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J L Salzer

Publications and source records attributed to J L Salzer.

49 records · Page 3Linked to original sources
PubMed1990

The myelin-associated glycoproteins: membrane disposition, evidence of a novel disulfide linkage between immunoglobulin-like domains, and posttranslational palmitylation.

The myelin-associated glycoproteins (MAG) are members of the immunoglobulin gene superfamily that function in the cell interactions of myelinating glial cells with axons. In this paper, we have characterized the structural features of these proteins. The disposition of MAG in the bilayer as a type 1 integral membrane protein (with an extracellularly disposed amino terminus, single transmembrane segment, and cytoplasmic carboxy terminus) was demonstrated in protease protection studies of MAG cotranslationally inserted into microsomes in vitro and in immunofluorescent studies with site specific antibodies. A genetically engineered MAG cDNA, which lacks the putative membrane spanning segment, was constructed and shown to encode a secreted protein. These results confirm the identify of this hydrophobic sequence as the transmembrane segment. Sequencing of the secreted protein demonstrated the presence of a cleaved signal sequence and the site of signal peptidase cleavage. To characterize the disulfide linkage pattern of the ectodomain, we cleaved MAG with cyanogen bromide and used a panel of antibodies to coprecipitate specific fragments under nonreducing conditions. These studies provide support for a novel disulfide linkage between two of the immunoglobulin domains of the extracellular segment. Finally, we report that MAG is posttranslationally palmitylated via an intramembranous thioester linkage. Based on these studies, we propose a model for the conformation of MAG, including its RGD sequence, which is considered with regard to its function as a cell adhesion molecule.

Animals↗
PubMed1990

Differential phosphorylation of myelin-associated glycoprotein isoforms in cell culture.

The alternative splicing of myelin-associated glycoprotein (MAG) mRNA generates two isoforms that harbor distinct potential phosphorylation sites in their cytoplasmic tails. Here we characterize the in vivo phosphorylation of MAG isoforms in NIH 3T3 cells transfected with the cDNAs encoding the two isoforms of MAG. Our results demonstrate that the longer isoform, L-MAG, is phosphorylated constitutively mainly on serine, but also on threonine and tyrosine residues. This phosphorylation is subject to change by 12-O-tetradecanoylphorbol 13-acetate (TPA) and ammonium vanadate, but not by dibutyryl-cyclic AMP. The shorter isoform, S-MAG, is constitutively phosphorylated only on serine residues. While TPA and dibutyryl-cyclic AMP have no detectable effect, ammonium vanadate induces tyrosine and threonine phosphorylation in S-MAG. 32P labeling of v-src-transformed NIH 3T3 cells that express L-MAG also show that L-MAG is likely to be an in vivo substrate for pp60v-src tyrosine kinase activity. These results demonstrate that both MAG isoforms are phosphorylated in a heterologous cell system and that this phosphorylation is subject to pharmacological manipulation.

Animals↗
PubMed1989

Mechanisms of cell adhesion in the nervous system: role of the immunoglobulin gene superfamily.

The amino acid sequences of a large number of the cell surface proteins known to mediate cell-cell interactions in the nervous system have recently been reported. Many of these proteins are members of the immunoglobulin gene superfamily and have remarkably similar structures and amino acid sequences in their extracellular segments. We have termed this family of immunoglobulin-related cell adhesion molecules the Ig-CAMs. In this article, we review the structural features of the Ig-CAMs, discuss how these features may relate to the role of these proteins in mediating cell adhesion and cell signaling events, and finally consider the evolutionary origins of this family of proteins.

Animals↗
PubMed1988

Chromosomal location of the mouse gene that encodes the myelin-associated glycoproteins.

