Myosin II Functions as a Direct Mechanosensor for Intercellular Invasion during Cell-Cell Fusion

How cells sense and react to external mechanical stimuli is a fundamental question in cellular biophysics. Here, we describe a previously unrecognized mechanosensory response to a localized cellular protrusive force during cell-cell fusion. Our previous studies of myoblast fusion in Drosophila embryos and cell fusion in a reconstituted culture system revealed that cell-cell fusion is an asymmetric process, in which one fusion partner (the attacking cell) extends invasive finger-like protrusions into the other (the receiving cell) to promote plasma membrane juxtaposition and fusion. Here, we demonstrate that the Rho-Rok-Myosin II (MyoII) pathway is specifically activated in the receiving cell in response to the invasive force from the attacking cell. Disrupting the function of this pathway renders less cortical resistance in the receiving cell and defects in cell-cell fusion, despite deeper invasion of the attacking cell. Increasing the cortical tension in the receiving cell by overexpressing an actin crosslinker significantly rescued such fusion defect. We show that MyoII accumulates to the cell cortex earlier than its upstream biochemical regulators Rok and Rho in response to applied force, and that MyoII is required for the steady-state accumulation of Rok and Rho. Furthermore, the motor domain of MyoII is indispensible for its cortical accumulation triggered by intercellular invasion. These results strongly suggest that MyoII functions as a direct mechanosensor that feeds back to its upstream regulators, and that the mechanosensory function of MyoII is mediated by its binding to actin filaments under mechanical stress. This newly discovered mechanosensory system, consisting of a well-defined source of protrusive force and the corresponding activation of the Rho-Rok-MyoII pathway in the neighboring cell, highlights a central role of MyoII in the mechanosensory response that ultimately leads to cortical tension generation in animal cells.