Transgenic Mice Reveal That Splice Variants Containing "human-specific" Exons Are Relatively Minor in Comparison to the Archetypal Transcript and That an Upstream Regulatory Element Bolsters Expression During Early Postnatal Brain Development

Much of what is known about the mechanisms that control the developmental expression of the myelin proteolipid protein gene () has been attained through use of transgenic animal models. In this study, we analyzed expression of related transgenes which utilize genomic DNA from either human or mouse to drive expression of a reporter. Human () sequence span either the proximal 6.2 or 2.7 kb of 5'-flanking DNA to an internal site in Exon 2, while those from mouse comprise the proximal 2.3 kb of 5'-flanking DNA to an analogous site in Exon 2. Transgenes with sequence were named, in part, to the amount of upstream sequence they have [6.2hPLP(+)Z/FL and 2.7hPLP(+)Z]. The transgene containing mouse sequence is referred to here as mPLP(+)Z, to denote the species origin of DNA. Mice which harbor the 6.2hPLP(+)Z/FL transgene were used as a model system to investigate the developmental expression of splice variants that incorporate supplementary exons from what is classically defined as intron 1. While expression of the splice variants were detected in brain through RT-PCR analysis, they are present at much lower levels relative to the archetypal (classic) transcript. Additionally, we show that mice which harbor the 6.2hPLP(+)Z/FL transgene demonstrate wide-ranging expression throughout brain at P2, whereas expression of mPLP(+)Z is quite limited at this age. Therefore, we generated new transgenic mouse lines with the 2.7hPLP(+)Z transgene, which contains sequence orthologous to just that in mPLP(+)Z. Of the seven lines analyzed, six showed higher levels of 2.7hPLP(+)Z expression in brain at P21 compared to P2; the other line expressed the transgene, only weakly, at either age. This trend, coupled with the robust expression observed for 6.2hPLP(+)Z/FL at P2, suggests that the distal 3.5 kb of 5'-flanking DNA specific to 6.2hPLP(+)Z/FL contains regulatory element(s) important for promoting early postnatal expression in brain.