A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Amyloid-induced neurodegeneration: A comprehensive review through aggregomics perception of proteins in health and pathology

Amyloidosis of protein caused by fibrillation and aggregation are some of the most exciting new edges not only in protein sciences but also in molecular medicines. The present review discusses recent advancements in the field of neurodegenerative diseases and therapeutic applications with ongoing clinical trials, featuring new areas of protein misfolding resulting in aggregation. The endogenous accretion of protein fibrils having fibrillar morphology symbolizes the beginning of neuro-disorders. Prognostic amyloidosis is prominent in numerous degenerative infections such as Alzheimer's and Parkinson's disease, Amyotrophic lateral sclerosis (ALS), etc. However, the molecular basis determining the intracellular or extracellular evidence of aggregates, playing a significant role as a causative factor in neurodegeneration is still unclear. Structural conversions and protein self-assembly resulting in the formation of amyloid oligomers and fibrils are important events in the pathophysiology of the disease. This comprehensive review sheds light on the evolving landscape of potential treatment modalities, highlighting the ongoing clinical trials and the potential socio-economic impact of novel therapeutic interventions in the realm of neurodegenerative diseases. Furthermore, many drugs are undergoing different levels of clinical trials that would certainly help in treating these disorders and will surely improve the socio-impact of human life.

TUBA4A downregulation as observed in ALS post-mortem motor cortex causes ALS-related abnormalities in zebrafish

Disease-associated variants of TUBA4A (alpha-tubulin 4A) have recently been identified in familial ALS. Interestingly, a downregulation of TUBA4A protein expression was observed in familial as well as sporadic ALS brain tissue. To investigate whether a decreased TUBA4A expression could be a driving factor in ALS pathogenesis, we assessed whether TUBA4A knockdown in zebrafish could recapitulate an ALS-like phenotype. For this, we injected an antisense oligonucleotide morpholino in zebrafish embryos targeting the zebrafish TUBA4A orthologue. An antibody against synaptic vesicle 2 was used to visualize motor axons in the spinal cord, allowing the analysis of embryonic ventral root projections. Motor behavior was assessed using the touch-evoked escape response. In post-mortem ALS motor cortex, we observed reduced TUBA4A levels. The knockdown of the zebrafish TUBA4A orthologue induced a motor axonopathy and a significantly disturbed motor behavior. Both phenotypes were dose-dependent and could be rescued by the addition of human wild-type TUBA4A mRNA. Thus, TUBA4A downregulation as observed in ALS post-mortem motor cortex could be modeled in zebrafish and induced a motor axonopathy and motor behavior defects reflecting a motor neuron disease phenotype, as previously described in embryonic zebrafish models of ALS. The rescue with human wild-type TUBA4A mRNA suggests functional conservation and strengthens the causal relation between TUBA4A protein levels and phenotype severity. Furthermore, the loss of TUBA4A induces significant changes in post-translational modifications of tubulin, such as acetylation, detyrosination and polyglutamylation. Our data unveil an important role for TUBA4A in ALS pathogenesis, and extend the relevance of TUBA4A to the majority of ALS patients, in addition to cases bearing TUBA4A mutations.

PEPITEM Treatment Ameliorates EAE in Mice by Reducing CNS Inflammation, Leukocyte Infiltration, Demyelination, and Proinflammatory Cytokine Production

To investigate the effect of the therapeutic treatment of the immunopeptide, peptide inhibitor of trans-endothelial migration (PEPITEM) on the severity of disease in a mouse model of experimental autoimmune encephalomyelitis (EAE) as a model for human multiple sclerosis (MS), a series of experiments were conducted. Using C57BL/6 female mice, we dosed the PEPITEM in the EAE model via IP after observing the first sign of inflammation. The disease was induced using MOG35-55 and complete Freund's adjuvants augmented with pertussis toxin. The EAE score was recorded daily until the end of the experiment (21 days). The histological and immunohistochemistry analysis was conducted on the spinal cord sections. A Western blot analysis was performed to measure the protein concentration of MBP, MAP-2, and N-Cadherin, and ELISA kits were used to measure IL-17 and FOXP3 in the serum and spinal cord lysate. The therapeutic treatment with PEPITEM reduced the CNS infiltration of T cells, and decreased levels of the protein concertations of MBP, MAP-2, and N-Cadherin were observed, in addition to reduced concertations of IL-17 and FOXP3. Using PEPITEM alleviated the severity of the symptoms in the EAE model. Our study revealed the potential of PEPITEM to control inflammation in MS patients and to reduce the harmful effects of synthetic drugs.

Preliminary PET Imaging of Microtubule-Based PET Radioligand [11C]MPC-6827 in a Nonhuman Primate Model of Alzheimer's Disease

The microtubule (MT) instability observed in Alzheimer's disease (AD) is commonly attributed to hyperphosphorylation of the MT-associated protein, tau. In vivo PET imaging offers an opportunity to gain critical information about MT changes with the onset and development of AD and related dementia. We developed the first brain-penetrant MT PET ligand, [11C]MPC-6827, and evaluated its in vivo imaging utility in vervet monkeys. Consistent with our previous in vitro cell uptake and in vivo rodent imaging experiments, [11C]MPC-6827 uptake increased with MT destabilization. Radioactive uptake was inversely related to (cerebrospinal fluid) CSF Aβ42 levels and directly related to age in a nonhuman primate (NHP) model of AD. Additionally, in vitro autoradiography studies also corroborated PET imaging results. Here, we report the preliminary results of PET imaging with [11C]MPC-6827 in four female vervet monkeys with high or low CSF Aβ42 levels, which have been shown to correlate with the Aβ plaque burden, similar to humans.

