Extracellular aaRSs drive autoimmune and inflammatory responses in rheumatoid arthritis via the release of cytokines and PAD4.

OBJECTIVES: Recent studies demonstrate that extracellular-released aminoacyl-tRNA synthetases (aaRSs) play unique roles in immune responses and diseases. This study aimed to understand the role of extracellular aaRSs in the pathogenesis of rheumatoid arthritis (RA). METHODS: Primary macrophages and fibroblast-like synoviocytes were cultured with aaRSs. aaRS-induced cytokine production including IL-6 and TNF-α was detected by ELISA. Transcriptomic features of aaRS-stimulated macrophages were examined using RNA-sequencing. Serum and synovial fluid (SF) aaRS levels in patients with RA were assessed using ELISA. Peptidyl arginine deiminase (PAD) 4 release from macrophages stimulated with aaRSs was detected by ELISA. Citrullination of aaRSs by themselves was examined by immunoprecipitation and western blotting. Furthermore, aaRS inhibitory peptides were used for inhibition of arthritis in two mouse RA models, collagen-induced arthritis and collagen antibody-induced arthritis. RESULTS: All 20 aaRSs functioned as alarmin; they induced pro-inflammatory cytokines through the CD14-MD2-TLR4 axis. Stimulation of macrophages with aaRSs displayed persistent innate inflammatory responses. Serum and SF levels of many aaRSs increased in patients with RA compared with control subjects. Furthermore, aaRSs released PAD4 from living macrophages, leading to their citrullination. We demonstrate that aaRS inhibitory peptides suppress cytokine production and PAD4 release by aaRSs and alleviate arthritic symptoms in a mouse RA model. CONCLUSIONS: Our findings uncovered the significant role of aaRSs as a novel alarmin in RA pathogenesis, indicating that their blocking agents are potent antirheumatic drugs.

Tryptophanyl-tRNA synthetase mediates high-affinity tryptophan uptake into human cells.

The tryptophan (Trp) transport system has a high affinity and selectivity toward Trp, and has been reported to exist in both human and mouse macrophages. Although this system is highly expressed in interferon-γ (IFN-γ)-treated cells and indoleamine 2,3-dioxygenase 1 (IDO1)-expressing cells, its identity remains incompletely understood. Tryptophanyl-tRNA synthetase (TrpRS) is also highly expressed in IFN-γ-treated cells and also has high affinity and selectivity for Trp. Here, we investigated the effects of human TrpRS expression on Trp uptake into IFN-γ-treated human THP-1 monocytes or HeLa cells. Inhibition of human TrpRS expression by TrpRS-specific siRNAs decreased and overexpression of TrpRS increased Trp uptake into the cells. Of note, the TrpRS-mediated uptake system had more than hundred-fold higher affinity for Trp than the known System L amino acid transporter, promoted uptake of low Trp concentrations, and had very high Trp selectivity. Moreover, site-directed mutagenesis experiments indicated that Trp- and ATP-binding sites, but not tRNA-binding sites, in TrpRS are essential for TrpRS-mediated Trp uptake into the human cells. We further demonstrate that the addition of purified TrpRS to cell culture medium increases Trp uptake into cells. Taken together, our results reveal that TrpRS plays an important role in high-affinity Trp uptake into human cells.

Identification of a residue crucial for the angiostatic activity of human mini tryptophanyl-tRNA synthetase by focusing on its molecular evolution.

Human tryptophanyl-tRNA synthetase (TrpRS) exists in two forms: a full-length TrpRS and a mini TrpRS. We previously found that human mini, but not full-length, TrpRS is an angiostatic factor. Moreover, it was shown that the interaction between mini TrpRS and the extracellular domain of vascular endothelial (VE)-cadherin is crucial for its angiostatic activity. However, the molecular mechanism of the angiostatic activity of human mini TrpRS is only partly understood. In the present study, we investigated the effects of truncated (mini) form of TrpRS proteins from human, bovine, or zebrafish on vascular endothelial growth factor (VEGF)-stimulated chemotaxis of human umbilical vein endothelial cells (HUVECs). We show that both human and bovine mini TrpRSs inhibited VEGF-induced endothelial migration, whereas zebrafish mini TrpRS did not. Next, to identify residues crucial for the angiostatic activity of human mini TrpRS, we prepared several site-directed mutants based on amino acid sequence alignments among TrpRSs from various species and demonstrated that a human mini K153Q TrpRS mutant cannot inhibit VEGF-stimulated HUVEC migration and cannot bind to the extracellular domain of VE-cadherin. Taken together, we conclude that the Lys153 residue of human mini TrpRS is a VE-cadherin binding site and is therefore crucial for its angiostatic activity.

Expression of the rodent-specific alternative splice variant of tryptophanyl-tRNA synthetase in murine tissues and cells.

Tryptophanyl-tRNA synthetase (TrpRS) catalyzes the aminoacylation of tRNA(Trp). mRNA of a rodent-specific alternative splice variant of TrpRS (SV-TrpRS), which results in the inclusion of an additional hexapeptide at the C-terminus of full-length TrpRS (FL-TrpRS), has been identified in murine embryonic stem (ES) cells. In the present study, we evaluated the expression of mouse TrpRS mRNA by real-time reverse transcription PCR. We show that SV-TrpRS and FL-TrpRS mRNAs are highly expressed in murine ES cells, embryo, spleen, lung, liver and uterus, and that the relative expression of SV-TrpRS compared to FL-TrpRS is significantly less in the brain. Moreover, we found that interferon-γ increases the expression of TrpRS in a mouse cell line. These results provide the first evidence for tissue-specific expression and alternative splicing of mouse TrpRS.