Regulation of antimicrobial peptide gene expression in Drosophila melanogaster : Involvement of POU and NF-kB/Rel factors in innate immunity

Abstract: The fruit fly, Drosophila melanogaster, has a well-developed immune response, and microbial assault induces a rapid production of potent antimicrobial peptides (AMPs). The aim of this thesis work was to gain deeper knowledge of the regulation of AMPs in Drosophila, by the isolation and characterization of transcription factors involved in AMP gene expression. A yeast screen was designed and used to isolate Drosophila cDNAs coding for novel regulators of the CecropinA1 (CecA1) gene. Three transcription factors belonging to the POU domain (Pdm) family were isolated, Pdm1, Pdm2, and Drifter (Dfr), and subsequently verified as regulators of CecA1 in Drosophila cells. POU proteins are known to regulate a range of developmental processes, but this is the first finding of POU factors controlling AMP gene expression. Dfr and Pdm1 were further analyzed with respect to their in vivo function as AMP gene regulators. Over-expression of Dfr activated several AMP genes in non-infected flies, suggesting that Dfr is involved in constitutive expression of AMP genes. Dfr was shown to bind to a CecA1 upstream enhancer, to which the homeodomain protein Caudal (Cad) previously had been shown to bind. Co-expression of Dfr and Cad promoted very high CecA1 expression, indicating that these two transcription factors act synergistically on CecA1 in tissues where both are expressed. In Pdm1 mutant flies, several AMP genes were highly expressed prior to infection, indicating that Pdm1 functions as a repressor of those genes. However, at least one gene, AttacinA, required Pdm1 for its expression suggesting that Pdm1 has dual functions, acting both as a repressor and activator. Finally, the post-translational activation of the NF-?B/Rel protein Relish in response to infection was investigated in detail. Deletion mapping revealed different functional domains of Relish, and site-directed mutagenesis was used to exactly determine the residues required for endoproteolytic cleavage by a caspase.