Research progress on the mollusc immunity in China

The chromosome level genome assembly of the Asian green mussel, Perna viridis

The Asian green mussel, Perna viridis is an important aquaculture species in the family Mytilidae contributing substantially to molluscan aquaculture. We generated a high-quality chromosome level assembly of this species by combining PacBio single molecule sequencing technique (SMRT), Illumina paired-end sequencing, high-throughput chromosome conformation capture technique (Hi-C) and Bionano mapping. The final assembly resulted in a genome of 723.49 Mb in size with a scaffold N50 of 49.74 Mb with 99% anchored into 15 chromosomes. A total of 49654 protein-coding genes were predicted from the genome. The presence of 634 genes associated with the cancer pathway and 408 genes associated with viral carcinogenesis indicates the potential of this species to be used as a model for cancer studies. The chromosome-level assembly of this species is also a valuable resource for further genomic selection and selective breeding for improving economically important aquaculture traits and augmenting aquaculture productivity.

Transcriptome-based network analysis reveals hub immune genes and pathways of hepatopancreas against LPS in Amphioctopus fangsiao

The hepatopancreas is the biggest digestive organ in Amphioctopus fangsiao (A. fangsiao), but also undertakes critical functions like detoxification and immune defense. Generally, pathogenic bacteria or endotoxin from the gut microbiota would be arrested and detoxified in the hepatopancreas, which could be accompanied by the inevitable immune responses. In recent years, studies related to cephalopods immune have been increasing, but the molecular mechanisms associated with the hepatopancreatic immunity are still unclear. In this study, lipopolysaccharide (LPS), a major component of the cell wall of Gram-negative bacteria, was used for imitating bacteria infection to stimulate the hepatopancreas of A. fangsiao. To investigate the immune process happened in A. fangsiao hepatopancreas, we performed transcriptome analysis of hepatopancreas tissue after LPS injection, and identified 2615 and 1943 differentially expressed genes (DEGs) at 6 and 24 h post-injection, respectively. GO and KEGG enrichment analysis showed that these DEGs were mainly involved in immune-related biological processes and signaling pathways, including ECM-receptor interaction signaling pathway, Phagosome signaling pathway, Lysosome signaling pathway, and JAK-STAT signaling pathways. The function relationships between these DEGs were further analyzed through protein-protein interaction (PPI) networks. It was found that Mtor, Mapk14 and Atm were the three top interacting DEGs under LPS stimulation. Finally, 15 hub genes involving multiple KEGG signaling pathways and PPI relationships were selected for qRT-PCR validation. In this study, for the first time we explored the molecular mechanisms associated with hepatopancreatic immunity in A. fangsiao using a PPI networks approach, and provided new insights for understanding hepatopancreatic immunity in A. fangsiao.

Molluscan pleiotropic FADD involved in innate immune signaling and induces apoptosis

Fas-associated protein with death domain (FADD) was initially identified as a crucial adaptor protein in the apoptotic pathway mediated by death receptor (DR). Subsequently, many studies have confirmed that FADD plays a vital role in innate immunity and inflammatory responses in animals. However, the function of this pleiotropic molecule in mollusk species has not been well explored. In this study, we successfully verified the gene sequence of FADD in the Zhikong scallop (Chlamys farreri) and designated it as CfFADD. The CfFADD protein contains a conserved death effector and death domains. Phylogenetic analysis showed that CfFADD is a novel addition to the molluscan FADD family with a close evolutionary relationship with molluscan FADD subfamily proteins. CfFADD mRNA expression in various scallop tissues was significantly induced by challenge with pathogen-associated molecular patterns (lipopolysaccharide, peptidoglycan, and poly(I:C)), suggesting its role in innate immunity in scallops. Co-immunoprecipitation showed that CfFADD interacted with the scallop DR (tumor necrosis factor receptor) and a signaling molecule involved in the Toll-like receptor pathway (interleukin-1 receptor-associated kinase), confirming that CfFADD may be involved in DR-mediated apoptosis and innate immune signaling pathways. Further studies showed that CfFADD interacted with CfCaspase-8 and activated caspase-3. HEK293T cells exhibited distinct apoptotic features after transfection with a CfFADD-expression plasmid, suggesting a functional DR-FADD-caspase apoptotic pathway in scallops. Overexpression of CfFADD led to a significant dose-dependent activation of interferon β and nuclear factor-κB reporter genes, demonstrating the key role of CfFADD in innate immunity. In summary, our research has confirmed the critical roles of CfFADD in innate immunity and apoptosis and provides valuable information for developing comparative immunology theories.

A newly identified scallop MyD88 interacts with TLR and functions in innate immunity

Myeloid differentiation factor-88 (MyD88) is a key adaptor of the toll-like receptor (TLR) signaling pathway and plays a crucial role in innate immune signal transduction in animals. However, the MyD88-mediated signal transduction mechanism in shellfish has not been well studied. In this study, a new MyD88 gene, CfMyD88-2, was identified in the Zhikong scallop, Chlamys farreri. The 1779 bp long open reading frame encodes 592 amino acids. The N-terminus of CfMyD88-2 contains a conserved death domain (DD), followed by a TIR (TLR/Interleukin-1 Receptor) domain. The results of the multi-sequence comparison showed that the TIR domain sequences were highly conserved. Phylogenetic analysis revealed that CfMyD88-2 was first associated with Mizuhopecten yessoensis MyD88-4 and Argopecten irradians MyD88-4. CfMyD88-2 mRNA was expressed in all scallop tissues, as detected by qRT-PCR, and the expression level was the highest in the mantle and hepatopancreas. In addition, CfMyD88-2 mRNA expression significantly increased after pathogen-associated molecular patterns (PAMPs, such as lipopolysaccharide, peptidoglycan, or polyinosinic-polycytidylic acid) stimulation. The results of the co-immunoprecipitation experiments in HEK293T cells showed that both CfMyD88-1 and CfMyD88-2 interacted with the TLR protein of scallops, suggesting the existence of more than one functional TLR-MyD88 signaling axis in scallops. Dual luciferase reporter gene assays indicated that the overexpressed CfMyD88-2 in HEK293T cells activated interferon (IFN) α, IFN-β, IFN-γ, and NF-κB reporter genes, indicating that the protein has multiple functions. The results of the subcellular localization experiment uncovered that CfMyD88-2 was mainly localized in the cytoplasm of human cells. In summary, the novel identified CfMyD88-2 can respond to the challenge of PAMPs, participate in TLR immune signaling, and may activate downstream effector genes such as NF-κB gene. These research results will be useful in advancing the theory of innate immunity in invertebrates and provide a reference for the selection of disease-resistant scallops in the future.

Combined effects of toxic Microcystis aeruginosa and high pH on antioxidant responses, immune responses, and apoptosis of the edible freshwater bivalve Corbicula fluminea

Due to increasing anthropogenic perturbation and water eutrophication, cyanobacterial blooms (CYBs) have become a global ecological and environmental problem. Toxic CYBs and elevated pH are considered to be the two key stressors associated with eutrophication in natural waters, particularly in the event of CO2 depletion induced by dense blooms. However, previous research has been focused on investigating the impacts of toxic CYBs or pH changes in isolation, whereas the interactive effects of such stressors on edible bivalves that inhabit CYB waters still lack information. In this study, the combined effects of toxic Microcystis aeruginosa and pH shifts on the antioxidant responses, immune responses, and apoptosis of the edible freshwater bivalve Corbicula fluminea were explored. The results showed that the activity of antioxidant enzymes was significantly impacted by the interactive effects between toxic M. aeruginosa exposure and time course, yet pH shifts showed no significant effects on the activities of these antioxidant enzymes, implying that the antioxidant response in C. fluminea was mainly triggered by toxic M. aeruginosa exposure. Toxic M. aeruginosa also induced an increased production of reactive oxygen species and malondialdehyde in treated clams, particularly under high pH settings. The elevated lysosomal enzyme activity helped C. fluminea defend against toxic M. aeruginosa exposure under high pH conditions. The principal component analysis (PCA) and the integrated biomarker response (IBR) results suggested that the treated clams were subjected to the elevated toxicity of toxic M. aeruginosa in conditions of high pH. The heat shock proteins-related genes might be triggered to resist the oxidative damage in treated clams. Moreover, the upregulation of TNF and casp8 genes indicated the potential activation of the caspase8-mediated apoptotic pathway through TNF receptor interaction, potentially resulting in apoptosis. The TUNEL assay results further confirmed that apoptosis appeared in treated clams. These findings improve our understanding of the combined toxicological effects of harmful algae and pH shifts on bivalves, which will provide insights into a comprehensive ecological risk assessment of toxic CYBs to edible bivalve species.

Antibacterial Properties of Peptide and Protein Fractions from Cornu aspersum Mucus

The discovery and investigation of new natural compounds with antimicrobial activity are new potential strategies to reduce the spread of antimicrobial resistance. The presented study reveals, for the first time, the promising antibacterial potential of two fractions from Cornu aspersum mucus with an MW < 20 kDa and an MW > 20 kDa against five bacterial pathogens-Bacillus cereus 1085, Propionibacterium acnes 1897, Salmonella enterica 8691, Enterococcus faecalis 3915, and Enterococcus faecium 8754. Using de novo sequencing, 16 novel peptides with potential antibacterial activity were identified in a fraction with an MW < 20 kDa. Some bioactive compounds in a mucus fraction with an MW > 20 kDa were determined via a proteomic analysis on 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and bioinformatics. High homology with proteins and glycoproteins was found, with potential antibacterial activity in mucus proteins named aspernin, hemocyanins, H-lectins, and L-amino acid oxidase-like protein, as well as mucins (mucin-5AC, mucin-5B, mucin-2, and mucin-17). We hypothesize that the synergy between the bioactive components determined in the composition of the fraction > 20 kDa are responsible for the high antibacterial activity against the tested pathogens in concentrations between 32 and 128 µg/mL, which is comparable to vancomycin, but without cytotoxic effects on model eukaryotic cells of Saccharomyces cerevisiae. Additionally, a positive effect, by reducing the levels of intracellular oxidative damage and increasing antioxidant capacity, on S. cerevisiae cells was found for both mucus extract fractions of C. aspersum. These findings may serve as a basis for further studies to develop a new antibacterial agent preventing the development of antibiotic resistance.

