Solubilization of UDPglucose-ceramide glucosyltransferase from the Golgi apparatus

Cloning and functional characterization of two key enzymes of glycosphingolipid biosynthesis in the amphibian Xenopus laevis

Gangliosides are a subfamily of complex glycosphingolipids (GSLs) with important roles in many biological processes. In this study, we report the cDNA cloning, functional characterization, and the spatial and temporal expression of Xlcgt and Xlgd3 synthase during Xenopus laevis development. Xlcgt was expressed both maternally and zigotically persisting at least until stage 35. Maternal Xlgd3 synthase mRNA could not be detected and showed a steady-state expression from gastrula to late tailbud stage. Xlcgt is mainly present in involuted paraxial mesoderm, neural folds, and their derivatives. Xlgd3 synthase transcripts were detected in the dorsal blastoporal lip, in the presumptive neuroectoderm, and later in the head region, branchial arches, otic and optic primordia. We determined the effect of glycosphingolipid depletion with 1-phenyl-2-palmitoyl-3-morpholino-1-propanol (PPMP) in mesodermal layer. PPMP-injected embryos showed altered expression domains in the mesodermal markers. Our results suggest that GSL are involved in convergent-extension movements during early development in Xenopus.

Ceramide glucosylation in bean hypocotyl microsomes: evidence that steryl glucoside serves as glucose donor

The formation of glucosylceramide, the predominant sphingolipid in plant tissues, was examined in microsomes from wax bean hypocotyls. Membranes were incubated with UDP-[14C]glucose in an assay mixture. The lipid extracts obtained from the assays were separated by thin-layer chromatography, and the radioactivity incorporated into glucosylceramide, steryl glucoside, and acylated steryl glucoside was determined. Although the formation of glucosylceramide was detected and characterized, several lines of evidence contradicted the assumption that UDP-glucose is the immediate glucose donor for glucosylceramide formation in plants: PDMP (DL-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol), an inhibitor of ceramide glucosyltransferase in animal tissues, did not inhibit glucosylceramide formation in bean microsomes. Addition of UDP-glucose pyrophosphorylase during the assay to degrade UDP-[14C]glucose blocked the further production of labeled steryl glucoside, but did not prevent the continued formation of labeled glucosylceramide. Omitting UDP-[14C]glucose and including steryl [14C]glucoside in the assay resulted in the formation of labeled glucosylceramide. Collectively, these results suggest that glucosylceramide formation in plants does not utilize UDP-glucose as the immediate glucose donor, as has been demonstrated for the reaction in animal tissues, and that steryl glucoside serves as glucose donor for ceramide formation. This study, the first to examine glucosylceramide formation in plants, provides evidence for a novel enzymatic reaction in sphingolipid synthesis as well as a new, metabolic role for steryl glucoside in plant tissues.

Induction of ceramide glucosyltransferase activity in cultured human keratinocytes. Correlation with culture differentiation

