bioRxiv2020
Myelin is composed of plasma membrane spirally wrapped around axons and compacted into dense sheaths by myelin associated proteins. In the central nervous system (CNS), myelin is elaborated by neuroepithelial derived oligodendrocytes and in the peripheral nervous system (PNS) by neural crest derived Schwann cells. While some myelin proteins are unique to only one lineage, myelin basic protein (Mbp) is expressed in both. Overlapping the Mbp gene is Golli, a transcriptional unit that is expressed widely both within and beyond the nervous system. A super-enhancer domain within the Golli/Mbp locus contains multiple enhancers shown previously to drive reporter construct expression specifically in oligodendrocytes or Schwann cells. In order to determine the contribution of each enhancer to the Golli/Mbp expression program and examine if interactions among these enhancers occur, we derived mouse lines in which enhancers were deleted, either singly or in different combinations, and relative mRNA accumulation was measured at key stages of development. Although super-enhancers have been shown to facilitate interaction among their component enhancers, the enhancers investigated here demonstrated functions that were largely additive. However, enhancers demonstrating autonomous activity strictly in one cell lineage, when missing, were found to significantly reduce output in the other thus revealing cryptic "stealth" activity. Further, Golli accumulation in all cell types investigated was markedly and uniformly attenuated by the absence of a key oligodendrocyte enhancer. Our observations expose a novel level of enhancer interaction and are consistent with a model in which enhancer-mediated DNA looping underlies higher-order Golli/Mbp regulatory organization. AUTHOR SUMMARYThe control of transcription is mediated through regulatory sequences that engage in a lineage and developmentally contextual manner. The Golli/Mbp locus gives rise to several mRNAs and while Mbp mRNAs accumulate exclusively in the two glial cell types that elaborate myelin, Golli mRNAs accumulate in diverse cell types both within and beyond the nervous system. To determine how the different Golli/Mbp enhancers distribute their activities and to reveal if they operate as autonomous agents or have functionally significant interactions with each other we derived multiple enhancer knock-out lines. Comparing the developmental accumulation of Mbp and Golli mRNAs revealed that the autonomous targeting capacity of multiple enhancers accurately predicted their in-situ contributions. Also, they acted in a largely additive manner indicating significant individual autonomy that can be accounted for by a simple chromatin looping model. Unexpectedly, we also uncovered cryptic "stealth" activity emanating from these same enhancers in lineages where they show no autonomous targeting capacity thus providing new insight into the control of lineage specific gene expression.