Glycoproteins And Glycolipids Associated With Myelin In Development And Disease

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Although there is clearly axonal degeneration in myelinated fibers in the PNS of MAG-null mice, early reports indicated the myelinated axons in the CNS of these mice were normal. However, a more recent report suggests that similar degeneration of myelinated axons occurs in the CNS of MAG-null mice as in the PNS, and this is an area that needs clarification by further investigation. The more recent report involved MAG-null mice that had been backcrossed to the C57BL/6 background, and it was suggested that the effect in the CNS was related to the genetic background of the mice. In the past few years, we completed more than 10 generations of backcrossing with C57BL/6J mice to obtain a congenic line of MAG-null mice exhibiting 99.5% C57BL/6J-derived genome content. This congenic line should substantially facilitate research on MAG in many ways. We are currently collaborating with the Neuronal Cytoskeletal Protein Regulation Section in NINDS to determine if there is degeneration of CNS myelinated axons in this line of congenic MAG-null mice. Another goal of our current research is to determine how findings about MAG receptors in immature plastic neurons relate to MAG-mediated signaling within the periaxonal compartment of myelinated axons that is essential for their normal maintenance. Recent experiments involve Western blotting with a membrane fraction isolated from white matter that is enriched in axolemma and periaxonal oligodendroglial membranes. MAG is enriched in this fraction, whereas the NgR and p75NtR are present, but not enriched. Levels of the NgR and p75NtR in the fraction are higher in developing brain than adult brain. These findings are consistent with the NgR and p75NtR being part of a functional receptor complex for MAG within myelinated axons. Experiments were also done with primary oligodendrocyte cultures and oligodendroglial cell lines to demonstrate and characterize MAG-mediated signaling pathways that promote the differentiation and/or survival of these cells. One approach for activating MAG-mediated signaling in cultured oligodendrocytes is cross-linking with anti-MAG antibodies. Under control conditions, little oligodendroglial MAG is associated with lipid rafts, but upon activation, much of the MAG shifts into the raft fraction. This is associated with a number of changes in the oligodendroctyes, including activation of Fyn tyrosine kinase, dephosphorylation of serine and threonine residues in some other proteins, and cleavage of alpha-fodrin followed by a transient depolymerization of actin. More recent experiments have indicated that activation of Fyn involves phosphorylation on tyrosine-416 and that oligodendroglial ERK 1/2 is also activated. These in vitro results support the hypothesis that MAG is a receptor for transmitting an axonal signal that activates a signaling cascade affecting the properties of oligodendrocytes.