Project Summary In this grant we propose three aims focused on exploring the predictions of these gene regulatory networks (GRN) by examining how critical transcription factors influence parvalbumin (PV) and somatostatin (SST) cortical interneurons (cINs) at early and late points in development (1a), as well as explore the role of candidate disease genes that affect these populations (1b). In the next two aims we then focus upon SST cINs as they settle within particular cortex laminae to explore how extrinsic signals contribute to their diversity (2a,b) how mutations in the laminar specific genes Fezf2 and Tbr1 affect SST cIN lamination, as well as using these methods in the context of our gene regulatory networks (GRN) strategy to identify molecules that affects these events(3a,b). This will allow us to test our hypothesis that cINs emerges from a combination of a genetic program initiated upon them becoming postmitotic combined with non-autonomous signals from pyramidal neurons at their settling position within cortical laminae. A chief goal of my laboratory is to determine how changes in gene regulation create cortical interneuron diversity and how perturbations in these programs result in brain disease. By joining efforts with the Bonneau laboratory, whose expertise is in computational biology, we propose to both create and test GRNs for parvalbumin and somatostatin cortical interneurons across development. Combining this with the Fishell laboratory’s expertise in development, genetics and circuit formation, we will explore in aims 2 and 3 the mechanisms by which non- autonomous cues from pyramidal neurons shape developmental programs in SST cINs, as well as their synaptic connectivity. Together, this grant represents a broad scale effort to both develop and test the robustness of GRNs by applying them to understanding gene regulation in developing cortical interneurons, as well as to determine the extrinsic environmental signals from pyramidal cells that shape both the development and connectivity of developing interneurons. From our preliminary data it is evident that canonical cortical circuits are much more precise and specific to particular subtypes. By defining their relationships, as well as the molecular mechanisms that dictate their synaptic specificity, this work will greatly increase both our understanding of cINs, as well as the microcircuits they contribute to. This in turn will greatly extend our understanding of how discrete cIN and pyramidal neurons interact in the obligate lock and key mechanisms that allow their assembly.