Date of Award
Doctor of Philosophy (PhD)
APOE4 is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). APOE4 increases brain amyloid-β (Aβ) pathology relative to other APOE isoforms. However, whether APOE independently influences tau pathology, the other pathological hallmark of AD and other tauopathies, or tau-mediated neurodegeneration, is not clear. By generating P301S tau transgenic mice on either a human APOE knock in (KI) or APOE knockout (KO) background, we show that the presence of human APOE, regardless of APOE isoforms, leads to various degrees of brain atrophy in 9-month old P301S mice, whereas APOE ablation strongly protects against neurodegeneration. In particular, P301S/E4 mice develop significantly more brain atrophy compared with P301S/E2 and P301S/E3 mice. Concomitantly, APOE-sufficient P301S mice develop ptau staining patterns distinct from P301S/EKO mice, with P301S/E4 mice displaying a yet another different pattern from P301S/E2 and P301S/E3 mice that indicates severe neurodegeneration. In diseased P301S mouse brain, APOE specifically binds to a high-molecular-weight tau species and may affect tau pathogenesis through direct APOE-tau interaction. APOE level strongly correlates with ptau levels, and is associated with less soluble tau and more insoluble tau. In addition to affecting tau pathology and neurodegeneration, APOE has potent immunomodulatory functions. In vitro, E4-expressing microglia produce more proinflammatory cytokines following LPS treatment compared with E2- and E3-expressing microglia. In vivo, P301S/E4 mice show markedly enhanced neuroinflammation compared to P301S/E2 and P301S/E3 mice, whereas P301S/EKO mice are largely protected from the change. Co-culturing P301S tau-expressing neurons with mixed glia (80-90% astrocytes + 10-20% microglia) derived from human APOE KI and APOE KO mice results in a marked neuronal loss in the neuron/E4 co-culture with a concomitant elevation of TNFα level in the medium compared to neuron/E2 and neuron/E3 co-cultures, whereas neurons co-cultured with EKO glia exhibit the greatest viability with the lowest level of secreted TNFα. In contrast, treatment of P301S neurons with recombinant APOE (E2, E3, E4) in the absence of glial cells only leads to a mild neuronal impairment compared to the absence of APOE, indicating that APOE regulates neurodegeneration in part through glia-mediated neuroinflammation. The effect of APOE on glia-mediated neuronal loss requires direct contact between neurons and glia cells, and APOE may serve as an opsonin bridging stressed neurons that present ‘eat-me’ signals on cell surfaces with microglia by binding to triggering receptor expressed on myeloid cells 2 (TREM2) to promote neuronal phagoptosis by microglia. APOE is also required for microglial polarization into a neurodegenerative phenotype characterized by a proinflammatory profile and enhanced phagocytotic activity. Depleting microglia from mouse brain using the colony-stimulating factor 1 receptor (CSF1R) inhibitor - PLX3397, rescues neurodegeneration in TE4 mice, but shows no impact on TEKO mice, indicating critical roles of microglia in neurodegeneration regulated by APOE. Reducing APOE levels in P301S mice by overexpressing an APOE receptor, the low-density lipoprotein receptor (LDLR), significantly attenuates neurodegeneration and shifts the ptau staining towards an early-disease pattern. In addition, LDLR overexpression leads to a significant reduction of ptau levels, similar to that observed in P301S /EKO mice.
Chair and Committee
David M. Holtzman
Celeste M. Karch, Jin-Moo Lee, Daniel S. Ory, Conrad C. Weihl,
Shi, Yang, "The Role of Apolipoprotein E in Regulating Tau Pathogenesis and Neurodegeneration in a Tauopathy Mouse Model" (2018). Arts & Sciences Electronic Theses and Dissertations. 1708.