The large isoform of myelin-associated glycoprotein is scarcely expressed in the quaking mouse brain

RNA-binding Protein Quaking Stabilizes Sirt2 mRNA during Oligodendroglial Differentiation

Myelination is controlled by timely expression of genes involved in the differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes (OLs). Sirtuin 2 (SIRT2), a NAD⁺-dependent deacetylase, plays a critical role in OL differentiation by promoting both arborization and downstream expression of myelin-specific genes. However, the mechanisms involved in regulating SIRT2 expression during OL development are largely unknown. The RNA-binding protein quaking (QKI) plays an important role in myelination by post-transcriptionally regulating the expression of several myelin specific genes. In quaking viable (qk^v/qk^v ) mutant mice, SIRT2 protein is severely reduced; however, it is not known whether these genes interact to regulate OL differentiation. Here, we report for the first time that QKI directly binds to Sirt2 mRNA via a common quaking response element (QRE) located in the 3' untranslated region (UTR) to control SIRT2 expression in OL lineage cells. This interaction is associated with increased stability and longer half-lives of Sirt2.1 and Sirt2.2 transcripts leading to increased accumulation of Sirt2 transcripts. Consistent with this, overexpression of qkI promoted the expression of Sirt2 mRNA and protein. However, overexpression of the nuclear isoform qkI-5 promoted the expression of Sirt2 mRNA, but not SIRT2 protein, and delayed OL differentiation. These results suggest that the balance in the subcellular distribution and temporal expression of QKI isoforms control the availability of Sirt2 mRNA for translation. Collectively, our study demonstrates that QKI directly plays a crucial role in the post-transcriptional regulation and expression of Sirt2 to facilitate OL differentiation.

Dysmyelination with preservation of transverse bands in a long-lived allele of the quaking mouse

The new mutant mouse shaking (shk) differs from other "myelin mutants" in having a more stable neurological impairment and a much longer lifespan. We have shown that transverse bands (TBs), the component of the paranodal junction (PNJ) that attaches the myelin sheath to the axon, are present in the shk central nervous system (CNS), in contrast to more severely affected mutants, in which TBs are absent or rare. We have proposed that TBs are the major determinant underlying shk neurological stability and longevity. Here we report that TBs are abundant not only in the shk CNS but also in its peripheral nervous system (PNS), which, as in other "myelin mutants", is not as severely dysmyelinated as the CNS but does display structural abnormalities likely to affect impulse propagation. In particular, myelin sheaths are thinner than normal, and some axonal segments lack myelin sheaths entirely. In addition, we establish that the shk mutation, previously localized to chromosome 17, is a quaking (qk) allele consisting of a 105-nucleotide insertion in the qk regulatory region that decreases qk transcription but does not extend to the Parkin and Parkin coregulated genes, which are affected in the qk allele. We conclude that: 1) dysmyelination is less severe in the shk PNS than in the CNS, but TBs, which are present in both locations, stabilize the PNJs and prevent the progressive neurological deficits seen in mutants lacking TBs; and 2) the insertional mutation in shk mice is sufficient to produce the characteristic neurological phenotype without involvement of the Parkin and Parkin coregulated genes.

RNA metabolism and dysmyelination in schizophrenia

Decreased expression of a subset of oligodendrocyte and myelin-related genes is the most consistent finding among gene expression studies of postmortem brain tissue from subjects with schizophrenia (SCZ), although heritable variants have yet to be found that can explain the bulk of this data. However, expression of the glial gene Quaking (QKI), encoding an RNA binding (RBP) essential for myelination, was recently found to be decreased in SCZ brain. Both oligodendrocyte/myelin related genes, and other RBPs that are known or predicted to be targets of QKI, are also decreased in SCZ. Two different quaking mutant mice share some pathological features in common with SCZ, including decreased expression of myelin-related genes and dysmyelination, without gross destruction of white matter. Therefore, although these mice are not a model of SCZ per se, understanding the similarities and differences in gene expression between brains from these mice and subjects with SCZ could help parse out distinct genetic pathways underlying SCZ.

