A novel big defensin identified in horseshoe crab hemocytes: isolation, amino acid sequence, and antibacterial activity

Biochemical characterization and antibacterial activity of lipopolysaccharide binding proteins of the jellyfish Aurelia aurita and Rhopilema asamushi

Proteins capable of binding lipopolysaccharides (LPSs) of Gram-negative bacteria were isolated from the jellyfish of the Sea of Japan. LPS-binding proteins (ILBP - invertebrate lipopolysaccharide-binding proteins) were found in lysates of the jellyfish mesogloea by immunofluorescence assay with fluorescein-labeled LPS (F-LPS). Cation exchange chromatography was used isolate and purify ILBPs from jellyfish lysate. Fractions of jellyfish lysate after cation exchange chromatography were shown to contain high-molecular-weight ILBPs. By ligand enzyme solid phase assay it was shown that isolated ILBP of jellyfishes was bound to LPS directly by concentration-dependent manner in saturation process. The Scatchard analysis plot of the binding data gave a biphasic curve, suggesting that there were two types of independent binding sites in ILBP. Both isolated R-LPS and free lipid A inhibited binding of LBP with B LPS. The O-specific side chain of LPS was not involved in the interaction with ILBP. This suggested that the binding proteins recognized the core oligosaccharide and the lipid A portion of LPS. The disaggregation of various forms of LPS upon interaction with ILBPs from two species of jellyfish was also studied by dynamic light scattering (DLS). The distribution pattern and particle size (34 nm and 88.8 nm) of R-LPS in complex with ILBP from R. asamushi indicate disaggregation of R-LPS. The negative charge of Rd-LPS (-42.2 mV) in the LPS-ILBP-Ropilema complex is neutralized to -4.4 mV. It was shown that the ILBP was involved in the defense mechanism of the jellyfish by agglutinating the Gram-negative bacteria Escherichia coli and Yersinia pseudotuberculosis. We studied the effects of ILBP concentration on the permeability of bacterial walls. Responses to ILBP varied across the studied Gram-negative bacteria. Y. pseudotuberculosis was more tolerant to the effects of ILBP compared to E. coli K-12. We attributed the observed distinctions to the different structure of LPSs. E. coli K-12 contains LPS with short carbohydrate chains, and lipid A is more easily available for binding to ILBP. These invertebrate species are abundant and are of interest as new sources of LPS binding proteins for potential antimicrobial agents. Proteins modulating the biological activity of endotoxin are important not only because they provide insights into the mechanisms of host defense against Gram-negative bacteria but also because they may suggest new therapeutic approaches to control the deleterious effects of endotoxins.

A role in intracellular K+ in protecting pathogenic dimorphic fungi against induced cell death by bioinspired antimicrobial peptides

Antimicrobial peptides (AMPs) are promising drugs, though their fungal combat mechanisms remain partly unclear. We designed three AMPs (dAMPs) based on the γ-core of the Vu-Def1 seed defensin from Vigna unguiculata L. Walp. named RR, D-RR, and WR, and assessed their actions on Candida tropicalis and Candida albicans. Amidst their actions are cell shrinkage caused by K+ efflux from fungal cells. K+ involvement in fungal death by these peptides was explored. We assessed cell shrinkage, oxidative stress, mitochondria hyperpolarization, membrane permeabilization, medium acidification, antimicrobial activity under hypoosmotic conditions, and cellular degradation. Viability assays were performed with channel blockers and K+ addition at various times. The interactions of dAMPs with salts and fungal cells were analyzed using circular dichroism and microscopy. K+ and Cl- channels were not directly involved in dAMPs-induced death. Supplementation with K+ protected fungal cells from death. In tests, cations often deactivated them through charge neutralization. Peptides maintained their conformation with K+ and were found in cell cytoplasm indicating K+ did not neutralize charges. K+ did not prevent oxidative stress, but protected from cell shrinkage and mitochondria hyperpolarization. dAMPs rapidly stimulated medium acidification, followed by inhibition after 1 min, and K+ prevented acidification. Membrane permeabilization occurred after 20 min, faster with WR, explaining lack of protection from blockers. Fungal death was accelerated under hypoosmotic conditions. Electrophoresis revealed protein degradation, while ultrastructural analysis of the cells showed vacuolization, indicative of cytoplasmic degradation. Thus, K+ prevented cell death by maintaining internal levels, averting activation of cell degradation process.

Aquatic Invertebrate Antimicrobial Peptides in the Fight Against Aquaculture Pathogens

The intensification of aquaculture has escalated disease outbreaks and overuse of antibiotics, driving the global antimicrobial resistance (AMR) crisis. Antimicrobial peptides (AMPs) provide a promising alternative due to their rapid, broad-spectrum activity, low AMR risk, and additional bioactivities, including immunomodulatory, anticancer, and antifouling properties. AMPs derived from aquatic invertebrates, particularly marine-derived, are well-suited for aquaculture, offering enhanced stability in high-salinity environments. This study compiles and analyzes data from AMP databases and over 200 scientific sources, identifying approximately 350 AMPs derived from aquatic invertebrates, mostly cationic and α-helical, across 65 protein families. While in vitro assays highlight their potential, limited in vivo studies hinder practical application. These AMPs could serve as feed additives, therapeutic agents, or in genetic engineering approaches like CRISPR/Cas9-mediated transgenesis to enhance resilience of farmed species. Despite challenges such as stability, ecological impacts, and regulatory hurdles, advancements in peptidomimetics and genetic engineering hold significant promise. Future research should emphasize refining AMP enhancement techniques, expanding their diversity and bioactivity profiles, and prioritizing comprehensive in vivo evaluations. Harnessing the potential of AMPs represents a significant step forward on the path to aquaculture sustainability, reducing antibiotic dependency, and combating AMR, ultimately safeguarding public health and ecosystem resilience.

Antifungal peptides from living organisms

In the post-COVID-19 era, people are increasingly concerned about microbial infections, including fungal infections that have risen in recent years. However, the currently available antifungal agents are rather limited. Worse still, the widespread use of the antifungal agents has caused the emergence of antifungal resistance in Candida, Cryptococcus, and Aspergillus species. Therefore, the development of novel antifungals is urgently needed. Antimicrobial peptides (AMPs), as components of the first-line defense of the host, are found to exhibit broad antimicrobial activity against bacteria, fungi, parasites, viruses, and protozoa. AMPs with antifungal activity are specifically referred to as antifungal peptides (AFPs). AFPs are currently regarded as the most promising alternative to conventional antifungal agents due to the fact that they are highly selective and less prone to facilitate the selection of drug resistance. In this review, we present an overview of the origin and classification of natural AFPs as well as their modes of action. Additionally, the production of natural, semisynthetic, and synthetic AFPs with a view to greater levels of exploitation is discussed. Finally, we evaluate the current and potential applications of AFPs in clinics and in the food industry.

