Region-specific and stage-dependent regulation of Olig gene expression and oligodendrogenesis by Nkx6.1 homeodomain transcription factor

During early neural development, the Nkx6.1 homeodomain neural progenitor gene is specifically expressed in the ventral neural tube, and its activity is required for motoneuron generation in the spinal cord. We report that Nkx6.1 also controls oligodendrocyte development in the developing spinal cord, possibly by regulating Olig gene expression in the ventral neuroepithelium. In Nkx6.1 mutant spinal cords, expression of Olig2 in the motoneuron progenitor domain is diminished, and the generation and differentiation of oligodendrocytes are significantly delayed and reduced. The regulation of Olig gene expression by Nkx6.1 is stage dependent, as ectopic expression of Nkx6.1 in embryonic chicken spinal cord results in an induction of Olig2 expression at early stages, but an inhibition at later stages. Moreover, the regulation of Olig gene expression and oligodendrogenesis by Nkx6.1 also appears to be region specific. In the hindbrain, unlike in the spinal cord, Olig1 and Olig2 can be expressed both inside and outside the Nkx6.1-expressing domains and oligodendrogenesis in this region is not dependent on Nkx6.1 activity.     

Multiple dorsoventral origins of oligodendrocyte generation in the spinal cord and hindbrain

Studies have indicated that oligodendrocytes in the spinal cord originate from a ventral progenitor domain defined by expression of the oligodendrocyte-determining bHLH proteins Olig1 and Olig2. Here, we provide evidence that progenitors in the dorsal spinal cord and hindbrain also produce oligodendrocytes and that the specification of these cells may result from a dorsal evasion of BMP signaling over time. Moreover, we show that the generation of ventral oligodendrocytes in the spinal cord depends on Nkx6.1 and Nkx6.2 function, while these homeodomain proteins in the anterior hindbrain instead suppress oligodendrocyte specification. The opposing roles for Nkx6 proteins in the spinal cord and hindbrain, in turn, appear to reflect that oligodendrocytes are produced by distinct ventral progenitor domains at these axial levels. Based on these findings, we propose that oligodendrocytes derive from several distinct positional origins and that the activation of Olig1/2 at different positions is controlled by distinct genetic programs.     

Complementary roles for Nkx6 and Nkx2 class proteins in the establishment of motoneuron identity in the hindbrain

The genetic program that underlies the generation of visceral motoneurons in the developing hindbrain remains poorly defined. We have examined the role of Nkx6 and Nkx2 class homeodomain proteins in this process, and provide evidence that these proteins mediate complementary roles in the specification of visceral motoneuron fate. The expression of Nkx2.2 in hindbrain progenitor cells is sufficient to mediate the activation of Phox2b, a homeodomain protein required for the generation of hindbrain visceral motoneurons. The redundant activities of Nkx6.1 and Nkx6.2, in turn, are dispensable for visceral motoneuron generation but are necessary to prevent these cells from adopting a parallel program of interneuron differentiation. The expression of Nkx6.1 and Nkx6.2 is further maintained in differentiating visceral motoneurons, and consistent with this the migration and axonal projection properties of visceral motoneurons are impaired in mice lacking Nkx6.1 and/or Nkx6.2 function. Our analysis provides insight also into the role of Nkx6 proteins in the generation of somatic motoneurons. Studies in the spinal cord have shown that Nkx6.1 and Nkx6.2 are required for the generation of somatic motoneurons, and that the loss of motoneurons at this level correlates with the extinguished expression of the motoneuron determinant Olig2. Unexpectedly, we find that the initial expression of Olig2 is left intact in the caudal hindbrain of Nkx6.1/Nkx6.2 compound mutants, and despite this, all somatic motoneurons are missing. These data argue against models in which Nkx6 proteins and Olig2 operate in a linear pathway, and instead indicate a parallel requirement for these proteins in the progression of somatic motoneuron differentiation. Thus, both visceral and somatic motoneuron differentiation appear to rely on the combined activity of cell intrinsic determinants, rather than on a single key determinant of neuronal cell fate.     

Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of Nkx6 regulation and Shh signaling

In the developing spinal cord, early progenitor cells of the oligodendrocyte lineage are induced in the motor neuron progenitor (pMN) domain of the ventral neuroepithelium by the ventral midline signal Sonic hedgehog (Shh). The ventral generation of oligodendrocytes requires Nkx6-regulated expression of the bHLH gene Olig2 in this domain. In the absence of Nkx6 genes or Shh signaling, the initial expression of Olig2 in the pMN domain is completely abolished. In this study, we provide the in vivo evidence for a late phase of Olig gene expression independent of Nkx6 and Shh gene activities and reveal a brief second wave of oligodendrogenesis in the dorsal spinal cord. In addition, we provide genetic evidence that oligodendrogenesis can occur in the absence of hedgehog receptor Smoothened, which is essential for all hedgehog signaling.     

Dorsally derived oligodendrocytes come of age

While oligodendrocyte precursors have previously been localized to the ventral midline, two papers in this issue of Neuron (Cai et al. and Vallstedt et al.) provide compelling evidence for a second dorsal origin of oligodendrocyte precursors in the spinal cord and hindbrain. These cells share expression of the PDGFalpha receptor and Olig2 with their ventral counterparts but differ in the requirement for Nkx6 and hedgehog signaling.