In vivo transition in chromatin accessibility during differentiation of deep-layer excitatory neurons in the neocortex.

During neuronal differentiation, gene transcription patterns change in response to both intrinsic and extrinsic cues. Chromatin regulation at regulatory elements plays a key role in this process. However, how chromatin accessibility evolves in vivo in cortical neurons remains unclear. Here, we established a method for labeling differentiating neurons with specific birthdates. Using this method, we traced the 4-day differentiation process of in vivo deep-layer excitatory neurons in the mouse embryonic cortex and examined changes in the genome-wide transcription pattern and chromatin accessibility using RNA sequencing and DNase sequencing, respectively. We found that genomic regions of genes linked to mature neuronal functions, including deep layer-specific and stimulus-responsive genes, became accessible even at the embryonic stage. Additionally, our results indicated the involvement of bivalent marks in neural precursor/stem cells and Dmrt3 and Dmrta2 in the regulation of chromatin accessibility during neuronal differentiation. These findings highlight the importance of chromatin regulation in embryonic neurons, enabling the timely activation of neuronal genes during maturation.