The morphogen Sonic Hedgehog (SHH) plays a critical role in the development of different tissues. In the central nervous system, SHH is well known to contribute to the patterning of the spinal cord and separation of the brain hemispheres. In addition, it has recently been shown that SHH signaling also contributes to the patterning of the telencephalon and establishment of adult neurogenic niches. In this work, we investigated whether SHH signaling influences the behavior of neural progenitors isolated from the dorsal telencephalon, which generate excitatory neurons and macroglial cells in vitro. We observed that SHH increases proliferation of cortical progenitors and generation of astrocytes, whereas blocking SHH signaling with cyclopamine has opposite effects. In both cases, generation of neurons did not seem to be affected. However, cell survival was broadly affected by blockade of SHH signaling. SHH effects were related to three different cell phenomena: mode of cell division, cell cycle length and cell growth. Together, our data in vitro demonstrate that SHH signaling controls cell behaviors that are important for proliferation of cerebral cortex progenitors, as well as differentiation and survival of neurons and astroglial cells.

Neocortical progenitor cells generate subtypes of excitatory projection neurons in sequential order followed by the generation of astrocytes. The transcription factor Zinc Finger and BTB Domain-Containing Protein 20 (ZBTB20) has been implicated in regulating cell specification during neocortical development. Here we show that ZBTB20 instructs the generation of a subset of callosal projections neurons in cortical layers II/III. Conditional deletion of Zbtb20 in cortical progenitors, and to a lesser degree in differentiating neurons, leads to an increase in the number of layer IV neurons at the expense of layer II/III neurons. Astrogliogenesis is also affected in the mutants with an increase in the number of a specific subset of astrocytes expressing GFAP. Astrogliogenesis is more severely disrupted by a ZBTB20 protein containing dominant mutations linked to Primrose Syndrome suggesting that ZBTB20 acts in concert with other ZBTB proteins that were also affected by the dominant negative protein to instruct astrogliogenesis. Overall, our data suggest that ZBTB20 acts both in progenitors and postmitotic cells to regulate cell-fate specification in the mammalian neocortex.

Differential expression levels of Sox9 in early neocortical radial glial cells regulate the decision between stem cell maintenance and differentiation.