Antibodies to Myelin/Oligodendrocyte-Specific Protein and Myelin/Oligodendrocyte Glycoprotein Signal Distinct Changes in the Organization of Cultured Oligodendroglial Membrane Sheets

Abstract: Cultured murine oligodendrocytes elaborate extensive membrane sheets that, unlike multilamellar myelin in vivo, allow the study of interactions between myelin proteins and cytoskeletal elements. This article describes the events that occur due to the interaction of specific antibodies with their respective antigens, myelin/oligodendrocyte-specific protein (MOSP) and myelin/oligodendrocyte glycoprotein (MOG), which are expressed uniquely by oligodendrocytes. After antibody binding, surface anti-MOSP:MOSP complexes redistribute over those cytoplasmic microtubular veins that have 2',3'-cyclic nucleotide 3'-phosphohydrolase colocalized along them. In contrast, surface anti-MOG-MOG complexes redistribute over internal myelin basic protein domains. Long-term anti-MOSP IgM exposure results in an apparent increase in number as well as thickness of microtubular structures in oligodendrocyte membrane sheets, whereas long-term anti-MOG exposure causes depolymerization of microtubular veins in membrane sheets. These data suggest that antibody binding to these two surface proteins elicits signals that have opposite effects on the cytoskeleton in oligodendroglial membrane sheets. Thus, it is possible that signals transduced via antibody binding may contribute to the pathogenesis of diseases affecting CNS myelin.     

Novel oligodendrocyte transmembrane signaling systems

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca²⁺ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.     

Apoptosis of neurons in rats with experimental allergic encephalomyelitis

NPP2, also known as phosphodiesterase-I alpha/autotaxin, is a type-II membrane protein that belongs to the nucleotide pyrophosphatase/phosphodiesterase family (NPP). We have recently demonstrated that NPP2 is expressed and released by differentiating oligodendrocytes during the critical stages of CNS myelination. The structural domains of this secreted macromolecule suggest a functional role in the regulation of oligodendrocyte adhesion. Here, we present data that demonstrates that NPP2 interferes with the ability of oligodendroglial cells to adhere to known CNS adhesion molecules present during the onset of myelination, such as fibronection, vitronectin, and merosin (laminin2). Responses to NPP2 appear to be regulated by a different mechanism depending on the developmental stage of the oligodendrocyte. Although the exact mechanisms for NPP2 mediated counter-adhesion are unknown, our studies have implicated that an active signalling mechanism involving heterotrimeric G proteins is responsible for adhesion modulation. These studies clearly define a role of NPP2 as a matricellular protein modulating oligodendrocyte adhesion and suggest that NPP2 function may represent the first step of oligodendrocyte remodelling when differentiating oligodendrocytes are actively involved in the formation of the myelin sheath.     

Oral Presentations OP02: Myelin and Diseases of Myelin. NPP2, A matricellular protein expressed during myelination,actively modulates oligodendrocyte adhesion

NPP2, also known as phosphodiesterase‐I alpha/autotaxin, is a type‐II membrane protein that belongs to the nucleotide pyrophosphatase/phosphodiesterase family (NPP). We have recently demonstrated that NPP2 is expressed and released by differentiating oligodendrocytes during the critical stages of CNS myelination. The structural domains of this secreted macromolecule suggest a functional role in the regulation of oligodendrocyte adhesion. Here, we present data that demonstrates that NPP2 interferes with the ability of oligodendroglial cells to adhere to known CNS adhesion molecules present during the onset of myelination, such as fibronection, vitronectin, and merosin (laminin2). Responses to NPP2 appear to be regulated by a different mechanism depending on the developmental stage of the oligodendrocyte. Although the exact mechanisms for NPP2 mediated counter‐adhesion are unknown, our studies have implicated that an active signalling mechanism involving heterotrimeric G proteins is responsible for adhesion modulation. These studies clearly define a role of NPP2 as a matricellular protein modulating oligodendrocyte adhesion and suggest that NPP2 function may represent the first step of oligodendrocyte remodelling when differentiating oligodendrocytes are actively involved in the formation of the myelin sheath.     

