Peptidoglycan recognition in innate immunity

Microbial Drug Resistance, 2012

Peptidoglycan recognition proteins (PGRPs) are conserved from insects to mammals and function in antibacterial immunity. We have revealed a novel mechanism of bacterial killing by innate immune system, in which mammalian PGRPs bind to bacterial cell wall or outer membrane and exploit bacterial stress defense response to kill bacteria. PGRPs enter Gram-positive cell wall at the site of daughter cell separation during cell division. In Bacillus subtilis PGRPs activate the CssR-CssS two-component system that detects and disposes of misfolded proteins exported out of bacterial cells. This activation results in membrane depolarization, production of hydroxyl radicals, and cessation of intracellular peptidoglycan, protein, RNA, and DNA synthesis, which are responsible for bacterial death. PGRPs also bind to the outer membrane in Escherichia coli and activate functionally homologous CpxA-CpxR two-component system, which also results in bacterial death. We excluded other potential bactericidal mechanisms, such as inhibition of extracellular peptidoglycan synthesis, hydrolysis of peptidoglycan, and membrane permeabilization. In vivo, mammalian PGRPs are expressed in polymorphonuclear leukocytes, skin, salivary glands, oral cavity, intestinal tract, eyes, and liver. They control acquisition and maintenance of beneficial normal gut microflora, which protects the host from enhanced inflammation, tissue damage, and colitis. Discovery and Functions of PGRPs P eptidoglycan recognition proteins (PGRPs or PGLYRPs) were discovered in 1996 in the silkworm, Bombyx mori, as the sensors of bacterial peptidoglycan that activate prophenoloxidase cascade. 44 This cascade is a part of innate immune response in invertebrates and generates quinones, which are toxic to microorganisms, and melanin pigments, which restrict spreading of the microorganisms within the host. The first PGRP was cloned in a moth Trichoplusia ni, which led to the discovery of highly conserved human and mouse orthologs. 19 Sequencing of the fruit fly, mosquito, and human genomes led to the discovery of diversified families of PGRPs, conserved from insects to mammals. 3,29,41 PGRPs function in antibacterial defenses and innate immunity. Insects have many PGRPs with diverse functions. Insect PGRPs sense bacteria and trigger host defense pathways, which generate antibacterial products, defend against infections, and regulate insects' microbiomes. Some insect PGRPs hydrolyze or nonenzymatically neutralize proinflammatory bacterial peptidoglycan and thus limit inflammation. 9,33,34 Mammals have four PGRP genes, Pglyrp1, Pglyrp2, Pglyrp3, and Pglyrp4, which were initially named PGRP-S, PGRP-L, PGRP-Ia, and PGRP-Ib (for ''short,'' ''long,'' or ''intermediate'' transcripts, respectively), by analogy to insect PGRPs. 29 PGLYRP1, PGLYRP3, and PGLYRP4 are directly bactericidal for both Gram-positive and Gram-negative bacteria, 11,28,30,37,39 and PGLYRP2 is a peptidoglycan-lytic amidase 14,40,45 and also has bactericidal activity. PGLYRP1 is mainly present in PMN's granules, PGLYRP2 is made in the liver and secreted into blood and is also induced in epithelial cells, and PGLYRP3 and PGLYRP4 are produced on the skin and mucous membranes, and in sweat, sebum, and saliva. 11,26,28-30,35-40,45 This review will first focus on the question how PGRPs kill bacteria, and then will briefly discuss the in vivo consequences of antibacterial activity of PGRPs-their effect on microbiome and inflammation. How do PGRPs Kill Bacteria?-The Initial Hypotheses Our first hypothesis was that PGRPs kill bacteria by inhibiting the transglycosylation or transpeptidation steps in peptidoglycan synthesis 10,30,39 because (a) PGRPs bind to the

Journal of Biological Chemistry, 2005

Skin and mucous membranes come in contact with external environment and protect tissues from infections by producing antimicrobial peptides. We report that human peptidoglycan recognition proteins 3 and 4 (PGLYRP3 and PGLYRP4) are secreted as 89 -115-kDa disulfide-linked homo-and heterodimers and are bactericidal against several pathogenic and nonpathogenic transient, but not normal flora, Gram-positive bacteria. PGLYRP3 and PGLYRP4 are also bacteriostatic toward all other tested bacteria, which include Gram-negative bacteria and normal flora Gram-positive bacteria. PGLYRP3 and PGLYRP4 are also active in vivo and protect mice against experimental lung infection. In contrast to antimicrobial peptides, PGLYRPs kill bacteria by interacting with their cell wall peptidoglycan, rather than permeabilizing their membranes. PGLYRP3 and PGLYRP4 are expressed in the skin, eyes, salivary glands, throat, tongue, esophagus, stomach, and intestine. Thus, we have identified the function of mammalian PGLYRP3 and PGLYRP4, and show that they are a new class of bactericidal and bacteriostatic proteins that have different structures, mechanism of actions, and expression patterns than antimicrobial peptides.

