Formation of intermediate filament protein aggregates with disparate effects in two transgenic mouse models lacking the neurofilament light subunit

The motor neuron m6A repertoire governs neuronal homeostasis and FTO inhibition mitigates ALS symptom manifestation

Amyotrophic lateral sclerosis (ALS) is a swiftly progressive and fatal neurodegenerative ailment marked by the degenerative motor neurons (MNs). Why MNs are specifically susceptible in predominantly sporadic cases remains enigmatic. Here, we demonstrated N6-methyladenosine (m6A), an RNA modification catalyzed by the METTL3/METTL14 methyltransferase complex, as a pivotal contributor to ALS pathogenesis. By conditional knockout Mettl14 in murine MNs, we recapitulate almost the full spectrum of ALS disease characteristics. Mechanistically, pervasive m6A hypomethylation triggers dysregulated expression of high-risk genes associated with ALS and an unforeseen reduction of chromatin accessibility in MNs. Additionally, we observed diminished m6A levels in induced pluripotent stem cell derived MNs (iPSC~MNs) from familial and sporadic ALS patients. Restoring m6A equilibrium via a small molecule or gene therapy significantly preserves MNs from degeneration and mitigates motor impairments in ALS iPSC~MNs and murine models. Our study presents a substantial stride towards identifying pioneering efficacious ALS therapies via RNA modifications.

ALS: A Silent Slayer of Motor Neurons. Traditional Chinese Herbal Medicine as an Effective Therapy

Amyotrophic lateral sclerosis (ALS), is a progressive neurodegenerative disease characterized by motor symptoms, and cognitive impairment. The complexity in treating ALS arises from genetic and environmental factors, contributing to the gradual decline of lower and upper motor neurons. The anticipated pharmaceutical market valuation for ALS is projected to reach $1,038.94 million by 2032. This projection underscores the escalating impact of ALS on global healthcare systems. ALS prevalence is expected to surge to 376,674 cases by 2040. In 2022, India ranked among the top 3 Asian-Pacific nations, while North America dominated the global ALS market. Ongoing investigations explore the potential of neuroprotective drugs like riluzole and edaravone in ALS treatment. Recently approved drugs, Relyvrio (sodium phenylbutyrate and taurursodiol) and Tofersen (Qalsody) have completed the trials, and others are currently undergoing extensive clinical trials. Continuous research and exploration of therapeutic avenues, including gene therapy and neuroprotective treatments, are imperative to address the challenges posed by ALS and other neurodegenerative diseases. Traditional Chinese medicine (TCM) approaches and clinical trials are being explored for treating ALS symptoms, targeting neuroinflammation, oxidative damage, and muscle weakness, showcasing the potential benefits of integrating traditional and modern approaches in ALS management.

Molecular mechanisms of neurofilament alterations and its application in assessing neurodegenerative disorders

Neurofilaments are intermediate filaments present in neurons. These provide structural support and maintain the size and shape of the neurons. Dysregulation, mutation, and aggregation of neurofilaments raise the levels of these proteins in the blood and cerebrospinal fluid (CSF), which are characteristic features of axonal damage and certain rare neurological diseases, such as Giant Axonal Neuropathy and Charcot-Mare-Tooth disease. Understanding the structure, dynamics, and function of neurofilaments has been greatly enhanced by a diverse range of biochemical and preclinical investigations conducted over more than four decades. Recently, there has been a resurgence of interest in post-translational modifications of neurofilaments, such as phosphorylation, aggregation, mutation, oxidation, etc. Over the past twenty years, several rare disorders have been studied from structural alterations of neurofilaments. These disorders are monitored by fluid biomarkers such as neurofilament light chains. Currently, there are many tools, such as Enzyme-Linked Immunosorbent Assay, Electrochemiluminescence Assay, Single-Molecule Array, Western/immunoblotting, etc., in use to assess the neurofilament proteins in Blood and CSF. However, all these techniques utilize expensive, non-specific, or antibody-based methods, which make them unsuitable for routine screening of neurodegenerative disorders. This provides room to search for newer sensitive, cost-effective, point-of-care tools for rapid screening of the disease. For a long time, the molecular mechanisms of neurofilaments have been poorly understood due to insufficient research attempts, and a deeper understanding of them remains elusive. Therefore, this review aims to highlight the available literature on molecular mechanisms of neurofilaments and the function of neurofilaments in axonal transport, axonal conduction, axonal growth, and neurofilament aggregation, respectively. Further, this review discusses the role of neurofilaments as potential biomarkers for the identification of several neurodegenerative diseases in clinical laboratory practice.

Neurofilament Biophysics: From Structure to Biomechanics

Neurofilaments (NFs) are multisubunit, neuron-specific intermediate filaments consisting of a 10-nm diameter filament "core" surrounded by a layer of long intrinsically disordered protein (IDP) "tails." NFs are thought to regulate axonal caliber during development and then stabilize the mature axon, with NF subunit misregulation, mutation, and aggregation featuring prominently in multiple neurological diseases. The field's understanding of NF structure, mechanics, and function has been deeply informed by a rich variety of biochemical, cell biological, and mouse genetic studies spanning more than four decades. These studies have contributed much to our collective understanding of NF function in axonal physiology and disease. In recent years, however, there has been a resurgence of interest in NF subunit proteins in two new contexts: as potential blood- and cerebrospinal fluid-based biomarkers of neuronal damage, and as model IDPs with intriguing properties. Here, we review established principles and more recent discoveries in NF structure and function. Where possible, we place these findings in the context of biophysics of NF assembly, interaction, and contributions to axonal mechanics.

Withania somnifera (L.) Dunal, a Potential Source of Phytochemicals for Treating Neurodegenerative Diseases: A Systematic Review

Withania somnifera (L.) Dunal is a medicinal plant belonging to the traditional Indian medical system, showing various therapeutic effects such as anti-cancer, anti-inflammatory, anti-microbial, anti-diabetic, and hepatoprotective activity. Of great interest is W. somnifera's potential beneficial effect against neurodegenerative diseases, since the authorized medicinal treatments can only delay disease progression and provide symptomatic relief and are not without side effects. A systematic search of PubMed and Scopus databases was performed to identify preclinical and clinical studies focusing on the applications of W. somnifera in preventing neurodegenerative diseases. Only English articles and those containing the keywords (Withania somnifera AND "neurodegenerative diseases", "neuroprotective effects", "Huntington", "Parkinson", "Alzheimer", "Amyotrophic Lateral Sclerosis", "neurological disorders") in the title or abstract were considered. Reviews, editorials, letters, meta-analyses, conference papers, short surveys, and book chapters were not considered. Selected articles were grouped by pathologies and summarized, considering the mechanism of action. The quality assessment and the risk of bias were performed using the Cochrane Handbook for Systematic Reviews of Interventions checklist. This review uses a systematic approach to summarize the results from 60 investigations to highlight the potential role of W. somnifera and its specialized metabolites in treating or preventing neurodegenerative diseases.

