Absence Of Myelin Basic Protein Research Articles

Shaping membrane interfaces in lipid vesicles mimicking the cytoplasmic leaflet of myelin through variation of cholesterol and myelin basic protein contents

Myelin basic protein (MBP) is an intrinsically disordered protein and in the central nervous system (CNS) mainly responsible for connecting the cytoplasmic surfaces of the multilamellar, compact myelin. Increased posttranslational modification of MBP is linked to both, the natural development (from adolescent to adult brains) of myelin, and features of multiple sclerosis. Here, we study how a combination of this intrinsically disordered myelin protein with varying the natural cholesterol content may alter the characteristics of myelin-like membranes and interactions between these membranes. Large unilamellar vesicles (LUVs) with a composition mimicking the cytoplasmic leaflet of myelin were chosen as the model system, in which different parameters contributing to the interactions between the lipid membrane and MBP were investigated. While we use cryo-transmission electron microscopy (TEM) for imaging, dynamic light scattering (DLS) and electrophoretic measurements through continuously-monitored phase-analysis light scattering (cmPALS) were used for a more global overview of particle size and charge, and electron paramagnetic resonance (EPR) spectroscopy was utilized for local behavior of lipids in the vesicles' membranes in aqueous solution. The cholesterol content was varied from 060 % in these LUVs and measurements were performed in the presence and absence of MBP. We find that the composition of the lipid layers is relevant to the interaction with MBP. Not only the size, the shape and the aggregation behavior of the vesicles depend on the cholesterol content, but also within each membrane, cholesterol's freedom of movement, its environmental polarity and its distribution were found to depend on the content using the EPR-active spin-labeled cholesterol (CSOSL). In addition, DLS and EPR measurements probing the transition temperatures of the lipid phases allow a correlation of specific behavior with the human body temperature of 37 °C. Overall, our results aid in understanding the importance of the native cholesterol content in the healthy myelin membrane, which serves as the basis for stable and optimum protein-bilayer interactions. Although studied in this specific myelin-like system, from a more general and materials science-oriented point of view, we could establish how membrane and vesicle properties depend on cholesterol and/or MBP content, which might be useful generally when specific membrane and vesicle characteristics are sought for.

The Absence of Myelin Basic Protein Reduces Non-Amyloidogenic Processing of Amyloid Precursor Protein.

The accumulation of amyloid β-protein (Aβ) in the brain is a pathological feature of Alzheimer's disease (AD). Aβ peptides originate from amyloid precursor protein (APP). APP can be proteolytically cleaved through amyloidogenic or non-amyloidogenic pathways. The molecular effects on APP metabolism/processing may be influenced by myelin and the breakdown of myelin basic protein (MBP) in AD patients and mouse models of AD pathology. We directly tested whether MBP can alter influence APP processing in MBP-/- mice, known as Shiverer (shi/shi) mice, in which no functional MBP is produced due to gene breakage from the middle of MBP exon ll. A significant reduction of the cerebral sAPPα level in Shiverer (shi/shi) mice was found, although the levels of both total APP and sAPPβ remain unchanged. The reduction of sAPPα was considered to be due to the changes in the expression levels of a disintegrin and metalloproteinase-9 (ADAM9) catalysis and non-amyloid genic processing of APP in the absence of MBP because it binds to ADAM9. MBP -/- mice exhibited increased Aβ oligomer production. These findings suggest that in the absence of MBP, there is a marked reduction of nonamyloidogenic APP processing to sAPPα, and targeting myelin of oligodendrocytes may be a novel therapy for the prevention and treatment of AD.

Pathological transitions in myelin membranes driven by environmental and multiple sclerosis conditions

Multiple sclerosis (MS) is an autoimmune disease, leading to the destruction of the myelin sheaths, the protective layers surrounding the axons. The etiology of the disease is unknown, although there are several postulated environmental factors that may contribute to it. Recently, myelin damage was correlated to structural phase transition from a healthy stack of lamellas to a diseased inverted hexagonal phase as a result of the altered lipid stoichiometry and low myelin basic protein (MBP) content. In this work, we show that environmental conditions, such as buffer salinity and temperature, induce the same pathological phase transition as in the case of the lipid composition in the absence of MBP. These phase transitions have different transition points, which depend on the lipid's compositions, and are ion specific. In extreme environmental conditions, we find an additional dense lamellar phase and that the native lipid composition results in similar pathology as the diseased composition. These findings demonstrate that several local environmental changes can trigger pathological structural changes. We postulate that these structural modifications result in myelin membrane vulnerability to the immune system attacks and thus can help explain MS etiology.