Anti-fungal peptides: an emerging category with enthralling therapeutic prospects in the treatment of candidiasis

Candida infections, particularly invasive candidiasis, pose a serious global health threat. Candida albicans is the most prevalent species causing candidiasis, and resistance to key antifungal drugs, such as azoles, echinocandins, polyenes, and fluoropyrimidines, has emerged. This growing multidrug resistance (MDR) complicates treatment options, highlighting the need for novel therapeutic approaches. Antifungal peptides (AFPs) are gaining recognition for their potential as new antifungal agents due to their diverse structures and functions. These natural or recombinant peptides can effectively target fungal virulence and viability, making them promising candidates for future antifungal development. This review examines infections caused by Candida species, the limitations of current antifungal treatments, and the therapeutic potential of AFPs. It emphasizes the importance of identifying novel AFP targets and their production for advancing treatment strategies. By discussing the therapeutic development of AFPs, the review aims to draw researchers' attention to this promising field. The integration of knowledge about AFPs could pave the way for novel antifungal agents with broad-spectrum activity, reduced toxicity, targeted action, and mechanisms that limit resistance in pathogenic fungi, offering significant advancements in antifungal therapeutics.

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.

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.

Applications of antimicrobial peptides (AMPs) as an alternative to antibiotic use in aquaculture – A mini-review

The use of antibiotics for the control of infections has not only been banned by FDA for use in food-producing animals, but also several countries have prohibited their use in aquaculture because of several reasons such as the occurrence of antibiotic-tolerant microorganisms, accumulation of antibiotic residues in fish and shrimp flesh, and aquatic environmental effluence concerns. These issues have led researchers and aquaculture scientists to conduct several studies to find antibiotic alternatives. Numerous substitutes have been evaluated, such as probiotics, synbiotics, prebiotics, postbiotics, phytogenics, essential oils, and several others. Results show that these supplements demonstrate proven efficacy in enhancing immune responses, reducing mortalities resulting from experimental infections, and reducing antibiotic usage in medicated aquafeed. Nonetheless, using antimicrobial peptides (AMPs) to control fish diseases and as antibiotic alternatives is a promising and interesting research topic. AMPs are a vital class of small peptides that could stimulate the innate immune system against challenging pathogens and also possess significant potent defensive responses against a variety of infectious and noninfectious pathogenic agents, including bacteria, parasites, fungi, and viruses. Regarding their source origin, AMPs can be classified into six main types: mammalian-, amphibian-, insect-, aquatic-, plant-, and microorganism-derived AMPs. On account of their unique structure, they can display an essential function in therapeutic strategies against infectious diseases affecting fish and shrimp. Reports showed several kinds of AMPs had a wide spectrum of antimicrobial properties. These effects are besides their prominent immunostimulatory functions. Thus, they may be considered a functional alternative to antibiotics in aquaculture. This article provides information on the current knowledge about the modes of action, sources, classification, functions, and potential applications for the development of aquatic animal health. The information included in this context will be valuable to enhance the sustainability of aquaculture.

The Role of Antimicrobial Peptides (AMPs) in Aquaculture Farming

 Antimicrobial peptides (AMPs) are the vital constituents that stimulate the innate immune defense system against pathogens and perform several biological activities, which provide the first defensive line against infectious diseases. Owing to their unique structure, they can be utilized as a therapeutic strategy for infectious diseases in fishes. Several kinds of AMPs are reported in fishes with broad-spectrum antimicrobial properties. Besides, the bacterial cells cannot develop resistance strains against these cationic compounds with low molecular weight. Thus, AMPs may be considered an alternative to antibiotics to prevent or control infectious diseases in aquaculture. It is essential to provide sufficient knowledge about the mode of action of AMPs against fish pathogenic agents and their future applications. 

Biomolecules of the Horseshoe Crab’s Hemolymph: Components of an Ancient Defensive Mechanism and Its Impact on the Pharmaceutical and Biomedical Industry

Without adaptive immunity, invertebrates have evolved innate immune systems that react to antigens on the surfaces of pathogens. These defense mechanisms are included in horseshoe crab hemocytes’ cellular responses to pathogens. Secretory granules, large (L) and small (S), are found on hemocytes. Once the invasion of pathogens is present, these granules release their contents through exocytosis. Recent data in biochemistry and immunology on the granular constituents of granule-specific proteins are stored in large and small granules which are involved in the cell-mediated immune response. L-granules contain most clotting proteins, which are necessary for hemolymph coagulation. They also include tachylectins; protease inhibitors, such as cystatin and serpins; and anti-lipopolysaccharide (LPS) factors, which bind to LPS and agglutinate bacteria. Big defensin, tachycitin, tachystatin, and tachyplesins are some of the essential cysteine-rich proteins in S-granules. These granules also contain tachycitin and tachystatins, which can agglutinate bacteria. These proteins in granules and hemolymph act synergistically to fight infections. These biomolecules are antimicrobial and antibacterial, enabling them to be drug resistant. This review is aimed at explaining the biomolecules identified in the horseshoe crab’s hemolymph and their application scopes in the pharmaceutical and biotechnology sectors.

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.

New insights into the hemolymph coagulation cascade of horseshoe crabs initiated by autocatalytic activation of a lipopolysaccharide-sensitive zymogen

The concept of a chain reaction of proteolytic activation of multiple protease zymogens was first proposed to explain the blood clotting system in mammals as an enzyme cascade. In multicellular organisms, similar enzyme cascades are widely present in signal transduction and amplification systems. The initiation step of the blood coagulation cascade often consists of autocatalytic activation of the corresponding zymogens located on the surfaces of host- or foreign-derived substances at injured sites. However, the molecular mechanism underlying the concept of autocatalytic activation remains speculative. In this review, we will focus on the autocatalytic activation of prochelicerase C on the surface of lipopolysaccharide as a potential initiator of hemolymph coagulation in horseshoe crabs. Prochelicerase C is presumed to have evolved from a common complement factor in Chelicerata; thus, evolutionary insights into the hemolymph coagulation and complement systems in horseshoe crabs will also be discussed.

