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J L Salzer

Publications and source records attributed to J L Salzer.

At least 37 records · Page 2Linked to original sources
PubMed1998

Myelin-associated glycoprotein in myelin and expressed by Schwann cells inhibits axonal regeneration and branching.

The mammalian CNS does not regenerate after injury due largely to myelin-specific inhibitors of axonal growth. The PNS, however, does regenerate once myelin is cleared and myelin proteins are down-regulated by Schwann cells. Myelin-associated glycoprotein (MAG), a sialic acid binding protein, is a potent inhibitor of neurite outgrowth when presented to neurons in culture. Here, we present additional evidence that strongly supports the suggestion that MAG contributes to the overall inhibitory properties of myelin. When myelin from MAG-/- mice is used as a substrate, axonal length is 100 and 60% longer for neonatal cerebellar and older DRG neurons, respectively, compared to MAG+/+ myelin. The converse is true for neurites from neonatal DRG neurons, which are twice as long on MAG+/+ relative to MAG-/- myelin, consistent with MAG's dual role of promoting axonal growth from neonatal DRG neurons but inhibiting growth in older DRG and all other postnatal neurons examined. Furthermore, desialylating neurons reverses inhibition by CNS myelin by 45%. Contrary to previous reports, under these conditions PNS myelin is also inhibitory for axonal regeneration. Importantly, results using PNS MAG-/- myelin as a substrate suggest that MAG contributes to this inhibition. Finally, when Schwann cells not expressing MAG and permissive for axonal growth are induced to express MAG by retroviral infection, not only is axonal outgrowth greatly inhibited by these cells but so also is neurite branching. This suggests for the first time that MAG not only affects axonal regeneration but may also play a role in the control of axonal sprouting.

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PubMed1997

The axonal membrane protein Caspr, a homologue of neurexin IV, is a component of the septate-like paranodal junctions that assemble during myelination.

We have investigated the potential role of contactin and contactin-associated protein (Caspr) in the axonal-glial interactions of myelination. In the nervous system, contactin is expressed by neurons, oligodendrocytes, and their progenitors, but not by Schwann cells. Expression of Caspr, a homologue of Neurexin IV, is restricted to neurons. Both contactin and Caspr are uniformly expressed at high levels on the surface of unensheathed neurites and are downregulated during myelination in vitro and in vivo. Contactin is downregulated along the entire myelinated nerve fiber. In contrast, Caspr expression initially remains elevated along segments of neurites associated with nascent myelin sheaths. With further maturation, Caspr is downregulated in the internode and becomes strikingly concentrated in the paranodal regions of the axon, suggesting that it redistributes from the internode to these sites. Caspr expression is similarly restricted to the paranodes of mature myelinated axons in the peripheral and central nervous systems; it is more diffusely and persistently expressed in gray matter and on unmyelinated axons. Immunoelectron microscopy demonstrated that Caspr is localized to the septate-like junctions that form between axons and the paranodal loops of myelinating cells. Caspr is poorly extracted by nonionic detergents, suggesting that it is associated with the axon cytoskeleton at these junctions. These results indicate that contactin and Caspr function independently during myelination and that their expression is regulated by glial ensheathment. They strongly implicate Caspr as a major transmembrane component of the paranodal junctions, whose molecular composition has previously been unknown, and suggest its role in the reciprocal signaling between axons and glia.

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PubMed1997

Myelin-associated glycoprotein interacts with neurons via a sialic acid binding site at ARG118 and a distinct neurite inhibition site.

