DNA polymerase ß decrement triggers death of olfactory bulb cells and impairs olfaction in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) involves the progressive degeneration of neurons critical for learning and memory. In addition, patients with AD typically exhibit impaired olfaction associated with neuronal degeneration in the olfactory bulb (OB). Because DNA base excision repair (BER) is reduced in brain cells during normal aging and AD, we determined whether inefficient BER due to reduced DNA polymerase-ß (Polß) levels renders OB neurons vulnerable to degeneration in the 3xTgAD mouse model of AD. We interrogated OB histopathology and olfactory function in wild-type and 3xTgAD mice with normal or reduced Polß levels. Compared to wild-type control mice, Polß heterozygous (Polß+/- ), and 3xTgAD mice, 3xTgAD/Polß+/- mice exhibited impaired performance in a buried food test of olfaction. Polß deficiency did not affect the proliferation of OB neural progenitor cells in the subventricular zone. However, numbers of newly generated neurons were reduced by approximately 25% in Polß+/- and 3xTgAD mice, and by over 60% in the 3xTgAD/Polß+/- mice compared to wild-type control mice. Analyses of DNA damage and apoptosis revealed significantly greater degeneration of OB neurons in 3xTgAD/Polß+/- mice compared to 3xTgAD mice. Levels of amyloid ß-peptide (Aß) accumulation in the OB were similar in 3xTgAD and 3xTgAD/Polß+/- mice, and cultured Polß-deficient neurons exhibited increased vulnerability to Aß-induced death. Olfactory deficit is an early sign in human AD, but the mechanism is not yet understood. Our findings in a new AD mouse model demonstrate that diminution of BER can endanger OB neurons, and suggest a mechanism underlying early olfactory impairment in AD.

Apo E4 Alleles and Impaired Olfaction as Predictors of Alzheimer's Disease.

Alzheimer's disease (AD) is the most common form of dementia that affects more than 5 million Americans. It is the only disease among the 10 causes of death that cannot be slowed or cured, thus raising the need for identification of early preclinical markers that could be the focus of preventative efforts. Although evidence is escalating that abnormalities in olfactory structure and function precede AD development and early cognitive impairments by one or more decades, the importance of olfaction is largely overlooked in AD, and such testing is not routinely performed in neurology clinics. Nevertheless, research using the olfactory model, has begun to advance our understanding of the preclinical pathophysiology of AD. Notably, an interesting series of studies is beginning to illuminate the relationship between Apolipoprotein E (ApoE) ε4 polymorphism and olfactory dysfunction and late-onset Alzheimer's disease. In this article, we reviewed present research on the significance of ApoE and olfaction to AD, summarized current studies on the associations and mechanisms of ApoE and olfactory dysfunction, and highlighted important gaps for future work to further advance the translational application of the olfactory paradigm to early, preclinical diagnosis and treatment of AD.

Mitochondrial SIRT3 Mediates Adaptive Responses of Neurons to Exercise and Metabolic and Excitatory Challenges.

The impact of mitochondrial protein acetylation status on neuronal function and vulnerability to neurological disorders is unknown. Here we show that the mitochondrial protein deacetylase SIRT3 mediates adaptive responses of neurons to bioenergetic, oxidative, and excitatory stress. Cortical neurons lacking SIRT3 exhibit heightened sensitivity to glutamate-induced calcium overload and excitotoxicity and oxidative and mitochondrial stress; AAV-mediated Sirt3 gene delivery restores neuronal stress resistance. In models relevant to Huntington's disease and epilepsy, Sirt3(-/-) mice exhibit increased vulnerability of striatal and hippocampal neurons, respectively. SIRT3 deficiency results in hyperacetylation of several mitochondrial proteins, including superoxide dismutase 2 and cyclophilin D. Running wheel exercise increases the expression of Sirt3 in hippocampal neurons, which is mediated by excitatory glutamatergic neurotransmission and is essential for mitochondrial protein acetylation homeostasis and the neuroprotective effects of running. Our findings suggest that SIRT3 plays pivotal roles in adaptive responses of neurons to physiological challenges and resistance to degeneration.

