Beta-Amyloid Monomer and Insulin/IGF-1 Signaling in Alzheimer's Disease

Alzheimer's disease is the most common form of dementia among older people and is still untreatable. While beta-amyloid protein is recognized as the disease determinant with a pivotal role in inducing neuronal loss and dementia, an impaired brain insulin signaling seems to account in part for the cognitive deficit associated with the disease. The origin of this defective signaling is uncertain. Accumulating toxic species of beta-amyloid, the so-called oligomers, has been proposed to be responsible for downregulation of neuronal insulin receptors. We have found that the nontoxic form of beta-amyloid, the monomer, is able to activate insulin/insulin-like growth factor-1 (IGF-1) receptor signaling and thus behaves as a neuroprotectant agent. Our suggestion is that depletion of beta-amyloid monomers, occurring in the preclinical phase of Alzheimer's disease, might be the cause of early insulin/IGF-1 signaling disturbances that anticipate cognitive decline.