Adult rat retina interneurons synthesize GD3: GD3 expression by these cells is regulated by cell-cell interactions

The good, the bad, and the unknown nature of decreased GD3 synthase expression

This paper explores the physiological consequences of decreased expression of GD3 synthase (GD3S), a biosynthetic enzyme that catalyzes the synthesis of b-series gangliosides. GD3S is a key factor in tumorigenesis, with overexpression enhancing tumor growth, proliferation, and metastasis in various cancers. Hence, inhibiting GD3S activity has potential therapeutic effects due to its role in malignancy-associated pathways across different cancer types. GD3S has also been investigated as a promising therapeutic target in treatment of various neurodegenerative disorders. Drugs targeting GD3 and GD3S have been extensively explored and underwent clinical trials, however decreased GD3S expression in mouse models, human subjects, and in vitro studies has demonstrated serious adverse effects. We highlight these negative consequences and show original mass spectrometry imaging (MSI) data indicating that inactivated GD3S can generally negatively affect energy metabolism, regulatory pathways, and mitigation of oxidative stress. The disturbance in several physiological systems induced by GD3S inhibition underscores the vital role of this enzyme in maintaining cellular homeostasis and should be taken into account when GD3S is considered as a therapeutic target.

Trans-activity of plasma membrane-associated ganglioside sialyltransferase in mammalian cells

Gangliosides are acidic glycosphingolipids that contain sialic acid residues and are expressed in nearly all vertebrate cells. They are synthesized at the Golgi complex by a combination of glycosyltransferase activities followed by vesicular delivery to the plasma membrane, where they participate in a variety of physiological as well as pathological processes. Recently, a number of enzymes of ganglioside anabolism and catabolism have been shown to be associated with the plasma membrane. In particular, it was observed that CMP-NeuAc:GM3 sialyltransferase (Sial-T2) is able to sialylate GM3 at the plasma membrane (cis-catalytic activity). In this work, we demonstrated that plasma membrane-integrated ecto-Sial-T2 also displays a trans-catalytic activity at the cell surface of epithelial and melanoma cells. By using a highly sensitive enzyme-linked immunosorbent assay combined with confocal fluorescence microscopy, we observed that ecto-Sial-T2 was able to sialylate hydrophobically or covalently immobilized GM3 onto a solid surface. More interestingly, we observed that ecto-Sial-T2 was able to sialylate GM3 exposed on the membrane of neighboring cells by using both the exogenous and endogenous donor substrate (CMP-N-acetylneuraminic acid) available at the extracellular milieu. In addition, the trans-activity of ecto-Sial-T2 was considerably reduced when the expression of the acceptor substrate was inhibited by using a specific inhibitor of biosynthesis of glycolipids, indicating the lipidic nature of the acceptor. Our findings provide the first direct evidence that an ecto-sialyltransferase is able to trans-sialylate substrates exposed in the plasma membrane from mammalian cells, which represents a novel insight into the molecular events that regulate the local glycosphingolipid composition.

The expression and functions of glycoconjugates in neural stem cells

The mammalian central nervous system is organized by a variety of cells such as neurons and glial cells. These cells are generated from a common progenitor, the neural stem cell (NSC). NSCs are defined as undifferentiated neural cells that are characterized by their high proliferative potential while retaining the capacity for self-renewal and multipotency. Glycoconjugates carrying carbohydrate antigens, including glycoproteins, glycolipids, and proteoglycans, are primarily localized on the plasma-membrane surface of cells and serve as excellent biomarkers at various stages of cellular differentiation. Moreover, they also play important functional roles in determining cell fate such as self-renewal, proliferation, and differentiation. In the present review, we discuss the expression pattern and possible functions of glycoconjugates and carbohydrate antigens in NSCs, with an emphasis on stage-specific embryonic antigen-1, human natural killer antigen-1, polysialic acid-neural cell-adhesion molecule, prominin-1, gp130, chondroitin sulfate proteoglycans, heparan sulfate proteoglycans, cystatin C, galectin-1, glycolipids, and Notch.

