<i>Neurog2</i> and <i>Ascl1</i> together regulate a postmitotic derepression circuit to govern laminar fate specification in the murine neocortex

Abstract

A derepression mode of cell-fate specification involving the transcriptional repressors Tbr1, Fezf2, Satb2, and Ctip2 operates in neocortical projection neurons to specify six layer identities in sequence. Less well understood is how laminar fate transitions are regulated in cortical progenitors. The proneural genes <i>Neurog2</i> and <i>Ascl1</i> cooperate in progenitors to control the temporal switch from neurogenesis to gliogenesis. Here we asked whether these proneural genes also regulate laminar fate transitions. Several defects were observed in the derepression circuit in <i>Neurog2</i>^<i>-/-</i><i>;Ascl1</i>^<i>-/-</i> mutants: an inability to repress expression of Tbr1 (a deep layer VI marker) during upper-layer neurogenesis, a loss of Fezf2⁺/Ctip2⁺ layer V neurons, and precocious differentiation of normally late-born, Satb2⁺ layer II-IV neurons. Conversely, in stable gain-of-function transgenics, <i>Neurog2</i> promoted differentiative divisions and extended the period of Tbr1⁺/Ctip2⁺ deep-layer neurogenesis while reducing Satb2⁺ upper-layer neurogenesis. Similarly, acute misexpression of <i>Neurog2</i> in early cortical progenitors promoted Tbr1 expression, whereas both <i>Neurog2</i> and <i>Ascl1</i> induced Ctip2. However, <i>Neurog2</i> was unable to influence the derepression circuit when misexpressed in late cortical progenitors, and <i>Ascl1</i> repressed only Satb2. Nevertheless, neurons derived from late misexpression of <i>Neurog2</i> and, to a lesser extent, <i>Ascl1</i>, extended aberrant subcortical axon projections characteristic of early-born neurons. Finally, <i>Neurog2</i> and <i>Ascl1</i> altered the expression of <i>Ikaros</i> and <i>Foxg1</i>, known temporal regulators. Proneural genes thus act in a context-dependent fashion as early determinants, promoting deep-layer neurogenesis in early cortical progenitors via input into the derepression circuit while also influencing other temporal regulators.