SummaryThe innate immune system comprises several classes of pattern-recognition receptors, including Toll-like receptors (TLRs) and nucleotide binding and oligomerization domain-like receptors (NLRs). TLRs recognize microbes on the cell surface and in endosomes, whereas NLRs sense microbial molecules in the cytosol. In this review, we focus on the role of NLRs in host defence against bacterial pathogens. Nod1 and Nod2 sense the cytosolic presence of molecules containing meso-diaminopimelic acid and muramyl dipeptide respectively, and drive the activation of mitogen-activated protein kinase and NF-kB. In contrast, Ipaf, Nalp1b and Cryopyrin/Nalp3 promote the assembly of inflammasomes that are required for the activation of caspase-1. Mutation in several NLR members, including NOD2 and Cryopyrin, is associated with the development of inflammatory disorders. Further understanding of NLRs should provide new insights into the mechanisms of host defence and the pathogenesis of inflammatory diseases.

Muramyl dipeptide (MDP), the microbial activator of nucleotide-binding oligomerization domain 2 (Nod2), induces NF-κB and MAPK activation, leading to the production of multiple anti-bacterial and proinflammatory molecules. In addition, MDP has been implicated in IL-1β secretion through the regulation of caspase-1. However, the mechanisms that mediate caspase-1 activation and IL-1β secretion in response to MDP stimulation remain poorly understood. We show here that fluorescent MDP molecules are internalized in primary macrophages and accumulate in granular structures that colocalize with markers of acidified endosomal compartments. The uptake of MDP was Nod2-independent. Upon ATP stimulation, labeled MDP was rapidly released from acidified vesicles into the cytosol, a process that required functional pannexin-1. Caspase-1 activation induced by MDP and ATP required pannexin-1 and Cryopyrin but was independent of Nod2. Conversely, induction of pro-IL-1β mRNA by MDP stimulation was abolished in Nod2-deficient macrophages but unimpaired in macrophages lacking Cryopyrin. These studies demonstrate a Nod2-independent mechanism mediated through pore-forming pannexin-1 that is required for intracellular delivery of MDP to the cytosol and caspase-1 activation. Furthermore, the work provides evidence for distinct roles of Nod2 and Cryopyrin in the regulation of MDP-induced caspase-1 activation and IL-1β secretion.

Muramyl dipeptide (MDP), the NOD2 agonist, induces NF-κB and MAPK activation leading to the production of antimicrobial and proinflammatory molecules. MDP is internalized into acidified vesicles in macrophages. However, the endocytic mechanism of MDP uptake that induces NOD2 signaling is unknown. We now report the identification of an endocytosis pathway dependent on clathrin and dynamin that mediates MDP internalization and NOD2 activation. Intracellular MDP uptake was inhibited by chlorpromazine, a drug that disrupts clathrin-dependent endocytosis, but not by compounds that block pinocytosis or cellular entry via scavenger or mannose receptors. In contrast, MDP uptake and NOD2-dependent signaling were unimpaired in macrophages deficient in PepT1, a peptide transporter previously implicated in MDP internalization. Both chlorpromazine and knockdown of clathrin expression by RNA interference attenuated MDP-induced NF-κB and MAPK activation. Furthermore, MDP uptake and NOD2-dependent signaling were impaired by inhibition of dynamin, a GTPase required for budding of clathrin-coated vesicles from the plasma membrane. Finally, bafilomycin A, a specific inhibitor of the vacuolar proton pump, blocked MDP accumulation in acidified vesicles and cytokine responses, suggesting that vacuolar maturation is important for MDP-induced NOD2 signaling. These studies provide evidence for a clathrin- and dynamin-dependent endocytosis pathway that mediates MDP uptake and NOD2 activation.

Background: Autophagy is triggered by NOD2 as an anti-bacterial response. Results: NOD2-stimulated autophagy requires RIP2-dependent activation of p38 MAPK and repression of the PP2A phosphatase in intestinal epithelial cell lines. Conclusion: RIP2 kinase activity is necessary for anti-bacterial autophagy induction by NOD2. Significance: These findings provide novel molecular targets for modulation of autophagy as an anti-bacterial response.

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