A novel JNK from Litopenaeus vannamei involved in white spot syndrome virus infection

MicroRNA sequencing analysis reveals immune responses in hepatopancreas of Fenneropenaeus penicillatus under white spot syndrome virus infection

White Spot Disease is one of the most harmful diseases of the red tail shrimp, which can cause devastating economic losses due to the highest mortality up to 100% within a few days. MicroRNAs (miRNAs) are large class of small noncoding RNAs with the ability to post-transcriptionally repress the translation of target mRNAs. MiRNAs are considered to have a significant role in the innate immune response of crustaceans, particularly in relation to antiviral defense mechanisms. Numerous crustacean miRNAs have been verified to be required in host immune defense against viral infection, however, till present, the miRNAs functions of F. penicillatus defense WSSV infection have not been studied yet. Here in this study, for the first time, miRNAs involved in the F. penicillatus immune defense against WSSV infection were identified using high-throughput sequencing platform. A total of 432 miRNAs were obtained including 402 conserved miRNAs and 30 novel predicted miRNAs. Comparative analysis between the WSSV-challenged group and the control group revealed differential expression of 159 microRNAs in response to WSSV infection. Among these, 48 were up-regulated and 111 were down-regulated. Ten candidate MicroRNAs associated with immune activities were randomly selected for qRT-PCR analysis, which confirming the expression profiling observed in the MicroRNA sequencing data. As a result, most differentially expressed miRNAs were down-regulated lead to increase the expression of various target genes that mediated immune reaction defense WSSV infection, including genes related to signal transduction, Complement and coagulation cascade, Phagocytosis, and Apoptosis. Furthermore, the genes expression of the key members in Toll and Imd signaling pathways and apoptotic signaling were mediated by microRNAs to activate host immune responses including apoptosis against WSSV infection. These results will help to understand molecular defense mechanism against WSSV infection in F. penicillatus and to develop an effective WSSV defensive strategy in shrimp farming.

c-Jun N-terminal kinase activation contributes to improving low temperature tolerance via regulating apoptosis in the Pacific white shrimp Penaeus vannamei

Temperature is an essential environmental factor for the survival of aquatic animals. Low temperature stress can induce mitochondria to produce excessive ROS and free radicals, and destroy homeostasis. c-Jun N-terminal kinase (JNK) is involved in regulating various physiological processes, including inflammatory responses, cell cycle, reproduction, and apoptosis. Here, we investigated the mechanism of ROS/JNK pathway under low temperature stress both in vitro and in vivo. In this study, transcriptome analysis revealed that apoptosis, autophagy, calcium channel, and antioxidant were involved in the mediation of low temperature tolerance in Pacific white shrimp (penaeus vannamei). PvJNK was activated in response to low temperature stress. Treatments with different temperature caused oxidative stress as demonstrated by increased intensity of the ROS indicator H2DCF-DA, and induced apoptosis as confirmed by indicator FITC. Pretreatment with N-acetylcysteine, an ROS scavenger, attenuated low temperature induced apoptosis, and inhibited the expression of PvJNK. In addition, we demonstrate that mediator PvJNK translocated to nuclear through interacting with PvRheb. By using flow cytometry, inhibiting PvJNK can increase the expression of apoptosis related genes, accelerate tissue damage, and induce ROS and cell apoptosis. The ultimate inhibition of PvJNK accelerates the mortality of shrimp under low temperature stress. Overall, these findings suggest that during low temperature stress, PvJNK was activated by ROS to regulates apoptosis via interacting with PvRheb to promote PvJNK into the nucleus and to improve low temperature tolerance of shrimp.

Single von Willebrand factor C-domain protein confers host defense against white spot syndrome virus by functioning as a pattern recognition receptor in Macrobrachium nipponense

The single von Willebrand factor C-domain proteins (SVWCs), also known as Vago, are primarily found in arthropods. Their expression was induced by nutritional status, bacterial and viral infections. Despite the prominence of SVWCs in antiviral immunity, the detailed molecular mechanisms remain poorly explained. SVWC has been proposed to elicit antiviral activities through its function as an interferon analog. In contrast, herein, we illustrate that an SVWC homolog from Macrobrachium nipponense (MnSVWC) confers host defense against white spot syndrome virus (WSSV) and covert mortality nodavirus (CMNV) as a pattern recognition receptor (PRR). qRT-PCR analyses demonstrated that the expression of MnSVWC was enhanced upon WSSV infection in all detected tissues, including gills, nerve cords, and hemocytes. Coating WSSV with recombinant MnSVWC (rMnSVWC) promoted the phagocytic activity of hemocytes and subsequent clearance of invasive WSSV from the prawn. On the other hand, the knockdown of MnSVWC with RNAi improved the proliferation ability of WSSV and CMNV in the prawn. Analysis of ELISA and Co-immunoprecipitation (Co-IP) showed that rMnSVWC could bind WSSV by interacting with the vesicle proteins VP26 and VP28. Co-IP analysis verified the interaction between MnSVWC and calmodulin, which implies a vesicle protein-SVWC-calmodulin-clathrin-dependent mechanism underlying the hemocyte-mediated phagocytosis against WSSV. Subsequently, MnSVWC was recognized to activate the expression of transcription factor STAT and an interferon-stimulating gene Viperin, illustrating its involvement in modulating humoral immunity via activation of the JAK/STAT pathway after WSSV infection. These findings indicate that MnSVWC could bind to WSSV as a PRR and participate in the promotion of hemocyte-mediated phagocytosis and the activation of the JAK/STAT pathway in prawns.

Identification of a Yorkie homolog from Litopenaeus vannamei as a negative regulator in anti-WSSV immune response

Hippo signaling pathway is a serine threonine kinase cascade that is evolutionary conserved with well-established roles in organ size control, development, tumorigenesis and immunity. As its core molecule, Yorkie also plays an important role against pathogen. In this study, we cloned and characterized a Yorkie homolog from Litopenaeus vannamei, designed as LvYKI, which has a 1650 bp open reading frame. It has the characterized domains of Yokie family, and displayed to be close to the insects and crustacean. Quantitative Real-time PCR showed that LvYKI had different regulatory mechanisms in different tissues. The transcriptional level of Lvyki was down-regulated in gill, while up-regulated in hepatopancreas post white spot syndrome virus (WSSV) infection. Moreover, the expression and phosphorylation of LvYKI was reduced upon WSSV infection, which indicated that LvYKI was involved in WSSV infection. Furthermore, RNAi was performed to evaluate the role of LvYKI in shrimp immune responses. Knocking down of Lvyki resulted in inhibition of the transcription of WSSV gene ie1 and vp28, and delayed mortality of shrimp post WSSV infection. Meanwhile, the apoptosis of hemocyte was increased as well. All results suggested that shrimp can promote apoptosis to resist WSSV infection mediated by down-regulation of LvYKI. In addition, it was found that LvYKI could interact with Lvβ-catenin, which cross-linked the Wnt and Hippo signaling pathway in innate immunity. Conclusively, our study provided clues that LvYKI plays an important role in the interaction between shrimp and virus. It will promote our understanding of the molecular mechanism in innate immunity.

