Biology and Biomedical Sciences: Neurosciences
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
Alzheimer's disease is a progressive, neurodegenerative disease characterized by several pathological lesions, one of which is the accumulation of the amyloid-β: Aβ) peptide into extracellular amyloid plaques. Several autosomal dominant mutations have been shown to cause familial forms of early-onset Alzheimer's disease, but factors that modulate the risk and onset for the more common sporadic, late-onset Alzheimer's disease are less understood. The strongest identified genetic risk factor for sporadic, late-onset Alzheimer's disease is the APOE e4 allele, the presence of which dramatically increases risk and hastens the onset of the disease relative to non-carriers of the allele. Evidence that APOE e4-carriers exhibit accelerated onset of amyloid accumulation has led to the hypothesis that APOE genotype differentially modulates AD risk and onset via regulation of Aβ metabolism. Thus, we sought to characterize the extent to which modulating the concentration and isoform of apoE regulates brain Aβ metabolism. To this end, we created transgenic mice overexpressing the low-density lipoprotein receptor: LDLR), a major receptor for apoE in the central nervous system, which led to a significant reduction of brain apoE concentration. After crossing these mice to a mouse model of β-amyloidosis, the resulting mice exhibited a marked reduction in Aβ deposition. To examine the mechanism by which Aβ deposition is reduced with increased LDLR expression, we performed in vivo microdialysis in young mice, finding that early decreases in the steady state concentration of Aβ in the brain interstitial fluid: ISF) could be explained by enhanced Aβ clearance. To further investigate the mechanism by which LDLR regulates Aβ metabolism prior to amyloid plaque deposition, we utilized a brain Aβ efflux method to show that LDLR overexpression increased Aβ clearance from the brain. To complement this approach, we developed a novel method to directly assess the plasma appearance rate of brain-derived Aβ, which revealed that LDLR overexpression increased brain to blood efflux of Aβ. We next examined the role of specific apoE isoforms in modulating amyloid accumulation in humans and in a mouse model of β-amyloidosis in which human apoE isoforms were expressed. We found that cerebral Aβ deposition varied in both humans and in mice in a manner that corresponded to APOE genotype. Using in vivo microdialysis, we found in both young and old mice that the concentration and clearance of Aβ from the ISF differed according to the isoform of apoE expressed. In vivo stable isotopic labeling kinetics experiments in young mice revealed that fractional synthesis rates of Aβ did not vary according to human apoE isoform. Moreover, we infused recombinant apoE particles during in vivo microdialysis to demonstrate that intrinsic differences in apoE isoforms contribute to differences in the steady state concentration of ISF Aβ. Together, these results suggest a mechanism by which APOE alleles modulate AD risk through differential regulation of brain Aβ clearance. Furthermore, our results suggest that apoE receptors and other molecules involved in Aβ clearance may represent useful therapeutic targets for AD prevention.
Castellano, Joseph, "The Role of Apolipoprotein E Concentration and Isoform in Amyloid-beta Metabolism In Vivo" (2011). All Theses and Dissertations (ETDs). 558.