Chapter 11 What is Functional Genomics Teaching us about Intraocular Pressure Regulation and Glaucoma?

The effects of myocilin expression on functionally relevant trabecular meshwork genes: a mini-review

Myocilin is a secreted glaucoma-associated protein, specifically induced by dexamethasone in human trabecular meshwork cells, where it was discovered. Myocilin is expressed in several tissues of the body, but it causes disease only in the eye. The protein contains two domains: an N-terminal region with significant homologies to nonmuscle myosin, and a C-terminal region, which is similar to the olfactomedin proteins. Forty percent of myocilin undergoes an intracellular endoproteolytic cleavage by calpain II, a calcium-dependent cysteine protease, which releases the 2 domains. The protein is known to interact with intracellular and extracellular matrix proteins, and some is released into the extracellular space associated with exosomes. Myocilin mutations are linked to glaucoma and induce elevated intraocular pressure. Most of the glaucoma-causative mutations map to the olfactomedin domain, which appears to be a critical domain for the function of the protein. Myocilin mutants are misfolded, aggregate in the endoplasmic reticulum, and are not secreted. Overexpression of myocilin and of its mutants in primary human trabecular meshwork cells triggers changes in the expression of numerous genes, many of which have been known to be involved in mechanisms important for the physiology and pathology of the tissue. Here we review recent studies from our laboratory and those of others that deal with trabecular meshwork genes, which are altered by the overexpression of wild-type and glaucoma-causative mutant myocilin genes.

Evidence for a role of angiopoietin-like 7 (ANGPTL7) in extracellular matrix formation of the human trabecular meshwork: implications for glaucoma

The trabecular meshwork tissue controls the drainage of the aqueous humor of the eye. A dysfunctional trabecular meshwork leads to an altered fluid resistance, which results in increased intraocular pressure (IOP). IOP is the major risk factor of glaucoma, the second-leading cause of blindness in the developed world. In the search for genes altered by glaucomatous insults, we identified angiopoietin-like7 (ANGPTL7), a member of the ANGPTL family. Although structurally related to the angiopoietins, ANGPTL7's function is poorly understood. Because ANGPTL7 is secreted and because extracellular matrix (ECM) deposition and organization is critical for aqueous humor resistance, we investigated the effect of ANGPTL7 on relevant trabecular meshwork ECM genes and proteins. We find that overexpression of ANGPTL7 in primary human trabecular meshwork cells altered the expression of fibronectin, collagens type I, IV & V, myocilin, versican, and MMP1. ANGPTL7 also interfered with the fibrillar assembly of fibronectin. Finally, we find that silencing ANGPTL7 during the glucocorticoid insult significantly affected the expression of other steroid-responsive proteins. These results indicate that ANGPTL7 modulates the trabecular meshwork's ECM as well as the response of this tissue to steroids. Together with previous findings, these properties strengthen ANGPTL7's candidacy for the regulation of IOP and glaucoma.