Metabolic relationships between myelin subfractions: entry of galactolipids and phospholipids

Nur7 is a nonsense mutation in the mouse aspartoacylase gene that causes spongy degeneration of the CNS

Aspartoacylase (ASPA) is an oligodendrocyte-restricted enzyme that catalyzes the hydrolysis of neuronally derived N-acetylaspartate (NAA) to acetate and aspartic acid. ASPA deficiency leads to the fatal childhood autosomal recessive leukodystrophy Canavan disease (CD). Here we demonstrate that the previously described ENU-induced nur7 mouse mutant is caused by a nonsense mutation, Q193X, in the Aspa gene (Aspa(nur7)). Homozygous Aspa(nur7nur7) mice do not express detectable Aspa protein and display an early-onset spongy degeneration of CNS myelin with increased NAA levels similar to that observed in CD patients. In addition, CNS regions rich in neuronal cell bodies also display vacuolization. Interestingly, distinct myelin rich areas, such as the corpus callosum, optic nerve, and spinal cord white matter appear normal in Aspa(nur7/nur7) mice. Reduced cerebroside synthesis has been demonstrated in CD patients and animal models. To determine the potential relevance of this observation in disease pathogenesis, we generated Aspa(nur7/nur7) mice that were heterozygous for a null allele of the gene that encodes the enzyme UDP-galactose:ceramide galactosyltransferase (Cgt), which is responsible for catalyzing the synthesis of the abundant myelin galactolipids. Despite reduced amounts of cerebrosides, the Aspa(nur7/nur7);Cgt(+/-) mice were not more severely affected than the Aspa(nur7) mutants, suggesting that diminished cerebroside synthesis is not a major contributing factor in disease pathogenesis. Furthermore, we found that myelin degeneration leads to significant axonal loss in the cerebellum of older Aspa(nur7) mutants. This finding suggests that axonal pathology caused by CNS myelin defects may underlie the neurological disabilities that CD patients develop at late stages of the disease.

The myelin-axolemmal complex: biochemical dissection and the role of galactosphingolipids

Myelin-axolemmal interactions regulate many cellular and molecular events, including gene expression, oligodendrocyte survival and ion channel clustering. Here we report the biochemical fractionation and enrichment of distinct subcellular domains from myelinated nerve fibers. Using antibodies against proteins found in compact myelin, non-compact myelin and axolemma, we show that a rigorous procedure designed to purify myelin also results in the isolation of the myelin-axolemmal complex, a high-affinity protein complex consisting of axonal and oligodendroglial components. Further, the isolation of distinct subcellular domains from galactolipid-deficient mice with disrupted axoglial junctions is altered in a manner consistent with the delocalization of axolemmal proteins observed in these animals. These results suggest a paradigm for identification of proteins involved in neuroglial signaling.

Normal metabolism but different physical properties of myelin from mice deficient in proteolipid protein

Proteolipid protein (PLP) is the primary protein component of CNS myelin, yet myelin from the PLP(null) mouse has only minor ultrastructural abnormalities. Might compensation for a potentially unstable structure involve increased myelin synthesis and turnover? This was not the case; neither accumulation nor in vivo synthesis rates for the myelin-specific lipid cerebroside was altered in PLP(null) mice relative to wild-type (wt) animals. However, the yield of myelin from PLP(null) mice, assayed as levels of cerebroside, was only about 55% of wt control levels. Loss of myelin occurred during initial centrifugation of brain homogenate at 20,000g for 20 min, which is sufficient to sediment almost all myelin from wt mice. Cerebroside-containing fragments from PLP(null) mice remaining in the supernatant could be sedimented by more stringent centrifugation, 100,000g for 60 min. Both the rapidly and the more slowly sedimenting cerebroside-containing membranes banded at the 0.85/0.32 M sucrose interface of a density gradient, as did myelin from wt mice. These results suggest at least some myelin from PLP(null) mice differs from wt myelin with respect to physical stability (fragmented into smaller particles during dispersion) and/or density. Alternatively, slowly sedimenting cerebroside-containing particles could be myelin precursor membranes that, lacking PLP, were retarded in their processing toward mature myelin and thus differ from mature myelin in physical properties. If this is so, recently synthesized cerebroside should be preferentially found in these "slower-sedimenting" myelin precursor fragments. Metabolic tracer experiments showed this was not the case. We conclude that PLP(null) myelin is physically less stable and/or less dense than wt myelin.

Turnover of myelin lipids in aging brain

Turnover rates of myelin membrane components in mouse brains were determined by a method using stable isotope-labeling and mass spectrometry. The half-replacement times based on incorporation rates of newly synthesized molecules for young adult mice were 359 days for cholesterol, 20 days for phosphatidylcholine, 25 days for phosphatidylethanolamine, 94 days for cerebroside and 102 days for ganglioside GM1. The turnover rates of half-lives of myelin components were calculated from the decay curves of initially labeled molecules, and they were about the same as the half-replacement times. Individual components were thus revealed to be metabolized at different rates, and their turnover rates were differently affected by aging. As was observed with phospholipids, myelin pools appeared to be compartmentalized into rapidly and slowly exchanging pools. The turnover rates of cerebroside and GM1 decreased between the young and adult periods and slightly increased in senescence. The latter phenomenon may indicate an enhanced myelin turnover in senescence. The present study reveals the dynamic aspects of myelin membrane turnover during the life span of mouse.

