Nucleic acid-induced antiviral immunity in shrimp

The interactions between CpG oligodeoxynucleotides and Toll-like receptors in Pacific white shrimp Litopenaeus vannamei

CpG oligodeoxynucleotides (ODNs), as a novel type of adjuvant with immunomodulatory effects, are recognized by Toll-like receptors (TLRs) in Litopenaeus vannamei. In the present study, eleven LvTLRs-pCMV recombinants (rLvTLRs) were constructed to investigate the relationships between various CpG ODNs and different LvTLRs in human embryonic kidney 293T (HEK293T) cells, which was further confirmed by bio-layer interferometry (BLI) technique. The results of dual luciferase reporter assay showed that every LvTLR could activate multiple downstream genes, mainly including NF-κB, CREB, ISRE, IL-6-promoter, TNF-α-promoter and Myc, thereby inducing main signaling pathways in shrimps. Most CpG ODNs possessed affinities to more than one LvTLR, while each LvTLR could recognize multiple CpG ODNs, and the widely recognized ligands within CpG ODNs are A-class and B-class. Moreover, BLI analysis showed that CpG 2216, Cpg 2006, CpG 2143 and CpG 21425 exhibited dose-dependent affinity to the expressed TLR protein, which were consistent with the results in HEK293T cells. It suggested that the interactions of CpG ODNs with LvTLRs were indispensable for the immune regulation triggered by CpG ODNs, and these findings would lay foundations for studying the activations of LvTLRs to immune signaling pathways and shedding lights on the immune functions and mechanisms of CpG ODNs.

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.

SARS-CoV-2 NSP5 and N protein counteract the RIG-I signaling pathway by suppressing the formation of stress granules

As a highly pathogenic human coronavirus, SARS-CoV-2 has to counteract an intricate network of antiviral host responses to establish infection and spread. The nucleic acid-induced stress response is an essential component of antiviral defense and is closely related to antiviral innate immunity. However, whether SARS-CoV-2 regulates the stress response pathway to achieve immune evasion remains elusive. In this study, SARS-CoV-2 NSP5 and N protein were found to attenuate antiviral stress granule (avSG) formation. Moreover, NSP5 and N suppressed IFN expression induced by infection of Sendai virus or transfection of a synthetic mimic of dsRNA, poly (I:C), inhibiting TBK1 and IRF3 phosphorylation, and restraining the nuclear translocalization of IRF3. Furthermore, HEK293T cells with ectopic expression of NSP5 or N protein were less resistant to vesicular stomatitis virus infection. Mechanistically, NSP5 suppressed avSG formation and disrupted RIG-I-MAVS complex to attenuate the RIG-I-mediated antiviral immunity. In contrast to the multiple targets of NSP5, the N protein specifically targeted cofactors upstream of RIG-I. The N protein interacted with G3BP1 to prevent avSG formation and to keep the cofactors G3BP1 and PACT from activating RIG-I. Additionally, the N protein also affected the recognition of dsRNA by RIG-I. This study revealed the intimate correlation between SARS-CoV-2, the stress response, and innate antiviral immunity, shedding light on the pathogenic mechanism of COVID-19.

Comparative transcriptome analysis of the gills of Procambarus clarkii provide novel insights into the response mechanism of ammonia stress tolerance

Procambarus clarkii is an important model crustacean organism in many researches. Ammonia nitrogen is one of common contaminants in aquatic environment, influencing the health of aquatic organisms. The primary objective of this study was to investigate molecular mechanisms on ammonia stress in gills of P. clarkii to provide new insights into the strategies of aquatic animals in responding to high concentration of ammonia in the environment. Procambarus clarkii were randomly assigned into two groups (ammonia stress group, AG; control group, CG), and gill samples were dependently excised from AG and CG. Then response mechanisms on ammonia stress were investigated based on transcriptome data of P. clarkii. 9237 differentially expressed genes were identified in ammonia stress group. The genes of ion transport enzymes (NKA and SLC6A5S) were significantly up-regulated. Whereas the immune-related genes (e.g. MAP3K7, HSP70, HSP90A, CTSF, CTSL1, CHI and CTL4) and pathways were significantly up-regulated, which played an important role in reacting to ammonia stress. Procambarus clarkii may enhance immune defense to counteract ammonia toxicity by the up-regulation of immune-related genes and signaling pathways. The activities of ion transport enzymes are changed to mobilise signal transduction and ion channel regulation for adapting to ammonia environment. These previous key genes play an important role in resistance to ammonia stress to better prepare for survival in high concentration of ammonia.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) membrane (M) protein inhibits type I and III interferon production by targeting RIG-I/MDA-5 signaling

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has quickly spread worldwide and has affected more than 10 million individuals. A typical feature of COVID-19 is the suppression of type I and III interferon (IFN)-mediated antiviral immunity. However, the molecular mechanism by which SARS-CoV-2 evades antiviral immunity remains elusive. Here, we reported that the SARS-CoV-2 membrane (M) protein inhibits the production of type I and III IFNs induced by the cytosolic dsRNA-sensing pathway mediated by RIG-I/MDA-5-MAVS signaling. In addition, the SARS-CoV-2 M protein suppresses type I and III IFN induction stimulated by SeV infection or poly (I:C) transfection. Mechanistically, the SARS-CoV-2 M protein interacts with RIG-I, MAVS, and TBK1, thus preventing the formation of the multiprotein complex containing RIG-I, MAVS, TRAF3, and TBK1 and subsequently impeding the phosphorylation, nuclear translocation, and activation of IRF3. Consequently, ectopic expression of the SARS-CoV-2 M protein facilitates the replication of vesicular stomatitis virus. Taken together, these results indicate that the SARS-CoV-2 M protein antagonizes type I and III IFN production by targeting RIG-I/MDA-5 signaling, which subsequently attenuates antiviral immunity and enhances viral replication. This study provides insight into the interpretation of SARS-CoV-2-induced antiviral immune suppression and illuminates the pathogenic mechanism of COVID-19.