Reproductive and neurological Quaking(viable) phenotypes in a severe combined immune deficient mouse background

The quaking(viable) (qkv) mutation, a spontaneous deletion of a multigenic region encompassing roughly 1 Mb at 5.9 cM on the proximal end of mouse chromosome 17, causes severe trembling in all homozygous animals and infertility in all homozygous males. Physiologically, quaking mice exhibit dysmyelination and postmeiotic spermatogenic arrest. Molecular defects in Qkv mice occur in the affected tissues, indicating the primary causes of these pathologies are cell autonomous. However, because both the reproductive and neurological defects are in immune-privileged sites and because some similar pathologies at both sites have been shown to be immune mediated, we tested whether the immune system participates secondarily in manifestation of Qkv phenotypes. The qkv mutation was bred into a severe combined immune-deficient mouse line (SCID; devoid of mature B and T cells) and penetrance of the neurological and the male sterile phenotypes was measured. Results showed that neither defect was ameliorated in the immune-deficient background. We conclude that the Qkv pathologies do not likely involve a B- or T-cell-dependent response against these immune-privileged sites.

Differential expression of L- and S-MAG upon cAMP stimulated differentiation in oligodendroglial cells

Myelin-associated glycoprotein (MAG), an immunoglobulin-like cell signaling protein involved in axon-glial interactions, displays two intracellular C-termini as a result of alternative mRNA splicing. During brain development, the two MAG mRNAs that encode L-MAG and S-MAG differ in their relative abundance. We have investigated the differential expression of L- and S-MAG upon cAMP treatment in the oligodendroglial cell line Oli-neu, a cell line able to differentiate in vitro. We have engineered GFP and VSVG fusions by small insertions into the alternatively spliced exons of the cloned MAG gene and reintroduced them into Oli-neu cells. The individually tagged MAG isoforms were expressed under the control of the MAG promoter and regulatory region. In this system, L-MAG was the predominant isoform before the stimulation of cells with cAMP, whereas upon cAMP treatment the S-MAG isoform was predominantly expressed in cells with a high degree of morphological differentiation. We suggest that the regulation of the MAG alternative splicing and the morphological differentiation in oligodendrocytes are controlled both by the same cAMP-responsive differentiation step.

Nuclear retention of MBP mRNAs in the quaking viable mice

Quaking viable (qk(v)) mice fail to properly compact myelin in their central nervous systems. Although the defect in the qk(v) mice involves a mutation affecting the expression of the alternatively spliced qk gene products, their roles in myelination are unknown. We show that the QKI RNA binding proteins regulate the nuclear export of MBP mRNAs. Disruption of the QKI nucleocytoplasmic equilibrium in oligodendrocytes results in nuclear and perikaryal retention of the MBP mRNAs and lack of export to cytoplasmic processes, as it occurs in qk(v) mice. MBP mRNA export defect leads to a reduction in the MBP levels and their improper cellular targeting to the periphery. Our findings suggest that QKI participates in myelination by regulating the mRNA export of key protein components.

Function of quaking in myelination: regulation of alternative splicing

Proteomic diversity is frequently achieved by alternative RNA-splicing events that can be fine-tuned in tissue-specific and developmentally regulated ways. Understanding this type of genetic regulation is compelling because of the extensive complexity of alternative splicing found in the nervous system. quaking (qk), one of the classical mouse dysmyelination mutants, is defective for the expression of myelin-associated glycoprotein (MAG), and the misregulation of MAG pre-mRNA alternative splicing is implicated as a causal factor. The qk locus encodes several RNA-binding proteins with heterogeneous nuclear ribonucleoprotein K-type homology, a characteristic of several known alternative splicing regulators. Here we test the nuclear-localized qk isoform (QKI-5) for its ability to regulate alternative splicing of MAG pre-mRNA in transient coexpression assays. QKI-5 exhibits properties of a negative regulator of MAG exon 12 alternative splicing. An intronic sequence element required for the repressive function and binding of QKI-5 is also identified. Direct evidence for irregularities in alternative splicing of MAG and other myelin protein transcripts in the qk mouse is demonstrated.

