Variation of proteins, enzyme markers and gangliosides in myelin subfractions

Laurent CD, Nakhaei-Nejad M, Fuchsberger FF, Streith L, Wang Q, Kawasaki N, Duan S, Bains A, Paulson JC, Rademacher C, Giuliani F, Sipione S, Macauley MS. Repression of phagocytosis by human CD33 is not conserved with mouse CD33

CD33 is an immunomodulatory receptor linked to Alzheimer's disease (AD) susceptibility via regulation of phagocytosis in microglia. Divergent features between human CD33 (hCD33) and murine CD33 (mCD33) include a unique transmembrane lysine in mCD33 and cytoplasmic tyrosine in hCD33. The functional consequences of these differences in restraining phagocytosis remains poorly understood. Using a new αmCD33 monoclonal antibody, we show that mCD33 is expressed at high levels on neutrophils and low levels on microglia. Notably, cell surface expression of mCD33 is entirely dependent on Dap12 due to an interaction with the transmembrane lysine in mCD33. In RAW264.7 cultured macrophages, BV-2 cultured microglia, primary neonatal and adult microglia, uptake of cargo - including aggregated Aβ1-42 - is not altered upon genetic ablation of mCD33. Alternatively, deletion of hCD33 in monocytic cell lines increased cargo uptake. Moreover, transgenic mice expressing hCD33 in the microglial cell lineage showed repressed cargo uptake in primary microglia. Therefore, mCD33 and hCD33 have divergent roles in regulating phagocytosis, highlighting the importance of studying hCD33 in AD susceptibility.

Transport of the major myelin proteolipid protein is directed by VAMP3 and VAMP7

CNS myelination by oligodendrocytes requires directed transport of myelin membrane components and a timely and spatially controlled membrane expansion. In this study, we show the functional involvement of the R-soluble N-ethylmaleimide-sensitive factor attachment protein receptor (R-SNARE) proteins VAMP3/cellubrevin and VAMP7/TI-VAMP in myelin membrane trafficking. VAMP3 and VAMP7 colocalize with the major myelin proteolipid protein (PLP) in recycling endosomes and late endosomes/lysosomes, respectively. Interference with VAMP3 or VAMP7 function using small interfering RNA-mediated silencing and exogenous expression of dominant-negative proteins diminished transport of PLP to the oligodendroglial cell surface. In addition, the association of PLP with myelin-like membranes produced by oligodendrocytes cocultured with cortical neurons was reduced. We furthermore identified Syntaxin-4 and Syntaxin-3 as prime acceptor Q-SNAREs of VAMP3 and VAMP7, respectively. Analysis of VAMP3-deficient mice revealed no myelination defects. Interestingly, AP-3δ-deficient mocha mice, which suffer from impaired secretion of lysosome-related organelles and missorting of VAMP7, exhibit a mild dysmyelination characterized by reduced levels of select myelin proteins, including PLP. We conclude that PLP reaches the cell surface via at least two trafficking pathways with distinct regulations: (1) VAMP3 mediates fusion of recycling endosome-derived vesicles with the oligodendroglial plasma membrane in the course of the secretory pathway; (2) VAMP7 controls exocytosis of PLP from late endosomal/lysosomal organelles as part of a transcytosis pathway. Our in vivo data suggest that exocytosis of lysosome-related organelles controlled by VAMP7 contributes to myelin biogenesis by delivering cargo to the myelin membrane.

Distinct endocytic recycling of myelin proteins promotes oligodendroglial membrane remodeling

The central nervous system myelin sheath is a multilayered specialized membrane with compacted and non-compacted domains of defined protein composition. How oligodendrocytes regulate myelin membrane trafficking and establish membrane domains during myelination is largely unknown. Oligodendroglial cells respond to neuronal signals by adjusting the relative levels of endocytosis and exocytosis of the major myelin protein, proteolipid protein (PLP). We investigated whether endocytic trafficking is common to myelin proteins and analyzed the endocytic fates of proteins with distinct myelin subdomain localization. Interestingly, we found that PLP, myelin-associated glycoprotein (MAG) and myelin-oligodendrocyte glycoprotein (MOG), which localize to compact myelin, periaxonal loops and abaxonal loops, respectively, exhibit distinct endocytic fates. PLP was internalized via clathrin-independent endocytosis, whereas MAG was endocytosed by a clathrin-dependent pathway, although both proteins were targeted to the late-endosomal/lysosomal compartment. MOG was also endocytosed by a clathrin-dependent pathway, but in contrast to MAG, trafficked to the recycling endosome. Endocytic recycling resulted in the association of PLP, MAG and MOG with oligodendroglial membrane domains mimicking the biochemical characteristics of myelin domains. Our results suggest that endocytic sorting and recycling of myelin proteins may assist plasma membrane remodeling, which is necessary for the morphogenesis of myelin subdomains.

Contactin-associated protein (Caspr) and contactin form a complex that is targeted to the paranodal junctions during myelination

Specialized paranodal junctions form between the axon and the closely apposed paranodal loops of myelinating glia. They are interposed between sodium channels at the nodes of Ranvier and potassium channels in the juxtaparanodal regions; their precise function and molecular composition have been elusive. We previously reported that Caspr (contactin-associated protein) is a major axonal constituent of these junctions (Einheber et al., 1997). We now report that contactin colocalizes and forms a cis complex with Caspr in the paranodes and juxtamesaxon. These proteins coextract and coprecipitate from neurons, myelinating cultures, and myelin preparations enriched in junctional markers; they fractionate on sucrose gradients as a high-molecular-weight complex, suggesting that other proteins may also be associated with this complex. Neurons express two contactin isoforms that differ in their extent of glycosylation: a lower-molecular-weight phosphatidylinositol phospholipase C (PI-PLC)-resistant form is associated specifically with Caspr in the paranodes, whereas a higher-molecular-weight form of contactin, not associated with Caspr, is present in central nodes of Ranvier. These results suggest that the targeting of contactin to different axonal domains may be determined, in part, via its association with Caspr. Treatment of myelinating cocultures of Schwann cells and neurons with RPTPbeta-Fc, a soluble construct containing the carbonic anhydrase domain of the receptor protein tyrosine phosphatase beta (RPTPbeta), a potential glial receptor for contactin, blocks the localization of the Caspr/contactin complex to the paranodes. These results strongly suggest that a preformed complex of Caspr and contactin is targeted to the paranodal junctions via extracellular interactions with myelinating glia.

