Possible role of sialocompounds in the uptake of choline into synaptosomes and nerve cell cultures

Abstract

Incubation of primary nerve cell cultures and of crude synaptosomal preparations with neuraminidase released sialic acid from both gangliosides and sialoglycoproteins. After this treatment, the pattern of ganglioside distribution was severely modified with a decrease of polysialogangliosides (GD1b, GT1b, Gt1L, GQ1) and a dramatic increase in monosialoganglioside GM1. The choline influx into neuraminidase treated cells and organelles was reduced by 30--50% but the efflux was unmodified. In particular the high affinity mechanism of choline uptake disappeared and the low affinity mechanism was modified in both cases. The disappearance of the high affinity uptake mechanism was not followed by a decreased acetylcholine synthesis as it should be if the current theories on choline uptake and acetylcholine synthesis are correct. Our present data thus confirm our previous hypothesis that choline metabolism regulates choline uptake rather than the other way round as is suggested by the theories most widely accepted at present. Choline uptake was unaffected by pretreatment of cells and organelles with tetanus toxin suggesting that the effect of neuraminidase on the choline uptake were either mediated through glycoproteins or through gangliosides other than those which bind to tetanus toxin (GD1b and GT1b). Several speculative models for explaining the effect of neuraminidase on choline uptake are proposed.

Surface-accessible GABA supports tonic and quantal synaptic transmission

“…Gangliosides and other membrane sialoglycoconjugates have been reported to contribute to neurotransmitter uptake (Massarelli et al, 1982;Zaleska and Ereciñ ska, 1987). This suggested the possibility of a direct interaction between GABA and the trisialoganglioside GT1b, which we tested using liposomes containing GT1b.…”

“…The surface matrix created by the local concentration of negative charges due to GT1b clusters in liposomes, or to clusters of gangliosides and other sialated and charged compounds in neurons, is likely to alter the biophysical properties such as the local acidity and/or the membrane surface potential, which could serve to trap GABA molecules. Such a trapping by sialic acid radicals has been proposed to contribute to a choline uptake mechanism (Massarelli et al, 1982). Furthermore, GABA uptake into synaptosomes was shown to depend on the presence of surface sialic acid moieties (Zaleska and Ereciñ ska, 1987).…”

Properties of endogenous, membrane-associated sialidase activity (N-acetylneuraminidase) of the goldfish visual system

“…Glycosidically-bound sialic acid (N-acetylneuraminic acid) in membrane proteins and lipids may play a critical role in diverse biological functions, including: recognition of serum glycoprotein (1); the lifetime of circulating lymphocytes (2) and erythrocytes (3); receptors for hormones (4), viruses (5) and bacterial toxins (6); and possibly in the malignant transformation of cells (7). In the nervous system, specific functions attributed to sialoglycoproteins and lipids (gangliosides) include choline uptake and acetylcholinesterase activity (8,9), susceptibility of dopamine 13-hydroxylase to proteolytic degradation (10), and Ca z+ mobilization during synaptic transmission (11,12).…”

Surface Glycosyltransferase Activities During Development of Neuronal Cell Cultures

“…Neuronal glycoconjugates are known to be located in high concentrations at the synaptic level and suggestions have been advanced to implicate these molecules in neurotransmitter function (Barondes, 1970;Rahmann et al, 1976; Kelly and Cotman, 1977;Svennerholm, 1980;Massarelli et al, 1982). Recently, Matsui et al (1983) showed the existence of glycosyltransferase activities at the external side of the neuronal plasma membrane using a methodology that has excluded possible sources of errors such as: (a) the uptake of free sugars, produced by the hydrolysis of sugar nucleotides, into neurons; (b) the release or the secretion of intracellular glycosyltransferases and glycoconjugates; and (c) the pinocytosis of exogenous acceptors.…”