Expression of a novel transcript of the myelin basic protein gene

Myelin management by the 18.5-kDa and 21.5-kDa classic myelin basic protein isoforms

The classic myelin basic protein (MBP) splice isoforms range in nominal molecular mass from 14 to 21.5 kDa, and arise from the gene in the oligodendrocyte lineage (Golli) in maturing oligodendrocytes. The 18.5-kDa isoform that predominates in adult myelin adheres the cytosolic surfaces of oligodendrocyte membranes together, and forms a two-dimensional molecular sieve restricting protein diffusion into compact myelin. However, this protein has additional roles including cytoskeletal assembly and membrane extension, binding to SH3-domains, participation in Fyn-mediated signaling pathways, sequestration of phosphoinositides, and maintenance of calcium homeostasis. Of the diverse post-translational modifications of this isoform, phosphorylation is the most dynamic, and modulates 18.5-kDa MBP's protein-membrane and protein-protein interactions, indicative of a rich repertoire of functions. In developing and mature myelin, phosphorylation can result in microdomain or even nuclear targeting of the protein, supporting the conclusion that 18.5-kDa MBP has significant roles beyond membrane adhesion. The full-length, early-developmental 21.5-kDa splice isoform is predominantly karyophilic due to a non-traditional P-Y nuclear localization signal, with effects such as promotion of oligodendrocyte proliferation. We discuss in vitro and recent in vivo evidence for multifunctionality of these classic basic proteins of myelin, and argue for a systematic evaluation of the temporal and spatial distributions of these protein isoforms, and their modified variants, during oligodendrocyte differentiation.

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.

Myelin tetraspan family proteins but no non-tetraspan family proteins are present in the ascidian (Ciona intestinalis) genome

Several of the proteins used to form and maintain myelin sheaths in the central nervous system (CNS) and the peripheral nervous system (PNS) are shared among different vertebrate classes. These proteins include one-to-several alternatively spliced myelin basic protein (MBP) isoforms in all sheaths, proteolipid protein (PLP) and DM20 (except in amphibians) in tetrapod CNS sheaths, and one or two protein zero (P0) isoforms in fish CNS and in all vertebrate PNS sheaths. Several other proteins, including 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP), myelin and lymphocyte protein (MAL), plasmolipin, and peripheral myelin protein 22 (PMP22; prominent in PNS myelin), are localized to myelin and myelin-associated membranes, though class distributions are less well studied. Databases with known and identified sequences of these proteins from cartilaginous and teleost fishes, amphibians, reptiles, birds, and mammals were prepared and used to search for potential homologs in the basal vertebrate, Ciona intestinalis. Homologs of lipophilin proteins, MAL/plasmolipin, and PMP22 were identified in the Ciona genome. In contrast, no MBP, P0, or CNP homologs were found. These studies provide a framework for understanding how myelin proteins were recruited during evolution and how structural adaptations enabled them to play key roles in myelination.

Unique clinical and pathological features in HLA-DRB1*0401-restricted MBP 111-129-specific humanized TCR transgenic mice

Amino acid residues 111–129 represent an immunodominant epitope of myelin basic protein (MBP) in humans with human leukocyte antigen (HLA)-DRB1*0401 allele(s). The MBP 111–129–specific T cell clone MS2-3C8 was repeatedly isolated from a patient with multiple sclerosis (MS), suggesting an involvement of MS2-3C8 T cells in the pathogenesis. To address the pathogenic potential of the MS2-3C8 T cell clone, we generated transgenic (Tg) mice expressing its T cell receptor and restriction element, HLA-DRB1*0401, to examine the pathogenic characteristics of MS2-3C8 Tg T cells by adoptive transfer into HLA-DRB1*0401 Tg mice. In addition to the ascending paralysis typical of experimental autoimmune encephalomyelitis, mice displayed dysphagia due to restriction in jaw and tongue movements and abnormal gait. In accordance with the clinical phenotype, infiltrates of MS2-3C8 Tg T cells and inflammatory lesions were predominantly located in the brainstem and the cranial nerve roots in addition to the spinal cord and spinal nerve roots. Together, these data suggest a pathogenic role of MBP-specific T cells in inflammatory demyelination within the brainstem and cranial nerve roots during the progression of MS. This notion may help to explain the clinical and pathological heterogeneity of MS.

