Monitoring mutant myocilin secretion and localization in trabecular meshwork cell cultures using a protein complementation-based luminescence assay

Abstract

Approximately 2–4 % of adult onset and 10 % of juvenile onset cases of primary open angle glaucoma can be attributed to non-synonymous coding mutations in MYOC. One of the key characteristics of a pathogenic MYOC mutant is the inability of the resulting protein to be secreted from trabecular meshwork cells. Instead, pathogenic myocilin variants accumulate in the endoplasmic reticulum. Typically, localization of MYOC mutants is compared to wild-type myocilin in cellular secretion assays that use immunoblot to detect myocilin in extracellular media, alongside intracellular soluble and insoluble (aggregated) fractions. Here, we implement a new method that utilizes a complement-based luminescence method in which an 11-residue HiBiT tag is appended to myocilin and complements a truncated nanoluciferase. The method allows for highly sensitive luminescence detection and does not require immunoblot. We tested non-synonymous coding variants T377R, D384G, D395ins, C433Y, T455K, and L486F, in an established immortalized trabecular meshwork cell line. Secretion was tested in 96-well plate format, revealing poor secretion for these mutants compared to wild-type myocilin. For assays conducted in 6-well plates, myocilin mutants were accumulated in intracellular fractions. HiBiT luminescence signals correlated well with immunofluorescence as well as immunoblot but is more sensitive than the latter. Overall, our study demonstrates that complement-based detection of mutant myocilin using luminescence allows for facile and sensitive detection of myocilin localization and has confirmed secretion defects for six variants.