Solution structure and activity of the synthetic four-disulfide bond Mediterranean mussel defensin (MGD-1)

Antimicrobial peptides: An alternative strategy to combat antimicrobial resistance

Antimicrobial peptides (AMPs) are a diverse group of naturally occurring molecules produced by eukaryotes and prokaryotes. They have an important role in innate immunity via their direct microbicidal properties or immunomodulatory activities against pathogens. With the widespread occurrence of antimicrobial resistance (AMR), AMPs are considered as viable alternatives for the treatment of multidrug-resistant microbes, inflammation, and, wound healing. The broad-spectrum antibacterial activity of AMPs is predominantly attributed to membrane disruption, leading to the formation of transmembrane pores and, eventually, cell lysis. However, mechanisms related to inhibition of cell wall synthesis, nucleic acid synthesis, protein synthesis, or enzymatic activity are also associated with these peptides. In this review, we discuss our current understanding, therapeutic uses and challenges associated with the clinical applications of AMPs.

Assignment of Disulfide Bonds in HNTX-XXI by Double-Enzymatic Digestion and Edman Degradation

HNTX-XXI, a peptide toxin derived from the venom of the spider Ornithoctonus hainana, comprises a 64-amino-acid protein architecture that notably incorporates eight cysteine residues positioned at positions 2, 10, 14, 16, 17, 23, 36, and 63. The close spatial proximity of Cys16 and Cys17 poses a challenge in resolving their disulfide bridge configurations using standard methodologies. In this study, we introduce an innovative and highly efficient approach for delineating disulfide pairings in peptides containing adjacent cysteines. Our methodology integrates a two-step proteolytic digestion strategy utilizing trypsin and Glu-specific staphylococcal V8 protease coupled with a subsequent round of Edman degradation. This multifaceted approach enables the precise characterization of the disulfide bonds within the peptide. Specifically, targeted proteolysis by trypsin and V8, followed by reversed-phase HPLC separation of the resulting peptides, facilitated the unambiguous identification of disulfide linkages between Cys10-Cys23 and Cys14-Cys63. For the fragment containing the four remaining cysteines, a single cycle of Edman degradation was employed, strategically breaking the peptide bond between the adjacent cysteines. This pivotal step enabled the isolation and analysis of the resulting fragments. Subsequently, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was utilized, revealing the presence of two additional disulfide bonds: Cys2-Cys17 and Cys16-Cys36. Collectively, these findings allow for the definitive assignment of the four disulfide linkages in HNTX-XXI as Cys2-Cys17, Cys10-Cys23, Cys14-Cys63, and Cys16-Cys36. This rapid and sensitive methodology represents a significant advancement in the structural characterization of peptide toxins with complex disulfide bond patterns, underscoring its potential for broad application in the field of venom peptide research.

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.

Comparative Genomics Reveals a Significant Sequence Variability of Myticin Genes in Mytilus galloprovincialis

Myticins are cysteine-rich antimicrobial peptides highly expressed in hemocytes of Mytilus galloprovincialis. Along with other antimicrobial peptides (AMPs), myticins are potent effectors in the mussel immune response to pathogenic infections. As intertidal filter-feeders, mussels are constantly exposed to mutable environmental conditions, as well as to the presence of many pathogens, and myticins may be key players in the great ability of these organisms to withstand these conditions. These AMPs are known to be characterized by a remarkable sequence diversity, which was further explored in this work, thanks to the analysis of the recently released genome sequencing data from 16 specimens. Altogether, we collected 120 different sequence variants, evidencing the important impact of presence/absence variation and positive selection in shaping the repertoire of myticin genes of each individual. From a functional point of view, both the isoelectric point (pI) and the predicted charge of the mature peptide show unusually low values compared with other cysteine-rich AMPs, reinforcing previous observations that myticins may have accessory functions not directly linked with microbe killing. Finally, we report the presence of highly conserved regulatory elements in the promoter region of myticin genes, which might explain their strong hemocyte-specific expression.

Molecular Diversity of Mytilin-Like Defense Peptides in Mytilidae (Mollusca, Bivalvia)

The CS-αβ architecture is a structural scaffold shared by a high number of small, cationic, cysteine-rich defense peptides, found in nearly all the major branches of the tree of life. Although several CS-αβ peptides involved in innate immune response have been described so far in bivalve mollusks, a clear-cut definition of their molecular diversity is still lacking, leaving the evolutionary relationship among defensins, mytilins, myticins and other structurally similar antimicrobial peptides still unclear. In this study, we performed a comprehensive bioinformatic screening of the genomes and transcriptomes available for marine mussels (Mytilida), redefining the distribution of mytilin-like CS-αβ peptides, which in spite of limited primary sequence similarity maintain in all cases a well-conserved backbone, stabilized by four disulfide bonds. Variations in the size of the alpha-helix and the two antiparallel beta strand region, as well as the positioning of the cysteine residues involved in the formation of the C1-C5 disulfide bond might allow a certain degree of structural flexibility, whose functional implications remain to be investigated. The identification of mytilins in Trichomya and Perna spp. revealed that many additional CS-αβ AMPs remain to be formally described and functionally characterized in Mytilidae, and suggest that a more robust scheme should be used for the future classification of such peptides with respect with their evolutionary origin.

Immunity in mussels: An overview of molecular components and mechanisms with a focus on the functional defenses

Bivalves' immunity has received much more attention in the last decade, which resulted to a valuable growth in the availability of its molecular components. Such data availability coupled with the economical importance of these organisms aimed to shift the increase in the number of immunological and stress-related studies. Unfortunately, the crowd of generated data deciphering the involved physiological processes, investigators' differential conceptualization and the aimed objectives, has complicated the sensu stricto outlining of immune-related mechanisms. Overall, this review tried to compiles a summary about the molecular components of the mussels' immune response, surveying an overview of the mussels' functional immunity through gathering the most recent-related topics of bivalves' immunity as apoptosis and autophagy which deserves a great attention as stress-related mechanisms, the disseminated neoplasia as outbreak transmissible disease, not only within the same specie but also among different species, the hematopoiesis as topic that still generating interesting debate in the scientific community, the mucosal immunity described as the interface where host-pathogen interactions would occurs and determinate the late immune response, and innate immune memory and transgenerational priming, which described as very recent research topic with extensive applications in shellfish farming industry.

