Molecular cloning and expression of a Toll receptor in the giant tiger shrimp, Penaeus monodon

Experimental Infection Models and Their Usefulness for White Spot Syndrome Virus (WSSV) Research in Shrimp

White spot syndrome virus (WSSV) is marked as one of the most economically devastating pathogens in shrimp aquaculture worldwide. Infection of cultured shrimp can lead to mass mortality (up to 100%). Although progress has been made, our understanding of WSSV's infection process and the virus-host-environment interaction is far from complete. This in turn hinders the development of effective mitigation strategies against WSSV. Infection models occupy a crucial first step in the research flow that tries to elucidate the infectious disease process to develop new antiviral treatments. Moreover, since the establishment of continuous shrimp cell lines is a work in progress, the development and use of standardized in vivo infection models that reflect the host-pathogen interaction in shrimp is a necessity. This review critically examines key aspects of in vivo WSSV infection model development that are often overlooked, such as standardization, (post)larval quality, inoculum type and choice of inoculation procedure, housing conditions, and shrimp welfare considerations. Furthermore, the usefulness of experimental infection models for different lines of WSSV research will be discussed with the aim to aid researchers when choosing a suitable model for their research needs.

Invertebrate Immunity, Natural Transplantation Immunity, Somatic and Germ Cell Parasitism, and Transposon Defense

While the vertebrate immune system consists of innate and adaptive branches, invertebrates only have innate immunity. This feature makes them an ideal model system for studying the cellular and molecular mechanisms of innate immunity sensu stricto without reciprocal interferences from adaptive immunity. Although invertebrate immunity is evolutionarily older and a precursor of vertebrate immunity, it is far from simple. Despite lacking lymphocytes and functional immunoglobulin, the invertebrate immune system has many sophisticated mechanisms and features, such as long-term immune memory, which, for decades, have been exclusively attributed to adaptive immunity. In this review, we describe the cellular and molecular aspects of invertebrate immunity, including the epigenetic foundation of innate memory, the transgenerational inheritance of immunity, genetic immunity against invading transposons, the mechanisms of self-recognition, natural transplantation, and germ/somatic cell parasitism.

LysM-containing proteins function in the resistance of Litopenaeus vannamei against Vibrio parahaemolyticus infection

Lysin motif (LysM) is a functional domain that can bind to peptidoglycans, chitin and their derivatives. The LysM-containing proteins participate in multiple biological processes, such as the hydrolysis of bacterial cell walls and the perception of PAMPs in plants and high animals. In the present study, two genes encoding LysM-containing proteins, designated as LvLysM1 and LvLysM2, were identified in the Pacific white shrimp, Litopenaeus vannamei, and their functions during Vibrio infection were analyzed. The open-reading frame (ORF) of LvLysM1 was 795 bp, only encoding a LysM domain at the N-terminal region. The ORF of LvLysM2 was 834 bp, encoding a LysM domain at the central region and a transmembrane region at the C-terminal region. Both LvLysM1 and LvLysM2 were widely transcribed in all tested shrimp tissues. Enzyme-linked immunosorbent assay (ELISA) showed that the recombinant protein of LvLysM2 could bind to different bacterial polysaccharides, while LvLysM1 showed no direct binding activity. The transcripts of LvLysMs in gills increased significantly after infection with Vibrio parahaemolyticus. When LvLysM1 or LvLysM2 was knocked down by dsRNA, the mortality of shrimp was significantly increased after infection with Vibrio parahaemolyticus. Interestingly, some SNPs existed in these two genes were apparently correlated with the VpAHPND resistance of shrimp. These results suggested that LvLysM1 and LvLysM2 might contribute to the disease resistance of shrimp. The data provide new knowledge about the function of LysM-containing proteins in shrimp and potential genetic markers for disease resistance breeding.

Effects of Sulfamethoxazole and Florfenicol on Growth, Antioxidant Capacity, Immune Responses and Intestinal Microbiota in Pacific White Shrimp Litopenaeus vannamei at Low Salinity

Antibiotic residue may pose a serious risk to aquaculture, and the culture of Litopenaeus vannamei in a low-salinity environment is a growing trend over the world. Here, we aimed to understand the combined effect of low salinity and sulfamethoxazole (SMZ) and florfenicol (FLO) antibiotics on L. vannamei. The growth performance, immune functions, antioxidant capacity and intestinal microbiota were investigated. Compared with the control group, the weight gain and survival rate significantly decreased (p < 0.05) in shrimp after they were exposed to low-salinity (salinity 3) water and the mixture of antibiotics and low-salt conditions for 28 days. The antioxidant activities of SOD and T-AOC, shown at low salinity and in the higher concentration of the SMZ treatment group (SMZH), were significantly decreased, while the GST activity was significantly increased in each treatment group in comparison with the control group. The expression of immune-related genes, including TOLL, LvIMD, PPO and HSP, in the low concentration of the SMZ treatment group (SMZL) was higher than that in the other groups. The diversity of intestine microbiota was disturbed with a lower Shannon index in the low-salinity and SMZH groups, and a higher Simpson index in the SMZH group. Proteobacteria, Actinobacteria and Bacteroidetes were the dominant phyla in the gut of L. vannamei. At the genus level, Microbacterium, Shewanella, Aeromonas, Acinetobacter, Gemmobacter, Paracoccus and Lysobacter were significantly decreased in the low-salinity group. However, the abundance of opportunistic pathogens belonging to the genus Aeromonas in the FLO group was increased. The predicted microbe-mediated functions showed that the pathway for “amino acid metabolism” and “replication and repair” was significantly inhibited in both the low-salinity and antibiotic-exposed groups. All the findings in this study indicate that the combined effect of antibiotics and low salinity on L. vannamei negatively impacted the physiological and intestinal microbiota functions.

Identification of three novel Spätzle genes in Eriocheir sinensis and their roles during white spot syndrome virus infection

Proteins of Spätzle family play an essential role in innate immunity in invertebrates by activating the Toll pathway to induce the expression of antimicrobial peptides. However, little is known about the function of Spätzle in in the immune response of the Chinese mitten crab. In the present study, three novel Spätzle genes (named as EsSpz1, EsSpz2, and EsSpz3) were identified from Eriocheir sinensis. The genome structure of EsSpz1 contains two exons and an intron. Three Spätzle proteins all contain a Pfam Spaetzle domain. In the evolution, EsSpz1-3 cluster with other Spätzle proteins from crustaceans. EsSpz1-3 were widely distributed in multiple immune tissues. The expression levels of EsSpz1-3 in the intestine were remarkably upregulated after white spot syndrome virus (WSSV) challenge. The knockdown of EsSpz1-3 remarkably decreased the expressions of crustins and anti-lipopolysaccharide factors during WSSV infection. Moreover, EsSpz1-3 silencing remarkably increased the expression of WSSV envelope protein VP28. These findings suggest that new-found EsSpz1-3 in E. sinensis could promote the synthesis of antimicrobial peptides and inhibit the expression of VP28 during WSSV infection. Our study indicates that EsSpz1-3 in E. sinensis may participate in the innate immune defenses against WSSV by inducing the expression of antimicrobial peptides. This study provides new knowledge for the function of Spätzle in the antiviral immune defense in crustacean.