Cerebroside synthesis as a measure of the rate of remyelination following cuprizone-induced demyelination in brain

We studied markers of myelin content and of the rate of myelination in brains of mice between 8 and 20 weeks of age. During the 12-week time-course, control animals showed slight increases in the content of oligodendroglial-specific cerebroside, as well as cholesterol (enriched in, but not specific to, myelin). In contrast, synthesis of these lipids, as assayed by in vivo incorporation of (3)H(2)O, was substantial, indicating turnover of 0.4% and 0.7% of total brain cerebroside and cholesterol, respectively, each day. We also studied mice exposed to a diet containing 0.2% of the copper chelator, cuprizone. After 6 weeks 20%, and by 12 weeks, over 30% of brain cerebroside was gone. Demyelination was accompanied by down-regulation of mRNA expression for enzymes controlling myelin lipid synthesis (ceramide galactosyl transferase for cerebroside; hydroxymethylglutaryl-CoA reductase for cholesterol), and for myelin basic protein. Synthesis of myelin lipids was also greatly depressed. The 20% cerebroside deficit consequent to 6 weeks of cuprizone exposure was restored 6 weeks after return to a control diet. During remyelination, expression of myelin-related mRNA species, as well as cerebroside and cholesterol synthesis were restored to normal. However, in contrast to the steady state metabolic turnover in the control situation, all the cerebroside and cholesterol made were accumulated. To the extent that accumulating cerebroside is targeted for eventual inclusion in myelin (discussed) the rate of its synthesis is proportional to remyelination. With our assay, in vivo rates of cerebroside synthesis can be determined for a time window of the order of hours. This offers greater temporal resolution and accuracy relative to classical methods assaying accumulation of myelin components at time intervals of several days. We propose this experimental design, and the reproducible cuprizone model, as appropriate for studies of how to promote remyelination.

Parameters related to lipid metabolism as markers of myelination in mouse brain

Myelination, during both normal development and with respect to disorders of myelination, is commonly studied by morphological and/or biochemical techniques that assay as their end-points the extent of myelination. The rate of myelination is potentially a more useful parameter, but it is difficult and time-consuming to establish, requiring a complete developmental study with labor-intensive methodology. We report herein development of methodology to assay the absolute rate of myelination at any desired time during development. This involves intraperitoneal injection of (3)H(2)O to label body water pools, followed by determination of label in the myelin-specific lipid, cerebroside. The absolute amount of cerebroside synthesized can then be calculated from the specific radioactivity of body water and knowledge of the number of hydrogens from water incorporated into cerebroside. During development, the rate of cerebroside synthesis correlated well with the rate of accumulation of the myelin-specific components, myelin basic protein and cerebroside. For purposes of control, we also tested other putative, albeit less quantitative, indices of the rate of myelination. Levels of mRNA for ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis) and for myelin basic protein did not closely correlate with myelination at all times. Cholesterol synthesis closely matched the rate of cholesterol accumulation but did not track well with myelination. Synthesis of fatty acids did not correlate well with accumulation of either fatty acids (phospholipids) or myelin markers. We conclude that measurement of cerebroside synthesis rates provides a good measure of the rate of myelination. This approach may be useful as an additional parameter for examining the effects of environmental or genetic alterations on the rate of myelination.

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.

Effects of maternal marginal zinc deficiency on myelin protein profiles in the suckling rat and infant rhesus monkey

In the current study, the effects of marginal Zn deficiency on myelin protein profiles in neonatal rats and rhesus monkeys were investigated. Following mating, rats were fed a Zn-adequate diet, ad libitum (50 micrograms Zn/g; 50 Zn AL), or a marginal Zn diet (10 micrograms Zn/g) from day 0 (10 Zn d0) or day 14 (10 Zn d14) of gestation to day 20 postnatal. An additional group of dams was restricted-fed the control diet to the food intake of the 10 Zn d0 group (50 Zn RF). Day 20 pup plasma and liver Zn concentrations in the 10 Zn groups were lower than in the 50 Zn groups. In a parallel experiment, rhesus monkeys were fed a Zn-adequate ad libitum diet (100 micrograms Zn/g) or a marginal Zn diet (4 micrograms Zn/g diet; MZD) throughout gestation and lactation. Day 30 monkey infant plasma and liver Zn levels were similar in the MZD and control groups. Rat brain and monkey brain cortex weights were similar among the dietary groups. The amount of myelin recovered (mg protein/g brain) from day 20 rat pups from the 10 Zn groups was lower than that recovered from the 50 Zn rat pups. Myelin recovery from the MZD and control monkey infants was similar. When myelin protein profiles were characterized, it was found that the percentages of high-molecular-weight (HMW) proteins and Wolfgram protein were higher, whereas the percentages of small and large basic proteins were lower in myelin from the 10 Zn d0 and 50 Zn RF pups compared to the distribution in the 50 Zn AL rat pups. Results for the 10 Zn d0 and 10 Zn d14 pups were similar for all of the parameters studied. The percentage of HMW proteins was higher and that of basic protein lower in myelin from MZD monkey infants compared to the percentage of these proteins in myelin from controls. Although the interpretation of the rat data is complicated because of the anorexia associated with Zn deficiency, the observed changes in monkey myelin protein profiles provide strong evidence that maternal Zn deficiency affects myelination in the offspring.

Calcium-activated neutral protease (CANP) in normal and dysmyelinating mutant paralytic tremor rabbit myelin

Calcium-activated neutral protease (CANP) in normal and dysmyelinating mutant, paralytic tremor (PT) rabbit myelin and premyelin fractions was studied using immature (4-5 wk) or adult animals. The enzyme was estimated by determination of its catalytic activity as well as by using immunoblot analysis after SDS-PAGE separation. The presence of two forms of CANP--one activated by calcium in the micromolar concentration (mu CANP) range and the other exhibiting low calcium sensitivity in the millimolar concentration range (m-CANP)--was found in the myelin and premyelin fractions. The developmental pattern of the enzyme activity was different for each of these two enzyme isoforms depending on the fraction studied. The higher activity on CANP (both isoforms) found in PT myelin and premyelin could be related to delayed myelination and/or to the higher turnover rate of already formed myelin. These results suggest complex and specific roles for these isoenzymes during myelin formation as is discussed further in this article. Our results confirm the extensive degradation of myelin basic protein (MBP), proteolipid protein (PLP), and, to a lesser extent, the other myelin proteins by endo- and exogenous CANP. This degradation process was significantly elevated in PT rabbit myelin. Moreover as was shown by two-dimensional gel electrophoresis, calcium-controlled proteolysis in nonmutant rabbits affected the net-charge of MBP in a manner similar to that reported for PT myelin, suggesting the possible involvement of CANP in the generation of charge isomers of MBP.

