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PubMed1989

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.

Animals↗
PubMed2005

3D motion adapted gating (3D MAG): a new navigator technique for accelerated acquisition of free breathing navigator gated 3D coronary MR-angiography.

This study aimed to evaluate the influence of a new navigator technique (3D MAG) on navigator efficiency, total acquisition time, image quality and diagnostic accuracy. Fifty-six patients with suspected coronary artery disease underwent free breathing navigator gated coronary MRA (Intera, Philips Medical Systems, 1.5 T, spatial resolution 0.9x0.9x3 mm3) with and without 3D MAG. Evaluation of both sequences included: 1) navigator scan efficiency, 2) total acquisition time, 3) assessment of image quality and 4) detection of stenoses >50%. Average navigator efficiencies of the LCA and RCA were 43+/-12% and 42+/-12% with and 36+/-16% and 35+/-16% without 3D MAG (P<0.01). Scan time was reduced from 12 min 7 s without to 8 min 55 s with 3D MAG for the LCA and from 12 min 19 s to 9 min 7 s with 3D MAG for the RCA (P<0.01). The average scores of image quality of the coronary MRAs with and without 3D MAG were 3.5+/-0.79 and 3.46+/-0.84 (P>0.05). There was no significant difference in the sensitivity and specificity in the detection of coronary artery stenoses between coronary MRAs with and without 3D MAG (P>0.05). 3D MAG provides accelerated acquisition of navigator gated coronary MRA by about 19% while maintaining image quality and diagnostic accuracy.

Aged↗
PubMed1982

Immunocytochemical study of myelin-associated glycoprotein (MAG) and basic protein (BP) in acute experimental allergic encephalomyelitis (EAE).

To compare distributions of oligodendroglial myelin-associated glycoprotein (MAG) and myelin basic protein (BP) during demyelination in acute experimental allergic encephalomyelitis (EAE). Lewis rats were sensitized with an emulsion containing guinea pig spinal cord and complete Freund's adjuvant. The rats were examined clinically and groups were perfused with fixative before symptoms appeared (7d), within 48 h of symptom onset (10d) and during more severe illness (14d, 21d). Paraffin sections of cerebrum, brainstem, and spinal cord were immunostained with antisera to MAG and BP. Other sections in each serially mounted series were stained with luxol fast blue, hematoxylin and eosin or with the Bodian method to correlate MAG and BP results with histological changes. No abnormalities were detected 7d after sensitization. After 10d, small perivenular inflammatory cell infiltrates were present in white matter, displaced myelinated fibers, but their MAG stained periaxonal regions and BP-stained myelin sheaths appeared normal. In pontine grey matter lesions, there were focal abnormalities in a few myelin sheaths. After 21d, demyelinating lesions were present that were largest in pontine grey matter. Decreased MAG staining was present in areas of myelin sheath loss. MAG-stained fragments were found in zones of active myelin breakdown but no decrease or other change in MAG staining extended beyond the margins of demyelinating lesions into areas with normally stained myelin sheaths. Thus, in contrast to multiple sclerosis (Itoyama et al. 1980), changes in periaxonal oligodendroglial MAG are not present in acute EAE before inflammatory cell infiltrates or myelin sheath changes appear. Our findings suggest that myelin sheaths are the primary targets in EAE-induced demyelination. Oligodendroglia appear to be relatively unaffected and are available to remyelinate axons.

Animals↗
PubMed1995

Streptococcal protein MAG--a protein with broad albumin binding specificity.

Protein MAG is a cell surface protein from Streptococcus dysgalactiae which binds alpha 2-macroglobulin (alpha 2M), serum albumin and immunoglobulin G (IgG). In this work protein MAG was expressed in Escherichia coli, purified and analysed for its albumin-binding specificity. The binding of protein MAG to serum albumins of different species origin was studied in a dot-blot assay and compared with the binding of streptococcal protein G, so far the best studied bacterial albumin receptor. The albumin-binding of protein MAG was also characterized using real-time biospecific interaction analysis (BIA), and the ka, kd and the Kaff values for different albumins were determined. Amino acid sequence alignment revealed homology between the albumin-binding domain of protein MAG and earlier described streptococcal albumin receptors including protein G. However, the MAG protein was reactive with serum albumin from bovine, dog, goat, horse, human, mouse, pig, rat and sheep origin and therefore displays a broader albumin-binding profile than protein G concerning the albumins tested in this work. Comparison of the albumin-binding spectrum of protein MAG with the earlier described albumin receptors of various S. dysgalactiae strains and other streptococci, suggests that protein MAG is a new type of albumin receptor.