The absence of myelin basic protein promotes neuroinflammation and reduces amyloid β-protein accumulation in Tg-5xFAD mice

BackgroundAbnormal accumulation of amyloid β-protein (Aβ) in the brain plays an important role in the pathogenesis \\of Alzheimer’s disease (AD). Aβ monomers assemble into oligomers and fibrils that promote neuronal dysfunction. This assembly pathway is influenced by naturally occurring brain molecules, the Aβ chaperone proteins, which bind to Aβ and modulate its aggregation. Myelin basic protein (MBP) was previously identified as a novel Aβ chaperone protein and a potent inhibitor for Aβ fibril assembly in vitro.MethodsIn this study, we determined whether the absence of MBP would influence Aβ pathology in vivo by breeding MBP knockout mice (MBP-/-) with Tg-5xFAD mice, a model of AD-like parenchymal Aβ pathology.ResultsThrough biochemical and immunohistochemical experiments, we found that bigenic Tg-5xFAD/MBP-/- mice had a significant decrease of insoluble Aβ and parenchymal plaque deposition at an early age. The expression of transgene encoded human AβPP, the levels of C-terminal fragments generated during Aβ production and the intracellular Aβ were unaffected in the absence of MBP. Likewise, we did not find a significant difference in plasma Aβ or cerebrospinal fluid Aβ, suggesting these clearance routes were unaltered in bigenic Tg-5xFAD/MBP-/- mice. However, MBP-/- mice and bigenic Tg-5xFAD/MBP-/- mice exhibited elevated reactive astrocytes and activated microglia compared with Tg-5xFAD mice. The Aβ degrading enzyme matrix metalloproteinase 9 (MMP-9), which is expressed by activated glial cells, was significantly increased in the Tg-5xFAD/MBP-/- mice.ConclusionsThese findings indicate that the absence of MBP decreases Aβ deposition in transgenic mice and that this consequence may result from increased glial activation and expression of MMP-9, an Aβ degrading enzyme.

The role of Zn in the interplay among Langmuir–Blodgett multilayer and myelin basic protein: a quantitative analysis of XANES spectra

We have performed a quantitative analysis of the X-ray absorption near-edge structure (XANES) spectra at the Zinc K-edge of systems formed by phospholipid Langmuir–Blodgett multilayers (LBMLs) in the presence and in the absence of myelin basic protein (MBP) and in two hydration conditions. These spectra have been analysed by a new procedure called Minuit XANes (MXAN) which is able to perform a quantitative fit of XANES data in terms of structural parameters. By this method, we have been able to correlate the relevant differences between the spectra observed in the XANES range with the coordination changes due to reduction of the space around the Zinc when the level of hydration is lowered and/or the myelin basic protein is added. These spectral differences are peculiar of the XANES energy range, and are not present in the extended X-ray absorption fine structure (EXAFS) energy range where the analysis was previously performed. With this investigation, we give an unambiguous answer to the question of the role of zinc in such complexes by showing that the metal interacts with both the phospholipid heads of the substrate and the myelin basic protein.

Antibodies to myelin basic protein, myelin oligodendrocytes peptides, alpha-beta-crystallin, lymphocyte activation and cytokine production in patients with multiple sclerosis.

