FUS-NLS/Transportin 1 complex structure provides insights into the nuclear targeting mechanism of FUS and the implications in ALS

Nuclear pore dysfunction and disease: a complex opportunity

The separation of genetic material from bulk cytoplasm has enabled the evolution of increasingly complex organisms, allowing for the development of sophisticated forms of life. However, this complexity has created new categories of dysfunction, including those related to the movement of material between cellular compartments. In eukaryotic cells, nucleocytoplasmic trafficking is a fundamental biological process, and cumulative disruptions to nuclear integrity and nucleocytoplasmic transport are detrimental to cell survival. This is particularly true in post-mitotic neurons, where nuclear pore injury and errors to nucleocytoplasmic trafficking are strongly associated with neurodegenerative disease. In this review, we summarize the current understanding of nuclear pore biology in physiological and pathological contexts and discuss potential therapeutic approaches for addressing nuclear pore injury and dysfunctional nucleocytoplasmic transport.

Investigation of a Fused in Sarcoma Splicing Mutation in a Chinese Amyotrophic Lateral Sclerosis Patient

OBJECTIVE

Genetic mutations of fused in sarcoma (FUS) causing amyotrophic lateral sclerosis (ALS) may disrupt mRNA splicing events. For example, the FUS c.1394-2delA variant was reported in two western ALS patients, but its molecular mechanism is unclear. In this study, we aim to investigate FUS splice site mutations in Chinese ALS patients.

METHODS

Sanger sequencing was used to identify FUS splicing mutations in Chinese ALS patients. We combined a deep learning tool (SpliceAI), RNA sequencing, and RT-PCR/RT-qPCR to analyze the effect of FUS c.1394-2delA mutation on RNA splicing and expression. AlphaFold was used to predict the protein structure of mutant FUS. In transfected cell lines, we used immunofluorescence to assess cytoplasmic mislocalization of mutant FUS protein.

RESULTS

We identified a de novo FUS splice acceptor site mutation (c.1394-2delA, p. Gly466Valfs*14) in one Chinese sporadic ALS patient, which is linked to exon 14 skipping, and upregulated total FUS mRNA expression. The FUS splice site mutation was predicted to be translated into a truncated protein product at C-terminal. In vitro studies revealed that the FUS mutation increased cytoplasmic mislocalization in both HEK293T and SH-SY5Y cells.

CONCLUSIONS

We identified a de novo FUS splicing mutation (c.1394-2delA, p. Gly466Valfs*14) in 1 out of 233 Chinese ALS patients. It caused abnormal RNA splicing, upregulated gene expression, truncated FUS translation, and cytosolic mislocalization. Our findings suggested that FUS splice site mutation is rare in Chinese ALS patients and extended our knowledge of molecular mechanisms of the FUS c.1394-2delA mutation.

Activation of Pgk1 Results in Reduced Protein Aggregation in Diverse Neurodegenerative Conditions

The prevention of protein condensates has emerged as a new drug target to treat diverse neurodegenerative disorders. We previously reported that terazosin (TZ), a prescribed antagonist of the α1 adrenergic receptor, is an activator of phosphoglycerate kinase 1 (Pgk1) and Hsp90. In this study, we aimed to determine whether TZ prevents the formation of diverse pathological condensates in cell cultures and animal disease models. In primary neuron culture, TZ treatment reduced both the protein density and abundance of fused in sarcoma (FUS)-P525L-GFP, a disease-associated mutant form of FUS. Regarding the mechanism, we found that increased intracellular ATP levels were critical for the reduction in protein aggregate density. In addition, Hsp90 activation by TZ enhanced Hsp90 interaction with ULK1, a master regulator of autophagy. Through in vivo studies, we examined neuron-specific overexpression of tau in Drosophila, mouse models of APP/PS1 Alzheimer's disease (AD), and a rat model of multiple system atrophy (MSA) via the viral expression of α-synuclein in the striatum. TZ prevented and reversed the formation of pathological protein condensates. Together, our results suggest that activation of Pgk1 in cytosol may dissolve pathological protein aggregates via increased ATP levels and degrade these proteins via autophagy; the FUS-P525L degradation pathway in nucleus is unclear.

The Impact of Dysregulated microRNA Biogenesis Machinery and microRNA Sorting on Neurodegenerative Diseases

MicroRNAs (miRNAs) are 22-nucleotide noncoding RNAs involved in the differentiation, development, and function of cells in the body by targeting the 3'- untranslated regions (UTR) of mRNAs for degradation or translational inhibition. miRNAs not only affect gene expression inside the cells but also, when sorted into exosomes, systemically mediate the communication between different types of cells. Neurodegenerative diseases (NDs) are age-associated, chronic neurological diseases characterized by the aggregation of misfolded proteins, which results in the progressive degeneration of selected neuronal population(s). The dysregulation of biogenesis and/or sorting of miRNAs into exosomes was reported in several NDs, including Huntington's disease (HD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Alzheimer's disease (AD). Many studies support the possible roles of dysregulated miRNAs in NDs as biomarkers and therapeutic treatments. Understanding the molecular mechanisms underlying the dysregulated miRNAs in NDs is therefore timely and important for the development of diagnostic and therapeutic interventions. In this review, we focus on the dysregulated miRNA machinery and the role of RNA-binding proteins (RBPs) in NDs. The tools that are available to identify the target miRNA-mRNA axes in NDs in an unbiased manner are also discussed.

A minimal construct of nuclear-import receptor Karyopherin-β2 defines the regions critical for chaperone and disaggregation activity

Karyopherin-β2 (Kapβ2) is a nuclear-import receptor that recognizes proline-tyrosine nuclear localization signals of diverse cytoplasmic cargo for transport to the nucleus. Kapβ2 cargo includes several disease-linked RNA-binding proteins with prion-like domains, such as FUS, TAF15, EWSR1, hnRNPA1, and hnRNPA2. These RNA-binding proteins with prion-like domains are linked via pathology and genetics to debilitating degenerative disorders, including amyotrophic lateral sclerosis, frontotemporal dementia, and multisystem proteinopathy. Remarkably, Kapβ2 prevents and reverses aberrant phase transitions of these cargoes, which is cytoprotective. However, the molecular determinants of Kapβ2 that enable these activities remain poorly understood, particularly from the standpoint of nuclear-import receptor architecture. Kapβ2 is a super-helical protein comprised of 20 HEAT repeats. Here, we design truncated variants of Kapβ2 and assess their ability to antagonize FUS aggregation and toxicity in yeast and FUS condensation at the pure protein level and in human cells. We find that HEAT repeats 8 to 20 of Kapβ2 recapitulate all salient features of Kapβ2 activity. By contrast, Kapβ2 truncations lacking even a single cargo-binding HEAT repeat display reduced activity. Thus, we define a minimal Kapβ2 construct for delivery in adeno-associated viruses as a potential therapeutic for amyotrophic lateral sclerosis/frontotemporal dementia, multisystem proteinopathy, and related disorders.

