Project Summary Axonal degeneration within the corticospinal tract leads to several neurological diseases, including hereditary spastic paraplegias (HSPs), which are a clinically and genetically heterogeneous group of gait disorders characterized by poor balance, spasticity, and progressive muscle weakness that can ultimately result in paralysis. Leveraging parallel animal (rat) and induced pluripotent stem cell (iPSC)-based models, our goal is to develop a better understanding of the pathomechanisms that underlie neurodegeneration resulting from mutations in genes that cause HSP, with a longer term goal of using these models as platforms to identify new therapeutics to combat disease. Using CRISPR-mediated genome editing, we have developed physiologically relevant models that recapitulate phenotypes exhibited by patients suffering from HSP. Specifically, CRISPR- modified rats expressing pathological variants of SPG4 (spastin) and SPG57 (TFG) demonstrate early onset hind limb spasticity and ataxia, which rapidly progresses to hind limb paralysis. Other rat models, including those harboring a truncation of SPG80 (UBAP1) identified previously in patients, exhibit later onset disease phenotypes, enabling us to examine disease progression in multiple, unique contexts. We now have an unprecedented opportunity to determine the mechanistic basis of the axonopathies observed. In particular, we plan to use high- resolution, live cell confocal imaging and electron tomography to test the hypothesis that changes in the trafficking of specific factors, including neurofilament proteins implicated previously in neurodegenerative disease, contribute to impaired neuronal function in HSP. We will also determine how neurofilament trafficking defects observed relate to disease onset based on a combination of electromyography studies, histopathology, and comprehensive gait and kinematic analysis of rodent movement as spasticity and muscle weakness ensues. Furthermore, we will determine