Proteomic investigation of ALS motor cortex identifies known and novel pathogenetic mechanisms

The key pathological feature in ALS is death of motor neurones from the brain and spinal cord, but the molecular mechanisms underlying this degeneration remain unknown. Quantifying the motor cortex proteome in autopsy brain and comparing tissues from ALS cases and non-ALS controls is critical to understanding these mechanisms. We used Sequential Window Acquisition of All Theoretical Mass Spectra (SWATH-MS) to characterize the proteomes of the motor cortex from ALS cases (n = 8) and control subjects (n = 8). A total of 1427 proteins were identified at a critical local false discovery rate < 5%; 187 of these exhibited significant expression differences between ALS cases and controls. Of these, 91 proteins were significantly upregulated and 96 proteins were significantly downregulated. Bioinformatics analysis revealed that these proteins are involved in molecular transport, protein trafficking, free radical scavenging, lipid metabolism, cell death and survival, nucleic acid metabolism, inflammatory response or amino acid metabolism and carbohydrate metabolism. Differentially expressed proteins were subjected to pathway analysis. This revealed abnormalities in pathways involving mitochondrial function, sirtuin signaling, oxidative phosphorylation, glycolysis, phagosome maturation, SNARE signaling, redox regulation and several others. Core analysis revealed mitochondrial dysfunction to be the top canonical pathway. The top-enriched networks involved JNK activation and inhibition of AKT signaling, suggesting that disruption of these signaling pathways could lead to demise of motor neurons in the ALS motor cortex.

A cellular taxonomy of the adult human spinal cord

The mammalian spinal cord functions as a community of cell types for sensory processing, autonomic control, and movement. While animal models have advanced our understanding of spinal cellular diversity, characterizing human biology directly is important to uncover specialized features of basic function and human pathology. Here, we present a cellular taxonomy of the adult human spinal cord using single-nucleus RNA sequencing with spatial transcriptomics and antibody validation. We identified 29 glial clusters and 35 neuronal clusters, organized principally by anatomical location. To demonstrate the relevance of this resource to human disease, we analyzed spinal motoneurons, which degenerate in amyotrophic lateral sclerosis (ALS) and other diseases. We found that compared with other spinal neurons, human motoneurons are defined by genes related to cell size, cytoskeletal structure, and ALS, suggesting a specialized molecular repertoire underlying their selective vulnerability. We include a web resource to facilitate further investigations into human spinal cord biology.

Cytoplasmic forces functionally reorganize nuclear condensates in oocytes

Cells remodel their cytoplasm with force-generating cytoskeletal motors. Their activity generates random forces that stir the cytoplasm, agitating and displacing membrane-bound organelles like the nucleus in somatic and germ cells. These forces are transmitted inside the nucleus, yet their consequences on liquid-like biomolecular condensates residing in the nucleus remain unexplored. Here, we probe experimentally and computationally diverse nuclear condensates, that include nuclear speckles, Cajal bodies, and nucleoli, during cytoplasmic remodeling of female germ cells named oocytes. We discover that growing mammalian oocytes deploy cytoplasmic forces to timely impose multiscale reorganization of nuclear condensates for the success of meiotic divisions. These cytoplasmic forces accelerate nuclear condensate collision-coalescence and molecular kinetics within condensates. Disrupting the forces decelerates nuclear condensate reorganization on both scales, which correlates with compromised condensate-associated mRNA processing and hindered oocyte divisions that drive female fertility. We establish that cytoplasmic forces can reorganize nuclear condensates in an evolutionary conserved fashion in insects. Our work implies that cells evolved a mechanism, based on cytoplasmic force tuning, to functionally regulate a broad range of nuclear condensates across scales. This finding opens new perspectives when studying condensate-associated pathologies like cancer, neurodegeneration and viral infections.

Cytoskeletal activity generates mechanical forces known to agitate and displace membrane-bound organelles in the cytoplasm. In oocytes, Al Jord et al. discover that these cytoplasmic forces functionally remodel nuclear RNA-processing condensates across scales for developmental success.

Potential proteomic alteration in the brain of Tg(SOD1*G93A)1Gur mice: A new pathogenesis insight of amyotrophic lateral sclerosis

The pathogenesis of amyotrophic lateral sclerosis (ALS) remains unclear. The recent studies have suggested that the protein abnormalities could play some important roles in ALS because several protein mutations were found in individuals with this disease. However, proteins that are currently known to be associated with ALS only explain the pathogenesis of this disease in a minority of cases, thus, further screening is needed to identify other ALS-related proteins. In this study, we systematically analyzed and compared the brain proteomic alterations between a mouse model of ALS, the Tg(SOD1*G93A)1Gur model, and wild-type mice using isobaric tags for relative and absolute quantitation (iTRAQ) as well as bioinformatics methods. The results revealed some significant up- and downregulated proteins at the different developmental stages in the ALS-like mice as well as the possibly related cellular components, molecular functions, biological processes, and pathways in the development of ALS. Our results identified some possible proteins that participate in the pathogenesis of ALS as well as the cellular components that are damaged by these proteins, we additionally identified the molecular functions, the biological processes, and the pathways of these proteins as well as the molecules that are associated with these pathways. This study represents an important preliminary investigation of the role of proteomic abnormalities in the pathogenesis of ALS, both in human patients and other animal models. We present some novel findings that may serve as a basis for further investigation of abnormal proteins that are involved in the pathogenesis of ALS.

Autophagy Dysfunction in ALS: from Transport to Protein Degradation

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting upper and lower motor neurons (MNs). Since the identification of the first ALS mutation in 1993, more than 40 genes have been associated with the disorder. The most frequent genetic causes of ALS are represented by mutated genes whose products challenge proteostasis, becoming unable to properly fold and consequently aggregating into inclusions that impose proteotoxic stress on affected cells. In this context, increasing evidence supports the central role played by autophagy dysfunctions in the pathogenesis of ALS. Indeed, in early stages of disease, high levels of proteins involved in autophagy are present in ALS MNs; but at the same time, with neurodegeneration progression, autophagy-mediated degradation decreases, often as a result of the accumulation of toxic protein aggregates in affected cells. Autophagy is a complex multistep pathway that has a central role in maintaining cellular homeostasis. Several proteins are involved in its tight regulation, and importantly a relevant fraction of ALS-related genes encodes products that directly take part in autophagy, further underlining the relevance of this key protein degradation system in disease onset and progression. In this review, we report the most relevant findings concerning ALS genes whose products are involved in the several steps of the autophagic pathway, from phagophore formation to autophagosome maturation and transport and finally to substrate degradation.

