On the Accessibility and Localisation of Cerebrosides in Central Nervous System Myelin

Abstract

SUMMARYCerebrosides are concentrated in the myelin sheath where they account for about 20% of the total lipid of the membrane. The present paper is concerned with the role and localisation of these glycolipids in the myelin lamellae. Isolated central nerve myelin preparations have been treated with two probes to investigate cerebroside accessibility in the membrane. The action of galactose oxidase on the galactose headgroups of cerebrosides is followed and quanti tated by recovery of the modified glycolipid and resoluGion of either the 6-aldehydo sugar or galactose remaining by gasliquid chromatography. With isolated myelin preparations only some 40-50% of the cerebroside galactose is attacked by galactose oxidase at 20 0 C. With periodate at 20 0 C over 90% of the galactose headgroups are oxidised in 3 h while the figure is 50-55% over the same time period at 4°c rising to 85% after 22 h. With multilamellar liposomes of mixed myelin ligids only some 20-25% of the available cerebroside is oxidised at 4 C, the reaction being complete in 2 h. The results are discussed in relation to the disposition of cerebroside in the myelin lamellae. A major location on the external face of the membrane system (intraperiod dense line) is favoured. A role for cerebroside in myelin in terms of increasing the stability and resistance of the lipid phase to ion movement is suggested.

Accessibility of Galactosyl Ceramides to Probe Reagents in Central Nervous System Myelin

“…The question of whether isolated myelin is a suitable material for probe labelling studies has been raised by Poduslo and Braun (1975). The present results confirm previous findings (Linington and Rumsby, 1978) that when myelin is isolated any splitting of lamellae occurring in the sheath takes place exclusively at the external apposition (intraperiod dense line) that arises from the close approach of external membrane surfaces of the oligodendrocyte plasma membrane (Davison and Peters, 1970). Isolated myelin fragments thus present a common external membrane surface of their lamellae to the incubation medium and thus to added probes.…”

“…Electron microscopic examination ( Fig. 1) of the membrane system revealed that during isolation separation of lamellae occurs at their intraperiod dense line (the external apposition surfaces), as has also been described previously (Linington and Rumsby, 1978) for isolated myelin preparations. The lamellae remain joined at the main dense lines (cytoplasmic apposition surfaces) and no evidence for separation of lamellar surfaces at this apposition has been obtained.…”

Interaction of cholesterol with galactocerebroside and galactocerebroside-phosphatidylcholine bilayer membranes

“…Note that glycosphingolipids in general tend to be asymmetrically disposed in biomembranes (34,35). Recent studies have shown that cerebroside is preferentially, if not totally, located on the external (extracellular) monolayer of myelin membrane (36,37). Such a concentration of galatocerebroside would result in -40 mol % cerebroside in the extracellular monolayer.…”

Effect of chain unsaturation on the structure and thermotropic properties of galactocerebrosides

“…Cerebrosides, specifically galactocerebrosides are found in relatively large amounts in the central and peripheral nerve myelin (Linington and Rumsby, 1978;Rumsby, 1978), although their exact structural/functional role is not known. The structure and properties of cerebrosides have been studied by spectroscopic (Bunow and Levin, 1980;Oldfield, 1979, 1982;Huang et al, 1980;Lee et al, 1986), calorimetric (Clowes et al, 1971;Bunow, 1979;Freire et al, 1980;Linington and Rumsby, 1981;Curatolo, 1982, Curatolo andJungalwala, 1985;Ruocco et al, 1981; Reed and Shipley, 1987), surface balance (Maggio et al, 1978a(Maggio et al, , b, 1981, and x-ray diffraction (Reiss-Husson, 1967;Abrahamsson et al, 1972, Fernandez-Bermudez et al, 1977Hosemann et al, 1979;Ruocco et al, 1981; Reed and Shipley, 1987) methods.…”