Characterisation of a novel Type I crustin involved in antibacterial and antifungal responses in the red claw crayfish, Cherax quadricarinatus

Characterization of a novel Type-I Crustin (carcininPm2) from black tiger shrimp Penaeus monodon

Carcinins are type-I crustins from crustaceans and play an important role in innate immune system. In this study, type-I crustins, carcininPm1 and carcininPm2, from the hemocytes of Penaeus monodon were identified. Comparison of their amino acid sequences and the phylogenetic tree revealed that they were closely related to the other crustacean carcinin proteins, but were clustered into different groups of the carcinin proteins. The full-length amino acids of carcininPm1 and carcininPm2 were 92 and 111 residues, respectively. CarcininPm1 and carcininPm2 were expressed mainly in hemocytes and intestine compared to the other tissues. The expression of carcininPm1 and carcininPm2 were dramatically increased in early time of bacterial challenged shrimp hemocytes. In contrast, the carcininPm1 and carcininPm2 were expressed in response to late state of YHV-infected shrimp hemocytes where the copy number of virus was high. The recombinant carcininPm2 (rcarcininPm2) but not its WAP domain (rcarcininPm2_WAP) exhibited antimicrobial activity against Vibrio harveyi and Vibrio parahaemolyticus AHPND but not other bacteria tested. The rcarcininPm2 was able to prolong the survival rate of VH-treated post larval shrimp from about 102 h to 156 h. These studies indicated that the carcininPm2 possessed the potential and challenges as antibacterial in innate immunity of shrimp.

The identification of a serpin with immune defense role in oriental river prawn Macrobrachium nipponense

Serpins are a protein superfamily of serine protease inhibitors. One of their functions is to participate in immune responses by inhibiting the activation of prophenoloxidase. To elucidate the immune role of serpin in Macrobrachium nipponense, a serpin gene (Mnserpin) was cloned from M. nipponense in this study. Mnserpin protein has an N-terminal signal peptide and a serpin domain that contains a hinge region, a signature sequence of serpin and a P1(arginine)-P1' scissile bond, and evolutionally closely related to the crustacean serpins. Mnserpin highly expressed in the hepatopancreas and gill. Mnserpin expression increased first and then decreased after Vibrio parahaemolyticus and Aeromonas hydrophila infection, and was knocked down by dsMnserpin injection with a maximum knockdown efficiency of 92 %. Mnserpin knockdown increased the expression of the clip domain serine protease and prophenoloxidase genes and phenoloxidase activity of M. nipponense as well as its mortality rate after V. parahaemolyticus and A. hydrophila infection. The recombinant Mnserpin (rMnserpin) showed bacteria-binding and bacteriostatic activity in vitro. Moreover, rMnserpin injection decreased the bacterial number and the mortality rate of M. nipponense post V. parahaemolyticus and A. hydrophila infection. These results suggested that Mnserpin plays a major role in the innate immune response of M. nipponense.

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.

Changes in the gene expression and gut microbiome to the infection of decapod iridescent virus 1 in Cherax quadricarinatus

As a new emerging viral pathogen, Decapod iridescent virus 1 (DIV1) seriously threatens crustacean farming in recent years. However, limited research progresses have been made on the immune mechanism between host and viral factors in response to DIV1 infection. In the current study, a natural occurrence of DIV1 infection with obvious clinical signs was found in farmed redclaw crayfish Cherax quadricarinatus, and confirmed by nested PCR detection and histopathological examination. Besides, gene expression profiles were analyzed after being challenged with DIV1, and results showed that 27 immune related genes were upregulated compared with the control group. Moreover, the gut microbiota from healthy and DIV1-infected crayfish were investigated by 16S rDNA high-throughput sequencing. Results showed that significant differences in the microbial composition and function were observed after DIV1 challenge. Furthermore, we discovered that changes in gene expression profiles were correlated with microbiota alterations under DIV1 challenge. Taken together, our findings will provide new insights into the immune response mechanism of DIV1 infection in crustaceans.