Localization and partial characterization of a 60 kDa citrulline-containing transport form of myelin basic protein from MO3-13 cells and human white matter

The localization of myelin basic proteins (MBPs) in an immortalized human-human hybrid cell line (MO3-13) formed by fusion of rhabdomyosarcoma TE671-TG6 with primary human oligodendrocytes, cultured from surgical specimens, demonstrated an intracellular localization in vesicles and vacuoles with an intricate internal membranous network and to the external surface of the cell by immunogold electron microscopy. The availability of antibodies to one of the components of MBP, i.e., the citrulline containing component ("C-8"), permitted us to localize this component of MBP to intracellular vacuoles and also on the external surface of the MO3-13 cells. Since the apposition of the external surfaces of the oligodendrocyte is responsible for the intraperiod line of the myelin sheath, localization of C-8 to the external surface of non-permeabilized cells by immunogold scanning electron microscopy is consistent with our observations that C-8 is localized to the intraperiod line of myelin (McLaurin et al.: J Neurosci Res 35:618-628, 1993). Western blots of isolated MBP from MO3-13 cells, probed with an antibody reactive with residues 130-137 of MBP, recognized a protein in the 60 kDa range. No immunoreactivity was found in the 18.5 kDa range. This 60 kDa protein also reacted with a monoclonal antibody raised with residues 70-84 of MBP, 2 different polyclonals raised with whole bovine MBP, an antibody to human MBP raised in monkeys, and the anti-citrulline antibody. These data strongly suggested that the 60 kDa protein contained MBP sequences within its primary structure. A similar protein has been isolated from human myelin-containing fractions but not from compact myelin demonstrating that the 60 kDa protein from MO3-13 cells was not an artefact related to fusion. Sequence determination of peptides obtained from enzymic and chemical cleavages revealed that the 60 kDa protein contained MBP sequences and peptides with 55-60% homology with dynamin, a protein involved in intracellular transport. These data suggest that the externalization of MBP in this cell involves transport by fusion of MBP with another protein. By sequestering MBP in a larger protein, the possibility of inducing autoimmune disease by MBP released, due to cell death, is minimized.

Metabolic turnover of myelin glycerophospholipids

The apparent half life for metabolic turnover of glycerophospholipids in the myelin sheath, as determined by measuring the rate of loss of label in a myelin glycerophospholipid following radioactive precursor injection, varies with the radioactive precursor used, age of animal, and time after injection during which metabolic turnover is studied. Experimental strategies for resolving apparent inconsistencies consequent to these variables are discussed. Illustrative data concerning turnover of phosphatidylcholine (PC) in myelin of rat brain are presented. PC of the myelin membrane exhibits heterogeneity with respect to metabolic turnover rates. There are at least two metabolic pools of PC in myelin, one with a half life of the order of days, and another with a half life of the order of weeks. To a significant extent biphasic turnover is due to differential turnover of individual molecular species (which differ in acyl chain composition). The two predominant molecular species of myelin PC turnover at very different rates (16:0, 18:1 PC turning over several times more rapidly than 18:0, 18:1 PC). Therefore, within the same membrane, individual molecular species of a phospholipid class are metabolized at different rates. Possible mechanisms for differential turnover of molecular species are discussed, as are other factors that may contribute to a multiphasic turnover of glycerophospholipids.

Isolation and characterization of axolemma-enriched fractions from rabbit and bovine peripheral nerve

Axolemma-enriched fractions were isolated from bovine spinal accessory nerves, bovine intradural dorsal roots, and rabbit sciatic nerve by differential centrifugation and separation on a linear 10-40% sucrose (w/w) gradient. The fractions were enriched 4 to 10 fold in acetylcholinesterase, a biochemical marker for axolemma. Axolemma-enriched fractions isolated from uniformly well-myelinated fibers (bovine spinal accessory nerve) contained lower CNPase activity and higher acetylcholinesterase activity than comparable fractions isolated from variably myelinated fibers (rabbit sciatic nerve and bovine intradural roots). Separation by polyacrylamide electrophoresis showed that the molecular weight distribution of all peripheral nerve axolemma-enriched fractions was similar and ranged from 20 to over 150 kilodaltons. All axolemma-enriched fractions appeared to contain a small but variable amount of myelin-specific proteins. Based on biochemical properties, peripheral nerves containing uniformly well-myelinated fibers yield an axolemma-enriched fraction which is least contaminated with myelin-related membranes.

Studies on NG-methylarginine derivatives in myelin basic protein from developing and mutant mouse brain

The amounts of NG-methylarginine derivatives in myelin basic protein (MBP) purified from dysmyelinating mutant and different stages of normal myelinating mouse brains have been studied by using h.p.l.c. with a highly sensitive post-column o-phthaldialdehyde derivative-formation method. All three naturally occurring derivatives (NG-monomethylarginine (MeArg), NGN'G-dimethylarginine [Me2(sym)Arg] and NGNG-dimethylarginine [Me2(asym)Arg]) were found in MBP; however, their relative concentrations varied significantly with the age of the animal. The amounts of MeArg and Me2(sym)Arg in MBP increased as a function of the age of the brain, whereas that of Me2(asym)Arg decreased. MBP from early-myelinating mouse brain was shown to contain a high proportion of Me2(asym)Arg, which was hardly detectable in older brain MBP. This derivative, Me2(asym)Arg, was also absent from MBP embedded in the most compact multilamellar myelin, but was present in MBP in the least compact myelin (P3B). Comparing the extent of total methylation in vivo (sum of all three arginine derivatives), MBP extracted from less-compact myelin (P3A and P3B) showed a level approx. 40% higher than that from compact myelin. MBPs isolated from dysmyelinating mutant mouse brains, such as jimpy (jp/y) and quaking (qk/qk), contained a much higher level of Me2(asym)Arg relative to the other two methyl derivatives and also in comparison with those levels in the mother brain MBP. SDS/PAGE analysis of MBPs extracted from the mutant (both jp/y and qk/qk) as well as young normal (6-13 days old) mouse brains indicated the presence of a high-molecular-mass isoform of MBP (about 32 kDa), but this isoform was not found in adult brains. These results therefore indicate that structural integrity of myelin membrane in which MBP is embedded appears to play a pivotal role in determining the extent and the kind of Me2Arg formation in MBP at the post-translational level.

A Rabbit Autoantibody Specific for the 46-kDa Form of 2',3'-Cyclic Nucleotide 3'-Phosphodiesterase

An autoantibody occurring in the serum of an apparently normal rabbit that immunocytochemically stains myelin sheaths and oligodendrocytes in rat brain was shown to react specifically with the 46-kDa isoform of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) (EC 3.1.4.37) in a number of species. Identification of the shorter isoform of the enzyme (CNP1) as the antigen was achieved by comparing the immunostaining of Western blots by the autoantibody with that of a well-characterized anti-CNP antiserum. The 46-kDa antigen reacting with the autoantibody exhibited the same Mr and pI as the small isoform of CNP on two-dimensional gels and showed a similar enrichment in purified CNS myelin. The autoantibody has very high affinity for CNP1 and is capable of detecting the very low amounts of this enzyme in peripheral nerve, spleen, adrenal gland, pancreas, testis, and intestine. Testing the reactivity of the autoantibody with synthetic peptides by enzyme-linked immunosorbent assay revealed that it reacted with the N-acetylated decapeptide corresponding to the N-terminus of CNP1, but did not react if the peptide was not acetylated or if the acetyl group was replaced with a palmityl group. The lack of reactivity with CNP2, which differs from CNP1 by a 20-amino acid extension at the N-terminus of the protein as a result of alternative splicing, may be due to the absence of the N-acetyl moiety that is part of the epitope and/or blocking of antibody binding to the decapeptide by extension of the polypeptide chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuraminidase activities in oligodendroglial cells of the rat brain

Neuraminidase activities in oligodendroglial cells were characterized using rats of different ages. Rat oligodendroglial cells had intrinsic neuraminidase activities directed toward GM3 and N-acetylneuramin(2-3)lactitol (NL). Developmental profiles of the neuraminidase activities toward the two substrates in oligodendroglial cells were different from each other. The neuraminidase activity toward GM3 increased rapidly with the onset of active myelination and, after 26 days of development, reached the adult level which was about 18 times higher than that in myelin. At the adult age, oligodendroglial cells had the highest neuraminidase activity toward GM3 among the individual brain cell types examined. The activity of NL-neuraminidase showed a less remarkable developmental profile, with a peak value at 26 days. The UDP-galactose:ceramide galactosyltransferase activity in oligodendroglial cells increased during the period of active myelination and, afterward, returned to the basal level. The enrichment and unique developmental profile in oligodendroglial cells of the neuraminidase activity toward GM3 suggest that this enzyme may play an important role in the formation and maintenance of the myelin sheath.

