Identification and functional analysis of a Hemolin like protein from Litopenaeus vannamei

CmHem, a hemolin-like gene identified from Cnaphalocrocis medinalis, involved in metamorphosis and baculovirus infection

Background

As a member of the immunoglobulin superfamily, hemolins play a vital role in insect development and defense against pathogens. However, the innate immune response of hemolin to baculovirus infection varies among different insects.

Methods and results

In this study, the hemolin-like gene from a Crambidae insect, Cnaphalocrocis medinalis, CmHem was cloned, and its role in insect development and baculovirus infection was analyzed. A 1,528 bp contig as potential hemolin-like gene of C. medinalis was reassembled from the transcriptome. Further, the complete hemolin sequence of C. medinalis (CmHem) was cloned and sequenced. The cDNA of CmHem was 1,515 bp in length and encoded 408 amino acids. The deduced amino acid of CmHem has relatively low identities (41.9-62.3%) to various insect hemolins. However, it contains four Ig domains similarity to other insect hemolins. The expression level of CmHem was the highest in eggs, followed by pupae and adults, and maintained a low expression level at larval stage. The synthesized siRNAs were injected into mature larvae, and the CmHem transcription decreased by 51.7%. Moreover, the abdominal somites of larvae became straightened, could not pupate normally, and then died. Infection with a baculovirus, C. medinalis granulovirus (CnmeGV), the expression levels of CmHem in the midgut and fat body of C. medinalis significantly increased at 12 and 24 h, respectively, and then soon returned to normal levels.

Conclusions

Our results suggested that hemolin may be related to the metamorphosis of C. medinalis. Exposure to baculovirus induced the phased expression of hemolin gene in the midgut and fat body of C. medinalis, indicated that hemolin involved in the immune recognition of Crambidae insects to baculovirus.

White Spot Syndrome Virus (WSSV) Inhibits Hippo Signaling and Activates Yki To Promote Its Infection in Penaeus vannamei

White spot syndrome virus (WSSV) is a serious threat to shrimp aquaculture, especially Pacific white shrimp, Penaeus vannamei, the most farmed shrimp in the world. Activation of the Hippo-Yki signaling pathway, characterized by the intracellular Hippo-Wts kinase cascade reactions and the phosphorylation and cytoplasmic retention of Yki, is widely involved in various life activities. The current work established the fundamental structure and signal transduction profile of the Hippo-Yki pathway in P. vannamei and further investigated its role in viral infection. We demonstrated that WSSV promoted the dephosphorylation and nuclear translocation of Yki, suggesting that Hippo signaling is impaired and Yki is activated after WSSV infection in shrimp. In vivo, Yki gene silencing suppressed WSSV infection, while Hippo and Wts silencing promoted it, indicating a positive role of Hippo signaling in antiviral response. Further analyses showed that Yki suppressed Dorsal pathway activation and inhibited hemocyte apoptosis in WSSV-infected shrimp, while Hippo and Wts showed opposite effects, which contributed to the role of Hippo signaling in WSSV infection. Therefore, the current study suggests that WSSV annexes Yki to favor its infection in shrimp by inhibiting Hippo signaling. IMPORTANCE White spot syndrome virus (WSSV) is one of the most harmful viral pathogens to shrimp. The pathological mechanism of WSSV infection remains unclear to date. The Hippo-Yki signaling pathway is important for various biological processes and is extensively involved in mammalian immunity, but little is known about its role in infectious diseases in invertebrates. Based on revealing the fundamental structure of the shrimp Hippo pathway, this study investigated its implication in the pathogenesis of WSSV disease. We demonstrated that WSSV enhanced Yki activation by inhibiting Hippo signaling in shrimp. The activated Yki promoted WSSV infection by inhibiting hemocyte apoptosis and suppressing the activation of Dorsal, an NF-κB family member in shrimp that is critical for regulating antiviral response. Therefore, this study suggests that WSSV can hijack the Hippo-Yki signaling pathway to favor its infection in shrimp.