Radiation-induced diffuse brain injury in the neonatal rat model--radiation-induced apoptosis of oligodendrocytes

The mechanism of radiation-induced diffuse brain injury was investigated using a model of delayed myelination in the irradiated neonatal rat brain in which the number of oligodendrocytes decreases without associated necrosis of the cerebral white matter. Immunohistochemical analysis using antibody against the large myelin-associated glycoprotein, a specific marker of oligodendrocytes at an early stage of development, showed that the number of the oligodendrocytes associated with myelination decreased in the irradiated hemisphere 1 day after irradiation and remained low until 5 days after irradiation. In situ terminal deoxynucleotidyl transferase-mediated nick end-labeling assay revealed that apoptosis mainly occurred in the cerebral white matter of the irradiated hemisphere. Three hours after irradiation, apoptotic cells were found in the subcortical white matter and the periventricular white matter. Six hours after irradiation, apoptotic cells were found in the internal capsule, and the numbers of apoptotic cells in the periventricular white matter and subcortical white matter increased. One day after irradiation, the number of apoptotic cells in the periventricular white matter decreased. Three days after irradiation, apoptotic cells were not observed in the cerebral white matter. These results suggest that the oligodendrocytes associated with myelination may be damaged via radiation-induced apoptosis, and depletion of the oligodendrocytes may cause delay of myelination.

Effects of galactolipid elimination on oligodendrocyte development and myelination

The galactolipids galactocerebroside and sulfatide, which require the enzyme UDP-galactose:ceramide galactosyltransferase (CGT) for their synthesis, are among the most prevalent molecules in the myelin sheath. Numerous studies, mainly using antibody perturbation methods in vitro, have suggested that these molecules are crucial mediators of oligodendrocyte differentiation and myelin formation. Although we have previously demonstrated that myelin formation occurs in CGT null mutant mice, which are incapable of synthesizing the myelin galactolipids, here we show that there are developmental alterations in the CNS of these animals. There is a significant decrease in the number of myelinated axon segments in the mutant spinal cord despite normal levels of myelin gene-specific mRNAs and proteins. Also, there is an increased cellularity in the mature mutant spinal cord and the distinctive morphology of the additional cells suggests that they are actively myelinating oligodendrocytes. Using in situ hybridization techniques, we show that there is a 50% increase in the number of oligodendrocytes in the mutant spinal cord. The data suggest that galactolipids play an important developmental role in regulating the maturation program and final number of oligodendrocytes.

The quaking I-5 protein (QKI-5) has a novel nuclear localization signal and shuttles between the nucleus and the cytoplasm

The mouse quaking (qk) gene is essential in both myelination and early embryogenesis. Its product, QKI, is an RNA-binding protein belonging to a growing protein family called STAR (signal transduction and activator of RNA). All members have an approximately 200-amino acid STAR domain, which contains a single extended heteronuclear ribonucleoprotein K homologue domain flanked by two domains called QUA1 and QUA2. We found that QKI isoforms could associate with each other, while one of the lethal mutations qkI(kt4) with a single amino acid change in QUA1 domain, leads to a loss of QKI self-interaction. This suggests that the QUA1 domain is responsible for QKI dimerization. Three QKI isoforms have different carboxyl termini and different subcellular localization. Here, using GFP fusion protein, we identified a 7-amino acid novel nuclear localization sequence in the carboxyl terminus of QKI-5, which is conserved in a subclass of STAR proteins containing SAM68 and ETLE/T-STAR. Thus, we name this motif STAR-NLS. In addition, the effects of active transcription, RNA-binding and self-interaction on QKI-5 localization were analyzed. Furthermore, using an interspecies heterokaryon assay, we found that QKI-5, but not another STAR protein ETLE, shuttles between the nucleus and the cytoplasm, which suggests that QKI-5 functions in both cell compartments.

Heterophilic binding of L1 on unmyelinated sensory axons mediates Schwann cell adhesion and is required for axonal survival

This study investigated the function of the adhesion molecule L1 in unmyelinated fibers of the peripheral nervous system (PNS) by analysis of L1- deficient mice. We demonstrate that L1 is present on axons and Schwann cells of sensory unmyelinated fibers, but only on Schwann cells of sympathetic unmyelinated fibers. In L1-deficient sensory nerves, Schwann cells formed but failed to retain normal axonal ensheathment. L1-deficient mice had reduced sensory function and loss of unmyelinated axons, while sympathetic unmyelinated axons appeared normal. In nerve transplant studies, loss of axonal-L1, but not Schwann cell-L1, reproduced the L1-deficient phenotype. These data establish that heterophilic axonal-L1 interactions mediate adhesion between unmyelinated sensory axons and Schwann cells, stabilize the polarization of Schwann cell surface membranes, and mediate a trophic effect that assures axonal survival.

