Recombinant expression and functional analysis of an isoform of anti-lipopolysaccharide factors (FcALF5) from Chinese shrimp Fenneropenaeus chinensis

Revolutionizing orthopedic healthcare: a systematic review unveiling recombinant antimicrobial peptides

The increasing demand for orthopedic surgeries, including joint replacements, is driven by an aging population and improved diagnosis of joint conditions. Orthopedic surgeries carry a risk of infection, especially in patients with comorbidities. The rise of antibiotic resistance exacerbates this issue, necessitating alternatives like in vitro bioengineered antimicrobial peptides (AMPs), offering broad-spectrum activity and multiple action mechanisms. This review aimed to assess the prevalence of antimicrobial potential and the yield after purification among recombinant AMP families. The antimicrobial potential was evaluated using the Minimum Inhibitory Concentration (MIC) values against the most common bacteria involved in clinical infections. This systematic review adhered to PRISMA guidelines, focusing on in vitro studies of recombinant AMPs. The search strategy was run on PubMed, Scopus and Embase up to 30th March 2023. The Population, Exposure and Outcome model was used to extract the data from studies and ToxRTool for the risk of bias analysis. This review included studies providing peptide production yield data and MIC values against pathogenic bacteria. Non-English texts, reviews, conference abstracts, books, studies focusing solely on chemical synthesis, those reporting incomplete data sets, using non-standard MIC assessment methods, or presenting MIC values as ranges rather than precise concentrations, were excluded. From 370 publications, 34 studies on AMPs were analyzed. These covered 46 AMPs across 18 families, with Defensins and Hepcidins being most common. Yields varied from 0.5 to 2,700 mg/L. AMPs were tested against 23 bacterial genera, with MIC values ranging from 0.125 to >1,152 μg/mL. Arenicins showed the highest antimicrobial activity, particularly against common orthopedic infection pathogens. However, AMP production yields varied and some AMPs demonstrated limited effectiveness against certain bacterial strains. This systematic review emphasizes the critical role of bioengineered AMPs to cope infections and antibiotic resistance. It meticulously evaluates recombinant AMPs, focusing on their antimicrobial efficacy and production yields. The review highlights that, despite the variability in AMP yields and effectiveness, Arenicins and Defensins are promising candidates for future research and clinical applications in treating antibiotic-resistant orthopedic infections. This study contributes significantly to the understanding of AMPs in healthcare, underscoring their potential in addressing the growing challenge of antibiotic resistance. Systematic review registration:https://osf.io/2uq4c/.

Host Defense Proteins and Peptides with Lipopolysaccharide-Binding Activity from Marine Invertebrates and Their Therapeutic Potential in Gram-Negative Sepsis

Sepsis is a life-threatening complication of an infectious process that results from the excessive and uncontrolled activation of the host's pro-inflammatory immune response to a pathogen. Lipopolysaccharide (LPS), also known as endotoxin, which is a major component of Gram-negative bacteria's outer membrane, plays a key role in the development of Gram-negative sepsis and septic shock in humans. To date, no specific and effective drug against sepsis has been developed. This review summarizes data on LPS-binding proteins from marine invertebrates (ILBPs) that inhibit LPS toxic effects and are of interest as potential drugs for sepsis treatment. The structure, physicochemical properties, antimicrobial, and LPS-binding/neutralizing activity of these proteins and their synthetic analogs are considered in detail. Problems that arise during clinical trials of potential anti-endotoxic drugs are discussed.

Antibacterial activity of an anti-lipopolysaccharide factor (MjALF-D) identified from kuruma prawn (Marsupenaeus japonicus)

Antimicrobial peptides (AMPs) play important roles in host innate immune systems. Anti-lipopolysaccharide factor (ALF), which is a primary AMP in crustaceans, is active against bacteria, fungi and some viruses. MjALF-D, an anionic peptide, is a group D ALF isolated from Marsupenaeus japonicus. In the present study, a series of experiments were performed to study its antibacterial spectrum and further explore its antibacterial and bacterial binding activities. Liquid growth inhibition data demonstrated that recombinant MjALF-D (rMjALF-D) possessed strong antibacterial activity against the gram-positive bacterium Micrococcus luteus and the gram-negative bacterium Photobacterium damselae, with a minimum inhibitory concentration (MIC) or minimum bactericidal concentration (MBC) lower than 1.25 μM. The kinetic analysis showed that the antibacterial activity of rMjALF-D was dose- and time-dependent. Additionally, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations the potential bactericidal process. rMjALF-D treatment resulted in a large number of unidentified filamentous structures wrapped around the bacteria, and during the incubation, the cell surface became obviously rough and disrupted. rMjALF-D showed distinct binding ability after direct incubation with M. luteus and P. damselae but no binding ability to Escherichia coli, which was weakly inhibited by rMjALF-D. These data suggest that MjALF-D displays modest antibacterial activity and may provide more insights into the function and role of ALF in shrimp immunity.

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.

