Glycoproteins And Glycolipids Associated With Myelin In Development And Disease

尽管MAG-null小鼠PNS有髓鞘纤维有明显的轴突变性，但早期报道表明这些小鼠CNS有髓鞘轴突是正常的。然而，最近的一份报告表明，在MAG-null小鼠的中枢神经系统中也发生了类似的髓鞘轴突退化，这是一个需要进一步研究来澄清的领域。最近的报道涉及到MAG-null小鼠，这些小鼠已经回交到C57BL/6背景，并且表明中枢神经系统的影响与小鼠的遗传背景有关。在过去的几年中，我们完成了与C57BL/6J小鼠的10多代回交，获得了MAG-null小鼠的基因系，其C57BL/6J衍生的基因组含量达到99.5%。这条基因系将在许多方面极大地促进MAG的研究。我们目前正在与NINDS的神经元细胞骨架蛋白调控组合作，以确定在这一基因mag缺失的小鼠系中是否存在中枢神经系统髓鞘轴突变性。我们当前研究的另一个目标是确定未成熟可塑性神经元中MAG受体的发现与髓鞘轴突轴周室中MAG介导的信号传导之间的关系，这对它们的正常维持至关重要。最近的实验涉及用从富含轴突膜和轴突周围少突胶质膜的白质中分离的膜组分进行Western blotting。MAG在这部分中富集，而NgR和p75NtR存在，但不富集。发育中的大脑中NgR和p75NtR的水平高于成人大脑。这些发现与NgR和p75NtR是髓鞘轴突MAG功能受体复合物的一部分相一致。

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.