Soft substrate priming erases fibrotic mechanical memory in mesenchymal stromal cells via YAP lysosomal degradation to improve therapeutic efficacy for spinal cord injury.

Mesenchymal stromal cells (MSCs) hold great promise for spinal cord injury (SCI) repair owing to their potent immunomodulatory and neuroprotective properties, yet their therapeutic efficacy collapses in the chronic phase. We identify a critical, overlooked barrier: the pathological stiffening of the fibrotic scar microenvironment hijacks the mechanotransduction machinery of transplanted MSCs, driving them toward a maladaptive fibroblast-like phenotype that compromises their immunomodulatory and regenerative functions, ultimately blunting their overall therapeutic potential. Here, we reveal that stiff substrates enforce MSCs into a pro-fibrotic state via a YAP/Smad-dependent mechanism. To overcome this, we developed a "soft-priming" strategy that effectively erases this fibrotic mechanical memory of MSCs. Mechanistically, soft priming does not merely prevent YAP nuclear translocation but promotes its cytoplasmic retention and subsequent lysosomal degradation, effectively dismantling the pro-fibrotic YAP signaling axis and rendering MSCs refractory to future stiffness challenges in vivo. Transplantation of these mechanically-reset MSCs into a SCI mouse model resulted in reduced scar formation, sustained immunomodulation and robust functional recovery by the early chronic stage, compared to standard-culture MSCs. By identifying soft mechanical priming as a scalable, chemical-free manufacturing step to insulate cells against hostile host environments, our findings provide a pragmatic translational pathway to revitalize MSC therapies for fibrosis-associated central nervous system (CNS) disorders.