Myelin basic protein deposition in the optic and sciatic nerves of dysmyelinating mutants quaking, jimpy, Trembler, mld, and shiverer during development

Developmental axon diameter growth of central nervous system axons does not depend on ensheathment or myelination by oligodendrocytes

Myelination facilitates the rapid conduction of action potentials along axons. In the central nervous system (CNS), myelinated axons vary over 100-fold in diameter, with conduction speed scaling linearly with increasing diameter. Axon diameter and myelination are closely interlinked, with axon diameter exerting a strong influence on myelination. Conversely, myelinating Schwann cells in the peripheral nervous system can both positively and negatively affect axon diameter. However, whether axon diameter is regulated by CNS oligodendrocytes is less clear. Here, we investigated CNS axon diameter growth in the absence of myelin using mouse (Mbp shi/shi and Myrf conditional knockout) and zebrafish (olig2 morpholino) models. We find that neither the ensheathment of axons, nor the formation of compact myelin are required for CNS axons to achieve appropriate and diverse diameters. This indicates that developmental CNS axon diameter growth is independent of myelination, and shows that myelinating cells of CNS and PNS differentially influence axonal morphology.

Resolving relaxometry and diffusion properties within the same voxel in the presence of crossing fibres by combining inversion recovery and diffusion-weighted acquisitions

Purpose

A comprehensive image‐based characterization of white matter should include the ability to quantify myelin and axonal attributes irrespective of the complexity of fibre organization within the voxel. While progress has been made with diffusion MRI‐based approaches to measure axonal morphology, to date available myelin metrics simply assign a single scalar value to the voxel, reflecting some form of average of its constituent fibres. Here, a new experimental framework that combines diffusion MRI and relaxometry is introduced. It provides, for the first time, the ability to assign to each unique fibre system within a voxel, a unique value of the longitudinal relaxation time, T₁, which is largely influenced by the myelin content.

Methods

We demonstrate the method through simulations, in a crossing fibres phantom, in fixed brains and in vivo.

Results

The method is capable of recovering unique values of T₁ for each fibre population.

Conclusion

The ability to extract fibre‐specific relaxometry properties will provide enhanced specificity and, therefore, sensitivity to differences in white matter architecture, which will be invaluable in many neuroimaging studies. Further the enhanced specificity should ultimately lead to earlier diagnosis and access to treatment in a range of white matter diseases where axons are affected. Magn Reson Med 75:372–380, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Rescuing qkV dysmyelination by a single isoform of the selective RNA-binding protein QKI

Alternative splicing of the qkI transcript generates multiple isoforms of the selective RNA-binding protein QKI, which play key roles in controlling the homeostasis of their mRNA targets. QKI deficiency in oligodendrocytes of homozygous quakingviable (qkV/qkV) mutant mice results in severe hypomyelination, indicating the essential function of QKI in myelinogenesis. However, the molecular mechanisms by which QKI controls myelination remain elusive. We report here that QKI-6 is the most abundant isoform in brain and is preferentially reduced in the qkV/qkV mutant during normal myelinogenesis. To test whether QKI-6 is the predominant isoform responsible for advancing CNS myelination, we developed transgenic mice that express Flag-QKI-6 specifically in the oligodendroglia lineage, driven by the proteolipid protein (PLP) promoter. When introduced into the qkV/qkV mutant, the QKI-6 transgene rescues the severe tremor and hypomyelination phenotype. Electron microscopic studies further revealed that the Flag-QKI-6 transgene is sufficient for restoring compact myelin formation with normal lamellar periodicity and thickness. Interestingly, Flag-QKI-6 preferentially associates with the mRNA encoding the myelin basic protein (MBP) and rescues MBP expression from the beginning of myelinogenesis. In contrast, Flag-QKI-6 binds the PLP mRNA with lower efficiency and has a minimal impact on PLP expression until much later, when the expression level of QKI-6 in the transgenic animal significantly exceeds what is needed for normal myelination. Together, our results demonstrate that QKI-6 is the major isoform responsible for CNS myelination, which preferentially promotes MBP expression in oligodendrocytes.

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.

A new ENU-induced allele of mouse quaking causes severe CNS dysmyelination

The mutant allelic series of the mouse quaking gene consists of the spontaneous quaking(viable) (qk(v)) allele, which is homozygous viable with a dysmyelination phenotype, and four ENU-induced alleles (qk(kt 1), qk(k2), qk(kt3/4), and qk(l-1)), which are homozygous embryonic lethal. Here we report the isolation of qk(e5), the first ENU-induced viable allele of quaking. Unlike qk(v)/qk(v), qk(e5)/qk(e5) animals have early-onset seizures, severe ataxia, and a dramatically reduced lifespan. Ultrastructural analysis of qk(e5)/qk(e5) brains reveals severe dysmyelination when compared with both wild-type and qk(v)/qk(v) brains. In addition, Calbindin detection in young adult qk(e5)/qk(e5) mice reveals Purkinje cell axonal swellings indicative of neurodegeneration , which is not seen in young adult qk(v)/qk(v) mice. Although the molecular defect in the qk(e5) allele is not evident by sequencing, protein expression studies show that qk(e5)/qk(e5) postnatal oligodendrocytes lack the QKI-6 and QKI-7 isoforms and have reduced QKI-5 levels. The oligodendrocyte developmental markers PDGF alpha R, NG 2, O4, CNP, and MBP are also present in the qk(e5)/qk(e5) postnatal brain although CNP and MBP levels are considerably reduced. Because the qk(v) allele is a large deletion that affects the expression of three genes, the new neurologic qk(e5) allele is an important addition to this allelic series.

