ALS-associated mutations in FUS disrupt the axonal distribution and function of SMN

The role of survival motor neuron protein (SMN) in protein homeostasis

Ever since loss of survival motor neuron (SMN) protein was identified as the direct cause of the childhood inherited neurodegenerative disorder spinal muscular atrophy, significant efforts have been made to reveal the molecular functions of this ubiquitously expressed protein. Resulting research demonstrated that SMN plays important roles in multiple fundamental cellular homeostatic pathways, including a well-characterised role in the assembly of the spliceosome and biogenesis of ribonucleoproteins. More recent studies have shown that SMN is also involved in other housekeeping processes, including mRNA trafficking and local translation, cytoskeletal dynamics, endocytosis and autophagy. Moreover, SMN has been shown to influence mitochondria and bioenergetic pathways as well as regulate function of the ubiquitin-proteasome system. In this review, we summarise these diverse functions of SMN, confirming its key role in maintenance of the homeostatic environment of the cell.

The hnRNP RALY regulates PRMT1 expression and interacts with the ALS-linked protein FUS: implication for reciprocal cellular localization

The RBP associated with lethal yellow mutation (RALY) is a member of the heterogeneous nuclear ribonucleoprotein family whose transcriptome and interactome have been recently characterized. RALY binds poly-U rich elements within several RNAs and regulates the expression as well as the stability of specific transcripts. Here we show that RALY binds PRMT1 mRNA and regulates its expression. PRMT1 catalyzes the arginine methylation of Fused in Sarcoma (FUS), an RNA-binding protein that interacts with RALY. We demonstrate that RALY down-regulation decreases protein arginine N-methyltransferase 1 levels, thus reducing FUS methylation. It is known that mutations in the FUS nuclear localization signal (NLS) retain the protein to the cytosol, promote aggregate formation, and are associated with amyotrophic lateral sclerosis. Confirming that inhibiting FUS methylation increases its nuclear import, we report that RALY knockout enhances FUS NLS mutants’ nuclear translocation, hence decreasing aggregate formation. Furthermore, we characterize the RNA-dependent interaction of RALY with FUS in motor neurons. We show that mutations in FUS NLS as well as in RALY NLS reciprocally alter their localization and interaction with target mRNAs. These data indicate that RALY’s activity is impaired in FUS pathology models, raising the possibility that RALY might modulate disease onset and/or progression.

mRNP assembly, axonal transport, and local translation in neurodegenerative diseases

The development, maturation, and maintenance of the mammalian nervous system rely on complex spatiotemporal patterns of gene expression. In neurons, this is achieved by the expression of differentially localized isoforms and specific sets of mRNA-binding proteins (mRBPs) that regulate RNA processing, mRNA trafficking, and local protein synthesis at remote sites within dendrites and axons. There is growing evidence that axons contain a specialized transcriptome and are endowed with the machinery that allows them to rapidly alter their local proteome via local translation and protein degradation. This enables axons to quickly respond to changes in their environment during development, and to facilitate axon regeneration and maintenance in adult organisms. Aside from providing autonomy to neuronal processes, local translation allows axons to send retrograde injury signals to the cell soma. In this review, we discuss evidence that disturbances in mRNP transport, granule assembly, axonal localization, and local translation contribute to pathology in various neurodegenerative diseases, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD).

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

Recapitulation of Pathological TDP-43 Features in Immortalized Lymphocytes from Sporadic ALS Patients

Amyotrophic lateral sclerosis (ALS) is a fatal progressive neurodegenerative disorder of still unknown etiology that results in loss of motoneurons, paralysis, and death, usually between 2 and 4 years from onset. There are no currently available ALS biomarkers to support early diagnosis and to facilitate the assessment of the efficacy of new treatments. Since ALS is considered a multisystemic disease, here we have investigated the usefulness of immortalized lymphocytes from sporadic ALS patients to study TDP-43 homeostasis as well as to provide a convenient platform to evaluate TDP-43 phosphorylation as a novel therapeutic approach for ALS. We report here that lymphoblasts from ALS patients recapitulate the hallmarks of TDP-43 processing in affected motoneurons, such as increased phosphorylation, truncation, and mislocalization of TDP-43. Moreover, modulation of TDP-43 by an in-house designed protein casein kinase-1δ (CK-1δ) inhibitor, IGS3.27, reduced phosphorylation of TDP-43, and normalized the nucleo-cytosol translocation of TDP-43 in ALS lymphoblasts. Therefore, we conclude that lymphoblasts, easily accessible cells, from ALS patients could be a useful model to study pathological features of ALS disease and a suitable platform to test the effects of potential disease-modifying drugs even in a personalized manner.

3'UTR Length-Dependent Control of SynGAP Isoform α2 mRNA by FUS and ELAV-like Proteins Promotes Dendritic Spine Maturation and Cognitive Function

FUS is an RNA-binding protein associated with frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Previous reports have demonstrated intrinsic roles of FUS in synaptic function. However, the mechanism underlying FUS's regulation of synaptic morphology has remained unclear. We found that reduced mature spines after FUS depletion were associated with the internalization of PSD-95 within the dendritic shaft. Mass spectrometry of PSD-95-interacting proteins identified SynGAP, whose expression decreased after FUS depletion. Moreover, FUS and the ELAV-like proteins ELAVL4 and ELAVL1 control SynGAP mRNA stability in a 3'UTR length-dependent manner, resulting in the stable expression of the alternatively spliced SynGAP isoform α2. Finally, abnormal spine maturation and FTLD-like behavioral deficits in FUS-knockout mice were ameliorated by SynGAP α2. Our findings establish an important link between FUS and ELAVL proteins for mRNA stability control and indicate that this mechanism is crucial for the maintenance of synaptic morphology and cognitive function.

Whole blood transcriptome analysis in amyotrophic lateral sclerosis: A biomarker study

The biological pathways involved in amyotrophic lateral sclerosis (ALS) remain elusive and diagnostic decision-making can be challenging. Gene expression studies are valuable in overcoming such challenges since they can shed light on differentially regulated pathways and may ultimately identify valuable biomarkers. This two-stage transcriptome-wide study, including 397 ALS patients and 645 control subjects, identified 2,943 differentially expressed transcripts predominantly involved in RNA binding and intracellular transport. When batch effects between the two stages were overcome, three different models (support vector machines, nearest shrunken centroids, and LASSO) discriminated ALS patients from control subjects in the validation stage with high accuracy. The models’ accuracy reduced considerably when discriminating ALS from diseases that mimic ALS clinically (N = 75), nor could it predict survival. We here show that whole blood transcriptome profiles are able to reveal biological processes involved in ALS. Also, this study shows that using these profiles to differentiate between ALS and mimic syndromes will be challenging, even when taking batch effects in transcriptome data into account.

