Impaired axonal development and degeneration are implicated in many debilitating disorders, such as hereditary spastic paraplegia (HSP), amyotrophic lateral sclerosis, and periphery neuropathy. HSP is caused by distal axonopathy involving the longest corticospinal tract axons, leading to spasticity and weakness of the lower extremities. The most common early-onset form of HSP, SPG3A, is caused by mutations in the atlastin-1 gene. This gene encodes atlastin-1 protein, which is a member of the dynamin-related large GTPase superfamily. Knockdown of atlastin-1 in rat cortical neuron in vitro cultures inhibits the axonal outgrowth and elongation. However, how altered atlastin-1 activity leads to axonal defects and why specific axons degenerate in HSP patients are largely unclear. The goal of this proposed study is to establish human neuronal models of SPG3A to delineate the mechanisms underlying the axonal defects in HSP. This study's hypothesis is that atlastin mutations result in axonal defects selectively in cortical projection neurons (cortical PNs), and this effect is mediated mainly by dysregulated bone morphogenetic protein (BMP) signaling. This hypothesis will be tested by pursuing the following two aims: 1) to examine the axonal outgrowth and transport in cortical PNs derived from iPSCs that are generated from SPG3A patients and normal individuals (as controls); 2) to delineate the role of BMP signaling in the axonal defects in SPG3A. By comparing the axonal defects, atlastin-1 activity, and BMP signaling alterations in cortical PNs, cortical interneurons, and spinal motor neurons derived from control and SPG3A iPSCs, this study will be able to delineate the cell type-specific defects in HSP and the underlying mechanisms. The cause-effect relationship between loss of atlastin function and axonal phenotypes will be confirmed by knocking down atlastin-1 in wild-type (WT) neurons and by expressing WT atlastin-1 in SPG3A iPSCs. Moreover, rescue experiments will be performed to identify the potential approaches for rescuing the axonal pathology, such as overexpression of atlastin or treatment with BMP antagonists. Together, this study will provide valuable insights into the roles of atlastin-1 and BMP signaling in HSP pathology and developing new therapeutics for rescuing the axonal degeneration in HSP.