Estrogen receptor-1 (Esr1) and -2 (Esr2) regulate the severity of clinical experimental allergic encephalomyelitis in male mice

Estrogens and estrogen-receptor signaling function in establishing and regulating the female immune system and it is becoming increasingly evident that they may play a similar role in males. We report that B10.PL/SnJ male mice with a disrupted estrogen receptor-1 (alpha) gene (Esr1(-/-)) develop less severe clinical experimental allergic encephalomyelitis (EAE) compared to either Esr1(+/-) or wild-type (Esr1(+/+)) controls when immunized with myelin basic protein peptide Ac1-11 (MBP(Ac1-11)). In contrast, the disease course in B10.PL/SnJ male mice with a disrupted estrogen receptor-2 (beta) gene (Esr2(-/-)) does not differ from that of wild-type (Esr2(+/+)) mice. However, Esr2(+/-) mice do develop more severe clinical disease with an earlier onset indicating that heterosis at Esr2 plays a significant role in regulating EAE in males. No significant differences in central nervous system histopathology or MBP(Ac1-11)-specific T-cell responses as assessed by proliferation and interleukin-2 production were observed as a function of either Esr1 or Esr2 genotype. An analysis of cytokine/chemokine secretion by MBP(Ac1-11)-specific T cells revealed unique Esr1 and Esr2 genotype-dependent regulation. Interferon-gamma secretion was found to be negatively regulated by Esr1 whereas interleukin-6 and tumor necrosis factor-alpha secretion exhibited classical Esr2 gene dose responses. Interestingly, MCP-1 displayed distinctively unique patterns of genotype-dependent regulation by Esr1 and Esr2. The contribution of the hematopoietic and nonhematopoietic cellular compartments associated with the heterotic effect at Esr2 in regulating the severity of clinical EAE was identified using reciprocal hematopoietic radiation bone marrow chimeras generated between male wild-type and Esr2(+/-) mice. Wild-type --> Esr2(+/-) mice exhibited EAE equivalent in severity to that seen in Esr2(+/-) --> Esr2(+/-) control constructs; both of which were more severe than the clinical signs observed in Esr2(+/-) --> wild-type and wild-type --> wild-type mice. These results indicate that the heterotic effect at Esr2 is a function of the nonhematopoietic compartment.

Identification of a protein that interacts with the golli-myelin basic protein and with nuclear LIM interactor in the nervous system

The myelin basic protein (MBP) gene encodes the classic MBPs and the golli proteins, which are related structurally to the MBPs but are not components of the myelin sheath. A yeast two-hybrid approach was used to identify molecular partners that interact with the golli proteins. A mouse cDNA was cloned that encoded a protein of 261 amino acids and called golli-interacting protein (GIP). Database analysis revealed that GIP was the murine homolog of human nuclear LIM interactor-interacting factor (NLI-IF), a nuclear protein whose function is just beginning to be understood. It is a member of a broad family of molecules, found in species ranging from yeast to human, that contain a common domain of approximately 100 amino acids. Immunocytochemical and Northern blot analyses showed co-expression of GIP and golli in several neural cell lines. GIP and golli also showed a similar developmental pattern of mRNA expression in brain, and immunohistochemical staining of GIP and golli showed co-expression in several neuronal populations and in oligodendrocytes in the mouse brain. GIP was localized predominantly in nuclei. GIP co-immunoprecipitated with golli in several in vitro assays as well as from PC12 cells under physiologic conditions. GIP was the first member of this family shown to interact with nuclear LIM interactor (NLI). NLI co-immunoprecipitated with GIP and golli from lysates of N19 cells transfected with NLI, further confirming an interaction between golli, GIP, and NLI. The ability of GIP to interact with both golli and NLI, and the nuclear co-localization of GIP and golli in many cells, indicates a role for the golli products of the MBP gene in NLI- associated regulation of gene expression.

