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Mitochondrial Defects and Dysfunction in Calcium Regulation in Glaucomatous Trabecular Meshwork Cells

¹From the State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China; the ²Department of Neural and Behavioral Sciences, Pennsylvania State University College of Medicine, Hershey, Pennsylvania; and the ⁴Department of Ophthalmology and Visual Science, Yale School of Medicine, New Haven, Connecticut.

PURPOSE. Disruption in intracellular calcium ion (Ca²⁺) homeostasis has major effects on health. Persistent Ca²⁺ overload induces mitochondrial permeability transition pore (MPTP) opening, which prompts mitochondrial release of calcium (mCICR) and reactive oxygen species (ROS) into the cytosol which, in turn, compromises mitochondrial function. This study was conducted to examine intracellular Ca²⁺ levels and mitochondrial vulnerability to Ca²⁺ stress in trabecular meshwork (TM) of individuals with primary open-angle glaucoma (POAG).

METHODS. Primary cultures of TM cells from POAG (GTM) and age-matched, nondiseased (NTM) eyes, obtained from postmortem donors eyes by standard surgical trabeculectomy, were treated with the following calcium regulators: the mitochondrial respiratory chain I inhibitor rotenone (ROT); the mitochondrial permeability transition pore (MPTP) inhibitors cyclosporine (Cys) and aristolochic acid (ArA); the Ca²⁺ chelators BAPTA/AM or EDTA; the mitochondrial Ca²⁺ uniporter inhibitor ruthenium red (RR); the Ca²⁺/Na⁺ exchanger inhibitor trifluoperazine; and the inositol 1,4,5-triphosphate receptor type 3 (IP3R) inhibitors 2-aminoethoxydiphenyl borane (2-APB) and xestospongin C (Xe-C). Ca²⁺ concentrations in the cytoplasm ([Ca²⁺]_c) and mitochondria ([Ca²⁺]_m) were determined by confocal microscopy and flow cytometry with the fluorescent Ca²⁺ indicators fluo-3/AM and rhod-2/AM, respectively. Mitochondrial membrane potential (m) was examined with the fluorescent probe tetramethylrhodamine ethyl ester (TMRE). The expression of cyclophilin D, a protein that induces MPTP opening was also measured.

RESULTS. There was increased [Ca²⁺]_c, [Ca²⁺]_m, mCICR, MPTP opening, and expression of cyclophilin D and decreased m in POAG TM cells compared with control cells. ROT artificially exacerbated these conditions in GTM cells. Chelation of [Ca²⁺]_c and inhibition of IP3R and MPTP opening suppressed mitochondrial dysfunction and reduced the additional effects of ROT in GTM cells.

CONCLUSIONS. POAG TM cells have defective mitochondrial function, which causes them to be abnormally vulnerable to Ca²⁺ stress. The dysfunction in calcium regulation by these cells may contribute to the failure of this tissue to control IOP. Pharmacologic inhibitors of IP3R, MPTP opening, and cyclophilin D could have clinical implications for primary open-angle glaucoma.