Biomedical subjects

M Schachner

Publications and source records attributed to M Schachner.

At least 91 records · Page 5Linked to original sources

PubMed1999

NrCAM, cerebellar granule cell receptor for the neuronal adhesion molecule F3, displays an actin-dependent mobility in growth cones.

The neuronal adhesion glycoprotein F3 is a multifunctional molecule of the immunoglobulin superfamily that displays heterophilic binding activities. In the present study, NrCAM was identified as the functional receptor mediating the inhibitory effect of F3 on axonal elongation from cerebellar granule cells. F3Fc-conjugated microspheres binding to neuronal growth cones resulted from heterophilic interaction with NrCAM but not with L1. Time-lapse video-microscopy indicated that F3Fc beads bind at the leading edge and move retrogradely to reach the base of the growth cone within a lapse of 30-60 seconds. Such velocity (5.7 microm/minute) is consistent with a coupling between F3 receptors and the retrograde flow of actin filaments. When actin filaments were disrupted by cytochalasin B, the F3Fc beads remained immobile at the leading edge. The retrograde mobility appeared to be dependent on NrCAM clustering since it was induced upon binding with cross-linked but not dimeric F3Fc chimera. These data indicate that F3 may control growth cone motility by modulating the linkage of its receptor, NrCAM, to the cytoskeleton. They provide further insights into the mechanisms by which GPI-anchored adhesion molecules may exert an inhibitory effect on axonal elongation.

Actins↗

PubMed1999

NCAM stimulates the Ras-MAPK pathway and CREB phosphorylation in neuronal cells.

The neural cell adhesion molecule NCAM plays an important role in axonal growth, learning, and memory. A signaling pathway has been elucidated in which clustering of the NCAM140 isoform in the neural plasma membrane stimulated the activating phosphorylation of mitogen-activated protein kinases (MAPKs) and the transcription factor cyclic AMP response-element binding protein (CREB). NCAM clustering transiently induced dual phosphorylation (activation) of the MAPKs ERK1 and ERK2 (extracellular signal-regulated kinases) by a pathway regulated by the focal adhesion kinase p125fak, p59fyn, Ras, and MAPK kinase. CREB phosphorylation at serine 133 induced by NCAM was dependent in part on an intact MAPK pathway. c-Jun N-terminal kinase, which is associated with apoptosis and cellular stress, was not activated by NCAM. Inhibition of the MAPK pathway in rat cerebellar neuron cultures selectively reduced NCAM-stimulated neurite outgrowth. These results define an NCAM signal transduction mechanism with the potential for modulating the expression of genes needed for axonal growth, survival, and synaptic plasticity.

Animals↗

PubMed1999

Inhibition of memory consolidation by antibodies against cell adhesion molecules after active avoidance conditioning in zebrafish.

Cell adhesion molecules are expected to play an important role in long-term storage of information in the central nervous system. Several of these glycoproteins, such as NCAM, L1, and the ependymins, express the HNK-1 carbohydrate structure, which is known to be involved in cell-cell and cell-matrix interactions. To investigate the contribution of the HNK-1 epitope and the secretory glycoproteins ependymins to memory formation in zebrafish (Brachydanio rerio), we developed an active avoidance conditioning paradigm. Zebrafish were trained in a shuttle-box to cross a hurdle, to avoid mild electric shocks following a conditioning light signal. One hour after acquisition of the task, zebrafish were injected intracerebroventricularly with monoclonal antibodies against the HNK-1 epitope or polyclonal antibodies against ependymins. Control fish received immunoglobulins G (IgGs) from nonimmune rat serum or the monoclonal antibody C183 against an unrelated cell-surface protein of the cyprinid brain. Two days later, injected zebrafish were tested for recall, and for quantitative evaluation a retention score (RS), ranging from 1.0 for immediate recall to 0.0, indicating no saving, was calculated. The average RS of anti-HNK-1-injected fish (RS = 0.30) and anti-ependymin-injected fish (0.24) were significantly different from the RS of uninjected fish (0.77), of controls injected with nonimmune serum IgGs (0.68), of C183-injected controls (0.78), and of overtrained fish injected with anti-HNK-1 antibodies (0.81). Anti-HNK-1 and anti-ependymin antibodies were characterized by Western blot analyses of subcellular brain fractions and immunohistochemical staining of the zebrafish optic tectum. Our data suggest that the antibodies influence cell recognition events at synaptic membranes and/or associated intracellular signaling cascades, and thereby block memory consolidation.

