Crustins from eyestalk cDNA library of swimming crab Portunus trituberculatus: molecular characterization, genomic organization and expression analysis

A2M possesses anti-bacterial functions by recruiting and enhancing phagocytosis through GRP78 in an echinoderm

Alpha-2-macroglobulin (A2M) is an extracellular macromolecule mainly known for its role as a broad-spectrum protease inhibitor in mammals. However, the immune recognition and regulation mechanisms of A2M in invertebrates are still not well investigated. In the current study, the role of sea cucumber Apostichopus japonicus A2M in the regulation of innate immune responses was explored. We found that AjA2M promotes phagocytosis of Vibrio splendidus in coelomocytes of sea cucumber. Then two major functional structural domains of AjA2M, the thioester domain (TED) and the receptor-binding structural domain (RBD) were cloned. It was found that the AjA2M-TED binds to pathogens while causing Vibrio splendidus aggregation; the AjA2M-RBD interacts with the Glucose Regulated Protein 78 (AjGRP78), subsequently AjGRP78 accelerates the degradation of Vibrio splendidus in lysosomes by facilitating polymerisation and rearrangement of the cytoskeleton. Collectively, the findings together suggest that A2M-GRP78 axis mediates immune signaling pathway of phagocytosis and AjA2M has been characterized to play an essential crucial role in antibacterial immune responses of invertebrates.

The antibacterial activity and antibacterial mechanism analyses of an LRR-IG protein in the Chinese mitten crab, Eriocheir sinensis

Leucine-rich repeat and immunoglobulin domain containing protein (LRR-IG) family is an important class of immune molecules in invertebrates. Herein, a novel LRR-IG, named as EsLRR-IG5, was identified from Eriocheir sinensis. It contained typical structures of LRR-IG including an N-terminal LRR region and three IG domains. EsLRR-IG5 was ubiquitously expressed in all the tested tissues, and its transcriptional levels increased after being challenged with Staphylococcus aureus and Vibrio parahaemolyticus. Recombinant proteins of LRR and IG domains from the EsLRR-IG5 (named as rEsLRR5 and rEsIG5) were successfully obtained. rEsLRR5 and rEsIG5 could bind to both gram-positive bacteria and gram-negative bacteria as well as lipopolysaccharide (LPS) and peptidoglycan (PGN). Moreover, rEsLRR5 and rEsIG5 exhibited antibacterial activities against V. parahaemolyticus and V. alginolyticus and displayed bacterial agglutination activities against S. aureus, Corynebacterium glutamicum, Micrococcus lysodeikticus, V. parahaemolyticus and V. alginolyticus. The scanning electron microscopy (SEM) observation revealed that the membrane integrity of V. parahaemolyticus and V. alginolyticus was destroyed by rEsLRR5 and rEsIG5, which may lead to the leakage of cell contents and death. This study provided clues for further studies on the immune defense mechanism mediated by LRR-IG in crustaceans and provided candidate antibacterial agents for prevention and control of diseases in aquaculture.

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.

Characterization of the Dual Functions of LvCrustinVII from Litopenaeus vannamei as Antimicrobial Peptide and Opsonin

Crustin are a family of antimicrobial peptides that play an important role in protecting against pathogens infection in the innate immune system of crustaceans. Previously, we identified several novel types of crustins, including type VI and type VII crustins. However, their immune functions were still unclear. In the present study, the immune function of type VII crustin LvCrustinVII were investigated in Litopenaeus vannamei. LvCrustinVII was wildly expressed in all tested tissues, with relatively high expression levels in hepatopancreas, epidermis and lymphoid organ. Upon Vibrio parahaemolyticus infection, LvCrustinVII was significantly upregulated in hepatopancreas. Recombinant LvCrustinVII (rLvCrustinVII) showed strong inhibitory activities against Gram-negative bacteria Vibrio harveyi and V. parahaemolyticus, while weak activities against the Gram-positive bacteria Staphylococcus aureus. Binding assay showed that rLvCrustinVII could bind strongly to V. harveyi and V. parahaemolyticus, as well as the cell wall components Glu, LPS and PGN. In the presence of Ca²⁺, rLvCrustinVII could agglutinate V. parahaemolyticus and enhance hemocyte phagocytosis. The present data partially illustrate the immune function of LvCrustinVII, which enrich our understanding on the functional mechanisms of crustins and provide useful information for application of this kind of antimicrobial peptides.

