Brain Aging and Late-Onset Alzheimer's Disease: A Matter of Increased Amyloid or Reduced Energy?

Alzheimer's disease (AD) represents the most common form of dementia in old age subjects, and despite decades of studies, the underlying etiopathogenetic mechanisms remain unsolved. The definition of AD has changed over the past years, offering an ever more detailed definition of pre-morbid and pre-clinical status, but without a similar strong emphasis on the role of aging as the main risk factor. In fact, while early-onset AD is a clear consequence of gene mutations, late-onset AD is more likely due to a gradual accumulation of age-related damages. The pathogenetic amyloid cascade hypothesis has been recently questioned due to multiple clinical failures. Furthermore, several studies reported that cognitively normal elderly have a high amyloid deposition in the brain comparable to the levels observed in old age subjects with AD. This suggests that amyloid accumulation enters into the normal process of aging and what really triggers neuronal death and clinical manifestation in late-onset AD still needs further explanation. In this context, 'normal brain aging' and AD might represent a different pathway of successful or failed capability to adapt brain structures and cerebral functions. Cellular senescence and age-related changes affecting the brain may be considered as biologic manifestations of increasing entropy. Bioenergetic deficits due to mitochondrial dysfunction may lead to progressive neuronal death and clinical expression of dementia. So, increased amyloid in the brain of old age subjects may represent the downstream event expression of a biological system that is cooling down because of its exhaustion and not the core causative factor of late-onset dementia.