C9orf72-derived arginine-containing dipeptide repeats associate with axonal transport machinery and impede microtubule-based motility

Amyotrophic Lateral Sclerosis: Focus on Cytoplasmic Trafficking and Proteostasis

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal motor neuron disease characterized by the pathological loss of upper and lower motor neurons. Whereas most ALS cases are caused by a combination of environmental factors and genetic susceptibility, in a relatively small proportion of cases, the disorder results from mutations in genes that are inherited. Defects in several different cellular mechanisms and processes contribute to the selective loss of motor neurons (MNs) in ALS. Prominent among these is the accumulation of aggregates of misfolded proteins or peptides which are toxic to motor neurons. These accumulating aggregates stress the ability of the endoplasmic reticulum (ER) to function normally, cause defects in the transport of proteins between the ER and Golgi, and impair the transport of RNA, proteins, and organelles, such as mitochondria, within axons and dendrites, all of which contribute to the degeneration of MNs. Although dysfunction of a variety of cellular processes combines towards the pathogenesis of ALS, in this review, we focus on recent advances concerning the involvement of defective ER stress, vesicular transport between the ER and Golgi, and axonal transport.

C21ORF2 mutations point towards primary cilia dysfunction in amyotrophic lateral sclerosis

Progressive loss of motor neurons is the hallmark of the neurodegenerative disease amyotrophic lateral sclerosis (ALS), but the underlying disease mechanisms remain incompletely understood. In this study, we investigate the effects of C21ORF2 mutations, a gene recently linked to ALS, and find that primary cilia are dysfunctional. Human patient-derived mutant C21ORF2 motor neurons have a reduced ciliary frequency and length. We report that C21ORF2 is located at the basal body of the primary cilium, and mutations associated with ALS alter this localization. Furthermore, we show that a reduction of C21ORF2 levels in cell lines and motor neurons is sufficient to cause fewer primary cilia and reduced cilial length. This ciliary dysfunction leads to defective downstream sonic hedgehog signalling and reduces the expression of cellular retinoic acid binding protein 1 (CRABP1), a protein involved in motor neuron maintenance and survival. In a compartmentalized co-culture system of motor neurons and muscle cells, these ciliary defects were associated with a reduced ability of neuromuscular junction formation. Interestingly, these cilia defects are seemingly not restricted to C21ORF2 ALS, as we also observed perturbed primary cilia in cultured motor neurons and post-mortem motor cortex from patients with the most common genetic subtype of ALS caused by repeat expansions in the C9ORF72 gene. Finally, overexpression of C21ORF2 in mutant C21ORF2 motor neurons rescued the ciliary frequency and length, CRAPBP1 expression and neuromuscular junction formation, confirming the importance of primary cilia for motor neuron function. These results point towards primary cilia dysfunction contributing to motor neuron degeneration in ALS and open new avenues for further research and interventions for this as yet untreatable disease.

Sporadic ALS hiPSC-derived motor neurons show axonal defects linked to altered axon guidance pathways

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by the selective and progressive loss of motor neurons, leading to gradual paralysis and death within 2 to 5 years after diagnosis. The exact underlying pathogenic mechanism(s) remain elusive. This is particularly the case for sporadic ALS (sALS), representing 90 % of cases, as modelling a sporadic disease is extremely difficult. We used human induced pluripotent stem cell (hiPSC)-derived motor neurons from sALS patients to investigate early disease mechanisms. The earliest phenotype that we observed were profound axonal defects including impaired axonal transport, defective axonal outgrowth and a reduced formation of neuromuscular junctions. Transcriptomic profiling revealed significant dysregulation in axon guidance pathways, with upregulation of specific axonal regeneration-inhibiting genes, such as EphA4 and DCC in sALS motor neurons. Our findings suggest that dysregulation of axon guidance pathways contributes to axonal defects and that this could play a crucial role in the pathogenesis of sALS.

Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics

GGGGCC repeat expansions in C9orf72 are a common genetic cause of amyotrophic lateral sclerosis in people of European ancestry; however, substantial variability in the penetrance of the mutation, age at disease onset, and clinical presentation can complicate diagnosis and prognosis. The repeat expansion is bidirectionally transcribed in the sense and antisense directions into repetitive RNAs and translated into dipeptide repeat proteins, and both accumulate in the cortex, cerebellum, and the spinal cord. Furthermore, neuropathological aggregates of phosphorylated TDP-43 are observed in motor cortex and other cortical regions, and in the spinal cord of patients at autopsy. C9orf72 repeat expansions can also cause frontotemporal dementia. The GGGGCC repeat induces a complex interplay of loss-of-function and gain-of-function pathological mechanisms. Clinical trials using antisense oligonucleotides to target the GGGGCC repeat RNA have not been successful, potentially because they only target a single gain-of-function mechanism. Novel therapeutic approaches targeting the DNA repeat expansion, multiple repeat-derived RNA species, or downstream targets of TDP-43 dysfunction are, however, on the horizon, together with the development of diagnostic and prognostic biomarkers.

Studying C9orf72 dipeptide repeat polypeptide aggregation using an analytical ultracentrifuge equipped with fluorescence detection

Sedimentation velocity, using an analytical ultracentrifuge equipped with fluorescence detection, and electrophoresis methods are used to study aggregation of proteins in transgenic animal model systems. Our previous work validated the power of this approach in an analysis of mutant huntingtin aggregation. We demonstrate that this method can be applied to another neurodegenerative disease studying the aggregation of three dipeptide repeats (DPRs) produced by aberrant translation of mutant c9orf72 containing large G4C2 hexanucleotide repeats. These repeat expansions are the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). We analyzed the aggregation patterns of (Gly-Pro)47, (Gly-Ala)50, and (Gly-Arg)50 fused to fluorescent proteins in samples prepared from D. melanogaster, and (Gly-Ala)50 in C. elegans, using AU-FDS and SDD-AGE. Results suggest that (GP)47 is largely monomeric. In contrast, (GA)50 forms both intermediate and large-scale aggregates. (GR)50 is partially monomeric with some aggregation noted in SDD-AGE analysis. The aggregation of this DPR is likely to represent co-aggregated states with DNA and/or RNA. The power of these methods is the ability to gather data on aggregation patterns and characteristics in animal model systems, which may then be used to interpret the mitigation of aggregation through genetic or molecular therapeutic interventions.

Exploring the Role of Axons in ALS from Multiple Perspectives

Amyotrophic lateral sclerosis (ALS), commonly known as motor neuron disease, is a neurodegenerative disorder characterized by the progressive degeneration of both upper and lower motor neurons. This pathological process results in muscle weakness and can culminate in paralysis. To date, the precise etiology of ALS remains unclear. However, a burgeoning body of research indicates that axonal dysfunction is a pivotal element in the pathogenesis of ALS and significantly influences the progression of disease. Dysfunction of axons in ALS can result in impediments to nerve impulse transmission, leading to motor impairment, muscle atrophy, and other associated complications that severely compromise patients' quality of life and survival prognosis. In this review, we concentrate on several key areas: the ultrastructure of axons, the mechanisms of axonal degeneration in ALS, the impact of impaired axonal transport on disease progression in ALS, and the potential for axonal regeneration within the central nervous system (CNS). Our objective is to achieve a more holistic and profound understanding of the multifaceted role that axons play in ALS, thereby offering a more intricate and refined perspective on targeted axonal therapeutic interventions.

