The turnover of myelin phospholipids in the adult and developing rat brain

Untargeted Lipidomics after D2O Administration Reveals the Turnover Rate of Individual Lipids in Various Organs of Living Organisms

The administration of low doses of D₂O to living organisms was used for decades for the investigation of metabolic pathways and for the measurement of the turnover rate for specific compounds. Usually, the investigation of the deuterium uptake in lipids is performed by measuring the deuteration level of the palmitic acid residue using GC-MS instruments, and to our knowledge, the application of the modern untargeted LC-MS/MS lipidomics approaches was only reported a few times. Here, we investigated the deuterium uptake for >500 lipids for 13 organs and body liquids of mice (brain, lung, heart, liver, kidney, spleen, plasma, urine, etc.) after 4 days of 100% D₂O administration. The maximum deuteration level was observed in the liver, plasma, and lung, while in the brain and heart, the deuteration level was lower. Using MS/MS, we demonstrated the incorporation of deuterium in palmitic and stearic fragments in lipids (PC, PE, TAG, PG, etc.) but not in the corresponding free forms. Our results were analyzed based on the metabolic pathways of lipids.

Late post-natal neurometabolic development in healthy male rats using 1 H and 31 P magnetic resonance spectroscopy

Brain metabolism evolves rapidly during early post-natal development in the rat. While changes in amino acids, energy metabolites, antioxidants or metabolites involved in phospholipid metabolism have been reported in the early stages, neurometabolic changes during the later post-natal period are less well characterized. Therefore, we aimed to assess the neurometabolic changes in male Wistar rats between post-natal days 29 and 77 (p29-p77) using longitudinal magnetic resonance spectroscopy (MRS) in vivo at 9.4 Tesla. ¹ H MRS was performed in the hippocampus between p29 and p77 at 1-week intervals (n = 7) and in the cerebellum between p35 and p77 at 2-week intervals (n = 7) using the SPECIAL sequence at ultra-short echo-time. NOE enhanced and ¹ H decoupled ³¹ P MR spectra were acquired at p35, p48 and p63 (n = 7) in a larger voxel covering cortex, hippocampus and part of the striatum. The hippocampus showed a decrease in taurine concentration and an increase in glutamate (with more pronounced changes until p49), seemingly a continuation of their well-described changes in the early post-natal period. A constant increase in myo-inositol and choline-containing compounds in the hippocampus (in particular glycero-phosphocholine as shown by ³¹ P MRS) was measured throughout the observation period, probably related to membrane metabolism and myelination. The cerebellum showed only a significant increase in myo-inositol between p35 and p77. In conclusion, this study showed important changes in brain metabolites in both the hippocampus and cerebellum in the later post-natal period (p29/p35-p77) of male rats, something previously unreported. Based on these novel data, changes in some neurometabolites beyond p28-35, conventionally accepted as the cut off for adulthood, should be taken into account in both experimental design and data interpretation in this animal model.

Multiple Sclerosis as a Syndrome-Implications for Future Management

We propose that multiple sclerosis (MS) is best characterized as a syndrome rather than a single disease because different pathogenetic mechanisms can result in the constellation of symptoms and signs by which MS is clinically characterized. We describe several cellular mechanisms that could generate inflammatory demyelination through disruption of homeostatic interactions between immune and neural cells. We illustrate that genomics is important in identifying phenocopies, in particular for primary progressive MS. We posit that molecular profiling, rather than traditional clinical phenotyping, will facilitate meaningful patient stratification, as illustrated by interactions between HLA and a regulator of homeostatic phagocytosis, MERTK. We envisage a personalized approach to MS management where genetic, molecular, and cellular information guides management.