A novel chitinase Chi6 with immunosuppressive activity promotes white spot syndrome virus (WSSV) infection in Penaeus vannamei

Chitinases, a group of glycosylase hydrolases that can hydrolyze chitin, are involved in immune regulation in animals. White spot syndrome virus (WSSV) causes huge losses to crustacean aquaculture every year. We identified a novel chitinase Chi6 from Pacific white shrimp Penaeus vannamei, which contains a catalytic domain but no chitin-binding domain. The Chi6 expression was regulated by multiple immune signaling pathways and increased after immune stimulations. Silencing of Chi6 by RNAi in vivo did not affect Vibrio parahaemolyticus infection, but significantly increased the survival rate of WSSV-infected shrimp. The expression of multiple WSSV immediate early and structural genes was also decreased upon Chi6 silencing. The recombinant Chi6 protein showed no effect on bacterial growth but could attenuate shrimp hemocyte phagocytosis. The mRNA levels of several key elements and downstream genes of the MAPK and Dorsal pathways in Chi6-silenced shrimp were significantly up-regulated, suggesting an inhibitory effect of Chi6 on humoral immune response. Moreover, Chi6 enhanced the regulatory effect of Dorsal on the expression of WSSV ie1 gene. Therefore, Chi6 promotes WSSV infection through immunosuppression and regulation of WSSV gene expression. Targeting Chi6 could be a potential strategy for controlling WSSV disease in shrimp farming.

A kelch motif-containing protein KLHDC2 regulates immune responses against Vibrio parahaemolyticus and white spot syndrome virus in Penaeus vannamei

The kelch motif-containing proteins are widely present in organisms and known to be involved in various biological processes, but their roles in immunity remain unclear. In this study, a kelch motif-containing protein KLHDC2 was identified from Pacific white shrimp Penaeus vannamei and its immune function was investigated. The klhdc2 gene was widely expressed in shrimp tissues and its protein product was mainly present in the nucleus. Expression of klhdc2 was regulated by shrimp NF-κB family members Dorsal and Relish, and changed after immune stimulation. KLHDC2 could enhance the immune defense against Vibrio parahaemolyticus in shrimp but inhibit that against white spot syndrome virus (WSSV). Further analyses showed that KLHDC2 did not affect the phagocytosis of hemocytes but regulated the expression of a series of immune effector genes. KLHDC2 has a complex regulatory relationship with Dorsal and Relish, which may partly contribute to its positive role in antibacterial response by regulating humoral immunity. Moreover, the regulatory effect of KLHDC2 on WSSV ie1 gene contributed to its negative effect on antiviral response. Therefore, the current study enrichs the knowledge on the Kelch family and helps to learn more about the regulatory mechanism of shrimp immunity.

Wnt5b plays a negative role in antibacterial response in Pacific white shrimp Penaeus vannamei

The Wnt family genes are essentially implicated in development and growth in animals. Accumulating clues have pointed to the importance of Wnts in invertebrate immunity, but the underlying mechanisms are still unclear to date. The Wnt5b has been known to promote white spot syndrome virus (WSSV) infection in shrimp but its role in antibacterial response remains unclear. In the current study, we focused on the involvement of Wnt5b in Vibrio parahaemolyticus infection in Pacific white shrimp Penaeus vannamei. We demonstrated that the expression of Wnt5b was regulated by the IMD-Relish and JAK-STAT pathways but not the Dorsal pathway and was suppressed upon bacterial infection. Although Wnt5b did not affect the cellular immunity in shrimp, it was involved in regulation of humoral immunity. Silencing of Wnt5b in vivo significantly increased expression of several antimicrobial peptides but decreased that of many immune functional proteins including C-type lectins and lysozymes. Treatment with recombinant Wnt5b protein increased the susceptibility of shrimp to V. parahaemolyticus infection, while silencing of Wnt5b in vivo showed an opposite result. These suggested that Wnt5b plays a negative role in antibacterial response in shrimp. Together with previous reports, the current study shows that Wnt5b functions as an inhibitor for shrimp immunity, which is a potential target for improving immune responses against infection.

The Non-Receptor Protein Tyrosine Phosphatase PTPN6 Mediates a Positive Regulatory Approach From the Interferon Regulatory Factor to the JAK/STAT Pathway in Litopenaeus vannamei

