Impaired Mitophagy and Mitochondrial Function in Alzheimer's Disease
Alzheimer's disease starts with an accumulation of amyloid-β, which disrupts cellular metabolism sufficiently to lay the grounds for the chronic inflammation and aggregation of tau protein that characterize the later, severe stage of the condition. Here, researchers make the argument that a fair degree of this progression is mediated via dysfunction of mitochondria and the quality control mechanisms of mitophagy, normally responsible for removing damaged mitochondria, and that this dysfunction is caused by amyloid-β.
Mitochondria are the power plants of the cell, and a faltering of their activity has profoundly disruptive effects. Needless to say, mitochondrial dysfunction is a characteristic feature of aging. This leads to the point that aging is a complex enough phenomenon for it to be possible to argue that mitochondrial dysfunction contributes to amyloid-β and tau aggregation, not vice versa. Or that both directions of causation are real phenomena. These are not simple, easily modeled systems. The fastest way to a definitive answer is likely that of building rejuvenation therapies capable of restoring mitochondrial function to youthful levels, and observing the result.
Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and multiple cognitive impairments. Several decades of intense research have revealed that multiple cellular changes are implicated in the development and progression of AD, including mitochondrial damage, synaptic dysfunction, amyloid beta (Aβ) formation and accumulation, hyperphosphorylated tau (P-Tau) formation and accumulation, deregulated microRNAs, synaptic damage, and neuronal loss in patients with AD. Among these, mitochondrial dysfunction and synaptic damage are early events in the disease process.
Recent research also revealed that Aβ and P-Tau-induced defective autophagy and mitophagy are prominent events in AD pathogenesis. Age-dependent increased levels of Aβ and P-Tau reduced levels of several autophagy and mitophagy proteins. In addition, abnormal interactions between (1) Aβ and mitochondrial fission protein Drp1; (2) P-Tau and Drp1; and (3) Aβ and PINK1/parkin lead to an inability to clear damaged mitochondria and other cellular debris from neurons. These events occur selectively in affected AD neurons.
In terms of rescuing and enhancing autophagy and mitophagy, reduced Drp1 and Aβ and P-tau levels and enhancing the levels of PINK1/parkin are proposed to rescue and/or maintain mitophagy and autophagy in affected AD neurons. The continuous clearance of cellular and mitochondrial debris is important for normal cellular function. We need more research on autophagy and mitophagy mechanisms and therapeutic aspects using cell cultures, animal models, and human AD clinical trials.