A Novel Peptide with Antifungal Activity from Red Swamp Crayfish Procambarus clarkii

The defense system of freshwater crayfish Procambarus clarkii as a diversified source of bioactive molecules with antimicrobial properties was studied. Antimicrobial activity of two polypeptide-enriched extracts obtained from hemocytes and hemolymph of P. clarkii were assessed against Gram positive (Staphylococcus aureus, Enterococcus faecalis) and Gram negative (Pseudomonas aeruginosa, Escherichia coli) bacteria and toward the yeast Candida albicans. The two peptide fractions showed interesting MIC values (ranging from 11 to 700 μg/mL) against all tested pathogens. Polypeptide-enriched extracts were further investigated using a high-resolution mass spectrometry and database search and 14 novel peptides were identified. Some peptides and their derivatives were chemically synthesized and tested in vitro against the bacterial and yeast pathogens. The analysis identified a synthetic derivative peptide, which showed an interesting antifungal (MIC and MFC equal to 31.2 μg/mL and 62.5 μg/mL, respectively) and antibiofilm (BIC₅₀ equal to 23.2 μg/mL) activities against Candida albicans and a low toxicity in human cells.

Antimicrobial peptides from freshwater invertebrate species: potential for future applications

Invertebrates are a significant source of antimicrobial peptides because they lack an adaptive immune system and must rely on their innate immunity to survive in a pathogen-infested environment. Various antimicrobial peptides that represent major components of invertebrate innate immunity have been described in a number of investigations over the last few decades. In freshwater invertebrates, antimicrobial peptides have been identified in arthropods, annelids, molluscs, crustaceans, and cnidarians. Freshwater invertebrate species contain antimicrobial peptides from the families astacidin, macin, defensin, and crustin, as well as other antimicrobial peptides that do not belong to these families. They show broad spectrum activities greatly directed against bacteria and to a less extent against fungi and viruses. This review focuses on antimicrobial peptides found in freshwater invertebrates, highlighting their features, structure-activity connections, antimicrobial processes, and possible applications in the food industry, animal husbandry, aquaculture, and medicine. The methods for their synthesis, purification, and characterization, as well as the obstacles and strategies for their development and application, are also discussed.

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

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

The identification of a nuclear factor Akirin with regulating the expression of antimicrobial peptides in red swamp crayfish (Procambarus clarkii)

Akirin is a highly conserved nuclear factor among different species. It is closely related to skeletal muscle development, innate immune response, and tumorigenesis in a variety of animals. In invertebrates, Akirin is mainly involved in gene transcription and NF-κB dependent natural immune response. In the present study, a nuclear factor Akirin was identified from Procambarus clarkii. The Akirin protein of crayfish consists of 204 amino acids and is conserved among its family members, especially the nuclear localization signal peptide motif (KRRR). PcAkirin was highly expressed in stomach, intestines, and hepatopancreas. After A. hydrophila challenge, the transcription level of Akirin significantly increased in hemocyte and hepatopancreas. In addition, the recombinant Akirin protein was produced successfully and helpful to resist WSSV infection by increasing the expression level of some immune related genes. On the contrary, after interfering with Akirin gene by dsRNA, the crayfish increased the sensitivity to A. hydrophila and WSSV infections. The results are more obvious in the accumulated mortality of P. clarkii infected with A. hydrophila and WSSV. All these results suggested that Akirin played a significant role in innate immune responses and protected it from WSSV and bacterial infection in crayfish.

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.

The immune function of a novel crustin with an atypical WAP domain in regulating intestinal microbiota homeostasis in Litopenaeus vannamei

Crustins are a family of antimicrobial peptides (AMP) with multiple functions, including antimicrobial activity, capability of protease inhibition, phagocytosis promotion, and wound healing in crustaceans. Till present, several members of crustins have been identified and their activities were studied. However, there are still less investigations on how they play functions in vivo. Here, we identified a novel crustin with an atypical WAP domain, LvCrustin Ⅰ-1, which is mainly distributed in tissues, including intestine, gill, epidermis and stomach of the shrimp Litopenaeus vannamei. The expression level of LvCrustin Ⅰ-1 was significantly up-regulated at 3 h, 6 h, 12 h, and 24 h after Vibrio parahaemolyticus infection. Knockdown of LvCrustin Ⅰ-1 with dsRNA resulted in a significant increase of the bacteria number in hepatopancreas of shrimp upon V. parahaemolyticus infection, showing that LvCrustin Ⅰ-1 participated in pathogen infection process. Recombinant LvCrustin Ⅰ-1 protein showed microorganism-binding activity rather than antibacterial activity against tested bacteria. Furthermore, significant difference existed between the intestinal microbiota in shrimp before and after LvCrustin Ⅰ-1 knockdown based on the result of alpha and NMDS analyses. Knockdown of LvCrustin Ⅰ-1 increased the proportion of Demequina, Nautella, Propionibacterium, Anaerospora and decreased the proportion of Bacteroidia and Vibrio. These data suggest that LvCrustin Ⅰ-1 might perform its immunological function through modulation of the intestinal microbiota homeostasis rather than direct inhibition of bacterial growth in shrimp.

