ApoE: the link between Alzheimer's-related glucose hypometabolism and Aβ deposition?

Alzheimer's disease (AD) is a complex, multifactorial progressive neurodegenerative disease. Pathologically, AD is characterized by extracellular deposits of amyloid beta (Aβ) protein and intracellular accumulation of neurofibrillary tangles (NFTs) of tau. The central role of Aβ protein in the AD etiology is well-established, and its increased deposition in AD brain is attributed to its decreased clearance from the brain. It is noteworthy that apolipoprotein E (ApoE), the most significant risk factor for late-onset AD, has been shown to play a vital role in brain Aβ clearance and the ability of ApoE to do this depends mainly upon its lipidation status. Thus, lower ApoE lipidation status leading to decreased Aβ clearance may underlie the increased Aβ deposition observed in AD brain. In addition to the pathophysiological Aβ deposits, AD is also characterized by certain metabolic changes. Among them, decreased cerebral glucose metabolism is one of the distinct characteristics of AD brain and is also observed in patients with Mild Cognitive Impairment (MCI) who subsequently develop AD. Thus, decreased cerebral glucose metabolism is an early event in AD pathology and may precede the neuropathological Aβ deposition associated with AD. In this context, we hypothesize here that the decreased glucose metabolism in pre-AD and early AD stages, may lead to lower ApoE lipidation status, which in turn may lead to decreased clearance and hence, increased deposition of Aβ protein in AD brain.