Regulation of protein kinase C activity by gangliosides

Chronic opioid treatment of neuroblastoma x dorsal root ganglion neuron hybrid F11 cells results in elevated GM1 ganglioside and cyclic adenosine monophosphate levels and onset of naloxone-evoked decreases in membrane K+ currents

Prolongation of the action potential duration of dorsal root ganglion (DRG) neurons by low (nM) concentrations of opioids occurs through activation of excitatory opioid receptors that are positively coupled via Gs regulatory protein to adenylate cyclase. Previous results suggested GM1 ganglioside to have an essential role in regulating this excitatory response, but not the inhibitory (APD-shortening) response to higher (microM) opioid concentrations. Furthermore, it was proposed that synthesis of GM1 is upregulated by prolonged activation of excitatory opioid receptor functions. To explore this possibility we have utilized cultures of hybrid F11 cells to carry out closely correlated electrophysiological and biochemical analyses of the effects of chronic opioid treatment on a homogeneous population of clonal cells which express many functions characteristic of DRG neurons. We show that chronic opioid exposure of F11 cells does, in fact, result in elevated levels of GM1 as well as cyclic adenosine monophosphate (AMP), concomitant with the onset of opioid excitatory supersensitivity as manifested by naloxone-evoked decreases in voltage-dependent membrane K+ currents. Such elevation of GM1 would be expected to enhance the efficacy of excitatory opioid receptor activation of the Gs/adenylate cyclase/cyclic AMP system, thereby providing a positive feedback mechanism that may account for the remarkable supersensitivity of chronic opioid-treated neurons to the excitatory effects of opioid agonists as well as antagonists. These in vitro findings may provide novel insights into the mechanisms underlying naloxone-precipitated withdrawal syndromes and opioid-induced hyperalgesia after chronic opiate addiction in vivo.

Inability to restore resting intracellular calcium levels as an early indicator of delayed neuronal cell death

The hippocampus is especially vulnerable to excitotoxicity and delayed neuronal cell death. Chronic elevations in free intracellular calcium concentration ([Ca2+]i) following glutamate-induced excitotoxicity have been implicated in contributing to delayed neuronal cell death. However, no direct correlation between delayed cell death and prolonged increases in [Ca2+]i has been determined in mature hippocampal neurons in culture. This investigation was initiated to determine the statistical relationship between delayed neuronal cell death and prolonged increases in [Ca2+]i in mature hippocampal neurons in culture. Using indo-1 confocal fluorescence microscopy, we observed that glutamate induced a rapid increase in [Ca2+]i that persisted after the removal of glutamate. Following excitotoxic glutamate exposure, neurons exhibited prolonged increases in [Ca2+]i, and significant delayed neuronal cell death was observed. The N-methyl-D-aspartate (NMDA) channel antagonist MK-801 blocked the prolonged increases in [Ca2+]i and cell death. Depolarization of neurons with potassium chloride (KCl) resulted in increases in [Ca2+]i, but these increases were buffered immediately upon removal of the KCl, and no cell death occurred. Linear regression analysis revealed a strong correlation (R = 0.973) between glutamate-induced prolonged increases in [Ca2+]i and delayed cell death. These data suggest that excitotoxic glutamate exposure results in an NMDA-induced inability to restore resting [Ca2+]i (IRRC) that is a statistically significant indicator of delayed neuronal cell death.

Possible role for protein kinase C in the pathogenesis of inborn errors of metabolism

Protein kinase C (PKC) is a ubiquitous enzyme family implicated in the regulation of a large number of short- and long-term intracellular processes. It is hypothesized that modulation of PKC activity may represent, at least in part, a functional link between mutations (genotype) that lead to the pathological accumulation of naturally occurring compounds that affect PKC activity and perturbation of PKC-mediated substrate phosphorylation and cellular function in the corresponding diseases (phenotype). This model provides a unifying putative mechanism by which the phenotypic expression of some inborn errors of metabolism may be explained. Recent studies in a cell-free system of human skin fibroblasts support the hypothesis that alteration of PKC activity may represent the functional link between accumulation of sphingolipids and fatty acyl-CoA esters, and perturbation of cell function in sphingolipidoses and fatty acid oxidation defects, respectively. Further studies will elucidate the effects of these alterations on PKC-mediated short- and long-term cellular functions in these diseases, as well as the possible role of PKC in the pathogenesis of other diseases.

Aggregation properties of semisynthetic GD1a ganglioside (IV3Neu5AcII3Neu5AcGgOse4Cer) containing an acetyl group as acyl moiety

GD1a ganglioside containing an acetyl group as acyl moiety, GD1a(acetyl), was synthesized from natural GD1a. The aggregative properties in aqueous solution of GD1a(acetyl) have been studied by static and dynamic laser light-scattering measurements. GD1a(acetyl) spontaneously aggregates as small micelles showing a hydrodynamic radius and molecular mass of 33 A and 96 kDa, respectively. Vibrio cholerae sialidase showed a very high activity on the micelles of GD1a(acetyl), compared to GD1a. This has been explained as a consequence of the high surface curvature of the the small micelles. High resolution proton NMR spectra were recorded from micelles of GD1a(acetyl) in deuterated water. The low overall correlation time of the GD1a(acetyl) micelles was calculated to be about 2 x 10(-8)s, a value one order of magnitude lower than that determined for natural GD1a.

Attenuation of glutamate-induced neurotoxicity in chronically ethanol-exposed cerebellar granule cells by NMDA receptor antagonists and ganglioside GM1

Ethanol, acutely, is a potent inhibitor of the function of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. After chronic exposure of animals to ethanol, however, the NMDA receptor in brain is upregulated. This upregulation is associated with the occurrence of ethanol withdrawal seizures. When cultured cerebellar granule neurons are exposed chronically to ethanol, the resulting upregulation of NMDA receptor function renders the cells more susceptible to glutamate-induced neurotoxicity. The present studies show that chronic ethanol exposure produces an increase in NMDA receptor number in the cells, measured by ligand binding to intact cells. Glutamate-induced excitotoxicity, both in control and ethanol-exposed cells, is blocked by the same NMDA receptor antagonists previously shown to block ethanol withdrawal seizures in animals. In addition, glutamate neurotoxicity is blocked by acute (2-hr) pretreatment of cells with ganglioside GM1 or by chronic (3 days) treatment with the ganglioside. Acute ganglioside treatment does not interfere with the initial rise in intracellular calcium caused by glutamate, whereas this response is downregulated after chronic ganglioside treatment. These results suggest that therapeutic agents can be developed to block both ethanol withdrawal signs and the neuronal damage that accompanies ethanol withdrawal. Furthermore, chronic ganglioside treatment during ethanol exposure has the potential to prevent changes in the NMDA receptor that lead to withdrawal seizures and enhanced susceptibility to excitotoxicity.

