Density and protein profiles of myelin from two regions of young and adult rat CNS

Neurotoxicant effects of bisphenol A, nonylphenol, and tert‑butyl phenol in the Nile tilapia (Oreochromis niloticus)

Worldwide production of alkyl phenols and ethoxylated alkyl phenols is high due to their broad industrial uses. It has been widely documented that they are endocrine disruptors, and it has been suggested that they could exert neurotoxic effects. However, a lack of information about the neurotoxic effects of APs and APEs prevails. In this study, the bisphenol A (BPA), 4-nonylphenol (NP), and 3‑tert-butylphenol (tertBP) effects on brain and spinal cord of Nile tilapia exposed to environmental concentrations were evaluated by assessing acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), and carboxylesterases (CES) activities, and γ-aminobutyric acid (GABA) levels and their effects were evaluated by molecular docking. BPA and NP, tertBP behave as agonists and antagonists of AChE, BuChE, CES, and GABA, with notable differences among organs. However, none of these compounds or their metabolites interact with the enzymes' catalytic triad, suggesting an indirect alteration of enzymatic activities. While inhibiting these enzymes stand out hydrophobic interactions with the peripheral anion site, contacts with the inner face of the active site and blocking the mouth of the gorge of the active site, and steric hindrance in the enzyme pocket of glutamate decarboxylase (GAD). In contrast, inductions probably are by homotropic pseudo-cooperative phenomenon, where APEs behave as anchors favoring the active site to remain open and interactions that confer a conservative stabilization of the regulatory domain. Although the results of this study are complex, with notable differences between organs and toxicants, they are some of the first evidence of the neurotoxicity of alkylphenols and their ethoxylated derivatives.

Myelin vesicles: what we know and what we do not know

In the following review, we address difficulties that have arisen when attempting to convert the myelin multilayers into vesicles. The emphasis is on CNS myelin of adult mammals although both central nervous system (CNS) and peripheral nervous system (PNS) myelin are considered. The ability to prepare vesicle of myelin membrane has yet not been feasible. We hope to clarify some aspect of this problem and offer some possible approaches. Special attention is paid to myelin swelling phenomena because these indicate ways in which the myelin multilayer can break down. Images of isolated myelin are reviewed with special attention to the ways in which the multilayer actually breaks down. Attempts at reproducing a procedure for vesiculating myelin are summarized, and a critique is given to account for the inability to reproduce the published results. Finally, novel approaches for vesiculating myelin are proposed, which are based on well-characterized swelling phenomena.

Characterization of a cytoskeletal matrix associated with myelin from rat brain

Extraction of rat brain myelin in a buffer containing Triton X-100 yielded a soluble fraction and an insoluble residue that was enriched in cytoskeletal elements. Immunoblot analysis of the detergent-soluble fraction and the insoluble cytoskeletal residue showed that all of the tubulin and more than half of the actin were found within the cytoskeletal fraction. The distribution of myelin-specific proteins was also examined, and revealed that 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) I and most of the myelin basic proteins (MBPs) were equally distributed between both fractions. By contrast, the large MBP (21.5 kDa) and CNPase II (50 kDa) were observed to partition almost entirely with the cytoskeletal fraction. Proteolipid protein was found predominantly in the detergent-soluble fraction, as was DM-20 protein. Analysis of the cytoskeletal fraction by sucrose-density-gradient centrifugation demonstrated that a distinct subset of lipids was tightly bound to the cytoskeletal protein residue. The cytoskeleton-associated lipid was considerably enriched in cerebroside and sphingomyelin by comparison with total myelin lipids. These results indicate that a cytoskeletal matrix is associated with multilamellar myelin, and suggest that this structure may play a fundamental role in myelinogenesis.

