Control of G₂ Phase Duration by CDC25B Modulates the Switch from Direct to Indirect Neurogenesis in the Neocortex

During development, cortical neurons are produced in a temporally regulated sequence from apical progenitors, directly or indirectly, through the production of intermediate basal progenitors. The balance between these major progenitor types is critical for the production of the proper number and types of neurons, and it is thus important to decipher the cellular and molecular cues controlling this equilibrium. Here we address the role of a cell cycle regulator, the CDC25B phosphatase, in this process. We show that, in the developing mouse neocortex of both sex, deleting CDC25B in apical progenitors leads to a transient increase in the production of TBR1⁺ neurons at the expense of TBR2⁺ basal progenitors. This phenotype is associated with lengthening of the G₂ phase of the cell cycle, the total cell cycle length being unaffected. Using in utero electroporation and cortical slice cultures, we demonstrate that the defect in TBR2⁺ basal progenitor production requires interaction with CDK1 and is because of the G₂ phase lengthening in CDC25B mutants. Together, this study identifies a new role for CDC25B and G₂ phase length in direct versus indirect neurogenesis at early stages of cortical development.SIGNIFICANCE STATEMENT This study is the first analysis of the function of CDC25B, a G₂/M regulator, in the developing neocortex. We show that removing CDC25B function leads to a transient increase in neuronal differentiation at early stages, occurring simultaneously with a decrease in basal intermediate progenitors (bIPs). Conversely, a CDC25B gain of function promotes production of bIPs, and this is directly related to CDC25B's ability to regulate CDK1 activity. This imbalance of neuron/progenitor production is linked to a G₂ phase lengthening in apical progenitors; and using pharmacological treatments on cortical slice cultures, we show that shortening the G₂ phase is sufficient to enhance bIP production. Our results reveal the importance of G₂ phase length regulation for neural progenitor fate determination.