The Drosophila Baramicin polypeptide gene protects against fungal infection

Abstract

The fruit fly Drosophila melanogaster combats microbial infection by producing a battery of effector peptides that are secreted into the haemolymph. Technical difficulties prevented the investigation of these short effector genes until the recent advent of the CRISPR/CAS era. As a consequence, many putative immune effectors remain to be formally described, and exactly how each of these effectors contribute to survival is not well characterized. Here we describe a novel Drosophila antifungal peptide gene that we name Baramicin A. We show that BaraA encodes a precursor protein cleaved into multiple peptides via furin cleavage sites. BaraA is strongly immune-induced in the fat body downstream of the Toll pathway, but also exhibits expression in other tissues. Importantly, we show that flies lacking BaraA are viable but susceptible to the entomopathogenic fungus Beauveria bassiana. Consistent with BaraA being directly antimicrobial, overexpression of BaraA promotes resistance to fungi and the IM10-like peptides produced by BaraA synergistically inhibit growth of fungi in vitro when combined with a membrane-disrupting antifungal. Surprisingly, BaraA mutant males but not females display an erect wing phenotype upon infection. Here, we characterize a new antifungal immune effector downstream of Toll signalling, and show it is a key contributor to the Drosophila antimicrobial response.

Drosophila immunity: The Drosocin gene encodes two host defence peptides with pathogen-specific roles

“…S3, similar sites noted in [20,46]), and ii) at a furin cleavage motif that separates the Drc and Btn peptides ("RVRR" in CG10816). Both cleavage motifs are common in AMP genes, including Drosophila Attacins, Defensins, Diptericins, and Baramicins, which all encode mature peptides separated by furin cleavage sites [1,20,25].…”

“…Even at 1mM, neither the Btn Thr nor Btn Ala showed any growth inhibition alone, and 100µM peptide combinations with Cecropin A showed no reduction of MIC over Cecropin A alone. 100µM represents the upper limit of AMP concentration in fly hemolymph after infection [70], and the concentration of Btn in vivo is likely much lower than this based on MALDI-TOF relative peak intensities [6,20,31,33]. As we tested Btn alone at 1mM, and at 100µM…”

“…Many AMP genes encode precursor proteins with multiple peptide products processed by furin cleavage [20]. This was initially shown for the Apidaecin gene of honey bees, which produces nine Apidaecin peptides from a single precursor [24].…”

Cecropins contribute to Drosophila host defense against a subset of fungal and Gram-negative bacterial infection

“…However, we could not identify a bacterial species or context for which flies mutant for cecropin genes alone succumb faster than wild-type. Studies of other AMPs have revealed that certain AMPs exhibit a high degree of specificity in determining host-pathogen interactions, as illustrated by the requirement of Diptericin in defense against P. rettgeri, Drosocin in defense against E. cloacae, and the recently described Daisho and Baramicin A genes in defense against Fusarium oxysporum and B. bassiana fungi, respectively (Unckless et al 2016;Hanson et al 2019aHanson et al , 2021Cohen et al 2020). Further studies may reveal bacteria for which the presence of Cecropins is essential for survival.…”

“…Drosophila AMPs also regulate the gut microbiota downstream of the Imd pathway, a function consistent with their bactericidal activity (Marra et al 2021). However, recent studies have suggested that AMP-like genes may play more subtle roles in other processes like memory formation (Barajas-Azpeleta et al 2018), an erect wing response upon infection (Hanson et al 2021), tumor control (Parvy et al 2019;Araki et al 2019), or regulation of JNK signaling in the salivary gland (Krautz et al 2020). While we confirm a primary importance for Cecropins and other AMPs in the systemic immune response, exploring the functions of AMPs in noncanonical roles is an exciting future direction of research.…”