Role of the Intermediate Filament Protein Peripherin in Health and Disease

Intermediate filaments are the most heterogeneous class among cytoskeletal elements. While some of them have been well-characterized, little is known about peripherin. Peripherin is a class III intermediate filament protein with a specific expression in the peripheral nervous system. Epigenetic modifications are involved in this cell-type-specific expression. Peripherin has important roles in neurite outgrowth and stability, axonal transport, and axonal myelination. Moreover, peripherin interacts with proteins involved in vesicular trafficking, signal transduction, DNA/RNA processing, protein folding, and mitochondrial metabolism, suggesting a role in all these processes. This review collects information regarding peripherin gene regulation, post-translational modifications, and functions and its involvement in the onset of a number of diseases.

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.

Modelling amyotrophic lateral sclerosis in rodents

The efficient study of human disease requires the proper tools, one of the most crucial of which is an accurate animal model that faithfully recapitulates the human condition. The study of amyotrophic lateral sclerosis (ALS) is no exception. Although the majority of ALS cases are considered sporadic, most animal models of this disease rely on genetic mutations identified in familial cases. Over the past decade, the number of genes associated with ALS has risen dramatically and, with each new genetic variant, there is a drive to develop associated animal models. Rodent models are of particular importance as they allow for the study of ALS in the context of a living mammal with a comparable CNS. Such models not only help to verify the pathogenicity of novel mutations but also provide critical insight into disease mechanisms and are crucial for the testing of new therapeutics. In this Review, we aim to summarize the full spectrum of ALS rodent models developed to date.

The Neglected Genes of ALS: Cytoskeletal Dynamics Impact Synaptic Degeneration in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that selectively affects motor neurons (MNs) of the cortex, brainstem, and spinal cord. Several genes have been linked to both familial (fALS) and sporadic (sALS) cases of ALS. Among all the ALS-related genes, a group of genes known to directly affect cytoskeletal dynamics (ALS2, DCTN1, PFN1, KIF5A, NF-L, NF-H, PRPH, SPAST, and TUBA4A) is of high importance for MN health and survival, considering that MNs are large polarized cells with axons that can reach up to 1 m in length. In particular, cytoskeletal dynamics facilitate the transport of organelles and molecules across the long axonal distances within the cell, playing a key role in synapse maintenance. The majority of ALS-related genes affecting cytoskeletal dynamics were identified within the past two decades, making it a new area to explore for ALS. The purpose of this review is to provide insights into ALS-associated cytoskeletal genes and outline how recent studies have pointed towards novel pathways that might be impacted in ALS. Further studies making use of extensive analysis models to look for true hits, the newest technologies such as CRIPSR/Cas9, human induced pluripotent stem cells (iPSCs) and axon sequencing, as well as the development of more transgenic animal models could potentially help to: differentiate the variants that truly act as a primary cause of the disease from the ones that act as risk factors or disease modifiers, identify potential interactions between two or more ALS-related genes in disease onset and progression and increase our understanding of the molecular mechanisms leading to cytoskeletal defects. Altogether, this information will give us a hint on the real contribution of the cytoskeletal ALS-related genes during this lethal disease.

Myelinated axons fail to develop properly in a genetically authentic mouse model of Charcot-Marie-Tooth disease type 2E

We have analyzed a mouse model of Charcot-Marie-Tooth disease 2E (CMT2E) harboring a heterozygous p.Asn98Ser (p.N98S) Nefl mutation, whose human counterpart results in a severe, early-onset neuropathy. Behavioral, electrophysiological, and pathological analyses were done on separate cohorts of Nefl^N98S/+ mutant mice and their wild type Nefl^+/+ littermates between 8 and 48 weeks of age. The motor performance of Nefl^N98S/+ mice, as evidenced by altered balance and gait measures, was impaired at every age examined (from 6 to 25 weeks of age). At all times examined, myelinated axons were smaller and contained markedly fewer neurofilaments in Nefl^N98S/+ mice, in all examined aspects of the PNS, from the nerve roots to the distal ends of the sciatic and caudal nerves. Similarly, the myelinated axons in the various tracts of the spinal cord and in the optic nerves were smaller and contained fewer neurofilaments in mutant mice. The myelinated axons in both the PNS and the CNS of mutant mice had relatively thicker myelin sheaths. The amplitude and the nerve conduction velocity of the caudal nerves were reduced in proportion with the diminished sizes of myelinated axons. Conspicuous aggregations of neurofilaments were only seen in primary sensory and motor neurons, and were largely confined to the cell bodies and proximal axons. There was evidence of axonal degeneration and regeneration of myelinated axons, mostly in distal nerves. In summary, the p.N98S mutation causes a profound reduction of neurofilaments in the myelinated axons of the PNS and CNS, resulting in substantially reduced axonal diameters, particularly of large myelinated axons, and distal axon loss in the PNS.

Abnormal neurofilament inclusions and segregations in dorsal root ganglia of a Charcot-Marie-Tooth type 2E mouse model

Charcot-Marie-Tooth (CMT) disease or hereditary motor and sensory neuropathy is the most prevalent inherited peripheral neuropathy and is associated with over 90 causative genes. Mutations in neurofilament light polypeptide gene, NEFL cause CMT2E, an axonal form of CMT that results in abnormal structures and/or functions of peripheral axons in spinal cord motor neurons and dorsal root ganglion neurons. We have previously generated and characterized a knock-in mouse model of CMT2E with the N98S mutation in Nefl that presented with multiple inclusions in spinal cord neurons. In this report, we conduct immunofluorescence studies of cultured dorsal root ganglia (DRG) from Nefl^N98S/+ mice, and show that inclusions found in DRG neurites can occur in embryonic stages. Ultrastructural analyses reveal that the inclusions are disordered neurofilaments packed in high density, segregated from other organelles. Immunochemical studies show decreased NFL protein levels in DRG, cerebellum and spinal cord in Nefl^N98S/+ mice, and total NFL protein pool is shifted toward the triton-insoluble fraction. Our findings reveal the nature of the inclusions in Nefl^N98S/+ mice, provide useful information to understand mechanisms of CMT2E disease, and identify DRG from Nefl^N98S/+ mice as a useful cell line model for therapeutic discoveries.

