Nuclear contour irregularity and abnormal transporter protein distribution in anterior horn cells in amyotrophic lateral sclerosis

Nuclear translocation of the LINE-1 encoded ORF1 protein alters nuclear envelope integrity in human neurons

LINE-1 retrotransposons are increasingly implicated in aging and neurodegenerative diseases, yet the precise pathogenic mechanisms remain elusive. While the endonuclease and reverse transcriptase activities of LINE-1-encoded ORF2p can induce DNA damage and inflammation, a role of LINE-1 ORF1p in cellular dysfunctions stays unassigned. Here we demonstrate, using a neuronal cellular model, that ORF1p translocates into the nucleus upon arsenite-induced stress, directly interacting with nuclear import (KPNB1), nuclear pore complex (NUP153), and nuclear lamina (Lamin B1) proteins. Nuclear translocation of ORF1p disrupts nuclear integrity, nucleocytoplasmic transport, and heterochromatin structure, features linked to neurodegeneration and aging. Elevated nuclear ORF1p levels induced either by arsenite-induced stress, ORF1p overexpression, or as observed in Parkinson's disease post-mortem brain tissues correlate with impaired nuclear envelope (NE) morphology. Stress-induced nuclear alterations are mitigated by blocking ORF1p nuclear import or with the anti-aging drug remodelin. This study thus reveals a pathogenic action of nuclear ORF1p in human neurons driving NE alterations and thereby contributing to LINE-1-mediated cell toxicity.

Using ALS to understand profilin 1's diverse roles in cellular physiology

Profilin is an actin monomer-binding protein whose role in actin polymerization has been studied for nearly 50 years. While its principal biochemical features are now well understood, many questions remain about how profilin controls diverse processes within the cell. Dysregulation of profilin has been implicated in a broad range of human diseases, including neurodegeneration, inflammatory disorders, cardiac disease, and cancer. For example, mutations in the profilin 1 gene (PFN1) can cause amyotrophic lateral sclerosis (ALS), although the precise mechanisms that drive neurodegeneration remain unclear. While initial work suggested proteostasis and actin cytoskeleton defects as the main pathological pathways, multiple novel functions for PFN1 have since been discovered that may also contribute to ALS, including the regulation of nucleocytoplasmic transport, stress granules, mitochondria, and microtubules. Here, we will review these newly discovered roles for PFN1, speculate on their contribution to ALS, and discuss how defects in actin can contribute to these processes. By understanding profilin 1's involvement in ALS pathogenesis, we hope to gain insight into this functionally complex protein with significant influence over cellular physiology.

Increased nuclear import characterizes aberrant nucleocytoplasmic transport in neurons from patients with spinocerebellar ataxia type 7

Introduction

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disorder characterized by cerebellar and retinal degeneration. SCA7 is caused by a CAG-polyglutamine repeat expansion in the ataxin-7 gene, which encodes a transcription factor protein that is a core component of the STAGA co-activator complex. As ataxin-7 protein regularly shuttles between the nucleus and the cytosol, we sought to test if polyglutamine-expanded ataxin-7 protein results in nuclear membrane abnormalities or defects in nucleocytoplasmic (N/C) transport.

Methods

We used SCA7 266Q knock-in mice and their wild-type (WT) littermate controls to assess nuclear membrane morphology and N/C transport. Additionally, induced pluripotent stem cells (iPSCs) from SCA7 patients were differentiated into neural progenitor cells (NPCs) and cortical neurons to measure nuclear import and export dynamics. The expression of nucleoporin POM121, a key regulator of N/C transport, was also analyzed in SCA7-derived NPCs.

Results

Our analysis revealed no significant differences in nuclear membrane morphology between SCA7 knock-in mice and WT controls, nor did we observe alterations in N/C transport within neurons from these mice. However, we documented significantly increased nuclear import in both NPCs and cortical neurons derived from SCA7 patient iPSCs. When we examined nuclear export function in SCA7 iPSC-derived cortical neurons, we noted a modest decrease that constituted only a trend. Furthermore, we identified a significant decrease in the expression of full-length POM121 in SCA7 NPCs.

Discussion

Our results reveal evidence for altered N/C transport in SCA7. The reduction in POM121 expression suggests a potential mechanism underlying these transport abnormalities. Importantly, our data suggests the N/C transport defect in SCA7 is distinctly different from other related neurodegenerative disorders.

Multiple lines of evidence for disruption of nuclear lamina and nucleoporins in FUS amyotrophic lateral sclerosis

Advanced pathological and genetic approaches have revealed that mutations in fused in sarcoma/translated in liposarcoma (FUS/TLS), which is pivotal for DNA repair, alternative splicing, translation and RNA transport, cause familial amyotrophic lateral sclerosis (ALS). The generation of suitable animal models for ALS is essential for understanding its pathogenesis and developing therapies. Therefore, we used CRISPR-Cas9 to generate FUS-ALS mutation in the non-classical nuclear localization signal (NLS), H517D (mouse position: H509D) and genome-edited mice. Fus WT/H509D mice showed progressive motor impairment (accelerating rotarod and DigiGait system) with age, which was associated with the loss of motor neurons and disruption of the nuclear lamina and nucleoporins and DNA damage in spinal cord motor neurons. We confirmed the validity of our model by showing that nuclear lamina and nucleoporin disruption were observed in lower motor neurons differentiated from patient-derived human induced pluripotent stem cells (hiPSC-LMNs) with FUS-H517D and in the post-mortem spinal cord of patients with ALS. RNA sequence analysis revealed that most nuclear lamina and nucleoporin-linking genes were significantly decreased in FUS-H517D hiPSC-LMNs. This evidence suggests that disruption of the nuclear lamina and nucleoporins is crucial for ALS pathomechanisms. Combined with patient-derived hiPSC-LMNs and autopsy samples, this mouse model might provide a more reliable understanding of ALS pathogenesis and might aid in the development of therapeutic strategies.

Morphometric analysis of spinal motor neuron degeneration in sporadic amyotrophic lateral sclerosis

OBJECTIVES

This study aimed to clarify the relationship between 43-kDa TAR DNA-binding protein (TDP-43) pathology and spinal cord anterior horn motor neuron (AHMN) atrophy in sporadic amyotrophic lateral sclerosis (SALS).

METHODS

Eight patients with SALS and 12 controls were included in this study. Formalin-fixed specimens of lumbar spinal cord samples were paraffin-embedded and sectioned at the level of the fourth lumbar spinal cord with a 4 μm thickness. Using a microscope, the long diameters of the neurons with nucleoli were measured in spinal AHMNs stained with an anti-SMI-32 antibody. AHMNs were divided into medial and lateral nuclei for statistical analysis. We also used previously reported data to measure the long diameter of AHMNs with initial TDP-43 pathology, in which TDP-43 was present both in the nucleus and cytoplasm.

RESULTS

The long diameter of the lumbar spinal AHMNs in patients with SALS was smaller in the medial nucleus (42.54 ± 9.33 μm, n = 24) and the lateral nucleus (49.41 ± 13.86 μm, n = 129) than in controls (medial nucleus: 55.84 ± 13.49 μm, n = 85, p < 0.001; lateral nucleus: 62.39 ± 13.29 μm, n = 756, p < 0.001, Mann-Whitney U test). All 21 motor neurons with initial TDP-43 pathology were in the lateral nucleus, and their long diameter (67.60 ± 18.3 μm, p = 0.352) was not significantly different from that of controls.

CONCLUSION

Motor neuron atrophy in SALS does not occur during the initial stages of TDP-43 pathology, and TDP-43 pathology is already advanced in the atrophied motor neurons.

