Inflammasome

History

The inflammasome was discovered by the team of Prof. Jürg Tschopp, at the University of Lausanne, in 2002.

Function

During an infection, one of the first forms of defense employed by the innate immune response is a group of pattern recognition receptors (PRRs) encoded in the germline to recognize molecular patterns expressed by invading pathogens. These may either be on the membrane surface e.g. Toll-like receptors (TLRs) and C-type Lectin Receptors (CLRs) or inside the cytoplasm e.g. Nod-like receptors (NLRs) and RIG-I-like receptors (RLRs). In 2002, it was first reported by Martinon et al. that a subset of NLRs named NLRP1 were able to assemble and oligomerize into a common structure which collectively activated the caspase-1 cascade, thereby leading to the production of pro-inflammatory cytokines especially IL-1B and IL-18. This NLRP1 multi-molecular complex was dubbed the ‘inflammasome’, which spurred much interest in the following years; since then, several other inflammasomes were discovered, two of which are also NLR subsets—NLRP3 and NLRC4. More recently, Hornung et al. classified an inflammasome of the PYHIN (pyrin and HIN domain-containing protein) family, termed absent in melanoma 2 (AIM2) which assembles upon sensing foreign cytoplasmic double-stranded DNA (dsDNA). Notably, the pyrin domain of the adaptor protein ASC has recently been shown to function as a prion-like domain, through a self-perpetuating manner upon activation.

Inflammatory cascade

Analogous to the apoptosome, which activates apoptotic cascades, the inflammasome activates an inflammatory cascade. Once active, the inflammasome binds to pro-caspase-1 (the precursor molecule of caspase-1), either homotypically via its own caspase activation and recruitment domain (CARD) or via the CARD of the adaptor protein ASC which it binds to during inflammasome formation. In its full form, the inflammasome appositions together many p45 pro-caspase-1 molecules, inducing their autocatalytic cleavage into p20 and p10 subunits. Caspase-1 then assembles into its active form consisting of two heterodimers with a p20 and p10 subunit each. Once active, it can then carry out a variety of processes in response to the initial inflammatory signal. These include the proteolytic cleavage of pro-IL-1B at Asp116 into IL1β, cleavage of pro-IL-18 into IL-18 to induce IFN-γ secretion and natural killer cell activation, cleavage and inactivation of IL-33, DNA fragmentation and cell pore formation, inhibition of glycolytic enzymes, activation of lipid biosynthesis and secretion of tissue-repair mediators such as pro-IL-1α. Additionally, AIM2 contains a HIN200 domain which senses and binds foreign cytoplasmic dsDNA and activates NF-κB, a role that is crucial in bacterial and viral infection.

NLR-subset inflammasomes

NLRP1, NLRP3 and NLRC4 are subsets of the NLR family and thus have two common features: the first is a nucleotide-binding domain (NBD) which is bound by ribonucleotide-phosphates (rNTP) and is important for self-oligomerization. The second is a C-terminus leucine-rich repeat (LRR), which serves as a ligand-recognition domain for other receptors (e.g. TLR) or microbial ligands.

NLRP1

Structure

In addition to NBD and LRR, NLRP1 contains at its N-terminal a pyrin domain (PYD) and at its C-terminal an FIIND motif and a CARD which distinguishes it from the other inflammasomes. Upon activation, the C-terminal CARD homotypically interacts with the CARD of procaspase-1 or procaspase-5, while its N-terminal PYD homotypically interacts with the PYD of adaptor protein ASC, whose CARD can then recruit another pro-caspase-1. The overall recruitment and cleavage of procaspase-1 can then activate all downstream caspase-1 pathways.

Activation

The mechanism of NLRP1 activation is unclear but has been proposed by Reed and colleagues to be a two-step process involving first activation by microbial ligands, followed by binding of an rNTP to the nucleotide-binding domain of NLRP1. NLRP1 has been shown to confer macrophage sensitivity to anthrax lethal toxin (LT), suggesting the role of bacterial toxins in inducing inflammasome formation.

NLRP1 activity is regulated by anti-apoptotic proteins Bcl-2 and Bcl-x(L) which, in resting cells, associate with and inhibit NLRP1 activity.

NLRP3

Structure

In addition to the NBD and LRR domains, NLRP3 contains a PYD domain like NLRP1 and thus activates caspase-1 the same way, using its PYD to recruit ASC. It forms only one oligomer per cell, and its oligomer is made of seven NLRP3 molecules. It is known to be the biggest inflammasome of all, covering about 2 um in diameter.

Activation

NLRP3 oligomerization is activated by a large number of stimuli, which has implicated studies into its activation pathway. Its activity has been shown to be induced and/or increased by low intracellular potassium concentrations, viruses e.g. influenza A and bacteria e.g. Neisseria gonorrhoeae, bacterial toxins e.g. nigericin and maitotoxin, liposomes, urban particulate matter, and most notably, crystallized endogenous molecules. Cholesterol crystals and monosodium urate (MSU) crystals increase NLRP3-induced IL-1β-production and this process is thought to be abrogated in atherosclerosis and gout, where these crystals form respectively in the cell. It has also been proven that inorganic particles like titanium dioxide, silicon dioxide and asbestos trigger the inflammasome-response. Pore-forming toxins and ATP-activated pannexin-1 may also trigger K+ efflux and grant access of toxins into the cell to directly activate NLRP3. Evidence indicates that NLRP3 inflammasome activation is involved in sleep regulation.

NLRC4

Structure

NLRC4 (also known as IPAF) is the only known subset of the NLRC family to form an inflammasome and contains only a CARD domain in addition to the NBD and LRR, which it uses to recruit procaspase-1 directly.

Activation

NLRC4 is activated by bacteria, a number of which have been identified using murine macrophage culture studies: Salmonella typhimurium, Legionella pneumophila and Pseudomonas aeruginosa. The activation process by these bacteria is unclear but is thought to require a type 3 or type 4 secretion system provided by bacterial flagellin, which gains entry through the cell membrane and is then detected by NLRC4, activating it.

AIM2

Main article: AIM2

AIM2 is an acronym for absent in melanoma 2, and is sometimes also referred to as Interferon-inducible protein.

Structure

AIM2 is a non-NLR family protein. It is a 343 amino acid protein with pyrin (DAPIN) and a HIN-200 domains, the former of which is activated in AIM2 by dsDNA.

Function

AIM2 is referred to as the DNA inflammasome for its ability to detect foreign dsDNA, using a HIN200 (hematopoietic interferon-inducible nuclear antigens with 200 amino acid repeats) domain (encoded by IFI16) attached to a PYD, which it uses to recruit the adaptor protein ASC during inflammasome formation. AIM2 binds dsDNA with its C-terminal domain. The PYDdomain of AIM2 homotypically interacts by PYD-PYD interactions with ASC. The ASC CARD domain recruits procaspase-1 into the complex. Caspase-1 activates maturation of proinflammatory cytokines (IL-1b, IL-18). AIM2 is activated by viral dsDNA, bacterial dsDNA and also aberrant host dsDNA. Activation of the AIM2 is supposed to play role in autoimmune responses during the autoimmune disease systematic lupus erythematosus. AIM2 inflammasome is also activated by pharmacological disruption of nuclear envelope integrity.