Matrix stiffness and stress relaxation regulate osteogenesis through histone demethylases KDM4B and KDM6B

Stem cells sense biophysical cues within their extracellular microenvironment and respond via mechanotransduction signaling pathways that induce changes in gene expression and associated cell fate outcomes. Histone modifying enzymes are known to drive stem cell differentiation through changes in chromatin accessibility, but little is understood as to how extracellular matrix (ECM) mechanics regulate epigenomic remodeling. Here, we utilize alginate hydrogels with tunable mechanical properties to investigate the role of both matrix stiffness and viscoelasticity on histone demethylase expression and activity during osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBMSCs). Our results reveal that the expression of two histone demethylases, KDM4B and KDM6B, are upregulated during osteogenesis in response to stiff, viscoelastic matrix conditions. Inhibition of mechanotransduction signaling pathways reduces expression of KDM4B and KDM6B and hinders osteogenic differentiation overall. Interestingly, phosphorylation of SMAD 1/5/8 was shown to increase in cells cultured in stiff, stress relaxing matrices, and pharmacological inhibition of SMAD 1/5/8 activation reduced expression of KDM4B and KDM6B and decreased osteogenic differentiation. Taken together, our results reveal novel impacts of stem cell mechanotransduction signaling events that promote osteogenesis through epigenetic remodeling.