In vivo acylation of rat brain myelin proteolipid protein

Myelin proteolipid protein is not esterified at the site of synthesis

Brain slices prepared from 20-day old rats were incubated with [(3)H]palmitic acid to study its incorporation into myelin proteins. After separation by SDS-PAGE, most of the label was found to be associated with the major proteolipid protein (PLP) and with the intermediate protein (I). The radioactivity measured in PLP at short incubation times was shown to be due to palmitic acid bound to the protein by ester linkages. Time-course incorporation of [(3)H]palmitic acid into PLP of fraction SN(4) (a myelin like membrane) and of purified myelin showed that the former was poorly labeled and no relationship of the type 'precursor-product' between these fractions could be detected. Incorporation of the fatty acid into PLP was not affected by inhibition of the synthesis or transport of myelin PLP with cycloheximide or colchicine, indicating that the pool of PLP that can be acylated must be larger than the extramyelin pool. Addition of unlabeled palmitic acid to the incubation medium, 30 min after the addition of [(3)H]palmitate, stopped the appearance of label in myelin PLP almost immediately, indicating that there is no significant extramyelin pool of PLP destined for transport into myelin. The results presented in this paper strongly suggest that esterification of PLP takes place in the myelin membrane or at a site very close to it.

Acylation of myelin proteolipid protein in subcellular fractions of rat brainstem

The acylation of myelin proteolipid protein (PLP) and intermediate protein (IP) was investigated in an in vitro system of tissue slices prepared from actively myelinating rat brainstem. The incorporation of [(3)H]palmitate into the proteins in nine subcellular fractions including myelin and other cellular membranes which are actively involved in the synthesis and intracellular transport of the proteins was measured. More than 80% of [(3)H]palmitate-labeled proteins were recovered in myelin. The incorporation was highest in the heavy myelin and lowest in the light myelin subfraction. Appreciable acylation was also detected in the myelin-like fraction. On the other hand, the remaining fractions comprising a variety of endo- and ectomembranes, which harbored over 90% of newly synthesized PLP and IP as seen from [(3)H]leucine labeling showed practically no [(3)H]palmitate incorporation. The results indicate that the acylation of PLP and IP is a late event in their posttranslational processing and occurs only at their entry into the myelin sheath.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Chemical deacylation reduces the adhesive properties of proteolipid protein and leads to decompaction of the myelin sheath

Myelin proteolipid protein (PLP) contains thioester-bound, long-chain fatty acids which are known to influence the structure of the molecule. To gain further insights into the role of this post-translational modification, we studied the effect that chemical deacylation of PLP had on the morphology of myelin and on the protein's ability to mediate the clustering of lipid vesicles. Incubation of rat optic nerves in isoosmotic solutions containing 100 mM hydroxylamine (HA) pH 7.4 led to deacylation of PLP and decompaction of myelin lamellae at the level of the intraperiod line. Incubation of nerves with milder nucleophilic agents (Tris and methylamine) or diluted HA, conditions that do not remove protein-bound fatty acids, caused no alterations in myelin structure. Other possible effects of HA which could have affected myelin compaction indirectly were ruled out. Incubation of optic nerves with 50 mM dithioerythritol (DTE) also led to the splitting of the myelin intraperiod line and this change again coincided with the removal of fatty acids. In addition, the apparently compacted CNS myelin in the PLP-less myelin-deficient rat, like that in tissue containing deacylated PLP, was readily decompacted upon incubation in isoosmotic buffers, suggesting that the function of PLP as a stabilizer of the interlamellar attachment is, at least in part, mediated by fatty acylation. Furthermore, in contrast to the native protein, PLP deacylated with either HA or DTE failed to induce the clustering of phosphatidylcholine/cholesterol vesicles in vitro. This phenomenon is not due to side-effects of the deacylation procedure since, upon partial repalmitoylation, the protein recovered most of its original vesicle-clustering activity. Collectively, these findings suggest that palmitoylation, by influencing the adhesive properties of PLP, is important for stabilizing the multilamellar structure of myelin.

