Insect anal droplets contain diverse proteins related to gut homeostasis.

Background: Insects share similar fundamental molecular principles with mammals in innate immunity. For modulating normal gut microbiota, insects produce phenoloxidase (PO), which is absent in all vertebrates, and reactive nitrogen species (ROS) and antimicrobial proteins (AMPs). However, reports on insect gut phagocytosis are very few. Furthermore, most previous studies measure gene expression at the transcription level. In this study, we provided proteomic evidence on gut modulation of normal microorganisms by investigating the anal droplets from a weevil, Cryptorhynchus lapathi.

Results: The results showed that the anal droplets contained diverse proteins related to physical barriers, epithelium renewal, pattern recognition, phenoloxidase activation, oxidative defense and phagocytosis, but AMPs were not detected. According to annotations, Scarb1, integrin βν, Dscam, spondin or Thbs2s might mediate phagocytosis. As a possible integrin βν pathway, βν activates Rho by an unknown mechanism, and Rho induces accumulation of mDia, which then promotes actin polymerization.

Conclusions: Our results well demonstrated that insect anal droplets can be used as materials to investigate the defense of a host to gut microorganisms and supported to the hypothesis that gut phagocytosis occurs in insects.