The RGG motif proteins: Interactions, functions, and regulations

Proteins with motifs rich in arginines and glycines were discovered decades ago and are functionally involved in a staggering range of essential processes in the cell. Versatile, specific, yet adaptable molecular interactions enabled by the unique combination of arginine and glycine, combined with multiplicity of molecular recognition conferred by repeated di-, tri-, and multiple peptide motifs, allow RGG motif proteins to interact with a broad range of proteins and nucleic acids. Furthermore, posttranslational modifications at the arginines in the motif extend the RGG protein's capacity for a fine-tuned regulation. In this review, we focus on the biochemical properties of the RGG motif, its molecular interactions with RNAs and proteins, and roles of the posttranslational modification in modulating their interactions. We discuss current knowledge of the RGG motif proteins involved in mRNA transport and translation, highlight our merging understanding of their molecular functions in translational regulation and summarize areas of research in the future critical in understanding this important family of proteins. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Mechanisms.

Regulating Phase Transition in Neurodegenerative Diseases by Nuclear Import Receptors

RNA-binding proteins (RBPs) with a low-complexity prion-like domain (PLD) can undergo aberrant phase transitions and have been implicated in neurodegenerative diseases such as ALS and FTD. Several nuclear RBPs mislocalize to cytoplasmic inclusions in disease conditions. Impairment in nucleocytoplasmic transport is another major event observed in ageing and in neurodegenerative disorders. Nuclear import receptors (NIRs) regulate the nucleocytoplasmic transport of different RBPs bearing a nuclear localization signal by restoring their nuclear localization. NIRs can also specifically dissolve or prevent the aggregation and liquid–liquid phase separation of wild-type or disease-linked mutant RBPs, due to their chaperoning activity. This review focuses on the LLPS of intrinsically disordered proteins and the role of NIRs in regulating LLPS in neurodegeneration. This review also discusses the implication of NIRs as therapeutic agents in neurogenerative diseases.

Progress in biophysics and molecular biology proteostasis impairment and ALS

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive and fatal neurodegenerative disease that results from the loss of both upper and lower motor neurons. It is the most common motor neuron disease and currently has no effective treatment. There is mounting evidence to suggest that disturbances in proteostasis play a significant role in ALS pathogenesis. Proteostasis is the maintenance of the proteome at the right level, conformation and location to allow a cell to perform its intended function. In this review, we present a thorough synthesis of the literature that provides evidence that genetic mutations associated with ALS cause imbalance to a proteome that is vulnerable to such pressure due to its metastable nature. We propose that the mechanism underlying motor neuron death caused by defects in mRNA metabolism and protein degradation pathways converges on proteostasis dysfunction. We propose that the proteostasis network may provide an effective target for therapeutic development in ALS.

Impaired nuclear transport induced by juvenile ALS causing P525L mutation in NLS domain of FUS: A molecular mechanistic study

Amyotrophic lateral sclerosis (ALS) and fronto-temporal lobar degeneration (FTLD) are progressive neurological disorders affecting motor neurons. Cellular aggregates of fused in sarcoma (FUS) protein are found in cytoplasm of ALS and FTLD patients. Nuclear localisation signal (NLS) domain of FUS binds to Karyopherin β2 (Kapβ2), which drives nuclear transport of FUS from cytoplasm. Several pathogenic mutations are reported in FUS NLS, which are associated with its impaired nuclear transport and cytoplasmic mis-localisation. P525L mutation in NLS is most commonly found in cases of juvenile ALS (jALS), which affects individuals below 25 years of age. jALS progresses aggressively causing death within a year of its onset. This study elucidates the molecular mechanism behind jALS-causing P525L mutation hindering nuclear transport of FUS. We perform multiple molecular dynamics simulations in aqueous and hydrophobic solvent to understand the effect of the mutation at molecular level. Dynamics of Kapβ2-FUS complex is better captured in hydrophobic solvent compared to aqueous solvent. P525 and Y526 (PY-motif) of NLS exhibit fine-tuned stereochemical arrangement, which is essential for optimum Kapβ2 binding. P525L causes loss of several native contacts at interface leading to weaker binding, which promotes self-aggregation of FUS in cytoplasm. Native complex samples closed conformation, while mutant complex exhibits open conformation exposing hydrophilic residues of Kapβ2 to hydrophobic solvent. Mutant complex also fails to exhibit spring-like motion essential for its transport through nuclear pore complex. This study provides a mechanistic insight of binding affinity between NLS and Kapβ2 that inhibits self-aggregation of FUS preventing the disease condition.

Emerging Roles for Phase Separation of RNA-Binding Proteins in Cellular Pathology of ALS

Liquid-liquid phase separation (LLPS) is emerging as a major principle for the mesoscale organization of proteins, RNAs, and membrane-bound organelles into biomolecular condensates. These condensates allow for rapid cellular responses to changes in metabolic activities and signaling. Nowhere is this regulation more important than in neurons and glia, where cellular physiology occurs simultaneously on a range of time- and length-scales. In a number of neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis (ALS), misregulation of biomolecular condensates leads to the formation of insoluble aggregates—a pathological hallmark of both sporadic and familial ALS. Here, we summarize how the emerging knowledge about the LLPS of ALS-related proteins corroborates with their aggregation. Understanding the mechanisms that lead to protein aggregation in ALS and how cells respond to these aggregates promises to open new directions for drug development.

Lysine acetylation regulates the RNA binding, subcellular localization and inclusion formation of FUS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the preferential death of motor neurons. Approximately 10% of ALS cases are familial and 90% are sporadic. Fused in sarcoma (FUS) is a ubiquitously expressed RNA-binding protein implicated in familial ALS and frontotemporal dementia (FTD). The physiological function and pathological mechanism of FUS are not well understood, particularly whether post-translational modifications play a role in regulating FUS function. In this study, we discovered that FUS was acetylated at lysine-315/316 (K315/K316) and lysine-510 (K510) residues in two distinct domains. Located in the nuclear localization sequence, K510 acetylation disrupted the interaction between FUS and Transportin-1, resulting in the mislocalization of FUS in the cytoplasm and formation of stress granule-like inclusions. Located in the RNA recognition motif, K315/K316 acetylation reduced RNA binding to FUS and decreased the formation of cytoplasmic inclusions. Treatment with deacetylase inhibitors also significantly reduced the inclusion formation in cells expressing ALS mutation P525L. More interestingly, familial ALS patient fibroblasts showed higher levels of FUS K510 acetylation as compared with healthy controls. Lastly, CREB-binding protein/p300 acetylated FUS, whereas both sirtuins and histone deacetylases families of lysine deacetylases contributed to FUS deacetylation. These findings demonstrate that FUS acetylation regulates the RNA binding, subcellular localization and inclusion formation of FUS, implicating a potential role of acetylation in the pathophysiological process leading to FUS-mediated ALS/FTD.