Nuclear Localization of Human SOD1 in Motor Neurons in Mouse Model and Patient Amyotrophic Lateral Sclerosis: Possible Links to Cholinergic Phenotype, NADPH Oxidase, Oxidative Stress, and DNA Damage

Amyotrophic lateral sclerosis (ALS) is a fatal disease that causes degeneration of motor neurons (MNs) and paralysis. ALS can be caused by mutations in the gene that encodes copper/zinc superoxide dismutase (SOD1). SOD1 is known mostly as a cytosolic antioxidant protein, but SOD1 is also in the nucleus of non-transgenic (tg) and human SOD1 (hSOD1) tg mouse MNs. SOD1's nuclear presence in different cell types and subnuclear compartmentations are unknown, as are the nuclear functions of SOD1. We examined hSOD1 nuclear localization and DNA damage in tg mice expressing mutated and wildtype variants of hSOD1 (hSOD1-G93A and hSOD1-wildtype). We also studied ALS patient-derived induced pluripotent stem (iPS) cells to determine the nuclear presence of SOD1 in undifferentiated and differentiated MNs. In hSOD1-G93A and hSOD1-wildtype tg mice, choline acetyltransferase (ChAT)-positive MNs had nuclear hSOD1, but while hSOD1-wildtype mouse MNs also had nuclear ChAT, hSOD1-G93A mouse MNs showed symptom-related loss of nuclear ChAT. The interneurons had preserved parvalbumin nuclear positivity in hSOD1-G93A mice. hSOD1-G93A was seen less commonly in spinal cord astrocytes and, notably, oligodendrocytes, but as the disease emerged, the oligodendrocytes had increased mutant hSOD1 nuclear presence. Brain and spinal cord subcellular fractionation identified mutant hSOD1 in soluble nuclear extracts of the brain and spinal cord, but mutant hSOD1 was concentrated in the chromatin nuclear extract only in the spinal cord. Nuclear extracts from mutant hSOD1 tg mouse spinal cords had altered protein nitration, footprinting peroxynitrite presence, and the intact nuclear extracts had strongly increased superoxide production as well as the active NADPH oxidase marker, p47phox. The comet assay showed that MNs from hSOD1-G93A mice progressively (6-14 weeks of age) accumulated DNA single-strand breaks. Ablation of the NCF1 gene, encoding p47phox, and pharmacological inhibition of NADPH oxidase with systemic treatment of apocynin (10 mg/kg, ip) extended the mean lifespan of hSOD1-G93A mice by about 25% and mitigated genomic DNA damage progression. In human postmortem CNS, SOD1 was found in the nucleus of neurons and glia; nuclear SOD1 was increased in degenerating neurons in ALS cases and formed inclusions. Human iPS cells had nuclear SOD1 during directed differentiation to MNs, but mutant SOD1-expressing cells failed to establish wildtype MN nuclear SOD1 levels. We conclude that SOD1 has a prominent nuclear presence in the central nervous system, perhaps adopting aberrant contexts to participate in ALS pathobiology.

Nuclear-import receptors as gatekeepers of pathological phase transitions in ALS/FTD

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal neurodegenerative disorders on a disease spectrum that are characterized by the cytoplasmic mislocalization and aberrant phase transitions of prion-like RNA-binding proteins (RBPs). The common accumulation of TAR DNA-binding protein-43 (TDP-43), fused in sarcoma (FUS), and other nuclear RBPs in detergent-insoluble aggregates in the cytoplasm of degenerating neurons in ALS/FTD is connected to nuclear pore dysfunction and other defects in the nucleocytoplasmic transport machinery. Recent advances suggest that beyond their canonical role in the nuclear import of protein cargoes, nuclear-import receptors (NIRs) can prevent and reverse aberrant phase transitions of TDP-43, FUS, and related prion-like RBPs and restore their nuclear localization and function. Here, we showcase the NIR family and how they recognize cargo, drive nuclear import, and chaperone prion-like RBPs linked to ALS/FTD. We also discuss the promise of enhancing NIR levels and developing potentiated NIR variants as therapeutic strategies for ALS/FTD and related neurodegenerative proteinopathies.

Nuclear pore complex and nucleocytoplasmic transport disruption in neurodegeneration

Nuclear pore complexes (NPCs) play a critical role in maintaining the equilibrium between the nucleus and cytoplasm, enabling bidirectional transport across the nuclear envelope, and are essential for proper nuclear organization and gene regulation. Perturbations in the regulatory mechanisms governing NPCs and nuclear envelope homeostasis have been implicated in the pathogenesis of several neurodegenerative diseases. The ESCRT-III pathway emerges as a critical player in the surveillance and preservation of well-assembled, functional NPCs, as well as nuclear envelope sealing. Recent studies have provided insights into the involvement of nuclear ESCRT-III in the selective reduction of specific nucleoporins associated with neurodegenerative pathologies. Thus, maintaining quality control of the nuclear envelope and NPCs represents a pivotal element in the pathological cascade leading to neurodegenerative diseases. This review describes the constituents of the nuclear-cytoplasmic transport machinery, encompassing the nuclear envelope, NPC, and ESCRT proteins, and how their structural and functional alterations contribute to the development of neurodegenerative diseases.

Therapeutic Effects of Combination of Nebivolol and Donepezil: Targeting Multifactorial Mechanisms in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive loss of motor neurons in the spinal cord. Although the disease's pathophysiological mechanism remains poorly understood, multifactorial mechanisms affecting motor neuron loss converge to worsen the disease. Although two FDA-approved drugs, riluzole and edaravone, targeting excitotoxicity and oxidative stress, respectively, are available, their efficacies are limited to extending survival by only a few months. Here, we developed combinatorial drugs targeting multifactorial mechanisms underlying key components in ALS disease progression. Using data analysis based on the genetic information of patients with ALS-derived cells and pharmacogenomic data of the drugs, a combination of nebivolol and donepezil (nebivolol-donepezil) was identified for ALS therapy. Here, nebivolol-donepezil markedly reduced the levels of cytokines in the microglial cell line, inhibited nuclear factor-κB (NF-κB) nucleus translocation in the HeLa cell and substantially protected against excitotoxicity-induced neuronal loss by regulating the PI3K-Akt pathway. Nebivolol-donepezil significantly promoted the differentiation of neural progenitor cells (NPC) into motor neurons. Furthermore, we verified the low dose efficacy of nebivolol-donepezil on multiple indices corresponding to the quality of life of patients with ALS in vivo using SOD1G93A mice. Nebivolol-donepezil delayed motor function deterioration and halted motor neuronal loss in the spinal cord. Drug administration effectively suppressed muscle atrophy by mitigating the proportion of smaller myofibers and substantially reducing phospho-neurofilament heavy chain (pNF-H) levels in the serum, a promising ALS biomarker. High-dose nebivolol-donepezil significantly prolonged survival and delayed disease onset compared with vehicle-treated mice. These results indicate that the combination of nebivolol-donepezil efficiently prevents ALS disease progression, benefiting the patients' quality of life and life expectancy.

Unraveling the impact of disrupted nucleocytoplasmic transport systems in C9orf72-associated ALS

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two adult-onset neurodegenerative diseases that are part of a common disease spectrum due to clinical, genetic, and pathological overlap. A prominent genetic factor contributing to both diseases is a hexanucleotide repeat expansion in a non-coding region of the C9orf72 gene. This mutation in C9orf72 leads to nuclear depletion and cytoplasmic aggregation of Tar DNA-RNA binding protein 43 (TDP-43). TDP-43 pathology is characteristic of the majority of ALS cases, irrespective of disease causation, and is present in ~50% of FTD cases. Defects in nucleocytoplasmic transport involving the nuclear pore complex, the Ran-GTPase cycle, and nuclear transport factors have been linked with the mislocalization of TDP-43. Here, we will explore and discuss the implications of these system abnormalities of nucleocytoplasmic transport in C9orf72-ALS/FTD, as well as in other forms of familial and sporadic ALS.

