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

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

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

Identification of toll-like receptor family and the immune function of new Sptlr-6 gene of Scylla paramamosain

As a crucial member of pattern-recognition receptors (PRRs), the Tolls/Toll-like receptors (TLRs) gene family has been proven to be involved in innate immunity in crustaceans. In this study, nine members of TLR gene family were identified from the mud crab (Scylla paramamosain) transcriptome, and the structure and phylogeny of different SpTLRs were analyzed. It was found that different SpTLRs possessed three conserved structures in the TIR domain. Meanwhile, the expression patterns of different Sptlr genes in examined tissues detected by qRT-PCR had wide differences. Compared with other Sptlr genes, Sptlr-6 gene was significantly highly expressed in the hepatopancreas and less expressed in other tissues. Therefore, the function of Sptlr-6 was further investigated. The expression of the Sptlr-6 gene was up-regulated by Poly I: C, PGN stimulation and Vibrio parahaemolyticus infection. In addition, the silencing of Sptlr-6 in hepatopancreas mediated by RNAi technology resulted in the significant decrease of several conserved genes involved in innate immunity in mud crab after V. parahaemolyticus infection, including relish, myd88, dorsal, anti-lipopolysaccharide factor (ALF), anti-lipopolysaccharide factor 2 (ALF-2) and glycine-rich antimicrobial peptide (glyamp). This study provided new knowledge for the role of the Sptlr-6 gene in defense against V. parahaemolyticus infection in S. paramamosain.

Diversity of MrTolls and their regulation of antimicrobial peptides expression during Enterobacter cloacae infection in Macrobrachium rosenbergii

Toll/Toll-like receptor (TLR) is an important pattern recognition receptor that plays an important role in the immunity of animals. Six Toll genes were identified in Macrobrachium rosenbergii, namely, MrToll, MrToll1, MrToll2, MrToll3, MrToll4, and MrToll5. SMART analysis showed that all six Tolls have a transmembrane domain, a TIR domain, and different number of LRR domains. The phylogenetic tree showed that six Tolls were located in six different branches. Among these six Tolls, only MrToll4 contains the QHR motif, which is similar to insect Toll9. MrToll4 belongs to V-type/scc Toll with only one LRRCT domain. MrToll1 and MrToll5 are classical P-type/mcc Toll with two LRRCT domains and an LRRNT. MrTolls were distributed in the hemocytes, heart, hepatopancreas, gills, stomach, and intestine. During the infection of Enterobacter cloacae, the expression level of MrToll and MrToll1-4 was upregulated in the intestine of M. rosenbergii. RNA interference experiments showed that the expression of most antimicrobial peptide (AMP) genes was negatively regulated by MrTolls during E. cloacae infection. On the contrary, crustin (Cru) 3 and Cru4 were inhibited after the knockdown of MrToll, and Cru1 and Cru4 were significantly downregulated with the knockdown of MrToll4 during E. cloacae challenge. These results suggest that MrTolls may be involved in the regulation of AMP expression in the intestine during E. cloacae infection.

Transcriptome analysis of Macrobrachium rosenbergii hemocytes in response to Staphylococcus aureus infection

The aquaculture industry has suffered significant financial losses as a result of disease outbreaks. In particular, disease outbreaks have become a major problem that can seriously affect the sustainable development of the Macrobrachium rosenbergii aquaculture industry. It is crucial to determine the defense mechanism of the host after pathogenic invasion in order to provide effective defense measures after disease outbreaks. Shrimp, like other invertebrates, primarily depend on their innate immune systems to defend against pathogens, and recognize and resist pathogens through humoral and cellular immune responses. In this investigation, we used RNA-seq technology to investigate the transcriptome of hemocytes from M. rosenbergii induced by Staphylococcus aureus. Our main targets were immune pathways and genes related to innate immunity. RNA-seq identified 209,069 and 204,775 unigenes in the control and experimental groups, respectively. In addition, we identified 547 and 1734 differentially expressed genes (DEGs) following S. aureus challenge after 6 and 12 h (h), respectively. GO and KEGG enrichment analysis revealed that the DEGs were significantly enriched in several biological signalling pathways, including NOD-like receptor, PI3K-Akt, Toll and Imd, IL-17, TGF-beta, RIG-I-like receptor, cAMP, apoptosis, and C-type lectin receptor. Sixteen DEGs were chosen at random for qPCR verification; these results concurred with those from sequencing. Our findings revealed that immune-related genes play an important role in antibacterial activities and have specific functions for gram-positive bacteria. These results provide more data for the prevention of M. rosenbergii diseases and offer a basis for the better prevention of diseases.