Potential Therapeutic Role of HDAC Inhibitors in FUS-ALS

Mutations in the FUS gene cause amyotrophic lateral sclerosis (ALS-FUS). However, the exact pathogenic mechanism of mutant fused in sarcoma (FUS) protein is not completely understood. FUS is an RNA binding protein (RBP) localized predominantly in the nucleus, but ALS-linked FUS mutations can affect its nuclear localization signal impairing its import into the nucleus. This mislocalization to the cytoplasm facilitates FUS aggregation in cytoplasmic inclusions. Therapies targeting post translational modifications are rising as new treatments for ALS, in particular acetylation which could have a role in the dynamics of RBPs. Research using histone deacetylase (HDAC) inhibitors in FUS-ALS models showed that HDACs can influence cytoplasmic FUS localization. Inhibition of HDACs could promote acetylation of the FUS RNA binding domain (RRM) and altering its RNA interactions resulting in FUS maintenance in the nucleus. In addition, acetylation of FUS RRMs might also favor or disfavor its incorporation into pathological inclusions. In this review, we summarize and discuss the evidence for the potential role of HDACs in the context of FUS-ALS and we propose a new hypothesis based on this overview.

Proteostatic imbalance and protein spreading in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder whose exact causative mechanisms are still under intense investigation. Several lines of evidence suggest that the anatomical and temporal propagation of pathological protein species along the neural axis could be among the main driving mechanisms for the fast and irreversible progression of ALS pathology. Many ALS-associated proteins form intracellular aggregates as a result of their intrinsic prion-like properties and/or following impairment of the protein quality control systems. During the disease course, these mutated proteins and aberrant peptides are released in the extracellular milieu as soluble or aggregated forms through a variety of mechanisms. Internalization by recipient cells may seed further aggregation and amplify existing proteostatic imbalances, thus triggering a vicious cycle that propagates pathology in vulnerable cells, such as motor neurons and other susceptible neuronal subtypes. Here, we provide an in-depth review of ALS pathology with a particular focus on the disease mechanisms of seeding and transmission of the most common ALS-associated proteins, including SOD1, FUS, TDP-43, and C9orf72-linked dipeptide repeats. For each of these proteins, we report historical, biochemical, and pathological evidence of their behaviors in ALS. We further discuss the possibility to harness pathological proteins as biomarkers and reflect on the implications of these findings for future research.

The RNA-binding protein FUS is chaperoned and imported into the nucleus by a network of import receptors

Fused in sarcoma (FUS) is a predominantly nuclear RNA-binding protein with key functions in RNA processing and DNA damage repair. Defects in nuclear import of FUS have been linked to severe neurodegenerative diseases; hence, it is of great interest to understand this process and how it is dysregulated in disease. Transportin-1 (TNPO1) and the closely related transportin-2 have been identified as major nuclear import receptors of FUS. They bind to the C-terminal nuclear localization signal of FUS and mediate the protein's nuclear import and at the same time also suppress aberrant phase transitions of FUS in the cytoplasm. Whether FUS can utilize other nuclear transport receptors for the purpose of import and chaperoning has not been examined so far. Here, we show that FUS directly binds to different import receptors in vitro. FUS formed stable complexes not only with TNPO1 but also with transportin-3, importin β, importin 7, or the importin β/7 heterodimer. Binding of these alternative import receptors required arginine residues within FUS-RG/RGG motifs and was weakened by arginine methylation. Interaction with these importins suppressed FUS phase separation and reduced its sequestration into stress granules. In a permeabilized cell system, we further showed that transportin-3 had the capacity to import FUS into the nucleus, albeit with lower efficiency than TNPO1. Our data suggest that aggregation-prone RNA-binding proteins such as FUS may utilize a network of importins for chaperoning and import, similar to histones and ribosomal proteins.

Transportin-1: A Nuclear Import Receptor with Moonlighting Functions

Transportin-1 (Trn1), also known as karyopherin-β2 (Kapβ2), is probably the best-characterized nuclear import receptor of the karyopherin-β family after Importin-β, but certain aspects of its functions in cells are still puzzling or are just recently emerging. Since the initial identification of Trn1 as the nuclear import receptor of hnRNP A1 ∼25 years ago, several molecular and structural studies have unveiled and refined our understanding of Trn1-mediated nuclear import. In particular, the understanding at a molecular level of the NLS recognition by Trn1 made a decisive step forward with the identification of a new class of NLSs called PY-NLSs, which constitute the best-characterized substrates of Trn1. Besides PY-NLSs, many Trn1 cargoes harbour NLSs that do not resemble the archetypical PY-NLS, which complicates the global understanding of cargo recognition by Trn1. Although PY-NLS recognition is well established and supported by several structures, the recognition of non-PY-NLSs by Trn1 is far less understood, but recent reports have started to shed light on the recognition of this type of NLSs. Aside from its principal and long-established activity as a nuclear import receptor, Trn1 was shown more recently to moonlight outside nuclear import. Trn1 has for instance been caught in participating in virus uncoating, ciliary transport and in modulating the phase separation properties of aggregation-prone proteins. Here, we focus on the structural and functional aspects of Trn1-mediated nuclear import, as well as on the moonlighting activities of Trn1.

