Identification and functional characterization of three myeloid differentiation factor 88 (MyD88) isoforms from thick shell mussel Mytilus coruscus

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.

Transcriptome profiling reveals the strategy of thermal tolerance enhancement caused by heat-hardening in Mytilus coruscus

The thick-shell mussel Mytilus coruscus serves as a common sessile intertidal species and holds economic significance as an aquatic organism. M. coruscus often endure higher temperatures than their ideal range during consecutive low tides in the spring. This exposure to elevated temperatures provides them with a thermal tolerance boost, enabling them to adapt to high-temperature events caused by extreme low tides and adverse weather conditions. This phenomenon is referred to as heat-hardening. Some related studies showed the phenomenon of heat-hardening in sessile intertidal species but not reported at the mechanism level based on transcriptome so far. In this study, physiological experiments, gene family identification and transcriptome sequencing were performed to confirm the thermotolerance enhancement based on heat-hardening and explore the mechanism in M. coruscus. A total of 2935 DEGs were identified and the results of the KEGG enrichment showed that seven heat-hardening relative pathways were enriched, including Toll-like receptor signal pathway, Arachidonic acid metabolism, and others. Then, 24 HSP70 members and 36 CYP2 members, were identified, and the up-regulated members are correlated with increasing thermotolerance. Finally, we concluded that the heat-hardening M. coruscus have a better thermotolerance because of the capability of maintaining the integrity and the phenomenon of vasodilation of the gill under thermal stress. Further, the physiological experiments yielded the same conclusions. Overall, this study confirms the thermotolerance enhancement caused by heat-hardening and reveals the survival strategy in M. coruscus. In addition, the conclusion provides a new reference for studying the intertidal species' heat resistance mechanisms to combat extreme heat events and the strategies for dealing with extreme weather in aquaculture under the global warming trend.

Molecular Characterization of MyD88 in Anodonta woodiana and Its Involvement in the Innate Immune Response to Bacterial Infection

Myeloid differentiation factor 88 (MyD88) is a key adapter molecule in Toll-like receptor signal transduction that triggers downstream immune cascades involved in the host defense response to exogenous pathogens. However, the function of MyD88s in mollusks, especially in freshwater shellfish, remains poorly understood. In this study, a novel freshwater shellfish MyD88 (denoted AwMyD88) was characterized from Anodonta woodiana. The present AwMyD88 protein consists of 474 amino acids and contains a conserved a typical death domain (DD) and a conservative Toll/IL-1R (TIR) domain with three typical boxes. Quantitative real-time PCR (qRT-PCR) analysis showed that AwMyD88 was broadly expressed in all the examined tissues, and the highest expression level was observed in hemocytes of A. woodiana. When challenged with Aeromonas hydrophila and lipopolysaccharide (LPS), the mRNA expression levels of AwMyD88 were significantly induced in hemocytes of A. woodiana in vivo and in vitro. In addition, in vivo injection experiments revealed that MyD88 signaling pathway genes showed strong responsiveness to A. hydrophila challenge, and their expression levels were significantly upregulated in hemocytes. Knockdown of AwMyD88 reduced the transcript levels of immune related transcription factors (AwNF-κB and AwAP-1) and effectors (AwTNF, AwLYZ, AwDefense and AwAIF) during A. hydrophila infection. Moreover, subcellular localization analysis indicated that AwMyD88 was mainly localized to the cytoplasm in HEK293T cells. Finally, luciferase reporter assays revealed that AwMyD88 associates with AwTLR to activate the NF-κB and AP-1 signaling pathways in HEK293T cells. These results suggested that AwMyD88 might be involved in the host defense response to bacterial challenge, providing new insight into the immune function of the MyD88 signaling pathway in freshwater shellfish.

A novel molluscan TLR molecule engaged in inflammatory response through MyD88 adapter recruitment

Toll-like receptors (TLRs) mediated signaling plays a vital role in activating innate and adaptive immunity. Although TLR mediated signaling has been comprehensively investigated in mammalian species, the mechanisms underlying TLR signaling in molluscs remain obscure. In the present study, a novel TLR isoform namely McTLR-like1 was identified in the thick shell mussel Mytilus coruscus. McTLR-like1 was highly expressed in molluscan immune-related tissues, and its transcriptional levels in hemocytes were significantly increased when challenged by V. alginolyticus. McTLR-like1 activated nuclear factor κB (NF-κB) and strengthened the transcription and phosphorylation of NF-κB subunit P65 in mammalian cells. Upon the silencing of McTLR-like1, the mRNA expression levels of pro-inflammatory cytokines were down-regulated, and the animals exhibited higher levels of resistance when challenged with V. alginolyticus. McMyD88a mRNA expression was also downregulated alongside McTLR-like1. Furthermore, GST-pull down assays revealed a visible affinity between McTLR-like1 and McMyD88a. Collectively, these results demonstrated that the newly identified gene affiliated to the molluscan TLR family and plays a role in the TLR-mediated activation of inflammatory response via its affinity with MyD88. The present study enhances our knowledge of TLR signaling mechanisms in molluscs and provides new insights into the evolution of TLRs.

