MBoC | ARTICLE ECM stiffness primes the TGFβ pathway to promote chondrocyte differentiation

ABSTRACT Cells encounter physical cues such as extracellular matrix (ECM) stiffness in a microenvironment replete with biochemical cues. However, the mechanisms by which cells integrate physical and biochemical cues to guide cellular decision making are not well de-fined. Here we investigate mechanisms by which chondrocytes generate an integrated re-sponse to ECM stiffness and transforming growth factor β (TGFβ), a potent agonist of chon-drocyte differentiation. Primary murine chondrocytes and ATDC5 cells grown on 0.5-MPa substrates deposit more proteoglycan and express more Sox9, Col2α1, and aggrecan mRNA relative to cells exposed to substrates of any other stiffness. The chondroinductive effect of this discrete stiffness, which falls within the range reported for articular cartilage, requires the stiffness-sensitive induction of TGFβ1. Smad3 phosphorylation, nuclear localization, and tran-scriptional activity are specifically increased in cells grown on 0.5-MPa substrates. ECM stiff-ness also primes cells for a synergistic response, such that the combination of ECM stiffness and exogenous TGFβ induces chondrocyte gene expression more robustly than either cue alone through a p38 mitogen-activated protein kinase–dependent mechanism. In this way, the ECM stiffness primes the TGFβ pathway to efficiently promote chondrocyte differentia-tion. This work reveals novel mechanisms by which cells integrate physical and biochemical cues to exert a coordinated response to their unique cellular microenvironment