The Glaucoma-Associated Olfactomedin Domain of Myocilin Forms Polymorphic Fibrils That Are Constrained by Partial Unfolding and Peptide Sequence

The glaucoma-associated olfactomedin domain of myocilin (myoc-OLF) is a recent addition to the growing list of disease-associated amyloidogenic proteins. Inherited, disease-causing myocilin variants aggregate intracellularly instead of being secreted to the trabecular meshwork, which is a scenario toxic to trabecular meshwork cells and leads to early onset of ocular hypertension, the major risk factor for glaucoma. Here we systematically structurally and biophysically dissected myoc-OLF to better understand its amyloidogenesis. Under mildly destabilizing conditions, wild-type myoc-OLF adopts non-native structures that readily fibrillize when incubated at a temperature just below the transition for tertiary unfolding. With buffers at physiological pH, two main endpoint fibril morphologies are observed: (a) straight fibrils common to many amyloids and (b) unique micron-length, ~ 300 nm or larger diameter, species that lasso oligomers, which also exhibit classical spectroscopic amyloid signatures. Three disease-causing variants investigated herein exhibit non-native tertiary structures under physiological conditions, leading to a variety of growth rates and a fibril morphologies. In particular, the well-documented D380A variant, which lacks calcium, forms large circular fibrils. Two amyloid-forming peptide stretches have been identified, one for each of the main fibril morphologies observed. Our study places myoc-OLF within the larger landscape of the amylome and provides insight into the diversity of myoc-OLF aggregation that plays a role in glaucoma pathogenesis.