Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality and the second most common autosomal recessive genetic disorder worldwide. SMA is characterized by gradual loss of motor neurons leading to failed innervation of muscles which results in progressive muscle weakness and eventual death due to respiratory distress. SMA results from either a deletion or mutation in the SMN1 gene, encoding the survival of motor neuron protein (SMN). SMN and its associated complex proteins, function to assemble spliceosomal small nuclear ribonucleoprotein (snRNP) complexes. Consistent with a role in snRNP assembly, splicing defects are seen in SMA animal models, however they alone unlikely explain the selective vulnerability of spinal motor neurons in SMA patients. Understanding the role for SMN in these spinal motor neurons will provide insight into the mechanisms underlying the pathophysiology and identify new targets for development of new SMA therapies. SMN localizes to axons and dendrites in spinal motor neurons. In the axon, SMN co- localizes and co-transports with multiple mRNA binding proteins that direct axonal mRNA transport. Preliminary data from our lab using a severe SMA mouse model demonstrate an axonal mRNA localization defect in these cultured spinal motor neurons, without effects on the steady state levels of mRNAs. Given the importance of mRNA transport in facilitating localized mRNA translation needed for axonal function, we predict that this defect in mRNA localization contributes to disease progression in SMA. We hypothesize that SMN mediates axonal localization of mRNAs required for growth and maintenance of the axon, through the assembly and/or transport of mRNA and associated protein (mRNP) granules. We will first define the role for SMN in mRNA localization. In specific aim one, using cultured spinal motor neurons derived from both wildtype and an SMA mouse model; we will use in situ approaches to test the assembly and transport of mRNP granules. In specific aim two we will expand the list of known mislocalized axonal mRNAs in SMN-deficient motor neurons, using stem cell-derived motor neurons combined with compartmentalized cultures and an RNA-sequencing approach. The RNA-seq method will give us a genome wide view of all transcripts with an altered steady-state level in the axon. Through this work we will further pursue our long-term goal of understanding the importance of mRNA localization as a regulator of axon growth and homeostasis and how defects in mRNA localization contribute to the pathophysiology of SMA. Through identification of novel mislocalized mRNA transcripts and a role for SMN in mRNA localization, this research will identify novel targets for the development of SMA therapies.