Involvement of a newly identified atypical type II crustin (SpCrus5) in the antibacterial immunity of mud crab Scylla paramamosain

Characterization of the antibacterial and opsonic functions of the antimicrobial peptide LvCrustinVI from Litopenaeus vannamei

Microbial drug resistance is becoming increasingly severe due to antibiotic abuse. The development and utilization of antimicrobial peptides is one of the important ways to solve this difficult problem. Crustins are a family of antimicrobial peptides that play important roles in the innate immune system of crustaceans. Several types of crustins exist in shrimp and their activities vary greatly. In the present study, we studied the immune function of one newly identified crustin and found that the type VI crustin encoding gene in Litopenaeus vannamei (LvCrustinVI) was mainly expressed in gills. Its expression was significantly up-regulated after Vibrio parahaemolyticus infection and knockdown of the gene promoted Vibrio proliferation in the hepatopancreas of shrimp, indicating that LvCrustinVI was involved in pathogens infection. The recombinant LvCrustinVI (rLvCrustinVI) showed strong inhibitory activities against both Gram-negative and Gram-positive bacteria, and exhibited binding activities with the bacteria and bacterial polysaccharides including Glu, LPS and PGN. In the presence of Ca2+, rLvCrustinVI showed a strong agglutination effect on V. parahaemolyticus and could significantly enhance the phagocytic ability of shrimp hemocytes against V. parahaemolyticus. In conclusion, LvCrustinVI played important roles as antimicrobial peptide and opsonin in the innate immune defense of L. vannamei. The study enriched our understanding of the functional activity of Crustin and provides an important basis for the development and utilization of antimicrobial peptides.

Identifying the function of the PI3K-AKT pathway during the pathogenic infection of Macrobrachium rosenbergii

The phosphoinositide-3-kinase/protein kinase b (PI3K-Akt) pathway is a signalling pathway based on protein phosphorylation and can be activated by a wide range of factors. To investigate the function of the PI3K-AKT signalling pathway in antibacterial immunity, we analysed the gene expression level of three key factors (PI3K, AKT and FoxO) and innate immune factors in immune tissues at different time points after Vibrio parahaemolyticus and Staphylococcus aureus infection. Tissues analysis showed that PI3K, AKT, and FoxO were expressed at high levels in the intestinal, hemocytes and hepatopancreas. Moreover, the expression levels of PI3K, AKT and FoxO can be regulated postinfection by different pathogens. In hemocytes and the intestine, V. parahaemolyticus infection was found to regulate the levels of PI3K, AKT, and FoxO more rapidly; however, an S. aureus infection regulated the levels of these factors more rapidly in the hepatopancreas and gills. Analysis showed that V. parahaemolyticus and S. aureus infection caused changes in the gene expression level of crustin, caspase 3 and NF-κB. Therefore, PI3K-AKT regulates the downstream immune pathway differentially in different immune tissues and participates in the regulation of cell apoptosis and the inflammatory response by activating caspase and NF-κB, respectively, following infection with V. parahaemolyticus and S. aureus.

Marine Invertebrate Antimicrobial Peptides and Their Potential as Novel Peptide Antibiotics

Marine invertebrates constantly interact with a wide range of microorganisms in their aquatic environment and possess an effective defense system that has enabled their existence for millions of years. Their lack of acquired immunity sets marine invertebrates apart from other marine animals. Invertebrates could rely on their innate immunity, providing the first line of defense, survival, and thriving. The innate immune system of marine invertebrates includes various biologically active compounds, and specifically, antimicrobial peptides. Nowadays, there is a revive of interest in these peptides due to the urgent need to discover novel drugs against antibiotic-resistant bacterial strains, a pressing global concern in modern healthcare. Modern technologies offer extensive possibilities for the development of innovative drugs based on these compounds, which can act against bacteria, fungi, protozoa, and viruses. This review focuses on structural peculiarities, biological functions, gene expression, biosynthesis, mechanisms of antimicrobial action, regulatory activities, and prospects for the therapeutic use of antimicrobial peptides derived from marine invertebrates.