Early responses in Penaeus vannamei during experimental infection with Enterocytozoon hepatopenaei (EHP) spores by injection and oral routes

Hepatopancreatic microsporidiosis caused by Enterocytozoon hepatopenaei (EHP) is associated with severe production losses in Penaeus vannamei farming. Early responses in P. vannamei experimentally infected with EHP was assessed in this study by feeding infected hepatopancreatic tissue and by injecting purified EHP spores (∼1 × 10⁵ Spores/shrimp). Immune responses to EHP infection were assessed in the haemolymph by analysing the total haemocyte count (THC), superoxide dismutase (SOD) activity, prophenoloxidase activity (proPO), respiratory burst activity (RBA), catalase activity (CAT), lysozyme activity (LYS) and Toll gene expression in hepatopancreas at 0, 6, 12, 24, 36, 48, 60 and 72 h post-infection (hpi). Experimental infection with EHP resulted in a significant (p < 0.05) reduction in the immune parameters such THC, CAT and LYS at 6, 24 and 24 hpi respectively while there was a significant increase (p < 0.05) in the levels of SOD, proPO and RBA at 6 hpi. The expression of the Toll gene was significantly upregulated (p < 0.05) after experimental infection with EHP from 6 hpi. These findings on immune responses in P. vannamei during EHP infection will assist in the development of suitable management measures to reduce the negative impacts of EHP in P. vannamei farming. This is the first report on early responses in P. vannamei during EHP infection.

Role of Cellular Receptors in the Innate Immune System of Crustaceans in Response to White Spot Syndrome Virus

Innate immunity is the only defense system for resistance against infections in crustaceans. In crustaceans, white spot diseases caused by white spot syndrome virus (WSSV) are a serious viral disease with high accumulative mortality after infection. Attachment and entry into cells have been known to be two initial and important steps in viral infection. However, systematic information about the mechanisms related to WSSV infection in crustaceans is still limited. Previous studies have reported that cellular receptors are important in the innate immune system and are responsible for the recognition of foreign microorganisms and in the stimulation of the immune responses during infections. In this review, we summarize the current understanding of the functions of cellular receptors, including Toll, C-type lectin, scavenger receptor, β-integrin, polymeric immunoglobulin receptor, laminin receptor, globular C1q receptor, lipopolysaccharide-and β-1,3-glucan-binding protein, chitin-binding protein, Ras-associated binding, and Down syndrome cell adhesion molecule in the innate immune defense of crustaceans, especially shrimp and crabs, in response to WSSV infection. The results of this study provide information on the interaction between viruses and hosts during infections, which is important in the development of preventative strategies and antiviral targets in cultured aquatic animals.

Identification and function analysis of an immune deficiency homolog in swimming crab, Portunus trituberculatus

The immune deficiency (IMD) pathway is involved in both antiviral and antibacterial immune responses in Drosophila. IMD protein is the key adaptor to link the extracellular signal and the intracellular reaction to initiate the signal transduction in IMD pathway. In present study, the cDNA of the IMD (Pt-IMD) was identified from a marine crab, Portunus trituberculatus. The Pt-IMD is predicted to encode 170 amino acids with a death domain. Real-Time quantitative PCR analysis showed that Pt-IMD was constitutively expressed in hemocytes, intestine, gill, heart, muscle and hepatopancreas in normal crab. Moreover, the transcript of Pt-IMD in large-granule hemocytes is approximately 6-fold higher than semi-granular cells and agranular cells. Intracellular localization showed Pt-IMD was distributed mainly in the cytoplasm when it was over-expressed in Drosophila Schneider 2 (S2) cell. Functionally, over-expression of Pt-IMD could activate the promoters of Drosophila antimicrobial peptide genes (AMPs) in S2 cell. Furthermore, Pt-IMD expression was also knock-down by RNAi to determine the function of Pt-IMD on regulation of the expression of different antimicrobial peptides (AMPs) in crab. In the primary cultured hemocytes challenged with or without Vibrio alginolyticus, after Pt-IMD was knocked-down by specific long double strand RNA, the expression of anti-lipopolysaccharide factor1 (ALF1), ALF3, crustin1, crustin3, arasin2, hyastatin1and hyastatin3 have been significantly inhibited in normal cell or bacterial infected cell, while the expression of lysozyme was normal in non-infected cells and was significantly induced in bacterial infected cells, which compared to the non-specific dsRNA treated cells.

Identification, characterization, and functional analysis of Toll and ECSIT in Exopalaemon carinicauda

Toll and evolutionary conserved signaling intermediate in Toll pathways (ECSIT) are two essential molecules in Toll/Toll-like receptor (TLR)-mediated signaling pathway. In this study, Toll and ECSIT (named as EcToll and EcECSIT) were identified for the first time from Exopalaemon carinicauda. EcToll mRNA transcripts were high expressed in hemocytes and gill, and EcECSIT was mainly expressed in gill. The expression levels of EcToll and EcECSIT in gills both responded rapidly to Vibrio parahaemolyticus and WSSV stimulations and three types of antimicrobial peptide (AMP) genes were significantly up-regulated by challenge with V. parahaemolyticus. Knockdown of EcToll or EcECSIT increased the sensitivity of E. carinicauda to V. parahaemolyticus challenge and double knockdown of both EcToll and EcECSIT significantly suppressed the bacterial clearance ability of E. carinicauda in vivo. Furthermore, suppressing EcToll restrained the upregulation of EcECSIT and AMPs and suppressing EcECSIT impaired expression of AMPs by V. parahaemolyticus injection, which indicated that EcToll restricted V. parahaemolyticus infection through activating EcECSIT to induce AMPs. This study provides valuable information about the function of Toll-ECSIT pathway in the innate immunity in crustacean.

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

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

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.