Amino Acid Sequence↗
PubMedDate not supplied

Shared antigen between the myelin-associated glycoprotein (MAG) and a cell line from human T cell leukemia (HSB-2).

HSB-2 is a cell line originated from human T-cell leukemia and its membrane antigen is recognized by anti-HNK-1, a monoclonal antibody against human natural killer (NK) cells. Enzyme linked immunosorbent assay (ELISA) revealed that not only anti-HNK-1 but also rabbit polyclonal and human monoclonal anti-myelin-associated glycoprotein (MAG) antibodies were absorbed by cell homogenate of HSB-2. Molt-4 and K-562 cell lines from human T-cell leukemia and human erythroleukemia did not absorb anti-HNK-1 and the anti-MAG antibodies. On the immunoblot of HSB-2 cell homogenate, two major molecules were stained by anti-HNK-1 and anti-MAG antibodies, while Molt-4 and K-562 were not stained by these antibodies. Comparing molecular weights of the two major molecules with those of MAG and derivative of MAG (dMAG), one is larger than MAG and the other is smaller than dMAG. The molecular weights of the two molecules were approximately 130 kDa and 80 kDa. The shared antigen in HSB-2 is not identical with MAG but a part of the molecule in the cells may be shared with a part of MAG molecule.

Animals↗
PubMed1999

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.

Amino Acid Sequence↗
PubMedDate not supplied

Improving the labeling of S-acetyl NHS-MAG(3)-conjugated morpholino oligomers.

S-Acetyl MAG(3) (S-acetylmercaptoacetyltriglycine) has been used as a chelator for the (99m)Tc labeling of a variety of biomolecules. The objective of this study was to improve upon the labeling of morpholino (MORF), a DNA analogue, as a model biomolecule. A 15mer MORF with a primary amine was conjugated with NHS-MAG(3) in the usual manner, and the MORF-MAG(3) was purified over a P4 column as before. The conjugate was radiolabeled using stannous ion as usual, and the impurities were identified using size exclusion high-performance liquid chromatography (SE HPLC). Various methods were then investigated to remove the impurities. With tartrate as the transchelator, two impurities were identified as labeled MAG(3) and labeled tartrate. The labeled MAG(3) could not be removed by simply repurifying the conjugate using the usual pH 5.2 NH(4)OAc buffer before labeling. However, this impurity could be completely removed if the conjugate was adjusted to pH 7.6 and heated before repurification. The labeled tartrate impurity was removed by heating during labeling. On the basis of these observations, the following procedure for purification of the conjugation mixture and subsequent labeling was adopted. After MORF was conjugated with NHS-MAG(3) and purified over P4 with pH 5.2 NH(4)OAc eluant, the oligomer fractions were combined, adjusted to pH 7.6, and heated in a boiling water bath for 20 min. The conjugated oligomer was then repurified over P4 for storage at refrigerator temperatures. Labeling is achieved simply by adding fresh stannous ion to a solution of the MORF-MAG(3) in pH 7.6 containing tartrate followed by (99m)Tc-pertechnetate. After the mixture is heated for 20 min in boiling water, the labeling efficiency is always over 90% as determined by size exclusion HPLC and paper chromatography and the specific activities can exceed 7 mCi/microg. By making several relatively simple changes to the routine procedure used to conjugate and radiolabel biomolecules with (99m)Tc via MAG(3), a modified procedure was developed that results in labeling efficiency high enough to avoid postlabeling purification.

Chelating Agents↗
PubMed2002

Immunocytological studies of L-MAG expression regulation during myelination of embryonic brain cell cocultures.

The myelin-associated glycoprotein (MAG) possesses two isoforms, known as S-MAG and L-MAG. We followed the expression and localization of L-MAG in comparison to myelin basic protein (MBP) during myelination in dissociated embryonic rat brain cell cocultures. Using immunofluorescence microscopy, we observed L-MAG and MBP in oligodendrocyte (OL) cell bodies and processes before and at the onset of myelination. At advanced stages of myelination, L-MAG immunostaining decreased, and subsequently disappeared from OL soma and myelinated internodes, whereas MBP immunoreactivity decreased only in the OL soma. These results are consistent with biochemical studies and provide the first clear demonstration of a transient expression of L-MAG during the early stages of myelination in vitro, providing further evidence that L-MAG plays a role during this period.

