A new non-phagocytic TLR6 with broad recognition ligands from Pacific oyster Crassostrea gigas

The involvement of CaMKKI in activating AMPKα in yesso scallop Patinopecten yessoensis under high temperature stress

Calcium/calmodulin dependent protein kinase kinase (CaMKK), a highly conserved protein kinase, is involved in the downstream processes of various biological activities by phosphorylating and activating 5'-AMP-activated protein kinase (AMPK) in response to the increase of cytosolic-free calcium (Ca2+). In the present study, a CaMKKI was identified from Yesso scallop Patinopecten yessoensis. Its mRNA was ubiquitously expressed in haemocytes and all tested tissues with the highest expression level in mantle. The expression level of PyCaMKKI mRNA in adductor muscle was significantly upregulated at 1, 3 and 6 h after high temperature treatment (25 °C), which was 3.43-fold (p < 0.05), 5.25-fold (p < 0.05), and 5.70-fold (p < 0.05) of that in blank group, respectively. At 3 h after high temperature treatment (25 °C), the protein level of PyAMPKα, as well as the phosphorylation level of PyAMPKα at Thr170 in adductor muscle, and the positive co-localized fluorescence signals of PyCaMKKI and PyAMPKα in haemocyte all increased significantly (p < 0.05) compared to blank group (18 °C). The pull-down assay showed that rPyCaMKKI and rPyAMPKα could bind each other in vitro. After PyCaMKKI was silenced by siRNA, the mRNA and protein levels of PyCaMKKI and PyAMPKα, and the phosphorylation level of PyAMPKα at Thr170 in adductor muscle were significantly down-regulated (p < 0.05) compared with the negative control group receiving an injection of siRNA-NC. These results collectively suggested that PyCaMKKI was involved in the activation of PyAMPKα in response to high temperature stress and would be helpful for understanding the function of PyCaMKKI-PyAMPKα pathway in maintaining energy homeostasis under high temperature stress in scallops.

The involvement of CgRHIM-containing protein in regulating haemocyte apoptosis after high temperature stress in Pacific oyster Crassostrea gigas

The interactions induced by RIP homotypic interaction motif (RHIM) are essential for the activation of inflammatory signaling and certain cell death pathways. In the present study, a RHIM-containing protein was identified from Pacific oyster Crassostrea gigas, which harbored a RHIM domain and a Death domain (designated CgRHIM-containing protein). The mRNA transcripts of CgRHIM-containing protein were constitutively expressed in all the examined tissues of oysters, with the highest expression level in mantle. The CgRHIM-containing protein was mainly distributed in the cytoplasm of oyster haemocytes. After high temperature stress, the expression levels of CgRel and CgBcl-2 increased significantly, and reached the peak level at 12 h, then decreased gradually. The transcripts of CgRHIM-containing protein, Cgcaspase-8 and Cgcaspase-3 in haemocytes up-regulated at 12 h after high temperature stress. Moreover, the protein abundance of CgRHIM-containing protein increased significantly, and the ubiquitination level of CgRHIM-containing protein in haemocytes showed an increasing trend at first and then decreased. After the expression of CgRHIM-containing protein was knocked down by siRNA, the mRNA expression levels of CgRel and CgBcl-2 decreased significantly at 6 h after high temperature stress, and those of CgFADD-like, Cgcaspase-8 and Cgcaspase-3, as well as the apoptosis rate of haemocytes also decreased significantly at 24 h. These results indicated that CgRHIM-containing protein might regulate haemocyte apoptosis in oysters upon high temperature stress via mediating the expression of Rel, Bcl-2 and caspase-8/3.

CgADAR1 involved in regulating the synthesis of interferon-like protein in Crassostrea gigas

Adenosine deaminases acting on RNA 1 (ADAR1) is a dsRNA adenosine (A)-to-inosine (I) editing enzyme that regulates the innate immune response against virus invasion. In the present study, a novel CgADAR1 was identified from the oyster Crassostrea gigas. The open reading frame (ORF) of CgADAR1 was of 3444 bp encoding a peptide of 1147 amino acid residues with two Zα domains, one dsRNA binding motif (DSRM) and one RNA adenosine deaminase domain (ADEAMc). The mRNA transcripts of CgADAR1 were detected in all the examined tissues, with higher expression levels in mantle and gill, which were 7.11-fold and 4.90-fold (p < 0.05) of that in labial palp, respectively. The mRNA transcripts of CgADAR1 in haemocytes were significantly induced at 24 h and 36 h after Poly (A: U) stimulation, which were 6.03-fold (p < 0.01) and 1.37-fold (p < 0.001) of that in control group, respectively. At 48 h after Poly (A:U) stimulation, the mRNA expression of CgRIG-Ⅰ, CgIRF8 and CgIFNLP significantly increased, which were 4.36-fold (p < 0.001), 1.82-fold (p < 0.05) and 1.92-fold (p < 0.05) of that in control group. After CgADAR1 expression was inhibited by RNA interference (RNAi), the mRNA expression levels of CgMDA5, CgRIG-Ⅰ, CgTBK1, CgIRF8 and CgIFNLP were significantly increased, which were 11.88-fold, 11.51-fold, 2.22-fold, 2.85-fold and 2.52-fold of that in control group (p < 0.001), and the phosphorylation level of CgTBK1 was also significantly increased. These results suggested that CgADAR1 played a regulation role in the early stages of viral infection by inhibiting the synthesis of interferon-like protein.

Genome-wide identification of toll-like receptors in Octopus sinensis and expression analysis in response to different PAMPs stimulation

Toll-like receptors (TLRs) are one of the extensively studied pattern recognition receptors (PRRs) and play crucial roles in the immune responses of vertebrates and invertebrates. In this study, 14 TLR genes were identified from the genome-wide data of Octopus sinensis. Protein structural domain analysis showed that most TLR proteins had three main structural domains: extracellular leucine-rich repeats (LRR), transmembrane structural domains, and intracellular Toll/IL-1 receptor domain (TIR). The results of subcellular localization prediction showed that the TLRs of O. sinensis were mainly located on the plasma membrane. The results of quantitative real-time PCR (qPCR) showed that the detected TLR genes were differentially expressed in the hemolymph, white bodies, hepatopancreas, gills, gill heart, intestine, kidney, and salivary gland of O. sinensis. Furthermore, the present study investigated the expression changes of O. sinensis TLR genes in hemolymph, white bodies, gills, and hepatopancreas in different phases (6 h, 12 h, 24 h, 48 h) after stimulation with PGN, poly(I: C) and Vibrio parahaemolyticus. The expression of most of the TLR genes was upregulated at different time points after infection with pathogens or stimulation with PAMPs, a few genes were unchanged or even down-regulated, and many of the TLR genes were much higher after V. parahaemolyticus infection than after PGN and poly(I:C) stimulation. The results of this study contribute to a better understanding of the molecular immune mechanisms of O. sinensis TLRs genes in resistance to pathogen stimulation.

Immunomodulatory effects of a probiotic combination treatment to improve the survival of Pacific oyster (Crassostrea gigas) larvae against infection by Vibrio coralliilyticus

Introduction

The culture of Pacific oysters (Crassostrea gigas) is of significant socio-economic importance in the U.S. Pacific Northwest and other temperate regions worldwide, with disease outbreaks acting as significant bottlenecks to the successful production of healthy seed larvae. Therefore, the current study aims to describe the mechanisms of a probiotic combination in improving the survival of C. gigas larvae. Specifically, we investigate changes in C. gigas larval gene expression in response to V. coralliilyticus infection with or without a pre-treatment of a novel probiotic combination.

Methods

Treatment groups consisted of replicates of Pacific oyster larvae exposed to a) a combination of four probiotic bacteria at a total concentration of 3.0 x 105 CFU/mL at 18 hours post-fertilization (hpf), b) pathogenic V. coralliilyticus RE22 at a concentration of 6.0 x 103 CFU/mL at 48 hpf, and c) the probiotic combination at 18 hpf and V. coralliilyticus RE22 at 48 hpf. RNA was extracted from washed larvae after 72 hpf, and transcriptome sequencing was used to identify significant differentially expressed genes (DEGs) within each treatment.

Results

Larvae challenged with V. coralliilyticus showed enhanced expression of genes responsible for inhibiting immune signaling (i.e., TNFAIP3, PSMD10) and inducing apoptosis (i.e., CDIP53). However, when pre-treated with the probiotic combination, these genes were no longer differentially expressed relative to untreated control larvae. Additionally, pre-treatment with the probiotic combination increased expression of immune signaling proteins and immune effectors (i.e., IL-17, MyD88). Apparent immunomodulation in response to probiotic treatment corresponds to an increase in the survival of C. gigas larvae infected with V. coralliilyticus by up to 82%.

Discussion

These results indicate that infection with V. coralliilyticus can suppress the larval immune response while also prompting cell death. Furthermore, the results suggest that the probiotic combination treatment negates the deleterious effects of V. coralliilyticus on larval gene expression while stimulating the expression of genes involved in infection defense mechanisms.