Mechanism-free repurposing of drugs for C9orf72-related ALS/FTD using large-scale genomic data

Repeat expansions in the C9orf72 gene are the most common genetic cause of (ALS) and frontotemporal dementia (FTD). Like other genetic forms of neurodegeneration, pinpointing the precise mechanism(s) by which this mutation leads to neuronal death remains elusive, and this lack of knowledge hampers the development of therapy for C9orf72-related disease. We used an agnostic approach based on genomic data (n = 41,273 ALS and healthy samples, and n = 1,516 C9orf72 carriers) to overcome these bottlenecks. Our drug-repurposing screen, based on gene- and expression-pattern matching and information about the genetic variants influencing onset age among C9orf72 carriers, identified acamprosate, a γ-aminobutyric acid analog, as a potentially repurposable treatment for patients carrying C9orf72 repeat expansions. We validated its neuroprotective effect in cell models and showed comparable efficacy to riluzole, the current standard of care. Our work highlights the potential value of genomics in repurposing drugs in situations where the underlying pathomechanisms are inherently complex. VIDEO ABSTRACT.

Axonopathy Underlying Amyotrophic Lateral Sclerosis: Unraveling Complex Pathways and Therapeutic Insights

Amyotrophic Lateral Sclerosis (ALS) is a complex neurodegenerative disorder characterized by progressive axonopathy, jointly leading to the dying back of the motor neuron, disrupting both nerve signaling and motor control. In this review, we highlight the roles of axonopathy in ALS progression, driven by the interplay of multiple factors including defective trafficking machinery, protein aggregation, and mitochondrial dysfunction. Dysfunctional intracellular transport, caused by disruptions in microtubules, molecular motors, and adaptors, has been identified as a key contributor to disease progression. Aberrant protein aggregation involving TDP-43, FUS, SOD1, and dipeptide repeat proteins further amplifies neuronal toxicity. Mitochondrial defects lead to ATP depletion, oxidative stress, and Ca2+ imbalance, which are regarded as key factors underlying the loss of neuromuscular junctions and axonopathy. Mitigating these defects through interventions including neurotrophic treatments offers therapeutic potential. Collaborative research efforts aim to unravel ALS complexities, opening avenues for holistic interventions that target diverse pathological mechanisms.

A pathogenic mutation in the ALS/FTD gene VCP induces mitochondrial hypermetabolism by modulating the permeability transition pore

Valosin-containing protein (VCP) is a ubiquitously expressed type II AAA+ ATPase protein, implicated in both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). This study aimed to explore the impact of the disease-causing VCPR191Q/wt mutation on mitochondrial function using a CRISPR/Cas9-engineered neuroblastoma cell line. Mitochondria in these cells are enlarged, with a depolarized mitochondrial membrane potential associated with increased respiration and electron transport chain activity. Our results indicate that mitochondrial hypermetabolism could be caused, at least partially, by increased calcium-induced opening of the permeability transition pore (mPTP), leading to mild mitochondrial uncoupling. In conclusion, our findings reveal a central role of the ALS/FTD gene VCP in maintaining mitochondrial homeostasis and suggest a model of pathogenesis based on progressive alterations in mPTP physiology and mitochondrial energetics.

A force-sensitive mutation reveals a non-canonical role for dynein in anaphase progression

The diverse roles of the dynein motor in shaping microtubule networks and cargo transport complicate in vivo analysis of its functions significantly. To address this issue, we have generated a series of missense mutations in Drosophila Dynein heavy chain. We show that mutations associated with human neurological disease cause a range of defects, including impaired cargo trafficking in neurons. We also describe a novel microtubule-binding domain mutation that specifically blocks the metaphase-anaphase transition during mitosis in the embryo. This effect is independent from dynein's canonical role in silencing the spindle assembly checkpoint. Optical trapping of purified dynein complexes reveals that this mutation only compromises motor performance under load, a finding rationalized by the results of all-atom molecular dynamics simulations. We propose that dynein has a novel function in anaphase progression that depends on it operating in a specific load regime. More broadly, our work illustrates how in vivo functions of motors can be dissected by manipulating their mechanical properties.

Molecular pathology, developmental changes and synaptic dysfunction in (pre-) symptomatic human C9ORF72-ALS/FTD cerebral organoids

A hexanucleotide repeat expansion (HRE) in C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Human brain imaging and experimental studies indicate early changes in brain structure and connectivity in C9-ALS/FTD, even before symptom onset. Because these early disease phenotypes remain incompletely understood, we generated iPSC-derived cerebral organoid models from C9-ALS/FTD patients, presymptomatic C9ORF72-HRE (C9-HRE) carriers, and controls. Our work revealed the presence of all three C9-HRE-related molecular pathologies and developmental stage-dependent size phenotypes in cerebral organoids from C9-ALS/FTD patients. In addition, single-cell RNA sequencing identified changes in cell type abundance and distribution in C9-ALS/FTD organoids, including a reduction in the number of deep layer cortical neurons and the distribution of neural progenitors. Further, molecular and cellular analyses and patch-clamp electrophysiology detected various changes in synapse structure and function. Intriguingly, organoids from all presymptomatic C9-HRE carriers displayed C9-HRE molecular pathology, whereas the extent to which more downstream cellular defects, as found in C9-ALS/FTD models, were detected varied for the different presymptomatic C9-HRE cases. Together, these results unveil early changes in 3D human brain tissue organization and synaptic connectivity in C9-ALS/FTD that likely constitute initial pathologies crucial for understanding disease onset and the design of therapeutic strategies.

The ALS-associated KIF5A P986L variant is not pathogenic for Drosophila motoneurons

Amyotrophic lateral sclerosis (ALS) is a devastating paralytic disorder caused by the death of motoneurons. Several mutations in the KIF5A gene have been identified in patients with ALS. Some mutations affect the splicing sites of exon 27 leading to its deletion (Δ27 mutation). KIF5A Δ27 is aggregation-prone and pathogenic for motoneurons due to a toxic gain of function. Another mutation found to be enriched in ALS patients is a proline/leucine substitution at position 986 (P986L mutation). Bioinformatic analyses strongly suggest that this variant is benign. Our study aims to conduct functional studies in Drosophila to classify the KIF5A P986L variant. When expressed in motoneurons, KIF5A P986L does not modify the morphology of larval NMJ or the synaptic transmission. In addition, KIF5A P986L is uniformly distributed in axons and does not disturb mitochondria distribution. Locomotion at larval and adult stages is not affected by KIF5A P986L. Finally, both KIF5A WT and P986L expression in adult motoneurons extend median lifespan compared to control flies. Altogether, our data show that the KIF5A P986L variant is not pathogenic for motoneurons and may represent a hypomorphic allele, although it is not causative for ALS.