SH2-domain-containing protein tyrosine phosphatases (PTPs), belonging to the class I PTP superfamily, are responsible for the dephosphorylation on the phosphorylated tyrosine residues in some proteins that are involved in multiple biological processes in eukaryotes. The Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway transduce signaling responding to interferons and initiate cellular antiviral responses. The activity of the JAK/STAT pathway is generally orchestrated by the de-/phosphorylation of the tyrosine and serine residues of JAKs and STATs, in which the dephosphorylation processes are mainly controlled by PTPs. In the present study, an SH2-domian-contianing PTP, temporally named as LvPTPN6, was identified in Litopenaeus vannamei. LvPTPN6 shares high similarity with PTPN6s from other organisms and was phylogenetically categorized into the clade of arthropods that differs from those of fishes and mammals. LvPTPN6 was constitutively expressed in all detected tissues, located mainly in the cytoplasm, and differentially induced in hemocyte and gill after the challenge of stimulants, indicating its complicated regulatory roles in shrimp immune responses. Intriguingly, the expression of LvPTPN6 was regulated by interferon regulatory factor (IRF), which could directly bind to the LvPTPN6 promoter. Surprisingly, unlike other PTPN6s, LvPTPN6 could promote the dimerization of STAT and facilitate its nuclear localization, which further elevated the expression of STAT-targeting immune effector genes and enhanced the antiviral immunity of shrimp. Therefore, this study suggests a PTPN6-mediated regulatory approach from IRF to the JAK/STAT signaling pathway in shrimp, which provides new insights into the regulatory roles of PTPs in the JAK/STAT signaling pathway and contributes to the further understanding of the mechanisms of antiviral immunity in invertebrates.

miR-10c Facilitates White Spot Syndrome Virus Infection by Targeting Toll3 in Litopenaeus vannemei

Toll-like receptors (TLRs) are canonical cell membrane receptors functioning to recognize pathogens and transduce signals to activate immune responses. It has been known that Toll3 in Pacific white shrimp Litopenaeus vannamei (LvToll3) plays a critical role in antiviral immunity by inducing the transcription of interferon regulatory factor (IRF), which mediates a signaling axis that is similar to the interferon system of vertebrates. However, the regulatory mechanism of the Toll3-IRF signaling is still unclear. In this study, a novel microRNA (miRNA) of miR-10 family, temporarily named as miR-10c, was identified from L. vannamei. miR-10c may play a nonnegligible regulatory role in shrimp immune responses since it was constitutively expressed in all detected tissues and transcriptionally induced by immune stimulation. Functional analysis validated that miR-10c could target LvToll3 to inhibit its expression, through which miR-10c blocked the nuclear translocation of IRF and facilitated white spot syndrome virus (WSSV) infection. To our knowledge, the present study revealed the first report of a Toll targeted by miRNA in crustaceans and provided a solid evidence base for supporting the role of LvToll3 in antiviral defense by activating IRF signaling in L. vannamei. Identification of the miR-10c/Toll3/IRF regulatory axis in shrimp provides new insights into the participation of miRNA in the regulation of immune responses and contributes to in-depth understanding of the mechanisms of Toll-induced immune responses in L. vannamei.

Effects of Nrf2-Keap1 signaling pathway on antioxidant defense system and oxidative damage in the clams Ruditapes philippinarum exposure to PAHs

NF-E2-related factor 2 (Nrf2) is a master regulator of antioxidant defense system which can maintain the oxidation balance in the cell. In our previous study, we first cloned the Nrf2 gene in clams and preliminarily explored the role of the Nrf2 at the transcription level. In this study, RNA interference (RNAi) technology was used to interfere with the expression of Nrf2 after being exposed to benzo(a)pyrene (BaP) for 5 days to verify the role of Nrf2 in the antioxidant defense system. Besides, we examined the mRNA expression and enzyme activities of antioxidases and the oxidative damage. The positive correlations between the Nrf2 with the mRNA expression and the enzyme activities of antioxidases indicated that Nrf2 was required for the induction of these antioxidant genes. Additionally, the mRNA expression and the enzyme activities of the glutathione peroxidase (GPx) in the Nrf2-dsRNA group were significantly higher than those in the control groups on the fifth day, indicating that the GPx is more sensitive to oxidative stress. Moreover, the oxidative damage in the RpNrf2-dsRNA group was markedly increased than control groups, indicating that Nrf2 transcriptional regulation may play an essential role in defending against oxidative damage. This study provides a foundation for further research on the mechanism of detoxification and antioxidation of polycyclic aromatic hydrocarbons (PAHs) in the clams at the transcription level and the protein level.

