toll-like receptor

toll-like receptor
Name	Toll-like receptor
Caption	Crystal structure of a TLR3 ectodomain
Width	200

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 usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have breached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses.

Structure and classification

Toll-like receptors are type I transmembrane proteins characterized by an extracellular leucine-rich repeat domain and an intracellular TIR domain. They are classified based on their cellular localization and ligand specificity, with TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 primarily located on the cell surface. In contrast, TLR3, TLR7, TLR8, and TLR9 are found within intracellular vesicles such as endosomes and lysosomes. The discovery of the first human homolog was reported by researchers including Bruce Beutler and Jules A. Hoffmann, work for which they shared the Nobel Prize in Physiology or Medicine.

Ligands and activation

These receptors are activated by binding to specific pathogen-associated molecular patterns. For instance, TLR4 recognizes lipopolysaccharide from Gram-negative bacteria, while TLR5 binds flagellin. Viral nucleic acids are detected by intracellular TLRs; TLR3 senses double-stranded RNA, and TLR9 responds to unmethylated CpG motifs common in bacteriophage and bacterial DNA. Activation often requires accessory proteins, such as MD-2 and CD14 for TLR4 signaling, which facilitate ligand binding and receptor dimerization.

Signaling pathways

Upon ligand binding, TLRs dimerize and undergo conformational changes that bring their intracellular TIR domains into proximity. This recruits adaptor proteins like MyD88, TRIF, TIRAP, and TRAM through homophilic TIR-TIR interactions. The MyD88-dependent pathway, used by all TLRs except TLR3, leads to the activation of NF-κB and MAPK pathways, promoting inflammatory cytokine production. The TRIF-dependent pathway, utilized by TLR3 and TLR4, can induce type I interferons via the transcription factors IRF3 and IRF7.

Role in immunity and disease

Toll-like receptors are crucial for initiating defense against pathogens like Streptococcus pneumoniae, Mycobacterium tuberculosis, and Influenza A virus. They bridge innate and adaptive immunity by activating dendritic cells for antigen presentation to T cells. However, dysregulated TLR signaling is implicated in numerous diseases. Overactivation can lead to sepsis, rheumatoid arthritis, and atherosclerosis, while defective signaling may increase susceptibility to infections or be linked to certain immunodeficiency disorders. Their role in sensing tissue damage via damage-associated molecular patterns also connects them to sterile inflammation.

Evolution and phylogeny

Toll-like receptors are evolutionarily ancient, with homologs found in organisms from fruit flies to humans. The prototypical Toll protein in Drosophila was first identified for its role in embryonic development before its immune function was discovered. Phylogenetic analyses suggest gene duplication events have expanded the family, with distinct TLR subfamilies evolving to recognize different microbial ligands. Comparative genomics in species like zebrafish and Xenopus has provided insights into the conservation and diversification of these pathways.

Clinical significance

Toll-like receptors are major targets for therapeutic intervention. Agonists like Imiquimod (a TLR7 agonist) are used to treat actinic keratosis and basal cell carcinoma, while Monophosphoryl lipid A (a TLR4 agonist) is an adjuvant in vaccines such as Cervarix. Conversely, antagonists are being investigated to treat inflammatory diseases; Eritoran was studied as a TLR4 antagonist for sepsis. Research into TLR polymorphisms, such as those affecting TLR4 in Crohn's disease, aims to enable personalized medicine approaches for infection and autoimmunity.

Category:Immune system Category:Protein families Category:Signal transduction