Gangliosides inhibit PDGF-induced signal transduction events in U-1242 MG human glioma cells

A mutation in a ganglioside biosynthetic enzyme, ST3GAL5, results in salt & pepper syndrome, a neurocutaneous disorder with altered glycolipid and glycoprotein glycosylation

'Salt & Pepper' syndrome is an autosomal recessive condition characterized by severe intellectual disability, epilepsy, scoliosis, choreoathetosis, dysmorphic facial features and altered dermal pigmentation. High-density SNP array analysis performed on siblings first described with this syndrome detected four shared regions of loss of heterozygosity (LOH). Whole-exome sequencing narrowed the candidate region to chromosome 2p11.2. Sanger sequencing confirmed a homozygous c.994G>A transition (p.E332K) in the ST3GAL5 gene, which encodes for a sialyltransferase also known as GM3 synthase. A different homozygous mutation of this gene has been previously associated with infantile-onset epilepsy syndromes in two other cohorts. The ST3GAL5 enzyme synthesizes ganglioside GM3, a glycosophingolipid enriched in neural tissue, by adding sialic acid to lactosylceramide. Unlike disorders of glycosphingolipid (GSL) degradation, very little is known regarding the molecular and pathophysiologic consequences of altered GSL biosynthesis. Glycolipid analysis confirmed a complete lack of GM3 ganglioside in patient fibroblasts, while microarray analysis of glycosyltransferase mRNAs detected modestly increased expression of ST3GAL5 and greater changes in transcripts encoding enzymes that lie downstream of ST3GAL5 and in other GSL biosynthetic pathways. Comprehensive glycomic analysis of N-linked, O-linked and GSL glycans revealed collateral alterations in response to loss of complex gangliosides in patient fibroblasts and in zebrafish embryos injected with antisense morpholinos that targeted zebrafish st3gal5 expression. Morphant zebrafish embryos also exhibited increased apoptotic cell death in multiple brain regions, emphasizing the importance of GSL expression in normal neural development and function.

Modulation of epidermal growth factor receptor phosphorylation by endogenously expressed gangliosides

The effect of changing the ganglioside composition of Chinese hamster ovary K1 cells on the function of the epidermal growth factor receptor (EGFr) was examined by studying the signalling pathway generated after the binding of epidermal growth factor (EGF) both in cells depleted of glycosphingolipids by inhibiting glucosylceramide synthase activity and in cell lines expressing different gangliosides as the result of stable transfection of appropriate ganglioside glycosyltransferases. After stimulation with EGF, cells depleted of glycolipids showed EGFr phosphorylation and extracellular signal-regulated protein kinase 2 (ERK2) activity as parental cells expressing GM3 [ganglioside nomenclature follows Svennerholm (1963) J. Neurochem. 10, 613-623] or as transfected cells expressing mostly GM2 and GD1a as the result of stable transfection of UDP-GalNAc:LacCer/GM3/GD3 N-acetylgalactosaminyltransferase. However, cells stably transfected with CMP-NeuAc:GM3 sialyltransferase and expressing GD3 at the cell surface showed both decreased EGFr phosphorylation and ERK2 activation after stimulation with EGF. Results suggest that changes in the ganglioside composition of cell membranes might be important in the regulation of the EGF signal transduction.

Early steps in termination of the immortalization state in Burkitt lymphoma: induction of genes involved in signal transduction, transcription, and trafficking by the ganglioside IV(3)NeuAc-nLcOse(4)Cer

Stimulation by the ganglioside IV(3)NeuAc-nLcOse(4)Cer leads to growth arrest in the Burkitt lymphoma cell line Raji. In order to analyze the primary response of Raji cells to that stimulus, a cDNA array screen and a suppression subtractive hybridization-PCR approach were performed. Twenty-four genes with assigned functions were confirmed to be induced by the ganglioside in reverse Northern blot experiments covering e.g. protein kinase B, phospholipase C, the MAP-kinase ERK3, the transcription factors YY1, DR1 and NSEP, the membrane traffic protein TAP, and the nuclear export protein CRM1. Most of the genes identified are involved in signal transduction, transcription, and cell trafficking. For selected genes, the induction of expression was quantified by semiquantitative RT-PCR.

