Cells alter their mechanical properties in response to their local microenvironment; this plays a role in determining cell function and can even influence stem cell fate. Here, we identify a robust and unified relationship between cell stiffness and cell volume. As a cell spreads on a substrate, its volume decreases, while its stiffness concomitantly increases. We find that both cortical and cytoplasmic cell stiffness scale with volume for numerous perturbations, including varying substrate stiffness, cell spread area, and external osmotic pressure. The reduction of cell volume is a result of water efflux, which leads to a corresponding increase in intracellular molecular crowding. Furthermore, we find that changes in cell volume, and hence stiffness, alter stem-cell differentiation, regardless of the method by which these are induced. These observations reveal a surprising, previously unidentified relationship between cell stiffness and cell volume that strongly influences cell biology.cell volume | cell mechanics | molecular crowding | gene expression | stem cell fate C ell volume is a highly regulated property that affects myriad functions (1, 2). It changes over the course of the cell life cycle, increasing as the cell plasma membrane grows and the amount of protein, DNA, and other intracellular material increases (3). However, it can also change on a much more rapid time scale, as, for example, on cell migration through confined spaces (4, 5); in this case, the volume change is a result of water transport out of the cell. This causes increased concentration of intracellular material and molecular crowding, having numerous important consequences (6, 7). Alternately, the volume of a cell can be directly changed through application of an external osmotic pressure. This forces water out of the cell, which also decreases cell volume, increases the concentration of intracellular material, and intensifies molecular crowding. Application of an external osmotic pressure to reduce cell volume also has other pronounced manifestations: For example, it leads to a significant change in cell mechanics, resulting in an increase in stiffness (8); it also impacts folding and transport of proteins (9), as well as condensation of chromatin (10). These dramatic effects of osmotic-induced volume change on cell behavior raise the question of whether cells ever change their volume through water efflux under isotonic conditions, perhaps to modulate their mechanics and behavior through changes in molecular crowding.Here, we demonstrate that when cells are cultured under the same isotonic conditions, but under stiffer extracellular environments, they reduce their cell volume through water efflux out of the cell, and this has a large and significant impact on cell mechanics and cell physiology. Specifically, as a cell spreads out on a stiff substrate, its volume decreases, and the cell behaves in a similar manner to that observed for cells under external osmotic pressure: Both the cortical and cytoplasmic stiffness increase as the vol...

No abstract

We study the growth and invasion of glioblastoma multiforme (GBM) in three-dimensional collagen I matrices of varying collagen concentration. Phase-contrast microscopy studies of the entire GBM system show that invasiveness at early times is limited by available collagen fibers. At early times, high collagen concentration correlates with more effective invasion. Conversely, high collagen concentration correlates with inhibition in the growth of the central portion of GBM, the multicellular tumor spheroid. Analysis of confocal reflectance images of the collagen matrices quantifies how the collagen matrices differ as a function of concentration. Studying invasion on the length scale of individual invading cells with a combination of confocal and coherent anti-Stokes Raman scattering microscopy reveals that the invasive GBM cells rely heavily on cell-matrix interactions during invasion and remodeling.

The cellular cytoskeleton is a dynamic network of filamentous proteins, consisting of filamentous actin (F-actin), microtubules, and intermediate filaments. However, these networks are not simple linear, elastic solids; they can exhibit highly nonlinear elasticity and athermal dynamics driven by ATP-dependent processes. To build quantitative mechanical models describing complex cellular behaviors, it is necessary to understand the underlying physical principles that regulate force transmission and dynamics within these networks. In this chapter, we review our current understanding of the physics of networks of cytoskeletal proteins formed in vitro. We introduce rheology, the technique used to measure mechanical response. We discuss our current understanding of the mechanical response of F-actin networks, and how the biophysical properties of F-actin and actin cross-linking proteins can dramatically impact the network mechanical response. We discuss how incorporating dynamic and rigid microtubules into F-actin networks can affect the contours of growing microtubules and composite network rigidity. Finally, we discuss the mechanical behaviors of intermediate filaments.