Animals↗
PubMed1983

Anti-MAG IgM antibodies in patients with neuropathy and IgM M proteins: detection by ELISA.

In some patients with plasma cell dyscrasia and neuropathy, there are IgM M proteins that react with the myelin-associated glycoprotein (MAG). We used an enzyme-linked immunosorbent assay (ELISA) system to detect anti-MAG IgM antibodies. Reactivity with human MAG by ELISA correlated with demonstration of anti-MAG IgM antibodies by the "immunoblot" technique. Human MAG was more effective than bovine MAG as antigen, and there was no significant reactivity with mouse MAG. The ELISA system is a simple and convenient method for detecting anti-MAG IgM antibodies.

Antibodies, Anti-Idiotypic↗
PubMed2000

Multiple functions of the myelin-associated glycoprotein MAG (siglec-4a) in formation and maintenance of myelin.

The myelin-associated glycoprotein, a minor component of myelin in the central and peripheral nervous system, has been implicated in the formation and maintenance of myelin. Although the analysis of MAG null mutants confirms this view, the phenotype of this mutant is surprisingly subtle. In the CNS of MAG-deficient mice, initiation of myelination, formation of morphologically intact myelin sheaths and to a minor extent, integrity of myelin is affected. In the PNS, in comparison, only maintenance of myelin is impaired. Recently, the large isoform of MAG has been identified as the functionally important isoform in the CNS, whereas the small MAG isoform is sufficient to maintain the integrity of myelinated fibers in the PNS. Remarkably, none of the different defects in the MAG mutant is consistently associated with each myelinated fiber. These observations suggest that other molecules performing similar functions as MAG might compensate, at least partially, for the absence of MAG in the null mutant.

Animals↗
PubMed2005

Differential diagnosis of chronic dysimmune demyelinating polyneuropathies with and without anti-MAG antibodies.

The distinction between chronic demyelinating polyneuropathies associated with IgM paraproteinemia and anti-myelin-associated glycoprotein (MAG) antibodies (MAG-PN) and chronic inflammatory demyelinating polyneuropathies (CIDPs) relies on the anti-MAG antibodies assay. The aim of the study was to identify clinical and electrophysiological features suggesting a diagnosis of MAG-PN. Fourteen patients with MAG-PN and 35 with CIDP were included, and a discriminant analysis was performed to identify the clinical and electrophysiological features suggestive of MAG-PN. Pure sensory clinical phenotype, low median and ulnar terminal latency index, and absence of M responses in the lower limbs were significantly associated with the diagnosis of MAG-PN, and indicate a moderate to large increase in probability of this diagnosis in patients with chronic dysimmune demyelinating polyneuropathies.

Adult↗
PubMed1997

Monitoring of Ca2+ release from intracellular stores in permeabilized rat parotid acinar cells using the fluorescent indicators Mag-fura-2 and calcium green C18.

The operation of intracellular Ca2+ stores in saponin-permeabilized rat parotid acinar cells was studied by monitoring the Ca2+ concentration within organelles loaded with the low affinity Ca2+ indicator Mag-fura-2. Inositol 1, 4, 5-trisphosphate (InsP3) caused a decrease in the Mag-fura-2 ratio in a dose-dependent manner, and this effect was reversed by a removal of InsP3 or by an addition of the InsP3 receptor antagonist heparin. The changes in Mag-fura-2 ratio indicate the Ca2+ release from InsP3-sensitive Ca2+ stores and Ca2+ re-uptake into the stores in permeabilized acinar cells. The decrease in Mag-fura-2 ratio induced by InsP3 was observed at all regions of the acinar cells, suggesting that the InsP3-sensitive Ca2+ stores are located throughout the cells. The InsP3-induced Ca2+ release was also monitored using the membrane-bound Ca2+ indicator Calcium Green C18 which is sensitive to the changes in Ca2+ concentration immediately adjacent to the membrane of intracellular Ca2+ stores. InsP3 caused a large increase in the Calcium Green C18 fluorescence reflecting Ca2+ release from the stores. The Ca2+ pump inhibitor thapsigargin (ThG) itself had little or no effect on the Mag-fura-2 ratio or Calcium Green C18 fluorescence, but combined application of ThG with a low concentration of InsP3 evoked a significant decrease in the Mag-fura-2 ratio. This result supports the hypothesis that the ThG-induced Ca2+ release is due to InsP3-sensitive Ca2+ release which is mediated by the resting levels of InsP3. Further, none of cyclic ADP-ribose, caffeine or ryanodine changed the Mag-fura-2 ratio and Calcium Green C18 fluorescence, leading to the assumption that the ryanodine-sensitive Ca2+ stores are minor in rat parotid acinar cells.

