Elsevier

Neurobiology of Aging

Volume 34, Issue 10, October 2013, Pages 2399-2407
Neurobiology of Aging

Regular article
Mitochondrial DNA damage in a mouse model of Alzheimer's disease decreases amyloid beta plaque formation

https://doi.org/10.1016/j.neurobiolaging.2013.04.014Get rights and content

Abstract

Mitochondrial DNA (mtDNA) damage and the generation of reactive oxygen species have been associated with and implicated in the development and progression of Alzheimer's disease. To study how mtDNA damage affects reactive oxygen species and amyloid beta (Aβ) pathology in vivo, we generated an Alzheimer's disease mouse model expressing an inducible mitochondrial-targeted endonuclease (Mito-PstI) in the central nervous system. Mito-PstI cleaves mtDNA causing mostly an mtDNA depletion, which leads to a partial oxidative phosphorylation defect when expressed during a short period in adulthood. We found that a mild mitochondrial dysfunction in adult neurons did not exacerbate Aβ accumulation and decreased plaque pathology. Mito-PstI expression altered the cleavage pathway of amyloid precursor protein without increasing oxidative stress in the brain. These data suggest that mtDNA damage is not a primary cause of Aβ accumulation.

Introduction

Alzheimer's disease (AD) is one of the most common age-related neurodegenerative diseases characterized by declines in memory and cognition. One pathological feature of AD is the presence of abnormal plaques composed of amyloid beta (Aβ) protein deposited in the cortex and hippocampus, regions that are affected in AD.

Mitochondrial dysfunction has been implicated in contributing to the development and progression of AD. Early studies reported defects in oxidative phosphorylation (OXPHOS), specifically cytochrome c oxidase, in postmortem AD brains (Chagnon et al., 1995; Long et al., 2012; Mutisya et al., 1994; Sheng et al., 2012). Mutations in mitochondrial DNA (mtDNA), which encodes proteins for several OXPHOS complexes, have also been found in affected patients (Coskun et al., 2004; Krishnan et al., 2012; Lin et al., 2002). Aging, the major risk factor in developing AD, is associated with declines in mitochondrial function in the central nervous system (Bowling et al., 1993; Hauptmann et al., 2009; Hong et al., 2008; Ross et al., 2010; Yao et al., 2009). Moreover, the cybrid model of AD, in which mtDNA from AD patients' platelets was transferred into cells lacking mtDNA, showed decreased mitochondrial mobility, increased oxidative stress, decreased cytochrome oxidase activity, and decreased mitochondrial membrane potential, suggesting that mitochondria and mtDNA abnormalities contribute to AD pathogenesis (Costa et al., 2012; Swerdlow, 2007).

In support of an alternative hypothesis, in which mitochondrial dysfunction is a consequence rather than a trigger of AD, Aβ negatively affects mitochondrial function. Aβ directly associates with the mitochondria inhibiting OXPHOS (Anandatheerthavarada et al., 2003; Calkins et al., 2011; Casley et al., 2002; Chen and Yan, 2010; Devi et al., 2006; Hansson Petersen et al., 2008; Manczak et al., 2006; Reddy and Beal, 2008). Mitochondrial dynamics are also affected in AD either by the downregulation of fission and/or fusion proteins, or by nitrosylation of dynamin-related protein 1 (DRP1), a mitochondrial fission protein (Manczak et al., 2011; Su et al., 2010; Wang et al., 2009). It is unknown to what extent OXPHOS defects affect and progress the pathophysiology of AD.

Oxidative stress and reactive oxygen species (ROS) damage also contribute to AD pathophysiology. Previous reports showed that ROS enhances β-secretase activity and exacerbates Aβ aggregation (Guglielmotto et al., 2009; Paola et al., 2000; Tamagno et al., 2002; Yao and Brinton, 2012). ROS also influence amyloid precursor protein (APP) processing, promoting Aβ through β- and γ-secretase activation (Leuner et al., 2012a, 2012b; Shen et al., 2008). The electron transport chain is a known source of ROS that can damage proteins, lipids, and DNA. Moreover, some functional alterations in the respiratory chain have been reported to increase ROS production. However, it is unclear whether mitochondria are the sole contributor to the ROS damage seen in AD, because alternative sources of ROS have also been identified (Abramov et al., 2004; Cutler et al., 2004; Yao and Brinton, 2012).

We examined the effect of mtDNA damage on Aβ accumulation and plaque formation to determine how a mild mitochondrial dysfunction affects the pathophysiological changes that occur in a mouse model of AD.

Section snippets

Mice procedures

The generation of Mito-PstI transgenic mice has been previously described (Fukui and Moraes, 2009). The AD transgenic mice, carrying mutant APP and mutant presenilin 1 (The Jackson Laboratory, Bar Harbor, Maine.), were first crossed with CaMKIIα-tTa mice (The Jackson Laboratory). Double-positive mice, AD/CaMKIIα-tTa, were then crossed with Mito-PstI mice to obtain AD/CaMKIIα-tTa/PstI mice that we called “AD-mito-PstI mice”. We called “AD mice” double-positive mice, either AD/CaMKIIα-tTa or

Generation of AD-mito-PstI mice

We previously described the generation and the characteristics of mito-PstI mice using the CaMKIIα promoter (Fukui and Moraes, 2009). Briefly, these mice express a mammalianized version of the bacterial PstI targeted to mitochondria, inserted downstream of a tetracycline response element promoter. Another transgenic allele expresses a tetracycline trans-activator (tTA) gene using the neuronal specific CaMKIIα promoter. When mice harbor both transgenic alleles (PstI+/tTA+), mito-PstI is

Discussion

In this study, we caused mtDNA damage in adult cortical and hippocampal neurons during the period of β-amyloid plaque formation by inducing mito-PstI expression at 6–8 months of age. This is the period when AD mice show marked accumulation of Aβ fragments and amyloid plaques. A 2-month induction period was sufficient to produce a decrease in CoxI content, an mtDNA encoded protein, indicating an mtDNA depletion consequent to PstI induction. We showed that mtDNA damage, which was associated with

Disclosure statement

The authors declare no conflicts of interest.

All experiments and animal husbandry were performed according to a protocol approved by the University of Miami Institutional Animal Care and Use Committee.

Acknowledgements

This work was supported in part by the National Institutes of Health Grants 1R01AG036871, 1R01NS079965, and 5R01EY010804 (CTM), 5T32NS007492, 5T32NS007459, American Heart Association Grant 11Pre7610007, and the Lois Pope LIFE Fellowship (AMP). The authors thank David Jackson (Neuroscience Program) for technical assistance, and Dr Beata Frydel and the Lois Pope LIFE Center Histology Core for the access and use of their microtome, reagents, and light microscopes.

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  • Cited by (0)

    1

    Contributed equally to this work.

    2

    Present address: Surgical Neurology Branch, NINDS/National Institutes of Health Bethesda, MD, USA.

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