The potential role of eyestalk in the immunity of Litopenaeus vannamei to Vibrio infection

The X-organ-sinus gland complex (XO-SG) in the eyestalk is an important neuroendocrine regulatory organ of crustaceans such as Litopenaeus vannamei, a prominent aquaculture species. The current study found significant changes in the enzyme activities of ALP, ACP, and T-SOD of hepatopancreatic in response to Vibrio parahaemolyticus exposure following eyestalk ablation, indicating that they were all involved in the immunological regulation of shrimps against V. parahaemolyticus infection. A total of 52,656 unigenes were obtained after RNA-Seq, with an average length of 1036 bp and an N50 of 1847 bp. Subsequently, 1899 eyestalk positive regulation genes (EPRGs), 745 eyestalk negative regulation genes (ENRGs), and 2077 non-eyestalk regulatory genes (NEGs) were identified. KEGG analysis of EPRGs revealed that eyestalk ablation might activate the neuroendocrine-immune (NEI) system. The RNA-Seq data were validated using quantitative real-time PCR (qRT-PCR). The findings suggested that eyestalk ablation might affect the expression of genes involved in the prophenoloxidase-activating system, the TLR signaling pathway, and numerous other immune-related genes in L. vannamei. All of these findings revealed that the eyestalk might have a role in the immune response of L. vannamei. The genes and pathways discovered in this study will help to elucidate the molecular mechanisms of hemocytes' immune response to V. parahaemolyticus following eyestalk ablation in shrimp, as well as provide the framework for building crustacean immunity theory.

Characterization and antimicrobial evaluation of a new Spgly-AMP, glycine-rich antimicrobial peptide from the mud crab Scylla paramamosain

Antimicrobial peptide (AMP) is a crucial component of the innate immune system in crustaceans. In mud crab, Scylla paramamosain, a commercially important species, a glycine-rich antimicrobial peptide (Spgly-AMP) gene was newly identified and putatively encoded a 26aa signal peptide and 37aa mature peptide. To understand the function of Spgly-AMP, the expression profile of Spgly-amp gene was characterized, which showed Spgly-amp was expressed widely in most tissues of adult crabs with the highest expression level in hemocytes. After Vibrio parahaemolyticus, PGN, or Poly I:C stimulations, the expression level of Spgly-amp was significantly up-regulated in the hemocytes. In antimicrobial assays, chemically synthesized Spgly-AMP peptides exhibited strong antibacterial activities against both Gram-positive and Gram-negative bacteria and high thermal stability after high-temperature heating. These findings in the present study verified the importance of the Spgly-AMP in defense of pathogenic bacteria infection in the mud crab and provided a promising candidate of antimicrobial agents in the crab aquaculture.

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 and Functional Diversity of Crustin-Like Genes in the Shrimp Litopenaeus vannamei

Crustins are crustacean cationic cysteine-rich antimicrobial peptides that contain one or two whey acidic protein (WAP) domain(s) at the carboxyl terminus and mainly show antimicrobial and/or proteinase inhibitory activities. Here, we performed genome and transcriptome screening and identified 34 full-length crustin-like encoding genes in Litopenaeus vannamei. Multiple sequence analysis of the deduced mature peptides revealed that these putative crustins included 10 type Ia, two type Ib, one type Ic, 11 type IIa, three type IIb, four type III, one type IV, one type VI, and one type VII. These putative crustins were clustered into different groups. Phylogenetic analysis, considering their domain composition, showed that different types of crustin-like genes in crustaceans might be originated from the WAP core region, along with sequence insertion, duplication, deletion, and amino acid substitution. Tissue distribution analysis suggested that most crustin-like genes were mainly detected in immune-related tissues while several crustin-like genes exhibited tissue-specific expression patterns. Quantitative PCR analysis on 15 selected crustin-like genes showed that most of them were apparently upregulated after Vibrio parahaemolyticus or white spot syndrome virus (WSSV) infection. One type Ib crustin-like gene, mainly expressed in the ovary, showed the highest expression levels before the gastrula stage and was hardly detected after the limb bud stage, suggesting that it was a maternal immune effector. Collectively, the present data revealed the molecular and functional diversity of crustins and their potential evolutionary routes in crustaceans.