PyLKB1 regulates glucose transport via activating PyAMPKα in Yesso Scallop Patinopecten yessoensis under high temperature stress

Liver kinase B1 (LKB1) is a classical serine/threonine protein kinase and plays an important role in maintaining energy homeostasis through phosphorylate AMP-activated protein kinase α subunit (AMPKα). In this study, a homologous molecule of LKB1 with a typical serine/threonine kinase domain and two nuclear localization sequences (NLSs) was identified in Yesso Scallop Patinopecten yessoensis (PyLKB1). The mRNA transcripts of PyLKB1 were found to be expressed in haemocytes and all the examined tissues, including gill, mantle, gonad, adductor muscle and hepatopancreas, with the highest expression level in hepatopancreas. PyLKB1 was mainly located in cytoplasm and nucleus of scallop haemocytes. At 3 h after high temperature stress treatment (25 °C), the mRNA transcripts of PyLKB1, PyAMPKα, and PyGLUT1 in hepatopancreas, the phosphorylation level of PyAMPKα at Thr170 in hepatopancreas, the positive fluorescence signals of PyLKB1 in haemocytes, glucose analogue 2-NBDG content in haemocytes, and glucose content in hepatopancreas, haemocytes and serum all increased significantly (p < 0.05) compared to blank group (15 °C). However, there was no significant difference at the protein level of PyLKB1 and PyAMPKα. After PyLKB1 was knockdown by siRNA, the mRNA expression level of PyGLUT1, and the glucose content in hepatopancreas and serum were significantly down-regulated (p < 0.05) compared with the negative control group receiving an injection of siRNA-NC. However, there were no significant difference in PyGLUT1 expression, glucose content and glucose analogue 2-NBDG content in haemocytes. These results collectively suggested that PyLKB1-PyAMPKα pathway was activated to promote glucose transport by regulating PyGLUT1 in response to high temperature stress. These results would be helpful for understanding the function of PyLKB1-PyAMPKα pathway in regulating glucose metabolism and maintaining energy homeostasis under high temperature stress in scallops.

The involvement of AMP-activated protein kinase α in regulating glycolysis in Yesso scallop Patinopecten yessoensis under high temperature stress

AMP-activated protein kinase α subunit (AMPKα), the central regulatory molecule of energy metabolism, plays an important role in maintaining energy homeostasis and helping cells to resist the influence of various adverse factors. In the present study, an AMPKα was identified from Yesso scallop Patinopecten yessoensis (PyAMPKα). The open reading frame (ORF) of PyAMPKα was of 1599 bp encoding a putative polypeptide of 533 amino acid residues with a typical KD domain, a α-AID domain and a α-CTD domain. The deduced amino acid sequence of PyAMPKα shared 59.89-74.78% identities with AMPKαs from other species. The mRNA transcripts of PyAMPKα were found to be expressed in haemocytes and all the examined tissues, including gill, mantle, gonad, adductor muscle and hepatopancreas, with the highest expression level in adductor muscle. PyAMPKα was mainly located in cytoplasm of scallop haemocytes. At 3 h after high temperature stress treatment (25 °C), the mRNA transcripts of PyAMPKα, the phosphorylation level of PyAMPKα at Thr170 and the lactic acid (LD) content in adductor muscle all increased significantly, while the glycogen content decreased significantly. The activity of pyruvate kinase (PyPK) and the relative mRNA expression level of phosphofructokinase (PyPFK) were significantly up-regulated at 3 h after high temperature stress treatment (25 °C). Furthermore, the PyAMPKα activator AICAR could effectively upregulate the phosphorylation level of PyAMPKα, and increase activities of PyPFK and pyruvate kinase (PyPK). Meanwhile the glycogen content also declined under AICAR treatment. These results collectively suggested that PyAMPKα was involved in the high temperature stress response of scallops by enhancing glycolysis pathway of glycogen. These results would be helpful for understanding the functions of PyAMPKα in maintaining energy homeostasis under high temperature stress in scallops.

Genome-wide identification and characterization of Toll-like receptors (TLR) genes in Haliotis discus hannai, H. rufescens, and H. laevigata

Toll-like receptors (TLRs) play essential roles in the innate immune system and have been extensively studied in mollusks. In this study, through a genome-wide search, TLR genes were identified as 29 in Haliotis discus hannai, 33 in H. rufescens, and 16 in H. laevigata. Domain analysis indicated that these TLR genes contain leucine-rich repeat (LRR) and Toll/IL-1 receptor (TIR) domains and exons ranging from 1 to 5. Polymorphism analysis showed that the TLRs in abalones did not have high diversities with 143 SNPs and no Indel in H. discus hannai, 92 SNPs and 3 Indels together with 6 missense mutations in H. rufescens, and no SNP or Indel in H. laevigata. The expression of 8 TLR genes in H. discus hannai was confirmed in the hepatopancreas, gill, hemolymph, gonads, intestine, muscle, and mantle. The expression of five TLR genes (out of 8) in gills (p < 0.05), three in hepatopancreas (p < 0.05), and three in hemolymph (p < 0.05) was upregulated separately in response to the infection caused by Vibrio parahaemolyticus. The findings in this study would contribute to a better understanding of the molecular immune mechanism of H. discus hannai against stimulation by V. parahaemolyticus and provide a basis for the study of TLRs in abalones.

Effect of high temperature stress on glycogen metabolism in gills of Yesso scallop Patinopecten yessoensis

Glycogen was the main energy storage material in mollusc, and the regulation of its metabolism is essential for the response against high temperature stress. In the present study, the alternation of lactic acid (LD) content, glycogen reserves, mRNA expression level of genes encoding glycogen metabolism enzymes and activities of glycogen metabolism enzymes in gills of Yesso scallop Patinopecten yessoensis after an acute high temperature treatment at 25 °C were examined to understand the effect of high temperature on glycogen metabolism. The activity of T-ATPase in gills of scallops presented a gradual increase trend especially at 6 h after an acute high temperature treatment (p < 0.05). The glycogen reserves did not change significantly even there was a downward trend at 24 h after the acute high temperature treatment (p > 0.05). The mRNA transcripts of glycogen synthase (PyGCS) in gills of scallops decreased significantly at 1, 3, 6 and 12 h (p < 0.05), and recovered to normal level at 24 h (p > 0.05) after the acute high temperature treatment, while that of glycogen phosphorylase a (PyGPa) and phosphoenol pyruvate carboxy kinase (PyPEPCK) were both significantly down-regulated from 1 h to 24 h (p < 0.05) after the acute high temperature treatment. The activity of PyGPa at 1, 12 and 24 h and the content of LD at 3 and 24 h in gills of scallops after the acute high temperature treatment both increased significantly (p < 0.05). Furthermore, the mRNA transcripts of hexokinase (PyHK) and pyruvate kinase (PyPK) in gills of scallops increased significantly (p < 0.05) after the acute high temperature treatment, and the response of PyHK was stronger. However, there was no significant difference on the activity of PyPK in gills of scallops between the experimental samples and the blank samples (p > 0.05). In addition, the mRNA transcripts of citrate synthase (PyCS) in gills of scallops were significantly down-regulated at 6 h and 12 h (p < 0.05), and finally returned to normal level at 24 h (p > 0.05) after the acute high temperature treatment. These results collectively indicated acute high temperature stress leaded the alternation of glycogen metabolism in the gills of Yesso scallop, glycogenesis, gluconeogenesis and TCA cycle were inhibited, and the glycolysis pathway of glycogen was enhanced to produce more energy for coping with environmental pressure.

The antibacterial activity and antibacterial mechanism analyses of an LRR-IG protein in the Chinese mitten crab, Eriocheir sinensis

Leucine-rich repeat and immunoglobulin domain containing protein (LRR-IG) family is an important class of immune molecules in invertebrates. Herein, a novel LRR-IG, named as EsLRR-IG5, was identified from Eriocheir sinensis. It contained typical structures of LRR-IG including an N-terminal LRR region and three IG domains. EsLRR-IG5 was ubiquitously expressed in all the tested tissues, and its transcriptional levels increased after being challenged with Staphylococcus aureus and Vibrio parahaemolyticus. Recombinant proteins of LRR and IG domains from the EsLRR-IG5 (named as rEsLRR5 and rEsIG5) were successfully obtained. rEsLRR5 and rEsIG5 could bind to both gram-positive bacteria and gram-negative bacteria as well as lipopolysaccharide (LPS) and peptidoglycan (PGN). Moreover, rEsLRR5 and rEsIG5 exhibited antibacterial activities against V. parahaemolyticus and V. alginolyticus and displayed bacterial agglutination activities against S. aureus, Corynebacterium glutamicum, Micrococcus lysodeikticus, V. parahaemolyticus and V. alginolyticus. The scanning electron microscopy (SEM) observation revealed that the membrane integrity of V. parahaemolyticus and V. alginolyticus was destroyed by rEsLRR5 and rEsIG5, which may lead to the leakage of cell contents and death. This study provided clues for further studies on the immune defense mechanism mediated by LRR-IG in crustaceans and provided candidate antibacterial agents for prevention and control of diseases in aquaculture.

