Cannabinoid type 2 receptor activation inhibits MPP+-induced M1 differentiation of microglia through activating PI3K/Akt/Nrf2 signal pathway

Background: Growing evidence indicates that cannabinoid type 2 (CB2) receptor activation inhibits neuroinflammation in the pathogenesis of Parkinson's disease (PD). Nonetheless, the precise mechanisms of CB2 receptor-mediated neuroprotection have not been fully elucidated. The differentiation of microglia from the M1 to M2 phenotype plays a vital role in neuroinflammation.

Methods: In the present study, we investigated the effect of CB2 receptor activation on the M1/M2 phenotypic transformation of microglia treated with 1-methyl-4-phenylpyridinium (MPP+). The M1 phenotype microglia markers, including inducible nitric oxide (iNOS), interleukin 6 (IL-6), and CD86, and the M2 phenotype microglia markers, including arginase-1 (Arg-1), IL-10, and CD206, were detected by western blots and flow cytometry. The levels of phosphoinositide-3-kinase (PI3K)/Akt and nuclear factor erythroid 2-related factor 2 (Nrf2) were determined by Western blots. Subsequent addition of Nrf2 inhibitors initially revealed the specific mechanism by which CB2 receptors affect phenotypic changes in microglia.

Results: Our results showed that pretreatment with JWH133 significantly inhibited the MPP⁺-induced up-regulation of M1 phenotype microglia markers. Meanwhile, JWH133 increased the levels of M2 phenotype microglia markers. JWH133-mediated effects were blocked by co-treatment with AM630. Mechanism studies found that MPP⁺ treatment downregulated PI3K, Akt phosphorylated proteins, and nuclear Nrf2 protein. JWH133 pretreatment promoted PI3K/Akt activation and facilitated nuclear translocation of Nrf2, which was reversed by the PI3K Inhibitor. Further studies showed that Nrf2 inhibitors inverted the effect of JWH133 on microglia polarization.

Conclusion: The results indicate that CB2 receptor activation promotes MPP⁺-induced microglia transformation from M1 to M2 phenotype through PI3K/Akt/Nrf2 signaling pathway.

Cannabinoid type 2 receptor; JWH133; M1/M2 transformation; Microglia.