It is becoming increasingly clear that the current view of Alzheimer's disease is incorrect: over 50,000 papers have now been published on the amyloid beta peptide(s) (Abeta), which is thought to mediate Alzheimer's disease by chemical and physical mechanisms, such as metal binding, reactive oxygen species production, and direct lysosomal membrane damage. However, this view does not explain why Abeta is produced constitutively by normal cells, nor what its physiological function is. Furthermore, these theories are incompatible with our recent finding that a point mutation in the intracytoplasmic domain of the amyloid precursor protein (APP), which has no effect on the accumulation of Abeta, nevertheless completely suppresses the AD phenotype in transgenic mice (Galvan et al., 2006;Saganich et al., 2006;Galvan et al., 2008). Our results suggest a completely new view of Alzheimer's disease as an imbalance in physiological signaling pathways that serve to mediate plasticity. APP interacts with competing ligands, mediating both neurite retraction and neurite extension: when APP interacts with netrin-1 (Calheiros et al., in press), it mediates neurite extension and cell survival;however, Abeta competes with netrin-1, and when Abeta binds APP it mediates neurite retraction, synaptic re-organization, and ultimately neuronal programmed cell death. Interestingly, the effect of Abeta shows positive feedback: when Abeta binds APP, the processing of APP leads to further production of Abeta, creating a "prionic" effect by a novel mechanism: inhibition of alpha-secretase, which would otherwise cleave APP and preclude Abeta formation. This finding also implies that netrin-1 is an "endogenous anti-prion" and a molecule that enhances this process, p75NTR, is "prionogenic". We propose to test the ramifications of this model and these preliminary data, using four specific aims: (1) Evaluate the "prionic" nature of Abeta via the proposed novel mechanism of protease inhibition. (2) Determine whether the observed reduction in sAPPalpha with Abeta, and increase in sAPP with netrin-1, represent direct or indirect effects of these ligands of APP. (3) Evaluate the structural effects of Abeta interaction with APP. (4) Evaluate the downstream signaling effects mediated by APP in response to the two antagonistic ligands, Abeta and netrin-1. These proposed studies are designed to test the model that has been developed based on the emerging data indicating a key role for APP signaling in AD. Since there is still no truly effective therapy for AD, it is critical that alternative models such as this be explored, and their tenets and implications confirmed or refuted. Furthermore, research in this area may ultimately lead to new insights into other neurodegenerative diseases, such as Parkinson's and amyotrophic lateral sclerosis, as well.
Alzheimer's disease represents one of the most significant healthcare problems in the U.S., and development of effective therapy will be facilitated by understanding its underlying mechanism. We have provided evidence that the prevailing view of Alzheimer's disease is incorrect, and have developed a new model that offers insight into the mechanism and potential treatment of this important and common disease.
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