Identification of genes in the oligodendrocyte lineage through the analysis of conditionally immortalized cell lines

The mouse oligodendrocyte cell lines, N19 and N20.1, were used as sources of potential stage-specific RNA in order to construct a subtraction library enriched in cDNAs expressed early in the oligodendrocyte (OL) lineage. From this library, 23 clones were examined and three were examined in most detail. The mRNAs of the three library clones were preferentially expressed in the N19 (progenitor) compared to the N20.1 (immature) OL line. One of these corresponded to the intermediate filament protein cytokeratin K19, which has not been reported to be expressed in OLs previously. Another was identified as the mouse homolog of T-cadherin, previously reported not to be present in OLs. Antisera raised against a T-cadherin peptide indicated the protein colocalized with the OL lineage markers A(2)B(5), A007, and 01 in mouse primary glial cultures. However, small round cells resembling OL precursors labeled intensely with T-cadherin, but were negative for the other markers, suggesting that this gene might be expressed earlier in the lineage. In early postnatal brain, in addition to the expected neuronal tracts, the T-cadherin antibody labeled small bipolar cells, approximately 8-10 microm in diameter, in white matter tracts. These cells had the morphology of OLs or their precursors and were identified within the cerebellar white matter and the corpus callosum, regions rich in OLs. The third clone, 3g5, was homologous to the P8 clone isolated from rat pancreas. It encoded an 80-amino-acid polypeptide with a protein kinase C domain suggesting a possible role in signal transduction. Antisera to this peptide also colocalized 3g5 with cells expressing A(2)B(5), A007, and 01 in culture and in cells within white matter tracts which had the same morphology as those labeled by T-cadherin in these regions. In addition to these, beta(10) thymosin and mevalonate kinase clones were also isolated from the screen.

Thymocytes express the golli products of the myelin basic protein gene and levels of expression are stage dependent

The golli products of the myelin basic protein gene have been shown to be expressed in mouse thymus and brain. The full repertoire of thymic cell types expressing golli products has not yet been determined, although immunoreactivity has been found in some macrophages. We have analyzed the cellular expression of golli mRNAs and proteins in the thymus. The results showed that MTS5(+) cortical/MTS10(+) medullary epithelial cells and NLDC145(+) dendritic cells did not express golli, while some macrophages did exhibit strong immunoreactivity. GOLLI: mRNAs were not detected in macrophages by in situ hybridization. Thymocytes expressed significant levels of golli mRNAs and proteins by in situ hybridization and immunohistochemistry. Interestingly, golli immunoreactivity varied with thymocyte stage of differentiation. For example, CD4(-)CD8(-) (double-negative) thymocytes expressed relatively high levels of golli. Upon further differentiation into CD4(-)CD8(-) (double-positive) thymocytes, golli protein expression declined dramatically. When thymocytes developed into CD8(-) or CD4(+) (single-positive) thymocytes, golli protein expression increased again, but it never achieved the levels found in double-negative thymocytes. Thus, the altered levels of expression of golli proteins in developing thymocytes correlated with the transitions from double-negative to double-positive and double-positive to single-positive stages. The lack of significant golli expression in thymic stromal cells may offer an alternative explanation for the mechanism of inefficient negative selection of those autoreactive thymocytes with specificity for myelin basic proteins.

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.

The major myelin protein genes are expressed in the human thymus

Demyelinating diseases, such as multiple sclerosis (MS) in man or experimental allergic encephalomyelitis (EAE) in rodents, may include an associated immune response directed against myelin protein antigens such as the proteolipid protein (PLP) and the myelin basic protein (MBP). Development of an immune response has been attributed, in part, to the sequestration of central nervous system antigens behind the blood-brain barrier. Recently, we identified a novel gene, the golli gene, which overlaps the mbp gene. The Golli transcription unit produces a family of mRNAs, and their corresponding proteins possess MBP epitopes known to be encephalitogenic in EAE. Transcription of the golli gene was detected in immune system tissue. Therefore, we wished to determine whether genes that encode the two major myelin protein components, PLP and MBP, were expressed in the human thymus. Our data demonstrate that both the plp and golli genes are transcribed in the fetal human thymus. Moreover, both the PLP and DM-20 transcripts are produced from the plp gene, and the HOG 7 and HOG 5 transcripts are produced from the golli gene. Confocal fluorescent immunohistochemistry using antibodies for the PLP/DM-20 and Golli proteins, co-localized expression of these antigens to thymic macrophages. Thus, the plp and golli genes are expressed, and their corresponding protein produced, in an antigen presenting cell in the human immune system.

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.

