Transcriptional Controls Over Specification of Neocortical Projection Neuron Subtype and Area Identity

Abstract

The complex and sophisticated neocortical circuits that mediate higher-order brain functions are assembled from an extraordinary variety of neuronal subtypes, each with distinct morphologies, output connectivity, and electrophysiological properties. These diverse neuronal subtypes are generated under precise molecular regulation during neocortical development, and elucidating programs that govern their specification will be critical toward more deeply understanding the organization, evolution, and function of the cerebral cortex. In recent years, several key transcriptional controls over specification of projection neuron subtype and area identity have been identified, providing substantial insight into the ‘molecular logic’ underlying cortical development. This work increasingly supports a model in which individual regulators are embedded within modular, but highly interconnected, networks that gate sequential developmental decisions. There is an emerging understanding that molecular controls over subtype and area identity are intimately interrelated, and that specification of neuronal identity relies on sequential molecular decision points in progenitors and postmitotic neurons. Despite substantial progress, however, the basic framework of transcriptional controls over neocortical projection neuron development has not been fully defined. The precise molecular mechanisms by which recently identified regulators act in parallel, synergistically, or cross-repressively to progressively delineate subtype and area identity are not well understood. Moreover, very few downstream-regulated genes responsible for executing specific aspects of neuronal differentiation have been identified. In this dissertation, I investigate functions of transcription factor Ctip1 in projection neuron subtype and area development. Using complementary genetic, molecular, and anatomic labeling approaches in mice, I find that Ctip1 is a novel control over 1) deep-layer projection neuron identity, promoting specification of corticothalamic and callosal projection neurons at the expense of subcerebral projection neurons; and 2) acquisition of sensory area identity, including area-specific gene expression, output connectivity, and afferent sensory map formation. In addition, I have developed recombinase-based strategies for dual-population mosaic analysis, which have enabled rigorous examination of cell autonomy in projection neuron subtype and area specification, and will have broader applications in other fields. Lastly, I have characterized two transgenic mouse lines produced by the GENSAT project, Rbp4-Cre and Ntsr1-Cre, which I find to specifically mediate recombination in subcerebral and corticothalamic projection neurons, respectively.