Toll-related receptors and the control of antimicrobial peptide expression inDrosophila

Abstract

Insects defend themselves against infectious microorganisms by synthesizing potent antimicrobial peptides. Drosophila has appeared in recent years as a favorable model to study this innate host defense. A genetic analysis of the regulation of the antifungal peptide drosomycin has demonstrated a key role for the transmembrane receptor Toll, which prompted the search for mammalian homologs. Two of these, Toll-like receptor (TLR)2 and TLR4, recently were shown to play a critical role in innate immunity against bacteria. Here we describe six additional Toll-related genes (Toll-3 to Toll-8) in Drosophila in addition to 18-wheeler. Two of these genes, Toll-3 and Toll-4, are expressed at a low level. Toll-6, -7, and -8, on the other hand, are expressed at high levels during embryogenesis and molting, suggesting that, like Toll and 18w, they perform developmental functions. Finally, Toll-5 is expressed only in larvae and adults. By using chimeric constructs, we have tested the capacity of the signaling Toll͞IL-1R homology domains of these receptors to activate antimicrobial peptide promoters and found that only Toll and Toll-5 can activate the drosomycin promoter in transfected cells, thus demonstrating specificity at the level of the Toll͞IL-1R homology domain. In contrast, none of these constructs activated antibacterial peptide promoters, suggesting that Toll-related receptors are not involved in the regulation of antibacterial peptide expression. This result was independently confirmed by the demonstration that a dominant-negative version of the kinase Pelle can block induction of drosomycin by the cytokine Spaetzle, but does not affect induction of the antibacterial peptide attacin by lipopolysaccharide.

How Drosophila combats microbial infection: a model to study innate immunity and host–pathogen interactions

“…Given that TLR4 mediates recognition of Gram-negative bacteria in mice [52], it is tempting to speculate that one of the eight other Tolls encoded by the Drosophila genome may function in microbial recognition in the Imd pathway. However, expression of seven of the eight Toll homologues in cultured cells did not provide a clear demonstration of their function in the antibacterial defence [53].…”

The road to Toll

“…The identification of TLRs as receptors for microbial and viral ligands was not predicted by studies carried out in flies because Toll clearly does not interact directly with microbial products but is activated by an endogenous ligand, Spätzle, during the immune response 30,52 . Although the Drosophila genome encodes eight additional Toll molecules, surprisingly, none of these has yet been shown to participate directly in microbial sensing, and only one (Toll-9) has been implicated in the regulation of immune responses (using a cell-culture assay) [53][54][55] . In contrast to the rapid identification of TLRs and their ligands, the Drosophila molecules involved in microbial recognition remained unknown until recently.…”

Drosophila Toll is activated by Gram-positive bacteria through a circulating peptidoglycan recognition protein

“…The best-documented examples are the various Toll-like receptors present on mammalian immune responsive cells, which bind distinct microbial patterns to activate NF-kB 21±26 . Toll-like receptors were discovered in the wake of the demonstration that Toll was involved in the immune response of Drosophila 27 , and it was expected that Toll (and the other receptors of this nine-member family 28 ) would also function as a pattern-recognition receptor in this species. As demonstrated here, this concept applies in the systemic immune response of Drosophila not to Toll itself, but to an upstream circulating protein.…”