Molecular cloning of cDNA for rat brain GD3-synthase

GD3 ganglioside-enriched extracellular vesicles stimulate melanocyte migration

Melanomas often accumulate gangliosides, sialic acid-containing glycosphingolipids found in the outer leaflet of plasma membranes, as disialoganglioside G_D3 and its derivatives. Here, we have transfected the G_D3 synthase gene (ST8Sia I) in a normal melanocyte cell line in order to evaluate changes in the biological behavior of non-transformed cells. G_D3-synthase expressing cells converted G_M3 into G_D3 and accumulated both G_D3 and its acetylated form, 9-O-acetyl-G_D3. Melanocytes were rendered more migratory on laminin-1 surfaces. Cell migration studies using the different transfectants, either treated or not with the glucosylceramide synthase inhibitor d-1-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (PPPP), allowed us to show that while G_M3 is a negative regulator of melanocyte migration, G_D3 increases it. We showed that gangliosides were shed to the matrix by migrating cells and that G_D3 synthase transfected cells shed extracellular vesicles (EVs) enriched in G_D3. EVs enriched in G_D3 stimulated cell migration of G_D3 negative cells, as observed in time lapse microscopy studies. Otherwise, EVs shed by G_M3^+veG_D3^-ve cells impaired migration and diminished cell velocity in cells overexpressing G_D3. The balance of antimigratory G_M3 and promigratory G_D3 gangliosides in melanocytes could be altered not only by the overexpression of enzymes such as ST8Sia I, but also by the horizontal transfer of ganglioside enriched extracellular vesicles. This study highlights that extracellular vesicles transfer biological information also through their membrane components, which include a variety of glycosphingolipids remodeled in disease states such as cancer.

Cloning and transcriptional regulation of genes responsible for synthesis of gangliosides

Ganglioside synthases are glycosyltransferases involved in the biosynthesis of glycoconjugates. A number of ganglioside synthase genes have been cloned and characterized. They are classified into different families of glycosyltransferases based on similarities of their amino acid sequences. Tissue-specific expression of these genes has been analyzed by hybridization using cDNA fragments. Enzymatic characterization with the expressed recombinant enzymes showed these enzymes differ in their donor and acceptor substrate specificities and other biochemical parameters. In vitro enzymatic analysis also showed that one linkage can be synthesized by multiple enzymes and one enzyme may be responsible for synthesis of multiple gangliosides. Following the cloning of the ganglioside synthase genes, the promoters of the key synthase genes in the ganglioside biosynthetic pathway have been cloned and analyzed. All of the promoters are TATA-less, lacking a CCAAT box but containing GC-rich boxes, characteristic of the house-keeping genes, although transcription of ganglioside synthase genes is subject to complex developmental and tissue-specific regulation. A set of cis-acting elements and transcription factors, including Sp1, AP2, and CREB, function in the proximal promoters. Negative-regulatory regions have also been defined in most of the promoters. We present here an overview of these genes and their transcriptional regulation.

Molecular cloning and expression of alpha2,8-sialyltransferase (ST8Sia I, GD3 Synthase) in Xenopus

GD3, a minor ganglioside in most normal tissues, is involved in important biological events and its expression could increase in pathological conditions. Organism integrity requires a tight balance between the anabolic and catabolic processes, thus it is important to control the intracellular expression of those "key" enzymes, which act at the "branching point" of ganglioside metabolism; one of these is the GD3-synthase (ST8Sia I). In this paper, we report the sequences of two ST8Sia I mRNAs found in Xenopus laevis and their genomic organization; the canonical form resulted constituted of 5 exons and 4 introns, while the "short" mRNA lacks of the exon 2. The expression of the two ST8Sia I mRNAs during embryo development and their tissue distribution in adult animals showed the single or simultaneous presence of the two forms. Experiments of in vitro expression and evaluation of enzymatic activity of the two hypothetical proteins turned out to be ST8Sia I. In the end, considering the growing interest toward the specie Xenopus tropicalis, due to its diploid genome that render it more suitable for genetic studies, we also cloned X. tropicalis ST8Sia I.

Glial-guided neuronal migration in P19 embryonal carcinoma stem cell aggregates

During development of the nervous system, neuronal precursors that originated in proliferative regions migrate along radial glial fibers to reach their final destination. P19 embryonal carcinoma (EC) stem cells exposed to retinoic acid (RA) differentiate into neurons, glia, and fibroblast-like cells. In this work, we induced P19 aggregates for 4 days with RA and plated them onto tissue culture dishes coated with poly-L-lysine. Several cells migrated out of and/or extended processes from the aggregates after 24 hr. Some cell processes were morphologically similar to radial glial fibers and stained for glial fibrillar acidic protein (GFAP) and nestin. Large numbers of migrating cells showed characteristics similar to those of bipolar migrating neurons and expressed the neuronal marker microtubule-associated protein 2. Furthermore, scanning electron microscopy analysis revealed an intimate association between the radial fibers and the migrating cells. Therefore, the migration of neuron-like cells on radial glia fibers in differentiated P19 aggregates resembled some of the migration models used thus far to study gliophilic neuronal migration. In addition, HPTLC analysis in this system showed the expression of 9-O-acetyl GD3, a ganglioside that has been associated with neuronal migration. Antibody perturbation assays showed that immunoblockage of 9-O-acetyl GD3 arrested neuronal migration in a reversible manner. In summary, we have characterized a new cell culture model for investigation of glial-guided neuronal migration and have shown that 9-O-acetyl GD3 ganglioside has an important role in this phenomenon.