A molluscan IRF interacts with IKKα/β family protein and modulates NF-κB and MAPK activity

Interferon regulatory factor (IRF) family proteins are key transcription factors involved in vital physiological processes such as immune defense. However, the function of IRF in invertebrates, especially in marine shellfish is not clear. In this study, a new IRF gene (CfIRF2) was identified in the Zhikong scallop, Chlamys farreri, and its immune function was analyzed. CfIRF2 has an open reading frame of 1107 bp encoding 368 amino acids. The N-terminus of CfIRF2 consists of a typical IRF domain, with conserved amino acid sequences. Phylogenetic analysis suggested close evolutionary relationship with shellfish IRF1 subfamily proteins. Expression pattern analysis showed that CfIRF2 mRNA was expressed in all tissues, with the highest expression in the hepatopancreas and gills. CfIRF2 gene expression was substantially enhanced by a pathogenic virus (such as acute viral necrosis virus) and poly(I:C) challenge. Co-immunoprecipitation assay identified CfIRF2 interaction with the IKKα/β family protein CfIKK1 of C. farreri, demonstrating a unique signal transduction mechanism in marine mollusks. Moreover, CfIRF2 interacted with itself to form homologous dimers. Overexpression of CfIRF2 in HEK293T cells activated reporter genes containing interferon stimulated response elements and NF-κB genes in a dose-dependent manner and promoted the phosphorylation of protein kinases (JNK, Erk1/2, and P38). Our results provide insights into the functions of IRF in mollusks innate immunity and also provide valuable information for enriching comparative immunological theory for the prevention of diseases in scallop farming.

A galectin-9 involved in the microbial recognition and haemocyte autophagy in the Pacific oyster Crassostrea gigas

Galectin-9 is a tandem-repeat type member of galectin family participating in various immune responses, such as cell agglutination, phagocytosis, and autophagy. In the present study, a tandem repeat galectin-9 (defined as CgGal-9) was identified from Pacific oyster Crassostrea gigas, which consisted of two conserved carbohydrate recognition domains (CRDs) joined by a linker peptide. CgGal-9 was closely clustered with CaGal-9 from C. angulata, and they were assigned into the branch of invertebrate galectin-9s in the phylogenetic tree. The mRNA transcripts of CgGal-9 were detected in all the tested tissues, with the highest expression level in haemocytes. The mRNA expressions of CgGal-9 in haemocytes increased significantly after lipopolysaccharide (LPS) and Vibrio splendidus stimulation. The recombinant CgGal-9 was able to bind all the examined pathogen-associated molecular patterns (LPS, peptidoglycan, and mannose) and microbes (V. splendidus, Escherichia coli, Micrococcus luteus, Staphylococcus aureus, Bacillus subtilis, and Pichia pastoris), and agglutinated most of them in the presence of Ca2+. In CgGal-9-RNAi oysters, the mRNA expressions of autophagy related genes (CgBeclin1, CgATG5, CgP62 and CgLC3) in haemocytes decreased significantly while that of CgmTOR increased significantly at 3 h after V. splendidus stimulation. The autophagy level and mRNA expressions of autophagy related genes decreased in haemocytes after CgGal-9 was blocked by the corresponding antibody. These results revealed that CgGal-9 was able to bind different microbes and might be involved in haemocyte autophagy in the immune response of oyster.

Identification and functional analysis of myeloid differentiation factor 88 (MyD88) in early development of Haliotis diversicolor

Myeloid differentiation factor 88 (MyD88) is a universal adaptor protein and plays an important role in the signal transduction of Toll like receptors (TLR) family. In this study, the MyD88 gene from the Haliotis diversicolor (hdMyD88) was identified. The full-length cDNA of hdMyD88 has a 1927 base pairs (bp), with an open reading frame of 1314 bp encoding 437 amino acids including a death domain (DD) at the N-terminus and TIR domain at the C-terminus which are typical features of MyD88 family proteins. Three conserved boxes are also found in the hdMyD88, which are similar to MyD88 in vertebrates. The expression levels of hdMyD88 mRNA at different early embryonic developmental stages of abalone were measured by qPCR revealed that their constitutive expression at all developmental stages analyzed with the considerably highest values at 8 cell stage and the lowest level at the trochosphere stage. Additionally, the mRNA expression of hdMyD88 decreased significantly (P < 0.05) after MyD88-dsRNA soak in the stage of trochosphere and veliger than EGFP-dsRNA group and blank control group. Whole embryo in situ hybridization showed that the positive signals of hdMyD88 were in visceral mass of trochophore larvae and veliger larvae. These results indicate hdMyD88 may could respond to pathogenic infection and may play an important role in early innate immunity in the process of abalone larval development.

A novel perlucin-like protein (PLP) protects Litopenaeus vannamei against Vibrio harveyi infection

C-type lectins (CTLs), as pattern recognition receptors (PRRs), play an important role in the innate immunity of Litopenaeus vannamei. In this study, a novel CTL, named perlucin-like protein (PLP), was identified from L. vannamei, which shared homology sequences of PLP from Penaeus monodon. PLP from L. vannamei was expressed in the hepatopancreas, eyestalk, muscle and brain and could be activated in the tissues (hepatopancreas, muscle, gill and intestine) after infection with the pathogen Vibrio harveyi. Bacteria (Vibrio alginolyticus, V. parahaemolyticus, V. harveyi, Streptococcus agalactiae and Bacillus subtilis) could be bound and agglutinated by the PLP recombinant protein in a Ca2+-dependent manner. Moreover, PLP could stabilise the expression of the immune-related genes (ALF, SOD, HSP70, Toll4 and IMD) and apoptosis gene (Caspase2). The RNAi of PLP could remarkably affect the expression of antioxidant gene, antimicrobial peptide genes, other CTLs, apoptosis genes, Toll signaling pathways, and IMD signaling pathways. Moreover, PLP reduced the bacterial load in the hepatopancreas. These results suggested that PLP was involved in the innate immune response against V. harveyi infection by recognising bacterial pathogens and activating the expression of immune-related and apoptosis genes.

Investigation of Immune Responses in Giant African Snail, Achatina immaculata, against a Two-Round Lipopolysaccharide Challenge

As one of the 100 most-threatening invasive alien species, the giant African snail (Achatina immaculata) has successfully invaded and established itself in most areas of southern China. Protection against recurrent pathogen infections is vital to biological invasion. Enhanced immune protection has been previously found in other invertebrates, but not in the unique immune system of the giant African snail. In the present study, the survival rate of the giant African snail was recorded following a second infection with lethal doses of Escherichia coli after a previous first injection using lipopolysaccharide (LPS), and the mechanism of immune enhancement was investigated by examining the cellular and transcriptomic response of the giant African snail after two successive stimuli using LPS. Snails injected first with LPS, sterilized physiologic (0.9%) saline (SPS), phosphate-buffered saline (PBS) or untreated (Blank) were rechallenged at 7d with E. coli (Ec), and were named as LPS + Ec, SPS + Ec, PBS + Ec, Ec, and Blank. The log-rank test shows the survival rate of the LPS + Ec group as significantly higher than that of other control groups after the second injection (p < 0.05). By performing cell counting and BrdU labeling on newly generated circulating hemocytes, we found that the total hemocyte count (THC) and the ratio of BrdU-positive cells to total cells increased significantly after primary stimulation with LPS and that they further increased after the second challenge. Then, caspase-3 of apoptosis protease and two antioxidant enzyme activities (CAT and SOD) increased significantly after infection, and were significantly higher in the second response than they had been in the first round. Moreover, transcriptome analysis results showed that 84 differentially expressed genes (DEGs) were expressed at higher levels in both the resting and activating states after the second immune response compared to the levels observed after the first challenge. Among them, some DEGs, including Toll-like receptor 4 (TLR4) and its downstream signaling molecules, were verified using qRT-PCR and were consistent with the transcriptome assay results. Based on gene expression levels, we proposed that these genes related to the TLR signaling cascade participate in enhanced immune protection. All results provide evidence that enhanced immune protection exists in the giant African snail.

Creb2 involved in innate immunity by activating PpMitf-mediated melanogenesis in Pteria penguin

cAMP response element binding protein 2 (CREB2) acts as an intracellular transcriptional factor and regulates many physiological processes, including melanogenesis and melanocyte differentiation. In our previous research, the Creb2 gene has been characterized from Pteria penguin (P. penguin), but its role and regulatory mechanism in P. penguin are still unclear. In this study, first, the function of PpCreb2 in melanogenesis and innate immunity were identified. PpCreb2 silencing significantly decreased the tyrosinase activity and melanin content, indicating PpCreb2 played an important role in melanogenesis. Meanwhile, PpCreb2 silencing visibly suppressed the antibacterial activity of hemolymph supernatant, indicating that PpCreb2 was involved in innate immunity of P. penguin. Second, the PpCreb2 was confirmed to perform immune function by regulating the melanogenesis. The decreased melanin oxidation product due to PpCreb2 silencing triggered the declining of antibacterial activity of hemolymph supernatant, which then could be rescued by adding exogenous melanin oxidation products. Third, the regulation pathway of PpCreb2 involved in innate immunity was analyzed. The promoter sequence analysis of PpMitf discovered 5 conserved cAMP response element (CRE), which were specifically recognized by basic Leucine zipper domain (bZIP) of upstream activation transcription factor. The luciferase activities analysis showed that PpCreb2 could activate the CRE in PpMitf promoter via highly conserved bZIP domain and regulate the expression of PpMitf, which further regulated the PpTyr expression. Therefore, the results collectively demonstrated that PpCreb2 participated in innate immunity by activating PpMitf-mediated melanogenesis in P. penguin.

