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

Identification of an Ortholog of MALT1 from Shrimp That Induces NF-κB-Mediated Antiviral Immunity

MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1) serves as a pivotal mediator for NF-κB activation in response to a wide spectrum of transmembrane receptor stimuli. In the present study, a homolog of MALT1, named LvMALT1, is cloned from the Pacific white shrimp (Litopenaeus vannamei) and its potential function in shrimp innate immunity is explored. The open reading frame of LvMALT1 is 2364 bp that encodes 787 amino acids. The predicted LvMALT1 protein structure comprises a death domain, three immunoglobulin domains, and a caspase-like domain, exhibiting remarkable similarity to other homologs. LvMALT1 is a cytoplasmic-localized protein and could interact with LvTRAF6. Overexpression of LvMALT1 induces the activation of promoter elements governing the expression of several key antimicrobial peptides (AMPs), including penaeidins (PENs) and crustins (CRUs). Conversely, silencing of LvMALT1 leads to a reduction in the phosphorylation levels of Dorsal and Relish, along with a concomitant decline in the in vivo expression levels of multiple AMPs. Furthermore, LvMALT1 is prominently upregulated in response to a challenge by the white spot syndrome virus (WSSV), facilitating the NF-κB-mediated expression of AMPs as a defense against viral infection. Taken together, we identified a MALT1 homolog from the shrimp L. vannamei, which plays a positive role in the TRAF6/NF-κB/AMPs axis-mediated innate immunity.

Pellino Proteins in Viral Immunity and Pathogenesis

Pellino proteins are a family of evolutionarily conserved ubiquitin ligases involved in intracellular signaling in a wide range of cell types. They are essential for microbe detection and the initiation of innate and adaptive immune responses. Some viruses specifically target the Pellino proteins as part of their immune evasion strategies. Through studies of mouse models of viral infections in the central nervous system, heart, lungs, and skin, the Pellino proteins have been linked to both beneficial and detrimental immune responses. Only in recent years have some of the involved mechanisms been identified. The objective of this review is to highlight the many diverse aspects of viral immunity and pathogenesis that the Pellino proteins have been associated with, in order to promote further research into their functions. After a brief introduction to the cellular signaling mechanisms involving Pellino proteins, their physiological roles in the initiation of immune responses, pathogenesis through excess inflammation, immune regulation, and cell death are presented. Known viral immune evasion strategies are also described. Throughout, areas that require more in-depth investigation are identified. Future research into the functions of the Pellino protein family may reveal fundamental insights into how our immune system works. Such knowledge may be leveraged in the fight against viral infections and their sequala.

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 new SVWC protein functions as a pattern recognition protein in antibacterial responses in Chinese mitten crab (Eriocheirsinensis)

Because invertebrates lack acquired immunity, they rely primarily on the innate immune system to defend themselves against viral and bacterial infections. SVWC, also called Vago, is a class of small-molecule proteins characterized by a single von Willebrand factor C-domain and appears to be restricted to arthropods. It has been reported that SVWC is involved in antiviral immunity in invertebrates, but whether it is involved in antimicrobial immunity and the mechanism of its involvement in antimicrobial immunity remains unclear. In this study, we identified a novel SVWC gene in Eriocheir sinensis and named it EsSVWC. EsSVWC was found to respond positively to bacterial stimulation and to regulate the expression of related antimicrobial peptides (AMPs). The EsSVWC protein recognized and bound to a variety of pathogen-associated molecular patterns (PAMPs) but did not exhibit direct bactericidal effects. Thus, the EsSVWC protein in crabs helps resist bacterial infection and improve survival rates. In summary, EsSVWC may regulate the innate immune system of crabs in response to microbial invasion in an indirect manner.

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.