Myelin dysfunction/degradation in the central nervous system: why are myelin sheaths susceptible to damage?

In the central nervous system, myelin sheaths are produced to electrically insulate axons and to increase the velocity of axonal conduction. They are highly complex structures, which are often destructed in neurological disorders. One possible reason for the vulnerability of myelin sheaths to damage became apparent from analyses of animals with altered amounts of otherwise normal myelin components: Due to limited redundance in function between different myelin proteins, dysfunction or loss of one protein may cause loss of function and instability of the entire myelin sheath.

The expression of recombinant large myelin-associated glycoprotein cytoplasmic domain and the purification of native myelin-associated glycoprotein from rat brain and peripheral nerve

The myelin-associated glycoprotein (MAG) is a transmembrane protein of the immunoglobulin superfamily existing as two isoforms (L-MAG and S-MAG) that are differentially expressed by myelinating glial cells of the central and peripheral nervous systems, where MAG represents 1 and 0.1% of the total myelin proteins, respectively. The polypeptide chains of the two isoforms differ only by the carboxy terminus of their respective cytoplasmic domains, which most probably determine the isoform-specific functions. Here, we describe the expression of the L-MAG cytoplasmic domain as a GST fusion protein. The recombinant protein was used to raise polyclonal antibodies against the L-MAG-specific carboxy terminus and against the region of the MAG cytoplasmic domain common to both S-MAG and L-MAG. These antibodies, which function in dot blotting, Western blotting, and immunoprecipitation, were used to immunopurify native MAG from both rat brain and peripheral nerves in quantities and purity sufficient for the realization of most biochemical and functional studies. The antibodies and the recombinant and native MAG proteins provide much needed tools for the study of the common and isoform-specific properties and functions of L-MAG and S-MAG.

Demyelination and altered expression of myelin-associated glycoprotein isoforms in the central nervous system of galactolipid-deficient mice

Vertebrate myelin is enriched in the lipid galactocerebroside (GalC) and its sulfated derivated sulfatide. To understand the in vivo function of these lipids, we analyzed myelination in mice that contain a null mutation in the gene encoding UDP-galactose:ceramide galactosyltransferase, the enzyme responsible for catalyzing the final step in GalC synthesis. Galactolipid-deficient myelin is regionally unstable and progressively degenerates. At postnatal day 30, demyelination is restricted to the midbrain and hindbrain, but by postnatal day 90, it spreads throughout the central nervous system. Activated microglial cells and reactive astrocytes appear with the loss of myelin in older animals. Nonetheless, major myelin protein gene mRNA levels are normal throughout the life of these animals, suggesting that widespread oligodendrocyte death is not the primary cause of demyelination. The developmental switch in myelin-associated glycoprotein isoform expression, however, does not occur normally in these mice, suggesting an alteration in oligodendrocyte maturation. Taken together, these findings indicate that GalC and sulfatide are required for the long-term maintenance of myelin and that their absence may have subtle effects on the development of oligodendrocytes.

Demyelination in the central nervous system mediated by an anti-oligodendrocyte antibody

The factors responsible for the major demyelinating disease of the central nervous system (CNS), multiple sclerosis, are poorly defined. Although T-cell-mediated immune responses play a pivotal role in establishing the inflammatory response, humoral factors also may be critical in disease progress. We have isolated a mouse monoclonal antibody (mAb 2B10) that recognizes a cell-surface molecule expressed exclusively by rat oligodendrocytes, the cells responsible for the formation and maintenance of CNS myelin. In cultures of neonatal rat spinal cord, mAb 2B10 specifically mediated oligodendrocyte cell death in the absence of complement. In the current study, mAb 2B10-producing hybridoma cells were implanted into adult rat brain ventricles, and the effect of mAb 2B10 on CNS cytoarchitecture was examined. In the optic nerves of mAb 2B10-treated animals, there was significant focal myelin degeneration near the optic chiasm. Axons in the myelin degenerate regions were largely healthy. There was no significant infiltration of hematopoietic-derived cells into the affected regions, but microglia were activated focally and phagocytosed the collapsed myelin. This study demonstrates that an antibody directed against myelin-forming cells induces CNS demyelination and supports the hypothesis that autoantibodies may play a role in CNS demyelinating diseases.