A novel type II crustin in the innate immune response of the freshwater crab (Sinopotamon henanense) against infection and its expression changes by cadmium

Antibacterial peptide (AMP), an effector of the innate immune system, is an essential component of invertebrate innate immunity. Crustin is a family of antimicrobial peptides that are widely studied in crustaceans. Here we report a novel crustin (designated Shcrustin) from the freshwater crab Sinopotamon henanense. The results revealed that the full-length cDNA of Shcrustin was 691 bp with an open reading frame (ORF) of 510 bp. Phylogenetic analysis of the Shcrustin sequence showed that it clustered with type II crustin. Shcrustin exists in different tissues, among which the highest expression level is found in the gills. After the bacterial challenge, the expression of Shcrustin increased in hemocytes or gills. However, crustin expression was suppressed in the presence of cadmium (Cd). To elucidate the biological activity of Shcrustin, we constructed a recombinant Shcrustin protein. Purified rShcrustin could bind to a variety of bacteria and inhibit the growth of different bacteria indicating that Shcrustin has inhibitory activity against gram-positive and gram-negative bacteria. In addition, the phagocytic rate of hemocytes toward bacteria decreased after the interference of Shcrustin expression by RNA interference, suggesting that Shcrustin may be involved in such a process. Therefore, we conclude that Shcrustin may be involved in the innate immunity of S. henanense by binding to bacteria and promoting hemolymph phagocytosis to clear invading pathogens. It is an important immune effector against pathogen infection. In the presence of Cd, it may alter the expression of Shcrustin and suppress its immune function.

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

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

A novel type I Crustin from Exopalaemon carinicauda: Antimicrobial ability related to conserved cysteine

Crustins are a kind of antibacterial peptides (AMP) existing in crustaceans, and their antibacterial abilities are considered to be related to the conserved WAP domain. In this study, a novel type I Crustin gene was identified in Exopalaemon carinicauda, named EcCru. The deduced amino acid sequence revealed that the conserved cysteine at position 7 in the WAP domain was replaced by aspartic acid. The gene is 405 bp in length, encoding 134 amino acids, and is mainly distributed in gills and hepatopancreas. After Vibrio parahaemolyticus and Aeromonas hydrophila stimulation, the expression of EcCru was significantly up-regulated within 12 h, and then returned to normal levels. The recombinant protein was obtained using the Pichia pastoris expression system, and the recombinant protein had neither antibacterial activity against gram-positive or gram-negative bacteria. But the antibacterial ability emerged when Asp¹⁰¹ was mutated to Cys. Notably, we also obtained a mutant that had a deletion at the 6 th conserved Cys in the WAP domain, and this mutant had antibacterial ability against gram-positive bacteria Bacillus subtilis and B. cereus. This indicates that the conserved cysteine with different positions in WAP domain can have different effects on the antibacterial ability of Crustins.

A New Crustin Gene Homolog SpCrus8 Identified in Scylla paramamosain Exerting In Vivo Protection Through Opsonization and Immunomodulation

Crustins are the most abundant class of antimicrobial peptides in crustaceans and are essential for protecting animals from infection. Among them, type II crustins usually exhibit potent antimicrobial activity. Interestingly, in this study, a newly identified type II crustin gene homolog (named SpCrus8) from mud crab Scylla paramamosain, the recombinant proteins of which (rSpCrus8 and rTrx-SpCrus8) showed no obvious antibacterial effects, but could significantly reduce the bacterial load in crab hemolymph and improve the survival rate of crabs infected with Vibrio alginolyticus. The immune-related function of SpCrus8 and the underlying mechanism deserve further investigation. It was found that the SpCrus8 gene was widely distributed in various tissues of adult crabs. In the hepatopancreas of crabs infected with V. alginolyticus or Staphylococcus aureus, transcripts of the SpCrus8 gene were remarkably induced, indicating that the SpCrus8 gene was involved in the immune response to bacterial infection in vivo. In addition, rSpCrus8 and rTrx-SpCrus8 had strong binding activity not only to microbial surface components (lipopolysaccharide, lipoteichoic acid, peptidoglycan, and glucan), but also to the tested bacteria (S. aureus, Pseudomonas aeruginosa and V. alginolyticus). Notably, rSpCrus8 and rTrx-SpCrus8 could significantly promote hemocyte phagocytosis. After rSpCrus8 and rTrx-SpCrus8 treatment, a large number of fluorescent microspheres were observed to aggregate into clusters and be phagocytosed by multiple hemocytes, while hemocytes in the control group phagocytosed only individual microspheres, indicating that SpCrus8 played an important role in opsonization. When the SpCrus8 gene was knocked down, the expression levels of the key phagocytosis-related genes SpRab5 and SpRab7 were significantly downregulated, as well as the IMD signaling pathway genes SpIKKβ and SpRelish, and another crustin gene SpCrus5. Correspondingly, all the SpIKKβ, SpRelish and SpCrus5 genes were significantly upregulated after rSpCrus8 treatment, suggesting that SpCrus8 might be involved in the immunomodulation of S. paramamosain. Taken together, this study revealed the immune-related functions of the SpCrus8 gene in opsonization and regulation, which will help us further understand the role of the crustin gene family in the immune system of mud crabs and provide new insights into the function of type II crutins.

