Actomyosin dynamics in detached cells: linking clutch model to cell migration and cytokinesis

Cortical retrograde flow of actomyosin plays a critical role in cell migration and cytokinesis. While such flows have been extensively studied in adherent cells, their underlying mechanisms in non-adherent conditions remain poorly understood. In this study, we investigated actomyosin dynamics in Dictyostelium cells detached from the substratum, with a focus on cytoskeletal turnover and the applicability of the clutch hypothesis. Detached cells exhibited robust retrograde flows of actin and myosin II filaments within the cell cortex, moving at identical velocities. In contrast, membrane components and particles attached to the cell surface showed no movement, suggesting that the flows are driven by cortical actomyosin rather than membrane trafficking. Inhibition of actin polymerization completely abolished the flows, while myosin II inhibition had only a minor effect, indicating that actin polymerization is the primary driver. Flow velocities gradually decreased from the anterior to the posterior, consistent with a model in which actin filaments are pushed rearward rather than pulled from the back. Flow velocity was inversely proportional to cell migration speed, and nascent adhesion foci also moved retrogradely, supporting the clutch hypothesis, although Dictyostelium cells have neither canonical integrins nor well-defined extracellular matrix. Photobleaching experiments revealed that actin and myosin II filaments undergo turnover and recycling near the posterior. During cytokinesis under non-adherent conditions, cortical flows from the poles to the equator were significantly enhanced, and membrane components remained static. The clutch hypothesis may explain also the cortical flow and contractile ring formation during cytokinesis. Our findings demonstrate that detachment from the substratum induces dynamic, turnover-dependent actomyosin flows. These flows are primarily driven by actin polymerization and are consistent with the clutch hypothesis, which helps explain both cell migration and contractile ring formation during cytokinesis. This study expands the applicability of the clutch model and highlights the dynamic nature of the cytoskeleton in non-adherent cells.