LXR agonist rescues synaptic dysfunction and degeneration in SPG3A patient-specific iPSC-derived neurons

Hereditary spastic paraplegias (HSPs) comprise a large, heterogeneous group of inherited disorders characterized by length-dependent axonal degeneration of corticospinal motor neurons, leading to lower extremity spasticity and gait impairment. Currently, there are no effective treatments for HSPs targeting axonal dysfunction. Our previous study showed that lipid defects in glial cells result in degeneration of iPSC-derived cortical projection neurons (PNs) in SPG3A, the most common early-onset form of HSP caused by autosomal dominant mutations in the ATL1 gene encoding atlastin-1. However, how cortical PNs degenerate and whether therapeutic compounds targeting lipid defects can effectively mitigate degeneration in human ATL1 neurons remain unclear. Here, by comparing SPG3A patient iPSC-derived neurons with control cells using RNA-sequencing, we identified synaptic dysfunction as a top-altered pathway in addition to lipid-related pathways. To examine the novel role of synaptic dysfunction in SPG3A, we generated patient-specific iPSCs from two SPG3A patients with distinct missense mutations and differentiated them into cortical PNs. We observed significant reductions of synaptic genes and proteins in cortical PNs from both SPG3A-P342S and SPG3A-M408T patient iPSCs, emphasizing synaptic dysfunction in SPG3A neurons. Calcium imaging revealed a significant reduction of activity in SPG3A cortical neurons compared to control neurons, further supporting functional deficits in SPG3A neurons. To further examine the role of these processes in HSP pathogenesis, we treated cells with LXR623, an orally bioavailable liver-X-receptor (LXR) agonist that can modulate lipid metabolism and transfer. LXR623 significantly mitigated the reduction in synaptic proteins and calcium activity and rescued axonal degeneration and apoptosis in SPG3A cortical PNs. Furthermore, analyses of lipid and synaptic genes and proteins revealed that LXR623 treatment effectively restored mRNA expression patterns for these pathways in SPG3A neurons. Taken together, our data demonstrate the role of synaptic dysfunction in degeneration of SPG3A neurons and highlight the therapeutic potential of an LXR agonist in mitigating human cortical neuron degeneration in HSP.