To measure neurone-specific humoral and cellular immune parameters in MRI-positive patients with multiple sclerosis (MS). It has been postulated from animal models for MS and in situ evidence in MS patients that antibodies, activated T cells and proinflammatory cytokines are involved in the destruction of myelin sheaths and loss of oligodendrocytes in active areas. Blood samples were obtained from 20 healthy control subjects and 20 patients with abnormal MRI and clinical diagnosis of MS. Sera were tested for levels of IgG, IgM and IgA against myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG) peptides, and a small heat-shock protein, alpha-beta-crystallin. Lymphocytes were isolated and cultured in the presence or absence of MBP, MOG peptides and alpha-beta-crystallin, measured for stimulated T cells, cytokine production and compared with controls. Patients with MS showed the highest levels of IgG, IgM or IgA antibodies against one or all three tested antigens. Moreover, in the presence of MBP, MOG peptides or alpha-beta-crystallin, a significant percent- age of lymphocytes from MS patients underwent blast transformation, which resulted in high levels of interferon gamma (IFN-gamma), tumour necrosis factor alpha (TNF-alpha) and tumour necrosis factor beta (TNF-beta) production. Sensitivity of these assays was 60-80% and specificity, 65-70%. Detection of antibodies against MBP, MOG peptides, alpha-beta-crystallin, lymphocyte stimulation and production of proinflammatory cytokines in response to these antigens could be used as surrogate markers for the confirmation of MS diagnosis. A combination of antibodies, lymphocyte activation or cytokine production with abnormal MRI may significantly increase the sensitivity and specificity of MS diagnosis.

An X-Ray Absorption Spectroscopy Study of the Zinc Environment in Langmuir-Blodgett Phospholipid Multilayers

For the first time x-ray absorption spectroscopy was used to investigate the Zn environment in Langmuir-Blodgett multilayers. The multilayers were taken as a model of the multilamellar structure of the myelin sheath, the membrane surrounding the nerve axon, which plays a crucial role for signal transduction along the axon. The layers were assembled from the phospholipid dilauroylphosphatidic acid, both in the presence and in the absence of myelin basic protein. The analysis of the extended x-ray absorption fine structure and of the near edge regions of the x-ray absorption spectra at the Zn K-edge provided an accurate description of the local structure showing that the Zn ions are bound to the heads of the phospholipid molecules. The myelin basic protein induces a distortion on the Zn local environment due to a steric constraint but does not substitute the phosphate headgroups. These findings represent an important step in understanding the interplay among myelin basic protein, Zn, and the lipids of the myelin sheath.

Multiple sclerosis: myelin basic protein induced mRNA expression of proinflammatory cytokines in mononuclear cells is suppressed by interferon‐β 1b in vitro

Beta‐interferon (IFN‐β) is a promising treatment in multiple sclerosis (MS), reducing the exacerbation rate and MRI lesion burden, as well as the disease progression in relapsing‐remitting MS. IFN‐β was originally defined by its antiviral effects, but the interest has recently been focused on its immunomodulatory properties. Myelin basic protein (MBP) is one of several autoantigens considered to be the target for autoaggressive immune responses, which eventually might lead to the development of MS. To study in‐vitro effects of IFN‐β1b on MBP induced cytokine expression, mRNA for the Th1 cytokines IFN‐γ and TNF‐α, the Th2 related IL‐4 and IL‐6, the cytolytic perforin and the immune response downregulating TGF‐β was measured with in situ hybridization after culture of blood mononuclear cells (MNC) in the presence and absence of MBP. Numbers of cells expressing IFN‐γ, TNF‐α, perforin and IL‐4 mRNA were significantly suppressed after culture with 10 U/ml IFN‐β1b. No such effect was seen on MBP induced IL‐6 or TGF‐β mRNA expression. These observations suggest that one of the major effects of IFN‐β1b is the induction of a shift in the cytokine mRNA profile towards a more immunosuppressive pattern. In parallel in vitro tests, the control substance dexametasone (40 μg/ml) reduced the numbers of cells expressing mRNA for all cytokines under study with the exception of TGF‐β, to an extent equal to or even more pronounced than IFN‐β1b.