The NF-κB family member dorsal plays a role in immune response against Gram-positive bacterial infection in mud crab (Scylla paramamosain)

The NF-κB family is a set of evolutionarily conserved transcription factors that play central roles in various biological events. Dorsal is an invertebrate NF-κB family member that is essential for the regulation of immune responses. In the current study, the Dorsal gene from Scylla paramamosain (SpDorsal) was identified, which showed high homology to other crustacean Dorsal proteins. Expression of SpDorsal was highest in hemocytes and could be significantly changed after immune stimulations. In expression vector-transfected S2 cells, SpDorsal was mainly localized in the cytoplasm and could be efficiently translocated into the nucleus upon immune stimulations with the Gram-positive bacteria Staphylococcus aureus and poly (I:C), but not the Gram-negative bacteria Vibrio parahaemolyticus. As a transcription factor, SpDorsal could activate the promoter of S. paramamosain Hyastatin (SpHyastatin) in vitro, while S. paramamosain Cactus (SpCactus), a homolog of IκB, could interact with SpDorsal to prevent its nuclear translocation and inhibit its transcription factor activity. Silencing of SpDorsal in vivo using RNAi strategy significantly increased the mortality of crabs infected with S. aureus but not that with V. parahaemolyticus. These indicated that the SpDorsal signaling pathway could be mainly implicated in immune responses against Gram-positive bacterial infection in S. paramamosain.

Identification and Molecular Characterization of a Pellino Protein in Kuruma Prawn (Marsupenaeus Japonicus) in Response to White Spot Syndrome Virus and Vibrio Parahaemolyticus Infection

Kuruma prawn, Marsupenaeus japonicus, has the third largest annual yield among shrimp species with vital economic significance in China. White spot syndrome virus (WSSV) is a great threat to the global shrimp farming industry and results in high mortality. Pellino, a highly conserved E3 ubiquitin ligase, has been found to be an important modulator of the Toll-like receptor (TLR) signaling pathways that participate in the innate immune response and ubiquitination. In the present study, the Pellino gene from Marsupenaeus japonicus was identified. A qRT-PCR assay showed the presence of MjPellino in all the tested tissues and revealed that the transcript level of this gene was significantly upregulated in both the gills and hemocytes after challenge with WSSV and Vibrio parahaemolyticus. The function of MjPellino was further verified at the protein level. The results of the three-dimensional modeling and protein-protein docking analyses and a GST pull-down assay revealed that the MjPellino protein was able to bind to the WSSV envelope protein VP26. In addition, the knockdown of MjPellino in vivo significantly decreased the expression of MjAMPs. These results suggest that MjPellino might play an important role in the immune response of kuruma prawn.

A signal transducers and activators of transcription (STAT) gene from Scylla paramamosain is involved in resistance against mud crab reovirus

A STAT gene from Scylla paramamosain, named SpSTAT, was cloned and characterized. The full length of SpSTAT mRNA contains a 5'untranslated region (UTR) of 238 bp, an open reading frame (ORF) of 2388 bp and a 3' UTR of 326 bp. The SpSTAT protein contains four characteristic STAT domains and showed 84% identity (90% similarity) and 44% identity (64% similarity) to Litopenaeus vannamei STAT protein and Human STAT5a/b protein, respectively. The mRNA of SpSTAT was high expressed in the intestine and eyestalk and low expressed in the heart and muscle. Moreover, expression of SpSTAT was significantly responsive to challenge of mud crab reovirus (MCRV), Poly(I:C), LPS and Staphylococcus aureus. SpSTAT could be activated by Poly(I:C) and LPS to translocate to the nucleus of Drosophila Schneider 2 (S2) cells. SpSTAT could be phosphorylated by interaction with JAK of S. paramamosain (SpJAK) and activated to translocate to the nucleus of S2 cells. Furthermore, Silencing of SpSTAT in vivo resulted in higher mortality rate of MCRV infected mud crab and increased the viral load in tissues, suggesting that SpSTAT could play an important role in defense against MCRV in mud crab.

A novel C-type lectin with microbiostatic and immune regulatory functions from Litopenaeus vannamei

C-type lectins (CTLs) are a group of lectins with at least one carbohydrate recognition domain (CRD), the binding of which to carbohydrates requires the presence of calcium ions. CTLs generally function as pattern recognition receptors (PRRs), essentially participating in innate immunity. In the current study, a novel CTL termed LvCTL5 was identified from Pacific white shrimp Litopenaeus vannamei, which shared sequence identities with other crustacean CTLs. LvCTL5 was highly expressed in hepatopancreas and could be activated by infection with bacteria, virus and fungi. The recombinant LvCTL5 protein purified from E. coli showed microbiostatic and agglutination activities against bacteria and fungi in vitro. Silencing of LvCTL5 in vivo could significantly affect expression of a series of immune effector genes and down-regulate the phagocytic activity of hemocytes. Compared with controls, the LvCTL5-silenced shrimp were highly susceptible to Vibrio parahaemolyticus and white spot syndrome virus (WSSV) infections. These suggest that LvCTL5 has microbiostatic and immune regulatory activities and is implicated in antiviral and antibacterial responses.

