Amyloid Beta

Amyloid beta (also written as amyloid-beta or A-beta) is a short protein fragment produced when a larger protein called amyloid precursor protein (APP) is cleaved by enzymes called secretases. Under normal circumstances, amyloid beta is produced and cleared continuously as part of normal brain metabolism. In Alzheimer's disease, this balance breaks down: either too much is produced, or too little is cleared, or both.

When amyloid beta accumulates beyond the brain's clearance capacity, it aggregates into increasingly large structures: first soluble oligomers (small clusters), then protofibrils, then insoluble plaques deposited between neurons. The different aggregate forms appear to have different toxicities. Soluble oligomers and protofibrils are currently thought to be particularly harmful to synaptic function, while larger plaques may represent a somewhat less toxic endpoint.

The dominant variant is amyloid beta 42 (A-beta 42), a 42-amino-acid fragment particularly prone to aggregation. The ratio of A-beta 42 to A-beta 40 in cerebrospinal fluid and blood is a key biomarker for Alzheimer's disease — a falling ratio indicates that A-beta 42 is being sequestered into plaques rather than cleared into cerebrospinal fluid.

Amyloid beta accumulation is currently the best-validated biomarker for Alzheimer's disease risk and is the primary target of the most advanced treatments — lecanemab and donanemab, both FDA-approved anti-amyloid antibodies. Amyloid positivity (detectable by PET scan or CSF/blood testing) is required for diagnosis of Alzheimer's disease under current research criteria and for eligibility for anti-amyloid treatments.

The amyloid cascade hypothesis — the proposal that amyloid accumulation is the initiating event that drives subsequent tau pathology, neurodegeneration, and cognitive decline — has been the dominant framework in Alzheimer's research for three decades. It remains influential, though it has been significantly refined. Most researchers now view amyloid as a necessary but not sufficient cause of Alzheimer's disease.

Amyloid accumulation begins approximately 15-20 years before clinical symptoms of Alzheimer's disease, during the preclinical phase. This creates the opportunity for early detection and potentially early intervention. Blood-based amyloid testing (measuring plasma A-beta 42/40 ratio or phosphorylated tau) can now detect amyloid pathology with increasing accuracy before symptoms appear.