Early and selective pathology of light chain neurofilament in the spinal cord and sciatic nerve of G86R mutant superoxide dismutase transgenic mice

Role of SNAREs in Neurodegenerative Diseases

Neurodegenerative diseases are pathologies of the central and peripheral nervous systems characterized by loss of brain functions and problems in movement which occur due to the slow and progressive degeneration of cellular elements. Several neurodegenerative diseases are known such as Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis and many studies on the molecular mechanisms underlying these pathologies have been conducted. Altered functions of some key proteins and the presence of intraneuronal aggregates have been identified as responsible for the development of the diseases. Interestingly, the formation of the SNARE complex has been discovered to be fundamental for vesicle fusion, vesicle recycling and neurotransmitter release. Indeed, inhibition of the formation of the SNARE complex, defects in the SNARE-dependent exocytosis and altered regulation of SNARE-mediated vesicle fusion have been associated with neurodegeneration. In this review, the biological aspects of neurodegenerative diseases and the role of SNARE proteins in relation to the onset of these pathologies are described.

TDP-43 mislocalization drives neurofilament changes in a novel model of TDP-43 proteinopathy

Mislocalization of the TAR DNA-binding protein 43 (TDP-43) from the nucleus to the cytoplasm is a common feature of neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). The downstream in vivo cellular effects of this mislocalization are not well understood. To investigate the impact of mislocalized TDP-43 on neuronal cell bodies, axons and axonal terminals, we utilized the mouse visual system to create a new model of TDP-43 proteinopathy. Mouse (C57BL/6J) retinal ganglion cells (RGCs) were transduced with GFP-tagged human wildtype TDP-43 (hTDP-WT-GFP) and human TDP-43 with a mutation in the nuclear localization sequence (hTDP-ΔNLS-GFP), to cause TDP-43 mislocalization, with ∼60% transduction efficiency achieved. Expression of both hTDP-WT-GFP and hTDP-ΔNLS-GFP resulted in changes to neurofilament expression, with cytoplasmic TDP-43 being associated with significantly (p<0.05) increased neurofilament heavy expression in the cell soma, and both forms of altered TDP-43 leading to significantly (p<0.05) decreased numbers of neurofilament-positive axons within the optic nerve. Alterations to neurofilament proteins were associated with significantly (p<0.05) increased microglial density in the optic nerve and retina. Furthermore expression of hTDP-WT-GFP was associated with a significant (p<0.05) increase in pre-synaptic input into RGCs in the retina. The current study has developed a new model allowing detailed examination of alterations to TDP-43 and will contribute to the knowledge of TDP-43-mediated neuronal alterations and degeneration.

A Human-Based Functional NMJ System for Personalized ALS Modeling and Drug Testing

Loss of the neuromuscular junction (NMJ) is an early and critical hallmark in all forms of ALS. The study design was to develop a functional NMJ disease model by integrating motoneurons (MNs) differentiated from multiple ALS-patients' induced pluripotent stem cells (iPSCs) and primary human muscle into a chambered system. NMJ functionality was tested by recording myotube contractions while stimulating MNs by field electrodes and a set of clinically relevant parameters were defined to characterize the NMJ function. Three ALS lines were analyzed, 2 with SOD1 mutations and 1 with a FUS mutation. The ALS-MNs reproduced pathological phenotypes, including increased axonal varicosities, reduced axonal branching and elongation and increased excitability. These MNs formed functional NMJs with wild type muscle, but with significant deficits in NMJ quantity, fidelity and fatigue index. Furthermore, treatment with the Deana protocol was found to correct the NMJ deficits in all the ALS mutant lines tested. Quantitative analysis also revealed the variations inherent in each mutant lines. This functional NMJ system provides a platform for the study of both fALS and sALS and has the capability of being adapted into subtype-specific or patient-specific models for ALS etiological investigation and patient stratification for drug testing.

The Peripheral Nervous System in Amyotrophic Lateral Sclerosis: Opportunities for Translational Research

Although amyotrophic lateral sclerosis (ALS) has been considered as a disorder of the motor neuron (MN) cell body, recent evidences show the non-cell-autonomous pathogenic nature of the disease. Axonal degeneration, loss of peripheral axons and destruction of nerve terminals are early events in the disease pathogenic cascade, anticipating MN degeneration, and the onset of clinical symptoms. Therefore, although ALS and peripheral axonal neuropathies should be differentiated in clinical practice, they also share damage to common molecular pathways, including axonal transport, RNA metabolism and proteostasis. Thus, an extensive evaluation of the molecular events occurring in the peripheral nervous system (PNS) could be fundamental to understand the pathogenic mechanisms of ALS, favoring the discovery of potential disease biomarkers, and new therapeutic targets.