Aberrantly Expressed Hsa_circ_0060762 and CSE1L as Potential Peripheral Blood Biomarkers for ALS

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive adult-onset neurodegenerative disease that is often diagnosed with a delay due to initial non-specific symptoms. Therefore, reliable and easy-to-obtain biomarkers are an absolute necessity for earlier and more accurate diagnostics. Circular RNAs (circRNAs) have already been proposed as potential biomarkers for several neurodegenerative diseases. In this study, we further investigated the usefulness of circRNAs as potential biomarkers for ALS. We first performed a microarray analysis of circRNAs on peripheral blood mononuclear cells of a subset of ALS patients and controls. Among the differently expressed circRNA by microarray analysis, we selected only the ones with a host gene that harbors the highest level of conservation and genetic constraints. This selection was based on the hypothesis that genes under selective pressure and genetic constraints could have a major role in determining a trait or disease. Then we performed a linear regression between ALS cases and controls using each circRNA as a predictor variable. With a False Discovery Rate (FDR) threshold of 0.1, only six circRNAs passed the filtering and only one of them remained statistically significant after Bonferroni correction: hsa_circ_0060762 and its host gene CSE1L. Finally, we observed a significant difference in expression levels between larger sets of patients and healthy controls for both hsa_circ_0060762 and CSE1L. CSE1L is a member of the importin β family and mediates inhibition of TDP-43 aggregation; the central pathogenicity in ALS and hsa_circ_0060762 has binding sites for several miRNAs that have been already proposed as biomarkers for ALS. In addition, receiver operating characteristics curve analysis showed diagnostic potential for CSE1L and hsa_circ_0060762. Hsa_circ_0060762 and CSE1L thus represent novel potential peripheral blood biomarkers and therapeutic targets for ALS.

Loss of C9orf72 perturbs the Ran-GTPase gradient and nucleocytoplasmic transport, generating compositionally diverse Importin β-1 granules

A hexanucleotide (GGGGCC)_n repeat expansion in C9orf72 causes amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), eliciting toxic effects through generation of RNA foci, dipeptide repeat proteins, and/or loss of C9orf72 protein. Defects in nucleocytoplasmic transport (NCT) have been implicated as a pathogenic mechanism underlying repeat expansion toxicity. Here, we show that loss of C9orf72 disrupts the Ran-GTPase gradient and NCT in vitro and in vivo. NCT disruption in vivo is enhanced by the presence of compositionally different types of cytoplasmic Importin β-1 granule that exhibit neuronal subtype-specific properties. We show that the abundance of Importin β-1 granules is increased in the context of C9orf72 deficiency, disrupting interactions with nuclear pore complex proteins. These granules appear to associate with the nuclear envelope and are co-immunoreactive for G3BP1 and K63-ubiquitin. These findings link loss of C9orf72 protein to gain-of-function mechanisms and defects in NCT.

Nuclear import receptors are recruited by FG-nucleoporins to rescue hallmarks of TDP-43 proteinopathy

BACKGROUND

Cytoplasmic mislocalization and aggregation of TAR DNA-binding protein-43 (TDP-43) is a hallmark of the amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) disease spectrum, causing both nuclear loss-of-function and cytoplasmic toxic gain-of-function phenotypes. While TDP-43 proteinopathy has been associated with defects in nucleocytoplasmic transport, this process is still poorly understood. Here we study the role of karyopherin-β1 (KPNB1) and other nuclear import receptors in regulating TDP-43 pathology.

METHODS

We used immunostaining, immunoprecipitation, biochemical and toxicity assays in cell lines, primary neuron and organotypic mouse brain slice cultures, to determine the impact of KPNB1 on the solubility, localization, and toxicity of pathological TDP-43 constructs. Postmortem patient brain and spinal cord tissue was stained to assess KPNB1 colocalization with TDP-43 inclusions. Turbidity assays were employed to study the dissolution and prevention of aggregation of recombinant TDP-43 fibrils in vitro. Fly models of TDP-43 proteinopathy were used to determine the effect of KPNB1 on their neurodegenerative phenotype in vivo.

RESULTS

We discovered that several members of the nuclear import receptor protein family can reduce the formation of pathological TDP-43 aggregates. Using KPNB1 as a model, we found that its activity depends on the prion-like C-terminal region of TDP-43, which mediates the co-aggregation with phenylalanine and glycine-rich nucleoporins (FG-Nups) such as Nup62. KPNB1 is recruited into these co-aggregates where it acts as a molecular chaperone that reverses aberrant phase transition of Nup62 and TDP-43. These findings are supported by the discovery that Nup62 and KPNB1 are also sequestered into pathological TDP-43 aggregates in ALS/FTD postmortem CNS tissue, and by the identification of the fly ortholog of KPNB1 as a strong protective modifier in Drosophila models of TDP-43 proteinopathy. Our results show that KPNB1 can rescue all hallmarks of TDP-43 pathology, by restoring its solubility and nuclear localization, and reducing neurodegeneration in cellular and animal models of ALS/FTD.

CONCLUSION

Our findings suggest a novel NLS-independent mechanism where, analogous to its canonical role in dissolving the diffusion barrier formed by FG-Nups in the nuclear pore, KPNB1 is recruited into TDP-43/FG-Nup co-aggregates present in TDP-43 proteinopathies and therapeutically reverses their deleterious phase transition and mislocalization, mitigating neurodegeneration.

Heat-shock chaperone HSPB1 regulates cytoplasmic TDP-43 phase separation and liquid-to-gel transition

While acetylated, RNA-binding-deficient TDP-43 reversibly phase separates within nuclei into complex droplets (anisosomes) comprised of TDP-43-containing liquid outer shells and liquid centres of HSP70-family chaperones, cytoplasmic aggregates of TDP-43 are hallmarks of multiple neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Here we show that transient oxidative stress, proteasome inhibition or inhibition of the ATP-dependent chaperone activity of HSP70 provokes reversible cytoplasmic TDP-43 de-mixing and transition from liquid to gel/solid, independently of RNA binding or stress granules. Isotope labelling mass spectrometry was used to identify that phase-separated cytoplasmic TDP-43 is bound by the small heat-shock protein HSPB1. Binding is direct, mediated through TDP-43's RNA binding and low-complexity domains. HSPB1 partitions into TDP-43 droplets, inhibits TDP-43 assembly into fibrils, and is essential for disassembly of stress-induced TDP-43 droplets. A decrease in HSPB1 promotes cytoplasmic TDP-43 de-mixing and mislocalization. HSPB1 depletion was identified in spinal motor neurons of patients with ALS containing aggregated TDP-43. These findings identify HSPB1 to be a regulator of cytoplasmic TDP-43 phase separation and aggregation.

Regulating Phase Transition in Neurodegenerative Diseases by Nuclear Import Receptors

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

NUP62 localizes to ALS/FTLD pathological assemblies and contributes to TDP-43 insolubility

A G4C2 hexanucleotide repeat expansion in the C9orf72 gene is the most common genetic cause of ALS and FTLD (C9-ALS/FTLD) with cytoplasmic TDP-43 inclusions observed in regions of neurodegeneration. The accumulation of repetitive RNAs and dipeptide repeat protein (DPR) are two proposed mechanisms of toxicity in C9-ALS/FTLD and linked to impaired nucleocytoplasmic transport. Nucleocytoplasmic transport is regulated by the phenylalanine-glycine nucleoporins (FG nups) that comprise the nuclear pore complex (NPC) permeability barrier. However, the relationship between FG nups and TDP-43 pathology remains elusive. Our studies show that nuclear depletion and cytoplasmic mislocalization of one FG nup, NUP62, is linked to TDP-43 mislocalization in C9-ALS/FTLD iPSC neurons. Poly-glycine arginine (GR) DPR accumulation initiates the formation of cytoplasmic RNA granules that recruit NUP62 and TDP-43. Cytoplasmic NUP62 and TDP-43 interactions promotes their insolubility and NUP62:TDP-43 inclusions are frequently found in C9orf72 ALS/FTLD as well as sporadic ALS/FTLD postmortem CNS tissue. Our findings indicate NUP62 cytoplasmic mislocalization contributes to TDP-43 proteinopathy in ALS/FTLD.

