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Accumulation of DNA damage in neurons is largely described in aging process and neurodegenerative diseases. Recently, RNA-binding proteins (RBPs) have emerged as important effectors of the cellular DNA damage response (DDR). The intrinsically disordered RBP FUS has been shown to play a role in multiple cellular functions, including mRNA transcription, splicing and transport, and in DDR. Besides that, FUS, together with some other RBPs, has been shown to undergo liquid-liquid phase separation (LLPS) under physiological conditions, but its precise molecular functions in DDR and the role of its LLPS remain poorly understood. The aims of this study are to: i) characterise the molecular functions of FUS in the DDR, ii) assess whether a proper repair of double strand breaks (DSB) is dependent on the LLPS of FUS. Using gene-edited HeLa cell lines, we showed that knock-out of FUS (FUS-KO) sensitised cells to genotoxic stress, impaired DSB repair, and caused changes in DDR pathways. At the molecular level, we found that FUS-KO impaired the recruitment of apical effectors of the DDR to laser-induced DNA damage sites. FUS itself is recruited within seconds to DSBs and is required for the retention of the apical DNA DSBs sensor KU. Mass spectrometry analysis revealed that FUS co-purifies with RBP implicated in DDR, including SFPQ. We demonstrate that LLPS occurs at DNA damage foci and is required for the efficient recruitment of key DDR factors. Finally, LLPS-deficient FUS variants impair the recruitment of KU and SFPQ. Overall, our findings establish for the first time that FUS-dependent LLPS contributes to the activation of the DDR and to the recruitment of DDR proteins at sites of DNA damage.

Liquid–liquid phase separation (LLPS) of proteins and RNAs has emerged as the driving force underlying the formation of membrane-less organelles. Such biomolecular condensates have various biological functions and have been linked to disease. The protein Fused in Sarcoma (FUS) undergoes LLPS and mutations in FUS have been causally linked to the motor neuron disease Amyotrophic Lateral Sclerosis (ALS-FUS). LLPS followed by aggregation of cytoplasmic FUS has been proposed to be a crucial disease mechanism. However, it is currently unclear how LLPS impacts the behaviour of FUS in cells, e.g. its interactome. Hence, we developed a method allowing for the purification of LLPS FUS-containing droplets from cell lysates. We observe substantial alterations in the interactome, depending on its biophysical state. While non-LLPS FUS interacts mainly with factors involved in pre-mRNA processing, LLPS FUS predominantly binds to proteins involved in chromatin remodelling and DNA damage repair. Interestingly, also mitochondrial factors are strongly enriched with LLPS FUS, providing a potential explanation for the observed changes in mitochondrial gene expression in mouse models of ALS-FUS. In summary, we present a methodology to investigate the interactomes of phase separating proteins and provide evidence that LLPS shapes the FUS interactome with implications for function and disease.

Amyotrophic lateral sclerosis (ALS) is a relentless and fatal neurological disease characterized by the selective degeneration of motor neurons. No effective therapy is available for this disease. Several lines of evidence indicate that alteration of RNA metabolism, including microRNA (miRNA) processing, is a relevant pathogenetic factor and a possible therapeutic target for ALS. Here, we showed that the abundance of components in the miRNA processing machinery is altered in a SOD1-linked cellular model, suggesting consequent dysregulation of miRNA biogenesis. Indeed, high-throughput sequencing of the small RNA fraction showed that among the altered miRNAs, miR-129-5p was increased in different models of SOD1-linked ALS and in peripheral blood cells of sporadic ALS patients. We demonstrated that miR-129-5p upregulation causes the downregulation of one of its targets: the RNA-binding protein ELAVL4/HuD. ELAVL4/HuD is predominantly expressed in neurons, where it controls several key neuronal mRNAs. Overexpression of pre-miR-129-1 inhibited neurite outgrowth and differentiation via HuD silencing in vitro, while its inhibition with an antagomir rescued the phenotype. Remarkably, we showed that administration of an antisense oligonucleotide (ASO) inhibitor of miR-129-5p to an ALS animal model, SOD1 (G93A) mice, result in a significant increase in survival and improved the neuromuscular phenotype in treated mice. These results identify miR-129-5p as a therapeutic target that is amenable to ASO modulation for the treatment of ALS patients.

Fused in sarcoma (FUS) is a ubiquitously expressed RNA-binding protein proposed to function in various RNA metabolic pathways, including transcription regulation, pre-mRNA splicing, RNA transport and microRNA processing. Mutations in the FUS gene were identified in patients with amyotrophic lateral sclerosis (ALS), but the pathomechanisms by which these mutations cause ALS are not known. Here, we show that FUS interacts with the minor spliceosome constituent U11 snRNP, binds preferentially to minor introns and directly regulates their removal. Furthermore, a FUS knockout in neuroblastoma cells strongly disturbs the splicing of minor intron-containing mRNAs, among them mRNAs required for action potential transmission and for functional spinal motor units. Moreover, an ALS-associated FUS mutant that forms cytoplasmic aggregates inhibits splicing of minor introns by trapping U11 and U12 snRNAs in these aggregates. Collectively, our findings suggest a possible pathomechanism for ALS in which mutated FUS inhibits correct splicing of minor introns in mRNAs encoding proteins required for motor neuron survival.

