Myeloid differentiation primary response 88 (MYD88) is a protein that, in humans, is encoded by the MYD88 gene. [5] [6] originally discovered in the laboratory of Dan A. Liebermann (Lord et al. Oncogene 1990) as a Myeloid differentiation primary response gene.
The MYD88 gene provides instructions for making a protein involved in signaling within immune cells. The MyD88 protein acts as an adapter, connecting proteins that receive signals from outside the cell to the proteins that relay signals inside the cell.
In innate immunity, the MyD88 plays a pivotal role in immune cell activation through Toll-like receptors (TLRs), which belong to large group of pattern recognition receptors (PRR). In general, these receptors sense common patterns which are shared by various pathogens – Pathogen-associated molecular pattern (PAMPs), or which are produced/released during cellular damage – damage-associated molecular patterns (DAMPs). [7]
TLRs are homologous to Toll receptors, which were first described in the onthogenesis of fruit flies Drosophila , being responsible for dorso-ventral development. Hence, TLRs have been proved in all animals from insects to mammals. TLRs are located either on the cellular surface (TLR1, TLR2, TLR4, TLR5, TLR6) or within endosomes (TLR3, TLR7, TLR8, TLR9) sensing extracellular or phagocytosed pathogens, respectively. TLRs are integral membrane glycoproteins with typical semicircular-shaped extracellular parts containing leucine-rich repeats responsible for ligand binding, and Intracellular parts containing Toll-Interleukin receptor (TIR) domain. [8]
After ligand binding, all TLRs, apart from TLR3, interact with adaptor protein MyD88. Another adaptor protein, which is activated by TLR3 and TLR4, is called TIR domain-containing adapter-inducing IFN-β (TRIF). Subsequently, these proteins activate two important transcription factors:
TLR7 and TLR9 activate both NF-κB and IRF3 through MyD88-dependent and TRIF-independent pathway, respectively. [8]
The human ortholog MYD88 seems to function similarly to mice, since the immunological phenotype of human cells deficient in MYD88 is similar to cells from MyD88 deficient mice. However, available evidence suggests that MYD88 is dispensable for human resistance to common viral infections and to all but a few pyogenic bacterial infections, demonstrating a major difference between mouse and human immune responses. [9] Mutation in MYD88 at position 265 leading to a change from leucine to proline have been identified in many human lymphomas including ABC subtype of diffuse large B-cell lymphoma [10] and Waldenström's macroglobulinemia. [11]
Myd88 has been shown to interact with:
Various single nucleotide polymorphisms (SNPs) of the MyD88 have been identified. and for some of them an association with susceptibility to various infectious diseases [22] and to some autoimmune diseases like ulcerative colitis was found. [23]
Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have reached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses. The TLRs include TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10, TLR11, TLR12, and TLR13. Humans lack genes for TLR11, TLR12 and TLR13 and mice lack a functional gene for TLR10. The receptors TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the cell membrane, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles.
Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed mainly by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils, as well as by epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.
IRAK-4, in the IRAK family, is a protein kinase involved in signaling innate immune responses from Toll-like receptors. It also supports signaling from T-cell receptors. IRAK4 contains domain structures which are similar to those of IRAK1, IRAK2, IRAKM and Pelle. IRAK4 is unique compared to IRAK1, IRAK2 and IRAKM in that it functions upstream of the other IRAKs, but is more similar to Pelle in this trait. IRAK4 has important clinical applications.
TIR domain containing adaptor molecule 1 is an adapter in responding to activation of toll-like receptors (TLRs). It mediates the rather delayed cascade of two TLR-associated signaling cascades, where the other one is dependent upon a MyD88 adapter.
Toll-like receptor 5, also known as TLR5, is a protein which in humans is encoded by the TLR5 gene. It is a member of the toll-like receptor (TLR) family. TLR5 is known to recognize bacterial flagellin from invading mobile bacteria. It has been shown to be involved in the onset of many diseases, including Inflammatory bowel disease due to the high expression of TLR in intestinal lamina propria dendritic cells. Recent studies have also shown that malfunctioning of TLR5 is likely related to rheumatoid arthritis, osteoclastogenesis, and bone loss. Abnormal TLR5 functioning is related to the onset of gastric, cervical, endometrial and ovarian cancers.
Toll-like receptor 4 (TLR4), also designated as CD284, is a key activator of the innate immune response and plays a central role in the fight against bacterial infections. TLR4 is a transmembrane protein of approximately 95 kDa that is encoded by the TLR4 gene.
TRAF6 is a TRAF human protein.
Toll-like receptor 6 is a protein that in humans is encoded by the TLR6 gene. TLR6 is a transmembrane protein, member of toll-like receptor family, which belongs to the pattern recognition receptor (PRR) family. TLR6 acts in a heterodimer form with toll-like receptor 2 (TLR2). Its ligands include multiple diacyl lipopeptides derived from gram-positive bacteria and mycoplasma and several fungal cell wall saccharides. After dimerizing with TLR2, the NF-κB intracellular signalling pathway is activated, leading to a pro-inflammatory cytokine production and activation of innate immune response. TLR6 has also been designated as CD286.