The two myelin-associated glycoprotein (MAG) species, designated large MAG (L-MAG) and small MAG (S-MAG), are believed to be generated by differential splicing from a single RNA transcript. We have now defined the genetic locus encoding the two MAG proteins in the mouse. Analysis of a panel of interspecies somatic cell hybrids indicated that all MAG coding sequences reside on chromosome 7. Following the inheritance of a restriction fragment length polymorphism associated with MAG coding sequences allowed the locus to be positioned 0.5 centimorgans from the locus Abpa (androgen binding protein alpha) on proximal chromosome 7. These data strongly support the hypothesis that a single gene encodes the two MAG proteins, and we propose the name Mag for this locus. This localization places Mag in close proximity to the neurological mutant locus qv (quivering) and raises the possibility of a functional relationship or identity between Mag and qv. However, an analysis of the MAG gene, its RNA transcripts, and its protein products revealed no abnormalities in homozygous qv mutant mice, suggesting that this chromosomal linkage is not etiologically significant.

Animals↗
PubMed1987

The amino acid sequences of the myelin-associated glycoproteins: homology to the immunoglobulin gene superfamily.

The myelin associated glycoproteins (MAG) are integral plasma membrane proteins which are found in oligodendrocytes and Schwann cells and are believed to mediate the axonal-glial interactions of myelination. In this paper we demonstrate the existence in central nervous system myelin of two MAG polypeptides with Mrs of 67,000 and 72,000 that we have designated small MAG (S-MAG) and large MAG (L-MAG), respectively. The complete amino acid sequence of L-MAG and a partial amino acid sequence of S-MAG have been deduced from the nucleotide sequences of corresponding cDNA clones isolated from a lambda gt11 rat brain expression library. Based on their amino acid sequences, we predict that both proteins have an identical membrane spanning segment and a large extracellular domain. The putative extracellular region contains an Arg-Gly-Asp sequence that may be involved in the interaction of these proteins with the axon. The extracellular portion of L-MAG also contains five segments of internal homology that resemble immunoglobulin domains, and are strikingly homologous to similar domains of the neural cell adhesion molecule and other members of the immunoglobulin gene superfamily. In addition, the two MAG proteins differ in the extent of their cytoplasmically disposed segments and appear to be the products of alternatively spliced mRNAs. Of considerable interest is the finding that the cytoplasmic domain of L-MAG, but not of S-MAG, contains an amino acid sequence that resembles the autophosphorylation site of the epidermal growth factor receptor.

Amino Acid Sequence↗
PubMed1980

Studies of Schwann cell proliferation. I. An analysis in tissue culture of proliferation during development, Wallerian degeneration, and direct injury.

In this paper the stimuli for and pattern of Schwann cell proliferation are defined under various experimental conditions. We used a tissue culture system in which fetal rat dorsal root ganglia, treated to eliminate contaminating fibroblasts (Wood, P., 1976, Brain Res. 115:361--375), appear to recapitulate many aspects of the developing peripheral nervous system. We observed that: (a) proliferation of Schwann cells on neurites is initially rapid, but, as each neurite becomes fully ensheathed, division slows considerably and is confined to the periphery of the outgrowth; (b) during the period of rapid proliferation, excision of the ganglion causes a rapid decay in the number of dividing cells; (c) excision of the ganglion from more established cultures in which there was little ongoing proliferation resulted in a small increase in labeling at the site of excision for all Schwann cells and a substantial increase in labeling for myelin-related cells with a peak labeling period at 4 d; (d) direct mechanical injury during Wallerian degeneration is mitogenic for Schwann cells; (e) a variety of potential mitogens failed to stimulate Schwann cell proliferation, and (f) replated cells have a slightly higher level of proliferation and show a small and variable response to the addition of cAMP.

Animals↗
PubMed1980

Studies of Schwann cell proliferation. II. Characterization of the stimulation and specificity of the response to a neurite membrane fraction.