The cytoplasmic domain of the large myelin-associated glycoprotein isoform is needed for proper CNS but not peripheral nervous system myelination

The myelin-associated glycoprotein (MAG) is a member of the immunoglobulin gene superfamily and is thought to play a critical role in the interaction of myelinating glial cells with the axon. Myelin from mutant mice incapable of expressing MAG displays various subtle abnormalities in the CNS and degenerates with age in the peripheral nervous system (PNS). Two distinct isoforms, large MAG (L-MAG) and small MAG (S-MAG), are produced through the alternative splicing of the primary MAG transcript. The cytoplasmic domain of L-MAG contains a unique phosphorylation site and has been shown to associate with the fyn tyrosine kinase. Moreover, L-MAG is expressed abundantly early in the myelination process, possibly indicating an important role in the initial stages of myelination. We have adapted the gene-targeting approach in embryonic stem cells to generate mutant mice that express a truncated form of the L-MAG isoform, eliminating the unique portion of its cytoplasmic domain, but that continue to express S-MAG. Similar to the total MAG knockouts, these animals do not express an overt clinical phenotype. CNS myelin of the L-MAG mutant mice displays most of the pathological abnormalities reported for the total MAG knockouts. In contrast to the null MAG mutants, however, PNS axons and myelin of older L-MAG mutant animals do not degenerate, indicating that S-MAG is sufficient to maintain PNS integrity. These observations demonstrate a differential role of the L-MAG isoform in CNS and PNS myelin.

STAR, a gene family involved in signal transduction and activation of RNA

A new subfamily of KH-domain-containing RNA-binding proteins is encoded by genes that are conserved from yeast to humans. Mutations with interesting developmental phenotypes have been identified in Caenorhabditis elegans, Drosophila and mouse. It is hypothesized that these bifunctional proteins provide a rich source of interesting molecular information about development and define a new cellular pathway that links signal transduction directly to RNA metabolism.

The quaking gene product necessary in embryogenesis and myelination combines features of RNA binding and signal transduction proteins

The mouse quaking gene, essential for nervous system myelination and survival of the early embryo has been positionally cloned. Its sequence implies that the locus encodes a multifunctional gene used in a specific set of developing tissues to unite signal transduction with some aspect of RNA metabolism. The quaking(viable) (qkv) mutation has one class of messages truncated by a deletion. An independent ENU-induced mutation has a nonconservative amino acid change in one of two newly identified domains that are conserved from the C. elegans gld-1 tumour suppressor gene to the human Src-associated protein Sam68. The size and conservation of the quaking gene family implies that the pathway defined by this mutation may have broad relevance for rapid conveyance of extracellular information directly to primary gene transcripts.

The myelin-associated glycoprotein of the peripheral nervous system in trembler mutants contains increased alpha 2-3 sialic acid and galactose

The myelin-associated glycoprotein (MAG) exhibits an abnormally high apparent molecular weight in sciatic nerve, but not in brain, of dysmyelinating trembler mutants (Inuzuka et al.: J Neurochem 44:793-797, 1985). Antibodies to the large and small isoforms of MAG (L- and S-MAG) and probes for oligosaccharide structure were used to determine if this was due to overexpression of L-MAG or increased glycosylation. Nerves from both control and trembler 36-day-old mice contained primarily S-MAG with only traces of L-MAG. The distribution of the two isoforms appeared normal in trembler mice, and both isoforms exhibited the higher apparent molecular weight. Lectin binding showed that, in contrast to brain in which most glycoproteins contain primarily alpha 2-3 linked sialic acid, most glycoproteins of both control and trembler nerve contained primarily alpha 2-6 linked sialic acid. Lectin binding and glycosidase treatments demonstrated that the higher molecular weight of MAG in trembler nerves was due to an increased content of alpha 2-3 linked sialic acid and galactose. The abnormal glycosylation of MAG in trembler mutants may contribute to the myelin pathology.

