Increased expression of Nkx2.2 and Olig2 identifies reactive oligodendrocyte progenitor cells responding to demyelination in the adult CNS

Within the adult CNS, a quiescent population of oligodendrocyte progenitor cells (OPCs) become activated in response to demyelination and give rise to remyelinating oligodendrocytes. During development, OPC differentiation is controlled by several transcription factors including Olig1 and Olig2, and Nkx2.2. We hypothesized that these genes may serve similar functions in activated adult OPCs allowing them to become remyelinating oligodendrocytes and tested this hypothesis by examining their expression during the remyelination of a toxin-induced rodent model of demyelination. During the acute phase of demyelination, OPCs within the lesion increased their expression of Nkx2.2 and Olig2, two transcription factors that in combination are critical for oligodendrocyte differentiation during developmental myelination. This activation was not associated with increases in Sonic hedgehog (Shh) expression, which does not appear essential for CNS remyelination. Consistent with a role in the activation and differentiation of OPCs, these increases were delayed in old adult animals where the rate of remyelination is slowed. Our data suggest the hypothesis that increased expression of Nkx2.2 and Olig2 plays a critically important role in the differentiation of adult OPCs into remyelinating oligodendrocytes and that these genes may present novel targets for therapeutic manipulation in cases where remyelination is impaired.

Introduction

Remyelination, the process by which new myelin sheaths are restored to demyelinated axons, provides one of the best examples in neurobiology of a multipotent progenitor cell contributing to regeneration in the adult CNS (Belachew et al., 2003, Gensert and Goldman, 1997, Kondo and Raff, 2000, Levine and Reynolds, 1999, Sim et al., 2002a, Watanabe et al., 2002). Oligodendrocyte progenitor cells (OPCs) in normal adult white matter constitute a stable population of quiescent cells that divide infrequently (Chang et al., 2000, Dawson et al., 2003, ffrench-Constant and Raff, 1986, Horner et al., 2000). However, following oligodendrocyte loss and demyelination, such as occurs in multiple sclerosis (MS), OPCs respond by proliferation and migration, and finally differentiation into myelin sheath forming oligodendrocytes (Franklin, 2002, Levine and Reynolds, 1999, Watanabe et al., 2002). This transition from a quiescent to a reactive population, responsive to mediators of repair and able to undergo differentiation, is critical to the success of remyelination. However, although there is increasing knowledge of the environmental signals that govern the behavior of OPCs following injury (Armstrong et al., 2002, Arnett et al., 2001, Arnett et al., 2003, Mason et al., 2000, Mason et al., 2001, Woodruff et al., 2003), very little is known about the intrinsic changes occurring within OPCs as they become activated and able to differentiate into remyelinating oligodendrocytes.

In this study, we have examined the hypothesis that the activation of adult OPCs required for oligodendrocyte differentiation in remyelination involves the increased expression of basic helix–loop–helix transcription factors Olig1 and Olig2, and the homeodomain transcription factor Nkx2.2 (Qi et al., 2001). Expression of the Olig genes defines a population of progenitor cells that in the later stages of development become oligodendrocytes (Lu et al., 2002, Zhou et al., 2000). Critical to the progenitor cells becoming oligodendrocytes is the developmental switch in which both Olig2 and Nkx2.2 are expressed in the same population of cells instead of being expressed in separate populations (Sun et al., 2001, Zhou et al., 2001). If the coincident expression of both genes in adult OPCs is critical to their becoming remyelinating oligodendrocytes, then one would predict that both genes would be expressed in the OPC population responding to demyelination. We have tested this prediction by examining the expression of both Olig2 and Nkx2.2 in adult OPCs, mostly identified using the marker platelet-derived growth factor-α receptor (PDGFRα) Armstrong et al., 2002, Sim et al., 2002a, Woodruff et al., 2003), following toxin-induced demyelination of deep cerebellar white matter.