Normal rate of Schwann cell proliferation in the MBP-deficient shiverer mouse during Wallerian degeneration

Myelin basic protein (MBP) processed by macrophages was reported to promote Schwann cell division during Wallerian degeneration. In the present study we have shown that there was no difference in the rate of Schwann cell proliferation between shiverer mice, which totally lack MBP, and control mice after nerve-transection. Furthermore, addition of autologous peritoneal macrophages in cultures led to enhanced thymidine uptake by Schwann cells, irrespective of the presence or absence of MBP. These results suggest that myelin components other than MBP play a role in Schwann cell proliferation induced by macrophages.

Absence of myelin basic protein in granular cell tumors

Thirty-one granular cell tumors were studied for the presence of myelin basic protein (MBP) using a monoclonal antibody with a characterized specificity and an immunoperoxidase technique. The tumors studied were from all areas of the body, from both sexes, and from nine decades of age. All the tumors stained negatively for MBP. One brief report using a monoclonal antibody had noted positive staining for MBP in granular cell tumors. A more recent study using polyclonal antibodies found negative staining for MBP in Schwann cell neoplasms including granular cell tumors. This study, using an immunohistochemical technique, demonstrated the absence of an MBP marker in granular cell tumors of diverse location despite the fact that granular cell tumors are widely held to be of Schwann cell origin.

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.

Shiverer*jimpy double mutant mice. I. Biochemical evidence for reciprocal intergenic suppression

Shiverer and jimpy are neurological mutations that cause hypomyelination in the mouse CNS. The 3 major protein components of CNS myelin are: myelin basic protein (MBP), proteolipid (PLP) and 2′,3′-cyclic nucleotide phosphohydrolase (CNP). Previous work has shown that in jimpy animals the CNS contains reduced levels of MBP and CNP while PLP is undetectable. In shiverer animals the major forms of MBP ae undetectable in either the CNS or PNS, but the level of CNP is unaffected by the mutation. In this study we have measured MBP, PLP and CNP in both the CNS (cerebral hemispheres) and PNS (sciatic nerve) of mice carrying the jimpy and shiverer mutations, individually and in combination. The results indicate that in the double mutant the levels of all 3 myelin proteins in both the CNS and PNS are intermediate between the levels in jimpy and the levels in shiverer animals. This means that part of the biochemical phenotype of the jimpy mutation (reduced levels of CNP and absence of PLP) is suppressed by the shiverer mutation, and part of the biochemical phenotype of the shiverer mutation (absence of MBP) is suppressed by the jimpy mutation. Possible mechanisms for this reciprocal intergenic suppression are discussed.

Absence of myelin basic protein in an improved purified duck embryo rabies vaccine

The encephalitogenic potential of rabies vaccines prepared from nervous tissue is a result of the presence of myelin basic protein. Vaccines prepared from duck embryos are economical and efficient, but, occasionally, cases of allergic encephalomyelitis have been reported. An improved rabies vaccine has been developed that contains the classical Pitman Moore strain of rabies virus grown in embryonated duck eggs. This vaccine has been highly purified and enriched in immunologically effective rabies virus glycoprotein antigen. We have searched for the presence of myelin basic protein using sensitive radioimmunological and immunoblotting techniques. Whereas the classical duck embryo rabies vaccine contained small amounts of myelin basic protein, in the improved purified duck embryo rabies vaccine, none could be detected.

Antigen presentation in the peripheral nervous system: Schwann cells present endogenous myelin autoantigens to lymphocytes

Schwann cells (SC) isolated from neonatal rat sciatic nerves are shown to immunogenically present foreign and exogenous autoantigen to antigen-specific syngeneic T line cells in vitro. The antigen-presenting SC express Ia antigens on their membranes upon treatment with interferon gamma and contact with syngeneic T line cells. Monoclonal antibodies against Ia block specific antigen presentation, but not polyclonal mitogenic T cell activation. The antigen-presenting SC bind antibodies specific for astrocytic glial fibrillary acidic protein and may thus be related to the nonmyelinating glia cells of the peripheral nerve. Furthermore, SC isolated from 6-day-old rats activate rat myelin basic protein (MBP)-specific syngeneic T line cells in the absence of exogenous MBP. In contrast, they activate purified protein derivative of tuberculin (PPD)-specific T cells only in the presence of PPD. Since the MBP-specific T line cells are not activated by syngeneic professional antigen-presenting cells in the absence of MBP, endogenous MBP produced in the 6-day-old sciatic nerves appears to be presented by autochthonous SC to the autoreactive T cells.

