Endogenous GM1 ganglioside of the plasma membrane promotes neuritogenesis by two mechanisms

Are There Lipid Membrane-Domain Subtypes in Neurons with Different Roles in Calcium Signaling?

Lipid membrane nanodomains or lipid rafts are 10-200 nm diameter size cholesterol- and sphingolipid-enriched domains of the plasma membrane, gathering many proteins with different roles. Isolation and characterization of plasma membrane proteins by differential centrifugation and proteomic studies have revealed a remarkable diversity of proteins in these domains. The limited size of the lipid membrane nanodomain challenges the simple possibility that all of them can coexist within the same lipid membrane domain. As caveolin-1, flotillin isoforms and gangliosides are currently used as neuronal lipid membrane nanodomain markers, we first analyzed the structural features of these components forming nanodomains at the plasma membrane since they are relevant for building supramolecular complexes constituted by these molecular signatures. Among the proteins associated with neuronal lipid membrane nanodomains, there are a large number of proteins that play major roles in calcium signaling, such as ionotropic and metabotropic receptors for neurotransmitters, calcium channels, and calcium pumps. This review highlights a large variation between the calcium signaling proteins that have been reported to be associated with isolated caveolin-1 and flotillin-lipid membrane nanodomains. Since these calcium signaling proteins are scattered in different locations of the neuronal plasma membrane, i.e., in presynapses, postsynapses, axonal or dendritic trees, or in the neuronal soma, our analysis suggests that different lipid membrane-domain subtypes should exist in neurons. Furthermore, we conclude that classification of lipid membrane domains by their content in calcium signaling proteins sheds light on the roles of these domains for neuronal activities that are dependent upon the intracellular calcium concentration. Some examples described in this review include the synaptic and metabolic activity, secretion of neurotransmitters and neuromodulators, neuronal excitability (long-term potentiation and long-term depression), axonal and dendritic growth but also neuronal cell survival and death.

Modulation of calcium signaling depends on the oligosaccharide of GM1 in Neuro2a mouse neuroblastoma cells

Recently, we demonstrated that the oligosaccharide portion of ganglioside GM1 is responsible, via direct interaction and activation of the TrkA pathway, for the ability of GM1 to promote neuritogenesis and to confer neuroprotection in Neuro2a mouse neuroblastoma cells. Recalling the knowledge that ganglioside GM1 modulates calcium channels activity, thus regulating the cytosolic calcium concentration necessary for neuronal functions, we investigated if the GM1-oligosaccharide would be able to overlap the GM1 properties in the regulation of calcium signaling, excluding a specific role played by the ceramide moiety inserted into the external layer of plasma membrane. We observed, by calcium imaging, that GM1-oligosaccharide administration to undifferentiated Neuro2a cells resulted in an increased calcium influx, which turned out to be mediated by the activation of TrkA receptor. The biochemical analysis demonstrated that PLCγ and PKC activation follows the TrkA stimulation by GM1-oligosaccharide, leading to the opening of calcium channels both on the plasma membrane and on intracellular storages, as confirmed by calcium imaging experiments performed with IP3 receptor inhibitor. Subsequently, we found that neurite elongation in Neuro2a cells was blocked by subtoxic administration of extracellular and intracellular calcium chelators, suggesting that the increase of intracellular calcium is responsible of GM1-oligosaccharide mediated differentiation. These results suggest that GM1-oligosaccharide is responsible for the regulation of calcium signaling and homeostasis at the base of the neuronal functions mediated by plasma membrane GM1.

Gangliosides in Nerve Cell Specification

The central nervous system is generated from progenitor cells that are recognized as neural stem cells (NSCs). NSCs are defined as undifferentiated neural cells that are characterized by the capacity for self-renewal and multipotency. Throughout neural development, NSCs undergo proliferation, migration, and cellular differentiation, and dynamic changes are observed in the composition of carbohydrate-rich molecules, including gangliosides. Gangliosides are sialic acid-containing glycosphingolipids with essential and multifaceted functions in brain development and NSC maintenance, which reflects the complexity of brain development. Our group has pioneered research on the importance of gangliosides for growth factor receptor signaling and epigenetic regulation of ganglioside biosynthesis in NSCs. We found that GD3 is the predominant ganglioside species in NSCs (>80%) and modulates NSC proliferation by interacting with epidermal growth factor receptor signaling. In postnatal brain, GD3 is required for long-term maintenance of NSCs. Deficiency in GD3 leads to developmental and behavioral deficits, such as depression. The synthesis of GD3 is switched to the synthesis of complex, brain-type gangliosides, namely, GM1, GD1a, GD1b, and GT1b, resulting in terminal differentiation and loss of "stemness" of NSCs. In this process, GM1 is augmented by a novel GM1-modulated epigenetic gene regulation mechanism of glycosyltransferases at a later differentiation stage. Consequently, our research suggests that stage-specific gangliosides play specific roles in maintaining NSC activities and in cell fate determination.