Buoyant density and lipid composition of purified myelin of aging human brain

Purified myelin of human brain from 15 young adult (below 50 years of age) and old (above 70 years of age) autopsy cases each was examined by isopycnic centrifugation in continuous sucrose gradients, and for lipid composition. The mean buoyant density of myelin was the same in both groups. Apparent features of old age were a wide range of density values, less compact myelin bands, and the dissociation of myelin into two bands in six of 15 old cases. Lipid analyses of randomly selected myelin samples of each group revealed an inverse relationship between the total lipid to protein ratio and density of myelin. In old age total lipids decreased by an average 10 mol lipid per mol protein. This decrease was accounted for by cholesterol, phosphatidylserine and cerebrosides. Changes in fatty acid moieties mainly affected sphingolipids. C20:0 and C24:0 of sphingomyelin increased, as did even more markedly the more hydrophilic OH-fatty acids of cerebrosides. Correlations with buoyant density existed for the ratios of cholesterol to protein in young adult cases, and those of galactolipids to protein in old cases. The results suggest that old age is associated with impaired stability and altered lipid composition of myelin.

Membrane interactions in nerve myelin: II. Determination of surface charge from biochemical data

In our accompanying paper (Inouye and Kirschner, 1988) we calculated the surface charge density at the extracellular surfaces in peripheral and central nervous system (PNS; CNS) myelins from observations on the dependency of the width of the extracellular space on pH and ionic strength. Here, we have determined the surface charge density of the membrane surfaces in myelin from its chemical composition and the localization of some of its molecular components. We then analyzed the attractive and repulsive forces between the apposed surfaces and calculated equilibrium periods for comparison with the measured values. The biochemical model accounts for the observed isoelectric range of the myelin period and, with the surface charge reduced (possibly by divalent cation binding or a space charge approximation), the model also accounts for the dependency of period on pH above the isoelectric range. At the extracellular (and cytoplasmic) surfaces the contribution of lipid (with pI approximately 2) to the net surface charge is about the same in both PNS and CNS myelin, whereas the contribution of protein depends on which ones are exposed at the two surfaces. The protein conformation and localization modulate the surface charge of the lipid, resulting in positively-charged cytoplasmic surfaces (pI approximately 9) and negatively-charged extracellular surfaces (pI approximately 2-4). The net negative charge at the extracellular surface is due in CNS myelin to lipid, and in PNS myelin to both lipid and (PO) glycoprotein. The net positive charge at the cytoplasmic surface is due in CNS myelin mostly to basic protein, and in PNS myelin to PO glycoprotein and basic protein. The invariance of the cytoplasmic packing may be due to specific short-range interactions. Our models demonstrate how the particular myelin proteins and their localization and conformation can account for the differences in inter-membrane interactions in CNS and PNS myelins.

Biochemical and physicochemical determinations in a premyelin fraction obtained by zonal centrifugation in normal mouse and in dysmyelinating mutants (quaking, shiverer, and myelin-deficient)

Myelin and premyelin material denser than myelin were obtained from quaking (Qk), shiverer (Shi), and myelin-deficient (mld) mutant and control mice, using zonal centrifugation on zonal rotor. On these fractions, we performed biochemical analysis (lipids and fatty acid), and, in parallel, we determined the physical structure of membranes by the spin-label method. The hyperfine splitting constant (2 Tll) was used to determine the order of membranes and their rigidity, and frequency of rotation (Vc) was used to measure fluidity. In control mice, the premyelin material contained a lesser amount of sphingolipids than pure myelin, but the relative proportions between hydroxy- and nonhydroxy-cerebroside and sulfatides were similar in the premyelin material and in pure myelin. The premyelin material contained half the alkanes found in the pure myelin and much less very-long-chain fatty acids. The (2 Tll) was lower in the premyelin material, but the (Vc) was similar in myelin and premyelin material. In mutants, the amount of material recovered in the premyelin fraction was reduced in qk, and increased in both shi and mld. The relative amount of sphingolipids were normal in mld, but not in shi mutants, especially in cerebrosides formed with alpha-hydroxylated fatty acids and sulfatides formed with unsubstituted fatty acids. The absolute amounts of sphingolipids were nearly normal in both shi and mld. In the premyelin fraction from qk mutants, both relative and absolute amounts of sphingolipids were drastically altered. In percentage, cerebrosides and sulfatides formed with nonhydroxyfatty acids were dramatically reduced, and, conversely, cerebrosides and sulfatides formed with hydroxyfatty acids were increased. In terms of absolute amount, only cerebrosides and sulfatides formed with nonhydroxyfatty acids were dramatically reduced. In the premyelin fraction, polyunsaturated fatty acids were increased in shi and mld, but decreased in qk. In this mutant, lignoceric (24:0) and nervonic (24:1) acids were drastically reduced. The amount of alkanes in the premylin material from qk and mld was reduced by 50%. The shi fraction was nearly free of alkanes. The maximal apparent coupling constant (hyperfine splitting constant, 2 Tll) was not affected in the mld and qk mutant, but was reduced in the shi mutant premyelin fraction. The Vc was dramatically increased in the qk, slightly decreased in the shi, and close to control in the mld. This work provides additional data on premyelin material prepared in various neurological mutants using continuous gradients in zonal rotor.(ABSTRACT TRUNCATED AT 400 WORDS)

