Early pathogenesis in the adult-onset neurodegenerative disease amyotrophic lateral sclerosis

Skeletal myotubes expressing ALS mutant SOD1 induce pathogenic changes, impair mitochondrial axonal transport, and trigger motoneuron death

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of motoneurons (MNs), and despite progress, there is no effective treatment. A large body of evidence shows that astrocytes expressing ALS-linked mutant proteins cause non-cell autonomous toxicity of MNs. Although MNs innervate muscle fibers and ALS is characterized by the early disruption of the neuromuscular junction (NMJ) and axon degeneration, there are controversies about whether muscle contributes to non-cell-autonomous toxicity to MNs. In this study, we generated primary skeletal myotubes from myoblasts derived from ALS mice expressing human mutant SOD1 G93A (termed hereafter mutSOD1). Characterization revealed that mutSOD1 skeletal myotubes display intrinsic phenotypic and functional differences compared to control myotubes generated from non-transgenic (NTg) littermates. Next, we analyzed whether ALS myotubes exert non-cell-autonomous toxicity to MNs. We report that conditioned media from mutSOD1 myotubes (mutSOD1-MCM), but not from control myotubes (NTg-MCM), induced robust death of primary MNs in mixed spinal cord cultures and compartmentalized microfluidic chambers. Our study further revealed that applying mutSOD1-MCM to the MN axonal side in microfluidic devices rapidly reduces mitochondrial axonal transport while increasing Ca2+ transients and reactive oxygen species (i.e., H 2 O 2 ). These results indicate that soluble factor(s) released by mutSOD1 myotubes cause MN axonopathy that leads to lethal pathogenic changes.

Inorganic polyphosphate: from basic research to diagnostic and therapeutic opportunities in ALS/FTD

Inorganic polyphosphate (polyP) is a simple, negatively charged biopolymer with chain lengths ranging from just a few to over a thousand ortho-phosphate (Pi) residues. polyP is detected in every cell type across all organisms in nature thus far analyzed. Despite its structural simplicity, polyP has been shown to play important roles in a remarkably broad spectrum of biological processes, including blood coagulation, bone mineralization and inflammation. Furthermore, polyP has been implicated in brain function and the neurodegenerative diseases amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease and Parkinson's disease. In this review, we first address the challenges associated with identifying mammalian polyP metabolizing enzymes, such as Nudt3, and quantifying polyP levels in brain tissue, cultured neural cells and cerebrospinal fluid. Subsequently, we focus on recent studies that unveil how the excessive release of polyP by human and mouse ALS/FTD astrocytes contributes to these devastating diseases by inducing hyperexcitability, leading to motoneuron death. Potential implications of elevated polyP levels in ALS/FTD patients for innovative diagnostic and therapeutic approaches are explored. It is emphasized, however, that caution is required in targeting polyP in the brain due to its diverse physiological functions, serving as an energy source, a chelator for divalent cations and a scaffold for amyloidogenic proteins. Reducing polyP levels, especially in neurons, might thus have adverse effects in brain functioning. Finally, we discuss how activated mast cells and platelets also can significantly contribute to ALS progression, as they can massively release polyP.

Exploring Relevant Time Windows in the Association Between PM2.5 Exposure and Amyotrophic Lateral Sclerosis: A Case-Control Study in Denmark

Studies suggest a link between particulate matter less than or equal to 2.5 μm in diameter (PM2.5) and amyotrophic lateral sclerosis (ALS), but to our knowledge critical exposure windows have not been examined. We performed a case-control study in the Danish population spanning the years 1989-2013. Cases were selected from the Danish National Patient Registry based on International Classification of Diseases codes. Five controls were randomly selected from the Danish Civil Registry and matched to a case on vital status, age, and sex. PM2.5 concentration at residential addresses was assigned using monthly predictions from a dispersion model. We used conditional logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs), adjusting for confounding. We evaluated exposure to averaged PM2.5 concentrations 12-24 months, 2-6 years, and 2-11 years pre-ALS diagnosis; annual lagged exposures up to 11 years prediagnosis; and cumulative associations for exposure in lags 1-5 years and 1-10 years prediagnosis, allowing for varying association estimates by year. We identified 3,983 cases and 19,915 controls. Cumulative exposure to PM2.5 in the period 2-6 years prediagnosis was associated with ALS (OR = 1.06, 95% CI: 0.99, 1.13). Exposures in the second, third, and fourth years prediagnosis were individually associated with higher odds of ALS (e.g., for lag 1, OR = 1.04, 95% CI: 1.00, 1.08). Exposure to PM2.5 within 6 years before diagnosis may represent a critical exposure window for ALS.

Mature iPSC-derived astrocytes of an ALS/FTD patient carrying the TDP43A90V mutation display a mild reactive state and release polyP toxic to motoneurons

Astrocytes play a critical role in the maintenance of a healthy central nervous system and astrocyte dysfunction has been implicated in various neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). There is compelling evidence that mouse and human ALS and ALS/FTD astrocytes can reduce the number of healthy wild-type motoneurons (MNs) in co-cultures or after treatment with astrocyte conditioned media (ACM), independently of their genotype. A growing number of studies have shown that soluble toxic factor(s) in the ACM cause non-cell autonomous MN death, including our recent identification of inorganic polyphosphate (polyP) that is excessively released from mouse primary astrocytes (SOD1, TARDBP, and C9ORF72) and human induced pluripotent stem cells (iPSC)-derived astrocytes (TARDBP) to kill MNs. However, others have reported that astrocytes carrying mutant TDP43 do not produce detectable MN toxicity. This controversy is likely to arise from the findings that human iPSC-derived astrocytes exhibit a rather immature and/or reactive phenotype in a number of studies. Here, we have succeeded in generating a highly homogenous population of functional quiescent mature astrocytes from control subject iPSCs. Using identical conditions, we also generated mature astrocytes from an ALS/FTD patient carrying the TDP43A90V mutation. These mutant TDP43 patient-derived astrocytes exhibit key pathological hallmarks, including enhanced cytoplasmic TDP-43 and polyP levels. Additionally, mutant TDP43 astrocytes displayed a mild reactive signature and an aberrant function as they were unable to promote synaptogenesis of hippocampal neurons. The polyP-dependent neurotoxic nature of the TDP43A90V mutation was further confirmed as neutralization of polyP in ACM derived from mutant TDP43 astrocytes prevented MN death. Our results establish that human astrocytes carrying the TDP43A90V mutation exhibit a cell-autonomous pathological signature, hence providing an experimental model to decipher the molecular mechanisms underlying the generation of the neurotoxic phenotype.

A Meta-Analysis Study of SOD1-Mutant Mouse Models of ALS to Analyse the Determinants of Disease Onset and Progression

A complex interaction between genetic and external factors determines the development of amyotrophic lateral sclerosis (ALS). Epidemiological studies on large patient cohorts have suggested that ALS is a multi-step disease, as symptom onset occurs only after exposure to a sequence of risk factors. Although the exact nature of these determinants remains to be clarified, it seems clear that: (i) genetic mutations may be responsible for one or more of these steps; (ii) other risk factors are probably linked to environment and/or to lifestyle, and (iii) compensatory plastic changes taking place during the ALS etiopathogenesis probably affect the timing of onset and progression of disease. Current knowledge on ALS mechanisms and therapeutic targets, derives mainly from studies involving superoxide dismutase 1 (SOD1) transgenic mice; therefore, it would be fundamental to verify whether a multi-step disease concept can also be applied to these animal models. With this aim, a meta-analysis study has been performed using a collection of primary studies (n = 137), selected according to the following criteria: (1) the studies should employ SOD1 transgenic mice; (2) the studies should entail the presence of a disease-modifying experimental manipulation; (3) the studies should make use of Kaplan-Meier plots showing the distribution of symptom onset and lifespan. Then, using a subset of this study collection (n = 94), the effects of treatments on key molecular mechanisms, as well as on the onset and progression of disease have been analysed in a large population of mice. The results are consistent with a multi-step etiopathogenesis of disease in ALS mice (including two to six steps, depending on the particular SOD1 mutation), closely resembling that observed in patient cohorts, and revealed an interesting relationship between molecular mechanisms and disease manifestation. Thus, SOD1 mouse models may be considered of high predictive value to understand the determinants of disease onset and progression, as well as to identify targets for therapeutic interventions.

The Cell Autonomous and Non-Cell Autonomous Aspects of Neuronal Vulnerability and Resilience in Amyotrophic Lateral Sclerosis

Simple Summary

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by a progressive paralysis due to the loss of particular neurons in our nervous system called motor neurons, that exert voluntary control of all our skeletal muscles. It is not entirely understood why motor neurons are particularly vulnerable in ALS, neither is it completely clear why certain groups of motor neurons, including those that regulate eye movement, are rather resilient to this disease. However, both vulnerability and resilience to ALS likely reflect cell intrinsic properties of different motor neuron subpopulations as well as non-cell autonomous events regulated by surrounding cell types. In this review we dissect the particular properties of different motor neuron types and their responses to disease that may underlie their respective vulnerabilities and resilience. Disease progression in ALS involves multiple cell types that are closely connected to motor neurons and we here also discuss their contributions to the differential vulnerability of motor neurons.