Single-Cell Sequencing Yields Insights in the Evolution of Foot-and-Mouth Disease Virus Persistent Infection

Foot-and-mouth disease virus (FMDV) could cause acute infection in host cells, or they could coexist with host cells to generate persistent infection. In persistent infection, the virus could survive for a long time in the host and could be transmitted between different host cells. In the case of FMDV-persistent infection cell line, there is a remarkable significant cellular heterogeneity in the FMDV-persistent infection cell line due to differences of viral load in the individual cells within the cell line. However, the mechanisms of FMDV-persistent infection are not well understood. It is now generally accepted that multiple factors contribute to the coevolution of viruses and cells during the course of persistent infection. The outcome would influence the development of persistent FMDV infection conjointly, reaching a state of equilibrium ultimately. Therefore, in order to elucidate the mechanism of cellular heterogeneity in FMDV-persistent infection cell line, single-cell sequencing was performed on BHK-Op, and pseudotime trajectory plot was draw through cell cluster. Based on the cell clusters, we predicted the development and progression of the FMDV-persistent infection. It could be well explained by the fact that, in BHK-Op cells, there are a fraction of infected cells and a fraction of virus-exposed but uninfected bystander cells. By further comparing the transcripts in cell clusters, we found that these genes were involved in changes in ribosome biogenesis, cell cycle, and intracellular signaling including the interferon signaling pathway and mitogen-activated protein kinase (MAPK) signaling pathway. Through comprehensive cross-tabulation analysis of differential expressed genes in various cluster of cells, we identified a high association of Fos, a downstream transcription factor of the MAPK/extracellular signal–regulated kinase (ERK) signaling pathway, with viral replication during the formation of FMDV-persistent infection. Through the further study of Fos, we found that downregulation of Fos facilitates viral clearance during FMDV-persistent infection. Upregulation of c-Raf, which is the upstream of the MAPK/ERK signaling pathway, could promote FMDV replication through downregulation of Fos. Our research is the first to provide insight into the mechanism of the formation FMDV-persistent infection through single-cell sequencing using persistent infection cell line. Pseudotime trajectory analysis was the first time to apply for FMDV-persistent infection cell line. Our work highlights the detailed overview of the evolution of FMDV-persistent infection. We also analyzed the differential expressed genes in the replication or elimination of FMDV within the host. We found that the MAPK/ERK signaling pathway and its downstream transcription factor Fos play an important role in FMDV-persistent infection.

TAK1 confers antibacterial protection through mediating the activation of MAPK and NF-κB pathways in shrimp

MAPK and NF-κB pathways are important components of innate immune system in multicellular animals. In some model organisms, the MAP3-kinase TGF-beta-activated kinase 1 (TAK1) have been shown to regulate both MAPK and NF-κB pathways activation to tailor immune responses to pathogens or infections. However, this process is not fully understood in shrimp. In this study, we investigated the effect of TAK1 on MAPK and NF-κB activation in shrimp Litopenaeus vannamei following Vibrio parahaemolyticus infection. We found that shrimp TAK1 could activate c-Jun and Relish, the transcription factors of MAPK pathway and NF-κB pathway, respectively. Specifically, over-expression of shrimp TAK1 was able to strongly induce the activities of both AP-1 and NF-κB reporters. TAK1 was shown to bind several MAP2-kinases, including MKK4, MKK6 and MKK7, and induced their phosphorylations, the hallmarks for MAPK pathways activation. TAK1 knockdown in vivo also inhibited the nuclear translocation of c-Jun and Relish during V. parahaemolyticus infection. Accordingly, ectopic expression of shrimp TAK1 in Drosophila S2 cells increased the cleavage of co-expressed shrimp Relish, and induced the promoter activity of Relish targeted gene Diptericin (Dpt). Furthermore, knockdown of c-Jun and Relish enhanced the sensitivity of shrimp to V. parahaemolyticus infection. These findings indicated that shrimp TAK1 conferred antibacterial protection through regulating the activation of both MAPK pathway and NF-κB pathway, and suggested that the TAK1-MAPK/NF-κB axis could be a potential therapeutic target for enhancing antibacterial responses in crustaceans.

Characterization and functional study of nuclear factor erythroid 2-related factor 2 (Nrf2) in black tiger shrimp (Penaeus monodon)

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a member of the Cap'n'collar basic region leucine zipper (CNC-bZIP) transcription factor family, and is activated by diverse oxidants, pro-oxidants, antioxidants and chemopreventive agents. The full-length cDNA of Nrf2 from Penaeus monodon (PmNrf2; 2024 bp long with 729 bp coding region, GenBank accession no. MW390830) was cloned. The 242-amino-acid polypeptide encoded by this gene had a predicted molecular mass of 27.80 kDa. Sequence homology and phylogenetic analysis showed that PmNrf2 was similar to the insect Cap'n'Collar (CNC) transcription factor and mammalian Nrf2. Tissue expression profile analyzed by quantitative real-time RT-PCR (qRT-PCR) demonstrated that PmNrf2 was constitutively expressed in all examined tissues, with the highest expression observed in the intestines and the weakest expression observed in the hemocyte. PmNrf2 expression profiles were detected in the hepatopancreas of shrimp after bacterial challenge. The results suggested that PmNrf2 was involved in the responses to bacterial challenge, but the temporal expression pattern trend of PmNrf2 differed between the gram-negative and gram-positive bacterial challenges in the shrimp hepatopancreas. The recombinant PmNrf2 protein was expressed and purified through affinity chromatography. Furthermore, an anti-PmNrf2 polyclonal antibody was obtained, which was able to clearly detect PmNrf2 protein expression in the hepatopancreas of shrimp. Knockdown of PmNrf2 by RNA interference (RNAi) resulted in a reduction in the expression of PmGPx gene. Taken together, the results of our study indicated that PmNrf2 played a role in regulation the transcription of PmGPx antioxidant enzyme genes.

Phosphorylation of Shrimp Tcf by a Viral Protein Kinase WSV083 Suppresses Its Antiviral Effect

Nuclear DNA-binding TCF proteins, which act as the main downstream effectors of Wnt signaling, are essential for the regulation of cell fate and innate immunity. However, their role during viral infection in shrimp remains unknown. Herein, we demonstrated that Litopenaeus vannamei TCF (LvTcf) acts independently of Lvβ-catenin to promote interferon-like protein LvVago1 production, thus mounting the response to WSSV infection. Further, we observed that WSV083, a WSSV serine/threonine protein kinase, bound to LvTcf and phosphorylated it. Phosphorylated LvTcf was then recognized and degraded via the ubiquitin-proteasome pathway. Moreover, mass spectrometry analyses indicated that the T39 and T104 residues of LvTcf were target sites phosphorylated by WSV083. Point mutation analyses suggested that additional sites of LvTcf may undergo phosphorylation via WSV083. Taken together, the current work provides valuable insights into host immunity and viral pathogenesis. LvTcf is not only a modulator of shrimp innate immunity but is also an important target for WSSV immune evasion. Thus, the current findings will help improve disease control in shrimps.

