The structure and function of myelin: from inert membrane to perfusion pump

A novel myelin basic protein transcript variant in the murine central nervous system

Myelin basic protein is a multifunctional protein whose primary role is to adhere membranes of the myelin sheath. There are various isoforms that have been identified, 6 distinct isoforms in human and 13 distinct isoforms in mice. These distinct isoforms are the product of alternative splicing of a single gene. The present study sought out to identify the different isoforms found in the murine central nervous system. Neuronal tissue (brain) from five different C57BL6/J mice at 2 months of age was harvested and used for mRNA extraction. mRNA was reversed transcribed to cDNA and transcripts were detected through PCR amplification and DNA agarose gel separation. Primers for exon 1, exon 5b and exon 11 of the myelin basic protein gene were used to capture all the possible transcripts that are naturally found in the murine central nervous system. Unknown transcript was sequenced at Genewiz facilities (South Plainfield, NJ) and mass spectrometry protein sequence analysis demonstrated the presence of a novel myelin basic protein transcript variant. We identified a novel transcript variant of myelin basic protein. This novel transcript variant corresponds to a myelin basic protein of 32.5 kDa which has not been previously reported. This novel transcript variant presents relevant clinical significance to various demyelinating diseases due to its contribution to the understanding of the natural state of the murine central nervous system.

Estrogen receptors and estetrol-dependent neuroprotective actions: a pilot study

Estetrol (E4) has strong antioxidative, neurogenic and angiogenic effects in neural system resulting in the attenuation of neonatal hypoxic-ischemic encephalopathy. We aimed to define the role of estrogen receptors in E4-dependent actions in neuronal cell cultures and prove the promyelinating effect of E4. In vitro the antioxidative and cell survival/proliferating effects of E4 on H₂O₂-induced oxidative stress in primary hippocampal cell cultures were studied using different combinations of specific inhibitors for ERα (MPP dihydrochloride), ERβ (PHTTP), GPR30 (G15) and palmytoilation (2-BR). LDH activity and cell survival assays were performed. In vivo the promyelinating role of different concentrations of E4 (1 mg/kg/day, 5 mg/kg/day, 10 mg/kg/day, 50 mg/kg/day) was investigated using the hypoxic-ischemic brain damage model in the 7-day-old immature rats before/after the induction of hypoxic-ischemic insult. Myelin basic protein (MBP) immunostaining was performed on brain coronal sections. Our results show that LDH activity is significantly upregulated in cell cultures where the E4's effect was completely blocked by concomitant treatment either with ERα and ERβ inhibitors (MPP and PHTPP, respectively), or ERα and ERβ inhibitors combined with 2-BR. Cell survival is significantly downregulated in cell cultures where the effect of E4 was blocked by ERβ inhibitor (PHTTP) alone. The blockage of GRP30 receptor did affect neither LDH activity nor cell survival. MBP immunostaining is significantly upregulated in E4-pretreated groups at a concentration of 5 mg/kg/day and 50 mg/kg/day E4, whereas the MBP-positive area OD ratio is significantly increased in all the E4-treated groups. E4's antioxidative actions mostly depend on ERα and ERβ, whereas neurogenesis and possibly promyelinating activities might be realized through ERβ.

Time-Dependent Progression of Demyelination and Axonal Pathology in MP4-Induced Experimental Autoimmune Encephalomyelitis

Background

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS) characterized by inflammation, demyelination and axonal pathology. Myelin basic protein/proteolipid protein (MBP-PLP) fusion protein MP4 is capable of inducing chronic experimental autoimmune encephalomyelitis (EAE) in susceptible mouse strains mirroring diverse histopathological and immunological hallmarks of MS. Limited availability of human tissue underscores the importance of animal models to study the pathology of MS.

Methods

Twenty-two female C57BL/6 (B6) mice were immunized with MP4 and the clinical development of experimental autoimmune encephalomyelitis (EAE) was observed. Methylene blue-stained semi-thin and ultra-thin sections of the lumbar spinal cord were assessed at the peak of acute EAE, three months (chronic EAE) and six months after onset of EAE (long-term EAE). The extent of lesional area and inflammation were analyzed in semi-thin sections on a light microscopic level. The magnitude of demyelination and axonal damage were determined using electron microscopy. Emphasis was put on the ventrolateral tract (VLT) of the spinal cord.

Results

B6 mice demonstrated increasing demyelination and severe axonal pathology in the course of MP4-induced EAE. In addition, mitochondrial swelling and a decrease in the nearest neighbor neurofilament distance (NNND) as early signs of axonal damage were evident with the onset of EAE. In semi-thin sections we observed the maximum of lesional area in the chronic state of EAE while inflammation was found to a similar extent in acute and chronic EAE. In contrast to the well-established myelin oligodendrocyte glycoprotein (MOG) model, disease stages of MP4-induced EAE could not be distinguished by assessing the extent of parenchymal edema or the grade of inflammation.

