Male sterility and enhanced radiation sensitivity in TLS(-/-) mice

Deficiency of IQCH causes male infertility in humans and mice

IQ motif-containing proteins can be recognized by calmodulin (CaM) and are essential for many biological processes. However, the role of IQ motif-containing proteins in spermatogenesis is largely unknown. In this study, we identified a loss-of-function mutation in the novel gene IQ motif-containing H (IQCH) in a Chinese family with male infertility characterized by a cracked flagellar axoneme and abnormal mitochondrial structure. To verify the function of IQCH, Iqch knockout (KO) mice were generated via CRISPR-Cas9 technology. As expected, the Iqch KO male mice exhibited impaired fertility, which was related to deficient acrosome activity and abnormal structures of the axoneme and mitochondria, mirroring the patient phenotypes. Mechanistically, IQCH can bind to CaM and subsequently regulate the expression of RNA-binding proteins (especially HNRPAB), which are indispensable for spermatogenesis. Overall, this study revealed the function of IQCH, expanded the role of IQ motif-containing proteins in reproductive processes, and provided important guidance for genetic counseling and genetic diagnosis of male infertility.

The crucial role of HFM1 in regulating FUS ubiquitination and localization for oocyte meiosis prophase I progression in mice

BACKGROUND

Helicase for meiosis 1 (HFM1), a putative DNA helicase expressed in germ-line cells, has been reported to be closely associated with premature ovarian insufficiency (POI). However, the underlying molecular mechanism has not been clearly elucidated. The aim of this study was to investigate the function of HFM1 in the first meiotic prophase of mouse oocytes.

RESULTS

The results suggested that the deficiency of HFM1 resulting in increased apoptosis and depletion of oocytes in mice, while the oocytes were arrested in the pachytene stage of the first meiotic prophase. In addition, impaired DNA double-strand break repair and disrupted synapsis were observed in the absence of HFM1. Further investigation revealed that knockout of HFM1 promoted ubiquitination and degradation of FUS protein mediated by FBXW11. Additionally, the depletion of HFM1 altered the intranuclear localization of FUS and regulated meiotic- and oocyte development-related genes in oocytes by modulating the expression of BRCA1.

CONCLUSIONS

These findings elaborated that the critical role of HFM1 in orchestrating the regulation of DNA double-strand break repair and synapsis to ensure meiosis procession and primordial follicle formation. This study provided insights into the pathogenesis of POI and highlighted the importance of HFM1 in maintaining proper meiotic function in mouse oocytes.

Recent advances in mechanisms ensuring the pairing, synapsis and segregation of XY chromosomes in mice and humans

Sex chromosome aneuploidies are among the most common variations in human whole chromosome copy numbers, with an estimated prevalence in the general population of 1:400 to 1:1400 live births. Unlike whole-chromosome aneuploidies of autosomes, those of sex chromosomes, such as the 47, XXY aneuploidy that causes Klinefelter Syndrome (KS), often originate from the paternal side, caused by a lack of crossover (CO) formation between the X and Y chromosomes. COs must form between all chromosome pairs to pass meiotic checkpoints and are the product of meiotic recombination that occurs between homologous sequences of parental chromosomes. Recombination between male sex chromosomes is more challenging compared to both autosomes and sex chromosomes in females, as it is restricted within a short region of homology between X and Y, called the pseudo-autosomal region (PAR). However, in normal individuals, CO formation occurs in PAR with a higher frequency than in any other region, indicating the presence of mechanisms that promote the initiation and processing of recombination in each meiotic division. In recent years, research has made great strides in identifying genes and mechanisms that facilitate CO formation in the PAR. Here, we outline the most recent and relevant findings in this field. XY chromosome aneuploidy in humans has broad-reaching effects, contributing significantly also to Turner syndrome, spontaneous abortions, oligospermia, and even infertility. Thus, in the years to come, the identification of genes and mechanisms beyond XY aneuploidy is expected to have an impact on the genetic counseling of a wide number of families and adults affected by these disorders.

Differential susceptibility of male and female germ cells to glucocorticoid-mediated signaling

While physiologic stress has long been known to impair mammalian reproductive capacity through hormonal dysregulation, mounting evidence now suggests that stress experienced prior to or during gestation may also negatively impact the health of future offspring. Rodent models of gestational physiologic stress can induce neurologic and behavioral changes that persist for up to three generations, suggesting that stress signals can induce lasting epigenetic changes in the germline. Treatment with glucocorticoid stress hormones is sufficient to recapitulate the transgenerational changes seen in physiologic stress models. These hormones are known to bind and activate the glucocorticoid receptor (GR), a ligand-inducible transcription factor, thus implicating GR-mediated signaling as a potential contributor to the transgenerational inheritance of stress-induced phenotypes. Here, we demonstrate dynamic spatiotemporal regulation of GR expression in the mouse germline, showing expression in the fetal oocyte as well as the perinatal and adult spermatogonia. Functionally, we find that fetal oocytes are intrinsically buffered against changes in GR signaling, as neither genetic deletion of GR nor GR agonism with dexamethasone altered the transcriptional landscape or the progression of fetal oocytes through meiosis. In contrast, our studies revealed that the male germline is susceptible to glucocorticoid-mediated signaling, specifically by regulating RNA splicing within the spermatogonia, although this does not abrogate fertility. Together, our work suggests a sexually dimorphic function for GR in the germline, and represents an important step towards understanding the mechanisms by which stress can modulate the transmission of genetic information through the germline.

FUS-dependent microRNA deregulations identify TRIB2 as a druggable target for ALS motor neurons

MicroRNAs (miRNAs) modulate mRNA expression, and their deregulation contributes to various diseases including amyotrophic lateral sclerosis (ALS). As fused in sarcoma (FUS) is a causal gene for ALS and regulates biogenesis of miRNAs, we systematically analyzed the miRNA repertoires in spinal cords and hippocampi from ALS-FUS mice to understand how FUS-dependent miRNA deregulation contributes to ALS. miRNA profiling identified differentially expressed miRNAs between different central nervous system (CNS) regions as well as disease states. Among the up-regulated miRNAs, miR-1197 targets the pro-survival pseudokinase Trib2. A reduced TRIB2 expression was observed in iPSC-derived motor neurons from ALS patients. Pharmacological stabilization of TRIB2 protein with a clinically approved cancer drug rescues the survival of iPSC-derived human motor neurons, including those from a sporadic ALS patient. Collectively, our data indicate that miRNA profiling can be used to probe the molecular mechanisms underlying selective vulnerability, and TRIB2 is a potential therapeutic target for ALS.