Abstract

Amyotrophic lateral sclerosis (ALS) is defined by the loss of upper motor neurons (MNs) that project from the cerebral cortex to the brain stem and spinal cord and of lower MNs in the brain stem and spinal cord which innervate skeletal muscles, leading to spasticity, muscle atrophy, and paralysis. ALS involves several disease stages, and multiple cell types show dysfunction and play important roles during distinct phases of disease initiation and progression, subsequently leading to selective MN loss. Why MNs are particularly vulnerable in this lethal disease is still not entirely clear. Neither is it fully understood why certain MNs are more resilient to degeneration in ALS than others. Brain stem MNs of cranial nerves III, IV, and VI, which innervate our eye muscles, are highly resistant and persist until the end-stage of the disease, enabling paralyzed patients to communicate through ocular tracking devices. MNs of the Onuf’s nucleus in the sacral spinal cord, that innervate sphincter muscles and control urogenital functions, are also spared throughout the disease. There is also a differential vulnerability among MNs that are intermingled throughout the spinal cord, that directly relate to their physiological properties. Here, fast-twitch fatigable (FF) MNs, which innervate type IIb muscle fibers, are affected early, before onset of clinical symptoms, while slow-twitch (S) MNs, that innervate type I muscle fibers, remain longer throughout the disease progression. The resilience of particular MN subpopulations has been attributed to intrinsic determinants and multiple studies have demonstrated their unique gene regulation and protein content in health and in response to disease. Identified factors within resilient MNs have been utilized to protect more vulnerable cells. Selective vulnerability may also, in part, be driven by non-cell autonomous processes and the unique surroundings and constantly changing environment close to particular MN groups. In this article, we review in detail the cell intrinsic properties of resilient and vulnerable MN groups, as well as multiple additional cell types involved in disease initiation and progression and explain how these may contribute to the selective MN resilience and vulnerability in ALS.

C. elegans as an Animal Model to Study the Intersection of DNA Repair, Aging and Neurodegeneration

Since its introduction as a genetic model organism, Caenorhabditis elegans has yielded insights into the causes of aging. In addition, it has provided a molecular understanding of mechanisms of neurodegeneration, one of the devastating effects of aging. However, C. elegans has been less popular as an animal model to investigate DNA repair and genomic instability, which is a major hallmark of aging and also a cause of many rare neurological disorders. This article provides an overview of DNA repair pathways in C. elegans and the impact of DNA repair on aging hallmarks, such as mitochondrial dysfunction, telomere maintenance, and autophagy. In addition, we discuss how the combination of biological characteristics, new technical tools, and the potential of following precise phenotypic assays through a natural life-course make C. elegans an ideal model organism to study how DNA repair impact neurodegeneration in models of common age-related neurodegenerative diseases.

Fluid Biomarkers in Alzheimer’s Disease and Other Neurodegenerative Disorders: Toward Integrative Diagnostic Frameworks and Tailored Treatments

The diagnosis of neurodegenerative diseases (NDDs) represents an increasing social burden, with the unsolved issue of disease-modifying therapies (DMTs). The failure of clinical trials treating Alzheimer′s Disease (AD) so far highlighted the need for a different approach in drug design and patient selection. Identifying subjects in the prodromal or early symptomatic phase is critical to slow down neurodegeneration, but the implementation of screening programs with this aim will have an ethical and social aftermath. Novel minimally invasive candidate biomarkers (derived from blood, saliva, olfactory brush) or classical cerebrospinal fluid (CSF) biomarkers have been developed in research settings to stratify patients with NDDs. Misfolded protein accumulation, neuroinflammation, and synaptic loss are the pathophysiological hallmarks detected by these biomarkers to refine diagnosis, prognosis, and target engagement of drugs in clinical trials. We reviewed fluid biomarkers of NDDs, considering their potential role as screening, diagnostic, or prognostic tool, and their present-day use in clinical trials (phase II and III). A special focus will be dedicated to novel techniques for the detection of misfolded proteins. Eventually, an applicative diagnostic algorithm will be proposed to translate the research data in clinical practice and select prodromal or early patients to be enrolled in the appropriate DMTs trials for NDDs.

Excessive release of inorganic phosphate by ALS/FTD astrocytes causes non-cell-autonomous toxicity to motoneurons

Non-cell-autonomous mechanisms contribute to neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), in which astrocytes release unidentified factors that are toxic to motoneurons (MNs). We report here that mouse and patient iPSC-derived astrocytes with diverse ALS/FTD-linked mutations (SOD1, TARDBP, and C9ORF72) display elevated levels of intracellular inorganic polyphosphate (polyP), a ubiquitous, negatively charged biopolymer. PolyP levels are also increased in astrocyte-conditioned media (ACM) from ALS/FTD astrocytes. ACM-mediated MN death is prevented by degrading or neutralizing polyP in ALS/FTD astrocytes or ACM. Studies further reveal that postmortem familial and sporadic ALS spinal cord sections display enriched polyP staining signals and that ALS cerebrospinal fluid (CSF) exhibits increased polyP concentrations. Our in vitro results establish excessive astrocyte-derived polyP as a critical factor in non-cell-autonomous MN degeneration and a potential therapeutic target for ALS/FTD. The CSF data indicate that polyP might serve as a new biomarker for ALS/FTD.

Electrical and Morphological Properties of Developing Motoneurons in Postnatal Mice and Early Abnormalities in SOD1 Transgenic Mice

In this chapter, we review electrical and morphological properties of lumbar motoneurons during postnatal development in wild-type (WT) and transgenic superoxide dismutase 1 (SOD1) mice, models of amyotrophic lateral sclerosis. First we showed that sensorimotor reflexes do not develop normally in transgenic SOD1^G85R pups. Fictive locomotor activity recorded in in vitro whole brainstem/spinal cord preparations was not induced in these transgenic SOD1^G85R mice using NMDA and 5HT in contrast to WT mice. Further, abnormal electrical properties were detected as early as the second postnatal week in lumbar motoneurons of SOD1 mice while they develop clinical symptoms several months after birth. We compared two different strains of mice (G85R and G93A) at the same postnatal period using intracellular recordings and patch clamp recordings of WT and SOD1 motoneurons. We defined three types of motoneurons according to their discharge firing pattern (transient, sustained and delayed onset firing) when motor units are not yet mature. The delayed-onset firing motoneurons had the higher rheobase compared to the transient and sustained firing groups in the WT mice. We demonstrated hypoexcitability in the delayed onset-firing motoneurons of SOD1 mice. Intracellular staining of motoneurons revealed dendritic overbranching in SOD1 lumbar motoneurons that was more pronounced in the sustained firing motoneurons. We suggested that motoneuronal hypoexcitability is an early pathological sign affecting a subset of lumbar motoneurons in the spinal cord of SOD1 mice.

RNA Editing: A New Therapeutic Target in Amyotrophic Lateral Sclerosis and Other Neurological Diseases

The conversion of adenosine to inosine in RNA editing (A-to-I RNA editing) is recognized as a critical post-transcriptional modification of RNA by adenosine deaminases acting on RNAs (ADARs). A-to-I RNA editing occurs predominantly in mammalian and human central nervous systems and can alter the function of translated proteins, including neurotransmitter receptors and ion channels; therefore, the role of dysregulated RNA editing in the pathogenesis of neurological diseases has been speculated. Specifically, the failure of A-to-I RNA editing at the glutamine/arginine (Q/R) site of the GluA2 subunit causes excessive permeability of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors to Ca²⁺, inducing fatal status epilepticus and the neurodegeneration of motor neurons in mice. Therefore, an RNA editing deficiency at the Q/R site in GluA2 due to the downregulation of ADAR2 in the motor neurons of sporadic amyotrophic lateral sclerosis (ALS) patients suggests that Ca²⁺-permeable AMPA receptors and the dysregulation of RNA editing are suitable therapeutic targets for ALS. Gene therapy has recently emerged as a new therapeutic opportunity for many heretofore incurable diseases, and RNA editing dysregulation can be a target for gene therapy; therefore, we reviewed neurological diseases associated with dysregulated RNA editing and a new therapeutic approach targeting dysregulated RNA editing, especially one that is effective in ALS.

