TARDBP mutations in individuals with sporadic and familial amyotrophic lateral sclerosis

BraInMap Elucidates the Macromolecular Connectivity Landscape of Mammalian Brain

Connectivity webs mediate the unique biology of the mammalian brain. Yet, while cell circuit maps are increasingly available, knowledge of their underlying molecular networks remains limited. Here, we applied multi-dimensional biochemical fractionation with mass spectrometry and machine learning to survey endogenous macromolecules across the adult mouse brain. We defined a global "interactome" comprising over one thousand multi-protein complexes. These include hundreds of brain-selective assemblies that have distinct physical and functional attributes, show regional and cell-type specificity, and have links to core neurological processes and disorders. Using reciprocal pull-downs and a transgenic model, we validated a putative 28-member RNA-binding protein complex associated with amyotrophic lateral sclerosis, suggesting a coordinated function in alternative splicing in disease progression. This brain interaction map (BraInMap) resource facilitates mechanistic exploration of the unique molecular machinery driving core cellular processes of the central nervous system. It is publicly available and can be explored here https://www.bu.edu/dbin/cnsb/mousebrain/.

A novel mutation in the cleavage site N291 of TDP-43 protein in a familial case of amyotrophic lateral sclerosis

Cytoplasmic aggregation of TAR-DNA binding protein (TDP-43) in Amyotrophic Lateral Sclerosis (ALS) and fronto-temporal lobar dementia (FTLD) is associated with post-translational modifications (PTM) and delocalization. Studies on postmortem brains of ALS and FTLD patients showed the existence of TDP-43 fragments that end at position N291. We report a new heterozygous mutation p.N291H in a familial case of ALS. Expression of the mutant protein in cell lines and primary motor neurons induces aggregate formation in the cytoplasm and reduces cell viability. The discovery of mutations at cleavage sites in TDP-43 in patients, which we reviewed here, is valuable for understanding the true role of the various TDP-43 fragments identified in patients and thus, for developing effective targeted therapies for ALS and FTLD treatment.

IN VITRO AND IN VIVO MODELS OF AMYOTROPHIC LATERAL SCLEROSIS: AN UPDATED OVERVIEW

Amyotrophic Lateral Sclerosis (ALS) is a progressive, neurodegenerative disease characterized by loss of upper motor neurons (UMN) and lower motor neurons (LMN). Disease affects people all over the world and is more prevalent in men. Patients with ALS develop extensive muscle wasting, paralysis and ultimately death, with a median survival of usually fewer than five years after disease onset. ALS may be sporadic (sALS, 90%) or familial (fALS, 10%). The large majority of fALS cases are associated with genetic alterations, which are mainly related to the genes SOD1, TDP-43, FUS, and C9ORF72. In vitro and in vivo models have helped elucidate ALS etiology and pathogenesis, as well as its molecular, cellular, and physiological mechanisms. Many studies in cell cultures and animal models, such as Caenorhabditis elegans, Drosophila melanogaster, zebrafish, rodents, and non-human primates have been performed to clarify the relationship of these genes to ALS disease. However, there are inherent limitations to consider when using experimental models. In this review, we provide an updated overview of the most used in vitro and in vivo studies that have contributed to a better understanding of the different ALS pathogenic mechanisms.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

Upregulation of ATG7 attenuates motor neuron dysfunction associated with depletion of TARDBP/TDP-43

A shared neuropathological hallmark in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is nuclear clearance and cytoplasmic aggregation of TARDBP/TDP-43 (TAR DNA binding protein). We previously showed that the ability of TARDBP to repress nonconserved cryptic exons was impaired in brains of patients with ALS and FTD, suggesting that its nuclear depletion contributes to neurodegeneration. However, the critical pathways impacted by the failure to repress cryptic exons that may contribute to neurodegeneration remain undefined. Here, we report that transcriptome analysis of TARDBP-deficient neurons revealed downregulation of ATG7, a critical gene required for macroautophagy/autophagy. Mouse and Drosophila models lacking TARDBP/TBPH in motor neurons exhibiting age-dependent neurodegeneration and motor deficits showed reduction of ATG7 and accumulation of SQSTM1/p62 inclusions. Importantly, genetic upregulation of the autophagy pathway improved motor function and survival in TBPH-deficient flies. Together with our observation that ATG7 is reduced in ALS-FTD brain tissues, these findings identify the autophagy pathway as one key effector of nuclear depletion of TARDBP that contributes to neurodegeneration. We thus suggest that the autophagy pathway is a therapeutic target for ALS-FTD and other disorders exhibiting TARDBP pathology.Abbreviations: ALS: amyotrophic lateral sclerosis; ANOVA: analysis of variance; ChAT: choline acetyltransferase; CTSD: cathepsin D; FTD: frontotemporal dementia; LAMP1: lysosomal associated membrane protein 1; NMJ: neuromuscular junction; RBFOX3/NeuN: RNA binding fox-1 homolog 3; SQSTM1: sequestosome 1; TARDBP/TDP-43: TAR DNA binding protein 43.

Omics Approach to Axonal Dysfunction of Motor Neurons in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is an intractable adult-onset neurodegenerative disease that leads to the loss of upper and lower motor neurons (MNs). The long axons of MNs become damaged during the early stages of ALS. Genetic and pathological analyses of ALS patients have revealed dysfunction in the MN axon homeostasis. However, the molecular pathomechanism for the degeneration of axons in ALS has not been fully elucidated. This review provides an overview of the proposed axonal pathomechanisms in ALS, including those involving the neuronal cytoskeleton, cargo transport within axons, axonal energy supply, clearance of junk protein, neuromuscular junctions (NMJs), and aberrant axonal branching. To improve understanding of the global changes in axons, the review summarizes omics analyses of the axonal compartments of neurons in vitro and in vivo, including a motor nerve organoid approach that utilizes microfluidic devices developed by this research group. The review also discusses the relevance of intra-axonal transcription factors frequently identified in these omics analyses. Local axonal translation and the relationship among these pathomechanisms should be pursued further. The development of novel strategies to analyze axon fractions provides a new approach to establishing a detailed understanding of resilience of long MN and MN pathology in ALS.