In silico analysis of the C-terminal domain of big defensin from the Pacific oyster

Defensins are antimicrobial peptides consisting of intramolecular disulphide bonds in a complex folded arrangement of two or three antiparallel β-sheets with or without an α-helical structure. They are produced by a vast range of organisms being constitutively expressed or induced in various tissues against different stimuli like infection, injury or other inflammatory factors. Two classes of invertebrate defensin exist, namely CS-αβ and big defensin, the latter being predominantly present in molluscs. Intriguingly, an invertebrate big defensin gene has been hypothesized as the most probable ancestor of vertebrate β-defensins. Here, conserved residues were identified for both big defensin and β-defensin. In silico mutation on conserved amino acid positions of the β-defensin-like domain of big defensin from Crassostrea gigas was carried out to understand the effects of mutation on the structure and function of the protein. R64A and E71A have been identified as deleterious as well as destabilizing for the protein. Changes in amino acid network and aggregation propensity were also observed upon mutating these two charged residues. 100 ns molecular dynamics simulations of wild-type, R64A and E71A structures revealed significant conformational changes in the case of mutants. Furthermore, molecular docking highlighted the significance of R64 in ligand interaction. In conclusion, these results provide the first in-depth understanding of the structural and functional importance imparted by two conserved charged residues in the C-terminal region of big defensin. It also enhances the existing knowledge about this antimicrobial peptide for application in therapeutics and other aspects of protein engineering.Communicated by Ramaswamy H. Sarma.

Identification, characterization, and antimicrobial activity of a novel big defensin discovered in a commercial bivalve mollusc, Tegillarca granosa

Molluscs, the second largest animal phylum on earth, primarily rely on cellular and humoral immune responses to fight against pathogen infection. Although antimicrobial peptides (AMPs) such as big defensin play crucial roles in the humoral immune response, it remains largely unknown in the ecological and economic important blood clam (Tegillarca granosa). In this study, a novel big defensin gene (TgBD) was identified in T. granosa through transcripts and whole genome searching. Bioinformatic analyses were conducted to explore the molecular characteristics of TgBD, and comparisons of TgBD with those reported in other molluscs were performed by multiple alignments and phylogenetic analysis. In addition, the expression patterns of TgBD in various tissues and upon bacterial challenge were investigated while the antimicrobial activity of synthetic N-terminal domain of TgBD was confirmed in vitro by radial diffusion experiment. Results obtained showed TgBD had an open reading frame (ORF) of 369 bp, encoding a prepropeptide containing a signal peptide and a propeptide. Similar to big defensins reported in other species, TgBD consists of a hydrophobic N-terminal domain containing β1-α1-α2-β2 folds and a cysteine-rich cationic C-terminal domain with three disulfide bonds between C1-C5, C2-C4, and C3-C6. Phylogenetic analysis showed that TgBD shared 76.80% similarity to its close relative ark shell (Scapharca broughtoni). In addition, TgBD expression was observed in all tissues investigated under normal conditions and was significantly induced by injection of Vibrio parahaemolyticus. Furthermore, synthetic N-terminal peptide of TgBD exhibited strong antimicrobial activity against Gram-positive bacteria tested. Our results indicated that TgBD is a constitutive and inducible acute phase AMP, which provides a universal and prompt protection for T. granosa.

Identification of a Novel Antimicrobial Peptide From the Ancient Marine Arthropod Chinese Horseshoe Crab, Tachypleus tridentatus

Humoral immunity is the first line of defense in the invertebrate immune system, and antimicrobial peptides play an important role in this biological process. A novel antimicrobial peptide, termed Tatritin, was identified and characterized in hemolymph of Chinese horseshoe crab, Tachypleus tridentatus, infected with Gram-negative bacteria via transcriptome analysis. Tatritin was significantly induced by bacterial infection in hemolymph and gill. The preprotein of Tatritin consists of a signal peptide (21 aa) and a mature peptide (47 aa) enriched by cysteine. The putative mature peptide was 5.6 kDa with a theoretical isoelectric point (pI) of 9.99 and showed a α-helix structure in the N-terminal and an anti-parallel β-sheet structure in the cysteine-stabilized C-terminal region. The chemically synthesized peptide of Tatritin exhibited a broad spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria and fungi. Furthermore, Tatritin may recognize and inhibit pathogenic microorganisms by directly binding to LPS, DNA, and chitin. In addition, administration of Tatritin reduced the mortality of zebrafish after bacterial infection. Due to its broad-spectrum antimicrobial activity in vivo and in vitro and the sensitivity to drug-resistant bacterial strains, Tatritin peptide can be used as a new type of drug for infection treatment or as an immune enhancer in animals.

Big defensin from the scallop Argopecten purpuratus ApBD1 is an antimicrobial peptide which entraps bacteria through nanonets formation

Big defensins is a large family of antimicrobial peptides found in restricted groups of invertebrates, in particular mollusks where they have highly diversified. Big defensins are composed of a highly hydrophobic N-terminal region and a C-terminal region containing six cysteine residues whose arrangement is identical to that of vertebrate β-defensins. They have been shown to be active against both Gram-positive and Gram-negative bacteria and fungi. Antimicrobial aggregates called nanonets entrapping and killing bacteria have been recently described for the hydrophobic N-terminal region of the Cg-BigDef1 from the oyster Crassostrea gigas. To determine whether nanonets formation is a conserved trait of mollusk big defensins, we assessed the potential entrapping of bacteria through nanonets of the big defensin from the scallop Argopecten purpuratus, ApBD1. Recombinant ApBD1 was produced with a thrombin-cleavable N-terminal His6 tag, followed by the mature peptide carrying a mutation of the last cysteine residue of the C-terminal region by and arginine, named rApBD1(C87R). This mutation did not apparently affect the three-dimensional structure and the biological properties of rApBD1(C87R), as evidenced by in silico modeling and in vitro antimicrobial assays. Strong immune staining of rApBD1(C87R) in numerous areas surrounding bacteria was observed by confocal microscopy, suggesting that rApBD1(C87R) entraps bacteria in peptide aggregates similar to those reported to the oyster big defensin. This study suggests the conservation of bactericidal activity and nanonet formation across big defensins from bivalve mollusks.

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.

In vivo immunostimulatory effect of the amoebocyte lysate and plasma of Asian horseshoe crab, Tachypleus gigas in a piscine model

Antimicrobial proteins/peptides are becoming a new generation of immunostimulants for prevention and disease control in human and animals, including aquatic animals. As the haemolymph of horseshoe crabs (Tachypleus) contains broad ranges of bioactive compounds, we have explored the in vivo immunostimulating potential of amoebocyte lysate and plasma using a fish model. Indian major carp, Labeo rohita, yearlings were injected intraperitoneally with two doses of lysate and plasma at 50 and 100 µg protein per fish. No abnormalities and/or mortalities were recorded in any group. L. rohita injected with 50 µg lysate and 100 µg plasma protein showed significant enhancement (P < 0.01) of various haematological and immunological parameters. There was a significant rise in the total protein and globulin content, myeloperoxidase and respiratory burst activity following injection with 50 µg lysate and 100 µg plasma protein. The agglutinating and haemagglutinating activities were increased albeit not significantly (P > 0.01) in any groups. On the contrary, a significantly high hemolysin titre was recorded in fish that received 100 µg plasma protein. Following challenge with Aeromonas hydrophila, both lysate and plasma protein(s) cross protected the fish after 30 days. The highest survival (50%) was recorded in group injected with 50 µg lysate protein, followed by 45% in both 100 µg lysate and plasma protein injected groups.