From the raw bar to the bench: Bivalves as models for human health

Bivalves, from raw oysters to steamed clams, are popular choices among seafood lovers and once limited to the coastal areas. The rapid growth of the aquaculture industry and improvement in the preservation and transport of seafood have enabled them to be readily available anywhere in the world. Over the years, oysters, mussels, scallops, and clams have been the focus of research for improving the production, managing resources, and investigating basic biological and ecological questions. During this decade, an impressive amount of information using high-throughput genomic, transcriptomic and proteomic technologies has been produced in various classes of the Mollusca group, and it is anticipated that basic and applied research will significantly benefit from this resource. One aspect that is also taking momentum is the use of bivalves as a model system for human health. In this review, we highlight some of the aspects of the biology of bivalves that have direct implications in human health including the shell formation, stem cells and cell differentiation, the ability to fight opportunistic and specific pathogens in the absence of adaptive immunity, as source of alternative drugs, mucosal immunity and, microbiome turnover, toxicology, and cancer research. There is still a long way to go; however, the next time you order a dozen oysters at your favorite raw bar, think about a tasty model organism that will not only please your palate but also help unlock multiple aspects of molluscan biology and improve human health.

Identification and bioactivity analysis of a newly identified defensin from the oyster Magallana gigas

The relatively conserved sequences of signal peptides and proregions that antimicrobial peptides (AMPs) contain have been successfully used to search for and identify novel AMPs from databases within the same lineages of fish and amphibians and across different animal classes. If such an approach is applicable to invertebrate species such as oyster has not yet been tested so far. In this study, we found a cDNA from the digestive gland of the oyster Magallana gigas, designated Mgdefdg, which contains two exons interspaced by one intron. Mgdefdg coded for a protein with features characteristic of defensins. The mature peptide had the cysteine-stabilized α-helix/β-sheet motif (CSαβ) and the consensus pattern C-X_5-6-C-X₃-C-X_4-6-C-X_3-4-C-X_7-8-C-X-C-X₂-C forming potential disulfide linkages C1-C5, C2-C6, C3-C7 and C4-C8 in the predicted tertiary structure. Functional assays revealed that recombinant mature MgDefdg (rmMgDefdg) was able to kill the Gram-negative bacterium Aeromonas hydrophila and the Gram-positive bacterium Staphylococcus aureus, and to induce bacterial membrane/cytoplasmic damage. ELISA showed that rmMgDefdg had high affinity to both A. hydrophila and S. aureus as well as the microbe-associated molecular pattern molecules LPS and LTA. Moreover, rmMgDefdg was capable of causing bacterial membrane permeabilization and depolarization, and intracellular ROS increase. Additionally, rmMgDefdg was not cytotoxic to human red blood cells and murine RAW264.7 cells. Taken together, our results indicate that MgDefdg is a previously uncharacterized defensin with membrane selectivity towards bacterial cells. It also shows that the use of conserved sequences of signal peptides of defensins can be an effective tool to identify potential defensins across different animal genera in invertebrates.

Identification and characterization of a novel defensin from Asian green mussel Perna viridis

Defensin is one of the most diversified groups of antimicrobial peptides in invertebrate. In the present study, a novel defensin member referred as Pv-Def was identified and characterized from Asian green mussel Perna viridis. Using in silico survey of several EST databases released from diverse tissues of P. viridis, a single peptide referred as Pv-Def was predicted as defensin homologue with Mytilus counterparts. Further analysis on gene structure revealed that Pv-Def was 1001 nt in length and consisted of 3 exons and 2 introns. The precursor of Pv-Def was composed of a signal peptide of 19 amino acids and a mature peptide of 45 amino acids. The mature Pv-Def peptide contains 6 cysteines which formed 3 disulfide bonds at ²⁷C₁- ⁵⁴C₄, ⁴⁰C₂- ⁶⁰C₅ and ⁴⁴C₃- ⁶²C₆. Like most of the defensin family members, mature Pv-Def peptide included an alpha helix and 2 beta strands. Pv-Def showed significantly tissue-specific expression pattern, while highest transcription level was observed in hepatopancreas, which was about 900 folds to that in hemocytes. Moreover, the expression of Pv-Def mRNA in hemocytes was significantly and accurately up-regulated at different time intervals by Vibrio parahaemolyticus challenge. Interestingly, phylogenetic analysis suggested that the Pv-Def possesses closest relationships with arthropods counterparts rather than other mollusk defensins. To our knowledge, this is the first time that a defensin member was reported in Asian green mussel P. viridis.

Microbial Diseases of Bivalve Mollusks: Infections, Immunology and Antimicrobial Defense

A variety of bivalve mollusks (phylum Mollusca, class Bivalvia) constitute a prominent commodity in fisheries and aquacultures, but are also crucial in order to preserve our ecosystem’s complexity and function. Bivalve mollusks, such as clams, mussels, oysters and scallops, are relevant bred species, and their global farming maintains a high incremental annual growth rate, representing a considerable proportion of the overall fishery activities. Bivalve mollusks are filter feeders; therefore by filtering a great quantity of water, they may bioaccumulate in their tissues a high number of microorganisms that can be considered infectious for humans and higher vertebrates. Moreover, since some pathogens are also able to infect bivalve mollusks, they are a threat for the entire mollusk farming industry. In consideration of the leading role in aquaculture and the growing financial importance of bivalve farming, much interest has been recently devoted to investigate the pathogenesis of infectious diseases of these mollusks in order to be prepared for public health emergencies and to avoid dreadful income losses. Several bacterial and viral pathogens will be described herein. Despite the minor complexity of the organization of the immune system of bivalves, compared to mammalian immune systems, a precise description of the different mechanisms that induce its activation and functioning is still missing. In the present review, a substantial consideration will be devoted in outlining the immune responses of bivalves and their repertoire of immune cells. Finally, we will focus on the description of antimicrobial peptides that have been identified and characterized in bivalve mollusks. Their structural and antimicrobial features are also of great interest for the biotechnology sector as antimicrobial templates to combat the increasing antibiotic-resistance of different pathogenic bacteria that plague the human population all over the world.

