Effect of gangliosides on membrane permeability studied by enzymic and fluorescence-spectroscopy techniques

Dietary Gangliosides EnhanceIn VitroGlucose Uptake in Weanling Rats

BACKGROUND

The intestine adapts to environmental stimuli, such as modifications in dietary lipids. Dietary lipids modify brush border membrane (BBM) permeability and nutrient transporter activities. Gangliosides (GANG) are glycolipids present in human milk, but they are present only in low amounts in infant formula. Exogenous GANG are incorporated into cell membranes and increase their permeability. This study was undertaken to determine if feeding a 0.2% GANG-enriched diet for 2 weeks alters in vitro intestinal sugar absorption in weanling rats compared with an isocaloric control diet or diet enriched with polyunsaturated long-chain fatty acids.

METHODS

In vitro uptake of 34-96 mm glucose and fructose and morphological measurements were assessed on intestinal tissue of weanling rats. Western blotting, immunohistochemistry, Northern blotting, and reverse transcription-polymerase chain reaction were performed to determine the mRNA and protein abundance of the sugar transporters SGLT-1, GLUT2 and GLUT5.

RESULTS

Feeding GANG did not alter the rates of animal weight gain or intestinal morphology. GANG did not affect fructose uptake. Depending on the concentration of glucose, GANG increased jejunal uptake of higher concentrations of glucose by approximately 20%-60%. There were no changes in GLUT5 or GLUT2 protein or mRNA abundance. Similarly, there were no changes in SGLT-1 mRNA and protein abundance, as determined by Northern and Western blotting. However, using immunohistochemistry, SGLT-1 was lower in GANG than in controls.

CONCLUSIONS

The results of this study suggest that the enhanced uptake of glucose that results from feeding 0.2% GANG for 2 weeks to weanling rats may be regulated posttranslationally. Clearly any adjustment of the content of GANG in infant formula must be studied carefully.

Dietary gangliosides enhance in vitro lipid uptake in weanling rats

BACKGROUND

The intestine adapts morphologically or functionally in response to environmental stimuli. Dietary lipids modify brush border membrane (BBM) permeability and nutrient transporter activities. Gangliosides (GANG) are glycolipids in human milk that are present only in low amounts in infant formula. Exogenous GANG are incorporated into cell membranes and increase their permeability. The objective of this study was to determine whether feeding a GANG-enriched diet alters in vitro intestinal lipid absorption.

METHODS

Weanling rats were fed either (1) GANG-enriched diet; (2) diet enriched with polyunsaturated long-chain fatty acids; or (3) isocaloric control diet for 2 weeks, after which in vitro intestinal lipid uptake was measured.

RESULTS

Feeding GANG did not alter weight gain or intestinal morphology. Enhanced uptake of stearic acid (18:0) in the ileum and stearic and linoleic acid (18:2) in the jejunum was not associated with a change in the abundance of the ileal lipid binding protein (ILBP), the intestinal fatty acid binding protein (I-FABP), or the liver fatty acid binding protein (L-FABP).

CONCLUSION

We speculate that the enhanced uptake of long-chain fatty acids in weanling rats fed GANG may be caused by a modification in physical properties of the BBM.

Direct cell-cell communication: a new approach derived from recent data on the nature and self-organisation of ultradian (circahoralian) intracellular rhythms

Recent data concerning ultradian (circahoralian) intracellular rhythms are used to assess the biochemical mechanisms of direct cell-cell communication. New results and theoretical considerations suggest a fractal nature of ultradian rhythms and their self-organisation. The fundamental and innate nature of these rhythms relates to their self-similarity at different levels of cell and tissue organisation. They can be detected in cell-free systems as well as in cells and organs in vivo. Such rhythms are a means of finding an optimal state of cell function rather than achieving a state of absolute stability. As a consequence, oscillations, being irregular and numerous by the set of periods, are resilient to functional overload and injury. Recent data on the maintenance of their fractal structure and, especially on the selection of optimal periods are discussed. The positive role of chaotic dynamics is stressed. The ultradian rhythm of protein synthesis in hepatocytes in vitro was used as a marker of direct cell-cell communication. The system demonstrates cell cooperation and synchronisation throughout the cell population, and suggests that the ultradian rhythms are self-organised. These observations also led to the detection of mechanisms of direct cell-cell communication in which extracellular factors have an essential role. Experimental evidence indicated the involvement of gangliosides and/or catecholamines in this large-scale synchronisation of protein synthesis. The response of all, or a major part, of the cell population is important; after the initial trigger effect, a periodic pattern is retained for some time. The influence of Ca2+-dependent protein kinases on protein phosphorylation can be a final step in the phase modulation of rhythms during cell-cell synchronisation. The intercellular medium plays an important role in self-synchronisation of ultradian rhythms between individual cells. Low cooperative activity of hepatocytes of old rats resulted from altered composition of the intercellular medium rather than direct effects of animal and cellular ageing. Similarly, in the whole body, changes in levels of gangliosides and catecholamines in the blood serum, a natural intercellular medium, can be critical events in age-dependent changes of the serum and accordingly cell-cell synchronisation. Hepatocytes of old rats exhibit some of the properties of young cells following an increase in blood serum ganglioside level, as well as, in in vitro conditions, after the addition of gangliosides to the culture medium. Together with data on ultradian functional and metabolic rhythms, all the material reviewed here allows us to propose a mechanism of direct cell-cell cooperation via the medium in which the cells exist, that supplements the nervous and hormonal central regulation of organ functions. Ultradian intracellular rhythms may thus provide a finer framework within which the integrated dynamics of respiration, heart rate, brain activity, and even behavioural patterns, are brought to an optimal functional pattern. Innate and direct cell-cell cooperation may have been employed as a means of intercellular regulation during the course of metazoan evolution, that preceded nervous regulation and is presently retained in mammals.

