Mutually exclusive apicobasolateral sorting of two oligodendroglial membrane proteins, proteolipid protein and myelin/oligodendrocyte glycoprotein, in Madin-Darby canine kidney cells.

Oligodendrocytes elaborate an extensive membrane network that ensheathes CNS axons in multilamellar wrappings. A compaction process excludes much of the cytoplasm in mature myelin membranes, giving rise to distinct lipid/protein compositions in two membrane compartments (compact myelin and membranes of the cell body and processes). Insofar as oligodendrocytes arise from neuroepithelial progenitors, it seems likely that some elements are shared for protein targeting by these two cell types. We hypothesized that certain membrane proteins targeting different oligodendroglial membrane compartments would preferentially sort to opposite domains when transfected into Madin-Darby canine kidney (MDCK) epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is found in uncompacted membrane (cell body, processes), and it sorts exclusively to MDCK basolateral membrane. Proteolipid protein (PLP) is found in compact myelin, and it sorts exclusively to MDCK apical membrane. Myelin-associated glycoprotein (MAG) is primarily in the periaxonal inner loop of myelin; however, it fails to target preferentially within MDCK cells. This inability of MAG to sort within MDCK cells suggests a lack of required oligodendroglial-specific targeting components. In contrast, the sorting machinery in both oligodendrocytes and MDCK cells recognizes targeting signals for MOG and PLP, and we propose that these oligodendroglial membrane proteins delineate cognate basolateral and apical domains, respectively.

Identification of a basolateral membrane targeting signal within the cytoplasmic domain of myelin/oligodendrocyte glycoprotein.

Oligodendrocytes possess two distinct membrane compartments--uncompacted plasma membrane (cell body, processes) and compact myelin. Specific targeting mechanisms must exist to establish and maintain these membrane domains. Polarized epithelial cells have the best characterized system for targeting components to apical and basolateral compartments. Since oligodendrocytes arise from neuroepithelial cells, we investigated whether they might utilize targeting paradigms similar to polarized epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is a transmembrane Ig-like molecule restricted to uncompacted oligodendroglial plasma membrane. We stably expressed MOG in Madin-Darby canine kidney (MDCK) Type II epithelial cells, which have been extensively used in protein-targeting studies. Data from surface biotinylation assays and confocal microscopy revealed that MOG sorts exclusively to the basolateral membrane of MDCK cells. Expression vectors containing progressive truncations of MOG from the cytoplasmic C-terminus were expressed in MDCK cells to localize basolateral sorting signals. A loss of only four C-terminal residues results in some MOG expression at the apical surface. More strikingly, removal of the C-terminal membrane associated hydrophobic domain from MOG results in complete loss of basolateral sorting and specific targeting to the apical membrane. These data suggest that myelinating oligodendrocytes may utilize a sorting mechanism similar to that of polarized epithelia.

Epitope specificity of demyelinating monoclonal autoantibodies directed against the human myelin oligodendrocyte glycoprotein (MOG).

We describe the epitope specificity of a panel of ten demyelinating monoclonal antibodies (mAb) that recognise the extracellular immunoglobulin-like domain of human myelin oligodendrocyte glycoprotein (hMOG(lgd)). All the mAbs bind to the surface of MOG-transfected fibroblasts as assessed in vitro by FACS and immunocytochemistry but failed to recognise overlapping 15-mer MOG peptides when assessed by ELISA. However, increasing peptide length to 25 amino acids revealed that four mAbs recognised epitopes within the amino acid sequence 63-100 of human MOG. In contrast, a non-demyelinating MOG-specific mAb recognised MOG by both ELISA and Western blotting but failed to stain MOG transfected fibroblasts. These observations suggest that assays based on the use of MOG-transfected cell lines will differentiate between pathogenic and non-pathogenic MOG-specific antibody responses in experimental models and human diseases of the nervous system.

IFN-gamma-activated primary murine astrocytes express B7 costimulatory molecules and prime naive antigen-specific T cells.