A janus kinase from Scylla paramamosain activates JAK/STAT signaling pathway to restrain mud crab reovirus

JAK/STAT signaling pathways are associated with the innate immune system and play important roles in mediating immune responses to virus infection. In this study, a Janus kinase gene from Scylla paramamosain (SpJAK) was cloned and characterized. The full length of SpJAK mRNA contains a 5' untranslated region (UTR) of 304 bp, an open reading frame of 3300 bp and a 3' UTR of 302 bp. The SpJAK protein contains seven characteristic JAK homology domains (JH1 to JH7) and showed 60% identity (78% similarity), 20% identity (35% similarity), and 21% identity (37% similarity) to the Litopenaeus vannamei JAK (LvJAK) protein, the Drosophila melanogaster hopscotch protein, and the Homo sapiens JAK2 protein, respectively. The mRNA of SpJAK showed high expression in the brain and nerve but low expression in the hemocyte and muscle. Moreover, the expression of SpJAK was significantly upregulated by stimulation with mud crab reovirus (MCRV), poly(I:C), and Vibrio parahaemolyticus. SpJAK significantly activated the STAT of S. paramamosain (SpSTAT) to translocate to the nucleus of Drosophila Schneider 2 cells. SpJAK significantly enhanced the activity of the promoter of the WSSV wsv069 gene that was activated significantly by SpSTAT by acting on the STAT-binding DNA motif. These results suggest that SpJAK activates the JAK/STAT pathway. Furthermore, silencing SpJAK in vivo resulted in the high mortality rate of MCRV-infected mud crabs and increased the viral load in tissues. Hence, SpJAK could play an important role in defense against MCRV in mud crab.

WSSV-host interaction: Host response and immune evasion

As invertebrates, shrimps rely on multiple innate defense reactions, including humoral immunity and cellular immunity to recognize and eliminate various invaders, such as viruses. White spot syndrome virus (WSSV) causes the most prevalent and devastating viral disease in penaeid shrimps, which are the most widely cultured species in the coastal waters worldwide. In the last couple of decades, studies about WSSV implicate a dual role of the immune system in protecting shrimps against the infection; these studies also explore on the pathogenesis of WSSV infection. Herein, we review our current knowledge of the innate immune responses of shrimps to WSSV, as well as the molecular mechanisms used by this virus to evade host immune responses or actively subvert them for its own benefit. Deciphering the interactions between WSSV and the shrimp host is paramount to understanding the mechanisms that regulate the balance between immune-mediated protection and pathogenesis during viral infection and to the development of a safe and effective WSSV defensive strategy.

Wntless, a conserved Wnt-transport protein, is involved in the innate immune response of Macrobrachium rosenbergii

Wnt signaling plays important roles in a variety of developmental and pathological processes. Here we show that Wntless, the main regulator for Wnt secretion, is involved in the innate immune response of the giant freshwater prawn, Macrobrachium rosenbergii. The full-length cDNA of the prawn Wntless (named MrWntless) is 2173 bp in length and contains a 1602-bp open reading frame (ORF), which is conceptually translated into a 533-amino acids sequence. MrWntless protein contains a highly conserved Wnt-binding domain which is required for secretion of Wnt ligands, and exhibits 57-67% identity with known Wntless proteins of other animals. MrWntless was found to be expressed in a variety of prawn tissues including heart, gill, muscle, gut, hepatopancreas and ovary. Moreover, MrWntless expression was significantly increased in the hepatopancreas and gill of the prawns challenged by the bacterial pathogen Aeromonas hydrophila and Vibrio parahaemolyticus. Knockdown of MrWntless by RNA interference in prawns led to dramatically decreased MrWntless expression of approximately 70%. Furthermore, the cumulative mortality rate of the prawn injected with MrWntless dsRNA was greatly increased in response to A. hydrophila challenge compared with the control prawns. Taken together, we provide evidence that prawn Wntless is important for their innate immune response against bacterial pathogens.

Identification of SNPs potentially related to immune responses and growth performance in Litopenaeus vannamei by RNA-seq analyses