Differential susceptibility of male and female germ cells to glucocorticoid-mediated signaling

While physiologic stress has long been known to impair mammalian reproductive capacity through hormonal dysregulation, mounting evidence now suggests that stress experienced prior to or during gestation may also negatively impact the health of future offspring. Rodent models of gestational physiologic stress can induce neurologic and behavioral changes that persist for up to three generations, suggesting that stress signals can induce lasting epigenetic changes in the germline. Treatment with glucocorticoid stress hormones is sufficient to recapitulate the transgenerational changes seen in physiologic stress models. These hormones are known to bind and activate the glucocorticoid receptor (GR), a ligand-inducible transcription factor, thus implicating GR-mediated signaling as a potential contributor to the transgenerational inheritance of stress-induced phenotypes. Here we demonstrate dynamic spatiotemporal regulation of GR expression in the mouse germline, showing expression in the fetal oocyte as well as the perinatal and adult spermatogonia. Functionally, we find that fetal oocytes are intrinsically buffered against changes in GR signaling, as neither genetic deletion of GR nor GR agonism with dexamethasone altered the transcriptional landscape or the progression of fetal oocytes through meiosis. In contrast, our studies revealed that the male germline is susceptible to glucocorticoid-mediated signaling, specifically by regulating RNA splicing within the spermatogonia, although this does not abrogate fertility. Together, our work suggests a sexually dimorphic function for GR in the germline, and represents an important step towards understanding the mechanisms by which stress can modulate the transmission of genetic information through the germline.

Brain region-specific synaptic function of FUS underlies the FTLD-linked behavioural disinhibition

Synaptic dysfunction is one of the earliest pathological processes that contribute to the development of many neurological disorders, including Alzheimer's disease and frontotemporal lobar degeneration. However, the synaptic function of many disease-causative genes and their contribution to the pathogenesis of the related diseases remain unclear. In this study, we investigated the synaptic role of fused in sarcoma, an RNA-binding protein linked to frontotemporal lobar degeneration and amyotrophic lateral sclerosis, and its potential pathological role in frontotemporal lobar degeneration using pyramidal neuron-specific conditional knockout mice (FuscKO). We found that FUS regulates the expression of many genes associated with synaptic function in a hippocampal subregion-specific manner, concomitant with the frontotemporal lobar degeneration-linked behavioural disinhibition. Electrophysiological study and molecular pathway analyses further reveal that fused in sarcoma differentially regulates synaptic and neuronal properties in the ventral hippocampus and medial prefrontal cortex, respectively. Moreover, fused in sarcoma selectively modulates the ventral hippocampus-prefrontal cortex projection, which is known to mediate the anxiety-like behaviour. Our findings unveil the brain region- and synapse-specific role of fused in sarcoma, whose impairment might lead to the emotional symptoms associated with frontotemporal lobar degeneration.

The RNA-binding protein FUS/TLS interacts with SPO11 and PRDM9 and localize at meiotic recombination hotspots

In mammals, meiotic recombination is initiated by the introduction of DNA double strand breaks (DSBs) into narrow segments of the genome, defined as hotspots, which is carried out by the SPO11/TOPOVIBL complex. A major player in the specification of hotspots is PRDM9, a histone methyltransferase that, following sequence-specific DNA binding, generates trimethylation on lysine 4 (H3K4me3) and lysine 36 (H3K36me3) of histone H3, thus defining the hotspots. PRDM9 activity is key to successful meiosis, since in its absence DSBs are redirected to functional sites and synapsis between homologous chromosomes fails. One protein factor recently implicated in guiding PRDM9 activity at hotspots is EWS, a member of the FET family of proteins that also includes TAF15 and FUS/TLS. Here, we demonstrate that FUS/TLS partially colocalizes with PRDM9 on the meiotic chromosome axes, marked by the synaptonemal complex component SYCP3, and physically interacts with PRDM9. Furthermore, we show that FUS/TLS also interacts with REC114, one of the axis-bound SPO11-auxiliary factors essential for DSB formation. This finding suggests that FUS/TLS is a component of the protein complex that promotes the initiation of meiotic recombination. Accordingly, we document that FUS/TLS coimmunoprecipitates with SPO11 in vitro and in vivo. The interaction occurs with both SPO11β and SPO11α splice isoforms, which are believed to play distinct functions in the formation of DSBs in autosomes and male sex chromosomes, respectively. Finally, using chromatin immunoprecipitation experiments, we show that FUS/TLS is localized at H3K4me3-marked hotspots in autosomes and in the pseudo-autosomal region, the site of genetic exchange between the XY chromosomes.

Amyotrophic Lateral Sclerosis Risk Genes and Suppressor

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that leads to death by progressive paralysis and respiratory failure within 2-4 years of onset. About 90-95% of ALS cases are sporadic (sALS), and 5-10% are inherited through family (fALS). Though the mechanisms of the disease are still poorly understood, so far, approximately 40 genes have been reported as ALS causative genes. The mutations in some crucial genes, like SOD1, C9ORF72, FUS, and TDP-43, are majorly associated with ALS, resulting in ROS-associated oxidative stress, excitotoxicity, protein aggregation, altered RNA processing, axonal and vesicular trafficking dysregulation, and mitochondrial dysfunction. Recent studies show that dysfunctional cellular pathways get restored as a result of the repair of a single pathway in ALS. In this review article, our aim is to identify putative targets for therapeutic development and the importance of a single suppressor to reduce multiple symptoms by focusing on important mutations and the phenotypic suppressors of dysfunctional cellular pathways in crucial genes as reported by other studies.