Astrocytes may serve as effectual APCs for T cell-mediated immune responses to myelin components during multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). Although astrocytes have been reported not to constitutively express MHC class II molecules, expression is up-regulated during active EAE and by in vitro incubation with IFN-gamma. Previous studies have reported that cytokine-activated astrocytes are able to activate Ag-specific previously activated T cells, but not naive alloreactive T cells. In the current study, we show that a subset of primary murine astrocytes constitutively expresses B7-2 molecules, as determined by FACS and PCR analyses, and up-regulates surface expression and mRNA levels of both B7-2 and B7-1 upon IFN-gamma stimulation. In contrast to earlier reports, we found that both untreated and IFN-gamma-treated astrocytes were able to stimulate proliferation of previously activated OVA-specific Th1 cells. In contrast, only IFN-gamma-treated astrocytes activated naive, transgenic OVA-specific T cells. Astrocyte-induced activation of both OVA-specific naive T cells and activated Th1 cells was dependent primarily on B7-2-mediated costimulation, as proliferation was inhibited by CTLA4-Ig and by anti-B7-2 mAbs. These results suggest that astrocytes in an inflammatory environment have the capacity to express the required MHC class II and B7 costimulatory molecules necessary for efficient activation of naive T cells. Since we have shown that T cells specific for endogenous myelin epitopes released during acute EAE play the major pathologic effector role in subsequent disease relapses (epitope spreading), astrocytes could play a role in the local activation and expansion of these responses.

Investigation of myelin/oligodendrocyte glycoprotein membrane topology.

Myelin/oligodendrocyte glycoprotein (MOG) is a CNS-specific integral membrane protein that is an atypical member of the immunoglobulin (Ig) superfamily with two potential transmembrane domains based upon hydropathy analysis. With only one other exception, all Ig family members possess a single or no membrane spanning region. In order to analyze MOG membrane topology, we prepared stably transfected cells that express mouse MOG and used three domain-specific antisera to ascertain the localization of these hydrophobic domains. As expected, MOG's glycosylated N-terminal Ig-like domain was identified as extracellular, because membrane permeabilization was not required for immunoreactivity with the MOG1-125 antiserum. In contrast, both MOG154-169 and MOG198-218 antisera stained cells only upon permeabilization. These data indicate that only MOG's N-terminal hydrophobic domain spans the lipid bilayer, and we propose that MOG's C-terminal hydrophobic domain associates with the cytoplasmic face of the plasma membrane. As for MOG's second hydrophobic domain, it is clear that either orientation (transmembrane versus membrane-associated) would be unique among Ig-like proteins, and the implications of our proposed topology for MOG in oligodendroglial plasma membrane are discussed.

Myelin/oligodendrocyte glycoprotein is alternatively spliced in humans but not mice.

Myelin/oligodendrocyte glycoprotein (MOG) is an integral membrane protein expressed on the oligodendrocyte cell surface and the outermost surface of myelin sheaths. Due to this localization, MOG is a primary target antigen involved in immune-mediated demyelination. We previously reported that MOG is a unique member of the immunoglobulin (Ig) superfamily in that it possesses two large hydrophobic domains. MOG is highly conserved between deduced peptide sequences of rodent and human MOG (approximately 89% identity). We have completed an investigation of alternative splicing within the human and mouse MOG genes. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of total cellular RNA isolated from both fetal and adult human central nervous system (CNS) tissues reveals a complex array of alternatively spliced MOG-specific variants and the presence of two novel exons. Exon 3 encodes a short hydrophilic domain containing multiple in-frame termination codons that would result in truncation of MOG prior to translation of its transmembrane domain. Exon 7 encodes an additional hydrophilic domain that replaces MOG's second hydrophobic domain in one splice variant. We also observed that five of our eight MOG variants exhibited an alternative internal 3' splice acceptor within MOG's terminal exon. Surprisingly, no splicing was observed in a developmental study using mouse brainstem RNA.

Myelin/oligodendrocyte glycoprotein (MOG) expression is associated with myelin deposition.

We investigated the onset of expression of the myelin/oligodendrocyte glycoprotein (MOG) mRNA and protein in the developing mouse central nervous system. In situ hybridization on brain sections at different stages of embryonic and postnatal development showed that MOG transcripts were first detected at birth in the medulla oblongata. During the first week after birth, cells expressing MOG mRNA were located in the ventral longitudinal funiculus. During the second postnatal week, the pattern of MOG mRNA expression extended rostrally to the mid-forebrain regions and reached completion by the beginning of the third week. MOG transcription was delayed by several days with respect to myelin basic protein (MBP), and it appeared that while the MBP probe labeled both non-myelinating and myelinating oligodendrocytes, only the latter were MOG-positive. In vitro, immunocytochemical analysis of MOG protein expression, performed on myelinating cultures derived from mouse brain embryos at 15 days of gestation, confirmed the strict restriction of MOG expression to myelinating oligodendrocytes. In particular, oligodendrocytes lining up their processes along axons, but not yet having started to deposit a myelin sheath, were still MOG negative. However, in the same cultures, pseudo-myelinating oligodendrocytes (i.e., cells not associated with neurites, but forming whorls of myelin-like figures) were MOG positive. Similarly, rat CG4 cells, an oligodendrocyte-like cell line, expressed MOG only after they had extended sheet-like processes, which suggested that the activation of MOG transcription depends more on an intrinsic oligodendroglial maturation program of myelination than on a neuronal signal.