Litopenaeus vannamei is one of the most important shrimp species for worldwide aquaculture. Despite this, little genomic information is available for this penaeid and other closely related taxonomic crustaceans. Consequently, genes, proteins and their respective polymorphisms are poorly known for these species. In this work, we used the RNA sequencing technology (RNA-seq) in L. vannamei shrimp evaluated for growth performance, and exposed to the White Spot Syndrome Virus (WSSV), in order to investigate the presence of Single Nucleotide Polymorphisms (SNPs) within genes related to innate immunity and growth, both features of great interest for aquaculture activity. We analyzed individuals with higher and lower growth rates; and infected (unhealthy) and non-infected (healthy), after exposure to WSSV. Approximately 7,000 SNPs were detected in the samples evaluated for growth, being 3,186 and 3,978 exclusive for individuals with higher and lower growth rates, respectively. In the animals exposed to WSSV we found about 16,300 unique SNPs, in which 9,338 were specific to non-infected shrimp, and 7,008 were exclusive to individuals infected with WSSV and symptomatic. In total, we describe 4,312 unigenes containing SNPs. About 60% of these unigenes returned GO blastX hits for Biological Process, Molecular Function and Cellular Component ontologies. We identified 512 KEGG unique KOs distributed among 275 pathways, elucidating the majority of metabolism roles related to high protein metabolism, growth and immunity. These polymorphisms are all located in coding regions, and certainly can be applied in further studies involving phenotype expression of complex traits, such as growth and immunity. Overall, the set of variants raised herein enriches the genomic databases available for shrimp, given that SNPs originated from nextgen are still rare for this relevant crustacean group, despite their huge potential of use in genomic selection approaches.

Transcriptome analysis of 'Huanghai No. 2' Fenneropenaeus chinensis response to WSSV using RNA-seq

White spot syndrome (WSS) is one of the most damaging phenomena in the culturing of shrimp. To characterize the mechanisms of the molecular responses to WSSV infection in 'Huanghai No. 2'' Fenneropenaeus chinensis, we used next-generation sequencing to observe the transcriptome after oral infection. A total of 108.6 million clean reads were obtained and assembled into 64,103 final unigenes with an average length of 845 bp (N50 = 1534 bp). The assembled unigenes contained 14,263 significant unigenes after BLASTX against the Nr database (E-value cut-off of 10^-5). After comparison of digital gene expression data between challenged and control shrimp, a total of 896 DEGs after WSSV infection were identified. Gene pathway analysis indicated that 92, 131 and 142 metabolic pathways were affected at early, peak and late phases respectively. Some pathways were related to the immune response, such as the phagosome, complement and coagulation cascades, the antigen processing and presentation pathway and so on. Many immune-related genes were also identified after pathway analysis. Interestingly, some growth-related genes, such as cathepsin L, myosin regulatory light chain 2 smooth muscle, and alpha-amylase were also differentially expressed after WSSV infection, and the correlation between growth trait and WSSV-resistance trait need further research. The expression patterns of eight DEGs were confirmed by quantitative real-time reverse transcription polymerase chain reaction, and there was good agreement between RNA-seq and qRT-PCR. These data will provide valuable information for characterizing the immune mechanism of the response of shrimp's to WSSV.

A cytosolic sensor, PmDDX41, mediates antiviral immune response in black tiger shrimp Penaeus monodon

DEAD (Asp-Glu-Ala-Asp)-box polypeptide 41 (DDX41), a receptor belonging to the DExD family, has recently been identified as an intracellular DNA sensor in vertebrates. Here, we report on the identification and functional characterization of PmDDX41, the first cytosolic DNA sensor in shrimp. By searching a Penaeus monodon expressed sequence tag (EST) database (http://pmonodon.biotec.or.th), three cDNA fragments exhibiting similarity to DDX41 in various species were identified and assembled, resulting in a complete open reading frame of PmDDX41 that contains 1863-bp and encodes a putative protein of 620 amino acids. PmDDX41 shares 83% and 79% similarity to DDX41 homolog from the bee Apis florea and fruit fly Drosophila melanogaster, respectively and contains three conserved domains in the protein: DEADc domain, HELICc domain, and zinc finger domain. The transcript of PmDDX41 was detected in all tested tissues and was up-regulated upon infection with a DNA virus, white spot syndrome virus (WSSV). However, PmDDX41 mRNA expression was not significantly changed and down-regulated in response to a bacterium, Vibrio harveyi, or an RNA virus, yellow head virus (YHV), respectively, compared with the control phosphate-buffered saline-injected shrimp. Furthermore, the suppression of PmDDX41 by dsRNA-mediated gene silencing resulted in more rapid death of WSSV-infected shrimp and a significant decrease in the mRNA expression levels of several immune-related genes (PmIKKβ, PmIKKɛ, PmRelish, PmCactus, PmDorsal, PmPEN3, PmPEN5, and ALFPm6). These results suggest that PmDDX41 is involved in the antiviral response, probably via a DNA-sensing pathway that is triggered through the IκB kinase complex and leads to the activation of several immune-related genes.

Comparative proteomic analysis of hepatopancreas in Macrobrachium rosenbergii responded to Poly (I:C)

Gel-enhanced liquid chromatography coupled with tandem mass spectrometry (GeLC-MS/MS) was used to analyze the proteome of Macrobrachium rosenbergii hepatopancreas responded to Poly (I:C). GeLC-MS/MS analysis identified 515 differentially-expressed proteins with ≥1.5 and ≤ -0.5 log₂ fold change. Of these, 195 differentially-expressed proteins were significantly matched to known proteins in the database, of which 102 proteins were up-regulated and 93 proteins were down-regulated. These proteins were classified into 21 categories, i.e. metabolic process, oxidative stress response, signaling, transcription, translation, cell cycle, transport, etc. Several immune factors were up-regulated upon Poly (I:C) injection. Protein-protein interaction network analysis of these immune factors identified three major protein clusters including RNAi, stress responses, and Toll pathway-proPO system, implying that Poly (I:C) activates immune responses in prawn through several mechanisms.

