Virtually every cell in our body spends at least a portion of its life time on adhering to the extracellular matrix (ECM). Such cell-to-ECM adhesion is primarily mediated by a class of heterodimeric (?/?) transmembrane receptors, integrins. Upon activation, integrins bind ECM proteins (adhesion) and then transmit signals to intracellular cytoskeleton via large multi-protein complexes called focal adhesions (FAs). This latter step is vital to promote dynamic cell adhesive responses such as cell shape change, cell migration, and proliferation. However, while much has been learned about integrin activation over the decades, the mechanism by which integrin initiates signaling to the cytoskeleton via FAs is still poorly understood. To this end, we have been focusing on integrin-linked kinase (ILK), a nascent mediator of FAs assembly and signaling. For >15 years, ILK was thought to function as a key Ser/Thr kinase to phosphorylate integrin cytoplasmic tail and initiate the receptor signaling. Highly upregulated in many cell adhesion-dependent diseases, ILK was also regarded as a ?hot? kinase target for drug development. However, structural studies from our laboratory uncovered that ILK contains a severely degraded active site incapable of performing kinase catalysis. The finding, corroborated by extensive biochemical and genetic data, led to a conceptual breakthrough as evidenced by dozens of reviews, hundreds of re-themed research articles, and grants. However, a key issue still remains largely unresolved: without kinase function, how does ILK mediate FAs assembly and signaling? This issue has broad relevance as there exist many poorly understood pseudoenzymes including pseudokinases that occupy ~10% of human kinome. In a most recent study, we discovered that by forming a tight complex with FA adaptors PINCH and Parvin (IPP), ILK triggers specific actin filament bundling ? a process known to generate force/mechanical signal to promote cytoskeleton reorganization and dynamic cell adhesion. We further found that such actin bundling is orchestrated by two previously unrecognized actin binding motifs within the ILK-centered IPP, one from PINCH and the other from Parvin. Strikingly, this process is also sensitized by Mg-ATP bound to the pseudoactive site of ILK and impaired by a novel inhibitor we have developed. Our findings thus signify a new milestone towards resolving the mystery of ILK as a pseudokinase in mediating cell adhesion and signaling. We propose to continue our investigation by focusing on three following aims: (i) to resolve a puzzle of how ILK is localized to nascent FAs ? the first step for the ILK action; (ii) to elucidate how ILK acts as a unique protein docking center to mediate different signaling pathways; (iii) to determine detailed molecular basis of non-catalytic roles of ILK-bound MgATP in fine-tuning the ILK-mediated signaling.
These aims reflect a strong momentum of our program, which has reached a critical phase towards establishing a new paradigm in ILK biology and cell adhesion. With newly discovered mechanisms and novel inhibitor for ILK, we believe that these studies have potential to transform the understanding and treatment of ILK-associated diseases such as heart failure and cancer.
ILK-mediated adhesion of cells to extracellular matrix is fundamentally important for many physiological processes such as heart development, tissue regeneration, and wound healing. Dysregulation of ILK has been linked to numerous major diseases such as heart failure and stroke. Our proposal is centered for elucidating the molecular basis of ILK function, which will not only provide mechanistic insight into cell adhesion at normal and disease states of our body but also transform our understanding and treatment of ILK-mediated diseases.
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