Conclusions

Our results complement our previous ultrastructural studies of B6 EAE models and suggest that B6 mice immunized with different antigens constitute useful instruments to study the diverse histopathological aspects of MS.

Altered interhemispheric and temporal lobe white matter microstructural organization in severe chronic schizophrenia

Diffusion MRI investigations in schizophrenia provide evidence of abnormal white matter (WM) microstructural organization as indicated by reduced fractional anisotropy (FA) primarily in interhemispheric, left frontal and temporal WM. Using tract-based spatial statistics (TBSS), we examined diffusion parameters in a sample of patients with severe chronic schizophrenia. Diffusion MRI data were acquired on 19 patients with chronic severe schizophrenia and 19 age- and gender-matched healthy controls using a 64 gradient direction sequence, (b=1300 s/mm(2)) collected on a Siemens 1.5T MRI scanner. Diagnosis of schizophrenia was determined by Diagnostic and Statistical Manual for Mental Disorders 4th Edition (DSM-IV) Structured Clinical Interview for DSM disorder (SCID). Patients were treatment resistance, having failed to respond to at least two antipsychotic medications, and had prolonged periods of moderate to severe positive or negative symptoms. Analysis of diffusion parameters was carried out using TBSS. Individuals with chronic severe schizophrenia had significantly reduced FA with corresponding increased radial diffusivity in the genu, body, and splenium of the corpus callosum, the right posterior limb of the internal capsule, right external capsule, and the right temporal inferior longitudinal fasciculus. There were no voxels of significantly increased FA in patients compared with controls. A decrease in splenium FA was shown to be related to a longer illness duration. We detected widespread abnormal diffusivity properties in the callosal and temporal lobe WM regions in individuals with severe chronic schizophrenia who have not previously been exposed to clozapine. These deficits can be driven by a number of factors that are indistinguishable using in vivo diffusion-weighted imaging, but may be related to reduced axonal number or packing density, abnormal glial cell arrangement or function, and reduced myelin.

Role of galactosylceramide and sulfatide in oligodendrocytes and CNS myelin: formation of a glycosynapse

The two major glycosphingolipids of myelin, galactosylceramide (GalC) and sulfatide (SGC), interact with each other by trans carbohydrate-carbohydrate interactions in vitro. They face each other in the apposed extracellular surfaces of the multilayered myelin sheath produced by oligodendrocytes and could also contact each other between apposed oligodendrocyte processes. Multivalent galactose and sulfated galactose, in the form of GalC/SGC-containing liposomes or silica nanoparticles conjugated to galactose and galactose-3-sulfate, interact with GalC and SGC in the membrane sheets of oligodendrocytes in culture. This interaction causes transmembrane signaling, loss of the cytoskeleton and clustering of membrane domains, similar to the effects of cross-linking by anti-GalC and anti-SGC antibodies. These effects suggest that GalC and SGC could participate in glycosynapses, similar to neural synapses or the immunological synapse, between GSL-enriched membrane domains in apposed oligodendrocyte membranes or extracellular surfaces of mature myelin. Formation of such glycosynapses in vivo would be important for myelination and/or oligodendrocyte/myelin function.

Myelin basic protein accumulation is impaired in a model of protein deficiency during development

During the development of the central nervous system (CNS) there is a great possibility of permanent effects in consequence of environmental disturbances. Nutritional deficiency is one of the factors that impair the normal CNS formation. In general, the protein deficiency evokes, beyond the damages in the maturation of nervous system, several consequences in body growth, biochemical maturation, motor function and the major cognitive functions. These effects were observed in undernourished children all over the world. Even in a restricted period, the malnutrition status may evoke permanent impairments in feeding behavior and in metabolism. Rats submitted to malnutrition during development, showed a marked decrease in the number of myelinated fibers. This condition may reflect a failure in the beginning of the wrapping of axons by oligodendroglial processes and/or a delay in the myelin synthesis. Myelin basic protein (MBP) is an intracellular oligodendrocyte protein that is directly related to the formation of the myelin sheath. In this study we verified the temporal pattern of MBP expression, by immunohistochemical and immunoblotting analyses, in a model of protein malnutrition induced during the first half of the lactation period. We showed that MBP expression was impaired in our malnutrition model and that some of the effects were maintained in adulthood, with possible consequences in the maturation of myelin sheath.

Normal myelination: a practical pictorial review

MRI is currently the best imaging modality to assess myelin maturation in the human brain. Myelin is the insulator for nerves and is present in both the peripheral nervous system and the central nervous system (CNS). In the CNS, it is a modified extension of the oligodendrocyte cell and is made up of multiple sheaths of protein-lipid-protein-lipid-protein. Standard T1-weighted and T2-weighted sequences can be performed on any MR imaging platform and with knowledge of normal age-related myelin maturation, myelin delay can be detected. Myelination progresses in a constant predetermined pattern from bottom to top, central to peripheral and back to front.