Ca2+ dysregulation in the pathogenesis of amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease without appropriate cure. One of the main reasons for the lack of a proper pharmacotherapy in ALS is the narrow knowledge on the molecular causes of the disease. In this respect, the identification of dysfunctional pathways in ALS is now considered a critical medical need. Among the causative factors involved in ALS, Ca²⁺ dysregulation is one of the most important pathogenetic mechanisms of the disease. Of note, Ca²⁺ dysfunction may induce, directly or indirectly, motor neuron degeneration and loss. Interestingly, both familial (fALS) and sporadic ALS (sALS) share the progressive dysregulation of Ca²⁺ homeostasis as a common noxious mechanism. Mechanicistically, Ca²⁺ dysfunction involves both plasma membrane and intracellular mechanisms, including AMPA receptor (AMPAR)-mediated excitotoxicity, voltage-gated Ca²⁺ channels (VGCCs) and Ca²⁺ transporter dysregulation, endoplasmic reticulum (ER) Ca²⁺ deregulation, mitochondria-associated ER membranes (MAMs) dysfunction, lysosomal Ca²⁺ leak, etc. Here, a comprehensive analysis of the main pathways involved in the dysregulation of Ca²⁺ homeostasis has been reported with the aim to focus the attention on new putative druggable targets.

Crosstalk between Different DNA Repair Pathways Contributes to Neurodegenerative Diseases

Simple Summary

Constant exposure to endogenous and environmental factors induces oxidative stress and DNA damage. Rare brain disorders caused by defects in DNA repair and DNA damage response (DDR) signaling establish that failure to process DNA damage may lead to neurodegeneration. In this review, we present mechanisms that link DDR with neurodegeneration in these disorders and discuss their relevance for common age-related neurodegenerative diseases (NDDs). Moreover, we highlight recent insight into the crosstalk between the DDR and other cellular processes known to be disturbed during NDDs.

Abstract

Genomic integrity is maintained by DNA repair and the DNA damage response (DDR). Defects in certain DNA repair genes give rise to many rare progressive neurodegenerative diseases (NDDs), such as ocular motor ataxia, Huntington disease (HD), and spinocerebellar ataxias (SCA). Dysregulation or dysfunction of DDR is also proposed to contribute to more common NDDs, such as Parkinson’s disease (PD), Alzheimer’s disease (AD), and Amyotrophic Lateral Sclerosis (ALS). Here, we present mechanisms that link DDR with neurodegeneration in rare NDDs caused by defects in the DDR and discuss the relevance for more common age-related neurodegenerative diseases. Moreover, we highlight recent insight into the crosstalk between the DDR and other cellular processes known to be disturbed during NDDs. We compare the strengths and limitations of established model systems to model human NDDs, ranging from C. elegans and mouse models towards advanced stem cell-based 3D models.

Early Hypoexcitability in a Subgroup of Spinal Motoneurons in Superoxide Dismutase 1 Transgenic Mice, a Model of Amyotrophic Lateral Sclerosis

In amyotrophic lateral sclerosis (ALS), large motoneurons degenerate first, causing muscle weakness. Transgenic mouse models with a mutation in the gene encoding the enzyme superoxide dismutase 1 (SOD1) revealed that motoneurons innervating the fast-fatigable muscular fibres disconnect very early. The cause of this peripheric disconnection has not yet been established. Early pathological signs were described in motoneurons during the postnatal period of SOD1 transgenic mice. Here, we investigated whether the early changes of electrical and morphological properties previously reported in the SOD1^G85R strain also occur in the SOD1^G93A-low expressor line with particular attention to the different subsets of motoneurons defined by their discharge firing pattern (transient, sustained, or delayed-onset firing). Intracellular staining and recording were performed in lumbar motoneurons from entire brainstem-spinal cord preparations of SOD1^G93A-low transgenic mice and their WT littermates during the second postnatal week. Our results show that SOD1^G93A-low motoneurons exhibit a dendritic overbranching similar to that described previously in the SOD1^G85R strain at the same age. Further we found an hypoexcitability in the delayed-onset firing SOD1^G93A-low motoneurons (lower gain and higher voltage threshold). We conclude that dendritic overbranching and early hypoexcitability are common features of both low expressor SOD1 mutants (G85R and G93A-low). In the high-expressor SOD1^G93A line, we found hyperexcitability in the sustained firing motoneurons at the same period, suggesting a delay in compensatory mechanisms. Overall, our results suggest that the hypoexcitability indicate an early dysfunction of the delayed-onset motoneurons and could account as early pathological signs of the disease.

The mechanism of non-blocking inhibition of sodium channels revealed by conformation-selective photolabeling

BACKGROUND AND PURPOSE

Sodium channel inhibitors can be used to treat hyperexcitability-related diseases, including epilepsies, pain syndromes, neuromuscular disorders and cardiac arrhythmias. The applicability of these drugs is limited by their nonspecific effect on physiological function. They act mainly by sodium channel block and in addition by modulation of channel kinetics. While channel block inhibits healthy and pathological tissue equally, modulation can preferentially inhibit pathological activity. An ideal drug designed to target the sodium channels of pathological tissue would act predominantly by modulation. Thus far, no such drug has been described.

EXPERIMENTAL APPROACH

Patch-clamp experiments with ultra-fast solution exchange and photolabeling-coupled electrophysiology were applied to describe the unique mechanism of riluzole on Nav1.4 sodium channels. In silico docking experiments were used to study the molecular details of binding.

KEY RESULTS

We present evidence that riluzole acts predominantly by non-blocking modulation. We propose that, being a relatively small molecule, riluzole is able to stay bound to the binding site, but nonetheless stay off the conduction pathway, by residing in one of the fenestrations. We demonstrate how this mechanism can be recognized.

CONCLUSIONS AND IMPLICATIONS

Our results identify riluzole as the prototype of this new class of sodium channel inhibitors. Drugs of this class are expected to selectively prevent hyperexcitability, while having minimal effect on cells firing at a normal rate from a normal resting potential.

Fine Particle Exposure and Clinical Aggravation in Neurodegenerative Diseases in New York State

BACKGROUND

Adult-onset neurodegenerative diseases affect millions and negatively impact health care systems worldwide. Evidence suggests that air pollution may contribute to aggravation of neurodegeneration, but studies have been limited.

OBJECTIVE

We examined the potential association between long-term exposure to particulate matter ≤ 2.5 μ m in aerodynamic diameter [fine particulate matter (PM 2.5 )] and disease aggravation in Alzheimer's (AD) and Parkinson's (PD) diseases and amyotrophic lateral sclerosis (ALS), using first hospitalization as a surrogate of clinical aggravation.

METHODS

We used data from the New York Department of Health Statewide Planning and Research Cooperative System (SPARCS 2000-2014) to construct annual county counts of first hospitalizations with a diagnosis of AD, PD, or ALS (total, urbanicity-, sex-, and age-stratified). We used annual PM 2.5 concentrations estimated by a prediction model at a 1 -km 2 resolution, which we aggregated to population-weighted county averages to assign exposure to cases based on county of residence. We used outcome-specific mixed quasi-Poisson models with county-specific random intercepts to estimate rate ratios (RRs) for a 1-y PM 2.5 exposure. We allowed for nonlinear exposure-outcome relationships using penalized splines and accounted for potential confounders.

RESULTS

We found a positive nonlinear PM 2.5 - PD association that plateaued above 11   μ g / m 3 (RR = 1.09 , 95% CI: 1.04, 1.14 for a PM 2.5 increase from 8.1 to 10.4   μ g / m 3 ). We also found a linear PM 2.5 - ALS positive association (RR = 1.05 , 95% CI: 1.01, 1.09 per 1 - μ g / m 3 PM 2.5 increase), and suggestive evidence of an association with AD. We found effect modification by age for PD and ALS with a stronger positive association in patients < 70  years of age but found insufficient evidence of effect modification by sex or urbanization level for any of the outcomes.

CONCLUSION

Our findings suggest that annual increase in county-level PM 2.5 concentrations may contribute to clinical aggravation of PD and ALS. Importantly, the average annual PM 2.5 concentration in our study was 8.1   μ g / m 3 , below the current American national standards, suggesting the standards may not adequately protect the aging population. https://doi.org/10.1289/EHP7425.

Human Motor Neurons With SOD1-G93A Mutation Generated From CRISPR/Cas9 Gene-Edited iPSCs Develop Pathological Features of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by gradual degeneration and elimination of motor neurons (MNs) in the motor cortex, brainstem, and spinal cord. Some familial forms of ALS are caused by genetic mutations in superoxide dismutase 1 (SOD1) but the mechanisms driving MN disease are unclear. Identifying the naturally occurring pathology and understanding how this mutant SOD1 can affect MNs in translationally meaningful ways in a valid and reliable human cell model remains to be established. Here, using CRISPR/Cas9 genome editing system and human induced pluripotent stem cells (iPSCs), we generated highly pure, iPSC-derived MNs with a SOD1-G93A missense mutation. With the wild-type cell line serving as an isogenic control and MNs from a patient-derived iPSC line with an SOD1-A4V mutation as a comparator, we identified pathological phenotypes relevant to ALS. The mutant MNs accumulated misfolded and aggregated forms of SOD1 in cell bodies and processes, including axons. They also developed distinctive axonal pathologies. Mutants had axonal swellings with shorter axon length and less numbers of branch points. Moreover, structural and molecular abnormalities in presynaptic and postsynaptic size and density were found in the mutants. Finally, functional studies with microelectrode array demonstrated that the individual mutant MNs exhibited decreased number of spikes and diminished network bursting, but increased burst duration. Taken together, we identified spontaneous disease phenotypes relevant to ALS in mutant SOD1 MNs from genome-edited and patient-derived iPSCs. Our findings demonstrate that SOD1 mutations in human MNs cause cell-autonomous proteinopathy, axonopathy, synaptic pathology, and aberrant neurotransmission.

