Schisandra chinensis lignans exerts endocannabinoids-like antidepressive effect: the phagocytotic relationship of activated CB2R-mediated M2 microglia and "stressed-but-viable" neuron.

Ethnopharmacological Relevance: Schisandra chinensis, a traditional functional Chinese medicine, is known for its ability to tonify the kidneys, calm the heart, and tranquilize the mind. Recent pharmacological research has demonstrated its anti-inflammatory and neuroprotective effects.

Aim Of The Study: We had previously demonstrated that Schisandra chinensis lignans (SCL) promote microglia polarization to M2 phenotype via targeting cannabinoid receptor type-2 (CB2R) to exert antidepressant effects. Considering the pathological features of abnormal microglial phagocytosis in chronic unpredictable mild stress (CUMS)-induced depression model, the current study aimed to build relationship between microglial phagocytosis and phenotype, further to explore whether SCL exerts antidepression by ameliorating abnormal phagocytotic "stressed-but-viable" neuron by targeting microglial CB2R.

Materials And Methods: Endocannabinoid levels were analyzed using Triple Quadrupole LC/MS. In vivo immunofluorescence assay was employed to evaluate the microglial abnormal phagocytosis. Then, we build models which one was microglia BV2 phagocytized FITC-IgG conjugated latex beads, the other was BV2 co-cultured with stressed-but-viable neurons. Phagocytosis was quantified using flow cytometry. The expression of calreticulin (CRT) in total, intracellular, and surface fractions was validated by western blot, flow cytometry, and immunofluorescence. The other proteins, such as LRP1, microglial phenotype markers and the PERK-eIF2α pathway, were assessed by western blot. The qRT-PCR was used to evaluate microglial phenotype markers. Based on the interaction between endocannabinoids and SCL with CB2R, we conducted a CB2R pharmacological antagonist in the CUMS model and used siRNA against CB2R in BV2 cells to verify the findings.

Results: SCL improved the disrupted levels of endocannabinoids induced by CUMS. In vivo studies revealed that the CB2R antagonist AM630 reversed the SCL-reduced efficiency of microglial mistakenly phagocytosed stressed-but-viable neurons and the up-regulated level of M2 phenotype. In the in vitro studies, we identified SCL activated M2 microglia via CB2R targeting, leading to a reduction in the neuronal cell-surface CRT, inhibition of the "eat-me" signaling, and alleviation abnormal phagocytosis. In-depth investigation performed in the co-culture model revealed that this mechanism involved the inactivation of the PERK-eIF2α pathway in neuronal cells by M2 microglia to exert the above-mentioned effects.

Conclusion: Overall, the improved abnormal phagocytotic process appears to be influenced by SCL and endocannabinoids, promoting microglial polarization toward the M2 phenotype in a CB2R-dependent manner. Specifically, this mechanism involves M2 microglia inactivation of the PERK-eIF2α pathway in stressed-but-viable neurons, thereby reducing CRT translocation to the cell surface and enhancing the regulation of abnormal phagocytosis, ultimately contributing to an antidepressant effect.