Toll receptor 2 (Toll2) positively regulates antibacterial immunity but promotes white spot syndrome virus (WSSV) infection in shrimp

The Toll family of receptors are a group of conserved pattern recognition receptors (PRRs) essentially controlling the initiation of innate immune responses. The white spot syndrome virus (WSSV) and Vibrio parahaemolyticus are major pathogens of aquaculture shrimp. Previous study has suggested that expression of the Toll2 receptor in Pacific white shrimp Penaeus vannamei was up-regulated by white spot syndrome virus (WSSV) infection but did not significantly changed upon infection with the bacterial pathogen Vibrio parahaemolyticus. The current study intends to investigate the role of P. vannamei Toll2 in antibacterial and antiviral immunity. We demonstrated that compared with the control, the Toll2-silenced shrimp was more susceptible to V. parahaemolyticus infection, suggesting that Toll2 may play a positive role in antibacterial immunity. However, silencing of Toll2 significantly enhanced survivorship of shrimp infected with WSSV and reduced the viral load in shrimp tissues. The expression of WSSV structural protein VP28 was also inhibited in Toll2-silenced shrimp. Histologic pathology analysis further showed that the WSSV infection was attenuated in stomach tissues from Toll2-silenced shrimp. These suggested that Toll2 could promote WSSV infection in shrimp. In Toll2-silenced shrimp, expression of antimicrobial peptides ALFs and PENs was significantly changed, which may contribute to the role of Toll2 in antibacterial immunity and WSSV infection.

The LARK protein is involved in antiviral and antibacterial responses in shrimp by regulating humoral immunity

The LARK proteins containing a C2HC-type zinc finger motif and two RNA recognition motifs are conserved across vertebrates and invertebrates. Previous studies have suggested that invertebrate LARKs and their mammalian counterparts, the RBM4 proteins, regulate gene expression by affecting RNA stability and post-transcriptional processing, participating in multiple life processes. In the current study, the LARK gene from Pacific white shrimp Litopenaeus vannamei was identified and functionally explored in the context of immunity. The LARK protein was mainly present in the nucleus of its expression vector-transfected S2 cells, and the LARK mRNA was detectable in all the tested shrimp tissues. Expression of LARK in gill was up-regulated by immune stimulation with various pathogens. In vivo experiments demonstrated that LARK played positive roles in both antiviral and antibacterial responses and silencing of LARK could make shrimp more susceptible to infection with Vibrio parahaemolyticus and white spot syndrome virus (WSSV). Although silencing of LARK did not affect the phagocytic activity of hemocytes, it regulated expression of many components of the NF-κB and JAK-STAT pathways and a series of immune function proteins. These suggested that LARK could be mainly involved in regulation of humoral immunity. The current study could help reveal the roles of LARK/RBM4 in immunity and further explore the regulatory mechanisms of shrimp immunity.

A double chitin catalytic domain-containing chitinase targeted by c-Jun is involved in immune responses in shrimp

Chitinases are a group of chitin-degrading enzymes widely distributed in organisms. Chitinases containing two chitin catalytic domains have been widely found in arthropods but their functions remain unclear. In this study, a member of these chitinases from Litopenaeus vannamei (dChi) was identified and functionally studied in the context of immunity. The promoter of dChi contained activator protein 1 (AP-1) binding sites and could be regulated by c-Jun. The recombinant dChi protein showed no bacteriostatic activity in vitro but knockdown of dChi in vivo increased the mortality of shrimp and the bacterial load in tissues after Vibrio parahaemolyticus infection, suggesting that dChi could play a positive role in antibacterial responses. However, silencing of dChi expression significantly decreased the mortality of WSSV-infected shrimp and down-regulated the viral load in tissues, indicating that dChi could facilitate WSSV infection. We further demonstrated that dChi was involved in regulation of the bacterial phagocytosis of hemocytes and expression of a series of immune related transcription factors and antimicrobial peptides. These indicated that the roles of dChi in antibacterial responses and anti-WSSV responses in vivo could result from its regulatory effects on the immune system. Taken together, the current study suggests that double chitin catalytic domain-containing chitinases could be important players in immune regulation in crustaceans.

The molecular mechanism of AhR-ARNT-XREs signaling pathway in the detoxification response induced by polycyclic aromatic hydrocarbons (PAHs) in clam Ruditapes philippinarum

The aryl hydrocarbon receptor (AhR) has been known primarily for its role in the regulation of several drug and xenobiotic metabolizing enzymes to mitigate environmental stresses. In this study, we interfere the expression of AhR gene to investigate the mechanism of AhR signaling pathway in the detoxification and antioxidation defense system that induced by Polycyclic Aromatic Hydrocarbons (PAHs) exposure by RNA interference (RNAi). The gene expressions of aryl hydrocarbon receptor nuclear translocator (ARNT), heat shock protein 90 (Hsp90) were evaluated after being exposed to benzo(a)pyrene (BaP) (4 μg/L) for 5 days and the positive correlations between AhR, ARNT, HSP90 indirectly indicating that AhR may have the ability to bind to ligands such as PAHs in Ruditapes philippinarum (R. philippinarum). Besides, the activities of detoxification enzymes were determined to investigate the role of AhR signaling pathway played in the metabolic detoxification. What's more, the gene expressions of protein kinase C (PKC) signaling pathway, mitogen-activated protein kinase (MAPKs) signaling pathway, NF-E2-related factor 2 (Nrf2) signaling pathway and antioxidant defense system indicated that AhR may regulate the Nrf2-Keap1 signaling pathway through Kelch-like ECH-associated protein-1 (Keap1) and MAPKs, PKC signaling pathways. In conclusion, adoption of RNA interference technology to explore the role of RpAhR gene played in the detoxification and antioxidation defense system under the PAHs stress at different time points can informe molecular endpoints for application towards ecotoxicology monitoring of bivalves.