A Type Ib Crustin from Deep-Sea Shrimp Possesses Antimicrobial and Immunomodulatory Activity

Crustins are small antimicrobial proteins produced by crustaceans. Of the many reported crustins, very few are from deep sea environments. Crustins are categorized into several types. Recently, the Type I crustin has been further classified into three subtypes, one of which is Type Ib, whose function is unknown. Here, we studied the function of a Type Ib crustin (designated Crus2) identified from a deep-sea crustacean. Crus2 has a whey acidic protein (WAP) domain and a long C-terminal region (named P58). Recombinant Crus2 bound to peptidoglycan (PGN), lipoteichoic acid (LTA), and lipopolysaccharide (LPS), and killed Gram-positive and Gram-negative bacteria by permeabilizing the bacterial cytomembrane. Consistently, Crus2 dramatically attenuated the inflammatory response induced by LPS and LTA. Disruption of the disulfide bonds in the WAP domain abolished the bactericidal ability of Crus2, but had no effect on the bacterial binding ability of Crus2. Deletion of the C-terminal P58 region moderately affected the antimicrobial activity of Crus2 against some bacteria. P58 as a synthesized peptide could bind bacteria and inhibit the bactericidal activity of Crus2. Taken together, these results revealed different roles played by the WAP domain and the P58 region in Type Ib crustin, and provided new insights into the antimicrobial and immunomodulatory functions of crustins.

SpSR-B2 functions as a potential pattern recognition receptor involved in antiviral and antibacterial immune responses of mud crab Scylla paramamosain

Although class B scavenger receptors (SR-Bs) in mammals are multifunctional molecules, the functions of SR-Bs in invertebrates remain largely unknown. In this study, we characterized an SR-B homolog, namely SpSR-B2, from Scylla paramamosain. SpSR-B2 shared high similarity with mammalian SR-Bs, and exhibited specific binding activity to ac-LDL, indicating that it may be a new member of SR-B class in invertebrates. SpSR-B2 was upregulated after challenge with white spot syndrome virus (WSSV) or bacteria. Binding assays showed that SpSR-B2 specifically interacted with WSSV envelope protein VP24. Besides, SpSR-B2 could bind to all tested bacterial cells and agglutinate these bacteria. SpSR-B2 also exhibited a strong binding activity to LPS but weak binding activities to other tested polysaccharides. These findings indicated that SpSR-B2 was a potential recognition molecule for viral protein VP24 and bacterial LPS. Knockdown of SpSR-B2 resulted in dramatically decreased expressions of certain antimicrobial peptides (AMPs), and overexpression of SpSR-B2 led to the increased expression of the AMP of SpALF2, suggesting that SpSR-B2 could regulate the expression of AMPs. Taken together, this study revealed that SpSR-B2 functioned as a potential pattern recognition receptor participating in antiviral and antibacterial immunity, and provided new insights into the immune functions of invertebrate SR-Bs.