In Vivo Translatome Profiling in Spinal Muscular Atrophy Reveals a Role for SMN Protein in Ribosome Biology

Genetic alterations impacting ubiquitously expressed proteins involved in RNA metabolism often result in neurodegenerative conditions, with increasing evidence suggesting that translation defects can contribute to disease. Spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of SMN protein, whose role in pathogenesis remains unclear. Here, we identified in vivo and in vitro translation defects that are cell autonomous and SMN dependent. By determining in parallel the in vivo transcriptome and translatome in SMA mice, we observed a robust decrease in translation efficiency arising during early stages of disease. We provide a catalogue of RNAs with altered translation efficiency, identifying ribosome biology and translation as central processes affected by SMN depletion. This was further supported by a decrease in the number of ribosomes in SMA motor neurons in vivo. Overall, our findings suggest ribosome biology as an important, yet largely overlooked, factor in motor neuron degeneration.

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Polysomal profiling reveals translation defects in SMA mice

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Translation defects are SMN dependent and cell autonomous

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Translation efficiency alterations highlight defects in ribosome biology

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The number of axonal ribosomes is decreased in SMA in vivo

Genetics of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating, uniformly lethal degenerative disorder of motor neurons that overlaps clinically with frontotemporal dementia (FTD). Investigations of the 10% of ALS cases that are transmitted as dominant traits have revealed numerous gene mutations and variants that either cause these disorders or influence their clinical phenotype. The evolving understanding of the genetic architecture of ALS has illuminated broad themes in the molecular pathophysiology of both familial and sporadic ALS and FTD. These central themes encompass disturbances of protein homeostasis, alterations in the biology of RNA binding proteins, and defects in cytoskeletal dynamics, as well as numerous downstream pathophysiological events. Together, these findings from ALS genetics provide new insight into therapies that target genetically distinct subsets of ALS and FTD.

Neurochondrin interacts with the SMN protein suggesting a novel mechanism for spinal muscular atrophy pathology

Spinal muscular atrophy (SMA) is an inherited neurodegenerative condition caused by a reduction in the amount of functional survival motor neuron (SMN) protein. SMN has been implicated in transport of mRNA in neural cells for local translation. We previously identified microtubule-dependent mobile vesicles rich in SMN and SNRPB, a member of the Sm family of small nuclear ribonucleoprotein (snRNP)-associated proteins, in neural cells. By comparing the interactomes of SNRPB and SNRPN, a neural-specific Sm protein, we now show that the essential neural protein neurochondrin (NCDN) interacts with Sm proteins and SMN in the context of mobile vesicles in neurites. NCDN has roles in protein localisation in neural cells and in maintenance of cell polarity. NCDN is required for the correct localisation of SMN, suggesting they may both be required for formation and transport of trafficking vesicles. NCDN may have potential as a therapeutic target for SMA together with, or in place of the targeting of SMN expression.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The essential neural protein neurochondrin interacts with the spinal muscular atrophy (SMA) protein SMN in cell lines and in mice. This might be relevant to the molecular pathology of SMA.

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.

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.

The emerging role of minor intron splicing in neurological disorders

Pre-mRNA splicing is an essential step in eukaryotic gene expression. Mutations in cis-acting sequence elements within pre-mRNA molecules or trans-acting factors involved in pre-mRNA processing have both been linked to splicing dysfunction that give rise to a large number of human diseases. These mutations typically affect the major splicing pathway, which excises more than 99% of all introns in humans. However, approximately 700-800 human introns feature divergent intron consensus sequences at their 5' and 3' ends and are recognized by a separate pre-mRNA processing machinery denoted as the minor spliceosome. This spliceosome has been studied less than its major counterpart, but has received increasing attention during the last few years as a novel pathomechanistic player on the stage in neurodevelopmental and neurodegenerative diseases. Here, we review the current knowledge on minor spliceosome function and discuss its potential pathomechanistic role and impact in neurodegeneration.

Advances in therapy for spinal muscular atrophy: promises and challenges

Spinal muscular atrophy (SMA) is a devastating motor neuron disease that predominantly affects children and represents the most common cause of hereditary infant mortality. The condition results from deleterious variants in SMN1, which lead to depletion of the survival motor neuron protein (SMN). Now, 20 years after the discovery of this genetic defect, a major milestone in SMA and motor neuron disease research has been reached with the approval of the first disease-modifying therapy for SMA by US and European authorities - the antisense oligonucleotide nusinersen. At the same time, promising data from early-stage clinical trials of SMN1 gene therapy have indicated that additional therapeutic options are likely to emerge for patients with SMA in the near future. However, the approval of nusinersen has generated a number of immediate and substantial medical, ethical and financial implications that have the potential to resonate beyond the specific treatment of SMA. Here, we provide an overview of the rapidly evolving therapeutic landscape for SMA, highlighting current achievements and future opportunities. We also discuss how these developments are providing important lessons for the emerging second generation of combinatorial ('SMN-plus') therapies that are likely to be required to generate robust treatments that are effective across a patient's lifespan.

Emerging understanding of the genotype-phenotype relationship in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, noncurable neurodegenerative disorder of the upper and lower motor neurons causing weakness and death within a few years of symptom onset. About 10% of patients with ALS have a family history of the disease; however, ALS-associated genetic mutations are also found in sporadic cases. There are over 100 ALS-associated mutations, and importantly, several genetic mutations, including C9ORF72, SOD1, and TARDBP, have led to mechanistic insight into this complex disease. In the clinical realm, knowledge of ALS genetics can also help explain phenotypic heterogeneity, aid in genetic counseling, and in the future may help direct treatment efforts.