Galactocerebroside and sulfatide independently mediate Ca2+ responses in oligodendrocytes

Galactocerebroside (GalC) and sulfated galactocerebroside (sulfatide) are sphingolipids highly enriched in myelin. The binding of antibodies reactive with either sulfatide or GalC to cultured oligodendrocytes causes a Ca2+ influx, followed by microtubule depolymerization; however, antisulfatide is less effective than anti-GalC in altering cytoskeleton. Typical Ca2+ responses are delayed for both antibodies but are transient for sulfatide-reactive antibodies in contrast to the sustained responses previously reported for anti-GalC (Dyer and Benjamins, J Cell Biol 111: 625-633, 1990). Approximately one-half as many oligodendrocytes respond to sulfatide-reactive antibodies (about 39%) as to anti-GalC (about 75%). Subpopulations of oligodendrocytes were identified that responded to neither antibody, only one antibody, or both antibodies, indicating that sulfatide and GalC independently mediate Ca2+ responses. These results suggest that sulfatide and GalC have different physiologic roles in regulating elaboration of myelin membrane by oligodendrocytes in vivo and support the possibility that viral or immune attack via GalC or sulfatide on oligodendrocytes may mimic normal signals in a manner that disrupts the sequence of events that coordinates myelination or maintenance of myelin in vivo.

Tellurium blocks cholesterol synthesis by inhibiting squalene metabolism: preferential vulnerability to this metabolic block leads to peripheral nervous system demyelination

Inclusion of 1.1% elemental tellurium in the diet of postweanling rats produces a peripheral neuropathy due to a highly synchronous primary demyelination of sciatic nerve; this demyelination is followed closely by remyelination. Sciatic nerves from animals fed tellurium for various times were removed and incubated ex vivo for 1 h with [14C]acetate, and radioactivity incorporated into individual lipid classes was determined. In nerves from rats exposed to tellurium, there was a profound and selective block in the conversion of radioactive acetate to cholesterol. Another radioactive precursor, [3H]water, gave similar results. We suggest that tellurium feeding inhibits squalene epoxidase activity and that the consequent lack of cholesterol destabilizes myelin, thereby causing destruction of the larger internodes. Ex vivo incubation experiments were also carried out with liver slices. As with nerve, tellurium feeding caused accumulation in squalene of label from radioactive acetate, whereas labeling of cholesterol was greatly inhibited. Unexpectedly, however, incorporation of label from [3H]water into both squalene and cholesterol was increased. Relevant is the demonstration that liver was the primary site of bulk accumulation of squalene, which accounted for 10% of liver dry weight at 5 days. Thus, accumulation of squalene (and other mechanisms, possibly including up-regulation of cholesterol biosynthetic pathways) drives squalene epoxidase activity at normal levels in liver even in the presence of inhibitors of this enzyme. This is reflected by continuing incorporation of [3H]water into cholesterol; incorporation of this precursor takes place at many of the postsqualene biosynthetic steps for sterol formation. [14C]Acetate entering the sterol pathway before squalene in liver is greatly diluted in specific activity when it reaches the large squalene pool, and thus increased squalene epoxidase activity does not transfer significant 14C label to sterols. In contrast to the situation with liver, synthesis of sterols is markedly depressed in sciatic nerve, and squalene does not accumulate to high levels.

Cholesterol from degenerating nerve myelin becomes associated with lipoproteins containing apolipoprotein E

Apolipoprotein E is synthesized and secreted by rat sciatic nerve consequent to several types of injury. It has been proposed that endoneurial apolipoprotein E, in analogy to its role in systemic cholesterol transport, is involved in the salvage and reutilization of myelin cholesterol during degeneration and regeneration. To test this hypothesis, nerve lipids were prelabeled via intraneural injection of [3H]acetate. Four weeks later the nerves were crushed. From 1 to 12 weeks later, crushed nerves were examined for extracellular lipoprotein-bound cholesterol label. By 2 weeks after injury, 10% of the endoneurial lipid label was in a soluble form that was releasable into incubation medium. This released fraction was enriched in labeled cholesterol, and its labeled lipid composition was constant, in contrast to the changing distribution of label in the nerve with time after injury. On a KBr gradient, the released lipid label cofractionated with the released apolipoprotein E at densities similar to that of lipoproteins. These data indicate that at least some myelin cholesterol in injured nerve becomes associated with apolipoprotein E-containing lipoproteins and thus is available for reutilization via the hypothesized model.

Cholesterol synthesis is down-regulated during regeneration of peripheral nerve

The discovery of apolipoprotein E synthesis and secretion by injured peripheral nerve to the hypothesis that endoneurial apolipoprotein E serves to salvage degenerating myelin cholesterol. This salvaged cholesterol could then be reutilized by Schwann cells during remyelination via uptake through low-density lipoprotein receptors. As a test of this hypothesis, we measured the rate of cholesterol synthesis in rat sciatic nerve endoneurium during development and at various times following a crush injury at 50 days of age. In control nerves [14C]acetate incorporation into cholesterol and 3-hydroxy-3-methylglutaryl-CoA reductase activity were closely linked throughout development, indicating that reductase activity in nerve, as in other tissues, is a good indicator of cholesterol's synthetic rate. In the crushed nerves cholesterol synthesis fell to nearly zero during the first week after the crush. There was a partial recovery during the second to fourth weeks, but unlike that of other lipids, cholesterol synthesis remained well below control nerve values throughout most of the 15-week post-crush period examined. Thus, cholesterol synthesis is at very low levels during the myelination of regenerating axons. These results are consistent with a receptor-mediated down-regulation of cholesterol synthesis by lipoproteins, and would be expected if Schwann cells were utilizing an external source of cholesterol as postulated above.