Inhibitory components in myelin are largely responsible for the lack of regeneration in the mammalian CNS. Myelin-associated glycoprotein (MAG), a sialic acid binding protein and a component of myelin, is a potent inhibitor of neurite outgrowth from a variety of neurons both in vitro and in vivo. Here, we show that MAG's sialic acid binding site is distinct from its neurite inhibitory activity. Alone, sialic acid-dependent binding of MAG to neurons is insufficient to effect inhibition of axonal growth. Thus, while soluble MAG-Fc (MAG extracellular domain fused to Fc), a truncated form of MAG-Fc missing Ig-domains 4 and 5, MAG(d1-3)-Fc, and another sialic acid binding protein, sialoadhesin, each bind to neurons in a sialic acid- dependent manner, only full-length MAG-Fc inhibits neurite outgrowth. These results suggest that a second site must exist on MAG which elicits this response. Consistent with this model, mutation of arginine 118 (R118) in MAG to either alanine or aspartate abolishes its sialic acid-dependent binding. However, when expressed at the surface of either CHO or Schwann cells, R118-mutated MAG retains the ability to inhibit axonal outgrowth. Hence, MAG has two recognition sites for neurons, the sialic acid binding site at R118 and a distinct inhibition site which is absent from the first three Ig domains.

Amino Acid Sequence↗
PubMed1997

Neural targeting of Mycobacterium leprae mediated by the G domain of the laminin-alpha2 chain.

We report that the molecular basis of the neural tropism of Mycobacterium leprae is attributable to the specific binding of M. leprae to the laminin-alpha2 (LN-alpha2) chain on Schwann cell-axon units. Using recombinant fragments of LN-alpha2 (rLN-alpha2), the M. leprae-binding site was localized to the G domain. rLN-alpha2G mediated M. leprae binding to cell lines and to sciatic nerves of dystrophic dy/dy mice lacking LN-alpha2, but expressing laminin receptors. Anti-beta4 integrin antibody attenuated rLN-alpha2G-mediated M. leprae adherence, suggesting that M. leprae interacts with cells by binding to beta4 integrin via an LN-alpha2G bridge. Our results indicate a novel role for the G domain of LN-2 in infection and reveal a model in which a host-derived bridging molecule determines nerve tropism of a pathogen.

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PubMed1996

Regional and ultrastructural distribution of the alpha 8 integrin subunit in developing and adult rat brain suggests a role in synaptic function.

Integrins are heterodimeric cell adhesion molecules comprised of alpha and beta subunits that have been implicated in the regulation of neuronal migration, differentiation, and process outgrowth. They mediate both cell-extracellular matrix and cell-cell interactions. The integrin alpha 8 beta 1 is a receptor for fibronectin, tenascin, and vitronectin that has been localized to axonal tracts and several types of non-neuronal cells in chick embryos and to smooth muscle cells in adult mammalian tissues. In this report, we describe the distribution of the alpha 8 subunit in the developing and adult mammalian brain. By light microscopy, alpha 8 labeling in the rat brain was found predominantly in neurons. It was primarily localized within perikarya and dendrites, but was also observed in certain fiber tracts. alpha 8 immunoreactivity was most concentrated in the olfactory bulb, hippocampal formation, substantia nigra, ventral tegmental area, and superior olivary complex, but was also found at moderate levels in several regions including layer 5 of the cerebral cortex. alpha 8 labeling was detected as early as E16, peaked in most areas during the first 3 postnatal weeks, and persisted in the adult. Electron microscopic analysis of the adult hippocampal formation revealed a striking concentration of alpha 8 immunoreactivity in the spines and postsynaptic densities of dendrites. These results suggest that alpha 8 is involved in the regulation of axonal and dendritic growth of some neurons in the developing central nervous system (CNS) and provide ultrastructural evidence that integrins may participate in the formation, maintenance, or plasticity of synapses.

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PubMed1996

GGF/neuregulin is a neuronal signal that promotes the proliferation and survival and inhibits the differentiation of oligodendrocyte progenitors.

We show that GGF/neuregulin is a mitogen for prooligodendrocytes (O4+/O1- cells), oligodendrocytes (O4+/O1+ cells), and type-2 astrocytes. Heregulin beta 1, another neuregulin isoform, is also mitogenic. The proliferative effect of glial growth factor (GGF) does not require, but is greatly potentiated by, serum factors. GGF also promotes the survival of pro-oligodendrocytes under serum-free conditions. High levels of GGF reversibly inhibit the differentiation and lineage commitment of oligodendrocyte progenitors and, in differentiated cultures, result in loss of O1 and myelin basic protein expression. All three erbB receptors are expressed by progenitors and are activated by GGF; the relative abundance of these receptors changes during differentiation. Finally, cortical neurons release a soluble mitogen for pro-oligodendrocytes that is specifically blocked by antibodies to GGF. These results implicate the neuregulins in the neuronal regulation of oligodendrocyte progenitor proliferation, survival, and differentiation.

