Mutant TDP-43 within motor neurons drives disease onset but not progression in amyotrophic lateral sclerosis

Nicotinamide Adenine Dinucleotide Precursor Supplementation Modulates Neurite Complexity and Survival in Motor Neurons from Amyotrophic Lateral Sclerosis Models

Aims: Increasing nicotinamide adenine dinucleotide (NAD+) availability has been proposed as a therapeutic approach to prevent neurodegeneration in amyotrophic lateral sclerosis (ALS). Accordingly, NAD+ precursor supplementation appears to exert neuroprotective effects in ALS patients and mouse models. The mechanisms mediating neuroprotection remain uncertain but could involve changes in multiple cell types. We investigated a potential direct effect of the NAD+ precursor nicotinamide mononucleotide (NMN) on the health of cultured induced pluripotent stem cell (iPSC)-derived human motor neurons and in motor neurons isolated from two ALS mouse models, that is, mice overexpressing wild-type transactive response DNA binding protein-43 (TDP-43) or the ALS-linked human superoxide dismutase 1 with the G93A mutation (hSOD1G93A). Results: NMN treatment increased the complexity of neuronal processes in motor neurons isolated from both mouse models and in iPSC-derived human motor neurons. In addition, NMN prevented neuronal death induced by trophic factor deprivation. In mouse and human motor neurons expressing ALS-linked mutant superoxide dismutase 1, NMN induced an increase in glutathione levels, but this effect was not observed in nontransgenic or TDP-43 overexpressing motor neurons. In contrast, NMN treatment normalized the TDP-43 cytoplasmic mislocalization induced by its overexpression. Innovation: NMN can directly act on motor neurons to increase the growth and complexity of neuronal processes and prevent the death induced by trophic factor deprivation. Conclusion: Our results support a direct beneficial effect of NAD+ precursor supplementation on the maintenance of the neuritic arbor in motor neurons. Importantly, this was observed in motor neurons isolated from two different ALS models, with and without involvement of TDP-43 pathology, supporting its therapeutic potential in sporadic and familial ALS. Antioxid. Redox Signal. 41, 573-589.

Frontotemporal dementia-like disease progression elicited by seeded aggregation and spread of FUS

RNA binding proteins have emerged as central players in the mechanisms of many neurodegenerative diseases. In particular, a proteinopathy of fused in sarcoma (FUS) is present in some instances of familial Amyotrophic lateral sclerosis (ALS) and about 10% of sporadic Frontotemporal lobar degeneration (FTLD). Here we establish that focal injection of sonicated human FUS fibrils into brains of mice in which ALS-linked mutant or wild-type human FUS replaces endogenous mouse FUS is sufficient to induce focal cytoplasmic mislocalization and aggregation of mutant and wild-type FUS which with time spreads to distal regions of the brain. Human FUS fibril-induced FUS aggregation in the mouse brain of humanized FUS mice is accelerated by an ALS-causing FUS mutant relative to wild-type human FUS. Injection of sonicated human FUS fibrils does not induce FUS aggregation and subsequent spreading after injection into naïve mouse brains containing only mouse FUS, indicating a species barrier to human FUS aggregation and its prion-like spread. Fibril-induced human FUS aggregates recapitulate pathological features of FTLD including increased detergent insolubility of FUS and TAF15 and amyloid-like, cytoplasmic deposits of FUS that accumulate ubiquitin and p62, but not TDP-43. Finally, injection of sonicated FUS fibrils is shown to exacerbate age-dependent cognitive and behavioral deficits from mutant human FUS expression. Thus, focal seeded aggregation of FUS and further propagation through prion-like spread elicits FUS-proteinopathy and FTLD-like disease progression.

Frontotemporal dementia-like disease progression elicited by seeded aggregation and spread of FUS

RNA binding proteins have emerged as central players in the mechanisms of many neurodegenerative diseases. In particular, a proteinopathy of fu sed in s arcoma (FUS) is present in some instances of familial Amyotrophic lateral sclerosis (ALS) and about 10% of sporadic FTLD. Here we establish that focal injection of sonicated human FUS fibrils into brains of mice in which ALS-linked mutant or wild-type human FUS replaces endogenous mouse FUS is sufficient to induce focal cytoplasmic mislocalization and aggregation of mutant and wild-type FUS which with time spreads to distal regions of the brain. Human FUS fibril-induced FUS aggregation in the mouse brain of humanized FUS mice is accelerated by an ALS-causing FUS mutant relative to wild-type human FUS. Injection of sonicated human FUS fibrils does not induce FUS aggregation and subsequent spreading after injection into naïve mouse brains containing only mouse FUS, indicating a species barrier to human FUS aggregation and its prion-like spread. Fibril-induced human FUS aggregates recapitulate pathological features of FTLD including increased detergent insolubility of FUS and TAF15 and amyloid-like, cytoplasmic deposits of FUS that accumulate ubiquitin and p62, but not TDP-43. Finally, injection of sonicated FUS fibrils is shown to exacerbate age-dependent cognitive and behavioral deficits from mutant human FUS expression. Thus, focal seeded aggregation of FUS and further propagation through prion-like spread elicits FUS-proteinopathy and FTLD-like disease progression.

Intra-condensate demixing of TDP-43 inside stress granules generates pathological aggregates

Cytosolic aggregation of the nuclear protein TDP-43 is associated with many neurodegenerative diseases, but the triggers for TDP-43 aggregation are still debated. Here, we demonstrate that TDP-43 aggregation requires a double event. One is up-concentration in stress granules beyond a threshold, and the other is oxidative stress. These two events collectively induce intra-condensate demixing, giving rise to a dynamic TDP-43 enriched phase within stress granules, which subsequently transitions into pathological aggregates. Mechanistically, intra-condensate demixing is triggered by local unfolding of the RRM1 domain for intermolecular disulfide bond formation and by increased hydrophobic patch interactions in the C-terminal domain. By engineering TDP-43 variants resistant to intra-condensate demixing, we successfully eliminate pathological TDP-43 aggregates in cells. We conclude that up-concentration inside condensates and simultaneous exposure to environmental stress could be a general pathway for protein aggregation, with intra-condensate demixing constituting a key intermediate step.

A transient protein folding response targets aggregation in the early phase of TDP-43-mediated neurodegeneration

Understanding the mechanisms that drive TDP-43 pathology is integral to combating amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD) and other neurodegenerative diseases. Here we generated a longitudinal quantitative proteomic map of the cortex from the cytoplasmic TDP-43 rNLS8 mouse model of ALS and FTLD, and developed a complementary open-access webtool, TDP-map ( https://shiny.rcc.uq.edu.au/TDP-map/ ). We identified distinct protein subsets enriched for diverse biological pathways with temporal alterations in protein abundance, including increases in protein folding factors prior to disease onset. This included increased levels of DnaJ homolog subfamily B member 5, DNAJB5, which also co-localized with TDP-43 pathology in diseased human motor cortex. DNAJB5 over-expression decreased TDP-43 aggregation in cell and cortical neuron cultures, and knockout of Dnajb5 exacerbated motor impairments caused by AAV-mediated cytoplasmic TDP-43 expression in mice. Together, these findings reveal molecular mechanisms at distinct stages of ALS and FTLD progression and suggest that protein folding factors could be protective in neurodegenerative diseases.

Changes in vocal fold gene expression and histology after injection augmentation in a recurrent laryngeal nerve injury model

OBJECTIVE

To investigate changes in neuroregenerative pathways with vocal fold denervation in response to vocal fold augmentation.

METHODS

Eighteen Yorkshire crossbreed swine underwent left recurrent laryngeal nerve transection, followed by observation or augmentation with carboxymethylcellulose or calcium hydroxyapatite at two weeks. Polymerase chain reaction expression of genes regulating muscle growth (MyoD1, MyoG and FoxO1) and atrophy (FBXO32) were analysed at 4 and 12 weeks post-injection. Thyroarytenoid neuromuscular junction density was quantified using immunohistochemistry.