ALS mouse model SOD1G93A displays early pathology of sensory small fibers associated to accumulation of a neurotoxic splice variant of peripherin

Growing evidence suggests that amyotrophic lateral sclerosis (ALS) is a multisystem neurodegenerative disease that primarily affects motor neurons and, though less evidently, other neuronal systems. About 75% of sporadic and familial ALS patients show a subclinical degeneration of small-diameter fibers, as measured by loss of intraepidermal nerve fibers (IENFs), but the underlying biological causes are unknown. Small-diameter fibers are derived from small-diameter sensory neurons, located in dorsal root ganglia (DRG), whose biochemical hallmark is the expression of type III intermediate filament peripherin. We tested here the hypothesis that small-diameter DRG neurons of ALS mouse model SOD1(G93A)suffer from axonal stress and investigated the underlying molecular mechanism. We found that SOD1(G93A)mice display small fiber pathology, as measured by IENF loss, which precedes the onset of the disease. In vitro small-diameter DRG neurons of SOD1(G93A)mice show axonal stress features and accumulation of a peripherin splice variant, named peripherin56, which causes axonal stress through disassembling light and medium neurofilament subunits (NFL and NFM, respectively). Our findings first demonstrate that small-diameter DRG neurons of the ALS mouse model SOD1(G93A)display axonal stress in vitro and in vivo, thus sustaining the hypothesis that the effects of ALS disease spread beyond motor neurons. These results suggest a molecular mechanism for the small fiber pathology found in ALS patients. Finally, our data agree with previous findings, suggesting a key role of peripherin in the ALS pathogenesis, thus highlighting that DRG neurons mirror some dysfunctions found in motor neurons.

Oligodendroglia metabolically support axons and contribute to neurodegeneration

Oligodendroglia support axon survival and function through mechanisms independent of myelination, and their dysfunction leads to axon degeneration in several diseases. The cause of this degeneration has not been determined, but lack of energy metabolites such as glucose or lactate has been proposed. Lactate is transported exclusively by monocarboxylate transporters, and changes to these transporters alter lactate production and use. Here we show that the most abundant lactate transporter in the central nervous system, monocarboxylate transporter 1 (MCT1, also known as SLC16A1), is highly enriched within oligodendroglia and that disruption of this transporter produces axon damage and neuron loss in animal and cell culture models. In addition, this same transporter is reduced in patients with, and in mouse models of, amyotrophic lateral sclerosis, suggesting a role for oligodendroglial MCT1 in pathogenesis. The role of oligodendroglia in axon function and neuron survival has been elusive; this study defines a new fundamental mechanism by which oligodendroglia support neurons and axons.

Degeneration of axons in spinal white matter in G93A mSOD1 mouse characterized by NFL and α-internexin immunoreactivity

Axonal degeneration is a prominent feature of amyotrophic lateral sclerosis (ALS) both in lower motor nerves as well as descending white matter axons in the spinal cord of human patients. Although the pathology of lower motor axonal degeneration has been described in both human ALS and related transgenic animal models, few studies have examined the pathological features of descending axon degeneration, particularly in mouse models of ALS. We have examined the degeneration of white matter tracts in the G93A mutant superoxide dismutase-1 (mSOD1+) mouse spinal cord white matter from 12 weeks of age to end-stage disease. In a G93A mSOD1 mouse model where green fluorescent protein was expressed in neurons (mSOD1+/GFP+), degeneration of white matter tracts was present from the ventral to dorsolateral funiculi. This pattern of axonal pathology occurred from 16 weeks of age. However, the dorsal funiculus, the site of the major corticospinal tract in mice, showed relatively less degeneration. Immunohistochemical analysis demonstrated that the neurofilament light chain (NFL) and neuronal intermediate filament protein alpha-internexin accumulated in axon swellings in the spinal white matter. Increased levels of alpha-internexin protein, in mSOD1+ mouse spinal cord tissue, were demonstrated by Western blotting. In contrast, degenerating axons did not show obvious accumulations of neurofilament medium and heavy chain proteins (NFM and NFH). These data suggest that white matter degeneration in this mouse model of ALS is widespread and involves a specific molecular signature, particularly the accumulation of NFL and alpha-internexin proteins.

