Dissociation of myelin from its 'enzyme markers' during ontogeny

Biochemical subtypes of oligodendrocyte in the anterior medullary velum of the rat as revealed by the monoclonal antibody Rip

Oligodendrocytes were studied in the anterior medullary velum (AMV) of the rat using the monoclonal antibody Rip, an oligodendrocyte marker of unknown function. Confocal microscopic imaging of double immunofluorescent labelling with antibodies to Rip and carbonic anhydrase II (CAII) revealed two biochemically and morphologically distinct populations of oligodendrocyte which were either Rip+CAII+ or Rip+CAII-. Double immunofluorescent labelling with Rip and myelin basic protein (MBP) or glial fibrillary acidic protein (GFAP) provided direct evidence that Rip-labelled cells were phenotypically oligodendrocytes and confirmed that Rip did not recognise astrocytes. Oligodendrocytes which were Rip+CAII+ supported numerous myelin sheaths for small diameter axons, whilst Rip+CAII- oligodendrocytes supported fewer myelin sheaths for large diameter axons. Morphologically, Rip+CAII+ oligodendrocytes corresponded to types I or II of classical nomenclature, whilst Rip+CAII- oligodendrocytes corresponded to types III and IV. The results demonstrated a biochemical difference between oligodendrocytes which myelinated small and large diameter fibres.

Biosynthesis of the myelin 2',3'-cyclic nucleotide 3'-phosphodiesterases

We have investigated the site of synthesis of the 2',3'-cyclic nucleotide 3'-phosphodiesterases (CNPs I and II) in rat brain. Rapid kinetics of incorporation of CNPs into oligodendrocyte plasma membrane in the intact brain are consistent with their synthesis on free polysomes. This hypothesis was confirmed by the translation in vitro of RNA isolated from free and bound polysomes, respectively. Unlike myelin basic protein (MBP) mRNAs, CNP mRNAs are not enriched in a myelin-associated pool of RNA. MBPs, but not CNPs, were found to readily associate in vitro with membrane vesicles derived from rough endoplasmic reticulum. The avidity of MBPs in binding to membranes is probably related to the previously observed spatial segregation of MBP mRNAs into actively myelinating cellular processes of the oligodendrocyte. Such a segregation would ensure that newly synthesized MBPs are immediately incorporated into myelin. In contrast, the CNPs probably associate with the cytoplasmic surface of the oligodendrocyte plasma membrane through interaction with a membrane-bound receptor.

Cellular and subcellular distribution of 2',3'-cyclic nucleotide 3'-phosphodiesterase and its mRNA in the rat central nervous system

The 2',3'-cyclic nucleotide 3'-phosphodiesterases (CNPs) are closely related oligodendrocyte proteins whose in vivo function is unknown. To identify subcellular sites of CNP function, the distribution of CNP and CNP mRNA was determined in tissue sections from rats of various developmental ages. Our results indicate that CNP gene products were expressed exclusively by oligodendrocytes in the CNS. CNP mRNA was concentrated around oligodendrocyte perinuclear regions during all stages of myelination. Developmentally, initial detection of CNP mRNA closely paralleled initial detection of its translation products. In electron micrographs of immunostained ultrathin cryosections, CNP was associated with oligodendrocyte membranes during the earliest phase of axonal ensheathment. In more mature fibers, immunocytochemistry established that the CNPs are not major components of compact myelin but are concentrated within specific regions of the oligodendrocyte and myelin internode. These include (a) the plasma membrane of oligodendrocytes and their processes, (b) the periaxonal membrane and inner mesaxon, (c) the outer tongue process, (d) the paranodal myelin loops, and (e) the "incisure-like" membranes found in many larger CNS myelin sheaths. A cytoplasmic pool of CNP was also detected in oligodendrocyte perikarya and larger oligodendrocyte processes. CNP was also enriched in similar locations in myelinated fibers of the PNS.