Cytoskeleton saga: Its regulation in normal physiology and modulation in neurodegenerative disorders

Cells are fundamental units of life. To ensure the maintenance of homeostasis, integrity of structural and functional counterparts is needed to be essentially balanced. The cytoskeleton plays a vital role in regulating the cellular morphology, signalling and other factors involved in pathological conditions. Microtubules, actin (microfilaments), intermediate filaments (IF) and their interactions are required for these activities. Various proteins associated with these components are primary requirements for directing their functions. Disruption of this organization due to faulty genetics, oxidative stress or impaired transport mechanisms are the major causes of dysregulated signalling cascades leading to various pathological conditions like Alzheimer's (AD), Parkinson's (PD), Huntington's disease (HD) or amyotrophic lateral sclerosis (ALS), hereditary spastic paraplegia (HSP) or any traumatic injury like spinal cord injury (SCI). Novel or conventional therapeutic approaches may be specific or non-specific, targeting either three basic components of the cytoskeleton or various cascades that serve as a cue to numerous pathways like ROCK signalling or the GSK-3β pathway. An enormous number of drugs have been redirected for modulating the cytoskeletal dynamics and thereby may pave the way for inhibiting the progression of these diseases and their complications.

Preclinical evaluation of a microtubule PET ligand [11C]MPC-6827 in tau and amyotrophic lateral sclerosis animal models

BACKGROUND

Microtubules are abundant in brain and their malfunctioning occurs in the early-to-advanced stages of neurodegenerative disorders. At present, there is no in vivo test available for a definitive diagnosis of most of the neurodegenerative disorders. Herein, we present the microPET imaging of microtubules using our recently reported Positron Emission Tomography (PET) tracer, [¹¹C]MPC-6827, in transgenic mice models of tau pathology (rTg4510) and amyotrophic lateral sclerosis pathology (SOD1*G93A) and compared to corresponding age-matched controls.

METHODS

Automated synthesis of [¹¹C]MPC-6827 was achieved in a GE-FX2MeI/FX2M radiochemistry module. In vivo PET imaging studies of [¹¹C]MPC-6827 (3.7 ± 0.8 MBq) were performed in rTg4510 and SOD1*G93A mice groups and their corresponding littermates (n = 5 per group). Dynamic PET images were acquired using a microPET Inveon system (Siemens, Germany) at 55 min for rTg4510 and 30 min for SOD1*G93A and corresponding controls. PET images were reconstructed using the 3D-OSEM algorithm and analyzed using VivoQuant version 4 (Invicro, MA). Tracer uptake in ROIs that included whole brain was measured as %ID/g over time to generate standardized uptake values (SUV) and time-activity curves (TACs).

RESULTS

[¹¹C]MPC-6827 exhibit a trend of lower tracer binding in mouse models of Alzheimer's disease (tau pathology, line rTg4510) and Amyotrophic Lateral Sclerosis (line SOD1*G93A) compared to wild-type littermates.

CONCLUSIONS

Our finding indicates a trend of loss of microtubule binding of [¹¹C]MPC-6827 in the whole brain of AD and ALS transgenic mice models compared to control mice. The pilot studies described herein show that [¹¹C]MPC-6827 could be used as a PET ligand for preclinical and human brain imaging of Alzheimer's disease, ALS, and other neurodegenerative diseases. Preclinical Evaluation of a Microtubule PET Ligand [¹¹C]MPC-6827 in Tau and Amyotrophic Lateral Sclerosis Animal Models. J. S. Dileep Kumar, Andrei Molotkov, Jongho Kim, Patrick Carberry, Sidney Idumonyi, John Castrillon, Karen Duff, Neil A. Shneider, Akiva Mintz.

Frontotemporal Lobar Degeneration Case with an N-Terminal TUBA4A Mutation Exhibits Reduced TUBA4A Levels in the Brain and TDP-43 Pathology

Recently, disease-associated variants of the TUBA4A gene were identified in patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we present the neuropathological report of a patient with the semantic variant of primary progressive aphasia with a family history of Parkinsonism, harboring a novel frameshift mutation c.187del (p.Arg64Glyfs*90) in TUBA4A. Immunohistochemistry showed abundant TAR DNA-binding protein 43 kDa (TDP-43) dystrophic neurite pathology in the frontal and temporal cortex and the dentate gyrus of the hippocampus, consistent with frontotemporal lobar degeneration (FTLD). The observed pathology pattern fitted best with that of FTLD-TDP Type C. qPCR showed the presence of mutant TUBA4A mRNA. However, no truncated TUBA4A was detected at the protein level. A decrease in total TUBA4A mRNA and protein levels suggests loss-of-function as a potential pathogenic mechanism. This report strengthens the idea that N-terminal TUBA4A mutations are associated with FTLD-TDP. These N-terminal mutations possibly exert their pathogenic effects through haploinsufficiency, contrary to C-terminal TUBA4A mutations which are thought to disturb the microtubule network via a dominant-negative mechanism.

In vivo and in silico investigations of the pegylated gold nanoparticle treatment of amyotrophic lateral sclerosis in mice

Amyotrophic lateral sclerosis (ALS) is a lethal disease that involves the progressive annihilation of motor neurons.

Nearly 30 Years of Animal Models to Study Amyotrophic Lateral Sclerosis: A Historical Overview and Future Perspectives

Amyotrophic lateral sclerosis (ALS) is a fatal, multigenic, multifactorial, and non-cell autonomous neurodegenerative disease characterized by upper and lower motor neuron loss. Several genetic mutations lead to ALS development and many emerging gene mutations have been discovered in recent years. Over the decades since 1990, several animal models have been generated to study ALS pathology including both vertebrates and invertebrates such as yeast, worms, flies, zebrafish, mice, rats, guinea pigs, dogs, and non-human primates. Although these models show different peculiarities, they are all useful and complementary to dissect the pathological mechanisms at the basis of motor neuron degeneration and ALS progression, thus contributing to the development of new promising therapeutics. In this review, we describe the up to date and available ALS genetic animal models, classified by the different genetic mutations and divided per species, pointing out their features in modeling, the onset and progression of the pathology, as well as their specific pathological hallmarks. Moreover, we highlight similarities, differences, advantages, and limitations, aimed at helping the researcher to select the most appropriate experimental animal model, when designing a preclinical ALS study.

