High-throughput assessment of mitochondrial membrane potential in situ using fluorescence resonance energy transfer

Mitochondrial dysfunction causes dozens of debilitating diseases, and is implicated in the etiology of type 2 diabetes, Parkinson's and Alzheimer's diseases, among others. However, development of mitochondrially targeted therapeutic agents has been impeded by the lack of high-throughput screening techniques that are capable of distinguishing in intact cells the mitochondrial membrane potential (Δψ_p) from the plasma membrane potential, (Δψ_p). We report here a fluorescence resonance energy transfer (FRET) assay that specifically monitors Δψ_p that is not confounded by background signal arising from potentiometric dye responding to Δψ_p. The technique relies on energy transfer between nonyl acridine orange (NAO), which stains diphosphatidyl glycerol (cardiolipin) that is indigenous to the inner mitochondrial membrane, and tetramethylrhodamine methyl ester (TMR), a potentiometric dye that is sequestered by mitochondria as a Nernstian function of Δψ_m and concentration. FRET occurs only when both dyes co-localize to the mitochondria, and results in quenching of NAO emission by TMR in proportion to Δψ_m. Validation studies using compounds with well-characterized mitochondrial effects, including oligomycin, CCCP⁺, bongkrekie acid, cyclosporin A, nigericin, ADP, and ruthenium red, demonstrate that the FRET-based Δψ_m assay responds in accord with the known pharmacology. Validation studies assessing the suitability of the technique for high-throughput compound screening indicate that the assay provides a sensitive and robust assessment not only of mitochondrial integrity in situ, but also, when used in conjunction with agents such as cyclosporin A, an indicator of permeability transition.