A novel vertebrates Toll-like receptor counterpart regulating the anti-microbial peptides expression in the freshwater crayfish, Procambarus clarkii

Research progress of pattern recognition receptors in red swamp crayfish (Procambarus clarkii)

Though Procambarus clarkii (red swamp crayfish) is a lower invertebrate, it has nonetheless developed a complex innate immune system. The crayfish farming industry has suffered considerable economic losses in recent years as a consequence of bacterial and viral diseases. Hence, perhaps the most effective ways to prevent microbial infections in P. clarkii are to examine and elucidate its innate immunity. The first step in the immune response is to recognize pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs). PRRs are expressed mainly on immune cell surfaces and recognize at least one PAMP. Thence, downstream immune responses are activated and pathogens are phagocytosed. To date, the PRRs identified in P. clarkii include Toll-like receptors (TLRs), lectins, fibrinogen-related proteins (FREPs), and β-1,3-glucan-binding proteins (BGRPs). The present review addresses recent progress in research on PRRs and aims to provide guidance for improving immunity and preventing and treating infectious diseases in P. clarkii.

Effects of Hizikia fusiforme polysaccharides on innate immunity and disease resistance of the mud crab Scylla paramamosain

In this study, we extracted the polysaccharides from Hizikia fusiforme (HFPs) and evaluated their effects on the immune response of the mud crab Scylla paramamosain. Compositional analysis revealed that HFPs were composed mainly of mannuronic acid (49.05%) and fucose (22.29%) as sulfated polysaccharides, and the sugar chain structure was β-type. These results indicated that HFPs have potential antioxidant and immunostimulation activity in vivo or in vitro assays. Through this research, we found that HFPs inhibited viral replication in white spot syndrome virus (WSSV)-infected crabs and promoted phagocytosis of Vibrio alginolyticus by hemocytes. Quantitative PCR results showed that HFPs up-regulated the expression levels of astakine, crustin, myosin, MCM7, STAT, TLR, JAK, CAP, and p53 in crab hemocytes. HFPs also promoted the activities of superoxide dismutase and acid phosphatase and the hemolymph antioxidant activities of crabs. HFPs maintained peroxidase activity after WSSV challenge, thereby providing protection against oxidative damage caused by the virus. HFPs also promoted apoptosis of hemocytes after WSSV infection. In addition, HFPs significantly enhanced the survival rate of WSSV-infected crabs. All results confirmed that HFPs improved the innate immunity of S. paramamosain by enhancing the expression of antimicrobial peptides, antioxidant enzyme activity, phagocytosis, and apoptosis. Therefore, HFPs have potential for use as therapeutic or preventive agents to regulate the innate immunity of mud crabs and protect them against microbial infection.

Sub-chronic ammonia exposure induces hepatopancreatic damage, oxidative stress, and immune dysfunction in red swamp crayfish (Procambarus clarkii)

Ammonia, as one of the primary water pollutants in aquaculture, has been shown to induce a wide range of ecotoxicological effects on aquatic animals. In order to investigate the antioxidant and innate immune responses in crustaceans disrupted by ammonia, red swamp crayfish (Procambarus clarkii) were exposed to 0, 15, 30, and 50 mg/L total ammonia nitrogen for 30 d, the alterations of antioxidant responses as well as innate immunity were studied. The results showed that the severity of hepatopancreatic injury were aggravated by the increasing ammonia levels, which were mainly characterized by tubule lumen dilatation and vacuolization. The swollen mitochondria and disappeared mitochondria ridges suggested that oxidative stress induced by ammonia targets the mitochondria. Concurrently, enhanced MDA levels, and decreased GSH levels as well as the decreased transcription and activity of antioxidant enzymes, including SOD, CAT, and GPx were noticed, which suggested that high concentrations of ammonia exposure induce oxidative stress in P. clarkii. Furthermore, a significant decrease of the hemolymph ACP, AKP, and PO along with the significant downregulation of immune-related genes (ppo, hsp70, hsp90, alf1, ctl) jointly indicated that ammonia stress inhibited the innate immune function. Our findings demonstrated that sub-chronic ammonia stress induced hepatopancreatic injury and exert suppressive effects on the antioxidant capacity as well as innate immunity of P. clarkii. Our results provide a fundamental basis for the deleterious effects of ammonia stress on aquatic crustaceans.

Distribution and influencing factors of antibiotic resistance genes of crayfish (Procambarus clarkii) intestine in main crayfish breeding provinces in China

The propagation of antibiotic resistance genes (ARGs) has become a global public health concern. However, the distribution and influencing factors of ARGs, especially high-risk ARGs, in the gut of aquaculture animals remain unclear. Here, we employed 16S rRNA gene sequencing and high-throughput quantitative PCR techniques to determine crayfish gut microbiota and ARGs collected from 40 culture ponds in major crayfish farming provinces of China. We detected 74 ARGs in crayfish gut. Among them, the beta-lactamase and tetracycline resistance genes were dominant. The total ARG abundance was the highest in Hubei Province. High-risk ARGs were also found in crayfish gut, and ermB had the highest abundance and distributed in Anhui, Hubei, Henan and Jiangxi Province. In addition, opportunistic pathogens (Streptococcus, Aeromonas and Acinetobacter) might be potential hosts for ARGs, including high-risk ARGs. Finally, habitat, environmental factors (NO₃-N, pH and temperature), microbial alpha diversity and mobile genetic elements (MGEs) showed significant influence on ARGs profiles. Generally, our results illustrate that ARGs are prevalent in crayfish gut and may pose potential risk to human health, which will help develop targeted strategies for the risk management and assessment of ARGs in the aquaculture.