Mechanisms through which PDGF alters intracellular calcium levels in U-1242 MG human glioma cells

PDGF-BB induces a rapid, sustained increase in intracellular calcium levels in U-1242 MG cells. We used several calcium channel blockers to identify the types of channels involved. L channel blockers (verapamil, nimodipine, nicardipine, nitrendipine and taicatoxin) had no effect on PDGF-BB induced alterations in intracellular calcium. Blockers of P, Q and N channels (omega-agatoxin-IVA, omega-conotoxin MVIIC and omega-conotoxin GVIA) also had no effect. This indicates that these channels play an insignificant role in supplying the Ca2+ necessary for PDGF stimulated events in U-1242 MG cells. However, a T channel blocker (NDGA) and the non-specific (NS) calcium channel blockers (FFA and SK&F 9365) abolished PDGF-induced increases in intracellular calcium. This indicates that PDGF causes calcium influx through both non-specific cationic channels and T channels. To study the participation of intracellular calcium stores in this process, we used thapsigargin, caffeine and ryanodine, all of which cause depletion of intracellular calcium stores. The PDGF effect was abolished using both thapsigargin and caffeine but not ryanodine. Collectively, these data indicate that in these human glioma cells PDGF-BB induces release of intracellular calcium from caffeine- and thapsigargin-sensitive calcium stores which in turn lead to further calcium influx through both NS and T channels.

Interaction of fibroblast growth factor-2 (FGF-2) with free gangliosides: biochemical characterization and biological consequences in endothelial cell cultures

Exogenous gangliosides affect the angiogenic activity of fibroblast growth factor-2 (FGF-2), but their mechanism of action has not been elucidated. Here, a possible direct interaction of sialo-glycolipids with FGF-2 has been investigated. Size exclusion chromatography demonstrates that native, but not heat-denatured, 125I-FGF-2 binds to micelles formed by gangliosides GT1b, GD1b, or GM1. Also, gangliosides protect native FGF-2 from trypsin digestion at micromolar concentrations, the order of relative potency being GT1b > GD1b > GM1 = GM2 = sulfatide > GM3 = galactosyl-ceramide, whereas asialo-GM1, neuraminic acid, and N-acetylneuramin-lactose were ineffective. Scatchard plot analysis of the binding data of fluorochrome-labeled GM1 to immobilized FGF-2 indicates that FGF-2/GM1 interaction occurs with a Kd equal to 6 microM. This interaction is inhibited by the sialic acid-binding peptide mastoparan and by the synthetic fragments FGF-2(112-129) and, to a lesser extent, FGF-2(130-155), whereas peptides FGF-2(10-33), FGF-2(39-59), FGF-2(86-96), and the basic peptide HIV-1 Tat(41-60) were ineffective. These data identify the COOH terminus of FGF-2 as a putative ganglioside-binding region. Exogenous gangliosides inhibit the binding of 125I-FGF-2 to high-affinity tyrosine-kinase FGF-receptors (FGFRs) of endothelial GM 7373 cells at micromolar concentrations. The order of relative potency was GT1b > GD1b > GM1 > sulfatide a = sialo-GM1. Accordingly, GT1b,GD1b, GM1, and GM2, but not GM3 and asialo-GM1, prevent the binding of 125I-FGF-2 to a soluble, recombinant form of extracellular FGFR-1. Conversely, the soluble receptor and free heparin inhibit the interaction of fluorochrome-labeled GM1 to immobilized FGF-2. In agreement with their FGFR antagonist activity, free gangliosides inhibit the mitogenic activity exerted by FGF-2 on endothelial cells in the same range of concentrations. Also in this case, GT1b was the most effective among the gangliosides tested while asialo-GM1, neuraminic acid, N-acetylneuramin-lactose, galactosyl-ceramide, and sulfatide were ineffective. In conclusion, the data demonstrate the capacity of exogenous gangliosides to interact with FGF-2. This interaction involves the COOH terminus of the FGF-2 molecule and depends on the structure of the oligosaccharide chain and on the presence of sialic acid residue(s) in the ganglioside molecule. Exogenous gangliosides act as FGF-2 antagonists when added to endothelial cell cultures. Since gangliosides are extensively shed by tumor cells and reach elevated levels in the serum of tumor-bearing patients, our data suggest that exogenous gangliosides may affect endothelial cell function by a direct interaction with FGF-2, thus modulating tumor neovascularization.

