Dysregulation of Protein Prenylation in the Pathogenesis of Alzheimer's Disease

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PROJECT SUMMARY The pathogenic mechanisms underlying the development of Alzheimer’s disease (AD) remain elusive. Intriguingly, emerging evidence indicates that a posttranslational lipid modification of proteins, known as prenylation, may play an important role in the pathogenesis of AD. Prenylation reactions are catalyzed by prenyltransferases that attach isoprenoids, either farnesyl or geranylgeranyl pyrophosphate, to proteins with a characteristic C-terminal motif. The lipid prenyl group facilitates anchoring of proteins in cell membranes and mediates protein-protein interactions. Prenylated proteins, including small GTPases comprising the Ras superfamily along with heterotrimeric G-proteins, are involved in regulating diverse cellular processes. In AD, the role of protein prenylation is underexplored. Several studies show that modulation of protein prenylation influences amyloid-β (Aβ) and tau levels, neuroinflammation, synaptic plasticity, and cognitive function. However, most of those studies were conducted in vitro and by pharmacological inhibition of isoprenoid production rather than direct modulation of protein prenylation. Using a direct genetic approach, our recent studies show that the two protein prenylation pathways play distinct neurophysiological roles. Reducing protein farnesylation rescues cognitive function as well as attenuates Aβ pathology whereas reducing protein geranylgeranylation results in adverse effects on synaptic function. In addition, our preliminary studies show that the level of membrane-associated H-Ras, an exclusively farnesylated protein, is significantly increased in the brain of patients with mild cognitive impairment (MCI) and AD compared to individuals with normal cognitive function; and the level of farnesylated H-Ras correlates significantly with the activation of ERK, a major downstream effector of H-Ras. Thus, we hypothesize that upregulation of protein farnesylation is an early event with primary importance in the pathogenic cascade of AD and activation of downstream signaling pathways contributes to the development of cognitive impairment and neuropathology. This central hypothesis will be tested rigorously by three specific aims using a combination of innovative genetic, behavioral, electrophysiological, and prenylomic approaches. Aim 1 is to test the hypothesis that neuron-specific deletion of protein farnesylation mitigates cognitive deficit, synaptic dysfunction, and pathology in a mouse model of AD. Aim 2 is to test the hypothesis that deletion of H-Ras improves whereas over-activation of H-Ras exacerbates cognitive/synaptic deficit and pathology in AD mice. Aim 3 is to elucidate the relationship between AD pathology and the levels of prenylated proteins in mouse models and human brain tissues. The overall goal of the project is to unravel the underexplored but potentially critical pathogenic mechanisms of AD involving protein prenylation and related pathways. Clarifying the linkage between prenylation and AD could be a transformative discovery and lead to the development of new therapeutic strategies.