Palmitoyl-protein thioesterase gene expression in the developing mouse brain and retina: implications for early loss of vision in infantile neuronal ceroid lipofuscinosis

Mutations in the palmitoyl-protein thioesterase (PPT) gene cause infantile neuronal ceroid lipofuscinosis (INCL), the clinical manifestations of which include the early loss of vision followed by deterioration of brain functions. To gain insight into the temporal onset of these clinical manifestations, we isolated and characterized a murine PPT (mPPT)-cDNA, mapped the gene on distal chromosome 4, and studied its expression in the eye and in the brain during development. Our results show that both cDNA and protein sequences of the murine and human PPTs are virtually identical and that the mPPT expression in the retina and in the brain is temporally regulated during development. Furthermore, the retinal expression of mPPT occurs much earlier and at a higher level than in the brain at all developmental stages investigated. Since many retinal and brain proteins are highly palmitoylated and depalmitoylation by PPT is essential for their effective recycling in the lysosomes, our results raise the possibility that inactivating mutations of the PPT gene, as occur in INCL, are likely to cause cellular accumulation of lipid-modified proteins in the retina earlier than in the brain. Consequently, the loss of vision occurs before the deterioration of brain functions in this disease.

Overview: protein palmitoylation in the nervous system: current views and unsolved problems

Palmitoylation refers to a dynamic post-translational modification of proteins involving the covalent attachment of long-chain fatty acids to the side chains of cysteine, threonine or serine residues. In recent years, palmitoylation has been identified as a widespread modification of both viral and cellular proteins. Because of its dynamic nature, protein palmitoylation, like phosphorylation, appears to have a crucial role in the functioning of the nervous system. Several important questions regarding the post-translational acylation of cysteine residues in proteins are briefly discussed: (a) What are the molecular mechanisms involved in dynamic acylation? (b) What are the determinants of the fatty acid specificity and the structural requirements of the acceptor proteins? (c) What are the physiological signals regulating this type of protein modification, and (d) What is the biological role(s) of this reaction with respect to the functioning of specific nervous system proteins? We also present the current experimental obstacles that have to be overcome to fully understand the biology of this dynamic modification.

Proteolipid protein and DM-20 are synthesized by Schwann cells, present in myelin membrane, but they are not fatty acylated

Proteolipid protein (PLP) and DM-20 were intensely labeled after immunoprecipitation of total cellular proteins and myelin proteins labeled with [35S]methionine in nerve slices. These results provided evidence that PLP and DM-20 are incorporated into the myelin membrane following their synthesis in Schwann cells. In contrast, PLP and DM-20 were not fatty acylated after incubation of the nerve slices with [3H]palmitic acid, however, P0 glycoprotein and 24 kDa protein were heavily fatty acylated. The lack of fatty acylation of PLP and DM-20 in the peripheral nervous system suggests that fatty acyltransferase responsible for their acylation is absent or non-functional in the peripheral nervous system.

Chorea-acanthocytosis: abnormal composition of covalently bound fatty acids of erythrocyte membrane proteins

Phospholipid class, peak profile of each phospholipid class, loosely bound fatty acids, covalently (tightly) bound fatty acids of the erythrocyte membranes, and plasma fatty acids were investigated using high-performance liquid chromatography in six patients with chorea-acanthocytosis and 14 age- and sex-matched normal control subjects. Additionally, six patients with Huntington's disease were included as disease control subjects in the study of covalently bound fatty acids. Study of covalently (tightly) bound fatty acids in erythrocyte membrane proteins after alkaline hydrolysis, hitherto undescribed in chorea-acanthocytosis, revealed that palmitic acid (C16:0) was significantly increased and stearic acid (C18:0) was decreased in the patients with chorea-acanthocytosis. Analyses for total covalently bound fatty acids disclosed that palmitic and docosahexaenoic (C22:6) acids were increased and stearic acid was decreased in chorea-acanthocytosis. Phospholipid class (phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, and phosphatidylserine) and peak profile of each phospholipid class from the erythrocyte membranes did not differ between the patients with chorea-acanthocytosis and the control subjects. Of the loosely bound fatty acids, linoleic acid (C18:2) was significantly decreased in those with chorea-acanthocytosis, which seemed to be nonspecific.