Prevention of apoptotic neuronal death by GM1 ganglioside. Involvement of Trk neurotrophin receptors

We have used serum-deprived cultures of wild type and genetically modified PC12 cells to investigate the molecular mechanisms by which monosialoganglioside (GM1) rescues neuronal cells from apoptotic death elicited by withdrawal of trophic support. Our findings indicate that GM1-promoted survival can be mediated in part by the Trk NGF receptor as well as by TrkB, and potentially by tyrosine kinase receptors for additional neurotrophic growth factors. Experiments employing K-252a, an inhibitor of Trk kinases, and PC12 cells overexpressing a dominant inhibitory form of Trk both indicate that a portion of the survival-promoting activity of GM1 is evoked by receptor dimerization and autophosphorylation. In consonance with this we find that GM1 stimulates Trk tyrosine autophosphorylation and Trk-associated protein kinase activity. These observations may provide a mechanism to account for the reported in vitro and in vivo trophic actions of GM1.

Stimulation of cyclic adenosine 3',5'-monophosphate-dependent protein kinase with brain gangliosides

The holoenzyme of cAMP-dependent protein kinase (cAMP-kinase) partially purified from the particulate fraction of rat brain was stimulated by gangliosides. Among various gangliosides tested, GM1 was most potent, giving Ka value of 19.5 microM. The maximal activation of the kinase was obtained with 100 microM GM1 using kemptide as substrate. Gangliosides inhibited the kinase activity of the catalytic subunit of cAMP-kinase. Of various substrates tested, the ganglioside-stimulated cAMP-kinase could phosphorylate microtubule-associated protein 2, synapsin I and myelin basic protein, but not histone H1 and casein. The molecular mechanisms of the stimulatory effect of gangliosides were investigated. The kinase activated with GM1 was inhibited by the addition of PKItide, a specific inhibitor for cAMP-kinase. However, GM1 did not dissociate the holoenzyme into the catalytic and regulatory subunits and did not interfere with the binding ability of cAMP to the holoenzyme. These results suggest that the gangliosides can directly activate cAMP-kinase in a different manner from cAMP.

Ganglioside modulation of the PDGF receptor. A model for ganglioside functions

Gangliosides are a family of glycolipids that are present at the cell surface of all mammalian cells. Patterns of gangliosides are different in gliomas than normal brain, and exogenously added gangliosides affect the growth of cultured glioma cells. Gangliosides inhibit the activities of several kinases, including protein kinase C (PKC) and cAMP-kinase. U-1242 MG cells (derived from a human malignant glioma) have receptors for platelet-derived growth factor (PDGF) that become phosphorylated on tyrosine when exposed to PDGF. Exposure of these cells to PDGF also causes an increase in intracellular calcium concentration ([Ca2+]i) and induces a translocation of PKC to the membrane. Preincubation of U-1242 MG cells with several species of gangliosides inhibits the increase in ([Ca2+]i) and PKC translocation in response to PDGF, but GM3 is much less effective than other species tested. This is due to a lack of activation of the receptor tyrosine kinase as monitored by phosphorylation of the receptor on tyrosine residues, but is not due to an inhibition of binding of PDGF to its receptors. The lack of activation of the PDGF receptor tyrosine kinase is due to an inhibition of dimerization of the receptor monomers by gangliosides GM1, GM2, GD1a, GT1b, but not GM3. Therefore, gangliosides may be involved in coordinating the activities of multiple trophic factors simultaneously acting on a cell by regulating the dimerization of their respective receptor monomers.

Geometrical and conformational properties of ganglioside GalNAc-GD1a, IV4GalNAcIV3Neu5AcII3Neu5AcGgOse4Cer

The aggregative properties of GalNAc-GD1a ganglioside, in comparison with those of GD1a, have been investigated and correlated to the intrinsic conformation and mobility of the oligosaccharide chain of the molecules. Micellar parameters in aqueous solution (molecular mass, hydrodynamic radius as well as the surface area at the lipid/water interface and the packing parameter of the monomer inserted in the aggregate) are measured by the laser light-scattering technique. The presence of a further GalNAc residue causes a 22% increase in molecular mass, contrary to expectation. Oligosaccharide moiety three-dimensional structures have been modeled using molecular mechanics and dynamics calculations, based on NOE interactions observed for native gangliosides dissolved in deuterated dimethylsulfoxide or, as mixed micelles with fully deuterated dodecylphosphocholine, in D2O. Compared with GD1a the GalNAc-GD1a is less mobile, thus influencing the surface area, this lower mobility together with the GalNAc-GD1a conformation leads to a larger number of monomers participating in the formation of the micelle. The results further substantiate the model in which the three-dimensional structure and the intrinsic dynamic properties of the oligosaccharide chain affect the geometrical properties of the aggregate.

Differential Effects of Glycosphingolipids n Protein Kinase C Activity in PC12D Pheochromocytoma Cells

Previous studies have shown that certain glycosphingolipids may function as modulators of protein kinase C (PKC) activity. To study the structure-activity relationship, we examined the effects of 17 gangliosides, 10 neutral glycolipids, as well as sulfatide, psychosine and ceramide on PKC activity in PC12D cells. Using an in vitro assay system, we found that all but one (GQ1b) ganglioside inhibited PKC activity at concentrations between 25 and 100 &mgr;M, and the potency was proportional to the number of sialic acid residues. However, at lower concentrations several gangliosides, including GM1 and LM1 behaved as mild activators of PKC activity. GQ1b had no effect within the range 0.1-10 &mgr;M, but acted as a mild activator of PKC activity at 25 &mgr;M. On the other hand, fucosyl-GM1 and GM1 containing blood group B determinant, which are abundant in PC12 cells, were potent inhibitors of PKC activity. Among the neutral glycosphingolipids tested, LacCer, Gb3, GalGb3, and GA1, all of which have a terminal galactose residue, were found to be ineffective or acted as mild activators of PKC activity. In contrast, GA2, Gb4 and Gb5 which have a terminal N-acetylgalactosamine residue, were potent inhibitors of the PKC activity. Thus, the terminal sugar residue may play a pivotal role in determining the effect of glycosphingolipids in modulating PKC activity. In addition, we also found that GalCer containing normal fatty acids acted as potent activators of PKC activity. Ceramide and GlcCer appeared to be ineffective in modulating PKC activity, wheras psychosine and sulfatides appeared to be inhibitory. We conclude that the carbohydrate head groups and the hydrophobic groups of gangliosides and neutral glycolipids may modulate the PKC system in unique manners, which may in turn affect various biological processes in the cell. Copyright 1994 S. Karger AG, Basel

Differentiation of oligodendrocytes cultured from developing rat brain is enhanced by exogenous GM3 ganglioside