A novel JNK induces innate immune response by activating the expression of antimicrobial peptides in Chinese mitten crab Eriocheir sinensis

c-Jun NH₂-terminal kinase (JNK) is a member of mitogen-activated protein kinases (MAPKs) that participates in the regulation of various physiological and pathological processes. In this study, we identified a novel JNK (EsJNK) and determined the cDNA sequence of its isoform (EsJNK-a) from the Chinese mitten crab Eriocheir sinensis. The open reading frame (ORF) of EsJNK was predicted to encode 421 peptides with a serine/threonine protein kinase, a catalytic (S_TKc) domain, and a low complexity region. The ORF of EsJNK-a was 1380 bp encoding a protein with 459 amino acids, which was 38 amino acids more than that of EsJNK. The predicted tertiary structure of EsJNK was conserved and contained 15 α-helices and 10 β-sheets. Phylogenetic tree analysis revealed that EsJNK was clustered with the JNK homologs of other crustaceans. Quantitative real-time PCR assays showed that EsJNK was expressed in all the tissues examined, but it was relatively higher in hemocytes, muscles, and intestines. The expression of EsJNK mRNA in the hemocytes was upregulated by lipopolysaccharides and peptidoglycans, as well as by Staphylococcus aureus or Vibrio parahaemolyticus challenge. Functionally, after silencing EsJNK by siRNA in crabs, the expression levels of two antimicrobial peptides (AMPs), namely, anti-lipopolysaccharide factor and crustin, were significantly inhibited. The purified recombinant EsJNK protein with His-tag accelerated the elimination of the aforementioned bacteria in vivo. However, knockdown of EsJNK had an opposite effect. These findings suggested that EsJNK might be involved in the antibacterial immune defense of crabs by regulating the transcription of AMPs.

Laodelphax striatellus Atg8 facilitates Rice stripe virus infection in an autophagy-independent manner

Rice stripe virus (RSV) is the causative agent of rice stripe disease and is completely dependent on insect vectors for its plant-to-plant transmission. Laodelphax striatellus is the major insect vector for RSV. In this study, we explored the interactions between RSV infection and L. striatellus autophagy, a potential intrinsic antiviral mechanism in insects. We found that L. striatellus autophagic activity did not affect RSV infection; however, the autophagy-related-8 (Atg8) gene significantly enhanced virus infection. During RSV initial infection within the L. striatellus midgut, silencing of Atg8 expression significantly decreased the phosphorylation of c-Jun N-terminal kinase (p-JNK); however, when RSV infection is absent, silencing of Atg8 did not alter p-JNK levels. These results indicated that Atg8 might activate the JNK machinery by allowing more virus infection into cells. We further revealed that Atg8-deficiency significantly decreased RSV accumulation on the surface of the insect midgut epithelial cells, suggesting a receptor trafficking function of the γ-aminobutyric acid receptor-associated protein family. Using the RSV ovary entry as a model, in which vitellogenin receptor (VgR) mediates RSV cell entry, we clarified that Atg8-deficiency decreased the abundance of VgR localizing on the cytomembrane and disturbed the attachment of RSV in the germarium zones. Collectively, these results revealed an autophagy-independent function of L. striatellus Atg8 that enhances RSV initial infection by increasing virus attachment on the infection sites.

Susceptibility of five different sizes of pathogenfree Litopenaeus vannamei to white spot syndrome virus (WSSV) by intramuscular inoculation

White spot syndrome virus (WWSV) has become one of the most widespread causes of mortality in commercial shrimp farming. In the present study, we used PCR to determine the shrimp infectious dose 50% endpoint (SID50 ml-1) of a Chinese isolate of WSSV in 5 different sizes of pathogen-free Litopenaeus vannamei inoculated intramuscularly. The lethal dose 50% endpoint (LD50 ml-1) was also determined from the percentage of dead shrimp. The LD50 ml-1 for 2, 4, 6, 8, and 10 cm shrimp were 104.68, 105.7, 106.70, 107.75, and 108.81, respectively, and the SID50 ml-1 were 104.68, 105.70, 106.90, 107.75, and 108.94, respectively. There was no significant difference between the LD50 ml-1 and SID50 ml-1 for each shrimp size, which indicated that all infected shrimp died. The lethal and infectious titer decreased about 1 log10 as shrimp size decreased 1 grade. These data clearly indicate that adult shrimp were more susceptible to WSSV than juvenile shrimp. The horizontal comparison showed that the amount of virus in the shrimp organs increased over the experimental period. The vertical comparison showed that virus quantity was lowest in the organs of 10 cm shrimp and highest in 2 cm shrimp, which indicates that the smaller shrimp had higher levels of viral replication. Hence, the optimal size for WSSV challenge in shrimp inoculated intramuscularly was 2 cm. The determination of virus titers in different sizes of shrimp represents a step towards creating strategies to reduce the negative impacts of WSSV in the aquaculture industry.

SpATF2 participates in maintaining the homeostasis of hemolymph microbiota by regulating dual oxidase expression in mud crab

Activating transcription factors 2 (ATF2) is a transcription factor of the members of ATF/CREB family that is phosphorylated and activated by the mitogen-activated protein kinase (MAPK) in responding to the stimulation of stimuli. In present study, SpATF2 from mud crab (Scylla paramamosain) was identified and studied. The open reading frame of SpATF2 with 2136 bp in length encodes a protein with 711 amino acids. The SpATF2 protein includes the putative zinc finger domain in the N-terminus and bZIP type DNA-binding domain in the C-terminal. Tissue distribution of SpATF2 transcripts showed that SpATF2 was ubiquitously expressed in all examined tissues of the untreated mud crabs, with the highest expression levels in muscle and hepatopancreas. The transcriptional level of SpATF2 was up-regulated in the hemocytes after Vibrio parahemolyticus or WSSV infection. Reporter gene assays indicated that SpATF2 could activate the expression of dual oxidase (SpDuox1) in S. paramamosain. The RNA interference (RNAi) of SpATF2 significantly decreased the expression of SpDuox1, and consequently reduced reactive oxygen species production thereby significantly increased the bacterial load in the hemolymph of mud crabs. Similarly, significant reduction in bacterial clearance of hemolymph was observed after the V. parahemolyticus infection in SpATF2 knockdown mud crabs. This study showed that SpATF2 played a vital role in maintaining homeostasis of the hemolymph microbiota through regulating the expression of dual oxidase of mud crab.

Fish c-Jun N-Terminal Kinase (JNK) Pathway Is Involved in Bacterial MDP-Induced Intestinal Inflammation

The c-Jun NH₂-terminal kinases (JNKs) are an evolutionarily conserved family of serine/threonine protein kinases that play critical roles in the pathological process in species ranging from insects to mammals. However, the function of JNKs in bacteria-induced intestinal inflammation is still poorly understood. In this study, a fish JNK (CiJNK) pathway was identified, and its potential roles in bacterial muramyl dipeptide (MDP)-induced intestinal inflammation were investigated in Ctenopharyngodon idella. The present CiJNK was found to possess a conserved dual phosphorylation motif (TPY) in a serine/threonine protein kinase (S_TKc) domain and to contain several potential immune-related transcription factor binding sites, including nuclear factor kappa B (NF-κB), activating protein 1 (AP-1), and signal transducer and activator of downstream transcription 3 (STAT3), in its 5′ flanking regions. Quantitative real-time PCR results revealed that the mRNA levels of the JNK pathway genes in the intestine were significantly upregulated after challenge with a bacterial pathogen (Aeromonas hydrophila) and MDP in a time-dependent manner. Additionally, the JNK pathway was found to be involved in regulating the MDP-induced expression levels of inflammatory cytokines (IL-6, IL-8, and TNF-α) in the intestine of grass carp. Moreover, the nutritional dipeptide carnosine and Ala–Gln could effectively alleviate MDP-induced intestinal inflammation by regulating the intestinal expression of JNK pathway genes and inflammatory cytokines in grass carp. Finally, fluorescence microscopy and dual-reporter assays indicated that CiJNK could associate with CiMKK4 and CiMKK7 involved in the regulation of the AP-1 signaling pathway. Overall, these results provide the first experimental demonstration that the JNK signaling pathway is involved in the intestinal immune response to MDP challenge in C. idella, which may provide new insight into the pathogenesis of inflammatory bowel disease.

