Pathophysiology of neuronal energy crisis in Alzheimer's disease

A large body of evidence indicates that sporadic Alzheimer's disease (AD) is a vascular disorder with neurodegenerative consequences and needs to be treated and managed as such. Epidemiologic studies of vascular risk factors, together with preclinical detection tools for AD are proof of concept that cerebral hypoperfusion is one of the earliest pathological signs in the development of cognitive failure. Vascular risk factors involving heart disease and stroke in the elderly individual who already possesses a dwindling cerebrovascular reserve due to advancing age contribute to further decline in cerebral blood flow (CBF) resulting in unrelenting brain hypoperfusion. Brain hypoperfusion, in turn, can reach a critically attained threshold of cerebral hypoperfusion (CATCH) giving rise to a neuronal energy crisis via reduced ATP synthesis. The ensuing metabolic energy crisis initially carves up ischemic-sensitive neurons in the hippocampus and posterior parietal cortex setting up cognitive meltdown and progressive neurodegenerative and atrophic changes in the brain. Neuronal energy compromise accelerates oxidative stress, excess production of reactive oxygen species, aberrant protein synthesis, ionic membrane pump dysfunction, signal transduction impairment, neurotransmitter failure, abnormal processing of amyloid precursor protein resulting in beta-amyloid deposition and axonal microtubule disruption from tau hyperphosphorylation. The high energy metabolic changes leading to oxidative stress and cellular hypometabolism precede clinical expression of AD. Regional CBF measurements using neuroimaging techniques can predict AD preclinically at the mild cognitive impairment stage or even before any clinical manifestation of dementia is expressed. Clinical diagnostic assessment of elderly persons who could develop or already present with memory complaints can prevent, reverse or slow down AD development. Although pathologic aging is the subject of thousands of studies, the question of why the elderly (and not younger people) succumb to AD has not been adequately addressed. The explanation(s) as to why vascular risk factors, for example, can trigger AD or vascular dementia usually in the elderly and not the young should provide vital clues in the search for a strategically effective dementia treatment. This review offers inductive hypothetical darts relative to that critical question.