Ceramides are the major component of the extracellular lipids that comprise the epidermal permeability barrier. They are derived from glucosylceramides (GlcCer) upon their extrusion from lamellar granules into the extracellular space in the upper layers of the epidermis. To better understand the regulation of the unique pathway for ceramide production in epidermis, we have studied the activity of the enzyme responsible for GlcCer synthesis, ceramide glucosyltransferase (CerGlc transferase), during keratinocyte culture differentiation. Human keratinocyte cultures were expanded in low calcium keratinocyte growth medium (KGM) and then switched to either normal calcium KGM (nKGM) or "complete" Dulbecco's modified Eagle's medium/Ham's F-12 (3:1) supplemented with 10% fetal bovine serum (cDMEM). At 7 and 10 days after the medium switch, electron microscopy revealed that cDMEM cultures were more fully differentiated morphologically and contained numerous lamellar granules. The GlcCer/DNA content of cDMEM cultures increased to 6 times that of day 0 cultures and was nearly 4 times greater than that of nKGM cultures, whereas the total lipid/DNA content of cDMEM cultures increased to only 1.8 times that of day 0 cultures and was approximately 1.2 times that of nKGM cultures. CerGlc transferase activity/DNA increased 6 times in cDMEM cultures but <1.5 times in nKGM cultures. By contrast, beta-glucocerebrosidase activity, which is responsible for the conversion of GlcCer to ceramide, increased to a similar extent in both differentiating culture systems. Treatment of cultures with the reversible CerGlc transferase inhibitor, DL-threo-1-phenyl-2-(palmitoylamino)-3-morpholino-1-propanol, prevented the increase of GlcCer in cDMEM cultures, and blocked conversion of exogenously added ceramide to GlcCer. A low level of CerGlc transferase activity, relative to that in differentiated keratinocytes, was detected in cultures of other human cell types. These results indicate that CerGlc transferase activity is induced during epidermal differentiation and that regulation of this enzyme may be an important determinant of the specialized production and compartmentalization of epidermal sphingolipids.

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.

A mouse B16 melanoma mutant deficient in glycolipids

Mouse B16 melanoma cell line, GM-95 (formerly designated as MEC-4), deficient in sialyllactosylceramide was examined for its primary defect. Glycolipids from the mutant cells were analyzed by high-performance TLC. No glycolipid was detected in GM-95 cells, even when total lipid from 10(7) cells was analyzed. In contrast, the content of ceramide, a precursor lipid molecule of glycolipids, was normal. Thus, the deficiency of glycolipids was attributed to the first glucosylation step of ceramide. The ceramide glucosyltransferase (EC 2.4.1.80) activity was not detected in GM-95 cells. There was no significant difference of sialyllactosylceramide synthase activity, however, between GM-95 and the parental cells. The deficiency of glycolipids in GM-95 cells was associated with changes of the cellular morphology and growth rate. The parental cells showed irregular shapes and tended to overlap each other. On the other hand, GM-95 cells exhibited an elongated fibroblastic morphology and parallel arrangement. The population-doubling times of GM-95 and the parental cells in serum-free medium were 28 hr and 19 hr, respectively.

Determination of the intracellular sites and topology of glucosylceramide synthesis in rat liver

We examined the intracellular site(s) and topology of glucosylceramide (GlcCer) synthesis in subcellular fractions from rat liver, using radioactive and fluorescent ceramide analogues as precursors, and compared these results with those obtained in our recent study of sphingomyelin (SM) synthesis in rat liver [Futerman, Stieger, Hubbard & Pagano (1990) J. Biol. Chem. 265, 8650-8657]. In contrast with SM synthesis, which occurs principally at the cis/medial Golgi apparatus, GlcCer synthesis was more widely distributed, with substantial amounts of synthesis detected in a heavy (cis/medial) Golgi-apparatus subfraction, a light smooth-vesicle fraction that is almost devoid of an endoplasmic-reticulum marker enzyme (glucose-6-phosphatase), and a heavy vesicle fraction. Furthermore, no GlcCer synthesis was detected in an enriched plasma-membrane fraction after accounting for contamination by Golgi-apparatus membranes. These results suggest that a significant amount of GlcCer may be synthesized in a pre- or early Golgi-apparatus compartment. Unlike SM synthesis, which occurs at the luminal surface of the Golgi apparatus, GlcCer synthesis appeared to occur at the cytosolic surface of intracellular membranes, since (i) limited proteolytic digestion of intact Golgi-apparatus vesicles almost completely inhibited GlcCer synthesis, and (ii) the extent of UDP-glucose translocation into the Golgi apparatus was insufficient to account for the amount of GlcCer synthesis measured. These findings imply that, after its synthesis, GlcCer must undergo transbilayer movement to the luminal surface to account for the known topology of higher-order glycosphingolipids within the Golgi apparatus and plasma membrane.