Figure 5.1 The extracellular matrix is a protein mesh surrounding cells that provides both mechanical and biochemical support. While matrix content varies with tissue and disease, certain core molecules are consistently present. Structural proteins such as collagen, fibronectin, and laminin define matrix mechanical properties and cell adhesion, whereas proteoglycans such as perlecan store signaling molecules like growth factors. The extracellular matrix, together with the cells embedded
in it, defines tissue mechanical properties, specifically the elastic modulus E or stiffness, k. Normal tissue moduli range from 1 kPa for soft tissue such as liver, to 10-100 kPa for intermediate tissues such as arteries, and more than 100 MPa for hard tissues such as bone. Early in vitro studies suggested that substrate modulus may affect cell function; however, it was difficult to independently modulate substrate biochemistry and mechanics. In 1997, Pelham and Wang used polyacrylamide gels coated with collagen to determine substrate stiffness effects on cell spreading and focal adhesion formation (Pelham and Wang, 1997). Polyacrylamide gel stiffness can be changed by varying the acrylamide-monomer to bisacrylamide-crosslinker concentration; gels do not adsorb proteins so specific matrix proteins can be linked to the gel surface to control the cell adhesion ligand; gels have superb optical quality that permits high resolution immunofluorescent microscopy; and the materials are familiar to biology and bioengineering laboratories that perform electrophoresis. With this new model system, the study of substrate stiffness effects on cell function rapidly expanded. Cell types ranging from endothelial cells to neurons to mesenchymal stem cells have been studied on gels of stiffness ranging from 15 Pa to 700 kPa. Cells sense substrate properties via integrins, transmembrane heterodimeric proteins that attach extracellularly to specific matrix protein domains and intracellularly to the actin cytoskeleton (Fig. 5.2) (Barczyk et al., 2010). The 24 integrins are formed of
different combinations of 18 a and 8 b subunits, with both subunits contributing to integrin-ligand specificity. For example, a1 b1 and a2 b1 integrins bind to the GFOGER sequence in collagen. av b3 and a5 b1 bind to the RGD sequences of vitronectin and fibronectin, respectively. Integrin-matrix interactions that depend on integrin and matrix protein density as well as the specific integrin-matrix protein pair control many cell processes. Integrins assemble into focal complexes that can mature into larger focal adhesions, which contain a plethora of linking and signaling proteins. Proteins such as vinculin and talin anchor actin stress fibers to provide a mechanical connection, whereas other proteins such as focal adhesion kinase provide biochemical signaling. When external forces are applied, cells exert traction forces on the substrate to which they are attached via focal adhesions. Cell traction forces deform softer materials, and the substrate resists these cellgenerated forces if it is sufficiently stiff to prevent deformation (Janmey and McCulloch, 2007).