Motor Neuron Gene Therapy: Lessons from Spinal Muscular Atrophy for Amyotrophic Lateral Sclerosis

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are severe nervous system diseases characterized by the degeneration of lower motor neurons. They share a number of additional pathological, cellular, and genetic parallels suggesting that mechanistic and clinical insights into one disorder may have value for the other. While there are currently no clinical ALS gene therapies, the splice-switching antisense oligonucleotide, nusinersen, was recently approved for SMA. This milestone was achieved through extensive pre-clinical research and patient trials, which together have spawned fundamental insights into motor neuron gene therapy. We have thus tried to distil key information garnered from SMA research, in the hope that it may stimulate a more directed approach to ALS gene therapy. Not only must the type of therapeutic (e.g., antisense oligonucleotide vs. viral vector) be sensibly selected, but considerable thought must be applied to the where, which, what, and when in order to enhance treatment benefit: to where (cell types and tissues) must the drug be delivered and how can this be best achieved? Which perturbed pathways must be corrected and can they be concurrently targeted? What dosing regime and concentration should be used? When should medication be administered? These questions are intuitive, but central to identifying and optimizing a successful gene therapy. Providing definitive solutions to these quandaries will be difficult, but clear thinking about therapeutic testing is necessary if we are to have the best chance of developing viable ALS gene therapies and improving upon early generation SMA treatments.

Pathogenic commonalities between spinal muscular atrophy and amyotrophic lateral sclerosis: Converging roads to therapeutic development

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are the two most common motoneuron disorders, which share typical pathological hallmarks while remaining genetically distinct. Indeed, SMA is caused by deletions or mutations in the survival motor neuron 1 (SMN1) gene whilst ALS, albeit being mostly sporadic, can also be caused by mutations within genes, including superoxide dismutase 1 (SOD1), Fused in Sarcoma (FUS), TAR DNA-binding protein 43 (TDP-43) and chromosome 9 open reading frame 72 (C9ORF72). However, it has come to light that these two diseases may be more interlinked than previously thought. Indeed, it has recently been found that FUS directly interacts with an Smn-containing complex, mutant SOD1 perturbs Smn localization, Smn depletion aggravates disease progression of ALS mice, overexpression of SMN in ALS mice significantly improves their phenotype and lifespan, and duplications of SMN1 have been linked to sporadic ALS. Beyond genetic interactions, accumulating evidence further suggests that both diseases share common pathological identities such as intrinsic muscle defects, neuroinflammation, immune organ dysfunction, metabolic perturbations, defects in neuron excitability and selective motoneuron vulnerability. Identifying common molecular effectors that mediate shared pathologies in SMA and ALS would allow for the development of therapeutic strategies and targeted gene therapies that could potentially alleviate symptoms and be equally beneficial in both disorders. In the present review, we will examine our current knowledge of pathogenic commonalities between SMA and ALS, and discuss how furthering this understanding can lead to the establishment of novel therapeutic approaches with wide-reaching impact on multiple motoneuron diseases.

Single-Cell Analysis of SMN Reveals Its Broader Role in Neuromuscular Disease

The mechanism underlying selective motor neuron (MN) death remains an essential question in the MN disease field. The MN disease spinal muscular atrophy (SMA) is attributable to reduced levels of the ubiquitous protein SMN. Here, we report that SMN levels are widely variable in MNs within a single genetic background and that this heterogeneity is seen not only in SMA MNs but also in MNs derived from controls and amyotrophic lateral sclerosis (ALS) patients. Furthermore, cells with low SMN are more susceptible to cell death. These findings raise the important clinical implication that some SMN-elevating therapeutics might be effective in MN diseases besides SMA. Supporting this, we found that increasing SMN across all MN populations using an Nedd8-activating enzyme inhibitor promotes survival in both SMA and ALS-derived MNs. Altogether, our work demonstrates that examination of human neurons at the single-cell level can reveal alternative strategies to be explored in the treatment of degenerative diseases.

HDAC6 inhibition reverses axonal transport defects in motor neurons derived from FUS-ALS patients

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disorder due to selective loss of motor neurons (MNs). Mutations in the fused in sarcoma (FUS) gene can cause both juvenile and late onset ALS. We generated and characterized induced pluripotent stem cells (iPSCs) from ALS patients with different FUS mutations, as well as from healthy controls. Patient-derived MNs show typical cytoplasmic FUS pathology, hypoexcitability, as well as progressive axonal transport defects. Axonal transport defects are rescued by CRISPR/Cas9-mediated genetic correction of the FUS mutation in patient-derived iPSCs. Moreover, these defects are reproduced by expressing mutant FUS in human embryonic stem cells (hESCs), whereas knockdown of endogenous FUS has no effect, confirming that these pathological changes are mutant FUS dependent. Pharmacological inhibition as well as genetic silencing of histone deacetylase 6 (HDAC6) increase α-tubulin acetylation, endoplasmic reticulum (ER)–mitochondrial overlay, and restore the axonal transport defects in patient-derived MNs.

Amyotrophic lateral sclerosis (ALS) leads to selective loss of motor neurons. Using motor neurons derived from induced pluripotent stem cells from patients with ALS and FUS mutations, the authors demonstrate that axonal transport deficits that are observed in these cells can be rescued by HDAC6 inhibition.

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.

Functional interaction between FUS and SMN underlies SMA-like splicing changes in wild-type hFUS mice

Several of the identified genetic factors in Amyotrophic Lateral Sclerosis (ALS) point to dysfunction in RNA processing as a major pathogenic mechanism. However, whether a precise RNA pathway is particularly affected remains unknown. Evidence suggests that FUS, that is mutated in familial ALS, and SMN, the causative factor in Spinal Muscular Atrophy (SMA), cooperate to the same molecular pathway, i.e. regulation of alternative splicing, and that disturbances in SMN-regulated functions, either caused by depletion of SMN protein (as in the case of SMA) or by pathogenic interactions between FUS and SMN (as in the case of ALS) might be a common theme in both diseases. In this work, we followed these leads and tested their pathogenic relevance in vivo. FUS-associated ALS recapitulates, in transgenic mice, crucial molecular features that characterise mouse models of SMA, including defects in snRNPs distribution and in the alternative splicing of genes important for motor neurons. Notably, altering SMN levels by haploinsufficiency or overexpression does not impact the phenotypes of mouse or Drosophila models of FUS-mediated toxicity. Overall, these findings suggest that FUS and SMN functionally interact and that FUS may act downstream of SMN-regulated snRNP assembly in the regulation of alternative splicing and gene expression.

