Sequence diversity and evolution of antimicrobial peptides in invertebrates

The mucosal immunity in crustaceans: inferences from other species

Crustaceans such as shrimps and crabs, hold significant ecological significance and substantial economic value within marine ecosystems. However, their susceptibility to disease outbreaks and pathogenic infections has posed major challenges to production in recent decades. As invertebrate, crustaceans primarily rely on their innate immune system for defense, lacking the adaptive immune system found in vertebrates. Mucosal immunity, acting as the frontline defense against a myriad of pathogenic microorganisms, is a crucial aspect of their immune repertoire. This review synthesizes insights from comparative immunology, highlighting parallels between mucosal immunity in vertebrates and innate immune mechanisms in invertebrates. Despite lacking classical adaptive immunity, invertebrates, including crustaceans, exhibit immune memory and rely on inherent "innate immunity factors" to combat invading pathogens. Drawing on parallels from mammalian and piscine systems, this paper meticulously explores the complex role of mucosal immunity in regulating immune responses in crustaceans. Through the extrapolation from well-studied models like mammals and fish, this review infers the potential mechanisms of mucosal immunity in crustaceans and provides insights for research on mucosal immunity in crustaceans.

Molecular characterization, expression and antibacterial function of a macin, HdMac, from Haliotis discus hannai

Macins are a family of antimicrobial peptides, which play multiple roles in the elimination of invading pathogens. In the present study, a macin was cloned and characterized from Pacific abalone Haliotis discus hannai (Designated as HdMac). Analysis of the conserved domain suggested that HdMac was a new member of the macin family. In non-stimulated abalones, HdMac transcripts were constitutively expressed in all five tested tissues, especially in hemocytes. After Vibrio harveyi stimulation, the expression of HdMac mRNA in hemocytes was significantly up-regulated at 12 hr (P < 0.01). RNAi-mediated knockdown of HdMac transcripts affected the survival rates of abalone against V. harveyi. Moreover, recombinant protein of HdMac (rHdMac) exhibited high antibacterial activities against invading bacteria, especially for Vibrio anguillarum. In addition, rHdMac possessed binding activities towards glucan, lipopolysaccharides (LPS), and peptidoglycan (PGN), but not chitin in vitro. Membrane integrity analysis revealed that rHdMac could increase the membrane permeability of bacteria. Meanwhile, both the phagocytosis and chemotaxis ability of hemocytes could be significantly enhanced by rHdMac. Overall, the results showed that HdMac could function as a versatile molecule involved in immune responses of H. discus hannai.

A novel family of defensin-like peptides from Hermetia illucens with antibacterial properties

BACKGROUND

The world faces a major infectious disease challenge. Interest in the discovery, design, or development of antimicrobial peptides (AMPs) as an alternative approach for the treatment of bacterial infections has increased. Insects are a good source of AMPs which are the main effector molecules of their innate immune system. Black Soldier Fly Larvae (BSFL) are being developed for large-scale rearing for food sustainability, waste reduction and as sustainable animal and fish feed. Bioinformatic studies have suggested that BSFL have the largest number of AMPs identified in insects. However, most AMPs identified in BSF have not yet undergone antimicrobial evaluation but are promising leads to treat critical infections.

RESULTS

Jg7197.t1, Jg7902.t1 and Jg7904.t1 were expressed into the haemolymph of larvae following infection with Salmonella enterica serovar Typhimurium and were predicted to be AMPs using the computational tool ampir. The genes encoding these proteins were within 2 distinct clusters in chromosome 1 of the BSF genome. Following removal of signal peptides, predicted structures of the mature proteins were superimposed, highlighting a high degree of structural conservation. The 3 AMPs share primary sequences with proteins that contain a Kunitz-binding domain; characterised for inhibitory action against proteases, and antimicrobial activities. An in vitro antimicrobial screen indicated that heterologously expressed SUMO-Jg7197.t1 and SUMO-Jg7902.t1 did not show activity against 12 bacterial strains. While recombinant SUMO-Jg7904.t1 had antimicrobial activity against a range of Gram-negative and Gram-positive bacteria, including the serious pathogen Pseudomonas aeruginosa.

CONCLUSIONS

We have cloned and purified putative AMPs from BSFL and performed initial in vitro experiments to evaluate their antimicrobial activity. In doing so, we have identified a putative novel defensin-like AMP, Jg7904.t1, encoded in a paralogous gene cluster, with antimicrobial activity against P. aeruginosa.

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.

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

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

DEVELOPMENT OF A HEMOLYMPH COLLECTION TECHNIQUE AND REPORT OF BIOCHEMICAL PARAMETERS OF HEMOLYMPH IN A MANAGED POPULATION OF FEMALE THORNY DEVIL STICK INSECTS (EURYCANTHA CALCARATA)

