Impairment of PGC-1α-mediated mitochondrial biogenesis precedes mitochondrial dysfunction and Alzheimer's pathology in the 3xTg mouse model of Alzheimer's disease

Highlights

• Alzheimer's disease, 3xTg-AD mice and mitochondrial biogenesis

• Reduction of PKA/CREB pathway at early ages of 3xTg-AD mice

• Mitochondrial biogenesis signaling mediated by PGC-1α is reduced at early ages of 3xTg-AD mice.

• Reduced PGC-1α-mediated mitochondrial biogenesis causes mitochondrial dysfunction at later ages in 3xTg-AD mice.

Abstract

Impairment of mitochondrial biogenesis and mitochondrial dysfunction is a prominent feature of Alzheimer's disease (AD). However, the extent to which the impairment of mitochondrial biogenesis influences mitochondrial dysfunction at the onset and during progression of AD is still unclear. Our study demonstrated that the protein expression pattern of the transcription factor pCREB/CREB, together with the protein expression of PGC-1α, NRF1 and TFAM are all significantly reduced in early ages of 3xTg-AD mice. We also found reduced mRNA expression levels of PKAC-α, CREB, PGC-1α, NRF1, NRF2 and TFAM as early as 1 month-of-age, an age at which there was no significant Aβ oligomer deposition, suggesting that mitochondrial biogenesis is likely impaired in ages preceding the development of the AD pathology. In addition, there was a decrease in VDAC2 expression, which is related to mitochondrial content and mitochondrial function, as demonstrated by protein expression of complex IV, as well as complex II + III, and complex IV activities, at later ages in 3xTg-AD mice. These results suggest that the impairment in mitochondrial biogenesis signaling mediated by PGC-1α at early ages of the AD mice model likely resulted in mitochondrial dysfunction and manifestation of the AD pathology at later ages. Taken together, enhancing mitochondrial biogenesis may represent a potential pharmacological approach for the treatment of AD.

Introduction

Alzheimer's disease (AD) is a debilitating neurodegenerative disorder which represents the most common cause of dementia and results in a major burden for patients and their families (Braak and Braak, 1997; Querfurth and Laferla, 2010). Although the formation of neuritic plaques (NPs) and neurofibrillary tangles (NFTs) have been considered as major AD hallmarks, some reports have shown weak correlations between amyloid burden and cognitive impairment in patients with AD (Beach et al., 2012; Jansen et al., 2015). Clinical studies using neuropathological and histological data, collected by the National Alzheimer's Coordinating Center (NACC), demonstrated that 29% of 919 subjects diagnosed with AD lacked the presence of postmortem NPs. In addition, these studies showed that 39% of 279 subjects who were not clinically diagnosed with AD presented high levels of postmortem NPs (Beach et al., 2012). These findings indicate the need to understand the molecular AD pathogenesis, since amyloid pathology alone cannot be used to confirm a diagnosis of AD during early stages (Beach et al., 2012; Jansen et al., 2015).

In recent years, increasing lines of evidence have implicated mitochondrial bioenergetics defects as a key pathological mechanism in neurodegenerative diseases (Fišar et al., 2019a; Swerdlow, 2018). Mitochondria are organelles that provide the majority of the cells' energy in the form of ATP, through oxidative phosphorylation (OXPHOS). Moreover, mitochondria also play a major role in calcium homeostasis, antioxidant defense signaling, release and re-uptake of neurotransmitters at synapses, cell death, and cell growth during development and aging (Friedman and Nunnari, 2014). Consequently, mitochondrial abnormalities can have an important impact in AD pathology by triggering and accelerating amyloid-β (Aβ) accumulation and tau hyperphosphorylation, which, in turn, further damage mitochondria (Fišar et al., 2019b; Swerdlow, 2018; Tyumentsev et al., 2018).

The appropriate function of mitochondria is fundamental to support the physiological needs of eukaryotic cells. In this regard, numerous pathways participate in the maintenance of mitochondrial integrity and/or proper restoration of mitochondrial function, including mitochondrial biogenesis (Golpich et al., 2017; Wu et al., 1999). This biogenesis process is regulated primarily by peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1α). Once being activated by either phosphorylation or deacetylation, PGC-1α activates different transcription factors, including nuclear respiratory factor 1 and 2 proteins (NRF1 and NRF2), which control mitochondrial protein expression, including mitochondrial transcription factor A (TFAM), which is critical for the initiation of mtDNA transcription and replication, and for the OXPHOS system (Golpich et al., 2017; Sheng et al., 2012; Wu et al., 1999). PGC-1α is expressed at high levels in mitochondria-rich cells with high energy demands, such as the brain. In this way, impairment in the activity of PGC-1α can trigger degeneration of neurons by inducing damage to mitochondrial biogenesis and, consequently, mitochondrial function, playing an important role in neurodegenerative diseases.

Recent research showed reduced PGC-1α levels in the brain of AD patients (Qin et al., 2009; Sheng et al., 2012) and in M17 cells overexpressing the familial form of AD-causing mutant amyloid precursor protein (APP_swe) (Sheng et al., 2012). Moreover, decreased levels of PGC-1α, NRF1, NRF2, and TFAM were also demonstrated in 12-month-old APP transgenic mice (Manczak et al., 2018) and cell lines derived from HT22 cells transfected with mutant mAPP complementary DNA (cDNA) (Reddy et al., 2018). Additionally, studies have shown that reduced expression of PGC-1α can contribute to pathological Aβ generation, leading to neurodegeneration in AD (Gong et al., 2013).

Although these studies demonstrated impaired mitochondrial biogenesis signaling in AD, it is not known when this signaling starts in the AD progression and whether it is a precursor or a consequence of Aβ aggregation. To explore the integrity of mitochondrial biogenesis and its contribution to mitochondrial dysfunction at the onset and during progression of AD, we investigated changes in mitochondrial biogenesis markers and their content and function in a triple-transgenic AD mouse model (3xTg-AD) with ages preceding the development and during the manifestation of amyloid pathology in AD.