Almost all animal cells maintain a thin layer of actin filaments and associated proteins underneath the cell membrane. The actomyosin cortex is subject to internal stress patterns which result from the spatiotemporally regulated activity of non-muscle myosin II motors in the actin network. We study how these active stresses drive changes in cell shape and flows within the cortical layer, and how these cytoskeletal deformations and flows govern processes such as cell migration, cell division and organelle transport. Following a continuum mechanics approach, we develop theoretical descriptions for three different cellular processes, to obtain - in collaboration with experimental groups - a detailed and quantitative understanding of the underlying cytoskeletal mechanics.
We investigate the forces and cortex flows involved in adhesion-independent cell migration in confinement. Many types of cell migration rely on the extension of protrusions at the leading edge, where the cells attach to the substrate with specific focal adhesions, and pull themselves forward, exerting stresses in the kPa range. In confined environments however, cells exhibit migration modes which are independent of specific adhesions. Combining hydrodynamic theory, microfluidics and quantitative imaging of motile, non-adherent carcinosarcoma cells, we analyze the mechanical behavior of cells during adhesion-independent migration. We find that the accumulation of active myosin motors in the rear part of these cells results in a retrograde cortical flow as well as the contraction of the cell body in the rear and expansion in the front, and we describe how both processes contribute to the translocation of the cells, depending on the geometric and mechanical parameters of the system. Importantly, we find that the involved propulsive forces are several orders of magnitude lower than during adhesive motility while the achieved migration velocities are similar. Moreover, the distribution of forces on the substrate during non-adhesive migration is fundamentally different, giving rise to a positive force dipole. In contrast to adhesive migration modes, non-adhesive cells move by exerting pushing forces at the rear, acting to expand rather than contract their substrate as they move. These differences may strongly affect hydrodynamic and/or deformational interactions between collectively migrating cells.
In addition to the work outlined above, we study contractile ring formation in the actin cytoskeleton before and during cell division. While in disordered actin networks, myosin motor activity gives rise to isotropic stresses, the alignment of actin filaments in the cortex during cell division introduces a preferred direction for motor-filament interactions, resulting in anisotropies in the cortical stress. Actin filaments align in myosin-dependent shear flows, resulting in possible feedback between motor activity, cortical flows and actin organization. We investigate how the mechanical interplay of these different cortical properties gives rise to the formation of a cleavage furrow during cell division, describing the level of actin filament alignment at different points on the cortex with a nematic order parameter, in analogy to liquid crystal physics. We show that cortical anisotropies arising from shear-flow induced alignment patterns are sufficient to drive the ingression of cellular furrows, even in the absence of localized biochemical myosin up-regulation. This mechanism explains the characteristic appearance of pseudocleavage furrows in polarizing cells.
Finally, we study the characteristic nuclear movements in pseudostratified epithelia during development. These tissues consist of highly proliferative, tightly packed and elongated cells, with nuclei actively travelling to the apical side of the epithelium before each cell division. We explore how cytoskeletal properties act together with the mechanics of the surrounding tissue to control the shape of single cells embedded in the epithelium, and investigate potential mechanisms underlying the observed nuclear movements. These findings form a theoretical basis for a more detailed characterization of processes in pseudostratified epithelia.
Taken together, we present a continuum mechanics description of the actomyosin cell cortex, and successfully apply it to several different cell biological processes. Combining our theory with experimental work from collaborating groups, we provide new insights into different aspects of cell mechanics.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:29250 |
| Date | 14 January 2016 |
| Creators | Erzberger, Anna |
| Contributors | Salbreux, Guillaume, Jülicher, Frank, Guck, Jochen, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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