Comparative expression analysis of immune-related factors in the sea cucumber Apostichopus japonicus

Vibrio splendidus infection promotes circRNA-FGL1-regulated coelomocyte apoptosis via competitive binding to Myc with the deubiquitinase OTUB1 in Apostichopus japonicus

Circular RNAs (circRNAs) are involved in various physiological and pathological processes in both vertebrates and invertebrates. However, most studies on circRNAs have focused on their roles as endogenous competitive RNAs. Here, we report a novel function of circRNA derived from the Fibrinogen-like protein 1 gene (circ-FGL1) that inhibits coelomocyte apoptosis via competing with the deubiquitinase AjOTUB1 to bind AjMyc in Apostichopus japonicus during Vibrio splendidus infection. The results showed that circ-FGL1 is significantly downregulated in coelomocytes of V. splendidus-induced A. japonicus and negatively regulates coelomocyte apoptosis through the AjBax-AjCyt c pathway. Mechanistically, the deubiquitinase AjOTUB1 and circ-FGL1 could interact with the transcription factor protein AjMyc in the same region with circ-FGL1/AjMyc having greater affinity. Under normal conditions, high levels of circ-FGL1 bind directly to AjMyc, inhibiting the deubiquitylation of AjMyc by AjOTUB1 and leading to the degradation of AjMyc. After V. splendidus infection, AjMyc disassociates from the depressed expression of circ-FGL1, promoting its deubiquitylation by binding to the induced deubiquitinase AjOTUB1 to inhibit its degradation. AjMyc is then transferred to the nucleus and promotes the transcription of AjCyt c and AjBax to induce coelomocyte apoptosis. The new finding will expand our present outstanding on the functional role of circRNAs and suggest new therapeutic targets for the treatment of echinoderms during bacterial invasion.

Effects of dietary arachidonic acid (ARA) on immunity, growth and fatty acids of Apostichopus japonicus

As an important aquaculture species, improving the immunity of cultured Apostichopus japonicus (A. japonicus) is vital for its health in aquaculture farming. It has been shown that ARA is an important metabolite for A. japonicus infected by Vibrio splendidus. In this study, we aimed to determine the effects of dietary exogenous ARA on healthy sea cucumber cultures, including assessments of immunity, growth, and fatty acid content. Five experimental diets containing 0.01%, 0.29%, 0.46%, 0.70%, and 1.09% ARA were tested. The specific growth rate (SGR) of sea cucumbers did not be significantly affected by exogenous ARA diet groups. The results showed that dietary ARA between 0.49 and 1.09% notably improved the survival rate of sea cucumbers infected by Vibrio splendidus compared with the control group without exogenous ARA. The results also showed the effects of dietary ARA on immune-related genes, enzymes, and oxidation indices; most of the exogenous ARA significantly upregulated the mRNA expression of the genes NFκB, TLR, TLR3, TRAF6, Toll, and MyD88. The activities of ACP, AKP, and lysozyme increased in the 0.49-1.09% ARA groups, especially the dietary 0.49% ARA group. The SOD1 and NOS activities were enhanced by dietary ARA between 0.29 and 0.70%. Compared with the control, the MDA content increased, but the 0.49% ARA-diet group had a lower MDA content. Based on these data, 0.49-0.70% ARA significantly enhanced immunity in cultured A. japonicus. Exogenous 0.49% and 0.70% ARA also elevated the ARA, total PUFA and n-6 PUFA in the body walls. In conclusion, the appropriate exogenous ARA (0.49%-0.70%) in diets could improve immunity and fatty acid content considerably. The results provide basic evidence that ARA can serve a useful immune enhancer for A. japonicus aquaculture.

Vibrio splendidus persister cells induced by host coelomic fluids show a similar phenotype to antibiotic-induced counterparts

Persister cells are dormant variants of regular cells that are multidrug tolerant and have heterogeneous phenotypes; these cells are a potential threat to hosts because they can escape the immune system or antibiotic treatments and reconstitute infectious. Skin ulcer syndrome (SUS) frequently occurs in the sea cucumber (Apostichopus japonicus), and Vibrio splendidus is one of the main bacterial pathogens of SUS. This study found that the active cells of V. splendidus became persister cells more readily in the presence of A. japonicus coelomic fluids. We showed that the A. japonicus coelomic fluids plus antibiotics induce 100-fold more persister cells in V. splendidus compared with antibiotics alone via nine sets of experiments including assays for antibiotic resistance, metabolic activity, and single-cell phenotypes. Furthermore, the coelomic fluids-induced persister cells showed similar phenotypes as the antibiotic-induced persister cells. Further investigation showed that guanosine pentaphosphate/tetraphosphate (henceforth ppGpp) and SOS response pathway involved in the formation of persister cells as determined using real-time RT-PCR. In addition, single-cell observations showed that, similar to the antibiotic-induced V. splendidus persister cells, the coelomic fluids-induced persister cells have five resuscitation phenotypes: no growth, expansion, elongation, elongation and then division, and elongation followed by death/disappearance. In addition, dark foci formed in the majority of persister cells for both the antibiotic-induced and coelomic fluids-induced persister cells. Our results highlight that the pathogen V. splendidus might escape from the host immune system by entering the persister state during the process of infection due to exposure to coelomic fluids.

