Inflammasome
Encyclopedia
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 cytokine
s 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
.
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).
, 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
.
, 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.
(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.
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.
, 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.
in addition to the NBD and LRR, which it uses to recruit procaspase-1 directly.
, 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 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.
Oligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...
consisting of caspase 1
Caspase 1
Caspase 1 is an enzyme that proteolytically cleaves other proteins, such as the precursor forms of the inflammatory cytokines interleukin 1-β and interleukin 18, into active mature peptides...
, PYCARD
PYCARD
Apoptosis-associated speck-like protein containing a CARD or ASC is a protein that in humans is encoded by the PYCARD gene.This gene encodes an adaptor protein that is composed of two protein–protein interaction domains: a N-terminal PYRIN-PAAD-DAPIN domain and a C-terminal caspase-recruitment...
, NALP
NALP
NALP is a type of NOD-like receptor. NALP proteins are part of the innate immunity and detect conserved pathogen characteristics such as peptodoglycan. It is thought that NALP proteins sense inherent danger, and link this with microbial products, creating a response under the concept of the...
and sometimes caspase 5
Caspase 5
Caspase 5 is an enzyme that proteolytically cleaves other proteins at an aspartic acid residue, and belongs to a family of cysteine proteases called caspases...
(also known as caspase 11 or ICH-3). It is expressed in myeloid cells
Myelocyte
A myelocyte is a young cell of the granulocytic series, occurring normally in bone marrow, but not in circulating blood .-Histology:...
and is a component of the innate immune system
Innate immune system
The innate immune system, also known as non-specific immune system and secondary line of defence, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner...
. 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 cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
s interleukin 1-β and interleukin 18
Interleukin 18
Interleukin-18 also known as IL18 is a protein which in humans is encoded by the IL18 gene. The protein encoded by this gene is a proinflammatory cytokine.- Function :...
.
The inflammasome is responsible for activation of inflammatory
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
processes, and has been shown to induce cell pyroptosis
Pyroptosis
Pyroptosis is a form of programmed cell death associated with antimicrobial responses during inflammation. In contrast to apoptosis, pyroptosis requires the function of caspase-1, and has been studied in the context of salmonella-infected macrophages....
, a process of programmed cell death distinct from apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
.
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)Toll-like receptor
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single, membrane-spanning, non-catalytic receptors that recognize structurally conserved molecules derived from microbes...
and C-type Lectin Receptors(CLRs)
C-type lectin
A C-type lectin is a type of carbohydrate-binding protein domain known as a lectin. The C-type designation is from their requirement for calcium for binding. Proteins that contain C-type lectin domains have a diverse range of functions including cell-cell adhesion, immune response to pathogens and...
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
Pyrin domain
A pyrin domain is a protein domain and a subclass of protein motif known as the death fold; it allows a pyrin domain containing protein to interact with other proteins that contain a pyrin domain...
and HIN domain-containing protein) family, termed absent in melanoma 2 (AIM2
AIM2
Interferon-inducible protein AIM2 also known as absent in melanoma 2 or simply AIM2 is a protein that in humans is encoded by the AIM2 gene...
) which assembles upon sensing foreign cytoplasmic double-stranded DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
(dsDNA).
Inflammatory cascade
Analogous to the apoptosomeApoptosome
The apoptosome is a large quaternary protein structure formed in the process of apoptosis. Its formation is triggered by the release of cytochrome c from the mitochondria in response to an internal or external cell death stimulus...
, which activates apoptotic
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
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)
CARD domain
Caspase recruitment domains, or Caspase activation and recruitment domains , are interaction motifs found in a wide array of proteins, typically those involved in processes relating to inflammation and apoptosis. These domains mediate the formation of larger protein complexes via direct...
or via the CARD of the adaptor protein ASC
PYCARD
Apoptosis-associated speck-like protein containing a CARD or ASC is a protein that in humans is encoded by the PYCARD gene.This gene encodes an adaptor protein that is composed of two protein–protein interaction domains: a N-terminal PYRIN-PAAD-DAPIN domain and a C-terminal caspase-recruitment...
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
IL1B
Interleukin-1 beta also known as catabolin, is a cytokine protein that in humans is encoded by the IL1B gene. IL-1β precursor is cleaved by caspase 1 . Cytosolic thiol protease cleaves the product to form mature IL-1β.- Function :Interleukin 1 was discovered by Gery in 1972...