Biosynthesis and insertion of Wolfgram protein into optic nerve membranes

Antibodies against pig brain Wolfgram protein (WP) were prepared and utilized in the analysis of WP biosynthesis in membranes from optic nerves of 20 day-old rats. Newly synthesized WP appeared rapidly (less than 5 min) in myelin and in a non-myelin microsome fraction and accumulated in both thereafter. Monensin did not affect the insertion of WP in either membrane fraction. These results are consistent with biosynthesis of WP on free ribosomes.

Characterization of myelin fractions from human brain white matter

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

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.

Distribution of lipid synthesizing enzymes, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and myelin proteins in rat forebrain subfractions during development

The distribution of UDP-galactose:ceramide galactosyltransferase (CGalT) was studied in subcellular fractions of rat forebrain during development using zonal centrifugation on linear gradients. Specialized subfractions: SN 1, a microsomal fraction, SN 4, a myelin-related fraction, and purified myelin were also used for this study. For comparison, two microsomal lipid synthesizing enzymes, a myelin-specific enzyme, 2',3'-cyclic nucleotide 3'-phosphodiesterase and myelin proteins were measured in the same subfractions. UDP-glucose:ceramide glucosyltransferase and cerebroside sulfotransferase were confined to microsomes. CGalT was localized in microsomes, but also in myelin and myelin-related fractions. The developmental change in distribution of CGalT in adult animals toward myelin containing fractions could indicate that the replacement of galactosylceramide in compact myelin could be carried out in close proximity to compact myelin (mesaxon, paranodal loops) rather than in the distant oligodendrocyte perikaryon.

Central nervous system myelin proteins and glycoproteins in vertebrates: a phylogenetic study

CNS myelin was isolated by a conventional method from a wide range of vertebrate classes and analyzed by SDS-PAGE for proteins (Coomassie blue) and glycoproteins (concanavalin A (Con-A)-peroxidase). Mammalian, avian and reptilian myelin shared similar protein patterns (basic protein, BP; intermediate protein, DM-20; proteolipid protein, PLP; Wolfgram protein, W). Amphibians lacked DM-20 but were characterized by specific activities of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) higher than those of the other classes examined. The Con A-binding profiles were similar in the high molecular weight (HMW) regions of the classes listed above, while the typical myelin proteins in the low molecular weight (LMW) regions were devoid of Con A-binding properties. In teleost myelin a putative BP band ran well ahead of rat small basic protein (SBP), whereas the region corresponding to rat PLP was covered by several closely spaced bands, most of which bound Con A. In elasmobranch myelin, apart from bands corresponding to BP, Con A-binding glycoproteins were detected migrating in the region of rat DM-20 and PLP as well as with mammalian PNS P0 protein. Cyclostomates yielded only very small amounts of material in the myelin preparation and displayed undifferentiated Coomassie blue- and Con A-binding in the HMW region, while typical LMW myelin proteins were absent. These results demonstrate that CNS myelin from bony and cartilaginous fishes is characterized by containing several major Con A-binding proteins of low molecular weight. This is in striking contrast to myelin from phylogenetically higher classes.