Single-cell RNA-seq analysis reveals penaeid shrimp hemocyte subpopulations and cell differentiation process

Crustacean aquaculture is expected to be a major source of fishery commodities in the near future. Hemocytes are key players of the immune system in shrimps; however, their classification, maturation, and differentiation are still under debate. To date, only discrete and inconsistent information on the classification of shrimp hemocytes has been reported, showing that the morphological characteristics are not sufficient to resolve their actual roles. Our present study using single-cell RNA sequencing revealed six types of hemocytes of Marsupenaeus japonicus based on their transcriptional profiles. We identified markers of each subpopulation and predicted the differentiation pathways involved in their maturation. We also predicted cell growth factors that might play crucial roles in hemocyte differentiation. Different immune roles among these subpopulations were suggested from the analysis of differentially expressed immune-related genes. These results provide a unified classification of shrimp hemocytes, which improves the understanding of its immune system.

Ubiquitination as an Important Host-Immune Response Strategy in Penaeid Shrimp: Inferences From Other Species

Shrimp aquaculture is an essential economic venture globally, but the industry faces numerous challenges, especially pathogenic infections. As invertebrates, shrimp rely mainly on their innate immune system for protection. An increasing number of studies have shown that ubiquitination plays a vital role in the innate immune response to microbial pathogens. As an important form of posttranslational modification (PTM), both hosts and pathogens have exploited ubiquitination and the ubiquitin system as an immune response strategy to outwit the other. This short review brings together recent findings on ubiquitination and how this PTM plays a critical role in immune modulation in penaeid shrimps. Key findings inferred from other species would help guide further studies on ubiquitination as an immune response strategy in shrimp-pathogen interactions.

Comprehensive expression analysis of the beta integrin from Penaeus monodon indicating its participation in innate immunity and ammonia nitrogen stress response

The aim of the present study was to investigate the function of the beta integrin (PmItgb) in Penaeus monodon. The 3011 bp cDNA sequence of PmItgb was cloned from P. monodon using rapid amplification of cDNA ends (RACE) PCR. Phylogenetic tree analyses indicated that the amino acid sequence of PmItgb should be merged into Fenneropenaeus chinensis (93%). Quantitative real-time PCR (q RT-PCR) revealed that PmItgb mRNA was highly expressed in the hemocytes. In addition, with regard to developmental stages, PmItgb showed significantly higher expression in oosperm, nauplius IV, zoea I and III, and post larval stages than that in other development stages. PmItgb expression in the shrimp epidermis was higher in the postmolt (B) stage, and lower in other molting stages. We also found that Vibrio harveyi and V. anguillarum challenge enhanced PmItgb expression in the hepatopancreas and gills. When PmItgb was inhibited, innate immunity-related genes such as ALF, crustin 1, crustin 7, penaeidin 3, and penaeidin 5 were significantly down-regulated. Furthermore, we demonstrated that PmItgb knock-down by specific dsRNA reduced bacterial clearance. In high ammonia nitrogen concentrations, PmItgb was significantly up-regulated in the hepatopancreas and gills. After PmItgb was silenced, the rate of mortality owing to high ammonia nitrogen concentrations decreased; the expression of related anti-apoptotic genes was up-regulated, and that of the apoptotic genes was slightly down-regulated. These results suggested that PmItgb may be involved in shrimp innate immunity and mediate apoptosis of hepatopancreatic cells induced by high ammonia nitrogen environments.

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

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

LvPPAE2 induced by WSV056 confers host defense against WSSV in Litopenaeus vannamei

Viral immediate early (IE) genes encode regulatory proteins that are critical for viral replication. WSV056 is an IE protein of white spot syndrome virus (WSSV), an important pathogen of farmed shrimp. It targets the host Rb protein(s) and, according to a previous study, may enhance the replication of the viral genome. However, the ectopic expression of WSV056 in transgenic Drosophila melanogaster exerted an inhibitory effect on the replication of Drosophila C virus (DCV). Transcriptome study using Affymetrix GeneChip suggested that the enrichment of serine proteases (SPs) likely accounts for DCV inhibition in WSV056-overexpressing Drosophila. Injection of recombinant WSV056 to the WSSV natural host Litopenaeus vannamei enhanced the expression of the SP family member prophenoloxidase-activating enzyme 2 (LvPPAE2) and conferred shrimp with more resistance to WSSV infection. LvPPAE2 knockdown contributed to decreased expression of antimicrobial peptides LvAlf1 and LvLyz1, reduced hemolymph phenoloxidase activity, and increased virus load, suggesting that LvPPAE2 is involved in the host defense against WSSV infection. Taken together, these results suggest that wsv056 plays a role in restricting viral replication by inducing the SP-mediated immune responses in the host.

