Emerging findings suggest that compromised cellular bioenergetics and DNA repair contribute to the pathogenesis of Alzheimer's disease (AD), but their role in disease-defining pathology is unclear. We developed a DNA repair-deficient 3xTgAD/Polβ mouse that exacerbates major features of human AD including phosphorylated Tau (pTau) pathologies, synaptic dysfunction, neuronal death, and cognitive impairment. Here we report that 3xTgAD/Polβ mice have a reduced cerebral NAD/NADH ratio indicating impaired cerebral energy metabolism, which is normalized by nicotinamide riboside (NR) treatment. NR lessened pTau pathology in both 3xTgAD and 3xTgAD/Polβ mice but had no impact on amyloid β peptide (Aβ) accumulation. NR-treated 3xTgAD/Polβ mice exhibited reduced DNA damage, neuroinflammation, and apoptosis of hippocampal neurons and increased activity of SIRT3 in the brain. NR improved cognitive function in multiple behavioral tests and restored hippocampal synaptic plasticity in 3xTgAD mice and 3xTgAD/Polβ mice. In general, the deficits between genotypes and the benefits of NR were greater in 3xTgAD/Polβ mice than in 3xTgAD mice. Our findings suggest a pivotal role for cellular NAD depletion upstream of neuroinflammation, pTau, DNA damage, synaptic dysfunction, and neuronal degeneration in AD. Interventions that bolster neuronal NAD levels therefore have therapeutic potential for AD.

We explore the role of DNA damage processing in the progression of cognitive decline by creating a new mouse model. The new model is a cross of a common Alzheimer's disease (AD) mouse (3xTgAD), with a mouse that is heterozygous for the critical DNA base excision repair enzyme, DNA polymerase β. A reduction of this enzyme causes neurodegeneration and aggravates the AD features of the 3xTgAD mouse, inducing neuronal dysfunction, cell death and impairing memory and synaptic plasticity. Transcriptional profiling revealed remarkable similarities in gene expression alterations in brain tissue of human AD patients and 3xTg/Polβ+/− mice including abnormalities suggestive of impaired cellular bioenergetics. Our findings demonstrate that a modest decrement in base excision repair capacity can render the brain more vulnerable to AD-related molecular and cellular alterations.

We have detected DNA polymerase beta (Pol␤), known as a key nuclear base excision repair (BER) protein, in mitochondrial protein extracts derived from mammalian tissue and cells. Manipulation of the N-terminal sequence affected the amount of Pol␤ in the mitochondria. Using Pol␤ fragments, mitochondrion-specific protein partners were identified, with the interactors functioning mainly in DNA maintenance and mitochondrial import. Of particular interest was the identification of the proteins TWINKLE, SSBP1, and TFAM, all of which are mitochondrion-specific DNA effectors and are known to function in the nucleoid. Pol␤ directly interacted functionally with the mitochondrial helicase TWINKLE. Human kidney cells with Pol␤ knockout (KO) had higher endogenous mitochondrial DNA (mtDNA) damage. Mitochondrial extracts derived from heterozygous Pol␤ mouse tissue and KO cells had lower nucleotide incorporation activity. Mouse-derived Pol␤ null fibroblasts had severely affected metabolic parameters. Indeed, gene knockout of Pol␤ caused mitochondrial dysfunction, including reduced membrane potential and mitochondrial content. We show that Pol␤ is a mitochondrial polymerase involved in mtDNA maintenance and is required for mitochondrial homeostasis.KEYWORDS DNA polymerase beta, mitochondrial DNA repair, TFAM, base excision repair, mitochondria, mitochondrial health, mutational studies C ellular DNA repair is critical for genomic stability, and the accumulation of DNA damage has been linked to many debilitating human disorders, including accelerated aging, cancer, and neurodegeneration (reviewed in references 1 and 2). Mammalian cells have two genomes, nuclear and mitochondrial, and both have the ability to replicate, accumulate DNA damage, and propagate mutations. The nucleus contains the vast majority of the mammalian genome and has extensive ability to repair complex bulky adducts, double-strand breaks (DSB), single-strand breaks (SSB), and hundreds of chemical DNA modifications. The ability to effectively repair this breadth of damage is achieved through multiple, often overlapping, DNA repair pathways. In contrast, the repair of mitochondrial DNA (mtDNA) is a more limited version of nuclear DNA (nDNA) repair. Mitochondria lack nucleotide excision repair, and the presence of double-strand break repair is debated (recently reviewed in reference 3). Despite the mitochondria having attenuated DNA repair capabilities compared to the nucleus, the accumulation of mtDNA damage is not without consequence. Ineffective mtDNA maintenance is the underlying cause of many human diseases, including Alpers syndrome and chronic progressive external ophthalmoplegia (CPEO) caused by mutations in mitochondrial polymerase gamma (Pol␥) or the TWINKLE helicase (4-6). The accu-

The common oxidatively generated lesion, 8-oxo-7,8-dihydroguanine (8-oxoGua), is removed from DNA by base excision repair. The glycosylase primarily charged with recognition and removal of this lesion is 8-oxoGuaDNA glycosylase 1 (OGG1). When left unrepaired, 8-oxodG alters transcription and is mutagenic. Individuals homozygous for the less active OGG1 allele, Ser326Cys, have increased risk of several cancers. Here, small molecule enhancers of OGG1 were identified and tested for their ability to stimulate DNA repair and protect cells from the environmental hazard paraquat (PQ). PQ-induced mtDNA damage was inversely proportional to the levels of OGG1 expression whereas stimulation of OGG1, in some cases, entirely abolished its cellular effects. The PQ-mediated decline of mitochondrial membrane potential or nuclear condensation were prevented by the OGG1 activators. In addition, in Ogg1 mouse embryonic fibroblasts complemented with hOGG1, there was increased cellular and mitochondrial reactive oxygen species compared to their wild type counterparts. Mitochondrial extracts from cells expressing hOGG1 were deficient in mitochondrial 8-oxodG incision activity, which was rescued by the OGG1 activators. These data demonstrate that small molecules can stimulate OGG1 activity with consequent cellular protection. Thus, OGG1-activating compounds may be useful in select humans to mitigate the deleterious effects of environmental oxidants and mutagens.