, cleavage of pro-IL-18 into 1L-18
Interleukin 18
Interleukin-18 also known as IL18 is a protein which in humans is encoded by the IL18 gene. The protein encoded by this gene is a proinflammatory cytokine.- Function :...
to induce IFN-γ
Interferon-gamma
Interferon-gamma is a dimerized soluble cytokine that is the only member of the type II class of interferons. This interferon was originally called macrophage-activating factor, a term now used to describe a larger family of proteins to which IFN-γ belongs...
secretion and natural killer cell
Natural killer cell
Natural killer cells are a type of cytotoxic lymphocyte that constitute a major component of the innate immune system. NK cells play a major role in the rejection of tumors and cells infected by viruses...
activation, cleavage and inactivation of IL-33
Interleukin 33
Interleukin 33 is a cytokine belonging to the IL-1 superfamily. IL-33 induces helper T cells, mast cells, eosinophils and basophils to produce type 2 cytokines. This cytokine was previously named NF-HEV 'nuclear factor in high endothelial venules' since it was originally identified in these...
, DNA fragmentation and cell pore formation, inhibition of glycolytic
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
enzymes, activation of lipid biosynthesis and secretion of tissue-repair mediators such as pro-IL-1α
IL1A
Interleukin-1 alpha is a protein that in humans is encoded by the IL1A gene.The protein encoded by this gene is a cytokine of the interleukin-1 family. Interleukin-1 alpha possesses a wide spectrum of metabolic, physiological, haematopoietic activities, and plays one of the central roles in the...
. 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
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
.
NLR-subset inflammasomes
NLRP1NLRP1
NACHT, LRR and PYD domains-containing protein 1 is a protein that in humans is encoded by the NLRP1 gene.-Interactions:NLRP1 has been shown to interact with caspase 9 and APAF1.-Further reading:...
, NLRP3
NALP3
NACHT, LRR and PYD domains-containing protein 3 or cryopyrin is a protein that in humans is encoded by the NLRP3 gene. The gene is also called cold induced autoinflammatory syndrome 1 and is located on the long arm of chromosome 1...
and NLRC4
NLRC4
NLR family CARD domain-containing protein 4 is a protein that in humans is encoded by the NLRC4 gene.-Interactions:NLRC4 has been shown to interact with caspase 1 and NOD2.-Further reading:...
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)
RNTP
A ribonucleotide tri-phosphate is a ribonucleotide with 3 phosphate groups. rNTPs are the building blocks of RNA synthesis as well as the synthesis of primers in DNA replication. They also serve as stores of chemical energy, chiefly Adenosine triphosphate ....
and is important for self-oligomerization. The second is a C-terminus leucine-rich repeat
Leucine-rich repeat
A leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...
(LRR), which serves as a ligand-recognition domain for other receptors(e.g. TLR
TLR
The three letter abbreviation TLR can mean* Toll-like receptors, proteins constituting an important part of the immune system* Tonic labyrinthine reflex, a primitive reflex found in newborn humans* Twin-lens reflex camera, a photo camera with two lenses...
) or microbial ligands.
Structure
In addition to NBD and LRR, NLRP1 contains at its N-terminal a pyrin domainPyrin domain
A pyrin domain is a protein domain and a subclass of protein motif known as the death fold; it allows a pyrin domain containing protein to interact with other proteins that contain a pyrin domain...
(PYD) and at its C-terminal an FIIND motif and a CARD
CARD domain
Caspase recruitment domains, or Caspase activation and recruitment domains , are interaction motifs found in a wide array of proteins, typically those involved in processes relating to inflammation and apoptosis. These domains mediate the formation of larger protein complexes via direct...
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
PYCARD
Apoptosis-associated speck-like protein containing a CARD or ASC is a protein that in humans is encoded by the PYCARD gene.This gene encodes an adaptor protein that is composed of two protein–protein interaction domains: a N-terminal PYRIN-PAAD-DAPIN domain and a C-terminal caspase-recruitment...
, 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 rNTPRNTP
A ribonucleotide tri-phosphate is a ribonucleotide with 3 phosphate groups. rNTPs are the building blocks of RNA synthesis as well as the synthesis of primers in DNA replication. They also serve as stores of chemical energy, chiefly Adenosine triphosphate ....
to the nucleotide-binding domain of NLRP1. NLRP1 has been shown to confer macrophage sensitivity to anthrax lethal toxin(LT)
Anthrax toxin
Anthrax toxin is a three-protein exotoxin secreted by virulent strains of the bacterium, Bacillus anthracis--the causative agent of anthrax. The toxin was first discovered by Harry Smith in 1954. Anthrax toxin is composed of a cell-binding protein, known as protective antigen , and two enzyme...