Gangliosides of the Nervous System

This review begins by attempting to recount some of the pioneering discoveries that first identified the presence of gangliosides in the nervous system, their structures and topography. This is presented as prelude to the current emphasis on physiological function, about which much has been learned but still remains to be elucidated. These areas include ganglioside roles in nervous system development including stem cell biology, membranes and organelles within neurons and glia, ion transport mechanisms, receptor modulation including neurotrophic factor receptors, and importantly the pathophysiological role of ganglioside aberrations in neurodegenerative disorders. This relates to their potential as therapeutic agents, especially in those conditions characterized by deficiency of one or more specific gangliosides. Finally we attempt to speculate on future directions ganglioside research is likely to take so as to capitalize on the impressive progress to date.

Genetics and Therapies for GM2 Gangliosidosis

Tay-Sachs disease, caused by impaired β-N-acetylhexosaminidase activity, was the first GM2 gangliosidosis to be studied and one of the most severe and earliest lysosomal diseases to be described. The condition, associated with the pathological build-up of GM2 ganglioside, has acquired almost iconic status and serves as a paradigm in the study of lysosomal storage diseases. Inherited as a classical autosomal recessive disorder, this global disease of the nervous system induces developmental arrest with regression of attained milestones; neurodegeneration progresses rapidly to cause premature death in young children. There is no effective treatment beyond palliative care, and while the genetic basis of GM2 gangliosidosis is well established, the molecular and cellular events, from diseasecausing mutations and glycosphingolipid storage to disease manifestations, remain to be fully delineated. Several therapeutic approaches have been attempted in patients, including enzymatic augmentation, bone marrow transplantation, enzyme enhancement, and substrate reduction therapy. Hitherto, none of these stratagems has materially altered the course of the disease. Authentic animal models of GM2 gangliodidosis have facilitated in-depth evaluation of innovative applications such as gene transfer, which in contrast to other interventions, shows great promise. This review outlines current knowledge pertaining the pathobiology as well as potential innovative treatments for the GM2 gangliosidoses.

Functional interplay between ganglioside GM1 and cross-linking galectin-1 induces axon-like neuritogenesis via integrin-based signaling and TRPC5-dependent Ca²⁺ influx

Axon‐like neuritogenesis in neuroblastoma (NG108‐15) cells and primary cerebellar granular neurons is furthered by the presence of ganglioside GM1. We describe here that galectin‐1 (Gal‐1), a homobivalent endogenous lectin, is an effector by cross‐linking the ganglioside and its associated glycoprotein α₅β₁‐integrin. The thereby triggered signaling cascade involves autophosphorylation of focal adhesion kinase and activation of phospholipase Cγ and phosphoinositide‐3 kinase. This leads to a transient increase in the intracellular Ca²⁺ concentration by opening of TRPC5 channels, which belong to the signal transduction‐gated cation channels. Controls with GM1‐defective cells (NG‐CR72 and neurons from ganglio‐series KO mice) were retarded in axonal growth, underscoring the relevance of GM1 as functional counterreceptor for Gal‐1. The lectin's presence was detected in the NG108‐15 cells, suggesting an autocrine mechanism of action, and in astrocytes in situ. Gal‐1, as cross‐linking lectin, can thus translate metabolic conversion of ganglioside GD1a to GM1 by neuraminidase action into axon growth.

Galectin‐1 (Gal‐1) was shown an effector of axonogenesis in cerebellar granule neurons (CGNs) and NG108‐15 cells by cross‐linking GM1 ganglioside and its associated glycoprotein α₅β₁‐integrin. The resulting signaling led to a transient increase in intracellular Ca²⁺ by opening TRPC5 channels. CGNs deficient in GM1 showed retarded axonogenesis, underscoring the relevance of GM1 as functional counterreceptor for Gal‐1 in this process. This Gal‐1/GM1‐induced signaling was manifest only at the earliest, initiating stage of axon development.

The multi-tasked life of GM1 ganglioside, a true factotum of nature

GM1 ganglioside occurs widely in vertebrate tissues, where it exhibits many essential functions, both in the plasma membrane and intracellular loci. Its essentiality is revealed in the dire consequences resulting from genetic deletion. This derives from its key roles in several signalosome systems, characteristically located in membrane rafts, where it associates with specific proteins that have glycolipid-binding domains. Thus, GM1 interacts with proteins that modulate mechanisms such as ion transport, neuronal differentiation, G protein-coupled receptors (GPCRs), immune system reactivities, and neuroprotective signaling. The latter occurs through intimate association with neurotrophin receptors, which has relevance to the etiopathogenesis of neurodegenerative diseases and potential therapies. Here, we review the current state of knowledge of these GM1-associated mechanisms.

