ASC proneural transcription factors mediate the timely initiation of the neural program during neuroectodermal to neuroblast transition ensuring progeny fidelity

Background ASC/ASCL proneural transcription factors are oncogenic and exhibit impressive reprogramming and pioneer activities. In both Drosophila and mammals, these factors are central in the early specification of the neural fate, where they act in opposition to Notch signalling. However, the role of ASC on the chromatin during CNS neural stem cells birth remains elusive.

Results We investigated the chromatin changes accompanying neural commitment using an integrative genetics and genomics methodology. We found that ASC factors bind equally strongly to two distinct classes of cis-regulatory elements: open regions remodeled earlier during maternal to zygotic transition by Zelda and Zelda-independent, less accessible regions. Both classes cis-elements exhibit enhanced chromatin accessibility during neural specification and correlate with transcriptional regulation of genes involved in many biological processes necessary for neuroblast function. We identified an ASC-Notch regulated TF network that most likely act as the prime regulators of neuroblast function. Using a cohort of ASC target genes, we report that ASC null neuroblasts are defectively specified, remaining initially stalled, lacking expression of many proneural targets and unable to divide. When they eventually start proliferating, they produce compromised progeny. Generation of lacZ reporter lines driven by proneural-bound elements display enhancer activity within neuroblasts and proneural dependency. Therefore, the partial neuroblast identity seen in the absence of ASC genes is driven by other, proneural-independent, cis-elements. Neuroblast impairment and the late differentiation defects of ASC mutants are corrected by ectodermal induction of individual ASC genes but not by individual members of the TF network downstream of ASC. However, in wild type embryos induction of individual members of this network induces CNS hyperplasia, suggesting that they synergize with the activating function of ASC to establish the chromatin dynamics that promote neural specification.

Conclusion ASC factors bind a large number of enhancers to orchestrate the timely activation of the neural chromatin program during neuroectodermal to neuroblast transition. This early chromatin remodeling is crucial for both neuroblast homeostasis as well as future progeny fidelity.