Enhanced skeletal muscle expression of extracellular superoxide dismutase mitigates streptozotocin-induced diabetic cardiomyopathy by reducing oxidative stress and aberrant cell signaling.

BACKGROUND: Exercise training enhances extracellular superoxide dismutase (EcSOD) expression in skeletal muscle and elicits positive health outcomes in individuals with diabetes mellitus. The goal of this study was to determine if enhanced skeletal muscle expression of EcSOD is sufficient to mitigate streptozotocin-induced diabetic cardiomyopathy. METHODS AND RESULTS: Exercise training promotes EcSOD expression in skeletal muscle and provides protection against diabetic cardiomyopathy; however, it is not known if enhanced expression of EcSOD in skeletal muscle plays a functional role in this protection. Here, we show that skeletal muscle-specific EcSOD transgenic mice are protected from cardiac hypertrophy, fibrosis, and dysfunction under the condition of type 1 diabetes mellitus induced by streptozotocin injection. We also show that both exercise training and muscle-specific transgenic expression of EcSOD result in elevated EcSOD protein in the blood and heart without increased transcription in the heart, suggesting that enhanced expression of EcSOD from skeletal muscle redistributes to the heart. Importantly, cardiac tissue in transgenic mice displayed significantly reduced oxidative stress, aberrant cell signaling, and inflammatory cytokine expression compared with wild-type mice under the same diabetic condition. CONCLUSIONS: Enhanced expression of EcSOD in skeletal muscle is sufficient to mitigate streptozotocin-induced diabetic cardiomyopathy through attenuation of oxidative stress, aberrant cell signaling, and inflammation, suggesting a cross-organ mechanism by which exercise training improves cardiac function in diabetes mellitus.

A novel MitoTimer reporter gene for mitochondrial content, structure, stress, and damage in vivo.

Mitochondrial dysfunction plays important roles in many diseases, but there is no satisfactory method to assess mitochondrial health in vivo. Here, we engineered a MitoTimer reporter gene from the existing Timer reporter gene. MitoTimer encodes a mitochondria-targeted green fluorescent protein when newly synthesized, which shifts irreversibly to red fluorescence when oxidized. Confocal microscopy confirmed targeting of the MitoTimer protein to mitochondria in cultured cells, Caenorhabditis elegans touch receptor neurons, Drosophila melanogaster heart and indirect flight muscle, and mouse skeletal muscle. A ratiometric algorithm revealed that conditions that cause mitochondrial stress led to a significant shift toward red fluorescence as well as accumulation of pure red fluorescent puncta of damaged mitochondria targeted for mitophagy. Long term voluntary exercise resulted in a significant fluorescence shift toward green, in mice and D. melanogaster, as well as significantly improved structure and increased content in mouse FDB muscle. In contrast, high-fat feeding in mice resulted in a significant shift toward red fluorescence and accumulation of pure red puncta in skeletal muscle, which were completely ameliorated by voluntary wheel running. Hence, MitoTimer allows for robust analysis of multiple parameters of mitochondrial health under both physiological and pathological conditions and will be highly useful for future research of mitochondrial health in multiple disciplines in vivo.