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Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative disorders characterized by the expansion of GGGGCC (G4C2) repeats in the C9orf72 gene and progressive motor neuron degeneration. A key pathological hallmark of these diseases is the accumulation and cytoplasmic mislocalization of dipeptide repeat (DPR) proteins, particularly poly(GR), which are neurotoxic. Enhancing the clearance of poly(GR) represents a promising therapeutic strategy; however, the molecular mechanisms regulating poly(GR) turnover are not fully understood. Our previous work demonstrated that translationally stalled poly(GR) is targeted by the ribosome-associated quality control (RQC) pathway. In the present study, we identify the IRE1/Xbp1s signaling axis as an essential regulator of poly(GR) degradation. Ectopic expression of IRE1 or its downstream effector Xbp1s, as well as pharmacological activation of IRE1 using IXA4, significantly reduces poly(GR) protein levels in a Drosophila disease model, mammalian cell lines, fibroblasts derived from patients with C9orf72-ALS, and a C9orf72 transgenic mouse model. Mechanistically, RNA-sequencing analysis reveals that IRE1/Xbp1s signaling upregulates heat shock protein Hsp70Ba, which plays a critical role in maintaining poly(GR) proteostasis. Additionally, we show that the Rictor/AKT/VCP pathway contributes to the translational regulation and turnover of poly(GR). Importantly, activation of IRE1, either through ectopic expression or IXA4 treatment, mitigates motor neuron loss in the C9orf72 mouse model. Collectively, our findings highlight the IRE1/Xbp1s axis as a key modulator of poly(GR) clearance and suggest its therapeutic potential in ALS/FTD.
Yu et al. (Wed,) studied this question.