Inactivation of Hippo and cJun-N-terminal Kinase (JNK) signaling mitigate FUS mediated neurodegeneration in vivo

Amyotrophic Lateral Sclerosis (ALS), a late-onset neurodegenerative disorder characterized by the loss of motor neurons in the central nervous system, has no known cure to-date. Disease causing mutations in human Fused in Sarcoma (FUS) leads to aggressive and juvenile onset of ALS. FUS is a well-conserved protein across different species, which plays a crucial role in regulating different aspects of RNA metabolism. Targeted misexpression of FUS in Drosophila model recapitulates several interesting phenotypes relevant to ALS including cytoplasmic mislocalization, defects at the neuromuscular junction and motor dysfunction. We screened for the genetic modifiers of human FUS-mediated neurodegenerative phenotype using molecularly defined deficiencies. We identified hippo (hpo), a component of the evolutionarily conserved Hippo growth regulatory pathway, as a genetic modifier of FUS mediated neurodegeneration. Gain-of-function of hpo triggers cell death whereas its loss-of-function promotes cell proliferation. Downregulation of the Hippo signaling pathway, using mutants of Hippo signaling, exhibit rescue of FUS-mediated neurodegeneration in the Drosophila eye, as evident from reduction in the number of TUNEL positive nuclei as well as rescue of axonal targeting from the retina to the brain. The Hippo pathway activates c-Jun amino-terminal (NH₂) Kinase (JNK) mediated cell death. We found that downregulation of JNK signaling is sufficient to rescue FUS-mediated neurodegeneration in the Drosophila eye. Our study elucidates that Hippo signaling and JNK signaling are activated in response to FUS accumulation to induce neurodegeneration. These studies will shed light on the genetic mechanism involved in neurodegeneration observed in ALS and other associated disorders.

Role of RNA Binding Proteins with prion-like domains in muscle and neuromuscular diseases

A number of neuromuscular and muscular diseases, including amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and several myopathies, are associated to mutations in related RNA-binding proteins (RBPs), including TDP-43, FUS, MATR3 or hnRNPA1/B2. These proteins harbor similar modular primary sequence with RNA binding motifs and low complexity domains, that enables them to phase separate and create liquid microdomains. These RBPs have been shown to critically regulate multiple events of RNA lifecycle, including transcriptional events, splicing and RNA trafficking and sequestration. Here, we review the roles of these disease-related RBPs in muscle and motor neurons, and how their dysfunction in these cell types might contribute to disease.

The debated toxic role of aggregated TDP-43 in amyotrophic lateral sclerosis: a resolution in sight?

Transactive response DNA-binding protein-43 (TDP-43) is an RNA/DNA binding protein that forms phosphorylated and ubiquitinated aggregates in the cytoplasm of motor neurons in amyotrophic lateral sclerosis, which is a hallmark of this disease. Amyotrophic lateral sclerosis is a neurodegenerative condition affecting the upper and lower motor neurons. Even though the aggregative property of TDP-43 is considered a cornerstone of amyotrophic lateral sclerosis, there has been major controversy regarding the functional link between TDP-43 aggregates and cell death. In this review, we attempt to reconcile the current literature surrounding this debate by discussing the results and limitations of the published data relating TDP-43 aggregates to cytotoxicity, as well as therapeutic perspectives of TDP-43 aggregate clearance. We point out key data suggesting that the formation of TDP-43 aggregates and the capacity to self-template and propagate among cells as a 'prion-like' protein, another pathological property of TDP-43 aggregates, are a significant cause of motor neuronal death. We discuss the disparities among the various studies, particularly with respect to the type of models and the different forms of TDP-43 used to evaluate cellular toxicity. We also examine how these disparities can interfere with the interpretation of the results pertaining to a direct toxic effect of TDP-43 aggregates. Furthermore, we present perspectives for improving models in order to better uncover the toxic role of aggregated TDP-43. Finally, we review the recent studies on the enhancement of the cellular clearance mechanisms of autophagy, the ubiquitin proteasome system, and endocytosis in an attempt to counteract TDP-43 aggregation-induced toxicity. Altogether, the data available so far encourage us to suggest that the cytoplasmic aggregation of TDP-43 is key for the neurodegeneration observed in motor neurons in patients with amyotrophic lateral sclerosis. The corresponding findings provide novel avenues toward early therapeutic interventions and clinical outcomes for amyotrophic lateral sclerosis management.

Gene therapy for overexpressing Neuregulin 1 type I in skeletal muscles promotes functional improvement in the SOD1G93A ALS mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motoneurons (MNs), with no effective treatment currently available. The molecular mechanisms that are involved in MN death are complex and not fully understood, with partial contributions of surrounding glial cells and skeletal muscle to the disease. Neuregulin 1 (NRG1) is a trophic factor highly expressed in MNs and neuromuscular junctions. Recent studies have suggested a crucial role of the isoform I (NRG1-I) in the collateral reinnervation process in skeletal muscle, and NRG1-III in the preservation of MNs in the spinal cord, opening a window for developing novel therapies for neuromuscular diseases like ALS. In this study, we overexpressed NRG1-I widely in the skeletal muscles of the SOD1^G93A transgenic mouse. The results show that NRG1 gene therapy activated the survival pathways in muscle and spinal cord, increasing the number of surviving MNs and neuromuscular junctions and reducing the astroglial reactivity in the spinal cord of the treated SOD1^G93A mice. Furthermore, NRG1-I overexpression preserved motor function and delayed the onset of clinical disease. In summary, our data indicates that NRG1 plays an important role on MN survival and muscle innervation in ALS, and that viral-mediated overexpression of NRG1 isoforms may be considered as a promising approach for ALS treatment.

Bridging biophysics and neurology: aberrant phase transitions in neurodegenerative disease

Biomolecular condensation arising through phase transitions has emerged as an essential organizational strategy that governs many aspects of cell biology. In particular, the role of phase transitions in the assembly of large, complex ribonucleoprotein (RNP) granules has become appreciated as an important regulator of RNA metabolism. In parallel, genetic, histopathological and cell and molecular studies have provided evidence that disturbance of phase transitions is an important driver of neurological diseases, notably amyotrophic lateral sclerosis (ALS), but most likely also other diseases. Indeed, our growing knowledge of the biophysics underlying biological phase transitions suggests that this process offers a unifying mechanism to explain the numerous and diverse disturbances in RNA metabolism that have been observed in ALS and some related diseases - specifically, that these diseases are driven by disturbances in the material properties of RNP granules. Here, we review the evidence for this hypothesis, emphasizing the reciprocal roles in which disease-related protein and disease-related RNA can lead to disturbances in the material properties of RNP granules and consequent pathogenesis. Additionally, we review evidence that implicates aberrant phase transitions as a contributing factor to a larger set of neurodegenerative diseases, including frontotemporal dementia, certain repeat expansion diseases and Alzheimer disease.

