Antimicrobial peptides (AMPs) are part of the armament that insects have developed to fight off pathogens. Insect AMPs are typically cationic and often made of less than 100 amino acid residues. Although their structures are diverse, most of the AMPs can be assigned to a limited number of families. The most common structures are represented by peptides assuming a α-helical conformation in organic solutions or disulfide-stabilized β-sheets with or without α- helical domains present. The diverse activity spectrum of these peptides may indicate different modes of action. Genetic analysis in the Drosophila model evidenced that multiple signal transduction pathways are activating the genes coding AMPs.

The production of antimicrobial peptides (AMPs) is an important aspect of host-defence in multicellular organisms. Biochemical analysis of the hemolymph of the fruit-fly Drosophila melanogaster and other Diptera has led to the discovery of eight classes of AMPs. These peptides can be grouped into three families based on their main biological targets, gram-positive bacteria (defensin), gram-negative bacteria (cecropins, drosocin, attacins, diptericin, MPAC), or fungi (drosomycin, metchnikowin). Drosophila AMPs are synthesized by the fat body in response to infection, and secreted into the blood. Most of them can also be induced in surface epithelia in a tissue-specific manner. Finally, some of them are constitutively expressed in defined tissues, such as the salivary glands or the reproductive tract. We review here the structures and activities of these AMPs, as well as the signalling cascades, which lead to their induction upon detection of infectious non-self.

We report here the isolation of three members of a new family of antimicrobial peptides from the hemolymph of shrimpsPenaeus vannamei in which immune response has not been experimentally induced. The three molecules display antimicrobial activity against fungi and bacteria with a predominant activity against Gram-positive bacteria. The complete sequences of these peptides were determined by a combination of enzymatic cleavages, Edman degradation, mass spectrometry, and cDNA cloning using a hemocyte cDNA library. The mature molecules (50 and 62 residues) are characterized by an NH₂-terminal domain rich in proline residues and a COOH-terminal domain containing three intramolecular disulfide bridges. One of these molecules is post-translationally modified by a pyroglutamic acid at the first position. Comparison of the data obtained from the cDNA clones and mass spectrometry showed that two of these peptides are probably COOH-terminally amidated by elimination of a glycine residue. These molecules with no evident homology to other hitherto described antimicrobial peptides were named penaeidins.

We have isolated from the blood of immune-challenged and untreated mussels (Mytilus edulis) antibacterial and antifungal peptides. We have characterized two isoforms of a novel 34-residue, cysteine-rich, peptide with potent bactericidal activity and partially characterized a novel 6.2-kDa antifungal peptide containing 12 cysteines. We report the presence of two members of the insect defensin family of antibacterial peptides and provide a phylogenetic analysis that indicates that mollusc and arthropod defensins have a common ancestry. Our data argue that circulating antimicrobial peptides represent an ancient host defense mechanism that predated the separation between molluscs and arthropods at the root of the Cambrian, about 545 million years ago.

We report the isolation of a novel antimicrobial peptide, bass hepcidin, from the gill of hybrid striped bass, white bass (Morone chrysops) × striped bass (M. saxatilis). After the intraperitoneal injection of Micrococcus luteus and Escherichia coli, the peptide was purified from HPLC fractions with antimicrobial activity against Escherichia coli. Sequencing by Edman degradation revealed a 21‐residue peptide (GCRFCCNCCPNMSGCGVCCRF) with eight putative cysteines. Molecular mass measurements of the native peptide and the reduced and alkylated peptide confirmed the sequence with four intramolecular disulfide bridges. Peptide sequence homology to human hepcidin and other predicted hepcidins, indicated that the peptide is a new member of the hepcidin family. Nucleotide sequences for cDNA and genomic DNA were determined for white bass. A predicted prepropeptide (85 amino acids) consists of three domains: a signal peptide (24 amino acids), prodomain (40 amino acids) and a mature peptide (21 amino acids). The gene has two introns and three exons. A TATA box and several consensus‐binding motifs for transcription factors including C/EBP, nuclear factor‐κB, and hepatocyte nuclear factor were found in the region upstream of the transcriptional start site. In white bass liver, hepcidin gene expression was induced 4500‐fold following challenge with the fish pathogen, Streptococcus iniae, while expression levels remained low in all other tissues tested. A novel antimicrobial peptide from the gill, bass hepcidin, is predominantly expressed in the liver and highly inducible by bacterial exposure.