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.