Animals↗

PubMed1999

Subtle roles of neural cell adhesion molecule and myelin-associated glycoprotein during Schwann cell spiralling in P0-deficient mice.

Peripheral nerves of P0-deficient mice display a severe dysmyelinating phenotype, confirming the view that P0 mediates myelin formation and compaction. In addition to the compromised myelin organization, an elevated expression of several cell recognition molecules was described in the axon-Schwann cell units of P0-deficient mice. The present study was performed to focus on the subcellular localization and functional roles of two of these up-regulated molecules, the neural cell adhesion molecule (N-CAM) and the myelin-associated glycoprotein (MAG). We show by postembedding immunoelectron microscopy that in peripheral nerves of P0-deficient mice both molecules are expressed in noncompacted myelin-like regions. In addition, N-CAM, but not MAG, is detected in partially compacted myelin. By the generation of P0/N-CAM- and P0/MAG-deficient double mutants, we investigated the roles of the dysregulated molecules in P0-deficient mice. In 4-week-old double mutants, the dysmyelinating phenotype of the axon-Schwann cell units was very similar to that seen in the P0-deficient single mutants, suggesting that neither N-CAM nor MAG are responsible for the poor myelin compaction in P0-deficient mice. However, the noncompacted turns surrounding the abnormally compacted regions were significantly reduced in number in P0/MAG mutants as compared to P0 or N-CAM/P0 mice. During formation of myelin sheaths, absence of N-CAM resulted in a transient retardation of Schwann cell spiralling in P0-deficient mice, whereas absence of MAG impaired Schwann cell spiralling for a more extended time period. Our findings demonstrate for the first time that MAG and also N-CAM can play significant roles during myelin formation in the peripheral nervous system. Because these functional roles are detectable only in the absence of P0, our results confirm the view that myelin-related molecules can play distinct, but also partially overlapping roles.

Animals↗

PubMed1999

Localization and functional roles of PMP22 in peripheral nerves of P0-deficient mice.

Peripheral nerves of P0-deficient (P0(-)) mice show a severe dysmyelination and altered expression of several cell surface molecules. In the present study we investigated the subcellular localization of the peripheral myelin protein (PMP)22 in the abnormal axon-Schwann cell units of the mutants. We show by postembedding immunoelectron microscopy that PMP22 is expressed in both noncompacted and abnormally compacted myelin-like regions of P0(-) mice. By the generation of mice deficient for both P0 and PMP22 (P0(-)/PMP22(-) double mutants) we investigated the functional role(s) of PMP22 in P0(-) mice. In 4-week-old double mutants, some abnormally compacted myelin-like sheaths showed slight alterations in compaction with collapsed intraperiod lines, whereas the totally uncompacted axon-Schwann cell units displayed a more irregular cytoarchitecture owing to the presence of more cytoplasm within the loose Schwann cell loops. These findings show an only subtle impact of PMP22 on the structure of P0-deficient myelin-like sheaths. During early stages of myelin formation, peripheral nerves of P0(-)/PMP22(-) mice resembled those of PMP22(-) mutants in that they were characterized by a strongly retarded spiralling of Schwann cell processes. Thus, P0 appears to be the major determinant of myelin structure, whereas PMP22 is the predominant regulator of the timely correct initiation of myelination.

