Major Depressive Disorder (MDD) is one of the most prevalent debilitating personal and public disorders worldwide. The huge burden caused by it is not shared equally among all patients with depression[1]. Moreover, the causative mechanism of major depression is still controversial, and most of the surviving therapeutic drugs have adverse effects, and the treatments are not fully effective[2]. Therefore, the search for effective antidepressants with few side effects is a pressing issue. Related studies have shown that depression and inflammation contribute to each other and play a key role in the pathogenesis of depression[3]. Depression recurrence and lack of therapeutic efficacy of antidepressant medications may be related to the overall activation of the inflammatory response, and thus, inflammation may be a key disease modifier that promotes susceptibility to depression[4], [5]. Increased inflammatory dysregulation alters neurotransmitter metabolism, impairs neuronal health, and affects neural activity in mood-related brain regions[3], [6]. It has been reported that acute peripheral inflammation leads to molecular and cellular alterations that result in reduced adult hippocampal neurogenesis[7] and that white matter integrity in the left hind limb of the intracerebral capsule decreases with elevated peripheral c-reactive protein levels in patients with major depression[8]. Thus, controlling inflammation may provide overall therapeutic benefits in the treatment of depression.

NLRP3 (NOD-like receptor thermal protein domain associated protein 3) inflammasome is a multiprotein complex. A detection protein called NLRP3 attaches to ASC and pre-caspase-1, generates inflammasomes, activates caspase-1, and leads to the production and release of pro-inflammatory cytokines, including interleukin-1β (IL-1β) and interleukin-18 (IL-18), causes inflammation, and promotes apoptotic cell death[9], [10]. NLRP3 inflammasomes activation is tightly regulated by the mechanism of deubiquitination. The E3 ubiquitin ligase MARCHF7 has been reported to promote ubiquitination and thus degradation of NLRP3, which can prevent inflammation dependent on NLRP3 inflammasomes[11]. Activation of NLRP3 inflammasomes leads to hippocampal neuroinflammation as well as depression and anxiety[12], but when its activity is inhibited, cysteine-1 and mature IL-1β levels are reduced and depression-like behavior in mice is improved[13]. Thus, the pathogenesis of depression involving NLRP3 inflammasomes may become a novel avenue for the treatment of depression.

SSd is a saponin monomer component with anti-inflammatory effects extracted from Saikosaponin. SSd significantly reduced the levels of TNF-α, IL-6, IL-1α and inhibited the activation of NF-κB, thereby preventing DSS-induced intestinal inflammation[14]. In addition, SSd significantly reduced endotoxin and RANKL-induced inflammatory bone loss as well as osteoclast production and bone resorption[15]. It has been reported in the literature that SSd reduces endotoxin-induced depression-like behavior by preventing microglia activation and neuroinflammation[16]. Based on the above, we speculate that SSd may reduce depressive-like behavior in mice by regulating NLRP3 ubiquitination through MARCHF7 protein. The CUMS model is often used in depression research because it can accurately characterize many cores behavioral features of human depression[17], [18]. Therefore, in this study, we used the CUMS model to investigate the antidepressant effects and potential pathogenic mechanisms of SSd.

We tested this speculation by establishing a mouse model of chronic unpredictable mild stress and found that SSd attenuated neuronal loss and depression-like behavior in mice through regulation of NLRP3 ubiquitination by MARCHF7 protein while suppressing the increase of inflammatory cytokines (TNF-α, IL-6, and IL-1β) and ameliorating the inflammatory response. In addition to this, we confirmed the key role of MARCHF7 gene mediated ubiquitination of NLRP3 by MARCHF7 gene silencing in alleviating depression-like behaviors in mice with SSd.