Cultured oligodendrocytes mimic in vivo phenotypic characteristics: Cell shape, expression of myelin-specific antigens, and membrane production

Primary cultures of neonatal mouse cerebra were maintained for up to 4 weeks in the absence of neurons. Oligodendrocytes in these cultures pass through a sequence of cytoarchitectural change and antigen expression which mimics the differentiation of oligodendrocytes in vivo. The cell bodies and processes of oligodendrocytes first express the myelin-specific antigen galactocerebroside (GC) by 2 days in vitro. Myelin basic protein (MBP) appears several days later. The majority of oligodendrocytes then proceed to elaborate large sheets of membranous material from the tips and lengths of cell processes. These membranous sheets, which contain GC and MBP, are reminiscent of unwrapped myelin profiles in vivo. As with the cell bodies and processes, GC is inserted into the sheets several days before MBP. Our results establish that oligodendrocytes cultured without neurons are able to produce extensive membranes containing myelin-specific antigens. They also suggest that oligodendrocyte shape and membrane production are, in part, regulated from within the oligodendrocyte itself.     

Neural progenitors, neurons and oligodendrocytes from human umbilical cord blood cells in a serum-free, feeder-free cell culture

We have previously demonstrated that lineage negative cells (Lin^neg) from umbilical cord blood (UCB) develop into multipotent cells capable of differentiation into bone, muscle, endothelial and neural cells. The objective of this study was to determine the optimal conditions required for Lin^neg UCB cells to differentiate into neuronal cells and oligodendrocytes. We demonstrate that early neural stage markers (nestin, neurofilament, A2B5 and Sox2) are expressed in Lin^neg cells cultured in FGF4, SCF, Flt3-ligand reprogramming culture media followed by the early macroglial cell marker O4. Early stage oligodendrocyte markers CNPase, GalC, Olig2 and the late-stage marker MOSP are observed, as is the Schwann cell marker PMP22. In summary, Lin^neg UCB cells, when appropriately cultured, are able to exhibit characteristics of neuronal and macroglial cells that can specifically differentiate into oligodendrocytes and Schwann cells and express proteins associated with myelin production after in vitro differentiation.     

Gas6 enhances axonal ensheathment by MBP+ membranous processes in human DRG/OL promyelinating co-cultures

The molecular requirements for human myelination are incompletely defined, and further study is needed to fully understand the cellular mechanisms involved during development and in demyelinating diseases. We have established a human co-culture model to study myelination. Our earlier observations showed that addition of human γ-carboxylated growth-arrest-specific protein 6 (Gas6) to human oligodendrocyte progenitor cell (OPC) cultures enhanced their survival and maturation. Therefore, we explored the effect of Gas6 in co-cultures of enriched OPCs plated on axons of human fetal dorsal root ganglia explant. Gas6 significantly enhanced the number of myelin basic protein-positive (MBP⁺) oligodendrocytes with membranous processes parallel with and ensheathing axons relative to co-cultures maintained in defined medium only for 14 days. Gas6 did not increase the overall number of MBP⁺ oligodendrocytes/culture; however, it significantly increased the length of MBP⁺ oligodendrocyte processes in contact with and wrapping axons. Multiple oligodendrocytes were in contact with a single axon, and several processes from one oligodendrocyte made contact with one or multiple axons. Electron microscopy supported confocal Z-series microscopy demonstrating axonal ensheathment by MBP⁺ oligodendrocyte membranous processes in Gas6-treated co-cultures. Contacts between the axonal and oligodendrocyte membranes were evident and multiple wraps of oligodendrocyte membrane around the axon were visible supporting a model system in which to study events in human myelination and aspects of non-compact myelin formation.     

Age-dependent myelin degeneration and proteolysis of oligodendrocyte proteins is associated with the activation of calpain-1 in the rhesus monkey