The immunostimulatory effects of Theobroma cacao L. pod husk extract via injection and dietary administrations on Macrobrachium rosenbergii and its resistance against Lactococcus garvieae

The extract from Theobroma cacao L, pod husk served as the immunostimulant to enhance the immunity and resistance against Lactococcus garvieae of Macrobrachium rosenbergii. In this study, we employed the injection method and dietary administration method to determine the effect of cocoa pod husk (CPH) on M. rosenbergii. The non-specific immune parameters and disease resistance were evaluated after the prawn injected with 1 μg prawn^-1 CPH extract (C1), 3 μg prawn^-1 CPH extract (C3), and 5 μg prawn^-1 CPH extract (C5) for 1, 3, and 7 days. The results showed a significant increase of total haemocyte count (THC), differential haemocyte count (DHC), phenoloxidase (PO) activity, respiratory bursts (RBs), and phagocytic activity and clearance efficiency to L. garvieae. The non-specific immune parameters, physiological parameters, and disease resistance and growth performance were evaluated after the prawn fed with 1 g kg^-1 CPH extract diet (CD1), 3 g kg^-1 CPH extract diet (CD3) and 5 g kg^-1 CPH extract diet (CD5). The results showed a significant increase in all immune parameters and showed a significant decrease in physiological parameters. No significant difference was observed in growth performance of prawn fed with the CPH containing diet. Both injection and dietary method showed a significant increase in disease resistance against to L. garvieae. We therefore recommend that CPH extract can be used as a immunostimulant for prawn by dietary administration to regulate immune responses, and carbohydrate metabolism lead to enhance resistance against pathogen.

Full-length transcriptome sequencing and comparative transcriptome analysis of Eriocheir sinensis in response to infection by the microsporidian Hepatospora eriocheir

As a new generation of high-throughput sequencing technology, PacBio Iso-Seq technology (Iso-Seq) provides a better alternative sequencing method for the acquisition of full-length unigenes. In this study, a total of 22.27 gigabyte (Gb) subread bases and 128,614 non-redundant unigenes (mean length: 2,324 bp) were obtained from six main tissues of Eriocheir sinensis including the heart, nerve, intestine, muscle, gills and hepatopancreas. In addition, 74,732 unigenes were mapped to at least one of the following databases: Non-Redundant Protein Sequence Database (NR), Gene Ontology (GO), Kyoto Encyclopaedia of Genes and Genomes (KEGG), KEGG Orthology (KO) and Protein family (Pfam). In addition, 6696 transcription factors (TFs), 28,458 long non-coding RNAs (lncRNAs) and 94,230 mRNA-miRNA pairs were identified. Hepatospora eriocheir is the primary pathogen of E. sinensis and can cause hepatopancreatic necrosis disease (HPND); the intestine is the main target tissue. Here, we attempted to identify the key genes related to H. eriocheir infection in the intestines of E. sinensis. By combining Iso-Seq and Illumina RNA-seq analysis, we identified a total of 12,708 differentially expressed unigenes (DEUs; 6,696 upregulated and 6,012 downregulated) in the crab intestine following infection with H. eriocheir. Based on the biological analysis of these DEUs, several key processes were identified, including energy metabolism-related pathways, cell apoptosis and innate immune-related pathways. Twelve selected genes from these DEUs were subsequently verified by quantitative real-time PCR (qRT-PCR) analysis. Our findings enhance our understanding of the E. sinensis transcriptome and the specific association between E. sinensis and H. eriocheir infection.

Genome-wide investigation of toll-like receptor genes (TLRs) in Procambarus clarkia and their expression pattern in response to black may disease

As a crucial component of pattern-recognition receptors (PRRs) that recognizing pathogen-associated molecular patterns (PAMPs) and defending against invading pathogens, the Toll-like receptors (TLRs) have been paid extensive attention. While the identification and functional roles of TLRs in innate immunity have been reported in a plenty of organisms, the systematic knowledge of TLRs is still lacking in the red swamp crayfish (Procambarus clarkia). In current study, a total of 7 tlr genes were identified in P. clarkia based on the published transcriptome and genome data. The PcTLRs length varied from 939 to 1517aa and contain typical domains of TLR protein, including transmembrane region, varied LRR and TIR domains. 7 Pctlr genes were distributed in 5 chromosomes and 2 scaffolds. The expression pattern of different Pctlr genes in different tissues (hepatopancreas, gill and muscle) and in response to black may disease (BMD) showed significant difference. In addition, 5 proteins that might interact with PcTLR-2 were predicted, among them the expression pattern of dorsal and relish was consistent with Pctlr-2 in three tissues, while the other genes were not. The PcTLR-2-Dorsal/Relish pathway might play crucial roles in response to BMD infection. The results provided a theoretical foundation for further studies on the molecular mechanisms of TLRs in BMD infection in the red swamp crayfish and provided reference for the research of other crustacean species.

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

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

Lysyl Oxidase-like Protein Recognizes Viral Envelope Proteins and Bacterial Polysaccharides against Pathogen Infection via Induction of Expression of Antimicrobial Peptides

Lysyl oxidases (LOXs) are copper-dependent monoamine oxidases, and they play critical roles in extracellular matrix (ECM) remodeling. The LOX and LOX-like (LOXL) proteins also have a variety of biological functions, such as development and growth regulation, tumor suppression, and cellular senescence. However, the functions of LOXLs containing repeated scavenger receptor cysteine-rich (SRCR) domains in immunity are rarely reported. In this study, we characterized the antiviral and antibacterial functions of a lysyl oxidase-like (LOXL) protein containing tandem SRCR domains in Marsupenaeus japonicus. The mRNA level of LoxL was significantly upregulated in the hemocytes and intestines of shrimp challenged using white spot syndrome virus (WSSV) or bacteria. After the knockdown of LoxL via RNA interference, WSSV replication and bacterial loads were apparently increased, and the survival rate of the shrimp decreased significantly, suggesting that LOXL functions against pathogen infection in shrimp. Mechanistically, LOXL interacted with the envelope proteins of WSSV or with lipopolysaccharide and peptidoglycan from bacteria in shrimp challenged using WSSV or bacteria, and it promoted the expression of a battery of antimicrobial peptides (AMPs) via the induction of Dorsal nuclear translocation against viral and bacterial infection. Moreover, LOXL expression was also positively regulated by Dorsal in the shrimp challenged by pathogens. These results indicate that, by acting as a pattern recognition receptor, LOXL plays vital roles in antiviral and antibacterial innate immunity by enhancing the expression of AMPs in shrimp.

