Release from UNC93B1 reinforces the compartmentalized activation of select TLRs

Abstract

Nucleic acid-sensing Toll-like receptors (TLRs) are subject to complex regulation to facilitate recognition of microbial DNA and RNA while limiting recognition of self-nucleic acids 1 . Failure to properly regulate these TLRs can lead to autoimmune and autoinflammatory disease [2][3][4][5][6] . Intracellular localization of these receptors is thought to be critical for self vs. non-self discrimination 7 , yet the molecular mechanisms that reinforce compartmentalized activation of intracellular TLRs remain poorly understood. Here we describe a new mechanism that prevents TLR9 activation from locations other than endosomes. This control is achieved through the regulated release of the receptor from its trafficking chaperone Unc93b1, which only occurs within endosomes and is required for ligand binding and signal transduction. Preventing TLR9 release from Unc93b1, either through mutations in Unc93b1 that increase affinity for TLR9 or through an artificial tether that impairs release, results in defective signaling. While TLR9 and TLR3 release from Unc93b1, TLR7 does not dissociate from Unc93b1 in endosomes and is regulated via distinct mechanisms. This work defines a new checkpoint that reinforces compartmentalized activation of TLR9 and provides a mechanism by which activation of individual endosomal TLRs may be distinctly regulated.The multi-pass transmembrane protein Unc93b1 contributes to the compartmentalized activation of nucleic acid-sensing TLRs by mediating their trafficking from the ER to endosomes 8 . Mutations in Unc93b1 that result in aberrant TLR trafficking can lead to autoimmune disease 3,9 . It has been suggested that Unc93b1 may regulate additional sorting steps that are distinct between individual TLRs 10 ; however, the mechanisms by which Reprints and permissions information is available at www.nature.com/reprints.Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

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“…66 The release of TLR9 from the ER is controlled by several mechanisms including tyrosine-based motifs in TLR9 cytoplasmic tail, [67][68][69][70] and phosphorylation of TLR9. 71 Key proteins required for TLR9 traffic from ER to endosomal compartments include glycoprotein 96 (gp96), 72 UNC93B1, 73,74 adapter protein 3 (AP-3), 75,76 a protein associated with TLR4 (PRAT4A), 77 and Slc15a4. 75 The unique localization and trafficking requirements of the nucleic acid-sensing TLRs 7-9 serve as a regulatory mechanism to limit immune responses to host nucleic acids; and indeed, artificial localization of TLR9 to cell surface causes autoimmune manifestations.…”

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“…It has been reported that a chaperone protein, UNC93B1, regulates the stability and/or transportation of these TLRs. Furthermore, TLR researchers have investigated whether UNC93B1 is related to the regulation of TLR functions Majer, Liu, Woo et al, 2019). As TLR7, TLR8 and TLR9 share some common features in sequence homology, function and structure, they constitute the TLR7 subfamily and harbor 26 LRRs, thus being the longest members of the TLR family, sense singlestranded nucleic acids and have a characteristic insertion between LRR14 and LRR15 called the Z-loop (Tanji et al, 2013;Ohto et al, 2015;Zhang et al, 2016;Fig.…”

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“…The human UNC93B1 consists of 597 amino acids and is a 12 transmembrane domain containing protein located in endosomal membranes [31]. It acts as a trafficking chaperone of the intracellular nucleic acid-sensing Toll-like receptors (TLRs) 3, 7 and 9 [32][33][34][35]. These TLRs are essential components of the innate immune system and activated when pathogen derived nucleic acids appear in endolysosomes.…”