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

A mutation in the Tubb4a gene leads to microtubule accumulation with hypomyelination and demyelination

OBJECTIVE

Our goal was to define the genetic cause of the profound hypomyelination in the taiep rat model and determine its relevance to human white matter disease.

METHODS

Based on previous localization of the taiep mutation to rat chromosome 9, we tested whether the mutation resided within the Tubb4a (β-tubulin 4A) gene, because mutations in the TUBB4A gene have been described in patients with central nervous system hypomyelination. To determine whether accumulation of microtubules led to progressive demyelination, we analyzed the spinal cord and optic nerves of 2-year-old rats by light and electron microscopy. Cerebral white matter from a patient with TUBB4A Asn414Lys mutation and magnetic resonance imaging evidence of severe hypomyelination were studied similarly.

RESULTS

As the taiep rat ages, there is progressive loss of myelin in the brain and dorsal column of the spinal cord associated with increased oligodendrocyte numbers with accumulation of microtubules. This accumulation involved the entire cell body and distal processes of oligodendrocytes, but there was no accumulation of microtubules in axons. A single point mutation in Tubb4a (p.Ala302Thr) was found in homozygous taiep samples. A similar hypomyelination associated with increased oligodendrocyte numbers and arrays of microtubules in oligodendrocytes was demonstrated in the human patient sample.

INTERPRETATION

The taiep rat is the first animal model of TUBB4 mutations in humans and a novel system in which to test the mechanism of microtubule accumulation. The finding of microtubule accumulation in a patient with a TUBB4A mutation and leukodystrophy confirms the usefulness of taiep as a model of the human disease. Ann Neurol 2017;81:690-702.

Myelin membrane wrapping of CNS axons by PI(3,4,5)P3-dependent polarized growth at the inner tongue

Central nervous system myelin is a multilayered membrane sheath generated by oligodendrocytes for rapid impulse propagation. However, the underlying mechanisms of myelin wrapping have remained unclear. Using an integrative approach of live imaging, electron microscopy, and genetics, we show that new myelin membranes are incorporated adjacent to the axon at the innermost tongue. Simultaneously, newly formed layers extend laterally, ultimately leading to the formation of a set of closely apposed paranodal loops. An elaborated system of cytoplasmic channels within the growing myelin sheath enables membrane trafficking to the leading edge. Most of these channels close with ongoing development but can be reopened in adults by experimentally raising phosphatidylinositol-(3,4,5)-triphosphate levels, which reinitiates myelin growth. Our model can explain assembly of myelin as a multilayered structure, abnormal myelin outfoldings in neurological disease, and plasticity of myelin biogenesis observed in adult life.

Diffusion weighted imaging of prefrontal cortex in prodromal Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disease with no effective disease-modifying treatments. There is considerable interest in finding reliable indicators of disease progression to judge the efficacy of novel treatments that slow or stop disease onset before debilitating signs appear. Diffusion-weighted imaging (DWI) may provide a reliable marker of disease progression by characterizing diffusivity changes in white matter (WM) in individuals with prodromal HD. The prefrontal cortex (PFC) may play a role in HD progression due to its prominent striatal connections and documented role in executive function. This study uses DWI to characterize diffusivity in specific regions of PFC WM defined by FreeSurfer in 53 prodromal HD participants and 34 controls. Prodromal HD individuals were separated into three CAG-Age Product (CAP) groups (16 low, 22 medium, 15 high) that indexed baseline progression. Statistically significant increases in mean diffusivity (MD) and radial diffusivity (RD) among CAP groups relative to controls were seen in inferior and lateral PFC regions. For MD and RD, differences among controls and HD participants tracked with baseline disease progression. The smallest difference was for the low group and the largest for the high group. Significant correlations between Trail Making Test B (TMTB) and mean fractional anisotropy (FA) and/or RD paralleled group differences in mean MD and/or RD in several right hemisphere regions. The gradient of effects that tracked with CAP group suggests DWI may provide markers of disease progression in future longitudinal studies as increasing diffusivity abnormalities in the lateral PFC of prodromal HD individuals.