Myelin proteolipid protein contains thioester-linked fatty acids

Myelin proteolipid protein (PLP) is known to contain long-chain, covalently bound fatty acids. Previous studies, including our own, have suggested the occurrence of an oxyester type of linkage between fatty acids and PLP. However, we found that protein-SH groups are required in the acylation reaction, suggesting the possible presence of thioesters. In the present study, we have examined the nature of the acyl-PLP linkages by determining whether free thiol groups are generated on removal of fatty acids. Incubation of reduced and carboxyamidomethylated proteolipid apoprotein (RCM-APL) with 0.2 M hydroxylamine and [14C]iodoacetamide at pH 7.5 and 37 degrees C resulted in the release of fatty acids and the concomitant labeling of newly formed thiol groups. Incubation with Tris or methylamine at pH 7.5 failed to remove fatty acids and generate free -SH groups. The possibility that on treatment buried thiol groups became exposed was essentially excluded because (1) similar results were obtained in 2-chloroethanol, a solvent in which acylated and deacylated PLP have the same conformation, and (2) small PLP peptides were labeled only in the presence of hydroxylamine. On incubation with [14C]methylamine at pH 9.0, RCM-APL was not labeled, thus excluding the occurrence of intramolecular thiol esters. On the other hand, fatty acids were released as radioactive N-methyl fatty acylamide, indicating the presence of intermolecular thioesters between fatty acids and protein. These results demonstrate that a large proportion of fatty acids covalently bound to PLP are liked to -SH groups.

Cleavage of the P0 glycoprotein of the rat peripheral nerve myelin: tentative identification of cleavage site and evidence for the precursor-product relationship

The incubation of sciatic nerve slices in Krebs Ringer bicarbonate (KRB) buffer (pH 7.4) at 37 degrees C, or the incubation of freshly isolated myelin in ammonium bicarbonate buffer (pH 8), resulted in the generation of a 24 kDa protein with a concomitant decrease of P0 protein. The conversion of P0 into 24 kDa protein was blocked by heating isolated myelin at 100 degrees C for 5 min suggesting that the reaction is enzyme mediated. Inclusion of the protease inhibitors and chelating agent to isolated myelin did not prevent the formation of 24 kDa protein. Similarly, addition of CaCl2 to isolated myelin did not accentuate the formation of 24 kDa protein suggesting that the conversion of P0 into 24 kDa protein may not be due to Ca2+ activated protease. It is postulated that the formation of 24 kDa protein may be due to neutral protease and/or metalloproteinase associated with the PNS myelin. 24 kDa protein was purified and characterized. The N-terminal sequence of 1-17 amino acid residues of 24 kDa protein was identical to P0. 24 kDa protein was immunostained and immunoprecipitated with anti-P0 antiserum indicating the immunological similarities between P0 and 24 kDa protein. Labeling of 24 kDa protein with [35S]methionine provided evidence that P0 may be in all probability cleaved between Met-168 and Met-193. Further studies were carried out to demonstrate that 24 kDa protein was phosphorylated, glycosylated and acylated like P0. Phosphorylation of 24 kDa protein in the nerve slices was increased five-fold by phorbol esters and phosphoserine was the only phosphoamino acid identified after partial acid hydrolysis of 24 kDa protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of galactosylsphingosine (psychosine) does not interfere with phosphorylation and methylation of myelin basic protein in the twitcher mouse

In attempts to elucidate mechanisms of demyelination in the twitcher mouse (Twi), phosphorylation and methylation of myelin basic protein (MBP) were examined in the brainstem and spinal cord of this species. Phosphorylation of MBP in isolated myelin by an endogenous kinase and an exogenous [32P]ATP was not impaired and protein kinase C activity in the brain cytosol was not reduced. When the methylation of an arginine residue of MBP was examined in slices of the brainstem and spinal cord, using [3H]methionine as a donor of the methyl groups, no difference was found between Twi and the controls. Radioactivity of the [3H] methionine residue of MBP of Twi was also similar to that of the controls. Thus, accumulation of psychosine in Twi does not interfere with the activity of endogenous kinase, methylation of MBP, and the synthesis and transport of MBP into myelin membrane.