Myelin provides important insulating properties to axons allowing for propagation of action potentials over large distances at high velocity. Disruption of the myelin sheath could therefore contribute to cognitive impairment, such as that observed during the normal aging process. In the present study, age-related changes in myelin, myelin proteins and oligodendrocyte proteins were assessed in relationship to calpain-1 expression and cognition in the rhesus monkey. Isolation of myelin fractions from brain white matter revealed that as the content of the intact myelin fraction decreased with age, there was a corresponding increase in the floating or degraded myelin fraction, suggesting an increased breakdown of intact myelin with age. Of the myelin proteins examined, only the myelin-associated glycoprotein decreased with age. Levels of the oligodendrocyte-specific proteins 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and myelin/oligodendrocyte-specific protein (MOSP) increased dramatically in white matter homogenates and myelin with age. Age-related increases in degraded CNPase also were demonstrable in white matter in association with increases in activated calpain-1. Degraded CNPase was also detectable in myelin fractions, with only the floating fraction containing activated calpain-1. The increases in the activated enzyme in white matter were much greater than those found in myelin fractions suggesting a source other than the myelin membrane for the marked overexpression of activated calpain-1 with age. In addition, CNPase was demonstrated to be a substrate for calpain in vitro. In summary, changes in myelin and oligodendrocyte proteins occur with age, and they appear to have a significant relationship to cognitive impairment. The overexpression of CNPase and MOSP suggests new formation of myelin by oligodendrocytes, which may occur in response to myelin degradation and injury caused by proteolytic enzymes such as calpain.     

Emergence of three myelin proteins in oligodendrocytes cultured without neurons

Oligodendrocytes, the myelin-forming cells of the central nervous system, were cultured from newborn rat brain and optic nerve to allow us to analyze whether two transmembranous myelin proteins, myelin-associated glycoprotein (MAG) and proteolipid protein (PLP), were expressed together with myelin basic protein (MBP) in defined medium with low serum and in the absence of neurons. Using double label immunofluorescence, we investigated when and where these three myelin proteins appeared in cells expressing galactocerebroside (GC), a specific marker for the oligodendrocyte membrane. We found that a proportion of oligodendrocytes derived from brain and optic nerve invariably express MBP, MAG, and PLP about a week after the emergence of GC, which occurs around birth. In brain-derived oligodendrocytes, MBP and MAG first emerge between the fifth and the seventh day after birth, followed by PLP 1 to 2 d later. All three proteins were confined to the cell body at that time, although an extensive network of GC positive processes had already developed. Each protein shows a specific cytoplasmic localization: diffuse for MBP, mostly perinuclear for MAG, and particulate for PLP. Interestingly, MAG, which may be involved in glial-axon interactions, is the first myelin protein detected in the processes at approximately 10 d after birth. MBP and PLP are only seen in these locations after 15 d. All GC-positive cells express the three myelin proteins by day 19. Simultaneously, numerous membrane and myelin whorls accumulate along the oligodendrocyte surface. The sequential emergence, cytoplasmic location, and peak of expression of these three myelin proteins in vitro follow a pattern similar to that described in vivo and, therefore, are independent of continuous neuronal influences. Such cultures provide a convenient system to study factors regulating expression of myelin proteins.     

Cellular Mechanism of Myelination in the Central Nervous System

A study of myelination with electron microscopy has been carried out on the spinal cord of young rats and cats. In longitudinal and transverse sections the intimate relationship of the growing axons with the oligodendrocytes was observed. Early naked axons appear to be embedded within the cytoplasm and processes of the oligodendrocytes from which they are limited only by the intimately apposed membranes of both elements (axon-oligocytic membrane). In a transverse section several axons are observed to be in a single oligodendrocyte. The process of myelination consists in the laying down, within the cytoplasm of the oligodendrocyte and around the axon, of concentric membranous myelin layers. The first of these layers is deposited at a certain distance (200 to 600 A or more) from the axon-oligocytic membrane. This and all the other subsequently formed membranes have higher electron density and are apparently formed by the coalescence and fusion of vesicles (of 200 to 800 A) and membranes found in large amounts within the cytoplasm of the oligodendrocytes. At an early stage the myelin layers may be discontinuous and some vesicular material may even be trapped among them or between the myelin proper and the axon-oligocytic membrane. Then, when the 8th to 10th layer is deposited, the complete coalescence and alignment of the lamellae leads to the characteristic orderly multilayered organization of the myelin sheath. Myelination in the central nervous system appears to be a process of membrane synthesis within the cytoplasm of the oligodendrocyte and not a result of the wrapping of the plasma membranes as postulated in Geren's hypothesis for the peripheral nerve fibers. The possible participation of Schwann cell cytoplasm in peripheral myelination is now being investigated.     