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.

Integrated Analysis of mRNA-Seq and MiRNA-Seq Reveals the Molecular Mechanism of the Intestinal Immune Response in Marsupenaeus japonicus Under Decapod Iridescent Virus 1 Infection

The intestine is not only an important digestive organ but also an important immune organ for shrimp; it plays a key role in maintaining homeostasis. Decapod iridescent virus 1 (DIV1) is a new type of shrimp-lethal virus that has received extensive attention in recent years. To date, most studies of the shrimp intestinal immune response under viral infections have relied on single omics analyses; there is a lack of systematic multi-omics research. In the current study, intestinal mRNA-seq and microRNA (miRNA)-seq analyses of Marsupenaeus japonicus under DIV1 infection were performed. A total of 1,976 differentially expressed genes (DEGs) and 32 differentially expressed miRNAs (DEMs) were identified. Among them, 21 DEMs were negatively correlated with 194 DEGs from a total of 223 correlations. Functional annotation analysis revealed that M. japonicus can regulate glycosaminoglycan biosynthesis (chondroitin sulfate, dermatan sulfate, and keratan sulfate), vitamin metabolism (retinol metabolism and ascorbate and aldarate metabolism), immune pathway activation (Toll and IMD signaling pathways, Wnt signaling pathway, IL-17 signaling pathway, and Hippo signaling pathway), immunity enzyme activity promotion (triose-phosphate isomerase), antimicrobial peptide (AMP) expression, reactive oxygen species (ROS) production, and cell apoptosis through miRNAs to participate in the host’s antiviral immune response, while DIV1 can influence Warburg effect-related pathways (pyruvate metabolism, glycolysis/gluconeogenesis, and citrate cycle), glycosphingolipid biosynthesis-related pathways (glycosphingolipid biosynthesis—globo and isoglobo series and glycosphingolipid biosynthesis—lacto and neolacto series), and the tight junction and adhesion junction of the intestinal mucosal epithelium through the host’s miRNAs and mRNA to promote its own invasion and replication. These results indicate that intestinal miRNAs play important roles in the shrimp immune response against DIV1 infection. This study provides a basis for further study of the shrimp intestinal antiviral immune response and for the formulation of effective new strategies for the prevention and treatment of DIV1 infection.

The potential of antimicrobial peptides isolated from freshwater crayfish species in new drug development: A review

The much-publicised increased resistance of pathogenic bacteria to conventional antibiotics has focused research effort on the characterization of new antimicrobial drugs. In this context, antimicrobial peptides (AMPs) extracted from animals are considered a promising alternative to conventional antibiotics. In recent years, freshwater crayfish species have emerged as an important source of bioactive compounds. In fact, these invertebrates rely on an innate immune system based on cellular responses and on the production of important effectors in the haemolymph, such as AMPs, which are produced and stored in granules in haemocytes and released after stimulation. These effectors are active against both Gram-positive and Gram-negative bacteria. In this review, we summarise the recent progress on AMPs isolated from the several species of freshwater crayfish and their prospects for future pharmaceutical applications to combat infectious agents.

Effects of Dietary Saccharomyces cerevisiae YFI-SC2 on the Growth Performance, Intestinal Morphology, Immune Parameters, Intestinal Microbiota, and Disease Resistance of Crayfish (Procambarus clarkia)

The present study aimed to evaluate the effect of the dietary supplementation of Saccharomyces cerevisiae YFI-SC2 on the growth performance, intestinal morphology, immune parameters, intestinal microbiota, and disease resistance of crayfish (Procambarus clarkia). Crayfish were randomly assigned to six different boxes and two different groups in triplicate. The control group received a basal diet and the treatment group received a diet containing S. cerevisiae at 10⁷ CFU/g. After feeding for 28 days, crayfish of the treatment group exhibited a significantly better weight gain ratio (WGR) and a specific growth rate (SGR) (p < 0.05) than crayfish of the control group. Compared to the treatment group, the control group intestines showed an oedema connective tissue layer and a weak muscle layer. For immune-related genes, Crustin2 expression was similar between the groups, whereas Lysozyme and prophenoloxidase from treatment group expression levels were upregulated significantly (p < 0.05) after 14 and 28 days of feeding. Prophenoloxidase showed the highest expression, with 10.5- and 8.2-fold higher expression than in the control group at 14 and 28 days, respectively. The intestinal microbiota community structure was markedly different between the two groups. After 14 and 28 days of feeding, the relative abundance of Cetobacterium and Lactobacillus increased, whereas Citrobacter and Bacteroides decreased in the treatment group compared with that of the control group. The challenge test showed that crayfish of the treatment group had a significantly enhanced resistance against Citrobacter freundii (p < 0.05). Our results suggest that a S. cerevisiae-containing diet positively influenced the health status, immune parameters, intestinal microbiota composition, and disease resistance of crayfish.

Hepatopancreas-Specific Lectin Participates in the Antibacterial Immune Response by Regulating the Expression of Antibacterial Proteins

The hepatopancreas is an important digestive and immune organ in crustacean. There were low but stable numbers of microbes living in the hemolymph of crustacean, whereas the organs (including hepatopancreas) of crustacean were immersed in the hemolymph. It is very important to study the immune mechanism of the hepatopancreas against bacteria. In this study, a novel CTL (HepCL) with two CRDs, which was mainly expressed in the hepatopancreas, was identified in red swamp crayfish (Procambarus clarkii). HepCL binds to bacteria in vitro and could enhance bacterial clearance in vivo. Compared with the C-terminal CRD of HepCL (HepCL-C), the N-terminal CRD (HepCL-N) showed weaker bacterial binding ability in vitro and stronger bacterial clearance activity in vivo. The expression of some antimicrobial proteins, such as FLP, ALF1 and ALF5, was downregulated under knockdown of HepCL or blocked with Anti-HepCL after challenge with Vibrio in crayfish. These results demonstrated that HepCL might be involved in the antibacterial immune response by regulating the expression of antimicrobial proteins.