The Dynamics of Gene Expression Unraveling the Immune Response of Macrobrachium rosenbergii Infected by Aeromonas veronii

To further investigate the immune response of Macrobrachium rosenbergii against Aeromonas veronii, comparative transcriptomic analyses of the M. rosenbergii hepatopancreas were conducted on challenge and control groups at 6, 12, and 24 h post-infection (hpi), independently. A total of 51,707 high-quality unigenes were collected from the RNA-seq data, and 8060 differentially expressed genes (DEGs) were discovered through paired comparisons. Among the three comparison groups, a KEGG pathway enrichment analysis showed that 173 immune-related DEGs were considerably clustered into 28 immune-related pathways, including the lysosome, the phagosome, etc. Moreover, the expression levels of the four key immune-related genes (TOLL, PAK1, GSK3β, and IKKα) were evaluated at various stages following post-infection in the hepatopancreas, hemolymph, and gills. Both PAK1 and GSK3β genes were highly up-regulated in all three tissues at 6 hpi with A. veronii; TOLL was up-regulated in the hepatopancreas and hemolymph but down-regulated in the gill at 6 hpi, and IKKα was up-regulated in hemolymph and gill, but down-regulated in the hepatopancreas at 6 hpi. These findings lay the groundwork for understanding the immune mechanism of M. rosenbergii after contracting A. veronii.

Transcriptome analysis of Macrobrachium rosenbergii hemocytes reveals in-depth insights into the immune response to Vibrio parahaemolyticus infection

Macrobrachium rosenbergii as one of the common freshwater prawn species in Southeast Asia, which breeding industry is seriously threatened by vibriosis and causes high mortality. In this study, the RNA-seq was employed for assessing the M. rosenbergii hemocytes transcriptomes following Vibrio parahaemolyticus challenge. After challenge for 6 h (h), there were overall 1849 DEGs or differentially expressed genes, including 1542 up-regulated and 307 down-regulated genes, and there was a total of 1048 DEGs, including 510 up-regulated genes and 538 down-regulated genes, after challenge for 12 h. Mitogen-activated protein kinase (MAPK) immune-related pathways, Toll, immune deficiency (IMD), and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) were among the immune pathways where a lot of the DEGs were connected. The expression patterns of 18 chosen immune-related genes were examined utilizing qRT-PCR or quantitative real-time polymerase chain reaction, which revealed that the V. parahaemolyticus infection activated the M. rosenbergii's immune response. Permutational multivariate analysis of variance (PERMANOVA) showed that V. parahaemolyticus infection modulated immune regulation and apoptosis pathways. The gathered information provided new insight into M. rosenbergii's immunity and suggested a novel approach to fight against bacterial infection.

Innate immune responses against viral pathogens in Macrobrachium

Some members of genus Macrobrachium are important economically prawns and valuable objects for studying the innate immune defense mechanism of crustaceans. Studies have focused on immune responses against bacterial and fungal infections and have expanded to include antiviral immunity over the past two decades. Similar to all living organisms, prawns are exposed to viruses, including white spot syndrome virus, Macrobrachium rosenbergii nodavirus, and Decapod iridescent virus 1 and develop effective defense mechanisms. Here, we review current understanding of the antiviral host defense in two species of Macrobrachium. The main antiviral defense of Macrobrachium is the activation of intracellular signaling cascades, leading to the activation of cellular responses (apoptosis) and humoral responses (immune-related signaling pathways, antimicrobial and antiviral peptides, lectins, and prophenoloxidase-activating system).

Identification and characterization of a Toll-like receptor gene from Macrobrachium nipponense

Outbreaks of infectious disease in shrimp pose a serious threat to shrimp agriculture worldwide. Shrimp lack adaptive immunity and depend only on innate immunity as a defense system against infectious disease. Toll-like receptors (TLR) are reported to play a critical role in the innate immune system. In this study, we identified a Toll-like receptor gene of a species of freshwater shrimp, Macrobrachium nipponense, designated MnToll, for the first time. The sequence of MnToll encoded 935 residues arranged as 10 leucine-rich repeat (LRR) domains, a leucine-rich repeat C-terminal (LRR CT) domain and a Toll/interleukin-1 receptor (TIR) domain and displayed 90% amino acid similarity to previously identified TLRs (Toll 1 and 2) of Macrobrachium rosenbergii. We additionally evaluated mRNA expression of MnToll in various tissues, including heart, gills, stomach, digestive gland, ventral nerve cord, antennal gland and muscle. Following infection with a viral pathogen, white spot syndrome virus (WSSV), MnToll expression was significantly upregulated between 12 and 72 h. Our data collectively suggest that the newly identified MnToll gene belongs to the TLR family in shrimp and is potentially involved in innate host defense, especially against WSSV.

