Paospora carinifang n. gen., n. sp. (Microsporidia: Spragueidae), a parasite of the ridgetail white prawn, Palaemon carinicauda.

A new microsporidian disease of the pond-reared ridgetail white prawn, Palaemon carinicauda, was found in China. Light microscopy, pathology, and scanning electron microscopy showed that the parasite infected the host's skeletal muscle tissue and formed spherical sporophorous vesicles (SPOVs). Electron microscopy revealed that its merogonic life stages developed in direct contact with the host cytoplasm. The sporogonic life stages underwent octosporoblastic sporogony with the formation of eight uninucleate spores in each SPOV. Fresh SPOVs were 5.4 ± 0.55 µm in diameter. The octospores were oval and measured 2.3 × 1.5 µm (fresh) and 1.96 × 1.17 µm (fixed). The isofilar polar filament was coiled with 9-10 turns and arranged in two rows. Phylogenetic analysis based on the SSU rRNA gene suggests that this microsporidium has close affinities with members of the genera Potaspora and Apotaspora, but represents an independent generic taxon. We therefore propose the establishment of a new genus and species (Paospora carinifang n. gen., n. sp.) within the family Spragueidae. We also propose a taxonomic revision to transfer Potaspora macrobrachium to this new genus and reclassify it as Paospora macrobrachium comb. nov.

Establishment of a TaqMan probe-based qPCR assay for detecting microsporidia Enterospora epinepheli in grouper.

Enterospora epinepheli is an intranuclear microsporidian parasite causing serious emaciative disease in hatchery-bred juvenile groupers (Epinephelus spp.). Rapid and sensitive detection is urgently needed as its chronic infection tends to cause emaciation as well as white faeces syndrome and results in fry mortality. This study established a TaqMan probe-based real-time quantitative PCR assays targeting the small subunit rRNA (SSU) gene of E. epinepheli. The relationship between the standard curve of cycle threshold (Ct) and the logarithmic starting quantity (SQ) was determined as Ct = -3.177 lg (SQ) + 38.397. The correlation coefficient (R2 ) was 0.999, and the amplification efficiency was 106.4%. The detection limit of the TaqMan probe-based qPCR assay was 1.0 × 101 copies/µL and that is 100 times sensitive than the traditional PCR method. There is no cross-reaction with other aquatic microsporidia such as Ecytonucleospora hepatopenaei, Nucleospora hippocampi, Potaspora sp., Ameson portunus. The intra-assay and inter-assay showed great repeatability and reproducibility. In addition, the test of clinical samples showed that this assay effectively detected E. epinepheli in the grouper's intestine tissue. The established TaqMan qPCR assays will be a valuable diagnostic tool for the epidemiological investigation as well as prevention and control of E. epinepheli.

A novel C-type lectin (SpccCTL) suppresses MCRV replication by binding viral protein and regulating antiviral peptides in Scylla paramamosain.

Pattern recognition receptors (PRRs) play a crucial role in the recognition and activation of innate immune responses against invading microorganisms. This study characterizes a novel C-type lectin (CTL), SpccCTL. The cDNA sequence of SpccCTL has a full length of 1744 bp encoding a 338-amino acid protein. The predicted protein contains a signal peptide, a coiled-coil (CC) domain, and a CLECT domain. It shares more than 50 % similarity with a few CTLs with a CC domain in crustaceans. SpccCTL is highly expressed in gills and hemocytes and upregulated after MCRV challenge, suggesting that it may be involved in antiviral immunity. Recombinant SpccCTL (rSpccCTL) as well as two capsid proteins of MCRV (VP11 and VP12) were prepared. Pre-incubating MCRV virions with rSpccCTL significantly suppresses the proliferation of MCRV in mud crabs, compared with the control (treatment with GST protein), and the survival rate of mud crabs is also significantly decreased. Knockdown of SpccCTL significantly facilitates the proliferation of MCRV in mud crabs. These results reveal that SpccCTL plays an important role in antiviral immune response. GST pull-down assay result shows that rSpccCTL interacts specifically with VP11, but not to VP12. This result is further confirmed by a Co-IP assay. In addition, we found that silencing SpccCTL significantly inhibits the expression of four antimicrobial peptides (AMPs). Considering that these AMPs are members of anti-lipopolysaccharide factor family with potential antiviral activity, they are likely involved in immune defense against MCRV. Taken together, these findings clearly demonstrate that SpccCTL can recognize MCRV by binding viral capsid protein VP11 and regulate the expression of certain AMPs, suggesting that SpccCTL may function as a potential PRR playing an essential role in anti-MCRV immunity of mud crab. This study provides new insights into the antiviral immunity of crustaceans and the multifunctional characteristics of CTLs.