The contributions of myelin and axonal caliber to transverse relaxation time in shiverer and neurofilament-deficient mouse models

White matter disorders can involve injury to myelin or axons but the respective contribution of each to clinical course is difficult to evaluate non-invasively. Here, to develop a paradigm for further investigations of axonal pathology by MRI, we compared two genetic mouse models exhibiting relatively selective axonal or myelin deficits using quantitative MRI relaxography of the transverse relaxation times (T2) in vivo and ultrastructural morphometry. In HM-DKO mice, which lack genes encoding the heavy (NF-H) and medium (NF-M) subunits of neurofilaments, neurofilament content of large myelinated axons of the central nervous system (CNS) is markedly reduced in the absence of changes in myelin thickness and volume. In shiverer mutant mice, which lack functional myelin basic protein, CNS myelin sheath formation is markedly reduced but neurofilament content is normal. We observed increases in T2 in nearly all white matter in shiverer mice compared to their wild type, while more subtle increases in T2 were observed in HM-DKO in the corpus callosum. White matter T2 was generally greater in shiverer mice than HM-DKO mice. Ultrastructural morphometry of the corpus callosum, which exhibited the greatest T2 differences, confirmed that total cross-sectional area occupied by axons was similar in the two mouse models and that the major ultrastructural differences, determined by morphometry, were an absence of myelin and larger unmyelinated axons in shiverer mice and absence of neurofilaments in HM-DKO mice. Our findings indicate that T2 is strongly influenced by myelination state and axonal volume, while neurofilament structure within the intra-axonal compartment has a lesser effect upon single compartment T2 estimates.

Myelination and long diffusion times alter diffusion-tensor-imaging contrast in myelin-deficient shiverer mice

Diffusion tensor imaging (DTI) using variable diffusion times (t(diff)) was performed to investigate wild-type (wt) mice, myelin-deficient shiverer (shi) mutant mice and shi mice transplanted with wt neural precursor cells that differentiate and function as oligodendrocytes. At t(diff) = 30 ms, the diffusion anisotropy "volume ratio" (VR), diffusion perpendicular to the fibers (lambda( perpendicular)), and mean apparent diffusion coefficient (<D>) of the corpus callosum of shi mice were significantly higher than those of wt mice by 12 +/- 2%, 13 +/- 2%, and 10 +/- 1%, respectively; fractional anisotropy (FA) and relative anisotropy (RA) were lower by 10 +/- 1% and 11 +/- 3%, respectively. Diffusion parallel to the fibers (lambda(//)) was not statistically different between shi and wt mice. Normalized T(2)-weighted signal intensities showed obvious differences (27 +/- 4%) between wt and shi mice in the corpus callosum but surprisingly did not detect transplant-derived myelination. In contrast, diffusion anisotropy maps detected transplant-derived myelination in the corpus callosum and its spatial distribution was consistent with the donor-derived myelination determined by immunohistochemical staining. Anisotropy indices (except lambda(//)) in the corpus callosum showed strong t(diff) dependence (30-280 ms), and the differences in lambda( perpendicular) and VR between wt and shi mice became significantly larger at longer t(diff), indicative of improved DTI sensitivity at long t(diff). In contrast, anisotropy indices in the hippocampus showed very weak t(diff) dependence and were not significantly different between wt and shi mice across different t(diff). This study provides insights into the biological signal sources and measurement parameters influencing DTI contrast, which could lead to developing more sensitive techniques for detection of demyelinating diseases.

Dysmyelination revealed through MRI as increased radial (but unchanged axial) diffusion of water

Myelin loss and axonal damage are both observed in white matter injuries. Each may have significant impact on the long-term disability of patients. Currently, there does not exist a noninvasive biological marker that enables differentiation between myelin and axonal injury. We describe herein the use of magnetic resonance diffusion tensor imaging (DTI) to quantify the effect of dysmyelination on water directional diffusivities in brains of shiverer mice in vivo. The principal diffusion eigenvalues of eight axonal fiber tracts that can be identified with certainty on DTI maps were measured. The water diffusivity perpendicular to axonal fiber tracts, lambda(perpendicular), was significantly higher in shiverer mice compared with age-matched controls, reflecting the lack of myelin and the increased freedom of cross-fiber diffusion in white matter. The water diffusivity parallel to axonal fiber tracts, lambda(parallel), was not different, which is consistent with the presence of intact axons. It is clear that dysmyelination alone does not impact lambda(parallel). The presence of intact axons in the setting of incomplete myelination was confirmed by electron microscopy. Although further validation is still needed, our finding suggests that changes in lambda(perpendicular) and lambda(parallel) may potentially be used to differentiate myelin loss versus axonal injury.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Evidence that CNS hypomyelination does not cause death of jimpy-msd mutant mice