Cultures consisting primarily of O-2A progenitor cells and immature oligodendrocytes with a few microglia and astrocytes were obtained by shaking primary cultures from neonatal rat brain after 12-14 days in vitro. Addition of 50 micrograms/ml exogenous Neu-NAc alpha 2-3Gal beta 1-4Glc beta 1-1'ceramide (GM3 ganglioside) to the cultures resulted in an increase in the number and thickness of cell processes that stained intensely for sulfatide and galactocerebroside (galC) in comparison to control cultures without added GM3. The treated cultures also contained fewer astrocytes than control cultures as revealed by immunostaining for glial fibrillary acidic protein (GFAP). Cells that immunostained for both GFAP and sulfatide/galC were very rare in control cultures but were frequently seen in the GM3-treated cultures, suggesting that these may represent cells changing their direction of differentiation away from type II astrocytes toward oligodendrocytes under the influence of GM3. These effects on the developing rat oligodendrocytes were specific for GM3 ganglioside and were not produced by adding GM1, GM2, GD3, or GD1a to the cultures. Lactosyl ceramide and neuraminyl lactose were also ineffective. When control cultures were initially plated on polylysine and incubated with [14C]galactose, GD3 was the principal labeled ganglioside. However, as the control cells differentiated over time in culture without the addition of exogenous GM3 and produced increasing amounts of myelin-related components, the incorporation of [14C]galactose into endogenous GM3 increased to become the predominant labeled ganglioside by 6 days after plating. Metabolic labeling of the GM3-treated oligodendrocytes with [14C]galactose revealed increased incorporation into galC and sulfatide in comparison to control cultures, but a decreased labeling of endogenous GM3. Similarly, incorporation of an amino acid precursor into the myelin-associated glycoprotein (MAG) was increased by GM3 treatment, but incorporation into myelin basic protein (MBP) was not affected. Although the overall effect of added GM3 was to decrease the phosphorylation of most proteins in the oligodendrocytes, including MBP, GM3 enhanced the phosphorylation of MAG. These findings indicate that GM3 ganglioside has an important role in the differentiation of cells of the O-2A lineage toward myelin production, since differentiation is associated with increased metabolic labeling of endogenous GM3 in control cultures and is enhanced by the addition of exogenous GM3.

Blockade effect of nerve growth factor on GM1 ganglioside-induced activation of transglutaminase in superior cervical sympathetic ganglia excised from adult rat

The activity of transglutaminase (TG), a Ca(2+)-dependent enzyme indicating tissue degradation or differentiation, showed in isolated adult rat superior cervical ganglia (SCG) a rapid (within 15 to 30 min) and marked (approx. 5- to 8-fold) increase with the addition of either GM1 ganglioside (GM1, 5 nM), which is rich in synapses, or sialyl cholesterol (SC, 20 microM), a synthetic sialic acid-containing compound, to the incubation medium at 37 degrees C. Under the same incubation conditions, addition of GM1 or SC decreased protein kinase C (PKC) activity (-26% to -39%) in the cytosolic fraction of the SCG, but increased the enzymic activity (+39% to +61%) in the particulate (cell membrane) fraction, suggesting that a sialic acid-containing compound (GM1 or SC) promotes PKC translocation from the cytosol to the membrane in ganglionic neurons. By contrast, addition of a promoting factor for survival of sympathetic neurons even in adulthood, nerve growth factor, (NGF, 0.25 micrograms/ml) to the medium significantly decreased ganglionic TG activity (-43%). This inhibition was completely antagonized by the co-addition of NGF-monoclonal antibody (0.75 microgram/ml). An effective blockade of GM1- or SC-induced stimulation of ganglionic TG activity was seen by further addition of NGF to the medium. Also, NGF almost abolished the translocation of ganglionic PKC activity induced by the sialic acid-containing compounds, although either NGF or 12-O-tetradecanoylphorbol ester (TPA) alone stimulated the cytosolic PKC activity (approx. +30%) in the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Aggregative properties of gangliosides in solution

The aggregative properties of gangliosides in diluted aqueous solutions are discussed on the basis of simple and well-established thermodynamic concepts. Theoretical assumptions are compared with experimental data obtained, mainly by scattering techniques, on GM3, GM2, GM1, GD1a, GalNAc-GD1a, GD1b, GD1b lactone and GT1b gangliosides, all containing ceramide portions of similar composition, and on GM1 molecular species containing different well-defined ceramide structures. We also report on mixed aggregates with amphiphilic compounds and on the ganglioside aggregate-soluble protein interaction effects which give rise to very stable lipoproteic complexes of well-defined ganglioside-protein composition.

Modulation of phospholipases A2 and C activities against dilauroylphosphorylcholine in mixed monolayers with semisynthetic derivatives of ganglioside and sphingosine

Most of the semisynthetic ganglioside and sphingosine derivatives studied here decreased the rate as well as the extent of hydrolysis of monomolecular layers of dilauroylphosphorylcholine (dlPC) by both phospholipase A2 (PLA2) and C (PLC). For PLA2, the rate of enzymatic activity was inversely correlated (p < 0.001) with the duration of the latency period of the enzymatic reaction. The correlation between the rate of activity and the latency period was not statistically significant for PLC. The potency to inhibit PLA2 and PLC was not significantly correlated with the presence of specific chemical groups. Also, the inhibitory effect is not correlated to changes of the substrate intermolecular spacing or surface potential caused by the sphingolipids (SLs). Conversely, for PLA2 the variation of the kinetic parameters of the reaction with the molecular polarization vector of the SLs are statistically significant (p < 0.01). The rate of phospholipid degradation by PLA2 decreased, and the lag times tended to become longer, with increasing values of the SLs' polarization vector. The kinetic parameters of the reaction with PLC did not show statistically significant correlation with the polarization vector of the SLs. Our results suggest that the configuration of the electrostatic field across the interface is more important than are formal charges or specific chemical moieties in modulating the activity of PLA2. Inhibition of phospholipase activities of these types by specific SLs or their metabolites may be important as supramolecular regulatory steps at the membrane level of the amplification of lipid-mediated signaling pathways.

Granulosa cell luteinizing hormone receptor expression is modulated by ganglioside-specific ligands

The ganglioside GM1 (Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3] Gal beta 1-->4Glc beta 1-->1Cer) was synthesized during granulosa cell development in vitro, and the effect of the interaction between cell-surface GM1 and its ligands on the luteinizing hormone (LH) receptor expression was investigated. GM1 synthesis, demonstrated by metabolic labeling of glycosphingolipids with [3H]galactose and binding studies using the 125I-B-subunit of cholera toxin, a specific ligand for GM1, was increased in follicle-stimulating hormone (FSH)-treated granulosa cells. When granulosa cells were cultured for 72 h in a medium containing the B-subunit of cholera toxin, FSH-induced LH-receptor contents determined by measuring the binding of 125I-deglycosylated human chorionic gonadotropin to intact cells, was augmented. The stimulatory effect of the B-subunit was dependent on the FSH concentration and culture duration. The augmentation was observed after culture for 48 h, and marked increases were evident after 72 h, which coincided with an increase of the 125I-B-subunit binding capacity. Scatchard analysis of the LH-receptor binding indicated that treatment with the B-subunit increased the number of LH-binding sites (6580 sites/cell after treatment with 20 ng/ml FSH; 11,290 sites/cell after FSH plus 100 ng/ml B-subunit), but did not alter the binding affinity. A specific antibody against GM1 mimicked the stimulatory effect of the B-subunit. The augmentation was not accompanied by granulosa cell proliferation. These findings suggest that binding of exogenous or possible endogenous ligands to cell-surface GM1 produces signals and modulates the cellular behavior during granulosa cell development.