Peroxisome proliferator-activated receptor γ agonists accelerate oligodendrocyte maturation and influence mitochondrial functions and oscillatory Ca(2+) waves

We have previously shown that natural (15-deoxy-Δ-prostaglandin J2) and synthetic (pioglitazone) agonists of peroxisome proliferator-activated receptor γ (PPAR-γ) strengthen the intrinsic cellular mechanisms protecting oligodendrocyte (OL) progenitors (OPs) from oxidative insults and promote their differentiation. Here, we demonstrate that repeated administrations of PPAR-γ agonists to OP cultures accelerate their differentiation to OLs, as indicated by increased numbers of O4- and O1-positive cells that show increased myelin basic protein expression, elaborated cholesterol-enrichedmembranes and have increased peroxisomes. Moreover, PPAR-γ agonist-treated OLs show increased activity of the mitochondrial respiratory chain Complex IV and an increased ability to respond to environmental signals, such as adenosine diphosphate (ADP), with oscillatory Ca waves; the latter closely correlated with the presence of mitochondria and were inhibited by the mitochondrial respiratory chain Complex I inhibitor rotenone. Because Ca oscillations and mitochondrial respiratory chain activity play crucial roles in OL differentiation, these findings suggest that PPAR-γ agonists could protect OLs and promote myelination through several mechanisms, including those involving mitochondrial functions. Our studies support the therapeutic potential of PPAR-γ agonists in brain diseases in which mitochondrial alteration, oxidative stress, and demyelination occur and point to the need for a better understanding of the role of PPAR-γ and its agonists in OL biology.

How histone deacetylases control myelination

Myelinated axons are a beautiful example of symbiotic interactions between two cell types: Myelinating glial cells organize axonal membranes and build their myelin sheaths to allow fast action potential conduction, while axons regulate myelination and enhance the survival of myelinating cells. Axonal demyelination, occurring in neurodegenerative diseases or after a nerve injury, results in severe motor and/or mental disabilities. Thus, understanding how the myelination process is induced, regulated, and maintained is crucial to develop new therapeutic strategies for regeneration in the nervous system. Epigenetic regulation has recently been recognized as a fundamental contributing player. In this review, we focus on the central mechanisms of gene regulation mediated by histone deacetylation and other key functions of histone deacetylases in Schwann cells and oligodendrocytes, the myelinating glia of the peripheral and central nervous systems.

Fatigue and cognition in patients with relapsing multiple sclerosis treated with interferon β

INTRODUCTION

Fatigue and cognitive deficits are common symptoms affecting patients with multiple sclerosis.

METHODS

The effects of interferon beta on fatigue and cognitive deficits were assessed in 50 patients with relapsing multiple sclerosis (recruited at a single center). The pre-treatment assessments were performed on visits 1 and 2 (Months 0 and 3). Patients started treatment with subcutaneous interferon beta-1a or beta-1b, or intramuscular interferon beta-1a at Month 3, with reassessment at visits 3 and 4 (6 and 12 months, respectively). Co-primary endpoints were change in fatigue (Modified Fatigue Impact Scale) and change in cognition (Brief Repeatable Battery of Neuropsychological Tests) from pre-treatment to visits 3 and 4. Follow-up data were obtained for 40 patients.

RESULTS

The pre-treatment demographic and disease characteristics did not differ between groups. Improvements in fatigue levels were reported for patients receiving subcutaneous interferon beta-1a versus patients in the intramuscular interferon beta-1a group (p = .04) and in the interferon beta-1b group (p = .09). Improvements were also reported in five out of 17 cognitive indices for all the treatment groups.

CONCLUSION

The data suggest that interferon beta may reduce fatigue and cognitive deficits in patients with relapsing multiple sclerosis. Larger, randomized, and controlled studies are required to confirm our findings.

Pathologic and phenotypic alterations in a mouse expressing a connexin47 missense mutation that causes Pelizaeus-Merzbacher-like disease in humans