Ecytonucleospora hepatopenaei n. gen. et comb. (Microsporidia: Enterocytozoonidae): A redescription of the Enterocytozoon hepatopenaei (Tourtip et al., 2009), a microsporidian infecting the widely cultivated shrimp Penaeus vannamei.

The microsporidian Enterocytozoon hepatopenaei from Penaeus vannamei (EHPPv) was redescribed on the basis of spore morphology, life cycle, pathology, and molecular character. Compared with the Enterocytozoon hepatopenaei isolated from Penaeus monodon (EHPPm), described by Tourtip et al. in 2009, new features were found in EHPPv. Electron microscopy demonstrated that EHPPv was closely associated with the nucleus of host cell. The merogony and sporogony phages were in direct contact with the cytoplasm of host cells, whereas some of the sporoblasts and the spores were surrounded by the interfacial envelope. Mature spores of EHPPv were oval and monokaryotic, measuring 1.65 ± 0.15 µm × 0.92 ± 0.05 µm. Spores possessed many polyribosomes around a bipartite polaroplast and the polar filament with 4-5 coils in two rows. Phylogenetic analyses showed all Enterocytozoon hepatopenaei isolates shared a common ancestor. Based on the morphological and molecular analyses, we propose the establishment of a new genus Ecytonucleospora and transferring Enterocytozoon hepatopenaei to the genus Ecytonucleospora, retaining the specific epithet hepatopenaei that Tourtip et al. proposed in recognition of their first research, as the new combination Ecytonucleospora hepatopenaei n. comb. Furthermore, it was suggested Enterospora nucleophila, Enterocytozoon sp. isolate RA19015_21, and Enterocytozoon schreckii be assigned into this new genus.

Transcriptome analysis of pacific white shrimp (Penaeus vannamei) intestines and hepatopancreas in response to Enterocytozoon hepatopenaei (EHP) infection.

Penaeus vannamei is the most economically important species of shrimp cultured worldwide. Enterocytozoon hepatopenaei (EHP) is an emerging pathogen that severely affects the growth and development of shrimps. In this study, the transcriptome differences between EHP-infected and uninfected shrimp were investigated through next-generation sequencing. The unigenes were assembled with the reads from all the four libraries. The differentially expressed genes (DEGs) of intestines and hepatopancreas were analyzed. There were 2,884 DEGs in the intestines and 2,096 DEGs in the hepatopancreas. The GO and KEGG enrichment analysis indicated that DEGs were significantly enriched in signaling pathways associated with nutritional energy metabolism and mobilizing autoimmunity. Moreover, the results suggested the downregulation of key genes in energy synthesis pathways contributed greatly to shrimp growth retardation; the upregulation of immune-related genes enhanced the resistance of shrimp against EHP infection. This study provided identified genes and pathways associated with EHP infection revealing the molecular mechanisms of growth retardation.

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.

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 ß-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.

Identification of Carassius auratus gibelio liver cell proteins interacting with the GABAA receptor γ2 subunit using a yeast two-hybrid system.

The γ-aminobutyric acid type A (GABAA) receptor is an important pentameric inhibitory neurotransmitter receptor, and the γ2 subunit of this receptor plays a key role in potentiation of the GABAA response. We previously detected that the expression of GABAA receptor in the livers of Carassius auratus gibelio significantly increased after medication (avermectin and difloxacin treatment). In order to better understand the mechanism of action of the GABAA receptor γ2 subunit in the livers of C. auratus gibelio, we constructed a C. auratus gibelio liver cDNA library (the titer value of 1.2 × 106 cfu/mL) and identified the proteins that interact with the GABAA receptor γ2 subunit by using a yeast two-hybrid assay. The yeast two-hybrid screening yielded seven positive clones, namely, prelid3b, cdc42, sgk1, spg21, proteasome, chia.5, and AP-3 complex subunit beta-1, all of which have been annotated by the NCBI database. The functions of these proteins are complex; therefore, additional studies are required to determine the specific interactions of these proteins with the GABAA receptor γ2 subunit in the liver of C. auratus gibelio. Although the interactions identified by the yeast two-hybrid system should be considered as preliminary results, the findings of this study may provide further direction and a foundation for future research focusing on the mechanisms of the GABAA receptor γ2 subunit in C. auratus gibelio livers.