The Actin Cytoskeleton in SMA and ALS: How Does It Contribute to Motoneuron Degeneration?

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are neurodegenerative diseases with overlapping clinical phenotypes based on impaired motoneuron function. However, the pathomechanisms of both diseases are largely unknown, and it is still unclear whether they converge on the molecular level. SMA is a monogenic disease caused by low levels of functional Survival of Motoneuron (SMN) protein, whereas ALS involves multiple genes as well as environmental factors. Recent evidence argues for involvement of actin regulation as a causative and dysregulated process in both diseases. ALS-causing mutations in the actin-binding protein profilin-1 as well as the ability of the SMN protein to directly bind to profilins argue in favor of a common molecular mechanism involving the actin cytoskeleton. Profilins are major regulat ors of actin-dynamics being involved in multiple neuronal motility and transport processes as well as modulation of synaptic functions that are impaired in models of both motoneuron diseases. In this article, we review the current literature in SMA and ALS research with a focus on the actin cytoskeleton. We propose a common molecular mechanism that explains the degeneration of motoneurons for SMA and some cases of ALS.

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.

Bioenergetic status modulates motor neuron vulnerability and pathogenesis in a zebrafish model of spinal muscular atrophy

Degeneration and loss of lower motor neurons is the major pathological hallmark of spinal muscular atrophy (SMA), resulting from low levels of ubiquitously-expressed survival motor neuron (SMN) protein. One remarkable, yet unresolved, feature of SMA is that not all motor neurons are equally affected, with some populations displaying a robust resistance to the disease. Here, we demonstrate that selective vulnerability of distinct motor neuron pools arises from fundamental modifications to their basal molecular profiles. Comparative gene expression profiling of motor neurons innervating the extensor digitorum longus (disease-resistant), gastrocnemius (intermediate vulnerability), and tibialis anterior (vulnerable) muscles in mice revealed that disease susceptibility correlates strongly with a modified bioenergetic profile. Targeting of identified bioenergetic pathways by enhancing mitochondrial biogenesis rescued motor axon defects in SMA zebrafish. Moreover, targeting of a single bioenergetic protein, phosphoglycerate kinase 1 (Pgk1), was found to modulate motor neuron vulnerability in vivo. Knockdown of pgk1 alone was sufficient to partially mimic the SMA phenotype in wild-type zebrafish. Conversely, Pgk1 overexpression, or treatment with terazosin (an FDA-approved small molecule that binds and activates Pgk1), rescued motor axon phenotypes in SMA zebrafish. We conclude that global bioenergetics pathways can be therapeutically manipulated to ameliorate SMA motor neuron phenotypes in vivo.

FUS affects circular RNA expression in murine embryonic stem cell-derived motor neurons

The RNA-binding protein FUS participates in several RNA biosynthetic processes and has been linked to the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Here we report that FUS controls back-splicing reactions leading to circular RNA (circRNA) production. We identified circRNAs expressed in in vitro-derived mouse motor neurons (MNs) and determined that the production of a considerable number of these circRNAs is regulated by FUS. Using RNAi and overexpression of wild-type and ALS-associated FUS mutants, we directly correlate the modulation of circRNA biogenesis with alteration of FUS nuclear levels and with putative toxic gain of function activities. We also demonstrate that FUS regulates circRNA biogenesis by binding the introns flanking the back-splicing junctions and that this control can be reproduced with artificial constructs. Most circRNAs are conserved in humans and specific ones are deregulated in human-induced pluripotent stem cell-derived MNs carrying the FUS^P525L mutation associated with ALS.

The RNA binding protein FUS functions in several RNA biosynthetic processes and has been linked to the pathogenesis of amyotrophic lateral sclerosis (ALS). Here the authors show that FUS controls back-splicing reactions leading to circular RNA (circRNA) production in stem cell-derived motor neurons and that ALS-associated FUS mutations affect the biogenesis of circRNAs.

ALS-linked FUS exerts a gain of toxic function involving aberrant p38 MAPK activation

Mutations in Fused in Sarcoma/Translocated in Liposarcoma (FUS) cause familial forms of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by progressive axonal degeneration mainly affecting motor neurons. Evidence from transgenic mouse models suggests mutant forms of FUS exert an unknown gain-of-toxic function in motor neurons, but mechanisms underlying this effect remain unknown. Towards this end, we studied the effect of wild type FUS (FUS WT) and three ALS-linked variants (G230C, R521G and R495X) on fast axonal transport (FAT), a cellular process critical for appropriate maintenance of axonal connectivity. All ALS-FUS variants impaired anterograde and retrograde FAT in squid axoplasm, whereas FUS WT had no effect. Misfolding of mutant FUS is implicated in this process, as the molecular chaperone Hsp110 mitigated these toxic effects. Interestingly, mutant FUS-induced impairment of FAT in squid axoplasm and of axonal outgrowth in mammalian primary motor neurons involved aberrant activation of the p38 MAPK pathway, as also reported for ALS-linked forms of Cu, Zn superoxide dismutase (SOD1). Accordingly, increased levels of active p38 MAPK were detected in post-mortem human ALS-FUS brain tissues. These data provide evidence for a novel gain-of-toxic function for ALS-linked FUS involving p38 MAPK activation.

Linking amyotrophic lateral sclerosis and spinal muscular atrophy through RNA-transcriptome homeostasis: a genomics perspective

In this review, we present our most recent understanding of key biomolecular processes that underlie two motor neuron degenerative disorders, amyotrophic lateral sclerosis, and spinal muscular atrophy. We focus on the role of four multifunctional proteins involved in RNA metabolism (TDP-43, FUS, SMN, and Senataxin) that play a causal role in these diseases. Recent results have led to a novel scenario of intricate connections between these four proteins, bringing transcriptome homeostasis into the spotlight as a common theme in motor neuron degeneration. We review reported functional and physical interactions between these four proteins, highlighting their common association with nuclear bodies and small nuclear ribonucleoprotein particle biogenesis and function. We discuss how these interactions are turning out to be particularly relevant for the control of transcription and chromatin homeostasis, including the recent identification of an association between SMN and Senataxin required to ensure the resolution of DNA-RNA hybrid formation and proper termination by RNA polymerase II. These connections strongly support the existence of common pathways underlying the spinal muscular atrophy and amyotrophic lateral sclerosis phenotype. We also discuss the potential of genome-wide expression profiling, in particular RNA sequencing derived data, to contribute to unravelling the underlying mechanisms. We provide a review of publicly available datasets that have addressed both diseases using these approaches, and highlight the value of investing in cross-disease studies to promote our understanding of the pathways leading to neurodegeneration.