Animals↗

PubMed1998

Interaction of voltage-gated sodium channels with the extracellular matrix molecules tenascin-C and tenascin-R.

The type IIA rat brain sodium channel is composed of three subunits: a large pore-forming alpha subunit and two smaller auxiliary subunits, beta1 and beta2. The beta subunits are single membrane-spanning glycoproteins with one Ig-like motif in their extracellular domains. The Ig motif of the beta2 subunit has close structural similarity to one of the six Ig motifs in the extracellular domain of the cell adhesion molecule contactin (also called F3 or F11), which binds to the extracellular matrix molecules tenascin-C and tenascin-R. We investigated the binding of the purified sodium channel and the extracellular domain of the beta2 subunit to tenascin-C and tenascin-R in vitro. Incubation of purified sodium channels on microtiter plates coated with tenascin-C revealed saturable and specific binding with an apparent Kd of approximately 15 nM. Glutathione S-transferase-tagged fusion proteins containing various segments of tenascin-C and tenascin-R were purified, digested with thrombin to remove the epitope tag, immobilized on microtiter dishes, and tested for their ability to bind purified sodium channel or the epitope-tagged extracellular domain of beta2 subunits. Both purified sodium channels and the extracellular domain of the beta2 subunit bound specifically to fibronectin type III repeats 1-2, A, B, and 6-8 of tenascin-C and fibronectin type III repeats 1-2 and 6-8 of tenascin-R but not to the epidermal growth factor-like domain or the fibrinogen-like domain of these molecules. The binding of neuronal sodium channels to extracellular matrix molecules such as tenascin-C and tenascin-R may play a crucial role in localizing sodium channels in high density at axon initial segments and nodes of Ranvier or in regulating the activity of immobilized sodium channels in these locations.

Animals↗

PubMed1998

Increase in proportion of hippocampal spine synapses expressing neural cell adhesion molecule NCAM180 following long-term potentiation.

Neural recognition molecules such as the neural cell adhesion molecule (NCAM) have been implicated in synaptic plasticity, including long-term potentiation (LTP), sensitization, and learning and memory. The major isoform of NCAM carrying the longest cytoplasmic domain of all NCAM isoforms (NCAM180) is predominantly localized in postsynaptic membranes and postsynaptic densities of hippocampal neurons, with only a proportion of synapses carrying detectable levels of NCAM180. To investigate whether this differential expression of NCAM180 may correlate with distinct states of synaptic activity, LTP was induced by high-frequency stimulation of the perforant path and the percentage of NCAM180 immunopositive spine synapses determined in the outer third of the dentate molecular layer of the dentate gyrus by immunoelectron microscopy. Twenty-four hours following induction of LTP by high-frequency stimulation, the percentage of spine synapses expressing NCAM180 increases from 37% (passive control) to 70%. This increase was inhibited by the noncompetitive N-methyl-D-aspartate receptor antagonist MK801. Following repeated LTP induction at 10 consecutive days with one tetanization each day, 60% of all spine synapses were NCAM180 immunoreactive. Compared to passive control animals, the percentage of NCAM180 expressing synapses in low-frequency stimulated animals decreased from 37% to 28%. Spine synapses in the inner part of the dentate molecular layer not contacted by the afferents of the perforant path did not change the percentage of NCAM180-expressing synapses. The results obtained by the postembedding immunogold staining technique confirmed the difference in NCAM180 expression of spine synapses between passive control and potentiated animals. These observations suggest a role for NCAM180 in synaptic remodeling accompanying LTP.

Animals↗

PubMed1998

Na,K-ATPase subunit beta1 knock-in prevents lethality of beta2 deficiency in mice.

