UPF1 reduces C9orf72 HRE-induced neurotoxicity in the absence of nonsense-mediated decay dysfunction

Abstract

SUMMARY Multiple cellular pathways have been suggested to be altered by the C9orf72 GGGGCC (G 4 C 2 ) hexanucleotide repeat expansion (HRE), including aspects of RNA regulation such as nonsense-mediated decay (NMD). Here, we investigate the role that overexpression of UPF1, a protein involved in NMD, plays in mitigating neurotoxicity in multiple models of C9orf72 ALS/FTD. First, we show that NMD is not altered in our endogenous induced pluripotent stem cell (iPSC)-derived spinal neuron (iPSN) model of C9orf72 ALS (C9-ALS) or postmortem motor cortex tissue from C9-ALS patients. Unexpectedly, we find that UPF1 overexpression significantly reduces the severity of known neurodegenerative phenotypes without altering NMD function itself. UPF1 overexpression reduces poly(GP) abundance without altering the amount of repeat RNA, providing a potential mechanism by which UPF1 reduces dipeptide repeat (DPR) protein-mediated toxicity. Together, these findings indicate that UPF1 is neuroprotective in the context of C9-ALS, albeit independent of known UPF1-mediated NMD pathways.

RNA Is a Double-Edged Sword in ALS Pathogenesis

“…Although this strongly refutes the presence of any NMD dysfunction in C9ALS, this study corroborates that UPF1 is neuroprotective in this context ( Zaepfel et al, 2021 ). Evidence is provided that UPF1 overexpression reduces the abundance of DPRs in multiple models, potentially explaining its neuroprotective effects ( Zaepfel et al, 2021 ). However, much is still left unanswered by these papers to precisely explain the role that UPF1 plays in supporting neuronal health.…”

“…Thus, it is difficult to conclude from these studies whether NMD function is at all altered by the HRE in human patients. A very new study has attempted to answer this question using an human induced pluripotent stem cell-derived spinal neuron (iPSN) model of C9ALS ( Zaepfel et al, 2021 ). Following validation of several bona fide transcripts as substrates for NMD, it was demonstrated that neither the steady-state abundance nor stability of these substrates is altered in C9ALS patient-derived iPSNs relative to age- and sex-matched controls ( Zaepfel et al, 2021 ).…”

“…A very new study has attempted to answer this question using an human induced pluripotent stem cell-derived spinal neuron (iPSN) model of C9ALS ( Zaepfel et al, 2021 ). Following validation of several bona fide transcripts as substrates for NMD, it was demonstrated that neither the steady-state abundance nor stability of these substrates is altered in C9ALS patient-derived iPSNs relative to age- and sex-matched controls ( Zaepfel et al, 2021 ). Although this strongly refutes the presence of any NMD dysfunction in C9ALS, this study corroborates that UPF1 is neuroprotective in this context ( Zaepfel et al, 2021 ).…”

“…Following validation of several bona fide transcripts as substrates for NMD, it was demonstrated that neither the steady-state abundance nor stability of these substrates is altered in C9ALS patient-derived iPSNs relative to age- and sex-matched controls ( Zaepfel et al, 2021 ). Although this strongly refutes the presence of any NMD dysfunction in C9ALS, this study corroborates that UPF1 is neuroprotective in this context ( Zaepfel et al, 2021 ). Evidence is provided that UPF1 overexpression reduces the abundance of DPRs in multiple models, potentially explaining its neuroprotective effects ( Zaepfel et al, 2021 ).…”

“…Since the discovery of mutations in TARDBP , FUS , and C9ORF72 that cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), numerous investigations have demonstrated the effect these mutations have on RNA metabolism, as well as how RNA itself contributes to pathogenicity ( Tollervey et al, 2011 ; Xiao et al, 2011 ; Daigle et al, 2013 ; Donnelly et al, 2013 ; Gendron et al, 2013 ; Ihara et al, 2013 ; Lagier-Tourenne et al, 2013 ; Lee et al, 2013 ; Nakaya et al, 2013 ; Shodai et al, 2013 ; Cooper-Knock et al, 2014 ; Coyne et al, 2014 , 2015 , 2017a , 2020 ; Highley et al, 2014 ; Liu-Yesucevitz et al, 2014 ; Mackness et al, 2014 ; Barmada et al, 2015 ; Burguete et al, 2015 ; Tran et al, 2015 ; Conlon et al, 2016 ; Dodd et al, 2016 ; Garnier et al, 2017 ; Liu et al, 2017 ; Lu et al, 2017 ; Kamelgarn et al, 2018 ; Tank et al, 2018 ; Chen et al, 2019 ; Xu et al, 2019 ; Ortega et al, 2020 ; Sun et al, 2020 ; Zaepfel et al, 2021 ). Mutations in all three of these genes have been linked to alterations in mRNA splicing and stress granule formation ( Daigle et al, 2013 ; Nakaya et al, 2013 ; Conlon et al, 2016 ; Donde et al, 2019 ; Klim et al, 2019 ).…”