SpCrus2 Glycine-Rich Region Contributes Largely to the Antiviral Activity of the Whole-Protein Molecule by Interacting with VP26, a WSSV Structural Protein

Crustins are cysteine-rich cationic antimicrobial peptides with diverse biological functions including antimicrobial and proteinase inhibitory activities in crustaceans. Although a few crustins reportedly respond to white spot syndrome virus (WSSV) infection, the detailed antiviral mechanisms of crustins remain largely unknown. Our previous research has shown that SpCrus2, from mud crab Scylla paramamosain, is a type II crustin containing a glycine-rich region (GRR) and a cysteine-rich region (CRR). In the present study, we found that SpCrus2 was upregulated in gills after WSSV challenge. Knockdown of SpCrus2 by injecting double-stranded RNA (dsSpCrus2) resulted in remarkably increased virus copies in mud crabs after infection with WSSV. These results suggested that SpCrus2 played a critical role in the antiviral immunity of mud crab. A GST pull-down assay showed that recombinant SpCrus2 interacted specifically with WSSV structural protein VP26, and this result was further confirmed by a co-immunoprecipitation assay with Drosophila S2 cells. As the signature sequence of type II crustin, SpCrus2 GRR is a glycine-rich cationic polypeptide with amphipathic properties. Our study demonstrated that the GRR and CRR of SpCrus2 exhibited binding activities to VP26, with the former displaying more potent binding ability than the latter. Interestingly, pre-incubating WSSV particles with recombinant SpCrus2 (rSpCrus2), rGRR, or rCRR inhibited virus proliferation in vivo; moreover, rSpCrus2 and rGRR possessed similar antiviral abilities, which were much stronger than those of rCRR. These findings indicated that SpCrus2 GRR contributed largely to the antiviral ability of SpCrus2, and that the stronger antiviral ability of GRR might result from its stronger binding activity to the viral structural protein. Overall, this study provided new insights into the antiviral mechanism of SpCrus2 and the development of new antiviral drugs.

Transcriptome analysis of immune-related genes in Sesarmops sinensis hepatopancreas in reaction to peptidoglycan challenge

Sesarmops sinensis is a dominant omnivorous crab species, which plays an important ecological function in salt marsh ecosystems. To better understand its immune system and immune related genes under pathogen infection, the transcriptome was analyzed by comparing the data of S. sinensis hepatopancreas stimulated by PBS and PGN. A set of assembly and annotation identified 39,039 unigenes with an average length of 1105 bp, obtaining 1300 differentially expressed genes (DEGs) in all, which included 466 remarkably up-regulated unigenes and 834 remarkably down-regulated unigenes. In addition, based on mensurable real time-polymerase chain reaction and high-throughput sequencing, several immune responsive genes were found to be markedly up-regulated under PGN stimulation. In conclusion, in addition to enriching the existing transcriptome data of S. sinensis, this study also clarified the immune response of S. sinensis to PGN stimulation, which will help us to further understand the crustacean's immune system.

Characterization of a gill-abundant crustin with microbiota modulating function in Litopenaeus vannamei

Gills as the respiratory tissue of crustacean encounter various threats in the environment. The immune defense in gills is essential to the health of shrimp. In the present study, we identified a novel gill-abundant crustin, LvCrustin Ⅱ-1, from the shrimp Litopenaeus vannamei. The full-length open reading frame of LvCrustin Ⅱ-1 is 522 bp, which encodes 173 amino acid residues. The amino acid sequence of LvCrustin Ⅱ-1 contained a signal peptide, a glycine rich region, a cysteine rich region and a whey acidic protein (WAP) domain. The expression level of LvCrustin Ⅱ-1 was significantly up-regulated at different time points after Vibrio parahaemolyticus immersion. When LvCrustin Ⅱ-1 was silenced by dsRNA interference, the count of bacteria colonies increased significantly in the hepatopancreas of shrimp after V. parahaemolyticus immersion, which indicated that the infection progress of pathogenic bacteria was accelerated after LvCrustin Ⅱ-1 knockdown. Compared with the microbiota of seawater, the lower proportion of aquatic bacteria and higher proportion of symbiont in the gills microbiota of shrimp indicated the bacterial colonization was modulated by the host. Knockdown of LvCrustin Ⅱ-1 changed the proportion of some potential pathogens and aquatic bacteria, which supported the idea that the new identified crustin in the gills played important roles in modulation of the microbiota community in shrimp. The present data provided new insights into the multiple functions of crustin in the immunity of shrimp.