Role of myelin-associated neuraminidase in the ganglioside metabolism of rat brain myelin

The role of myelin-associated neuraminidase in ganglioside metabolism was examined using rats of ages ranging from 17 to 97 days. The neuraminidase activity directed toward the ganglioside GM3 in the total myelin fraction was high during the period of active myelination and, thereafter, decreased rapidly to the adult level. The ganglioside composition became simpler during development with an increasing amount of GM1 and decreasing percentages of di- and polysialogangliosides. The decrease in the proportion of GD1a was most prominent, whereas relative amounts of GD1b and GT1b increased transiently before reducing to the adult levels. The heavy myelin subfraction contained higher percentages of di- and polysialo-species compared to the light myelin fraction at young and adult ages. The in vitro incubation of myelin of young rats under an optimal condition for neuraminidase action produced a profile of ganglioside changes similar to that observed in in vivo development. These results strongly suggest that myelin-associated neuraminidase may play a pivotal role in the developmental changes in the ganglioside composition of rat brain myelin.

Adenosine receptors in myelin fractions and subfractions: the effect of the agonist (R)-phenylisopropyladenosine on myelin membrane microviscosity

In this article the existence of A1 adenosine receptors and the absence of A2 adenosine receptors in myelin membranes purified from pig brain white matter are demonstrated. The characterization of (R)-[3H]phenylisopropyladenosine ([3H]R-PIA) binding to purified myelin fractions was performed. The distribution of high- and low-affinity species of the A1 adenosine receptor was different in heavy, medium, and light myelin. The fluidity of myelin subfractions and of pig brain cortical membranes was estimated; the microviscosity of heavy myelin (5.4 poises) and of cortical membranes (5.1 poises) was similar and less than that of medium (7.8 poises) and light (8.2 poises) myelin. It was also demonstrated that the agonist R-PIA modifies the microviscosity of myelin membranes and that the degree of modification depends on the fluidity of the membrane assayed. These results suggest that adenosine receptors may have an important role in the functionality of myelin membranes.

Insulin-like growth factor I stimulates oligodendrocyte development and myelination in rat brain aggregate cultures

Insulin-like growth factor I (IGF-I) and high concentrations of insulin have been shown to stimulate an increase in the number of oligodendrocytes that appear in developing monolayer cultures of rat brain cells (McMorris et al., Proc Natl Acad Sci USA 83: 822-826, 1986; McMorris et al., Ann NY Acad Sci 605:101-109, 1990; McMorris and Dubois-Dalcq, J Neurosci Res 21:199-209, 1988). In the present study, we investigated whether IGF-I or insulin treatment induces a corresponding increase in the synthesis and accumulation of myelin. Aggregate cultures, established from 16-day-old fetal rat brains, were treated with either 100 ng/ml IGF-I or 5,000 ng/ml insulin and analyzed for the number of oligodendrocytes, activity of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), total amount of myelin, and synthesis rate of myelin proteins. Cultures treated with IGF-I beginning on day 2 after explantation contained 35-80% more oligodendrocytes and had 60-160% higher CNP activity than controls when tested on day 13, 20, or 27. By day 27, treated cultures had 35-90% more myelin than controls. Similar results were observed in response to 5,000 ng/ml insulin, a concentration at which insulin binds to IGF receptors and acts as an analogue of IGF-I. The synthesis rate of myelin proteins was measured in experiments using 5,000 ng/ml insulin. When treatment was begun at day 20 rather than day 2, cultures did not exhibit an increased number of oligodendrocytes over control during the following 4-6 days.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro changes in the fine structure and protein composition of light myelin fractions isolated from guinea pig brain

To find out if in vitro maintenance produces changes in the electron microscopic appearance, protein composition and phosphorylation properties of guinea pig CNS myelin fractions, we incubated them for 10 min, 4 hr, 24 hr, and 48 hr in phosphate-buffered saline (pH 7.4) or in 20 mM Hepes, 2 mM EDTA, 0.5 mM EGTA, 0.5 mM dithiothreitol, and 20 mM NaCl at 4 and 30 degree C. Aliquots were processed for electron microscopic study, were analyzed for protein content by gel electrophoresis, and were assayed for endogenous protein phosphorylation. Before incubation, electron micrographs of fractions contained two types of multilamellar whorls with the periodicity of CNS myelin sheaths. The first type of whorl was separated from nearby whorls; the other type had surface lamellae that were connected to other multilayered membrane fragments. After incubation at 4 degree C for 24 hr, the number of both types of multilamellar whorls in micrographs had increased approximately 3- to 4- fold. Counts per unit area showed that the observed increase was both time- and temperature-dependent. In aliquots studied by gel electrophoresis, only minor degradation of myelin proteins was observed. The endogenous protein phosphorylation properties of the myelin fragments also remained functional, suggesting that the activities of protein phosphotransferases were not altered. We conclude that the incubation conditions described here favor interactions of proteins and lipids that lead to the formation of multilayered aggregates of CNS myelin membranes.

Enzymic methylation of myelin basic protein in myelin

Myelin fractions with different degrees of compaction were isolated from bovine brain, and post-translational methylation of membrane-associated proteins was studied. When the purified myelin-basic-protein-specific protein methylase I and S-adenosyl-L-[methyl-14C]methionine were added exogenously, the most compact myelin fraction exhibited higher methyl-accepting activity than the less compact dense fractions. The methylated protein was identified as myelin basic protein (18.4 kDa) exclusively among the several myelin proteins from all membrane fractions, by SDS/PAGE/radioautography of methyl-14C-labelled membrane proteins. The methyl-14C-labelled amino acid residue in the basic protein was identified by h.p.l.c. as NG-methylarginine, indicating the high degree of specificity for the arginine residue as well as the myelin basic protein in the intact myelin membranes. The possibility of a charge alteration of myelin basic protein resulting from its arginine methylation was investigated by using the purified component 1 of myelin basic protein. The methylated component was shown to be less cationic than the unmethylated component by Bio-Rex 70 cation-exchange chromatography, since the former preceded the latter. However, in the presence of the denaturant (guanidinium chloride), the two species were co-eluted, indicating that the charge difference between methylated and unmethylated myelin basic protein can only be shown under the renatured condition.

Lipid profile of rat myelin subfractions

Myelin from adult rat brains was separated on a discontinuous sucrose gradient into three subfractions. Analysis of "light", "heavy" and "membrane fraction" lipid classes was performed by HPTLC and densitometry while fatty acid composition was determinated by GLC. The more interesting results observed are: i) the "membrane fraction" resembles in its lipid and fatty acid composition other cell membranes (particularly oligodentrocytes); ii) "light" and "heavy" myelin are quite similar between them but the former has a higher content of sphingomyelin, a lower hydroxy/nonhydroxy cerebrosides ratio and a lower content of monoenoic fatty acids than the "heavy" subfraction. The results obtained could explain the different structures observed in each myelin subfraction since fatty acid composition, hydroxy fatty acids, sphingomyelin and cholesterol play a key role in the stability and structure of membranes.

