PROJECT SUMMARY Congenital ataxias generally result from dysfunctions and malformations of the cerebellum, particularly the medial vermis. These disorders may result from the lack of proper developmental signaling cascades which dictate the proliferation and formation of neurons. Primary cilia provide a hub for various developmental signaling proteins such as SHH or WNT. Dysfunction in ciliary proteins leads to rare genetic disorders affecting human development in the nervous system, optical system, and liver, kidney, and skeletal systems. Patients with ciliopathies like Joubert Syndrome and related disorders display cerebellar vermis hypoplasia, thickened superior cerebellar peduncles, and a deepened interpeduncular fossa. Components of the cytoskeleton, such as actin and microtubules, play a vital role in ciliogenesis and the maintenance of existing ciliary components and supporting scaffold. Although many cilia-related genes have been found to be causal for these disorders, cytoskeletal regulators of the formin family and their relationship with cilia has not yet been fully defined, nor have these molecules been previously associated with abnormal brain development. Our lab uses a forward genetic approach to identify pathways critical to cerebellar development and degeneration of neurons in this brain region. Through a chemical mutagenesis screening, we discovered an ataxic mouse mutant with phenotypes similar to those observed in some ciliopathies: cerebellar hippocampal hypoplasia, abnormal foliation, cerebellar elongation along the anterior-posterior axis, as well as the failure of the superior cerebellar peduncle to decussate. By positional cloning, we identified a mutation at a splice acceptor in Fmnl2, leading to exon skipping in Fmnl2 transcripts. Interestingly, levels of Fmnl2 transcripts in the brain of mutant mice are unchanged compared to WT, but protein levels are reduced, suggesting that the in-frame deletion encoded by this exon are necessary for stability of this protein. FMNL2 is an autoinhibited cytoskeletal effector that has been previously shown to drive actin polymerization at filopodia and lamellipodia tips of cultured cells. Although other proteins in this family have shown to bind and regulate microtubules, actin, and influence cilia formation, whether this protein functions in microtubules and actin during brain development is unknown. Using this novel mouse model, I will investigate the role of FMNL2 in actin and microtubule stabilization and determine how the hypomorphic loss of this protein may impact ciliogenesis and cilia maintenance. These studies will enlighten our understanding of cerebellar malformations and impact our understanding of the mechanisms underlying the role of microtubules and actin in human ciliopathies.