Aging is a natural phenomenon characterized by progressive decline in tissue and organ function leading to increased risk of disease and mortality. Among diverse factors that contribute to human aging, the mitochondrial dysfunction has emerged as one of the key hallmarks of aging process and is linked to the development of numerous age-related pathologies including metabolic syndrome, neurodegenerative disorders, cardiovascular diseases and cancer. Mitochondria are central in the regulation of energy and metabolic homeostasis, and harbor a complex quality control system that limits mitochondrial damage to ensure mitochondrial integrity and function. The intricate regulatory network that balances the generation of new and removal of damaged mitochondria forms the basis of aging and longevity. Here, I will review our current understanding on how mitochondrial functional decline contributes to aging, including the role of somatic mitochondrial DNA (mtDNA) mutations, reactive oxygen species (ROS), mitochondrial dynamics and quality control pathways. I will further discuss the emerging evidence on how dysregulated mitochondrial dynamics, mitochondrial biogenesis and turnover mechanisms contribute to the pathogenesis of age-related disorders. Strategies aimed to enhance mitochondrial function by targeting mitochondrial dynamics, quality control, and mitohormesis pathways might promote healthy aging, protect against age-related diseases, and mediate longevity.

Mitochondrial DNA (mtDNA) deletions are a common cause of mitochondrial disorders and have been found to accumulate during normal aging. Despite the fact that hundreds of deletions have been characterized at the molecular level, their mechanisms of genesis are unknown. We tested the effect of double-strand breaks of muscle mtDNA by developing a mouse model in which a mitochondrially targeted restriction endonuclease (PstI) was expressed in skeletal muscle of mice. Because mouse mtDNA harbors two PstI sites, transgenic founders developed a mitochondrial myopathy associated with mtDNA depletion. The founders showed a chimeric pattern of transgene expression and their residual level of wild-type mtDNA in muscle was approximately 40% of controls. We were able to identify the formation of large mtDNA deletions in muscle of transgenic mice. A family of mtDNA deletions was identified, and most of these rearrangements involved one of the PstI sites and the 3' end of the D-loop region. The deletions had no or small direct repeats at the breakpoint region. These features are essentially identical to the ones observed in humans with multiple mtDNA deletions in muscle, suggesting that double-strand DNA breaks mediate the formation of large mtDNA deletions.

Mutations in the mitochondrial DNA (mtDNA) can cause a variety of human diseases. In most cases, such mutations are heteroplasmic (i.e. mutated and wild-type mtDNA coexist) and a small percentage of wild-type sequences can have a strong protective effect against a metabolic defect. Because a genetic approach to correct mtDNA mutations is not currently available, the ability to modulate heteroplasmy would have a major impact in the phenotype of many patients with mitochondrial disorders. We show here that a restriction endonuclease targeted to mitochondria has this ability. A mitochondrially targeted PstI degraded mtDNA harboring PstI sites, in some cases leading to a complete loss of mitochondrial genomes. Recombination between DNA ends released by PstI was not observed. When expressed in a heteroplasmic rodent cell line, containing one mtDNA haplotype with two sites for PstI and another haplotype having none, the mitochondrial PstI caused a significant shift in heteroplasmy, with an accumulation of the mtDNA haplotype lacking PstI sites. These experiments provide proof of the principle that restriction endonucleases are feasible tools for genetic therapy of a sub-group of mitochondrial disorders. Although this approach is limited by the presence of mutation-specific restriction sites, patients with neuropathy, ataxia and retinitis pigmentosa (NARP) could benefit from it, as the T8399G mutation creates a unique restriction site that is not present in wild-type human mitochondrial DNA.

Nicotinamide adenine dinucleotide (NAD + ) is a central metabolic cofactor in eukaryotic cells that plays a critical role in regulating cellular metabolism and energy homeostasis. NAD + in its reduced form (i.e. NADH) serves as the primary electron donor in mitochondrial respiratory chain, which involves adenosine triphosphate production by oxidative phosphorylation. The NAD + /NADH ratio also regulates the activity of various metabolic pathway enzymes such as those involved in glycolysis, Kreb’s cycle, and fatty acid oxidation. Intracellular NAD + is synthesized de novo from l -tryptophan, although its main source of synthesis is through salvage pathways from dietary niacin as precursors. NAD + is utilized by various proteins including sirtuins, poly ADP-ribose polymerases (PARPs) and cyclic ADP-ribose synthases. The NAD + pool is thus set by a critical balance between NAD + biosynthetic and NAD + consuming pathways. Raising cellular NAD + content by inducing its biosynthesis or inhibiting the activity of PARP and cADP-ribose synthases via genetic or pharmacological means lead to sirtuins activation. Sirtuins modulate distinct metabolic, energetic and stress response pathways, and through their activation, NAD + directly links the cellular redox state with signaling and transcriptional events. NAD + levels decline with mitochondrial dysfunction and reduced NAD + /NADH ratio is implicated in mitochondrial disorders, various age-related pathologies as well as during aging. Here, I will provide an overview of the current knowledge on NAD + metabolism including its biosynthesis, utilization, compartmentalization and role in the regulation of metabolic homoeostasis. I will further discuss how augmenting intracellular NAD + content increases oxidative metabolism to prevent bioenergetic and functional decline in multiple models of mitochondrial diseases and age-related disorders, and how this knowledge could be translated to the clinic for human relevance.