Systematic proximal mapping of the classical RAD51 paralogs unravel functionally and clinically relevant interactors for genome stability

Homologous recombination (HR) plays an essential role in the maintenance of genome stability by promoting the repair of cytotoxic DNA double strand breaks (DSBs). More recently, the HR pathway has emerged as a core component of the response to replication stress, in part by protecting stalled replication forks from nucleolytic degradation. In that regard, the mammalian RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3) have been involved in both HR-mediated DNA repair and collapsed replication fork resolution. Still, it remains largely obscure how they participate in both processes, thereby maintaining genome stability and preventing cancer development. To gain better insight into their contribution in cellulo, we mapped the proximal interactome of the classical RAD51 paralogs using the BioID approach. Aside from identifying the well-established BCDX2 and CX3 sub-complexes, the spliceosome machinery emerged as an integral component of our proximal mapping, suggesting a crosstalk between this pathway and the RAD51 paralogs. Furthermore, we noticed that factors involved RNA metabolic pathways are significantly modulated within the BioID of the classical RAD51 paralogs upon exposure to hydroxyurea (HU), pointing towards a direct contribution of RNA processing during replication stress. Importantly, several members of these pathways have prognostic potential in breast cancer (BC), where their RNA expression correlates with poorer patient outcome. Collectively, this study uncovers novel functionally relevant partners of the different RAD51 paralogs in the maintenance of genome stability that could be used as biomarkers for the prognosis of BC.

Status of ALS Treatment, Insights into Therapeutic Challenges and Dilemmas

Amyotrophic lateral sclerosis (ALS) is an extremely heterogeneous disease of motor neurons that eventually leads to death. Despite impressive advances in understanding the genetic, molecular, and pathological mechanisms of the disease, the only drug approved to date by both the FDA and EMA is riluzole, with a modest effect on survival. In this opinion view paper, we will discuss how to address some challenges for drug development in ALS at the conceptual, technological, and methodological levels. In addition, socioeconomic and ethical issues related to the legitimate need of patients to benefit quickly from new treatments will also be addressed. In conclusion, this brief review takes a more optimistic view, given the recent approval of two new drugs in some countries and the development of targeted gene therapies.

DNA Double-Strand Breaks as Pathogenic Lesions in Neurological Disorders

The damage and repair of DNA is a continuous process required to maintain genomic integrity. DNA double-strand breaks (DSBs) are the most lethal type of DNA damage and require timely repair by dedicated machinery. DSB repair is uniquely important to nondividing, post-mitotic cells of the central nervous system (CNS). These long-lived cells must rely on the intact genome for a lifetime while maintaining high metabolic activity. When these mechanisms fail, the loss of certain neuronal populations upset delicate neural networks required for higher cognition and disrupt vital motor functions. Mammalian cells engage with several different strategies to recognize and repair chromosomal DSBs based on the cellular context and cell cycle phase, including homologous recombination (HR)/homology-directed repair (HDR), microhomology-mediated end-joining (MMEJ), and the classic non-homologous end-joining (NHEJ). In addition to these repair pathways, a growing body of evidence has emphasized the importance of DNA damage response (DDR) signaling, and the involvement of heterogeneous nuclear ribonucleoprotein (hnRNP) family proteins in the repair of neuronal DSBs, many of which are linked to age-associated neurological disorders. In this review, we describe contemporary research characterizing the mechanistic roles of these non-canonical proteins in neuronal DSB repair, as well as their contributions to the etiopathogenesis of selected common neurological diseases.

Modelling amyotrophic lateral sclerosis in rodents

The efficient study of human disease requires the proper tools, one of the most crucial of which is an accurate animal model that faithfully recapitulates the human condition. The study of amyotrophic lateral sclerosis (ALS) is no exception. Although the majority of ALS cases are considered sporadic, most animal models of this disease rely on genetic mutations identified in familial cases. Over the past decade, the number of genes associated with ALS has risen dramatically and, with each new genetic variant, there is a drive to develop associated animal models. Rodent models are of particular importance as they allow for the study of ALS in the context of a living mammal with a comparable CNS. Such models not only help to verify the pathogenicity of novel mutations but also provide critical insight into disease mechanisms and are crucial for the testing of new therapeutics. In this Review, we aim to summarize the full spectrum of ALS rodent models developed to date.

Liquid-Liquid Phase Separation of TDP-43 and FUS in Physiology and Pathology of Neurodegenerative Diseases

Liquid-liquid phase separation of RNA-binding proteins mediates the formation of numerous membraneless organelles with essential cellular function. However, aberrant phase transition of these proteins leads to the formation of insoluble protein aggregates, which are pathological hallmarks of neurodegenerative diseases including ALS and FTD. TDP-43 and FUS are two such RNA-binding proteins that mislocalize and aggregate in patients of ALS and FTD. They have similar domain structures that provide multivalent interactions driving their phase separation in vitro and in the cellular environment. In this article, we review the factors that mediate and regulate phase separation of TDP-43 and FUS. We also review evidences that connect the phase separation property of TDP-43 and FUS to their functional roles in cells. Aberrant phase transition of TDP-43 and FUS leads to protein aggregation and disrupts their regular cell function. Therefore, restoration of functional protein phase of TDP-43 and FUS could be beneficial for neuronal cells. We discuss possible mechanisms for TDP-43 and FUS aberrant phase transition and aggregation while reviewing the methods that are currently being explored as potential therapeutic strategies to mitigate aberrant phase transition and aggregation of TDP-43 and FUS.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

FUS driven circCNOT6L biogenesis in mouse and human spermatozoa supports zygote development

Circular RNA (circRNA) biogenesis requires a backsplicing reaction, promoted by inverted repeats in cis-flanking sequences and trans factors, such as RNA-binding proteins (RBPs). Among these, FUS plays a key role. During spermatogenesis and sperm maturation along the epididymis such a molecular mechanism has been poorly explored. With this in mind, we chose circCNOT6L as a study case and wild-type (WT) as well as cannabinoid receptor type-1 knock-out (Cb1^−/−) male mice as animal models to analyze backsplicing mechanisms. Our results suggest that spermatozoa (SPZ) have an endogenous skill to circularize mRNAs, choosing FUS as modulator of backsplicing and under CB1 stimulation. A physical interaction between FUS and CNOT6L as well as a cooperation among FUS, RNA Polymerase II (RNApol2) and Quaking (QKI) take place in SPZ. Finally, to gain insight into FUS involvement in circCNOT6L biogenesis, FUS expression was reduced through RNA interference approach. Paternal transmission of FUS and CNOT6L to oocytes during fertilization was then assessed by using murine unfertilized oocytes (NF), one-cell zygotes (F) and murine oocytes undergoing parthenogenetic activation (PA) to exclude a maternal contribution. The role of circCNOT6L as an active regulator of zygote transition toward the 2-cell-like state was suggested using the Embryonic Stem Cell (ESC) system. Intriguingly, human SPZ exactly mirror murine SPZ.