A novel C-type lectin with a YPD motif from Portunus trituberculatus (PtCLec1) mediating pathogen recognition and opsonization

C-type lectins are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins that function as pattern recognition receptors (PRRs) in innate immune system. In this study, a new C-type lectin was identified from the swimming crab Portunus trituberculatus (PtCLec1). The full-length cDNA of PtCLec1 was 873 bp encoding 176 amino acids. The predicted PtCLec1 protein contained a signal peptide and a single carbohydrate-recognition domain with a special YPD motif. The PtCLec1 transcripts were mainly detected in hepatopancreas and its relative expression levels were significantly up-regulated after the challenges of Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris. The recombinant PtCLec1 (rPtCLec1) could bind all the tested pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS), peptidoglycan (PGN) and glucan (GLU), and microorganisms, including V. alginolyticus, V. parahaemolyticus, Pseudomonas aeruginosa, Staphylococcus aureus, M. luteus and P. pastoris. It also exhibited strong activity to agglutinate bacteria and yeast in a Ca²⁺-dependent manner, and such agglutinating activity could be inhibited by d-galactose and LPS. Moreover, rPtCLec1 revealed antimicrobial activity against the tested Gram-negative (V. alginolyticus, V. parahaemolyticus and P. aeruginosa) and Gram-positive bacteria (S. aureus and M. luteus), and promoted the clearance of V. alginolyticus in vivo and hemocyte phagocytosis in vitro. Knockdown of PtCLec1 could down-regulate the expression of phagocytosis-related genes, but enhance the expression levels of prophenoloxidase (proPO) system-related genes, mannose-binding lectin (MBL), antimicrobial peptides (AMPs), MyD88 and Relish. All these results indicate that PtCLec1 might act as a PRR in immune recognition and an opsonin in pathogen elimination.

An MBT domain containing anti-lipopolysaccharide factor (PtALF8) from Portunus trituberculatus is involved in immune response to bacterial challenge

Anti-lipopolysaccharide factors are effective antimicrobial peptides that can bind and neutralize lipopolysaccharide (LPS). In the present study, a new sequence encoding for ALF (designated as PtALF8) was cloned by suppression subtractive hybridization method using ovary of swimming crab Portunus trituberculatus as material. The full-length cDNA of PtALF8 consisted of 531 bp with an ORF of 348 bp encoding a peptide of 115 amino acids containing a putative signal peptide of 19 amino acids. The mature PtALF8 had a predicted molecular weight (MW) of 11.28 kDa and theoretical isoelectricpoint (pI) of 5.11. The PtALF8 contains an MBT domain which was not found in the other 7 isoforms of ALF reported in P. trituberculatus. Unlike most ALFs expressed in hemocytes, PtALF8 transcript was predominantly detected in hepatopancreas. After challenge with Vibrio alginolyticus, the temporal expression level of PtALF8 transcript in hemocytes reached the highest level at 3 h, then decreased to the lowest level at 24 h, and started to increase at 48 h. The recombinant protein showed antimicrobial and bactericidal activity against several bacteria, such as Gram-positive bacteria, Staphylococcus aureus, Micrococcus luteus and Gram-negative bacteria, V. alginolyticus, indicated that the PtALF8 isoform might play protective function against invading bacteria in P. trituberculatus.

Transcriptional responses to low-salinity stress in the gills of adult female Portunus trituberculatus

The swimming crab (Portunus trituberculatus, Portunus) can tolerate low salinity, but the mechanism of its varied salinity adaptation at the molecular level remains unclear. In this study, we prepared four mRNA and microRNA (miRNA) libraries using the gills collected from four salinity groups and performed RNA-sequencing (RNA-Seq) to identify the genes related to the low salinity. We set 25 ppt as the control group. A total of 659 genes were differentially expressed in at least one of the six comparison groups (25 ppt vs. 20 ppt, 25 ppt vs. 15 ppt, 25 ppt vs. 10 ppt, 20 ppt vs. 15 ppt, 20 ppt vs. 10 ppt and 15 ppt vs. 10 ppt). A total of 15 and 9 unigenes were downregulated and upregulated under low salinity compared with that in 25 ppt, respectively. Six genes, namely, aminopeptidase, centromere protein, cytochrome b5 reductase, bone morphogenetic protein, and two carbonic anhydrases, were selected for verification through quantitative real-time PCR. The results were consistent with the RNA-Seq results. Furthermore, 95 conserved miRNAs and 16 novel miRNAs were differentially expressed in at least one of the six comparison groups. Analysis of the miRNA-mRNA interaction showed that miR-2 and miR-317 regulated >50 mRNA targets. In addition, let-7c was downregulated in all groups under low salinity compared with that in the control group. This study helped elucidate the adaptation mechanism of the swimming crab in low-saline environments.

