Mitoflash altered by metabolic stress in insulin-resistant skeletal muscle

ABSTRACT: Central to bioenergetics and reactive oxygen species (ROS) signaling, the mitochondrion plays pivotal roles in the pathogenesis of metabolic diseases. Recent advances have shown that mitochondrial flash (“mitoflash”) visualized by the biosensor mt-cpYFP affords a frequency-coded, optical readout linked to mitochondrial ROS production and energy metabolism, at the resolution of a single mitochondrion. To investigate possible mitoflash responses to metabolic stress in insulin resistance (IR), we generated an mt-cpYFP-expressing db/db mouse model with the obesity and IR phenotypes unaltered. In conjunction with in vivo imaging of skeletal muscles, we uncovered a progressive increase of mitoflash frequency along with its morphological changes. Interestingly, enhanced mitochondrial networking occurred at 12 weeks of age, and this was followed by mitochondrial fragmentation at 34 weeks. Pioglitazone treatment normalized mitoflash frequency and morphology while restored mitochondrial respiratory function and insulin sensitivity in 12 weeks mt-cpYFP db/db mice. Mechanistic study revealed that the mitoflash remodeling was associated with altered expression of proteins involved in mitochondrial dynamics and quality control. These findings indicate that mitoflash activity may serve as an optical functional readout of the mitochondria, a robust and sensitive biomarker to gauge IR stresses and their amelioration by therapeutic interventions. KEY MESSAGE: In vivo detection of mitochondrial flashes in mt-cpYFP-expressing db/db mouse. Mitoflash frequency increased progressively with disease development. Mitoflash morphology revealed a biphasic change in mitochondrial networking. Mitoflash abnormalities and mitochondrial defects are restored by pioglitazone. Mitoflash may serve as a unique biomarker to gauge metabolic stress in insulin resistance. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00109-015-1278-y) contains supplementary material, which is available to authorized users.