A lysin motif-containing protein (SpLysMD3) functions as a PRR involved in the antibacterial responses of mud crab, Scylla paramamosain

The role of pattern recognition receptors in the innate immune system of Chinese mitten crab (Eriocheir sinensis)

Eriocheir sinensis (Chinese mitten crab) is one of the main economic species in China, which has evolved an extremely sophisticated innate immune system to fend off disease invasions. However, bacterial and viral infections have caused significant financial losses for the E. sinensis aquaculture in recent years. Making well-informed judgments for the control microbial infections would require a thorough understanding and clarification of the intricate innate immune system of E. sinensis. Innate immunity is essential for the host's defense against invasive pathogens. Pattern recognition receptors (PRRs) initially recognize pathogen-associated molecular patterns (PAMPs) and trigger an innate immune response, causing the generation of inflammatory cytokine and promoting the clearance and control of pathogens. In E. sinensis, Toll/Toll-like receptors, lipopolysaccharide and β-1,3-glucan binding proteins, C-type lectins, galactoside-binding lectins, L-type lectins, scavenger receptors, and down syndrome cell adhesion molecules have been identified to be PRRs that are involved in the recognition of bacteria, fungi, and viruses. In this review, we give a comprehensive overview of the literature regarding PRRs' roles in the immunological defenses of E. sinensis, with the aim of providing clues to the mechanisms of innate immunity.

LysM-containing proteins function in the resistance of Litopenaeus vannamei against Vibrio parahaemolyticus infection

Lysin motif (LysM) is a functional domain that can bind to peptidoglycans, chitin and their derivatives. The LysM-containing proteins participate in multiple biological processes, such as the hydrolysis of bacterial cell walls and the perception of PAMPs in plants and high animals. In the present study, two genes encoding LysM-containing proteins, designated as LvLysM1 and LvLysM2, were identified in the Pacific white shrimp, Litopenaeus vannamei, and their functions during Vibrio infection were analyzed. The open-reading frame (ORF) of LvLysM1 was 795 bp, only encoding a LysM domain at the N-terminal region. The ORF of LvLysM2 was 834 bp, encoding a LysM domain at the central region and a transmembrane region at the C-terminal region. Both LvLysM1 and LvLysM2 were widely transcribed in all tested shrimp tissues. Enzyme-linked immunosorbent assay (ELISA) showed that the recombinant protein of LvLysM2 could bind to different bacterial polysaccharides, while LvLysM1 showed no direct binding activity. The transcripts of LvLysMs in gills increased significantly after infection with Vibrio parahaemolyticus. When LvLysM1 or LvLysM2 was knocked down by dsRNA, the mortality of shrimp was significantly increased after infection with Vibrio parahaemolyticus. Interestingly, some SNPs existed in these two genes were apparently correlated with the VpAHPND resistance of shrimp. These results suggested that LvLysM1 and LvLysM2 might contribute to the disease resistance of shrimp. The data provide new knowledge about the function of LysM-containing proteins in shrimp and potential genetic markers for disease resistance breeding.

Functions of lysin motif (LysM)-containing protein in antibacterial responses of sea cucumbers, Apostichopus japonicus

The lysin motif (LysM)-containing protein is one of widespread pattern-recognition receptors in prokaryotes and eukaryotes. Numerous LysM-containing gene sequences are present in gene databases; however, few have been well characterized, especially in echinoderms. In this study, the full-length cDNA of a novel LysM-containing gene was obtained from the sea cucumber Apostichopus japonicus, named AjLysM-1, using polymerase chain reaction (PCR) combined with rapid amplification of cDNA ends. We prepared and expressed recombinant AjLysM-1 protein (rAjLysM-1) and determined its pathogen-recognition ability by enzyme-linked immunosorbent and immunofluorescence assays. We also analyzed the tissue expression pattern and response to immune challenges of AjLysM-1 using quantitative real-time reverse transcription-PCR and in situ hybridization. The AjLysM-1 protein was predicted to be an intracellular non-secreted LysM-containing protein, highly homologous to the same protein in other marine echinoderms. AjLysM-1 transcripts were highest expressed in coelomocytes and were strikingly induced by challenge with representative bacterial and fungal polysaccharides. rAjLysM-1 showed weak binding to mannan, Pseudoalteromonas nigrifaciens, and Shewanella baltica, implying that AjLysM-1 might provide inadequate defense against Gram-negative bacteria and fungi. Notably, rAjLysM-1 also interacted with tyrosine protein kinase and filamin-B, indicating that it could be involved in focal adhesion in A. japonicus. These findings improve our understanding of the functions of LysM-containing proteins in marine echinoderms.

