PROJECT SUMMARY Spinal muscular atrophy is a monogenic motor neuron disease and is historically a leading inherited cause of death in infancy and childhood. While development of SMN-dependent therapies has significantly improved patient outcomes, continued neurological deficits in most patients highlights the need for SMN- independent therapeutics to improve motor neuron (MN) function and survival. This effort is hindered by the lack of understanding of how SMN deficiency impacts MN development and degeneration. Our laboratory recently demonstrated that severely impaired proximal motor axon radial growth beginning at mid-late embryogenesis and rapid degeneration of the entire MN postnatally are prominent features of type I SMA patients, but the molecular mechanisms driving these cellular events and the importance of these pathologies to milder forms of SMA are unknown. Historically, it has been difficult to obtain transcriptomic data from MNs in vivo due to their sparseness within the spinal cord, and while in vitro models can provide detailed transcriptomic data, these models forgo MN heterogeneity and the complexity of the spinal cord environment. To circumvent these difficulties, we have collaborated with Dr. Le Pichon at the NIH to produce two SMA mouse models that express GFP-tagged Sun1, a nuclear envelope protein, in choline acetyltransferase (ChAT) positive neurons of the spinal cord for FACS sorting and subsequent single nucleus RNA sequencing (snRNAseq). Using snRNAseq, we aim to elucidate the aberrant developmental pathways that drive early stages of SMA pathogenesis, and further compare the magnitude and temporality of the identified mechanisms in severe and mild models of SMA. We aim to do this by characterizing and performing snRNAseq on two SMA mouse models, the severe model (Smn-/- SMN2+/+) and a mild model (Smn2B/-). Each model will be behaviorally and histologically characterized in Aim 1 to identify appropriate time points for snRNAseq in Aim 2. In Aim 3, genes and pathways of interest will be validated using qRT-PCR, in situ hybridization, and protein quantification in both mouse models and in human autopsy tissues. Together, these studies aim to identify shared and divergent developmental and degenerative mechanisms across the spectrum of disease severity to inform future therapeutic development.