Myelin-associated glycoprotein in multiple sclerosis lesions: a quantitative and qualitative analysis

Myelinodegeneration vs. Neurodegeneration in MS Progressive Forms

In MS patients with a progressive form of the disease, the slow deterioration of neurological functions is thought to result from a combination of neuronal cell death, axonal damages and synaptic dysfunctions [...].

Irrespective of Plaque Activity, Multiple Sclerosis Brain Periplaques Exhibit Alterations of Myelin Genes and a TGF-Beta Signature

In a substantial share of patients suffering from multiple sclerosis (MS), neurological functions slowly deteriorate despite a lack of radiological activity. Such a silent progression, observed in either relapsing-remitting or progressive forms of MS, is driven by mechanisms that appear to be independent from plaque activity. In this context, we previously reported that, in the spinal cord of MS patients, periplaques cover large surfaces of partial demyelination characterized notably by a transforming growth factor beta (TGF-beta) molecular signature and a decreased expression of the oligodendrocyte gene NDRG1 (N-Myc downstream regulated 1). In the present work, we re-assessed a previously published RNA expression dataset in which brain periplaques were originally used as internal controls. When comparing the mRNA profiles obtained from brain periplaques with those derived from control normal white matter samples, we found that, irrespective of plaque activity, brain periplaques exhibited a TGF-beta molecular signature, an increased expression of TGFB2 (transforming growth factor beta 2) and a decreased expression of the oligodendrocyte genes NDRG1 (N-Myc downstream regulated 1) and MAG (myelin-associated glycoprotein). From these data obtained at the mRNA level, a survey of the human proteome allowed predicting a protein-protein interaction network linking TGFB2 to the down-regulation of both NDRG1 and MAG in brain periplaques. To further elucidate the role of NDRG1 in periplaque-associated partial demyelination, we then extracted the interaction network linking NDRG1 to proteins detected in human central myelin sheaths. We observed that such a network was highly significantly enriched in RNA-binding proteins that notably included several HNRNPs (heterogeneous nuclear ribonucleoproteins) involved in the post-transcriptional regulation of MAG. We conclude that both brain and spinal cord periplaques host a chronic process of tissue remodeling, during which oligodendrocyte myelinating functions are altered. Our findings further suggest that TGFB2 may fuel such a process. Overall, the present work provides additional evidence that periplaque-associated partial demyelination may drive the silent progression observed in a subset of MS patients.

Healthy attachments: Cell adhesion molecules collectively control myelin integrity

Many cell adhesion molecules are present along myelinated axons and in myelinating glia, but functional interactions among these proteins have not been fully elucidated. In this issue, Elazar et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201906099) report that distinct adhesion proteins act in coordination to ensure accurate myelination.

Computer design, synthesis, and bioactivity analyses of drugs like fingolimod used in the treatment of multiple sclerosis

Multiple sclerosis (MS) is a very common disease of vital importance. In the MS treatment, some drugs such as fingolimod which help to protect nerves from damage are used. The main goal of the drug therapy in MS is to take control of the inflammation which leads to the destruction of myelin and axons in nerve cell and thus prevent and stop the progression of the disease. Fingolimod (FTY720) is an orally active immunomodulatory drug that has been used for the treatment of relapsing-remitting multiple sclerosis. It is a sphingosine-1-phosphate receptor modulator which prevents lymphocytes from contributing to an autoimmune reaction by inhibiting egress of lymphocytes them from lymph nodes. In this study, we have computer designed, synthesized and characterized two novel derivatives of FTY720, F1-12h and F2-9, and have determined their underlying mechanism of their beneficial effect in SH-SY5Y, SK-N-SH, and U-118 MG cell lines. For this purpose, we first determined the regulation of the cAMP response element (CRE) activity and cAMP concentration by F1-12h and F2-9 together with FTY720 using pGL4.29 luciferase reporter assay and cAMP immunoassay, respectively. Then, we have determined their effect on MS- and GPCR-related gene expression profiles using custom arrays along with FTY720 treatment at non-toxic doses (EC10). It was found that both derivatives significantly activate CRE and increase cAMP concentration in all three cell lines, indicating that they activate cAMP pathway through cell surface receptors as FTY720 does. Furthermore, F1-12h and F2-9 modulate the expression of the pathway related genes that are important in inflammatory signaling, cAMP signaling pathway, cell migration as well as diverse receptor and transcription factors. Expression of the genes involved in myelination was also increased by the treatment with F1-12h and F2-9. In summary, our data demonstrate that the two novel FTY720 derivatives act as anti-inflammatory ultimately by influencing the gene expression via the cAMP and downstream transcription factor CRE pathway. In conclusion, F1-12h and F2-9 might contribute future therapies for autoimmune diseases such as multiple sclerosis.

