Peptidoglycan recognition proteins: modulators of the microbiome and inflammation

Purification and HPLC Analysis of Cell Wall Muropeptides from Caulobacter crescentus

The peptidoglycan sacculus, or cell wall, is what defines bacterial cell shape. Cell wall composition can be best characterized at the molecular level by digesting the peptidoglycan murein polymer into its muropeptide subunits and quantifying the abundance of muropeptides using high-pressure liquid chromatography. Certain features of the cell wall including muropeptide composition, glycan strand length, degree of crosslinking, type of crosslinking and other peptidoglycan modifications can be quantified using this approach. Well-established protocols provide us with highly-resolved and quantitatively reproducible chromatographic data, which can be used to investigate bacterial cell wall composition under a variety of environmental or genetic perturbations. The method described here enables the purification of muropeptide samples, their quantification by HPLC, and fraction collection for peak identification by mass spectrometry. Although the methods for peptidoglycan purification and HPLC analysis have been previously published, our method includes important details on how to re-equilibrate the column between runs to allow for automated analysis of multiple samples.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Identification of cells expressing two peptidoglycan recognition proteins in the gill of the vent mussel, Bathymodiolus septemdierum

In symbiotic systems in which symbionts are transmitted horizontally, hosts must accept symbionts from the environment while defending themselves against invading pathogenic microorganisms. How they distinguish pathogens from symbionts and how the latter evade host immune defences are not clearly understood. Recognition of foreign materials is one of the most critical steps in stimulating immune responses, and pattern recognition receptors (PRRs) play vital roles in this process. In this study, we focused on a group of highly conserved PRRs, peptidoglycan recognition proteins (PGRPs), in the deep-sea mussel, Bathymodiolus septemdierum, which harbours chemosynthetic bacteria in their gill epithelial cells. We isolated B. septemdierum PGRP genes BsPGRP-S and BsPGRP-L, which encode a short- and a long-type PGRP, respectively. The short-type PGRP has a signal peptide and was expressed in the asymbiotic goblet mucous cells in the gill epithelium, whereas the long-type PGRP was predicted to include a transmembrane domain and was expressed in gill bacteriocytes. Based on these findings, we hypothesize that the secreted and transmembrane PGRPs are engaged in host defence against pathogenic bacteria and/or in the regulation of symbiosis via different cellular localizations and mechanisms.

Characterization of immune-related PGRP gene expression and phenoloxidase activity in Cry1Ac-susceptible and -resistant Plutella xylostella (L.)

Peptidoglycan recognition proteins (PGRPs) are important recognition receptors which play a critical role in signal identification and transmission in Toll or immune deficiency (IMD) pathways, particularly when pathogens evade and circumvent reactive oxygen species. Antimicrobial peptides (AMPs) synthesis can be activated by these signals to further eliminate pathogens. In this study, we cloned and characterized three different PGRP genes in Plutella xylostella strains, DBM1Ac-S, DBM1Ac-R and a field strain (DBMF). The results showed that PGRP1 belongs to the PGRP-SA family, PGRP2 to PGRP-LB, and PGRP3 to PGRP-LF. Moreover, PGRP1 expressed the highest transcript level, followed by PGRP3 and PGRP2, in two tissues including the gut and the larval carcass tissues of the DBM1Ac-S strain. Furthermore, altered expression levels of PGRP1-3 genes were detected in both gut and carcass tissues. Moreover, the DBM1Ac-R strain had the highest phenol oxidase (PO) activity among these three strains. The characterization of PGRP gene expression and PO activity in DBM1Ac-S, DBM1Ac-R and DBM-F provides insights into their important physiological roles in the immune system of P. xylostella exposed to Bt Cry1Ac toxin.

Bacterial peptidoglycans as novel signaling molecules from microbiota to brain

Mounting evidence indicates that gut microbiota exerts a broad range of effects on host physiology and development beyond the gastrointestinal tract, including the modulation of brain development. However, the mechanisms mediating the interactions between the microbiota and the developing brain are still poorly understood. Pattern recognition receptors of the innate immune system that recognize microbial products, such as peptidoglycans have emerged as potential key regulators of gut microbiome-brain interactions. Peptidoglycan-sensing molecules are expressed in the placenta and brain during specific time windows of development. Moreover, peptidoglycans are ubiquitously present in circulation and can cross the blood brain barrier. This review brings together the current evidence supporting a broad function of peptidoglycans well beyond host's immunity, extending to neurodevelopment and behavior.

Peptidoglycan Recognition Protein 4 Limits Bacterial Clearance and Inflammation in Lungs by Control of the Gut Microbiota

Streptococcus pneumoniae is the most frequent cause of community-acquired pneumonia. Endogenous host defense molecules such as peptidoglycan recognition protein 4 (PGLYRP4) might influence the course of this disease. To the best of our knowledge, there are no reports on the relevance of PGLYRP4 in pneumonia. Therefore, wild type (WT) and PGLYRP4-deficient (PGLYRP4KO) mice were analyzed in an in vivo and in vitro experimental setting to examine the influence of PGLYRP4 on the course of pneumococcal pneumonia. Furthermore, caecal 16S rRNA microbiome analysis was performed, and microbiota were transferred to germfree WT mice to assess the influence of microbiotal communities on the bacterial burden. Mice lacking PGLYRP4 displayed an enhanced bacterial clearance in the lungs, and fewer mice developed bacteremia. In addition, an increased recruitment of immune cells to the site of infection, and an enhanced bacterial killing by stronger activation of phagocytes could be shown. This may depend partly on the detected higher expression of complement factors, interferon-associated genes, and the higher pro-inflammatory cytokine response in isolated primary PGLYRP4KO vs. WT cells. This phenotype is underlined by changes in the complexity and composition of the caecal microbiota of PGLYRP4KO compared to WT mice. Strikingly, we provided evidence, by cohousing and stable transfer of the respective WT or PGLYRP4KO mice microbiota into germfree WT mice, that the changes of the microbiota are responsible for the improved clearance of S. pneumoniae lung infection. In conclusion, the deficiency of PGLYRP4, a known antibacterial protein, leads to changes in the gut microbiota. Thus, alterations in the microbiota can change the susceptibility to S. pneumoniae lung infection independently of the host genotype.

Molecular and functional characterization of a short-type peptidoglycan recognition protein, PGRP-S in the amphibian Xenopus laevis

Peptidoglycan recognition proteins (PGRPs) are a family of pattern recognition receptors (PRRs) involved in host antibacterial responses, and their functions have been characterized in most invertebrate and vertebrate animals. However, little information is available regarding the function of frog PGRPs. In this study, a short-type PGRP (termed Xl-PGRP-S) gene was identified in the African clawed frog, Xenopus laevis. The predicted protein of Xl-PGRP-S contains several structural features known in PGRPs, including a typical PGRP domain and two closely spaced conserved cysteines. Xl-PGRP-S gene was constitutively expressed in all tissues examined, with the highest expression level observed in muscle. As a typical PRR, Xl-PGRP-S is inducible after peptidoglycan (PGN) stimulation, and has an ability to bind PGN. In addition, Xl-PGRP-S has been proven to have Zn²⁺-dependent amidase activity and antibacterial activity against Edwardsiella tarda. The present study represents the first discovery on the function of frog PGRPs, thus contributing to a better understanding of the functional evolution of PGRPs in early tetrapods.