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.

Inhibition of neurite outgrowth of neuroblastoma Neuro-2a cells by cholera toxin B-subunit and anti-GM1 antibody

The role of cell surface GM1 ganglioside in neurite outgrowth of Neuro-2a neuroblastoma cells was investigated by application of anti-GM1 antibody and the B subunit of cholera toxin (cholera B) to cultured cells stimulated to grow neurites in various ways. When the cells were simultaneously treated with stimulatory agent and cholera B, inhibition, as measured by percent of neurite-bearing cells, was observed with most stimuli: neuraminidase; GD1a ganglioside, retinoic acid, and low serum. However, with dibutyryl cyclic AMP the small reduction observed was not statistically significant. The inhibitory effect of cholera B on neurite outgrowth induced by low serum was dose-dependent, reaching a maximum at 200 ng/mL; 48 h after washout of cholera B the cells were released from inhibition and regrew neurites at nearly the previous rate in the presence of low serum. When the cells were exposed to stimulus for 6 h or more the inhibitory effect of subsequent addition of cholera B was reduced or eliminated; inhibition thus occurs during an early stage of neurite initiation. Anti-GM1 antibody at dilutions of 1:100-1:400 had the same inhibitory effect as cholera B with cells stimulated by GD1a or retinoic acid, whereas anti-GM2 antibody had no effect at 1:200 or 1:400; inhibition by the latter antibody at 1:100 dilution was similar to that attained with control ascites fluid. These results point to a pivotal role for cell surface GM1 in Neuro-2a differentiation induced by many (but not all) neuritogenic agents.

Synthesis and aggregative properties of GM1 ganglioside (IV3Neu5AcGgOse4Cer) containing D-(+)-2-hydroxystearic acid

GM1 ganglioside containing a hydroxylated fatty acid moiety, GM1(OH), was synthesized starting from lyso-GM1 and D-(+)-2-hydroxystearic acid. The aggregative, geometrical and distribution properties of GM1(OH) were compared with those of stearic acid containing GM1 ganglioside; laser light scattering measurements, differential scanning calorimetry and fluorescence spectroscopy were used. GM1 and GM1(OH) are present in solution as micelles with a hydrodynamic radius of 58.7 and 60.0 A, and molecular mass of 470 and 570 kDa, respectively. The surface area occupied by the monomer of GM1(OH) at the lipid-water interface of the aggregate was calculated to be 117 A2, which is 3 A2 lower than that determined for GM1. Proton NMR analyses of GM1 and GM1(OH) suggest different three-dimensional structures at the ganglioside lipid-water interface. Both GM1(OH) and GM1 inserted into dipalmitoylphosphatidylcholine (DPPC) vesicles undergo segregation phenomena, with the formation of ganglioside-enriched microdomains, but GM1(OH) shows a higher degree of dispersion in the DPPC matrix and exerts a lower rigidifying effect than does GM1.

Effects of gangliosides GM1 and GD1a on GAP-43 phosphorylation and dephosphorylation in isolated growth cones

Phosphorylation of the nervous system-specific protein GAP-43 in growth cones in vivo increases as the growth cones near their targets, at a time when the gangliosides GM1 and GD1a are being accumulated in the growth cone membrane, thus raising the possibility that the gangliosides could modulate GAP-43 behavior. We used a subcellular fraction of intact isolated growth cones to show that both GM1 and GD1a affected the calcium-dependent posttranslational regulation of GAP-43 in several similar ways. Both gangliosides induced rapid incorporation of phosphate into GAP-43; however, the induction was undetectable with our antibody 2G12 that is specific for kinase C-phosphorylated GAP-43. Furthermore, neither ganglioside stimulated kinase C activity in isolated growth cones, suggesting that the rapid phosphorylation may not be on Ser41, the kinase C site. However, both gangliosides did induce a slower accumulation of GAP-43 phosphorylated on Ser41, apparently by inhibiting a phosphatase. Finally, calcium-dependent proteolysis of GAP-43 was also stimulated by both GM1 and GD1a. In contrast, GD1a, but not GM1, caused the redistribution of GAP-43 into the isolated growth cone cytoskeleton. The results demonstrate that both gangliosides can modulate the calcium-dependent regulation of GAP-43.

Nephron segment and cell-type specific expression of gangliosides in the developing and adult kidney

Despite the increasing knowledge of the role of gangliosides in normal and diseased tissues, little is known of the presence, distribution and functions of these molecules in the kidney. In this study we analyzed the main gangliosides of isolated glomeruli and cortical, medullary and papillary fractions of the human, rat and bovine kidneys biochemically. In addition, we used immunohistochemistry to visualize the distribution of GM1/GM2, GD2, GD3 and O-acetyl GD3 gangliosides along the nephron. Furthermore, we explored the species specific expression of gangliosides by comparing those from the rat, bovine and human kidney, and studied the pattern of ganglioside expression during development. In glomeruli, cortical tubuli, medullae and papillae, a relatively simple pattern of main gangliosides was observed as revealed by thin layer chromatographic (TLC) analysis in each species studied. Furthermore, considerable changes in the glomerular gangliosides during maturation were observed, with a complex type of gangliosides predominating during the fetal age and with a preference to more simple precursors upon maturation. Interestingly, the immunohistochemical detection revealed a distinct pattern of ganglioside compartmentation to various nephron segments or cell types. These findings provide a basis for studying the role of segment- and cell type-specific gangliosides for local functions.

The effects of methylprednisolone and the ganglioside GM1 on acute spinal cord injury in rats

Recent clinical trials have reported that methylprednisolone sodium succinate (MP) or the monosialic ganglioside GM1 improves neurological recovery in human spinal cord injury. Because GM1 may have additive or synergistic effects when used with MP, the authors compared MP, GM1, and MP+GM1 treatments in a graded rat spinal cord contusion model. Spinal cord injury was caused by dropping a rod weighing 10 gm from a height of 1.25, 2.5, or 5.0 cm onto the rat spinal cord at T-10, which had been exposed via laminectomy. The lesion volumes were quantified from spinal cord Na and K shifts at 24 hours after injury and the results were verified histologically in separate experiments. A single dose of MP (30 mg/kg), given 5 minutes after injury, reduced 24-hour spinal cord lesion volumes by 56% (p = 0.0052), 28% (p = 0.0065), and 13% (p > 0.05) in the three injury-severity groups, respectively, compared to similarly injured control groups treated with vehicle only. Methylprednisolone also prevented injury-induced hyponatremia and increased body weight loss in the spine-injured rats. When used alone, GM1 (10 to 30 mg/kg) had little or no effect on any measured variable compared to vehicle controls; when given concomitantly with MP, GM1 blocked the neuroprotective effects of MP. At a dose of 3 mg/kg, GM1 partially prevented MP-induced reductions in lesion volumes, while 10 to 30 mg/kg of GM1 completely blocked these effects of MP. The effects of MP on injury-induced hyponatremia and body weight loss were also blocked by GM1. Thus, GM1 antagonized both central and peripheral effects of MP in spine-injured rats. Until this interaction is clarified, the authors recommend that MP and GM1 not be used concomitantly to treat acute human spinal cord injury. Because GM1 modulates protein kinase activity, protein kinases inhibit lipocortins, and lipocortins mediate anti-inflammatory effects of glucocorticoids, it is proposed that the neuroprotective effects of MP are partially due to anti-inflammatory effects and that GM1 antagonizes the effects of MP by inhibiting lipocortin. Possible beneficial effects of GM1 reported in central nervous system injury may be related to the effects on neural recovery rather than acute injury processes.

