Topographic studies of glycoproteins of intact synaptosomes from rat brain cortex

GM1 ganglioside reduces glutamate toxicity to cortical cells. Lowered LDH release and preserved membrane integrity

As an in vitro model of CNS excitatory amino acid (EAA) injury, rat cortical neuronal cultures were challenged with glutamate (0.5 or 10 mM) and the levels of released lactate dehydrogenase (LDH) were monitored at 1 h, 1, 2, and 7 d. LDH release is correlated with levels of plasma membrane damage. GM1 has been shown to be continuously distributed on the outer surface of CNS cellular membranes. By staining for the distribution of endogenous GM1 ganglioside using cholera toxin/antitoxin immunohistochemistry, we were able to assess morphologically cellular plasma membrane integrity after damage. We used these two measures (LDH and GM1 localization) to study the neuroprotective effects of exogenous GM1 ganglioside to further elucidate its mechanism. Cortical cultures derived from 15-d rat fetuses were subjected to the glutamate challenge for 30 min. Parallel cultures were either pre- or post-treated with 80 microM of GM1. Exposure to 10 mM glutamate caused a highly significant increase in LDH release at 1-48 h. Pretreatment with GM1 reduced the release, whereas posttreatment reduced the LDH release even more. Plasma membrane changes observed by the GM1 immunohistochemistry reflected the LDH release data. All cultures treated with GM1 evidenced substantial structural integrity (continuous staining of GM1 along perikarya and processes) as compared to untreated cultures. These data support our hypothesis that GM1 treatment (pre- and post-) reduces plasma membrane damage.

Role of gangliosides in behavioral and biochemical actions of alcohol: cell membrane structure and function

Alcohol exerts its pharmacological effects in adult brain by altering the physicochemical properties of cellular plasma membranes. Although alcohol does induce changes in membrane lipid composition, studies to relate these alterations to the development of behavioral tolerance to alcohol and the withdrawal effects have been unsuccessful. Actions of alcohol on developing brain are even more complex. Some of the reported effects include inhibition of embryogenesis, cell migration, and differentiation, including synaptogenesis. Gangliosides have neuroprotective action against a variety of neural insults (e.g., mechanical injury, drug toxicity, or hypoxic insult). This review addresses the role and significance of gangliosides in the CNS pathophysiology of alcohol exposure, as well as the effect of changes in endogenous gangliosides on membrane structure and function. We also describe the role of exogenous gangliosides in prevention of alcohol (acute and/or chronic)-induced CNS (prenatal and postnatal) neurotoxicity through their action on cellular plasma membranes. We propose that ganglioside's neuroprotective effects against alcohol neurotoxicity involve protection and restoration of plasma membrane structure (proteins and lipids) and thereby its function (ionic homeostasis, neurotransmitter receptor-mediated signal transduction). Thus gangliosides may have potential therapeutic use in treatment of alcohol-related problems.

Ganglioside-induced regeneration and reestablishment of axonal continuity in spinal cord-transected rats

In this study we examined the effect of chronic GM-1 ganglioside treatment on the reestablishment of axonal continuity and functional recovery in spinal cord-transected rats. Previous studies have shown that chronic treatment with GM-1 ganglioside is effective in producing regeneration of lesioned mesostriatal dopaminergic neurons in the central nervous system [1, 2]. In addition, GM-1 ganglioside advances peripheral nerve regeneration following nerve crush injury [12]. Axonal continuity was determined by the ability of the spinal cord to transport horseradish peroxidase across the region of transection. Comparisons between ganglioside-treated and saline-treated controls showed that ganglioside treatment resulted in the reestablishment of axonal continuity between the spinal cord distal to the level of the transection and the brainstem. Saline-treated controls showed little evidence of axonal continuity between these two regions. Thus gangliosides induce reestablishment of axonal continuity and thereby could advance functional recovery in rats following spinal cord transection.

Exogenous gangliosides enhance recovery from cns injury

Reports indicate that exogenous gangliosides can accelerate neurite outgrowth in vitro and facilitate peripheral nerve regeneration in vivo. An experiment was designed to assess whether ganglioside administration alters functional recovery and neuronal regeneration after a CNS lesion. Rats trained on an alternation behavior and subjected to a unilateral entorhinal cortical lesion were given daily (i.m.) injections of either total brain ganglioside or GM1 ganglioside. Results show that ganglioside administration reduces the extent of behavioral deficit caused by the lesions and facilitated the course of functional recovery. It is hypothesized that gangliosides are enhancing hippocampal sprouting which occurs subsequent to the entorhinal lesion.