Cleavage of myelin associated glycoprotein by matrix metalloproteinases

Derivative myelin associated glycoprotein (dMAG) results from proteolysis of transmembrane MAG and can inhibit axonal growth. We have tested the ability of certain matrix metalloproteinases (MMPs) elevated with inflammatory and demyelinating diseases to cleave MAG. We show MMP-2, MMP-7 and MMP-9, but not MMP-1, cleave recombinant human MAG. Cleavage by MMP-7 occurs at Leu 509, just distal to the transmembrane domain and, to a lesser extent, at Met 234. We also show that MMP-7 cleaves MAG expressed on the external surface of CHO cells, releasing fragments that accumulate in the medium over periods of up to 48 h or more and that are able to inhibit outgrowth by dorsal root ganglion (DRG) neurons. We conclude that MMPs may have the potential both to disrupt MAG dependent axon-glia communication and to generate bioactive fragments that can inhibit neurite growth.

Degraded myelin-associated glycoprotein (dMAG) formation from pure human brain myelin-associated glycoprotein (MAG) is not mediated by calpain or cathepsin L-like activities

The myelin-associated glycoprotein (MAG) is a transmembrane cell adhesion molecule participating in myelin formation and maintenance. Calcium-activated/-dependent proteolysis of myelin-associated glycoprotein by calpain and cathepsin L-like activities has already been detected in purified myelin fractions, producing a soluble fragment, called degraded (d)MAG, characterized by the loss of the transmembrane and cytoplasmic domains. Here, we demonstrate and analyze dMAG formation from pure human brain myelin-associated glycoprotein. The activity never exhibited the high rate previously reported in human myelin fractions. Degradation is time-, temperature-, buffer- and structure-dependent, is inhibited at 4 degrees C and by denaturation of the sample, and is mediated by a trans-acting factor. There is no strict pH dependency of the proteolysis. Degradation was inhibited by excess aprotinin, but not by 1-10 micro g/mL aprotinin and was not eliminated by the use of an aprotinin-sepharose matrix during the purification. dMAG formation was not enhanced by calcium, nor inhibited by a wide variety of protease inhibitors, including specific calpain and cathepsin L inhibitors. Therefore, while cysteine proteases may be present in human myelin membrane fractions, they are not involved in dMAG formation from highly purified human brain myelin-associated glycoprotein preparations.

Preferential loss of myelin-associated glycoprotein reflects hypoxia-like white matter damage in stroke and inflammatory brain diseases

Destruction of myelin and oligodendrocytes leading to the formation of large demyelinated plaques is the hallmark of multiple sclerosis (MS) pathology. In a subset of MS patients termed pattern III, actively demyelinating lesions show preferential loss of myelin-associated glycoprotein (MAG) and apoptotic-like oligodendrocyte destruction, whereas other myelin proteins remain well preserved. MAG is located in the most distal periaxonal oligodendrocyte processes and primary "dying back" oligodendrogliopathy may be the initial step of myelin degeneration in pattern III lesions. In the present study, various human white matter pathologies, including acute and chronic white matter stroke, virus encephalitis, metabolic encephalopathy, and MS were studied. In addition to a subset of MS cases, a similar pattern of demyelination was found in some cases of virus encephalitis as well as in all lesions of acute white matter stroke. Brain white matter lesions presenting with MAG loss and apoptotic-like oligodendrocyte destruction, irrespective of their primary disease cause, revealed a prominent nuclear expression of hypoxia inducible factor-1alpha in various cell types, including oligodendrocytes. Our data suggest that a hypoxia-like tissue injury may play a pathogenetic role in a subset of inflammatory demyelinating brain lesions.

