Peptidoglycan Recognition Proteins: Major Regulators of Drosophila Immunity

All eukaryotic organisms have an innate immune system characterized by germ-line encoded receptors and effector molecules, which mediate detection and clearance of microbes such as bacteria, fungi, and parasites. Vertebrate animals have, in addition to innate immune responses, evolved an adaptive immune system characterized by antibodies and T-cell receptors. Insects in general and the fruit fly in particular, have in recent years become an attractive model for the study of innate immunity. This is partly because it shares a great deal of similarity with the mammalian innate immune system. The versatility of Drosophila as a genetically tractable organism with a multitude of molecular-biology techniques available makes it an excellent tool for studying innate immunity. This thesis concerns the peptidoglycan recognition protein (PGRP) gene family in the fruit fly. The family consists of thirteen genes, of which a few have been reported to be part of the signaling pathways that regulates immune gene expression. PGRP proteins have affinity for peptidoglycan, an essential bacterial cell wall molecule. The main goal of this thesis has been to investigate which PGRP variants are required for immune pathway induction and which ligands are being recognized. Data presented show that the putative receptors have affinity for peptidoglycan, but not for lipopolysaccharide, or the fungal cell wall polymer beta-glucan. PGRP-SA, receptor of the Toll pathway, has a preference for peptidoglycan with a lysine in the crosslinking peptide. Furthermore PGRP-LCx, receptor of the Imd pathway, shows strong affinity for all types of peptidoglycan, both polymeric and monomeric. Activation of the Imd-pathway by polymeric, DAP-type, peptidoglycan is dependent on PGRP-LCx, whereas activation by monomeric peptidoglycan is dependent on both PGRP-LCx and PGRP-LCa. This result suggests that receptor dimerization is required for activation. Indeed, monomeric peptidoglycan can induce dimerization of the extracellular parts of PGRP-LCx and PGRP-LCa. However, PGRP-LCa does not have peptidoglycan affinity but is apparently only involved as an adapter molecule. These findings provide an explanation of the activation data for which a model is presented. In a search for novel PGRP receptors I found two PGRP proteins that instead displayed enzymatic activity towards peptidoglycan. They are of the N-actylmuramoyl Lalanine amidase type, which degrades peptidoglycan by splitting the crosslinking peptides from the glycan strands. The PGRP-SC1B-degraded Staphylococcus aureus peptidoglycan looses its immune elicitor capacity. This is in contrast to lysozyme-degraded peptidoglycan, which is still immune stimulating. Amino acid sequence homology showed as many as six of the thirteen Drosophila PGRPs to be potential enzymes. PGRP-SB1 is the other enzymatic PGRP described within this thesis. It has a more narrow activity spectrum as compared to SC1B implying that the fine-structure of peptidoglycan is important for the activity. In addition, PGRP-SB1 has antibacterial activity against Bacillus megaterium. In conclusion, receptor PGRP proteins binds bacterial peptidoglycan and triggers immune gene pathways and enzymatic PGRPs have the capacity to reduce the elicitor property of peptidoglycan. © 2005 Peter Mellroth ISBN 91-7155-105-0 (pp 1-58) Intellecta docusys AB, Stockholm 2005 The scientific theory I like best is that the rings of Saturn are composed entirely of lost airline luggage Mark Russell