A Comprehensive Review on Crustaceans' Immune System With a Focus on Freshwater Crayfish in Relation to Crayfish Plague Disease

Freshwater crayfish immunity has received great attention due to the need for urgent conservation. This concern has increased the understanding of the cellular and humoral defense systems, although the regulatory mechanisms involved in these processes need updating. There are, however, aspects of the immune response that require clarification and integration. The particular issues addressed in this review include an overall description of the oomycete Aphanomyces astaci, the causative agent of the pandemic plague disease, which affects freshwater crayfish, and an overview of crustaceans' immunity with a focus on freshwater crayfish. It includes a classification system of hemocyte sub-types, the molecular factors involved in hematopoiesis and the differential role of the hemocyte subpopulations in cell-mediated responses, including hemocyte infiltration, inflammation, encapsulation and the link with the extracellular trap cell death pathway (ETosis). In addition, other topics discussed include the identity and functions of hyaline cells, the generation of neoplasia, and the emerging topic of the role of sessile hemocytes in peripheral immunity. Finally, attention is paid to the molecular execution of the immune response, from recognition by the pattern recognition receptors (PRRs), the role of the signaling network in propagating and maintaining the immune signals, to the effector elements such as the putative function of the Down syndrome adhesion molecules (Dscam) in innate immune memory.

The miR-155 regulates cytokines expression by SOSC1 signal pathways of fish in vitro and in vivo

MiR-155 is reported as immune regulated miRNA in mammalian corresponding to immunity, antibacterial and antiviral effects regulation. However, the roles and mechanisms of the miRNA have remained largely undefined. We herein comprehensively investigated the functions of miR-155 in vitro and in vivo by miR-155 mimics, agomir and antagomir in Cyprinus carpio and Ictalurus punctatus, with the target genes in the SOSC1 pathway certified in I. punctatus via luciferase reporter assays. Results showed that the miR-155 regulated the expressions of cytokines, including TNF-α, IFN-γ, IL-1β, IL-6 and IL-10. Further research confirmed SOSC1 as one of the targets of the miRNA, and the JAK1/STAT3/SOSC1 signal pathway involved in the miR-155 effects on the expression of immune cytokines as well. Additionally, the changes of TLR2 in fish may also be related to miR-155 along with its target SOCS1, and the TLR2/MyD88 pathway may partly participate in the effects of the miR-155 on the cytokines. The research here confirmed that the miR-155 can regulate cytokines expression by SOSC1 signal pathways of fish in vitro and in vivo, which would provide resources for understanding and studying about immune regulation in fish.

Injection of an extract of fresh cacao pod husks into Litopenaeus vannamei upregulates immune responses via innate immune signaling pathways

Immunostimulation is a novel method and a promising development in aquaculture. Products derived from plants exhibit various biological activities. In this study, the hot-water extract isolated from fresh cacao (Theobroma cacao) pod husks (CPHs) was administered by injection to evaluate cellular signaling pathways of innate immunity, the immunostimulating potential, disease resistance, and hypothermal tolerance of white shrimp, Litopenaeus vannamei. Results showed significant increases in the total hemocyte count, semigranular cells, granular cells, phenoloxidase activity, and respiratory bursts (RBs) of hemocytes per unit of hemolymph at 1 day, and in phagocytic activity toward and the clearance efficiency of Vibrio alginolyticus at 1-3 days after shrimp were injected with fresh CPH extract at 40 μg shrimp^-1. However, only RBs per hemocyte had significantly decreased at 1 day after the injection. All immune parameters had returned to control levels by 3 days after receiving fresh CPH extract except for RBs, phagocytic activity, and the clearance efficiency, which had returned to control values by 7 days. Furthermore, at 1 day after the injection, the peroxinectin, prophenoloxidase (proPO) II, toll-like receptors (TLR) 3, signal transducer and activator of transcription (STAT), and crustin in shrimp receiving fresh CPH extract at 20 μg shrimp^-1, and the lipopolysaccharide and β-1,3-glucan-binding protein, proPO II, TLR1, and STAT in shrimp receiving fresh CPH extract at 40 μg shrimp^-1 were significantly higher than those of shrimp receiving saline. After injecting fresh CPH extract at 10-40 μg shrimp^-1 and 40 μg shrimp^-1, the mortality of shrimp challenged with V. alginolyticus and exposed to 14 °C for 96 h significantly decreased, respectively. These results suggest that fresh CPH extract can be used as an immunostimulant and a physiological regulator for shrimp through injection administration to enhance immunological and physiological responses, which can elevate the resistance against V. alginolyticus and tolerance against hypothermal stress in L. vannamei.

The feeding of dietary Codonopsis pilosula polysaccharide enhances the immune responses, the expression of immune-related genes and the growth performance of red swamp crayfish (Procambarus clarkii)