Transcriptome analysis reveals roles of polian vesicle in sea cucumber Apostichopus japonicus response to Vibrio splendidus infection

Polian vesicle is originally regarded as a hematopoietic and inflammatory response organ in sea cucumber by the operations of cell depletion and heterogeneous cells injection, respectively. In the present study, to reveal the role and immune mechanisms of polian vesicle in response to pathogen, Vibrio splendidus, we first performed a comparative transcriptome analysis for the cells from polian vesicle wall in V. splendidus-challenged Apostichopus japonicus through RNA high-throughput sequencing technology. Briefly, 465,356,848 clean reads were obtained after cleaning up low-quality reads in total. Approximately 73% of the sequenced reads could be aligned to the reference genome of A. japonicus. The DEGs of CG (control group) vs TG 24 h (24 h post-infection group), CG vs TG 72 h (72 h post-infection group) and TG 24 h vs TG 72 h were 3762, 1391 and 3258, respectively. Gene Ontology (GO) annotation assay revealed that those genes associated with the processes such as cell process, cell, binding and catalytic activity were significantly induced in all three groups post V. splendidus infection. KEGG enrichment analysis suggested the DEGs in TG 24 h were enriched in Toll-like receptor (TLR) signaling pathway, complement and coagulation cascades, antigen processing and presentation and IL-17 signaling pathway compared with that in CG, while the pathways including ribosome biogenesis in eukaryotes, DNA replication, and cell cycle related with cell proliferation were mainly enriched in TG 72 h than that of CG. Furthermore, six important DEGs were chosen and showed the consistent expression patterns with the results of RNA-seq by qPCR. Overall, our analysis towards the current data demonstrates that polian vesicle may play an essential role in the regulation of immune response in A. japonicus and provide new insights into hematopoietic function of polian vesicle in response to pathogen infection.

Molecular Aspects of Regeneration Mechanisms in Holothurians

Holothurians, or sea cucumbers, belong to the phylum Echinodermata. They show good regenerative abilities. The present review provides an analysis of available data on the molecular aspects of regeneration mechanisms in holothurians. The genes and signaling pathways activated during the asexual reproduction and the formation of the anterior and posterior parts of the body, as well as the molecular mechanisms that provide regeneration of the nervous and digestive systems, are considered here. Damage causes a strong stress response, the signs of which are recorded even at late regeneration stages. In holothurian tissues, the concentrations of reactive oxygen species and antioxidant enzymes increase. Furthermore, the cellular and humoral components of the immune system are activated. Extracellular matrix remodeling and Wnt signaling play a major role in the regeneration in holothurians. All available morphological and molecular data show that the dedifferentiation of specialized cells in the remnant of the organ and the epithelial morphogenesis constitute the basis of regeneration in holothurians. However, depending on the type of damage, the mechanisms of regeneration may differ significantly in the spatial organization of regeneration process, the involvement of different cell types, and the depth of reprogramming of their genome (dedifferentiation or transdifferentiation).

Transcriptomic analysis of sea cucumber (Holothuria leucospilota) coelomocytes revealed the echinoderm cytokine response during immune challenge

Background

The sea cucumber Holothuria leucospilota belongs to echinoderm, which is evolutionally the most primitive group of deuterostomes. Sea cucumber has a cavity between its digestive tract and the body wall that is filled with fluid and suspended coelomic cells similar to blood cells. The humoral immune response of the sea cucumber is based on the secretion of various immune factors from coelomocytes into the coelomic cavity. The aim of this study is to lay out a foundation for the immune mechanisms in echinoderms and their origins in chordates by using RNA-seq.

Results

Sea cucumber primary coelomocytes were isolated from healthy H. leucospilota and incubated with lipopolysaccharide (LPS, 10 μg/ml), polyinosinic-polycytidylic acid [Poly (I:C), 10 μg/ml] and heat-inactived Vibrio harveyi (10⁷ cell/ml) for 24 h, respectively. After high-throughput mRNA sequencing on an Illumina HiSeq2500, a de novo transcriptome was assembled and the Unigenes were annotated. Thirteen differentially expressed genes (DEGs) were selected randomly from our data and subsequently verified by using RT-qPCR. The results of RT-qPCR were consistent with those of the RNA-seq (R² = 0.61). The top 10 significantly enriched signaling pathways and immune-related pathways of the common and unique DEGs were screened from the transcriptome data. Twenty-one cytokine candidate DEGs were identified, which belong to 4 cytokine families, namely, BCL/CLL, EPRF1, IL-17 and TSP/TPO. Gene expression in response to LPS dose-increased treatment (0, 10, 20 and 50 μg/ml) showed that IL-17 family cytokines were significantly upregulated after 10 μg/ml LPS challenge for 24 h.