RESULTS

Denervated vocal folds demonstrated reduced expression of MyoD1, MyoG, FoxO1 and FBXO32, but overexpression after augmentation. Healthy vocal folds showed increased early and late MyoD1, MyoG, FoxO1 and FBXO32 expression in all animals. Neuromuscular junction density had a slower decline in augmented compared to untreated denervated vocal folds, and was significantly reduced in healthy vocal folds contralateral to augmentation.

CONCLUSION

Injection augmentation may slow neuromuscular degeneration pathways in denervated vocal folds and reduce compensatory remodelling in contralateral healthy vocal folds.

Astrocyte-Neuron Interactions Contributing to Amyotrophic Lateral Sclerosis Progression

Amyotrophic lateral sclerosis (ALS) is a complex disease impacting motor neurons of the brain, brainstem, and spinal cord. Disease etiology is quite heterogeneous with over 40 genes causing the disease and a vast ~90% of patients having no prior family history. Astrocytes are major contributors to ALS, particularly through involvement in accelerating disease progression. Through study of genetic forms of disease including SOD1, TDP43, FUS, C9orf72, VCP, TBK1, and more recently patient-derived cells from sporadic individuals, many biological mechanisms have been identified to cause intrinsic or glial-mediated neurotoxicity to motor neurons. Overall, many of the normally supportive and beneficial roles that astrocytes contribute to neuronal health and survival instead switch to become deleterious and neurotoxic. While the exact pathways may differ based on disease-origin, altered astrocyte-neuron communication is a common feature of ALS. Within this chapter, distinct genetic forms are examined in detail, along with what is known from sporadic patient-derived cells. Overall, this chapter highlights the interplay between astrocytes and neurons in this complex disease and describes the key features underlying: astrocyte-mediated motor neuron toxicity, excitotoxicity, oxidative/nitrosative stress, protein dyshomeostasis, metabolic imbalance, inflammation, trophic factor withdrawal, blood-brain/blood-spinal cord barrier involvement, disease spreading, and the extracellular matrix/cell adhesion/TGF-β signaling pathways.

Pathological insights from amyotrophic lateral sclerosis animal models: comparisons, limitations, and challenges

In order to dissect amyotrophic lateral sclerosis (ALS), a multigenic, multifactorial, and progressive neurodegenerative disease with heterogeneous clinical presentations, researchers have generated numerous animal models to mimic the genetic defects. Concurrent and comparative analysis of these various models allows identification of the causes and mechanisms of ALS in order to finally obtain effective therapeutics. However, most genetically modified rodent models lack overt pathological features, imposing challenges and limitations in utilizing them to rigorously test the potential mechanisms. Recent studies using large animals, including pigs and non-human primates, have uncovered important events that resemble neurodegeneration in patients' brains but could not be produced in small animals. Here we describe common features as well as discrepancies among these models, highlighting new insights from these models. Furthermore, we will discuss how to make rodent models more capable of recapitulating important pathological features based on the important pathogenic insights from large animal models.

TDP-43 pathology in Drosophila induces glial-cell type specific toxicity that can be ameliorated by knock-down of SF2/SRSF1

Accumulation of cytoplasmic inclusions of TAR-DNA binding protein 43 (TDP-43) is seen in both neurons and glia in a range of neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and Alzheimer's disease (AD). Disease progression involves non-cell autonomous interactions among multiple cell types, including neurons, microglia and astrocytes. We investigated the effects in Drosophila of inducible, glial cell type-specific TDP-43 overexpression, a model that causes TDP-43 protein pathology including loss of nuclear TDP-43 and accumulation of cytoplasmic inclusions. We report that TDP-43 pathology in Drosophila is sufficient to cause progressive loss of each of the 5 glial sub-types. But the effects on organismal survival were most pronounced when TDP-43 pathology was induced in the perineural glia (PNG) or astrocytes. In the case of PNG, this effect is not attributable to loss of the glial population, because ablation of these glia by expression of pro-apoptotic reaper expression has relatively little impact on survival. To uncover underlying mechanisms, we used cell-type-specific nuclear RNA sequencing to characterize the transcriptional changes induced by pathological TDP-43 expression. We identified numerous glial cell-type specific transcriptional changes. Notably, SF2/SRSF1 levels were found to be decreased in both PNG and in astrocytes. We found that further knockdown of SF2/SRSF1 in either PNG or astrocytes lessens the detrimental effects of TDP-43 pathology on lifespan, but extends survival of the glial cells. Thus TDP-43 pathology in astrocytes or PNG causes systemic effects that shorten lifespan and SF2/SRSF1 knockdown rescues the loss of these glia, and also reduces their systemic toxicity to the organism.

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

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

X-linked SBMA model mice display relevant non-neurological phenotypes and their expression of mutant androgen receptor protein in motor neurons is not required for neuromuscular disease

X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease) is a rare neuromuscular disorder characterized by adult-onset proximal muscle weakness and lower motor neuron degeneration. SBMA was the first human disease found to be caused by a repeat expansion mutation, as affected patients possess an expanded tract of CAG repeats, encoding polyglutamine, in the androgen receptor (AR) gene. We previously developed a conditional BAC fxAR121 transgenic mouse model of SBMA and used it to define a primary role for skeletal muscle expression of polyglutamine-expanded AR in causing the motor neuron degeneration. Here we sought to extend our understanding of SBMA disease pathophysiology and cellular basis by detailed examination and directed experimentation with the BAC fxAR121 mice. First, we evaluated BAC fxAR121 mice for non-neurological disease phenotypes recently described in human SBMA patients, and documented prominent non-alcoholic fatty liver disease, cardiomegaly, and ventricular heart wall thinning in aged male BAC fxAR121 mice. Our discovery of significant hepatic and cardiac abnormalities in SBMA mice underscores the need to evaluate human SBMA patients for signs of liver and heart disease. To directly examine the contribution of motor neuron-expressed polyQ-AR protein to SBMA neurodegeneration, we crossed BAC fxAR121 mice with two different lines of transgenic mice expressing Cre recombinase in motor neurons, and after updating characterization of SBMA phenotypes in our current BAC fxAR121 colony, we found that excision of mutant AR from motor neurons did not rescue neuromuscular or systemic disease. These findings further validate a primary role for skeletal muscle as the driver of SBMA motor neuronopathy and indicate that therapies being developed to treat patients should be delivered peripherally.

TDP-43 pathology in Drosophila induces glial-cell type specific toxicity that can be ameliorated by knock-down of SF2/SRSF1

Accumulation of cytoplasmic inclusions of TAR-DNA binding protein 43 (TDP-43) is seen in both neurons and glia in a range of neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and Alzheimer's disease (AD). Disease progression involves non-cell autonomous interactions among multiple cell types, including neurons, microglia and astrocytes. We investigated the effects in Drosophila of inducible, glial cell type-specific TDP-43 overexpression, a model that causes TDP-43 protein pathology including loss of nuclear TDP-43 and accumulation of cytoplasmic inclusions. We report that TDP-43 pathology in Drosophila is sufficient to cause progressive loss of each of the 5 glial sub-types. But the effects on organismal survival were most pronounced when TDP-43 pathology was induced in the perineural glia (PNG) or astrocytes. In the case of PNG, this effect is not attributable to loss of the glial population, because ablation of these glia by expression of pro-apoptotic reaper expression has relatively little impact on survival. To uncover underlying mechanisms, we used cell-type-specific nuclear RNA sequencing to characterize the transcriptional changes induced by pathological TDP-43 expression. We identified numerous glial cell-type specific transcriptional changes. Notably, SF2/SRSF1 levels were found to be decreased in both PNG and in astrocytes. We found that further knockdown of SF2/SRSF1 in either PNG or astrocytes lessens the detrimental effects of TDP-43 pathology on lifespan, but extends survival of the glial cells. Thus TDP-43 pathology in astrocytes or PNG causes systemic effects that shorten lifespan and SF2/SRSF1 knockdown rescues the loss of these glia, and also reduces their systemic toxicity to the organism.