Primary central white matter hypoplasia of the neocortex

Leukodystrophies, or diseases of the white matter, represent acute or ongoing damage to the oligodendrocytes of the central nervous system. Early childhood white matter disease is most commonly observed after hypoxic ischemic insults, with acute magnetic resonance imaging changes followed by atrophy or periventricular leukomalacia. Dysmyelination occurring in the setting of inborn errors of metabolism is characterized by progressive changes with high signal intensity in white matter on magnetic resonance imaging. This report presents a patient whose magnetic resonance imaging scans demonstrated hypoplasia of myelin in the telencephalon, without clinical or magnetic resonance imaging evidence of inflammatory dysmyelination. Clinical features included intractable seizures, severe hypotonia, and dysmorphic facial features coupled with a static failure to gain developmental milestones. Together, the clinical and magnetic resonance imaging findings are evidence of a primary failure of myelination in the neocortex.

Destabilization and mislocalization of myelin basic protein mRNAs in quaking dysmyelination lacking the QKI RNA-binding proteins

Quakingviable (qk(v)) is a well known dysmyelination mutation. Recently, the genetic lesion of qk(v) has been defined as a deletion 5' to the qkI gene, which results in the severe reduction of the qkI-encoded QKI RNA-binding proteins in myelin-producing cells. However, no comprehensive model has been proposed regarding how the lack of QKI leads to dysmyelination. We hypothesized that QKI binds to myelin protein mRNAs, and the lack of QKI causes posttranscriptional misregulation, which in turn leads to the loss of the corresponding myelin proteins. To test this hypothesis, we developed an RNase protection assay to directly measure the mRNA isoforms encoding the myelin basic proteins (MBPs) in the brain. Our result suggested that isoform-preferential destabilization of MBP mRNAs in the cytoplasm was responsible for the reduced MBPs in the qk(v)/qk(v) brain during early myelination. In addition, we detected markedly reduced MBP mRNAs in the qk(v)/qk(v) myelin fraction with concomitant accumulation of MBP mRNAs associated with membrane-free polyribosomes. Presumably, the impaired localization of MBP mRNAs to the myelin membrane may cause insufficient incorporation of the newly synthesized MBPs into the myelin sheath. Finally, we observed interactions between QKI and MBP mRNAs, and removing MBP 3'UTR significantly reduced QKI-binding. Taken together, these observations suggest that misregulation at multiple posttranscriptional steps is responsible for the severe reduction of MBPs in qk(v) dysmyelination, presumably because of the lack of interactions between MBP mRNAs and the QKI RNA-binding proteins.

Understanding glial abnormalities associated with myelin deficiency in the jimpy mutant mouse

Jimpy is a shortened life-span murine mutant showing recessive sex-linked inheritance. The genetic defect consists of a point mutation in the PLP gene and produces a severe CNS myelin deficiency that is associated with a variety of complex abnormalities affecting all glial populations. The myelin deficiency is primarily due to a failure to produce the normal amount of myelin during development. However, myelin destruction and oligodendrocyte death also account for the drastic myelin deficit observed in jimpy. The oligodendroglial cell line shows complex abnormalities in its differentiation pattern, including the degeneration of oligodendrocytes through an apoptotic mechanism. Oligodendrocytes seem to be the most likely candidate to be primarily altered in a disorder affecting myelination, but disturbances affecting astrocytes and microglia are also remarkable and may have a crucial significance in the development of the jimpy disorder. In fact, the jimpy phenotype may not be attributed to a defect in a single cell but rather to a deficiency in the normal relations between glial cells. Evidences from a variety of sources indicate that the jimpy mutant could be a model for disturbed glial development in the CNS. The accurate knowledge of the significance of PLP and its regulation during development must be of vital importance in order to understand glial abnormalities in jimpy.

Upregulation of the endosomal-lysosomal pathway in the trembler-J neuropathy

A nonconservative leucine to proline mutation in peripheral myelin protein 22 (PMP22) causes the Trembler-J (TrJ) neuropathy in mice and humans. The expression levels and localization of the PMP22 protein in the TrJ mouse have not been previously determined. The aim of our studies was to reevaluate the extent of myelin deficit in genotyped heterozygous and homozygous animals and to examine how the TrJ mutation alters the normal in vivo post-translational processing of PMP22. Morphological studies show evidence for primary dysmyelination and myelin instability in affected animals. As expected, Western blot analysis indicates that in adult heterozygous TrJ animals, the level of PMP22 is markedly decreased, similar to myelin basic protein and protein zero, whereas myelin-associated glycoprotein is largely unaffected. The decrease in myelin protein expression is associated with an increase in lysosomal biogenesis, suggestive of augmented endocytosis or autophagy. Double-immunolabeling experiments show the accumulation of PMP22 in endosomal/lysosomal structures of TrJ Schwann cells, and chloroquine treatment of nerve segments indicates that the degradation of protein zero, PMP22, and myelin basic protein is augmented in TrJ nerves. These studies suggest that the TrJ mutation alters myelin stability and that the mutant protein is likely degraded via the lysosomal pathway.