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.

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.

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

Crustins, the main AMP family in Crustacea, are generated as isoforms in many species and implicated in innate immune responses, but their detailed molecular mechanisms on susceptible bacteria remain largely unclear. Type II and type I crustins are distinguished by glycine-rich region (GRR), which is a major marker motif, and some type II crustins exhibit stronger antibacterial activities than their GRR deletion mutants. In the present study, a novel crustin, namely, SpCrus5, was functionally characterized from a commercially valuable crab Scylla paramamosain. SpCrus5 contained a typical cysteine-rich domain at the N-terminus, a conserved WAP domain in the center, and a special GRR at the C-terminus, which is located in a site that differs from that of GRRs in typical type II crustins found between signal peptides and cysteine-rich domains. SpCrus5 shared high similarities with most type II crustins, and it was more closely related to type II crustins than to other retrieved crustins. SpCrus5 was predominantly expressed in gills and remarkably upregulated after the crabs were challenged with Vibrio parahemolyticus or Staphylococcus aureus, suggesting that SpCrus5 might participate in antibacterial immune responses. To further elucidate how this C-terminal GRR affects the function of SpCrus5, we harvested a GRR deletion mutant (SpCrus5-ΔGRR) by deleting the GRR. Liquid growth inhibition assays demonstrated that the antimicrobial activity of SpCrus5 was stronger than that of SpCrus5-ΔGRR, and the antibacterial spectrum of the former toward Gram-negative bacteria was broader than that of the latter. Binding assays revealed that the microorganism-binding ability and polysaccharide-binding activity of SpCrus5 were stronger than those of SpCrus5-ΔGRR. SpCrus5 or SpCrus5-ΔGRR agglutinated all tested Gram-positive bacteria. Therefore, the antibacterial activities of SpCrus5 were stronger and broader than those of SpCrus5-ΔGRR, and the binding ability and agglutination activity might contribute to the antimicrobial activity of SpCrus5. These results revealed that the C-terminal GRR was necessary to produce an efficient antibacterial activity of SpCrus5. SpCrus5 was highly identical with most type II crustins and it functioned as many type II crustins did, indicating that SpCrus5 was more likely an atypical type II crustin than a type I crustin. This study revealed that SpCrus5 participated as an essential antimicrobial effector in immune responses and provided new insights into the underlying mechanisms of the sequence and function diversity of crustins.

Unusual tissue distribution of carcinin, an antibacterial crustin, in the crab, Carcinus maenas, reveals its multi-functionality

Crustins are whey acidic four-disulphide core (WFDSC) domain-containing proteins in decapods that are widely regarded as antimicrobial agents that contribute to host defence. Whilst there have been many analyses of crustin gene expression in tissues, few studies have been made of the distribution of the natural proteins. Here we report an immunostaining investigation of carcinin, a native crustin from Carcinus maenas, in the body organs. The results show that the protein is largely confined to the haemocytes with only a weak signal detected in the heart, hepatopancreas and midgut caecum where it is restricted to the outer surfaces. Importantly, carcinin was seen to be deposited by the haemocytes on these surfaces. Higher levels of staining were detected in the gonads with carcinin particularly abundant in the capsule of ovary as well as some oocytes. Conspicuous staining was further evident in the cuticle of the eyestalk peduncles. Ablation of the eyestalks resulted in a reduction of carcinin in the maturing ovary with the mature eggs rarely displaying a strong signal for the protein. Interestingly, the degree of carcinin also strongly increased in the healing peduncle, indicating that the protein may be associated with wounding, cell damage and/or tissue regeneration.

