Regulation of plasma membrane tension through hydrostatic pressure and actin protrusion forces

The plasma membrane and its associated proteins form a critical signaling hub, mediating communication between the extracellular environment and the intracellular space. Previous research suggests that both membrane trafficking and signaling activity are influenced by mechanical tension in the plasma membrane. Despite its importance, the mechanisms by which cells regulate membrane tension remain poorly understood. Using the optical tension sensor FliptR and AFM-assisted tether force measurements, we investigate plasma membrane tension regulation in mitotic cells by measuring tension changes following cytoskeletal and cell shape perturbations. Our findings show that in both assays, reported tensions are critically influenced by the cytoskeleton, however, with partially deviating trends highlighting the conceptual differences between bare and apparent membrane tension. By integrating experimental data with theoretical modeling, we demonstrate that the actin cytoskeleton regulates bare membrane tension through two distinct mechanisms: (i) modulation of intracellular hydrostatic pressure and (ii) adjustment of polymerization forces in actin-rich finger-like protrusions. SIGNIFICANCE STATEMENTThe plasma membrane is a vital cell interface, facilitating communication with the environment. Mechanical tension in the membrane plays a crucial role in tuning this communication, yet its regulation remains poorly understood. This is partly due to the lack of precise measurement tools and the misleading use of membrane tether forces as proxies. In this study, we use advanced techniques -- FliptR tension sensors, AFM-based tether force measurements, and theoretical modeling -- to explore membrane tension in mitotically arrested cells. We find that the actin cytoskeleton regulates membrane tension via intracellular pressure control and polymerization forces in protrusions. Moreover, we show that tether forces do not reliably reflect in-plane membrane tension, making them unsuitable as direct reporters of membrane mechanics.