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

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

The lipid composition of light and heavy myelin subfractions isolated from rabbit sciatic nerve

Centrifugation of adult rabbit sciatic nerve homogenates on continuous sucrose density gradients yields three distinct maxima at 0.12, 0.33 and 0.57 M sucrose (labeled A, B and C), respectively. Fraction A is absent in homogenates of immature rabbit sciatic nerve and the material isolated from adult animals is found to contain large amounts of triglycerides. It is proposed that fraction A results from the presence of adipose tissue adhering to the adult nerve preparations and should not be regarded as a true myelin subfraction. The concentration of the major membrane lipid classes (phospholipid, cholesterol and galactosyl ceramide) decreases from the light to the heavy side of the gradient. The ethanolamine phosphatide to sphingomyelin ratio increases from 1.06 to 1.42 between fractions B and C, due to an increase in the phosphatidal ethanolamine content at the expense of sphingomyelin.

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

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

Dissociation of myelin from its 'enzyme markers' during ontogeny

To avoid any loss of membranes, total particulate material prepared from the forebrain and spinal cord of rats of different ages was fractionated on linear sucrose gradients. Particle distribution, proteins and enzyme activities were measured. Beginning at early ages, optical density increases were observed around 0.60 M-sucrose, which corresponded to myelin deposition, and were expressed by the appearance of typical myelin proteins, which paralleled the peak activity of myelin-associated enzymes. During later development myelin proteins were found over a broad density range together with optical density shifts to higher (forebrain) and lower (spinal cord) values. In both regions myelin-associated enzymes shifted to heavier densities and were dissociated from the density region commonly considered to be compacted myelin.

Myelination in rabbit optic nerves is accelerated by artificial eye opening

Artificial opening of the eyes of young rabbits on the 5th postnatal day led to accelerated myelination: the myelin-specific basic and proteolipid proteins nearly doubled between the 7th and the 10th postnatal days when compared to controls; 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) activity also increased by about 60%. Conversely, lowered AChE activities presumably reflected elevated myelin/axolemma ratios. Myelination in treated animals normalized during later ontogenetic stages (greater than 20th postnatal day).

Distribution of phosphoinositides among subfractions of rat brain myelin

Rat brain myelin was separated into three subfractions, heavy, medium, and light, and the concentrations of phosphatidic acids (PA), phosphatidylinositol (PI), di- (DPI), and triphosphoinositide (TPI) in these fractions were determined. PI was evenly distributed among the fractions, and PA, DPI, and TPI occurred in highest concentrations in the "light" myelin. This result indicates that these fast metabolizing lipids play an important role in the tightly packed central lamellae of the myelin sheath.

Distribution of PNS myelin proteins and membrane enzymes in fractions isolated by continuous gradient zonal centrifugation

Myelin was purified from adult rabbit sciatic nerve by two procedures: discontinuous gradient centrifugation and continuous gradient zonal centrifugation. Two fractions were obtained from the discontinuous gradient. The fraction floating on 0.32 M sucrose and the fraction recovered from the 0.32/0.85 M sucrose interface showed typical myelin membranes by electron microscopy and typical myelin proteins by gel electrophoresis. The specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) decreased from the top to the bottom of the discontinuous gradient. The myelin separated by zonal centrifugation on a continuous sucrose gradient showed three distinct peaks (on monitoring optical density) at 0.10, 0.30 and 0.57 M sucrose. The latter peak yielded 92% of the material applied. The two minor peaks of low density exhibited high CNP and acetylcholinesterase (AChE) activities but the specific activity of both enzymes increased markedly at the heavy end of the gradient. The zonal fractions showed typical myelin proteins in all fractions by polyacrylamide gel electrophoresis but with important quantitative differences. These results indicate that PNS myelin shows significant heterogeneity.

Carbonic anhydrase activity in myelin fractions from rat optic nerves

The carbonic anhydrase activity of myelin fractions isolated from the optic nerves of adult and immature (20-day-old) rats was examined. The specific activity in both total homogenate and myelin fractions was about 2-fold higher in adult than in immature animals and at both ages, the activity in the homogenate was higher than in myelin. After subfractionation by zonal gradient centrifugation, it was shown that carbonic anhydrase activity was greatest in the heaviest myelin particles at both ages. These data are consistent with the hypothesis that a small proportion of the total enzyme activity is localised in myelin.