WSV152 induces apoptosis and promotes viral replication in Litopenaeus vannamei

Previous studies have indicated that white spot syndrome virus (WSSV) infection induces apoptosis in many shrimp organs. However, the mechanism by which WSSV causes host apoptosis remains largely unknown. In this study, we demonstrated the function of wsv152, the first mitochondrial protein identified as encoded by WSSV. Glutathione S-transferase pulldown and co-immunoprecipitation analysis revealed that wsv152 interacts with the shrimp mitochondrial protein cytochrome c oxidase 5a (COX5a), a subunit of the COX complex. We also found that wsv152 expression significantly increased the rate of apoptosis, suggesting a role of wsv152 in WSSV-induced apoptosis in shrimp. Knockdown of wsv152 in vivo led to downregulation of several apoptosis-related shrimp genes, including cytochrome c, apoptosis-inducing factor and caspase-3. Suppression of wsv152 also resulted in significant reductions in the number of WSSV genome copies in tissues and in the mortality of WSSV-infected shrimp. Together, these results suggest that wsv152 targets host COX5a and is associated with the expression profiles of apoptosis-related shrimp genes. Wsv152 is likely also involved in WSSV-induced apoptosis, thereby facilitating virus infection and playing a complex role in WSSV pathogenesis.

A shrimp gene encoding a single WAP domain (SWD)-containing protein regulated by JAK-STAT and NF-κB pathways

Regulation of immune responses in animals is largely governed by the JAK-STAT and NF-κB pathways, which are conserved across vertebrates and invertebrates. At present, the relationship between these two pathways in invertebrates remains unclear. In the current study, a novel antimicrobial peptide termed LvSWD5 belonging to the Crustin family was identified from Pacific white shrimp Litopenaeus vannamei. The mature LvSWD5 peptide containing a single WAP domain (SWD) could directly bind bacteria and fungi and inhibit the growth of both Gram-positive and -negative bacteria in vitro. The LvSWD5 promoter was predicted to contain binding sites for STAT and NF-κB and could be regulted by the JAK-STAT and Relish pathways. The expression of LvSWD5 was up-regulated during bacterial, viral and fungal infections and silencing of LvSWD5 in vivo affected the expression of a series of immune related genes and decreased the phagocytic activity of hemocytes against V. Parahaemolyticus. Moreover, the susceptibility of shrimp to V. parahaemolyticus and white spot syndrome virus (WSSV) was significantly increased after silencing of LvSWD5, indicating that LvSWD5 could be involved in antibacterial and antiviral responses. These suggested that the JAK-STAT and NF-κB pathways could converge at the promoter level of a common target gene to regulate the immunity in shrimp.

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 JAK-STAT pathway target gene encoding a single WAP domain (SWD)-containing protein from Litopenaeus vannamei

In shrimp, the JAK-STAT pathway is essentially implicated in both antiviral and antibacterial responses. However, few regulatory target genes of the JAK-STAT pathway in shrimp have been reported so far. In this study, a novel single WAP domain-containing peptide (LvSWD4) was identified from Pacific white shrimp Litopenaeus vannamei. The promoter of LvSWD4 was predicted to harbor multiple STAT-binding DNA motifs. Over-expression of the JAK-STAT pathway components STAT, JAK and Domeless in vitro significantly enhanced the transcriptional activity of the LvSWD4 promoter, and in vivo silencing of STAT and the the JAK-STAT pathway upstream regulator IRF down-regulated the expression of LvSWD4, suggesting that LvSWD4 could be a target gene of the JAK-STAT pathway. The expression of LvSWD4 was significantly increased after infection with Gram-negative and positive bacteria, fungi and virus, and silencing of LvSWD4 increased the susceptibility of shrimp to V. parahaemolyticus and WSSV infections. In vitro experiments also demonstrated that the recombinant LvSWD4 protein had significant inhibitory activities against Gram negative bacteria V. parahaemolyticus and E. coli and Gram positive bacteria S. aureus and B. subtilis. Furthermore, silencing of LvSWD4 in vivo significantly affected expression of various immune functional genes and attenuated the phagocytic activity of hemocytes. These suggested that as a target gene of STAT, LvSWD4 was essentially implicated in shrimp immunity, which could constitute part of the mechanism underlying the immune function of the shrimp JAK-STAT pathway.