The regulation of immune responses against white spot syndrome virus or Vibrio alginolyticus in toll-like receptors silenced giant freshwater prawn (Macrobrachium rosenbergii)

Toll-like receptors, which are a class of cell-surface proteins, have been regarded as the most important pattern recognition receptors in the innate immunity and play a vital role in multiple innate immune responses against pathogen invasion. The full-length cDNA of a novel Toll-like receptor (MrToll3) was identified from Macrobrachium rosenbergii in the current research. The nucleotide sequence of MrToll3 is 4481 bp long and contains a 3726-bp open reading frame encoding a putative protein of 1241 amino acids. MrToll3 was constitutively expressed in all the examined tissues, and high expression of MrToll3 was detected in gill, heart, and ganglion. The result of RNA interference assay revealed that silencing of MrToll1 remarkably suppressed the prophenoloxidase (proPO) expression and phenoloxidase (PO) activities while enhancing MrToll2 expression in the prawns. Furthermore, the expression of myeloid differentiation factor 88 (MyD88), anti-lipopolysaccharide factor (ALF) and crustin was remarkably down-regulated in the MrToll1-silenced prawns after white spot syndrome virus (WSSV) or Vibrio alginolyticus challenge. MrToll2-silenced prawns exhibited the significant decline of ALF and crustin expression post the pathogen challenges, and silencing of MrToll3 obviously improved the immune deficiency (IMD) expression during the whole RNA interference assay. Additionally, higher mortality was observed in MrToll1-or MrToll2-silenced prawns after V. alginolyticus challenge, and the MrToll1-silenced prawns also showed the obviously enhanced susceptibility to WSSV. These results suggested that MrToll1, 2, and 3 were involved in the innate immune responses against WSSV and V. alginolyticus in M. rosenbergii.

Feasibility of vaccination against Macrobrachium rosenbergii nodavirus infection in giant freshwater prawn

The giant freshwater prawn/giant river prawn, Macrobrachium rosenbergii is one of the high market value crustaceans cultured worldwide. The intensified aquaculture of the species has led to the outbreak of infectious diseases, prominently, the white tail disease (WTD). It is caused by the infection of Macrobrachium rosenbergii nodavirus (MrNV), which was classified in the family of Nodaviridae. To-date, there are no effective prophylactic and therapeutic agents available against MrNV infection. Vaccination is known to be the most effective prophylactic agent in disease prevention. However, vaccine development against virus infection in crustaceans is equivocal. The feasibility of vaccination in conferring immune protection in crustaceans against infectious diseases is disputable. The argument lies in the fact that crustaceans do not possess adaptive immunity, which is the main immune component that functions to establish immunological memory upon vaccination. Nevertheless, an increasing number of literatures has been documented, which concerns the development of vaccines against infectious diseases in crustaceans. The current review deliberates different approaches in vaccine development against MrNV, which were documented in the past years. It is noteworthy that the live-attenuated MrNV vaccine has not been experimented by far. Thus, the potential of live-attenuated MrNV vaccine in conferring long-term immune protection through the establishment of innate immune memory is currently being discussed.

Clone, identification and functional characterization of a novel toll (Shtoll1) from the freshwater crab Sinopotamon henanense in response to cadmium exposure and Aeromonas hydrophila infection

Toll is essential in innate immune system which is important for defense against bacterial, fungal and viral infections in invertebrates. Our previous study showed that cadmium (Cd) could change the expression pattern of ShToll3 in the epithelium (gills and midgut from the freshwater crab Sinopotamon henanense) infected by Aeromonas hydrophila. To investigate the diverse innate immune roles of crustacean homolog Tolls, in this study, we cloned Shtoll1 from S. henanense. The full-length cDNA of Shtoll1 was 4746 bp, with an ORF of 3033 bp encoding a putative protein of 111 amino acids, a 5'-untranslated region of 255 bp and a 3'-untranslated region of 1713 bp. Phylogenetic analysis showed that ShToll1 was clustered into the group of DmToll1, DmToll 4 and DmToll 5. In addition, the tissue distribution results showed that Shtoll1 was expressed widely in different tissues, with the highest expression in heamocytes. Besides, Shtoll1 expressions were upregulated in heamocytes and hepatopancreas after A. hydrophila infection. At the same time, the increase of Shtoll1 expressions were examined in heamocytes in response to Cd exposure and A. hydrophila infection in combination. Through western blotting and immunohistochemical analysis, the ShToll1 expressions in heamocytes were increased in response to A. hydrophila and Cd independently as well as in combination. Moreover, the mRNA level of three antimicrobial peptides (AMPs) alf5, alf6, and c-lys, which possibly responded to Cd and A. hydrophila stimulation through Shtoll1, were analyzed. Thus, we conclude that Cd expand the susceptibility of ShToll1 to A. hydrophila infection in heamocytes. This suggest that ShToll1 may contribute to the innate immune defense of S. henanense against A. hydrophila and Cd in heamocytes.