Mice expressing three of the proteolipid protein (Plp) mutations in the mouse (jimpy, jimpy-msd, and jimpy-4J) all have a severe deficiency of CNS myelin and oligodendrocytes (OLs), and die sometime in their 4th postnatal week. The prevailing view has been that the animals' shortened life span and lack of myelin are causally related. Here we describe the survival of jimpy-msd males for as long as postnatal day (P) 210. Although these spontaneously occurring longer-lived jimpy-msd males show a 2- to 8-fold increase in numbers of myelinated axons in many CNS regions, this does not protect them from a later but still premature death. Investigating the cause of premature death may reveal previously undiscovered properties of the myelin genes or the cells that express them, or perhaps additional unsuspected cellular responses that contribute to the disease. This study identifies small accumulations of inflammatory cells in the brain parenchyma of jimpy-msd mice as young as P14 and as old as P60, suggesting that the pathology of the disease produced by at least this Plp mutation may be far more complex than has been previously recognized.

Quaking*shiverer double-mutant mice survive for at least 100 days with no CNS myelin

Mice expressing mutations that produce CNS hypomyelination often die prematurely: the more severe the hypomyelination, the shorter the life span. However, we have previously described jimpy-msd mice that survive twice as long as usual; although they acquire significantly increased amounts of myelin, they still succumb long before their unaffected littermates. This result contradicts any postulated causal relationship between extent of CNS hypomyelination and premature death of the animal. Here we have addressed this question in another way, by using an animal model that does not involve a proteolipid protein (Plp) gene mutation. We demonstrate that quaking*shiverer double-mutant mice can survive for at least 100 days without any CNS myelin whatsoever. Therefore, at least for a mouse, absence of CNS myelin is not lethal per se.

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

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

The taiep rat: a myelin mutant with an associated oligodendrocyte microtubular defect

This report describes a new inherited disorder of myelination in the rat, named taiep, in which failure of normal myelination of the CNS and subsequent demyelination result in a progressive neurological disturbance. At two months of age, myelin is present throughout the spinal cord, but is immature in the fasciculus gracilis and corticospinal tracts despite the presence of abundant oligodendrocytes. By 12 months, myelin has largely been lost in these spinal cord tracts and also in more rostral parts of the CNS, such as the cerebellum and optic nerves. Other funiculi of the spinal cord show a more diffuse lack of myelin. Oligodendrocytes develop a unique cellular abnormality, most obviously in older rats, which is characterized by the accumulation of microtubules throughout their cytoplasm. As the mutant rats age, there is a continued protracted breakdown of myelin throughout the CNS, with evidence suggesting either persistent hypomyelination or attempts at remyelination of affected axons. It is proposed that the microtubular defect in oligodendrocytes results in a disruption of the normal myelination process in certain areas of the CNS of this mutant, and eventually leads to failure of maintenance of the myelin sheath.

Molecular genetic analysis of the mldr mouse: a spontaneous revertant at the mld locus containing a recombinant myelin basic protein gene