Association between ALFPm3 single nucleotide polymorphism and white spot syndrome virus resistance in black tiger shrimp Penaeus monodon

Here single nucleotide polymorphisms (SNPs) were associated with white spot syndrome virus (WSSV) resistance in black tiger shrimp Penaeus monodon. SNPs were identified by single-strand conformation polymorphism (SSCP) screening and DNA sequencing of shrimp sampled from 3 families (100 shrimp per family) challenged with WSSV. Shrimp that died over the 14 d challenge trial were designated susceptible, with those remaining alive on Day 14 designated resistant. To compare SNPs, 10 samples from the susceptible and resistant groups, each comprising DNA pooled from 3 shrimp, were amplified by polymerase chain reaction (PCR) using primers to 12 selected genes and screened by SSCP. SNPs were only identified in the anti-lipopolysaccharide factor 3 (ALFPm3) gene product. Analysis of complete ALFPm3 gene sequences confirmed the existence of 3 SNPs (g.934C>G, g.1186A>G, and g.1898C>G) that were polymorphic between the susceptible and resistant groups. Further analyses using specific tetra-primer amplification refractory mutation system PCR primer sets associated these 3 SNPS, and particularly the g.1186A>G SNP, with WSSV resistance. This SNP thus has potential for use as a DNA marker to select for WSSV resistance in P. monodon breeding programs.

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.

Anthraquinone extract from Rheum officinale Bail improves growth performance and Toll-Relish signaling-regulated immunity and hyperthermia tolerance in freshwater prawn Macrobrachium nipponense

This study was aimed to investigate the facilitation of anthraquinone extract on growth performance, immunity, and antioxidant capacity of the oriental river prawn (Macrobrachium nipponense), and whether it could ameliorate the hyperthermia stress. A 12-week rearing experiment was conducted with 0, 125, 250, 500, and 1000 mg kg^-1 anthraquinone extract from Rheum officinale Bail-supplemented diets (AE0, AE125, AE250, AE500, and AE1000), and followed a 48-h thermal stress with 32℃ incubation. Results indicate AE250 and AE500 significantly improved the growth performance and feed utilization, the optimum level was estimated to be 251.22 mg kg^-1 based on the regression analysis of special growth ratio (SGR). Meanwhile, AE250 and AE500 improved antioxidant enzyme activity and immune-related protein concentration of iNOS-NO signaling. Under thermal stress, AE250 and AE500 improved the heat tolerance, and Toll-Relish signaling was active to the resistance. These results indicate anthraquinone extract could be used as an effective immunostimulant to improve growth performance, physiological balance and protect organism form environmental hyperthermia stress. This may provide insights for immunostimulant development in aquaculture production.

The involvement of crustaceans toll-like receptors in pathogen recognition

Crustacean species are considered as a major sector in the aquaculture industry that plays a fundamental role in the world's economy. However, with a wide range of various epidemic diseases in the industry, studies of immune-related genes such as toll-like receptor genes are of great importance. Recently, the TLR in crustacean species has been described to perform a vital role in defense of crustacean against the pathogens. Meanwhile, many TLR genes from crustacean were characterized, and their contribution discovered in innate immunity against several pathogens. This review was aimed to present an overview of the crustacean TLRs including structural features that contained three major domains: a leucine-rich repeat (LRR) domains, a transmembrane area (TM), and a conserved region called Toll/interleukin-1 receptor (TIR) domain. The tissue distribution patterns of TLR genes, which act as a guide for future research on which TLR gene or genes that can be expressed, at which tissue or tissues. We also described recent works on the expression of the TLR gene that evaluated the immune function after pathogen stimulation in shrimp, crab, and crayfish. Furthermore, we recommended a prospective for future investigation plan that might contribute to the development and management systems in the global crustacean aquaculture industry. Lastly, we assumed that a clear understanding of the expression pattern and biological function of crustacean TLR genes could serve as a baseline for future immunological studies.

Isolation and transcriptome analysis of three subpopulations of shrimp hemocytes reveals the underlying mechanism of their immune functions

Hemocytes in shrimp play important roles in innate immune responses against pathogens. Although three types of hemocytes including hyalinocytes, semi-granulocytes and granulocytes were identified based on their morphological characters in penaeid shrimp, knowledge about the molecular basis of their functions in the immunity is still very limited. In the present study, three subpopulations of hemocytes were firstly separated by Percoll gradient centrifugation, and their transcriptomes were analyzed. The data showed that significantly differential gene expression patterns existed in different types of hemocytes. The genes encoding phagocytic receptors, lectins and actin cytoskeleton involved in phagocytosis were highly expressed in hyalinocytes, while genes involved in the humoral immunity signaling pathways were highly expressed in semi-granulocytes, and genes encoding prophenoloxidase (proPO)-activating enzyme and serine proteases involved in proPO system activation were highly expressed in granulocytes. Further flow cytometry analysis indicated that hyalinocytes were the main hemocytes subpopulation responsible for ingesting foreign fluorescent beads, and this ingestion process mainly depends on the endocytic way of macropinocytosis. These data provide valuable information for understanding the molecular basis of distinct shrimp hemocytes subpopulations of shrimp in cellular and humoral immunity.

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.

miRNAs induced by white spot syndrome virus involve in immunity pathways in shrimp Litopenaeus vannamei

White shrimp Litopenaeus vannamei are widely cultured in the world and white spot syndrome virus (WSSV) led to huge economic losses in the shrimp industry every year. In the present study, miRNAs involved in the response of shrimp L. vannamei to WSSV infection were obtained through the Illumina HiSeq 2500 high-throughput next-generation sequencing technique. A total number of 7 known miRNAs and 54 putative novel miRNAs were obtained. Among them, 14 DEMs were identified in the shrimp infected with WSSV. The putative target genes of these DEMs were related to host immune response or signaling pathways, indicating the importance of miRNAs in shrimp against WSSV infection. The results will provide information for further research on shrimp response to virus infection and contribute to the development of new strategies for effective protection against WSSV infections.

The white spot syndrome virus hijacks the expression of the Penaeus vannamei Toll signaling pathway to evade host immunity and facilitate its replication

The white spot syndrome virus (WSSV), the most lethal pathogen of shrimp, is a dsDNA virus with approximately a 300,000 base pairs and contains approximately 180-500 predicted open reading frames (ORFs), of which only 6% show homology to any known protein from other viruses or organisms. Although most of its ORFs encode enzymes for nucleotide metabolism, DNA replication, and protein modification, the WSSV uses some of its encoded proteins successfully to take control of the metabolism of the host and avoid immune responses. The contribution of the shrimp innate immune response to prevent viral invasions is recognized but yet not fully understood. Thus, the role of several components of Toll pathway of the shrimp Penaeus vannamei against WSSV has been previously described, and the consequential effects occurring through the cascade remain unknown. In the current study the effects of WSSV over various components of the shrimp Toll pathway were studied. The gene expression of Spätzle, Toll, Tube, Cactus and Dorsal was altered after 6-12 h post inoculation. The expression of LvToll3, LvCactus, LvDorsal, decreased ~4.4-, ~3.7- and ~7.3-fold at 48, 24 and 48 hpi, respectively. Furthermore, a remarkable reduction (~18-fold) in the expression of the gene encoding LvCactus in WSSV infected specimens was observed at 6 hpi. This may be a sophisticated strategy exploited by WSSV to evade the Toll-mediated immune action, and to promote its replication, thereby contributing to viral fitness.