Poly-GR repeats associated with ALS/FTD gene C9ORF72 impair translation elongation and induce a ribotoxic stress response in neurons

Hexanucleotide repeat expansion in the C9ORF72 gene is the most frequent inherited cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The expansion results in multiple dipeptide repeat proteins, among which arginine-rich poly-GR proteins are highly toxic to neurons and decrease the rate of protein synthesis. We investigated whether the effect on protein synthesis contributes to neuronal dysfunction and degeneration. We found that the expression of poly-GR proteins inhibited global translation by perturbing translation elongation. In iPSC-differentiated neurons, the translation of transcripts with relatively slow elongation rates was further slowed, and stalled, by poly-GR. Elongation stalling increased ribosome collisions and induced a ribotoxic stress response (RSR) mediated by ZAKα that increased the phosphorylation of the kinase p38 and promoted cell death. Knockdown of ZAKα or pharmacological inhibition of p38 ameliorated poly-GR-induced toxicity and improved the survival of iPSC-derived neurons from patients with C9ORF72-ALS/FTD. Our findings suggest that targeting the RSR may be neuroprotective in patients with ALS/FTD caused by repeat expansion in C9ORF72.

Cellular and axonal transport phenotypes due to the C9ORF72 HRE in iPSC motor and sensory neurons

Induced pluripotent stem cell (iPSC)-derived motor neurons (MNs) from patients with amyotrophic lateral sclerosis (ALS) and the C9ORF72 hexanucleotide repeat expansion (HRE) have multiple cellular phenotypes, but which of these accurately reflect the biology underlying the cell-specific vulnerability of ALS is uncertain. We therefore compared phenotypes due to the C9ORF72 HRE in MNs with sensory neurons (SNs), which are relatively spared in ALS. The iPSC models were able to partially reproduce the differential gene expression seen between adult SNs and MNs. We demonstrated that the typical hallmarks of C9ORF72-ALS, including RNA foci and dipeptide formation, as well as specific axonal transport defects, occurred equally in MNs and SNs, suggesting that these in vitro phenotypes are not sufficient to explain the cell-type selectivity of ALS in isolation.

Tropomyosin 1-I/C coordinates kinesin-1 and dynein motors during oskar mRNA transport

Dynein and kinesin motors mediate long-range intracellular transport, translocating towards microtubule minus and plus ends, respectively. Cargoes often undergo bidirectional transport by binding to both motors simultaneously. However, it is not known how motor activities are coordinated in such circumstances. In the Drosophila female germline, sequential activities of the dynein-dynactin-BicD-Egalitarian (DDBE) complex and of kinesin-1 deliver oskar messenger RNA from nurse cells to the oocyte, and within the oocyte to the posterior pole. We show through in vitro reconstitution that Tm1-I/C, a tropomyosin-1 isoform, links kinesin-1 in a strongly inhibited state to DDBE-associated oskar mRNA. Nuclear magnetic resonance spectroscopy, small-angle X-ray scattering and structural modeling indicate that Tm1-I/C suppresses kinesin-1 activity by stabilizing its autoinhibited conformation, thus preventing competition with dynein until kinesin-1 is activated in the oocyte. Our work reveals a new strategy for ensuring sequential activity of microtubule motors.

The sense of antisense therapies in ALS

Treatment of patients with amyotrophic lateral sclerosis (ALS) has entered a new era now that encouraging results about antisense oligonucleotides (ASOs) are becoming available and a first ASO therapy for ALS has been approved by the FDA. Moreover, there is hope not only that ALS can be stopped but also that symptoms can be reversed. Until now, degrading ASOs seemed to be successful mostly for rarer forms of familial ALS. However, the first attempts to correct mis-splicing events in sporadic ALS are underway, as well as a clinical trial examining interference with a genetic modifier. In this review, we discuss the current status of using ASOs in ALS and the possibilities and pitfalls of this therapeutic strategy.

Generation of Human Induced Pluripotent Stem Cell (hiPSC)-Derived Astrocytes for Amyotrophic Lateral Sclerosis and Other Neurodegenerative Disease Studies

Astrocytes are increasingly recognized for their important role in neurodegenerative diseases like amyotrophic lateral sclerosis (ALS). In ALS, astrocytes shift from their primary function of providing neuronal homeostatic support towards a reactive and toxic role, which overall contributes to neuronal toxicity and cell death. Currently, our knowledge on these processes is incomplete, and time-efficient and reproducible model systems in a human context are therefore required to understand and therapeutically modulate the toxic astrocytic response for future treatment options. Here, we present an efficient and straightforward protocol to generate human induced pluripotent stem cell (hiPSC)-derived astrocytes implementing a differentiation scheme based on small molecules. Through an initial 25 days, hiPSCs are differentiated into astrocytes, which are matured for 4+ weeks. The hiPSC-derived astrocytes can be cryopreserved at every passage during differentiation and maturation. This provides convenient pauses in the protocol as well as cell banking opportunities, thereby limiting the need to continuously start from hiPSCs. The protocol has already proven valuable in ALS research but can be adapted to any desired research field where astrocytes are of interest. Key features • This protocol requires preexisting experience in hiPSC culturing for a successful outcome. • The protocol relies on a small molecule differentiation scheme and an easy-to-follow methodology, which can be paused at several time points. • The protocol generates >50 × 106 astrocytes per differentiation, which can be cryopreserved at every passage, ensuring a large-scale experimental output.

PP2A and GSK3 act as modifiers of FUS-ALS by modulating mitochondrial transport

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease which currently lacks effective treatments. Mutations in the RNA-binding protein FUS are a common cause of familial ALS, accounting for around 4% of the cases. Understanding the mechanisms by which mutant FUS becomes toxic to neurons can provide insight into the pathogenesis of both familial and sporadic ALS. We have previously observed that overexpression of wild-type or ALS-mutant FUS in Drosophila motor neurons is toxic, which allowed us to screen for novel genetic modifiers of the disease. Using a genome-wide screening approach, we identified Protein Phosphatase 2A (PP2A) and Glycogen Synthase Kinase 3 (GSK3) as novel modifiers of FUS-ALS. Loss of function or pharmacological inhibition of either protein rescued FUS-associated lethality in Drosophila. Consistent with a conserved role in disease pathogenesis, pharmacological inhibition of both proteins rescued disease-relevant phenotypes, including mitochondrial trafficking defects and neuromuscular junction failure, in patient iPSC-derived spinal motor neurons (iPSC-sMNs). In FUS-ALS flies, mice, and human iPSC-sMNs, we observed reduced GSK3 inhibitory phosphorylation, suggesting that FUS dysfunction results in GSK3 hyperactivity. Furthermore, we found that PP2A acts upstream of GSK3, affecting its inhibitory phosphorylation. GSK3 has previously been linked to kinesin-1 hyperphosphorylation. We observed this in both flies and iPSC-sMNs, and we rescued this hyperphosphorylation by inhibiting GSK3 or PP2A. Moreover, increasing the level of kinesin-1 expression in our Drosophila model strongly rescued toxicity, confirming the relevance of kinesin-1 hyperphosphorylation. Our data provide in vivo evidence that PP2A and GSK3 are disease modifiers, and reveal an unexplored mechanistic link between PP2A, GSK3, and kinesin-1, that may be central to the pathogenesis of FUS-ALS and sporadic forms of the disease.