VIH from the mud crab is specifically expressed in the eyestalk and potentially regulated by transactivator of Sox9/Oct4/Oct1

Vitellogenesis-inhibiting hormone (VIH) is known to regulate ovarian maturation by suppressing the synthesis of vitellogenin (Vtg) in crustaceans, which belongs to a member of crustacean hyperglycemic hormone (CHH) family synthesized and secreted from the X-organ/sinus gland complex of eyestalks. In this study, the cDNA, genomic DNA (gDNA) and the 5'-upstream regulatory (promoter region) sequences of VIH gene were obtained by conventional PCR, genome walker and tail-PCR techniques according to our transcriptomic database of Scylla paramamosain. The full-length cDNA of SpVIH is 634bp including 105bp 5'UTR, 151bp 3'UTR and 378bp ORF that encodes a peptide of 125 amino acids. The full length gDNA of SpVIH is 790bp containing two exons and one intron. The 5'-flanking promoter regions of SpVIH we isolated are 3070bp from the translation initiation (ATG) and 2398bp from the predicted transcription initiation (A), which consists of putative core promoter region and multiple potential transcription factor binding sites. SpVIH was only expressed in eyestalk. The expression level of SpVIH in eyestalk of female crab decreased gradually along with the development of ovary. As there is not cell line of crabs available, we chose the mature transfection system HEK293FT cell lines to explore the mechanism of transcription regulation of SpVIH in crabs. Sequential deletion assays using luciferase reporter gene in HEK293FT cells revealed that the possible promoter activity regions (including positive and negative transcription factors binding sites simultaneously) presented between pSpVIH-4 and pSpVIH-6. In order to further identify the crucial transcription factors binding site in this region, the site-directed mutagenesis of Sox9/Oct4/Oct1 binding site of pSpVIH-4 was created. The results demonstrated that the transcriptional activity of pSpVIH-4△ decreased significantly (p<0.05). Thus, it is reasonable to deduce that the Sox9/Oct4/Oct1 may be the essential positive transcription factors which regulate the expression of SpVIH.

Toll-receptor 9 gene in the black tiger shrimp (Penaeus monodon) induced the activation of the TLR-NF-κB signaling pathway

Toll receptors are important pathogen recognition receptors (PRRs) in shrimps, which play a vital role in defending against virus and bacterial challenge. In this paper, the characterization and functional analysis of a Toll9 receptor gene from Penaeus monodon was performed in HEK293T cells. Data showed that PmToll9 can activate the NF-κB promoter activities of TLR pathway, while ISRE and IFN-β promoter cannot be activated obviously in HEK293T cells using dual-luciferase reporter system. The downstream immune factors of IL-8, IκB-α, and TRAF6 were activated by PmToll9 and IL-8 showed the most significant up-regulation in expression levels, indicating the activities of NF-κB can be mediated by PmToll9. Six LRRs-deletion mutants were constructed and results showed these mutants had obvious declines in luciferase activities, among which the mutant pCMV-DeLRR4 showed the most significant decline. qPCR data indicated LRRs-deletion mutants efficiently impaired the activities of the downstream immune factors IL-8, IκB-α, and TRAF6. It demonstrates that LRRs-deletion mutants could result in the weaken abilities of PmToll9 in signaling transduction. Overexpression of PmToll9-GFP fusion protein in Hela cells revealed the primary cellular localization of PmToll9 is in the cytoplasm.

Cloning, identification and functional analysis of a β-catenin homologue from Pacific white shrimp, Litopenaeus vannamei

Wnt signaling is known to control multiple of cellular processes such as cell differentiation, communication, apoptosis and proliferation, and is also reported to play a role during microbial infection. β-catenin is a key regulator of the Wnt signaling cascade. In the present study, we cloned and identified a β-catenin homologue from Litopenaeus vannamei termed Lvβ-catenin. The full-length of Lvβ-catenin transcript was 2797 bp in length within a 2451 bp open reading frame (ORF) that encoded a protein of 816 amino acids. Lvβ-catenin protein was comprised of several characteristic domains such as an N-terminal region of GSK-β consensus phosphorylation site and Coed coil section, a central region of 12 continuous Armadillo/β-Catenin-like repeat (ARM) domains and a C-terminal region. Real-time PCR showed Lvβ-catenin expression was responsive to Vibrio parahaemolyticus and white spot syndrome virus (WSSV) infection. Dual-reporter analysis showed that over-expression of Lvβ-catenin could induce activation of the promoter activities of several antimicrobial peptides (AMPs) such as shrimp PEN4, suggesting that Lvβ-catenin could play a role in regulating the production of AMPs. Knockdown of Lvβ-catenin enhanced the sensitivity of shrimps to V. parahaemolyticus and WSSV challenge, suggesting Lvβ-catenin could play a positive role against bacterial and viral pathogens. In summary, the results presented in this study provided some insights into the function of Wnt/β-catenin of shrimp in regulating AMPs and the host defense against invading pathogens.

