Les peptides antimicrobiens de la drosophile.

Abstract

En reponse a une infection experimentale, le corps gras des insectes (equivalent fonctionnel du foie des mammiferes) synthetise plusieurs peptides antimicrobiens. Chez la drosophile, on a caracterise sept peptides antimicrobiens qui sont secretes dans le sang a des concentrations variant de 1 <mu>M a 100 <mu>M. Ces molecules appartiennent aux quatre familles de peptides antimicrobiens d' insectes: (1) les peptides constitues d' helices a; (2) les peptides a ponts disulfure; (3) les peptides riches en residus proline; et (4) les peptides riches en residus glycine. La reponse immunitaire humorale des insectes presente des similitudes avec la reponse inflammatoire de phase aigue des mammiferes et avec la reponse immunitaire des plantes. En effet, les defensines d' insectes presentent des similitudes structurales avec les defensines des mammiferes ainsi qu' avec des peptides antifongiques de plantes.

Insects are remarkably resistant to microbial infections. Their host defense relies on cellular and humoral responses. The cellular mechanism involves phagocytosis and encapsulation of pathogens. The humoral response includes activation of proteolytic cascades leading to coagulation and melanization and synthesis of antimicrobial peptides acting in the hemolymph to fight the infection. Over the last years, Drosophila melanogaster has become a favourite model to investigate the molecular mechanisms of insect immunity. In Drosophila, seven distinct molecules plus isoforms have been characterized, the antibacterial peptides cecropins, defensin, drosocin, diptericin and attacins, the antifungal drosomycin and metchnikowins which are active against both bacteria and fungi. These molecules belong to the 4 families of insect antimicrobial peptides namely (1) peptides forming alpha helices, (2) cysteine-rich peptides, (3) proline-rich peptides and (4) peptides with a high content in glycine residues. These antibiotics have in common a positive net charge which allows them to interact with the membrane of the microorganisms. In response to an experimental infection, the overall hemolymph concentration of Drosophilla antimicrobial peptides synthesized by the fat body (a functional equivalent of the mammalian liver) reaches the value of 200 muM, half of which is accounted for drosomycin. Recent studies on the regulation of the antimicrobial peptide gene expression during the systemic Drosophila immune response has revealed striking similarities with vertebrate innate immunity. [References: 18]