Signaling events induced by lipopolysaccharide-activated Toll in response to bacterial infection in shrimp

Toll-like receptors (TLR) play a crucial role in the detection of microbial infections in vertebrates and invertebrates. Mammalian TLRs directly recognize a variety of structurally conserved microbial components. However, invertebrates such as Drosophila indirectly recognize microbial products by binding to the cytokine-like ligand Spätzle, which activates signaling cascades that are not completely understood. In this study, we investigated the signaling events triggered by Toll in response to lipopolysaccharide (LPS), a cell wall component of gram-negative bacteria, and Vibrio parahaemolyticus infection in the arthropod shrimp Litopenaeus vannamei. We found that five of the nine Tolls from L. vannamei bound to LPS and the RNAi of LvToll1, LvToll2, LvToll3, LvToll5, and LvToll9 weakened LvDorsal-L phosphorylation induced by V. parahaemolyticus. All nine Tolls combined with MyD88 via the TIR domain, thereby conferring signals to the tumor necrosis factor receptor-associated factor 6 (TRAF6)-transforming growth factor-β activated kinase 1 binding protein 2 (TAB2)-transforming growth factor-β activated kinase 1 (TAK1) complex. Further examination revealed that the LvTRAF6-LvTAB2-LvTAK1 complex contributes to Dorsal-L phosphorylation and nuclear translocation during V. parahaemolyticus infection. Overall, shrimp Toll1/2/3/5/9-TRAF6/TAB2/TAK1-Dorsal cascades protect the host from V. parahaemolyticus infection, which provides a better understanding of how the innate immune system recognizes and responds to bacterial infections in invertebrates.

The expression profile of a multi-stress inducible transient receptor potential vanilloid 4 (TRPV4) in Pacific oyster Crassostrea gigas

1

CgTRPV4 with typical structural characteristics was indentified from Crassostrea gigas.

2

CgTRPV4 was located in both endoplasmic reticulum and cytoplasmic membrane of oyster haemocytes.

3

CgTRPV4 mRNA was ubiquitously expressed with the highest level in gill.

4

The expression of CgTRPV4 mRNA was significantly up-regulated after high temperature stress at 30°C or V. splendidus stimulation.

Transient receptor potential vanilloid 4 (TRPV4) is one of the major non-selective cation channel proteins, which plays a crucial role in sensing biotic and abiotic stresses, such as pathogen infection, temperature, mechanical pressure and osmotic pressure changes by regulating Ca²⁺ homeostasis. In the present study, a TRPV4 homologue was identified in Pacific oyster Crassostrea gigas, designated as CgTRPV4. The open reading frame (ORF) of CgTRPV4 was of 2298 bp encoding a putative polypeptide of 765 amino acid residues with three typical ankyrin domains and six conserved transmembrane domains of TRPV4 subfamily proteins, as well as multiple N-glycosylation sites, cAMP- and cGMP-dependent protein kinase phosphorylation sites, protein kinase C phosphorylation sites, casein kinase II phosphorylation sites, and prokaryotic membrane lipoprotein lipid attachment site. The deduced amino acid sequence of CgTRPV4 shared 20.5%-26.2% similarity with TRPV4s from other species. During the early ontogenesis stages of oyster, the mRNA transcripts of CgTRPV4 were detectable in all the stages with the highest expression level in fertilized eggs and the lowest in D-hinged larvae. In adult oyster, the CgTRPV4 mRNA could be detected in all the examined tissues, including gill, hepatopancreas, adductor muscle, labial palp, mantle and haemocyte, with the highest expression level in gill (45.08-fold of that in hepatopancreas, p < 0.05). In immunocytochemical assay, the CgTRPV4 positive signals were distributed in both endoplasmic reticulum and cytoplasmic membrane of oyster haemocytes. The mRNA expression of CgTRPV4 in gill was significantly up-regulated after high temperature stress at 30°C (p < 0.05) and after Vibrio splendidus stimulation (p < 0.05). These results indicated that CgTRPV4 was a classical member of TRPV4 family in oyster, which was induced by either biotic or abiotic stimulations and involved in mediating the stress response of oysters.

The truncated MyD88s negatively regulates TLR2 signal on expression of IL17-1 in oyster Crassostrea gigas

Toll like receptor (TLR) signaling plays a key role in the innate immune recognition and inflammatory regulation in both vertebrates and invertebrates. The expanded TLR signaling components, including 83 TLRs and 10 MyD88s, have been reported in the genome of the Pacific oyster Crassostrea gigas. In the present study, one endogenous TLR (designated CgTLR2) and two MyD88s (including a full-length CgMyD88-2 containing intact TIR domain and Death-domain, and a truncated CgMyD88s with only TIR domain) were identified from oyster C. gigas. CgTLR2 was highly expressed in haemocytes, especially in granulocytes. The recombinant protein of the extracellular LRR domains of CgTLR2 recognized and bound a variety of PAMPs with the strongest binding capability to LPS. The recombinant protein of intracellular TIR domain of CgTLR2 was able to bind the recombinant proteins of rCgMyD88-2 (K_D = 1.96 × 10^-9 M) and rCgMyD88s (K_D = 4.84 × 10^-8 M), with higher affinity towards rCgMyD88-2. After Vibrio splendidus stimulation, the mRNA expression levels of CgTLR2 and CgMyD88-2 were rapidly up-regulated at early stage of immune response (from the 3rd hours after V. splendidus stimulation), while that of CgMyD88s did not change until 24 h post stimulation. When CgTLR2 was knocked-down by siRNA interference, the expression levels of CgMyD88-2 and CgMyD88s decreased significantly, concomitant with the down-regulation of expression of CgIL17-1. After the expression of CgMyD88-2 was interfered, the expressions of CgMyD88s and CgIL17-1 were all decreased. In contrast, after the expression of CgMyD88s was interfered, the expressions of CgMyD88-2 and CgIL17-1 all increased. The results showed that CgMyD88s played a negative role in the regulation of CgTLR2 on inflammatory factor CgIL17-1.

Cortisol modulates glucose metabolism and oxidative response after acute high temperature stress in Pacific oyster Crassostrea gigas

Cortisol is the main stress hormone that plays crucial roles in energy metabolism and immune response in vertebrates. In the present study, the homologues of 11β-hydroxysteroid dehydrogenase type 1 (designated Cg11β-HSD1) and 5α-reductase 1 (designated Cg5αR1), the key enzymes related to cortisol metabolism, were identified from Pacific oyster Crassostrea gigas. The Cg11β-HSD1 harbored a conserved SDR domain, and Cg5αR1 contained a Steroid_dh domain and three transmembrane domains. The mRNA transcripts of Cg11β-HSD1 and Cg5αR1 were constitutively expressed in all the examined tissues of oysters, with the highest expression level in haemocytes and labial palp, respectively. After acute high temperature stress (28 °C), the mRNA expression level of Cg11β-HSD1 in hepatopancreas significantly up-regulated at 6 h and 12 h, and that of Cg5αR1 significantly up-regulated at 6 h, compared with the Blank group (11 °C). The concentration of cortisol and glucose, as well as the activities of superoxide dismutase (SOD) and catalase (CAT) in hepatopancreas all significantly up-regulated after acute high temperature stress, while the glycogen concentration in adductor muscle decreased significantly at 6 h and 12 h. After the blockage of Cg11β-HSD1 with metyrapone, the cortisol concentration and the activities of SOD and CAT significantly decreased after acute high temperature stress, the glucose concentration in hepatopancreas significantly increased at 24 h, and the glycogen concentration in adductor muscle significantly increased at 6 h. These results collectively suggested that cortisol played a crucial role in regulating glucose metabolism and oxidative response in oysters upon acute high temperature stress.

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 ECSIT Mediated Toll3-Dorsal-ALFs Pathway Inhibits Bacterial Amplification in Kuruma Shrimp

The Toll signaling pathway plays an important role in animal innate immunity. However, its activation and signal transmission greatly differ across species and need to be investigated. Shrimp farming is a worldwide economic activity affected by bacterial disease from the 1990s, which promoted research on shrimp immunity. In this study, we first proved that, among the three identified Toll receptors in Marsupenaeus japonicus kuruma shrimp, Toll 3 plays a pivotal role in initiating the antibacterial response in vivo, especially upon anti-Staphylococcus aureus infection. Further research showed that this result was due to the activation of the Dorsal transcription factor, which induced the expression of two anti-lipopolysaccharide factors (Alfs). Moreover, the evolutionarily conserved signaling intermediate in Toll pathways, ECSIT, was proved to be needed for signal transmission from Toll 3 to Dorsal and the expression of anti-lipopolysaccharide factors. Finally, the mortality assay showed that a Toll3-ECSIT-Dorsal-Alf axis was functional in the anti-S.aureus immunity of M. japonicus shrimp. The results provide new insights into the function and signal transduction of the Toll pathway in aquatic species and offer basic knowledge for shrimp disease control and genetic breeding.

