The broad long-range objective of our proposal is to obtain a detailed molecular understanding of cell adhesion and to elucidate how it mediates various physiological and pathological processes. Over the past funding period, we have been focusing on studying the mechanism of integrins, a class of heterodimeric (alpha/beta) transmembrane receptors that play key roles in the cell adhesion, particularly the cell-ECM adhesion. By performing vigorous structural studies, we and others have made a major progress toward the molecular understanding of how integrins are activated to trigger the cell-ECM adhesion. In this continuation proposal, we wish to begin the investigation on the events following the integrin activation, i.e., outside-in signaling where integrin extracellular domain adheres to ECM and elicits signals to the cytoplasmic face triggering cascades of intracellular signaling pathways that link to cytoskeleton. Extensive genetic and biochemical studies have shown that such outside-in signaling process is essential for regulating dynamic adhesion events such as cell shape change (spreading), cell migration, proliferation, and survival, yet the underlying mechanism remains poorly understood. As a first step of our investigation, we will focus on some critical yet representative pathways by addressing the following immediate questions: (i) Upon integrin activation and its subsequent ECM engagement, how exactly do the small integrin alpha/beta cytoplasmic tails (CTs, mostly 20-50 residues) recognize different effectors leading to different intracellular events? (ii) how are these different integrin-effector interactions modulated to mediate the dynamic cell adhesion events? In preliminary studies, we have found that while integrin effectors specifically interact with various integrin CTs, these interactions are surprisingly mutually exclusive with their corresponding downstream ones. This has led us to hypothesize a feedback mechanism for regulating the on/off switch of the integrin-effector interactions by their downstream targets. Such feedback regulation may promote efficient integrin-cytoskeleton assembly/reassembly during the cyclic cell spreading and migration processes. Hence our goals are 2 fold: (a) to define the molecular specificity of these interactions;and (b) to vigorously evaluate the hypothesized feedback mechanism. We will continue to use the combined structural and functional approach to pursue our goals. We believe that the results will not only provide key molecular insights into how different integrin-effector interactions mediate distinct outside-in signaling pathways but also unravel a unifying zip code, i.e., the feedback control, for directing the highly coordinated and dynamic cell spreading and migration processes. Since dysfunctions of components involved in these pathways are correlated with numerous human disorders, the specific results derived here may be also of therapeutic value.
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