5-HT7R Enhances Neuroimmune Resilience and Alleviates Meningitis by Promoting CCR5 Ubiquitination

Academic Background

Bacterial meningitis is a rapidly onset, highly fatal disease with epidemic potential, especially when caused by Streptococcus pneumoniae (S.p.) infection. The destruction of the blood-brain barrier (BBB) leads to the release of inflammatory factors and chemokines, triggering an excessive immune response known as the “cytokine storm.” This overactive immune response not only causes tissue damage but can also lead to neurological sequelae such as cognitive decline and learning impairments. Despite advances in treatment, effective prevention and therapeutic methods for bacterial meningitis remain lacking.

The chemokine receptor CCR5 (Chemokine Receptor 5) is a G protein-coupled receptor (GPCR) that plays a crucial role in the migration and functional regulation of immune cells. Studies have shown that CCR5 is involved in regulating inflammatory responses in various diseases, including cytokine storms and autoimmune damage induced by SARS-CoV-2 infection. However, the regulatory mechanisms of CCR5 itself remain unclear. Meanwhile, 5-HT7R (5-Hydroxytryptamine Receptor 7), a member of the serotonin receptor family, is expressed in both the nervous and immune systems, but its specific role in infectious diseases has not been fully elucidated.

This study aims to explore the interaction mechanisms between 5-HT7R and CCR5 in bacterial meningitis, revealing how serotonin signaling alleviates infection-induced excessive immune responses and cognitive impairments by regulating CCR5 expression, providing a new theoretical basis and therapeutic targets for neuroimmune regulation.

Source of the Paper

This paper was co-authored by Zhenfang Gao, Yang Gao, and other researchers from the Beijing Institute of Basic Medical Sciences and the Beijing Institute of Brain Disorders, Capital Medical University. The paper was published in the Journal of Advanced Research (2019 Impact Factor: 6.992) in 2025, titled “5-HT7R Enhances Neuroimmune Resilience and Alleviates Meningitis by Promoting CCR5 Ubiquitination.”

Research Process

1. Animal Model Construction and Drug Treatment

The study first constructed a mouse model of bacterial meningitis induced by Streptococcus pneumoniae. C57BL/6 mice were infected via intracranial injection of S.p., followed by intraperitoneal injection of fluoxetine (FLX, 2.5 mg/kg) or saline control on days 1, 2, and 3. FLX, a selective serotonin reuptake inhibitor (SSRI), is known to modulate mood and immune function by increasing serotonin levels. The study also included treatment with LP-211, a specific agonist of 5-HT7R, to validate the protective role of 5-HT7R in meningitis.

2. Survival Rate and Clinical Symptom Assessment

The researchers assessed the protective effects of FLX and LP-211 on meningitis by recording the survival rate and clinical symptom scores of the mice. The results showed that FLX significantly improved the survival rate and reduced clinical symptom scores, indicating its effectiveness in alleviating the severity of meningitis.

3. Immune Response and Tissue Inflammation Analysis

Through flow cytometry (FACS) and histopathological analysis, the research team found that FLX significantly reduced the expression of inflammatory factors (e.g., TNF-α, IL-1β, and IL-6) in brain tissue and inhibited the activation of microglia. Additionally, FLX decreased the bacterial load in the brain and blocked the migration of peripheral macrophages into the brain.

4. Cognitive Function Assessment

To evaluate the protective effects of FLX on cognitive impairments induced by meningitis, the research team conducted several behavioral experiments, including the Morris water maze test (to assess spatial learning and memory), the open-field test (to assess locomotor activity and spatial memory), and the novel object recognition test (to assess cognition and recognition memory). The results demonstrated that FLX significantly improved cognitive function in mice with meningitis.