Withania somnifera Reverses Transactive Response DNA Binding Protein 43 Proteinopathy in a Mouse Model of Amyotrophic Lateral Sclerosis/Frontotemporal Lobar Degeneration

Abnormal cytoplasmic mislocalization of transactive response DNA binding protein 43 (TARDBP or TDP-43) in degenerating neurons is a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). Our previous work suggested that nuclear factor kappa B (NF-κB) may constitute a therapeutic target for TDP-43-mediated disease. Here, we investigated the effects of root extract of Withania somnifera (Ashwagandha), an herbal medicine with anti-inflammatory properties, in transgenic mice expressing a genomic fragment encoding human TDP-43A315T mutant. Ashwagandha extract was administered orally to hTDP-43A315T mice for a period of 8 weeks starting at 64 and 48 weeks of age for males and females, respectively. The treatment of hTDP-43A315T mice ameliorated their motor performance on rotarod test and cognitive function assessed by the passive avoidance test. Microscopy examination of tissue samples revealed that Ashwagandha treatment of hTDP-43A315T mice improved innervation at neuromuscular junctions, attenuated neuroinflammation, and reduced NF-κB activation. Remarkably, Ashwagandha treatment reversed the cytoplasmic mislocalization of hTDP-43 in spinal motor neurons and in brain cortical neurons of hTDP-43A315T mice and it reduced hTDP-43 aggregation. In vitro evidence is presented that the neuronal rescue of TDP-43 mislocalization may be due to the indirect effect of factors released from microglial cells exposed to Ashwagandha. These results suggest that Ashwagandha and its constituents might represent promising therapeutics for TDP-43 proteinopathies.

α-Internexin and Peripherin: Expression, Assembly, Functions, and Roles in Disease

α-Internexin and peripherin are neuronal-specific intermediate filament (IF) proteins. α-Internexin is a type IV IF protein like the neurofilament triplet proteins (NFTPs, which include neurofilament light chain, neurofilament medium chain, and neurofilament high chain) that are generally considered to be the primary components of the neuronal IFs. However, α-internexin is often expressed together with the NFTPs and has been proposed as the fourth subunit of the neurofilaments in the central nervous system. α-Internexin is also expressed earlier in the development than the NFTPs and is a maker for neuronal IF inclusion disease. α-Internexin can self-polymerize in vitro and in transfected cells and it is present in the absence of the NFTP in development and in granule cells in the cerebellum. In contrast, peripherin is a type III IF protein. Like α-internexin, peripherin is specific to the nervous system, but it is expressed predominantly in the peripheral nervous system (PNS). Peripherin can also self-assemble both in vitro and in transfected cells. It is as abundant as the NFTPs in the sciatic nerve and can be considered a fourth subunit of the neurofilaments in the PNS. Peripherin has multiple isoforms that arise from intron retention, cryptic intron receptor site or alternative translation initiation. The functional significance of these isoforms is not clear. Peripherin is a major component found in inclusions of patients with amyotrophic lateral sclerosis (ALS) and peripherin expression is upregulated in ALS patients.

Antisense Morpholino Oligonucleotides Reduce Neurofilament Synthesis and Inhibit Axon Regeneration in Lamprey Reticulospinal Neurons

The sea lamprey has been used as a model for the study of axonal regeneration after spinal cord injury. Previous studies have suggested that, unlike developing axons in mammal, the tips of regenerating axons in lamprey spinal cord are simple in shape, packed with neurofilaments (NFs), and contain very little F-actin. Thus it has been proposed that regeneration of axons in the central nervous system of mature vertebrates is not based on the canonical actin-dependent pulling mechanism of growth cones, but involves an internal protrusive force, perhaps generated by the transport or assembly of NFs in the distal axon. In order to assess this hypothesis, expression of NFs was manipulated by antisense morpholino oligonucleotides (MO). A standard, company-supplied MO was used as control. Axon retraction and regeneration were assessed at 2, 4 and 9 weeks after MOs were applied to a spinal cord transection (TX) site. Antisense MO inhibited NF180 expression compared to control MO. The effect of inhibiting NF expression on axon retraction and regeneration was studied by measuring the distance of axon tips from the TX site at 2 and 4 weeks post-TX, and counting the number of reticulospinal neurons (RNs) retrogradely labeled by fluorescently-tagged dextran injected caudal to the injury at 9 weeks post-TX. There was no statistically significant effect of MO on axon retraction at 2 weeks post-TX. However, at both 4 and 9 weeks post-TX, inhibition of NF expression inhibited axon regeneration.

Neurofilament light polypeptide gene N98S mutation in mice leads to neurofilament network abnormalities and a Charcot-Marie-Tooth Type 2E phenotype

Charcot-Marie-Tooth disease (CMT) is the most commonly inherited neurological disorder with a prevalence of 1 in 2500 people worldwide. Patients suffer from degeneration of the peripheral nerves that control sensory information of the foot/leg and hand/arm. Multiple mutations in the neurofilament light polypeptide gene, NEFL, cause CMT2E. Previous studies in transfected cells showed that expression of disease-associated neurofilament light chain variants results in abnormal intermediate filament networks associated with defects in axonal transport. We have now generated knock-in mice with two different point mutations in Nefl: P8R that has been reported in multiple families with variable age of onset and N98S that has been described as an early-onset, sporadic mutation in multiple individuals. Nefl(P8R/+) and Nefl(P8R/P8R) mice were indistinguishable from Nefl(+/+) in terms of behavioral phenotype. In contrast, Nefl(N98S/+) mice had a noticeable tremor, and most animals showed a hindlimb clasping phenotype. Immunohistochemical analysis revealed multiple inclusions in the cell bodies and proximal axons of spinal cord neurons, disorganized processes in the cerebellum and abnormal processes in the cerebral cortex and pons. Abnormal processes were observed as early as post-natal day 7. Electron microscopic analysis of sciatic nerves showed a reduction in the number of neurofilaments, an increase in the number of microtubules and a decrease in the axonal diameters. The Nefl(N98S/+) mice provide an excellent model to study the pathogenesis of CMT2E and should prove useful for testing potential therapies.

