Neocortical neurogenesis: a proneural gene perspective

The neocortex, which is the site of higher-order cognitive functioning, is comprised of two main neuronal types: excitatory (E) and inhibitory (I). Neurodevelopmental disorders that disrupt the balance of E:I neurotransmission predispose individuals to atypical brain function, highlighting the importance of generating the correct numbers of each neuronal type. During development, neurons with E and I neurotransmission profiles are primarily generated from neural stem and progenitor cells (NPCs), located in the dorsal and ventral telencephalon, respectively. To ensure that correct numbers of each neuronal type are generated, NPC differentiation dynamics vary depending on positional and temporal information and host species. Despite variations in NPC differentiation kinetics and outcomes, proneural genes encoding basic helix-loop-helix (bHLH) transcription factors (TFs) have remained constant as the core drivers of neurogenesis and neuronal subtype specification from fly to human. This high degree of functional conservation raises the question of how proneural TF activity is regulated to control precise neurogenic patterns. In the neocortex, the proneural genes neurogenin 1 (Neurog1) and Neurog2 specify an excitatory neuronal identity in dorsal telencephalic NPCs, whereas achaete-scute family bHLH transcription factor 1 (Ascl1) specifies an inhibitory neurotransmission fate in ventral NPCs, generating interneurons that then migrate tangentially to enter the neocortex. Here, we review our current knowledge of how Neurog1/Neurog2 and Ascl1 functions are regulated to ensure that E:I balance is ultimately achieved in the lissencephalic murine cortex and in gyrencephalic species. Together, these studies point to emergent and conserved features of proneural gene regulation and function across evolutionary time.