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Notes: Abstract: We have measured levels and synthesis of proteolipid protein (PLP) and its transport into myelin in female mice heterozygous for the jimpy gene and in their normal female littermates. In both cord and cerebrum, jimpy carriers show deficits in PLP during development followed by compensation in adulthood. Recovery of PLP occurs earlier in cord than in brain. At 13 days levels of PLP in carriers compared to controls are reduced to 0.60 and 0.44, respectively, in cord and cerebrum. By 100 days, normal levels of PLP are attained in cord (1.13) whereas levels of PLP in cerebrum are only 0.78 of control. By 200 days full recovery occurs in cerebrum, with a ratio of 1.21, suggesting a possible overcompensation. The yield of myelin from cerebrum was reduced to 0.78 in carriers compared to controls at 17 days. In brain slices, incorporation of [3H]leucine into homogenate PLP from carriers is the same as in controls, whereas [3H]Ieucine incorporation into myelin PLP is reduced to 0.68 of control. These results indicate that synthesis of PLP in the carriers is normal at 17 days, but transport of PLP into myelin is reduced. Similarly, acylation of homogenate PLP is normal, whereas acylation of myelin PLP is reduced, as measured by incorporation of [3H]palmitic acid. Transport of PLP into myelin was compared to transport of MBP; transport of both proteins was equally decreased as indicated by the similar ratio of labeled PLP to MBP in myelin from carriers compared to noncarriers. Thus, total synthesis of PLP is not reduced in the carriers, whereas transport of both PLP and MBP into myelin is decreased, as are levels of myelin. Our data from the metabolic studies and measurement of PLP show that myelination is retarded in the carriers. Recovery from the deficit in myelin appears complete by later ages. We find, however, no evidence for a selective defect in synthesis or acylation of PLP, or transport of PLP relative to MBP. Mechanisms that could be employed by the oligodendrocyte to compensate for the early deficits in the carriers are discussed relative to glial differentiation and the possible formation of defective PLP.