A new insight to biomarkers related to resistance in survived-white spot syndrome virus challenged giant tiger shrimp, Penaeus monodon

The emergence of diseases such as white spot disease has become a threat to Penaeus monodon cultivation. Although there have been a few studies utilizing RNA-Seq, the cellular processes of host-virus interaction in this species remain mostly anonymous. In the present study, P. monodon was challenged with WSSV by intramuscular injection and survived for 12 days. The effect of the host gene expression by WSSV infection in the haemocytes, hepatopancreas and muscle of P. monodon was studied using Illumina HiSeq 2000. The RNA-Seq of cDNA libraries was developed from surviving WSSV-challenged shrimp as well as from normal healthy shrimp as control. A comparison of the transcriptome data of the two groups showed 2,644 host genes to be significantly up-regulated and 2,194 genes significantly down-regulated as a result of the infection with WSSV. Among the differentially expressed genes, our study discovered HMGB, TNFSF and c-Jun in P. monodon as new potential candidate genes for further investigation for the development of potential disease resistance markers. Our study also provided significant data on the differential expression of genes in the survived WSSV infected P. monodon that will help to improve understanding of host-virus interactions in this species.

White Spot Syndrome Virus Establishes a Novel IE1/JNK/c-Jun Positive Feedback Loop to Drive Replication

Viruses need to hijack and manipulate host proteins to guarantee their replication. Herein, we uncovered that the DNA virus white spot syndrome virus (WSSV) established a novel positive feedback loop by hijacking the host JNK pathway via its immediate-early 1 (IE1) protein to drive replication. Specifically, the WSSV IE1 bound to host JNK, and enhanced JNK autoactivation by autophosphorylation, and in turn, elevated JNK kinase activity to its substrate c-Jun and induced IE1, which resulted in a viral gene-mediated positive feedback loop. Moreover, the activation of this loop is able to induce wsv056, wsv249, and wsv403, in addition to IE1 itself. Disruption of this loop during WSSV infection by knockdown of JNK, c-Jun or IE1 led to an increased survival rate and lower viral burdens in shrimp. Taken together, this loop may provide a potential target for the development of specific antiviral strategies or agents against WSSV infection.

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Lvc-Jun promotes WSSV IE1 induction via interacting with the promoter of IE1 gene

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The interaction of IE1-LvJNK enhances the autophosphorylation of LvJNK

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IE1 hijacks the JNK/c-Jun cascade to create a feedback loop to drive replication

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wsv056, wsv249, and wsv403 are also benefit from this positive feedback loop

Molecular characterization of a novel white spot syndrome virus response protein (dubbed LvWRP) from Litopenaeus vannamei

White spot syndrome, which is caused by white spot syndrome virus (WSSV), is a highly contagious disease of penaeid shrimp. However, there is currently incomplete understanding of the infection mechanism and pathogenesis of WSSV. In this study, a novel gene of a previously uncharacterized WSSV response protein (LvWRP) in Litopenaeus vannamei was identified and characterized. The LvWRP gene has an open reading frame (ORF) of 879 bp encoding a putative protein of 292 amino acids. Sequence analysis revealed that LvWRP shared 24.9% identity with an uncharacterized protein of Penaeus monodon nudivirus. Real-time qPCR analysis showed that LvWRP was ubiquitously expressed in shrimp tissues, with transcript levels induced in hemocytes upon immune challenge with Vibrio parahaemolyticus, Streptoccocus iniae, lipopolysaccharide (LPS), and WSSV. In addition, RNA interference-mediated knockdown of LvWRP followed by WSSV challenge revealed significant decrease in the transcript levels of WSSV IE1 and VP28 genes coupled with a reduction in WSSV copies in shrimp hemocytes. Moreover, depletion of LvWRP followed by WSSV challenge significantly increased the transcript levels of Vago4 and Vago5 as well as increased the phosphorylation of STAT, while hemocytes apoptosis in terms of caspase 3/7 activity was decreased. These results suggest that LvWRP is important for WSSV replication in shrimp, and therefore one of the vital host factors in WSSV infection.

The Two NF-κB Pathways Regulating Bacterial and WSSV Infection of Shrimp

The outbreak of diseases ordinarily results from the disruption of the balance and harmony between hosts and pathogens. Devoid of adaptive immunity, shrimp rely largely on the innate immune system to protect themselves from pathogenic infection. Two nuclear factor-κB (NF-κB) pathways, the Toll and immune deficiency (IMD) pathways, are generally regarded as the major regulators of the immune response in shrimp, which have been extensively studied over the years. Bacterial infection can be recognized by Toll and IMD pathways, which activate two NF-κB transcription factors, Dorsal and Relish, respectively, to eventually lead to boosting the expression of various antimicrobial peptides (AMPs). In response to white-spot-syndrome-virus (WSSV) infection, these two pathways appear to be subverted and hijacked to favor viral survival. In this review, the recent progress in elucidating microbial recognition, signal transduction, and effector regulation within both shrimp Toll and IMD pathways will be discussed. We will also highlight and discuss the similarities and differences between shrimps and their Drosophila or mammalian counterparts. Understanding the interplay between pathogens and shrimp NF-κB pathways may provide new opportunities for disease-prevention strategies in the future.

Identification of Functional Gene Modules Associated With STAT-Mediated Antiviral Responses to White Spot Syndrome Virus in Shrimp

White spot syndrome virus (WSSV) is one of the major threats to shrimp aquaculture. It has been found that the signal transducer and activator of transcription (STAT) protein plays an important role in the antiviral immunity of shrimp with a WSSV infection. However, the mechanism that underlies the STAT-mediated antiviral responses in shrimp, against WSSV infection, remains unclear. In this work, based on the gene expression profiles of shrimp with an injection of WSSV and STAT double strand RNA (dsRNA), we constructed a gene co-expression network for shrimp and identified the gene modules that are possibly responsible for STAT-mediated antiviral responses. These gene modules are found enriched in the regulation of the viral process, JAK-STAT cascade and the regulation of immune effector process pathways. The gene modules identified here provide insights into the molecular mechanism that underlies the STAT-mediated antiviral response of shrimp, against WSSV.

A low-density lipoprotein receptor (LDLR) class A domain-containing C-type lectin from Litopenaeus vannamei plays opposite roles in antibacterial and antiviral responses

C-type lectins (CTLs) are a group of pattern recognition receptors (PRRs) that contain carbohydrate recognition domains and play important roles in innate immunity. CTLs that contain an additional low-density lipoprotein receptor (LDLR) class A domain (LdlrCTL) have been identified in many crustaceans, but their functions in immune responses are mostly unknown. In this study, a novel LdlrCTL was identified from pacific white shrimp Litopenaeus vannamei (LvLdlrCTL), which showed high homology with previously reported crustacean LdlrCTLs. LvLdlrCTL was highly expressed in hemocytes and its expression was up-regulated after immune stimulations. Silencing of LdlrCTL significantly promoted infection of shrimp by Vibrio parahaemolyticus but inhibited infection by white spot syndrome virus (WSSV), suggesting that LdlrCTL could play opposite roles in antibacterial and antiviral responses. LdlrCTL exhibited agglutination activity against bacteria and fungi and could potentiate the phagocytosis of hemocytes. Moreover, the expression of many immune effector genes and signalling pathway components was significantly changed in LdlrCTL-silenced shrimp, indicating that LdlrCTL could be involved in immune regulation.

