Cellular Microbiology (2008) 10(1), 1–8 doi:10.1111/j.1462-5822.2007.01059.x First published online 18 October 2007 Microreview Intracellular NOD-like receptors in innate immunity, infection and disease Luigi Franchi, Jong-Hwan Park, Michael H. Shaw, germline-encoded pattern-recognition receptors (PRRs) Noemi Marina-Garcia, Grace Chen, Yun-Gi Kim and that sense highly conserved microbial motifs, so-called Gabriel Núñez* pathogen-associated molecular patterns (PAMPs) (Kawai Department of Pathology and Comprehensive Cancer and Akira, 2006). PRRs can be found in the extracellular Center, University of Michigan Medical School, space, integrated in cellular membranes or in the cytosol. Ann Arbor, MI 48109, USA. Among the membrane-bound PRRs, the best-known PRRs are the Toll-like receptors (TLRs) that sense a wide array of microbial ligands at the cell surface or within Summary endosomes (Kawai and Akira, 2006). Cytoplasmic PRRs The innate immune system comprises several include the caspase-recruiting domain (CARD) helicases, classes of pattern-recognition receptors, including such as retinoic acid-inducible protein I and melanoma- Toll-like receptors (TLRs) and nucleotide binding and differentiation-associated protein 5, which are involved in oligomerization domain-like receptors (NLRs). TLRs antiviral responses (Kawai and Akira, 2006), and the recognize microbes on the cell surface and in endo- nucleotide binding oligomerization domain (NOD)-like somes, whereas NLRs sense microbial molecules in receptor (NLR) family that recognize primarily microbial the cytosol. In this review, we focus on the role of molecules of bacterial origin (Inohara et al., 2005). In NLRs in host defence against bacterial pathogens. humans, the NLR family is composed of 23 cytosolic Nod1 and Nod2 sense the cytosolic presence of mol- proteins characterized by the presence of a conserved ecules containing meso-diaminopimelic acid and NOD domain and leucine-rich repeats (LRRs) (Inohara muramyl dipeptide respectively, and drive the activa- et al., 2005). The general domain structure of the NLR tion of mitogen-activated protein kinase and NF-kB. In family members includes an amino-terminal effector contrast, Ipaf, Nalp1b and Cryopyrin/Nalp3 promote region that consists of a protein–protein interaction the assembly of inflammasomes that are required for domain such as the CARD, Pyrin or BIR domain, a cen- the activation of caspase-1. Mutation in several NLR trally located NOD domain, and carboxyl-terminal LRRs members, including NOD2 and Cryopyrin, is associ- that are involved in microbial sensing (Inohara et al., ated with the development of inflammatory disorders. 2005). Some members of the NLR family, namely Nod1 Further understanding of NLRs should provide new and Nod2, mediate activation of NF-kB and mitogen- insights into the mechanisms of host defence and the activated protein kinases (MAPKs) in response to pathogenesis of inflammatory diseases. peptidoglycan-related molecules (McDonald et al., 2005). A different set of NLRs, including Nalp1, Cryopyrin/Nalp3 and Ipaf, are involved in the activation of the protease Introduction caspase-1 (Franchi et al., 2006a). Ipaf is activated by Upon encountering pathogenic microorganisms, the bacterial flagellin (Franchi et al., 2006b; Miao et al., immunocompetent host activates two distinct effector 2006); mouse Nalp1b by lethal toxin produced by Bacillus mechanisms, the innate and the adaptive immune anthracis (Boyden and Dietrich, 2006); Cryopyrin is acti- defences, to ensure effective elimination of the invading vated in response to a variety of microbial molecules microbe. Unlike adaptive immune responses, the innate (Kanneganti et al., 2006; 2007; Mariathasan et al., 2006; immune system relies on phagocytic and non- Sutterwala et al., 2006) as well as endogenous ligands, haematopietic cells to sense the presence of pathogens. such as uric acid crystals (Martinon et al., 2006). The initial recognition of microbes is mediated by a set of While certain microbial molecules, such as meso- diaminopimelic acid (iE-DAP) and muramyl dipeptide Received 1 August, 2007; revised 6 September, 2007; accepted 13 September, 2007. *For correspondence. E-mail [email protected]; (MDP) (McDonald et al., 2005), are exclusively recog- Tel. (+1) 734 764 8514; Fax (+1) 734 647 9654. nized by NLRs, other PAMPs are also sensed by TLR © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd 2 L. Franchi et al. Fig. 1. Model for activation of Nod1 and Nod2 signalling pathways. The NLR proteins NOD1 and NOD2 sense intracellular iE-DAP and MDP respectively, leading to recruitment of the adaptor proteins RICK. Extracellular PAMPs are recognized by TLRs, which signal through MyD88, IRAK proteins and TRAF members independently