The mld mutation is a complex genetic lesion affecting the myelin basic protein (MBP) locus in the mouse. The mutation consists of a variety of DNA rearrangements including: tandem duplication of the MBP structural gene, partial inversion of the 3' end of the upstream gene copy, duplication of a region flanking the rearrangement junction in the upstream copy and insertion between the two gene copies of a segment of extraneous DNA not associated with the wild-type MBP locus. The net result of the mutation is a dysfunctional MBP locus. Homozygous mld/mld mice produce very little MBP and consequently very little myelin. They exhibit a clinical phenotype characteristic of hypomyelination (shaking, convulsions). We have discovered a revertant mld mouse which does not exhibit clinical symptoms of hypomyelination. Genetic analysis indicates that the reversion is allelic to mld. We have designated the revertant locus mldr. Restriction analysis of mldr genomic DNA indicates that there is a single intact MBP gene. Analysis of various junction regions using the polymerase chain reaction indicates that the single MBP gene in mldr is derived by recombination from the 5' end of the upstream gene and the 3' end of the downstream gene. Studies on MBP expression in mldr mice indicate that the developmental regulation, level of expression and pattern of post-transcriptional processing of MBP gene products in mldr are similar to wild type. These results indicate that the recombinant MBP gene in mldr is fully functional. From this we infer that the MBP-deficient phenotype of the original mld mutant is attributable to the complex rearrangements in the upstream gene copy which render the locus dysfunctional.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin basic protein gene and the function of antisense RNA in its repression in myelin-deficient mutant mouse

The myelin-deficient (mld) mouse is an autosomal recessive mutant characterized by hypomyelination of the CNS due to reduced expression of the myelin basic protein (MBP) gene. In the mld mutant, the MBP gene is duplicated in tandem. One gene is intact, but a large portion is inverted upstream of the other copy, and its transcription yields the antisense RNA. This antisense RNA was shown to be localized in the nucleus and to form an RNA:RNA duplex with sense RNA. These findings suggested that inhibition of transport from the nucleus or selective degradation of the duplex is responsible for the reduced expression of the MBP gene in the mld mutant. The mechanism of gene rearrangement at the MBP locus was also characterized. Cosmid clones encompassing whole MBP gene loci from control and mld genomic DNA libraries were isolated. The recombination points indicated that the duplication and inversion observed in mld occurred due to nonhomologous recombination.

Radial component of CNS myelin: junctional subunit structure and supramolecular assembly

The radial component is a structural specialization within CNS myelin that is believed to stabilize the apposition of membranes in the internode. Previous observations on thin sections and freeze-fracture replicas show that this junctional complex consists of linear, particulate strands that run parallel to the nerve fibre axis and radially through the myelin sheath, but details on its molecular organization are lacking. The objective of our current study was to gain further insight into its arrangement and composition by examining its fine-structure and incidence in: myelin with known deficits in protein composition (e.g., shiverer, transgenic shiverer, myelin deficient and jimpy mutant mice); isolated CNS myelin, which has been shown by X-ray diffraction to be more stable than intact CNS myelin; and human white matter, in which this junctional complex has not yet been described. Our results confirm the localization and general appearance of the radial component as previously reported. In addition, we found that: (1) the radial component occurs abundantly in human CNS myelin where it has a complex subunit structure; (2) the constituent junctional unit of this structure is organized as a pair of globular domains (each approximately 40 A diameter) at the extracellular apposition which is linked by approximately 15 A diameter filaments extending through the bilayer to approximately 25 A globular domains in the adjacent cytoplasmic apposition; (3) the radial component is present with apparently normal structure in the sparse, compact myelin of murine mutants containing either different amounts of MBP or no PLP which indicates that neither of these proteins is necessary for junctional integrity; (4) the radial component is present in purified CNS myelin membranes which may account for the stability of these membranes; and (5) the radial component is structurally resistant to Triton, which suggests a method for its further biochemical characterization. Finally, from an analysis of images from tilted transverse and longitudinal sections, we have reconstructed a model of its three-dimensional, supramolecular organization.

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.

Proton magnetic resonance in myelin deficient brains of mutant mice

The role of myelin in determining the magnetic resonance (MR) characterization of the central nervous system (CNS) was investigated in unmyelinated brains of normal fetal mice, as well as myelin-deficient adult mutant mice (shi, qk, mld) and their age-matched controls. In vitro NMR relaxation time measurements at 10 MHz for whole brains showed consistently longer T1 (range 558 +/- 8 to 580 +/- 27 msec) and T2 (range 81 +/- 3 to 89 +/- 3 msec) values for the adult myelin-deficient animals than the age-matched controls (T1 = 496 +/- 31, T2 = 79 +/- 4 msec). The fetal brains exhibited even more prolonged relaxation times (T1 = 976 +/- 60, T2 = 158 +/- 7 msec). MR images obtained at 81 MHz using spin echo (SE) sequences, which unlike the in vitro approach allowed discrimination between white and gray matter areas, revealed an absence of gray-white matter contrast in the brains of mutant mice, consistent with longer than normal relaxation of the myelin-deficient white matter. While larger tissue water components such as those present in the immature brain and edematous white matter contribute a greater effect, myelin and its associated bound water may still play an important role in the MR characterization of normal gray and white matter.

