Pro-inflammatory and Homeostatic Functions of Huntington’s Disease Microglia. Modulation by Ganglioside GM1
Creators: Steinberg, Noam
Publisher: name: University of Alberta Library
Year: 2023

<p>Huntington&#39;s disease (HD) is a hereditary neurodegenerative disorder characterized by motor, cognitive, and psychiatric symptoms, and caused by an expanded polyglutamine track in the huntingtin (HTT) gene. While early studies mainly focused on neurons, emerging evidence has highlighted the involvement of other cell types in the disease. Microglia, the immune cells of the brain, play a crucial role in maintaining brain health by fighting photogenes, removing damaged cells, remodeling synapses, and secreting molecules that modulate various neuronal functions and survival. In HD, they have been shown to be activated throughout the course of the disease, including prior to its onset. However, it remains unclear whether this activation is due to microglial expression of mutant HTT (mHTT) or is secondary to neurodegeneration.<br>
To address this question, we isolated neonatal primary microglia from Q140/140 knock-in mice, which express full-length mHTT from its endogenous gene locus and thus closely resemble the genetic makeup of the human disease. Our findings demonstrated that Q140/140 and wild-type (WT) Q7/7 microglia display comparable activation and deactivation kinetics in response to a range of pro-inflammatory stimuli. Similarly, monocyte-derived macrophages from HD patients and control subjects had similar response to a pro-inflammatory stimulus.<br>
Within the brain, microglia are exposed to repetitive stimuli rather than isolated events, highlighting a critical role for the development of innate immune tolerance in limiting chronic inflammation and potentially reducing tissue damage in neuroinflammation and neurodegeneration. Our experiments revealed that while Q7/7 microglia can acquire a tolerant state in response to repeated stimulations, Q140/140 microglia fail to develop full tolerance. <br>
Alongside the exaggerated pro-inflammatory response, it has been proposed that in neurodegenerative conditions, microglia also lose other functions necessary for brain homeostasis and recovery. Therefore, we investigated whether 1) chemotaxis, 2) phagocytosis and 3) response to anti-inflammatory cytokines are affected in HD microglia prepared from Q140/140 and transgenic YAC128 mice.<br>
Microglial chemotaxis and phagocytosis were affected by the strain and genotype of the HD models used. Although no differences in the wound closure kinetics were observed between Q7/7 and Q140/140 microglia in a wound scratch assay, chemotaxis toward ATP was impaired in Q140/140 microglia. Phagocytic activity toward different substrates was generally normal in HD microglia, although YAC128 microglia exhibited a specific defect in the phagocytosis of apoptotic bodies. Finally, treatment with anti-inflammatory cytokines elicited a similar expression of anti-inflammatory genes in Q140/140 microglia compared to Q7/7 microglia, and in human HD macrophages compared to healthy controls, but a specific increase in Ym-1 expression in Q140/140 microglia suggest that subtle differences between control and HD microglia might exist that warrant further investigation. <br>
Neuroinflammatory changes occurring in HD brains might contribute to disease progression and are considered a potential target for intervention in HD. We recently showed that ganglioside GM1, a sialic acid containing glycosphingolipid that is highly enriched in the brain, has anti-inflammatory effects on both murine and human control microglia, suggesting that GM1 could potentially reduce neuroinflammation in HD. <br>
Our data support this hypothesis and show that GM1 can attenuate the expression of pro-inflammatory cytokines in both HD microglia and human HD macrophages. It also prevented re-activation of microglia exposed to repeated pro-inflammatory stimuli. On the other hand, GM1 did not promote the acquisition of an anti-inflammatory phenotype. GM1 also promoted migration and chemotaxis in both WT and HD microglia, although these effects appeared to be mouse strain- and genotype-dependent. Additionally, GM1 modulated microglia phagocytic activity in different directions depending on the material being phagocytosed and the species and mouse strains from which microglia were obtained.  </p>

<p>Collectively, our findings suggest that a defect in the ability of HD microglia to develop tolerance, rather than cell-autonomous spontaneous activation or stimulus-induced overactivation of naïve HD microglia, may contribute to chronic inflammation in HD brains. Additionally, impaired microglia mobility and decreased phagocytosis in inflammatory conditions might further hinder microglial response to neuronal damage. Treatment with GM1 reduces microglial activation, promotes mobility, and modulates phagocytosis. These combined effects of GM1 may reduce the contribution of microglia to HD pathogenesis and may hold therapeutic potential not only in HD but also in other neurodegenerative conditions with an inflammatory component.</p>