Divergent FUS phosphorylation in primate and mouse cells following double-strand DNA damage

Fused in sarcoma (FUS) is a RNA/DNA protein involved in multiple nuclear and cytoplasmic functions including transcription, splicing, mRNA trafficking, and stress granule formation. To accomplish these many functions, FUS must shuttle between cellular compartments in a highly regulated manner. When shuttling is disrupted, FUS abnormally accumulates into cytoplasmic inclusions that can be toxic. Disrupted shuttling of FUS into the nucleus is a hallmark of ~10% of frontotemporal lobar degeneration (FTLD) cases, the neuropathology that underlies frontotemporal dementia (FTD). Multiple pathways are known to disrupt nuclear/cytoplasmic shuttling of FUS. In earlier work, we discovered that double-strand DNA breaks (DSBs) trigger DNA-dependent protein kinase (DNA-PK) to phosphorylate FUS (p-FUS) at N-terminal residues leading to the cytoplasmic accumulation of FUS. Therefore, DNA damage may contribute to the development of FTLD pathology with FUS inclusions. In the present study, we examined how DSBs effect FUS phosphorylation in various primate and mouse cellular models. All cell lines derived from human and non-human primates exhibit N-terminal FUS phosphorylation following calicheamicin γ1 (CLM) induced DSBs. In contrast, we were unable to detect FUS phosphorylation in mouse-derived primary neurons or immortalized cell lines regardless of CLM treatment, duration, or concentration. Despite DNA damage induced by CLM treatment, we find that mouse cells do not phosphorylate FUS, likely due to reduced levels and activity of DNA-PK compared to human cells. Taken together, our work reveals that mouse-derived cellular models regulate FUS in an anomalous manner compared to primate cells. This raises the possibility that mouse models may not fully recapitulate the pathogenic cascades that lead to FTLD with FUS pathology.

Characteristic Features of FUS Inclusions in Spinal Motor Neurons of Sporadic Amyotrophic Lateral Sclerosis

Alterations of RNA metabolism caused by mutations in RNA-binding protein genes, such as transactivating DNA-binding protein-43 (TDP-43) and fused in sarcoma (FUS), have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Unlike the accumulation of TDP43, which is accepted as a pathological hall mark of sporadic ALS (sALS), FUS pathology in sALS is still under debate. Although immunoreactive inclusions of FUS have been detected in sALS patients previously, the technical limitation of signal detection, including the necessity of specific antigen retrieval, restricts our understanding of FUS-associated ALS pathology. In this study, we applied a novel detection method using a conventional antigen retrieval technique with Sudan Black B treatment to identify FUS-positive inclusions in sALS patients. We classified pathological motor neurons into 5 different categories according to the different aggregation characteristics of FUS and TDP-43. Although the granular type was more dominant for inclusions with TDP-43, the skein-like type was more often observed in FUS-positive inclusions, suggesting that these 2 proteins undergo independent aggregation processes. Moreover, neurons harboring FUS-positive inclusions demonstrated substantially reduced expression levels of dynactin-1, a retrograde motor protein, indicating that perturbation of nucleocytoplasmic transport is associated with the formation of cytoplasmic inclusions of FUS in sALS.

FUS mutation is probably the most common pathogenic gene for JALS, especially sporadic JALS

Juvenile amyotrophic lateral sclerosis (JALS) is a rare and severe form of ALS. The development of gene sequencing methods has resulted in increased reports of JALS cases in recent years, and additional gene mutations in FUS have been identified. Fused in sarcoma (FUS) mutations, appeared rarely in classical ALS but indeed were the most frequent pathogenic mutations in JALS, especially in sporadic JALS. After studied the reports in the last 10 years about JALS cases, the case characteristics caused by FUS mutations and the commonality of the mutation sites were summarized in this review. FUS mutation associated with more than half of JALS and the very majority of sporadic JALS. It's worth noting that almost all of the mutations occur in nuclear localization signal (NLS) of FUS in sporadic JALS. This discovery emphasized a new perspective focus on NLS for the diagnosis and etiology of sporadic JALS as well as for further study about new treatment.

Karyopherins and condensates

Several aggregation-prone RNA-binding proteins, including FUS, EWS, TAF15, hnRNP A1, hnRNP A2, and TDP-43, are mutated in neurodegenerative diseases. The nuclear-cytoplasmic distribution of these proteins is controlled by proteins in the karyopherin family of nuclear transport factors (Kaps). Recent studies have shown that Kaps not only transport these proteins but also inhibit their self-association/aggregation, acting as molecular chaperones. This chaperone activity is impaired for disease-causing mutants of the RNA-binding proteins. Here, we review physical data on the mechanisms of self-association of several disease-associated RNA-binding proteins, through liquid-liquid phase separation and amyloid fiber formation. In each case, we relate these data to biophysical, biochemical, and cell biological data on the inhibition of self-association by Kaps. Our analyses suggest that Kaps may be effective chaperones because they contain large surfaces with diverse physical properties that enable them to engage multiple different regions of their cargo proteins, blocking self-association.

Metabolomics: A Tool to Understand the Impact of Genetic Mutations in Amyotrophic Lateral Sclerosis

Metabolomics studies performed in patients with amyotrophic lateral sclerosis (ALS) reveal a set of distinct metabolites that can shed light on the pathological alterations taking place in each individual. Metabolites levels are influenced by disease status, and genetics play an important role both in familial and sporadic ALS cases. Metabolomics analysis helps to unravel the differential impact of the most common ALS-linked genetic mutations (as C9ORF72, SOD1, TARDBP, and FUS) in specific signaling pathways. Further, studies performed in genetic models of ALS reinforce the role of TDP-43 pathology in the vast majority of ALS cases. Studies performed in differentiated cells from ALS-iPSC (induced Pluripotent Stem Cells) reveal alterations in the cell metabolism that are also found in ALS models and ultimately in ALS patients. The development of metabolomics approaches in iPSC derived from ALS patients allow addressing and ultimately understanding the pathological mechanisms taking place in any patient. Lately, the creation of a "patient in a dish" will help to identify patients that may benefit from specific treatments and allow the implementation of personalized medicine.

ALS Genetics, Mechanisms, and Therapeutics: Where Are We Now?

The scientific landscape surrounding amyotrophic lateral sclerosis (ALS) continues to shift as the number of genes associated with the disease risk and pathogenesis, and the cellular processes involved, continues to grow. Despite decades of intense research and over 50 potentially causative or disease-modifying genes identified, etiology remains unexplained and treatment options remain limited for the majority of ALS patients. Various factors have contributed to the slow progress in understanding and developing therapeutics for this disease. Here, we review the genetic basis of ALS, highlighting factors that have contributed to the elusiveness of genetic heritability. The most commonly mutated ALS-linked genes are reviewed with an emphasis on disease-causing mechanisms. The cellular processes involved in ALS pathogenesis are discussed, with evidence implicating their involvement in ALS summarized. Past and present therapeutic strategies and the benefits and limitations of the model systems available to ALS researchers are discussed with future directions for research that may lead to effective treatment strategies outlined.