Regulation of ganglioside biosynthesis in the nervous system

Ganglioside biosynthesis is strictly regulated by the activities of glycosyltransferases and is necessarily controlled at the levels of gene transcription and posttranslational modification. Cells can switch between expressing simple and complex gangliosides or between different series within these two groups during brain development. The sequential biosynthesis of gangliosides in parallel enzymatic pathways, however, requires fine-tuned subcellular sequestration and orchestration of glycosyltransferases. A popular model predicts that this regulation is achieved by the vectorial organization of ganglioside biosynthesis: sequential biosynthetic steps occur with the traffic of ganglioside intermediates through subsequent subcellular compartments. Here, we review current models for the subcellular distribution of glycosyltransferases and discuss results that suggest a critical role of N-glycosylation for the processing, transport, and complex formation of these enzymes. In this context, we attempt to illustrate the regulation of ganglioside biosynthesis as well as the biological significance of N-glycosylation as a posttranslational regulatory mechanism. We also review the results of analyses of the 5' regulatory sequences of several glycosyltransferases in ganglioside biosynthesis and provide insights into how their synthesis can be regulated at the level of transcription.

Down-regulation of GD3 ganglioside and its O-acetylated derivative by stable transfection with antisense vector against GD3-synthase gene expression in hamster melanoma cells: effects on cellular growth, melanogenesis, and dendricity

The expression of gangliosides in hamster melanoma cells is closely related to cellular growth and degree of differentiation, with slow-growing, highly differentiated melanotic melanoma cells expressing GM3 and fast-growing, undifferentiated amelanotic Ab melanoma cells having a preponderance of GD3 and O-acetyl-GD3. We recently showed that down-regulation of O-acetyl-GD3 expression in hamster melanoma cells by introducing the influenza C virus O-acetylesterase cDNA into the cells resulted in induction of dendricity, with a concomitant increased expression of GD3. To examine the effect of the increased GD3 expression in the plasma membrane on the dendricity of the AbC-1 cells, we first established the cDNA coding for hamster GD3-synthase. We then targeted the sialyltransferase gene expression by the antisense knockdown experiment, and the results showed that inhibition of the expression of gangliosides GD3 and O-acetyl-GD3 induced dendricity in the hamster melanoma AbC-1 cell line. These GD3- and O-acetyl-GD3-depleted cells also demonstrated a decreased rate of cell growth, but their melanogenic potential was not affected. These results rule out the possibility that GD3 may serve as an active molecule for dendrite outgrowth in this cell line and suggest that the enhanced expression of O-acetyl-GD3 ganglioside may stimulate cellular growth and suppress certain differentiated phenotypes such as dendrite formation, but not melanogenesis, in our system.

Molecular cloning and functional expression of two members of mouse NeuAcalpha2,3Galbeta1,3GalNAc GalNAcalpha2,6-sialyltransferase family, ST6GalNAc III and IV

Two cDNA clones encoding NeuAcalpha2,3Galbeta1,3GalNAc GalNAcalpha2, 6-sialyltransferase have been isolated from mouse brain cDNA libraries. One of the cDNA clones is a homologue of previously reported rat ST6GalNAc III according to the amino acid sequence identity (94.4%) and the substrate specificity of the expressed recombinant enzyme, while the other cDNA clone includes an open reading frame coding for 302 amino acids. The deduced amino acid sequence is not identical to those of other cloned mouse sialyltransferases, although it shows the highest sequence similarity with mouse ST6GalNAc III (43.0%). The expressed soluble recombinant enzyme exhibited activity toward NeuAcalpha2, 3Galbeta1, 3GalNAc, fetuin, and GM1b, while no significant activity was detected toward Galbeta1,3GalNAc or asialofetuin, or the other glycoprotein substrates tested. The sialidase sensitivity of the 14C-sialylated residue of fetuin, which was sialylated by this enzyme with CMP-[14C]NeuAc, was the same as that of ST6GalNAc III. These results indicate that the expressed enzyme is a new type of GalNAcalpha2,6-sialyltransferase, which requires sialic acid residues linked to Galbeta1,3GalNAc residues for its activity; therefore, we designated it mouse ST6GalNAc IV. Although the substrate specificity of this enzyme is similar to that of ST6GalNAc III, ST6GalNAc IV prefers O-glycans to glycolipids. Glycolipids, however, are better substrates for ST6GalNAc III.

