Evaluation of the inhibition of Neuroinflammation by Quassin

The over-activation of microglia is a common feature of neuroinflammation. This is associated with neuronal damage and the release of inflammatory mediators within the CNS in response to an injury or infection. The release of pro-and anti-inflammatory cytokines, chemokines and other mediators plays a vital role in the pathogenesis of neurodegenerative diseases. Studies have demonstrated the use of natural compounds possessing anti-inflammatory and neuroprotective properties in inhibiting neuroinflammation. Quassin is a bitter constituent derived from the bark of the quassia amara tree; it is used as a traditional medicine in several cultures. Several studies have reported quassin to possess antiulcerogenic, antiplasmodial, antileishmanial and anti-malarial properties. However, there is limited research on the anti-inflammatory properties of quassin. This study therefore aimed at investigating the inhibition of neuroinflammation by quassin through evaluating the actions of quassin on underlying mechanisms of neuroinflammation in LPS-stimulated Bv2 microglia. The inhibitory effects of quassin (10, 20, 40 and 80 μM) against neuroinflammation were investigated in LPS (100ng/ml) stimulated BV2 microglia.

Griess assay was used to measure the levels of nitrite in BV2 microglia cells. The levels of pro-inflammatory cytokines TNF, IL-1 and IL-6; chemokines CCL2 and CCL5; and anti-inflammatory cytokine IL-10 were measured using ELISA kits. COX-2 and iNOS protein expressions were measured using western blotting. Luciferase reporter gene assay and ELISA were used to measure quassin’s activity on NF-B -phospho-p65 and NF-B promoter.

Results from this study demonstrated that quassin inhibited IL-1, and IL-6 at a concentration of 80 μM. Additionally, quassin suppressed NO's production and the expression of iNOS protein at 20 and 80 μM. The anti-inflammatory effects of quassin were not sufficient at inhibiting the production of TNF, CCL2, CCL5, and COX-2 protein expression, alongside increasing the release of IL-10 in activated microglia. On the other hand, quassin was able to inhibit the production of pro-inflammatory cytokines IL-1 and IL-6 released in the NF-B signalling pathway. Western blotting and Luciferase reporter gene assay showed that quassin did not inhibit phosphorylation p65 and NF-B dependent gene expression. Findings from this study suggest that quassin’s inhibitory effects on IL-1, IL-6, NO and the expression of iNOS are not mediated by targeting the phosphorylation of NF-B -phospho-p65 subunit in LPS-stimulated BV2 microglia.

In conclusion, this study has contributed to our understanding of the effect of quassin in inhibiting neuroinflammation mediation in neurodegenerative diseases. Quassin showed promising results through the inhibition of proinflammatory cytokines IL-1, and IL-6 and also in the suppression of NO production. Further investigations need to be carried out in order to determine the anti-inflammatory activity of quassin.