A novel toll-like receptor from Crassostrea gigas is involved in innate immune response to Vibrio alginolyticus

Based on previous reports，toll-like receptors (TLRs) are recognition molecules common in various aquatic animals and play a vital role in innate immunity. In this study, a novel TLR CgToll-3 with leucine-rich repeats (LRRs) and a TIR (Toll-interleukin 1-resistance) domain was cloned in Crassostrea gigas. CgToll-3 with sixteen potential extracellular N-linked glycosylation sites and shares the closest phylogenic relationship with molluscan TLRs. Alignment of LRRs and TIR domains indicated that CgToll-3 was highly conserved compared to other LRRs of mollusks which could respond against Vibrio or other bacterial molecules, and contained three conserved functionally important motifs (Box 1, Box 2, and Box 3). The Hex Molecular Docking result showed that CgToll-3 could interact with CgMyd88 via the TIR domain. Subcellular Co-localization and BiFC Assay confirmed this interaction, and they could induce NF-κB activation. CgToll-3 was moderately expressed in the digestive gland, and its expression level was significantly up-regulated after Vibrio alginolyticus challenge. Taken together, CgToll-3 might be involved in the innate immune response to V. alginolyticus for C. gigas through a MyD88-dependent TLR mediated signaling pathway.

The Elevated Expressions of Anti-lipopolysaccharide Factors After Priming Stimulation Confer Lastingly Humoral Protection in Crab Eriocheir sinensis

Evidence of immune memory in invertebrates (immune priming) has accumulated in various organisms, and both cellular and humoral immune reactions are speculated to be involved in immune priming. However, there is a lack of understanding of the molecular mechanisms involved. In the present study, the protective effect of primed haemolymph was further validated by the increased survival rate of naïve crabs receiving a transfusion of primed haemolymph. By proteomic analysis, there were 474 proteins identified from the primed haemolymph, and most of them were functionally annotated in transport and metabolism classes. A total of 70 proteins were found to be differentially expressed in haemolymph at 12 hours and 7 days after priming stimulation with Aeromonas hydrophila, among which anti-lipopolysaccharide factor 1 (EsALF-1) and 3 (EsALF-3) were identified as the most significant (p < 0.05). After being challenged with A. hydrophila, EsALF-1 and EsALF-3 were highly expressed at both mRNA (in haemocytes) and protein (in haemolymph) levels compared with blank crabs, and the mRNA expressions of components in the EsTLR1-EsMyd88-EsPelle-EsALF pathway also increased significantly (p < 0.05). The EsALF-3 and EsMyd88 were even significantly higher expressed in response to the second A. hydrophila challenge, but their expressions all decreased (p < 0.05) when EsTLR1 was knocked down by RNAi. After the naïve crabs received an injection with the recombinant protein of EsALF-1 (rEsALF-1) or EsALF-3 (rEsALF-3), their survival rate increased significantly (p < 0.05) upon A. hydrophila stimulation. In contrast, the survival rate of the primed crabs reduced significantly (p < 0.05) after they received an injection with the antibody of EsALF-1 or EsALF-3. The enhanced expressions of EsALF-1 and EsALF-3 after A. hydrophilap riming stimulation could sustain for four weeks. All the results suggested that the EsTLR1-mediated productions of EsALF-1 and EsALF-3 in haemolymph played an indispensable role in the month-long humoral immune protection induced by A. hydrophila, which provides solid evidence of immune priming in crabs and a valuable reference for further understanding immune memory in invertebrates.

PDGFRβ Recognizes and Binds Bacteria to Activate Src/Stat Pathway in Oysters

The Stat signaling pathway plays important roles in mediating the secretions of a large number of cytokines and growth factors in vertebrates, which is generally triggered by the growth factor receptor, cytokine receptor, G protein coupled receptor, and receptor protein tyrosine kinase. In the current study, a platelet-derived growth factor receptor (defined as CgPDGFRβ) was identified from the Pacific oyster Crassostrea gigas, with a signal peptide, three Ig domains, a transmembrane domain, and an intracellular Ser/Thr/Tyr kinase domain. The two N-terminal Ig domains of CgPDGFRβ showed relatively higher binding activity to Gram-negative bacteria and LPS compared with Gram-positive bacteria and peptidoglycan. Upon binding bacteria, CgPDGFRβ in hemocytes formed a dimer and interacted with protein tyrosine kinase CgSrc to induce the phosphorylation of CgSrc at Tyr416. The activated CgSrc interacted with CgStat to induce the translocation of CgStat into the nucleus of hemocytes, which then promoted the expressions of Big defensin 1 (CgBigdef1), IL17-4 (CgIL17-4), and TNF (CgTNF1). These findings together demonstrated that the Src/Stat signaling was activated after the binding of CgPDGFRβ with bacteria to induce the expressions of CgBigdef1, CgIL17-4, and CgTNF1.

The DNA cytosine-5-methyltransferase 3 (DNMT3) involved in regulation of CgIL-17 expression in the immune response of oyster Crassostrea gigas

DNA methyltransferase, a key enzyme mediating DNA methylation, is involved in numerous processes including genomic imprinting, X chromosome inactivation, transposable element suppression, and immune defense in vertebrates. In the present study, a DNA cytosine-5-methyltransferase 3 was identified from oyster Crassostrea gigas (designed as CgDNMT3). There were a PWWP domain, a PHD domain and a DNA-methylase domain in the deduced amino acid sequences of CgDNMT3, and the conserved motifs I, IV, VI, Ⅷ, IX and X were identified in its C-terminal catalytic DNA-methylase domain. The mRNA transcripts of CgDNMT3 were detected in haemocytes, mantle, gill, adductor muscle, digestive gland and labial palp, with higher expression level in haemocytes (6.54 folds of those in gill, p < 0.01). The expression level of CgDNMT3 mRNA in haemocytes increased significantly after LPS primed (2.87 folds of that in control group, p < 0.05) in vitro or Vibrio splendidus challenging (1.94 folds of that in control group, p < 0.05) in vivo. Immunocytochemical analysis revealed that CgDNMT3 protein was distributed mainly in cytoplasm and partial in nucleus of oyster haemocytes. After CgDNMT3 was transfected and expressed in HEK293T cells, the DNA 5-methylcytosine (5-mc) level in the transfected group was significantly increased, which was 1.22 folds (p < 0.05) of the pcDNA-3.1 group. The expressions of oyster CgIL17-1, CgIL17-2 and CgIL17-5 in haemocytes increased (13.05 folds, 4.78 folds and 9.41 folds of that in control group, respectively) at 12 h after V. splendidus challenging, but the increase were significantly inhibited when the oysters were pre-treated with DNA methyltransferase inhibitor 5-Azacytidine, which were 9 folds, 1.93 folds and 3.22 folds of that in control group, respectively. These results collectively suggested that CgDNMT3 was a conserved member of DNA methyltransferase 3 family in oyster, and participated in regulating the expression of cytokines during immune response.

Yarrowia lipolytica, health benefits for animals

The yeast Yarrowia lipolytica has been industrially adopted for docosahexaenoic acid and eicosapentaenoic acid production under good manufacturing practices over 2 decades. In recent years, it has claimed attention for novel biotechnological applications, such as a functional feed additive for animals. Studies have demonstrated that this yeast is safe and has probiotic and nutritional properties for mammals, birds, fish, crustaceans, and molluscs. Animals fed Y. lipolytica enhanced productive and immune parameters, as well as modulated microbiome, fatty acid composition, and biochemical profiles. Additionally, some Y. lipolytica-derived compounds have improved productive performance, immune status, and disease resistance in animals. Therefore, the aim of this review is to identify and discuss research advances on the potential use of this yeast for animals of economic interest. Challenges, opportunities, and trends were identified and envisioned in the near future for this industrially produced yeast. KEY POINTS: • Yarrowia lipolytica has probiotic and nutritional effects in animals. • Lipase2, EPA, and β-glucan from Y. lipolytica have health benefits for animals. • Y. lipolytica is envisioned in terrestrial and aquatic animal production systems.

