Activation of Plp Gene Expression in Oligodendrocytes

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The myelin proteolipid protein (PLP) gene (Plp1) encodes the most abundant protein present in CNS myelin. Its expression in oligodendrocytes is tightly regulated. Mutations in the human gene have been shown to be associated with the X-linked dysmyelinating disorder Pelizaeus-Merzbacher disease (PMD) and some types of spastic paraplegia (SPG2). Dysmyelination can arise from either elevated levels of Plp1 gene expression or the lack, thereof. Hence accurate expression of the gene in is critical. With transgenic mice that harbor Plp1- lacZ fusion genes we have shown that Plp1 intron 1 DNA is essential for regulating the dramatic increase in Plp1 gene transcription in oligodendrocytes during the active myelination period of CNS development. The intron accounts for roughly one-half of the Plp1 gene, encompassing over 8 kb of DNA. We have identified a regulatory element within Plp1 intron 1 DNA and named it ASE for antisilencer/enhancer. We believe the ASE participates in the formation of a multi-protein nucleoprotein complex which acts as a potent transcriptional activator called an enhanceosome. Enhanceosomes are a new class of transcriptional regulators that are characterized by a higher-order nucleoprotein complex which functions as a unit. The precise three- dimensional structure permits synergy between factors and is obligatory to elicit gene activation. The intron also contains another enhancer, designated as wmN1, which has been roughly localized to a 1.2 kb segment of DNA. We hypothesize that the dramatic increase in Plp1 gene activity in oligodendrocytes during development is governed by the ASE and wmN1 enhancers and their associated (cognate) DNA-binding factors. Elucidation of the mechanisms controlling Plp1 gene expression will be an important step in the design of therapies aimed at enhancing remyelination since at the time of myelination, 10% of the total mRNA in oligodendrocytes is generated from the Plp1 gene. Developmental differences in factors present early in childhood versus those in adults may explain the rather poor levels of remyelination observed in MS patients. Cognate factors which bind the ASE and wmN1 enhancers may provide new therapeutic targets to enhance remyelination in these patients. As well, critical mutations in the ASE or wmN1 enhancer, or a mutation in a gene which encodes a cognate factor obligatory for enhancer function, would lead to under-expression of the Plp1 gene and consequently may result in a dysmyelinating disorder. In this study, deletion-transfection analysis with Plp1-lacZ constructs will be used to minimally map the wmN1 enhancer within the 1.2 kb segment of Plp1 intron 1 DNA. Transgenic mice will also be generated to test whether the ASE and wmN1 enhancer work independently, or in combination with one another, to significantly increase the levels of Plp1 gene expression during development.