Glycobiology of ion transport in the nervous system

The nervous system is richly endowed with large transmembrane proteins that mediate ion transport, including gated ion channels as well as energy-consuming pumps and transporters. Transport proteins undergo N-linked glycosylation which can affect expression, location, stability, and function. The N-linked glycans of ion channels are large, contributing between 5 and 50 % of their molecular weight. Many contain a high density of negatively charged sialic acid residues which modulate voltage-dependent gating of ion channels. Changes in the size and chemical composition of glycans are responsible for developmental and cell-specific variability in the biophysical and functional properties of many ion channels. Glycolipids, principally gangliosides, exert considerable influence on some forms of ion transport, either through direct association with ion transport proteins or indirectly through association with proteins that activate transport through appropriate signaling. Examples of both pumps and ion channels have been revealed which depend on ganglioside regulation. While some of these processes are localized in the plasma membrane, ganglioside-regulated ion transport can also occur at various loci within the cell including the nucleus. This chapter will describe ion channel and ion pump structures with a focus on the functional effects of glycosylation on ion channel availability and function, and effects of alterations in glycosylation on nervous system function. It will also summarize highlights of the research on glycolipid/ganglioside-mediated regulation of ion transport.

Structural analysis of brain ganglioside acetylation patterns in mice with altered ganglioside biosynthesis

Gangliosides are sialylated membrane glycosphingolipids especially abundant in mammalian brain tissue. Sialic acid O-acetylation is one of the most common structural modifications of gangliosides which considerably influences their chemical properties. In this study, gangliosides extracted from brain tissue of mice with altered ganglioside biosynthesis (St8sia1 null and B4galnt1 null mice) were structurally characterized and their acetylation pattern was analyzed. Extracted native and alkali-treated gangliosides were resolved by high performance thin layer chromatography. Ganglioside mixtures as well as separated individual ganglioside fractions were further analyzed by tandem mass spectrometry. Several O-acetylated brain ganglioside species were found in knockout mice, not present in the wild-type mice. To the best of our knowledge this is the first report on the presence of O-acetylated GD1a in St8sia1 null mice and O-acetylated GM3 species in B4galnt1 null mice. In addition, much higher diversity of abnormally accumulated brain ganglioside species regarding the structure of ceramide portion was observed in knockout versus wild-type mice. Obtained findings indicate that the diversity of brain ganglioside structures as well as acetylation patterns in mice with altered ganglioside biosynthesis, is even higher than previously reported. Further investigation is needed in order to explore the effects of acetylation on ganglioside interactions with other molecules and consequently the physiological role of acetylated ganglioside species.

Interorganellar membrane microdomains: dynamic platforms in the control of calcium signaling and apoptosis

The dynamic interplay among intracellular organelles occurs at specific membrane tethering sites, where two organellar membranes come in close apposition but do not fuse. Such membrane microdomains allow for rapid and efficient interorganelle communication that contributes to the maintenance of cell physiology. Pathological conditions that interfere with the proper composition, number, and physical vicinity of the apposing membranes initiate a cascade of events resulting in cell death. Membrane contact sites have now been identified that tether the extensive network of the endoplasmic reticulum (ER) membranes with the mitochondria, the plasma membrane (PM), the Golgi and the endosomes/lysosomes. Thus far, the most extensively studied are the MAMs, or mitochondria associated ER membranes, and the ER-PM junctions that share functional properties and crosstalk to one another. Specific molecular components that define these microdomains have been shown to promote the interaction in trans between these intracellular compartments and the transfer or exchange of Ca²⁺ ions, lipids, and metabolic signaling molecules that determine the fate of the cell.

Expression of tyrosine kinase receptors in cultured dorsal root ganglion neurons in the presence of monosialoganglioside and skeletal muscle cells

The neurotrophic factor-like activity of monosialoganglioside (GM1) has been shown to activate tyrosine kinase receptors (Trk). Targets of neuronal innervation play a vital role in regulating the survival and differentiation of innervating neurotrophin-responsive neurons. Both GM1 and target skeletal muscle (SKM) cells are essential for the maintenance of the function of neurons. However, much less is known about the effects of GM1 or/and target SKM cells on the expression of Trk receptors in dorsal root ganglion (DRG) neurons. Here we have tested what extent to the expression of TrkA, TrkB, and TrkC receptors in primary cultured of DRG neurons in absence or presence of GM1 or/and SKM cells. In this experiment, we found that: (1) GM1 promoted expression of TrkA and TrkB but not TrkC in primary cultured DRG neurons; (2) target SKM cells promoted expression of TrkC but not TrkA and TrkB in neuromuscular cocultures without GM1 treatment; and (3) GM1 and target SKM cells had additional effects on expression of these three Trk receptors. The results of the present study offered new clues for a better understanding of the association of GM1 and target SKM on the expression of Trk receptors.

Functional roles of gangliosides in neurodevelopment: an overview of recent advances

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nervous system. They are localized primarily in the outer leaflets of plasma membranes and participated in cell-cell recognition, adhesion, and signal transduction and are integral components of cell surface microdomains or lipid rafts along with proteins, sphingomyelin and cholesterol. Ganglioside-rich lipid rafts play an important role in signaling events affecting neural development and the pathogenesis of certain diseases. Disruption of gangloside synthase genes in mice induces developmental defects and neural degeneration. Targeting ganglioside metabolism may represent a novel therapeutic strategy for intervention in certain diseases. In this review, we focus on recent advances on metabolic and functional studies of gangliosides in normal brain development and in certain neurological disorders.