Nuclear mRNA Export and Aging

The relationship between transcription and aging is one that has been studied intensively and experimentally with diverse attempts. However, the impact of the nuclear mRNA export on the aging process following its transcription is still poorly understood, although the nuclear events after transcription are coupled closely with the transcription pathway because the essential factors required for mRNA transport, namely TREX, TREX-2, and nuclear pore complex (NPC), physically and functionally interact with various transcription factors, including the activator/repressor and pre-mRNA processing factors. Dysregulation of the mediating factors for mRNA export from the nucleus generally leads to the aberrant accumulation of nuclear mRNA and further impairment in the vegetative growth and normal lifespan and the pathogenesis of neurodegenerative diseases. The optimal stoichiometry and density of NPC are destroyed during the process of cellular aging, and their damage triggers a defect of function in the nuclear permeability barrier. This review describes recent findings regarding the role of the nuclear mRNA export in cellular aging and age-related neurodegenerative disorders.

Karyopherin abnormalities in neurodegenerative proteinopathies

Neurodegenerative proteinopathies are characterized by progressive cell loss that is preceded by the mislocalization and aberrant accumulation of proteins prone to aggregation. Despite their different physiological functions, disease-related proteins like tau, α-synuclein, TAR DNA binding protein-43, fused in sarcoma and mutant huntingtin, all share low complexity regions that can mediate their liquid-liquid phase transitions. The proteins' phase transitions can range from native monomers to soluble oligomers, liquid droplets and further to irreversible, often-mislocalized aggregates that characterize the stages and severity of neurodegenerative diseases. Recent advances into the underlying pathogenic mechanisms have associated mislocalization and aberrant accumulation of disease-related proteins with defective nucleocytoplasmic transport and its mediators called karyopherins. These studies identify karyopherin abnormalities in amyotrophic lateral sclerosis, frontotemporal dementia, Alzheimer's disease, and synucleinopathies including Parkinson's disease and dementia with Lewy bodies, that range from altered expression levels to the subcellular mislocalization and aggregation of karyopherin α and β proteins. The reported findings reveal that in addition to their classical function in nuclear import and export, karyopherins can also act as chaperones by shielding aggregation-prone proteins against misfolding, accumulation and irreversible phase-transition into insoluble aggregates. Karyopherin abnormalities can, therefore, be both the cause and consequence of protein mislocalization and aggregate formation in degenerative proteinopathies. The resulting vicious feedback cycle of karyopherin pathology and proteinopathy identifies karyopherin abnormalities as a common denominator of onset and progression of neurodegenerative disease. Pharmacological targeting of karyopherins, already in clinical trials as therapeutic intervention targeting cancers such as glioblastoma and viral infections like COVID-19, may therefore represent a promising new avenue for disease-modifying treatments in neurodegenerative proteinopathies.

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.

A system-wide mislocalization of RNA-binding proteins in motor neurons is a new feature of ALS

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease characterized by progressive degeneration of motor neurons. Mislocalization of TAR DNA-binding protein 43 (TDP-43) is an early event in the formation of cytoplasmic TDP-43-positive inclusions in motor neurons and a hallmark of ALS. However, the underlying mechanism and the pathogenic impact of this mislocalization are relatively unexplored. We previously reported that abnormal AMPK activation mediates TDP-43 mislocalization in motor neurons of humans and mice with ALS. In the present study, we hypothesized that other nuclear proteins are mislocalized in the cytoplasm of motor neurons due to the AMPK-mediated phosphorylation of importin-α1 and subsequently contribute to neuronal degeneration in ALS. To test this hypothesis, we analyzed motor neurons of sporadic ALS patients and found that when AMPK is activated, importin-α1 is abnormally located in the nucleus. Multiple integrative molecular and cellular approaches (including proteomics, immunoprecipitation/western blot analysis, immunohistological evaluations and gradient analysis of preribosomal complexes) were employed to demonstrate that numerous RNA binding proteins are mislocalized in a rodent motor neuron cell line (NSC34) and human motor neurons derived from iPSCs during AMPK activation. We used comparative proteomic analysis of importin-α1 complexes that were immunoprecipitated with a phosphorylation-deficient mutant of importin-α1 (importin-α1-S105A) and a phosphomimetic mutant of importin-α1 (importin-α1-S105D) to identify 194 proteins that have stronger affinity for the unphosphorylated form than the phosphorylated form of importin-α1. Furthermore, GO and STRING analyses suggested that RNA processing and protein translation is the major machinery affected by abnormalities in the AMPK-importin-α1 axis. Consistently, the expression of importin-α1-S105D alters the assembly of preribosomal complexes and increases cell apoptosis. Collectively, we propose that by impairing importin-α1-mediated nuclear import, abnormal AMPK activation in motor neurons alters the cellular distribution of many RNA-binding proteins, which pathogenically affect multiple cellular machineries in motor neurons and contribute to ALS pathogenesis.

Measuring and Interpreting Nuclear Transport in Neurodegenerative Disease-The Example of C9orf72 ALS

Transport from and into the nucleus is essential to all eukaryotic life and occurs through the nuclear pore complex (NPC). There are a multitude of data supporting a role for nuclear transport in neurodegenerative diseases, but actual transport assays in disease models have provided diverse outcomes. In this review, we summarize how nuclear transport works, which transport assays are available, and what matters complicate the interpretation of their results. Taking a specific type of ALS caused by mutations in C9orf72 as an example, we illustrate these complications, and discuss how the current data do not firmly answer whether the kinetics of nucleocytoplasmic transport are altered. Answering this open question has far-reaching implications, because a positive answer would imply that widespread mislocalization of proteins occurs, far beyond the reported mislocalization of transport reporters, and specific proteins such as FUS, or TDP43, and thus presents a challenge for future research.

Nanoscopic investigation of C9orf72 poly-GA oligomers on nuclear membrane disruption by a photoinducible platform

Glycine-alanine dipeptide repeats (GA DPRs) translated from the mutated C9orf72 gene have recently been correlated with amyotrophic lateral sclerosis (ALS). While GA DPRs aggregates have been suggested as amyloid, the biophysical features and cytotoxicity of GA DPRs oligomers has not been explored due to its unstable nature. In this study, we develop a photoinducible platform based on methoxynitrobenzene chemistry to enrich GA DPRs that allows monitoring the oligomerization process of GA DPRs in cells. By applying advanced microscopies, we examined the GA DPRs oligomerization process nanoscopically in a time-dependent manner. We provided direct evidences to demonstrate GA DPRs oligomers rather than nanofibrils disrupt nuclear membrane. Moreover, we found GA DPRs hamper nucleocytoplasmic transport in cells and cause cytosolic retention of TAR DNA-binding protein 43 in cortical neurons. Our results highlight the toxicity of GA DPRs oligomers, which is a key step toward elucidating the pathological roles of C9orf72 DPRs.