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.

Functional Insights From the Evolutionary Diversification of Big Defensins

Big defensins are antimicrobial polypeptides believed to be the ancestors of β-defensins, the most evolutionary conserved family of host defense peptides (HDPs) in vertebrates. Nevertheless, big defensins underwent several independent gene loss events during animal evolution, being only retained in a limited number of phylogenetically distant invertebrates. Here, we explore the evolutionary history of this fascinating HDP family and investigate its patchy distribution in extant metazoans. We highlight the presence of big defensins in various classes of lophotrochozoans, as well as in a few arthropods and basal chordates (amphioxus), mostly adapted to life in marine environments. Bivalve mollusks often display an expanded repertoire of big defensin sequences, which appear to be the product of independent lineage-specific gene tandem duplications, followed by a rapid molecular diversification of newly acquired gene copies. This ongoing evolutionary process could underpin the simultaneous presence of canonical big defensins and non-canonical (β-defensin-like) sequences in some species. The big defensin genes of mussels and oysters, two species target of in-depth studies, are subjected to gene presence/absence variation (PAV), i.e., they can be present or absent in the genomes of different individuals. Moreover, big defensins follow different patterns of gene expression within a given species and respond differently to microbial challenges, suggesting functional divergence. Consistently, current structural data show that big defensin sequence diversity affects the 3D structure and biophysical properties of these polypeptides. We discuss here the role of the N-terminal hydrophobic domain, lost during evolution toward β-defensins, in the big defensin stability to high salt concentrations and its mechanism of action. Finally, we discuss the potential of big defensins as markers for animal health and for the nature-based design of novel therapeutics active at high salt concentrations.

Purification and Assays of Tachylectin-5

Tachylectin-5, a 41-kDa protein with a common fold of the C-terminal globular domain of the γ-chain of fibrinogen, is purified from horseshoe crab hemolymph plasma by affinity column chromatography, using acetyl-group-immobilized resin. Two types of isolectins, tachylectin-5A and tachylectin-5B, are obtained by stepwise elution with GlcNAc at 25 and 250 mM, respectively. Tachylectins-5A and -5B exhibit extraordinarily strong hemagglutinating activity against all types of human erythrocytes (the minimum agglutinating concentration of 0.004-0.008 μg/mL for tachylectin-5A and 0.077-0.27 μg/mL for tachylectin-5B). Their hemagglutinating activities are inhibited by acetyl group-containing sugars and noncarbohydrates such as sodium acetate, acetylcholine, and acetyl CoA (the minimum inhibitory concentrations of 1.3-1.6 mM), indicating that the acetyl group is required and sufficient for recognition by tachylectins-5A and -5B. EDTA inhibits their hemagglutinating activity, whereas the inhibition is overcome by adding an excess amount of Ca²⁺. Tachylectins-5A and -5B also exhibit bacterial agglutinating activity against both Gram-negative bacteria (the minimum agglutinating concentrations of 0.04-0.08 μg/mL for tachylectin-5A and 0.05-0.11 μg/mL for tachylectin-5B) and Gram-positive bacteria (the minimum agglutinating concentrations of 0.3-2.4 μg/mL for tachylectin-5A and 15.1-26.8 μg/mL for tachylectin-5B). Interestingly, tachylectins-5A and -5B enhance the antimicrobial activity of a hemocyte-derived peptide, big defensin.

Antifungal Peptides as Therapeutic Agents

Fungi have been used since ancient times in food and beverage-making processes and, more recently, have been harnessed for the production of antibiotics and in processes of relevance to the bioeconomy. Moreover, they are starting to gain attention as a key component of the human microbiome. However, fungi are also responsible for human infections. The incidence of community-acquired and nosocomial fungal infections has increased considerably in recent decades. Antibiotic resistance development, the increasing number of immunodeficiency- and/or immunosuppression-related diseases and limited therapeutic options available are triggering the search for novel alternatives. These new antifungals should be less toxic for the host, with targeted or broader antimicrobial spectra (for diseases of known and unknown etiology, respectively) and modes of actions that limit the potential for the emergence of resistance among pathogenic fungi. Given these criteria, antimicrobial peptides with antifungal properties, i.e., antifungal peptides (AFPs), have emerged as powerful candidates due to their efficacy and high selectivity. In this review, we provide an overview of the bioactivity and classification of AFPs (natural and synthetic) as well as their mode of action and advantages over current antifungal drugs. Additionally, natural, heterologous and synthetic production of AFPs with a view to greater levels of exploitation is discussed. Finally, we evaluate the current and potential applications of these peptides, along with the future challenges relating to antifungal treatments.

Review: Antibacterial components of the Bivalve's immune system and the potential of freshwater bivalves as a source of new antibacterial compounds

Antibacterial research is reaching new heights due to the increasing demand for the discovery of new substances capable of inhibiting bacteria, especially to respond to the appearance of more and more multi-resistant strains. Bivalves show enormous potential for the finding of new antibacterial compounds, although for that to be further explored, more research needs to be made regarding the immune system of these organisms. Beyond their primary cellular component responsible for bacterial recognition and destruction, the haemocytes, bivalves have various other antibacterial units dissolved in the haemolymph that intervene in the defense against bacterial infections, from the recognition factors that detect different bacteria to the effector molecules carrying destructive properties. Moreover, to better comprehend the immune system, it is important to understand the different survival strategies that bacteria possess in order to stay alive from the host's defenses. This work reviews the current literature regarding the components that intervene in a bacterial infection, as well as discussing the enormous potential that freshwater bivalves have in the discovery of new antibacterial compounds.

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.

The draft genome of horseshoe crab Tachypleus tridentatus reveals its evolutionary scenario and well-developed innate immunity

BACKGROUND

Horseshoe crabs are ancient marine arthropods with a long evolutionary history extending back approximately 450 million years, which may benefit from their innate immune systems. However, the genetic mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear.

RESULTS

Here, we describe the 2.06 Gbp genome assembly of Tachypleus tridentatus with 24,222 predicted protein-coding genes. Comparative genomics shows that T. tridentatus and the Atlantic horseshoe crab Limulus polyphemus have the most orthologues shared among two species, including genes involved in the immune-related JAK-STAT signalling pathway. Divergence time dating results show that the last common ancestor of Asian horseshoe crabs (including T. tridentatus and C. rotundicauda) and L. polyphemus appeared approximately 130 Mya (121-141), and the split of the two Asian horseshoe crabs was dated to approximately 63 Mya (57-69). Hox gene analysis suggests two clusters in both horseshoe crab assemblies. Surprisingly, selective analysis of immune-related gene families revealed the high expansion of conserved pattern recognition receptors. Genes involved in the IMD and JAK-STAT signal transduction pathways also exhibited a certain degree of expansion in both genomes. Intact coagulation cascade-related genes were present in the T. tridentatus genome with a higher number of coagulation factor genes. Moreover, most reported antibacterial peptides have been identified in T. tridentatus with their potentially effective antimicrobial sites.