Increased surface glycoconjugates of synaptic membranes in mice during chronic ethanol treatment

Synaptosomal plasma membranes from mice treated chronically with ethanol were incubated with galactose oxidase and [3H]-sodium borohydride, in order to label the exposed galactose and N-acetylgalactosamine groups of glycoconjugates. The ethanol treatment approximately doubled the amount of exposed sugars. This change may be related to previously observed alterations in the physical properties of neuronal membranes in ethanol-treated mice.

A new brain cell surface glycoprotein identified by monoclonal antibody

Of 207 monoclonal antibodies produced against cultured mouse cerebellar cells, 16 reacted with cerebellar cell surfaces and 4 reacted with glycoproteins. One of them, called an anti-BSP-3 (Brain cell Surface Protein-3) defines a 48,000 molecular weight protein which can be iodinated at the surface of cultured cerebellar cells. Lectin-binding and sugar incorporation studies established the glycoprotein nature of the antigen. Astroglia (glial fibrillary acidic protein-positive cells) in primary cerebellar cultures were labelled intensely for this antigen by the indirect immunofluorescence method while neuronal cells and their processes were more weakly labelled. Fibronectin-positive cells were negative for BSP-3. In cerebellar sections using the immunoperoxidase method at both the optical and electron microscope levels, the difference in staining intensity between astrocytes and neuronal cells was not significant: in Purkinje cells and in the large neurones present in the deep cerebellar nuclei the immunoperoxidase percipitate was confined to the plasma, membrane while in both astrocytes and granule cells cytoplasmic labelling was also observed. Oligodendrocytes do not appear to react with the anti-BSP-3 monoclonal antibody; neither do endothelial or leptomeningeal cells. The availability of a monoclonal antibody produced by a stable hybridoma line will be a powerful tool in attempts to purify the BSP-3 antigen and to elucidate its function.

The organization of gangliosides and other lipid components in synaptosomal plasma membranes and modifying effects of calcium ion

Synaptosomes were prepared from bovine brain by zonal rotor sucrose density centrifugation. While a major fraction of lipid-bound sialic acid is included uniformly within the synaptosomal distribution profile, the sialoglycoproteins and some gangliosides do not follow this pattern. Exposure to extrasynaptosomal calcium results in alterations in the surface labeling properties of some gangliosides and membrane plasmalogens, suggesting that extrasynaptic Ca2+ may influence the conformation of complex lipids in synaptic plasma membranes. The level of intrinsic membrane-associated sialidase activity that liberates sialic acid from these sialoglycoconjugates parallels the synaptosomal buoyant density distribution profile, supporting a view that this enzyme resides in synaptosomal membranes in close association with a sialolipid substrate.

Fucose incorporation and identification of fucosylglycoproteins in synaptosomes

A synaptosome-enriched fraction from sheep cortex was incubated with L-fucose. The uptake of the sugar into this preparation was dependent on time, temperature, and concentration. A Kmapp of 0.94 mM-L-fucose and a Vmaxapp value of 0.24 nM-L-fucose/mg synaptosomal soluble protein/20 min was determined. After incubation for 10 min at 25 degrees C with L-[3H]fucose, 70% of the radioactive label was found in the soluble fraction. DEAE-cellulose chromatography resulted in the elution of three fucosylprotein peaks which were then characterised by gel filtration and sodium dodecyl sulfate-polyacrylamide electrophoresis (SDS-PAGE). At least eleven 3H protein-staining bands were identified with M. W. 13,000--115,000. Control experiments involving the incubation of the hexose with heat-treated synaptosomes and myelin, mitochondria, and microsomes indicted that the tritiated material associated with the synaptosomal soluble fraction was not due to nonspecific binding or to the presence of contaminating subcellular material. A 3H glycopeptide was identified, and on analysis the carbohydrate moiety was found to be rich in sialic acid, fucose, galactose, mannose, and N-acetylglucosamine. Mild acid treatment of the glycopeptide released fucose, which implies that this carbohydrate occupies a terminal position in the oligosaccharide chain. From these results it is proposed that synthesis or the modification of soluble fucosylglycoproteins is possible in synaptosomes.