Anti-GM3 antibodies activate calcium inflow and inhibit platelet-derived growth factor beta receptors (PDGFbetar) in T51B rat liver epithelial cells

Glycolipids expressed in the plasma membrane regulate a variety of cellular processes including intracellular calcium dynamics. We used flow cytometry to characterize the glycoconjugates on the plasma membrane of T51B liver epithelial cells. Antibodies against glycolipids found to be present were tested for their ability elevate intracellular calcium. An antibody against GM3 (DH2) nearly doubles intracellular calcium while an antibody against type II chains (1B2) increases calcium to nearly four times the baseline level, similar to levels obtained with epidermal growth factor (EGF). The antibodies stimulated calcium inflow but did not trigger calcium release from internal stores. In addition DH2 but not 1B2 inhibited platelet-derived growth factor beta receptor (PDGFbetar) function. This is the first demonstration of activation of calcium inflow by agents that bind GM3 and type II chains. The ganglioside-mediated calcium inflow is likely to stimulate secretion by these liver cells.

Sphingolipids as receptor modulators. An overview

Glycosphingolipids are amphipathic compounds that exist mainly in the plasmalemma with their oligosaccharide portion protruding into the extracellular environment. In this position they are admirably situated for interacting with both ligands and receptors. Binding studies have demonstrated that specific glycolipids function as receptors for some microorganisms and bacterial toxins. Specific oligosaccharides on both glycolipids and glycoproteins bind members of the selection families, and some gangliosides facilitate integrins binding to their ligands. Gangliosides modulate the trophic factor-stimulated dimerization, tyrosine phosphorylation, and subsequent signal transduction events of several tyrosine kinase receptors. GM3 inhibits both the epidermal growth factor receptor and basic fibroblast factor receptor; several gangliosides except GM3 inhibit the platelet-derived growth-factor receptor; GM1 enhances nerve growth-factor-stimulated activation of TrkA; insulin receptor is inhibited to varying degrees by several gangliosides, but 2-->3 sialosylparagloboside is most effective. Activities of the beta(1)-adrenergic and delta-opioid receptors are modulated by GM1. Available information suggests that glycolipids serve as coordinators of multiple receptor functions.

Effects of U73122 and U73343 on human platelet calcium signalling and protein tyrosine phosphorylation

We have investigated the actions of the PLC inhibitor, U73122, and its close analogue, U73343, which does not inhibit PLC, in Fura-2-loaded human platelets. Rises in [Ca2+]i evoked by thrombin and collagen, and the TxA2-dependent rise in [Ca2+]i evoked by thapsigargin, were abolished by U73122, indicating that it inhibits the activity of both beta and gamma isoforms of PLC. The supposed control compound U73343, was found to inhibit TxA2 formation; it therefore partially inhibited the rise in [Ca2+]i evoked by low concentrations of thrombin, by thapsigargin or by collagen. U73343 had a greater effect than aspirin on the action of collagen, indicating an action on the TxA2-independent component of the signal, via PLC gamma-U73343 lowered TxA2 production by inhibiting the activation of cPLA2, probably at a tyrosine phosphorylation step. U73343 seems to inhibit only the tyrosine kinases involved in the activation of PLC gamma and the generation of TxA2. In contrast, U73122 increased tyrosine phosphorylation of platelet proteins, perhaps by inhibiting receptor independent tyrosine phosphatases, but inhibited all further tyrosine phosphorylation on addition of thrombin or other agonists.