The turnover of myelin phosphatidylcholine and sphingomyelin in the adult 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.

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.

Phospholipid metabolism in mouse sciatic nerve in vivo

To probe the activities of various pathways of lipid metabolism in peripheral nerve, six phospholipid-directed precursors were individually injected into the exposed sciatic nerves of adult mice, and their incorporation into phospholipids and proteins was studied over a 2-week period. Tritiated choline, inositol, ethanolamine, serine, and glycerol were mainly used in phospholipid synthesis; in contrast, methyl-labeled methionine was primarily incorporated into protein. Phosphatidylcholine was the main lipid formed from tritiated choline, glycerol, and methionine precursors. Phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol were the main lipids formed from serine, ethanolamine, and inositol, respectively. With time there was a shift in label among phospholipids, with higher proportions of choline appearing in sphingomyelin, glycerol in phosphatidylserine, ethanolamine in phosphatidylethanolamine (plasmalogen), and inositol in polyphosphoinositides, especially phosphatidylinositol 4,5-bisphosphate. We suggest that the delay in formation of these phospholipids, which are concentrated in peripheral nerve myelin, may, at least in part, be due to their formation at a site(s) distant from the sites where the bulk of Schwann cell lipids are made. We propose that separating the synthesis of these myelin-destined lipids to near the Schwann cell's plasma membrane would facilitate their concentration in peripheral nerve myelin sheaths. At earlier labeling times, ethanolamine and glycerol were more actively incorporated into phosphatidylcholine and phosphatidylinositol, respectively, than later. The transient labeling of these phospholipids may reflect some unique role in peripheral nerve function.

UDP-galactose: ceramide galactosyltransferase of rat central nervous system myelin during development

The activity of UDP-galactose:hydroxy fatty acid containing ceramide galactosyltransferase was studied in the myelin and microsomal fractions of rat cerebral hemispheres, cerebellum and spinal cord during development. In all three regions, the specific activity of the enzyme reached a maximum in myelin prior to that in the microsomal membranes. This temporal relationship between myelin and microsomal fraction was similar in all the three regions, although the overall timing was shifted corresponding to known differential timing of myelin deposition in these regions. The activity of the enzyme from both the membranes, during development, increased in parallel with temperature up to 45 degrees C. Specific localization of galactosyltransferase in early myelin may suggest specific role of the enzyme in the myelination process.

The phospholipid-N-methyltransferase of rat brain microsomes

The phospholipid-N-methyltransferase activity of rat brain microsomes had an optimum pH of 11.0 in the absence or presence of phosphatidylethanolamine (PE) but pH 10.0 in the presence of phosphatidylmonomethylethanolamine (PMME) or phosphatidyldimethylethanolamine (PDME). An apparent Km for S-adenosyl methonine from 0.10 to 0.12 mM was observed with exogenous methylated phospholipids PMME or PDME. Methylated neutral lipid was the major lipid produced in the absence of the exogenous acceptors. Two exogenous phospholipids, PMME and PDME, significantly stimulated microsomal phospholipid-N-methyltransferase activity and the predicted methylated phospholipids were the major products. PE additions did not cause any stimulation of methylated lipid formation. Preincubation of particles at temperatures from 40 to 100 degrees C resulted in a loss in the microsomal phospholipid-N-methyltransferase activity that was stimulated by PMME and PDME.

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)

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.

Contribution of axonal transport to the renewal of myelin phospholipids in peripheral nerves. II. Biochemical study

The classes of radioactive phospholipids appearing in the ciliary ganglion (CG) and especially in the myelin sheath of the intraorbital part of the oculomotor nerve (OMN) were determined after the intracerebral injection of [2-3H]glycerol and [methyl-14C]choline to chickens. Analysis of the radioactive compounds in water-soluble fractions and chloroform-methanol extracts was performed by thin-layer chromatography (TLC). The water-soluble content of the OMN and CG was much poorer in [2-3H]glycerol and metabolites than in [methyl-14C]choline and derivatives. All classes of glycerophospholipids were found to be axonally transported along the OMN and into the CG, but choline-phosphoglycerides (CPG) were largely predominant. In myelin fractions from the OMN, the specific radioactivity (SRA) of CPG labeled with [2-3H]glycerol reached a maximum earlier (40 h) than the SRA of CPG labeled with [methyl-14C]choline. A 25-fold enhancement of the [14C]SRA of sphingomyelin (SM) was observed between 12 h and 7 days. These results indicate that: (1) axonally transported phospholipids labeled with [2-3H]glycerol consist mainly of CPG; (2) small amounts of CPG are translocated from the axon to myelin; and (3) the progressive enrichment of myelin in [14C]CPG and, to a greater extent, SM draws attention to the importance of the base recycling for local synthesis of myelin phospholipids. Thus the axonal supply of Schwann cells with choline and the transfer of axonal phospholipids to myelin would probably contribute to the metabolic interdependence existing between neuron and glia.