TLR4 involved in immune response against Vibrio Parahaemolyticus by MyD88-dependent pathway in Crassostrea hongkongensis

Vibrio parahaemolyticus (V. parahaemolyticus) is a salt-loving gram-negative bacterium, and is the leading cause of mortality in cultured shellfish in recent years. Toll-like Receptor 4 (TLR4) is a classical pattern recognition receptor (PRRs) that recognizes pathogen-associated molecular patterns (PAMPs) of pathogenic microorganism and activates the immune response. However, the function and signal pathway of TLR4 in oyster are still unknown. In this study, a new TLR4 gene was identified from the Crassostrea hongkongensis (C. hongkongensis). The ChTLR4 contained an open reading frame of 2643 bp, encoding 880 amino acids with seven leucine-rich repeat (LRR) domains and a Toll/IL-1R (TIR) domain. The ChTLR4 shared the highest sequence identity (83.0%) with TLR4 of Crassostrea gigas. Tissue expression analysis revealed that ChTLR4 showed the highest constitutive expression in the gill and hepatopancreas, and was significantly upregulated in immune tissues post V. parahaemolyticus infection, especially in gill and hemocytes. Moreover, TLR4 silencing significantly inhibited the immune-enzyme activities, including SOD, CAT, ACP, AKP in gill and LZM in hemolymph supernatant, and increased MDA content in hemolymph supernatant. Meanwhile, the antimicrobial activities of the hemolymph supernatant were also significantly inhibited by TLR4 silencing. These data demonstrated that the ChTLR4 involved in innate immune response of C. hongkongensis against V. parahaemolyticus challenge. Finally, qRT-PCR analysis showed that ChTLR4 silencing clearly inhibited the expression of genes in TLR4-MyD88 pathway, indicating that MyD88-dependent pathway played a crucial role in ChTLR4-mediated immune response against V. parahaemolyticus.

Identification and Characterization of Three New Antimicrobial Peptides from the Marine Mollusk Nerita versicolor (Gmelin, 1791)

Mollusks have been widely investigated for antimicrobial peptides because their humoral defense against pathogens is mainly based on these small biomolecules. In this report, we describe the identification of three novel antimicrobial peptides from the marine mollusk Nerita versicolor. A pool of N. versicolor peptides was analyzed with nanoLC-ESI-MS-MS technology, and three potential antimicrobial peptides (Nv-p1, Nv-p2 and Nv-p3) were identified with bioinformatical predictions and selected for chemical synthesis and evaluation of their biological activity. Database searches showed that two of them show partial identity to histone H4 peptide fragments from other invertebrate species. Structural predictions revealed that they all adopt a random coil structure even when placed near a lipid bilayer patch. Nv-p1, Nv-p2 and Nv-p3 exhibited activity against Pseudomonas aeruginosa. The most active peptide was Nv-p3 with an inhibitory activity starting at 1.5 µg/mL in the radial diffusion assays. The peptides were ineffective against Klebsiella pneumoniae, Listeria monocytogenes and Mycobacterium tuberculosis. On the other hand, these peptides demonstrated effective antibiofilm action against Candida albicans, Candida parapsilosis and Candida auris but not against the planktonic cells. None of the peptides had significant toxicity on primary human macrophages and fetal lung fibroblasts at effective antimicrobial concentrations. Our results indicate that N. versicolor-derived peptides represent new AMP sequences and have the potential to be optimized and developed into antibiotic alternatives against bacterial and fungal infections.

Transcriptome analysis preliminary reveals the immune response mechanism of golden cuttlefish (Sepia esculenta) larvae exposed to Cd

As a well-known marine metal element, Cd can significantly affect bivalve mollusk life processes such as growth and development. However, the effects of Cd on the molecular mechanisms of the economically important cephalopod species Sepia esculenta remain unclear. In this study, S. esculenta larval immunity exposed to Cd is explored based on RNA-Seq. The analyses of GO, KEGG, and protein-protein interaction (PPI) network of 1,471 differentially expressed genes (DEGs) reveal that multiple immune processes are affected by exposure such as inflammatory reaction and cell adhesion. Comprehensive analyses of KEGG signaling pathways and the PPI network are first used to explore Cd-exposed S. esculenta larval immunity, revealing the presence of 16 immune-related key and hub genes involved in exposure response. Results of gene and pathway functional analyses increase our understanding of Cd-exposed S. esculenta larval immunity and improve our overall understanding of mollusk immune functions.

Characteristic and antibacterial effect of a histone H2A and its preliminary roles in extracellular traps in manila clam Ruditapes philippinarum

In the present study, a histone H2A (designed as RpH2A) was identified and characterized from clam Ruditapes philippinarum, and its open reading frame (ORF) was of 387 bp encoding a polypeptide of 128 amino acids. The deduced amino acid sequence of RpH2A shared high identities ranging from 57.1% to 96.1% with that of other identified H2A. The mRNA expression of RpH2A was up-regulated significantly after Vibrio anguillarum challenge. The recombinant RpH2A protein (rRpH2A) displayed significantly binding affinity to lipopolysaccharide (LPS) and peptidoglycan (PGN) in vitro, and also exhibited antimicrobial properties against Escherichia coli. In addition, the antimicrobial RpH2A was shown to co-localize with extracellular traps (ETs) released from hemocytes induced by E. coli, suggesting that RpH2A might contribute to eliminate invading bacteria in clam ETs. Altogether, our data revealed that RpH2A could function as antimicrobial peptides, which might play a crucial role in the immune responses of hemocytes ETs in clams.

Comprehensive analysis of the lncRNAs, mRNAs, and miRNAs implicated in the immune response of Pinctada fucata martensii to Vibrio parahaemolyticus

Non-coding RNAs (ncRNAs) have been implicated in a variety of biological processes. However, most ncRNAs are of unknown function and are as-yet unannotated. The immune-related functions of ncRNAs in the pearl oyster Pinctada fucata martensii were explored based on transcriptomic differences in the expression levels of long non-coding RNAs (lncRNAs), microRNAs (miRNAs), and messenger RNAs (mRNAs) in the hemocytes of P.f. martensii after challenge by the pathogenic bacterium Vibrio parahaemolyticus. Across the challenged and control pearl oysters, 144 miRNAs and 14,571 lncRNAs were identified. In total, 13,375 ncRNAs were differentially expressed between the challenged and control pearl oysters; in the challenged pearl oysters as compared to the controls, 15 miRNAs and 5147 lncRNAs were upregulated, while 51 miRNAs and 8162 lncRNAs were downregulated. The sequencing results were validated using quantitative real-time polymerase chain reaction (qRT-PCR) analysis. GO and KEGG pathway analysis showed that genes targeted by the differentially expressed ncRNAs were associated with the vascular endothelial growth factor (VEGF) signaling pathway and the nuclear factor kappa-B (NF-κB) signaling pathway. An lncRNA-mRNA-miRNA network that was developed based on the transcriptomic results of this study suggested that lncRNAs may compete with miRNAs for mRNA binding sites. This study may provide a useful framework for the detection of additional novel ncRNAs, as well as new insights into the pathogenic mechanisms underlying the response of P.f. martensii to V. parahaemolyticus.

Molecular characterization of an LRR-only protein gene in Pacific white shrimp Litopenaeus vannamei: Sequence feature, expression pattern, and protein activity

Leucine-rich repeat (LRR)-only proteins have been proved to be involved in the innate immune responses as they could mediate protein-protein or protein-ligand interactions. In the present study, a novel LRR-only protein (LvLRRop-1) was identified and characterized from Pacific white shrimp Litopenaeus vannamei. The complete cDNA sequence of LvLRRop-1 contains an open reading frame (ORF) of 1488 bp, which encoded a polypeptide of 495 amino acids with a predicted molecular mass of 55.67 kDa and a calculated theoretical isoelectric point of 6.435. There are five LRR motifs, six LRR_TYP motifs in the protein sequence of LvLRRop-1 with consensus signature sequences of LxxLxxLxLxxNxL. The LvLRRop-1 mRNA transcripts could be detected in all the tested tissues, including eyestalk, gill, gonad, heart, hemocytes, hepatopancreas, intestine, muscle, nerve and stomach, especially highest in hemocytes and hepatopancreas. The mRNA transcripts of LvLRRop-1 increased within the first 6 h in hemocytes and hepatopancreas after Vibrio parahaemolyticus or white spot syndrome virus (WSSV) challenges. The recombinant LvLRRop-1 could bind four typical pathogen-associated molecular patterns (PAMPs), including lipopolysaccharide (LPS), peptidoglycan (PGN), glucan (GLU) and polycytidine-polycytidylic acid (poly IC), in a dose-dependent manner, and inhibit the growth of bacteria Micrococcus luteus. These data indicated that LvLRRop-1 could play a pivotal role in the innate immune response of shrimps as a kind of pattern recognition receptor (PRR).

Integrated RNA-seq and RNAi Analysis of the Roles of the Hsp70 and SP Genes in Red-Shell Meretrix meretrix Tolerance to the Pathogen Vibrio parahaemolyticus

The "Wanlihong" Meretrix meretrix (WLH-M) clam is a new variety of this species that has a red shell and stronger Vibrio tolerance than ordinary M. meretrix (ORI-M). To investigate the molecular mechanisms responsible for the WLH-M strain's tolerance to Vibrio, we challenged clams with Vibrio parahaemolyticus and then assessed physiological indexes and conducted transcriptome analysis and RNA interference experiments. The mortality, tissue bacterial load, and hemocyte reactive oxygen species level of ORI-M were significantly higher than those of WLH-M, whereas the content and activity of lysozyme were significantly lower. Gene Ontology functional annotation analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis revealed that immune and metabolic pathways were enriched in Vibrio-challenged clams. The expressions of the heat shock protein 70 (Hsp70) and serine protease (SP) genes, which are involved in antibacterial immunity, were significantly upregulated in WLH-M but not in ORI-M, while the expression of the kynurenine 3-monooxygenase gene, a proinflammatory factor, was significantly downregulated in WLH-M. RNA interference experiments confirmed that Hsp70 and SP downregulation could result in increased mortality of WLH-M. Therefore, we speculate that Hsp70 and SP may be involved in the antibacterial immunity of WLH-M in vivo. Our data provided a valuable resource for further studies of the antibacterial mechanism of WLH-M and provided a foundation for the breeding of pathogen-resistant strains.