Ganglioside expression during differentiation of chick retinal cells in vitro

The neural retina has been widely used to study the developmental patterns of ganglioside metabolism. Recent findings about in vitro differentiating chick embryo retina cells showed that: a) GD3 and GD1a ganglioside patterns undergo the most dramatic changes; b) when the cells emit neurites, GD3 ganglioside and a group of complex gangliotetraosylgangliosides (GTOG) are transiently coexpressed; c) synchronized developmental phenomena are dissociated by anti-GM1 antibodies; d) GD3 remains as a major ganglioside in differentiated neurons, though it is almost not immunoexpressed; e) GTOG affect antibody binding to GD3; f) the content of gangliosides involved in neural differentiation modifies their immunostain localization on cell membrane; g) after exogenous GTOG uptake, immature neurons mimic GD3 immunofluorescent localization of mature cells; h) a subset of purified retinal ganglion cells express GTOG characteristic of mature neurons.

Changes in ganglioside composition of photoreceptors during postnatal maturation of the rat retina

To examine at which stage the unusual ganglioside composition observed in adult retinal photoreceptor cells was established, and to see whether ganglioside changes could be correlated to distinct maturational events, quantitative and qualitative variations in gangliosides within pure sheets of photoreceptors during postnatal differentiation and aging of retina were studied. Retinas were separated into their component layers, (particularly photoreceptor layers uncontaminated by other neuronal types) by exploiting a technique of mechanical separation by vibratome. We extracted lipids from the cell membranes and analyzed the ganglioside composition by high performance thin layer chromatography. The data show that from the earliest recordable postnatal age (6 days) until late in life (18 months), photoreceptors contain low quantities of lipid-bound N-acetyl neuraminic acid and a simplified ganglioside profile compared to inner retinal neurons. Specific ganglioside changes occur within photoreceptor cells during postnatal maturation and aging, with downregulation of a-pathway GM1 and overlapping upregulation of b-pathway GD1b taking place during the period corresponding to outer segment formation, correlating with the onset of retinal function.

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).

Peptide growth factors but not ganglioside protect against excitotoxicity in rat retinal neurons in vitro

Glutamate is the major excitatory neurotransmitter in the retina, but excessive stimulation of its receptors leads to widespread neuronal stress and death. Both growth factors and gangliosides display important influences on responses to neuronal injury and degeneration. In this study, we have investigated the potential protective effects of two well characterized growth factors, epidermal and basic fibroblast growth factor (EGF and bFGF respectively), and the monosialoganglioside GM1, on cultured rat retinal neurons submitted to toxic levels of excitatory amino acids. Application of 1 mM glutamic acid reduced global neuronal viability by 80% when compared to control untreated cultures, whereas treatment with the glutamic acid agonist kainic acid (1 mM) led to specific, large decreases (75% reduction) in amacrine cell numbers. 24 h pretreatment with either EGF or bFGF (500 pM each) prevented the majority of excitatory amino acid-induced neuronal death, whereas similar treatment with 10(-5) M GM1 did not block neuronal degeneration. These findings demonstrate that EGF and bFGF act as neuroprotective agents against retinal excitotoxicity in vitro, whereas ganglioside GM1 is not effective in this particular paradigm.