Biomolecular condensates at sites of DNA damage: More than just a phase

Protein recruitment to DNA break sites is an integral part of the DNA damage response (DDR). Elucidation of the hierarchy and temporal order with which DNA damage sensors as well as repair and signaling factors assemble around chromosome breaks has painted a complex picture of tightly regulated macromolecular interactions that build specialized compartments to facilitate repair and maintenance of genome integrity. While many of the underlying interactions, e.g. between repair factors and damage-induced histone marks, can be explained by lock-and-key or induced fit binding models assuming fixed stoichiometries, structurally less well defined interactions, such as the highly dynamic multivalent interactions implicated in phase separation, also participate in the formation of multi-protein assemblies in response to genotoxic stress. Although much remains to be learned about these types of cooperative and highly dynamic interactions and their functional roles, the rapidly growing interest in material properties of biomolecular condensates and in concepts from polymer chemistry and soft matter physics to understand biological processes at different scales holds great promises. Here, we discuss nuclear condensates in the context of genome integrity maintenance, highlighting the cooperative potential between clustered stoichiometric binding and phase separation. Rather than viewing them as opposing scenarios, their combined effects can balance structural specificity with favorable physicochemical properties relevant for the regulation and function of multilayered nuclear condensates.

Fused in sarcoma regulates DNA replication timing and kinetics

Fused in sarcoma (FUS) encodes an RNA-binding protein with diverse roles in transcriptional activation and RNA splicing. While oncogenic fusions of FUS and transcription factor DNA-binding domains are associated with soft tissue sarcomas, dominant mutations in FUS can cause amyotrophic lateral sclerosis. FUS has also been implicated in genome maintenance. However, the underlying mechanisms of its actions in genome stability are unknown. Here, we applied gene editing, functional reconstitution, and integrated proteomics and transcriptomics to illuminate roles for FUS in DNA replication and repair. Consistent with a supportive role in DNA double-strand break repair, FUS-deficient cells exhibited subtle alterations in the recruitment and retention of double-strand break-associated factors, including 53BP1 and BRCA1. FUS-/- cells also exhibited reduced proliferative potential that correlated with reduced speed of replication fork progression, diminished loading of prereplication complexes, enhanced micronucleus formation, and attenuated expression and splicing of S-phase-associated genes. Finally, FUS-deficient cells exhibited genome-wide alterations in DNA replication timing that were reversed upon re-expression of FUS complementary DNA. We also showed that FUS-dependent replication domains were enriched in transcriptionally active chromatin and that FUS was required for the timely replication of transcriptionally active DNA. These findings suggest that alterations in DNA replication kinetics and programming contribute to genome instability and functional defects in FUS-deficient cells.

ALS-linked FUS mutants affect the localization of U7 snRNP and replication-dependent histone gene expression in human cells

Genes encoding replication-dependent histones lack introns, and the mRNAs produced are a unique class of RNA polymerase II transcripts in eukaryotic cells that do not end in a polyadenylated tail. Mature mRNAs are thus formed by a single endonucleolytic cleavage that releases the pre-mRNA from the DNA and is the only processing event necessary. U7 snRNP is one of the key factors that determines the cleavage site within the 3ʹUTR of replication-dependent histone pre-mRNAs. We have previously showed that the FUS protein interacts with U7 snRNA/snRNP and regulates the expression of histone genes by stimulating transcription and 3ʹ end maturation. Mutations in the FUS gene first identified in patients with amyotrophic lateral sclerosis (ALS) lead to the accumulation of the FUS protein in cytoplasmic inclusions. Here, we report that mutations in FUS lead to disruption of the transcriptional activity of FUS and mislocalization of U7 snRNA/snRNP in cytoplasmic aggregates in cellular models and primary neurons. As a consequence, decreased transcriptional efficiency and aberrant 3ʹ end processing of histone pre-mRNAs were observed. This study highlights for the first time the deregulation of replication-dependent histone gene expression and its involvement in ALS.

New Faces of old Friends: Emerging new Roles of RNA-Binding Proteins in the DNA Double-Strand Break Response

DNA double-strand breaks (DSBs) are highly cytotoxic DNA lesions. To protect genomic stability and ensure cell homeostasis, cells mount a complex signaling-based response that not only coordinates the repair of the broken DNA strand but also activates cell cycle checkpoints and, if necessary, induces cell death. The last decade has seen a flurry of studies that have identified RNA-binding proteins (RBPs) as novel regulators of the DSB response. While many of these RBPs have well-characterized roles in gene expression, it is becoming increasingly clear that they also have non-canonical functions in the DSB response that go well beyond transcription, splicing and mRNA processing. Here, we review the current understanding of how RBPs are integrated into the cellular response to DSBs and describe how these proteins directly participate in signal transduction, amplification and repair at damaged chromatin. In addition, we discuss the implications of an RBP-mediated DSB response for genome instability and age-associated diseases such as cancer and neurodegeneration.

FUS-dependent liquid-liquid phase separation is important for DNA repair initiation

RNA-binding proteins (RBPs) are emerging as important effectors of the cellular DNA damage response (DDR). The RBP FUS is implicated in RNA metabolism and DNA repair, and it undergoes reversible liquid-liquid phase separation (LLPS) in vitro. Here, we demonstrate that FUS-dependent LLPS is necessary for the initiation of the DDR. Using laser microirradiation in FUS-knockout cells, we show that FUS is required for the recruitment to DNA damage sites of the DDR factors KU80, NBS1, and 53BP1 and of SFPQ, another RBP implicated in the DDR. The relocation of KU80, NBS1, and SFPQ is similarly impaired by LLPS inhibitors, or LLPS-deficient FUS variants. We also show that LLPS is necessary for efficient γH2AX foci formation. Finally, using superresolution structured illumination microscopy, we demonstrate that the absence of FUS impairs the proper arrangement of γH2AX nanofoci into higher-order clusters. These findings demonstrate the early requirement for FUS-dependent LLPS in the activation of the DDR and the proper assembly of DSB repair complexes.