Toll-like receptor 10 is a protein that in humans is encoded by the TLR10 gene. TLR10 has also been designated as CD290 . TLR10 has not been extensively studied because it is a pseudogene in mice, though all other mammalian species contain an intact copy of the TLR10 gene. Unlike other TLRs, TLR10 does not activate the immune system and has instead been shown to suppress inflammatory signaling on primary human cells. This makes TLR10 unique among the TLR family. TLR10 was thought to be an "orphan" receptor, however, recent studies have identified ligands for TLR10 and these include HIV-gp41. Ligands for TLR2 are potential ligands for TLR10.
Interleukin-1 receptor-associated kinase 1 (IRAK-1) is an enzyme in humans encoded by the IRAK1 gene. IRAK-1 plays an important role in the regulation of the expression of inflammatory genes by immune cells, such as monocytes and macrophages, which in turn help the immune system in eliminating bacteria, viruses, and other pathogens. IRAK-1 is part of the IRAK family consisting of IRAK-1, IRAK-2, IRAK-3, and IRAK-4, and is activated by inflammatory molecules released by signaling pathways during pathogenic attack. IRAK-1 is classified as a kinase enzyme, which regulates pathways in both innate and adaptive immune systems.
Interleukin 1 receptor, type I (IL1R1) also known as CD121a, is an interleukin receptor. IL1R1 also denotes its human gene.
Interleukin-1 receptor accessory protein is a protein that in humans is encoded by the IL1RAP gene.
Toll interacting protein, also known as TOLLIP, is an inhibitory adaptor protein that in humans is encoded by the TOLLIP gene.
Interleukin-1 receptor-associated kinase 3 is an enzyme that in humans is encoded by the IRAK3 gene. Using in vivo liposome-mediated delivery of CRISPR/Cas9 plasmid expressing IRAK3 gRNA, IRAK3 was shown to be responsible for endotoxin-induced expression of A20 and VE-cadherin in endothelial cells. Thus, IRAK3 is crucial for maintenance and repair of endothelial barrier after endotoxin-induced lung injury.
Interleukin-1 receptor-associated kinase-like 2 is an enzyme that in humans is encoded by the IRAK2 gene.
The toll-interleukin-1 receptor (TIR) homology domain is an intracellular signaling domain found in MyD88, SARM1, interleukin-1 receptors, toll receptors and many plant R proteins. It contains three highly conserved regions, and mediates protein-protein interactions between the toll-like receptors (TLRs) and signal-transduction components. TIR-like motifs are also found in plant proteins where they are involved in resistance to disease and in bacteria where they are associated with virulence. When activated, TIR domains recruit cytoplasmic adaptor proteins MyD88 (UniProt Q99836) and TOLLIP (toll-interacting protein, UniProt Q9H0E2). In turn, these associate with various kinases to set off signaling cascades. Some TIR domains have also been found to have intrinsic NAD+ cleavage activity, such as in SARM1. In the case of SARM1, the TIR NADase activity leads to the production of Nam, ADPR and cADPR and the activation of downstream pathways involved in Wallerian degeneration and neuron death.
Members of the very wide interleukin-1 receptor (IL-1R) family are characterized by extracellular immunoglobulin-like domains and intracellular Toll/Interleukin-1R (TIR) domain. It is a group of structurally homologous proteins, conserved throughout the species as it was identified from plants to mammals. Proteins of this family play important role in host defence, injury and stress. There are four main groups of TIR domain-containing proteins in animals; Toll-like receptors, Interleukin-1 receptor (IL-1R), cytosolic adaptor proteins and insect and nematode Toll. Each of these groups is involved mainly in host defence; Toll receptors are also involved in embryogenesis.
The interleukin-1 receptor (IL-1R) associated kinase (IRAK) family plays a crucial role in the protective response to pathogens introduced into the human body by inducing acute inflammation followed by additional adaptive immune responses. IRAKs are essential components of the Interleukin-1 receptor signaling pathway and some Toll-like receptor signaling pathways. Toll-like receptors (TLRs) detect microorganisms by recognizing specific pathogen-associated molecular patterns (PAMPs) and IL-1R family members respond the interleukin-1 (IL-1) family cytokines. These receptors initiate an intracellular signaling cascade through adaptor proteins, primarily, MyD88. This is followed by the activation of IRAKs. TLRs and IL-1R members have a highly conserved amino acid sequence in their cytoplasmic domain called the Toll/Interleukin-1 (TIR) domain. The elicitation of different TLRs/IL-1Rs results in similar signaling cascades due to their homologous TIR motif leading to the activation of mitogen-activated protein kinases (MAPKs) and the IκB kinase (IKK) complex, which initiates a nuclear factor-κB (NF-κB) and AP-1-dependent transcriptional response of pro-inflammatory genes. Understanding the key players and their roles in the TLR/IL-1R pathway is important because the presence of mutations causing the abnormal regulation of Toll/IL-1R signaling leading to a variety of acute inflammatory and autoimmune diseases.
Interleukin 17 receptor D is a protein that in humans is encoded by the IL17RD gene.
An innate immune defect is a defect in the innate immune response that blunts the response to infection. These defects may occur in monocytes, neutrophils, natural killer cells, basophils, mast cells or complement proteins.
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