Differential Loss of Spinal Interneurons in a Mouse Model of ALS

Amyotrophic lateral sclerosis (ALS) leads to a loss of specific motor neuron populations in the spinal cord and cortex. Emerging evidence suggests that interneurons may also be affected, but a detailed characterization of interneuron loss and its potential impacts on motor neuron loss and disease progression is lacking. To examine this issue, the fate of V1 inhibitory neurons during ALS was assessed in the ventral spinal cord using the SOD^G93A mouse model. The V1 population makes up ∼30% of all ventral inhibitory neurons, ∼50% of direct inhibitory synaptic contacts onto motor neuron cell bodies, and is thought to play a key role in modulating motor output, in part through recurrent and reciprocal inhibitory circuits. We find that approximately half of V1 inhibitory neurons are lost in SOD^G93A mice at late disease stages, but that this loss is delayed relative to the loss of motor neurons and V2a excitatory neurons. We further identify V1 subpopulations based on transcription factor expression that are differentially susceptible to degeneration in SOD^G93A mice. At an early disease stage, we show that V1 synaptic contacts with motor neuron cell bodies increase, suggesting an upregulation of inhibition before V1 neurons are lost in substantial numbers. These data support a model in which progressive changes in V1 synaptic contacts early in disease, and in select V1 subpopulations at later stages, represent a compensatory upregulation and then deleterious breakdown of specific interneuron circuits within the spinal cord.

Silence superoxide dismutase 1 (SOD1): a promising therapeutic target for amyotrophic lateral sclerosis (ALS)

Introduction: Amyotrophic lateral sclerosis (ALS) is a progressive and incurable neurodegenerative disorder that targets upper and lower motor neurons and leads to fatal muscle paralysis. Mutations in the superoxide dismutase 1 (SOD1) gene are responsible for 15% of familial ALS cases, but several studies have indicated that SOD1 dysfunction may also play a pathogenic role in sporadic ALS. SOD1 induces numerous toxic effects through the pathological misfolding and aggregation of mutant SOD1 species, hence a reduction of the levels of toxic variants appears to be a promising therapeutic strategy for SOD1-related ALS. Several methods are used to modulate gene expression in vivo; these include RNA interference, antisense oligonucleotides (ASOs) and CRISPR/Cas9 technology.Areas covered: This paper examines the current approaches for gene silencing and the progress made in silencing SOD1 in vivo. It progresses to shed light on the key results and pitfalls of these studies and highlights the future challenges and new perspectives for this exciting research field.Expert opinion: Gene silencing strategies targeting SOD1 may represent effective approaches for familial and sporadic ALS-related neurodegeneration; however, the risk of off-target effects must be minimized, and effective and minimally invasive delivery strategies should be fine-tuned.

Relaxation of synaptic inhibitory events as a compensatory mechanism in fetal SOD spinal motor networks

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease affecting motor neurons (MNs) during late adulthood. Here, with the aim of identifying early changes underpinning ALS neurodegeneration, we analyzed the GABAergic/glycinergic inputs to E17.5 fetal MNs from SOD1G93A (SOD) mice in parallel with chloride homeostasis. Our results show that IPSCs are less frequent in SOD animals in accordance with a reduction of synaptic VIAAT-positive terminals. SOD MNs exhibited an EGABAAR10 mV more depolarized than in WT MNs associated with a KCC2 reduction. Interestingly, SOD GABAergic/glycinergic IPSCs and evoked GABAAR-currents exhibited a slower decay correlated to elevated [Cl-]i. Computer simulations revealed that a slower relaxation of synaptic inhibitory events acts as compensatory mechanism to strengthen GABA/glycine inhibition when EGABAAR is more depolarized. How such mechanisms evolve during pathophysiological processes remain to be determined, but our data indicate that at least SOD1 familial ALS may be considered as a neurodevelopmental disease.

Human Huntington's Disease iPSC-Derived Cortical Neurons Display Altered Transcriptomics, Morphology, and Maturation

Huntington's disease (HD) is a neurodegenerative disease caused by an expanded CAG repeat in the Huntingtin (HTT) gene. Induced pluripotent stem cell (iPSC) models of HD provide an opportunity to study the mechanisms underlying disease pathology in disease-relevant patient tissues. Murine studies have demonstrated that HTT is intricately involved in corticogenesis. However, the effect of mutant Hungtintin (mtHTT) in human corticogenesis has not yet been thoroughly explored. This examination is critical, due to inherent differences in cortical development and timing between humans and mice. We therefore differentiated HD and non-diseased iPSCs into functional cortical neurons. While HD patient iPSCs can successfully differentiate toward a cortical fate in culture, the resulting neurons display altered transcriptomics, morphological and functional phenotypes indicative of altered corticogenesis in HD.

Safe and effective superoxide dismutase 1 silencing using artificial microRNA in macaques

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disease caused by degeneration of motor neurons leading to rapidly progressive paralysis. About 10% of cases are caused by gain-of-function mutations that are transmitted as dominant traits. A potential therapy for these cases is to suppress the expression of the mutant gene. Here, we investigated silencing of SOD1, a gene commonly mutated in familial ALS, using an adeno-associated virus (AAV) encoding an artificial microRNA (miRNA) that targeted SOD1 In a superoxide dismutase 1 (SOD1)-mediated mouse model of ALS, we have previously demonstrated that SOD1 silencing delayed disease onset, increased survival time, and reduced muscle loss and motor and respiratory impairments. Here, we describe the preclinical characterization of this approach in cynomolgus macaques (Macaca fascicularis) using an AAV serotype for delivery that has been shown to be safe in clinical trials. We optimized AAV delivery to the spinal cord by preimplantation of a catheter and placement of the subject with head down at 30° during intrathecal infusion. We compared different promoters for the expression of artificial miRNAs directed against mutant SOD1 Results demonstrated efficient delivery and effective silencing of the SOD1 gene in motor neurons. These results support the notion that gene therapy with an artificial miRNA targeting SOD1 is safe and merits further development for the treatment of mutant SOD1-linked ALS.

Circuit-Specific Early Impairment of Proprioceptive Sensory Neurons in the SOD1G93A Mouse Model for ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in which motor neurons degenerate, resulting in muscle atrophy, paralysis, and fatality. Studies using mouse models of ALS indicate a protracted period of disease development with progressive motor neuron pathology, evident as early as embryonic and postnatal stages. Key missing information includes concomitant alterations in the sensorimotor circuit essential for normal development and function of the neuromuscular system. Leveraging unique brainstem circuitry, we show in vitro evidence for reflex circuit-specific postnatal abnormalities in the jaw proprioceptive sensory neurons in the well-studied SOD1^G93A mouse. These include impaired and arrhythmic action potential burst discharge associated with a deficit in Nav1.6 Na⁺ channels. However, the mechanoreceptive and nociceptive trigeminal ganglion neurons and the visual sensory retinal ganglion neurons were resistant to excitability changes in age-matched SOD1^G93A mice. Computational modeling of the observed disruption in sensory patterns predicted asynchronous self-sustained motor neuron discharge suggestive of imminent reflexive defects, such as muscle fasciculations in ALS. These results demonstrate a novel reflex circuit-specific proprioceptive sensory abnormality in ALS.SIGNIFICANCE STATEMENT Neurodegenerative diseases have prolonged periods of disease development and progression. Identifying early markers of vulnerability can therefore help devise better diagnostic and treatment strategies. In this study, we examined postnatal abnormalities in the electrical excitability of muscle spindle afferent proprioceptive neurons in the well-studied SOD1^G93A mouse model for neurodegenerative motor neuron disease, amyotrophic lateral sclerosis. Our findings suggest that these proprioceptive sensory neurons are exclusively afflicted early in the disease process relative to sensory neurons of other modalities. Moreover, they presented Nav1.6 Na⁺ channel deficiency, which contributed to arrhythmic burst discharge. Such sensory arrhythmia could initiate reflexive defects, such as muscle fasciculations in amyotrophic lateral sclerosis, as suggested by our computational model.

