Early molecular changes in Alzheimer disease: can we catch the disease in its presymptomatic phase?

Alzheimer disease (AD) is the most common form of dementia and characterized by deposition of amyloid-β (Aβ) plaques, neurofibrillary tangles consisting of hyperphosphorylated tau, atrophy, and progressive neurodegeneration. While the familial, early onset form of AD is known to be caused by specific mutations in genes encoding presenilin 1, presenilin 2, or amyloid-β protein precursor, the underlying mechanisms leading to the development of sporadic AD are still not known. The major risk factors are, however, aging and APOE ε4. Here we review the latest evidence for the involvement of malfunctioning insulin signaling, dysfunction of mitochondria-associated membranes, cerebrovascular changes, increased oxidative stress and free radical formation, DNA damage, disturbed energy metabolism, and synaptic dysfunction in early stages of AD. We focus on whether the changes in these processes precede or succeed the earliest symptoms in AD patients, i.e., minimal cognitive impairment. Since changes in Aβ processing are probably a key event in AD we also highlight the relationship of the above mentioned processes with the formation, secretion, aggregation, and toxicity of Aβ. Based on our literature findings we propose a model in which insulin dysfunction, pathological cerebrovascular changes, dysfunction of mitochondria-associated membranes, and/or synaptic changes are likely to interact with each other, thereby initiating and facilitating the development of AD pathology by accelerating the production and deposition of Aβ. Increased oxidative stress and free radical formation, DNA damage, disturbed energy metabolism, and synaptic loss follow these events, but still occur very early in AD.