A haemocyte-expressed Methyltransf_FA domain containing protein (MFCP) exhibiting microbe binding activity in oyster Crassostrea gigas

The Methyltransf_FA domain is well-known as a key protein domain of enzyme synthesizing juvenile hormone, and Methyltransf_FA domain containing proteins (MFCPs) are widely existed in vertebrates and invertebrates. In the present study, a CgMFCP with a single Methyltransf_FA domain was screened from oyster Crassostrea gigas, and its open reading frame of CgMFCP was of 1128 bp, encoding a polypeptide of 376 amino acids with a signal peptide, a Methyltransf_FA domain and a transmembrane region. CgMFCP was clustered with FAMeTs from insecta and crustacea of arthropod. The mRNA transcripts of CgMFCP were detected in different tissues, with the extremely high expression level in haemocytes, which was 131.36-fold (p < 0.05) of that in gills. The expression level of CgMFCP protein was verified to be highly expressed in haemocytes. The expression level of CgMFCP mRNA in primarily cultured haemocytes significantly up-regulated at 3 h, 24 h and 48 h post LPS stimulation, which was 3.25-fold (p < 0.01), 2.04-fold (p < 0.05) and 3.59-fold (p < 0.01) compared to that in blank group. After the oysters were stimulated with Vibrio splendidus in vivo, the expression level of CgMFCP mRNA in haemocytes was also significantly up-regulated at 3 h, 12 h, and 24 h, which was 4.22-fold (p < 0.05), 4.39-fold (p < 0.05) and 6.35-fold (p < 0.01) of that in control group, respectively. By flow cytometry analysis, anti-rCgMFCP can label 95% of oyster haemocytes. And by fluorescence microscope analysis, CgMFCP was mainly distributed in cytomembrane of haemocytes. The recombinant CgMFCP (rCgMFCP) exhibited higher affinity towards MAN and LPS in a dose-dependent manner, while relatively lower affinity to PGN and poly (I:C). rCgMFCP also displayed binding activities towards Gram-negative bacteria (Vibrio anguillarum and V. splendidus), Gram-positive bacteria (Staphylococcu aureu) and fungi (Pichia pastoris). These results collectively indicated that CgMFCP specifically expressed in haemocytes and functioned as a pattern recognition receptor by binding to various microbes in oyster C. gigas, which provided insight into the function of Methyltransf_FA domain containing proteins.

The involvement of PyBeclin 1 and PyLC3 in regulating the activation of autophagy in scallop Patinopecten yessoensis after acute high temperature stress

Beclin 1 and LC3 are important autophagy regulation proteins involved in vesicle nucleation and extension stage, respectively. In the present study, a Beclin 1 and a LC3 were identified from Yesso scallop Patinopecten yessoensis (PyBeclin 1 and PyLC3). The open reading frame (ORF) of PyBeclin 1 was of 1335 bp encoding a putative polypeptide of 444 amino acid residues with an N-terminal BCL-2 homology 3 (BH3) domain, a central coiled-coil domain (CCD), and a C-terminal evolutionarily conserved domain (ECD). The ORF of PyLC3 was of 369 bp encoding a putative polypeptide of 122 amino acid residues with an APG12 domain. The deduced amino acid sequences of PyBeclin 1 and PyLC3 shared 31.92-74.09% and 68.38-79.50% identities with Beclin 1s and LC3s from other species, respectively. The mRNA transcripts of PyBeclin 1 and PyLC3 were found to be expressed in all the examined tissues, including adductor muscle, gonad, gill, haemocytes and mantle, with the highest expression level in gill and haemocytes. The mRNA expression level of PyBeclin 1 in haemocytes increased significantly at 1, 3, 6, 12 and 24 h (2.98-4.07 fold of that in the Blank group, p < 0.05), and returned to normal level at 48 h after acute high temperature stress at 25 °C. Unlike PyBeclin 1, the mRNA transcripts of PyLC3 in haemocytes were significantly up-regulated at1, 3, 6 and 12 h (1.80-2.54 fold of that in the Blank group, p < 0.05), then decreased to blank level at 24 h (p > 0.05), and increased significantly again at 48 h (3.70 fold of that in the Blank group, p < 0.05) after high temperature. PyBeclin 1 and PyLC3 were mainly located in the cytoplasm and a small amount in the nucleus with few puncta, and the numbers of PyBeclin 1 and PyLC3 puncta increased at 3 h after acute high temperature stress. The LC3-II levels in gill and haemocytes were significantly up-regulated at 1 h and 3 h after acute high temperature stress. These results collectively suggested that PyBeclin 1 and PyLC3 were conserved members of Beclin 1 and LC3 family in scallops, and involved in regulating the activation of autophagy in scallops after acute high temperature stress.

Characterization of a novel toll-like receptor and activation NF-κB signal pathway in triangle sail mussel Hyriopsis cumingii

Toll-like receptor (TLR) family plays an important role in innate immunity for detection of and defense against microbial pathogens. In this study, a novel toll-like receptor (HcTLRn) was characterized from freshwater pearl mussel H. cumingii. The complete sequence of HcTLRn was 3725 bp, and the open reading frame (ORF) encoded 718 amino acid residues. Predicted HcTLRn protein possessed seven atypical leucine-rich repeat (LRR) domains, two typical LRR subfamily domains, a C-terminal domain LRR, a transmembrane domain and an intracellular Toll/interleukin-1 (IL-1) receptor domain. Transcripts of HcTLRn were constitutive expressed in the tissues of healthy mussels and were markedly induced in hepatopancreas and gills after lipopolysaccharide (LPS), peptidoglycan (PGN) and polyinosinic polycytidylic acid (ploy I: C) stimulation. Knockdown of HcTLRn in vivo significantly decreased the mRNA levels of TLR pathway transcription factors p65 and p105 as well as antimicrobial peptides (AMPs) including lysozyme (HcLys), theromacin (HcTher), whey acidic protein (HcWAP), LPS-binding protein/bactericidal permeability increasing protein (HcLBP/BPI) 1 and 2 after mussels challenged by LPS. In situ hybridization results showed that HcTLRn mRNA was significantly increased in hemocytes after LPS, PGN and poly I:C stimulation. HcTLRn protein was mainly expressed in hepatopancreas and gills and was significantly increased after LPS stimulation. Moreover, recombinant extracellular domain of HcTLRn (HcTLRn-ECD) proteins could bind to a variety of bacterial and pathogen-associated molecular patterns such as LPS, PGN, and poly I:C in vitro. Subcellular localization results showed that HcTLRn was mainly distributed near the cell membrane and in cytoplasm. Over-expression of HcTLRn activated the NF-κB luciferase reporter in HEK293T cells. Collectively, these results suggested that HcTLRn was a TLR family member that might play an important role in activation of NF-κB signal pathway in Mollusca.

Transcriptome-wide identification and characterization of toll-like receptors response to Vibrio anguillarum infection in Manila clam (Ruditapes philippinarum)

The Manila clam (Ruditapes philippinarum), one of the major marine aquaculture species in China, is susceptible to infection with the pathogen Vibrio, which results in massive mortality and economic losses. Toll-like receptors (TLRs) are significant pattern recognition receptors (PRRs) of innate immunity that are involved in immune regulation against pathogenic invasion. Molecular characterization of Manila clam TLRs and investigations of their immune functions are essential to prevent and control Vibrio infection. In the present research, eight cDNA sequences of R. philippinarum TLRs (RpTLRs) were identified from previous transcriptome libraries and then classified into four groups, namely, P-TLR (one sequence), V-TLR (one sequence), Ls-TLR (two sequences) and sP-TLR (four sequences), based on the corresponding LRR domain arrangement of their protein structures within the typical TLR motifs. A selective pressure test firstly suggested that the molluscan P-TLR, V-TLR, Ls-TLR and sP-TLR families underwent positive selection, and different numbers of positive selection sites (PSSs) were identified in different domains of the four types of RpTLRs, as determined by PAML and analysis of website data. These findings indicated that the evolution of RpTLRs may be associated with their immune recognition and function. Furthermore, tissue-specific expression analysis showed that all RpTLRs were ubiquitously expressed in all test tissues and were dominant in hemocytes. Quantitative real-time PCR revealed that the cDNA expression of all eight RpTLRs was upregulated after injection with Vibrio anguillarum (P < 0.01) in R. philippinarum hemocytes, revealing that these RpTLRs play important roles in responding to pathogenic stimulation. In summary, these findings provide a foundation for future investigations of the molecular classification and evolutionary patterns of Toll-like receptors in invertebrates, and the innate immune responses of TLR signaling pathways in Mollusca.

Stimulation of Mytilus galloprovincialis Hemocytes With Different Immune Challenges Induces Differential Transcriptomic, miRNomic, and Functional Responses

Mediterranean mussels (Mytilus galloprovincialis) are marine bivalve molluscs with high resilience to biotic and abiotic stress. This resilience is one of the reasons why this species is such an interesting model for studying processes such as the immune response. In this work, we stimulated mussel hemocytes with poly I:C, β-glucans, and LPS and then sequenced hemocyte mRNAs (transcriptome) and microRNAs (miRNome) to investigate the molecular basis of the innate immune responses against these pathogen-associated molecular patterns (PAMPs). An immune transcriptome comprising 219,765 transcripts and an overview of the mussel miRNome based on 5,175,567 non-redundant miRNA reads were obtained. The expression analyses showed opposite results in the transcriptome and miRNome; LPS was the stimulus that triggered the highest transcriptomic response, with 648 differentially expressed genes (DEGs), while poly I:C was the stimulus that triggered the highest miRNA response, with 240 DE miRNAs. Our results reveal a powerful immune response to LPS as well as activation of certain immunometabolism- and ageing/senescence-related processes in response to all the immune challenges. Poly I:C exhibited powerful stimulating properties in mussels, since it triggered the highest miRNomic response and modulated important genes related to energy demand; these effects could be related to the stronger activation of these hemocytes (increased phagocytosis, increased NO synthesis, and increased velocity and accumulated distance). The transcriptome results suggest that after LPS stimulation, pathogen recognition, homeostasis and cell survival processes were activated, and phagocytosis was induced by LPS. β-glucans elicited a response related to cholesterol metabolism, which is important during the immune response, and it was the only stimulus that induced the synthesis of ROS. These results suggest a specific and distinct response of hemocytes to each stimulus from a transcriptomic, miRNomic, and functional point of view.