Pharmacological intervention of early neuropathy in neurodegenerative diseases

Extensive studies have reported the significant roles of numerous cellular features and processes in properly maintaining neuronal morphology and function throughout the lifespan of an animal. Any alterations in their homeostasis appear to be strongly associated with neuronal aging and the pathogenesis of various neurodegenerative diseases, even before the occurrence of prominent neuronal death. However, until recently, the primary focus of studies regarding many neurodegenerative diseases has been on the massive cell death occurring at the late stages of disease progression. Thus, our understanding on early neuropathy in these diseases remains relatively limited. The complicated nature of various neuropathic features manifested early in neurodegenerative diseases suggests the involvement of a system-wide transcriptional regulation and epigenetic control. Epigenetic alterations and consequent changes in the neuronal transcriptome are now begun to be extensively studied in various neurodegenerative diseases. Upon the catastrophic incident of neuronal death in disease progression, it is utterly difficult to reverse the deleterious defects by pharmacological treatments, and therefore, therapeutics targeting the system-wide transcriptional dysregulation associated with specific early neuropathy is considered a better option. Here, we review our current understanding on the system-wide transcriptional dysregulation that is likely associated with early neuropathy shown in various neurodegenerative diseases and discuss the possible future developments of pharmaceutical therapeutics.

Copy Number Variations in Amyotrophic Lateral Sclerosis: Piecing the Mosaic Tiles Together through a Systems Biology Approach

Amyotrophic lateral sclerosis (ALS) is a devastating and still untreatable motor neuron disease. Despite the molecular mechanisms underlying ALS pathogenesis that are still far from being understood, several studies have suggested the importance of a genetic contribution in both familial and sporadic forms of the disease. In addition to single-nucleotide polymorphisms (SNPs), which account for only a limited number of ALS cases, a consistent number of common and rare copy number variations (CNVs) have been associated to ALS. Most of the CNV-based association studies use a traditional candidate-gene approach that is inadequate for uncovering the genetic architectures of complex traits like ALS. The emergent paradigm of “systems biology” may offer a new perspective to better interpret the wide spectrum of CNVs in ALS, enabling the characterization of the complex network of gene products underlying ALS pathogenesis. In this review, we will explore the landscape of CNVs in ALS, putting specific emphasis on the functional impact of common CNV regions and genes consistently associated with increased risk of developing disease. In addition, we will discuss the potential contribution of multiple rare CNVs in ALS pathogenesis, focusing our attention on the complex mechanisms by which these proteins might impact, individually or in combination, the genetic susceptibility of ALS. The comprehensive detection and functional characterization of common and rare candidate risk CNVs in ALS susceptibility may bring new pieces into the intricate mosaic of ALS pathogenesis, providing interesting and important implications for a more precise molecular biomarker-assisted diagnosis and more effective and personalized treatments.

Sequestration of PRMT1 and Nd1-L mRNA into ALS-linked FUS mutant R521C-positive aggregates contributes to neurite degeneration upon oxidative stress

Mutations in fused in sarcoma (FUS), a DNA/RNA binding protein, are associated with familial amyotrophic lateral sclerosis (ALS). However, little is known about how ALS-causing mutations alter protein-protein and protein-RNA complexes and contribute to neurodegeneration. In this study, we identified protein arginine methyltransferase 1 (PRMT1) as a protein that more avidly associates with ALS-linked FUS-R521C than with FUS-WT (wild type) or FUS-P525L using co-immunoprecipitation and LC-MS analysis. Abnormal association between FUS-R521C and PRMT1 requires RNA, but not methyltransferase activity. PRMT1 was sequestered into cytosolic FUS-R521C-positive stress granule aggregates. Overexpression of PRMT1 rescued neurite degeneration caused by FUS-R521C upon oxidative stress, while loss of PRMT1 further accumulated FUS-positive aggregates and enhanced neurite degeneration. Furthermore, the mRNA of Nd1-L, an actin-stabilizing protein, was sequestered into the FUS-R521C/PRMT1 complex. Nd1-L overexpression rescued neurite shortening caused by FUS-R521C upon oxidative stress, while loss of Nd1-L further exacerbated neurite shortening. Altogether, these data suggest that the abnormal stable complex of FUS-R521C/PRMT1/Nd1-L mRNA could contribute to neurodegeneration upon oxidative stress. Overall, our study provides a novel pathogenic mechanism of the FUS mutation associated with abnormal protein-RNA complexes upon oxidative stress in ALS and provides insight into possible therapeutic targets for this pathology.

Comparative interactomics analysis of different ALS-associated proteins identifies converging molecular pathways

Amyotrophic lateral sclerosis (ALS) is a devastating neurological disease with no effective treatment available. An increasing number of genetic causes of ALS are being identified, but how these genetic defects lead to motor neuron degeneration and to which extent they affect common cellular pathways remains incompletely understood. To address these questions, we performed an interactomic analysis to identify binding partners of wild-type (WT) and ALS-associated mutant versions of ATXN2, C9orf72, FUS, OPTN, TDP-43 and UBQLN2 in neuronal cells. This analysis identified several known but also many novel binding partners of these proteins. Interactomes of WT and mutant ALS proteins were very similar except for OPTN and UBQLN2, in which mutations caused loss or gain of protein interactions. Several of the identified interactomes showed a high degree of overlap: shared binding partners of ATXN2, FUS and TDP-43 had roles in RNA metabolism; OPTN- and UBQLN2-interacting proteins were related to protein degradation and protein transport, and C9orf72 interactors function in mitochondria. To confirm that this overlap is important for ALS pathogenesis, we studied fragile X mental retardation protein (FMRP), one of the common interactors of ATXN2, FUS and TDP-43, in more detail in in vitro and in vivo model systems for FUS ALS. FMRP localized to mutant FUS-containing aggregates in spinal motor neurons and bound endogenous FUS in a direct and RNA-sensitive manner. Furthermore, defects in synaptic FMRP mRNA target expression, neuromuscular junction integrity, and motor behavior caused by mutant FUS in zebrafish embryos, could be rescued by exogenous FMRP expression. Together, these results show that interactomics analysis can provide crucial insight into ALS disease mechanisms and they link FMRP to motor neuron dysfunction caused by FUS mutations.