Astrocytic TDP-43 dysregulation impairs memory by modulating antiviral pathways and interferon-inducible chemokines

Transactivating response region DNA binding protein 43 (TDP-43) pathology is prevalent in dementia, but the cell type-specific effects of TDP-43 pathology are not clear, and therapeutic strategies to alleviate TDP-43-linked cognitive decline are lacking. We found that patients with Alzheimer's disease or frontotemporal dementia have aberrant TDP-43 accumulation in hippocampal astrocytes. In mouse models, induction of widespread or hippocampus-targeted accumulation in astrocytic TDP-43 caused progressive memory loss and localized changes in antiviral gene expression. These changes were cell-autonomous and correlated with impaired astrocytic defense against infectious viruses. Among the changes, astrocytes had elevated levels of interferon-inducible chemokines, and neurons had elevated levels of the corresponding chemokine receptor CXCR3 in presynaptic terminals. CXCR3 stimulation altered presynaptic function and promoted neuronal hyperexcitability, akin to the effects of astrocytic TDP-43 dysregulation, and blockade of CXCR3 reduced this activity. Ablation of CXCR3 also prevented TDP-43-linked memory loss. Thus, astrocytic TDP-43 dysfunction contributes to cognitive impairment through aberrant chemokine-mediated astrocytic-neuronal interactions.

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

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

NR1D1 downregulation in astrocytes induces a phenotype that is detrimental to cocultured motor neurons

Nuclear receptor subfamily 1 group D member 1 (NR1D1, also known as Rev-erbα) is a nuclear transcription factor that is part of the molecular clock encoding circadian rhythms and may link daily rhythms with metabolism and inflammation. NR1D1, unlike most nuclear receptors, lacks a ligand-dependent activation function domain 2 and is a constitutive transcriptional repressor. Amyotrophic lateral sclerosis (ALS) is the most common adult-onset motor neuron disease, caused by the progressive degeneration of motor neurons in the spinal cord, brain stem, and motor cortex. Approximately 10%-20% of familial ALS is caused by a toxic gain-of-function induced by mutations of the Cu/Zn superoxide dismutase (SOD1). Dysregulated clock and clock-controlled gene expression occur in multiple tissues from mutant hSOD1-linked ALS mouse models. Here we explore NR1D1 dysregulation in the spinal cord of ALS mouse models and its consequences on astrocyte-motor neuron interaction. NR1D1 protein and mRNA expression are significantly downregulated in the spinal cord of symptomatic mice expressing mutant hSOD1, while no changes were observed in age-matched animals overexpressing wild-type hSOD1. In addition, NR1D1 downregulation in primary astrocyte cultures induces a pro-inflammatory phenotype and decreases the survival of cocultured motor neurons. NR1D1 orchestrates the cross talk between physiological pathways identified to be disrupted in ALS (e.g., metabolism, inflammation, redox homeostasis, and circadian rhythms) and we observed that downregulation of NR1D1 alters astrocyte-motor neuron interaction. Our results suggest that NR1D1 could be a potential therapeutic target to prevent astrocyte-mediated motor neuron toxicity in ALS.

SYNGR4 and PLEKHB1 deregulation in motor neurons of amyotrophic lateral sclerosis models: potential contributions to pathobiology

Amyotrophic lateral sclerosis is the most common adult-onset neurodegenerative disease affecting motor neurons. Its defining feature is progressive loss of motor neuron function in the cortex, brainstem, and spinal cord, leading to paralysis and death. Despite major advances in identifying genes that can cause disease when mutated and model the disease in animals and cellular models, it still remains unclear why motor symptoms suddenly appear after a long pre-symptomatic phase of apparently normal function. One hypothesis is that age-related deregulation of specific proteins within key cell types, especially motor neurons themselves, initiates disease symptom appearance and may also drive progressive degeneration. Genome-wide in vivo cell-type-specific screening tools are enabling identification of candidates for such proteins. In this minireview, we first briefly discuss the methodology used in a recent study that applied a motor neuron-specific RNA-Seq screening approach to a standard model of TAR DNA-binding protein-43 (TDP-43)-driven amyotrophic lateral sclerosis. A key finding of this study is that synaptogyrin-4 and pleckstrin homology domain-containing family B member 1 are also deregulated at the protein level within motor neurons of two unrelated mouse models of mutant TDP-43 driven amyotrophic lateral sclerosis. Guided by what is known about molecular and cellular functions of these proteins and their orthologs, we outline here specific hypotheses for how changes in their levels might potentially alter cellular physiology of motor neurons and detrimentally affect motor neuron function. Where possible, we also discuss how this information could potentially be used in a translational context to develop new therapeutic strategies for this currently incurable, devastating disease.

Blood-spinal cord barrier leakage is independent of motor neuron pathology in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease involving progressive degeneration of upper and lower motor neurons. The pattern of lower motor neuron loss along the spinal cord follows the pattern of deposition of phosphorylated TDP-43 aggregates. The blood-spinal cord barrier (BSCB) restricts entry into the spinal cord parenchyma of blood components that can promote motor neuron degeneration, but in ALS there is evidence for barrier breakdown. Here we sought to quantify BSCB breakdown along the spinal cord axis, to determine whether BSCB breakdown displays the same patterning as motor neuron loss and TDP-43 proteinopathy. Cerebrospinal fluid hemoglobin was measured in living ALS patients (n = 87 control, n = 236 ALS) as a potential biomarker of BSCB and blood–brain barrier leakage. Cervical, thoracic, and lumbar post-mortem spinal cord tissue (n = 5 control, n = 13 ALS) were then immunolabelled and semi-automated imaging and analysis performed to quantify hemoglobin leakage, lower motor neuron loss, and phosphorylated TDP-43 inclusion load. Hemoglobin leakage was observed along the whole ALS spinal cord axis and was most severe in the dorsal gray and white matter in the thoracic spinal cord. In contrast, motor neuron loss and TDP-43 proteinopathy were seen at all three levels of the ALS spinal cord, with most abundant TDP-43 deposition in the anterior gray matter of the cervical and lumbar cord. Our data show that leakage of the BSCB occurs during life, but at end-stage disease the regions with most severe BSCB damage are not those where TDP-43 accumulation is most abundant. This suggests BSCB leakage and TDP-43 pathology are independent pathologies in ALS.

The Skeletal Muscle Emerges as a New Disease Target in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder that leads to progressive degeneration of motor neurons (MNs) and severe muscle atrophy without effective treatment. Most research on ALS has been focused on the study of MNs and supporting cells of the central nervous system. Strikingly, the recent observations of pathological changes in muscle occurring before disease onset and independent from MN degeneration have bolstered the interest for the study of muscle tissue as a potential target for delivery of therapies for ALS. Skeletal muscle has just been described as a tissue with an important secretory function that is toxic to MNs in the context of ALS. Moreover, a fine-tuning balance between biosynthetic and atrophic pathways is necessary to induce myogenesis for muscle tissue repair. Compromising this response due to primary metabolic abnormalities in the muscle could trigger defective muscle regeneration and neuromuscular junction restoration, with deleterious consequences for MNs and thereby hastening the development of ALS. However, it remains puzzling how backward signaling from the muscle could impinge on MN death. This review provides a comprehensive analysis on the current state-of-the-art of the role of the skeletal muscle in ALS, highlighting its contribution to the neurodegeneration in ALS through backward-signaling processes as a newly uncovered mechanism for a peripheral etiopathogenesis of the disease.