Genome-Wide SNPs and InDels Characteristics of Three Chinese Cattle Breeds

Simple Summary

Whole-genome resequencing is an important tool to reveal the in-depth genomic characteristics of a genome. Adaptability traits are key to the survival of the south Chinese zebu cattle. However, the potential genetic information behind these remarkable traits still remains uncertain and needs to be addressed. In the current study, we utilized a total of 15 local south Chinese cattle samples (Leiqiong (LQ), Wannan (WN), Wenshan (WS)) from one of our previous studies mapped to the old reference genome (Btau_5.0.1) and remapped them to the latest reference genome (ARS-UCD1.2) to explore potential single nucleotide polymorphisms (SNPs) and insertions-deletions (InDels) responsible for some important immune related traits. The present study emphasizes and illustrates the genetic diversity, extending our previous study. The InDel annotation show that WS cattle had more enriched genes associated with immune functions than the other two breeds. Our findings provide valuable resources for further investigation of the functions of SNP- and InDels-related genes and help to determine the molecular basis of adaptive mutations in Chinese zebu cattle.

Abstract

We report genome characterization of three native Chinese cattle breeds discovering ~34.3 M SNPs and ~3.8 M InDels using whole genome resequencing. On average, 10.4 M SNPs were shared amongst the three cattle breeds, whereas, 3.0 M, 4.9 M and 5.8 M were specific to LQ, WN and WS breeds, respectively. Gene ontology (GO)analysis revealed four immune response-related GO terms were over represented in all samples, while two immune signaling pathways were significantly over-represented in WS cattle. Altogether, we found immune related genes (PGLYRP2, ROMO1, FYB2, CD46, TSC1) in the three cattle breeds. Our study provides insights into the genetic basis of Chinese indicine adaptation to the tropic and subtropical environment, and provides a valuable resource for further investigations of genetic characteristics of the three breeds.

Paraimmunobiotic Bifidobacteria Modulate the Expression Patterns of Peptidoglycan Recognition Proteins in Porcine Intestinal Epitheliocytes and Antigen Presenting Cells

Peptidoglycan recognition proteins (PGLYRPs) are a family of pattern recognition receptors (PRRs) that are able to induce innate immune responses through their binding to peptidoglycan (PGN), lipopolysaccharide, or lipoteichoic acid, or by interacting with other PRR-ligands. Recently, progress has been made in understanding the immunobiology of PGLYRPs in human and mice, however, their functions in livestock animals have been less explored. In this study, we characterized the expression patterns of PGLYRPs in porcine intestinal epithelial (PIE) cells and antigen-presenting cells (APCs) and their modulation by the interactions of host cells with PRR-ligands and non-viable immunomodulatory probiotics referred to as paraimmunobiotics. We demonstrated that PGLYRP-1, -2, -3, and -4 are expressed in PIE cells and APCs from Peyer’s patches, being PGLYPR-3 and -4 levels higher than PGLYRP-1 and -2. We also showed that PGLYRPs expression in APCs and PIE cells can be modulated by different PRR agonists. By using knockdown PIE cells for TLR2, TLR4, NOD1, and NOD2, or the four PGLYRPs, we demonstrated that PGLYRPs expressions would be required for activation and functioning of TLR2, TLR4, NOD1, and NOD2 in porcine epitheliocytes, but PGLYRPs activation would be independent of those PRR expressions. Importantly, we reported for the first time that PGLYRPs expression can be differentially modulated by paraimmunobiotic bifidobacteria in a strain-dependent manner. These results provide evidence for the use of paraimmunobiotic bifidobacteria as an alternative for the improvement of resistance to intestinal infections or as therapeutic tools for the reduction of the severity of inflammatory damage in diseases in which a role of PGLYRPs-microbe interaction has been demonstrated.

A general protein O-glycosylation machinery conserved in Burkholderia species improves bacterial fitness and elicits glycan immunogenicity in humans

The Burkholderia genus encompasses many Gram-negative bacteria living in the rhizosphere. Some Burkholderia species can cause life-threatening human infections, highlighting the need for clinical interventions targeting specific lipopolysaccharide proteins. Burkholderia cenocepacia O-linked protein glycosylation has been reported, but the chemical structure of the O-glycan and the machinery required for its biosynthesis are unknown and could reveal potential therapeutic targets. Here, using bioinformatics approaches, gene-knockout mutants, purified recombinant proteins, LC-MS-based analyses of O-glycans, and NMR-based structural analyses, we identified a B. cenocepacia O-glycosylation (ogc) gene cluster necessary for synthesis, assembly, and membrane translocation of a lipid-linked O-glycan, as well as its structure, which consists of a β-Gal-(1,3)-α-GalNAc-(1,3)-β-GalNAc trisaccharide. We demonstrate that the ogc cluster is conserved in the Burkholderia genus, and we confirm the production of glycoproteins with similar glycans in the Burkholderia species: B. thailandensis, B. gladioli, and B. pseudomallei Furthermore, we show that absence of protein O-glycosylation severely affects bacterial fitness and accelerates bacterial clearance in a Galleria mellonella larva infection model. Finally, our experiments revealed that patients infected with B. cenocepacia, Burkholderia multivorans, B. pseudomallei, or Burkholderia mallei develop O-glycan-specific antibodies. Together, these results highlight the importance of general protein O-glycosylation in the biology of the Burkholderia genus and its potential as a target for inhibition or immunotherapy approaches to control Burkholderia infections.

Innate Immune Recognition: An Issue More Complex Than Expected

Primary interaction of an intracellular bacterium with its host cell is initiated by activation of multiple signaling pathways in response to bacterium recognition itself or as cellular responses to stress induced by the bacterium. The leading molecules in these processes are cell surface membrane receptors as well as cytosolic pattern recognition receptors recognizing pathogen-associated molecular patterns or damage-associated molecular patterns induced by the invading bacterium. In this review, we demonstrate possible sequences of events leading to recognition of Francisella tularensis, present findings on known mechanisms for manipulating cell responses to protect Francisella from being killed, and discuss newly published data from the perspective of early stages of host-pathogen interaction.

Phagocytosis in Drosophila: From molecules and cellular machinery to physiology

Phagocytosis is an evolutionarily conserved mechanism that plays a key role in both host defence and tissue homeostasis in multicellular organisms. A range of surface receptors expressed on different cell types allow discriminating between self and non-self (or altered) material, thus enabling phagocytosis of pathogens and apoptotic cells. The phagocytosis process can be divided into four main steps: 1) binding of the phagocyte to the target particle, 2) particle internalization and phagosome formation, through remodelling of the plasma membrane, 3) phagosome maturation, and 4) particle destruction in the phagolysosome. In this review, we describe our present knowledge on phagocytosis in the fruit fly Drosophila melanogaster, assessing each of the key steps involved in engulfment of both apoptotic cells and bacteria. We also assess the physiological role of phagocytosis in host defence, development and tissue homeostasis.