Glycolipids isolated from Aplysia kurodai can activate cyclic adenosine 3',5'-monophosphate-dependent protein kinase from rat brain

Cyclic AMP (cAMP)-dependent protein kinase (cAMP-kinase) partially purified from the membrane fractions of rat brains was stimulated by novel phosphonoglycosphingolipids (glycolipids) derived from the skin and nerve fibers of Aplysia kurodai. Among various glycolipids tested, a major glycolipid from the skin, 3-O-MeGal beta 1-->3GalNAc alpha 1-->3[6'-O-(2-aminoethylphosphonyl)Gal alpha 1-->2](2-aminoethylphosphonyl-->6)Glc beta 1-->4Glc beta 1-->1ceramide (SGL-II), was most potent, giving half-maximal activation at 32.2 microM. Activation of cAMP-kinase was maximal with 250 microM SGL-II using kemptide as substrate. The effect of SGL-II was additive on kinase activity at submaximal concentrations of cAMP. The kinase activity activated with SGL-II was inhibited by the addition of protein kinase inhibitor peptide, a specific peptide inhibitor for cAMP-kinase. Its inhibitory pattern was similar to that for the catalytic subunit. Of the various substrates tested, the glycolipid-stimulated cAMP-kinase could phosphorylate microtubule-associated protein 2, synapsin I, and myelin basic protein but not histone H1 and casein. The regulatory subunit strongly inhibited the activity of purified catalytic subunit of cAMP-kinase. This inhibition was reversed by addition of SGL-II, as observed for cAMP. SGL-II was capable of partially dissociating cAMP-kinase, which was observed by gel filtration column chromatography. However, the binding activity of cAMP to the holoenzyme was not inhibited with SGL-II. These results demonstrate that the glycolipids can directly activate cAMP-kinase in a manner similar, but not identical, to that of cAMP.

[Brain gangliosides and memory formation]

It is generally accepted that the process of molecular facilitation (= Bahnung) of neuronal circuits by means of stabilization of synaptic contacts represents the structural basis for memory formation. On the basis of physiological, electron microscopic, biochemical, and physicochemical data, a concept is presented speaking in favor of the idea that interactions between brain gangliosides (= amphiphilic sialic acid-containing glycosphingolipids with peculiar physicochemical properties), calcium, and functional membraneous proteins (ion channels, ion pumps, receptors, kinases) play a crucial modulatory role in the transmission and storage of information.

Protein kinase C substrates and ganglioside inhibitors in bovine mammary nuclei

In cow mammary gland, unlike in other tissues, gangliosides (putative physiologic regulators of protein kinase C) may be distributed in nuclei and on the cell surface. This study was designed to determine whether gangliosides and the protein kinase C system (the enzyme and its substrate proteins) are present in cow mammary gland nuclei and to examine the effect of gangliosides detected in nuclei on protein phosphorylation catalyzed by protein kinase C. Gangliosides GM3, GD3, and GT1b were detected in the highly purified nuclear fraction. The nuclear ganglioside pattern was different from those of whole tissue and cytosol, thereby suggesting the presence of the gangliosides in nuclei. Protein kinase C and its substrate proteins (120, 97, 56, 43, 38, and 36 kDa) were extracted by Triton X-100 treatment of nuclei. Both protein kinase C activity (histone phosphorylation) and the nuclear substrate phosphorylation were effectively inhibited by the three gangliosides. Of the gangliosides, GT1b was the most potent in inhibiting phosphorylation, followed by GD3 and GM3. These results suggest that signal transduction mediated by protein kinase C in cow mammary gland nuclei may be regulated by gangliosides.

Promotion of granule cell survival by high K+ or excitatory amino acid treatment and Ca2+/calmodulin-dependent protein kinase activity

Cerebellar granule cells in culture develop survival requirements which can be met either by chronic membrane depolarization (25 mM K+) or by stimulation of ionotropic excitatory amino acid receptors. We observed previously that this trophic effect is mediated via Ca2+ influx, either through dihydropyridine-sensitive, voltage-dependent calcium channels (activated directly by high K+ or indirectly by kainate) or through N-methyl-D-aspartate receptor-linked ion channels. Steps after Ca2+ entry in the transduction cascade mediating the survival-supporting effect of high K+ and excitatory amino acids have now been examined. Using protein kinase inhibitors (H-7, polymixin B and gangliosides), and modulating protein kinase C activity by treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, we obtained evidence against the involvement of protein kinase C and cyclic nucleotide-dependent protein kinases in the transduction cascade. On the other hand, calmidazolium (employed as a calmodulin inhibitor) counteracted the trophic effect of elevated K+ with high potency (IC50 0.3 microM), which exceeded by approximately 10-fold the potency for the blockade by the drug of voltage-sensitive calcium channels. The potency of calmidazolium in interfering with the N-methyl-D-aspartate rescue of cells was also much higher in comparison with the inhibition of 45Ca2+ influx through N-methyl-D-aspartate receptor-linked channels. Our results indicated that after calmodulin the next step in the trophic effects involves Ca2+/calmodulin-dependent protein kinase II activity. KN-62, a fairly specific antagonist of this enzyme, compromised elevated K+ or excitatory amino acid-supported cell survival with high potency (IC50 2.5 microM). In the relevant concentration range, KN-62 had little or no effect on Ca2+ entry through either voltage- or N-methyl-D-aspartate receptor-gated channels. Combining information on the toxic action of glutamate in "mature" granule cells with the trophic effect of either excitatory amino acids or high K+ treatment on "young" cells, we conclude that after the initial steps involving calcium in both cases the respective transduction pathways diverge. The toxic action of glutamate seems to be mediated through protein kinase C [Favaron et al. (1990) Proc. natn. Acad. Sci. U.S.A. 87, 1983-1987 whereas a Ca2+/calmodulin-dependent protein kinase, which can be inhibited by KN-62 (but is resistant to gangliosides and to inhibitors whose potency is higher for protein kinase C than for Ca2+ calmodulin-dependent protein kinases, such as H-7 and polymixin B), is involved critically in the trophic effect.

