Glucosylceramide in the nervous system--a mini-review

Neuronal activity induces glucosylceramide that is secreted via exosomes for lysosomal degradation in glia

Recessive variants in GBA1 cause Gaucher disease, a prevalent form of lysosome storage disease. GBA1 encodes a lysosomal enzyme that hydrolyzes glucosylceramide (GlcCer) into glucose and ceramide. Its loss causes lysosomal dysfunction and increased levels of GlcCer. We generated a null allele of the Drosophila ortholog Gba1b by inserting the Gal4 using CRISPR-Cas9. Here, we show that Gba1b is expressed in glia but not in neurons. Glial-specific knockdown recapitulates the defects found in Gba1b mutants, and these can be rescued by glial expression of human GBA1. We show that GlcCer is synthesized upon neuronal activity, and it is transported from neurons to glia through exosomes. Furthermore, we found that glial TGF-β/BMP induces the transfer of GlcCer from neurons to glia and that the White protein, an ABCG transporter, promotes GlcCer trafficking to glial lysosomes for degradation.

Ceramide glycosylation catalyzed by glucosylceramide synthase and cancer drug resistance

Glucosylceramide synthase (GCS), converting ceramide to glucosylceramide, catalyzes the first reaction of ceramide glycosylation in sphingolipid metabolism. This glycosylation by GCS is a critical step regulating the modulation of cellular activities by controlling ceramide and glycosphingolipids (GSLs). An increase of ceramide in response to stresses, such as chemotherapy, drives cells to proliferation arrest and apoptosis or autophagy; however, ceramide glycosylation promptly eliminates ceramide and consequently, these induced processes, thus protecting cancer cells. Further, persistently enhanced ceramide glycosylation can increase GSLs, participating in selecting cancer cells to drug resistance. GCS is overexpressed in diverse drug-resistant cancer cells and in tumors of breast, colon, and leukemia that display poor response to chemotherapy. As ceramide glycosylation by GCS is a rate-limiting step in GSL synthesis, inhibition of GCS sensitizes cancer cells to anticancer drugs and eradicates cancer stem cells. Mechanistic studies indicate that uncoupling ceramide glycosylation can modulate gene expression, decreasing MDR1 through the cSrc/β-catenin pathway and restoring p53 expression via RNA splicing. These studies not only expand our knowledge in understanding how ceramide glycosylation affects cancer cells but also provide novel therapeutic approaches for targeting refractory tumors.

Development of Adamantan-1-yl-methoxy-Functionalized 1-Deoxynojirimycin Derivatives as Selective Inhibitors of Glucosylceramide Metabolism in Man

In this article, we present a straightforward synthesis of adamantan-1-yl-methoxy-functionalized 1-deoxynojirimycin derivatives. The used synthetic routes are flexible and can be used to create a wide variety of lipophilic mono- and difunctionalized 1-deoxynojirimycin derivatives. The compounds reported here are lipophilic iminosugar based on lead compound 4, a potent inhibitor of the three enzymes involved in the metabolism of the glycosphingolipid glucosylceramide. Iminosugar-based inhibitors of glucosylceramide synthase, one of these three enzymes, have attracted increasing interest over the past decade due to the crucial role of this enzyme in glycosphingolipid biosynthesis. Combined with the fact that an increasing number of pathological processes are being linked to excessive glycosphingolipid levels, glucosylceramide synthase becomes a very attractive therapeutic and research target. Our results presented here demonstrate that relocating the lipophilic moiety from the nitrogen atom to other positions on the 1-deoxynojirimycin ring system does not lead to a more potent or selective inhibitor of glucosylceramide synthase. The beta-aza-C-glycoside analogue (17) retained the best inhibitory potency for glucosylceramide synthase and is a more potent inhibitor than the therapeutic agent N-butyl-1-deoxynojirimycin (3), marketed as treatment for Gaucher disease under the commercial name Zavesca.

Glucosylceramide synthase activity and ceramide levels are modulated during cerebral ischemia after ischemic preconditioning

After 24-hour middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats, brain ceramide level increased from baseline reached 595% (ischemic core) and 460% (perifocal/penumbral areas); brain glucosylceramide synthase (GCS) activities in these areas simultaneously decreased by 70% and 50%, respectively. Ten-minute MCAO preconditioning significantly attenuated 24-hour MCAO-induced ceramide accumulation by 40% to 60% in ischemic core and perifocal areas, and GCS activities improved by 60% to 70% in both areas. Thus, potentially toxic levels of brain ceramide induced by MCAO were attenuated to intermediate levels in preconditioned animals; brain GCS activity was relatively preserved. In ischemic tolerance, GCS appears to modulate otherwise high levels of brain ceramide.

Determination of galactose and galactocerebroside using a galactose oxidase column and electrochemical detector

A method has been developed to measure galactose and galactocerebroside using galactose oxidase immobilized on a solid resin. Galactose oxidase converts galactose and galactocerebroside to their corresponding aldehydes and hydrogen peroxide, the latter being electroactive and measurable by electrochemical detection using DC amperometric detection. The minimal detection limits of galactose and galactocerebroside were 1 and 2 microM, respectively. The linear response to galactose and galactocerebroside was to at least 300 microM. About 100 samples can be measured per hour using flow injection analysis. The activity of sulfatidase (cerebroside-3-sulfate-3-sulfohydrolase), which converts sulfatide (sulfogalactocerebroside) to galactocerebroside, was measured, and its inhibition by O-phospho-L-tyrosine was determined.