Acetyl-CoA carboxylase in rat brain. I. Activities in homogenates and isolated fractions

Acetyl-CoA carboxylase (ACC) catalyzes the rate-limiting and/or first committed step in fatty acid biosynthesis. Because fatty acids must be synthesized as components of the galactolipids and phospholipids in myelin, high specific activities of ACC would be expected in brain during myelination and in the myelinating cells, the oligondendroglia, in particular. Under reaction conditions where ACC was linear with time and protein concentration, we found specific activities of 1.7 and 3.1 nmol/min/mg protein in supernatants from forebrains and brainstems, respectively, of 20-day-old rats. In both regions, ACC declined during development, particularly after the age of 20 days. To separate forebrain into discrete fractions containing cells, membrane vesicles, and other components, without destroying the ACC, it was necessary to modify the published methods by adding citrate to the isolation medium and by omitting trypsin. A fraction which sedimented over 1.2 M sucrose showed the highest specific activities and recoveries of ACC. This fraction was rich in small cells, many of which immunostained with antibodies against galactocerebroside and carbonic anhydrase, both of which are localized in oligodendrocytes and immature glial cells. The cells in this fraction also immunostained with antibodies against ACC. The results are consistent with the hypothesis that ACC is an oligodendrocyte-associated enzyme, although it probably is not exclusive to cells of that type.

Localization of 5'-nucleotidase in bovine brain myelin fraction and myelin subfractions

Purified myelin from fresh calf brain white matter was subfractionated in a discontinuous sucrose gradient; significant recovery of protein and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and 5'-nucleotidase (5'N) activities occurred in all three obtained subfractions, the highest recovery being in the light subfraction; highest 5'N and CNP specific activities were in medium myelin. Purified myelin was also subfractionated in a continuous sucrose gradient, with a similar localization of protein; CNP activity and 5'N activity maxima suggest that myelin may be a predominant locus of 5'N in bovine brain white matter. Freezing of brain white matter caused an increase in protein and in CNP and 5'N total activity recoveries in denser myelin subfractions. Cytochemistry showed the reaction product of 5'N in the whole myelin fraction to be associated with the innermost, outermost and medial compact myelin layers. Effects of non-ionic detergent (LUbrol WX) on 5'N activity were studied, and the results also suggest the intrinsic nature of 5'N as an ectoenzyme in myelin membranes. Lubrol WX was viewed as an advisable detergent for the stimulation of myelin 5'N activity, but not for the solubilization of this enzyme.

Regulation of sterol synthesis and of 3-hydroxy-3-methylglutaryl coenzyme A reductase by lipoproteins in glial cells in primary culture

Although plasma lipoproteins have been demonstrated to have a major role in regulating cholesterol biosynthesis in extraneural cells, no data concerning such regulation are available for developing brain, when cholesterol synthesis is especially active. Glial primary cultures derived from neonatal rat brain and by morphological and biochemical criteria essentially exclusively composed of astrocytes were utilized to examine such regulation. When the primary cultures, which had been maintained in 10% fetal calf serum, were placed in 10% lipoprotein-poor serum on day 7 of culture, an induction of sterol synthesis (1.6-2.2-fold) and of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase-specific activity (1.5-2-fold) resulted after 24 hr. Addition of low-density lipoprotein (LDL) to the 10% lipoprotein-poor serum prevented the induction of both sterol synthesis and HMG-CoA reductase. However, addition of high-density lipoprotein (HDL) to the 10% lipoprotein-poor serum caused a 1.5-2-fold further induction of sterol synthesis relative to that in cultures containing 10% lipoprotein-poor serum alone. In contrast to the glial primary cultures, cultures of C-6 glioma cells responded to replacement of 10% fetal calf serum with 10% lipoprotein-poor serum with much more marked increases of sterol synthesis and HMG-CoA reductase. Although, as with the primary cultures, addition of LDL to the C-6 glioma cell cultures prevented the increases in sterol synthesis and reductase activity, addition of HDL had no effect. Thus, these results indicate that in developing glial cells in primary culture, cholesterol synthesis and HMG-CoA reductase are capable of responsiveness to both LDL and HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin instability and oligodendrocyte metabolism in myelin-deficient mutant mice

During the active phase of myelination in myelin-deficient mutant mice (mld), myelin basic protein (MBP) synthesis is defective and the myelin lamellae are uncompacted. In these mutants, we found a fast metabolism of the myelin-associated glycoprotein (MAG) and of sulfatides, and the presence of cholesterol esters and a degradation product of MAG, dMAG, indicating that mld myelin was unstable. The increased synthesis of MAG and Wolfgram protein, two proteins present in uncompacted myelin sheath and paranodal loops, was demonstrated by high levels of messengers. Simultaneously, we found an accumulation of inclusion bodies, vacuoles, and rough endoplasmic reticulum in mld oligodendrocytes. This material was heavily immunostained for MAG. Furthermore, the developmental change between the two molecular forms of MAG (p72MAG/p67MAG) was delayed in mld mice. In 85-d-old mld mice, the MBP content increased and myelin lamellae became better compacted. In these mutants, dMAG was absent and MAG mRNAs were found in normal amounts. Furthermore, the fine structure of mld oligodendrocytes was normal and the MAG immunostaining was similar to age-matched controls. These results support a functional role for MBP in maintaining the metabolic stability and the compact structure of myelin. Furthermore, in the absence of MBP and myelin compaction, the regulation of the synthesis of at least two membrane proteins related to myelin cannot proceed.

Relation of cellular phospholipid composition to oligodendroglial differentiation in C-6 glial cells

The relation of the polar head group composition of cellular phospholipids to a biochemical expression of oligodendroglial differentiation was studied in cultured C-6 glial cells. Induction of the oligodendroglial enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), was determined after alteration of the polar head group composition of phospholipids by exposure of the cells to choline analogues, especially N,N'-dimethylethanolamine. To accomplish the phospholipid alteration, cells were grown in the presence of the analogue in medium free of exogenous lipid, i.e., first for 24 h in 10% delipidated serum and then for 48 h in serum-free medium. The 48-h exposure to serum-free medium resulted in untreated C-6 cells in a several fold increase in CNP activity, but in cells treated with 2.5 mM N,N'-dimethylethanolamine, total inhibition of this induction was observed. A graded, concentration-dependent inhibitory effect of the analogue on the induction of CNP was defined. The effect of the analogue was relatively specific, e.g., the activity of another plasma membrane enzyme of C-6 cells, (Na+ + K+)-activated ATPase, was not affected. Morever, there was no evidence of a toxic effect of the analogue; thus, total protein synthesis and cell growth were not altered, and the induction of CNP in serum-free medium recurred after removal of the analogue. N,N'-Dimethylethanolamine was shown to be incorporated into cellular phospholipids, primarily at the expense of phosphatidylcholine. The data define an important role for the polar head group composition of membrane phospholipids in oligodendroglial differentiation in this model system.

