The innate immune system is the first line of defense against microbial invasion, relying on pattern recognition receptors to recognize external pathogenic microorganisms and then remove them. Toll-like receptors (TLRs) and Nod-like receptors (NLRs) are important receptors that mediate immune recognition. By identifying pathogen-associated molecular patterns, they not only initiate innate immune responses, but also activate adaptive immune responses. These processes are bridges between innate immunity and adaptive immunity.
Nucleotide-binding oligomerization domain-like receptors, in short NOD-like receptors (NLRs), are intracellular sensors of pathogen-associated molecular patterns (PAMPs) that enter the cell via phagocytosis or pores and damage-associated molecular patterns (DAMPs) that are associated with cell stress. The Nod-like receptor is a cytoplasmic recognition receptor that specifically recognizes "non-self-components" such as pathogenic microorganisms, non-microorganisms, and some dangerous signals in different parts of the organism with TLR, and interacts with each other to regulate the immune response in the body.
Studies show that NLR is mainly composed of three different domains: the leucine rich repeat (LRR) of the C-terminus, which plays an important role in the recognition of ligands; the N-terminal effector domain such as CARD (activating and recruitment domain) and PYD (Pyrin domain), mainly linked to NLR receptor molecules and downstream adaptor proteins and effector molecules; in the middle is the NACHT domain (neuronal apoptosis inhibitory protein cIITA HET-E TP1), which is very important for oligomerization and activation of NLR. According to the NACHT domain, human NLR is divided into NOD (nucleotide-binding oligomerization domain), NALP (neuronal apoptosis LRR pyrin-domain protein), CIITA (class II trasactivator), NAlP (neuronal apoptosis inhibitory protein), IPAF (ICE-protease-activating factor) and so on. Among them, NOD and NALP are the main members, and NOD refers to the oligomeric domain of the binding nucleotide. It is the first discovered NLR, which contains five CARD domains. The difference between NALP and NOD is that PYD replaces CARD with 14 members. It has been found that humans have more than 20 NLR members, most of which are widely expressed, such as NOD1, which is widely expressed in adult tissue cells, and NOD2, which is expressed in bone marrow-derived cells, especially immune cells such as macrophages and neutrophils (PMN). However, some expressions are restricted, for example, NALP3 is mainly expressed in immune cells, while NALP5 is mainly expressed in germ cells.
According to the phylogenetic relationships, NLRs can be divided into 3 subfamilies.
NODs subfamily consists of NOD1, NOD2, NOD3, NOD4 with CARD domain, CIITA containing acidic transactivator domain and NOD5 without any N-terminal domain.
The well-described receptors are NOD1 and NOD2. NOD1 and NOD2 recognize peptidoglycan motifs from bacterial cells which consist of N-acetylglucosamine and N-acetylmuramic acid. These sugar chains are cross-linked by peptide chains that can be sensed by NODs. NOD1 recognizes a molecule called meso-diaminopimelic acid (meso-DAP), which is mostly found in Gram-negative bacteria (for example Helicobacter pylori, Pseudomonas aeruginosa). NOD2 proteins can sense intracellular muramyl dipeptide (MDP), typical for bacteria such as Streptococcus pneumoniae or Mycobacterium tuberculosis. After NOD1 or NOD2 recognizes their ligands, it will recruit oligomerization of NACHT domain and CARD-CARD interaction with CARD-containing serine-threonin kinase RIP2 which leads to activation of RIP2. RIP2 mediates the recruitment of kinase TAK1 which phosphorylates and activates IκB kinase. The activation of IκB kinase results in the phosphorylation of inhibitor IκB which releases NF-κB and its nuclear translocation. NF-κB then activates expression of inflammatory cytokines.
NLRPs subfamily contains NLRP1-NLRP14 that is characterized by the presence of PYD domain. IPAF subfamily has two members – IPAF with CARD domain and NAIP with BIR domain.
The best characterized inflammasome is NLRP3, and the activation through PAMPs (such as microbial toxins (for example alpha-toxin Staphylococcus aureus) or whole pathogens, for instance Candida albicans, Saccharomyces cerevisiae, Sendai virus, Influenza.) or DAMPs (cell stress, such as extracellular ATP, extracellular glucose, crystals of monosodium urate (MSU), calcium pyrophosphate dihydrate (CPPD), alum, cholesterol or environmental irritants – silica, asbestos, UV irradiation and skin irritants.) leads to the oligomerization. The pyrin domain of NLRs binds to an adaptor protein ASC (PYCARD) via PYD-PYD interaction. ASC contains PYD and CARD domain and links the NLRs to inactive forms of caspase 1 through the CARD domain. All of these protein-protein interactions form a complex called the inflammasome. The aggregation of the pro-caspase-1 causes the auto-cleavage and formation of an active enzyme. Caspase-1 is important for the proteolytic processing of the pro-inflammatory cytokines IL-1β and IL-18.
NLR activates downstream molecules (such as NF-κB) through signal transduction to mediate the production of pro-inflammatory cytokines such as IL-1β, and the activation of NF-κB can up-regulate the expression of certain proteins (NOD2 and TLR2, etc.). It has recently been discovered that the domain of CARD1+2 polymer of NOD2 specifically interacts with NLRP1, NLRP3, NLRP12 and RIPK2, but has no effect on NLRP2, NLRP7, NLRP10 and NLRP11. A complex network between NLR and inflammatory factors and between NLR members forms a complex immune response that enhances the body's inflammatory response and antimicrobial infection.
Under physiological conditions, NLR plays a crucial role in the activation and regulation of various immune response genes. However, persistent activation or pathological conditions can lead to pathological damage to the tissue, and may even cause organ failure, septic shock, and death. Therefore, negative regulation of NLR signaling is important to maintain the health of the body. McDonald et al. found that over expression of Erbin protein can inhibit NOD2-dependent activation of NF-KB, which provides a potential mechanism for the treatment of related diseases. Mutations in the NLR can cause genetic diseases, mostly immunological or inflammatory diseases. A recent study by Negroni found that mutations in NOD2 are closely related to the susceptibility of individual Crohn's disease. Mutations in the domains of the NALP3 molecule cause Muckle wells syndrome.
NLR mediates the massive release of inflammatory factors through signaling pathways and is involved in tissue and organ damage. Its activity is closely related to many clinical diseases, and has received more and more attention and research has achieved notable results. There are still many problems that remain unresolved. Further research on NLR will provide new therapeutic strategies for the diagnosis, treatment and even prevention of stubborn antibiotic-resistant infectious diseases, inflammation-related diseases, autoimmune diseases, transplant rejection, tumors and the preparation of novel vaccine adjuvants.
Figure 1. NOD-like receptor signaling pathway.