The unfolded protein response (UPR) is an adaptive stress response that responds to the accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER) and that adjusts the protein-folding capacity to the needs of the cell. Perturbation of cellular lipids also activates the UPR. Lipid-induced UPR has attracted much attention because it is associated with the pathology of some metabolic diseases. However, how the lipid-induced UPR is activated remains unclear. We previously showed that palmitic acid treatment or knockdown of stearoyl-CoA desaturase in HeLa cells promotes membrane lipid saturation and activates the UPR. In this study, we compared UPR activation by membrane lipid saturation with UPR activation by conventional ER stressors that cause the accumulation of unfolded proteins such as tunicamycin and thapsigargin. Membrane lipid saturation induced autophosphorylation of inositol-requiring 1a (IRE1a) and protein kinase RNA-like ER kinase, but not the conversion of activating transcription factor-6a to the active form. A conventional ER stressor induced clustering of fluorescently tagged IRE1a fusion protein, but palmitic acid treatment did not, suggesting that IRE1a was activated without large cluster formation by membrane lipid saturation. Together, these results suggest membrane lipid saturation, and unfolded proteins activate the UPR through different mechanisms.

The giant clam Tridacna crocea inhabits shallow tropical seas with poorly nourished water and severe sun irradiation. They harbor symbiotic algae zooxanthellae (dinoflagellate family Symbiodiniaceae) in the mantle tissue and are thought to thrive in this extreme environment by utilizing photosynthetic products from the algae. However, there is no measure of the detailed metabolic flow between the host and symbiont to evaluate one of the most successful symbiotic relationships in nature. Here, we employed liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based lipidomics and Fourier-transform ion cyclotron resonance MS imaging on T. crocea tissues, revealing a unique lipid composition and localization with their symbiont algae. We discovered that the non-phosphorous microalgal betaine lipid diacylglycerylcarboxy-hydroxymethylcholine (DGCC) was present in all tissues and organs of T. crocea to approximately the same degree as phosphatidylcholine (PC). The fatty acid composition of DGCC was similar to that of PC, which is thought to have physiological roles similar to that of DGCC. MS imaging showed co-localization of these lipids throughout the clam tissues. Glycerylcarboxy-hydroxymethylcholine (GCC), the deacylated derivative of DGCC, was found to be a free form of DGCC in the clams and was isolated and characterized from cultured Symbiodiniaceae strains that were isolated from giant clams. These results strongly suggest that giant clams have evolved to smartly utilize DGCCs, phosphorus-free polar lipids of symbiont algae, as essential membrane components to enable them to thrive in oligotrophic coral reef milieu.

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