Journal of Biological Chemistry, 2003

Innate immune recognition of microbes is a complex process that can be influenced by both the host and the microbe. Drosophila uses two distinct immune signaling pathways, the Toll and immune deficiency (Imd) pathways, to respond to different classes of microbes. The Toll pathway is predominantly activated by Gram-positive bacteria and fungi, while the Imd pathway is primarily activated by Gram-negative bacteria. Recent work has suggested that this differential activation is achieved through peptidoglycan recognition protein (PGRP)-mediated recognition of specific forms of peptidoglycan (PG). In this study, we have further analyzed the specific PG molecular requirements for Imd activation through the pattern recognition receptor PGRP-LC in both cultured cell line and in flies. We found that two signatures of Gram-negative PG, the presence of diaminopimelic acid in the peptide bridge and a 1,6-anhydro form of N-acetylmuramic acid in the glycan chain, allow discrimination between Gram-negative and Grampositive bacteria. Our results also point to a role for PG oligomerization in Imd activation, and we demonstrate that elements of both the sugar backbone and the peptide bridge of PG are required for optimum recognition. Altogether, these results indicate multiple requirements for efficient PG-mediated activation of the Imd pathway and demonstrate that PG is a complex immune elicitor.

Proceedings of the National Academy of Sciences, 2007

Peptidoglycan recognition proteins (PGRPs) are highly conserved pattern-recognition molecules of the innate immune system that bind bacterial peptidoglycans (PGNs), which are polymers of alternating N -acetylglucosamine (NAG) and N -acetylmuramic acid (NAM) cross-linked by short peptide stems. Human PRGPs are bactericidal against pathogenic and nonpathogenic Gram-positive bacteria, but not normal flora bacteria. Like certain glycopeptide antibiotics (e.g., vancomycin), PGRPs kill bacteria by directly interacting with their cell wall PGN, thereby interfering with PGN maturation. To better understand the bactericidal mechanism of PGRPs, we determined the crystal structure of the C-terminal PGN-binding domain of human PGRP-Iβ in complex with NAG-NAM- l -Ala-γ- d -Glu- l -Lys- d -Ala- d -Ala, a synthetic glycopeptide comprising a complete PGN repeat. This structure, in conjunction with the previously reported NMR structure of a dimeric PGN fragment, permitted identification of major con...

Nature Reviews Immunology, 2011

All multicellular eukaryotes live in symbiotic (mutualistic or commensal) association with microorganisms, which often form complex communities on and/or in the body of the host. These microorganisms make up the host microbiome. All eukaryotes evolved in the presence of prokaryotes, and thus eukaryotes and prokaryotes have often co-evolved to use each other to their advantage. Eukaryotes acquired prokaryotes to form two of their organelles: mitochondria and chloroplasts. Multicellular eukaryotes have developed

Critical Reviews in Eukaryotic Gene Expression, 2012

Peptidoglycan recognition protein (PGRP) is an important host innate immunity arm capable of peptidoglycan and allied bacteria recognition. PGRP belongs to host pattern recognition receptors (PRRs) responsible for pathogen associated molecular patterns recognition, such as lipopolysaccharide, lipoteichoic acid, PGN, and mannose. As an essential host PRR, PGRP is well conserved from insects to mammals. The distribution, structure, function, regulation of gene expression, and evolution of PGRP from insects and mammals were summarized to furnish insights into this important molecule family.