Research progress in innate immunity of freshwater crustaceans

Invertebrates lack adaptive immunity and innate immunity plays important roles in combating foreign invasive pathogens. Freshwater crustaceans, which are invertebrates, depend completely on their innate immune system. In recent years, many immune-related molecules in freshwater crustaceans, as well as their functions, have been identified. Three main immune signaling pathways, namely, Toll, immune deficiency (IMD), and Janus kinase-signal transducer activator of transcription (JAK/STAT) pathways, were found in freshwater crustaceans. A series of pattern recognition receptors (PRRs), including Toll receptors, lectins, lipopolysaccharide and β-1,3-glucan binding protein, scavenger receptors, Down syndrome cell adhesion molecules, and thioester-containing proteins, were reported. Prophenoloxidase activation system and antimicrobial peptide synthesis are two important immune effector systems. These components are involved in the innate immunity of freshwater crustaceans, and they function in the innate immune defense against invading pathogens. This review mainly summarizes innate immune signaling pathways, PRRs, and effector molecules in freshwater crustaceans.

Transcriptomic analysis of Macrobrachium rosenbergii (giant fresh water prawn) post-larvae in response to M. rosenbergii nodavirus (MrNV) infection: de novo assembly and functional annotation

Background

Macrobrachium rosenbergii, is one of a major freshwater prawn species cultured in Southeast Asia. White tail disease (WTD), caused by Macrobrachium rosenbergii nodavirus (MrNV), is a serious problem in farm cultivation and is responsible for up to 100% mortality in the post larvae stage. Molecular data on how M. rosenbergii post-larvae launches an immune response to an infection with MrNV is not currently available. We therefore compared the whole transcriptomic sequence of M. rosenbergii post-larvae before and after MrNV infection.

Results

Transcriptome for M. rosenbergii post-larvae demonstrated high completeness (BUSCO Complete: 83.4%, fragmentation: 13%, missing:3.3%, duplication:16.2%; highest ExN50 value: 94%). The assembled transcriptome consists of 96,362 unigenes with N₅₀ of 1308 bp. The assembled transcriptome was successfully annotated against the NCBI non-redundant arthropod database (33.75%), UniProt database (26.73%), Gene Ontology (GO) (18.98%), Evolutionary Genealogy of Genes: Non-supervised Orthologous Groups (EggNOG) (20.88%), and Kyoto Encyclopedia of Genes and Genome pathway (KEGG) (20.46%). GO annotations included immune system process, signaling, response to stimulus, and antioxidant activity. Differential abundance analysis using EdgeR showed 2413 significantly up-regulated genes and 3125 significantly down-regulated genes during the infection of MrNV.

Conclusions

This study reported a highly complete transcriptome from the post-larvae stage of giant river prawn, M. rosenbergii. Differential abundant transcripts during MrNV infection were identified and validated by qPCR, many of these differentially abundant transcripts as key players in antiviral immunity. These include known members of the innate immune response with the largest expression change occurring in the M. rosenbergii post-larvae after MrNV infection such as antiviral protein, C-type lectin, prophenol oxidase, caspase, ADP ribosylation factors, and dicer.