The N-terminal domain of the myelin oligodendrocyte glycoprotein (MOG) induces acute demyelinating experimental autoimmune encephalomyelitis in the Lewis rat.

Using a highly purified recombinant protein, mMOG, we demonstrated that autoimmune responses to the N-terminal domain (a.a 1-125) of the myelin oligodendrocyte glycoprotein (MOG) induce an acute demyelinating variant of experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. Immunisation with 100 micrograms of mMOG in adjuvant at the base of the tail induced mild clinical disease in 9 of 11 animals (mean clinical score 1.1). The disease was characterised histopathologically by the presence of inflammation and focal demyelinating lesions in the central nervous system (CNS). Adoptive transfer experiments suggest that this inflammatory demyelinating pathology is mediated by synergy between a weakly encephalitogenic, MOG-specific T cell response and a demyelinating, MOG-specific autoantibody response. Using in vitro selected mMOG-reactive T cell lines, the encephalitogenic T cell response to this domain of MOG was found to recognise two distinct epitopes, MOG1-20 and MOG35-55; whereas ELISA demonstrated that the immunodominant B cell epitope was located within the amino acid sequence MOG1-25. However although active immunisation with synthetic peptides corresponding to the T cell epitopes, MOG1-20 or MOG35-55, induced an inflammatory response in the CNS, this was not associated with demyelination indicating that the demyelinating antibody response recognises other, possibly conformation dependent epitopes. This study unequivocally demonstrates that MOG-specific autoimmune responses are alone sufficient to induce a demyelinating disease of the CNS and supports the proposal that MOG may play an important role in the immunopathogenesis of multiple sclerosis.

Identification of epitopes of myelin oligodendrocyte glycoprotein for the induction of experimental allergic encephalomyelitis in SJL and Biozzi AB/H mice.

A recombinant protein corresponding to the Ig-like domain of myelin oligodendrocyte glycoprotein (MOG) and synthetic 15-mer peptides of the whole MOG molecule with eight amino acid overlaps were screened for their ability to induce experimental allergic encephalomyelitis (EAE) in Biozzi AB/H (H-2dq1) and SJL (H-2S) mice. Clinical and histologic evidence of EAE developed after sensitization with the recombinant MOG protein in both AB/H and SJL mice. In AB/H mice at least three MOG epitopes within residues 1-22, 43-57, and 134-148 induced clinical and histologic EAE, whereas only the sequence 92-106 was encephalitogenic in SJL mice. Histologically, the inflammatory response in the central nervous system consisted of perivascular accumulations of CD5+ T cells and F4/80+ macrophage/microglia cells equally distributed in the brain and spinal cord. The subpial/meningeal infiltration, characteristic of mouse EAE induced with spinal cord homogenate, was only observed in cases of severe clinical disease in SJL mice in which the cellular infiltrates predominated in the spinal cord. In spite of the presence of histologic lesions in AB/H mice immunized with MOG, clinical disease either rapidly resolved or was clinically silent. In contrast to immunization of SJL mice with recombinant MOG, sensitization to MOG 92-106 induced severe clinical paralysis. After recovery these animals relapsed and exhibited demyelinated lesions. This study is the first to describe encephalitogenic epitopes of MOG that induce both clinical and histologic signs of EAE in mice. These and previous findings implicating MOG as a target Ag for Ab-mediated attack in EAE suggest that such autoreactivity to MOG may be significant in the development of human demyelinating diseases such as multiple sclerosis.

Cloning and cDNA sequence analysis of myelin/oligodendrocyte glycoprotein: a novel member of the immunoglobulin gene superfamily.

Myelin/oligodendrocyte glycoprotein (MOG) is a CNS-specific protein that has been identified on the external myelin sheath and oligodendrocyte processes. MOG is the primary target autoantigen for demyelinating antibodies in experimental autoimmune encephalomyelitis, a widely used animal model for autoimmune demyelinating diseases such as multiple sclerosis. We have isolated a number of rat MOG cDNA clones as tools to begin studies to ascertain MOG function. A full-length cDNA clone (1.6 kb) was sequenced and the amino acid sequence for MOG deduced. This cDNA clone encodes a signal peptide of 27 amino acids, followed by 218 residues for mature MOG (24962 MW). A single site for N-glycosylation is found at Asn-31. The N-terminal half of mature MOG shares 52% identity with bovine butyrophilin, a possible lipid receptor, and 39% identity with chicken B-G antigen, a major histocompatibility complex antigen. This homology with Ig-like chicken MHC B-G antigens raises the issue of MOG involvement in autoimmune demyelination. Both MOG and butyrophilin meet criteria for inclusion in the immunoglobulin gene superfamily. Moreover, MOG appears to represent a novel member of this superfamily in that it possesses two potential transmembrane domains, in contrast to a single membrane-spanning domain or glycophospholipid anchor found in all other members of this superfamily. MOG mRNA expression is restricted to CNS tissue, and peak expression occurs during active myelination.

