Ropinirole Functions Through a Dopamine Receptor <scp>D</scp>2‐Independent Mechanism to Ameliorate Amyotrophic Lateral Sclerosis Phenotypes in <scp><i>TARDBP</i></scp>‐Mutant i<scp>PSC</scp>‐Derived Motor Neurons

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron (MN) degeneration. Ropinirole hydrochloride (ROPI), a dopamine receptor D2 (DRD2) agonist, was identified through phenotypic screening of MNs derived from patient-derived induced pluripotent stem cells (iPSCs) as a disease model and has emerged as a promising candidate drug for ALS treatment. The ROPALS trial, a phase I/IIa trial in patients with ALS, suggested the safety and efficacy of ROPI, albeit in a small sample size. Interestingly, a DRD2 antagonist and modulator only partially mitigated the suppressive effect of ROPI on the ALS phenotype, and the detailed mechanism of ROPI activity remains unclear. Therefore, in this study, we investigated whether the therapeutic effects of ROPI in ALS are dependent on DRD2. For this purpose, we generated DRD2-deficient iPSCs and showed that ROPI effectively reduced neuronal cell death, reactive oxygen species (ROS) production, and neuronal hyperexcitation, independently of DRD2. Further analyses revealed that ROPI corrected aberrant RNA splicing and restored the mRNA expression of mitochondrial proteins in a DRD2-independent manner. Our findings suggest that ROPI not only functions as a canonical DRD2 agonist but also has pleiotropic DRD2-independent effects, offering a novel avenue for treatment strategies that target multiple pathways involved in ALS pathology.