Quantitative analysis of a novel glucosylated phospholipid by liquid chromatography-mass spectrometry

A world of sphingolipids and glycolipids in the brain--novel functions of simple lipids modified with glucose

Glycosphingolipids (GSLs) are present on cell surface membranes and are particularly abundant in the brain. Since over 300-400 GSLs are synthesized from glucosylceramide (GlcCer), GlcCer is believed to only serve as the source of most GSLs, including sialic acid-containing GSLs or gangliosides, in the brain. Recent studies, however, suggest that GlcCer itself plays a role in the heat stress response, as it functions as a glucose donor for the synthesis of cholesterylglucoside, a lipid mediator in heat stress responses in animals. GlcCer in adipose tissues is also thought to be involved in mechanisms that regulate energy (sugar and lipid) metabolism. Our extensive structural study revealed an additional novel glucosylated membrane lipid, called phosphatidylglucoside, in developing rodent brains and human neutrophils. These lipids, all modified with glucose, are enriched in lipid rafts and play important roles in basic cellular processes. Here, I summarize the recent progress regarding these glucosylated lipids and their biosynthesis and regulation in the central nervous system (CNS).

Quantification of human uridine-diphosphate glucuronosyl transferase 1A isoforms in liver, intestine, and kidney using nanobore liquid chromatography-tandem mass spectrometry

Uridine-disphosphate glucuronosyl transferase (UGT) enzymes catalyze the formation of glucuronide conjugates of phase II metabolism. Methods for absolute quantification of UGT1A1 and UGT1A6 were previously established utilizing stable isotope peptide internal standards with liquid chromatography-tandem mass spectrometry (LC-MS/MS). The current method expands upon this by quantifying eight UGT1A isoforms by nanobore high-performance liquid chromatography (HPLC) coupled with a linear ion trap time-of-flight mass spectrometer platform. Recombinant enzyme digests of each of the isoforms were used to determine assay linearity and detection limits. Enzyme expression level in human liver, kidney, and intestinal microsomal protein was determined by extrapolation from spiked stable isotope standards. Intraday and interday variability was <25% for each of the enzyme isoforms. Enzyme expression varied from 3 to 96 pmol/mg protein in liver and intestinal microsomal protein digests. Expression levels of UGT1A7, 1A8, and 1A10 were below detection limits (<1 pmol/mg protein) in human liver microsome (HLMs). In kidney microsomes the expression of UGT1A3 was below detection limits, but levels of UGT1A4, 1A7, 1A9, and 1A10 protein were higher relative to that of liver, suggesting that renal glucuronidation could be a significant factor in renal elimination of glucuronide conjugates. This novel method allows quantification of all nine UGT1A isoforms, many previously not amenable to measurement with traditional methods such as immunologically based assays. Quantitative measurement of proteins involved in drug disposition, such as the UGTs, significantly improves the ability to evaluate and interpret in vitro and in vivo studies in drug development.

Lipid rafts enriched in phosphatidylglucoside direct astroglial differentiation by regulating tyrosine kinase activity of epidermal growth factor receptors

Membrane lipid rafts provide a specialized microenvironment enriched with sphingolipids and phospholipids containing saturated fatty acids and serve as a platform for various intracellular signalling pathways. PtdGlc (phosphatidylglucoside) is a type of glycophospholipid localized in the outer leaflet of the plasma membrane. Owing to PtdGlc's unique fatty acid composition, exclusively composed of C18:0 at sn-1 and C20:0 at sn-2 of the glycerol backbone, it tends to form PGLRs (PtdGlc-enriched lipid rafts). Previously, we demonstrated that PGLRs reside on the cell surface of astroglial cells from fetal rat brain [Nagatsuka, Horibata, Yamazaki, Kinoshita, Shinoda, Hashikawa, Koshino, Nakamura and Hirabayashi (2006) Biochemistry 45, 8742–8750]. In the present study, we observed PGLRs in astroglial lineage cells at mid-embryonic to early-postnatal stages of developing mouse cortex. This suggests that PGLRs are developmentally correlated with astroglial differentiation during fetal cortical development. Our cell culture studies with multipotent neural progenitor cells prepared from fetal mouse telencephalon demonstrated that treatment with EGF (epidermal growth factor) or anti-PtdGlc antibody caused recruitment of EGFRs (EGF receptors) into lipid raft compartments, leading to activation of EGFRs. Moreover, the activation of EGFRs by antibody triggered downstream tyrosine kinase signalling and induced marked GFAP (glial fibrillary acidic protein) expression via the JAK (Janus kinase)/STAT (signal transducer and activator of transcription) signalling pathway. These findings strongly suggest that PGLRs are physiologically coupled to activated EGFRs on neural progenitor cells during fetal cortical development, and thereby play a distinct role in mediating astrogliogenesis.