Anti-lipopolysaccharide factors regulated by Stat, Dorsal, and Relish are involved in anti-WSSV innate immune defense in Macrobrachium nipponense

Anti-lipopolysaccharide factors (ALF) is an important antimicrobial peptide and critical effector molecule with a broad spectrum of antimicrobial activities in crustaceans. In addition to the previously reported five ALFs (MnALF1-5), another three ALFs [MnALF1, which is different from MnALF1 (ALF02818) that has been reported; MnALF6; and MnALF7] and an isoform of MnALF4 (MnALF4-isoform2) were newly identified from Macrobrachium nipponense in this study. MnALF6 has 134 amino acids and one single nucleotide polymorphism (SNP) in MnALF6 resulted in the change of 107th amino acid from E to D. Intron 1 retention produced longer transcript of MnALF6. The full length of MnALF7 has 691 bp with a 363 bp ORF encoding 120 amino acid protein. Three SNPs in MnALF2 resulted in the conversion of amino acids at positions 70, 73, and 91 from T⁷⁰I⁷³P⁹¹ to K⁷⁰L⁷³S⁹¹. The deletion of 13 bp in MnALF4 resulted in early termination of ORF, resulting in MnALF4-isoform2 with only 98 amino acids. The gDNAs of MnALF1, MnALF2, MnALF5, and MnALF6 contain three exons and two introns, while those of MnALF3 and MnALF7 contain three exons, one known intron, and one unknown intron. The MnALF1-7 in M. nipponense were widely distributed in multiple tissues. After white spot syndrome virus (WSSV) stimulation, the expression levels of MnALF1-7 changed. Knockdown of MnALF1-7 could evidently increase the expression of the envelope protein VP28 and the copy number of WSSV during viral infection. Further studies found that silencing of three transcription factors (Stat, Dorsal, and Relish) in M. nipponense significantly inhibit the synthesis of MnALF1-7 during the process of WSSV challenge. This study adds to the knowledge about the roles of ALFs in the innate immune responses to WSSV infection in M. nipponense.

Extracellular traps involved in invertebrate immune mechanisms

The invertebrate immune system possesses a mechanism named extracellular traps (ETs), it has been identified that this mechanism immobilizes and kills pathogens. ETs formation induces modification of histones, chromatin decondensation, and mixes with granule molecules, releasing them into the extracellular space as a defense mechanism. In the present review, we provide an overview on the identification of triggering stimuli such as pathogens, PAMPs, DAMPs, and chemical stimuli, discuss the participation of potential signaling pathways involving MAPK, PI3K, PKC, and ERK molecules that lead to NADPH oxidase or mitochondrial ROS production, and explore the potential relationship with several proteins such as myeloperoxidase, heat sock proteins, peroxinectin, elastase, and apolipoproteins. Furthermore, we also discuss the association of ETs with other immune mechanisms that could collaborate in the elimination of pathogens.

Identification and characterization of two highly homologous lysozymes from red swamp crayfish, Procambarus clarkii

Lysozyme is an important immune effector in innate immunity against pathogen infection. But the study on the active region of lysozyme is limited. In this study, two highly homologous lysozymes were identified from crayfish (designated as PcLysi4 and PcLysi5). The molecular structures of PcLysi4 and PcLysi5 were predicted by SWISS-MODEL with the structure of lysozyme (PDB accession No. 4PJ2.2.B) as model. The results suggested that the structure of PcLysi4 and PcLysi5 were highly similar, but there were more α-helices at positions (127–139) and longer β-sheet at positions (49–57) in the structure of PcLysi5 than in that of PcLysi4. The antibacterial and antiviral functions of the two lysozymes were investigated. PcLysi4 and PcLysi5 could promote the bacterial clearance ability of crayfish, and increase the survival rate of Vibrio-infected crayfish. Further study showed that PcLysi5 inhibited WSSV replication, and enhanced the survival rate of WSSV-infected crayfish. There was no evidence that PcLysi4 has an influence on WSSV replication. Furthermore, PcLysi5 was detected to interact with envelope protein VP24 of WSSV. Our results would provide a new reference for the study on active region of lysozyme.

Herbicide atrazine exposure induce oxidative stress, immune dysfunction and WSSV proliferation in red swamp crayfish Procambarus clarkii

Atrazine is considered as a potential environmental endocrine disruptors and exhibits various toxic effects on animals. It has a great impact in the aquatic ecosystems, but there are few studies on its immunotoxicity in crustaceans. In the present study, the Procambarus clarkii were utilized to assess the immune toxicity after 0.5 mg/L and 5 mg/L atrazine exposure. A significant decrease in total hemocytes count (THC) was observed at 5 mg/L atrazine exposure throughout the experiment. The activities of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were significantly inhibited, but the content of reactive oxygen species (ROS) and malondialdehyde (MDA) were up-regulated, indicating the potential oxidative stress. The analysis of the integrated biomarker response (IBR) showed the induction of oxidative stress biomarkers and the inhibition of antioxidants. After 5 mg/L atrazine exposure for 144 h, the integrity of crayfish hepatopancreas was destroyed with disappeared connections between tubules and increased liver tubules vacuoles. The relative expression levels of different immune genes in hepatopancreas after atrazine exposure were measured. Most of these genes were suppressed and exhibited a certain dose-dependent effect. The results of crayfish white spot syndrome virus (WSSV) replication shown the amount of virus in muscle was significantly higher and exhibited a higher mortality rate at 5 mg/L group than other groups. The present study determined the impact of atrazine exposure on WSSV outbreaks, and also provide an important basis for further assessing the occurrence of pesticides on diseases of P. clarkii.

