Fat finding in disease: Lipids coming 'ome

Lipidomics Gateway (24 June 2009) [doi:10.1038/lipidmaps.2009.12]

New lipidomics techniques define specific lipid profiles produced during disease states or found on invading pathogens, opening new avenues for treatment.

A huge, dynamic array of highly related lipid species inhabits cells. Previously, technical difficulties with separation and detection of lipid molecules made the goal of an integrated picture of lipid biology impossible. Now, researchers are developing new techniques to comprehensively analyze entire classes of lipids or lipid-modified proteins, providing invaluable insights into disease processes. As reported in the Journal of Biological Chemistry, Blaho et al. chose one such process, the inflammatory response in Lyme arthritis, to demonstrate the utility of their eicosanoid monitoring protocols. Similarly, Nakayasu et al. have developed a technique to identify glycosylphosphatidylinositol (GPI)–anchored molecules and used it to characterize a pathogenic parasite, published in Molecular Systems Biology. Both approaches could be applied to other diseases or infectious organisms, with the potential to identify novel therapeutic targets.

The Ixodes ricinus tick is a vector for Lyme disease, caused by Borrelia bacteria.
Richard Bartz

Lyme disease is caused by infection with Borrelia species of Gram-negative bacteria, transmitted by ticks. If untreated, the disease will usually develop into an inflammatory arthritis. Eicosanoids, a large class of signaling lipids including prostaglandins and lipoxins, participate in inflammatory responses. However, an integrated picture of their involvement from initiation to resolution of inflammation has been lacking. As part of the LIPID MAPS initiative, Blaho et al. developed a complete eicosanoid extraction protocol. Pulverizing snap-frozen joints from Lyme arthritis-resistant and -susceptible mouse strains with and without infection, the authors found that a key incubation step was critical for maximum eicosanoid extraction. Deuterated standards representing all major eicosanoid biosynthetic pathways were added before purified extracts were separated by reverse-phase liquid chromatography (LC) and analyzed by multiple-reaction monitoring (MRM) mass spectrometry (MS).

Using this strategy, the authors were able to quantify about 90% of known eicosanoid species, compared with fewer than 50% possible with previous methods which also involved longer run times. They identified significant differences between basal and infection-induced eicosanoid production. During disease progression, pro-inflammatory species that promote neutrophil recruitment were counterbalanced by others that may facilitate infiltration of anti-inflammatory macrophages. Unexpectedly, they found that products of cyclooxygenase 2 (COX-2), the enzyme responsible for prostaglandin production, were involved in the resolution of inflammation as well as the onset.

The LC–MS/MS approach is now the preferred methodology for lipid analysis and was also used by Nakayasu et al. in their analysis of the parasite Trypanosoma cruzi. This protozoan causes the potentially fatal Chagas disease and is extensively coated with GPI-anchored molecules. The complex, amphiphilic lipid–glycan structure of GPI has hindered high-resolution separations. To address this, the authors used a nanocapillary column packed with a hydrophobic resin, coupled to an LC–MS system. Quantification was achieved by a total ion current MS2 approach, standardized with synthetic GPIs. Their system identified 79 species of protein-free GPIs (or glycoinositolphospholipids, GIPLs), 70 of them novel, and 12 different species of GPI attached to proteins, of which only three had been previously characterized. The diversity of GPI-anchored molecules on T. cruzi is thought to contribute to its immune evasion: thus a baseline profile is imperative for screening drugs targeted against parasite GPI biosynthesis.

Both of these studies demonstrate how lipidomic analyses of specific lipid classes are becoming increasingly comprehensive as methodologies improve. Establishing baseline profiles allows analysis of changes during disease or after drug administration, whereas understanding how bioactive lipids participate in disease processes presents a whole extra level of potential interventions, compared with targeting genes or proteins alone. Seeing the whole picture also places these interventions in context, for example demonstrating that interfering with COX-2 function to prevent swelling has the undesired effect of impeding resolution of inflammation by altering global eicosanoid profiles.

Emma Leah