GLYCOPROTEINS AND GLYCOLIPIDS ASSOCIATED WITH MYELIN IN DEVELOPMENT AND DISEASE

本项目主要研究雪旺细胞和少突胶质细胞在髓鞘形成和脱髓鞘过程中的糖蛋白和糖脂。这项研究的一个主要方面涉及到髓鞘相关糖蛋白(MAG)，它定位于有髓纤维的轴突周围神经胶质膜上，并在轴突和髓鞘形成细胞之间传递信号。MAG属于免疫球蛋白超家族的I型凝集素亚群，并与含有末端α2-3唾液酸的糖偶联物结合，提示其配体可能是一种糖蛋白，其他糖蛋白上的唾液酸部分和MAG本身可能调节其功能。虽然众所周知，脑糖蛋白上的唾液酸键大多是α2-3，但我们发现小鼠和人类周围神经中的唾液酸键主要是α2-6。此外，在一些动物和人类神经疾病中，MAG和其他糖蛋白上的α2-3唾液酸的表达增加，我们的结果表明，这可能通过干扰MAG介导的信号转导而参与病理过程。我们继续调查去年报道的MAG与微管相关蛋白1B(MAP1B)结合的发现，令人惊讶的结果表明，神经元中的一些MAP1B以唾液酸化的表膜糖蛋白的形式表达。如果MAP1B是MAG的一个生理配体，它可能在MAG介导的信号传递中发挥作用，这些信号已知会影响有髓轴突的细胞骨架结构。MAG基因敲除小鼠的详细生化研究以及表达MAG的细胞和神经元共培养的实验正在进行中，以进一步确定MAG功能所涉及的分子机制。之前关于中枢神经系统髓鞘紊乱(Z01 NS 02848 LMCN)和神经疾病糖共轭抗体(Z01 NS 02786 LMCN)的项目报告已经终止，这些领域的研究成果将从今年开始纳入该项目。我们以前对多发性硬化症(MS)的研究表明，MAG被与髓鞘相关的组织蛋白酶L样中性蛋白酶转化为可溶的90kD衍生物(DMAG)，与其他髓鞘蛋白相比，MAG优先从MS活性斑块的边缘丢失。由于在成人脑中MAG仅定位于髓鞘少突胶质细胞的轴突周围膜，MS中MAG的选择性定性和定量改变表明，MS的轴突周围膜存在损害，提示其病理方面可能是垂死性少突胶质细胞病。正在对MS组织进行实验，以确定这种疾病中还发生了哪些轴突周围异常，并将结果与MAG基因敲除小鼠的轴突周围变化进行比较，MAG基因敲除小鼠也表现出垂死性的少突胶质细胞病。MAG也与自身免疫性脱髓鞘周围神经病有关，在这些疾病中，抗MAG IgM单抗针对MAG中的碳水化合物表位，并与PNS髓鞘的其他糖蛋白包括PO和PMP-22，以及与糖鞘糖脂-3-葡萄糖醛酸基副糖苷(SGPG)发生交叉反应。然而，我们最近的结果表明，这些人类抗体在患者中表现出显著的异质性，因为它们与这些潜在的靶抗原结合的相对强度可能会影响致病机制并导致临床变异性。-自身免疫、神经节苷脂、糖脂、糖蛋白、多发性硬化症、髓鞘、神经病变、少突胶质细胞、雪旺细胞

This project focuses on glycoproteins and glycolipids of Schwann cells and oligodendrocytes during myelination and demyelination. A major aspect of the research concerns the myelin-associated glycoprotein (MAG) that is localized in periaxonal glial membranes of myelinated fibers and functions in transmitting signals between axons and myelin- forming cells. MAG is in the I-type lectin subgroup of the immunoglobulin superfamily and binds to glycoconjugates containing terminal alpha2-3 sialic acid, suggesting that its ligand could be a glycoprotein and that sialic acid moieties on other glycoproteins and on MAG itself could modulate its function. Whereas most of the sialic acid linkages on brain glycoproteins are well known to be alpha 2-3, we have found that sialic acid linkages in mouse and human peripheral nerve are predominantly alpha 2-6 Furthermore, the expression of alpha 2-3 sialic acid on MAG and other glycoproteins of nerve is increased in some animal and human neuropathies, and our results suggest that this contributes to pathology by interfering with MAG-mediated signaling. We have continued to investigate the finding reported last year that MAG binds to microtubule-associated protein 1B (MAP1B) and the surprising results suggesting some MAP1B in neurons is expressed as a sialylated surface membrane glycoprotein. If MAP1B is a physiological ligand for MAG, it could function in the transmission of MAG-mediated signals that are known to affect the cytoskeletal structure of myelinated axons. A detailed biochemical investigation of MAG knockout mice and experiments with cocultures of MAG-expressing cells and neurons are underway to further define the molecular mechanisms involved in MAG function. The previous project reports entitled Disorders of CNS Myelin(Z01 NS 02848 LMCN) and Antibodies to Glycoconjugates in Neurological Diseases (Z01 NS 02786 LMCN) describing our clinically-related research on myelin have been terminated, and results in those areas will be incorporated into this project beginning this year. Our previous research on multiple sclerosis (MS) had shown that MAG is converted to a soluble 90 kD derivative (dMAG) by a cathepsin L-like neutral protease associated with myelin and is preferentially lost from the edge of active MS plaques in comparison to other myelin proteins. Since in adult brain, MAG is localized exclusively in periaxonal membranes of myelinating oligodendroctyes, the selective qualitative and quantitative alterations of MAG in MS indicate that there is damage to the periaxonal membranes, suggesting that one aspect of the pathology may be a dying-back oligodendrogliopathy. Experiments are being undertaken in MS tissue to determine what other periaxonal abnormalities occur in this disease and to compare the results to periaxonal changes in MAG knockout mice that also exhibit a dying-back oligodendrogliopathy. MAG has also been implicated in autoimmune demyelinating peripheral neuropathies in which anti-MAG IgM monoclonal antibodies are directed toward carbohydrate epitopes in MAG and cross react with other glycoproteins of PNS myelin including PO and PMP-22, as well as with the glycosphingolipid, sulfate-3-glucuronyl paragloboside (SGPG). However, our recent results have shown that these human antibodies exhibit substantial heterogeneity among patients with regard to the relative strengths with which they bind to these potential target antigens that could affect pathogenic mechanisms and contribute to clinical variability. - autoimmunity, ganglioside, glycolipid, glycoprotein, multiple sclerosis, myelin, neuropathy, oligodendrocyte, Schwann cell