A novel RING finger protein CqRNF152-like with self-ubiquitination activity inhibits white spot syndrome virus infection in a crustacean Cherax quadricarinatus

Really Interesting New Gene (RING) finger proteins are highly conserved molecules that participate in a variety of biological processes such as regulation of development, apoptosis and antiviral immunity in vertebrates. However, the functions of RING finger proteins are still poorly understood in crustaceans. Previously, we found that the transcript of a homolog of RING finger protein 152 (CqRNF152-like) was up-regulated in a differentially expressed transcriptome library of the haematopietic tissue (Hpt) cells from red claw crayfish Cherax quadricarinatus upon white spot syndrome virus (WSSV) infection, which is one of the most devastating viral diseases for crustaceans like shrimp and crayfish. The full-length cDNA sequence of CqRNF152-like was then identified with 975 bp, including an ORF of 685 bp that encoded a 195 amino acids protein, a 5'- UTR of 180 bp, and a 3'-UTR with a poly (A) tail of 207 bp. The conserved domain prediction showed that CqRNF152-like contained a conserved RING-finger domain. Gene expression analysis showed that CqRNF152-like was distributed in all tissues examined and the transcript is significantly up-regulated after WSSV challenge both in vivo in Hpt tissue and in vitro in cultured Hpt cells. Furthermore, the transcripts of both an immediate early gene ie1 and a late envelope protein gene vp28 of WSSV were clearly increased in the Hpt tissues, hemocytes and cultured Hpt cells after gene silencing of CqRNF152-like, which were further proved to be significantly decreased after overloading of recombinant CqRNF152-like protein in Hpt cell cultures. Meanwhile, CqRNF152-like was found to bind with WSSV envelope protein VP28 by proteins pull-down assay. Similar to most of RNF proteins, CqRNF152-like protein sequence contained a conserved RING-finger domain and showed self-ubiquitination activity in a RING finger domain dependent manner. Taken together, CqRNF152-like is likely to function as an antiviral molecular against WSSV infection through interaction with the envelope protein VP28 in a crustacean C. quadricarinatus. This is the first report that a RING finger protein with directly antiviral functions via interaction with viral protein and self-ubiquitination activity in crustacean, which sheds new light on the molecular mechanism of WSSV infection and the control of white spot disease.

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.

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.

Molecular cloning and functional characterization of a homolog of the transcriptional regulator CSL in Litopenaeus vannamei

The Notch signaling pathway transcriptional regulator, CSL (also called as CBF1, Suppressor of Hairless or Lag-1 in different species, generally designated as CSL1), is not only associated with cell proliferation and differentiation but also involved in tumorigenesis, inflammation and immune regulation in vertebrates. We recently showed that Notch signaling was involved in the immune response of Litopenaeus vannamei shrimp. However, as an important transcriptional regulator of this pathway, whether or not shrimp CSL was also involved in immune response had not been explored. Here, we cloned and characterized the CSL gene in L. vannamei (LvCSL), which has a 2271 bp open reading frame (ORF) encoding a putative protein of 756 amino acids, and contains two conserved Lag1-DNA bind as well as beta trefoil domains (BTD). LvCSL clustered with invertebrates in the phylogenetic tree and closely related to the RBP Jk X1 of Parasteatoda tepidariorum. The transcript level of LvCSL analyzed by quantitative polymerase chain reaction (qPCR) showed that LvCSL was widely expressed in all tissues tested, with induced levels observed in the hepatopancreas and hemocytes following immune challenge with Vibrio parahaemolyticus, Streptoccocus iniae, lipopolysaccharide (LPS), and white spot syndrome virus (WSSV), therefore, suggesting LvCSL involvement in shrimp immune response to pathogens. Besides, LvCSL knockdown decreased the expression of proliferation-related genes (LvHey2 and LvAstakine), and attenuated the expression of immune-related genes L. vannamei hypoxia inducible factor alpha (LvHIF-α), LvLectin and L. vannamei small subunit hemocyanin (LvHMCS) in shrimp hemocytes, as well as significantly decreased total hemocyte count. Moreover, high cumulative mortality was observed in LvCSL depleted shrimp challenged with V. parahaemoliticus. In conclusion, our present data strongly suggest that LvCSL is an important factor in shrimp, vital for shrimp survival and contributing to immune resistance to pathogens.