New insights into the regulation of ion channels by integrins

By controlling cell adhesion to the extracellular matrix, integrin receptors regulate processes as diverse as cell migration, proliferation, differentiation, apoptosis, and synaptic stability. Because the underlying mechanisms are generally accompanied by changes in transmembrane ion flow, a complex interplay occurs between integrins, ion channels, and other membrane transporters. This reciprocal interaction regulates bidirectional signal transduction across the cell surface and may take place at all levels of control, from transcription to direct conformational coupling. In particular, it is becoming increasingly clear that integrin receptors form macromolecular complexes with ion channels. Besides contributing to the membrane localization of the channel protein, the integrin/channel complex can regulate a variety of downstream signaling pathways, centered on regulatory proteins like tyrosine kinases and small GTPases. In turn, the channel protein usually controls integrin activation and expression. We review some recent advances in the field, with special emphasis on hematology and neuroscience. Some oncological implications are also discussed.

Role of gangliosides in brain aging and neurodegeneration

Gangliosides are membrane glycosphingolipids bearing sialic acid residues. Within membranes, gangliosides are specifically enriched in highly organized domains, lipid rafts, and are attributed with diverse functions such as intercellular interactions, cell recognition, neurotransmission, and signal transduction. The highest concentration and variability of ganglioside structures are found in the human brain. Specific temporal and regional distribution of brain gangliosides has been reported; moreover, gangliosides may serve as markers of neurodevelopmental stages, aging and neurodegeneration. Brain ganglioside content and composition as well as ganglioside metabolism are altered in Alzheimer’s disease. It appears that the alterations of ganglioside metabolism leading to changes in membrane physico-chemical properties are not merely a consequence of primary pathology, but may also be involved in the early pathogenesis of Alzheimer’s disease through documented effects on APP proteolytic processing and amyloid aggregation. Investigations of glycolipid metabolic alterations which accompany neurodegenerative disorders provide insight into pathogenetic mechanisms and enable recognition of diagnostic markers as well as molecular structures acting as therapeutic tools interfering with cascade of pathological events.

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.

Regulation of axonal development by plasma membrane gangliosides

Gangliosides present in the plasma membrane participate in fundamental processes during neuronal development. From the determination and the outgrowth of the axon, to the growth inhibitory activity produced after CNS injury, local interconversion of these glycosphingolipids regulate actin dynamics in a spatially restricted manner by modulating membrane receptors and their downstream signaling pathways. Here, we will review the possible mechanisms underlying these modulations and the potential importance of gangliosides and ganglioside-transforming enzymes as therapeutic targets.

Maternal alcohol consumption increases sphingosine levels in the brains of progeny mice

The effect of 'binge' alcohol upon sphingolipid metabolism in the fetal alcohol syndrome (FAS) was examined in pregnant mice (C57BL/6J) by administering a single dose of alcohol during the third trimester (gestational day 15-16). The control mice were administered a sucrose solution of equal caloric value. Brains from progeny at postnatal days 5, 15, 21 and 30 were dissected into three regions, and sphingolipid concentrations of the brain regions were determined including assay of monoglycosylceramide, ceramide, sphingosine and sphingomyelin. We found that a single dose of ethanol induces an elevation of sphingosine (2-3.5-fold) in the brain of progeny. The level of brain ceramide at a dose of 1.5 g/kg was significantly higher than control. Alcohol consumption during pregnancy induces neuronal loss in progeny brains. Our result suggests that the elevation of sphingosine in progeny brain induced by maternal alcohol consumption may be responsible for observed neuronal loss in FAS.

Induction of calcium influx through TRPC5 channels by cross-linking of GM1 ganglioside associated with alpha5beta1 integrin initiates neurite outgrowth

Previous studies demonstrated that cross-linking of GM1 ganglioside with multivalent ligands, such as B subunit of cholera toxin (CtxB), induced Ca2+ influx through an unidentified, voltage-independent channel in several cell types. Application of CtxB to undifferentiated NG108-15 cells resulted in outgrowth of axon-like neurites in a Ca2+ influx-dependent manner. In this study, we demonstrate that CtxB-induced Ca2+ influx is mediated by TRPC5 channels, naturally expressed in these cells and primary neurons. Both Ca2+ influx and neurite induction were blocked by TRPC5 small interfering RNA (siRNA). Pretreatment of NG108-15 cells with neuraminidase increased cell-surface GM1 and greatly enhanced the signal. GM1 was not directly associated with TRPC5 but rather with alpha5beta1 integrin, which opened the channel through a signaling sequence after cross-linking of the GM1/integrin complex. This cascade included autophosphorylation of focal adhesion kinase and subsequent activation of phospholipase Cgamma (PLCgamma) and phosphoinositide-3 kinase [PI(3)K]. Pharmacological blockers that inhibited tyrosine kinase, PLC, and PI(3)K suppressed both CtxB-induced Ca2+ influx and neurite outgrowth. These were also suppressed by SK&F96365, a nonspecific transient receptor potential channel blocker. Confocal immunocytochemistry revealed that GM1 cross-linking induced colocalization of GM1 with these signaling elements in sprouting regions of plasma membrane. In primary cerebellar granular neurons (CGNs), TRPC5 was detected at 2 d in vitro (2 DIV), a stage corresponding to CtxB-stimulated Ca2+ influx. Neurite outgrowth in CGNs, determined at 3 DIV, was accelerated by CtxB and suppressed by TRPC5 siRNA and the above blockers. The crucial role of GM1 was indicated with CGNs from ganglio-series null mice, in which growth of axons was significantly retarded.