Insects are increasingly common in households, zoological collections, research, and food industries. Increased knowledge of insect health parameters is necessary to ensure proper welfare. The study goal was to develop a hemolymph collection technique and report hemolymph serum biochemical parameters for the thorny devil stick insect (Eurycantha calcarata). Clinically healthy adult stick insects (5 males and 14 females, 15.1-24.7 g) were enrolled. Four collection techniques were evaluated. Hemolymph collection was unsuccessful in males, but was successful in females by using a single technique. The insect was manually restrained in an elevated position and an 18-ga × 2.54-cm needle was used to puncture the membrane just caudal to the third pair of legs. With the puncture site directed ventrally, ≤1 ml of hemolymph was collected via gravity and allowed to visibly clot. The sample was then centrifuged, and the serum was separated. Serum samples were individually analyzed (Avian/Reptilian Profile Plus, VetScan VS2, Abaxis, Inc, Union City, CA 94587, USA). Fourteen samples (0.2-1.0 ml) were collected from 14 females. Median (minimum-maximum) parameters included the following: aspartate aminotransferase 12 (0-45) U/L, creatinine kinase 25 (0-76) U/L, uric acid 7.5 (3.1-13.7) mg/dl, glucose 12 (8-22) mg/dl, calcium 18.6 (17.2-19.4) mg/dl, phosphorus 15.0 (n = 1) or >30.0 (n = 13) mg/dL, total protein 2.7 (1.6-2.9) g/dL, albumin 0.9 (0.2-1.2) g/dL, globulin 1.7 (1.6-1.8) g/dL (n = 6) or not quantified (n = 8), potassium 10.6 (9.0-11.8) mmol/L, sodium < 100 mmol/L, and bile acids 0 lmol/L. This is the first report of biochemistry parameters in clinically healthy female stick insects. Larger sample sizes are needed to establish statistically valid reference ranges. Hemolymph collection techniques for male stick insects warrant further investigation.

Invertebrate Immunity, Natural Transplantation Immunity, Somatic and Germ Cell Parasitism, and Transposon Defense

While the vertebrate immune system consists of innate and adaptive branches, invertebrates only have innate immunity. This feature makes them an ideal model system for studying the cellular and molecular mechanisms of innate immunity sensu stricto without reciprocal interferences from adaptive immunity. Although invertebrate immunity is evolutionarily older and a precursor of vertebrate immunity, it is far from simple. Despite lacking lymphocytes and functional immunoglobulin, the invertebrate immune system has many sophisticated mechanisms and features, such as long-term immune memory, which, for decades, have been exclusively attributed to adaptive immunity. In this review, we describe the cellular and molecular aspects of invertebrate immunity, including the epigenetic foundation of innate memory, the transgenerational inheritance of immunity, genetic immunity against invading transposons, the mechanisms of self-recognition, natural transplantation, and germ/somatic cell parasitism.

Marine Invertebrate Antimicrobial Peptides and Their Potential as Novel Peptide Antibiotics

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

Where are the Penaeids crustins?

Crustins are antimicrobial peptides and members of the four-disulfide core (4-DSC) domain-containing proteins superfamily. To date, crustins have only been reported in crustaceans and possess a structural signature characterized by a single 4-DSC domain and one cysteine-rich region. The high-throughput sequencing technologies have produced vastly valuable genomic information that sometimes dilutes information about previously sequenced molecules. This study aimed (1) to corroborate the loss of valuable descriptive information regarding crustin identification when high throughput sequencing carries out automatic annotation processes and (2) to detect possible crustin sequences reported in Penaeids to attempt a list considering structural similarities, which allows the establishment of phylogenetic relationships based on molecular characteristics. All crustins sequences reported in Penaeids and registered in the databases were obtained. The first list was made with the proteins reported as crustin or carcinin, excluding those that did not meet the structural characteristics. Subsequently, using local alignments, sequences were sought with high similarity even if they had been reported with a different name of crustin but with a probability of being crustin. This broader list, including proteins with high structural similarity, can help establish phylogenetic relationships of shrimp genes and the evolutionary trajectory of this antimicrobial distributed exclusively among crustaceans. Results revealed that in most sequences obtained by Sanger or transcriptomics, which met the structural criteria, the identification was correctly established as crustin. Contrarily, the sequences corresponding to crustins obtained by whole genome sequencing projects were incorrectly classified or not characterized, being momentarily "buried" in the information generated. In addition, the sequences that complied with the criteria of crustin tended to be grouped into species separated by geographical regions; for example, the crustins of the inhabitant shrimp of the American coasts differ from those corresponding to the natives of the Asian coasts. Finally, the results suggest the convenience of annotations considering the previous but correct information, even if such information was generated with previous technologies.

Novel integrated computational AMP discovery approaches highlight diversity in the helminth AMP repertoire

Antimicrobial Peptides (AMPs) are immune effectors that are key components of the invertebrate innate immune system providing protection against pathogenic microbes. Parasitic helminths (phylum Nematoda and phylum Platyhelminthes) share complex interactions with their hosts and closely associated microbiota that are likely regulated by a diverse portfolio of antimicrobial immune effectors including AMPs. Knowledge of helminth AMPs has largely been derived from nematodes, whereas the flatworm AMP repertoire has not been described. This study highlights limitations in the homology-based approaches, used to identify putative nematode AMPs, for the characterisation of flatworm AMPs, and reveals that innovative algorithmic AMP prediction approaches provide an alternative strategy for novel helminth AMP discovery. The data presented here: (i) reveal that flatworms do not encode traditional lophotrochozoan AMP groups (Big Defensin, CSαβ peptides and Myticalin); (ii) describe a unique integrated computational pipeline for the discovery of novel helminth AMPs; (iii) reveal >16,000 putative AMP-like peptides across 127 helminth species; (iv) highlight that cysteine-rich peptides dominate helminth AMP-like peptide profiles; (v) uncover eight novel helminth AMP-like peptides with diverse antibacterial activities, and (vi) demonstrate the detection of AMP-like peptides from Ascaris suum biofluid. These data represent a significant advance in our understanding of the putative helminth AMP repertoire and underscore a potential untapped source of antimicrobial diversity which may provide opportunities for the discovery of novel antimicrobials. Further, unravelling the role of endogenous worm-derived antimicrobials and their potential to influence host-worm-microbiome interactions may be exploited for the development of unique helminth control approaches.