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.

A novel JNK is involved in immune response by regulating IL expression in oyster Crassostrea gigas

The c-Jun N-terminal kinase (JNK) is a member of mitogen-activated protein kinases (MAPK) highly conserved from yeast to mammals and participates in regulating many physiological and pathological processes. In the present study, a novel JNK was identified from oyster Crassostrea gigas (designated as CgJNK) and its biological functions were investigated in response against lipopolysaccharide (LPS) stimulation. The CgJNK consists of 415 amino acids, which includes a serine/threonine protein kinase (S_TKc) domain with a conserved Thr-Pro-Tyr (TPY) motif. Phylogenetic analysis revealed that CgJNK shared high similarity with other members of the JNK subfamily. CgJNK mRNA was detected in all the tested tissues and CgJNK mRNA expression levels in hemocytes were significantly up-regulated from 6 to 72 h after LPS stimulation and reached the highest level (16.1-fold, p < 0.01) at 24 h. The phosphorylation level of CgJNK in C. gigas hemocytes was increased at 2 h after LPS stimulation. The subcellular localization of CgJNK phosphorylation in hemocytes was analyzed after LPS stimulation, and CgJNK phosphorylation could be detected in both cytoplasm and nucleus of oyster hemocytes at 2 h post LPS stimulation. Additionally, the interleukins (CgILs) were detected in hemocytes of CgJNK-knockdown oysters. CgIL17-1, CgIL17-2, CgIL17-4 and CgIL17-6 transcripts were decreased significantly in CgJNK-knockdown oysters at 24 h post LPS stimulation. In summary, these results suggested that CgJNK played an important role in the immune response of oysters by regulating IL expression.

A transcriptome study on Macrobrachium nipponense hepatopancreas experimentally challenged with white spot syndrome virus (WSSV)

White spot syndrome virus (WSSV) is one of the most devastating pathogens of cultured shrimp, responsible for massive loss of its commercial products worldwide. The oriental river prawn Macrobrachium nipponense is an economically important species that is widely farmed in China and adult prawns can be infected by WSSV. However, the molecular mechanisms of the host pathogen interaction remain unknown. There is an urgent need to learn the host pathogen interaction between M. nipponense and WSSV which will be able to offer a solution in controlling the spread of WSSV. Next Generation Sequencing (NGS) was used in this study to determin the transcriptome differences by the comparison of control and WSSV-challenged moribund samples, control and WSSV-challenged survived samples of hepatopancreas in M. nipponense. A total of 64,049 predicted unigenes were obtained and classified into 63 functional groups. Approximately, 4,311 differential expression genes were identified with 3,308 genes were up-regulated when comparing the survived samples with the control. In the comparison of moribund samples with control, 1,960 differential expression genes were identified with 764 genes were up-regulated. In the contrast of two comparison libraries, 300 mutual DEGs with 95 up-regulated genes and 205 down-regulated genes. All the DEGs were performed GO and KEGG analysis, overall a total of 85 immune-related genes were obtained and these gene were groups into 13 functions and 4 KEGG pathways, such as protease inhibitors, heat shock proteins, oxidative stress, pathogen recognition immune receptors, PI3K/AKT/mTOR pathway, MAPK signaling pathway and Ubiquitin Proteasome Pathway. Ten genes that valuable in immune responses against WSSV were selected from those DEGs to furture discuss the response of host to WSSV. Results from this study contribute to a better understanding of the immune response of M. nipponense to WSSV, provide information for identifying novel genes in the absence of genome of M. nipponense. Furthermore, large number of transcripts obtained from this study could provide a strong basis for future genomic research on M. nipponense.

Cloning, identification and function analysis of a Chibby homolog from Litopenaeus vannamei

Chibby, a vital inhibitor molecule of Wnt/β-catenin signaling pathway, participates in development and stem cell differentiation through the regulation of β-catenin. Our previous studies have demonstrated that Litopenaeus vannamei β-catenin (Lv-β-catenin) was involved in WSSV infection and could inhibit virus replication by modulating the host immune system. In the study, a Chibby homolog from L. vannamei (designed as Lv-Chibby) was isolated and its role in WSSV infection was investigated. Sequence analysis suggested that Lv-Chibby was a novel homolog of Chibby family. It could transcript in all examined tissues, including hemocyte, gill, intestine, hepatopancreas, muscle and heart. Real-time quantitative PCR demonstrated that Lv-Chibby could take part in WSSV infection and be down-regulated by WSSV. Further studies confirmed that Lv-Chibby was able to interact with Lv-β-catenin. Moreover, the relationship of Lv-β-catenin, Lv-Chibby and WSSV069 was investigated. It was shown that Lv-Chibby enhanced the interaction between Lv-β-catenin and WSSV069. Interestingly, WSSV069 promoted the interaction between Lv-β-catenin and Lv-Chibby under high concentration, while low concentration of WSSV069 inhibited their interaction. A subsequent immunofluorescence assay revealed that WSSV069 appeared to reduce the nuclear entry of Lv-β-catenin. In sum, these results implied that Wnt/β-catenin signal pathway plays an important role in the defense against virus, and Chibby could be modulated by WSSV to regulate the signal pathway.

Mechanisms for type-II vitellogenesis-inhibiting hormone suppression of vitellogenin transcription in shrimp hepatopancreas: Crosstalk of GC/cGMP pathway with different MAPK-dependent cascades

Vitellogenesis is the process of yolk formation via accumulating vitellin (Vn) with nutrients in the oocytes. Expression of vitellogenin (Vg), the precursor of Vn, is one of the indicators for the start of vitellogenesis. In Pacific white shrimp (Litopenaeus vannamei), the type-II vitellogenesis-inhibiting hormone (VIH-2) effectively suppresses hepatopancreatic Vg mRNA expression. In this study, we demonstrate the increasing transcript levels of hepatopancreatic Vg during L. vannamei ovarian development, suggesting that the hepatopancreas-derived Vg/Vn may also contribute to vitellogenesis in this species. Using a combination of in vivo injections and in vitro primary cell cultures, we provide evidences that the inhibition of VIH-2 on hepatopancreatic Vg gene expression is mediated through a functional coupling of the GC/cGMP pathway with different MAPK-dependent cascades in female shrimp. In VIH-2 signaling, the NO-independent GC/cGMP/PKG cascades were upstream of the MAPKs. Activations of the MAPK signal by VIH-2 include the phosphorylation of JNK and the mRNA/protein expression of P³⁸MAPK. Additionally, the cAMP/PKA pathway is another positive intracellular signal for hepatopancreatic Vg mRNA expression but is independent of its VIH-2 regulation. Our findings establish a model for the signal transduction mechanism of Vg regulation by VIH and shed light on the biological functions and signaling of the CHH family in crustaceans.

