Toll-Like Receptor and Signaling Pathway - Essay Example

Toll like receptor (TLR) and signaling pathway Experiments reported in last few years reveled conserved, wide spread distributionof TLR among living system starting from plants to mammals. It was also shown that there are significant diversity found among different group of organisms but still they have similar function. Differences among pathway components are mainly due to evolution where mammals lost one of the three functioning of TLR namely development while Drosophila retains all three. Diversity among conserved pathway of TLR makes it best choice for immune system over other pathways due to variability among pathogens and its products. Introduction: In 1989, Charles Jeneway published it classical paper discussing inate immunity and establishes the fact that recognition of pathogen is done by germline-encoded receptors and not the recombinant antigen receptors (Janeway, 1989) He establishes the role of co stimulatory molecules or signals responsible for T cell activation. After extensive research of almost 20 years we have identified that co- stimulatory signal named as Toll receptor and its pathway. Surprisingly people working with different model system recognize that Toll receptor was not only responsible for human innate immunity but also found in wide range of living systems starting from earthworm to plants. Initially Toll receptor was recognized for its role in innate immunity but later on many unique role of toll was appeared in different model system like In drosophila where Toll have key roles in development. Investigation of Toll receptors and pathway related or activated by this receptor open ups new era of innate immunity and give us insight in to evolutionary conservation. Toll like receptors: Toll like receptors (TLR) are PRRs (pattern recognition receptor) having unique and essential role in innate immunity. It comprises of family of type I transmembrane receptors characterized by an extracellular leucine rich repeat domain and intracellular Toll/IL-1 receptor (TIR) domain. LRR founds in diverse set of proteins having role in ligand recognition and signal transduction (Ruslan Medzhitov, 2001). The signature sequence of LRR is L(X2) LXL(X2) NXL(X2) L(X7) L(X2). Where X2 is any amino acids. The TIR domain of Toll receptor is conserved protein-protein interaction module found in different transmembrane and cytoplasmic proteins in animals and plants. Interestingly, most of these TIR associated proteins in animals and plants have a role in host defense. Figure:1 Schematic of Toll likes receptor (TLR) (www.icampus.ucl.ac.be) TLR in mammalian Immunity: In mammalian species there are at least 10 TLR, and having distinct role in innate immunity. There are almost dozens of ligand for TLR were identified and many more yet to be established. TOL ligand are quite diverse in nature but having some common characteristics like most of TLR ligands are conserved microbial products (PAMPs), TLR recognize different structurally unrelated ligand and some TLR requires accessory proteins to recognize its ligand . Fig 2 shows different ligand molecules recognized by mammalian TLRs. (Ruslan Medzhitov Nature Reviews Immunology 1, 135-145 November 2001) Figure: 2 Ligand specification of TLR TLR Pathways: Activation of signal transduction pathway by TLR leads to expression of several genes have key role in host defense. For example TLR activated signaling pathway activates expression of cytokines, chemokines, MHC and various co stimulatory molecules (Ruslan Medzhitov, 2001). Initially it was proposed that all TLR have similar signaling cascade but growing evidences indicates that even though there are conserved signaling molecules in TLR pathway, there are differences among different types of TLR mediated pathways. The common molecules found in all TLR mediated signaling are the Adaptor protein MyD88 and TOLLIP (Toll interacting protein), Protein kinas IRAK (IL-IR associated kinase) and another adaptor, TRAF6(TNF-receptor associated kinase). MyD88 interact with TIR by its carboxy- terminal domain while amino-terminal interacts with death domain of IRAK and recruits it to the receptor complex, similarly TOLLP also intercept with IRAK and recruits its to receptor complex but with different kinetics. The functional differences among these two molecules are not very clear yet. After recruitment IRAK is autophosphorylates and associates with TRAF6 which subsequently activates TAK1 and MKK6 (MAP kinase kinase 6) which in turns activates of NF-B, JNK (c-jun N-terminal kinase) and p38 MAP kinase. In addition to this standard pathways there are some variation among different TLR signaling like in case of TLR2 involves protein kinase B By interacting with RHO family GTPase, RAC1 and phosphotidylinositol 3 kinase (PI3K). Similarly it was shown that when MyD88 deficient mice challenged with LPS and poly IC induces NF-B, JNK and p38 indicates alternative pathway utilized by TLR4 and TLR3. Fig 3 shows differences between TLR mediated pathways in mammals, (Roberto Baccala e' tal, Nature Medicine 13, 543-551(2007) Figure:3 TLR pathways in Mammals Toll in Drosophila: The origin of name Toll was came from identification of Toll gene as Maternal-effect gene that functions in a pathway that controls dosrsoventral axis formation in fruit fly embryo. While SptZle identified as Toll ligand, the adaptor protein Tube, the protein kinase Pelle, nuclear factor B, family transcription factor Dorsal and inhibitor of Dorsal Cactus( homologue to I B). In developmental signaling SptZle secreted as precursor which cleaved by protease and than binds to Toll. Activated Toll leads to generation of TIR based signaling to a complex of Cactus and Dorsal dimmer by unknown mechanism. It was known that interaction between death domain of pelle and Tube are required for the same. Signaling to Cactus and Dorsal leads to degradation of Cactus and nuclear migration of Dorsal. During developmental studies and to investigate role of TLR mediated signaling various mutants were created like TLR mutant. The surprising event was observed where Toll mutant of Drosophila succumbed to fungal infection due to failure to induced anti-fungal peptide called