Matrix elasticity, cytoskeletal forces and physics of the nucleus: how deeply do cells 'feel' outside and in?

Abstract

Cellular organization within a multicellular organism requires that a cell assess its relative location, taking in multiple cues from its microenvironment. Given that the extracellular matrix (ECM) consists of the most abundant proteins in animals and contributes both structure and elasticity to tissues, ECM probably provides key physical cues to cells. In vivo, in the vicinity of many tissue cell types, fibrous characteristics of the ECM are less discernible than the measurably distinct elasticity that characterizes different tissue microenvironments. As a cell engages matrix and actively probes, it senses the local elastic resistance of the ECM and nearby cells via their deformation, and--similar to the proverbial princess who feels a pea placed many mattresses below--the cell seems to possess feedback and recognition mechanisms that establish how far it can feel. Recent experimental findings and computational modeling of cell and matrix mechanics lend insight into the subcellular range of sensitivity. Continuity of deformation from the matrix into the cell and further into the cytoskeleton-caged and -linked nucleus also supports the existence of mechanisms that direct processes such as gene expression in the differentiation of stem cells. Ultimately, cells feel the difference between stiff or soft and thick or thin surroundings, regardless of whether or not they are of royal descent.

Fig. 1.

Mesenchymal cells, matrix elasticity and culture models. (A) Left: development of mesenchyme into both cartilage and bone (images courtesy of Marc D. McKee, McGill University, Quebec, Canada). Right: transmission electron micrographs of nascent bone with a layer of cells and matrix on top of bone (upper) or mature cartilage (lower) with a single cell surrounded by matrix. E denotes the elasticity of matrix (see main text). (B) Left: longitudinal view of muscle stem cells (pink marker) on striated skeletal muscle. Right: transverse section showing stem cells adhering to matrix (Cerletti et al., 2008). (C) Culture models use gels to mimic the thinness and elasticity of natural matrices.

Fig. 2.

Elasticity of normal tissues. (A) Articular cartilage imaged by rastering with an AFM, either at the micron scale using a 5-μm-diameter spherical tip (left) or at the nanometer scale with a sharp pyramidal tip (right) (Stolz et al., 2004). Cartilage matrix elasticity E_M depends on length scale: micron-scale and macroscopic measurements probe the stiff collagen-fiber network, whereas the nm-sharp tip probes the softer proteoglycans. (B) Tissue cells are located in a wide range of elastic microenvironments that vary from soft brain and fat through the intermediate stiffness of muscle to relatively rigid cartilage and osteoid. (C) Mesenchymal stem cells attach to elastic substrates and take on phenotypic morphologies within hours of plating. Days later, the cells begin to express lineage markers that are determined in part by the elasticity of the substrate on which they are grown (Engler et al., 2006).

Fig. 3.

Cell-induced substrate deformations show that decreasing substrate thickness effectively stiffens the substrate. Cardiac fibroblasts contract and deform thin (A, 6.6 μm) or thick (B, 79 μm) bottom-fixed substrates (Merkel et al., 2007). The white lines indicate surface displacements of magnitude indicated by the leftmost scale bars; the rightmost scale bars indicate image size. (C) Cell spreading and cell stress (e.g. stress-fiber assembly) increase with substrate stiffness, which increases with substrate elasticity E and decreases with substrate thickness H_M.

Fig. 4.

Computing cell-induced matrix deformations, depth sensing and matrix-mediated cell-cell interactions. (A) Cells on gels are modeled as axisymmetric and homogeneously pre-stressed (i.e. with uniform tension) coupled continuously to compliant gels (Sen et al., 2009). The cell height and radius are H_cell and R_cell, respectively, and matrix thickness is H_gel. (B) Matrix displacements in finite-element computations are maximal at the cell edge and include vertical displacements of the cell-matrix interface (inset). The color scales are for the indicated variable. Matrix and cell elasticity are E_gel and E_cell, respectively; the vertical displacement of the gel is u_z. (C) The strain field propagates across thin gels (top) to the rigid base at bottom, whereas strain localizes to the cell edge for thick gels (bottom). (D) Cell-cell interactions are facilitated by elastic deformations of the matrix. Matrix deformations are suppressed in the middle of a string of cells (four cells in this case) due to tractions applied by adjacent cells of width L_cell, separated by distance L_sep. With increasing cell number, mean displacement decreases owing to opposite tractions at the middle of the string, and this trend saturates to a value that is independent of string length. The color scales are for the indicated variable, which is the mean interfacial displacement.

Fig. 5.

The nucleus: caging by the cytoskeleton, deformability, and molecular components of nuclear adhesions. (A) Typical cytoskeletal architecture around the nucleus of a mesenchymal stem cell on a 34-kPa collagen-coated polyacrylamide gel substrate. The actin cytoskeleton is shown in red (phalloidin staining; i), microtubules are shown in green (anti-α-tubulin; ii) and nuclear DNA is stained blue with Hoechst. (B) Micropipette aspiration mimics the large deformations observed in vivo (e.g. Fig. 1A). Experimental aspiration pressures (i) are typical of the stresses that cells generate (Pajerowski et al., 2007). Such pressures deform the nucleus, as evidenced by chromatin flow (ii) and lamina stretching (iii). Photobleaching is indicated by the lightning bolt and reveals the respective flow and displacement profiles. (C) Schematic of nuclear organization and interaction with selected major cell components. The nuclear envelope consists of the inner and outer nuclear membrane, the latter of which is continuous with the endoplasmic reticulum (ER). Trafficking of ER on the microtubule network is depicted. The lamina interacts with chromatin and inner membrane proteins such as SUN proteins, which also bind nesprins that span the outer membrane and cross into the perinuclear space. The nesprins link to the various cytoskeletal components and provide a means to transmit cell stress to the chromatin via a nuclear adhesion.