Rhein Inhibits Microglia-Mediated Neuroinflammation and Neuronal Damage of Alzheimer's Disease via Regulating the Glutamine-Aspartate-Arginine-NO Metabolic Pathway.

Microglia-mediated neuroinflammation is a key driver of Alzheimer's disease (AD). In AD, microglia are activated and trigger an increased secretion of pro-inflammatory factors. Rhein, an anthraquinone compound extracted from rhubarb, has been shown to reduce the secretion of pro-inflammatory cytokines including TNF-α and IL-1β in activated microglia. However, the mechanism of rhein on microglia-mediated neuroinflammation and neuronal damage in AD remains unclear. In this study, we found that rhein improved behavioral abnormalities in AD rats and reduced the levels of inflammatory factors such as IL-1β, iNOS, and NO in the brain of AD rats. In the LPS-induced microglial model, rhein significantly reduced the levels of inflammatory factors to improve neuroinflammation. Untargeted metabolomics showed that the reprogramming of glutamine metabolism occurred in M1 microglia. Targeted metabolomics and (13)C, (15)N isotope tracing experiments demonstrated that rhein regulated the metabolite levels in the glutamine-aspartate-arginine metabolic pathway. Meanwhile, the upregulated expression of proteins such as GLS1 and GOT1 within this pathway was reversed by rhein. Furthermore, we found that the glutamine-aspartate-arginine metabolic pathway regulates the production of nitric oxide (NO, a neuroinflammatory mediator). Rhein alleviates neuronal damage by inhibiting the glutamine-aspartate-arginine-NO metabolic pathway. In conclusion, our study shows that rhein may inhibits NO production by regulating the glutamine-aspartate-arginine metabolic pathway in activated microglia, thereby inhibiting the neuroinflammation and neuronal damage in AD.