Biochemistry of Toll‐Like Receptors

Abstract

Recognition of potentially pathogenic organisms is a critical first step in an immune response. To prevent pathogen outgrowth, this detection must be rapid and initiate a robust inflammatory response. However, inflammation is dangerous, and spurious activation in response to nonthreatening stimuli has the potential to cause autoimmunity and other inflammatory disorders. For this reason, the pattern recognition receptors (PRRs) of the innate immune system have evolved to recognise pathogen‐associated molecular patterns to distinguish between self and potentially infectious nonself. The first discovered and most well‐studied PRRs are the toll‐like receptors (TLRs), which are transmembrane receptors that detect a diverse set of microbe‐specific products. Extensive work has uncovered the proteins required in TLR signalling, but a more complete understanding of the biochemistry of signalling molecules in their cellular context is required to understand the role of TLRs in pathogen detection and clearance.

Key Concepts:

  • Pathogen‐associated molecular patterns are molecular patterns unique to potentially pathogenic microbes, conserved among many species, and essential to the fitness of the microbes that use them.

  • Pattern recognition receptors distinguish self from nonself.

  • Toll‐like receptor ‘sorting adaptors’ define the subcellular location of signal transduction.

  • Toll‐like receptor ‘signalling adaptors’ link the cytosolic TIR domain of the receptor to signalling enzymes.

Keywords: toll‐like receptor; pathogen‐associated molecular pattern; pattern recognition receptor; lipopolysaccharide; innate immunity

Figure 1.

(a) PRRs are positioned at cell membranes and within the cytosol to detect the presence of pathogens. (b) TLRs use sorting adaptors (TIRAP or TRAM) to recruit signalling adaptors (MyD88 or TRIF) to the site of signal transduction. Signalling adaptors bridge the cytosolic TIR domain of the receptor to downstream enzymes. (c) Most TLRs signal through the assembly of the ‘myddsome,’ a multi‐protein complex consisting of TIRAP, MyD88 and IRAKs. (d) Negative regulators such as SOCS1, MyD88s and SARM are cell‐intrinsic negative regulators that target adaptor proteins in TLR signalling. Pathogens such as Brucella and HSV also encode negative regulators of these pathways to evade detection.

Figure 2.

TLRs at the cell surface recognise structural components of bacteria and fungi such as LPS (TLR4), Flagellin (TLR5) or cell wall glycoproteins (TLR2). Endosomal TLRs typically recognise nucleic acid ligands such as unmethylated CpG DNA (TLR9), double or single stranded RNA (TLRs 3 or 7 respectively) or bacterial ribosomal RNA (TLR13).

Figure 3.

(a) The sorting adaptor TIRAP recruits MyD88 to TLRs at the cell surface and within endosomes. TIRAP's lipid binding domain is promiscuous, binding to lipids such as PI(4,5)P2 (at the plasma membrane) and PI(3)P (on early endosomes). (b) TLR4 signals from both the plasma membrane and within endosomes, utilising a difference sorting/signalling adaptor pair in each location. At the cell surface, TIRAP and MyD88 form a myddosome downstream of LPS binding. After undergoing CD14‐dependent endocytosis, TLR4 engages TRAM and TRIF.

Figure 4.

(1) TLRs are synthesised in the ER and loaded into COPII‐coated vesicles, dependent on gp96 and other chaperones. TLRs destined for endosomes (right) require Unc93B1. (2) After glycosylation in the golgi, some TLRs are exported to the cell surface. (3) Upon ligand binding, TLR4 undergoes CD14‐dependent endocytosis. (4) TLR7 translocates directly from the golgi to lysosomes through its interaction with AP‐4. (5) TLR9 is translocated to the cell surface by Unc93B1. At the cell surface, Unc93B1 interacts with AP‐2, which mediates endocytosis and translocation to lysosomes.

close

References

Akira S and Takeda K (2004) Toll‐like receptor signalling. Nature Reviews Immunology 4: 499–511.

Anderson KV, Jürgens G and Nüsslein‐Volhard C (1985) Establishment of dorsal‐ventral polarity in the Drosophila embryo: genetic studies on the role of the Toll gene product. Cell 42: 779–789.

Barbalat R, Lau L, Locksley RM and Barton GM (2009) Toll‐like receptor 2 on inflammatory monocytes induces type I interferon in response to viral but not bacterial ligands. Nature Immunology 10(11): 1200–1207. doi: 10.1038/ni.1792.

Barton GM and Kagan JC (2009) A cell biological view of Toll‐like receptor function: regulation through compartmentalization. Nature Reviews Immunology 9: 535–542.

Barton GM and Medzhitov R (2002) Toll‐like Receptor Family Members and Their Ligands. Berlin, Heidelberg: Springer.

