Heliomicin

Cationic host-defence antimicrobial peptides are recognised as an important component of the innate immune response in most multicellular organisms. In humans, several antimicrobial peptides have recently been recognised as key factors in the pathology of diseases such as cystic fibrosis, septic shock, atopic dermatitis and morbus Kostmann. To date, several hundred cationic antimicrobial peptides have been characterised. They are amphipathic peptides, comprising 20 to 50 amino acids, and exhibiting large structural diversity. These peptides display a broad spectrum of activity against bacterial, fungal and viral pathogens. Their mode of action is best known for cecropins and magainins, which act upon the cytoplasmic membrane of microorganisms, causing its disruption by a detergent-like activity and pore formation. In the last few years, several of these peptides or analogues (derived from magainin, protegrin, indolicidin and histatin) were in advanced clinical development, especially for localised infections (oral and cutaneous infections, pneumonias etc.). Several other molecules (rBPI, heliomicin and thanatin) are currently under development for various systemic infections (Staphylococcus sp., Aspergillus sp., Candida sp. etc.) and may represent important additions to the anti-infectious therapeutic arsenal.

In response to an experimental infection, the lepidopteran Heliothis virescens produces an antifungal protein named heliomicin. Heliomicin displays sequence similarities with antifungal plant defensins and antibacterial or antifungal insect defensins. To gain information about the structural elements required for either antifungal or antibacterial activity, heliomicin and selected point-mutated variants were expressed in yeast as fusion proteins. The effects of mutations, defined by comparing the primary structure of heliomicin with the sequences of members of the insect defensin family, were analyzed using antibacterial and antifungal assays. One of the variants shows significant activity against Gram-positive bacteria while remaining efficient against fungi. The three-dimensional structures of this variant and of the wild-type protein were determined by two-dimensional (1)H NMR to establish a correlation between structure and antibacterial or antifungal activity. Wild-type and mutated heliomicins adopt a similar scaffold, including the so-called cysteine-stabilized alphabeta motif. A comparison of their structures with other defensin-type molecules indicates that common hydrophobic characteristics can be assigned to all the antifungal proteins. A comparative analysis of various structural features of heliomicin mutant and of antibacterial defensins enables common properties to be assessed, which will help to design new mutants with increased antibacterial activity.

ETD151, an analogue of the antifungal insect defensin heliomicin, is an antifungal peptide active against yeasts and filamentous fungi. To decipher the mechanisms underlying its molecular action on the phytopathogenic fungus Botrytis cinerea, a necrotrophic pathogen responsible for gray mold disease, we investigated the changes in 3 day-old mycelia upon treatment with different concentrations of ETD151. Optical and fluorescence microscopies were used prior to establishing the peptide/protein profiles through two mass spectrometry approaches: MALDI profiling, to generate molecular mass fingerprints as peptide signatures, and a gel-free bottom-up proteomics approach. Our results show that a concentration of ETD151 above the half-maximal inhibitory concentration can alter the integrity of the mycelial structure of B. cinerea. Furthermore, reproducible modifications of the peptide/protein composition were demonstrated in the presence of ETD151 within a 1500-16,000 mass (m/z) range. After the robustness of LC-ESI-MS/MS analysis on B. cinerea mycelial extracts was confirmed, our analyses highlighted 340 significantly modulated proteins upon treatment with ETD151 within a 4.8-466 kDa mass range. Finally, data mapping on KEGG pathways revealed the molecular impact of ETD151 on at least six pathways, namely, spliceosome, ribosome, protein processing in endoplasmic reticulum, endocytosis, MAPK signaling pathway, and oxidative phosphorylation.