Blocking RhoA Relieves Nuclear Stress and Rejuvenates Aged Blood Stem Cells in Mice

Aging weakens hematopoietic stem cells (HSCs), the cells that rebuild blood and immune systems, and this decline contributes to age-related disease. Epigenetic drift is a hallmark of aged stem cells, but what causes it is poorly understood. This study tests whether mechanical stress on the cell nucleus, and the RhoA signaling protein that responds to it, helps drive that drift and whether it can be reversed.

The researchers used mouse HSCs, sorted from young (8–20 weeks) and aged (>80 weeks) mice. They squeezed the cells under micro-pillar confinement, used hypotonic shock and the chromatin-loosening drug sodium butyrate to raise nuclear envelope (NE) tension, and cultured cells on hydrogels of different stiffness. They also used inducible Rhoa knockout cells, 3D confocal imaging, and machine-learning-style feature analysis (UMAP clustering) of DAPI-stained nuclei. Marrow stiffness was mapped by nanoindentation.

In young HSCs, nuclear stretching activated RhoA through a cell-intrinsic route: phosphorylated cPLA2 moved to the NE and released arachidonic acid, a known RhoA activator. Rhoa-deficient HSCs could not survive moderate (5 µm) confinement, and their nuclei broke. Substrate stiffness did not change RhoA activity, and RhoA loss did not alter responses to stiffness. Aged HSCs showed higher RhoA activity, more NE P-cPLA2, increased nuclear TAZ and NE smoothing consistent with higher tension. Aged marrow was softer, not stiffer, which argues for an intrinsic cause. The RhoA inhibitor Rhosin and the cPLA2 inhibitor AACOCF3 both lowered RhoA activity. Rhosin also restored NE folding and a youthful distribution of DAPI-dense chromatin regions near the nuclear border.

Per the abstract, RhoA inhibition was also linked to restored H3K9me2, reduced chromatin accessibility and transcription at retrotransposons, and increased Klf4 expression and activity. In vivo, this improved aged HSC regenerative capacity and corrected lymphoid/myeloid skewing. This suggests that targeting nuclear mechanosignaling could be a route to rejuvenating aged stem cells.

Caveats are substantial. All data come from mice and isolated cells, with modest cell numbers in imaging assays. Rhosin also inhibits RhoB/RhoC, so effects are not strictly RhoA-specific. The text supplied was truncated, so the chromatin, retrotransposon, Klf4 and transplantation results here rely on the abstract. Safety and durability in aging humans are untested, and RhoA is essential for normal hematopoiesis.