Molecular cloning and expression analysis of two type II crustin genes in the oriental river prawn, Macrobrachium nipponense

Innate immunity is the primary defense of crustaceans against pathogens. Crustins, as antimicrobial peptides, are important to crustacean innate immunity. In this study, two kinds of Gly-rich crustin genes were cloned from Macrobrachium nipponense and were referred to as Mn-Gly-Cru1 and Mn-Gly-Cru2. These crustins belong to type II crustins with typical type II crustin structures. The full-length cDNA of Mn-Gly-Cru1 is 677 bp and contains a 576 bp open reading frame (ORF) encoding 191 amino acids. The full-length cDNA of Mn-Gly-Cru2 is 727 bp, with 573 bp ORF encoding 190 amino acids. The constructed phylogenetic tree indicated that Mn-Gly-Cru1 and Mn-Gly-Cru2 belong to the type IIa subfamily. RT-PCR analysis showed that Mn-Gly-Cru1 and Mn-Gly-Cru2 are widely distributed in various tissues. qRT-PCR results indicated that Mn-Gly-Cru1 is mainly expressed in the gills, whereas Mn-Gly-Cru2 is expressed at the highest level in hemocytes. The transcripts of Mn-Gly-Cru1 and Mn-Gly-Cru2 respond to bacterial or white spot syndrome virus (WSSV) stimuli. After injection of 48 h dsMnRelish, the expression of MnRelish, Mn-Gly-Cru1, and Mn-Gly-Cru2 were all inhibited. After WSSV, Vibrio parahaemolyticus, or Staphylococcus aureus challenge, MnRelish, Mn-Gly-Cru1, and Mn-Gly-Cru2 were all upregulated. However, the expression levels of MnRelish, Mn-Gly-Cru1, and Mn-Gly-Cru2 at 6 h bacteria or 36 h WSSV challenge were downregulated in Relish-silenced prawns when compared with the control (bacteria or WSSV challenge only, bacteria or WSSV challenge plus dsGFP injection). Results suggest that Mn-Gly-Cru1 and Mn-Gly-Cru2 play essential roles in M. nipponense innate immunity against bacteria or WSSV, and the expression levels of both genes are regulated by Relish transcriptional factor.

Temperature potentially induced distinctive flavor of mud crab Scylla paramamosain mediated by gut microbiota

Many factors affect the flavor of crabs. However, impact of temperature on flavor has not been reported. Here, we examined Scylla paramamosain collected within the main four producing areas in China from north sampling point (NP) and south sampling point (SP), respectively. The contents of flavouring-related substances in hepatopancreas, muscles and gonads were determined by high-performance liquid chromatography (HPLC). Meanwhile, high-throughput sequencing of 16S RNA gene was used to reveal the diversity distribution of gut microbiota at each sample collection point. Comparisons among flavor substances of edible parts, the implied higher temperature in SP may be beneficial to the accumulation of flavor substances in gonads, while lower temperature in NP may be beneficial to the accumulation of flavor substances in muscles and hepatopancreas. The gut microbiota of crabs, was analyzed via 16S rRNA gene sequencing. The results of gut microbiota showed that there were significant differences in the distribution of gut microbiota in NP and SP. The microbiota composition of SP has a high distribution richness and no absolute dominant bacteria, while NP has absolute dominant bacteria and its microbiota richness was lower than SP. The results of redundancy analysis (RDA) showed that there was a significant correlation between temperature and the relative abundance of gut microbiota, and a significant correlation between gut microbiota and the content of flavor substances. This study indicates that temperature may be one of the main factors for the differences of flavor substances between SP and NP, which was most probably mediated by gut microbiota. Further exploration is needed with laboratory experiments in which the environment is more precisely controlled if these views are to be determined.