ALS Genetics: Gains, Losses, and Implications for Future Therapies

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder caused by the loss of motor neurons from the brain and spinal cord. The ALS community has made remarkable strides over three decades by identifying novel familial mutations, generating animal models, elucidating molecular mechanisms, and ultimately developing promising new therapeutic approaches. Some of these approaches reduce the expression of mutant genes and are in human clinical trials, highlighting the need to carefully consider the normal functions of these genes and potential contribution of gene loss-of-function to ALS. Here, we highlight known loss-of-function mechanisms underlying ALS, potential consequences of lowering levels of gene products, and the need to consider both gain and loss of function to develop safe and effective therapeutic strategies.

miR-4478 sensitizes ovarian cancer cells to irradiation by inhibiting Fus and attenuating autophagy

Ovarian cancer (OC) is a type of cancer with high prevalence and shocking mortality in women around the world. Radioresistance is a major reason for OC relapse. Mounting studies have shown the significant function of dysregulated microRNAs (miRNAs) in cancer progression and the cellular response to irradiation. The present study inquired about the function and mechanism of microRNA (miR)-4478 in regulating radiosensitivity of OC cells. Results showed that miR-4478 was downregulated in OC, and a low miR-4478 level indicated a disappointing prognosis for OC patients. Besides, in OC cells exposed to irradiation, the expression of miR-4478 decreased over time. Functionally, the upregulation of miR-4478 retarded OC cell proliferation and sensitized OC cells to irradiation. Mechanistically, miR-4478 targeted and inhibited fused in sarcoma (Fus). Additionally, Fus was upregulated in OC and its expression further elevated in OC cells under irradiation. Furthermore, miR-4478 targeted Fus to inhibit autophagy, therefore sensitizing OC cells to irradiation. Collectively, our study uncovered miR-4478 as a novel radiosensitizer by targeting Fus in OC cells, which may shed a new light on developing targets for treating patients with OC, particularly those with radioresistance.

Fused in Sarcoma (FUS) in DNA Repair: Tango with Poly(ADP-ribose) Polymerase 1 and Compartmentalisation of Damaged DNA

The fused in sarcoma (FUS) protein combines prion-like properties with a multifunctional DNA/RNA-binding domain and has functions spanning the regulation of RNA metabolism, including transcription, pre-mRNA splicing, mRNA transport and translation. In addition to its roles in RNA metabolism, FUS is implicated in the maintenance of DNA integrity. In this review, we examine the participation of FUS in major DNA repair pathways, focusing on DNA repair associated with poly(ADP-ribosyl)ation events and on how the interaction of FUS with poly(ADP-ribose) may orchestrate transient compartmentalisation of DNA strand breaks. Unravelling how prion-like RNA-binding proteins control DNA repair pathways will deepen our understanding of the pathogenesis of some neurological diseases and cancer as well as provide the basis for the development of relevant innovative therapeutic technologies. This knowledge may also extend the range of applications of poly(ADP-ribose) polymerase inhibitors to the treatment of neurodegenerative diseases related to RNA-binding proteins in the cell, e.g., amyotrophic lateral sclerosis and frontotemporal lobar degeneration.

Review: Modelling the pathology and behaviour of frontotemporal dementia

Frontotemporal dementia (FTD) encompasses a collection of clinically and pathologically diverse neurological disorders. Clinical features of behavioural and language dysfunction are associated with neurodegeneration, predominantly of frontal and temporal cortices. Over the past decade, there have been significant advances in the understanding of the genetic aetiology and neuropathology of FTD which have led to the creation of various disease models to investigate the molecular pathways that contribute to disease pathogenesis. The generation of in vivo models of FTD involves either targeting genes with known disease-causative mutations such as GRN and C9orf72 or genes encoding proteins that form the inclusions that characterize the disease pathologically, such as TDP-43 and FUS. This review provides a comprehensive summary of the different in vivo model systems used to understand pathomechanisms in FTD, with a focus on disease models which reproduce aspects of the wide-ranging behavioural phenotypes seen in people with FTD. We discuss the emerging disease pathways that have emerged from these in vivo models and how this has shaped our understanding of disease mechanisms underpinning FTD. We also discuss the challenges of modelling the complex clinical symptoms shown by people with FTD, the confounding overlap with features of motor neuron disease, and the drive to make models more disease-relevant. In summary, in vivo models can replicate many pathological and behavioural aspects of clinical FTD, but robust and thorough investigations utilizing shared features and variability between disease models will improve the disease-relevance of findings and thus better inform therapeutic development.

Functional characterization of a FUS mutant zebrafish line as a novel genetic model for ALS

Mutations in Fused in sarcoma (FUS), an RNA-binding protein, are known to cause Amyotrophic Lateral Sclerosis (ALS). However, molecular mechanisms due to loss of FUS function remain unclear and controversial. Here, we report the characterization and phenotypic analysis of a deletion mutant of the unique FUS orthologue in zebrafish where Fus protein levels are depleted. The homozygous mutants displayed a reduced lifespan as well as impaired motor abilities associated with specific cellular deficits, including decreased motor neurons length and neuromuscular junctions (NMJ) fragmentation. Furthermore, we demonstrate that these cellular impairments are linked to the misregulation of mRNA expression of acetylcholine receptor (AChR) subunits and histone deacetylase 4, markers of denervation and reinnervation processes observed in ALS patients. In addition, fus loss of function alters tau transcripts favoring the expression of small tau isoforms. Overall, this new animal model extends our knowledge on FUS and supports the relevance of FUS loss of function in ALS physiopathology.

Biomolecular condensates in neurodegeneration and cancer

The intracellular environment is partitioned into functionally distinct compartments containing specific sets of molecules and reactions. Biomolecular condensates, also referred to as membrane-less organelles, are diverse and abundant cellular compartments that lack membranous enclosures. Molecules assemble into condensates by phase separation; multivalent weak interactions drive molecules to separate from their surroundings and concentrate in discrete locations. Biomolecular condensates exist in all eukaryotes and in some prokaryotes, and participate in various essential house-keeping, stress-response and cell type-specific processes. An increasing number of recent studies link abnormal condensate formation, composition and material properties to a number of disease states. In this review, we discuss current knowledge and models describing the regulation of condensates and how they become dysregulated in neurodegeneration and cancer. Further research on the regulation of biomolecular phase separation will help us to better understand their role in cell physiology and disease.

Phenotypes and malignancy risk of different FUS mutations in genetic amyotrophic lateral sclerosis

OBJECTIVE

Mutations in Fused in Sarcoma (FUS or TLS) are the fourth most prevalent in Western European familial amyotrophic lateral sclerosis (ALS) populations and have been associated with causing both early and very late disease onset. FUS aggregation, DNA repair deficiency, and genomic instability are contributors to the pathophysiology of FUS-ALS, but their clinical significance per se and their influence on the clinical variability have yet to be sufficiently investigated. The aim of this study was to analyze genotype-phenotype correlations and malignancy rates in a newly compiled FUS-ALS cohort.

