Further Characterization of a Myelin‐Associated Neuraminidase: Properties and Substrate Specificity

We have examined the ganglioside composition of 30-day and 60-day postnatal rat oligodendroglia, adult bovine oligodendroglia, gray matter, white matter, and myelin and also the total lipid composition of the oligodendroglial preparations. The ganglioside patterns of rat and bovine oligodendroglia, as previously found for human oligodendroglia, were more complex than those of myelin. These data indicate that oligodendroglial perikarya can synthesize many brain type gangliosides, not all of which are incorporated into the compact myelin. Alternatively, the ganglioside composition of myelin may be altered in situ by the myelin-associated neuraminidase. In these two species, as in human, GM4 appears specific to oligodendroglia and myelin, while GD3 and GM3 are enriched in oligodendroglia but not myelin. In bovine oligodendrocytes GD3 is the major ganglioside. The total lipid concentration, as well as the percentage of cholesterol, sphingomyelin, phosphatidylinositol, and phosphatidylserine, differ for 30- and 60-day-old rat oligodendroglia and may be developmentally correlated with changes in myelin composition during myelinogenesis. There are also marked differences in the lipid composition of bovine oligodendroglia compared to rat oligodendroglia, with the former having more galactolipid and less ethanolamine phosphoglycerides.

Ganglioside and lipid composition of bulk‐isolated rat and bovine oligodendroglia

Macala

Farooq

et al. 1989

Possible role of myelin‐associated neuraminidase in membrane adhesion

Saito

1993

Previous studies from our laboratory have demonstrated the presence of a specific interaction between myelin-associated neuraminidase and GM1 (Saito and Yu, J Neurochem 47:632-641, 1986). In the present study, we further characterized this neuraminidase-GM1 interaction and examined its role in the adhesion of rat oligodendroglial cells to GM1. Hydrolysis of N-acetylneuramin-lactitol by the enzyme was inhibited by GM1 in a competitive manner; GM1 itself was not hydrolyzed, suggesting that GM1 may serve as a competitive inhibitor of the enzyme. Asialo-GM1 had no inhibitory effect. When a soluble enzyme preparation was applied to a GM1-linked affinity column, the enzyme activity was retained on the column and was recovered from the column only by elution with a buffer containing 5 mM 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (Neu2en5Ac), a competitive inhibitor of neuraminidase. A binding study with 51Cr-labeled rat oligodendroglial cells showed that oligodendroglial cells bound preferentially to GM1 developed on a thin-layer plate, but not to other gangliosides such as GM3, GD1a, GD1b, and GT1b. The binding reaction to GM1 was inhibited by Neu2en5Ac (5 mM). These results suggest that myelin-associated neuraminidase specifically interacts with GM1 and may be involved in adhesion of oligodendroglial cells to GM1. This neuraminidase-GM1 interaction may play an important role in the formation and stabilization of the multilamellar structure of the myelin sheath.

Characterization of sialidase activity in mouse synaptic plasma membranes and its age‐related changes

Saito

Tanaka

Tang

et al. 1995

Sialidase activity in synaptic plasma membranes (SPM) isolated from C57BL/6 mouse brain was examined using exogenous ganglioside substrates. The enzyme activity directed toward GM3 showed sharp pH dependency with optimal pH of 4.0, and was greatly enhanced by Triton CF-54, Nonidet P-40 or CHAPS. The apparent Km and Vmax values for enzyme activity in SPM were 11 microM and 164 pmol/mg protein/min, respectively. Examination of sialidase activities in subcellular fractions of brain tissues showed the enrichment of enzyme activity in SPM prepared from either young adult or senescent mice. Substrate specificity of SPM sialidase was compared with that of myelin sialidase using delipidated, solubilized enzyme preparations. The SPM sialidase hydrolyzed GD1a more effectively as compared with the myelin enzyme. While SPM sialidase could hydrolyze GM1, the hydrolytic rate by the SPM enzyme was significantly lower than that by the myelin enzyme. The sialidase activity in SPM decreased with increasing age; activity was highest between the ages of 4-7 months, decreased to a relatively constant level between 13-25 months, and reached its lowest level at 31 months. These results demonstrate that SPM contain a distinct sialidase activity which is regulated in an age-dependent manner.