DNA polymerase ß deficiency leads to neurodegeneration and exacerbates Alzheimer disease phenotypes.

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 ß. 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/Polß(+/-) 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.

Loss of NEIL1 causes defects in olfactory function in mice.

Oxidative DNA damage accumulation has been implicated in neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. The base excision repair pathway is a primary responder to oxidative DNA damage. Effects of loss of base excision repair on normal brain function is a relatively nascent area of research that needs further exploration for better understanding of related brain diseases. Recently, we found that loss of a versatile DNA glycosylase endonuclease 8-like 1 (NEIL1) causes deficits in spatial memory retention using the Morris water maze test. Furthermore, we found that there is a significant loss of NEIL1 enzyme levels and its activity in postmortem Alzheimer's disease brains. Based on the Allen Brain Atlas in situ hybridization data, the expression levels of Neil1 messenger RNA are higher in the olfactory bulb compared with other areas of the brain. Olfaction in mice is a central brain function that involves many central nervous system pathways. Here, we studied the effect of complete loss of Neil1 gene on olfactory function. We explored olfactory function in mice with 3 different behavioral tests namely, olfactory sensitivity, performance, and buried food tests. Neil1(-/-) mice performed poorly compared with wild-type mice in all 3 tests. Our data indicate that loss of Neil1 causes olfactory function deficits supporting our previous findings and that normal brain function requires robust DNA repair.

The role of DNA repair in brain related disease pathology.

Oxidative DNA damage is implicated in brain aging, neurodegeneration and neurological diseases. Damage can be created by normal cellular metabolism, which accumulates with age, or by acute cellular stress conditions which create bursts of oxidative damage. Brain cells have a particularly high basal level of metabolic activity and use distinct oxidative damage repair mechanisms to remove oxidative damage from DNA and dNTP pools. Accumulation of this damage in the background of a functional DNA repair response is associated with normal aging, but defective repair in brain cells can contribute to neurological dysfunction. Emerging research strongly associates three common neurodegenerative conditions, Alzheimer's, Parkinson's and stroke, with defects in the ability to repair chronic or acute oxidative damage in neurons. This review explores the current knowledge of the role of oxidative damage repair in preserving brain function and highlights the emerging models and methods being used to advance our knowledge of the pathology of neurodegenerative disease.

Brain region-specific vulnerability of astrocytes in response to 3-nitropropionic acid is mediated by cytochrome c oxidase isoform expression.

Brain region specificity is a feature characteristic of neurodegenerative disorders, such as Huntington's disease (HD). We have studied the brain region-specific vulnerability of striatal compared with cortical and mesencephalic astrocytes treated with 3-nitropropionic acid (NPA), an in vitro model of HD. Mitochondrial dysfunction is involved in neurodegenerative processes. We have previously demonstrated a causal relationship between NPA-induced transcription of the cytochrome c oxidase (COX) subunit IV isoform (cox4i2) and increased oxidative stress leading to higher rates of necrotic cell death in striatal astrocytes by the application of a small interfering RNA knockdown system. Here, we have investigated the correlation of COX IV-2 protein expression with intracellular ATP content, mitochondrial peroxide production, and viability of astrocytes from three different brain regions. In cortical and mesencephalic astrocytes, NPA caused an elevation of cox4i2 transcription as in striatal astroglia. However, increased COX IV-2 and decreased COX IV-1 protein expression levels have been observed only in striatal astrocytes. In agreement with our hypothesis, Striatal astrocytes showed the highest levels of peroxide production and necrotic cell death rates compared with cortical and mesencephalic astroglia. Thus, we suggest that the higher vulnerability of astrocytes from the striatum in our in vitro model of HD is, at least in part, based on brain region-specific differences of the COX IV-2/COX IV-1 protein ratios and accompanied elevated oxidative stress.