Small junction, big problems: Neuromuscular junction pathology in mouse models of amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease with an extremely heterogeneous clinical and genetic phenotype. In our efforts to find therapies for ALS, the scientific community has developed a plethora of mouse models, each with their own benefits and drawbacks. The peripheral nervous system, specifically the neuromuscular junction (NMJ), is known to be affected in ALS patients and shows marked dysfunction across mouse models. Evidence of pathology at the NMJ includes denervated NMJs, changes in endplate size and loss of terminal Schwann cells. This review compares the temporal disease progression with severity of disease at the NMJ in mouse models with the most commonly mutated genes in ALS patients (SOD1, C9ORF72, TARDBP and FUS). Despite variability, early NMJ dysfunction seems to be a common factor in models with SOD1, TARDBP and FUS mutations, while C9ORF72 models do not appear to follow the same pattern of pathology. Further work into determining the timing of NMJ pathology, particularly in newer ALS mouse models, will confirm its pivotal role in ALS pathogenesis and therefore highlight the NMJ as a potential therapeutic target.

Non-neuronal cells in amyotrophic lateral sclerosis - from pathogenesis to biomarkers

The prevailing motor neuron-centric view of amyotrophic lateral sclerosis (ALS) pathogenesis could be an important factor in the failure to identify disease-modifying therapy for this neurodegenerative disorder. Non-neuronal cells have crucial homeostatic functions within the CNS and evidence of involvement of these cells in the pathophysiology of several neurodegenerative disorders, including ALS, is accumulating. Microglia and astrocytes, in crosstalk with peripheral immune cells, can exert both neuroprotective and adverse effects, resulting in a highly nuanced range of neuronal and non-neuronal cell interactions. This Review provides an overview of the diverse roles of non-neuronal cells in relation to the pathogenesis of ALS and the emerging potential of non-neuronal cell biomarkers to advance therapeutic development.

Defective Oligodendroglial Lineage and Demyelination in Amyotrophic Lateral Sclerosis

Motor neurons and their axons reaching the skeletal muscle have long been considered as the best characterized targets of the degenerative process observed in amyotrophic lateral sclerosis (ALS). However, the involvement of glial cells was also more recently reported. Although oligodendrocytes have been underestimated for a longer time than other cells, they are presently considered as critically involved in axonal injury and also conversely constitute a target for the toxic effects of the degenerative neurons. In the present review, we highlight the recent advances regarding oligodendroglial cell involvement in the pathogenesis of ALS. First, we present the oligodendroglial cells, the process of myelination, and the tight relationship between axons and myelin. The histological abnormalities observed in ALS and animal models of the disease are described, including myelin defects and oligodendroglial accumulation of pathological protein aggregates. Then, we present data that establish the existence of dysfunctional and degenerating oligodendroglial cells, the chain of events resulting in oligodendrocyte degeneration, and the most recent molecular mechanisms supporting oligodendrocyte death and dysfunction. Finally, we review the arguments in support of the primary versus secondary involvement of oligodendrocytes in the disease and discuss the therapeutic perspectives related to oligodendrocyte implication in ALS pathogenesis.

Neuromuscular junction-on-a-chip: ALS disease modeling and read-out development in microfluidic devices

Amyotrophic lateral sclerosis (ALS) is a fatal and progressive neurodegenerative disease affecting upper and lower motor neurons with no cure available. Clinical and animal studies reveal that the neuromuscular junction (NMJ), a synaptic connection between motor neurons and skeletal muscle fibers, is highly vulnerable in ALS and suggest that NMJ defects may occur at the early stages of the disease. However, mechanistic insight into how NMJ dysfunction relates to the onset and progression of ALS is incomplete, which hampers therapy development. This is, in part, caused by a lack of robust in vitro models. The ability to combine microfluidic and induced pluripotent stem cell (iPSC) technologies has opened up new avenues for studying molecular and cellular ALS phenotypes in vitro. Microfluidic devices offer several advantages over traditional culture approaches when modeling the NMJ, such as the spatial separation of different cell types and increased control over the cellular microenvironment. Moreover, they are compatible with 3D cell culture, which enhances NMJ functionality and maturity. Here, we review how microfluidic technology is currently being employed to develop more reliable in vitro NMJ models. To validate and phenotype such models, various morphological and functional read-outs have been developed. We describe and discuss the relevance of these read-outs and specifically illustrate how these read-outs have enhanced our understanding of NMJ pathology in ALS. Finally, we share our view on potential future directions and challenges.

Riluzole Exhibits No Therapeutic Efficacy on a Transgenic Rat model of Amyotrophic Lateral Sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a neurological disorder clinically characterized by motor system dysfunction, with intraneuronal accumulation of the TAR DNAbinding protein 43 (TDP-43) being a pathological hallmark. Riluzole is a primarily prescribed medicine for ALS patients, while its therapeutical efficacy appears limited. TDP-43 transgenic mice are existing animal models for mechanistic/translational research into ALS.

METHODS

We developed a transgenic rat model of ALS expressing a mutant human TDP-43 transgene (TDP-43M337V) and evaluated the therapeutic effect of Riluzole on this model. Relative to control, rats with TDP-43M337V expression promoted by the neurofilament heavy subunit (NEF) gene or specifically in motor neurons promoted by the choline acetyltransferase (ChAT) gene showed progressive worsening of mobility and grip strength, along with loss of motor neurons, microglial activation, and intraneuronal accumulation of TDP-43 and ubiquitin aggregations in the spinal cord.

RESULTS

Compared to vehicle control, intragastric administration of Riluzole (30 mg/kg/d) did not mitigate the behavioral deficits nor alter the neuropathologies in the transgenics.

CONCLUSION

These findings indicate that transgenic rats recapitulate the basic neurological and neuropathological characteristics of human ALS, while Riluzole treatment can not halt the development of the behavioral and histopathological phenotypes in this new transgenic rodent model of ALS.

Early-Onset Familial Alzheimer Disease Variant PSEN2 N141I Heterozygosity is Associated with Altered Microglia Phenotype

BACKGROUND

Early-onset familial Alzheimer disease (EOFAD) is caused by heterozygous variants in the presenilin 1 (PSEN1), presenilin 2 (PSEN2), and APP genes. Decades after their discovery, the mechanisms by which these genes cause Alzheimer's disease (AD) or promote AD progression are not fully understood. While it is established that presenilin (PS) enzymatic activity produces amyloid-β (Aβ), PSs also regulate numerous other cellular functions, some of which intersect with known pathogenic drivers of neurodegeneration. Accumulating evidence suggests that microglia, resident innate immune cells in the central nervous system, play a key role in AD neurodegeneration.

OBJECTIVE

Previous work has identified a regulatory role for PS2 in microglia. We hypothesized that PSEN2 variants lead to dysregulated microglia, which could further contribute to disease acceleration. To mimic the genotype of EOFAD patients, we created a transgenic mouse expressing PSEN2 N141I on a mouse background expressing one wildtype PS2 and two PS1 alleles.

RESULTS

Microglial expression of PSEN2 N141I resulted in impaired γ-secretase activity as well as exaggerated inflammatory cytokine release, NFκB activity, and Aβ internalization. In vivo, PS2 N141I mice showed enhanced IL-6 and TREM2 expression in brain as well as reduced branch number and length, an indication of "activated" morphology, in the absence of inflammatory stimuli. LPS intraperitoneal injection resulted in higher inflammatory gene expression in PS2 N141I mouse brain relative to controls.

CONCLUSION

Our findings demonstrate that PSEN2 N141I heterozygosity is associated with disrupted innate immune homeostasis, suggesting EOFAD variants may promote disease progression through non-neuronal cells beyond canonical dysregulated Aβ production.

