Voltage-operated Ca(2+) and Na(+) channels in the oligodendrocyte lineage.

It is becoming increasingly clear that expression of Ca(2+) and Na(+) channels in the OL lineage is highly regulated and may be functionally related to different stages of development and myelination. Characterization of the mechanisms of voltage-dependent Ca(2+) and Na(+) entry are important because changes in intracellular Ca(2+) and Na(+) are central to practically all cellular activities. In nonexcitable cells, voltage-dependent Ca(2+) influx plays a key role in several important processes, including proliferation, apoptosis, and cell migration. It has been demonstrated that Ca(2+) signaling is essential in the development and functioning of OLs. For example, Ca(2+) uptake is required for the initiation of myelination, and perturbation of Ca(2+) homeostasis, e.g., overwhelming influxes of Ca(2+), leads to demyelination. Although OL progenitor cell Na(+) channels are present at a much lower density, their physiological properties appear to be indistinguishable from those recorded in neurons. Interestingly, recent data indicate that, as with neurons, some white matter OPCs possess the ability to generate Na(+)-dependent action potentials. This Mini-Review focuses on the mechanisms of Ca(2+) and Na(+) signaling in cells within the OL lineage mediated by voltage-operated ion channels, with a particular focus on the relevance of these voltage-dependent currents to oligodendroglial development, myelination, and demyelination. Overall, it is clear that cells in the OL lineage exhibit remarkable plasticity with regard to the expression of voltage-gated Ca(2+) and Na(+) channels and that perturbation of Ca(2+) and Na(+) homeostasis likely plays an important role in the pathogenesis underlying demyelinating diseases.

Oligodendrocyte adhesion activates protein kinase C-mediated phosphorylation of myelin basic protein.

When isolated adult oligodendrocytes adhere to a substratum myelinogenesis occurs. Investigation of the mechanism by which this happens indicated that the oligodendrocyte-substratum interaction activated protein kinase C-dependent phosphorylation of myelin basic protein and promoted the synthesis of myelin basic protein. In addition, when agents that activate protein kinase C (second messenger diacylglycerol or a tumor-promoting phorbol ester) were added to nonattached oligodendrocytes, they mimicked the influence of the substratum by inducing phosphorylation of myelin basic protein; and reagents that increase cellular adenosine 3', 5'-monophosphate (cyclic AMP) inhibited phosphorylation of myelin basic protein. Thus, at least in vitro, the interaction between oligodendrocytes and the substratum may mediate myelinogenic events, and phosphorylation of myelin basic protein may be an early requirement in the sequence of steps that ultimately results in myelin formation.

Tbr1 regulates differentiation of the preplate and layer 6.

During corticogenesis, early-born neurons of the preplate and layer 6 are important for guiding subsequent neuronal migrations and axonal projections. Tbr1 is a putative transcription factor that is highly expressed in glutamatergic early-born cortical neurons. In Tbr1-deficient mice, these early-born neurons had molecular and functional defects. Cajal-Retzius cells expressed decreased levels of Reelin, resulting in a reeler-like cortical migration disorder. Impaired subplate differentiation was associated with ectopic projection of thalamocortical fibers into the basal telencephalon. Layer 6 defects contributed to errors in the thalamocortical, corticothalamic, and callosal projections. These results show that Tbr1 is a common genetic determinant for the differentiation of early-born glutamatergic neocortical neurons and provide insights into the functions of these neurons as regulators of cortical development.

Expression of oligodendrocyte-associated genes in cell lines derived from human gliomas and neuroblastomas.

