Pax6 Limits the Competence of Developing Cerebral Cortical Cells

Abstract

The development of stable specialized cell types in multicellular organisms relies on mechanisms controlling inductive intercellular signals and the competence of cells to respond to such signals. In developing cerebral cortex, progenitors generate only glutamatergic excitatory neurons despite being exposed to signals with the potential to initiate the production of other neuronal types, suggesting that their competence is limited. Here we tested the hypothesis that this limitation is due to their expression of transcription factor Pax6. We used bulk and single cell RNAseq to show that conditional cortex-specific Pax6 deletion from the onset of cortical neurogenesis allowed some progenitors to generate abnormal lineages resembling those normally found outside the cortex. Analysis of selected gene expression showed that the changes occurred in specific spatiotemporal patterns. We then compared the responses of control and Pax6-deleted cortical cells to in vivo and in vitro manipulations of extracellular signals. We found that Pax6 loss increased cortical progenitors’ competence to generate inappropriate lineages in response to extracellular factors normally present in developing cortex, including the morphogens Shh and Bmp4. Regional variation in the levels of these factors could explain spatiotemporal patterns of fate change following Pax6 deletion in vivo. We propose that Pax6’s main role in developing cortical cells is to minimize the risk of their development being derailed by the potential side-effects of morphogens engaged contemporaneously in other essential functions.

Pax6 mutant cerebral organoids partially recapitulate phenotypes of Pax6 mutant mouse strains

“…Interestingly, we saw a much larger variation in the amount of Tuj1+ neurons produced in Pax6 -/organoids than in controls. One possible explanation for this is that Pax6 acts to protect cells from the influence of signalling molecules in the extracellular environment as has recently been shown in mice (Manuel et al, 2022). It is possible that the variation in the extent of precocious neural differentiation in Pax6 -/organoids is due to variability in the presence of environmental signals that promote cell cycle exit.…”

Thirty Years’ History since the Discovery of Pax6: From Central Nervous System Development to Neurodevelopmental Disorders

“…A mathematical analysis of gene regulatory networks using Boolean and computational models has revealed an underpinning expression gradient in the mammalian cortex [ 65 , 66 ]. Although upstream secretary molecules that can induce the expression of Pax6 have not been identified yet, it is of interest that Pax6 controls the responsiveness to secreted molecules such as sonic hedgehog and BMP in the cortex [ 67 ]. Future analyses may reveal more about the relationship between Pax6 and secreted molecules.…”

“…In this section, we have discussed that Pax6 and its downstream transcriptional factors and signaling molecules are important for neural fate determination, where these transcriptional factors have stage- and cell type-specific regulatory mechanisms. A recent preprint article has reported that a conditional knock-out of Pax6 ectopically induces GABAergic interneurons, as well as oligodendrocyte precursor cells in the cortex [ 67 ]. This might be reasonable because cyclin D2 is upregulated in the Sey/Sey mouse cortex ([ 94 , 134 ] and our unpublished data).…”