A robust TDP-43 knock-in mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset degenerative disorder of motor neurons. The diseased spinal cord motor neurons of more than 95% of amyotrophic lateral sclerosis (ALS) patients are characterized by the mis-metabolism of the RNA/DNA-binding protein TDP-43 (ALS-TDP), in particular, the presence of cytosolic aggregates of the protein. Most available mouse models for the basic or translational studies of ALS-TDP are based on transgenic overexpression of the TDP-43 protein. Here, we report the generation and characterization of mouse lines bearing homologous knock-in of fALS-associated mutation A315T and sALS-associated mutation N390D, respectively. Remarkably, the heterozygous TDP-43 (N390D/+) mice but not those heterozygous for the TDP-43 (A315T/+) mice develop a full spectrum of ALS-TDP-like pathologies at the molecular, cellular and behavioral levels. Comparative analysis of the mutant mice and spinal cord motor neurons (MN) derived from their embryonic stem (ES) cells demonstrates that different ALS-associated TDP-43 mutations possess critical ALS-causing capabilities and pathogenic pathways, likely modified by their genetic background and the environmental factors. Mechanistically, we identify aberrant RNA splicing of spinal cord Bcl-2 pre-mRNA and consequent increase of a negative regulator of autophagy, Bcl-2, which correlate with and are caused by a progressive increase of TDP-43, one of the early events associated with ALS-TDP pathogenesis, in the spinal cord of TDP-43 (N390D/+) mice and spinal cord MN derived from their ES cells. The TDP-43 (N390D/+) knock-in mice appear to be an ideal rodent model for basic as well as translational studies of ALS- TDP.

[Molecular mechanism of amyotrophic lateral sclerosis (ALS) from the viewpoint of the formation and degeneration of transactive response DNA-binding protein 43 kDa (TDP-43) inclusions]

Sporadic amyotrophic lateral sclerosis (SALS) and many cases of familial ALS (FALS) demonstrate cytoplasmic transactive response DNA-binding protein 43 kDa (TDP-43)-positive inclusion bodies. Thus, TDP-43 plays a vital role in ALS pathogenesis. Functional analysis of the ALS causative genes advanced the elucidation of the mechanism associated with the formation and degradation of TDP-43 aggregates. Stress granules, which are non-membranous organelles, are attracting attention as sites of aggregate formation, with involvement of FUS and C9orf72. Concurrently, ALS causative genes related to the ubiquitin-proteasome and autophagy systems, which are aggregate degradation mechanisms, have also been reported. Therefore, therapeutic research based on the molecular pathology common to SALS and FALS has been advanced.

To Be or Not To Be…Toxic-Is RNA Association With TDP-43 Complexes Deleterious or Protective in Neurodegeneration?

TAR DNA binding protein (TDP-43) is a nucleic acid binding protein associated with insoluble cytoplasmic aggregates in several neurodegenerative disorders, including 97% of the ALS cases. In healthy individuals, TDP-43 is primarily localized to the nucleus; it can shuttle between the nucleus and the cytoplasm, and is involved in several aspects of RNA processing including transcription, splicing, RNA stability, transport, localization, stress granule (SG) formation, and translation. Upon stress, TDP-43 aggregates in the cytoplasm and associates with several types of RNA and protein assemblies, resulting in nuclear depletion of TDP-43. Under conditions of prolonged stress, cytoplasmic TDP-43 undergoes liquid-liquid phase separation (LLPS) and becomes less mobile. Evidence exists to support a scenario in which insoluble TDP-43 complexes sequester RNA and/or proteins causing disturbances in both ribostasis and proteostasis, which in turn contribute to neurodegeneration. However, the relationship between RNA binding and TDP-43 toxicity remains unclear. Recent studies provide conflicting views on the role of RNA in TDP-43 toxicity, with some finding RNA as a toxic factor whereby RNA binding contributes to TDP-43 toxicity, while others find RNA to be a protective factor that inhibits TDP-43 aggregation. Here we review and discuss these recent reports, which ultimately highlight the importance of understanding the heterogeneity of TDP-43 assemblies and collectively point to solubilizing TDP-43 as a potential therapeutic strategy.

Alternative Splicing of ALS Genes: Misregulation and Potential Therapies

Neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), Parkinson's, Alzheimer's, and Huntington's disease affect a rapidly increasing population worldwide. Although common pathogenic mechanisms have been identified (e.g., protein aggregation or dysfunction, immune response alteration and axonal degeneration), the molecular events underlying timing, dosage, expression, and location of RNA molecules are still not fully elucidated. In particular, the alternative splicing (AS) mechanism is a crucial player in RNA processing and represents a fundamental determinant for brain development, as well as for the physiological functions of neuronal circuits. Although in recent years our knowledge of AS events has increased substantially, deciphering the molecular interconnections between splicing and ALS remains a complex task and still requires considerable efforts. In the present review, we will summarize the current scientific evidence outlining the involvement of AS in the pathogenic processes of ALS. We will also focus on recent insights concerning the tuning of splicing mechanisms by epigenomic and epi-transcriptomic regulation, providing an overview of the available genomic technologies to investigate AS drivers on a genome-wide scale, even at a single-cell level resolution. In the future, gene therapy strategies and RNA-based technologies may be utilized to intercept or modulate the splicing mechanism and produce beneficial effects against ALS.

Role of Dynein and Dynactin (DCTN-1) in Neurodegenerative Diseases

The pathophysiology and concept of degeneration in central nervous system is very complex and overwhelming at times. There is a complex mechanism which exists among different molecules in the cytoplasm of cell bodies of neurons, antegrade and retrograde axonal transport of cargoes and accumulation of certain substances and proteins which can influence the excitatory neurotransmitter like glutamate initiating the process of neurodegeneration. Neurons have extensive processes and communication between those processes and the cell body is crucial to neuronal function, viability and survival over time with progression of age. Researchers believe neurons are uniquely dependent on microtubule-based cargo transport. There is enough evidence to support that deficits in retrograde axonal transport contribute to pathogenesis in multiple neurodegenerative diseases. Cytoplasmic dynein and its regulation by Dynactin (DCTN1) is the major molecular motor cargo involved in autophagy, mitosis and neuronal cell survival. Mutation in dynactin gene located in 2p13.1,is indeed studied very extensively and is considered to be involved directly or indirectly to various conditions like Perry syndrome, familial and sporadic Amyotrophic lateral sclerosis, Hereditary spastic paraplegia, Spinocerebellar Ataxia (SCA-5), Huntingtons disease, Alzheimers disease, Charcot marie tooth disease, Hereditary motor neuropathy 7B, prion disease, parkinsons disease, malformation of cortical development, polymicrogyria to name a few with exception of Multiple Sclerosis (MS).

The first case of the TARDBP p.G294V mutation in a homozygous state: is a single pathogenic allele sufficient to cause ALS?