Mitogen-activated protein kinase signaling is involved in suramin-induced neurite outgrowth in a neuronal cell line

Suramin is a well-known antitrypanosomal drug and a novel experimental agent for the treatment of several cancers. Previous study showed that suramin is an activator of extracellular signal-regulated kinase (ERK1/2) signaling in several cell lines including Chinese hamster ovary cells, although the physiological relevance of this activation remains uncertain. Here, it was shown that suramin enhances neurite outgrowth concomitant with activation of ERK1/2 in Neuro-2a cells, a neuronal cell line. These neurite outgrowth and ERK1/2 activation were significantly inhibited by PD98059, an inhibitor of mitogen-activated protein kinase kinase, as well as by activation of endogenous adenosine A2A receptors. The suramin-induced phosphorylation of ERK1/2 was also inhibited by inhibitors of Src family kinases. This attenuation of ERK1/2 activity was accompanied by a significant decrease in suramin-induced neurite outgrowth. These results suggest that suramin activates the Src/ERK1/2 signaling pathway that induces neurite outgrowth, both of which are negatively regulated by cAMP produced in response to activation of endogenous adenosine A2A receptors.

Induction of axonal differentiation by silencing plasma membrane-associated sialidase Neu3 in neuroblastoma cells

A reduction of 70% of the plasma membrane-associated sialidase Neu3 activity, due to a corresponding reduction of the enzyme expression by transducing cells with a short hairpin RNA encoding a sequence target (complementary messenger of mouse Neu3), caused neurite elongation in Neuro2a murine neuroblastoma cells. The differentiation process was accompanied in parallel by an increase of the acetylcholinesterase activity, a moderate increase of the c-Src expression and by the presence of the axonal marker tau protein on the neurites. The sphingolipid pattern and turnover in transduced and control cells were characterized by thin layer chromatography, mass spectrometry and metabolic radiolabeling after feeding cells with tritiated sphingosine. Control cells contained about 2 nmol of gangliosides/mg cell protein. GM2 was the main compound, followed by GD1a, GM3 and GM1. In Neu3 silenced cells, the total ganglioside content remained quite similar, but GM2 increased by 54%, GM3 remain constant, and GM1 and GD1a decreased by 66% and 50%, respectively. Within the organic phase sphingolipids, ceramide decreased by 50%, whereas the sphingomyelin content did not change in Neu3 silenced cells.

Dependence of neurotrophic factor activation of Trk tyrosine kinase receptors on cellular sialidase

A direct link between receptor glycosylation and activation following natural ligand interaction has not been observed. Here, we discover a membrane sialidase-controlling mechanism that depends on ligand binding to its receptor to induce enzyme activity which targets and desialylates the receptor and, consequently, causes the induction of receptor dimerization and activation. We also identify a specific sialyl alpha-2,3-linked beta-galactosyl sugar residue of TrkA tyrosine kinase receptor, which is rapidly targeted and hydrolyzed by the sialidase. Trk-expressing cells and primary cortical neurons following stimulation with specific neurotrophic growth factors express a vigorous membrane sialidase activity. Neuraminidase inhibitors, Tamiflu, BCX1812, and BCX1827, block sialidase activity induced by nerve growth factor (NGF) in TrkA-PC12 cells and by brain-derived neurotrophic factor (BDNF) in primary cortical neurons. In contrast, the neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid, specific for plasma membrane ganglioside Neu3 and Neu2 sialidases has no inhibitory effect on NGF-induced pTrkA. The GM1 ganglioside specific cholera toxin subunit B applied to TrkA-PC12 cells has no inhibitory effect on NGF-induced sialidase activity. Neurite outgrowths induced by NGF-treated TrkA-PC12 and BDNF-treated PC12(nnr5) stably transfected with TrkB receptors (TrkB-nnr5) cells are significantly inhibited by Tamiflu. Our results establish a novel mode of regulation of receptor activation by its natural ligand and define a new function for cellular sialidases.

Activation of Src/kinase/phospholipase C/mitogen-activated protein kinase and induction of neurite expression by ATP, independent of nerve growth factor