Gap junction channels are intercellular conduits that allow diffusional exchange of ions, second messengers, and metabolites. Human oligodendrocytes express the gap junction protein connexin47 (Cx47), which is encoded by the GJC2 gene. The autosomal recessive mutation hCx47M283T causes Pelizaeus-Merzbacher-like disease 1 (PMLD1), a progressive leukodystrophy characterized by hypomyelination, retarded motor development, nystagmus, and spasticity. We introduced the human missense mutation into the orthologous position of the mouse Gjc2 gene and inserted the mCx47M282T coding sequence into the mouse genome via homologous recombination in embryonic stem cells. Three-week-old homozygous Cx47M282T mice displayed impaired rotarod performance but unchanged open-field behavior. 10-15-day-old homozygous Cx47M282T and Cx47 null mice revealed a more than 80% reduction in the number of cells participating in glial networks after biocytin injections into oligodendrocytes in sections of corpus callosum. Homozygous expression of mCx47M282T resulted in reduced MBP expression and astrogliosis in the cerebellum of ten-day-old mice which could also be detected in Cx47 null mice of the same age. Three-month-old homozygous Cx47M282T mice exhibited neither altered open-field behavior nor impaired rotarod performance anymore. Adult mCx47M282T expressing mice did not show substantial myelin alterations, but homozygous Cx47M282T mice, additionally deprived of connexin32, which is also expressed in oligodendrocytes, died within six weeks after birth and displayed severe myelin defects accompanied by astrogliosis and activated microglia. These results strongly suggest that PMLD1 is caused by the loss of Cx47 channel function that results in impaired panglial coupling in white matter tissue.

A role for hemopexin in oligodendrocyte differentiation and myelin formation

Myelin formation and maintenance are crucial for the proper function of the CNS and are orchestrated by a plethora of factors including growth factors, extracellular matrix components, metalloproteases and protease inhibitors. Hemopexin (Hx) is a plasma protein with high heme binding affinity, which is also locally produced in the CNS by ependymal cells, neurons and glial cells. We have recently reported that oligodendrocytes (OLs) are the type of cells in the brain that are most susceptible to lack of Hx, as the number of iron-overloaded OLs increases in Hx-null brain, leading to oxidative tissue damage. In the current study, we found that the expression of the Myelin Basic Protein along with the density of myelinated fibers in the basal ganglia and in the motor and somatosensory cortex of Hx-null mice were strongly reduced starting at 2 months and progressively decreased with age. Myelin abnormalities were confirmed by electron microscopy and, at the functional level, resulted in the inability of Hx-null mice to perform efficiently on the Rotarod. It is likely that the poor myelination in the brain of Hx-null mice was a consequence of defective maturation of OLs as we demonstrated that the number of mature OLs was significantly reduced in mutant mice whereas that of precursor cells was normal. Finally, in vitro experiments showed that Hx promotes OL differentiation. Thus, Hx may be considered a novel OL differentiation factor and the modulation of its expression in CNS may be an important factor in the pathogenesis of human neurodegenerative disorders.

Simvastatin interferes with process outgrowth and branching of oligodendrocytes

Statins have attracted interest as a treatment option for multiple sclerosis (MS) because of their pleiotropic antiinflammatory and immunomodulatory effects. However, contradictory results have been described when they are applied to oligodendrocytes (OLGs), the cell type predominantly affected in MS. In this study we focus on the in vitro effect of statins on process outgrowth in OLN-93 cells, a well-characterized OLG-derived cell line, and primary cultures of neonatal rat OLGs. Application of the lipophilic simvastatin, as low as 0.1-1 μM, disturbs process formation of both cell types, leading to less ramified cells. We show that both protein isoprenylation and cholesterol synthesis are required for the normal differentiation of OLGs. It is further demonstrated that the expression of 2',3'-cyclic-nucleotide-3' phosphodiesterase (CNP) and tubulin is lowered, concomitant with a reduction of membrane-bound CNP as well as tubulin. Therefore, we propose that lack of isoprenylation of CNP could help to explain the altered morphological and biochemical differentiation state of treated OLGs. Moreover, expression of specific myelin markers, such as myelin basic protein, myelin-associated glycoprotein, and myelin oligodendrocyte glycoprotein, was compromised after treatment. We conclude that simvastatin treatment has detrimental effects on OLG process outgrowth, the prior step in (re)myelination, thereby mortgaging long-term healing of MS lesions.

Visualization of cytoplasmic diffusion within living myelin sheaths of CNS white matter axons using microinjection of the fluorescent dye Lucifer Yellow

The compactness of myelin allows for efficient insulation defining rapid propagation of action potentials, but also raises questions about how cytoplasmic access to its membranes is achieved, which is critical for physiological activity. Understanding the organization of cytoplasmic ('water') spaces of myelin is also important for diffusion MRI studies of CNS white matter. Using longitudinal slices of mature rat spinal cord, we monitored the diffusion of the water-soluble fluorescent dye Lucifer Yellow injected into individual oligodendrocytes or internodal myelin. We show that living myelin sheaths on CNS axons are fenestrated by a network of diffusionally interconnected cytoplasmic 'pockets' (1.9 ± 0.2 pockets per 10μm sheath length, n=58) that included Schmidt-Lanterman clefts (SLCs) and numerous smaller compartments. 3-D reconstructions of these cytoplasmic networks show that the outer cytoplasmic layer of CNS myelin is cylindrically 'encuffing', which differs from EM studies using fixed tissue. SLCs were found in different 'open states' and remained stable within a 1-2hour observation period. Unlike the peripheral nervous system, where similarly small (<500Da) molecules diffuse along the whole myelin segment within a few minutes, in mature CNS this takes more than one hour. The slower cytoplasmic diffusion in CNS myelin possibly contributes to its known vulnerability to injury and limited capacity for repair. Our findings point to an elaborate cytoplasmic access to compact CNS myelin. These results could be of relevance to MRI studies of CNS white matter and to CNS repair/regeneration strategies.