Multiple MAG peptides are recognized by circulating T and B lymphocytes in polyneuropathy and multiple sclerosis

Abnormal immune responses to myelin associated glycoprotein (MAG), a component of myelin of the central and peripheral nervous system, have been suggested to play a role in the pathogenesis of multiple sclerosis (MS) and certain types of inflammatory polyneuropathy. To identify possible immunodominant MAG peptides in neuroinflammation, we examined T and B cell responses to five selected synthetic MAG peptides and myelin proteins in 21 patients with non-inflammatory polyneuropathy, 26 patients with MS, 10 optic neuritis patients and 17 healthy subjects. Enzyme-linked immunosorbent spot-forming cell assays were adopted, allowing the detection and enumeration of individual antigen responsive T and B cells in body fluids. Patients with polyneuropathy as well as those with MS had elevated levels of T and B cells recognizing MAG and its peptides. Any of the five MAG peptides under study functioned as immunodominant T and/or B cell epitope in individual subjects. None of the MAG peptides elicited a specific disease-associated T or B cell response. The enhanced T and B cell response to myelin components like MAG may play some role in initiation and/or progression of these diseases, but they could also represent secondary responses associated with myelin damage and indicate tolerization rather than autoaggressive immunity.

Identification of single nucleotide variations in the coding and regulatory regions of the myelin-associated glycoprotein gene and study of their association with multiple sclerosis

The myelin-associated glycoprotein (MAG) gene is an appealing candidate in the 19q13 Multiple Sclerosis (MS) candidate region. Using denaturing high performance liquid chromatography (DHPLC), we identified 14 single nucleotide polymorphisms (SNPs) in MAG coding and regulatory regions, and we tested their possible association with MS in Italian patient and control DNA pools. Eight variations had a frequency <0.05, i.e. below the detection limit in the pools. Of these, Arg537Cys was further studied with individually genotyped individuals and was detected in 1/189 patients and 0/85 controls. The frequency of the six remaining SNPs were not significantly different in pools including a total of 1266 patient and 1612 control chromosomes. Considering the statistical power of the experimental design, these results exclude the MAG gene as an MS susceptibility factor with an odds ratio (OR) equal or higher than 1.3.

Oligodendrocytes in aging mice lacking myelin-associated glycoprotein are dystrophic but not apoptotic

Although MAG-null mice myelinate relatively normally except for subtle structural abnormalities in the periaxonal region of myelin sheaths, they develop more severe pathological changes as they age. The purpose of this study was to further define the biochemical aspects of CNS pathology caused by an absence of MAG. Proteins associated with myelin and oligodendrocytes were quantified by densitometry of western blots in whole brain homogenates, as well as in isolated myelinated axons and myelin. Neither myelin yields, nor levels of myelin basic protein and proteolipid protein, were decreased in comparison to control levels in 14-month-old MAG-null mice. On the other hand, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and the 120 kD neural cell adhesion molecule (N-CAM) were substantially reduced in whole brain, myelinated axons, and myelin. Tubulin, Na(+)K(+)ATPase and Fyn tyrosine kinase were also reduced significantly in myelin-related fractions, but not in whole brain homogenate. The decreased levels of these proteins suggest pathological abnormalities in oligodendrocytes. Furthermore, significant reductions of CNPase and 120 kD NCAM were also present at 2 months, indicating that the oligodendroglial abnormalities begin at a relatively early age. Neither TUNEL assays nor multiplex RT-PCR for mRNAs of apoptosis-related proteins in the aging MAG-null mice provided evidence for apoptotic oligodendrocytes. These biochemical findings suggest oligodendroglial damage in MAG-null mice and support the morphological observations pointing to a progressive "dying-back oligodendrogliopathy" as a consequence of MAG deficiency.

A screen of candidates from peaks of linkage: evidence for the involvement of myeloperoxidase in multiple sclerosis

We tested 11 microsatellite markers for evidence of transmission distortion in 744 trio families with multiple sclerosis. Ten of the markers lie within or near to candidate genes selected on the basis that they map within the regions of potential linkage identified in our previously reported linkage genome screen, while the eleventh is an anonymous marker which had previously shown modest evidence for transmission distortion in our sibling pair families. Only the marker related to the myeloperoxidase (MPO) gene revealed tentative evidence for linkage disequilibrium and further work on this gene is clearly needed in order to resolve the status of this region in conferring susceptibility to multiple sclerosis.

