The turnover of myelin proteins in adult rat brain

Activation of mTORC1 and c-Jun by Prohibitin1 loss in Schwann cells may link mitochondrial dysfunction to demyelination

Schwann cell (SC) mitochondria are quickly emerging as an important regulator of myelin maintenance in the peripheral nervous system (PNS). However, the mechanisms underlying demyelination in the context of mitochondrial dysfunction in the PNS are incompletely understood. We recently showed that conditional ablation of the mitochondrial protein Prohibitin 1 (PHB1) in SCs causes a severe and fast progressing demyelinating peripheral neuropathy in mice, but the mechanism that causes failure of myelin maintenance remained unknown. Here, we report that mTORC1 and c-Jun are continuously activated in the absence of Phb1, likely as part of the SC response to mitochondrial damage. Moreover, we demonstrate that these pathways are involved in the demyelination process, and that inhibition of mTORC1 using rapamycin partially rescues the demyelinating pathology. Therefore, we propose that mTORC1 and c-Jun may play a critical role as executioners of demyelination in the context of perturbations to SC mitochondria.

Transcriptional profile of the human peripheral nervous system by serial analysis of gene expression

The peripheral nerve contains both nonmyelinating and myelinating Schwann cells. The interactions between axons, surrounding myelin, and Schwann cells are thought to be important for the correct functioning of the nervous system. To get insight into the genes involved in human myelination and maintenance of the myelin sheath and nerve, we performed a serial analysis of gene expression of human sciatic nerve and cultured Schwann cells. In the sciatic nerve library, we found high expression of genes encoding proteins related to lipid metabolism, the complement system, and the cell cycle, while cultured Schwann cells showed mainly high expression of genes encoding extracellular matrix proteins. The results of our study will assist in the identification of genes involved in maintenance of myelin and peripheral nerve and of genes involved in inherited peripheral neuropathies.

Myelin-associated changes in mouse brain following irradiation

The goals of this study were to quantify myelin-associated changes in the brain following single doses of radiation and to determine their relationship to the dose limits that this tissue can tolerate. Mice developed a transient loss of balance 1 month after 60 Gy doses 250 kVp X-rays to the brain and 3-4 months after 30-45 Gy radiation, but not after lower doses. The symptoms were transient and lasted approximately 1 month. The ED50/300 for radiation-induced brain death, which occurred large between 200 and 240 days, was 32.4 Gy (29.1, 35.5 Gy, 95% confidence limit of mean). At the time that animals developed neurological symptoms, 3-4 months after irradiation with doses of 30-45 Gy, biochemical assays of myelin-associated proteins showed decreases in 2',3'-cyclic nucleotide phosphohydrolase (CNPase) and myelin basic protein (MBP) levels that were not seen with lower radiation doses. By 120-180 days, further dose-dependent decreases in both CNPase and MBP levels were found after 20-45 Gy irradiation that preceded and correlated with death. The correlation of the decrease in CNPase and MBP levels with the incidence of transient neurological malfunction and animal death, together with histological evidence, suggests that demyelination is responsible for these phenomena.

Alteration in myelin-associated proteins following spinal cord irradiation in guinea pigs

The aim of this study was to investigate the pathological and cellular basis for radiation-induced myelopathy in guinea pigs by monitoring biochemical alterations in levels of myelin basic protein and 2',3'-cyclic nucleotide phosphohydrolase. Guinea pigs were irradiated to the lumbar region with various doses of neutrons or cobalt gamma irradiation. The ED50s for paralysis were 17.2 Gy and 67.5 Gy for neutron and cobalt irradiation, respectively, and was histologically associated with demyelination. In spinal cords taken from animals at the onset of paralysis myelin basic protein levels were decreased in direct relationship to the radiation dose. The lowest doses to cause paralysis led to a 25% decrease in MBP levels. In a separate experiment, alterations in MBP were measured in the spinal cords over the time period leading up to paralysis. Surprisingly, decreases in MBP were found immediately after the end of the 4 week irradiation period. These early changes in MBP were not markedly dose dependent and occurred with nonparalyzing doses. Dose-dependent decreases were found only just before the onset of paralysis. CNPase activity measured in the same specimens showed changes that were essentially similar to those for MBP. In the CSF, MBP levels were essentially constant until onset of paralysis. This study showed that demyelination, as assessed by the levels of the myelin-associated proteins MBP and CNPase, can occur soon after spinal cord irradiation but that profound dose-dependent changes are seen only immediately preceding the onset of paralysis. Although increases in MBP in the CSF were associated with the onset of radiation-induced myelopathy, its assay is unlikely to predict this complication of irradiation.

