Availability to neuraminidase of gangliosides and sialoglycoproteins in neuronal membranes

Involvement of sialic acid in high-affinity uptake of dopamine by synaptosomes from rat brain

The high-affinity, sodium-dependent uptake of dopamine (DA) was inhibited by the pretreatment of synaptosomes with neuraminidase from Vibrio cholerae. The inhibition was of a non-competitive type, resulting in a 40% decrease of Vmax. Neither basal nor depolarization-stimulated release of DA was affected. Treatment of synaptosomes with neuraminidase caused a 48% loss of sialic acid from the lipid-bound pool and a 80% decrease in the protein-bound fraction. The inhibition of DA uptake was found to be related linearly to the loss of sialic acid from the protein pool. It is postulated that a sialic acid moiety is involved in DA transport across the synaptosomal membrane.

Role of sialic acid in synaptosomal transport of amino acid transmitters

Active, high-affinity, sodium-dependent uptake of gamma-aminobutyric acid and of the acidic amino acid D-aspartate was inhibited by pretreatment of synaptosomes with neuraminidase from Vibrio cholerae. Inhibition was of a noncompetitive type and was related to the amount of sialic acid released. The maximum accumulation ratios of both amino acids (intracellular [amino acid]/extracellular [amino acid]) remained largely unaltered. Treatment with neuraminidase affected neither the synaptosomal energy levels nor the concentration of internal potassium. It is suggested that the gamma-aminobutyric acid and acidic amino acid transporters are glycosylated and that sialic acid is involved in the operation of the carrier proteins directly and not through modification of driving forces responsible for amino acid uptake.

Glycolipid crypticity in membranes--not a simple shielding effect of macromolecules

The potential of membrane-bound macromolecules for shielding glycolipids from involvement in specific binding events was considered in model membranes. Serum albumin and several Dextrans were covalently derivatized with oleic acid so that they adsorbed irreversibly to lipid bilayers. This provided a means of generating bilayer membranes with a considerable layer of attached material. Gangliosides dispersed in such membranes were subjected to attack by the enzyme, neuraminidase, in order to assess their "accessibility'. We were surprised to find that we could not demonstrate any significant reduction in ganglioside hydrolysis in phosphatidylcholine bilayers bearing extensive surface coats of protein or polysaccharide. We conclude that non-specific, physical shielding by macromolecules is an unlikely source of the often-observed "crypticity' of glycolipids at the cell surface. Consistent with this interpretation was a relative lack of headgroup motional restriction seen for spin-labelled ganglioside headgroups in the same bilayers and in cell membranes.

Disposition of gangliosides and sialosylglycoproteins in neuronal membranes

Labeled gangliosides and glycoproteins were obtained by incubation of homogenized neuronal perikarya from rat brain with CMP-[3H]N-acetyl neuraminic acid. The highest degree of labelling was observed in a subcellular fraction that also showed the highest specific activities for several ganglioside glycosyltransferases. The [3H]sialosylglycoconjugates of this fraction remained associated with the membranes after treatment with 1 M-KCl, 125 mM-EDTA, repeated freezing and thawing, or controlled sonication, but were solubilized by sodium deoxycholate (DOC) at a concentration high enough to solubilize the choline phospholipids. About 75% of th neuraminidase-labile sialosyl residues of these labeled endogenous gangliosides and glycoproteins were protected from the action of added neuraminidase or pronase or both enzymes added together. The protection was not abolished by pretreatment of the membranes with high ionic strength or with EDTA but was abolished by sonication or low concentration of DOC. Between 50 and 80% of the neuraminidase-labile sialosyl residues of the gangliosides of the neuronal perikaryon membrane fraction labelled in vivo by an intracerebral injection of N-[3H]acetylmannosamine were, at 3 h after the injection, also protected from the action of added neuraminidase. The protection was abolished by the addition of DOC. In contrast with behaviour of the labeled glycoconjugates of this neuronal perikaryon fraction, the gangliosides and sialosylglycoproteins from intact synaptosomes were accessible to neuraminidase. It is suggested that most gangliosides and sialosylglycoproteins are sialosylated as intrinsic components of the neuronal perikaryon membrane fraction and that at some stage of the process of transport through the axon and incorporation into the synaptic plasma membrane they change their accessibility to added enzymes.

Biochemical evidence on the role of gangliosides in nerve-endings

From the biochemical and developmental data in the literature, there appears to be some specific relationship of gangliosides to synapses. Whether the concentration of polysialo-gangliosides is located in the immediately pre-synaptic part of the neuronal plasma membrane is not known. There are many unanswered questions about the gangliosides, which we have attempted to pose in this article. There are as many hypotheses about roles for gangliosides, and virtually no data to back them up. In fact there are not even real indications of possible functions. We believe that it will be very difficult to define the functions of gangliosides until more of the basic questions are answered-even if their synaptic localization is a constant temptation to speculate.

Rat brain microsomal gangliosides. Accessibility to a neuraminidase preparation and the possible existence of different pools in relation to their biosynthesis

1. Treatment of rat brain microsomal membranes with a neuraminidase preparation from Clostridium perfringens resulted in an almost complete conversion of polysialogangliosides into monosialogangliosides. 2. Neuraminidase treatment of the membranes did not increase the incorporation of N-[(3)H]acetylneuraminic acid from CMP-N-[(3)H]acetylneuraminic acid into the gangliosidic fraction, indicating that a monosialoganglioside is an acceptor of N-acetylneuraminic acid in these membranes only if, in addition to having the right chemical structure, it is in a proper position, probably in relation to the endogenous sialyltransferases. 3. These experiments also indicated that no independent turnover of the neuraminidase-labile N-acetylneuraminyl groups of gangliosides occurred in vitro. 4. N-[(3)H]Acetylneuraminic acid from endogenous polysialogangliosides labelled in vitro was released by neuraminidase at a slower rate than N-acetylneuraminic acid from unlabelled gangliosides of the same membranes. From this it was concluded that recently synthesized polysialogangliosides (completed in vitro) are in the membranes in a position less accessible to neuraminidase than are those synthesized earlier which were present in the membranes at the start of the labelling experiment.