Fig. 3.

Classification of nucleotide-binding oligomerization domain proteins (Nods). Similar to TLRs, Nod1 and Nod2 are pattern recognition receptors (PRRs) responding to pathogen-associated molecular patterns (PAMPs) of bacterial origin (Newton and Dixit, 2012). Both Nods are localized to the cytoplasm, requiring either phagocytosis of bacteria and subsequent peptidoglycan entry into the cytoplasm or uptake of peptidoglycan by endocytosis, peptide transporters or pore-forming toxins. Nod1 is distributed across tissues and cell types whereas Nod2 is localized principally to leucocytes but can be induced in epithelium (Clarke and Weiser, 2011; Newton and Dixit, 2012). The primary difference between TLRs and Nods (and Nod-like receptors, NLRs) is the identity of the ligand and intracellular pathway. RICK or RIPK/RIP-2 initiate the eventual activation of NF-κB, as compared to MyD88 or TRIF. Similar to TLRs, Nods induce the expression and secretion of cytokines. For definitions, see List of abbreviations.