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PubMed1995

Biosynthesis and regulation of expression of the HNK-1 epitope on myelin-associated glycoprotein in a transfected cell model system.

The HNK-1 antibody recognizes a carbohydrate epitope expressed by many cell adhesion molecules in the nervous system that has been proposed to be an important adhesive determinant. This epitope is particularly prominent on the myelin-associated glycoprotein (MAG) and is related to the antigenic target in an autoimmune mediated demyelinating neuropathy. Elucidation of the mechanisms underlying the biosynthesis and regulation of expression of the HNK-1 epitope is therefore likely to have important functional and clinical implications. In order to investigate its biosynthesis and the regulation of its expression, we have expressed both human and rat MAG in several different cell lines by retroviral infection. These studies indicate that the cellular milieu determines whether the HNK-1 epitope is expressed on the MAG polypeptide and provide an explanation for the significant variation in HNK-1 levels that has been noted in different species. Using a transfected human neuroblastoma line, we have determined that this epitope is present on the fourth and/or fifth immunoglobulin-like domain of rat MAG and that it is added intracellularly, probably in the trans Golgi. Finally we have found that expression of the HNK-1 epitope is increased by activation of different second messenger systems, providing direct evidence that its expression can be regulated independently from that of the MAG polypeptide.

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PubMed1995

Transforming growth factor-beta 1 regulates axon/Schwann cell interactions.

We have investigated the potential regulatory role of TGF-beta in the interactions of neurons and Schwann cells using an in vitro myelinating system. Purified populations of neurons and Schwann cells, grown alone or in coculture, secrete readily detectable levels of the three mammalian isoforms of TGF-beta; in each case, virtually all of the TGF-beta activity detected is latent. Expression of TGF-beta 1, a major isoform produced by Schwann cells, is specifically and significantly downregulated as a result of axon/Schwann cell interactions. Treatment of Schwann cells or Schwann cell/neuron cocultures with TGF-beta 1, in turn, has dramatic effects on proliferation and differentiation. In the case of purified Schwann cells, treatment with TGF-beta 1 increases their proliferation, and it promotes a pre- or nonmyelinating Schwann cell phenotype characterized by increased NCAM expression, decreased NGF receptor expression, inhibition of the forskolin-mediated induction of the myelin protein P0, and induction of the Schwann cell transcription factor suppressed cAMP-inducible POU protein. Addition of TGF-beta 1 to the cocultures inhibits many of the effects of the axon on Schwann cells, antagonizing the proliferation induced by contact with neurons, and, strikingly, blocking myelination. Ultrastructural analysis of the treated cultures confirmed the complete inhibition of myelination and revealed only rudimentary ensheathment of axons. Associated defects of the Schwann cell basal lamina and reduced expression of laminin were also detected. These effects of TGF-beta 1 on Schwann cell differentiation are likely to be direct effects on the Schwann cells themselves which express high levels of TGF-beta 1 receptors when cocultured with neurons. The regulated expression of TGF-beta 1 and its effects on Schwann cells suggest that it may be an important autocrine and paracrine mediator of neuron/Schwann cell interactions. During development, TGF-beta 1 could serve as an inhibitor of Schwann cell proliferation and myelination, whereas after peripheral nerve injury, it may promote the transition of Schwann cells to a proliferating, nonmyelinating phenotype, and thereby enhance the regenerative response.

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PubMed1995

Cloning of neurotrimin defines a new subfamily of differentially expressed neural cell adhesion molecules.