METHODS

We cross-sectionally reviewed FUS-ALS patient histories in a multicenter cohort with 36 novel cases and did a meta-analysis of published FUS-ALS cases reporting the largest genotype-phenotype correlation of FUS-ALS.

RESULTS

The age of onset (median 39 years, range 11-80) was positively correlated with the disease duration. C-terminal domain mutations were found in 90%. Among all, P525L and truncating/ frameshift mutations most frequently caused juvenile onset, rapid disease progression, and atypical ALS often associated with negative family history while the R521 mutation site was associated with late disease onset and pure spinal phenotype. Malignancies were found in one of 40 patients.

INTERPRETATION

We report the largest genotype-phenotype correlation of FUS-ALS, which enables a careful prediction of the clinical course in newly diagnosed patients. In this cohort, FUS-ALS patients did not have an increased risk for malignant diseases.

The RNA binding protein EWS is broadly involved in the regulation of pri-miRNA processing in mammalian cells

The Ewing Sarcoma protein (EWS) is a multifaceted RNA binding protein (RBP) with established roles in transcription, pre-mRNA processing and DNA damage response. By generating high quality EWS-RNA interactome, we uncovered its specific and prevalent interaction with a large subset of primary microRNAs (pri-miRNAs) in mammalian cells. Knockdown of EWS reduced, whereas overexpression enhanced, the expression of its target miRNAs. Biochemical analysis revealed that multiple elements in target pri-miRNAs, including the sequences flanking the stem-loop region, contributed to high affinity EWS binding and sequence swap experiments between target and non-target demonstrated that the flanking sequences provided the specificity for enhanced pri-miRNA processing by the Microprocessor Drosha/DGCR8. Interestingly, while repressing Drosha expression, as reported earlier, we found that EWS was able to enhance the recruitment of Drosha to chromatin. Together, these findings suggest that EWS may positively and negatively regulate miRNA biogenesis via distinct mechanisms, thus providing a new foundation to understand the function of EWS in development and disease.

Fused in Sarcoma: Properties, Self-Assembly and Correlation with Neurodegenerative Diseases

Fused in sarcoma (FUS) is a DNA/RNA binding protein that is involved in RNA metabolism and DNA repair. Numerous reports have demonstrated by pathological and genetic analysis that FUS is associated with a variety of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and polyglutamine diseases. Traditionally, the fibrillar aggregation of FUS was considered to be the cause of those diseases, especially via its prion-like domains (PrLDs), which are rich in glutamine and asparagine residues. Lately, a nonfibrillar self-assembling phenomenon, liquid–liquid phase separation (LLPS), was observed in FUS, and studies of its functions, mechanism, and mutual transformation with pathogenic amyloid have been emerging. This review summarizes recent studies on FUS self-assembling, including both aggregation and LLPS as well as their relationship with the pathology of ALS, FTLD, and other neurodegenerative diseases.

FUS (fused in sarcoma) is a component of the cellular response to topoisomerase I-induced DNA breakage and transcriptional stress

This work shows that the ALS-associated protein FUS is a component of the cellular response to transcriptional stress induced by topoisomerase I–induced DNA breakage, thereby accumulating at sites of nucleolar rRNA synthesis.

FUS (fused in sarcoma) plays a key role in several steps of RNA metabolism, and dominant mutations in this protein are associated with neurodegenerative diseases. Here, we show that FUS is a component of the cellular response to topoisomerase I (TOP1)–induced DNA breakage; relocalising to the nucleolus in response to RNA polymerase II (Pol II) stalling at sites of TOP1-induced DNA breaks. This relocalisation is rapid and dynamic, reversing following the removal of TOP1-induced breaks and coinciding with the recovery of global transcription. Importantly, FUS relocalisation following TOP1-induced DNA breakage is associated with increased FUS binding at sites of RNA polymerase I transcription in ribosomal DNA and reduced FUS binding at sites of RNA Pol II transcription, suggesting that FUS relocates from sites of stalled RNA Pol II either to regulate pre-mRNA processing during transcriptional stress or to modulate ribosomal RNA biogenesis. Importantly, FUS-mutant patient fibroblasts are hypersensitive to TOP1-induced DNA breakage, highlighting the possible relevance of these findings to neurodegeneration.

Self-assembly of FUS through its low-complexity domain contributes to neurodegeneration

Aggregation of fused in sarcoma (FUS) protein, and mutations in FUS gene, are causative to a range of neurodegenerative disorders including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. To gain insights into the molecular mechanism whereby FUS causes neurodegeneration, we generated transgenic Drosophila melanogaster overexpressing human FUS in the photoreceptor neurons, which exhibited mild retinal degeneration. Expression of familial ALS-mutant FUS aggravated the degeneration, which was associated with an increase in cytoplasmic localization of FUS. A carboxy-terminally truncated R495X mutant FUS also was localized in cytoplasm, whereas the degenerative phenotype was diminished. Double expression of R495X and wild-type FUS dramatically exacerbated degeneration, sequestrating wild-type FUS into cytoplasmic aggregates. Notably, replacement of all tyrosine residues within the low-complexity domain, which abolished self-assembly of FUS, completely eliminated the degenerative phenotypes. Taken together, we propose that self-assembly of FUS through its low-complexity domain contributes to FUS-induced neurodegeneration.

The Fusion Oncogene FUS-CHOP Drives Sarcomagenesis of High-Grade Spindle Cell Sarcomas in Mice

Myxoid liposarcoma is a malignant soft tissue sarcoma characterized by a pathognomonic t(12;16)(q13;p11) translocation that produces a fusion oncoprotein, FUS-CHOP. This cancer is remarkably sensitive to radiotherapy and exhibits a unique pattern of extrapulmonary metastasis. Here, we report the generation and characterization of a spatially and temporally restricted mouse model of sarcoma driven by FUS-CHOP. Using different Cre drivers in the adipocyte lineage, we initiated in vivo tumorigenesis by expressing FUS-CHOP in Prrx1+ mesenchymal progenitor cells. In contrast, expression of FUS-CHOP in more differentiated cells does not form tumors in vivo, and early expression of the oncoprotein during embryogenesis is lethal. We also employ in vivo electroporation and CRISPR technology to rapidly generate spatially and temporally restricted mouse models of high-grade FUS-CHOP-driven sarcomas for preclinical studies.