Progressive changes in microglia and macrophages in spinal cord and peripheral nerve in the transgenic rat model of amyotrophic lateral sclerosis

Background

The role of neuroinflammation in motor neuron death of amyotrophic lateral sclerosis (ALS) is unclear. The human mutant superoxide dismutase-1 (hmSOD1)-expressing murine transgenic model of ALS has provided some insight into changes in microglia activity during disease progression. The purpose of this study was to gain further knowledge by characterizing the immunological changes during disease progression in the spinal cord and peripheral nerve using the more recently developed hmSOD1 rat transgenic model of ALS.

Methods

Using immunohistochemistry, the extent and intensity of tissue CD11b expression in spinal cord, lumbar nerve roots, and sciatic nerve were evaluated in hmSOD1 rats that were pre-clinical, at clinical onset, and near disease end-stage. Changes in CD11b expression were compared to the detection of MHC class II and CD68 microglial activation markers in the ventral horn of the spinal cord, as well as to the changes in astrocytic GFAP expression.

Results

Our study reveals an accumulation of microglia/macrophages both in the spinal cord and peripheral nerve prior to clinical onset based on CD11b tissue expression. The microglia formed focal aggregates in the ventral horn and became more widespread as the disease progressed. Hypertrophic astrocytes were not prominent in the ventral horn until after clinical onset, and the enhancement of GFAP did not have a strong correlation to increased CD11b expression. Detection of MHC class II and CD68 expression was found in the ventral horn only after clinical onset. The macrophages in the ventral nerve root and sciatic nerve of hmSOD1 rats were observed encircling axons.

Conclusions

These findings describe for the first time in the hmSOD1 rat transgenic model of ALS that enhancement of microglia/macrophage activity occurs pre-clinically both in the peripheral nerve and in the spinal cord. CD11b expression is shown to be a superior indicator for early immunological changes compared to other microglia activation markers and astrogliosis. Furthermore, we suggest that the early activity of microglia/macrophages is involved in the early phase of motor neuron degeneration and propose that studies involving immunomodulation in hmSOD1transgenic models need to consider effects on macrophages in peripheral nerves as well as to microglia in the spinal cord.

NEOSTRIATAL CYTOSKELETON CHANGES FOLLOWING PERINATAL ASPHYXIA: EFFECT OF HYPOTHERMIA TREATMENT

Long-term changes of different types of neurofilaments (NF) and glial fibrillar acid protein (GFAP) were studied in neostriatal rat subjected to perinatal asphyxia (PA) under normothermic and hypothermic (15 degrees C) conditions, using immunohistochemistry for light and electron microscopy. Neostriatal neurons of 6-month-old rats that were subjected to 19 and 20 min of PA, showed an increase of NF 200 kDa immunostaining mainly in the axon fascicles in comparison with the control and hypothermia groups. In contrast, no alterations were seen with NF68 and NF160 neurofilament antibodies. Furthermore, the same PA groups showed astroglial cells with enhanced GFAP immunoreactivity, evidencing a typical astroglial reaction with a clear hypertrophy of these cells. A quantitative image analysis confirmed these observations. Hypothermic treated animals did show neither astroglial nor neuronal cytoskeletal changes in comparison to the control group. These findings showed that PA produces chronic cytoskeletal alterations in the neostriatum cells that can be prevented by hypothermia.

Gene profiling of skeletal muscle in an amyotrophic lateral sclerosis mouse model

Muscle atrophy is a major hallmark of amyotrophic lateral sclerosis (ALS), the most frequent adult-onset motor neuron disease. To define the full set of alterations in gene expression in skeletal muscle during the course of the disease, we used the G86R superoxide dismutase-1 transgenic mouse model of ALS and performed high-density oligonucleotide microarrays. We compared these data to those obtained by axotomy-induced denervation. A major set of gene regulations in G86R muscles resembled those of surgically denervated muscles, but many others appeared specific to the ALS condition. The first significant transcriptional changes appeared in a subpopulation of mice before the onset of overt clinical symptoms and motor neuron death. These early changes affected genes involved in detoxification (e.g., ALDH3, metallothionein-2, and thioredoxin-1) and regeneration (e.g., BTG1, RB1, and RUNX1) but also tissue degradation (e.g., C/EBPdelta and DDIT4) and cell death (e.g., ankyrin repeat domain-1, CDKN1A, GADD45alpha, and PEG3). Of particular interest, metallothionein-1 and -2, ATF3, cathepsin-Z, and galectin-3 genes appeared, among others, commonly regulated in both skeletal muscle (our present data) and spinal motor neurons (as previously reported) of paralyzed ALS mice. The importance of these findings is twofold. First, they designate the distal part of the motor unit as a primary site of disease. Second, they identify specific gene regulations to be explored in the search for therapeutic strategies that could alleviate disease before motor neuron death manifests clinically.