CONCLUSIONS

The draft genome of T. tridentatus would provide important evidence for further clarifying the taxonomy and evolutionary relationship of Chelicerata. The expansion of conserved immune signalling pathway genes, coagulation factors and intact antimicrobial peptides in T. tridentatus constitutes its robust and effective innate immunity for self-defence in marine environments with an enormous number of invading pathogens and may affect the quality of the adaptive properties with regard to complicated marine environments.

Purification and Assays of Tachycitin

An antimicrobial peptide tachycitin (73 amino acids) is purified by steps of chromatography, including Sephadex G-50 and S Sepharose FF, from the acid extract of hemocyte debris of horseshoe crabs. Tachycitin is present in monomer form in solution, revealed by ultracentrifugation analysis. Tachycitin exhibits bacterial agglutination activity and inhibits the growth of both Gram-negative bacteria, Gram-positive bacteria, and fungus Candida albicans. Interestingly, tachycitin shows synergistic antimicrobial activity in corporation with another antimicrobial peptide, big defensin. Tachycitin shows a specific binding activity to chitin but not to cellulose, mannan, xylan, and laminarin. Tachycitin is composed of the N-terminal three-stranded β-sheet and the C-terminal two-stranded β-sheet following a short helical turn, and the C-terminal structural motif shares a significant structural similarity with the chitin-binding domain derived from a plant chitin-binding protein, hevein.

Transcriptome analysis of larval immune defence in the lamprey Lethenteron japonicum

The lamprey is a primitive jawless vertebrate that occupies a critical phylogenetic position, and its larval stage represents the major portion of its life cycle [1]. Lamprey larvae have been proven to be an important model organism for studying numerous biological problems, such as the immune system, due to their unique biological features [2]. In addition, early-stage larvae have never been obtained from the wild [3]; therefore, it is necessary to establish artificial breeding of lampreys in the laboratory. However, during early development, the larvae exhibit susceptibility to saprolegniasis, and the immune responses of lamprey larvae to this infection remain poorly understood. Here, we established a model of fungal infection in lamprey larvae and then used RNA sequencing to investigate the transcript profiles of lamprey larvae and their immune responses to Saprolegnia ferax. Among the profiled molecules, genes involved in pathogen recognition, inflammation, phagocytosis, lysosomal degradation, soluble humoral effectors, and lymphocyte development were significantly upregulated. The results were validated by analysis of several genes by quantitative real-time PCR and whole-mount in situ hybridization. Finally, we performed a Western blot for VLRs in infected and uninfected lampreys. This work not only provides an animal model for studying fungal infection but also suggests a molecular basis for developing defensive strategies to manage Saprolegnia ferax infection.

The Ancestral N-Terminal Domain of Big Defensins Drives Bacterially Triggered Assembly into Antimicrobial Nanonets

β-Defensins are host defense peptides controlling infections in species ranging from humans to invertebrates. However, the antimicrobial activity of most human β-defensins is impaired at physiological salt concentrations. We explored the properties of big defensins, the β-defensin ancestors, which have been conserved in a number of marine organisms, mainly mollusks. By focusing on a big defensin from oyster (Cg-BigDef1), we showed that the N-terminal domain lost during evolution toward β-defensins confers bactericidal activity to Cg-BigDef1, even at high salt concentrations. Cg-BigDef1 killed multidrug-resistant human clinical isolates of Staphylococcus aureus. Moreover, the ancestral N-terminal domain drove the assembly of the big defensin into nanonets in which bacteria are entrapped and killed. This discovery may explain why the ancestral N-terminal domain has been maintained in diverse marine phyla and creates a new path of discovery to design β-defensin derivatives active at physiological and high salt concentrations.

Big defensins, ancestors of β-defensins, are composed of a β-defensin-like C-terminal domain and a globular hydrophobic ancestral N-terminal domain. This unique structure is found in a limited number of phylogenetically distant species, including mollusks, ancestral chelicerates, and early-branching cephalochordates, mostly living in marine environments. One puzzling evolutionary issue concerns the advantage for these species of having maintained a hydrophobic domain lost during evolution toward β-defensins. Using native ligation chemistry, we produced the oyster Crassostrea gigas BigDef1 (Cg-BigDef1) and its separate domains. Cg-BigDef1 showed salt-stable and broad-range bactericidal activity, including against multidrug-resistant human clinical isolates of Staphylococcus aureus. We found that the ancestral N-terminal domain confers salt-stable antimicrobial activity to the β-defensin-like domain, which is otherwise inactive. Moreover, upon contact with bacteria, the N-terminal domain drives Cg-BigDef1 assembly into nanonets that entrap and kill bacteria. We speculate that the hydrophobic N-terminal domain of big defensins has been retained in marine phyla to confer salt-stable interactions with bacterial membranes in environments where electrostatic interactions are impaired. Those remarkable properties open the way to future drug developments when physiological salt concentrations inhibit the antimicrobial activity of vertebrate β-defensins.

Medicinal Potentialities of Plant Defensins: A Review with Applied Perspectives

Plant-based secondary metabolites with medicinal potentialities such as defensins are small, cysteine-rich peptides that represent an imperative aspect of the inherent defense system. Plant defensins possess broad-spectrum biological activities, e.g., bactericidal and insecticidal actions, as well as antifungal, antiviral, and anticancer activities. The unique structural and functional attributes provide a nonspecific and versatile means of combating a variety of microbial pathogens, i.e., fungi, bacteria, protozoa, and enveloped viruses. Some defensins in plants involved in other functions include the development of metal tolerance and the role in sexual reproduction, while most of the defensins make up the innate immune system of the plants. Defensins are structurally and functionally linked and have been characterized in various eukaryotic microorganisms, mammals, plants, gulls, teleost species of fish, mollusks, insect pests, arachnidan, and crustaceans. This defense mechanism has been improved biotechnologically as it helps to protect plants from fungal attacks in genetically modified organisms (GMO). Herein, we review plant defensins as secondary metabolites with medicinal potentialities. The first half of the review elaborates the origin, structural variations, and mechanism of actions of plant defensins. In the second part, the role of defensins in plant defense, stress response, and reproduction are discussed with suitable examples. Lastly, the biological applications of plant defensins as potential antimicrobial and anticancer agents are also deliberated. In summary, plant defensins may open a new prospect in medicine, human health, and agriculture.

