METTL3 induces bone marrow mesenchymal stem cells osteogenic differentiation and migration through facilitating M1 macrophage differentiation

Despite the crucial role of m⁶A methyltransferase METTL3 in multiple diseases onset and progression, there are still lacking hard evidence proving that METTL3 could affect macrophage polarization in the stage of bone repair. Here, we aimed to explore the potential involvement of METTL3 in bone repair through modulating macrophage polarization and decipher the underlying cellular/molecular mechanisms. Here we treated RAW 264.7 cells and BM-derived primary macrophages (BMDM) with lipopolysaccharide (LPS) to induce M1 differentiation. METTL3 expression was upregulated in pro-inflammatory macrophages (M1) as compared with macrophages (M0). And overexpression of METTL3 promoted the expression of IL-6 and iNOS secretion by M1 macrophage. In the coculture condition, M1 macrophages with forced expression of METTL3 significantly enhanced migration ability of BMSCs, and also remarkably facilitated osteogenesis ability of BMSCs; the opposite was true when expression of METTL3 was knockdown. In addition, the m⁶A-RIP microarray suggested that METTL3 silencing significantly reduce the m⁶A modification of DUSP14, HDAC5 and Nfam1. Furthermore, the findings showed that expression of HADC5 was downregulated in M1 macrophages with METTL3 knockdown, while the DUSP14 expression had slight change and Nfam1 expression was very low. In contrast, METTL3 overexpression promoted HDAC5 expression, indicating that HDAC5 is the critical target gene of METTL3. Under such a theme, we proposed that METTL3 overexpression might be a new approach of replacement therapy for the treatment of bone repair.