Substrate reduction intervention by L-cycloserine in twitcher mice (globoid cell leukodystrophy) on a B6;CAST/Ei background

Globoid cell leukodystrophy (GCL) is usually a fatal demyelinating disease caused by mutations in galactosylceramidase, which normally recycles galactosylceramide, a predominant glycolipid of myelin, and psychosine. The initial pathology is thought to be due to the accumulation of psychosine in myelin-forming cells leading to their death. In this study, substrate reduction therapy using L-cycloserine, an inhibitor of 3-ketodihydrosphingosine synthase, was examined in twitcher mice on a C57BL/6xCAST/Ei (B6;CAST/Ei) background, which mimics a late onset variant of GCL. A graded dose regimen of L-cycloserine initiated before the onset of symptoms increased the lifespan by approximately 45% and delayed the onset of weight loss while the administration of L-cycloserine beginning after the onset of symptoms had no effect. Despite the pronounced effect for the early treatment regimen, B6;CAST/Ei twitcher mice still displayed a progressive disease leading to an early death.

Substrate-reduction therapy enhances the benefits of bone marrow transplantation in young mice with globoid cell leukodystrophy

Globoid cell leukodystrophy is an autosomal recessive disease with progressive demyelination caused by a deficiency of the lysosomal enzyme galactosylceramidase. Bone marrow transplantation (BMT) is a therapeutic option for patients with late-onset disease and for patients with early onset disease that had an early diagnosis owing to an affected sibling. This therapy, however, typically is not effective for early onset disease when the diagnosis occurs after several months of life. In an effort to enable a broader range of patients to benefit from BMT, we tested whether combining substrate-reduction therapy with BMT would result in a greater benefit than either treatment alone in the twitcher mouse model of globoid cell leukodystrophy. Twitcher mice treated with L-cycloserine, an inhibitor of 3-ketodyhydrosphingosine synthase, and transplanted with 50 +/- 5 x 10(6) bone marrow cells on d 10 had a mean life-span of 112 d compared with 51 d for BMT alone (p < 0.001) or L-cycloserine alone, which was previously reported to be 56 d. L-Cycloserine treatment also was initiated neonatally to determine whether it would allow for a delayed BMT to have therapeutic value. Twitcher mice given only BMT at 18 d or only a short course of L-cycloserine died at 36 and 37 d, respectively. Twitcher mice given a short course of L-cycloserine + BMT at 18 d lived to 58 d (p = 0.0006). In conclusion, substrate-reduction therapy enhanced the value of BMT in twitcher mice, suggesting that this combination strategy might benefit patients with globoid cell leukodystrophy.

L-cycloserine slows the clinical and pathological course in mice with globoid cell leukodystrophy (twitcher mice)

Globoid cell leukodystrophy (Krabbe's disease) is an autosomal recessive disease that affects the lysosomal enzyme galactosylceramidase. Galactosylceramidase removes galactose from galactosylceramide and psychosine, which are derived from sphingosine. In the present study, L-cycloserine (an inhibitor of 3-ketodyhydrosphingosine synthase) was administered to the twitcher mouse, an authentic model of globoid cell leukodystrophy. Twitcher mice treated with L-cycloserine had a significantly longer life span and a delayed onset of weight loss than vehicle-injected twitcher mice. Pathological features such as macrophage infiltration and astrocyte gliosis also were less in treated twitcher mice. These results indicate that substrate reduction therapy may have therapeutic value for individuals with residual enzymatic activity, e.g., individuals with late onset disease or individuals with partial enzyme replacement via bone marrow transplantation. In these cases, a reduction in galactosylceramide and psychosine synthesis would enable residual enzymatic activity to keep up with the accumulation of these substrates that would otherwise lead to pathology.

Relationship between fatty acid accretion, membrane composition, and biologic functions

Dietary fat affects metabolic pathways for phospholipid biosynthesis in tissues in a coordinated fashion. This may be important to aspects of development that concern phosphatidylcholine metabolism or regulatory processes that depend on signals from a changing milieu in the microenvironment of the membrane. Dietary fat influences the phosphatidylethanolamine (PE) composition in many membranes of the brain and retina and may by altered by small changes in the content of 20:4(6) and 22:6(3). Membrane PE fatty acids that contain one, four, or six double bonds and the ratio of 22:5(6) to 22:6(3) in PE that contains four to six double bonds are also affected. An increase in the omega 6 fatty acid content of membranes is associated with increased PE methyltransferase activity and decreased phosphocholine transferase activity, thus indicating a mechanism by which change in an exogenous factor (e.g., dietary fat intake) may alter neural phospholipid biosynthesis. Small changes in the composition of dietary fat intake change the composition of brain membranes during development. It is provocative to ponder whether diet could be used to induce formation of membrane structures that are more resistant to specific insults that cause degeneration of brain structural material, to ensure optimal functional compositions, or to reverse degenerative changes that occur in neural membrane structure and function.