A novel decalcification method for adult rodent bone for histological analysis of peripheral-central nervous system connections

Histological analysis of bone encased tissue is severely hampered by technical difficulties associated with sectioning calcified tissue. Cryosectioning of bone is possible but requires significant technical adaptation and expensive materials and is often time-consuming. Some decalcifying reagents in common use result in successful cryosectioning in less time but the integrity of the soft tissue of the spinal column is often compromised during processing. This can result in significant loss of cellular detail. In order to find a method that would allow cryosectioning of the bone without loss of structural integrity of the underlying soft tissue we assessed the efficacy of four different decalcifying reagents with respect to their effects on the cellular structure of the myelin of the grey and white matter of the spinal cord. The antigenic integrity of the spinal cord white matter was evaluated using tissue structural integrity and quality of myelin basic protein immunostaining. The result of this research shows that 6% TCA not only decalcifies intact spinal column suitably for cryosectioning but does so without compromising the antigenic integrity of the tissue. The ease of application, speed of processing and a favorable cost-effective profile were secondary benefits noted with the use of the 6% TCA decalcifying solution. The ability to utilize a decalcifying solution that allows for both histomorphometry and immunohistochemistry in the same spinal column segment represents a novel technique that will provide new insights into pathophysiological aspects and therapeutic approaches ispinal cord damage or disease.

Magnetic resonance imaging of myelin

The ability to measure myelin in vivo has great consequences for furthering our knowledge of normal development, as well as for understanding a wide range of neurological disorders. The following review summarizes the current state of myelin imaging using MR. We consider five MR techniques that have been used to study myelin: 1) conventional MR, 2) MR spectroscopy, 3) diffusion, 4) magnetization transfer, and 5) T2 relaxation. Fundamental studies involving peripheral nerve and MR/histology comparisons have aided in the interpretation and validation of MR data. We highlight a number of important findings related to myelin development, damage, and repair, and we conclude with a critical summary of the current techniques available and their potential to image myelin in vivo.

Ultrastructural identification of oligodendrocyte/myelin proteins in corpus callosum of hypothyroid animals

Thyroid hormone (T3) deficiency impairs the development of the CNS, particularly myelination. We have previously described an increase in the frequency of morphological abnormalities in the central myelin sheath in a hypothyroidism model, which reinforced the hypothesis of a role for T3 in myelin compaction. However, there are no data concerning the cellular distribution of myelin proteins in hypothyroid animals. In the present work, we describe the distribution of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), myelin basic protein (MBP) and proteolipid protein (PLP) throughout the central myelin sheath of a hypothyroidism model. We used euthyroid and hypothyroid adult rats at 90 days of age. In order to induce hypothyroid status, animals received 0.02% methimazol from the 19th gestation day onwards. After perfusion with a fixative mixture, small pieces of corpus callosum were obtained, dehydrated and embedded in LR White resin. Ultrathin sections were immunoreacted, using specific antibodies revealed by a secondary antibody coupled to colloidal gold particles of 10nm. Gold particle density per region of myelin sheath for each one of these proteins was obtained. In normal animals, CNPase, PLP and MBP were identified in sites that had already been described in previous studies. In hypothyroid animals, CNPase was identified in the region corresponding to compact lamellae, which normally does not contain this protein, while, in this same region, PLP and MBP immunolabeling were decreased. These results suggest that thyroid hormone deficiency impairs the distribution of the major oligodendrocyte/myelin markers. This effect may justify the reduction in myelin sheath compaction previously demonstrated in a similar model of hypothyroidism.