A Type IIa crustin from the pink shrimp Farfantepenaeus paulensis (crusFpau) is constitutively synthesized and stored by specific granule-containing hemocyte subpopulations

Crustins are cysteine-rich antimicrobial peptides (AMPs) widely distributed across crustaceans. From the four described crustin Types (I to IV), crustins from the subtype IIa are the most abundant and diverse members found in penaeid shrimp. Despite the critical role of Type IIa crustins in shrimp antimicrobial defenses, there is still limited information about their synthesis and antimicrobial properties. Here, we report the subcellular localization and the antibacterial spectrum of crusFpau, a Type IIa crustin from the pink shrimp Farfantepenaeus paulensis. The recombinantly expressed crusFpau showed antimicrobial activity against both Gram-positive and Gram-negative bacteria at low concentrations. Results from immunofluorescence using anti-rcrusFpau antiserum revealed that crusFpau is synthetized and stored by both granular and semigranular hemocytes, but not by hyaline cells. Interestingly, not all granular and semigranular hemocytes stained for crusFpau, revealing that this crustin is produced by specific granule-containing hemocyte subpopulations. Finally, we showed that the granule-stored peptides are not constitutively secreted into the plasma of healthy animals.

A lysin motif-containing protein (SpLysMD3) functions as a PRR involved in the antibacterial responses of mud crab, Scylla paramamosain

Lysin motif (LysM)-containing proteins function as pattern-recognition receptors in plants to recognize different N-acetylglucosamine-containing ligands, thereby triggering specific defense responses against pathogens. However, the biological functions of these proteins in animals remain unclear. In this study, we characterized a novel LysM protein, designated as SpLysMD3, in mud crab Scylla paramamosain. The cDNA sequence of SpLysMD3 had 1058 bp with an open reading frame of 840 bp encoding a protein with 279 amino acid residues. The deduced protein contained a LysM domain and a transmembrane region. SpLysMD3 was highly expressed in gills, intestine, muscle, and hemocytes and upregulated after challenges with bacteria, suggesting that it may be involved in antibacterial defense. Binding assay showed that SpLysMD3 possessed specific binding activities to all tested microorganisms as well as bacterial cell wall components lipopolysaccharide (LPS) and peptidoglycan (PGN), indicating that SpLysMD3 was an important LPS- and PGN-binding protein in mud crab. Bacterial clearance assay revealed that coating bacteria with SpLysMD3 accelerated bacterial clearance in vivo. The promotion of bacterial clearance by SpLysMD3 was further determined by using SpLysMD3-silenced crabs injected with S. aureus or V. parahemolyticus. Silencing SpLysMD3 dramatically suppressed the bacterial clearance. Meanwhile, knockdown of SpLysMD3 also severely impaired the expression of a specific set of antimicrobial peptides (AMPs); moreover, SpLysMD3 overexpression can enhance the promoter activity of SpALF2. These results suggested that SpLysMD3 affected bacterial clearance by regulating AMPs. Collectively, all the results demonstrated that SpLysMD3 may function as a potential receptor involved in innate immunity by binding to LPS and PGN and by regulating AMPs to eliminate invading pathogen. This study provided new insights into the biological functions of LysM proteins in animals and the mechanisms underlying the antibacterial activity of crustaceans.

Pattern recognition receptors and their roles on the innate immune system of mud crab (Scylla paramamosain)

The innate immune system is the first line of defense protecting the hosts against invading pathogens. Mud crab (Scylla paramamosain) is widely distributed in China and Indo-west Pacific countries, which develops a very complicated innate immune system against pathogen invasions. Innate immunity involves the humoral and cellular responses that are linked to the pattern recognition receptors (PRRs). PRRs initially recognize the infection and trigger the activation of signaling cascades, leading to transcriptional regulation of inflammatory mediators that function in pathogenic control and clearance. In mud crab S. paramamosain, the Toll/Toll-like receptors, lipopolysaccharide and β-1,3-glucan binding proteins, C-type lectins, scavenger receptors, and down syndrome cell adhesion molecules have been identified as receptor families responsible for the recognition of bacteria, fungi, and viruses, and are important components in the innate immune system. In this review, we summarize the literature on the current knowledge and the roles of PRRs in the immune defenses of mud crab, which in an effort to provide much information for further researches.

High-throughput sequencing reveals the core gut microbiota of the mud crab (Scylla paramamosain) in different coastal regions of southern China

Background

Scylla paramamosain is a commercially important mud crab. The microbiota is a community that inhabits the crab intestine, and is important for physiological functional and host health.