Preliminary Evaluations of [11C]Verubulin: Implications for Microtubule Imaging With PET

[¹¹C]Verubulin (a.k.a.[¹¹C]MCP-6827), [¹¹C]HD-800 and [¹¹C]colchicine have been developed for imaging microtubules (MTs) with positron emission tomography (PET). The objective of this work was to conduct an in vivo comparison of [¹¹C]verubulin for MT imaging in mouse and rat brain, as well as an in vitro study with this radiotracer in rodent and human Alzheimer’s Disease tissue. Our preliminary PET imaging studies of [¹¹C]verubulin in rodents revealed contradictory results between mouse and rat brain uptake under pretreatment conditions. In vitro autoradiography with [¹¹C]verubulin showed an unexpected higher uptake in AD patient tissue compared with healthy controls. We also conducted the first comparative in vivo PET imaging study with [¹¹C]verubulin, [¹¹C]HD-800 and [¹¹C]colchicine in a non-human primate. [¹¹C]Verubulin and [¹¹C]HD-800 require pharmacokinetic modeling and quantification studies to understand the role of how these radiotracers bind to MTs before translation to human use.

Novel TUBA4A Variant Associated With Familial Frontotemporal Dementia

OBJECTIVE

Despite the strong genetic component of frontotemporal dementia (FTD), a substantial proportion of patients remain genetically unresolved. We performed an in-depth study of a family with an autosomal dominant form of FTD to investigate the underlying genetic cause.

METHODS

Following clinical and pathologic characterization of the family, genetic studies included haplotype sharing analysis and exome sequencing. Subsequently, we performed immunohistochemistry, immunoblotting, and a microtubule repolymerization assay to investigate the potential impact of the candidate variant in tubulin alpha 4a (TUBA4A).

RESULTS

The clinical presentation in this family is heterogeneous, including behavioral changes, parkinsonian features, and uncharacterized dementia. Neuropathologic examination of 2 patients revealed TAR DNA binding protein 43 (TDP-43) pathology with abundant dystrophic neurites and neuronal intranuclear inclusions, consistent with frontotemporal lobar degeneration-TDP type A. We identified a likely pathogenic variant in TUBA4A segregating with disease. TUBA4A encodes for α-tubulin, which is a major component of the microtubule network. Variants in TUBA4A have been suggested as a rare genetic cause of amyotrophic lateral sclerosis (ALS) and have sporadically been reported in patients with FTD without supporting genetic segregation. A decreased trend of TUBA4A protein abundance was observed in patients compared with controls, and a microtubule repolymerization assay demonstrated disrupted α-tubulin function. As opposed to variants found in ALS, TUBA4A variants associated with FTD appear more localized to the N-terminus, indicating different pathogenic mechanisms.

CONCLUSIONS

Our findings support the role of TUBA4A variants as rare genetic cause of familial FTD.

Amyotrophic Lateral Sclerosis and Frontotemporal Lobar Degenerations: Similarities in Genetic Background

Amyotrophic lateral sclerosis (ALS) is a devastating, uniformly lethal progressive degenerative disorder of motor neurons that overlaps with frontotemporal lobar degeneration (FTLD) clinically, morphologically, and genetically. Although many distinct mutations in various genes are known to cause amyotrophic lateral sclerosis, it remains poorly understood how they selectively impact motor neuron biology and whether they converge on common pathways to cause neuronal degeneration. Many of the gene mutations are in proteins that share similar functions. They can be grouped into those associated with cell axon dynamics and those associated with cellular phagocytic machinery, namely protein aggregation and metabolism, apoptosis, and intracellular nucleic acid transport. Analysis of pathways implicated by mutant ALS genes has provided new insights into the pathogenesis of both familial forms of ALS (fALS) and sporadic forms (sALS), although, regrettably, this has not yet yielded definitive treatments. Many genes play an important role, with TARDBP, SQSTM1, VCP, FUS, TBK1, CHCHD10, and most importantly, C9orf72 being critical genetic players in these neurological disorders. In this mini-review, we will focus on the molecular mechanisms of these two diseases.

Genomic Portrait of a Sporadic Amyotrophic Lateral Sclerosis Case in a Large Spinocerebellar Ataxia Type 1 Family

Background: Repeat expansions in the spinocerebellar ataxia type 1 (SCA1) gene ATXN1 increases the risk for amyotrophic lateral sclerosis (ALS), supporting a relationship between these disorders. We recently reported the co-existence, in a large SCA1 family, of a clinically definite ALS individual bearing an intermediate ATXN1 expansion and SCA1 patients with a full expansion, some of which manifested signs of lower motor neuron involvement. Methods: In this study, we employed a systems biology approach that integrated multiple genomic analyses of the ALS patient and some SCA1 family members. Results: Our analysis identified common and distinctive candidate genes/variants and related biological processes that, in addition to or in combination with ATXN1, may contribute to motor neuron degeneration phenotype. Among these, we distinguished ALS-specific likely pathogenic variants in TAF15 and C9ORF72, two ALS-linked genes involved in the regulation of RNA metabolism, similarly to ATXN1, suggesting a selective role for this pathway in ALS pathogenesis. Conclusions: Overall, our work supports the utility to apply personal genomic information for characterizing complex disease phenotypes.

