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 utilized alginate hydrogels with tunable mechanical properties to investigate the role of both matrix stiffness and stress relaxation on histone demethylase expression and activity during osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBMSCs). Our results revealed that the expression of two histone demethylases, KDM4B and KDM6B, was upregulated during osteogenesis in response to stiff and fast stress-relaxing matrix conditions. Additionally, CUT&Tag profiling coupled with RNA-sequencing demonstrated that repressive histone methylation was decreased at osteogenic-specific loci in stiff, fast-relaxing matrices. Further, inhibition of mechanotransduction signaling pathways reduced expression of KDM4B and KDM6B and hindered osteogenic differentiation overall. Interestingly, phosphorylation of SMAD 1/5/8 increased in cells cultured in stiff, stress-relaxing matrices, and pharmacological inhibition of SMAD 1/5/8 activation reduced expression of KDM4B and KDM6B. Together, our results establish novel impacts of stem cell mechanotransduction signaling events that promote osteogenesis through epigenetic remodeling.