ALS/FTD-Linked Mutation in FUS Suppresses Intra-axonal Protein Synthesis and Drives Disease Without Nuclear Loss-of-Function of FUS

Abstract

SummaryThrough the generation of humanized FUS mice expressing full-length human FUS, we identify that when expressed at near endogenous murine FUS levels, both wild-type and ALS-causing and frontotemporal dementia (FTD)-causing mutations complement the essential function(s) of murine FUS. Replacement of murine FUS with mutant, but not wild-type, human FUS causes stress-mediated induction of chaperones, decreased expression of ion channels and transporters essential for synaptic function, and reduced synaptic activity without loss of nuclear FUS or its cytoplasmic aggregation. Most strikingly, accumulation of mutant human FUS is shown to activate an integrated stress response and to inhibit local, intra-axonal protein synthesis in hippocampal neurons and sciatic nerves. Collectively, our evidence demonstrates that human ALS/FTD-linked mutations in FUS induce a gain of toxicity that includes stress-mediated suppression in intra-axonal translation, synaptic dysfunction, and progressive age-dependent motor and cognitive disease without cytoplasmic aggregation, altered nuclear localization, or aberrant splicing of FUS-bound pre-mRNAs.Video Abstract

Low level of expression of C-terminally truncated human FUS causes extensive changes in spinal cord transcriptome of asymptomatic transgenic mice

“…1c). The latter observation is consistent with only partial cytoplasmic mislocalisation of FUS carrying point mutations in or completely lacking the NLS reported in neurons of mice expressing low levels of these human FUS variants [38][39][40] .…”

“…However, this phenotype is typical for mouse lines expressing high levels or/and highly pathogenic variants of human FUS. In several recently produced models a problem of artificially high levels of endogenous FUS expression was solved by using knock-in techniques 40,41 , a relevant promoter 38 40,41,43 . These data were compared to data obtained in the same studies by RNA sequencing analysis of similar neural samples of FUS knockout mice, which allowed to discriminate changes caused by the loss of FUS function and the gain of function by FUS lacking NLS and therefore partially mislocalised to the cytoplasm.…”

“…RNAs and therefore cannot directly affect RNA metabolism. Therefore, it was not surprising that mRNA encoding proteins involved in RNA metabolism were barely represented in L-FUS DEGs in contrast to high representation of these mRNA in DEGs identified in studies of mouse lines expressing low levels of FUS variants with intact RNA-binding domains 38,40 Humphrey et al,…”

Review: Modelling the pathology and behaviour of frontotemporal dementia

“…In a mouse model overexpressing FUS lacking its nuclear localization signal (and thus mislocalized to the cytoplasm) with the pan-neuronal Thy1.2 promoter, symptoms associated with FTD are seen prior to the onset of motor deficits; phenotypes included social interactional deficits and impaired fear memory retrieval, accompanied by changes in synaptic density in the frontal cortex [204,205]. However, when FUS is expressed in mice using its endogenous promoter, mutants, but not wildtype, cause early motor phenotypes and late cognitive and social deficits; early motor symptoms were associated with reduced translation in axons [206]. In summary, FTD-related phenotypes have been identified in both gain-and loss-of-function FUS models in varying coincidence with motor phenotypes.…”

Fused in sarcoma regulates DNA replication timing and progression

“…Originally described as an oncogenic fusion to the CHOP transcription factor in myxoid liposarcoma (MLS) (Crozat et al, 1993;Rabbitts et al, 1993), FUS, rose to prominence with the discovery that inherited and de novo mutations in its open-reading frame cause dominant forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (Corrado et al, 2010;Kwiatkowski et al, 2009;Vance et al, 2009). Although the underlying mechanisms are still unclear, the preponderance of ALS/FTD-associated mutations in FUS interfere with its nuclear import and folding, leading to the accumulation of cytosolic FUS aggregates that disrupt cellular function through loss-and gain-of function mechanisms impacting protein translation and nuclear transport among other processes (Dormann et al, 2010;Kamelgarn et al, 2018;Ling et al, 2019;Lopez-Erauskin et al, 2018;Shang and Huang, 2016).…”