PcToll3 was involved in anti-Vibrio response by regulating the expression of antimicrobial peptides in red swamp 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.

A pattern recognition receptor CgTLR3 involves in regulating the proliferation of haemocytes in oyster Crassostrea gigas

Toll-like receptors (TLRs) are expressed on various immune cells as key elements of innate and adaptive immunity, and they also play significant roles in regulating cell proliferation and differentiation. In the present study, the binding activity of CgTLR3 to PAMPs and CgMyD88-2, and its role in mediating the proliferation of haemocytes was investigated. The recombinant proteins of the extracellular six LRR domains (rCgTLR3-LRR) and intracellular TIR domain (rCgTLR3-TIR) of CgTLR3 were obtained respectively. rCgTLR3-LRR exhibited binding activity to lipopolysaccharide (LPS), peptidoglycan (PGN), mannan (MAN) and Poly (I:C), with the highest affinity for LPS. While rCgTLR3-TIR displayed binding activity to the recombinant protein of rCgMyD88-2, with KD value of 7.22 × 10-7 M. The CgTLR3 mRNA and protein were detected in three subpopulations of oyster haemocytes, and they were mainly concentrated in granulocytes, which was 7.27-fold (p < 0.05) of that in semi-granulocytes and 8.51-fold (p < 0.01) of that in agranulocytes. The percentage of CgTLR3 positive cells (FITC+ haemocytes) in granulocytes was 4.45-fold (p < 0.01) and 2.57-fold (p < 0.05) of that in agranulocytes and semi-granulocytes, respectively. After Vibrio splendidus stimulation, the mRNA expression level of CgTLR3 in haemocytes significantly upregulated at 6 h and 12 h, which was 2.93-fold (p < 0.05) and 4.15-fold (p < 0.05) of that in the control group. After the expression of CgTLR3 was inhibited by the injection of si-CgTLR3, the expression levels of transcription factors associated with hematopoiesis (CgGATA, CgRunx), cell cycle-related genes (CgPCNA, CgCDC-45, CgCDK-2), the agranulocyte marker CgCD-9, the granulocyte marker CgAATase, and the inflammatory factor CgIL17-1 significantly decreased (p < 0.05) after the V. splendidus stimulation, which were 0.43-fold, 0.83-fold, 0.48-fold, 0.44-fold, 0.53-fold, 0.7-fold, 0.62-fold, and 0.47-fold of that in NC + V. s group in vivo, respectively. Meanwhile, the percentage of EdU+ haemocytes in si-CgTLR3+V. s group was significantly reduced by 0.54-fold (p < 0.05) compared to the control group (2.7%). These results collectively indicated that CgTLR3 was involved in modulating the proliferation of haemocytes by regulating the expression of proliferation-related genes and inflammatory factor in oyster C. gigas.

Antioxidation, immunity and hepatopancreatic histology of red swamp crayfish (Procambarus clarkii) subjected to low-temperature aerial exposure stress and re-immersion

Desiccation is a stressful situation that decapods often experience during live transportation. This study investigated the effects of low-temperature aerial exposures (LTAEs) (dry exposure (DL) and moist exposure (ML) at 6 °C) and re-immersion on the antioxidative and immune responses and hepatopancreatic histopathology in P. clarkii. Compared to the control group (normally feeding at 24.0 °C water temperature), the crayfish under LTAEs showed overall severe hepatopancreatic oxidative damage, with significantly increased malondialdehyde (MDA) contents and significantly reduced total antioxidant capacity (T-AOC), and oxidant damage was not fully recovered even after 12 h of re-immersion; the expression of hsp70 was significantly increased within 24-48 h of stress and re-immersion. The activity of hemolymphatic acid phosphatase (ACP) was significantly increased during 24-48 h of the stress and at 12 h of re-immersion; the activity of aspartic aminotransferase (AST) and alanine aminotransferase (ALT) was significantly increased throughout the experiment; and the gene expression of proPO or TLR was significantly increased during 12-48 h of the stress. Severe histopathological changes (lumen dilatation, vacuolation of epithelial cells and reduced cell numbers) were observed in hepatopancreas at 48 h of stress and 12 h of re-immersion. These results indicated that 48 h of low-temperature aerial exposure stress stimulated the non-specific immunity but adversely affected the antioxidation and hepatopancreatic histomorphology of P. clarkii, whereas 12 h of re-immersion was not sufficient to restore crayfish from stress to a normal state.

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.

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.