The First Genome Survey of the Antarctic Krill (Euphausia superba) Provides a Valuable Genetic Resource for Polar Biomedical Research

The world-famous Antarctic krill (Euphausia superba) plays a fundamental role in the Antarctic food chain. It resides in cold environments with the most abundant biomass to support the Antarctic ecology and fisheries. Here, we performed the first genome survey of the Antarctic krill, with genomic evidence for its estimated genome size of 42.1 gigabases (Gb). Such a large genome, however, is beyond our present capability to obtain a good assembly, although our sequencing data are a valuable genetic resource for subsequent polar biomedical research. We extracted 13 typical protein-coding gene sequences of the mitochondrial genome and analyzed simple sequence repeats (SSRs), which are useful for species identification and origin determination. Meanwhile, we conducted a high-throughput comparative identification of putative antimicrobial peptides (AMPs) and antihypertensive peptides (AHTPs) from whole-body transcriptomes of the Antarctic krill and its well-known counterpart, the whiteleg shrimp (Penaeus vannamei; resident in warm waters). Related data revealed that AMPs/AMP precursors and AHTPs were generally conserved, with interesting variations between the two crustacean species. In summary, as the first report of estimated genome size of the Antarctic krill, our present genome survey data provide a foundation for further biological research into this polar species. Our preliminary investigations on bioactive peptides will bring a new perspective for the in-depth development of novel marine drugs.

Functional characterization of an ER-stress responding Crustin gene in Litopenaeus vannamei

Shrimp in culture ponds are challenged by various pathogens as well as harsh water environment. The innate immune system and environmental stress response system of shrimp paly an important role in shrimp survival and growth. For remission the endoplasmic reticulum (ER)-stress caused by environmental stress, unfolded protein response (UPR) may reduce the synthesis of most proteins, including great mass of immune factors, which could weaken the immune function of shrimp. Therefore, how cells keep appropriate amount of immune factor synthesis under such a situation is critical important for shrimp health and growth. In this study, we cloned a new Crustin gene (LvCruU) from Litopenaeus vannamei. We showed that LvCruU has antibacterial activity, and reducing its expression would increase the cumulative mortality of L. vannamei upon the Vibrio parahemolyticus infection. In addition, we found that promoter activity of LvCruU was enhanced not only by the deformed epidermal autoregulatory factor-1 (Deaf1), but also by activating transcription factor 3 (LvATF3) of shrimp UPR. Real-time RT-PCR showed that LvCruU and LvATF3 both were induced upon UPR activation. And moreover, in Thapsigargin plus dsLvCruU injection test, we showed that down-regulation of LvCruU increased the cumulative mortality of V. parahemolyticus-infected shrimp under ER-stress. These results suggest that LvCruU work as a downstream effector of UPR, and contribute to antimicrobic immune response upon ER-stress in L. vannamei.

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.

Efficient production of a recombinant Venerupis philippinarum defensin (VpDef) in Pichia pastoris and characterization of its antibacterial activity and stability

VpDef is a novel defensin isolated from the clam Venerupis philippinarum. Previously it was expressed in Escherichia coli; however, the E. coli-derived recombinant VpDef did not show effective antimicrobial activity against Staphyloccocus aureus or the Gram-negative bacteria tested. As such, the goal of this study was to design, express, and purify a recombinant VpDef (rVpDef) in Pichia pastoris and to determine its antibacterial potency and stability. A 6.9 KDa rVpDef was successfully expressed as a secreted peptide in P. pastoris, and the amount of rVpDef accumulation was shown to reach as high as approximate 60 μg per 1 ml of culture medium only after an initial optimization was performed. The purified rVpDef demonstrated a broad antibacterial spectrum and was active against six typical common bacteria, both Gram-positive and Gram-negative. A minimal inhibition concentration of as low as 50 μg/ml was observed for rVpDef against the growth of E. coli O157 (ATCC 35150). Moreover, rVpDef was tolerant to temperature shock and proteinase digestion and maintained a high stability over a relatively broad pH range. In addition, rVpDef had a low hemolytic activity against rabbit erythrocytes. Taken together, this study demonstrated that rVpDef could be produced in a large-scale manner in P. pastoris and has a good antibacterial activity and suitable stability. This is the first report on heterologous expression of a biologically active VpDef in P. pastoris, supporting its use for both research and application purposes.

Crayfish immunity - Recent findings

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

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.

