Project 2- Mechanistic Role of Talin in Cellular Signaling

Project Summary The long-term objective of this proposal is to understand the activation and signaling mechanisms of integrins – a class of heterodimeric transmembrane receptors that control cell-extracellular matrix (ECM) adhesion and diverse adhesion-dependent physiological and pathological processes such as angiogenesis, blood clotting,

thrombosis, and stroke. Discovered four decades ago, integrins have been intensively studied with nearly 75,000 articles in PubMed. Tremendous effort has been made by many laboratories to investigate the mechanism of

integrin activation – the first key step to initiate the cell-ECM adhesion, which led to the discovery and elucidation of a key integrin activator talin – a large actin-binding protein containing an N-terminal head domain (talin-H) and a C-terminal rod domain (Talin-R). It is now widely believed that talin utilizes its talin-H to bind integrin

cytoplasmic face to elicit an “inside-out” conformational signal to activate the receptor. However, a recent pilot study from our laboratory revealed unexpectedly that a constitutively active full length talin is much more potent than talin-H to activate integrins, challenging the talin-H-only premise for controlling the integrin activation.

Moreover, intact talin is autoinhibited in the cytosol and how it is recruited/activated to activate integrin and link integrin to actin (outside-in signaling) for promoting cytoskeleton reassembly and dynamic adhesion remains elusive. We propose to resolve these puzzling issues in three highly integrated aims. Aim1 will elucidate how a

small GTPase Rap1 cooperatively binds talin F0 and F1 domains of talin-H to facilitate the talin recruitment and how a nascent adhesion adaptor paxillin synergizes with PIP2 to activate talin. Aim2 will investigate that after

activating talin, how paxillin further regulates the talin function by bridging its talin-H as well as talin-R to another integrin activator kindlin-2 to promote potent integrin activation. The results would provide novel insight into how talin R contributes significantly to the integrin activation. Aim3 will examine how talin mediates integrin signaling

to actin cytoskeleton – a long speculated integrin outside-in process that has remained poorly elucidated. Together, our proposed studies are expected to lead to a new paradigm for understanding the cell adhesion mechanisms by providing a comprehensive view of how talin is regulated to undergo a sequence of dynamic

events to activate integrin and link it to actin filaments leading to cytoskeleton reassembly and dynamic adhesion. Given the critical involvement of talin and its associated proteins in thrombosis, stroke, etc., our studies may ultimately lead to the development of better diagnosis and therapeutics for these diseases.