Disruption of neurofilament network with aggregation of light neurofilament protein: a common pathway leading to motor neuron degeneration due to Charcot-Marie-Tooth disease-linked mutations in NFL and HSPB1

Mutations in neurofilament light (NFL) subunit and small heat-shock protein B1 (HSPB1) cause autosomal-dominant axonal Charcot-Marie-Tooth disease type 2E (CMT2E) and type 2F (CMT2F). Previous studies have shown that CMT mutations in NFL and HSPB1 disrupt NF assembly and cause aggregation of NFL protein. In this study, we investigate the role of aggregation of NFL protein in the neurotoxicity of CMT mutant NFL and CMT mutant HSPB1 in motor neurons. We find that expression of CMT mutant NFL leads to progressive degeneration and loss of neuronal viability of cultured motor neurons. Degenerating motor neurons show fragmentation and loss of neuritic processes associated with disruption of NF network and aggregation of NFL protein. Co-expression of wild-type HSPB1 diminishes aggregation of CMT mutant NFL, induces reversal of CMT mutant NFL aggregates and reduces CMT mutant NFL-induced loss of motor neuron viability. Like CMT mutant NFL, expression of S135F CMT mutant HSPB1 also leads to progressive degeneration of motor neurons with disruption of NF network and aggregation of NFL protein. Further studies show that wild-type and S135F mutant HSPB1 associate with wild-type and CMT mutant NFL and that S135F mutant HSPB1 has dominant effect on disruption of NF assembly and aggregation of NFL protein. Finally, we show that deletion of NFL markedly reduces degeneration and loss of motor neuron viability induced by S135F mutant HSPB1. Together, our data support the view that disruption of NF network with aggregation of NFL is a common triggering event of motor neuron degeneration in CMT2E and CMT2F disease.

Age-related Changes in Tau Expression in Transgenic Mouse Model of Amyotrophic Lateral Sclerosis

The work is a continuation of studies on tau expression and alternative splicing in the central nervous system of transgenic mice harboring human SOD1 with G93A amyotrophic lateral sclerosis (ALS)-associated mutation. Since age is an important risk factor for ALS, we expanded the studies into younger animals (age 5 and 25 days). We also included cerebellum, a structure not studied in the context of neurodegeneration in ALS. We found decreased total tau-mRNA expression in hippocampus but not in cortex and spinal cord of young transgenics, and a lack of exon 10 in 5-day-old mice. In cerebellum, the total tau-mRNA expression was increased in transgenic animals during the whole period of life, however at the symptomatic stage of ALS (age 120 days) the level of protein was decreased. It can be concluded that the SOD1 G93A mutation causes early alterations of tau expression in cns, which are not exclusively restricted to the upper and lower motor neuron.

Role of neurofilament aggregation in motor neuron disease

A major question in the pathogenesis of motor neuron disease is why motor neurons are selectively susceptible to mutations in widely expressed gene products. Reexamination of motor neuron degeneration due to alterations of neurofilament (NF) expression suggests that disruption of assembly with aggregation of the light neurofilament (NFL) protein may be an upstream event and contributing factor leading to the preferential degeneration of motor neurons. The implications of these findings are that aggregation of NFL is not only a triggering mechanism to account for the hallmark aggregates of NF protein in sporadic and familial forms of amyotrophic lateral sclerosis, but that aggregates of NFL may also promote aggregation of wildly expressed proteins that are destabilized by missense mutations, such as by mutations in superoxide dismutase-1 protein. This review examines the potential role of NFs in determining and promoting the preferential degeneration of motor neurons in motor neuron disease. The underlying premise is that motor neurons are selectively susceptible to alterations in NF expression, that alterations in NF expression lead to NF aggregates in motor neurons, and that elevated levels of NF aggregates provide a favorable microenvironment for the formation of neurotoxic aggregation and degeneration of motor neurons.

RNA-binding protein is involved in aggregation of light neurofilament protein and is implicated in the pathogenesis of motor neuron degeneration

