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. [5] [6]
AIM2 is a cytoplasmic sensor found in hematopoietic cells that recognizes the presence of double-stranded DNA (dsDNA) of microbial or host cellular origin. [7] AIM2-like receptor (ALR) family was founded on AIM2 and now consists of four members in human genome. [8] Activated AIM2 recruits apoptosis-associated speck-like protein containing a CARD (ASC), resulting in caspase-1 binding, and forming of AIM2 inflammasome. This signaling contributes to the defense against bacterial and viral DNA. The AIM2 inflammasome can also be an integral component of the AIM2-PANoptosome to drive PANoptosis. [9] [10]
Proteins belonging to ALR family usually contain an N-terminal pyrin (PYD) domain, and one or two HIN domains. AIM2 consists of two domains connected through a long linker: an N-terminal PYD domain (amino acids 1-87), and a C-terminal HIN-200 domain (amino acids 138–337). The PYD domain mediates homotypic protein-protein interaction, while the HIN domain binds to DNA with its two tandem oligonucleotide/oligosaccharide binding (OB) folds. [11]
AIM2 is a component of the innate immune system that functions as a cytoplasmic dsDNA sensor playing a role in antiviral and antibacterial defenses, as well as in autoimmune diseases involving self DNA. Together with the adaptor ASC protein AIM2 forms a caspase-1 activating complex known as the AIM2 inflammasome. This AIM2 inflammasome can also be an integral component of a larger cell death-inducing complex called the AIM2-PANoptosome that drives PANoptosis. [9] [10]
The first step in the formation of AIM2 inflammasome is DNA binding. The HIN domain of AIM2 binds to both strands of B-form dsDNA (either viral, bacterial, or even host) in a sequence-independent manner. However, the DNA sequence must be at least 80 base pairs in length. [12] The interaction is mainly electrostatic, where positively charged amino acid residues are coordinating with phosphates and sugar moieties on DNA backbone. Binding of dsDNA displaces PYD domain, which then engages the downstream inflammasome adaptor protein ASC through homotypic PYD-PYD interactions. [13] ASC is a bipartite PYD-CARD-containing protein. CARD domain of ASC recruits procaspase-1 (CARD-CARD interaction) to the complex creating the basic structural elements of the AIM2 inflammasome. Caspase-1 autoactivates and processes cleavage of pro-IL-1β, pro-IL-18, and gasdermin D. The N-terminal fragment of gasdermin D induces pyroptosis that allows mature cytokines IL-1β, and IL-18 to be released from the cell.
AIM2 can also induce PANoptosis, a prominent innate immune, inflammatory, and lytic cell death pathway initiated by innate immune sensors and driven by caspases and receptor-interacting protein kinases (RIPKs) through PANoptosomes. [14] [15] PANoptosomes are multi-protein complexes assembled by germline-encoded pattern-recognition receptor(s) (PRRs) (innate immune sensor(s)) in response to pathogens, including bacterial, viral, and fungal infections, as well as pathogen-associated molecular patterns, damage-associated molecular patterns, cytokines, and homeostatic changes during infections, inflammatory conditions, and cancer. [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] To form the PANoptosome, the AIM2 inflammasome further interacts with caspase-8, FADD, RIPK3, and RIPK1 in response to specific pathogens, including Francisella novicida and herpes simplex virus 1 (HSV1), to drive PANoptosis.
Regulation of inflammasome assembly is essential for cellular homeostasis maintenance. AIM2 activation is inhibited by the mouse protein p202 that consists of two HIN domains and lacks the PYD. ASC protein is not recruited due to the absence of PYD domain. The HIN1 domain binds to DNA, whereas HIN2 domain interacts with AIM2. HIN2 domain does not block the DNA binding surface of AIM2, hence, DNA binding affinity of AIM2 remains unaffected. It is believed that binding of p202 to DNA and AIM2 might attain a balance between host defense and pathological DNA-induced inflammation. When both p202 and AIM2 are present in equal amounts, there is a competition for dsDNA binding. [30]
A novel transcript isoform of human IFI16-designated IFI16-β has been also shown to inhibit the AIM2 inflammasome assembly. Its domain structure is similar to that of mouse p202 as it contains two HIN domains. Analogously it interacts with AIM2, competes in dsDNA binding, and disrupts ASC recruitment. [31] According to studies of p202 and IFI16-β, it appears that proteins expressing two HIN domains bind to dsDNA more robustly than proteins containing a single HIN domain. [32]
Regarding post-translational modifications, there is limited knowledge. However, it has been reported that TRIM11 binds AIM2 and leads to its degradation. Hence, it might be a negative regulator of the AIM2 inflammasome. [33]
A broad range of microbes is sensed by AIM2, leading to protective inflammasome- or PANoptosome-mediated host responses. Recent publications have shown that AIM2 inflammasome also plays roles in non-infectious diseases.