Quaking shiverer double mutant mice: morphological phenotypes support possible dual actions of the shiverer locus

Mice doubly homozygous for the two different hypomyelination mutations, quaking (qk) and shiverer (shi) or shiverer myelin-deficient (shimld) (abbreviations: qk*shi and qk*shimld), both have much less myelin than either single mutant ancestor, myelin morphology resembling shi or shimld rather than qk, and abundant shi-type oligodendrocytic microprocesses. The qk*shimld double mutant differs from qk*shi only in having small amounts of normal or abnormal major dense line, in keeping with the morphologic difference between the shi and shimld single mutants. By contrast, shi*jp and shimld*jp have clearly different morphological phenotypes; unexpectedly the major dense line is present in the CNS myelin of shi*jp but not shimld*jp. When shi and shimld act alone, their different DNA abnormalities produce similar protein abnormalities. We speculate that the two mutations interact with qk at a different, later step of DNA expression than they interact with jp. In the interaction with qk, the similar proteins produce similar morphologies. In the interaction with jp, the different DNAs are somehow caused to produce protein differences that are reflected in different morphologies. In this study we have observed for the first time a morphological effect of these mutant genes in heterozygous animals. Of particular importance, animals whose genomes combine shi/+ or shimld/+ with qk/qk produce qk-type, compacted myelin but abundant shi-type oligodendrocyte microprocesses. We consider this as evidence that both shi and shimld have two effects: non-production of a normal structural protein, myelin basic protein, and production of an abnormal protein which perturbs the cytogogic function we postulate to be normally exercised by the myelin basic protein gene.

Shiverer jimpy double mutant mice. IV. Five combinations of allelic mutations produce three morphological phenotypes

Mice which simultaneously express mutant genes at both the shi and jp loci (double mutant mice) have phenotypes much more complex than simple addition of individual mutant characteristics. Morphological study of shi jpmsd, shimld jp, and shi/shimld jp, and comparison with previously studied shi jp and shimld jpmsd, shows that 3 classes of central nervous system (CNS) white matter morphology are produced. (1) shi jp shows suppression of most jp characteristics: it is like shi except for more major dense line, and possibly more myelin, than shi alone. No other combination has as much myelin or any major dense line at all. (2) shi jpmsd has qualitative and quantitative characteristics intermediate between the two single mutants. (3) All combinations studied involving shimld have much less myelin than either single mutant. Qualitatively they express most jp locus features but suppress all shi locus features except abnormalities of myelin compaction. The difference between shi and shimld has more influence on the double mutant morphology than the difference between jp and jpmsd. In the 3-mutant combination, shi/shimld jp, the influence of shimld completely overrides that of shi. These morphological phenotypes resist assignment to any hierarchy of normalcy, and their specific features have no simple explanation in presently known molecular biology of the shi and jp locus mutations. They suggest the possibility of multiple copies and multiple primary functions of the messages at these loci.

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.

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.

Genotype-specific myelin formation around normal axons in cytosine arabinoside-treated organotypic cultures injected with normal or shiverer optic nerve

Organotypic cultures of newborn mouse cerebellum, treated with 'antimitotic' drugs to prevent myelination, contain abundant large neurons and axons but no mature oligodendroglia and no myelin. When oligodendroglia, contained in optic nerve fragments from 7-12-day-old normal or shiverer mutant mouse, are added to the cultures, myelin is formed in the vicinity of the optic nerve explant. The phenotype of the added myelin corresponds to the genotype of the optic nerve added, indicating that the added oligodendroglia are making the myelin themselves rather than contributing a diffusible factor which stimulates native cerebellar oligodendroglia or their precursors. This system will be valuable for direct, detailed comparisons of myelin formation by normal and mutant oligodendrocytes.