Extracellular ATP has been reported to potentiate the neurite outgrowth induced by nerve growth factor. In the present study the neurotrophic effect of ATP and other nucleotides was examined in mouse neuroblastoma neuro2a cells which lack nerve growth factor receptor. Exposure of neuro2a cells to ATP resulted in a dramatic increase in neurite bearing cells as compared with untreated control cells. Experiments performed with purinergic receptor agonists and antagonists suggest that the ATP stimulates neurite outgrowth via P2 receptors. Neurite outgrowth was completely blocked by P2 receptor antagonist suramin whereas the P1 receptor antagonist CGS15943 was ineffective. P1 receptor agonist 5'-(N-ethylcarboxamido)adenosine failed to induce neurite outgrowth. The potency order of different P2 receptor agonists was ATP=ATPgammaS>ADP>>2Me-S-ATP. It was insensitive to UTP and antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid) suggesting the involvement of P2Y11 receptor in the observed neuritogenic effect. The signaling pathway leading to ATP-induced neuritogenesis was investigated. The neuritogenic effect of ATP is independent of rise in intracellular Ca(2+) as pharmacological profile of neuritogenic P2Y receptor does not match with that of P2Y2 receptor associated with [Ca(2+)](i) signaling cascade. Exposure of cells to ATP caused activation of Src kinase, phospholipase Cgamma and extracellular signal-regulated kinases ERK1/2. Mitogen-activated protein kinase (MAPK) inhibitor U0126 drastically reduced the number of neurite bearing cells in ATP-treated cultures implying that the neurotrophic effect of ATP is mediated by MAPK. Our results demonstrate that ATP can stimulate neurite outgrowth independent of other neurotrophic factors and can be an effective trophic agent.

Synaptic function of cholinergic-specific Chol-1alpha ganglioside

The function of a cholinergic-specific ganglioside, Chol-1alpha, was investigated. The release of acetylcholine from synaptosomes was inhibited by anti-Chol-1alpha monoclonal antibody but not by monoclonal antibodies against other brain gangliosides tested. Chol-1alpha ganglioside stimulated the high-affinity choline uptake by synaptosomes and consequently enhanced acetylcholine synthesis, resulting in an increased release of acetylcholine from synaptosomes. The memory and learning abilities of rats given anti-Chol-1alpha antibody were remarkably suppressed. These in vitro and in vivo studies suggest that Chol-1alpha ganglioside plays a pivotal role in cholinergic synaptic transmission and participates in cognitive function.

Expression of gangliosides in an immortalized neural progenitor/stem cell line

Glycosphingolipids (GSLs) are known to play important roles in cellular growth and differentiation in the nervous system. The change in expression of gangliosides is correlated with crucial developmental events and is evolutionarily conserved among many vertebrate species. The emergence of neural progenitors represents a crucial step in neural development, but little is known about the exact composition and subcellular localization of gangliosides in neural progenitor cells. The C17.2 cell line was derived after v-myc transformation of neural progenitor cells isolated from neonatal mouse cerebellar cortex. The developmental potential of C17.2 cells is similar to that of endogenous neural progenitor/stem cells in that they are multipotential and capable of differentiating into all neural cell types. We characterized the GSL composition of C17.2 cells and found the presence of only a-series gangliosides. Subcellular localization studies revealed that GM1 and GD1a are localized mainly on the plasma membrane and partly in the cytoplasm, both as punctate clusters. Reverse transcription-polymerase chain reaction revealed the absence of ST-II transcripts in C17 cells, which most likely accounts for the lack of expression of b- and c-series complex gangliosides in this cell line. These data suggest that the divergence in ganglioside expression in C17.2 cells is regulated at the transcriptional level.

Involvement of endogenous ceramide in the inhibition of telomerase activity and induction of morphologic differentiation in response to all-trans-retinoic acid in human neuroblastoma cells

In this study, we examined the role of endogenous ceramide in the inhibition of telomerase and induction of morphologic differentiation in response to all-trans-retinoic acid (ATRA) in the SK-N-SH and SK-N-AS human neuroblastoma cell lines. The results showed that ATRA inhibited the activity of telomerase significantly in a time- and dose-dependent manner, as determined by telomere repeat amplification protocol (TRAP). The inhibition of telomerase by ATRA was maximum (about 50-80% of untreated controls) at 5-10 microM for 4-8 days. Treatment of cells with ATRA (5 microM) also resulted in the inhibition of growth by about 30-70% after 4 and 8 days of treatment, respectively, which was measured by trypan blue exclusion method. Measurement of accumulation of endogenous ceramide by high pressure liquid chromatography coupled with mass spectroscopy (LC/MS) showed that treatment of cells with ATRA resulted in increased levels of mainly C24:0 and C24:1 ceramides at days 2, 4, and 8, respectively. Also, treatment of cells with ATRA in the presence of myriocin blocked the accumulation of ceramide significantly, and more importantly, presence of myriocin partially prevented the inhibition of telomerase. Mechanistically, inhibition of telomerase by endogenous ceramide in response to ATRA treatment involves, at least in part, down-regulation of the expression of telomerase reverse transcriptase (hTERT) mRNA, as determined by semi-quantitative RT-PCR, in these cells. In addition, the modulation of telomerase activity by ATRA correlated with the induction of morphologic differentiation, which was also blocked by myriocin, as determined by extension of neurites using phase-contrast microscopy. These results, therefore, reveal an important effect of ATRA on telomerase inhibition and induction of morphologic differentiation in human neuroblastoma cells. These data also demonstrate that endogenous ceramide is one of the upstream regulators of telomerase activity in human neuroblastoma cells in response to ATRA.

