PHENOTYPIC SEVERITY OF MURINE PLP MUTANTS REFLECTS IN-VIVO AND IN-VITRO VARIATIONS IN TRANSPORT OF PLP ISOPROTEINS

Mutations of the major myelin gene, proteolipid protein (Plp), cause P elizaeus-Merzbacher disease and some forms of spastic paraplegia in ma n and dysmyelinating phenotypes in animals. The clinical severity is m arkedly heterogeneous, ranging from relatively mild to severe and fata l. Point mutations, or frame shifts, which are predicted to result in translation of structurally altered proteins account for many of these cases, including 3 of the allelic murine conditions. Plp(jp-rsh), plp (jp-msd), and Plp(jp) represent an increasing severity of clinical and pathological phenotypes, respectively. In this study we determined wh ether there was any correlation between the severity of phenotype and the transport of the predicted abnormal protein. We examined the abili ty of the two products of the Plp gene, PLP and DM20, to insert into t he plasma membrane of transfected BHK or COS-7 cells, and into the mye lin sheath of oligodendrocytes. With these complementary in vitro and in vivo approaches we find that proteins of Plp(jp-rsh), associated wi th the mildest phenotype, have a far greater ability to insert into th e cell membrane or myelin than those associated with the severe phenot ypes. Additionally altered DM20 is more readily transported to the cel l surface and to myelin than the PLP isoprotein. Interestingly, the tw o clonal cell lines chosen for transient transfection differ in their ability to fold DM20 from Plp(jp-rsh) and Plp(jp-msd) mice correctly, as inferred by staining for the conformation-sensitive O10 epitope. In the case of Plp(jp), which is associated with the most severe phenoty pe, no PLP or O10 staining is present at the cell surface or in myelin . The perturbation in trafficking observed for altered Plp(jp) PLP and DM20 in oligodendrocytes does not extend to other myelin membrane pro teins, such as MAG and MOG, nor to wild type PLP co-expressed in the s ame cell, all of which are correctly inserted into myelin. As Plp-knoc kout mice do not have a dysmyelinating phenotype it seems unlikely tha t absence of PLP and/or DM20 in the membrane is responsible for the pa thology. It remains to be determined whether the perturbation in prote in trafficking is associated with the dysmyelination, or if the altere d product of the mutant alleles acquire a novel function which is dele terious to myelin production by oligodendrocytes. (C) 1997 Wiley-Liss, Inc.