Spp38 MAPK participates in maintaining the homeostasis of hemolymph microbiota in Scylla paramamosain

The p38 mitogen-activated protein kinases (MAPKs) are evolutionally conserved from yeasts to mammals, and are involved in the regulation of cells response to various extracellular stimuli. In this study, the p38 MAPK gene (designated as Spp38) of mud crab (Scylla paramamosain) was identified and studied. Spp38 contained the conserved Thr-Gly-Tyr (TGY) motif and a Ala-Thr-Arg-Trp (ATRW) substrate-binding site. Spp38 transcript was ubiquitously expressed in all tissues examined, with the highest expression found in muscle and hepatopancras. Quantitative real-time PCR revealed that Spp38 was upregulated in hemocytes and hepatopancras after infection with Vibrio parahemolyticus and Lipopolysaccharides (LPS). Reporter gene assays indicated that Spp38 activated the expression of anti-lipopolysaccharides (SpALF1 - SpALF6) in S. paramamosian. RNA interference (RNAi)-mediated knockdown of Spp38 or inhibition of Spp38 by SB203580 decreased the expression levels of SpALF1-6 and dual oxidase (SpDuox1 and SpDuox2) in S. paramamosian, which consequently reduced reactive oxygen species (ROS) production thereby significantly increasing the bacterial count in the hemolymph of mud crabs. Similarly, there was a significant reduction in bacterial clearance ability of hemolymph after Spp38 knockdown followed by V. parahemolyticus infection. Taken together, the current data indicated that Spp38 could play a vital role in maintaining the homeostasis of hemolymph microbiota in S. paramamosain.

Functional characterization of short-type peptidoglycan recognition proteins (PGRPs) from silkworm Bombyx mori in innate immunity

Peptidoglycan recognition proteins (PGRPs) are members of an important class of pattern recognition receptors in insects that can specifically recognize peptidoglycan (PGN) in bacterial cell walls and participate in immune regulation and bacterial clearance. Although the role of PGRPs in regulating the innate immune response in Drosophila melanogaster has been studied, little is known regarding PGRPs in Lepidoptera species. In this study, five short (S)-type Bombyx mori PGRPs (BmPGRPs) were cloned, expressed, and evaluated for their function in innate immunity. B. mori larvae that were injected with the gram-positive bacterium Bacillus megaterium or the gram-negative bacterium Escherichia coli exhibited a rapid and significant upregulation in S-type BmPGRP expression. The results showed that the five evaluated BmPGRPs have significant agglutination activity toward E. coli and B. megaterium and more notable amidase activity toward meso-diaminopimelic acid peptidoglycan (DAP-PGN). Furthermore, only in the presence of BmPGRP-S5 did B. mori larval hemocytes exhibit significant phagocytosis against E. coli and B. megaterium.

Peptidoglycan Muropeptides: Release, Perception, and Functions as Signaling Molecules

Peptidoglycan (PG) is an essential molecule for the survival of bacteria, and thus, its biosynthesis and remodeling have always been in the spotlight when it comes to the development of antibiotics. The peptidoglycan polymer provides a protective function in bacteria, but at the same time is continuously subjected to editing activities that in some cases lead to the release of peptidoglycan fragments (i.e., muropeptides) to the environment. Several soluble muropeptides have been reported to work as signaling molecules. In this review, we summarize the mechanisms involved in muropeptide release (PG breakdown and PG recycling) and describe the known PG-receptor proteins responsible for PG sensing. Furthermore, we overview the role of muropeptides as signaling molecules, focusing on the microbial responses and their functions in the host beyond their immunostimulatory activity.

Microbial recognition by GEF-H1 controls IKKε mediated activation of IRF5

During infection, transcription factor interferon regulatory factor 5 (IRF5) is essential for the control of host defense. Here we show that the microtubule-associated guanine nucleotide exchange factor (GEF)-H1, is required for the phosphorylation of IRF5 by microbial muramyl-dipeptides (MDP), the minimal structural motif of peptidoglycan of both Gram-positive and Gram-negative bacteria. Specifically, GEF-H1 functions in a microtubule based recognition system for microbial peptidoglycans that mediates the activation of IKKε which we identify as a new upstream IKKα/β and IRF5 kinase. The deletion of GEF-H1 or dominant-negative variants of GEF-H1 prevent activation of IKKε and phosphorylation of IRF5. The GEF-H1-IKKε-IRF5 signaling axis functions independent of NOD-like receptors and is critically required for the recognition of intracellular peptidoglycans and host defenses against Listeria monocytogenes.

The transcription factor IRF5 is essential for immune defense against pathogens. Here, the authors show that the microtubule-associated factor GEF-H1 plays a critical role in host defense against Listeria monocytogenes in macrophages via activation of the IRF5 kinase IKKε.

Weevil pgrp-lb prevents endosymbiont TCT dissemination and chronic host systemic immune activation

Long-term intracellular symbiosis (or endosymbiosis) is widely distributed across invertebrates and is recognized as a major driving force in evolution. However, the maintenance of immune homeostasis in organisms chronically infected with mutualistic bacteria is a challenging task, and little is known about the molecular processes that limit endosymbiont immunogenicity and host inflammation. Here, we investigated peptidoglycan recognition protein (PGRP)-encoding genes in the cereal weevil Sitophilus zeamais's association with Sodalis pierantonius endosymbiont. We discovered that weevil pgrp-lb generates three transcripts via alternative splicing and differential regulation. A secreted isoform is expressed in insect tissues under pathogenic conditions through activation of the PGRP-LC receptor of the immune deficiency pathway. In addition, cytosolic and transmembrane isoforms are permanently produced within endosymbiont-bearing organ, the bacteriome, in a PGRP-LC-independent manner. Bacteriome isoforms specifically cleave the tracheal cytotoxin (TCT), a peptidoglycan monomer released by endosymbionts. pgrp-lb silencing by RNAi results in TCT escape from the bacteriome to other insect tissues, where it chronically activates the host systemic immunity through PGRP-LC. While such immune deregulations did not impact endosymbiont load, they did negatively affect host physiology, as attested by a diminished sexual maturation of adult weevils. Whereas pgrp-lb was first described in pathogenic interactions, this work shows that, in an endosymbiosis context, specific bacteriome isoforms have evolved, allowing endosymbiont TCT scavenging and preventing chronic endosymbiont-induced immune responses, thus promoting host homeostasis.

Characterization and Function of Two Short Peptidoglycan Recognition Proteins Involved in the Immunity of Bactrocera dorsalis (Hendel)

Peptidoglycans (PGNs) are major bacterial components recognized by the immune systems of insects and mammals. PGN recognition proteins (PGRPs) are widely distributed and highly conserved in vertebrates and invertebrates. PGRPs are a family of pattern recognition receptors that recognize peptidoglycan and regulate immune responses. In this study, we cloned two PGRP genes (BdPGRP-SA and BdPGRP-SD) from Bactrocera dorsalis (Hendel), which encode 192 and 196 amino acid residues, respectively. Both genes were highly expressed in adults, especially in the fat body and midgut. These two genes were up-regulated when challenged by the immune triggers, PGN-EB (Escherichia coli O111:B4) and PGN-SA (Staphylococcus aureus). The suppression of transcriptional expression of either gene by RNA interference (RNAi) resulted in increased sensitivities to Gram-negative E. coli and Gram-positive S. aureus PGNs. Suppression of BdPGRP-SA and -SD expression by RNAi resulted in weak expressions of four antimicrobial peptides (AMPs) upon injected with E. coli or S. aureus. BdPGRP-SA and -SD are involved in recognizing both Gram-negative and Gram-positive bacteria independently to activate the downstream AMP's response to bacterial infection.