Expression of beta 1-4 N-acetylgalactosaminyltransferase gene in the developing rat brain and retina: mRNA, protein immunoreactivity and enzyme activity

The developmental pattern of expression of the UDP-GalNAc:GM3 N-acetylgalactosaminyltransferase (GalNAc-T) gene was examined in the rat brain and retina. A GalNAc-T cDNA cloned from a rat olfactory bulb cDNA library was used as a probe for Northern blot and in situ hybridization experiments and a rabbit polyclonal antibody to rat GalNAc-T peptide was used for Western blot analysis. In Northern blot experiments, a single approximately 3 kb transcript was detected both in brain and retina. In brain, the abundance of this transcript increased from E15 to PN1-5 and then declined while, in retina, it increased steadily from PN1 to PN13-24. The developmental trends of GalNAc-T mRNA expression, GalNAc-T immunoreactive protein and GalNAc-T activity were comparable in brain. In retina, however, GalNAc-T activity and GalNAc-T peptide immunoreactivity followed developmental patterns that were similar between them and different from that of the specific mRNA. Results suggest that post-transcriptional controls of the GalNAc-T gene expression operate in the rat CNS, which are particularly evident in retina. The expression of the GalNAc-T gene in glial and neuronal cells was examined in rat retina cell cultures by in situ hybridization. The GalNAc-T mRNA was abundant in GM1+/GD3+ neurons and almost absent in the flat, GM1-/GD3+ Müller glia-derived cells.

Successive isolation and separation of the major lipid fractions including gangliosides from single biological samples

Currently available techniques concerning extraction and characterization of the different lipids from biological specimens are designed for particular families and do not address consecutive isolation of lipid constituents in their globality. We describe here a simple, nondestructive chromatographic procedure that allows efficient elution and further analysis of the major lipid classes (neutral lipids, phospholipids, nonsialylated sphingolipids, and gangliosides) in their natural states from the same starting material. The procedure describes the use of solvent mixtures adapted to silicic acid column chromatography and permits 90-97% recovery of each of the above lipid groups. We have particularly concentrated on optimizing the efficient recovery of the diverse minor forms of gangliosides, free of other contaminants, from relatively small amounts of neural tissue. As model systems we have used in vivo and in vitro preparations of mammalian retina for which only fragmentary data are available on lipid composition. We show that relative to brain, retina contains, for example, twofold more sphingomyelin and sixfold more GD3 ganglioside. In turn, cultured retinal glial cells contain twofold higher levels of globoside and eightfold higher amounts of GM3 ganglioside with respect to intact retina. Compared to previously published techniques, we obtain improved total ganglioside recovery, with enrichment of poly-sialogangliosides. The technique presented here should be widely applicable to analyze global lipid composition of diverse biological samples.

Differential modulation of basic fibroblast and epidermal growth factor receptor activation by ganglioside GM3 in cultured retinal Müller glia

Polypeptide growth factors and membrane-bound gangliosides are involved in cell signaling, including that observed in cells of neural origin. To analyze possible interactions between these two systems, we investigated the modulation of short- and long-term responses to basic fibroblast and epidermal growth factor (bFGF and EGF, respectively) in cultured retinal Müller glial cells following experimental modification of their ganglioside composition. These glial cells readily incorporated exogenously administered GM3 ganglioside, which was not substantially metabolized within 24 h. Such treatments significantly inhibited bFGF-induced DNA replication and cell migration, while having much less effect on analogous EGF-mediated behaviors. To explore GM3/growth factor interactions further, different aspects of glial metabolism in response to bFGF or EGF stimulation were examined: membrane fluidity, growth factor binding, global and individual changes in growth factor-induced phosphotyrosine levels, and growth factor-induced activation of mitogen-activated protein kinase. GM3 reduced the intensity of immunocytochemical labeling of phosphotyrosine-containing proteins within bFGF-stimulated cells and down-regulated FGF receptor activation and tyrosine phosphorylation of its cellular substrates, whereas similar parameters in EGF-stimulated cells were much less affected. Hence the data reveal a complex relationship in normal neural cells between polypeptide growth factors and membrane-bound gangliosides, which may participate in retinal cellular physiology in vivo.