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

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

Axonal mRNA translation in neurological disorders

It is increasingly recognized that local protein synthesis (LPS) contributes to fundamental aspects of axon biology, in both developing and mature neurons. Mutations in RNA-binding proteins (RBPs), as central players in LPS, and other proteins affecting RNA localization and translation are associated with a range of neurological disorders, suggesting disruption of LPS may be of pathological significance. In this review, we substantiate this hypothesis by examining the link between LPS and key axonal processes, and the implicated pathophysiological consequences of dysregulated LPS. First, we describe how the length and autonomy of axons result in an exceptional reliance on LPS. We next discuss the roles of LPS in maintaining axonal structural and functional polarity and axonal trafficking. We then consider how LPS facilitates the establishment of neuronal connectivity through regulation of axonal branching and pruning, how it mediates axonal survival into adulthood and its involvement in neuronal stress responses.

TUBB4A mutations result in both glial and neuronal degeneration in an H-ABC leukodystrophy mouse model

Mutations in TUBB4A result in a spectrum of leukodystrophy including Hypomyelination with Atrophy of Basal Ganglia and Cerebellum (H-ABC), a rare hypomyelinating leukodystrophy, often associated with a recurring variant p.Asp249Asn (D249N). We have developed a novel knock-in mouse model harboring heterozygous (Tubb4a^D249N/+) and the homozygous (Tubb4a^D249N/D249N) mutation that recapitulate the progressive motor dysfunction with tremor, dystonia and ataxia seen in H-ABC. Tubb4a^D249N/D249N mice have myelination deficits along with dramatic decrease in mature oligodendrocytes and their progenitor cells. Additionally, a significant loss occurs in the cerebellar granular neurons and striatal neurons in Tubb4a^D249N/D249N mice. In vitro studies show decreased survival and dysfunction in microtubule dynamics in neurons from Tubb4a^D249N/D249N mice. Thus Tubb4a^D249N/D249N mice demonstrate the complex cellular physiology of H-ABC, likely due to independent effects on oligodendrocytes, striatal neurons, and cerebellar granule cells in the context of altered microtubule dynamics, with profound neurodevelopmental deficits.

PET Imaging of [11C]MPC-6827, a Microtubule-Based Radiotracer in Non-Human Primate Brains

Dysregulation of microtubules is commonly associated with several psychiatric and neurological disorders, including addiction and Alzheimer's disease. Imaging of microtubules in vivo using positron emission tomography (PET) could provide valuable information on their role in the development of disease pathogenesis and aid in improving therapeutic regimens. We developed [11C]MPC-6827, the first brain-penetrating PET radiotracer to image microtubules in vivo in the mouse brain. The aim of the present study was to assess the reproducibility of [11C]MPC-6827 PET imaging in non-human primate brains. Two dynamic 0-120 min PET/CT imaging scans were performed in each of four healthy male cynomolgus monkeys approximately one week apart. Time activity curves (TACs) and standard uptake values (SUVs) were determined for whole brains and specific regions of the brains and compared between the "test" and "retest" data. [11C]MPC-6827 showed excellent brain uptake with good pharmacokinetics in non-human primate brains, with significant correlation between the test and retest scan data (r = 0.77, p = 0.023). These initial evaluations demonstrate the high translational potential of [11C]MPC-6827 to image microtubules in the brain in vivo in monkey models of neurological and psychiatric diseases.

Mice Carrying ALS Mutant TDP-43, but Not Mutant FUS, Display In Vivo Defects in Axonal Transport of Signaling Endosomes

Amyotrophic lateral sclerosis (ALS) is a fatal, progressive neurodegenerative disease resulting from a complex interplay between genetics and environment. Impairments in axonal transport have been identified in several ALS models, but in vivo evidence remains limited, thus their pathogenetic importance remains to be fully resolved. We therefore analyzed the in vivo dynamics of retrogradely transported, neurotrophin-containing signaling endosomes in nerve axons of two ALS mouse models with mutations in the RNA processing genes TARDBP and FUS. TDP-43M337V mice, which show neuromuscular pathology without motor neuron loss, display axonal transport perturbations manifesting between 1.5 and 3 months and preceding symptom onset. Contrastingly, despite 20% motor neuron loss, transport remained largely unaffected in FusΔ14/+ mice. Deficiencies in retrograde axonal transport of signaling endosomes are therefore not shared by all ALS-linked genes, indicating that there are mechanistic distinctions in the pathogenesis of ALS caused by mutations in different RNA processing genes.

Natural and pathogenic protein sequence variation affecting prion-like domains within and across human proteomes

BACKGROUND

Impaired proteostatic regulation of proteins with prion-like domains (PrLDs) is associated with a variety of human diseases including neurodegenerative disorders, myopathies, and certain forms of cancer. For many of these disorders, current models suggest a prion-like molecular mechanism of disease, whereby proteins aggregate and spread to neighboring cells in an infectious manner. The development of prion prediction algorithms has facilitated the large-scale identification of PrLDs among "reference" proteomes for various organisms. However, the degree to which intraspecies protein sequence diversity influences predicted prion propensity has not been systematically examined.

RESULTS

Here, we explore protein sequence variation introduced at genetic, post-transcriptional, and post-translational levels, and its influence on predicted aggregation propensity for human PrLDs. We find that sequence variation is relatively common among PrLDs and in some cases can result in relatively large differences in predicted prion propensity. Sequence variation introduced at the post-transcriptional level (via alternative splicing) also commonly affects predicted aggregation propensity, often by direct inclusion or exclusion of a PrLD. Finally, analysis of a database of sequence variants associated with human disease reveals a number of mutations within PrLDs that are predicted to increase prion propensity.

CONCLUSIONS

Our analyses expand the list of candidate human PrLDs, quantitatively estimate the effects of sequence variation on the aggregation propensity of PrLDs, and suggest the involvement of prion-like mechanisms in additional human diseases.

