Cytoskeleton in myelin-basic-protein-deficient shiverer oligodendrocytes.

Production of CNS myelin by oligodendrocytes requires the regulated synthesis and assembly of cytoskeletal components. However, the molecular signals that mediate this process are not known. Here we use the shiverer mutant mouse, which is missing a large segment of the myelin basic protein (MBP) gene, to investigate the possible role in cytoskeletal assembly of an MBP gene product or of other myelin components whose expression may be linked to that of MBP. In axon-free cultures, we find that approximately half of shiverer oligodendrocytes have enlarged cell bodies, abnormal processes and fail to elaborate extensive membrane sheets. In those membrane sheets that are elaborated by shiverer oligodendrocytes, microtubular structures are abnormal in size and distribution. Additionally, 2',3'-cyclic nucleotide 3'-phosphohydrolase and microfilaments are not colocalized with microtubular structures as they would be in mature wild-type membrane sheets. These observations suggest that an MBP gene product has a direct or indirect role in regulating various aspects of cytoskeleton assembly in wild-type oligodendrocytes. In the absence of this signal, oligodendrocytes apparently do not normally assemble cytoskeleton; this may be one important basis for the abnormal morphology of intact shiverer CNS.

Myelin basic protein mediates extracellular signals that regulate microtubule stability in oligodendrocyte membrane sheets.

Treatment of cultured oligodendrocytes with a monoclonal antibody to galactocerebroside (GalC) triggers a cascade of events including the redistribution of membrane surface GalC over internal domains of MBP and loss of microtubular structures within the sheets (Dyer and Benjamins: J Neurosci 8:4307-4318, 1988; Dyer and Benjamins: J Neurosci Res 24:212-221, 1989). In this report, wild type and myelin basic protein (MBP)-deficient shiverer oligodendrocytes were used to study the possible relationships between these events, and specifically to determine if MBP mediates signals which destabilize microtubular assemblies in cultured oligodendrocytes. We now show that MBP and GalC, which are both initially Triton X-100 soluble, become Triton X-100 insoluble following anti-GalC binding and anti-GalC:GalC complex redistribution, suggesting that the surface anti-GalC: GalC complexes become associated with cytoplasmic MBP. Mediation of the signaling event by MBP is further demonstrated by 1) a decreased phosphorylation of MBP in wild type oligodendrocytes after antibody binding, and 2) the absence of responses, such as GalC redistribution and microtubule loss, in MBP-deficient shiverer oligodendrocytes treated with anti-GalC. Continuous activation of the GalC/MBP pathway for 7 days in wild type oligodendrocytes results in enlarged cell bodies and production of numerous microprocesses, a morphology that is similar to MBP-deficient shiverer oligodendrocytes. A second signaling pathway which produces an opposite effect, i.e., the stabilization and apparent up-regulation of microtubular structures in cultured oligodendrocyte membrane sheets, remains functional in shiverer oligodendrocytes. Thus, MBP appears to be important for mediating extracellular signals that cause a loss of microtubular structures in oligodendrocyte membrane sheets and abnormal morphology.