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.

Neonatal hyperthyroidism in the rat produces an increase in the activity of microperoxisomal marker enzymes coincident with biochemical signs of accelerated myelination

The effect of neonatal hyperthyroidism produced by injection of tri-iodothyronine (T3) on myelination and on the microperoxisomal population of the brain was studied in young rats. Data on the lipid composition of myelin show that myelinogenesis starts earlier in treated animals. In coincidence with the early appearance of myelin, there is an increase in the population of brain microperoxisomes, indicated by the increase in the activity of two enzymes that have been shown to be located in these organelles: catalase and acyl CoA-dihydroxyacetone phosphate acyl transferase. Double-label experiments using (1,2,3-3H) and (2-3H) glycerol to study the synthesis of glycerophospholipids through the dihydroxyacetone phosphate (DHAP) pathway give further support to the above-mentioned findings and suggest that there is an active participation of microperoxisomes in the synthesis of myelin lipids during the period of myelin formation.

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.

Intracellular transport and neuronal activation of phospholipid and glycoprotein synthesis during axonal regeneration of cranio-spinal nerves

In the present work, the hypothesis that the increased rapid intracellular transport of newly-synthesized material along the axons of a regenerating system is sustained by an alteration of the transport of proteolipid complexes through subcellular compartments of a neuronal cell body was tested by a biochemical methodology. The motoneurons of spinal cord ventral horn, 4 wk after unilateral lesion (crush) of cervico-thoracic nerves of the rabbit at the level of brachial plexus, were chosen as the model system of regeneration. A time-staggered procedure of in vivo and in vitro double labeling with metabolic precursors, such as [3H]-choline, [14C]-choline, [3H]-fucose, and [14C]-fucose, was used. Subcellular fractions (RER, SER, Golgi apparatus, and plasma membranes) of ventral horn tissue, taken from spinal cord hemisections (regenerating and contralateral side), were further isolated. Twenty-eight days after axotomy, we did not observe any change of intracellular transport kinetics (14C/3H ratio) of newly-synthesized choline-phospholipids and glycoproteins in regenerating motoneurons compared to controls. However, associated with regenerating phenomenon in Golgi apparatus, we observed an increase of labeled choline-phospholipid and glycoprotein material that could contribute to the increased fast axonal transport and delivery of membrane proteolipid complexes to plasma membrane and axonal compartments. The increase of glycoprotein labeling was more pronounced in the SER portion (vesicles and elements of smooth membranes). This result is in favor of the hypothesis that membrane-bound proteins are transported from the Golgi to the axon through the perikaryal SER.

Lipid metabolism during early stages of Wallerian degeneration in the rat sciatic nerve

We examined changes in biosynthetic capacity of sciatic nerve during the early stages of Wallerian degeneration, utilizing a model that permits exclusion of nonresident cells from degenerating nerve. Sciatic nerve segments were placed in either 5-microns pore (allowing infiltration of nonresident cells) or 0.22-microns pore (excluding nonresident cells) Millipore diffusion chambers and then implanted in the peritoneal cavity of the same 32-34-day-old rat. At times up to 7 days postsurgery, nerve segments from the chambers, as well as control segments from the contralateral sciatic nerve, were removed and their capacity to incorporate radioactive precursors into lipids and proteins assayed in vitro. In nerve segments from both the 0.22- and 5-microns pore chambers, incorporation of [14C]acetate into total lipids was decreased relative to control by 50% at 12 h postsurgery and by 85% at day 3. This decreased incorporation of [14C]acetate reflects primarily decreased de novo synthesis of cholesterol and of fatty acyl residues incorporated into glycerolipids and sphingolipids. There was a preferentially decreased synthesis of cerebrosides and cholesterol (components enriched in myelin) relative to other lipids, while cholesterol esters and free fatty acids (products of membrane degradation) accounted for a greater proportion of the greatly reduced levels of total lipid label. In contrast to [14C]acetate, incorporation of [3H]glycerol into lipids was increased up to fourfold, relative to control, 1 day after nerve transection.(ABSTRACT TRUNCATED AT 250 WORDS)

Tellurium-induced neuropathy: metabolic alterations associated with demyelination and remyelination in rat sciatic nerve

Rats fed a diet containing 1.25% elemental tellurium initiated on postnatal day 20 undergo a transient neuropathy characterized by synchronous demyelination of peripheral nerves. In sciatic nerve, the extent of demyelination was maximal after 5 days of tellurium exposure; there was a loss of 25% of the myelin, as assayed by concentration of myelin-specific P0 protein. Tellurium-induced alterations in the metabolic capacity of Schwann cells were examined by measuring the synthesis of myelin lipids in vitro in isolated sciatic nerve segments. Exposure to tellurium resulted in an early marked decrease of approximately 50% in overall incorporation of [14C]acetate into lipids, with a preferential depression in synthesis of cerebrosides, cholesterol, and ethanolamine plasmalogens (components enriched in myelin). Most dramatically, within 1 day of initiation of tellurium exposure, there was a profound increase in [14C]acetate-derived radioactivity in squalene; 23% of incorporated label was in this intermediate of cholesterol biosynthesis, compared to less than 0.5% in controls. In association with the remyelinating phase seen after 5 days of tellurium exposure, synthesis of myelin components gradually returned to normal levels. After 30 days, metabolic and morphologic alterations were no longer apparent. We suggest that the sequence of metabolic events in sciatic nerve following tellurium treatment initially involves inhibition of the conversion of squalene to 2,3-epoxysqualene, and that this block in the cholesterol biosynthesis pathway results, either directly or indirectly, in the inhibition of the synthesis of myelin components and breakdown of myelin.