Characterization of a crustin-like peptide involved in shrimp immune response to bacteria and Enterocytozoon hepatopenaei (EHP) infection in Palaemon carinicauda

Crustins represent one type of antimicrobial peptides (AMPs) that are key components of the innate immune process of crustaceans. This study successfully identified a novel crustin-like peptide, EcCrustin2, in ridgetail white prawn, Palaemon carinicauda (formerly Exopalaemon carinicauda). EcCrustin2 was found to be 1082 bp in length, with a 378 bp open reading frame (ORF) encoding 125 amino acids. The deduced amino acid sequence of EcCrustin2 exhibited characteristics of crustins in crustacean, including a Cys-rich region at the N-terminus as well as a whey acidic protein domain at the C-terminus. Phylogenetic analysis revealed that the EcCrustin2 was first clustered with Type I crustins, then with other crustins. Expression of EcCrustin2 was mainly detected in immune tissues, including hemocytes, gill and stomach. The expression level of EcCrustin2 was also significantly up-regulated after being exposed to lipopolysaccharide (LPS), lipoteichoic acid (LTA), Vibrio parahaemolyticus and Staphylococcus aureus. EHP infection could also induce EcCrustin2 expression in P. carinicauda. Knockdown of EcCrustin2 with siRNA increased the mortality of V. parahaemolyticus challenged shrimp. Finally, the recombinant EcCrustin2 protein was obtained and demonstrated a wide spectrum of antibacterial activity in vitro. These results indicated that EcCrustin2 takes part in the immune response against bacteria and EHP infection.

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.

Signaling events induced by lipopolysaccharide-activated Toll in response to bacterial infection in shrimp

Toll-like receptors (TLR) play a crucial role in the detection of microbial infections in vertebrates and invertebrates. Mammalian TLRs directly recognize a variety of structurally conserved microbial components. However, invertebrates such as Drosophila indirectly recognize microbial products by binding to the cytokine-like ligand Spätzle, which activates signaling cascades that are not completely understood. In this study, we investigated the signaling events triggered by Toll in response to lipopolysaccharide (LPS), a cell wall component of gram-negative bacteria, and Vibrio parahaemolyticus infection in the arthropod shrimp Litopenaeus vannamei. We found that five of the nine Tolls from L. vannamei bound to LPS and the RNAi of LvToll1, LvToll2, LvToll3, LvToll5, and LvToll9 weakened LvDorsal-L phosphorylation induced by V. parahaemolyticus. All nine Tolls combined with MyD88 via the TIR domain, thereby conferring signals to the tumor necrosis factor receptor-associated factor 6 (TRAF6)-transforming growth factor-β activated kinase 1 binding protein 2 (TAB2)-transforming growth factor-β activated kinase 1 (TAK1) complex. Further examination revealed that the LvTRAF6-LvTAB2-LvTAK1 complex contributes to Dorsal-L phosphorylation and nuclear translocation during V. parahaemolyticus infection. Overall, shrimp Toll1/2/3/5/9-TRAF6/TAB2/TAK1-Dorsal cascades protect the host from V. parahaemolyticus infection, which provides a better understanding of how the innate immune system recognizes and responds to bacterial infections in invertebrates.

Non-targeted metabolomics and microbial analyses of the impact of oat antimicrobial peptides on rats with dextran sulfate sodium-induced enteritis

To study the prevention and mechanism of oat antimicrobial peptides (AMPs) on enteritis. Oat protein was hydrolyzed by alkaline protease and isolated to obtain oat antimicrobial peptides. Rat enteritis models were constructed using dextran sodium sulfate (DSS), and a blank group, a negative control group, a positive control group, and an experimental group (low dose, medium dose, and high dose) were established. Through pathological test, antioxidant test, intestinal microbial and metabolite determination, it was found that AMPS can improve the antioxidant capacity of colon, reduce the production of inflammatory cells, and have the effect of preventing enteritis. In addition, the AMPS group is able to change and reduce the abundance of Bacteroides-eggerthii-DSM-20697 and Desulfovibrionaceae, increase the abundance of probiotics such as roboutsia and Ruminococcus and optimize the diversity of intestinal microorganisms. Then, the combined analysis of microorganism and metabolites showed that Romboutsia and Ruminococcus reduced the contents of amino acid and glucose and promoted the production of phospholipid, while Bacteroides promoted the synthesis of amino acid in the body. From the above, it can be seen that DSS causes damage to the mechanical barrier of the gut. Oat antimicrobial peptides provide a microbial barrier for the gut microbes, which produce acetic acid and succinic acid with small amounts of isobutyric acid, isovaleric acid, and lactic acid. The acidic metabolites produced reduce the pH of the gut and produce substances with antibacterial effects (such as lipophilic molecules, antibiotics, and hydroperoxides). Inhibit the growth and reproduction of other harmful bacteria, Vibrio desulphuris, from adhering to and colonizing the intestinal mucosa. Secreted short-chain fatty acids, such as acetate and butyric acid, maintain tight connections between the epithelial cells of the intestinal mucosa, thus protecting the mechanical barrier of the intestinal mucosa. Moreover, amino acids are converted into phospholipid metabolism through protein digestion and absorption to promote the production of phospholipid in the intestine and repair damaged cell membranes.

