Transient upregulation of Nkx2.2 expression in oligodendrocyte lineage cells during remyelination

While numerous oligodendrocyte progenitor cells (OPCs) exist in the adult central nervous system (CNS), the molecular signals that promote or inhibit their differentiation into mature oligodendrocytes (OLs) are not known. To investigate whether remyelination in the adult CNS is regulated by the same mechanisms that promote developmental myelination, we used an acute demyelinating/remyelinating lesion in the adult rat spinal cord to examine the expression of the homeodomain transcription factor Nkx2.2, which has previously been implicated in oligodendrocyte differentiation during embryonic development. After a demyelinating insult, Nkx2.2 expression was upregulated first in NG2-expressing OPCs surrounding the lesion and subsequently in both precursors and OLs that appeared inside the lesion prior to the onset of remyelination. The temporal and spatial pattern of Nkx2.2 upregulation coincided with that of oligodendrocyte differentiation characterized in our previous study. A similar increase in the level of Nkx2.2 expression was observed in the postnatal developing optic nerve in a wave from the proximal to the distal retinal end. In vitro Nkx2.2 was expressed in OPCs and immature OLs isolated from postnatal rat spinal cord but was absent from mature OLs. These observations indicate that the process of generating new OLs in a remyelinating lesion recapitulates the developmental program involving activation of the Nkx2.2 gene, which may trigger the existing NG2-expressing precursors in the adult CNS to undergo terminal differentiation into remyelinating OLs.