Detection and partial biochemical characterization of a novel 57,500 dalton protein in rat brain myelin

Using a monoclonal antibody (P6C3) derived from mice immunized with partially delipidated rat myelin, we detected a single 57.5 kDa protein species (BT57.5) positioned between alpha- and beta-tubulin on nitrocellulose containing electrophoretically separated rat brain myelin proteins. The kinetics of incorporation of 14C-labeled BT57.5 and myelin basic protein into myelin was similar in rats injected intracranially with [14C]amino acids to radioactively label these proteins. Immunoblotting of the separated proteins of the myelinlike fraction of rat brain with the anti-BT57.5 monoclonal antibody occasionally revealed a faint band corresponding to BT57.5 but consistently showed a more prominent protein band that migrated slightly ahead of BT57.5. When the myelin and myelinlike fraction were isolated from rats injected with [14C] amino acids, labeling of the prominent anti-BT57.5 binding protein in the myelinlike fraction preceded that of BT57.5 in myelin, reaching a peak of labeling and then decreasing before BT57.5 reached its peak level. Thus, the prominent anti-BT57.5 protein in the myelinlike fraction might be a metabolic precursor of BT57.5 in myelin or the myelinlike fraction is simply turning over more rapidly and is distinct from the myelin fraction. BT57.5 was detectable in rat, mouse, guinea pig, bovine, and human CNS myelin but not rabbit and was detectable in the peripheral nervous system only in myelin of rats and humans. Finally, in extensively purified myelin, BT57.5 appeared to copurify with myelin basic protein, suggesting that BT57.5 is a constituent of myelin rather than an artifact arising during brain homogenization procedures.

Aspects of the protein and the lipid composition of myelinoid Marchi-positive bodies from mammalian spinal cord

The fraction floating on 0.32 M sucrose was isolated from normal mammalian spinal cord and analyzed with regard to protein and lipid composition. Comparisons were made with the myelin fraction isolated from the same spinal cord. A close relationship between the two fractions was indicated by a similar protein banding on SDS-polyacrylamide gel electrophoresis. The relative amounts of various proteins however were different and some high molecular weight proteins appeared unique to the floating fraction. The phospho- and galactolipid patterns, as revealed by thin-layer chromatography, were similar in the floating and the myelin fractions. The proportion of hydrophobic lipids, such as sterols and isoprenyl derivatives, was higher in the floating fraction. Bands co-migrating with cholesterol esters were detected only in the floating fraction from guinea pigs. Marchi-positive material of possible paranodal origin is enriched in the floating fraction. The present findings of a biochemical composition of the floating fraction closely resembling that of myelin is in line with the view that myelin turnover includes a step of degradation localized to the paranodal regions.

Myelin composition and activities of CNPase and Na+,K+-ATPase in hypomyelinated "pt" mutant rabbit

A disorder of CNS myelination was found in paralytic tremor ("pt") rabbits. The condition is inherited in a sex-linked recessive mode. Ultrastructurally, an obvious myelin deficiency with aberration of myelin sheath formation is observed. The yield of myelin isolation was reduced to 20-30% of control. Myelin isolated from 4-week-old "pt" rabbits contained reduced amounts of galactosphingolipids and of several myelin protein markers. Moreover, myelin basic protein, analyzed by two-dimensional gel electrophoresis, showed a deficit in its more basic components. All these facts suggest a delay in myelin maturation. Ganglioside content was increased as well as Na+,K+-ATPase specific activity. 2',3'-Cyclic nucleotide phosphodiesterase (CNPase) specific activity was the same in "pt" as in control myelin but differed by having greater sensitivity to detergent activation.

Metabolic studies on dysmyelinating mutant "pt" rabbit brain in vitro

"Paralytic tremor" (pt) rabbit mutant is characterized by a severe hypomyelination of the CNS, however, it is not defined if the defect in myelinogenesis is an "assembly" or "synthesis" type. In this study, we have compared the general metabolic and biosynthetic properties of the myelinating mutant brain with unaffected controls of the same age. In the brain slices of 4 wk old "pt" rabbits the incorporation of U-[14C]glucose, 6-[3H] galactose, and U-[14C] leucine into macromolecules (total lipids and proteins, galactolipids, and myelin basic protein) was substantially elevated. In isolated myelin fraction, the total reduction of the radioactivity was followed by the increased specific activity of all examined macromolecules. The myelin to homogenate specific activity ratio was similar in control and "pt" rabbits. Distribution of the label and myelin marker, cyclic nucleotide 3'-phosphodiesterase (CNP-ase) among the membranous fractions suggests the partial inhibition of myelin formation in "pt" rabbits on the step of premyelin, unilamellar membranes. 14CO2 yields derived from differently labeled glucose were used for the evaluation of the basal oxidative metabolism in "pt" brain slices. 14CO2 production from U-[14C] glucose was normal. The depolarization of the slices by 50 mM K+ stimulated glucose oxidation to a higher extent in "pt" than in control. Hexose monophosphate pathway (HMP), the route providing much of NADPH required for lipid biosynthesis, did not change significantly by mutation. The activity of glucose 6-phosphate dehydrogenase (Glc-6-P DH), an oligodendroglia enriched, HMP connected enzyme, was slightly lower in "pt" homogenates by 13-17%, whereas CNP-ase was lowered more than 30% in the same samples. All this data suggest that the capacity for the synthesis of myelin constituents is well preserved in the mutant brain and the impairment of myelogenesis is probably caused by increased elimination of already synthesized, myelin-related components.