Antibacterial activity and immunomodulatory role of a proline-rich antimicrobial peptide SpPR-AMP1 against Vibrio campbellii infection in shrimp Litopenaeus vannamei

Antimicrobial peptides (AMPs) constitute one of the most promising sources of natural molecules used for the design of effective antimicrobial agents alternative to antibiotics. Previously, we have showed that a crab proline-rich AMP designated as SpPR-AMP1 is a potent AMP that exhibited antimicrobial activity against both Gram-positive and Gram-negative bacteria. Here, we demonstrated the importance of SpPR-AMP1 peptide in treating a virulent acute hepatopancreatic necrosis disease (AHPND) Vibrio campbellii VH-639 isolate and eliciting the innate immune response to counter the AHPND infection in shrimp Litopenaeus vannamei. SpPR-AMP1 exhibited a strong antimicrobial activity against V. campbellii VH-639 at MIC value of 0.195-0.39 μM. Scanning electron microscopy (SEM) revealed the membrane disruption potential of SpPR-AMP1 against the V. campbellii VH-639 cells. The in vivo effect of SpPR-AMP1 in shrimp L.vannamei was investigated and the results showed that SpPR-AMP1 was capable of modulating the innate immune response by stimulating the expression levels of AMP transcripts in shrimp hemocytes. Moreover, treatments with SpPR-AMP1 could promote the resistance of shrimp against V. campbellii VH-639 infection as demonstrated by a significant increase in shrimp survival rate and decrease in both the bacterial load and the expression levels of bacterial PirA and PirB toxin gene transcripts in the infected shrimp. These results suggest the potential of SpPR-AMP1 peptide with the combined antimicrobial and immunoenhancing capabilities as promising antimicrobial agent to treat V. campbellii VH-639 causing AHPND infection in shrimp aquaculture.

Characterisation of a novel crustin isoform from mud crab, Scylla serrata (Forsskål, 1775) and its functional analysis in silico

A 336-base pair (bp) sized mRNA sequence encoding 111 amino acid size crustin isoform (MC-crustin) was obtained from the gill sample of the green mud crab, Scylla serrata. MC-crustin possessed an N-terminal signal peptide region comprising of 21 amino acid residues, followed by a 90 amino acid mature peptide region having a molecular weight of 10.164 kDa, charge + 4.25 and theoretical pI of 8.27. Sequence alignment and phylogenetic tree analyses revealed the peptide to be a Type I crustin, with four conserved cysteine residues forming the cysteine rich region, followed by WAP domain. MC-crustin was cationic with cysteine/proline rich structure and was predicted with antimicrobial, anti-inflammatory, anti-angiogenic and anti-hypertensive property making it a potential molecule for possible therapeutic applications.

An Overview of the Potentialities of Antimicrobial Peptides Derived from Natural Sources

Antimicrobial peptides (AMPs) are constituents of the innate immune system in every kind of living organism. They can act by disrupting the microbial membrane or without affecting membrane stability. Interest in these small peptides stems from the fear of antibiotics and the emergence of microorganisms resistant to antibiotics. Through membrane or metabolic disruption, they defend an organism against invading bacteria, viruses, protozoa, and fungi. High efficacy and specificity, low drug interaction and toxicity, thermostability, solubility in water, and biological diversity suggest their applications in food, medicine, agriculture, animal husbandry, and aquaculture. Nanocarriers can be used to protect, deliver, and improve their bioavailability effectiveness. High cost of production could limit their use. This review summarizes the natural sources, structures, modes of action, and applications of microbial peptides in the food and pharmaceutical industries. Any restrictions on AMPs' large-scale production are also taken into consideration.

Identification and functional characterization of C-type lectins and crustins provide new insights into the immune response of Portunus trituberculatus

A meticulous understanding of the immune characteristics of aquaculture animals is the basis for developing precise disease prevention and control strategies. In this study, four novel C-type lectins (PtCTL-5, PtCTL-6, PtCTL-7 and PtCTL-8) including a single carbohydrate-recognition domain (CRD), and four novel crustins (Ptcrustin-1, Ptcrustin-2, Ptcrustin-3 and Ptcrustin-4) with a single whey acidic protein (WAP) domain were identified from the swimming crab Portunus trituberculatus. Tissue distribution analysis indicated that most of the target genes were predominantly expressed in the hepatopancreas in all examined tissues, except for Ptcrustin-1 which were mainly expressed in the gills. Our results showed that the eight genes displayed various transcriptional profiles across different tissues. In hemocytes, the PtCTL-7 responded quickly to Vibrio alginolyticus and exhibited much more strongly up-regulation than other three PtCTLs. The Ptcrustin-1 rapidly responded to V. alginolyticus within 3 h in all the three tested tissues. Furthermore, recombinant proteins of PtCTL-5 and PtCTL-8 were successfully obtained, and both of them displayed bacterial binding activities toward V. alginolyticus, V. harveyi and Staphylococcus aureus, and only showed antibacterial activity against V. harveyi. These findings provided new insights into the diverse immune response of P. trituberculatus and laid theoretical foundations for the development of precise disease prevention and control strategies in P. trituberculatus farming. Moreover, the specific anti-V. harveyi activities exhibited by rPtCTL-5 and rPtCTL-8 suggested their promising application prospects for controlling diseases caused by V. harveyi.