Surface-Charge-Driven Ferroptosis and Mitochondrial Dysfunction Is Involved in Toxicity Diversity in the Marine Bivalve Exposed to Nanoplastics

Nanoplastics (NPs) pervade daily life, posing serious threats to marine ecosystems. Despite the crucial role that surface charge plays in NP effects, there is a substantial gap in our understanding of how surface charge influences NP toxicity. Herein, by exposing Ruditapes philippinarum (R. philippinarum) to both positively charged NPs (p-NPs) and negatively charged NPs (n-NPs) at environmentally relevant particle number levels for a duration of 35 days, we unequivocally demonstrate that both types of NPs had discernible impacts on the clams depending on their surface charge. Through transcriptomic and proteomic analyses, we unveiled the primary mechanisms behind p-NP toxicity, which stem from induced mitochondrial dysfunction and ferroptosis. In contrast, n-NPs predominantly stimulated innate immune responses, influencing salivary secretion and modulating the complement and coagulation cascades. Furthermore, in vitro tests on clam immune cells confirmed that internalized p-NPs triggered alterations in mitochondrial morphology, a decrease in membrane potential, and the initiation of ferroptosis. Conversely, n-NPs, to a certain extent, moderated the expression of genes related to immune responses, thus mitigating their adverse effects. Taken together, these findings indicate that the differential surface-charge-driven ferroptosis and mitochondrial dysfunction in clams play a critical role in the toxicity profile of NPs, providing an insightful reference for assessing the ecological toxicity associated with NPs.

A toxic gain-of-function mechanism in C9orf72 ALS impairs the autophagy-lysosome pathway in neurons

BACKGROUND

Motor neurons (MNs), which are primarily affected in amyotrophic lateral sclerosis (ALS), are a specialized type of neurons that are long and non-dividing. Given their unique structure, these cells heavily rely on transport of organelles along their axons and the process of autophagy to maintain their cellular homeostasis. It has been shown that disruption of the autophagy pathway is sufficient to cause progressive neurodegeneration and defects in autophagy have been associated with various subtypes of ALS, including those caused by hexanucleotide repeat expansions in the C9orf72 gene. A more comprehensive understanding of the dysfunctional cellular mechanisms will help rationalize the design of potent and selective therapies for C9orf72-ALS.

METHODS

In this study, we used induced pluripotent stem cell (iPSC)-derived MNs from C9orf72-ALS patients and isogenic control lines to identify the underlying mechanisms causing dysregulations of the autophagy-lysosome pathway. Additionally, to ascertain the potential impact of C9orf72 loss-of-function on autophagic defects, we characterized the observed phenotypes in a C9orf72 knockout iPSC line (C9-KO).

RESULTS

Despite the evident presence of dysfunctions in several aspects of the autophagy-lysosome pathway, such as disrupted lysosomal homeostasis, abnormal lysosome morphology, inhibition of autophagic flux, and accumulation of p62 in C9orf72-ALS MNs, we were surprised to find that C9orf72 loss-of-function had minimal influence on these phenotypes. Instead, we primarily observed impairment in endosome maturation as a result of C9orf72 loss-of-function. Additionally, our study shed light on the pathological mechanisms underlying C9orf72-ALS, as we detected an increased TBK1 phosphorylation at S172 in MNs derived from C9orf72 ALS patients.

CONCLUSIONS

Our data provides further insight into the involvement of defects in the autophagy-lysosome pathway in C9orf72-ALS and strongly indicate that those defects are mainly due to the toxic gain-of-function mechanisms underlying C9orf72-ALS.

Amyotrophic lateral sclerosis: translating genetic discoveries into therapies

Recent advances in sequencing technologies and collaborative efforts have led to substantial progress in identifying the genetic causes of amyotrophic lateral sclerosis (ALS). This momentum has, in turn, fostered the development of putative molecular therapies. In this Review, we outline the current genetic knowledge, emphasizing recent discoveries and emerging concepts such as the implication of distinct types of mutation, variability in mutated genes in diverse genetic ancestries and gene-environment interactions. We also propose a high-level model to synthesize the interdependent effects of genetics, environmental and lifestyle factors, and ageing into a unified theory of ALS. Furthermore, we summarize the current status of therapies developed on the basis of genetic knowledge established for ALS over the past 30 years, and we discuss how developing treatments for ALS will advance our understanding of targeting other neurological diseases.

A force-sensitive mutation reveals a spindle assembly checkpoint-independent role for dynein in anaphase progression

The cytoplasmic dynein-1 (dynein) motor organizes cells by shaping microtubule networks and moving a large variety of cargoes along them. However, dynein's diverse roles complicate in vivo studies of its functions significantly. To address this issue, we have used gene editing to generate a series of missense mutations in Drosophila Dynein heavy chain (Dhc). We find that mutations associated with human neurological disease cause a range of defects in larval and adult flies, including impaired cargo trafficking in neurons. We also describe a novel mutation in the microtubule-binding domain (MTBD) of Dhc that, remarkably, causes metaphase arrest of mitotic spindles in the embryo but does not impair other dynein-dependent processes. We demonstrate that the mitotic arrest is independent of dynein's well-established roles in silencing the spindle assembly checkpoint. In vitro reconstitution and optical trapping assays reveal that the mutation only impairs the performance of dynein under load. In silico all-atom molecular dynamics simulations show that this effect correlates with increased flexibility of the MTBD, as well as an altered orientation of the stalk domain, with respect to the microtubule. Collectively, our data point to a novel role of dynein in anaphase progression that depends on the motor operating in a specific load regime. More broadly, our work illustrates how cytoskeletal transport processes can be dissected in vivo by manipulating mechanical properties of motors.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

A human proteogenomic-cellular framework identifies KIF5A as a modulator of astrocyte process integrity with relevance to ALS

Genome-wide association studies identified several disease-causing mutations in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). However, the contribution of genetic variants to pathway disturbances and their cell type-specific variations, especially in glia, is poorly understood. We integrated ALS GWAS-linked gene networks with human astrocyte-specific multi-omics datasets to elucidate pathognomonic signatures. It predicts that KIF5A, a motor protein kinesin-1 heavy-chain isoform, previously detected only in neurons, can also potentiate disease pathways in astrocytes. Using postmortem tissue and super-resolution structured illumination microscopy in cell-based perturbation platforms, we provide evidence that KIF5A is present in astrocyte processes and its deficiency disrupts structural integrity and mitochondrial transport. We show that this may underly cytoskeletal and trafficking changes in SOD1 ALS astrocytes characterised by low KIF5A levels, which can be rescued by c-Jun N-terminal Kinase-1 (JNK1), a kinesin transport regulator. Altogether, our pipeline reveals a mechanism controlling astrocyte process integrity, a pre-requisite for synapse maintenance and suggests a targetable loss-of-function in ALS.

Poly(A)-binding protein is an ataxin-2 chaperone that regulates biomolecular condensates

Biomolecular condensation underlies the biogenesis of an expanding array of membraneless assemblies, including stress granules (SGs), which form under a variety of cellular stresses. Advances have been made in understanding the molecular grammar of a few scaffold proteins that make up these phases, but how the partitioning of hundreds of SG proteins is regulated remains largely unresolved. While investigating the rules that govern the condensation of ataxin-2, an SG protein implicated in neurodegenerative disease, we unexpectedly identified a short 14 aa sequence that acts as a condensation switch and is conserved across the eukaryote lineage. We identify poly(A)-binding proteins as unconventional RNA-dependent chaperones that control this regulatory switch. Our results uncover a hierarchy of cis and trans interactions that fine-tune ataxin-2 condensation and reveal an unexpected molecular function for ancient poly(A)-binding proteins as regulators of biomolecular condensate proteins. These findings may inspire approaches to therapeutically target aberrant phases in disease.