Altered Phase Separation and Cellular Impact in C9orf72-Linked ALS/FTD

Since the discovery of the C9orf72 repeat expansion mutation as causative for chromosome 9-linked amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in 2011, a multitude of cellular pathways have been implicated. However, evidence has also been accumulating for a key mechanism of cellular compartmentalization—phase separation. Liquid-liquid phase separation (LLPS) is fundamental for the formation of membraneless organelles including stress granules, the nucleolus, Cajal bodies, nuclear speckles and the central channel of the nuclear pore. Evidence has now accumulated showing that the formation and function of these membraneless organelles is impaired by both the toxic arginine rich dipeptide repeat proteins (DPRs), translated from the C9orf72 repeat RNA transcript, and the repeat RNA itself. Both the arginine rich DPRs and repeat RNA themselves undergo phase separation and disrupt the physiological phase separation of proteins involved in the formation of these liquid-like organelles. Hence abnormal phase separation may explain a number of pathological cellular phenomena associated with C9orf72-ALS/FTD. In this review article, we will discuss the principles of phase separation, phase separation of the DPRs and repeat RNA themselves and how they perturb LLPS associated with membraneless organelles and the functional consequences of this. We will then discuss how phase separation may impact the major pathological feature of C9orf72-ALS/FTD, TDP-43 proteinopathy, and how LLPS may be targeted therapeutically in disease.

Nucleocytoplasmic Transport: Regulatory Mechanisms and the Implications in Neurodegeneration

Nucleocytoplasmic transport (NCT) across the nuclear envelope is precisely regulated in eukaryotic cells, and it plays critical roles in maintenance of cellular homeostasis. Accumulating evidence has demonstrated that dysregulations of NCT are implicated in aging and age-related neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease (AD), and Huntington disease (HD). This is an emerging research field. The molecular mechanisms underlying impaired NCT and the pathogenesis leading to neurodegeneration are not clear. In this review, we comprehensively described the components of NCT machinery, including nuclear envelope (NE), nuclear pore complex (NPC), importins and exportins, RanGTPase and its regulators, and the regulatory mechanisms of nuclear transport of both protein and transcript cargos. Additionally, we discussed the possible molecular mechanisms of impaired NCT underlying aging and neurodegenerative diseases, such as ALS/FTD, HD, and AD.

Nuclear Envelope and Nuclear Pore Complexes in Neurodegenerative Diseases-New Perspectives for Therapeutic Interventions

Transport of proteins, transcription factors, and other signaling molecules between the nucleus and cytoplasm is necessary for signal transduction. The study of these transport phenomena is particularly challenging in neurons because of their highly polarized structure. The bidirectional exchange of molecular cargoes across the nuclear envelope (NE) occurs through nuclear pore complexes (NPCs), which are aqueous channels embedded in the nuclear envelope. The NE and NPCs regulate nuclear transport but are also emerging as relevant regulators of chromatin organization and gene expression. The alterations in nuclear transport are regularly identified in affected neurons associated with human neurodegenerative diseases. This review presents insights into the roles played by nuclear transport defects in neurodegenerative disease, focusing primarily on NE proteins and NPCs. The subcellular mislocalization of proteins might be a very desirable means of therapeutic intervention in neurodegenerative disorders.

Nuclear Transport Deficits in Tau-Related Neurodegenerative Diseases

Tau is a cytosolic microtubule binding protein that is highly abundant in the axons of the central nervous system. However, alternative functions of tau also in other cellular compartments are suggested, for example, in the nucleus, where interactions of tau with specific nuclear entities such as DNA, the nucleolus, and the nuclear envelope have been reported. We would like to review the current knowledge about tau-nucleus interactions and lay out possible neurotoxic mechanisms that are based on the (pathological) interactions of tau with the nucleus.

The ataxin-1 interactome reveals direct connection with multiple disrupted nuclear transport pathways

The expanded polyglutamine (polyQ) tract form of ataxin-1 drives disease progression in spinocerebellar ataxia type 1 (SCA1). Although known to form distinctive intranuclear bodies, the cellular pathways and processes that polyQ-ataxin-1 influences remain poorly understood. Here we identify the direct and proximal partners constituting the interactome of ataxin-1[85Q] in Neuro-2a cells, pathways analyses indicating a significant enrichment of essential nuclear transporters, pointing to disruptions in nuclear transport processes in the presence of elevated levels of ataxin-1. Our direct assessments of nuclear transporters and their cargoes confirm these observations, revealing disrupted trafficking often with relocalisation of transporters and/or cargoes to ataxin-1[85Q] nuclear bodies. Analogous changes in importin-β1, nucleoporin 98 and nucleoporin 62 nuclear rim staining are observed in Purkinje cells of ATXN1[82Q] mice. The results highlight a disruption of multiple essential nuclear protein trafficking pathways by polyQ-ataxin-1, a key contribution to furthering understanding of pathogenic mechanisms initiated by polyQ tract proteins.

Nucleo-cytoplasmic transport defects and protein aggregates in neurodegeneration

In the ongoing process of uncovering molecular abnormalities in neurodegenerative diseases characterized by toxic protein aggregates, nucleo-cytoplasmic transport defects have an emerging role. Several pieces of evidence suggest a link between neuronal protein inclusions and nuclear pore complex (NPC) damage. These processes lead to oxidative stress, inefficient transcription, and aberrant DNA/RNA maintenance. The clinical and neuropathological spectrum of NPC defects is broad, ranging from physiological aging to a suite of neurodegenerative diseases. A better understanding of the shared pathways among these conditions may represent a significant step toward dissecting their underlying molecular mechanisms, opening the way to a real possibility of identifying common therapeutic targets.

Traffic jam at the nuclear pore: All roads lead to nucleocytoplasmic transport defects in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a fatal late-onset neurodegenerative disease that specifically affects the function and survival of spinal and cortical motor neurons. ALS shares many genetic, clinical, and pathological characteristics with frontotemporal dementia (FTD), and these diseases are now recognized as presentations of a disease spectrum known as ALS/FTD. The molecular determinants of neuronal loss in ALS/FTD are still debated, but the recent discovery of nucleocytoplasmic transport defects as a common denominator of most if not all forms of ALS/FTD has dramatically changed our understanding of the pathogenic mechanisms of this disease. Loss of nuclear pores and nucleoporin aggregation, altered nuclear morphology, and impaired nuclear transport are some of the most prominent features that have been identified using a variety of animal, cellular, and human models of disease. Here, we review the experimental evidence linking nucleocytoplasmic transport defects to the pathogenesis of ALS/FTD and propose a unifying view on how these defects may lead to a vicious cycle that eventually causes neuronal death.

The Hitchhiker's Guide to Nucleocytoplasmic Trafficking in Neurodegeneration

The widespread nature of nucleocytoplasmic trafficking defects and protein accumulation suggests distinct yet overlapping mechanisms in a variety of neurodegenerative diseases. Detailed understanding of the cellular pathways involved in nucleocytoplasmic transport and its dysregulation are essential for elucidating neurodegenerative pathogenesis and pinpointing potential areas for therapeutic intervention. The transport of cargos from the nucleus to the cytoplasm is generally regulated by the structure and function of the nuclear pore as well as the karyopherin α/β, importin, exportin, and mRNA export mechanisms. The disruption of these crucial transport mechanisms has been extensively described in the context of neurodegenerative diseases. One common theme in neurodegeneration is the cytoplasmic aggregation of proteins, including nuclear RNA binding proteins, repeat expansion associated gene products, and tau. These cytoplasmic aggregations are partly a consequence of failed nucleocytoplasmic transport machinery, but can also further disrupt transport, creating cyclical feed-forward mechanisms that exacerbate neurodegeneration. Here we describe the canonical mechanisms that regulate nucleocytoplasmic trafficking as well as how these mechanisms falter in neurodegenerative diseases.