Immune tolerance to myelin proteins

Multiple sclerosis (MS) is a demyelinating disorder of the central nervous system. It is believed to be an autoimmune disease arising from a breakdown of immune tolerance in T cells specific for myelin antigens. The heterogeneity in clinical signs and pathology observed in MS patients suggests a complex pathogenesis in which the specificity of the pathogenic T cells and the tolerance mechanisms that are compromised vary among individual patients. In this review, we summarize some of the features of the diverse immune pathology observed in MS and the animal models used to study this disease. We then describe the current state of knowledge regarding the expression of the major myelin protein antigens believed to be targeted in MS and the mechanisms of immune tolerance that operate on T cells that recognize these antigens.

Expression and regulation of golli products of myelin basic protein gene during in vitro development of oligodendrocytes

The myelin basic protein (MBP) gene produces two families of proteins, the classic MBPs, important for myelination of the CNS, and the golli proteins, whose biological role in oligodendrocytes (OLs) is still unknown. The goals of this work were to study the in vitro pattern of expression of the golli products during OL differentiation and to compare it with that of the classic MBP products of the gene. Mouse primary glial cultures were analyzed at the mRNA and protein levels with an array of techniques. We found that OLs express golli mRNA primarily during intermediate stages of differentiation, which was confirmed by immunocytochemical analysis. Golli expression was low in proliferating OL progenitors as well as in terminally mature OLs. Golli proteins were found associated with the OL cell soma and nuclei and, to a lesser extent, with the cellular processes. We also found that golli proteins are not targeted to myelin in vitro and in vivo, in contrast to the classic MBPs. Finally, we found that golli expression is regulated during OL development and can be manipulated by growth factors such as basic fibroblast growth factor, neurotrophin-3, and retinoic acid.

The specificity of the myelin basic protein gene promoter studied in transgenic mice

The myelin basic proteins (MBPs) are a family of polypeptides that are predominantly expressed in the nervous system where they play a major role in myelination. We have generated four lines of transgenic mice carrying a transgene in which 1.34 kb of the 5'-flanking sequence of the mouse MBP gene was fused upstream of the coding region of the Escherichia coli lac Z gene in order to investigate developmental and tissue-specific expression of the MBP gene. Expression of both the lacZ transgene and the endogenous MBP gene followed a common developmental pattern in mouse brain. Transgene expression was detected in primary oligodendrocytes, but not in type 2 astrocytes. In addition, the lacZ gene product was expressed in epithelial cells of certain nonneural tissues, namely kidney, epididymis, ureter, and seminal vesicles. The ectopic expression of the transgene was associated with the development of DNase I hypersensitive sites at the site of insertion which was found to be within the intron 1 region of the endogenous MBP gene. The results reported here strongly suggest that the 1.34-kb 5'-flanking region of the MBP gene contains cis-regulatory elements that confer developmental regulation of the MBP gene, although this region appears to lack elements that restrict its expression to the nervous system.

Overexpression of a minor component of myelin basic protein isoform (17.2 kDa) can restore myelinogenesis in transgenic shiverer mice

Shiverer (shi) mice, which are neurologically mutant, lack a large portion of the gene for the myelin basic proteins (MBPs), have virtually no myelin in their central nervous system (CNS), and shiver, undergo seizures, and die early. At least five types of MBPs (21.5, 18.5, 17.3, 17.2 and 14.0 kDa) are known to be generated through alternative splicing from a single MBP gene. We have produced transgenic shi mice carrying a cDNA encoding mouse 14-kDa MBP isoform, the most abundant form of MBPs, under control of a mouse MBP gene promoter, and showed that expression of the 14-kDa MBP can restore CNS myelination. To test whether the 17.2-kDa MBP isoform, one of the minor components of MBPs, can also elicit myelination in homozygous shi mutants, we produced seven independent transgenic shi mice carrying cDNA encoding the mouse 17.2-kDa MBP isoform, and the transcription of which was driven by a mouse MBP gene promoter. The axons in the cerebellum of one transgenic line, which exhibited the highest expression of transgene-derived mRNA ( approximately 50% of the level of total MBP mRNA in the normal mouse brain), were myelinated. This mouse exhibited nearly normal behavior. These findings indicate that the 17.2-kDa MBP isoform, even when the only 17.2-kDa MBP isoform is present, has the ability to elicit CNS myelination in transgenic shi mice. This transgenic strategy will be useful for elucidating the role of each type of MBP isoform in CNS myelinogenesis.