Glial differentiation in dissociated cell cultures of neonatal rat brain: noncoordinate and density-dependent regulation of oligodendroglial enzymes

The mixed glial system of primary cultures of cells dissociated from neonatal rat brain was utilized to study glial differentiation. The investigation was addressed specifically to the possibility of noncoordinate regulation of two manifestations of oligodendroglial differentiation, i.e., activities of glycerol-3-phosphate dehydrogenase (GPDH) and of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), as well as the effects of initial cell density on the time of onset and the intensity of expression of these aspects of oligodendroglial differentiation. Simultaneously, glutamine synthetase activity was studied to determine effects on astrocytic differentiation. GPDH exhibited a major developmental increase in specific activity between 20 and 32 days in culture. However, CNP activity exhibited a major developmental increase that commenced approximately 2 weeks earlier. The onset of these expressions of oligodendroglial differentiation was not affected by such environmental factors as initial cell density. However, the intensity of expression of the temporally separate increases in GPDH and CNP activities was markedly density-dependent. The highest activities were attained in cultures plated at the lowest cell densities. The astrocytic enzyme, glutamine synthetase, also exhibited a striking developmental increase (approximately tenfold between 13 and 30 days in culture), but initial cell density affected neither the time of onset nor the intensity of expression of this aspect of astrocytic differentiation. The data demonstrate a striking developmental increase in GPDH activity that is not coordinate with that in CNP. The noncoordinate manifestations of oligodendroglial differentiation commence as a function of time in culture, whereas the intensity of expression of this differentiation can be influenced by such environmental factors as initial cell density.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of myelin fractions from human brain white matter

Myelin and myelin-containing (P3) fractions were prepared from human white matter by discontinuous sucrose gradient centrifugation. The myelin isolated from each of the fractions of different densities was morphologically and biochemically distinct. Light myelin fractions consisted of compact, multilamellar myelin, whereas the denser fractions consisted predominantly of loose myelin with fewer lamellae. The amounts of both basic protein and lipophilin (proteolipid protein) were reduced in the denser fractions. In contrast, the high-molecular-weight components were elevated in the dense fractions. The lipid composition was similar in all the fractions studied. Analysis of basic protein by gel electrophoresis at pH 10.6 revealed differences in basic protein microheterogeneity among the fractions. The light myelin fraction was enriched in the more positively charged basic protein components (components 1, 2, and 3), whereas these components were reduced in the denser fractions. Myelin in the dense fractions was enriched in the more modified forms of basic protein (components 6, 7, and 8). The pattern of microheterogeneity was different for basic protein isolated from myelins of a 2-year-old and an adult brain; the former showed fewer components and mainly the most cationic species. On the other hand, the pattern of microheterogeneity of basic protein isolated from the different density gradient fractions was similar for both ages.

Early adrenalectomy increases myelin content of the rat brain

Rats were adrenalectomized (ADX) or sham-operated (SHAM) on the 11th day of life and killed on days 35-36, 63, or 151-153 for the isolation of cerebral myelin from each animal. Despite having lower overall body weights, ADX rats had heavier cerebra than SHAM control rats at all ages. Mean cerebral weight increases were 10.0% at day 35-36, 15.3% at day 63, and 16.7% at day 151-153. Recovered myelin dry weights were even more elevated in the ADX rats, but only at day 63 (41.7% increase) and 151-153 (42.1% increase). At both of these ages, there was a clear linear relationship between cerebral wet weights and the amount of myelin recovered from the cerebra. Analysis of the day-63 myelin samples showed no group differences in total cholesterol or protein concentration or in the specific activity of the myelin marker enzyme 2':3'-cyclic-nucleotide 3'-phosphodiesterase (CNP). However, myelin isolated from the ADX rats appeared to be deficient in both galactolipid and phospholipid. Optic nerve myelination was assessed in all animals by measuring CNP activity in homogenates prepared from this tissue. No difference between ADX and SHAM rats was observed at any age. These results indicate that early adrenalectomy stimulates myelin deposition in the rat brain as part of a more general, long-lasting enhancement of brain growth. Myelin from the brains of ADX animals may be slightly abnormal in its lipid composition. Finally, the optic nerve data may mean that myelination is not affected equally in all areas of the CNS by the loss of adrenal glands.

Myelination process in the rat sciatic nerve during regeneration and development: molecular species composition and acyl group biosynthesis of choline-, ethanolamine-, and serine-glycerophospholipids of myelin fractions

The content of alkenyl-acyl, alkyl-acyl and diacyl types of the three major myelin glycerophospholipids such as PtdCho, PtdEtn and PtdSer was determined in myelin fractions prepared from sciatic nerve segments of rats at 12, 25 and 45 days after birth, and of adult rats (6-month-old) 90 days after crush injury. The biosynthesis and metabolic heterogeneity of lipid classes and types were also studied by incubation with [1-14C] acetate of nerve segments of young rats at different ages as well as crushed and sham-operated control nerve segments of adult rats. The analysis of composition and positional distribution in major individual molecular species extracted from light myelin and myelin-related fraction suggest that the metabolism of alkenyl-acyl-glycerophosphorylethanolamines and unsaturated species of PtdCho and PtdSer may not be regulated in the same manner during peripheral nerve myelination of developing rat and remyelination of regenerating nerve in the adult animal. The 14C-radioactivity incorporation into lipid classes and alkyl and acyl moieties of the three major phospholipids of sciatic nerve segments during the developmental period investigated revealed that Schwann cells were capable of synthesizing acyl-linked fatty acids in both myelin fractions at a decreasing rate and with different patterns during development. In regenerating sciatic nerve of adult animals the labeling of myelin lipid classes and types of remyelinating nerve segment distal to the crush site was markedly higher than that of sham-operated normal one; however, the magnitude and the pattern of the specific radioactivity never approached those observed during active myelination of the nerve in young animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical abnormalities in spinal cord myelin and CNS homogenates in heterozygotes affected by the shiverer mutation

Myelin was purified from the spinal cords of normal mice and mice heterozygous for the shiverer mutation, and measurements were made of the major myelin proteins and lipids and the specific activities of three myelin-associated enzymes. The myelin purified from the spinal cords of the heterozygotes (shi/+) was deficient by 30-40% in yield and had an apparently unique composition. In particular, when compared with normal mouse spinal cord myelin, there were more high-molecular-weight protein, less myelin basic protein, a higher protein-to-lipid ratio, and higher specific activities of 2',3'-cyclic nucleotide-3'-phosphohydrolase (EC 3.1.4.37) and carbonic anhydrase (EC 4.2.1.1) in the myelin purified from the shi/+ animals. These abnormalities were reflected in the composition of shi/+ whole spinal cord, where the protein-to-lipid ratio was intermediate between the respective values for +/+ and shi/shi spinal cords. Whole brains from shi/+ mice showed deficiencies in galactocerebroside and galactocerebroside sulfate and an increase in total phospholipid, and the lipid composition in the brains of the shi/shi mice was similar to that reported for another dysmyelinating mutant, quaking. The findings provide the first values for the lipids in normal mouse spinal cord myelin and show that heterozygotes are affected by the shiverer mutation. The observations imply that there can be considerable deviation from the normal CNS myelin content and composition without apparent qualitative morphological abnormalities or loss of function and that the amount of myelin basic protein available during myelination may influence the incorporation of other constituents into the myelin membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoskeletal structures and oligodendroglial differentiation in C-6 glial cells

