Pinching the cortex of live cells reveals thickness instabilities caused by myosin II motors

Abstract

The cell cortex is a contractile actin meshwork, which determines cell shape and is essential for cell mechanics, migration, and division. Because its thickness is below optical resolution, there is a tendency to consider the cortex as a thin uniform two-dimensional layer. Using two mutually attracted magnetic beads, one inside the cell and the other in the extracellular medium, we pinch the cortex of dendritic cells and provide an accurate and time-resolved measure of its thickness. Our observations draw a new picture of the cell cortex as a highly dynamic layer, harboring large fluctuations in its third dimension because of actomyosin contractility. We propose that the cortex dynamics might be responsible for the fast shape-changing capacity of highly contractile cells that use amoeboid-like migration.

WASp triggers mechanosensitive actin patches to facilitate immune cell migration in dense tissues

“…Formation and dynamics of cortical actin heavily depends on the Arp2/3 nucleation complex ( Bovellan et al., 2014 ; Laplaud et al., 2021 ). Upon pharmacological inhibition of Arp2/3 with CK666 actin patches in DCs under intermediate-stiff agarose were abolished, and treated cells spent most of their time in arrest ( Figure S2 A).…”

Active gels, heavy tails, and the cytoskeleton

“…Dynamics in quasi-2D systems may be even more complicated than what we have addressed here: because the Saffman-Delbrück length scale will, in a viscoelastic system, depend on frequency, it is possible that a material may act as effectively two-dimensional at some frequencies -but three-dimensional at others [50]. We also have not addressed recently observed thickness fluctuations arising from activity within the cortex [42]. Altogether, we view the quasi-two-dimensional geometry as one giving a huge amount of control over the scaling of the Oseen tensor T i j (r) -and therefore the predicted tails, making it an ideal testbed for finding a rich zoo of different types of rare events in active gels.…”

“…Other deviations from our results here may arise from short-range deviations from linear elasticity [63,64], which can change the appropriate power law exponent of T (r). Cortex turnover and dynamic thickness variations may also be relevant [42].…”

“…However, both experiments that show heavy power-law tails are measuring the displacement of tracers at the cell surface [16,17]. The cortex cannot be treated as a three-dimensional material -typical cortex thicknesses are ∼ 200 nm [41,42], while typical minifilament sizes are on the order of 400 nm. We instead move to an opposite extreme, treating the cortex as quasi-twodimensional (Fig.…”

How dynamic prestress governs the shape of living systems, from the subcellular to tissue scale

“…Observing the system simultaneously at different scales, especially combining global-scale stress and strain measurements with very local measurements could enable better characterization of how the microstructure dynamics give rise to emergent properties. For instance, tools measuring strain at the molecular scale, like Förster resonance energy transfer [198], or at the meso-scale, like micro-magnets [199], micro-droplets [200] or optical tweezers [116], could be coupled to cell-scale techniques, like parallel plate rheometry or TFM, to decipher the subcellular contributions to cell shape changes. On the other hand, cell-scale in parallel to tissue-scale mechanical testing appears necessary to understand the cellular origins of tissue flows [201].…”