Cell-specific fatty acylation of proteins in cultured cells of neuronal and glial origin

Distinct sets of cellular proteins were labeled with [3H]myristic and [3H]palmitic acids in primary (rat neurons and astroglia) and continuous (murine N1E-115 neuroblastoma and rat C6 glioma) cell cultures derived from the nervous system. Both soluble and membrane proteins were modified by myristate in a hydroxylamine-stable (amide) linkage, while palmitoylated proteins were ester-linked and almost exclusively membrane bound. Chain elongation of both labeled fatty acids prior to acylation was observed, but no protein amide-linked [3H]myristate originating from [3H]palmitate was detected. Fatty acylation profiles differed considerably among most of the cell lines, except for rat astroglial and glioma cells in which myristoylated proteins appeared to be almost identical based on SDS gel electrophoresis. An unidentified 47 kDa myristoylated protein was labeled to a significantly greater extent in astroglial than in glioma cells; the expression of this protein could be related to transformation or development in cells of glial origin.

Decreased fatty acylation of myelin proteolipid protein in the twitcher mouse

We examined chronological changes of myelin proteins of the brainstem and spinal cord of the twitcher mouse (15, 20, and 30 days old), a murine model of human globoid cell leukodystrophy caused by a genetic deficiency of galactosylceramidase I activity. The yield of myelin was normal until postnatal day 20, whereas galactosylsphingosine (psychosine) accumulated with age in myelin. The protein profiles of myelin and the activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase in the myelin remained normal throughout the experimental period. Fatty acylation of proteolipid protein (PLP) was examined in a cell-free system by incubation of myelin with [3H]palmitic acid, CoA, and ATP, and was normal at postnatal day 15, but decreased after postnatal day 20. Decreased fatty acylation of PLP was also observed in the twitcher mouse at postnatal day 20 when the isolated myelin was incubated with [14C]palmitoyl-CoA in the absence of ATP and CoA, or the slices of brainstem and spinal cord were incubated with [3H]palmitic acid. The activity of fatty acid:CoA ligase was reduced in myelin. These data suggest that decreased acylation of PLP in twitcher mouse myelin is probably due to reduced activities for both activation and transfer of fatty acid into PLP and that metabolic disturbance is present in myelin because acylation of PLP has been shown to occur in myelin membrane. Although psychosine (200 microM) inhibited only 17% of the acylation in vitro, it may be responsible for the reduced acylation of PLP in vivo.

In vivo labeling of myelin lipids and proteolipid protein with [3H]myristate, [14C]linoleate, and [14C]linolenate

To investigate the incorporation of essential fatty acids into myelin components, 24-day-old rabbits were injected intracerebrally with [14C]linoleate, [14C]linolenate, or [3H]myristate for comparison. Animals were killed 22 hr later and myelin was isolated. [3H]myristate labeled all myelin lipids including monogalactosyl diglyceride, with the exception of sulfatides. With 14C-essential fatty acids, only glycerophospholipids were efficiently labeled and their specific activities were in the following decreasing orders: PC greater than PI greater than PE greater than PS with [14C]linoleate, and PE greater than PC greater than PI = PS with [14C]linolenate. Among myelin proteins, PLP and DM-20 were labeled with all 3 precursors. PLP was purified from myelin labeled with 14C-essential fatty acids. The label was then cleaved from the protein by alkaline methanolysis and was identified as a dienoic [( 14C]linoleate) or a tetraenoic [( 14C]linolenate) fatty acid. MBP was not labeled with [3H]myristate, but was slightly labeled with both 14C-essential fatty acids. The signification of the latter result is discussed.

