Separate Proteolipid Protein/DM20 Enhancers Serve Different Lineages and Stages of Development

Abstract

The gene encoding DM20 emerged in cartilaginous fish, descending from a bilaterian ancestor of the M6 proteolipid gene family. Its proteolipid protein (PLP) isoform appeared in amphibians, contains an additional 35 amino acids, and, in the mammalian CNS, is the dominant myelin protein in which it confers an essential neuroprotective function. During development, the DM20 isoform is prominent in a number of tissues, and plp/DM20 transcripts are detected in multiple progenitor populations, including those that continue to express plp/DM20 as they differentiate into myelinating oligodendrocytes. The locus also encodes isoforms with extended leader sequences that accumulate in the cell bodies of several types of neurons. Here, to locate and characterize regulatory sequences controlling the complex plp/DM20 transcription program, putative regulatory sequences, suggested by interspecies conservation, were ligated individually to a minimally promoted eGFPlacZ reporter gene. These constructs were inserted in single copy at a common site adjacent to the hypoxanthine-guanine phosphoribosyltransferase locus in embryonic stem cells and their in vivo expression programs were compared in transgenic mice. Most expressed developmental and cell-specific subprograms accommodated within the known expression phenotype of the endogenous plp/DM20 locus, thus defining multiple components of the combinatorial mechanism controlling its normal temporal and cell-specific program. Along with previously characterized nervous system enhancers, those described here should help expose the content and configuration of elements that are operational in multiple glial and neuronal lineages. The transgenic lines derived here also provide effective markers for multiple stages of glial and neuronal lineage progression.

The Proteolipid Protein Promoter Drives Expression outside of the Oligodendrocyte Lineage during Embryonic and Early Postnatal Development

“…We demonstrate the Plp promoter cassette's capability to drive expression of a Cre recombinase transgene within the CNS, but in a more widespread pattern than initially anticipated. This was particularly true at early developmental stages when the transgene was expressed in neural tissues, and not restricted to the oligodendrocyte lineage, results corroborated by the work of others [3], [14], [16], [21]. During this time, transgene expression was also localized to the PNS (sciatic nerve, ventral and dorsal roots), most likely in Schwann cells.…”

“…Although the promoter is generally considered oligodendrocyte lineage specific, accumulating evidence suggests a Plp expression pattern outside of defined white matter zones; included are both glial and neuronal CNS subpopulations, implying a role for Plp in activities independent of myelination [11], [12], [13], [14], [15], [16].…”

The wmN1 enhancer region in intron 1 is required for expression of human PLP1

“…The wmN2 region was identified in the same study as wmN1, and is believed to mediate high level expression in Schwann cells and their progenitors, dorsal root ganglion (DRG) satellite cells and OECs, and weak expression in oligodendrocyte lineage cells (Tuason et al, 2008). Results presented here suggest that activity of the wmN1 enhancer extends beyond the cell types initially identified by Tuason and coworkers (2008), and includes those ascribed as being regulated by the wmN2 enhancer. It is possible that there is partial redundancy in the cell types affected by the two enhancers, or alternatively, species-specific differences may exist between the enhancers in human and mouse.…”

Early Postnatal Proteolipid Promoter-Expressing Progenitors Produce Multilineage CellsIn Vivo

“…The plp gene encodes myelin proteolipid protein in myelinating oligodendrocytes and a smaller isoform, DM-20, which, in the mouse, is expressed in embryonic neuroepithelial cells (embryonic day 9.1, E9.5), radial glial cells (E13.5) and progenitors expressing the proteoglycan, NG2 (E14.5) (Ikenaka et al, 1992; Timsit et al, 1992; Timsit et al, 1995; Spassky et al, 1998; Belachew et al, 2001; Tuason et al, 2008; Delaunay et al, 2008). These plp promoter expressing embryonic progenitors are multipotent both in vivo and in vitro , giving rise to oligodendrocytes, astrocytes and neurons (Le Bras et al, 2005; Delaunay et al, 2008).…”