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.

Effectiveness of traditional Chinese herbal medicine, San-Huang-San, in combination with enrofloxacin to treat AHPND-causing strain of Vibrio parahaemolyticus infection in Litopenaeus vannamei

The effects of oral administration of enrofloxacin (ENR) and San-Huang-San (SHS), singly or in combination, on the survival performance, disease resistance, and immunity of Litopenaeus vannamei were investigated. After challenge with an AHPND-causing strain of Vibrio parahaemolyticus (VP_AHPND), shrimp were immediately fed a drug-free diet, diets containing only ENR (20 mg·kg^-1) or SHS (500 mg·kg^-1) or diets containing low-dose (10 mg·kg^-1 ENR + 250 mg ·kg^-1 SHS), medium-dose (20 mg·kg^-1 ENR + 500 mg ·kg^-1 SHS), and high-dose (40 mg·kg^-1 ENR + 1000 mg ·kg^-1 SHS) drug combinations for 5 days. The cumulative shrimp mortality over 5 days after injection of VP_AHPND in the ENR + SHS combination groups was significantly lower than that in the ENR or SHS alone groups (p < 0.05). Immune parameters, including the vibrio density, total hemocyte counts (THCs), hemocyanin (HEM) concentration, antibacterial activity, activity levels of lysozyme (LZM), acid phosphatase (ACP), alkaline phosphatase (AKP), and phenoloxidase (PO) in cell-free hemolymph, and the expression levels of the immune-related genes anti-lipopolysaccharide factor (ALF), cathepsin B (catB), crustin, lectin (Lec), lysozyme (LZM), and Toll-like receptor (TLR) in hemocytes were determined in the shrimp. The results showed that the shrimp in drug combination groups cleared more VP_AHPND than that in the ENR or SHS group in the same time. The values for other immune parameters in the drug combination groups were higher than those in the ENR or SHS group (p < 0.05). Finally, in the histological examinations, the histological structural alignment and integrity of the hepatopancreatic tubules in the drug combination groups were better than that in the ENR and SHS groups. Under the experimental conditions, compared with ENR or SHS used alone, the combination use of ENR and SHS could improve immunity and disease resistance in shrimp after VP_AHPND infection, and could reduce the use of ENR when the better therapeutic effect was achieved.

Evaluation of combination effects of Astragalus polysaccharides and florfenicol against acute hepatopancreatic necrosis disease-causing strain of Vibrio parahaemolyticus in Litopenaeus vannamei

The effects of oral administration of Astragalus polysaccharides (APS) and florfenicol (FFC), singly or in combination, on the survival performance, disease resistance, and immunity of Litopenaeus vannamei were investigated. After challenge with an AHPND-causing strain of Vibrio parahaemolyticus (VP_AHPND), shrimp were immediately fed a drug-free diet, diets containing only APS (200 mg·kg^-1) or FFC (15 mg·kg^-1), or diets containing low-dose (7.5 mg·kg^-1 FFC + 100 mg·kg^-1 APS), medium-dose (15 mg·kg^-1 FFC + 200 mg·kg^-1 APS), and high-dose (30 mg·kg^-1 FFC+400 mg·kg^-1 APS) drug combinations for 5 days. The cumulative shrimp mortality over 5 days after injection of VP_AHPND in the APS + FFC combination groups was significantly lower than that in the APS or FFC alone groups (p < 0.05). Immune parameters, including the total hemocyte counts (THCs), hemocyanin (HEM) concentration, antibacterial activity, activity levels of lysozyme (LZM), and levels of acid phosphatase (ACP), alkaline phosphatase (AKP), and phenoloxidase (PO) in cell-free hemolymph, and the expression levels of the immune-related genes anti-lipopolysaccharide factor (ALF), cathepsin B (catB), crustin, lectin (Lec), lysozyme (LZM), and Toll-like receptor (TLR) in hemocytes and hepatopancreas were determined in the shrimp. The values for these immune parameters in the drug combination groups were higher than those in the APS or FFC group (p < 0.05). Finally, in the histological examinations, the histological structural alignment and integrity of the hepatopancreatic tubules in the drug combination groups was better than that in the APS and FFC groups. Under the experimental conditions, dietary APS and FFC had a synergistic effect on immunity and disease resistance among shrimp after VP_AHPND infection.

Transcriptome of white shrimp Litopenaeus vannamei induced with rapamycin reveals the role of autophagy in shrimp immunity

Autophagy plays a vital role in innate and adaptive immunity against invading microorganisms, such as virus and bacteria. However, the mechanism underlying autophagy in shrimp is still limited. In our study, we challenged white shrimp L. vannamei with rapamycin to induce autophagy and employed Solexa/Illumina high-throughput RNA-seq method to examine the differences of transcriptome from gills of shrimps treated with or without rapamycin. More than 22.64 Gb raw data were produced, which were assembled into 62, 503 unigenes, with 14,126 unigenes over 1 kb in length. We then performed differential expression analysis and identified a total of 3050 differentially expressed genes (DEGs). Among them, 1456 were upregulated and 1594 were downregulated. We further annotated DEGs by matching against non-redundant protein sequence (Nr), Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes (KEGG), Clusters of Orthologous Groups of proteins (COG), euKaryotic Orthologous Groups (KOG), Gene ontology (GO), and Pfam databases. The assembled and annotated DEGs will facilitate our understanding of the molecular mechanism underlying autophagy and promote the studies on the role of autophagy in innate immunity of L. vannamei and other crustaceans.