Effects of ZnCl 2 on membrane interactions in myelin of normal and shiverer mice

X-ray diffraction was used to record the effects of metal cations on the structure of peripheral nerve myelin. Acidic saline (pH 5.0) either with or without added metal cations caused myelin to swell by 10–20 Å from its native period of 178 Å. The X-ray patterns usually showed broad reflections, and higher orders were either weak or unobserved. With added ZnCl 2, however, the swollen myelin gave diffraction patterns that retained sharp reflections to approx. 15 Å spacing. Alkaline saline (pH 9.7) containing ZnCl 2 produced a reduction of the myelin period by approx. 5 Å which was at least twice as much as that produced by other metals. To examine the underlying chemical basis for these unique interactions of Zn 2+ with myelin, we carried out parallel X-ray experiments on sciatic nerve from the shiverer mutant mouse, which lacks the major myelin basic proteins. Shiverer myelin responded like normal myelin to ZnCl 2 in acidic saline; however, in alkaline saline shiverer myelin showed broadened X-ray reflections which indicated disordering of the regularity of the membrane arrays, and additional reflections were recorded which indicated lipid phase separation. This breakdown may come about by the binding of Zn 2+ to negatively-charged lipids which could be more exposed due to the absence of myelin basic proteins. Electron density profiles were calculated on the assumption that, except for changes in their packing, the myelin membranes were minimally altered in structure. For both normal and shiverer myelins, treatments under acidic conditions resulted in swelling at the extracellular apposition and a slight narrowing of the cytoplasmic space. This swelling is likely due to adsorption of protons and divalent cations. Interaction between Zn 2+ and myelin P 0 glycoprotein could preserve an ordered arrangement of the apposed membrane surfaces. Alkaline saline containing ZnCl 2 produced compaction at the cytoplasmic apposition in both normal and shiverer myelins possibly through interactions with a portion of P 0 glycoprotein which extends into the cytoplasmic space between membranes.

Cell lineage analysis of Schwann cells in the PNS determined by shiverer-normal mouse chimeras.

The myelin of the peripheral nervous system from the shiverer mutant mice is characterized by the absence of myelin basic protein, while the other myelin protein components are present at normal levels. Myelin lamella formation is normal in the shiverer mutant. Therefore, by using antiserum against myelin basic protein, we can distinguish the shiverer from the wild-type control myelin immunohistochemically. To study the cell lineage of Schwann cells, chimeras produced by the aggregation of eight-cell embryos from wild-type mice and shiverer mice have been used. Using myelin basic protein as a marker, it was observed that Schwann cells in the sciatic nerve existed as patches of cells with like-genotype. The patches occurred in a linear array along the axons with some intermingling of Schwann cells. Complete randomization by intermingling of Schwann cells was not observed and clones of Schwann cells may persist as contiguous groups throughout peripheral nerve development.

Lipid/myelin basic protein multilayers: A model for the cytoplasmic space in central nervous system myelin

A multilayered complex forms when a solution of myelin basic protein is added to single-bilayer vesicles formed by sonicating myelin lipids. Vesicles and multilayers have been studied by electron microscopy, biochemical analysis, and X-ray diffraction. Freeze-fracture electron microscopy shows well-separated vesicles before myelin basic protein is added, but afterward there are aggregated, possibly multilayered, vesicles and extensive planar multilayers. The vesicles aggregate and fuse within seconds after the protein is added, and the multilayers form within minutes. No intra-bilayer particles are seen, with or without the protein. Some myelin basic protein, but no lipid, remains in the supernatant after the protein is added and the complex sedimented for X-ray diffraction. A rather variable proportion of the protein is bound. X-ray diffraction patterns show that the vesicles are stable in the absence of myelin basic protein, even under high g-forces. After the protein is added, however, lipid/myelin basic protein multilayers predominate over single-bilayer vesicles. The protein is in every space between lipid bilayers. Thus the vesicles are torn open by strong interaction with myelin basic protein. The inter-bilayer spaces in the multilayers are comparable to the cytoplasmic spaces in central nervous system myelins. The diffraction indicates the same lipid bilayer thickness in vesicles and multilayers, to within 1 Å. By comparing electron-density profiles of vesicles and multilayers, most of the myelin basic protein is located in the inter-bilayer space while up to one-third may be inserted between lipid headgroups. When cytochrome c is added in place of myelin basic protein, multilayers also form. In this case the protein is located entirely outside the unchanged bilayer. Comparison of the various profiles emphasizes the close and extensive apposition of myelin basic protein to the lipid bilayer. Numerous bonds may form between myelin basic protein and lipids. Cholesterol may enhance binding by opening gaps between diacyl-lipid headgroups.