Nature Medicine, 2011

Peptidoglycan recognition proteins (PGRPs) are conserved from insects to mammals and function in antibacterial immunity. We have revealed a novel mechanism of bacterial killing by innate immune system, in which mammalian PGRPs bind to bacterial cell wall or outer membrane and exploit bacterial stress defense response to kill bacteria. PGRPs enter Gram-positive cell wall at the site of daughter cell separation during cell division. In Bacillus subtilis PGRPs activate the CssR-CssS two-component system that detects and disposes of misfolded proteins exported out of bacterial cells. This activation results in membrane depolarization, production of hydroxyl radicals, and cessation of intracellular peptidoglycan, protein, RNA, and DNA synthesis, which are responsible for bacterial death. PGRPs also bind to the outer membrane in Escherichia coli and activate functionally homologous CpxA-CpxR two-component system, which also results in bacterial death. We excluded other potential bactericidal mechanisms, such as inhibition of extracellular peptidoglycan synthesis, hydrolysis of peptidoglycan, and membrane permeabilization. In vivo, mammalian PGRPs are expressed in polymorphonuclear leukocytes, skin, salivary glands, oral cavity, intestinal tract, eyes, and liver. They control acquisition and maintenance of beneficial normal gut microflora, which protects the host from enhanced inflammation, tissue damage, and colitis. Discovery and Functions of PGRPs P eptidoglycan recognition proteins (PGRPs or PGLYRPs) were discovered in 1996 in the silkworm, Bombyx mori, as the sensors of bacterial peptidoglycan that activate prophenoloxidase cascade. 44 This cascade is a part of innate immune response in invertebrates and generates quinones, which are toxic to microorganisms, and melanin pigments, which restrict spreading of the microorganisms within the host. The first PGRP was cloned in a moth Trichoplusia ni, which led to the discovery of highly conserved human and mouse orthologs. 19 Sequencing of the fruit fly, mosquito, and human genomes led to the discovery of diversified families of PGRPs, conserved from insects to mammals. 3,29,41 PGRPs function in antibacterial defenses and innate immunity. Insects have many PGRPs with diverse functions. Insect PGRPs sense bacteria and trigger host defense pathways, which generate antibacterial products, defend against infections, and regulate insects' microbiomes. Some insect PGRPs hydrolyze or nonenzymatically neutralize proinflammatory bacterial peptidoglycan and thus limit inflammation. 9,33,34 Mammals have four PGRP genes, Pglyrp1, Pglyrp2, Pglyrp3, and Pglyrp4, which were initially named PGRP-S, PGRP-L, PGRP-Ia, and PGRP-Ib (for ''short,'' ''long,'' or ''intermediate'' transcripts, respectively), by analogy to insect PGRPs. 29 PGLYRP1, PGLYRP3, and PGLYRP4 are directly bactericidal for both Gram-positive and Gram-negative bacteria, 11,28,30,37,39 and PGLYRP2 is a peptidoglycan-lytic amidase 14,40,45 and also has bactericidal activity. PGLYRP1 is mainly present in PMN's granules, PGLYRP2 is made in the liver and secreted into blood and is also induced in epithelial cells, and PGLYRP3 and PGLYRP4 are produced on the skin and mucous membranes, and in sweat, sebum, and saliva. 11,26,28-30,35-40,45 This review will first focus on the question how PGRPs kill bacteria, and then will briefly discuss the in vivo consequences of antibacterial activity of PGRPs-their effect on microbiome and inflammation. How do PGRPs Kill Bacteria?-The Initial Hypotheses Our first hypothesis was that PGRPs kill bacteria by inhibiting the transglycosylation or transpeptidation steps in peptidoglycan synthesis 10,30,39 because (a) PGRPs bind to the

Journal of Biological Chemistry, 2005

Cell Host & Microbe, 2010

Journal of Biological Chemistry, 2006

Drosophila peptidoglycan recognition protein (PGRP)-LCx and -LCa are receptors that preferentially recognize meso-diaminopimelic acid (DAP)-type peptidoglycan (PGN) present in Gram-negative bacteria over lysine-type PGN of Gram-positive bacteria and initiate the IMD signaling pathway, whereas PGRP-LE plays a synergistic role in this process of innate immune defense. How these receptors can distinguish the two types of PGN remains unclear. Here the structure of the PGRP domain of Drosophila PGRP-LE in complex with tracheal cytotoxin (TCT), the monomeric DAP-type PGN, reveals a buried ionic interaction between the unique carboxyl group of DAP and a previously unrecognized arginine residue. This arginine is conserved in the known DAP-type PGN-interacting PGRPs and contributes significantly to the affinity of the protein for the ligand. Unexpectedly, TCT induces infinite head-to-tail dimerization of PGRP-LE, in which the disaccharide moiety, but not the peptide stem, of TCT is positioned at the dimer interface. A sequence comparison suggests that TCT induces heterodimerization of the ectodomains of PGRP-LCx and -LCa in a closely analogous manner to prime the IMD signaling pathway, except that the heterodimer formation is nonperpetuating.