Every cell contains a herd of hundreds of mitochondria, organelles descended from ancient symbiotic bacteria. The primary purpose of mitochondria is to package the chemical energy store molecule adenosine triphosphate (ATP) that is needed to power cellular processes. Each mitochondrion contains one more copies of a small circular genome, the mitochondrial DNA. This mitochondrial DNA is unfortunately poorly protected and repaired in comparison to nuclear DNA. Accumulation of damage in the form of mutations is thought to be an important contributing cause of mitochondrial dysfunction in aging, leading to less ATP and thus disruption of cell and tissue function.
The data of recent years indicates that not all mutations in mitochondrial DNA are equal when it comes to causing problems. Point mutations seem to be quite well tolerated, as illustrated by the heterozygous PolG mutator mice. These mice exhibit very high levels of point mutations in mitochondrial DNA due to a loss of function mutation in one of the two copies of PolG, an enzyme involved in mitochondrial DNA replication and repair. Deletion mutations, on the other hand, are the path to sizable and detrimental changes, as they can remove or disable electron transport chain proteins. This can result in mitochondria that outcompete their undamaged peers in replication efficiency or resistance to the quality control mechanisms of mitophagy, take over a cell, render it dysfunctional, and export harmful reactive molecules into surrounding tissue.
Mitochondria are essential for respiration and the regulation of diverse cellular processes; thus, mitochondrial dysfunction is believed to underlie a variety of metabolic and aging-related diseases. Mutations in the mitochondrial genome are thought to drive mitochondrial dysfunction and have been implicated in aging-related diseases; however, whether mtDNA mutations are causal or consequent of metabolic dysfunction remains unclear. The polymerase gamma (PolG) "mutator" mouse is a model of intrinsic mitochondrial dysfunction and was employed for this study to determine whether mtDNA mutations are sufficient to drive metabolic abnormalities and aging-associated insulin resistance and adiposity.
Mice harboring a homozygous PolG loss of proofreading 3′-5′ exonuclease function mutation (PolGmut/mut) develop mtDNA point mutations at a rate that far exceeds mutations observed in aged wild-type (WT) animals and humans. The mtDNA point mutations that accumulate in young PolGmut/mut mice (~136-fold increase versus WT mice) manifest a variety of preadolescent phenotypic abnormalities including progeroid-like symptoms throughout maturation as well as premature death (~12-16 months of age). Because of the complexity of the early-onset aging, we studied the PolG heterozygous (PolG+/mut) mouse, which lacks progeroid-like symptoms despite a supraphysiological mtDNA point mutation frequency (~30-fold greater mutation load in PolG+/mut versus WT mice). Furthermore, male and female PolG+/mut mice show no significant difference in lifespan versus WT animals (tested up to 800 days of age).
Based on previous reports, we hypothesized that an increased mtDNA point mutation frequency in PolG+/mut mice would promote mitochondrial dysfunction and accelerate the development of insulin resistance during aging. We examined specific aspects of metabolism in male PolG+/mut mice at 6 and 12 months of age under three dietary conditions: normal chow (NC) feeding, high-fat feeding (HFD), and 24-hr starvation. We performed mitochondrial proteomics and assessed dynamics and quality control signaling in muscle and liver to determine whether mitochondria respond to mtDNA point mutations by altering morphology and turnover. In the current study, we observed that the accumulation of mtDNA point mutations failed to disrupt metabolic homeostasis and insulin action in male mice, but with aging, metabolic health was likely preserved by countermeasures against oxidative stress and compensation by the mitochondrial proteome.