Antimicrobial peptides appear to be ubiquitous and multipotent components of the innate immune defense arsenal used by both prokaryotic and eukaryotic organisms. During the past 15 years a multitude of these peptides have been isolated largely from insects. In spite of great differences in size, amino acid composition and structure, most of the antimicrobial peptides from insects can be grouped into one of three categories. The largest category in number contains peptides with intramolecular disulfide bonds forming hairpin-like β-sheets or α-helical–β-sheet mixed structures. The second most important group is composed of peptides forming amphipathic α-helices. The third group comprises peptides with an overrepresentation in proline and/or glycine residues. In general, the insect antimicrobial peptides have a broad range of activity and are not cytotoxic. Despite a wealth of information on structural requirements for their antimicrobial activity, the mode of action of these peptides is not yet fully understood. However, some data suggest the existence of two types of mode of action: 1. through peptide–lipid interaction or 2. through receptor-mediated recognition processes. This review presents the main results obtained during the last four years in the field of antimicrobial peptides from insects with a special focus on the proline-rich and cysteine-rich peptides.

Antimicrobial peptides in insects; structure and function

Bioactive peptides from marine processing waste and shellfish: A review

Penaeidins, a New Family of Antimicrobial Peptides Isolated from the Shrimp Penaeus vannamei (Decapoda)

Defense mechanisms and disease prevention in farmed marine invertebrates

Antimicrobial peptides are ancient components of the innate immune system and have been isolated from organisms spanning the phylogenetic spectrum. Over an evolutionary time span, these peptides have retained potency, in the face of highly mutable target microorganisms. This fact suggests important coevolutionary influences in the host-pathogen relationship. Despite their diverse origins, the majority of antimicrobial peptides have common biophysical parameters that are likely essential for activity, including small size, cationicity, and amphipathicity. Although more than 900 different antimicrobial peptides have been characterized, most can be grouped as belonging to one of three structural classes: (1) linear, often of alpha-helical propensity; (2) cysteine stabilized, most commonly conforming to beta-sheet structure; and (3) those with one or more predominant amino acid residues, but variable in structure. Interestingly, these biophysical and structural features are retained in ribosomally as well as nonribosomally synthesized peptides. Therefore, it appears that a relatively limited set of physicochemical features is required for antimicrobial peptide efficacy against a broad spectrum of microbial pathogens. During the past several years, a number of themes have emerged within the field of antimicrobial peptide immunobiology. One developing area expands upon known microbicidal mechanisms of antimicrobial peptides to include targets beyond the plasma membrane. Examples include antimicrobial peptide activity involving structures such as extracellular polysaccharide and cell wall components, as well as the identification of an increasing number of intracellular targets. Additional areas of interest include an expanding recognition of antimicrobial peptide multifunctionality, and the identification of large antimicrobial proteins, and antimicrobial peptide or protein fragments derived thereof. The following discussion highlights such recent developments in antimicrobial peptide immunobiology, with an emphasis on the biophysical aspects of host-defense polypeptide action and mechanisms of microbial resistance.

Advances in antimicrobial peptide immunobiology

Plasma from the mussel Mytilus galloprovincialis previously immunized by injecting them with bacteria contains several bactericidal proteins. One protein, MGD-1, was purified by reverse-phase HPLC of supernatant from acidified cell-free hemolymph. Its biological activity is directed against both gram-positive and gram-negative bacteria but it is not cytotoxic towards human erythrocytes nor protozoa. As determined by mass spectrometry, the molecular mass of MGD-1 is 4418 Da. Primary-structure analysis revealed 38 amino acids including 8 cysteines and a modified amino acid residue in position 28. Computer searches unambiguously recognized the signature of an arthropod defensin, but the presence of two extra cysteines and of one modified amino acid suggest that it is a previously unknown member of that family.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1996.0302h.x

A member of the arthropod defensin family from edible Mediterranean mussels (Mytilus galloprovincialis)

Cytotoxic and antibacterial properties of Mytilus galloprovincialis, Ostrea edulis and Crassostrea gigas (bivalve molluscs) hemolymph [hemocyte lysate]

Cytotoxic and antibacterial properties of Mytilus galloprovincialis, Ostrea edulis and Crassostrea gigas (bivalve molluscs) hemolymph

Mussel hemolymph possessed naturally occurring cytotoxic activity. Vertebrate erythrocytes, mouse tumor cell line and parasite Bonamia were killed, but not bacteria. Two step chromatography revealed a complex multimeric protein of 320 kDa, acting through a polymerization process on the target cell membrane. Of naive animals from the two oyster species Crassostrea gigas and Ostrea eclulis, a low proportion of plasmas and hemocyte lysates presented naturally occurring antibacterial activity. Induction assays by injection of saline solution or bacterial suspensions resulted in increased activity in virtually all oysters. Identification of active polypeptides included acidic solubility followed by reverse phase chroma‐tographies. Oyster hemocytes responded to heat‐shock by the synthesis of particular proteins. Treated in the same way, the protozoan parasite Perkinsus marinus also synthesised long‐lasting 70 to 30 kDa proteins which are different from the oyster ones. The question arises of the biolog...

Thierry Noël

Papers

A member of the arthropod defensin family from edible Mediterranean mussels (Mytilus galloprovincialis)

Cytotoxic and antibacterial properties of Mytilus galloprovincialis, Ostrea edulis and Crassostrea gigas (bivalve molluscs) hemolymph [hemocyte lysate]

Cytotoxic and antibacterial properties of Mytilus galloprovincialis, Ostrea edulis and Crassostrea gigas (bivalve molluscs) hemolymph

Present knowledge on the molecular basis of cytotoxicity, antibacterial activity and stress response in marine bivalves