Myelin dysfunction drives amyloid-β deposition in models of Alzheimer's disease

Alzheimer's disease (AD), the leading cause of dementia, shows a strong correlation with age, yet the underlying mechanisms remain unclear. The amyloid hypothesis posits that amyloid-β (Aβ) plaque accumulation initiates a cascade of events leading to neuronal dysfunction. While aging is the primary risk factor for AD, the connection between brain aging and amyloid deposition isn't fully understood. Myelin, the insulating sheath around axons, plays a crucial role in axonal conduction speed and metabolic support. The unique structure of myelin makes protein and lipid turnover slow, and the long lifespan of oligodendrocytes (myelin-producing cells) might contribute to age-related myelin degradation. This study investigates the hypothesis that myelin breakdown in the aging brain drives Aβ deposition. This is a significant research question because establishing a causal link between myelin dysfunction and AD could open new avenues for therapeutic interventions targeting myelin health to prevent or delay the onset of AD. The existing literature suggests a correlation between myelin damage and AD but lacks conclusive evidence of causality. This research aims to fill this gap by employing mouse models with manipulated myelin function to determine if myelin dysfunction can trigger amyloid deposition, providing direct evidence for a causal relationship. The importance of this study lies in its potential to revolutionize our understanding of AD pathogenesis and pave the way for novel therapeutic strategies.

Macroscopic brain imaging studies have indicated cortical myelin damage in the preclinical phase of AD. However, microscopic evidence supporting this has been scarce. Previous research has shown that myelin, primarily composed of lipids and proteins, is essential for efficient neuronal transmission and metabolic support of axons. Age-related deterioration of myelin has been observed in various studies, and this deterioration can lead to secondary neuroinflammation. Studies have investigated the interaction between amyloid pathology and myelin alteration, but most have focused on the effects of Aβ on oligodendrocytes and myelin, often reporting secondary demyelination or hypomyelination. Single-cell RNA sequencing studies of AD mouse models have revealed altered signatures in oligodendrocytes, primarily near amyloid plaques. Analysis of human AD autopsy samples have identified myelin-related transcripts among the most altered gene clusters. However, these findings might mainly reflect downstream effects of Aβ and tau pathology, lacking direct evidence for a causal link between myelin defects and amyloid deposition.

The study used multiple approaches to investigate the relationship between myelin dysfunction and amyloid deposition. First, immunofluorescence was used to analyze myelin integrity in the trans-entorhinal area of autopsy samples from AD patients and age-matched controls. This allowed for a direct visualization and quantification of myelin density and its association with microglia in human samples. Second, two different mouse models of AD (5xFAD and APP^NL-G-F) were used, and these models were crossed with mice that develop genetic myelin defects (CNP and PLP mutants). This genetic manipulation allowed for the investigation of the effects of myelin dysfunction in the context of AD amyloidogenesis. Light sheet microscopy (LSM) after Congo red staining was employed for unbiased quantification of amyloid plaque load in whole brains. This provided a comprehensive assessment of amyloid burden across different brain regions. To investigate the effect of acute demyelination, 5xFAD mice were fed a cuprizone diet for 4 weeks followed by a 4-week recovery period. Cuprizone induces demyelination, allowing researchers to study the impact of acute myelin loss on amyloid deposition. The use of experimental autoimmune encephalomyelitis (EAE) in 5xFAD mice provided an alternative model of demyelination to further test the hypothesis. Furthermore, forebrain-specific shiverer mice, which lack cortical myelin, were crossbred with 5xFAD mice to examine the effect of near-complete absence of cortical myelin on amyloidosis. High-pressure freezing electron microscopy (EM) was used to analyze ultrastructural changes in myelin-damage associated axonal swellings. Western blot analysis quantified protein levels of APP, its cleavage products, and BACE1 in different brain regions. Microglia were isolated from mouse brains via magnetic activated cell sorting (MACS) for bulk RNA sequencing (RNA-seq) to study changes in gene expression caused by myelin defects. Single-nuclei RNA sequencing (snRNA-seq) further delved into the microglial responses to both amyloid plaques and myelin dysfunction. Behavioral tests, such as the Y-maze and elevated plus maze, evaluated potential behavioral effects of myelin dysfunction and amyloid pathology.

