Availability of TGF-1 in the Myofibroblast Matrix: A Matter of Strain and Fibronectin

Fibrosis is a detrimental disease that causes organ failure with no effective therapy available to date. Activated fibroblasts -myofibroblasts- are responsible for the excessive secretion of extracellular matrix (ECM) and contraction into the stiff collagenous scar tissue that characterizes fibrosis. Transforming growth factor 1 (TGF-1) is the main cytokine that activates myofibroblasts from a variety of different precursor cells and is central regulator of all fibrotic processes. Because TGF-1 also positively controls vascular, inflammatory, and epithelial cell homeostasis, global TGF-1 inhibition strategies failed to treat fibrosis due to severe side-effects on other cell types. My thesis aims to identify myofibroblastic-specific mechanisms of TGF-1 activation as potential therapeutic targets to treat fibrosis. I am revealing how the mechanical and compositional properties of the ECM contribute to the extracellular storage and activation of latent TGF-1 by myofibroblast contraction. We are the first to establish a direct link between the ECM remodeling activity of myofibroblasts and TGF-1 bio-availability. Analogous to Toronto squirrels storing chestnuts for the winter, myofibroblasts establish stores of latent TGF-1 in the ECM for later activation to gain the contractile strength required to remodel scar tissue. My results demonstrate that: (1) Direct application of mechanical force to latent TGF-1 through cell surface receptor integrins induces a conformational change in the TGF-1-associated pro-peptide and releases the active growth factor. (2) Akin to loading a mechanical spring, myofibroblasts gradually strain the ECM that stores TGF-1 and thereby mechanically prime the latent complex for more efficient release of active TGF-1. (3) The presence of extracellular domain-A (ED-A), being characteristically spliced into cellular fibronectin in conditions of tissue remodeling, enhances binding of latent TGF-1 to the myofibroblast-specific ECM. In the course of these studies, I further show that inhibiting myofibroblast force production and integrin force transmission, blocking the binding of latent TGF-1 to ED-A fibronectin, and releasing the myofibroblast ECM from strain all result in reduced amounts of active TGF-1 being available to bind receptors in the cell membrane. Consequently, these interventions interrupt the feed-forward loop of myofibroblast activation by TGF-1, enhanced contraction, and enhanced auto-activation of TGF-1. Eventually, these findings are poised to lead to novel strategies to attenuate fibrosis progression in vivo using the myofibroblast and its ECM as specific target.Ph.D.2016-11-16 00:00:0