Vicious Cycles Within the Neuropathophysiologic Mechanisms of Alzheimer's Disease

Rigorous scientific research has identified multiple interactive mechanisms that parallel and are likely causative of the development of Alzheimer's disease (AD). Causative mechanisms include genomics, the creation of amyloid beta (Abeta), factors inhibiting the Abeta removal process, the transformation of Abeta to its toxic forms (various forms of Abeta aggregation), and lastly the oxidative, inflammatory, and other effects of toxic Abeta. Fibrillar beta-amyloid peptide, a major component of senile plaques in AD brain, is known to induce microglial-mediated neurotoxicity under certain conditions, but some recent studies support the notion that Abeta oligomers are the primary neurotoxins. Abeta-42 oligomers that are soluble and highly neurotoxic, referred to as Abeta-derived diffusible ligands (ADDLs), assemble under conditions that block fibril formation. These oligomers bind to dendrite surfaces in small clusters with ligand-like specificity and are capable of destroying hippocampal neurons at nanomolar concentrations. Evidence is presented that AD is triggered by these soluble, neurotoxic assemblies of Abeta rather than the late stage pathology landmarks of amyloid plaques and tangles. The premise is that AD symptoms stem from aberrant nerve cell signaling and synaptic failure rather than nerve cell death, which nevertheless follows and exacerbates the initial pathologies of AD. The defective clearance of amyloid leads to amyloid angiopathy that in turn perpetuates hypoperfusion that affects formation as well as absorption of CSF thereby altering clearance of amyloid and promoting vascular and parenchymal deposition[ 1]. Hypoperfusion, the defective clearance of amyloid, and resultant increase in amyloid deposition thus represent a vicious cycle. Chronic vascular hypoperfusion-induced mitochondrial failure results in oxidative damage, which drives caspase 3-mediated Abeta peptide secretion and enhances amyloidogenic APP processing. Intracellular Abeta accumulation in turn promotes a significant oxidative and inflammatory mechanism that generates a vicious cycle of Abeta generation and oxidation, each accelerating the other. Abeta activates astrocytes that add to the oxidative imbalance, upregulate the expression of APP via TGF-beta, and are capable of expressing BACE1. Each of these 3 actions accelerates the larger cycle of cholinergic neuron destruction. As oxidative stress induces lesions of cholinergic nuclei producing a reduction in cholinergic neurotransmission, a subsequent increase in cortical APP involving PKCepsilon leads to accelerated amyloidogenic APP metabolism. The linkage of cholinergic activation and APP metabolism completes an additional feedback loop wherein the damage wrought by Abeta accelerates further Abeta production. A comprehensive vision of the neuropathophysiologic mechanisms that result in AD reveals several vicious cycles within a larger vicious cycle, that is to say, a number of interactive systems that each, once set in motion, amplify their own processes, thus accelerating the development of AD.