Nanoparticle Assisted Remodeling of Proteotoxic SOD1 Mutants Alters the Biointerface of the Functional Interaction of Microtubules and Kinesin Motors

Transport deficits with motor neuron degeneration have been implicated in amyotrophic lateral sclerosis (ALS). We report a biomimetic system composed of microtubules/kinesin motor that mimics the dysregulated motor dynamics of ALS. Pathogenic ALS mutants A4V SOD1 completely shut off motility. Treatment with 5 nm citrate coated gold nanoparticles recovers the impaired motor stepping by remodeling the A4V SOD1 rather than stabilizing microtubules or protein folding. Instead, gold nanoparticles alter the protein by a mechanism that reforms protein elements of A4V SOD1, in turn fixing the aberrant interaction of kinesin with microtubules. Reinstating kinesin motility holds potential for managing debilitating ALS.

LOXL1 folding in exfoliation glaucoma

Exfoliation syndrome (XFS) is an age-related disease defined by the deposition of aggregated fibrous material (XFM) in the peri-cellular space. Principal morbidity occurs in the eye, where XFM accumulates on the anterior ocular tissues. GWAS have found that certain genetic variants of lysyl oxidase-like 1 (LOXL1), a matrix cross-linking enzyme that is required for elastic fiber formation confer risk for the development of XFS, but are not a single causative factor as many genetically affected individuals do not develop XFS or subsequent glaucoma (XFG). We have found that XFG cells display defects in lysosomes, microtubules, autophagy, and mitochondria resembling defects found in cells from age-related syndromes, such as the main neurodegenerative diseases. In the majority of these diseases, the determining cellular factor is a protein containing intrinsically disordered regions (IDRs) and displaying a high propensity for aggregation. We have found that in XFG patient-derived cells, LOXL1 protein is actively subjected to autophagic clearance, suggesting that LOXL1 is undergoing aggregation. In silico analysis demonstrates that LOXL1's first 369 aa constitute an IDR with the highest disorder probability peak centering around the known risk positions. Experimentally, we have found over-expression of either unmodified LOXL1 or fluorescent chimeras preserving the well-structured N-terminus cause copious intracellular aggregation and that aggregation wanes when the high IDR peaks are deleted. Overall, our work suggests that XFS/G results from the aggregation of the LOXL1 protein coupled with a reduction of cellular proteostasis capabilities in aging, resulting in a chronic build-up of LOXL1-containing protein aggregates.

Class I and II histone deacetylase expression is not altered in human amyotrophic lateral sclerosis: Neuropathological and positron emission tomography molecular neuroimaging evidence

INTRODUCTION

There is an unmet need for mechanism-based biomarkers and effective disease modifying treatments in amyotrophic lateral sclerosis (ALS). Previous findings have provided evidence that histone deacetylases (HDAC) are altered in ALS, providing a rationale for testing HDAC inhibitors as a therapeutic option.

METHODS

We measured class I and II HDAC protein and transcript levels together with acetylation levels of downstream substrates by using Western blotting in postmortem tissue of ALS and controls. [¹¹ C]Martinostat, a novel HDAC positron emission tomography ligand, was also used to assess in vivo brain HDAC alterations in patients with ALS and healthy controls (HC).

RESULTS

There was no significant difference in HDAC levels between patients with ALS and controls as measured by Western blotting and reverse-transcription quantitative polymerase chain reaction. Similarly, no differences were detected in [¹¹ C]Martinostat-positron emission tomography uptake in ALS participants compared with HCs.

DISCUSSION

These findings provide evidence that alterations in HDAC isoforms are not a dominant pathological feature at the bulk tissue level in ALS.

Influence of selenium on the emergence of neuro tubule defects in a neuron-like cell line and its implications for amyotrophic lateral sclerosis

Impairment of the axonal transport system mediated by intracellular microtubules (MTs) is known to be a major drawback in neurodegenerative processes. Due to a growing interest on the neurotoxic effects of selenium in environmental health, our study aimed to assess the relationship between selenium and MTs perturbation, that may favour disease onset over a genetic predisposition to amyotrophic lateral sclerosis. We treated a neuron-like cell line with sodium selenite, sodium selenate and seleno-methionine and observed that the whole cytoskeleton was affected. We then investigated the protein interactome of cells overexpressing αTubulin-4A (TUBA4A) and found that selenium increases the interaction of TUBA4A with DNA- and RNA-binding proteins. TUBA4A ubiquitination and glutathionylation were also observed, possibly due to a selenium-dependent increase of ROS, leading to perturbation and degradation of MTs. Remarkably, the TUBA4A mutants R320C and A383 T, previously described in ALS patients, showed the same post-translational modifications to a similar extent. In conclusion this study gives insights into a specific mechanism characterizing selenium neurotoxicity.

TDP-43 enhances translation of specific mRNAs linked to neurodegenerative disease

The RNA-binding protein TDP-43 is heavily implicated in neurodegenerative disease. Numerous patient mutations in TARDBP, the gene encoding TDP-43, combined with data from animal and cell-based models, imply that altered RNA regulation by TDP-43 causes Amyotrophic Lateral Sclerosis and Frontotemporal Dementia. However, underlying mechanisms remain unresolved. Increased cytoplasmic TDP-43 levels in diseased neurons suggest a possible role in this cellular compartment. Here, we examined the impact on translation of overexpressing human TDP-43 and the TDP-43A315T patient mutant protein in motor neuron-like cells and primary cultures of cortical neurons. In motor-neuron like cells, TDP-43 associates with ribosomes without significantly affecting global translation. However, ribosome profiling and additional assays revealed enhanced translation and direct binding of Camta1, Mig12, and Dennd4a mRNAs. Overexpressing either wild-type TDP-43 or TDP-43A315T stimulated translation of Camta1 and Mig12 mRNAs via their 5'UTRs and increased CAMTA1 and MIG12 protein levels. In contrast, translational enhancement of Dennd4a mRNA required a specific 3'UTR region and was specifically observed with the TDP-43A315T patient mutant allele. Our data reveal that TDP-43 can function as an mRNA-specific translational enhancer. Moreover, since CAMTA1 and DENND4A are linked to neurodegeneration, they suggest that this function could contribute to disease.