Crustins are distinctive members of the WAP-containing protein superfamily: An improved classification approach

Crustins are considered effector molecules of innate immunity in arthropods, and classification schemes have been proposed over the last 10 years. However, classification problems have emerged: for example, proteins that have been well identified as members of a particular category have also been classified as crustins. Therefore, the objective of this manuscript was to analyze and, based on solid arguments, improve the original proposed nomenclature to make crustins a distinctive group of antibacterial proteins. The presence of WAP or 4DSC domain has been considered a distinctive feature of crustins; however, several antibacterial proteins containing WAP domains have been detected in diverse taxonomic groups (including mammals). Here, we present evidence supporting the idea that the Cys-rich region and the 4DSC domain can be considered a signature of crustins and, together with some distance arrangements occurring within this 12-Cys region, yield enough information for the classification of these proteins. Herein, the core characteristics to be considered for classification purposes are the length of the Gly-rich region and the repetitive tetrapeptides occurring within this region; these characteristics are then hierarchically followed by the F and A distances located within the 4DSC domain. Finally, the proposed system considers the crustin signature as the common structure in all members, which is a differentiator from other proteins containing WAP domains, separating crustins as a well-distinguished member of the superfamily of WAP-domain containing proteins.

A chymotrypsin-like serine protease from Portunus trituberculatus involved in pathogen recognition and AMP synthesis but not required for prophenoloxidase activation

Clip domain serine proteases (clip-SPs) play critical roles in various immune responses in arthropods, such as hemolymph coagulation, antimicrobial peptide (AMP) synthesis, cell adhesion and melanization. In the present study, we report the molecular and functional characterization of a clip domain serine protease (PtcSP2) from the swimming crab Portunus trituberculatus. The N-terminal clip domain and the C-terminal SP-like domain of PtcSP2 were expressed in Escherichia coli system, and assayed for their activities. Sequence similarity and phylogenetic analysis revealed that PtcSP2 may belong to the chymotrypsin family, which was confirmed by protease activity assay of the recombinant SP-like domain. The clip domain of PtcSP2 exhibited strong antibacterial activity and microbial-binding activity, suggesting the potential role in immune defense and recognition. Knockdown of PtcSP2 by RNA interference could significantly reduce PtcSP2 transcript levels, but neither decrease the total phenoloxidase (PO) activity in crab nor significantly alter the expression levels of serine protease inhibitors PtPLC and PtSerpin. These results indicate that PtcSP2 is not involved in the proPO system. However, suppression of PtcSP2 led to a significant change in the expression of AMP genes PtALFs and PtCrustin but not PtALF5. All these findings suggest that PtcSP2 is a multifunctional chymotrypsin-like serine protease and may participate in crab innate immunity by its antibacterial activity, immune recognition or regulation of AMP expression.

Marine Antimicrobial Peptides: Nature Provides Templates for the Design of Novel Compounds against Pathogenic Bacteria

The discovery of antibiotics for the treatment of bacterial infections brought the idea that bacteria would no longer endanger human health. However, bacterial diseases still represent a worldwide treat. The ability of microorganisms to develop resistance, together with the indiscriminate use of antibiotics, is mainly responsible for this situation; thus, resistance has compelled the scientific community to search for novel therapeutics. In this scenario, antimicrobial peptides (AMPs) provide a promising strategy against a wide array of pathogenic microorganisms, being able to act directly as antimicrobial agents but also being important regulators of the innate immune system. This review is an attempt to explore marine AMPs as a rich source of molecules with antimicrobial activity. In fact, the sea is poorly explored in terms of AMPs, but it represents a resource with plentiful antibacterial agents performing their role in a harsh environment. For the application of AMPs in the medical field limitations correlated to their peptide nature, their inactivation by environmental pH, presence of salts, proteases, or other components have to be solved. Thus, these peptides may act as templates for the design of more potent and less toxic compounds.

