Tenebrio molitor PGRP-LE Plays a Critical Role in Gut Antimicrobial Peptide Production in Response to Escherichia coli.

Maryam Keshavarz,Yong Hun Jo,Tariku Tesfaye Edosa,Yeon Soo Han

doi:10.3389/fphys.2020.00320

Abstract

Invading pathogens are recognized by peptidoglycan recognition proteins (PGRPs) that induce translocation of NF-κB transcription proteins and expression of robust antimicrobial peptides (AMPs). Tenebrio molitor PGRP-LE (TmPGRP-LE) has been previously identified as a key sensor of Listeria monocytogenes infection. Here, we present that TmPGRP-LE is highly expressed in the gut of T. molitor larvae and 5-day-old adults in the absence of microbial infection. In response to Escherichia coli and Candida albicans infections, TmPGRP-LE mRNA levels are significantly upregulated in both the fat body and gut. Silencing of TmPGRP-LE by RNAi rendered T. molitor significantly more susceptible to challenge by E. coli infection and, to a lesser extent, Staphylococcus aureus and C. albicans infections. Reduction of TmPGRP-LE levels in the larval gut resulted in downregulation of eight AMP genes following exposure to E. coli, S. aureus, and C. albicans. However, the transcriptional levels of AMPs more rapidly reached a higher level in the dsEGFP-treated larval gut after challenge with E. coli, which may suggest that AMPs induction were more sensitive to E. coli than S. aureus and C. albicans. In addition, TmPGRP-LE RNAi following E. coli and C. albicans challenges had notable effects on TmRelish, TmDorsal X1 isoform (TmDorX1), and TmDorX2 expression level in the fat body and gut. Taken together, TmPGRP-LE acts as an important gut microbial sensor that induces AMPs via Imd activation in response to E. coli, whereas involvement of TmPGRP-LE in AMPs synthesize is barely perceptible in the hemocytes and fat body.

Highlights

Invading infectious agents stimulate the innate immune system through complex intracellular pathways
TmPGRPLE mRNA was detected in all T. molitor larval tissues, with the highest expression level observed in the gut, followed by that in hemocytes and Malpighian tubules, while the lowest mRNA quantities were found in the integument and fat body (Figure 1A)
We further show that the involvement of TmPGRP-LE in the control of the immune deficiency (Imd) pathway is limited to only some antimicrobial peptides (AMPs) in the fat body and hemocytes of S. aureus-challenged larvae

Summary

Introduction

Invading infectious agents stimulate the innate immune system through complex intracellular pathways. There are some considerable differences in the innate immune systems of vertebrates and invertebrates, including the mechanism of interaction between terminal molecules and receptors, the signal transduction pathways involved, and the effect of signaling pathway activation on the secretion or production of specific molecules (Sheehan et al, 2018). Mammalian Toll-like receptor (TLR) and tumor necrosis factor alpha (TNFα) signaling pathways are activated in response to invading microorganisms and this results in the translocation of NF-κB transcription factors to the nucleus, which stimulate the production of pro-inflammatory cytokines and chemokines, induce apoptosis, and connect innate and adaptive immunities (Ganesan et al, 2011; Ting and Bertrand, 2016). Insect NFκB, Toll, and immune deficiency (Imd) signaling pathways, induce production of antimicrobial peptides (AMPs) (Lemaitre and Hoffmann, 2007)

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