Molecular Insights into Myocilin and Its Glaucoma-Causing Misfolded Olfactomedin Domain Variants

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Numerous human disorders arise due to the inability of a particular protein to adopt its correct threedimensional structure in the context of the cell, leading to aggregation. A new addition to the list of such protein conformational disorders is the inherited subtype of glaucoma. Different and rare coding mutations in myocilin, found in families throughout the world, are causal for early onset ocular hypertension, a key glaucoma risk factor. Myocilin is expressed at high levels in the trabecular meshwork (TM) extracellular matrix. The TM is the anatomical region of the eye that regulates intraocular pressure, and its dysfunction is associated with most forms of glaucoma. Disease variants, distributed across the 30 kDa olfactomedin domain (mOLF), cause myocilin to be sequestered intracellularly instead of being secreted to the TM extracellular matrix. The working hypothesis is that the intracellular aggregates cause a toxic gain of function: TM cell death is thought to lead to TM matrix dysfunction, hastening elevated intraocular pressure and subsequent vision loss.Our lab has provided molecular underpinnings for myocilin structure and misfolding, placing myocilin-associated glaucoma within the context of amyloid diseases like Alzheimer and diabetes. We have dissected complexities of the modular wild-type (WT) myocilin structure and associated misfolded states. Our data support the model that full-length WT myocilin adopts a Y-shaped dimer-of-dimers conferred by two different coiled-coil regions, generating new hypotheses regarding its mysterious function. The mOLF β-propellers are paired at each tip of the Y. Disease-associated variants aggregate because mOLFs are less stable, leading to facile aggregation under physiological conditions (37 °C, pH 7.2). Mutant myocilin aggregates exhibit numerous characteristics of amyloid in vitro and in cells, and aggregation proceeds from a partially folded state accessed preferentially by disease variants at physiological conditions. Interestingly, destabilization is not a universal consequence of mutation. We identified counterintuitive, stabilizing point variants that adopt a non-native structure and do not aggregate; however, these variants have not been identified in glaucoma patients. An ongoing effort is predicting the consequence of any given mutation. This effort is relevant to interpreting data from large-scale sequencing projects where clinical and family history data are not available. Finally, our work suggests avenues to develop disease-modifying precision medicines for myocilin-associated glaucoma.

Common and rare myocilin variants: Predicting glaucoma pathogenicity based on genetics, clinical, and laboratory misfolding data

“…Pathogenic variants are destabilized (Burns et al, 2010(Burns et al, , 2011Donegan et al, 2015), a feature that explains their misfolding propensity and downstream GOF pathogenic mechanism. However, as underscored by the numerous surprises we encountered in the lab (Lieberman & Ma, 2021), the effect of a mutation on OLF structure and stability cannot be predicted with a high degree of confidence, despite applying reasonable physicochemical intuition. Sequence conservation mapped onto the structure demonstrates that across the OLF sequence, the surface residues are least conserved, and internal sequences are largely conserved (Donegan et al, 2015).…”

Amyloid fibrillation of the glaucoma associated myocilin protein is inhibited by epicatechin gallate (ECG)

“…36 Myocilin associated POAG can thus be considered an inclusion to the expanding catalogue of ailments arising due to protein misfolding. 26,35,37 Regulation of IOP levels has remained the primary therapeutic strategy for managing glaucoma patients. Eye drop formulation of drugs that help in reducing the production of AH and/or enhancing its removal are customarily employed to regulate IOP levels.…”

“…Preservation of the native protein state can mitigate the progression of misfolding and associated amyloidogenesis. 37,[46][47][48][49][50][51][52] Despite the clinical relevance of myocilin, only limited studies have been performed to inspect the aggregation nature of this protein. Production of recombinant myocilin in amounts enough for carrying out in vitro aggregation assays is a likely hindrance.…”

“…36 Myocilin associated POAG can thus be considered an inclusion to the expanding catalogue of ailments arising due to protein misfolding. 26,35,37…”

“…Preservation of the native protein state can mitigate the progression of misfolding and associated amyloidogenesis. 37,46–52…”