Physiological roles of amyloid-β and implications for its removal in Alzheimer's disease

The underlying pathological cause of Alzheimer's disease has been postulated to be an excess of amyloid-beta (Abeta) which aggregates into toxic fibrillar deposits within the extracellular space of the brain, thereby disrupting neuronal and synaptic function and eventually leading to neuronal degeneration and dementia. As a result, therapeutic strategies have been developed that are designed to remove Abeta from the brain. Caution needs to be exercised concerning such strategies because, in addition to its presence in neuritic plaques, Abeta has a widespread distribution through the brain and body, even in cognitively normal individuals. Evidence indicates that instead of being a toxic peptide, soluble Abeta serves a variety of physiological functions, including modulation of synaptic function, facilitation of neuronal growth and survival, protection against oxidative stress, and surveillance against neuroactive compounds, toxins and pathogens. These physiological functions must be taken into account when strategies are developed to reduce Abeta load in Alzheimer's disease. Ideally, such strategies should target forms of Abeta that are not bioavailable, such as fibrillar Abeta, or forms that are regarded to be overexpressed in Alzheimer's disease (such as oligomers) while leaving normal soluble Abeta(1-40) and Abeta(1-42) intact. At present none of the available therapeutic strategies appears to have such selectivity. Until these technical limitations and the uncertainties regarding the effect of depletion of Abeta from the brain are resolved, it would not be prudent to begin further clinical trials.