GM3 regulates protein kinase systems in cultured brain microvascular endothelial cells

The barrier function of endothelial cells is known to be positively regulated by protein kinase A (PKA) and negatively regulated by protein kinase C (PKC). We found that exogenously administered GM3(NeuAc) promoted PKA activity in cultured brain microvascular endothelial cells (BMECs). Other glycolipids, including GM1, sulfoglucuronyl paragloboside, and GM3(NeuGc), did not have any effect on the PKA activity of BMECs. PC12 cells did not respond to exogenously applied GM3(NeuAc). GM3(NeuAc) also suppressed the PKC activity of BMECs. Thus, GM3(NeuAc) may function as a modulator of blood-brain barrier function via the two different kinase systems.

Molecular parameters of semisynthetic derivatives of gangliosides and sphingosine in monolayers at the air-water interface

The molecular parameters (molecular area, surface potential, collapse pressure, dipole moment contributions) of semisynthetic derivatives of ganglioside GM1 and of sphingosine were studied in lipid monolayers at the air-NaCl (145 mM, pH 5.6) interface at 22 +/- 0.3 degrees C. The chemical modifications included alterations of the fatty acyl chain moiety linked to the 2-amino position of the sphingosine (Sph) base. The compounds studied were PKS-1 (N-acetyl Sph), PKS-2 (N-chloroacetyl Sph), PKS-3 (N-dichloroacetyl Sph), PKS-4 (N-trichloroacetyl Sph), Lyso-GM1 (ganglioside GM1 lacking the N-linked fatty acyl chain and the N-acetyl group on the sialic acid), Liga-4 (N-acetyl, lyso[NeuAc]GM1) and Liga-20 (N-dichloroacetyl, lyso[NeuAc]GM1). Relatively small modifications of the chemical structure of sphingolipids introduce dramatic consequences on their surface molecular properties. The absence of the long chain fatty acyl moiety and of the N-acetyl group on the neuraminic acid in Lyso-GM1 leads to a more condensed behavior and to an increase of the collapse pressure compared with GM1. The acetylation or chloroacetylation at the 2-amino position in Liga-4 and Liga-20 induce an expansion of the surface pressure-area isotherm and a decrease of the collapse pressure. The limiting molecular areas of GM1 derivatives, taken at the collapse pressure point, are consistent with the oligosaccharide chain being oriented approximately perpendicularly to the interface. Sphingosine shows a liquid expanded isotherm. The acetylation and successive chlorination of the acetyl residue at the 2-amino position of Sph cause a progressive increase in the limiting molecular area. The variation of the resultant dipole moment under compression, calculated from the surface potential values, suggests the reorientation of selective groups within these molecules that depend on the degree of intermolecular packing. Thermodynamic-geometric correlations on the basis of the molecular parameters of these derivatives suggest that small alterations of the substituent group at the 2-amino position of Sph could have large and amplified consequences on the type, curvature and stability of the possible self-aggregated structure that these lipids may form in aqueous medium.

Time course of the translocation and inhibition of protein kinase C during complete cerebral ischemia in the rat

The time course for the ischemia-induced changes in the subcellular distribution of protein kinase C (PKC) (alpha), (beta II), and (gamma) and the activity of PKC were studied in the neocortex of rats subjected to 1, 2, 3, 5, 10, and 15 min of global cerebral ischemia. In the particulate fraction, a 14-fold increase in PKC (gamma) levels was seen at 3 min of ischemia, which further increased at 5-15 min of ischemia. At 15 min of ischemia, PKC (alpha) and (beta II) levels had increased two- and six-fold, respectively. In the cytosolic fraction, a transient early 1.4-fold increase in PKC (beta II) and PKC (gamma) levels was seen, whereas no change in the levels PKC (alpha) was noted. PKC (gamma) levels then progressively declined, reaching 50% at 15 min of ischemia. At 5 min of ischemia, a 43% decrease in PKC activity was seen in the particulate fraction, reaching 50% at 15 min of ischemia concomitant with a 27% decrease in the cytosolic fraction. There was no change in the activator-independent PKC activity. Pretreatment with the ganglioside AGF2 prevented the redistribution of PKC (gamma) in the particulate fraction at 5 min, but not at 10 min of ischemia. The observed time course for the translocation of PKC (gamma) parallels the ischemia-induced release of neurotransmitters and increased levels of diacylglycerols, arachidonate, and increased levels of diacylglycerols, arachidonate, and intracellular calcium and delineates this subspecies as especially ischemia-sensitive. Ganglioside pretreatment delayed the translocation of PKC (gamma), possibly by counter-acting the effects of ischemia-induced factors that favor PKC binding to cell membranes.

The influence of exogenous gangliosides on the dynamics of the development of prolonged posttetanic potentiation

The influence of various gangliosides on the dynamics of the development of prolonged posttetanic potentiation (PPTP) in the layer of pyramidal cells of field CA3 during the stimulation of mossy fibers was investigated in rat hippocampus section. It was demonstrated that the GM1 and GD1b gangliosides stimulate the induction and maintenance of PPTP to varying degrees and that GD1a and GT1b inhibit the phase of maintenance as compared with the active control. A conclusion was reached regarding the participation of gangliosides in the mechanisms of synaptic plasticity.

Inhibition of protein kinase C activity in mucolipidosis type-4: a model for a new pathogenetic mechanism in inborn errors of metabolism

Inhibition of protein kinase C [PK-C] activity by sphingosine and its derivatives has been suggested to play a role in the pathogenesis of sphingolipidoses. In the present study, PK-C activity and PK-C-mediated phosphorylation of endogenous substrates were studied in skin fibroblasts from patients with mucolipidosis type 4 [ML-4], in which there is accumulation of the phospholipids phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine as well as gangliosides. Cytosolic PK-C activity in 5 ML-4 cell lines was comparable to that in control cells. PK-C activity in the particulate fraction of these cells was 84 +/- 14 pmol 32P/mg protein per min compared with 267 +/- 26 in control cells. Increasing the concentrations of the activating lipids in the reaction mixture did not enhance PK-C activity in ML-4 cells, suggesting a non-competitive inhibition of the kinase. Following partial purification of the enzyme from the particulate fraction PK-C activity increased to 288 +/- 14 and 339 +/- 12 pmol 32P/mg protein per min in ML-4 and control cells, respectively. The phosphorylation pattern of endogenous substrates in the particulate fraction of ML-4 cells differed from that in control cells both in the absence and in the presence of calcium and activating lipids. We suggest that PK-C may be involved in the pathogenesis of sphingolipidoses and that this may represent an example for a new type of pathogenetic mechanisms in inborn errors of metabolism.

