Regulation of cytoskeleton by myelin components: studies on shiverer oligodendrocytes carrying an Mbp transgene.

Oligodendrocytes from the shiverer mutant mouse are missing most of the myelin basic protein (Mbp) gene. In axon-free cultures, they produce membrane sheets with abnormally assembled microtubule and actin-based structures. This suggests that an Mbp gene product may have an important role in regulating the organization and stability of the wild-type oligodendrocyte cytoskeleton. We now present evidence extending these observations, using cultured oligodendrocytes that carry both the shiverer mutation and the Mbp1 transgene which partially corrects their deficit. Shiverer oligodendrocytes that carry one dose of the Mbp1 transgene abnormally express MBP along major cytoskeletal vein-like structures in processes and sheets. Shiverer oligodendrocytes that carry two doses of the Mbp1 transgene contain two types of membrane sheet regions, i.e. regions filled with aberrant punctate foci of MBP, and regions with normal domains of MBP. Immunocytochemical staining data show that the distribution of cytoskeleton and associated 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) is dependent upon how MBP is organized. Bundling of actin filaments occurs only around MBP domains, and the colocalization of CNPase along microtubular structures also appears to be regulated by MBP domains in sheets. Multinucleated oligodendrocytes are observed, a likely result of the inability of dividing pro-oligodendrocytes to bundle actin filaments. In addition, the ability of MBP to mediate extracellular signals that modulate cytoskeleton appears to be dependent upon MBP's organization. Transduction of the galactocerebroside signaling pathway, which results in the destabilization of microtubules but not actin filaments, occurs only in sheets containing MBP domains. The distribution of MBP, however, does not affect the myelin/oligodendrocyte-specific protein signaling pathway, which results in growth of microtubular structures and extensive destabilization of the actin cytoskeleton.