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.

Impact of dietary fatty acid balance on membrane structure and function of neuronal tissues

Neural tissue has generally been viewed as resistant to structural changes induced by exogenous factors. Research has shown that the brain responds to changes in diet by altering neurotransmitter synthesis, and by shifting neuroendocrine controls over a variety of physiological events. Animal model research also indicates that fatty acid constituents and synthesis of brain structural lipid in membranes undergoing turnover can be altered by changing the composition of dietary fat. In growing animals, the balance between dietary omega 6 and omega 3 fatty acids influences brain phospholipid fatty acid composition, phosphatidylethanolamine methyltransferase activity, and rate of phosphatidylcholine biosynthesis via the CDP-choline pathway. It is concluded that biosynthetic control mechanisms regulating synthesis of brain structural lipid, in particular phosphatidylcholine, respond to exogenous factors and represent a normal physiological response by the brain. This response may provide a mechanism for therapeutic treatment of disorders involving degeneration of brain structural lipid.

In vivo metabolism of fluorescent ceramide in central nervous system myelin of adult rats

The metabolism of sphingolipids in the central nervous system (CNS) has been studied in adult rats by intraventricular administration of fluorescent ceramide (CER). Rats were sacrificed at various time points post inoculation and the fluorescence of CER, cerebrosides (CB), sulfatides (SULF) and sphingomyelin (SPM) was determined in the CNS myelin and in the pellet, containing mainly microsomes, obtained by Norton myelin preparation. The incorporation of fluorescence was more in the pellet than in the myelin at all times studied. Initially the fluorescence present in the pellet was prevalently due to untransformed CER but an increase of fluorescent products with time was observed. CB was the main product up to 2 h post inoculation, then it decreased with concomitant increase of fluorescent SULF. In the myelin we did not observe differences in incorporation and transformation of fluorescent CER with time: CB was the main fluorescent product at all times studied. At 0.5 h post inoculation the fluorescence, observed by fluorescence microscope, was located in the cell lining the ventricles while after 24 h it appeared also in the paraventricular areas.

Effect of neonatal undernutrition upon cerebroside sulfate degradation in the developing rat brain

In order to find out if the decreased accumulation of cerebroside sulfates observed in 21-d-old undernourished rats was in part the result of an increased rate of catabolism of these galactolipids, the in vivo degradation of brain cerebroside sulfates was studied in 18-d-old normal and undernourished rats. Two hours after the intracranial injection of the precursor (0 time), the animals were injected intraperitoneally with unlabeled sodium sulfate. Labeled cerebroside sulfates were measured in the brain up to 48 h after the chase. In normal animals, the radioactivity decreased at 24 h and 48 h to 55% and 41%, respectively, of the value obtained at 0 time. In undernourished animals, degradation was negligible, since the radioactivity attained at 0 time remained almost constant up to 48 h. The lack of in vivo degradation of cerebroside sulfates observed in the starved rats cannot be explained by a deficiency of Arylsulfatase A, since the pattern of activity of the enzyme was similar in both groups of animals.

In vivo 13C nuclear magnetic resonance investigations of choline metabolism in rabbit brain

The metabolism of choline in rabbit brain was studied by the noninvasive approach of in vivo 13C NMR spectroscopy. 13C-Enriched precursors were introduced into the brain. Surgery of the head skin was avoided through controlled localization of the surface coil. For long-term accumulation studies in brain, repeated subcutaneous injections proved to be advantageous over other forms of application. The resorption kinetics was calculated to be zero order which suggests slow delivery from the subcutaneous depots. Choline metabolism was studied by two approaches: [N-13CH3]choline and S-[13CH3]methionine were administered separately to adult and myelinating rabbits (Days 5 to 32), respectively, over 4 weeks. [N-13CH3]Choline and the 13CH3 group of methionine were incorporated into lecithin and sphingomyelin of brain myelin. In vivo kinetic studies of the turnover of these labeled structures were carried out. Choline and methionine are readily transported through the blood-brain barrier and utilized by the myelinating brain for the biosynthesis of its phospholipids.