Distribution of P0 protein and the myelin-associated glycoprotein in peripheral nerves from Trembler mice

The Trembler mouse has a dysymelination of peripheral nerves that includes hypomyelination, failure of myelin compaction, and demyelination/remyelination. We have localized the myelin proteins P0 and myelin associated glycoprotein in Trembler peripheral nerve and correlated their distributions with the ultrastructure of myelin internodes. Immunocytochemically, myelin-associated glycoprotein was localized in Schwann cell periaxonal membranes, Schmidt-Lanterman incisures, paranodal loops, and internal and external mesaxons. P0 staining was located over compact myelin and regions of Schwann cell cytoplasm rich in Golgi membranes. An unusual abundance of small, P0-stained, Golgi-related vesicles was found in some Schwann cells. P0 protein was also detected in multiple spiral wraps of myelin-associated glycoprotein-positive mesaxon membranes. At some sites the periodicity of the myelin membranes was intermediate to that found in mesaxon membranes and compact myelin. The distance between apposing extracellular leaflets was similar to that found in mesaxon membranes, while the cytoplasmic leaflets were fused but twice as thick as normal major dense lines. These intermediate membranes were stained by P0 and myelin-associated glycoprotein antiserum. These studies suggest that altered transport and/or translocation of P0 and myelin-associated glycoprotein results in defective myelin compaction in Trembler peripheral nerve.

Developmental expression of myelin protein genes in dysmyelinating mutant mice: analysis by nuclear run-off transcription assay, in situ hybridization, and immunohistochemistry

Gene expression for myelin proteolipid protein (PLP) and myelin basic protein (MBP) in the dysmyelinating mutant mice shiverer and jimpy was analyzed by nuclear run-off transcription assay, in situ hybridization, and immunohistochemistry. The level of PLP transcription in shiverer brains was lower than that in controls at postnatal day 18 but relatively higher at later stages. In spite of the considerable amount of hybridization with PLP cDNA, immunoreaction for PLP was greatly reduced in shiverer mice throughout their lives, probably owing to a defect in the assembly of PLP into myelin. Abnormal deposition of PLP in oligodendroglial cell bodies suggested that transport of PLP to myelin is delayed in shiverer brains. The number of oligodendrocytes expressing PLP mRNA was drastically reduced in jimpy mice. MBP mRNA in jimpy mice is localized preferentially in oligodendroglial cell bodies, a result suggesting that oligodendrocytes in jimpy are mostly the immature type. Although transcriptional activity of the MBP gene in jimpy was greatly reduced, a finding reflecting the decrease in the number of mature oligodendrocytes, that of the PLP gene remained high at early stages. The discrepancy of the two gene expressions is discussed relative to the role of PLP transcripts at early stages of myelination.

Quaking*jimpy double mutant mice: additional evidence for independence of primary deficits in jimpy

Mice which have the genotype qk/qk*Tajp/Y, and therefore simultaneously express both the quaking (qk) and jimpy (jp) mutations, have CNS white matter morphology intermediate between qk and jp with respect to amount of myelin, myelin structure, and oligodendrocyte number. The level of myelin basic protein in the CNS is also intermediate; however, myelin proteolipid protein (PLP) is virtually absent. Thus in the qk/qk*Tajp/Y double mutant mouse the PLP deficit is as severe as in jp alone but the oligodendrocyte survival deficit (reflected in number and myelin production) of jp alone is rendered less severe. The observation that these two cardinal deficits of the jp mutation can be independently altered in double mutant combinations is consistent with our previous suggestion that the PLP genetic locus may encode at least two independently regulated primary gene functions: a structural protein and signal influencing oligodendrocyte behavior.

The dysmyelinating mouse mutations shiverer (shi) and myelin deficient (shimld)

Shiverer (shi/shi) is an autosomal recessive mouse mutation that produces a shivering phenotype in affected mice. A shivering gait can be seen from a few weeks after birth until their early death, which occurs between 50 and 100 days. The central nervous system of the mutant mouse is hypomyelinated but the peripheral nervous system appears normal. The myelin of the CNS, wherever present, is not well compacted and lacks the major dense line. Myelin basic protein (MBP), which is associated with the major dense line, is absent, and this is due to a deletion of the major part of the gene encoding MBP. Transgenic shiverer mice that have integrated and express the wild-type mouse MBP transgene no longer shiver and have normal life spans. Conversely, normal mice that have integrated an antisense MBP transgene, shiver. Myelin deficient shimld/shimld is allelic to shiverer (shi/shi) but the mutant mouse is less severely affected. Although MBP is present in the CNS, it is low in quantity and is not developmentally regulated. The gene encoding MBP has been both duplicated and inverted. Transgenic shimld/shimld mice with the wild-type MBP transgene have normal phenotypes.