Transcriptome analysis of growth heterosis in pearl oyster Pinctada fucata martensii

Heterosis improves growth and survival of shellfish species. Although breeders have widely exploited heterosis, its underlying molecular mechanisms remain unclear. In this study, a 2 × 2 complete diallel cross was facilitated between two full-sib families to produce two inbred families (A and D) and their reciprocal hybrid families (B and C) of pearl oyster Pinctada fucata martensii. Growth traits of the four families were compared at the adult stages. Transcriptome analysis was conducted on the four families using an Illumina sequencing platform. The results revealed that the growth traits of the four families significantly varied (P < 0.05). The mid-parent heterosis values of shell length, shell height, shell width, shell weight, and total weight were 12.9%, 14.9%, 18.2%, 17.2%, and 33.2%, respectively. The B- and C-inbred (A and D) triads had 79 and 68 differentially expressed genes (DEGs), respectively, which were dominantly nonadditive, including overdominance, underdominance, and low-parent dominance. Gene ontology term analysis showed that the DEGs in the B- and C-inbred triads were enriched for metabolic process, cellular process cell part, binding, and catalytic activity. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis indicated that the DEGs in the B- and C-inbred triads were involved in focal adhesion, the P13K-Akt signaling pathway, the mRNA surveillance pathway, and the focal adhesion pathway. The reliability of the sequencing data was confirmed by real-time polymerase chain reaction analysis of six growth-related genes. The findings of this study provide new insights into heterosis for growth traits and the design of genetic breeding programs for this species.

Silencing of a Kazal-type serine proteinase inhibitor SPIPm2 from Penaeus monodon affects YHV susceptibility and hemocyte homeostasis

In shrimp, the Kazal-type serine proteinase inhibitors (KPIs) are involved in host innate immune defense system against pathogenic microorganisms. A five-Kazal-domain SPIPm2 is the most abundant KPIs in the black tiger shrimp Penaeus monodon and up-regulated in response to yellow head virus (YHV) infection. In this study, the role of SPIPm2 in YHV infection was investigated. The expression of SPIPm2 in hemocytes, gill and heart from 48-h YHV-infected shrimp was increased. The expression of SPIPm2 in hemocytes was significantly increased after 12 h of infection and gradually increased higher afterwards. Silencing of SPIPm2 by dsRNA interference resulted in the increased expression of different apoptosis-related genes, the increased expression of transcriptional factors of antimicrobial synthesis pathways, the reduction of circulating hemocytes in the shrimp hemolymph, and the increased susceptibility of the silenced shrimp to YHV infection. The activities of caspase-3 and caspase-7 in the hemocytes of SPIPm2-silenced shrimp was also increased by 5.32-fold as compared with those of the control shrimp. The results suggested that the SPIPm2 was involved in the hemocyte homeostasis.

Toll-Like Receptors, Associated Biological Roles, and Signaling Networks in Non-Mammals

The innate immune system is the first line of defense against pathogens, which is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs). Among all the PRRs identified, the toll-like receptors (TLRs) are the most ancient class, with the most extensive spectrum of pathogen recognition. Since the first discovery of Toll in Drosophila melanogaster, numerous TLRs have been identified across a wide range of invertebrate and vertebrate species. It seems that TLRs, the signaling pathways that they initiate, or related adaptor proteins are essentially conserved in a wide variety of organisms, from Porifera to mammals. Molecular structure analysis indicates that most TLR homologs share similar domain patterns and that some vital participants of TLR signaling co-evolved with TLRs themselves. However, functional specification and emergence of new signaling pathways, as well as adaptors, did occur during evolution. In addition, ambiguities and gaps in knowledge still exist regarding the TLR network, especially in lower organisms. Hence, a systematic review from the comparative angle regarding this tremendous signaling system and the scenario of evolutionary pattern across Animalia is needed. In the current review, we present overview and possible evolutionary patterns of TLRs in non-mammals, hoping that this will provide clues for further investigations in this field.

