FBXL16: A New Regulator of Neuroinflammation and Cognition in Alzheimer’s Disease through Ubiquitination-Dependent Degradation of Amyloid Precursor Protein

Academic Background

Alzheimer’s Disease (AD) is an age-related neurodegenerative disorder characterized by progressive cognitive decline. With the global aging population, the incidence of neurodegenerative diseases is gradually increasing, placing a heavy burden on families and society. According to the 2021 World Health Organization’s global report, there are currently 40 to 50 million AD patients worldwide, and this number is expected to exceed 100 million by 2050. Although the exact etiology and pharmacological targets of AD remain unclear, research has identified multiple factors contributing to its pathogenesis, including genetic defects, β-amyloid (Aβ) cascade, free radical damage, abnormal calcium metabolism, cholinergic system imbalance, neuroinflammation, and aluminum toxicity. Current treatments for AD primarily focus on modulating neurotransmitter levels to alleviate symptoms but fail to address the underlying disease progression.

The Ubiquitin-Proteasome System (UPS) plays a crucial role in preventing intracellular protein accumulation, particularly in the central nervous system, by degrading and clearing misfolded and mutant proteins such as tau, APP, and α-synuclein, thereby preventing neurodegenerative diseases. Studies have shown that damage to the UPS pathway can lead to the aggregation of disease-related proteins, contributing to neurodegenerative diseases like AD. FBXL16 (F-box and leucine-rich repeat protein 16) is an E3 ubiquitin ligase belonging to the F-box protein family, which is involved in protein degradation, cell cycle regulation, cell proliferation, apoptosis, migration, and invasion. However, the specific molecular and physiological roles of FBXL16 in AD remain poorly understood.

Source of the Paper

This paper was co-authored by Liqun Qu, Yong Tang, and Jianhui Wu, among others, from institutions such as Macau University of Science and Technology and Southwest Medical University. The paper was published in 2024 in the journal Biomarker Research under the title “FBXL16: A New Regulator of Neuroinflammation and Cognition in Alzheimer’s Disease through the Ubiquitination-Dependent Degradation of Amyloid Precursor Protein.”

Research Process

1. Proteomic Analysis

The study began with proteomic analysis using protein lysates from HEK293 cells overexpressing FBXL16 to identify proteins interacting with FBXL16. Co-immunoprecipitation (Co-IP) experiments, combined with MG132 and cycloheximide (CHX) treatments, confirmed that FBXL16 promotes the proteasomal degradation of the APP protein. Additionally, immunohistochemistry (IHC) and immunocytochemistry (ICC) experiments further validated the role of FBXL16 in AD models.

2. Behavioral Experiments

Using stereotaxic injection, the study constructed a brain-specific AD mouse model overexpressing FBXL16 and conducted behavioral tests such as the Morris water maze and Y-maze to assess the cognitive improvements in AD mice.

3. FBXL16 Conditional Knockout Mouse Model

To further validate the role of FBXL16 in AD, the study generated a brain-specific conditional knockout (CKO) FBXL16 mouse model and performed behavioral tests and pathological analyses to confirm the protective effects of FBXL16 in AD.

Key Findings

1. Interaction Between FBXL16 and APP

Through proteomic analysis, the study identified 141 proteins interacting with FBXL16, of which 19 were upregulated and 62 were downregulated. STRING functional protein correlation network analysis revealed a potential interaction network involving FBXL16 and these proteins, suggesting that FBXL16 plays a significant role in age-related cognitive function and ubiquitination.

2. FBXL16 Promotes APP Degradation via Ubiquitination

Molecular docking analysis revealed that FBXL16 directly binds to APP and promotes its degradation through the ubiquitin-dependent proteasomal pathway. Co-immunoprecipitation experiments further confirmed that FBXL16 significantly enhances the ubiquitination of the APP protein.

3. FBXL16 Improves Cognitive Function in AD Mice

In the 3×Tg AD mouse model, overexpression of FBXL16 significantly improved spatial learning and memory. Morris water maze and Y-maze tests showed that mice overexpressing FBXL16 performed better in terms of platform exploration time, distance traveled, and time spent in the target quadrant compared to controls.

4. FBXL16 Attenuates Neuroinflammation in AD Mice

Immunohistochemical analysis revealed that FBXL16 significantly suppressed the overactivation of astrocytes and microglia in the brains of AD mice, indicating a protective role of FBXL16 in mitigating neuroinflammation.

Conclusion

This study demonstrates that FBXL16 improves cognitive function and neuroinflammation in AD mice through the ubiquitination-dependent degradation of APP. The expression of FBXL16 decreases with age in AD mice, and its overexpression promotes APP degradation, reducing Aβ accumulation and alleviating AD pathology. These findings provide a new potential therapeutic target for AD and lay the foundation for developing strategies to combat other neurodegenerative proteinopathies.

Research Highlights

1. Novel Function of FBXL16: This study is the first to reveal the protective role of FBXL16 in AD through the ubiquitination-dependent degradation of APP, improving cognitive function and neuroinflammation in AD mice.
2. Innovative Experimental Methods: The study constructed a brain-specific AD mouse model overexpressing FBXL16 using stereotaxic injection and comprehensively evaluated its function through behavioral tests and pathological analyses.
3. Potential Therapeutic Target: As an E3 ubiquitin ligase, FBXL16’s regulation of APP degradation offers new insights for AD treatment, paving the way for the development of FBXL16-targeted drugs.

Research Significance

This study not only elucidates the molecular mechanisms of FBXL16 in AD but also provides a new potential therapeutic target. By modulating FBXL16 expression, effective treatment strategies for AD and other neurodegenerative proteinopathies may be developed. Additionally, the study highlights the potential of gene therapy and cell replacement therapy in AD treatment, offering valuable insights for future clinical research.

Other Valuable Information

A limitation of this study is the lack of validation of FBXL16 expression levels in healthy individuals and AD patients, which warrants further investigation. Moreover, the study found that FBXL16 expression is regulated by the transcription factor E2F1, and future research could explore whether E2F1 influences APP degradation through FBXL16.