Site restricted and neuron dominant expression of alpha 2,8sialyltransferase gene in the adult mouse brain and retina

Gene expression of the alpha 2,8sialyltransferase (alpha 2,8S-T) responsible for GD3 synthesis in the adult mouse brain and retina was analysed by reverse transcription-polymerase chain reaction/Southern blotting (RT-PCR/Southern) and in situ hybridization. Among various portions of the brain, high levels of 9.5 kb mRNA were observed in the retina and midbrain. Results of RT-PCR/Southern did not necessarily correlate with the enzyme activities in the individual sites. In situ hybridization analysis revealed that this gene was characteristically expressed in the inner segment of photoreceptor cells, some nuclei in the midbrain, cranial nerve nuclei in the pons-medulla, Purkinje cells in the cerebellum, pyramidal cells of the hippocampus and granular cells of the dentate gyrus. In the retina, the alpha 2,8S-T gene was broadly expressed over the layers during development, and retained high expression levels in the photoreceptor cells of adult mice consistent with high expression of GD3. Destruction of neurons in the hippocampus and dentate gyrus by injection of kainic acid and colchicine respectively resulted in the disappearance of the hybridization signal, suggesting that the alpha 2,8S-T gene was mainly expressed by neurons.

A reappraisal of ganglioside GD3 expression in the CNS

GD3 ganglioside is a major glycolipid component of the developing central nervous system but diminishes considerably as the CNS matures. Despite consistent biochemical data, the cellular localization of GD3 expression has been controversial. In this commentary we will review the cellular expression of GD3 during CNS development and in neuropathological circumstances as determined by studies with the two most commonly used anti GD3 monoclonal antibodies, R24 and LB1. GD3 is not restricted to any one cell lineage, being expressed in development to varying degrees by immature neuroectodermal cells, oligodendrocyte progenitors, ameboid microglia, and subpopulations of developing neurons and astrocytes. In the adult CNS, GD3 is expressed in low amounts by some neuronal subpopulations, on reactive and resting microglia, and by reactive astrocytes. In the appropriate contexts of development or neuropathology, anti-GD3 antibodies are useful for cell type identification and for cell isolation, but caution should be exercised because of the lack of cellular specificity.

Stimulation of endogenous ganglioside metabolism by neurotrophic growth factors in cultured retinal Müller glia

Neurotrophic factors such as basic fibroblast and epidermal factor (bFGF and EGF respectively) are known to influence many differentiative processes, but their effects on an important group of glycosylated signalling molecules involved in neural differentiation, the gangliosides, are unknown. To study this possibility, we analyzed the effects of exogenously added bFGF and EGF upon the amount and type of endogenous gangliosides extracted from purified cultures of retinal Müller glial cells. A single addition of 500 pM bFGF or EGF for 48 h to such cultures led to significant increases in total ganglioside levels of 30-40%. Analysis of the distribution of specific ganglioside species within control and growth factor treated cells revealed that the precursor form GM3 formed 50-60% of the total ganglioside pool in all cases, the remainder being composed principally of GD1a (20%) with no detectable tri-sialogangliosides. Growth factor treatment for 48 h led to increases mainly in GM3, whereas longer exposure (96 h) of confluent glial cultures to growth factors additionally stimulated synthesis of GT1b. Furthermore, growth factor-induced ganglioside increases were dose-dependent, reaching maximal stimulation at 500 pM for bFGF. Incorporation of radiolabelled [3H]-glucosamine into glial cultures showed that ganglioside synthesis was stimulated 2-fold by the growth factors. To our knowledge these data constitute the first demonstration of neurotrophic factor stimulation of ganglioside levels in cells of central nervous system origin. Such complex interactions between peptide growth factors and gangliosides, if occurring in vivo, could have important consequences for retinal cell behaviour.