Developmental expression of the P0 glycoprotein and basic protein mRNAs in normal and trembler mutant mice

Mice affected by the autosomal dominant Trembler mutation exhibit a severe hypomyelinization of the PNS. Previous biochemical studies have shown that the accumulation of the major PNS myelin proteins, P0 and myelin basic protein (MBP), is strongly diminished in Trembler sciatic nerves during postnatal development. We performed Northern blots which showed that the size of mRNA species for P0 and MBP in normal and mutant mice are indistinguishable. Densitometric analysis of Northern blots showed that, in normal mice, the proportion of P0 mRNA increases up to the 12th day, then decreases slowly. At day 40, the proportion is 60% of the maximal value. In the mutant, the proportion of P0 mRNA increases up to the 12th day and then decreases much faster than in the control. At days 12 and 40, the P0 mRNA proportion measured in Trembler sciatic nerves represents only 40% and 7%, respectively, of the proportion measured in control littermates. The MBP mRNA proportion in the normal mice increases up to the 16th day, and then decreases to attain 45% of the maximum level at day 40. In the Trembler mouse, there is a maximum level at day 12, representing 25% of the normal level, but the MBP mRNA is barely detectable at days 8 or 40. Thus, these data seem to indicate that in the Trembler sciatic nerves, the proportions of P0 and MBP mRNAs are too small to allow the synthesis of normal levels of the corresponding proteins.

Restoration of myelin formation by a single type of myelin basic protein in transgenic shiverer mice

A minigene containing mouse cDNA coding for the smallest type of myelin basic protein and including the native promoter was constructed and used to produce transgenic shiverer mice. The hypomyelinating mouse, the shiverer, has a deletion in its myelin basic protein gene, lacks all four types of myelin basic protein in its myelin, and shows abnormal behavior such as violent tremors. Five of twenty-one transgenic shiverer mice showed recovered protein synthesis, compact myelin formation, and normal behavior. These results suggest that a single type of myelin basic protein restores myelin formation and returns the shivering phenotype to normal in the transgenic shiverer mouse.

Myelin-oligodendrocyte glycoprotein (MOG) is a surface marker of oligodendrocyte maturation

The myelin-oligodendrocyte glycoprotein (MOG) is a minor component of central nervous system myelin. Using neonatal rat optic nerve oligodendrocyte cultures we have compared the development in vitro of MOG with galactocerebroside, myelin basic protein and 2' ,3'-cyclic-nucleotide 3'-phosphodiesterase. MOG appears on the surface of oligodendrocytes 1-2 days later than these other oligodendrocyte markers, suggesting that MOG may be a useful indicator of oligodendrocyte maturation. The relevance of these findings for investigating mechanisms of myelin injury in vitro and the role of oligodendrocyte damage in demyelinating disease is discussed.

Leukoencephalopathy among native Indian infants in northern Quebec and Manitoba

We report 14 cases of a severe familial leukoencephalopathy among native North American Indian infants in northern Quebec and Manitoba. Affected infants have hypotonia and mild motor delay, followed by seizures, hypotonia or spasticity, eye deviation, and abnormal posture during a febrile illness around 6 months of age. Death follows a rigid, vegetative state that manifests days to months after disease onset and is marked in some cases by prominent autonomic disturbances, blindness, and cessation of head growth. Symmetrical hemispheric white matter lucencies and diffuse hypomyelination of the cerebral hemispheres and brainstem are the radiological and pathological hallmarks. This disease differs from the known diseases of cerebral myelin. An autosomal recessive pattern of inheritance awaits statistical confirmation. The proposed cause is a delay in development or abnormal turnover of central nervous system myelin.

Expression of galactocerebroside in developing normal and jimpy oligodendrocytes in situ

Intense and specific immunostaining of oligodendrocytes in vivo has been obtained for the first time using antibodies to galactocerebroside. We have examined the differentiation of oligodendrocytes in normal mice and then compared their differentiation to the myelin-deficient mouse jimpy, using immunoperoxidase, immunogold and immunofluorescence labelling techniques. We also compared staining for galactocerebroside with staining obtained using antibodies to myelin basic protein, carbonic anhydrase II, 2', 3'-cyclic nucleotide 3'-phosphohydrolase and proteolipid protein. The results of this comparative study confirm previous tissue culture studies and show that galactocerebroside is specific for oligodendrocytes in situ. As in tissue culture, galactocerebroside is one of the earliest oligodendrocyte markers to be expressed, making it an important marker for studying the differentiation of this cell type. The shape of oligodendrocytes in situ changes distinctly with time, shifting from an early stellate form with numerous spidery processes to a cell with a few processes radiating from the perikaryon. These morphological changes are observed for both normal and jimpy mice and they parallel those described in vitro. Oligodendrocytes in jimpy mice express most myelin markers, but the staining within the cells is generally less intense than in normal oligodendrocytes and the antigens are restricted to the cell body and processes without being incorporated into myelin sheaths. Quantification of the number of oligodendrocytes stained for galactocerebroside in normal and jimpy mice show that their number is not reduced in the corpus callosum and cerebellum during the first 2 weeks postnatal. This finding shows that many cells in jimpy mice which were considered to be unclassifiable by the application of morphological criteria have, in fact, differentiated to the stage where they are galactocerebroside-positive.