Convergent evolution of defensin sequence, structure and function

Defensins are a well-characterised group of small, disulphide-rich, cationic peptides that are produced by essentially all eukaryotes and are highly diverse in their sequences and structures. Most display broad range antimicrobial activity at low micromolar concentrations, whereas others have other diverse roles, including cell signalling (e.g. immune cell recruitment, self/non-self-recognition), ion channel perturbation, toxic functions, and enzyme inhibition. The defensins consist of two superfamilies, each derived from an independent evolutionary origin, which have subsequently undergone extensive divergent evolution in their sequence, structure and function. Referred to as the cis- and trans-defensin superfamilies, they are classified based on their secondary structure orientation, cysteine motifs and disulphide bond connectivities, tertiary structure similarities and precursor gene sequence. The utility of displaying loops on a stable, compact, disulphide-rich core has been exploited by evolution on multiple occasions. The defensin superfamilies represent a case where the ensuing convergent evolution of sequence, structure and function has been particularly extreme. Here, we discuss the extent, causes and significance of these convergent features, drawing examples from across the eukaryotes.

Establishing a reference array for the CS-αβ superfamily of defensive peptides

BACKGROUND

"Invertebrate defensins" belong to the cysteine-stabilized alpha-beta (CS-αβ), also known as the scorpion toxin-like, superfamily. Some other peptides belonging to this superfamily of defensive peptides are indistinguishable from "defensins," but have been assigned other names, making it unclear what, if any, criteria must be met to qualify as an "invertebrate defensin." In addition, there are other groups of defensins in invertebrates and vertebrates that are considered to be evolutionarily unrelated to those in the CS-αβ superfamily. This complicates analyses and discussions of this peptide group. This paper investigates the criteria for classifying a peptide as an invertebrate defensin, suggests a reference cysteine array that may be helpful in discussing peptides in this superfamily, and proposes that the superfamily (rather than the name "defensin") is the appropriate context for studying the evolution of invertebrate defensins with the CS-αβ fold.

METHODS

CS-αβ superfamily sequences were identified from previous literature and BLAST searches of public databases. Sequences were retrieved from databases, and the relevant motifs were identified and used to create a conceptual alignment to a ten-cysteine reference array. Amino acid sequences were aligned in MEGA6 with manual adjustments to ensure accurate alignment of cysteines. Phylogenetic analyses were performed in MEGA6 (maximum likelihood) and MrBayes (Bayesian).

RESULTS

Across invertebrate taxa, the term "defensin" is not consistently applied based on number of cysteines, cysteine spacing pattern, spectrum of antimicrobial activity, or phylogenetic relationship. The analyses failed to reveal any criteria that unify "invertebrate defensins" and differentiate them from other defensive peptides in the CS-αβ superfamily. Sequences from various groups within the CS-αβ superfamily of defensive peptides can be described by a ten-cysteine reference array that aligns their defining structural motifs.

CONCLUSIONS

The proposed ten-cysteine reference array can be used in addition to current nomenclature to compare sequences in the CS-αβ superfamily and clarify their features relative to one another. This will facilitate analysis and discussion of "invertebrate defensins" in an appropriate evolutionary context, rather than relying on nomenclature.

Characterization of the antimicrobial peptide family defensins in the Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus), and tammar wallaby (Macropus eugenii)

Defensins comprise a family of cysteine-rich antimicrobial peptides with important roles in innate and adaptive immune defense in vertebrates. We characterized alpha and beta defensin genes in three Australian marsupials: the Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus), and tammar wallaby (Macropus eugenii) and identified 48, 34, and 39 defensins, respectively. One hundred and twelve have the classical antimicrobial peptides characteristics required for pathogen membrane targeting, including cationic charge (between 1+ and 15+) and a high proportion of hydrophobic residues (>30%). Phylogenetic analysis shows that gene duplication has driven unique and species-specific expansions of devil, koala, and tammar wallaby beta defensins and devil alpha defensins. Defensin genes are arranged in three genomic clusters in marsupials, whereas further duplications and translocations have occurred in eutherians resulting in four and five gene clusters in mice and humans, respectively. Marsupial defensins are generally under purifying selection, particularly residues essential for defensin structural stability. Certain hydrophobic or positively charged sites, predominantly found in the defensin loop, are positively selected, which may have functional significance in defensin-target interaction and membrane insertion.

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

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

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.

Hit 'em where it hurts: The growing and structurally diverse family of peptides that target lipid-II

Understanding the mode of action of antibiotics is becoming more and more important in the time that microorganisms start to develop resistance. One very well validated target of several classes of antibiotics is the peptidoglycan precursor lipid II. In this review different classes of lipid II targeting antibiotics will be discussed in detail, including the lantibiotics, human invertebrate defensins and the recently discovered teixobactin. By hitting bacteria where it hurts, at the level of lipid II, we expect to be able to develop efficient antibacterial agents in the future. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

An intimate link between antimicrobial peptide sequence diversity and binding to essential components of bacterial membranes

Antimicrobial peptides and proteins (AMPs) are widespread in the living kingdom. They are key effectors of defense reactions and mediators of competitions between organisms. They are often cationic and amphiphilic, which favors their interactions with the anionic membranes of microorganisms. Several AMP families do not directly alter membrane integrity but rather target conserved components of the bacterial membranes in a process that provides them with potent and specific antimicrobial activities. Thus, lipopolysaccharides (LPS), lipoteichoic acids (LTA) and the peptidoglycan precursor Lipid II are targeted by a broad series of AMPs. Studying the functional diversity of immune effectors tells us about the essential residues involved in AMP mechanism of action. Marine invertebrates have been found to produce a remarkable diversity of AMPs. Molluscan defensins and crustacean anti-LPS factors (ALF) are diverse in terms of amino acid sequence and show contrasted phenotypes in terms of antimicrobial activity. Their activity is directed essentially against Gram-positive or Gram-negative bacteria due to their specific interactions with Lipid II or Lipid A, respectively. Through those interesting examples, we discuss here how sequence diversity generated throughout evolution informs us on residues required for essential molecular interaction at the bacterial membranes and subsequent antibacterial activity. Through the analysis of molecular variants having lost antibacterial activity or shaped novel functions, we also discuss the molecular bases of functional divergence in AMPs. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Cysteine-stabilized αβ defensins: From a common fold to antibacterial activity

