A Chemical Screen Identifies Compounds Limiting the Toxicity of C9ORF72 Dipeptide Repeats

Chung

et al. 2019

Preprint

25Accumulation of RNA in the nucleus is one of the pathological features of C9orf72-associated 26 amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD), yet its potential 27 42 Staufen, may also be an important pathological feature of C9-ALS/FTD. 43 44 45 46 47 3 48 Author summary 49 Cytoplasmic accumulation of nuclear RNA-binding proteins (RBPs) is one of the common 50 pathological features of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia 51 (FTD). In C9orf72-associated ALS/FTD fly model, we found that Staufen, a double-stranded 52 (ds) RBP normally localized mostly in cytoplasm, accumulates in the nucleus in an RNA-53 dependent manner. Next, we checked wherein the nucleus Staufen accumulates and found that 54 Staufen partially co-localizes with heterochromatin and nucleolus. Interestingly, the expression 55 of Fibrillarin, a nucleolar protein, was significantly increased by C9orf72-derived PR toxicity 56 and further augmented by reduction in stau dosage, the gene encoding Staufen. When we 57 knocked down fib, the gene encoding Fibrillarin, PR-induced retinal degeneration was 58 exacerbated. This indicates that increased Fibrillarin expression by stau dosage reduction is 59 protective. Furthermore, when we reduced stau dosage in flies presenting PR toxicity, their 60 retinal degeneration and viability were largely rescued. Based on these data, we suggest that 61 nuclear accumulation of Staufen is an important feature of C9-ALS/FTD and suggest that 62 reducing stau dosage is a promising therapeutic target.

Section: Resultssupporting

confidence: 94%

C9orf72-associated arginine-rich dipeptide repeats induce RNA-dependent accumulation of Staufen in nucleus

Chung

et al. 2019

Preprint

“…S8A and B ). These data are in line with a previous report showing increased nucleolar size determined by Fibrillarin staining in U2OS and NSC-34 cells expressing PR20 ( 54 ).…”

Section: Resultssupporting

confidence: 93%

C9orf72-associated arginine-rich dipeptide repeats induce RNA-dependent nuclear accumulation of Staufen in neurons

Human Molecular Genetics

Chung

Park

et al. 2021

RNA-binding proteins (RBPs) play essential roles in diverse cellular processes through post-transcriptional regulation of RNAs. The subcellular localization of RBPs is thus under tight control, the breakdown of which is associated with aberrant cytoplasmic accumulation of nuclear RBPs such as TDP-43 and FUS, well-known pathological markers for amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). Here, we report in Drosophila model for ALS/FTD that nuclear accumulation of a cytoplasmic RBP, Staufen, may be a new pathological feature. We found that in Drosophila C4da neurons expressing PR36, one of the arginine-rich dipeptide repeat proteins (DPRs), Staufen accumulated in the nucleus in Importin- and RNA-dependent manner. Notably, expressing Staufen with exogenous NLS—but not with mutated endogenous NLS—potentiated PR-induced dendritic defect, suggesting that nuclear-accumulated Staufen can enhance PR toxicity. PR36 expression increased Fibrillarin staining in the nucleolus, which was enhanced by heterozygous mutation of stau (stau+/−), a gene that codes Staufen. Furthermore, knockdown of fib, which codes Fibrillarin, exacerbated retinal degeneration mediated by PR toxicity, suggesting that increased amount of Fibrillarin by stau+/− is protective. Stau+/− also reduced the amount of PR-induced nuclear-accumulated Staufen and mitigated retinal degeneration and rescued viability of flies expressing PR36. Taken together, our data show that nuclear accumulation of Staufen in neurons may be an important pathological feature contributing to the pathogenesis of ALS/FTD.

“…While our genetic studies are consistent with this model, additional cell biological and biochemical tests are needed to more rigorously test its predictions. Interestingly, in a recent small molecule screen, two BET inhibitors were identified as suppressors of exogenous PR20 toxicity (48). These observations are nominally the opposite effect our model would predict.…”

Section: Discussionmentioning

confidence: 99%

The nuclear ubiquitin ligase adaptor SPOP is a conserved regulator of C9orf72 dipeptide toxicity

Snoznik

Medvedeva²,

Mojsilovic-Petrovic³

et al. 2021

Preprint

A hexanucleotide repeat expansion in the C9orf72 gene is the most common cause of inherited amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Unconventional translation of the C9orf72 repeat produces dipeptide repeat proteins (DPRs). Previously, we showed that the DPRs (PR)50 and (GR)50 are highly toxic when expressed in C. elegans and this toxicity depends on nuclear localization of the DPR. In an unbiased genome-wide RNAi screen for suppressors of (PR)50 toxicity, we identified 12 genes that consistently suppressed either the developmental arrest and/or paralysis phenotype evoked by (PR)50 expression. All of these genes have vertebrate homologs and 7/12 contain predicted nuclear localization signals. One of these genes was spop-1, the C. elegans homolog of SPOP, a nuclear localized E3 ubiquitin ligase adaptor only found in metazoans. SPOP is also required for (GR)50 toxicity and functions in a genetic pathway that includes cul-3, which is the canonical E3 ligase partner for SPOP. Genetic or pharmacological inhibition of SPOP in mammalian primary spinal cord motor neurons suppressed DPR toxicity without affecting DPR expression levels. Finally, we find that genetic inhibition of bet-1, the C. elegans homolog of the known SPOP ubiquitination targets BRD2/3/4, suppresses the protective effect of SPOP mutations. Together, these data suggest a model in which SPOP promotes the DPR-dependent ubiquitination and degradation of BRD proteins. We speculate the pharmacological manipulation of this pathway, which is currently underway for multiple cancer subtypes, could also represent a novel entry point for therapeutic intervention to treat C9 FTD/ALS.