Three clustered serine proteases, Hayan, Persephone and Skanda, regulate Toll pathway-mediated immunity in Drosophila

Abstract

Drosophila melanogaster is an ideal model system for characterising innate immune pathways. Studying the immune response of Drosophila allows us to understand their functioning in mammals and other insects, providing us with a multitude of environmental and clinical applications. Two of these immune pathways, Toll signalling and melanisation, involved in combatting Gram-positive bacteria and fungi, are activated by extracellular serine protease cascades. Two clustered serine proteases, Hayan and Psh, play similar roles in regulating these cascades. Skanda (CG15046) is an uncharacterised serine protease homolog also clustered with Hayan and Psh. Its expression is upregulated in the fly upon infection, suggesting that it might be involved in the Drosophila immune response. In this study, we investigate whether Skanda, like its neighbours Hayan and Psh, does indeed function in Drosophila immunity. We reveal that Skanda is required for resistance to Gram-positive bacteria, particularly Staphylococcus aureus. We use compound mutants of the Hayan-psh-Skanda gene cluster to uncover interesting phenotypes that might be missed while using single mutants. We reveal that Skanda redundantly regulates the Toll pathway along with Psh, and psh-Skanda double mutants have no functional Toll response, similar to Hayan-psh double mutants. Therefore, we hypothesize that the Hayan-psh-Skanda cluster allows for the differential regulation and precise tuning of the Toll and melanisation pathways.