Chromatin Acetylation Overrides Substrate Stiffness-Dependent Mechanoresponse and Lineage Specification in Human Mesenchymal Stem Cells

Abstract

Human mesenchymal stem cells (hMSCs) exhibit a stiffness dependent cellular phenotype and lineage bias. Cells on soft substrate display reduced cell spreading, less cellular traction, smaller and fewer focal adhesions and decreased actin stress fibre organization, whereas cells on stiff substrate exhibit the converse behaviour. Further, hMSCs preferentially undergo adipogenic differentiation on soft substrate and osteogenic differentiation on stiff substrate. These stiffness dependent changes are accompanied by global chromatin condensation/decondensation on soft/stiff substrates. Despite these well documented correlations, it remains unclear whether modulating chromatin condensation alone can be sufficient to reprogram stiffness dependent cellular phenotype. To answer whether chromatin organization acts as an active regulatory element in rigidity sensing, we induced chromatin condensation/decondensation in hMSCs cultured on soft/stiff polyacrylamide gels using histone deacetylase inhibitor (HDACi) and histone acetyltransferase inhibitor (HATi) respectively. We found that across conditions, phenotypic outcomes including nuclear localization of mechanosignalling protein YAP correlated strongly with chromatin compaction state rather than with substrate stiffness when the cells were presented with combination of signals. Mechanistically pharmacological inhibition of ERK phosphorylation attenuated chromatin dependent lineage specification while leaving chromatin compaction unchanged, indicating that ERK phosphorylation operates downstream of chromatin state. Collectively, these findings suggests an inside-out signalling mechanism in which chromatin compaction state reprograms the stiffness dependent mechanoresponse and fate decisions in hMSCs. Altogether, this works provides a fundamental framework for integrating mechanical and epigenetic cues in designing biomaterial microenvironment for controlling hMSCs fate in tissue engineering and regenerative medicine.