Toxicity and accumulation of zinc pyrithione in the liver and kidneys of Carassius auratus gibelio: association with P-glycoprotein expression.

Zinc pyrithione (ZPT) is a broad-spectrum antibacterial and antifungal agent; therefore, it is widely used in industry and civilian life. It is discharged into the aquatic environment with industrial and civilian waste water. Carassius sp. is one of the most widely distributed and farmed fish in China. The effects of aquatic ZPT on Carassius sp. remain unknown. In this study, we determined the acute toxicity of ZPT on Carassius sp. The results showed that the median lethal concentration (LC50 96 h) of ZPT on Carassius sp. cultivated in freshwater or water with 1.5 or 3 ‰ salinity was 0.163, 0.126, and 0.113 mg/L, respectively. ZPT has a higher affinity to the liver than the kidney, with a prolonged tissue residual time. P-glycoprotein (P-gp), an ATP-binding cassette transporter, was found to be induced in the liver and kidney tissues of these Carassius spp. after ZPT treatment, based on the determination of its mRNA and protein levels by quantitative real-time reverse transcription polymerase chain reaction and immunohistochemistry, respectively. The ZPT accumulation and magnitude of P-gp induction were also affected by the salinity of the cultivation water. These results suggest that aquatic ZPT is potentially toxic to Carassius sp. We speculate that P-gp induction may play a protective role for Carassius sp. Our findings provide a basis for assessing the potential risk of ZPT to aquatic animals including Carassius sp.

SpALF4: a newly identified anti-lipopolysaccharide factor from the mud crab Scylla paramamosain with broad spectrum antimicrobial activity.

Anti-lipopolysaccharide factors (ALFs) are antimicrobial peptides with binding and neutralizing activities to lipopolysaccharide (LPS) in crustaceans. This study identified and characterized a novel ALF homolog (SpALF4) from the mud crab Scylla paramamosain. The complete cDNA of SpALF4 had 756 bp with a 381 bp open reading frame encoding a protein with 126 aa. The deduced protein contained a signal peptide and a LPS-binding domain. SpALF4 shared the highest identity with PtALF5 at amino acid level but exhibited low similarity with most of other crustacean ALFs. Furthermore, different from the previously identified three SpALF homologs and most of other ALFs, SpALF4 had a low isoelectric point (pI) for the mature peptide and the LPS-binding domain with the values of 6.93 and 6.74, respectively. These results indicate that SpALF4 may be a unique ALF homolog with special biological function in the mud crab. Similar to the spatial structure of ALFPm3, SpALF4 contains three α-helices packed against a four-strand ß-sheet, and an amphipathic loop formed by a disulphide bond between two conserved cysteine residues in LPS-binding domain. SpALF4, mainly distributed in hemocytes, could be upregulated by Vibrio harveyi, Staphylococcus aureus, or white spot syndrome virus. Recombinant SpALF4 could inhibit the growth of Gram-negative bacteria (V. harveyi, Vibrio anguillarum, Vibrio alginolyticus, Aeromonas hydrophila, Pseudomonas putida), Gram-positive bacteria (S. aureus and Bacillus megaterium), and a fungus Candida albicans to varying degrees. Further study showed that it could also bind to all the aforementioned microorganisms except S. aureus. These results demonstrate that SpALF4 is a unique ALF homolog with potent antimicrobial activity against bacteria and fungi. This characteristic suggests SpALF4 plays an essential function in immune defense against pathogen invasion in mud crab.

A novel peroxinectin involved in antiviral and antibacterial immunity of mud crab, Scylla paramamosain.