Endocytic depletion of L-MAG from CNS myelin in quaking mice

Quaking is an autosomal recessive hypo/dysmyelinating mutant mouse which has a 1-Mbp deletion on chromosome 17. The mutation exhibits pleiotrophy and does not include genes encoding characterized myelin proteins. The levels of the 67-kD isoform of the myelin-associated glycoprotein (S-MAG) relative to those of the 72-kD isoform (L-MAG) are increased in the quaking CNS, but not in other dysmyelinating mutants. Abnormal expression of MAG isoforms in quaking may result from altered transcription of the MAG gene or from abnormal sorting, transport, or targeting of L-MAG or S-MAG. To test these hypotheses, we have determined the distribution of L-MAG and S-MAG in cervical spinal cord of 7-, 14-, 21-, 28-, and 35-d-old quaking mice. In 7-d-old quaking and control spinal cord, L- and S-MAG was detectable in periaxonal regions of myelinated fibers and in the perinuclear cytoplasm of oligodendrocytes. Between 7 and 35 d, L-MAG was removed from the periaxonal membrane of quaking but not control mice. Compared to control mice, a significant increase in MAG labeling of endosomes occurred within oligodendrocyte cytoplasm of 35-d-old quaking mice. S-MAG remained in periaxonal membranes of both quaking and control mice. Analysis of the cytoplasmic domain of L-MAG identifies amino acid motifs at tyrosine 35 and tyrosine 65 which meet the criteria for "tyrosine internalization signals" that direct transmembrane glycoproteins into the endocytic pathway. These results establish that L-MAG is selectively removed from the periaxonal membrane of CNS-myelinated fibers by receptor-mediated endocytosis. The loss of L-MAG from quaking periaxonal membranes results from increased endocytosis of L-MAG and possibly a decrease in L-MAG production.

Abnormal expression and glycosylation of the large and small isoforms of myelin-associated glycoprotein in dysmyelinating quaking mutants

The relative expression of large (L) and small (S) isoforms of the myelin-associated glycoprotein (MAG) and their glycosylation were compared in developing spinal cord of quaking and control mice. Using antisera specific for L- and S-MAG, respectively, it was shown that S-MAG is the principal isoform in quaking mice at all ages between 13 and 72 days, although L-MAG was just detectable by western blotting at the early ages. Both L- and S-MAG have higher apparent molecular weights in quaking mice than in controls. Experiments involving lectin binding and glycosidase treatment demonstrated that the higher molecular weight of MAG in the quaking mutant was due to a higher content of N-acetylneuraminic acid residues linked alpha 2-3 to galactose as well as to more branching of oligosaccharide moieties indicated by a higher content of subterminal galactose residues. The total sialic acid measured by HPAE-chromatography in purified quaking MAG was 40% higher than in control MAG. By contrast, quaking MAG contained less of the adhesion-related, HNK-1 carbohydrate epitope. Another difference was that a lower molecular weight form of MAG with predominantly high mannose oligosaccharides was prominent in young quaking mice, but not in controls. The abnormalities of MAG expression related to splicing of its mRNA and glycosylation may contribute to the myelin pathology in quaking mutants.

Initial events of myelination involve Fyn tyrosine kinase signalling

Myelin is the lipoprotein multimembrane that functions as an insulator preventing the flow of ion currents across the axonal membrane and facilitating the conduction of nerve impulses. It is synthesized by oligodendrocytes in the central nervous system at about the time of birth in mammals. During the initial stages of myelination, several proteins are phosphorylated on tyrosine. Among these proteins, we identified Fyn tyrosine kinase, one of the non-receptor-type tyrosine kinases of the Src family. Here we report that Fyn tyrosine kinase is activated during the initial stages of myelination and that it is associated with the large myelin-associated glycoprotein (MAG), an adhesion molecule that has been implicated in myelinogenesis. The Fyn-large MAG association requires amino-terminal domains of Fyn that include SH2 and SH3 (Src homology domains 2 and 3). Crosslinking of large MAG with antibody induces a rapid increase in the specific activity of Fyn kinase. These results indicate that Fyn participates in the initial events of myelination as a signalling molecule downstream of large MAG; indeed, we find that fyn-deficient mice exhibit impaired myelination.

