Investigation of selected nutraceuticals to protect against mitochondrial dysfunction: Potential therapeutic role in Alzheimer’s disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder that affects approximately 60-80% of all clinically diagnosed dementia cases worldwide. It is the second major cause of death in Australia. The progressive nature of the disease is characterized by cognitive deficits that worsen over time, usually leading to death within about a decade from their first manifestation. The AD brain is classically characterized by extracellular deposition of amyloid-β (Aβ) protein aggregates, as senile plaques, and intracellular neurofibrillary tangles (NFTs), composed of hyper-phosphorylated forms of the microtubule-associated protein Tau. There is no effective treatment for AD and the drugs currently being used to treat the disease only alleviate symptoms for a limited period in most cases. There has been limited success in clinical trials for some proposed therapies, so attention has been drawn towards using alternative approaches, including prevention strategies. As a result, nutraceuticals, the bioactive components of foods, have become attractive compounds for their potential neuroprotective capabilities. The overall objective of the present study was to derive a nutraceutical compound combination in vitro that has the potential to be used as a preventative therapy for AD. The first part of this project was aimed at screening eleven nutraceutical compounds derived from pomegranate, turmeric and fish oil, for their neuroprotective action using an in vitro cell model of Aβ cytotoxicity. Of these eleven compounds, punicalagin (PUN), docosahexaenoic acid (DHA), luteolin (LUT) and urolithin A (UA) were identified to effectively inhibit Aβ1-42-induced toxicity. The next aim was to determine whether Aβ1-42-induced toxicity could be blocked by the identified compounds in a synergistic manner. For this aim, the compounds PUN, DHA, LUT and UA were tested in two-compound combinations using the established cell model. In silico prediction (CompuSyn v.1.0 software) was used to predict suitable combinations based upon data from the initial screening phase. Experimental validation of these compound pairs against Aβ1-42-induced toxicity allowed the prediction of a three-compound combination. This three-compound combination made up of DHA, LUT and UA, (D5L5U5), was experimentally shown to be very effective at inhibiting Aβ1-42-induced toxicity. The third aim was to determine a pathway of action for the three-compound combination in inhibiting Aβ1-42-induced toxicity. It is well-established that Aβ1-42 causes mitochondrial dysfunction. Moreover, one of the compounds in this combination UA, is known to act as a mitophagy-inducing compound. Mitophagy is the selective degradation of dysfunctional mitochondria and is a key mitochondrial quality control mechanism in cells. Due to accumulating evidence of mitophagy deficits in AD, this represents a possible therapeutic target for AD. Therefore, part of this aim was to examine the effects of Aβ1-42 cytotoxicity in cells towards impairment in the mitochondrial quality control mechanisms. The results indicated that chronic Aβ1-42 exposure promotes mitochondrial dysfunction by increasing ROS levels and severely diminishing mitochondrial ATP levels, while affecting mitochondrial dynamics, mitophagy and mitobiogenesis processes. Additionally, it was revealed that these changes were time dependent due to the cell responses against the Aβ1-42 cytotoxicity. The final aim of this thesis was to compare the three-compound combination with the component compounds in their optimal concentrations against Aβ1-42-induced mitochondrial dysfunction. The findings indicated that the three-compound nutraceutical combination, D5L5U5 precedes its component compounds for its protective ability on all aspects tested, namely, by reducing ROS levels, increasing ATP levels, and inducing mitophagy and mitobiogenesis. The compounds, LUT, DHA and UA were also independently protective of mitochondria. Overall, it was determined that the three-compound nutraceutical combination, D5L5U5 has strong inhibitory effects against Aβ1-42-induced toxicity through its mitoprotective activities by minimizing oxidative stress and inducing mitophagy and mitobiogenesis. However, it warrants further investigations in other in vitro and in vivo AD models to confirm its potential to be used as a preventative therapy for AD