NOD1 and NOD2: signaling, host defense, and inflammatory disease

Abstract

The nucleotide-binding oligomerization domain (NOD) proteins NOD1 and NOD2, the founding members of the intracellular NOD-like receptor family, sense conserved motifs in bacterial peptidoglycan and induce proinflammatory and antimicrobial responses. Here, we discuss recent developments about the mechanisms by which NOD1 and NOD2 are activated by bacterial ligands, the regulation of their signaling pathways, and their role in host defense and inflammatory disease. Several routes for the entry of peptidoglycan ligands to the host cytosol to trigger activation of NOD1 and NOD2 have been elucidated. Furthermore, genetic screens and biochemical analyses have revealed mechanisms that regulate NOD1 and NOD2 signaling. Finally, recent studies have suggested several mechanisms to account for the link between NOD2 variants and susceptibility to Crohn's disease. Further understanding of NOD1 and NOD2 should provide new insight into the pathogenesis of disease and the development of new strategies to treat inflammatory and infectious disorders.

Figure 1. Potential mechanisms for bacterial recognition by NOD1 and NOD2

NOD1 and NOD2 sense intracellular PGN fragments from bacteria. Host cells can internalize PGN by multiple routes such as phagocytosis of bacteria and subsequent bacterial degradation (1), uptake of PGN fragments from bacteria derived extracellular outer membrane vesicles (2), transport across host membranes via channels, pore-forming molecules or bacterial secretion systems (3), endocytosis (4), or from neighboring cells (5). Once inside the cell, NOD1 and NOD2 activation typically involves their re-localization to various intracellular locations such as the plasma and endosomal membranes via different adaptor molecules that are differentially expressed in host cells. For example, the activation of NOD2 by intracellular pathogens induces the formation of an autophagosome, which is mediated by ATG16L1.

Figure 2. NOD2 signaling pathways for gene activation

NOD2 interacts directly with intracellular bacterial PGN fragments containing the MDP motif. Ligand recognition relieves intra-molecular autoinhibitory interactions leading to NOD oligomerization. Recruitment of the downstream RIPK2 Ser/Thr kinase occurs through CARD-CARD domain interactions. Subsequent activation of the NF-κB and MAPK pathways results in the transcriptional up-regulation of pro-inflammatory and host defense genes. Multiple steps in the pathway are regulated either positively or negatively by post-translational modifications such as phosphorylation and pUb events. Multiple regulatory genes act to influence various steps in the pathway often in a cell-type dependent manner.

Figure 3. NOD2 and Crohn’s disease

NOD2 plays a crucial role in regulating intestinal homeostasis. By sensing microbiota-derived PGN fragments, NOD2 activates NF-κB, which, in turn, leads to the production of antimicrobial peptides (AMPs) in Paneth cells (PC) that provide a barrier between the microorganisms and the epithelial layer. Activation of NOD2 in dendritic cells (DCs) leads to production of the interleukin-23 (IL-23) thus promoting an early mucosal T helper 17 (Th17) cell response that enhances barrier protection by inducing IL-22 and regenerating islet-derived protein IIIγ (REGIIIγ). NOD2 activation in stromal cells also promotes CC-ligand 2 (CCL2)-mediated recruitment of inflammatory monocytes (Mo) to the intestine. Interaction between NOD2 and ATG16L1 promotes autophagosome formation in intestinal epithelial cells (IEC) and intraepithelial bacterial clearance. Crohn’s disease-associated NOD2 variants perturb many aspects of immune homeostasis including reduced MDP sensing in both macrophages (Mac) and DCs, impaired anti-microbial responses in Paneth cells, and altered autophagy leading to defective barrier function and/or bacterial clearance. These alterations along with the development of dysbiosis may lead to enhanced mucosal adherence and translocation of bacteria. MDP, muramyl-dipeptide, GC, globet cells, IELs, intraepithelial lymphocytes, N, neutrophils, Th1, T helper 1.