Shrimp humoral responses against pathogens: antimicrobial peptides and melanization

Diseases have caused tremendous economic losses and become the major problem threatening the sustainable development of shrimp aquaculture. The knowledge of host defense mechanisms against invading pathogens is essential for the implementation of efficient strategies to prevent disease outbreaks. Like other invertebrates, shrimp rely on the innate immune system to defend themselves against a range of microbes by recognizing and destroying them through cellular and humoral immune responses. Detection of microbial pathogens triggers the signal transduction pathways including the NF-κB signaling, Toll and Imd pathways, resulting in the activation of genes involved in host defense responses. In this review, we update the discovery of components of the Toll and Imd pathways in shrimp and their participation in the regulation of shrimp antimicrobial peptide (AMP) synthesis. We also focus on a recent progress on the two most powerful and the best-studied shrimp humoral responses: AMPs and melanization. Shrimp AMPs are mainly cationic peptides with sequence diversity which endues them the broad range of activities against microorganisms. Melanization, regulated by the prophenoloxidase activating cascade, also plays a crucial role in killing and sequestration of invading pathogens. The progress and emerging research on mechanisms and functional characterization of components of these two indispensable humoral responses in shrimp immunity are summarized and discussed. Interestingly, the pattern recognition protein (PRP) crosstalk is evidenced between the proPO activating cascade and the AMP synthesis pathways in shrimp, which enables the innate immune system to build up efficient immune responses.

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.

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

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

Identification of two p53 isoforms from Litopenaeus vannamei and their interaction with NF-κB to induce distinct immune response

p53 is a transcription factor with capability of regulating diverse NF-κB dependent biological progresses such as inflammation and host defense, but the actual mechanism remains unrevealed. Herein, we firstly identified two novel alternatively spliced isoforms of p53 from Litopenaeus vannamei (LvΔNp53 and the full-length of p53, LvFLp53). We then established that the two p53 isoforms exerted opposite effects on regulating NF-κB induced antimicrobial peptides (AMPs) and white spot syndrome virus (WSSV) immediate-early (IE) genes expression, suggesting there could be a crosstalk between p53 and NF-κB pathways. Of note, both of the two p53 isoforms could interact directly with LvDorsal, a shrimp homolog of NF-κB. In addition, the activation of NF-κB mediated by LvDorsal was provoked by LvΔNp53 but suppressed by LvFLp53, and the increased NF-κB activity conferred by LvΔNp53 can be attenuated by LvFLp53. Furthermore, silencing of LvFLp53 in shrimp caused higher mortalities and virus loads under WSSV infection, whereas LvΔNp53-knockdown shrimps exhibited an opposed RNAi phenotype. Taken together, these findings present here provided some novel insight into different roles of shrimp p53 isoforms in immune response, and some information for us to understand the regulatory crosstalk between p53 pathway and NF-κB pathway in invertebrates.

Characterization and function analysis of interleukin-1 receptor-associated kinase-1 (IRAK-1) from Fenneropenaeus penicillatus