Acylation in vitro of the myelin proteolipid protein and comparison with acylation in vivo: acylation of a cysteine occurs nonenzymatically

Characteristics of fatty acylation of myelin proteolipid protein (PLP) in vitro were compared with the corresponding process in vivo. Rapid and efficient separation of labelled PLP from other proteins and lipids was effected by extraction into chloroform/methanol/0.1 N HCl (10/10/1) and chromatography on Sephadex LH-60 in the same solvent. Covalent linkage of [3H]-palmitate to PLP was demonstrated by repetitive chromatography on LH-60, thin layer chromatography, and polyacrylamide gel electrophoresis. Reductive cleavage with sodium borohydride of PLP acylated in vitro or in vivo yielded [3H]-hexadecanol, identifying at least one of the acyl linkages as a thiolester bond. When PLP was acylated with acyl-CoA as the fatty acid donor, the reaction occurred non-enzymatically as supported by the following observations: 1) acylation activity increased with increasing pH above pH 7.5, 2) acylation activity was heat stable, 3) acylation activity was not removed from PLP during purification in organic solvents or in Triton X-100-containing buffers, and 4) acylation of tryptic fragments occurred in the absence of an exogenously added enzyme source. The relevance of in vitro fatty acylation of PLP to that in vivo was confirmed by comparison of proteolytically derived peptide maps that showed that likely the same domain of PLP was acylated in vitro and in vivo.

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

In vitro acylation of myelin PLP and DM-20 in the quaking mouse brain

Both proteolipid proteins (PLP) and DM-20 were found to be present by the immunoblot technique in myelin isolated from quaking mouse brain; however, the relative concentration of these proteins in myelin from quaking brain was substantially reduced when compared to the control. Brain slices from littermate control and quaking mice were incubated with [3H]palmitic acid to determine the incorporation of fatty acid into myelin proteolipid proteins. Fluorography of gels containing myelin proteins from control and quaking mice brain revealed that both PLP and DM-20 were acylated. The incorporation of [3H]palmitic acid into quaking myelin PLP and DM-20 was reduced by 75% and 20% respectively of those in control brain. The significance of differential acylation of quaking myelin PLP and DM-20 is discussed with respect to availability of non-acylated pools of proteolipid proteins and the activities of acylating enzymes.

The acylation of rat rhodopsin in vitro and in vivo

Rat retinas were incubated with [3H] palmitate and [14C] leucine and subsequently detergent-extracted. Glycoproteins were isolated on Con A-Sepharose columns and separated by gel electrophoresis. Leucine labeled the newly synthesized opsin but palmitate was esterified to both mature rhodopsin and newly synthesized opsin which migrated more slowly because of its untrimmed oligosaccharide chains. Crude rod outer segments were found to contain most of the palmitate-labeled mature rhodopsin, while the retinal debris contained most of the doubly labeled newly synthesized opsin. Homogenization of double-labeled retinas followed by centrifugation in a linear sucrose gradient gave rise to several bands of particles including purified rod outer segments, a Golgi-enriched fraction and a pellet enriched in endoplasmic reticulum. Newly synthesized opsin was first found in the pellet at the earliest incubation times and subsequently appeared in the Golgi fraction and finally in the rod outer segments. Palmitate-labeled mature rhodopsin was found only in the rod outer segments. It appeared at the earliest time points and increased with time. Thus the acylation of opsin occurs in the endoplasmic reticulum, shortly after polypeptide synthesis, and in the rod outer segments, the latter possibly as an exchange reaction. Most of the newly synthesized opsin remained in the pellet and did not pass through the Golgi to the rod outer segments. Intravitreal injection of [3H] palmitate and [14C] leucine gave rise to doubly labeled opsin that appeared to remain untrimmed for at least 6 hr in vivo. After 17 hr, both labels were found only in mature rhodopsin, thus accumulation of new molecules in the endoplasmic reticulum may occur in vivo. In addition, leucine maximally labeled the opsin-rhodopsin pool early in the first day whereas palmitate did not maximally label rhodopsin until 2- or 3 days post injection. Moreover, while leucine label was lost at day 9 because of rod outer-segment renewal and shedding, the palmitate label in rhodopsin remained unchanged. Thus, palmitate labeling in vivo reflects the pattern seen in vitro with a prolonged equilibration of rod outer-segment rhodopsin with the fatty-acid pool, probably mediated by a fatty acyl exchange reaction.