Isolation and characterization of a cDNA encoding the zebra finch myelin proteolipid protein

A cDNA was isolated from a zebra finch telencephalon cDNA library that encodes the myelin proteolipid protein. The clone was 2874 nucleotides long containing an open reading frame of 831 nucleotides that encoded a 277 amino acid myelin proteolipid protein. The 5'- and 3' untranslated regions were 112 and 1931 nucleotides, respectively. In Northern blots the clone hybridized to 3 bands of 3.5, 2.4 and 1.5 Kb in mouse brain RNA, but to only a single band of 3.0 kb in zebra finch brain RNA, suggesting the lack of alternative polyadenylation sites within the 3' untranslated region of the zebra finch PLP mRNAs. There was a small degree of homology between the zebra finch and chicken PLP 5' untranslated regions, but relatively little homology of the 5' untranslated regions of the zebra finch PLP cDNA clone with the homologous regions of PLP cDNAs of many mammalian species. Except for a small stretch of considerable homology, there was little overall homology with the 3' untranslated regions of mammalian PLP mRNAs. Approximately 10% (i.e. 28) of the amino acids in the zebra finch PLP differed from mammalian PLP, with most of these changes located within exon 3. There were 16 amino acid changes between zebra finch and chicken, suggesting that greater sequence variation in PLP structure is tolerated among avian species than among mammalian species.

Structural features of myelin basic protein mRNAs influence their translational efficiencies

The myelin basic protein (MBP) gene expresses several alternatively spliced products with the same 5' and 3' untranslated regions (UTRs). It has been reported that its expression may be regulated not only at the transcriptional level but also at the translational level during development. We engineered several MBP mRNA deletion mutants with 5' (-48, -37, -27, -22, and -10) and 3' UTRs of differing lengths and examined the translational efficiencies of these constructs in cell-free systems. The translational efficiencies of the constructs differed significantly over a range of almost 10-fold. A deletion of 11 nucleotides from the 5' end of the natural (i.e., -48) MBP mRNA resulted in an approximate fourfold reduction in translational efficiency. Further truncation of the 5' UTR increased the translational efficiencies of the constructs as has been observed with many RNAs. These results suggest that there may be a positive control element between -48 and -37 nucleotides in the 5' UTR of MBP mRNA. The effects of modifying the lengths of the 5' UTR on the translational efficiency of mRNAs encoding the 21.5-kDa and 14-kDa MBPs were the same, suggesting that the effect observed was not unique to the 21.5-kDa MBP mRNA. Truncating the 3' UTR of four different alternatively spliced MBP mRNAs also altered their translational efficiencies. Thus, the 5' and 3' UTRs of MBP mRNAs appear to influence the translation of these mRNAs, and such factors may be involved in the translational regulation of MBP gene expression.

The human myelin basic protein gene is included within a 179-kilobase transcription unit: expression in the immune and central nervous systems

Two human Golli (for gene expressed in the oligodendrocyte lineage)-MBP (for myelin basic protein) cDNAs have been isolated from a human oligodendroglioma cell line. Analysis of these cDNAs has enabled us to determine the entire structure of the human Golli-MBP gene. The Golli-MBP gene, which encompasses the MBP transcription unit, is approximately 179 kb in length and consists of 10 exons, seven of which constitute the MBP gene. The human Golli-MBP gene contains two transcription start sites, each of which gives rise to a family of alternatively spliced transcripts. At least two Golli-MBP transcripts, containing the first three exons of the gene and one or more MBP exons, are produced from the first transcription start site. The second family of transcripts contains only MBP exons and produces the well-known MBPs. In humans, RNA blot analysis revealed that Golli-MBP transcripts were expressed in fetal thymus, spleen, and human B-cell and macrophage cell lines, as well as in fetal spinal cord. These findings clearly link the expression of exons encoding the autoimmunogen/encephalitogen MBP in the central nervous system to cells and tissues of the immune system through normal expression of the Golli-MBP gene. They also establish that this genetic locus, which includes the MBP gene, is conserved among species, providing further evidence that the MBP transcription unit is an integral part of the Golli transcription unit and suggest that this structural arrangement is important for the genetic function and/or regulation of these genes.

Structure and developmental regulation of Golli-mbp, a 105-kilobase gene that encompasses the myelin basic protein gene and is expressed in cells in the oligodendrocyte lineage in the brain

We have identified a novel transcription unit of 105 kilobases (called the Golli-mbp gene) that encompasses the mouse myelin basic protein (MBP) gene. Three unique exons within this gene are alternatively spliced into MBP exons and introns to produce a family of MBP gene-related mRNAs that are under individual developmental regulation. These mRNAs are temporally expressed within cells of the oligodendrocyte lineage at progressive stages of differentiation. Thus, the MBP gene is a part of a more complex gene structure, the products of which may play a role in oligodendrocyte differentiation prior to myelination. One Golli-mbp mRNA that encodes a protein antigenically related to MBP is also expressed in the spleen and other non-neural tissues.