Characterization and function of a β-catenin homolog from Litopenaeus vannamei in WSSV infection

As a conserved signaling pathway, Wnt/β-catenin signaling pathway participates in many physiological activities, including cell differentiation, apoptosis and so on. β-catenin is the key molecule of Wnt/β-catenin signaling pathway and plays a pivotal role. In this article, a β-catenin homolog from Litopenaeus vannamei (designed as Lv-β-catenin) was cloned and its role in WSSV infection was investigated. Sequence analysis suggested that Lv-β-catenin had characters of β-catenin family. Semi-quantitative RT-PCR showed that Lv-β-catenin transcripted in all detected tissues. In the subsequent WSSV infection experiments, it was found that the transcription levels of Lv-β-catenin were down-regulated, as well as the expression levels. Immunofluorescence assay further confirmed that WSSV could reduce the amount of Lv-β-catenin and promoted Lv-β-catenin to translocate into the nucleus. Moreover, we found that WSSV could influence the amount of Lv-β-catenin by ubiquitination. While Lv-β-catenin was up-regulated by a β-catenin activator GSK-3 Inhibitor IX, the transcription of virus immediate early gene WSSV069 was significantly inhibited. In addition, it was found that Lv-β-catenin could interact with WSSV069. Conclusively, our study provided evidences that β-catenin may participate in the WSSV infection, and Wnt/β-catenin signal pathway may play an important role in immune regulation.

Experimental inoculation of oriental river prawn Macrobrachium nipponense with white spot syndrome virus (WSSV)

The oriental river prawn Macrobrachium nipponense is an economically important species that is widely farmed in China. White spot syndrome virus (WSSV) is one of the most devastating pathogens of the cultured shrimp Litopenaeus vannamei, responsible for massive loss of its commercial products worldwide. We investigated the infectivity and pathogenicity of WSSV in adult M. nipponense using standardized conditions for L. vannamei. The median lethal dose of WSSV in adult M. nipponense was 103.84±0.06 copies g-1, which was about 1000-fold higher than in L. vannamei (100.59±0.22 copies g-1). WSSV was detected by 2-step PCR in the gills, hepatopancreas, muscle, stomach, heart, gut, nerve, integument, pereopod, eyestalk, testis, and ovary of experimentally infected dead M. nipponense. Lesions were observed histologically following WSSV injection, showing basophilic intranuclear inclusion bodies in the hepatopancreas and subsequently in the gills. The clearance of WSSV was observed in hepatopancreas and gills at 48 and 96 h post-inoculation, respectively. No histological lesions were detected in muscle from 0-96 h post-injection. The results show that the oriental river prawn M. nipponense can be infected by WSSV and the infections are self limiting over time; therefore, M. nipponense may serve as a useful model for studying resistance to WSSV.

Identification and characterization of an interleukin-16-like gene from pacific white shrimp Litopenaeus vannamei

Interleukins are a group of cytokines that play essential roles in immune regulation. Almost all interleukin genes are only found in vertebrates. In this study, an interleukin-16-like gene (LvIL-16L) was identified from Pacific white shrimp, Litopenaeus vannamei. LvIL-16L was predicted to encode a precursor (pro-LvIL-16L) with 1378 amino acids, sharing similarities with predicted pro-IL-16-like proteins from insects. The C-terminus of pro-LvIL-16L protein contained two PDZ domains homologous to the mature IL-16 cytokine of vertebrates. In tissues, LvIL-16L could be processed into a ∼36 kDa mature peptide through a caspase-3 cleavage site, which was verified by in vitro site mutation analysis and in vivo RNA interference (RNAi) experiments. The LvIL-16L mRNA could be detected in all the analyzed tissues and the expression of LvIL-16L was significantly up-regulated after immune stimulation. Using RNAi strategy, the role of LvIL-16L in immune responses was initially investigated. Interestingly, knockdown of LvIL-16L could significantly increase the mortality of the Vibro parahaemolyticus infected shrimps but reduce that of the WSSV infected shrimps, suggesting that LvIL-16L could have opposite effects on the antiviral and antibacterial immune responses in shrimp. To our knowledge, this is the first study of an IL-16-like gene in invertebrates, which could help to elucidate interleukin evolution and regulatory mechanisms of shrimp immune responses.

The c-Jun N-terminal kinase pathway of a vector insect is activated by virus capsid protein and promotes viral replication

No evidence has shown whether insect-borne viruses manipulate the c-Jun N-terminal kinase (JNK) signaling pathway of vector insects. Using a system comprising the plant virus Rice stripe virus (RSV) and its vector insect, the small brown planthopper, we have studied the response of the vector insect’s JNK pathway to plant virus infection. We found that RSV increased the level of Tumor Necrosis Factor-α and decreased the level of G protein Pathway Suppressor 2 (GPS2) in the insect vector. The virus capsid protein competitively bound GPS2 to release it from inhibiting the JNK activation machinery. We confirmed that JNK activation promoted RSV replication in the vector, whereas JNK inhibition caused a significant reduction in virus production and thus delayed the disease incidence of plants. These findings suggest that inhibition of insect vector JNK may be a useful strategy for controling the transmission of plant viruses.

DOI:http://dx.doi.org/10.7554/eLife.26591.001

There are over a thousand different viruses that infect plants. Many plant viruses are transmitted by insects that feed on the plants, much as mosquitoes spread diseases between people when feeding on blood. Often the plant virus can replicate inside the cells of the insect. However, unlike in the plant hosts, the viruses do not seem to cause disease in the insects that carry them.

Rice stripe disease is a major viral disease of rice that can reduce the crop’s yield by more than 50% in some areas. An insect called the small brown planthopper spreads the rice stripe virus between plants. Like other animals, insects have an immune system that protects them against viral infections. This means that the rice stripe virus must manipulate the planthopper’s immune system in order to replicate inside the insect’s cells. It was not clear how the virus did this, but answering this question could provide important clues to help scientists develop new ways to protect crops against plant viruses.

Wang, Zhao, Li et al. now show that rice stripe virus manipulates its insect host to produce more of a protein called TNF-α and less of a protein called GPS2. Moreover, a protein that makes up part of the virus also binds to GPS2. This stops GPS2 from inhibiting a conserved signaling pathway that involves an enzyme known as JNK. When the JNK signaling pathway becomes active, replication of the rice stripe virus inside the insect is accelerated. Further experiments showed that inhibiting JNK made it harder for the virus to replicate, which meant that it took longer for the disease to develop in rice plants.

These findings uncover a host of proteins that could be manipulated in insects to benefit rice agriculture. Such alterations could possibly be achieved through breeding or otherwise genetically modifying the insects to make them less able to carry viruses and then releasing them into wild populations. Alternatively, if further studies can identify chemicals that cause insect cells to alter the levels of the proteins, such chemicals could be administered to farmland to reduce the spread of viruses.

DOI:http://dx.doi.org/10.7554/eLife.26591.002

The immediate early protein WSV187 can influence viral replication via regulation of JAK/STAT pathway in Drosophila

The world production of shrimp is seriously affected by the white spot syndrome virus (WSSV). Viral immediate-early (IE) genes encode regulatory proteins critical for the viral lifecycle. In spite of their importance, only five out of the 21 identified WSSV IE genes are functionally characterized. Here, we report the use of Drosophila melanogaster as a model to explore the role of WSSV IE gene wsv187. In vivo expression of WSV187 in transgenic flies show WSV187 localized in the cytoplasm. Overexpression of wsv187 results wing defects consistent with phenotypes observed in JAK/STAT exacerbated flies. After artificial infection of the DCV virus, the flies expressing wsv187 showed a lower viral load, a higher survival rate and an up-regulated STAT92E expression. These data demonstrate wsv187 plays a role in the controlling of virus replication by activating host JAK/STAT pathway.