Expression of myelin-associated glycoprotein isoforms after sciatic nerve crush injury in mice

The large myelin-associated glycoprotein isoform (L-MAG) protein and small myelin-associated glycoprotein isoform (S-MAG) protein were demonstrated after sciatic nerve crush injury in mice by an immunoblotting technique using specific antibodies to the L-MAG protein and the S-MAG protein, respectively. Immunoblots indicated a rapid decrease in expression of both isoform proteins in the crushed sciatic nerves to < 10% of the control side. By 13 d after injury, L-MAG protein expression had quickly recovered to 100% of the control level. Following the increase in L-MAG protein expression, S-MAG protein expression recovered to 100% by 20 d after injury. It has been reported that the developmental maximum expression of L-MAG protein precedes that of S-MAG protein in both central and peripheral nervous system (CNS and PNS). Our previous work demonstrated that L-MAG mRNA was characteristically induced at the time of most active myelination, including remyelination in the CNS. We here have shown the expression of L-MAG protein precedes that of S-MAG protein during active remyelination in the PNS. This suggests that it plays an important role in the early stage of myelin formation.

Expression of the large myelin-associated glycoprotein isoform in rat oligodendrocytes around cerebral infarcts

An immunohistochemical method that uses two specific antisera-distinguishable myelin-associated glycoprotein (MAG) isoforms showed an expression of large MAG isoform (L-MAG) protein in the oligodendrocyte cytoplasms in the white matter around experimental cerebral infarcts produced by occlusion of the left middle cerebral artery in the rat. L-MAG protein also was detected in the white matter on the contralateral side. This protein appears prior to S-MAG protein in myelination and remyelination in the central and peripheral nervous systems, and is believed to function in myelin formation. Because L-MAG protein has been found in the oligodendrocyte cytoplasm only in the early development period, its appearance in this cytoplasm after ischemic insult is evidence of MAG regeneration.

Gene expression in cells of the central nervous system

This review summarized a part of our studies over a long period of time, relating them to the literature on the same topics. We aimed our research toward an understanding of the genetic origin of brain specific proteins, identified by B. W. Moore and of the high complexity of the nucleotide sequence of brain mRNA, originally investigated by W. E. Hahn, but have not completely achieved the projected goal. According to our studies, the reason for the high complexity in the RNA of brain nuclei might be the high complexity in neuronal nuclear RNA as described in the Introduction. Although one possible explanation is that it results from the summation of RNA complexities of several neuronal types, our saturation hybridization study with RNA from the isolated nuclei of granule cells showed an equally high sequence complexity as that of brain. It is likely that this type of neuron also contains numerous rare proteins and peptides, perhaps as many as 20,000 species which were not detectable even by two-dimensional PAGE. I was possible to gain insight into the reasons for the high sequence complexity of brain RNA by cloning the cDNA and genomic DNA of the brain-specific proteins as described in the previous sections. These data provided evidence for the long 3'-noncoding regions in the cDNA of the brain-specific proteins which caused the mRNA of brain to be larger than that from other tissues. During isolation of such large mRNAs, a molecule might be split into a 3'-poly(A)+RNA and 5'-poly(A)-RNA. In the studies on genomic DNA, genes with multiple transcription initiation sites were found in brain, such as CCK, CNP and MAG, in addition to NSE which was a housekeeping gene, and this may contribute to the high sequence complexity of brain RNA. Our studies also indicated the presence of genes with alternative splicing in brain, such as those for CNP, MAG and NGF, suggesting a further basis for greater RNA nucleotide sequence complexity. It is noteworthy that alternative splicing of the genes for MBP and PLP also produced multiple mRNAs. Such a mechanism may be a general characteristic of the genes for the myelin-specific proteins produced by oligodendrocytes. In considering the high nucleotide sequence complexity, it is interesting that MAG and S-100 beta genes etc. possess two additional sites for poly(A).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Immunohistochemical localization of myelin-associated glycoprotein isoforms during the development in the mouse brain

The developmental changes in localization of myelin-associated glycoprotein (MAG) isoforms in the mouse brain were demonstrated by an immunohistochemical method using antisera specific to two MAG isoforms. The antiserum to the large isoform of MAG (L-MAG) stained the myelin sheaths and the cytoplasm of oligodendroglia in the active myelinating stage in the mouse central nervous system. However, the antiserum to the small isoform of MAG (S-MAG) stained only myelin sheaths in the adult stage. These findings suggest that L-MAG plays an important role in active myelination.