Identification and characterization of two novel vascular endothelial growth factor genes in Litopenaeus vannamei

Vascular endothelial growth factor (VEGF) signaling pathway induces endothelial cell proliferation, promotes cell migration, and inhibits apoptosis. Although three VEGF and two VEGF receptor genes have been identified in Litopenaeus vannamei and demonstrated their roles in WSSV infection, another two novel VEGF genes (LvVEGF4, LvVEGF5) were isolated and their involvements in the WSSV infection of shrimp were studied in the present study. The deduced amino acid sequences of both LvVEGF4 and LvVEGF5 contained a signal peptide, a typical PDGF/VEGF domain and a cysteine knot motif (CXCXCX). Tissue distribution analysis showed that LvVEGF4 was predominantly expressed in gill and hemocytes, while LvVEGF5 was mainly detected in hemocytes and intestine. WSSV infection could cause up-regulation of the transcriptional levels of LvVEGF4 and LvVEGF5. Their functions were studied by double-strand RNA interference. The results showed that knock-down of LvVEGF4 and LvVEGF5 led to a decrease of the viral copy number in WSSV infected shrimp. Yeast two-hybrid analysis showed that both LvVEGF4 and LvVEGF5 could interact with LvVEGFR1 rather than LvVEGFR2. In addition, knock-down of LvVEGF4 and LvVEGF5 could reduce the expressional levels of downstream genes FAK and PI3K. The present study provides new clues in demonstrating that the VEGF signaling pathway is involved in the process of WSSV infection in shrimp.

Transcriptome-wide identification, functional characterization, and expression analysis of two novel invertebrate-type Toll-like receptors from disk abalone (Haliotis discus discus)

Toll-like receptors (TLRs) are well-known pattern recognition receptors that play key immunological roles in a diverse range of organisms. In this study, two novel invertebrate TLRs from disk abalone (designated as AbTLR-A and AbTLR-B) were identified and functionally characterized for the first time. AbTLR-A and AbTLR-B comprised the typical TLR domain architecture containing an extracellular leucine-rich repeat domain, transmembrane domain, and Toll/interleukin-1 receptor domain. Expressional analysis revealed that both TLRs were constitutively expressed at all the early embryonic stages of disk abalone analyzed, with the highest level of AbTLR-A found at the 16-cell stage and AbTLR-B at the trochophore stage. According to tissue distribution analysis, prominent mRNA expression of AbTLR-A and AbTLR-B was detected in the hemocytes and gills, respectively. AbTLR-A and AbTLR-B mRNAs were significantly up-regulated in response to Gram-negative Vibrio parahemolyticus, Gram-positive Listeria monocytogenes, and viral hemorrhagic septicemia virus injections in abalone hemocytes and gills. Overexpression of AbTLR-A and AbTLR-B in HEK293T cells directly activated nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) responsive reporters. Neither TLRs showed a high response to pathogen-associated molecular patterns in vitro. Co-expression of AbTLR-A and AbTLR-B with AbMyD88-2 and AbMyD88-X activated NF-κB-responsive reporters in a synergetic manner. These findings demonstrate the involvement of AbTLR-A and AbTLR-B in abalone innate immunity.

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.

Identification of Litopenaeus vannamei BiP as a novel cellular attachment protein for white spot syndrome virus by using a biotinylation based affinity chromatography method

White spot syndrome virus (WSSV) is a dangerous threat to shrimp farming that also attacks a wide range of crustaceans. Knowledge of the surface protein-protein interactions between the pathogen and host is very crucial to unraveling the molecular pathogenesis mechanisms of WSSV. In this study, LvBiP (Litopenaeus vannamei immunoglobulin heavy-chain-binding protein) was identified as a novel WSSV binding protein of L. vannamei by a biotinylation based affinity chromatography method. By using pull-down and ELISA assays, the binding of recombinant LvBiP to WSSV was proved to be specific and ATP- dependent. The interaction was also confirmed by the result of co-immunoprecipitation assay. Immunofluorescence studies revealed the co-localization of LvBiP with WSSV on the cell surface of shrimp haemocytes. Additionally, LvBiP is likely to play an important role in WSSV infection. Treatment of gill cellular membrane proteins (CMPs) with purified rLvBiP and antibody that specifically recognizes LvBiP, led to a significant reduction in the binding of WSSV to gill CMPs. In the in vivo neutralization assay, rLvBiP and anti-LvBiP polyclonal antibody partially blocked the infection of WSSV. Taken together, the results indicate that LvBiP, a molecular chaperon of the HSP70 family, is a novel host factor involved at the step of attachment of the WSSV to the host cells and a potential candidate of therapeutic target.

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.

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.

The silkrose of Bombyx mori effectively prevents vibriosis in penaeid prawns via the activation of innate immunity

We previously identified novel bioactive polysaccharides from Bactrocera cucurbitae and Antheraea yamamai that activate innate immunity in RAW264 murine macrophages. However, in terms of potential applications in the cultivation of prawns, there were problems with the availability of these insects. However, we have now identified a polysaccharide from Bombyx mori that activates innate immunity in RAW264 cells and penaeid prawns. This purified polysaccharide, termed silkrose of B. mori (silkrose-BM), has a molecular weight of 1,150,000 and produces a single symmetrical peak on HPLC. Eight of nine constitutive monosaccharides of silkrose-BM are concomitant with dipterose of B. cucurbitae (dipterose-BC) and silkrose of A. yamamai (silkrose-AY). The major differences are found in the molar ratios of the monosaccharides. Silkrose-BM is approximately 500-fold less potent than silkrose-AY (EC₅₀: 2.5 and 0.0043 μg/mL, respectively) in a nitrite oxide (NO) production assay using RAW264 cells. However, the maximum NO production for silkrose-BM and AY were comparable and higher than that of the lipopolysaccharide of Escherichia coli. The survival of penaeid prawns (Litopenaeus vannamei and Marsupenaeus japonicus) after infection with Vibrio penaecida was significantly improved by both dietary silkrose-BM and B. mori pupae. This suggests that silkrose-BM effectively prevents vibriosis in penaeid prawns via the activation of innate immunity.

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.

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.

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.

Activation of Toll Pathway Is Different between Kuruma Shrimp and Drosophila

The Toll pathway is essential for inducing an immune response to defend against bacterial invasion in vertebrates and invertebrates. Although Toll receptors and the transcription factor Dorsal were identified in different shrimp, relatively little is known about how the Toll pathway is activated or the function of the pathway in shrimp antibacterial immunity. In this study, three Tolls (Toll1–3) and the Dorsal were identified in Marsupenaeus japonicus. The Toll pathway can be activated by Gram-positive (G⁺) and Gram-negative (G⁻) bacterial infection. Unlike Toll binding to Spätzle in Drosophila, shrimp Tolls could directly bind to pathogen-associated molecular patterns from G⁺ and G⁻ bacteria, resulting in Dorsal translocation into nucleus to regulate the expression of different antibacterial peptides (AMPs) in the clearance of infected bacteria. These findings suggest that shrimp Tolls are pattern recognition receptors and the Toll pathway in shrimp is different from the Drosophila Toll pathway but identical with the mammalian Toll-like receptor pathway in its activation and antibacterial functions.