, suggesting the role of bacterial toxins in inducing inflammasome formation.
NLRP1 activity is regulated by anti-apoptotic proteins Bcl-2
Bcl-2
Bcl-2 is the founding member of the Bcl-2 family of apoptosis regulator proteins encoded by the BCL2 gene. Bcl-2 derives its name from B-cell lymphoma 2, as it is the second member of a range of proteins initially described in chromosomal translocations involving chromosomes 14 and 18 in...
and Bcl-x(L) which, in resting cells, associate with and inhibit NLRP1 activity.
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 gonorrhoeaeNeisseria gonorrhoeae
Neisseria gonorrhoeae, also known as gonococci , or gonococcus , is a species of Gram-negative coffee bean-shaped diplococci bacteria responsible for the sexually transmitted infection gonorrhea.N...
, bacterial toxins e.g. nigericin
Nigericin
Nigericin is an antibiotic derived from Streptomyces hygroscopicus. Its isolation was described in the 1950s, and in 1968 the structure could be elucidated by X-ray crystallography. The structure and properties of nigericin are similar to the antibiotic monensin. Commercially it is obtained as a...
and maitotoxin
Maitotoxin
Maitotoxin is an extremely potent toxin produced by Gambierdiscus toxicus, a dinoflagellate species. Maitotoxin is so potent that it has been demonstrated that an intraperitoneal injection of 0.13 µg/kg was lethal in mice...
, and most notably, crystallized endogenous molecules. Cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
crystals and monosodium urate(MSU)
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...
crystals increase NLRP3-induced IL-1β-production and this process is thought to be abrogated in atherosclerosis
Atherosclerosis
Atherosclerosis is a condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol...
and gout
Gout
Gout is a medical condition usually characterized by recurrent attacks of acute inflammatory arthritis—a red, tender, hot, swollen joint. The metatarsal-phalangeal joint at the base of the big toe is the most commonly affected . However, it may also present as tophi, kidney stones, or urate...
, 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.
Structure
NLRC4 (also known as IPAF) is the only known subset of the NLRC family to form an inflammasome and contains only a CARD domainCARD domain
Caspase recruitment domains, or Caspase activation and recruitment domains , are interaction motifs found in a wide array of proteins, typically those involved in processes relating to inflammation and apoptosis. These domains mediate the formation of larger protein complexes via direct...
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 typhimuriumSalmonella enterica enterica
Salmonella enterica enterica is a subspecies of Salmonella enterica, the rod shaped, flagellated, aerobic, Gram-negative bacterium. It is a member of the genus Salmonella. Many of the pathogenic serovars of the S...
, Legionella pneumophila
Legionella pneumophila
Legionella pneumophila is a thin, ærobic, pleomorphic, flagellated, non-spore forming, Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of legionellosis or Legionnaires' disease.-Characterization:L...
and Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...
. The activation process by these bacteria is unclear but is thought to require a type 3
Type three secretion system
Type three secretion system is a protein appendage found in several Gram-negative bacteria....
or type 4 secretion system provided by bacterial flagellin
Flagellin
Flagellin is a protein that arranges itself in a hollow cylinder to form the filament in bacterial flagellum. It has a mass of about 30,000 to 60,000 daltons...
, which gains entry through the cell membrane and is then detected by NLRC4, activating it.
AIM2
Main article: AIM2AIM2
Interferon-inducible protein AIM2 also known as absent in melanoma 2 or simply AIM2 is a protein that in humans is encoded by the AIM2 gene...
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
IFI16
Gamma-interferon-inducible protein Ifi-16 also known as interferon-inducible myeloid differentiation transcriptional activator is a protein that in humans is encoded by the IFI16 gene.- Function :...
) attached to a PYD
Pyrin domain
A pyrin domain is a protein domain and a subclass of protein motif known as the death fold; it allows a pyrin domain containing protein to interact with other proteins that contain a pyrin domain...
, which it uses to recruit the adaptor protein ASC
PYCARD
Apoptosis-associated speck-like protein containing a CARD or ASC is a protein that in humans is encoded by the PYCARD gene.This gene encodes an adaptor protein that is composed of two protein–protein interaction domains: a N-terminal PYRIN-PAAD-DAPIN domain and a C-terminal caspase-recruitment...
during inflammasome formation. It is thought to oligomerize upon sensing bacterial and viral DNA, as well as aberrant host DNA in autoimmunity.