A Toll pathway effector protectsDrosophilaspecifically from distinct toxins secreted by a fungus or a bacterium

Abstract

The host defense against several Gram-positive bacterial and fungal infections is mostly mediated by the Toll pathway in Drosophila, which regulates the expression of multiple genes including effectors of the innate immune response. One such potential effector is IMPPP/BaraA, a precursor protein that is processed at furin cleavage sites into an armamentarium of small DIM peptides that display a high degree of sequence similarity. We report here the generation of multiple mutants affecting this gene. Our phenotypic analysis revealed a specific sensitivity to pathogens belonging to distinct kingdoms, the Gram-positive bacterium Enterococcus faecalis and the entomopathogenic fungus Metarhizium anisopliae, only in septic injury models of infection. Unexpectedly, we failed to reveal a consistently increased microbial burden in the mutant flies infected with either of these microorganisms, opening the possibility for a role of BaraA in resilience rather than in resistance, which we were however unable to confirm. We also found that some BaraA-derived DIM peptides display an antimicrobial activity at millimolar concentrations in vitro. BaraA is additionally required for an efficient cleavage of pro-phenol oxidase into an active enzyme. BaraA is also involved in the cellular host defense, but through distinct mechanisms: it needs to be expressed in hemocytes for an efficient response solely to E. faecalis infection whereas it is required for the uptake by plasmatocytes of M. robertsii conidia. We propose a model whereby BaraA secreted by hemocytes may act at a very short range and protect host tissues or organs targeted specifically by E. faecalis. This study thus reveals an unexpected functional complexity of a potential effector of the Toll pathway in the host defense against specific infections.

Dense time-course gene expression profiling of the Drosophila melanogaster innate immune response

“…Dso1 and I were also reported to be regulated by Toll signaling and aid defense against specific filamentous fungi [49]. Finally, CG33470 (BaraA1) and IMPPP (BaraA2) were also shown to be regulated by Toll signaling [50][51][52]. These two genes show nearly identical gene counts in our dataset, which fits with the finding that they likely arose through gene duplication [52].…”

Repeated truncation of a modular antimicrobial peptide gene for neural context

“…Second, the Dmel\BaraB protein has lost the otherwise conserved signal peptide structure of the Baramicin gene family, suggesting its cellular localization differs from that of other Baramicins. Certainly BaraA IM24 is secreted, as this peptide is robustly detected in immune-induced hemolymph [24,25,38]. Likewise, Dmel\BaraC bears a couple key signatures indicating it is likely to reach the plasma membrane: i) it encodes a signal peptide that should enable entry into the secretory pathway, and ii) the IM24 domain is preceded by an "SA" dipeptidyl peptidase site; XA or XP motifs are cleaved by dipeptidyl peptidases at the cell membrane to maturate secreted peptides [39].…”

“…1A). BaraA mutants are susceptible to infection by fungi, and preliminary in vitro experiments suggest the BaraA IM10-like peptides have antifungal activity [28,30]. The remaining Baramicin domains encoding IM22 and IM24 remain uncharacterized.…”

The Drosophila Baramicin polypeptide gene protects against fungal infection

“…The name “ Baramicin ” was partly inspired by Eichero Oda’s character “Buggy,” a Bara-Bara superhuman. Finally, we further thank Dominique Ferrandon and Jianqiong Huang et al [ 57 ] for their cooperation in publishing initial descriptions of the BaraA gene, and for stimulating discussion.…”