Expression of the amino acid dimorphism in the small myelin-associated glycoprotein cytoplasmic domain in rat peripheral nerves during postnatal development

Myelin-associated glycoprotein and galactosylcerebroside expression in Schwann cells during myelination

The temporo-spatial expression profiles of the myelin-associated glycoprotein (MAG) and galactosylcerebrosides (Gal-CBs) were analysed during Schwann cell differentiation and myelination in Schwann cell cultures and in dorsal root ganglion/Schwann cell cocultures. The temporal expression profiles demonstrate that the mechanisms triggering and regulating MAG and Gal-CBs are different and independent. They also provide further support for a role of MAG, but not Gal-CBs, in the establishment of the Schwann cell/axon interaction and in the formation of the primary mesaxon, while both MAG and Gal-CBs may participate in the further maturation of the mesaxon. The spatial expression profiles demonstrate that although Gal-CBs are concentrated in the same structures (mesaxonal spirals, paranodes and Schmidt-Lanterman incisures) as MAG in both developing and mature sheaths, where it is therefore possible that the two might have complementary functions, they are also present in regions from which MAG is excluded.

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

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

The small myelin-associated glycoprotein binds to tubulin and microtubules

The myelin-associated glycoprotein (MAG) exists as two isoforms, differing only by their respective cytoplasmic domains, that have been suggested to function in the formation and maintenance of myelin. In the present study, a 50 kDa protein binding directly to the small MAG (S-MAG) cytoplasmic domain was detected and identified as tubulin, the core component of the microtubular cytoskeleton. In vitro, the S-MAG cytoplasmic domain slowed the polymerization rate of tubulin and co-purified with assembled microtubules. A significant sequence homology was found between the tau family tubulin-binding repeats and the carboxy-terminus of S-MAG. Our results indicate that S-MAG is the first member of the Ig superfamily that can be classified as a microtubule-associated protein, and place S-MAG in a dynamic structural complex that could participate in linking the axonal surface and the myelinating Schwann cell cytoskeleton.

S100beta inhibits the phosphorylation of the L-MAG cytoplasmic domain by PKA

The myelin-associated glycoprotein (MAG) is a cell adhesion molecule expressed by myelinating glia, existing as two isoforms that differ only by their cytoplasmic domains. We have studied the in vitro phosphorylation of recombinant rat MAG cytoplasmic domains by three kinases for which consensus sequences exist within this domain, revealing phosphorylation of the L-MAG-specific domain by protein kinase A (PKA). Phosphorylation of the L-MAG cytoplasmic domain by PKA was decreased in the presence of S100beta, providing a functional significance to the interaction between L-MAG and S100beta, and further indicating that L-MAG may play a role in myelinating glial cell signalling processes.

Estimation of total ribonucleic acid quantity from dilute samples by nondenaturing electrophoresis and silver staining

Often, the amount of RNA that can be isolated from a defined tissue is very small. A method consisting of nondenaturing polyacrylamide gel electrophoresis and silver staining is described that can be used to evaluate the concentration of very dilute RNA samples. The method is a good starting point for assays dealing with small amounts of RNA, such as semiquantitative reverse transcription polymerase chain reaction (RT-PCR), making it possible to perform parallel assays from similar amounts of total RNA when quantitation by other methods is too insensitive. The method has been used successfully to monitor the amount of total RNA isolated from rat sciatic nerve and the rat C6 glioma cell line.

Calcium-dependent interaction between the large myelin-associated glycoprotein and S100beta

The myelin-associated glycoprotein is a transmembrane cell adhesion molecule expressed by myelinating glial cells of the nervous system. So far, only protein kinases have been reported to interact with the cytoplasmic domains of the two isoforms of the myelin-associated glycoprotein. We report here the identification of the first nonkinase intracellular ligand for the large isoform of the myelin-associated glycoprotein as the S100beta protein. The interaction is dependent on the presence of calcium. We have also localized the S100beta-binding site in the cytoplasmic domain specific to the large myelin-associated glycoprotein isoform to a putative basic amphipathic alpha-helix. A synthetic peptide corresponding to this region bound to S100beta in a calcium-dependent manner with a stoichiometric ratio of 1:1 (K(D) approximately 7 microM). We suggest that the observed interaction may play a role in the regulation of the myelinating glial cell cytoskeleton and the divalent cation-dependent signal transduction events during myelin formation and maintenance.

The expression of recombinant large myelin-associated glycoprotein cytoplasmic domain and the purification of native myelin-associated glycoprotein from rat brain and peripheral nerve

The myelin-associated glycoprotein (MAG) is a transmembrane protein of the immunoglobulin superfamily existing as two isoforms (L-MAG and S-MAG) that are differentially expressed by myelinating glial cells of the central and peripheral nervous systems, where MAG represents 1 and 0.1% of the total myelin proteins, respectively. The polypeptide chains of the two isoforms differ only by the carboxy terminus of their respective cytoplasmic domains, which most probably determine the isoform-specific functions. Here, we describe the expression of the L-MAG cytoplasmic domain as a GST fusion protein. The recombinant protein was used to raise polyclonal antibodies against the L-MAG-specific carboxy terminus and against the region of the MAG cytoplasmic domain common to both S-MAG and L-MAG. These antibodies, which function in dot blotting, Western blotting, and immunoprecipitation, were used to immunopurify native MAG from both rat brain and peripheral nerves in quantities and purity sufficient for the realization of most biochemical and functional studies. The antibodies and the recombinant and native MAG proteins provide much needed tools for the study of the common and isoform-specific properties and functions of L-MAG and S-MAG.