Glycolipid sialosyltransferase activity in synaptosomes exhibits a product specificity for (2-8)disialosyl lactosyl ceramide (ganglioside GD3)

Chronic ethanol consumption down-regulates CMP-NeuAc:GM3 α2,8-sialyltransferase (ST8Sia-1) gene in the rat brain

Alcoholics have an increase in sialic acid-deficient glycoconjugates such as carbohydrate-deficient transferrin, sialic acid-deficient gangliosides and free sialic acids. The elevated presence of these asialoconjugates could be a consequence of alcohol-mediated impaired sialylation rate or due to increased desialylation rate. Chronic ethanol-induced brain abnormalities and behavioral changes could be mediated through these asialogangliosides. We have therefore determined the level of brain CMP-NeuAc:GM(3) alpha2,8-sialyltransferase (ST8Sia-1) and Gal-beta1,3GalNAc alpha2,3-sialyltransferase (ST3Gal-11) messenger RNA (mRNA) and correlated with the activity of these key enzymes in male Wistar rats as a function of increasing dietary concentration of ethanol after 8 weeks of feeding. The relative level of brain synaptosomal ST8Sia-1 and ST3Gal-11 mRNA were determined by real-time quantitative polymerase chain reaction (RT-PCR). We compared the observed ST8Sia-1 gene expression with its enzymatic activity in the synaptosomal membrane fraction isolated from the rat brain in the ethanol and pair-fed control groups. The results showed that the relative level of brain ST8Sia-1 mRNA expression was down-regulated by 13% (p<0.05) in 10.6%, by 40% (p<0.01) in 20.8% and by 57% (p<0.01) in the 36% ethanol-calorie groups, compared to the control (0% ethanol-calorie) group. In addition, ethanol at 36% dietary calories caused a significant 61% (p<0.01) decrease in the brain synaptosomal ST8Sia-1 activity compared to the control group. However, ethanol (10.6, 20.8 or 36% level) did not significantly affect the relative level of brain ST3Gal-11 mRNA as compared to the control (0% ethanol-calorie) group. Thus, our findings imply that chronic ethanol exposure preferentially down-regulates brain ST8Sia-1 mRNA accompanied by a concomitant decrease in its activity in a dose-dependent manner. Therefore, the selective loss of 2,8-sialic acid residues from gangliosides might contribute towards the appearance of asialogangliosides and related brain-abnormalities associated with ethanol abuse.

Effects of chronic ethanol administration on the activities and relative synthetic rates of myelin and synaptosomal plasma membrane-associated sialidase in the rat brain

In an attempt to understand the possible mechanism of chronic ethanol-induced generation of asialoconjugates in the brain and consequent behavioral abnormalities, we have studied the effects of chronic ethanol feeding to rats on the plasma membrane sialidase status in the various subcellular fractions of the brain. We determined sialidase activity using 3H-monosialoganglioside (3H-GM3), 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid (4-MU-NeuAC) substrates and Amplex Red (Sialidase) kit. We determined the plasma membrane sialidase protein by Western blot using the anti-plasma membrane sialidase. We also determined its relative synthetic rate (RSR) by the 60 min incorporation of intracranially infused [35S]-methionine (50 microCi/100 g) into immunoprecipitable plasma membrane sialidase. Chronic ethanol administration stimulated the sialidase activity in the total brain homogenate as well as the myelin and synaptosomal membrane fractions, respectively, in all the three experimental models. Chronic ethanol also increased the concentration of the rat brain plasma membrane sialidase protein relative to that of glyceraldehyde-3-phosphate dehydrogenase by 2.4-, 1.62- and 1.51-fold in the total brain homogenate, myelin and synaptosomal membrane fractions, respectively. These increases in plasma membrane sialidase activity and its protein content were due to concomitant increases in their relative synthetic rates by 115% (p < 0.01) and 72% (p < 0.01) in the myelin and synaptosomal membrane fractions, respectively. Thus, our studies clearly show that chronic ethanol induced deglycosylation of brain gangliosides is in part, due to specific up-regulation of plasma membrane sialidase in the myelin and synaptosomal membrane fractions of the brain. This increase in plasma membrane sialidase may be responsible for chronic-ethanol-induced physiological and neurological impairment in the brain, presumably due to deglycosylation of gangliosides that are essential for crucial cellular and metabolic activities.

A ganglioside-specific sialyltransferase localizes to axons and non-Golgi structures in neurons

To investigate the tissue distribution and subcellular localization of ST3GalV (CMP-NeuAc:lactosylceramide alpha2,3 sialyltransferase/GM3 synthase) in the adult mouse, we generated two antisera against mouse ST3GalV that were designated CS2 (directed against amino acids K227-I272) and CS14 (directed against amino acids D308-H359). We previously reported that CS2 antiserum stains medial and trans-Golgi cisternae in all cell types investigated. In neural tissue, however, CS14 antiserum reveals a subpopulation of ST3GalV with a subcellular distribution complementary to CS2 antiserum. CS14 antiserum strongly stains axons in cortical, cerebellar, brainstem, and spinal cord tissue sections. The subcellular localization of neuronal ST3GalV is maintained in primary cultures of rat hippocampal neurons and in PC12 cells. In PC12 cells, ST3GalV localization evolves during NGF-induced differentiation such that a pool of enzyme leaves the Golgi for a distal compartment in conjunction with neurite outgrowth. In PC12 cells transfected with an epitope-tagged form of ST3GalV, staining for the epitope tag coincides with expression of endogenous enzyme. The non-Golgi pool of ST3GalV does not colocalize with markers for the trans-Golgi network, endosome, or synaptic vesicles, nor is it detected on the cell surface. Distinct subpopulations of ST3GalV imply that ganglioside synthesis can occur outside of the Golgi or, alternatively, that a portion of the total ST3GalV pool subserves a nonenzymatic function. Significantly fewer transfected cells were found in PC12 cultures treated with plasmid encoding ST3GalV than in cultures treated with control plasmid, indicating that the expression of ST3GalV in excess of endogenous levels results in either cell death or a decreased rate of cell division.