A Systematic Review of Suggested Molecular Strata, Biomarkers and Their Tissue Sources in ALS

Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease, is an incurable neurodegenerative condition, characterized by the loss of upper and lower motor neurons. It affects 1–1.8/100,000 individuals worldwide, and the number of cases is projected to increase as the population ages. Thus, there is an urgent need to identify both therapeutic targets and disease-specific biomarkers–biomarkers that would be useful to diagnose and stratify patients into different sub-groups for therapeutic strategies, as well as biomarkers to follow the efficacy of any treatment tested during clinical trials. There is a lack of knowledge about pathogenesis and many hypotheses. Numerous “omics” studies have been conducted on ALS in the past decade to identify a disease-signature in tissues and circulating biomarkers. The first goal of the present review was to group the molecular pathways that have been implicated in monogenic forms of ALS, to enable the description of patient strata corresponding to each pathway grouping. This strategy allowed us to suggest 14 strata, each potentially targetable by different pharmacological strategies. The second goal of this review was to identify diagnostic/prognostic biomarker candidates consistently observed across the literature. For this purpose, we explore previous biomarker-relevant “omics” studies of ALS and summarize their findings, focusing on potential circulating biomarker candidates. We systematically review 118 papers on biomarkers published during the last decade. Several candidate markers were consistently shared across the results of different studies in either cerebrospinal fluid (CSF) or blood (leukocyte or serum/plasma). Although these candidates still need to be validated in a systematic manner, we suggest the use of combinations of biomarkers that would likely reflect the “health status” of different tissues, including motor neuron health (e.g., pNFH and NF-L, cystatin C, Transthyretin), inflammation status (e.g., MCP-1, miR451), muscle health (miR-338-3p, miR-206) and metabolism (homocysteine, glutamate, cholesterol). In light of these studies and because ALS is increasingly perceived as a multi-system disease, the identification of a panel of biomarkers that accurately reflect features of pathology is a priority, not only for diagnostic purposes but also for prognostic or predictive applications.

GM604 regulates developmental neurogenesis pathways and the expression of genes associated with amyotrophic lateral sclerosis

Background

Amyotrophic lateral sclerosis (ALS) is currently an incurable disease without highly effective pharmacological treatments. The peptide drug GM604 (GM6 or Alirinetide) was developed as a candidate ALS therapy, which has demonstrated safety and good drug-like properties with a favorable pharmacokinetic profile. GM6 is hypothesized to bolster neuron survival through the multi-target regulation of developmental pathways, but mechanisms of action are not fully understood.

Methods

This study used RNA-seq to evaluate transcriptome responses in SH-SY5Y neuroblastoma cells following GM6 treatment (6, 24 and 48 h).

Results

We identified 2867 protein-coding genes with expression significantly altered by GM6 (FDR < 0.10). Early (6 h) responses included up-regulation of Notch and hedgehog signaling components, with increased expression of developmental genes mediating neurogenesis and axon growth. Prolonged GM6 treatment (24 and 48 h) altered the expression of genes contributing to cell adhesion and the extracellular matrix. GM6 further down-regulated the expression of genes associated with mitochondria, inflammatory responses, mRNA processing and chromatin organization. GM6-increased genes were located near GC-rich motifs interacting with C2H2 zinc finger transcription factors, whereas GM6-decreased genes were located near AT-rich motifs associated with helix-turn-helix homeodomain factors. Such motifs interacted with a diverse network of transcription factors encoded by GM6-regulated genes (STAT3, HOXD11, HES7, GLI1). We identified 77 ALS-associated genes with expression significantly altered by GM6 treatment (FDR < 0.10), which were known to function in neurogenesis, axon guidance and the intrinsic apoptosis pathway.

Conclusions

Our findings support the hypothesis that GM6 acts through developmental-stage pathways to influence neuron survival. Gene expression responses were consistent with neurotrophic effects, ECM modulation, and activation of the Notch and hedgehog neurodevelopmental pathways. This multifaceted mechanism of action is unique among existing ALS drug candidates and may be applicable to multiple neurodegenerative diseases.

Electronic supplementary material

The online version of this article (10.1186/s40035-018-0135-7) contains supplementary material, which is available to authorized users.

A De Novo RAPGEF2 Variant Identified in a Sporadic Amyotrophic Lateral Sclerosis Patient Impairs Microtubule Stability and Axonal Mitochondria Distribution

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that is frequently linked to microtubule abnormalities and mitochondrial trafficking defects. Whole exome sequencing (WES) of patient-parent trios has proven to be an efficient strategy for identifying rare de novo genetic variants responsible for sporadic ALS (sALS). Using a trio-WES approach, we identified a de novo RAPGEF2 variant (c.4069G>A, p.E1357K) in a patient with early-onset sALS. To assess the pathogenic effects of this variant, we have used patient-derived skin fibroblasts and motor neuron-specific overexpression of the RAPGEF2-E1357K mutant protein in Drosophila. Patient fibroblasts display reduced microtubule stability and defective microtubule network morphology. The intracellular distribution, ultrastructure, and function of mitochondria are also impaired in patient cells. Overexpression of the RAPGEF2 mutant in Drosophila motor neurons reduces the stability of axonal microtubules and disrupts the distribution of mitochondria to distal axons and neuromuscular junction (NMJ) synapses. We also show that the recruitment of the pro-apoptotic protein BCL2-associated X (BAX) to mitochondria is significantly increased in patient fibroblasts compared with control cells. Finally, increasing microtubule stability through pharmacological inhibition of histone deacetylase 6 (HDAC6) rescues defects in the intracellular distribution of mitochondria and BAX. Overall, our data suggest that the RAPGEF2 variant identified in this study can drive ALS-related pathogenic effects through microtubule dysregulation.

Maintenance mechanisms of circuit-integrated axons

Adult, circuit-integrated neurons must be maintained and supported for the life span of their host. The attenuation of either maintenance or plasticity leads to impaired circuit function and ultimately to neurodegenerative disorders. Over the last few years, significant discoveries of molecular mechanisms were made that mediate the formation and maintenance of axons. Here, we highlight intrinsic and extrinsic mechanisms that ensure the health and survival of axons. We also briefly discuss examples of mutations associated with impaired axonal maintenance identified in specific neurological conditions. A better understanding of these mechanisms will therefore help to define targets for therapeutic interventions.