Host-pathogen coevolution drives innate immune response to Aphanomyces astaci infection in freshwater crayfish: transcriptomic evidence

BACKGROUND

For over a century, scientists have studied host-pathogen interactions between the crayfish plague disease agent Aphanomyces astaci and freshwater crayfish. It has been hypothesised that North American crayfish hosts are disease-resistant due to the long-lasting coevolution with the pathogen. Similarly, the increasing number of latent infections reported in the historically sensitive European crayfish hosts seems to indicate that similar coevolutionary processes are occurring between European crayfish and A. astaci. Our current understanding of these host-pathogen interactions is largely focused on the innate immunity processes in the crayfish haemolymph and cuticle, but the molecular basis of the observed disease-resistance and susceptibility remain unclear. To understand how coevolution is shaping the host's molecular response to the pathogen, susceptible native European noble crayfish and invasive disease-resistant marbled crayfish were challenged with two A. astaci strains of different origin: a haplogroup A strain (introduced to Europe at least 50 years ago, low virulence) and a haplogroup B strain (signal crayfish in lake Tahoe, USA, high virulence). Here, we compare the gene expression profiles of the hepatopancreas, an integrated organ of crayfish immunity and metabolism.

RESULTS

We characterised several novel innate immune-related gene groups in both crayfish species. Across all challenge groups, we detected 412 differentially expressed genes (DEGs) in the noble crayfish, and 257 DEGs in the marbled crayfish. In the noble crayfish, a clear immune response was detected to the haplogroup B strain, but not to the haplogroup A strain. In contrast, in the marbled crayfish we detected an immune response to the haplogroup A strain, but not to the haplogroup B strain.

CONCLUSIONS

We highlight the hepatopancreas as an important hub for the synthesis of immune molecules in the response to A. astaci. A clear distinction between the innate immune response in the marbled crayfish and the noble crayfish is the capability of the marbled crayfish to mobilise a higher variety of innate immune response effectors. With this study we outline that the type and strength of the host immune response to the pathogen is strongly influenced by the coevolutionary history of the crayfish with specific A. astaci strains.

Marine Arthropods as a Source of Antimicrobial Peptides

Peptide therapeutics play a key role in the development of new medical treatments. The traditional focus on endogenous peptides has shifted from first discovering other natural sources of these molecules, to later synthesizing those with unique bioactivities. This review provides concise information concerning antimicrobial peptides derived from marine crustaceans for the development of new therapeutics. Marine arthropods do not have an adaptive immune system, and therefore, they depend on the innate immune system to eliminate pathogens. In this context, antimicrobial peptides (AMPs) with unique characteristics are a pivotal part of the defense systems of these organisms. This review covers topics such as the diversity and distribution of peptides in marine arthropods (crustacea and chelicerata), with a focus on penaeid shrimps. The following aspects are covered: the defense system; classes of AMPs; molecular characteristics of AMPs; AMP synthesis; the role of penaeidins, anti-lipopolysaccharide factors, crustins, and stylicins against microorganisms; and the use of AMPs as therapeutic drugs. This review seeks to provide a useful compilation of the most recent information regarding AMPs from marine crustaceans, and describes the future potential applications of these molecules.

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

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

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.

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

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

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.

Discovery and Characterization of a New Crustin Antimicrobial Peptide from Amphibalanus amphitrite

Crustins are an antimicrobial peptide (AMP) family that plays an important role in innate immunity in crustaceans. It is important to discover new AMPs from natural sources to expand the current database. Here, we identified and characterized a new crustin family member, named AaCrus1, from Amphibalanus amphitrite. AaCrus1 shares high identity (48.10%) with PvCrus, a Type I crustin of Penaeus vannamei that possesses a whey acidic protein (WAP) domain. AaCrus1 contains 237 amino acids and eight cysteine residues forming conserved ‘four-disulfide core’ structure. Our recombinant AaCrus1 (rAaCrus 1) could inhibit the growth of two Gram-positive bacteria (Staphylococcus aureus, Bacillus sp. T2) and four Gram-negative bacteria (Vibrio parahaemolyticus, Vibrio harveyi, Vibrio anguillarum, Vibrio alginolyticus) with a minimum inhibitory concentration of 3.5–28 μM. It can further induce agglutination of both Gram-positive and Gram-negative bacteria. rAaCrus1 can bind to bacteria and damage bacterial cell membranes. Furthermore, rAaCrus1 disrupted biofilm development of S. aureus and V. parahaemolyticus. Our discovery and characterization of this new crustin can be further optimized as a good alternative to antibiotics.

Identification, antibacterial activities and action mode of two macins from manila clam Venerupis philippinarum

In this study, two macins were identified from clam Venerupis philippinarum (designated as VpMacin-1 and VpMacin-2). They showed 64.71% similarity with each other. The highest mRNA expression of VpMacin-1 and VpMacin-2 was detected in gills and hepatopancreas, respectively, in non-stimulated clams, and their expression could be induced significantly in hemocytes after Vibrio anguillarum infection. Silencing of VpMacin-1 and VpMacin-2 led to 22% and 49% mortality 6 days post infection. Escherichia coli cells were killed by recombinant protein rVpMacin-1 and rVpMacin-2 within 1000 and 400 min, respectively, at a concentration of 1.0 × MIC. Compared with rVpMacin-1, rVpMacin-2 not only showed higher broad-spectrum antimicrobial activities towards Vibrio strains, but possessed stronger abilities to inhibit the formation of bacterial biofilm. Both membrane integrity and electrochemical assay indicated that rVpMacins were capable of causing bacterial membrane permeabilization, especially for rVpMacin-2. Besides, rVpMacin-1 significantly induced both phagocytic (0.1 and 1.0 × MIC, p < 0.05) and chemotactic effects (0.1 × MIC, p < 0.01) of hemocytes, while there was no significant increase for rVpMacin-2. Overall, our results suggested that VpMacin-1 and VpMacin-2 play important roles in host defense against invasive pathogens.

