Xiaoyaosan against depression through suppressing LPS mediated TLR4/NLRP3 signaling pathway in “microbiota-gut-brain” axis

Depression impairs not only central nervous system, but also peripheral systems of the host. Gut microbiota have been proved to be involved in the pathogenesis of depression. Xiaoyaosan (XYS) has a history of over a thousand years in China for treating depression, dramatically alleviating anxiety, cognitive disorders, and especially gastrointestinal dysfunctions. Yet, it still just scratches the surface of the anti-depression mechanisms of XYS. This study aims to elucidate the mechanism of actions of XYS from the perspective of "microbiota-gut-brain" axis. We firstly evaluated the effects of XYS on the macroscopic behaviors of depressed rats that induced by chronic unpredictable mild stress (CUMS). Secondly, the effects of XYS on intestinal homeostasis of depressed rats were revealed by using dysbacteriosis model. Subsequently, the underlying mechanisms were demonstrated by 16S rRNA gene sequencing technology and molecular biology methods. Finally, correlation analysis and visualization of the anti-depression effects of XYS were performed from the "microbiota - gut - brain" perspective. Our data indicated that XYS ameliorated the depression-like symptoms of CUMS rats, partly depending on the presence of gut microbiota. Furthermore, we illustrated that XYS reversed CUMS-induced gut dysbiosis of depressed rats in terms of decreasing the Bacteroidetes/Firmicutes ratio and the abundances of Bacteroides, and Corynebacterium, while increasing the abundances of Lactobacillus and Adlercreutzia. The significant enrichment of Bacteroides and the level of lipopolysaccharides (LPS) suggested that depression damaged the immune responses and gut barrier. Mechanistically, XYS significantly down-regulated the expression levels of factors that involved in TLR4/NLRP3 signaling pathway in the colon and brain tissues of depressed rats. In addition, XYS significantly increased the levels of claudin 1 and ZO-1, showing that XYS positively maintained the integrity of gut and blood-brain barriers (BBB). Our study offers insights into the anti-depression effects of XYS through a lens of "microbiota-TLR4/NLRP3 signaling pathway-barriers", providing a foundation for enhancing clinical efficiency and enriching drug selection, and contributing to our understanding of the mechanisms of traditional Chinese medicines (TCMs) in treating depression.