Effects of sulfated β-glucan from Saccharomyces cerevisiae on growth performance, antioxidant ability, nonspecific immunity, and intestinal flora of the red swamp crayfish (Procambarus clarkii)

The aim of this study was to examine the combined effects of sulfated β-Glucan from Saccharomyces cerevisiae (sGSC) on growth performance, antioxidant ability, nonspecific immunity, and intestinal flora of the red swamp crayfish (Procambarus clarkii). Four experimental diets (sGSC25, sGSC50, sGSC100 and sGSC200) with different levels of sGSC (0.025%, 0.05%, 0.1% and 0.2% in diet, respectively) were fed to juvenile crayfish (average weight: 2.5 ± 0.5 g) for 8 weeks. The control diet was given with 2000 mg/kg GSC (GSC200 group). The based control diet was given without sGSC or GSC (blank group). Each group had 3 parallel test pools, 20 crayfish were reared in each pool. At the end of the growth trial, adding dietary 0.025%-0.1% sGSC could significantly improve the growth performance, antioxidant capacity and immunity of crayfish. Compared with GSC, sGSC had a better effect at lower concentration. Higher concentration of sGSC (>0.1%) would cause some side effects. sGSC also could improve the structure of the intestinal flora and optimize the function of the flora. sGSC would increase the abundances of probiotics such as Hafnia and Acinetobacter, and decreases the abundances of maleficent bacteria such as Enterobacteriaceae. Higher concentration of sGSC (>0.1%) would increase the abundance of Aeromonas. To conclude, 0.025%-0.1% sGSC can be used as a supplement in crayfish feed to increase growth, immunity, and antioxidant capacity and improve the structure of intestinal flora. These results provided a theoretical basis for the application of sGSC instead of GSC in crayfish breeding. It will be necessary to further study the optimal concentration of sGSC in feed additives in different growth stages of crayfish in the future.

A new insight into the SNP genotyping using high-resolution melting method after the correlation analysis of the SNPs with WSSV-resistant traits

Procambarus clarkii is an important freshwater cultured crayfish in China. With the gradual development of its aquaculture industry, research on white spot disease, which is harmful to healthy culture of P. clarkii, increases gradually. The prophenoloxidase (proPO) system is an important part of crayfish's innate immunity and plays a role in virus resistance. In this study, based on the early discovery of three SNP sites in the intron of proPO gene, the linkage disequilibrium and haplotype were analyzed for the SNPs, and it was found that there was a strong linkage disequilibrium relationship among them. Through the analysis on association between the haplotypes and genotype of each SNP site with the WSSV-resistant traits, the detection of the SNP_7081 genotype was considered as the most convenient and efficient way for WSSV-resistant group selection. Furtherly, the high-resolution melting curve (HRM), which is a rapid and economic genotyping method, was chosen to establish for SNP_7081 site genotyping. The 68 bp target fragment with 27.94% GC content was amplified and melting curve analysis were performed. However, the appearance of false negatives which led to unable automatically grouped although the melting curves of genotypes CC, C>T and T>C were obviously different, and could be treated as standard to manually genotype the samples with an accuracy rate of 97.61%. The low GC content which correlated with the Tm value, was confirmed as the reason for the false negatives by the assay about the recombinant plasmid PMD18-T-SNP_7081 constructed with 45.24% GC content. Eventually, the adaptor primers were used to increase the GC content of the target fragment, and a modified HRM method for genotyping SNP_7081 site that could group automatically was established, which could provide a new insight for the HRM method to genotype SNPs.

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.

Recent insights into anti-WSSV immunity in crayfish

White spot syndrome virus (WSSV) is currently the most severely viral pathogen for farmed crustaceans such as shrimp and crayfish, which has been causing huge economic losses for crustaceans farming worldwide every year. Unfortunately, study on the molecular mechanisms of WSSV has been restricted by the lack of crustacean cell lines for WSSV propagation as well as the incompletely annotated genomes for host species, resulting in limited elucidation for WSSV pathogenesis at present. In addition to the findings of anti-WSSV response in shrimp, some of novel cellular events involved in WSSV infection have been recently revealed in crayfish, including endocytosis and intracellular transport of WSSV, innate immune pathways in response to WSSV infection, and regulation of viral gene expression by host genes. Despite these advances, many fundamental gaps in WSSV pathogenesis are still remaining, for example, how WSSV genome enters into nucleus and how the progeny virions are fully assembled in the host cell nucleus. In this review, recent findings in WSSV infection mechanism and the antiviral immunity against WSSV in crayfish are summarized and discussed, which may provide us a better understanding of the WSSV pathogenesis as well as new ideas for the target design of antiviral drugs against WSSV in crustaceans farming.

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.

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.

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.

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.

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.

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

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

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

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

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.

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

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

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.

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.