BRM preserves sinusoidal niche integrity to prevent microenvironmental senescence and sustain hematopoietic stem cells

During aging and stress, remodeling of the bone marrow (BM) microenvironment compromises hematopoietic stem cell (HSC) maintenance, contributing to an extrinsic HSC aging phenotype. Although niche-derived Notch signaling is essential for hematopoietic regeneration following myelosuppressive injury, the upstream epigenetic mechanisms that regulate this stress-responsive signaling remain poorly understood. Here, we identify the chromatin remodeler BRM (SMARCA2) as a critical regulator of the BM sinusoidal niche that preserves vascular integrity and hematopoietic regeneration. Using reciprocal BM transplantation, we demonstrate that a Brm-deficient microenvironment impairs HSC repopulating capacity and imposes an aging-like myeloid bias characterized by expansion of granulocyte-monocyte progenitors. Following 5-fluorouracil (5-FU)-induced myelosuppression, BrmKO mice exhibit defective sinusoidal regeneration accompanied by endothelial degeneration. Mechanistically, BRM deficiency attenuates endothelial Notch signaling by impairing stress-induced Notch2 expression in sinusoidal endothelial cells (SECs), while simultaneously reducing Jag2 ligand pool through persistent depletion of SECs and impaired stress-induced expansion of Jag2-producing LepR-positive stromal cells. These alterations attenuate endothelial Notch signaling, resulting in defective sinusoidal regeneration, loss of mesenchymal niche support, and progressive displacement of HSCs from the sinusoidal vasculature. Notably, Brm expression is physiologically reduced in aged wild-type SECs and LepR-positive stromal cells. Collectively, our findings identify BRM as a key epigenetic regulator of bone marrow niche integrity and suggest that age-associated BRM decline contributes to niche dysfunction and hematopoietic aging.