ECM Regulation of Ocular Surface Disease

ABSTRACT Bacterial keratitis is a serious public health threat associated with significant ocular morbidity and is one of the major causes of blindness worldwide. By one estimate, the annual incidence of bacterial keratitis is approximately 500,000 patients worldwide. Even with modern day treatment, corneal infections can result in poor vision in 50% and surgical intervention in 12% of patients. Several Gram-positive and Gram-negative bacterial pathogens can infect the cornea and cause keratitis. Bacterial pathogens use all resources available to survive in the hostile host environment. Subversion of host extracellular matrix (ECM) components and their receptors as attachment sites is thought to be a common virulence mechanism shared by many bacteria. However, there are few data that clearly support this idea in vivo. We found in preliminary studies that deletion of syndecan-1 (Sdc1), a major cell surface heparan sulfate proteoglycan (HSPG) of epithelial cells, causes a gain of function in a mouse model of scarified corneal infection, where Sdc1-/- corneas are significantly less susceptible to Streptococcus pneumoniae infection. Topical administration of excess Sdc1 ectodomains or heparan sulfate (HS) significantly inhibits S. pneumoniae corneal infection, suggesting that HS chains of Sdc1 promote infection as a cell surface attachment receptor. However, S. pneumoniae does not interact with Sdc1 and Sdc1 is shed upon S. pneumoniae infection, indicating that Sdc1 does not directly support S. pneumoniae adhesion. Instead, Sdc1 promotes S. pneumoniae adhesion by driving the assembly of fibronectin (FN) fibrils in the corneal basement membrane to which S. pneumoniae attaches when infecting injured corneas. Excess Sdc1 ectodomains inhibit S. pneumoniae corneal infection by binding to the heparin-binding domain in FN, and interfering with S. pneumoniae binding to FN. Based on these data, this proposal will examine the overall hypothesis that specific ECM interactions coordinate the assembly of corneal basement membranes, and that certain bacterial pathogens of the ocular surface exploit these normal biological processes to promote their pathogenesis. This hypothesis will be tested in 3 Specific Aims. Aim 1 will define the structural basis of how HS inhibits bacterial corneal infection. Aim 2 will determine the significance and relevance of bacteria-induced Sdc1 shedding in corneal infection, and Aim 3 will elucidate the underlying mechanisms of how Sdc1 regulates FN fibrillogenesis in the corneal basement membrane. These studies are expected to uncover previously unknown functions of the ECM in the cornea and to establish a new integrated virulence pathway in bacterial keratitis.