Isolation and functional analysis of the promoter of the rat CMP-NeuAc:GM3 alpha2,8 sialyltransferase gene 1

A 2.1-kb 5'-flanking fragment of the rat CMP-NeuAc:GM3 alpha2,8 sialyltransferase (GD3-synthase) gene was cloned by the genomic walking procedure. The promoter activity of the fragment was assessed in F-11 cells by transient transfection and the locations for the basal and maximal promoter activities were defined. Primer extension analysis identified a transcription start site approximately 98 bp upstream of the ATG start codon. DNA sequence analysis of the promoter revealed a number of consensus binding sites for known transcription factors such as SP1, AP1, NFkappaB, C/EBP and TFIID, and a repeat GC-GT sequence motif seen for the formation of Z-type DNA. Both TATA and CCAAT boxes were not found in the promoter. Our results from deletion constructs suggested that both positive and negative cis-acting regulatory regions were present in this TATA-less promoter of the rat GD3-synthase gene.

Influence of N-glycosylation and N-glycan trimming on the activity and intracellular traffic of GD3 synthase

GD3 synthase (ST8Sia I) transfers a sialic acid in alpha-2-->8 linkage to the sialic acid moiety of GM3 to form the ganglioside GD3. The cDNAs of GD3 synthases predict several putative N-glycosylation sites. In this work we have examined the occupancy of these sites in a chicken GD3 synthase and how they affect its activity and intracellular traffic. COS-7 cells were transfected with an influenza virus hemagglutinin (HA) epitope-tagged form of GD3 synthase (GD3 synthase-HA). Cells acquired GD3 synthase activity, cell surface GD3 immunoexpression, and GD3 synthase-HA immunoreactivity in the Golgi complex. In Western blots, a main GD3 synthase-HA band of 47 kDa was detected, which was radioactive upon metabolic labeling with [2-3H] mannose. Tunicamycin prevented the incorporation of [2-3H]mannose into GD3 synthase-HA, blocked the enzyme activity, and promoted a reduction of the enzyme molecular mass of 6-7 kDa. Timed deglycosylation with N-glycosidase F showed that all three potential N-glycosylation sites of GD3 synthase-HA were glycosylated. The deglycosylated forms were enzymatically more unstable than the native form. Tunicamycin treatment of cells led to retention of GD3 synthase-HA immunoreactivity in the endoplasmic reticulum (ER). Castanospermine and deoxynojirimycin, inhibitors of the ER-processing enzymes alpha-glucosidases I and II, also prevented the exit from the ER but did not essentially affect the enzyme specific activity. 1-Deoxymannojirimycin and swainsonine, inhibitors of mannosidases, did not affect either the enzyme activity or the Golgi localization. Results indicate that (a) N-glycosylation is necessary for GD3 synthase to attain and to maintain a catalytically active folding, and for exiting the ER; and (b) N-glycan trimming in the ER, while not required for enzyme activity, is necessary for proper trafficking of GD3 synthase to the Golgi complex.

Cloning, characterization and developmental expression of alpha2,8 sialyltransferase (GD3 synthase, ST8Sia I) gene in chick brain and retina

GD3 and GM2 synthases act on ganglioside GM3 at the branching point of the pathway of synthesis of gangliosides in which the "a", "b" and "c" families are produced. The relative activities of these enzymes are important for regulating the ganglioside composition of a given tissue. In the present work, we report the cloning and characterization of a chick GD3 synthase cDNA. The cloned cDNA directed the synthesis of a functionally active enzyme in transiently transfected CHO-K1 cells and was highly homologous to mammalian GD3 synthases. In Northern blot experiments the cDNA detected a single specific GD3 synthase mRNA of about 9.0 kb both in the chicken brain and retina. The abundance of the specific mRNA transcript declined steadily from E7-E9 to very low values around PN2. The levels of enzyme activities measured at the same developmental stages roughly followed the changes of specific mRNA levels in both tissues. In situ hybridization of embryonic neural retina cells in culture showed that both glial- and neuron-like cells expressed the specific GD3 synthase mRNA, although with different intensities. Results indicate that transcription and/or stability of the specific GD3 synthase mRNA constitute a level of control of the expression of GD3 synthase and indirectly of the ganglioside composition in the developing chicken central nervous system (CNS).

Cloning of the cDNA coding for rat brain CMP-NeuAc:GD3 alpha2-8 sialyltransferase

The cDNA coding for GT3-synthase has been cloned from a rat fetal brain cDNA library. The complete nucleotide sequence and the deduced amino acid sequence of the rat GT3-synthase cDNA were highly homologous to those of the mouse GT3-synthase. Quantitative RT-PCR showed that, as a key enzyme for the synthesis of 'c'-series gangliosides, the expression of GT3-synthase was developmentally regulated in embryonic rat brains.