Restoration of FcRγ/Fyn signaling repairs central nervous system demyelination

Disruption of myelin causes severe neurological diseases. An understanding of the mechanisms that control myelination and remyelination is needed to develop therapeutic strategies for demyelinating diseases such as multiple sclerosis (MS). Our previous finding indicating the critical involvement of the gamma chain of immunogloblin Fc receptors (FcRgamma) and Fyn signaling in oligodendrocyte differentiaion and myelination demands a fundamental revision of the strategies used for MS therapy, because antigen-antibody complexes in MS patients may induce the direct dysregulation of myelination process as well as the inflammatory destruction of myelin sheath. Here we show that the FcRgamma/Fyn signaling cascade is critically involved in cuprizone-induced demyelination/remyelination, with no lymphocytic response. The levels of phosphorylated myelin basic proteins (p-MBPs), especially the 21.5-kDa isoform, but not the levels of total MBPs, decreased markedly during demyelination induced by aging, cuprizone treatment, and double knockout of FcRgamma/Fyn genes. We also showed that the recovery from demyelination in cuprizone-treated and aged mice is achieved after administration of the herbal medicine Ninjin'yoeito, an effective therapy targeting the FcRgamma/Fyn-Rho (Rac1)-MAPK (P38 MAPK)-p-MBPs signaling cascade. These results suggest that the restoration of FcRgamma/Fyn signaling represents a new approach for the treatment of demyelinating diseases.

Coated Glass and Vicryl Microfibers as Artificial Axons

The complex interactions that occur between oligodendrocytes and axons during the process of central nervous system myelination and remyelination remain unclear. Elucidation of the cell-biological and -biochemical mechanisms supporting myelin production and elaboration requires a robust in vitro system that recapitulates the relationship between axons and oligodendrocytes in a manner that is open to molecular dissection. We provide evidence for an artificial axon culture system in which we observed oligodendrocytes extending large plasma membrane projections that frequently completely ensheathed fibers coated with a variety of extracellular matrix molecules. These membrane projections varied in extent and thickness depending upon the substrate and upon the diameter of the coated fiber. Matrigel-coated glass microfibers were found to support the development of thick membrane sheaths that extended for hundreds of microns and exhibited many features suggestive of the potential for true myelin deposition. Likewise, Matrigel-coated Vicryl fibers supported plasma membrane extensions that covered extremely large surface areas and occasionally wrapped the coated Vicryl fibers in more than one membrane layer. Our findings suggest that the deposition of molecular cues onto glass or polymer fibers either via adsorption or chemical modification may be a useful tool for the discovery or validation of axonal factors critical for myelination and remyelination. Herein, we provide evidence that polyglactin 910 and glass microfibers coated with adhesion factors may provide a reasonable system for the in vitro analysis of myelination, and may eventually serve a role in engineering artificial systems for neural repair.

Partitioning of myelin basic protein into membrane microdomains in a spontaneously demyelinating mouse model for multiple sclerosisThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

We have characterized the lipid rafts in myelin from a spontaneously demyelinating mouse line (ND4), and from control mice (CD1 background), as a function of age and severity of disease. Myelin was isolated from the brains of CD1 and ND4 mice at various ages, and cold lysed with 1.5% CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulphonate). The lysate was separated by low-speed centrifugation into supernatant and pellet fractions, which were characterized by Western blotting for myelin basic protein (MBP) isoforms and their post-translationally modified variants. We found that, with maturation and with disease progression, there was a specific redistribution of the 14–21.5 kDa MBP isoforms (classic exon-II-containing vs exon-II-lacking) and phosphorylated forms into the supernatant and pellet. Further fractionation of the supernatant to yield detergent-resistant membranes (DRMs), representing coalesced lipid rafts, showed these to be highly enriched in exon-II-lacking MBP isoforms, and deficient in methylated MBP variants, in mice of both genotypes. The DRMs from the ND4 mice appeared to be enriched in MBP phosphorylated by MAP kinase at Thr95 (murine 18.5 kDa numbering). These studies indicate that different splice isoforms and post-translationally modified charge variants of MBP are targeted to different microdomains in the myelin membrane, implying multifunctionality of this protein family in myelin maintenance.

Thyroid hormone deficiency changes the distribution of oligodendrocyte/myelin markers during oligodendroglial differentiation in vitro

Myelination depends on the proper differentiation of oligodendrocytes and several factors may influence this event. For instance, thyroid hormone (T3) affects the timing of differentiation and regulates the expression of several enzymes involved in the synthesis of complex lipids and in the expression of some myelin structural proteins. We investigated the effect of T3 deficiency on oligodendroglial differentiation and in the distribution of oligodendrocyte/myelin proteins 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and myelin basic protein (MBP). Oligodendroglial-enriched cultures were obtained from cerebra of neonate rats grown in a modified medium. The T3-deficient status was induced by using medium devoid of T3. We observed a delay, in T3-deficient cultures, in oligodendroglial maturation characterized by less extensive processes and membrane vellum than in controls. In control cultures, CNPase immunoreactivity was punctated, showing cell bodies and processes at earlier stages and redistribution to cytoskeleton vein-like structures in later stages. In T3-deficient cultures, CNPase remained in a punctated pattern and only at 10 days in vitro we observed CNPase redistribution to the presumptive cytoskeleton vein-like structures. MBP in control cultures was distributed through the whole cell body and processes whereas in T3-deficient cultures, MBP immunoreactivity was concentrated in the perinuclear region. These results reinforce the hypothesis that T3 is an important factor in oligodendrocyte differentiation, particularly regarding the distribution of myelin proteins.

