Uncoupling Protein 2 Inhibition Exacerbates Glucose Fluctuation-Mediated Neuronal Effects

Though glucose fluctuations have been considered as an adverse factor for the development of several diabetes-related complications, their impact in the central nervous system is still not fully elucidated. This study was conducted to evaluate the responses of neuronal cells to different glycemic exposures alongside to elucidate the role of uncoupling protein 2 (UCP2) in regulating such responses. To achieve our goals, primary cortical neurons were submitted to constant high (HG)/low (LG) or glucose level variations (GVs), and the pharmacological inhibition of UCP2 activity was performed using genipin. Results obtained show that GV decreased neuronal cells' viability, mitochondrial membrane potential, and manganese superoxide dismutase activity and increased reactive oxygen species (ROS) production. GV also caused an increase in the glutathione/glutathione disulfide ratio and in the protein expression levels of nuclear factor E2-related factor 2 (NRF2), UCP2, NADH-ubiquinone oxidoreductase chain 1 (ND1), and mitochondrially encoded cytochrome c oxidase I (MTCO1), both mitochondrial DNA encoded subunits of the electron transport chain. Contrariwise, genipin abrogated all those compensations and increased the levels of caspase 3-like activity, potentiated mitochondrial ROS levels, and the loss of neuronal synaptic integrity, decreased the protein expression levels of NRF1, and increased the protein expression levels of UCP5. Further, in the control and LG conditions, genipin increased mitochondrial ROS and the protein expression levels of UCP4, postsynaptic density protein 95 (PSD95), ND1, and MTCO1. Overall, these observations suggest that UCP2 is in the core of neuronal cell protection and/or adaptation against GV-mediated effects and that other isoforms of neuronal UCPs can be upregulated to compensate the inhibition of UCP2 activity.