Integral membrane proteins significantly decrease the molecular motion in lipid bilayers: a deuteron NMR relaxation study of membranes containing myelin proteolipid apoprotein

The influence of the myelin proteolipid apoprotein on lipid chain order and dynamics was studied by 2H NMR of membranes reconstituted with specifically deuterated dimyristoyl phosphatidylcholines. Quadrupolar echo and saturation recovery experiments were fitted by numerical solution of the stochastic Liouville equation, using a model that includes both inter- and intramolecular motions [Meier, P., Ohmes, E. & Kothe, G. (1986) J. Chem. Phys. 85, 3598-3614]. Combined simulations of both the relaxation times and the quadrupolar echo line shapes as a function of pulse spacing allowed unambiguous assignment of the various motional modes and a consistent interpretation of data from lipids labeled on the C-6, C-13, and C-14 positions of the sn-2 chain. In the fluid phase, the protein has little influence on either the chain order or the population of gauche rotational isomers but strongly retards the chain dynamics. For 1-myristoyl-2-[13-2H2] myristoyl-sn-glycero-3-phosphocholine at 35 degrees C, the correlation time for chain fluctuation increases from 20 nsec to 650 nsec and for chain rotation from 10 nsec to 180 nsec, and the gauche isomer lifetime increases from 0.15 nsec to 1.75 nsec, on going from the lipid alone to a recombinant of protein/lipid ratio 0.073 mol/mol. The results are essentially consistent with spin-label ESR studies on the same system [Brophy, P.J., Horvath, L.I. & Marsh, D. (1984) Biochemistry 23, 860-865], when allowance is made for the different time scales of the two spectroscopies.

Glycerolipid metabolism during early amphibian embryogenesis. Neutral lipid involvement in fertilization triggered events

In order to study glycerolipid metabolism during early embryonic development, Bufo arenarum Hensel female toads were administered [1-3-(14)C]glycerol together with hormonal stimulus. Triglycerides comprised 60% of the total label in all developmental stages analyzed. The highest specific activity was observed in phosphatidic acid which underwent a net decrease as a function of development. Phosphatidylcholine was the most actively synthesized phosphoglyceride. A significant decrease in the total lipid labeling, mainly accounted for by triglycerides, was detected at fertilization time concomitant with a net diminution in the content of these neutral lipids. Subcellular studies showed that yolk platelets contain about 99% of the total oocyte triglycerides suggesting that massive breakdown would be confined to this fraction. Present findings evidence that the de novo lipid biosynthesis is operating during the developmental analyzed period. In addition, they would suggest the active role of triglycerides in the energetic events taking place around fertilization.

Normal myelin composition and phospholipid synthesis in adrenalectomized rats with reduced brain myelin and glycerol 3-phosphate dehydrogenase activity

The composition of CNS myelin was investigated in rats adrenalectomized at day 14 and killed 7 days later, previously shown to result in a 25% reduction in the amount of bulk-isolated myelin and a 40% decrease in brain glycerol 3-phosphate dehydrogenase activity. The proportions of the major myelin proteins, as well as the specific activity of 2',3'-cyclic nucleotide 3'-phosphohydrolase, were the same in the myelin from both adrenalectomized and control animals. The amount of total phospholipid and the proportions of individual phospholipids were also normal in myelin from the adrenalectomized animals. The amount of nonmyelin phospholipid in whole brain was unchanged by adrenalectomy. Labeling studies carried out 4 days after adrenalectomy of 14-day-old animals showed no change in the synthesis rates of the major myelin phospholipids as compared with the synthesis rate of nonmyelin phospholipids. Furthermore, incorporation of [1,(3)-14C]glycerol into the glycerol moiety of ethanolamine plasmalogen, which requires glycerol 3-phosphate dehydrogenase, was also normal, showing that the reduced oligodendroglial glycerol 3-phosphate dehydrogenase activity following adrenalectomy was not rate-limiting for myelin phospholipid synthesis.

Myelin-associated galactolipids in primary cultures from dissociated fetal rat brain: biosynthesis, accumulation, and cell surface expression

Galactolipid metabolism was investigated as a function of development in primary cultures initiated from 19-21-day-old dissociated fetal rat brain. Significant amounts of galactocerebrosides, sulfatides, and monogalactosylglycerides were synthesized and accumulated by 8 days in culture. Thereafter the synthetic rates and levels of these galactolipids increased rapidly, reaching maximal values approximately 22-29 days in culture. Galactolipids containing nonhydroxy or 2-hydroxy fatty acid were both synthesized at approximately equal rates. The initial rates of synthesis, investigated at 15, 29, and 50 days in culture, were three- to fivefold higher for galactocerebrosides than for sulfatides and two- to threefold higher than for monogalactosylglycerides. The total number of cells staining with antisera against galactocerebroside of sulfatide also increased very rapidly between 8 and 22 days in culture, reaching levels of 4-5 million cells per seeded fetal brain. The amount of galactocerebroside or sulfatide per cell stained with the corresponding antiserum increased severalfold from 10 to 27 days in culture and remained high until at least 36 days in culture (the latest time point examined). Thus, the temporal expression of galactolipid accumulation in the cell cultures was comparable to that occurring in rat brain, but some important quantitative reductions in the levels of accumulation per cell in culture were noted. In addition, in contrast to normal brain in which galactolipid synthetic rates are reduced after the period of most active myelination, in culture both synthesis and turnover of these galactolipids remained high, suggestive of a partial arrest in myelin maturation.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of cerebrosides and sulfatides in subcellular fractions of developing rat brain

Previous studies on myelinating rat brain indicated that microsomes, Golgi-enriched and cytosol fractions may process galactolipids destined for myelin. To extend these findings we labeled brain galactolipids in vivo and determined the specific radioactivity of cerebrosides and sulfatides in several subcellular fractions. 17-day-old rats were treated by intracranial injection with [14C]galactose 60 min prior to and [3H]galactose 15 min prior to killing. Subcellular fractions were prepared from brain stem, and concentrations of cerebrosides and sulfatides were determined, their radioactivity measured and the 3H/14C ratio compared. Our results showed that the heavier Golgi-enriched fraction (designated Fraction 2) is unique in its low galactolipid content and high specific radioactivities of cerebrosides and sulfatides. The low ratio of the specific activity of cerebroside to that of sulfatide in Fraction 2 compared to other fractions indicates that it may be the site of most rapid conversion of newly synthesized cerebrosides to sulfatides. The specific radioactivities of cerebrosides and sulfatides in cytosol are intermediate between those in Golgi-enriched Fraction 2 and microsomes and those in myelin, consistent with the role postulated for cytoplasmic elements in the transport of cerebrosides and sulfatides to myelin.