Two putative human oligodendroglioma cell lines were examined for the expression of the oligodendrocyte-associated genes, 2',3'-cyclic nucleotide-3'-phosphodiesterase, myelin basic protein, myelin proteolipid proteins, and myelin-associated glycoprotein. The expression of these genes also was examined in control astrocytoma and neuroblastoma cell lines. In addition, the expression of the non-oligodendrocyte-specific genes, glial fibrillary acidic protein (GFAP), neuron-specific enolase and neurofilaments (NF) NF-L and NF-M also were examined. All the cell lines expressed 2',3'-cyclic nucleotide 3'-phosphodiesterase, neuron-specific enolase, and vimentin, and none expressed myelin-associated glycoprotein. The "oligodendrocyte-specific" myelin proteolipid protein mRNAs and the "neuron-specific" NF-L mRNA were expressed in the two astrocytoma cell lines, which also expressed GFAP. Expression of intermediate filament protein genes was more restricted. The astrocytoma, neuroblastoma, and oligodendroglioma cell lines expressed only GFAP, NF-M, and cytokeratin K7, respectively. These results: (a) provide molecular data confirming the classification of the two cell lines as oligodendrogliomal and suggest that their molecular profiles are indicative of immature oligodendrocytes; (b) demonstrate the expression of cytokeratins in oligodendrogliomal cell lines and suggest that apparent GFAP expression in oligodendrogliomas detected by immunocytochemical methods may be due to cross-reactivity with cytokeratins, with which they share common polypeptide sequence; and (c) indicate that astrocytoma cell lines can exhibit a "mixed" phenotype, expressing genes associated with fully differentiated oligodendrocytes and neurons.

Expression of oligodendrocytic mRNAs in glial tumors: changes associated with tumor grade and extent of neoplastic infiltration.

We examined the expression of glial- and neuronal-specific mRNAs within human gliomas using in situ hybridization. We found that low-grade astrocytomas contained a high number of proteolipid protein (PLP) mRNA-positive cells and that the number of PLP-stained cells decreased markedly with increasing tumor grade. Interestingly, the ratio of PLP mRNA-stained cells:myelin basic protein (MBP) mRNA-stained cells in normal white matter and low-grade astrocytoma was about 2:1 but approached 1:1 with increasing tumor grade. This parameter appeared to be a good indicator of tumor infiltration in astrocytomas, so we tested this in the analysis of other gliomas. Unlike astrocytomas, oligodendrogliomas were found consistently to contain few PLP mRNA- or MBP mRNA-expressing cells. In contrast, gemistocytic astrocytomas, typically highly invasive tumors, contained high numbers of PLP-positive cells and a ratio of PLP mRNA:MBP mRNA-stained cells of about 1.5:1, similar to low-grade astrocytomas. Nonradioactive in situ hybridization also enabled the morphological identification of specific cells. For example, gemistocytic astrocytes, which were found to be strongly vimentin mRNA positive, contained little glial fibrillary acidic protein mRNA and did not stain for PLP or MBP mRNAs. Neuronal mRNAs, such as neurofilament 68, were observed in small numbers of entrapped neurons within gliomas but were uninformative with respect to predicting tumor grade. Our results suggest that oligodendrocytes survive low-grade tumor infiltration and that glial tumor cells, unlike cell lines derived from them, do not express oligodendrocyte or neuronal mRNAs. In addition, the expression of mRNAs for the two major myelin protein genes, PLP and MBP, could be used to predict the grade and extent of tumor infiltration in astrocytomas.

Golli Myelin Basic Proteins Modulate Voltage-Operated Ca(++) Influx and Development in Cortical and Hippocampal Neurons.

The golli proteins, products of the myelin basic protein gene, are widely expressed in oligodendrocyte progenitor cells and neurons during the postnatal development of the brain. While golli appears to be important for oligodendrocyte migration and differentiation, its function in neuronal development is completely unknown. We have found that golli proteins function as new and novel modulators of voltage-operated Ca(++) channels (VOCCs) in neurons. In vitro, golli knock-out (KO) neurons exhibit decreased Ca(++) influx after plasma membrane depolarization and a substantial maturational delay. Increased expression of golli proteins enhances L-type Ca(++) entry and processes outgrowth in cortical neurons, and pharmacological activation of L-type Ca(++) channels stimulates maturation and prevents cell death in golli-KO neurons. In situ, Ca(++) influx mediated by L-type VOCCs was significantly decreased in cortical and hippocampal neurons of the golli-KO brain. These Ca(++) alterations affect cortical and hippocampal development and the proliferation and survival of neural progenitor cells during the postnatal development of the golli-KO brain. The CA1/3 sections and the dentate gyrus of the hippocampus were reduced in the golli-KO mice as well as the density of dendrites in the somatosensory cortex. Furthermore, the golli-KO mice display abnormal behavior including deficits in episodic memory and reduced anxiety. Because of the expression of the golli proteins within neurons in learning and memory centers of the brain, this work has profound implication in neurodegenerative diseases and neurological disorders.