Molecular biology of myelin proteins from the central nervous system

Within the past several years, several of the genes coding for the major myelin proteins have been isolated, characterized, and mapped to specific chromosomes. In all cases, it has been clearly established that these proteins exist as multiple isoforms, and their structures have been established through an analysis of the cDNA clones encoding them. In each case, the isoforms appear to arise through the translation of individual mRNAs produced by alternative splicing of the primary transcript of a single gene. In several cases, the expression of the individual isoforms appears to be developmentally and/or regionally regulated, probably at the level of the splicing of the primary transcript. In the case of the dysmyelinating mutants shiverer and jimpy, the molecular defects involve the MBP gene and PLP gene, respectively; most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin protein genes.

A novel mutation in myelin-deficient mice results in unstable myelin basic protein gene transcripts

Mice homozygous for the myelin-deficient (mld) mutation have an unusual phenotype in which the gene encoding myelin basic protein (MBP) is expressed at low levels and on an abnormal developmental schedule. In this report we describe the organization of the mld MBP locus, which results in this alteration of MBP expression. The mld MBP locus consists of two tandem MBP genes, with the upstream gene containing an inversion of its 3' region. We also demonstrate that although there are low steady-state levels of MBP RNA in mld mice, the mld MBP locus is transcribed at a rate comparable to that of the wild-type MBP gene, indicating that the MBP transcripts are abnormally unstable.

Po, MBP, histone, and DNA levels in sciatic nerve. Postnatal accumulation studies in normal and trembler mice

We studied the quantitative changes in proteins (total, Po, MBP, and histones) and DNA from sciatic nerves of normal and Trembler mice during postnatal development. Polyacrylamide gel electrophoresis and immunoblotting procedures allowed an accurate characterization of Po, MBP, and histones, as well as the comparison of their respective amounts from d 2 to d 120 after birth. It was found that 1. The immunoblotting procedure ascertains the presence of Po in the sciatic nerve of Trembler. In the 2-d-old mice, Po is detected in essentially similar amounts in Trembler and normal PNS, whereas its level in adult mutant sciatic nerves is never greater than 20% of the control. The sharp increase in Po levels observed during the third week in the normal nerves is not observed in those of the mutant; 2. MBP species are at most 4% of the control in the 10- to 12-d-old Trembler mice, whereas they were not detectable in adult nerves. The distribution of the different MBP species is the same in both mutant and control mice; 3. In normal mice, Po and MBP accumulate at similar rates, but the 14 kDa MBP accumulates faster than the 18.5 kDa MBP; and 4. Histone and DNA contents decrease 3- to 5-fold in normal nerves, whereas they remain constant, or increase slightly, in the mutant.

Stage-specific cell-surface antigens of oligodendrocytes in the peripheral nervous system. Expression during development and regeneration and in myelin-deficient mutants

Monoclonal antibodies to stage-specific cell surface antigens of oligodendrocytes have been used to investigate the expression of antigens 05 through 011 in the peripheral nervous system of the mouse by immunohistology. In the adult sciatic nerve antigens 05 through 09 and 011 were diffusely positive. 010 antigen was not detectable in the peripheral nervous system at any age tested. During development antigens 05, 06 and 07 were first detectable at birth in tracts at the proximal part of the sciatic nerve. At day 2 the whole diameter of the nerve was positive for 05 antigen, while antigens 06 and 07 were detectable only in part of the nerve and antigens 08 and 09 were just about to appear. At day 4 antigen 011 was the last to appear. At day 7 all antigens were strongly detectable throughout the nerve. After transection of adult sciatic nerve expression of antigens 05 through 09 and 011 was studied at the proximal and distal ends of the cut. Three days after transection all antigens were fully detectable in the degenerating myelin and its debris. After 15 days residual debris was still distinctly positive, while Schwann cells in the bands of Bünger were antigen-negative. At approximately two weeks a connecting bridge between proximal and distal ends of the cut nerve had developed, but the 0 antigens were not detectable in this bridge until day 21. At day 42 all antigens were again fully detectable in the regenerating nerve. In hypomyelinating mouse mutants no difference to the normal control littermates was seen in staining pattern and intensity for jimpy and shiverer, while quaking showed an increase in staining intensity for 05 through 08 antigens. In trembler antigens 05, 06 and 07, but not 08, 09 and 011 appeared associated with non-myelin-forming Schwann cells, while the few recognizable myelin-forming Schwann cells expressed all antigens. These observations show that we have characterized 4 new monoclonal antibodies as further reagents to look at developmentally distinct steps in myelination of the peripheral nervous system.

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

Merosin, a protein specific for basement membranes of Schwann cells, striated muscle, and trophoblast, is expressed late in nerve and muscle development

We have identified a tissue-specific basement membrane-associated protein by using monoclonal antibodies prepared against a protein fraction of human placenta. In immunofluorescence, the monoclonal antibodies stained basement membranes of Schwann cells, striated muscle, and trophoblast, whereas no reaction was seen with any other basement membrane or tissue structure. In antibody-affinity chromatography of proteolytic digests of human placenta, a 65-kDa polypeptide was bound by these monoclonal antibodies. Rabbit antisera and monoclonal antibodies raised against the isolated 65-kDa polypeptide stained human and monkey tissues identically to the original monoclonal antibodies and reacted with an 80-kDa polypeptide in tissue extracts prepared without proteolysis. The 65-kDa and 80-kDa polypeptides were shown to be immunologically distinct from laminin, type IV collagen, fibronectin, and major serum proteins. They presumably represent a novel basement membrane-associated protein, which we have named merosin. No merosin immunoreactivity could be detected in cultures of any of 28 established cell lines. In developing mouse tissues, merosin staining first appeared at the newborn stage. The restricted tissue distribution and late developmental appearance of merosin suggest that the protein has a tissue-specific function associated with a high level of differentiation.