Radiosynthesis and Evaluation of [11C]HD-800, a High Affinity Brain Penetrant PET Tracer for Imaging Microtubules

Microtubules (MTs) are highly abundant throughout the cytoskeleton, and their dysfunction is implicated in the pathogenesis of malignancies, various neurodegenerative disorders, and brain injuries. Validated radiotracers reported so far for MTs are [¹¹C]paclitaxel, [¹⁸F]fluoropaclitaxel, and [¹¹C]docetaxel; however, they are well-characterized substrates of efflux transporters and consequently have poor uptake into the brain due to minimal blood brain barrier (BBB) penetration. PET imaging of MT expression requires radiolabeled BBB penetrating MT ligands, and it may offer a direct and more sensitive approach for early diagnosis, monitoring disease progression, and treatment effects in brain diseases and assessing the clinical potential of targeted therapeutics and treatments. We have identified N-(4-methoxyphenyl)-N-5-dimethylfuro[2,3-d]pyrimidin-4-amine (HD-800) as a high affinity and selective colchicine site tubuline inhibitor amenable to radiolabel with C-11, a positron emitting isotope. HD-800 and desmethyl-HD-800 were synthesized in one step with 75% and 80% yields respectively from commercial synthons. The radiosynthesis of [¹¹C]HD-800 was achieved in 45 ± 5% yield at EOS. Ex vivo biodistribution binding data of [¹¹C]HD-800 indicate that the radioligand penetrated the BBB and it was retained in brain with 75% specific binding. Apart from the brain, specific binding was observed in muscle (55%), heart (50%), lungs (43%), blood (37%), and pancreas (30%). MicroPET imaging in mice showed excellent binding in brain that was blocked by preadministration of unlabeled HD-800 and a colchicine site binding MT ligand MPC-6827. The above results indicate that [¹¹C]HD-800 may be a suitable PET ligand for the in vivo quantification of MT inside and outside the brain.

Epothilone D accelerates disease progression in the SOD1G93A mouse model of amyotrophic lateral sclerosis

AIMS

Degeneration of the distal neuromuscular circuitry is a hallmark pathology of Amyotrophic Lateral Sclerosis (ALS). The potential for microtubule dysfunction to be a critical pathophysiological mechanism in the destruction of this circuitry is increasingly being appreciated. Stabilization of microtubules to improve neuronal integrity and pathology has been shown to be a particularly favourable approach in other neurodegenerative diseases. We present evidence here that treatment with the microtubule-targeting compound Epothilone D (EpoD) both positively and negatively affects the spinal neuromuscular circuitry in the SOD1^G93A mouse model of ALS.

METHODS

SOD1^G93A mice were treated every 5 days with 2 mg/kg EpoD. Evaluation of motor behaviour, neurological phenotype and survival was completed, with age-dependent histological characterization also conducted, using the thy1-YFP mouse. Motor neuron degeneration, axonal integrity, neuromuscular junction (NMJ) health and gliosis were also assessed.

RESULTS

EpoD treatment prevented loss of the spinal motor neuron soma, and distal axon degeneration, early in the disease course. This, however, was not associated with protection of the NMJ synapse and did not improve motor phenotype or clinical progression. EpoD administration was also found to be neurotoxic at later disease stages. This was evidenced by accelerated motor neuron cell body loss, increasing gliosis, and was associated with detrimental outcomes to motor behaviour, clinical assessment and survival.

CONCLUSIONS

The results suggest that EpoD accelerates disease progression in the SOD1^G93A mouse model of ALS, and highlights that the pathophysiological involvement of microtubules in ALS is an evolving and underappreciated phenomenon.

Radiosynthesis and in Vivo Evaluation of [11C]MPC-6827, the First Brain Penetrant Microtubule PET Ligand

Abnormalities of microtubules (MTs) are implicated in the pathogenesis of many CNS diseases. Despite the potential of an MT imaging agents, no PET ligand is currently available for in vivo imaging of MTs in the brain. We radiolabeled [¹¹C]MPC-6827, a high affinity MTA, and demonstrated its specific binding in rat and mice brain using PET imaging. Our experiments show that [¹¹C]MPC-6827 has specific binding to MT in brain, and it is the first MT-binding PET ligand.

Profilin Directly Promotes Microtubule Growth through Residues Mutated in Amyotrophic Lateral Sclerosis

Profilin is an abundant actin monomer-binding protein with critical actin regulatory roles in vivo [1, 2]. However, profilin also influences microtubule dynamics in cells, which may be mediated in part through its interactions with formins that in turn bind microtubules [3, 4]. Specific residues on human profilin-1 (PFN1) are mutated in patients with amyotrophic lateral sclerosis (ALS) [5, 6]. However, the observation that some ALS-linked PFN1 mutants fail to alter cellular actin organization or dynamics [5-8] or in vitro actin-monomer affinity [9] has been perplexing, given that profilin is best understood as an actin regulator. Here, we investigated direct effects of profilin on microtubule dynamics and whether ALS-linked mutations in PFN1 disrupt such functions. We found that human, fly, and yeast profilin homologs all directly enhance microtubule growth rate by several-fold in vitro. Microtubule stimulatory effects were unaffected by mutations in the canonical actin- or poly-proline-binding sites of profilin. Instead, microtubule activities depended on specific surface residues on profilin mutated in ALS patients. Furthermore, microtubule effects were attenuated by increasing concentrations of actin monomers, suggesting competition between actin and microtubules for binding profilin. Consistent with these biochemical observations, a 2-fold increase in the expression level of wild-type PFN1, but not the ALS-linked PFN1 mutants, increased microtubule growth rates in cells. Together, these results demonstrate that profilin directly enhances the growth rate of microtubules. They further suggest that ALS-linked mutations in PFN1 may perturb cellular microtubule dynamics and/or the coordination between the actin and microtubule cytoskeletons, leading to motor neuron degeneration.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).