Alzheimer’s disease is a complex condition because the brain is a complex environment. Neurodegeneration is caused by the accumulation of two forms of protein aggregate, amyloid-β and tau. There is evidence to suggest that each can spur the generation of the other, and that they act in synergy to cause worse harm to the brain than either would alone, but the present consensus is that amyloid-β precedes tau in the development of the condition. It may even turn out to be the case that tau causes the majority of the damage in the later stages of the condition, not amyloid-β.
Whether this means that amyloid-β causes tau aggregration is another question entirely, and one that is unlikely to be adequately answered without the development of reliable means to clear amyloid-β from the brain. That has so far proven to be more challenging than was originally hoped, and even those clinical efforts that did remove amyloid-β to some degree failed to show benefits in patients. Varied factions within the research community have their theories as to why this might be the case, and scientists here note one of them – that
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