Gain-of-function mutations in the
olfactomedin domain of the MYOC gene facilitate the toxic accumulation of
amyloid-containing
myocilin aggregates, hastening the onset of the prevalent ocular disorder
primary open-angle glaucoma. Aggregation of wild-type
myocilin has been reported in other
glaucoma subtypes, suggesting broader relevance of misfolded
myocilin across the disease spectrum, but the absence of
myocilin does not cause disease. Thus, strategies aimed at eliminating
myocilin could be therapeutically relevant for
glaucoma. Here, a novel and selective
Grp94 inhibitor reduced the levels of several mutant
myocilin proteins as well as wild-type
myocilin when forced to misfold in cells. This inhibitor rescued mutant
myocilin toxicity in primary human trabecular meshwork cells. Mechanistically, in vitro kinetics studies demonstrate that
Grp94 recognizes on-pathway aggregates of the
myocilin olfactomedin domain (myoc-OLF), accelerates rates of aggregation and co-precipitates with myoc-OLF. These results indicate that aberrant
myocilin quaternary structure drives
Grp94 recognition, rather than
peptide motifs exposed by unfolded
protein. Inhibition of
Grp94 ameliorates the effects of Grp94-accelerated myoc-OLF aggregation, and
Grp94 remains in
solution. In cells, when wild-type
myocilin is driven to misfold and aggregate, it becomes a client of
Grp94 and sensitive to
Grp94 inhibition. Taken together, the interaction of
Grp94 with
myocilin aggregates can be manipulated by cellular environment and genetics; this process can be exploited with
Grp94 inhibitors to promote the clearance of toxic forms of
myocilin.