Drosomycine. This observation clearly indicated role of TLR in immunity. Subsequent experimentation with loss of function mutant of SptZle, tube or pelle demonstrated same effect where flies become sensitive to fungal infection. The major difference between two pathways is in case of immunity pathway Dif (Drosophila immunity factor) replaces the Dorsal. The second interesting difference is in case of Immunity pathway Toll dose not act as PRR but infection activation SptZle get cleaved and which subsequently activates Toll pathway. This observation along with some mutation studies related to necrotic gene (protease which cleaves SptZle) indicates that there is pattern recognition upstream to Toll which activates cascade of events leads to activation of protein kinase and hence Toll pathway via SptZle cleavage. Similarly it was observed that Toll mutant shows immunity against gram negative bacterial infection like wild type which supports above mention argument of up stream pattern recognition and some alternative pathway. Drosophila mounts anti bacterial immune response by producing anti bacterial peptides called Diptericin. Mutant studies indicated important role played by imd/ird pathways in anti bacterial response generation. The target of imd pathway is Relish (p105 homologue active in humoral immunity).The major difference between p105 mediated pathway and Relish mediated path way is incase of p105 proteosome act on it and cleave to release IB, while in case of Relish proteosome independent activation occurs leads to generation of stable Rel and IB like fragment. The major difference between Toll and imd mediated path way is both pathway uses different NF-B transactivators that are activated by two different mechanism. Dif in Toll pathway activated by it degradation of its inhibitor Cactus while Relish activated by removal of its auto inhibitory ankyrin repeats. In conclusion even though there are many similarities between Mammalian Toll/ NF-B pathway there are several differences at different stages of signaling cascade. Fig 5 shows differences in Toll and IMD pathway in Drosophila. (Bruno Lemaitre, Nature Reviews Immunology 4, 521-527,July 2004) Figure: 5 Toll and IMD pathway in Drosophila Toll in plants and other systems: The extraordinary reach of Toll pathway conservation became clear when it was found that N gene of tobacco was related in sequences to Toll and many of the disease resistance gene code for Pelle homologous such as Pto. Absence of transmembrane Domain in Plant Toll makes it difficult to understand as both LRR repeats and TIR domain were found to be cytoplasmic (Wasserman S.A, 2000). Soon after this problem researcher realizes that in plant most of viral and bacterial pathogen injects it virulence gene in to the cytoplasm and hence there is no need for transmembrane domain. Similarly absence of Rel homologous indicates that Toll pathway is quite divergent from their Human and Drosophila counterpart. On same note nematode Caenorhabditis elegans appears to have Toll pathway component but no Rel-like protein. It has only one TLR gene along with single gene belong to the Pelle, IB, and TRAF families. Most of them are dispensable for nematode morphological development and hence presume to have role in Immunity. Fig 6 gives comparison between plant Drosophila and mammalian Toll path ways. (Ruslan Medzhitov Nature Reviews Immunology 1, 135-145 November 2001) Figure : 6 Toll in different systems . Differences and similarity among drosophila and Mammalian TLR: The major difference among two Toll systems is in drosophila Toll mediated pathway is key in development while in case of mammals there is no evidence which suggest similar kind of functioning. This observation indicates that gene responsible for development in Drosophila are either diverted toward other function or eliminated in process of development. For example upstream component of Dorsal group GD (proteiolytic enzyme) closely resembles mammalian prothrombine. While Snake and Easter distantly resembles kallikeein and complement component C1s. most of this proteins are involved in inflammatory response. While homologous of Sptzle is completely absent in mammalian Toll pathway. Based on evolutionary facts out of three Basic line of TLR domain all three are represented in Drosophila while mammals have retained only two. Hence the "lost" developmental function of mammalian TLR may have been subserved by the "Lost" line, to which eight of the nine Drosophila TLR belongs (Bruce Beutler, 2001) Conclusion: In recent years there was great deal of interest to understand functional and mechanistic details of different pathways to understand evolution of complex signaling network which is a key process in homeostasis in living systems. A classical example is Wingless and Ras signaling cassettes are highly conserved at biochemical level but have diverged role in developmental processes. While in case of TLR or Toll pathways the overall functioning is same but the events in signaling process are different. Conserved function and diverged mechanism makes sense for pathway that deals with immune system as there are constant evolution in pathogenic organism and its specificity. Diversity in Toll mediated pathways makes it right candidate for innate immune system to fight against different group of pathogens and creates links between specific ligand to specific response. References: 1) Approaching the asymptote Evolution and revolution in immunology Janeway, C.A., Jr., Cold Spring Harb. Symp. Quant. Biol. 54, 1-13, 1989). 2) Toll-like receptors and innate immunity Ruslan Medzhitov Nature Reviews Immunology 1, 135-145 November 2001 3) Toll signaling: the enigma variations Wasserman S.A Current Opinion in Genetics & Development, Volume 10,Number 5, 1 October 2000 , pp. 497-502(6) 4) TLR-dependent and TLR-independent pathways of type I interferon induction in systemic autoimmunity Roberto Baccala, Kasper Hoebe, Dwight H Kono, Bruce Beutler & Argyrios N Theofilopoulos Nature Medicine 13, 543 - 551 May (2007) 5) The road to Toll Bruno Lemaitre, Nature Reviews Immunology 4, 521-527,July 2004 6)Toll We mate again Bruce Beutler and Alexander Poltorak Nature Immunology 2, 9-10 ,january 2001 Read More