Polysaccharides have many functions in aquatic animals and are widely used as immunopotentiators. However, despite the emergence of serious diseases, few studies have explored the effects of Codonopsis pilosula polysaccharide (CPP) on crustaceans. We studied the effects of CPP on the growth performance, nonspecific immunity, antioxidant activity and disease resistance of red swamp crayfish (Procambarus clarkii). Healthy crayfish (5.80 ± 0.1 g) were fed diets supplemented with 0% (control), 0.05%, 0.1%, 0.15%, 0.20%, and 0.30% CPP for 8 weeks. At the end of the 8-week feeding trial, the optimal final body weight (FBW), weight gain (WG), specific growth rate (SGR), and feed conversion ratio (FCR) were observed in the crayfish fed the diets with 0.15% and 0.20% CPP, followed by those fed the diet with 0.30% CPP and then those fed the diet with 0.10% CPP, whereas the values of these parameters were obtained with the control crayfish (P < 0.05). The crayfish fed the diets with 0.15% and 0.20% CPP exhibited a significantly higher total hemocyte count (THC) and significantly increased phenoloxidase (PO), lysozyme (LZM), hemocyte (Hc), acid phosphatase (ACP) and alkaline phosphatase (AKP) compared with those belonging to the other groups (P < 0.05). The crayfish fed the diets with 0.15% and 0.2% CPP exhibited significantly higher total superoxide dismutase (T-SOD) and glutathione peroxidase (GPx) activities, a significantly increased total antioxidant capacity (T-AOC) and a significantly lower malondialdehyde (MDA) content compared with the other groups (P < 0.05), which indicated that antioxidant capacity was significantly induced by the CPP-supplemented diets. Significantly upregulated expression of immune-related genes (anti-lipopolysaccharide factors (alf), peroxiredoxin (prx5), cathepsin B (ctsb), mitochondrial manganese superoxide dismutase (mtMnsod), cyclophilin A (cypa), glutathione peroxidase (gpx), Toll-like receptor 3 (tlr3), and heat shock protein 70 (hsp70)) was detected in the crayfish fed the diets supplemented with 0.15% and 0.20% CPP diet compared with the levels observed in the control crayfish. These results showed that dietary CPP supplementation greatly improved the growth, immunity and antioxidant capacities of crayfish, and according to the observed results, 0.15%-0.2% is the recommended optimal level of CPP dietary supplementation for crayfish.

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.

Effects of hesperidin on the growth performance, antioxidant capacity, immune responses and disease resistance of red swamp crayfish (Procambarus clarkii)

We evaluated the effects of hesperidin on the nonspecific immunity, antioxidant capacity and growth performance of red swamp crayfish (Procambarus clarkii). A total of 900 healthy crayfish were randomly divided into six groups: the control group (fed the basal diet) and the HES25, HES50, HES75, HES100 and HES150 groups, which were fed the basal diet supplemented with 25, 50, 75, 100 and 150 mg kg^-1 hesperidin, respectively. The feeding experiment lasted 8 weeks. The results indicated that compared with the control group, the crayfish groups supplemented with 50-150 mg kg^-1 hesperidin had a decreased feed conversion ratio (FCR) and increased final body weight (FBW), specific growth rate (SGR) and weight gain (WG) (P < 0.05). The protein carbonyl content (PCC), reactive oxygen species (ROS) level and malondialdehyde (MDA) level in the hepatopancreas and hemocytes were significantly lower, while the total antioxidant capacity (T-AOC), glutathione peroxidase (GPx) activity, and superoxide dismutase (SOD) activity were significantly higher in the crayfish groups supplemented with 50-150 mg kg^-1 hesperidin than in the control group. Supplementation with 50-150 mg kg^-1 hesperidin significantly increased the activities of acid phosphatase (ACP), alkaline phosphatase (AKP), lysozyme (LZM), and phenoloxidase (PO) compared with the control group (P < 0.05); upregulated the mRNA expression of cyclophilin A (CypA), extracellular copper-zinc superoxide dismutase (ecCuZnSOD), GPxs, crustin, astacidin, Toll3 and heat shock protein 70 (HSP70) (P < 0.05); and decreased crayfish mortality following white spot syndrome virus (WSSV) infection. These findings indicate that dietary hesperidin supplementation at an optimum dose of 50-150 mg kg^-1 may effectively improve nonspecific immunity, antioxidant capacity and growth performance in crayfish.

Induction of innate immune response in whiteleg shrimp (Litopenaeus vannamei) embryos

The immune response of commercially relevant marine invertebrates has been extensively studied, in search of new disease-control strategies. Immune training is considered a novel approach that could help improve resistance to different pathogens. Here, we stimulated the white shrimp (Litopenaeus vannamei) during embryo development by exposure to heat-killed bacteria and evaluated their effect on hatching, larval development, and the expression of immune-related genes. In addition, we evaluated its impact on the response of shrimp nauplii during a challenge with Vibrio parahaemolyticus. We observed that the percentage of hatching and the resistance to bacterial infection increased due to the treatment of embryos with heat-killed cells of Vibrio and Bacillus. Apparently different stimuli could generate a differential pattern of gene expression, e.g., Vibrio induced a strong effector immune response whereas Bacillus elicited a protective immune profile. In addition, each response was triggered by molecular patterns detected in the environment. The results obtained in this study provide new insights for immune training to improve shrimp farming.

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.

CgRel involved in antibacterial immunity by regulating the production of CgIL17s and CgBigDef1 in the Pacific oyster Crassostrea gigas

The NF-κB/Rel transcription factors play essential roles in the induction and regulation of innate immune responses. In the present study, the full-length cDNA of CgRel from the Pacific oyster Crassostrea gigas was of 2,647 bp with an RHD and an IPT domain. The mRNA of CgRel was found to be constitutively expressed in all the tested tissues including gills, hepatopancreas, gonad, adductor muscle, labial palps, mantle, hemocytes, and ganglion. After lipopolysaccharide (LPS) stimulation, the expression level of CgRel mRNA in hemocytes was up-regulated to the first peak at 3 h (3.06-fold compared to the control group, p < 0.001) and second peak at 48 h (1.96-fold, p < 0.05). It increased significantly at 3 h (7.68-fold compared to the control group, p < 0.001), 24 h (3.63-fold, p < 0.05) and 48 h (1.99-fold, p < 0.05) post Vibrio splendidus stimulation, respectively. The protein of CgRel was translocated from cytoplasm into nucleus of oyster hemocytes after LPS stimulation. The mRNA expression levels of interleukin17s (CgIL17s) and big defensin (CgBigDef1) in hemocytes were examined after the expression of CgRel was silenced by RNAi. The transcripts of CgIL17-1 (0.25-fold of the control group, p < 0.01), CgIL17-2 (0.12-fold, p < 0.01), CgIL17-4 (0.33-fold, p < 0.01), CgIL17-6 (0.27-fold, p < 0.05) and CgBigDef1 (0.38-fold, p < 0.01) in CgRel-knockdown oysters decreased significantly at 12 h after LPS stimulation. The results indicated that CgRel played important roles in the immune defense against bacteria by regulating the expression of CgIL17 and CgBigDef1.