Abnormal protein aggregation is emerging as a common theme in the pathogenesis of neurodegenerative disease. Our previous studies have shown that overexpression of untranslated light neurofilament (NF-L) RNA causes motor neuron degeneration in transgenic mice, leads to accumulation of ubiquitinated aggregates in degenerating cultured motor neurons and triggers aggregation of NF-L protein and co-aggregation of mutant SOD1 protein in neuronal cells. Here, we report that p190RhoGEF, an RNA-binding protein that binds to a destabilizing element in NF-L mRNA, is involved in aggregation of NF-L protein and is implicated in the pathogenesis of motor neuron degeneration. We show that p190RhoGEF co-aggregates with unassembled NF-L protein and that co-aggregation is associated with down-regulation of parent NF-L mRNA in neuronal cells. Co-expression of NF-M increases NF assembly and reduces RNA-triggered aggregation as well as loss of solubility of NF-L protein. siRNA-induced down-regulation of p190RhoGEF not only reduces aggregation and promotes assembly of NF-L and NF-M, but also causes reversal of aggregation and recovery of NF assembly in transfected cells. Examination of transgenic models of motor neuron disease shows that prominent aggregates of p190RhoGEF and NF-L and down-regulation of NF-L expression occur in degenerating motor neurons of mice expressing untranslated NF-L RNA or a G93A mutant SOD1 transgene. Moreover, aggregates of p190RhoGEF and NF-L appear as early pathological changes in presymptomatic G93A mutant SOD1 transgenic mice. Together, the findings indicate that p190RhoGEF is involved in aggregation of NF-L protein and support a working hypothesis that aggregation of p190RhoGEF and NF-L is an upstream event triggering neurotoxicity in motor neuron disease.

A NUDEL-dependent mechanism of neurofilament assembly regulates the integrity of CNS neurons

The cytoskeleton controls the architecture and survival of central nervous system (CNS) neurons by maintaining the stability of axons and dendrites. Although neurofilaments (NFs) constitute the main cytoskeletal network in these structures, the mechanism that underlies subunit incorporation into filaments remains a mystery. Here we report that NUDEL, a mammalian homologue of the Aspergillus nidulans nuclear distribution molecule NudE, is important for NF assembly, transport and neuronal integrity. NUDEL facilitates the polymerization of NFs through a direct interaction with the NF light subunit (NF-L). Knockdown of NUDEL by RNA interference (RNAi) in a neuroblastoma cell line, primary cortical neurons or post-natal mouse brain destabilizes NF-L and alters the homeostasis of NFs. This results in NF abnormalities and morphological changes reminiscent of neurodegeneration. Furthermore, variations in levels of NUDEL correlate with disease progression and NF defects in a mouse model of neurodegeneration. Thus, NUDEL contributes to the integrity of CNS neurons by regulating NF assembly.

3' untranslated region in a light neurofilament (NF-L) mRNA triggers aggregation of NF-L and mutant superoxide dismutase 1 proteins in neuronal cells

The pathogenesis of neurodegenerative diseases is believed to involve abnormal aggregation of proteins, but the mechanisms initiating protein aggregation are unclear. Here we report a novel phenomenon that could be instrumental in triggering protein aggregation in neurodegenerative diseases. We show that the 3' untranslated region (3'UTR) of a light neurofilament (NF-L) transcript enhances the reactivity of its own translated product and leads to loss of solubility and aggregation of NF-L protein and to coaggregation of mutant superoxide dismutase 1 (SOD1) protein. Full-length mouse NF-L cDNAs, with and without NF-L 3'UTR, were fused to the C terminus of a green fluorescent protein (GFP) reporter gene, and the GFP-tagged NF-L proteins were examined in transfected Neuro2a cells. The GFP-tagged NF-L protein expressed from the transgene containing NF-L 3'UTR, but not from the transgene lacking NF-L 3'UTR, colocalizes with endogenous heavy neurofilament protein and, at high-level expression, leads to loss of solubility and aggregation of GFP-tagged NF-L protein. Aggregation of GFP-tagged NF-L protein triggers coaggregation and loss of solubility of coexpressed DsRed-tagged mutant (G93A) SOD1 protein but not wild-type SOD1 protein. Deletional mutagenesis maps the RNA sequence causing aggregation of GFP-tagged NF-L protein to the proximal 45 nucleotides of NF-L 3'UTR. This is the site of a major destabilizing element in NF-L RNA and binding site for RNA-binding proteins. Our findings support a working model whereby NF-L RNA, or cognate RNA-binding factors, enhances the reactivity of NF-L protein and provides a triggering mechanism leading to aggregation of NF-L and other proteins in neurodegenerative diseases.

Mécanismes moléculaires de la sclérose latérale amyotrophique : apports récents de l’analyse de modèles animaux

Amyotrophic Lateral Sclerosis is a neurodegenerative condition defined by loss of both upper and lower motor neurons. The molecular mechanisms underlying this pathology are currently elucidated using transgenic mice lines expressing mutated alleles of the copper-zinc superoxide dismutase, an enzyme mutated in about 2 p. cent of ALS cases. These transgenic mice also provide a valuable animal model to set up new therapeutic tools.