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.

Characterization and immune function of the thioredoxin-interacting protein (TXNIP) from Litopenaeus vannamei

The thioredoxin (Trx) system plays essential roles in maintenance and regulation of the redox state of cysteine residues in cellular proteins. The Trx-interacting protein (TXNIP) is a TRX inhibitory protein that works as a negative regulator in the TRX system. The function of TXNIP in invertebrates, in particular in immunity, remains unclear to date. In the current study, a novel TXNIP from Pacific white shrimp Litopenaeus vannamei was identified and characterized and its roles in immune responses was investigated. TXNIP could interact with Trx and inhibit its redox regulatory activity, suggesting that TXNIP was involved in regulation of the cellular redox state in shrimp. The expression of TXNIP was high in the stomach, gill, scape, eyestalk, epithelium, pyloric and muscle and low in the hepatopancreas, intestine, nerve, hemocytes and heart. Stimulations with pathogens white spot syndrome virus (WSSV) and Vibrio parahaemolyticus and immune stimulants poly (I:C) and LPS could significantly increase the expression of TXNIP in vivo. Silencing of TXNIP using RNAi strategy significantly facilitated the infection of V. parahaemolyticus but inhibited the infection of WSSV in shrimp. These indicated that TXNIP could be positively involved in antibacterial responses but negatively involved in antiviral responses in shrimp. Moreover, knockdown of TXNIP in vivo exerted opposite effects on expression of antimicrobial peptides anti-lipopolysaccharide factors and penaeidins and enhanced the phagocytic activity of hemocytes against bacteria. These suggested that TXNIP could play a complex role in regulation of humoral and cellular immune responses in shrimp.

A single C4 Zinc finger-containing protein from Litopenaeus vannamei involved in antibacterial responses

The Zinc finger domains (ZnFs), which contain finger-like protrusions stabilized by zinc ions and function to bind DNA, RNA, protein and lipid substrates, are ubiquitously present in a large number of proteins. In this study, a novel protein containing a single C4 type Znf domain (SZnf) was identified from Pacific white shrimp, Litopenaeus vannamei and its role in immunity was further investigated. The ZnF domain of SZnF but not other regions shared high homology with those of fushi tarazu-factor 1 (FTZ-F1) proteins. The SZnF protein was mainly localized in the cytoplasm and was also present in the nucleus at a small level. SZnF was high expressed in the scape and muscle tissues of healthy shrimp and its expression in gill and heptopancreas was strongly up-regulated during bacterial infection. Silencing of SZnf in vivo could strongly increase the susceptibility of shrimp to infection with Vibrio parahaemolyticus but not white spot syndrome virus (WSSV), suggesting that SZnf could be mainly involved in antibacterial responses. Both dual luciferase reporter assays and real-time PCR analysis demonstrated that SZnf could positively regulate the expression of various antimicrobial peptides in vitro and in vivo, which could be part of the mechanism underlying its antibacterial effects. In summary, the current study could help learn more about the function of ZnF-containing proteins and the regulatory mechanisms of immune responses against pathogen infection in crustaceans.

Identification of the thioredoxin-related protein of 14 kDa (TRP14) from Litopenaeus vannamei and its role in immunity

The thioredoxin system plays essential roles in maintenance and regulation of the redox state of cysteine residues in cellular proteins. The thioredoxin-related protein of 14 kDa (TRP14) is an important member of the TRX superfamily which acts on various substrate proteins, some of which are not overlapped with those of thioredoxin. The knowledge on the function of TRP14 in invertebrates is limited to date. In this study, a TRP14 gene was identified from Pacific white shrimp Litopenaeus vannamei (LvTRP14) and its role in immune responses was investigated. We demonstrated that the expression level of LvTRP14 was high in hepatopancreas and intestine, low in eyestalk, and medium in other tissues of healthy shrimp. The transcription of LvTRP14 in vivo was significantly down-regulated in Relish-silencing shrimp but up-regulated in STAT-silencing shrimp, indicating a complex regulation of LvTRP14 expression. Although the LvTRP14 expression showed little change after immune stimulation with different type of pathogens, knockdown of LvTRP14 expression using RNAi strategy could significantly facilitate the infection of white spot syndrome virus (WSSV) and Vibrio parahaemolyticus in shrimp. Dual luciferase reporter assays demonstrated that LvTRP14 enhanced the transcription factor activity of Relish but attenuated that of Dorsal. Furthermore, silencing of LvTRP14 in vivo had opposite effects on expression of different type of antimicrobial peptides. These suggested that LvTRP14 could play a complex role in shrimp immunity.