Subcellular fractionation of rat sciatic nerve and specific localization of ganglioside LM1 in rat nerve myelin

Subcellular fractionation of rat sciatic nerve was developed to determine the specific localization of gangliosides in the nerve membrane fractions. Myelin, microsomal, and a plasma membrane-like fraction were isolated and purified by sucrose density gradient centrifugation. These subfractions were characterized by electron microscopy, marker enzyme assays, and their protein and lipid profile. In rat sciatic nerve myelin, 90 mol% of the total gangliosides were monosialogangliosides. LM1 (sialosyl-lactoneotetraosylceramide) (61 mol%) and GM3 (21%) were the major gangliosides of the rat nerve myelin. Two other neolacto series of gangliosides, viz., sialosyl-lactoneonorhexaosylceramide and sialosyl-lactoneooctaosylceramide, were also localized mostly in the myelin fraction. GM1 was only a minor (less than 2%) ganglioside in myelin. The ganglioside patterns of the microsomal and plasma membrane-like fractions were similar with minor quantitative differences and were entirely different from that of myelin. Monosialogangliosides were approximately 70-75 mol% of the total in these fractions. The major gangliosides of the microsomal and plasma membrane-like fractions were GM3 (approximately 40%) and GM1 (approximately 20%). LM1 in these fractions was minimal (less than approximately 5%). Significant amounts of GM3 with N-glycolylneuraminic acid (approximately 10%) and GM1b (4-14%) were also identified in the microsomal and plasma membrane-like fractions but not in myelin. These and the higher lactoneo series of gangliosides have not been previously reported to be present in the rat nervous system. Almost exclusive localization of LM1 in myelin in rat peripheral nervous system is consistent with our previous observation that deposition of LM1 in the nerve with age was very similar to that of myelin marker lipids cerebrosides and sulfatides.

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.

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.

Early adrenalectomy stimulates subsequent growth and development of the rat brain

Rats were adrenalectomized (ADX) or sham-operated (SHAM) on the 11th day of life and subsequent brain development (cerebrum and cerebellum) studied in terms of tissue weight and biochemical composition. Measured at about 65 days of age, early ADX subjects had significantly heavier brains (in terms of both wet and dry weights) than SHAMs, despite being lighter in overall body weight. Brain protein and DNA contents were elevated, as was the activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP), a myelin marker enzyme. Glycerol-3-phosphate dehydrogenase (GPDH), a glucocorticoid-regulated enzyme, was reduced in activity. Although both the cerebrum and cerebellum showed the growth-enhancing effects of early adrenalectomy, the DNA and CNP changes were most pronounced in the cerebrum. Finally, the effect of adrenal removal on myelinogenesis was confirmed by subcellular fractionation experiments demonstrating that more myelin could be recovered from the brains of ADX than from SHAM animals. These results are significant in terms of the influence of adrenal secretions on normal brain development and the role of GPDH in myelin lipid biosynthesis.

The nutritional significance, metabolism, and function of myo-inositol and phosphatidylinositol in health and disease

Recent advances in nutritional and biochemical research have substantiated the importance of inositol as a dietary and cellular constituent. The processes involved in the metabolism of inositol and its derivatives in mammalian tissues have been characterized both in vivo and at the enzyme level. Biochemical functions elucidated for phosphatidylinositol in biological membranes include the mediation of cellular responses to external stimuli, nerve transmission, and the regulation of enzyme activity through specific interactions with various proteins. Inositol deficiency in animals has been shown to produce an accumulation of triglyceride in liver, intestinal lipodystrophy, and other abnormalities. The metabolic mechanisms giving rise to these latter phenomena have been extensively studied as a function of dietary inositol. Altered metabolism of inositol has been documented in patients with diabetes mellitus, chronic renal failure, galactosemia, and multiple sclerosis. A moderate increase in plasma and nerve inositol levels by dietary supplementation has been suggested as a means of treating diabetic neuropathy, although excessively high levels, such as are found in uremic patients, may be neurotoxic. A thorough consideration of the biochemical functions of inositol and a further characterization of various diseases with the aid of appropriate animal models may suggest a possible role for inositol and other dietary components in their prevention and treatment

Turnover rate of molecular species of sphingomyelin in rat brain

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

Is Na + ATPase a myelin-associated enzyme?