Results

Proteobacteria, Firmicutes, Bacteroidetes, Tenericutes, Spirochaetae and Fusobacteria were the dominant phyla of the 36 representative phyla. Eleven genera of the 820 representative genera were considered as core gut microbiota and were distributed in the five dominant phyla. The core genus of the Proteobacteria included Arcobacter, Photobacterium, Vibrio, Shewanella and Desulfovibrio. The other four phyla contained one or two genera. Male and female crab samples had two different core genera, (male samples: Psychrilyobacter & Lactococcus; female samples: Clostridium_sensu_stricto_11 and Candidatus_Bacilloplasma).

Conclusions

This is the first time core intestinal microbiota have been identified in crab from nine coastal regions of southern China. This study provides sequencing data related to the gut microbiota of S. paramamosain, and may contribute to probiotic development for S. paramamosain aquaculture industries.

SpBark Suppresses Bacterial Infection by Mediating Hemocyte Phagocytosis in an Invertebrate Model, Scylla paramamosain

Scavenger receptors are cell surface membrane-bound receptors that typically bind multiple ligands and promote the removal of endogenous proteins and pathogens. In this study, we characterized a novel scavenger receptor-like protein, namely, SpBark. SpBark was upregulated in hemocytes after challenges with bacteria, suggesting that it might be involved in antibacterial defense. SpBark is a type I transmembrane protein with four extracellular domains, including three scavenger receptor cysteine-rich domains (SRCRDs) and a C-type lectin domain (CTLD). Western blot assay showed that SpBark CTLD possessed a much stronger binding activity to tested microbes than the three SRCRDs. It also exhibited apparent binding activities to lipopolysaccharide (LPS) and acetylated low-density lipoprotein (ac-LDL), whereas the other SRCRDs showed much lower or no binding activities to these components. Agglutination activities were observed in the presence of Ca²⁺ by incubating microorganisms with SpBark CTLD instead of SRCRDs. These results suggested that SpBark CTLD was the major binding site for ac-LDL and LPS. Coating Vibrio parahemolyticus with SpBark CTLD promoted bacterial clearance in vivo. This finding indicated that SpBark might participate in the immune defenses against Gram-negative bacteria through a certain mechanism. The promotion of bacterial clearance by SpBark was further determined using SpBark-silenced crabs injected with V. parahemolyticus. SpBark knockdown by injection of SpBark dsRNA remarkably suppressed the clearance of bacteria in hemolymph. Meanwhile, it also severely restrained the phagocytosis of bacteria. This finding suggested that SpBark could modulate the phagocytosis of bacteria, and the promotion of bacterial clearance by SpBark was closely related to SpBark-mediated phagocytosis activity. The likely mechanism of bacterial clearance mediated by SpBark was as follows: SpBark acted as a pattern recognition receptor, which could sense and bind to LPS on the surface of invading bacteria with its CTLD in hemolymph. The binding to LPS made the bacteria adhere to the surface of hemocytes. This process would facilitate phagocytosis of the bacteria, resulting in their removal. This study provided new insights into the hemocyte phagocytosis mechanisms of invertebrates and the multiple biological functions of Bark proteins.

A new crustin homologue (SpCrus6) involved in the antimicrobial and antiviral innate immunity in mud crab, Scylla paramamosain

