Sterol Biosynthesis Contributes to Brefeldin-A-Induced Endoplasmic Reticulum Stress Resistance in Chlamydomonas reinhardtii.

The endoplasmic reticulum (ER) stress response is an evolutionarily conserved mechanism in most eukaryotes. In this response, sterols in the phospholipid bilayer play a crucial role in controlling membrane fluidity and homeostasis. Despite the significance of both the ER stress response and sterols in maintaining ER homeostasis, their relationship remains poorly explored. Our investigation focused on Chlamydomonas strain CC-4533 and revealed that free sterol biosynthesis increased in response to ER stress, except in mutants of the ER stress sensor IRE1. Transcript analysis of Chlamydomonas experiencing ER stress unveiled the regulatory role of the IRE1/bZIP1 pathway in inducing the expression of ERG5, which encodes C-22 sterol desaturase. Through the isolation of three erg5 mutant alleles, we observed a defect in the synthesis of Chlamydomonas' sterol end products, ergosterol and 7-dehydroporiferasterol. Furthermore, these erg5 mutants also exhibited increased sensitivity to ER stress induced by brefeldin A (BFA, an inhibitor of ER-Golgi trafficking), whereas tunicamycin (Tm, an inhibitor of N-glycosylation) and dithiothreitol (DTT, an inhibitor of disulfide-bond formation) had no such effect. Intriguingly, the sterol biosynthesis inhibitors fenpropimorph (Fp) and fenhexamid (Fh), which impede steps upstream of the ERG5 enzyme in sterol biosynthesis, rescued BFA hypersensitivity in CC-4533 cells. Collectively, our findings support the conclusion that the accumulation of intermediates in the sterol biosynthetic pathway influences ER stress in a complex manner. This study highlights the significance and complexity of regulating sterol biosynthesis during the ER stress response in microalgae.

Effect of Common ER Stress-Inducing Drugs on the Growth and Lipid Phenotypes of Chlamydomonas and Arabidopsis.

Endoplasmic reticulum (ER) stress is caused by the stress-induced accumulation of unfolded proteins in the ER. Several compounds are used to induce the unfolded protein response (UPR) in animals, with different modes of action, but which ER stress-inducing drugs induce ER stress in microalgae or land plants is unclear. In this study, we examined the effects of seven chemicals that were reported to induce ER stress in animals on the growth, UPR gene expression and fatty acid profiles of Chlamydomonas reinhardtii (Chlamydomonas) and Arabidopsis thaliana (Arabidopsis): 2-deoxyglucose, dithiothreitol (DTT), tunicamycin (TM), thapsigargin, brefeldin A (BFA), monensin (MON) and eeyarestatin I. In both model photosynthetic organisms, DTT, TM, BFA and MON treatment induced ER stress, as indicated by the induction of spliced bZIP1 and bZIP60, respectively. In Chlamydomonas, DTT, TM and BFA treatment induced the production of transcripts related to lipid biosynthesis, but MON treatment did not. In Arabidopsis, DTT, TM, BFA and MON inhibited seed germination and seedling growth with the activation of bZIP60. These findings lay the foundation for using four types of ER stress-inducing drugs in photosynthetic organisms, and they help uncover the mode of action of each compound.

Biotechnological Approaches for Biomass and Lipid Production Using Microalgae Chlorella and Its Future Perspectives.

Heavy reliance on fossil fuels has been associated with increased climate disasters. As an alternative, microalgae have been proposed as an effective agent for biomass production. Several advantages of microalgae include faster growth, usage of non-arable land, recovery of nutrients from wastewater, efficient CO2 capture, and high amount of biomolecules that are valuable for humans. Microalgae Chlorella spp. are a large group of eukaryotic, photosynthetic, unicellular microorganisms with high adaptability to environmental variations. Over the past decades, Chlorella has been used for the large-scale production of biomass. In addition, Chlorella has been actively used in various food industries for improving human health because of its antioxidant, antidiabetic, and immunomodulatory functions. However, the major restrictions in microalgal biofuel technology are the cost-consuming cultivation, processing, and lipid extraction processes. Therefore, various trials have been performed to enhance the biomass productivity and the lipid contents of Chlorella cells. This study provides a comprehensive review of lipid enhancement strategies mainly published in the last five years and aimed at regulating carbon sources, nutrients, stresses, and expression of exogenous genes to improve biomass production and lipid synthesis.