Acylation of rat brain myelin proteolipid protein with different fatty acids

The acylation of rat brain proteolipid protein (PLP) with tritiated palmitic, oleic, and myristic acids was studied in vivo and in vitro and compared with the acylation of lipids. Twenty-four hours after intracranial injection of [3H]myristic acid, only 16% of the PLP-bound label appeared as myristic acid, with 66% as palmitic, 9% as stearic, and 6% as oleic acid, whereas greater than 63% of the label in total or myelin phospholipid was in the form of myristic acid. In contrast, after labelling with [3H]palmitic or oleic acids, 75% and 86%, respectively, of the radioactivity in PLP remained in the original form. When brain tissue slices were incubated for short periods of time, the incorporation of palmitic and oleic acids into PLP exceeded that of myristic acid by a factor of 8. In both systems and with all precursors studied, the label associated with PLP was shown to be in ester linkage. The results suggest a preferential acylation of PLP with palmitic and oleic acids as compared with myristic acid. This is consistent with the fatty acid composition of the isolated PLP.

Fatty acid acylation of rat brain myelin proteolipid protein in vitro: identification of the lipid donor

The immediate acyl chain donor for fatty acid esterification of proteolipid protein (PLP) was identified in an in vitro system. Rat brain total membranes, after removal of crude nuclear and mitochondrial fractions, were incubated with radioactive acyl donors, extracted with chloroform/methanol, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of [3H]palmitic acid, CoA, ATP, and Mg2+, acylation of endogenous PLP occurred at a linear rate for at least 2 h. The radioactivity was associated with the protein via an ester linkage, mainly as palmitic acid. Omission of ATP, CoA, Mg2+, or all three reduced fatty acid incorporation into PLP to 44, 27, 8, and 4%, respectively, of the values in the complete system. Incubation of the membrane fraction with [3H]palmitoyl-CoA in the absence of CoA and ATP led to highly labeled PLP. These data demonstrate that activation of free fatty acid is required for acylation. Phospholipids and glycolipids were not able to acylate the PLP directly. Finally, when isolated myelin was incubated with [3H]palmitoyl-CoA in the absence of cofactors, only PLP was labeled, thus confirming the identity of palmitoyl-CoA as the direct acyl chain donor and suggesting that the acylating activity and the PLP pool available for acylation are both in the myelin.

Electron microscopic immunocytochemical localization of myelin proteolipid protein and myelin basic protein to oligodendrocytes in rat brain during myelination

Electron microscopic immunocytochemical studies were carried out to localize myelin basic protein and myelin proteolipid protein during the active period of myelination in the developing rat brain using antisera to purified rat brain myelin proteolipid protein and large basic protein. The anti-large basic protein serum was shown by the immunoblot technique to cross-react with all five forms of basic protein present in the myelin of 8-day-old rat brain. Basic protein was localized diffusely in oligodendrocytes and their processes at very early stages in myelination. The immunostaining for basic protein was not specifically associated with any subcellular structures or organelles. The ultrastructural localization of basic protein suggests that it may be involved in fusion of the cytoplasmic faces of the oligodendrocyte processes during compaction of myelin. Immunoreactivity in the oligodendrocyte and myelin due to proteolipid protein appeared at a later stage of myelination than did that due to basic protein. Staining for proteolipid protein in the oligodendrocyte was restricted to the membranes of the rough endoplasmic reticulum, the Golgi apparatus, and apparent Golgi vesicles. The early, uncompacted periaxonal wrappings of oligodendrocyte processes were well stained with antiserum to large basic protein whereas staining for proteolipid protein was visible only after the compaction of myelin sheaths had begun. Our evidence indicates that basic protein and proteolipid protein are processed differently by the oligodendrocytes with regard to their subcellular localization and their time of appearance in the developing myelin sheath.