Properties of Glial Cell at the Neuromuscular Junction Are Incompatible with Synaptic Repair in the SOD1G37R ALS Mouse Model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motoneurons (MNs) in a motor-unit (MU)-dependent manner. Glial dysfunction contributes to numerous aspects of the disease. At the neuromuscular junction (NMJ), early alterations in perisynaptic Schwann cell (PSC), glial cells at this synapse, may impact their ability to regulate NMJ stability and repair. Indeed, muscarinic receptors (mAChRs) regulate the repair phenotype of PSCs and are overactivated at disease-resistant NMJs [soleus muscle (SOL)] in SOD1^G37R mice. However, it remains unknown whether this is the case at disease-vulnerable NMJs and whether it translates into an impairment of PSC-dependent repair mechanisms. We used SOL and sternomastoid (STM) muscles from SOD1^G37R mice and performed Ca²⁺-imaging to monitor PSC activity and used immunohistochemistry to analyze their repair and phagocytic properties. We show that PSC mAChR-dependent activity was transiently increased at disease-vulnerable NMJs (STM muscle). Furthermore, PSCs from both muscles extended disorganized processes from denervated NMJs and failed to initiate or guide nerve terminal sprouts at disease-vulnerable NMJs, a phenomenon essential for compensatory reinnervation. This was accompanied by a failure of numerous PSCs to upregulate galectin-3 (MAC-2), a marker of glial axonal debris phagocytosis, on NMJ denervation in SOD1 mice. Finally, differences in these PSC-dependent NMJ repair mechanisms were MU type dependent, thus reflecting MU vulnerability in ALS. Together, these results reveal that neuron-glia communication is ubiquitously altered at the NMJ in ALS. This appears to prevent PSCs from adopting a repair phenotype, resulting in a maladapted response to denervation at the NMJ in ALS.SIGNIFICANCE STATEMENT Understanding how the complex interplay between neurons and glial cells ultimately lead to the degeneration of motor neurons and loss of motor function is a fundamental question to comprehend amyotrophic lateral sclerosis (ALS). An early and persistent alteration of glial cell activity takes place at the neuromuscular junction (NMJ), the output of motor neurons, but its impact on NMJ repair remains unknown. Here, we reveal that glial cells at disease-vulnerable NMJs often fail to guide compensatory nerve terminal sprouts and to adopt a phagocytic phenotype on denervated NMJs in SOD1^G37R mice. These results show that glial cells at the NMJ elaborate an inappropriate response to NMJ degeneration in a manner that reflects motor-unit (MU) vulnerability and potentially impairs compensatory reinnervation.

The role of TDP-43 mislocalization in amyotrophic lateral sclerosis

Since its discovery as a primary component in cytoplasmic aggregates in post-mortem tissue of patients with Amyotrophic Lateral Sclerosis (ALS), TAR DNA Binding Protein 43 kDa (TDP-43) has remained a central focus to understand the disease. TDP-43 links both familial and sporadic forms of ALS as mutations are causative for disease and cytoplasmic aggregates are a hallmark of nearly all cases, regardless of TDP-43 mutational status. Research has focused on the formation and consequences of cytosolic protein aggregates as drivers of ALS pathology through both gain- and loss-of-function mechanisms. Not only does aggregation sequester the normal function of TDP-43, but these aggregates also actively block normal cellular processes inevitably leading to cellular demise in a short time span. Although there may be some benefit to therapeutically targeting TDP-43 aggregation, this step may be too late in disease development to have substantial therapeutic benefit. However, TDP-43 pathology appears to be tightly linked with its mislocalization from the nucleus to the cytoplasm, making it difficult to decouple the consequences of nuclear-to-cytoplasmic mislocalization from protein aggregation. Studies focusing on the effects of TDP-43 mislocalization have demonstrated both gain- and loss-of-function consequences including altered splicing regulation, over responsiveness to cellular stressors, increases in DNA damage, and transcriptome-wide changes. Additionally, mutations in TARDBP confer a baseline increase in cytoplasmic TDP-43 thus suggesting that small changes in the subcellular localization of TDP-43 could in fact drive early pathology. In this review, we bring forth the theme of protein mislocalization as a key mechanism underlying ALS, by highlighting the importance of maintaining subcellular proteostasis along with the gain- and loss-of-functional consequences when TDP-43 localization is dysregulated. Additional research, focusing on early events in TDP-43 pathogenesis (i.e. to the protein mislocalization stage) will provide insight into disease mechanisms, therapeutic targets, and novel biomarkers for ALS.

FABP7 upregulation induces a neurotoxic phenotype in astrocytes

Fatty acid binding proteins (FABPs) are key regulators of lipid metabolism, energy homeostasis, and inflammation. They participate in fatty acid metabolism by regulating their uptake, transport, and availability of ligands to nuclear receptors. In the adult brain, FABP7 is especially abundant in astrocytes that are rich in cytoplasmic granules originated from damaged mitochondria. Mitochondrial dysfunction and oxidative stress have been implicated in the neurodegenerative process observed in amyotrophic lateral sclerosis (ALS), either as a primary cause or as a secondary component of the pathogenic process. Here we investigated the expression of FABP7 in animal models of human superoxide dismutase 1 (hSOD1)-linked ALS. In the spinal cord of symptomatic mutant hSOD1-expressing mice, FABP7 is upregulated in gray matter astrocytes. Using a coculture model, we examined the effect of increased FABP7 expression in astrocyte-motor neuron interaction. Our data show that FABP7 overexpression directly promotes an NF-κB-driven pro-inflammatory response in nontransgenic astrocytes that ultimately is detrimental for motor neuron survival. Addition of trophic factors, capable of supporting motor neuron survival in pure cultures, did not prevent motor neuron loss in cocultures with FABP7 overexpressing astrocytes. In addition, astrocyte cultures obtained from symptomatic hSOD1-expressing mice display upregulated FABP7 expression. Silencing endogenous FABP7 in these cultures decreases the expression of inflammatory markers and their toxicity toward cocultured motor neurons. Our results identify a key role of FABP7 in the regulation of the inflammatory response in astrocytes and identify FABP7 as a potential therapeutic target to prevent astrocyte-mediated motor neuron toxicity in ALS.

Presynaptic Homeostasis Opposes Disease Progression in Mouse Models of ALS-Like Degeneration: Evidence for Homeostatic Neuroprotection

Progressive synapse loss is an inevitable and insidious part of age-related neurodegenerative disease. Typically, synapse loss precedes symptoms of cognitive and motor decline. This suggests the existence of compensatory mechanisms that can temporarily counteract the effects of ongoing neurodegeneration. Here, we demonstrate that presynaptic homeostatic plasticity (PHP) is induced at degenerating neuromuscular junctions, mediated by an evolutionarily conserved activity of presynaptic ENaC channels in both Drosophila and mouse. To assess the consequence of eliminating PHP in a mouse model of ALS-like degeneration, we generated a motoneuron-specific deletion of Scnn1a, encoding the ENaC channel alpha subunit. We show that Scnn1a is essential for PHP without adversely affecting baseline neural function or lifespan. However, Scnn1a knockout in a degeneration-causing mutant background accelerated motoneuron loss and disease progression to twice the rate observed in littermate controls with intact PHP. We propose a model of neuroprotective homeostatic plasticity, extending organismal lifespan and health span.

The vulnerability of motor and frontal cortex-dependent behaviors in mice expressing ALS-linked mutation in TDP-43

TDP-43 aggregates are the defining pathological hallmark for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Strikingly, these TDP-43 proteinopathies are also found in other neurodegenerative diseases, including Alzheimer's disease and are prevalent in the brains of old-aged humans. Furthermore, disease-causal mutations in TDP-43 have been identified for ALS and FTD. Collectively, the evidence indicates that TDP-43 dysfunctions lead to motor and cognitive deficits. To determine whether the mouse line expressing an ALS-linked mutation in TDP-43 (Q331K) can be used to study ALS-FTD spectrum disorders, we performed a systematic and longitudinal behavioral assessment that covered motor and cognitive functions. Deficits in motor and cognitive abilities were observed as early as 3 months of age and persisted through to 12 months of age. Within the cognitive modalities, the hippocampus-mediated spatial learning and memory, and contextual fear conditioning, were normal; whereas the frontal cortex-mediated working memory and cognitive flexibility were impaired. Biochemically, the human TDP-43 transgene downregulates endogenous mouse TDP-43 mRNA and protein, resulting in human TDP-43 protein that is comparable with the physiological level in cerebral cortex and hippocampus. Furthermore, Q331K TDP-43 is largely retained at the nucleus without apparent aggregates. Taken together, our data suggest that motor and frontal cortex may be more vulnerable to disease-linked mutation in TDP-43 and, this mouse model may be used to assess ALS-FTD-related spectrum diseases and the molecular underpinnings associated with the phenotypes.