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

Through 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.

FUS causes synaptic hyperexcitability in Drosophila dendritic arborization neurons

Mutations in the nuclear localization signal of the RNA binding protein FUS cause both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These mutations result in a loss of FUS from the nucleus and the formation of FUS-containing cytoplasmic aggregates in patients. To better understand the role of cytoplasmic FUS mislocalization in the pathogenesis of ALS, we identified a population of cholinergic neurons in Drosophila that recapitulate these pathologic hallmarks. Expression of mutant FUS or the Drosophila homolog, Cabeza (Caz), in class IV dendritic arborization neurons results in cytoplasmic mislocalization and axonal transport to presynaptic terminals. Interestingly, overexpression of FUS or Caz causes the progressive loss of neuronal projections, reduction of synaptic mitochondria, and the appearance of large calcium transients within the synapse. Additionally, we find that overexpression of mutant but not wild type FUS results in a reduction in presynaptic Synaptotagmin, an integral component of the neurotransmitter release machinery, and mutant Caz specifically disrupts axonal transport and induces hyperexcitability. These results suggest that FUS/Caz overexpression disrupts neuronal function through multiple mechanisms, and that ALS-causing mutations impair the transport of synaptic vesicle proteins and induce hyperexcitability.

Role of Ionizing Radiation in Neurodegenerative Diseases

Ionizing radiation (IR) from terrestrial sources is continually an unprotected peril to human beings. However, the medical radiation and global radiation background are main contributors to human exposure and causes of radiation sickness. At high-dose exposures acute radiation sickness occurs, whereas chronic effects may persist for a number of years. Radiation can increase many circulatory, age related and neurodegenerative diseases. Neurodegenerative diseases occur a long time after exposure to radiation, as demonstrated in atomic bomb survivors, and are still controversial. This review discuss the role of IR in neurodegenerative diseases and proposes an association between neurodegenerative diseases and exposure to IR.

Importance of Functional Loss of FUS in FTLD/ALS

Fused in sarcoma (FUS) is an RNA binding protein that regulates RNA metabolism including alternative splicing, transcription, and RNA transportation. FUS is genetically and pathologically involved in frontotemporal lobar degeneration (FTLD)/amyotrophic lateral sclerosis (ALS). Multiple lines of evidence across diverse models suggest that functional loss of FUS can lead to neuronal dysfunction and/or neuronal cell death. Loss of FUS in the nucleus can impair alternative splicing and/or transcription, whereas dysfunction of FUS in the cytoplasm, especially in the dendritic spines of neurons, can cause mRNA destabilization. Alternative splicing of the MAPT gene at exon 10, which generates 4-repeat Tau (4R-Tau) and 3-repeat Tau (3R-Tau), is one of the most impactful targets regulated by FUS. Additionally, loss of FUS function can affect dendritic spine maturations by destabilizing mRNAs such as Glutamate receptor 1 (GluA1), a major AMPA receptor, and Synaptic Ras GTPase-activating protein 1 (SynGAP1). Moreover, FUS is involved in axonal transport and morphological maintenance of neurons. These findings indicate that a biological link between loss of FUS function, Tau isoform alteration, aberrant post-synaptic function, and phenotypic expression might lead to the sequential cascade culminating in FTLD. Thus, to facilitate development of early disease markers and/or therapeutic targets of FTLD/ALS it is critical that the functions of FUS and its downstream pathways are unraveled.

Synaptic Paths to Neurodegeneration: The Emerging Role of TDP-43 and FUS in Synaptic Functions

TAR DNA-binding protein-43 KDa (TDP-43) and fused in sarcoma (FUS) as the defining pathological hallmarks for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), coupled with ALS-FTD-causing mutations in both genes, indicate that their dysfunctions damage the motor system and cognition. On the molecular level, TDP-43 and FUS participate in the biogenesis and metabolism of coding and noncoding RNAs as well as in the transport and translation of mRNAs as part of cytoplasmic mRNA-ribonucleoprotein (mRNP) granules. Intriguingly, many of the RNA targets of TDP-43 and FUS are involved in synaptic transmission and plasticity, indicating that synaptic dysfunction could be an early event contributing to motor and cognitive deficits in ALS and FTD. Furthermore, the ability of the low-complexity prion-like domains of TDP-43 and FUS to form liquid droplets suggests a potential mechanism for mRNP assembly and conversion. This review will discuss the role of TDP-43 and FUS in RNA metabolism, with an emphasis on the involvement of this process in synaptic function and neuroprotection. This will be followed by a discussion of the potential phase separation mechanism for forming RNP granules and pathological inclusions.

Mouse models of ALS: Past, present and future

Genome sequencing of both sporadic and familial patients of Amyotrophic Lateral Sclerosis (ALS) has led to the identification of new genes that are both contributing and causative in the disease. This gene discovery has come at an unprecedented rate, and much of it in recent years. Knowledge of these genetic mutations provides us with opportunities to uncover new and related mechanisms, increasing our understanding of the disease and bringing us closer to defined therapies for patients. Mouse models have played an important role in our current understanding of the pathophysiology of ALS and have served as important preclinical models in testing new therapeutics. With these new gene discoveries, new mouse models will follow. The information derived from these new models will depend on the careful construction and importantly, an understanding of the capabilities and limitations of each of the models. The genetic discovery in ALS comes at a time when genetic engineering technologies in mice are highly efficient through CRISPR/Cas9 and can be applied to a wide array of genetic backgrounds. New mouse resources in the forms of the Collaborative Cross and Diversity Outbred panels provide us with unique opportunities to study these mutations on diverse genetic backgrounds, and importantly in the context of a population. This review focuses on the mouse models of the past and present, and discusses exciting new opportunities for mouse models of the future.

The Role of Post-Translational Modifications on Prion-Like Aggregation and Liquid-Phase Separation of FUS

Subcellular mislocalization and aggregation of the human FUS protein occurs in neurons of patients with subtypes of amyotrophic lateral sclerosis and frontotemporal dementia. FUS is one of several RNA-binding proteins that can functionally self-associate into distinct liquid-phase droplet structures. It is postulated that aberrant interactions within the dense phase-separated state can potentiate FUS's transition into solid prion-like aggregates that cause disease. FUS is post-translationally modified at numerous positions, which affect both its localization and aggregation propensity. These modifications may influence FUS-linked pathology and serve as therapeutic targets.