Identification of a new exon in the myelin proteolipid protein gene encoding novel protein isoforms that are restricted to the somata of oligodendrocytes and neurons.

The myelin proteolipid protein (PLP) gene (i.e., the PLP/DM20 gene) has been of some interest because of its role in certain human demyelinating diseases, such as Pelizaeus-Merzbacher disease. A substantial amount of evidence, including neuronal pathology in knock-out and transgenic animals, suggests the gene also has functions unrelated to myelin structure, but the products of the gene responsible for these putative functions have not yet been identified. Here we report the identification of a new exon of the PLP/DM20 gene and at least two new products of the gene that contain this exon. The new exon, located between exons 1 and 2, is spliced into PLP and DM20 mRNAs creating a new translation initiation site that generates PLP and DM20 proteins with a 12 amino acid leader sequence. This leader sequence appears to target these proteins to a different cellular compartment within the cell bodies of oligodendrocytes and away from the myelin membranes. Furthermore, these new products are also expressed in a number of neuronal populations within the postnatal mouse brain, including the cerebellum, hippocampus, and olfactory system. We term these products somal-restricted PLP and DM20 proteins to distinguish them from the classic PLP and DM20 proteolipids. They represent putative candidates for some of the nonmyelin-related functions of the PLP/DM20 gene.

Interaction of the mouse and bovine myelin basic proteins and two cleavage fragments with anionic detergents.

The binding of deoxycholate and dodecyl sulfate to the mouse and bovine myelin basic proteins and two peptide fragments, obtained by cleavage of the bovine basic protein at its single tryptophan residue, was examined. Complete equilibrium binding isotherms for both detergents were obtained by examining their binding to each of the polypeptides immobilized on agarose. The bulk of the binding of dodecyl sulfate was found to be highly cooperative, and at saturation all four polypeptides bound far more detergent than globular, water-soluble proteins. The sum of the dodecyl sulfate bound by each of the two bovine basic protein cleavage fragments was almost twice that bound by the intact protein at saturation, suggesting that cleavage of the bovine basic protein exposes sites for additional binding of dodecyl sulfate. At pH values below pH 8.0, an additional cooperative transition was observed below the critical micelle concentration of sodium dodecyl sulfate in the binding isotherms of all four polypeptides. The midpoint of this transition corresponded to an apparent pK of approximately 5.5; however, the destruction of 90% of the histidine residues in the bovine basic protein had no effect on this transition. At pH 9.2 and moderate ionic strength (I = 0.1), the bulk of the binding of deoxycholate to the mouse and bovine basic proteins occurred at and above the critical micelle concentration of the detergent; and saturation values of deoxycholate binding to these two proteins were considerably higher than that reported for globular, water-soluble proteins. In marked contrast to the results with dodecyl sulfate, neither cleavage fragment was observed to bind deoxycholate. The results suggest that the higher ordered structure of the bovine basic protein may play an important role in the binding of anionic amphiphiles to the protein.

Unexpected expression of intermediate filament protein genes in human oligodendroglioma cell lines.

From a human oligodendroglioma cell line cDNA library, ten intermediate filament (IF) cDNA clones were isolated. Five clones corresponded to vimentin mRNA, two corresponded to cytokeratin K7 mRNA, and two corresponded to cytokeratin K8 mRNA. One clone encoded a novel IF mRNA. The expression of these and other IF protein genes was examined in five cell lines derived from human oligodendroglioma, astrocytoma and neuroblastoma tumors. Vimentin mRNA and K18 mRNA were expressed in all the cell lines. The K7 and K8 genes were expressed only in the oligodendroglioma cell lines. Surprisingly, nestin mRNA was expressed in the astrocytoma lines and the neuroblastoma line, but was not expressed in the oligodendroglioma lines. These results indicate that oligodendroglioma cell lines express Types I and II cytokeratin genes. This pattern of IF gene expression was different from that of the astrocytoma and neuroblastoma cell lines, which expressed IF genes usually associated with the mature cell types or with differentiating fetal neural precursor cells, i.e. GFAP and neurofilament-L. The results also suggest that the oligodendroglioma cell lines are more epithelial in character and do not reflect the gene expression of mature oligodendrocytes.