Bonham KS, Orzalli MH, Hayashi K et al. (2014) A promiscuous lipid‐binding protein diversifies the subcellular sites of toll‐like receptor signal transduction. Cell 156: 705–716.

Brown V, Brown RA, Ozinsky A, Hesselberth JR and Fields S (2006) Binding specificity of Toll‐like receptor cytoplasmic domains. European Journal of Immunology 36: 742–753.

Burns K, Janssens S, Brissoni B et al. (2003) Inhibition of interleukin 1 receptor/Toll‐like receptor signaling through the alternatively spliced, short form of MyD88 is due to its failure to recruit IRAK‐4. Journal of Experimental Medicine 197: 263–268.

Cao W, Manicassamy S, Tang H et al. (2008) Toll‐like receptor‐mediated induction of type I interferon in plasmacytoid dendritic cells requires the rapamycin‐sensitive PI(3)K‐mTOR‐p70S6K pathway. Nature Immunology 9(10): 1157–1164. doi: 10.1038/ni.1645.

Carty M, Goodbody R, Schröder M et al. (2006) The human adaptor SARM negatively regulates adaptor protein TRIF‐dependent Toll‐like receptor signaling. Nature Immunology 7: 1074–1081.

Cervantes JL, Dunham‐Ems SM, La Vake CJ et al. (2011) Phagosomal signaling by Borrelia burgdorferi in human monocytes involves Toll‐like receptor (TLR) 2 and TLR8 cooperativity and TLR8‐mediated induction of IFN‐beta. Proceedings of the National Academy of Sciences of the USA 108: 3683–3688.

Choe J, Kelker MS and Wilson IA (2005) Crystal structure of human toll‐like receptor 3 (TLR3) ectodomain. Science (New York, NY) 309: 581–585.

Choi YJ, Jung J, Chung HK, Im E and Rhee SH (2013) PTEN regulates TLR5‐induced intestinal inflammation by controlling Mal/TIRAP recruitment. FASEB Journal 27: 243–254.

Covert MW, Leung TH, Gaston JE and Baltimore D. (2005) Achieving stability of lipopolysaccharide‐induced NF‐kappaB activation. Science (New York, NY) 309: 1854–1857.

Ewald SE and Barton GM (2011) Nucleic acid sensing Toll‐like receptors in autoimmunity. Current Opinion in Immunology 23: 3–9.

Ewald SE, Lee BL, Lau L et al. (2008) The ectodomain of Toll‐like receptor 9 is cleaved to generate a functional receptor. Nature 456: 658–662.

Gay NJ and Keith FJ (1991) Drosophila Toll and IL‐1 receptor. Nature 351: 355–356.

Janeway CA (1989) Approaching the asymptote? Evolution and revolution in immunology. Cold Spring Harbor Symposia on Quantitative Biology 54 (Pt 1): 1–13.

Janssens S, Burns K, Vercammen E, Tschopp J and Beyaert R (2003) MyD88S, a splice variant of MyD88, differentially modulates NF‐kappaB‐ and AP‐1‐dependent gene expression. FEBS Letters 548: 103–107.

Jin MS and Lee J‐O (2008) Structures of the toll‐like receptor family and its ligand complexes. Immunity 29: 182–191.

Kagan JC (2012) Signaling organelles of the innate immune system. Cell 151: 1168–1178.

Kagan JC and Medzhitov R (2006) Phosphoinositide‐mediated adaptor recruitment controls Toll‐like receptor signaling. Cell 125: 943–955.

Kagan JC, Su T, Horng T et al. (2008) TRAM couples endocytosis of Toll‐like receptor 4 to the induction of interferon‐beta. Nature Immunology 9(4): 361–368. doi: 10.1038/ni1569.

Kerkmann M, Rothenfusser S, Hornung V et al. (2003) Activation with CpG‐A and CpG‐B oligonucleotides reveals two distinct regulatory pathways of type I IFN synthesis in human plasmacytoid dendritic cells. Journal of Immunology 170: 4465–4474.

Lee BL, Moon JE, Shu JH et al. (2013) UNC93B1 mediates differential trafficking of endosomal TLRs. Elife 2: e00291.

Lemaitre B, Nicolas E, Michaut L, Reichhart JM and Hoffman JA (1996) The dorsoventral regulatory gene cassette sp{ä}tzle/Toll/cactus controls the potent antifungal response in Drosophila adults. Cell 86(6): 973–983.

Li X‐D and Chen ZJ (2012) Sequence specific detection of bacterial 23S ribosomal RNA by TLR13. Elife 1: e00102.