Crustins play important roles in defending against bacteria in the innate immunity system of crustaceans. In present study, we identified a crustin gene in Scylla paramamosain, which was named as SpCrus6. The ORF of SpCrus6 possessed a signal peptide sequence (SPS) at the N-terminus and a WAP domain at the C-terminus. And there were 5 Proline residues, 5 Glycine and 4 Cysteine residues between SPS and WAP domain in SpCrus6. These features indicated that SpCrus6 was a new member of crustin family. The SpCrus6 mRNA transcripts were up-regulated obviously after bacteria or virus challenge. These changes showed that SpCrus6 was involved in the antimicrobial and antiviral responses of Scylla paramamosain. Recombinant SpCrus6 (rSpCrus6) showed strong inhibitory abilities against Gram-positive bacteria (Bacillus megaterium, Staphylococcus aureus, and Bacillus subtilis). But the inhibitory abilities against four Gram-negative bacteria (Vibrio parahemolyticus, Vibrio alginolyticus, Vibrio harveyi and Escherichia coli) and two fungi (Pichia pastoris and Candida albicans) were not strong enough. Besides, rSpCrus6 could strongly bind to two Gram-positive bacteria (B. subtilis and B. megaterium) and three Gram-negative bacteria (V. alginolyticus, V. parahemolyticus, and V. harveyi). And the binding levels to S. aureus and two fungi (P. pastoris and C. albicans) were weak. The polysaccharides binding assays' results showed rSpCrus6 had superior binding activities to LPS, LTA, PGN and β-glucan. Through agglutinating assays, we found rSpCrus6 could agglutinate well three Gram-positive bacteria (S. aureus, B. subtilis and B. megaterium). And the agglutinating activities to Gram-negative bacteria and fungi were not found. In the aspect of antiviral functions, rSpCrus6 could bind specifically to the recombinant envelop protein 26 (rVP26) of white spot syndrome virus (WSSV) but not to recombinant envelop protein 28 (rVP28), whereas GST protein could not bind to rVP26 or rVP28. Besides, rSpCrus6 could suppress WSSV reproduction to some extent. Taken together, SpCrus6 was a multifunctional immunity effector in the innate immunity defending response of S. paramamosain.

Transcriptomic analysis of immune-related genes in the lipopolysaccharide-stimulated hepatopancreas of the mudflat crab Helice tientsinensis

The mudflat crab Helice tientsinensis is one of the most economically important aquaculture species in China. Nevertheless, it is susceptible to various diseases caused by viruses, bacteria and rickettsia-like organisms. A better understanding of the immune system and genes related to the responses to bacterial and viral infection is required. Herein, the hepatopancreas transcriptome of H. tientsinensis was analyzed by comparing control and lipopolysaccharide (LPS)-stimulated RNA-Seq data, yielding 91,885,038 bp and 13.78 Gb of clean reads. Following assembly and annotation, 93,207 unigenes with an average length of 883 bp were identified, of which 31,674 and 13,700 were annotated in Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, respectively. Following LPS, 4845 differentially expressed genes (DEGs) were identified, of which 2491 and 2354 were up- and down-regulated, respectively. To further investigate immune-related DEGs, KEGG enrichment analysis identified immune response pathways, most notably the peroxisome and Toll-like receptor signaling pathways. Quantitative real time-PCR (qRT-PCR) confirmed the up-regulation of a random selection of DEGs. This systematic transcriptomic analysis of the innate immune pathway in H. tientsinensis expands our understanding of the immune system in crabs.

Comparative sequence analysis of crustin isoform MjCRS7 and MjWFDC-like gene from kuruma shrimp Marsupenaeus japonicus shows variant of the WFDC domain

Crustins are well known cysteine-rich cationic antimicrobial peptides (AMPs) in crustaceans that have WFDC [WAP (whey acidic protein) four-disulfide core] domain at the carboxyl terminus. Proteins containing a WFDC domain have been discovered in many invertebrates and vertebrates. Although, there have been many WFDC domain containing nucleotide sequences found in NCBI GenBank database, their distinct sequential characteristics and their role in the innate immune system is not well understood. Here, we identified a new crustin isoform from Marsupenaeus japonicus by transcriptome analysis. The full-length cDNA of this isoform (MjCRS7) consists of 537 bp that include a 489 bp open reading frame (ORF) encoding 162 deduced amino acids (aa). The sequence contains the eight conserved cysteine residues characteristic of the WFDC domain. A phylogenetic analysis showed that MjCRS7 is a type II crustin. We also identified the full-length cDNA of a M. japonicus MjWFDC-like gene. MjWFDC-like has a 543 bp ORF encoding 180 aa. In an RT-PCR analysis, MjCRS7 and MjWFDC-like transcripts were mainly detected in gill tissue. An alignment of MjCRS7 and MjWFDC-like with previously reported M. japonicus crustin isoform 1-5 (MjCRS1-5) showed variation in the WFDC-like domain. Neither of the genes was responsive to Vibrio parahaemolyticus, Vibrio penaeicida or white spot syndrome virus (WSSV) either by immersion or injection challenge test. Although crustins are mainly antimicrobial peptides, the present results suggest that MjCRS7 may have other roles in M. japonicus.