Abstract
Lipid microdomains ('lipid rafts') are plasma membrane subregions, enriched in cholesterol and glycosphingolipids, which participate dynamically in cell signaling and molecular trafficking operations. One strategy for the study of the physicochemical properties of lipid rafts in model membrane systems has been the use of nuclear magnetic resonance (NMR), but until now this spectroscopic method has not been considered a clinically relevant tool. We performed a proof-of-concept study to test the feasibility of using NMR to study lipid rafts in human tissues. Platelets were selected as a cost-effective and minimally invasive model system in which lipid rafts have previously been studied using other approaches. Platelets were isolated from plasma of medication-free adult research participants (n=13) and lysed with homogenization and sonication. Lipid-enriched fractions were obtained using a discontinuous sucrose gradient. Association of lipid fractions with GM1 ganglioside was tested using HRP-conjugated cholera toxin B subunit dot blot assays. (1)H high resolution magic-angle spinning nuclear magnetic resonance (HRMAS NMR) spectra obtained with single-pulse Bloch decay experiments yielded spectral linewidths and intensities as a function of temperature. Rates of lipid lateral diffusion that reported on raft size were measured with a two-dimensional stimulated echo longitudinal encode-decode NMR experiment. We found that lipid fractions at 10-35% sucrose density associated with GM1 ganglioside, a marker for lipid rafts. NMR spectra of the membrane phospholipids featured a prominent 'centerband' peak associated with the hydrocarbon chain methylene resonance at 1.3 ppm; the linewidth (full width at half-maximum intensity) of this 'centerband' peak, together with the ratio of intensities between the centerband and 'spinning sideband' peaks, agreed well with values reported previously for lipid rafts in model membranes. Decreasing temperature produced decreases in the 1.3 ppm peak intensity and a discontinuity at ~18 °C, for which the simplest explanation is a phase transition from L(d) to L(o) phases indicative of raft formation. Rates of lateral diffusion of the acyl chain lipid signal at 1.3 ppm, a quantitative measure of microdomain size, were consistent with lipid molecules organized in rafts. These results show that HRMAS NMR can characterize lipid microdomains in human platelets, a methodological advance that could be extended to other tissues in which membrane biochemistry may have physiological and pathophysiological relevance.