Identifying targets that lower APP as a therapeutic strategy for Alzheimer's disease

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Project Summary AD, the most common neurodegenerative disease, is characterized pathologically by extracellular aggregation of Aβ [a cleavage product of amyloid precursor protein (APP)] and intraneuronal neurofibrillary tangles consisting of hyperphosphorylated tau. As such, AD belongs to the larger family of neurodegenerative proteopathies, which includes diseases such as Parkinson's (PD) and polyglutamine diseases such as Huntington's (HD) and spinocerebellar ataxia type 1 (SCA1). One of the many challenges facing drug development for AD is the lack of new targets capable of modifying the disease course. Ab is the main target for drugs currently in the pipeline, but it has not yielded any tangible benefits in clinical trials. The goal of this work is to elucidate the possible pathogenic role of a lipid pathway which regulates the levels of APP and may contribute to the development of AD. The idea that the brain is sensitive to the steady-state levels of APP is based on two major lines of evidence. First, studies of other neurodegenerative proteopathies such as PD and SCA1 have established that elevated levels of the disease-driving protein are pathogenic. Second, human genetic studies show that individuals carrying an extra copy of APP (e.g., those with Trisomy 21 or APP locus duplication) develop early-onset AD, but rare cases of partial trisomy 21(PT21), in which APP is not included in the trisomic segment, show no AD neuropathology. Because lowering the levels of a disease-driving protein have shown benefits in mouse models of SCA1, it is worth testing whether lowering abnormally elevated APP levels could prevent Aβ generation and mitigate disease. To identify novel regulators of APP, the Zoghbi lab performed parallel high-throughput shRNA screens in human cells and flies. Combined data from both screens identified a number of genes that, when inhibited, lowered full- length APP levels in a transgenic cell line expressing a fluorescently-labeled APP and mitigated toxicity in flies overexpressing human APP. These candidates were then further tested for their ability to regulate endogenous APP levels in human cells and surprisingly, I identified two genes, ACSL3 and SLC27A1, which function in the same pathway to regulate fatty acid metabolism. To determine if there were additional genes that fell into this pathway, I went back to the original primary cell-based screen data and uncovered a dozen additional genes involved in lipid metabolism which are absent in the fly genome and were therefore not previously validated. Three of these, ACOT8, ACADL, and ACAD10 have now been successfully validated in human cells. The overall objective of this proposal is to test these five candidates in a human cellular model of AD (Aim 1) and in mouse models of AD (Aim 2) to determine if full-length APP levels can be lowered, APP processing altered, amyloid pathology mitigated, and a potential mechanistic link between lipid metabolism and APP uncovered. The discovery of APP regulators will open new areas to understand APP biology, will provide potential therapeutic targets for cases of AD in which APP is increased, and will afford a rich source for genes that might increase vulnerability to AD.