The Role of the Cortex and the Cytoplasm in Deformations of the Plasma Membrane

Abstract

Recently, the role of the actin cortex has been shown to play a considerable role in resisting mechanical deformations of the membrane. Not only does this membrane-tethered network withstand large magnitude, high-aspect ratio, localized loads, it also responds to deformations by aiding in recovering their pre-deformed morphologies. Cytoplasmic flow, has also recently gained attention, and may aid in this shape-recovery process. Considered a biphasic material, the cytoplasm has been shown to manoeuvre its liquid cytosol through its solid filamentous networks, in response to force. Here, we examine the short and long-term dynamics of membrane-cortex deformations, and investigate the influence of the membrane, cytoskeletal components, and osmotic pressure. Atomic force microscopy was used to induce localized cellular deformations while resonant scanning laser confocal microscopy was used to quantify and observe the deformation and recovery response within the plane of deformation. Following both short (seconds) and long (minutes) perturbations, HeLa cells displayed two distinct responses: cells either recovered immediately (milliseconds), or gradually (seconds). While 90% of untreated cells recovered quickly, anti-cytoskeletal inhibitors led to a drastic reduction in this response (only 10-20% fast recovery), demonstrating the main role of actin and microtubule networks in the recovery process. Despite cytoskeletal disruption, this small percentage of cells still recovered quickly, suggestive of osmotic pressure-driven shape changes. By subjecting cells to hyper- and hypo-osmotic conditions, we demonstrated that cytosolic flow, in concert with an intact cytoskeleton, is largely responsible for the quick return to pre-deformed morphologies. Altogether, we demonstrate that the cytoskeleton and cytoplasm act in concert in the recovery of cell shape, and that there exists a duality in the recovery response of cells, regardless of the cell having an intact cytoskeleton, the reason for which remains unknown.