Apolipoprotein E in Alzheimer's Disease: Cellular Mechanisms

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Apolipoprotein (apo) E4 is a major risk factor or susceptibility gene for Alzheimer's disease (AD). Although the pathogenic mechanisms are unclear, our findings during the preceding funding period, and findings reported by others, suggest that apoE4¿with its multiple cellular origins and multiple structural and biophysical properties¿contributes to AD by interacting with different factors through various pathways, some of which are amyloid-(3 (A|3) dependent and others are not. Although the A|3-dependent roles of apoE4 in AD pathogenesis have been widely studied and much valuable information generated, the A|3-independent roles of apoE4¿the focus of the current proposal¿have drawn less attention and have been understudied. In the central nervous system (CNS), apoE is produced by several types of cells, including astrocytes, activated microglia, and injured neurons. Emerging evidence suggests that neuron-generated apoE and astroycte-generated apoE have distinct roles in physiological and pathophysiological pathways, including AD pathogenesis. Thus, understanding how apoE expression is regulated in CNS neurons should provide fundamental insights into the varied effects of apoE isoforms in neurobiology and neurodegeneration. This proposal builds on four findings during the preceding funding period. First, CNS neurons express apoE in response to injury. Second, neuronal expression of apoE after injury is regulated by an astrocytederived factor (or factors) that controls intron-3 retention/splicing of the apoE gene. Third, apoE4 is more susceptible than apoES to neuron-specific proteolysis, and the resulting fragments cause AD-like neurodegeneration and behavioral deficits in transgenic mice. Fourth, in transgenic mice, pan-neuronal expression of apoE4 or its fragment causes learning and memory deficits and early neuronal deficits in the entorhinal cortex, subiculum, and hippocampus, suggesting selective vulnerability of specific CNS neurons. The goal of this project is to study the regulation of apoE expression in CNS neurons and to explore Apindependent, isoform-specific roles of apoE in the pathogenesis of AD. Specifically, we will explore the regulation of apoE expression in CNS neurons (Aim 1); determine if inhibition of proteolysis reduces or abolishes apoE4-related detrimental effects in transgenic mice (Aim 2); and explore the mechanisms underlying the selective vulnerability of different types of CNS neurons to apoE4 and its fragments (Aim 3). These studies, involving both in vitro and in vivo approaches, will provide insights into the regulation and role of apoE4 in both health and disease and may identify new therapeutic targets for apoE4-associated neurodegenerative disorders, particularly AD.