The primitive interferon-like system and its antiviral function in molluscs

The phylum mollusca is a very important group in the animal kingdom for the large number and diversified species. Recently, interest in molluscan immunity has increased due to their phylogenetic position and importance in worldwide aquaculture and aquatic environment. As the main aquaculture animal, most molluscs live in the water environment and they have to cope with many pathogen challenges, in which virus is one of the primary causes for the mass mortality. In vertebrates, interferon (IFN) system is generally recognized as the first line of defence against viral infection, while the antiviral mechanisms in molluscs remain to be clearly illuminated. Recently, some IFN-like proteins and IFN-related components have been characterized from molluscs, such as pattern recognition receptors (PRRs), interferon regulatory factors (IRFs), IFN-like receptors, JAK/STAT and IFN-stimulated genes (ISGs), which reinforce the existence of IFN-like system in molluscs. This system can be activated by virus or poly (I:C) challenges and further regulate the antiviral response of haemocytes in molluscs. This review summarizes the research progresses of IFN-like system in molluscs with the emphases on the uniformity and heterogeneity of IFN-like system of molluscs compared to that of other animals, which will be helpful for elucidating the antiviral modulation in molluscs and understanding the origin and evolution of IFN system.

Transcriptome analysis and histopathological observations of Geloina erosa gills upon Cr(VI) exposure

The heavy metal contamination like Cr(VI) has been increased by human activities and that threats the ecosystem health of mangrove areas. Bioindicator is an emerging tool in the environmental contamination assessment. The objective of this study was to investigate the Geloina erosa response mechanisms and sensitivities of several biomarkers in the Cr(VI) exposure and identify the G. erosa capability of being used as heavy metals bioindicator. In this study, G. erosa was exposed to 100 μmol·L^-1 Cr(VI) for 48 h. After transcriptome sequencing, a total of 134,817 unigenes were obtained, including 12,555 up-regulated and 18,829 down-regulated differentially expressed genes and were validated through quantitative real-time PCR. In addition, a total of 12,185 SSRs and 1,428,214 candidate SNPs were identified from all the G. erosa transcriptome libraries. Histopathology of the gill indicated the Cr(VI) exposure induced damage of the organ leading to its immunization, detoxification or apoptosis reactions. Among eight genes of the selected biomarkers, Calm, HSP70, CYP450, ATG5, TLR2, MYD88 and CASP8 were up-regulated, while TLR4 was down-regulated in response to the Cr(VI) exposure.

Interactive effects of salinity variation and exposure to ZnO nanoparticles on the innate immune system of a sentinel marine bivalve, Mytilus edulis

ZnO nanoparticles (nZnO) are released into the coastal environment from multiple sources, yet their toxicity to marine organisms is not well understood. We investigated the interactive effects of salinity (normal 15, low 5, and fluctuating 5-15) and nZnO (100 μg l-1) on innate immunity of the blue mussels Mytilus edulis from a brackish area of the Baltic Sea. Exposure to ionic Zn (100 μg l-1) was used to test whether the toxic effects of nZnO can be attributed to the potential release of Zn2+. Functional parameters and the expression of key immune-related genes were investigated in the mussels exposed to nZnO or ionic Zn under different salinity regimes for 21 days. nZnO exposures elevated hemocyte mortality, suppressed adhesion, stimulated phagocytosis, and led to an apparent increase in lysosomal volume. At salinity 15, nZnO suppressed the mRNA expression of the Toll-like receptors TLRb and c, C-lectin, and the complement system component C3q indicating impaired ability for pathogen recognition. In contrast, the mRNA levels of an antimicrobial peptide defensin increased during nZnO exposure at salinity 15. At fluctuating salinity (5-15), nZnO exposure increased expression of multiple immune-related genes in hemocytes including the complement system components C1 and C3q, and the Toll-like receptors TLRa, b and c. Low salinity (5) had strong immunosuppressive effects on the functional and molecular immune traits of M. edulis that overshadowed the effects of nZnO. The salinity-dependent modulation of immune response to nZnO cannot be attributed to the differences in the aggregation or solubility of nZnO, and likely reflects the interaction of the toxic effects of nanoparticles and physiological effects of the osmotic stress. These findings have implications for the environmental risk assessment of nanomaterials and the development of the context-specific biomarker baselines for coastal pollution monitoring.