Peroxinectin (PX) with cell adhesion and peroxidase activities is important in invertebrate immune responses. We identified a novel PX homolog from Scylla paramamosain (designated as Sp-PX) through transcriptome sequencing. The full-length of cDNA sequence was 3,165 bp. And there was a peroxidase domain in the deduced protein sequence. A cell-adhesive sequence (KGD motif) was also found in the N-terminus. The predicted molecular mass of the mature protein is 83.9 kDa, with an estimated pI of 6.21. At the amino acid level, Sp-PX shared much higher similarities with other crustaceans PX proteins. And Sp-PX also exhibited some similarities with other peroxidase family members. According to real-time polymerase chain reaction, Sp-PX was mainly distributed in the hemocytes. The gene expression levels in the hemocytes of the normal and white spot syndrome virus (WSSV)-challenged crabs were compared via high-throughput RNA sequencing technology, and the results showed that Sp-PX was upregulated at 48 h post-WSSV challenge. Subsequently, how Sp-PX responds to WSSV stimulus was explored through time-course experiments. The Sp-PX transcripts dramatically increased and reached the highest level at 12 h post-injection, whereas Sp-PX transcripts were recovered at 96 h post-challenge. Meanwhile, it was found that the WSSV copies proliferated significantly after a period of latent viral infection for 48 h. In addition,Sp-PX transcripts were also upregulated after Vibrio harveyi or Staphylococcus aureus challenge. Overall, Sp-PX not only participates in antibacterial immunity but also plays a crucial role in the antiviral immune responses of mud crab at the early stage of WSSV infection.

PvML1 suppresses bacterial infection by recognizing LPS and regulating AMP expression in shrimp.

Toll and Toll-like receptors (TLRs) play essential roles in the innate immunity of Drosophila and mammals. Recent studies have revealed the presence of Toll-mediated immune signaling pathways in shrimp. However, the recognition and activation mechanism of Toll signaling pathways in crustaceans remain poorly understood due to the absence of key recognition molecules, such as peptidoglycan recognition proteins. Here, a novel MD2-related lipid-recognition (ML) member named PvML1 was characterized in Penaeus vannamei. We found that PvML1 shared a similar 3D structure with human MD2 that could specifically recognize lipopolysaccharides (LPS) participating in LPS-mediated TLR4 signaling. PvML1 was highly expressed in hemocytes and remarkably upregulated after Vibrio parahemolyticus challenge. Furthermore, the binding and agglutinating assays showed that PvML1 possessed strong binding activities to LPS and its key portion lipid A as well as Vibrio cells, and the binding of PvML1 with bacterial cells led to the agglutination of bacteria, suggesting PvML1 may act as a potential pathogen recognition protein upon interaction with LPS. Besides, coating V. parahemolyticus with recombinant PvML1 promoted bacterial clearance in vivo and increased the survival rate of bacterium-challenged shrimp. This result was further confirmed by RNAi experiments. The knockdown of PvML1 remarkably suppressed the clearance of bacteria in hemolymph and decreased the survival rate of infected shrimp. Meanwhile, the silencing of PvML1 severely impaired the expression of a few antimicrobial peptides (AMPs). These results demonstrated the significant correlation of bacterial clearance mediated by PvML1 with the AMP expression. Interestingly, we found that PvML1 interacted with the extracellular region of PvToll2, which had been previously shown to participate in bacterial clearance by regulating AMP expression. Taken together, the proposed antibacterial model mediated by PvML1 might be described as follows. PvML1 acted as a potential recognition receptor for Gram-negative bacteria by binding to LPS, and then it activated PvToll2-mediated signaling pathway by interacting with PvToll2 to eliminate invading bacteria through producing specific AMPs. This study provided new insights into the recognition and activation mechanism of Toll signaling pathways of invertebrates and the defense functions of ML members.

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.

A novel ML domain-containing protein (SpMD2) functions as a potential LPS receptor involved in anti-Vibrio immune response.

