Type 2 diabetes mellitus (T2DM) is a worldwide scourge, with over 6 million new cases reported each year. T2DM is caused by unbalanced insulin secretion and lack of responsiveness of peripheral cells to insulin. Mitochondria have been thought to contribute to insulin resistance, but their role has been unclear. A mechanism suggested by some studies is that impaired oxidative phosphorylation (OxPhos) is the mechanism leading to T2DM and obesity.
Genetic studies have shown that mitochondrial genes (PGC-1α and NRF-1) related to OxPhos are downregulated in insulin resistant subjects. NMR studies have tracked a correlation between OxPhos and insulin resistance in skeletal muscle.
EPR measurements performed on a Bruker X-band EPR spectrometer showed that there were no significant changes in internally or externally generated ROS in preparations from the engineered mice. That suggests that deletion of AIF does not cause accumulation of ROS or induce inflammation-important for the next stage of their experiments.
Initial metabolic analysis of the mice who had OxPhos deficient skeletal muscle at 8 weeks revealed a surprise. The mice had increased oral glucose tolerance and an increased insulin sensitivity compared to normal mice. These mice were also resistant to diet-induced obesity and diabetes when fed a high fat diet. Mice with a mosaic (mixed) muscle-specific knockout pattern also showed the same qualities of increased glucose tolerance and resistance to obesity and diabetes.
Experiments repeated with liver-specific AIF knockout mice showed the same result--increased glucose tolerance and insulin sensitivity. A multisystem AIF knockout mouse, the Harlequin mouse, was also found to have the same qualities of improved insulin sensitivity, glucose tolerance, and resistance to diet-induced obesity and diabetes.
The AIF knockout can be reversed, as well. Using an adenoviral transgene delivery system, the researchers restored the missing AIF gene. The experiment returned the AIF knockout mice to normal glucose tolerance and plasma peak insulin levels. This proved that changing AIF expression can induce and reverse mitochondrial dysfunction, and that the changes correlated with glucose tolerance.
Joza et al. (2005) have shown that deletion of mitochondrial flavoprotein apoptosis inducing factor (AIF) in mice leads to progressive OxPhos dysfunction. Maintenance of the mitochondrial respiratory chain is thought to be the main function of AIF. Pospisilik et al. (2007) investigate whether defects in mitochondrial respiration cause T2DM and obesity using AIF to generate models of OxPhos deficiency in mice.
They engineered AIF knockout mice with muscle- or liver-specific OxPhos deficiency and confirmed that the animals were a valid model for OxPhos dependency of insulin resistance. As part of that validation, they determined that OxPhos was dissociable from reactive oxygen species (ROS), which are produced by electron flux through OxPhos and are known to cause insulin resistance.
The conclusion of these studies was that reduced OxPhos led to reduced adiposity and increased insulin sensitivity--a contradiction of the theory that reduced OxPhos was the cause of T2DM. The difference from previous studies may be due to absence of accumulation of ROS in this particular model of OxPhos deficiency.
The mechanism proposed by the authors to explain their findings was that reduction of OxPhos produced a compensatory increase in anaerobic glucose metabolism, leading to a net increase in fuel utilization to meet energy needs. Basically, they made the mice very inefficient consumers of fuel, which prevented them from gaining weight on a high fat diet or developing insulin resistance. Because the AIF knockout shifted the metabolite balance--decreased ATP, decreased NAD, and increased AMP--activation of AMPKinase might have stimulated catabolic processes including glucose uptake and fatty acid oxidation. As well, lower NAD levels might have reduced activation of a pathway promoting glycolysis.
This study shows that a primary OxPhos defect does not cause T2DM in mice. It actually made the mice resistant to the disease--a finding with potential implications for development of therapies for diabetes.