Detection of a novel frameshift mutation and regions with homozygosis within ARHGEF28 gene in familial amyotrophic lateral sclerosis

Rho guanine nucleotide exchange factor (RGNEF) is a novel NFL mRNA destabilizing factor that forms neuronal cytoplasmic inclusions in spinal motor neurons in both sporadic (SALS) and familial (FALS) ALS patients. Given the observation of genetic mutations in a number of mRNA binding proteins associated with ALS, including TDP-43, FUS/TLS and mtSOD1, we analysed the ARHGEF28 gene (approx. 316 kb) that encodes for RGNEF in FALS cases to determine if mutations were present. We performed genomic sequencing, copy number variation analysis using TaqMan real-time PCR and spinal motor neuron immunohistochemistry using a novel RGNEF antibody. In this limited sample of FALS cases (n=7) we identified a heterozygous mutation that is predicted to generate a premature truncated gene product. We also observed extensive regions of homozygosity in the ARHGEF28 gene in two FALS patients. In conclusion, our findings of genetic alterations in the ARHGEF28 gene in cases of FALS suggest that a more comprehensive genetic analysis would be warranted.

Rho guanine nucleotide exchange factor is an NFL mRNA destabilizing factor that forms cytoplasmic inclusions in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset progressive disorder of unknown etiology characterized by the selective degeneration of motor neurons. Recent evidence supports the hypothesis that alterations in RNA metabolism in motor neurons can explain the development of protein inclusions, including neurofilamentous aggregates, observed in this pathology. In mice, p190RhoGEF, a guanine nucleotide exchange factor, is involved in neurofilament protein aggregation in an RNA-triggered transgenic model of motor neuron disease. Here, we observed that rho guanine nucleotide exchange factor (RGNEF), the human homologue of p190RhoGEF, binds low molecular weight neurofilament mRNA and affects its stability via 3' untranslated region destabilization. We observed that the overexpression of RGNEF in a stable cell line significantly decreased the level of low molecular weight neurofilament protein. Furthermore, we observed RGNEF cytoplasmic inclusions in ALS spinal motor neurons that colocalized with ubiquitin, p62/sequestosome-1, and TAR (trans-active regulatory) DNA-binding protein 43 (TDP-43). Our results provide further evidence that RNA metabolism pathways are integral to ALS pathology. This is also the first described link between ALS and an RNA binding protein with aggregate formation that is also a central cell signaling pathway molecule.

ALS pathogenesis: recent insights from genetics and mouse models

For the vast majority of cases of amyotrophic lateral sclerosis (ALS) the etiology remains unknown. After the discovery of missense mutations in the gene coding for the Cu/Zn superoxide dismutase 1 (SOD1) in subsets of familial ALS, several transgenic mouse lines have been generated with various forms of SOD1 mutants overexpressed at different levels. Studies with these mice yielded complex results with multiple targets of damage in disease including mitochondria, proteasomes, and secretory pathways. Many unexpected discoveries were made. For instance, the toxicity of mutant SOD1 seems unrelated to copper-mediated catalysis but rather to formation of misfolded SOD1 species and aggregates. Transgenic studies revealed a potential role of wtSOD1 in exacerbating mutant SOD1-mediated disease. Another key finding came from chimeric mouse studies and from Cre-lox mediated gene deletion experiments which have highlighted the importance of non-neuronal cells in the disease progression. Involvement of cytoskeletal components in ALS pathogenesis is supported by several mouse models of motor neuron disease with neurofilament abnormalities and with genetic defects in microtubule-based transport. Recently, the generation of new animal models of ALS has been made possible with the discovery of ALS-linked mutations in other genes encoding for alsin, dynactin, senataxin, VAPB, TDP-43 and FUS. Following the discovery of mutations in the TARDBP gene linked to ALS, there have been some reports of transgenic mice with high level overexpression of WT or mutant forms of TDP-43 under strong gene promoters. However, these TDP-43 transgenic mice do not exhibit all pathological features the human ALS disease. Here, we will describe these new TDP-43 transgenic mice and discuss their validity as animal models of human ALS.

Human low molecular weight neurofilament (NFL) mRNA interacts with a predicted p190RhoGEF homologue (RGNEF) in humans

In the mouse, p190RhoGEF is a low molecular weight neurofilament (NFL) mRNA stability factor that is involved in NF aggregate formation in neurons. A human homologue of this protein has not been described. Our objective was to identify a human homologue of p190RhoGEF, and to determine its interaction with human NFL mRNA. We used sequence homology searches to predict a human homologue (RGNEF), and RT-PCR to determine the expression of mRNA in ALS and neuropathologically normal control tissues. Gel shift assays determined the interaction of RGNEF with human NFL mRNA in vitro, while IP-RT-PCR and gel shift assays were used to confirm the interaction in tissue lysates. We determined that RGNEF is a human homologue of p190RhoGEF, and that its RNA is expressed in both brain and spinal cord. While RGNEF and NFL mRNA interact directly in vitro, interestingly they only appear to interact in ALS lysates and not in controls. These data add another player to the family of NFL mRNA stability regulators, and raise the intriguing possibility that the mechanism by which p190RhoGEF contributes to murine neuronal NF aggregate formation may be important to human ALS NF aggregate formation.

Type III intermediate filament peripherin inhibits neuritogenesis in type II spiral ganglion neurons in vitro

Peripherin, a type III intermediate filament protein, forms part of the cytoskeleton in a subset of neurons, most of which have peripheral fibre projections. Studies suggest a role for peripherin in axon outgrowth and regeneration, but evidence for this in sensory and brain tissues is limited. The exclusive expression of peripherin in a sub-population of primary auditory neurons, the type II spiral ganglion neurons (SGN) prompted our investigation of the effect of peripherin gene deletion (pphKO) on these neurons. We used confocal immunofluorescence to examine the establishment of the innervation of the cochlear outer hair cells by the type II SGN neurites in vivo and in vitro, in wildtype (WT) and pphKO mice, in the first postnatal week. The distribution of the type II SGN nerve fibres was normal in pphKO cochleae. However, using P1 spiral ganglion explants under culture conditions where the majority of neurites were derived from type II SGN, pphKO resulted in increased numbers of neurites/explant compared to WT controls. Type II SGN neurites from pphKO explants extended approximately double the distance of WT neurites, and had reduced complexity based on greater distance between turning points. Addition of brain-derived neurotrophic factor (BDNF) to the culture media increased neurite number in WT and KO explants approximately 30-fold, but did not affect neurite length or distance between turning. These results indicate that peripherin may interact with other cytoskeletal elements to regulate outgrowth of the peripheral neurites of type II SGN, distinguishing these neurons from the type I SGN innervating the inner hair cells.

A novel recessive Nefl mutation causes a severe, early-onset axonal neuropathy

OBJECTIVE

To report the first cases of a homozygous recessive mutation in NEFL, the gene that encodes the light subunit of neurofilaments.

