The lipid composition of a membrane greatly influences how it functions, yet we know little about the mechanisms that regulate its maintenance. Although there is a continual flux of new fatty acids to maintain its composition, there are no current methods that allow an understanding of the dynamics and underlying mechanisms. Here, we developed mass spectrometry based approaches to compare multiple lipid populations simultaneously and found different dynamics of fatty acid replacement in phospholipids and neutral lipids. Because Stearoyl-CoA desaturases are known regulators of membrane composition, we used adult-only SCD RNAi to look for changes in membrane dynamics when the proper fatty acids are not available. Strikingly, we also found a reduction in the turnover of fatty acids produced independently of SCDs, demonstrating a more general role for these enzymes in membrane maintenance. Furthermore, we extended our technique to analyze intact phospholipids with a novel 15N approach by LC-MS/MS and found that SCD RNAi led to a significant reduction in the turnover of nearly all the phospholipid molecules examined. Taken together, the 13C and 15N feeding approaches have revealed a novel role for SCDs in influencing the overall rates of membrane maintenance. Finally, we identified compensatory responses in overall phospholipid structure as SCD RNAi results in major adaptations to the membrane structure by LC-MS/MS, most notably a pairing of saturated fat with highly polyunsaturated fatty acid that suggests active remodeling. The implementation of stable isotopes to monitor membrane maintenance in C. elegans has identified novel facets of membrane regulation particularly a role for Stearoyl-CoA desaturases in controlling rates of membrane rejuvenation.
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