Efficient Oral Priming of Tenebrio molitor Larvae Using Heat-Inactivated Microorganisms

Microbial resistance is a global health problem that will increase over time. Advances in insect antimicrobial peptides (AMPs) offer a powerful new approach to combat antimicrobial resistance. Invertebrates represent a rich group of animals for the discovery of new antimicrobial agents due to their high diversity and the presence of adaptive immunity or “immune priming”. Here, we report a priming approach for Tenebrio molitor that simulates natural infection via the oral route. This oral administration has the advantage of minimizing the stress caused by conventional priming techniques and could be a viable method for mealworm immunity studies. When using inactivated microorganisms for oral priming, our results showed an increased survival of T. molitor larvae after exposure to various pathogens. This finding was consistent with the induction of antimicrobial activity in the hemolymph of primed larvae. Interestingly, the hemolymph of larvae orally primed with Escherichia coli showed constitutive activity against Staphylococcus aureus and heterologous activity for other Gram-negative bacteria, such as Salmonella enterica. The priming of T. molitor is generally performed via injection of the microorganism. To our knowledge, this is the first report describing the oral administration of heat-inactivated microorganisms for priming mealworms. This technique has the advantage of reducing the stress that occurs with the conventional methods for priming vertebrates.

Transcriptome profiling based on larvae at different time points after hatching provides a core set of gene resource for understanding the immune response mechanisms of the egg-protecting behavior against Vibrio anguillarum infection in Amphioctopus fangsiao

Mollusks have recently received increasing attention because of their unique immune systems. Mollusks such as Amphioctopus fangsiao are economically important cephalopods, and the effects of their egg-protecting behavior on the larval immune response are unclear. Meanwhile, little research has been done on the resistance response of cephalopod larvae infected with pathogenic bacteria such as Vibrio anguillarum. In this study, V. anguillarum was used to infect the primary hatching A. fangsiao larvae under different egg-protecting behaviors for 24 h, and a total of 7156 differentially expressed genes (DEGs) were identified at four time points after hatching based on transcriptome analysis. GO and KEGG enrichment analyses showed that multiple immune-related GO terms and KEGG signaling pathways were enriched. Protein-protein interaction networks (PPI networks) were used to search functional relationships between immune-related DEGs. Finally, 20 hub genes related to multiple gene functions or involved in multiple signaling pathways were identified, and their accuracy was verified using quantitative RT-PCR. PPI networks were first used to study the effects A. fangsiao larvae after infection with V. anguillarum under different egg-protecting behaviors. The results provide significant genetic resources for exploring invertebrate larval immune processes. The data lays a foundation for further study the immune response mechanisms for invertebrates after infection.

Effect and Mechanisms of Antibacterial Peptide Fraction from Mucus of C. aspersum against Escherichia coli NBIMCC 8785

Peptides isolated from the mucus of Cornu aspersum could be prototypes for antibiotics against pathogenic bacteria. Information regarding the mechanisms, effective concentration, and methods of application is an important tool for therapeutic, financial, and ecological regulation and a holistic approach to medical treatment. A peptide fraction with MW < 10 kDa was analyzed by MALDI-TOF-TOF using Autoflex™ III. The strain Escherichia coli NBIMCC 8785 (18 h and 48 h culture) was used. The changes in bacterial structure and metabolic activity were investigated by SEM, fluorescent, and digital image analysis. This peptide fraction had high inhibitory effects in surface and deep inoculations of E. coli of 1990.00 and 136.13 mm2/mgPr/µMol, respectively, in the samples. Thus, it would be effective in the treatment of infections involving bacterial biofilms and homogenous cells. Various deformations of the bacteria and inhibition of its metabolism were discovered and illustrated. The data on the mechanisms of impact of the peptides permitted the formulation of an algorithm for the treatment of infections depending on the phase of their development. The decrease in the therapeutic concentrations will be more sparing to the environment and will lead to a decrease in the cost of the treatment.

Transcriptome analysis reveals potential key immune genes of Hong Kong oyster (Crassostrea hongkongensis) against Vibrio parahaemolyticus infection

Hong Kong oyster (Crassostrea hongkongensis) is one of the main species of economic shellfish cultivated in the coastal areas of southern China. The cultivation of this shellfish may be adversely impacted by Vibrio parahaemolyticus, a harmful pathogenic bacterium for many mariculture species, as it usually exists on the surface of Hong Kong oysters. Although previous studies have discovered that oysters rely on non-specific immune system to fight pathogen invasion, the genes corresponding to the complex immune system against Vibrio is still not fully elucidated. Therefore, we conducted a transcriptome analysis on the gill from Hong Kong oysters at two time points (i.e., 12 h and 24 h after V. parahaemolyticus or PBS challenge) to identify potential immune genes against V. parahaemolyticus infection. A total of 61779 unigenes with the average length of 1221 bp were obtained, and the annotation information of 39917 unigenes were obtained from Nr, SwissProt, KEGG and COG/KOG. After a pairwise comparison between V. parahaemolyticus or PBS challenge at the two time points, three groups of differentially expressed genes induced by V. parahaemolyticus were captured and analyzed. GO and KEGG analyses showed that multiple immune-related genes played an important role in pathogen infection, including HSP70, PCDP3 and TLR4. Furthermore, genes annotation indicated that LITAF, TNFSF10, Duox2 and big defensin family are also involved in immune regulation. Our study provides a reference for further exploration the molecular mechanism that defenses the pathogen infection regarding the identified immune-related genes in Hong Kong oysters.

Yorkie Negatively Regulates the Expression of Antimicrobial Proteins by Inducing Cactus Transcription in Prawns Macrobrachium nipponense

The Hippo signaling pathway controls organ size and immune system in Drosophila and mammals. Yorkie acts as a transcriptional co-activator in the Hippo pathway and cross-talks with other essential pathways. In this study, a Yorkie gene and two Cactus isoforms (designated as MnYorkie, MnCactus-a, and MnCactus-b, respectively) were isolated and characterized from oriental river prawns (Macrobrachium nipponense). Results showed that MnYorkie includes 1620 bp open reading frame and encodes a protein of 539 amino acids (aa). MnCactus-a (377 aa) and MnCactus-b (471 aa) were produced by alternative splicing. MnYorkie and MnCactus were continuously expressed in all selected tissues. Upon Gram-positive bacterium Staphylococcus aureus and Gram-negative bacterium Vibrio parahaemolyticus stimulation, the mRNA levels of MnYorkie and MnCactus in hemocytes and intestines underwent time-dependent enhancement. RNA interference studies showed that MnYorkie silencing remarkably downregulated the transcription of MnCactus but upregulated the expression of seven immune-related genes. In addition, MnYorkie silencing in vivo decreased the susceptibility of prawns to bacterial challenge. After S. aureus and V. parahaemolyticus infection, the survival rate of prawns increased significantly from 2 to 6 days, which corresponded to the period of MnYorkie knockdown. All these findings suggested that MnYorkie in the Hippo pathway might exhibit remarkable biological roles in the immune defense of M. nipponense by negatively regulating the expression of immune-related genes and promoting the transcription of MnCactus.

The mechanism of apoptosis of Chlamys farreri hemocytes under benzopyrene stress in vitro

Hemocytes are critical to the immune defense system of bivalves, and polycyclic aromatic hydrocarbons (PAHs) can mediate the immunity of bivalves by affecting the apoptosis of hemocytes. However, the underlying mechanism is still unclear. Chlamys farreri, as an important economic bivalve, was selected as the research subject for this experimentation. The hemocytes were exposed to typical PAHs-benzopyrene (B[a]P) in vitro to explore the apoptosis mechanism through detecting oxidative stress and oxidative damage-related indicators, apoptosis pathway factors, and apoptosis rate within 24 h. The results showed that the reactive oxygen species (ROS) and benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE) content in hemocytes increased significantly under B[a]P exposure, while antioxidant genes, glutathione peroxidase content and total antioxidant capacity all showed a trend of first rising and subsequent falling. B[a]P also caused serious damage to DNA and lysosomal membrane stability. The proapoptotic factors genes in the mitochondrial apoptosis pathway were significantly up-regulated, and the anti-apoptotic gene Bcl-2 was significantly down-regulated. Besides, mitochondrial membrane potential stability was significantly reduced and caspase 9 enzyme activity was significantly improved with the B[a]P stimulation. The factors of death receptor pathway were also significantly up-regulated by B[a]P. Moreover, the expression levels of Mitogen-Activated Protein Kinases were also induced. The gene expression and enzyme activity of the caspase 3 and the apoptosis rate were significantly increased under B[a]P exposure. In conclusion, these results indicated that ROS was induced by B[a]P, and further triggered the oxidative stress and oxidative damage in hemocytes. B[a]P induced hemocyte apoptosis was mediated by both mitochondrial apoptosis pathway and death receptor apoptosis, and the activation of mitochondrial apoptosis pathway was affected by ROS. In addition, BPDE and MAPKs may play important roles in the B[a]P-mediated apoptosis pathway. This study deepens understanding of the apoptosis pathway and the immunotoxicity mechanism in bivalves hemocytes stimulated by persistent organic pollutants.