Disruption of axonal transport in neurodegeneration

Neurons are markedly compartmentalized, which makes them reliant on axonal transport to maintain their health. Axonal transport is important for anterograde delivery of newly synthesized macromolecules and organelles from the cell body to the synapse and for the retrograde delivery of signaling endosomes and autophagosomes for degradation. Dysregulation of axonal transport occurs early in neurodegenerative diseases and plays a key role in axonal degeneration. Here, we provide an overview of mechanisms for regulation of axonal transport; discuss how these mechanisms are disrupted in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, hereditary spastic paraplegia, amyotrophic lateral sclerosis, and Charcot-Marie-Tooth disease; and discuss therapeutic approaches targeting axonal transport.

Neurofilament accumulations in amyotrophic lateral sclerosis patients' motor neurons impair axonal initial segment integrity

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease in adults with no curative treatment. Neurofilament (NF) level in patient' fluids have recently emerged as the prime biomarker of ALS disease progression, while NF accumulation in MNs of patients is the oldest and one of the best pathological hallmarks. However, the way NF accumulations could lead to MN degeneration remains unknown. To assess NF accumulations and study the impact on MNs, we compared MNs derived from induced pluripotent stem cells (iPSC) of patients carrying mutations in C9orf72, SOD1 and TARDBP genes, the three main ALS genetic causes. We show that in all mutant MNs, light NF (NF-L) chains rapidly accumulate in MN soma, while the phosphorylated heavy/medium NF (pNF-M/H) chains pile up in axonal proximal regions of only C9orf72 and SOD1 MNs. Excitability abnormalities were also only observed in these latter MNs. We demonstrate that the integrity of the MN axonal initial segment (AIS), the region of action potential initiation and responsible for maintaining axonal integrity, is impaired in the presence of pNF-M/H accumulations in C9orf72 and SOD1 MNs. We establish a strong correlation between these pNF-M/H accumulations, an AIS distal shift, increased axonal calibers and modified repartition of sodium channels. The results expand our understanding of how NF accumulation could dysregulate components of the axonal cytoskeleton and disrupt MN homeostasis. With recent cumulative evidence that AIS alterations are implicated in different brain diseases, preserving AIS integrity could have important therapeutic implications for ALS.

Downregulation of Hsp90 and the antimicrobial peptide Mtk suppresses poly(GR)-induced neurotoxicity in C9ORF72-ALS/FTD

GGGGCC repeat expansion in the C9ORF72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Repeat RNAs can be translated into dipeptide repeat proteins, including poly(GR), whose mechanisms of action remain largely unknown. In an RNA-seq analysis of poly(GR) toxicity in Drosophila, we found that several antimicrobial peptide genes, such as metchnikowin (Mtk), and heat shock protein (Hsp) genes are activated. Mtk knockdown in the fly eye or in all neurons suppresses poly(GR) neurotoxicity. These findings suggest a cell-autonomous role of Mtk in neurodegeneration. Hsp90 knockdown partially rescues both poly(GR) toxicity in flies and neurodegeneration in C9ORF72 motor neurons derived from induced pluripotent stem cells (iPSCs). Topoisomerase II (TopoII) regulates poly(GR)-induced upregulation of Hsp90 and Mtk. TopoII knockdown also suppresses poly(GR) toxicity in Drosophila and improves survival of C9ORF72 iPSC-derived motor neurons. These results suggest potential novel therapeutic targets for C9ORF72-ALS/FTD.

CRISPR/Cas9 screen in human iPSC-derived cortical neurons identifies NEK6 as a novel disease modifier of C9orf72 poly(PR) toxicity

INTRODUCTION

The most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are hexanucleotide repeats in chromosome 9 open reading frame 72 (C9orf72). These repeats produce dipeptide repeat proteins with poly(PR) being the most toxic one.

METHODS

We performed a kinome-wide CRISPR/Cas9 knock-out screen in human induced pluripotent stem cell (iPSC) -derived cortical neurons to identify modifiers of poly(PR) toxicity, and validated the role of candidate modifiers using in vitro, in vivo, and ex-vivo studies.

RESULTS

Knock-down of NIMA-related kinase 6 (NEK6) prevented neuronal toxicity caused by poly(PR). Knock-down of nek6 also ameliorated the poly(PR)-induced axonopathy in zebrafish and NEK6 was aberrantly expressed in C9orf72 patients. Suppression of NEK6 expression and NEK6 activity inhibition rescued axonal transport defects in cortical neurons from C9orf72 patient iPSCs, at least partially by reversing p53-related DNA damage.

DISCUSSION

We identified NEK6, which regulates poly(PR)-mediated p53-related DNA damage, as a novel therapeutic target for C9orf72 FTD/ALS.

Lost in local translation: TDP-43 and FUS in axonal/neuromuscular junction maintenance and dysregulation in amyotrophic lateral sclerosis

Key early features of amyotrophic lateral sclerosis (ALS) are denervation of neuromuscular junctions and axonal degeneration. Motor neuron homeostasis relies on local translation through controlled regulation of axonal mRNA localization, transport, and stability. Yet the composition of the local transcriptome, translatome (mRNAs locally translated), and proteome during health and disease remains largely unexplored. This review covers recent discoveries on axonal translation as a critical mechanism for neuronal maintenance/survival. We focus on two RNA binding proteins, transactive response DNA binding protein-43 (TDP-43) and fused in sarcoma (FUS), whose mutations cause ALS and frontotemporal dementia (FTD). Emerging evidence points to their essential role in the maintenance of axons and synapses, including mRNA localization, transport, and local translation, and whose dysfunction may contribute to ALS. Finally, we describe recent advances in omics-based approaches mapping compartment-specific local RNA and protein compositions, which will be invaluable to elucidate fundamental local processes and identify key targets for therapy development.

Aberrant phase separation is a common killing strategy of positively charged peptides in biology and human disease

Positively charged repeat peptides are emerging as key players in neurodegenerative diseases. These peptides can perturb diverse cellular pathways but a unifying framework for how such promiscuous toxicity arises has remained elusive. We used mass-spectrometry-based proteomics to define the protein targets of these neurotoxic peptides and found that they all share similar sequence features that drive their aberrant condensation with these positively charged peptides. We trained a machine learning algorithm to detect such sequence features and unexpectedly discovered that this mode of toxicity is not limited to human repeat expansion disorders but has evolved countless times across the tree of life in the form of cationic antimicrobial and venom peptides. We demonstrate that an excess in positive charge is necessary and sufficient for this killer activity, which we name 'polycation poisoning'. These findings reveal an ancient and conserved mechanism and inform ways to leverage its design rules for new generations of bioactive peptides.

Why should we care about astrocytes in a motor neuron disease?