Mislocated FUS is sufficient for gain-of-toxic-function amyotrophic lateral sclerosis phenotypes in mice

Mutations in RNA-binding proteins, including fused in sarcoma (FUS) and TAR DNA-binding protein 43 (TDP-43, encoded by TARDBP), are associated with sporadic and familial amyotrophic lateral sclerosis. A major question is whether neuronal loss is caused by toxic gain-of-function cytoplasmic aggregates or loss of nuclear RNA-binding protein function. We generated a transgenic mouse overexpressing exogenous FUS without a nuclear localization signal (ΔNLS-FUS), which developed progressive spastic motor deficits and neuronal loss in the motor cortex. The ΔNLS-FUS protein was restricted to the cytoplasm and formed ubiquitin/p62-positive aggregates. Endogenous FUS expression, nuclear localization, and splicing activity were not altered, indicating that mislocated FUS is sufficient for proteinopathy. Crossing ΔNLS-FUS with wild-type human TDP-43 transgenic mice exacerbated pathological and behavioural phenotypes, suggesting that both proteins are involved in a common cascade. RNA-sequence analysis revealed specific transcriptome alterations, including genes regulating dynein-associated molecules and endoplasmic reticulum stress. ΔNLS-FUS mice are promising tools for understanding amyotrophic lateral sclerosis pathogenesis and testing new therapeutic approaches.

Altered mRNP granule dynamics in FTLD pathogenesis

In neurons, RNA-binding proteins (RBPs) play a key role in post-transcriptional gene regulation, for example alternative splicing, mRNA localization in neurites and local translation upon synaptic stimulation. There is increasing evidence that defective or mislocalized RBPs - and consequently altered mRNA processing - lead to neuronal dysfunction and cause neurodegeneration, including frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Cytosolic RBP aggregates containing TAR DNA-binding protein of 43 kDa (TDP-43) or fused in sarcoma (FUS) are a common hallmark of both disorders. There is mounting evidence that translationally silent mRNP granules, such as stress granules or transport granules, play an important role in the formation of these RBP aggregates. These granules are thought to be 'catalytic convertors' of RBP aggregation by providing a high local concentration of RBPs. As recently shown in vitro, RBPs that contain a so-called low-complexity domain start to 'solidify' and eventually aggregate at high protein concentrations. The same may happen in mRNP granules in vivo, leading to 'solidified' granules that lose their dynamic properties and ability to fulfill their physiological functions. This may result in a disturbed stress response, altered mRNA transport and local translation, and formation of pathological TDP-43 or FUS aggregates, all of which may contribute to neuronal dysfunction and neurodegeneration. Here, we discuss the general functional properties of these mRNP granules, how their dynamics may be disrupted in frontotemporal lobar degeneration/amyotrophic lateral sclerosis, for example by loss or gain of function of TDP-43 and FUS, and how this may contribute to the development of RBP aggregates and neurotoxicity. In this review, we discuss how dynamic mRNP granules, such as stress granules or neuronal transport granules, may be converted into pathological aggregates containing misfolded RNA-binding proteins (RBPs), such as TDP-43 and FUS. Abnormal interactions between low-complexity domains in RBPs may cause dynamic mRNP granules to solidify and become dysfunctional. This may result in a disturbed stress response, altered mRNA transport and local translation, as well as RBP aggregation, all of which may contribute to neuronal dysfunction and neurodegeneration.

Toxic gain of function from mutant FUS protein is crucial to trigger cell autonomous motor neuron loss

FUS is an RNA-binding protein involved in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Cytoplasmic FUS-containing aggregates are often associated with concomitant loss of nuclear FUS Whether loss of nuclear FUS function, gain of a cytoplasmic function, or a combination of both lead to neurodegeneration remains elusive. To address this question, we generated knockin mice expressing mislocalized cytoplasmic FUS and complete FUS knockout mice. Both mouse models display similar perinatal lethality with respiratory insufficiency, reduced body weight and length, and largely similar alterations in gene expression and mRNA splicing patterns, indicating that mislocalized FUS results in loss of its normal function. However, FUS knockin mice, but not FUS knockout mice, display reduced motor neuron numbers at birth, associated with enhanced motor neuron apoptosis, which can be rescued by cell-specific CRE-mediated expression of wild-type FUS within motor neurons. Together, our findings indicate that cytoplasmic FUS mislocalization not only leads to nuclear loss of function, but also triggers motor neuron death through a toxic gain of function within motor neurons.

A role for the survival of motor neuron protein in mRNP assembly and transport

Localization and local translation of mRNA plays a key role in neuronal development and function. While studies in various systems have provided insights into molecular mechanisms of mRNA transport and local protein synthesis, the factors that control the assembly of mRNAs and mRNA binding proteins into messenger ribonucleoprotein (mRNP) transport granules remain largely unknown. In this review we will discuss how insights on a motor neuron disease, spinal muscular atrophy (SMA), is advancing our understanding of regulated assembly of transport competent mRNPs and how defects in their assembly and delivery may contribute to the degeneration of motor neurons observed in SMA and other neurological disorders.

Dysregulated axonal RNA translation in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult‐onset motor neuron disease that has been associated with a diverse array of genetic changes. Prominent among these are mutations in RNA‐binding proteins (RBPs) or repeat expansions that give rise to toxic RNA species. RBPs are additionally central components of pathologic aggregates that constitute a disease hallmark, suggesting that dysregulation of RNA metabolism underlies disease progression. In the context of neuronal physiology, transport of RNAs and localized RNA translation in axons are fundamental to neuronal survival and function. Several lines of evidence suggest that axonal RNA translation is a central process perturbed by various pathogenic events associated with ALS. Dysregulated translation of specific RNA groups could underlie feedback effects that connect and reinforce disease manifestations. Among such candidates are RNAs encoding proteins involved in the regulation of microtubule dynamics. Further understanding of axonally dysregulated RNA targets and of the feedback mechanisms they induce could provide useful therapeutic insights. WIREs RNA 2016, 7:589–603. doi: 10.1002/wrna.1352