Ancestral Physical Stress and Later Immune Gene Family Expansions Shaped Bivalve Mollusc Evolution

Bivalve molluscs comprise 20,000 species occupying a wide diversity of marine habitats. As filter feeders and detritivores they act as ecosystem engineers clarifying water, creating reefs, and protecting coastlines. The global decline of natural oyster reefs has led to increased restoration efforts in recent years. Bivalves also play an important role in global food security contributing to >20% of worldwide aquaculture production. Despite this importance, relatively little is known about bivalve evolutionary adaptation strategies. Difficulties previously associated with highly heterozygous and repetitive regions of bivalve genomes have been overcome by long-read sequencing, enabling the generation of accurate bivalve assemblies. With these resources we have analyzed the genomes of 32 species representing each molluscan class, including 15 bivalve species, to identify gene families that have undergone expansion during bivalve evolution. Gene family expansions across bivalve genomes occur at the point of evolutionary pressures. We uncovered two key factors that shape bivalve evolutionary history: expansion of bivalvia into environmental niches with high stress followed by later exposure to specific pathogenic pressures. The conserved expansion of protein recycling gene families we found across bivalvia is mirrored by adaptations to a sedentary lifestyle seen in plants. These results reflect the ability of bivalves to tolerate high levels of environmental stress and constant exposure to pathogens as filter feeders. The increasing availability of accurate genome assemblies will provide greater resolution to these analyses allowing further points of evolutionary pressure to become clear in other understudied taxa and potentially different populations of a single species.

Arginase plays an important role in ammonia detoxification of yellow catfish Pelteobagrus fulvidraco

A two-stage study was carried out to test the mechanism of arginase in ammonia detoxification of yellow catfish. At stage 1, fish was injected lethal half concentration ammonium acetate and 0.9% sodium chloride respectively every 12 h in six replicates for 72 h. The result found that no significant different in serum ammonia contents of fish in ammonium acetate group at hours 12, 24, 36, 48, 60 and 72. At stage 2, ammonium acetate group was split in two, one continued to injected with ammonium acetate (NH₃ group) and the other with ammonium acetate and valine (an inhibitor of arginase; Val group); Sodium chloride group also was split in two, one continued to injected with sodium chloride (NaCl group) and the other with sodium chloride and valine (NaCl + Val group). The experiment continued for 12 h. Serum ammonia and liver arginine contents of fish in Val group were higher than those of fish in NH₃ group; Compared with NaCl group, arginase activity and ARG 1 expression in liver of fish in Val group were lower; Fish in NaCl and NaCl + Val groups had the lowest serum superoxide dismutase activities, malondialdehyde, tumor necrosis factor-α, interleukin 1 and 8 contents, TNF-α, IL-1 and IL-8 expressions than fish in NH₃ and Val groups, and had the higher lysozyme activities, complement 3 and 4 contents. This study indicates that ammonia poisoning would lead to oxidative damage, immunosuppression and inflammation in yellow catfish; Arginase may be an important target of ammonia toxicity in yellow catfish; Exogenous arginine supplementation might alleviate the symptoms of ammonia poisoning in yellow catfish.

Microplastics alter digestive enzyme activities in the marine bivalve, Mytilus galloprovincialis

Microplastics are eaten by many invertebrates, particularly filter-feeding organisms like mussels. Since microplastics can be retained in the digestive system for extended periods, there is ample opportunity for them to interact with the functions of digestive enzymes. This study determined how the polymer type, size and concentration of ingested spherical microplastics affects the activities of seven key digestive enzymes in the digestive gland of Mytilus galloprovincialis, a common marine mussel. Polymer type significantly affected the activities of carbohydrase enzymes: polystyrene reduced amylase and xylanase activities, and increased cellulase activity. High concentrations of microplastics (5 × 10⁴ microplastics L^-1) caused a 2.5-fold increase in total protease activity. The activities of laminarinase, lipases and lipolytic esterases were unaffected by the polymer type, size or concentration of microplastics. Microplastics-induced changes to digestive enzyme activities can affect mussels' ability to acquire energy from food and reduce their energy reserves.

Dorsal regulates the expression of two phage lysozymes acquired via horizontal gene transfer in triangle sail mussel Hyriopsis cumingii

Dorsal is a Rel/NF-κB transcription factor, which forms a key part of the Toll pathway. Lysozyme is a ubiquitous enzyme that degrades bacterial cell walls. In this study, a Dorsal homolog was cloned and characterized from triangle sail mussel Hyriopsis cumingii, namely, HcDorsal. Dorsal consisted of 3041 bp, including a 1938 bp open reading frame encoding a 645 amino acid protein. The deduced HcDorsal protein contained a Rel homology domain and an Ig-like, plexin, transcription factor domain. Analysis of expression patterns showed that HcDorsal was highly expressed in the hepatopancreas of H. cumingii. The expression level of HcDorsal continuously increased after Vibrio parahaemolyticus stimulation. When HcDorsal was knocked down by siRNA interference, two phage lysozyme genes (HcLyso1 and HcLyso2) obtained by horizontal gene transfer were significantly downregulated in hemocytes of mussels. Furthermore, knockdown of HcLyso1 and HcLyso2 could weaken V. parahaemolyticus clearance ability. Recombinant HcLyso1 and HcLyso2 proteins accelerated the bacterial clearance in vivo in mussels and evidently inhibited the growth of V. parahaemolyticus. These results suggested that HcDorsal could be activated after V. parahaemolyticus stimulation and then modulate the immune response through the transcriptional regulation of HcLyso1 and HcLyso2, thereby playing a protective role in mussels.

Potential of genomic technologies to improve disease resistance in molluscan aquaculture

Molluscan aquaculture is a major contributor to global seafood production, but is hampered by infectious disease outbreaks that can cause serious economic losses. Selective breeding has been widely used to improve disease resistance in major agricultural and aquaculture species, and has clear potential in molluscs, albeit its commercial application remains at a formative stage. Advances in genomic technologies, especially the development of cost-efficient genomic selection, have the potential to accelerate genetic improvement. However, tailored approaches are required owing to the distinctive reproductive and life cycle characteristics of molluscan species. Transgenesis and genome editing, in particular CRISPR/Cas systems, have been successfully trialled in molluscs and may further understanding and improvement of genetic resistance to disease through targeted changes to the host genome. Whole-organism genome editing is achievable on a much greater scale compared to other farmed species, making genome-wide CRISPR screening approaches plausible. This review discusses the current state and future potential of selective breeding, genomic tools and genome editing approaches to understand and improve host resistance to infectious disease in molluscs. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

The glutaminase (CgGLS-1) mediates anti-bacterial immunity by prompting cytokine synthesis and hemocyte apoptosis in Pacific oyster Crassostrea gigas

Glutaminase, an amidohydrolase enzyme that hydrolyzes glutamine to glutamate, plays crucial roles in various immunomodulatory processes such as cell apoptosis, proliferation, migration, and secretion of cytokines. In the present study, a glutaminase homologue (designated as CgGLS-1) was identified from Pacific oyster Crassostrea gigas, whose open reading frame was of 1836 bp. CgGLS-1 exhibited high sequence identity with vertebrate kidney-type GLS, and closely clustered with their homologues from mollusc C. virginica. The enzyme activity of recombinant CgGLS-1 protein (rCgGLS-1) was estimated to be 1.705 U/mg. CgGLS-1 mRNA was constitutively expressed in all the tested tissues of oysters, with the highest expression level in hemocytes. CgGLS-1 mRNA expression in hemocytes was significantly up-regulated and peaked at 6 h (2.07-fold, p < 0.01) after lipopolysaccharide (LPS) stimulation. The CgGLS-1 protein was mainly distributed in the cytoplasm with a significant co-location with mitochondria in oyster hemocytes. The content of Glu in the oyster serum was significantly decreased after the inhibition of CgGLS-1 using specific inhibitor Bis-2- [5-(phenyl acetamido)-1,3,4-thiadiazol-2-yl] ethyl sulfide (BPTES), and the expression levels of CgmGluR6, CgAP-1, cytokines CgIL17-5 and CgTNF-1 were significantly decreased after BPTES and LPS stimulation. The transcripts of CgCaspase3 as well as the apoptosis index of hemocytes were also decreased. These results collectively suggest that CgGLS-1 is the enzyme to synthesize Glu in oyster, which can modulate anti-bacterial immunity by regulating the secretion of pro-inflammatory cytokines CgIL17-5 and CgTNF-1, as well as hemocyte apoptosis.

Transcriptomic analysis reveals the immune changes associated with reproduction in the clam Meretrix petechialis

Substantial mortality and economic losses in marine mollusk culture has drawn considerable attention in recent years. The changes in immune status and environmental stress are thought to be the main causes of shellfish summer mortality. The reproduction and immune defense are both physiologically demanding processes, therefore, the immune status of mollusk is likely to be affected by reproduction during breeding. In present study, we performed transcriptome and gene expression analyses in the clam Meretrix petechialis pre-/post-spawning. DEGs enrichment analysis revealed important immune signaling pathways and key genes changed after spawning. Further analysis showed females up-regulated genes involved in apoptosis, TLR signal pathway and heat shock, whereas males down-regulated complement-related genes after spawning. Additionally, both genders of clams up-regulated its immune response level to against Vibrio infection after spawning revealed by the changes of four immune-related DEGs. The up-regulation of two marker genes at the transcription and protein levels further confirmed that pathogen reinforced the expression differences of immune-related genes between the two groups. Our study provides a new insight into the understanding of molecular mechanisms underlying reproduction influenced immune differences in M. petechialis.

Extracellular Vesicles and Post-Translational Protein Deimination Signatures in Mollusca-The Blue Mussel (Mytilus edulis), Soft Shell Clam (Mya arenaria), Eastern Oyster (Crassostrea virginica) and Atlantic Jacknife Clam (Ensis leei)

Simple Summary

Oysters and clams form an important component of the food chain and food security and are of considerable commercial value worldwide. They are affected by pollution and climate change, as well as a range of infections, some of which are opportunistic. For aquaculture purposes they are furthermore of great commercial value and changes in their immune responses can also serve as indicators of changes in ocean environments. Therefore, studies into understanding new factors in their immune systems may aid new biomarker discovery and are of considerable value. This study assessed new biomarkers relating to changes in protein function in four economically important marine molluscs, the blue mussel, soft shell clam, Eastern oyster, and Atlantic jacknife clam. These findings indicate novel regulatory mechanisms of important metabolic and immunology related pathways in these mollusks. The findings provide new understanding to how these pathways function in diverse ways in different animal species as well as aiding new biomarker discovery for Mollusca aquaculture.