Gill transcriptome alterations in Macrobrachium rosenbergii under copper exposure

Copper is one of common contaminants in estuaries and coastal zones, which may cause physiological dysfunction in aquatic organisms. However, molecular response triggered by Cu have remained largely unknown in freshwater prawn Macrobrachium rosenbergii. In the present study, we performed transcriptomic analysis to characterize molecular mechanisms of copper immunotoxicity in gills from M. rosenbergii. A large number of potential simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) loci in the transcriptome were identified. 19,417 and 8989 differentially expressed genes (DEGs) were obtained at 3 h and 48 h after exposure, respectively. Most of these DEGs were down-regulated implying that gene expressions were largely inhibited by Cu, which might lead to impairments of biological functions. Functional enrichment analysis of these DEGs revealed that immune, detoxification and apoptosis were the differentially regulated processes by Cu stress. 12 DGEs involved in immune response and heavy metal detoxification were discovered and validated by qRT-PCR. The results indicated that the M. rosenbergii might counteract the toxicity of Cu at the transcriptomic level by increasing expressions of immune- and heavy metal detoxification-related genes, and these selected genes could be used as molecular indicators for Cu stress. Our study firstly reported the stress response at transcriptional level in M. rosenbergii during Cu exposure. The genes and pathways identified here not only give us new insight into molecular mechanisms underlying Cu toxicity effects in prawn, but facilitate biomarker identification and stress-resistant breeding studies.

Role of Clathrin Assembly Protein-2 Beta Subunit during White Spot Syndrome Virus Infection in Black Tiger Shrimp Penaeus monodon

White spot syndrome virus (WSSV) is one of the most lethal viruses severely affecting shrimp industry. This disease can cause 100% mortality of farmed shrimp within a week. This work aims to characterize clathrin assembly proteins in Penaeus monodon and investigate their roles in WSSV entry. In general, clathrin assembly proteins form complexes with specific receptors and clathrins, leading to clathrin-mediated endocytosis. Adaptor protein 2 (AP-2), which is responsible for endocytosis at plasma membrane, consists of four subunits including α, β2, μ2 and σ2. Knockdown of clathrin coat AP17, or σ subunit of AP-2 dramatically reduced WSSV infectivity. Similar results were observed, when shrimp were pre-treated with chlorpromazine (CPZ), an inhibitor of clathrin-dependent endocytosis. The complete open reading frames of AP-2β and μ subunits of P. monodon are reported. PmAP-2 β was up-regulated about 4-fold at 6 and 36 h post-WSSV infection. Knockdown of PmAP-2β delayed shrimp mortality during WSSV infection, of which WSSV intermediate early 1 gene expression was also down-regulated. Immunogold-labelling and transmission electron microscopy revealed that PmAP-2β co-localized with WSSV particles at plasma membrane. In addition, PmAP-2β-silencing significantly affected the expression levels of PmSTAT, PmDOME, PmDorsal and ALFPm3 during WSSV infection. It is possible that PmAP-2β is associated with the JAK/STAT and the Toll pathway.

A toll receptor is involved in antibacterial defense in the oriental river prawn, Macrobrachium nipponense

Toll-like receptors (TLRs) play an important role in the activation of innate immune response in animals. In this study, we identified a TLR from the oriental river prawn, Macrobrachium nipponense (MnToll1) and investigated its functions in immunity. The MnToll1 protein shares similar structural characteristics with other known Toll family proteins. MnToll1 transcripts are broadly distributed in all of the examined tissues, and its expression level was significantly up-regulated by bacterial challenge. RNAi-mediated knockdown of MnToll1 significantly impaired the survivability of Vibrio-challenged prawns. RNAi experiments also revealed that the expression of several antimicrobial peptide genes were regulated by MnToll1. Moreover, we found the extracellular region of MnToll1 could directly bind to bacteria and bacterial glycoconjugates. These findings suggest that MnToll1 function as a pattern recognition receptor to recognize invading pathogen and initiate downstream gene expression, to participate in antibacterial defense of M. nipponense.

Pathogenicity of non-O1/ O139 Vibrio cholerae and its induced immune response in Macrobrachium rosenbergii

Outbreaks of mass mortalities occurred in Macrobrachium rosenbergii farms in Gaoyou county, Jiangsu Province of China. The bacterial isolates from M. rosenbergii exhibited the same phenotypic traits and biochemical characteristics, and were identified as non-O1/O139 Vibrio cholerae according to biochemical characteristics and molecular identification. In challenge test, M. rosenbergii infected with non-O1/O139 V. cholerae GXFL1-4 developed similar pathological signs to the naturally diseased prawns, and LD₅₀ of the strain to M. rosenbergii was 4.5 × 10⁶ CFU/mL at 96 h post-infection. Histopathological analysis revealed that hepatopancreas and intestines of diseased M. rosenbergii exhibited obvious inflammatory responses to non-O1/O139 V. cholerae infection. Detection virulence factors of the strain GXFL1-4 showed that the bacteria produced caseinase, lipase, amylase, lecithinase and hemolysin, and carried toxR, hlyA, ompW, ompU, hap, rtxA and rtxC virulence related genes, supporting the strong virulence to M. rosenbergii. Additionally, the immune related gene expression in M. rosenbergii evaluated by qRT-PCR analysis showed that HSP70, Crustin, Lysozyme, TRL1, ALF1, Lectin, Peroxinectin, proPO and SOD immune related genes were significantly up-regulated at 6 and 12 h after infection with GXFL1-4. The results of our study suggested that non-O1/O139 V. cholerae was an etiological element in the mass mortalities of M. rosenbergii and this study provided preliminary insights into the diversity in the immune response of M. rosenbergii to the bacterial invasion.