Antimicrobial peptides (AMPs) seem to be promising alternatives to common antibiotics, which are facing increasing bacterial resistance. Among them are the cysteine-stabilized αβ defensins. These peptides are small, with a length ranging from 34 to 54 amino acid residues, cysteine-rich and extremely stable, normally composed of an α-helix and three β-strands stabilized by three or four disulfide bonds and commonly found in several organisms. Moreover, animal and plant CSαβ defensins present different specificities, the first being mainly active against bacteria and the second against fungi. The role of the CSαβ-motif remains unknown, but a common antibacterial mechanism of action, based on the inhibition of the cell-wall formation, has already been observed in some fungal and invertebrate defensins. In this context, the present work aims to group the data about CSαβ defensins, highlighting their evolution, conservation, structural characteristics, antibacterial activity and biotechnological perspectives.

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.

Isolation, characterization and mechanism of action of an antimicrobial peptide from Lecythis pisonis seeds with inhibitory activity against Candida albicans

Antimicrobial peptides (AMPs) are produced by a range of organisms as a first line of defense against invaders or competitors. Owing to their broad antimicrobial activity, AMPs have attracted attention as a potential source of chemotherapeutic drugs. The increasing prevalence of infections caused by Candida species as opportunistic pathogens in immunocompromised patients requires new drugs. Lecythis pisonis is a Lecythydaceae tree that grows in Brazil. The AMPs produced by this tree have not been described previously. We describe the isolation of 12 fractions enriched in peptides from L. pisonis seeds. Of the 12 fractions, at 10 μg/ml, the F4 fraction had the strongest growth inhibitory effect (53.7%) in Candida albicans, in addition to a loss of viability of 94.9%. The F4 fraction was separated into seven sub-fractions by reversed-phase chromatography. The F4.7' fraction had the strongest activity at 10 μg/ml, inhibiting C. albicans growth by 38.5% and a 69.3% loss of viability. The peptide in F4.7' was sequenced and was found to be similar to plant defensins. For this reason, the peptide was named L. pisonis defensin 1 (Lp-Def1). The mechanism of action that is responsible for C. albicans inhibition by Lp-Def1 includes a slight increase of reactive oxygen species induction and a significant loss of mitochondrial function. The results described here support the future development of plant defensins, specifically Lp-Def1, as new therapeutic substances against fungi, especially C. albicans.

A defensin from clam Venerupis philippinarum: Molecular characterization, localization, antibacterial activity, and mechanism of action

Antimicrobial peptides (AMPs) are important mediators of the primary host defense system against microbial invasion. In the present study, we cloned and characterized a member of the invertebrate defensin from the clam Venerupis philippinarum, designated VpDef. Amino acid sequence analysis showed that VpDef was similar to defensins from marine mollusks and ticks. In non-stimulated clams, RT-PCR and immunohistochemical analysis revealed that both VpDef mRNA and the encoding peptide were constitutively expressed in hemocytes and mantles, as well as in other major tissues. VpDef transcripts were significantly induced in hemocytes at different time intervals post Vibrio anguillarum infection. The recombinant VpDef (rVpDef) showed the highest activity against Gram-positive bacteria Micrococcus luteus and less effective to Gram-negative bacteria. In addition, incubation of rVpDef with M. luteus at 1 × and 3 × MIC could induce an obvious decrease of the membrane potential and notable changes of membrane permeability in a dose-dependent manner. Membrane integrity and bacterial viability analysis also revealed that rVpDef increased the membrane permeability of M. luteus and then resulted in cell death at 2 × and 10 × MIC. Overall, these results suggest that VpDef has an important function in host defense against invasive pathogens, probably killing microbes by inducing membrane lesions.

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.

Copsin, a novel peptide-based fungal antibiotic interfering with the peptidoglycan synthesis

Fungi and bacteria compete with an arsenal of secreted molecules for their ecological niche. This repertoire represents a rich and inexhaustible source for antibiotics and fungicides. Antimicrobial peptides are an emerging class of fungal defense molecules that are promising candidates for pharmaceutical applications. Based on a co-cultivation system, we studied the interaction of the coprophilous basidiomycete Coprinopsis cinerea with different bacterial species and identified a novel defensin, copsin. The polypeptide was recombinantly produced in Pichia pastoris, and the three-dimensional structure was solved by NMR. The cysteine stabilized α/β-fold with a unique disulfide connectivity, and an N-terminal pyroglutamate rendered copsin extremely stable against high temperatures and protease digestion. Copsin was bactericidal against a diversity of Gram-positive bacteria, including human pathogens such as Enterococcus faecium and Listeria monocytogenes. Characterization of the antibacterial activity revealed that copsin bound specifically to the peptidoglycan precursor lipid II and therefore interfered with the cell wall biosynthesis. In particular, and unlike lantibiotics and other defensins, the third position of the lipid II pentapeptide is essential for effective copsin binding. The unique structural properties of copsin make it a possible scaffold for new antibiotics.

Bioactive compounds from marine mussels and their effects on human health

The consumption of marine mussels as popular seafood has increased steadily over the past decades. Awareness of mussel derived molecules, that promote health, has contributed to extensive research efforts in that field. This review highlights the bioactive potential of mussel components from species of the genus Mytilus (e.g. M. edulis) and Perna (e.g. P. canaliculus). In particular, the bioactivity related to three major chemical classes of mussel primary metabolites, i.e. proteins, lipids, and carbohydrates, is evaluated. Within the group of proteins the focus is mainly on mussel peptides e.g. those obtained by bio-transformation processes, such as fermentation. In addition, mussel lipids, comprising polyunsaturated fatty acids (PUFAs), are discussed as compounds that are well known for prevention and treatment of rheumatoid arthritis (RA). Within the third group of carbohydrates, mussel polysaccharides are investigated. Furthermore, the importance of monitoring the mussel as food material in respect to contaminations with natural toxins produced by microalgae is discussed.