A Lymphoid Organ Specific Anti-Lipopolysaccharide Factor from Litopenaeus vannamei Exhibits Strong Antimicrobial Activities

Different shrimp species are known to possess apparent distinct resistance to different pathogens in aquaculture. However, the molecular mechanism underlying this finding still remains unknown. One kind of important antimicrobial peptides, anti-lipopolysaccharide factors (ALF), exhibit broad-spectrum antimicrobial activities. Here, we reported a newly identified ALF from the shrimp Litopenaeus vannamei and compared the immune function with its counterpart in the shrimp Fenneropenaeus chinensis. The ALF, designated as LvALF8, was specifically expressed in the lymphoid organ of L. vannamei. The expression level of LvALF8 was apparently changed after white spot syndrome virus (WSSV) or Vibrio parahaemolyticus challenges. The synthetic LBD peptide of LvALF8 (LvALF8-LBD) showed strong antibacterial activities against most tested Gram-negative and Gram-positive bacteria. LvALF8-LBD could also inhibit the in vivo propagation of WSSV similar as FcALF8-LBD, the LBD of LvALF8 counterpart in F. chinensis. However, LvALF8-LBD and FcALF8-LBD exhibited apparently different antibacterial activity against V. parahaemolyticus, the main pathogen causing acute hepatopancreatic necrosis disease (AHPND) of affected shrimp. A structural analysis showed that the positive net charge and amphipathicity characteristics of LvALF8-LBD peptide were speculated as two important components for its enhanced antimicrobial activity compared to those of FcALF8-LBD. These new findings may not only provide some evidence to explain the distinct disease resistance among different shrimp species, but also lay out new research ground for the testing and development of LBD-originated antimicrobial peptides to control of shrimp diseases.

A trypsin-like serine protease domain of masquerade gene in crayfish Procambarus clarkii could activate prophenoloxidase and inhibit bacterial growth

Masquerade (Mas) is a secreted trypsin-like serine protease (SPs) and involved in immune response in some arthropods. However, according to previous studies, Mas presents different functional activities. In the present study, the functional mechanisms of Mas in crayfish Procambarus clarkii immune defense were studied. A fragment cDNA sequence of PcMas was identified and characterized. From the structural analysis, it contains a trypsin-like serine protease domain. The highest expression level of PcMas was detected in hepatopancreas. The infection of A. hydrophila could induce the expression of PcMas, while the WSSV infection did not cause changes in the expression of PcMas. Through the prokaryotic expression system, the PcMas protein was expressed in E. coli. It was verified that PcMas can bind to bacteria in vitro and inhibit the growth of the bacteria. By dsRNA interference with the expression of PcMas, the decrease expression of PcMas led to a decrease in the activity of phenoloxidase in hemolymph and an increase of mortality caused by A. hydrophila infection. The injection of recombinant protein can enhance the activity of phenoloxidase and reduce mortality caused by A. hydrophila infections. Therefore, the present study confirmed that PcMas could improve the body's immune response to eliminate bacterial pathogens by binding with bacteria and activating the prophenoloxidase system. The results will enrich the molecular mechanisms of crustaceans immune defense.

Molecular and Functional Characterization of an Anti-lipopolysaccharide Factor Mm-ALF from Speckled Shrimp Metapenaeus monoceros

Anti-lipopolysaccharide factors (ALFs) are antimicrobial peptides of approximately 100 amino acid residues with a broad spectrum of antimicrobial activity. It is an amphipathic peptide with an N-terminal hydrophobic region and a lipopolysaccharide binding domain (LBD). In the present study, we report an isoform of the anti-lipopolysaccharide factor (Mm-ALF) from the speckled shrimp, Metapenaeus monoceros. A 359 bp cDNA encoded 119 amino acids, and the sequence showed 99.16% similarity to ALF from the shrimp Fenneropenaeus indicus. The mature peptide of 94 amino acids has a net charge of +8, molecular weight 10.62 kDa, and pI 10.11. The mature peptide Mm-ALF was recombinantly expressed in E. coli Rosetta-gami cells, and the peptide was isolated and purified. The rMm-ALF exhibited notable antibacterial activity against Gram-positive (Staphylococcus aureus and Bacillus cereus) and Gram-negative (Escherichia coli, Edwardsiella tarda, Aeromonas hydrophila, Pseudomonas aeruginosa, Vibrio parahaemolyticus, Vibrio harveyi, Vibrio alginolyticus, Vibrio proteolyticus, Vibrio cholerae and Vibrio fluvialis) bacteria.

Anti-lipopolysaccharide Factor from Crucifix Crab Charybdis feriatus, Cf-ALF2: Molecular Cloning and Functional Characterization of the Recombinant Peptide

Antilipopolysaccharide factors (ALFs) are important effectors of innate immunity in crustaceans with broad spectrum antimicrobial activity. Present study deals with the molecular and functional characterization of a 98-amino acid ALF isoform from, crucifix crab, Charybdis feriatus termed as Cf-ALF2. The ALF isoform Cf-ALF2 exhibits characteristic features of an AMP including a cationic net charge of + 9 and a total hydrophobic ratio of 34%. Recombinant peptide rCf-ALF2 showed remarkable antimicrobial activity against Gram-negative and Gram-positive bacteria especially against Staphylococcus aureus (minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 5 µM) and Escherichia coli (MIC 10 µM and MBC 20 µM). Using scanning electron microscopy, bacterial membrane blebbing, disruption, and cell content leakage were observed in peptide treated E. coli. The recombinant peptide was found to be non-hemolytic and non-cytotoxic in NCI-H460 cell line at the highest tested concentration (20 µM). Thus, this study identified a novel isoform of ALF from C. feriatus and revealed the potent antimicrobial property of the recombinant peptide Cf-ALF2 and the future prospects of using the peptide for therapeutic applications in the future.