The relationship of the cytoskeleton to a biochemical expression of oligodendroglial differentiation was studied in cultured C-6 glial cells. Specifically, we investigated the effect of the cytoskeletal perturbants, colchicine and cytochalasin D, on the induction of the oligodendroglial marker enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), caused by removal of serum from the culture medium. Each drug inhibited CNP induction in a concentration-dependent manner, and essentially complete inhibition of induction was observed with 0.25 microM colchicine or 2.0 microM cytochalasin D. Detailed study of the effect of colchicine was carried out. This antimicrotubular agent not only totally prevented induction if added at the onset of serum removal, but also prevented further induction when added at various times after serum removal. That the effect of colchicine related to the drug's effect on microtubules was supported by the demonstration that lumicolchicine, a colchicine isomer which has no effect on microtubules, had no effect on the CNP induction. Moreover, colchicine, but not lumicolchicine, prevented the morphological signs of differentiation provoked by serum removal. The effect of colchicine was reversible and relatively specific. Thus, no concomitant effect of colchicine on the activity of another plasma membrane enzyme of C-6 cells, i.e., (Na+ + K+)-activated ATPase, or on the rate of incorporation of [3H]leucine into total protein of intact cells could be discerned. The possibility that the site of the effect of colchicine is on intracellular events was suggested by the observation that the drug inhibited the induction of CNP by dibutyryl cyclic AMP. The data suggest that the cytoskeleton is involved in oligodendroglial differentiation.

Myelination in the CNS of mld mutant mice: comparison between composition and structure

Myelination was studied between 15 and 135 days postnatally in the brain and optic nerves of myelin deficient (mld) mutant mice. Between 15 and 30 days almost no myelin basic protein (MBP) could be detected in mld myelin. The axons were loosely wrapped by membranes which only fused at the extracellular sites forming the intraperiod line. At this age the major dense line was absent. At 25-30 days, purified myelin contained extremely high 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) (EC 3.1.4.37) activities which could be related to the redundant paranodal-like structures observed at this age in mld CNS. Therefore, it can be suggested that CNP is probably localized in such paranodal loops. After the active phase of myelin deposition was completed in controls, mld mutants showed important increases of MBP concentration in myelin with the concomitant appearance of the major electron dense line and better compaction of the myelin lamellae. The yield of myelin increased from 5 to 14% of control values during the period of 30 to 135 days. Since the recovery phase occurred at the time when myelin lipid synthesizing enzymes are at low residual activities, the myelin deficit could only be partially corrected. This study indicates that there is a delay of MBP synthesis in mld mice and the decrease of other myelin proteins could be secondary to the assumed primary defect involving MBP.

Myelin-associated glycoprotein (MAG) distribution in human central nervous tissue studied immunocytochemically with monoclonal antibody

Recent biochemical data show that myelin-associated glycoprotein (MAG) is the antigen for a monoclonal antibody found in sera of patients with IgM paraproteinemia and neuropathy (Braun et al. 1982). Immunoreactivity of this antibody with CNS has not been described. To study this, monoclonal anti-MAG was used in the avidin-biotin-peroxidase complex method (Hsu et al. 1981) to immunostain paraffin and epon sections of human CNS. Well characterized polyclonal MAG antiserum (Quarles et al. 1981) was employed in comparison tests. In paraffin sections of developing CNS, both monoclonal and polyclonal MAG antisera stained oligodendroglia and myelin. In adult CNS, periaxonal regions of myelin sheaths were immunostained in paraffin sections and semithin epon sections treated with monoclonal and polyclonal anti-MAG. In electron-microscopic experiments that included milder pretreatment of epon thin sections and more precise reaction product localization, entire thickness of myelin sheaths were immunostained. Thus, in electron micrographs, monoclonal and polyclonal anti-MAG immunoreactivity also have the same localization. In other electron-microscopic experiments, the same reaction product localization was observed with antiserum to myelin basic protein (MBP), a known constituent of compact myelin. Thus, results with this monoclonal anti-MAG provide important new evidence to support the localization of MAG in compact CNS myelin. Our data also suggest that monoclonal antibodies against MAG will be useful in studies of the pathogenesis of multiple sclerosis and other demyelinating diseases.

Comparison of the major proteins of shark myelin with the proteins of higher vertebrates

On gel electrophoresis in dodecyl sulphate solutions shark CNS myelin showed four bands close in mobility to the proteolipid protein of bovine CNS myelin. They had apparent molecular weights of 21,000, 26,000, 27,000, and 31,500. Unlike bovine proteolipid protein, all of these shark proteins were shown to be glycosylated by staining gels with the periodate-Schiff reagent. Amino acid analyses of the polypeptides eluted from polyacrylamide gels indicated a high content of apolar amino acids and a composition approximating that of the Po protein of bovine peripheral nervous system (PNS) myelin, rather than that of the CNS proteolipid protein. The shark polypeptide of apparent molecular weight 31,500 was obtained by elution from dodecyl sulphate gels and antibodies raised against it in rabbits. By probing of electroblots with this antiserum the four shark CNS bands were shown to share common determinants with each other, with a major shark PNS protein and with sheep and chicken major PNS glycoproteins (Po). The binding of antibody was unaffected by deglycosylation of the shark CNS polypeptides with anhydrous hydrogen fluoride. Together, these results appeared to establish that shark CNS myelin contains four proteins that are closely related to a major shark PNS protein and to the Po protein of higher species.

Intracellular localization of 2',3'-cyclic nucleotide 3'-phosphohydrolase in rat oligodendrocytes and C6 glioma cells, and effect of cell maturation and enzyme induction on localization

We investigated whether the membrane-associated myelin enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP; EC 3.1.4.37), is localized primarily inside the cell or exposed on the cell surface of rat oligodendrocytes and rat C6 glioma cells. Determinations were made by enzyme assays of intact, viable cells vs cells broken by freezing and thawing. Assay of both oligodendrocytes and C6 cells showed that the great majority of the CNP activity was localized inside the cells. Oligodendrocytes were also tested by immunofluorescence staining of unfixed, living cells whose membranes had been made permeable to antibody by fixation. Fixed oligodendrocytes showed intense fluorescence when incubated with rabbit anti-CNP antiserum and fluorescein-conjugated second antibody whereas unfixed cells were not stained. We then tested the possible influence on CNP localization of 3 conditions known to increase CNP specific activity: maturation of oligodendrocytes in vitro during a period when CNP specific activity increases 8-fold or more; growth of C6 cultures to high cell density; and induction of CNP activity in oligodendrocytes and C6 cells by dibutyryl cyclic AMP. Under all conditions, most CNP activity was intracellular. These results show that both the catalytic and major antigenic sites of CNP are localized primarily inside the cell, and suggest an intracellular role for CNP in oligodendrocytes. The results with C6 cells also show that these cells resemble oligodendrocytes with respect to CNP localization.

