Cell stretching, the application of physical force to cells in a laboratory setting, has transformed the study of cell biology. This technique allows researchers to apply precise mechanical cues to cultured cells, moving beyond the traditional view of cells as static, purely chemical entities. Cells constantly respond to their physical environment, and their behavior is governed by both biochemical signals and mechanical forces. Controlled stretching mimics the forces cells experience within the body, providing insight into how these physical stimuli direct cellular behavior.
How Cells Naturally Sense Mechanical Force
The mechanism by which cells translate physical force into a biological response is called mechanotransduction. This process begins when mechanical signals, such as the rhythmic tension on heart muscle cells or the strain on blood vessel walls, are detected. The extracellular matrix (ECM), the network of proteins surrounding cells, acts as the first layer of resistance and transmission for these forces.
Specialized protein complexes, particularly integrins, function as mechanosensors embedded in the cell membrane. These sensors physically link the external ECM to the cell’s internal scaffolding, or cytoskeleton. When the ECM is stretched, integrins relay this tension across the cell membrane, initiating a cascade of internal events. The internal scaffolding, composed of a dynamic network of filaments, propagates the mechanical signal throughout the cell’s interior.
This internal structure, anchored by the actomyosin network, is constantly under tension, allowing it to sense even subtle changes in external strain. Mechanical forces cause structural changes within the cytoskeleton, which then triggers biochemical reactions. This process rapidly converts a physical change in the cell’s environment into a chemical message that influences cell function.
Laboratory Methods for Controlled Cell Strain
To replicate these complex physical interactions, researchers utilize specialized bioreactor systems that apply precise mechanical strain to cultured cells. These systems rely on flexible membranes, commonly made from a silicone elastomer such as Polydimethylsiloxane (PDMS), onto which cells are grown. The membrane is coated with ECM proteins, allowing the cells to adhere and sense the substrate stiffness.
One prominent method uses a computer-regulated pneumatic system that applies a vacuum beneath the flexible membrane. When the vacuum is generated, the membrane is pulled down over fixed cylindrical posts, causing it to stretch and deform. This results in an equibiaxial strain, meaning the cells are stretched uniformly in all directions on the horizontal plane.
Alternatively, motor-driven systems use a stepper motor to mechanically pull the edges of the cell-seeded membrane. This technique allows for the application of uniaxial strain, stretching the cells primarily along one axis, which better mimics forces like longitudinal muscle contraction. Researchers precisely control the magnitude (the percentage of elongation, e.g., 5% or 20% strain) and the frequency (how often the stretch occurs, e.g., 0.1 Hz for slow cycles or 1 Hz for rapid cycles).
The Impact of Stretching on Cellular Structure and Function
Once the mechanical signal is perceived by the cell, it initiates a reorganization of the cell’s internal architecture. The first visible change is often the rapid reorganization of the cytoskeleton’s microfilaments and microtubules. This internal scaffolding restructures itself, frequently aligning perpendicular to the direction of the applied stretch to better withstand the force.
This physical reorganization triggers complex signaling cascades that communicate the mechanical stress to the cell’s nucleus. For example, mechanical strain often causes the phosphorylation of specific tyrosine residues on focal adhesion proteins, creating docking sites for other signaling molecules. Key pathways involving Rho-family GTPases, such as Rac1 and Rho-associated protein kinase (ROCK), become activated, further regulating the tension and structure of the actomyosin network.
The final consequence of mechanotransduction is a change in gene expression, altering the cell’s long-term function. When the mechanical signal reaches the nucleus, it influences which genes are turned on or off. For example, in fibroblasts, stretch can increase the production of collagen and other ECM components, leading to tissue remodeling. This alteration is sometimes mediated by epigenetic modifications, such as changes in histone occupancy, allowing the mechanical force to reprogram the cell’s identity and function.
Using Cell Stretching to Model Human Disease
The ability to precisely control mechanical forces has made cell stretching an indispensable tool for modeling human pathology in the laboratory. In the context of vascular disease, researchers use stretch systems to study atherosclerosis, a condition exacerbated by dysfunctional endothelial cells lining the blood vessels. Modeling disturbed cyclical stretch, characterized by low magnitude or multidirectional forces, causes endothelial cells to adopt a disease-prone state.
This pro-atherogenic response is specifically linked to an increase in the nuclear expression of the transcription factor NF-κB, which promotes inflammation and plaque formation. By contrast, uniform, high-magnitude stretch, which mimics healthy blood flow, encourages a protective, anti-inflammatory phenotype.
Cell stretching also provides a model for understanding cardiac hypertrophy, the pathological enlargement of heart muscle cells (cardiomyocytes) often caused by chronic high blood pressure. Applying cyclic stretch (e.g., 10% to 21% elongation in two-second cycles) successfully mimics the mechanical overload experienced by the heart. This overload induces hypertrophic hallmarks in human stem cell-derived cardiomyocytes, including increased cell size and a shift in gene expression profiles to those found in fetal heart cells.
Cell stretching is also used in modeling ventilator-induced lung injury (VILI), a complication of mechanical ventilation. By subjecting lung epithelial cells to high-amplitude cyclic stretch, often exceeding 18% elongation, scientists replicate the pathological mechanical forces of over-distension. This stretch leads to cellular damage, the production of reactive oxygen species, and the activation of inflammatory signaling pathways, allowing researchers to test protective ventilation strategies and potential drug therapies.