Glycyrrhizic acid improves cognitive levels of aging mice by regulating T/B cell proliferation

Licorice, the root of Glycyrrhiza glabra L. (Leguminosae), is one of the oldest known medicinal herbs. Glycyrrhizic acid (GA), a triterpenoid saponin isolated from licorice, has a variety of pharmacological activities. Previous studies indicate that GA produces neuroprotection via the modulation of anti-apoptotic and pro-apoptotic factors, primarily through the ERK signaling pathway and the suppression of inflammatory cytokine induction. However, the mechanisms of these neuroprotective effects and the effects of neuroimmunomodulation on cognition are not fully elucidated. This study investigated the effects of GA in preventing age-related immune involution and cognitive disorders as well as the relationship between immune involution and cognition and the underlying molecular mechanisms. Twelve-month-old female C57BL/6 mice were injected with GA (5mg/kg i.v. once every other day) for 6 weeks. Then, we tested the behavioral effects of GA using the Morris water maze task to evaluate spatial learning and memory. Age-related learning and memory impairments were prevented by GA treatment. This effect was accompanied by improved cerebral blood flow. Transcriptome analysis of C57BL/6 mice blood by RNA-sequencing and KEGG pathway analysis showed that GA could significantly influence hematopoietic cell lineage. The differential genes in the pathway were clustered using heat mapping, and RNA-seq results showed that GA altered gene expression. To correlate modulated gene expression and cognitive effects, we analyzed the levels of CD3e+ positive T cells, CD45R/B220+ positive B cells, and CD49b+ positive NK cells in the blood and spleen after 22 days of GA treatment. Flow cytometry showed that GA could increase T and B cells in the blood and spleen. To further investigate the relationship between neuroprotective effects of GA and effects on T and B cells, we tested behaviors after immunological reconstitution and GA treatment using a novel object recognition task in B-NDG mice. We found that, following immunological reconstitution and GA treatment, mice showed an increased preference for novel objects compared with controls as well as increased numbers of T, B, CD4, and CD8 cells. Increased neural stem cell markers accompanied these changes. Thus, our study suggests that GA may be a potential treatment for alleviating aging-associated cognitive declines and that these effects may be related to immune system modulation.