Biomolecular condensates in neurodegeneration and cancer

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

Oncogenic potential of nucleoporins in non-hematological cancers: recent update beyond chromosome translocation and gene fusion

INTRODUCTION

The nuclear pore complex is comprised of approximately 30 proteins named nucleoporins (Nups) and tightly regulates nucleocytoplasmic transport of macromolecules across the nuclear membrane. Genetic alterations in many NUP genes are associated with many human maladies, such as neurological disease, autoimmune disorders and cancer.

METHODS

We reviewed the status quo of recent advancement of the knowledge of oncogenic role of nucleoporins in human carcinogenesis, focusing on major non-hematological malignancies in the recent literature. Both clinical study-derived and experiment-based reports were critically reviewed. We have also discussed the potential of nucleoporins as novel cancer biomarkers and promising therapeutic target against human malignancies.

RESULTS

Several Nups such as Nup53, Nup88, Nup98, Nup160 and Nup214 modulated a plethora of cellular and physiological pathways involved in tumorigenesis such as GSK3β-Snail, Wnt/β-Catenin and RanGap1/RanBP2 signaling axes, DNA damage response, resistance to apoptosis and chemotherapy.

CONCLUSION

Although classically, majority of studies have shown oncogenic roles of nucleoporins as genetic fusion partners in several types of leukemia, emerging evidence suggests that nucleoporins also modulate many cellular signaling pathways that are associated with several major non-hematological malignancies, such as carcinomas of skin, breast, lung, prostate and colon. Hence, nucleoporins are emerging as novel therapeutic targets in human tumors.

Microglial activation in an amyotrophic lateral sclerosis-like model caused by Ranbp2 loss and nucleocytoplasmic transport impairment in retinal ganglion neurons

Nucleocytoplasmic transport is dysregulated in sporadic and familial amyotrophic lateral sclerosis (ALS) and retinal ganglion neurons (RGNs) are purportedly involved in ALS. The Ran-binding protein 2 (Ranbp2) controls rate-limiting steps of nucleocytoplasmic transport. Mice with Ranbp2 loss in Thy1+-motoneurons develop cardinal ALS-like motor traits, but the impairments in RGNs and the degree of dysfunctional consonance between RGNs and motoneurons caused by Ranbp2 loss are unknown. This will help to understand the role of nucleocytoplasmic transport in the differential vulnerability of neuronal cell types to ALS and to uncover non-motor endophenotypes with pathognomonic signs of ALS. Here, we ascertain Ranbp2's function and endophenotypes in RGNs of an ALS-like mouse model lacking Ranbp2 in motoneurons and RGNs. Thy1+-RGNs lacking Ranbp2 shared with motoneurons the dysregulation of nucleocytoplasmic transport. RGN abnormalities were comprised morphologically by soma hypertrophy and optic nerve axonopathy and physiologically by a delay of the visual pathway's evoked potentials. Whole-transcriptome analysis showed restricted transcriptional changes in optic nerves that were distinct from those found in sciatic nerves. Specifically, the level and nucleocytoplasmic partition of the anti-apoptotic and novel substrate of Ranbp2, Pttg1/securin, were dysregulated. Further, acetyl-CoA carboxylase 1, which modulates de novo synthesis of fatty acids and T-cell immunity, showed the highest up-regulation (35-fold). This effect was reflected by the activation of ramified CD11b+ and CD45+-microglia, increase of F4\80+-microglia and a shift from pseudopodial/lamellipodial to amoeboidal F4\80+-microglia intermingled between RGNs of naive mice. Further, there was the intracellular sequestration in RGNs of metalloproteinase-28, which regulates macrophage recruitment and polarization in inflammation. Hence, Ranbp2 genetic insults in RGNs and motoneurons trigger distinct paracrine signaling likely by the dysregulation of nucleocytoplasmic transport of neuronal-type selective substrates. Immune-modulators underpinning RGN-to-microglial signaling are regulated by Ranbp2, and this neuronal-glial system manifests endophenotypes that are likely useful in the prognosis and diagnosis of motoneuron diseases, such as ALS.

Modulation of actin polymerization affects nucleocytoplasmic transport in multiple forms of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of unknown etiology. Although defects in nucleocytoplasmic transport (NCT) may be central to the pathogenesis of ALS and other neurodegenerative diseases, the molecular mechanisms modulating the nuclear pore function are still largely unknown. Here we show that genetic and pharmacological modulation of actin polymerization disrupts nuclear pore integrity, nuclear import, and downstream pathways such as mRNA post-transcriptional regulation. Importantly, we demonstrate that modulation of actin homeostasis can rescue nuclear pore instability and dysfunction caused by mutant PFN1 as well as by C9ORF72 repeat expansion, the most common mutation in ALS patients. Collectively, our data link NCT defects to ALS-associated cellular pathology and propose the regulation of actin homeostasis as a novel therapeutic strategy for ALS and other neurodegenerative diseases.

Disrupted neuronal trafficking in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, adult-onset neurodegenerative disease caused by degeneration of motor neurons in the brain and spinal cord leading to muscle weakness. Median survival after symptom onset in patients is 3-5 years and no effective therapies are available to treat or cure ALS. Therefore, further insight is needed into the molecular and cellular mechanisms that cause motor neuron degeneration and ALS. Different ALS disease mechanisms have been identified and recent evidence supports a prominent role for defects in intracellular transport. Several different ALS-causing gene mutations (e.g., in FUS, TDP-43, or C9ORF72) have been linked to defects in neuronal trafficking and a picture is emerging on how these defects may trigger disease. This review summarizes and discusses these recent findings. An overview of how endosomal and receptor trafficking are affected in ALS is followed by a description on dysregulated autophagy and ER/Golgi trafficking. Finally, changes in axonal transport and nucleocytoplasmic transport are discussed. Further insight into intracellular trafficking defects in ALS will deepen our understanding of ALS pathogenesis and will provide novel avenues for therapeutic intervention.

The coming-of-age of nucleocytoplasmic transport in motor neuron disease and neurodegeneration

The nuclear pore is the gatekeeper of nucleocytoplasmic transport and signaling through which a vast flux of information is continuously exchanged between the nuclear and cytoplasmic compartments to maintain cellular homeostasis. A unifying and organizing principle has recently emerged that cements the notion that several forms of amyotrophic lateral sclerosis (ALS), and growing number of other neurodegenerative diseases, co-opt the dysregulation of nucleocytoplasmic transport and that this impairment is a pathogenic driver of neurodegeneration. The understanding of shared pathomechanisms that underpin neurodegenerative diseases with impairments in nucleocytoplasmic transport and how these interface with current concepts of nucleocytoplasmic transport is bound to illuminate this fundamental biological process in a yet more physiological context. Here, I summarize unresolved questions and evidence and extend basic and critical concepts and challenges of nucleocytoplasmic transport and its role in the pathogenesis of neurodegenerative diseases, such as ALS. These principles will help to appreciate the roles of nucleocytoplasmic transport in the pathogenesis of ALS and other neurodegenerative diseases, and generate a framework for new ideas of the susceptibility of motoneurons, and possibly other neurons, to degeneration by dysregulation of nucleocytoplasmic transport.