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

Effect of hypoxia on the incorporation of [2-3H] glycerol and [1-14C[-palmitate into lipids of various brain regions

The lipid metabolism in guinea pig brain after intermittent hypoxia, prolonged for 80 hrs, was markedly impaired. The in vivo incorporation of [2-3H] glycerol adn [1-14C] palmitate into lipids of microsomes, mitochondria, myelin, and synaptosomes, purified form cerebral hemispheres, was significantly lower in the hypoxic animals than in the controls. The same effect was observed on the incorporation of labeled precursors into lipids of mitochondria purified from cerebellum and brainstem. In particular, the labeling of th major phospholipids present - ie, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) - in the mitochondria of the three brain regions examined decreased after hypoxic treatment.

Retention of a dialkylphosphatidylcholine in myelin and other membranes

We describe an attempt to incorporate a metabolically inert phospholipid analog into animal membranes, especially myelin, in vivo, with the view of eventual long-term membrane modification or membrane engineering. A sonicated suspension of a mixture of [14C]phosphatidylcholine and its dialkyl analog, [3H] tetradecyloctadecano(1)phosphocholine, was injected into the brain of weanling rats. Samples were counted of whole brain, myelin, liver, and carcass, at intervals from 1 to 63 days, and the composition of the extracted lipid was determined by thin-layer chromatography. Both lipid labels were found to be cleared from the body at similar rates, but while phosphatidylcholine was metabolized within a day, with the label appearing mainly in the phosphatidylethanolamine fraction and in nonpolar lipids, the dialkylphosphatidylcholine remained intact, with retention in myelin of a small but almost constant amount for a month. Ways will have to be found to enhance uptake of the lipids by the brain.

UDP-galactose-ceramide galactosyltransferase in rat brain myelin subfractions during development

The localization and activity of the enzyme UDP-galactose-hydroxy fatty acid-containing ceramide galactosyltransferase is described in rat brain myelin subfractions during development. Other lipid-synthesizing enzymes, such as cerebroside sulphotransferase, UDP-glucose-ceramide glucosyltransferase and CDP-choline-1,2-diacylglycerol cholinephosphotransferase, were also studied for comparison in myelin subfractions and microsomal membranes. The purified myelin was subfractionated by isopycnic sucrose-density-gradient centrifugation. Four myelin subfractions, three floating respectively on 0.55 M- (light-myelin fraction), 0.75 M- (heavy-myelin fraction) and 0.85 M-sucrose (membrane fraction), and a pellet, were isolated and purified. At all ages, 70--75% of the total myelin proteins was found in the heavy-myelin fraction, whereas 2--5% of the protein was recovered in the light-myelin fraction, and about 7--12% in the membrane fraction. Most of the galactosyltransferase was associated with the heavy-myelin and membrane fractions. Other lipid-synthesizing enzymes studied appeared not to associate with purified myelin or myelin subfractions, but were enriched in the microsomal-membrane fraction. During development, the specific activity of the microsomal galactosyltransferase reached a maximum when the animals were about 20 days old and then declined. By contrast the specific activity of the galactosyltransferase in the heavy-myelin and membrane fractions was 3--4 times higher than that of the microsomal membranes in 16-day-old animals. The specific activity of the enzyme in the heavy-myelin fraction sharply declined with age. Chemical and enzymic analyses of the heavy-myelin and membrane myelin subfractions at various ages showed that the membrane fraction contained more proteins in relation to lipids than the heavy-myelin fraction. The membrane fraction was also enriched in phospholipids compared with cholesterol and contrined equivalent amounts of 2':3'-cyclic nucleotide 3'-phosphohydrolase compared with heavy- and light-myelin fractions. The membrane fraction was deficient in myelin basic protein and proteolipid protein and enriched in high-molecular-weight proteins. The specific localization of galactosyltransferase in heavy-myelin and membrane fractions at an early age when myelination is just beginning suggests that it may have some role in the myelination process.

