Mechanisms of Focal Adhesions

Focal adhesions are dynamic multiprotein structures connecting cells to their surrounding microenvironment. Cells receive critical mechanical signals from adhesions that control many cellular processes including wound healing, differentiation, development, and cancer. Proteins that form adhesions are called adhesion proteins and some of these proteins can be mechanosensitive, meaning that they respond to mechanical stimuli. During spreading and migration, cells mechanically test extracellular matrix rigidity by contracting matrix to a constant displacement. Transmission and processing of such mechanical signals rely upon the dynamic regulation of the adhesions, which is tightly coordinated with activation of intracellular signaling cascades involving various adhesion molecules. However, the molecular mechanisms of mechanical signals that are transmitted through the adhesions to control cell behavior are poorly understood. In this thesis, we discovered novel phenomenon and mechanisms to elucidate roles of mechanical signals for multiple key aspects of basic cell behavior, especially cell growth. We performed live cell imaging of cells spreading on fibronectin coated micropillars to understand adhesion formation, adhesion regulation, and their impact on cell behavior. One of the earliest molecules to arrive at an adhesion formation site is a mechanosensitive protein called talin which binds to several other entities to form the backbone of focal adhesions. We found a novel role of talin cleavage, which previously was thought to play a role only in focal adhesion turnover. We found that talin cleavage is a force dependent process that regulates proper adhesion formation, thereby governing several critical cellular processes. In the absence of this talin cleavage, cells formed abnormal adhesions and showed inhibited growth. Further, we found that upon inhibition of talin cleavage, one of the key cellular behaviors of increased cellular motility upon stimulation by epidermal growth factor seemed to disappear. Epidermal growth factor receptor is a transmembrane protein and has previously been shown to play important role in various cancers where cells exhibit altered rigidity sensing. Surprisingly, we found that epidermal growth factor receptor was required for cellular rigidity sensing only on rigid substrates, highlighting the importance of the interplay between mechanical and biochemical signals in determining cell behavior.