TDP-43 regulates early-phase insulin secretion via CaV1.2-mediated exocytosis in islets

TAR DNA-binding protein 43 kDa (TDP-43), encoded by TARDBP, is an RNA-binding protein, the nuclear depletion of which is the histopathological hallmark of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disorder affecting both upper and lower motor neurons. Besides motor symptoms, patients with ALS often develop nonneuronal signs including glucose intolerance, but the underlying pathomechanism is still controversial, i.e., whether it is impaired insulin secretion and/or insulin resistance. Here, we showed that ALS subjects reduced early-phase insulin secretion and that the nuclear localization of TDP-43 was lost in the islets of autopsied ALS pancreas. Loss of TDP-43 inhibited exocytosis by downregulating CaV1.2 calcium channels, thereby reducing early-phase insulin secretion in a cultured β cell line (MIN6) and β cell-specific Tardbp knockout mice. Overexpression of CaV1.2 restored early-phase insulin secretion in Tardbp knocked-down MIN6 cells. Our findings suggest that TDP-43 regulates cellular exocytosis mediated by L-type voltage-dependent calcium channels and thus plays an important role in the early phase of insulin secretion by pancreatic islets. Thus, nuclear loss of TDP-43 is implicated in not only the selective loss of motor neurons but also in glucose intolerance due to impaired insulin secretion at an early stage of ALS.

Thy-1/CD90 a Bidirectional and Lateral Signaling Scaffold

Thy-1/CD90 is a glycoprotein attached to the outer face of the plasma membrane with various functions, which depend on the context of specific physiological or pathological conditions. Many of these reported functions for Thy-1/CD90 arose from studies by our group, which identified the first ligand/receptor for Thy-1/CD90 as an integrin. This finding initiated studies directed toward unveiling the molecular mechanisms that operate downstream of Thy-1/CD90 activation, and its possible interaction with proteins in the membrane plane to regulate their function. The association of Thy-1/CD90 with a number of cell surface molecules allows the formation of extra/intracellular multiprotein complexes composed of various ligands and receptors, extracellular matrix proteins, intracellular signaling proteins, and the cytoskeleton. The complexes sense changes that occur inside and outside the cells, with Thy-1/CD90 at the core of this extracellular molecular platform. Molecular platforms are scaffold-containing microdomains where key proteins associate to prominently influence cellular processes and behavior. Each component, by itself, is less effective, but when together with various scaffold proteins to form a platform, the components become more specific and efficient to convey the messages. This review article discusses the experimental evidence that supports the role of Thy-1/CD90 as a membrane-associated platform (ThyMAP).

Neonatal Brain Injury and Genetic Causes of Adult-Onset Neurodegenerative Disease in Mice Interact With Effects on Acute and Late Outcomes

Neonatal brain damage and age-related neurodegenerative disease share many common mechanisms of injury involving mitochondriopathy, oxidative stress, excitotoxicity, inflammation, and neuronal cell death. We hypothesized that genes causing adult-onset neurodegeneration can influence acute outcome after CNS injury at immaturity and on the subsequent development of chronic disability after early-life brain injury. In two different transgenic (Tg) mouse models of adult-onset neurodegenerative disease, a human A53T-α-synuclein (hαSyn) model of Parkinson's disease (PD) and a human G93A-superoxide dismutase-1(hSOD1) model of amyotrophic lateral sclerosis (ALS), mortality and survivor morbidity were significantly greater than non-Tg mice and a Tg mouse model of Alzheimer's disease after neonatal traumatic brain injury (TBI). Acutely after brain injury, hαSyn neonatal mice showed a marked enhancement of protein oxidative damage in forebrain, brain regional mitochondrial oxidative metabolism, and mitochondriopathy. Extreme protein oxidative damage was also observed in neonatal mutant SOD1 mice after TBI. At 1 month of age, neuropathology in forebrain, midbrain, and brainstem of hαSyn mice with neonatal TBI was greater compared to sham hαSyn mice. Surviving hαSyn mice with TBI showed increased hαSyn aggregation and nitration and developed adult-onset disease months sooner and died earlier than non-injured hαSyn mice. Surviving hSOD1 mice with TBI also developed adult-onset disease and died sooner than non-injured hSOD1 mice. We conclude that mutant genes causing PD and ALS in humans have significant impact on mortality and morbidity after early-life brain injury and on age-related disease onset and proteinopathy in mice. This study provides novel insight into genetic determinants of poor outcomes after acute injury to the neonatal brain and how early-life brain injury can influence adult-onset neurodegenerative disease during aging.

Driven to decay: Excitability and synaptic abnormalities in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease and is clinically characterised by the death of corticospinal motor neurons (CSMNs), spinal and brainstem MNs and the degeneration of the corticospinal tract. Degeneration of CSMNs and MNs leads inexorably to muscle wastage and weakness, progressing to eventual death within 3-5 years of diagnosis. The CSMNs, located within layer V of the primary motor cortex, project axons constituting the corticospinal tract, forming synaptic connections with brainstem and spinal cord interneurons and MNs. Clinical ALS may be divided into familial (∼10% of cases) or sporadic (∼90% of cases), based on apparent random incidence. The emergence of transgenic murine models, expressing different ALS-associated mutations has accelerated our understanding of ALS pathogenesis, although precise mechanisms remain elusive. Multiple avenues of investigation suggest that cortical electrical abnormalities have pre-eminence in the pathophysiology of ALS. In addition, glutamate-mediated functional and structural alterations in both CSMNs and MNs are present in both sporadic and familial forms of ALS. This review aims to promulgate debate in the field with regard to the common aetiology of sporadic and familial ALS. A specific focus on a nexus point in ALS pathogenesis, namely, the synaptic and intrinsic hyperexcitability of CSMNs and MNs and alterations to their structure are comprehensively detailed. The association of extramotor dysfunction with neuronal structural/functional alterations will be discussed. Finally, the implications of the latest research on the dying-forward and dying-back controversy are considered.

Functional up-regulation of the M-current by retigabine contrasts hyperexcitability and excitotoxicity on rat hypoglossal motoneurons

KEY POINTS

Excessive neuronal excitability characterizes several neuropathological conditions, including neurodegenerative diseases such as amyotrophic lateral sclerosis. Hypoglossal motoneurons (HMs), which control tongue muscles, are extremely vulnerable to this disease and undergo damage and death when exposed to an excessive glutamate extracellular concentration that causes excitotoxicity. Our laboratory devised an in vitro model of excitotoxicity obtained by pharmacological blockade of glutamate transporters. In this paradigm, HMs display hyperexcitability, collective bursting and eventually cell death. The results of the present study show that pharmacological up-regulation of a K⁺ current (M-current), via application of the anti-convulsant retigabine, prevented all hallmarks of HM excitotoxicity, comprising bursting, generation of reactive oxygen species, expression of toxic markers and cell death. ○Our data may have translational value to develop new treatments against neurological diseases by using positive pharmacological modulators of the M-current.

ABSTRACT

Neuronal hyperexcitability is a symptom characterizing several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). In the ALS bulbar form, hypoglossal motoneurons (HMs) are an early target for neurodegeneration because of their high vulnerability to metabolic insults. In recent years, our laboratory has developed an in vitro model of a brainstem slice comprising the hypoglossal nucleus in which HM neurodegeneration is achieved by blocking glutamate clearance with dl-threo-β-benzyloxyaspartate (TBOA), thus leading to delayed excitotoxicity. During this process, HMs display a set of hallmarks such as hyperexcitability (and network bursting), reactive oxygen species (ROS) generation and, finally, cell death. The present study aimed to investigate whether blocking early hyperexcitability and bursting with the anti-convulsant drug retigabine was sufficient to achieve neuroprotection against excitotoxicity. Retigabine is a selective positive allosteric modulator of the M-current (I_M ), an endogenous mechanism that neurons (comprising HMs) express to dampen excitability. Retigabine (10 μm; co-applied with TBOA) contrasted ROS generation, release of endogenous toxic factors into the HM cytoplasm and excitotoxicity-induced HM death. Electrophysiological experiments showed that retigabine readily contrasted and arrested bursting evoked by TBOA administration. Because neuronal I_M subunits (Kv7.2, Kv7.3 and Kv7.5) were expressed in the hypoglossal nucleus and in functionally connected medullary nuclei, we suggest that they were responsible for the strong reduction in network excitability, a potent phenomenon for achieving neuroprotection against TBOA-induced excitotoxicity. The results of the present study may have translational value for testing novel positive pharmacological modulators of the I_M under pathological conditions (including neurodegenerative disorders) characterized by excessive neuronal excitability.

Parvalbumin fast-spiking interneurons are selectively altered by paediatric traumatic brain injury

KEY POINTS

Traumatic brain injury (TBI) in children remains a leading cause of death and disability and it remains poorly understood why children have worse outcomes and longer recover times. TBI has shown to alter cortical excitability and inhibitory drive onto excitatory neurons, yet few studies have directly examined changes to cortical interneurons. This is addressed in the present study using a clinically relevant model of severe TBI (controlled cortical impact) in interneuron cell type specific Cre-dependent mice. Mice subjected to controlled cortical impact exhibit specific loss of parvalbumin (PV) but not somatostatin immunoreactivity and cell density in the peri-injury zone. PV interneurons are primarily of a fast-spiking (FS) phenotype that persisted in the peri-injury zone but received less frequent inhibitory and stronger excitatory post-synaptic currents. The targeted loss of PV-FS interneurons appears to be distinct from previous reports in adult mice suggesting that TBI-induced pathophysiology is dependent on the age at time of impact.