Uptake and metabolism of exogenous glycosphingolipids by cultured cells

Exogenous glycosphingolipids, especially gangliosides, are used to study transport and metabolism of their endogenous counterparts as well as their role in cell adhesion, cell recognition and signal transduction. Unlike monodispersed solutes, in aqueous media ganglioside molecules aggregate into micelles (or bilayer structures) with a very low critical micellar concentration. Upon addition to cells in culture, exogenous gangliosides bind to the cell surface in three operationally defined modes: loosely associated micelles removable by serum; tightly attached micelles removable by proteases such as trypsin; and ganglioside molecules inserted into the outer leaflet of the plasma membrane. As shown by a biotin-labeled derivative of the ganglioside GM1 these inserted molecules are endocytosed and transported to intralysosomal membranes for catabolism. The benefit from using (partially) nondegradable as well as semi-truncated glycosphingolipids in transport studies is discussed.

Apoptogenic ganglioside GD3 directly induces the mitochondrial permeability transition

Early events in apoptotic cascades initiated by ceramides or by activation of the surface receptor CD95 (Fas/APO-1) include the formation of ganglioside GD3. GD3 appears to be both necessary and sufficient to propagate this lipid-mediated apoptotic pathway. Later events common to many apoptotic pathways include induction of the mitochondrial permeability transition (PT) and cytochrome c release, which in turn triggers downstream caspases and cell death. The links between GD3 formation and downstream stages of apoptosis are unknown. We report that ganglioside GD3 directly induces the PT in isolated rat liver mitochondria at 30-100 microM in the presence of exogenous substrate (succinate) and at approximately 3 microM in the absence of exogenous substrate. In contrast, other gangliosides tested (e.g. GM1) have only weak stimulatory effects in the presence of succinate and protect against PT induction in the absence of respiratory substrates. GD3-mediated induction of PT was antagonized by known PT inhibitors, namely cyclosporin A, ADP, trifluoperazine, and Mg(2+). GD3 induced PT even in the presence of submicromolar Ca(2+); GD3 is therefore the first biological PT inducer identified that does not require elevated Ca(2+). Exposure to GD3 also led to mitochondrial cytochrome c release. In contrast, C(2)-ceramide, which can initiate the lipid-mediated apoptotic cascade in susceptible cells, failed to either induce PT or release cytochrome c. These observations suggest that GD3 propagates apoptosis by inducing the PT and cytochrome c release. This model provides a mechanistic link between the earlier and later stages of CD95-induced/ceramide-mediated apoptosis.

Commitment to apoptosis by GD3 ganglioside depends on opening of the mitochondrial permeability transition pore

We have studied the effects of GD3 ganglioside on mitochondrial function in isolated mitochondria and intact cells. In isolated mitochondria, GD3 ganglioside induces complex changes of respiration that depend on the substrate being oxidized. However, these effects are secondary to opening of the cyclosporin A-sensitive permeability transition pore and to the ensuing swelling and cytochrome c depletion rather than to an interaction with the respiratory chain complexes. By using a novel in situ assay based on the fluorescence changes of mitochondrially entrapped calcein (Petronilli, V., Miotto, G., Canton, M., Colonna, R., Bernardi, P., and Di Lisa, F. (1999) Biophys. J. 76, 725-734), we unequivocally show that GD3 ganglioside also induces the mitochondrial permeability transition in intact cells and that this event precedes apoptosis. The mitochondrial effects of GD3 ganglioside are selective, in that they cannot be mimicked by either GD1a or GM3 gangliosides, and they are fully sensitive to cyclosporin A, which inhibits both the mitochondrial permeability transition in situ and the onset of apoptosis induced by GD3 ganglioside. These results provide compelling evidence that opening of the permeability transition pore is causally related to apoptosis.

