Calycosin activates Nrf2/Keap1 signaling to ameliorate hydrogen peroxide-induced spinal cord neuron death and mitochondrial dysfunction

Oxidative stress is a hallmark of secondary injury of spinal cord injuries. Controlling oxidative stress is crucial for mitigating secondary injury and promoting functional recovery after spinal cord injuries. Calycosin is an O-methylated isoflavone with antioxidant activity. To evaluate the effect of calycosin on spinal cord neurons under oxidative stress and clarify the molecular mechanism underlying the effect, we tested the neuroprotective activity of calycosin in a primary spinal cord neuron culture model. We found that calycosin protected neurons from H₂O₂-induced neuronal death in a dose-dependent manner. Further experiments revealed that calycosin decreased H₂O₂-induced mitochondrial fragmentation and mitochondrial membrane potential loss, and subsequently reduced H₂O₂-triggered release of mitochondrial cytochrome c into the cytoplasm. In addition, calycosin inhibited H₂O₂-induced reactive oxygen species generation and activation of NF-κB signaling in spinal cord neurons. Furthermore, the expression of several antioxidant enzymes such as HO-1, NQO1, GCLC, GCLM, TrxR1, and Trx1 was significantly promoted by calycosin. More importantly, we revealed that the Nrf2/Keap1 signal is crucial for the effect of calycosin, because calycosin increased the amount of nuclear Nrf2 while decreasing the amount of cytoplasmic Nrf2. Nrf2 knockdown with siRNA transfection abolished the neuroprotective effect of calycosin. Taken together, this study disclosed a novel mechanism by which calycosin combats oxidative stress. Our study thus sheds light on the potential clinical application of calycosin in SCI treatment.