TDP-43 and FUS-structural insights into RNA recognition and self-association

RNA-binding proteins TDP-43 and FUS play essential roles in pre-mRNA splicing, localization, granule formation and other aspects of RNA metabolism. Both proteins are implicated in neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Despite their apparent similarities, each protein has unique structural characteristics. Here we present the current structural understanding of RNA-binding and self-association mechanisms. Both globular and intrinsically disordered domains contribute to RNA binding, each with different specificities, affinities and kinetics. Self-associating Prion-like domains in each protein form multivalent interactions and labile cross-β structures. These interactions are modulated by distinctive additional domains including a globular oligomerization domain in TDP-43 and synergistic interactions with intrinsically disordered Arginine-Glycine rich domains in FUS. These insights contribute to a better understanding of native biological functions of TDP-43 and FUS and potential molecular pathways in neurodegenerative diseases.

The physiological and pathological biophysics of phase separation and gelation of RNA binding proteins in amyotrophic lateral sclerosis and fronto-temporal lobar degeneration

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Some intrinsically disordered proteins undergo reversible phase separation/gelation.

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Reversible phase separation/gelation underpins function of membraneless organelles.

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fALS-FUS mutations increase propensity of FUS to form highly stable condensates.

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Changes in arginine methylation and FUS chaperones in FTLD-FUS have similar effects.

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Stable fibrillar condensates sequester cargo and impair RNP granule function.

Many RNA binding proteins, including FUS, contain moderately repetitive, low complexity, intrinsically disordered domains. These sequence motifs have recently been found to underpin reversible liquid: liquid phase separation and gelation of these proteins, permitting them to reversibly transition from a monodispersed state to liquid droplet- or hydrogel-like states. This function allows the proteins to serve as scaffolds for the formation of reversible membraneless intracellular organelles such as nucleoli, stress granules and neuronal transport granules. Using FUS as an example, this review examines the biophysics of this physiological process, and reports on how mutations and changes in post-translational state alter phase behaviour, and lead to neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal lobar degeneration.

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.

Fused in Sarcoma Neuropathology in Neurodegenerative Disease

Abnormal intracellular accumulation of the fused in sarcoma (FUS) protein is the characteristic pathological feature of cases of familial amyotrophic lateral sclerosis (ALS) caused by FUS mutations (ALS-FUS) and several uncommon disorders that may present with sporadic frontotemporal dementia (FTLD-FUS). Although these findings provide further support for the concept that ALS and FTD are closely related clinical syndromes with an overlapping molecular basis, important differences in the pathological features and results from experimental models indicate that ALS-FUS and FTLD-FUS have distinct pathogenic mechanisms.

Genetic mutations in RNA-binding proteins and their roles in ALS

Mutations in genes that encode RNA-binding proteins (RBPs) have emerged as critical determinants of neurological diseases, especially motor neuron disorders such as amyotrophic lateral sclerosis (ALS). RBPs are involved in all aspects of RNA processing, controlling the life cycle of RNAs from synthesis to degradation. Hallmark features of RBPs in neuron dysfunction include misregulation of RNA processing, mislocalization of RBPs to the cytoplasm, and abnormal aggregation of RBPs. Much progress has been made in understanding how ALS-associated mutations in RBPs drive pathogenesis. Here, we focus on several key RBPs involved in ALS—TDP-43, HNRNP A2/B1, HNRNP A1, FUS, EWSR1, and TAF15—and review our current understanding of how mutations in these proteins cause disease.

Clinical and experimental studies of a novel P525R FUS mutation in amyotrophic lateral sclerosis

Objective:

To describe the clinical features of a novel fused in sarcoma (FUS) mutation in a young adult female amyotrophic lateral sclerosis (ALS) patient with rapid progression of weakness and to experimentally validate the consequences of the P525R mutation in cellular neuronal models.

Methods:

We conducted sequencing of genomic DNA from the index patient and her family members. Immunocytochemistry was performed in various cellular models to determine whether the newly identified P525R mutant FUS protein accumulated in cytoplasmic inclusions. Clinical features of the index patient were compared with 19 other patients with ALS carrying the P525L mutation in the same amino acid position.

Results:

A novel mutation c.1574C>G (p.525P>R) in the FUS gene was identified in the index patient. The clinical symptoms are similar to those in familial ALS patients with the P525L mutation at the same position. The P525R mutant FUS protein showed cytoplasmic localization and formed large stress granule–like cytoplasmic inclusions in multiple cellular models.

Conclusions:

The clinical features of the patient and the cytoplasmic inclusions of the P525R mutant FUS protein strengthen the notion that mutations at position 525 of the FUS protein result in a coherent phenotype characterized by juvenile or young adult onset, rapid progression, variable positive family history, and female preponderance.

A novel mutation of the C-terminal amino acid of FUS (Y526C) strengthens FUS gene as the most frequent genetic factor in aggressive juvenile ALS

Although amyotrophic lateral sclerosis (ALS) typically occurs around 60 years, numerous publications report an onset of ALS before the age of 25 years that define juvenile ALS (jALS). Over the last decade, growing literature mentioned jALS with an aggressive evolution which are mainly linked to the FUS gene. We report here the case of a 25-year-old woman with a bulbar onset ALS that progressed in less than 12 months to invasive ventilation due to respiratory failure; Genetic screening identified a new mutation in the FUS gene that lies within the last codon. After reading the literature, it might be legitimate to consider that jALS linked to FUS mutations represent a specific entity different from both classical jALS and adult ALS linked to FUS gene. This should encourage clinician to firstly screen the FUS gene in the presence of a sporadic ALS that occurs before the age of 25 and with an aggressive profile of evolution.

TDP-43/FUS in motor neuron disease: Complexity and challenges

Amyotrophic lateral sclerosis (ALS), a common motor neuron disease affecting two per 100,000 people worldwide, encompasses at least five distinct pathological subtypes, including, ALS-SOD1, ALS-C9orf72, ALS-TDP-43, ALS-FUS and Guam-ALS. The etiology of a major subset of ALS involves toxicity of the TAR DNA-binding protein-43 (TDP-43). A second RNA/DNA binding protein, fused in sarcoma/translocated in liposarcoma (FUS/TLS) has been subsequently associated with about 1% of ALS patients. While mutations in TDP-43 and FUS have been linked to ALS, the key contributing molecular mechanism(s) leading to cell death are still unclear. One unique feature of TDP-43 and FUS pathogenesis in ALS is their nuclear clearance and simultaneous cytoplasmic aggregation in affected motor neurons. Since the discoveries in the last decade implicating TDP-43 and FUS toxicity in ALS, a majority of studies have focused on their cytoplasmic aggregation and disruption of their RNA-binding functions. However, TDP-43 and FUS also bind to DNA, although the significance of their DNA binding in disease-affected neurons has been less investigated. A recent observation of accumulated genomic damage in TDP-43 and FUS-linked ALS and association of FUS with neuronal DNA damage repair pathways indicate a possible role of deregulated DNA binding function of TDP-43 and FUS in ALS. In this review, we discuss the different ALS disease subtypes, crosstalk of etiopathologies in disease progression, available animal models and their limitations, and recent advances in understanding the specific involvement of RNA/DNA binding proteins, TDP-43 and FUS, in motor neuron diseases.