The interleukin-1 receptor-associated kinase-1 (IRAK-1) is an important adapter protein which links downstream of MyD88, and involved in the complex composed of MyD88 and TRAF6 to activate TLRs signaling pathway. In this study, an IRAK-1 homolog (FpIRAK-1) was cloned from the red tail shrimp Fenneropenaeus penicillatus. The ORF of FpIRAK-1 consisted of 2874 bp encoding a protein of 957 amino acids which contains a death domain (DD) and a catalytic domain of serine/threonine kinases (STKc). Homology analysis revealed that the predicted amino acid sequence of FpIRAK-1 shared 71% similarities with IRAK-1 of Litopenaeus vannamei. Real-time RT-PCR indicated that FpIRAK-1 was constitutively expressed in various tissues of F. penicillatus. The expression level of FpIRAK-1 mRNA was significantly up-regulated and then decreased gradually after white spot syndrome virus (WSSV) and Vibrio alginolyticus challenge. Gene knockdown of FpIRAK-1 enhanced the sensitivity of shrimps to WSSV and V. alginolyticus challenge, suggesting FpIRAK-1 could play a positive role against bacterial and viral pathogens. In conclusion, the results of this study provide some insights into the function of FpIRAK-1 in activating Toll signaling pathway and the host defense against invading pathogens.

Tumor necrosis factor receptor-associated factor 6 (TRAF6) participates in anti-lipopolysaccharide factors (ALFs) gene expression in mud crab

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a key cytoplasm signal adaptor that mediates signals activated by tumor necrosis factor receptor (TNFR) superfamily and the Interleukin-1 receptor/Toll-like receptor (IL-1/TLR) superfamily. The full-length 2492 bp TRAF6 (Sp-TRAF6) from Scylla paramamosain contains 1800 bp of open reading frame (ORF) encoding 598 amino acids, including an N-terminal RING-type zinc finger, two TRAF-type zinc fingers and a conserved C-terminal meprin and TRAF homology (MATH) domain. Multiple alignment analysis shows that the putative amino acid sequence of Sp-TRAf6 has highest identity of 88% with Pt-TRAF6 from Portunus trituberculatus, while the similarity of Sp-TRAF6 with other crustacean sequences was 54-55%. RT-PCR analysis indicated that Sp-TRAF6 transcripts were predominantly expressed in the hepatopancreas and stomach, whereas it was barely detected in the heart and hemocytes in our study. Moreover, Sp-TRAF6 transcripts were significantly up-regulated after Vibrio parahemolyticus and LPS challenges. RNA interference assay was carried out used by siRNA to investigate the genes expression patterns regulated by Sp-TRAF6. The qRT-PCR results showed that silencing Sp-TRAF6 gene could inhibit SpALF1, SpALF2, SpALF5 and SpALF6 expression in hemocytes, while inhibit SpALF1, SpALF3, SpALF4, SpALF5 and SpALF6 expression in hepatopancreas. Taken together, the acute-phase response to immune challenges and the inhibition of SpALFs gene expression indicate that Sp-TRAF6 plays an important role in host defense against pathogen invasions via regulation of ALF gene expression in S. paramamosain.

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.

Toll receptor response to white spot syndrome virus challenge in giant freshwater prawns (Macrobrachium rosenbergii)

Toll receptors are evolutionary ancient families of pattern recognition receptors with crucial roles in invertebrate innate immune response. In this study, we identified a Toll receptor (MrToll) from giant freshwater prawns (Macrobrachium rosenbergii). The full-length cDNA of MrToll is 4257 bp, which encodes a putative protein of 1367 amino acids. MrToll contains 17 LRR domains, a transmembrane domain, and a TIR domain. Phylogenetic analysis showed that MrToll was grouped with Drosophila Toll7 and other arthropod Tolls. The transcripts of MrToll are mainly distributed in the heart, hepatopancreas, gills, stomach, and intestine. A low level of MrToll expression can be detected in hemocytes and the lymphoid organ. MrToll expression in gills was gradually upregulated to the highest level from 24 h to 48 h during the white spot syndrome virus (WSSV) challenge. The expression levels of the crustin (Cru) genes Cru3 and Cru7 in gills were relatively lower than those of Cru2 and Cru4. The expression levels of Cru3 and Cru7 were inhibited after the RNA interference of MrToll in gills during the WSSV challenge. The anti-lipopolysaccharide factor (ALF) genes ALF2, ALF3, ALF4, and ALF5 were also regulated by MrToll in gills during the virus challenge. These findings suggest that MrToll may contribute to the innate immune defense of M. rosenbergii against WSSV.