The formation of defined 3 ′ ends is an important step in the biogenesis of mRNAs. In eukaryotic cells, all mRNA 3 ′ ends are generated by endonucleolytic cleavage of primary transcripts in reactions that are essentially posttranscriptional. Nevertheless, 3 ′ end formation is tightly connected to transcription in vivo, and a link with mRNA export to the cytoplasm has been postulated. Here, we briefly review the current knowledge about the two types of mRNA 3 ′ end processing reactions, cleavage/polyadenylation and histone RNA processing. We then focus on factors shared between these two reactions. In particular, we discuss evidence for new functions of the mammalian cleavage factor I subunit CF I m68 in histone RNA 3 ′ processing and in the export of mature mRNAs from the nucleus to the cytoplasm. © 2010 John Wiley & Sons, Inc.

Metazoan replication-dependent histone pre-mRNAs undergo a unique 3'-cleavage reaction which does not result in mRNA polyadenylation. Although the cleavage site is defined by histone-specific factors (hairpin binding protein, a 100-kDa zinc-finger protein and the U7 snRNP), a large complex consisting of cleavage/polyadenylation specificity factor, two subunits of cleavage stimulation factor and symplekin acts as the effector of RNA cleavage. Here, we report that yet another protein involved in cleavage/polyadenylation, mammalian cleavage factor I 68-kDa subunit (CF I(m)68), participates in histone RNA 3'-end processing. CF I(m)68 was found in a highly purified U7 snRNP preparation. Its interaction with the U7 snRNP depends on the N-terminus of the U7 snRNP protein Lsm11, known to be important for histone RNA processing. In vivo, both depletion and overexpression of CF I(m)68 cause significant decreases in processing efficiency. In vitro 3'-end processing is slightly stimulated by the addition of low amounts of CF I(m)68, but inhibited by high amounts or by anti-CF I(m)68 antibody. Finally, immunoprecipitation of CF I(m)68 results in a strong enrichment of histone pre-mRNAs. In contrast, the small CF I(m) subunit, CF I(m)25, does not appear to be involved in histone RNA processing.

Export of mRNA from the nucleus is linked to proper processing and packaging into ribonucleoprotein complexes. Although several observations indicate a coupling between mRNA 3′ end formation and export, it is not known how these two processes are mechanistically connected. Here, we show that a subunit of the mammalian pre-mRNA 3′ end processing complex, CF Im68, stimulates mRNA export. CF Im68 shuttles between the nucleus and the cytoplasm in a transcription-dependent manner and interacts with the mRNA export receptor NXF1/TAP. Consistent with the idea that CF Im68 may act as a novel adaptor for NXF1/TAP, we show that CF Im68 promotes the export of a reporter mRNA as well as of endogenous mRNAs, whereas silencing by RNAi results in the accumulation of mRNAs in the nucleus. Moreover, CF Im68 associates with 80S ribosomes but not polysomes, suggesting that it is part of the mRNP that is remodeled in the cytoplasm during the initial stages of translation. These results reveal a novel function for the pre-mRNA 3′ end processing factor CF Im68 in mRNA export.

Paraquat (PQ) is a neurotoxic herbicide that induces superoxide formation. Although it is known that its toxic properties are linked to ROS production, the cellular response to PQ is still poorly understood. We reported previously that treatment with PQ induced genome-wide changes in pre-mRNA splicing. Here, we investigated the molecular mechanism underlying PQ-induced pre-mRNA splicing alterations. We show that PQ treatment leads to the phosphorylation and nuclear accumulation of SRPK2, a member of the family of serine/arginine (SR) protein-specific kinases. Concomitantly, we observed increased phosphorylation of SR proteins. Site-specific mutagenesis identified a single serine residue that is necessary and sufficient for nuclear localization of SRPK2. Transfection of a phosphomimetic mutant modified splice site selection of the E1A minigene splicing reporter similar to PQ-treatment. Finally, we found that PQ induces DNA damage and vice versa that genotoxic treatments are also able to promote SRPK2 phosphorylation and nuclear localization. Consistent with these observations, treatment with PQ, cisplatin or γ-radiation promote changes in the splicing pattern of genes involved in DNA repair, cell cycle control, and apoptosis. Altogether, our findings reveal a novel regulatory mechanism that connects PQ to the DNA damage response and to the modulation of alternative splicing via SRPK2 phosphorylation.