Bacterial DNA is released into the cytoplasm during infection of a host cell, where it is recognized by AIM2 and other cytoplasmic DNA sensors. AIM2 has been shown to recognize a number of pathogenic bacteria – Francisella tularensis , Listeria monocytogenes , Streptococcus pneumoniae , Mycobacterium species, Porphyromonas gingivalis , Staphylococcus aureus , Brucella abortus , and Chlamydia muridarum . [7] Type I IFNs augment the activity of the AIM2 inflammasome during bacterial infection. [34] [35] In addition, AIM2 assembles the AIM2-PANoptosome complex in response to Francisella novicida, inducing inflammatory cell death, PANoptosis. [9]
AIM2 inflammasome plays a crucial role in the defense against viral infection as genetic material from DNA viruses that enter the cytoplasm can be recognized. However, AIM2 does not respond to all DNA viruses. To date, only mouse cytomegalovirus (MCMV), vaccinia virus, and human papillomaviruses have been observed to induce AIM2 inflammasome. [7] AIM2 also responds to herpes simplex virus 1 (HSV1), forming the AIM2-PANoptosome, which leads to PANoptosis. [9]
Moreover, AIM2 has been shown to mediate host defense to the fungal pathogen Aspergillus fumigatus [36] and the protozoan Plasmodium berghei . [37]
The gene encoding AIM2 was originally isolated from human melanoma cells. [5] AIM2 has been shown to suppress the development of tumors. However, the expression of AIM2 was observed to be differential in a range of tumor tissues suggesting that it may have unique roles in different cancer types. Recent studies investigating AIM2 function in cancer highlight the potential role of therapies inhibiting the AKT pathway in the treatment of cancer associated with AIM2 mutations. [7]
The accumulation of DNA in the cytosol can serve as an endogenous danger signal triggering AIM2 inflammasome. Aberrant activation of AIM2 from self-DNA is a key driver of inflammatory and autoimmune diseases. Elevated levels of AIM2 expression are found in skin cells from people with acute and chronic skin conditions, including psoriasis, atopic dermatitis, and contact dermatitis. Increased expression of AIM2 has also been reported in patients with inflammatory bowel disease and liver inflammation. Moreover, AIM2 might be involved in inflammation and cell death of the brain. [7] In systemic lupus erythematosus, lysosome dysfunction allows DNA to gain access to the cytosol and activate AIM2 resulting in increased type 1 interferon production. [38]
Caspases are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cysteine protease activity – a cysteine in its active site nucleophilically attacks and cleaves a target protein only after an aspartic acid residue. As of 2009, there are 12 confirmed caspases in humans and 10 in mice, carrying out a variety of cellular functions.
Caspase-1/Interleukin-1 converting enzyme (ICE) is an evolutionarily conserved enzyme that proteolytically cleaves other proteins, such as the precursors of the inflammatory cytokines interleukin 1β and interleukin 18 as well as the pyroptosis inducer Gasdermin D, into active mature peptides. It plays a central role in cell immunity as an inflammatory response initiator. Once activated through formation of an inflammasome complex, it initiates a proinflammatory response through the cleavage and thus activation of the two inflammatory cytokines, interleukin 1β (IL-1β) and interleukin 18 (IL-18) as well as pyroptosis, a programmed lytic cell death pathway, through cleavage of Gasdermin D. The two inflammatory cytokines activated by Caspase-1 are excreted from the cell to further induce the inflammatory response in neighboring cells.
NLR family pyrin domain containing 3 (NLRP3), is a protein that in humans is encoded by the NLRP3 gene located on the long arm of chromosome 1.
Pyroptosis is a highly inflammatory form of lytic programmed cell death that occurs most frequently upon infection with intracellular pathogens and is likely to form part of the antimicrobial response. This process promotes the rapid clearance of various bacterial, viral, fungal and protozoan infections by removing intracellular replication niches and enhancing the host's defensive responses. Pyroptosis can take place in immune cells and is also reported to occur in keratinocytes and some epithelial cells.
Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) functions in a variety of cellular pathways related to both cell survival and death. In terms of cell death, RIPK1 plays a role in apoptosis, necroptosis, and PANoptosis Some of the cell survival pathways RIPK1 participates in include NF-κB, Akt, and JNK.
PYCARD, often referred to as ASC, is a protein that in humans is encoded by the PYCARD gene. It is localized mainly in the nucleus of monocytes and macrophages. In case of pathogen infection, however, it relocalizes rapidly to the cytoplasm, perinuclear space, endoplasmic reticulum and mitochondria and it is a key adaptor protein in activation of the inflammasome.
Gamma-interferon-inducible protein Ifi-16 (Ifi-16) also known as interferon-inducible myeloid differentiation transcriptional activator is a protein that in humans is encoded by the IFI16 gene.
NLR family CARD domain-containing protein 4 is a protein that in humans is encoded by the NLRC4 gene.
Nucleotide-binding oligomerization domain-like receptor (NLR) pyrin domain (PYD)-containing protein 12 is a protein that in humans is encoded by the NLRP12 gene.
Z-DNA-binding protein 1, also known as DNA-dependent activator of IFN-regulatory factors (DAI) and DLM-1, is a protein that in humans is encoded by the ZBP1 gene.
