Conformational regulation of TGF-β activation by integrin αvβ6

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Summary/Abstract: TGF-b is a key driver of lung fibrosis. The long-term goal of our research is to acquire a deep understanding of the regulation of TGF-b activity to develop new strategies and treatments for fibrosing lung disease. Currently, there are no effective therapies to treat lung fibrosis. The multifunctional cytokine TGF- b is a potent mediator of fibrosis and is a potential therapeutic target in fibrosing lung disease. However, targeting TGF-b itself or its receptors is associated with demonstrated risks as evidenced by toxicities in rodents, primates and humans. More selective targeting of the fibroinflammatory effects of TGF-b, without perturbing its normal essential functions are highly desirable. One property of TGF-b that may allow more selective targeting is to target its “activation” since it is always produced in a latent form (L-TGF-b) that requires activation in order to function. Another feature of L-TGF-b that could facilitate more specific targeting is to that it is normally covalently bound to the extracellular matrix or to the surface of cells by association with TGF-b binding proteins such as latent-TGF-b binding protein (LTBP) or glycoprotein-A repetitions predominant (GARP). We and others have shown that L-TGF-b binding to two integrins, a 8 and a vb vb 6 is essential for TGF-b activation in vivo. For the integrin a 6 activation mechanism, it has long been assumed that TGF-b vb must be released from LAP so that free TGF-b can diffuse and bind its receptors on target cells. The structural mechanism for such release of TGF-b is incompletely understood since full-length a vb 6 has not been studied in complex with TGF-b bound to TGF-b binding proteins. Based on recent structural data obtained using single particle electron cryomicroscopy (cryo-EM), we have recently proposed a new model whereby a vb 8 can bind to L-TGF-b on cells presenting the L-TGF-b/GARP complex and induce signaling without release and diffusion of TGF-b. These two different models of TGF-b activation may be able to be separately targeted, which could help mitigate therapeutic risk. Here in four aims, we will employ a structure-based approach to address the mechanism of a vb 6-mediated TGF-b activation. We will use the technique of electron cryomicroscopy (cryo- EM), which is a technique that allows for high-resolution structures of proteins and protein complexes in physiologically relevant conditions. Cryo-EM will be used to examine the role of integrin a vb 6 conformation in the mechanism of TGF-b activation. These studies will improve mechanistic understanding of TGF-b activation and therapeutic targeting strategies to more selectively and safely inhibit it.