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Active or passive deformation of the cell membrane supplied by actin assembly. Perpectives on nucleus deformation
In all cell functions, a common observation is that cytoskeleton assembly correlates with membrane deformation based on active forces. The exact role, in particular, of the actin cytoskeleton in cell membrane deformation, with pushing or pulling forces, is what we address both experimentally and theoretically. We conceive stripped-down experimental systems that reproduce cellular behaviours in simplified conditions : cytoskeleton dynamics are reproduced on liposome membranes. Actin polymerization through the growth of a branched actin network is able to initiate membrane tubules and spikes by pushing or pulling, and mimics cellular deformations. By changing experimentally membrane tension and the structural details of the cytoskeleton architecture, we displace the system within a phase diagram where inward or outward deformations are favoured [1]. Moreover, shells of branched actin networks grown around liposomes display buckling and wrinkling under osmotic deflation, thereby confirming their elastic properties. The time during which we let the network grow around liposomes allows us to vary the shell thickness, and to specify the length scale of buckling versus wrinkling [2]. Our results illustrate the generic mechanism of buckling and wrinkling found in various systems spanning from pollen grains to the development of the gut or the brain.
Inspired by actin forces exerted on membranes and organelles, we address now how the nucleus, which is the most rigid cell organelle, is deformed by the actin cytoskeleton during cell translocation. When cells move through narrow spaces that are smaller than their nuclei, we find that proteins of the nuclear membrane, such as nesprins, accumulate at the nucleus front and pull the nucleus forward [3]. We want to address in the future how we could characterize this active process and infer its molecular details.
[1] Simon et al Nature Physics 2019
[2] Kusters et al, Soft Matter 2019
[3] Davidson et al, in revision