Peptidoglycan Metabolite Photoaffinity Reporters Reveal Direct Binding to Intracellular Pattern Recognition Receptors and Arf GTPases

The peptidoglycan fragments γ-d-glutamyl- meso-diaminopimelic acid (iE-DAP) and muramyl-dipeptide (MDP) are microbial-specific metabolites that activate intracellular pattern recognition receptors and stimulate immune signaling pathways. While extensive structure-activity studies have demonstrated that these bacterial cell wall metabolites trigger NOD1- and NOD2-dependent signaling, their direct binding to these innate immune receptors or other proteins in mammalian cells has not been established. To characterize these fundamental microbial metabolite-host interactions, we synthesized a series of peptidoglycan metabolite photoaffinity reporters and evaluated their cross-linking to NOD1 and NOD2 in mammalian cells. We show that active iE-DAP and MDP photoaffinity reporters selectively cross-linked NOD1 and NOD2, respectively, and not their inactive mutants. We also discovered MDP reporter cross-linking to Arf GTPases, which interacted most prominently with GTP-bound Arf6 and coimmunoprecipitated with NOD2 upon MDP stimulation. Notably, MDP binding to NOD2 and Arf6 was abrogated with loss-of-function NOD2 mutants associated with Crohn's disease. Our studies demonstrate peptidoglycan metabolite photoaffinity reporters can capture their cognate immune receptors in cells and reveal unpredicted ligand-induced interactions with other cellular cofactors. These photoaffinity reporters should afford useful tools to discover and characterize other peptidoglycan metabolite-interacting proteins.

Molecular cloning and functional characterization of a short peptidoglycan recognition protein from triangle-shell pearl mussel (Hyriopsis cumingii)

Peptidoglycan (PGN) is an important target of recognition in invertebrate innate immunity. PGN recognition proteins (PGRPs) are responsible for PGN recognition. In this study, we cloned and functionally analyzed a short PGRP (HcPGRP2) from the triangle-shell pearl mussel Hyriopsis cumingii. The full-length cDNA sequence of HcPGRP2 gene was 1185 bp containing an open reading frame of 882 bp encoding a 293 amino acid protein. HcPGRP2 was predicted to have two SH3b domains and a conserved C-terminal PGRP domain. Quantitative real-time RT-PCR showed that HcPGRP2 was expressed in all examined tissues and its expression was induced most significantly by Staphylococcus aureus and Vibrio parahaemolyticus in the hepatopancreas and gills. RNA interference by siRNA results revealed that HcPGRP2 was involved in the regulation of whey acidic protein, theromacin, and defensin expression. As a pattern-recognition receptor, recombinant HcPGRP2 (rHcPGRP2) protein can bind and agglutinate (Ca²⁺ dependent) all tested bacteria. rHcPGRP2 exhibited specific binding to PGN but not to lipopolysaccharide. Moreover, rHcPGRP2 inhibited the growth activities of S. aureus and V. parahaemolyticus in vitro and accelerated the clearance of V. parahaemolyticus in vivo. Overall, our results indicated that HcPGRP2 may play an important role in the antibacterial immune mechanisms of H. cumingii.

Structure and function of the Ts2631 endolysin of Thermus scotoductus phage vB_Tsc2631 with unique N-terminal extension used for peptidoglycan binding

To escape from hosts after completing their life cycle, bacteriophages often use endolysins, which degrade bacterial peptidoglycan. While mesophilic phages have been extensively studied, their thermophilic counterparts are not well characterized. Here, we present a detailed analysis of the structure and function of Ts2631 endolysin from thermophilic phage vB_Tsc2631, which is a zinc-dependent amidase. The active site of Ts2631 consists of His30, Tyr58, His131 and Cys139, which are involved in Zn²⁺ coordination and catalysis. We found that the active site residues are necessary for lysis yet not crucial for peptidoglycan binding. To elucidate residues involved in the enzyme interaction with peptidoglycan, we tested single-residue substitution variants and identified Tyr60 and Lys70 as essential residues. Moreover, substitution of Cys80, abrogating disulfide bridge formation, inactivates Ts2631, as do substitutions of His31, Thr32 and Asn85 residues. The endolysin contains a positively charged N-terminal extension of 20 residues that can protrude from the remainder of the enzyme and is crucial for peptidoglycan binding. We show that the deletion of 20 residues from the N-terminus abolished the bacteriolytic activity of the enzyme. Because Ts2631 exhibits intrinsic antibacterial activity and unusual thermal stability, it is perfectly suited as a scaffold for the development of antimicrobial agents.

Peptidoglycan binding protein (PGBP)-modified magnetic nanobeads for efficient magnetic capturing of Staphylococcus aureus associated with sepsis in blood

Peptidoglycan-binding protein-modified magnetic nanobeads (PGBP-MNBs) were prepared for efficient magnetic capturing of Staphylococcus aureus (S. aureus), which is associated with sepsis, using the binding affinity of PGBP for the peptidoglycan (PG) layer on S. aureus. These PGBP-MNBs can simply capture S. aureus in plasma within 1 hr or even 15 min. Importantly, they also can capture various types of Gram-positive bacteria, such as Bacillus cereus and methicillin-resistant and methicillin-susceptible S. aureus (MRSA and MSSA). We believe that PGBP-based systems will be used to develop diagnostic systems for Gram-positive bacteria-related diseases.

Linkages between Epithelial Microbiota and Host Transcriptome in the Ileum during High-Grain Challenges: Implications for Gut Homeostasis in Goats

A high-grain (HG) diet can result in ruminal subacute acidosis, which is detrimental to gut health and can lead to decreased productivity. This study investigated the ileal epithelial microbiota and its relationship with host epithelial function in goats fed a HG diet (concentrate/hay, 90:10) and a control diet (concentrate/hay, 55:45), aiming to elucidate the mechanisms involved in ileal adaptation to subacute acidosis. The HG challenge increased the ileal volatile fatty acid concentration ( p = 0.030) and altered the ileal epithelial microbiota by increasing (FDR < 0.05) relative abundances of active carbohydrate and protein degraders Synergistetes, Prevotella, Fibrobacter, Clostridium, Treponema, and unclassified Ruminococcaceae by 20.1-, 6.3-, 16.8-, 8.5-, 19.9-, and 7.1-fold, respectively. However, the HG diet tended to reduce (FDR < 0.10) the relative abundance of Candidatus Arthromitus (38.8 ± 36.1 versus 2.1 ± 3.1). Microbial functional potentials inferred using PICRUSt indicated that the HG challenge elevated abundances of pathways associated with metabolism of amino acid, glycan, cofactors, and vitamins, whereras it decreased pathways associated with signal transduction, xenobiotic biodegradation, and metabolism. Additionally, in the ileal epithelium of HG goats, transcriptome analysis identified the increment (FDR < 0.10) of candidate genes involved in metabolism of carbohydrates, lipids, proteins, vitamins, and the proinflammatory cytokine pathway, while downregulating genes encoding antimicrobials and complements (FDR < 0.05). Collectively, the HG challenge shifted the structure and functional potentials of the ileal microbial community and affected the host responses in the ileum of goats toward increased metabolic activities of macro- and micronutrients, together with an increased risk of gut inflammation.