Cellular mechanism of myelination in the central nervous system

A study of myelination with electron microscopy has been carried out on the spinal cord of young rats and cats. In longitudinal and transverse sections the intimate relationship of the growing axons with the oligodendrocytes was observed. Early naked axons appear to be embedded within the cytoplasm and processes of the oligodendrocytes from which they are limited only by the intimately apposed membranes of both elements (axon-oligocytic membrane). In a transverse section several axons are observed to be in a single oligodendrocyte. The process of myelination consists in the laying down, within the cytoplasm of the oligodendrocyte and around the axon, of concentric membranous myelin layers. The first of these layers is deposited at a certain distance (200 to 600 A or more) from the axon-oligocytic membrane. This and all the other subsequently formed membranes have higher electron density and are apparently formed by the coalescence and fusion of vesicles (of 200 to 800 A) and membranes found in large amounts within the cytoplasm of the oligodendrocytes. At an early stage the myelin layers may be discontinuous and some vesicular material may even be trapped among them or between the myelin proper and the axon-oligocytic membrane. Then, when the 8th to 10th layer is deposited, the complete coalescence and alignment of the lamellae leads to the characteristic orderly multilayered organization of the myelin sheath. Myelination in the central nervous system appears to be a process of membrane synthesis within the cytoplasm of the oligodendrocyte and not a result of the wrapping of the plasma membranes as postulated in Geren's hypothesis for the peripheral nerve fibers. The possible participation of Schwann cell cytoplasm in peripheral myelination is now being investigated.     

Identification of New Oligodendrocyte- and Myelin-Specific Genes by a Differential Screening Approach

Abstract: We have isolated several new genes that are specifically expressed by oligodendrocytes in the CNS. This was achieved by differential screening of a rat spinal cord cDNA library with probes derived from normal and from oligodendrocyte-free spinal cord mRNAs. Four of these genes are exclusively expressed by oligodendrocytes: Three of these are not related to known genes, whereas one encodes the myelin oligodendrocyte glycoprotein (MOG). Four other genes are expressed by oligodendrocytes as well as by Schwann cells. One gene codes for apolipoprotein D, which is thought to be involved in lipid metabolism. A second cDNA sequence codes for the recently identified galactosylceramide-synthesizing enzyme UDP-galactose:ceramide galactosyl-transferase. The third gene encodes a small protein with four putative transmembrane domains that is related to a T-lymphocyte-specific membrane protein, MAL. The fourth gene encodes the rat homologue of the stearyl-CoA-desaturase 2 (SCD2) gene, which is specifically expressed in the nervous system and involved in the synthesis and regulation of long-chain unsaturated fatty acids essential for myelination. Finally, we found that a member of the β-tubulin family is highly expressed in oligodendrocytes as well as neurons. The identification of several new proteins that may play a role in myelin synthesis and sheath formation will lead to new insight into this complex mechanism.     

Glutamine synthetase expression in rat oligodendrocytes in culture: regulation by hormones and growth factors.

Glutamine synthetase (GS, EC 6.3.1.2.) has long been considered as a protein specific for astrocytes in the brain, but recently GS immunoreactivity has been reported in oligodendrocytes both in mixed primary glial cell cultures and in vivo. We have investigated its expression and regulation in "pure" oligodendrocyte cultures. "Pure" oligodendrocyte secondary cultures were derived from newborn rat brain primary cultures enriched in oligodendrocytes as described by Besnard et al. (1987) and were grown in chemically defined medium. These cultures contain more than 90% galactocerebroside-positive oligodendrocytes and produce "myelin" membranes (Fressinaud et al., 1990) after 6-10 days in subcultures (30-35 days, total time in culture). The presence of GS in oligodendrocytes from both primary glial cell cultures and "pure" oligodendrocyte cultures was confirmed by double immunostaining with a rabbit antisheep GS and guinea pig antirat brain myelin 2', 3'-cyclic nucleotide 3'-phosphodiesterase. In "pure" oligodendrocyte cultures, about half of cells were labeled with anti-GS antibody. Furthermore, on the immunoblot performed with a rabbit antisheep GS, the GS protein in "pure" oligodendrocyte secondary cultures was visualized as a single band with an apparent molecular mass of about 43 kDa. In contrast, two protein bands for GS were observed in cultured astrocytes. On the immunoblot performed with a rabbit antichick GS, two immunopositive protein bands were observed: a major one migrating as the purified adult chick brain GS and a minor one with a lower molecular mass. Two similar immunoreactive bands were also observed in pure rat astrocyte cultures. Compared to pure rat astrocyte cultures, "pure" oligodendrocyte cultures of the same age displayed an unexpectedly high GS specific activity that could not be explained by astrocytic contamination of the cultures (less than 5%). As for cultured astrocytes, treatment of oligodendrocyte cultures with dibutyryl-adenosine 3':5'-cyclic monophosphate, triiodothyronine, or hydrocortisone increased significantly GS specific activity. Interestingly, epidermal growth factor, basic fibroblast growth factor, and platelet-derived growth factor that increase the GS activity in astrocytes do not affect this activity in oligodendrocytes. Thus we confirm the finding of Warringa et al. (1988) that GS is also expressed in oligodendrocytes. We show that its activity is regulated similarly in astrocytes and oligodendrocytes by hormones, but that it is regulated differently by growth factors in these two cell types.     