Expression of the large myelin-associated glycoprotein isoform during the development in the mouse peripheral nervous system

The developmental maximum expression of the large myelin-associated glycoprotein isoform (L-MAG) protein prior to that of the small myelin-associated glycoprotein isoform (S-MAG) in both the central and peripheral nervous systems (CNS, PNS) in mice was shown by immunoblotting techniques using specific antibodies to the L-MAG protein and the S-MAG protein. Both the L-MAG protein and the S-MAG protein were expressed earlier in the PNS than in the CNS, which reflects earlier myelination in the PNS. The peak of the L-MAG protein concentration was 8 days in the sciatic nerve and 15 days in the brainstem. The concentration of the S-MAG protein in the sciatic nerve reached a peak at 15 days, whereas in the brainstem it increased rapidly between 15 and 20 days and gradually thereafter. Thus, the preceding maximum expression of the L-MAG during active myelination in the PNS demonstrated here as well as in the CNS strongly suggests an important role for L-MAG in myelin formation.

Antibody to a zinc finger protein in a patient with paraneoplastic cerebellar degeneration

In some patients with paraneoplastic cerebellar degeneration (PCD), autoantibodies against neural components have been identified. Here, we demonstrate a major 58 kd protein antigen in an immunoblot of human cerebellum by serum from a patient with PCD. Immunohistochemically, the serum recognized neural cells especially Purkinje cells in a human brain. To identify the details of the target antigens for the antibody, we isolated a cDNA clone from a human cerebellar library. Homology searches revealed a similarity with the zinc finger proteins. PCD related proteins reported here may be important to maintain neural cells especially those in the cerebellum, and further studies on this molecule may help us elucidate the causes of degenerative or autoimmune diseases in the cerebellum.

Structure of mouse myelin-associated glycoprotein gene

The mouse myelin-associated glycoprotein gene was isolated from a mouse gene library. This gene was split into 13 exons distributed about 15 kb in length. Each extracellular immunoglobulin-related domain was encoded by a single exon, and RNA splicing between those exons occurred between the first and second nucleotides of the junctional codon, the features of which are conserved in most of the genes of the immunoglobulin superfamily. The sequence of the 5'-flanking region appeared to have some regions homologous to other myelin proteins, which suggested that they were possible cis-elements for specific expression of oligodendrocytes.

Differential expression of MAG isoforms during development

The myelin-associated glycoproteins (MAG) mediate the cell interactions of oligodendrocytes and Schwann cells with axons that are myelinated. MAG exists in two developmentally regulated isoforms: large MAG (L-MAG) and small MAG (S-MAG). In this paper, we have studied the tissue-specific and developmentally regulated alternative splicing of these isoforms using monospecific antibodies that recognize epitopes common to both isoforms or that are present only on L-MAG. In the central nervous system (CNS), L-MAG is the major form synthesized early in development, and it persists as a significant proportion of the MAG present in the adult. In the peripheral nervous system (PNS), L-MAG is expressed at modest levels during development; it is virtually absent in the adult. Thus, the expression of L-MAG is not limited to the CNS, as was formerly believed, suggesting that it plays a common role during the early stages of myelin formation by both oligodendrocytes and Schwann cells. In both the CNS and PNS, S-MAG is the predominant isoform in the adult. A higher-molecular-weight form of MAG is present in the PNS at low abundance, that is developmentally regulated, and appears to be a glycosylation variant. An analysis of the carbohydrate residues on MAG demonstrates that it contains both N-linked and O-linked sugars that could be modulated during development. These results suggest a possible mechanism for the regulation of MAG function during myelinogenesis via the expression of alternative isoforms and carbohydrate modifications.