CqToll participates in antiviral response against white spot syndrome virus via induction of anti-lipopolysaccharide factor in red claw crayfish Cherax quadricarinatus

It is well known that Tolls/Toll like receptors (TLRs), a family of pattern recognition receptors, play important roles in immune responses. Previously, we found that a Toll transcript was increased in a transcriptome library of haematopoietic tissue (Hpt) cells from the red claw crayfish Cherax quadricarinatus post white spot syndrome virus infection. In the present study, a full-length cDNA sequence of Toll receptor (named as CqToll) was identified with 3482 bp which contained an open reading frame of 3021 bp encoding 1006 amino acids. The predicted structure of CqToll protein was composed of three domains, including an extracellular domain of 19 leucine-rich repeats residues, a transmembrane domain and an intracellular domain of 138 amino acids. Tissue distribution analysis revealed that CqToll was expressed widely in various tissues determined from red claw crayfish with highest expression in haemocyte but lowest expression in eyestalk. Importantly, significant lower expression of the anti-lipopolysacchride factor (CqALF), an antiviral antimicrobial peptide (AMP) in crustaceans, but not CqCrustin was observed after gene silencing of CqToll in crayfish Hpt cell cultures, indicating that the CqALF was likely to be positively regulated via Toll pathway in red claw crayfish. Furthermore, the transcription of both an immediate early gene and a late envelope protein gene VP28 of WSSV were clearly enhanced in Hpt cells if silenced with CqToll, suggesting that the increase of WSSV replication was likely to be caused by the lower expression of the CqALF resulted from the loss-of-function of CqToll. Taken together, these data implied that CqToll might play a key role in anti-WSSV response via induction of CqALF in a crustacean C. quadricarinatus.

Newly identified PcToll4 regulates antimicrobial peptide expression in intestine of red swamp crayfish Procambarus clarkii

Tolls or Toll-like receptors (TLRs) have an essential role in initiating innate immune responses against pathogens. In this study, a novel Toll gene, PcToll4, was first identified from the intestinal transcriptome of the freshwater crayfish, Procambarus clarkii. The PcToll4 cDNA is 4849bp long with a 3036bp open reading frame that encodes a 1011-amino acid protein. PcToll4 contains a signal peptide, 13 LRR domains, 3 LRR TYP domains, 2 LRR CT domains, an LRR NT domain, a transmembrane region, and a TIR domain. Quantitative RT-PCR analysis revealed that PcToll4 mRNA was detected in all tested tissues, and the expression of PcToll4 in the intestine was significantly upregulated after white spot syndrome virus (WSSV) challenge. Overexpression of PcToll4 in Drosophila Schneider 2 (S2) cells activates the antimicrobial peptides (AMPs) of Drosophila, including metchnikowin, drosomycin, attacin A, and shrimp Penaeidin-4. Results of RNA interference by siRNA also showed that PcToll4 regulates the expressions of 5 anti-lipopolysaccharide factors (ALFs) in the intestine of crayfish. Our findings suggest that PcToll4 is important for the innate immune responses of P. clarkii because this gene regulates the expressions of AMPs against WSSV.

PcToll2 positively regulates the expression of antimicrobial peptides by promoting PcATF4 translocation into the nucleus

Drosophila Toll and mammalian Toll-like receptors (TLRs) are a family of evolutionarily conserved immune receptors that play a crucial role in the first-line defense against intruded pathogens. Activating transcription factor 4 (ATF4), a member of the ATF/CREB transcription factor family, is an important factor that participates in TLR signaling and other physiological processes. However, in crustaceans, whether ATF4 homologs were involved in TLR signaling remains unclear. In the current study, we identified a Toll homolog PcToll2 and a novel ATF4 homolog PcATF4 from Procambarus clarkii, and analyzed the likely regulatory activity of PcATF4 in PcToll2 signaling. The complete cDNA sequence of PcToll2 was 4175 bp long containing an open reading frame of 2820 bp encoding a 939-amino acid protein, and the cDNA sequence of PcATF4 was 2027 bp long with an open reading frame of 1296 bp encoding a 431-amino acid protein. PcToll2 and human TLR4 shared the high identity and they were grouped into a cluster. Furthermore, PcToll2 had a close relationship with other shrimp TLRs that possessed potential antibacterial activity. PcToll2 was highly expressed in the hemocytes, heart and gills, while PcATF4 mainly distributed in gills. Upon challenge with Vibrio parahemolyticus, PcToll2 and PcATF4 together with the antimicrobial peptides of ALF1 and ALF2 were significantly up-regulated in the hemocytes, and the PcATF4 was translocated into the nucleus. After PcToll2 silencing and challenge with Vibrio, the translocation of PcATF4 into the nucleus was inhibited and the expression of ALF1 and ALF2 was reduced, but the expression of PcDorsal and PcSTAT was not affected. Furthermore, after PcATF4 knockdown and challenge with or without Vibrio, the expression of ALF1 and ALF2 was also decreased while the expression of PcToll2 was upregulated. These results suggested that PcToll2 might regulate the expression of ALF1 and ALF2 by promoting the import of PcATF4, instead of the routine transcription factor PcDorsal, into the nucleus participating in the immune defense against Gram-negative bacteria.

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.

Innate immune-stimulating and immune genes up-regulating activities of three types of alginate from Sargassum siliquosum in Pacific white shrimp, Litopenaeus vannamei

The Total Haemocyte Count (THC), phenoloxidase (PO), Superoxide Dismutase (SOD) activity, Phagocytic Activity/Index and Total Protein Plasma (TPP) were examined after feeding the white shrimp Litopenaeus vannamei with diets supplemented with three different types of alginates (acid, calcium and sodium alginates). Immune-related genes expression was evaluated by quantitative Real Time PCR (qRT-PCR). Results indicated that the immune parameters directly increased according to the doses of alginates and time. The 2.0 g kg(-1) of acid and sodium alginate treatments were gave better results. Four immune-related genes expression i.e. LGBP, Toll, Lectin, proPO were up regulated. It is therefore concluded that the supplementation of alginate of Sargassum siliquosum on the diet of L. vannamei enhanced the innate immunity as well as the expression of immune-related genes. It is the first report on the simultaneous evaluation of three alginate types to enhance innate immune parameters and immune-related genes expression in L. vannamei.