Postnatal localization and morphogenesis of cells expressing the dopaminergic D2 receptor gene in rat brain: expression in non-neuronal cells

The cellular localization of the dopaminergic D2 receptor (D2R) mRNA and protein was determined during postnatal development, from birth to 35 days, in the rat neostriatum by in situ hybridization histochemistry and immunohistochemistry. To localize and identify more precisely the morphology of cells expressing the D2R mRNA, nonradioactive, digoxigenin in situ hybridization was performed. Throughout this period of development, D2R mRNA and protein were widely expressed by neostriatal cells, adjoining forebrain cells and small cellular processes. Within morphologically identifiable neurons, the expression of the D2 receptor appeared to occur after cell division ceased. D2R gene expression appeared during neuronal migration and followed the developmental pattern of neuronal settling within the neostriatum. Both D2R mRNA and protein appeared to colocalize in neostriatal cells and the labeling of both appeared to accumulate within the cells progressively with age. The structural phenotypes of neostriatal neurons bearing D2R mRNA and protein were diverse throughout postnatal development. The most frequently stained cells were a heterogeneous group of medium spiny and aspiny neurons. Large cells corresponding to aspiny neurons were less frequently stained. Both phenotypes exhibited considerable postnatal growth of their cell bodies. In addition to neurons, other cell types were also observed to express the D2R mRNA and protein over the developmental period studied. These other cells included patches of ciliated ependymal cells lining the lateral ventricles and many interfascicular oligodendroglia of forebrain fiber tracts. These results demonstrate the unexpected expression of the dopaminergic D2 receptor in non-neuronal cells within the brain. They provide a novel morphologic suggestion that the dopaminergic D2 receptor may support unrecognized, nonsynaptic functions in specific non-neuronal cell populations in the nervous system.

The shiverer mutation affects the persistence of Theiler's virus in the central nervous system

Theiler's virus persists in the white matter of the spinal cord of genetically susceptible mice and causes primary demyelination. The virus persists in macrophages/microglial cells, but also in oligodendrocytes, the myelin-forming cells. Susceptibility/resistance to this chronic infection has been mapped to several loci including one tentatively located in the telomeric region of chromosome 18, close to the myelin basic protein locus (Mbp locus). To determine if the MBP gene influences viral persistence, we inoculated C3H mice bearing the shiverer mutation, a 20-kb deletion in the gene. Whereas control C3H mice were of intermediate susceptibility, C3H mice heterozygous for the mutation were very susceptible, and those homozygous for the mutation were completely resistant. This resistance was not immune mediated. Furthermore, C3H/101H mice homozygous for a point mutation in the gene coding for the proteolipid protein of myelin, the rumpshaker mutation, were resistant. These results strongly support the view that oligodendrocytes are a necessary viral target for the establishment of a persistent infection by Theiler's virus.

Alterations in the spontaneous release of dopamine and the density of the DA D2 receptor mRNA after chronic postnatal exposure to cocaine

The influence of cocaine administration on dopamine (DA) release and D2 dopamine receptor mRNA levels was examined in developing rat brain. In the rat pup, cocaine (25 mg/ kg SC) was administered daily from postnatal days 1-9 and extracellular DA measured 24 h after the last injection of cocaine, using in vivo micro dialysis. Twenty-four hours after discontinuing cocaine administration, a decrease in the extracellular concentration of DA of more than 100% was found in treated pups compared to control pups. Pups were tested on postnatal days 10-12, 20-21, or 35-36. After 1 month, basal release of DA returned to control levels. To examine the structural basis of the alteration in basal release of DA, in situ hybridization studies were performed to access the effect of chronic administration of cocaine on the mRNA encoding the D2 DA receptor. These preliminary studies, on postnatal day 10, indicate that drug treatment alters the developmental pattern of D2 mRNA. The changes in D2 mRNA expression were accompanied by delayed disaggregation of neostriatal cells and diminished growth of neostriatal neurons. These structural changes may lead to functional impairment in the development of dopamine target cells, thus altering the balance of synaptic and trophic effects of DA.