Encephalitogenic and neuritogenic T cell responses to the myelin-associated glycoprotein (MAG) in the Lewis rat

Autoimmune responses to the myelin-associated glycoprotein (MAG) are implicated in the immunopathogenesis of both multiple sclerosis (MS) and certain peripheral neuropathies. In this study we demonstrate that T cell responses to defined epitopes of MAG mediate a pathological inflammatory response in the nervous system of the Lewis rat. Peptide-specific T cells were generated against four different MAG epitopes, three of which are common to both L- and S-isoforms of MAG (amino acid (a.a.) sequence: 20-34, 124-137, 354-377) whilst the fourth epitope (a.a. sequence: 570-582) is located in the C-terminal sequence of S-MAG. The adoptive transfer of T cells specific for these epitopes initiated a mild but dose-dependent inflammatory response in the central nervous system (CNS) of naive recipients. Clinical disease was only observed in those animals injected with T cells specific for the a.a. sequence 20-34 (MP1.1), which also initiated an inflammatory response in the peripheral nervous system (PNS). Co-transfer of MP1.1 (a.a. sequence 20-34)-specific T cells with the myelin oligodendrocyte glycoprotein (MOG)-specific monoclonal antibody 8-18C5 enhanced disease severity and induced widespread demyelination in the CNS. In contrast, co-transfer of T cells with the MAG-specific mAb 513 failed to induce demyelination, but had a moderate effect on the local inflammatory response. The ability of MAG to initiate an autoaggressive T cell response in the Lewis rat supports the concept that MAG-specific autoimmune responses may play a role in the pathogenesis of immune mediated diseases of the nervous system in man.

Characterization of myelin-associated glycoprotein (MAG) proteolysis in the human central nervous system

Purified human central nervous system myelin contains an endogenous cysteine protease which degrades the 100-kDa myelin-associated glycoprotein into a slightly smaller 90-kDa derivative called dMAG, and which has been implicated in demyelinating diseases. The native proteolytic site in human MAG was determined in order to characterize this cysteine protease in humans further. This was accomplished by identifying the carboxy-terminus of purified dMAG. The results of these experiments, in conjunction with peptidolysis assays of myelin, demonstrated that the enzyme which proteolyses MAG is extracellular and has cathepsin L-like specificity. Furthermore, it was shown that this cathepsin L-like activity potentially was regulated by the endogenous extracellular inhibitor cystatin C.

Soluble myelin-associated glycoprotein (MAG) found in vivo inhibits axonal regeneration

Myelin-associated glycoprotein (MAG) is a potent inhibitor of axonal regeneration when used as a substrate for growth. However, to be characterized definitively as inhibitory rather than nonpermissive, MAG must also inhibit axonal regeneration when presented in solution. Here, we show that soluble dMAG (extracellular domain only), released in abundance from myelin and found in vivo and chimeric MAG-Fc, can potently inhibit axonal regeneration. For both dMAG and MAG-Fc, inhibition is dose-dependent. If myelin-conditioned medium is immunodepleted of dMAG, or if a MAG antibody is included with MAG-Fc, inhibition is completely neutralized. Together with MAG's ability to induce growth cone collapse, these results demonstrate that MAG is an inhibitory molecule and not merely nonpermissive. The results also suggest that MAG binds to a specific receptor and initiates a signal transduction cascade to effect inhibition. Importantly, these results indicate that soluble dMAG detected in vivo could contribute to the lack of regeneration in the mammalian CNS after injury.

