Bone Marrow-Derived Macrophages' Response to Anisotropic Forces through Distinct Sensing and Transduction Pathways

The mechano-sensitivity of bone marrow-derived macrophages (BMDMs) is crucial for bone remodeling. In addition to force strength, BMDMs also showed a force-direction-dependent response. However, how anisotropic force regulates the function and differentiation of BMDMs is still under debate. Herein, a single-cell-level force-application system was developed to manipulate cells in a noncontact model based on biospecific magnetic microbeads. By adjusting the magnetic field parameters, the microbeads attached to the cell surface can generate controllable forces with specific directions. BMDMs exhibited differential responses to tensile and compressive forces regarding cell spreading. Surprisingly, although less potent than tensile force, compressive force demonstrated a significant suppressive effect on the osteoclast differentiation of BMDMs. The results suggest that this cellular behavior results from distinct pathways through which BMDMs sense and transduce tensile and compressive forces. BMDMs sense tensile force signals through α5β1 integrin and transduce them via the Rac-pPAK pathway. Compressive force, however, initially activates αvβ3 integrin on BMDMs, leading to signal transduction through the RhoA/ROCK-pMLC signaling axis that regulates BMDM differentiation. Furthermore, both in vitro coculture and in vivo subcutaneous ectopic osteogenesis studies suggest that tensile and compressive forces not only individually regulate the fate specification of BMDMs and BMSCs but also simultaneously mediate the crosstalk between these cell types. These findings provide novel insight into the mechanoresponsive mechanisms of BMDMs, deepening our understanding of mechanically induced bone remodeling.