Karyopherin-β2 Recognition of a PY-NLS Variant that Lacks the Proline-Tyrosine Motif

Karyopherin-β2 or Transportin-1 binds proline-tyrosine nuclear localization signals (PY-NLSs) in its cargos. PY-NLSs are described by structural disorder, overall positive charge, and binding epitopes composed of an N-terminal hydrophobic or basic motif and a C-terminal R-X_2-5P-Y motif. The N-terminal tail of histone H3 binds Kapβ2 with high affinity but does not contain a recognizable PY-NLS. The crystal structure of the Kapβ2-H3 tail shows residues 11-27 of H3 binding to the PY-NLS site of Kapβ2. H3 residues ¹¹TGGKAPRK¹⁸ bind the site for PY-NLS Epitope 1 (N-terminal hydrophobic/basic motif), which is most important for Kapβ2-binding. H3 residue Arg26 occupies the PY-NLS Epitope 2 position (usually arginine of R-X_2-5P-Y) but PY-NLS Epitope 3 (proline-tyrosine motif) is missing in the H3 tail. Histone H3 thus provides an example of a PY-NLS variant with no proline-tyrosine or homologous proline-hydrophobic motif. The H3 tail uses a very strong Epitope 1 to compensate for loss of the often-conserved proline-tyrosine epitope.

Variants in HNRNPH2 on the X Chromosome Are Associated with a Neurodevelopmental Disorder in Females

Via whole-exome sequencing, we identified six females from independent families with a common neurodevelopmental phenotype including developmental delay, intellectual disability, autism, hypotonia, and seizures, all with de novo predicted deleterious variants in the nuclear localization signal of Heterogeneous Nuclear Ribonucleoprotein H2, encoded by HNRNPH2, a gene located on the X chromosome. Many of the females also have seizures, psychiatric co-morbidities, and orthopedic, gastrointestinal, and growth problems as well as common dysmorphic facial features. HNRNPs are a large group of ubiquitous proteins that associate with pre-mRNAs in eukaryotic cells to produce a multitude of alternatively spliced mRNA products during development and play an important role in controlling gene expression. The failure to identify affected males, the severity of the neurodevelopmental phenotype in females, and the essential role of this gene suggests that male conceptuses with these variants may not be viable.

Nuclear trafficking in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Amyotrophic lateral sclerosis and frontotemporal lobar degeneration are two ends of a phenotypic spectrum of disabling, relentlessly progressive and ultimately fatal diseases. A key characteristic of both conditions is the presence of TDP-43 (encoded by TARDBP) or FUS immunoreactive cytoplasmic inclusions in neuronal and glial cells. This cytoplasmic mislocalization of otherwise predominantly nuclear RNA binding proteins implies a perturbation of the nucleocytoplasmic shuttling as a possible event in the pathogenesis. Compromised nucleocytoplasmic shuttling has recently also been associated with a hexanucleotide repeat expansion mutation in C9orf72, which is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal lobar degeneration, and leads to accumulation of cytoplasmic TDP-43 inclusions. Mutation in C9orf72 may disrupt nucleocytoplasmic shuttling on the level of C9ORF72 protein, the transcribed hexanucleotide repeat RNA, and/or dipeptide repeat proteins translated form the hexanucleotide repeat RNA. These defects of nucleocytoplasmic shuttling may therefore, constitute the common ground of the underlying disease mechanisms in different molecular subtypes of amyotrophic lateral sclerosis and frontotemporal lobar degeneration.

Delineating the relationship between amyotrophic lateral sclerosis and frontotemporal dementia: Sequence and structure-based predictions

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are related neurodegenerative disorders which are characterized by a rapid decline in cognitive and motor functions, and short survival. Both syndromes may be present within the same family or even in the same person. The genetic findings for both diseases also support the existence of a continuum, with mutations in the same genes being found in patients with ALS, FTD or FTD/ALS. Little is known about the molecular mechanisms underlying the differences in mutations of the same protein causing either ALS or FTD. Here, we shed light on 348 ALS and FTD missense mutations in 14 genes focusing on genic intolerance and protein stability based on available 3D structures. Using EvoTol, we prioritized the disease-causing genes and their domain. The most intolerant genes predicted by EvoTol are SQSTM1 and OPTN which are involved in protein homeostasis. Further, using ENCoM (Elastic Network Contact Model) that predicts stability based on vibrational entropy, we predicted that most of the missense mutations with destabilizing energies are in the structural regions that control the protein-protein interaction, and only a few mutations affect protein folding. We found a trend that energy changes are higher for ALS compared to FTD mutations. The stability of the ALS mutants correlated well with the duration of disease progression as compared to FTD-ALS mutants. This study provides a comprehensive understanding of the mechanism of ALS and illustrates the significance of structure-energy based studies in differentiating ALS and FTD mutations.

Monomethylated and unmethylated FUS exhibit increased binding to Transportin and distinguish FTLD-FUS from ALS-FUS

Deposition of the nuclear DNA/RNA-binding protein Fused in sarcoma (FUS) in cytosolic inclusions is a common hallmark of some cases of frontotemporal lobar degeneration (FTLD-FUS) and amyotrophic lateral sclerosis (ALS-FUS). Whether both diseases also share common pathological mechanisms is currently unclear. Based on our previous finding that FUS deposits are hypomethylated in FTLD-FUS but not in ALS-FUS, we have now investigated whether genetic or pharmacological inactivation of Protein arginine methyltransferase 1 (PRMT1) activity results in unmethylated FUS or in alternatively methylated forms of FUS. To do so, we generated FUS-specific monoclonal antibodies that specifically recognize unmethylated arginine (UMA), monomethylated arginine (MMA) or asymmetrically dimethylated arginine (ADMA). Loss of PRMT1 indeed not only results in an increase of UMA FUS and a decrease of ADMA FUS, but also in a significant increase of MMA FUS. Compared to ADMA FUS, UMA and MMA FUS exhibit much higher binding affinities to Transportin-1, the nuclear import receptor of FUS, as measured by pull-down assays and isothermal titration calorimetry. Moreover, we show that MMA FUS occurs exclusively in FTLD-FUS, but not in ALS-FUS. Our findings therefore provide additional evidence that FTLD-FUS and ALS-FUS are caused by distinct disease mechanisms although both share FUS deposits as a common denominator.