Conclusion

A de novo transcriptome was sequenced and assembled to generate the gene expression profiling across the sea cucumber coelomocytes treated with LPS, Poly (I:C) and V. harveyi. The cytokine genes identified in DEGs could be classified into 4 cytokine families, in which the expression of IL-17 family cytokines was most significantly induced after 10 μg/ml LPS challenge for 24 h. Our findings have laid the foundation not only for the research of molecular mechanisms related to the immune response in echinoderms but also for their origins in chordates, particularly in higher vertebrates.

Physiological and immunological responses of sea cucumber Apostichopus japonicus during desiccation and subsequent resubmersion

Desiccation is one of the extremely stressful situations experienced by aquatic animals, and sea cucumber usually suffers from desiccation stress during transportation without water. The present study was conducted to evaluate the effect of desiccation and subsequent resubmersion on physiological stress, oxidative damage, antioxidant status and non-specific immune response of Apostichopus japonicus, providing valuable information on the health management of sea cucumber culturing. Control and desiccation groups were set up, and each group has three replicates. After 1, 3 and 6 h of desiccation, individuals were resubmersed in aerated seawater for a 24 h recovery in three batches, which were represented as D1, D3 and D6, respectively. The results showed that glucose level in coelomic fluid of sea cucumber significantly decreased after desiccation, whereas lactate, cortisol and osmolality showed remarkable ascending trends. Thereafter, all stress parameters gently recovered towards normal levels as control group during 24 h resubmersion. The prolonged desiccation at D6 treatment induced the significant increases of malondialdehyde (MDA) and reactive oxygen species (ROS) contents, as well as relatively lower superoxide dismutase (SOD) and catalase (CAT) activities. During the period of desiccation and subsequent resubmersion, sea cucumber adjusted antioxidant defense to reduce the concentrations of MDA and ROS as a strategy for protecting against oxidative damage. Desiccation also had significant effects on non-specific immune parameters (total coelomocytes counts, TCC; complement C3; total nitric oxide synthase, T-NOS; lysozyme, LSZ; alkaline phosphatase, AKP) of A. japonicus, which could be recovered to some extent during resubmersion. In conclusion, less than 6 h of desiccation did not induce irreparable damage to sea cucumber, and was recommended for handling and shipping live sea cucumbers.

Gender specific differences of immune competence in the sea cucumber Apostichopus japonicus before and after spawning

The gender differences of immunity have been elucidated in many vertebrates and invertebrates. However, the information of this difference was still not clear in the sea cucumber Apostichopus japonicus, which is one of the most valuable aquaculture species and susceptible to diseases caused by pathogen infection. In the present study, the transcriptome of coelomocytes from female and male A. japonicus before and after spawning was obtained by RNA-sequencing technology. A total of 4,538 and 8,248 differentially expressed genes were identified between female and male A. japonicus before and after spawning, respectively, indicating that the gender differences of gene expression profiles in A. japonicus were more remarkable after spawning. Further KEGG enrichment analyses were conducted for both male and female up-regulated genes before and after spawning. The results revealed that the capacity to kill pathogens in female A. japonicus might be more powerful than that in males no matter before and after spawning; the antioxidant ability in male A. japonicus was probably stronger than that in females after spawning; the complement system in male A. japonicus might be more effective than that in females after spawning; and the apoptosis was likely to be more serious in male A. japonicus before spawning. Moreover, we speculated that the fatty acid composition might be one of the inducements for gender specific immune differences of A. japonicus. Overall, the results of our study illustrated the global gender specific immune differences of A. japonicus and contributed to understanding of the molecular mechanisms underlying sea cucumber immune regulation.

Saponins from Sea Cucumber and Their Biological Activities

Sea cucumbers, belonging to the phylum Echinodermata, have been valued for centuries as a nutritious and functional food with various bioactivities. Sea cucumbers can produce highly active substances, notably saponins, the main secondary metabolites, which are the basis of their chemical defense. The saponins are mostly triterpene glycosides with triterpenes or steroid in aglycone, which possess multiple biological properties including antitumor, hypolipidemic activity, improvement of nonalcoholic fatty liver, inhibition of fat accumulation, antihyperuricemia, promotion of bone marrow hematopoiesis, antihypertension, etc. Sea cucumber saponins have received attention due to their rich sources, low toxicity, high efficiency, and few side effects. This review summarizes current research on the structure and activities of sea cucumber saponins based on the physiological and pharmacological activities from source, experimental models, efficacy, and mechanisms, which may provide a valuable reference for the development of sea cucumber saponins.