The nucleotide-binding oligomerization domain-like receptors, or 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. They are types of pattern recognition receptors (PRRs), and play key roles in the regulation of innate immune response. NLRs can cooperate with toll-like receptors (TLRs) and regulate inflammatory and apoptotic response.
Inflammasomes are cytosolic multiprotein complexes of the innate immune system responsible for the activation of inflammatory responses and cell death. They are formed as a result of specific cytosolic pattern recognition receptors (PRRs) sensing microbe-derived pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs) from the host cell, or homeostatic disruptions. Activation and assembly of the inflammasome promotes the activation of caspase-1, which then proteolytically cleaves pro-inflammatory cytokines, interleukin 1β (IL-1β) and interleukin 18 (IL-18), as well as the pore-forming molecule gasdermin D (GSDMD). The N-terminal GSDMD fragment resulting from this cleavage induces a pro-inflammatory form of programmed cell death distinct from apoptosis, referred to as pyroptosis, which is responsible for the release of mature cytokines. Additionally, inflammasomes can act as integral components of larger cell death-inducing complexes called PANoptosomes, which drive another distinct form of pro-inflammatory cell death called PANoptosis.
A pyrin domain is a protein domain and a subclass of protein motif known as the death fold, the 4th and most recently discovered member of the death domain superfamily (DDF). It was originally discovered in the pyrin protein, or marenostrin, encoded by MEFV. The mutation of the MEFV gene is the cause of the disease known as Familial Mediterranean Fever. The domain is encoded in 23 human proteins and at least 31 mouse genes.
NLRC5, short for NOD-like receptor family CARD domain containing 5, is an intracellular protein that plays a role in the immune system. NLRC5 is a pattern recognition receptor implicated in innate immunity to viruses potentially by regulating interferon activity. It also acts as an innate immune sensor to drive inflammatory cell death, PANoptosis. In humans, the NLRC5 protein is encoded by the NLRC5 gene. It has also been called NOD27, NOD4, and CLR16.1.
NOD-like receptor family pyrin domain containing 11 is a protein that in humans is encoded by the NLRP11 gene located on the long arm of human chromosome 19q13.42. NLRP11 belongs to the NALP subfamily, part of a large subfamily of CATERPILLER. It is also known as NALP11, PYPAF6, NOD17, PAN10, and CLR19.6
NLRP (Nucleotide-binding oligomerization domain, Leucine rich Repeat and Pyrin domain containing), also abbreviated as NALP, is a type of NOD-like receptor. NOD-like receptors are a type of pattern recognition receptor that are found in the cytosol of the cell, recognizing signals of antigens in the cell. NLRP proteins are part of the innate immune system and detect conserved pathogen characteristics, or pathogen-associated molecular patterns, such as such as peptidoglycan, which is found on some bacterial cells. It is thought that NLRP proteins sense danger signals linked to microbial products, initiating the processes associated with the activation of the inflammasome, including K+ efflux and caspase 1 activation. NLRPs are also known to be associated with a number of diseases. Research suggests NLRP proteins may be involved in combating retroviruses in gametes. As of now, there are at least 14 different known NLRP genes in humans, which are named NLRP1 through NLRP14. The genes translate into proteins with differing lengths of leucine-rich repeat domains.
In molecular biology, the guanylate-binding proteins family is a family of GTPases that is induced by interferon (IFN)-gamma. GTPases induced by IFN-gamma are key to the protective immunity against microbial and viral pathogens. These GTPases are classified into three groups: the small 47-KD immunity-related GTPases (IRGs), the Mx proteins, and the large 65- to 67-kd GTPases. Guanylate-binding proteins (GBP) fall into the last class.
Stimulator of interferon genes (STING), also known as transmembrane protein 173 (TMEM173) and MPYS/MITA/ERIS is a protein that in humans is encoded by the STING1 gene.
Thirumala-Devi Kanneganti is an immunologist and is the Rose Marie Thomas Endowed Chair, Vice Chair of the Department of Immunology, and Member at St. Jude Children's Research Hospital. She is also Director of the Center of Excellence in Innate Immunity and Inflammation at St. Jude Children's Research Hospital. Her research interests include investigating fundamental mechanisms of innate immunity, including inflammasomes and inflammatory cell death, PANoptosis, in infectious and inflammatory disease and cancer.
PANoptosis is a prominent innate immune, inflammatory, and lytic cell death pathway initiated by innate immune sensors and driven by caspases and receptor-interacting protein kinases (RIPKs) through multiprotein PANoptosome complexes. The assembly of the PANoptosome cell death complex occurs in response to germline-encoded pattern-recognition receptors (PRRs) sensing pathogens, including bacterial, viral, and fungal infections, as well as pathogen-associated molecular patterns, damage-associated molecular patterns, and cytokines that are released during infections, inflammatory conditions, and cancer. Several PANoptosome complexes, such as the ZBP1-, AIM2-, RIPK1-, and NLRC5- and NLRP12-PANoptosomes, have been characterized so far.