Brain region specificity of 3-nitropropionic acid-induced vulnerability of neurons involves cytochrome c oxidase.

Mitochondria play a pivotal role in the regulation of energy metabolism and apoptotic pathways. Properties and functions of mitochondria might render subsets of selectively vulnerable neurons intrinsically susceptible to a different extent to cellular stress and degeneration. We have investigated the effect of 3-nitropropionic acid (NPA), a mitochondrial toxin and mimicking symptoms of Huntington's disease (HD) when applied systemically, on mitochondrial function and viability of primary neurons isolated from mouse brain striatum and cortex. We observed a higher vulnerability of striatal compared with cortical neurons in response to NPA treatment. This effect might be correlated with the transcription pattern of cytochrome c oxidase (EC 1.9.3.1.; COX) subunit IV isoforms. In cortical neurons, NPA induced a down-regulation of the COX IV-2/COX IV-1 ratio, whereas an up-regulation was found in striatal neurons. Previously, we have shown that an increased COX IV-2/COX IV-1 ratio is responsible for a higher enzyme activity which is paralleled by elevated intracellular ATP levels at the expense of increased mitochondrial peroxide production. These effects could also be demonstrated in striatal neurons. On the contrary, a decreased COX IV-2/COX IV-1 ratio was observed in cortical neurons which was accompanied by a decrease in intracellular ATP content and no significant changes in mitochondrial peroxide production. We propose that COX isoform IV-2 mediates increased oxidative stress that is, at least in part, responsible for a higher vulnerability of striatal compared with cortical neurons against NPA. This mechanism, in turn, may serve as an explanation for brain region-specific differences in the neuronal susceptibility to toxic conditions.

Gender-specific role of mitochondria in the vulnerability of 6-hydroxydopamine-treated mesencephalic neurons.

Many neurodegenerative diseases, such as Morbus Parkinson, exhibit a gender-dependency showing a higher incidence in men than women. Most of the neurodegenerative disorders involve either causally or consequently a dysfunction of mitochondria. Therefore, neuronal mitochondria may demonstrate a gender-specificity with respect to structural and functional characteristics of these organelles during toxic and degenerative processes. The application of 6-OHDA (6-hydroxydopamine) in vitro and in vivo represents a well-accepted experimental model of Parkinson's disease causing Parkinsonian symptoms. Besides the known effects of 6-OHDA on mitochondria and neuronal survivability, we aimed to demonstrate that the mitochondrial neurotoxin affects the morphology and survival of primary dopaminergic and non-dopaminergic neurons in the mesencephalon in a gender-specific manner by influencing the transcription of mitochondrial genes, ATP and reactive oxygen species production. Our data suggest that cell death in response to 6-OHDA is primarily caused due to increased oxidative stress which is more pronounced in male than in female mesencephalic neurons.

Cytochrome c oxidase isoform IV-2 is involved in 3-nitropropionic acid-induced toxicity in striatal astrocytes.

Astrocyte mitochondria play an important role for energy supply and neuronal survival in the brain. Toxic and degenerative processes are largely associated with mitochondrial dysfunction. We, therefore, investigated the effect of 3-nitropropionic acid (NPA), a mitochondrial toxin and in vitro model of Huntington's disease (HD), on mitochondrial function and viability of primary striatal astrocytes. Although NPA is known as an irreversible inhibitor of succinate dehydrogenase, we observed an increase of astrocyte ATP levels after NPA treatment. This effect could be explained by NPA-mediated alterations of cytochrome c oxidase subunit IV isoform (COX IV) expression. The up-regulation of COX isoform IV-2 caused an increased enzyme activity at the expense of elevated mitochondrial peroxide production causing increased cell death. The application of a small interfering RNA against COX IV-2 revealed the causal implication of COX isoform IV-2 in NPA-mediated elevation of oxidative stress and necrotic cell death. Thus, we propose a novel, additional mechanism of NPA-induced cell stress and death which is based on structural and functional changes of astrocyte COX and which could indirectly impair neuronal survival.