Abstract

Oysters and clams are important for food security and of commercial value worldwide. They are affected by anthropogenic changes and opportunistic pathogens and can be indicators of changes in ocean environments. Therefore, studies into biomarker discovery are of considerable value. This study aimed at assessing extracellular vesicle (EV) signatures and post-translational protein deimination profiles of hemolymph from four commercially valuable Mollusca species, the blue mussel (Mytilus edulis), soft shell clam (Mya arenaria), Eastern oyster (Crassostrea virginica), and Atlantic jacknife clam (Ensis leei). EVs form part of cellular communication by transporting protein and genetic cargo and play roles in immunity and host–pathogen interactions. Protein deimination is a post-translational modification caused by peptidylarginine deiminases (PADs), and can facilitate protein moonlighting in health and disease. The current study identified hemolymph-EV profiles in the four Mollusca species, revealing some species differences. Deiminated protein candidates differed in hemolymph between the species, with some common targets between all four species (e.g., histone H3 and H4, actin, and GAPDH), while other hits were species-specific; in blue mussel these included heavy metal binding protein, heat shock proteins 60 and 90, 2-phospho-D-glycerate hydrolyase, GTP cyclohydrolase feedback regulatory protein, sodium/potassium-transporting ATPase, and fibrinogen domain containing protein. In soft shell clam specific deimination hits included dynein, MCM3-associated protein, and SCRN. In Eastern oyster specific deimination hits included muscle LIM protein, beta-1,3-glucan-binding protein, myosin heavy chain, thaumatin-like protein, vWFA domain-containing protein, BTB domain-containing protein, amylase, and beta-catenin. Deiminated proteins specific to Atlantic jackknife clam included nacre c1q domain-containing protein and PDZ domain-containing protein In addition, some proteins were common as deiminated targets between two or three of the Bivalvia species under study (e.g., EP protein, C1q domain containing protein, histone H2B, tubulin, elongation factor 1-alpha, dominin, extracellular superoxide dismutase). Protein interaction network analysis for the deiminated protein hits revealed major pathways relevant for immunity and metabolism, providing novel insights into post-translational regulation via deimination. The study contributes to EV characterization in diverse taxa and understanding of roles for PAD-mediated regulation of immune and metabolic pathways throughout phylogeny.

New Antibacterial Peptides from the Freshwater Mollusk Pomacea poeyana (Pilsbry, 1927)

Antimicrobial peptides (AMPs) are biomolecules with antimicrobial activity against a broad group of pathogens. In the past few decades, AMPs have represented an important alternative for the treatment of infectious diseases. Their isolation from natural sources has been widely investigated. In this sense, mollusks are promising organisms for the identification of AMPs given that their immune system mainly relies on innate response. In this report, we characterized the peptide fraction of the Cuban freshwater snail Pomacea poeyana (Pilsbry, 1927) and identified 37 different peptides by nanoLC-ESI-MS-MS technology. From these peptide sequences, using bioinformatic prediction tools, we discovered two potential antimicrobial peptides named Pom-1 (KCAGSIAWAIGSGLFGGAKLIKIKKYIAELGGLQ) and Pom-2 (KEIERAGQRIRDAIISAAPAVETLAQAQKIIKGG). Database search revealed that Pom-1 is a fragment of Closticin 574 previously isolated from the bacteria Clostridium tyrobutyrium, and Pom-2 is a fragment of cecropin D-like peptide first isolated from Galleria mellonella hemolymph. These sequences were chemically synthesized and evaluated against different human pathogens. Interestingly, structural predictions of both peptides in the presence of micelles showed models that comprise two alpha helices joined by a short loop. The CD spectra analysis of Pom-1 and Pom-2 in water showed for both structures a high random coil content, a certain content of α-helix and a low β-sheet content. Like other described AMPs displaying a disordered structure in water, the peptides may adopt a helical conformation in presence of bacterial membranes. In antimicrobial assays, Pom-1 demonstrated high activity against the Gram-negative bacteria Pseudomonas aeruginosa and moderate activity against Klebsiella pneumoniae and Listeria monocytogenes. Neither of the two peptides showed antifungal action. Pom-1 moderately inhibits Zika Virus infection but slightly enhances HIV-1 infectivion in vitro. The evaluation of cell toxicity on primary human macrophages did not show toxicity on THP-1 cells, although slight overall toxicity was observed in high concentrations of Pom-1. We assume that both peptides may play a key role in innate defense of P. poeyana and represent promising antimicrobial candidates for humans.

Multiple functions of thymosin β4 in the pearl oyster Pinctada fucata suggest its multiple potential roles in artificial pearl culture

Thymosin β4 is a multifunctional protein in vertebrates that participates in physiological processes, such as wound healing, immune response, cell proliferation and migration. We assessed the multifarious roles of this small peptide in Pinctada fucata, an oyster commonly used in pearl culture in China. Our results showed that when P. fucata was challenged by bacterial pathogens or LPS, the relative expression level of Pfthymosin β4 mRNA was significantly up-regulated, suggesting its involvement in immune response of the animal. Recombinant Pfthymosin β4 (rPfthymosin β4) was produced and showed in vitro different antibacterial activities against different pathogenic bacteria; the inhibitory effect of rPfthymosin β4 on bacterial growth was relatively stronger in the broth culture than agar culture. The overexpression of Pfthymosin β4 in Escherichia coli BL21(DE3) cells could improve their resistance to Cu²⁺, Zn²⁺, Cd²⁺, and H₂O₂, suggesting that Pfthymosin β4 is likely involved with antioxidant. rPfthymosin β4 also significantly promoted the proliferation and migration of mouse aortic vascular smooth muscle cells as indicated by MTT assay and cell scratch assay, respectively. In addition, chemically synthesized or recombinant Pfthymosin β4 could transiently increase the circulating total hemocytes counts but down-regulated by RNAi in P. fucata. Taking together above results and previous studies suggested that Pfthymosin β4 is potentially able to promote wound healing through enhancing antibacterial activity and antioxidant capacity, promotion of cell proliferation and migration, and increase of circulating hemocytes in P. fucata due to nucleus implantation injury. Thus, the future of recombinant Pfthymosin β4 should be promising in the culture of pearls in P. fucata.

Ammonia toxicity in the yellow catfish (Pelteobagrus fulvidraco): The mechanistic insight from physiological detoxification to poisoning

Ammonia is toxic to fishes. Different fish have different defense strategies against ammonia, so the mechanism of ammonia poisoning is different. In this study, yellow catfish were exposed to three levels of ammonia (0, 5.70 and 57.00 mg L^-1) for 96 h. The results showed that ammonia poisoning could lead to free amino acid imbalance (ornithine and citrulline contents declined; arginine content elevated), urea cycle enzymes deficiency (carbamyl phosphate synthetase and arginase contents declined), oxidative stress (superoxide dismutase, catalase and glutathione peroxidase activities declined), immunosuppression (lysozyme activity, 50% hemolytic complement and total immunoglobulin contents and phagocytic index declined) and cytokines release (TNF, IL 1 and IL 8 contents elevated). In addition, ammonia poisoning could induce up-regulation of antioxidant enzymes (Cu/Zn-SOD, Mn-SOD, CAT and GPx), cytokines (TNFα, IL 1 and IL 8) and apoptosis (p53, Bax, cytochrome c, Caspase 3 and Caspase 9) genes transcription. This study suggesting that the urea cycle and glutamine synthesis both were involved in the ammonia detoxification of yellow catfish, and the immunosuppression, inflammation and apoptotic induced by ammonia poisoning in yellow catfish are related to oxidative stress.

A Signaling Pathway to Mediate the Combined Immunomodulation of Acetylcholine and Enkephalin in Oyster Crassostrea gigas

Molluscs have evolved a primitive but complete neuroendocrine-immune (NEI) system with a vast array of neurotransmitters to conduct both humoral and cellular immunomodulation. Previous studies have illustrated the immune functions of several key neurotransmitters. However, the combined effects of multiple neurotransmitters and the signaling pathway to mediate such immunomodulation have not been well-understood. In the present study, iTRAQ and LC-ESI-MS/MS approaches were employed to investigate the combined immunomodulation functions of two crucial neurotransmitters, acetylcholine (ACh), and [Met⁵]-enkephalin (ENK), in oyster Crassostrea gigas. A total number of 5,379 proteins were identified from hemocytes of oysters after the treatments with Ach and ENK separately or simultaneously, and 1,475 of them were found to be significantly up-regulated, while 1,115 of them were significantly down-regulated. The protein expression patterns in the groups treated by ACh and ENK separately were quite similar, which were dramatically different from that in the group treated by ACh+ENK. One hundred seventy-two proteins were found to be differentially expressed in all the three neurotransmitter treatment groups. Functional validation suggested that ACh and ENK possibly modulate the immune response in oyster hemocytes by enhancing pathogen recognition, cell apoptosis, and the enzyme activities of superoxide dismutase (SOD). Moreover, GO enrichment and co-expression network analyses implied that the combined immunomodulation of ACh and ENK might be mediated by p53, EGF-R–ErbB, and Fc gamma R (FcγR) signaling pathways. These results collectively indicated that multiple neurotransmitters executed a combined and ordered immune regulation through common signaling cascades in molluscs, which was under delicate control to maintain the homeostasis.

