Modelling Hereditary Spastic Paraplegias using Human Pluripotent Stem Cells

Hereditary spastic paraplegias (HSP) comprise a large, heterogeneous group of genetic neurodegenerative disorders. The unifying symptom present among HSP patients is lower limb spasticity, which is caused by the degeneration of cortical spinal motor neuron (CSMN) axons. Research on various HSP subtypes has identified a number of common cellular pathways that are weak points for long projection neurons, particularly CSMNs. The focus of this work has been to generate novel cellular models of several HSP subtypes using human pluripotent stem cells (hPSCs). Stem cell models were chosen because of their ability to use cells that have the same genetic background as patients generate affected neuronal subtypes. The most common HSP subtype SPG4, which is caused by dominant mutations in the SPAST gene affecting the microtubule-severing enzyme spastin, was first analyzed. This revealed that SPG4-derived telencephalic glutamatergic neurons possess a number of abnormalities, including axonal swellings, reduced fast axonal transport, and increased microtubule stabilization. Treatment with the microtubule targeting drug vinblastine rescued the axonal swelling phenotype in neurons with reduced spastin. Next SPG3A was analyzed using patient-specific iPSC-derived telencephalic neurons. SPG3A cells with a novel P342S mutation in atlastin-1 had reduced endoplasmic reticulum (ER) complexity, and neurons had reduced neurite outgrowth and fast axonal transport. Similar to SPG4, vinblastine was able to rescue the neurite outgrowth phenotype in SPG3A neurons. Lastly, two rare, autosomal recessive forms of HSP, SPG15 and SPG48, which lack spastizin and AP5Z1 expression respectively, were examined. Telencephalic glutamatergic and midbrain dopaminergic neurons showed a number of defects, including reduced neurite outgrowth, increased apoptosis, abnormal mitochondrial morphology and membrane potential. Inhibition of mitochondrial fission with the small molecule mdivi-1 rescued mitochondrial and neurite outgrowth defects, offering a potential therapeutic avenue in the future. All of these studies emphasize the utility of hPSC-derived neurons to study human monogenic disorders and for testing therapeutic compounds. In the future, these models will allow further investigation of the mechanisms that cause the degeneration of a subset of neurons, which will be valuable not just for HSP, but for other neurodegenerative disorders