Complement factor I from flatfish half-smooth tongue (Cynoglossus semilaevis) exhibited anti-microbial activities

Complement factor I (Cfi) is a soluble serine protease which plays a crucial role in the modulation of complement cascades. In the presence of substrate modulating cofactors (such as complement factor H, C4bp, CR1, etc), Cfi cleaves and inactivates C3b and C4b, thereby controlling the complement-mediated processes. In this study, we sequenced and characterized Cfi gene from Cynoglossus Semilaevis (designated as CsCfi) for the first time. The full-length cDNA of CsCfi was 2230 bp in length, including a 98 bp 5'-untranslated region (UTR), a 164 bp 3'-UTR and a 1968 bp open reading frame (ORF). It encoded a polypeptide of 656 amino acids, with a molecular mass of 72.28 kDa and an isoelectric point of 7.71. A signal peptide was defined at N-terminus, resulting in a 626-residue mature protein. Multiple sequence alignment revealed that Cfi proteins were well conserved with the typical modular architecture and identical active sites throughout the vertebrates, which suggested the conserved function of Cfi. Phylogenetic analysis indicated that CsCfi and the homologous Cfi sequences from teleosts clustered into a clade, separating from another clade from the cartilaginous fish and other vertebrates. Tissue expression profile analysis by quantitative real-time PCR (qRT-PCR) showed that CsCfi mRNA constitutively expressed in all tested tissues, with the predominant expression in liver and the lowest in stomach. Temporal expression levels of CsCfi after challenging with Vibrio anguillarum showed different expression patterns in intestine, spleen, skin, blood, head kidney and liver. The recombinant CsCfi (rCsCfi) protein showed broad-spectrum antimicrobial activities against the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa and Shewanella putrefaciens. The research revealed that CsCfi plays an important role in C. Semilaevis immunity.

Antimicrobial proteins: From old proteins, new tricks

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

Sequence diversity and evolution of antimicrobial peptides in invertebrates

Antimicrobial peptides (AMPs) are evolutionarily ancient molecules that act as the key components in the invertebrate innate immunity against invading pathogens. Several AMPs have been identified and characterized in invertebrates, and found to display considerable diversity in their amino acid sequence, structure and biological activity. AMP genes appear to have rapidly evolved, which might have arisen from the co-evolutionary arms race between host and pathogens, and enabled organisms to survive in different microbial environments. Here, the sequence diversity of invertebrate AMPs (defensins, cecropins, crustins and anti-lipopolysaccharide factors) are presented to provide a better understanding of the evolution pattern of these peptides that play a major role in host defense mechanisms.

Type I and type II crustins from Penaeus monodon, genetic variation and antimicrobial activity of the most abundant crustinPm4

An antimicrobial protein, crustin, is involved in the innate immunity of crustacean by defending the host directly against the microbial pathogens. By data mining the Penaeus monodon EST database, two type I crustins, carcininPm1 and 2, and ten type II crustins, crustinPm1-10, were identified. The abundant crustins were crustinPm1, 4 and 7, each with variation in the length of Gly-rich repeat among its members. A few crustinPm1, 4 and 7 with deletion in the Cys-rich region were also observed. Furthermore, the crustinPm4 with the longest N-terminal Gly-rich region was characterized. The crustinPm4 allelic genes were expressed mainly from the hemocytes. Its expression was up-regulated readily by WSSV infection and gradually decreased to normal level afterwards. The recombinant crustinPm4-1 (rcrustinPm4-1) isoform was produced using the Escherichia coli expression system and tested for its antimicrobial activity. The rcrustinPm4-1 was able to inhibit the growth of a Gram-positive bacterium, Bacillus megaterium but not Bacillus subtilis, Micrococcus luteus and Staphylococcus aureus. It also inhibited the growth of two Gram-negative bacteria, E. coli 363 and Vibrio harveyi 639 at lower potency. The rcrustinPm4-1 affected the WSSV infection because the expression of an intermediate early gene ie1 in WSSV-infected hemocyte cell culture was reduced. It was shown further that the rcrustinPm4-1 could delay by about one and a half days the manifestation of disease by WSSV.

Transcriptome analysis of the Portunus trituberculatus: de novo assembly, growth-related gene identification and marker discovery

Background

The swimming crab, Portunus trituberculatus, is an important farmed species in China, has been attracting extensive studies, which require more and more genome background knowledge. To date, the sequencing of its whole genome is unavailable and transcriptomic information is also scarce for this species. In the present study, we performed de novo transcriptome sequencing to produce a comprehensive transcript dataset for major tissues of Portunus trituberculatus by the Illumina paired-end sequencing technology.

