PMC

Time-dependent stiffness of rat brain at 10% shear strain. The shear modulus measured after imposition of a 10% strain decays over time as the tissue rearranges to relieve stress. The images show the structure of rat astrocytes, visualized by staining for the intermediate filament glial fibrillary acidic protein. The astrocytes were cultured on polyacrylamide gels of constant time-independent stiffnesses that mimic either the rapid elastic response (1.1 kPa, top) or the long-time elastic response (150 Pa, bottom) of the intact brain.

Comparison of biochemical and mechanical signaling. Biochemical signals can diffuse from a point source, such as a cell or a gland, or can be released into the circulation (top two illustrations on the left). In both cases, the strength of the signal decreases with distance from its origin at a rate of 1/r² (in a 2D system) or becomes diluted in the blood flow in circulation, as shown in the graph on the left. By contrast, mechanical signals are transmitted through the effects of the force on the target cell, typically causing its deformation. The mechanical signal can be applied directly to a single cell (top image on the right). Here, the cell is anchored to a matrix through focal adhesions (red squares) and its mechanical properties are determined by its cytoskeleton (actin fibers in green) and protein interactions with its plasma membrane (cell membrane in blue surrounding cell). Force could also be applied indirectly, from one cell to another, as shown in the second illustration on the right (cells in blue, cell–cell adhesion in red), or, alternatively, at a distance and transmitted through another structure such as the fibers in a matrix. This is shown in the third illustration on the right, whereby the cell on the right contracts, thus pulling the matrix through focal adhesions (red squares). The matrix fibers are displaced, immediately exerting force on the left cell through its focal adhesions (red squares). In principle, signals of this type do not significantly lose their intensity with distance (shown schematically in the graph below, red line). However, in reality, signal strength might be altered by the elastic properties of the transmitting substance. If the transmitting substance (here, the basement membrane with collagen fibers) is not rigid but elastic, the full force of the initial deformation might not be transmitted. Nevertheless, the strength of the signal does not decrease over distance or time to the same extent as chemical signals (blue line in the graph). The directionality of biochemical and mechanical signaling also differs. Cells respond to, and migrate in response to, chemical gradients by chemotaxis, as shown in the bottom image on the left, in which the intensity of the gradient is represented by the thickness of the vertical lines. Generally speaking, chemotaxis represents a simple 2D response. In a tissue, a cell responds to spatially complex mechanical stimuli (bottom illustration on the right). The arrows shown represent the direction and magnitude of forces impinging on a cell through its focal adhesions (red squares), resulting in a net force vector that is directed to the right.

Sensation of, and responses to, matrix-generated mechanical signals. (A) The basic molecular machinery that senses and responds to matrix-generated mechanical signals. When a cell encounters a matrix, integrins bind molecules in the matrix and additional proteins aggregate, forming a focal complex (top image). A focal complex contains integrins that connect the ECM to actin fibers, as well as additional essential proteins that participate in the activation and aggregation of integrins to link them to actin fibers. These proteins include talin, paxillin and vinculin. Other factors, such as kinases and phosphatases, which are also important for these processes, are not shown for simplicity. In the presence of force, probably generated by actin polymerization, additional integrins and other proteins aggregate and bind to F-actin fibers and non-muscle myosins, resulting in formation of a focal adhesion (middle image). The cell surveys its mechanical environment with periodic contraction of actin and non-muscle myosin stress fibers, which are attached to the integrins that pull against the matrix. Focal complexes and focal adhesions differ significantly in the content and phosphorylation state of their proteins, as well as their stability over time, focal contacts being transient unless they mature into focal adhesions. An important difference between the two structures is the presence of non-muscle myosin in stress fibers that join focal adhesions. The presence of non-muscle myosin permits generation of significantly more force than can occur with actin polymerization alone; cells without non-muscle myosins cannot sense matrix stiffness. Over time, and as a result of mechanical force acting on the integrins from the actin fibers and non-smooth muscle myosins, the aggregation of these additional proteins results in development of mature focal adhesions (bottom image). Additionally, proteins including α-actinin, filamin and cortactin cross-link actin fibers, thereby adding mechanical strength to the actin cytoskeleton and, consequently, the cell overall. Filamin and α-actinin also participate in linking actin fibers to integrin β subunits. If the cell finds itself on a matrix with increased stiffness (as indicated by thicker and longer force arrows at the bottom of the integrins in the bottom illustration), the cell senses the increased stiffness through reduced ability of the non-muscle myosin–actin fibers to contract against the focal adhesions attached to the matrix and displace it. This process leads to an increase in integrin aggregation and thus enlarged focal adhesions through further aggregation of proteins and additional actin fibers with more contraction force. The cellular cytoskeletal and contractile elements increase their force of contraction to match the new increased stiffness of the matrix. The cell spreads on the matrix by pulling against it and sending out lamellipodia that establish new focal complexes that mature into focal adhesions under stress (transition from the middle to the bottom image; see also transition between top and middle illustration in B). At some point, which is characteristic of each cell type, the cell reaches its maximum stiffness value; this might be less than that of the matrix. As the stiffness of the cell approaches its maximum value, thick bundles of actin, often called stress fibers, form. Stress fibers bridge focal adhesions and result in a stiffer cell. Their formation is illustrated by the transition from the middle to the bottom image here, and also in the transition between the top and the middle illustration in B. (B) The responses of normal cells to soft and stiff matrices (top two illustrations) and of an abnormal cell (bottom image) that is unable to sense matrix stiffness. In the top illustration, a cell is shown on a soft matrix, represented by a wavy black line, indicating that the cell can contract against the matrix and deform it. This cell has only a few focal adhesions (red squares) and actin fibers (green arrows). When the same cell is placed on a stiffer matrix, which the contractile apparatus of the cell cannot deform (middle), the number of focal adhesions increases. The number of actin–non-muscle myosin stress fibers and their thickness also increase, leading to cell spreading and stiffening. In disease states such as cancer and scarring, cells might encounter abnormally stiff matrix and therefore take on abnormal mechanical and cell biological characteristics. The illustration at the bottom of the panel shows a cell that cannot sense or respond to matrix-generated mechanical signals on a stiff matrix. The cell remains soft with only a few focal adhesions and actin fibers. Cells with these characteristics are found, for example, in filamin-null M2 melanoma cells, in cells that lack integrins, in glomerular podocytes from a mouse model of HIV-associated nephropathy, in α-actinin-4-null mice and in cells without functional non-muscle myosins. These cells all have defects in adhesion and migration, and show increased susceptibility to injury by mechanical force.