Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by preferential motor neuron death. Approximately 15% of ALS cases are familial, and mutations in the fused in sarcoma (FUS) gene contribute to a subset of familial ALS cases. FUS is a multifunctional protein participating in many RNA metabolism pathways. ALS-linked mutations cause a liquid–liquid phase separation of FUS protein in vitro, inducing the formation of cytoplasmic granules and inclusions. However, it remains elusive what other proteins are sequestered into the inclusions and how such a process leads to neuronal dysfunction and degeneration. In this study, we developed a protocol to isolate the dynamic mutant FUS-positive cytoplasmic granules. Proteomic identification of the protein composition and subsequent pathway analysis led us to hypothesize that mutant FUS can interfere with protein translation. We demonstrated that the ALS mutations in FUS indeed suppressed protein translation in N2a cells expressing mutant FUS and fibroblast cells derived from FUS ALS cases. In addition, the nonsense-mediated decay (NMD) pathway, which is closely related to protein translation, was altered by mutant FUS. Specifically, NMD-promoting factors UPF1 and UPF3b increased, whereas a negative NMD regulator, UPF3a, decreased, leading to the disruption of NMD autoregulation and the hyperactivation of NMD. Alterations in NMD factors and elevated activity were also observed in the fibroblast cells of FUS ALS cases. We conclude that mutant FUS suppresses protein biosynthesis and disrupts NMD regulation, both of which likely contribute to motor neuron death.
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This study was supported, in part, by National Institutes of Neurological Disorder and Stroke GrantR01NS077284, Muscular Dystrophy Association Grant MDA352743, ALS Association Grant 6SE340, and Department of Veteran Affairs Merit Review Award I01 BX002149 (to H.Z.). Support from the Multidisciplinary Value Program initiative at the University of Kentucky College of Medicine is appreciated.M.K. is supported by National Institute of Environmental Health Sciences Training Grant T32ES007266 and the University of Kentucky College of Medicine Fellowship for Excellence in Graduate Research.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1810413115/-/DCSupplemental.
Kamelgarn, Marisa; Chen, Jing; Kuang, Lisha; Jin, Huan; Kasarskis, Edward J.; and Zhu, Haining, "ALS Mutations of FUS Suppress Protein Translation and Disrupt the Regulation of Nonsense-Mediated Decay" (2018). Toxicology and Cancer Biology Faculty Publications. 75.