Myelin-associated glycoprotein: electron microscopic immunocytochemical localization in compact developing and adult central nervous system myelin

Light microscopic immunocytochemical studies have shown that myelin-associated glycoprotein (MAG) is localized in myelin of the developing CNS; but in the adult, MAG appears to be restricted to periaxonal regions of myelinated fibers. To extend these observations, we embedded optic nerves of 15-day-old rats, adult rats, and an adult human in epon after aldehyde and osmium tetroxide fixation. After 5% H2O2 pretreatment, thin sections were immunostained with 1:250-1:5,000 rabbit antiserum to rat CNS MAG according to the avidin-biotin-peroxidase complex (ABC) method. Dense deposits of reaction product covered compact myelin in both developing and adult optic nerves. When we used 1:500, 1:1,000, and 1:2,000 anti-MAG, less intense immunostaining of myelin was found. We also obtained the same localization in compact myelin with the peroxidase-antiperoxidase (PAP) method. With 1:250 anti-MAG, dense deposits of reaction product were not observed on axolemmal membranes or on oligodendroglial membranes located periaxonally and paranodally. In thin sections of adult human optic nerve, anti-MAG also stained compact myelin intensely. When thin sections of rat and human optic nerves were treated with preimmune or absorbed serum, no immunostaining was observed. Immunoblot tests showed that our MAG antisera did not react with any non-MAG myelin proteins. In contrast with earlier light microscopic data, this study shows that MAG localization does not change during CNS development; both developing and adult compact myelin sheaths contain MAG. As many biochemical studies also show that MAG is present in compact myelin, we suggest that this 100,000 dalton glycoprotein now be called myelin glycoprotein (MGP) instead of MAG.

Identification of an axolemma-enriched fraction from peripheral nerve

A method has been devised for the fractionation of whole peripheral nerve. The procedure utilizes differential centrifugation and separation on a linear sucrose gradient (10-40%, wt/wt). A membrane fraction localized between 26% and 29% sucrose was not only enriched for the plasma membrane markers, 5'-nucleotidase and acetylcholinesterase (AChE), but also possessed the highest binding of [3H]saxitoxin, a specific marker for sodium channels. Neurons in the lumbar dorsal roots and ventral horns of rats were injected with [3H]fucose to label glycoproteins associated with the axolemma from sciatic nerve. Fractionation of the labeled nerves demonstrated a coincidence in the distribution of [3H]fucose-labeled material and AChE activity in the sucrose density gradient. The increase in the specific activity of marker enzymes for plasma membrane, sodium channels, and labeled membrane, previously demonstrated to be of axolemmal origin, identified the 26-29% region of the sucrose gradient as enriched for axolemma derived from peripheral nerve.

Phosphate groups modifying myelin basic proteins are metabolically labile; methyl groups are stable

Young and adult rats received intracranial injections of [33P]orthophosphoric acid. The time course of the appearance and decay of the radioactive label on basic proteins in isolated myelin was followed for 1 mo. Incorporation was maximal by 1 h, followed by a decay phase with a half-life of approximately 2 wk. However, radioactivity in the acid-soluble precursor pool (which always constituted at least half of the total radioactivity) decayed with a similar half-life, suggesting that the true turnover time of basic protein phosphates might be masked by continued exchange with a long-lived radioactive precursor pool. Calculations based on the rate of incorporation were made to more closely determine the true turnover time; it was found that most of the phosphate groups of basic protein turned over in a matter of minutes. Incorporation was independent of the rate of myelin synthesis but was proportional to the amount of myelin present. Experiments in which myelin was subfractionated to yield fractions differing in degree of compaction suggested that even the basic protein phosphate groups of primarily compacted myelin participated in this rapid exchange. Similar studies were carried out on the metabolism of radioactive amino acids incorporated into the peptide backbone of myelin basic proteins. The metabolism of the methyl groups of methylarginines also was monitored using [methyl-3H]methionine as a precursor. In contrast to the basic protein phosphate groups, both the peptide backbone and the modifying methyl groups had a metabolic half-life of months, which cannot be accounted for by reutilization from a pool of soluble precursor. The demonstration that the phosphate groups of myelin basic protein turn over rapidly suggests that, in contrast to the static morphological picture, basic proteins may be readily accessible to cytoplasm in vivo.

Cyclic AMP induction of the myelin enzyme 2',3'-cyclic nucleotide 3'-phosphohydrolase in rat oligodendrocytes

Cyclic AMP (cAMP) is known to induce the activity of the myelin enzyme 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP; EC 3.1.4.37) in C6 rat glioma cells. This report shows that CNP is also inducible in oligodendrocytes explanted from 1-day-old rat cerebrum and grown in tissue culture. Induction was observed after a 1-day treatment with 1 mM N6, O2-dibutyryl cyclic AMP (dbcAMP) and was maximal after 5 days, reaching 200-240% of control. Induction was observed both in mixed cerebral cell cultures containing oligodendrocytes and astrocytes, and in purified cultures of oligodendrocytes prepared by a differential shakeoff procedure. Addition of dbcAMP to the cultures 3-9 days after the cells were explanted from rat brain induced CNP activity, but no induction was observed when dbcAMP treatment was begun 13 or more days after explanation. These results demonstrate that one component of myelin, CNP, is inducible in oligodendrocytes by a cAMP-mediated mechanism, and suggest a role for cAMP in the regulation of the myelin-associated functions of oligodendrocytes.

Relation of cholesterol to oligodendroglial differentiation in C-6 glial cells

The relation of cellular cholesterol content of a biochemical expression of oligodendroglial differentiation was studied in cultured C-6 glial cells. Induction of the oligodendroglial marker enzyme 2':3'-cyclic nucleotide 3'-phosphohydrolase (CNP) was determined after alteration of the sterol content of cellular membranes by exposure to compactin, a specific inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol synthesis. The sterol content and, as a consequence, the sterol/phospholipid molar ratio of C-6 glial cells were decreased by treating the cells, in 10% lipoprotein-poor serum, with various concentrations of compactin for 24 h. The degrees of sterol depletion thus produced were maintained for 48 h after removal of the compactin if the cells were maintained in serum-free medium, the culture conditions necessary for induction of CNF in untreated cells. Forty-eight hours after removal of serum, no induction of CNP occurred in cells previously treated with 0.5 micrograms/ml of compactin, whereas untreated cells exhibited a three- to fourfold increase in CNP activity. Intermediate degree of sterol depletion resulted in intermediate degrees of inhibition of the CNP induction. Moreover, the morphological expressions of glial differentiation observed in the untreated cells did not occur in the sterol-depleted cells. That the effect of compactin on the induction of CNP relates to depletion of sterol was indicated by the finding that when low-density lipoprotein was added to the compactin-treated cells, the induction of CNP, the morphological expressions of differentiation, and the sterol/phospholipid molar ratios were preserved. The degree of sterol depletion that totally prevented the induction of CNP had no effect on (Na+ R K+)-activated ATPase activity, total protein synthesis, and cell viability. The data define a critical role for sterol in oligodendroglial differentiation in this model system.

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.