New insights into the Manila clam and PAMPs interaction based on RNA-seq analysis of clam through in vitro challenges with LPS, PGN, and poly(I:C)

Background

Manila clam (Ruditapes philippinarum) is a worldwide commercially important marine bivalve species. In recent years, however, microbial diseases caused high economic losses and have received increasing attention. To understand the molecular basis of the immune response to pathogen-associated molecular patterns (PAMPs) in R. philippinarum, transcriptome libraries of clam hepatopancreas were constructed at 24 h post-injection with Lipopolysaccharide (LPS), peptidoglycan (PGN), and polyinosinic-polycytidylic acid (poly(I:C)) and phosphate-buffered saline (PBS) control by using RNA sequencing technology (RNA-seq).

Results

A total of 832, 839, and 188 differentially expressed genes (DEGs) were found in LPS, PGN, and poly(I:C) challenge group compared with PBS control, respectively. Several immune-related genes and pathways were activated in response to the different PAMPs, suggesting these genes and pathways might specifically participate in the immune response to pathogens. Besides, the analyses provided useful complementary data to compare different PAMPs challenges in vivo. Functional enrichment analysis of DEGs demonstrated that PAMPs responsive signal pathways were related to apoptosis, signal transduction, immune system, and signaling molecules and interaction. Several shared or specific DEGs response to different PAMPs were revealed in R. philippinarum, including pattern recognition receptors (PRRs), antimicrobial peptides (AMPs), interferon-induced proteins (IFI), and some other immune-related genes were found in the present work.

Conclusions

This is the first study employing high throughput transcriptomic sequencing to provide valuable genomic resources and investigate Manila clam response to different PAMPs through in vivo challenges with LPS, PGN, and poly(I:C). The results obtained here provide new insights to understanding the immune characteristics of R. philippinarum response to different PAMPs. This information is critical to elucidate the molecular basis of R. philippinarum response to different pathogens invasion, which potentially can be used to develop effective control strategies for different pathogens.

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.

The sensing pattern and antitoxic response of Crassostrea gigas against extracellular products of Vibrio splendidus

Serious juvenile oyster disease induced by pathogenic Vibrio splendidus has resulted in tremendous economic loss, but the molecular mechanisms underlying this killing mechanism remain unclear. The resistance of adult oyster to V. splendidus or its virulence factors might provide a possible access to cognize the interaction between pathogen and host. In the present study, the extracellular products (ECP) from less virulent V. splendidus JZ6 were injected into adult Pacific oyster Crassostrea gigas, and the cellular and humoral immune response induced by ECP were investigated. The phagocytosis rate of hemocytes was significantly up-regulated (30.57%) at 6 h after ECP injection compared with that (21%) of control groups. And significantly high level of ROS production was also observed from 3 h to 12 h in ECP-injected oysters, concomitant with increased apoptosis rate of hemocytes (16.4% in ECP-injected group, p < 0.01) compared with control group (6.7%). By RT-PCR analysis, the expression level of antioxidant CgSOD in hemocytes significantly increased to 6.41-fold of that in control groups (p < 0.01) at 12 h post ECP injection. The expression levels of anti-toxic metalloprotease inhibitors CgTIMP629 and CgTIMP628 were also significantly up-regulated at the early (3-6 h) and late (6-24 h) stage of immune response, respectively. Moreover, after the ECP were incubated with serum proteins isolated from the ECP-injected oysters in vitro, the metalloprotease activity of ECP significantly declined by 21.39%, and less degraded serum proteins were detected by SDS-PAGE. When the primarily cultured hemocytes were stimulated with heat-inactivated ECP or fragments derived from ECP-degraded serum proteins, the expressions of CgTIMP629 (13.64 and 7.03-fold of that in saline group, respectively, p < 0.01) and CgTIMP628 (5.07 and 6.08-fold of that in saline group, respectively, p < 0.01) in hemocytes were all significantly induced. All the results indicated that the adult oysters could launch phagocytosis, antioxidant and anti-toxic response to resist the virulence of ECP, possibly by sensing heterologous ECP and ECP-induced endogenous alarm signals. These results provided a possible clue for the resistance mechanism of adult oysters towards the ECP of less virulent V. splendidus, which might be valuable for exploring strategies for the control of oyster disease.

An LRR-domain containing protein identified in Bathymodiolus platifrons serves as intracellular recognition receptor for the endosymbiotic methane-oxidation bacteria

As domain species in seep and vent ecosystem, Bathymodioline mussels has been regarded as a model organism in investigating deep sea chemosymbiosis. However, mechanisms underlying their symbiosis with chemosynthetic bacteria, especially how the host recognizes symbionts, have remained largely unsolved. In the present study, a modified pull-down assay was conducted using enriched symbiotic methane-oxidation bacteria as bait and gill proteins of Bathymodiolus platifrons as a target to isolate pattern recognition receptors involved in the immune recognition of symbionts. As a result, a total of 47 proteins including BpLRR-1 were identified from the pull-down assay. It was found that complete cDNA sequence of BpLRR-1 contained an open reading frame of 1479 bp and could encode a protein of 492 amino acid residues with no signal peptide or transmembrane region but eight LRR motif and two EFh motif. The binding patterns of BpLRR-1 against microbial associated molecular patterns were subsequently investigated by surface plasmon resonance analysis and LPS pull-down assay. Consequently, BpLRR-1 was found with high binding affinity with LPS and suggested as a key molecule in recognizing symbionts. Besides, transcripts of BpLRR-1 were found decreased significantly during symbiont depletion assay yet increased rigorously during symbionts or nonsymbiotic Vibrio alginolyticus challenge, further demonstrating its participation in the chemosynthetic symbiosis. Collectively, these results suggest that BpLRR-1 could serve as an intracellular recognition receptor for the endosymbionts, providing new hints for understanding the immune recognition in symbiosis of B. platifrons.

A toll receptor is involved in antibacterial defense in the oriental river prawn, Macrobrachium nipponense

Toll-like receptors (TLRs) play an important role in the activation of innate immune response in animals. In this study, we identified a TLR from the oriental river prawn, Macrobrachium nipponense (MnToll1) and investigated its functions in immunity. The MnToll1 protein shares similar structural characteristics with other known Toll family proteins. MnToll1 transcripts are broadly distributed in all of the examined tissues, and its expression level was significantly up-regulated by bacterial challenge. RNAi-mediated knockdown of MnToll1 significantly impaired the survivability of Vibrio-challenged prawns. RNAi experiments also revealed that the expression of several antimicrobial peptide genes were regulated by MnToll1. Moreover, we found the extracellular region of MnToll1 could directly bind to bacteria and bacterial glycoconjugates. These findings suggest that MnToll1 function as a pattern recognition receptor to recognize invading pathogen and initiate downstream gene expression, to participate in antibacterial defense of M. nipponense.

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.

Molecular Characterization of Two Toll Receptors in Hyriopsis cumingii and Their Potential Roles in Antibacterial Response

Tolls/Toll-like receptors (TLRs) play a key role in innate immunity by detecting the invading microbes and subsequently activating downstream signaling cascades. In this study, two new molluscan Toll members (designed as HcToll6 and HcToll7) were identified from triangle-shell pearl mussel (Hyriopsis cumingii). The obtained HcToll6 full-length cDNA was 3207 bp consisting of a 2223 bp open reading frame (ORF) that encoded a peptide of 740 amino acids. HcToll7 cDNA is a 3216 bp molecule that contains an ORF of 2139 bp encoding a protein of 712 amino acids. The deduced HcToll6 and HcToll7 proteins share two common structures: extracellular leucine-rich repeat (LRR) domains and intracellular Toll/interleukin-1 receptor (TIR) domain. Quantitative real-time PCR results showed that HcToll6 and HcToll7 were mainly expressed in the hepatopancreas and the gills, and they responded rapidly to bacterial stimulation. RNA interference by dsRNA results revealed that HcToll6 and HcToll7 RNAi strongly decreased the expression of lysozyme (HcLyso) and defensin (HcDef) in the gills of RNAi-treated mussels with Vibrio parahaemolyticus challenge. As a pattern recognition receptor, the prokaryotic expressed the recombinant LRR domains of HcToll6 and HcToll7 (rHcToll6-LRR and rHcToll7-LRR) could bind to Gram-positive and Gram-negative bacteria and had a strong tendency to recognize lipopolysaccharide (LPS) and peptidoglycan (PNG). rHcToll6-LRR and rHcToll7-LRR exhibited a significant in vitro bactericidal activity against V. parahaemolyticus and Staphylococcus aureus. These findings provide useful information to characterize Tolls in mussels.