Characterization of PGRP-S1 from the oriental armyworm, Mythimna separata

Peptidoglycan is the key component forming the backbone of bacterial cell wall. It can be recognized by a group of pattern recognition receptors, known as peptidoglycan recognition proteins (PGRPs) in insects and higher animals. PGRPs may serve as immune receptors or N-acetylmuramoyl-L-alanine amidases (EC 3.5.1.28). Here, we report the characterization of a short PGRP, PGRP-S1, from the oriental armyworm, Mythimna separata. MsePGRP-S1 cDNA encodes a protein of 197 amino acids (aa) with a PGRP domain of about 150 aa. MsePGRP-S1 was expressed in several tissues of naïve larvae, including hemocytes, midgut, fat body and epidermis. Bacterial challenges caused variable changes in different tissues at the mRNA level. The recombinant protein bound strongly to Staphylococcus aureus and purified peptidoglycans from Staphylococcus aureus and Bacillus subtilis. It can inhibit the growth of gram-negative and gram-positive bacteria by disrupting bacterial surface. It can degrade peptidoglycans from Escherichia coli and Staphylococcus aureus. Taken together, these data demonstrate that M. separata PGRP-S1 is involved in defending against bacteria.

Peptidoglycan recognition proteins in insect immunity

Insects lack an acquired immune system and rely solely on the innate immune system to combat microbial infection. The innate immunity of insects mainly depends on the interaction between the host's pattern recognition receptor (PRR) and pathogen-associated molecular pattern (PAMP). The peptidoglycan recognition proteins (PGRPs) family is the most important pattern recognition receptor (PRR) for insects. It can recognize the main component of the cell wall of the pathogenic microorganism, peptidoglycan (PGN), and plays an important role in the innate immunity of insects. In this paper, the structure, classification, and function of PGRPs is summarized, and the role of PGRPs in the innate immunity of insects is also discussed.

PGlyRP3 concerts with PPARγ to attenuate DSS-induced colitis in mice

Nutrients may modulate immunity through their transcription factors that act on both metabolic and immunity genes. It has been shown that the transcription factor of lipid ligands PPARγ physically binds the gene promoter of the peptidoglycan recognition protein (PGlyRP3), which showed anti-inflammatory action in vitro. It is hypothesized in the present work that olive oil feeding protects against toxicity of DSS-induced colitis via activation of the lipid transcription factor PPARγ that stimulates the anti-inflammatory PGlyRP3. Results: PGlyRP3 is expressed in mouse colon and up-regulated by olive oil feeding. Olive oil reduced mortality and severity scores of DSS-induced colitis and down-regulated the proinflammatory IL-1b, IL-6 and TNFα genes. This protective effect was accompanied by up-regulation of both PPARγ and PGlyRP3. Inhibition of PPARγ by its antagonist BADGE down-regulated PGlyRP3 and abolished the anti-inflammatory effect of olive oil feeding in this DSS-induced colitis model, reflecting the pivotal role of PPARγ binding nutrition and inflammation. Activation of PGlyRP3 by its ligand peptidoglycan was not responsible for the inflammation caused by peptidoglycan, since neutralization of TLR2 attenuated this inflammatory response without affecting the peptidoglycan-induced PGlyRP3 level. Olive oil activated the IκBα and inhibited NF-κB and cox-2 gene expressions, and p65 nuclear translocation in DSS-colitis mice, reflecting the involvement of the inhibition of NF-κB signaling pathway in the anti-inflammatory olive oil - PPARγ - PGlyRP3 access. This pathway was reactivated by the PPARγ antagonist BADGE. Conclusions: Olive oil regulates by the same transcription factor (PPARγ) both lipid metabolic and immune gene (PGlyRP3) expressions, exerting the anti-inflammatory effect, and protecting against DSS-induced colitis in mice.

Evolution of host support for two ancient bacterial symbionts with differentially degraded genomes in a leafhopper host

Plant sap-feeding insects (Hemiptera) rely on bacterial symbionts for nutrition absent in their diets. These bacteria experience extreme genome reduction and require genetic resources from their hosts, particularly for basic cellular processes other than nutrition synthesis. The host-derived mechanisms that complete these processes have remained poorly understood. It is also unclear how hosts meet the distinct needs of multiple bacterial partners with differentially degraded genomes. To address these questions, we investigated the cell-specific gene-expression patterns in the symbiotic organs of the aster leafhopper (ALF), Macrosteles quadrilineatus (Cicadellidae). ALF harbors two intracellular symbionts that have two of the smallest known bacterial genomes: Nasuia (112 kb) and Sulcia (190 kb). Symbionts are segregated into distinct host cell types (bacteriocytes) and vary widely in their basic cellular capabilities. ALF differentially expresses thousands of genes between the bacteriocyte types to meet the functional needs of each symbiont, including the provisioning of metabolites and support of cellular processes. For example, the host highly expresses genes in the bacteriocytes that likely complement gene losses in nucleic acid synthesis, DNA repair mechanisms, transcription, and translation. Such genes are required to function in the bacterial cytosol. Many host genes comprising these support mechanisms are derived from the evolution of novel functional traits via horizontally transferred genes, reassigned mitochondrial support genes, and gene duplications with bacteriocyte-specific expression. Comparison across other hemipteran lineages reveals that hosts generally support the incomplete symbiont cellular processes, but the origins of these support mechanisms are generally specific to the host-symbiont system.

Synthetic Immunotherapeutics against Gram-negative Pathogens

While traditional drug discovery continues to be an important platform for the search of new antibiotics, alternative approaches should also be pursued to complement these efforts. We herein designed a class of molecules that decorate bacterial cell surfaces with the goal of re-engaging components of the immune system toward Escherichia coli and Pseudomonas aeruginosa. More specifically, conjugates were assembled using polymyxin B (an antibiotic that inherently attaches to the surface of Gram-negative pathogens) and antigenic epitopes that recruit antibodies found in human serum. We established that the spacer length played a significant role in hapten display within the bacterial cell surface, a result that was confirmed both experimentally and via molecular dynamics simulations. Most importantly, we demonstrated the specific killing of bacteria by our agent in the presence of human serum. By enlisting the immune system, these agents have the potential to pave the way for a potent antimicrobial modality.