Localization of 5'-nucleotidase in bovine brain myelin fraction and myelin subfractions

Purified myelin from fresh calf brain white matter was subfractionated in a discontinuous sucrose gradient; significant recovery of protein and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and 5'-nucleotidase (5'N) activities occurred in all three obtained subfractions, the highest recovery being in the light subfraction; highest 5'N and CNP specific activities were in medium myelin. Purified myelin was also subfractionated in a continuous sucrose gradient, with a similar localization of protein; CNP activity and 5'N activity maxima suggest that myelin may be a predominant locus of 5'N in bovine brain white matter. Freezing of brain white matter caused an increase in protein and in CNP and 5'N total activity recoveries in denser myelin subfractions. Cytochemistry showed the reaction product of 5'N in the whole myelin fraction to be associated with the innermost, outermost and medial compact myelin layers. Effects of non-ionic detergent (LUbrol WX) on 5'N activity were studied, and the results also suggest the intrinsic nature of 5'N as an ectoenzyme in myelin membranes. Lubrol WX was viewed as an advisable detergent for the stimulation of myelin 5'N activity, but not for the solubilization of this enzyme.

Vitamin B12 deficiency alters the distribution of membrane proteins on linear sucrose gradients in the fruit bat brain

Total particulate material prepared by homogenization in water and centrifugation at 100,000 g for 60 min from the brains of normal and vitamin B12-deficient fruit bats was fractionated on linear sucrose gradients (0.1 M-1.4 M sucrose). Animals were made vitamin B12-deficient by dietary deprivation or as a result of exposure to nitrous oxide. Based on absorbance at 280 nm three peaks were seen in material derived from the B12-deficient fruit bat brain and only two peaks in the normal animal. Myelin proteins were observed over a larger range of molarities of sucrose in the deficient brain than in the control tissue. Animals rendered vitamin B12-deficient by nitrous oxide treatment showed membrane protein patterns similar to those observed in the control animal.

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.

Bulk separation and long-term culture of oligodendrocytes from adult pig brain. II. Some biochemical data

Oligodendroglial proteins labeled with radioactive amino acids were subjected to one- and two-dimensional polyacrylamide electrophoresis. Bands comigrating with myelin proteins, the basic protein (MBP), the proteolipid protein (PLP), and the Wolfgram protein (WP) doublet, were detected by Coomassie Blue staining and by autoradiography. The identity of the MBP and WP in the cellular material is evidenced by immunoblotting with specific antibodies. A comparative study of myelin samples from rat and pig CNS reveals that WP can be detected immunochemically in both species. Different protein patterns, however, are observed. Three protein bands are found with antibodies against the myelin-associated glycoprotein (MAG). The high-molecular-weight component prevails in pig myelin, whereas the medium-molecular-weight component is predominant in rat myelin. Moreover, two protein bands, of molecular weights 35,000 and 33,000 (Ol 1 and Ol 2), are present in high amounts in oligodendroglial particulate material but are not detectable in myelin. These oligodendroglial characteristic proteins are not species-specific, since they are found in preparations of cat oligodendrocytes as well. Activities of cerebroside sulfotransferase (EC 2.8.2.11) are low in freshly isolated cells and increase during the first week of culture. A reverse course of enzyme activities is observed with 2',3'-cyclic nucleotide 3'-phosphohydrolase (EC 3.1.4.37). Values reach a minimum about day 5 in culture and recover their initial values. At day 10 they remain stable until the end of the third week of the culture period.

Intracellular localization of 2',3'-cyclic nucleotide 3'-phosphohydrolase in rat oligodendrocytes and C6 glioma cells, and effect of cell maturation and enzyme induction on localization

We investigated whether the membrane-associated myelin enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP; EC 3.1.4.37), is localized primarily inside the cell or exposed on the cell surface of rat oligodendrocytes and rat C6 glioma cells. Determinations were made by enzyme assays of intact, viable cells vs cells broken by freezing and thawing. Assay of both oligodendrocytes and C6 cells showed that the great majority of the CNP activity was localized inside the cells. Oligodendrocytes were also tested by immunofluorescence staining of unfixed, living cells whose membranes had been made permeable to antibody by fixation. Fixed oligodendrocytes showed intense fluorescence when incubated with rabbit anti-CNP antiserum and fluorescein-conjugated second antibody whereas unfixed cells were not stained. We then tested the possible influence on CNP localization of 3 conditions known to increase CNP specific activity: maturation of oligodendrocytes in vitro during a period when CNP specific activity increases 8-fold or more; growth of C6 cultures to high cell density; and induction of CNP activity in oligodendrocytes and C6 cells by dibutyryl cyclic AMP. Under all conditions, most CNP activity was intracellular. These results show that both the catalytic and major antigenic sites of CNP are localized primarily inside the cell, and suggest an intracellular role for CNP in oligodendrocytes. The results with C6 cells also show that these cells resemble oligodendrocytes with respect to CNP localization.