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.

Peroxiredoxin 6 modulates Toll signaling pathway and protects DNA damage against oxidative stress in red swamp crayfish (Procambarus clarkii)

Peroxiredoxin 6 (Prx6) is an important member of the peroxiredoxin family that plays critical roles in protecting host against the toxicity of oxidative stress and participates in cell signaling. Herein, we report Prx6 gene from red swamp crayfish, Procambarus clarkii. The cDNA fragment of PcPrx6 was 660 bp, encoding a 219 amino acid residues protein. The quantitative real time PCR analysis showed ubiquitous expression of PcPrx6 mRNA in the tested tissues. The challenge with peptidoglycan and Poly I:C remarkably suppressed the mRNA level of PcPrx6 in hepatopancreas at 3, 12, 48 h compared with the PBS control. However, the expression level significantly increased after 36 h of their treatment. The knockdown of PcPrx6 by small interference RNA significantly enhanced the transcript levels of Toll pathway-responsive genes at 24 h. Recombinant PcPrx6 protein was purified using affinity chromatography and analyzed for its biological role. The results revealed that the recombinant PcPrx6 protein manifested the ability to protect supercoiled DNA damage from oxidative stress elicited by mixed function oxidative assay. Altogether, PcPrx6 may have multiple functional roles in the physiology of P. clarkii, since it negatively regulates the Toll signaling transduction and protects supercoiled DNA damage from oxidative stress.

Gene expression profiles at different stages for formation of pearl sac and pearl in the pearl oyster Pinctada fucata

Background

The most critical step in the pearl formation during aquaculture is issued to the proliferation and differentiation of outer epithelial cells of mantle graft into pearl sac. This pearl sac secretes various matrix proteins to produce pearls by a complex physiological process which has not been well-understood yet. Here, we aimed to unravel the genes involved in the development of pearl sac and pearl, and the sequential expression patterns of different shell matrix proteins secreted from the pearl sac during pearl formation by pearl oyster Pinctada fucata using high-throughput transcriptome profiling.

Results

Principal component analysis (PCA) showed clearly different gene expression profiles between earlier (before 1 week) and later stages (1 week to 3 months) of grafting. Immune-related genes were highly expressed between 0 h – 24 h (donor dependent) and 48 h – 1 w (host dependent), and in the course of wound healing process pearl sac was developed by two weeks of graft transplantation. Moreover, for the first time, we identified some stem cell marker genes including ABCG2, SOX2, MEF2A, HES1, MET, NRP1, ESR1, STAT6, PAX2, FZD1 and PROM1 that were expressed differentially during the formation of pearl sac. The expression profiling of 192 biomineralization-related genes demonstrated that most of the shell matrix proteins (SMPs) involved in prismatic layer formation were first up-regulated and then gradually down-regulated indicating their involvement in the development of pearl sac and the onset of pearl mineralization. Most of the nacreous layer forming SMPs were up-regulated at 2 weeks after the maturation of pearl sac. Nacrein, MSI7 and shematrin involved in both layer formation were highly expressed during 0 h – 24 h, down-regulated up to 1 week and then up-regulated again after accomplishment of pearl sac formation.

Conclusions

Using an RNA-seq approach we unraveled the expression pattern of the key genes involved in the development of pearl sac and pearl as a result of host immune response after grafting. These findings provide valuable information in understanding the molecular mechanism of pearl formation and immune response in P. fucata.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5579-3) contains supplementary material, which is available to authorized users.

Transcriptome-wide identification, functional characterization, and expression analysis of two novel invertebrate-type Toll-like receptors from disk abalone (Haliotis discus discus)

Toll-like receptors (TLRs) are well-known pattern recognition receptors that play key immunological roles in a diverse range of organisms. In this study, two novel invertebrate TLRs from disk abalone (designated as AbTLR-A and AbTLR-B) were identified and functionally characterized for the first time. AbTLR-A and AbTLR-B comprised the typical TLR domain architecture containing an extracellular leucine-rich repeat domain, transmembrane domain, and Toll/interleukin-1 receptor domain. Expressional analysis revealed that both TLRs were constitutively expressed at all the early embryonic stages of disk abalone analyzed, with the highest level of AbTLR-A found at the 16-cell stage and AbTLR-B at the trochophore stage. According to tissue distribution analysis, prominent mRNA expression of AbTLR-A and AbTLR-B was detected in the hemocytes and gills, respectively. AbTLR-A and AbTLR-B mRNAs were significantly up-regulated in response to Gram-negative Vibrio parahemolyticus, Gram-positive Listeria monocytogenes, and viral hemorrhagic septicemia virus injections in abalone hemocytes and gills. Overexpression of AbTLR-A and AbTLR-B in HEK293T cells directly activated nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) responsive reporters. Neither TLRs showed a high response to pathogen-associated molecular patterns in vitro. Co-expression of AbTLR-A and AbTLR-B with AbMyD88-2 and AbMyD88-X activated NF-κB-responsive reporters in a synergetic manner. These findings demonstrate the involvement of AbTLR-A and AbTLR-B in abalone innate immunity.

SpToll1 and SpToll2 modulate the expression of antimicrobial peptides in Scylla paramamosain

Tolls and Toll-like receptors (TLRs) were the first pattern recognition receptors (PRRs) identified to play key roles in host innate immunity. However, relatively little is known about other types of Toll-like receptors in Scylla paramamosain, although a Toll-like receptor (SpToll1) has recently been cloned. In this study, we cloned and characterized another novel Toll-like receptor 2 (SpToll2) from S. paramamosain. The full-length cDNA of SpToll2 is 3391 bp with a 2646 bp open reading frame (ORF) encoding a putative protein of 881 amino acids, and predicted to contain six extracellular leucine-rich repeat (LRR) domains, a transmembrane domain and an intracellular Toll/IL-1 receptor (TIR) domain. Phylogenetic analysis revealed that SpToll2 clustered with Drosophila Toll1, and shared high homology with PtToll4. Real-time qPCR analysis showed that SpToll2 was widely expressed in all tissues tested, with the highest level found in hemocytes and hepatopancreas while the lowest in heart and muscle. The transcript levels of both SpToll1 and SpToll2 in mud crabs hemocytes was induced following challenge with Vibrio parahaemolyticus, Staphylococcus aureus, Polyinosinic: polycytidylic acid (Poly I:C) and white spot syndrome virus (WSSV). In addition, recombinant SpToll1-LRR and SpToll2-LRR proteins could bind to V. parahaemolyticus, S. aureus, Escherichia coli, and Beta Streptococcus. In order to study the signaling pathway of AMPs' expression in mud crab, RNA interference were used to test the expression of SpAMPs after the challenges with V. parahaemolyticus or S. aureus. The data suggested that SpToll1and SpToll2 could regulate the transcripts of several AMPs and four immune related mediators (SpMyD88, SpTube, SpPelle and SpTRAF6) at different scale. While silencing of SpToll1 post pathogens challenge attenuated the expression of SpHistin, SpALF1 and SpALF5 in mud crab's hemocytes, depletion of SpToll2 post pathogens challenge inhibited the expression of SpALF1-6, SpGRP, SpArasin and SpHyastastin. Furthermore, the results of overexpression assay also showed SpToll1 and SpToll2 could enhance the promoter activities of SpALFs in mud crab. Taken together, these results indicated that SpToll1 and SpToll2 might play important roles in host defense against pathogen invasions in S. paramamosain.

A novel toll-like receptor from Mytilus coruscus is induced in response to stress

Toll-like receptor (TLR) is considered to be an evolutionarily conserved transmembrane protein which promotes the Toll signal pathway to active the expression of transcription factors in the innate immunity of the organism. In this study, a full length of TLR homologue of 2525bp in Mytilus coruscus (named as McTLR-a, GenBank accession no: KY940571) was characterized. Its ORF was 1815 bp with a 5'untranslated region (UTR) of 128 bp and a 3'UTR of 582 bp, encoding 602 amino acid residues with a calculated molecular weight of 70.870 kDa (pI = 6.10). BLASTn analysis and phylogenetic relationship strongly suggested that this cDNA sequence was a member of TLR family. Quantitative real time RT-PCR showed that constitutive expression of McTLR-a was occurred, with increasing order in hemocyte, gonad, mantle, adducter, gill and hepatopancreas. Bacterial infection and heavy metals stimulation up-regulated the expression of McTLR-a mRNA in hepatopancreas with time-dependent manners. The maximum expression appeared at 12 h after pathogenic bacteria injection, with approximately 22-fold in Aeromonas hydrophila and 17-fold in Vibrio parahemolyticus higher than that of the blank group. In heavy metals stress group, they all reached peaks at 3d, while the diverse concentration caused the maximum expression were different. The highest expression reached approximately 7-fold higher than the blank in low concentration of Pb²⁺ exposure. In Cu²⁺ treated group, it reached the peak (approximately 12-fold higher than the blank)in middle concentration. These results indicated that McTLR-a might be involved in the defense response and had a significant role in mediating the environmental stress.

