More on the Amyloid Cascade Hypothesis 2.0

Researchers recently proposed a version 2.0 of the amyloid cascade hypothesis regarding the development of Alzheimer's disease. This was provoked by the failure of amyloid-clearing immunotherapies to produce meaningful benefits in patients. Those results require rethinking the role of amyloid-β in Alzheimer's disease. Some researchers theorize that amyloid-β aggregation is a side-effect of the real disease process, which is more a matter of persistent viral infection and consequent chronic inflammation in brain tissue. The amyloid hypothesis 2.0 keeps amyloid-β front and center as the primary early stage disease mechanism, however. The question is whether the right type or localization of amyloid-β is being targeted by current research programs: almost certainly not, given the poor results to date.

Recently, we proposed the Amyloid Cascade Hypothesis 2.0 (ACH2.0), a reformulation of the original Amyloid Cascade Hypothesis (ACH). In ACH2.0, in contrast to ACH, Alzheimer's disease (AD) is driven by intraneuronal amyloid-β (iAβ) rather than extraneuronal amyloid-β and occurs in two stages. In the first, relatively benign stage, Aβ protein precursor (AβPP)-derived iAβ activates. Then upon reaching a critical threshold, the AβPP-independent iAβ-generating pathway triggers a devastating second stage resulting in neuronal death.

While the ACH2.0 remains aligned with the ACH premise that Aβ is toxic, the toxicity is exerted because of intracellular rather than extracellular Aβ. In this framework, a once-in-a-lifetime-only iAβ depletion treatment via transient activation of BACE1 and/or BACE2 (exploiting their Aβ-cleaving activities) or by any means appears to be the best therapeutic strategy for AD.

Whereas the notion of differentially derived iAβ being the principal moving force at both AD stages is both plausible and elegant, a possibility remains that the second AD stage is enabled by an AβPP-derived iAβ-activated self-sustaining mechanism producing a yet undefined deleterious "substance X" which anchors the second AD stage. The present study generalizes the ACH2.0 by incorporating this possibility and shows that, in this scenario, the iAβ depletion therapy may be ineffective at symptomatic AD stages but fully retains its preventive potential for both AD and the aging-associated cognitive decline, which is defined in the ACH2.0 framework as the extended first stage of AD.