White Matter Rafting––Membrane Microdomains in Myelin

The myelin membrane comprises a plethora of regions that are compositionally, ultrastructurally, and functionally distinct. Biochemical dissection of oligodendrocytes, Schwann cells, and central and peripheral nervous system myelin by means such as cold-detergent extraction and differential fractionation has led to the identification of a variety of detergent-resistant membrane assemblies, some of which represent putative signalling platforms. We review here the different microdomains that have hitherto been identified in the myelin membrane, particularly lipid rafts, caveolae, and cellular junctions such as the tight junctions that are found in the radial component of the CNS myelin sheath.

Expression profiling reveals multiple myelin alterations in murine succinate semialdehyde dehydrogenase deficiency

Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare genetic defect of GABA degradation recently modelled in mice (SSADH(-/-) mice), manifests early absence seizures that evolve into generalized convulsive seizures and lethal status epilepticus in gene-ablated mice. Disrupted GABA homeostasis, in conjunction with the epileptic phenotype and increased gamma-hydroxybutyric acid (GHB), suggested that expression profiling with the U74Av2 Affymetrix system would reveal dysregulation of receptor genes associated with GABAergic and glutamatergic neurotransmission. Unexpectedly, we found significant downregulation for genes associated with myelin biogenesis and compaction, predominantly in hippocampus and cortex. These results were confirmed by: (1) myelin basic protein (MBP) immunohistochemistry; (2) western blotting of myelin-associated glycoprotein (MAG) and MBP; (3) qRT-PCR analyses of myelin-associated oligodendrocytic basic protein (MOBP), MAG, MBP and proteolipid protein (PLP) in hippocampus, cortex and spinal cord; (4) quantitation of ethanolamine and choline plasmalogens, all core myelin components; (5) evaluation of myelin content in brain sections employing toluidine blue staining; and (6) ultrastructural evaluation of myelin sheath thickness via electron microscopy. We speculate that increased GABA/GHB, acting through GABAergic systems, results in decreased levels of the neurosteroids progesterone and allopregnanolone [Gupta et al (2003) Ann Neurol 54(Supplement 6): S81-S90] and phosphorylation of mitogen-activated protein (MAP) kinase, with resulting myelin protein abnormalities primarily in the cortex of SSADH(-/-) mice.

Markers for OLN-93 oligodendroglia differentiation

Oligodendrocytes are target cells in the pathogenesis of multiple sclerosis (MS), a chronic demyelinating disease of the central nervous system (CNS). During the course of the disease, inflammatory mediators may damage oligodendrocytes and their myelin sheaths. Differentiation of oligodendrocyte progenitors is an important step in the process of remyelination. In the present study, OLN-93 differentiation was studied in co-culture with C6 astrocytes as a natural source of growth and differentiation factors as well as after exposure to insulin-like growth factor-I (IGF-I). Morphological evaluation showed an increased degree of differentiation of OLN-93 cells after IGF-I administration, but not after co-culture with astrocytes. During early differentiation, 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and zonula occludens-1 (ZO-1) tight junction protein expression were significantly increased. However, neither astrocyte co-culture nor exposure to IGF-I further increased the expression of these markers. Although reverse transcriptase-polymerase chain reaction revealed myelin basic protein (MBP) mRNA expression not to be affected during differentiation, we did find increased MBP protein expression by Western blotting. ZO-1 protein and DM20 mRNA levels were increased during the course of differentiation and after IGF-I administration. The present findings suggest that ZO-1 may be used as a marker for OLN-93 oligodendroglia differentiation.

Charge effects modulate actin assembly by classic myelin basic protein isoforms

Myelin basic protein (MBP), a highly cationic structural protein of the myelin sheath, is believed to be associated with the cytoskeleton in vivo and interacts with actin in vitro, but little is known about the regulation of this interaction. The rate and extent of actin polymerization induced by 18.5 kDa MBP charge isomers were correlated to charge reduction by post-translational modifications. Increased ionic strength attenuated the initial rate but not the final extent of polymerization achieved. Reduced pH enhanced the rate and extent of polymerization, presumably via partial protonation of intrinsic histidyl residues. The polymerizing activities of the 21.5, 17, and 14 kDa MBP splice variants were not proportionate to their net charges or charge densities. The presence of at least one region derived from exon II or VI of the "classic" MBP gene was required for effective bundling as assessed by light scattering and transmission electron microscopy.