Here, we described the first amyotrophic lateral sclerosis patient presenting the c.881 G > T p.G294V TARDBP mutation in homozygous status. The patient belongs to a large pedigree from Morocco. Except for one older affected brother his parents and remaining 8 sibs are referred to be healthy and do not show any neurological sign or symptom. The lack of evidence of TARDBP deletions of any sizes, together with the presence of several AOH segments, strongly suggests that the homozygosity status of p.G294V in the proband derived from parental consanguinity. A revision of the literature and our cohorts indicates that the p.G294V mutation has been detected in only 15 additional ALS patients in heterozygosity and, except for one additional Moroccan patient, all were of Italian origin. The analysis of microsatellite markers surrounding the TARDBP gene in 8 individuals carrying the p.G294V mutation showed that the haplotypic context of the Moroccan proband is shared with most patients of European origin indicating that they carry the p.G294V mutation inherited from a common ancestor. The analysis of the 15 ALS pedigrees (from literature data and present study), strongly suggests a reduced penetrance of the p.G294V mutation since for 13 of the 15 described p.G294V ALS cases the parents did not show any neurological symptoms. This result has potentially important implications in genetic counseling, since genetic testing of a reduced penetrance mutation on pre-symptomatic individuals proves very difficult to predict the outcome based on the genotype.

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.

Molecular Dissection of FUS Points at Synergistic Effect of Low-Complexity Domains in Toxicity

RNA-binding protein aggregation is a pathological hallmark of several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). To gain better insight into the molecular interactions underlying this process, we investigated FUS, which is mutated and aggregated in both ALS and FTLD. We generated a Drosophila model of FUS toxicity and identified a previously unrecognized synergistic effect between the N-terminal prion-like domain and the C-terminal arginine-rich domain to mediate toxicity. Although the prion-like domain is generally considered to mediate aggregation of FUS, we find that arginine residues in the C-terminal low-complexity domain are also required for maturation of FUS in cellular stress granules. These data highlight an important role for arginine-rich domains in the pathology of RNA-binding proteins.

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Both QGSY and RGG2 domains are necessary for FUS-induced neurodegeneration in flies

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Arginine-rich domains interact with QGSY hydrogels and liquid droplets

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RGG2 arginines promote phase separation of FUS in vitro and in cells

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FUS phase separation behavior in vitro correlates with neurodegeneration in vivo

RRM adjacent TARDBP mutations disrupt RNA binding and enhance TDP-43 proteinopathy

Amyotrophic lateral sclerosis (ALS) presents with focal muscle weakness due to motor neuron degeneration that becomes generalized, leading to death from respiratory failure within 3-5 years from symptom onset. Despite the heterogeneity of aetiology, TDP-43 proteinopathy is a common pathological feature that is observed in >95% of ALS and tau-negative frontotemporal dementia (FTD) cases. TDP-43 is a DNA/RNA-binding protein that in ALS and FTD translocates from being predominantly nuclear to form detergent-resistant, hyperphosphorylated aggregates in the cytoplasm of affected neurons and glia. Mutations in TARDBP account for 1-4% of all ALS cases and almost all arise in the low complexity C-terminal domain that does not affect RNA binding and processing. Here we report an ALS/FTD kindred with a novel K181E TDP-43 mutation that is located in close proximity to the RRM1 domain. To offer predictive gene testing to at-risk family members, we undertook a series of functional studies to characterize the properties of the mutation. Spectroscopy studies of the K181E protein revealed no evidence of significant misfolding. Although it is unable to bind to or splice RNA, it forms abundant aggregates in transfected cells. We extended our study to include other ALS-linked mutations adjacent to the RRM domains that also disrupt RNA binding and greatly enhance TDP-43 aggregation, forming detergent-resistant and hyperphosphorylated inclusions. Lastly, we demonstrate that K181E binds to, and sequesters, wild-type TDP-43 within nuclear and cytoplasmic inclusions. Thus, we demonstrate that TDP-43 mutations that disrupt RNA binding greatly enhance aggregation and are likely to be pathogenic as they promote wild-type TDP-43 to mislocalize and aggregate acting in a dominant-negative manner. This study highlights the importance of RNA binding to maintain TDP-43 solubility and the role of TDP-43 aggregation in disease pathogenesis.

Dysfunctional RNA-binding protein biology and neurodegeneration in experimental autoimmune encephalomyelitis in female mice

Altered stress granule (SG) and RNA-binding protein (RBP) biology have been shown to contribute to the pathogenesis of several neurodegenerative diseases, yet little is known about their role in multiple sclerosis (MS). Pathological features associated with dysfunctional RBPs include RBP mislocalization from its normal nuclear location to the cytoplasm and the formation of chronic SGs. We tested the hypothesis that altered SG and RBP biology might contribute to the neurodegeneration in experimental autoimmune encephalomyelitis (EAE). C57BL/6 female mice were actively immunized with MOG_35-55 to induce EAE. Spinal cords were examined for mislocalization of the RBPs, heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) and TAR-DNA binding protein-43 (TDP-43), SGs, neurodegeneration (SMI-32), T cells (CD3), and macrophages (CD68). In contrast to naive mice, mice with EAE showed SG formation (p < 0.0001) and mislocalization of hnRNP A1 (p < 0.05) in neurons of the ventral spinal cord gray matter, which correlated with clinical score (R = 0.8104, p = 0.0253). In these same areas, there was a neuronal loss (p < 0.0001) and increased SMI-32 immunoreactivity (both markers of neurodegeneration) and increased staining for CD3⁺ T cells and IFN-gamma. These findings recapitulate the SG and RBP biology and markers of neurodegeneration in MS tissues and suggest that altered SG and RBP biology contribute to the neurodegeneration in EAE, which might also apply to the pathogenesis of MS.

Modeling cell-autonomous motor neuron phenotypes in ALS using iPSCs

Amyotrophic lateral sclerosis (ALS) is an aggressive and uniformly fatal degenerative disease of the motor nervous system. In order to understand underlying disease mechanisms, researchers leverage a host of in vivo and in vitro models, including yeast, worms, flies, zebrafish, mice, and more recently, human induced pluripotent stem cells (iPSCs) derived from ALS patients. While mouse models have been the main workhorse of preclinical ALS research, the development of iPSCs provides a new opportunity to explore molecular phenotypes of ALS within human cells. Importantly, this technology enables modeling of both familial and sporadic ALS in the relevant human genetic backgrounds, as well as a personalized or targeted approach to therapy development. Harnessing these powerful tools requires addressing numerous challenges, including different variance components associated with iPSCs and motor neurons as well as concomitant limits of reductionist approaches. In order to overcome these obstacles, optimization of protocols and assays, confirmation of phenotype robustness at scale, and validation of findings in human tissue and genetics will cement the role for iPSC models as a valuable complement to animal models in ALS and more broadly among neurodegenerative diseases.

Inherited and Sporadic Amyotrophic Lateral Sclerosis and Fronto-Temporal Lobar Degenerations arising from Pathological Condensates of Phase Separating Proteins

Recent work on the biophysics of proteins with low complexity, intrinsically disordered domains that have the capacity to form biological condensates has profoundly altered the concepts about the pathogenesis of inherited and sporadic neurodegenerative disorders associated with pathological accumulation of these proteins. In the present review, we use the FUS, TDP-43 and A11 proteins as examples to illustrate how missense mutations and aberrant post-translational modifications of these proteins cause amyotrophic lateral sclerosis (ALS) and fronto-temporal lobar degeneration (FTLD).