Myelin/oligodendrocyte glycoprotein is a unique member of the immunoglobulin superfamily.

Myelin/oligodendrocyte glycoprotein (MOG) is a primary target autoantigen in experimental autoimmune encephalomyelitis, a widely used animal model for autoimmune demyelinating diseases such as multiple sclerosis. We have isolated several rat MOG cDNAs and confirmed their identity by comparison with MOG N-terminal peptide sequence. As expected, MOG mRNA expression is CNS-specific and peaks during active myelination. Our studies show that full length MOG mRNA is approximately 1.6 kb and encodes a signal peptide of 27 amino acids, followed by 218 residues for mature MOG (24,962 MW). A single site for N-glycosylation is found at Asn-31. Rather than the ubiquitous AAUAAA polyadenylation signal, a series of three overlapping, rare poly A signals were identified. The N-terminal half of mature MOG shares 52% identity with bovine butyrophilin, a possible lipid receptor. This same region has 39% identity with chicken B-G antigen, a major histocompatibility complex antigen involved in B cell selection and immune repertoire development. We show that both MOG and butyrophilin, each exhibiting a single Ig-like variable region domain, meet criteria for inclusion in the immunoglobulin superfamily. Moreover, MOG appears to represent a unique member of this superfamily in that it possesses two potential transmembrane domains, in contrast to a single membrane-spanning domain or glycophospholipid anchor found in all other members of Ig superfamily members.

Purification and partial structural and functional characterization of mouse myelin/oligodendrocyte glycoprotein.

The myelin/oligodendrocyte glycoprotein (MOG) is found exclusively in the CNS, where it is localized on the surface of myelin and oligodendrocyte cytoplasmic membranes. The monoclonal antibody 8-18C5 identifies MOG. Several studies have shown that anti-MOG antibodies can induce demyelination, thus inferring an important role in myelin stability. In this study, we demonstrate that MOG consists of two polypeptides, with molecular masses of 26 and 28 kDa. This doublet becomes a single 25-kDa band after deglycosylation with trifluoromethanesulfonic acid or peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, indicating that there are no or few O-linked sugars and that the doublet band represents differential glycosylation. Partial trypsin cleavage, which also gave a doublet band of lower molecular weight, confirmed this idea. MOG was purified by polyacrylamide gel electrophoresis, followed by electroelution. Three N-terminal sequences of eight to 26 amino acids were obtained. By western blot analysis, no binding was found between MOG and cerebellar soluble lectin. MOG does not seem to belong to the signal-transducing GTP-binding proteins. Reduced MOG concentrations were observed in jimpy and quaking dysmyelinating mutant mice, giving further support to its localization in compact myelin of the CNS.

Mutations in the myelin proteolipid protein gene alter oligodendrocyte gene expression in jimpy and jimpymsd mice.

The mouse myelin proteolipid protein (PLP) gene has been studied in normal and jimpymsd mice. Potential upstream regulatory regions of the normal gene have been cloned and mapped, but when these regions were studied in jimpymsd mice by Southern blots, no alterations were observed, relative to the normal gene. To assess whether the low ratio of PLP to DM20 proteins in this mutant reflected an altered PLP/DM20 ratio mRNAs, S1 nuclease analyses were undertaken, which demonstrated that at all ages studied in both jimpy and jimpymsd mice, PLP mRNA was elevated above DM20 mRNA. When exon 3 (the site of the alternative splice signal for DM20 mRNA) of the jimpymsd PLP gene was sequenced, no mutation was identified. The transcription of the PLP gene in normal and mutant animals was studied. The transcription rate increases in normal animals with development, and in very young jimpymsd or jimpy mice, the transcription rate of the PLP gene was close to that of age-matched normal animals. However, by 10 days of age, the transcription rate of this gene in both mutants was significantly below that of age-matched controls. The transcription rate of the myelin basic protein (MBP) gene was also reduced, indicating that expression of both genes is affected by this mutation. In contrast, the transcription rate of the glycerol phosphate dehydrogenase (GPDH) gene, an early marker of oligodendrocytes, is equal to or greater than normal in both mutants. We have confirmed an earlier report of a point mutation in exon 6 of the jimpymsd PLP gene, which converts an alanine to a valine.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin proteolipid protein gene expression in jimpy and jimpy(msd) mice.

