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Jun 1, 2013 (Vol. 33, No. 11)

Deciphering the Mysteries in Lipid Biology

  • Eicosanoid Signatures

    Mounting evidence indicates that saturated fatty acids amplify inflammation. Fatty acids can activate G-protein–coupled receptors and initiate the arachidonic acid cascade. This in turn leads to production of eicosanoids, lipid mediators of inflammation.

    “Global lipid analysis is indispensable in quantitation of all products in the inflammatory cascade,” says Dr. Dennis. “One of the major achievements of the LIPID MAPS consortium is development of over 500 standards covering eight main lipid categories in plasma, as well as 150 authentic eicosanoid standards. Now we can follow production of eicosanoids in response to inflammatory stimuli and correlate the molecular profiles with physiological manifestations of inflammation.”

    In the study of inflammation caused by Borrelia (an agent of Lyme disease), the team identified a number of unusual eicosanoids, including resolvins and protectins thought to help resolve inflammation.

    “We are just beginning to explore ways to increase the levels of beneficial anti-inflammatory eicosanoids,” says Dr. Dennis. “By applying lipidomic tools to model systems we can follow eicosanoid fluxes in response to inflammatory stimuli. Then we overlay the effects of dietary supplements.”

    While dietary fish oil, rich in omega-3 fatty acids, was long known to elicit anti-inflammatory effects, the exact mechanism of action was not understood. Using the comprehensive analysis of the whole lipid cascade, Dr. Dennis and colleagues were able to demonstrate the direct mechanistic effects of omega-3 fatty acids on the production of mediators.

    “Current technological achievements in lipidomics offer incredible opportunities to understand diseases and then to find mechanisms to treat them,” explains Dr. Dennis. “Investigation of hyperalgesia, or hypersensitivity caused by previous trauma, such as a burn or mechanical injury, is an example of such lipidomic discovery.”

    Peripheral injury generates eicosanoid “memory signatures” in neurons. A new discovery showed that hepoxilins, metabolites in these signatures, act on well-known receptors of calcium signaling. These findings now enable development of anti-inflammatory therapeutics targeting these receptors.

  • Global Triglyceride Profiles

    Click Image To Enlarge +
    Fecal lipidomics profiling opens a new avenue into the study of nutrition, gastrointestinal health, and disease, especially in high risk, difficult to sample populations such as premature infants. Ongoing studies are using this approach to study nutrition, development, and disease in premature infants at Brigham and Women’s Hospital. [Figure adapted from and mass chromatogram reprinted with permission from Gregory et al., Analytical Chemistry, 2013. Infant and test tube picture from Creative Commons. Baby photo by tamakisono, test tube photo by Trondheim Havn.]

    “The field of lipidomics is opening up,” says Bruce Kristal, Ph.D., associate professor of surgery, department of neurosurgery at Brigham and Women’s Hospital/Harvard Medical School. “Development of enabling technologies provides an opportunity to examine broad lipid profiles while maintaining the concurrent ability to accurately identify and quantify each one. We can now reach deep into the lipidome in a single MS run.”

    Dr. Kristal’s team applies global lipidomic profiling to probe lipidomic changes mediated by diet and biological processes. Although LIPID MAPS developed many standards for lipid identification, these remain insufficient to cover the entire structural diversity. “We often have to identify the metabolites de novo, using mass, retention time, and fragmentation pattern as inputs. Combined with biological knowledge, this gives us a pretty good idea what we are looking at,” says Dr. Kristal.

    This discovery paradigm was most recently adapted to study premature infants. This high-risk population commonly suffers from GI morbidities. The ability to recognize problems with intestinal functions in early infancy would help to optimize their care. Lipids were collected from the fecal matter using an extraction technique developed specifically for this purpose.

    “The fecal lipidome is very complex,” continues Dr. Kristal. “We found over 300 known endogenous lipid metabolites, as well as others that seem to be quite different from plasma species. Most of these are not yet known.” The profiling method developed is broadly applicable to biological samples where lipids play a key role. This includes the analysis of triglycerides, which is standard in medicine, but provides a rather crude disease diagnostic based on the sum total of triglycerides.

    “This will change in the near future,” predicts Dr. Kristal. “We will conceivably profile all 120+ triglycerides and follow each triglyceride along with, for example, dietary changes in each individual patient.”

  • Sterol Analytics

    Cholesterol is the most well-known member of sterol family of lipids, and it serves as a precursor of many key molecules. Sterols participate in nearly every important cellular function, but are mostly recognized for their causative effect on atherosclerotic plaque.

    “We are after the undiscovered roles of sterol metabolites,” says David Russell, Ph.D., professor of molecular genetics, University of Texas Southwestern Medical Center. “Mutations in genes encoding sterol metabolic enzymes lead to serious liver and endocrine problems. And yet, until now we did not possess a robust ability to evaluate all cholesterol precursors and derivatives at once.”

    Dr. Russell’s team developed a method to reproducibly detect and analyze >60 sterols in just 100 microliters of blood, plasma, or urine. “To develop this method, we first reviewed the collective scientific wisdom for sterol purification. Next, we optimized each analytical step using dozens of different sterol standards to ensure maximum recovery at each step,” continues Dr. Russell. “The initial method development was lengthy, but now we have a rapid and quantitative method to analyze a majority of sterols in human plasma.”

    This analytical method was deployed to evaluate an unbiased population sample, the Dallas Heart Study. The sample consists of “typical” residents of Dallas county aged 18 to 65, who underwent multiple blood tests, diagnostic imaging, and health surveys. The team focused on 20 consistently detected sterols and created sterol profiles for over 3,000 samples. “Our approach combines sterol analytics with SNP genotyping and clinical profiles,” adds Dr. Russell. In collaboration with Merck, the Russell team is searching for correlations between sterol profiles and genetic alleles. If found, such correlations may reveal a new biomarker of disease.

    Simultaneously, the team is evaluating samples from patients with clearly defined diseases, such as West Nile Virus, common influenza, and septicemia. The researchers hope that sterol profiles may provide a means to detect an infection before its clinical manifestation.

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