Deacylation of myelin proteolipid protein in organic solvents

A procedure has been developed for the deacylation of the hydrophobic, myelin proteolipid apoprotein using hydroxylamine in an alkaline organic solvent medium. Complete removal of covalently bound fatty acids was obtained after 4 hr of treatment. After deacylation, no changes could be detected in the electrophoretic pattern or in the number of free sulfhydryl groups. The deacylated apoprotein remains soluble in chloroform-methanol mixtures and is suitable for further physicochemical characterization.

In vitro synthesis of myelin proteolipid protein from rat brain using a total homogenate system

In vitro synthesis of myelin proteolipid protein (PLP) was explored at different ages using rat brain total homogenates, incubated for 30 min with [3H]glycine. Total proteolipids, extracted from the incubated samples, were separated by SDS-PAGE and the radioactivity was measured in the band corresponding to myelin PLP. The incorporation into PLP in relation to the incorporation into brain total proteins increased from 0.04% at 10 days of age to 0.63% at 20 days, and declined slowly thereafter. Time course experiments were carried out using brain homogenates obtained from rats of 20 days of age (i.e. at the period of maximal synthesis of PLP). Labeled PLP molecules were already found at 2.5 min of incubation and the incorporation of the label into this protein, relative to the incorporation into total proteins, did not vary throughout the entire incubation time (30 min). Pulse-chase experiments using a similar system and adding cycloheximide at different incubation times showed that the appearance of label into mature PLP was immediately blocked by the inhibitor of protein synthesis. These data suggest that PLP is synthesized as such and not as a pre-protein which is subsequently processed to render mature PLP.

Biochemical characterization of the central nervous system myelin proteins of the rainbow trout, Salmo gairdneri

Central nervous system myelin isolated from the rainbow trout (Salmo gairdneri) displays a very low median density on zonal gradient centrifugation, banding at approximately 0.32 M sucrose. Its proteins consist of a 36 K (36,000 mol.wt.) component, two Concanavalin A-reactive intermediate proteins IP1 (23,000 mol.wt.) and IP2 (26,200 mol.wt.), and two basic proteins BP1 and BP2, of which the latter co-migrates with rat SBP while BP1 is of slightly smaller size. The trout myelin proteins electrofocus at pH positions similar to those of their mammalian counterparts. Immunoblotting shows that antibodies against rat PNS myelin P0 glycoprotein are bound by IP1 and IP2, but not by 36K. None of the trout myelin proteins react with anti-rat CNS myelin proteolipid protein (PLP) antiserum. The basic proteins BP1 and BP2 bind strongly to antibodies directed against human myelin basic protein. In vivo injection of tritiated fucose or palmitate leads to radiolabeling of IP1 and IP2. Under autolytic in situ conditions the appearance of a glycosylated 20,000 mol.wt. component (IP0) is noted, with parallel reduction of both IP1 and IP2, indicating sequence homologies between IP1 and IP2. The 36K protein is not affected by autolysis.

Identification of antibodies in anti-CNS and anti-PNS myelin sera by immunoblot, characterization by immunohistochemistry, and their effect in tissue culture

Immunoblot analysis of antiserum to rat central nervous system (CNS) myelin revealed antibodies to myelin basic protein (MBP), proteolipid protein (PLP), and numerous high molecular weight proteins. In addition, anti-CNS myelin serum exclusively immunostained 4 basic proteins of rat peripheral nervous system (PNS) myelin. Similarly, anti-PNS myelin sera immunostained many high molecular weight proteins in both CNS and PNS myelin in addition to P0 and 4 basic proteins. Purified MBP and PLP were immunostained by anti-CNS myelin sera and MBP and P0 by anti-PNS myelin sera, indicating that antigenic sites are preserved during protein purification. Immunohistochemical localization with antisera was confined to the myelin sheath except that antisera to CNS myelin also stained oligodendrocytes during the active period of myelination. While anti-CNS myelin sera specifically demyelinated centrally myelinated fibers in culture, none of the anti-PNS myelin sera used here demyelinated organotypic spinal cord-dorsal root ganglion cultures.

