Pro370Leu mutant myocilin impairs mitochondrial functions in human trabecular meshwork cells

Purpose: Oxidative stress is a risk factor for the onset and progression of primary open-angle glaucoma (POAG), but the exact molecular basis remains unknown. Here, we investigated the mechanisms for Pro370Leu mutant myocilin to induce mitochondrial dysfunction and subsequent reactive oxygen species (ROS) generation in trabecular meshwork (TM) cells obtained from POAG individuals. Methods: Primary non-diseased human TM cultures were transfected with pIRES-EGFP (Mock), pIRES-wild-type (WT), or pIRES-Pro370Leu mutant myocilin. Transfection efficiency and myocilin subcellular localization were determined by polymerase chain reaction (PCR), western blot analysis, and confocal microscopy. ROS levels as well as free Ca²⁺ concentrations in cytoplasm ([Ca²⁺]_c) and mitochondria ([Ca²⁺]m) were examined by 2′7′-dichlorofluorescein diacetate (H₂-DCF-DA), Fluo-3 acetoxymethyl ester (Fluo-3/AM), and Rhod-2 acetoxymethyl ester (rhod-2/AM), respectively, using flow cytometry. Mitochondrial functions were revealed by changes in mitochondrial membrane potential (ΔΨm) and ATP production, which were found by fluorescent probe 5,5′,6,6′-tetrachloro- 1,1′3,3′-tetraethylbenzimid azolocarbocyanine iodide (JC-1) and a luciferin/luciferase-based ATP assay, respectively. Results: Both WT and Pro370Leu mutant myocilin are localized in the mitochondria of TM cells as indicated using confocal microscopy and western blot analysis. Overexpression of WT myocilin decreases ΔΨm, which is further reduced by Pro370Leu mutant myocilin. TM cells that overexpressed Pro370Leu mutant myocilin have greater cell death, higher endogenous ROS, [Ca²⁺]_c, and [Ca²⁺]_m levels, and lower ATP production, and yet, these effects are not seen in the overexpression of WT myocilin. Conclusions: Our findings suggested that Pro370Leu mutant myocilin causes mitochondrial defects, which may lead to TM cell dysfunction and even cell death. Therefore, preventive measures targeting mitochondrial protection may delay the onset of glaucoma in individuals carrying the Pro370Leu myocilin mutation.