Nuclear localization signals for four distinct karyopherin-β nuclear import systems

The Karyopherin-β family of proteins mediates nuclear transport of macromolecules. Nuclear versus cytoplasmic localization of proteins is often suggested by the presence of NLSs (nuclear localization signals) or NESs (nuclear export signals). Import-Karyopherin-βs or Importins bind to NLSs in their protein cargos to transport them through nuclear pore complexes into the nucleus. Until recently, only two classes of NLS had been biochemically and structurally characterized: the classical NLS, which is recognized by the Importin-α/β heterodimer and the PY-NLS (proline-tyrosine NLS), which is recognized by Karyopherin-β2 or Transportin-1. Structures of two other Karyopherin-βs, Kap121 and Transportin-SR2, in complex with their respective cargos were reported for the first time recently, revealing two new distinct classes of NLSs. The present paper briefly describes the classical NLS, reviews recent literature on the PY-NLS and provides in-depth reviews of the two newly discovered classes of NLSs that bind Kap121p and Transportin-SR respectively.

Phosphorylation of C-terminal tyrosine residue 526 in FUS impairs its nuclear import

Aberrant cytoplasmic aggregation of FUS, which is caused by mutations primarily in the C-terminal nuclear localisation signal, is associated with 3% of cases of familial amyotrophic lateral sclerosis (ALS). FUS aggregates are also pathognomonic for 10% of all frontotemporal lobar degeneration (FTLD) cases; however, these cases are not associated with mutations in the gene encoding FUS. This suggests that there are differences in the mechanisms that drive inclusion formation of FUS in ALS and FTLD. Here, we show that the C-terminal tyrosine residue at position 526 of FUS is crucial for normal nuclear import. This tyrosine is subjected to phosphorylation, which reduces interaction with transportin 1 and might consequentially affect the transport of FUS into the nucleus. Furthermore, we show that this phosphorylation can occur through the activity of the Src family of kinases. Our study implicates phosphorylation as an additional mechanism by which nuclear transport of FUS might be regulated and potentially perturbed in ALS and FTLD.

Mechanisms of amyloid fibril formation

Amyloid and amyloid-like aggregates are elongated unbranched fibrils consisting of β-structures of separate monomers positioned perpendicular to the fibril axis and stacked strictly above each other. In their physicochemical properties, amyloid fibrils are reminiscent of synthetic polymers rather than usual proteins because they are stable to the action of denaturing agents and proteases. Their mechanical stability can be compared to a spider's web, that in spite of its ability to stretch, is stronger than steel. It is not surprising that a large number of diseases are accompanied with amyloid fibril depositing in different organs. Pathologies provoked by depositing of incorrectly folded proteins include Alzheimer's, Parkinson's, and Huntington's diseases. In addition, this group of diseases involves mucoviscidosis, some types of diabetes, and hereditary cataracts. Each type of amyloidosis is characterized by aggregation of a certain type of protein that is soluble in general, and thus leads to specific distortions of functions of the corresponding organs. Therefore, it is important to understand the process of transformation of "native" proteins to amyloid fibrils to clarify how these molecules acquire such strength and what key elements of this process determine the pathway of erroneous protein folding. This review presents our analysis of complied information on the mechanisms of formation and biochemical properties of amyloid fibrils.

Stepwise acquirement of hallmark neuropathology in FUS-ALS iPSC models depends on mutation type and neuronal aging

Autosomal-dominant mutations within the gene FUS (fused in sarcoma) are responsible for 5% of familial cases of amyotrophic lateral sclerosis (ALS). The FUS protein is physiologically mainly located in the nucleus, while cytoplasmic FUS aggregates are pathological hallmarks of FUS-ALS. Data from non-neuronal cell models and/or models using heterologous expression of FUS mutants suggest cytoplasmic FUS translocation as a pivotal initial event which leads to neurodegeneration depending on a second hit. Here we present the first human model of FUS-ALS using patient-derived neurons carrying endogenous FUS mutations leading to a benign (R521C) or a more severe clinical phenotype (frameshift mutation R495QfsX527). We thereby showed that the severity of the underlying FUS mutation determines the amount of cytoplasmic FUS accumulation and cellular vulnerability to exogenous stress. Cytoplasmic FUS inclusions formed spontaneously depending on both, severity of FUS mutation and neuronal aging. These aggregates showed typical characteristics of FUS-ALS including methylated FUS. Finally, neurodegeneration was not specific to layer V cortical neurons perfectly in line with the current model of disease spreading in ALS. Our study highlights the value and usefulness of patient-derived cell models in FUS-ALS.

Oxr1 improves pathogenic cellular features of ALS-associated FUS and TDP-43 mutations

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the loss of motor neuron-like cells. Mutations in the RNA- and DNA-binding proteins, fused in sarcoma (FUS) and transactive response DNA-binding protein 43 kDa (TDP-43), are responsible for 5–10% of familial and 1% of sporadic ALS cases. Importantly, aggregation of misfolded FUS or TDP-43 is also characteristic of several neurodegenerative disorders in addition to ALS, including frontotemporal lobar degeneration. Moreover, splicing deregulation of FUS and TDP-43 target genes as well as mitochondrial abnormalities are associated with disease-causing FUS and TDP-43 mutants. While progress has been made to understand the functions of these proteins, the exact mechanisms by which FUS and TDP-43 cause ALS remain unknown. Recently, we discovered that, in addition to being up-regulated in spinal cords of ALS patients, the novel protein oxidative resistance 1 (Oxr1) protects neurons from oxidative stress-induced apoptosis. To further understand the function of Oxr1, we present here the first interaction study of the protein. We show that Oxr1 binds to Fus and Tdp-43 and that certain ALS-associated mutations in Fus and Tdp-43 affect their Oxr1-binding properties. We further demonstrate that increasing Oxr1 levels in cells expressing specific Fus and Tdp-43 mutants improves the three main cellular features associated with ALS: cytoplasmic mis-localization and aggregation, splicing changes of a mitochondrial gene and mitochondrial defects. Taken together, these findings suggest that OXR1 may have therapeutic benefits for the treatment of ALS and related neurodegenerative disorders with TDP-43 pathology.

