Expression profile of the matricellular protein osteopontin in primary open-angle glaucoma and the normal human eye.

Elevated intraocular pressure (IOP) in primary open-angle glaucoma (POAG) is caused by increased outflow resistance within the trabecular meshwork, the tissue that drains most of the aqueous humor (AH) from the anterior segment of the human eye.1 Changes that occur within the trabecular cells, in protein secretions, or within the extracellular matrix have been identified as the most likely causes for increased outflow resistance in POAG.2,3 Traditionally, standard treatment for POAG has focused on reducing IOP surgically, through medications, or both. Pharmaceutical agents such as the prostaglandin analogs (e.g., latanoprost) enhance the flow of AH by stimulating extracellular matrix (ECM) turnover by expression changes of matrix metalloproteinases, tissue inhibitor of metalloproteinases, and matricellular proteins.4–7 Matricellular proteins are nonstructural ECM proteins that regulate cellular function and matrix production by interacting with cellular receptors, growth factors, and cytokines. Matricellular proteins are involved in a wide range of cell modulatory functions, including cell adhesion, migration, survival, and apoptosis.8–11 Several members of the matricellular protein family have been identified as potential regulators of aqueous outflow resistance. For example, mice lacking the matricellular protein SPARC (secreted protein, acidic and rich in cysteine) have lower IOP than wild-type controls.12 Hevin, a matricellular protein with homology to SPARC, interacts with a mutant form of the glaucoma-associated protein myocilin.13 Thrombospondin 1, a multifunctional matricellular protein, is elevated at the mRNA level in primary cultures of lamina cribrosa cells obtained from POAG patients and is also a key activator of transforming growth factor beta (TGFβ), which is elevated in 50% of POAG AH.14–19 Tenascin C, another member of the matricellular protein family, is upregulated in response to mechanical stretch and has been shown to alter cell shape, migration, and adhesion.20–22 However, though somewhat defined, the role of matricellular proteins in IOP regulation and glaucoma is still relatively unknown. Osteopontin (OPN), another member of the matricellular protein family, is a key regulator of ECM mineralization and bone formation, is considered a strong marker of calcification, vascular diseases (e.g., atherosclerosis), and biomineralization, and is a prognostic marker for inflammatory heart diseases.23–26 OPN is associated with a wide range of biological processes, including cell adhesion, migration, cell survival, activation of immune cells, and ECM remodeling, and is an inhibitor of vascular calcification.8,9,11,27–29 In the eye, OPN modulates wound healing and neovascularization in the mouse cornea, controls epithelial-mesenchymal transition in the mouse lens, and is upregulated in rat retinal ganglion cells in response to excitotoxic and ischemic injury.30–33 In the normal trabecular meshwork, the expression of OPN mRNA and several other genes associated with calcification inhibition indicates a possible role of these molecules in maintaining a healthy trabecular meshwork.34–38 Imbalance of these inhibitors may lead to a less pliable trabecular meshwork that eventually may affect outflow and IOP. This is consistent with increased trabecular meshwork stiffness in glaucoma.39 We recently identified OPN as one of the most abundant proteins in human AH.40 Given the high level of OPN expression in AH, its potential role as an inhibitor of tissue calcification, and the involvement of other matricellular proteins in IOP regulation, we evaluated the distribution profile of OPN in human ocular tissues and compared OPN concentrations between normal and POAG AH samples. In addition, OPN expression was analyzed in normal human trabecular meshwork (NTM) cells after latanoprost-free acid (LFA) treatment to determine whether OPN levels were influenced by IOP-lowering medications. Finally, the role of OPN in IOP regulation was examined in OPN−/− mice.