Cytoplasmic TDP-43 De-mixing Independent of Stress Granules Drives Inhibition of Nuclear Import, Loss of Nuclear TDP-43, and Cell Death

While cytoplasmic aggregation of TDP-43 is a pathological hallmark of amyotrophic lateral sclerosis and frontotemporal dementia, how aggregates form and what drives its nuclear clearance have not been determined. Here we show that TDP-43 at its endogenous level undergoes liquid-liquid phase separation (LLPS) within nuclei in multiple cell types. Increased concentration of TDP-43 in the cytoplasm or transient exposure to sonicated amyloid-like fibrils is shown to provoke long-lived liquid droplets of cytosolic TDP-43 whose assembly and maintenance are independent of conventional stress granules. Cytosolic liquid droplets of TDP-43 accumulate phosphorylated TDP-43 and rapidly convert into gels/solids in response to transient, arsenite-mediated stress. Cytoplasmic TDP-43 droplets slowly recruit importin-α and Nup62 and induce mislocalization of RanGap1, Ran, and Nup107, thereby provoking inhibition of nucleocytoplasmic transport, clearance of nuclear TDP-43, and cell death. These findings identify a neuronal cell death mechanism that can be initiated by transient-stress-induced cytosolic de-mixing of TDP-43.

Mitochondria, ER, and nuclear membrane defects reveal early mechanisms for upper motor neuron vulnerability with respect to TDP-43 pathology

Insoluble aggregates containing TDP-43 are widely observed in the diseased brain, and defined as "TDP-43 pathology" in a spectrum of neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease and ALS with frontotemporal dementia. Here we report that Betz cells of patients with TDP-43 pathology display a distinct set of intracellular defects especially at the site of nuclear membrane, mitochondria and endoplasmic reticulum (ER). Numerous TDP-43 mouse models have been generated to discern the cellular and molecular basis of the disease, but mechanisms of neuronal vulnerability remain unknown. In an effort to define the underlying causes of corticospinal motor neuron (CSMN) degeneration, we generated and characterized a novel CSMN reporter line with TDP-43 pathology, the prp-TDP-43^A315T-UeGFP mice. We find that TDP-43 pathology related intracellular problems emerge very early in the disease. The Betz cells in humans and CSMN in mice both have impaired mitochondria, and display nuclear membrane and ER defects with respect to TDP-43 pathology.

C9orf72-FTD/ALS pathogenesis: evidence from human neuropathological studies

What are the most important and treatable pathogenic mechanisms in C9orf72-FTD/ALS? Model-based efforts to address this question are forging ahead at a blistering pace, often with conflicting results. But what does the human neuropathological literature reveal? Here, we provide a critical review of the human studies to date, seeking to highlight key gaps or uncertainties in our knowledge. First, we engage the C9orf72-specific mechanisms, including C9orf72 haploinsufficiency, repeat RNA foci, and dipeptide repeat protein inclusions. We then turn to some of the most prominent C9orf72-associated features, such as TDP-43 loss-of-function, TDP-43 aggregation, and nuclear transport defects. Finally, we review potential disease-modifying epigenetic and genetic factors and the natural history of the disease across the lifespan. Throughout, we emphasize the importance of anatomical precision when studying how candidate mechanisms relate to neuronal, regional, and behavioral findings. We further highlight methodological approaches that may help address lingering knowledge gaps and uncertainties, as well as other logical next steps for the field. We conclude that anatomically oriented human neuropathological studies have a critical role to play in guiding this fast-moving field toward effective new therapies.

Impaired Nucleoporins Are Present in Sporadic Amyotrophic Lateral Sclerosis Motor Neurons that Exhibit Mislocalization of the 43-kDa TAR DNA-Binding Protein

BACKGROUND AND PURPOSE

Disruption of nucleoporins has been reported in the motor neurons of patients with sporadic amyotrophic lateral sclerosis (sALS). However, the precise changes in the morphology of nucleoporins associated with the pathology of the 43-kDa TAR DNA-binding protein (TDP-43) in the disease process remain unknown. We investigated the expression of nucleoporins that constitute the nuclear pore complex (NPC) in spinal motor neurons that exhibit sALS in relation to TDP-43 pathology, which is a reliable neuropathological hallmark of sALS.

METHODS

Paraffin-embedded sections of the lumbar spinal cord were obtained for immunofluorescence analysis from seven control subjects and six sALS patients. Anti-TDP-43 antibody, anti-nucleoporin p62 (NUP62) antibody, and anti-karyopherin beta 1 (KPNB1) antibody were applied as primary antibodies, and then visualized using appropriate secondary antibodies. The sections were then examined under a fluorescence microscope.

RESULTS

NUP62 and KPNB1 immunoreactivity appeared as a smooth round rim bordering the nuclear margin in normal spinal motor neurons that exhibited nuclear TDP-43 immunoreactivity. sALS spinal motor neurons with apparent TDP-43 mislocalization demonstrated irregular, disrupted nuclear staining for NUP62 or KPNB1. Some atrophic sALS spinal motor neurons with TDP-43 mislocalization presented no NUP62 immunoreactivity.

CONCLUSIONS

Our findings suggest a close relationship between NPC alterations and TDP-43 pathology in the degenerative process of the motor neurons of sALS patients.

Perturbations in Traffic: Aberrant Nucleocytoplasmic Transport at the Heart of Neurodegeneration

Neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Huntington’s disease (HD), are characterized by intracellular aggregation of proteins. In the case of ALS and FTD, these protein aggregates are found in the cytoplasm of affected neurons and contain certain RNA-binding proteins (RBPs), namely the TAR DNA-binding protein of 43 kDa (TDP-43) and the fused in sarcoma (FUS) gene product. TDP-43 and FUS are nuclear proteins and their displacement to the cytoplasm is thought to be adverse in at least two ways: loss-of-function in the nucleus and gain-of-toxicity in the cytoplasm. In the case of HD, expansion of a polyglutamine (polyQ) stretch within the N-terminal domain of the Huntingtin (HTT) protein leads to nuclear accumulation of polyQ HTT (or mHTT) and a toxic gain-of-function phenotype resulting in neurodegeneration. Numerous studies in recent years have provided evidence that defects in nucleocytoplasmic transport critically contribute to the pathology of these neurodegenerative diseases. A new mechanistic view is emerging, implicating three types of perturbations in normal cellular pathways that rely on nucleocytoplasmic transport: displacement of nuclear transport receptors and nucleoporins from nuclear pore complexes (NPCs), mislocalization and aggregation of RNA-binding proteins, and weakening of the chaperone activity of nuclear import receptors.

RNA Nucleocytoplasmic Transport Defects in Neurodegenerative Diseases

In eukaryotic cells, transcription and translation are compartmentalized by the nuclear membrane, requiring an active transport of RNA from the nucleus into the cytoplasm. This is accomplished by a variety of transport complexes that contain either a member of the exportin family of proteins and translocation fueled by GTP hydrolysis or in the case of mRNA by complexes containing the export protein NXF1. Recent evidence indicates that RNA transport is altered in a number of different neurodegenerative diseases including Huntington's disease, Alzheimer's disease, frontotemporal dementia, and amyotrophic lateral sclerosis. Alterations in RNA transport predominately fall into three categories: Alterations in the nuclear membrane and mislocalization and aggregation of the nucleoporins that make up the nuclear pore; alterations in the Ran gradient and the proteins that control it which impacts exportin based nuclear export; and alterations of proteins that are required for the export of mRNA leading nuclear accumulation of mRNA.

Moonlighting nuclear pore proteins: tissue-specific nucleoporin function in health and disease

The nuclear pore complex is the main transportation hub for exchange between the cytoplasm and the nucleus. It is built from nucleoporins that form distinct subcomplexes to establish this huge protein complex in the nuclear envelope. Malfunctioning of nucleoporins is well known in human malignancies, such as gene fusions of NUP214 and NUP98 in hematological neoplasms and overexpression of NUP88 in a variety of human cancers. In the past decade, the incremental utilization of next-generation sequencing has unraveled mutations in nucleoporin genes in the context of an increasing number of hereditary diseases, often in a tissue-specific manner. It emerges that, on one hand, the central nervous system and the heart are particularly sensitive to mutations in nucleoporin genes. On the other hand, nucleoporins forming the scaffold structure of the nuclear pore complex are eminently mutation-prone. These novel and exciting associations between nucleoporins and human diseases emphasize the need to shed light on these unanticipated tissue-specific roles of nucleoporins that may go well beyond their role in nucleocytoplasmic transport. In this review, the current insights into altered nucleoporin function associated with human hereditary disorders will be discussed.