Molecular cloning, inducible expression and antibacterial analysis of a novel i-type lysozyme (lyz-i2) in Pacific white shrimp, Litopenaeus vannamei

The full-length cDNA coding for a novel invertebrate (i-type) lysozyme was identified in Pacific white shrimp (Litopenaeus vannamei). The newly obtained L. vannamei lysozyme is similar to the Penaeus monodon i-type lysozyme 2, but it is distant from the known L. vannamei c-type lysozyme and i-type lysozyme 1 in protein sequence; therefore, it was defined as L. vannamei i-type lysozyme 2 (lyz-i2). Expression of L. vannamei lyz-i2 transcripts were ubiquitously detected in all tissues we selected, with the highest abundance observed in the hemolymph. Challenge with Vibrio harveyi might elicit L. vannamei lyz-i2 mRNA expression in the hepatopancreas, intestine, muscle, gill and hemolymph. In the themolymph, specifically, the stimulatory effects of Vibrio and lipopolysaccharide (LPS) on lyz-i2 transcript levels were durable and transient, respectively; while Polyinosinic:polycytidylic acid [Poly (I:C)] treatment did not affect lyz-i2 expression. L. vannamei lyz-i2 recombinant protein was generated in an Escherichia coli system. By lysoplate and turbidimetric assays, the L. vannamei lyz-i2 recombinant protein showed a broad spectrum of antimicrobial properties with high activities against Micrococcaceae lysodeikticus and various Vibrio species and relatively low activity against E. coli. In conclusion, L. vannamei lyz-i2 might be a potent antibacterial protein with a role in innate immunity in Penaeid shrimp.

The nucleic acid-binding protein PcCNBP is transcriptionally regulated during the immune response in red swamp crayfish Procambarus clarkii

Gene family encoding cellular nucleic acid binding proteins (CNBP) is well conserved among vertebrates; however, there is limited knowledge in lower organisms. In this study, a CNBP homolog from the red swamp crayfish Procambarus clarkii was characterised. The full-length cDNA of PcCNBP was of 1257 bp with a 5'-untranslated region (UTR) of 63 bp and a 3'-UTR of 331 bp with a poly (A) tail, and an open-reading frame (ORF) of 864 bp encoding a polypeptide of 287 amino acids with the predicted molecular weight of about 33 kDa. The predicted protein possesses 7 tandem repeats of 14 amino acids containing the CCHC zinc finger consensus sequence, two RGG-rich single-stranded RNA-binding domain and a nuclear localization signal, strongly suggesting that PcCNBP was a homolog of vertebrate CNBP. The PcCNBP transcript was constitutively expressed in all tested tissues of unchallenged crayfish, including hepatopancreas, gill, eyestalk, haemocytes, intestine, stomach and cuticle with highest expression in haemocytes, intestine, gills and hepatopancreas. The mRNA expression of PcCNBP in haemocytes was modulated at transcriptional level by different immune challenges, suggesting its involvement in the immune response of P. clarkii during both bacteria and viruses infection.

Characterization of Amphioxus IFN Regulatory Factor Family Reveals an Archaic Signaling Framework for Innate Immune Response

The IFN regulatory factor (IRF) family encodes transcription factors that play important roles in immune defense, stress response, reproduction, development, and carcinogenesis. Although the origin of the IRF family has been dated back to multicellular organisms, invertebrate IRFs differ from vertebrate IRFs in genomic structure and gene synteny, and little is known about their functions. Through comparison of multiple amphioxus genomes, in this study we suggested that amphioxus contains nine IRF members, whose orthologs are supposed to be shared among three amphioxus species. As the orthologs to the vertebrate IRF1 and IRF4 subgroups, Branchiostoma belcheri tsingtauense (bbt)IRF1 and bbtIRF8 bind the IFN-stimulated response element (ISRE) and were upregulated when amphioxus intestinal cells were stimulated with poly(I:C). As amphioxus-specific IRFs, both bbtIRF3 and bbtIRF7 bind ISRE. When activated, they can be phosphorylated by bbtTBK1 and then translocate into nucleus for target gene transcription. As transcriptional repressors, bbtIRF2 and bbtIRF4 can inhibit the transcriptional activities of bbtIRF1, 3, 7, and 8 by competing for the binding of ISRE. Interestingly, amphioxus IRF2, IRF8, and Rel were identified as target genes of bbtIRF1, bbtIRF7, and bbtIRF3, respectively, suggesting a dynamic feedback regulation among amphioxus IRF and NF-κB. Collectively, to our knowledge we present for the first time an archaic IRF signaling framework in a basal chordate, shedding new insights into the origin and evolution of vertebrate IFN-based antiviral networks.

Activation of Vago by interferon regulatory factor (IRF) suggests an interferon system-like antiviral mechanism in shrimp

There is a debate on whether invertebrates possess an antiviral immunity similar to the interferon (IFN) system of vertebrates. The Vago gene from arthropods encodes a viral-activated secreted peptide that restricts virus infection through activating the JAK-STAT pathway and is considered to be a cytokine functionally similar to IFN. In this study, the first crustacean IFN regulatory factor (IRF)-like gene was identified in Pacific white shrimp, Litopenaeus vannamei. The L. vannamei IRF showed similar protein nature to mammalian IRFs and could be activated during virus infection. As a transcriptional regulatory factor, L. vannamei IRF could activate the IFN-stimulated response element (ISRE)-containing promoter to regulate the expression of mammalian type I IFNs and initiate an antiviral state in mammalian cells. More importantly, IRF could bind the 5′-untranslated region of L. vannamei Vago4 gene and activate its transcription, suggesting that shrimp Vago may be induced in a similar manner to that of IFNs and supporting the opinion that Vago might function as an IFN-like molecule in invertebrates. These suggested that shrimp might possess an IRF-Vago-JAK/STAT regulatory axis, which is similar to the IRF-IFN-JAK/STAT axis of vertebrates, indicating that invertebrates might possess an IFN system-like antiviral mechanism.