Next-Generation Sequencing and the Crustacean Immune System: The Need for Alternatives in Immune Gene Annotation

Next-generation sequencing has been a huge benefit to investigators studying non-model species. High-throughput gene expression studies, which were once restricted to animals with extensive genomic resources, can now be applied to any species. Transcriptomic studies using RNA-Seq can discover hundreds of thousands of transcripts from any species of interest. The power and limitation of these techniques is the sheer size of the dataset that is acquired. Parsing these large datasets is becoming easier as more bioinformatic tools are available for biologists without extensive computer programming expertise. Gene annotation and physiological pathway tools such as Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) Orthology enable the application of the vast amount of information acquired from model organisms to non-model species. While noble in nature, utilization of these tools can inadvertently misrepresent transcriptomic data from non-model species via annotation omission. Annotation followed by molecular pathway analysis highlights pathways that are disproportionately affected by disease, stress, or the physiological condition being examined. Problems occur when gene annotation procedures only recognizes a subset, often 50% or less, of the genes differently expressed from a non-model organisms. Annotated transcripts normally belong to highly conserved metabolic or regulatory genes that likely have a secondary or tertiary role, if any at all, in immunity. They appear to be disproportionately affected simply because conserved genes are most easily annotated. Evolutionarily induced specialization of physiological pathways is a driving force of adaptive evolution, but it results in genes that have diverged sufficiently to prevent their identification and annotation through conventional gene or protein databases. The purpose of this manuscript is to highlight some of the challenges faced when annotating crustacean immune genes by using an American lobster (Homarus americanus) transcriptome as an example. Immune genes have evolved rapidly over time, facilitating speciation and adaption to highly divergent ecological niches. Complete and proper annotation of immune genes from invertebrates has been challenging. Modulation of the crustacean immune system occurs in a variety of physiological responses including biotic and abiotic stressors, molting and reproduction. A simple method for the identification of a greater number of potential immune genes is proposed, along with a short introductory primer on crustacean immune response. The intended audience is not the advanced bioinformatic user, but those investigating physiological responses who require rudimentary understanding of crustacean immunological principles, but where immune gene regulation is not their primary interest.

Identification and functional characterization of a solute carrier family 15, member 4 gene in Litopenaeus vannamei

Innate immunity in shrimp is important in resisting bacterial infection. The NF-κB pathway is pivotal in such an immune response. This study cloned and functionally characterized the solute carrier family (SLC) 15 member A 4 (LvSLC15A4) gene in Litopenaeus vannamei. The open reading frame of LvSLC15A4 is 1, 902 bp long and encodes a putative 633-amino acid protein, which is localized in the plasma membrane and intracellular vesicular compartments. Results of the reporter gene assay showed that LvSLC15A4 upregulated NF-κB target genes, including the immediate-early gene 1 of white spot syndrome virus, as well as several antimicrobial peptide genes, such as pen4, CecA, AttA, and Mtk in S2 cells. Moreover, knocked-down expression of LvSLC15A4 reduced pen4 expression in L. vannamei. LvSLC15A4 down-regulation also increased the cumulative mortality of Vibrio parahemolyticus-infected L. vannamei. Furthermore, LvSLC15A4 expression was induced by unfolded protein response (UPR) in L. vannamei hematocytes. These results suggest that LvSLC15A4 participates in L. vannamei innate immunity via the NF-κB pathway and thus may be related to UPR.

YHV-responsive gene expression under the influence of PmRelish regulation

In animals, infection by Gram-negative bacteria and certain viruses activates the Imd signaling pathway wherein the a NF-κB transcription factor, Relish, is a key regulatory protein for the synthesis of antimicrobial proteins. Infection by yellow head virus (YHV) activates the Imd pathway. To investigate the expression of genes involved in YHV infection and under the influence of PmRelish regulation, RNA interference and suppression subtractive hybridization (SSH) are employed. The genes in forward library expressed in shrimp after YHV infection and under the activity of PmRelish were obtained by subtracting the cDNAs from YHV-infected and PmRelish-knockdown shrimp with cDNAs from YHV-infected shrimp. Opposite subtraction gave a reverse library whereby an alternative set of genes under YHV infection and no PmRelish expression were obtained. Nucleotide sequences of 252 and 99 cDNA clones from the forward and reverse libraries, respectively, were obtained and annotated through blast search against the GenBank sequences. Genes involved in defense and homeostasis were abundant in both libraries, 31% and 23% in the forward and reverse libraries, respectively. They were predominantly antimicrobial proteins, proteinases and proteinase inhibitors. The expression of antimicrobial protein genes, ALFPm3, crustinPm1, penaeidin3 and penaeidin5 were tested under PmRelish silencing and Gram-negative bacterium Vibrio harveyi infection. Together with the results using YHV infection previously reported, the expression of penaeidin5 and also penaeidin3 but not ALFPm3 and crustinPm1 were under the regulation of PmRelish in the Imd pathway.

Three novel Toll genes (PtToll1-3) identified from a marine crab, Portunus trituberculatus: Different tissue expression and response to pathogens

The Toll signaling pathway is one of the most important regulators of the immune response in both vertebrates and invertebrates. Herein, three novel Toll (PtToll1-3) cDNA sequences were cloned from the swimming crab, Portunus trituberculatus. PtToll1 has 1003 amino acid residues and consists of an extracellular domain containing 15 leucine-rich repeats (LRRs) and a cytoplasmic Toll/interleukin-1 receptor (TIR) domain of 139 residues. PtToll2 encodes 1196 peptides, with an extracellular domain containing 28 LRRs and a cytoplasmic TIR domain. PtToll3 is 1229 residues long and contains 26 LRRs and a cytoplasmic TIR domain. Based on sequence and phylogenetic analyses, PtToll1 distinctly clustered with almost all crustacean Tolls, except Litopenaeus vannamei Toll3. However, PtToll2 and PtToll3 were separated from most reported crustacean Tolls, which mostly clustered with Drosophila melanogaster (Dm) Toll8, L. vannamei Toll3, and DmToll6. Reverse transcription PCR and real-time quantitative PCR analyses showed that PtToll1 and PtToll3 were constitutively expressed in all tissues tested, but PtToll2 mRNA was only highly enriched in gills. Upon challenges with Vibrio alginolyticus, Candida lusitaniae, or white spot syndrome virus (WSSV), the three Tolls exhibited different responses: the PtToll1 transcript was up-regulated in response to C. lusitaniae or V. alginolyticus challenge, but did not respond to WSSV challenge; both PtToll2 and PtToll3 mRNAs were down-regulated 12 h after C. lusitaniae or V. alginolyticus infection. However, WSSV elicited the expression of PtToll2 at 6 h post-infection, but suppressed transcription of PtToll3 at 24 h post-infection. The study provides valuable data for understanding the role of Toll pathways in the host defense against microbial pathogens, which will facilitate future studies on host-pathogen interactions in crabs.