IN VITRO AND IN VIVO MODELS OF AMYOTROPHIC LATERAL SCLEROSIS: AN UPDATED OVERVIEW

Amyotrophic Lateral Sclerosis (ALS) is a progressive, neurodegenerative disease characterized by loss of upper motor neurons (UMN) and lower motor neurons (LMN). Disease affects people all over the world and is more prevalent in men. Patients with ALS develop extensive muscle wasting, paralysis and ultimately death, with a median survival of usually fewer than five years after disease onset. ALS may be sporadic (sALS, 90%) or familial (fALS, 10%). The large majority of fALS cases are associated with genetic alterations, which are mainly related to the genes SOD1, TDP-43, FUS, and C9ORF72. In vitro and in vivo models have helped elucidate ALS etiology and pathogenesis, as well as its molecular, cellular, and physiological mechanisms. Many studies in cell cultures and animal models, such as Caenorhabditis elegans, Drosophila melanogaster, zebrafish, rodents, and non-human primates have been performed to clarify the relationship of these genes to ALS disease. However, there are inherent limitations to consider when using experimental models. In this review, we provide an updated overview of the most used in vitro and in vivo studies that have contributed to a better understanding of the different ALS pathogenic mechanisms.

Aberrant Development Corrected in Adult-Onset Huntington's Disease iPSC-Derived Neuronal Cultures via WNT Signaling Modulation

Aberrant neuronal development and the persistence of mitotic cellular populations have been implicated in a multitude of neurological disorders, including Huntington's disease (HD). However, the mechanism underlying this potential pathology remains unclear. We used a modified protocol to differentiate induced pluripotent stem cells (iPSCs) from HD patients and unaffected controls into neuronal cultures enriched for medium spiny neurons, the cell type most affected in HD. We performed single-cell and bulk transcriptomic and epigenomic analyses and demonstrated that a persistent cyclin D1+ neural stem cell (NSC) population is observed selectively in adult-onset HD iPSCs during differentiation. Treatment with a WNT inhibitor abrogates this NSC population while preserving neurons. Taken together, our findings identify a mechanism that may promote aberrant neurodevelopment and adult neurogenesis in adult-onset HD striatal neurons with the potential for therapeutic compensation.

Role of RNA Binding Proteins with prion-like domains in muscle and neuromuscular diseases

A number of neuromuscular and muscular diseases, including amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and several myopathies, are associated to mutations in related RNA-binding proteins (RBPs), including TDP-43, FUS, MATR3 or hnRNPA1/B2. These proteins harbor similar modular primary sequence with RNA binding motifs and low complexity domains, that enables them to phase separate and create liquid microdomains. These RBPs have been shown to critically regulate multiple events of RNA lifecycle, including transcriptional events, splicing and RNA trafficking and sequestration. Here, we review the roles of these disease-related RBPs in muscle and motor neurons, and how their dysfunction in these cell types might contribute to disease.

The debated toxic role of aggregated TDP-43 in amyotrophic lateral sclerosis: a resolution in sight?

Transactive response DNA-binding protein-43 (TDP-43) is an RNA/DNA binding protein that forms phosphorylated and ubiquitinated aggregates in the cytoplasm of motor neurons in amyotrophic lateral sclerosis, which is a hallmark of this disease. Amyotrophic lateral sclerosis is a neurodegenerative condition affecting the upper and lower motor neurons. Even though the aggregative property of TDP-43 is considered a cornerstone of amyotrophic lateral sclerosis, there has been major controversy regarding the functional link between TDP-43 aggregates and cell death. In this review, we attempt to reconcile the current literature surrounding this debate by discussing the results and limitations of the published data relating TDP-43 aggregates to cytotoxicity, as well as therapeutic perspectives of TDP-43 aggregate clearance. We point out key data suggesting that the formation of TDP-43 aggregates and the capacity to self-template and propagate among cells as a 'prion-like' protein, another pathological property of TDP-43 aggregates, are a significant cause of motor neuronal death. We discuss the disparities among the various studies, particularly with respect to the type of models and the different forms of TDP-43 used to evaluate cellular toxicity. We also examine how these disparities can interfere with the interpretation of the results pertaining to a direct toxic effect of TDP-43 aggregates. Furthermore, we present perspectives for improving models in order to better uncover the toxic role of aggregated TDP-43. Finally, we review the recent studies on the enhancement of the cellular clearance mechanisms of autophagy, the ubiquitin proteasome system, and endocytosis in an attempt to counteract TDP-43 aggregation-induced toxicity. Altogether, the data available so far encourage us to suggest that the cytoplasmic aggregation of TDP-43 is key for the neurodegeneration observed in motor neurons in patients with amyotrophic lateral sclerosis. The corresponding findings provide novel avenues toward early therapeutic interventions and clinical outcomes for amyotrophic lateral sclerosis management.

A robust TDP-43 knock-in mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset degenerative disorder of motor neurons. The diseased spinal cord motor neurons of more than 95% of amyotrophic lateral sclerosis (ALS) patients are characterized by the mis-metabolism of the RNA/DNA-binding protein TDP-43 (ALS-TDP), in particular, the presence of cytosolic aggregates of the protein. Most available mouse models for the basic or translational studies of ALS-TDP are based on transgenic overexpression of the TDP-43 protein. Here, we report the generation and characterization of mouse lines bearing homologous knock-in of fALS-associated mutation A315T and sALS-associated mutation N390D, respectively. Remarkably, the heterozygous TDP-43 (N390D/+) mice but not those heterozygous for the TDP-43 (A315T/+) mice develop a full spectrum of ALS-TDP-like pathologies at the molecular, cellular and behavioral levels. Comparative analysis of the mutant mice and spinal cord motor neurons (MN) derived from their embryonic stem (ES) cells demonstrates that different ALS-associated TDP-43 mutations possess critical ALS-causing capabilities and pathogenic pathways, likely modified by their genetic background and the environmental factors. Mechanistically, we identify aberrant RNA splicing of spinal cord Bcl-2 pre-mRNA and consequent increase of a negative regulator of autophagy, Bcl-2, which correlate with and are caused by a progressive increase of TDP-43, one of the early events associated with ALS-TDP pathogenesis, in the spinal cord of TDP-43 (N390D/+) mice and spinal cord MN derived from their ES cells. The TDP-43 (N390D/+) knock-in mice appear to be an ideal rodent model for basic as well as translational studies of ALS- TDP.

Evidence for a protective role of the CX3CL1/CX3CR1 axis in a model of amyotrophic lateral sclerosis

Aberrant microglial activation and neuroinflammation is a pathological hallmark of amyotrophic lateral sclerosis (ALS). Fractalkine (CX3CL1) is mostly expressed on neuronal cells. The fractalkine receptor (CX3CR1) is predominantly expressed on microglia. Many progressive neuroinflammatory disorders show disruption of the CX3CL1/CX3CR1 communication system. But the exact role of the CX3CL1/CX3CR1 in ALS pathology remains unknown. F1 nontransgenic/CX3CR1+/- females were bred with SOD1G93A/CX3CR1+/- males to produce F2 SOD1G93A/CX3CR1-/-, SOD1G93A/CX3CR1+/+. We analyzed end-stage (ES) SOD1G93A/CX3CR1-/- mice and progression-matched SOD1G93A/CX3CR1+/+ mice. Our study showed that the male SOD1G93A/CX3CR1-/- mice died sooner than male SOD1G93A/CX3CR1+/+ mice. In SOD1G93A/CX3CR1-/- mice demonstrated more neuronal cell loss, more microglial activation and exacerbated SOD1 aggregation at the end-stage of ALS. The NF-κB pathway was activated; the autophagy-lysosome degradation pathway and the autophagosome maturation were impaired. Our results indicated that the absence of CX3CR1/CX3CL1 signaling in the central nervous system (CNS) may worsen neurodegeneration. The CX3CL1/CX3CR1 communication system has anti-inflammatory and neuroprotective effects and plays an important role in maintaining autophagy activity. This effort may lead to new therapeutic strategies for neuroprotection and provide a therapeutic target for ALS patients.