Accumulation of RNA-dependent protein kinase (PKR) in the nuclei of lung cancer cells mediates radiation resistance

We have previously demonstrated that radiation induced cell death in PKR (−/−) deficient mouse embryo fibroblasts (MEFs) but not in PKR (+/+) wild type MEFs. Our study indicated that PKR can also be involved in survival pathways following radiation therapy through activation of the AKT survival pathways in these MEFs is mediated in part through PKR. The role of PKR on radiation sensitivity in cancer cells has not been evaluated. In this study, we demonstrated that radiation treatment causes nuclear translocation of PKR in human lung cancer cells. The transduction of lung cancer cells with a dominant negative adenoviral PKR vector blocks nuclear translocation of PKR and leads to the reversal of radiation resistance. Plasmid transduction of lung cancer cells with nuclear targeted wild type PKR vectors also increased radiation resistance. This effect is selectively abrogated by plasmid transduction of dominant negative PKR vectors which restore radiation sensitivity. These findings suggest a novel role for PKR in lung cancer cells as a mediator of radiation resistance possibly through translocation of the protein product to the nucleus.

Impaired DNA damage response signaling by FUS-NLS mutations leads to neurodegeneration and FUS aggregate formation

Amyotrophic lateral sclerosis (ALS) is the most frequent motor neuron disease. Cytoplasmic fused in sarcoma (FUS) aggregates are pathological hallmarks of FUS-ALS. Proper shuttling between the nucleus and cytoplasm is essential for physiological cell function. However, the initial event in the pathophysiology of FUS-ALS remains enigmatic. Using human induced pluripotent stem cell (hiPSCs)-derived motor neurons (MNs), we show that impairment of poly(ADP-ribose) polymerase (PARP)-dependent DNA damage response (DDR) signaling due to mutations in the FUS nuclear localization sequence (NLS) induces additional cytoplasmic FUS mislocalization which in turn results in neurodegeneration and FUS aggregate formation. Our work suggests that a key pathophysiologic event in ALS is upstream of aggregate formation. Targeting DDR signaling could lead to novel therapeutic routes for ameliorating ALS.

Abnormal cytoplasmic aggregates of FUS are a hallmark of some forms of amyotrophic lateral sclerosis (ALS). Here, using neurons derived from patients with FUS-ALS, the authors demonstrate that impairment of PARP-dependent DNA damage signaling is an event that occurs upstream of neurodegeneration and cytoplasmic aggregate formation in FUS-ALS.

RBM45 competes with HDAC1 for binding to FUS in response to DNA damage

DNA damage response (DDR) is essential for genome stability and human health. Recently, several RNA binding proteins (RBPs), including fused-in-sarcoma (FUS), have been found unexpectedly to modulate this process. The role of FUS in DDR is closely linked to the pathogenesis of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord. Given that RBM45 is also an ALS-associated RBP, we wondered whether RBM45 plays any function during this process. Here, we report that RBM45 can be recruited to laser microirradiation-induced DNA damage sites in a PAR- and FUS-dependent manner, but in a RNA-independent fashion. Depletion of RBM45 leads to abnormal DDR signaling and decreased efficiency in DNA double-stranded break repair. Interestingly, RBM45 is found to compete with histone deacetylase 1 (HDAC1) for binding to FUS, thereby regulating the recruitment of HDAC1 to DNA damage sites. A common familial ALS-associated FUS mutation (FUS-R521C) is revealed to prefer to cooperate with RBM45 than HDAC1. Our findings suggest that RBM45 is a key regulator in FUS-related DDR signaling whose dysfunction may contribute to the pathogenesis of ALS.

Dysregulated molecular pathways in amyotrophic lateral sclerosis-frontotemporal dementia spectrum disorder

Frontotemporal dementia (FTD), the second most common form of dementia in people under 65 years of age, is characterized by progressive atrophy of the frontal and/or temporal lobes. FTD overlaps extensively with the motor neuron disease amyotrophic lateral sclerosis (ALS), especially at the genetic level. Both FTD and ALS can be caused by many mutations in the same set of genes; the most prevalent of these mutations is a GGGGCC repeat expansion in the first intron of C9ORF72 As shown by recent intensive studies, some key cellular pathways are dysregulated in the ALS-FTD spectrum disorder, including autophagy, nucleocytoplasmic transport, DNA damage repair, pre-mRNA splicing, stress granule dynamics, and others. These exciting advances reveal the complexity of the pathogenic mechanisms of FTD and ALS and suggest promising molecular targets for future therapeutic interventions in these devastating disorders.

TDP-43 and FUS en route from the nucleus to the cytoplasm

Misfolded or mislocalized RNA-binding proteins (RBPs) and, consequently, altered mRNA processing, can cause neuronal dysfunction, eventually leading to neurodegeneration. Two prominent examples are the RBPs TAR DNA-binding protein of 43 kDa (TDP-43) and fused in sarcoma (FUS), which form pathological messenger ribonucleoprotein aggregates in patients suffering from amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), two devastating neurodegenerative disorders. Here, we review the multiple functions of TDP-43 and FUS in mRNA processing, both in the nucleus and in the cytoplasm. We discuss how TDP-43 and FUS may exit the nucleus and how defects in both nuclear and cytosolic mRNA processing events, and possibly nuclear export defects, may contribute to neurodegeneration and ALS/FTD pathogenesis.

Characterization of genetic loss-of-function of Fus in zebrafish

The RNA-binding protein FUS is implicated in transcription, alternative splicing of neuronal genes and DNA repair. Mutations in FUS have been linked to human neurodegenerative diseases such as ALS (amyotrophic lateral sclerosis). We genetically disrupted fus in zebrafish (Danio rerio) using the CRISPR-Cas9 system. The fus knockout animals are fertile and did not show any distinctive phenotype. Mutation of fus induces mild changes in gene expression on the transcriptome and proteome level in the adult brain. We observed a significant influence of genetic background on gene expression and 3'UTR usage, which could mask the effects of loss of Fus. Unlike published fus morphants, maternal zygotic fus mutants do not show motoneuronal degeneration and exhibit normal locomotor activity.