Animals↗
PubMed1994

Determination of cytosolic Mg2+ activity and buffering in BC3H-1 cells with mag-fura-2.

The magnesium buffer coefficient (BMg) was calculated for BC3H-1 cells from the rise in cytosolic Mg2+ activity observed when magnesium was released from ATP after iodoacetate (IAA) and NaCN treatment. The basal cytosolic Mg2+ activity (0.54 +/- 0.1 mM) measured with mag-fura-2 doubled when 4.54 mM magnesium was liberated from ATP: BMg was 12.9 indicating that a 1 mM increase in Mg2+ activity requires an addition of about 13 mM magnesium. The accuracy of this value depends on these assumptions: (a) all of the magnesium released from ATP stayed in the cells; (b) the rise in Mg2+ was not secondary to pH-induced changes in BMg; (c) mag-fura-2 measured Mg2+ and not Ca2+; and (d) the accuracy of the mag-fura-2 calibration. Total magnesium did not change in response to IAA/CN treatment, thus the change in Mg2+ activity reflected a redistribution of cell magnesium. pH changes induced by NH4Cl pulse and removal had little effect on Mg2+ activity and the changes were slower than and opposite to pH-induced changes in Ca2+ activity measured by fura-2. Ca2+ responses were temporally uncoupled from Mg2+ responses when the cells were treated with IAA only and in no cases did Ca2+ levels rise above 1 microM, showing that the mag-fura-2 is responding to Mg2+. Additional studies demonstrated that approximately 90% of the mag-fura-2 signal was cytosolic in origin. The remaining non-diffusible mag-fura-2 either was bound to cytosolic membranes or sequestered in organelles with the fluorescence characteristics of the Mg2(+)-complexed form, even when cytosolic free Mg2+ activity was approximately 0.5 mM. This bound mag-fura-2 would appear to increase the Kd and thus clearly limits the accuracy of our estimmate for BMg. Despite this limitation, we demonstrate that Mg2+ is tightly regulated in face of large changes in extracellular Mg2+, and that interplay observed between pH, Ca2+ and Mg2+ activities strongly supports the hypothesis that these factors interact through a shared buffer capacity of the cell.

Adenosine Triphosphate↗
PubMed1997

Occurrence of myelin-associated glycoprotein (MAG)-like immunoreactivity in some nervous, endocrine, and immune-related cells of the rat. An immunohistochemical study.

The occurrence and distribution of myelin-associated glycoprotein (MAG)-like immunoreactivity was investigated in the rat using a polyclonal antibody to MAG purified from rat brain. In the nervous system, MAG immunoreactivity was found in the periaxonal portion of the myelinated fibers and in a small number of oligodendroglia in the cortex, hippocampus, and the spinal cord. The sheath of Schwann cells in unmyelinated fibers and satellite cells in the spinal ganglia were also immunoreactive for MAG. In the endocrine system, the noradrenaline-containing cells in the adrenal medulla and some endocrine cells in the duodenum showed MAG immunoreactivity. In the immune system, numerous reticular cells with slender cytoplasmic processes, which formed a dense network, were immunopositive for MAG within the germinal center in the lymph nodes and spleen. In the thymus, a number of epithelial reticular cells within the medulla showed variation in staining intensity. These findings provide new information on the wide distribution of MAG immunoreactivity in the nervous, endocrine, and immune systems, and may contribute to the further understanding of the biological roles of this protein.

Adrenal Glands↗
PubMed1987

Comparison of the distribution of Na+,K+-ATPase and myelin-associated glycoprotein (MAG) in the optic nerve, spinal cord and trigeminal ganglion of shiverer (shi/shi) and control (+/+) mice.