Gangliosides asymmetrically alter the membrane order in cultured PC-12 cells

Exogenous gangliosides readily associate with the cell membranes and produce marked effects on cell growth and differentiation. We have studied the effect of bovine brain gangliosides (BBG) on the membrane dynamics of intact cells. The structural and dynamic changes in the cell membrane were monitored by the fluorescence probes DPH, TMA-DPH and laurdan. Incorporation of BBG into the cell membrane decreased the fluorescence intensity, lifetime and the steady state anisotropy of TMA-DPH. Analysis of the time resolved anisotropy decay by wobbling in the cone model revealed that BBG decreased the order parameter, and increased the cone angle without altering the rotational relaxation rate. The fluorescence intensity and lifetime of DPH were unaffected by BBG incorporation, however, a modest increase was observed in the steady state anisotropy. BBG incorporation reduced the total fluorescence intensity of laurdan with pronounced quenching of the 440-nm band. The wavelength sensitivity of generalized polarization of laurdan manifested an ordered liquid crystalline environment of the probe in the cell membrane. BBG incorporation reduced the GP values and augmented the liquid crystalline behavior of the cell membrane. BBG incorporation also influenced the permeability of cell membranes to cations. An influx of Na+ and Ca2+ and an efflux of K+ was observed. The data demonstrate that incorporation of gangliosides into the cell membrane substantially enhances the disorder and hydration of the lipid bilayer region near the exoplasmic surface. The inner core region near the center of the bilayer becomes slightly more ordered and remains highly hydrophobic. Such changes in the structure and dynamics of the membrane could play an important role in modulation of transmembrane signaling events by the gangliosides.

Structural transitions associated with the interaction of Alzheimer beta-amyloid peptides with gangliosides

Alzheimer's disease is characterized pathologically by the presence of neurofibrillary tangles and amyloid plaques. The principal component of the plaque is the beta-amyloid peptide (Abeta), a 39-43-residue peptide. The conformational change required for the conversion of soluble peptide into amyloid fibrils is modulated by pH, Abeta concentration, addition of kinetic and thermodynamic enhancers, and alterations in the primary sequence of Abeta. We report here the ability of gangliosides to induce an alpha-helical structure in Abeta and thereby diminish fibrillogenesis. Circular dichroism and a fluorescence dye release assay data indicate that gangliosides interact with and induce alpha-helix formation in Abeta. We find that the sialic acid moiety of gangliosides is necessary for the induction of alpha-helical structure. Differences in the amount and the position of the sialic acid on the carbohydrate backbone also affect the conformational switch. The Abeta-ganglioside interaction at pH 7.0, monitored by CD, is stable over time and resistant to high concentrations of NaCl. The induction of alpha-helical structure is greater with Abeta1-40 than Abeta1-42. The ability of gangliosides to sequester Abeta from fibril formation was also evaluated by electron microscopy.

Requirement for GD3 ganglioside in CD95- and ceramide-induced apoptosis

Gangliosides participate in development and tissue differentiation. Cross-linking of the apoptosis-inducing CD95 protein (also called Fas or APO-1) in lymphoid and myeloid tumor cells triggered GD3 ganglioside synthesis and transient accumulation. CD95-induced GD3 accumulation depended on integral receptor "death domains" and on activation of a family of cysteine proteases called caspases. Cell-permeating ceramides, which are potent inducers of apoptosis, also triggered GD3 synthesis. GD3 disrupted mitochondrial transmembrane potential (DeltaPsim), and induced apoptosis, in a caspase-independent fashion. Transient overexpression of the GD3 synthase gene directly triggered apoptosis. Pharmacological inhibition of GD3 synthesis and exposure to GD3 synthase antisense oligodeoxynucleotides prevented CD95-induced apoptosis. Thus, GD3 ganglioside mediates the propagation of CD95-generated apoptotic signals in hematopoietic cells.

Calcium- and pH-linked oligomerization of sorcin causing translocation from cytosol to membranes

Sorcin, a cytosolic calcium-binding protein containing a pair of EF-hand motifs, undergoes a Ca2(+)-dependent translocation to the cell membrane. The underlying conformational change is similar at pH 6.0 and 7.5 and consists in an increase in overall hydrophobicity that involves the aromatic residues and in particular the two tryptophan residues which become less exposed to solvent. The concomitant association from dimers to tetramers indicates that the tryptophan residues, which are located between the EF-hand sites, become buried at the dimer-dimer interface. Ca2(+)-bound sorcin displays a striking difference in solubility as a function of pH that has been ascribed to the formation of calcium-stabilized aggregates.