Repeat-associated non-ATG (RAN) translation

Microsatellite expansions cause more than 40 neurological disorders, including Huntington's disease, myotonic dystrophy, and C9ORF72 amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD). These repeat expansion mutations can produce repeat-associated non-ATG (RAN) proteins in all three reading frames, which accumulate in disease-relevant tissues. There has been considerable interest in RAN protein products and their downstream consequences, particularly for the dipeptide proteins found in C9ORF72 ALS/FTD. Understanding how RAN translation occurs, what cellular factors contribute to RAN protein accumulation, and how these proteins contribute to disease should lead to a better understanding of the basic mechanisms of gene expression and human disease.

Identification and characterization of ubiquitinylation sites in TAR DNA-binding protein of 43 kDa (TDP-43)

TAR DNA-binding protein of 43 kDa (TDP-43) forms pathological aggregates in neurodegenerative diseases, particularly in certain forms of frontotemporal dementia and amyotrophic lateral sclerosis. Pathological modifications of TDP-43 include proteolytic fragmentation, phosphorylation, and ubiquitinylation. A pathognomonic TDP-43 C-terminal fragment (CTF) spanning amino acids 193-414 contains only four lysine residues that could be potentially ubiquitinylated. Here, serial mutagenesis of these four lysines to arginine revealed that not a single residue is responsible for the ubiquitinylation of mCherry-tagged CTF. Removal of all four lysines was necessary to suppress ubiquitinylation. Interestingly, Lys-408 substitution enhanced the pathological phosphorylation of the immediately adjacent serine residues 409/410 in the context of mCherry-CTF. Thus, Lys-408 ubiquitinylation appears to hinder Ser-409/410 phosphorylation in TDP-43 CTF. However, we did not observe the same effect for full-length TDP-43. We extended the mutagenesis study to full-length TDP-43 and performed MS. Ubiquitinylated lysine residues were identified in the nuclear localization sequence (NLS; Lys-84 and Lys-95) and RNA-binding region (mostly Lys-160, Lys-181, and Lys-263). Mutagenesis of Lys-84 confirmed its importance as the major determinant for nuclear import, whereas Lys-95 mutagenesis did not significantly affect TDP-43's nucleo-cytoplasmic distribution, solubility, aggregation, and RNA-processing activities. Nevertheless, the K95A mutant had significantly reduced Ser-409/410 phosphorylation, emphasizing the suspected interplay between TDP-43 ubiquitinylation and phosphorylation. Collectively, our analysis of TDP-43 ubiquitinylation sites indicates that the NLS residues Lys-84 and Lys-95 have more prominent roles in TDP-43 function than the more C-terminal lysines and suggests a link between specific ubiquitinylation events and pathological TDP-43 phosphorylation.

Cell death cascade and molecular therapy in ADAR2-deficient motor neurons of ALS

TAR DNA-binding protein (TDP-43) pathology in the motor neurons is the most reliable pathological hallmark of amyotrophic lateral sclerosis (ALS), and motor neurons bearing TDP-43 pathology invariably exhibit failure in RNA editing at the GluA2 glutamine/arginine (Q/R) site due to down-regulation of adenosine deaminase acting on RNA 2 (ADAR2). Conditional ADAR2 knockout (AR2) mice display ALS-like phenotype, including progressive motor dysfunction due to loss of motor neurons. Motor neurons devoid of ADAR2 express Q/R site-unedited GluA2, and AMPA receptors with unedited GluA2 in their subunit assembly are abnormally permeable to Ca²⁺, which results in progressive neuronal death. Moreover, analysis of AR2 mice has demonstrated that exaggerated Ca²⁺ influx through the abnormal AMPA receptors overactivates calpain, a Ca²⁺-dependent protease, that cleaves TDP-43 into aggregation-prone fragments, which serve as seeds for TDP-43 pathology. Activated calpain also disrupts nucleo-cytoplasmic transport and gene expression by cleaving molecules involved in nucleocytoplasmic transport, including nucleoporins. These lines of evidence prompted us to develop molecular targeting therapy for ALS by normalization of disrupted intracellular environment due to ADAR2 down-regulation. In this review, we have summarized the work from our group on the cell death cascade in sporadic ALS and discussed a potential therapeutic strategy for ALS.

Stress Granule Assembly Disrupts Nucleocytoplasmic Transport

Defects in nucleocytoplasmic transport have been identified as a key pathogenic event in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) mediated by a GGGGCC hexanucleotide repeat expansion in C9ORF72, the most common genetic cause of ALS/FTD. Furthermore, nucleocytoplasmic transport disruption has also been implicated in other neurodegenerative diseases with protein aggregation, suggesting a shared mechanism by which protein stress disrupts nucleocytoplasmic transport. Here, we show that cellular stress disrupts nucleocytoplasmic transport by localizing critical nucleocytoplasmic transport factors into stress granules, RNA/protein complexes that play a crucial role in ALS pathogenesis. Importantly, inhibiting stress granule assembly, such as by knocking down Ataxin-2, suppresses nucleocytoplasmic transport defects as well as neurodegeneration in C9ORF72-mediated ALS/FTD. Our findings identify a link between stress granule assembly and nucleocytoplasmic transport, two fundamental cellular processes implicated in the pathogenesis of C9ORF72-mediated ALS/FTD and other neurodegenerative diseases.

Ageing as a risk factor for ALS/FTD

Like many other neurodegenerative diseases, age is a major risk factor in the development of ALS/FTD. But why is this the case? Recent genetic advances have highlighted some of pathways involved in the development of disease, and, strikingly, they appear to substantially overlap with those known to directly modulate the ageing process. Many ALS/FTD linked genes play a direct role in autophagy/lysosomal degradation, one of the most important pathways linked to ageing. However, systemic processes such as inflammation, as well as cellular maintenance pathways, including RNA splicing and nuclear-cytoplasmic transport have been increasingly linked both to disease and ageing. We highlight some of the shared mechanisms between the ageing process itself and emerging pathogenic mechanisms in ALS/FTD.

Importin-β and exportin-5 are strong biomarkers of productive reoviral infection of cancer cells

Acute reoviral infection has been extensively studied given the virus's propensity to target malignant cells and activate caspase-3 mediated apoptosis. Reovirus infection of malignant N1E-115 mouse neuroblastoma cells led to significant increased expression of importin-β and exportin-5 mRNAs (qRTPCR) and proteins (immunohistochemistry) which was partially blocked by small interfering LNA oligomers directed against the reoviral genome. Co-expression analysis showed that the N1E-115 cells that contained reoviral capsid protein had accumulated importin-β and exportin-5, as well as activated caspase 3. Reoviral oncolysis using a syngeneic mouse model of multiple myeloma similarly induced a significant increase in importin-β and exportin-5 proteins that were co-expressed with reoviral capsid protein and caspase-3. Apoptotic proteins (BAD, BIM, PUMA, NOXA, BAK, BAX) were increased with infection and co-localized with reoviral capsid protein. Surprisingly the anti-apoptotic MCL1 and bcl2 were also increased and co-localized with the capsid protein suggesting that it was the balance of pro-apoptotic molecules that correlated with activation of caspase-3. In summary, productive reoviral infection is strongly correlated with elevated importin-β and exportin-5 levels which may serve as biomarkers of the disease in clinical specimens.