Physiological and immunological responses to mass mortality in noble scallop Chlamys nobilis cultured in Nan'ao waters of Shantou, China

The noble scallop Chlamys nobilis has been a commercially important marine cultured bivalve in the Southern Sea of China for decades. Mass mortality events, however, often occur during scallops' cultivation. Mortality of up to 67%-90% was recorded at the beginning of March in 2017 in some culture areas of Nan'ao (Shantou, China), spreading to all scallops within a week. In the present study, in order to investigate the response of the noble scallop at the physiological and molecular level during mass mortality, scallops with different mortalities of 90%, 67%, and 6% were sampled from three sites at Hunter bay, Baisha bay, and Longhai, respectively. Total carotenoids content (TCC), total antioxidant capacity (TAC), malondialdehyde (MDA) content and the expression levels of three immune-related genes (toll-like receptor, C-type lectin receptor and big defensing) in different scallop tissues were determined. The scallops were divided into three groups of sub-health, lesion, and health. TAC, TCC, as well as transcript levels of CnTLR-1, Cnlec-1 and CnBD in sub-health and lesion scallops were all significantly lower (P < 0.05) than those in health scallops, while MDA in sub-health and lesion scallops were significantly higher than those in health scallops (P < 0.05). Similarly, TCC and TAC in lesion scallops were both higher than sub-health scallops. Moreover, significantly positive correlations were found between TCC and TAC (P < 0.05) and between CnTLR-1 and Cnlec-1 (P < 0.05), while significantly negative correlations were found between TCC and CnTLR-1 (P < 0.05), TCC and Cnlec-1 (P < 0.05), TAC and CnBD (P < 0.05), as well as between MDA and Cnlec-1 (P < 0.001). All the results indicate that noble scallops significantly change their physiological and molecular levels when suffering from stress, and that their antioxidant and immune response systems play important defense functions.

Molecular characterization and expression analysis of CSαβ defensin genes from the scorpion Mesobuthus martensii

Defensins are important components of innate host defence system against bacteria, fungi, parasites and viruses. Here, we predicted six potential defensin genes from the genome of the scorpion Mesobuthus martensii and then validated four genes from them via the combination of PCR and genomic sequence analysis. These four scorpion defensin genes share the same gene organization and structure of two exons and one phase-I intron with the GT-AG rule. Conserved motif and phylogenetic analysis showed that they belonged to the members of the invertebrate cysteine-stabilized α-helix/β-sheet motif defensin (CSαβ) defensin family. All these four CSαβ defensin genes have the expression feature of constitutive transcription (CON) by the whole scorpion infection model, promoter sequence analysis and dual luciferase assays. Further evolution and comparison analysis found that the invertebrate CSαβ defensin genes from most of arachnids and mollusks appear to share the expression pattern of CON, but those from insects and lower invertebrates (nematodes, annelids, cnidarians and sponges) seem to have identical inducible transcription (IND) after being challenged by microorganisms. Together, we identified four scorpion CSαβ defensin genes with the expression feature of CON, and characterized the diversified expression patterns of the invertebrate CSαβ defensin genes, which will shed insights into the evolution of the invertebrate CSαβ defensin genes and their expression patterns.

Antimicrobial peptides in marine invertebrate health and disease

Aquaculture contributes more than one-third of the animal protein from marine sources worldwide. A significant proportion of aquaculture products are derived from marine protostomes that are commonly referred to as 'marine invertebrates'. Among them, penaeid shrimp (Ecdysozosoa, Arthropoda) and bivalve molluscs (Lophotrochozoa, Mollusca) are economically important. Mass rearing of arthropods and molluscs causes problems with pathogens in aquatic ecosystems that are exploited by humans. Remarkably, species of corals (Cnidaria) living in non-exploited ecosystems also suffer from devastating infectious diseases that display intriguing similarities with those affecting farmed animals. Infectious diseases affecting wild and farmed animals that are present in marine environments are predicted to increase in the future. This paper summarizes the role of the main pathogens and their interaction with host immunity, with a specific focus on antimicrobial peptides (AMPs) and pathogen resistance against AMPs. We provide a detailed review of penaeid shrimp AMPs and their role at the interface between the host and its resident/pathogenic microbiota. We also briefly describe the relevance of marine invertebrate AMPs in an applied context.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

The role of natural antimicrobial peptides during infection and chronic inflammation

Natural antimicrobial peptides (AMPs), a family of small polypeptides that are produced by constitutive or inducible expression in organisms, are integral components of the host innate immune system. In addition to their broad-spectrum antibacterial activity, natural AMPs also have many biological activities against fungi, viruses and parasites. Natural AMPs exert multiple immunomodulatory roles that may predominate under physiological conditions where they lose their microbicidal properties in serum and tissue environments. Increased drug resistance among microorganisms is occurring far more quickly than the discovery of new antibiotics. Natural AMPs have shown promise as 'next generation antibiotics' due to their broad-spectrum curative effects, low toxicity, the fact that they are not residual in animals, and the low rates of resistance exhibited by many pathogens. Many types of synthetic AMPs are currently being tested in clinical trials for the prevention and treatment of various diseases such as chemotherapy-associated infections, diabetic foot ulcers, catheter-related infections, and other conditions. Here, we provide an overview of the types and functions of natural AMPs and their role in combating microorganisms and different infectious and inflammatory diseases.

Panusin represents a new family of β-defensin-like peptides in invertebrates

Beta_defensin have been solely found in vertebrates until β-defensin-like peptides were described as transcript isoforms in two species of Panulirus genus. They were considered as putative antimicrobials since their biological activity have not been demonstrated. Here we purified and characterized a defensin-like peptide from the hemocytes of spiny lobster P. argus, hereafter named panusin. Structurally, panusin presents a cysteine-stabilized α/β motif, and is prone to form homodimers. Biological activity of panusin showed broad-spectrum antimicrobial activity, characterized for being strikingly salt-resistant. Panusin did not showed hemolytic activity but was demonstrated its binding capacity to different lipid membrane models, indicating amphipathicity of β-sheet core as driving force for its antimicrobial activity. Panusin is considered a new kind of arthropod defensin which share structural and biological features with beta-defensin from vertebrates. The presence of beta-defensin like peptides in crustacean might suggest the emergence of the evolutionary relationship of β-defensins from vertebrates.

Anti-Candida albicans natural products, sources of new antifungal drugs: A review

INTRODUCTION

Candida albicans is the most prevalent fungal pathogen in humans. Due to the development of drug resistance, there is today a need for new antifungal agents for the efficient management of C. albicans infections. Therefore, we reviewed antifungal activity, mechanisms of action, possible synergism with antifungal drugs of all natural substances experimented to be efficient against C. albicans for future.

METHODS

An extensive and systematic review of the literature was undertaken and all relevant abstracts and full-text articles analyzed and included in the review.

