TLR4 (Toll-like receptor 4) is a type-1 transmembrane glycoprotein that is a pattern recognition receptor (PRR) belonging to the TLR family (1-3). TLR4 is expressed in many tissues and is most abundantly expressed in the placenta, spleen, and peripheral blood leukocytes (1). Human TLR4 is synthesized as a 839 amino acid (aa) protein containing a signal sequence (1-23 aa), an extracellular domain (ECD) (24-631 aa), a transmembrane domain (632-652 aa), and Toll/interleukin-1 receptor (TIR) cytoplasmic domain (652-839 aa) with a theoretical molecular weight of 95 kDa (3, 4). The ECD contains 21 leucine-rich repeats (LRRs) and has a horseshoe-shaped structure (3, 4). TLR4 requires binding with the co-receptor myeloid differentiation protein 2 (MD2) largely via hydrophilic interactions for proper ligand sensing and signaling (2-4). In general, the TLR family plays a role in activation of innate immunity and responds to a variety of pathogen-associated molecular patterns (PAMPs) (5). TLR4 is specifically responsive to lipopolysaccharide (LPS), which is found on the outer-membrane of most ram-negative bacteria (3-5). Activation of TLR4 requires binding of a ligand, such as LPS to MD2, followed by MD2-LPS complex binding to TLR4, resulting in a partial complex (TLR4-MD2/LPS) (3, 5). To become fully active, two partial complexes must dimerize thereby allowing the TIR domains of TLR4 to bind other adapter molecular and initiate signaling, triggering an inflammatory response and cytokine production (3, 5).
TLR4 signaling occurs through two distinct pathways: The MyD88 (myeloid differentiation primary response gene 88)-dependent pathway and the MyD88-independent (TRIF-dependent, TIR domain-containing adaptor inducing IFN-beta) pathway (3, 5-7). The MyD88-dependent pathway occurs mainly at the plasma membrane and involves the binding of MyD88-adaptor-like (MAL) protein followed by a signaling cascade that results in the activation of transcription factors including nuclear factor-kappaB (NF-kappaB) that promote the secretion of inflammatory molecules and increased phagocytosis (5-7). Conversely, the MyD88-independent pathway occurs after TLR4-MD2 complex internalization in the endosomal compartment. This pathway involves the binding of adapter proteins TRIF and TRIF-related adaptor molecule (TRAM), a signaling activation cascade resulting in IFN regulatory factor 3 (IRF3) translocation into the nucleus, and secretion of interferon-beta (INF-beta) genes and increased phagocytosis (5-7).
Given its expression on immune-related cells and its role in inflammation, TLR4 activation can contribute to various diseases (6-8). For instance, several studies have found that TLR4 activation is associated with neurodegeneration and several central nervous system (CNS) pathologies, including Alzheimer's disease, Parkinson's disease, and Huntington's disease (6, 7). Furthermore, TLR4 mutations have been shown to lead to higher rates of infections and increased susceptibility to sepsis (7-8). One potential therapeutic approach aimed at targeting TLR4 and neuroinflammation is polyphenolic compounds which include flavonoids and phenolic acids and alcohols (8).
Alternative names for TLR4 includes 76B357.1, ARMD10, CD284 antigen, CD284, EC 126.96.36.199, homolog of Drosophila toll, hToll, toll like receptor 4 protein, TOLL, toll-like receptor 4.
1. Vaure, C., & Liu, Y. (2014). A comparative review of toll-like receptor 4 expression and functionality in different animal species. Frontiers in immunology. https://doi.org/10.3389/fimmu.2014.00316
2. Park, B. S., & Lee, J. O. (2013). Recognition of lipopolysaccharide pattern by TLR4 complexes. Experimental & molecular medicine. https://doi.org/10.1038/emm.2013.97
3. Krishnan, J., Anwar, M.A., & Choi, S. (2016) TLR4 (Toll-Like Receptor 4). In: Choi S. (eds) Encyclopedia of Signaling Molecules. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-6438-9_592-1
4. Botos, I., Segal, D. M., & Davies, D. R. (2011). The structural biology of Toll-like receptors. Structure. https://doi.org/10.1016/j.str.2011.02.004
5. Lu, Y. C., Yeh, W. C., & Ohashi, P. S. (2008). LPS/TLR4 signal transduction pathway. Cytokine. https://doi.org/10.1016/j.cyto.2008.01.006
6. Leitner, G. R., Wenzel, T. J., Marshall, N., Gates, E. J., & Klegeris, A. (2019). Targeting toll-like receptor 4 to modulate neuroinflammation in central nervous system disorders. Expert opinion on therapeutic targets. https://doi.org/10.1080/14728222.2019.1676416
7. Molteni, M., Gemma, S., & Rossetti, C. (2016). The Role of Toll-Like Receptor 4 in Infectious and Noninfectious Inflammation. Mediators of inflammation. https://doi.org/10.1155/2016/6978936
8. Rahimifard, M., Maqbool, F., Moeini-Nodeh, S., Niaz, K., Abdollahi, M., Braidy, N., Nabavi, S. M., & Nabavi, S. F. (2017). Targeting the TLR4 signaling pathway by polyphenols: A novel therapeutic strategy for neuroinflammation. Ageing research reviews. https://doi.org/10.1016/j.arr.2017.02.004