Effects of monensin and colchicine on myelin galactolipids

Monensin and colchicine have been used in a variety of systems to disrupt functioning of the Golgi apparatus and transport of Golgi-derived vesicles to the plasma membrane. In this study the effects of monensin and colchicine on the synthesis of cerebroside and sulfatide and their appearance in myelin were examined to determine whether these myelin components are processed through the Golgi apparatus. Brain slices from rats 17 days old were incubated with [3H]galactose and [35S]-sulfate to label cerebroside and sulfatide. Myelin was isolated on sucrose density gradients. Fractions highly enriched in cerebroside and sulfatide were prepared from homogenates and myelin fractions by lipid extraction, alkaline methanolysis, and in some cases TLC. Monensin at 0.1 microM had no significant effect on synthesis of these galactolipids as measured by incorporation of [3H]-galactose into cerebroside or [35S]sulfate into sulfatide in homogenates. However, appearance of [35S]sulfatide in the myelin fraction was reduced to 49% of control, while appearance of [3H]cerebroside was not significantly reduced. Colchicine from 1 mM to 0.1 microM had effects similar to monensin, that is, appearance of [35S]sulfatide in myelin was depressed, but again [3H]cerebroside was not affected. Incorporation of [35S]sulfate into sulfatide in homogenate was 93% of control, while appearance of [35S]sulfatide in the myelin fraction was depressed to 58% of control. The inhibition of appearance of sulfatide in myelin by colchicine and monensin is consistent with the view that sulfation of cerebroside occurs in the Golgi and that sulfatide is transported via Golgi-derived vesicles to the forming myelin membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical expression of mosaicism in female mice heterozygous for the jimpy gene

Expression of the jimpy gene in heterozygous females was analyzed by measuring galactolipid synthesis in brain during early myelination. Sulfatide labeling in brains of heterozygous females at 13-14 days is decreased to 40-80% that of female littermates who do not carry the jimpy gene. The activities of ceramide galactosyl transferase and cerebroside sulfotransferase and levels of myelin basic protein were similarly depressed. Since the jimpy gene was maintained with the Tabby gene in these studies, the effect of the Tabby gene on these parameters was examined and found to have no effect. These biochemical findings indicate that myelination is retarded in the brains of heterozygous jimpy females during the second week of development. However, at 18 days and thereafter, sulfatide labeling is less reduced, suggesting that the oligodendrocytes in brain attempt to compensate, in agreement with morphologic studies which show that myelin is decreased in brain during early development, but appears normal in adult animals.

Phospholipid exchange activity in developing rat brain

Phospholipid exchange activity has been determined in the supernatant fraction of rat brain from birth through to maturity by measuring the protein-catalysed transfer of total and individual 32P-labelled phospholipids from microsomal membranes to mitochondria, and the transfer of [14C]phosphatidylcholine from liposomes to mitochondria. Transfer activity has also been compared in brain and liver supernatant. Overall phospholipid exchange activity in the brain increased only slightly with age. The activity at birth was 75% of the adult value. However, the transfer of individual phospholipids showed markedly different trends during postnatal brain development. The transfer of phosphatidylinositol (PI) and ethanolamine phospholipids increased postnatally to a maximum at 9 days of age, with lowest values in adult brain. Phosphatidylcholine (PC) transfer increased from 9 days to reach maximum values in the mature brain. The transfer of sphingomyelin was highest immediately after birth. PI transfer activity was higher in brain than liver, while PC and ethanolamine phospholipid transfer activity was higher in liver. The heterogeneity of phospholipid exchange proteins in central nervous system tissue is reflected in the developmental changes in exchange activity towards individual phospholipids. The various exchange proteins appear to have separate induction mechanisms. The presence of exchange-protein activity from birth in the rat indicates the functional importance of phospholipid transport during cell acquisition and membrane proliferation. Activity is not primarily associated with membrane formation such as the formation of the myelin sheath, and therefore is more likely to be involved in the process of phospholipid turnover.

The role of phosphatidylserine decarboxylase in brain phospholipid metabolism

In brain, phosphatidylethanolamine can be synthesized from free ethanolamine either by a pathway involving the formation of CDP-ethanolamine and its transfer to diglyceride, or by base-exchange of ethanolamine with existing phospholipids. Although de novo synthesis from serine has also been demonstrated, the metabolic pathway involved is not known. The enzyme phosphatidylserine decarboxylase appears to be involved in the synthesis of much of the phosphatidylethanolamine in liver, but the significance of this route in brain has been challenged. Our in vitro studies demonstrate the existence of phosphatidylserine decarboxylase activity in rat brain and characterize some of its properties. This enzyme is localized in the mitochondrial fraction, whereas the enzymes involved in base-exchange and the cytidine pathway are localized to microsomal membranes. Parallel in vivo studies showed that after the intracranial injection of L-[G-3H]serine, the specific activity of phosphatidylserine was greater in the microsomal fractions than in the mitochondrial fraction, whereas the opposite was true for phosphatidylethanolamine. When L-[U-14C]serine and [1-3H]ethanolamine were simultaneously injected, the 14C/3H ratio in mitochondrial phosphatidylethanolamine was 10 times that in microsomal phosphatidylethanolamine. The results demonstrate that serine is incorporated into the base moiety of phosphatidylethanolamine primarily through the decarboxylation of phosphatidylserine in brain mitochondria. A minimal value of 7% for the contribution of phosphatidylserine decarboxylase to whole-brain phosphatidylethanolamine synthesis can be estimated from the in vivo data.

