Inflammasome

The inflammasome is a multiprotein oligomer consisting of caspase 1, PYCARD, NALP and sometimes caspase 5 (also known as caspase 11 or ICH-3). It is expressed in myeloid cells and is a component of the innate immune system. The exact composition of an inflammasome depends on the activator which initiates inflammasome assembly i.e. dsRNA will trigger one inflammasome composition whereas asbestos will assemble a different variant. The inflammasome promotes the maturation of inflammatory cytokines interleukin 1-β and interleukin 18.

The inflammasome is responsible for activation of inflammatory processes, and has been shown to induce cell pyroptosis, a process of programmed cell death distinct from apoptosis.

Introduction
During an infection, one of the first forms of defence 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-like helicase 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 coined 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).

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-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 IL1B, cleavage of pro-IL-18 into 1L-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 nuceotide-binding domain (NBD) which is bound to 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

 * See NLRP1 for gene information

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

 * See NALP3 for gene information

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 Neisseria gonorrhoeae, bacterial toxins e.g. nigericin and maitotoxin, 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. 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.

NLRC4

 * See NLRC4 for gene information

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 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. It is thought to oligomerize upon sensing bacterial and viral DNA, as well as aberrant host DNA in autoimmunity.