Antimicrobial Peptides: An Update on Classifications and Databases

Antimicrobial peptides (AMPs) are distributed across all kingdoms of life and are an indispensable component of host defenses. They consist of predominantly short cationic peptides with a wide variety of structures and targets. Given the ever-emerging resistance of various pathogens to existing antimicrobial therapies, AMPs have recently attracted extensive interest as potential therapeutic agents. As the discovery of new AMPs has increased, many databases specializing in AMPs have been developed to collect both fundamental and pharmacological information. In this review, we summarize the sources, structures, modes of action, and classifications of AMPs. Additionally, we examine current AMP databases, compare valuable computational tools used to predict antimicrobial activity and mechanisms of action, and highlight new machine learning approaches that can be employed to improve AMP activity to combat global antimicrobial resistance.

Characteristics of Two Crustins from Alvinocaris longirostris in Hydrothermal Vents

Crustins are widely distributed among different crustacean groups. They are characterized by a whey acidic protein (WAP) domain, and most examined Crustins show activity against Gram-positive bacteria. This study reports two Crustins, Al-crus 3 and Al-crus 7, from hydrothermal vent shrimp, Alvinocaris longirostris. Al-crus 3 and Al-crus 7 belong to Crustin Type IIa, with a similarity of about 51% at amino acid level. Antibacterial assays showed that Al-crus 3 mainly displayed activity against Gram-positive bacteria with MIC₅₀ values of 10–25 μM. However, Al-crus 7 not only displayed activity against Gram-positive bacteria but also against Gram-negative bacteria Imipenem-resistant Acinetobacter baumannii, in a sensitive manner. Notably, in the effective antibacterial spectrum, Methicillin-sensitive Staphylococcus aureus, Escherichia coli (ESBLs) and Imipenem-resistant A. baumannii were drug-resistant pathogens. Narrowing down the sequence to the WAP domain, Al-crusWAP 3 and Al-crusWAP 7 demonstrated antibacterial activities but were weak. Additionally, the effects on bacteria did not significantly change after they were maintained at room temperature for 48 h. This indicated that Al-crus 3 and Al-crus 7 were relatively stable and convenient for transportation. Altogether, this study reported two new Crustins with specific characteristics. In particular, Al-crus 7 inhibited Gram-negative imipenem-resistant A. baumannii.

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

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

Taxonomic bias in AMP prediction of invertebrate peptides

Invertebrate antimicrobial peptides (AMPs) are at the forefront in the search for agents of therapeutic utility against multi-resistant microbial pathogens, and in recent years substantial advances took place in the in silico prediction of antimicrobial function of amino acid sequences. A yet neglected aspect is taxonomic bias in the performance of these tools. Owing to differences in the prediction algorithms and used training data sets between tools, and phylogenetic differences in sequence diversity, physicochemical properties and evolved biological functions of AMPs between taxa, notable discrepancies may exist in performance between the currently available prediction tools. Here we tested if there is taxonomic bias in the prediction power in 10 tools with a total of 20 prediction algorithms in 19 invertebrate taxa, using a data set containing 1525 AMP and 3050 non-AMP sequences. We found that most of the tools exhibited considerable variation in performance between tested invertebrate groups. Based on the per-taxa performances and on the variation in performances across taxa we provide guidance in choosing the best-performing prediction tool for all assessed taxa, by listing the highest scoring tool for each of them.

The American lobster genome reveals insights on longevity, neural, and immune adaptations

The American lobster, Homarus americanus, is integral to marine ecosystems and supports an important commercial fishery. This iconic species also serves as a valuable model for deciphering neural networks controlling rhythmic motor patterns and olfaction. Here, we report a high-quality draft assembly of the H. americanus genome with 25,284 predicted gene models. Analysis of the neural gene complement revealed extraordinary development of the chemosensory machinery, including a profound diversification of ligand-gated ion channels and secretory molecules. The discovery of a novel class of chimeric receptors coupling pattern recognition and neurotransmitter binding suggests a deep integration between the neural and immune systems. A robust repertoire of genes involved in innate immunity, genome stability, cell survival, chemical defense, and cuticle formation represents a diversity of defense mechanisms essential to thrive in the benthic marine environment. Together, these unique evolutionary adaptations contribute to the longevity and ecological success of this long-lived benthic predator.

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.

Antimicrobial Peptides and Copper(II) Ions: Novel Therapeutic Opportunities

The emergence of new pathogens and multidrug resistant bacteria is an important public health issue that requires the development of novel classes of antibiotics. Antimicrobial peptides (AMPs) are a promising platform with great potential for the identification of new lead compounds that can combat the aforementioned pathogens due to their broad-spectrum antimicrobial activity and relatively low rate of resistance emergence. AMPs of multicellular organisms made their debut four decades ago thanks to ingenious researchers who asked simple questions about the resistance to bacterial infections of insects. Questions such as "Do fruit flies ever get sick?", combined with pioneering studies, have led to an understanding of AMPs as universal weapons of the immune system. This review focuses on a subclass of AMPs that feature a metal binding motif known as the amino terminal copper and nickel (ATCUN) motif. One of the metal-based strategies of hosts facing a pathogen, it includes wielding the inherent toxicity of copper and deliberately trafficking this metal ion into sites of infection. The sudden increase in the concentration of copper ions in the presence of ATCUN-containing AMPs (ATCUN-AMPs) likely results in a synergistic interaction. Herein, we examine common structural features in ATCUN-AMPs that exist across species, and we highlight unique features that deserve additional attention. We also present the current state of knowledge about the molecular mechanisms behind their antimicrobial activity and the methods available to study this promising class of AMPs.