Membrane adhesion and other functions for the myelin basic proteins

The myelin basic proteins are a set of peripheral membrane polypeptides which play an essential role in myelination. Their most well-documented property is the unique ability to 'seal' the cytoplasmic aspects of the myelin membrane, but this is probably not the only function for these highly charged molecules. Despite extensive homology, the individual myelin basic proteins (MBPs) exhibit different expression patterns and biochemical properties, and so it is now believed that the various isoforms are not functionally equivalent in myelinating cells. We now think that while the major MBPs are intracellular adhesion molecules, some of the quantitatively less abundant isoforms that are expressed very early in development may have regulatory effects on the myelination program.

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 structural complexities of the myelin basic protein gene from mouse are also present in shark

The Golli-mbp gene complex contains two overlapping transcription units with two distinct promoters, of which the downstream (myelin basic protein [mbp]) promoter is more frequently used. A previous comparison of the downstream promoter sequences from shark and mouse allowed the identification of two DNA sequences called the boxes I and II and the wobble zone. The boxes I and II sequence is a composite cis-acting motif that is thought to be involved in the regulation of the downstream promoter. It contains sequences similar to T-antigen, MyoD/E2A, and glucocorticoid receptor-binding sites. The wobble zone codes for an exon (5a in the nomenclature of Campagnoni et al., 1993) that is included in messenger RNAs transcribed from the upstream promoter. The polypeptides encoded by this exon from shark and mouse are 86 and 84 amino acids long, respectively. These polypeptides are overall 59% identical and include a region (residues 41-75 in shark and 39-73 in mouse) that is 89% identical between the two species. A primary sequence analysis showed that each of these polypeptides contains an N-glycosylation site, phosphorylation sites for Ca2+/calmodulin-dependent protein kinase, protein kinase C and casein kinase II, and partial ATP- and GTP-binding sites. The shark polypeptide also contains a phosphorylation site for proline-directed protein kinase. These observations are consistent with the notion that the intricate structure and regulation of the Golli-mbp gene complex arose during vertebrate evolution within a common ancestor to sharks and mammals.

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.

Identification of an embryonic isoform of myelin basic protein that is expressed widely in the mouse embryo

We have identified a myelin basic protein (MBP) isoform in mouse embryos that includes an exon upstream of the usual transcription initiation site. This isoform, embryonic-neonatal MBP (E-MBP), is expressed at the protein level in the embryonic nervous system at a time when other MBP isoforms are not detected. In addition to the central and peripheral nervous systems of the embryo and neonate, the thymus, spleen, and testes also express E-MBP at the protein level. The expression of E-MBP in cell types distinct from the nervous system strongly suggests that this MBP isoform has a role apart from the formation of myelin.

The proximal region of the MBP gene promoter is sufficient to induce oligodendroglial-specific expression in transgenic mice

To characterize regulatory DNA sequences involved in oligodendroglial expression of myelin basic protein (MBP), transgenic mice carrying a 256 bp fragment of the mouse MBP promoter fused to an Escherichia coli lacZ gene were generated. Of four transgenic families, two (lines 2 and 4) expressed beta-galactosidase activity in the nervous system but not in most other tissues. Histochemical and immunohistochemical analysis of adult brain from these two lines showed oligodendroglial-specific expression of the transgene. In line 2, only a small proportion of oligodendrocytes expressed the transgene, and in labelled cells the product of the enzymatic reaction with beta-galactosidase was confined to a small round vesicle in the vicinity of the nucleus. In contrast, in tissue sections from line 4 adult brain and spinal cord beta-galactosidase activity was much more intense and at least 80-90% of oligodendrocytes expressed the transgene. Detection of the MBP-lacZ transcript by in situ hybridization showed that the transgene mRNA was confined to the oligodendrocyte cell body. These results suggest that cis-acting regulatory elements, specifying oligodendrocytes identity, are located within 256 bp upstream from the MBP gene.

