Innate immune responses through Toll-like receptor 3 require human-antigen-R-mediated Atp6v0d2 mRNA stabilization

Abstract

Toll-like receptor 3 (TLR3) recognizes double-stranded RNA derived from virus and its synthetic analogue, polyinosinic–polycytidylic acid [poly(I:C)]. Upon poly(I:C) binding, TLR3 activates transcription factors to express inflammatory cytokines and type I interferon. TLR3 is located in the endosomes and its recognition of poly(I:C) and activation of downstream signaling is regulated by endosomal acidification. However, the mechanism of post-transcriptional regulation in TLR3-mediated innate responses remains unclear. Here, we focused on Human antigen R (HuR, also known as ELAVL1) that recognizes and binds to the 3′ untranslated regions (3′UTRs) of target mRNAs, thereby protecting them from mRNA degradation, and found that HuR-deficient murine macrophage cells showed significantly reduced Ifnb1 mRNA expression after poly(I:C) stimulation. HuR-deficient cells also showed a marked reduction in the expression of Atp6v0d2 mRNA, which encodes a subunit of vacuolar-type H+ ATPase (V-ATPase), and therefore reduced endosomal acidification. HuR associated with the 3′UTR of Atp6v0d2 mRNA and the stability of Atp6v0d2 mRNA was maintained by its association with HuR. Taken together, our results suggest that HuR stabilizes Atp6v0d2 mRNA, which is required for the TLR3-mediated innate immune responses.

Leishmania infection-induced multinucleated giant cell formation via upregulation of ATP6V0D2 expression

“…First, the upregulated expression of Atp6v0d2 in BMDMs infected with L. donovani was not accompanied by the upregulation of the other V-ATPase components (Figure 4). This unorchestrated change of Atp6v0d2 expression among V-ATPase components was also reported in macrophages treated with a TLR3 agonist or osteopontin (Zainol et al, 2019;Dai et al, 2022). Second, the location of upregulated ATP6V0D2 did not overlap with a lysosomal marker LAMP1 (Figure 4), as opposed to a previous finding in BMDMs (Xia et al, 2019).…”

“…Although Atp6v0d2 upregulation is reported on macrophages infected with L. amazonesis (Kikuchi et al, 2001), the magnitude of upregulation was less than two-fold and much weaker than that found in L. donovani infection in vitro and in vivo, indicating the involvement of species-specific molecules of the parasites for induction of ATP6V0D2 in macrophages. It is reported that Atp6v0d2 expression is controlled by TFEB (Palmieri et al, 2011;Liu et al, 2019), NFATc1 (Kim et al, 2008;Oh et al, 2015;Kim et al, 2022), and ELAVL1 (Zainol et al, 2019). In the spleen of L. donovani-infected mice, we did not find transcriptional changes in any of Tfeb, Nfatc1, or Elavl1 (data not shown).…”

Signaling Through Nucleic Acid Sensors and Their Roles in Inflammatory Diseases

“…The pH of intracellular vesicles might also be important for proteolytic processing and ligand recognition of endosomal TLRs. Blockade of the acidification of intracellular vesicles resulted in impaired innate immune responses mediated by TLR3, TLR7, and TLR9 ( 22 , 23 ). The localization of nucleic acid-sensing TLRs in cellular compartments is also important for the initiation of cell-type-specific signaling pathways.…”

Host Components That Modulate the Disease Caused by hMPV

“…Depending on the cell type, TLR3 can be located both on the surface of the cell and inside endosomes, as is the case for fibroblasts, or only within endosomes, as is the case for cDCs [ 96 ]. Once TLR3 binds to dsRNA, the adaptor molecule TIR-domain-containing adapter-inducing interferon-β (TRIF) becomes activated and initiates the phosphorylation and nuclear translocation of the transcription factors nuclear factor κB (NF-κB) and interferon regulatory factor 3 (IRF3) ( Figure 2 B) [ 97 , 98 , 99 ]. While NF-κB induces proinflammatory cytokines, IRF3 promotes the expression of IFN-I, such as IFN-β [ 97 ].…”

“…Once TLR3 binds to dsRNA, the adaptor molecule TIR-domain-containing adapter-inducing interferon-β (TRIF) becomes activated and initiates the phosphorylation and nuclear translocation of the transcription factors nuclear factor κB (NF-κB) and interferon regulatory factor 3 (IRF3) ( Figure 2 B) [ 97 , 98 , 99 ]. While NF-κB induces proinflammatory cytokines, IRF3 promotes the expression of IFN-I, such as IFN-β [ 97 ]. Notably, the interaction between TLR3 and hMPV has not been studied thoroughly yet.…”