Developmental Expression of the Myelin Proteolipid Protein and Basic Protein mRNAs in Normal and Dysmyelinating Mutant Mice

Expression of the myelin proteolipid protein (PLP) was examined in the nuclei and polysomes of 12-27-day-old quaking, jimpy, and shiverer mouse brains and in 2-27-day-old normal brains and compared with expression of the myelin basic proteins (MBPs). Northern blots showed the presence of multiple mouse PLP RNAs, the developmental expression of which coincided with myelination. Two major mouse PLP RNAs, 3.5 and 2.6 kilobases in length, were observed in both cytoplasmic polyribosomes and nuclei, and, in addition, a larger 4.6-kilobase PLP RNA was observed in nuclei. Quantitative measurements with slot blot analyses showed that the levels of PLP and MBP RNAs peaked simultaneously at 18 days in nuclei but that maximal levels of PLP RNA lagged behind MBP RNA by several days in the polysomes. The developmental expression of both major classes of myelin protein mRNAs was affected in all three mutants. In shiverer brains, the levels of PLP mRNA in polysomes and nuclei were only 30-55% of control levels after 15 days. Thus, the deletion of a portion of the MBP gene appeared to have a major effect on the expression of the PLP gene in this mutant. In jimpy mice, where the mutation has been shown to involve the PLP gene, expression of MBP mRNA was also severely reduced, to less than 25% of control values. In quaking brains, the expression of each gene followed its own developmental course, different from each other and different from the normal mouse. The extent to which the expression of PLP and MBP was affected by the quaking mutation depended on the age at which it was examined.