Nitrous oxide alters the pattern of myelin proteins in the nervous system of the fruit bat Roussettus aegyptiacus

Proteins of the myelin membrane were separated by SDS-polyacrylamide electrophoresis. The protein patterns obtained from myelin from the forebrain, medulla, cervical and thoracic spinal cord and phrenic nerve of normal animals were compared with those obtained from animals given nitrous oxide or nitrous oxide plus folinic acid. A shift from high molecular weight proteins to lower molecular weight proteins was seen in all regions studied in animals given N2O and folinic acid. These changes were seen also in the medulla and thoracic spinal cord of animals given N2O alone. Peripheral nerve showed a slight increase in the amounts of Po at the expense of P3 and P2 in N2O and folinic acid-treated animals.

ATPase activities in myelin and oligodendrocytes isolated from the brains of developing rats and from bovine brain white matter

Of the Na,K-ATPase activity in brain homogenates from 20-, 60-, and 120-day-old rats, 1.4 to 2.6% was recovered in myelin. The relative specific activities, at 0.2 to 0.3 times the specific activities in the rat brain homogenates, did not decrease during development, and myelin from bovine brain white matter had a similar relative specific activity. Oligodendrocytes from rat brains and bovine white matter had approximately one third the Na,K-ATPase specific activities found in myelin from the respective sources. The Mg-ATPase activity in rat brain myelin decreased during development and, in myelin from adult rats, was much lower than the Na,K-ATPase activity. Notably, oligodendrocytes from both the forebrains of 10- to 120-day-old rats and from bovine white matter had high Mg-ATPase activities. Whereas Na,K-ATPase may be intrinsic to certain regions of the myelin sheath, the Mg-ATPase in isolated myelin probably arises from fragments of oligodendrocyte membranes.

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.

Rat brain 5'-nucleotidase: developmental changes in myelin and activities in subcellular fractions and myelin subfractions

The activities of 5'-nucleotidase, measured in brain homogenates and myelin isolated from rats at 21, 60 and greater than 90 days of age, were compared to values for two other myelin-associated enzymes, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and carbonic anhydrase. Whereas the activities of all 3 enzymes were higher in brain homogenates from 60-day-old rats than in those from 21-day-old rats, only 5'-nucleotidase increased significantly in specific activity in both homogenates and myelin after the age of 60 days. The ratios of 5'-nucleotidase to the myelin basic and proteolipid proteins in subcellular fractions from adult rat brain suggested that the microsomal fraction was the only fraction containing 5'-nucleotidase levels not attributable to contamination by myelin membranes. Like carbonic anhydrase, 5'-nucleotidase had a greater distribution than CNP into microsomes of adult rats. When purified myelin was fractionated on a density gradient, the specific activity of 5'-nucleotidase was highest in the heaviest subfraction, with recovery of significant activity occurring, however, in all 3 subfractions. In rats over 60 days of age the recovery of 5'-nucleotidase in myelin was almost as high as that of the relatively myelin-specific enzyme CNP, suggesting that myelin may be the predominant, although not exclusive locus of 5'-nucleotidase in the adult rat brain.

The glycoprotein composition of peripheral nervous system myelin subfractions

Two fractions were isolated by continuous density gradient centrifugation from total particulate matter of rabbit sciatic nerves: a minor fraction, B, consisting of small-sized membrane fragments and a major fraction, C, of characteristic multilayered myelin figures, with maxima at 0.33 and 0.58 M-sucrose, respectively. In comparison with C, fraction B was enriched in CNPase and alkaline phosphatase activities and the P0, 23K and Z proteins, but was virtually devoid of basic protein. The glycoprotein composition of all fractions was examined with four fluorescein isothiocyanate-labelled lectins (WGA, Con A, RCA-60, U.E.). These revealed the presence of six glycoproteins in all fractions with similar lectin binding capacities and molecular weights ranging from 35,500 to 16,000, of which P0 was the predominant component. Material found on the heavy side of fraction C was characterized by the presence of a multitude of glycoproteins which bound variable proportions of the four different lectins, suggesting substantial variations in their carbohydrate moieties. Their absence from the central portion of fraction C points to a location other than that of compact PNS myelin.