Cells actively sense and process mechanical information that is provided by the extracellular environment to make decisions about growth, motility and differentiation. It is important to understand the underlying mechanisms given that deregulation of the mechanical properties of the extracellular matrix (ECM) is implicated in various diseases, such as cancer and fibrosis. Moreover, matrix mechanics can be exploited to program stem cell differentiation for organ-on-chip and regenerative medicine applications. Mechanobiology is an emerging multidisciplinary field that encompasses cell and developmental biology, bioengineering and biophysics. Here we provide an introductory overview of the key players important to cellular mechanobiology, taking a biophysical perspective and focusing on a comparison between flat versus three dimensional substrates. This article is part of a Special Issue entitled: Mechanobiology.

The docking protein p130Cas is a major Src substrate involved in integrin signaling and mechanotransduction. Tyrosine phosphorylation of p130Cas in focal adhesions (FAs) has been linked to enhanced cell migration, invasion, proliferation, and survival. However, the mechanism of p130Cas targeting to FAs is uncertain, and dynamic aspects of its localization have not been explored. Using live cell microscopy, we show that fluorophore-tagged p130Cas is a component of FAs throughout the FA assembly and disassembly stages, although it resides transiently in FAs with a high mobile fraction. Deletion of either the N-terminal Src homology 3 (SH3) domain or the Cas-family C-terminal homology (CCH) domain significantly impaired p130Cas FA localization, and deletion of both domains resulted in full exclusion. Focal adhesion kinase was implicated in the FA targeting function of the p130Cas SH3 domain. Consistent with their roles in FA targeting, both the SH3 and CCH domains were found necessary for p130Cas to fully undergo tyrosine phosphorylation and promote cell migration. By revealing the capacity of p130Cas to function in FAs throughout their lifetime, clarifying FA targeting mechanism, and demonstrating the functional importance of the highly conserved CCH domain, our results advance the understanding of an important aspect of integrin signaling.p130Cas (Crk-associated substrate) is a Src substrate that functions in integrin signaling to promote cell motility, invasion, proliferation, and survival (1, 2). p130Cas was first recognized as a prominent tyrosine-phosphorylated protein in cells transformed by v-crk (3) and v-src (4). The observation that p130Cas interacts directly with the Src homology 2 (SH2) 2 domain of the v-Crk protein (5, 6) contributed to the recognition of SH2 domains as phosphotyrosine-binding modules in signal transduction. The primary structure of p130Cas (7) indicated a function as a docking/scaffolding protein, lacking domains indicative of intrinsic enzymatic activity but having various domains and motifs for mediating interactions with other proteins.p130Cas was independently identified in a screen for proteins that interact with focal adhesion kinase (FAK) (8), a tyrosine kinase named for its prominent localization to sites of integrin-mediated cell adhesion. A Src homology 3 (SH3) domain at the N terminus of p130Cas mediates the FAK interaction. Like FAK, p130Cas localizes to focal adhesions (FAs) and undergoes tyrosine phosphorylation in response to adhesion (9 -11). Thus, p130Cas is a signaling component of the FA protein complex ("adhesome") that assembles to bring about cellular responses to integrin engagement. A primary role for p130Cas in integrin signaling is consistent with the phenotype of p130Cas-deficient mice, which die during embryonic development due to defects associated with a disorganized actin cytoskeleton (12). Despite the direct interaction with FAK, tyrosine phosphorylation of p130Cas is attributed to Src-family kinases (13-15). However, FAK can act as a scaffold to recru...

Integrin adhesion receptors connect the extracellular matrix (ECM) to the cytoskeleton and serve as bidirectional mechanotransducers. During development, angiogenesis, wound healing and cancer progression, the relative abundance of fibronectin receptors, including integrins a5b1 and avb3, changes, thus altering the integrin composition of cell-matrix adhesions. Here, we show that enhanced avb3 expression can fully compensate for loss of a5b1 and other b1 integrins to support outside-in and inside-out force transmission. a5b1 and avb3 each mediate actin cytoskeletal remodeling in response to stiffening or cyclic stretching of the ECM. Likewise, a5b1 and avb3 support cellular traction forces of comparable magnitudes and similarly increase these forces in response to ECM stiffening. However, cells using avb3 respond to lower stiffness ranges, reorganize their actin cytoskeleton more substantially in response to stretch, and show more randomly oriented traction forces. Centripetal traction force orientation requires long stress fibers that are formed through the action of Rho kinase (ROCK) and myosin II, and that are supported by a5b1. Thus, altering the relative abundance of fibronectin-binding integrins in cell-matrix adhesions affects the spatiotemporal organization of force transmission.