Deletion of Microglia PDCD4 Alleviates Neuroinflammation-Associated Depression by Promoting DAXX-Mediated PPARγ/IL-10 Signaling

Background

In recent years, neuroinflammatory processes have been demonstrated to be associated with various psychiatric disorders, including schizophrenia, depression, and anxiety. A common feature of these diseases is the imbalance between pro-inflammatory and anti-inflammatory processes in the brain. Accumulating evidence suggests that inflammatory factors such as interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNF-α) are involved in the pathophysiology of affective disorders. However, despite the extensive research focusing on the role of pro-inflammatory factors in depression, the mechanisms of anti-inflammatory factors and their imbalance with pro-inflammatory factors remain to be further explored.

Inflammatory stimuli are particularly sensitive to the resident macrophages in the brain—microglia. Microglia are activated under stress conditions and participate in the regulation of emotional behaviors through activity-dependent synaptic pruning. Peripheral lipopolysaccharide (LPS) acts as an activator of microglia, induces pro-inflammatory response and produces depression-like behavior in mice. Long-term use of microglial inhibitors, such as minocycline, can prevent depression-like behaviors induced by neuroinflammation in mice. Additionally, Peroxisome proliferator-activated receptor gamma (PPARγ), a member of the nuclear receptor family, has gained attention for its important role in depression.

Programmed cell death 4 (PDCD4) is an apoptosis-related molecule widely involved in tumorigenesis and inflammatory diseases. Previous studies have indicated that PDCD4 expression is elevated in the brains of depressed patients and acts by blocking brain-derived neurotrophic factor (BDNF) and upregulating the pro-inflammatory response in chronic stress-induced depression-like behaviors. However, the specific function of PDCD4 in neuroinflammation is still unclear.

This study aims to explore the role of microglia-specific PDCD4 in inflammation-related depression and its potential mechanisms.

Source of the Paper

This paper was written by Yuan Li, Bing Zhan, Xiao Zhuang, Ming Zhao, Xiaotong Chen, Qun Wang, Qiji Liu, and Lining Zhang (corresponding author), and published in the Journal of Neuroinflammation, 2024, Volume 21, Issue 143.

Research Process

1. Alleviation of Neuroinflammation-Associated Depression by Deletion of Microglia PDCD4

a. Experimental Design and Model

To investigate microglia PDCD4 in mediating inflammation-related depression, researchers administered LPS via intraperitoneal injection to adult male mice to simulate a depressive state. Depression-like behaviors in mice were validated using the tail suspension test (TST) and forced swimming test (FST).

Using genetic engineering techniques, researchers generated conditional microglia PDCD4 knockout mice (PDCD4 mcko mice) and conducted a series of biochemical and behavioral tests.

b. Experimental Results

The results showed that the PDCD4 mRNA and protein levels were significantly elevated in the prefrontal cortex (PFC) of mice with LPS-induced depression-like behavior. However, the deletion of PDCD4 in microglia significantly alleviated LPS-induced depression-like behavior. This protective effect was achieved through microglia-specific PDCD4 deletion.

Immunofluorescence staining experiments revealed increased expression levels of PDCD4 in the PFC and hippocampus with LPS treatment. Behavioral tests showed that LPS-treated PDCD4 mcko mice exhibited significantly reduced immobility time in both the forced swimming test and tail suspension test, and a significantly increased sucrose preference in the sucrose preference test, indicating alleviated depression-like behavior.

2. Microglia PDCD4 Deletion Alleviates LPS-Induced Microglial Activation by Promoting PPARγ Signaling

a. Exploration of Molecular Mechanisms

To explore the molecular mechanisms underlying the observed anti-depressive effects of microglia PDCD4 deletion, researchers conducted RNA sequencing analysis. The results showed 586 differentially expressed genes in the PFC of LPS-treated PDCD4 mcko mice. KEGG pathway analysis revealed significant changes in heat generation and oxidative phosphorylation. Gene Set Enrichment Analysis (GSEA) indicated that the PPAR signaling pathway was enriched in LPS-treated PDCD4 mcko mice, suggesting PPARγ may be a key regulatory gene.

Further experiments showed that PPARγ mRNA and protein levels were significantly reduced in the PFC of LPS-treated control mice, but did not decrease in PDCD4 mcko mice. The PPARγ inhibitor GW9662 was able to eliminate the anti-depressive effect of microglia-specific PDCD4 deletion under LPS treatment.

3. PDCD4 Inhibits PPARγ Nuclear Expression by Blocking PPARγ and DAXX Interaction

To investigate how PDCD4 regulates PPARγ function, researchers conducted experiments using primary mouse microglial cells. The results showed that PPARγ protein levels did not change with PDCD4 deletion, but significantly increased after 24 hours of LPS stimulation.

Immunoprecipitation experiments showed that DAXX interacted with PPARγ, and further explored PDCD4’s effect on PPARγ-DAXX complex formation. The results indicated that PDCD4 overexpression decreased the PPARγ and DAXX interaction, while PDCD4 deletion enhanced their interaction, thereby promoting PPARγ nuclear translocation.

4. Microglia PDCD4 Deletion Alleviates LPS-Induced Depression-like Behavior by Rescuing IL-10 Expression

Researchers injected IL-10 neutralizing antibody IL-10rα via intracerebral ventricular injection and found that IL-10 neutralization significantly prevented the improvement in depression-like behavior in LPS-treated microglia PDCD4 mcko mice, indicating the important role of IL-10 in PDCD4 deletion’s antidepressant effect.

Research Conclusion

This study provides direct evidence indicating that microglia PDCD4 mediates neuroinflammation-associated microglial activation and subsequent depression-like behaviors by interrupting PPARγ-mediated IL-10 transcription.

Scientific Value and Application Prospects

This study reveals the role and molecular mechanisms of microglia PDCD4 in neuroinflammation and depression, highlighting the potential of PDCD4 as a therapeutic target for neuroinflammation-associated depression, providing new ideas for developing novel antidepressant treatments.

Research Highlights

1. Mechanism Innovation: Revealed the key role of PDCD4 via DAXX-mediated PPARγ/IL-10 signaling pathway in inflammation-related depression.
2. Behavioral Verification: Confirmed the alleviation effect of microglia PDCD4 deletion on LPS-induced depression-like behavior across multiple behavioral tests.
3. Biochemical Experiments: Explored the related molecular mechanisms through RNA sequencing, immunofluorescence, immunoprecipitation, and other techniques.