Characterization of cholera toxin B subunit-induced Ca(2+) influx in neuroblastoma cells: evidence for a voltage-independent GM1 ganglioside-associated Ca(2+) channel

The role of endogenous GM1 ganglioside in neurite outgrowth has been studied in N18 and NG108-15 neuroblastoma cells with the GM1-specific ligand cholera toxin B subunit (Ctx B), which stimulates Ca(2+) influx together with neuritogenesis. Our primary goal has been to identify the nature of the calcium channel that is modulated by GM1. An L-type voltage-operated Ca(2+) channel (VOCC) was previously proposed as the mediator of this phenomenon. This investigation, employing fura-2 fluorescent measurements and specific channel blockers and other agents, revealed that GM1 modulates a hitherto unidentified Ca(2+) channel not of the L type. It was opened by Ctx B; was permeable to Ca(2+) and Ba(2+) but not Mn(2+); and was blocked by Ni(2+), Cd(2+), and La(3+). Although most dihydropyridines inhibited Ctx B-induced Ca(2+) influx as well as neurite outgrowth at higher concentrations, they and other VOCC blockers at normally employed concentrations failed to do so, suggesting uninvolvement of VOCC. In addition, Ca(2+) influx induced by Ctx B was not mediated by cGMP-dependent or G-protein-coupled nonselective cation channels, as demonstrated by the cGMP antagonist Rp-cGMPS or the G-protein/receptor uncoupling agent suramin, respectively. Finally, Ca(2+) influx was unlikely to be due to inhibition or reversal of Na(+)-Ca(2+) exchanger via Ctx B induction of Na(+) uptake, insofar as no effect was seen on blocking Na(+) channels, inhibiting Na(+)-K(+)-ATPase, or eliminating extracellular Na(+). The results suggest that this novel channel is gated by interaction with GM1, which, when associated with the channel and bound by appropriate ligand, promotes Ca(2+) influx. This in turn induces signaling for the onset of neuritogenesis.

Ganglioside function in calcium homeostasis and signaling

Ganglioside function in eukaryotic cells encompasses a variety of modulatory interactions related to both development and mature cellular behavior. In relation to the nervous system this includes induction of neurite outgrowth and trophic/neuroprotective phenomena; more generally this applies to ganglioside effects on receptor function, adhesion reactions, and signal transduction mechanisms in neural and extraneural systems. Underlying many of these trophic effects are ganglioside-induced changes in cellular calcium, accomplished through modulation of Ca2+ influx channels, Ca2+ exchange proteins, and various Ca2+-dependent enzymes that are altered through association with gangliosides. A clear distinction needs to be drawn between intrinsic functions of gangliosides as naturally expressed by the cell and activities created by application of exogenous ganglioside(s) that may or may not reflect natural function. This review attempts to summarize findings in this area and point to possible future directions of research.

Potentiation of a sodium-calcium exchanger in the nuclear envelope by nuclear GM1 ganglioside

Calcium is recognized as an important intracellular messenger with a pivotal role in the regulation of many cytosolic and nuclear processes. Gangliosides of various types, especially GM1, are known to have a role in some aspects of Ca2+ regulation, operating through a variety of mechanisms that are gradually coming to light. The present study provides evidence for a sodium-calcium exchanger in the nuclear envelope of NG108-15 neuroblastoma cells that is potently and specifically activated by GM1. Immunoblot analysis revealed an unusually tight association of GM1 with the exchanger in the nuclear envelope but not with that in the plasma membrane. Exchanger and associated GM1 were located in the inner membrane of the nuclear envelope, suggesting this system could function to transfer Ca2+ between nucleoplasm and the envelope lumen. The GM1-enhanced exchange was blocked by cholera toxin B subunit while C2-ceramide, a recently discovered inhibitor of the exchanger, blocked all transfer. Exchanger activity was significantly elevated in nuclei isolated from cells that were induced to differentiate by KCl + dibutyryl-cAMP, a treatment previously shown to promote up-regulation of nuclear GM1 in conjunction with axonogenesis. Similar enhancement was achieved by addition of exogenous GM1 to nuclei from undifferentiated cells. These results suggest a prominent role for nuclear GM1 in regulation of nuclear Ca2+ homeostasis.

Cholera toxin-B subunit blocks excitatory opioid receptor-mediated hyperalgesic effects in mice, thereby unmasking potent opioid analgesia and attenuating opioid tolerance/dependence