Effects of acute ammonia toxicity on oxidative stress, DNA damage and apoptosis in digestive gland and gill of Asian clam (Corbicula fluminea)

Ammonia is one of the major pollutants associated with the main river basins due to ammonification of uneaten food and animal excretion, which usually brings detrimental health effects to aquatic invertebrate. However, the mechanisms of ammonia toxicity in aquatic invertebrate have rarely been reported. In this study, C. fluminea was exposed to different levels of ammonia (control group, 10 mg/L, and 25 mg/L) for 24 h and 48 h, and digestive gland and gill were collected to explore toxic effects on oxidative stress, DNA damage and apoptosis under ammonia stress. The results showed that ammonia poisoning could increase the activity of oxidative stress enzyme (SOD and CAT), inducing differentially expressed genes (DRAM2, GADD45, P53, BAX, BCL2, CASP8, CASP9, CASP3, HSP70 and HSP90) and different cytokines (IL-1 beta, IL-8, IL-17 and TNF-alpha) of DNA damage and apoptosis. The difference of toxic effects induced by ammonia among digestive gland and gill were also observed by real-time PCR and TUNEL staining. Our results will be helpful to understand the mechanism of aquatic toxicology induced by ammonia in C. fluminea.

Myticusin-beta, antimicrobial peptide from the marine bivalve, Mytilus coruscus

We isolated and purified an antimicrobial peptide (AMP) from the mantle of the hard-shelled mussel, Mytilus coruscus. The peptide was purified through C₁₈ reversed-phase high-performance liquid chromatography, and displayed antibacterial activity. Total molecular mass of 11,182 Da was determined using matrix-assisted laser desorption ionization time-of-flight mass spectrophotometry. The N-terminal 23-amino acid sequence of its purified peak was obtained through Edman degradation, revealing 82% identity with myticusin-1 of M. coruscus. Complete sequence of the target peptide was determined through cDNA cloning and rapid amplification of cDNA ends. The complete sequence comprised 574 bp with a 387-bp open reading frame (ORF) encoding 24 amino acids of a signal peptide and 104 amino acids of a mature peptide, which was named myticusin-beta. Furthermore, we discovered two novel isoforms of myticusin-beta. We constructed and expressed recombinant myticusin-beta, which displayed antimicrobial activity against gram-positive (Bacillus cereus, Bacillus subtilis, Clostridium perfringens, Staphylococcus aureus, Streptococcus iniae, Streptococcus mutans) and gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Vibrio alginolyticus, Klebsiella pneumoniae). Purified recombinant myticusin-beta also showed anti-parasitic activity at various concentrations. A short AMP analog was designed and synthesized based on the sequence of myticusin-beta, with markedly improved antimicrobial activity. Expression of myticusin-beta was detected in the mantle at the highest level, followed by hemocytes. The results obtained in this work suggest that myticusin-beta is an immune-related AMP of M. coruscus and an effective alternative to antibiotics.

Molecular cloning, characterization, and expression of two TNFRs from the pearl oyster Pinctada fucata martensii

Proteins in the tumor necrosis factor receptor (TNFR) superfamily play significant roles in many physiological and pathological events, such as inflammation, apoptosis, autoimmunity, and organogenesis. Here, two TNFR gene homologs (PmTNFR1 and PmTNFR5) were identified in the pearl oyster Pinctada fucata martensii. The predicted PmTNFR1 and PmTNFR5 protein sequences were 406 and 533 amino acids long, respectively, and both possessed motifs characteristic of the TNFR family, including a TNFR homology domain (CRD), a transmembrane domain (TM), and death domains. However, the predicted amino acid sequences of PmTNFR1 and PmTNFR5 had low identity (~16-23%) with sequences of vertebrate TNFR family proteins. Furthermore, PmTNFR5 had a death domain at the C-terminal, indicating that this protein may be a novel member of the TNFR superfamily. Constitutive PmTNFR1 and PmTNFR5 mRNA expression was detected in all six pearl oyster tissues tested, with comparatively greater transcript abundance in the hepatopancreas and gill. The gene expression levels of PmTNFR1 and PmTNFR5, as well as those of downstream signaling molecules related to the NF-κB pathway (RIP, TRAF2, TRAF3, IKK, and NF-κB), were quantified in the gill after LPS challenge and in the hemocytes after nucleus insertion surgery using real-time PCR (qRT-PCR). We found that all genes were significantly upregulated at 6 h and 12 h post-injection, as well as at 15 d post-insertion. We used RNAi to inhibit the expression of the PmTNFR1 and PmTNFR5 genes. We then quantified the expression levels of PmTNFR1 and PmTNFR5, as well as downstream genes, using qRT-PCR. We found that RNAi inhibition of PmTNFR1 and PmTNFR5 downregulated the downstream genes (RIP, TRAF2, TRAF3, IKK, and NF-κB). Therefore, our results suggested that PmTNFR1 and PmTNFR5 mediate the NF-κB signaling pathway, and are closely related to immune defense, particularly allograft immunity, in the pearl oyster P. fucata martensii.

A membrane-bound dopamine β-hydroxylase highly expressed in granulocyte of Pacific oyster Crassostrea gigas

Dopamine β-hydroxylase (DBH) is one of key rate-limiting enzymes converting dopamine to norepinephrine. It locates not only in catecholaminergic neuron system, but also in immunocytes and plays roles in the immune response of vertebrates. However, the knowledge about the function of DBH in immune system is still very limited in invertebrates. In the present study, the DBH gene family with seven members was screened from Crassostrea gigas genome, and their mRNA expressions in various tissues were recorded. Among them, one DBH (designated CgDBH-1) with high expression level in oyster hemocytes was further characterized. The deduced amino acid sequence of CgDBH-1 was predicted to contain a transmembrane domain and shared 30.1% and 30.9% similarity with that in Mus musculus and Homo sapiens, respectively. CgDBH-1 was closely clustered with DBH from Aplysia californica in the phylogenetic tree. The recombinant protein of CgDBH-1 (rCgDBH-1) exhibited significant enzymatic activity (0.54 ± 0.019 pmol L^-1 min^-1) to synthesize norepinephrine. Importantly, the mRNA transcript of CgDBH-1 was highly expressed in oyster hemocytes, and the highest expression level was observed in granulocytes among the three types of hemocytes, which was 8.18-fold (p < 0.01) of that in agranulocytes. Moreover, the expression of CgDBH-1 in hemocytes was significantly increased at the late stage of immune response. The CgDBH-1 protein was mainly co-localized with the granules and endoplasmic reticulum (ER) of granulocytes. These results collectively suggested that CgDBH-1, as a novel molluscan norepinephrine synthesizing enzyme highly expressed in granulocytes, involved in the late-stage immune response of oysters, which provided vital insight to understand the crosstalk between neuroendocrine and immune systems in invertebrates.

Functional characterization of a putative lipopolysaccharide-induced TNF-alpha factor (LITAF) from blood clam Tegillarca granosa in innate immunity

Lipopolysaccharide-induced TNF-alpha factor (LITAF), as a transcription factor, activates the transcription of TNF and other cytokines in inflammatory response upon lipopolysaccharide (LPS) stimulation. In the present study, we cloned and identified the full-length cDNA of LITAF homolog from blood clam Tegillarca granosa for the first time. The full-length cDNA of TgLITAF was 1801 bp encoding a polypeptide of 147 amino acids with an estimated molecular mass of 16.13 kDa. TgLITAF contained a zf-LITAF-like zinc ribbon domain at the C-terminal of the protein and the TgLITAF domain showed 48-74% amino acid sequence identity with other known LITAFs from other species. Subcellular localization study showed that TgLITAF was mainly expressed in the nucleus. qRT-PCR analysis showed that the TgLITAF transcription expressed constitutively in all the examined tissues with the highest expression level in the gills. After LPS or V. alginolyticus treatment, expression of TgLITAF in hemocytes was both up-regulated significantly at 3-6 h. Furthermore, in vitro study indicated that overexpression of TgLITAF in HeLa cells resulted in the activation of TNFα, p53, and influenced the expression levels of apoptotic-related genes Bax, Bcl-2, Caspase-3, Caspase-6, and Caspase-7. The proliferation of HeLa cells was inhibited by overexpression of TgLITAF. Apoptotic fluorescence assay further revealed that TgLITAF participated in the apoptotic process of HeLa cells. Western blotting analysis showed that overexpression of TgLITAF increased endogenous level of cleaved Caspase-7. Taken together, these results revealed that TgLITAF participates in the innate immune response to the pathogen invasion in blood clams and induces apoptosis in HeLa cells.

The involvement of TLR signaling and anti-bacterial effectors in enhanced immune protection of oysters after Vibrio splendidus pre-exposure

The phenomena of enhanced protection of innate immunity responding to a pre-exposed pathogen have been reported in invertebrates. The underpinning molecular basis and mechanism for the enhanced immune protection are still missing. In order to explore the possible molecular basis for enhanced immune protection in molluscs, the transcriptomic analysis of oysters Crassostrea gigas hemocytes after twice stimulation of Vibrio splendidus were conducted, and a total of 403 M clean reads and 34254 differentially expressed genes (DEGs) were collected. There were 2964 common DEGs up-regulated in hemocytes after both the first and second immune stimulation, which were mostly enriched in metabolic processes and immune related pathways, such as endocytosis, MAPK signaling pathway and TLR signal pathway. Moreover, 187 and 55 DEGs were higher expressed at resting (0 h after stimulation) and activating state (12 h after stimulation) of the second immune response than that of the first response, respectively, mainly including immune recognition receptor scavenger receptor 2, signal molecule MAPK2, immune regulator IL17-d, apoptosis inhibitor IAP and effector cathepsin. More importantly, 13 DEGs were long-lastingly higher expressed at both the resting and activating state within the second immune response than that of the first, including TLR signal molecule MyD88, anti-virulent tissue inhibitor of metalloproteinase, anti-bacterial proline-rich transmembrane protein, which might play indispensable roles in enhanced immune protection against V. splendidus re-infection. The expression patterns of TLR signals (CgTLR6 and CgMyD88) and effector molecules (CgTIMP and CgPRTP) were further validated by RT-PCR, which were consistent to transcriptomic results. All the results provided an overall molecular basis of enhanced immune protection for hemocytes defensing against the second stimulation of V. splendidus in oyster, which would be valuable for understanding the protection mechanisms of pre-exposure in invertebrates.

