Trimethylamine-N-oxide damages astrocytes and lymphatic endothelial cells in the cerebral lymphatic system.

Background: Trimethylamine-N-oxide (TMAO), as a gut microbiota dependent metabolite, is involved in the occurrence and progression of many neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease, which are related to the disruption of the cerebral lymphatic system. However, the relationship between TMAO and cerebral lymphatic system remains to be elucidated. This study aimed to investigate the effects of TMAO on the astrocytes of the brain glymphatic system, and endothelial cells of the meningeal lymphatic vessels. Methods: Astrocytes and lymphatic endothelial cells were treated with different concentrations of TMAO. Alterations in cell proliferation or apoptosis, inflammatory cytokines, the nuclear factor-kappaB (NF-κB) signaling pathway, reactive oxygen species (ROS), and functional proteins such as aquaporin-4 (AQP4), glial fibrillary acidic protein (GFAP), S100β, claudin-5, and Ocln were analyzed. C57BL/6 male mice were treated with TMAO after which alpha-synuclein (SNCA) was injected intracranially. Neuronal damage and expressions of above functional proteins in the ventral midbrain, and levels of SNCA and inflammatory factors in the cerebrospinal fluid (CSF) of mice were assessed. Results: TMAO activated the NF-κB signaling pathway, increased nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3), tumor necrosis factor-alpha, interleukin (IL)-6, IL-1β, and ROS levels in astrocytes and lymphatic endothelial cells and promoted their apoptosis; increased the expression of GFAP and S100β, decreased the expression of AQP4 in astrocytes; decreased the expression of claudin-5 and Ocln in lymphatic endothelial cells. However, NF-κ B signaling pathway inhibitor BAY11-7082 improved the above indicators. Animal studies revealed that TMAO induced intracranial inflammation, affected the expression of functional proteins in the cerebral lymphatic system, and intensified SNCA aggregation in the mouse brain. Conclusion: TMAO can activate the NF-κB signaling pathway and damage the cellular function of brain glymphatic system and meningeal lymphatic vessels, and promote intracranial inflammation and SNCA deposition in mice, which may be a potential mechanism for TMAO involvement in neurodegenerative diseases.