Proteolipid Promoter Activity Distinguishes Two Populations of NG2-Positive Cells throughout Neonatal Cortical Development

Abstract

Transgenic mice expressing enhanced green fluorescent protein (EGFP) driven by the mouse myelin proteolipid protein (PLP) gene promoter have been developed to investigate cells in the oligodendrocyte lineage. Transgene expression is consistent with the developmental expression of PLP, with cells at all stages of oligodendrocyte differentiation clearly visualized. These animals were analyzed to establish the time course of oligodendrocyte progenitor migration, proliferation, and differentiation in neonatal cortex. In these animals, two populations of NG2 proteoglycan-positive oligodendrocyte progenitor cells were identified that exist in postnatal subventricular zone and cortex. These two populations are distinguished by the presence or absence of PLP gene expression. Thus, PLP gene expression defines a subpopulation of NG2-positive cells from very early postnatal ages, which migrates toward the pial surface and proliferates in situ. EGFP ϩ /NG2 ϩ cells are present in the cortex from postnatal day 1, and they remain in the cortex as undifferentiated oligodendrocyte progenitors for up to 3 weeks before myelination begins. These data could be explained by the presence of an important inhibitor of oligodendrocyte differentiation in the cortex during this period, which is downregulated in a region-specific manner to allow myelination. On the other hand, it is possible that oligodendrocyte progenitor cells remain undifferentiated in cortex until an essential signal is produced in situ to induce differentiation.

Expression of a Myelin Proteolipid Protein (Plp)-lacZ Transgene is Reduced in both the CNS and PNS of Plp jp Mice

“…This was true in both the CNS and PNS, and may result from greater stability of Plp/Dm20 transcripts than transgene mRNA. Previous studies indicate that the Plp-specific sequence, which is absent from Dm20 mRNA, and the 3′-untranslated region of the Plp/Dm20 mRNA are important for stability of Plp gene transcripts in glial cells [15,43,44]. Since neither of these sequences is present in the transgene mRNA, amounts of the PLP(+)Z transcript are a true reflection of the transcriptional activity.…”

“…The PLP(+)Z transgene contains the proximal 2.4 kb of 5′-flanking DNA, exon 1, intron 1 and the first 37 bp of exon 2 of the Plp gene, and encodes a fusion protein between the first 13 amino acids of PLP and β-galactosidase (β-gal), which is trafficked to the myelin membrane [12]. A series of transgenic animals have subsequently been derived that further corroborate the ability of these Plp sequences to promote transgene expression, and to serve as a reliable readout of endogenous Plp gene activity [13][14][15][16][17][18], with inclusion of Plp intron 1 sequences being important [19]. In the current studies, the PLP(+)Z transgene was crossed into the Plp jp background and used to monitor the effects of the Plp jp mutation on Plp promoter activity via lacZ reporter gene expression.…”

Electrical signalling properties of oligodendrocyte precursor cells

“…This result was suggested by Mallon et al (2002) to indicate that there were two types of NG2-expressing OPCs of similar morphology, one which turned into myelinating oligodendrocytes (which need to express PLP to make myelin) and the other (lacking PLP expression) which may become the OPCs found in the adult CNS after myelination is complete. Alternative interpretations might be that the cells lacking EGFP expression are either simply OPCs that do not yet express PLP at a high level (because they are not yet myelinating axons), or are leaving the oligodendrocyte lineage to become astrocytes or neurons, as discussed below (Dimou et al, 2008;Rivers et al, 2008; Zhu et al, 2008a,b).…”

“…When EGFP is expressed under the control of the promoter for proteolipid protein (PLP, a myelin protein expressed only in oligodendrocytes), two populations of proliferating NG2-expressing cells were revealed in the neocortex, one that expresses EGFP and one that does not (Mallon et al, 2002). This result was suggested by Mallon et al (2002) to indicate that there were two types of NG2-expressing OPCs of similar morphology, one which turned into myelinating oligodendrocytes (which need to express PLP to make myelin) and the other (lacking PLP expression) which may become the OPCs found in the adult CNS after myelination is complete.…”

“…2G of Káradóttir et al, 2008). At present it is unclear how these two OPC classes (which are apparently morphologically indistinguishable) relate to the two classes reported by Mallon et al (2002) and Horner et al (2000). The segregation of OPCs into two electrically distinct classes has since been confirmed to exist for OPCs in two separate transgenic mice expressing YFP under the control of the oligodendrocyte-specific Sox10 or PDGFRa promoters (Clarke et al, 2009).…”