Rho under pressure

High blood pressure and pulse rate cause some dramatic changes to the human heart – cardiac fibroblasts proliferate, cardiomyocytes enlarge and fibroblasts differentiate into myofibroblasts. Myofibroblasts are characterised by their expression of α-smooth muscle actin (SMA), and McCulloch and colleagues are interested in signalling events that lead from haemodynamic stress to SMA expression (see p. 1801). The researchers already knew that mechanical stress causes the small GTPase RhoA to reorganise actin filaments by stimulating phosphorylation of the actin regulators LIM kinase and cofilin. They also knew that actin reorganisation promotes the expression of myocardin-related transcription factor (MRTF), which regulates the SMA promoter. To explore the connection between these pathways, the researchers used collagen-coated magnetic beads to apply stress to fibroblasts in vitro. As expected, force increased RhoA activity in the fibroblasts, causing the phosphorylation of LIM kinase and cofilin, and the polymerization of actin filaments. It also caused the nuclear translocation of an MRTF. But, crucially, using a Rho-kinase inhibitor and actin-polymerization inhibitor, they showed that MRTF translocation requires both Rho kinase and intact actin filaments, demonstrating that force-induced SMA expression occurs through a RhoA–Rho-kinase–LIMK–cofilinpathway.