[Amyloid beta: functional protein or biological junk?].

During a decade there was a dogma that Alzheimer's amyloid beta (Ap) is produced only upon the disease, and that this protein is neurotoxic for neurons and brain tissue. Current scientific evidence demonstrate that AP is an essential molecule in synaptic plasticity that underlie learning and memory. Therefore, it was hypothesized that the change of AP biology in Alzheimer's disease (as well as in a number of other human pathologies, including cardiovascular disease, Niemann-Pick type C disease and Down syndrome) represents a physiological mechanism serving to compensate the impaired brain structure or function. This review summarizes experimental evidence on Abeta as functional player in synaptic plasticity and neurochemical pathways.