Innate immune recognition of microbial cell wall components and microbial strategies to evade such recognitions

Pathogen recognition receptors in channel catfish: IV. Identification, phylogeny and expression analysis of peptidoglycan recognition proteins

Peptidoglycan recognition proteins (PGRPs) can recognize bacterial cell wall (peptidoglycan) and activate innate immune system. In addition to its function as pathogen recognition receptors (PRRs), PGRPs are also involved in directly killing bacteria, and regulating multiple signaling pathways. Recently, we have reported catfish PRRs including nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs), retinoic acid inducible gene I (RIG-I) like receptors (RLRs), and Toll-like receptors (TLRs). In this study, we identified and characterized the PGRP gene family in channel catfish which included two members, PGLYRP-5 and PGLYRP-6. Phylogenetic analysis, syntenic analysis and protein structural analysis were conducted to determine their identities and evolutionary relationships. In order to gain insight into the roles of PGRPs in catfish innate immune responses, quantitative real-time PCR was used to investigate the expression profiles in catfish healthy tissues and after bacterial infection. Both PGLYRP-5 and PGLYRP-6 were ubiquitously expressed in all 12 healthy tissues, and most highly expressed in gill and spleen, respectively. Distinct expression patterns were observed for PGRPs after infection with Edwardsiella ictaluri and Flavobacterium columnare, both Gram-negative bacteria. After infection with E. ictaluri, both PGLYRP-5 and PGLYRP-6 were significantly down-regulated at a certain time-point, while both genes were generally up-regulated in the gill after infection with F. columnare. Collectively, these findings suggested that PGRPs may play complex roles in the host immune response to bacterial pathogens in catfish.

Characterization of a novel Manduca sexta beta-1, 3-glucan recognition protein (βGRP3) with multiple functions

Recognition of pathogens by insect pattern recognition receptors is critical to mount effective immune responses. In this study, we reported a new member (βGRP3) of the β-1, 3-glucan recognition protein (βGRP) family from the tobacco hornworm Manduca sexta. Unlike other members of the M. sexta βGRP family proteins, which contain an N-terminal small glucan binding domain and a C-terminal large glucanase-like domain, βGRP3 is 40-45 residues shorter at the N-terminus and lacks the small glucan binding domain. The glucanase-like domain of βGRP3 is most similar to that of M. sexta microbe binding protein (MBP) with 78% identity. βGRP3 transcript was mainly expressed in the fat body, and both its mRNA and protein levels were not induced by microorganisms in larvae. Recombinant βGRP3 purified from Drosophila S2 cells could bind to several Gram-negative and Gram-positive bacteria and yeast, as well as to laminarin (β-1, 3-glucan), mannan, lipopolysaccharide (LPS), lipoteichoic acid (LTA), and meso-diaminopimelic acid (DAP)-type peptidoglycan (PG), but did not bind to Lysine-type PG. Binding of βGRP3 to laminarin could be competed well by free laminarin, mannan, LPS and LTA, but almost not competed by free PGs. Recombinant βGRP3 could agglutinate Bacillus cereus and Escherichia coli in a calcium-dependent manner and showed antibacterial (bacteriostatic) activity against B. cereus, novel functions that have not been reported for the βGRP family proteins before. M. sexta βGRP3 may serve as an immune surveillance receptor with multiple functions.

Defining the conformation of human mincle that interacts with mycobacterial trehalose dimycolate

Trehalose dimycolate, an unusual glycolipid in the outer membrane of Mycobacterium tuberculosis, stimulates macrophages by binding to the macrophage receptor mincle. This stimulation plays an important role both in infection by mycobacteria and in the use of derivatives of mycobacteria as adjuvants to enhance the immune response. The mechanism of trehalose dimycolate binding to the C-type carbohydrate-recognition domain in human mincle has been investigated using a series of synthetic analogs of trehalose dimycolate and site-directed mutagenesis of the human protein. The results support a mechanism of binding acylated trehalose derivatives to human mincle that is very similar to the mechanism of binding to bovine mincle, in which one glucose residue in the trehalose headgroup of the glycolipid is ligated to the principle Ca²⁺-binding site in the carbohydrate-recognition domain, with specificity for the disaccharide resulting from interactions with the second glucose residue. Acyl chains attached to the 6-OH groups of trehalose enhance affinity, with the affinity dependent on the length of the acyl chains and the presence of a hydrophobic groove adjacent to the sugar-binding sites. The results indicate that the available crystal structure of the carbohydrate-recognition domain of human mincle is unlikely to be in a fully active conformation. Instead, the ligand-binding conformation probably resembles closely the structure observed for bovine mincle in complex with trehalose. These studies provide a basis for targeting human mincle as a means of inhibiting interactions with mycobacteria and as an approach to harnessing the ability of mincle to stimulate the immune response.

Surface-associated microbes continue to surprise us in their sophisticated strategies for assembling biofilm communities

Microorganisms are rarely found in isolation. Frequently, they live as complex consortia or communities known as biofilms. The microbes within these complex structures are typically enmeshed in a matrix of macromolecules collectively known as the extracellular polymeric substances (EPS). The last decade has seen enormous growth in the breadth and depth of biofilm-related research. An important area of focus has been the study of pure culture biofilms of different model species. This work has informed us about the different genetic determinants involved in biofilm formation and the environmental conditions that influence the process. These studies have also highlighted both species-specific aspects of biofilm development and common trends observed across many different organisms. This report highlights some exciting findings in recent biofilm-related research.

Does bacterial infection cause genome instability and cancer in the host cell?

Research of the past several decades suggests that bacterial infection can lead to genome instability of the host cell often resulting in cancer development. However, there is still a substantial lack of knowledge regarding possible mechanisms involved in the development of genomic instability. Several questions remain unanswered, namely: Why has the causative relationship between the bacterial infection and cancer been established only for a small number of cancers? What is the mechanism responsible for the induction of genome instability and cancer? Is the infection process required to cause genome instability and cancer? In this review, we present a hypothesis that the bacterial infection, exposure to heat-killed bacteria or even some bacterial determinants may trigger genome instability of exposed and distal cells, and thus may cause cancer. We will discuss the mechanisms of host responses to the bacterial infection and present the possible pathways leading to genome instability and cancer through exposure to bacteria.