A Cell Cycle Hypothesis for the Development of Alzheimer's Disease
The development of treatments for Alzheimer's disease based on clearance of amyloid continues to be a struggle, and it is still unclear as to whether this is because it is an inherently hard task, or because amyloid aggregates are not the most important contributing cause of this condition. Given that theorizing is a lot easier than building therapies, all delays in evident progress tend to give rise to a lot of theorizing. There is a prolific construction of alternative hypotheses regarding the biochemistry of Alzheimer's disease, its causes and progression. This example of one such a hypothesis has little support for its thesis in the broader research community, but is interesting as an example of the range of thinking taking place on this topic:
Early-onset familial Alzheimer's disease (EOFAD) and late-onset sporadic AD (LOSAD) both follow a similar pathological and biochemical course that includes: neuron and synapse loss and dysfunction, microvascular damage, microgliosis, extracellular amyloid-β deposition (Aβ), and the deposition of phosphorylated tau protein in the form of intracellular neurofibrillary tangles in affected brain regions. Any mechanistic explanation of AD must accommodate these biochemical and neuropathological features for both forms of the disease.
Cell cycle abnormalities represent another major biochemical and neuropathological feature common to both EOFAD and LOSAD, and 1) appear very early in the disease process, prior to the appearance of plaques and tangles, and 2) explain the biochemical (e.g., tau phosphorylation), neuropathological (e.g., neuron hypertrophy) and cognitive changes observed in EOFAD and LOSAD. Since neurogenesis after the formation of a memory is sufficient to induce forgetting, any stimulus that promotes cell cycle re-entry will be a negative event for memory. In this insight paper, we propose that aberrant re-entry of terminally differentiated, post-mitotic neurons into the cell cycle is a common pathway that explains both early and late-onset forms of AD. In the case of EOFAD, mutations in APP, PSEN1, and PSEN2 that alter AβPP and Notch processing drive reactivation of the cell cycle, while in LOSAD, age-related reproductive endocrine dyscrasia that upregulates mitogenic TNF signaling, AβPP processing toward the amyloidogenic pathway and tau phosphorylation drives reactivation of the cell cycle. Inhibition of cell cycle reentry of post-mitotic neurons may be a useful therapeutic strategy to prevent or halt disease progression.