When prepared by methods utilized in our laboratory, pure populations of Schwann cells in culture do not divide, but, after recombination with peripheral sensory neurons or their processes, proliferate rapidly (Wood and Bunge, 1975, Nature (Lond.) 256:661--664). In this paper, we demonstrate that a membrane fraction prepared from sensory ganglion neurites is also mitogenic for Schwann cells and increases the labeling index (assessed by autoradiography after incubation of cells with tritiated thymidine) from less than 0.2 to 10% for primary cells, and from 0.4 to 18--19% for replated cells. The increased responsiveness of replated cells may reflect their greater access to the neurite membranes which is a consequence of the elimination of multiple cell layers after replating and the removal of the basal lamina. This stimulation was specific; addition of membrane preparations from other cell types (3T3, C1300, etc.) was not mitogenic. Ultrastructural analysis demonstrated apparent binding of neurite membranes to Schwann cells as well as significant phagocytosis of the membranes by the cells. The uptake of nonmitogenic membranes suggests that phagocytosis per se is not the stimulus of proliferation.

Axons↗
PubMed1980

Studies of Schwann cell proliferation. III. Evidence for the surface localization of the neurite mitogen.

In the preceding paper (Salzer et al., 1980, J. Cell Biol. 84:753--766), evidence was presented that a neurite membrane fraction could be used to stimulate Schwann cell proliferation in culture. In this study, we present evidence that the mitogenic signal by which intact neurites or neurite membranes stimulate Schwann cell proliferation is located at the neurite surface. This conclusion is based on the following observations: (a) stimulation of Schwann cell proliferation by neurons requires direct contact between neurites and Schwann cells, separation of the two cells by a permeable collagen diaphragm 6 microns thick prevents Schwann cell proliferation; (b) treatment of intact neurites with trypsin before preparation of neurite membranes abolishes the ability of these membranes to be mitogenic for Schwann cells; and (c) the mitogenic activity of neurite homogenates is exclusively localized in the particulate rather than the soluble fraction of the homogenate. The mitogenic component on the neurite surface is heat labile, and is inactivated by aldehyde fixation. Preliminary data suggest that the mitogenic effect of neurite on Schwann cells is not mediated by 3',5'-cyclic AMP.

1-Methyl-3-isobutylxanthine↗
PubMedDate not supplied

Clustering of neuronal sodium channels requires contact with myelinating Schwann cells.

Efficient and rapid conduction of action potentials by saltatory conduction requires the clustering of voltage-gated sodium channels at nodes of Ranvier. This clustering results from interactions between neurons and myelinating glia, although it has not been established whether this glial signal is contact-dependent or soluble. To investigate the nature of this signal, we examined sodium channel clustering in co-cultures of embryonic rat dorsal root ganglion neurons and Schwann cells. Cultures maintained under conditions promoting or preventing myelination were immunostained with antibodies against the alpha subunit of the sodium channel and against ankyrin(G), a cytoskeletal protein associated with these channels. Consistent with previous in vivo studies (Vabnick et al., 1996), sodium channels and ankyrin G cluster at the onset of myelination. These clusters form adjacent to the ends of the myelinating Schwann cells and appear to fuse to form mature nodes. In contrast, sodium channels and ankyrin G do not cluster in neurons grown alone or in co-cultures where myelination is precluded by growing cells in defined media. Conditioned media from myelinating co-cultures also failed to induce sodium channel or ankyrin G clusters in cultures of neurons alone. Finally, no clusters develop in the amyelinated portions of suspended fascicles of dorsal root ganglia explants despite being in close proximity to myelinated segments in other areas of the dish. These results indicate that clustering of sodium channels requires contact with myelinating Schwann cells.

Animals↗
PubMedDate not supplied

A new developmental screening test. The Denver II.

To maximize the potential of a child with a developmental delay, early detection and intervention is essential (Frankenburg and Thornton, 1989). The nurse practitioner who includes a developmental screening with the child's general assessment may detect a developmental delay during a routine office visit. The purpose of this article is to provide an overview of the Denver II, a new developmental screening test.

Child, Preschool↗