In a previous study we demonstrated that injection (i.p.) of low doses of GM1 ganglioside in mice rapidly attenuates morphine's analgesic effects. This result is consonant with our electrophysiologic studies in nociceptive types of dorsal root ganglion (DRG) neurons in culture, which showed that exogenous GM1 rapidly increased the efficacy of excitatory (Gs-coupled) opioid receptor functions. By contrast, treatment of DRG neurons with the non-toxic B-subunit of cholera toxin (CTX-B) which binds selectively to GM1, blocked the excitatory, but not inhibitory, effects of morphine and other bimodally-acting opioid agonists, thereby resulting in a net increase in inhibitory opioid potency. The present study provides more direct evidence that endogenous GM1 plays a physiologic role in regulating excitatory opioid receptor functions in vivo by demonstrating that cotreatment with remarkably low doses of CTX-B (10 ng/kg, s.c.) selectively blocks hyperalgesic effects elicited by morphine or by a kappa opioid agonist, thereby unmasking potent opioid analgesia. These results are comparable to the effects of cotreatment of mice with morphine plus an ultra-low dose of the opioid antagonist, naltrexone (NTX) which blocks opioid-induced hyperalgesic effects, unmasking potent opioid analgesia. Low-dose NTX selectively blocks excitatory opioid receptors at their recognition site, whereas CTX-B binds to, and interferes with, a putative allosteric GM1 regulatory site on excitatory opioid receptors. Furthermore, chronic cotreatment of mice with morphine plus CTX-B attenuates development of opioid tolerance and physical dependence, as previously shown to occur during cotreatment with low-dose NTX.

Human IgM anti-GM1 autoantibodies modulate intracellular calcium homeostasis in neuroblastoma cells

Increased titers of IgM anti-GM1 antibodies are present in some patients with Lower Motor Neuron Disease (LMND) or Motor Neuropathy (MN), but their pathogenic role and the mechanism of action are unclear. Previous studies have shown that the B subunit of Cholera Toxin (CT), which binds and crosslinks ganglioside GM1, modulate intracellular calcium in murine neuroblastoma cells via the activation of L-type voltage-dependent calcium channels (VGCC). Therefore, using a fluorimetric approach, we have examined the hypothesis that the pentameric IgM anti-GM1 antibodies, could similarly alter calcium concentration in N18 neuroblastoma cells. Sera with human IgM anti-GM1 antibodies were obtained from 5 patients with LMND and 2 patients with MN. Human IgG anti-GM1, IgM anti-Myelin Associated Glycoprotein (MAG), IgM anti-sulfatide antibodies and lectin peanut agglutinin (PNA), that recognizes specifically the Gal(betal-3)GalNAc epitope, were used as control sera. Direct application of either human IgM anti-GM1 antibodies or the B subunit of CT to N18 neuroblastoma cells induced a sustained influx of manganese ions, as indicated by a quench of the intracellular fura-2 fluorescence. Furthermore, the dihydropyridine L-type channel antagonists completely inhibited the manganese influx, suggesting that it is due to activation of an L-type VGCC. The magnitude of the influx was correlated with antibody titers. None of human IgG anti-GM1, IgM anti-MAG, IgM anti-sulfatide antibodies or PNA induce an ion influx, pointing to the selective participation of the pentameric IgM isotype of anti-GM1 in the modulation of L-type calcium channels opening. Given that L-type calcium channels are present on motor neurons, the modulation of L-type calcium channels by IgM GM1 antisera may have important implications in diseases such as LMND and MN.

Cerebellar neurons lacking complex gangliosides degenerate in the presence of depolarizing levels of potassium

Mice engineered to lack GM2/GD2 synthase (GalNAc-T), with resultant deficit of GM2, GD2, and all gangliotetraose gangliosides, were originally described as showing a relatively normal phenotype with only a slight reduction in nerve conduction. However, a subsequent study showed that similar animals suffer axonal degeneration, myelination defects, and impaired motor coordination. We have examined the behavior of cerebellar granule neurons from these neonatal knockouts in culture and have found evidence of impaired capacity for Ca2+ regulation. These cells showed relatively normal behavior when grown in the presence of physiological or moderately elevated K+ but gradually degenerated in the presence of high K+. This degeneration in depolarizing medium was accompanied by progressive elevation of intracellular calcium and onset of apoptosis, phenomena not observed with normal cells. No differences were detected in cells from normal vs. heterozygous mice. These findings suggest that neurons from GalNAc-T knockout mice are lacking a calcium regulatory mechanism that is modulated by one or more of the deleted gangliosides, and they support the hypothesis that maintenance of calcium homeostasis is one function of complex gangliosides during, and perhaps subsequent to, neuronal development.

Cerebellar neurons lacking complex gangliosides degenerate in the presence of depolarizing levels of potassium

Mice engineered to lack GM2/GD2 synthase (GalNAc-T), with resultant deficit of GM2, GD2, and all gangliotetraose gangliosides, were originally described as showing a relatively normal phenotype with only a slight reduction in nerve conduction. However, a subsequent study showed that similar animals suffer axonal degeneration, myelination defects, and impaired motor coordination. We have examined the behavior of cerebellar granule neurons from these neonatal knockouts in culture and have found evidence of impaired capacity for Ca2+ regulation. These cells showed relatively normal behavior when grown in the presence of physiological or moderately elevated K+ but gradually degenerated in the presence of high K+. This degeneration in depolarizing medium was accompanied by progressive elevation of intracellular calcium and onset of apoptosis, phenomena not observed with normal cells. No differences were detected in cells from normal vs. heterozygous mice. These findings suggest that neurons from GalNAc-T knockout mice are lacking a calcium regulatory mechanism that is modulated by one or more of the deleted gangliosides, and they support the hypothesis that maintenance of calcium homeostasis is one function of complex gangliosides during, and perhaps subsequent to, neuronal development.