PPARδ activation in microglia drives a transcriptional response that primes phagocytic function while countering inflammatory activation

Introduction/Background:

As the resident immune cells of the central nervous system, microglia serve a protective role in the normal functioning of the brain. However, microglia have also been shown to contribute to the progression of neurodegenerative disease, such as Alzheimer’s disease (AD). The characterization of microglia in these disease contexts is needed, as it is becoming more clear that microglia can exhibit many different functional states– meaning they can take on various roles or behaviors– based on their morphology and gene expression profile. 

 

PPARδ is a member of the PPAR transcription factor family that is highly expressed by microglia. PPARs are lipid sensors and mediate inflammation. Agonists for PPARδ specifically have been tested as treatments for Huntington’s disease (HD) and sporadic AD. Although PPARδ has shown potential in this regard, it is still not entirely clear how it modifies the function of microglia. The authors performed several different experiments to establish the effects of PPARδ activation on microglia.

 

Why I highlight this work:

Microglia identity is an area of research that has evolved extensively over recent years, transitioning from the idea of dichotomic categories to the understanding that a spectrum of microglia states exists. Research on microglia phenotypes in the context of various diseases has always been interesting to me, especially because I study cellular changes in the brain during the process of aging and in vascular diseases. I found that this study thoroughly investigates the role of the transcription factor PPARδ, and presents it as a promising therapeutic target in microglia for AD.

 

Key Findings:

PPARδ agonism in mice increases markers of phagocytosis while decreasing markers of inflammation and migration

In their initial in vivo experiment, mice treated with a PPARδ agonist showed significantly up- and down-regulated genes in bulk RNA sequencing. From gene ontology analysis, they determined that many down-regulated genes were overall related to immune activation and cell migration, suggesting that these processes lessen. Looking at specific regulated genes that were significantly regulated also demonstrated an increase in phagocytosis and decrease in neuroinflammation.

 

PPARδ agonism reduces inflammation and migration in vitro

In a complementary in vitro study, the authors differentiated induced pluripotent stem cells (iPSCs) into microglia. They treated the resulting microglia with lipopolysaccharide (LPS) to induce an inflammatory response. Microglia that were co-treated with PPARδ exhibited a decrease in the release of inflammatory cytokines. A migration assay with the cultured cells also revealed less migration in microglia pre-treated with PPARδ, consistent with their in vivo RNA sequencing results. They also found that PPARδ agonist-treated microglia showed increased phagocytosis of fluorescently labelled rat synaptosomes and amyloid beta. 

 

Additionally, the authors used a  novel approach to assess the microglial response to a broader neurodegenerative injury. Induced microglia exposed to “brain powder” in combination with PPARδ agonist showed significant and opposing transcriptome differences compared to those treated with PPARδ antagonist. Again, genes upregulated with PPARδ agonism were associated with decreased inflammatory pathways and migration and increased lipid metabolism.

 

PPARδ interacts with and modulates PU.1 in vitro

The authors took a closer look at the relationship between PPARδ and the transcription factor PU.1, which regulates many of the inflammatory genes they found to be downregulated by PPARδ. Co-expression and co-immunoprecipitation revealed an interaction between the two proteins. Further, PPARδ agonist reduced the interaction between PPARδ and PU.1 and the activity of PU.1 in vitro. 

 

Inflammatory markers decrease with PPARδ agonism in HD and tauopathy mice 

Bringing in more clinical relevance, the authors returned to an in vivo system to examine how PPARδ agonism affects inflammation in both a HD and tauopathy mouse model. In both models, as well as in wildtype mice, they found a decrease in inflammatory genes upon treatment with PPARδ agonist.

 

Questions and Future Directions:

1. Do you have any idea whether there may be proteins that could interact with PPARδ to mediate the changed expression of genes related to other processes of interest, such as migration, lipid processing, and phagocytosis?
2. Have you done in vitro experiments with microglia co-cultured with neurons and other glial cells? What differences would you expect to observe when including these other cell types? 
3. Given your results looking at phagocytosis of AD-relevant substrates, do you plan to look at PPARδ agonism in an amyloid beta mouse model?

Tags: alzheimer's, inflammation, microglia, phagocytosis

doi: https://doi.org/10.1242/prelights.42921

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