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Myocilin, a protein associated with the development of glaucoma, is expressed in most eye tissues with highest expression observed in trabecular meshwork cells. In culture, primary human trabecular meshwork cells incubated in 10% fetal bovine serum have reduced myocilin expression compared to in vivo, but incubation in human aqueous humor, their normal in vivo nutrient source, restores myocilin expression to near in vivo levels. To investigate the mechanism by which human aqueous humor stimulates myocilin accumulation in conditioned media from normal human trabecular meshwork cells, three independent trabecular meshwork cell lines were cultured in Dulbecco's Modified Eagle's Medium (DMEM) containing various supplements: fetal bovine serum (10%), human serum (0.2%), porcine aqueous humor (50%), bovine serum albumin (0.1%), dexamethasone (10(-7)M), human aqueous humor (50%) or heat-inactivated human aqueous humor (50%). Conditioned media from cultured primary trabecular meshwork cells following incubation in human aqueous humor showed significant accumulation of myocilin in a time- (15 min) and dose-dependent manner (half maximal effective concentration ∼ 30%) while intracellular myocilin levels decreased. Minimal myocilin accumulation was observed in conditioned media isolated from trabecular meshwork cells cultured in DMEM containing fetal bovine or human serum, bovine serum albumin, porcine aqueous humor, dexamethasone or DMEM alone. Heat inactivation of human aqueous humor nearly eliminated human aqueous humor-stimulated myocilin secretion. Inhibitors of new protein synthesis, gene transcription, the endoplasmic reticulum/Golgi system and endocytic/exocytic secretory pathways failed to inhibit human aqueous humor-stimulated myocilin secretion. Using immunolabeling and transmission electron microscopy, myocilin was found associated with 70-90 nm vesicle-like structures within the cytoplasm of human aqueous humor treated trabecular meshwork cells. These studies suggest that myocilin secretion from trabecular meshwork cells occurs in a Golgi-independent manner following human aqueous humor treatment. Heat-labile factors in human aqueous humor are responsible for the time- and dose-dependent release of myocilin from vesicle-like structures within the cytoplasm of trabecular meshwork cells., (Published by Elsevier Ltd.)

Over a decade has passed since myocilin was identified as the first gene linked to early and late-onset primary open-angle glaucoma. During this time, considerable effort has been put forth to understand the functional role myocilin has in normal and glaucomatous eyes. Myocilin is expressed in many ocular and non-ocular tissues, is found in both intracellular and extracellular spaces, and has been linked to elevations in intraocular pressure. Mutations in the myocilin gene that have been associated with glaucoma appear to confer a gain-of-functional activity rather than loss of function. Unfortunately, what the normal function of myocilin is and how alterations in the function can confer a glaucoma phenotype have yet to be elucidated. We will review the current understanding of myocilin with special emphasis on the structural makeup of the myocilin gene and protein, its possible physiological roles internal and external to ocular cells, the regulation of intraocular pressure as evidenced through the use of perfusion culture systems and animal models, and as a causative agent in some forms of glaucoma.