METHODS

Clinical and electrophysiologic data were evaluated, and a sural nerve biopsy from one affected child was examined by immunohistochemistry and electron microscopy. The ability of the mutant protein to form filaments was characterized in an established cell culture system.

RESULTS

Four of five siblings developed of a severe, progressive neuropathy beginning in early childhood. Serial nerve conduction studies showed progressively reduced amplitudes with age and pronounced slowing at all ages. Visual-evoked responses were slowed in three children, indicating that central nervous system axons were subclinically involved. All four affected children were homozygous for a nonsense mutation at glutamate 210 (E210X) in the NEFL gene; both parents were heterozygous carriers. A sural nerve biopsy from an affected patient showed markedly reduced numbers of myelinated axons; the remaining myelinated axons were small and lacked intermediate filaments. The E210X mutant protein did not form an intermediate filament network and did not interfere with the filament formation by wild-type human light subunit of neurofilaments in SW-13 vim(-) cells.

INTERPRETATION

This is the first demonstration of a recessive NEFL mutation, which appears to cause a simple loss of function, resulting in a severe, early-onset axonal neuropathy with unique features. These results confirm that neurofilaments are the main determinant of axonal caliber and conduction velocity, and demonstrate for the first time that neurofilaments are required for the maintenance of myelinated peripheral nervous system axons.

Tar DNA binding protein of 43 kDa (TDP-43), 14-3-3 proteins and copper/zinc superoxide dismutase (SOD1) interact to modulate NFL mRNA stability. Implications for altered RNA processing in amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disease characterized by progressive motor neuron degeneration in association with neurofilament (NF) aggregate formation. This process is accompanied by an alteration in the stoichiometry of NF subunit protein expression such that the steady state levels of the low molecular weight NF (NFL) mRNA levels are selectively suppressed. We have previously shown that each of TDP-43, 14-3-3 and mutant SOD1 can function as NFL mRNA 3'UTR binding proteins that directly affect the stability of NFL transcripts. In this study, we demonstrate that the interaction of TDP-43 with the NFL mRNA 3' UTR involves ribonucleotide (UG) motifs present on stem loops of the 3'UTR as well as the RRM1 and RRM2 motifs of TDP-43. Ex vivo, TDP-43, 14-3-3 and SOD1 proteins interact to modulate NFL mRNA stability, although in vivo, only TDP-43 and either mutant or wild-type SOD1 co-localize in ALS motor neurons. TDP-43 was observed to co-localize to RNA transport granules (Staufen immunoreactive) in both control and ALS spinal motor neurons. In contrast, both stress granules (TIA-1 immunoreactive) and processing bodies (P-bodies; XRN-1 immunoreactive) were more prevalent in ALS motor neurons than in controls and demonstrated strong co-localization with TDP-43. Using RNA-IP-PCR, we further demonstrate that NFL mRNA is preferentially sequestered to both stress granules and P-bodies in ALS. These data suggest that NFL mRNA processing is fundamentally altered in ALS spinal motor neurons to favour compartmentalization within both stress granules and P-bodies, and that TDP-43 plays a fundamental role in this process.

Neuronal intermediate filaments and neurodegenerative disorders

Intermediate filaments represent the most abundant cytoskeletal element in mature neurons. Mutations and/or accumulations of neuronal intermediate filament proteins are frequently observed in several human neurodegenerative disorders. Although it is now admitted that disorganization of the neurofilament network may be directly involved in neurodegeneration, certain type of perikaryal intermediate filament aggregates confer protection in motor neuron disease. The use of various mouse models provided a better knowledge of the role played by the disorganization of intermediate filaments in the pathogenesis of neurodegenerative disorders, but the mechanisms leading to the formation of these aggregates remain elusive. Here, we will review some neurodegenerative diseases involving intermediate filaments abnormalities and possible mechanisms susceptible to provoke them.

Real-time imaging reveals defects of fast axonal transport induced by disorganization of intermediate filaments

Intermediate filament (IF) abnormalities frequently appear in neurodegenerative disorders, but how they may contribute to neuronal dysfunction remains unclear. Here, we examined the effects of IF disorganization on the fast axonal transport using time-lapse microscopy. We studied the axonal transport of mitochondria and lysosomes in cultured primary dorsal root ganglion (DRG) neurons derived from mice deficient for neurofilament light (NFL(-/-)), mice overexpressing peripherin (Per), and mice double transgenic Per;NFL(-/-). Unexpectedly, a net retrograde transport of mitochondria was detected in Per;NFL(-/-) neurons, opposite to the net anterograde transport of these organelles observed in wild-type (Wt), NFL(-/-), and Per neurons. A detailed analysis of the kinetic properties of mitochondrial movements revealed an increased frequency of retrograde movements and an increase of their velocity in Per;NFL(-/-) neurons compared to Wt, NFL(-/-), and Per neurons. We also noticed that the depletion of axonal neurofilaments (NFs) in NFL(-/-) and Per;NFL(-/-) neurons induced longer and more persistent movements of mitochondria and lysosomes in both directions, which suggests that the NF network hampers the traffic of these organelles. The finding that an up-regulation of peripherin in context of NFL deficiency can provoke a net retrograde transport of mitochondria is a phenomenon that may contribute to pathogenic changes in some neurodegenerative disorders with IF protein accumulations.

Gamma-synucleinopathy: neurodegeneration associated with overexpression of the mouse protein

The role of α-synuclein in pathogenesis of familial and idiopathic forms of Parkinson’s disease, and other human disorders known as α-synucleinopathies, is well established. In contrast, the involvement of two other members of the synuclein family, β-synuclein and γ-synuclein, in the development and progression of neurodegeneration is poorly studied. However, there is a growing body of evidence that α-synuclein and β-synuclein have opposite neuropathophysiological effects. Unlike α-synuclein, overexpressed β-synuclein does not cause pathological changes in the nervous system of transgenic mice and even ameliorates the pathology caused by overexpressed α-synuclein. To assess the consequences of excess expression of the third family member, γ-synuclein, on the nervous system we generated transgenic mice expressing high levels of mouse γ-synuclein under control of Thy-1 promoter. These animals develop severe age- and transgene dose-dependent neuropathology, motor deficits and die prematurely. Histopathological changes include aggregation of γ-synuclein, accumulation of various inclusions in neuronal cell bodies and processes, and astrogliosis. These changes are seen throughout the nervous system but are most prominent in the spinal cord where they lead to loss of spinal motor neurons. Our data suggest that down-regulation of small heat shock protein HSPB1 and disintegration of neurofilament network play a role in motor neurons dysfunction and death. These findings demonstrate that γ-synuclein can be involved in neuropathophysiological changes and the death of susceptible neurons suggesting the necessity of further investigations of the potential role of this synuclein in disease.

