tissue, leading to a notable increase in the activities of trehalase and phosphofructokinase. Overall, our findings suggest that AjGPER1 is directly activated by BPA and affects sea cucumber reproduction by disrupting ovarian tissue metabolism, suggesting that marine pollutants pose a threat to the conservation of sea cucumber resources. [ABSTRACT FROM AUTHOR]

Aestivation is a state of dormancy in diverse animal species characterized by a depressed metabolism. High temperatures in the summer trigger aestivation in the adult sea cucumber, Apostichopus japonicus. In the present study, the glycolytic suppression in aestivating A. japonicus was verified by metabolite quantification and activity and a gene expression analysis of rate-limiting enzymes (hexokinase and pyruvate kinase). Significant decreases in glucose concentrations were discovered in the respiration tree ( p < 0.01), intestine ( p < 0.05) and muscle ( p < 0.01) of aestivating A. japonicus; likewise, over this period, pyruvic acid concentrations significantly decreased in the respiration tree ( p < 0.05) and intestine ( p < 0.01), but were unchanged in muscle. Enzymatic activity variation of hexokinase ( Aj-HK) and pyruvate kinase ( Aj-PK) in aestivating A. japonicus revealed that Aj-HK remained at high levels in the respiration tree and muscle but significantly decreased in intestine ( p < 0.01); the activity of Aj-PK also declined significantly in intestine ( p < 0.01), whereas it increased significantly in the respiration tree ( p < 0.05) and muscle ( p < 0.01). The gene sequences of Aj-HK and Aj-PK were identified, and their transcriptional quantification revealed that the expression of Aj-HK was depressed in all of the examined tissues, whereas Aj-PK expression was depressed in muscle only. These results suggest that glycolytic suppression occurred in aestivating A. japonicus, especially in the intestine, and transcriptional and post-transcriptional modifications are involved in the regulation of glycolytic suppression in this marine species. [ABSTRACT FROM AUTHOR]

Cathepsin L, a lysosomal endopeptidase, has been noted for its involvement in the innate immune response in invertebrates. Here, the cathepsin L cDNA of the sea cucumber Apostichopus japonicus ( Aj CatL) is identified from an EST library and then cloned by the rapid amplification of the cDNA ends (RACE) PCR. The full-length cDNA is 1678 bp long containing an open reading frame (ORF) of 1002 bp, an 80 bp 5′ UTR and a 599 bp 3′ UTR. The cDNA encodes 333 amino acid residues with a predicted molecular mass of 37.07 kDa and a theoretical isoelectric point (pI) of 5.01. The full-length Aj CatL contains three active sites of eukaryotic thiol (cysteine) protease at positions 133–144, 278–288 and 295–314. Analysis of the predicted tertiary structure of prepro-CatL (17–333 aa) and mature-CatL (116–333 aa) reveals that the propeptide region (17–115 aa) blocks access to the substrate-binding cleft. Phylogenetic analysis shows that the Aj CatL is clustered together with two other CatLs from Strongylocentrotus purpuratus . The enzymatic activity of Aj CatL was verified using a substrate hydrolyzing assay with recombinant m Aj CatL. Further analysis of real time-PCR demonstrates that the expression of Aj CatL mRNA is significantly up-regulated in the coelomocytes in cases of infection with the common bacterial pathogen, Vibrio splendidus . This suggests that the Aj CatL is likely to be involved in the immune response. [ABSTRACT FROM AUTHOR]

The sea cucumber Apostichopus japonicus is dioecious, with seasonal reproduction. G protein-coupled receptor (GPCR)-mediated signaling systems might play critical roles in the reproductive control of A. japonicus. Here, we classified GPCR from the genome in silico and used transcriptomic analyses to further mine those that function in gonadal-development control. Totally, 487 GPCRs were predicted from A. japonicus , and 183 of these were further annotated to molecular pathways. Transcriptome analysis revealed 327 GPCRs expressed in gonads, and these were classified into four families and 19 subfamilies. Three pathways were apparently associated with reproduction, including neuroactive ligand-receptor interaction, the mTOR and Wnt signaling pathways. Seven and eight ovary- and testis-specific GPCRs were filtered, and the gene expression profiles were determined in multiple tissues and gonads at different developmental stages by qPCR. These results provide new insights into the discovery of GPCR-mediated signaling control in sea cucumber reproduction, especially in gonadal development control. • 487 G protein-coupled receptors are predicted and classified from Apostichopus japonicus. • A variety of G protein-coupled receptors are detected to be expressed in the gonads of Apostichopus japonicus. • Gonad-specific G protein-coupled receptors have been screened out. [ABSTRACT FROM AUTHOR]