Expression of myelin protein genes and other myelin components in an oligodendrocytic cell line conditionally immortalized with a temperature-sensitive retrovirus

We have conditionally immortalized oligodendrocytes isolated from normal and shiverer primary mouse brain cultures through the use of the retroviral vector ZIPSVtsA58. This vector encodes an immortalizing thermolabile simian virus 40 large T antigen (Tag) and allows for clonal selection by conferring neomycin (G418) resistance. We isolated 14 shiverer and 10 normal lines that expressed the early oligodendrocyte marker 2',3'-cyclic nucleotide 3'-phosphodiesterase mRNA. These cell lines grew continuously at the permissive temperature (34 degrees C) and displayed Tag nuclear immunostaining. On shifting to nonpermissive temperatures (39 degrees C), the cells showed rapid arrested cell growth and loss of Tag staining. One line (N20.1) engineered from normal oligodendrocytes also expressed myelin basic protein (MBP) and proteolipid protein (PLP) mRNAs, genes normally expressed by mature, differentiated oligodendrocytes. No differences in any of the myelin-specific protein mRNA levels were observed in N20.1 cells grown at 39 degrees C for > 9 days compared with cells maintained at 34 degrees C. Immunocytochemical staining revealed N20.1 cells to be positive for the oligodendrocyte surface markers--galactocerebroside, A007, and A2B5. However, MBP and PLP polypeptides could not be detected by western blot or immunocytochemical staining at either the permissive or nonpermissive temperature. Cell-free protein synthesis experiments indicated that the MBP mRNAs isolated from N20.1 cells were translatable and directed the synthesis of the 17-, 18.5-, and 21.5-kDa MBP isoforms. Analysis of the PLP/DM20 gene splice products by polymerase chain reaction indicated that the expression of DM20 mRNA predominated over that of PLP mRNA in this cell line. Because the cell line expressed the MBP and PLP genes, it represents a "mature" oligodendrocyte, but the splicing patterns of these genes indicate that it is at an early stage of "maturation." This cell line has now been passaged > 40 times with fidelity of phenotype and genotype.

DM20 mRNA splice product of the myelin proteolipid protein gene is expressed in the murine heart

Northern blot analysis of poly A(+) RNA isolated from mouse heart revealed the expression of 3.3 and 2.4 kb mRNAs that hybridized with a cDNA for the mouse proteolipid protein (PLP). In order to examine the relationship of these RNAs to the myelin PLP/DM20 mRNAs, a mouse heart cDNA library was prepared and screened with a mouse PLP cDNA. A cDNA was isolated, sequenced, and found to encode the DM20 variant of PLP. Polymerase chain reaction (PCR) analysis of heart cDNA with three sets of primers confirmed the presence of DM20 mRNA in mouse heart and indicated that it is the major splice product of the PLP gene expressed in that tissue. In situ hybridization localized the expression of the DM20 mRNA to the myocardial cells. Northern blot analysis indicated that expression of the DM20 mRNA is developmentally regulated in the murine heart, increasing significantly in concentration after 12 days postpartum. Northern analysis also revealed the expression of the DM20 mRNA in the hearts of the jimpy and quaking mutants. These results indicate that the PLP gene is expressed in tissues other than brain and support the concept that products of the PLP gene may have some biological role other than as structural components of myelin.

Altered excitatory amino acid function and morphology of the cerebellum of the spastic Han-Wistar rat

A mutant strain of Han-Wistar rat carries an autosomal recessive gene producing spastic paresis which is characterized by ataxia, tremor and hind limb rigidity. Brains of affected rats and unaffected littermate controls were transected at the mesencephalon into rostral and caudal portions (the caudal portion contained the cerebellum and brainstem). Poly(A)+ mRNA was isolated from pooled rostral or caudal portions and injected into Xenopus oocytes. The oocytes were voltage-clamped and exposed to 1 mM L-glutamate, 500 microM kainate, 500 microM quisqualate, 200 microM N-methyl-D-aspartate (NMDA) or 1 mM gamma-aminobutyric acid (GABA). Oocytes injected with mRNA isolated from the caudal portions of the affected rat brains exhibited statistically significant increases in glutamate and kainate peak current responses compared to oocytes injected with mRNA from other brain samples. No differences were noted in the responses of the groups when exposed to quisqualate, NMDA or GABA. Cerebellar and brain stem mRNA were also isolated separately in different groups of mutants and unaffected littermates. Only oocytes injected with cerebellar mRNA from mutants displayed statistically significant increases in responses to glutamate and kainate. In parallel morphological studies changes in the cerebellum of mutants were also observed. These consisted of a loss of Purkinje cells and an asymmetrical disarrangement of the granule cell layer of cerebellar cortex. Taken together, the physiological and morphological results suggest that alterations in glutamate/kainate receptors in the cerebellum are phenotypic manifestations of the Han-Wistar mutation. The results are consistent with the hypothesis that this mutant rat might serve as a model of glutamate/kainate excitotoxicity in the brain.

Expression of a novel transcript of the myelin basic protein gene

A cDNA (M41) corresponding to a mouse myelin basic protein (MBP) mRNA with a longer 5'-untranslated region than predicted from earlier studies of MBP gene structure has been isolated and characterized. The additional 5'-untranslated region is encoded by two previously unidentified exons upstream of the major transcription start site of the gene. Using a DNA probe specific for M41-MBP mRNAs, Northern blot analysis indicated that expression of this transcript follows a developmental course in mouse brain similar to that of the majority of MBP mRNAs, but that the level of expression varies between brain and spinal cored. Expression of MBP mRNAs similar to the mouse M41-MBP also was identified in rat brain. The results suggest that the structure of the MBP gene is more complex than originally thought, containing at least two more exons. There appears to be at least one more MBP gene promoter that directs the synthesis of a subset of MBP mRNAs with a unique 5'-untranslated region.