The turnover of myelin proteins in adult rat brain

The turnover of proteins of myelin of brain was followed after an intracerebral injections of [1-14C] leucine into adult rats and compared with the turnover of proteins of total brain homogenate, microsomal and supernatant fractions. The myelin proteins were separated into three major fractions, viz., the Wolfgram, proteolipid and basic proteins as fluorescamine derivatives by a new preparative sodium-dodecylsulfate-polyacrylamide slab gel electrophoresis method. The bands were visualized, without staining, by their fluoresence under UV light. The maximal incorporation into myelin proteins was about 11% of that in the microsomal proteins. The maximum amoount ofnto myelin proteins was about 11% of that in the microsomal proteins. The incorporation of the label in myelin proteins reached a moximum about six hr after the injection, while the microsomal and supernatant and myelin proteins declined at multiphasic rates with half-lives varying from three hours to several days. Individual myelin proteins also appeared to turn over at multiphasic rates, with half-lives of 5, 20 and 90-100 days for the Wolfgram proteins, 7 and 100-120 days for the proteolipid proteins and 5 and 80-110 days for the basic proteins.

Membrane turnover in imbibed dormant embryos of the wild oat (Avena fatua L.) : II. Phospholipid turnover and membrane replacement

Germinating non-dormant (ND) embryos of wild oat incorporate [(3)H]glycerol into phospholipid, and a 250% increase in total extractable phospholipid occurs within 72 h. During germination, leveles of phosphatidyl inositol showed the greatest change, increasing approximately 5-fold.Imbibed dormant (D) embryos of the wild oat also incorporate [(3)H]gycerol into phospholipids, but there is no net synthesis. A continuous turnover of membrane phospholipids could be demonstrated in pulse chase experiments, and although the proportions of most phospholipids does not change, there was a decrease of 50% in phosphatidyl serine.The half-life of [(3)H]glycerol in the extracted phospholipids of D and ND embryos varies between 35 and 57 h, and in membrane fractions separated on sucrose density gradients the half-lives vary between 26 and 56 h.D embryos induced to germinate with GA and ND embryos in which germination is repressed by ABA show similar phospholipid changes to ND and D embryos respectively, with the exception that the proportion of phosphatidyl serine remained unchanged in the ND-ABA embryos.It is concluded that the continual turnover of membranes of imbibed dormant embryos is consistent with the maintenance of cellular integrity determining the longevity of the seed under natural conditions.

Membrene turnover in imbibed and dormant embryos of the wild oat (Avena fatua L.) : I. Protein turnover and membrane replacement

A comparative study of protein synthesis has been carried out with embryos excised from dormant (D) and non-dormant (ND) caryopses of the wild oat. Although D embryos imbibed in water or ND embryos imbibed in abscisic acid do not germinate, they incorporate [(14)C]leucine into TCA-insoluble material for the first 48 h as readily as embryos that do germinate (ND embryos imbibed in water, or D embryos imbibed in gibberellic acid). Pulsechase experiments with [(14)]leucine show that in both D and ND embryos the proteins associated with the membranes undergo turnover. The rates of decay of incorporated radioactivity are similar in both dormant and germinating embryos up to 98 h following embryo excision. Fractionation of the membrane proteins in SDS-polyacrylamide gels indicates that the different polypeptides have different rates of turnover. It is concluded that membrane proteins in imbibed D embryos are in a state of constant turnover, and that this is a part of the replacement processes necessary to maintain the integrity of hydrated cells. The continuation of such synthetic events could account for long term survival of dormant Avena fatua in the imbibed state.