Pellino-1 derived cationic antimicrobial prawn peptide: Bactericidal activity, toxicity and mode of action

The antimicrobial peptides (AMPs) are multifunctional molecules which represent significant roles in the innate immune system. These molecules have been well known for decades because of their role as natural antibiotics in both invertebrates and vertebrates. The development of multiple drug resistance against conventional antibiotics brought a greater focus on AMPs in recent years. The cationic peptides, in particular, proven as host defense peptides and are considered as effectors of innate immunity. Among the various innate immune molecules, functions of pellino-1 (Peli-1) have been recently studied for its remarkable role in specific immune functions. In our study, we have identified Peli-1 from the cDNA library of freshwater prawn Macrobrachium rosenbergii (Mr) and analyzed its features using various in-silico methods. Real time PCR analysis showed an induced expression of MrPeli-1 during white spot syndrome virus (WSSV), bacteria (Vibrio harveyi) and lipopolysaccharide (LPS) from Escherichia coli challenge. Also, a cationic AMP named MrDN was derived from MrPeli-1 protein sequence and its activity was confirmed against various pathogenic bacteria. The mode of action of MrDN was determined to be its membrane permeabilization ability against Bacillus cereus ATCC 2106 as well as its DNA binding ability. Further, scanning electron microscopic (SEM) images showed the membrane disruption and leakage of cellular components of B. cereus cells induced by MrDN. The toxicity of MrDN against normal cells (HEK293 cells) was demonstrated by MTT and hemolysis assays. Overall, the results demonstrated the innate immune function of MrPeli-1 with a potential cationic AMP in prawn.

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

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

Identification and functional characterization of the TAB2 gene from Litopenaeus vannamei

In Drosophila, TAB2, an important intermediate in the IMD signaling pathway, plays critical roles in the innate immune response in response to bacterial and viral infection. However, the role of TAB-related proteins in the immune response of shrimp has not yet been established. Here, we reported the identification of a TAB2-like gene in Litopenaeus vannamei designated as LvTAB2. The full-length cDNA of LvTAB2 was 2160 bp with an open reading frame of 1827 bp, which encoded a putative protein of 608 amino acids including a ubiquitin binding domain (CUE) at the N-terminal and a Zinc Finger domain (ZnF) at the C-terminus. Real-time RT-PCR analysis showed that LvTAB2 was expressed in all tested tissues and the expression levels of LvTAB2 in gills and hemocytes were positively induced in response to LPS, Vibrio parahemolyticus and White Spot Syndrome Virus (WSSV) challenges. Dual luciferase reporter assays demonstrated that LvTAB2 was able to induce the expression of antimicrobial peptide (AMP) genes, including Drosophila Attacin A and shrimp Penaeidins. Interestingly, over-expression of LvTAB2 could up-regulate the promoter activities of L. vannamei Vago1, Vago3 and Vago4 genes in S2 cells. To our knowledge, it was the first report that TAB2 participated in innate immune signaling to regulate the expression of Vago genes in invertebrates. Moreover, RNAi-mediated knockdown of LvTAB2 enhanced sensitivity of L. vannamei to Vibrio parahaemolyticus infection and caused elevated virus loads after WSSV infection. We suggested that the LvTAB2 may play important roles in the shrimp innate immunity.

Daxx from Pacific white shrimp Litopenaeus vannamei is involved in activation of NF-κB pathway

Death domain-associated factor 6 (Daxx) is a Fas-binding protein that mediates the activation of Jun amino-terminal kinase (JNK) pathway and Fas-induced apoptosis. In this study, a crustacean Daxx (LvDaxx) was firstly cloned and identified from Pacific white shrimp Litopenaeus vannamei. The LvDaxx cDNA was 2644 bp in length with an Open Reading Frame (ORF) of 2217 bp. Sequence analysis indicated that LvDaxx contained a single Daxx domain and two nuclear localization signals (NLSs) and shared a similarity with Drosophila melanogaster Daxx. LvDaxx was a nuclear-localized protein that was expressed highest in hemocytes and could be up-regulated in pathogen- and stimulant-challenge shrimps. LvDaxx could activate the artificial promoter containing an NF-κB binding site and the promoters of white spot syndrome virus (WSSV) ie1 gene and arthropod antimicrobial peptides (AMPs), suggesting LvDaxx could be involved in the activation of the NF-κB pathway. Knock-down of LvDaxx in vivo resulted in down-regulation of shrimp AMPs and reduction of WSSV copies in tissues. Furthermore, suppression of LvDaxx significantly decreased the mortality of WSSV-infected shrimps, but increased the mortality of Vibrio Parahaemolyticus-infected shrimps. Thus, these suggested that LvDaxx could play a role in the innate immunity against Vibrio parahaemolyticus in L. vannamei, while in the antiviral response, LvDaxx may be hijacked by WSSV and play a complex role in WSSV pathogenesis.