Results

Total RNA was isolated from eyestalk, gill, heart, hepatopancreas and muscle. Equal quantities of RNA from each tissue were pooled to construct a cDNA library. Using the Illumina paired-end sequencing technology, we generated a total of 120,137 transcripts with an average length of 1037 bp. Further assembly analysis showed that all contigs contributed to 87,100 unigenes, of these, 16,029 unigenes (18.40% of the total) can be matched in the GenBank non-redundant database. Potential genes and their functions were predicted by GO, KEGG pathway mapping and COG analysis. Based on our sequence analysis and published literature, many putative genes with fundamental roles in growth and muscle development, including actin, myosin, tropomyosin, troponin and other potentially important candidate genes were identified for the first time in this specie. Furthermore, 22,673 SSRs and 66,191 high-confidence SNPs were identified in this EST dataset.

Conclusion

The transcriptome provides an invaluable new data for a functional genomics resource and future biological research in Portunus trituberculatus. The data will also instruct future functional studies to manipulate or select for genes influencing growth that should find practical applications in aquaculture breeding programs. The molecular markers identified in this study will provide a material basis for future genetic linkage and quantitative trait loci analyses, and will be essential for accelerating aquaculture breeding programs with this species.

Differences in gene organization between type I and type II crustins in the morotoge shrimp, Pandalopsis japonica

Crustins are cysteine-rich cationic antimicrobial peptides (AMPs) found in decapod crustaceans. Six novel crustin genes (Paj-CrusIc, Id, Ie, If, IIb and IIc) were identified in the morotoge shrimp, Pandalopsis japonica. Deduced amino acid sequences of isolated Paj-Crus genes ranged from 99 to 178 amino acid residues (10.6-17.8 kDa). Sequence analysis of nine isolated Paj-Crus genes and 100 different crustins from various decapod crustaceans revealed that a splice site and KXXXCP motif within the WAP domain may be the main criteria for classifying type I and II crustins, suggesting that the two types of crustin genes may have been generated by different processes. We also identified three intron-less crustin I genes (Paj-Crus Id, Ie and If) for the first time, which may have been generated by gene duplication. The tissue distribution profiles showed that Paj-CrusI genes were expressed predominantly in the gill and epidermis, whereas Paj-CrusII genes were expressed ubiquitously, suggesting that the two types of crustins may play different roles in various tissues or under different physiological conditions. Differing from previous results, hemocyte-specific crustin was not isolated from Pandalopsis japonica. This study showed that both types of crustin genes (types I and II) exist in decapod crustaceans and their primary structure and expression profiles differ from each other, suggesting that they may play different biological roles. This will help to extend our knowledge of the crustacean innate immune response, which will provide important basic information of shrimp immunity against various pathogens.

Molecular cloning, genomic structure and antimicrobial activity of PtALF7, a unique isoform of anti-lipopolysaccharide factor from the swimming crab Portunus trituberculatus

Anti-lipopolysaccharide factors (ALFs), as the potent antimicrobial peptides, are becoming predominant candidates for potential therapeutic agents of bacterial and viral diseases. In this study, a unique isoform of ALF (PtALF7) was identified from hemocytes cDNA library of the swimming crab Portunus trituberculatus. The PtALF7 cDNA contained an open reading frame (ORF) of 372 bp encoding 123 amino acids. The deduced peptide of PtALF7 shared high similarity with our previously reported PtALF1-3 but low with PtALF4-6. The PtALF7 gene consisted of three exons interrupted by two introns, and was clearly transcribed from different genomic loci compared with other PtALF isoforms. Totally 128 SNPs including 12 in coding region and 116 in noncoding region were detected in PtALF7 gene by direct sequencing of 20 samples. The mRNA expression of PtALF7 transcript was primarily observed in hemocytes followed by gill and eyestalk, but barely detectable in hepatopancreas. After challenge with Vibrio alginolyticus, a main pathogen causing high mortality in P. trituberculatus, the PtALF7 transcript in hemocytes showed a clear time-dependent response expression pattern with obvious decrease at 6 h and significant increase at 24 h. The recombinant PtALF7 protein exhibited antimicrobial activity against the test Gram-negative and Gram-positive bacteria, but did not inhibit the growth of fungus Pichia pastoris. These results together indicate a potential involvement for PtALF7 in the innate immune response of P. trituberculatus.