A novel cuticle protein involved in WSSV infection to the Pacific white shrimp Litopenaeus vannamei

As the most productive crustacean species in aquaculture, Litopenaeus vannamei is seriously threatened by white spot syndrome virus (WSSV), which has caused huge economic damage in the past decades. Shrimp cuticle proteins are the important components in the frontier target tissues, including cuticle and the chitinous lining of the digestive tract. In present study, a novel cuticle protein gene, named LvCPAP1, was isolated and demonstrated to play an important role in WSSV infection. The deduced amino acid sequence of LvCPAP1 contained a signal peptide and a conserved chitin-binding domain type 2 (ChBD2). Tissue distribution analysis revealed that LvCPAP1 was predominantly expressed in epidermis and stomach. The transcription levels of LvCPAP1 in epidermis and stomach were significantly regulated upon WSSV challenge. DsRNA silencing of LvCPAP1 decreased the in vivo WSSV copy numbers and the death rate of shrimp after WSSV infection, indicating that LvCPAP1 might facilitate WSSV invasion. In addition, the interaction between LvCPAP1 and the major envelop protein VP24 of WSSV was revealed by yeast two-hybrid system and further confirmed by dot blot and pull-down assays. The present study implied that cuticle protein LvCPAP1 might favor the entry process of WSSV, which provided new clues for understanding the role of cuticle proteins during virus infection.

The Anti-lipopolysaccharide Factors in Crustaceans

Anti-lipopolysaccharide factors (ALFs) are a type of antimicrobial peptide (AMP) which show broad-spectrum antimicrobial activity against Gram-positive bacteria, Gram-negative bacteria, fungi and viruses. In this chapter, we review the discovery and classification of this kind of antimicrobial peptide in crustaceans. The structure and function, as well as the mechanism of antibacterial and antiviral activities of ALFs will be summarized and discussed. We will then describe the expression and regulation of various ALF genes in different crustacean species. Finally, the application prospects of ALFs in drug development and disease-resistant genetic breeding will be pointed out and discussed. The review will also discuss several key questions such as the systematic classification and expression regulation of the ALF genes, as well as the future application of ALFs and ALF-derived peptides.

Molecular and antimicrobial characterization of a group G anti-lipopolysaccharide factor (ALF) from Penaeus monodon

Anti-lipopolysaccharide factors (ALFs) are important host-defense molecules of crustaceans. They all contain a lipopolysaccharide-binding domain (LBD) and some ALFs exhibit strong antimicrobial activity. In this research, a Group G ALF from Penaeus monodon (ALFPm11) was studied. It is an anionic peptide specifically having a cationic and highly amphipathic LBD, with five positively charged residues separated by aromatic residues. It was abundantly expressed in the hepatopancreas of P. monodon normally but the expression level in other tissues was relatively low or undetectable. However, in the shrimps challenged by Vibrio, expression of ALFPm11 could be detected in all tissues. Chemically synthesized ALFPm11-LBD displayed high inhibitory activity (minimum inhibition concentration≤ 4 μM) against various bacteria, e.g. Exiguobacterium sp. L33, Bacillus sp. T2, and Acinetobacter sp. L32. It also displayed apparent activity in the agar well diffusion assay. Furthermore, it could efficiently induce agglutination of both Gram-positive and Gram-negative bacteria and cause significant membrane permeabilization of the bacteria. As a comparative study, ALFPm11-LBD showed a better or equal antimicrobial function to ALFPm3-LBD which was reported to possess strong antimicrobial activity against Gram-positive, Gram-negative bacteria and fungi. Thus, this research found a new effective ALF in P. monodon and demonstrated its antimicrobial mechanism, suggesting its potential applications in the future.

A new antimicrobial peptide isoform, Pc-crustin 4 involved in antibacterial innate immune response in fresh water crayfish, Procambarus clarkii

The main advantage of antimicrobial peptides (AMPs) used as the effectors in the innate immunity system of invertebrates is that the high specificity is not indispensable. And they play important roles in the systemic defenses against microbial invasion. In this study, a new full-length cDNA of the crustins molecule was identified in red swamp crayfish, P. clarkii (named Pc-crustin 4). The ORF of Pc-crustin 4 contained 369 bp which encoded a protein of 122 amino acids, with a 20-amino-acid signal peptide sequence. On the base of the classification method established by Smith et al., Pc-crustin 4 belonged to Type Ⅰ crustin molecule. The Pc-crustin 4 transcripts were expressed in hemocytes at relatively high level, and relatively low level in hepatopancreas, gills, and intestine in normal crayfish. After respectively challenged with S. aureus or E. ictaluri, the expression levels of Pc-crustin 4 showed up-regulation trends at different degrees in the hemocytes, hepatopancreas, gills, and intestine tissues. Besides, the results of liquid antibacterial assay showed that rPc-crustin 4 inhibited obviously the growth of S. aureus and E. ictaluri. The results of bacteria binding assay showed that rPc-crustin 4 could bind strongly to S. aureus and E. ictaluri. Finally, RNAi assay was performed to study the immunity roles of Pc-crustin 4 in crayfish in vivo. Taken together, Pc-crustin 4 is an important immunity effector molecule, which plays crucial roles in defending against bacterial infection in crayfish.

