Myelin protein expression in the myelin-deficient rat brain and cultured oligodendrocytes.

The myelin-deficient (MD) rat does not express the major protein of CNS myelin, proteolipid protein (PLP). Here we further analyze whether this defect is reflected at the level of mRNA and whether the expression of other myelin proteins is affected in oligodendrocytes in vivo and in vitro. Both myelin basic protein (MBP) and PLP message levels were reduced in MD rats to 10-20% of the normal littermate controls, while the level of expression of an astrocyte-specific gene, glial fibrillary acidic protein (GFAP), was normal. Although MBP and PLP mRNAs were equally depressed, only MBP was detected with immunolabeling of corpus callosum, while PLP was absent in oligodendrocytes both in vivo and in vitro. A reduced number of MD rat oligodendrocytes express MBP in vitro compared to controls. The MD rat optic nerve contains normal numbers of 0-2A progenitors, but they tend to differentiate into GC-positive oligodendrocytes faster than oligodendrocytes from control littermates. In conclusion, the absence of PLP and reduced levels of MBP in the MD rats point to similarities with the jimpy mouse lesion. Moreover, the defect influences the expression of other myelin proteins and the oligodendrocyte developmental pathway.