Functional relationship between CgMyD88-1 and CgMyD88-2 in the Pacific oyster

MyD88 is a universal adapter protein for the Toll-like receptor/interleukin-1 receptor (TLR/IL-1R) signaling pathway. Since invertebrates are believed to lack MyD88-independent pathways, MyD88 appears more critical in oyster TLR signaling pathway. In the Pacific oyster (Crassostrea gigas), two complete paralogues, named as CgMyD88-1 and CgMyD88-2, have been identified. In the current study, we indicated that CgMyD88-1 and CgMyD88-2 might act synergistically to increase the efficiency of immune signaling by activating NF-κB transcription factor. However, we found that upon stimulation with lipopolysaccharide (LPS) or polyinosinic:polycytidylic acid [poly (I:C)], CgMyD88-1 and CgMyD88-2 show differences in their response: CgMyD88-1 accumulated as large spots in the cytoplasm, while CgMyD88-2 assembled in the cytoplasm and in the membrane. Our results support the theory that expansion of these immune genes is associated with functional diversity.

Characterization, subcellular localization and function analysis of myeloid differentiation factor 88 (Pt-MyD88) in swimming crab, Portunus trituberculatus

Myeloid differentiation factor 88 (MyD88) is a universal and essential adaptor protein required for the Toll-like receptors (TLRs) pathway activation in invertebrates as well as in vertebrates. Herein, we characterized a MyD88 (Pt-MyD88) cDNA sequence in the swimming crab (Portunus trituberculatus). The Pt-MyD88 ORF is predicted to encode 469 peptides with an N-terminal death domain and a typical C-terminal TIR domain. Real-Time quantitative PCR analysis showed that the Pt-MyD88 transcriptions were constitutively expressed in hemocytes, gill, intestine, heart and muscle in normal crab. The expressions of Pt-MyD88 would be down-regulated by V. alginolyticus or LPS challenge, and be up-regulated by WSSV infection in hemocytes. Intracellular localization showed Pt-MyD88 was distributed mainly in the cytoplasm when it was over-expressed in human cell HEK293T or in Drosophila Schneider 2 (S2). Functionally, over-expression of Pt-MyD88 could either activate the NF-κB in HEK293T cells or activate the promoters of Drosophila antimicrobial peptide genes (AMPs) in S2 cell. In primary cultured hemocytes of swimming crab, after Pt-MyD88 was knocked-down by specific long double strand RNA, the expression of anti-lipopolysaccharide factor1 (ALF1), hyastatin3, crustin1 and crustin3 have been significantly inhibited, while the expression of other AMPs is normal compared to non-specific dsRNA treated cells.

A novel invertebrate toll-like receptor is involved in TLR mediated signal pathway of thick shell mussel Mytilus coruscus

Toll-like receptors (TLRs) are the most well studied pattern recognition receptors (PRRs) that play a crucial role in both innate and adaptive immunity in animals. In the present study, a novel toll-like receptor (McTLRj) was identified and characterised in thick shell mussel Mytilus coruscus. McTLRj possessed a signal peptide, a transmembrane domain, leucine-rich repeats and an intracellular Toll/interleukin-1 receptor domain that were conserved in typical TLRs. McTLRj transcripts were constitutively expressed in all of the examined tissues with high expression level in immune-related tissues, and significantly induced in haemocytes upon live Vibrio alginolyticus, lipopolysaccharide, polyinosinic-polycytidylic acid and peptidoglycans challenge. The overexpression of the McTLRj TLR fragment in Drosophila S2 cells could induce the expression of Drosophila attacin A, drosomycin, cecropin A, and metchnikowin expression. The expression of McTLRj was obviously repressed by dsRNA-mediated RNA interference, and downstream TLR pathway factors, such as MyD88a, IRAK4, and TRAF6 were significantly repressed in McTLRj-silenced mussels upon LPS challenge. These results collectively indicated that McTLRj is a TLR family member that may play a potential PRR role in TLR-mediated signalling pathway. This research contributed to the clarification of innate immune response in molluscs.