REVIEW

A total of 111 documents were published and highlighted 142 anti-C. albicans natural products. These products are mostly are reported in Asia (44.37%) and America (28.17%). According to in vitro model criteria, from the 142 natural substances, antifungal activity can be considered as important for 40 (28.20%) and moderate for 24 (16.90%). Sixteen products have their antifungal activity confirmed by in vivo gold standard experimentation. Microbial natural products, source of antifungals, have their antifungal mechanism well described in the literature: interaction with ergosterol (polyenes), inhibition 1,3-β-d-glucan synthase (Echinocandins), inhibition of the synthesis of cell wall components (chitin and mannoproteins), inhibition of sphingolipid synthesis (serine palmitoyltransferase, ceramide synthase, inositol phosphoceramide synthase) and inhibition of protein synthesis (sordarins). Natural products from plants mostly exert their antifungal effects by membrane-active mechanism. Some substances from arthropods are also explored to act on the fungal membrane. Interestingly, synergistic effects were found between different classes of natural products as well as between natural products and azoles.

CONCLUSION

Search for anti-C. albicans new drugs is promising since the list of natural substances, which disclose activity against this yeast is today long. Investigations must be pursued not only to found more new anti-Candida compounds from plants and organisms but also to carried out details on molecules from already known anti-Candida compounds and to more elucidate mechanisms of action.

Yeast recombinant Factor C from horseshoe crab binds endotoxin and causes bacteriostasis

Carcinoscorpius rotundicauda Factor C cDNA has been cloned and expressed in Pichia pastoris to produce a recombinant full-length Factor C (rFC) which is both immunoreactive and functional. The presence of a functional endotoxin-binding domain on rFC was ascertained by LPS-binding assays. One involved the relative binding affinity of rFC to electroblotted lipid A moiety of LPS. The second assay showed that rFC competed against native Factor C contained in C. rotundicauda amebocyte lysate (CAL) to bind LPS. Purification of rFC enhanced its binding affinity to LPS. By agglutination, rFC caused bacteriostasis of Gram-negative bacteria within 2 h. In an in vivo system, rFC also decreased the mortality of actinomycin D-sensitized/LPS-challenged mice. The rFCEE, bearing the 5' terminal LPS binding domain displayed a lowered affinity for LPS. This is in contrast to the rFCSN subclone that is devoid of the 5' end of Factor C, and which does not bind LPS. The presence of a fully-functional endotoxin binding domain in rFC probably requires a full-length protein for co-operative interaction of its downstream sequences. Thus, rFC has potential in the detection and removal of contaminating LPS from biological specimens and fluids for injection, since it is capable of binding both free and bound lipid A.

Cloning of a big defensin gene and its response to Vibrio parahaemolyticus challenge in the noble scallop Chlamys nobilis (Bivalve: Pectinidae)

The noble scallop Chlamys nobilis has been an important marine cultured bivalve in the Southern Sea of China for decades. However, large-scale mortality events often occurred during the scallop' cultivation. As one of AMPs (antimicrobial peptides), big defensin is an important component of the innate immunity against pathogenic microorganisms in invertebrates. In order to investigate whether the big defensin can play a role in the immune defense against pathogenic microorganisms in noble scallop, a big defensin gene from the hemocytes of Chlamys nobilis (CnBD) was cloned, and the mRNA level was measured after an acute Vibrio parahaemolyticus challenge of 36 h. The CnBD cDNA contains an open reading frame (ORF) of 381 bp encoding a peptide of 126 amino acids residues. The deduce amino acid sequence of CnBD shows a high similarity with that from Argopecten irradians and displays common features of big defensin, indicating that CnBD is a new member of the big defensin family. Compared with the control group, the relative mRNA level of CnBD was significantly up-regulated at 3, 24 and 36 h. The present result indicated that CnBD played an immune role against bacterial infection in noble scallop.

Immunomodulatory properties of shellfish derivatives associated with human health

Some vital components of marine shellfish are documented as an important source for both nutritional and pharmacological applications.

Comparative genomic study of arachnid immune systems indicates loss of beta-1,3-glucanase-related proteins and the immune deficiency pathway

Analyses of arthropod genomes have shown that the genes in the different innate humoral immune responses are conserved. These genes encode proteins that are involved in immune signalling pathways that recognize pathogens and activate immune responses. These immune responses include phagocytosis, encapsulation of the pathogen and production of effector molecules for pathogen elimination. So far, most studies have focused on insects leaving other major arthropod groups largely unexplored. Here, we annotate the immune-related genes of six arachnid genomes and present evidence for a conserved pattern of some immune genes, but also evolutionary changes in the arachnid immune system. Specifically, our results suggest that the family of recognition molecules of beta-1,3-glucanase-related proteins (βGRPs) and the genes from the immune deficiency (IMD) signalling pathway have been lost in a common ancestor of arachnids. These findings are consistent with previous work suggesting that the humoral immune effector proteins are constitutively produced in arachnids in contrast to insects, where these have to be induced. Further functional studies are needed to verify this. We further show that the full haemolymph clotting cascade found in the horseshoe crab is retrieved in most arachnid genomes. Tetranychus lacks at least one major component, although it is possible that this cascade could still function through recruitment of a different protein. The gel-forming protein in horseshoe crabs, coagulogen, was not recovered in any of the arachnid genomes; however, it is possible that the arachnid clot consists of a related protein, spätzle, that is present in all of the genomes.

An updated molecular basis for mussel immunity

Non-self recognition with the consequent tolerance or immune reaction is a crucial process to succeed as living organisms. At the same time the interactions between host species and their microbiome, including potential pathogens and parasites, significantly contribute to animal life diversity. Marine filter-feeding bivalves, mussels in particular, can survive also in heavily anthropized coastal waters despite being constantly surrounded by microorganisms. Based on the first outline of the Mytilus galloprovincialis immunome dated 2011, the continuously growing transcript data and the recent release of a draft mussel genome, we explored the available sequence data and scientific literature to reinforce our knowledge on the main gene-encoded elements of the mussel immune responses, from the pathogen recognition to its clearance. We carefully investigated molecules specialized in the sensing and targeting of potential aggressors, expected to show greater molecular diversification, and outlined, whenever relevant, the interconnected cascades of the intracellular signal transduction. Aiming to explore the diversity of extracellular, membrane-bound and intracellular pattern recognition receptors in mussel, we updated a highly complex immune system, comprising molecules which are described here in detail for the first time (e.g. NOD-like receptors) or which had only been partially characterized in bivalves (e.g. RIG-like receptors). Overall, our comparative sequence analysis supported the identification of over 70 novel full-length immunity-related transcripts in M. galloprovincialis. Nevertheless, the multiplicity of gene functions relevant to immunity, the involvement of part of them in other vital processes, and also the lack of a refined mussel genome make this work still not-exhaustive and support the development of more specific studies.

