Abstract
Peptidoglycan recognition proteins (PGRPs) are a family of innate immune molecules that recognize bacterial peptidoglycan. PGRP-LE, a member of the PGRP family, selectively binds to diaminopimelic acid (DAP)-type peptidoglycan to activate both the immune deficiency (Imd) and proPhenoloxidase (proPO) pathways in insects. A PGRP-LE-dependent induction of autophagy to control Listeria monocytogenes has also been reported. We identified and partially characterized a novel PGRP-LE homologue, from Tenebrio molitor and analyzed its functional role in the survival of the insect against infection by a DAP-type PGN containing intracellular pathogen, L. monocytogenes. The cDNA is comprised of an open reading frame (ORF) of 990 bp and encodes a polypeptide of 329 residues. TmPGRP-LE contains one PGRP domain, but lacks critical residues for amidase activity. Quantitative RT-PCR analysis showed a broad constitutive expression of the transcript at various stages of development spanning from larva to adult. RNAi mediated knockdown of the transcripts, followed by a challenge with L. monocytogenes, showed a significant reduction in survival rate of the larvae, suggesting a putative role of TmPGRP-LE in sensing and control of L. monocytogenes infection in T. molitor. These results implicate PGRP-LE as a defense protein necessary for survival of T. molitor against infection by L. monocytogenes.
Highlights
Innate immunity serves as a major line of defense in insects and recognizes, modulates and signals effector functions against invading pathogens
A single expressed sequence tag (EST) homologous to known and fully characterized Peptidoglycan recognition proteins (PGRPs) of other organisms was identified from the sequencing of random clones of T. molitor cDNA library
The TmPGRP-LE open reading frame (ORF) is comprised of 990 nucleotides encoding a polypeptide of 329 amino acids with a predicted molecular weight of 37.3 kDa
Summary
Innate immunity serves as a major line of defense in insects and recognizes, modulates and signals effector functions against invading pathogens. The first important step in innate immunity is the recognition of pathogen-associated molecular patterns (PAMPs), which are exclusively found in microbes as dangerous, non-self entities This role is accomplished through the activity of various molecular families capable of sensing the presence of invading microbial pathogens in either hemolymph or various immune cells (hemocytes, fat cells, gut epithelial cells) of host organisms [1]. PGRP-LE preferentially activates Imd signaling pathway in a non-cell autonomous manner and depends mainly on the activity of another member of the family, PGRP-LC [11] Such a synergistic action of PGRP-LE and PGRP-LC has been reported in Drosophila, wherein the single mutants (PGRP-LE112 and PGRP-LC7454) showed complete resistance but the double mutants (PGRP-LE112/PGRP-LC7454) were found susceptible against E. coli. L. monocytogenes is not a natural pathogen of insects, it is generally accepted as a convenient tool for addressing innate immune and antibacterial defense of insect hosts
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