Microtubule alterations in cultured taiep rat oligodendrocytes lead to deficits in myelin membrane formation

The taiep rat is a myelin mutant in which hypomyelination and progressive demyelination of the CNS are accompanied by an accumulation of microtubules within oligodendrocytes. To investigate whether and how the myelin defects were caused by microtubule abnormalities, we have established a taiep oligodendrocyte culture system in which mutant cells produce abnormally high levels of tubulin and microtubule-associated proteins and exhibit myelin defects. The studies show that abnormal microtubule accumulation and tight microtubule bundles developed in the taiep oligodendrocytes, with a higher ratio of minus-end-distal to plus-end-distal microtubules in their processes. Initially, in culture, immature taiep oligodendrocytes which have higher levels of tubulin than controls extend roughly twice as much membrane sheet as controls. The membrane sheets of the mature taiep oligodendrocytes which display the microtubule accumulation, however, grew much less rapidly compared to controls. By the fifth day in culture, a majority of the taiep oligodendrocytes had ceased the expansion of their membrane sheets and in some cases the sheets retracted. The levels of the myelin proteins, proteolipid protein and myelin-associated glycoprotein, were also markedly diminished in the mature taiep oligodendrocytes. Treatment with the microtubule depolymerizing drug nocodazole prevented not only the accumulation of microtubules but also restored the normal distribution of proteolipid proteins within the taiep oligodendrocytes. These data demonstrate that myelin synthesis in the oligodendrocyte cultures relies on the formation of a normal microtubule array, and the microtubule abnormalities are directly responsible for the myelin deficit in the taiep oligodendrocytes.     

Rnh1 promotes differentiation and myelination via RhoA in oligodendrocytes

Increases in Rattus norvegicus ribonuclease/angiogenin inhibitor 1 (Rnh1) are observed in rat primary neuron injury and/or the regeneration process and in differentiated oligodendrocytes. However, the roles of Rnh1 in the central nervous system are still largely unexplored. RhoA is an important signaling protein that has been implicated in oligodendrocyte differentiation and myelination. We demonstrate enhanced differentiation and myelination of oligodendrocytes mediated by Rnh1 in vitro. We further show that Rnh1 is expressed in oligodendrocyte precursors and oligodendrocytes. Importantly, Rnh1 strongly affects oligodendrocyte differentiation through RhoA-ROCK signaling. Moreover, changes in Rnh1 expression in oligodendrocytes regulates the expression and phosphorylation of Fyn, a regulator of RhoA activity. Finally, Rnh1 promotes myelination in vitro. These results show that Rnh1-mediated RhoA inactivation enhances the differentiation and myelination in oligodendrocytes. Overall, Rnh1 might contribute to oligodendrocyte differentiation and myelination processes in vitro.     

