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

Significance

Projection neurons of the neocortex represent a diverse neuronal population that can be conveniently classified into three broad categories based on axon projection patterns: corticothalamic, subcerebral, and callosal. These neuronal subtypes are known to be specified postmitotically by a cortical derepression circuit involving the key transcription factors Tbr1, Fezf2, Satb2, and Ctip2. However, projection neuron identities are also known to be determined at the progenitor cell level, but the molecular mechanisms are poorly understood. Here we reveal that the proneural genes Neurog2 and Ascl1, which are together expressed in neocortical progenitors, cooperate to regulate the expression of components of the cortical derepression circuit to specify corticothalamic and subcerebral identities while repressing a callosal fate.

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 Neurog2 and Ascl1 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 Neurog2 ^−/− ;Ascl1 ^−/− 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, Neurog2 promoted differentiative divisions and extended the period of Tbr1 ⁺/Ctip2 ⁺ deep-layer neurogenesis while reducing Satb2 ⁺ upper-layer neurogenesis. Similarly, acute misexpression of Neurog2 in early cortical progenitors promoted Tbr1 expression, whereas both Neurog2 and Ascl1 induced Ctip2. However, Neurog2 was unable to influence the derepression circuit when misexpressed in late cortical progenitors, and Ascl1 repressed only Satb2. Nevertheless, neurons derived from late misexpression of Neurog2 and, to a lesser extent, Ascl1, extended aberrant subcortical axon projections characteristic of early-born neurons. Finally, Neurog2 and Ascl1 altered the expression of Ikaros and Foxg1, 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.