The cerebral cortex is the most complex structure of the mammalian brain and the seat of higher sensory, motor, and cognitive functions. Its formation relies on a finely tuned balance between proliferation and differentiation of progenitor cells, which determines the number and identity of the neurons specific to each cortical layer and area. A key checkpoint of this balance lies in the duration of the G1 phase of the cell cycle: lengthening of this phase increases the propensity of a dividing cell to exit the cycle and commit to neuronal differentiation. Proliferative divisions are characterized by a short G1 phase, whereas differentiative divisions are characterized by a long G1 phase. Progression through G1 depends primarily on the activity of Cyclin/Cdk complexes, among which Cyclin D1 (Ccnd1) occupies a central position in the regulatory network governing the fate of cortical progenitors.
To investigate the transcriptional regulation of Ccnd1 at the early stages of mouse corticogenesis, the authors combined genome-wide chromatin immunoprecipitation (ChIP-Seq), in vitro luciferase-based transcriptional activity assays, and mouse genetics, drawing in particular on transgenic gain- and loss-of-function models of Sp8 as well as RNA sequencing data. This approach aimed to identify the transcription factors controlling expression of the gene and the genomic regions on which they act.
The work shows that the transcription factor SP8 binds to the Ccnd1 locus at exonic regions, and not solely at the classically studied promoter. In vitro, SP8 exhibits binding activity at the 3′ end of the gene, at a fragment corresponding to exon 5, where a cluster of putative binding sites coincides with the peak identified by ChIP-Seq. SP8 also appears to modulate PAX6-mediated repression of Ccnd1 along the dorso-ventral axis of the developing pallium, contributing to the establishment of a neuronal differentiation gradient that is low medially and high laterally. By contrast, activation of Ccnd1 through the promoter and 5′ end of the gene does not depend on SP8 but on β-catenin (CTNNB1), an effector of the Wnt pathway, with no cooperation between these two factors over this region. Most importantly, altering the expression level of Sp8 in vivo modifies Ccnd1 expression during early corticogenesis.
Taken together, these findings indicate that the regulation of Ccnd1 in the mouse pallium results from multiple signals leading to a complex expression pattern, and that SP8 plays a leading role therein. The authors emphasize that the 3′ end of the gene, already known as a critical regulatory element at the post-transcriptional level, may also be subject to transcriptional control, revealing an unexpected activation mechanism of the SP factor family that remains to be further explored.