Proteolipid protein (PLP) gene expression was studied in the dysmyelinating mouse mutant jimpy(msd) (jpmsd; myelin synthesis deficient) and compared with that in wild-type mice and the allelic mutant, jimpy (jp). Southern analyses of genomic DNA from jpmsd mice revealed no major rearrangements of the PLP gene relative to the wild-type mouse PLP gene. PLP-specific mRNA levels were significantly reduced in these mutant mice, although both the 3.2- and 2.4-kilobase PLP-specific mRNAs were seen. Also, no size differences in either PLP or DM20 mRNAs were found by S1 nuclease assays of brain RNA from either jpmsd or wild-type mice. Both PLP and DM20 protein were detectable at low levels in jpmsd brain homogenates, and these proteins comigrated with PLP and DM20 protein from normal mice. Western analyses showed an altered PLP:DM20 ratio in jpmsd mice relative to wild-type mice; DM20 levels exceeded PLP levels. It is surprising that a similar pattern of expression was seen in normal mice at less than 10 days of age: DM20 protein expression preceding PLP expression. Thus, jpmsd mice are capable of synthesizing normal PLP and DM20 protein; however, the PLP gene defect has affected the normal developmental pattern of expression for these two proteins.

An AG----GG transition at a splice site in the myelin proteolipid protein gene in jimpy mice results in the removal of an exon.

The myelin proteolipid protein gene was characterized in jimpy mice to identify the specific mutation that produces dysmyelination, oligodendrocyte cell death, and death of the animal by 30 days of age. Exon 5 and flanking intron segments were isolated from jimpy proteolipid protein genomic clones and sequenced. A single nucleotide difference was noted between the normal and jimpy proteolipid protein genes: the conversion of an AG/GT to a GG/GT in the splice acceptor signal preceding exon 5, which apparently destroys the splice signal. Thus, exon 5 of the mouse myelin proteolipid protein gene is skipped during the processing of mRNA, producing a shortened proteolipid protein mRNA.

Structure and expression of the mouse myelin proteolipid protein gene.

The gene for the mouse myelin proteolipid protein has been isolated and the seven exons have been sequenced. Since the sequence of a rat proteolipid protein cDNA and partial sequence of the human proteolipid protein gene have been determined, it was possible to demonstrate a very high degree of conservation for the proteolipid protein gene exons among species. While there are some nucleotide changes, the protein coding region of the mouse gene encodes protein that is totally conserved relative to both rat and human proteolipid proteins. The regulatory and noncoding regions of the proteolipid protein gene are also highly conserved. The upstream regulatory and 5'-noncoding region of the gene is 92% homologous to the comparable region of the human proteolipid protein gene, and the 3'-noncoding region of the mouse gene is approximately 90% homologous to a rat proteolipid protein cDNA through 2,200 nucleotides of 3'-noncoding DNA. S1 nuclease protection experiments indicated that the major 5'-end for proteolipid protein mRNAs from mouse, rat, human, or baboon is approximately 147-160 nucleotides upstream from the initial methionine codon of the protein coding region. Other S1 nuclease protection experiments indicated the possible existence of an alternative splice site within exon 3, which may produce mRNA for DM20. This mRNA is approximately 100 nucleotides shorter than that for the proteolipid protein, and it is missing the latter half of exon 3, that is, amino acids 116-150 of the proteolipid protein sequence.

Characterization of myelin proteolipid mRNAs in normal and jimpy mice.

A clone specific for the rat myelin proteolipid protein (PLP) was isolated from a cDNA library made in pUC18 from 17-day-old rat brain stem mRNA. This clone corresponded to the carboxyl-terminal third of the PLP-coding region. The clone was used to identify PLP-specific mRNAs in mouse brain and to establish the time course of PLP mRNA expression during mouse brain development. Three PLP-specific mRNAs were seen, approximately 1,500, 2,400, and 3,200 bases in length, of which the largest was the most abundant. During brain development, the maximal period of PLP mRNA expression was from 14 to 25 days of age, and this was a similar time course to that for myelin basic protein mRNA expression. When the jimpy mouse, an X-linked dysmyelination mutant, was studied for PLP mRNA expression, low levels of PLP mRNA were seen which were approximately 5% of wild-type levels at 20 days of age. When jimpy brain RNA was analyzed by Northern blotting, the PLP-specific mRNA was shown to be 100 to 200 bases shorter than the wild-type PLP-specific mRNA. This size difference was seen in the two major PLP mRNAs, and it did not result from a loss of polyadenylation of these mRNAs.