Glaucoma is a leading cause of preventable blindness, particularly among persons of African and Latino descent. Elevated intraocular pressure (IOP) is a causative risk factor and is due to altered drainage through the trabecular meshwork (TM). Extracellular matrix (ECM) turnover is known to be one of the factors that influence IOP regulation in the TM, but little is known of how ECM turnover is controlled. Abnormal accumulations of ECM in the TM are seen in both untreated and treated eyes with POAG. Matricellular proteins are non-structural secreted glycoproteins that facilitate cellular control over their surrounding ECM. Matricellular proteins, and in particular SPARC, generally cause an increase in ECM. We hypothesize that matricellular proteins, which are known to affect ECM turnover, will regulate ECM turnover in the TM, alter IOP, and that dysregulation contributes to the pathophysiology of glaucoma. As a new investigator, I have defined a research program to elucidate the regulatory role of matricellular proteins in the TM. The matricellular family includes SPARC, thrombospondins (TSP)-1 and -2, tenascins C and X, hevin, and osteopontin. To focus the start of our in-depth investigations, we screened the IOPs of transgenic mice with single deletions of SPARC, TSP-1, TSP-2, and hevin. SPARC deletion (SPARC- null) resulted in the greatest impact on IOP (lowering of approximately 15%). SPARC is one of the most highly expressed genes by TM cells at baseline and is one of the most highly upregulated in response to physiologic stress (mechanical stretch). We were the first to define SPARC's expression pattern and intracellular location in the TM. We believe that SPARC increases ECM deposition and IOP. We will support our hypothesis by increasing (via adenoviral transfer of cloned human SPARC) and decreasing (lentiviral transfer of sequences to express shRNA targeting SPARC) in human TM cell culture, anterior-segment organ perfusion (to show the effects on TM outflow facility and structural correlations), and restore the SPARC-null phenotype to baseline by replenishing SPARC expression. Our preliminary data indicate the SPARC overexpression increases collagen IV, laminin, and fibronectin production and shifts the balance of matrix metalloproteinases and tissue inhibitors of metalloproteinases towards greater ECM deposition. Furthermore, our early findings show SPARC overexpression increases IOP in the anterior-segment perfusion model and restores the IOP in SPARC-null mice to levels equivalent to wild-type mice. At this early stage, suppressing SPARC seems to have the opposite effects. Our work will provide critical new information on the molecular events that control IOP and provide primary therapeutic targets for new anti-glaucoma treatments based on ECM modulation.
Glaucoma is a leading cause of preventable blindness, particularly among persons of African and Latino descent that is caused by an elevated eye pressure. The fundamental control of eye pressure is unknown. If successful, this project will reveal a previously unappreciated system leading to both a greater understanding of normal physiology and a potential avenue for treatment.
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