The myeloid differentiation protein 2 (MD2)-related lipid-recognition (ML) proteins display diverse biological functions in host immunity and lipid metabolism by interacting with different lipids. Human MD2, an indispensable accessory protein in TLR4 signaling pathway, specifically recognizes lipopolysaccharides (LPS), thereby leading to the activation of TLR4 signaling pathway to produce many effectors that participate in inflammatory and immuneresponses against Gram-negative bacteria. Toll and immune deficiency (IMD) pathways are first characterized in Drosophila and are reportedly present in crustaceans, but the recognition and activation mechanism of these signaling pathways in crustaceans remains unclear. In the present study, a novel ML protein was characterized in mud crab (Scylla paramamosain) and designated as SpMD2. The complete SpMD2 cDNA sequence is 1114 bp long with a 465 bp open reading frame; it encodes a protein that contains 154 amino acids (aa). In the deduced protein, a signal peptide (1-21 aa residues) and a ML domain (43-151 aa residues) were predicted. SpMD2 shared a similar three-dimensional structure and a close evolutionary relationship with human MD2. SpMD2 was highly expressed in gills, hemocytes, intestine, and hepatopancreas and was upregulated in gills and hemocytes after challenges with bacteria, thereby suggesting its involvement in antibacterial defense. Western blot assay showed that SpMD2 possesses strong binding activities to different bacteria and two fungi. ELISA demonstrated that SpMD2 exhibits binding abilities to LPS, lipid A, peptidoglycan (PGN), and lipoteichoic acid (LTA). Its binding ability to LPS and lipid A were stronger than to PGN or LTA, implying that SpMD2 was an important LPS-binding protein in mud crab. Bacterial clearance assay revealed that the pre-incubation of Vibrio parahemolyticus with SpMD2 facilitates bacterial clearance in vivo and that knockdown of SpMD2 dramatically suppresses the bacterial clearance and decreases the expression of several antimicrobial peptides (AMPs). Furthermore, SpMD2 overexpression could enhance the promoter activity of SpALF2. These results revealed that SpMD2 affects bacterial clearance by regulating AMPs. Thus, by binding to LPS and by regulating AMPs, SpMD2 may function as a potential receptor, which is involved in the recognition and activation of a certain immune signaling pathway against Gram-negative bacteria. This study provides new insights into the diverse functions of ML proteins and into the antibacterial mechanisms of crustaceans.

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.

SpCrus3 and SpCrus4 share high similarity in mud crab (Scylla paramamosain) exhibiting different antibacterial activities.

Type I crustins are crucial effectors of crustacean immune system. Various type I crustins with high sequence diversity possess different antimicrobial activities. To date, the mechanism on how the sequence diversity of type I crustins affects their antimicrobial activities is largely unclear, and how different crustins function together against bacterial invasion still remains unknown. In this study, we identified two novel type I crustins, namely, SpCrus3 and SpCrus4, from an economically important crab, Scylla paramamosain. Either SpCrus3 or SpCrus4 was highly expressed in gill. After challenges with Vibrio parahemolyticus or Staphylococcus aureus, SpCrus4 was up-regulated, whereas SpCrus3 was down-regulated. No significant expression change of SpCrus3 and SpCrus4 was observed after white spot syndrome virus injection, suggesting that these two genes may not participate in the antiviral immune responses. SpCrus3 and SpCrus4 had the common 5' terminus and high similarity of 66.06%, but SpCrus4 exhibited stronger antimicrobial activity than that of SpCrus3. Microorganism-binding assay results revealed that both SpCrus3 and SpCrus4 exhibited binding ability to all tested microorganisms. Furthermore, the polysaccharide-binding assay showed that these two proteins exhibited strong binding activity to bacterial polysaccharides, such as lipopolysaccharide (LPS), lipoteichoic acid (LTA), and peptidoglycan (PGN). SpCrus3 and SpCrus4 exhibited stronger binding activity to LPS or LTA than to PGN. Moreover, SpCrus4 showed stronger binding activity to LTA than that of SpCrus3, which may be responsible for the significantly distinct antimicrobial activity between these two proteins. In addition, SpCrus4 displayed stronger agglutination activity against several kinds of microorganisms than that of SpCrus3. This increased agglutination activity may also contribute to the strong antibacterial activity of SpCrus4. On the basis of all these results, a possible antibacterial mode exerted by SpCrus3 and SpCrus4 was proposed as follows. SpCrus3 was highly expressed in normal crabs to maintain low-level antibacterial activity without bacterial challenges. When crabs were challenged with bacteria, large amount of SpCrus4 was generated to exhibit strong antibacterial activity against bacterial invasion. This study provides new insights to understand the antibacterial functions and mechanisms of type I crustins.

Newly identified type II crustin (SpCrus2) in Scylla paramamosain contains a distinct cysteine distribution pattern exhibiting broad antimicrobial activity.