The beta2 subunit of the Na,K-ATPase displays functional properties of both an integral constituent of an ion pump and an adhesion and neurite outgrowth-promoting molecule in vitro. To investigate whether the beta1 subunit of the Na,K-ATPase can functionally substitute for the beta2 isoform in vivo, we have generated beta2/beta1 knock-in mice by homologous recombination in embryonic stem cells. In beta2/beta1 knock-in mice, expression of beta2 was abolished, whereas beta1 mRNA expression from the mutated gene amounted to approximately 15% of the normal expression of beta2 in the adult mouse brain and prevented the juvenile lethality observed for beta2 null mutant mice. In contrast to beta2 null mutant mice, the overall morphological structure of all analyzed brain regions was normal. By immunohistochemical analysis, beta1 expression was detected in photoreceptor cells in the retina of knock-in mice at an age when expression of beta1 and beta2, respectively, is downregulated and persisting in the wild-type mice. Morphological analysis by light and electron microscopy revealed a progressive degeneration of photoreceptor cells. Apoptotic death of photoreceptor cells determined quantitatively by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling analysis increased in beta2/beta1 knock-in mice with age. These observations suggest that the beta1 subunit of the Na,K-ATPase can substitute sufficiently, at least in certain cell types, for the role of the beta2 subunit as a component of a functional Na,K-ATPase, but they do not allow us to determine the possible role of the beta2 subunit as an adhesion molecule in vivo.

Amino Acid Sequence↗

PubMed1998

Long-term potentiation in vivo increases rat hippocampal tenascin-C expression.

We investigated the expression of the extracellular matrix glycoprotein tenascin-C after induction of long-term potentiation (LTP) by high-frequency tetanization (HFT) in the rat dentate gyrus in vivo. Expression of tenascin-C was evaluated at the mRNA and protein levels by in situ hybridization and immunocytochemistry, respectively. Whereas no tenascin-C mRNA was detectable in control animals, an increase in tenascin-C mRNA levels was observed in the granule cell layer of the dentate gyrus 4 h after HFT. At 24 h after HFT, tenascin-C mRNA had returned to control levels. Expression of tenascin-C protein 4 h after HFT followed that of controls in that tenascin was detectable in the strata oriens and radiatum of CA1, in the molecular layer, and within a narrow area at the inner surface of the granule cell layer in the dentate gyrus. However, 24 h after HFT, additional patches of tenascin-C immunoreactivity were observed in the molecular layer of the dentate gyrus. No increase in tenascin mRNA or protein levels was detected in control animals that received no stimulation, low-frequency stimulation, or HFT in the presence of the N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid or the metabotropic glutamate receptor antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine. These observations implicate a role for tenascin-C in N-methyl-D-aspartate and metabotropic glutamate receptor-dependent changes accompanying induction and/or maintenance of LTP.

2-Amino-5-phosphonovalerate↗

PubMed1998

Expression of L1 and PSA during sprouting and regeneration in the adult hippocampal formation.

The objective of the present study was to evaluate the expression of polysialic acid (PSA) and the cell adhesion molecule L1 during axonal regeneration and sprouting after injury to the adult rat brain. All animals received a complete lesion of the fimbria-fornix (FF). Grafts of nerve growth factor (NGF)- or beta-galactosidase (betaGal)-producing fibroblasts were placed in the FF lesion cavity and induced septohippocampal cholinergic regeneration or sympathetic tyrosine hydroxylase (TH)-positive sprouting, respectively. Cholinergic regeneration was evaluated from 2 to 8 weeks following grafting of NGF-producing fibroblasts in the FF lesion cavity. In the graft area, choline acetyltransferase (ChAT)-positive fibers expressed L1 and PSA. Once cholinergic axons reached the hippocampal formation (HF), they no longer expressed L1 or PSA. Eight weeks after a lesion of the FF and transplantation of betaGal-producing fibroblasts, TH-positive fibers sprouted in the denervated HF and expressed L1 but not PSA. At the zone of reactive gliosis, PSA but not L1 expression was increased following a lesion of the FF and transplantation of NGF- or betaGal-producing fibroblasts. In animals that received a graft of NGF-producing fibroblasts in the FF lesion cavity, numerous ChAT-positive axons were observed along these areas rich in PSA and reactive astrocytes. Taken together, these results suggest that the expression of PSA and L1 is upregulated on regenerating cholinergic axons during axonal elongation and downregulated upon target innervation. In contrast, TH-positive fibers that sprout in the denervated HF express and maintain their expression of L1. Finally, the expression of PSA in the area of reactive gliosis may contribute to a permissive environment for axonal regrowth.

