In vivo magnetic resonance studies of muscle mitochondrial function in transgenic mice

Alterations in muscle mitochondrial function have been implicated in the pathogenesis of numerous metabolic disorders, including insulin resistance, type 2 diabetes, obesity, and the deleterious effects of aging. However, the precise role for mitochondrial function in these processes remains to be established. In vivo 31P Magnetic Resonance Spectroscopy (31P-MRS) is an effective technique that permits the non-invasive investigation of skeletal muscle metabolism and transgenic mice are a novel tool for examining the effects of specific genes on mitochondrial function. Combining these two approaches would be a powerful methodology for studying mitochondrial function but its implementation has been limited due to the small volume of muscle from which the MR signal can be obtained and the requirement that the region of interest (ROI) must remain still for the entire duration of these lengthy studies. A new MR-compatible experimental set-up was developed to perform these experiments under low-dose anesthesia to minimize movement with constant physiological monitoring to ensure that the animal remained viable throughout the study. The unidirectional flux of ATP synthesis (Pi → ATP) was measured using 31P saturation-transfer MRS in two different transgenic mouse models overexpressing PGC-1α or UCP3. While PGC-1α has been shown to be a potent promoter of mitochondrial biogenesis and fiber-type remodelling, UCP3 seems to play a critical role in regulating mitochondrial activity, but, whether this might be its primary role is still a matter of debate. The rates of ATP production (VATP) were 19% lower in UCP3 +/+ mice with respect to their wild-type (WT) littermates (P=0.02) accompanied by a significant increase in energy expenditure and food intake, confirming a contributing role played by this protein in regulating mitochondrial energy production. In PGC-1α +/+ mice, VATP was increased in mice fed a regular chow (by 50%, P<0.01) or a high fat diet (by 58%, P<0.001) with respect to wild-type littermate mice, with no significant difference in energy expenditure, food intake or locomotor activity. In the mouse model overexpressing PGC-1α, higher rates of VATP suggest enhanced efficiency in ATP production. This thesis has demonstrated that 31P-MRS can successfully be applied to the investigation of muscle mitochondrial function in transgenic mice in vivo.