Identification of Single Nucleotide Polymorphisms Related to the Resistance Against Acute Hepatopancreatic Necrosis Disease in the Pacific White Shrimp Litopenaeus vannamei by Target Sequencing Approach

Acute hepatopancreatic necrosis disease (AHPND) is a major bacterial disease in Pacific white shrimp Litopenaeus vannamei farming, which is caused by Vibrio parahaemolyticus. AHPND has led to a significant reduction of shrimp output since its outbreak. Selective breeding of disease-resistant broodstock is regarded as a key strategy in solving the disease problem. Understanding the relationship between genetic variance and AHPND resistance is the basis for marker-assisted selection in shrimp. The purpose of this study was to identify single nucleotide polymorphisms (SNPs) associated with the resistance against AHPND in L. vannamei. In this work, two independent populations were used for V. parahaemolyticus challenge and the resistant or susceptible shrimp were evaluated according to the survival time after Vibrio infection. The above two populations were genotyped separately by a SNP panel designed based on the target sequencing platform using a pooling strategy. The SNP panel contained 508 amplicons from DNA fragments distributed evenly along the genome and some immune-related genes of L. vannamei. By analyzing the allele frequency in the resistant and susceptible groups, 30 SNPs were found to be significantly associated with the resistance of the shrimp against V. parahaemolyticus infection (false discovery rate corrected at P < 0.05). Three SNPs were further validated by individual genotyping in all samples of population 1. Our study illustrated that target sequencing and pooling sequencing were effective in identifying the markers associated with economic traits, and the SNPs identified in this study could be used as molecular markers for breeding disease-resistant shrimp.

Characterization of a group D anti-lipopolysaccharide factor (ALF) involved in anti-Vibrio response in Penaeus monodon

Antimicrobial peptides (AMPs) are an essential component of innate immunity of invertebrates. Anti-lipopolysaccharide factor (ALF), as a main type of AMPs in crustaceans, attends in the disease prevention in general. In this research, a novel Group D ALF was identified and characterized from Penaeus monodon, named PenmonALF8. It was an anionic peptide, with both the full-length peptide and lipopolysaccharide binding domain (LBD) a low isoelectric point. PenmonALF8, composed of a signal peptide of 26 amino acids and a mature peptide of 98 amino acids, probably contained three alpha helixes and four beta sheets. Moreover, PenmonALF8 was detected in all tested tissues of P. monodon, and the expression level in hemocyte and intestine was relatively high. When challenged by Vibrio parahaemolyticus, PenmonALF8 showed 30-100 times higher expression level in all the tissues except in hemocyte and intestine, indicating that PenmonALF8 played a very important role in the immune response of P. monodon. By fusing to a SUMO protein, PenmonALF8 was successfully over-expressed in E. coli and purified by affinity chromatography. Additionally, the reconstituted PenmonALF8 and its LBD region displayed modest antimicrobial activity. This is the first research about the Group D ALF in P. monodon, which provides more information for humoral immunity study of shrimps.

The pathogenicity characterization of non-O1 Vibrio cholerae and its activation on immune system in freshwater shrimp Macrobrachium nipponense

Outbreaks of mass mortalities among cultured Macrobrachium nipponense occurred in a commercial hatchery during the autumn of 2017 in Jiangsu province, P. R. China, and non-O1 Vibrio cholerae was isolated and identified as causal agents of M. nipponense, with a LD₅₀ value 4.09 × 10⁴ CFU/mL. Detection of virulence-associated genes by PCR indicated that XL1 was positive for Mp, HlyA, RtxA, OmpU, Ace, Zot and T6SS. Furthermore, the results of extracellular enzyme analysis revealed that the strain can produce lecithinase, amylase, gelatinase and hemolysin. Histopathological analysis revealed that the hepatic tubule lumen and the gap between the hepatic tubules became larger, and the brush border disappeared in the hepatopancreas. Quantitive real-time PCR (qRT-PCR) was undertaken to measure mRNA expression levels for thirteen immune related genes in M. nipponense after non-O1 V. cholerae infection. The transcriptional analysis of these immune related genes demonstrated that the expression levels of dorsal, relish, p38, crustin1, crustin2, crustin3, hemocyanin, i-lysozyme, anti-lipopolysaccharide factors 1, anti-lipopolysaccharide factors 2, prophenoloxidase were significantly up-regulated in hemolymph of M. nipponense post-infection. These results revealed varying expression profiles and clear transcriptional activation of these immune related genes in hemolymph, which will contribute to better understand the pathogenesis and host defensive system in non-O1 V. cholerae invasion.

