Harnessing engineered exosomes as METTL3 carriers: Enhancing osteogenesis and suppressing lipogenesis in bone marrow mesenchymal stem cells for postmenopausal osteoporosis treatment

Postmenopausal osteoporosis (PMOP), a prevalent skeletal disorder among women post-menopause, has emerged as a pressing global public health concern. Exosomes derived from serum have exhibited encouraging therapeutic potential in addressing PMOP, albeit with underlying mechanisms requiring deeper exploration. To elucidate these mechanisms, we devised a mouse model by surgically inducing ovariectomy and isolated exosomes from serum samples. Subsequently, we employed qRT-PCR, Western blotting, and immunofluorescence analysis to quantify relevant gene and protein expression patterns. To assess the biological effects on treated cells and tissues, we utilized ARS staining, oil red O staining, and micro-CT analysis. Additionally, we examined the METTL3/FOXO1 m6A site interaction and the FOXO1/YTHDF1 complex using dual-luciferase reporter assays and RIP assays. The m6A modification levels of FOXO1 were quantified via MeRIP-PCR. Furthermore, we engineered bone marrow mesenchymal stem cell exosomes by loading abundant METTL3 mRNA and decorating their surfaces with bone-targeting peptides. The successful synthesis and bone-targeting capabilities of these modified exosomes were validated through electron microscopy, <i>in vivo</i> imaging, and immunofluorescence staining. Our findings reveal that METTL3, in collaboration with YTHDF1 within serum-derived exosomes, enhances FOXO1 gene transcription by fostering m6A modification of FOXO1. This, in turn, promotes osteogenic differentiation of bone marrow mesenchymal stem cells while inhibiting lipogenic differentiation. Notably, our engineered exosomes, BT-oe-METTL3-EXO, not only harbor high levels of METTL3 but also demonstrate exceptional bone-targeting efficiency. <i>In vitro</i> studies demonstrated that BT-oe-METTL3-EXO significantly mitigated bone mass loss induced by ovariectomy in mice, bolstered osteogenic differentiation of mouse bone marrow mesenchymal stem cells, and inhibited lipogenic differentiation. Collectively, our research underscores the pivotal regulatory function of serum-derived exosomes in human bone marrow stem cells (hBMSCs) and underscores the promising therapeutic potential of BT-oe-METTL3-EXO for combating postmenopausal osteoporosis.