Reptilian β-defensins: Expanding the repertoire of known crocodylian peptides

One of the major families of host defense peptides (HDPs) in vertebrates are β-defensins. They constitute important components of innate immunity and have remained an interesting topic of research for more than two decades. While many β-defensin sequences in mammals and birds have been identified and their properties and functions characterized, β-defensin peptides from other groups of vertebrates, particularly reptiles, are still largely unexplored. In this review, we focus on reptilian β-defensins and summarize different aspects of their biology, such as their genomic organization, evolution, structure, and biological activities. Reptilian β-defensin genes exhibit similar genomic organization to birds and their number and gene structure are variable among different species. During the evolution of reptiles, several gene duplication and deletion events have occurred and the functional diversification of β-defensins has been mainly driven by positive selection. These peptides display broad antimicrobial activity in vitro, but a deeper understanding of their mechanisms of action in vivo, including their role as immunomodulators, is still lacking. Reptilian β-defensins constitute unique polypeptide sequences to expand our current understanding of innate immunity in these animals and elucidate core biological functions of this family of HDPs across amniotes.

Molecular characterization and evaluation of the antibacterial activity of a plant defensin peptide derived from a gene of oat (Avena sativa L.)

Plant defensins are a group of small disulfide-rich cationic peptides that exhibit a broad spectrum of antimicrobial activities. In the present study, an antibacterial plant defensin peptide was successfully identified and characterized from the transcriptome of the oat (Avena sativa L.), and called AsDef1. The complete nucleotide sequence of AsDef1 was determined (321 bp) and found to contain an open reading frame (ORF) encoding a peptide of 77 aa with a putative 22 aa signal peptide sequence that addresses the mature defensin to the apoplast. Further in silico analyses revealed that the structure of the identified defensin (AsDef1) consists of the Knot1 functional domain with eight conserved cysteine residues and four disulfide bonds. The highest expression of AsDef1 was observed in the developing seeds of the A. sativa plant. AsDef1 also showed antibacterial activity against both Gram-positive and Gram-negative bacteria. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values ranged from 0.15625 μM to 0.625 μM. In this study, we identified and characterized an antibacterial defensin from A. sativa for the first time. The findings of the present study offer insights that can be used in producing pathogen-resistant transgenic plants and in developing potential antibacterial agents in the future using AsDef1 from A. sativa.

Cysteine-Rich Peptides: Hyperstable Scaffolds for Protein Engineering

Cysteine-rich peptides (CRPs) are small proteins of less than 100 amino acids in length characterized by the presence of disulfide bridges and common end-to-end macrocyclization. These properties confer hyperstability against high temperatures, salt concentration, serum presence, and protease degradation to CRPs. Moreover, their intercysteine domains (loops) are susceptible to residue hypervariability. CRPs have been successfully applied as stable scaffolds for molecular grafting, a protein engineering process in which cysteine-rich structures provide higher thermodynamic and metabolic stability to an epitope and acquire new biological function(s). This review describes the successes and limitations of seven cysteine-rich scaffolds, their bioactive epitopes, and the resulting grafted peptides.

The molecular characterization, expression analysis and antimicrobial activity of theromacin from Asian polychaeta (Perinereis linea)

Antimicrobial peptides (AMPs) are molecular factors in innate immunity and are believed to play a key role in invertebrate host defence. We identified theromacin (TM) from an Asian polychaeta, Perinereis linea, using de novo RNA-seq analysis. TM, a typical AMP of invertebrates, is a cysteine-rich AMP with five disulfide bonds consisting of ten cysteine residues. In gene expression analysis, TM genes were constantly upregulated after lipopolysaccharide (LPS) stimulation. In contrast, after peptidoglycan (PGN) stimulation, it was upregulated initially and downregulated after 12 h. We synthesized a peptide based on the macin AMP in the TM amino acid sequence. The synthetic peptide showed antibacterial activity against some Gram-positive and Gram-negative bacteria. Therefore, the AMPs of P. linea might have broad roles in host defence and exhibit different degrees of activity.

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

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

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 involvement of ecdysone and ecdysone receptor in regulating the expression of antimicrobial peptides in Chinese mitten crab, Eriocheir sinensis

