Late Control of Cortical Histogenesis by Transcription Factors Patterning the Early Pallial Field

A number of transcription factor genes implicated in primary regional patterning of the pallial primordium continue to be active or are specifically reactivated upon completion of such patterning. This late expression may be required for proper tuning of advanced cerebro- cortical histogenesis. It may account for key neurological aberrancies associated to mutations affecting such transcription factor genes. We have previously shown that a transcription factor implicated in primary tangential patterning of the pallial primordium, Foxg1, shapes the temporal progression of the rates at which pallial stem cells give rise to astrogenic committed progenitors and the latter progress to astrocytes. Inspired by these findings, we hypothesized that these two rates might be also differentially regulated in space, in distinctive regions of the murine developing pallium, and that factors patterning pallium might be implicated in such regulation. This prediction turned out to be correct. We found that, compared to their age-matched rostro-lateral counterparts, early caudo-medial pallial stem cells are more biased to astrogenesis, and generate astroblasts less prone to proliferation and more keen to early differentiation. We got evidence that Emx2, a transcription factor gene mastering caudo-medial cortical specification, drives regionally patterned articulation of astrogliogenesis. Moreover, we demonstrated that three established pro-astrogenic factors, Sox9, Nfia and Couptf1, mediate Emx2 impact on neural stem cells commitment to astrogenesis. On the other hand, the two mutant FOXG1 alleles FOXG1G224S and FOXG1W308X have been found in children affected by severe varieties of the FOXG1 Syndrome. Previous functional characterization of these alleles in murine wild type cells showed that they act as gain- and loss-of-function variants of their healthy counterpart, respectively. Before any possible therapeutic exploitation, results of this study require a stringent validation in human neural cells, harboring gene configurations matching the patients' ones. To this aim, we obtained human pluripotent stem cells heterozygous for these mutations and their co-isogenic healthy controls, and we set up a protocol suitable to reproducibly generate dorsal telencephalic tissue starting from healthy human pluripotent stem cells. We discovered that, applying such protocol to heterozygous FOXG1W308X mutants, the generation of pallial neuroepithelium is defective. To allow robust interrogation of more advanced derivatives of FOXG1W308X/+ neuroepithelium, we are now managing to mitigate this issue.