Distinct sites of origin of oligodendrocytes and somatic motoneurons in the chick spinal cord: oligodendrocytes arise from Nkx2.2-expressing progenitors by a Shh-dependent mechanism

In the vertebrate spinal cord, oligodendrocytes arise from the ventral part of the neuroepithelium, a region also known to generate somatic motoneurons. The emergence of oligodendrocytes, like that of motoneurons, depends on an inductive signal mediated by Sonic hedgehog. We have defined the precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick embryo. We show that ventral neuroepithelial explants, isolated at various development stages, are unable to generate oligodendrocytes in culture until E5 but become able to do so in an autonomous way from E5.5. This indicates that the induction of oligodendrocyte precursors is a late event that occurs between E5 and E5.5, precisely at the time when the ventral neuroepithelium stops producing somatic motoneurons. Analysis of the spatial restriction of oligodendrocyte progenitors, evidenced by their expression of O4 or PDGFR(&agr;), indicate that they always lie within the most ventral Nkx2.2-expressing domain of the neuroepithelium, and not in the adjacent domain characterized by Pax6 expression from which somatic motoneurons emerge. We then confirm that Shh is necessary between E5 and E5.5 to specify oligodendrocyte precursors but is no longer required beyond this stage to maintain ongoing oligodendrocyte production. Furthermore, Shh is sufficient to induce oligodendrocyte formation from ventral neuroepithelial explants dissected at E5. Newly induced oligodendrocytes expressed Nkx2.2 but not Pax6, correlating with the in vivo observation. Altogether, our results show that, in the chick spinal cord, oligodendrocytes originate from Nkx2.2-expressing progenitors.     

Dysmyelination, demyelination and reactive astrogliosis in the optic nerve of the taiep rat

Taiep is an autosomal recessive mutant rat that shows a highly hypomyelinated central nervous system (CNS). Oligodendrocytes accumulate microtubules (MTs) in association with endoplasmic reticulum (ER) membranes forming MT-ER complexes. The microtubular defect in oligodendrocytes, the abnormal formation of CNS myelin and the astrocytic reaction were characterized by immunocytochemical and ultrastructural methods during the first year of life. Optic nerves of both control and taiep rats were processed by the immunoperoxidase method using antibodies against tubulin, myelin basic protein (MBP) and glial fibrillary acidic protein (GFAP). Taiep oligodendrocytes are strongly immunoreactive against tubulin, indicative of a significant accumulation of microtubules. Early differentiated oligodendrocytes observed with electron microscopy show that MT-ER complexes are mainly present in the cell body. This defect increases during the first year of life; oligodendrocytes show large MT-ER complexes projected within oligodendrocyte processes. Using anti-MBP, there was a progressive reduction of immunolabeling in the myelin sheaths as taiep rats grew older. Ultrastructural analysis revealed severely dysmyelinated axons with a frequently collapsed periaxonal collar. However, through age the myelin sheath became gradually infiltrated by MTs, suggesting their contribution to premature loss of myelin in the taiep rat. Axons of one-year-old taiep rats were severely demyelinated. Modifications in astrocytes revealed by the GFAP antibody showed a strong hypertrophy with increased immunostaining in their processes. As demyelination of axons progressed, taiep rats developed a strong astrogliosis. The present findings suggest that in taiep rats the early abnormal myelination of axons affects the adequate maintenance of myelin, leading to a progressive loss of myelin components and severe astrogliosis, features that should be considered in the pathogenesis of dysmyelinating diseases.     

Developmental Expression of Neural Cell Adhesion Molecules of Oligodendrocytes In Vivo and in Culture

Abstract: Previously, we have shown that oligodendrocyte adhesion molecules are related to the 120,000–M_r neural cell adhesion molecule (NCAM-120). In this report, we present further evidence that the oligodendrocyte adhesion molecule is NCAM-120. Studies on the expression of NCAM-120 and other molecular forms of NCAM in vivo in rat brain, in vitro in primary mixed cultures, and in cultures enriched for oligodendrocytes are described. Western blot analysis of rat brain using anti-NCAM showed that NCAM-120 first appears at postnatal day 7 and increases in quantity thereafter, coincident with the development of oligodendrocytes in vivo and comparable to the expression of myelin basic protein. Purified oligodendrocytes from 4-week-old rat brains expressed only NCAM-120. Quantitation of various forms of NCAMs in rat brain showed marked age-related differences in the expression of three molecular forms of NCAM. Immunofluorescence analysis showed that oligodendrocytes, at all ages tested, expressed NCAM, but in older oligodendrocytes, the intensity of staining was less. Western blot analysis of oligodendrocyte-enriched cultures showed that from day 1 after isolation (12 days of age) through day 7 after isolation (18 days of age) only NCAM-120 is seen. A possible role for NCAM in myelination and remyelination is discussed.