Previous studies in the laboratory indicated that glycosylphosphatidylinositol (GPI)-anchored proteins may generate diversity of the cell surface of different neuronal populations (Rosen et al., 1992). In this study, we have extended these findings and surveyed the expression of GPI-anchored proteins in the developing rat CNS. In addition to several well characterized GPI-anchored cell adhesion molecules (CAMs), we detected an unidentified broad band of 65 kDa that is the earliest and most abundantly expressed GPI-anchored species in the rat CNS. Purification of this protein band revealed that it is comprised of several related proteins that define a novel subfamily of immunoglobulin-like (Ig) CAMs. One of these proteins is the opiate binding-cell adhesion molecule (OBCAM). We have isolated a cDNA encoding a second member of this family, that we have termed neurotrimin, and present evidence for the existence of additional family members. Like OBCAM, with which it shares extensive sequence identity, neurotrimin contains three immunoglobulin-like domains. Both proteins are encoded by distinct genes that may be clustered on the proximal end of mouse chromosome 9. Characterization of the expression of neurotrimin and OBCAM in the developing CNS by in situ hybridization reveals that these proteins are differentially expressed during development. Neurotrimin is expressed at high levels in several developing projection systems: in neurons of the thalamus, subplate, and lower cortical laminae in the forebrain and in the pontine nucleus, cerebellar granule cells, and Purkinje cells in the hindbrain. Neurotrimin is also expressed at high levels in the olfactory bulb, neural retina, dorsal root ganglia, spinal cord, and in a graded distribution in the basal ganglia and hippocampus. OBCAM has a much more restricted distribution, being expressed at high levels principally in the cortical plate and hippocampus. These results suggest that these proteins, together with other members of this family, provide diversity to the surfaces of different neuronal populations that could be important in the specification of neuronal connectivity.

Amino Acid Sequence↗
PubMed1993

Axonal regulation of Schwann cell integrin expression suggests a role for alpha 6 beta 4 in myelination.

Ensheathment and myelination of axons by Schwann cells in the peripheral nervous system requires contact with a basal lamina. The molecular mechanism(s) by which the basal lamina promotes myelination is not known but is likely to reflect the activity of integrins expressed by Schwann cells. To initiate studies on the role of integrins during myelination, we characterized the expression of two integrin subunits, beta 1 and beta 4, in an in vitro myelination system and compared their expression to that of the glial adhesion molecule, the myelin-associated glycoprotein (MAG). In the absence of neurons, Schwann cells express significant levels of beta 1 but virtually no beta 4 or MAG. When Schwann cells are cocultured with dorsal root ganglia neurons under conditions promoting myelination, expression of beta 4 and MAG increased dramatically in myelinating cells, whereas beta 1 levels remained essentially unchanged. (In general agreement with these findings, during peripheral nerve development in vivo, beta 4 levels also increase during the period of myelination in sharp contrast to beta 1 levels which show a striking decrease.) In cocultures of neurons and Schwann cells, beta 4 and MAG appear to colocalize in nascent myelin sheaths but have distinct distributions in mature sheaths, with beta 4 concentrated in the outer plasma membrane of the Schwann cell and MAG localized to the inner (periaxonal) membrane. Surprisingly, beta 4 is also present at high levels with MAG in Schmidt-Lanterman incisures. Immunoprecipitation studies demonstrated that primary Schwann cells express beta 1 in association with the alpha 1 and alpha 6 subunits, while myelinating Schwann cells express alpha 6 beta 4 and possibly alpha 1 beta 1. beta 4 is also downregulated during Wallerian degeneration in vitro, indicating that its expression requires continuous Schwann cell contact with the axon. These results indicate that axonal contact induces the expression of beta 4 during Schwann cell myelination and suggest that alpha 6 beta 4 is an important mediator of the interactions of myelinating Schwann cells with the basal lamina.

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PubMed1992

Expression of unique sets of GPI-linked proteins by different primary neurons in vitro.