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.

Full-length transcriptome analysis of Litopenaeus vannamei reveals transcript variants involved in the innate immune system

To better understand the immune system of shrimp, this study combined PacBio isoform sequencing (Iso-Seq) and Illumina paired-end short reads sequencing methods to discover full-length immune-related molecules of the Pacific white shrimp, Litopenaeus vannamei. A total of 72,648 nonredundant full-length transcripts (unigenes) were generated with an average length of 2545 bp from five main tissues, including the hepatopancreas, cardiac stomach, heart, muscle, and pyloric stomach. These unigenes exhibited a high annotation rate (62,164, 85.57%) when compared against NR, NT, Swiss-Prot, Pfam, GO, KEGG and COG databases. A total of 7544 putative long noncoding RNAs (lncRNAs) were detected and 1164 nonredundant full-length transcripts (449 UniTransModels) participated in the alternative splicing (AS) events. Importantly, a total of 5279 nonredundant full-length unigenes were successfully identified, which were involved in the innate immune system, including 9 immune-related processes, 19 immune-related pathways and 10 other immune-related systems. We also found wide transcript variants, which increased the number and function complexity of immune molecules; for example, toll-like receptors (TLRs) and interferon regulatory factors (IRFs). The 480 differentially expressed genes (DEGs) were significantly higher or tissue-specific expression patterns in the hepatopancreas compared with that in other four tested tissues (FDR <0.05). Furthermore, the expression levels of six selected immune-related DEGs and putative IRFs were validated using real-time PCR technology, substantiating the reliability of the PacBio Iso-seq results. In conclusion, our results provide new genetic resources of long-read full-length transcripts data and information for identifying immune-related genes, which are an invaluable transcriptomic resource as genomic reference, especially for further exploration of the innate immune and defense mechanisms of shrimp.

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.

SpToll1 and SpToll2 modulate the expression of antimicrobial peptides in Scylla paramamosain

Tolls and Toll-like receptors (TLRs) were the first pattern recognition receptors (PRRs) identified to play key roles in host innate immunity. However, relatively little is known about other types of Toll-like receptors in Scylla paramamosain, although a Toll-like receptor (SpToll1) has recently been cloned. In this study, we cloned and characterized another novel Toll-like receptor 2 (SpToll2) from S. paramamosain. The full-length cDNA of SpToll2 is 3391 bp with a 2646 bp open reading frame (ORF) encoding a putative protein of 881 amino acids, and predicted to contain six extracellular leucine-rich repeat (LRR) domains, a transmembrane domain and an intracellular Toll/IL-1 receptor (TIR) domain. Phylogenetic analysis revealed that SpToll2 clustered with Drosophila Toll1, and shared high homology with PtToll4. Real-time qPCR analysis showed that SpToll2 was widely expressed in all tissues tested, with the highest level found in hemocytes and hepatopancreas while the lowest in heart and muscle. The transcript levels of both SpToll1 and SpToll2 in mud crabs hemocytes was induced following challenge with Vibrio parahaemolyticus, Staphylococcus aureus, Polyinosinic: polycytidylic acid (Poly I:C) and white spot syndrome virus (WSSV). In addition, recombinant SpToll1-LRR and SpToll2-LRR proteins could bind to V. parahaemolyticus, S. aureus, Escherichia coli, and Beta Streptococcus. In order to study the signaling pathway of AMPs' expression in mud crab, RNA interference were used to test the expression of SpAMPs after the challenges with V. parahaemolyticus or S. aureus. The data suggested that SpToll1and SpToll2 could regulate the transcripts of several AMPs and four immune related mediators (SpMyD88, SpTube, SpPelle and SpTRAF6) at different scale. While silencing of SpToll1 post pathogens challenge attenuated the expression of SpHistin, SpALF1 and SpALF5 in mud crab's hemocytes, depletion of SpToll2 post pathogens challenge inhibited the expression of SpALF1-6, SpGRP, SpArasin and SpHyastastin. Furthermore, the results of overexpression assay also showed SpToll1 and SpToll2 could enhance the promoter activities of SpALFs in mud crab. Taken together, these results indicated that SpToll1 and SpToll2 might play important roles in host defense against pathogen invasions in S. paramamosain.

