Author ORCID Identifier

https://orcid.org/0000-0003-0170-9014

Date Available

9-1-2024

Year of Publication

2024

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department/School/Program

Neuroscience

First Advisor

Adam Bachstetter

Abstract

Microglia are the primary immune cells of the central nervous system and are crucial in maintaining homeostasis and defense from disease and injury in the brain. Recent genome-wide association studies (GWAS) have also highlighted microglia as a central player in Alzheimer’s disease (AD). However, the mechanisms of microglia’s influence are poorly understood. Traditionally, microglial subtypes characterized physiologically as “reactive” or “activated” have been linked with disease and injury in the brain. However, morphologically, a recently described microglia category known as “dystrophic,” characterized structurally by fragmented processes and cytoplasmic decay is believed to be more strongly associated with aging and neurodegeneration. To test whether age is a mediating factor driving microglia dystrophy, we conducted stereological counts of hypertrophic and dystrophic morphologies across the normally aging lifespan within the hippocampus and frontal cortex. We hypothesized that if age leads to the development of dystrophic microglia, we would observe an age-related increase in counts without disease pathology. As age is a prominent risk factor for developing disease pathology, we also tested whether dystrophic microglia are a disease-associated phenotype that merely correlates with age. To do so, we compared non-diseased cases to cases of Alzheimer's disease (AD), Lewy Body Disease (LBD), and limbic predominant age-related TDP-43 encephalopathy (LATE). The most robust method of identifying dystrophic microglia is brightfield immunohistochemistry. Yet, post-mortem human brain tissue presents distinct technical problems due to fixation procedures and autofluorescence. To address these technical challenges, we developed a multiplex immunohistochemistry approach known as Quantitative multiplex Immunohistochemistry with Visual colorimetric staining to Enhance Regional protein localization (QUIVER). Using this technique, we tested if the presence of amyloid plaques and neurofibrillary tangles (NFTs), characteristic of AD, has any bearing on microglia’s cellular and molecular phenotypes. Dystrophic microglia are a prominent feature in neurodegenerative disease, potentially serving as both markers and mediators. To test the hypothesis that dystrophic microglia are involved in AD progression, we assessed ramified, hypertrophic, and dystrophic microglia populations using stereological sampling and digital pathology from five brain regions of 66 individuals across six disease states, from healthy controls to advanced AD stages, including comparative conditions such as Lewy Body Disease (LBD) and LATE. If the presence of dystrophic microglia is unique to AD neuropathologic change (ADNC), there would be substantially more than in neurodegenerative diseases characterized by other proteins, such as α-synuclein or TDP-43. Recent studies suggest that the interaction between tau proteins and amyloid-β might induce dystrophic changes in microglia. To test whether dystrophic microglia are a symptom or driver of disease pathology, we used mediation and pathway models of analysis. We hypothesized that dystrophic microglia occur in the presence of disease pathology but also mediate the spread of tau pathology throughout the brain in worsening stages of ADNC. Our results show that age is not a driving factor for microglial dystrophy; rather, they are a disease-associated morphology. We propose iron dyshomeostasis in microglia as a potential molecular mechanism driving the degeneration of microglia in neurodegenerative disease. In line with these results, using our QUIVER method, we observed a specific microglia subtype located around amyloid plaques immunoreactive for IBA1, ferritin, and CD68, that are absent around NFTs representing a population of microglia transitioning between reactive and dystrophic. We found a significant increase in dystrophic microglia in areas early affected by ADNC, indicating a disease-specific role in neuropathology. Mediation analysis and structural equation modeling revealed that dystrophic microglia substantially affect the regional spread of ADNC, suggesting their mechanistic role in neurodegeneration. Tau was found to be the primary initiating factor leading to the development of dystrophic microglia, which then was associated with the spread of amyloid-β and tau. These results further support that a loss of microglia's protective role contributes to the spread of ADNC.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.391

Funding Information

This work was supported by National Institutes of Health award numbers: T32 AG078110, R21 AG066865, R01 AG057187, P30 AG072946 and RF1 NS118584

Download

Share

COinS