Suppression of Rho-associated kinase 1 (ROCK1) promotes human hematopoietic stem cell expansion by attenuating mitochondrial fission

Ex vivo expansion of hematopoietic stem cells (HSCs) is limited by mitochondrial stress-induced loss of stemness. To identify protective mechanisms resembling the hypoxic bone marrow niche, we performed single-cell transcriptomics on hypoxia-collected HSCs, revealing significant downregulation of Rho-associated kinase 1 (ROCK1). This observation suggested ROCK1 as a potential regulator of HSC homeostasis. We tested this by genetically or pharmacologically inhibiting ROCK1 with shRNA or Y27632, which reduced mitochondrial reactive oxygen species, mitochondrial mass, and membrane potential while promoting expansion of phenotypic HSCs (Lin⁻CD34⁺CD38⁻CD45RA⁻CD49f⁺CD90⁺). Mechanistically, ROCK1 inhibition attenuated DRP1-mediated mitochondrial fission by decreasing p-DRP1(Ser616) and increasing p-DRP1(Ser637), thereby reducing mitochondrial fragmentation. Additionally, ROCK1 inhibition elevated BCL2 expression and reduced active Caspase-3 levels, indicating suppressed apoptosis. Limiting dilution transplants demonstrated a fourfold increase in functional HSC frequency following ROCK1 inhibition, with enhanced long-term engraftment in secondary recipients. Our findings identify ROCK1 as a critical regulator of mitochondrial dynamics in HSCs and provide a mechanistic basis for targeting ROCK1 to enhance functional HSC expansion, offering a promising strategy to improve outcomes in HSC transplantation by mimicking hypoxic niche signals ex vivo.