RNAi screening identifies a new Toll from shrimp Litopenaeus vannamei that restricts WSSV infection through activating Dorsal to induce antimicrobial peptides

The function of Toll pathway defense against bacterial infection has been well established in shrimp, however how this pathway responds to viral infection is still largely unknown. In this study, we report the Toll4-Dorsal-AMPs cascade restricts the white spot syndrome virus (WSSV) infection of shrimp. A total of nine Tolls from Litopenaeus vannamei namely Toll1-9 are identified, and RNAi screening in vivo reveals the Toll4 is important for shrimp to oppose WSSV infection. Knockdown of Toll4 results in elevated viral loads and renders shrimp more susceptible to WSSV. Furthermore, Toll4 could be a one of upstream pattern recognition receptor (PRR) to detect WSSV, and thereby leading to nuclear translocation and phosphorylation of Dorsal, the known NF-κB transcription factor of the canonical Toll pathway. More importantly, silencing of Toll4 and Dorsal contributes to impaired expression of a specific set of antimicrobial peptides (AMPs) such as anti-LPS-factor (ALF) and lysozyme (LYZ) family, which exert potent anti-WSSV activity. Two AMPs of ALF1 and LYZ1 as representatives are demonstrated to have the ability to interact with several WSSV structural proteins to inhibit viral infection. Taken together, we therefore identify that the Toll4-Dorsal pathway mediates strong resistance to WSSV infection by inducing some specific AMPs.

The TLR pathway mediated antiviral immune response is well identified in mammals, yet, Toll pathway governing this protection in invertebrates remains unknown. In the present study, we uncover that a shrimp Toll4 from a total of nine Tolls in L. vannamei confers resistance to WSSV thought inducing the NF-κB transcription factor Dorsal to inspire the production of some antimicrobial peptides (AMPs) with antiviral activity. The anti-LPS-factor (ALF) and lysozyme (LYZ) family are identified as the Toll4-Dorsal pathway targeted genes with the ability to interact with viral structural proteins in response to WSSV infection. These results suggest that the Toll receptor induces the expression of AMPs with antiviral activity could be a general antiviral mechanism in invertebrates and Toll pathway established antiviral defense could be conserved during evolution.

Cloning and Characterization of Two Toll Receptors (PcToll5 and PcToll6) in Response to White Spot Syndrome Virus in the Red Swamp Crayfish Procambarus clarkii

Toll/Toll-like receptors are key components in the innate immune responses of invertebrates. In this study, we identified two novel Toll receptors (PcToll5 and PcToll6) from the red swamp crayfish Procambarus clarkii. The complete cDNA sequence of PcToll5 is 4247 bp, encoding a 1293 amino acid polypeptide. The full-length 4688 bp PcToll6 encodes a putative protein of 1195 amino acids. Quantitative RT-PCR analysis indicated that PcToll5 and PcToll6 were constitutively expressed in all tissues studied. The highest expression levels of PcToll5 and PcToll6 were found in the intestine and gills, respectively, and were significantly upregulated from 24 to 48 h during white spot syndrome virus (WSSV) challenge. siRNA-mediated RNA interference results showed that PcToll5 and PcToll6 might regulate the expression of anti-lipopolysaccharide factors (PcALF2 and PcALF3) in vivo. Overexpression of PcToll5 and PcToll6 in Drosophila Schneider 2 (S2) cells activated the transcription of Drosophila antimicrobial peptides, including drosomycin (Drs), metchnikowin (Mtk), and attacin A (AttA), and shrimp Penaeidin-4 (Pen4). These findings provide significant information that PcToll5 and PcToll6 may contribute to host immune defense against WSSV in P. clarkii.