The c-Fos and c-Jun from Litopenaeus vannamei play opposite roles in Vibrio parahaemolyticus and white spot syndrome virus infection

Growing evidence indicates that activator protein-1 (AP-1) plays a major role in stimulating the transcription of immune effector molecules in cellular response to an incredible array of stimuli, including growth factors, cytokines, cellular stresses and bacterial and viral infection. Here, we reported the isolation and characterization of a cDNA from Litopenaeus vannamei encoding the full-length c-Fos protein (named as Lvc-Fos). The predicted amino acid sequences of Lvc-Fos contained a basic-leucine zipper (bZIP) domain, which was characteristic of members of the AP-1 family. Immunoprecipitation and native-PAGE assays determined that Lvc-Fos could interact with the Lvc-Jun, a homolog of c-Jun family in L. vannamei, in a heterodimer manner. Further investigation demonstrated that Lvc-Fos and Lvc-Jun were expressed in all tested tissues and located in the nucleus. Real-time RT-PCR analysis showed both Lvc-Fos and Lvc-Jun in gills were up-regulated during Vibrio parahaemolyticus and white spot syndrome virus (WSSV) challenges. In addition, reporter gene assays indicated Lvc-Fos and Lvc-Jun could activate the expression of antimicrobial peptides (AMPs) of Drosophila and shrimp, as well as WSSV immediate early (IE) genes wsv069 and wsv249, in a different manner. Knockdown of Lvc-Fos or Lvc-Jun by RNA interference (RNAi) resulted in higher mortalities of L. vannamei after infection with V. parahaemolyticus, suggesting that Lvc-Fos and Lvc-Jun might play protective roles in bacterial infection. However, silencing of Lvc-Fos or Lvc-Jun in shrimp caused lower mortalities and virus loads under WSSV infection, suggesting that Lvc-Fos and Lvc-Jun could be engaged for WSSV replication and pathogenesis. In conclusion, our results provided experimental evidence and novel insight into the roles of L. vannamei AP-1 in bacterial and viral infection.

Identification and functional analysis of a novel IFN-like protein (CgIFNLP) in Crassostrea gigas

Interferons (IFNs) belong to class II helical cytokines family with pleiotropic biological activities, which have been demonstrated to play crucial roles in innate and adaptive immunity in vertebrates. In the present study, a novel IFN-like protein (designed CgIFNLP) was identified from oyster Crassostrea gigas, which contained an interferon domain from 14 to 97 amino acids showing low sequence similarities with vertebrates IFNs, but shared a similar three-dimensional structure with class II helical cytokines. The mRNA transcripts of CgIFNLP was detected in all the tested tissues including gonad, adductor muscle, hemocytes, mantle, gills, and hepatopancreas, with the highest expression level in gills (39-fold, P < 0.05). Moreover, the expression level of CgIFNLP mRNA in hemocytes increased significantly at 12 h (8.35-fold, P < 0.01) and 24 h (4.95-fold, P < 0.01) after poly (I: C) stimulation. After the treatments by recombinant CgIFNLP protein (rCgIFNLP) at different concentrations, the apoptosis and phagocytosis rates of oyster hemocytes increased obviously. The proliferation rate of L929 did not change obviously after incubation with rCgIFNLP for 72 h, but the proliferation rate of A549 abated significantly at 36 h and 48 h after incubation with rCgIFNLP. The results collectively suggested that the IFN-like molecule existed in oyster and it tended to present conserved functions rather than conserved amino acid sequence in comparison with vertebrate IFNs.

A Janus Kinase in the JAK/STAT signaling pathway from Litopenaeus vannamei is involved in antiviral immune response

The JAK/STAT signaling pathways are conserved in evolution and mediate diversity immune responses to virus infection. In the present study, a Janus kinase (designated as LvJAK) gene was cloned and characterized from Litopenaeus vannamei. LvJAK contained the characteristic JAK homology domain (JH domain) from JH1 to JH7 and showed 19% identity (34% similarity) and 21% identity (35% similarity) to Drosophila Hopscotch protein and Human JAK2 protein, respectively. The mRNA of LvJAK was highly expressed in hepatopancreas of L. vannamei and its expression level was prominently upregulated after the stimulation of Poly (I:C) and white spot syndrome virus (WSSV) challenges. There were 10 putative STAT binding motifs in the promoter region of LvJAK, and it could be regulated by LvJAK self or (and) LvSTAT, suggesting that LvJAK is the JAK/STAT pathway target gene and could function as a positive regulator to form a positive feedback loop. In addition, the silencing of LvJAK caused higher mortality rate and virus load, suggesting that LvJAK could play an important role in defense against WSSV. This is the first report about the complete set of JAK/STAT proteins in shrimp and the results provide the evidence of the positive feedback loop mediated by JAK protein present in the JAK/STAT pathway in invertebrates.