Identification of TYW3/CRYZ and FGD4 as susceptibility genes for amyotrophic lateral sclerosis

Objective

A 2-stage genome-wide association was conducted to explore the genetic etiology of amyotrophic lateral sclerosis (ALS) in the Chinese Han population.

Methods

Totally, 700 cases and 4,027 controls were genotyped in the discovery stage using Illumina Human660W-Quad BeadChips. Top associated single nucleotide polymorphisms from the discovery stage were then genotyped in an independent cohort with 884 cases and 5,329 controls. Combined analysis was conducted by combining all samples from the 2 stages.

Results

Two novel loci, 1p31 and 12p11, showed strong associations with ALS. These novel loci explained 2.2% of overall variance in disease risk. Expression quantitative trait loci searches identified TYW/CRYZ and FGD4 as risk genes at 1p13 and 12p11, respectively.

Conclusions

This study identifies novel susceptibility genes for ALS. Identification of TYW3/CRYZ in the current study supports the notion that insulin resistance may be involved in ALS pathogenesis, whereas FGD4 suggests an association with Charcot-Marie-Tooth disease.

Overexpression of human wtTDP-43 causes impairment in hippocampal plasticity and behavioral deficits in CAMKII-tTa transgenic mouse model

AIMS

The current study utilizes the adeno-associated viral gene transfer system in the CAMKIIα-tTA mouse model to overexpress human wild type TDP-43 (wtTDP-43) and α-synuclein (α-Syn) proteins. The co-existence of these proteins is evident in the pathology of neurodegenerative disorders such as frontotemporal lobar degeneration (FTLD), Parkinson disease (PD), and dementia with Lewy bodies (DLB).

METHODS

The novel bicistronic recombinant adeno-associated virus (rAAV) serotype 9 drives wtTDP-43 and α-Syn expression in the hippocampus via "TetO" CMV promoter. Behavior, electrophysiology, and biochemical and histological assays were used to validate neuropathology.

RESULTS

We report that overexpression of wtTDP-43 but not α-Syn contributes to hippocampal CA2-specific pyramidal neuronal loss and overall hippocampal atrophy. Further, we report a reduction of hippocampal long-term potentiation and decline in learning and memory performance of wtTDP-43 expressing mice. Elevated wtTDP-43 levels induced selective degeneration of Purkinje cell protein 4 (PCP-4) positive neurons while both wtTDP-43 and α-Syn expression reduced subsets of the glutamate receptor expression in the hippocampus.

CONCLUSIONS

Overall, our findings suggest the significant vulnerability of hippocampal neurons toward elevated wtTDP-43 levels possibly via PCP-4 and GluR-dependent calcium signaling pathways. Further, we report that wtTDP-43 expression induced selective CA2 subfield degeneration, contributing to the deterioration of the hippocampal-dependent cognitive phenotype.

The basis of clinicopathological heterogeneity in TDP-43 proteinopathy

Transactive response DNA-binding protein 43 kDa (TDP-43) was identified as a major disease-associated component in the brain of patients with amyotrophic lateral sclerosis (ALS), as well as the largest subset of patients with frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-U), which characteristically exhibits cytoplasmic inclusions that are positive for ubiquitin but negative for tau and α-synuclein. TDP-43 pathology occurs in distinct brain regions, involves disparate brain networks, and features accumulation of misfolded proteins in various cell types and in different neuroanatomical regions. The clinical phenotypes of ALS and FTLD-TDP (FTLD with abnormal intracellular accumulations of TDP-43) correlate with characteristic distribution patterns of the underlying pathology across specific brain regions with disease progression. Recent studies support the idea that pathological protein spreads from neuron to neuron via axonal transport in a hierarchical manner. However, little is known to date about the basis of the selective cellular and regional vulnerability, although the information would have important implications for the development of targeted and personalized therapies. Here, we aim to summarize recent advances in the neuropathology, genetics and animal models of TDP-43 proteinopathy, and their relationship to clinical phenotypes for the underlying selective neuronal and regional susceptibilities. Finally, we attempt to integrate these findings into the emerging picture of TDP-43 proteinopathy, and to highlight key issues for future therapy and research.

Splicing repression is a major function of TDP-43 in motor neurons

Nuclear depletion of TDP-43, an essential RNA binding protein, may underlie neurodegeneration in amyotrophic lateral sclerosis (ALS). As several functions have been ascribed to this protein, the critical role(s) of TDP-43 in motor neurons that may be compromised in ALS remains unknown. We show here that TDP-43 mediated splicing repression, which serves to protect the transcriptome by preventing aberrant splicing, is central to the physiology of motor neurons. Expression in Drosophila TDP-43 knockout models of a chimeric repressor, comprised of the RNA recognition domain of TDP-43 fused to an unrelated splicing repressor, RAVER1, attenuated motor deficits and extended lifespan. Likewise, AAV9-mediated delivery of this chimeric rescue repressor to mice lacking TDP-43 in motor neurons delayed the onset, slowed the progression of motor symptoms, and markedly extended their lifespan. In treated mice lacking TDP-43 in motor neurons, aberrant splicing was significantly decreased and accompanied by amelioration of axon degeneration and motor neuron loss. This AAV9 strategy allowed long-term expression of the chimeric repressor without any adverse effects. Our findings establish that splicing repression is a major function of TDP-43 in motor neurons and strongly support the idea that loss of TDP-43-mediated splicing fidelity represents a key pathogenic mechanism underlying motor neuron loss in ALS.

TDP-43 inhibitory peptide alleviates neurodegeneration and memory loss in an APP transgenic mouse model for Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia in the elderly, characterized clinically by progressive decline in cognitive function and neuropathologically by the presence of senile plaques and neuronal loss in the brain. While current drugs for AD are always employed as symptomatic therapies with variable benefits, there is no treatment to delay its progression or halt neurodegeneration. TAR DNA-binding protein 43 (TDP-43) proteinopathy has increasingly been implicated as a prominent histopathological feature of AD and related dementias. Our recent studies have implicated mitochondria as critical targets of TDP-43 neurotoxicity. Here, we demonstrate that the suppression of mitochondrial-associated TDP-43 protects against neuronal loss and behavioral deficits in 5XFAD transgenic mice recapitulating AD-related phenotypes. In AD patients and 5XFAD mice, the level of TDP-43 is increased in mitochondria, and TDP-43 highly co-localizes with mitochondria in brain neurons exhibiting TDP-43 proteinopathy. Chronic administration of a TDP-43 mitochondrial localization inhibitory peptide, PM1, significantly alleviates TDP-43 proteinopathy, mitochondrial abnormalities, microgliosis and even neuronal loss without effect on amyloid plaque load in 12-month-old 5XFAD mice well after the onset of symptoms. Additionally, PM1 also improves the cognitive and motor function in 12-month-old 5XFAD mice and completely prevents the onset of mild cognitive impairment in 6-month-old 5XFAD mice. These data indicate that mitochondria-associated TDP-43 is likely involved in AD pathogenesis and that the inhibitor of mitochondria-associated TDP-43 may be a valuable drug to treat underlying AD.

