Stress relaxation in epithelial monolayers is controlled by the actomyosin cortex

Epithelial monolayers are one-cell thick tissue sheets that separate internal and external environments. As part of their function, they have to withstand extrinsic mechanical stresses applied at high strain rates. However, little is known about how monolayers respond to mechanical deformations. Here, by subjecting suspended epithelial monolayers to stretch, we find that they dissipate stresses on a minute time-scale in a process that involves an increase in monolayer length, pointing to active remodelling of cell architecture during relaxation. Strikingly, monolayers consisting of tens of thousands of cells relax stress with similar dynamics to single rounded cells and both respond similarly to perturbations of actomyosin. By contrast, cell-cell junctional complexes and intermediate filaments do not relax tissue stress, but form stable connections between cells, allowing monolayers to behave rheologically as single cells. Taken together our data show that actomyosin dynamics governs the rheological properties of epithelial monolayers, dissipating applied stresses, and enabling changes in monolayer length.Peer ReviewedPostprint (published version

Impact of Narrow Constraint on Single Cell Motion

Die extrazelluläre Mikroumgebung spielt eine grundlegende Rolle bei der Entwicklung von Metastasen und hat einen großen Einfluss auf die Wahl der Migrationsstrategien, die von Karzinomzellen während der Invasion angewandt werden. In-vitro Anordnungen sind hilfreiche Instrumente für die Untersuchung von Zellmigration und -invasion, da sie grundlegende Merkmale von In-vivo-Geweben reproduzieren können. Ziel dieser Forschungsarbeit ist es, die Fähigkeit von mesenchymalen und epithelialen Brusttumorzellen zu untersuchen, sich zu verflüssigen und durch enge und starre Mikrostrukturen zu navigieren. Wir verwendeten eine mikrofluidische Vorrichtung mit trichterförmigen Mikroverengungen und verglichen das Verhalten von fünf verschiedenen menschlichen Brustkrebszelllinien in der Mikrovorrichtung bei Stimulation durch Chemoattraktoren. Wir fanden heraus, dass grundsätzlich verschiedene Zelllinien das gleiche invasive Potenzial haben, da normale Epithelzellen in der Lage waren, durch die stark komprimierenden Trichter zu wandern, ähnlich wie die invasiveren mesenchymalen Zellen. Wir fanden auch heraus, dass die Migration der normalen Epithelzellen auch ohne einen chemo-attraktiven Stimulus stattfindet. Wir konzentrierten unsere Beobachtungen auf die Rolle des Aktin- und Intermediärfilament-Zytoskeletts während der eingeschränkten Migration und zeigten, dass das Aktin-Zytoskelett eine starke und langanhaltende Reorganisation erfährt, damit die Zellen durch die engen Verengungen kriechen können. Wir sahen keinen Hinweis darauf, dass das Keratin- und Vimentin-Zwischenfilament- Zytoskelett während der Invasion in die Mikroverengungen eine aktive mechanische Rolle spielte. Insbesondere die Expression des Vimentin-Zwischenfilamentproteins korrelierte in unserem Versuchsaufbau nicht mit der Invasionsfähigkeit einzelner Zellen. Unter diesen Voraussetzungen wurden die passiven (Elastizität und Viskosität) und aktiven (Kontraktilität) viskoelastischen Eigenschaften der Zellen weiter untersucht und quantifiziert. Wir fanden keinen signifikanten Unterschied in der passiven viskoelastischen Reaktion der Zellen, nachdem sie oszillierenden Druckkräften mittels AFM-Sondierung ausgesetzt waren, was darauf hindeutet, dass Elastizität und Viskosität nicht zur Unterscheidung zwischen invasiven und nicht-invasiven Zellen verwendet werden können. Es wurde kein Hinweis darauf gefunden, dass die Kompressionsversteifung die Invasion durch die Mikroverengungen entweder behindert oder fördert. Schließlich haben wir bei der Betrachtung aktiver viskoelastischer Parameter die kontraktile Reaktion unserer Zelllinien verglichen, wenn sie mit dem mikrofluidischen optischen Strecker Laser-Streckkräften ausgesetzt wurden. Hier fanden wir eine klare Korrelation zwischen den Zelllinien, die ein invasives Verhalten in den Mikroverengungen zeigten, und denjenigen, die eine aktive (substratunabhängige) kontraktile Reaktion in der optischen Streckvorrichtung zeigten. Wir kommen zu dem Schluss, dass ein entscheidender Faktor für eine erfolgreiche Migration durch hohe räumliche Enge die Fähigkeit der Zellen ist, aktiv Aktin-Stressfasern zu erzeugen und abzubauen, was sich in der Fähigkeit manifestiert, von einer substratabhängigen und stressfaserbasierten Kontraktilität zu einer substratunabhängigen kortikalen Kontraktilität zu wechseln

