We introduce a simple mechanical model for adherent cells that quantitatively relates cell shape, internal cell stresses and cell forces as generated by an anisotropic cytoskeleton. We perform ex-... Show moreWe introduce a simple mechanical model for adherent cells that quantitatively relates cell shape, internal cell stresses and cell forces as generated by an anisotropic cytoskeleton. We perform ex- periments on the shape and traction forces of different types of cells with anisotropic morphologies, cultured on microfabricated elastomeric pillar arrays. We demonstrate that, irrespectively of the cell type, the shape of the cell edge between focal adhesions is accurately described by elliptical arcs, whose eccentricity expresses the ratio between directed and isotropic stresses. Our work paves the way toward the reconstruction of cellular forces from geometrical data available via optical microscopy. Show less
Shape and biological function are tightly connected. Physical descriptions are used to connect the shape of a biological system with its function. One system investigated here is the dendritic... Show moreShape and biological function are tightly connected. Physical descriptions are used to connect the shape of a biological system with its function. One system investigated here is the dendritic spine, which is the connection between neurons. The dendritic spine is mimicked in an artificial system. In this way, I was able to show that the shape of the dendritic spine is important in memory and learning. The shape of a cell itself is governed by its actin cytoskeleton. I showed that a simple model can be used to describe the shape of adherent cells. An adherent cell attaches itself at discrete points to the substrate. The edge of the cell in between these points can be described as a part of an ellipse. I show that all edges of a single cell can all be described with a single ellipse. On a small scale, the shape of the membrane of a cell changes with changing lipid content. I used light to change the lipid content, triggering phase unmixing in an artificial lipid membrane. I show that various important parameters change significantly. Show less
Understanding of the regulation mechanisms of CXCR4 signaling is essential for revealing its role in physiological and pathological processes. Though biochemical pathways following CXCR4 activation... Show moreUnderstanding of the regulation mechanisms of CXCR4 signaling is essential for revealing its role in physiological and pathological processes. Though biochemical pathways following CXCR4 activation by its ligand CXCL12 are well established, knowledge about the receptor dynamics on the plasma membrane remains limited. Here we used Ewing sarcoma-derived cells to unravel the processes that are involved in regulating CXCR4 dynamics on the plasma membrane during receptor signaling. Single-molecule epi-fluorescence microscopy showed that CXCR4 was present in monomeric state on the plasma membrane independent of receptor stimulation. However, upon activation freely diffusing receptors were immobilized in a ligand concentration-dependent manner. CXCR4 immobilization was strongly correlated with the ability for G-protein signaling and was a precursor of subsequent endocytotic events. Our data suggest that, a balanced regulation of G-protein dependent and independent pathways is required for controlling CXCR4 receptor mobility, and potentially subsequent controlled signal transduction. (C) 2015 Published by Elsevier B.V. Show less
Pomp, W.; Schakenraad, K.K.; Hoorn, H. van; Balciglu, H.E.; Danen, E.H.J.; Giomi, L.; Schmidt, T. 2016
The shape of a cell membrane is largely defined by the underlying actin cytoskeleton and membrane mechanics. The actin cytoskeleton asserts contractile forces on the membrane that can be divided... Show moreThe shape of a cell membrane is largely defined by the underlying actin cytoskeleton and membrane mechanics. The actin cytoskeleton asserts contractile forces on the membrane that can be divided in isotropic and directed forces. We present a theory which is an extension of the Young-Laplace equation. It models cell edges as parts of one uniform ellipse, which changes from cell to cell. The ellipse parameters are characterized by the ratio of isotropic to directed contractility of the cell. We demonstrate the capabilities of this model using fibroblasts seeded on an elastic micro-pillar array. In this way adhesion forces exerted by the cell at single adhesion sites are measured. We show that isotropic and directed forces balance the line tension in cortical actin. Furthermore, for cells with homogeneous contractile forces and a single orientation of stress-fibers any part of the cell edge follows a universal ellipse, enabling us to calculate the magnitude of isotropic and directed contractility in a single cell. We show that in 3T3 fibroblasts the directed contractility is about three times as strong as the isotropic contractility. If myosin motors are inhibited, however, directed contractility decreases, effectively disabling forces generated by stress-fibers, and the elliptical cell cortex turns into a circular shape predicted for an isotropic contractile cytoskeleton. Our analysis shows that a simple two-parameter model explains polarity, shape of the cell cortex and cellular forces as experimentally observed. Potentially this model can be used to predict stresses and forces on the extracellular matrix and tissue. Show less