The broad pattern recognition spectrum of the Toll-like receptor in mollusk Zhikong scallop Chlamys farreri

Toll-like receptors (TLRs) are among the most studied pattern recognition receptors (PRRs) playing essential roles in innate immune defenses. In the present study, the basic features of CfTLR in mollusk Zhikong scallop Chlamys farreri, including sequence homology, tissue distribution, subcellular localization and ligands spectrum, were investigated to elucidate its pattern recognition. The elements of extracellular domains (ECD) in CfTLR displayed high homology to the corresponding parts of the ECDs in TLRs from Homo sapiens. CfTLR protein was detected in hemocytes, mantle, gills, hepatopancreas, kidney and gonad of the scallops, and it was localized in both the plasma membranes and the lysosomes in HEK293T cells. CfTLR could activate NFκB in response to multiple HsTLR ligands including Pam3CSK4, glucan (GLU), peptidoglycan (PGN), polyriboinosinic:polyribocytidylic acid (poly I:C), Imiquimod and three types of CpG. Additionally, the scallop serum could enhance the induction of NFκB in the CfTLR expressing cells elicited by most PAMPs, including GLU, PGN, Imiquimod and four types of CpG. It could be concluded that this primitive mollusk TLR shared a hybrid function in pattern recognition and could recognize broader ligands than mammalian TLRs, and its mosaic capability of pathogen associated molecular pattern (PAMP) recognition might be based on the basic features of its structure, ligand properties and the assistance of some components in scallop serum.

Antimicrobial proteins: From old proteins, new tricks

This review describes the main types of antimicrobial peptides (AMPs) synthesised by crustaceans, primarily those identified in shrimp, crayfish, crab and lobster. It includes an overview of their range of microbicidal activities and the current landscape of our understanding of their gene expression patterns in different body tissues. It further summarises how their expression might change following various types of immune challenges. The review further considers proteins or protein fragments from crustaceans that have antimicrobial properties but are more usually associated with other biological functions, or are derived from such proteins. It discusses how these unconventional AMPs might be generated at, or delivered to, sites of infection and how they might contribute to crustacean host defence in vivo. It also highlights recent work that is starting to reveal the extent of multi-functionality displayed by some decapod AMPs, particularly their participation in other aspects of host protection. Examples of such activities include proteinase inhibition, phagocytosis, antiviral activity and haematopoiesis.

First description and expression analysis of tumor necrosis factor receptor-associated factor 6 (TRAF6) from the swimming crab, Portunus trituberculatus

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a cytoplasmic adapter protein that mediates signals induced by the tumor necrosis factor receptor (TNFR) superfamily and the interleukin-1 receptor (IL-1R). In the present study, the full-length cDNA of TRAF6 (Pt-TRAF6) was identified in a marine crab, Portunus trituberculatus. Pt-TRAF6 ORF is predicted to encode a 599-amino acid protein, including a RING type zinc finger, two TRAF-type zinc fingers, and a meprin and TRAF homology (MATH) domain. The overall amino acid sequence identity between Pt-TRAF6 and other TRAF6s ranged from 50.9 to 51.3% for shrimp and from 16.1 to 19.4% for insects. The Pt-TRAF6 gene contains six exons and five introns, which is different from the organization of the insect TRAF6 gene. Pt-TRAF6 transcripts were broadly expressed in all tissues tested, and their expression was higher in hemocytes, gills, the intestine, and heart than in muscle. Interestingly, the level of Pt-TRAF6 transcript differed between male and female crabs. After Vibrio alginolyticus or lipopolysaccharide (LPS) challenge, the Pt-TRAF6 transcript was down-regulated in hemocytes and up-regulated in gills. Moreover, Pt-TRAF6 expression was altered sooner in the LPS challenge group than in the V. alginolyticus challenge group. These results indicate that Pt-TRAF6 may respond to Gram-negative bacterial infections.

Molecular characterization and expression of a novel Toll gene from the swimming crab Portunus trituberculatus

Tolls/Toll-like receptors (TLRs) are important cell-surface receptors serving as pattern recognition receptors (PRRs) in the Tolls/TLRs signaling pathway of innate immune responses. In the present study, we isolated and characterized a novel Toll gene (PtToll) from Portunus trituberculatus, and further investigated its expression in various tissues of crab hosts challenged with the parasitic dinoflagellate Hematodinium. The full-length cDNA of PtToll was 3745 bp, with a 3012 bp open reading frame (ORF) encoding 1003 amino acids. Conserved domains consist of 15 tandem leucine-rich repeats (LRRs), a single-pass transmembrane segment (TM) and a cytoplasmic Toll/interleukin-1R (TIR) domain. The PtToll protein shared high similarity to other crustacean Tolls and was clustered with the crustacean Tolls in the phylogenetic tree. The PtToll gene was constitutively expressed in various tissues of P. trituberculatus, with the highest expression in hemocytes. After being challenged with the parasite, the transcripts of PtToll reacted immediately with significant alterations in all the tested tissues, and decreased consistently in most of the detected tissues (e.g., hemocytes, gill, heart, and muscle) within 24h. Then the transcripts of PtToll were significantly up-regulated in hemocytes and heart at 48 h, and in hepatopancreas at 48 and 96 h post the parasitic challenge. By 192 h post challenge, the transcriptional level of PtToll indicated a significant suppression or a decreasing trend. The fluctuations of PtToll gene expression suggested that PtToll was closely associated with intrusion of the Hematodinium parasites, and may possess a vital and systematic function in the innate immunity of P. trituberculatus against the parasitic infection.

Recent insights into host-pathogen interaction in white spot syndrome virus infected penaeid shrimp

Viral disease outbreaks are a major concern impeding the development of the shrimp aquaculture industry. The viral disease due to white spot syndrome virus (WSSV) observed in early 1990s still continues unabated affecting the shrimp farms and cause huge economic loss to the shrimp aquaculture industry. In the absence of effective therapeutics to control WSSV, it is important to understand viral pathogenesis and shrimp response to WSSV at the molecular level. Identification and molecular characterization of WSSV proteins and receptors may facilitate in designing and development of novel therapeutics and antiviral drugs that may inhibit viral replication. Investigations into host-pathogen interactions might give new insights to viral infectivity, tissue tropism and defence mechanism elicited in response to WSSV infection. However, due to the limited information on WSSV gene function and host immune response, the signalling pathways which are associated in shrimp pathogen interaction have also not been elucidated completely. In the present review, the focus is on those shrimp proteins and receptors that are potentially involved in virus infection or in the defence mechanism against WSSV. In addition, the major signalling pathways involved in the innate immune response and the role of apoptosis in host-pathogen interaction is discussed.