Development of novel antimicrobial peptides derived from anti-lipopolysaccharide factor of the swimming crab, Portunus trituberculatus

Anti-lipopolysaccharide factors (ALFs) are a representative host defense protein in crustaceans. In this study, we successfully developed two novel antimicrobial peptides (AMPs), named crab-ALF2A and crab-ALF6A, which contain changes to the amino acid sequences of the lipopolysaccharide binding domain and signal peptide, respectively, of the ALF of the swimming crab Portunus trituberculatus. The crab-ALF2A peptide showed potent antimicrobial activity against the Gram-positive bacteria Bacillus cereus, Staphylococcus aureus, and Streptococcus iniae (minimal effective concentration [MEC] 1.51-1.93 μg/mL) and the Gram-negative bacteria Pseudomonas aeruginosa and Escherichia coli (MEC 1.87-1.98 μg/mL), with maximal bactericidal activity at a peptide concentration of 5 μg/mL. The crab-ALF6A peptide also showed potent antimicrobial activity against B. cereus, S. aureus, and S. iniae (MEC 1.49-2.3 μg/mL) and P. aeruginosa and E. coli (MEC 1.72-1.19 μg/mL) at a peptide concentration of 5 μg/mL. Notably, the crab-ALF2A and crab-ALF6A peptides exhibited strong activity against Candida albicans (MECs of 2.11 and 1.95 μg/mL, respectively). These activities were stable following heat treatment. Moreover, the effect of crab-ALF2A and crab-ALF6A peptide treatment on microbe cell morphology was confirmed by scanning electron microscopy. Membrane disruption and damage, and the leakage of cytoplasmic content were clearly observed. A downsizing peptide approach illustrated that the hexapeptide ALF6A8 (RVLLRL) was the shortest peptide showing significant antimicrobial activity. Our approach allows for the generation of novel antimicrobial peptides in a cost effective manner as potential next-generation antibiotics.

Comparative transcriptome analysis of red swamp crayfish (Procambarus clarkia) hepatopancreas in response to WSSV and Aeromonas hydrophila infection

To better study the immune system of shrimp and understand the similarities and differences between the host's immune defense against viral and bacterial infections, this study used a comparative transcriptomics method to systematically analyze the hepatopancreas of the crayfish Procambarus clarkia in response to WSSV and A. hydrophila infection. After assembly, there was an average of 24,404,837 clean reads were obtained after filtering out low-quality reads. Unigenes were annotated by comparing against nr, Swiss-Prot\KEGG\COG\KOG\GO and Pfam databases, and 17,954 unigenes were annotated in at least one database. 2600 and 2073 differentially expressed genes (DEGs) in the hepatopancreas in response to WSSV and A. hydrophila infections were identified respectively. The GO and KEGG enrichment analyses of DEGs were conducted to further explore their functions. The pathways like PI3K-Akt signaling pathway, mTOR signaling pathway, Jak-STAT signaling pathway, NF-κB signaling pathway, VEGF signaling pathway, Ras signaling pathway, were the most prominent for immunity-related DEGs in C-/W-Groups, while Endocytosis, Lysozyme, Focal adhesion, Phagosome, Peroxisome, MAPK signaling pathway were observed in C-/A-Groups. Furthermore, the expression levels of nine selected immune-related DEGs were validated by qRT-PCR, substantiating the reliability of RNA-Seq results. This study not only provides effective data support to reveal the different immune defense strategies by P. clarkia to cope with bacterial and WSSV infections, but also to provide new information about the immune system and defense mechanisms of shrimp.

RNAi screening identifies a new Toll from shrimp Litopenaeus vannamei that restricts WSSV infection through activating Dorsal to induce antimicrobial peptides

The function of Toll pathway defense against bacterial infection has been well established in shrimp, however how this pathway responds to viral infection is still largely unknown. In this study, we report the Toll4-Dorsal-AMPs cascade restricts the white spot syndrome virus (WSSV) infection of shrimp. A total of nine Tolls from Litopenaeus vannamei namely Toll1-9 are identified, and RNAi screening in vivo reveals the Toll4 is important for shrimp to oppose WSSV infection. Knockdown of Toll4 results in elevated viral loads and renders shrimp more susceptible to WSSV. Furthermore, Toll4 could be a one of upstream pattern recognition receptor (PRR) to detect WSSV, and thereby leading to nuclear translocation and phosphorylation of Dorsal, the known NF-κB transcription factor of the canonical Toll pathway. More importantly, silencing of Toll4 and Dorsal contributes to impaired expression of a specific set of antimicrobial peptides (AMPs) such as anti-LPS-factor (ALF) and lysozyme (LYZ) family, which exert potent anti-WSSV activity. Two AMPs of ALF1 and LYZ1 as representatives are demonstrated to have the ability to interact with several WSSV structural proteins to inhibit viral infection. Taken together, we therefore identify that the Toll4-Dorsal pathway mediates strong resistance to WSSV infection by inducing some specific AMPs.

The TLR pathway mediated antiviral immune response is well identified in mammals, yet, Toll pathway governing this protection in invertebrates remains unknown. In the present study, we uncover that a shrimp Toll4 from a total of nine Tolls in L. vannamei confers resistance to WSSV thought inducing the NF-κB transcription factor Dorsal to inspire the production of some antimicrobial peptides (AMPs) with antiviral activity. The anti-LPS-factor (ALF) and lysozyme (LYZ) family are identified as the Toll4-Dorsal pathway targeted genes with the ability to interact with viral structural proteins in response to WSSV infection. These results suggest that the Toll receptor induces the expression of AMPs with antiviral activity could be a general antiviral mechanism in invertebrates and Toll pathway established antiviral defense could be conserved during evolution.

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.