SpSR-B2 functions as a potential pattern recognition receptor involved in antiviral and antibacterial immune responses of mud crab Scylla paramamosain

Although class B scavenger receptors (SR-Bs) in mammals are multifunctional molecules, the functions of SR-Bs in invertebrates remain largely unknown. In this study, we characterized an SR-B homolog, namely SpSR-B2, from Scylla paramamosain. SpSR-B2 shared high similarity with mammalian SR-Bs, and exhibited specific binding activity to ac-LDL, indicating that it may be a new member of SR-B class in invertebrates. SpSR-B2 was upregulated after challenge with white spot syndrome virus (WSSV) or bacteria. Binding assays showed that SpSR-B2 specifically interacted with WSSV envelope protein VP24. Besides, SpSR-B2 could bind to all tested bacterial cells and agglutinate these bacteria. SpSR-B2 also exhibited a strong binding activity to LPS but weak binding activities to other tested polysaccharides. These findings indicated that SpSR-B2 was a potential recognition molecule for viral protein VP24 and bacterial LPS. Knockdown of SpSR-B2 resulted in dramatically decreased expressions of certain antimicrobial peptides (AMPs), and overexpression of SpSR-B2 led to the increased expression of the AMP of SpALF2, suggesting that SpSR-B2 could regulate the expression of AMPs. Taken together, this study revealed that SpSR-B2 functioned as a potential pattern recognition receptor participating in antiviral and antibacterial immunity, and provided new insights into the immune functions of invertebrate SR-Bs.

SpCrus2 Glycine-Rich Region Contributes Largely to the Antiviral Activity of the Whole-Protein Molecule by Interacting with VP26, a WSSV Structural Protein

Crustins are cysteine-rich cationic antimicrobial peptides with diverse biological functions including antimicrobial and proteinase inhibitory activities in crustaceans. Although a few crustins reportedly respond to white spot syndrome virus (WSSV) infection, the detailed antiviral mechanisms of crustins remain largely unknown. Our previous research has shown that SpCrus2, from mud crab Scylla paramamosain, is a type II crustin containing a glycine-rich region (GRR) and a cysteine-rich region (CRR). In the present study, we found that SpCrus2 was upregulated in gills after WSSV challenge. Knockdown of SpCrus2 by injecting double-stranded RNA (dsSpCrus2) resulted in remarkably increased virus copies in mud crabs after infection with WSSV. These results suggested that SpCrus2 played a critical role in the antiviral immunity of mud crab. A GST pull-down assay showed that recombinant SpCrus2 interacted specifically with WSSV structural protein VP26, and this result was further confirmed by a co-immunoprecipitation assay with Drosophila S2 cells. As the signature sequence of type II crustin, SpCrus2 GRR is a glycine-rich cationic polypeptide with amphipathic properties. Our study demonstrated that the GRR and CRR of SpCrus2 exhibited binding activities to VP26, with the former displaying more potent binding ability than the latter. Interestingly, pre-incubating WSSV particles with recombinant SpCrus2 (rSpCrus2), rGRR, or rCRR inhibited virus proliferation in vivo; moreover, rSpCrus2 and rGRR possessed similar antiviral abilities, which were much stronger than those of rCRR. These findings indicated that SpCrus2 GRR contributed largely to the antiviral ability of SpCrus2, and that the stronger antiviral ability of GRR might result from its stronger binding activity to the viral structural protein. Overall, this study provided new insights into the antiviral mechanism of SpCrus2 and the development of new antiviral drugs.

