Sphingolipids and their Impact in Corneal Wound Healing

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Up to one fourth of all cases of blindness worldwide are attributable to corneal opacities generated by scarring, or fibrosis, representing a huge economic and societal burden. No effective therapies for corneal fibrosis have been developed and the most reliable treatment option is corneal transplantation, which has numerous limitations/ and complications, including post-surgical corneal fibrosis. Corneal fibrosis is characterized by the formation of corneal scars from over-accumulation of disorganized extracellular matrix (ECM) produced by fibroblasts and myofibroblasts after they are activated by injury or infection. The corneal wound-healing, as well as the process of fibrosis, is driven by multiple complex pathways involving many cytokines, growth factors, and chemokines, which are not completely understood. The lack of knowledge regarding this process is a critical barrier to developing new treatment strategies to minimize scarring and retain or restore corneal transparency. Our recent advances, with the aid of an NIH/NEI R21 (EY025256), have led us to discover a novel potential mechanism that combines sphingolipid (SPL) signaling with classical transforming growth factor-β (TGF-β) pathways to mediate corneal fibrosis via activation/differentiation of keratocytes into myofibroblasts. We have also demonstrated that SPL metabolism is altered in “injured” corneal stromal cells, and that stimulation of healthy corneal stromal cells with Sphingosine 1-Phosphate (S1P), a bioactive SPL, induces TGF-β1 expression and fibrosis, signaling through the S1P receptor 3 (S1P3). Furthermore, we found that TGF-β isoforms can increase S1P3 signaling by increasing expression of Sphingosine kinase 1 (SPHK1; an enzyme that synthesizes S1P) and S1P3. Based on these discoveries, we hypothesize that S1P is a key mediator of corneal fibrosis via activation of TGF-β, and TGF-β in turn induces expression of S1P signaling proteins and thus forms a positive feedback loop which drives irreversible activation of corneal fibroblasts and differentiation to myofibroblasts. The proposed studies are designed to clearly define the key players in these pathways and delineate how they interact in the context of corneal fibrosis using our in vitro 2D and 3D models of human and mouse corneal stromal cells (Aim 1); and in vivo models of corneal wound healing in wild type, Sphk1, and S1P3 knockout mice, along with testing the therapeutic potential of targeting S1P and TGF-β signaling using selective inhibitors in these models (Aim 2). The role of S1P in corneal fibrosis has not received substantial attention and we are currently the only group pursuing SPL-based processes as part of the potential mechanism. If successful, the results from the proposed studies could have far-reaching scientific and clinical significance, as the understanding of corneal fibrotic mechanisms and the role of S1P as an important mediator would not only be important for clinical management/treatment of corneal fibrosis, but could also be applicable to many diseases in which fibrosis is a major pathological outcome, such as liver, lung, and cardiac fibrosis.