Synthesis of non-hydroxy-galactosylceramides and galactosyldiglycerides by hydroxy-ceramide galactosyltransferase

Galactosylceramide (GalCer) is the major glycolipid in brain. In order to characterize the activity of brain UDPgalactose: ceramide galactosyltransferase (CGalT), it has been stably expressed in CGalT-negative Chinese hamster ovary (CHO) cells. After fractionation of transfected cells, CHO-CGT, on sucrose gradients, the activity resides at the density of endoplasmic reticulum and not of Golgi. A lipid chromatogram from CHO-CGT cells revealed two new iodine-staining spots identified as GalCer, since they comigrate with GalCer standards, can be metabolically labelled with [3H]galactose, are recognized by anti-GalCer antibodies, and are resistant to alkaline hydrolysis. A third [3H]galactose lipid was identified as galactosyldiglyceride. In the homogenate CGalT displays a 25-fold preference for hydroxy fatty acid-containing ceramides. Remarkably, endogenous GalCer of transfected cells contains exclusively non-hydroxy fatty acids: fast atom bombardment and collision-induced dissociation mass spectrometric analysis revealed mainly C16:0 in the lower GalCer band on TLC and mainly C22:0 and C24:0 in the upper band. Our results suggest that CGalT galactosylates both hydroxy- and non-hydroxy fatty acid-containing ceramides and diglycerides, depending on their local availability. Thus, CGalT alone may be responsible for the synthesis of hydroxy- and non-hydroxy-GalCer, and galactosyldiglyceride in myelin.

Expression cloning of a cDNA for human ceramide glucosyltransferase that catalyzes the first glycosylation step of glycosphingolipid synthesis

We have isolated a cDNA encoding human ceramide glucosyltransferase (glucosylceramide synthase, UDP-glucose:N-acylsphingosine D-glucosyltransferase, EC 2.4.1.80) by expression cloning using as a recipient GM-95 cells lacking the enzyme. The enzyme catalyzes the first glycosylation step of glycosphingolipid synthesis and the product, glucosylceramide, serves as the core of more than 300 glycosphingolipids. The cDNA has a G+C-rich 5' untranslated region of 290 nucleotides and the open reading frame encodes 394 amino acids (44.9 kDa). A hydrophobic segment was found near the N terminus that is the potential signal-anchor sequence. In addition, considerable hydrophobicity was detected in the regions close to the C terminus, which may interact with the membrane. A catalytically active enzyme was produced from Escherichia coli transfected with the cDNA. Northern blot analysis revealed a single transcript of 3.5 kb, and the mRNA was widely expressed in organs. The amino acid sequence of ceramide glucosyltransferase shows no significant homology to ceramide galactosyltransferase, which indicates different evolutionary origins of these enzymes.

Sorting of newly synthesized galactosphingolipids to the two surface domains of epithelial cells

The high concentration of glycosphingolipids on the apical surface of epithelial cells may be generated by selective transport from their site of synthesis to the cell surface. Previously, we showed that canine kidney MDCK and human intestinal Caco-2 cells converted a ceramide carrying the short fluorescent fatty acid C6-NBD to glucosylceramide (GlcCer) and sphingomyelin (SM), and that GlcCer was preferentially transported to the apical surface as compared to SM. Here, we address the point that not all glycosphingolipid classes are apically enriched in epithelia. We show that a ceramide containing the 2-hydroxy fatty acid C6OH was preferentially converted by MDCK and Caco-2 cells to galactosylceramide (GalCer) and its derivatives galabiosylceramide (Ga2Cer) and sulfatide (SGalCer) as compared to SM and GlcCer--all endogenous lipid classes of these cells. Transport to the apical and basolateral cell surface was monitored by a BSA-depletion assay. In MDCK cells, GalCer reached the cell surface with two- to sixfold lower apical/basolateral polarity than GlcCer. Remarkably, in Caco-2 cells GalCer and GlcCer displayed the same apical/basolateral polarity, but it was sixfold lower for lipids with a C6OH chain than for C6-NBD lipids. Therefore, the sorting of a sphingolipid appears to depend on lipid structure and cell type. We propose that the different ratios of gluco- and galactosphingolipid synthesis in the various epithelial tissues govern lipid sorting in the membrane of the trans Golgi network by dictating the composition of the domains from where vesicles bud to the apical and basolateral cell surface.

Evidence for disruption of sphingolipid metabolism as a contributing factor in the toxicity and carcinogenicity of fumonisins

Fumonisins are inhibitors of the biosynthesis of sphingosine and more complex sphingolipids. In eucaryotic cells, fumonisin inhibition of sphingolipid biosynthesis is a result of inhibition of the enzyme ceramide synthase. Large increase in free sphinganine concentration in plant and animal cells are observed within a few hours after exposure to fumonisins and/or Alternaria toxins (AAL-toxins). Some of the sphinganine is metabolized to other bioactive intermediates, and some is released from cells. In animals, free sphinganine accumulates in tissues and quickly appears in blood and urine. Free sphingoid bases are toxic to most cells, and complex sphingolipids are essential for normal cell growth. Fumonisin B1 stimulates sphinganine-dependent DNA synthesis in Swiss 3T3 cells, but is mitoinhibitory in other cell types. In cultured cells the accumulation of bioactive long-chain sphingoid bases and depletion of complex sphingolipids are clearly contributing factors in growth inhibition, increased cell death, and (in Swiss 3T3 cells) mitogenicity of fumonisins. While disruption of sphingolipid metabolism directly affects cells, it may indirectly affect some tissues. For example, fumonisin B1 impairs the barrier function of endothelial cells in vitro. Adverse effects on endothelial cells could indirectly contribute to the neurotoxicity and pulmonary edema caused by fumonisins. It is hypothesized that fumonisin-induced changes in the sphingolipid composition of target tissues could directly or indirectly contribute to all Fusarium moniliforme-associated diseases.