Characterization and expression analysis of a thioredoxin-like protein gene in the sea cucumber Apostichopus japonicus

As the most important disulfide bond reducates of intracellular oxidordeuctase, thioredoxin (TRX) plays a crucial role in maintaining reducing state of intracellular proteins to normally perform their function. In this study, a cDNA of TRX-like protein gene from Apostichopus japonicus (denoted as AjTRX) was cloned and characterized. The full-length cDNA of AjTRXwas of 1870 bp, consisting of a 5'-UTR of 101 bp, a long 3'-UTR of 887 bp and a 882 bp open reading frame (ORF) encoding a 293 amino acids. The predicted molecular mass and the theoretical PI of the deduced amino acids of AjTRX were 32.3 kDa and 5.52, respectively. Phylogenetic trees showed that AjTRX had a closer evolution relationship with TRX from Strongylocentrotus purpuratus. AjTRX was found to be ubiquitously expressed in all examined tissues including longitudinal muscle, coelomocytes, tube feet, intestine, respiratory tree and body wall indicating a general role in physiological processes. Temporal expression pattern of AjTRX in coelomocytes showed that AjTRX reached two peak expression levels at 8 h and 48 h after Vibrio splendidus challenge with a 8.6 and 9.3-fold increase compared to their control groups, respectively. The recombinant AjTRX protein (rAjTRX) displayed obvious antioxidant activity in a dose-dependent manner, and the higher reducing activity was detected in 20 μM experimental group. All these results strongly suggested that AjTRX could play an important role as an antioxidant in a physiological context, and might be involved in the process of bacterial challenge.

Identification of an anti-lipopolysacchride factor possessing both antiviral and antibacterial activity from the red claw crayfish Cherax quadricarinatus

It is well-known that anti-lipopolysacchride factors (ALFs) are involved in the recognition and elimination of invading pathogens. In this study, the full-length ALF cDNA sequence of the red claw crayfish Cherax quadricarinatus (termed CqALF) was cloned from a suppression subtractive hybridization library constructed using red claw crayfish hematopoietic tissue cell (Hpt cell) cultures following challenge with white spot syndrome virus (WSSV). The full-length cDNA sequence of CqALF was 863 bp, and the open reading frame encoded 123 amino acids with a signal peptide in the N-terminus and a conserved LPS-binding domain. Unlike most ALFs, which are highly expressed in haemocytes, high expression levels of CqALF were detected in epithelium, the stomach and eyestalks, while lower expression was detected in Hpt, nerves, the heart, muscle tissue, gonads, haemocytes, intestines, gills and the hepatopancreas. To further explore the biological activities of CqALF, mature recombinant CqALF protein (rCqALF) was expressed and purified using a eukaryotic expression system, and an antimicrobial activity test was carried out. rCqALF clearly exerted antiviral activity, as evidenced by the severe disruption of the envelope of intact WSSV virions following co-incubation of virions with rCqALF. Additionally, pre-incubation of WSSV with rCqALF resulted in both a significant reduction in WSSV replication in red claw crayfish Hpt cell cultures and an increased survival rate among animals. Furthermore, rCqALF was effective against both Gram-negative bacteria and Gram-positive bacteria, particularly Shigella flexneri and Staphylococcus aureus. A membrane integrity assay suggested that rCqALF was unlikely to disrupt bacterial membrane integrity compared to cecropin P1. Taken together, these data suggest that CqALF may play an important role in immune defence in the crustacean C. quadricarinatus.

Characterization of a type-I crustin with broad-spectrum antimicrobial activity from red swamp crayfish Procambarus clarkii

Crustins are a family of antimicrobial peptides mainly identified in crustaceans and characterized by a whey acidic protein (WAP) domain and an additional glycine-, cysteine-, or proline-rich region. In this study, we identified and characterized PcCru, a new crustin isolated from red swamp crayfish Procambarus clarkii. The open reading frame of PcCru was 333 base pairs long and encoded a 110-residue polypeptide, which contained a signal peptide, a cysteine-rich region, and a WAP domain. The architecture and phylogenetic analysis suggested that PcCru was a new member of the type-I crustin family. PcCru was highly expressed in hemocytes and was significantly induced by viral and bacterial stimulations at both the translational and transcriptional levels. The titer of PcCru in circulating plasma was also increased considerably by bacterial challenge. Recombinant PcCru from both prokaryotic and eukaryotic expression systems were generated, and the proteins exhibited broad-spectrum antimicrobial activity. Furthermore, PcCru protected crayfish from infection by pathogenic bacteria Aeromonas hydrophila in vivo. This study provided new information emphasizing the important role of the crustin family in the crustacean antibacterial immune response.

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.