Reactive Astrocytes Worsen Alzheimer’s Disease Pathology

Post by Elisa Guma

What's the science?

Reactive astrocytes are a cellular component of gliosis and often detected in neurodegenerative diseases such as Alzheimer’s disease. There is diversity in their degree of astrocyte reactivity ranging from mild or moderate to severe, reflected in both their morphology and function. Although they are known to play a role in the pathogenesis of Alzheimer’s disease, their function remains elusive due to the lack of appropriate experimental models. This week in Nature Neuroscience, Chun and colleagues demonstrate the importance of these cells in Alzheimer’s disease pathogenesis using a newly developed animal model.        

How did they do it?

The authors developed a novel animal model of toxin-triggered reactive astrocytes by crossing two mouse lines; one expressing receptors sensitive to diphtheria toxin (DTR mouse), the other localizing that expression only to astrocytes (GFAP-CreER mouse) resulting in what they refer to as the GiD mouse. Systemically administering diphtheria toxin to the GiD mice (which binds to the diphtheria toxin receptors on astrocytes) allowed the authors to experimentally control when, and to what severity, reactive astrocytes are induced. First, the authors investigated the dose-dependency of astrocyte reactivity in these mice by either administering the diphtheria toxin for 2 days to induce moderate reactivity, or 16 days to induce severe hypertrophy, and used immunohistochemistry to evaluate astrocyte morphology.

Next, the authors wanted to understand what neurotrophic or neurotoxic factors were released by the moderate or severe reactive astrocytes. Reactive astrocytes have previously been shown to activate the monoamine oxidase B (MAO-B pathway), which causes increased GABA release and increases oxidative stress; to test this, the authors performed microdialysis in the hippocampus to measure GABA and hydrogen peroxide (H2O2; a by-product of oxidative stress) levels following induction of reactive astrocytes. To determine whether hydrogen peroxide was necessary for inducing neuroinflammatory reactions, they administered a hydrogen peroxide blocker and examined the downstream effects on reactive astrocytes.

The authors also wanted to investigate whether reactive astrocyte production of hydrogen peroxide could be associated with Alzheimer’s disease pathology. They evaluated neuronal function (by staining for neuronal markers), the presence of phosphorylated tau protein (involved in neurodegeneration in Alzheimer’s disease), and memory performance in GiD mice who had severe reactive astrocytes. They also overexpressed reactive astrocytes in a commonly used Alzheimer’s disease animal model (APP/PS1 model) which typically lacks many important hallmarks of Alzheimer’s disease such as tauopathy, atrophy, and neuronal death, and measured the downstream molecular and behavioural effects. Finally, to further validate their findings, the authors examined hydrogen peroxide-mediated reactive astrocytes, tauopathy, and neurodegeneration in an in vitro human Alzheimer’s disease brain model, which recapitulates human amyloid-beta pathology, and by immunostaining tissue from the temporal cortex of individuals with AD to determine whether markers of reactive astrocytes and oxidative stress were present.

What did they find?

The authors confirmed that following diphtheria toxin injection, their GiD mouse model presented reactive astrocyte hypertrophy, rather than cell death, in several brain regions including the cortex, hippocampus, striatum, and amygdala. Astrocyte hypertrophy was sensitive to dose such that longer activation via diphtheria toxin administration (16 days) led to a greater number of reactive astrocytes with a greater degree of branching than a short administration (2 days) resulting in severe vs. moderate reactivity. 

Next, when severe reactive astrocytes activate the MAO-B pathway, the authors detected an increase in hydrogen peroxide production, but not GABA, in the hippocampus, reflective of an increase in oxidative stress. Further, the authors found that hydrogen peroxide production was necessary for reactive astrocyte hypertrophy, increased oxidative stress, and for the activation of microglia, as blocking hydrogen peroxide impeded astrocyte reactivity. Furthermore, they found that mice who had severe reactive astrocytes also had disrupted hippocampal pyramidal layer neurons, an increase in phosphorylated tau, and significant memory impairments. These effects were all prevented by blocking hydrogen peroxide, suggesting that it plays a role in neurodegeneration.

Increasing astrocyte reactivity in the APP/PS1 mice caused neuronal degeneration and impaired memory function. This indicates that by introducing severe reactive astrocytes in the APP/PS1 mouse line the missing neurodegeneration-related hallmarks of Alzheimer’s disease can be precipitated, suggesting that severe reactive astrocytes are sufficient for neurodegeneration. To further validate the relevance of their findings to Alzheimer’s disease pathology, they observed that induction of reactive astrocytes in an in vitro human Alzheimer’s disease model led to an increase in hydrogen peroxide production and phosphorylated Tau, which were reversed by administration of a hydrogen peroxide blocker. Finally, the authors detected an increased presence of reactive astrocytes and markers of oxidative stress in post-mortem samples of the temporal cortex from Alzheimer’s disease patients.

What's the impact?

The authors present compelling evidence for the causal relationship between reactive astrocytes and neurodegeneration. They show that excessive hydrogen peroxide production (from monoamine oxidase B) in severe reactive astrocytes leads to many pathological changes relevant to Alzheimer's disease, such as glial activation, tauopathy, neuronal death, and memory deficits. These findings were recapitulated in an Alzheimer’s disease culture model, another Alzheimer’s disease mouse model, and in the brain of Alzheimer’s disease patients, providing further evidence for reactive astrocytes’ critical role in neurodegeneration.

Chun et al. Severe reactive astrocytes precipitate pathological hallmarks of Alzheimer’s disease via H202-production. Nature Neuroscience (2020). Access to the original publication can be found here.