Effect of gangliosides on diacylglycerol content and molecular species in nerve from diabetic rats

The effects of ganglioside treatment on 1,2-diacylglycerol content and on molecular species in 1,2-diacylglycerol, phosphatidic acid and total diacylglycerolipids, as well as Na+,K(+)-ATPase activity, were examined in sciatic nerves from streptozotocin-induced diabetic rats. Beginning 2 weeks after induction of diabetes, animals were administered mixed bovine brain gangliosides, AGF1, an inner ester derivative of this mixture, or saline for 5 weeks. The levels of 1,2-diacylglycerol and arachidonoyl-containing molecular species in age-matched non-diabetic animals were not affected by ganglioside treatment. In nerves from saline-treated diabetic animals, 1,2-diacylglycerol levels were not reduced, but both Na+,K(+)-ATPase activity and all arachidonyl-containing species except for 18:0/20:4 1,2-diacylglycerol were significantly decreased. The content of 1,2-diacylglycerol was lowered by 23 and 16% in bovine brain ganglioside and AGF1-treated diabetic animals, respectively, and the quantity of 18:0/20:4 1,2-diacylglycerol was also selectively reduced. Ganglioside administration did not affect the diminished levels of arachidonoyl-containing molecular species in 1,2-diacylglycerol, phosphatidic acid or diacylglycerolipids in nerve from diabetic rats. In the same nerves, bovine brain gangliosides partially and AGF1 completely restored Na+,K(+)-ATPase activity. The results suggest that gangliosides depress the content of total 1,2-diacylglycerol and the quantity of 18:0/20:4 1,2-diacylglycerol, specifically, in diabetic nerve. The possible relationship between the corrective action of gangliosides on Na+,K(+)-ATPase activity and the effect of these substances on 1,2-diacylglycerol molecular species composition and metabolism is discussed.

Gangliosides inhibit platelet-derived growth factor-stimulated receptor dimerization in human glioma U-1242MG and Swiss 3T3 cells

We previously showed that gangliosides inhibit DNA synthesis in Swiss 3T3 cells stimulated with platelet-derived growth factor (PDGF) in a dose-responsive manner. This correlated with the inhibitory effects of several gangliosides (except GM3) on tyrosine phosphorylation of the PDGF receptor (PDGFR). [35S]Methionine-labeled Swiss 3T3 cells were incubated either with or without gangliosides and stimulated with PDGF, and proteins were cross-linked with bis(sulfosuccinimidyl) suberate. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that two protein bands (170 and 350 kDa) were specifically immunoprecipitated with an anti-PDGFR antibody. Using both Swiss 3T3 and human glioma U-1242MG cells, western blots with anti-PDGFR and anti-phosphotyrosine antibodies confirmed that these bands were the PDGFR monomer and dimer, respectively, and that phosphotyrosine was present in these bands only after cells were stimulated with PDGF. Of the gangliosides tested, GM1, GM2, GD1a, GD1b, GD3, and GT1b, but not GM3, inhibited the formation of the 350-kDa band. These results demonstrate that all gangliosides tested, except GM3, probably inhibit PDGF-mediated growth by preventing dimerization of PDGFR monomers. Loss of more complex gangliosides in human gliomas would permit unregulated activation of the PDGFR, contributing to uncontrolled growth stimulation. We propose that ganglioside inhibition of receptor dimerization is a novel mechanism for regulating and coordinating several trophic factor-mediated cell functions.

Gangliosides protect from TNF alpha-induced apoptosis

The modulation of tumor necrosis factor alpha-mediated cytotoxicity by gangliosides was analyzed. When cells of the TNF alpha-sensitive fibrosarcoma cell line L929 were incubated for 16 h with recombinant TNF alpha (156 or 312 pg/ml), they were lysed to 65.2% or 78.0%, respectively. The presence of a bovine brain ganglioside mixture (BBG, Cronassial) inhibited the TNF alpha-mediated lysis in a dose-dependent manner (IC50 approximately 1 mg/ml). Maximum inhibition was achieved with 2 mg/ml BBG (82.6% inhibition for 156 pg/ml TNF alpha and 88.5% inhibition for 312 pg/ml TNF alpha). In situ nick translation revealed that BBG (2 mg/ml) significantly reduces the number of cells with TNF alpha-induced DNA-strand breaks from 71.2% to 6.6% (P < 0.0001), indicating a protection against TNF alpha-mediated apoptosis. We conclude that BBG prevents the cytotoxic action of TNF alpha on tumor cells. The inhibitory effect may be due to an interference of gangliosides with intracellular signal transduction pathways, resulting in an inhibition of the activation of DNA-cleaving endonucleases.

Inhibitors of protein kinase C prevent the toxicity of glutamate in primary neuronal cultures

Glutamate-induced neurotoxicity has been proposed to depend on a sustained increase of intracellular free Ca2+ levels. However, the molecular mechanism(s) involved are not well understood. Some results suggest that activation of protein kinase C by the increased levels of Ca2+ could play a role in the mediation of glutamate neurotoxicity. To assess this hypothesis we have tested if the 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine (H7) and calphostin C, inhibitors of protein kinase C, are able to protect neurons in primary culture from glutamate-induced cell death. It is shown that both H7 and calphostin C prevent nearly completely the death of neurons from cerebellum, even when 2 mM glutamate was used. HA-1004, an inhibitor of cyclic nucleotide-dependent protein kinases, did not protect neurons. The protective effect was maximum at approximately 10 microM H7 and at approximately 10 nM calphostin C. The results reported support the hypothesis that protein kinase C plays a key role in the mediation of glutamate neurotoxicity.

Characterization of the neutral pH-optimum sphingomyelinase from rat brain: inhibition by copper II and ganglioside GM3

A neutral pH-optimum sphingomyelinase (N-SMase), solubilized from rat brain membranes, was characterized with respect to metal and membrane lipid effects. Chromatofocusing chromatography, which separates proteins according to pI, showed two N-SMase activities. One eluted at pH 4.7 and the other required 0.4 M NaCl before elution. Kinetically, the two preparations appeared similar. The N-SMase eluting at pH 4.7 was most extensively studied here. Of the phospholipids studied, only phosphatidylserine showed any influence on N-SMase and that was to increase its activity by as much as 50%. Neither serine nor phosphatidic acid had any effect. Of the cations tested, none was able to replace Mg2+ as a required activator. However, it was found that several metals were inhibitory, with Cu2+ being most effective (IC50 = 5 microM). Gangliosides, particularly the monosialoganglioside, GM3 (IC50 approximately 50 microM), inhibited N-SMase. Other glycolipids showed little effect on activity, even the immediate precursor to GM3 - lactosylceramide. The ganglioside sugar, N-acetylneuraminic acid, also had no effect on N-SMase activity. None of these inhibitors affected the acidic pH-optimum sphingomyelinase. Other sphingolipid compounds such as ceramide - the enzymatic product - and sphingosylphosphorylcholine (lysosphingomyelin) showed no capacity to inhibit N-Smase, implying that the enzyme may have a selective substrate-binding site.