PI3K signaling pathways modulated white spot syndrome virus (WSSV) replication in Procambarus clarkii

The PI3K/AKT signaling pathway is commonly exploited to regulate viral replication and affect the fate of infected cells. In the present study, a PI3K-specific inhibitor (LY294002) was employed to pretreat crayfish to evaluate the effects of PI3K/AKT signaling pathway in WSSV replication. The results showed that the WSSV copy numbers in crayfish pretreated with LY294002 were significantly lower than those in Tris-HCl pretreatment crayfish on the sixth and tenth day after WSSV infection. In semigranular cells, the apoptosis rates were up-regulated on the third day post-WSSV infection, and a significantly lower proportion of apoptosis cells were observed in LY294002-pretreatment group. The expression level of Bax, Bax inhibitor-1 and lectin mRNA in haemocytes of crayfish were increased after WSSV infection. After the secondary stimulation with Tris-HCl, the Bax expression level in LY294002-pretreatment crayfish was significantly higher than that of crayfish pretreated with Tris-HCl on the third or sixth day, but the Toll and lectin mRNA expression decreased significantly on the third, sixth and tenth day. The Bax mRNA expression levels in LY294002-WSSV group were significantly higher than those in Tris-HCl-WSSV group on the third and tenth day. The Bax inhibitor-1 mRNA expression levels in LY294002-WSSV group were significantly lower than those in Tris-HCl-WSSV crayfish on the third day. These results together indicated that the hosts PI3K/AKT signaling pathway play positive roles in WSSV replication through the balance between host cell apoptois and innate immune responses. This information is helpful to further understand the role of PI3K/AKT signaling pathway on WSSV replication in Decapoda crustaceans.

Effects of dietary inulin and mannan oligosaccharide on immune related genes expression and disease resistance of Pacific white shrimp, Litopenaeus vannamei

The effects of inulin and mannan oligosaccharide (MOS) at different doses (2.5, 4 and 10 mg/g) in singular or combined diet on growth rate, immune related genes expression, and resistance to white spot syndrome virus (WSSV) and Vibrio alginolyticus in Pacific white shrimp (Litopenaeus vannamei) were investigated. At the end of 28-day singular feeding experiment, the highest values of specific growth rate (SGR) and the expression of toll-like receptor1, 2 and 3 (TLR1, 2, 3), signal transducer and activator of transcription (STAT), crustin, anti-lipopolysaccharide factor (ALF) as well as prophenoloxidase (proPO) were observed in shrimp individually fed with 5 mg/g dietary inulin or MOS, respectively. Compared with individual treatments, diet containing combined prebiotics (5 mg/g inulin and MOS) significantly improved the expression of TLRs, STAT, proPO, crustin and ALF in L. vannamei after four-week feeding. Additionally, Pacific white shrimp fed with combined dietary prebiotics showed significantly higher expression of immune related genes and lower cumulative mortality in WSSV and Vibrio alginolyticus challenges, compared to the singular feeding groups and control. These results in the present study demonstrated that the combined supplementation of inulin (5 mg/g) and MOS (5 mg/g) remarkably enhanced innate immune response and pathogen resistance of shrimp, and should be considered as a promising immunostimulatory additive for the culture of Pacific white shrimp.

Identification of a novel toll gene (Shtoll3) from the freshwater crab Sinopotamon henanense and its expression pattern changes in response to cadmium followed by Aeromonas hydrophila infection

Toll signaling is essential for expression of immune genes which are important for defense against bacterial, fungal and viral infections in invertebrates. Although several toll genes have been identified in the crustaceans, none of them has been investigated in freshwater crab Sinopotamon henanense. Moreover, the effect of cadmium (Cd) on toll gene expression has never been examined on the freshwater crabs which live in the sediments and are prone to heavy metal bioaccumulation. Our transcriptomic analysis of hepatopancreas tissue reveals that toll3 gene expression has been decreased when treated with Cd. In this study, we cloned one toll gene (hereby designated Shtoll3) from the crab. The full-length cDNA of Shtoll3 was 4488 bp, with an ORF of 3693 bp encoding a putative protein of 1230 amino acids, a 5'-untranslated region of 414 bp and a 3'-untranslated region of 781 bp. Phylogenetic analysis showed that ShToll3 was clustered into the group of DmToll8. The tissue distribution results showed that Shtoll3 was expressed widely in different tissues, with the highest in gills, and the lowest in hemocytes. Shtoll3 expression was down-regulated only in midguts after Aeromonas hydrophila infection. With Cd presence, Shtoll3 expression in response to A. hydrophila were up-regulated in midguts and gills, which was further confirmed by western blotting analysis. Moreover, the mRNA level of two antimicrobial peptides (AMPs) crustin and c-lys, which possibly responded to Cd and A. hydrophila stimulation through Shtoll3, were analysised. Thus, we conclude that Cd changes the susceptibility of Shtoll3 to A. hydrophila infection in gills and midguts. This suggest that Shtoll3 may contribute to the innate immune defense of S. henanense to A. hydrophila and Cd can modify the immune function in epithelium.