Rapid conversion of myelin-associated glycoprotein to a soluble derivative in primates

Myelin-associated glycoprotein (MAG) is susceptible to proteolysis by a calcium-activated neutral protease which is located in myelin. The conversion of MAG (M(r) 100,000) to its soluble derivative dMAG (M(r) 90,000) occurs much more rapidly in myelin from human white matter than in myelin from rat brain, and the rate of formation of dMAG is increased even more in myelin from white matter of patients with multiple sclerosis (MS). The MAG to dMAG conversion was studied in several species, ranging from mice to non-human primates and humans to determine what animal model would be the most appropriate for investigating the MAG to dMAG reaction in demyelinating disorders. Myelin fractions from brains of these species were prepared and incubated at 37 degrees C in 0.2 M NH4HCO3, pH 7.4 for time periods ranging from 5 min to 24 h. Western blot analysis of the samples, taken at the end points of the different incubation periods, showed that the time required for a 50% conversion of MAG to dMAG was 18-24 h in myelin from rodents to bovine. The non-human primate studies revealed a 50% conversion at 2 h for marmoset samples and rhesus monkey samples, 20 min for gorilla samples and 10 min for chimpanzee samples. Human myelin samples needed only 5 min for a 50% conversion of MAG to dMAG. The reason for the significantly faster formation of dMAG in primate myelin is unknown and currently is being investigated.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Virus-reactive and autoreactive T cells are accumulated in cerebrospinal fluid in multiple sclerosis

Elevated numbers of B cells--plasma cells secreting antibodies to measles and mumps virus, and to myelin associated glycoprotein (MAG), one of several putative myelin autoantigens--have previously been reported in cerebrospinal fluid (CSF) from patients with multiple sclerosis (MS), while it is unknown if corresponding T cell reactivities occur. We have defined the T cell reactivities to measles and mumps virus and to MAG in an immunospot assay which is based on the detection of secretion of interferon-gamma (IFN-gamma) by single cells upon stimulation with specific antigen in short term cultures. Patients with MS had higher numbers of MAG-reactive T cells in blood compared to controls, while no differences were observed for measles or mumps virus-reactive T cells. In CSF, elevated numbers of MAG-reactive T cells and also of measles- and mumps-reactive T cells were found in patients with MS compared to other neurological diseases. A strong accumulation of antigen-reactive T cells was observed in the MS patients' CSF compared to blood. The magnitude of these T cell reactivities did not correlate with clinical MS variables. The T cell repertoire in MS thus includes, besides myelin basic protein, proteolipid protein and myelin oligodendrocyte glycoprotein, also MAG and, in addition, measles and mumps virus. It is not clear whether these T cell reactivities accumulated in the CSF have importance for the pathogenesis of MS or reflect phenomena secondary to myelin damage, or result from both these alternatives.

Down-regulation of myelin-associated glycoprotein on Schwann cells by interferon-gamma and tumor necrosis factor-alpha affects neurite outgrowth

To investigate the influence of inflammatory cytokines on the potential of peripheral nerves to regenerate, we analyzed the effect of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on the ability of immortalized Schwann cells to mediate outgrowth of neurites from primary DRG neurons. We found that IFN-gamma and TNF-alpha synergistically inhibited the neurite outgrowth-promoting properties of the Schwann cells by specifically down-regulating myelin-associated glycoprotein (MAG) at the levels of mRNA and cell surface protein by approximately 60%. Antibodies to MAg inhibited the outgrowth of neurites on Schwann cells to the same extent as treatment with the two cytokines. Since MAG appears to be involved in both neurite outgrowth and myelination, our findings may provide evidence for a mechanism, by which inflammatory cytokines interfere with Schwann cell-neuron interactions.

Differential expression of MAG isoforms during development

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

Central nervous system myelin: structure, function, and pathology

Multiple sclerosis (MS) and a number of related distinctive diseases are characterized by the active degradation of central nervous system (CNS) myelin, an axonal sheath comprised essentially of proteins and lipids. These demyelinating diseases appear to arise from complex interactions of genetic, immunological, infective, and biochemical mechanisms. While circumstances of MS etiology remain hypothetical, one persistent theme involves recognition by the immune system of myelin-specific antigens derived from myelin basic protein (MBP), the most abundant extrinsic myelin membrane protein, and/or another equally susceptible myelin protein or lipid component. Knowledge of the biochemical and physical-chemical properties of myelin proteins and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to the understanding of how and why these antigens become selected during the development of MS. This review focuses on current understanding of the molecular basis underlying demyelinating disease as it may relate to the impact of the various protein and lipid components on myelin morphology; the precise molecular architecture of this membrane as dictated by protein-lipid and lipid-lipid interactions; and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Multiple sclerosis: cells secreting antibodies against myelin-associated glycoprotein are present in cerebrospinal fluid