Immunohistochemical, biochemical and electron microscopic analysis of myelin formation in the central nervous system of myelin deficient (mld) mutant mice

Myelin deficiency (mld) is an autosomal recessive mutation in mice and is considered to be allelic to the shiverer (shi) mutation. Mld mice are characterized by hypomyelination of the central nervous system (CNS). They show typical symptoms such as tremor, tonic convulsion and ataxic movement. Subcellular fractionation of the CNS revealed that the MBP bands were greatly decreased in the P2A (myelin) fraction and the total content of myelin basic protein (MBP) was much lower than that in the control in all parts of the CNS. Sections from mld mice were examined by immunohistochemical tests with MBP antiserum, and a mosaic expression of MBP was found in the myelin of the mld mice. Since the major dense line is considered to be composed mainly of MBP, we investigated the myelin of mld mice by electron microscopy and found that there were 3 types of myelin: (1) a normal type compact myelin with a major dense line, (2) a shiverer-type myelin with no major dense line, and (3) a mixed-type myelin, in which within a myelin lamella the major dense line abruptly changes to cytoplasm of oligodendrocytes.

Myelin-associated glycoprotein in the central and peripheral nervous system of quaking mice

The myelin-associated glycoprotein (MAG) was quantitated in the CNS and PNS of quaking mice and the levels compared to the levels of myelin basic protein (MBP) and 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) activity. In the brainstems of 36-day-old quaking mice, MBP, MAG, and CNPase were reduced to 12, 16, and 29% of control levels, respectively. In the sciatic nerves of the 36-day-old quaking mice, MBP and CNPase were 38 and 75% of control levels, respectively, whereas the concentration of MAG was unchanged or slightly increased. Similar quantitative results were obtained for the sciatic nerves and spinal roots of 7-month-old quaking mice. Immunoblots showed that the principal MAG band from the brainstems, sciatic nerves, and spinal roots of the quaking mice had a higher than normal apparent Mr. In addition, there was a minor component reacting with anti-MAG antiserum in the brainstems of the quaking mice that had a slightly lower Mr than control MAG and was not detected in the normal mice. The results for the quaking mice are compared with those from similar studies on other mutants with dysmyelination of the CNS and PNS.

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.

Inefficient transcription of the myelin basic protein gene possibly causes hypomyelination in myelin-deficient mutant mice

A hereditary dysmyelination mutation, named myelin deficient (mld), is considered to be allelic to shiverer, a deletion mutation of the myelin basic protein (MBP) gene. The present study showed that MBP expression is greatly reduced in mld, but that it is still detectable. Northern blot analysis revealed that the pronounced decrease in the MBP level in mld resulted from a reduced mRNA level and was not caused by deletion of a large portion of the MBP gene as in shiverer. Southern blot studies with BamHI-digested chromosomal DNA suggested some part of the MBP gene, at least the 5'-portion, was duplicated in mld. These results indicated that the mld and shiverer mutations were different from each other, even though genetic allelism between the two was reconfirmed. We also examined the developmental pattern of the gene expression of MBP and that of another protein, myelin proteolipid protein (PLP), specifically expressed in the oligodendrocyte, in mld by RNA dot blot study. The mRNA level of MBP in mld was greatly reduced during the active myelination stages, gradually increasing and remaining constant in the later stages. The PLP-mRNA content in mld was almost normal (60-80% that of control) at any stage of development. All these findings imply that the primary defect in mld is due to reduced transcriptional activity of the MBP gene.

Myelin instability and oligodendrocyte metabolism in myelin-deficient mutant mice

During the active phase of myelination in myelin-deficient mutant mice (mld), myelin basic protein (MBP) synthesis is defective and the myelin lamellae are uncompacted. In these mutants, we found a fast metabolism of the myelin-associated glycoprotein (MAG) and of sulfatides, and the presence of cholesterol esters and a degradation product of MAG, dMAG, indicating that mld myelin was unstable. The increased synthesis of MAG and Wolfgram protein, two proteins present in uncompacted myelin sheath and paranodal loops, was demonstrated by high levels of messengers. Simultaneously, we found an accumulation of inclusion bodies, vacuoles, and rough endoplasmic reticulum in mld oligodendrocytes. This material was heavily immunostained for MAG. Furthermore, the developmental change between the two molecular forms of MAG (p72MAG/p67MAG) was delayed in mld mice. In 85-d-old mld mice, the MBP content increased and myelin lamellae became better compacted. In these mutants, dMAG was absent and MAG mRNAs were found in normal amounts. Furthermore, the fine structure of mld oligodendrocytes was normal and the MAG immunostaining was similar to age-matched controls. These results support a functional role for MBP in maintaining the metabolic stability and the compact structure of myelin. Furthermore, in the absence of MBP and myelin compaction, the regulation of the synthesis of at least two membrane proteins related to myelin cannot proceed.

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.