Cellular Hydraulics Suggests a Poroelastic Cytoplasm Rheology

The cytoplasm represents the largest part of the cell by volume and hence its rheology sets the rate at which cellular shape change can occur. Recent experimental evidence suggests that cytoplasmic rheology can be described using a poroelastic formulation in which the cytoplasm is considered a biphasic material constituted of a porous elastic solid meshwork (cytoskeleton, organelles, macromolecules) bathed in an interstitial fluid (cytosol). In this picture, the rate of cellular deformation is limited by the rate at which intracellular water can redistribute within the cytoplasm. Though this is a conceptually attractive model, direct supporting evidence has been lacking. Here we present such evidence and directly validate this concept to explain cellular rheology at physiologically relevant time-scales using microindentation tests in conjunction with mechanical, chemical and genetic treatments. Our results show that water redistribution through the solid phase of cytoplasm (cytoskeleton and crowders) plays a fundamental role in setting cellular rheology.Engineering and Applied Science

Focus on the Physics of Cancer

Despite the spectacular achievements of molecular biology in the second half of the twentieth century and the crucial advances it permitted in cancer research, the fight against cancer has brought some disillusions. It is nowadays more and more apparent that getting a global picture of the very diverse and interlinked aspects of cancer development necessitates, in synergy with these achievements, other perspectives and investigating tools. In this undertaking, multidisciplinary approaches that include quantitative sciences in general and physics in particular play a crucial role. This `focus on' collection contains 19 articles representative of the diversity and state-of-the-art of the contributions that physics can bring to the field of cancer research.Comment: Invited editorial review for the `Focus on the Physics of Cancer' published by the New journal of Physics in 2011--201

Intermediate Filament Mechanics Across Scales – From Single Filaments to Single Interactions and Networks in Cells

The mechanical properties of cells are largely determined by the cytoskeleton. The cytoskeleton is an intricate and complex structure formed by protein filaments, motor proteins, and crosslinkers. The three main types of protein filaments are microtubules, actin filaments, and intermediate filaments ( IFs ). Whereas the proteins that form microtubules and actin filaments are exceptionally conserved throughout cell types and organisms, the family of IFs is diverse. For example, the IF protein vimentin is expressed in relatively motile fibroblasts, and keratin IFs are found in epithelial cells. This variety of IF proteins might therefore be linked to the various mechanical properties of different cell types. In the scope of this thesis, I combine studies of IF mechanics on different time scales and in systems of increasing complexity, from single filaments to networks in cells. This multiscale approach allows for the simplification necessary to interpret observations while adding increasing physiological context in subsequent experiments. We especially focus on the tunability of the IF mechanics by environmental cues in these increasingly complex systems. In a series of experiments, including single filament elongation studies, single filament stretching measurements with optical tweezers, filament-filament interaction measurements with four optical tweezers, microrheology, and isotropic cell stretching, we characterize how electrostatic (pH and ion concentration) and hydrophobic interactions (detergent) provide various mechanisms by which the mechanics of the IF cytoskeleton can be tuned. These studies reveal how small changes, such as charge shifts, influence IF mechanics on multiple scales. In combination with simulations, we determine the mechanisms by which charge shifts alter single vimentin filament mechanics and we extract energy landscapes for interactions between single filaments. Such insights will provide a deeper understanding of the mechanisms by which cells can maintain their integrity and adapt to the mechanical requirements set by their environment

Mesoscale physical principles of collective cell organization

We review recent evidence showing that cell and tissue dynamics are governed by mesoscale physical principles. These principles can be understood in terms of simple state diagrams in which control variables include force, density, shape, adhesion and self-propulsion. An appropriate combination of these physical quantities gives rise to emergent phenomena such as cell jamming, topological defects and underdamped waves. Mesoscale physical properties of cell assemblies are found to precede and instruct biological functions such as cell division, extrusion, invasion and gradient sensing. These properties are related to properties of biomolecules, but cannot be predicted from biochemical principles. Thus, biological function is governed by emergent mesoscale states that can be predicted by a simple set of physical properties