Oxidative stress contributes to autophagy induction in response to endoplasmic reticulum stress in Chlamydomonas

The accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER) results in the activation of stress responses such as the unfolded protein response or the catabolic process of autophagy to ultimately recover cellular homeostasis. ER stress also promotes the production of reactive oxygen species (ROS), which play an important role in autophagy regulation. However, it remains unknown whether ROS are involved in ER stress-induced autophagy. In the present study, we provide evidence connecting redox imbalance caused by ER stress and autophagy activation in the model unicellular green alga Chlamydomonas reinhardtii. Treatment of Chlamydomonas cells with the ER stressors tunicamycin or dithiothreitol resulted in upregulation of the expression of genes encoding ER resident ERO1 oxidoreductase and protein disulfide isomerases. ER stress also triggered autophagy in Chlamydomonas based on the protein abundance, lipidation, cellular distribution and mRNA levels of the autophagy marker ATG8. Moreover, an increase in the oxidation of the glutathione pool and in the expression of oxidative stress-related genes were detected in tunicamycin-treated cells. Our results revealed that the antioxidant glutathione partially suppressed ER stress-induced autophagy and decreased the toxicity of tunicamycin, suggesting that oxidative stress participates in the control of autophagy in response to ER stress in Chlamydomonas. In close agreement, we also found that autophagy activation by tunicamycin was more pronounced in the Chlamydomonas sor1 mutant, which shows increased expression of oxidative stress-related gene