A C-type lectin (LvCTL4) from Litopenaeus vannamei is a downstream molecule of the NF-κB signaling pathway and participates in antibacterial immune response

C-type lectins (CTLs) play multiple roles in innate immune defense against invading pathogens in both vertebrates and invertebrates. In this study, a new C-type lectin gene from pacific white shrimp Litopenaeus vannamei (designated as LvCTL4) was cloned by rapid amplification of the cDNA ends (RACE) method. The full-length cDNA of LvCTL4 was 563 bp with open reading frame (ORF) of 471 bp encoding a polypeptide of 156 amino acids, including a putative signal sequence and a single C-type lectin-like domain (CTLD). The CTLD of 137 amino acid residues contained a mutated 'EPA' (Glu(121)-Pro(122)-Ala(123)) motif in the calcium-binding site 2 and three conserved disulfide bonds involved in structure maintenance. Tissue expression analysis showed LvCTL4 was ubiquitously distributed with high levels in gill, intestine, epithelium and hepatopancreas. The expression of LvCTL4 in gill was up-regulated in response to Vibrio parahaemolyticus challenge. RNAi knock-down of the LvCTL4 gene significantly increased mortality after V. parahaemolyticus infection. A 103 bp 5' flanking promoter sequence was obtained using the genome walking method and it contained a conserved NF-κB binding motif. Dual-Luciferase assay showed both LvDorsal and LvRelish could up regulate the promoter activity of LvCTL4. This is the first report that a shrimp C-type lectin can be regulated by both LvDorsal and LvRelish. These findings provided novel insights into the regulation of shrimp CTLs expression.

Nucleic acid-induced antiviral immunity in invertebrates: an evolutionary perspective

Nucleic acids derived from viral pathogens are typical pathogen associated molecular patterns (PAMPs). In mammals, the recognition of viral nucleic acids by pattern recognition receptors (PRRs), which include Toll-like receptors (TLRs) and retinoic acid-inducible gene (RIG)-I-like receptors (RLRs), induces the release of inflammatory cytokines and type I interferons (IFNs) through the activation of nuclear factor κB (NF-κB) and interferon regulatory factor (IRF) 3/7 pathways, triggering the host antiviral state. However, whether nucleic acids can induce similar antiviral immunity in invertebrates remains ambiguous. Several studies have reported that nucleic acid mimics, especially dsRNA mimic poly(I:C), can strongly induce non-specific antiviral immune responses in insects, shrimp, and oyster. This behavior shows multiple similarities to the hallmarks of mammalian IFN responses. In this review, we highlight the current understanding of nucleic acid-induced antiviral immunity in invertebrates. We also discuss the potential recognition and regulatory mechanisms that confer non-specific antiviral immunity on invertebrate hosts.

Antiviral defense in shrimp: from innate immunity to viral infection

The culture of penaeid shrimp is rapidly developing as a major business endeavor worldwide. However, viral diseases have caused huge economic loss in penaeid shrimp culture industries. Knowledge of shrimp innate immunity and antiviral responses has made important progress in recent years, allowing the design of better strategies for the prevention and control of shrimp diseases. In this study, we have updated information on shrimp antiviral immunity and interactions between shrimp hosts and viral pathogens. Current knowledge and recent progress in immune signaling pathways (e.g., Toll/IMD-NF-κB and JAK-STAT signaling pathways), RNAi, phagocytosis, and apoptosis in shrimp antiviral immunity are discussed. The mechanism of viral infection in shrimp hosts and the interactions between viruses and shrimp innate immune systems are also analyzed.

Pellino protein from pacific white shrimp Litopenaeus vannamei positively regulates NF-κB activation

Pellino, named after its property that binds Pelle (the Drosophila melanogaster homolog of IRAK1), is a highly conserved E3 class ubiquitin ligase in both vertebrates and invertebrates. Pellino interacts with phosphorylated IRAK1, causing polyubiquitination of IRAK1, and plays a critical upstream role in the toll-like receptor (TLR) pathway. In this study, we firstly cloned and identified a crustacean Pellino from pacific white shrimp Litopenaeus vannamei (LvPellino). LvPellino contains a putative N-terminal forkhead-associated (FHA) domain and a C-terminal ring finger (RING) domain with a potential E3 ubiquitin-protein ligase activity, and shows a high similarity with D. melanogaster Pellino. LvPellino could interact with L. vannamei Pelle (LvPelle) and over-expression of LvPellino could increase the activity of LvDorsal (a L. vannamei homolog of NF-κB) on promoters containing NF-κB binding motifs and enhance the expression of arthropod antimicrobial peptides (AMPs). The LvPellino protein was located in the cytoplasm and nucleus and LvPellino mRNA was detected in all the tissues examined and could be up-regulated after lipopolysaccharides, white spot syndrome virus (WSSV), Vibrio parahaemolyticus, and Staphylococcus aureus challenges, suggesting a stimulation response of LvPellino to bacterial and immune stimulant challenges. Knockdown of LvPellino in vivo could significantly decrease the expression of AMPs and increase the mortality of shrimps caused by V. parahaemolyticus challenge. However, suppression of the LvPellino expression could not change the mortality caused by WSSV infection, and dual-luciferase reporter assays demonstrated that over-expression of LvPellino could enhance the promoters of WSSV genes wsv069 (ie1), wsv303, and wsv371, indicating a complex role of LvPellino in WSSV pathogenesis and shrimp antiviral mechanisms.