The pathobiology of myelin mutants reveal novel biological functions of the MBP and PLP genes.

Substantial biological data indicate that the myelin basic protein (MBP) and myelin proteolipid protein (PLP/DM20) genes produce products with functions beyond that of serving as myelin structural proteins. Much of this evidence comes from studies on naturally-occurring and man-made mutations of these genes in mice and other species. This review focuses upon recent evidence showing the existence of other products of these genes that may account for some of these other functions, and recent studies providing evidence for alternative biological functions of PLP/DM20. The MBP and PLP/DM20 genes each encode the classic MBP and PLP isoforms, as well as a second family of proteins that are not involved in myelin structure. The biological roles of these other products of the genes are becoming clarified. The non-classic MBP gene products appear to be components of transcriptional complexes in the nucleus, and they also may be involved in signaling pathways in T-cells and in neural cells. The non-classic PLP/DM20 gene products appear to be components of intracellular transport vesicles in oligodendrocytes. There is evidence for other functions of the classic PLP/DM20 proteins, including a role in neural cell death mechanisms, autocrine and paracrine regulation of oligodendrocytes and neurons, intracellular transport and oligodendrocyte migration.

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.

Identification of a novel silencer that regulates the myelin basic protein gene in neural cells.

The myelin basic protein gene produces two families of proteins, the golli proteins and the 'classic' myelin basic proteins from three transcription start sites (tsp). The golli proteins are expressed from the first tsp, and little is known about genetic elements that control its activity. We have examined elements that may regulate the expression of the golli products produced from this promoter in neural cell lines with constructs containing upstream portions of the first tsp by transient transfection assays. Three putative regulatory elements were identified, among them a 345bp novel silencer region, termed the golli silencer region (GSR), which was characterized in detail. This silencer was responsible for a significant (approx. 60%) inhibition of luciferase expression in PC12 cells. It was orientation-dependent and a double dose of this GSR completely abolished expression of the luciferase reporter activity. Transfections with deleted constructs identified three critical sites that bind at least two repressor proteins. We postulate that the silencer activity is the result of synergistic interactions between these repressor proteins and might involve the formation of a high-ordered protein-DNA structure.

Zebra finch aromatase gene expression is regulated in the brain through an alternate promoter.

The learned singing behavior in songbirds is sex-steroid-dependent and sexually dimorphic. Estrogen plays a major role in masculinizing the song system in these songbirds. The songbird brain synthesizes large amounts of estrogen, which, in the case of zebra finches, have been found to enter the systemic circulation. Aromatase cytochrome P450 is the key enzyme catalyzing the conversion of androgens to estrogens. We have cloned a novel alternatively spliced form of aromatase cDNA expressed predominantly in the zebra finch brain. We have also isolated and characterized the gene coding for zebra finch aromatase which spans 20kb in length. The alternate forms of aromatase mRNA (ARO) differ in their 5'-untranslated regions encoded by either exon 1a or 1b. The putative promoter sequences controlling the regulation of the alternate forms of ARO in zebra finches contain consensus binding sites for various transcription factors. While both the promoters have binding sites for SRY-like transcription factor, a binding site for SF-1 is present only in the promoter 1b active in the ovary. Intriguingly, a 55bp segment within the promoter 1a sequence appears to be highly conserved among zebra finch, mouse and human aromatase promoters active in the brain.

An atlas of aromatase mRNA expression in the zebra finch brain.

Neural conversion of androgen to estrogen by aromatase is an important step in the development and expression of masculine behavior in mammals and birds. In contrast to the low telencephalic levels of aromatase in adult mammals and nonsongbirds, the zebra finch telencephalon possesses high aromatase activity. This study maps, by in situ hybridization, cells that express aromatase mRNA in the adult zebra finch telencephalon, diencephalon, midbrain, and pons. High aromatase mRNA expression was observed in the caudal neostriatum, limbic archistriatum, and hypothalamus. The hippocampus, parahippocampal area, and hyperstriatum accessorium contained cells expressing moderate amounts of aromatase message. Weakly labeled cells were found in the rostral neostriatum, lobus parolfactorius, and mesencephalic reticular formation. These findings are consistent with aromatase activity measurements of zebra finch tissue and document with anatomical precision both the widespread expression of aromatase mRNA in the brain and novel sites of brain aromatase. This study identifies the caudal neostriatum as a major site of telencephalic aromatase. A previous survey (Gahr et al., 1993: J. Comp. Neurol. 327:112-122) of several avian species found that the presence of estrogen receptors in parts of the caudal neostriatum is unique to songbirds, which are the only birds to possess the elaborated telencephalic song system. Together, these findings suggest that the heightened estrogen synthesis and estrogen sensitivity of the passerine caudal neostriatum may have some functional relation with the telencephalic circuits responsible for song.

