PROJECT SUMMARY/ABSTRACT Alternative splicing is a key process in shaping the functional complexity of the brain. Accordingly, many neurologic and psychiatric disorders are caused by mutations in RNA binding proteins or their targets in alternative splicing. The Rbfox family of RNA Binding proteins regulate alternative splicing during neuronal development and their mutations have been implicated in autism spectrum disorder and various familial epileptic disorders. To understand the role of Rbfox in these diseases, many studies have focused on how individual Rbfox proteins bind to their targets and regulate their splicing. However, Rbfox proteins are also known to interact with cofactors that influence their function. We previously found that nuclear Rbfox proteins in the mouse brain are almost exclusively bound to a large assembly of splicing regulators (LASR). However, it remains unknown what the full subunits of the neuronal LASR complex are and how they interact with Rbfox to regulate alternative splicing in neurons. In this proposal, I will define the neuronal components of LASR, determine the transcriptome-wide targets of the LASR/Rbfox complex and its subunits in the mouse brain, and analyze how components of LASR affect Rbfox’s splicing regulatory activity in neurons. These studies will elucidate how combinatorial interactions between RNA binding proteins in a novel splicing regulatory protein complex shape the gene regulatory circuit of the brain and will further our understanding of the function of the Rbfox proteins which have been implicated in epilepsy and autism spectrum disorder.