Saikosaponin-d Alleviates Depression by Promoting NLRP3 Ubiquitination and Inhibiting Inflammasome Activation

Saikosaponin-d (SSd) is a triterpene saponin from the roots of Bupleurum chinese. Recent studies have revealed its antidepressant activity, but its mechanism involved is unclear. This study's objective was to ascertain how SSd may reduce depression in depressed mice subjected to chronic unpredictable animal stress (CUMS) and to investigate the mechanisms underlying these effects. Models of CUMS depression were established and different groups were treated with SSd and escitalopram. After the last day of administration of the treatment, behavioral tests were performed. ELISA was used to measure the expression of IL-1β, TNF-α, and IL-18, and western blot was used to measure the presence of proteins associated with NLRP3. Hippocampal neuronal damage was observed using Nissl staining, and NLRP3 ubiquitination assay was performed by immunoprecipitation and gene silencing. An inflammatory cell model was constructed by treating BV2 cells with lipopolysaccharides (LPS) and adenosine triphosphate (ATP) to verify the ubiquitination modification of NLRP3 by SSd. Behavioral tests demonstrated that SSd effectively alleviated depression-like symptoms. SSd should substantially limit the degrees of proteins associated with NLRP3, as properly as limit the harm to hippocampal neurons. Gene silencing results showed that SSd regulates NLRP3 through the E3 ubiquitin ligase MARCHF7. In vitro, SSd remarkably increased the protein expression of K48-linked ubiquitin in inflammatory BV2 cells, while decreasing the protein levels of NLRP3. Our findings suggest that SSd has antidepressant effects in CUMS mice by promoting ubiquitination of NLRP3 to inhibit inflammasome activation and improve the inflammatory state.