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Mechanisms of FUS mutations in familial amyotrophic lateral sclerosis

Recent advances in the genetics of amyotrophic lateral sclerosis (ALS) have provided key mechanistic insights to the pathogenesis of this devastating neurodegenerative disease. Among many etiologies for ALS, the identification of mutations and proteinopathies in two RNA binding proteins, TDP-43 (TARDBP or TAR DNA binding protein 43) and its closely related RNA/DNA binding protein FUS (fused in sarcoma), raises the intriguing possibility that perturbations to the RNA homeostasis and metabolism in neurons may contribute to the pathogenesis of these diseases. Although the similarities between TDP-43 and FUS suggest that mutations and proteinopathy involving these two proteins may converge on the same mechanisms leading to neurodegeneration, there is increasing evidence that FUS mutations target distinct mechanisms to cause early disease onset and aggressive progression of disease. This review focuses on the recent advances on the molecular, cellular and genetic approaches to uncover the mechanisms of wild type and mutant FUS proteins during development and in neurodegeneration. These findings provide important insights to understand how FUS mutations may perturb the maintenance of dendrites through fundamental processes in RNA splicing, RNA transport and DNA damage response/repair. These results contribute to the understanding of phenotypic manifestations in neurodegeneration related to FUS mutations, and to identify important directions for future investigations. This article is part of a Special Issue entitled SI:RNA Metabolism in Disease.

Copy Number Variations in the Survival Motor Neuron Genes: Implications for Spinal Muscular Atrophy and Other Neurodegenerative Diseases

Proximal spinal muscular atrophy (SMA), a leading genetic cause of infant death worldwide, is an early-onset, autosomal recessive neurodegenerative disease characterized by the loss of spinal α-motor neurons. This loss of α-motor neurons is associated with muscle weakness and atrophy. SMA can be classified into five clinical grades based on age of onset and severity of the disease. Regardless of clinical grade, proximal SMA results from the loss or mutation of SMN1 (survival motor neuron 1) on chromosome 5q13. In humans a large tandem chromosomal duplication has lead to a second copy of the SMN gene locus known as SMN2. SMN2 is distinguishable from SMN1 by a single nucleotide difference that disrupts an exonic splice enhancer in exon 7. As a result, most of SMN2 mRNAs lack exon 7 (SMNΔ7) and produce a protein that is both unstable and less than fully functional. Although only 10–20% of the SMN2 gene product is fully functional, increased genomic copies of SMN2 inversely correlates with disease severity among individuals with SMA. Because SMN2 copy number influences disease severity in SMA, there is prognostic value in accurate measurement of SMN2 copy number from patients being evaluated for SMA. This prognostic value is especially important given that SMN2 copy number is now being used as an inclusion criterion for SMA clinical trials. In addition to SMA, copy number variations (CNVs) in the SMN genes can affect the clinical severity of other neurological disorders including amyotrophic lateral sclerosis (ALS) and progressive muscular atrophy (PMA). This review will discuss how SMN1 and SMN2 CNVs are detected and why accurate measurement of SMN1 and SMN2 copy numbers is relevant for SMA and other neurodegenerative diseases.

ALS-associated mutant FUS induces selective motor neuron degeneration through toxic gain of function

Mutations in FUS cause amyotrophic lateral sclerosis (ALS), including some of the most aggressive, juvenile-onset forms of the disease. FUS loss-of-function and toxic gain-of-function mechanisms have been proposed to explain how mutant FUS leads to motor neuron degeneration, but neither has been firmly established in the pathogenesis of ALS. Here we characterize a series of transgenic FUS mouse lines that manifest progressive, mutant-dependent motor neuron degeneration preceded by early, structural and functional abnormalities at the neuromuscular junction. A novel, conditional FUS knockout mutant reveals that postnatal elimination of FUS has no effect on motor neuron survival or function. Moreover, endogenous FUS does not contribute to the onset of the ALS phenotype induced by mutant FUS. These findings demonstrate that FUS-dependent motor degeneration is not due to loss of FUS function, but to the gain of toxic properties conferred by ALS mutations.

The mechanism by which FUS mutations cause familial ALS remains unclear. Here, the authors use mouse transgenic models to show that a toxic gain-of-function underlies motor neuron degeneration, and that the toxicity of mutant FUS does not depend on a loss or excess of FUS activity.

Lrig2 Negatively Regulates Ectodomain Shedding of Axon Guidance Receptors by ADAM Proteases

Many guidance receptors are proteolytically cleaved by membrane-associated metalloproteases of the ADAM family, leading to the shedding of their ectodomains. Ectodomain shedding is crucial for receptor signaling and function, but how this process is controlled in neurons remains poorly understood. Here, we show that the transmembrane protein Lrig2 negatively regulates ADAM-mediated guidance receptor proteolysis in neurons. Lrig2 binds Neogenin, a receptor for repulsive guidance molecules (RGMs), and prevents premature Neogenin shedding by ADAM17 (TACE). RGMa reduces Lrig2-Neogenin interactions, providing ADAM17 access to Neogenin and allowing this protease to induce ectodomain shedding. Regulation of ADAM17-mediated Neogenin cleavage by Lrig2 is required for neurite growth inhibition by RGMa in vitro and for cortical neuron migration in vivo. Furthermore, knockdown of Lrig2 significantly improves CNS axon regeneration. Together, our data identify a unique ligand-gated mechanism to control receptor shedding by ADAMs and reveal functions for Lrigs in neuron migration and regenerative failure.

The pathogenicity of splicing defects: mechanistic insights into pre-mRNA processing inform novel therapeutic approaches

Removal of introns from pre-mRNA precursors (pre-mRNA splicing) is a necessary step for the expression of most genes in multicellular organisms, and alternative patterns of intron removal diversify and regulate the output of genomic information. Mutation or natural variation in pre-mRNA sequences, as well as in spliceosomal components and regulatory factors, has been implicated in the etiology and progression of numerous pathologies. These range from monogenic to multifactorial genetic diseases, including metabolic syndromes, muscular dystrophies, neurodegenerative and cardiovascular diseases, and cancer. Understanding the molecular mechanisms associated with splicing-related pathologies can provide key insights into the normal function and physiological context of the complex splicing machinery and establish sound basis for novel therapeutic approaches.