The Anti-lipopolysaccharide Factors in Crustaceans

Anti-lipopolysaccharide factors (ALFs) are a type of antimicrobial peptide (AMP) which show broad-spectrum antimicrobial activity against Gram-positive bacteria, Gram-negative bacteria, fungi and viruses. In this chapter, we review the discovery and classification of this kind of antimicrobial peptide in crustaceans. The structure and function, as well as the mechanism of antibacterial and antiviral activities of ALFs will be summarized and discussed. We will then describe the expression and regulation of various ALF genes in different crustacean species. Finally, the application prospects of ALFs in drug development and disease-resistant genetic breeding will be pointed out and discussed. The review will also discuss several key questions such as the systematic classification and expression regulation of the ALF genes, as well as the future application of ALFs and ALF-derived peptides.

Effects of dietary n-3 highly unsaturated fatty acids on growth, non-specific immunity, expression of some immune-related genes and resistance to Vibrio harveyi in hybrid grouper (♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatu)

This study was conducted to investigate the effects of dietary n-3 highly unsaturated fatty acids (n-3 HUFA) on growth performance, non-specific immunity, expression of some immune-related genes and resistance to Vibrio harveyi in juvenile hybrid grouper (♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatu). Six isoproteic and isolipidic experimental diets were formulated with graded levels of n-3 HUFA (0.65, 1.00, 1.35, 1.70, 2.05 and 2.40% of dry matter, respectively), and the 0.65% group was used as control group. Each diet was randomly allocated to triplicate groups of fish in 1000 L fiberglass tank, and each tank was stocked with 40 fish (initial weight 12.06 ± 0.01 g) for 8 weeks. Results showed that feed conversion ratio (FCR), survival rate (SR), hepatosomatic index (HSI) and condition factor (CF) were all not significantly affected by dietary n-3 HUFA levels (P > 0.05). Weight gain (WG) and specific growth rate (SGR) in 1.35% group were significantly higher than those in 2.40% group (P < 0.05). Crude lipid of body in 1.00% group was significantly lower than that in 1.70% and 2.40% groups (P < 0.05). Liver and muscle fatty acid profiles reflected that of diets. Before challenge with Vibrio harveyi, the activity of serum superoxide dismutase (SOD), catalase (CAT) and content of complement 3 (C3) in 1.35% and 1.70% groups significantly higher than those of control group (P < 0.05). After challenge with Vibrio harveyi, serum CAT, glutathione peroxidase (GSH-PX), lysozyme (LZM) and C3 all increased sharply, while SOD showed the opposite trend. Before challenge with Vibrio harveyi, the expression levels of intestine toll-like receptor 22 (TLR22) and myeloid differentiation factor 88 (MyD88) mRNA in 2.40% group were significantly increased, and the expression levels of tumour necrosis factor α (TNF-α) and interleukin 1β (IL-1β) mRNA in 2.05% group were significantly higher than those in 1.00% and 1.35% groups (P < 0.05). In addition, the TLR22 and IL-1β mRNA levels in kidney of 1.70% group were significantly lower than those in control group (P < 0.05). After challenge with Vibrio harveyi, the expression level of MyD88 mRNA in intestine of 1.35% group was significantly higher than that in 1.00% group and from 1.70% to 2.40% groups (P < 0.05), while TNF-α and IL-1β obtained minimum values in 1.70% group. In the kidney, the interleukin 10 (IL10) mRNA expression was significantly higher in 1.70% group than that in other groups, while the IL-1β expression in 1.70% group was on the contrary and significantly lower than that in 2.40% group (P < 0.05). Results of this study suggested that moderate dietary n-3 HUFA (1.47%-1.70% HUFA) could improve the growth performance, non-specific immunity and inhibit the inflammatory response of hybrid grouper.

A novel invertebrate toll-like receptor is involved in TLR mediated signal pathway of thick shell mussel Mytilus coruscus

Toll-like receptors (TLRs) are the most well studied pattern recognition receptors (PRRs) that play a crucial role in both innate and adaptive immunity in animals. In the present study, a novel toll-like receptor (McTLRj) was identified and characterised in thick shell mussel Mytilus coruscus. McTLRj possessed a signal peptide, a transmembrane domain, leucine-rich repeats and an intracellular Toll/interleukin-1 receptor domain that were conserved in typical TLRs. McTLRj transcripts were constitutively expressed in all of the examined tissues with high expression level in immune-related tissues, and significantly induced in haemocytes upon live Vibrio alginolyticus, lipopolysaccharide, polyinosinic-polycytidylic acid and peptidoglycans challenge. The overexpression of the McTLRj TLR fragment in Drosophila S2 cells could induce the expression of Drosophila attacin A, drosomycin, cecropin A, and metchnikowin expression. The expression of McTLRj was obviously repressed by dsRNA-mediated RNA interference, and downstream TLR pathway factors, such as MyD88a, IRAK4, and TRAF6 were significantly repressed in McTLRj-silenced mussels upon LPS challenge. These results collectively indicated that McTLRj is a TLR family member that may play a potential PRR role in TLR-mediated signalling pathway. This research contributed to the clarification of innate immune response in molluscs.

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.

A novel invertebrate toll-like receptor with broad recognition spectrum from thick shell mussel Mytilus coruscus

Toll-like receptors (TLRs) are a category of most well recognized pattern recognition molecules that act on a vital role in both innate and adaptive immunity. In the present study, a novel toll-like receptor (McTLRw) was identified and characterized in thick shell mussel Mytilus coruscus. McTLRw possesses one intracellular Toll/interleukin-1 (IL-1) receptor (TIR) domain, one transmembrane region (TM), one leucine rich repeat N-terminal domain (LRR_NT) and a few of leucine-rich repeats (LRRs), which all are common in typical TLRs. McTLRw transcripts were constitutively expressed in all examined tissues with higher expression levels in immune related tissues, and were significantly induced in haemocytes with the challenges of live Vibrio alginolyticus, lipopolysaccharide (LPS), peptidoglycans (PGN) and β-glucan (GLU), but not induced by polyinosinic-polycytidylic acid (poly I:C). rMcTLRw exhibited affinity to LPS, PGN and GLU while no affinity to poly I:C. Further, the downstream of TLR signaling pathway myeloid differentiation factor 88a (MyD88a), interleukin-1 receptor-associated kinase-4 (IRAK4) and tumor necrosis factor receptor-associated factor 6 (TRAF6) were significantly repressed in McTLRw silenced mussels while challenged with LPS. These results collectively indicated that McTLRw is one member of TLR family and involved in immune response to against invaders by taking participate in TLR mediated signaling pathway.

