Diversity of Innate Immune Recognition Mechanism for Bacterial Polymeric meso-Diaminopimelic Acid-type Peptidoglycan in Insects

Abstract

In Drosophila, the synthesis of antimicrobial peptides in response to microbial infections is under the control of the Toll and immune deficiency (Imd) signaling pathway. The Toll signaling pathway responds mainly to the lysine-type peptidoglycan of Gram-positive bacteria and fungal ␤-1,3-glucan, whereas the Imd pathway responds to the meso-diaminopimelic acid (DAP)-type peptidoglycan of Gram-negative bacteria and certain Gram-positive bacilli. Recently we determined the activation mechanism of a Toll signaling pathway biochemically using a large beetle, Tenebrio molitor. However, DAP-type peptidoglycan recognition mechanism and its signaling pathway are still unclear in the fly and beetle. Here, we show that polymeric DAP-type peptidoglycan, but not its monomeric form, formed a complex with Tenebrio peptidoglycan recognition protein-SA, and this complex activated the three-step proteolytic cascade to produce processed Spätzle, a Toll receptor ligand, and induced Drosophila defensin-like antimicrobial peptide in Tenebrio larvae similarly to polymeric lysine-type peptidoglycan. Monomeric DAP-type peptidoglycan induced Drosophila diptericin-like antimicrobial peptide in Tenebrio hemocytes. In addition, both polymeric and monomeric DAP-type peptidoglycans induced expression of Tenebrio peptidoglycan recognition protein-SC2, which is DAP-type peptidoglycan-selective N-acetylmuramyl-L-alanine amidase that functions as a DAPtype peptidoglycan scavenger, appearing to function as a negative regulator of the DAP-type peptidoglycan signaling by cleaving DAP-type peptidoglycan in Tenebrio larvae. Taken together, these results demonstrate that molecular recognition mechanism for polymeric DAP-type peptidoglycan is different between Tenebrio larvae and Drosophila adults, providing biochemical evidences of biological diversity of innate immune responses in insects.

Endogenous egg immune defenses in the yellow mealworm beetle (Tenebrio molitor)

“…The induction of PGRP-SA and PGRP-SC2 in eggs and adults is in line with their function as a pattern recognition protein and scavenger receptor, respectively (Park et al, 2006;Yu et al, 2010). PGRP-SA in T. molitor is able to recognize both Lys-and DAP-type peptidoglycans (Park et al, 2006), which are associated with Gram-positive and Gram-negative bacteria.…”

“…This species is well known as a model for innate immunity (Chae et al, 2012;Dobson et al, 2012;Goltsev et al, 2009;Johnston et al, 2014;Lee et al, 1996;Moon et al, 1994;Park et al, 2007Park et al, , 2006Yu et al, 2010;Zhang et al, 2003;Zhu et al, 2013Zhu et al, , 2014, but has also received considerable attention with respect to TGIP (Dubuffet et al, 2015;Moreau et al, 2012;Moret, 2006;Zanchi et al, 2012). Here we show that the eggs of T. molitor can induce immune genes, including antimicrobial peptides (AMPs), to high levels upon bacterial challenge.…”

“…In this study we choose to adopt attacin D and defensin 1, respectively, for these genes, as these names confer information about the gene families discussed. Furthermore, we were able to identify coleoptericin A-C (accession All26031.1, All26032.1 and All26033.1; Zhu et al, 2014), attacin C (Dobson et al, 2012), tenecin 3 (accession Q27270.1; Lee et al, 1996), PGRP-SA (accession BAE78510.1; Park et al, 2006) and PGRP-SC2 (accession BAJ23047.1; Yu et al, 2010). We also found attacin A and B (described as attacin A and C in Johnston et al, 2014), cecropin, defensin 2 and 3, thaumatin 1-3, a putative defense protein hdd11, a GNBP and a toll receptor.…”

Transcription profiling of immune genes during parasite infection in susceptible and resistant strains of the flour beetles (Tribolium castaneum)

“…Consequently, they were highly upregulated in our treatments. PGRP-SA, a member of the peptidoglycan recognition proteins (PGRPs), serves as an important surveillance mechanism for microbial infection by binding to Lys-and diaminopimelate-type peptidoglycans of bacteria (Steiner, 2004;Yu et al, 2010). Moreover, the Toll-dependent defense against Gram-positive bacterial infections is mediated through PGRP-SA (Gobert et al, 2003), which supports the role of gene PGRP-SA in response to parasite infection in the beetle.…”

93-kDa Twin-domain Serine Protease Inhibitor (Serpin) Has a Regulatory Function on the Beetle Toll Proteolytic Signaling Cascade

“…In recent years, we have analyzed the serine protease cascade that activates the Toll signaling pathway in the larvae of a beetle, Tenebrio molitor, in considerable detail (15)(16)(17)(18). The large size of this insect enables us to collect enough hemolymph (insect blood) to purify serine proteases and serpins by biochemical methods.…”