Distribution of six synaptic membrane antigens in subcellular fractions of rat brain cortex

The subcellular distribution in rat brain cortex of six synaptic membrane antigens (56K, 58K, 62K, 63K, 64K, 66K) was studied by rocket immunoelectrophoresis, using antiserum to a highly purified synaptic plasma membrane fraction. Initial analysis of the insoluble portion of subcellular fractions showed that these antigens were also present in smooth microsomes, rough microsomes, and synaptic vesicles; that only traces were present in synaptic junctions; and that none was present in nuclei, mitochondria, and myelin. A trace amount of activity was also present in synaptic vesicle cytosol, but none in whole brain cytosol. Quantitative measurements of synaptic plasma membranes, smooth microsomes, and synaptic vesicles showed that all six antigens were present in synaptic plasma membranes and smooth microsomes, but that the 66K antigen was absent from synaptic vesicles. The 56K, 58K, 62, 63K, and 64K antigens were present in highest concentration in synaptic plasma membranes, whereas the 66K antigen content was highest in smooth microsomes. Only the 58K, 62K, and 63K antigen were detectable in the membrane fraction of whole brain. Their enrichments in synaptic plasma membranes were 10.9, 5.4, and 5.9, respectively. We conclude that the 56K, 58K, 62K, 63K and 64K antigens are primary components of synaptic plasma membranes. The presence of synaptic plasma membrane antigens in smooth microsomes and synaptic vesicles probably represents material being actively transported, consistent with the hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are transported via smooth endoplasmic reticulum.

Topographic studies of gangliosides of intact synaptosomes from rat brain cortex

Gangliosides in the external surface of intact synaptosomes from rat brain cortex have been studied by oxidation of exposed galactose and galactosamine groups with galactose oxidase followed by reduction with labeled sodium borohydride. Purified synaptosomes were labeled, disrupted by osmotic shock, and the particulate components fractionated on diatrizoate to give four synaptosomal membrane fractions (A-D) and a mitochondrial pellet (E). Fractions A and B represent synaptosomal plasma membranes. When intact synaptosomes were labeled, the major portion of the total radioactivity incorporated into ganglioside fraction was found to be in GM1 3 species. With isolated membrane fractions little selectivity was seen: (1) more label was present compared to intact synaptosomes, and (2) zones corresponding to GM2, GM1, GD1a, GD1b were the major gangliosides labeled. The results confirm the conclusion that membrane fractions A and B are derived from the exposed synaptosome surface and also show that GM1 is the major ganglioside species available for enzyme oxidation at the surface.

Membrane asymmetry. A survey and critical appraisal of the methodology. I. Methods for assessing the asymmetric orientation and distribution of proteins

This and the companion article are aimed at surveying the methods used for the study of membrane asymmetry. The techniques employed for the assessment of the asymmetric distribution and orientation of membrane proteins are reviewed in this article, whereas those pertaining to the unequal distribution of lipids are detailed in the companion paper. The use of immunological techniques and lectins, functions of proteins and their perturbations, chemical reagents, enzymatic isotopic labeling and enzymatic cleavage of membrane proteins and physical techniques are discussed and illustrated using recent examples of their application. Whenever appropriate, problems involving crypticity and non-availability or non-reactivity of functional sites, relevant chemical functions or protein fragments to appropriate ligands, reagents or modifying enzymes are envisaged and possible modification of the exposure of proteins during preparation of ghosts and other drawbacks are discussed, the use of different techniques and control experiments in conjunction is recommended for a more realistic assessment of the distribution and orientation of proteins.

Membrane asymmetry. A survey and critical appraisal of the methodology. II. Methods for assessing the unequal distribution of lipids