Response of a novel selenium-dependent glutathione peroxidase from thick shell mussel Mytilus coruscus exposed to lipopolysaccharide, copper and benzo[α]pyrene

Glutathione peroxidase (GPx) plays an important antioxidant role in cellular defense against environmental stress. In the present study, a novel selenium-dependent glutathione peroxidase termed McSeGPx firstly identified in thick shell mussel Mytilus coruscus. McSeGPx consists of 197 amino acid residues, characterized with one selenocysteine residue encoded by an opal stop codon TGA, one selenocysteine insertion sequence (SECIS) in the 3' untranslated region (UTR), two active site motifs and one signature sequence motif. McSeGPx transcripts were constitutively expressed in all examined tissues, and were significantly induced in gills and digestive glands with the stimulations of lipopolysaccharide (LPS), copper (Cu) and benzo[α]pyrene (B[α]P). Additionally, rough increases in McSeGPx activity were detected in both tissues under the challenge of LPS, Cu and B[α]P. Collectively, these results suggested that McSeGPx affiliate to selenocysteine dependent GPx (SeGPx) family and might play an important role in mediating the environmental stressors and antioxidant response in M. coruscus.

A novel invertebrate toll-like receptor with broad recognition spectrum from thick shell mussel Mytilus coruscus

Toll-like receptors (TLRs) are a category of most well recognized pattern recognition molecules that act on a vital role in both innate and adaptive immunity. In the present study, a novel toll-like receptor (McTLRw) was identified and characterized in thick shell mussel Mytilus coruscus. McTLRw possesses one intracellular Toll/interleukin-1 (IL-1) receptor (TIR) domain, one transmembrane region (TM), one leucine rich repeat N-terminal domain (LRR_NT) and a few of leucine-rich repeats (LRRs), which all are common in typical TLRs. McTLRw transcripts were constitutively expressed in all examined tissues with higher expression levels in immune related tissues, and were significantly induced in haemocytes with the challenges of live Vibrio alginolyticus, lipopolysaccharide (LPS), peptidoglycans (PGN) and β-glucan (GLU), but not induced by polyinosinic-polycytidylic acid (poly I:C). rMcTLRw exhibited affinity to LPS, PGN and GLU while no affinity to poly I:C. Further, the downstream of TLR signaling pathway myeloid differentiation factor 88a (MyD88a), interleukin-1 receptor-associated kinase-4 (IRAK4) and tumor necrosis factor receptor-associated factor 6 (TRAF6) were significantly repressed in McTLRw silenced mussels while challenged with LPS. These results collectively indicated that McTLRw is one member of TLR family and involved in immune response to against invaders by taking participate in TLR mediated signaling pathway.

Immunity in mussels: An overview of molecular components and mechanisms with a focus on the functional defenses

Bivalves' immunity has received much more attention in the last decade, which resulted to a valuable growth in the availability of its molecular components. Such data availability coupled with the economical importance of these organisms aimed to shift the increase in the number of immunological and stress-related studies. Unfortunately, the crowd of generated data deciphering the involved physiological processes, investigators' differential conceptualization and the aimed objectives, has complicated the sensu stricto outlining of immune-related mechanisms. Overall, this review tried to compiles a summary about the molecular components of the mussels' immune response, surveying an overview of the mussels' functional immunity through gathering the most recent-related topics of bivalves' immunity as apoptosis and autophagy which deserves a great attention as stress-related mechanisms, the disseminated neoplasia as outbreak transmissible disease, not only within the same specie but also among different species, the hematopoiesis as topic that still generating interesting debate in the scientific community, the mucosal immunity described as the interface where host-pathogen interactions would occurs and determinate the late immune response, and innate immune memory and transgenerational priming, which described as very recent research topic with extensive applications in shellfish farming industry.

Spotted knifejaw (Oplegnathus punctatus) MyD88: Intracellular localization, signal transduction function and immune responses to bacterial infection

Myeloid differentiation factor 88 (MyD88) links members of the toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) superfamily to the downstream activation of NF-κB as a "bridge" molecular in response to exogenous pathogen, but the function in spotted knifejaw (Oplegnathus. punctatus), a commercial fish in China, is still unknown. We present a functional analysis of spotted knifejaw MyD88 (OppMyD88) with a typical death domain (DD) at the N-terminus and a conservative Toll/IL-1R (TIR) domain at the C-terminus and suggest that MyD88 is important for the activation of TLR-mediated NF-κB with the synergy between domains. Subcellular localization showed that OppMyD88 was distributed in the cytoplasm in a condensed form. Tissues expression profiling analysis showed that OppMyD88 ubiquitously expressed in all tested tissues with the highest expression in the liver, as determined by real-time PCR. The expression of OppMyD88 significantly upregulated in the liver, spleen, kidney and gills within 120 h post Vibrio anguillarum infection. Moreover, we further confirmed that over-expressed OppMyD88 could also induce apoptosis. These results indicate that OppMyD88 might possess important roles in defense against microbial infection and other biological processes in spotted knifejaw similar to those in mammals, which will deepen our understandings in innate immunity of spotted knifejaw.