Altered cerebral protein turnover in rats following prolonged in vivo treatment with nicotine

Turnover rates of cerebral proteins were examined in control adult rats and in those subjected to prolonged in vivo treatment with "low" (0.02 mg/ml) or "high" (0.04 mg/ml) doses of nicotine (added to drinking water), using [14C]bicarbonate as the label. It was found that the turnover of proteins in various subcellular fractions consisted of two distinct components turning over at a "fast" or a "slow" rate and having relatively short or long half-lives, respectively. Thus in control animals the half-lives of the protein components turning over at a fast rate ranged from 1.31 to 3.61 days whereas for those turning over at a slow rate the half-lives ranged from 8.56 to 24.28 days. Treatment with low doses of nicotine resulted in a more rapid turnover of nuclear fast turning over component with a concomitant decreased turnover of homogenate, cytosol, mitochondrial, and microsomal proteins; in the synaptosomal membranes this component disappeared altogether. The half-lives of the slow turning over components decreased in general from 14.3 to 33.3% with the exception of the nuclear proteins, where the half-live increased by 71.1%. Turnover of microsomal proteins was not affected. When the animals were given a high dose of nicotine, the turnover of fast components became even more rapid for nuclear, myelin, and microsomal proteins with a decrease in half-life from 26.6 to 32.3%. By contrast, half-lives of synaptosomal and mitochondrial proteins increased by 16.1-89.3%. These changes were not reflected in the turnover rate of whole homogenate proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Resistance to disruption of multilamellar fragments of central nervous system myelin

Single-bilayer vesicles of myelin are desirable for studying myelin development and metabolism. Accordingly, our interest was drawn to a procedure for vesiculating myelin (Steck et al., Biochim, Biophys. Acta 509, 397-408, 1978). We used X-ray diffraction analysis to examine these putative vesicle preparations because much larger amounts of material can be surveyed by this method than by electron microscopy. The sharpness (width) of the rings in the X-ray diffraction pattern varies inversely with the number of bilayers per multilayer structure. We therefore expected to see the diffuse diffraction pattern characteristic of single bilayers. Diffraction patterns were recorded from isolated rat brain myelin before and after the vesiculation procedure. Both patterns showed sharp rings, indicating numerous multilayered structures. Average values ranging from 7 to 10 bilayers per multilayer were calculated in both cases. This procedure did produce a small fraction of single-bilayer structures, which were isolated by differential centrifugation; however, these accounted for only about 1% of the total myelin present. The diffraction pattern of this material showed the diffuse band typical of single-bilayer structures, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated it had the same protein composition as in normal myelin. Similar results were also obtained using either fresh or frozen bovine brain myelin. Variations of the published vesiculation procedure (incubation in 0.1 M NaCl or in buffers containing glycerol; disruption by sonication or use of a Tissumizer) also were not effective in breaking down the multilamellar fragments into thinner structures. The conclude that the multilamellar fragments of isolated CNS myelin resist disruption into single-bilayer structures.

Effect of experimental hyperphenylalaninemia on myelin metabolism at later stages of brain development