Immunoreactive myelin basic proteins are not detected when shiverer mutant Schwann cells and fibroblasts are co-cultured with normal neurons.

Shiverer (shi) is an autosomal recessive mutation in mice that results in hypomyelination in the central nervous system (CNS) but normal myelination in the peripheral nervous system (PNS). Myelin basic proteins (MBPs) are virtually absent in both PNS and CNS. It is not known whether the cellular target in the PNS is the myelin-forming Schwann cell or another cell type which secondarily affects the Schwann cell. To determine the cellular target of the shi gene, we have adapted tissue culture techniques that allow co-culture of pure populations of mouse sensory neurons of one genotype with Schwann cells and fibroblasts of another genotype under conditions that permit myelin formation. These cultures were stained immunocytochemically as whole mounts to determine whether MBPs were expressed under various in vitro conditions. In single-genotype cultures, presence or absence of MBPs was consistent with earlier in vivo results: +/+ cultures were MBP-positive and shi/shi cultures were MBP-negative. In mixed-genotype cultures, visualization of MBPs in myelin accorded with the genotype of the non-neuronal Schwann cells and fibroblasts and not with the neurons--those cultures that contained +/+ non-neuronal cells were MBP-positive and those with shi/shi non-neuronal cells were MBP-negative, independent of the neuronal genotype. These results rule out neurons or circulating substances as mediators of the influence of the shi genetic locus on MBP synthesis and deposition in peripheral myelin.

Transplantation of CNS fragments into the brain of shiverer mutant mice: extensive myelination by implanted oligodendrocytes. I. Immunohistochemical studies.

Solid fragments of olfactory bulb from new-born normal (B6CBA and C57BL6) mice were implanted into new-born shiverer (shi/shi) brains. The shiverer mouse being characterized by the absence of myelin basic protein (MBP), myelination due to implanted oligodendrocytes can be detected in the shiverer brain using an antiserum anti-MBP. Observation of sagittal sections of the host brains revealed very extensive areas of normal myelination from the level of the graft (rostral thalamus) up to the caudal brain (diencephalon, cerebellum, pons). Thus, oligodendrocytes contained in the implant migrate out of the graft over long distances in the host brain, before they differentiate and synthesize myelin. These results raise the question of the behaviour of oligodendrocytes in normal development.

Density distribution of 2′,3′-cyclic nucleotide 3′-phosphodiesterase and myelin proteins in particulate material from myelin deficient ( mld) mutant and control brains

Total particulate material from control and myelin deficient ( mld) brains was subjected to density centrifugation on a continuous sucrose gradient. Particles from control brains distributed in a bell-shaped mode with a peak density near 0.64 M-sucrose. In mld material only a slight elevation of optical density was observed near 0.8 M-sucrose. The highest specific activities of 2′,3′-cyclic nucleotide 3′-phosphodiesterase were observed at densities of 0.63 and 0.71 M-sucrose for mld and control brains, respectively. The peak of myelin basic protein in control fractions was near 0.60 M-sucrose. In mld fractions no peak was observed. Proteolipid and Wolfgram proteins had a maximum near 0.65 and 0.73 M-sucrose in control and mld fractions, respectively. The absence of myelin basic proteins in all the fractions makes it unlikely that, in mld mice, myelin basic proteins are synthesized but not incorporated into myelin.