Na+,K+ ATPase and myelin-associated glycoprotein (MAG) were studied by immunocytochemistry on paraffin sections of the spinal cord, optic nerve and trigeminal ganglion of adult control (+/+) and CNS myelin-deficient shiverer (shi/shi) mice. Immunostaining for Na+, K+-ATPase outlined the periphery of nerve fibers in the spinal cord white matter, optic nerve and trigeminal ganglion of +/+ and shi/shi mice. Immunostaining for Na+,K+-ATPase appeared somewhat denser in the optic nerve and spinal cord lateral funiculi of shi/shi than in +/+ mice. In addition, immunostaining for Na+,K+-ATPase was demonstrated at the plasmalemma of presumed satellite cells situated at the periphery of ganglion cell bodies in the trigeminal ganglion of both species of mice. Immunostaining for MAG was localized along the periphery of nerve fibers in the spinal cord funiculi (with little immunostaining within gray horns), optic nerve and trigeminal ganglion of both +/+ and shi/shi mice. The major differences between shi/shi and +/+ mice were that the number of MAG-immunostained nerve fibers was greatly reduced in the spinal cord funiculi and the density of immunostaining was slightly increased in the optic nerve of shi/shi mice. The numbers of MAG-immunostained nerve fibers in trigeminal ganglion were similar in both species. Also, the cytoplasm of some oligodendrocyte-like cells was found densely immunostained for MAG in the spinal cord and optic nerve of shi/shi mice, but not of +/+ mice. This light microscopic study provides evidence that the defective shiverer gene leads to a decrease in MAG deposition and to aggregations of MAG-like material within perikarya of oligodendrocyte-like cells in regions of the CNS.

Animals↗
PubMed1996

Differential expression of MAG, MBP and L1 in the developing lateral superior olive.

The aim of this study was to investigate whether glial-associated molecules exhibit a pattern of expression that could influence oriented dendrite outgrowth in the gerbil lateral superior olive (LSO). In particular, we have previously noted that axon fascicles are oriented parallel to isofrequency laminae in the medial limb of the LSO, as are LSO dendrites, a phenotype that emerges postnatally. Therefore, we examined the immunocytochemical staining pattern of antibodies directed against three proteins that are found along axons: myelin basic protein (MBP), myelin-associated glycoprotein (MAG), and neuron-glia cell adhesion molecule (L1). MAG staining was first observed at postnatal day (P) 4 on the axon fibers surrounding the LSO. By P7 there was a differential pattern of MAG staining within the LSO, and immunopositive fibers were observed solely in the medial limb (e.g., high frequency projection region). Between P7 and P12, MAG staining was restricted largely to fascicles in the medial limb, and these were oriented parallel to the isofrequency axes. Few positive fibers of irregular orientation were observed in the lateral limb (e.g., low frequency projection region). Significant MAG-staining was not observed in the lateral limb until P15. The MAG immunoreactivity extended throughout the LSO by P21, although it was no longer restricted to axon fascicles. In contrast, MBP-positive fibers were uniformly distributed within the LSO by P12. Finally, L1 was found on oriented axon fascicles at P0, but became sparsely distributed throughout the LSO neuropil after P7, and was restricted to neuron cell bodies in the adult. Taken together, the results suggest that oriented axon fascicles bearing MAG and L1 may contribute to the developmental refinement of dendrite and axon arbors within the LSO.

Aging↗
PubMed1989

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.

Antibodies↗
PubMed1988

Differential splicing of MAG transcripts during CNS and PNS development.

Myelin-associated glycoprotein (MAG) is expressed on the surface of glial cells and is thought to act as a glial-neuronal adhesion molecule during early stages in the myelination process. Sequencing of several cDNA clones predicted the existence of two classes of MAG mRNAs which differ in the presence or absence of a 45 nucleotide insert near the 3' end. These two mRNAs are sufficient to encode the two MAG proteins previously described (p67MAG and p72MAG) and show that they differ only in their carboxyl terminal regions. The results of RNAse protection experiments reported here confirm the existence of two mRNAs for MAG which arise by alternative splicing of exon 12, as shown by Lai et al. Our results show that the p72MAG mRNA is expressed during the time of active myelin formation in the CNS, reaching a peak by post-natal day 22 and thereafter declining to adult levels by day 62. Conversely, p67MAG mRNA is produced as the minor species during myelin formation, but becomes the predominant form in adult brain. Cultures of oligodendrocytes express both forms of MAG. In the PNS, mRNA coding for p67MAG is predominant throughout development, reaching peak levels at day 6-10, whereas p72MAG mRNA is a very minor species. Alternative splicing also occurs at the 5' terminus. One form of mRNA lacking exon 2 from the 5' non-coding region is predominant in PNS, whereas mRNA containing exon 2 predominates in the CNS. Therefore, at least two and possibly four different mRNA species encode MAG. These results confirm the hypothesis that the two forms of MAG (p67 and p72) are generated by alternative splicing and show that each form is differentially regulated during development in the CNS and PNS.

Aging↗