Type II crustins are the most abundant type of crustins in shrimps that exhibit remarkable sequence diversities and broad antibacterial activities. This study characterized a novel type II crustin, SpCrus2, in the mud crab Scylla paramamosain. The SpCrus2 cDNA sequence is 620-bp long with a 495-bp open reading frame encoding a 164-amino acid protein. In the deduced protein, a 17-amino acid signal peptide, a glycine-rich hydrophobic region (GRR), and a cysteine-rich region (CRR) containing a whey acidic protein domain were predicted. SpCrus2 shares high similarity with most type II crustins (types IIa and IIb crustins) in shrimps but has a novel distribution pattern of cysteine residues that is distinct from most crustins. SpCrus2 and PlCrus3 from Pacifastacus leniusculus share high similarity and the same distribution pattern of cysteine residues. Thus, we proposed them as type IIc crustins. SpCrus2 is mainly distributed in the gills and can be up-regulated through Vibrio parahemolyticus or Staphylococcus aureus challenge. To investigate the biological functions of SpCrus2 and the underlying mechanisms, SpCrus2, GRR, CRR, and the mutant of CRR (CRR-M, the cysteine distribution pattern is mutated into that in most conventional crustins) were all overexpressed and purified. SpCrus2 GRR itself, as a glycine-rich amphiphilic peptide, exhibited evident antibacterial ability against Gram-negative bacteria, whereas CRR possessed potent antibacterial activity against Gram-positive bacteria. Either GRR or CRR exhibited weaker antibacterial activity than the whole protein of SpCrus2, indicating that GRR and CRR synergized to exert their potential antibacterial functions. In addition, CRR exhibited slightly stronger antimicrobial activity than CRR-M, suggesting that SpCrus2 containing this novel cysteine distribution pattern may exhibit stronger antimicrobial activity than most type II crustins with the conventional distribution pattern of cysteine residues. The likely antimicrobial ability of SpCrus2 may result from its microbial polysaccharide-binding and agglutination activities. Overall, this study characterized the first type II crustin in crabs and provided new insights into understanding the sequence and functional diversity of crustins and their immune functions in crustaceans.

A novel anti-lipopolysaccharide factor SpALF6 in mud crab Scylla paramamosain exhibiting different antimicrobial activity from its single amino acid mutant.

In crustaceans, anti-lipopolysaccharide factors (ALFs) are important immune effectors that have sequence diversity and exhibit broad antimicrobial activities. In this study, we characterized a novel ALF homolog SpALF6 from mud crab Scylla paramamosain and its variant SpALF6-V, which was generated by mutations of two amino acids (H46 to R and A110 to P) due to the presence of two single nucleotide polymorphisms (SNPs). SpALF6 was an anionic peptide with isoelectric point (pI) 6.79, whereas SpALF6-V was a cationic protein with pI 7.98. These two proteins shared a common lipopolysaccharide (LPS)-binding domain (LBD) with pI 6.05. SpALF6 was expressed mainly in hemocytes and up-regulated by Vibrio parahaemolyticus or Staphylococcus aureus challenge, indicating that SpALF6 may participate in the antibacterial immune responses. To investigate the likely functional differences between SpALF6 and SpALF6-V and elucidate the underlying mechanisms, a single amino acid mutant SpALF6-M (from H46 to R, outside but very close to LBD), which had the same pI as SpALF6-V, was harvested by a fusion PCR. Then, both SpALF6 and SpALF6-M were overexpressed and purified to test antimicrobial activity and binding activity to microbial cells or polysaccharides. SpALF6-M exhibited more potent antimicrobial and cell-binding activity on Gram-positive bacteria and fungi than SpALF6. Furthermore, SpALF6-M possessed stronger lipoteichoic acid (LTA)-binding activity than SpALF6, demonstrating that this particular positively charged amino acid outside but close to LBD contributed to the increase in SpALF6-M antibacterial activity. In addition, SpALF6 LBD peptide and its biotin-labeled form were synthesized in this study. Results showed that this anionic LBD peptide itself did not exhibit any significant antimicrobial activity against 10 kinds of microorganisms but it possessed strong binding activity to LPS, LTA, and peptidoglycan. These findings suggested that this anionic LBD was still an important active center and required collaboration with some particular positively charged amino acids outside LBD to exhibit antibacterial activity. Thus, SpALF6-M antimicrobial activity was increased by the mutation of H46 to R instead of A110 to P, which did not change the protein charge, suggesting that SpALF6-V may have more potent antimicrobial activity than SpALF6 and play more important roles in antibacterial immunity. This study provided a new insight into the mechanisms of how ALF amino acid sequence diversity resulted in their functional divergence.

Newly identified invertebrate-type lysozyme (Splys-i) in mud crab (Scylla paramamosain) exhibiting muramidase-deficient antimicrobial activity.

