Increased NOD2-mediated recognition of N-glycolyl muramyl dipeptide

Abstract

Peptidoglycan-derived muramyl dipeptide (MDP) activates innate immunity via the host sensor NOD2. Although MDP is N-acetylated in most bacteria, mycobacteria and related Actinomycetes convert their MDP to an N-glycolylated form through the action of N-acetyl muramic acid hydroxylase (NamH). We used a combination of bacterial genetics and synthetic chemistry to investigate whether N-glycolylation of MDP alters NOD2-mediated immunity. Upon infecting macrophages with 12 bacteria, tumor necrosis factor (TNF) α secretion was NOD2 dependent only with mycobacteria and other Actinomycetes (Nocardia and Rhodococcus). Disruption of namH in Mycobacterium smegmatis obrogated NOD2-mediated TNF secretion, which could be restored upon gene complementation. In mouse macrophages, N-glycolyl MDP was more potent than N-acetyl MDP at activating RIP2, nuclear factor κB, c-Jun N-terminal kinase, and proinflammatory cytokine secretion. In mice challenged intraperitoneally with live or killed mycobacteria, NOD2-dependent immune responses depended on the presence of bacterial namH. Finally, N-glycolyl MDP was more efficacious than N-acetyl MDP at inducing ovalbumin-specific T cell immunity in a model of adjuvancy. Our findings indicate that N-glycolyl MDP has a greater NOD2-stimulating activity than N-acetyl MDP, consistent with the historical observation attributing exceptional immunogenic activity to the mycobacterial cell wall.

The protein Nod2: An innate receptor more complex than previously assumed

“…They showed that N-glycolyl MDP is more potent than N-acetyl MDP in activating Nod2-mediated signalling in vitro [75]. In mice challenged intraperitoneally with live wild-type Mycobacterium smegmatis or namH-deficient M. smegmatis, Nod2-dependent immune responses were disrupted in the absence of NamH [75]. Altogether, these data suggest that Nod2 may be exquisitely tuned to detect mycobacterial infections.…”

“…An interesting obervation is that mycobacteria convert their MDP into an N-glycolylated form through the action of N-acetyl muramic acid hydroxylase (NamH) while MDP is N-acetylated in most bacteria [74]. They showed that N-glycolyl MDP is more potent than N-acetyl MDP in activating Nod2-mediated signalling in vitro [75]. In mice challenged intraperitoneally with live wild-type Mycobacterium smegmatis or namH-deficient M. smegmatis, Nod2-dependent immune responses were disrupted in the absence of NamH [75].…”

Vitamin D, NOD2, autophagy and Crohn’s disease

“…NOD2 recognizes modified forms of muramyl didpeptide (MDP), a lysosomal breakdown product of bacterial peptidoglycan. Recent work by one of our team has shown that NOD2 is particularly sensitive to the N-glycolyl form of MDP produced by mycobacteria [14]. Remarkably, major NOD2 variants associated with CD contain frameshift or point mutations that encode proteins incapable of recognizing MDP, and are thus inactive signaling molecules.…”

Pathogenesis of Inflammatory Bowel Diseases

“…The NOD2 ligand muramyl dipeptide (MDP) is ubiquitous, indicating that broad classes of bacteria are capable of activating NOD2. However, the N-glycolyated form of muramyl dipeptide found in mycobacteria and actinomycetes more potently activates NOD2 compared to the Nacetylated form, found more frequently in gram-positive and gram-negative bacteria (Coulombe et al, 2009). Autophagy 16-like 1 (ATG16L1) has been strongly associated with CD and encodes a protein component of the autophagy complex (Levine & Deretic, 2007).…”