Toll-like receptors: Less is more
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Proteolytic cleavage of the Toll-like receptor TLR9 creates a functional receptor that is retained at intracellular endolysosome compartments.
Toll-like receptors (TLRs) sense microbial nucleic acids and stimulate an immune response. However, it is not known how TLRs distinguish between self and foreign genetic material. There is some evidence that their subcellular localization governs selectivity as ligand recognition appears to occur only at endolysosomes, but this hypothesis is complicated by an incomplete understanding of TLR trafficking. In Nature, Gregory Barton and colleagues now report that nascent TLR9 transits through the Golgi en route to endolysosomes, where it is cleaved. Surprisingly, only cleaved TLR9 binds to the adaptor MyD88 to affect an immune response.
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Exogenous expression of TLR9 in a variety of macrophage and dendritic cell lines resulted in the phagosomal accumulation of an 80-kDa TLR9 fragment comprised of the cytoplasmic and transmembrane domains and half of the ectodomain. The protease cathepsin produced an 80-kDa TLR9 fragment in vitro; however, neither specific cathepsin inhibitors nor broad-scale protease inhibitors prevented in vivo processing, suggesting that TLR9 is the target of other or multiple proteases. TLR7 was also cleaved, suggesting that receptor cleavage may be conserved amongst many nucleic-acid-sensing receptors.
Previous studies had indicated that full-length TLRs transit from the endoplasmic reticulum (ER) to endolysosomes without passing through the Golgi. However, the authors found that the TLR9 fragments had acquired Golgi-specific glycosyl modifications and confirmed that a small pool of full-length TLR9 trafficked through the Golgi and was then cleaved. Depletion of the membrane protein UNC93B1 — which delivers nucleic-acid-sensing TLRs to endolysosomes — inhibited TLR9 cleavage and phagosome accumulation, and blocked TLR9-mediated TNF-α production. However, production of full-length TLR9 was unchanged in the absence UNC93B1. Furthermore, although both full-length and cleaved TLR9 were able to bind ligand, only the processed form interacted with the MyD88 adaptor protein, suggesting that it is the functional form of the receptor.
Interestingly, the authors were unable to force plasma membrane localization of the TLR9 fragment. A TLR9 construct with a plasma-membrane-targeting signal passed through the Golgi but was not cleaved and had no response to ligand. Point mutations in the membrane-targeting sequence restored processing and ligand sensitivity, but destroyed plasma membrane localization. Thus, the stringent endolysosomal localization of cleaved TLR9 greatly reduces the possibility of mature TLR9 trafficking to the plasma membrane and encountering 'self' genetic material. These data identify a novel mechanism by which proteolytic processing and subcellular localization tightly regulate the activation of nucleic-acid-sensing TLRs.
Emily J. Chenette
Signaling Gateway
References
- Ewald, S. E. et al. The ectodomain of Toll-like receptor 9 is cleaved to generate a functional receptor. Nature 456, 658-662 (2008) | Article | PubMed |
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