Challenging the amyloid cascade hypothesis: Senile plaques and amyloid-β as protective adaptations to Alzheimer disease

Ever since their initial description over a century ago, senile plaques and their major protein component, amyloid-β, have been considered key contributors to the pathogenesis of Alzheimer disease. However, counter to the popular view that amyloid-β represents an initiator of disease pathogenesis, we herein challenge dogma and propose that amyloid-β occurs secondary to neuronal stress and, rather than causing cell death, functions as a protective adaptation to the disease. By analogy, individuals suffering from altitude sickness nearly always have elevated levels of hemoglobin. However, while hemoglobin is toxic to cells in culture and increased erythropoiesis at sea level can be deadly, it is clear that the increases in hemoglobin occurring at altitude are beneficial. Amyloid, like hemoglobin, may also be beneficial, in this case, following neuronal stress or disease. Although controversial, a protective function for amyloid-β is supported by all of the available literature to date and also explains why many aged individuals, despite the presence of high numbers of senile plaques, show little or no cognitive decline. With this in mind, we suspect that current therapeutic efforts targeted toward lowering amyloid-β production or removal of deposited amyloid-β will only serve to exacerbate the disease process.