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.

Choline antagonism of methotrexate liver toxicity in the rat

Because of the frequent reports of hepatic toxicity associated with long-term administration of methotrexate, a rat model was developed utilizing daily methotrexate administration. This model revealed an incidence of fatty metamorphosis of over 80 percent, atrophy and necrosis of 30 percent, and fibrosis of 10 percent. Fatty liver changes did not differ substantially from control animals in those animals receiving long-term thydroxyurea, an agent which, like methotrexate, inhibits DNA synthesis but unlike methotrexate, does not impair methylation reactions. Because choline has a lipotropic effect and because its synthesis requires methylation, an attempt was made to block the liver toxicity of methotrexate by simultaneous administration of choline. Animals so treated did not show the pathologic changes in the liver characteristic of methotrexate treatment alone. Furthermore, the accumulation of triglycerides in the liver which was characteristic of methotrexate administration was markedly reduced in those animals receiving choline. These data strongly suggest that, in the rat model, methotrexate produced liver toxicity by virtue of an effect other than inhibition of DNA synthesis; and that this toxicity can be blocked without impairing methotrexate effect on bone marrow by the administration of choline, a lipotropic agent requiring methylation for its synthesis. It is suggested that these results may have implications for human therapeutic situations involving long-term administration of methotrexate.

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.

Effects of chemotherapy on the central nervous system. A study of parenteral methotrexate in long-term survivors of leukemia and lymphoma in childhood

Twenty-three children surviving more than 5 years from the diagnosis of leukemia, lymphoma, or malignant histiocytosis were evaluated for clinical evidence of central nervous system disease. Severe impairment, consisting of seizures, paraplegia, and dementia was present in 4, all of whom received methotrexate (MTX) and other agents for 2 to 7 years. Brain biopsies in 3 of these children showed white-matter gliosis and no evidence of viral or other infections or leukemic infiltrates. Of the remaining children 10/19 were found to have mild clinical or EEG abnormalities. All had received i.v. MTX with other drugs for 2-6 years; 5 did not receive cranial irradiation. Common to all impaired patients was the administration of intravenous methotrexate in relatively high doses over a prolonged period of time. Impairment in nervous system function may present as a spectrum of deficiencies, with the most severe resulting in death, or as in the 4 patients described here, profound dementia and dependence. Less dramatic changes in functioning, may, however, result from various combinations of chemotherapy and radiation therapy. Methods of assessing their etiology and impact on survivors need now to be devised.

Synthesis and turnover of brain phosphoglycerides- results, methods of calculation and interpretation

The important problem of membrane assembly and disassembly can be studied by measurements of rates of tunrover of labeled components. After intracerebral injections of ethanolamine and glycerol into mice, we have found rapid and slow turnover pools of 1,2-diacyl-sn-glycero-3-phosphorylethanolamine (diacyl-GPE) and 1,2-diacyl-sn-glycero-3-phosphorylcholine (diacyl-GPC). We have described the methods for calculation of half-lives for two or more pools and for the calculation of the relative size of the pools. For mice injected between 5 and 8 weeks of age, the rapid turnover pools have a half-life of 1.5 and 1.8 days for diacyl-GPC and diacyl-Ge respectively. Corresponding half-lives for the slow turnover pools are 20 and 27 days. The slow turnover pools include 74% of the diacyl-GPC and 86% of the diacyl-Ge. These turnover rates are in agreement with the flux of fatty acids through the diacylglycerol pools. We have proposed that the rapid turnover pool may be the phosphoglycerides that are exchangable with cytosol carrier proteins and that the slow turnover may represent the catabolism of membrane segments including the intrinsic proteins.