Toll-like receptors (TLRs) are pattern recognition receptors, which play a pivotal role in the innate immune response and recognize danger- (or damage-) associated molecular patterns (DAMPs), and pathogen-associated molecular patterns (PAMPs). The most studied receptor among the TLR family is TLR4 with its canonical ligand lipopolysaccharide (LPS), a major component of the cell wall of gram-negative bacteria. We are interested to understand how different chemotypes of LPS promote different activation modes of MyD88-dependent and MyD88-independent signaling responses by the host cell, a phenomenon termed biased signaling. To date, the exact mechanisms underlying this biased signaling are not fully understood.
We applied quantitative single-molecule localization microscopy (super-resolution microscopy) to study the stoichiometry of TLR4 in signaling complexes. We found co-receptors MD2 and CD14 a prerequisite for active TLR4 signaling complexes, which contain dimers of TLR4. We also found that LPS chemotypes act differently on TLR4 signaling complexes, either promoting the formation of TLR4 monomers or dimers.
Additionally, we found for glioma cells which endogenously express TLR4, that biased signaling is not dependent on the formation of higher order clusters of TLR4 or a change in receptor density on the plasma membrane. Treatment with different LPS chemotypes did not alter the organization of TLR4 clusters, whilst the used LPS chemotypes promoted biased signaling.
Krüger, C. L., Zeuner, M.-T., Cottrell, G. S., Widera, D. & Heilemann., M. (2017) Quantitative single-molecule imaging of TLR4 reveals ligand-specific receptor dimerization. Sci Signal. 10 (503), eaan1308.
Zeuner, M.-T., Krüger, C. L., Volk, K., Bieback, K., Cottrell, G. S., Heilemann M. & Widera D. (2016) Biased signalling is an essential feature of TLR4 in glioma cells. BBA Molecular Cell Research. 1863 (12), 3084-3095.