We evaluated the B-cell response in cerebrospinal fluid (CSF) and blood by enumerating cells secreting antibodies to myelin-associated glycoprotein (MAG) and, for reference, to myelin basic protein (MBP), two myelin components which may constitute targets for autoimmune attack in multiple sclerosis (MS). Among 25 untreated MS patients, 12 had cells in CSF secreting anti-MAG IgG antibodies (mean value 1 per 1429 CSF cells) and three also had cells secreting anti-MAG antibodies of the IgM isotype but at lower levels. In CSF from 2 out of 10 MS patients examined, anti-MAG and anti-MBP IgG antibody-secreting cells were present concurrently. Antibody-secreting cells were less frequent in blood and bone marrow, reflecting compartmentalization to CSF. Anti-MAG antibody-secreting cells were found in CSF from only 1 out of 27 control patients. The intrathecal production of anti-MAG and anti-MBP antibodies may be important in the pathogenesis of MS.

Changes within the "normal" cerebral white matter of multiple sclerosis patients during acute attacks and during high-dose cortisone therapy assessed by means of quantitative MRI

Changes in the apparently unaffected cerebral white matter of multiple sclerosis (MS) patients were studied during acute attacks as well as during high-dose prednisolone therapy. Serial MR scans of patients with a clinically definite diagnosis were performed on four defined occasions: before an episode, within three days after its onset, after 10 days of therapy as well as four weeks later. Thirteen patients agreed to cooperate in forming a MRI data base and to be rescanned immediately after the onset of an acute relapse. Within one year, six patients had such episodes, one of them had a second bout. Both T1 and T2 relaxation times within the apparently normal white matter were significantly prolonged in all cerebral lobes compared to a control group of healthy volunteers. During the acute attacks as well as during therapy the T1 values remained as before. The T2 values were elevated only in two out of six cases during the episode. After therapy a considerable clinical improvement was seen in all cases, but a significant T2 decrease as a possible effect of cortisone was noted in only one case. We conclude that the prolonged relaxation times T1 and T2 within the apparently normal cerebral white matter of MS patients are the result of a number of molecular events differing considerably among individual patients and that serial measurements of these relaxation times do not consistently change during an acute relapse and do not reflect clinical improvement after high dose prednisolone therapy.

Myelin-associated glycoprotein (MAG) and rat brain-specific 1B236 protein: mapping of epitopes and demonstration of immunological identity

The myelin-associated glycoprotein (MAG) and the brain 1B236 protein are 100-kDa glycoproteins containing 30% carbohydrate that exist in two developmentally regulated forms and are specific to the nervous system. Recent cDNA cloning experiments in several laboratories using primarily immunological means of identification have determined the complete primary sequence of a rat brain glycoprotein that seems to correspond to both MAG and 1B236, suggesting that these proteins are identical. However, MAG was previously considered to be an oligodendrocyte/myelin specific component in the CNS at all ages, whereas 1B236 was thought to be primarily a neuronal component in adult rats but synthesized by oligodendrocytes at the time of active myelination. The composite term 1B236/MAG was proposed to describe the molecule identified by the cDNAs. In order to explore further the relationship between MAG and 1B236, as well as their developmentally regulated forms, experiments were carried out on rat samples utilizing synthetic peptides corresponding to sequences throughout the 1B236 molecule, antisera raised to synthetic peptides in the C-terminus of 1B236 that distinguish between the two developmentally regulated forms, and well-characterized polyclonal and monoclonal antibodies raised to purified MAG. Epitope mapping demonstrated that reactive sites were distributed throughout the extracellular and intracellular domains of 1B236/MAG. Only antibodies reacting with the smaller of the two forms of 1B236/MAG detected the glycoprotein in the peripheral nervous system. Both anti-MAG and anti-1B236 antibodies revealed a drastic reduction of the level of 1B236/MAG in 25-day-old myelin-deficient rats and in adult quaking mice, and both types of antibodies revealed a slight shift of 1B236/MAG toward higher apparent Mr in quaking mice as had previously been reported for MAG. The results indicate that MAG and 1B236 are almost certainly identical since they cannot be distinguished immunologically by the reagents available and that quantitatively most of the glycoprotein is associated with oligodendrocytes and myelin rather than neurons at all ages.

