Identification of the Critical Residues Involved in Peptidoglycan Detection by Nod1

Stephen E Girardin,Muguette Jéhanno,Dominique Mengin-Lecreulx,Philippe J Sansonetti,Pedro M Alzari,Dana J Philpott

doi:10.1074/jbc.m509537200

Abstract

Nod1 is an intracellular pattern recognition molecule activated following bacterial infection, which senses a specific muropeptide (l-Ala-d-Glu-meso-DAP (diaminopimelic acid); "Tri(DAP)") from peptidoglycan. Here we investigated the molecular basis of Tri(DAP) sensing by human (h) Nod1. Our results identified the domain responsible for Tri(DAP) detection in the center of the concave surface of hNod1 leucine-rich repeat domain. Amino acid residues critical for sensing define a contiguous surface patch that is largely conserved in Nod1 proteins from different species. Accordingly, the distinct specificities of human versus murine Nod1 toward muropeptide detection were also found to lie in this central cleft. Several splicing variants of Nod1 lacking repeats 7-9 have been characterized recently, the relative balance of which is thought to correlate with the onset of asthma or inflammatory bowel disease. We demonstrated that these isoforms failed to transduce NF-kappaB activation upon muropeptide stimulation. This study provided insights into the molecular mechanisms responsible for the detection of bacterial peptidoglycan by Nod1 and suggested that defects in Nod1-dependent peptidoglycan sensing may contribute to elicit certain inflammatory disorders.

Highlights

The innate immune system has evolved means to mediate recognition of microbes through the specific detection of highly conserved structures
It is becoming clear that Nod proteins, via the specific detection of muropeptide motifs, are key molecules involved in innate immunity and inflammation
We have undertaken an approach based on mutagenesis to gain more insight into the molecular basis of peptidoglycan detection by Nod1

Summary

Introduction

The innate immune system has evolved means to mediate recognition of microbes through the specific detection of highly conserved structures. Studies have identified polymorphisms in Nod associated with genetic predisposition to inflammatory bowel disease [22], atopic eczema [23], and asthma [24] In the latter case, a link between the genetic observation and any functional significance remains to be clearly addressed, because the polymorphisms found lie within the ninth intron of the Nod gene. Because recent evidence has identified the existence of several Nod splicing variants and their correlation with an increased risk for asthma and inflammatory bowel disease [22, 24], we analyzed these variants for their ability to sense peptidoglycan and muropeptides These isoforms all contained only a part of the TriDAP sensing domain, and we observed that only the full-length molecule could transduce NF-␬B activation upon stimulation. This study illustrates the importance of defining with precision the molecular determinants responsible for detection of peptidoglycan by Nod and suggests a link between defective Nod signaling and the onset of inflammatory disorders

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Conclusion