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Study of the role of apolipoprotein E in amyloid plaque deposition in transgenic mice (Bachelor thesis)

Σαμολαδά, Γεωργία - Μαρία/ Samolada, Georgia - Maria

Apolipoprotein E (ApoE) is an amphiphilic glycoprotein with a molecular weight of approximately 34kDa (299 amino acids), which is generated following the cleavage of an 18-residue signal peptide and belongs to the apolipoprotein family. Structurally, this protein consists of an N-terminal receptor-binding domain, a C-terminal lipid-binding domain and a flexible hinge region connecting these two domains. In the body, it is primarily found in very-low-density lipoprotein (VLDL) particles in plasma and highdensity lipoprotein-like (HDL-like) particles in the Central Nervous System (CNS). In the brain, ApoE is mainly secreted by astrocytes, and to a lesser extent by neurons and microglia under stressful conditions, and its lipidation is facilitated by transporters of the ABC family. Furthermore, ApoE mediates the transition of microglia from a homeostatic to a degenerative phenotype via the ApoE-TREM2 signaling pathway. The first evidence of ApoE's involvement in Alzheimer's Disease (AD) emerged from the pioneering study by Wisniewksi et. al., who discovered the co-localization of ApoE within amyloid plaques, demonstrating that its binding to amyloid-beta (Αβ) exhibits an isoform-dependent character. The ApoE4 allele stands as the strongest genetiv risk factor for the sporadic form of AD and excessive research is currently investigating its implication in the pathogenesis of the disease. Therefore, it is deemed essential to study the role of the ApoE protein in Alzheimer's Disease, elucidate the mechanisms involved, and investigate the correlation between ApoE and immune responses, along with their impact on the disease phenotype. In the present study, the impact of ApoE deficiency on the progression of amyloid pathology, neuroglial response, neuronal integrity, and the peripheral immune system were investigated. For this purpose, 4-month-old female mice of the 5XFAD transgenic model carrying a deletion of the ApoE gene (ApoE-/-) were studied, yielding the final genotype 5XFAD/ApoE-/-. Subsequent analyses revealed that these mice exhibit a reduced deposition of amyloid plaques in both the hippocampus and the cerebral cortex compared to 5XFAD control mice. Additionally, decreased astrocyte activation was observed in 5XFAD/ApoE-/- mice. A particularly significant finding was the reduction of microglia, their randomized spatial distribution, and the altered morphology of the amyloid plaques. No difference was observed in neuronal integrity between the models. Finally, splenocyte analysis was performed, revealing a progressive increase in the percentage of peripheral inflammatory myeloid cells (CD11b+Gr1hi). In conclusion, this thesis demonstrates that the absence of ApoE reduces the amyloid load and limits the activation of astrocytes. Simultaneously, it prevents the formation of the protective microglial barrier and induces a more pronounced systemic peripheral immune response. Further research is needed in order to investigate the precise role of this protein and identify the underlying mechanisms.