Studies on subcellular fractions which are involved in myelin assembly: labeling of myelin proteins by a double radioisotope approach indicates developmental relationships

The question of developmental relationships amongst myelin-related membranes in subfractions of myelinating mouse brain (15 days) was investigated by a time-staggered double isotope protocol using [3H]leucine and [14C]leucine. Preliminary results are interpreted and discussed in the context of a mathematical conceptualization of pulse-labeling kinetic analyses of myelin proteins in subcellular membrane compartments. Differences in ratio of the two leucine labels among proteins of myelin-containing subfractions are interpreted as confirming metabolic differences relating to various stages of development rather than precursor-product relationships. The incorporation into myelin of 14K, 17K, and 18.5K basic proteins (MBPs) occurs with relatively short delay times, following their synthesis (less than 5 min), and seems to occur simultaneously into all compartments. The 21.5K MBP and the proteolipid protein, on the other hand, require 10-14 min and 14-20 min, respectively. A scheme is presented to illustrate the probable assignment of subfractions to various myelin "compartments" during myelination, and to serve as a working hypothesis for studies on precursor-product relationships.

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.

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.

Differences in the kinetics of axonal transport for individual lipid classes in rat sciatic nerve

Lipid precursors ([2-3H]glycerol for phospholipids and [3H]acetate for cholesterol) were injected into the L-5 dorsal root ganglion of adult rats. At various times, animals were killed, the ganglion and consecutive 5-mm segments of sciatic nerve were dissected, and lipids were extracted and analyzed by TLC. Individual lipid classes exhibited markedly different transport patterns. The crest of radioactive phosphatidylcholine moved as a sharply defined front at about 300 mm/day, with a relatively flat plateau behind the moving crest. Although some radioactive phosphatidylethanolamine also moved at the same rate, the crest was continually attenuated as it moved so that a gradient of radioactive phosphatidylethanolamine along the axon was maintained for several days. Transported diphosphatidylglycerol exhibited a defined crest, as did phosphatidylcholine, but moved at about half the rate. Labeled cholesterol was transported at a rapid rate similar to that for phosphatidylcholine and phosphatidylethanolamine, but like phosphatidylethanolamine, the initial moving crest of radioactivity was continually attenuated. Relative to the phospholipids, cholesterol showed a more prolonged period of accumulation in the axons and was more metabolically stable. We propose that most labeled phosphatidylcholine, phosphatidylethanolamine, and cholesterol is transported in similar (or the same) rapidly moving membranous particles. Once incorporated into these particles, molecules of phosphatidylcholine tend to maintain associated with them during transport. In contrast, molecules of phosphatidylethanolamine and cholesterol in these transported particles exchange extensively with unlabeled molecules in stationary axonal structures. Diphosphatidylglycerol, localized in a specialized organelle, the mitochondrion, is transported at a slower rate than other phospholipids, and does not exchange with other structures.

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.

Sidedness of phospholipid synthesis on brain membranes

We have investigated the localization of the site of incorporation and the subsequent equilibration of newly synthesized phospholipids in brain membranes. Rats were injected intracranially with [3H]glycerol; the animals were killed at varying times afterwards, and microsomal fractions were isolated from the brains. In some cases, microsomes were subfractionated on sucrose gradients. Initially, most of the radioactive phosphatidylethanolamine appeared in a pool that reacted with the impermeable reagent trinitrobenzene sulfonic acid (TNBS). This probe presumably modified only the lipid on the outer face of microsomal vesicles (which may, in large part, consist of pinched-off endoplasmic reticulum). At 5 min after injection, the specific radioactivity of the TNBS-modified phosphatidylethanolamine (cytoplasmic face) was four times that of the unmodified (luminal or inner face) phosphatidylethanolamine. With time, the ratio of the specific activities in the modified and unmodified pools of phosphatidylethanolamine approached 1.0, with kinetics that suggested a half-time on the order of 30 min for in vivo conversion of the TNBS-accessible to the -inaccessible pool. This equilibration in specific activities could be the result of either translocation of phospholipids across endoplasmic reticulum membranes or conversion with time of initially labeled endoplasmic reticulum to other membranous organelles which form randomly oriented vesicles upon homogenization. A similar experimental design, using phospholipase C to hydrolyze outer face phospholipids preferentially, verified this conclusion for phosphatidylethanolamine and yielded similar results for phosphatidylcholine. Control studies measuring radioactive sucrose permeability indicated that neither TNBS nor phospholipase C treatment significantly disrupted microsomal vesicles under the conditions used.

Cerebroside sulfotransferase in Golgi-enriched fractions from rat brain

Golgi-enriched fractions were prepared from brainstems of 17-day-old rats by first floating off myelin, then fractionating the remaining pellet by a series of differential and density gradient centrifugations in sucrose. Fractions enriched in Golgi membranes were recovered at 0.46/0.76 M and 0.76/0.87 M interfaces on the final sucrose gradient as indicated by morphology and the biochemical markers thiamine pyrophosphatase and [3H]fucose-labeled glycoprotein. Morphology of the two fractions indicated very little contamination with myelin lamellae; however, the presence of significant levels of 2',3'-cyclic nucleotidase in the lighter fraction suggested a contribution from oligodendroglial or myelin-related membranes. Cerebroside sulfotransferase was highly enriched in the lighter Golgi-enriched fraction relative to the denser fraction, the post-34,880 X g microsomes, and the myelin-like fraction. In contrast, ceramide galactosyl transferase was more evenly distributed among the fractions. Our results show a more highly localized distribution of sulfatide synthesis than of galactocerebroside synthesis, probably in Golgi membranes or oligodendroglia-related membranes with similar properties.

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.