Shrimp TAB1 interacts with TAK1 and p38 and activates the host innate immune response to bacterial infection

Mammalian TAB1 has been previously identified as transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) binding protein, which functions as the activator of TAK1 and p38. This report, for the first time, identified and characterized the homolog of TAB1 in shrimp, to be specific, the homolog gene from Litopenaeus vannamei, containing a 1560-bp open reading frame (ORF) that encoded a putative protein of 519 amino acids with the conserved PP2Cc (Serine/threonine phosphatases, family 2C, catalytic) domain in N-terminal and a TAK1 binding motif in C-terminus, has been cloned and named LvTAB1. LvTAB1 was most abundant in gills and its expression could respond significantly to a series of stimuli, including LPS, Vibrio parahemolyticus and Staphylococcus aureus. Moreover, Co-immunoprecipitation (Co-IP) experiments showed that LvTAB1 could combine with LvTAK1 as well as Lvp38, two members of IMD-NF-κB/MAPK pathway, which meant LvTAB1 could have a role in regulating the activities of these kinases. Over-expression of LvTAB1 in drosophila S2 cells could improve the transcriptional levels of antimicrobial peptide genes (AMPs) such as Diptericin (Dpt), the hallmark of drosophila NF-κB activated genes, indicating its activation effect on NF-κB pathway. Furthermore, suppression of LvTAB1 expression in vivo by RNA-interference increased the sensibility of shrimps to V. parahaemolyticus infection, implying its protective role against bacterial infection. In conclusion, these results provide some insight into the function of LvTAB1 during bacterial infection.

c-Jun N-terminal kinase (JNK) is involved in immune defense against bacterial infection in Crassostrea hongkongensis

c-Jun N-terminal kinase (JNK) is a universal and essential subgroup of the mitogen-activated protein kinase (MAPK) superfamily, which is highly conserved from yeast to mammals and functions in a variety of physiological and pathological processes. In this study, we report the first oyster JNK gene homolog (ChJNK) and its biological functions in the Hong Kong oyster Crassostrea hongkongensis. The ChJNK protein consists of 383 amino acids and contains a conserved serine/threonine protein kinase (S_TKc) domain with a typical TPY motif. Phylogenetic analysis revealed that ChJNK shared a close evolutionary relationship with Crassostrea gigas JNK. Quantitative RT-PCR analyses revealed broad expression patterns of ChJNK mRNA in various adult tissues and different embryonic and larval stages of C. hongkongensis. When exposed to Vibrio alginolyticus or Staphylococcus haemolyticus, ChJNK mRNA expression levels were significantly up-regulated in the hemocytes and gills in a time-dependent manner. Additionally, subcellular localization studies that ChJNK is a cytoplasm-localized protein, and that its overexpression could significantly enhance the transcriptional activities of AP-1-Luc in HEK293T cells. In summary, this study provided the first experimental demonstration that oysters possess a functional JNK that participates in host defense against bacterial infection in C. hongkongensis.

Down-regulation apoptosis signal-regulating kinase 1 gene reduced the Litopenaeus vannamei hemocyte apoptosis in WSSV infection

Apoptosis signal-regulating kinase 1 (ASK1), a mitogen-activated protein kinase kinase kinase, is crucial in various cellular responses. In the present study, we identified and characterized an ASK1 homolog from Litopenaeus vannamei (LvASK1). The full-length cDNA of LvASK1 was 5400 bp long, with an open reading frame encoding a putative 1420 amino acid protein. LvASK1 was highly expressed in muscle, hemocyte, eyestalk and heart. Real-time RT-PCR analysis showed that the expression of the LvASK1 was upregulated during the white spot syndrome virus (WSSV) challenge. The knocked-down expression of LvASK1 by RNA interference significantly reduced the apoptotic ratio of the hemocytes collected from WSSV-infected L. vannamei. Furthermore, the down-regulation of LvASK1 also decreased the cumulative mortality of WSSV-infected L. vannamei. These results suggested that down-regulation of LvASK1 decreased the apoptotic rate of hemocytes in WSSV-infected shrimp, and that it could contribute to the reduction of cumulative mortality in WSSV-infected L. vannamei.

Molecular Mechanisms of White Spot Syndrome Virus Infection and Perspectives on Treatments

Since its emergence in the 1990s, White Spot Disease (WSD) has had major economic and societal impact in the crustacean aquaculture sector. Over the years shrimp farming alone has experienced billion dollar losses through WSD. The disease is caused by the White Spot Syndrome Virus (WSSV), a large dsDNA virus and the only member of the Nimaviridae family. Susceptibility to WSSV in a wide range of crustacean hosts makes it a major risk factor in the translocation of live animals and in commodity products. Currently there are no effective treatments for this disease. Understanding the molecular basis of disease processes has contributed significantly to the treatment of many human and animal pathogens, and with a similar aim considerable efforts have been directed towards understanding host-pathogen molecular interactions for WSD. Work on the molecular mechanisms of pathogenesis in aquatic crustaceans has been restricted by a lack of sequenced and annotated genomes for host species. Nevertheless, some of the key host-pathogen interactions have been established: between viral envelope proteins and host cell receptors at initiation of infection, involvement of various immune system pathways in response to WSSV, and the roles of various host and virus miRNAs in mitigation or progression of disease. Despite these advances, many fundamental knowledge gaps remain; for example, the roles of the majority of WSSV proteins are still unknown. In this review we assess current knowledge of how WSSV infects and replicates in its host, and critique strategies for WSD treatment.

Identification and characterization of MKK7 as an upstream activator of JNK in Litopenaeus vannamei

Mitogen-activated protein kinase kinase 7 (MKK7) is a key signal transduction regulator in c-Jun N-terminal kinase (JNK) signaling pathway, which is involved in a wide range of physiological and pathological processes. In this study, we described the molecular cloning of a new member of MKK7 group from Litopenaeus vannamei named as LvMKK7. The full-length cDNA of LvMKK7 was 3093 bp in length, with an open reading frame (ORF) of 1440bp encoding a putative protein of 479 amino acids. LvMKK7 contained a conserved kinase domain of 261 amino acids in which there was a characteristic S-K-A-K-T motif as a potential target site of phosphorylation by MKKK. Moreover, subcellular localization showed LvMKK7 was located in both the cytoplasm and the nucleus of Drosophila S2 cells. Real-time PCR indicated that LvMKK7 was universally expressed in all tested tissues and its expression in hepatopancreas was responsive to the challenge of LPS, Poly (I:C), Vibrio parahaemolyticus, Staphhylococcus aureus and white spot syndrome virus (WSSV). In addition, co-immunoprecipitation assay demonstrated that LvJNK was phosphorylated and activated by LvMKK7, which suggested LvMKK7 was the upper regulator of LvJNK. Furthermore, RNAi-mediated knockdown of LvMKK7 enhanced the sensitivity of shrimps to V. parahaemolyticus infection. Overall, our results suggested that LvMKK7 may play important roles in the shrimp innate immunity.

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.

