Biosynthesis of myelin proteins in vitro

Inhibition of protein synthesis during CNS myelination produces focal accumulations of membrane vesicles in oligodendrocytes

Optic nerves of Xenopus tadpoles were exposed to cycloheximide to identify changes that occur during CNS myelin membrane formation when protein synthesis is inhibited. Groups of stage 51-56 tadpoles were immersed in either 10 or 20 micrograms ml-1 cycloheximide, and at specified times between 12 and 18 h after initial immersion tadpoles were killed and their optic nerves prepared for ultrastructural analysis. As early as 12 h there were alterations in oligodendrocytes from treated animals compared with control animals. The number of polyribosomes in the perikarya and cell processes was greatly reduced and the rough endoplasmic reticulum was disorganized. Mitochondria and microtubules were normal in appearance. Many oligodendroglial tongue processes at the inner margin of the myelin sheath were enlarged, occasionally indented the axon and were filled with vesicular profiles. Vesicles were noted in other cytoplasmic regions of oligodendrocytes and focal changes in the lamellar structure of myelin were found in paranodal regions. The internodal portions of the myelin sheath, axons and astrocytes appeared normal. Polyacrylamide gels of optic nerves showed that the incorporation of 35S-methionine into polypeptides had been almost completely inhibited by treatment with cycloheximide. These observations suggest that cycloheximide, by inhibiting synthesis of myelin proteins, alters the ability of oligodendrocytes to incorporate membrane components into CNS myelin sheaths.

Effects of monensin on posttranslational processing of myelin proteins

Rat brain slices were incubated with [3H]palmitic acid and [14C]glycine to label the lipid and protein moieties, respectively, of myelin proteolipid protein (PLP). The effects of monensin on posttranslational processing of proteins were examined by measuring the appearance of [14C]glycine- and [3H]palmitate-labeled proteins in myelin and myelin-like fractions. At 0.01 and 0.10 microM, monensin did not appreciably affect total lipid or protein synthesis; higher concentrations caused increased inhibition. Monensin at 0.10 microM markedly decreased the appearance of [14C]glycine-labeled PLP in myelin, but had little effect on the 14C basic proteins or the incorporation of [3H]palmitic acid into total or myelin PLP. The same relative effect was apparent at higher monensin concentrations. In the myelin-like fraction, monensin at 0.10 microM also depressed entry of [14C]glycine into protein comigrating with PLP, and again had no effect on incorporation of [3H]palmitic acid. In addition, monensin increased the [3H]palmitate label associated with two high-molecular-weight proteins in the myelin-like fraction with no concomitant increase in [14C]glycine label.

Distribution of radioactivity among total myelin protein amino acids following administration of labeled glycine, leucine, or methionine

This paper analyzes the distribution of radioactivity in the different amino acids of brain myelin protein for up to 6-7 weeks after an intracranial pulse administration of radioactive leucine, glycine, or methionine. Results show that there is no significant accumulation or reutilization of protein radioactivity in any form other than the one administered.

Cell-free synthesis of the myelin basic proteins in a wheat germ system programmed with brain messenger RNA

Poly A(+) messenger RNA (mRNA) was isolated from the brains of 3-week-old mice and translated in a cell-free system derived from wheat germ. Maximal stimulation of the system by brain mRNA was observed at a relatively low K+ concentration (45 mM) and low mRNA concentration (1-10 microgram/ml). The translational system was dependent on an energy-generating system and stimulated by the addition of spermidine and transfer RNA. Under optimal conditions, incorporation was linear for almost 45 min, but the overall stimulation with brain mRNA was relatively low (about twofold). In spite of the low stimulation, analysis of the translation products indicated that in the presence of brain mRNA polypeptides which co-chromatographed and co-electrophoresed with the two mouse myelin basic proteins could be detected. In control experiments with liver poly A(+) mRNA, which stimulated the translational system to a greater extent than brain mRNA, no such polypeptides could be detected. In this system the ratio of synthesis of small myelin basic protein to large myelin basic protein was found to be about 4.0, which correlates well with that found in vivo.

Effects of anti-white matter serum on myelin and lipid synthesis in brain prisms

Tissue prisms prepared by choping whole mouse brain maintained respiratory capacity and ultrastructural integrity of 3 h in vitro. Normal rabbit serum (ca. 25%) caused no morphological change but inhibited the synthesis of galactolipids by the prisms. Heating the serum abolished the inhibition. Complement containing anti-white matter rabbit serum destroyed myelin and inhibited galactolipid synthesis to a greater degree than did normal serum. Structures other than myelin were unaffected by the antiserum. Incubation in the presence of heated anti-white matter serum eliminated the myelin destruction but resulted in specific morphological changes characterized by the doubling of the myelin lamellae at the intraperiod line. Immunoperoxidase studies suggest specific binding of immunoglobulin to components of myelin located at the intraperiod lone. These changes were similar to those found in organotypic cultures. Heated antiserum did not inhibit galactolipid synthesis but addition of complement (normal guinea pig serum) to the heated antiserum restored only that portion of the inhibition which exceeded that caused by normal serum. Heat labile factors in normal rabbit serum which inhibit myelin lipid synthesis in the prisms must be corrected for in studies in which the heating of serum is used to demonstrate that the effect is complement dependent. The prism system is simpler than that of organotypic cultures and may be useful in the study of myelinotoxic factors.

Effects of ceramide analogs on myelinating organ cultures

Analogs of ceramide which inhibit galactocerebrosidase also demyelinate or inhibit myelination in organ cultures of rat cerebellum. The potency of the analogs in culture correlated with their effectiveness as inhibitors of cerebrosidase, but not with their effectiveness as inhibitors of galactosyl transferase. The most effective compound was the decanoyl amide of 3-phenyl-2-amino-1,3-propanediol with erythroconformation. Stimulators of cerebrosidase also demyelinated cultures. With both groups of compounds, myelin sheaths became distorted, then broke into lipid droplets. Axons were preserved, but neurons showed some nuclear changes and granularity. Metabolic studies with the most effective inhibitor showed that glucose incorporation into cerebroside and other alkali-stable lipids was initially depressed compared to proteins and total lipids.