ABSTRACT

Paediatric traumatic brain injury (TBI) is a leading cause of death and disability in children. Traditionally, ongoing neurodevelopment and neuroplasticity have been considered to confer children with an advantage following TBI. However, recent findings indicate that the paediatric brain may be more sensitive to brain injury. Inhibitory interneurons are essential for proper cortical function and are implicated in the pathophysiology of TBI, yet few studies have directly investigated TBI-induced changes to interneurons themselves. Accordingly, in the present study, we examine how inhibitory neurons are altered following controlled cortical impact (CCI) in juvenile mice with targeted Cre-dependent fluorescence labelling of interneurons (Vgat:Cre/Ai9 and PV:Cre/Ai6). Although CCI failed to alter the number of excitatory neurons or somatostatin-expressing interneurons in the peri-injury zone, it significantly decreased the density of parvalbumin (PV) immunoreactive cells by 71%. However, PV:Cre/Ai6 mice subjected to CCI showed a lower extent of fluorescence labelled cell loss. PV interneurons are predominantly of a fast-spiking (FS) phenotype and, when recorded electrophysiologically from the peri-injury zone, exhibited intrinsic properties similar to those of control neurons. Synaptically, CCI induced a decrease in inhibitory drive onto FS interneurons combined with an increase in the strength of excitatory events. The results of the present study indicate that CCI induced both a loss of PV interneurons and an even greater loss of PV expression. This suggests caution is required when interpreting changes in PV immunoreactivity alone as direct evidence of interneuronal loss. Furthermore, in contrast to reports in adults, TBI in the paediatric brain selectively alters PV-FS interneurons, primarily resulting in a loss of interneuronal inhibition.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Motor Neuron Gene Therapy: Lessons from Spinal Muscular Atrophy for Amyotrophic Lateral Sclerosis

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are severe nervous system diseases characterized by the degeneration of lower motor neurons. They share a number of additional pathological, cellular, and genetic parallels suggesting that mechanistic and clinical insights into one disorder may have value for the other. While there are currently no clinical ALS gene therapies, the splice-switching antisense oligonucleotide, nusinersen, was recently approved for SMA. This milestone was achieved through extensive pre-clinical research and patient trials, which together have spawned fundamental insights into motor neuron gene therapy. We have thus tried to distil key information garnered from SMA research, in the hope that it may stimulate a more directed approach to ALS gene therapy. Not only must the type of therapeutic (e.g., antisense oligonucleotide vs. viral vector) be sensibly selected, but considerable thought must be applied to the where, which, what, and when in order to enhance treatment benefit: to where (cell types and tissues) must the drug be delivered and how can this be best achieved? Which perturbed pathways must be corrected and can they be concurrently targeted? What dosing regime and concentration should be used? When should medication be administered? These questions are intuitive, but central to identifying and optimizing a successful gene therapy. Providing definitive solutions to these quandaries will be difficult, but clear thinking about therapeutic testing is necessary if we are to have the best chance of developing viable ALS gene therapies and improving upon early generation SMA treatments.

Motor Areas Show Altered Dendritic Structure in an Amyotrophic Lateral Sclerosis Mouse Model

Objective: Motor neurons (MNs) die in amyotrophic lateral sclerosis (ALS), a clinically heterogeneous neurodegenerative disease of unknown etiology. In human or rodent studies, MN loss is preceded by increased excitability. As increased neuronal excitability correlates with structural changes in dendritic arbors and spines, we have examined longitudinal changes in dendritic structure in vulnerable neuron populations in a mouse model of familial ALS.

Methods: We used a modified Golgi-Cox staining method to determine the progressive changes in dendritic structure of hippocampal CA1 pyramidal neurons, striatal medium spiny neurons, and resistant (trochlear, IV) or susceptible (hypoglossal, XII; lumbar) MNs from brainstem and spinal cord of mice over-expressing the human SOD1^G93A (SOD1) mutation, in comparison to wild-type (WT) mice, at four postnatal (P) ages of 8–15, 28–35, 65–75, and 120 days.

Results: In SOD1 mice, dendritic changes occur at pre-symptomatic ages in both XII and spinal cord lumbar MNs. Spine loss without dendritic changes was present in striatal neurons from disease onset. Spine density increases were present at all ages studied in SOD1 XII MNs. Spine density increased in neonatal lumbar MNs, before decreasing to control levels by P28-35 and was decreased by P120. SOD1 XII MNs and lumbar MNs, but not trochlear MNs showed vacuolization from the same time-points. Trochlear MN dendrites were unchanged.

Interpretation: Dendritic structure and spine alterations correlate with the neuro-motor phenotype in ALS and with cognitive and extra-motor symptoms seen in patients. Prominent early changes in dendritic arbors and spines occur in susceptible cranial and spinal cord MNs, but are absent in MNs resistant to loss in ALS.

αVβ3 Integrin regulates astrocyte reactivity

Background

Neuroinflammation involves cytokine release, astrocyte reactivity and migration. Neuronal Thy-1 promotes DITNC1 astrocyte migration by engaging α_Vβ₃ Integrin and Syndecan-4. Primary astrocytes express low levels of these receptors and are unresponsive to Thy-1; thus, inflammation and astrocyte reactivity might be necessary for Thy-1-induced responses.

Methods

Wild-type rat astrocytes (TNF-activated) or from human SOD1^G93A transgenic mice (a neurodegenerative disease model) were used to evaluate cell migration, Thy-1 receptor levels, signaling molecules, and reactivity markers.

Results

Thy-1 induced astrocyte migration only after TNF priming. Increased expression of α_Vβ₃ Integrin, Syndecan-4, P2X7R, Pannexin-1, Connexin-43, GFAP, and iNOS were observed in TNF-treated astrocytes. Silencing of β₃ Integrin prior to TNF treatment prevented Thy-1-induced migration, while β₃ Integrin over-expression was sufficient to induce astrocyte reactivity and allow Thy-1-induced migration. Finally, hSOD1^G93A astrocytes behave as TNF-treated astrocytes since they were reactive and responsive to Thy-1.

Conclusions

Therefore, inflammation induces expression of α_Vβ₃ Integrin and other proteins, astrocyte reactivity, and Thy-1 responsiveness. Importantly, ectopic control of β₃ Integrin levels modulates these responses regardless of inflammation.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-0968-5) contains supplementary material, which is available to authorized users.

Neurotoxicity of the Cyanotoxin BMAA Through Axonal Degeneration and Intercellular Spreading

β-Methylamino-L-alanine (BMAA) is implicated in neurodegeneration and neurotoxicity, particularly in ALS-Parkinson Dementia Complex. Neurotoxic properties of BMAA have been partly elucidated, while its transcellular spreading capacity has not been examined. Using reconstructed neuronal networks in microfluidic chips, separating neuronal cells into two subcompartments-(1) the proximal, containing first-order neuronal soma and dendrites, and (2) a distal compartment, containing either only axons originating from first-order neurons or second-order striatal neurons-creates a cortico-striatal network. Using this system, we investigated the toxicity and spreading of BMAA in murine primary neurons. We used a newly developed antibody to detect BMAA in cells. After treatment with 10 μM BMAA, the cyanotoxin was incorporated in first-degree neurons. We also observed a rapid trans-neuronal spread of BMAA to unexposed second-degree neurons in 48 h, followed by axonal degeneration, with limited somatic death. This in vitro study demonstrates BMAA axonal toxicity at sublethal concentrations and, for the first time, the transcellular spreading abilities of BMAA. This neuronal dying forward spread that could possibly be associated with progression of some neurodegenerative diseases especially amyotrophic lateral sclerosis.

