Two distinct receptor-binding domains of human glycyl-tRNA synthetase 1 displayed on extracellular vesicles activate M1 polarization and phagocytic bridging of macrophages to cancer cells.

Macrophages play important roles in cancer microenvironment. Human cytosolic glycyl-tRNA synthetase (GARS1) was previously shown to be secreted via extracellular vesicles (EVs) from macrophages to trigger cancer cell death. However, the effects of GARS1-containing EVs (GARS1-EVs) on macrophages as well as on cancer cells and the working mechanisms of GARS1 in cancer microenvironment are not yet understood. Here we show that GARS1-EVs induce M1 polarization and facilitate phagocytosis of macrophages. GARS1-EVs triggers M1 polarization of macrophage via the specific interaction of the extracellular cadherin subdomains 1-4 of the cadherin EGF LAG seven-pass G-type receptor 2 (CELSR2) with the N-terminal WHEP domain containing peptide region of GARS1, and activates the RAF-MEK-ERK pathway for M1 type cytokine production and phagocytosis. Besides, GARS1 interacted with cadherin 6 (CDH6) of cancer cells via its C-terminal tRNA-binding domain to induce cancer cell death. In vivo model, GARS1-EVs showed potent suppressive activity against tumor initiation via M1 type macrophages. GARS1 displayed on macrophage-secreted extracellular vesicles suppressed tumor growth in dual mode, namely through pro-apoptotic effect on cancer cells and M1 polarization effect on macrophages. Collectively, these results elucidate the unique tumor suppressive activity and mechanism of GARS1-EVs by activating M1 macrophage via CELSR2 as well as by direct killing of cancer cells via CDH6.

A novel endogenous damage signal, glycyl tRNA synthetase, activates multiple beneficial functions of mesenchymal stem cells.

During tissue repair, the injury site releases various bioactive molecules as damage signals to actively recruit stem cells to the damaged region. Despite convincing evidence that mesenchymal stem cells (MSCs) can sense damage signals and promote repair processes, the identity of these signals and how these signals regulate stem cell-mediated tissue repair remain unknown. Glycyl tRNA synthetase (GRS) is a ubiquitously expressed enzyme that catalyzes the first step of protein synthesis in all organisms. In addition to this canonical function, we identified for the first time that GRS is released by damaged tissues or cells in response to various injury signals and may function as a damage signal that activates the proliferative, differentiation, and migratory potential of MSCs, possibly through its identified receptor, cadherin-6 (CDH-6). Binding between GRS and CDH-6 activates survival signals, such as those of the PI3K/Akt and/or FAK/ERK1/2 pathways. More importantly, we also found that MSCs stimulated with GRS show significantly improved homing and differentiation potential and subsequent in vivo therapeutic effects, in a liver fibrosis animal model. Collectively, our findings provide compelling evidence for a novel function of GRS in enhancing the multiple beneficial functions of stem cells via a non-canonical mechanism as a damage signal.