Non-Native Structural Properties of the Glaucoma-Associated Olfactomedin Domain of Myocilin Lead to Amyloid Fibrils

Abstract

Approximately 4% of adult-onset and 10-33% of juvenile-onset primary open angle glaucoma cases are caused by mutations in the gene encoding for myocilin. The vast majority of mutations are within the 30 kDa C-terminal olfactomedin (OLF) domain. Disease-causing myocilin variants aggregate within human trabecular meshwork (HTM) cells instead of secretion to the extracellular matrix. This accumulation taxes the cells and leads to HTM cell death, resulting in increased intraocular pressure, and a hastening of glaucoma-associated vision loss. In spite of reports on the intracellular accumulation of myocilin in HTM cells and model organisms, these aggregates have not been structurally characterized. Our lab recently provided biophysical evidence for the hallmarks of amyloid fibrils in aggregated forms of WT and mutant myocilin localized to the C-terminal OLF domain (myoc-OLF). Under mildly destabilizing conditions, WT myoc-OLF has a distinct non-native structure capable of forming amyloid fibrils. In the case of disease-causing myocilin variants, this non-native structure is accessible under physiological conditions, leading to accelerated fibril growth. Building on the hypothesis that partial unfolding of myoc-OLF is necessary for fibril growth, this study also investigates the structural and biophysical effects of adding stabilizing agents, such as osmolytes and Ca2+, to the myoc-OLF domain as a strategy to prevent myoc-OLF amyloid aggregation. The results of this study suggest a novel protein-based hypothesis for glaucoma pathogenesis and the need for further testing in a clinical setting.