Investigating the Associations Between Hmga2 Overexpression, R-Loop Reduction, and Bone Loss in Aging Mice.

Background and Objectives: Aging-related bone loss still lacks interventions. As bone marrow-derived mesenchymal stem cells (BMSCs) undergo aging, R-loop-induced DNA replication stress impairs the osteogenic ability of BMSCs. High-mobility group A-2 (Hmga2) acts as a DNA-binding protein, and the understanding of its underlying mechanisms is crucial for developing effective preventive and therapeutic strategies. Materials and Methods: Aging mice were used as the experimental model, and mouse BMSCs were isolated from their femurs. Hmga2 was achieved through specific gene delivery methods. R-loop formation was detected using dot blotting, chromatin immunoprecipitation (ChIP), and DNA-RNA immunoprecipitation (DRIP) assays. Osteogenic differentiation was evaluated. Results: R-loops were highly accumulated in aging BMSCs. Notably, the key regulator Hmga2 reversed the accumulation of R-loops in aging BMSCs. Hmga2 overexpression significantly decreased the senescence and improved the osteogenic differentiation of aging mBMSCs. Mechanistically, R-loop-forming sequence (RLFS) regions were confirmed in key osteogenesis-related genes, including runt-related transcription factor 2 (Runx2). Hmga2 bound to the RLFS region of Runx2 and promoted its expression by reducing the R-loop level. More, Hmga2 treatment delivered via the AAV system effectively decreased bone loss in aging mice and increased the serum bone turnover biomarkers and collagen remodeling. Conclusions: Our study demonstrates that Hmga2 acts as an activator of aging BMSCs, significantly promoting their osteogenic ability by eliminating the aging-induced DNA replication stress caused by R-loops. Our findings provide new insights into the mechanisms of aging-related bone loss, suggesting that Hmga2 may be a new strategy for alleviating the bone loss phenotype in aging individuals.