Database-Guided Discovery of Potent Peptides to Combat HIV-1 or Superbugs

Antimicrobial peptides (AMPs), small host defense proteins, are indispensable for the protection of multicellular organisms such as plants and animals from infection. The number of AMPs discovered per year increased steadily since the 1980s. Over 2,000 natural AMPs from bacteria, protozoa, fungi, plants, and animals have been registered into the antimicrobial peptide database (APD). The majority of these AMPs (>86%) possess 11–50 amino acids with a net charge from 0 to +7 and hydrophobic percentages between 31–70%. This article summarizes peptide discovery on the basis of the APD. The major methods are the linguistic model, database screening, de novo design, and template-based design. Using these methods, we identified various potent peptides against human immunodeficiency virus type 1 (HIV-1) or methicillin-resistant Staphylococcus aureus (MRSA). While the stepwise designed anti-HIV peptide is disulfide-linked and rich in arginines, the ab initio designed anti-MRSA peptide is linear and rich in leucines. Thus, there are different requirements for antiviral and antibacterial peptides, which could kill pathogens via different molecular targets. The biased amino acid composition in the database-designed peptides, or natural peptides such as θ-defensins, requires the use of the improved two-dimensional NMR method for structural determination to avoid the publication of misleading structure and dynamics. In the case of human cathelicidin LL-37, structural determination requires 3D NMR techniques. The high-quality structure of LL-37 provides a solid basis for understanding its interactions with membranes of bacteria and other pathogens. In conclusion, the APD database is a comprehensive platform for storing, classifying, searching, predicting, and designing potent peptides against pathogenic bacteria, viruses, fungi, parasites, and cancer cells.

Molecular characterization of a novel antimicrobial peptide from Mytilus coruscus

Antimicrobial peptides (AMPs) are components of the innate immune responses that form the first line of host defense against pathogens. Marine mussels can produce a surprising abundance of cysteine-rich AMPs pertaining to the defensin, myticin, mytilin and mytimycin families, particularly in the circulating hemocytes. In the current study, we purified and characterized a novel cysteine-rich peptide with remarkable antibacterial activity from Mytilus coruscus and designated with myticusin-1, a 104-amino acid long polypeptide including 10 cysteine residues forming an unusual cysteine pattern. Antimicrobial assays demonstrated that myticusin-1 exhibited stronger anti-microbial properties against Gram-positive bacteria more than Gram-negative bacteria and fungus. Furthermore, myticusin-1 caused significant morphological alterations in both Sarcina luteus and Escherichia coli as shown by transmission electron microscopy (TEM). The cDNA of myticusin-1 was cloned and sequenced from the hemocytes cDNA library of M. coruscus. The mRNA transcripts of myticusin-1 are mainly detected in hemocyte, which indicates that myticusin-1 are specifically synthesized and stored in circulating hemocytes. The expression level of myticusin-1 in hemocytes was up-regulated and reached the highest level at 36 h after S. luteus challenge, which was 20-fold increase compared to that of the control group. These results indicated that myticusin-1 was involved in the host immune response against bacterial infection and might contribute to the clearance of invading bacteria.

Molecular characterization and expression of a novel big defensin (Sb-BDef1) from ark shell, Scapharca broughtonii

Big defensins, endogenous cysteine-rich antimicrobial peptides (AMPs) with antimicrobial activity and immunomodulatory property, play crucial roles in host defense against various microbial pathogens. A novel big defensin (Sb-BDef1) of ark shell Scapharca broughtonii was identified by expressed sequence tag (EST) and RACE techniques. The Sb-BDef1 cDNA contained an open reading frame (ORF) of 336-bp encoding a polypeptide of 111 amino acids with a putative signal peptide of 21 amino acid residues, followed by a putative propeptide of 11 residues and a putative mature peptide of 79 residues. The mature peptide shared the common features of big defensins, including a high hydrophobic residues region (59%) in the N-terminus, a defensin domain in the C-terminus, which perfectly corresponds to the six conserved disulfide-bonded cysteine residues involved in the formation of the internal disulfide bridges (C1-C5, C2-C4 and C3-C6) in all big defensins from mollusk, horseshoe crab and amphioxus. Quantitative real-time PCR analysis revealed that the expression of Sb-BDef1 transcript was detected in all the tissues examined from normal ark shells, and the temporal expression of Sb-BDef1 mRNA was remarkably up-regulated at 8, 16 h in hemocytes, and at 16, 24 h in hepatopancreas after Vibrio anguillarum-challenge, respectively. These results suggested that Sb-BDef1 was a constitutive and inducible acute-phase protein and should be involved in immune response of Gram-negative microbial infection in ark shell S. broughtonii.

Eurocin, a new fungal defensin: structure, lipid binding, and its mode of action

Antimicrobial peptides are a new class of antibiotics that are promising for pharmaceutical applications because they have retained efficacy throughout evolution. One class of antimicrobial peptides are the defensins, which have been found in different species. Here we describe a new fungal defensin, eurocin. Eurocin acts against a range of Gram-positive human pathogens but not against Gram-negative bacteria. Eurocin consists of 42 amino acids, forming a cysteine-stabilized α/β-fold. The thermal denaturation data point shows the disulfide bridges being responsible for the stability of the fold. Eurocin does not form pores in cell membranes at physiologically relevant concentrations; it does, however, lead to limited leakage of a fluorophore from small unilamellar vesicles. Eurocin interacts with detergent micelles, and it inhibits the synthesis of cell walls by binding equimolarly to the cell wall precursor lipid II.