PGRP-LB homolog acts as a negative modulator of immunity in maintaining the gut-microbe symbiosis of red palm weevil, Rhynchophorus ferrugineus Olivier

Many notorious insect pests live in the symbiotic associations with gut microbiota. However, the mechanisms underlying how they host their gut microbiota are unknown. Most gut bacteria can release peptidoglycan (PGN) which is an important antigen to activate the immune response. Therefore, how to keep the appropriate gut immune intensity to host commensals while to efficiently remove enteropathogens is vital for insect health. This study is aimed at elucidating the roles of an amidase PGRP, Rf PGRP-LB, in maintaining the gut-microbe symbiosis of Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier. RfPGRP-LB is a secreted protein containing a typical PGRP domain. The existence of five conservative amino acid residues, being required for amidase activity, showed that RfPGRP-LB is a catalytic protein. Expression analysis revealed abundance of RfPGRP-LB transcripts in gut was dramatically higher than those in other tissues. RfPGRP-LB could be significantly induced against the infection of Escherichia coli. In vitro assays revealed that rRfPGRP-LB impaired the growth of E. coli and agglutinated bacteria cells obviously, suggesting RfPGRP-LB is a pathogen recognition receptor and bactericidal molecule. RfPGRP-LB knockdown reduced the persistence of E. coli in gut and load of indigenous gut microbiota significantly. Furthermore, the community structure of indigenous gut microbiota was also intensively altered by RfPGRP-LB silence. Higher levels of the antimicrobial peptide, attacin, were detected in guts of RfPGRP-LB silenced larvae than controls. Collectively, RfPGRP-LB plays multiple roles in modulating the homeostasis of RPW gut microbiota not only by acting as a negative regulator of mucosal immunity through PGN degradation but also as a bactericidal effector to prevent overgrowth of commensals and persistence of noncommensals.

Regulation of the Peptidoglycan Amidase PGLYRP2 in Epithelial Cells by Interleukin-36γ

Interleukin-36 (IL-36) cytokines are important regulators of mucosal homeostasis and inflammation. We have previously established that oral epithelial cells upregulate IL-36γ expression in response to the bacterial pathogen Porphyromonas gingivalis Here, we have established that IL-36γ can stimulate the gene expression of mechanistically distinct antimicrobial proteins, including the peptidoglycan amidase PGLYRP2, in oral epithelial cells (e.g., TIGK cells). PGLYRP2 gene expression was not stimulated by either IL-17 or IL-22, thus demonstrating selectivity in the regulation of PGLYRP2 by IL-36γ. The IL-36γ-inducible expression of PGLYRP2 was shown to be mediated by IRAK1- and p38 mitogen-activated protein (MAP) kinase-dependent signaling. Furthermore, our finding that IL-36γ-inducible PGLYRP2 expression was reduced in proliferating TIGK cells but increased in terminally differentiating cells suggests that control of PGLYRP2 expression is associated with the maturation of the oral epithelium. PGLYRP2 expression in TIGK cells can also be directly stimulated by oral bacteria. However, the extracellular gingipain proteases (Kgp and RgpA/B) produced by P. gingivalis, which are critical virulence factors, can antagonize PGLYRP2 expression. Thus, the expression of IL-36γ by oral epithelial cells in response to P. gingivalis might enable the subsequent autocrine stimulation of PGLYRP2 expression. In summary, our data identify how IL-36γ may promote oral mucosal homeostasis by regulating PGLYRP2 expression.

Comparative immunological study of the snail Physella acuta (Hygrophila, Pulmonata) reveals shared and unique aspects of gastropod immunobiology

The freshwater snail Physella acuta was selected to expand the perspective of comparative snail immunology. Analysis of Physella acuta, belonging to the Physidae, taxonomic sister family to Planorbidae, affords family-level comparison of immune features characterized from Biomphalaria glabrata, the model snail often used to interpret general gastropod immunity. To capture constitutive and induced immune sequences, transcriptomes of an individual Physella acuta snail, 12 h post injection with bacteria (Gram -/+) and one sham-exposed snail were recorded with 454 pyrosequencing. Assembly yielded a combined reference transcriptome containing 24,288 transcripts. Additionally, genomic Illumina reads were obtained (∼15-fold coverage). Recovery of transcripts for two macin-like antimicrobial peptides (AMPs), 12 aplysianins, four LBP/BPIs and three physalysins indicated that Physella acuta shares a similar organization of antimicrobial defenses with Biomphalaria glabrata, contrasting a modest AMP arsenal with a diverse set of antimicrobial proteins. The lack of predicted transmembrane domains in all seven Physella acuta PGRP transcripts supports the notion that gastropods do not employ cell-bound PGRP receptors, different from ecdysozoan invertebrates yet similar to mammals (vertebrate deuterostomes). The well-documented sequence diversification by Biomphalaria glabrata FREPs (immune lectins comprising immunoglobulin superfamily domains and fibrinogen domains), resulting from somatic mutations of a large FREP gene family is hypothesized to be unique to Planorbidae; Physella acuta revealed just two bonafide FREP genes and these were not diversified. Furthermore, the flatworm parasite Echinostoma paraensei, confirmed here to infect both snail species, did not evoke from Physella acuta the abundant expression of FREP proteins at 2, 4 and 8 days post exposure that was previously observed from Biomphalaria glabrata. The Physella acuta reference transcriptome also revealed 24 unique transcripts encoding proteins consisting of a single fibrinogen-related domain (FReDs), with a short N-terminal sequence encoding either a signal peptide, transmembrane domain or no predicted features. The Physella acuta FReDs are candidate immune genes based on implication of similar sequences in immunity of bivalve molluscs. Overall, comparative analysis of snails of sister families elucidated the potential for taxon-specific immune features and investigation of strategically selected species will provide a more comprehensive view of gastropod immunity.

Immune functions of insect βGRPs and their potential application

Insects rely completely on the innate immune system to sense the foreign bodies and to mount the immune responses. Germ-line encoded pattern recognition receptors play crucial roles in recognizing pathogen-associated molecular patterns. Among them, β-1,3-glucan recognition proteins (βGRPs) and gram-negative bacteria-binding proteins (GNBPs) belong to the same pattern recognition receptor family, which can recognize β-1,3-glucans. Typical insect βGRPs are comprised of a tandem carbohydrate-binding module in the N-terminal and a glucanase-like domain in the C-terminal. The former can recognize triple-helical β-1,3-glucans, whereas the latter, which normally lacks the enzymatic activity, can recruit adapter proteins to initiate the protease cascade. According to studies, insect βGRPs possess at least three types of functions. Firstly, some βGRPs cooperate with peptidoglycan recognition proteins to recognize the lysine-type peptidoglycans upstream of the Toll pathway. Secondly, some directly recognize fungal β-1,3-glucans to activate the Toll pathway and melanization. Thirdly, some form the 'attack complexes' with other immune effectors to promote the antifungal defenses. The current review will focus on the discovery of insect βGRPs, functions of some well-characterized members, structure-function studies and their potential application.

Peptidoglycan recognition proteins in hematophagous arthropods

Hematophagous arthropods are medically important disease vectors that transmit a variety of pathogens. Unlike mammals that employ both innate and adaptive immunity to clear invading pathogens, these vectors rely mainly on an innate immune system to combat pathogens. Peptidoglycan recognition proteins (PGRPs) are important components of innate immune signaling pathways and are responsible for recognizing microbe-associated molecular patterns (MAMPs), thus regulating host immune interactions with both harmful and helpful microbes. Here we review a number of recent studies in different vectors that address the function of PGRPs in immune regulation. Further, we discuss the variation of PGRPs between vectors and Drosophila.

Synthetic Immunobiotics: A Future Success Story in Small Molecule-Based Immunotherapy?

Drug resistance to our current stock of antibiotics is projected to increase to levels that threaten our ability to reduce and eliminate bacterial infections, which is now considered one of the primary health care crises of the 21st century. Traditional antibiotic agents (e.g., penicillin) paved the way for massive advances in human health, but we need novel strategies to maintain the upper hand in the battle against pathogenic bacteria. Nontraditional strategies, such as targeted immunotherapies, could prove fruitful in complementing our antibiotic arsenal.