Expression of the myelin basic protein gene locus in neurons and oligodendrocytes in the human fetal central nervous system.

The myelin basic protein (MBP) gene locus is composed of two overlapping transcription units that share all of the MBP exons. One of these transcription units expresses the MBPs and the other expresses a family of proteins structurally related to the MBPs. This second transcription unit is called the Golli gene, and the entire complex is called the Golli-mbp gene. In this study, the expression of the Golli gene was examined in the human fetal central nervous system (CNS). By using reverse transcriptase-polymerase chain reaction cloning we have identified eight new members of the Golli gene family of transcripts expressed in the human CNS. Golli gene expression was examined by in situ hybridization and immunohistochemistry, and surprisingly, Golli products were found to be expressed in neurons as well as oligodendrocytes. Furthermore, the subcellular distribution of Golli immunoreactivity in fetal spinal cord interneurons shifted between the various laminae. Golli protein was localized within the nuclei of interneurons in the posterior horn, but was found in the cell bodies and processes of interneurons in the anterior horn. Within oligodendrocytes, Golli protein was detected in the cell bodies and processes, including processes which were wrapping axonal segments. Golli mRNA expression was also observed in neurons within the cerebral cortex between 18 and 20 weeks postconception, prior to myelination of this brain region. During this period, there was a striking developmental increase in the numbers and in the locations of neurons expressing Golli mRNAs within the cortical plate. The diverse distribution of Golli proteins within neurons and oligodendrocytes indicates that their function is quite different from that of the MBPs to which they are closely related.

Characterization of a novel monoclonal anti-myelin basic protein antibody: use in immunoblotting and immunohistochemical studies.

Monoclonal antibodies (MAbs) to the myelin basic protein (MBP) were produced in CAF1 (BALB/c x A/J) mice immunized with intact bovine MBP. A number of MAbs were obtained, one of which was characterized in detail with respect to its isotype, antigenic determinant on the MBP, the spectrum of antigens with which it reacted in mouse brain, and its immunohistochemical staining characteristics. This monoclonal, GB-1 (an IgG1), recognized an epitope within residues 30-51 of bovine MBP. It also reacted with a family of MBP-related proteins present in brain homogenates of mice from 7-35 days. Immunohistochemically, GB-1 stained myelinated fibers and oligodendrocytes in the rodent CNS. A second monoclonal (GB-2, and IgM) was partially characterized. It reacted with intact MBP when it was immobilized to plastic or nitrocellulose, but it was not found to be useful for immunoblots or immunohistochemistry.

Strain-related differences in the ability of T lymphocytes to recognize proteins encoded by the golli-myelin basic protein gene.

Protein products of the golli-MBP gene complex, expressed in the nervous and lymphoid systems, contain sequences in common with sequences in 'classic' MBP, expressed exclusively in the nervous system. In this report, it was determined whether T cell lines (TCLs) specific for encephalitogenic epitopes of 'classic' MBP were able to recognize sequences within golli-MBP. TCLs derived from SJL mice specific for the immunodominant 83-102 sequence and the subdominant 19-27 sequence of 'classic' MBP recognized golli-MBP J37 and BG21, respectively. In contrast, TCLs derived from PL and B10.PL mice specific for the immunodominant 1-9 sequence of 'classic' MBP did not recognize this sequence within either J37 or BG21. These strain-related differences in the ability of golli-MBP proteins to stimulate 'classic' MBP-specific TCLs are discussed with respect to a possible influence on whether the course of EAE is relapsing (SJL) or monophasic (PL and B10.PL).

Expression of the myelin proteolipid protein gene in the human fetal thymus.