A new family of transcripts of the myelin basic protein gene: expression in brain and in immune system

A cDNA clone (MBP2) corresponding to a novel mouse myelin basic protein (MBP) mRNA has been isolated from an adult mouse bone marrow cDNA library. It contains the MBP exons 1a-7 except exon 5. Using PCR experiments we have determined that this MBP2 mRNA belongs to a new MBP mRNA family initiated upstream from exon 1b. Their 5' end extends into exon 1a and/or the region 0' previously described. These mRNAs are generated by alternative splicing of the primary transcript involving excision of exon 1a, 1b, 2, 5, or 6. Thus, these new mRNAs are produced from a promoter(s) located upstream from the major promoter 1b. They are expressed in brain (at least from embryonic day 15), in bone marrow, and in other hemolymphopoietic tissues, particularly in macrophage cells. As their expression is not restricted to myelinating cells, the function of these novel MBP mRNAs and putative proteins might not be related to myelination.

Novel isoforms of mouse myelin basic protein predominantly expressed in embryonic stage

Myelin basic protein (MBP), a major protein of myelin, is thought to play an important role in myelination, which occurs postnatally in mouse. Here we report that the MBP gene is expressed from the 12th embryonic day in mouse brain and that most of the predominant embryonic isoforms are not those reported previously. These isoforms have a deletion of a sequence encoded by exon 5 from the well-known isoforms. These isoforms show a unique developmental profile, i.e., they peak in the embryonic stage and decrease thereafter. In jimpy, a dysmyelinating mutant, the level of these isoforms remains high even in the older ages. These results suggest that MBPs have heretofore unknown functions unrelated to myelination before myelinogenesis begins. The possible presence of 18 isoforms of MBP mRNA, which are classified into at least three groups with different developmental profiles, is also reported here.

Sequence similarities of myelin basic protein promoters from mouse and shark: implications for the control of gene expression in myelinating cells

To better understand the cell type-specific and coordinated regulation of the myelin protein genes, we cloned and sequenced the shark myelin basic protein (MBP) promoter. An alignment of the shark sequence with the corresponding mouse sequence showed striking similarities. These similarities, together with the results from expression experiments, define two major regions (A and B) within the MBP promoter. Region A is located immediately 5' to the transcription initiation sites and includes five sequences thought to be cis-acting domains. These domains include two boxes of 13 and 12 nucleotides, respectively, separated from each other by 10 nucleotides, an MBP enhancer, and GC, CCAAT, and TATA boxes. Region A also contains a putative exon that codes for 35 amino acids of an unidentified polypeptide. Region B, which is located adjacent to the 5' end of region A, contains two boxes that are 10 and 11 nucleotides long, respectively, and are identical in mouse and shark. We have previously cloned and sequenced the shark glycoprotein zero (P0) promoter. A comparison between the sequences of the rat and shark P0 promoters shows three conserved regions in addition to CCAAT and TATA boxes. The shark P0 promoter is active in the CNS and PNS, and contains a sequence of 13 nucleotides that is located at -159 from the initiation of transcription and is similar to that of the MBP enhancer. The mammalian P0 promoter is active exclusively in the adult PNS and contains a sequence similar to that of the MBP enhancer located adjacent to the 3' side of the transcription initiation site. Sequence similarities and differences between the promoters of the mammalian and shark myelin protein genes will help to identify the basis for the cell type-specific and coordinated expression of these genes.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

A novel transcript overlapping the myelin basic protein gene

Myelin basic protein (MBP) is a major constituent of myelin synthesized by oligodendrocytes and Schwann cells. We have investigated the expression of mouse MBP RNAs outside the nervous system. Nuclease protection experiments indicate that RNAs containing exon 1 and not the six downstream exons of the MBP gene are transcribed in various hemopoietic tissues. We have isolated a hemopoietic MBP-related (HMBPR) cDNA clone from a mouse bone marrow cDNA library screened with an MBP cDNA probe. This clone contains exons 1a and 1b and a part of intron 1 of the MBP gene. An additional 5' region, encoded by at least three unidentified exons, lies upstream of exon 1a. The HMBPR clone corresponds to a 5-kb RNA expressed in bone marrow, spleen, thymus, and macrophagic cells. This transcript is expressed at a similar level in brain, although at a lower level than the classical 2-kb mRNA. These data indicate that a new transcript, overlapping the MBP transcription unit and controlled by a distinct promoter, is expressed in hemopoietic tissues. This RNA might encode a 21-kDa protein sharing a common domain with MBP.