Age Factors↗

PubMed1998

Accelerated demyelination of peripheral nerves in mice deficient in connexin 32 and protein zero.

Mutant mice that lack either protein zero (P0) or connexin 32 (Cx32) were generated previously to investigate the function of these myelin proteins in peripheral nerves and to assess the value of these mice as animal models for hereditary human peripheral neuropathies. Mice that are completely devoid of P0 expression (P0(+/0)) show a complex phenotype that is characterized by hypomyelination, compromised myelin compaction, and degeneration of myelin and axons early in life. In contrast, young mouse mutants that have retained one wild-type allele of the P0 gene (P0(+/0)) reveal morphologically normal myelin but start to develop signs of demyelination and remyelination at 4 months of age. A similar late-onset myelin deficiency was observed in Cx32-deficient mice (Cx32(0/0)). We have now generated mice deficient for Cx32 and P0. In animals that lack both proteins (Cx32(0/0)/P0(0/0), the phenotype is morphologically identical to mice that solely lack P0. Animals that lack Cx32 and carry one functional P0 allele (Cx32(0/0/P0(+/0)) revealed demyelination and remyelination as evidenced by thin myelin and Schwann cell onion bulb formation already at the age of 4 weeks, a time point when no pathology was observed in the single mutants. These morphological deficits were also more prominent in 4-month-old Cx32(0/0)/P0(+/0)animals compared to the single mutants. Our data support the view that Cx32 and P0 are crucial molecules for the maintenance of myelin. Furthermore, the function of Cx32 in the peripheral nervous system appears to be largely dispensable when myelin compaction is impaired.

Alleles↗

PubMed1998

Readiness of zebrafish brain neurons to regenerate a spinal axon correlates with differential expression of specific cell recognition molecules.

We analyzed changes in the expression of mRNAs for the axonal growth-promoting cell recognition molecules L1.1, L1.2, and neural cell adhesion molecule (NCAM) after a rostral (proximal) or caudal (distal) spinal cord transection in adult zebrafish. One class of cerebrospinal projection nuclei (represented by the nucleus of the medial longitudinal fascicle, the intermediate reticular formation, and the magnocellular octaval nucleus) showed a robust regenerative response after both types of lesions as determined by retrograde tracing and/or in situ hybridization for GAP-43. A second class (represented by the nucleus ruber, the nucleus of the lateral lemniscus, and the tangential nucleus) showed a regenerative response only after proximal lesion. After distal lesion, upregulation of L1.1 and L1.2 mRNAs, but not NCAM mRNA expression, was observed in the first class of nuclei. The second class of nuclei did not show any changes in their mRNA expression after distal lesion. After proximal lesion, both classes of brain nuclei upregulated L1.1 mRNA expression (L1.2 and NCAM were not tested after proximal lesion). In the glial environment distal to the spinal lesion, labeling for L1.2 mRNA but not L1.1 or NCAM mRNAs was increased. These results, combined with findings in the lesioned retinotectal system of zebrafish (Bernharnhardt et al., 1996), indicate that the neuron-intrinsic regulation of cell recognition molecules after axotomy depends on the cell type as well as on the proximity of the lesion to the neuronal soma. Glial reactions differ for different regions of the CNS.

Animals↗

PubMed1998

L1 neural cell adhesion molecule is a survival factor for fetal dopaminergic neurons.

