mTORC2 regulates mechanically induced cytoskeletal reorganization and lineage selection in marrow-derived mesenchymal stem cells.

The cell cytoskeleton interprets and responds to physical cues from the microenvironment. Applying mechanical force to mesenchymal stem cells induces formation of a stiffer cytoskeleton, which biases against adipogenic differentiation and toward osteoblastogenesis. mTORC2, the mTOR complex defined by its binding partner rictor, is implicated in resting cytoskeletal architecture and is activated by mechanical force. We asked if mTORC2 played a role in mechanical adaptation of the cytoskeleton. We found that during bi-axial strain-induced cytoskeletal restructuring, mTORC2 and Akt colocalize with newly assembled focal adhesions (FA). Disrupting the function of mTORC2, or that of its downstream substrate Akt, prevented mechanically induced F-actin stress fiber development. mTORC2 becomes associated with vinculin during strain, and knockdown of vinculin prevents mTORC2 activation. In contrast, mTORC2 is not recruited to the FA complex during its activation by insulin, nor does insulin alter cytoskeletal structure. Further, when rictor was knocked down, the ability of mesenchymal stem cells (MSC) to enter the osteoblastic lineage was reduced, and when cultured in adipogenic medium, rictor-deficient MSC showed accelerated adipogenesis. This indicated that cytoskeletal remodeling promotes osteogenesis over adipogenesis. In sum, our data show that mTORC2 is involved in stem cell responses to biophysical stimuli, regulating both signaling and cytoskeletal reorganization. As such, mechanical activation of mTORC2 signaling participates in mesenchymal stem cell lineage selection, preventing adipogenesis by preserving β-catenin and stimulating osteogenesis by generating a stiffer cytoskeleton.