Transcriptional profiling of differentially vulnerable motor neurons at pre-symptomatic stage in the Smn (2b/-) mouse model of spinal muscular atrophy

INTRODUCTION

The term motor neuron disease encompasses a spectrum of disorders in which motor neurons are the lost. Importantly, while some motor neurons are lost early in disease and others remain intact at disease end-stage. This creates a valuable experimental paradigm to investigate the factors that regulate motor neuron vulnerability. Spinal muscular atrophy is a childhood motor neuron disease caused by mutations or deletions in the SMN1 gene. Here, we have performed transcriptional analysis on differentially vulnerable motor neurons from an intermediate mouse model of Spinal muscular atrophy at a presymptomatic time point.

RESULTS

We have characterised two differentially vulnerable populations, differing in the level neuromuscular junction loss. Transcriptional analysis on motor neuron cell bodies revealed that reduced Smn levels correlate with a reduction of transcripts associated with the ribosome, rRNA binding, ubiquitination and oxidative phosphorylation. Furthermore, P53 pathway activation precedes neuromuscular junction loss, suggesting that denervation may be a consequence, rather than a cause of motor neuron death in Spinal muscular atrophy. Finally, increased vulnerability correlates with a decrease in the positive regulation of DNA repair.

CONCLUSIONS

This study identifies pathways related to the function of Smn and associated with differential motor unit vulnerability, thus presenting a number of exciting targets for future therapeutic development.

Developing therapies for spinal muscular atrophy

Spinal muscular atrophy is an autosomal-recessive pediatric neurodegenerative disease characterized by loss of spinal motor neurons. It is caused by mutation in the gene survival of motor neuron 1 (SMN1), leading to loss of function of the full-length SMN protein. SMN has a number of functions in neurons, including RNA splicing and snRNP biogenesis in the nucleus, and RNA trafficking in neurites. The expression level of full-length SMN protein from the SMN2 locus modifies disease severity. Increasing full-length SMN protein by a small amount can lead to significant improvements in the neurological phenotype. Currently available interventions for spinal muscular atrophy patients are physical therapy and orthopedic, nutritional, and pulmonary interventions; these are palliative or supportive measures and do not address the etiology of the disease. In the past decade, there has been a push for developing therapeutics to improve motor phenotypes and increase life span of spinal muscular atrophy patients. These therapies are aimed primarily at restoration of full-length SMN protein levels, but other neuroprotective treatments have been investigated as well. Here, we discuss recent advances in basic and clinical studies toward finding safe and effective treatments of spinal muscular atrophy using gene therapy, antisense oligonucleotides, and other small molecule modulators of SMN expression.

C9orf72 ablation in mice does not cause motor neuron degeneration or motor deficits

Objective

How hexanucleotide (GGGGCC) repeat expansions in C9ORF72 cause amyotrophic lateral sclerosis (ALS) remains poorly understood. Both gain‐ and loss‐of‐function mechanisms have been proposed. Evidence supporting these mechanisms in vivo is, however, incomplete. Here we determined the effect of C9orf72 loss‐of‐function in mice.

Methods

We generated and analyzed a conditional C9orf72 knockout mouse model. C9orf72^fl/fl mice were crossed with Nestin‐Cre mice to selectively remove C9orf72 from neurons and glial cells. Immunohistochemistry was performed to study motor neurons and neuromuscular integrity, as well as several pathological hallmarks of ALS, such as gliosis and TDP‐43 mislocalization. In addition, motor function and survival were assessed.

Results

Neural‐specific ablation of C9orf72 in conditional C9orf72 knockout mice resulted in significantly reduced body weight but did not induce motor neuron degeneration, defects in motor function, or altered survival.

Interpretation

Our data suggest that C9orf72 loss‐of‐function, by itself, is insufficient to cause motor neuron disease. These results may have important implications for the development of therapeutic strategies for C9orf72‐associated ALS. Ann Neurol 2015;78:426–438

Amyotrophic lateral sclerosis: mechanisms and therapeutics in the epigenomic era

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease of the motor neurons, which results in weakness and atrophy of voluntary skeletal muscles. Treatments do not modify the disease trajectory effectively, and only modestly improve survival. A complex interaction between genes, environmental exposure and impaired molecular pathways contributes to pathology in patients with ALS. Epigenetic mechanisms control the hereditary and reversible regulation of gene expression without altering the basic genetic code. Aberrant epigenetic patterns-including abnormal microRNA (miRNA) biogenesis and function, DNA modifications, histone remodeling, and RNA editing-are acquired throughout life and are influenced by environmental factors. Thus, understanding the molecular processes that lead to epigenetic dysregulation in patients with ALS might facilitate the discovery of novel therapeutic targets and biomarkers that could reduce diagnostic delay. These achievements could prove crucial for successful disease modification in patients with ALS. We review the latest findings regarding the role of miRNA modifications and other epigenetic mechanisms in ALS, and discuss their potential as therapeutic targets.

U1 snRNP is mislocalized in ALS patient fibroblasts bearing NLS mutations in FUS and is required for motor neuron outgrowth in zebrafish

Mutations in FUS cause amyotrophic lateral sclerosis (ALS), but the molecular pathways leading to neurodegeneration remain obscure. We previously found that U1 snRNP is the most abundant FUS interactor. Here, we report that components of the U1 snRNP core particle (Sm proteins and U1 snRNA), but not the mature U1 snRNP-specific proteins (U1-70K, U1A and U1C), co-mislocalize with FUS to the cytoplasm in ALS patient fibroblasts harboring mutations in the FUS nuclear localization signal (NLS). Similar results were obtained in HeLa cells expressing the ALS-causing FUS R495X NLS mutation, and mislocalization of Sm proteins is RRM-dependent. Moreover, as observed with FUS, knockdown of any of the U1 snRNP-specific proteins results in a dramatic loss of SMN-containing Gems. Significantly, knockdown of U1 snRNP in zebrafish results in motor axon truncations, a phenotype also observed with FUS, SMN and TDP-43 knockdowns. Our observations linking U1 snRNP to ALS patient cells with FUS mutations, SMN-containing Gems, and motor neurons indicate that U1 snRNP is a component of a molecular pathway associated with motor neuron disease. Linking an essential canonical splicing factor (U1 snRNP) to this pathway provides strong new evidence that splicing defects may be involved in pathogenesis and that this pathway is a potential therapeutic target.