Neural progenitors in the embryonic neocortex must be tightly regulated in order to generate the correct number and projection neuron subtypes necessary for the formation of functional neocortical circuits. In this study, we show that the intracellular protein Suppressor of Fused (Sufu) regulates the proliferation of intermediate progenitor (IP) cells at later stages of corticogenesis to affect the number of Cux1+ upper layer neurons in the postnatal neocortex. This correlates with abnormal levels of the repressor form of Gli3 (Gli3R) and the ectopic expression of Patched 1 (Ptch1), a Sonic Hedgehog (Shh) target gene. These studies reveal that the canonical role of Sufu as an inhibitor of Shh signaling is conserved at later stages of corticogenesis and that Sufu plays a crucial role in regulating neuronal number by controlling the cell cycle dynamics of IP cells in the embryonic neocortex.
Transactivation response element RNA-binding protein (TRBP) is known to play important roles in human immunodeficiency virus (HIV) replication and microRNA biogenesis. However, recent studies implicate TRBP in a variety of biological processes as a mediator for cross-talk between signal transduction pathways. Here, we provide the first evidence that TRBP is required for efficient neurosphere formation, and expression of neural stem cell markers and Notch target genes in primary neural progenitor cells in vitro. Consistent with this, introduction of TRBP into the mouse embryonic brain in utero increased the fraction of cells expressing Sox2 in the ventricular zone (VZ). We also show TRBP physically interacts with the Notch transcriptional coactivation complex through C promoter binding factor 1 (CBF1) and strengthens the association between the Notch intracellular domain (NICD) and CBF1, resulting in increased NICD recruitment to the promoter region of a Notch target gene. Our data indicate that TRBP is a novel transcriptional coactivator of the Notch signaling pathway playing an important role in neural stem cell regulation during mammalian brain development.
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