Cell-Type-Specific Contributions of the Transcription Factor FOXP1 to Striatal Development and Function

Mutations in FOXP1, a member of the forkhead box protein (FOXP) family of transcription factors, have been identified as among the most significantly recurring de novo mutations associated with autism spectrum disorder (ASD). ASD is a genetically complex disorder, however, recent studies have identified distinct neuronal cell-types particularly vulnerable in this disorder. These cell-types include deep layer cortical neurons and dopamine receptor 1 (D1) and 2 (D2) expressing striatal spiny projection neurons (SPNs) where FOXP1 is highly expressed. However, the role of Foxp1 within these cell-types was largely unknown. Using a Foxp1 heterozygous mouse model and a human in vitro model system, I reported that FoxP1 regulates conserved pathways within the striatum based on a module preservation analysis between human and rodent gene co-expression networks. I also found a cell-type-specific functional consequence of reduced Foxp1 expression in Foxp1 heterozygous mice, whereby D2 SPNs had increased intrinsic excitability with no significant changes in dSPNs. Together, these data strongly support a conserved, cell-type-specific role for Foxp1 in striatal development and function. The striatum is a critical forebrain structure for integrating cognitive, sensory, and motor information from diverse brain regions into meaningful behavioral output. Therefore, the overarching goal of my project is to investigate the cell-type specific molecular pathways regulated by Foxp1 within distinct striatal SPNs and link these molecular pathways to functional and behavioral outcomes. To do this, I generated mice with deletion of Foxp1 from D1 SPNs, D2 SPNs, or both populations, and used a combination of single-cell RNA-sequencing (scRNA-seq), serial-two-photon tomography, and behavioral assays to delineate the contribution of Foxp1 to striatal development and function. I show that Foxp1 is crucial for maintaining the cellular composition of the striatum, especially D2 SPN specification, and proper formation of the striosome-matrix compartments at early postnatal and adult timepoints. I uncover downstream targets regulated by Foxp1 within D1 and D2 SPNs and connect these molecular findings to cell-type-specific deficits in motor and limbic system-associated behaviors, including motor-learning, ultrasonic vocalizations, and fear conditioning. Moreover, I identify non-cell autonomous molecular and functional effects produced by disruption of Foxp1 within one SPN subpopulation and the molecular compensation that occurs. Using the scRNA-seq data, I also examined gene expression changes within neuronal and non-neuronal cell-types of the developing striatum. Using my above findings, I attempted to pharmacologically rescue motor-learning deficits in Foxp1 cKO mice by targeting dopaminergic and mTOR-regulated pathways. Finally, I discuss the current challenges and future strategies for therapeutic intervention in cases of FOXP1 mutations. Overall, the findings presented in this thesis provide an important molecular window into striatal development and furthers our understanding of striatal circuits underlying ASD-relevant phenotypes.