NOD1 and NOD2 in inflammatory and infectious diseases

Abstract

It has been long recognized that NOD1 and NOD2 are critical players in the host immune response, primarily by their sensing bacterial peptidoglycan-conserved motifs. Significant advances have been made from efforts that characterize their upstream activators, assembly of signaling complexes, and activation of downstream signaling pathways. Disruption in NOD1 and NOD2 signaling has also been associated with impaired host defense and resistance to the development of inflammatory diseases. In this review, we will describe how NOD1 and NOD2 sense microbes and cellular stress to regulate host responses that can affect disease pathogenesis and outcomes.

Keywords: NOD1; NOD2; Nod-like receptor; immunity; inflammation; innate.

FIGURE 1

Schematic representation of NOD-like receptors 1 and 2. NOD1 consists of one N-terminal caspase activation and recruitment domain (CARD), and NOD2 has two in tandem. In both receptors, the domain CARD is followed by a core nucleotide-binding domain (NBD) and a C-terminal leucine-rich repeat domain (LRR). In the absence of ligand, NOD1 and NOD2 are in an inactive monomeric form, maintained by the binding of LRR domain into NBD and stabilized by chaperone proteins, such as HSP70 or HSP90. Upon recognition of PGN (peptidoglycan) ligands, a conformational change occurs, resulting in homo-oligomerization of two NOD molecules that once activated trigger inflammatory signaling pathways

FIGURE 2

Basic structure of peptidoglycan (PGN) motifs recognized by NOD1 and NOD2. Lys-PGN is present in Gram-positive bacteria, whereas DAP-PGN constitutes the cell wall of Gram-negative bacteria. Abbreviations: D-Ala: D-alanine; D-Glu: D-glutamic acid; GlcNAc: N-acetylglucosamine; L-Ala: L-alanine; L-Lys: L-lysine; mDAP: meso-diaminopimelic acid; MurNAc: N-acetylmuramic acid; iE-DAP: γ-D-glutamyl-meso-diaminopimelic acid; M-Tetra-DAP: MurNAc-L-Ala-D-Glu-mDAP-D-Ala; M-TriDAP: MurNAc-L-Ala-D-Glu-mDAP

FIGURE 3

Mechanisms of PGN entrance into host cells to trigger NOD signaling. Host cells can internalize PGN, such as MDP through different mechanisms: (A) Phagocytosis of bacteria can release PGN in the cytoplasm after degradation of bacteria on the phagosome. Some pathogenic bacteria can evade the phagosome and replicate in the host cell, thus releasing PGN in the cytoplasm; (B) extracellular PGN fragments can enter the host cell through endocytosis and transported to the cytosol through SLC15A3/4, lysosomal membrane transporters. Alternatively, the dipeptide transporter hPepT1, expressed in the intestine, can be also carrier-free PGN fragments toward the host cell; (C) some bacteria can deliver PGN into the host cell cytoplasm through its secretion system; (D) uptake of outer membrane vesicles (OMVs) released by Gram-negative bacteria facilitates internalization of PGN

FIGURE 4

NOD1 and NOD2 signaling pathways. Sensing of iE-DAP and/or MDP occurs via the LRR domains by NOD1 and NOD2, respectively. Once activated, NOD receptors recruit the kinase RIP2 which can interact with LUBAC and the kinase complex TAK1:TAB. This complex of proteins can activate two different pathways: (1) activation of the IKK complex (NEMO: IKKα: IKKβ) which in turn activates NF-κB nuclear translocation; (2) activation of MAP kinases (eg, p38, ERK, and JNK) will activate the transcription factor AP1. Both NF-κB and MAPK pathways will induce gene expression of pro-inflammatory cytokines and mediators, including anti-microbial peptides (AMP). Alternatively, ER stress triggers the unfolded protein response (UPR) caused by accumulation of unfolded or misfolded proteins and bacterial infection. ER stress activates IRE1α (inositol-requiring enzyme 1α) which recruits TRAF2, NOD receptors, and RIPK2 to the ER membrane and initiates inflammatory response through NF-κB signaling; however, the exact mechanism of NF-κB signaling activation by ER stress is still unclear. A cytosolic UPR (cUPR) has been described that is required for NOD1 and NOD2 complex formation and activation of NF-κB. Upon PGN recognition, the heat-shock protein HSPBB8 is released from the complex with HRI and binds NOD1 or NOD2 allowing the folding and release of these receptors from endomembranes. This pathway is associated with NF-κB activation, although the mechanism is not totally known. At the bacterial entry site on the plasma membrane, NOD2 can recruit the autophagy protein ATG16L1, leading to elimination of intracellular pathogens. NOD1 and NOD2 signaling can also induce type I interferon expression. NOD2-RIPK2 activates TRAF3 which recruits TBK1 and induces the IFN-β transcription factor IRF7. In addition, NOD2 is also activated by sensing of virus-derived single-strand RNA (ssRNA). Binding of NOD2/TRAF3 to mitochondrial antiviral signaling (MAVS) induces activation of IRF3 which induces IFN-β gene expression

FIGURE 5

NOD2 Polymorphisms in IBD. (A) Mutations in the NBD are associated with Blau syndrome, and single nucleotide polymorphisms in the LRR domain are associated with Crohn’s disease. (B) NOD2 mutations result in poor sensing of MDP fragments impairing NF-κB activation and decrease production of anti-microbial peptides (AMPs) by Paneth cells. NOD2 variants also fail to recruit ATG16L1 resulting in impaired autophagy by epithelial cells. Dysregulation of these mechanisms leads to reduced bacterial clearance and loss of mucosal barrier function. NOD2 is also important for maintenance of the goblet cell number and mucus secretion. In addition, loss of commensal bacteria homeostasis possibly related to defects in NOD2 surveillance can lead to dysbiosis, which is associated with increased mucosal adherence and consequent bacterial translocation. NOD2 stimulation by MDP also maintains stem cell survival through protection against oxidative stress-mediated cell death, and NOD2 depletion results in reduced stem cell survival and proliferation. Finally, NOD2 variants can cause dysregulation of immune responses in the lamina propria. Defective NOD2 expressed in macrophages and DC is not able to suppress TLR2 signaling leading to overactivation of NF-κB and increased expression of IL-12, IL-1β, and IFN-γ, which in turn can lead to damage of the epithelial layer. In addition, DC autophagy-induced T_reg cells are reduced favoring a dysregulated inflammation. Abbreviations: ATG16L1: autophagy-related protein 16-like 1; DC: dendritic cell; MDP: muramyl dipeptide; NBD: Nucleotide-binding domain; NOD2: nucleotide-binding oligomerization domain 2; NF-κB: nuclear factor kappa B; Th1: T-helper 1; TLR: Toll-like receptor; Treg: T regulatory cell