The cyclin-dependent kinase 2 (CDK2) mediates hematopoiesis through G1-to-S transition in Chinese mitten crab Eriocheir sinensis

Cyclin-dependent kinases (CDKs), a family of cell cycle-related serine/threonine kinases, participate in various biological processes, and play crucial roles in the innate immunity. In the present study, a CDK2 (designed as EsCDK2) with a serine/threonine protein kinase catalytic domain was identified from Chinese mitten crab (Eriocheir sinensis). The full-length cDNA sequence of EsCDK2 was of 2405 bp with an open reading frame (ORF) of 909 bp. EsCDK2 shared 66%-81% sequence similarities with previously identified CDK2s. It was clustered with the CDK2 from Penaeus monodon in the invertebrate branch of the phylogenetic tree. The mRNA transcripts of EsCDK2 were highly expressed in hematopoietic tissue (HPT) and gonad, while lower in hemocytes, heart, gills, and muscle. EsCDK2 protein distributed in both cytoplasm and nucleus of HPT cells. The expression of EsCDK2 mRNA in HPT was significantly up-regulated and peaked at 3 h post stimulations with Aeromonas hydrophila (2.31-fold, p < 0.05) and Lipopolysaccharide (LPS) (2.02-fold, p < 0.05). After exsanguination, the total hemocyte counts (THC) decreased significantly to 0.42 × 10⁷/ml (0.39-fold, p < 0.05) at 0.5 h, then returned to a normal level at 6 h, while the mRNA expression of EsCDK2 in HPT cells was up-regulated at the early phase from 0.5 h to 6 h. After injection of EsCDK2-dsRNA, the mRNA expression level of EsCDK2 in HPT and THC both decreased to 0.53-fold (p < 0.01) and 0.78-fold (p < 0.05) at 24 h, respectively, and the percentage of new-born hemocytes in HPT also decreased significantly from 37.7% to 16.3% (0.43-fold, p < 0.01). After knocking down of EsCDK2, THC decreased dramatically at 6 h (0.65-fold, p < 0.01) post exsanguination, while returned normal at 6 h in PBS group. After interference of EsCDK2 mRNA expression, the percentage of G0-G1 phase cells significantly increased to 85.01% (1.26-fold, p < 0.01), while S phase and G2-M phase cells significantly decreased to 7.92% (0.46-fold, p < 0.01) and 7.07% (0.43-fold, p < 0.01) respectively, indicating that the cell cycle of HPT cells arrested at G1 phase. These results collectively demonstrated that EsCDK2 participated in the regeneration of hemocytes or hematopoiesis by regulating the transition from G1 to S phase in the cell cycle, and involves in the innate immune responses of E. sinensis.

Pathogen-Derived Carbohydrate Recognition in Molluscs Immune Defense

Self-nonself discrimination is a common theme for all of the organisms in different evolutionary branches, which is also the most fundamental step for host immune protection. Plenty of pattern recognition receptors (PRRs) with great diversity have been identified from different organisms to recognize various pathogen-associated molecular patterns (PAMPs) in the last two decades, depicting a complicated scene of host-pathogen interaction. However, the detailed mechanism of the complicate PAMPs–PRRs interactions at the contacting interface between pathogens and hosts is still not well understood. All of the cells are coated by glycosylation complex and thick carbohydrates layer. The different polysaccharides in extracellular matrix of pathogen-host are important for nonself recognition of most organisms. Coincidentally, massive expansion of PRRs, majority of which contain recognition domains of Ig, leucine-rich repeat (LRR), C-type lectin (CTL), C1q and scavenger receptor (SR), have been annotated and identified in invertebrates by screening the available genomic sequence. The phylum Mollusca is one of the largest groups in the animal kingdom with abundant biodiversity providing plenty of solutions about pathogen recognition and immune protection, which might offer a suitable model to figure out the common rules of immune recognition mechanism. The present review summarizes the diverse PRRs and common elements of various PAMPs, especially focusing on the structural and functional characteristics of canonical carbohydrate recognition proteins and some novel proteins functioning in molluscan immune defense system, with the objective to provide new ideas about the immune recognition mechanisms.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

LRFN (leucine-rich repeat and fibronectin type-III domain-containing protein) recognizes bacteria and promotes hemocytic phagocytosis in the Pacific oyster Crassostrea gigas

In bivalve mollusks, circulating hemocyte mediated phagocytosis is one of the primary ways to eliminate invading microbes. Here, we have identified one CgLRFN (leucine-rich repeat and fibronectin type-III domain-containing protein) in the Crassostrea gigas as a novel transmembrane LRR (Leucine-rich repeat) domain containing protein in C. gigas, homologous to the jawless fish VLR protein, that plays an important role in recognizing bacteria and promoting hemocytic phagocytosis. Tissue distribution analysis of CgLRFN in Pacific oyster showed that it is widely expressed in various tissues like the gills, adductor muscles, digestive glands, gonads, heart and in the hemocytes. Furthermore, infection of Pacific oysters with two marine Vibrio strains V. alginolyticus and V. parahaemolyticus was found to significantly increase CgLRFN expression in the hemocytes. Analysis of subcellular localization showed that CgLRFN is primarily localized in the cell membrane. Additionally, CgLRFN was found to be able to bind both the bacterial strains, indicating its possible role as a cell surface receptor. Flow cytometry analysis revealed that CgLRFN coated bacteria was phagocytosed by oyster hemocytes at a significantly higher rate compared to the uncoated bacteria. Finally, RNAi mediated knockdown of CgLRFN in vivo resulted in reduced clearance of both the bacterial strains from the oyster hemolymph. Overall, our study demonstrates that CgLRFN acts as a pattern recognition receptor for Vibrio spp. and promotes hemocytic phagocytosis in the Pacific oyster, which is critical for understanding the mechanism of bacterial infection in lower invertebrates, and also contributes to disease management of this economically and ecologically important marine mollusk.

Comparative and evolutionary analysis of an adapter molecule MyD88 in invertebrate metazoans

The myeloid differentiation factor 88 (MyD88) is an essential adapter in Toll-like receptor (TLR) signalling pathways, with TLR the first pattern-recognition receptor (PRR) that was discovered in Drosophila. In the present study, a MyD88 gene was identified and characterized from a commercially important shellfish, Scapharca subcrenata, including a DEATH domain and TIR domain conserved within other molluscs. Furthermore, comparative genomic evidence revealed that MyD88 was of different lengths and contained quantitative exon and intron regions, which might be involved in specific mechanisms. To further explore the phylogenetic relationships of invertebrate metazoan MyD88, we applied MrBayes and PhyML software to construct phylogenetic trees using Bayesian and maximum likelihood approaches, respectively, which suggested that the MyD88 of Arthropoda was closely related to lower invertebrates, in contrast to morphological taxonomy. Finally, we investigated the evolutionary patterns and location of positive selection sites (PSSs) in the MyD88 gene from Arthropoda, Mollusca and Insecta using PAML software with the maximum likelihood method. The data showed that positive selection sites were detected in these groups, and partial sites were located in the TIR domain but were not found in the DEATH domain. To summarize, in this study, we report on the diversification of MyD88 in invertebrate metazoans, the specific evolutionary position of Arthropoda MyD88, and the positive selection pressures on MyD88 of Arthropoda, Mollusca and Insecta. These results are a valuable contribution to understand and clarify the evolutionary pattern of TLR/MyD88 signalling pathways in invertebrate and vertebrate taxa.

Activation of Toll Pathway Is Different between Kuruma Shrimp and Drosophila

The Toll pathway is essential for inducing an immune response to defend against bacterial invasion in vertebrates and invertebrates. Although Toll receptors and the transcription factor Dorsal were identified in different shrimp, relatively little is known about how the Toll pathway is activated or the function of the pathway in shrimp antibacterial immunity. In this study, three Tolls (Toll1–3) and the Dorsal were identified in Marsupenaeus japonicus. The Toll pathway can be activated by Gram-positive (G⁺) and Gram-negative (G⁻) bacterial infection. Unlike Toll binding to Spätzle in Drosophila, shrimp Tolls could directly bind to pathogen-associated molecular patterns from G⁺ and G⁻ bacteria, resulting in Dorsal translocation into nucleus to regulate the expression of different antibacterial peptides (AMPs) in the clearance of infected bacteria. These findings suggest that shrimp Tolls are pattern recognition receptors and the Toll pathway in shrimp is different from the Drosophila Toll pathway but identical with the mammalian Toll-like receptor pathway in its activation and antibacterial functions.