Mice heterozygous for the mld mutation have intermediate levels of myelin basic protein mRNA and its translation products

Myelin-deficiency (mld) is an autosomal recessive mutation in mice exhibiting a severe deficit in the synthesis of myelin basic protein (MBP). In order to understand the mechanisms involved in the regulation of MBP synthesis in the mld mutation, we examined the amount of MBP and MBP-specific mRNA in control, heterozygous and homozygous mld brains. In vitro translation of poly(A)+ RNA in a cell-free system, in situ hybridization, and filter hybridization with a radiolabelled probe pMBP-1 after dot or Northern blotting were used in this study. The levels of MBP and MBP-specific mRNA were very low but detectable in mld homozygotes, and intermediate in heterozygotes. MBP specific mRNA from mutants, and its translation products, were of normal size. These results show that the mld mutation is expressed co-dominantly in heterozygotes and affects a cis-acting regulatory element controlling the MBP gene.

Myelination by transplanted fetal and neonatal oligodendrocytes in a dysmyelinating mutant

Demyelination is a major feature of CNS injury and disease, including multiple sclerosis. To examine the potential for myelination by transplanted oligodendrocytes, initially described by Gumpel et al., we have transplanted neonatal cortex of mice with normal myelin into a dysmyelinating mutant, the shiverer mouse. We have found that oligodendrocyte precursors mature and synthesize myelin following transplantation. Immunostaining with antibodies to myelin basic protein (MBP), neurofilament protein and glial fibrillary acidic protein, demonstrates myelination both within the graft and extending out into the host, axonal sprouting from the graft which parallels the MBP-reactivity, and minimal astrocytic proliferation in response to the transplant.

Transplantation of bulk-separated oligodendrocytes into the brains of shiverer mutant mice: immunohistochemical and electron microscopic studies on the myelination

Normal oligodendrocytes were separated from 7-day-old mouse (BALB/c) brains by the Percoll gradient method. Immunohistochemical staining with an anti-galactocerebroside serum revealed that about 85% of the separated cells were oligodendrocytes. The oligodendrocytes were transplanted into the corpus striatum of 4-week-old shiverer mutant mice which are characterized by the lack of myelin basic protein (MBP). Myelination by the implanted oligodendrocytes was investigated immunohistochemically and electron microscopically 6 weeks after the operation. Certain areas in the corpus striatum were intensely stained with antiserum to MBP. Electron microscopic examination showed that some axons were surrounded by normal type myelin sheaths with major dense lines. These results clearly indicate that matured oligodendrocytes are able to survive and myelinate the host axons even in the adult brain.

Fatty acid biosynthesis in the peripheral nervous system of normal and Trembler mice

De novo fatty acid biosynthesis was demonstrated in a particle-free supernatant from normal and Trembler mouse sciatic nerves. In both systems, it required acetyl-CoA and malonyl-CoA, and led chiefly to the formation of free palmitic acid. No palmitoyl-CoA formation was detected. The ability of the cell-free extract of the mutant to form palmitic acid in vitro was greater than that of the control extracts when the results were expressed as the total activity per 100 mg of freshly excised sciatic nerves.

The generation and regeneration of oligodendroglia. A short review

During postnatal development of the higher vertebrate CNS, large populations of oligodendroglia are generated from precursor cells in a very dependable way. In adult lesioned CNS tissues, local populations of oligodendroglia are replenished by proliferation of this replenishment varies from one species to another and also from one lesion type another. Studies on the developmental generation of oligodendroglia are reviewed here, delineating what is known of the early relationships between the CNS glial lineages and of what regulates this development. Contributions from recent cell biological work are considered against the background of morphological and radioautographic results. The quiescent condition of extremely slow turnover in the normal adult CNS is noted, and the dramatic effects of lesions on the neural cell environment are considered. Lesions can trigger proliferation at a much greater rate in the mature oligodendroglial population, as observed both in situ and in tissue culture; in addition to persisting stem cells, the mature cells participate in replenishing the local oligodendroglial population. This regeneration from cells already committed to the oligodendroglial lineage may minimise such disturbing effects of the lesion environment as might distort replenishment of the population from precursor cells.

Myelin deficient (shimld) mutant allele: morphological comparison with shiverer (shi) allele on a B6C3 mouse stock

A new B6C3 stock of shimld mutant mice is compared in terms of behavior and CNS morphology with both a B6C3 shi stock and reports on other shimld animals. Defects of B6C3 shimld myelination seen at postnatal day 21 (P-21) are comparable to those in B6C3 shi with respect to % axons myelinated, sheath thickness, errors in the wrapping and targeting of myelin and abnormal oligodendrocyte shape. The two mutations are similarly expressed in cerebellar organotypic cultures. However, the major dense line (MDL) is present in a few shimld myelin sheaths at P-21 and a few sheaths show myelin basic protein by immunocytochemistry, while neither phenomenon is seen in shi at this age in the same CNS regions. Shimld mice survive their disease significantly better than shi. The shimld stock currently under study elsewhere differs from this B6C3 stock in that MDL was reported only in older animals, and behavior and survival were severely compromised.