Unusual low proton permeability of liposomes prepared from the endogenous myelin lipids

In contrast with most other lipid substrates, in this article we show that liposomes prepared from the total myelin lipids exhibited a negligible proton permeability. Neither the generation of valinomycin-induced potassium diffusion potentials as high as -177 mV nor the imposition of large pH gradients (up to three units) was able to produce a substantial flux of protons through liposomal membranes, as determined by the distribution of [14C]-methylamine, or the changes in the fluorescence of the probes 9-aminoacridine, acridine orange, and pyranine. The presence of cations (Na+, K+, Ca2+) did not alter this behavior. Voltage clamping did not increase the transmembrane delta pH-driven proton permeability. However, liposome diameter was found to be critical because small unilamellar vesicles displayed a much higher proton permeability than large unilamellar or multilamellar vesicles. This abnormally low proton permeability is interpreted by virtue of the characteristic biochemical composition of myelin lipid matrix, with a high content of cholesterol and sphingolipids and a very low level of free fatty acids. These results could be important for elucidating the role of myelin in the regulation of pH in the brain. In addition, the myelin lipid extract could be useful for reconstituting proteins that participate in the transport of H+ through the membrane.

Exogenous gangliosides modulate calcium fluxes in cultured neuronal cells

Previous work from this and other laboratories has shown that the neuritogenic effect due to exogenous gangliosides on primary neurons in culture is accompanied by several morphological and biochemical modifications. The present results indicate that the treatment of these neurons with gangliosides, under the experimental conditions which are known to produce a sprouting effect, inhibited the influx of 45Ca2+ and increased the release of 45Ca2+ from the cells. No significant differences were noted using concentrations of gangliosides (10(-8)-10(-5) M) either below or above the critical micellar concentrations. No apparent specificity was observed among various species of individual sialocompounds (GM1, GD1a). Moreover the presence or absence of fetal calf serum in the culture medium influenced the levels of 45Ca2+ fluxes. This study confirms the hypothesis that gangliosides may be considered as Ca2+ flux modulators in neuronal cells.

Preparation of a protein-free total brain white matter lipid fraction: characterization of liposomes

A method of preparing a total lipid extract (TLE), free of protein, by extracting brain white matter with tetrahydrofuran is presented. The optimal conditions of extraction were found to be 50 ml of THF per gram of lyophilized tissue, though fresh tissue can also be used if larger volumes of solvent are employed. The method allowed, in a short time and in a single step, a yield of TLE of 50% on a dry weight basis. Its analytical characterization revealed a qualitative and quantitative composition very similar to the lipid composition of CNS myelin, including all the phospholipid and galactolipid species, cholesterol and gangliosides, but it contained only traces (0.1%) of protein. TLE has been used to prepare liposomes, either multilamellar (MLVs) or unilamellar (LUVs, SUVs), characterized by freeze-fracture electron microscopy. A multilayered, heterogeneous population of liposomes is observed in the MLVs preparation. When these samples were submitted to a freezing and thawing procedure the resulting liposomes were single-walled, and their intravesicular volume was increased. They were quite impermeable to the monovalent cation 86Rb+ and, by contrast, rather permeable to 45Ca+ +. Their complex lipid composition, together with their permeability properties and their response to ionophores, make them very useful to study protein-lipid interactions occurring within the myelin membrane as well as the functional properties of myelin proteins in reconstitution experiments.

Effect of the different supramolecular organization on the uptake and metabolization of exogenous GM1 ganglioside by human fibroblasts

In this report we have investigated the differences in the uptake and metabolization of exogenous GM1 by human fibroblasts, as a function of its supramolecular organization in solution. For this we used a tritium labelled GM1, given alone or inserted in dispersions of phosphatidylcholine (PC) or sulphatide. The addition of fetal calf serum (FCS) to these dispersions was also studied. With respect to GM1 pure micelles, the presence in the medium of a sulphatide/GM1, 10:1 molar ratio, greatly increased the incorporation of GM1-associated radioactivity by the cultured cells. Conversely, the presence of PC dramatically diminished the GM1 incorporation values. The metabolization of exogenous GM1 was favoured by the presence of FCS, regardless of the presence of sulphatide. The obtained data provide useful information on the appropriate procedure for feeding cultured fibroblasts with gangliosides.