Myelination is the most important process in postnatal maturation of the nervous system and during this period the growing infant passes through a "vulnerable period" during which irreversible brain damage can occur if the neonate is subjected to a potential neurotoxin. This study was undertaken to investigate the mechanisms by which chronic hyperphenylalaninemia interferes with myelin metabolism, beyond the neonatal period of rapid myelination, at a time when myelin continues to accumulate. Rats of 25 days of age were placed on a hyperphenylalanimenia (HyPhe) inducing diet of 5% phenylalanine plus 0.4% alpha-methylphenylalanine (alpha MP) at 25 days of age to approximate plasma phenylalanine levels of an untreated human PKU patient (1.5 mM). The HyPhe group exhibited approximately a 15% decrease in the amount of myelin protein throughout the 70 days of the study. The rate of incorporation of 3H-lysine into both TCA precipitable whole brain proteins or myelin proteins did not vary from the HyPhe group and a weight matched control group (WMC). Therefore, this loss of myelin could not be attributed to a hypo-myelination. The turnover of whole brain proteins also was unaffected by the HyPhe treatment; however, the turnover of myelin proteins in the HyPhe group was dramatically different (t 1/2 = 3 days) from that of the WMC group (t 1/2 = 36 days) or a group treated with only alpha MP (t 1/2 = 26 days).

Turnover of the fast components of myelin and myelin proteins in experimental hyperphenylalaninaemia. Relevance to termination of dietary treatment in human phenylketonuria

The turnover of myelin and of myelin protein fractions has been measured in the central nervous system of rats who were placed on a hyperphenylalaninaemia-inducing diet (3% L-phenylalanine and 0.12% p-chlorophenylalanine added to the normal laboratory chow) when they were 25 days of age. A considerably increased turnover of the fast component of myelin and of myelin protein fractions was observed, which was not found in weight-matched controls or in controls fed the normal laboratory chow supplemented with 0.12% p-chlorophenylalanine. The increased turnover is therefore due to the hyperphenylalaninaemic condition and not due to the slow-down in growth or the presence of p-chlorophenylalanine. Furthermore, an inhibition of myelin synthesis due to the hyperphenylalaninaemic condition has been observed. Since these effects on myelin metabolism can be demonstrated to occur even when the brain has matured considerably, prudence should be exercised in considering the termination of the dietary treatment of patients with phenylketonuria.

Turnover rate of molecular species of sphingomyelin in rat brain

Turnover rate of individual molecular species of sphingomyelin of adult rat brain myelin and microsomal membranes was determined after an intracerebral injection of 100 microCi of [C3H3]choline. Myelin and microsomal membrane sphingomyelins were isolated from the rest of the lipids. The individual molecular species of benzoylated sphingomyelin were separated and quantitated by reversed-phase high performance liquid chromatography. All individual major molecular species of microsomal and myelin sphingomyelin had maximum incorporation at 6 and 15 days, respectively, after the injection. The specific radioactivity of all the various molecular species of both myelin and microsomal sphingomyelin declined at a similar rate after reaching a maximum. There was no significant difference in the turnover rate of short chain (16:0, 18:0) and long chain (greater than 22:0) fatty acid containing sphingomyelin. The average apparent turnover rate of myelin and microsomal sphingomyelin molecular species was about 14-16 days for the fast pool and about 45 days for the slow pool. It is concluded that individual molecular species of sphingomyelin of myelin and microsomal membranes turned over at a similar rate. Thus, turnover rate of sphingomyelin in myelin and microsomal membranes is not affected by the fatty acyl composition of the lipid.

Is Na + ATPase a myelin-associated enzyme?

The Na + K ATPase activity associated with purified myelin has been investigated. On the basis of marker enzyme studies, the Na + K ATPase activity of myelin was higher than could be accounted for by microsomal contamination. Fractions prepared from white matter-enriched areas of rat brain showed a threefold enrichment in Na + K ATPase activity in myelin as compared with the white matter homogenate. The ATPase activity in myelin was stimulated fourfold by treatment with sodium deoxycholate, but the activity in the whole brain homogenate and the microsomal fraction was only doubled. This discontinuity temperature for Na + K ATPase activity was significantly higher for the myelin fraction (29 degrees C) than for the microsomal fraction (21 degrees C), but the energies of activation, both above and below the discontinuity temperature, were the same for both fractions, Myelin Na + K ATPase had a lower affinity for strophanthidin than the microsomal enzyme, but both fractions were inhibited to the same extent by 10-3 M-strophanthidin. The evidence thus indicated that much of the ATPase activity of myelin is not the result of microsomal contamination. Although the possibility of axolemmal contamination cannot be ruled out conclusively, indirect evidence suggest that this is not a significant factor and that Na + K ATPase may be a myelin-associated enzyme.