Localizing lipids: Subcellular distribution

Lipidomics Gateway (26 August 2010) [doi:10.1038/lipidmaps.2010.25]

Lipidomic study analyzes the subcellular localization of major membrane lipids in resting and activated macrophages.

Modern mass spectrometry methods have revolutionized the analysis of lipids, but detailed localization data lag behind proteomic research. As lipids can adopt either signaling or structural roles, in which they function remotely or locally, respectively, understanding how lipids affect cellular responses requires knowledge of their distribution within the cell.

Now, using a combination of subcellular fractionation and liquid chromatography mass spectrometry, the LIPID MAPS consortium has analyzed the subcellular lipidome of a mammalian macrophage, in both resting and activated states, and reports its findings in the Journal of Lipid Research.

Researchers from the consortium isolated nuclei, mitochondria, endoplasmic reticulum (ER), dense microsomes, plasmalemma and cytoplasm from RAW 264.7 macrophages, and identified 163 glycerophospholipids, 48 sphingolipids, 13 sterols and 5 prenols in these cells, with the highest abundance in the plasmalemma, followed by decreasing amounts in the ER, mitochondria, nuclei, dense microsomes and cytoplasm.

Differences were apparent, however, in the distribution of individual lipids and classes, and the lipid profiles, between organelles. For example, ubiquinones were found almost exclusively in mitochondria, as would be expected; and the most abundant phospholipids in these organelles were phosphatidylcholines PC(36:2), PC(36:1), whereas in the ER, high levels of phosphatidylethanolamine PE(36:2) and ether-linked phosphatidylethanolamine (plasmalogen) PE(P-38:4) were present.

For more accurate information about the spatial specificity of lipid distribution, the researchers excluded contributions from organellar and species factors (the relative amount of membrane in the organelle and the total amount of the particular lipid in the whole cell, respectively). When applied to the class of prenol lipids, this analysis showed, for example, that the specific localization of dolichol was even more pronounced for nuclei than for the ER.

Next, members of the consortium analyzed the response of the macrophages to activation of Toll-like receptor-4 by Kdo2-Lipid A (KLA). KLA induced considerable remodeling of the subcellular lipidome. Some of the reported changes in lipid composition affected all organelles to the same extent. For instance, the levels of ceramides increased throughout the cell, consistent with a previous report; levels of the cholesterol precursors lanosterol and desmosterol also increased globally. By contrast, other changes affected particular organelles. In mitochondria, for example, the levels of highly unsaturated cardiolipin species (those with four or more double bonds) decreased, whereas those that were less unsaturated increased, as did levels of oxidized sterols. In the ER, levels of ether-linked phospholipids decreased, despite increasing in all other organelles.

As the authors indicate, their resulting dynamic lipidome is "a powerful hypothesis-building engine", enabling them to propose a number of testable theories. Describing their report as "the first step on a long road to the complete subcellular lipid mapping of the cell", the consortium acknowledges that extending the level of lipidomics analysis to include minor, modified, and even more individual molecular species of lipids, as well as suborganellar analysis, would expedite this journey.

Katrin Legg