Treadmill exercise attenuates CUMS-induced depressive behaviors by modulating the UPRmt via the Nrf2/Keap1 pathway

Depression is a very common brain disorder worldwide. It is associated with damaging behaviors such as self-harm and suicide that hurt the brain and body. Mitochondrial dysfunction and dysregulation of the mitochondrial unfolded protein response (UPRmt) are increasingly seen as playing a key role in chronic stress-induced pathophysiology of depression. Aerobic exercise is a very effective non-pharmacological intervention, however, the specific mechanisms of how it modifies hippocampal UPRmt is poorly understood. This effect may be attributed to the Nrf2/Keap1 pathway, a master regulator of cellular antioxidant defense that mediates exercise-induced neuroprotection. This study aimed to investigate whether treadmill exercise modulates UPRmt dysregulation in CUMS-induced depressive mice by activating the hippocampal Nrf2/Keap1 signaling pathway, thereby ameliorating mitochondrial dysfunction and depression-like behaviors.

Male C57BL/6 J mice were subjected to a chronic unpredictable model of stress for the induction of a depression model which through exercise on a treadmill for 6 weeks was tested for therapeutic effects. Behaviors consistent with a model of depressive behavior were assessed by OFT, SPT, and TST. Hippocampal mitochondrial function was assessed by transmission electron microscopy, flow cytometry, biochemical, and ELISA. Oxidative stress markers were assessed with biochemical kits and ELISA. The mRNA and protein levels of key markers in the Nrf2/Keap1 pathway and UPRmt were analyzed using RT-qPCR, Western blotting, and immunofluorescence. Pathway dependence was determined using the Nrf2 inhibitor, ML385 and the Nrf2 activator Bardoxolone methyl.

The six-week treadmill exercise program significantly reduced depression-like behaviors (e.g., anxiety-like behaviors, anhedonia, and behavioral despair), restored mitochondrial functions (mitochondrial cristae morphology, Δψm, ATP, and ROS levels) and eliminated oxidative stress (SOD, T-AOC, and MDA levels). Moreover, treadmill exercise significantly increased the expression of proteins in the Nrf2/Keap1 pathway (Nrf2, Keap1, NQO1, and HO-1), which attenuated CUMS-induced UPRmt markers (HSP60, ClpP, HSP70, LONP1, and ATF5) and the associated stress transcription factor CHOP.

The treadmill exercise activates the Nrf2/Keap1 pathway in the hippocampus, thereby reducing CUMS-induced excessive and dysregulated endoplasmic reticulum stress (UPRmt) in an Nrf2-dependent manner, which leads to a recovery of mitochondrial function, suppression of oxidative stress, and improvement of depressive-like behaviors.