Enzymes and non-enzymatic antioxidants responses to sequential cold stress in polymorphic noble scallop Chlamys nobilis with different total carotenoids content

Noble scallop, an economically important edible marine bivalve displays polymorphism in shells (golden and brown) and flesh colors (orange and white). Mass mortality of noble scallops usually occurs during the winter months. Interestingly, carotenoid-rich golden scallops demonstrated much higher survival rates than brown scallops in winter. In order to understand the response of polymorphic noble scallops to sequential cold stress, the present study aimed to investigate the enzyme and non-enzymatic antioxidant responses of golden and brown scallops under sequential cold stress. Parameters evaluated included total carotenoid content (TCC), fatty acid composition, total antioxidant capacity (TAC), methylenedioxyamphetamine (MDA) content, catalase (CAT) activity, and superoxide dismutase (SOD) enzyme activity. The results of the present study revealed that golden scallops have higher cold tolerance than brown scallops. Golden and brown scallops are well adapted to low water temperature of above 12 °C, but in areas where winter water temperatures are below 12 °C, golden scallops are more suitable for aquaculture than brown scallops. The findings of this study are crucial to understanding the physiological responses of polymorphic scallops to cold stress and identify suitable candidates for winter aquaculture.

Oxidative stress responses of golden and brown noble scallops Chlamys nobilis to acute cold stress

The noble scallop Chlamys nobilis is an important edible marine bivalve that is widely cultivated in the sea of southern China. Unfortunately, the mass mortality of noble scallops frequently occurs during the winter months. The present study investigated the effects of acute cold stress (8 °C) to the physiological responses of polymorphic noble scallops, by assessing the HSP70 gene expression, total carotenoid content (TCC), total antioxidant capacity (TAC), malondialdehyde (MDA) content, catalase (CAT) activity and superoxide dismutase (SOD) enzymatic activity in different tissues of golden and brown scallops. The results of the present study revealed that MDA, TCC and CAT increased drastically in most tissues in the early stage of acute cold stress (0-3 h), but TCC, SOD and CAT generally showed a downward trend. Within 3-6 h of acute cold stress, MDA content decreased in most tissues and the SOD content increased significantly in most tissues, while TCC and CAT remained at peak. After 6 h of acute cold stress, MDA content continued to increase in most tissues, while TCC, CAT, SOD and TAC decreased or remained at a lower level. For HSP70 expression, up-regulation of the HSP70 gene was observed only in mantle of brown scallops and hemolymph of golden scallops at 3 h and 24 h, respectively. The findings of the present study can better understand the physiological response of noble scallops to acute cold stress.

De novo assembly, characterization of tissue-specific transcriptomes and identification of immune related genes from the scallop Argopecten purpuratus

The scallop Argopecten purpuratus is one of the most economically important cultured mollusks on the coasts from Chile and Peru but its production has declined, in part, due to the emergence of mass mortality events of unknown origin. Driven by this scenario, increasing progress has been made in recent years in the comprehension of immune response mechanisms in this species. However, it is still not entirely understood how different mucosal interfaces participate and cooperate with the immune competent cells, the hemocytes, in the immune defense. Thus, in this work we aimed to characterize the transcriptome of three tissues with immune relevance from A. purpuratus by next-generation sequencing and de novo transcriptome assembly. For this, 18 cDNA libraries were constructed from digestive gland, gills and hemocytes tissues of scallops from different immune conditions and sequenced by the Illumina HiSeq4000 platform. A total of 967.964.884 raw reads were obtained and 967.432.652 clean reads were generated. The clean reads were de novo assembled into 46.601 high quality contigs and 32.299 (69.31%) contigs were subsequently annotated. In addition, three de novo specific assemblies were performed from clean reads obtained from each tissue cDNA libraries for their comparison. Gene ontology (GO) and KEGG analyses revealed that annotated sequences from digestive gland, gills and hemocytes could be classified into both general and specific subcategory terms and known biological pathways, respectively, according to the tissue nature. Finally, several immune related candidate genes were identified, and the differential expression of tissue-specific genes was established, suggesting they could display specific roles in the host defense. The data presented in this study provide the first insight into the tissue specific transcriptome profiles of A. purpuratus, which should be considered for further research on the interplay between the hemocytes and mucosal immune responses.

A novel LRR and Ig domain-containing protein could function as an immune effector in Crassostrea gigas

A variety of combinations of leucine-rich repeat (LRR) and immunoglobulin-like (Ig) domains have been found and discovered in invertebrates and vertebrates, but the functions remain largely unexplored. In the present study, a novel LRR and Ig domain-containing protein (LRRIG), CgLRRIG-3, was identified and characterized from oyster Crassostrea gigas. It contained two typical LRR motifs, a LRRNT motif and an Ig domain and PSI-BALST and phylogeny analysis revealed that the sequence of CgLRRIG-3 was most related with leucine-rich repeat neuronal 1 proteins from vertebrate. Its mRNA transcripts were constitutively expressed in muscle, gill, hepatopancreas, mantle, gonad and hemocytes with the highest level in hepatopancreas. The mRNA expression level of CgLRRIG-3 in hemocytes could respond to the stimulations of variety PAMPs including lipopolysaccharide (LPS), peptidoglycan (PGN), glucan (GLU) and polyinosinic-polycytidylic acid (poly I:C). The recombinant proteins exhibited a wide PAMP binding repertoire to four typical PAMPs and could significantly induce the expression of CgTNF-1 and CgIL17-5 as well as increase phagocytosis in primary cultured oyster hemocytes. In hepatopancreas, CgLRRIG-3 was mainly distributed in the basolateral membrane of digestive tubule and the hemocoel sinusoid between the digestive tubules. And in hemocytes, the positive signal was mainly distributed in a special group of granulocytes. These results collectively indicated that CgLRRIG-3 could not only function as an immune effector.

Responses of HSP70 Gene to Vibrio parahaemolyticus Infection and Thermal Stress and Its Transcriptional Regulation Analysis in Haliotis diversicolor

Heat-shock protein 70 (HSP70) is a molecular chaperone that plays critical roles in cell protein folding and metabolism, which helps to protect cells from unfavorable environmental stress. Haliotis diversicolor is one of the most important economic breeding species in the coastal provinces of south China. To date, the expression and transcriptional regulation of HSP70 in Haliotis diversicolor (HdHSP70) has not been well characterized. In this study, the expression levels of HdHSP70 gene in different tissues and different stress conditions were detected. The results showed that the HdHSP70 gene was ubiquitously expressed in sampled tissues and was the highest in hepatopancreas, followed by hemocytes. In hepatopancreas and hemocytes, the HdHSP70 gene was significantly up-regulated by Vibrio parahaemolyticus infection, thermal stress, and combined stress (Vibrio parahaemolyticus infection and thermal stress combination), indicating that HdHSP70 is involved in the stress response and the regulation of innate immunity. Furthermore, a 2383 bp of 5'-flanking region sequence of the HdHSP70 gene was cloned, and it contains a presumed core promoter region, a CpG island, a (TG)39 simple sequence repeat (SSR), and many potential transcription factor binding sites. The activity of HdHSP70 promoter was evaluated by driving the expression of luciferase gene in HEK293FT cells. A series of experimental results indicated that the core promoter region is located between -189 bp and +46 bp, and high-temperature stress can increase the activity of HdHSP70 promoter. Sequence-consecutive deletions of the luciferase reporter gene in HEK293FT cells revealed two possible promoter activity regions. To further identify the binding site of the key transcription factor in the two regions, two expression vectors with site-directed mutation were constructed. The results showed that the transcriptional activity of NF-1 site-directed mutation was significantly increased (p < 0.05), whereas the transcriptional activity of NF-κB site-directed mutation was significantly reduced. These results suggest that NF-1 and NF-κB may be two important transcription factors that regulate the expression of HdHSP70 gene.

A novel globular C1q domain containing protein (C1qDC-7) from Crassostrea gigas acts as pattern recognition receptor with broad recognition spectrum

The globular C1q domain containing (C1qDC) proteins are a family of versatile pattern recognition receptors (PRRs) to bind various ligands by their globular C1q (gC1q) domain. In the present study, a novel globular C1qDC (CgC1qDC-7) was characterized from Pacific oyster Crassostrea gigas. The open reading frame of CgC1qDC-7 was of 555 bp, encoding a polypeptide of 185 amino acids. Phylogenetic analysis indicated that CgC1qDC-7 shared high homology with C1qDCs from Crassostrea virginica, Mytilus galloprovincialis, and Mizuhopecten yessoensis. The mRNA transcripts of CgC1qDC-7 were widely expressed in all the tested tissues including mantle, gonad, gills, adductor muscle, hemocytes, hepatopancreas and labial palps, with the highest expression level in hemocytes and gills. The recombinant protein of CgC1qDC-7 (rCgC1qDC-7) exhibited binding activity towards Gram-negative bacteria (Vibrio splendidus, V. anguillarum, Escherichia coli, V. alginolyticus, and Aeromonas hydrophila), Gram-positive bacteria (Micrococcus luteus and Staphylococcus aureus) and fungi (Pichia pastoris and Yarrowia lipolytica), and displayed strongest binding affinity towards Gram-negative bacteria V. splendidus and V. anguillarum. It also exhibited affinity to vital pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), peptidoglycan (PGN), mannan (MAN) and Poly (I:C) with high affinity towards LPS and PGN, and low affinity to MAN and Poly (I:C). These results collectively indicated that CgC1qDC-7 was a novel PRR in C. gigas with high binding affinity towards LPS and PGN as well as Gram-negative bacteria.