The study revealed several key findings: 1. Immunofluorescence analysis of human AD autopsy samples showed decreased intracortical myelin density in areas beyond immediate plaque vicinity, alongside increased IBA1+ microglia. 2. In mouse models, genetic myelin defects (CNP and PLP mutants crossed with 5xFAD mice) significantly exacerbated amyloid plaque load, particularly in the alveus (hippocampal white matter) and cortex. This effect was most pronounced at 6 months, suggesting a time-dependent relationship between myelin dysfunction and amyloid deposition. 3. Acute demyelination induced by cuprizone or EAE also increased amyloid deposition, particularly in the alveus of cuprizone-treated 5xFAD mice and the spinal cord of EAE-induced 5xFAD mice. 4. The near-complete absence of cortical myelin in forebrain shiverer;5xFAD mice initially reduced amyloid deposition at 3 months, but this effect diminished by 6 months. 5. Myelin defects promoted the accumulation of amyloidogenic machinery (APP, BACE1, and γ-secretase) within axonal swellings, enhancing APP processing and Aβ generation. Western blot confirmed increased BACE1 (white matter) and APP CTFs (cortex). 6. In mouse models with myelin dysfunction, microglia failed to form the typical barrier around amyloid plaques, despite increased microglia numbers. Bulk RNA-seq showed a more inflammatory microglial profile in myelin-defective mice, with increased DAM signature genes. SnRNA-seq identified two DAM clusters: amyloid-DAM (5xFAD) and myelin-DAM (CNP). Microglia appeared diverted towards myelin phagocytosis, neglecting amyloid plaques. 7. Behavioral tests in myelin mutant mice showed hyperactivity, which was further increased in myelin mutant;5xFAD mice suggesting a synergistic effect of myelin defects and amyloid pathology on behavior. Analysis of human AD snRNA-seq data revealed an MS4A gene cluster upregulation in activated microglia, indicating a shared myelin-clearance related signature in both mice and humans.

The findings strongly support the hypothesis that myelin dysfunction acts as an upstream risk factor for amyloid deposition in AD. The exacerbation of amyloid burden in mouse models with myelin defects, even in cases of acute demyelination, provides a clear causal link. The mechanistic insights highlight two crucial aspects: 1) Myelin dysfunction enhances Aβ production by accumulating the amyloidogenic machinery in axonal swellings. 2) Myelin damage distracts microglia from plaque clearance, further contributing to amyloid buildup. The observed altered microglia response, characterized by a distinct DAM-like signature and a lack of plaque corralling, highlights the interplay between myelin damage and the innate immune response in AD pathogenesis. The initial protective effect of near-complete myelin absence in forebrain shiverer;5xFAD mice suggests that healthy myelin initially plays an inhibitory role in plaque formation, but with progression, other mechanisms contribute to increased amyloid. The synergistic effect of myelin defects and amyloid pathology on behavioral deficits further emphasizes the importance of myelin health in the overall AD phenotype. These findings provide a compelling explanation for the strong correlation between age and AD risk, as age-related myelin breakdown could initiate a cascade leading to amyloid deposition.

This study provides strong evidence that age-related myelin dysfunction is an upstream risk factor for amyloid deposition in AD. Myelin defects enhance Aβ production through axonal swellings and impair amyloid clearance by diverting microglia. This suggests that improving oligodendrocyte health and myelin integrity may be a promising therapeutic target for delaying or preventing AD. Future research should focus on translating these findings into human studies and developing therapeutic strategies aimed at preserving myelin health in the aging brain. Further exploration into the specific molecular mechanisms driving microglial diversion and the interaction between myelin damage and amyloid pathology is crucial.

The study primarily utilized mouse models, which may not perfectly replicate the complex pathophysiology of human AD. The sample size for human autopsy analysis was relatively small, limiting the generalizability of these findings. While the study provides strong evidence for a causal link between myelin dysfunction and amyloid deposition, further research is needed to fully elucidate the underlying mechanisms. Moreover, the study focused primarily on the impact of myelin dysfunction on amyloid-β pathology, and the interaction with other AD-related pathologies, like tau tangles, warrants further exploration.