Myelin subfractions isolated from mouse brain: analysis of the lipid composition at three developmental stages

Lipids were examined in whole myelin and 8 myelin subfractions isolated from mouse brain at 18-24, 44-48 and 80-90 days of age. Relative to protein, total lipid was lowest in whole myelin isolated from the oldest animals as well as from subfractions isolated at greater sucrose densities, thus partially accounting for the observed myelin subfraction distribution pattern which shifted during development and an average peak density between 0.55 and 0.65 M sucrose to one banding between 0.60 and 0.70 M sucrose. Whole myelin and each myelin subfraction isolated at one age contained nearly the same ratio of sterol and phospholipid to galactolipid; these ratios decreased uniformly during development suggesting enrichment with galactolipid in all myelin subfractions. Sulfatide, as percentage of total galactolipid, was relatively constant during development and appeared to be slightly enriched in the denser myelin subfractions. The findings suggest that regardless of the origin(s) of the subfractions, an age-related mechanism exists in the central nervous system which modified myelin lipid composition relatively uniformly.

Myelin proteins, glycoproteins, and myelin-related enzymes in experimental demyelination of the rabbit optic nerve: sequence of events

Wallerian degeneration of the rabbit optic nerve was investigated by the technique of retinal ablation which precludes edema, hemorrhage, or macrophage infiltration. After 8 days of degeneration, marked degradation of axons and some myelin abnormalities appeared in the optic nerve, optic chiasma, and optic tract. Myelin lesions were maximal 32 days after retinal destruction. The amount of material stained with a myelin dye decreased drastically between 32 and 90 days after the operation. Biochemical parameters gave the following sequence of events. The concentration of the major periodic acid--Schiff staining glycoproteins was decreased after 2 days, and 6 days later the presence of cholesterol esters was detected in the optic tissue. After 16 days of Wallerian degeneration, the specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase not associated with myelin decreased, indicating a possible de-differentiation of oligodendrocytes. Degradation of myelin basic protein became significant at 32 days and the amount of myelin isolated decreased later. The loss of myelin basic protein coincided with a reduction of myelin periodicity as measured in purified fractions by electron microscopy. These results show that secondary myelin destruction in the absence of edema, hemorrhage, or macrophages is a very slow process, and in this situation myelin undergoes a selective and sequential loss of its constituents.

Rat brain 5'-nucleotidase: developmental changes in myelin and activities in subcellular fractions and myelin subfractions

The activities of 5'-nucleotidase, measured in brain homogenates and myelin isolated from rats at 21, 60 and greater than 90 days of age, were compared to values for two other myelin-associated enzymes, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and carbonic anhydrase. Whereas the activities of all 3 enzymes were higher in brain homogenates from 60-day-old rats than in those from 21-day-old rats, only 5'-nucleotidase increased significantly in specific activity in both homogenates and myelin after the age of 60 days. The ratios of 5'-nucleotidase to the myelin basic and proteolipid proteins in subcellular fractions from adult rat brain suggested that the microsomal fraction was the only fraction containing 5'-nucleotidase levels not attributable to contamination by myelin membranes. Like carbonic anhydrase, 5'-nucleotidase had a greater distribution than CNP into microsomes of adult rats. When purified myelin was fractionated on a density gradient, the specific activity of 5'-nucleotidase was highest in the heaviest subfraction, with recovery of significant activity occurring, however, in all 3 subfractions. In rats over 60 days of age the recovery of 5'-nucleotidase in myelin was almost as high as that of the relatively myelin-specific enzyme CNP, suggesting that myelin may be the predominant, although not exclusive locus of 5'-nucleotidase in the adult rat brain.

Demonstration of five major glycoproteins in myelin and myelin subfractions

Myelin was found to contain five major glycoproteins with molecular weights of 120000, 95000, 88000, 43000 and 38000. Light myelin contained only 5-7% of the amount of these glycoproteins in whole myelin, whereas heavy myelin and the membrane fraction contained amounts nearly identical with whole myelin. Since all the major and minor glycoproteins, with the exception of 120000-mol-wt. glycoprotein, were detected only after treating the myelin membrane with neuraminidase, N-acetylneuraminic acid is a terminal sugar residue in these glycoproteins.

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.

Myelin gangliosides: an unusual pattern in the avian central nervous system

Gangliosides were isolated from purified myelin obtained from brain and spinal cord of mature chickens and pigeons. Total concentrations were approximately two- to fivefold greater than for previously reported mammalian species, and their patterns also differed in containing significantly more sialosylgalactosylceramide (GM4). The latter comprised one-third to one-fourth of total myelin ganglioside, approximately equivalent to GM1 (II3NeuNAcGgOse4Cer). As in mammals, GM4 of avian CNS appeared to be localized in myelin. Fatty acids of this ganglioside included both the hydroxy- and unsubstituted types, and long-chain bases were almost entirely C18. Ganglioside GM1 split into two closely migrating bands on TLC, the slower of which resembled mammalian GM1 in having stearate as the main fatty acid with a measurable amount (10%) of C20-sphingosine; the faster band had predominantly longer-chain fatty acids and very little C20-sphingosine.

The protein antigens of peripheral nerve myelin

Peripheral nervous system (PNS) tissue contains a variety of proteins, lipids, and carbohydrates that may serve as immunogens in proving immune responses, as antigens participating in immunological reactions, or as both types of agents. Three proteins P0, P1, and P2, account for approximately 70% of PNS myelin proteins. P0 is the major PNS myelin protein and is restricted to the PNS. P1 is similar to, if not identical with, myelin basic protein, the component of central nervous system myelin which induces experimental allergic encephalomyelitis. P2 has neuritogenic properties for inducing experimental allergic neuritis and may be involved in immune-mediated PNS myelin injury in humans. The complete amino acid sequence for P2 has recently been delineated, and its neuritogenic, immunogenic, and antigenic features can now be further characterized.

Evidence for the association of 2',3'-cyclic-nucleotide 3'-phosphodiesterase with myelin-related membranes in peripheral nervous system

In PNS, the specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in myelin was not enriched over the starting homogenate. Nevertheless, most of the total activity was recovered in myelin. In myelin-deficient mutants, low CNP activities were measured in sciatic nerves. CNP specific activities were similar in myelinated and non-myelinated nerves but in non-nervous tissues, they were significantly lower than in nervous tissue. There was no indication for the presence of an isoenzyme of CNP in peripheral nerves. These results indicate that CNP is present in PNS myelin and preferentially localized in Schwann cell plasma membranes.

Studies on the Wolfgram high molecular weight CNS myelin proteins: relationship to 2',3'-cyclic nucleotide 3'-phosphodiesterase

Evidence is presented that the major protein components of the high molecular weight CNS myelin proteins designated as the Wolfgram protein doublet (W1 and W2) contain the enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37, CNP). CNP is a basic hydrophobic protein containing about 830 to 840 amino acid residues. When electrophoresed on SDS polyacrylamide gels, CNP appears as a protein doublet, separated by a molecular weight difference of about 2500-3000 in bovine, human, rat, guinea pig, and rabbit. A similar protein doublet has been identified as the Wolfgram proteins W2 and W1 in myelin and in the chloroform-methanol-insoluble pellet obtained from myelin. Moreover, the relative Coomassie blue staining intensity of the CNP2 plus CNP1 protein doublet among the species examined was remarkably similar to that observed for electrophoresed myelin and chloroform-methanol-insoluble pellet derived from myelin. Antisera raised against purified bovine CNP recognized the W1 and W2 proteins isolated from bovine and human brain. The amino acid composition of pure bovine CNP is presented and compared with the compositions of several rat and bovine Wolfgram proteins obtained by other investigators. Our electrophoretic, compositional, and immunological data support the contention that the enzyme CNP is a major component of the Wolfgram protein doublet.