The ecdysone, 20-hydroxyecdysone (20E) and ecdysone receptor (EcR), are regarded as the key regulators of development, metamorphosis, and growth in arthropods. In the present study, the role of 20E and EsEcR in regulating the expression of antimicrobial peptides (AMPs) was investigated in Chinese mitten crab, Eriocheir sinensis. The concentration of 20E in plasma was significantly (p < 0.05) up-regulated from 3 h to 12 h after lipopolysaccharide (LPS) stimulation. The mRNA expression level of EsEcR-4 in hemocytes was significantly (p < 0.01) up-regulated from 6 h to 24 h after LPS stimulation, while no significant changes of EsEcR-2 and EsEcR-3 transcripts were observed. After 20E injection, EsEcR-4 expression level was significantly increased from 12 h to 48 h with the highest level at 24 h (4.34-fold compared to the control group, p < 0.01), and the mRNA expression levels of AMPs (EsALF-2, EsLYZ and EsCrus) in hemocytes were significantly increased from 6 h to 24 h with the peak level of 2.93-fold (p < 0.01), 2.33-fold (p < 0.01) and 2.75-fold (p < 0.01) at 12 h, respectively. After EsEcR-4 expression was interfered with specific dsRNA, a significant reduction of EsALF-2 (0.56-fold compared to the control group, p < 0.01), EsLYZ (0.27-fold, p < 0.01) and EsCrus (0.41-fold, p < 0.01) mRNA expression level was observed in dsEsEcR-4+LPS group at 12 h post LPS stimulation. Moreover, the mRNA expression levels of EsDorsal and EsJNK in hemocytes were significantly (p < 0.05) increased from 6 h to 24 h post 20E injection, and the phosphorylation of Dorsal and JNK in the hemocytes were significantly (p < 0.01) up-regulated at 3 h post 20E injection, while that in dsEsEcR-4+LPS group were significantly decreased after LPS stimulation compared to dsEsEGFP+LPS group. Taken together, these results suggested that 20E and EsEcR-4 play important roles in regulating the expression level of AMPs in the immune responses of E. sinensis by regulating the mRNA expression level and phosphorylation of Dorsal and JNK.

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

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

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

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

Defensins: Transcriptional regulation and function beyond antimicrobial activity

Defensins are one the largest group of antimicrobial peptides and are part of the innate defence. Defensins are produced by animals, plants and fungi. In animals and plants, defensins can be constitutively or differentially expressed both locally or systemically which confer defence before and a stronger response after infection. Immune signalling pathways regulate the gene expression of defensins. These pathways include cellular receptors, which recognise pathogen-associated molecular patterns and are found both in plants and animals. After recognition, signalling pathways and, subsequently, transcriptional factors are activated. There is an increasing number of novel functions in defensins, such as immunomodulators and immune cell attractors. Identification of defensin triggers could help us to elucidate other new functions. The present article reviews the different elicitors of defensins with a main focus on human, fish and marine invertebrate defensins.

Research progress in innate immunity of freshwater crustaceans

Invertebrates lack adaptive immunity and innate immunity plays important roles in combating foreign invasive pathogens. Freshwater crustaceans, which are invertebrates, depend completely on their innate immune system. In recent years, many immune-related molecules in freshwater crustaceans, as well as their functions, have been identified. Three main immune signaling pathways, namely, Toll, immune deficiency (IMD), and Janus kinase-signal transducer activator of transcription (JAK/STAT) pathways, were found in freshwater crustaceans. A series of pattern recognition receptors (PRRs), including Toll receptors, lectins, lipopolysaccharide and β-1,3-glucan binding protein, scavenger receptors, Down syndrome cell adhesion molecules, and thioester-containing proteins, were reported. Prophenoloxidase activation system and antimicrobial peptide synthesis are two important immune effector systems. These components are involved in the innate immunity of freshwater crustaceans, and they function in the innate immune defense against invading pathogens. This review mainly summarizes innate immune signaling pathways, PRRs, and effector molecules in freshwater crustaceans.

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

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

Shedding Light on the Antimicrobial Peptide Arsenal of Terrestrial Isopods: Focus on Armadillidins, a New Crustacean AMP Family

In crustaceans, antimicrobial peptides (AMPs) are clustered into four major groups according to their amino acid composition and structure: (1) single-domain peptides containing cysteine residues such as anti-lipopolysaccharide-factor (ALF), (2) multi-domain or chimeric AMPs such as crustins, (3) non-conventional AMPs, and (4) linear single-domain AMPs. The majority of AMPs has been described in commercially exploited crustaceans, particularly decapods living in aquatic environments (crab, shrimp, lobster, and crayfish). Here, we aimed at establishing the AMPs repertoire of terrestrial isopods (Oniscidea), an original suborder of crustaceans adapted to life outside of the aquatic environment. Using transcriptomic data from 21 species, we identified 110 ALF and 73 crustin sequences. We also characterized the full-length sequence of armadillidins from 17 species, similar to the AMP previously described in the terrestrial isopod Armadillidium vulgare. Furthermore, we tested the antimicrobial activity of three armadillidin peptides characterized from three distantly related species. This analysis revealed similar activity spectra against pathogens, despite extensive structural variation among the tested peptides. In addition to conventional crustacean AMPs, our work highlights armadillidins as a new and independent family of AMPs specific to the Oniscidea, thus opening new perspectives concerning the study of the immune system of terrestrial isopods.

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.

ADAPTABLE: a comprehensive web platform of antimicrobial peptides tailored to the user's research

Antimicrobial peptides (AMPs) are part of the innate immune response to pathogens in all of the kingdoms of life. They have received significant attention because of their extraordinary variety of activities, in particular, as candidate drugs against the threat of super-bacteria. A systematic study of the relation between the sequence and the mechanism of action is urgently needed, given the thousands of sequences already in multiple web resources. ADAPTABLE web platform (http://gec.u-picardie.fr/adaptable) introduces the concept of "property alignment" to create families of property and sequence-related peptides (SR families). This feature provides the researcher with a tool to select those AMPs meaningful to their research from among more than 40,000 nonredundant sequences. Selectable properties include the target organism and experimental activity concentration, allowing selection of peptides with multiple simultaneous actions. This is made possible by ADAPTABLE because it not only merges sequences of AMP databases but also merges their data, thereby standardizing values and handling non-proteinogenic amino acids. In this unified platform, SR families allow the creation of peptide scaffolds based on common traits in peptides with similar activity, independently of their source.