Neuroimmunological basis of Alzheimer's disease and related dementias

Student thesis: Doctoral ThesisDoctor of Medicine (MD)

Abstract

Background: Neuroinflammation is a common feature of Alzheimer’s disease pathology, which is characterized by the presence of activated astrocytes and activation of the microglia (the brain’s resident macrophages), as well as expression of inflammatory cytokines & complement system activation. Recently, genome-wide association studies (GWAS) have shown an association of risk of sporadic AD with genes linked to innate immunity such as genes related to microglial functions and complement genes. The complement system plays an integral role in the innate immune response; however, the contribution of the immune system and complement activation in glial cell activation and other immune-related changes, such as cytokine release in AD, are not completely understood. Abnormally accumulated misfolded proteins in the brain stimulate different parts of the immune response to clear them. Objectives: The aim of this project was to study the potential contribution of the innate immune response in AD compared with control brain. This involved work to investigate the differences between a number of complement system proteins, astrocyte markers and cytokines using quantitative protein measurements and quantification of gene expression. Another aim was to explore the correlations of these inflammatory changes with each other and with AD-related protein changes and other related confounding factors. Method: To achieve these objectives, a range of specific inflammatory proteins and genes were quantified in frontal and temporal cortex post-mortem brain tissue from South West Dementia Brain Bank (SWDBB) from cases of neuropathologically confirmed AD and elderly non-demented controls. Tissue was homogenized and used in ELISA assays and qPCR for those measurement. To ensure all of the changes seen were originated from brain tissue, fibrinogen was measured as a marker of Blood-brain barrier disruption. Innate immune markers such as complement cascade proteins (C1q, C3), activation products (iC3b and TCC) and regulators (Clusterin and CFH), astrocyte marker (GFAP), pro-inflammatory cytokines (IL-1β) were measured. Relative gene expression quantification was done to a variety of genes (C1QA, C3, CFH, CLU, CR1 GFAP, IL-1β and TNF-α) in relation to different calibrator genes. Results: There was no significant difference in fibrinogen levels between different groups of the study. Some of the complement proteins and regulators, such as C3, CFH iC3b & clusterin, are increased in one or both brain areas examined in the AD group compared to age-matched controls while others show no significant difference between the groups such as C1q and TCC, as shown in the figure. These changes were related to Aβ load or increased with later Braak stages. Complement genes were also upregulated in frontal brain areas in AD such as (C3 and CLU), while other genes were upregulated in the temporal cortex (C1QA, C3 and CR1). GFAP as a marker of astrocytes was found to be increased in the temporal cortex of AD patients compared to controls, while GFAP relative gene expression was upregulated in both frontal and temporal brain areas. Other inflammatory markers tested as pro-inflammatory cytokines showed a significant decrease in IL-1β levels in AD in both brain areas compared to controls. Moreover, IL-1β relative gene expression was also downregulated in AD brain mRNA. Conclusion: Data from this study support the role of neuroinflammation as an important contributor to AD pathology. Results from human brain tissue are more relevant than those obtained from cell or animal models which may not reflect the complexity of human pathological changes in AD brain. The findings provide novel data in one of the largest cohorts selected to study the inflammatory markers by sensitive and accurate methods for the quantitative measurement of proteins and genes. Moreover, the main conclusion is that the complement system is dysregulated in AD brain, some of the complement system components were found to be correlated with the disease pathology according to Braak staging, however, further studies would be needed to confirm these preliminary results and to try and find neuroinflammation related therapeutic options for treating AD.
Date of Award21 Jan 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SponsorsBritish council in Egypt-Egyptian Government (Newton-Mosharafa Fund)
SupervisorPatrick Gavin Kehoe (Supervisor), Shelley J Allen Allen-Birt (Supervisor) & B Paul Morgan (Supervisor)

Keywords

  • Alzheimer's disease
  • Immunity
  • Complement system
  • Astrocyte
  • Neuroinflammation
  • Cytokines
  • Microglia
  • Innate immune response
  • Blood-brain barrier
  • Fibrinogen
  • Complement activation
  • Alzheimer's brain
  • post-mortem tissue
  • molecular immunology

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