The effects of postmortem autolysis in situ on myelin proteins and glycoproteins were studied in 25- and 125-day-old mouse brain and in adult bovine brainstem. In bovine myelin a loss of the major myelin glycoprotein was the only difference observed when the tissue was left at 19 degrees C for 24 hours compared to immediately frozen material. In the autolysed mouse brain, the myelin major glycoprotein was the most affected component with a 55% decrease. Both myelin basic protein components were degraded with a 35% loss. The other myelin proteins did not change under the conditions used for this study. There was also no change in the specific activity of 2',3'-cyclic nucleotide 3'-phosphohydrolase, a myelin-associated enzyme. Using the double labelling technique with [3H]fucose and [3 5S] sulfate as precursors injected intracranially, a shift of the major myelin glycoprotein labelled with radioactive sulfate towards a smaller apparent molecular size was observed as a result of the autolysis whereas the electrophoretic mobility of the fucose labelled major peak was unaffected.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗

PubMedDate not supplied

Characterization of two subcellular fractions isolated from myelinated axons.

Myelin and a heavy membrane fraction (1.0/1.2 fraction) were isolated from rabbit white matter by a slight modification of the procedure for bovine CNS. The specific activities of acetylcholinesterase and Na+, K+-ATPase were higher in the 1.0/1.2 fraction than in myelin. In contrast, the cerebroside content and 2'3'-cyclic nucleotide 3'-phosphohydrolase activity in the 1.0/1.2 fraction were 4.5 and 3.4 times lower than in myelin. Total lipids accounted for only 30% of the 1.0/1.2 fracton's dry weight; for myelin, they represented 70%. Polacrylamide gel electrophoresis showed the presence of many high molecular weight proteins and glycoproteins in the 1.0/1.2 fraction but myelin components were practically missing. Cytochrome c oxidase and NADPH-cytochrome c reductase activities suggested about 15% contamination in the 1.0/1.2 fraction but less than 5% for myelin. In electron micrographs of the 1.0/1.2 fraction, there were many membraneous profiles that varied in size, some mitochondrial fragments, and only a few lamellar whorls of compact myelin. The results suggest that the 1.0/1.2 fraction is different from other myelin-related fractions and is probably enriched in axolemma.

Animals↗

PubMedDate not supplied

Myelin basic protein deficit in the PNS of mld mutant mice recovers during development.

Myelination was studied between 15 and 135 days postnatally in peripheral nerves of myelin deficient (mld) mice and in unaffected littermates. The nerve weights were not affected by the mutation and showed a 4-fold increase during the developmental period studied. The amounts of myelin present in peripheral nerves, as shown by biochemical and morphological techniques, were slightly reduced in mld in comparison to control mice. In controls, the concentration of myelin doubled during the investigation period. The increase of myelin basic protein (MBP) in total nerve homogenate paralleled the deposition of myelin, but the MBP concentration remained constant in normal myelin. In contrast, in mld myelin MBP concentrations were extremely low until 60 days of age and increased thereafter to reach almost normal values at 135 days. Similarly, the amounts of myelin isolated at 85 and 135 days were normal. 2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP; EC 3.1.4.37), the myelin-specific enzyme, showed normal specific activities in mld nerves. In mld and control myelin, CNP-specific activities decreased during development suggesting a preferential localization of CNP in Schwann cell plasma membranes. In contrast to the central nervous system, other myelin proteins were not altered in mld peripheral nervous system (PNS) and the very low MBP content had no severe repercussions on the composition and structure of the myelin sheath. Furthermore, Schwann cells appeared normal in mld PNS. Nevertheless, more subtle alterations could be detected. Slightly decreased amounts of myelin were observed in young mld mice and preliminary results indicate discrete alterations of the myelin periodicity.(ABSTRACT TRUNCATED AT 250 WORDS)

2',3'-Cyclic-Nucleotide Phosphodiesterases↗

PubMedDate not supplied

Intrinsic myelin proteins are normally synthesized in vitro in the myelin-deficient (mld) mutant mouse.

The synthesis of myelin basic protein is severely reduced in the central and peripheral nervous system of myelin-deficient (mld) mutant mice. Using an in vitro system and immunoprecipitation, we found a normal rate of synthesis in mld mice for two intrinsic myelin proteins: proteolipid protein in central nervous system myelin and P0 in peripheral nervous system myelin. These results indicate that protein synthesis on membrane-bound ribosomes is not affected by the mutation.

Animals↗

PubMedDate not supplied

Myelin basic protein and the stability of the multilamellar myelin structure.

A purified myelin fraction was prepared from myelin deficient (mld) mutant mice and normal littermates (20 to 25 days old). Mld brains contained only 5% of the amount of myelin present in controls. Myelin isolated from mld brains had a normal lipid and protein content. The lipid composition of mld myelin was also normal. Myelin basic protein (MBP) was nearly missing in mld myelin. At the electron microscope, in mld myelin, the major dense line was often missing and the myelin lamellae were uncompacted. The high specific radioactivity of the myelin associated glycoprotein and sulfatides, two myelin constituents which are not affected by the mutation, indicate a high turnover rate of myelin membranes in the mld mutant. Furthermore, the presence of increased amounts of cholesterol esters in mld brains suggests that myelin in mld mutants is unstable. We propose that MBP, which is responsible in the central nervous system for myelin compaction, is responsible for the stability of this membrane.

Animals↗