Tissue- and Ligand-Specific Sensing of Gram-Negative Infection in Drosophila by PGRP-LC Isoforms and PGRP-LE

Abstract

The Drosophila antimicrobial response is one of the best characterized systems of pattern recognition receptor-mediated defense in metazoans. Drosophila senses Gram-negative bacteria via two peptidoglycan recognition proteins (PGRPs), membrane-bound PGRP-LC and secreted/cytosolic PGRP-LE, which relay diaminopimelic acid (DAP)-type peptidoglycan sensing to the Imd signaling pathway. In the case of PGRP-LC, differential splicing of PGRP domain-encoding exons to a common intracellular domain-encoding exon generates three receptor isoforms, which differ in their peptidoglycan binding specificities. In this study, we used Phi31-mediated recombineering to generate fly lines expressing specific isoforms of PGRP-LC and assessed the tissue-specific roles of PGRP-LC isoforms and PGRP-LE in the antibacterial response. Our in vivo studies demonstrate the key role of PGRP-LCx in sensing DAP-type peptidoglycan-containing Gram-negative bacteria or Gram-positive bacilli during systemic infection. We also highlight the contribution of PGRP-LCa/x heterodimers to the systemic immune response to Gram-negative bacteria through sensing of tracheal cytotoxin (TCT), whereas PGRP-LCy may have a minor role in antagonizing the immune response. Our results reveal that both PGRP-LC and PGRP-LE contribute to the intestinal immune response, with a predominant role of cytosolic PGRP-LE in the midgut, the central section of endodermal origin where PGRP-LE is enriched. Our in vivo model also definitively establishes TCT as the long-distance elicitor of systemic immune responses to intestinal bacteria observed in a loss-of-tolerance model. In conclusion, our study delineates how a combination of extracellular sensing by PGRP-LC isoforms and intracellular sensing through PGRP-LE provides sophisticated mechanisms to detect and differentiate between infections by different DAP-type bacteria in Drosophila.

Dermatophagoides pteronyssinus Major Allergen 1 Activates the Innate Immune Response of the Fruit Fly Drosophila melanogaster

“…Flies feeding on these fruits may ingest the corresponding peptidases, where they immediately come into contact with the intestinal epithelium. Presumably to counteract the unwanted activation of this arm of the immune system by ingested peptidases, the major PRR PGRP-LC is not present or is present at only very low levels in the midgut of the fly (36). Our own observations support this finding.…”

The Drosophila Imd Signaling Pathway

“…Ectopic expression of PGRP-LE in the fat body is sufficient to activate AMP expression, in a cell-autonomous fashion, in the absence of infection. It also was shown that cytoplasmic PGRP-LE can activate the Imd pathway, independently of PGRP-LC, by interacting with Imd (40)(41)(42)(43). PGRP-LE is the only intracellular pathogen receptor identified in Drosophila.…”

“…PGRP-LC mutant flies are still responsive toward TCT. This is due to PGRP-LE, which is found in two forms (40,41). The short form is secreted and binds PGN in the hemolymph (42) and is thought to assist Imd signaling by presenting PGN to PGRP-LC, although the export mechanism of PGRP-LE has not been characterized.…”

Friend, foe or food? Recognition and the role of antimicrobial peptides in gut immunity and Drosophila –microbe interactions

“…The Imd pathway, which is similar to the mammalian TNF and TIR-domaindependent TLR pathway, recognizes diaminopimelic acid (DAP)-type peptidoglycan found in Gram-negative bacteria and Gram-positive Bacilli, as well as peptidoglycan monomers (also referred to as TCT, for tracheal cytotoxin) of Gram-negative bacteria. The surface-bound receptor PGRP-LC and cytosolic receptor PGRP-LE are the main Imd receptors in the fat body and, as described in more detail below, are also found in the gut [29][30][31][32][33].…”

“…As in the systemic response, both the transmembrane receptor PGRP-LC and intracellular PGRP-LE recognize peptidoglycan in the gut and induce the Imd pathway cascade [31,32]. However, whereas both receptors are associated with fat body cells in the haemocoel, in the gut the activity of these receptors is regionalized, with PGRP-LC being localized to the fore-and hindgut, while PGRP-LE is restricted to the midgut [32]. AMPs are most highly expressed in the anterior midgut [67,89,90], where they are thought to reduce or eliminate potential pathogens ingested with the food.…”