Gene expression in the jimpy mutant: evidence for fewer oligodendrocytes expressing myelin protein genes and impaired translocation of myelin basic protein mRNA.

Myelin basic protein (MBP) and proteolipid protein (PLP) gene expression was investigated in the murine dysmyelinating mutant, jimpy and age-matched normal mice. MBP and PLP mRNA expression was examined in several brain regions by in situ hybridization histochemistry between 10 and 20 days postpartum. The results showed a general reduction in both PLP and MBP mRNA expression throughout in jimpy brain due primarily to fewer numbers of jimpy oligodendrocytes expressing these genes. A less severe, but significant, reduction in the level of PLP expression per oligodendrocyte was also observed in the mutant brain. Because of the diffuse pattern of MBP mRNA distribution in the controls it was not possible to determine whether MBP expression was reduced on a per cell basis in the jimpy mutant. Silver grains representing MBP mRNAs were diffusely distributed over myelinating regions in normal mice but, in contrast, the grains were primarily clustered over oligodendrocyte cell bodies in the jimpy brains. No significant differences in jimpy oligodendrocyte morphology were evident by galactocerebroside immunohistochemistry. These results suggested that the apparent MBP mRNA clustering was not due to truncated or reduced numbers of oligodendrocyte processes. Rather, they suggest that the inability of jimpy mice to incorporate MBP into myelin may be due, in part, to a disruption in the translocation of MBP mRNAs within the oligodendrocyte.