The Antimicrobial Defense of the Pacific Oyster, Crassostrea gigas. How Diversity may Compensate for Scarcity in the Regulation of Resident/Pathogenic Microflora

Healthy oysters are inhabited by abundant microbial communities that vary with environmental conditions and coexist with immunocompetent cells in the circulatory system. In Crassostrea gigas oysters, the antimicrobial response, which is believed to control pathogens and commensals, relies on potent oxygen-dependent reactions and on antimicrobial peptides/proteins (AMPs) produced at low concentrations by epithelial cells and/or circulating hemocytes. In non-diseased oysters, hemocytes express basal levels of defensins (Cg-Defs) and proline-rich peptides (Cg-Prps). When the bacterial load dramatically increases in oyster tissues, both AMP families are driven to sites of infection by major hemocyte movements, together with bactericidal permeability/increasing proteins (Cg-BPIs) and given forms of big defensins (Cg-BigDef), whose expression in hemocytes is induced by infection. Co-localization of AMPs at sites of infection could be determinant in limiting invasion as synergies take place between peptide families, a phenomenon which is potentiated by the considerable diversity of AMP sequences. Besides, diversity occurs at the level of oyster AMP mechanisms of action, which range from membrane lysis for Cg-BPI to inhibition of metabolic pathways for Cg-Defs. The combination of such different mechanisms of action may account for the synergistic activities observed and compensate for the low peptide concentrations in C. gigas cells and tissues. To overcome the oyster antimicrobial response, oyster pathogens have developed subtle mechanisms of resistance and evasion. Thus, some Vibrio strains pathogenic for oysters are equipped with AMP-sensing systems that trigger resistance. More generally, the known oyster pathogenic vibrios have evolved strategies to evade intracellular killing through phagocytosis and the associated oxidative burst.

Macin family of antimicrobial proteins combines antimicrobial and nerve repair activities

The tertiary structures of theromacin and neuromacin confirmed the macin protein family as a self-contained family of antimicrobial proteins within the superfamily of scorpion toxin-like proteins. The macins, which also comprise hydramacin-1, are antimicrobially active against Gram-positive and Gram-negative bacteria. Despite high sequence identity, the three proteins showed distinct differences with respect to their biological activity. Neuromacin exhibited a significantly stronger capacity to permeabilize the cytoplasmic membrane of Bacillus megaterium than theromacin and hydramacin-1. Accordingly, it is the only macin that displays pore-forming activity and that was potently active against Staphylococcus aureus. Moreover, neuromacin and hydramacin-1 led to an aggregation of bacterial cells that was not observed with theromacin. Analysis of the molecular surface properties of macins allowed confirmation of the barnacle model as the mechanistic model for the aggregation effect. Besides being antimicrobially active, neuromacin and theromacin, in contrast to hydramacin-1, were able to enhance the repair of leech nerves ex vivo. Notably, all three macins enhanced the viability of murine neuroblastoma cells, extending their functional characteristics. As neuromacin appears to be both a functional and structural chimera of hydramacin-1 and theromacin, the putative structural correlate responsible for the nerve repair capacity in leech was located to a cluster of six amino acid residues using the sequence similarity of surface-exposed regions.

Distribution and characteristics of ABFs, cecropins, nemapores, and lysozymes in nematodes

Several groups of antimicrobial effector molecules have been identified in nematodes, but most studies have been limited to Caenorhabditis elegans and, to a lesser extent, Ascaris suum. Although these two species are not closely related, they are not representative of overall nematode diversity. This study utilized available sequence information to investigate whether four groups of antimicrobial effectors (defensin-like antibacterial factors [ABFs], cecropins, saposin domain-containing proteins, and lysozymes) are components of an archetypal nematode immune system or more narrowly restricted. Saposin domain-containing proteins (caenopores in C. elegans) and lysozymes were widely distributed and found in most taxa, but likely have digestive as well as defensive functions. ABFs were widely distributed in fewer taxa, suggesting selective loss in some lineages. In contrast, cecropins were identified in only three related species, suggesting acquisition of this effector molecule in their common ancestor.

Post-translational Modifications of Natural Antimicrobial Peptides and Strategies for Peptide Engineering

Natural antimicrobial peptides (AMPs) are gene-coded defense molecules discovered in all the three life domains: Eubacteria, Archaea, and Eukarya. The latter covers protists, fungi, plants, and animals. It is now recognized that amino acid composition, peptide sequence, and post-translational modifications determine to a large extent the structure and function of AMPs. This article systematically describes post-translational modifications of natural AMPs annotated in the antimicrobial peptide database (http://aps.unmc.edu/AP). Currently, 1147 out of 1755 AMPs in the database are modified and classified into more than 17 types. Through chemical modifications, the peptides fold into a variety of structural scaffolds that target bacterial surfaces or molecules within cells. Chemical modifications also confer desired functions to a particular peptide. Meanwhile, these modifications modulate other peptide properties such as stability. Elucidation of the relationship between AMP property and chemical modification inspires peptide engineering. Depending on the objective of our design, peptides may be modified in various ways so that the desired features can be enhanced whereas unwanted properties can be minimized. Therefore, peptide design plays an essential role in developing natural AMPs into a new generation of therapeutic molecules.

Big defensins and mytimacins, new AMP families of the Mediterranean mussel Mytilus galloprovincialis

Antimicrobial peptides (AMPs) play a fundamental role in the innate immunity of invertebrates, preventing the invasion of potential pathogens. Mussels can express a surprising abundance of cysteine-rich AMPs pertaining to the defensin, myticin, mytilin and mytimycin families, particularly in the circulating hemocytes. Based on deep RNA sequencing of Mytilus galloprovincialis, we describe the identification, molecular diversity and constitutive expression in different tissues of five novel transcripts pertaining to the macin family (named mytimacins) and eight novel transcripts pertaining to the big defensins family (named MgBDs). The predicted antimicrobial peptides exhibit a N-terminal signal peptide, a positive net charge and a high content in cysteines, allegedly organized in intra-molecular disulfide bridges. Mytimacins and big defensins therefore represent two novel AMP families of M. galloprovincialis which extend the repertoire of cysteine-rich AMPs in this bivalve mollusk.