Cell adhesion molecules play a central role in neural development and are also critically involved in axonal regeneration and synaptic plasticity in the adult nervous system. We investigated whether the neural cell adhesion molecule L1 was capable of stimulating survival and differentiation in the mid-brain dopaminergic neurons which degenerate in Parkinson's disease. Monoclonal L1 antibodies, known to enhance neurite outgrowth, were substrate-coated or added at the time of plating to medium of cultures containing mid-brain dopaminergic neurons from 14-day-old fetal rats. Tritiated dopamine uptake per well and the number of tyrosine hydroxylase-immunopositive neurons increased in a dose-dependent manner with increasing concentrations of L1 antibody, suggesting that L1 acts directly or indirectly as a growth factor for dopaminergic neurons. A monoclonal L1 antibody not enhancing neurite outgrowth was ineffective. The growth-promoting effects of L1 antibodies on dopaminergic neurons in culture did not appear to be mediated by the cAMP-activated protein kinase A pathway, since combined treatment with a phosphodiesterase inhibitor had only additive effects on the L1-induced increase of dopamine uptake, and in addition, antibodies against L1 failed to protect cultures of dopaminergic neurons against the neurotoxin MPP+, whereas pretreatment with forskolin and phosphodiesterase type-IV inhibitors was strongly protective.

1-Methyl-4-phenylpyridinium↗

PubMed1998

L1 antibodies block lymph node fibroblastic reticular matrix remodeling in vivo.

L1 is an immunoglobulin superfamily adhesion molecule highly expressed on neurons and involved in cell motility, neurite outgrowth, axon fasciculation, myelination, and synaptic plasticity. L1 is also expressed by nonneural cells, but its function outside of the nervous system has not been studied extensively. We find that administration of an L1 monoclonal antibody in vivo disrupts the normal remodeling of lymph node reticular matrix during an immune response. Ultrastructural examination reveals that reticular fibroblasts in mice treated with L1 monoclonal antibodies fail to spread and envelop collagen fibers with their cellular processes. The induced defect in the remodeling of the fibroblastic reticular system results in the loss of normal nodal architecture, collapsed cortical sinusoids, and macrophage accumulation in malformed sinuses. Surprisingly, such profound architectural abnormalities have no detectable effects on the primary immune response to protein antigens.

Animals↗

PubMed1998

Defasciculation of neurites is mediated by tenascin-R and its neuronal receptor F3/11.

Fasciculation and defasciculation of axons are major morphogenetic events in the formation of neuronal pathways during development. We have identified the extracellular matrix glycoprotein tenascin-R (TN-R) and its neuronal receptor, the immunoglobulin superfamily recognition molecule F3, as promoters of neurite defasciculation in cerebellar explant cultures. Perturbation of the interaction between these two molecules using both antibodies and an antisense oligonucleotide resulted in increased neurite fasciculation. The domains involved in defasciculation were identified as the N-terminal region of TN-R containing the cysteine-rich stretch and the 4.5 epidermal growth factor-like repeats and the immunoglobulin-like domains of F3. Fasciculation induced by antibodies and the antisense oligonucleotide could be reverted by a phorbol ester activator of protein kinase C, whereas the protein kinase inhibitor staurosporine increased fasciculation. Our observations indicate that defasciculated neurite outgrowth does not only depend on the reduction of the expression of fasciculation enhancing adhesion molecules, such as L1 and the neural cell adhesion molecule (NCAM), but also on recognition molecules that actively induce defasciculation by triggering second messenger systems.

Animals↗

PubMed1998

Three time windows for amnestic effect of antibodies to cell adhesion molecule L1 in chicks.