We have analyzed human fetal thymus and spleen for expression of the proteolipid protein (PLP) gene. We demonstrate that the PLP gene is transcribed in both tissues, and that both the PLP and DM-20 mRNAs are produced. Western blot analyses revealed that both the PLP and DM-20 protein isoforms were present in the fetal thymus and spleen. Immunohistochemical analyses indicated that the PLP/DM-20 proteins were detected in cells which have the distribution and morphology of thymic macrophages. These results provide further evidence that the PLP and DM-20 proteins are expressed in cell types other than myelin forming cells and possess function(s) unrelated to myelin structure. Furthermore, these data demonstrate that the PLP and DM-20 proteins are not shielded from the immune system behind the blood-brain barrier. These observations directly impinge upon the debate concerning acquisition of tolerance and the recognition that the encephalitogenic nature of PLP in diseases, such as Multiple Sclerosis, may not simply be related to its 'sequestration' from a 'naive' immune system.

Developmental expression of myelin proteolipid, basic protein, and 2',3'-cyclic nucleotide 3'-phosphodiesterase transcripts in different rat brain regions.

RNA was extracted from five different rat brain regions during development, starting from embryonic day 15 (E15) until postnatal day 60 (P60). These RNA preparations were analyzed by both Northern and dot blot for their content of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), myelin proteolipid protein (PLP), and myelin basic protein (MBP) -specific transcripts. CNPase mRNA was readily detectable at E15 and PLP mRNA at P1 in all brain regions examined. In contrast, expression of MBP mRNA followed a caudorostral gradient. It was first observed at P1 in the mesencephalon and at P9-P11 in the olfactory bulb. Expression of these three transcripts displayed two types of developmental profiles. One was termed biphasic because the specific mRNA level increased regularly and then reached a plateau level. The other developmental profile was termed triphasic, because there was a gradual increase in the level of specific transcripts with a sudden appearance of a sharp peak followed by a decline to a plateau level. When the triphasic pattern was observed, the date of the peak appearance was probe-, but not region-, dependent. It was P15 for CNPase, P18 for MBP, and P21 for PLP. As these peaks occurred at a time during development when myelination was the most active, we postulate the existence of a transient external signal, perhaps neuronal, which would be responsible for this increased amount of myelin-related transcripts.

Cellular influences on RNA sorting in neurons and glia: an in situ hybridization histochemical study.

The unique structures of process-bearing cells in the central nervous system (CNS) present an ideal model with which to study the differential distribution of mRNA. We conducted a side-by-side examination of the intracellular distribution of nine neural mRNAs by in situ hybridization histochemistry in mammalian brain and observed four general types of mRNA distributions. (1) Some mRNA species were confined to cell somas and included those encoding the glial proteins, myelin proteolipid protein and 2'3'-cyclic nucleotide-3'-phosphodiesterase and the neuronal enzymes, neuron-specific enolase and glutamate decarboxylase-67. (2) Some mRNAs were found abundantly within the cell soma and were also located throughout cellular processes. These included myelin basic protein (MBP) mRNA, which was localized to the cell soma and myelin sheaths of oligodendrocytes, and glial fibrillary acidic protein (GFAP) mRNA, which was localized to the cell soma and processes of reactive and some non-reactive astrocytes in the adult brain and radial glia in embryonic brain. (3) Some mRNAs were found primarily in perinuclear cytoplasm but in some cells were also observed in cell processes. These included mRNAs encoding the protein kinase C/calmodulin-binding substrates, RC3 (neurogranin) and GAP-43, which were identified in the somas as well as within the proximal dendritic branches of specific forebrain neurons. (4) Some mRNAs were localized primarily within cell processes. These included MAP2 mRNA, which was identified by deep staining within dendritic fields but by only light staining within neuronal cell bodies. The data also indicated that the stage of cellular development and the regional location of a cell within the CNS had a profound influence on translocation events. MAP2 mRNA was found in the dendritic processes of most neurons but was confined to the soma of neurons in specific brainstem nuclei. MBP mRNA was confined to the perinuclear cytoplasm of immature oligodendrocytes and was then transported into the myelin sheath at a developmental stage corresponding to myelination. The distribution patterns of these mRNAs are likely to reflect the mechanism by which the protein products of these molecules are targeted within neurons and glia. In addition, mRNA movement may be influenced by cellular and regional factors not encoded solely within the structure of the translocated mRNA.