Downregulation of the Musca domestica peptidoglycan recognition protein SC (PGRP-SC) leads to overexpression of antimicrobial peptides and tardy pupation

PGRP (peptidoglycan recognition protein) is a conserved protein family that recognizes the peptidoglycan in bacterial cell wall and causes the activation of various innate immune responses. Previous studies have reported that PGRP-SCs in Drosophila dampen the activation of Immune Deficiency (Imd) pathway to microbial infection, and participate in the lifespan extension of the insects. To facilitate understanding the function of PGRP-SCs from an evolutionary angle, we identified and functionally characterized the PGRP-SC gene in the housefly Musca domestica, a species that has adapted to a septic environment much harsher than the natural habitat of Drosophila. The gene designated as MdPGRP-SC was found most abundantly expressed in the 3rd instar larvae, and is expressed at this developmental stage predominantly in the gut. MdPGRP-SC was virtually unchanged in whole larvae after a septic injury at the second larval instar, while two antimicrobial peptides (AMPs), diptericin and attacin, were upregulated in the first 24h but not later. Through dsRNA microinjection, MdPGRP-SC was knocked down by RNA interference (RNAi), and caused the significant increased expression of diptericin and attacin. The pupation of MdPGRP-SC-depleted larvae was severely suppressed compared to controls. Opposite to the expression trend of MdPGRP-SC, a spontaneous active expression of diptericin and attacin was found in pre-pupae but not in third instar larvae. Taken together, our study reveals that downregulation of MdPGRP-SC leads to the overexpression of the AMPs, and is involved in the larvae-to-pupa transition of housefly.