Lysozymes are widely distributed immune effectors exerting muramidase activity against the peptidoglycan of the bacterial cell wall to trigger cell lysis. However, some invertebrate-type (i-type) lysozymes deficient of muramidase activity still exhibit antimicrobial activity. To date, the mechanism underlying the antimicrobial effect of muramidase-deficient i-type lysozymes remains unclear. Accordingly, this study characterized a novel i-type lysozyme, Splys-i, in the mud crab Scylla paramamosain. Splys-i shared the highest identity with the Litopenaeus vannamei i-type lysozyme (Lvlys-i2, 54% identity) at the amino acid level. Alignment analysis and 3D structure comparison show that Splys-i may be a muramidase-deficient i-type lysozyme because it lacks the two conserved catalytic residues (Glu and Asp) that are necessary for muramidase activity. Splys-i is mainly distributed in the intestine, stomach, gills, hepatopancreas, and hemocytes, and it is upregulated by Vibrio harveyi or Staphylococcus aureus challenge. Recombinant Splys-i protein (rSplys-i) can inhibit the growth of Gram-negative bacteria (V. harveyi, Vibrio alginolyticus, Vibrio parahemolyticus, and Escherichia coli), Gram-positive bacteria (S. aureus, Bacillus subtilis, and Bacillus megaterium), and the fungus Candida albicans to varying degrees. In this study, two binding assays and a bacterial agglutination assay were conducted to elucidate the potential antimicrobial mechanisms of Splys-i. Results demonstrated that rSplys-i could bind to all nine aforementioned microorganisms. It also exhibited a strong binding activity to lipopolysaccharide from E. coli and lipoteichoic acid and peptidoglycan (PGN) from S. aureus but a weak binding activity to PGN from B. subtilis and ß-glucan from fungi. Moreover, rSplys-i could agglutinate these nine types of microorganisms in the presence of Ca2+ at different protein concentrations. These results suggest that the binding activity and its triggered agglutinating activity might be two major mechanisms of action to realize the muramidase-deficient antibacterial activity. In addition, rSplys-i can hydrolyze the peptidoglycan of some Gram-positive bacteria because it exhibits weak isopeptidase activities in salt and protein concentration-dependent manner. This result indicates that such an isopeptidase activity may contribute to the muramidase-deficient antimicrobial activity to a certain degree. In conclusion, Splys-i is upregulated by pathogenic bacteria, and it inhibits bacterial growth by binding and agglutination activities as well as isopeptidase activity, suggesting that Splys-i is involved in immune defense against bacteria through several different mechanisms of action.

Analysis of Saprolegnia parasitica Transcriptome following Treatment with Copper Sulfate.

BACKGROUND: Massive infection caused by oomycete fungus Saprolegnia parasitica is detrimental to freshwater fish. Recently, we showed that copper sulfate demonstrated good efficacy for controlling S. parasitica infection in grass carp. In this study, we investigated the mechanism of inhibition of S. parasitica growth by copper sulfate by analyzing the transcriptome of copper sulfate-treated S. parasitica. To examine the mechanism of copper sulfate inhibiting S. parasitica, we utilized RNA-seq technology to compare differential gene expression in S. parasitica treated with or without copper sulfate. RESULTS: The total mapped rates of the reads with the reference genome were 90.50% in the control group and 73.50% in the experimental group. In the control group, annotated splice junctions, partial novel splice junctions and complete novel splice junctions were about 83%, 3% and 14%, respectively. In the treatment group, the corresponding values were about 75%, 6% and 19%. Following copper sulfate treatment, a total 310 genes were markedly upregulated and 556 genes were markedly downregulated in S. parasitica. Material metabolism related GO terms including cofactor binding (33 genes), 1,3-beta-D-glucan synthase complex (4 genes), carboxylic acid metabolic process (40 genes) were the most significantly enriched. KEGG pathway analysis also determined that the metabolism-related biological pathways were significantly enriched, including the metabolic pathways (98 genes), biosynthesis of secondary metabolites pathways (42 genes), fatty acid metabolism (13 genes), phenylalanine metabolism (7 genes), starch and sucrose metabolism pathway (12 genes). The qRT-PCR results were largely consistent with the RNA-Seq results. CONCLUSION: Our results indicate that copper sulfate inhibits S. parasitica growth by affecting multiple biological functions, including protein synthesis, energy biogenesis, and metabolism.