Examining the relationship between astrocyte dysfunction and neurodegeneration in ALS using hiPSCs

Amyotrophic lateral sclerosis (ALS) is a complex and fatal neurodegenerative disease for which the causes of disease onset and progression remain unclear. Recent advances in human induced pluripotent stem cell (hiPSC)-based models permit the study of the genetic factors associated with ALS in patient-derived neural cell types, including motor neurons and glia. While astrocyte dysfunction has traditionally been thought to exacerbate disease progression, astrocytic dysfunction may play a more direct role in disease initiation and progression. Such non-cell autonomous mechanisms expand the potential targets of therapeutic intervention, but only a handful of ALS risk-associated genes have been examined for their impact on astrocyte dysfunction and neurodegeneration. This review summarizes what is currently known about astrocyte function in ALS and suggests ways in which hiPSC-based models can be used to more effectively study the role of astrocytes in neurodegenerative disease.

Motor dysfunction and neurodegeneration in a C9orf72 mouse line expressing poly-PR

A GGGGCC hexanucleotide repeat expansion in intron 1 of chromosome 9 open reading frame 72 (C9ORF72) gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Repeat-associated non-ATG translation of dipeptide repeat proteins (DPRs) contributes to the neuropathological features of c9FTD/ALS. Among the five DPRs, arginine-rich poly-PR are reported to be the most toxic. Here, we generate a transgenic mouse line that expresses poly-PR (GFP-PR₂₈) specifically in neurons. GFP-PR₂₈ homozygous mice show decreased survival time, while the heterozygous mice show motor imbalance, decreased brain weight, loss of Purkinje cells and lower motor neurons, and inflammation in the cerebellum and spinal cord. Transcriptional analysis shows that in the cerebellum, GFP-PR₂₈ heterozygous mice show differential expression of genes related to synaptic transmission. Our findings show that GFP-PR₂₈ transgenic mice partly model neuropathological features of c9FTD/ALS, and show a role for poly-PR in neurodegeneration.

Could an Impairment in Local Translation of mRNAs in Glia be Contributing to Pathogenesis in ALS?

One of the key pathways implicated in amyotrophic lateral sclerosis (ALS) pathogenesis is abnormal RNA processing. Studies to date have focussed on defects in RNA stability, splicing, and translation, but this review article will focus on the largely overlooked RNA processing mechanism of RNA trafficking, with particular emphasis on the importance of glia. In the central nervous system (CNS), oligodendrocytes can extend processes to myelinate and metabolically support up to 50 axons and astrocytes can extend processes to cover up to 100,000 synapses, all with differing local functional requirements. Furthermore, many of the proteins required in these processes are large, aggregation-prone proteins which would be difficult to transport in their fully translated, terminally-folded state. This, therefore, highlights a critical requirement in these cells for local control of protein translation, which is achieved through specific trafficking of mRNAs to each process and local translation therein. Given that a large number of RNA-binding proteins have been implicated in ALS, and RNA-binding proteins are essential for trafficking mRNAs from the nucleus to glial processes for local translation, RNA misprocessing in glial cells is a likely source of cellular dysfunction in ALS. To date, neurons have been the focus of ALS research, but an intrinsic deficit in glia, namely astrocytes and oligodendrocytes, could have an additive effect on declining neuronal function in ALS. This review article aims to highlight the key evidence that supports the contention that RNA trafficking deficits in astrocytes and oligodendrocytes may contribute to in ALS.

Circuit Dysfunction in SOD1-ALS Model First Detected in Sensory Feedback Prior to Motor Neuron Degeneration Is Alleviated by BMP Signaling

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease for which the origin and underlying cellular defects are not fully understood. Although motor neuron degeneration is the signature feature of ALS, it is not clear whether motor neurons or other cells of the motor circuit are the site of disease initiation. To better understand the contribution of multiple cell types in ALS, we made use of a Drosophila Sod1G85R knock-in model, in which all cells harbor the disease allele. End-stage dSod1G85R animals of both sexes exhibit severe motor deficits with clear degeneration of motor neurons. Interestingly, earlier in dSod1G85R larvae, motor function is also compromised, but their motor neurons exhibit only subtle morphological and electrophysiological changes that are unlikely to cause the observed decrease in locomotion. We analyzed the intact motor circuit and identified a defect in sensory feedback that likely accounts for the altered motor activity of dSod1G85R We found cell-autonomous activation of bone morphogenetic protein signaling in proprioceptor sensory neurons which are critical for the relay of the contractile status of muscles back to the central nerve cord, completely rescues early-stage motor defects and partially rescue late-stage motor function to extend lifespan. Identification of a defect in sensory feedback as a potential initiating event in ALS motor dysfunction, coupled with the ability of modified proprioceptors to alleviate such motor deficits, underscores the critical role that nonmotor neurons play in disease progression and highlights their potential as a site to identify early-stage ALS biomarkers and for therapeutic intervention.SIGNIFICANCE STATEMENT At diagnosis, many cellular processes are already disrupted in the amyotrophic lateral sclerosis (ALS) patient. Identifying the initiating cellular events is critical for achieving an earlier diagnosis to slow or prevent disease progression. Our findings indicate that neurons relaying sensory information underlie early stage motor deficits in a Drosophila knock-in model of ALS that best replicates gene dosage in familial ALS (fALS). Importantly, studies on intact motor circuits revealed defects in sensory feedback before evidence of motor neuron degeneration. These findings strengthen our understanding of how neural circuit dysfunctions lead to neurodegeneration and, coupled with our demonstration that the activation of bone morphogenetic protein signaling in proprioceptors alleviates both early and late motor dysfunction, underscores the importance of considering nonmotor neurons as therapeutic targets.

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

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

CNS-derived extracellular vesicles from superoxide dismutase 1 (SOD1)G93A ALS mice originate from astrocytes and neurons and carry misfolded SOD1

Extracellular vesicles (EVs) are secreted by myriad cells in culture and also by unicellular organisms, and their identification in mammalian fluids suggests that EV release also occurs at the organism level. However, although it is clearly important to better understand EVs' roles in organismal biology, EVs in solid tissues have received little attention. Here, we modified a protocol for EV isolation from primary neural cell culture to collect EVs from frozen whole murine and human neural tissues by serial centrifugation and purification on a sucrose gradient. Quantitative proteomics comparing brain-derived EVs from nontransgenic (NTg) and a transgenic amyotrophic lateral sclerosis (ALS) mouse model, superoxide dismutase 1 (SOD1)^G93A, revealed that these EVs contain canonical exosomal markers and are enriched in synaptic and RNA-binding proteins. The compiled brain EV proteome contained numerous proteins implicated in ALS, and EVs from SOD1^G93A mice were significantly depleted in myelin-oligodendrocyte glycoprotein compared with those from NTg animals. We observed that brain- and spinal cord-derived EVs, from NTg and SOD1^G93A mice, are positive for the astrocyte marker GLAST and the synaptic marker SNAP25, whereas CD11b, a microglial marker, was largely absent. EVs from brains and spinal cords of the SOD1^G93A ALS mouse model, as well as from human SOD1 familial ALS patient spinal cord, contained abundant misfolded and nonnative disulfide-cross-linked aggregated SOD1. Our results indicate that CNS-derived EVs from an ALS animal model contain pathogenic disease-causing proteins and suggest that brain astrocytes and neurons, but not microglia, are the main EV source.