Sarm1 deletion suppresses TDP-43-linked motor neuron degeneration and cortical spine loss

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative condition that primarily affects the motor system and shares many features with frontotemporal dementia (FTD). Evidence suggests that ALS is a 'dying-back' disease, with peripheral denervation and axonal degeneration occurring before loss of motor neuron cell bodies. Distal to a nerve injury, a similar pattern of axonal degeneration can be seen, which is mediated by an active axon destruction mechanism called Wallerian degeneration. Sterile alpha and TIR motif-containing 1 (Sarm1) is a key gene in the Wallerian pathway and its deletion provides long-term protection against both Wallerian degeneration and Wallerian-like, non-injury induced axonopathy, a retrograde degenerative process that occurs in many neurodegenerative diseases where axonal transport is impaired. Here, we explored whether Sarm1 signalling could be a therapeutic target for ALS by deleting Sarm1 from a mouse model of ALS-FTD, a TDP-43Q331K, YFP-H double transgenic mouse. Sarm1 deletion attenuated motor axon degeneration and neuromuscular junction denervation. Motor neuron cell bodies were also significantly protected. Deletion of Sarm1 also attenuated loss of layer V pyramidal neuronal dendritic spines in the primary motor cortex. Structural MRI identified the entorhinal cortex as the most significantly atrophic region, and histological studies confirmed a greater loss of neurons in the entorhinal cortex than in the motor cortex, suggesting a prominent FTD-like pattern of neurodegeneration in this transgenic mouse model. Despite the reduction in neuronal degeneration, Sarm1 deletion did not attenuate age-related behavioural deficits caused by TDP-43Q331K. However, Sarm1 deletion was associated with a significant increase in the viability of male TDP-43Q331K mice, suggesting a detrimental role of Wallerian-like pathways in the earliest stages of TDP-43Q331K-mediated neurodegeneration. Collectively, these results indicate that anti-SARM1 strategies have therapeutic potential in ALS-FTD.

Phase separation-deficient TDP43 remains functional in splicing

Intrinsically disordered regions (IDRs) are often fast-evolving protein domains of low sequence complexity that can drive phase transitions and are commonly found in many proteins associated with neurodegenerative diseases, including the RNA processing factor TDP43. Yet, how phase separation contributes to the physiological functions of TDP43 in cells remains enigmatic. Here, we combine systematic mutagenesis guided by evolutionary sequence analysis with a live-cell reporter assay of TDP43 phase dynamics to identify regularly-spaced hydrophobic motifs separated by flexible, hydrophilic segments in the IDR as a key determinant of TDP43 phase properties. This heuristic framework allows customization of the material properties of TDP43 condensates to determine effects on splicing function. Remarkably, even a mutant that fails to phase-separate at physiological concentrations can still efficiently mediate the splicing of a quantitative, single-cell splicing reporter and endogenous targets. This suggests that the ability of TDP43 to phase-separate is not essential for its splicing function.

Astrocytes in Motor Neuron Diseases

Motor neuron disorders are highly debilitating and mostly fatal conditions for which only limited therapeutic options are available. To overcome this limitation and develop more effective therapeutic strategies, it is critical to discover the pathogenic mechanisms that trigger and sustain motor neuron degeneration with the greatest accuracy and detail. In the case of Amyotrophic Lateral Sclerosis (ALS), several genes have been associated with familial forms of the disease, whilst the vast majority of cases develop sporadically and no defined cause can be held responsible. On the contrary, the huge majority of Spinal Muscular Atrophy (SMA) occurrences are caused by loss-of-function mutations in a single gene, SMN1. Although the typical hallmark of both diseases is the loss of motor neurons, there is increasing awareness that pathological lesions are also present in the neighbouring glia, whose dysfunction clearly contributes to generating a toxic environment in the central nervous system. Here, ALS and SMA are sequentially presented, each disease section having a brief introduction, followed by a focussed discussion on the role of the astrocytes in the disease pathogenesis. Such a dissertation is substantiated by the findings that built awareness on the glial involvement and how the glial-neuronal interplay is perturbed, along with the appraisal of this new cellular site for possible therapeutic intervention.

Functionalization of the TMEM175 p.M393T variant as a risk factor for Parkinson disease

Multiple genome-wide association studies (GWAS) in Parkinson disease (PD) have identified a signal at chromosome 4p16.3; however, the causal variant has not been established for this locus. Deep investigation of the region resulted in one identified variant, the rs34311866 missense SNP (p.M393T) in TMEM175, which is 20 orders of magnitude more significant than any other SNP in the region. Because TMEM175 is a lysosomal gene that has been shown to influence α-synuclein phosphorylation and autophagy, the p.M393T variant is an attractive candidate, and we have examined its effect on TMEM175 protein and PD-related biology. After knocking down each of the genes located under the GWAS peak via multiple shRNAs, only TMEM175 was found to consistently influence accumulation of phosphorylated α-synuclein (p-α-syn). Examination of the p.M393T variant showed effects on TMEM175 function that were intermediate between the wild-type (WT) and knockout phenotypes, with reduced regulation of lysosomal pH in response to starvation and minor changes in clearance of autophagy substrates, reduced lysosomal localization, and increased accumulation of p-α-syn. Finally, overexpression of WT TMEM175 protein reduced p-α-syn, while overexpression of the p.M393T variant resulted in no change in α-synuclein phosphorylation. These results suggest that the main signal in the chromosome 4p16.3 PD risk locus is driven by the TMEM175 p.M393T variant. Modulation of TMEM175 may impact α-synuclein biology and therefore may be a rational therapeutic strategy for PD.

Pleiotropic requirements for human TDP-43 in the regulation of cell and organelle homeostasis

TDP-43 is an RNA-binding protein that forms cytoplasmic aggregates in multiple neurodegenerative diseases. Although the loss of normal TDP-43 functions likely contributes to disease pathogenesis, the cell biological consequences of human TDP-43 depletion are not well understood. We, therefore, generated human TDP-43 knockout (KO) cells and subjected them to parallel cell biological and transcriptomic analyses. These efforts yielded three important discoveries. First, complete loss of TDP-43 resulted in widespread morphological defects related to multiple organelles, including Golgi, endosomes, lysosomes, mitochondria, and the nuclear envelope. Second, we identified a new role for TDP-43 in controlling mRNA splicing of Nup188 (nuclear pore protein). Third, analysis of multiple amyotrophic lateral sclerosis causing TDP-43 mutations revealed a broad ability to support splicing of TDP-43 target genes. However, as some TDP-43 disease-causing mutants failed to fully support the regulation of specific target transcripts, our results raise the possibility of mutation-specific loss-of-function contributions to disease pathology.