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

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

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

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

Flightless-I (FliI) is a potential negative regulator of the Toll pathway in Litopenaeus vannamei

Flightless-I (FliI) is a protein negatively modulates the Toll-like receptor (TLR) pathway through interacting with Myeloid differentiation factor 88 (MyD88). To investigate the function of FliI in innate immune responses in invertebrates, Litopenaeus vannamei FliI (LvFliI) was identified and characterized. The full-length cDNA of LvFliI is 4, 304 bp long, with an open reading frame (ORF) encoding a putative protein of 1292 amino acids, including 12 leucine-rich repeat (LRR) domains at the N-terminus and 6 gelsolin homology (GEL) domains at the C-terminus. The LvFliI protein was located in the cytoplasm and LvFliI mRNA was constitutively expressed in healthy L. vannamei, with the highest expression level in the muscle. LvFliI could be up-regulated in hemocytes after lipopolysaccharide (LPS), poly I:C, CpG-ODN2006, Vibrio parahaemolyticus, Staphylococcus aureus, and white spot syndrome virus (WSSV) challenges, suggesting a stimulation response of LvFliI to bacterial and immune stimulant challenges. Upon LPS stimulation, overexpression of LvFliI in Drosophila Schneider 2 cells led to downregulation of Drosophila and shrimp antimicrobial peptide (AMP) genes. Knockdown of LvFliI by RNA interference (RNAi) resulted in an increase of the expression of three shrimp AMP genes (PEN2, crustin, and Lyz1). However, the mortality rates of LvFliI-knockdown shrimp in response to V. parahaemolyticus, S. aureus or WSSV infections were not significantly different from those of the control group. Taken together, all the results suggested that LvFliI may play a negative role in TLR signaling response in L. vannamei.

Molecular characterization and functional analysis of Cactin gene from Litopenaeus vannamei

Cactin (Cactus interactor) is a conserved protein which was initially discovered as a novel interactor of Drosophila IkB protein Cactus. Cactin was always characterized as a negative regulator of many different developmental processes, but only found to play an immune role in humans. To better know the immune function of Cactin gene, Litopenaeus vannamei Cactin (LvCactin) was identified and characterized in this study. The full-length cDNA of LvCactin is 3, 150 bp long, with an open reading frame (ORF) encoding a Cactin_mid domain in the N-terminus 356–547 residues and a CactinC_cactus domain in the C-terminal 731–855 residues. The LvCactin protein was located in the cytoplasm and LvCactin mRNA was constitutively expressed in healthy L. vannamei, with the highest expression level in the eyestalk. LvCactin could be regulated in hemocytes after lipopolysaccharide (LPS), poly I:C, CpG-ODN2006, Vibrio parahaemolyticus, Staphylococcus aureus, and white spot syndrome virus (WSSV) challenges. Dual-luciferase reporter assays in Drosophila Schneider 2 cells revealed that LvCactin inhibited the promoters of Drosophila and shrimp antimicrobial peptide (AMP) genes. Knockdown of LvCactin by RNA interference (RNAi) increased the expression of shrimp AMP genes PEN4, crustin and ALF2 but not Lyz2. However, the mortality rates of LvCactin-knockdown shrimp in response to V. parahaemolyticus or WSSV infections were not significantly different from those of the control group. Taken together, all the results suggested that LvCactin may play a role in innate immune in L. vannamei.