Murray, Christina Elizabeth;
(2018)
The role of TREM2 in neurodegeneration.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Introduction: Alzheimer’s disease (AD) is the most common neurodegenerative disease and has a high prevalence worldwide. Neuroinflammation has long been known to play a role in AD. However, the findings that several genes associated with inflammation were identified as hits in AD GWAS studies brought closer attention to neuroinflammatory mechanisms in AD. TREM2 was identified as a genetic risk factor for late onset AD with a similar odds ratio to that of APOE4. TREM2 is expressed on microglia, and has been shown to be upregulated on the microglia surrounding amyloid plaques both in human post mortem tissue and AD mouse models. In this thesis, the AD pathology, microglial phenotype, genetic inflammatory profile and proteomic profile of six TREM2 variant cases (5 R47H and 1 D87N variant) were investigated and compared to sporadic AD (SAD), familial AD (FAD) and neurologically normal control cases with the hypothesis that the TREM2 variant cases will differ from both SAD and FAD cases. Materials and Methods: Immunohistochemistry was performed on the frontal cortex, temporal cortex, hippocampus, putamen and cerebellum of SAD (n=19), FAD (n=11), TREM2 variant SAD (n=3), TREM2 variant controls (no AD pathology, n=2) and neurologically normal controls (n=6) using antibodies against Aβ, tau (AT8) and microglia (Iba1, CD68, CR3-43 and P2RY12). Microglial load/area, circularity and perimeter scores were calculated for all microglial markers. The frontal cortex was homogenised from a subset of each group and RNA and protein extracted. Nanostring’s Human Inflammation panel with their nCounter Technology was used to determine the genetic profile. The proteomic profile was assessed using label-free quantitative mass spectrometry. The pathological and proteomic profile of the presubiculum was investigated using immunohistochemistry, matrix-assisted laser desorption ionisation mass spectrometry, laser-capture microdissection and further label free quantitative mass spectrometry and compared to the neighbouring area, the entorhinal cortex to assess whether it has protective properties against neurodegeneration. Results: TREM2 variant AD cases differed from other SAD and FAD cases with a significantly increased CD68 load, more circular Iba1, CR3-43 and CD68 microglial positivity suggesting the microglia were in a phenotype more consistent with phagocytosis. Furthermore, these cases showed an increased upregulation of neuroinflammatory processes and neurodegenerative processes at the genetic and proteomic level than SAD cases. TREM2 variant controls however, showed large levels of downregulation in these processes compared to all groups. APOE genotyping identified the TREM2 variant cases correlated with the presence of the ApoE4 isoform. Investigation of the presubiculum area identified a large non-fibrillar Aβ deposit that contained significantly less NFT’s, activated microglia and N-terminally truncated Aβ peptides than in the entorhinal cortex and had an altered proteomic profile more comparable to the TREM2 variant controls than any other AD cases. Conclusions: Overall, this thesis has shown that TREM2 variant cases posess differences in microglial phenotype, genetic and proteomic expression compared to either sporadic or familial AD cases. TREM2 variant controls show altered pathology and genetic profiles compared to TREM2 variant SAD cases and it can be hypothesised that these cases may use similar mechanisms to the neuroprotection observed in the presubiuclum of AD cases. TREM2’s link to APOE and the fact that the APOE genotype lacks an ApoE4 allele in TREM2 variant controls, indicates that APOE may be exerting this change between TREM2 variants, leading us to question whether the TREM2 R47H variant is acting independently. Further investigations into these pathways and the differences between TREM2 variants that develop disease and those that don’t may lead us to the mechanisms that can be targeted for treatments.
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