The active ingredient Monotropein in Morinda officinalis alleviates neuroinflammation via inhibiting cGAS/STING signaling pathway

Ethnopharmacological relevance: Neuroinflammation is recognized as a key pathological mechanism in the onset and progression of depression. Inhibiting neuroinflammation represents a promising strategy for effective depression treatment. Monotropein (Mon), a major bioactive cyclohexene ether terpene glycoside extracted from Morinda officinalis, a medicinal and edible herb, exhibits potent anti-inflammatory activity.

Aim of the study: The aim of this study is to investigate whether Mon exerts its antidepressant effects in a lipopolysaccharide (LPS)-induced mouse model of depression via the cGAS/STING signaling pathway.

Methods: A model of neuroinflammation induced by LPS was established in C57BL/6 J mice, which received varying doses of Mon via gavage, with fluoxetine (FXT) serving as a positive control. Depressive-like behaviors in the neuroinflammatory mice were evaluated using the open field test (OFT), sucrose preference test (SPT), forced swimming test (FST), and tail suspension test (TST). Subsequently, neuronal damage and inflammatory responses were assessed through histopathological examination, enzyme-linked immunosorbent assay (ELISA), Reverse transcription quantitative PCR (RT-qPCR)., and immunofluorescence techniques. RNA Sequencing (RNA seq) was conducted to identify potential targets, and molecular docking was employed to illustrate the interaction between Mon and the cGAS/STING proteins. Furthermore, in vivo experiments combining C176 with Mon were performed to further validate the underlying mechanism.

Results: Studies have demonstrated that Mon significantly alleviates LPS-induced depression-like behaviors and hippocampal pathological injury. Specifically, rats treated with Mon exhibited increased sucrose preference, reduced immobility time in the FST and TST, and enhanced locomotor activity and travel distance in the OFT. Additionally, Mon treatment was associated with attenuated neuronal damage and structural disruption in the hippocampus. This protective effect correlates with a marked reduction in neuroinflammatory responses, as evidenced by significantly decreased levels of pro-inflammatory cytokines (IL-1β, TNF-α, IL-6, and IFN-β) and reduced expression of glial activation markers, indicating suppression of glial cell activation. Furthermore, Mon effectively modulates microglial activity and improves the neuroinflammatory microenvironment both in vivo and in LPS-induced BV-2 microglial cells. RNA seq analysis revealed significant enrichment of inflammation-related signaling pathways, with the cGAS/STING pathway being prominently regulated. Molecular experiments further confirmed that Mon downregulates the expression of cGAS and p-STING proteins in both LPS-induced animal tissues and BV-2 cells. Additionally, further experiments demonstrated that the anti-inflammatory effect of Mon on LPS-induced neuroinflammation in mice was significantly attenuated when co-administered with C176, indicating that Mon ameliorates neuroinflammation via the cGAS/STING signaling pathway.

Conclusions: Our findings indicate that Mon significantly ameliorates LPS-induced depressive-like symptoms in mice, potentially through inhibition of the cGAS/STING signaling pathway.