Role of Microglia in Neurodegeneration -Effect of ApoE

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Project Summary Like Alzheimer's disease, prion diseases (prionoses) are conformational disorders, in which deposition of misfolded proteins is accompanied by microglia neurodegenerative phenotype (MGnD) displaying mixed phagocytic and inflammatory properties. Prion mouse models with misfolded protein driven neurodegeneration and activated glia response can be used to uncover Alzheimer's disease relevant pathomechanisms. APOE ε4 allele is the foremost risk factor in sporadic Alzheimer's disease, increasing its odds by 3 and 15-fold in hetero and homozygotes, respectively. APOE encodes apolipoprotein (apo) E, which controls brain lipid homeostasis and modulates Alzheimer's disease risk through isoform-specific effect on clearance and deposition of soluble Aβ. ApoE is expressed by astrocytes in form of apoE/HDLs and also by MGnD as lipid-poor particles. Differential contributions of these two apoE pools to MGnD properties remain unclear. While lipidated apoE may facilitate clearance of misfolded proteins, expression of lipid-poor apoE by MGnD is linked to their pro-inflammatory properties, which also are differentially controlled by APOE genotype. In prionoses, accumulation of toxic PrPSc protein is the culprit of pathogenesis. Microglia undergo activation early in the course of disease and exert opposing roles in PrPSc mediated neurodegeneration. While clearance of PrPSc has a disease-limiting effect, microglia-driven neuroinflammation is deleterious to neurons. Involvement of apoE in prion pathogenesis has not been established, though there is evidence for ~2-fold higher risk of sporadic Creutzfeldt-Jakob disease (CJD) in ε4 carriers. Our preliminary work shows increased brain apoE level in prion infected mice along with reduced apoE expression in astrocytes and increased expression in activated microglia. The overall effect of apoE in prionoses is beneficial as global Apoe KO exacerbates prion pathology by aggravating the vicious cycle of neuronal death and neuroinflammation. In Apoe-/- mice, clearance of PrPSc and dying neurons by MGnD becomes inefficient while neuronal debris exaggerate MGnD phenotype, release of inflammatory cytokines, and induce A1 neurotoxic astrocytes. Our studies also suggest, apoE effect in prionoses is isoform dependent. ε4/ε4 targeted replacement (TR) mice have shorter disease incubation time, increased pathology load and microglia hyperactivation compared to ε3/ε3 and ε2/ε2 mice. This preliminary work led us to hypothesize 1) apoE involvement in prion pathogenesis is by control of microglia response to PrPSc mediated neurodegeneration; 2) apoE-based approaches have therapeutic merit in prionoses; and 3) effect of apoE in human prionoses is isoform dependent. These hypotheses shall be explored in grant's specific aims. Aim I will assess the role of astrocyte vs. microglia-expressed apoE on PrPSc mediated neurodegeneration in mice with cell-specific conditional Apoe knock out. Aim II will assess whether regulating lipidation level of astrocytic and microglial apoE and the total brain apoE level modulates neurodegeneration. Aim III will explore effects of APOE genotype on microglia activation phenotype in prion infected APOE TR mice and in sporadic CJD and Alzheimer's disease patients.