Changes in the Excitability of Neocortical Neurons in a Mouse Model of Amyotrophic Lateral Sclerosis Are Not Specific to Corticospinal Neurons and Are Modulated by Advancing Disease

Cell type-specific changes in neuronal excitability have been proposed to contribute to the selective degeneration of corticospinal neurons in amyotrophic lateral sclerosis (ALS) and to neocortical hyperexcitability, a prominent feature of both inherited and sporadic variants of the disease, but the mechanisms underlying selective loss of specific cell types in ALS are not known. We analyzed the physiological properties of distinct classes of cortical neurons in the motor cortex of hSOD1^G93A mice of both sexes and found that they all exhibit increases in intrinsic excitability that depend on disease stage. Targeted recordings and in vivo calcium imaging further revealed that neurons adapt their functional properties to normalize cortical excitability as the disease progresses. Although different neuron classes all exhibited increases in intrinsic excitability, transcriptional profiling indicated that the molecular mechanisms underlying these changes are cell type specific. The increases in excitability in both excitatory and inhibitory cortical neurons show that selective dysfunction of neuronal cell types cannot account for the specific vulnerability of corticospinal motor neurons in ALS. Furthermore, the stage-dependent alterations in neuronal function highlight the ability of cortical circuits to adapt as disease progresses. These findings show that both disease stage and cell type must be considered when developing therapeutic strategies for treating ALS.SIGNIFICANCE STATEMENT It is not known why certain classes of neurons preferentially die in different neurodegenerative diseases. It has been proposed that the enhanced excitability of affected neurons is a major contributor to their selective loss. We show using a mouse model of amyotrophic lateral sclerosis (ALS), a disease in which corticospinal neurons exhibit selective vulnerability, that changes in excitability are not restricted to this neuronal class and that excitability does not increase monotonically with disease progression. Moreover, although all neuronal cell types tested exhibited abnormal functional properties, analysis of their gene expression demonstrated cell type-specific responses to the ALS-causing mutation. These findings suggest that therapies for ALS may need to be tailored for different cell types and stages of disease.

Differential activation of neuronal and glial STAT3 in the spinal cord of the SOD1G93A mouse model of amyotrophic lateral sclerosis

Signal transducer and activator of transcription (STAT) proteins are activated by phosphorylation in the spinal cord of patients suffering from amyotrophic lateral sclerosis (ALS). The major scope of our study is a comprehensive histological characterization of the mechanisms underlying neuronal and glial STAT3 activation in the pathogenesis of ALS using SOD1^G93A mice. We calculated the fold changes (FCs, ratios vs. appropriate controls) of the numerical densities of the following phosphorylated STAT3-positive (pSTAT3)⁺ cells - choline acetyltransferase (ChAT)⁺ α-motoneurons, ionized calcium-binding adapter molecule 1 (Iba1)⁺ microglia, and S100β⁺ astrocytes in SOD1^G93A mice. The FCs of pSTAT3⁺ microglia and pSTAT3⁺ astrocytes were increased from 9 to 15 weeks of age and then plateaued until 21 weeks. In contrast, the FCs of pSTAT3⁺ α-motoneurons peaked at 9 weeks and then decreased until 21 weeks. The immunoreactivity for nonphosphorylated neurofilament protein (SMI-32), a marker of axonal impairment, was decreased in pSTAT3⁺ α-motoneurons compared with pSTAT3^- α-motoneurons at 9 weeks of age. We then compared the following pharmacological models - the chronic administration of 3,3'-iminodipropionitrile (IDPN), which models axonal impairment, and the acute administration of lipopolysaccharide (LPS), which is a model of neuroinflammation. The FCs of pSTAT3⁺ α-motoneurons were increased in IDPN-treated mice, while those of pSTAT3⁺ microglia were increased in LPS-treated mice. The FCs of pSTAT3⁺ astrocytes were higher in SOD1^G93A mice at 9 weeks compared with IDPN- and LPS-treated mice. Our results indicate that axonopathy and neuroinflammation may trigger the respective activation of neuronal and glial STAT3, which is observed during ALS pathogenesis.

Chronic infusion of SOD1G93A astrocyte-secreted factors induces spinal motoneuron degeneration and neuromuscular dysfunction in healthy rats

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease and studies in vitro show that motoneuron degeneration is triggered by non-cell-autonomous mechanisms. However, whether soluble toxic factor(s) released by mutant superoxide dismutase 1 (SOD1) expressing astrocytes induces death of motoneurons and leads to motor dysfunction in vivo is not known. To directly test this, healthy adult rats were treated with conditioned media derived from primary mouse astrocytes (ACM) that express human (h) SOD1^G93A (ACM-hG93A) via chronic osmotic pump infusion in the lumbar spinal cord. Controls included ACM derived from transgenic mice expressing hSOD1^WT (ACM-hWT) or non-transgenic mouse SOD1^WT (ACM-WT) astrocytes. Rats chronically infused with ACM-hG93A started to develop motor dysfunction at 8 days, as measured by rotarod performance. Additionally, immunohistochemical analyses at day 16 revealed reactive astrogliosis and significant loss of motoneurons in the ventral horn of the infused region. Controls did not show significant motor behavior alterations or neuronal damage. Thus, we demonstrate that factors released in vitro from astrocytes derived from ALS mice cause spinal motoneuron death and consequent neuromuscular dysfunction in vivo.

Gamma motor neurons survive and exacerbate alpha motor neuron degeneration in ALS

The molecular and cellular basis of selective motor neuron (MN) vulnerability in amyotrophic lateral sclerosis (ALS) is not known. In genetically distinct mouse models of familial ALS expressing mutant superoxide dismutase-1 (SOD1), TAR DNA-binding protein 43 (TDP-43), and fused in sarcoma (FUS), we demonstrate selective degeneration of alpha MNs (α-MNs) and complete sparing of gamma MNs (γ-MNs), which selectively innervate muscle spindles. Resistant γ-MNs are distinct from vulnerable α-MNs in that they lack synaptic contacts from primary afferent (I_A) fibers. Elimination of these synapses protects α-MNs in the SOD1 mutant, implicating this excitatory input in MN degeneration. Moreover, reduced I_A activation by targeted reduction of γ-MNs in SOD1^G93A mutants delays symptom onset and prolongs lifespan, demonstrating a pathogenic role of surviving γ-MNs in ALS. This study establishes the resistance of γ-MNs as a general feature of ALS mouse models and demonstrates that synaptic excitation of MNs within a complex circuit is an important determinant of relative vulnerability in ALS.

Cortical synaptic and dendritic spine abnormalities in a presymptomatic TDP-43 model of amyotrophic lateral sclerosis

Layer V pyramidal neurons (LVPNs) within the motor cortex integrate sensory cues and co-ordinate voluntary control of motor output. In amyotrophic lateral sclerosis (ALS) LVPNs and spinal motor neurons degenerate. The pathogenesis of neural degeneration is unknown in ALS; 10% of cases have a genetic cause, whereas 90% are sporadic, with most of the latter showing TDP-43 inclusions. Clinical and experimental evidence implicate excitotoxicity as a prime aetiological candidate. Using patch clamp and dye-filling techniques in brain slices, combined with high-resolution confocal microscopy, we report increased excitatory synaptic inputs and dendritic spine densities in early presymptomatic mice carrying a TDP-43^Q331K mutation. These findings demonstrate substantive alterations in the motor cortex neural network, long before an overt degenerative phenotype has been reported. We conclude that increased excitatory neurotransmission is a common pathophysiology amongst differing genetic cases of ALS and may be of relevance to the 95% of sporadic ALS cases that exhibit TDP-43 inclusions.

Therapeutic rAAVrh10 Mediated SOD1 Silencing in Adult SOD1(G93A) Mice and Nonhuman Primates

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease; survival in ALS is typically 3-5 years. No treatment extends patient survival by more than three months. Approximately 20% of familial ALS and 1-3% of sporadic ALS patients carry a mutation in the gene encoding superoxide dismutase 1 (SOD1). In a transgenic ALS mouse model expressing the mutant SOD1(G93A) protein, silencing the SOD1 gene prolongs survival. One study reports a therapeutic effect of silencing the SOD1 gene in systemically treated adult ALS mice; this was achieved with a short hairpin RNA, a silencing molecule that has raised multiple safety concerns, and recombinant adeno-associated virus (rAAV) 9. We report here a silencing method based on an artificial microRNA termed miR-SOD1 systemically delivered using adeno-associated virus rAAVrh10, a serotype with a demonstrated safety profile in CNS clinical trials. Silencing of SOD1 in adult SOD1(G93A) transgenic mice with this construct profoundly delayed both disease onset and death in the SOD1(G93A) mice, and significantly preserved muscle strength and motor and respiratory functions. We also document that intrathecal delivery of the same rAAVrh10-miR-SOD1 in nonhuman primates significantly and safely silences SOD1 in lower motor neurons. This study supports the view that rAAVrh10-miR-SOD1 merits further development for the treatment of SOD1-linked ALS in humans.

Aberrant Subcellular Dynamics of Sigma-1 Receptor Mutants Underlying Neuromuscular Diseases

The sigma-1 receptor (σ-1R) is an endoplasmic reticulum resident chaperone protein involved in a plethora of cellular functions, and whose disruption has been implicated in a wide range of diseases. Genetic analysis has revealed two σ-1R mutants involved in neuromuscular disorders. A point mutation (E102Q) in the ligand-binding domain results in the juvenile form of amyotrophic lateral sclerosis (ALS16), and a 20 amino-acid deletion (Δ31-50) in the putative cytosolic domain leads to a form of distal hereditary motor neuropathy. We investigated the localization and functional properties of these mutants in cell lines using confocal imaging and electrophysiology. The σ-1R mutants exhibited a significant increase in mobility, aberrant localization, and enhanced block of the inwardly rectifying K(+) channel Kir2.1, compared with the wild-type σ-1R. Thus, these σ-1R mutants have different functional properties that could contribute to their disease phenotypes.

