Studying the Molecular Regulation of MGE Projection Neuron Identity by St18

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PROJECT SUMMARY: The medial ganglionic eminence (MGE) is a neural progenitor domain located in the ventral telencephalon. It is well-studied for its production of cortical interneurons, a broadly distributed population of locally-pro- jecting neurons that regulate cortical rhythmicity. The MGE also produces projection neurons that remain in the ventral forebrain, arranged in discrete nuclei that project distally point-to-point. Interneuron and projection neuron identity represent two fundamentally-distinct populations, with distinct functions. How the MGE delineates between the production of IN and PN, however, is not understood. We have identified a transcription factor, St18 that appears to regulate this fundamental delineation. We previously described a model system to study MGE development using induced expression of candidate factors. When St18 was intro- duced in this system, the cells largely adopted a projection neuron identity. Further, when these cells were transplanted in utero, they remained in the ventral forebrain and adopted long-range axonal projections reminiscent of globus pallidus pars externa (GPe). Moreover, in our initial analysis of the St18 conditional loss-of-function mutant mouse, we observed a spe- cific loss of GPe neurons, while all other MGE lineages appear spared. In this proposal, we will; Aim 1) Determine whether St18 induced expression is sufficient to direct MGE progenitors towards projection neuron identity (Techniques: ESC directed differentiation, neuronal culture, in vitro migration assays, UBM transplantation, RNA-seq). Aim 2) Test whether St18 is necessary for MGE projection neuron fate by analysis of St18 conditional loss-of-function mouse (Techniques: mouse genetics, viral and dye tracing of axon projections, scRNA- seq). Aim 3) Manipulate St18 during MGE progenitor differentiation to probe the time window of its action and test how it regulates gene expression in directing projection neuron identity (Techniques: ESC directed differentiation, in vitro mor- phology and migration assays, ChIP-seq). Importantly, in the course of our study, we will perform a multi-level expression and ChIP-seq analysis to uncover the gene networks and cellular pathways under the regulation of St18 that collectively govern aspects of projection neuron identity: neuronal migration, polarity, process morphogenesis. In summary, we are uniquely positioned to address how MGE progenitors regulate projection neuron versus interneuron identity in the MGE lineage. Through these efforts, we aim to inform more broadly as to how projection and interneuron identity is assigned throughout the nervous system.