Treatment of Alzheimer's Disease: The Legacy of the Cholinergic Hypothesis, Neuroplasticity, and Future Directions

In this issue, an article by Waring et al. provides a meta-analysis of the effects of apo-lipo-protein E (APOE) genotype on the beneficial effect of acetyl-cholinesterase inhibitors (AChEIs) in patients with Alzheimer's disease (AD). There was no significant effect found. As of 2015, AChEI medications are the mainstay of AD treatment, and APOE genotype is the most significant factor associated with AD causation. This lack of a significant effect of APOE is analyzed with respect to the "Cholinergic Hypothesis" of AD, dating from 1976, through the recognition that cholinergic neurons are not the sole target of AD, but rather that AD attacks all levels of neuroplasticity in the brain, an idea originated by Ashford and Jarvik in 1985 and which still provides the clearest explanation for AD dementia. The "Amyloid Hypothesis" is dissected back to the alpha/beta pathway switching mechanism affecting the nexin-amyloid pre-protein (NAPP switch). The NAPP switch may be the critical neuroplasticity component of all learning involving synapse remodeling and subserve all learning mechanisms. The gamma-secretase cleavage is discussed, and its normal complementary products, beta-amyloid and the NAPP intracellular domain (NAICD), appear to be involved in natural synapse removal, but the link to AD dementia may involve the NAICD rather than beta-amyloid. Understanding neuroplasticity and the critical pathways to AD dementia are needed to determine therapies and preventive strategies for AD. In particular, the effect of APOE on AD predisposition needs to be established and a means found to adjust its effect to prevent AD.