5. CCR5 Expression and Regulatory Mechanism Study

Using Western Blot and co-immunoprecipitation (Co-IP) techniques, the researchers found that CCR5 expression was significantly upregulated in the meningitis model, while FLX and LP-211 markedly suppressed CCR5 expression. Further experiments revealed that FLX induced CCR5 ubiquitination and degradation via the 5-HT7R signaling pathway. 5-HT7R signaling recruits the E3 ubiquitin ligase STUB1 to initiate K48-linked ubiquitination of CCR5 at lysine 138 (K138) and lysine 322 (K322), promoting its proteasome-dependent degradation.

6. In Vitro Cell Experiments

To further validate the regulatory mechanism of 5-HT7R signaling on CCR5, the researchers conducted in vitro experiments in RAW264.7 macrophage and PC12 neuronal cells. The results showed that FLX and LP-211 dose-dependently reduced CCR5 expression, and this effect was blocked by the proteasome inhibitor MG132, indicating that CCR5 degradation is proteasome-dependent. Additionally, silencing STUB1 or inhibiting the mTOR signaling pathway reversed 5-HT7R-mediated CCR5 ubiquitination and degradation.

Main Results and Logical Relationships

1. FLX significantly improves survival rates and alleviates clinical symptoms in mice with meningitis: This result suggests that FLX, by modulating serotonin signaling, may protect against infection-induced excessive immune responses.
2. FLX inhibits the release of inflammatory factors and microglial activation: This finding further confirms the anti-inflammatory effects of FLX in alleviating meningitis.
3. FLX improves cognitive function in mice with meningitis: This result indicates that FLX not only mitigates infection-induced tissue damage but also protects the nervous system from cognitive impairments.
4. FLX induces CCR5 ubiquitination and degradation via 5-HT7R signaling: This mechanism reveals the critical role of 5-HT7R signaling in regulating CCR5 expression, providing new insights into neuroimmune regulation.
5. The key roles of STUB1 and mTOR signaling in CCR5 ubiquitination: This discovery further elucidates the molecular mechanism of CCR5 degradation, offering a theoretical foundation for developing therapeutic strategies targeting CCR5.

Conclusion and Significance

This study uncovers a novel mechanism by which 5-HT7R alleviates excessive immune responses and cognitive impairments induced by bacterial meningitis through the induction of CCR5 ubiquitination and degradation. This discovery not only provides a new perspective on neuroimmune regulation but also offers potential therapeutic targets for infectious diseases. Specifically, the 5-HT7R signaling pathway recruits the E3 ubiquitin ligase STUB1 to initiate K48-linked ubiquitination of CCR5, providing a new molecular basis for regulating CCR5 expression.

Moreover, this study is the first to report the ubiquitination modification of CCR5, offering new insights into the regulatory mechanisms of CCR5 in various diseases. Future drug development targeting the 5-HT7R and CCR5 signaling pathways may open new avenues for treating infectious and neuroimmune-related diseases.

Research Highlights

1. First report of the ubiquitination modification mechanism of CCR5: This study is the first to describe the molecular mechanism by which CCR5 is degraded through K48-linked ubiquitination, providing new insights into the regulation of CCR5 in various diseases.
2. Key role of 5-HT7R signaling in neuroimmune regulation: The study reveals that 5-HT7R signaling alleviates infection-induced excessive immune responses and cognitive impairments by regulating CCR5 expression.
3. Critical roles of STUB1 and mTOR signaling: The study found that STUB1 and mTOR signaling play key roles in 5-HT7R-mediated CCR5 ubiquitination, offering new targets for developing therapeutic strategies targeting CCR5.
4. Potential clinical applications: The findings of this study provide a theoretical basis for developing therapeutic strategies for infectious and neuroimmune-related diseases, highlighting its significant clinical value.

Other Valuable Information

The study also found through database analysis that critically ill COVID-19 patients exhibit lower levels of 5-HT7R expression, further supporting the crucial role of 5-HT7R in infectious diseases. In the future, drug development targeting the 5-HT7R signaling pathway may provide new directions for treating various infectious and neuroimmune-related diseases.

Additionally, the research team plans to further validate the role of the 5-HT7R-CCR5 axis in other neuroimmune diseases through gene knockout experiments, offering new perspectives for in-depth research in this field.