Abnormal cellular translocation of alpha-internexin in spinal motor neurons of Dystonia musculorum mice

Dystonia musculorum (dt) is a mutant mouse with hereditary neuropathy where the dysfunction is mainly found in the dorsal root ganglia (DRG) neurons but not in the spinal motor neurons. However, the accumulation of intermediate filament (IF) proteins in the swelling axons of spinal motor neurons could be found in dt/dt mice. In order to understand the pathological role of neuronal IFs in the swelling axons of spinal motor neurons from dt/dt mice, we extensively examined the distribution of neuronal IF proteins. By immunofluorescence staining, our results indicated that alpha-internexin was a major component in the swelling axon and showed abnormal translocation in the nuclei of spinal motor neurons in dt/dt mice. This abnormal translocation of alpha-internexin in the nuclei of spinal motor neurons was also confirmed by Western blotting and immunoelectron microscopy. Instead of the 10-nm filamentous structure, a diffuse immunopositive pattern of alpha-internexin was observed in the nucleus of spinal motor neurons in dt/dt mutants. We further examined the cell death of spinal motor neurons by TUNEL assay, and no TUNEL-positive cells could be identified from spinal motor neurons in dt/dt mice. From these observations we suggest that abnormal accumulation of neuronal IFs in the swelling axons and abnormal translocation of alpha-internexin in the nuclei of the spinal motor neurons from dt/dt mice may not directly cause cell death of the spinal motor neurons.

Amyotrophic lateral sclerosis models and human neuropathology: similarities and differences

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that primarily involves the motor neuron system. The author initially summarizes the principal features of human ALS neuropathology, and subsequently describes in detail ALS animal models mainly from the viewpoint of pathological similarities and differences. ALS animal models in this review include strains of rodents that are transgenic for superoxide dismutase 1 (SOD1), ALS2 knockout mice, and mice that are transgenic for cytoskeletal abnormalities. Although the neuropathological results obtained from human ALS autopsy cases are valuable and important, almost all of such cases represent only the terminal stage. This makes it difficult to clarify how and why ALS motor neurons are impaired at each clinical stage from disease onset to death, and as a consequence, human autopsy cases alone yield little insight into potential therapies for ALS. Although ALS animal models cannot replicate human ALS, in order to compensate for the shortcomings of studies using human ALS autopsy samples, researchers must inevitably rely on ALS animal models that can yield very important information for clarifying the pathogenesis of ALS in humans and for the establishment of reliable therapy. Of course, human ALS and all ALS animal models share one most important similarity in that both exhibit motor neuron degeneration/death. This important point of similarity has shed much light on the pathomechanisms of the motor neuron degeneration/death at the cellular and molecular levels that would not have been appreciated if only human ALS autopsy samples had been available. On the basis of the aspects covered in this review, it can be concluded that ALS animal models can yield very important information for clarifying the pathogenesis of ALS in humans and for the establishment of reliable therapy only in combination with detailed neuropathological data obtained from human ALS autopsy cases.

Human mesenchymal stromal cells ameliorate the phenotype of SOD1-G93A ALS mice

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a progressive, lethal, neurodegenerative disease, currently without any effective therapy. Multiple advantages make mesenchymal stromal cells (MSC) a good candidate for cellular therapy in many intractable diseases such as stroke and brain injury. Until now, no irrefutable evidence exists regarding the outcome of MSC transplantation in the mouse model of ALS. The present study was designed to investigate the therapeutic potential of human MSC (hMSC) in the mouse model of ALS (SOD1-G93A mice).

METHODS

hMSC were isolated from iliac crest aspirates from healthy donors and kept in cell cultures. hMSC of the fifth passage were delivered intravenously into irradiated pre-symptomatic SOD1-G93A mice. Therapeutic effects were analyzed by survival analysis, rotarod test, motor neuron count in spinal cord and electrophysiology. The engraftment and in vivo differentiation of hMSC were examined in the brain and spinal cord of hMSC-transplanted mice.

RESULTS

After intravenous injection into irradiated pre-symptomatic SOD1-G93A mice, hMSC survived more than 20 weeks in recipient mice, migrated into the parenchyma of brain and spinal cord and showed neuroglia differentiation. Moreover, hMSC-transplanted mice showed significantly delayed disease onset (14 days), increased lifespan (18 days) and delayed disease progression compared with untreated mice.

DISCUSSION

Our data document the positive effects of hMSC transplantation in the mouse model of ALS. It may signify the potential use of hMSC in treatment of ALS.

Neuronal expression of peripherin, a type III intermediate filament protein, in the mouse hindbrain

Peripherin is a 57 kDa Type III intermediate filament protein associated with neurite extension, neuropathies such as amyotrophic lateral sclerosis, and cranial nerve and dorsal root projections. However, knowledge of peripherin expression in the CNS is limited. We have used immunoperoxidase histochemistry to characterise peripherin expression in the mouse hindbrain, including the inferior colliculus, pons, medulla and cerebellum. Peripherin immunolabelling was observed in the nerve fibres and nuclei that are associated with all cranial nerves [(CN) V-XII] in the hindbrain. Peripherin expression was prominent in the cell bodies and axons of the mesenchephalic trigeminal nucleus and the pars compacta region of nucleus ambiguus, and in the fibres that comprise the solitary tract, the descending spinal trigeminal tract and the trigeminal and facial nerves. A small proportion of peripherin positive fibres in CN VIII likely arise from cochlear type II spiral ganglion neurons. Peripherin positive fibres were also observed in the inferior cerebellar peduncle and folia in the intermediate zone of the cerebellum. Antibody specificity was confirmed by absence of labelling in hindbrain tissue from peripherin knockout mice. This study shows that in the adult mouse hindbrain, peripherin is expressed in discrete neuronal subpopulations that have sensory, motor and autonomic functions.

