MRTF-A signaling regulates the acquisition of the contractile phenotype in dedifferentiated chondrocytes.

Chondrocyte culture as a monolayer for cell number expansion results in dedifferentiation whereby expanded cells acquire contractile features and increased actin polymerization status. This study determined whether the actin polymerization based signaling pathway, myocardin-related transcription factor-a (MRTF-A) is involved in regulating this contractile phenotype. Serial passaging of chondrocytes in monolayer culture to passage 2 resulted in increased gene and protein expression of the contractile molecules alpha-smooth muscle actin, transgelin and vinculin compared to non-passaged, primary cells. This resulted in a functional change as passaged 2, but not primary, chondrocytes were capable of contracting type I collagen gels in a stress-relaxed contraction assay. These changes were associated with increased actin polymerization and MRTF-A nuclear localization. The involvement of actin was demonstrated by latrunculin B depolymerization of actin which reversed these changes. Alternatively cytochalasin D which activates MRTF-A increased gene and protein expression of α-smooth muscle actin, transgelin and vinculin, whereas CCG1423 which deactivates MRTF-A decreased these molecules. The involvement of MRTF-A signaling was confirmed by gene silencing of MRTF or its co-factor serum response factor. Knockdown experiments revealed downregulation of α-smooth muscle actin and transgelin gene and protein expression, and inhibition of gel contraction. These findings demonstrate that passaged chondrocytes acquire a contractile phenotype and that this change is modulated by the actin-MRTF-A-serum response factor signaling pathway.