Six defensins from the triangle-shell pearl mussel Hyriopsis cumingii

Antimicrobial peptides (AMPs) are the first line of defense of invertebrates against invading pathogens. Defensins, unique AMPs, have a cysteine-stabilized α-helix and β-sheet (CSαβ) motif. In invertebrates, defensins have been reported in arthropods and mussels. Recently, six defensins were identified from Hyriopsis cumingii for the first time, and were designated as HcDef1, HcDef2, HcDef3, HcDef4, HcDef5, and HcDef6. HcDef1 and HcDef2 encode a protein containing 61 and 60 amino acids, respectively. HcDef3, HcDef4, and HcDef6 have 65 amino acids each. HcDef5 is longer than the other five defensins, comprising 83 amino acids. HcDef3 and HcDef4 have three pairs of disulfide bonds. HcDef1, HcDef5, and HcDef6 are exceptions; each has four pairs of disulfide bonds. Evolutionary analysis revealed that only purifying selection and no positive selection could be detected in defensin genes; purifying selection might be the major evolutionary driving force in the evolution of defensin genes. The present study reveals for the first time that the defensins from H. cumingii are diverse and phylogenetic analysis showed that these 6 defensins from H. cumingii were clustered into one group. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed that HcDef1-HcDef4 could be detected in the hepatopancreas and gills whereas HcDef5-HcDef6 could only be detected in gills. In addition, the expression levels of HcDef2, HcDef3, and HcDef5 in H. cumingii with pearls were higher than that in H. cumingii without pearls. Quantitative RT-PCR analysis showed that HcDef1, HcDef2, HcDef3, and HcDef5 were downregulated by Vibrio anguillarum challenge whereas HcDef4 and HcDef6 were upregulated under Vibrio challenge. Our results suggest the roles of defensins in the innate immunity of H. cumingii.

Massively parallel amplicon sequencing reveals isotype-specific variability of antimicrobial peptide transcripts in Mytilus galloprovincialis

Background

Effective innate responses against potential pathogens are essential in the living world and possibly contributed to the evolutionary success of invertebrates. Taken together, antimicrobial peptide (AMP) precursors of defensin, mytilin, myticin and mytimycin can represent about 40% of the hemocyte transcriptome in mussels injected with viral-like and bacterial preparations, and unique profiles of myticin C variants are expressed in single mussels. Based on amplicon pyrosequencing, we have ascertained and compared the natural and Vibrio-induced diversity of AMP transcripts in mussel hemocytes from three European regions.

Methodology/Principal Findings

Hemolymph was collected from mussels farmed in the coastal regions of Palavas (France), Vigo (Spain) and Venice (Italy). To represent the AMP families known in M. galloprovincialis, nine transcript sequences have been selected, amplified from hemocyte RNA and subjected to pyrosequencing. Hemolymph from farmed (offshore) and wild (lagoon) Venice mussels, both injected with 10⁷Vibrio cells, were similarly processed. Amplicon pyrosequencing emphasized the AMP transcript diversity, with Single Nucleotide Changes (SNC) minimal for mytilin B/C and maximal for arthropod-like defensin and myticin C. Ratio of non-synonymous vs. synonymous changes also greatly differed between AMP isotypes. Overall, each amplicon revealed similar levels of nucleotidic variation across geographical regions, with two main sequence patterns confirmed for mytimycin and no substantial changes after immunostimulation.

Conclusions/Significance

Barcoding and bidirectional pyrosequencing allowed us to map and compare the transcript diversity of known mussel AMPs. Though most of the genuine cds variation was common to the analyzed samples we could estimate from 9 to 106 peptide variants in hemolymph pools representing 100 mussels, depending on the AMP isoform and sampling site. In this study, no prevailing SNC patterns related to geographical origin or Vibrio injection emerged. Whether or not the contact with potential pathogens can increase the amount of AMP transcript variants in mussels requires additional study.

Big defensins, a diverse family of antimicrobial peptides that follows different patterns of expression in hemocytes of the oyster Crassostrea gigas

Background

Big defensin is an antimicrobial peptide composed of a highly hydrophobic N-terminal region and a cationic C-terminal region containing six cysteine residues involved in three internal disulfide bridges. While big defensin sequences have been reported in various mollusk species, few studies have been devoted to their sequence diversity, gene organization and their expression in response to microbial infections.

Findings

Using the high-throughput Digital Gene Expression approach, we have identified in Crassostrea gigas oysters several sequences coding for big defensins induced in response to a Vibrio infection. We showed that the oyster big defensin family is composed of three members (named Cg-BigDef1, Cg-BigDef2 and Cg-BigDef3) that are encoded by distinct genomic sequences. All Cg-BigDefs contain a hydrophobic N-terminal domain and a cationic C-terminal domain that resembles vertebrate β-defensins. Both domains are encoded by separate exons. We found that big defensins form a group predominantly present in mollusks and closer to vertebrate defensins than to invertebrate and fungi CSαβ-containing defensins. Moreover, we showed that Cg-BigDefs are expressed in oyster hemocytes only and follow different patterns of gene expression. While Cg-BigDef3 is non-regulated, both Cg-BigDef1 and Cg-BigDef2 transcripts are strongly induced in response to bacterial challenge. Induction was dependent on pathogen associated molecular patterns but not damage-dependent. The inducibility of Cg-BigDef1 was confirmed by HPLC and mass spectrometry, since ions with a molecular mass compatible with mature Cg-BigDef1 (10.7 kDa) were present in immune-challenged oysters only. From our biochemical data, native Cg-BigDef1 would result from the elimination of a prepropeptide sequence and the cyclization of the resulting N-terminal glutamine residue into a pyroglutamic acid.

Conclusions

We provide here the first report showing that big defensins form a family of antimicrobial peptides diverse not only in terms of sequences but also in terms of genomic organization and regulation of gene expression.

Functional characterization of two defensin isoforms of the hard tick Ixodes ricinus

BACKGROUND

The immune system of ticks is stimulated to produce many pharmacologically active molecules during feeding and especially during pathogen invasion. The family of cationic peptides - defensins - represents a specific group of antimicrobial compounds with six conserved cysteine residues in a molecule.