The role of UhpA in regulating the virulence gene expression in Edwardsiella piscicida

Edwardsiella piscicida (E. piscicida) is an important fish pathogen. However, the mechanism of Glu6P transport regulatory protein UhpA how to affect the virulence gene expression in E. piscicida is still unclear. The results in this study showed that the metabolism-related gene expression of cysteine synthase (orf 1134) and sulphate transporter (ychM) in the uhpA mutant strain ΔuhpA was 0.76-fold and 0.68-fold lower than the ones in the wild strains (p < .05). The gene expression of ethA and ethB in the ΔuhpA strain was 0.80-fold and 0.72-fold lower than the ones in the wild strains (p < .05). However, the gene expression of fliC and flgN in the ΔuhpA was 1.51-fold and 1.21-fold higher than the ones in the wild strains (p < .05). The gene expression of T3SS (esrB and esrC) and T6SS (evpB and evpC) in the ΔuhpA was 1.27-fold, 1.13-fold, 1.28-fold and 1.23-fold higher than the ones in the wild strains (p < .05). This suggested that the uhpA gene could regulate the key virulence gene expression, and the uhpA gene was associated with the pathogenicity of E. piscicida in fish.

Molecular identification and functional analysis of MyD88 in giant freshwater prawn (Macrobrachium rosenbergii) and expression changes in response to bacterial challenge

Myeloid differentiation factor 88 (MyD88) is a crucial adaptor protein for Toll-like receptor (TLR)-mediated signaling pathways and plays an important role in immune response. In this study, the full-length cDNA of MyD88 from Macrobrachium rosenbergii (MRMyD88) was cloned. The MRMyD88 cDNA is 1758 bp long and contains a 1398-bp open reading frame. Multiple sequence alignment and phylogenetic analysis revealed that the amino acid sequence of MRMyD88 shared high identity with the known MyD88 proteins. The MRMyD88 mRNA was widely expressed in all examined tissues, with highest level in intestine, followed by gonad and pleopod. Furthermore, the MRMyD88 promoter region, spanning 1622 bp, contains several transcription factor-binding sites, including nine GATA-1 box motifs. Electrophoretic mobility shift assay showed that Gfi-1, SRF, and Oct-1 bind to the upstream region of MRMyD88. Additionally, the results showed that the expression levels of TLR1, TLR2 and TLR3 were different in response to Vibrio anguillarum, Lactobacillus plantarum and Aeromonas hydrophila infections. However, these bacteria significantly increased the expression levels of MyD88 and prophenoloxidase. These data suggest that the TLR-mediated signaling pathway is MyD88-dependent in response to pathogenic and probiotic bacteria in M. rosenbergii.

The UhpA mutant of Edwardsiella piscicida enhanced its motility and the colonization in the intestine of tilapia

Edwardsiella piscicida (E. piscicida) is a significant bacterial pathogen of cultured fish, which infected fish meanly through the intestine. Glucose 6-phosphate (Glu6P) in the intestine is nutritious to the pathogen, Meanwhile, Glu6P was found using as a virulent regulating signal for bacteria. The UhpA, one of the Glu6P transport system regulatory proteins could down-regulate the uhpC/uhpB/uhpA system and decrease its pathogenicity. However, the motility and the colonization of E. piscicida affected by UhpA were still unclear. In this study, the motility and the colonization of E. piscicida were monitored. The result demonstrated that the motility of EIB202 was significantly stronger than that of in ΔuhpA according to fractions 4, 8 and 9. However, the motility of ΔuhpA was significantly stronger than that of EIB202 according to the total number at the whole experiment. Although, there was no difference in the number of bacteria in the posterior intestine of tilapia after infected with E. piscicida EIB202 and ΔuhpA. The number of bacteria in the anterior and the middle intestine of fish infected with ΔuhpA were significantly higher than that of in fish infected with EIB202 at the whole experiment (P < 0.05). Interestingly, both E. piscicida strains colonized in the anterior intestine than that of in the middle and posterior intestines of tilapia. Besides, the gene expression of IL-1β and TNF-α in the head-kidney of fish infected with ΔuhpA showed significantly higher (p < 0.05) than fish infected with EIB202 during the whole experimental period. Most importantly, the survival rate of E. piscicida EIB202 and ΔuhpA were 57% and 37% respectively. All results indicate that the uhpA gene mutant in E. piscicida could enhance its motility and the colonization in the intestine of tilapia, this illustrates the mechanism of UhpA decreases the pathogenesis of E. piscicida in fish.

