Mesenchymal stem cells differentiate dynamic from static load through variable regulation of the Hippo pathway

The mechanical environment of a cell, which profoundly influences cell behaviors, includes loading parameters such as magnitude, rate cycle number and duration. Time-varying, or dynamic, levels of load are generally associated with anabolic responses, while time invariant, or static, forces often increase catabolic pathophysiology. How cells differentiate between static and dynamic loading parameters is unclear. In mesenchymal stem cells (MSC), we found that static strain (SS) increased nuclear YAP at 3 h, whereas 200 cycles of dynamic strain (DS) did not. Differences in nuclear YAP were directed by phosphorylation of Hippo enzymes: SS promoted association of PP2Ac dephosphorylase with nuclear MST1/2. In contrast, DS activated Hippo signaling with phosphorylation of MST1/2 and YAP. Actin remodeling was differentially affected by DS and SS: DS generated thicker fibers with greater cross-sectional area and enhanced parallel alignment. In contrast, static strain reinforced the existing isotropic actin network by increasing fiber density and focal adhesion count. DS induced actin remodeling was associated with phosphorylation of both LATS1/2 and AMOT, which binds both actin and nuclear MST1/2. When LATS1/2 was inhibited, or AMOT was knocked down, nuclear PP2Ac increased and YAP was dephosphorylated. This suggests that DS induced actin remodeling provokes release of phosphorylated AMOT from actin binding sites, while SS induced actin reinforcement sequesters AMOT. In sum, our data shows that MSC are equipped to differentiate between load components, resulting in nuanced mechanical effects on the Hippo pathway.