In the companion paper, I have reviewed the techniques employed for assessment of the asymmetric distribution and orientation of membrane proteins. This article deals with methods applicable to the investigation of the unequal distribution of lipids between the two membrane leaflets. Among the techniques I will discuss are the use of immunological techniques and lectins, chemical reagents, enzymatic isotopic labeling and degradation of membrane lipids, exchange proteins and physical techniques. Whenever appropriate, problems of crypticity and non-availability of lipids to interact with the appropriate ligands, reagents, modifying enzymes or exchange proteins have been envisaged. It appears that in many case, highly discordant results, sometimes with the same biological material, have been obtained. Some of the difficulties encountered presumably stem from the reported existence of non-bilayer arrangements and isotropic movement of lipids as evidenced by freeze-fracture and NMR studies. Other problems may be related to the induction of such arrangements, especially the inverted micellar arrangement, by the modifying agents, particularly degradation enzymes or exchange proteins when they cause severe unilateral modification of the lipids of the exposed leaflet. In addition, the situation is complicated by the role of the induced increase in the flip-flop rate under different experimental conditions and by modification of the rearrangement of lipid molecules as a result of the metabolic state of the cell or ghost preparation and of the reactivity of lipids as a consequence of temperature changes. Here, more so than with proteins, one must be cautious in interpreting experimental results. Moreover, it would appear that the use of different techniques in conjunction and the consequent comparison of results should be recommended. It has been emphasized that 'general rules' do not hold and that each new material should be assay again. To give one example, it is not pertinent to state that proteins enhance the flip-flop rate in lipid vesicles (and hence in membranes). This holds true for glycophorin from erythrocyte membrane, but could not be proved when mitochondrial cytochrome oxidase was used. There seems to be no rule for the distribution of lipids between the two leaflets of different membranes. For example, even for different strains of the same bacterial species, highly divergent results have been reported. It is generally (and probably under the influence of different studies with erythrocytes) believed that in mammalian plasma membranes, choline phospholipids are enriched in the outer leaflet and aminophospholipids in the inner leaflet. Though this contention may prove to be correct, different instances of contradictory results have been given in the text. This shows that if rules do exist, they remain to be discovered or established...

Synaptic junctional glycoconjugates from chick brain. Glycoprotein identification and carbohydrate composition

Forebrains from day-old chicks were homogenized and fractionated by differential sedimentation and density gradient centrifugation to yield subcellular fractions. The synaptosomal plasma membrane fraction was further treated with Triton X-100 to yield subsynaptic membrane fractions including synaptic junctions. Glycoproteins from these subsynaptic membrane fractions were identified after separation by SDS-polyacrylamide gel electrophoresis by incubating the gel slabs with radioiodinated concanavalin A. Two lectin-binding proteins were discerned in the synaptic junction fraction while none were observed in the Triton-soluble portion of the synaptic plasma membrane. The carbohydrate content of the glycoproteins from each subcellular fraction was quantitated after methanolysis and derivatization as o-methyl-trifluoroacetyl analogs by gas-liquid chromatography. The lowest concentration of glycoprotein sugars was found in the synaptic junction, mitochondrial, and soluble fractions while the greatest concentration was found in the myelin, light-synaptic plasma membrane, and the Triton-soluble portion of the synaptic plasma membrane. Of the subcellular fractions, the synaptic junction contained the highest porportion of mannose and lowest proportion of sialic acid. Moreover, this fraction's content of galactose and N-acetylglucosamine, relative to mannose was the lowest while its content of fucose was low. The oligosaccharide chains extending into the synaptic cleft therefore are predominantly of the "neutral, mannose-rich" type and are attached to a limited number of high-molecular-weight glycoproteins.

Identification and characterisation of two surface glycoproteins on cultured cerebellar cells

Two high molecular weight surface glycoproteins of cerebellar cells which are selectively labeled by lactoperoxidase-catalyzed iodination of monolayer cultures from the developing mouse cerebellum, have been identified and partially characterised. Both molecules, called peak 2 and peak 3 proteins, were the major glycoprotein species detected in cerebellar cell cultures after labelling with various radioactive sugars. The freshly iodinated molecules were firmly bound to the cells, but they were released into the medium upon prolonged incubation on the cultures. The soluble peak 2 and peak 3 proteins recovered from the medium comigrated on SDS-polyacrylamide gels with their cell-bound counterparts. Thus, their release results from mechanisms other than extensive degradation. The soluble proteins eluted from gel columns corresponding to molecular weights of over 500,000 and around 300,000, for peaks 2 and 3, respectively. They bound to and were specifically eluted from concanavalin A-Sepharose columns. Peak 3 protein could be easily identified as the most prominent iodinatable polypeptide in cerebellar cell cultures. Its surface expression depended on the presence of neuronal cells. After degeneration of neuron-like cells, a component of greater molecular weight than peak 3 or 2 was predominantly labeled by surface iodination. Peak 2 protein was quantitatively precipitated from labeled culture medium by two heterologous antiseRA. Anti-peak 2 activity was removed from the antiserum by absorption with adult mouse brain, but not by liver, spleen, thymus, kidney, heart and lung. Thus, peak 2 protein may be considered as a brain-specific glycoprotein.