Modulation of high-threshold Ca current and spontaneous postsynaptic transient currents by phorbol 12,13-diacetate, 1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7), and monosialoganglioside (GM1) in CA1 pyramidal neurons of rat hippocampus in vitro

Phorbol esters, which activate protein kinase C (PKC), enhance synaptic transmission in the CA1 subfield of hippocampus, both in situ and in vitro. The increase in synaptic transmission could be the consequence of enhanced Ca influx into nerve terminals, and perhaps a more general increase in voltage-dependent Ca currents. The effects of phorbol 12,13-diacetate (PDAc) on the high-voltage activated (HVA) Ca currents, as well as spontaneous transient currents were therefore investigated by intracellular recording in hippocampal slices. PDAc selectively augmented, by 45% +/- 10%, the early peak of the HVA Ca current (but not its sustained component), and also spontaneous inhibitory postsynaptic currents. The inactive phorbol ester, 4 alpha-PDAc, had no comparable effects. The actions of PDAc were reversible on prolonged washing, and they were antagonized by the PKC inhibitors (1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7) and monosialoganglioside (GM1). In addition, GM1, which also activates the Ca/calmodulin-dependent kinase, enhanced spontaneous excitatory postsynaptic currents, while inhibiting the IPSCs. It is concluded that activation of PKC increases HVA (probably N-type) Ca current and facilitates ongoing GABAergic IPSCs.

Enhancement of ganglioside GM3 synthesis in okadaic-acid-treated granulosa cells

Okadaic acid is a potent inhibitor of type-2A (PP2A) and type-1 (PP1) protein phosphatases and has been proved to be a valuable tool for studies on the protein phosphorylation. We have investigated the effects of okadaic acid on rat granulosa cells in order to determine whether the regulation of ganglioside synthesis involves protein phosphorylation via inhibition of PP2A and PP1. Granulosa cells expressed luteinizing hormone (LH) receptors, measured as the binding of 125I-deglycosylated human chorionic gonadotropin (hCG) to intact cells, and synthesized the gangliosides NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1Cer (GM3) and Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3]Gal beta 1-->4Glc beta 1-->1Cer (GM1), demonstrated by metabolic labeling of glycosphingolipids with [3H]galactose, in response to follicle-stimulating hormone (FSH). When FSH-stimulated granulosa cells were treated with 10 nM okadaic acid for 15 h, down-regulation of LH receptors, dissociation of LH receptor-effector coupling and significant decreases of intracellular and extracellular 3',5'-cyclic adenosine monophosphate (cAMP) levels were observed. The okadaic acid-induced desensitization to gonadotropin in granulosa cells was accompanied by increased ganglioside synthesis. The amount of 3H-labeled ganglioside GM3, the major ganglioside (about 95% of the total) synthesized by mature granulosa cells, was enhanced in okadaic acid-desensitized cells (to 215% of the control value) and in those desensitized by hCG (by 354%), forskolin (by 190%) and 12-O-tetradecanoylphorbol 13-acetate (by 143%). The results of this study suggest that an increase in the phosphorylation state of cells is accompanied by enhancement of ganglioside synthesis.

Biphasic effects of exogenous ganglioside GM3 on follicle-stimulating hormone-dependent expression of luteinizing hormone receptor in cultured granulosa cells

The major ganglioside NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1Cer (GM3) present in cultured rat granulosa cells was examined for potential function in the expression of luteinizing hormone (LH) receptor on the cell surface in response to follicle-stimulating hormone (FSH). Synthesis of GM3 was stimulated concentration-dependently by FSH, and the stimulation was enhanced synergistically by insulin, as revealed by metabolic labeling of glycosphingolipids with [3H]galactose. When granulosa cells were cultured in the media containing GM3 (0.2-20 microM), biphasic changes in FSH-dependent expression of LH receptor were observed, as measured by the binding of 125I-deglycosylated human choriogonadotropin to the intact cells. Exogenous GM3 suppressed expression of LH receptor in the cells treated with a low dose of FSH (20 ng/ml), which was characterized by a low GM3 level, to 30% of control at 10 microM, with a half-maximal inhibitory concentration of 8 microM. In contrast, in the cells treated with a high dose of FSH (100 ng/ml) and insulin, which was characterized by a high GM3 level, expression of LH receptor was enhanced by exogenous GM3, to 148% of control at 10 microM, with a half-maximal effective concentration of 2 microM. Exogenous GM3 produced concomitant changes in the levels of extracellular cAMP. These effects of exogenous GM3 were not accompanied by changes in granulosa cell proliferation. Exogenous GM3 also modulated the LH receptor expression by the synergistic action of 12-O-tetradecanoylphorbol 13-acetate with insulin, with no significant changes in cellular DNA contents, suggesting that exogenous GM3 does not modulate directly the action of FSH at its receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics and regulation of ganglioside-induced elevation of free cytoplasmic Ca2+ in human blood platelets

We have found that gangliosides GD3 and GM3 induced rapid, reversible elevation of free cytoplasmic Ca2+ in fura-2-loaded human blood platelets. The effect persisted in Ca(2+)-free medium, indicating that gangliosides stimulated mobilization of intracellular stores. The action of gangliosides was concentration-dependent with EC50 of about 1 microM. The Ca(2+)-mobilizing effects of gangliosides were potentiated by epinephrine and inhibited by substances inducing activation of protein kinase C and cAMP-dependent protein kinases. Acidic phospholipids partially mimicked the Ca(2+)-mobilizing effects of gangliosides indicating that lipid head charge is essential for this activity. While the elevation of [Ca2+]i produced by arachidonic acid was almost completely blocked by aspirin pretreatment, the effects of gangliosides were diminished only 2-fold, indicating that gangliosides activate both aspirin-sensitive and aspirin-insensitive mechanisms of [Ca2+]i elevation.

Gangliosides as modulators of cell function

Gangliosides, sialic acid-containing glycosphingolipids, are found in the outer leaflet of the plasma membrane of all vertebrate tissues and species. This report presents a brief introduction to the gangliosides and reviews the chemistry and topography of their biosynthesis. It also presents an overview of the present evidence supporting a physiological significance for the gangliosides in a variety of experimental systems. This includes consideration of their potential roles in development and cell adhesion. In addition, experimental examples in which gangliosides appear to influence signal transduction processes through their interactions with plasma membrane proteins are discussed.

Inhibition of protein kinase C restores Na+,K(+)-ATPase activity in sciatic nerve of diabetic mice

We have tested if inhibition of protein kinase C is able to prevent and/or to restore the decrease of Na+,K(+)-ATPase activity in the sciatic nerve of alloxan-induced diabetic mice. Mice were made diabetic by subcutaneous injection of 200 mg of alloxan/kg of body weight. The activity of Na+,K(+)-ATPase decreased rapidly (43% after 3 days) and slightly thereafter (58% at 11 days). We show that intraperitoneal injection of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), an inhibitor of protein kinase C, prevents completely the loss of Na+,K(+)-ATPase activity produced by alloxan. Also, H7 injected into diabetic mice, 4-9 days after the injection of alloxan, restores the activity of the enzyme. The amount of activity recovered depends on the dose of H7 administered; complete recovery was reached with injection of 15 mg of H7/kg of body weight. The effect of H7 is transient, with a half-life of approximately 1 h.