Cortical interneurons are implicated in numerous neurological disorders, including autism, schizophrenia and epilepsy. At present however we have little understanding of how this diverse population is generated. Work from my laboratory has demonstrated that Hedgehog-signaling between embryonic day 9 and 10 is fundamental for patterning of the mouse ventral telencephalon, from where all cortical interneurons arise. Moreover, our recent work has demonstrated that in mice where Hedgehog-signaling has been ablated at E9.0 cortical neurons as a whole fail to be specified. In this regard, work from spinal cord indicates that Hedgehog-signaling functions to establish diversity within ventral spinal cord. In this proposal we wish to determine the contribution of Hedgehog signaling to the establishment of cortical interneuron diversity. To this end we have assembled a comprehensive array of null and conditional alleles that allow us to selectively disrupt different aspects of Hedgehog signaling as well as a number of key Hedgehog response genes. In the first part of the grant, we with use conditional methods to specifically remove Gli3 from the ventral telencephalon. This is based on our previous work showing that the Gli3 protein is the Gli gene required for most aspects of Hedgehog signaling. We next examine the contribution of activators using inducible genetic fate mapping of Gli1 and Nkx6.2 cells, which function as reporters of cells in which Gli-activators are acting. We follow this up in the second portion of aim 2 by examining the requirement for Nkx6.2 through gain and loss of function analysis. In the final aim of the grant we investigate the phenotype resulting from the removal of a selection of the Hedgehog-responsive gene, Nkx2.1. This gene requires Hedgehog signaling for its expression and previous loss of function analysis of a null allele of Nkx2.1 demonstrates its function is required in cortical interneuron MGE progenitors. In the second portion of this aim, we will explore the role of Nkx2.1 through a set of gain of function experiments where we use genetic means to precisely control the place and time of ectopic expression and are able to genetically fate map the fate of cells after ectopic expression. Taken together this grant will provide a comprehensive analysis of the requirement for Hedgehog-signaling and its downstream effector genes for the generation of cortical interneuron diversity. PUBLIC HEALTH RELEVANCE: Developmental genetic disorders underlie some of the most prevalent forms of brain disease. The failure to generate or correctly specify specific neuronal subtypes is the primary etiology for many of the most devasting brain disorders. In this regard, cortical interneurons although representing only 20% of the neurons within the cerebrum are a particularly attractive target for therapy. If we can understand how this population is generated, we will take the first step towards assessing whether we can use gene or cell therapies to ameliorate the devasting consequences associated with cortical interneuron dysfunction.