MEF2D Mediates the Neuroprotective Effect of Methylene Blue Against Glutamate-Induced Oxidative Damage in HT22 Hippocampal Cells

Methylene blue (MB) can ameliorate behavioral, neurochemical, and neuropathological impairments in animal models of acute and chronic neurodegenerative disorders, but the underlying mechanism remains unclear. Myocyte enhancer factor 2 (MEF2D) is known to promote neuronal survival in several models, and several survival and death signals converge on MEF2D and regulate its activity. Here, we investigated the role of MEF2D in the neuroprotective effect of MB against glutamate-induced toxicity in HT22 neuronal cells. Our results showed that MB, event at less than 100 nM, improved the viability of HT22 cells exposed to 2 mM glutamate. MB attenuated the mitochondrial impairment and quenches the reactive oxygen species (ROS) induced by glutamate. Surprisingly, MB at 50–200 nM did not affect the Nrf2/HO-1 pathway, an important endogenous anti-oxidative system. Further study showed that MB increased the transcription and translation of MEF2D. In addition, MB upregulated the expression of mitochondrial NADH dehydrogenase 6 (ND6) in a MEF2D-dependent manner. Knockdown of MEF2D abolished both MB-medicated increase of ND6 and MB-induced neuroprotection against glutamate-induced toxicity. Moreover, we showed that MB promoted Akt function activity, suppressed GSK-3β activity, and increased MEF2D level in hippocampus of mice and HT22 cells. These findings for the first time demonstrate that MB protects HT22 neuronal cells against glutamate-induced cell death partially via the regulation of MEF2D-associated survival pathway.