Tenascin-C inhibits beta1 integrin-dependent cell adhesion and neurite outgrowth on fibronectin by a disialoganglioside-mediated signaling mechanism

We have previously shown that the extracellular matrix molecule tenascin-C inhibits fibronectin-mediated cell adhesion and neurite outgrowth by an interaction with a cellular RGD-independent receptor which interferes with the adhesion and neurite outgrowth promoting activities of the fibronectin receptor(s). Here we demonstrate that the inhibitory effect of tenascin-C on beta1integrin-dependent cell adhesion and neurite outgrowth is mediated by the interaction of the protein with membrane-associated disialogangliosides, which interferes with protein kinase C-related signaling pathways. First, in substratum mixtures with fibronectin, an RGD sequence-containing fragment of the molecule or synthetic peptide, tenascin-C inhibited cell adhesion and spreading by a disialoganglioside-dependent, sialidase-sensitive mechanism leading to an inhibition of protein kinase C. Second, the interaction of intact or trypsinized, i.e., cell surface glycoprotein-free, cells with immobilized tenascin-C was strongly inhibited by gangliosides or antibodies to gangliosides and tenascin-C. Third, preincubation of immobilized tenascin-C with soluble disialogangliosides resulted in a delayed cell detachment as a function of time. Similar to tenascin-C, immobilized antibody to GD2 (3F8) or sphingosine, a protein kinase C inhibitor, strongly inhibited RGD-dependent cell spreading. Finally, the degree of tenascin-C-induced inhibition of cell adhesion was proportional to the degree of disialoganglioside levels of expression by different cells suggesting the relevance of such mechanism in modulating integrin-mediated cell-matrix interactions during pattern formation or tumor progression.

Deleterious actions of chronic ethanol treatment on the glycosylation of rat brain clusterin

Clusterin is a N-glycosylated sialoglycoprotein present in rat brain cells. Clusterin, which elicits aggregation in a wide variety of cells, has been suggested to play an important role in synaptic remodeling through its cell adhesion property or lipid transport capacity in the brain. Sialic acid residues in clusterin may be responsible for its structural conformation, stability and functional ability. Maturation of clusterin is governed by the relative actions of sialyltransferases and sialidases that are present in brain microsomes, golgi bodies, cytosol and plasma membranes. We have earlier reported that chronic ethanol treatment in rats has a damaging effect on the hepatic glycosylation machinery. Others have reported increased hydrolysis of brain sialoconjugates in rats following chronic ethanol administration. Specificity of the effects of chronic ethanol treatment in the brain in relation to the glycosylation process, is still obscure. Therefore, in this investigation, we have studied the specific effects of chronic ethanol treatment on the glycosylation of rat brain clusterin and the causes that may lead to any possible defects in the glycosylation process. We have determined the effects of chronic ethanol treatment on (i) the incorporation of labeled leucine and N-acetylmannosamine into immunoprecipitable clusterin in whole brain homogenate, microsomes, golgi, cytosol, plasma membrane and synaptosomes, (ii) enzymatic activities of sialyltransferases in golgi and synaptosomes, and sialidase in brain cytosol and plasma membranes, and (iii) de novo synthetic rate of rat brain cytosolic sialidase. Our results showed that chronic ethanol treatment in rats resulted in (1) a decreased sialation index of brain clusterin by 47. 2% (p<0.001), 56.7% (p<0.05), 51.7% (p<0.05), 64.8% (p<0.001), and 54.5% (p<0.05), respectively, in whole brain homogenate, golgi, cytosol, plasma membranes, and synaptosomes; (2) a 46.1% (p<0.05) and 12.5% (p<0.05) decreased activities of brain sialyltransferases, respectively, in the golgi and the synaptosomal fractions; (3) a 70. 1% (p<0.05) and 42.6% (p<0.05) increased activities of sialidases, respectively, in the cytosol and plasma membrane fractions; and (4) a 22.2%-64.3% (p<0.001) increased incorporation of labeled leucine into brain cytosolic sialidase. Our findings have clearly established that long-term ethanol treatment in rats leads to a marked impairment in the glycosylation of rat brain clusterin as a result of altered activities of brain sialation and desialation enzymes. In particular, the specific increase noted in brain sialidase activity was due to concomitant increases in its synthetic rate. These defects in the glycosylation of brain clusterin may lead to changes in the molecular conformation of clusterin, and thus, may result in its structural instability and/or functional impairment.

Ethanol attenuation of ganglioside sialylation and neuritogenesis

Neurite-producing cultured embryonic chick brain neurons in 96-h culture were exposed chronically to 25 mM ethanol for a 72-h period. Neurital plasma membrane extension was markedly attenuated in the ethanol-exposed neurons as compared with ethanol-free control cultures. The rate of biosynthetic sialylation of gangliosides, which are major structural sialoglycosphingolipid components in the exofacial lipid bilayer of the neurital plasma membrane, decreased to about half that in the alcohol-free neurons. The findings show that ganglioside sialylation in the morphodifferentiating central nervous system neuron is significantly attenuated by chronic exposure to a moderate dose of ethanol. Depression of ganglioside-dependent neuritogenesis may represent a mechanism for development of the central nervous system component of the fetal alcohol syndrome.