TDP-43 enhances translation of specific mRNAs linked to neurodegenerative disease

The RNA-binding protein TDP-43 is heavily implicated in neurodegenerative disease. Numerous patient mutations in TARDBP, the gene encoding TDP-43, combined with data from animal and cell-based models, imply that altered RNA regulation by TDP-43 causes Amyotrophic Lateral Sclerosis and Frontotemporal Dementia. However, underlying mechanisms remain unresolved. Increased cytoplasmic TDP-43 levels in diseased neurons suggest a possible role in this cellular compartment. Here, we examined the impact on translation of overexpressing human TDP-43 and the TDP-43A315T patient mutant protein in motor neuron-like cells and primary cultures of cortical neurons. In motor-neuron like cells, TDP-43 associates with ribosomes without significantly affecting global translation. However, ribosome profiling and additional assays revealed enhanced translation and direct binding of Camta1, Mig12, and Dennd4a mRNAs. Overexpressing either wild-type TDP-43 or TDP-43A315T stimulated translation of Camta1 and Mig12 mRNAs via their 5'UTRs and increased CAMTA1 and MIG12 protein levels. In contrast, translational enhancement of Dennd4a mRNA required a specific 3'UTR region and was specifically observed with the TDP-43A315T patient mutant allele. Our data reveal that TDP-43 can function as an mRNA-specific translational enhancer. Moreover, since CAMTA1 and DENND4A are linked to neurodegeneration, they suggest that this function could contribute to disease.

TDP-43 proteinopathy and mitochondrial abnormalities in neurodegeneration

Genetic mutations in TAR DNA-binding protein 43 (TDP-43) cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Importantly, TDP-43 proteinopathy, characterized by aberrant phosphorylation, ubiquitination, cleavage or nuclear depletion of TDP-43 in neurons and glial cells, is a common prominent pathological feature of various major neurodegenerative diseases including ALS, FTD, and Alzheimer's disease (AD). Although the pathomechanisms underlying TDP-43 proteinopathy remain elusive, pathologically relevant TDP-43 has been repeatedly shown to be present in either the inside or outside of mitochondria, and functionally involved in the regulation of mitochondrial morphology, trafficking, and function, suggesting mitochondria as likely targets of TDP-43 proteinopathy. In this review, we first describe the current knowledge of the association of TDP-43 with mitochondria. We then review in detail multiple mitochondrial pathways perturbed by pathological TDP-43, including mitochondrial fission and fusion dynamics, mitochondrial trafficking, bioenergetics, and mitochondrial quality control. Lastly, we briefly discuss how the study of TDP-43 proteinopathy and mitochondrial abnormalities may provide new avenues for neurodegeneration therapeutics.

The impact of histone post-translational modifications in neurodegenerative diseases

Every year, neurodegenerative disorders take more than 5000 lives in the US alone. Cures have not yet been found for many of the multitude of neuropathies. The majority of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and Parkinson's disease (PD) cases have no known genetic basis. Thus, it is evident that contemporary genetic approaches have failed to explain the etiology or etiologies of ALS/FTD and PD. Recent investigations have explored the potential role of epigenetic mechanisms in disease development. Epigenetics comprises heritable changes in gene utilization that are not derived from changes in the genome. A main epigenetic mechanism involves the post-translational modification of histones. Increased knowledge of the epigenomic landscape of neurodegenerative diseases would not only further our understanding of the disease pathologies, but also lead to the development of treatments able to halt their progress. Here, we review recent advances on the association of histone post-translational modifications with ALS, FTD, PD and several ataxias.

Neuronal ribonucleoprotein granules: Dynamic sensors of localized signals

Membrane-less organelles, because of their capacity to dynamically, selectively and reversibly concentrate molecules, are very well adapted for local information processing and rapid response to environmental fluctuations. These features are particularly important in the context of neuronal cells, where synapse-specific activation, or localized extracellular cues, induce signaling events restricted to specialized axonal or dendritic subcompartments. Neuronal ribonucleoprotein (RNP) particles, or granules, are nonmembrane bound macromolecular condensates that concentrate specific sets of mRNAs and regulatory proteins, promoting their long-distance transport to axons or dendrites. Neuronal RNP granules also have a dual function in regulating the translation of associated mRNAs: while preventing mRNA translation at rest, they fuel local protein synthesis upon activation. As revealed by recent work, rapid and reversible switches between these two functional modes are triggered by modifications of the networks of interactions underlying RNP granule assembly. Such flexible properties also come with a cost, as neuronal RNP granules are prone to transition into pathological aggregates in response to mutations, aging, or cellular stresses, further emphasizing the need to better understand the mechanistic principles governing their dynamic assembly and regulation in living systems.

Key role of UBQLN2 in pathogenesis of amyotrophic lateral sclerosis and frontotemporal dementia

Ubiquilin-2 (UBQLN2) is a member of the ubiquilin family, actively implicated in the degradation of misfolded and redundant proteins through the ubiquitin-proteasome system and macroautophagy. UBQLN2 received much attention after the discovery of gene mutations in amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). The abnormal presence of positive UBQLN2 inclusion in the cytosol of degenerating motor neurons of familial and sporadic forms of ALS patients has been newly related to neurodegeneration. Only recently, data have emerged on its role in liquid-liquid phase separation, in stress granule development and in the formation of secondary amyloid structures. Furthermore, several animal models are available to investigate its involvement in TDP-43 pathology and neuroinflammation in ALS. This review addresses the molecular pathogenetic pathways involving UBQLN2 abnormalities which are converging toward defects in clearance mechanisms. UBQLN2.

Rare variants in MYH15 modify amyotrophic lateral sclerosis risk

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by progressive muscular atrophy and respiratory failure. The G4C2 repeat expansion in the C9orf72 gene is the most prevalent genetic risk for ALS. Mutation carriers (C9ALS) display variability in phenotypes such as age-at-onset and duration, suggesting the existence of additional genetic factors. Here we introduce a three-step gene discovery strategy to identify genetic factors modifying the risk of both C9ALS and sporadic ALS (sALS) using limited samples. We first identified 135 candidate genetic modifiers of C9ALS using whole-genome sequencing (WGS) of extreme C9ALS cases diagnosed ~30 years apart. We then performed an unbiased genetic screen using a Drosophila model of the G4C2 repeat expansion with the genes identified from WGS analysis. This genetic screen identified the novel genetic interaction between G4C2 repeat-associated toxicity and 18 genetic factors, suggesting their potential association with C9ALS risk. We went on to test if 14 out of the 18 genes, those which were not known to be risk factors for ALS previously, are also associated with ALS risk in sALS cases. Gene-based-statistical analyses of targeted resequencing and WGS were performed. These analyses together reveal that rare variants in MYH15 represent a likely genetic risk factor for ALS. Furthermore, we show that MYH15 could modulate the toxicity of dipeptides produced from expanded G4C2 repeat. Our study presented here demonstrates the power of combining WGS with fly genetics to facilitate the discovery of fundamental genetic components of complex traits with a limited number of samples.