Antibodies to myelin-associated glycoprotein (MAG) in the cerebrospinal fluid of multiple sclerosis patients

Cerebrospinal fluids (CSF) and sera from patients with multiple sclerosis (MS), other neurological diseases (ONDs) and healthy controls were tested for antibodies to myelin-associated glycoprotein (MAG) by several different assays. Using a very sensitive, solid-phase radioimmunoassay with radioiodinated protein A, a statistically significant elevation of anti-MAG antibodies was detected in MS CSFs in comparison to those from ONDs and healthy controls. The antibodies reacted with human MAG, but not with rat MAG, and appeared to be directed towards carbohydrate determinants in the glycoprotein. The CSFs from high IgG producers had significantly greater anti-MAG antibody levels than those from low IgG producers, even though the assays were done on CSF samples that had been normalized to the same IgG concentration. The elevated antibodies were not detected when the same samples were tested with a liquid-phase radioimmunoassay or an enzyme-linked immunosorbent assay, and the antibodies in the MS CSF also could not be detected by Western blotting. An elevated level of antibodies was not found in sera from MS patients by any of the assays, possibly because these samples gave higher and more variable background. The results suggest that there is a low level of humoral immunity to MAG in MS patients that can only be detected by the most sensitive assays. This weak immune response to MAG may be secondary to the demyelinating process, but could play a role in the progression of the disease.

Multiple sclerosis: an overview

Demyelinating lesions of MS are infiltrated by activated T-lymphocytes and macrophages with secretion of soluble factors. This results in the synthesis of oligoclonal immunoglobulin (IgG) by plasma cells. The activated T-lymphocytes migrate from the peripheral blood to the CNS. This hyperactive state is linked to a selective loss of the suppressor/inducer T-cell subset. Administration of a soluble factor--interferon gamma--enhances the immune response by promoting class II antigen expression on macrophages or astrocytes, resulting in a relapse. However, the reason for T-cell activation in peripheral blood is not known, nor is the antigen. Myelin basic protein (MBP) has been considered to be the target since MBP is able to induce chronic relapsing allergic encephalomyelitis (CRAE) in an animal model of MS. Yet other myelin antigens have succeeded in inducing CRAE in animal models, and anti-MBP antibodies have been found in healthy individuals. The possibility that the hyperimmune state results from a viral infection has not yet been proven. It is known that in Caucasians, a genetic susceptibility factor is linked to class II MHC. Using MRI it has been found that the presence of new plaques was not regularly correlated with relapses, which indicates that MS is an ongoing pathology process. Most drugs used in MS influence the immune response but have potential toxicity. Monoclonal antibodies offer the opportunity of specific targeting of T-cells and are promising for the future.

Myelin-associated calpain II

Anti-chicken muscle calpain (calcium-activated neutral protease) antibody (ACAb) was found to be absorbed by purified human brain myelin when titrated by enzyme-linked immunosorbent assay, suggesting the close association of the protease with myelin. To confirm this, calcium-dependent protease was extracted from myelin membrane and purified on a phenyl Sepharose CL 4B column. It was activated by calcium ion in the millimolar range, and therefore was determined to be calpain II. This enzyme fraction was electrophoresed and immunostained with ACAb, resulting in staining as a single band with apparent molecular weight of 80K. This protease degraded exogenous myelin-associated glycoprotein. From the present results, it is suggested that calpain is bound to myelin membrane and involved in the turnover of myelin proteins.

Developmental and pathophysiological aspects of the myelin-associated glycoprotein

1. A glycoslylated sulfate-containing protein known as myelin-associated glycoprotein (MAG) appears to be unique to the central and peripheral nervous systems. This component has been characterized and cDNA clones have been isolated. 2. MAG is a member of the immunoglobulin superfamily. The principal form of MAG synthesized in brain during active myelination has an apparent molecular weight of 100,000. Alternate exon splicing leads to an additional 5000-dalton smaller form with a different C terminus. 3. In patients with multiple sclerosis, MAG is rapidly lost in areas of active disease. It is immunologically reactive in patients with benign monoclonal gammopathy associated with peripheral neuropathy. 4. The role of MAG in the formation of the myelin sheath and its participation in autoimmune neurological disorders are outlined.