Shrimp humoral responses against pathogens: antimicrobial peptides and melanization

Diseases have caused tremendous economic losses and become the major problem threatening the sustainable development of shrimp aquaculture. The knowledge of host defense mechanisms against invading pathogens is essential for the implementation of efficient strategies to prevent disease outbreaks. Like other invertebrates, shrimp rely on the innate immune system to defend themselves against a range of microbes by recognizing and destroying them through cellular and humoral immune responses. Detection of microbial pathogens triggers the signal transduction pathways including the NF-κB signaling, Toll and Imd pathways, resulting in the activation of genes involved in host defense responses. In this review, we update the discovery of components of the Toll and Imd pathways in shrimp and their participation in the regulation of shrimp antimicrobial peptide (AMP) synthesis. We also focus on a recent progress on the two most powerful and the best-studied shrimp humoral responses: AMPs and melanization. Shrimp AMPs are mainly cationic peptides with sequence diversity which endues them the broad range of activities against microorganisms. Melanization, regulated by the prophenoloxidase activating cascade, also plays a crucial role in killing and sequestration of invading pathogens. The progress and emerging research on mechanisms and functional characterization of components of these two indispensable humoral responses in shrimp immunity are summarized and discussed. Interestingly, the pattern recognition protein (PRP) crosstalk is evidenced between the proPO activating cascade and the AMP synthesis pathways in shrimp, which enables the innate immune system to build up efficient immune responses.

Recent Advances in Antibacterial and Antiendotoxic Peptides or Proteins from Marine Resources

Infectious diseases caused by Gram-negative bacteria and sepsis induced by lipopolysaccharide (LPS) pose a major threat to humans and animals and cause millions of deaths each year. Marine organisms are a valuable resource library of bioactive products with huge medicinal potential. Among them, antibacterial and antiendotoxic peptides or proteins, which are composed of metabolically tolerable residues, are present in many marine species, including marine vertebrates, invertebrates and microorganisms. A lot of studies have reported that these marine peptides and proteins or their derivatives exhibit potent antibacterial activity and antiendotoxic activity in vitro and in vivo. However, their categories, heterologous expression in microorganisms, physicochemical factors affecting peptide or protein interactions with bacterial LPS and LPS-neutralizing mechanism are not well known. In this review, we highlight the characteristics and anti-infective activity of bifunctional peptides or proteins from marine resources as well as the challenges and strategies for further study.

Identification of the first cathelicidin gene from skin of Chinese giant salamanders Andrias davidianus with its potent antimicrobial activity

Cathelicidins, as effector molecules, play important roles against infections and represent a crucial component of the innate immune system in vertebrates. They are widely studied in mammals, but little is known in amphibians. In the present study, we report the identification and characterization of a novel cathelicidin from Chinese giant salamander Andrias davidianus, which is the first study in Caudata amphibian. The cDNA sequence encodes a predicted 148-amino-acid polypeptide, which composed of a 20-residue signal peptide, a 94-residue conserved cathelin domain and a 34-residue mature peptide. From the multiple sequence alignments and phylogenetic analysis, AdCath shared conserved structure with other orthologs and clustered with other amphibian peptides. The tissue expression profiles revealed AdCath was highly expressed in skin. To study the function of AdCath gene, the AdCath precursor protein and mature peptide were recombinantly expressed and chemical synthesized respectively. The rAdCath protein could bind to LPS in a dose-dependent manner. When the concentrations of rAdCath protein and mature peptide were up to 22 μg/mL, they showed significantly cytotoxicity to human 293T cell lines. The rAdCath protein and synthetic peptide could exhibit antibacterial activities detected by the minimum inhibitory concentrations assay. From the SEM assay, the synthetic mature peptide could destroy the membrane of bacteria and cause loss of membrane integrity. Collectively, these findings characterized the first cathelicidin from A. davidianus, and highlighted its potential antimicrobial activities, indicating its important roles in the skin immune response against different bacteria.

Analysis on the expression and function of a chicken-type and goose-type lysozymes in Chinese giant salamanders Andrias davidianus

Lysozymes as an important immune factor, play vital roles in innate immune response against pathogen infection. In the present study, one c-type and g-type lysozymes were identified from Chinese giant salamander (Andrias davidianus). They shared highly conserved structural features with lysozymes from other species. Spatial expression analysis revealed that AdlysC transcript was most abundant in liver and stomach, and least in muscle and brain. In contrast, the expression level of AdlysG was most abundant in liver and least in muscle and skin. The transcription level of c-type and g-type lysozymes were up-regulated after Aeromonas hydrophila infection in liver and spleen, indicating their participations in the immune response. Moreover, the recombinant AdlysC and AdlysG protein were produced and purified, and were used to investigate the lysozyme activity at different pH and temperatures. The optimal lytic activity was determined at pH 6.0 and at a temperature of 30 °C. Through the minimal inhibitory concentration test, the rAdlysC and rAdlysG exhibited apparent antibacterial activity against both Gram-positive and Gram-negative bacteria with a variable concentration. In conclusion, it is the first report of lysozymes in A. davidianus, and c-type and g-type lysozymes should be involved in the innate immune response of A. davidianus.

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.