Expression of myelin basic protein genes in several dysmyelinating mouse mutants during early postnatal brain development

Northern blot and "dot" blot analyses using a myelin basic protein (MBP) specific cDNA probe and in vitro translation techniques were utilized to estimate the relative levels of myelin basic protein messenger RNA (mRNA) in the brains of C57BL/6J control mice, three dysmyelinating mutants (qk/qk, jp/Y, and shi/shi), and three heterozygote controls (qk/+, jp/+, and shi+) during early postnatal development. In general, the MBP mRNA levels measured directly by Northern blot and "dot" blot analyses correlated well with the indirect in vitro translation measurements. The Northern blots indicated that the size of MBP mRNAs in quaking and jimpy brain polysomes appeared to be similar to controls. Very low levels of MBP mRNAs were observed in shi/shi brain polyribosomes throughout early postnatal development. Compared to C57BL/6J controls, accumulation of MBP mRNAs in qk/qk and qk/+ brain polyribosomes was delayed by several days. That is, whereas MBP mRNA levels were below normal between 12 and 18 days, normal levels of message had accumulated in both qk/qk and qk/+ brain polyribosomes by 21 days. Furthermore, normal levels of MBP mRNAs were observed to be maintained until at least 27 days. MBP mRNA levels remained well below control levels in jp/Y brain polyribosomes throughout early postnatal development. The levels did, however, fluctuate slightly and peaked at 15 days in both jp/Y and jp/+ brains, 3 days earlier than in normal mice. Thus, it appears that jimpy and quaking mice exhibit developmental patterns of MBP expression different from each other and from C57BL/6J control mice.

Identification and partial purification of fibroblast growth factor from the brains of developing rats and leucodystrophic mutant mice

High titres of fibroblast growth factor activity (assessed by mitogenicity for Balb/c 3T3 fibroblast cells in vitro) have been extracted from the brains of foetal and neonatal rats long before myelinogenesis commences, from the brains of hypomyelinated, leucodystrophic, murine mutants and from normal adult rats. Partial purification, by ion-exchange chromatography and gel filtration, indicates that the brain fibroblast growth factor activity from these sources is associated with very similar basic protein fractions. These results, together with the observation that none of the samples of partially purified basic fibroblast growth factor elicits the experimental allergic encephalomyelitis response attributed to myelin basic protein in the rat, suggest that basic fibroblast growth factor is not a degradation product of myelin basic protein.

Molecular genetic analysis of myelin-deficient mice: shiverer mutant mice show deletion in gene(s) coding for myelin basic protein

The gene expression of myelin basic proteins (MBPs) in shiverer mutant mice was investigated by the Northern and Southern hybridization techniques. In the control mice RNA molecules from the brains which were about 2,300 nucleotides in length were hybridized to cDNA of 1.8 kb encoding for a mouse MBP, but RNA from the brains of 3-week-old shiverer mutant mice contained no detectable amount of MBP transcripts hybridizing to this probe. Moreover the shiverer mutant mice lost several restriction fragments that hybridized to the same probe in the control mice when each of the five restriction enzymes, i.e., HindIII, PstI, PvuII, AccI, and StuI, was used. These data suggest that the shiverer mutation may correspond to the deletion of a large portion of MBP exon(s) in the gene, and this deletion causes inefficient transcription leading to the depletion of MBPs in the myelin and the dysmyelination observed in these mice.

Myelin-associated glycoprotein and other proteins in Trembler mice

The myelin-associated glycoprotein (MAG) and other myelin proteins were quantitated in homogenates of whole sciatic nerve from adult and 20-day-old Trember mice. In the nerves of adult mice, the concentration of MAG was increased from 1.1 ng/micrograms of total protein in the controls to 1.4 ng/micrograms protein in the Tremblers. By contrast, the concentrations of P0 glycoprotein and myelin basic proteins were reduced to 27% and 20% of control levels, respectively. Immunoblots demonstrated that P2 was also greatly reduced in the Trembler nerves. The specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) was 65% of the control level. Immunoblot analysis showed that MAG had a higher than normal apparent Mr in the sciatic nerves of the Trembler mice, but its apparent Mr was normal in the brains of these mutants. In 20-day-old Tremblers, the P0 and myelin basic protein were reduced slightly less to about 40% of the level in the nerves of age-matched controls. CNP and MAG levels were not significantly different from those in controls, and MAG exhibited a shift toward higher apparent Mr similar to that in the adults. The maintenance of high MAG levels despite the severe deficit of myelin, as reflected by the decrease of the major myelin proteins, is consistent with the immunocytochemical localization of MAG in periaxonal Schwann cell membranes, Schmidt-Lantermann incisures, lateral loops, and the outer mesaxon and its absence from compact myelin. The abnormal form of MAG in the peripheral nervous system (PNS) of the Trembler mice may contribute to the pathology in this mutant.

Anticonvulsive treatment of myelin-deficient (mld) mice improves survival and confirms the delayed increase of myelin basic protein

Myelin-deficient (mld) mutant mice were treated with phenobarbital between 60 and 90 d of age. The survival rate at 90 d increased from 1.4% in untreated mutants to 46% in those who received phenobarbital. This is evidence that apneic spells during tonic seizures are a major cause of death in mld mice. Myelin basic protein (MBP) content of brain homogenates from treated mld mice increased significantly between 30 and 90 d. MBP was present in myelin purified from the 90-d-old treated mld mice. These results demonstrate that the MBP increase, which occurs after the active phase of myelin formation is completed, is a general phenomenon and is not caused by the selection of a small and mildly affected subpopulation of mutants.