RESULTS

Two isoforms of the defensin gene (def1 and def2) were identified in the European tick Ixodes ricinus. Expression of both genes was induced in different tick organs by a blood feeding or pathogen injection. We have tested the ability of synthetic peptides def1 and def2 to inhibit the growth or directly kill several pathogens. The antimicrobial activities (expressed as minimal inhibition concentration and minimal bactericidal concentration values) against Gram positive bacteria were confirmed, while Gram negative bacteria, yeast, Tick Borne Encephalitis and West Nile Viruses were shown to be insensitive. In addition to antimicrobial activities, the hemolysis effect of def1 and def2 on human erythrocytes was also established.

CONCLUSIONS

Although there is nothing known about the realistic concentration of defensins in I. ricinus tick body, these results suggest that defensins play an important role in defence against different pathogens. Moreover this is a first report of a one amino acid substitution in a defensins molecule and its impact on antimicrobial activity.

Primary structure and cellular localization of callinectin, an antimicrobial peptide from the blue crab

We report the complete amino acid sequence of callinectin, a 32 amino acid, proline-, arginine-rich antimicrobial peptide (AMP) with four cysteines and having the sequence WNSNRRFRVGRPPVVGRPGCVCFRAPCPCSNY-amide. The primary structure of callinectin is highly similar to arasins, AMPs recently identified in the small spider crab (Hyas araneus). Callinectin exists in three isomers that vary in the functional group on the tryptophan (W) residue. The most prevalent isomer had a hydroxy-N-formylkynurenine group, while the other two isomers had either N-formylkynurenine or hydroxy-tryptophan. Using a sequence highly similar to native callinectin, we chemically synthesized a peptide which we called callinectin-like peptide (CLP). Via immuno-electron microscopy, affinity-purified rabbit antibodies raised to CLP successfully localized the site of callinectin in blue crab hemocytes to the large electron-dense granules that are found primarily in large granule hemocytes.

Expression of defensins in non-infected araneomorph spiders

Defensins are a major family of antimicrobial peptides found throughout the phylogenetic tree. From the spider species: Cupiennius salei, Phoneutria reidyi, Polybetes pythagoricus, Tegenaria atrica, and Meta menardi, defensins belonging to the 'ancestral' class of invertebrate defensins were cloned and sequenced. The deduced amino acid sequences contain the characteristic six cysteines of this class of defensins and reveal precursors of 60 or 61 amino acid residues. The mature peptides consist of 37 amino acid residues, showing up to 70% identities with tick and scorpion defensins. In C. salei, defensin mRNA was found to be constitutively expressed in hemocytes, ovaries, subesophageal nerve mass, hepatopancreas, and muscle tissue. This is the first report presenting and comparing antimicrobial peptides belonging to the family of defensins from spiders.

Insight into invertebrate defensin mechanism of action: oyster defensins inhibit peptidoglycan biosynthesis by binding to lipid II

Three oyster defensin variants (Cg-Defh1, Cg-Defh2, and Cg-Defm) were produced as recombinant peptides and characterized in terms of activities and mechanism of action. In agreement with their spectrum of activity almost specifically directed against Gram-positive bacteria, oyster defensins were shown here to be specific inhibitors of a bacterial biosynthesis pathway rather than mere membrane-active agents. Indeed, at lethal concentrations, the three defensins did not compromise Staphylococcus aureus membrane integrity but inhibited the cell wall biosynthesis as indicated by the accumulation of the UDP-N-acetylmuramyl-pentapeptide cell wall precursor. In addition, a combination of antagonization assays, thin layer chromatography, and surface plasmon resonance measurements showed that oyster defensins bind almost irreversibly to the lipid II peptidoglycan precursor, thereby inhibiting the cell wall biosynthesis. To our knowledge, this is the first detailed analysis of the mechanism of action of antibacterial defensins produced by invertebrates. Interestingly, the three defensins, which were chosen as representative of the oyster defensin molecular diversity, bound differentially to lipid II. This correlated with their differential antibacterial activities. From our experimental data and the analysis of oyster defensin sequence diversity, we propose that oyster defensin activity results from selective forces that have conserved residues involved in lipid II binding and diversified residues at the surface of oyster defensins that could improve electrostatic interactions with the bacterial membranes.

Conventional and unconventional antimicrobials from fish, marine invertebrates and micro-algae

All eukaryotic organisms, single-celled or multi-cellular, produce a diverse array of natural anti-infective agents that, in addition to conventional antimicrobial peptides, also include proteins and other molecules often not regarded as part of the innate defences. Examples range from histones, fatty acids, and other structural components of cells to pigments and regulatory proteins. These probably represent very ancient defence factors that have been re-used in new ways during evolution. This review discusses the nature, biological role in host protection and potential biotechnological uses of some of these compounds, focusing on those from fish, marine invertebrates and marine micro-algae.

Antimicrobial peptides from marine invertebrates as a new frontier for microbial infection control

Antimicrobial peptides are widely expressed in organisms and have been linked to innate and acquired immunities in vertebrates. These compounds are constitutively expressed and rapidly induced at different cellular levels to interact directly with infectious agents and/or modulate immunoreactions involved in defense against pathogenic microorganisms. In invertebrates, antimicrobial peptides represent the major humoral defense system against infection, showing a diverse spectrum of action mechanisms, most of them related to plasma membrane disturbance and lethal alteration of microbial integrity. Marine invertebrates are widespread, extremely diverse, and constantly under an enormous microbial challenge from the ocean environment, itself altered by anthropic influences derived from industrialization and transportation. Consequently, this study reexamines the peptides isolated over the past 2 decades from different origins, bringing phyla not previously reviewed up to date. Moreover, a promising novel use of antimicrobial peptides as effective drugs in human and veterinary medicine could be based on their unusual properties and synergic counterparts as immune response humoral effectors, in addition to their direct microbicidal activity. This has been seen in many other marine proteins that are sufficiently immunogenic to humans, not necessarily in terms of antibody generation but as inflammation promoters and recruitment agents or immune enhancers.