Silencing strategies for therapy of SOD1-mediated ALS

Amyotrophic lateral sclerosis (ALS) is an adult-onset, lethal, paralytic disorder caused by the degeneration of motor neurons. Our understanding of this disease has been greatly facilitated by studies of familial ALS caused by mutations in the gene encoding superoxide dismutase 1 (SOD1). Evidence indicates that misfolded wild-type SOD1 may also be pathogenic in sporadic ALS. Mutant SOD1 is neurotoxic through multiple mechanisms. Because the pathogenicity of mutant SOD1 is proportional to the dose of the toxic protein, a rational approach to treating SOD1-related ALS is to reduce levels of the toxic SOD1 species. An advantage of this strategy is that it potentially obviates intervening in multiple, downstream pathological cascades. In recent years, several strategies to silence gene expression have been developed. The most clinically promising are predicated on approaches that enhance degradation of RNA, such as anti-sense oligonucleotides (ASO) and RNA interference (RNAi); the latter include small inhibitory RNA (siRNA), short hairpin RNA (shRNA) and microRNA (miR). Agents such as shRNA and either native or synthetic miR are capable of permeating the central nervous system (CNS) and efficiently silencing genes in the brain and spinal cord. Here we review recent progress in silencing SOD1, focusing on studies using artificial shRNA or miRNA in combination with potent viral vector delivery systems to mediate SOD1 silencing within the CNS in transgenic SOD1^G93A mice and non-human primates.

Marked changes in dendritic structure and spine density precede significant neuronal death in vulnerable cortical pyramidal neuron populations in the SOD1(G93A) mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterised by the death of upper (corticospinal) and lower motor neurons (MNs) with progressive muscle weakness. This incurable disease is clinically heterogeneous and its aetiology remains unknown. Increased excitability of corticospinal MNs has been observed prior to symptoms in human and rodent studies. Increased excitability has been correlated with structural changes in neuronal dendritic arbors and spines for decades. Here, using a modified Golgi-Cox staining method, we have made the first longitudinal study examining the dendrites of pyramidal neurons from the motor cortex, medial pre-frontal cortex, somatosensory cortex and entorhinal cortex of hSOD1^G93A (SOD1) mice compared to wild-type (WT) littermate controls at postnatal (P) days 8–15, 28–35, 65–75 and 120. Progressive decreases in dendritic length and spine density commencing at pre-symptomatic ages (P8-15 or P28-35) were observed in layer V pyramidal neurons within the motor cortex and medial pre-frontal cortex of SOD1 mice compared to WT mice. Spine loss without concurrent dendritic pathology was present in the pyramidal neurons of the somatosensory cortex from disease-onset (P65-75). Our results from the SOD1 model suggest that dendritic and dendritic spine changes foreshadow and underpin the neuromotor phenotypes present in ALS and may contribute to the varied cognitive, executive function and extra-motor symptoms commonly seen in ALS patients. Determining if these phenomena are compensatory or maladaptive may help explain differential susceptibility of neurons to degeneration in ALS.

Voltage-gated calcium channels are abnormal in cultured spinal motoneurons in the G93A-SOD1 transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motoneurons. Hyperexcitability and excitotoxicity have been implicated in the early pathogenesis of ALS. Studies addressing excitotoxic motoneuron death and intracellular Ca(2+) overload have mostly focused on Ca(2+) influx through AMPA glutamate receptors. However, intrinsic excitability of motoneurons through voltage-gated ion channels may also have a role in the neurodegeneration. In this study we examined the function and localization of voltage-gated Ca(2+) channels in cultured spinal cord motoneurons from mice expressing a mutant form of human superoxide dismutase-1 with a Gly93→Ala substitution (G93A-SOD1). Using whole-cell patch-clamp recordings, we showed that high voltage activated (HVA) Ca(2+) currents are increased in G93A-SOD1 motoneurons, but low voltage activated Ca(2+) currents are not affected. G93A-SOD1 motoneurons also have altered persistent Ca(2+) current mediated by L-type Ca(2+) channels. Quantitative single-cell RT-PCR revealed higher levels of Ca1a, Ca1b, Ca1c, and Ca1e subunit mRNA expression in G93A-SOD1 motoneurons, indicating that the increase of HVA Ca(2+) currents may result from upregulation of Ca(2+) channel mRNA expression in motoneurons. The localizations of the Ca1B N-type and Ca1D L-type Ca(2+) channels in motoneurons were examined by immunocytochemistry and confocal microscopy. G93A-SOD1 motoneurons had increased Ca1B channels on the plasma membrane of soma and dendrites. Ca1D channels are similar on the plasma membrane of soma and lower on the plasma membrane of dendrites of G93A-SOD1 motoneurons. Our study demonstrates that voltage-gated Ca(2+) channels have aberrant functions and localizations in ALS mouse motoneurons. The increased HVA Ca(2+) currents and PCCa current could contribute to early pathogenesis of ALS.

INaP selective inhibition reverts precocious inter- and motorneurons hyperexcitability in the Sod1-G93R zebrafish ALS model

The pathogenic role of SOD1 mutations in amyotrophic lateral sclerosis (ALS) was investigated using a zebrafish disease model stably expressing the ALS-linked G93R mutation. In addition to the main pathological features of ALS shown by adult fish, we found remarkably precocious alterations in the development of motor nerve circuitry and embryo behavior, and suggest that these alterations are prompted by interneuron and motor neuron hyperexcitability triggered by anomalies in the persistent pacemaker sodium current I_NaP. The riluzole-induced modulation of I_NaP reduced spinal neuron excitability, reverted the behavioral phenotypes and improved the deficits in motor nerve circuitry development, thus shedding new light on the use of riluzole in the management of ALS. Our findings provide a valid phenotype-based tool for unbiased in vivo drug screening that can be used to develop new therapies.

Mutant SOD1 protein increases Nav1.3 channel excitability

Amyotrophic lateral sclerosis (ALS) is a lethal paralytic disease caused by the degeneration of motor neurons in the spinal cord, brain stem, and motor cortex. Mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) are present in ~20% of familial ALS and ~2% of all ALS cases. The most common SOD1 gene mutation in North America is a missense mutation substituting valine for alanine (A4V). In this study, we analyze sodium channel currents in oocytes expressing either wild-type or mutant (A4V) SOD1 protein. We demonstrate that the A4V mutation confers a propensity to hyperexcitability on a voltage-dependent sodium channel (Nav1.3) mediated by heightened total Na(+) conductance and a hyperpolarizing shift in the voltage dependence of Nav1.3 activation. To estimate the impact of these channel effects on excitability in an intact neuron, we simulated these changes in the program NEURON; this shows that the changes induced by mutant SOD1 increase the spontaneous firing frequency of the simulated neuron. These findings are consistent with the view that excessive excitability of neurons is one component in the pathogenesis of this disease.

Reactive oxygen species trigger motoneuron death in non-cell-autonomous models of ALS through activation of c-Abl signaling

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease in which pathogenesis and death of motor neurons are triggered by non-cell-autonomous mechanisms. We showed earlier that exposing primary rat spinal cord cultures to conditioned media derived from primary mouse astrocyte conditioned media (ACM) that express human SOD1^G93A (ACM-hSOD1^G93A) quickly enhances Na_v channel-mediated excitability and calcium influx, generates intracellular reactive oxygen species (ROS), and leads to death of motoneurons within days. Here we examined the role of mitochondrial structure and physiology and of the activation of c-Abl, a tyrosine kinase that induces apoptosis. We show that ACM-hSOD1^G93A, but not ACM-hSOD1^WT, increases c-Abl activity in motoneurons, interneurons and glial cells, starting at 60 min; the c-Abl inhibitor STI571 (imatinib) prevents this ACM-hSOD1^G93A-mediated motoneuron death. Interestingly, similar results were obtained with ACM derived from astrocytes expressing SOD1^G86R or TDP43^A315T. We further find that co-application of ACM-SOD1^G93A with blockers of Na_v channels (spermidine, mexiletine, or riluzole) or anti-oxidants (Trolox, esculetin, or tiron) effectively prevent c-Abl activation and motoneuron death. In addition, ACM-SOD1^G93A induces alterations in the morphology of neuronal mitochondria that are related with their membrane depolarization. Finally, we find that blocking the opening of the mitochondrial permeability transition pore with cyclosporine A, or inhibiting mitochondrial calcium uptake with Ru360, reduces ROS production and c-Abl activation. Together, our data point to a sequence of events in which a toxic factor(s) released by ALS-expressing astrocytes rapidly induces hyper-excitability, which in turn increases calcium influx and affects mitochondrial structure and physiology. ROS production, mediated at least in part through mitochondrial alterations, trigger c-Abl signaling and lead to motoneuron death.