Synthesis and turnover of cerebrosides and phosphatidylserine of myelin and microsomal fractions of adult and developing rat brain

The synthesis and turnover of cerebrosides and phospholipids was followed in microsomal and myelin fractions of developing and adult rat brains after an intracerebral injection of [U-14C]serine. The kinetics of incorporation of radioactivity into microsomal and myelin cerebrosides indicate the possibility of a precursor-product relationship between cerebrosides of these membranes. The specific radioactivity of myelin cerebrosides was corrected for the deposition of newly formed cerebrosides in myelin. Multiphasic curves were obtained for the decline in specific radioactivity of myelin and microsomal cerebrosides, suggesting different cerebroside pools in these membranes. The half-life of the fast turning-over pool of cerebrosides of myelin was 7 and 22 days for the developing and adult rat brain respectively. The half-life of the slowly turning-over pool of myelin cerebrosides was about 145 days for both groups of animals. The half-life of the rapidly turning-over microsomal cerebrosides was calculated to be 20 and 40 h for the developing and adult animals respectively. The half-life of the intermediate and slowly turning-over microsomal cerebrosides was 11 and 60 days respectively, for both groups of animals. The amount of incorporation of radioactivity into microsomal cerebrosides from L-serine was greatly decreased in the adult animals, and greater amounts of the precursor were directed towards the synthesis of phosphatidylserine. In the developing animals, considerable amounts of cerebrosides were synthesized from L-serine, besides phosphatidylserine. The time-course of incorporation indicated that a precursor-product relationship exists between microsomal and myelin phosphatidylserine. The half-life of microsomal phosphatidylserine was calculated to be about 8 h for the fast turning-over pool in both groups of animals.

Incorporation of newly formed lecithin into peripheral nerve myelin

Radioactive choline was used to study the metabolism and movement of choline-containing phospholipids in peripheral nerve myelin of adult mice. Incorporation at various times after intraperitoneal injection was measured in serial segments of sciatic nerve as well as in myelin isolated from those segments. At no time (1 h to 35 days) could a proximal-distal difference in the extent of labeling be demonstrated. This finding suggests that incorporation of precursor choline phospholipids into nerve membranes is a local event, with little contribution from the neuronal perikaryon via axoplasmic transport. Autoradiographic investigations were undertaken to elucidate the pattern of movement of radioactive choline-labeled phospholipids, predominantly lecithin, into the myelin sheaths of the sciatic nerve. A sequence of autoradiographs was prepared from animals sacrificed between 20 min and 35 days after a microinjection of precursor directly into the nerve. Analysis of these autoradiograms revealed that labeling is initially concentrated in the Schwann cell cytoplasm. Later, the label moves first into the outer regions of the myelin sheaths and is eventually distributed evenly throughout the inner and outer layers of the sheath. At no time is there a build-up of label in the axon. The rate of uptake of precursor and subsequent redistribution of lecithin into the myelin were also examined in frog sciatic nerve (18 degrees C). Both uptake and redistribution processes were considerably slower in the cold-blooded animal.

Sensitive analysis of ethanolamine- and serine-containing phosphoglycerides by high-performance liquid chromatography

A highly sensitive method for the separation and quantitative measurement of phospholipids containing primary amino groups, such as phosphatidylethanolamine, phosphatidylserine and lysophosphatidylethanolamine, is described. The method involves a simple and quantitative derivative formation of the phospholipids containing amino groups to their u.v.-absorbing biphenylcarbonyl derivatives. These have molar extinction coefficients of about 23,000 at 268nm. The phospholipid derivatives are then separated and non-destructively determined by high-performance liquid chromatography. The amino phospholipids containing vinyl ether bonds (plasmalogens) can be determined separately from the diacyl- and alkylacyl-amino phospholipids. The lower limit of detection by high-performance liquid-chromatographic analysis of the phospholipid derivatives is about 10-13pmol or 0.3-0.4ng of phospholipid P. The quantitative range of derivative formation and analysis by high-performance liquid chromatography of the phospholipids containing amino groups was shown to be 10-500nmol. The method was shown to be applicable to the analysis of phospholipids containing amino groups in tissue samples.