Rearing Water Treatment Induces Microbial Selection Influencing the Microbiota and Pathogen Associated Transcripts of Cod (Gadus morhua) Larvae

We have previously shown that K-selection and microbial stability in the rearing water increases survival and growth of Atlantic cod (Gadus morhua) larvae, and that recirculating aquaculture systems (RAS) are compatible with this. Here, we have assessed how water treatment influenced the larval microbiota and host responses at the gene expression level. Cod larvae were reared with two different rearing water systems: a RAS and a flow-through system (FTS). The water microbiota was examined using a 16S rDNA PCR/DGGE strategy. RNA extracted from larvae at 8, 13, and 17 days post hatching was used for microbiota and microarray gene expression analysis. Bacterial cDNA was synthesized and used for 16S rRNA amplicon 454 pyrosequencing of larval microbiota. Both water and larval microbiota differed significantly between the systems, and the larval microbiota appeared to become more dissimilar between systems with time. In total 4 phyla were identified for all larvae: Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. The most profound difference in larval microbiota was a high abundance of Arcobacter (Epsilonproteobacteria) in FTS larvae (34 ± 9% of total reads). Arcobacter includes several species that are known pathogens for humans and animals. Cod larval transcriptome responses were investigated using an oligonucleotide gene expression microarray covering approximately 24,000 genes. Interestingly, FTS larvae transcriptional profiles revealed an overrepresentation of upregulated transcripts associated with responses to pathogens and infections, such as c1ql3-like, pglyrp-2-like and zg16, compared to RAS larvae. In conclusion, distinct water treatment systems induced differences in the larval microbiota. FTS larvae showed up-regulation of transcripts associated with responses to microbial stress. These results are consistent with the hypothesis that RAS promotes K-selection and microbial stability by maintaining a microbial load close to the carrying capacity of the system, and ensuring long retention times for both bacteria and water in the system.

Pathogen-Derived Carbohydrate Recognition in Molluscs Immune Defense

Self-nonself discrimination is a common theme for all of the organisms in different evolutionary branches, which is also the most fundamental step for host immune protection. Plenty of pattern recognition receptors (PRRs) with great diversity have been identified from different organisms to recognize various pathogen-associated molecular patterns (PAMPs) in the last two decades, depicting a complicated scene of host-pathogen interaction. However, the detailed mechanism of the complicate PAMPs–PRRs interactions at the contacting interface between pathogens and hosts is still not well understood. All of the cells are coated by glycosylation complex and thick carbohydrates layer. The different polysaccharides in extracellular matrix of pathogen-host are important for nonself recognition of most organisms. Coincidentally, massive expansion of PRRs, majority of which contain recognition domains of Ig, leucine-rich repeat (LRR), C-type lectin (CTL), C1q and scavenger receptor (SR), have been annotated and identified in invertebrates by screening the available genomic sequence. The phylum Mollusca is one of the largest groups in the animal kingdom with abundant biodiversity providing plenty of solutions about pathogen recognition and immune protection, which might offer a suitable model to figure out the common rules of immune recognition mechanism. The present review summarizes the diverse PRRs and common elements of various PAMPs, especially focusing on the structural and functional characteristics of canonical carbohydrate recognition proteins and some novel proteins functioning in molluscan immune defense system, with the objective to provide new ideas about the immune recognition mechanisms.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

PGRP-LD mediates A. stephensi vector competency by regulating homeostasis of microbiota-induced peritrophic matrix synthesis

Peptidoglycan recognition proteins (PGRPs) and commensal microbes mediate pathogen infection outcomes in insect disease vectors. Although PGRP-LD is retained in multiple vectors, its role in host defense remains elusive. Here we report that Anopheles stephensi PGRP-LD protects the vector from malaria parasite infection by regulating gut homeostasis. Specifically, knock down of PGRP-LD (dsLD) increased susceptibility to Plasmodium berghei infection, decreased the abundance of gut microbiota and changed their spatial distribution. This outcome resulted from a change in the structural integrity of the peritrophic matrix (PM), which is a chitinous and proteinaceous barrier that lines the midgut lumen. Reduction of microbiota in dsLD mosquitoes due to the upregulation of immune effectors led to dysregulation of PM genes and PM fragmentation. Elimination of gut microbiota in antibiotic treated mosquitoes (Abx) led to PM loss and increased vectorial competence. Recolonization of Abx mosquitoes with indigenous Enterobacter sp. restored PM integrity and decreased mosquito vectorial capacity. Silencing PGRP-LD in mosquitoes without PM didn’t influence their vector competence. Our results indicate that PGPR-LD protects the gut microbiota by preventing hyper-immunity, which in turn promotes PM structurally integrity. The intact PM plays a key role in limiting P. berghei infection.

Malaria parasites must overcome several obstacles to complete their development in mosquito. Understanding the interactions between parasites and mosquitoes will provide potential targets to control malaria transmission. PGRP-LD is a peptidoglycan recognition protein, of which limit information is available in insects. Here we show that A. stephensi PGRP-LD mediates malaria parasite infection outcomes by influencing homeostasis of the gut microbiota. Reduction of the gut microbiota density, resulting from upregulation of immune activities in PGRP-LD knock down mosquitoes, changes expression of PM genes and causes PM fragmentation. The compromised PM leads to increasing susceptibility to parasite infection. We also discovered that the PM is lost in mosquitoes in which the gut microbiota is removed by antibiotic treatment. Knock down of PGRP-LD in these mosquitoes doesn’t increase their vector competence. Altogether, these results indicate that capacity of Anopheles mosquito to transmit parasites is determined by a finely tuned balance between host immunity, gut microbiota and peritrophic matrix. PGRP-LD is a key mediator in regulating this balance. Our results expand knowledge on interactions between immune system, gut microbiota and Plasmodium, and will shed light on equivalent processes in other disease transmitting vectors.

A Balancing Act: PGRPs Preserve and Protect

How does the immune system maintain a balance between preserving a beneficial microbiome and protecting against pathogens while also inducing effective, yet not damaging, responses? In this issue of Cell Host & Microbe, Charroux et al. (2018) reveal that, in Drosophila, this task is performed by three isoforms of PGRP-LB, a peptidoglycan-hydrolyzing amidase.

Transcriptomic and Proteomic Profiling Revealed High Proportions of Odorant Binding and Antimicrobial Defense Proteins in Olfactory Tissues of the House Mouse

Mammalian olfaction depends on chemosensory neurons of the main olfactory epithelia (MOE), and/or of the accessory olfactory epithelia in the vomeronasal organ (VNO). Thus, we have generated the VNO and MOE transcriptomes and the nasal cavity proteome of the house mouse, Mus musculus musculus. Both transcriptomes had low levels of sexual dimorphisms, while the soluble proteome of the nasal cavity revealed high levels of sexual dimorphism similar to that previously reported in tears and saliva. Due to low levels of sexual dimorphism in the olfactory receptors in MOE and VNO, the sex-specific sensing seems less likely to be dependent on receptor repertoires. However, olfaction may also depend on a continuous removal of background compounds from the sites of detection. Odorant binding proteins (OBPs) are thought to be involved in this process and in our study Obp transcripts were most expressed along other lipocalins (e.g., Lcn13, Lcn14) and antimicrobial proteins. At the level of proteome, OBPs were highly abundant with only few being sexually dimorphic. We have, however, detected the major urinary proteins MUP4 and MUP5 in males and females and the male-biased central/group-B MUPs that were thought to be abundant mainly in the urine. The exocrine gland-secreted peptides ESP1 and ESP22 were male-biased but not male-specific in the nose. For the first time, we demonstrate that the expression of nasal lipocalins correlates with antimicrobial proteins thus suggesting that their individual variation may be linked to evolvable mechanisms that regulate natural microbiota and pathogens that regularly enter the body along the ‘eyes-nose-oral cavity’ axis.