Developmental partitioning of myelin basic protein into membrane microdomains

Specific membrane microdomains (including lipid rafts) exist in myelin but have not been fully characterized. Myelin basic protein (MBP) maintains the compactness of the myelin sheath and is highly posttranslationally modified. Thus, it has been suggested that MBP might also have other functions, e.g., in signal transduction. Here, the distribution of MBP and its modified forms was studied, spatially and temporally, by detailed characterization of membrane microdomains from developing and mature bovine myelin. Myelin membranes were extracted with three different detergents (Brij 96V, CHAPS, or Triton X-100) at 4 degrees C. The detergent-resistant membranes (DRMs), representing coalesced lipid rafts, were isolated as low-buoyant-density fractions on a sucrose density gradient. These myelin rafts were disrupted when cholesterol was depleted with methyl-beta-cyclodextrin. The use of CHAPS detergent led to enrichment of several myelin proteins, including phospho-Thr97-MBP, in the DRMs from mature myelin. Citrullinated and methylated MBP remained in "nonraft" microdomains. In contrast, the DRMs from early myelin were enriched in Golli-MBP, Fyn, Lyn, and CNP. The localization of various proteins in DRMs was further supported by the colocalization of these lipid raft components in cultured mouse oligodendrocytes. Thus, there is a developmental regulation of posttranslationally modified forms of MBP into specific membrane microdomains.

Electron paramagnetic resonance spectroscopy and molecular modelling of the interaction of myelin basic protein (MBP) with calmodulin (CaM)—diversity and conformational adaptability of MBP CaM-targets

The classic 18.5 kDa isoform of murine myelin basic protein (mMBP) has been shown to bind calmodulin (CaM) strongly and specifically in vitro. Here, we have used site-directed spin labelling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy to map more precisely the sites of interaction of recombinant mMBP (rmMBP) with CaM. On the basis of these and previous experimental data, and the predictions of CaM-binding motifs using the Calmodulin Target Database (), three main segments of MBP were suggested for the interaction. The first site is located at the C-terminus; the second one lies in the central portion of the protein and forms an amphipathic alpha-helix in reconstituted myelin-mimetic systems; the third is quite close to the N-terminus. The murine Golli-MBP isoform J37 has also been shown to bind CaM in vitro, and an interaction site was predicted in the N-terminal Golli-specific portion of the protein. From these four segments, we selected peptide fragments of 12-14 residues in length, chosen on the bases of their amphipathicity and CaM-target characteristics. We modelled each of these peptides as alpha-helices, and performed docking simulations to investigate their interactions with the CaM peptide-binding tunnel. Different yet almost equally favourable CaM-binding modes were found for each of them. The experimental SDSL/EPR and theoretical modelling results were in good agreement, and supported the conjecture that there are several plausible CaM-binding sites in MBP, that could be induced into an alpha-helical conformation by their interaction with CaM and account for strong immobilisation of spin-labeled residues in all three segments. Phosphorylation and deimination were also emulated and simulated for known sites of MBP post-translational modification. The results obtained confirmed the appropriate utilisation of simple residue substitutions to mimic the natural modifications, and demonstrated molecular mechanisms by which MBP-CaM interactions could be modulated in vivo.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

The myelinated axon is dependent on the myelinating cell for support and maintenance: molecules involved

The myelin-forming cells, oligodendrocytes and Schwann cells, extend processes that spirally wrap axons and provide the insulation that allows rapid saltatory conduction. Recent data suggest a further role for the myelin-forming cells in axonal support and maintenance. This Mini-Review summarises some of the data that support this view and highlights the molecules involved.

Comparison of in vivo and in vitro subcellular localization of estrogen receptors alpha and beta in oligodendrocytes

The existence of estrogen receptors (ERs) in oligodendrocytes (OLGs) in vivo and in vitro is unresolved, as their presence has been reported in some studies and their absence in others. Using molecular and immunocytochemical techniques, we describe the subcellular localization of ERalpha and ERbeta in OLGs in vivo and in vitro. Both ERalpha and ERbeta are detected in an immortalized OLG cell line and in enriched OLG cultures by RT-PCR and western blot. Immunocytochemistry of OLGs from enriched cultures shows ERalpha receptors are nuclear, whereas ERbeta receptors are cytoplasmic. Confocal and deconvolution microscopy of enriched OLG cultures reveals ERbeta immunoreactivity is concentrated in perikarya and veins of OLG membrane sheets; lesser reactivity is present in their plasma membranes and nuclei. In vivo, we readily detect ERalpha in neurons but not in OLGs, even though we used different fixation procedures and different ERalpha antibodies. The presence of ERalpha in cultured OLGs may be due to culture media that contains factors stimulating ERalpha expression but are reduced in normal brain. In vivo, ERbeta immunoreactivity is readily detectable in OLG cytoplasm and in myelin sheaths. Incubation of glial cultures without or with increasing concentrations of 17beta-estradiol (E2) shows that E2 significantly accelerates OLG process formation.