Motor neuron disease in transgenic mice with an H46R mutant SOD1 gene

Human familial amyotrophic lateral sclerosis with an H46R mutant Cu/Zn superoxide dismutase (SOD1) gene is characterized by initial muscle weakness and atrophy in the legs and a very long-term clinical course (approximately 15 years). Transgenic mice with this mutation generated in our laboratory occasionally showed aggregates in the anterior horns and axonal degeneration in all white matter sections of the spinal cord on plastic sections at the presymptomatic stages (12 and 16 weeks old), although conventional staining revealed no pathologic changes. At the symptomatic stages (20 and 24 weeks), loss of anterior horn neurons was observed. On plastic sections, aggregates were frequently seen not only in the anterior horns but also in the posterior horns and in all sections of white matter. Degenerated fibers were observed in the anterior and posterior roots as well as in white matter. Electron and immunoelectron microscopic observation revealed human SOD1- and ubiquitin-positive aggregates consisting of intermediate filaments in the anterior horn even from an early presymptomatic stage. Thus, H46R mutant SOD1 transgenic mice are characterized by widespread pathologic changes of the spinal cord that extend beyond the motor system, including many aggregates lacking vacuoles. The close pathologic similarity makes this animal model suitable for the investigation of human familial amyotrophic lateral sclerosis with the mutation.

Post-transcriptional control of neurofilaments in development and disease

Tight coordination of the expression of neurofilament subunits is integral to the normal development and function of the nervous system. Imbalances in their expression are increasingly implicated in the induction of neurodegeneration in which formation of neurofilamentous aggregates is central to the pathology. Neurofilament expression can be controlled not only at the transcriptional level but also through post-transcriptional regulation of mRNA localization, stability, and translational efficiency. The critical role that post-transcriptional mechanisms play in maintaining neurofilament homeostasis is highlighted, for example, by the human disease amyotrophic lateral sclerosis, in which selective destabilization of NF-L mRNA (or failure to stabilize it) is associated with the formation of neurofilamentous aggregates - a hallmark of the disease process. This review discusses the post-transcriptional regulatory mechanisms and associated ribonucleoproteins that have been implicated to date in controlling neurofilament expression during normal development and in disrupting neurofilament homeostasis during neurodegenerative disease.

Amyotrophic lateral sclerosis – The tools of the trait

The aim of this review is to analyze how our knowledge on the etiology, pathology, and treatment of amyotrophic lateral sclerosis (ALS) has profited from the application of biotechnology tools for the identification of disease markers, the development of animal disease models, and the design of innovative therapeutics. In humans, ALS-specific clinical, genetic or protein biomarkers, or panels of biomarkers stemming from genomics and proteomics analyses can be critical for early diagnosis, monitoring of disease progression, drug validation in clinical trials, and identification of therapeutic targets for subsequent drug development. At the same time, animal models representing a number of human superoxide dismutase 1 mutations, intermediate-filament disorganization or axonal-transport defects have been invaluable in unraveling aspects of the pathophysiology of the disease; in each case, these only represent a small proportion of all ALS patients. Preclinical and clinical trials, although at present heavily concentrating on pharmacological approaches, are embracing the emerging alternative strategies of stem-cell and gene therapy. In combination with a further subcategorization of patients and the development of corresponding model systems for functional analyses, they will significantly influence the already changing face of ALS therapy.

Neuronal intermediate filaments and ALS: a new look at an old question

One of the pathological hallmarks of ALS is the presence of axonal spheroids and perikaryal accumulations/aggregations comprised of the neuronal intermediate filament proteins, neurofilaments and peripherin. These abnormalities represent a point of convergence of both familial and sporadic forms of the disease and understanding their formation may reveal shared pathways in what is otherwise considered a highly heterogeneous disorder. Here we provide a review of the basic biology of neurofilaments and peripherin and the evidence linking them with ALS disease pathogenesis.

Topography of alpha-internexin-positive neuronal aggregates in 10 patients with neuronal intermediate filament inclusion disease

Abnormal neuronal intermediate filament (IF) inclusions immunopositive for the type IV IF alpha-internexin have been identified as the pathological hallmark of neuronal intermediate filament inclusion disease (NIFID). We studied the topography of these inclusions in the frontal and temporal lobe in 68 areas from 10 cases of NIFID. In the cerebral cortex, CA sectors of the hippocampus, and dentate gyrus granule cell layer, the inclusions were distributed mainly in regularly distributed clusters, 50-800 microm in diameter. In seven cortical areas, there was a more complex pattern in which the clusters of inclusions were aggregated into larger super clusters. In 11 cortical areas, the size of the clusters approximated to those of the cells of origin of the cortico-cortical pathways but in the majority of the remaining areas, cluster size was smaller than 400 microm. The topography of the lesions suggests that there is degeneration of the cortico-cortical projections in NIFID with the formation of alpha-internexin-positive aggregates within vertical columns of cells. Initially, only a subset of cells within a vertical column develops inclusions but as the disease progresses, the whole of the column becomes affected. The corticostriate projection appears to have little effect on the cortical topography of the inclusions.

Temporal profiles of neuronal degeneration, glial proliferation, and cell death in hNFL(+/+) and NFL(-/-) mice

Neurofilament (NF) aggregate formation within motor neurons is a pathological hallmark of both the sporadic and familial forms of amyotrophic lateral sclerosis (ALS). The relationship between aggregate formation and both microglial and astrocytic proliferation, as well as additional neuropathological features of ALS, is unknown. To examine this, we have used transgenic mice that develop NF aggregates, through either a lack of the low-molecular-weight NF subunit [NFL (-/-)] or the overexpression of human NFL [hNFL (+/+)]. Transgenic and wild-type C57bl/6 mice were examined from 1 month to 18 months of age, and the temporal pattern of motor neuron degeneration, microglial and astrocytic proliferation, and heat shock protein-70 (HSP-70) expression characterized. We observed three overlapping phases in both transgenic mice, including transient aggregate formation, reactive microgliosis, and progressive motor neuron loss. However, only NFL (-/-) mice demonstrated significant astrogliosis and HSP-70 upregulation in both motor neurons and astrocytes. These in vivo models suggest that the development of NF aggregates in motor neurons leads to motor neuron death, but that the interaction between the degenerating motor neurons and the adjacent non-neuronal cells may differ significantly depending on the etiology of the NF aggregate itself.

Defective axonal transport of neurofilament proteins in neurons overexpressing peripherin

Peripherin is a type III neuronal intermediate filament detected in motor neuron inclusions of amyotrophic lateral sclerosis (ALS) patients. We previously reported that overexpression of peripherin provokes late-onset motor neuron dysfunction in transgenic mice. Here, we show that peripherin overexpression slows down axonal transport of neurofilament (NF) proteins, and that the transport defect precedes by several months the appearance of axonal spheroids in adult mice. Defective NF transport by peripherin up-regulation was further confirmed with dorsal root ganglia (DRG) neurons cultured from peripherin transgenic embryos. Immunofluorescence microscopy and western blotting revealed that excess peripherin provokes reduction in levels of hyperphosphorylated NF-H species in DRG neurites. Similarly the transport of a green fluorescent protein (GFP)-tagged NF-M, delivered by means of a lentiviral construct, was impaired in DRG neurites overexpressing peripherin. These results demonstrate that peripherin overexpression can cause defective transport of type IV NF proteins, a phenomenon that may account for the progressive formation of ALS-like spheroids in axons.

