Sigma-1 Receptor Mitigates Poly-GR-Associated Redox Dysregulation by Restoring NRF2/HO1 Signaling in C9orf72-Associated ALS.

Hexanucleotide GGGGCC repeat expansions in C9orf72 produce arginine-rich dipeptide repeat proteins, including poly-GR and poly-PR, that contribute to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Although poly-GR has been linked to oxidative stress and mitochondrial dysfunction, the mechanisms that determine whether antioxidant defenses are preserved or suppressed in poly-GR-expressing motor neurons remain incompletely understood. Here, we investigated whether Sigma-1 receptor (Sigma-1R) modulates poly-GR-associated redox dysregulation in cellular and mouse models. In NSC34 cells, both EGFP-PR42 and EGFP-GR50 expression reduced NRF2 protein levels. EGFP-GR50 expression was also associated with reduced NRF1 and HO1 levels and increased reactive oxygen species (ROS) accumulation. Sigma-1R overexpression or pharmacological activation with fluvoxamine or PRE084 restored NRF2 and HO1 expression and attenuated both cellular and mitochondrial oxidative stress in EGFP-GR50-expressing cells. Co-treatment with the Sigma-1R antagonist NE100 attenuated the agonist-induced restoration of NRF2 and HO1. Furthermore, the NRF2 inhibitor ML385 attenuated agonist-mediated HO1 restoration, supporting the involvement of NRF2 activity in this downstream response. In an AAV-mediated EGFP-GR50 mouse model, Sigma-1R deficiency was associated with shortened survival, reduced NeuN-positive neuronal counts, and decreased NRF2 immunoreactivity in the brain. Together, these findings identify Sigma-1R as a modulator of poly-GR-associated redox dysregulation and neuronal vulnerability and support further investigation of Sigma-1R modulation as a potential therapeutic strategy for C9orf72-associated ALS.