The Negative Regulative Roles of BdPGRPs in the Imd Signaling Pathway of Bactrocera dorsalis.

Ping Zhang,Hongyu Zhang,Shuai Bai,Zhichao Yao

doi:10.3390/cells11010152

Abstract

Peptidoglycan recognition proteins (PGRPs) are key regulators in insects’ immune response, functioning as sensors to detect invading pathogens and as scavengers of peptidoglycan (PGN) to reduce immune overreaction. However, the exact function of PGRPs in Bactrocera dorsalis is still unclear. In this study, we identified and functionally characterized the genes BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2 in B. dorsalis. The results showed that BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2 all have an amidase-2 domain, which has been shown to have N-Acetylmuramoyl-l-Alanine amidase activity. The transcriptional levels of BdPGRP-LB and BdPGRP-SC2 were both high in adult stages and midgut tissues; BdPGRP-SB1 was found most abundantly expressed in the 2nd instar larvae stage and adult fat body. The expression of BdPGRP-LB and BdPGRP-SB1 and AMPs were significantly up-regulated after injury infected with Escherichia coli at different time points; however, the expression of BdPGRP-SC2 was reduced at 9 h, 24 h and 48 h following inoculation with E. coli. By injection of dsRNA, BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2 were knocked down by RNA-interference. Silencing of BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2 separately in flies resulted in over-activation of the Imd signaling pathway after bacterial challenge. The survival rate of the ds-PGRPs group was significantly reduced compared with the ds-egfp group after bacterial infection. Taken together, our results demonstrated that three catalytic PGRPs family genes, BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2, are important negative regulators of the Imd pathway in B. dorsalis.

Highlights

Insects come into contact with many kinds of pathogenic microorganisms from their habitat, and insects have involved a strong innate immune system to resist microbial challenge
BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2 all have conserved amino acid Arg, which is necessary for the recognition of DAP-type peptidoglycan [32]
BdPGRP-SC2 has conserved H61, H75, H169, W81 and Y86 for Zn2+ binding and amidase activity (Figure 1C). These results indicate that BdPGRP-LB, BdPGRP-SB1 and BdPGRP-SC2 belong to the catalytic peptidoglycan recognition proteins (PGRPs)

Summary

Introduction

Insects come into contact with many kinds of pathogenic microorganisms from their habitat, and insects have involved a strong innate immune system to resist microbial challenge. This system immediately responds against invading pathogens, and consists of cellular and humoral immune responses [1]. The activation of a series of antimicrobial defense mechanisms relies on a microbial sensing system of pattern-recognition receptors (PRRs) [2]. Peptidoglycan recognition proteins (PGRPs) are a major class of PRRs that can recognize peptidoglycan (PGN), the specific component of the cell wall in both Gram-positive and Gram-negative bacteria [3,4]. With the progress of genome projects for different species, PGRP and its homologues have been identified in animals ranging from insects to mammals [8,9,10,11]

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Conclusion