Lin S‐C, Lo Y‐C and Wu H (2010) Helical assembly in the MyD88‐IRAK4‐IRAK2 complex in TLR/IL‐1R signalling. Nature 465: 885–890.

van Lint AL, Murawski MR, Goodbody RE et al. (2010) Herpes simplex virus immediate‐early ICP0 protein inhibits Toll‐like receptor 2‐dependent inflammatory responses and NF‐kappaB signaling. Journal of Virology 84: 10802–10811.

Mansell A, Smith R, Doyle SL et al. (2006) Suppressor of cytokine signaling 1 negatively regulates Toll‐like receptor signaling by mediating Mal degradation. Nature Immunology 7: 148–155.

Medzhitov R (2009) Approaching the asymptote: 20 years later. Immunity 30: 766–775.

Medzhitov R, Preston‐Hurlburt P and Janeway CA (1997) A human homologue of the Drosophila Toll protein signals activation of adaptive immunity. Nature 388: 394–397.

Naugler WE, Sakurai T, Kim S et al. (2007) Gender disparity in liver cancer due to sex differences in MyD88‐dependent IL‐6 production. Science 317: 121–124.

Ngo VN, Young RM, Schmitz R et al. (2011) Oncogenically active MyD88 mutations in human lymphoma. Nature 470: 115–119.

O'Neill LA and Greene C (1998) Signal transduction pathways activated by the IL‐1 receptor family: ancient signaling machinery in mammals, insects, and plants. Journal of Leukocyte Biology 63: 650–657.

O'Neill LAJ, Fitzgerald KA and Bowie AG (2003) The Toll–IL‐1 receptor adaptor family grows to five members. Trends in Immunology 24(6): 286–290.

Ohnishi H, Tochio H, Kato Z et al. (2012) TRAM is involved in IL‐18 signaling and functions as a sorting adaptor for MyD88. PLoS One 7: e38423.

Oldenburg M, Krüger A, Ferstl R et al. (2012) TLR13 recognizes bacterial 23S rRNA devoid of erythromycin resistance‐forming modification. Science (New York, NY) 337: 1111–1115.

Park B, Brinkmann MM, Spooner E et al. (2008) Proteolytic cleavage in an endolysosomal compartment is required for activation of Toll‐like receptor 9. Nature Immunology 9: 1407–1414.

Poltorak A, Smirnova I, He X et al. (1998) Genetic and physical mapping of the Lps locus: identification of the Toll‐4 receptor as a candidate gene in the critical region. Blood Cells, Molecules and Diseases 24: 340–355.

Radhakrishnan GK, Yu Q, Harms JS and Splitter GA (2009) Brucella TIR domain‐containing protein mimics properties of the Toll‐like receptor adaptor protein TIRAP. Journal of Biological Chemistry 284: 9892–9898.

Rakoff‐Nahoum S, Paglino J, Eslami‐Varzaneh F, Edberg S and Medzhitov R (2004) Recognition of commensal microflora by toll‐like receptors is required for intestinal homeostasis. Cell 118: 229–241.

Regan T, Nally K, Carmody R et al. (2013) Identification of TLR10 as a key mediator of the inflammatory response to Listeria monocytogenes in intestinal epithelial cells and macrophages. Journal of Immunology 191: 6084–6092.

Sasai M, Linehan MM and Iwasaki A (2010) Bifurcation of Toll‐like receptor 9 signaling by adaptor protein 3. Science (New York, NY) 329: 1530–1534.

Sun D and Ding A (2006) MyD88‐mediated stabilization of interferon‐gamma‐induced cytokine and chemokine mRNA. Nature Immunology 7: 375–381.

Uematsu S, Sato S, Yamamoto M et al. (2005) Interleukin‐1 receptor‐associated kinase‐1 plays an essential role for Toll‐like receptor (TLR)7‐ and TLR9‐mediated interferon‐{alpha} induction. Journal of Experimental Medicine 201: 915–923.

Ulrichts P, Peelman F, Beyaert R and Tavernier J (2007) MAPPIT analysis of TLR adaptor complexes. FEBS Letters 581: 629–636.

Wan Y, Kim TW, Yu M et al. (2011) The dual functions of IL‐1 receptor‐associated kinase 2 in TLR9‐mediated IFN and proinflammatory cytokine production. Journal of Immunology 186(5): 3006–3014.

Werner SL, Barken D and Hoffmann A (2005) Stimulus specificity of gene expression programs determined by temporal control of IKK activity. Science (New York, NY) 309: 1857–1861.

Zanoni I, Ostuni R, Marek LR et al. (2011) CD14 controls the LPS‐induced endocytosis of toll‐like receptor 4. Cell 147: 868–880.

Further Reading

Kagan JC (2012) Defining the subcellular sites of innate immune signal transduction. Trends in Immunology 33(9): 442–448.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Bonham, Kevin Scott, and Kagan, Jonathan C(Oct 2014) Biochemistry of Toll‐Like Receptors. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024234]