Bhlhb5 Regulates the Post-Mitotic Acquisition of Area Identities in Layers II–V of the Developing Neocortex

While progenitor-restricted factors broadly specify area identities in developing neocortex, the downstream regulatory elements involved in acquisition of those identities in post-mitotic neurons are largely unknown. Here, we identify Bhlhb5, a transcription factor expressed in layers II–V, as a post-mitotic regulator of area identity. Bhlhb5 is initially expressed in a high caudomedial to low rostrolateral gradient that transforms into a sharp border between sensory and rostral motor cortices. Bhlhb5 -null mice exhibit aberrant expression of area-specific genes and structural organization in the somatosensory and caudal motor cortices. In somatosensory cortex, Bhlhb5 -null mice display post-synaptic disorganization of vibrissal barrels. In caudal motor cortex, Bhlhb5 -null mice exhibit anomalous differentiation of corticospinal motor neurons, accompanied by failure of corticospinal tract formation. Together, these results demonstrate Bhlhb5’s function as an area-specific transcription factor that regulates the post-mitotic acquisition of area identities and elucidate the genetic hierarchy between progenitors and post-mitotic neurons driving neocortical arealization.