Proliferation dynamics of WSSV in crayfish, Procambarus clarkii, and the host responses at different temperatures

The replication profile of white spot syndrome virus (WSSV) in crayfish, Procambarus clarkii, at different water temperature was investigated in this study. The WSSV detections were negative at 15 ± 1°C, and the natural infection ratio increased at 19 ± 1°C (24.2% ± 2.25%), reached 100% at 25 ± 1°C and decreased at 30 ± 1°C (93.2% ± 3.37%). The WSSV genome copies number was much higher at 25 ± 1°C (≥5 × 10^{6.45 ± 0.35} /mg) than at 15 ± 1°C (≤5 × 10^{1.13 ± 0.12} /mg), 19 ± 1°C (≤5 × 10^{2.74 ± 0.48} /mg) and 32 ± 1°C (≤5 × 10^{3.18 ± 0.27} /mg). Meanwhile, the significant transcription signals of immediate early gene ie1 and late gene vp28 and a large number of virus particles were detected in epitheliums of stomach, gut and gill, hepatopancreas, heart and muscle cells at 25 ± 1°C by using in situ hybridization (ISH) and transmission electron microscopy. The experimental infection of P. clarkii with WSSV infection showed reduced mortality and lower virus copies number at 19 ± 1°C (23.51% ± 0.84%, ≤5 × 10^{3.41 ± 0.11} /mg) and 32 ± 1°C (38.42% ^±  1.21%, ≤5 × 10^{3.72 ± 0.13} /mg) compared to 25 ± 1°C (100%, ≥5 × 10^{4.99 ± 0.24} /mg). The water temperature regulated the transcription of immune-related genes (crustin2, prophenoloxidase (proPO) and heat shock protein70 (Hsp70)), with some differences between WSSV treatments and control treatments. These results demonstrate that water temperature has effect on WSSV proliferation, which may due to transcriptional response of immune-related genes to temperature.

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.

Cottonseed protein concentrate (CPC) suppresses immune function in different intestinal segments of hybrid grouper ♀Epinephelus fuscoguttatus×♂Epinephelus lanceolatu via TLR-2/MyD88 signaling pathways

Cottonseed protein concentrate (CPC) has similar amino acid composition compared with fish meal, and has the characteristics of low gossypol and low toxicity. The present study was conducted to investigate the growth performance, antioxidant capacity and different intestinal segments immune responses of hybrid grouper to replacement dietary fish meal ofCPC. Six iso-nitrogenous (50% crude protein) and iso-lipidic (10% crude lipid) diets were formulated: a reference diet (FM) containing 60% fishmeal and five experimental diets (12%, 24%, 36%, 48 and 60%) in which fishmeal protein was substituted at different levels by CPC to feed fish (initial body weight: 11 ± 0.23 g) for 8 weeks. Thena challenge test with injection of Vibrio parahaemolyticus was conducted for 7 days until the fish stabilized. The results showed that specific growth rate (SGR) was the highest with 24% replacement level and feed conversion ratio (FCR)was significantly increased when the replacement level reached 48% (P < 0.05). The content of malonaldehyde (MDA) in the serum was significantly increased when the replacement level reached 36% (P < 0.05). The plica height in the proximal, mid and distal intestine were significantly decreased with the replacement level up to 48% (P < 0.05). Hepatic fat deposition wasaggravatedwhen the replacement level reached 36% (P < 0.05). The expression of IL-6, TNF-α, and IL-1β mRNAs were significantly up-regulated (P < 0.05). The hepcidin mRNA expression was significantly down-regulated (P < 0.05). In proximal intestine (PI) and mid intestine (MI), IFN-γ mRNA expression was significantly up-regulated (P < 0.05). These results suggested that the CPC decreased hybrid grouper growth performance and inflammation function, and different inflammation function responses in PI,MI, and distal intestine (DI) were mediated partly by the TLR-2/MyD88 signaling pathway. According to the analysis of specific growth rate, the dietary optimum replacement level and maximum replacement level were estimated to be 17% and 34%, respectively.

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.

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.

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.

Crayfish immunity - Recent findings

Freshwater crayfish is an important commodity as well as a successful model for studies on crustacean immunity. Due to the ease with which they are kept and the available methods for hemocyte separation and culture they have proven to be very useful. Here, recent progress regarding pattern recognition, immune effector production and antiviral mechanisms are discussed. Several cases of functional resemblance between vertebrate complement and the crayfish immune reactions are highlighted.

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.

Host MicroRNA-217 Promotes White Spot Syndrome Virus Infection by Targeting Tube in the Chinese Mitten Crab (Eriocheir sinensis)

MicroRNAs (miRNAs), a group of small molecule non-encoding RNAs, are key post-transcriptional regulators of gene expression that are implicated in many biological processes. In the current study, miR-217 from Eriocheir sinensis was selected for studying its roles during host-virus interaction. Overexpression or silencing of miR-217 led to considerable effects on white spot syndrome virus (WSSV) replication, implying that miR-217 played a positive role in WSSV infection. In insect High Five cells, miR-217 significantly inhibited Tube gene expression by binding to the 3'-untranslated region of the Tube. Overexpression of miR-217 in crab led to downregulation of tube expression. Knockdown of Tube in vivo led to significant enhancement of WSSV infection and inhibited the expression of five antimicrobial peptide (AMP) genes (Anti-lipopolysaccharide factor ALF1, ALF2, ALF3; Crustin Crus1, Crus2) in WSSV-challenged crabs. Overexpression of miR-217 also led to downregulation of these AMP genes in WSSV-challenged crabs. Our results showed that host miRNA played positive roles in virus infection by regulation of host tube gene, which is the key component of Toll signaling pathway.