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease in adults, causing progressive degeneration of motor neurons, which results in muscle atrophy, respiratory failure and ultimately death of the patients. The pathogenesis of ALS is complex, and extensive efforts have focused on unravelling the underlying molecular mechanisms with a large emphasis on the dying motor neurons. However, a recent shift in focus towards the supporting glial population has revealed a large contribution and influence in ALS, which stresses the need to explore this area in more detail. Especially studies into astrocytes, the residential homeostatic supporter cells of neurons, have revealed a remarkable astrocytic dysfunction in ALS, and therefore could present a target for new and promising therapeutic entry points. In this review, we provide an overview of general astrocyte function and summarize the current literature on the role of astrocytes in ALS by categorizing the potentially underlying molecular mechanisms. We discuss the current efforts in astrocyte-targeted therapy, and highlight the potential and shortcomings of available models.

Common mechanisms underlying axonal transport deficits in neurodegenerative diseases: a mini review

Many neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis are characterized by the accumulation of pathogenic proteins and abnormal localization of organelles. These pathological features may be related to axonal transport deficits in neurons, which lead to failures in pathological protein targeting to specific sites for degradation and organelle transportation to designated areas needed for normal physiological functioning. Axonal transport deficits are most likely early pathological events in such diseases and gradually lead to the loss of axonal integrity and other degenerative changes. In this review, we investigated reports of mechanisms underlying the development of axonal transport deficits in a variety of common neurodegenerative diseases, such as Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease and Huntington's disease to provide new ideas for therapeutic targets that may be used early in the disease process. The mechanisms can be summarized as follows: (1) motor protein changes including expression levels and post-translational modification alteration; (2) changes in microtubules including reducing stability and disrupting tracks; (3) changes in cargoes including diminished binding to motor proteins. Future studies should determine which axonal transport defects are disease-specific and whether they are suitable therapeutic targets in neurodegenerative diseases.

Arginine-rich C9ORF72 ALS proteins stall ribosomes in a manner distinct from a canonical ribosome-associated quality control substrate

Hexanucleotide expansion mutations in C9ORF72 are a frequent cause of amyotrophic lateral sclerosis. We previously reported that long arginine-rich dipeptide repeats (DPRs), mimicking abnormal proteins expressed from the hexanucleotide expansion, caused translation stalling when expressed in cell culture models. Whether this stalling provides a mechanism of pathogenicity remains to be determined. Here, we explored the molecular features of DPR-induced stalling and examined whether known mechanisms such as ribosome quality control (RQC) regulate translation elongation on sequences that encode arginine-rich DPRs. We demonstrate that arginine-rich DPRs lead to stalling in a length-dependent manner, with lengths longer than 40 repeats invoking severe translation arrest. Mutational screening of 40×Gly-Xxx DPRs shows that stalling is most pronounced when Xxx is a charged amino acid (Arg, Lys, Glu, or Asp). Through a genome-wide knockout screen, we find that genes regulating stalling on polyadenosine mRNA coding for poly-Lys, a canonical RQC substrate, act differently in the case of arginine-rich DPRs. Indeed, these findings point to a limited scope for natural regulatory responses to resolve the arginine-rich DPR stalls, even though the stalls may be sensed, as evidenced by an upregulation of RQC gene expression. These findings therefore implicate arginine-rich DPR-mediated stalled ribosomes as a source of stress and toxicity and may be a crucial component in pathomechanisms.

C9ORF72-derived poly-GA DPRs undergo endocytic uptake in iAstrocytes and spread to motor neurons

Dipeptide repeat (DPR) proteins are aggregation-prone polypeptides encoded by the pathogenic GGGGCC repeat expansion in the C9ORF72 gene, the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. In this study, we focus on the role of poly-GA DPRs in disease spread. We demonstrate that recombinant poly-GA oligomers can directly convert into solid-like aggregates and form characteristic β-sheet fibrils in vitro. To dissect the process of cell-to-cell DPR transmission, we closely follow the fate of poly-GA DPRs in either their oligomeric or fibrillized form after administration in the cell culture medium. We observe that poly-GA DPRs are taken up via dynamin-dependent and -independent endocytosis, eventually converging at the lysosomal compartment and leading to axonal swellings in neurons. We then use a co-culture system to demonstrate astrocyte-to-motor neuron DPR propagation, showing that astrocytes may internalise and release aberrant peptides in disease pathogenesis. Overall, our results shed light on the mechanisms of poly-GA cellular uptake and propagation, suggesting lysosomal impairment as a possible feature underlying the cellular pathogenicity of these DPR species.

HNRNPK alleviates RNA toxicity by counteracting DNA damage in C9orf72 ALS

A 'GGGGCC' repeat expansion in the first intron of the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The exact mechanism resulting in these neurodegenerative diseases remains elusive, but C9 repeat RNA toxicity has been implicated as a gain-of-function mechanism. Our aim was to use a zebrafish model for C9orf72 RNA toxicity to identify modifiers of the ALS-linked phenotype. We discovered that the RNA-binding protein heterogeneous nuclear ribonucleoprotein K (HNRNPK) reverses the toxicity of both sense and antisense repeat RNA, which is dependent on its subcellular localization and RNA recognition, and not on C9orf72 repeat RNA binding. We observed HNRNPK cytoplasmic mislocalization in C9orf72 ALS patient fibroblasts, induced pluripotent stem cell (iPSC)-derived motor neurons and post-mortem motor cortex and spinal cord, in line with a disrupted HNRNPK function in C9orf72 ALS. In C9orf72 ALS/FTD patient tissue, we discovered an increased nuclear translocation, but reduced expression of ribonucleotide reductase regulatory subunit M2 (RRM2), a downstream target of HNRNPK involved in the DNA damage response. Last but not least, we showed that increasing the expression of HNRNPK or RRM2 was sufficient to mitigate DNA damage in our C9orf72 RNA toxicity zebrafish model. Overall, our study strengthens the relevance of RNA toxicity as a pathogenic mechanism in C9orf72 ALS and demonstrates its link with an aberrant DNA damage response, opening novel therapeutic avenues for C9orf72 ALS/FTD.

Defective axonal transport of endo-lysosomes and dense core vesicles in a Drosophila model of C9-ALS/FTD

A GGGGCC (G₄ C₂ ) repeat expansion in the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Although disruptions in axonal transport are implicated in the pathogenesis of multiple neurodegenerative diseases, the underlying mechanisms causing these defects remain unclear. Here, we performed live imaging of Drosophila motor neurons expressing expanded G₄ C₂ repeats in third-instar larvae and investigated the axonal transport of multiple organelles in vivo. Expression of expanded G₄ C₂ repeats causes an increase in static axonal lysosomes, while it impairs trafficking of late endosomes (LEs) and dense core vesicles (DCVs). Surprisingly, however, axonal transport of mitochondria is unaffected in motor axons expressing expanded G₄ C₂ repeats. Thus, our data indicate that expanded G₄ C₂ repeat expression differentially impacts axonal transport of vesicular organelles and mitochondria in Drosophila models of C9orf72-associated ALS/FTD. This article is protected by copyright. All rights reserved.