A chitinase from pacific white shrimp Litopenaeus vannamei involved in immune regulation

Chitinases are a group of hydrolytic enzymes that hydrolyze chitin and widely exist in organisms. Studies in mammals have demonstrated that chitinases play important roles in regulation of humoral and cellular immune responses. In arthropods, although it is well known that chitinases are involved in growth, molting and development, the current knowledge on the role of chitinases in immunity, especially in immune regulation, remains largely unknown. In this study, a chitinase (LvChi5) from Litopenaeus vannamei was representatively selected for studying its immune function. The start codon of LvChi5 was corrected by 5'RACE analysis and its protein sequence was reanalyzed. LvChi5 contains a catalytic domain and a chitin binding domain and shows no inhibitory effect on growth of bacteria in vitro. However, in vivo experiments demonstrated that silencing of LvChi5 increased the mortality of shrimp infected with white spot syndrome virus (WSSV) and Vibro parahaemolyticus and significantly upregulated the load of pathogens in tissues. The expression of various immune related genes, including transcription factors, antimicrobial peptides and other functional proteins with antibacterial and antiviral activities, was widely changed in LvChi5 silencing shrimp. Moreover, the recombinant LvChi5 protein could enhance the phagocytic activity of hemocytes against bacteria. These suggested that shrimp chitinase could play a role in regulation of both humoral and cellular immune responses in shrimp.

A single WAP domain (SWD)-containing protein with antiviral activity from Pacific white shrimp Litopenaeus vannamei

The single whey acidic protein (WAP) domain (SWD)-containing proteins, also called type III crustins, are a group of antimicrobial peptides (AMPs) in crustaceans. At present, a number of SWDs have been identified in shrimp, which showed essential antibacterial activities. However, the roles of SWDs in antiviral immune responses have not been reported up to now. In this study, a novel SWD (LvSWD3) was identified from Pacific white shrimp, Litopenaeus vannamei, which contained a typical single WAP domain homologous to those of other crustacean SWDs. Although lacking the pro and arg-rich region between the signal peptide and the WAP domain, LvSWD3 was closely clustered with other shrimp SWDs in the phylogenetic tree. Similar to many shrimp SWDs, the highest expression of LvSWD3 was detected in hemocytes. The LvSWD3 expression exhibited only limited changes after challenges with Vibrio parahaemolyticus, Poly (I:C) and lipopolysaccharide, but was significantly up-regulated after white spot syndrome virus (WSSV) infection. Silencing of LvSWDs significantly accelerated the death of the WSSV-infected but not the V. parahaemolyticus-infected shrimp. The recombinant LvSWD3 protein did not show proteinase inhibitory and antibacterial activities but could significantly postpone the death of WSSV-infected shrimp and reduce the viral load in tissues. These suggested that LvSWD3 was a novel SWD with antiviral activity.

A molting-inhibiting hormone-like protein from Pacific white shrimp Litopenaeus vannamei is involved in immune responses

The molting-inhibiting hormones (MIHs) from the crustacean hyperglycemic hormone (CHH) family are a group of neuropeptides that are implicated in regulation of molting and reproduction in crustaceans. In this study, a novel protein containing a typical crustacean neuropeptide domain was identified from Litopenaeus vannamei. The protein showed high homology with other shrimp MIHs and was then designated as a MIH-like protein (MIHL). Among the detected tissues, the heart expressed the highest level of MIHL. The expression of MIHL could be significantly up-regulated after infection with white spot syndrome virus (WSSV), gram-negative bacterium Vibro parahaemolyticus and gram-positive bacterium Staphylococcus aureus, indicating that MIHL could be involved in immune responses. The promoter of MIHL was predicted to contain two NF-κB binding sites and could be regulated by the NF-κB family protein Relish but not Dorsal, suggesting that MIHL could be an effector gene of the IMD/Relish pathway. Silencing of MIHL in vivo by RNAi strategy significantly down-regulated the expression of many immune effector genes and increased the mortalities of shrimp infected by V. parahaemolyticus and WSSV and their copy numbers in tissues. These confirmed that MIHL could play a role in antiviral and antibacterial immune responses in shrimp.