Fatty-acid-binding proteins. Occurrence of two fatty-acid-binding proteins in bovine liver cytosol and their binding of fatty acids, cholesterol, and other lipophilic ligands

Fatty-acid-binding proteins (FABPs) are known as cytosolic binding sites for fatty acids and their CoA esters. Radioactively labeled and fluorescent fatty acids were used to locate and identify these proteins in bovine liver cytosol. The occurrence of two species of FABPs was demonstrated and these were designated pI6.0-FABP and pI7.0-FABP according to their isoelectric points in the delipidated state. Oleic acid/FABP binding ratios were 1 with pI6.0-FABP and 2 with pI7.0-FABP. Upon binding of oleic acid the isoelectric points of liganded FABPs shifted to pH 5.0-5.1 in each case. Both proteins were purified by removing nonbinding proteins by acid and heat denaturation and subsequent gel filtration. By making use of the pI shifts observed upon lipidation and delipidation of the binding proteins with ligand fatty acids, final purification was achieved in two fractionations by isoelectric focusing. The binding proteins (Mr 11 800 +/- 1 000) had similar amino-acid compositions (no Trp) and were not covalently modified by carbohydrate and fatty acid. Fatty acids and their CoA esters were complexed by either FABP, cholesterol only by pI-7.0-FABP, though non-stoichiometrically. 16-(9-Anthroyloxy)palmitic acid was bound by pI-7.0-FABP in a 1:1 ratio and precluded the additional binding of a straight-chain fatty acid. Electrophoretic titration curves indicated dissociation of the oleic acid/pI7.0-FABP complex below pH 5.0. It appears that fatty acids and their CoA esters are the foremost binding partners of FABPs in vivo. The results are discussed in terms of a single binding site for fatty acids per molecule FABP.

The primary structure of bovine brain myelin lipophilin (proteolipid apoprotein)

The amino-acid sequence of bovine myelin lipophilin (proteolipid apoprotein, Folch-protein) has been completed. Lipophilin is a 276 amino acid residues containing, extremely hydrophobic membrane protein with molecular mass 30,000 Da. The sequence determination was based on automated Edman degradation of four tryptophan and four cyanogen bromide fragments and of proteolytic peptides of complete lipophilin as well as the fragments obtained by chemical cleavage. Four additional sequences were determined which led to the completion of the primary structure. Lipophilin is esterified at threonine-198 by long chain fatty acids (palmitic, stearic and oleic acid). The attachment site has been established at the same threonine residue in three different peptides isolated from thermolysinolytic, papainolytic and chymotrypsinolytic hydrolysates. This threonine residue is part of a hydrophilic segment of lipophilin. The covalent fatty acyl bond is being discussed together with important structural and functional properties of this membrane protein which can be derived from sequence information. New separation and purification methods of hydrophobic and hydrophilic polypeptides for this sequence determination (fractional solubilization, silica gel exclusion, high-performance liquid chromatography) had to be elaborated as indispensable tools. They are generally applicable to the structural analysis of hydrophobic membrane proteins. Four long (26, 29, 40 and 36 residues) and one medium long (12 residues) hydrophobic segments are separated by four predominantly positively and one negatively charged hydrophilic segments. On the basis of structural data a model for the membrane integration of lipophilin is proposed.

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.

In vivo acylation of proteolipid protein and DM-20 in myelin and myelin subfractions of developing rat brain: immunoblot identification of acylated PLP and DM-20

The acylation of proteolipid protein (PLP) was examined in myelin and myelin subfractions from rat brain during the active period of myelination. Proteolipid protein and DM-20 in myelin and myelin subfractions were readily acylated in developing rat brain 22 hours after intracerebral injection of [3H]palmitic acid. No differences in the relative specific activity of PLP in myelin from 9-, 15-, and 30-day-old rat brains was observed; however, the relative specific activity of PLP in the heavy myelin subfraction tended to be higher than that in the light myelin subfraction. The acylation of PLP was confirmed by fluorography of immuno-stained cellulose nitrate sheets, clearly establishing that the acylated protein is in fact the oligodendroglial cell- and myelin-specific protein, PLP. Since PLP is acylated in the 9-day-old animal, when little compact myelin is present, it is possible that the acylation of PLP is a prerequisite for the incorporation of this protein into the myelin membrane.