Identification and function analysis of an anti-lipopolysaccharide factor from the ridgetail prawn Exopalaemon carinicauda

Anti-lipopolysaccharide factor is a kind of antimicrobial peptide (AMP) with broad-spectrum activities against bacteria, virus, and fungi in crustacean. Different isoforms of ALFs showed different activities to virus or bacteria. Therefore, discovery of more ALFs will provide new insights into drug development and disease control. Here we reported an ALF gene, EcALF1, isolated from the ridgetail prawn Exopalaemon carinicauda. Its transcripts was mainly detected in hemocytes. Silencing of EcALF1 caused a lesion of hepatopancreas and finally led to death of the prawn. In vivo bacteria detection to the hepatopancreas showed that the colonies of thiosulphate citrate bile salts (TCBS) cultured bacteria in EcALF1-silenced prawn increased dramatically. We identified the dominant bacteria cultured in TCBS and found that Vibrio alginolyticus and Vibrio parahaemolyticus were dominant bacteria which were pathogenic species to prawn. We further synthesized the functional domain, LPS binding domain (LBD) of EcALF1, and its antibacterial and antiviral activities were detected. The synthetic EcLBD1 peptide showed very strong antibacterial activity against Micrococcus luteus and V. alginolyticus. Pre-incubation of WSSV with EcLBD1 peptide reduced the pathogenicity of WSSV infection to the prawn. The present data showed the important roles of an ALF gene in modulating the in vivo bacterial proliferation in E. carinicauda and provided new insight in disease control in aquaculture.

Characterization of galectin-1 from Chinese giant salamanders Andrias davidianus and its involvements during immune response

Galectins are considered as a multifunctional protein which play essential roles in cell adhesion and apoptosis, inflammation, tumor progression and immune response. In spite of extensive studies of galectin importance in immune system among different animals, few studies have been devoted to their functions in amphibian. In the present study, we characterized one proto type of galectin (named AdGal1) from Chinese giant salamander Andrias davidianus and studied its function in immune response. AdGal1 cDNA possesses an open reading frame of 598 bp, which encodes a putative galectin of 134 amino acids containing one carbohydrate recognition domains (CRDs). The constitutive expression of mRNA transcripts was detected in a wide range of tissues, with the highest expression in kidney. Immune challenges with Aeromonas hydrophila and Chinese giant salamander iridovirus (GSIV), the transcript level of AdGal1 in kidney was significantly upregulated. The mature protein of AdGal1 was successfully expressed and purified in Escherichia coli BL21 (DE3). The recombinant AdGal1 (rAdGal1) could show bind activity to different Gram negative and Gram positive bacteria. It could also strongly agglutinate different kinds of bacteria at different concentrations. Collectively, these data from the present study indicate that AdGal1 is a vital pattern recognition receptor to recognize different microbes in the innate immune system of Andrias davidianus.

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.

Recombinant Expression of a Modified Shrimp Anti-Lipopolysaccharide Factor Gene in Pichia pastoris GS115 and Its Characteristic Analysis

Anti-lipopolysaccharide factors (ALFs) with a LPS-binding domain (LBD) are considered to have broad spectrum antimicrobial activities and certain antiviral properties in crustaceans. FcALF2 was one isoform of ALFs isolated from the Chinese shrimp Fenneropenaeus chinensis. Our previous study showed that a modified LBD domain (named LBDv) of FcALF2 exhibited a highly enhanced antimicrobial activity. In the present study, a modified FcALF2 gene (mFcALF2), in which the LBD was substituted by LBDv, was designed and synthesized. This gene was successfully expressed in yeast Pichia pastoris GS115 eukaryotic expression system, and the characteristics of the recombinant protein mFcALF2 were analyzed. mFcALF2 exhibited apparent antibacterial activities against Gram-negative bacteria, including Escherichia coli, Vibrio alginolyticus, Vibrio harveyi, and Vibrio parahaemolyticus, and Gram-positive bacteria, including Bacillus licheniformis and Staphylococcus epidermidis. In addition, mFcALF2 could reduce the propagation of white spot syndrome virus (WSSV) in vivo by pre-incubation with virus. The present study paves the way for developing antimicrobial drugs in aquaculture.

Structure and Bioactivity of a Modified Peptide Derived from the LPS-Binding Domain of an Anti-Lipopolysaccharide Factor (ALF) of Shrimp

The lipopolysaccharide binding domain (LBD) in anti-lipopolysaccharide factor (ALF) is the main functional element of ALF, which exhibits antimicrobial activities. Our previous studies show that the peptide LBDv, synthesized based on the modified sequence of LBD (named LBD2) from FcALF2, exhibited an apparently enhanced antimicrobial activity. To learn the prospect of LBDv application, the characteristics of LBDv were analyzed in the present study. The LBDv peptide showed higher antimicrobial and bactericidal activities compared with LBD2. These activities of the LBDv peptide were stable after heat treatment. LBDv could also exhibit in vivo antimicrobial activity to Vibrio harveyi. The LBDv peptide was found to bind bacteria, quickly cause bacterial agglutination, and kill bacteria by damaging their membrane integrity. Structure analysis showed that both LBDv and LBD2 held the β-sheet structure, and the positive net charge and amphipathicity characteristic were speculated as two important components for their antimicrobial activity. The cytotoxicity of LBDv was evaluated in cultured Spodoptera frugiperda (Sf9) cells and Cherax quadricarinatus hemocytes. More than 80% cells could survive with the LBDv concentration up to 16 μM. Collectively, these findings highlighted the potential antimicrobial mechanism of LBD peptides, and provided important information for the commercial use of LBDv in the future.