Subpial delivery of adeno-associated virus 9-synapsin-caveolin-1 (AAV9-SynCav1) preserves motor neuron and neuromuscular junction morphology, motor function, delays disease onset, and extends survival in hSOD1G93A mice

Elevating neuroprotective proteins using adeno-associated virus (AAV)-mediated gene delivery shows great promise in combating devastating neurodegenerative diseases. Amyotrophic lateral sclerosis (ALS) is one such disease resulting from loss of upper and lower motor neurons (MNs) with 90-95% of cases sporadic (SALS) in nature. Due to the unknown etiology of SALS, interventions that afford neuronal protection and preservation are urgently needed. Caveolin-1 (Cav-1), a membrane/lipid rafts (MLRs) scaffolding and neuroprotective protein, and MLR-associated signaling components are decreased in degenerating neurons in postmortem human brains. We previously showed that, when crossing our SynCav1 transgenic mouse (TG) with the mutant human superoxide dismutase 1 (hSOD1G93A) mouse model of ALS, the double transgenic mouse (SynCav1 TG/hSOD1G93A) exhibited better motor function and longer survival. The objective of the current study was to test whether neuron-targeted Cav-1 upregulation in the spinal cord using AAV9-SynCav1 could improve motor function and extend longevity in mutant humanized mouse and rat (hSOD1G93A) models of familial (F)ALS. Methods: Motor function was assessed by voluntary running wheel (RW) in mice and forelimb grip strength (GS) and motor evoked potentials (MEP) in rats. Immunofluorescence (IF) microscopy for choline acetyltransferase (ChAT) was used to assess MN morphology. Neuromuscular junctions (NMJs) were measured by bungarotoxin-a (Btx-a) and synaptophysin IF. Body weight (BW) was measured weekly, and the survival curve was determined by Kaplan-Meier analysis. Results: Following subpial gene delivery to the lumbar spinal cord, male and female hSOD1G93A mice treated with SynCav1 exhibited delayed disease onset, greater running-wheel performance, preserved spinal alpha-motor neuron morphology and NMJ integrity, and 10% increased longevity, independent of affecting expression of the mutant hSOD1G93A protein. Cervical subpial SynCav1 delivery to hSOD1G93A rats preserved forelimb GS and MEPs in the brachial and gastrocnemius muscles. Conclusion: In summary, subpial delivery of SynCav1 protects and preserves spinal motor neurons, and extends longevity in a familial mouse model of ALS without reducing the toxic monogenic component. Furthermore, subpial SynCav1 delivery preserved neuromuscular function in a rat model of FALS. The latter findings strongly indicate the therapeutic applicability of SynCav1 to treat ALS attributed to monogenic (FALS) and potentially in sporadic cases (i.e., SALS).

Multiple roles for the cytoskeleton in ALS

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease caused by more than sixty genes identified through classic linkage analysis and new sequencing methods. Yet no clear mechanism of onset, cure, or effective treatment is known. Popular discourse classifies the proteins encoded from ALS-related genes into four disrupted processes: proteostasis, mitochondrial function and ROS, nucleic acid regulation, and cytoskeletal dynamics. Surprisingly, the mechanisms detailing the contribution of the neuronal cytoskeletal in ALS are the least explored, despite involvement in these cell processes. Eight genes directly regulate properties of cytoskeleton function and are essential for the health and survival of motor neurons, including: TUBA4A, SPAST, KIF5A, DCTN1, NF, PRPH, ALS2, and PFN1. Here we review the properties and studies exploring the contribution of each of these genes to ALS.

The Interplay Between Autophagy and RNA Homeostasis: Implications for Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis and frontotemporal dementia are neurodegenerative disorders that lie on a disease spectrum, sharing genetic causes and pathology, and both without effective therapeutics. Two pathways that have been shown to play major roles in disease pathogenesis are autophagy and RNA homeostasis. Intriguingly, there is an increasing body of evidence suggesting a critical interplay between these pathways. Autophagy is a multi-stage process for bulk and selective clearance of malfunctional cellular components, with many layers of regulation. Although the majority of autophagy research focuses on protein degradation, it can also mediate RNA catabolism. ALS/FTD-associated proteins are involved in many stages of autophagy and autophagy-mediated RNA degradation, particularly converging on the clearance of persistent pathological stress granules. In this review, we will summarise the progress in understanding the autophagy-RNA homeostasis interplay and how that knowledge contributes to our understanding of the pathobiology of ALS/FTD.

ALS-associated KIF5A mutations abolish autoinhibition resulting in a toxic gain of function

Understanding the pathogenic mechanisms of disease mutations is critical to advancing treatments. ALS-associated mutations in the gene encoding the microtubule motor KIF5A result in skipping of exon 27 (KIF5AΔExon27) and the encoding of a protein with a novel 39 amino acid residue C-terminal sequence. Here, we report that expression of ALS-linked mutant KIF5A results in dysregulated motor activity, cellular mislocalization, altered axonal transport, and decreased neuronal survival. Single-molecule analysis revealed that the altered C terminus of mutant KIF5A results in a constitutively active state. Furthermore, mutant KIF5A possesses altered protein and RNA interactions and its expression results in altered gene expression/splicing. Taken together, our data support the hypothesis that causative ALS mutations result in a toxic gain of function in the intracellular motor KIF5A that disrupts intracellular trafficking and neuronal homeostasis.

The role of inflammation in neurodegeneration: novel insights into the role of the immune system in C9orf72 HRE-mediated ALS/FTD

Neuroinflammation is an important hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). An inflammatory reaction to neuronal injury is deemed vital for neuronal health and homeostasis. However, a continued activation of the inflammatory response can be detrimental to remaining neurons and aggravate the disease process. Apart from a disease modifying role, some evidence suggests that neuroinflammation may also contribute to the upstream cause of the disease. In this review, we will first focus on the role of neuroinflammation in the pathogenesis of chromosome 9 open reading frame 72 gene (C9orf72) hexanucleotide repeat expansions (HRE)-mediated ALS/FTD (C9-ALS/FTD). Additionally, we will discuss evidence from ex vivo and in vivo studies and finally, we briefly summarize the trials and progress of anti-inflammatory therapies.

Frontotemporal Lobar Degeneration Case with an N-Terminal TUBA4A Mutation Exhibits Reduced TUBA4A Levels in the Brain and TDP-43 Pathology

Recently, disease-associated variants of the TUBA4A gene were identified in patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we present the neuropathological report of a patient with the semantic variant of primary progressive aphasia with a family history of Parkinsonism, harboring a novel frameshift mutation c.187del (p.Arg64Glyfs*90) in TUBA4A. Immunohistochemistry showed abundant TAR DNA-binding protein 43 kDa (TDP-43) dystrophic neurite pathology in the frontal and temporal cortex and the dentate gyrus of the hippocampus, consistent with frontotemporal lobar degeneration (FTLD). The observed pathology pattern fitted best with that of FTLD-TDP Type C. qPCR showed the presence of mutant TUBA4A mRNA. However, no truncated TUBA4A was detected at the protein level. A decrease in total TUBA4A mRNA and protein levels suggests loss-of-function as a potential pathogenic mechanism. This report strengthens the idea that N-terminal TUBA4A mutations are associated with FTLD-TDP. These N-terminal mutations possibly exert their pathogenic effects through haploinsufficiency, contrary to C-terminal TUBA4A mutations which are thought to disturb the microtubule network via a dominant-negative mechanism.