An emerging role of dysfunctional axon-oligodendrocyte coupling in neurodegenerative diseases

Myelin is made by highly specialized glial cells and enables fast axonal impulse propagation. Recent studies show that oligodendrocytes in the central nervous system are, in addition to myelination, required for the integrity and survival of axons, independent of the presence or absence of myelin itself. The underlying mechanism of this support is given by glycolytic oligodendrocytes which provide axons with energy-rich metabolites. These findings represent a paradigm shift for the physiological function of axon-associated glia, and open the intriguing possibility that oligodendrocytes are important contributors to neurodegenerative diseases in which myelinated axons are lost, such as in Alzheimer disease, amyotrophic lateral sclerosis, and multiple system atrophy. Understanding the role of axon-oligodendrocyte coupling in neurodegenerative diseases may pave the way for the development of metabolism-based therapeutic approaches.

RNA Splicing and Disease: Animal Models to Therapies

Alternative splicing of pre-mRNA increases genetic diversity, and recent studies estimate that most human multiexon genes are alternatively spliced. If this process is not highly regulated and accurate, it leads to mis-splicing events, which may result in proteins with altered function. A growing body of work has implicated mis-splicing events in a range of diseases, including cancer, neurodegenerative diseases, and muscular dystrophies. Understanding the mechanisms that cause aberrant splicing events and how this leads to disease is vital for designing effective therapeutic strategies. In this review, we focus on advances in therapies targeting splicing, and highlight the animal models developed to recapitulate disease phenotypes as a model for testing these therapies.

From Mouse Models to Human Disease: An Approach for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal adult-onset neurodegenerative disorder. There are several genetic mutations that lead to ALS development, such as chromosome 9 hexanucleotide repeat 72 (C9ORF72), transactive response DNA-binding protein (TARDBP), superoxide dismutase 1 (SOD1) and fused in sarcoma (FUS). ALS is associated with disrupted gene homeostasis causing aberrant RNA processing or toxic pathology. Several animal models of ALS disease have been developed to understand whether TARDBP-mediated neurodegeneration results from a gain or a loss of function of the protein, however, none exactly mimic the pathophysiology and the phenotype of human ALS. Here, the pathophysiology of specific ALS-linked gene mutations is discussed. Furthermore, some of the generated mouse models, as well as the similarities and differences between these models, are comprehensively reviewed. Further refinement of mouse models will likely aid the development of a better form of model that mimics human ALS. However, disrupted gene homeostasis that causes mutation can result in an ALS-like syndrome, increasing concerns about whether neurodegeneration and other effects in these models are due to the mutation or to gene overexpression. Research on the pleiotropic role of different proteins present in motor neurons is also summarized. The development of better mouse models that closely mimic human ALS will help identify potential therapeutic targets for this disease.

SecinH3 Attenuates TDP-43 p.Q331K-Induced Neuronal Toxicity by Suppressing Endoplasmic Reticulum Stress and Enhancing Autophagic Flux

Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset, neurodegenerative disease. The transactivating response region DNA binding protein 43 (TDP-43) p.Q331K mutation (TDP-43 Q331K) has previously been identified in ALS as a disease-causing mutation with neurotoxicity. SecinH3, a cytohesin inhibitor, has neuroprotective effects against mutant superoxide dismutase 1 (SOD1) toxicity. However, whether SecinH3 protects against mutant TDP-43 p.Q331K protein toxicity and its potential molecular mechanisms have not yet been investigated. To determine whether TDP-43 Q331K induces neuronal toxicity, SH-SY5Y, a human derived neuronal cell line were selected as an in vitro model of neuronal function. SH-SY5Y cells were transiently transfected with TDP-43 wild-type or TDP-43 Q331K. Remarkably, TDP-43 Q331K induced neuronal damage via endoplasmic reticulum (ER) stress-mediated apoptosis and the impairment of the autophagic flux. SecinH3 was demonstrated to successfully attenuate the TDP-43 Q331K-induced neuronal toxicity by suppressing ER stress-mediated apoptosis and enhancing the autophagic flux. Taken together, our in vitro study provided evidence that SecinH3 exerts neuroprotective effects against TDP-43 Q331K-mediated neuronal toxicity and was able to elucidate its mode of action. SecinH3 could, therefore, be considered a promising candidate as a therapeutic agent of ALS. © 2018 IUBMB Life, 71(1):192-199, 2019.

Cell-autonomous requirement of TDP-43, an ALS/FTD signature protein, for oligodendrocyte survival and myelination

TDP-43 aggregates in neurons and glia are the defining pathological hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), raising the possibility of glial damage in the disease pathogenesis. However, the normal physiological functions of TDP-43 in glia are largely unknown. To address how TDP-43 may be required for oligodendroglial functions we selectively deleted TDP-43 in mature oligodendrocytes in mice. Although mice with TDP-43 deleted in oligodendrocytes are born in an expected Mendelian ratio, they develop progressive neurological phenotypes leading to early lethality accompanied by a progressive reduction in myelination. The progressive myelin reduction is likely due to a combination of the cell-autonomous RIPK1-mediated necroptosis of mature oligodendrocytes and the TDP-43-dependent reduction in the expression of myelin genes. Strikingly, enhanced proliferation of NG2-positive oligodendrocyte precursor cells within the white matter, but not the gray matter, was able to replenish the loss of mature oligodendrocytes, indicating an intrinsic regeneration difference between the gray and white matter oligodendrocytes. By contrast, there was no loss of spinal cord motor neurons and no sign of denervation at the neuromuscular synapses. Taken together, our data demonstrate that TDP-43 is indispensable for oligodendrocyte survival and myelination, and loss of TDP-43 in oligodendrocytes exerts no apparent toxicity on motor neurons.

FTLD/ALS-linked TDP-43 mutations do not alter TDP-43's ability to self-regulate its expression in Drosophila

TDP-43 is a major disease-causing protein in amyotrophic lateral sclerosis (ALS) and Frontotemporal Lobar Degeneration (FTLD). Today, >50 missense mutations in the TARDBP/TDP-43 gene have been described in patients with FTLD/ALS. However, the functional consequences of FTLD/ALS-linked TDP-43 mutations are not fully elucidated. In the physiological state, TDP-43 expression is tightly regulated through an autoregulatory negative feedback loop. Maintaining normal TDP-43 protein levels is critical for proper physiological functions of the cells. In the present study, we investigated whether the FTLD/ALS-associated mutations could interfere with TDP-43 protein's capacity to modulate its own protein levels using Drosophila as an experimental model. Our data show that FTLD/ALS-associated mutant proteins regulate TDP-43 production with the same efficiency as the wild-type form of the protein. Thus, FTLD/ALS-linked TDP-43 mutations do not alter TDP-43's ability to self-regulate its expression and consequently of the homeostasis of TDP-43 protein levels.

Role of Extracellular Vesicles in Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is the most common motor neuron disease in adults and primarily targets upper and lower motor neurons. The progression of the disease is mostly mediated by altered intercellular communication in the spinal cord between neurons and glial cells. One of the possible ways by which intercellular communication occurs is through extracellular vesicles (EVs) that are responsible for the horizontal transfer of proteins and RNAs to recipient cells. EVs are nanoparticles released by the plasma membrane and this review will describe all evidence connecting ALS, intercellular miscommunication and EVs. We mainly focus on mutant proteins causing ALS and their accumulation in EVs, along with the propensity of mutant proteins to misfold and propagate through EVs in prion-like behavior. EVs are a promising source of biomarkers and the state of the art in ALS will be discussed along with the gaps and challenges still present in this blooming field of investigation.

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.