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.

PcToll3 was involved in anti-Vibrio response by regulating the expression of antimicrobial peptides in red swamp crayfish, Procambarus clarkii

Tolls and Toll-like receptors (TLRs) play an important role in host immune defenses by regulating the expression of antimicrobial peptides (AMPs) and cytokines, but the functional differences of crustacean Tolls from Drosophila Tolls or Mammal TLRs are largely unknown. A novel Toll receptor, named PcToll3, was identified from red swamp crayfish, Procambarus clarkii. It was widely expressed in all detected tissues, and its transcript in hemocytes was up-regulated at 12 h after Vibrio parahemolyticus (Vibrio) injection or at 24 h post white spot syndrome virus (WSSV) challenge. After knockdown of PcToll3, the activity of bacterial clearance was inhibited, and the expression levels of AMPs including Crustin1 (Cru1), Anti-lippopolysaccharide factor 1 (ALF1), and Lysozymes1 (Lys1), which could be up-regulated by Vibrio, were all affected. Meanwhile, PcToll3 silencing influenced the expression of myeloid differentiation factor 88 (PcMyd88), tumor necrosis factor-associated factor 6 (PcTRAF6), and PcDorsal, which were the counterparts of Drosophila Toll signaling pathway. Interestingly, PcToll3 silencing inhibited translocation of PcDorsal from cytoplasm to nucleus. Furthermore, the knockdown of PcDorsal also impaired the expression of AMPs after Vibrio challenge. Hence, we concluded that, besides participating in antiviral immunity, PcToll3 might also regulate the expression of Cru1 and Lys1 to participate in anti-Vibrio immune responses by promoting PcDorsal translocation into nucleus.

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.

BAX, a novel cell pro-apoptotic protein, involved in hemocytes early antiviral immune response in fresh water crayfish, Procambarus clarkii

Apoptosis plays an important role in various biological processes and acts as a host defending mechanism by which infected cells are eliminated to restrict the virus propagation scale. Bax is a crucial pro-apoptotic protein, which mediates the release of cytochrome c from mitochondrion to cytosol in mammalian. However, its role in invertebrate is still obscure. Here, a novel pro-apoptotic protein gene was identified from hemocytes of red swamp crayfish. There was a Bcl-2 domain in the C-terminus of Pc-Bax, which possessed 497 amino acids residues. And an important transmembrane region existed in the C-terminus of Pc-Bax, which implied that Pc-Bax located in mitochondrial membrane. Besides, Pc-Bax was expressed at a relative high level in hemocytes, and a relative low expression levels in hepatopancreas, gills, and intestine. In hemocytes, Pc-Bax transcript was rapidly up-regulated from 12 h to 36 h after WSSV infection. And there was the same trend for Pc-Bax protein expression level in hemocytes after WSSV infection. Results of qRT-PCR testing for VP28 gene showed WSSV replication was obviously enhanced after Pc-Bax knockdown. Meantime, hemocytes apoptosis was suppressed in Pc-Bax knockdown crayfish after WSSV injection, compared with the dsGFP injection group and normal group. Taken together, these results revealed that crayfish hemocytes apoptosis scale was enhanced to suppress WSSV replication by up-regulating Bax protein expression level after WSSV infection.

Enhanced antimicrobial peptide-induced activity in the mollusc Toll-2 family through evolution via tandem Toll/interleukin-1 receptor

Toll receptors play an important role in the innate immunity of invertebrates. All reported Tolls have only one Toll/interleukin-1 receptor (TIR) domain at the C-terminal. In this study, numerous Tolls with tandem TIRs at the C-terminal were found in molluscs. Such Tolls presented an extra TIR (TIR-1) compared with Toll-I. Thus, Toll-I might be the ancestor of tandem TIRs containing Toll. To test this hypothesis, 83 Toll-I and Toll-2 (most have two TIRs, but others seem to be the evolutionary intermediates) genes from 29 shellfish species were identified. These Tolls were divided into nine groups based on phylogenetic analyses. A strong correlation between phylogeny and motif composition was found. All Toll proteins contained the TIR-2 domain, whereas the TIR-1 domain only existed in some Toll-2 protein, suggesting that TIR-1 domain insertion may play an important role in Toll protein evolution. Further analyses of functional divergence and adaptive evolution showed that some of the critical sites responsible for functional divergence may have been under positive selection. An additional intragenic recombination played an important role in the evolution of the Toll-I and Toll-2 genes. To investigate the functional difference of Toll-I and Toll-2, over expression of Hcu_Toll-I or Hcu_Toll-2-2 in Drosophila S2 cells was performed. Results showed that Hcu_Toll-2-2 had stronger antimicrobial peptide (AMP) activity than Hcu_Toll-I. Therefore, enhanced AMP-induced activity resulted from tandem TIRs in Toll-2s of molluscs during evolution history.

De Novo assembly and annotation of the freshwater crayfish Astacus astacus transcriptome

We generated RNA-seq data to assemble the transcriptome of the noble crayfish (Astacus astacus) from four combined tissues (abdominal muscle, hepatopancreas, ovaries, green glands). A total of 194 million read pairs with a length of 100 bp were generated. The transcriptome was assembled de novo using Trinity software, producing 158,649 non-redundant transcripts. Lowly expressed transcripts were filtered out leaving 45,415 transcripts of which 14,559 were found to contain open reading frames with predicted gene function. The Transrate software revealed that 91% of the total reads were realigned to the assembly. Furthermore, BUSCO analysis indicated that our assembly is 64% complete. A total of 13,770 transcripts were assigned at least one GO term. This first de novo transcriptome assembly is an important foundation for future genomic research on the noble crayfish and adds to the general knowledge and further characterization of transcriptomes of non-model organisms.