Doctor of Philosophy
Prado, Vania F.
Prado, Marco AM.
Microglia, the resident immune cells of the central nervous system, maintain brain homeostasis and respond to injury and disease. In Alzheimer’s disease (AD), microglia play complex roles, having both positive and negative effects on disease progression. For instance, microglial phagocytosis of toxic amyloid peptides may be beneficial while their excessive release of inflammatory mediators can exacerbate the pathology. Modulating microglial activity to promote the uptake of amyloid peptides while limiting their inflammatory response could provide an effective therapeutic approach to treating AD. Different G protein-coupled receptors (GPCRs) have been shown to modulate microglial activity such as phagocytosis and the release of cytokines in vitro, however their roles in vivo are not entirely understood. To study the role of microglial GPCRs in vivo, we generated mice expressing either hM3Dq or hM4Di Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) selectively in microglia. We aimed to explore whether chemogenetics can be employed to manipulate microglial activity in vivo and whether this approach can be used to modulate neuroinflammation and Ab-clearance in a mouse model of AD. Interestingly, activation of hM3Dq increased the phagocytic activity of primary microglia and promoted their uptake of amyloid oligomers. Furthermore, acute activation of hM3Dq increased the expression of pro-inflammatory cytokines in the brain while repeated clozapine N-oxide (CNO) treatment had an anti-inflammatory effect. Remarkably, repeated hM3Dq activation prior to lipopolysaccharide-induced neuroinflammation attenuated pro-inflammatory cytokine expression and the associated behavioural deficits. In contrast, activation of hM4Di did not modulate the phagocytic activity of microglial nor neuroinflammation in mice. In applying this chemogenetic approach to the AppNL-G-F AD mouse model, chronic activation of hM3Dq or hM4Di did not affect cortical amyloid-beta levels nor plaque deposition. Furthermore, chronic CNO treatment had little effect on the expression of pro-inflammatory cytokines in the cortex of hM3Dq- and hM4Di-AppNL-G-F mice. Interestingly, the subset of microglia expressing hM3Dq or hM4Di were associated closely with amyloid plaques and displayed altered morphology, suggesting that plaques may influence the phenotypes of microglia. Overall, we used chemogenetics to further the understanding of microglial function in vivo under homeostatic and disease conditions.
Summary for Lay Audience
This research uses an innovative technique to study microglia, the brain’s immune cells, and how they respond to changes in the brain.
Microglia, the immune cells of the central nervous system, act as the first line of defense against injury or infection in the brain. Recently, these cells have emerged as key players in neurodegenerative diseases where their function is impaired. For instance, in Alzheimer’s disease, microglia fail to remove toxic protein aggregates from the brain which causes them to mount an inflammatory response. The resulting neuroinflammation and accumulation of protein aggregates in the brain ultimately leads to the death of brain cells.
This research aims to alter microglia function in various contexts with the goal of manipulating microglial activity in a mouse model of Alzheimer’s disease to decrease inflammation and promote the clearance of toxic protein aggregates in the brain.
Microglia constantly survey their surroundings and respond to external stimuli via receptors on their surface. Different types of chemical messengers activate different receptors which in turn initiate specific changes within the cells, altering their function. Here we use mice that have been genetically modified such that their microglia have receptors that respond solely to a drug which can be administered by scientists. Upon treatment with this drug, the receptor activates a specific signaling pathway within microglia, potentially modulating the cell’s function, such as its release of inflammatory mediators or its destruction of harmful pathogens/toxic proteins.
Pharmacological activation of a specific signaling pathway within microglia facilitated the uptake of foreign material by cultured cells and prevented neuroinflammation in mice. However, despite these promising results, activation of this microglial signaling pathway in an Alzheimer’s mouse model had no effect on the levels of aggregated protein clumps or inflammatory mediators in the brain.
This work demonstrates that this pharmacological approach can be used to manipulate microglial activity. This technique could be used to alter the response of microglial in various conditions where their function is abnormal. Overall, this work has furthered the understanding of how microglia function in healthy and diseased brains.
Hogan-Cann, Aja, "Chemogenetic modulation of microglial activity in vivo" (2023). Electronic Thesis and Dissertation Repository. 9322.
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Available for download on Monday, June 23, 2025