Accumulated Amyloid-beta Peptide and Hyperphosphorylated Tau Protein: Relationship and Links in Alzheimer's Disease

The neuropathology associated with Alzheimer's disease (AD) is characterized by the presence of extracellularly neuritic plaques, intracellularly neurofibrillary tangles and the loss of basal forebrain cholinergic neurons. The neuritic plaque is composed of a core of amyloid-beta peptide (A beta) while the neurofibrillary tangles contain phosphorylated tau protein, and, as such, both A beta and tau are important molecules associated with AD. In healthy human bodies, clearance mechanisms for A beta are available; yet if clearance fails, A beta accumulates, increasing the risk of neurotoxicity in the brain. Tau, one of the main microtubule-associated proteins, will be hyperphosphorylated and lose the ability to bind microtubules when the homeostasis of phosphorylation and dephosphorylation is disturbed in neurons. Accumulated A beta and hyperphosphorylated tau are thought to be coexistent. Research on the pathological changes in AD indicates that accumulated A beta in vivo may initiate the hyperphosphorylation of tau. Also, the signal transduction pathways of tau hyperphosphorylation may be related to accumulated A beta. In this review, we will discuss how A beta accumulates, how tau protein is hyperphosphorylated, and how accumulated A beta initiates hyperphosphorylation of tau protein in AD.