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BACKGROUND: We studied whether regulation of Cdca7 (Cell division cycle associated 7) expression by transcription factor Pax6 contributes to Pax6's cellular actions during corticogenesis. The function of Cdca7 in mediating Pax6's effects during corticogenesis has not been explored. Pax6 is expressed by radial glial progenitors in the ventricular zone of the embryonic cortical neuroepithelium, where it is required for the development of a normal complement of Tbr2-expressing intermediate progenitor cells in the subventricular zone. Pax6's expression levels are graded across the ventricular zone, with highest levels laterally where Tbr2-expressing progenitors are generated in greatest numbers at early stages of corticogenesis.METHODS: We used in situ hybridization and immunohistochemistry to analyse patterns of Cdca7 and Pax6 expression in cortical tissue from wild-type and Pax6 (-/-) embryos. In each genotype we compared the graded expression of the two genes quantitatively at several ages. To test whether defects in Cdca7 expression in lateral cortical cells might contribute to the cellular defects in this region caused by Pax6 loss, we electroporated a Cdca7 expression vector into wild-type lateral cortex and examined the effect on the production of Tbr2-expressing cells.RESULTS: We found that Cdca7 is co-expressed with Pax6 in cortical progenitors, at levels opposite to those of Pax6. Lowest levels of Cdca7 are found in the radial glial progenitors of lateral cortex, where Pax6 levels are highest. Higher levels of Cdca7 are found in ventral telencephalon, where Pax6 levels are low. Loss of Pax6 causes Cdca7 expression to increase in the lateral cortex. Elevating Cdca7 in normal lateral cortical progenitors to levels close to those normally found in ventral telencephalon reduces their production of Tbr2-expressing cells early in lateral cortical formation.CONCLUSION: Our results suggest that Pax6 normally represses Cdca7 expression in the lateral cortex and that repression of Cdca7 in cells of this region is required for their production of a normal complement of Tbr2-expressing intermediate progenitors.

The recent development of organoid techniques, in which embryonic brain-like tissue can be grown from human or mouse stem cells in vitro offers the potential to transform the way in which brain development is studied. In this review, we summarize key aspects of the embryonic development of mammalian forebrains, focussing in particular on the cerebral cortex and highlight significant differences between mouse and primates, including human. We discuss recent work using cerebral organoids that has revealed key similarities and differences between their development and that of the brain in vivo. Finally, we outline the ways in which cerebral organoids can be used in combination with CRISPR/Cas9 genome editing to unravel genetic mechanisms that control embryonic development of the cerebral cortex, how this can help us understand the causes of neurodevelopmental disorders and some of the key challenges which will have to be resolved before organoids can become a mainstream tool to study brain development.

The intrinsic mechanisms that link extracellular signalling to the onset of neural differentiation are not well understood. In pluripotent mouse cells, BMP blocks entry into the neural lineage via transcriptional upregulation of inhibitor of differentiation (Id) factors. We have previously identified the major binding partner of Id proteins in pluripotent cells as the basic helix-loop-helix (bHLH) transcription factor (TF) E2A. Id1 can prevent E2A from forming heterodimers with bHLH TFs or from forming homodimers. Here, we show that overexpression of a forced E2A homodimer is sufficient to drive robust neural commitment in pluripotent cells, even under non-permissive conditions. Conversely, we find that E2A null cells display a defect in their neural differentiation capacity. E2A acts as an upstream activator of neural lineage genes, including Sox1 and Foxd4, and as a repressor of Nodal signalling. Our results suggest a crucial role for E2A in establishing neural lineage commitment in pluripotent cells., Summary: E2A is usually a permissive partner for instructive transcription factors. Expression and genome editing assays revealed that E2A goes it alone as a homodimer to dominantly impose neural identity on pluripotent cells.

The transcription factor Pax6 controls multiple aspects of forebrain development. Conditional deletion of Pax6 in embryonic mouse cortex causes increased proliferation of cortical progenitor cells and a concomitant decrease in neural differentiation. Notch signaling regulates the balance between proliferation and differentiation of cortical progenitor cells, suggesting a possible connection between Pax6 and Notch signaling. We investigated how expression of the Notch ligand delta-like 1 (Dll1) is altered by loss of Pax6. Acute cortex-specific deletion of Pax6 resulted in a widespread decrease in the density of Dll1+ cells at embryonic days 12.5 and 13.5 (E12.5 and E13.5). In constitutive loss-of-function mutants, decreases in the densities of Dll1+ cells were more limited both spatially and temporally. Controlled over-expression of Pax6 had no detectable effect on Dll1 expression. The proneural transcription factor Neurog2 is a target of Pax6 that can activate Dll1 expression and we found clear co-expression of Neurog2 and Dll1 in radial glial progenitors, suggesting that Pax6’s effect on Dll1 could be mediated through Neurog2. However, we found no change in Dll1+ cells in Neurog2−/− cortex suggesting either that Neurog2 is not directly involved, or that its loss of function in embryonic cortex can be compensated for.