Molecular Mechanisms of Mechanosensitivity in Focal Adhesions

Physical environment guides tissue regeneration and morphology in both health and disease. In the past three decades, several experiments illustrated that mechanical cues are captured and transduced to biochemical signals in the cellular level (mechanotransduction) mediated by cell adhesion. Cells adhere to their microenvironment through large protein assemblies known as focal adhesions that directly couple intra- and extra-cellular matrices and play a critical role in many vital cell functions including proliferation, differentiation and cell fate. It is inherently difficult to investigate the molecular basis of focal adhesion formation and growth using current experimental methodologies due to the fine time- and length-scales of protein-protein interactions. Here, I used molecular dynamics simulations to investigate the underlying molecular mechanisms of focal adhesion formation and maturation with atomic resolution. Integrins are key focal adhesion receptors that reside on the cell membrane and mediate bi-directional signaling between cell cytoskeleton and ECM. Focal adhesions are a mixture of integrin-associated protein complexes known as integrin modules that forms the basic adhesion units. Integrin module formation is initiated by talin binding to the integrin tail, which is shown to be sufficient for integrin activation. A few other focal adhesion proteins can also directly engage with integrin’s cytoplasmic tail and link it to the actin cytoskeleton. It is not yet clear how simultaneous (cooperative) versus sequential (competitive) binding of focal adhesion proteins to integrin with respect to talin result in different functionalities of integrin modules. In the first part of this study, competitive versus cooperative integrin binding between two important focal adhesion proteins –filamin and α-actinin– with talin were studied. The purpose of this aim was to gain insight on integrin module formation that eventually determines its functional properties. Maturation of focal adhesions follows an increase in local forces. A well-established hypothesis on force transmission across focal adhesion complexes is the presence of mechanosensitive elements that change their conformations in response to force. In the second part of this study, we investigated and argued force-induced conformational changes of two important focal adhesion proteins –vinculin and α-actinin– in order to shed light on their role in transmission of forces across focal adhesions leading to adhesion maturation and growth. In conclusion, this study unravels the structural basis of mechanosensitivity of key focal adhesions. Furthermore, important molecular interactions that give rise to mechanosensitivite characteristics of focal adhesions were studies. Important impacts of the current study include but are not limited to the following: 1) our results was used to complement previous experimental studies and also construct new hypotheses for future experiments; 2) Understanding regulatory mechanisms of focal adhesions is critical for developing novel therapeutics for many diseases involving cell adhesion including cancer as it enhances target recognition and the accuracy of drug delivery systems. 3) In addition, performing extensive simulations on protein complexes will contribute to improving the accuracy of various aspects of computational methods including empirical force fields that is indicative of our understanding of fundamental physical and chemical principles governing protein-protein interactions. 4) And most importantly, this work provides a fundamental insight into the relation between structure and function of mechanosensitive proteins in focal adhesions