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.

An immunochemical comparison of human myelin basic protein and its modified, citrullinated form, C8

An immunochemical analysis was conducted to compare the C1 isomer of human myelin basic protein (MBP) with the newly described and less cationic, citrullinated isomer of MBP referred to as C8. Ten polyclonal antisera directed at multiple epitopes or restricted regions of MBP were used in radioimmunoassays to examine MBP-C1 and MBP-C8. Antisera reactive with MBP peptide 1-14 clearly distinguished MBP-C1 from MBP-C8. Antisera to human MBP peptides 10-19 and 90-170, but not to MBP peptide 69-89, showed modest differences between MBP-C1 and MBP-C8. The MBP-C8s from multiple sclerosis (MS) and non-MS brain reacted essentially the same. With murine monoclonal antibodies and enzyme-linked immunosorbent assay (ELISA), differences between MBP-C8 and other isomers were shown for anti-MBP 10-19 but not for anti-MBP 1-9 or anti-MBP 80-89. These findings imply differences in sequence or conformation in the structure of MBP-C7 compared to MBP-C1, most notably near the amino terminus.

Posttranscriptional regulation of myelin protein gene expression

Regulation of myelin protein gene expression occurs at many different levels including transcription, mRNA translocation, translation, and posttranslational modification of myelin proteins prior to their assembly into the membrane. Translocation of myelin basic protein (MBP) mRNAs into oligodendrocyte processes was observed in vivo and in primary cultures, but no such translocation was observed for the mRNAs encoding the proteolipid protein (PLP) or myelin-associated glycoprotein. More than 99% of the mRNAs encoding 2'3'-cyclic nucleotide phosphodiesterase (CNP) remained associated with cell bodies. In the jimpy mutant, MBP mRNA translocation appeared to be impaired, but translocation occurred normally in quaking brains in vivo. We have found that steroids, such as glucocorticoids, stimulate the translation of MBP and PLP mRNAs in cell-free systems and inhibit the translation of CNP mRNA. This pattern of regulation is consistent with compositional changes noted in myelin during development. We have localized a nine nucleotide segment within the 5'-untranslated region of the MBP mRNA that is involved in the action of steroids on translation of this mRNA. We have also determined that the protein synthetic step modulated by the steroids is chain initiation, enhancing the rate at which new ribosomal subunits bind to the MBP mRNAs.

Posttranscriptional events in the expression of myelin protein genes

A number of posttranscriptional events may be involved in regulating the expression of the myelin protein genes. One such event in the expression of the myelin basic protein (MBP) gene is the translocation of MBP mRNAs from oligodendrocyte cell bodies to their processes. This translocation can be observed in vivo and in primary mixed glial cell cultures. In jimpy brains the translocation of MBP mRNA appears to be disrupted, so that most of the mRNA remains associated with cell bodies. This apparent failure of translocation may account for the lack of incorporation of newly synthesized MBP into jimpy myelin. In quaking myelin, where MBP assembly is also defective, translocation appears to be normal, suggesting that incorporation of MBP into the membrane also is regulated posttranslationally. We have identified a number of the structural features of MBP mRNAs that influence the efficiencies with which they are translated and may be involved in regulating the levels of individual MBP produced. We also found that glucocorticoids stimulate the translation of MBP and PLP mRNAs and inhibit the translation of CNP mRNA in cell-free systems. Our results suggest that this pattern of translational regulation may be physiologically meaningful, especially during maturation of myelin. The mechanism by which the steroids modulate translation of these messages appears to be novel. Analysis of the effect of steroids on cRNAs produced from engineered MBP cDNA constructs has permitted the identification of a nine nucleotide element involved in this steroid modulation within the 5' untranslated region of the MBP mRNA.