Mitochondria in Alzheimer's Disease Pathogenesis

Abstract

Alzheimer's disease (AD) is a progressive and incurable neurodegenerative disorder that primarily affects persons aged 65 years and above. It causes dementia with memory loss and deterioration in thinking and language skills. AD is characterized by specific pathology resulting from the accumulation in the brain of extracellular plaques of amyloid-β and intracellular tangles of phosphorylated tau. The importance of mitochondrial dysfunction in AD pathogenesis, while previously underrecognized, is now more and more appreciated. Mitochondria are an essential organelle involved in cellular bioenergetics and signaling pathways. Mitochondrial processes crucial for synaptic activity such as mitophagy, mitochondrial trafficking, mitochondrial fission, and mitochondrial fusion are dysregulated in the AD brain. Excess fission and fragmentation yield mitochondria with low energy production. Reduced glucose metabolism is also observed in the AD brain with a hypometabolic state, particularly in the temporo-parietal brain regions. This review addresses the multiple ways in which abnormal mitochondrial structure and function contribute to AD. Disruption of the electron transport chain and ATP production are particularly neurotoxic because brain cells have disproportionately high energy demands. In addition, oxidative stress, which is extremely damaging to nerve cells, rises dramatically with mitochondrial dyshomeostasis. Restoring mitochondrial health may be a viable approach to AD treatment.

Keywords: Alzheimer’s disease; electron transport chain; inflammation; mitochondria.

Figure 1

Diagram demonstrating the flow of electrons through the mitochondrial electron transport chain (ETC). Electrons initially enter the ETC by NADH at complex I, and FADH2 from complex II. Ubiquinone transports the electrons to complex III, and then through cytochrome (Cyt) C to complex IV where oxygen is reduced into water. A proton gradient pumped across the inner mitochondrial membrane caused by the translocation of protons synthesizes ATP.

Figure 2

Schematic representation of factors involved in mitochondrial dysfunction and AD: Mitochondrial dysfunction in the AD brain results in elevated oxidative stress, increased mitochondrial fragmentation, mitophagy, impaired mitochondrial trafficking, mitochondrial DNA damage, defective mitochondrial biogenesis and dynamics and reduced glucose metabolism. Accumulation of Aβ can contribute to mitochondrial oxidative stress while tau protein, present in extracellular tangles, can interfere with axonal movement of mitochondria.