Saikosaponin B2 ameliorates depression-induced microglia activation by inhibiting ferroptosis-mediated neuroinflammation and ER stress

Saikosaponins B2 (SSB2) is one of the main active components isolated from Radix Bupleuri (Bupleurum chinense DC.), a herb widely used of traditional Chinese medicine. It has been used for the treatment of depression for more than two thousand years. However, the molecular mechanisms remain to be determined. In this study, we evaluated the anti-inflammatory effect and elucidated underlying molecular mechanisms of SSB2 in LPS-induced primary microglia and CUMS-induced mice model of depression. The effects of SSB2 treatment were investigated both in vitro and in vivo. The chronic unpredictable mild stimulation (CUMS) procedure was applied to establish the animal model of depression. Behavioural tests were used to evaluate the depressive-like behaviors in CUMS-exposed mice, including sucrose preference test, open field test, tail suspension test, and forced swimming test. The GPX4 gene of microglia was silenced using shRNA, and inflammatory cytokines were determined by Western Blot and immunofluorescence analysis. Endoplasmic reticulum stress and ferroptosis-related markers were detected by qPCR, flow cytometry and confocal microscopy. SSB2 reversed depressive-like behaviours in CUMS-exposed mice and relieved central neuroinflammation and ameliorated hippocampal neural damage. SSB2 alleviated LPS-induced activation of microglia through the TLR4/NF-κB pathway. LPS-induced ferroptosis, with increased levels of ROS, intracellular Fe2+, mitochondrial membrane potential, lipid peroxidation, GSH, SLC7A11, FTH, GPX4 and Nrf2, and decreased transcription levels of ACSL4 and TFR1, was attenuated with SSB2 treatment in primary microglia cells. GPX4 knockdown activated ferroptosis, induced endoplasmic reticulum (ER) stress, and abrogated the protective effects of SSB2. Further, SSB2 attenuated ER stress, balanced calcium homeostasis, reduced lipid peroxidation and intracellular Fe2+ content by regulating the level of intracellular Ca2+. Our study suggested that SSB2 treatment can inhibit ferroptosis, maintain calcium homeostasis, relieve endoplasmic reticulum stress and attenuate central neuroinflammation. SSB2 exhibited anti-ferroptosis and anti-neuroinflammatory effects through the TLR4/NF-κB pathway in a GPX4-dependent manner.