The molecular mechanism of species-specific recognition of lipopolysaccharides by the MD-2/TLR4 receptor complex☆
Introduction
When microbes developed pathogenic factors that enabled them to infect eukaryotic hosts, the hosts in turn evolved different mechanisms that enabled them to recognize the invasion of the pathogen and to fight the infections. The direct way for the host to recognize an invading pathogen is to look for specific molecules that distinguish the pathogen from host's own structures. These molecules are named PAMPs or MAMPs (pathogen- or microbe-associated molecular patterns, respectively) and represent structures that are essential for microbial survival and therefore unlikely to be circumvented by rapid evolutionary modifications, e.g. components of the bacterial cell wall (lipopolysaccharides, lipopeptides), proteins of the bacterial flagellum or viral double stranded RNA.
The initial recognition of microbial PAMPs is mediated by membrane-bound host receptors, one of the most prominent group of which is the Toll-like receptor family (TLRs), named after the Toll receptor, responsible for antifungal response in Drosophila (Lemaitre et al., 1996). Today at least 15 distinct TLRs have been identified in different animal species (Temperley et al., 2008).
TLR4, the human Toll homolog originally known as hToll (Medzhitov et al., 1997), is responsible for the recognition of lipopolysaccharide (i.e. LPS, endotoxin) and enables immune response against invading Gram-negative bacteria. It has the most complex activation mechanism among the TLR family. It is the only TLR member that does not directly bind its respective ligand and requires a co-receptor, the MD-2 protein (i.e. myeloid differentiation 2 protein), which is indispensable for LPS recognition and is directly involved in the activated receptor complex (Shimazu et al., 1999). Moreover, TLR4 is the only TLR that can convey the activating signal through two distinct intracellular signaling pathways, one requiring the MyD88 (myeloid differentiation primary response gene 88) adapter protein and the other using the TRIF (TIR-domain-containing adapter-inducing interferon-β) adapter protein (Kawai et al., 2001, Yamamoto et al., 2003).
Several crystal structures and extensive mutagenesis studies of the receptor proteins involved in the LPS recognition have shed much light on the molecular aspects of MD-2/TLR4 activation (Ohto et al., 2007, Park et al., 2009, Ohto et al., 2012, Resman et al., 2009). Nonetheless, several interesting questions are still not completely clear. This review focuses on the molecular aspects of species-specific recognition of different lipopolysaccharides by the MD-2/TLR4 complex of different animal species with emphasis on the role of the lipid A structure species-specific MD-2/TLR4 activation.
Section snippets
The structure of lipopolysaccharides
Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria. The polysaccharide portion is the antigenic part of the molecule and varies from strain to strain. On the other hand, the lipid A portion is highly conserved and represents the PAMP that is recognized by the MD-2/TLR4 complex (Fig. 1) (Raetz and Whitfield, 2002).
The majority of the MD-2/TLR4 research is based on the “canonical” Escherichia coli-type lipid A, common to most enterobacteria, which has six acyl
The molecular structure of the MD-2/TLR4 complex
Like other members of the TLR family, TLR4 is a type-I transmembrane protein that comprises a characteristic horse-shoe/solenoid shaped ectodomain that is responsible for ligand recognition, a transmembrane domain and a cytoplasmic TIR (Toll/interleukin 1R) domain (Fig. 2). When TLRs recognize their respective agonists they dimerize, which brings two TIR domains together and forms a scaffold for the binding and activation of several cytoplasmic adapter proteins (Fekonja et al., 2012), like
The species-specificities of the MD-2/TLR4 complex
What are therefore the structural determinants of the MD-2 and the TLR4 receptors that govern the ligand specificity between different species (e.g. human, mouse or horse) and enable the same ligand to act as an agonist for some and an antagonist for receptors from other species? Several mutational, docking and other studies have attempted to answer this question (Table 2, Table 3, Fig. 4).
The most conclusive answers were provided by the crystal structures of the murine MD-2/TLR4 complex with
How bacteria (try to) evade immune recognition – lipid A modifications
Anti-microbial defenses are found throughout the animal as well as plant kingdoms. Indeed, even plants possess a diverse array of innate defense strategies, from production of anti-microbial peptides to activation of specific plant resistance proteins (Jones and Dangl, 2006). The initial recognition of microbial PAMPs by animals is based on similar membrane-bound host receptors as those that are also found in plants, indicating high evolutionary conservation. It is conceivable that the
Conclusions
Lipopolysaccharide recognition by the MD-2/TLR4 receptor complex is essential for the prevention of Gram-negative bacterial infections in most vertebrate animals. Despite the highly conserved lipid A biosynthetic pathway, which is common to most bacteria, several modification mechanisms have evolved that enable alterations of the most prominent lipid A features, i.e. the number of acyl chains and their length, the acylation and the phosphorylation pattern. These attempts of bacteria to evade
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This article belongs to Special Issue on Endotoxin, TLR4.