Transgenic mouse models of amyotrophic lateral sclerosis

The discovery of missense mutations in the gene coding for the Cu/Zn superoxide dismutase 1 (SOD1) in subsets of familial cases was rapidly followed by the generation of transgenic mice expressing various forms of SOD1 mutants. The mice overexpressing high levels of mutant SOD1 mRNAs do develop motor neuron disease but unraveling the mechanisms of pathogenesis has been very challenging. Studies with mouse lines suggest that the toxicity of mutant SOD1 is unrelated to copper-mediated catalysis but rather to propensity of a subfraction of mutant SOD1 proteins to form misfolded protein species and aggregates. However, the mechanism of toxicity of SOD1 mutants remains to be elucidated. Involvement of cytoskeletal components in ALS pathogenesis is supported by several mouse models of motor neuron disease with neurofilament abnormalities and with genetic defects in microtubule-based transport. Here, we describe how transgenic mouse models have been used for understanding pathogenic pathways of motor neuron disease and for pre-clinical drug testing.

HoxB2 binds mutant SOD1 and is altered in transgenic model of ALS

Mutations in Cu/Zn superoxide dismutase (SOD1) cause approximately 20% of familial amyotrophic lateral sclerosis by a toxic gain of function; however, the precise mechanisms remain unclear. Here, we report the identification of HoxB2, a homeodomain-containing transcription factor, as a G93A mutant SOD1 interactive protein in a yeast two-hybrid screen. We show that HoxB2 co-precipitates and co-localizes with mutant SOD1 in neuronal cell lines, as well as in brain and spinal cord of G93A mutant SOD1 transgenic mice. Mutagenesis further shows that this interaction is mediated by the central homeodomain of HoxB2. In motor neuron-like NSC-34 cells, overexpression of HoxB2 or its homeodomain decreases the insolubility of mutant SOD1 and inhibits G93A or G86R mutant SOD1-induced neuronal cell death. In human and mouse tissues, we show that expression of HoxB2 persists in adult spinal cord and is primarily localized in nuclei of motor neurons. In G93A transgenic mice, HoxB2 co-localizes with mutant SOD1 and is redistributed to perikarya and proximal neurites of motor neurons. In addition, there is progressive accumulation of HoxB2 and mutant SOD1 as punctate inclusions in the neuropil surrounding motor neurons. Taken together, our findings demonstrate that interaction of HoxB2 with mutant SOD1 occurs in motor neurons of G93A mutant SOD1 transgenic mice and suggest that this interaction may modulate the neurotoxicity of mutant SOD1.

Mutations in the neurofilament light gene linked to Charcot-Marie-Tooth disease cause defects in transport

Neurofilament light gene mutations have been linked to a subset of patients with Charcot-Marie-Tooth disease, the most common inherited motor and sensory neuropathy. We have previously shown that Charcot-Marie-Tooth-linked mutant neurofilament light assembles abnormally in non-neuronal cells. In this study, we have characterized the effects of expression of mutant neurofilament light proteins on axonal transport in a neuronal cell culture model. We demonstrated that the Charcot-Marie-Tooth-linked neurofilament light mutations: (i) affect the axonal transport of mutant neurofilaments; (ii) have a dominant-negative effect on the transport of wild-type neurofilaments; (iii) affect the transport of mitochondria and the anterograde axonal transport marker human amyloid precursor protein; (iv) result in alterations of retrograde axonal transport and (v) cause fragmentation of the Golgi apparatus. Increased neuritic degeneration was observed in neuronal cells overexpressing neurofilament light mutants. Our results suggest that these generalized axonal transport defects could be responsible for the neuropathy in Charcot-Marie-Tooth disease.

Ultrastructural study of aggregates in the spinal cord of transgenic mice with a G93A mutant SOD1 gene

The ultrastructural features of SOD1-positive aggregates were determined to clarify whether these aggregates are associated with the pathogenesis of SOD1 mutant mice. We examined the spinal cord of transgenic mice expressing a G93A mutant human SOD1 gene with fewer copies (gene copy 10). At the early presymptomatic stage (age 24 weeks), SOD1- and ubiquitin-positive granular, linear, or round deposits were found occasionally in the neuropil of the anterior horns. Ultrastructurally, small filamentous aggregates were observed occasionally in the neuronal processes including the axons in the anterior horns. At the late presymptomatic stage (28 weeks), SOD1- and ubiquitin-positive deposits and Lewy body-like inclusions (LIs) were frequently demonstrated in the neuronal processes including cord-like swollen axons and in some remaining anterior horn neurons. Ultrastructurally, larger filamentous aggregates were frequent, predominating in the neuronal processes of the anterior horns including the proximal axons, but were rare in the somata and dendrites. The aggregates usually consisted of interwoven intermediate filaments (about 10-15 nm in diameter) and frequently contained electron-dense cores in the center resembling LIs. Occasionally the aggregates consisted mainly of granular, amorphous, or vesicular substance, showing fewer filamentous structures. At the symptomatic stages (32 and 35 weeks), LIs were frequently demonstrated within the neuronal processes in the anterior horns, particularly in the cord-like swollen axons. Many more prominent SOD1- and ubiquitin-positive deposits were observed over the whole white matter columns and in the gray matter of the anterior and posterior horns than at the previous stage. Ultrastructurally, aggregates frequently contained electron-dense cores, and were frequently observed in cord-like swollen axons consisting of accumulated neurofilaments. A high level of human SOD1-and ubiquitin-immunogold labeling was present in small to large aggregates even at the presymptomatic stages, and the aggregates increased in size and frequency with time. Compactly packed filaments and electron-dense cores of aggregates showed SOD1-and ubiquitin-immunogold labeling more prominently than in loosely packed filaments. These findings suggest that the accumulation of SOD1-positive aggregates in the neuronal processes, predominantly in the axons, constitutes an important determinant of neurotoxicity and the pathogenesis of this animal model, probably causing impairment of axonal transport by the sequestration of mutant SOD1 protein within aggregates, or in part by physically blocking the axonal transport.