The cytoarchitecture of the mammalian cortex is formed during embryonic and postnatal development through the synchronization of progenitor proliferation and specification, neuronal differentiation and neuronal migration (reviewed by Shoemaker

INTRODUCTION The cytoarchitecture of the mammalian cortex is formed during embryonic and postnatal development through the synchronization of progenitor proliferation and specification, neuronal differentiation and neuronal migration (reviewed by Shoemaker and Arlotta, 2010; Molyneaux et al., 2007; Marin and Rubenstein, 2003). These tightly regulated events lead to a multilaminar structure containing layer-specific classes of pyramidal neurons that form local networks and long-range connections throughout the brain and spinal cord (Rakic, 2007). Lineage tracing and transplantation experiments suggest that the time when progenitors exit the cell cycle defines their eventual fate such that neurons that contribute to deep cortical layers are generated first, whereas laterborn cortical neurons occupy more superficial positions (Luskin et al., 1988; Bayer and Altman, 1991; McConnell and Kaznowski, 1991; Reid et al., 1995). This ordered sequence of neuronal differentiation is consolidated further by a progressive restriction in progenitor competence whereby progenitors later in development preferentially give rise to superficial cortical neurons (Desai and McConnell, 2000; Frantz and McConell, 1996). Thus, the timing of cortical neurogenesis is a key regulatory nexus to ensure cortical neuronal diversity; however, the molecular mechanisms that control this process are not well understood. Studies in the Drosophila embryonic nervous system have provided major insight into the mechanisms that link birthdate and neuronal fate. In Drosophila, neural progenitors (neuroblasts) express a set of transcription factors in a characteristic sequence that correlates precisely with the ordered generation of particular sets of neurons (reviewed by Pearson and Doe, 2004; Brody and Odewald, 2002; Jacob et al., 2008). This sequence of events is recapitulated in vitro using isolated neuroblasts and is linked to the number of cell cycles that have progressed, suggesting that the derivation of different neuronal cell types from multipotent progenitors depends primarily upon an intrinsic cell-cycle clock mechanism. One interpretation of these observations is that neuronal diversity is controlled by the extent of cell-cycle progression rather than the simple passage of time (Isshiki et al., 2001; Pearson and Doe, 2004). In the mammalian cortex, layer-specific neuronal fates are ultimately shaped by a combination of intrinsic mechanisms and environmental signals; however, recent experiments suggest that a similar intrinsic timing mechanism operates to ensure the sequence of layer-specific pyramidal neuron generation (Qian et al., 2000; Shen et al., 2006; Gaspard et al., 2008). Clonal analyses of isolated cortical progenitors show that neurogenesis in vitro proceeds as observed in vivo. Specifically, Cajal-Retzius cells are generated first, followed by deep-layer and superficial cortical neurons. Moreover, this sequence of differentiation is accompanied by a corresponding restriction in competence (Shen et al., 2006). Studies aiming to understand the molecular mechanisms involved in this regulation suggest that the cell-intrinsic mechanisms that control the timing of neurogenesis are phylogenetically conserved; for example, the transcription factors hunchback (hb) and seven up regulate the ratio of earlyto late-born neurons in the fly nervous system, and their vertebrate counterparts Ikaros (Ikzf1 – Mouse Genome Informatics) and CoupTFI/II (Nr2f1 and Nr2f2 – Mouse Genome Informatics) play similar roles in the mammalian cortex and retina (Elliott et al., 2008; Maurange et al., 2008; Faedo et al., 2008; reviewed by Okano and Temple, 2009). We considered the possibility that, in addition to cell intrinsic mechanisms, regulatory pathways that directly stimulate progenitor cell-cycle exit would constitute important control modules that could affect progenitor competence and the generation of neuronal The Solomon Snyder Department of Neuroscience, The Johns Hopkins University School of Medicine, PCTB1004, 725 N Wolfe Street, Baltimore, MD 21205, USA.