We have surveyed the proteins expressed at the surface of different primary neurons as a first step in elucidating how axons regulate their ensheathment by glial cells. We characterized the surface proteins of dorsal root ganglion neurons, superior cervical ganglion neurons, and cerebellar granule cells which are myelinated, ensheathed but unmyelinated, and unensheathed, respectively. We found that the most abundant proteins are common to all three types of neurons. Reproducible differences in the composition of the integral membrane proteins (enriched by partitioning into a Triton X-114 detergent phase) were detected. These differences were most striking when the expression of glycosylphosphatidyl-inositol (GPI)-anchored membrane proteins by these different neurons was compared. Variations in the relative abundance and degree of glycosylation of several well known GPI-anchored proteins, including Thy-1, F3/F11, and the 120-kD form of the neural cell adhesion molecule (N-CAM), and an abundant 60-kD GPI-linked protein were observed. In addition, we have identified several potentially novel GPI-anchored glycoproteins on each class of neurons. These include a protein that is present only on superior cervical ganglion neurons and is 90 kD; an abundant protein of 69 kD that is essentially restricted in its expression to dorsal root ganglion neurons; and proteins of 38 and 31 kD that are expressed only on granule cell neurons. Finally, the relative abundance of the three major isoforms of N-CAM was found to vary significantly between these different primary neurons. These results are the first demonstration that nerve fibers with diverse ensheathment fates differ significantly in the composition of their surface proteins and suggest an important role for GPI-anchored proteins in generating diversity of the neuronal cell surface.

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PubMed1991

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.

Animals↗
PubMed1991

Cell adhesion properties of myelin-associated glycoprotein in L cell fibroblasts.

Myelin-associated glycoprotein (MAG) is a cell surface molecule expressed by oligodendrocytes and Schwann cells. In order to determine whether MAG expression can confer adhesive properties to cells which normally do not aggregate in suspension, the cDNA encoding the long form of MAG (L-MAG) was introduced into L cell fibroblasts by retroviral infection. Clonal L cell lines expressing MAG were then subjected to a cell aggregation assay. Our results indicate that L-MAG can function as an intercellular adhesion molecule in a heterologous cell system. A critical threshold value of L-MAG expression was required for cell aggregation to occur. The adhesive properties of these cells were specific to MAG, since monoclonal antibodies directed against its extracellular domain inhibited aggregation. Furthermore, the adhesion was found to be calcium- and temperature-independent. Cell sorting experiments demonstrated that L-MAG-expressing cells bind in a heterotypic fashion to parental L cell fibroblasts. These results suggest that L-MAG can function as a heterotypic cell adhesion molecule recognizing a cell surface molecule(s) expressed by L cells.

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PubMed1990

Expression of recombinant myelin-associated glycoprotein in primary Schwann cells promotes the initial investment of axons by myelinating Schwann cells.

Myelin-associated glycoprotein (MAG) is an integral membrane protein expressed by myelinating glial cells that occurs in two developmentally regulated forms with different carboxyterminal cytoplasmic domains (L-MAG and S-MAG). To investigate the role of MAG in myelination a recombinant retrovirus was used to introduce a MAG cDNA (L-MAG form) into primary Schwann cells in vitro. Stably infected populations of cells were obtained that constitutively expressed MAG at the cell surface without the normal requirement for neuronal contact to induce expression. Constitutive expression of L-MAG did not affect myelination. In long term co-culture with purified sensory neurons, the higher level of MAG expression on infected Schwann cells was reduced to control levels on cells that formed myelin. On the other hand, unlike normal Schwann cells, infected Schwann cells associated with nonmyelinated axons or undergoing Wallerian degeneration expressed high levels of MAG. This suggests that a posttranscriptional mechanism modulates MAG expression during myelination. Immunostaining myelinating cultures with an antibody specific to L-MAG showed that L-MAG was normally transiently expressed at the earliest stages of myelination. In short term co-culture with sensory neurons, infected Schwann cells expressing only L-MAG segregated and ensheathed larger axons after 4 d in culture provided that an exogenous basal lamina was supplied. Similar activity was rarely displayed by control Schwann cells correlating with the low level of MAG induction after 4 d. These data strongly suggest that L-MAG promotes the initial investment by Schwann cells of axons destined to be myelinated.

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