We showed in previous work that asperosaponin VI inhibits NF-κB signaling to mitigate lipopolysaccharide-induced depression-like behaviors in mice by reducing microglia-mediated acute neuroinflammation . Here, using a classical animal model of depression, we showed that asperosaponin VI induces a PPAR-γ-dependent neuroprotective microglial phenotype that mitigates depression-like behaviors induced by CMS, which is associated with restoration of hippocampal synaptic function. Our work extends the list of conditions where asperosaponin VI can exert therapeutic anti-inflammatory and neuroprotective effects in the brain, a list that already includes Alzheimer's disease and optic nerve damage [19, 24].
Depression usually manifests as diverse debilitating symptoms, including hopelessness and anhedonia . Anhedonia, a core symptom of MDD, can be assessed in the SPT . In addition to the SPT, we used the FST and TST to assess passive stress-coping behavioral despair [27, 28]. As expected, CMS caused depression-like behaviors in all these tests, which subsequent asperosaponin VI treatment improved, to an even greater extent than the classic monoamine antidepressant imipramine. In fact, asperosaponin VI but not imipramine partially restored weight loss caused by CMS, suggesting that the former may lack serious side effects at the dose of 40 mg/kg. We believe that these results indicate genuine antidepressant effects of asperosaponin VI, because the compound did not significantly alter performance in the OFT.
Our previous research indicated that the dysregulation of pro- and anti-inflammatory cytokines plays a crucial role in depression . In the present study, CMS upregulated the pro-inflammatory IL-1β, IL-6, iNOS and Tnf-α in hippocampus of mice, and asperosaponin VI reversed these changes while also upregulating the anti-inflammatory cytokines Arg-1, IL-10, and Tgf-β as well as BDNF. These results establish a link between the neuroprotective and anti-inflammatory effects of asperosaponin VI.
Stress or immunostimulation has been shown to induce neuroinflammation, which appears to involve microglial activation, particularly in the hippocampus [29, 30]. Consistent with these previous studies, we found here that CMS caused morphological changes in hippocampal microglia indicative of microglial activation. Asperosaponin VI did not reduce the extent of overall microglial activation in hippocampus of CMS mice; instead, it altered the type of such activation, from a pro-inflammatory to neuroprotective phenotype. The decrease in proportion of pro-inflammatory microglia translates to lower production of pro-inflammatory cytokines and neurotoxic products (such as nitric oxide and quinolinic acid) , which increase neuropathic pain and inhibit hippocampal neurogenesis, contributing to cognitive deficits and depression-like behaviors [10, 32, 33].
We found that the anti-inflammatory effects of asperosaponin VI are mediated by PPAR-γ, a ligand-dependent transcription factor belonging to the nuclear hormone receptor superfamily . PPAR-γ regulates the expression of anti-inflammatory cytokines , and the PPAR-γ agonists pioglitazone or rosiglitazone can switch activated microglia cells from a pro-inflammatory to anti-inflammatory state [36, 37]. Our previous research showed that asperosaponin VI acts via PPAR-γ to switch activated microglia from a pro-inflammatory to anti-inflammatory phenotype in vitro . In present study, we further demonstrated that asperosaponin VI acts via PPAR-γ to induce a neuroprotective phenotype in hippocampal microglia of CMS-exposed mice and mitigate depressive-like mouse behaviors. Conversely, blocking the PPAR-γ signaling pathway abolished the neuroprotective microglia in induced by asperosaponin VI in hippocampus of CMS-exposed mice, as well as the antidepressant effect of asperosaponin VI. Thus, we speculated that asperosaponin VI exerts its antidepressant and anti-inflammatory effects via the PPAR-γ signaling pathway to regulating the phenotype of microglia.
How proinflammatory microglia lead to depression is a hot topic of current research. A great deal of researches showed the dysfunctional microglia can lead to abnormal neuron-microglia communication and disrupt synaptic function [39–41]. In this study, we found CMS-induced decrease in the intercommunicating molecules between neuron and microglia (Cx3cl1 / Cx3cr1 and CD200 / CD200R) in hippocampus of CMS-exposed mice was reversed by ASA VI treatment. Meanwhile, the PSD-95, CamKII α and CamKII β as well as decreased levels of phosphorylated GluA 2 in hippocampus of CMS-exposed mice, these are thought to be crucial for morphological maturation and synaptic development of hippocampal neurons [42–47], were partially reversed by asperosaponin VI via PPAR-γ-dependent pathway. These results reveal for the first time the role of asperosaponin VI in maintaining normal communication between neurons and microglia as well as in repairing CMS-induced damage to synaptic plasticity in hippocampus.