The role of uhpA in Edwardsiella piscicida and the inflammatory cytokine response in tilapia

Edwardsiella piscicida (E. piscicida) is an important zoonotic pathogen that infects fish by colonizing the intestines. The intestine provides nutrition including Glucose 6-phosphate (Glu6P) and a competitive environment for the microbiota. Although the transport system regulatory protein gene uhpA has been reported in E. piscicida genomes, whether the uhpA gene is involved in the pathogenicity of E. piscicida remains largely unknown. Therefore, the uhpA gene mutants strain E. piscicida ΔuhpA was constructed to elucidate the functions of Glu6P and the uhpA gene in E. piscicida. The results demonstrated that Glu6P significantly increased the gene expression of uhpC/uhpB/uhpA than without adding Glu6P in the culture. The gene expression of uhpC and uhpB was down regulated in the mutant strain than that of in the wild type strain. E. piscicida ΔuhpA exhibited an increase in virulence compared to that of E. piscicida EIB202 [LD₅₀ value: (3.98 × 10⁶ CFU/fish) and LD₅₀ value: (1.45 × 10⁷ CFU/fish) respectively]. Besides, although TNF-α did not show significant differences (p > 0.05) in the spleen of tilapia infected with ΔuhpA and EIB202 in the whole observed period, the gene expression of IL-1β and TGF-β in the spleen of tilapia infected with ΔuhpA showed significantly higher (p < 0.05) than that of in tilapia infected with EIB202. Meanwhile, the gene expression of IL-1β and TGF-β in spleen of tilapia infected with ΔuhpA showed significantly higher (p < 0.05) than that of in fish infected with EIB202 when zebrafish used as the control in the whole observed period. All these results suggested that Glu6P up-regulated the gene expression of uhpC/uhpB/uhpA; most important, the uhpA gene deletion in E. piscicida down-regulated the gene expression of uhpC and uhpB, enhanced its pathogenicity and its role in inducing the inflammatory cytokine responses in tilapia.

LuxS/AI-2 Quorum Sensing System in Edwardsiella piscicida Promotes Biofilm Formation and Pathogenicity

LuxS/AI-2 is an important quorum sensing system which affects the growth, biofilm formation, virulence, and metabolism of bacteria. LuxS is encoded by the luxS gene, but how this gene is associated with a diverse array of physiological activities in Edwardsiella piscicida (E. piscicida) is not known. Here, we constructed an luxS gene mutant strain, the △luxS strain, to identify how LuxS/AI-2 affects pathogenicity. The results showed that LuxS was not found in the luxS gene mutant strain, and this gene deletion decreased E. piscicida growth compared to that of the wild-type strain. Meanwhile, the wild-type strain significantly increased penetration and motility in mucin compared to levels with the △luxS strain. The 50% lethal dose (LD₅₀) of the E. piscicida △luxS strain for zebrafish was significantly higher than that of the wild-type strain, which suggested that the luxS gene deletion could attenuate the strain's virulence. The AI-2 activities of EIB202 were 56-fold higher than those in the △luxS strain, suggesting that the luxS gene promotes AI-2 production. Transcriptome results demonstrated that between cells infected with the △luxS strain and those infected with the wild-type strain 46 genes were significantly differentially regulated, which included 34 upregulated genes and 12 downregulated genes. Among these genes, the largest number were closely related to cell immunity and signaling systems. In addition, the biofilm formation ability of EIB202 was significantly higher than that of the △luxS strain. The supernatant of EIB202 increased the biofilm formation ability of the △luxS strain, which suggested that the luxS gene and its product LuxS enhanced biofilm formation in E. piscicida All results indicate that the LuxS/AI-2 quorum sensing system in E. piscicida promotes its pathogenicity through increasing a diverse array of physiological activities.