Disruption of ER-mitochondria tethering and signalling in C9orf72-associated amyotrophic lateral sclerosis and frontotemporal dementia

Hexanucleotide repeat expansions in C9orf72 are the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The mechanisms by which the expansions cause disease are not properly understood but a favoured route involves its translation into dipeptide repeat (DPR) polypeptides, some of which are neurotoxic. However, the precise targets for mutant C9orf72 and DPR toxicity are not fully clear, and damage to several neuronal functions has been described. Many of these functions are regulated by signalling between the endoplasmic reticulum (ER) and mitochondria. ER‐mitochondria signalling requires close physical contacts between the two organelles that are mediated by the VAPB‐PTPIP51 ‘tethering’ proteins. Here, we show that ER‐mitochondria signalling and the VAPB‐PTPIP51 tethers are disrupted in neurons derived from induced pluripotent stem (iPS) cells from patients carrying ALS/FTD pathogenic C9orf72 expansions and in affected neurons in mutant C9orf72 transgenic mice. In these mice, disruption of the VAPB‐PTPIP51 tethers occurs prior to disease onset suggesting that it contributes to the pathogenic process. We also show that neurotoxic DPRs disrupt the VAPB‐PTPIP51 interaction and ER‐mitochondria contacts and that this may involve activation of glycogen synthase kinases‐3β (GSK3β), a known negative regulator of VAPB‐PTPIP51 binding. Finally, we show that these DPRs disrupt delivery of Ca²⁺ from ER stores to mitochondria, which is a primary function of the VAPB‐PTPIP51 tethers. This delivery regulates a number of key neuronal functions that are damaged in ALS/FTD including bioenergetics, autophagy and synaptic function. Our findings reveal a new molecular target for mutant C9orf72‐mediated toxicity.

Assessment of Mitochondrial Trafficking as a Surrogate for Fast Axonal Transport in Human Induced Pluripotent Stem Cell-Derived Spinal Motor Neurons

Axonal transport is essential for the development, function, and survival of the nervous system. In an energy-demanding process, motor proteins act in concert with microtubules to deliver cargoes, such as organelles, from one end of the axon to the other. Perturbations in axonal transport are a prominent phenotype of many neurodegenerative diseases, including amyotrophic lateral sclerosis. Here, we describe a simple method to fluorescently label mitochondrial cargo, a surrogate for fast axonal transport, in human induced pluripotent stem cell-derived motor neurons. This method enables the sparse labeling of axons to track directionality of movement and can be adapted to assess not only the cell autonomous effects of a genetic mutation on axonal transport but also the cell non-autonomous effects, through the use of conditioned medium and/or co-culture systems.

RAN proteins in neurodegenerative disease: Repeating themes and unifying therapeutic strategies

Microsatellite-expansion mutations cause >50 neurological diseases but there are no effective treatments. Mechanistic studies have historically focused on protein loss-of-function and protein or RNA gain-of-function effects. It is now clear that many expansion mutations are bidirectionally transcribed producing two toxic expansion RNAs, which can produce up to six mutant proteins by repeat associated non-AUG (RAN) translation. Multiple types of RAN proteins have been shown to be toxic in cell and animal models, to lead to common types of neuropathological changes, and to dysregulate key pathways. How RAN proteins are produced without the canonical AUG or close-cognate AUG-like initiation codons is not yet completely understood but RNA structure, flanking sequences and stress pathways have been shown to be important. Here, we summarize recent progress in understanding the role of RAN proteins, mechanistic insights into their production, and the identification of novel therapeutic strategies that may be applicable across these neurodegenerative disorders.

DNA Damage and Repair Deficiency in ALS/FTD-Associated Neurodegeneration: From Molecular Mechanisms to Therapeutic Implication

Emerging studies reveal that neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), are commonly linked to DNA damage accumulation and repair deficiency. Neurons are particularly vulnerable to DNA damage due to their high metabolic activity, relying primarily on oxidative phosphorylation, which leads to increased reactive oxygen species (ROS) generation and subsequent DNA damage. Efficient and timely repair of such damage is critical for guarding the integrity of genomic DNA and for cell survival. Several genes predominantly associated with RNA/DNA metabolism have been implicated in both ALS and FTD, suggesting that the two diseases share a common underlying pathology with varied clinical manifestations. Recent studies reveal that many of the gene products, including RNA/DNA binding proteins (RBPs) TDP-43 and FUS are involved in diverse DNA repair pathways. A key question in the etiology of the ALS/FTD spectrum of neurodegeneration is the mechanisms and pathways involved in genome instability caused by dysfunctions/mutations of those RBP genes and their consequences in the central nervous system. The understanding of such converging molecular mechanisms provides insights into the underlying etiology of the rapidly progressing neurodegeneration in ALS/FTD, while also revealing novel DNA repair target avenues for therapeutic development. In this review, we summarize the common mechanisms of neurodegeneration in ALS and FTD, with a particular emphasis on the DNA repair defects induced by ALS/FTD causative genes. We also highlight the consequences of DNA repair defects in ALS/FTD and the therapeutic potential of DNA damage repair-targeted amelioration of neurodegeneration.

Order controls disordered droplets: structure-function relationships in C9orf72-derived poly(PR)

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have been thought as two distinct neurodegenerative diseases. However, recent genetic screening and careful investigations found the genetic and pathological overlap among these disorders. Hexanucleotide expansions in intron 1 of C9orf72 are a leading cause of familial ALS and familial FTD. These expansions facilitate the repeat-associated non-ATG initiated translation (RAN translation), producing five dipeptide repeat proteins (DRPs), including Arg-rich poly(PR: Pro-Arg) and poly-(GR: Gly-Arg) peptides. Arg is a positively charged, highly polar amino acid that facilitates interactions with anionic molecules such as nucleic acids and acidic amino acids via electrostatic forces and aromatic amino acids via cation-pi interaction, suggesting that Arg-rich DRPs underlie the pathophysiology of ALS via Arg-mediated molecular interactions. Arg-rich DRPs have also been reported to induce neurodegeneration in cellular and animal models via multiple mechanisms; however, it remains unclear why the Arg-rich DRPs exhibit such diverse toxic properties, because not all Arg-rich peptides are toxic. In this mini-review, we discuss the current understanding of the pathophysiology of Arg-rich C9orf72 DRPs and introduce recent findings on the role of Arg distribution as a determinant of the toxicity and its contribution to the pathogenesis of ALS.

The Role of Nucleocytoplasmic Transport Defects in Amyotrophic Lateral Sclerosis

There is ample evidence that nucleocytoplasmic-transport deficits could play an important role in the pathology of amyotrophic lateral sclerosis (ALS). However, the currently available data are often circumstantial and do not fully clarify the exact causal and temporal role of nucleocytoplasmic transport deficits in ALS patients. Gaining this knowledge will be of great significance in order to be able to target therapeutically nucleocytoplasmic transport and/or the proteins involved in this process. The availability of good model systems to study the nucleocytoplasmic transport process in detail will be especially crucial in investigating the effect of different mutations, as well as of other forms of stress. In this review, we discuss the evidence for the involvement of nucleocytoplasmic transport defects in ALS and the methods used to obtain these data. In addition, we provide an overview of the therapeutic strategies which could potentially counteract these defects.