The Sarda Sheep Host Fecal Proteome

The first characterization of the sheep fecal microbiota was recently reported, as obtained by using a multi meta-omic approach. Here, the mass spectra generated by single-run LC/high-resolution MS in the context of that study were reanalyzed using a host-specific database, in order to gain insights for the first time into the host fecal proteome of healthy Sarda sheep. On the whole, 5349 non-redundant tryptic peptide sequences were identified, belonging to 1046 different proteins. The "core" fecal proteome (common to all animals) comprised 431 proteins, mainly related to biological processes as immune response and proteolysis. Proteins involved in the immune/inflammatory response and peptidases were specifically investigated. This dataset provides novel insights into the repertoire of proteins secreted in the sheep intestinal lumen, and constitutes the basis for future shotgun and targeted proteomics studies aimed at monitoring changes in the sheep fecal proteome in response to production variables, infectious/inflammatory states, and variations in the gut microbiota. Data are available via ProteomeXchange with identifier PXD006145.

RNA-Sequencing of Drosophila melanogaster Head Tissue on High-Sugar and High-Fat Diets

Obesity has been shown to increase risk for cardiovascular disease and type-2 diabetes. In addition, it has been implicated in aggravation of neurological conditions such as Alzheimer’s. In the model organism Drosophila melanogaster, a physiological state mimicking diet-induced obesity can be induced by subjecting fruit flies to a solid medium disproportionately higher in sugar than protein, or that has been supplemented with a rich source of saturated fat. These flies can exhibit increased circulating glucose levels, increased triglyceride content, insulin-like peptide resistance, and behavior indicative of neurological decline. We subjected flies to variants of the high-sugar diet, high-fat diet, or normal (control) diet, followed by a total RNA extraction from fly heads of each diet group for the purpose of Poly-A selected RNA-Sequencing. Our objective was to identify the effects of obesogenic diets on transcriptome patterns, how they differed between obesogenic diets, and identify genes that may relate to pathogenesis accompanying an obesity-like state. Gene ontology analysis indicated an overrepresentation of affected genes associated with immunity, metabolism, and hemocyanin in the high-fat diet group, and CHK, cell cycle activity, and DNA binding and transcription in the high-sugar diet group. Our results also indicate differences in the effects of the high-fat diet and high-sugar diet on expression profiles in head tissue of flies, despite the reportedly similar phenotypic impacts of the diets. The impacted genes, and how they may relate to pathogenesis in the Drosophila obesity-like state, warrant further experimental investigation.

Peptidoglycan recognition by the innate immune system

The innate immune system recognizes microbial products using germline-encoded receptors that initiate inflammatory responses to infection. The bacterial cell wall component peptidoglycan is a prime example of a conserved pathogen-associated molecular pattern (PAMP) for which the innate immune system has evolved sensing mechanisms. Peptidoglycan is a direct target for innate immune receptors and also regulates the accessibility of other PAMPs to additional innate immune receptors. Subtle structural modifications to peptidoglycan can influence the ability of the innate immune system to detect bacteria and can allow bacteria to evade or alter host defences. This Review focuses on the mechanisms of peptidoglycan recognition that are used by mammalian cells and discusses new insights into the role of peptidoglycan recognition in inflammation, metabolism, immune homeostasis and disease.

Sex-dependent alterations in motor and anxiety-like behavior of aged bacterial peptidoglycan sensing molecule 2 knockout mice

Peptidoglycan recognition proteins (PGRPs) are key sensing-molecules of the innate immune system that specifically detect bacterial peptidoglycan (PGN) and its derivates. PGRPs have recently emerged as potential key regulators of normal brain development and behavior. To test the hypothesis that PGRPs play a role in motor control and anxiety-like behavior in later life, we used 15-month old male and female peptidoglycan recognition protein 2 (Pglyrp2) knockout (KO) mice. Pglyrp2 is an N-acetylmuramyl-l-alanine amidase that hydrolyzes PGN between the sugar backbone and the peptide chain (which is unique among the mammalian PGRPs). Using a battery of behavioral tests, we demonstrate that Pglyrp2 KO male mice display decreased levels of anxiety-like behavior compared with wild type (WT) males. In contrast, Pglyrp2 KO female mice show reduced rearing activity and increased anxiety-like behavior compared to WT females. In the accelerated rotarod test, however, Pglyrp2 KO female mice performed better compared to WT females (i.e., they had longer latency to fall off the rotarod). Further, Pglyrp2 KO male mice exhibited decreased expression levels of synaptophysin, gephyrin, and brain-derived neurotrophic factor in the frontal cortex, but not in the amygdala. Pglyrp2 KO female mice exhibited increased expression levels of spinophilin and alpha-synuclein in the frontal cortex, while exhibiting decreased expression levels of synaptophysin, gephyrin and spinophilin in the amygdala. Our findings suggest a novel role for Pglyrp2asa key regulator of motor and anxiety-like behavior in late life.

Unravelling the relationship between the tsetse fly and its obligate symbiont Wigglesworthia: transcriptomic and metabolomic landscapes reveal highly integrated physiological networks

Insects with restricted diets rely on obligate microbes to fulfil nutritional requirements essential for biological function. Tsetse flies, vectors of African trypanosome parasites, feed exclusively on vertebrate blood and harbour the obligate endosymbiont Wigglesworthia glossinidia. Without Wigglesworthia, tsetse are unable to reproduce. These symbionts are sheltered within specialized cells (bacteriocytes) that form the midgut-associated bacteriome organ. To decipher the core functions of this symbiosis essential for tsetse's survival, we performed dual-RNA-seq analysis of the bacteriome, coupled with metabolomic analysis of bacteriome and haemolymph collected from normal and symbiont-cured (sterile) females. Bacteriocytes produce immune regulatory peptidoglycan recognition protein (pgrp-lb) that protects Wigglesworthia, and a multivitamin transporter (smvt) that can aid in nutrient dissemination. Wigglesworthia overexpress a molecular chaperone (GroEL) to augment their translational/transport machinery and biosynthesize an abundance of B vitamins (specifically B₁-, B₂-, B₃- and B₆-associated metabolites) to supplement the host's nutritionally deficient diet. The absence of Wigglesworthia's contributions disrupts multiple metabolic pathways impacting carbohydrate and amino acid metabolism. These disruptions affect the dependent downstream processes of nucleotide biosynthesis and metabolism and biosynthesis of S-adenosyl methionine (SAM), an essential cofactor. This holistic fundamental knowledge of the symbiotic dialogue highlights new biological targets for the development of innovative vector control methods.