Activities of five different sialyltransferases in fish and rat brains

To investigate the role of sialyltransferases in the metabolism of brain gangliosides, we examined activities of five different sialyltransferases (GM3-, GD3-, GT3-, GD1a-, and GT1a-synthase) using total membrane preparations from cichlid fish and Sprague-Dawley rat brains, and analyzed the relationship between the enzyme activities and the ganglioside compositions. The patterns of sialyltransferase activities in fish and rat brains differed from each other. In fish brain, the GM3-synthase activity was lower than GD3-synthase activity, whereas the opposite relationship was observed in rat brain. The GT3-synthase reaction with fish brain membranes produced radiolabeled GM3, GD3, and a ganglioside that was identified as GT3 based on mobility on TLC using two different solvent systems. No GT3-synthase activity was detected in rat brain. The GD1a- and GT1a-synthase activities in fish brain were higher than those in rat brain. Although GT1a was a single radiolabeled ganglioside in fish GT1a-synthase reaction, this ganglioside could not be detected in rat brain. The ratios of GM3-, GD3-, GT3-, GD1a-, and GT1a-synthase activities in fish and rat brain were 23:31:4:28:14 and 61:21:0:18:0, respectively. Ganglioside analysis showed that fish brain was enriched with c-series gangliosides including GT3 and polysialo-species, whereas a- and b-series gangliosides were major components in rat brain. These results suggest that the species-specific expression of gangliosides in brain tissues may be regulated, at least in part, at the level of sialyltransferase activities.

In vitro modulation of changes in ganglioside patterns of differentiating neurons in the presence of an anti-GM1 antibody

Retinal cells from 7-day-old chicken embryos were cultured in the presence of a polyclonal anti-GM1 antibody, at low and high density in a "sandwich cell culture". Cells that were about 80% neurofilament positive at all times, changed their morphology and emitted processes as controls. By examining immunocytochemical expression of gangliosides, cells cultured in the presence of the antibody maintained GD3 expression longer than controls, albeit the expression of the gangliotetraosylgangliosides (GTOG) was not considerably affected. This leads to an extension of the transient period in which differentiating cells coexpressed both types of gangliosides (GD3 and GTOG). At 3-4 days in vitro the relative synthesis of GD3 was about 30% higher and that of GD1a about 40% lower than in controls, indicating a delay in the shift of the synthesis pattern. Nevertheless, the pattern of ganglioside composition resembled at 4 days in vitro. Results indicate that the anti-GM1 antibody may modulate the expression and synthesis of gangliosides without a detectable decrease in neuritogenesis. Considering that the emission of neurites occurs in coexpressing GD3 and GTOG neurons, it is suggested that neuritogenesis could be irrespective of losing the GD3 expression.

Regulation of ganglioside composition and synthesis is different in developing chick retinal pigment epithelium and neural retina

We examined the immunocytochemical expression of GM3 and GD3 in 3-day-old chick embryo retinal pigment epithelium (RPE) and neural retina (NR). We also compared the composition of gangliosides and the activities of key ganglioside glycosyltransferases of the RPE and NR of 8-, 12-, and 15-day old embryos. The immunocytochemical studies in 3-day-old embryos showed heavy expression of GM3 and GD3 at the inner and outer layers of the optic vesicle that are the precursors of the RPE and NR, respectively. The compositional and enzymatic studies showed pronounced differences between RPE and NR of 8-day and older embryos. HPTLC showed that at 8 days the major species were GM3 and GD3 in RPE and GD3 and GT3 in NR. As development proceeded, GD3 decreased in both tissues, GM3 became the major ganglioside in RPE, and ganglio-series gangliosides (mainly GD1a) became the major species in NR. At 15 days the major species were GD1a in NR and GM3 in RPE. Enzyme determinations showed that whereas in RPE from 12-day-old embryos GM2 synthase was under the limit of detection and GD3 synthase activity was about sixfold lower than GM3 synthase, in NR the activities of GM3 and GD3 synthases were similar and both six- to ninefold lower than GM2 synthase. These results evidence a markedly different modulation of the ganglioside glycosylating system in cells of a common origin that through distinct differentiation pathways originate two closely related tissues of the optic system.(ABSTRACT TRUNCATED AT 250 WORDS)