The L1/NgCAM cell adhesion molecule is involved in neurite outgrowth, axonal fasciculation and cell migration in the nervous system. We studied the effects of antibodies against L1 injected intracranially at various times before and after training 2-day-old chicks in a visual categorization task. Memory retention was tested 24 h post-training. Anti-L1 antibodies impaired task retention only when administered in three restricted time windows: immediately before training, at about 5.5 h after training and from 15 to 18 h after training. No amnesia was produced by injections before, between or after these sensitive periods (from -1 to +21 h relative to training). These results indicate that there are multiple post-training periods during which L1 is involved in the formation of long-term memory.

Amnesia↗

PubMed1998

High affinity binding and overlapping localization of neurocan and phosphacan/protein-tyrosine phosphatase-zeta/beta with tenascin-R, amphoterin, and the heparin-binding growth-associated molecule.

We have studied the interactions of the nervous tissue-specific chondroitin sulfate proteoglycans neurocan and phosphacan with the extracellular matrix protein tenascin-R and two heparin-binding proteins, amphoterin and the heparin-binding growth-associated molecule (HB-GAM), using a radioligand binding assay. Both proteoglycans show saturable, high affinity binding to tenascin-R with apparent dissociation constants in the 2-7 nM range. Binding is reversible, inhibited in the presence of unlabeled proteoglycan, and increased by approximately 60% following chondroitinase treatment of the proteoglycans, indicating that the interactions are mediated via the core (glyco)proteins rather than by the glycosaminoglycan chains, which may in fact partially shield the binding sites. In contrast to their interactions with tenascin-C, in which binding was decreased by approximately 75% in the absence of calcium, binding of phosphacan to tenascin-R was not affected by the absence of divalent cations in the binding buffer, although there was a small but significant decrease in the binding of neurocan. Neurocan and phosphacan are also high affinity ligands of amphoterin and HB-GAM (Kd = 0.3-8 nM), two heparin-binding proteins that are developmentally regulated in brain and functionally involved in neurite outgrowth. The chondroitin sulfate chains on neurocan and phosphacan account for at least 80% of their binding to amphoterin and HB-GAM. The presence of amphoterin also produces a 5-fold increase in phosphacan binding to the neural cell adhesion molecule contactin. Immunocytochemical studies showed an overlapping localization of the proteoglycans and their ligands in the embryonic and postnatal brain, retina, and spinal cord. These studies have therefore revealed differences in the interactions of neurocan and phosphacan with the two major members of the tenascin family of extracellular matrix proteins, and also suggest that chondroitin sulfate proteoglycans play an important role in the binding and/or presentation of differentiation factors in the developing central nervous system.

Animals↗

PubMed1998

The cytoplasmic domain of the large myelin-associated glycoprotein isoform is needed for proper CNS but not peripheral nervous system myelination.

The myelin-associated glycoprotein (MAG) is a member of the immunoglobulin gene superfamily and is thought to play a critical role in the interaction of myelinating glial cells with the axon. Myelin from mutant mice incapable of expressing MAG displays various subtle abnormalities in the CNS and degenerates with age in the peripheral nervous system (PNS). Two distinct isoforms, large MAG (L-MAG) and small MAG (S-MAG), are produced through the alternative splicing of the primary MAG transcript. The cytoplasmic domain of L-MAG contains a unique phosphorylation site and has been shown to associate with the fyn tyrosine kinase. Moreover, L-MAG is expressed abundantly early in the myelination process, possibly indicating an important role in the initial stages of myelination. We have adapted the gene-targeting approach in embryonic stem cells to generate mutant mice that express a truncated form of the L-MAG isoform, eliminating the unique portion of its cytoplasmic domain, but that continue to express S-MAG. Similar to the total MAG knockouts, these animals do not express an overt clinical phenotype. CNS myelin of the L-MAG mutant mice displays most of the pathological abnormalities reported for the total MAG knockouts. In contrast to the null MAG mutants, however, PNS axons and myelin of older L-MAG mutant animals do not degenerate, indicating that S-MAG is sufficient to maintain PNS integrity. These observations demonstrate a differential role of the L-MAG isoform in CNS and PNS myelin.

Alleles↗