of NOD1/NOD2. For clarity, the TLR pathway has been simplified. The subsequent activation of NF-kB and MAPKs results in the transcriptional upregulation of pro-inflammatory and host defence genes. family members. Flagellin, for example, is recognized by recruitment of RICK (RIP2), a serine threonine kinase that both Ipaf (Franchi et al., 2006b; Miao et al., 2006) and is required for Nod1- and Nod2-mediated NF-kB and TLR5 (Hayashi et al., 2001), although the amino acid MAPK activation (Inohara et al., 2000; Girardin et al., residues of flagellin that are recognized by these two 2001; Park et al., 2007a,b) (Fig. 1). Whereas Nod1 is PRRs appear to be different (Franchi et al., 2007a). ubiquitously expressed in various cell types, Nod2 is expressed at higher levels in phagocytic cells and Paneth cells of the small intestine (Inohara et al., 2005). Admin- Nod1 and Nod2 istration of Nod1 ligands to cells and mice induce chemok- Early studies revealed that Nod1 and Nod2 induce NF-kB ine production and recruitment of neutrophils in vivo activation when overexpressed in mammalian cells and (Masumoto et al., 2006). Furthermore, Nod1 stimulation enhance the response to specific microbial products inde- contributes to adaptive immune responses, although the pendently of TLRs (Inohara et al., 2001). Subsequent mechanism involved remains unclear (Fritz et al., 2007). studies revealed that Nod1 recognizes peptidoglycan- In vitro studies have demonstrated that many bacteria related molecules containing iE-DAP, which is found in express Nod1-stimulatory activity, which is highest in many Gram-negative and certain Gram-positive bacteria, Bacillus species (Hasegawa et al., 2007). Moreover, including the genus Listeria and Bacillus (Chamaillard infection of host cells by several pathogenic bacteria, et al. 2003; Girardin et al., 2003a). In contrast, Nod2 is including Shigella flexneri (Girardin et al., 2001), entero- activated by MDP, a peptidoglycan motif which is present invasive Escherichia coli (Kim et al., 2004), Listeria mono- in all Gram-positive and Gram-negative bacteria (Girardin cytogenes (Park et al., 2007b) and Campylobacter jejuni et al., 2003b; Inohara et al., 2003). Upon ligand recogni- (Zilbauer et al., 2007), results in Nod1-dependent NF-kB tion, Nod1 and Nod2 undergo conformational changes, activation. However, the role of Nod1 during in vivo infec- resulting in self-oligomerization via the NOD domain and tion with the exception of Helicobacter pylori remains © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd, Cellular Microbiology, 10, 1–8 NLRs and innate immunity and infection 3 poorly understood (Viala et al., 2004; Boughan et al., 2005). In contrast to CD, the Nod2 mutations associated 2006). with BS and EOS represent gain-of-function mutations Similar to Nod1, Nod2 has been implicated in the (Tanabe et al., 2004), which is consistent with the domi- detection of several pathogenic bacteria and induction of nant mode of inheritance of these diseases. innate immune responses to Streptococcus pneumoniae (Opitz et al., 2004), Mycobacterium tuberculosis (Fer- The inflammasome: a molecular machinery for werda et al., 2005), Staphylococcus aureus (Kapet- caspase-1 activation anovic et al. 2007) and L. monocytogenes (Kobayashi et al., 2005; Herskovits et al., 2007). The mechanism Caspase-1 is the prototypical inflammatory caspase and involved in the entry of MDP into the host cytosol is still mediates the proteolytic maturation of the cytokines IL-1b unknown, although in epithelial cells, an active transport and IL-18 (Lamkanfi et al., 2007a). Upon detection of mechanism for MDP through the peptide transporter specific microbial motifs, some NLRs switch conformation hPepT1 has been proposed (Ismair et al., 2006). MDP and assemble a molecular platform, the inflammasome, might also be generated from phagocytosed bacteria which is responsible for the processing and activation of and recognized via Nod2 in the cytosol (Herskovits pro-caspase-1 into the enzymatically active heterodimer et al., 2007). Stimulation of TLRs and Nod1 or Nod2 by composed of a 10 kDa and a 20 kDa chain (Fig. 2). Ipaf their respective agonists results in synergistic production senses cytosolic flagellin (Amer et al., 2006; Franchi of pro-inflammatory cytokines (Fritz et al., 2005; Koba- et al., 2006b; Miao et al., 2006), while the detection of yashi et al., 2005; Tada et al., 2005). This type of coop- microbial molecules by Cryopyrin depends on membrane erative signalling may increase the sensitivity of bacterial pore formation induced by several toxins, including mai- detection and reduce the threshold for Nod1/Nod2 and totoxin and nigericin, which are thought to aid the trans- TLR activation. location of microbial products into the