A Bird's-Eye View of the Multiple Biochemical Mechanisms that Propel Pathology of Alzheimer's Disease: Recent Advances and Mechanistic Perspectives on How to Halt the Disease Progression Targeting Multiple Pathways

Abstract

Neurons consume the highest amount of oxygen, depend on oxidative metabolism for energy, and survive for the lifetime of an individual. Therefore, neurons are vulnerable to death caused by oxidative-stress, accumulation of damaged and dysfunctional proteins and organelles. There is an exponential increase in the number of patients diagnosed with neurodegenerative diseases such as Alzheimer's (AD) as the number of elderly increases exponentially. Development of AD pathology is a complex phenomenon characterized by neuronal death, accumulation of extracellular amyloid-β plaques and neurofibrillary tangles, and most importantly loss of memory and cognition. These pathologies are most likely caused by mechanisms including oxidative stress, mitochondrial dysfunction/stress, accumulation of misfolded proteins, and defective organelles due to impaired proteasome and autophagy mechanisms. Currently, there are no effective treatments to halt the progression of this disease. In order to treat this complex disease with multiple biochemical pathways involved, a complex treatment regimen targeting different mechanisms should be investigated. Furthermore, as AD is a progressive disease-causing morbidity over many years, any chemo-modulator for treatment must be used over long period of time. Therefore, treatments must be safe and non-interfering with other processes. Ideally, a treatment like medicinal food or a supplement that can be taken regularly without any side effect capable of reducing oxidative stress, stabilizing mitochondria, activating autophagy or proteasome, and increasing energy levels of neurons would be the best solution. This review summarizes progress in research on different mechanisms of AD development and some of the potential therapeutic development strategies targeting the aforementioned pathologies.

Keywords: Alzheimer’s disease; amyloid-β plaques; ashwagandha; autophagy; mitochondrial dysfunction; oxidative stress; presenilin-1; water-soluble coenzyme-Q10.

Fig.1

Presenilin-1 (PS-1) mutation leads to several changes in the cell due to improper cleavage of the amyloid-β protein precursor (AβPP). PS-1 mutation causes the improper cleavage of AβPP leading to aberrant forms of amyloid-β (Aβ) which occurs in both the cell and mitochondrial membrane. Accumulation of aberrant Aβ proteins form Aβ plaques which cause increased intracellular oxidative stress and influx of calcium ion. Both of these events lead to mitochondrial dysfunction which in turn increases oxidative stress and cellular damage.

Fig.2

Possible mechanism for autophagic reintroduction by WS-CoQ₁₀ in PS-1 mutated fibroblasts. mTOR is related to the maintenance of cellular senescence as well as the inhibition of autophagy through the induction hyper-phosphorylation of ATG13. Beclin-1 a known inducer of autophagy is degraded by caspase 8 which is activated during times of oxidative stress produced in huge quantities by the damaged mitochondria of PS-1 mutated fibroblasts. Aβ has been seen to lead to mitochondrial damage and can increase the influx of Ca²⁺ which can activate the antiapoptotic protein calpain. WS-CoQ₁₀ is able to considerably lessen mitochondrial damage and the levels of ROS that lead to oxidative stress. Its possible the reduction of oxidative stress may lead to reduced degradation of beclin-1 via caspase 8 leading to increased autophagy. This may act as a positive feedback loop as the reintroduction of autophagy would allow the degradation of dysfunctional mitochondria and oxidized proteins.