Polygala oligosaccharide esters improve memory disorder by restoring gut microbiota homeostasis through the regulation of the “gut-brain” axis

Background

Yuanzhi (Polygala tenuifolia Willd.) possesses the effects of calming the spirit, enhancing intelligence, regulating the heart-kidney connection, eliminating phlegm, and reducing swelling. It is commonly used in the treatment of insomnia and forgetfulness. Previous studies have indicated that the oligosaccharide esters (OE) derived from Yuanzhi exhibit neuroprotective and memory-enhancing activities. However, its underlying mechanisms, particularly those involving the gut-brain axis, remain unclear.

Purpose of the research

This study aimed to investigate the therapeutic efficacy and underlying mechanisms of Oligosaccharide Esters (OE) from Polygala tenuifolia Willd. against memory dysfunction in mice, with a specific focus on the gut-brain axis.

Methods

A mouse model of memory dysfunction was induced using D-galactose combined with AlCl₃. Behavioral tests, molecular biology techniques (histopathology, enzyme-linked immunosorbent assay (ELISA), immunohistochemistry, and Western blot), and multi-omics approaches (16S rRNA sequencing and lipidomic analysis) were employed to investigate the therapeutic efficacy of OE against memory dysfunction. Meanwhile, with the aid of fecal microbiota transplantation (FMT) assay, we observed the repair of brain and colonic tissues, inflammatory responses and intestinal permeability, further clarified the regulatory effect of OE on gut microbiota, and ultimately revealed the underlying mechanisms of OE mediated by the gut-brain axis.

Results

OE administration significantly enhanced learning and memory in MD mice, repaired neuronal damage in the hippocampal regions (CA1, CA3, DG) of the MD mouse brain, and increased the number of Nissl bodies. OE elevated the serum levels of BDNF and CREB and reduced the TMAO level; simultaneously, it enhanced the activities of SOD and GSH-Px and decreased the MDA content in the brain tissue. OE treatment modulated the relative abundance of the gut microbiota in MD mice, restored the microbial imbalance induced by memory deficits, and particularly affected the abundances of Firmicutes, Bacteroidetes, their ratio (F/B), and genera such as Ligilactobacillus. Lipidomics analysis indicated that OE exerts its therapeutic effects primarily by regulating the glycerophospholipid metabolism pathway, and a total of 17 key differential lipid metabolites were identified. Correlation analysis further revealed that the levels of key differential lipid metabolites, LysoPC(22:2) and PC(38:4), were significantly positively correlated with the levels of neuroprotective factors (CREB, BDNF) and the activities of antioxidant enzymes (SOD, GSH-Px), but were significantly negatively correlated with the harmful metabolite TMAO and the oxidative damage product MDA. In contrast, the lipid metabolite GPEA exhibited a trend opposite to that of LysoPC(22:2) and PC(38:4). Further investigation results demonstrated that OE could repair pathological damage in colon tissue, regulate the levels of the microbial metabolite TMAO and the neurotransmitter 5-HT, reduce the levels of pro-inflammatory factors (LPS, TNF-α, IL-6) in both the brain and colon, and inhibit the abnormal activation of astrocytes and the abnormal hyperphosphorylation of Tau protein. The results of correlation analysis indicated that beneficial bacteria [e.g., Ligilactobacillus) and beneficial lipids (e.g., LysoPC(22:2) and PC(38:4)] were collectively significantly negatively correlated with key pathological indicators (e.g., TMAO and TNF-α) and were positively correlated with the neurotransmitter (e.g., 5-HT). OE also significantly up-regulated the expression of tight junction proteins (Occludin, Claudin-5) in both brain and colon tissues, thereby structurally repairing the damaged gut-brain barrier. FMT experiments showed that FMT improved the learning and memory abilities of mice, repaired neuronal damage in the hippocampus (CA1, CA3, DG), and increased the number of Nissl bodies. In addition, FMT alleviated colonic tissue injury, attenuated inflammatory responses in the brain and colon, and reduced intestinal permeability in MD mice, exerting a therapeutic effect similar to that of OE.

Conclusion

OE exerted anti-amnestic effects via the gut-brain axis, primarily by alleviating neuroinflammation and oxidative stress, restoring gut microbiota homeostasis, and regulating glycerophospholipid metabolism, ultimately improving learning and memory abilities in MD mice.

Graphical Abstract