Essential roles of Cdc42 and MAPK in cadmium-induced apoptosis in Litopenaeus vannamei

Cadmium, one of the most toxic heavy metals in aquatic environments, has severe effects on marine invertebrates and fishes. The MAPK signaling pathway plays a vital role in stress responses of animals. The mitogen-activated protein kinase (MAPK) signaling pathway plays a vital role in animals' stress responses, including mediation of apoptosis induced by the Rho GTPase Cdc42. However, there is limited knowledge about its function in shrimps, although disorders exacerbated by environmental stresses (including heavy metal pollution) have caused serious mortality in commercially cultured shrimps. Thus, we probed roles of Cdc42 in Litopenaeus vannamei shrimps (LvCdc42) during cadmium exposure by inhibiting its expression using dsRNA-mediated RNA interference. The treatment successfully reduced expression levels of MAPKs (including p38, JNK, and ERK). Cadmium exposure induced significant increases in expression levels of LvCdc42 and MAPKs, accompanied by reductions in total hemocyte counts (THC) and increases in apoptotic hemocyte ratios and ROS production. However, all of these responses were much weaker in LvCdc42-suppressed shrimps, in which mortality rates were higher than in controls. Our results suggest that the MAPK pathway plays a vital role in shrimps' responses to Cd(2+). They also indicate that LvCdc42 in shrimps participates in its regulation, and thus plays key roles in ROS production, regulation of apoptosis and associated stress responses.

Identification of a c-Jun homolog from Litopenaeus vannamei as a downstream substrate of JNK in response to WSSV infection

c-Jun, a major substrate of c-Jun N-terminal kinase (JNK), participates in regulating gene transcription in response to various stimuli, including cytokines, stress signals, bacterial and viral infection. Results from our previous studies suggested that Litopenaeus vannamei JNK (LvJNK) could be utilized by white spot syndrome virus (WSSV) to facilitate viral replication and gene expression. In this article, a c-Jun homolog from Litopenaeus vannamei (designated as Lvc-Jun) was cloned and its role in WSSV infection was studied. Sequence analysis displayed that Lvc-Jun was a novel homolog of c-Jun family, which contained characteristic Jun and basic leucine zipper (bZIP) domains, and two conserved serine phosphorylation sites (Ser49/59). Semi-quantitative RT-PCR analysis showed that Lvc-Jun mRNAs were expressed in all examined tissues. Further investigation determined that Lvc-Jun was located in the nucleus through self-interaction and its phosphorylation levels could be reduced by JNK inhibitor, suggesting that Lvc-Jun could be regulated by LvJNK through phosphorylation and function as a transcription regulator in a homodimer. During the process of WSSV infection, the transcription levels of Lvc-Jun were up-regulated associating with the raising expression and phosphorylation levels of its protein. Moreover, TPA (12-O-tetradecanoylphorbol-13-acetate), a potent inducer of c-Jun, could remarkably promote viral immediate-early gene wsv069 transcription in crayfish hemocytes. Conclusively, our results provided experimental evidences that Lvc-Jun was engaged in WSSV infection and further implied that JNK-c-Jun signaling pathway might be important for WSSV replication and viral gene expression.

eIF2α of Litopenaeus vannamei involved in shrimp immune response to WSSV infection

The alpha subunit of Eukaryotic Initiation Factor 2 (eIF2α) is a key translation regulator that plays an important role in cellular stress responses, which including virus infection. To investigate whether WSSV infection can activate the PERK-eIF2α pathway, the eIF2α in shrimp Litopenaeus vannamei, designed as LveIF2α, was analyzed. The LveIF2α, a 332-amino acid polypeptide, shares a high degree of similarity with eIF2α from other species, having two eIF2α protein signatures at the 13-88 aa and 192-243 aa. The WSSV challenge experiment showed that the protein level of the total LveIF2α was decreased after infection, while the phosphorylation of LveIF2α has no significant change, which indicated that the phosphorylation ratio of LveIF2α was increased after infection. Furthermore, inhibitor treatment led to a significant decrease of WSSV loads. Moreover, the Binding immunoglobulin protein (BiP), an endoplasmic reticulum (ER) stress sensor, and PERK were also investigated during virus infection and it was shown that they were both up-regulated. Taken together, these results suggested that WSSV infection can induce ER stress and activated the unfolded protein response (UPR), and the PERK-eIF2α pathway is important for innate immune during WSSV infection in shrimp.

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.

Bioinformatic prediction of WSSV-host protein-protein interaction

WSSV is one of the most dangerous pathogens in shrimp aquaculture. However, the molecular mechanism of how WSSV interacts with shrimp is still not very clear. In the present study, bioinformatic approaches were used to predict interactions between proteins from WSSV and shrimp. The genome data of WSSV (NC_003225.1) and the constructed transcriptome data of F. chinensis were used to screen potentially interacting proteins by searching in protein interaction databases, including STRING, Reactome, and DIP. Forty-four pairs of proteins were suggested to have interactions between WSSV and the shrimp. Gene ontology analysis revealed that 6 pairs of these interacting proteins were classified into “extracellular region” or “receptor complex” GO-terms. KEGG pathway analysis showed that they were involved in the “ECM-receptor interaction pathway.” In the 6 pairs of interacting proteins, an envelope protein called “collagen-like protein” (WSSV-CLP) encoded by an early virus gene “wsv001” in WSSV interacted with 6 deduced proteins from the shrimp, including three integrin alpha (ITGA), two integrin beta (ITGB), and one syndecan (SDC). Sequence analysis on WSSV-CLP, ITGA, ITGB, and SDC revealed that they possessed the sequence features for protein-protein interactions. This study might provide new insights into the interaction mechanisms between WSSV and shrimp.

The role of Litopenaeus vannamei p38 in white spot syndrome virus infection

p38 as a member of MAPK family, has been conversed from yeast to mammals. It has been implicated in numerous biological processes, including the responses to stress and inflammation. In this study, three closely related p38 MAPK homologs, lvp38a, lvp38b and lvp38c, which differ only in an internal 25-amino acid sequence, have been cloned from Litopenaeus vannamei. Three isoforms shared p38 conserved TGY motif and catalytic center, as well as had maximum identities to human p38α and Drosophila p38b. Tissue distribution revealed that lvp38a and lvp38b were ubiquitously expressed in most tissues, while lvp38c showed at relatively low levels and in a tissue-specific manner. Western blotting analysis showed that lvp38 was activated by phosphorylation during WSSV infection. Furthermore, silencing lvp38 mediated by specific dsRNA in shrimps promoted white spot syndrome virus (WSSV) replication and viral gene transcription at the early stage. These results demonstrated that lvp38 was involved in WSSV infection and might participate in host defense at the early stage.

Molecular characterization of a p38 MAPK from Litopenaeus vannamei and its expression during the molt cycle and following pathogen infection

The p38 mitogen-activated protein kinase (MAPK), a serine/threonine-specific protein kinase, has been reported to be involved in innate immunity, development and muscle differentiation. To explore the function of p38 in shrimp, partial cDNA sequence of p38 in Litopenaeus vannamei (designated as Lv-p38) was characterized and the expression of Lv-p38 in hepatopancreas of the shrimp after being infected with Vibrio parahaemolyticus and in muscle of the shrimp at different molt stages was detected by quantitative RT-PCR in this study. The results showed that the open reading frame of Lv-p38 was 1098 bp and encoded a protein of 365 amino acids. The protein of Lv-p38 which showed close phylogenetic relationship to Marsupenaeus japonicus p38 had a conserved TGY motif and serine/threonine protein kinase (S_TKc) domain. The expression of Lv-p38 was detected in all tested tissues, especially in the hepatopancreas and muscle. The expression of Lv-p38 in the hepatopancreas was different from that of the control at the 24th hour after the injection of V. parahaemolyticus and in the muscle significantly increased at stage C but decreased at other stages during molt, illustrating that Lv-p38 could be involved in pathogen infection and the molt cycle of shrimp. In conclusion, we identified Lv-p38 and studied its role in pathogen infection and molting, which might facilitate our understanding of the function of p38 in innate immunity and growth during molt of shrimp.