Identification, characterization, and function analysis of the NF-κB repressing factor (NKRF) gene from Litopenaeus vannamei

The NF-κB family transcription factors regulate a wide spectrum of biological processes, in particular immune responses. The studies in human suggest that the NF-κB repressing factor (NKRF) negatively regulates the activity of NF-κB through a direct protein-protein interaction. However, the function of NKRF has not been studied outside mammals up to now. The current study identified a NKRF gene (LvNKRF) from the Pacific white shrimp, Litopenaeus vannamei, which showed homology with NKRFs from insects, fishes and mammals. LvNKRF was high expressed in intestine, stomach and muscle tissues and was localized in the nucleus. LvNKRF could interact with both Dorsal and Relish, the two members of the shrimp NF-κB family. Interestingly, although sharing a similar protein structure with that of human NKRF, LvNKRF showed no inhibitory but instead enhancing effects on activities of Dorsal and Relish, which was contrary to those of mammalian NKRFs. The expression of LvNKRF could not be induced by Gram-positive and -negative bacteria and immunostimulants lipopolysaccharide (LPS) and poly (I:C) but was significantly up-regulated after white spot syndrome virus (WSSV) infection. Silencing of LvNKRF significantly decreased the mortalities of shrimp caused by WSSV infection and down-regulated the WSSV copies and the expression of WSSV structural gene in tissues. These suggested that LvNKRF could facilitate the infection of shrimp by WSSV, which may be an additional strategy for WSSV to hijack the host NF-κB pathway to favor its own replication. The current study could provide a valuable context for further investigating the evolutionary derivation of NKRFs and facilitate the study of regulatory mechanisms of invertebrate NF-κB pathways.

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.

Fatty acid synthase plays a positive role in shrimp immune responses against Vibrio parahaemolyticus infection

Fatty acid synthase (FAS) is an important enzyme that catalyzes the synthesis of fatty acids. In this study, the role of the FAS gene from pacific white shrimp Litopenaeus vannamei (LvFAS) in immune responses against Vibrio parahaemolyticus infection was studied. The expression of LvFAS could be up-regulated upon infection of V. parahaemolyticus and stimulation of lipopolysaccharide and poly (I:C). The promoter of LvFAS was predicted to harbor a NF-κB binding site and dual-luciferase reporter assays demonstrated that the NF-κB family proteins Relish, sRelish and Dorsal could activate the transcription of LvFAS. After knockdown of LvFAS expression using RNAi strategy, both the mortality of V. parahaemolyticus infected shrimps and the bacterial load in shrimp tissues were significantly increased. Meanwhile, the expression of many immune-responsive genes, such as antimicrobial peptides, C-type lectins (CTLs), lysozyme and hemolin, was down-regulated. These suggested that LvFAS could play a positive role in anti-V. parahaemolyticus responses in shrimp. To our knowledge, this is the first study that investigates the role of FAS in antibacterial immunity in animals, which may indicate the relationship between the anabolism of fatty acids and immune responses in crustaceans.

The Dorsal/miR-1959/Cactus feedback loop facilitates the infection of WSSV in Litopenaeus vannamei

miR-1959, a novel microRNA identified from Litopenaeus vannamei, mediates a positive feedback loop between Dorsal and Cactus that can continuously maintain the activation of the NF-κB pathway. It has been known that miR-1959 is involved in antibacterial immunity in shrimp, but its function in antiviral responses is still unknown. In this study, we focused on the role of miR-1959 in infection of white spot syndrome virus (WSSV), the major viral pathogen in shrimp worldwide. The expression of miR-1959 in shrimp hemocytes, gill, and hepatopancreas was significantly up-regulated upon WSSV infection. Dual-luciferase reporter assays demonstrated that miR-1959 could enhance the activity of the promoter of WSSV immediate early gene ie1. In vivo experiments also showed that inhibition of miR-1959 led to decrease of the mortality of WSSV-infected shrimp and the genome copies of WSSV in tissues, meanwhile the expression of WSSV ie1 and VP28 genes was down-regulated. In contrast, increase of the miR-1959 level in shrimp by injection of miR-1959 mimics produced opposite results. These suggested that the Dorsal/miR-1959/Cactus feedback loop could favor the infection of WSSV in shrimp. Thus, our study helps further reveal the interaction between WSSV and shrimp immune system.