Non-genetically modified models exhibit TARDBP mRNA increase due to perturbed TDP-43 autoregulation

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by accumulation of fragmented insoluble TDP-43 and loss of TDP-43 from the nucleus. Increased expression of exogenous TARDBP (encoding TDP-43) induces TDP-43 pathology and cytotoxicity, suggesting the involvement of aberrant expression of TDP-43 in the pathogenesis of ALS. In normal conditions, however, the amount of TDP-43 is tightly regulated by the autoregulatory mechanism involving alternative splicing of TARDBP mRNA. To investigate the influence of autoregulation dysfunction, we inhibited the splicing of cryptic intron 6 using antisense oligonucleotides in vivo. This inhibition doubled the Tardbp mRNA expression, increased the fragmented insoluble TDP-43, and reduced the number of motor neurons in the mouse spinal cord. In human induced pluripotent stem cell-derived neurons, the splicing inhibition of intron 6 increased TARDBP mRNA and decreased nuclear TDP-43. These non-genetically modified models exhibiting rise in the TARDBP mRNA levels suggest that TDP-43 autoregulation turbulence might be linked to the pathogenesis of ALS.

Myelin degeneration induced by mutant superoxide dismutase 1 accumulation promotes amyotrophic lateral sclerosis

Myelin is a specialized membrane that wraps around nerve fibers and is essential for normal axonal conduction in neurons. In the central nervous system, oligodendrocytes are responsible for myelin formation. Recent studies have reported pathological abnormalities in oligodendrocytes in human patients with amyotrophic lateral sclerosis (ALS) and a mouse model of ALS expressing the G93A mutation of the human superoxide dismutase 1 (mtSOD1). However, it is unclear whether oligodendrocyte pathology in ALS represents the primary dysfunction induced by mtSOD1 and how mtSOD1 contributes to oligodendrocyte degeneration and ALS pathogenesis. We analyzed GAL4-VP16-UAS transgenic zebrafish selectively expressing mtSOD1 in mature oligodendrocytes. We observed that mtSOD1 directly induced oligodendrocyte degeneration by disrupting the myelin sheath and downregulating monocarboxylate transporter 1 (MCT1), thereby causing spinal motor neuron degeneration. Pathological changes observed in this transgenic zebrafish were similar to the pathology observed in the SOD1^G93A mouse model of ALS, which is characterized by expression of mtSOD1 in all cells. In addition, oligodendrocyte dysfunction induced by mtSOD1 was associated with anxiety-related behavioral abnormalities, learning impairments, and motor defects in the early symptomatic stage. We also found that treatment with potassium channel inhibitors rescued behavioral abnormalities without rescuing MCT1 expression, suggesting that myelin disruption induces behavioral abnormalities independently of MCT1. These results indicate that mtSOD1-induced dysfunction of mature oligodendrocytes is sufficient to induce motor neuron degeneration, thus informing future therapeutic strategies targeted at oligodendrocytes in ALS.

Extracellular vesicles and their diagnostic potential in amyotrophic lateral sclerosis

Extracellular vesicles, small reservoirs that carry various biomolecules, have gained significant interest from the clinical field in recent years based on the diagnostic, therapeutic and prognostic possibilities they offer. While information abound regarding the clinical potential of such vesicles in diverse conditions, the information demonstrating their likely importance in amyotrophic lateral sclerosis (ALS) is more limited. This review will thus provide a brief introduction to extracellular vesicles, highlight their diagnostic significance in various diseases with a focus on ALS and explore additional applications of extracellular vesicles in the medical field. Overall, this work sheds further light on the clinical importance of extracellular vesicles in diagnostic applications as well as supports the need to better characterize their roles and signatures in patients diagnosed with ALS.

DNA repair and neurological disease: From molecular understanding to the development of diagnostics and model organisms

In both replicating and non-replicating cells, the maintenance of genomic stability is of utmost importance. Dividing cells can repair DNA damage during cell division, tolerate the damage by employing potentially mutagenic DNA polymerases or die via apoptosis. However, the options for accurate DNA repair are more limited in non-replicating neuronal cells. If DNA damage is left unresolved, neuronal cells die causing neurodegenerative disorders. A number of pathogenic variants of DNA repair proteins have been linked to multiple neurological diseases. The current challenge is to harness our knowledge of fundamental properties of DNA repair to improve diagnosis, prognosis and treatment of such debilitating disorders. In this perspective, we will focus on recent efforts in identifying novel DNA repair biomarkers for the diagnosis of neurological disorders and their use in monitoring the patient response to therapy. These efforts are greatly facilitated by the development of model organisms such as zebrafish, which will also be summarised.

ALS and FTD: Where RNA metabolism meets protein quality control

Recent genetic and biochemical evidence has improved our understanding of the pathomechanisms that lead to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), two devastating neurodegenerative diseases with overlapping symptoms and causes. Impaired RNA metabolism, enhanced aggregation of protein-RNA complexes, aberrant formation of ribonucleoprotein (RNP) granules and dysfunctional protein clearance via autophagy are emerging as crucial events in ALS/FTD pathogenesis. Importantly, these processes interact at the molecular level, converging on a common pathogenic cascade. In this review, we summarize key principles underlying ALS and FTD, and we discuss how mutations in genes involved in RNA metabolism, protein quality control and protein degradation meet mechanistically to impair the functionality and dynamics of RNP granules, and how this leads to cellular toxicity and death. Finally, we describe recent advances in understanding signaling pathways that become dysfunctional in ALS/FTD, partly due to altered RNP granule dynamics, but also with stress granule-independent mechanisms and, thus could be promising targets for future therapeutic intervention.

Tau tubulin kinases in proteinopathy

A number of neurodegenerative diseases are characterized by deposition of abnormally phosphorylated tau or TDP-43 in disease-affected neurons. These diseases include Alzheimer's disease, frontotemporal lobar degeneration, and amyotrophic lateral sclerosis. No disease-modifying therapeutics is available to treat these disorders, and we have a limited understanding of the cellular and molecular factors integral to disease initiation or progression. Phosphorylated tau and TDP-43 are important markers of pathology in dementia disorders and directly contribute to tau- and TDP-43-related neurotoxicity and neurodegeneration. Here, we review the scope of tau and TDP-43 phosphorylation in neurodegenerative disease and discuss recent work demonstrating the kinases TTBK1 and TTBK2 phosphorylate both tau and TDP-43, promoting neurodegeneration.