The new insights into the oyster antimicrobial defense: Cellular, molecular and genetic view

Oysters are sessile filter feeders that live in close association with abundant and diverse communities of microorganisms that form the oyster microbiota. In such an association, cellular and molecular mechanisms have evolved to maintain oyster homeostasis upon stressful conditions including infection and changing environments. We give here cellular and molecular insights into the Crassostrea gigas antimicrobial defense system with focus on antimicrobial peptides and proteins (AMPs). This review highlights the central role of the hemocytes in the modulation and control of oyster antimicrobial response. As vehicles for AMPs and other antimicrobial effectors, including reactive oxygen species (ROS), and together with epithelia, hemocytes provide the oyster with local defense reactions instead of systemic humoral ones. These reactions are largely based on phagocytosis but also, as recently described, on the extracellular release of antimicrobial histones (ETosis) which is triggered by ROS. Thus, ROS can signal danger and activate cellular responses in the oyster. From the current literature, AMP production/release could serve similar functions. We provide also new lights on the oyster genetic background that underlies a great diversity of AMP sequences but also an extraordinary individual polymorphism of AMP gene expression. We discuss here how this polymorphism could generate new immune functions, new pathogen resistances or support individual adaptation to environmental stresses.

Biologically active and antimicrobial peptides from plants

Bioactive peptides are part of an innate response elicited by most living forms. In plants, they are produced ubiquitously in roots, seeds, flowers, stems, and leaves, highlighting their physiological importance. While most of the bioactive peptides produced in plants possess microbicide properties, there is evidence that they are also involved in cellular signaling. Structurally, there is an overall similarity when comparing them with those derived from animal or insect sources. The biological action of bioactive peptides initiates with the binding to the target membrane followed in most cases by membrane permeabilization and rupture. Here we present an overview of what is currently known about bioactive peptides from plants, focusing on their antimicrobial activity and their role in the plant signaling network and offering perspectives on their potential application.

Sequence diversity and evolution of antimicrobial peptides in invertebrates

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

Transcriptional changes in Manila clam (Ruditapes philippinarum) in response to Brown Ring Disease

Brown Ring Disease (BRD) is a bacterial infection affecting the economically-important clam Ruditapes philippinarum. The disease is caused by a bacterium, Vibrio tapetis, that colonizes the edge of the mantle, altering the biomineralization process and normal shell growth. Altered organic shell matrices accumulate on the inner face of the shell leading to the formation of the typical brown ring in the extrapallial space (between the mantle and the shell). Even though structural and functional changes have been described in solid (mantle) and fluid (hemolymph and extrapallial fluids) tissues from infected clams, the underlying molecular alterations and responses remain largely unknown. This study was designed to gather information on clam molecular responses to the disease and to compare focal responses at the site of the infection (mantle and extrapallial fluid) with systemic (hemolymph) responses. To do so, we designed and produced a Manila clam expression oligoarray (15K Agilent) using transcriptomic data available in public databases and used this platform to comparatively assess transcriptomic changes in mantle, hemolymph and extrapallial fluid of infected clams. Results showed significant regulation in diseased clams of molecules involved in pathogen recognition (e.g. lectins, C1q domain-containing proteins) and killing (defensin), apoptosis regulation (death-associated protein, bcl-2) and in biomineralization (shell matrix proteins, perlucin, galaxin, chitin- and calcium-binding proteins). While most changes in response to the disease were tissue-specific, systemic alterations included co-regulation in all 3 tested tissues of molecules involved in microbe recognition and killing (complement-related factors, defensin). These results provide a first glance at molecular alterations and responses caused by BRD and identify targets for future functional investigations.

Deep transcriptome sequencing of Pecten maximus hemocytes: a genomic resource for bivalve immunology

Pecten maximus, the king scallop, is a bivalve species with important commercial value for both fisheries and aquaculture, traditionally consumed in several European countries. Major problems in larval rearing, however, still limit hatchery-based seed production. High mortalities during early larval stages, likely related to bacterial pathogens, represent the most relevant bottleneck. To address this issue, understanding host defense mechanisms against microbes is extremely important. In this study next-generation RNA-sequencing was carried on scallop hemocytes. To enrich for immune-related transcripts, cDNA libraries from hemocytes challenged in vivo with inactivated-Vibrio anguillarum and in vitro with pathogen-associated molecular patterns, as well as unchallenged controls, were sequenced yielding 216,444,674 sequence reads. De novo assembly of the scallop hemocyte transcriptome consisted of 73,732 contigs (31% annotated). A total of 934 contigs encoded proteins with a known immune function, grouped into several functional categories. Particular attention was reserved to Toll-like receptors (TLRs), a family of pattern recognition receptors (PRRs) involved in non-self recognition. Through mining the scallop hemocyte transcriptome, at least four TLRs could be identified. The organization of canonical TLR domains demonstrated that single cysteine cluster and multiple cysteine cluster TLRs co-exist in this species. In addition, preliminary data concerning their mRNA level following bacterial challenge suggested that different members of this family could exhibit opposite responses to pathogenic stimuli. Finally, a global analysis of differential expression comparing gene-expression levels in in vitro and in vivo stimulated hemocytes against controls provided evidence on a large set of transcripts involved in the great scallop immune response.

Properties and mechanisms of action of naturally occurring antifungal peptides

Antimicrobial peptides are a vital component of the innate immune system of all eukaryotic organisms and many of these peptides have potent antifungal activity. They have potential application in the control of fungal pathogens that are a serious threat to both human health and food security. Development of antifungal peptides as therapeutics requires an understanding of their mechanism of action on fungal cells. To date, most research on antimicrobial peptides has focused on their activity against bacteria. Several antimicrobial peptides specifically target fungal cells and are not active against bacteria. Others with broader specificity often have different mechanisms of action against bacteria and fungi. This review focuses on the mechanism of action of naturally occurring antifungal peptides from a diverse range of sources including plants, mammals, amphibians, insects, crabs, spiders, and fungi. While antimicrobial peptides were originally proposed to act via membrane permeabilization, the mechanism of antifungal activity for these peptides is generally more complex and often involves entry of the peptide into the cell.

Defensin-based anti-infective strategies

Cationic and amphiphilic peptides are widely distributed in eukaryotic organisms and constitute a first line of host defense against invading pathogens. Some of these host defense peptides (HDPs) combine specific antibiotic activities with modulation of immune responses. Moreover, they are active against bacteria resistant to conventional antibiotics and show only modest resistance development under in vitro selection pressure. Based on these features, HDPs and particularly defensins are considered a promising source of novel anti-infective agents. This review summarizes the current knowledge about defensins from different kingdoms and discusses their potential for therapeutic application.