Highly efficient cell-type-specific gene inactivation reveals a key function for the Drosophila FUS homolog cabeza in neurons

To expand the rich genetic toolkit of Drosophila melanogaster, we evaluated whether introducing FRT or LoxP sites in endogenous genes could allow for cell-type-specific gene inactivation in both dividing and postmitotic cells by GAL4-driven expression of FLP or Cre recombinase. For proof of principle, conditional alleles were generated for cabeza (caz), the Drosophila homolog of human FUS, a gene implicated in the neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Upon selective expression in neurons or muscle, both FLP and Cre mediated caz inactivation in all neurons or muscle cells, respectively. Neuron-selective caz inactivation resulted in failure of pharate adult flies to eclose from the pupal case, and adult escapers displayed motor performance defects and reduced life span. Due to Cre-toxicity, FLP/FRT is the preferred system for cell-type-specific gene inactivation, and this strategy outperforms RNAi-mediated knock-down. Furthermore, the GAL80 target system allowed for temporal control over gene inactivation, as induction of FLP expression from the adult stage onwards still inactivated caz in >99% of neurons. Remarkably, selective caz inactivation in adult neurons did not affect motor performance and life span, indicating that neuronal caz is required during development, but not for maintenance of adult neuronal function.

Nuclear import factor transportin and arginine methyltransferase 1 modify FUS neurotoxicity in Drosophila

Inclusions containing Fused in Sarcoma (FUS) are found in familial and sporadic cases of the incurable progressive motor neuron disease amyotrophic lateral sclerosis and in a common form of dementia, frontotemporal dementia. Most disease-associated mutations are located in the C-terminal proline-tyrosine nuclear localization sequence (PY-NLS) of FUS and impair its nuclear import. It has been shown in cell culture that the nuclear import of FUS is mediated by transportin, which binds the PY-NLS and the last arginine/glycine/glycine-rich (RGG) domain of FUS. Methylation of this last RGG domain by protein arginine methyltransferases (PRMTs) weakens transportin binding and therefore impairs nuclear translocation of FUS. To investigate the requirements for the nuclear import of FUS in an in vivo model, we generated different transgenic Drosophila lines expressing human FUS wild type (hFUS wt) and two disease-related variants P525L and R495X, in which the NLS is mutated or completely absent, respectively. To rule out effects caused by heterologous hFUS expression, we analysed the corresponding variants for the Drosophila FUS orthologue Cabeza (Caz wt, P398L, Q349X). Expression of these variants in eyes and motor neurons confirmed the PY-NLS-dependent nuclear localization of FUS/Caz and caused neurodegenerative effects. Surprisingly, FUS/Caz toxicity was correlated to the degree of its nuclear localization in this overexpression model. High levels of nuclear FUS/Caz became insoluble and reduced the endogenous Caz levels, confirming FUS autoregulation in Drosophila. RNAi-mediated knockdown of the two transportin orthologues interfered with the nuclear import of FUS/Caz and also enhanced the eye phenotype. Finally, we screened the Drosophila PRMT proteins (DART1-9) and found that knockdown of Dart1 led to a reduction in methylation of hFUS P525L and aggravated its phenotype. These findings show that the molecular mechanisms controlling the nuclear import of FUS/Caz and FUS autoregulation are conserved between humans and Drosophila. In addition to the well-known neurodegenerative effects of FUS loss-of function, our data suggest toxic potential of overexpressed FUS in the nucleus and of insoluble FUS.

NLS-tagging: an alternative strategy to tag nuclear proteins

The characterization of transcription factor complexes and their binding sites in the genome by affinity purification has yielded tremendous new insights into how genes are regulated. The affinity purification requires either the use of antibodies raised against the factor of interest itself or by high-affinity binding of a C- or N-terminally added tag sequence to the factor. Unfortunately, fusing extra amino acids to the termini of a factor can interfere with its biological function or the tag may be inaccessible inside the protein. Here, we describe an effective solution to that problem by integrating the ‘tag’ close to the nuclear localization sequence domain of the factor. We demonstrate the effectiveness of this approach with the transcription factors Fli-1 and Irf2bp2, which cannot be tagged at their extremities without loss of function. This resulted in the identification of novel proteins partners and a new hypothesis on the contribution of Fli-1 to hematopoiesis.

Transportin-1 and Transportin-2: protein nuclear import and beyond

Nearly 20 years after its identification as a new β-karyopherin mediating the nuclear import of the RNA-binding protein hnRNP A1, Transportin-1 is still commonly overlooked in comparison with its best known cousin, Importin-β. Transportin-1 is nonetheless a considerable player in nucleo-cytoplasmic transport. Over the past few years, significant progress has been made in the characterization of the nuclear localization signals (NLSs) that Transportin-1 recognizes, thereby providing the molecular basis of its diversified repertoire of cargoes. The recent discovery that mutations in the Transportin-dependent NLS of FUS cause mislocalization of this protein and result in amyotrophic lateral sclerosis illustrates the importance of Transportin-dependent import for human health. Besides, new functions of Transportin-1 are emerging in processes other than nuclear import. Here, we summarize what is known about Transportin-1 and the related β-karyopherin Transportin-2.

Rodent models of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterised by the degeneration of upper and lower motor neurons. Recent advances in our understanding of some of the genetic causes of ALS, such as mutations in SOD1, TARDBP, FUS and VCP have led to the generation of rodent models of the disease, as a strategy to help our understanding of the pathophysiology of ALS and to assist in the development of therapeutic strategies. This review provides detailed descriptions of TDP-43, FUS and VCP models of ALS, and summarises potential therapeutics which have been recently trialled in rodent models of the disease. This article is part of a Special Issue entitled: Animal Models of Disease.

ALS mutant FUS disrupts nuclear localization and sequesters wild-type FUS within cytoplasmic stress granules

Mutations in the gene encoding Fused in Sarcoma (FUS) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disorder. FUS is a predominantly nuclear DNA- and RNA-binding protein that is involved in RNA processing. Large FUS-immunoreactive inclusions fill the perikaryon of surviving motor neurons of ALS patients carrying mutations at post-mortem. This sequestration of FUS is predicted to disrupt RNA processing and initiate neurodegeneration. Here, we demonstrate that C-terminal ALS mutations disrupt the nuclear localizing signal (NLS) of FUS resulting in cytoplasmic accumulation in transfected cells and patient fibroblasts. FUS mislocalization is rescued by the addition of the wild-type FUS NLS to mutant proteins. We also show that oxidative stress recruits mutant FUS to cytoplasmic stress granules where it is able to bind and sequester wild-type FUS. While FUS interacts with itself directly by protein-protein interaction, the recruitment of FUS to stress granules and interaction with PABP are RNA dependent. These findings support a two-hit hypothesis, whereby cytoplasmic mislocalization of FUS protein, followed by cellular stress, contributes to the formation of cytoplasmic aggregates that may sequester FUS, disrupt RNA processing and initiate motor neuron degeneration.