Osmotic niche changes as multifaceted trigger of cellular regenerative processes in organ injury

Summary Cell niches are organ-specific and characterized by a variety of distinct biophysical cues, including mechanics and osmolality. Injury disrupts this cellular environment and marks the start of regenerative processes. It remains unclear whether bone fracture alters the osmolality of bone marrow, and how associated changes in the extracellular matrix (ECM) affect marrow-resident cells in the onset of regeneration. Here we present analyses of human tissue samples indicating that osmolality differences among tissue types lead to a sudden drop in bone marrow osmolality upon fracture, which in turn enhances ECM viscoelasticity. We reveal that a sudden osmolality drop, mimicked in vitro by lowering ion concentrations, triggers bone regenerative processes in mesenchymal stromal cells (MSCs), markedly enhancing their spreading, proliferation, and osteogenic differentiation while residing in osmolality- responsive viscoelastic ECM. Conversely, in non-physiologically elastic ECM, similarly increased osmolality augments MSC osteogenic differentiation, suggesting that ECM viscous dissipation redirects cellular responses to osmotic changes. Mechanistically, the regenerative function of the osmolality drop depends on the matrix providing cell-adhesion ligands and physiological viscoelastic properties. Sequencing data show altered gene expression already two hours after differentiation start, with distinct characteristics related to chromatin structural changes specifically associated with hypoosmolality. Our results suggest that the osmolality drop serves as fast-acting regenerative stimulus for MSCs by extracellularly enhancing matrix viscoelasticity, while altering chromatin structure intracellularly. This stimulus upon injury potentially orchestrates the individual responses of multiple cell types within a niche, facilitating a collective action towards regeneration. Learning how to leverage osmotic cues to induce regenerative cascades may eventually advance local and personalized therapeutic strategies for patients with impaired healing capacity. We anticipate that the integration of osmotic and mechanical ECM properties, as demonstrated in our assay, will catalyze advanced 3D cell culture systems and offer new perspectives on material design in tissue engineering, disease modeling, and mechanobiology.